REVIEWS

Engineering Aspects of Industrial Liquid-Phase Air Oxidation ofHydrocarbons

Akkihebbal Krishnamurthy SureshDepartment of Chemical Engineering, IIT Bombay, Powai, Mumbai 400 076, India

Man Mohan Sharma† and Tamarapu Sridhar*Department of Chemical Engineering, Monash University, Clayton, Victoria 3168, Australia

Liquid-phase air oxidation of hydrocarbons, notably p-xylene, cumene, ethylbenzene/isobutane,cyclohexane, and n-butane, is of great scientific, technological, and commercial importance. Thisstate-of-the-art paper covers the chemistry and engineering science aspects of these reactions.The role of uncatalyzed reactions and metal ion and mixed metal ion catalysts with bromideactivation is discussed. An analysis is presented for the role of mass transfer in influencing therate of reaction and selectivity for the desired product. Different types of reactors that are used,notably bubble-column reactors and mechanically agitated reactors, are analyzed, and a simplebasis is provided for selection of reactors. Some emerging oxidation systems, notably oxidationof cycloalkenes (cyclohexene/cyclooctene/cyclododecene) and oxidation of isobutane under su-percritical conditions, are presented. New strategies for conducting air oxidations, such as inbiphasic systems (including fluorous biphasic systems), biocatalysis, photocatalysis, etc., areemerging and illustrate the considerable tailoring of the reaction microenvironment that isbecoming possible. In some cases, it may be possible to manipulate chemo-, regio-, and enantio-selectivity in these reactions.

Contents

1. Introduction 39592. Scope and Structure of the Review 39593. Historical Development 39604. A Survey of Industrial

Hydrocarbon Oxidations3963

4.1. p-Xylene and Other MCOxidations

3964

4.2. Oxidation of Cyclohexane, OtherSaturated Hydrocarbons, andTerpenes

3965

4.3. LPO-Based Routes to Phenoland Benzylic Alcohols

3967

4.4. The Oxirane Process: Oxidationof iso-Butane and Ethylbenzene

3967

4.5. Acetic Acid from ParaffinOxidation

3968

5. Mechanism of HydrocarbonOxidation

3969

5.1. Initiation 39695.2. Propagation 39695.3. Termination 39705.4. Degenerate Chain Branching 39705.5. Overall Kinetic Features Based

on the Mechanism3971

5.6. Catalysis of Organic Oxidations 39715.7. Co-oxidations 3972

6. Chemistry of Selected Oxidations 3973

6.1. p-Xylene and Other MCOxidations

3973

6.2. Oxidation of Cyclohexane 39746.3. Oxidation of Cumene 39746.4. Oxidation of Isobutane 39756.5. Oxidation of Cycloalkenes 39776.6. Oxidation of Vinyl Cyclohexene

and Vinyl Cyclohexane3979

6.7. Miscellaneous Oxidations 39797. Kinetics of Hydrocarbon Oxidation 3979

7.1. Laboratory Reactors 39807.2. Kinetic Models from Laboratory

Studies3981

8. Processing Options 39828.1. The Case for Liquid-Phase Air

Oxidation3982

8.2. Reactor Configurations andMaterials

3983

9. Role of Mass Transfer inLiquid-Phase Oxidations

3984

9.1. Mass-Transfer Rates at ElevatedTemperatures and Pressuresand under Actual OxidationConditions

3984

9.2. Influence of Mass Transfer onLiquid-Phase Oxidations

3985

10. Rate Oscillations and OtherNonlinear Phenomena

3986

3958 Ind. Eng. Chem. Res. 2000, 39, 3958-3997

10.1021/ie0002733 CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 10/11/2000

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11. Safety Issues in Organic Oxidations 398712. New Developments in Organic

Oxidations3988

12.1. Biphasic Mode of Operation 398812.2. The Role of Ultrasound in

Oxidation Reactions3989

12.3. Oxidation in SupercriticalMedia

3989

12.4. Photochemical Activation ofOxidation Reactions

3990

12.5. Enzyme-Catalyzed Reactions 399012.6. Use of O2 + H2 and O2 + CO as

Oxidants3991

12.7. Catalyst Developments 399112.8. Stereospecificity in Organic

Oxidations3992

13. Conclusions 3992References 3993

1. Introduction

Processes involving the oxidation of hydrocarbons inthe liquid phase, using air or oxygen, are of greatimportance to industrialized economies because of theirrole in converting petroleum hydrocarbon feedstockssuch as alkanes, olefins, and aromatics into industrialorganic chemicals important in the polymer and petro-chemical industries. Some notable examples are theoxidations of p-xylene to terephthalic acid and dimethylterephthalate, of cyclohexane to cyclohexyl hydroper-oxide and cyclohexanol/cyclohexanone, of cumene tocumene hydroperoxide, and of iso-butane to tert-butylhydroperoxide and tert-butyl alcohol. Some of the majorinefficiencies in the production of such chemicals canoften be traced to the operation of the reactor. Oxidativetransformations of functional groups are also basic toorganic chemistry and are used extensively in thelaboratory and industrial synthesis of a variety of fineorganic chemicals. It is therefore not surprising that thisclass of reactions has spawned much study and re-search. In recent times, increasing environmental con-cerns have also acted as a significant driving force ofresearch. The mechanisms that operate in organicoxidations and the theoretical underpinnings of suchmechanisms have thus been studied in detail, andextensive treatises on the chemistry of oxidation pro-cesses are available.1-3 What is perhaps surprising isthat, barring a few exceptions,4,5 an exhaustive reviewof the engineering aspects of such reactions is lacking.Information about these aspects is still largely to befound scattered in various places, amidst discussionsof specific processes. The main objective of the presentpaper is therefore to bring together and discuss, in aunified manner, the literature on hydrocarbon oxida-tions, with a strong emphasis on the engineering aspectsof relevance to industrial practice. The last few decadeshave seen several advances in this area, much of it onlyavailable in patents. A second objective of this paper

is, therefore, to review these developments and analyzetheir implications for future industrial practice in thisarea.

2. Scope and Structure of the Review

The review addresses hydrocarbon “autoxidations”,that is, oxidations in which molecular oxygen is acti-vated via a free-radical chain process. These are alsoclassified as “homolytic” processes,4 insofar as theyoriginate from the homolytic breakage of a C-H bond.Thus, those processes (“heterolytic”) in which oxygenactivation occurs by direct reaction with a reducingmetal ion (the Wacker process is an example) are largelyexcluded from the scope of this review. As alreadystated, the emphasis is on the engineering aspects, anddiscussion of the mechanistic aspects is kept brief.Although the attempt is to study hydrocarbon oxidationsin a unified manner, individual peculiarities are oftenimportant to industrial practice, and these are high-lighted where relevant. Examples from various hydro-carbon oxidations are liberally used, but the commer-cially important oxidations form the center of attention.The commonality of mechanisms and industrial impor-tance both make for an emphasis on processes involvingthe oxidation of secondary and tertiary carbon atoms.Within this perimeter, both catalyzed and uncatalyzedoxidations are discussed. Air has tended to be thepreferred source of oxygen in the past for reasons of bothsafety and economics, and the focus here is naturallyon air oxidations. However, some recent developmentson the use of enriched air or even pure oxygen areincluded as they could be of interest to future designs.Even though gas- and liquid-phase homolytic oxidationsshare common mechanisms, the reaction rate andselectivity are usually better in the case of liquid-phaseoxidations. The reasons, at least partly, have to do withthe higher density of the liquid phase. Hence, except inthe case of lighter hydrocarbons for which very highpressures become necessary to maintain a liquid phaseat reaction temperatures, oxidation in the liquid phaseturns out to be the natural choice. Isobutane presentsan interesting case for which a minor shift in reactionconditions can make the reaction occur under gas orliquid or supercritical conditions. This case is consideredin some detail, but for the rest, it is the liquid-phaseoxidations that form the focus here. Finally, emphasishas been placed on the developments of the last twodecades or so, and references are made to earlierliterature wherever relevant.

Historically, a fundamental understanding of thenature and mechanisms of oxidation reactions hasproceeded in parallel with industrial practice, as hasbeen the case with so many other areas of humanendeavor. We begin our discussion of hydrocarbonoxidations, with a brief historical account in section 3.This is followed, in section 4, by a broad survey of thefield of industrial oxidations, which provides the back-drop for further discussion.

Because oxidation of hydrocarbons ultimately givescarbon dioxide and water, it is clear that it is partialoxidations that are of greatest interest to the industry,and the success of an industrial oxidation processdepends on proper control of the reaction to yield thedesired intermediates with reasonable selectivities. Thecomplex chemistry of hydrocarbon oxidations leads toa multiplicity of products even at fairly early stages inthe conversion. An understanding of the chemistry is,

* Author to whom correspondence should be addressed.Tel: 613 9905 3427. Fax: 613 9905 9649. E-mail: tam.sridhar@eng.monash.edu.au.

† Permanent address: 502, “Saurabh”, Plot 39, SwastikPark, Chembur, Munibai 400 071, India.

Ind. Eng. Chem. Res., Vol. 39, No. 11, 2000 3959

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therefore, essential if one is to appreciate the motivationbehind much of the research and development in thisarea. Section 5 describes the salient aspects of themechanisms that operate in hydrocarbon oxidations.Over the years, several catalysts have found applicationin such reactions for various reasons, and section 5includes a discussion of the role of particular catalystsin modifying the reaction mechanism.

Notwithstanding the unity of the mechanistic prin-ciples that underlie hydrocarbon oxidations, it is oftenthe minor differences among them that are of interestfrom the point of view of industrial exploitation. Thus,whereas one expects from chemistry that the oxidationof an aldehyde (which occurs as an intermediate inoxidations leading to carboxylic acid) should be rapid,unoxidized aldehydic impurities are a major problemin terephthalic acid manufacture, because the productspecifications for fiber-grade terephthalic acid are verystringent. Again, in the oxidation of cymene, the smallextent of oxidation of the secondary C atom that occursin parallel with the desired oxidation of the tertiary Catom has important implications for downstream pro-cessing. (The oxidation of ethylbenzene provides anothersimilar example.) The section on general mechanismsis, therefore, followed by a section on the chemistry ofcommercially important oxidations. This section alsotreats some important emerging oxidation systems suchas cycloalkenes and vinylcyclohexane/vinylcyclohexene.

Given the complexity of the chemistry involved inhydrocarbon oxidation, and the fact that one often mustestimate the kinetics from rate data in heterogeneous(gas-liquid) systems, the planning and execution oflaboratory studies to establish true kinetics is usuallya demanding task. Furthermore, the engineer mustdecide the level of detail at which he needs to establishthe kinetics for the purpose at hand. Thus, the engineeris often forced to resort to an empirical approach toestablish kinetics, with some basis from the knownmechanisms. One therefore sees, in the literature,several lumped kinetic models for oxidation systems.These models and the issues involved in planning andexecuting kinetic investigations in this area are dis-cussed in section 7.

In many hydrocarbon oxidations, the desired inter-mediates have a tendency to undergo further reactionsin the oxidizing medium. The limitations imposed bythe chemistry on the selectivity to the desired interme-diates that is achieved has often meant that conversionsare kept low to minimize the formation of unwantedproducts. The need to achieve better selectivity atreasonable conversions has naturally been a majordriving force for research. This, coupled with otherissues such as safety and the need for control, presentsa number of process options. Some of the issues involvedand the processing options that emerge are consideredin section 8.

The high temperatures at which hydrocarbon oxida-tions are carried out and the need to maintain a liquidphase under these conditions results in high-pressureoperation. Furthermore, the oxygen for the reactionmust be supplied by a process of gas-liquid masstransfer. Thus, the oxidation reactors are usually high-pressure gas-liquid contactors such as bubble columnsand sparged, agitated reactors. Rational design of suchequipment requires a knowledge, under conditions ofpressure and temperature, of important mass-transfer-related quantities such as mass-tranfer coefficients, gas

holdup, and interfacial area. Available information onthese aspects is summarized in section 9.

The kinetics of gas-liquid reactions such as hydro-carbon oxidations are, in general, subject to mass-tranfer limitations. Under appropriate conditions, therate as well as the selectivity of such reactions can beinfluenced by mass transfer. Theories of mass transferwith chemical reaction have been fairly well developed,but there are only a few instances of their applicationto oxidation reactions. These studies are summarizedin section 9.2.

The complexity of chemical mechanisms in organicoxidations leads to highly nonlinear kinetics, which, inturn, often result in complex dynamic behavior. Manyfeatures of such behavior have been documented inexperimental studies. The available information on suchaspects is summarized in section 10.

Safety is clearly a major consideration in hydrocarbon-air contact, both in the design of industrial processesand in the planning of experimental research. Apartfrom hazardous possibilities of gas-phase oxidation suchas autoignition, the dangers of ignition from an externalsource also deserve attention. Additional considerationsassume importance in the design and operation ofexperimental rigs, which are usually designed so as toprovide flexibility in operation and choice of operatingparameters. The relevant aspects of this issue areelaborated in section 11.

The usual motivating factors such as a search forbetter rates and selectivities, together with the recentemphasis on cleaner and safer processes, has meant thatnew processes and processing alternatives are alwayson the horizon. Section 12 discusses some major newdevelopments in the area of organic oxidations, whichhold promise for further development. Alternative ca-talysis and heterogeneous strategies (whether for ca-talysis or for simultaneous separation of reaction inter-mediates) are among the important themes. Much ofthis progress has appeared in the patent literature.

Salient conclusions on the state of our understandingof industrial liquid-phase oxidations are summarizcd insection 13.

3. Historical Development

History shows that the major developments in hy-drocarbon oxidations have most often been motivatedby the need for appropriate feedstocks for the ever-growing polymer industry. In the following section, weseek to examine the development of the more practicalaspects of technology, on one hand, and the evolutionof theoretical insights, on the other, in this field.Although this historical account does not claim to beexhaustive, it does show the complementary manner inwhich advances along the two fronts have been made,and it serves to highlight areas in which understandingstill lags behind practice.

From early days, the functionalization of naturallyoccurring petroleum components through reaction withair was naturally seen as the simplest way to deriveuseful chemicals. The research of Semenov (and laterBolland and others) in the first half of the twentiethcentury clarified the concepts of chain reactions and putthe theory of free-radical autoxidations on a firmbasis.1,3 Industrial practice also developed alongside.Whereas early processes for the oxidative transforma-tion of petroleum feedstocks involved vapor-phase oxi-dations (ethylene to ethylene oxide, for example), liquid-

3960 Ind. Eng. Chem. Res., Vol. 39, No. 11, 2000

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phase oxidation processes began from the 1950s. TheWacker process for the conversion of terminal olefinsto carbonyl compounds and the Hock process for theproduction of phenol from cumene (via the hydroperox-ide) were commercialized during this period. Thecumene-phenol process was developed, by Distillers Co.in the U.K. and Hercules Powder Co. in the U.S., froma reaction discovered by Hock and Lang during the war.(A parallel development by A. Farkas in the Wilmingtonresearch labs of Union Oil Co., which went unpublishedbecause of the company’s interest in naphthenic hydro-peroxides, is described by Farkas6.) At that time, thefree-radical chemistry of such reactions had just beenestablished. Several major contributions to that field,such as the concept of activation of a tertiary H atomby a phenyl ring (which leads to the specificity ofoxidation in this process) and an understanding of therole of impurities, came during the process-developmenteffort.7 Several developments of an engineering naturealso played a part in the final commercialization, suchas the improvement in rate and selectivity throughstaging of reactors and the imposition of a temperatureprofile (with the temperature decreasing as the hydro-peroxide concentration increases) on the reactor cascade.The cumene-phenol process accounts for most of theinstalled capacity for phenol in the world today and isexpected to maintain its dominant position as long asthe coproduct acetone has a market.8

Homogeneous catalysis of liquid-phase oxidations hasplayed a major role in the development of oxidationprocesses. The catalytic effect of transition metal ionssuch as cobalt on oxidations had been known for sometime (for example by the Du Pont work on the oxidationof toluene9). Whereas catalysis is important to theindustrial process, adventitious catalysis, say by thereactor wall material and impurities in the chemicalsused, can be a major problem in the understanding andinterpretation of reaction kinetics. Twigg7 describeshow, in the kinetic studies leading to the processdevelopment for phenol, the most elaborate purificationprocedures for cumene had to be followed in order toavoid adventitious catalysis. The role of wall catalysishad been recognized in the Russian work on cyclohexaneoxidation,10 but the need for passivating reactor surfacesin kinetic work seems to have been recognized andprocedures developed in the work of Winkler andHearne11 on isobutane oxidation. Wall catalysis remainsa problem with the interpretation of many kineticstudies reported in the literature and is especially aproblem in laboratory reactors with large surface-to-volume ratios.

Systematic studies of the industrially importanthomogeneous catalysis of oxidation processes began inthe 1950s.2 The discovery of the (appropriately named)“Mid-Century” catalysts and the subsequent develop-ment of the Mid-Century (MC) process for the oxidationof p-xylene to terephthalic acid belong to this period.12

The impetus for the discovery of the Mid-Centurycatalyst was provided by the need for a cheap source ofaromatic acids for the industrial production of aromaticpolyesters, in particular, poly(ethylene terephthalate)(PET). In searching for an alternative to esterification(Witten process of 1951) for overcoming the inhibitingeffect of the first carboxyl group on further oxidation,Landau and co-workers at Scientific Design discoveredthe promoting effect of the bromide anion13 and im-mediately obtained qualitative improvements in yield

over the norm using cobalt catalyst alone. Standard Oilof Indiana (later Amoco) purchased the patent rightsto the new catalyst and proceeded to develop theoxidation technology. Landau14 has recently given afascinating account of this development. In the subse-quent decades, many aspects of the action of Mid-Century catalysts have been clarified, and the principlehas been extended to over 200 other aromatic, alkylaro-matic, and other systems.12 In particular, improvementsin the technology for purification have today put PureTerephthalic Acid (PTA) in a dominant position in theworld market as the preferred raw material for PETfiber.8

The processes for the oxidation of cyclohexane, likethose for the oxidation of p-xylene, were driven bydevelopments in the synthetic fiber industry, as rawmaterials for the manufacture of Nylon-6 and Nylon-6,6 (caprolactam, adipic acid, and hexamethylenedi-amine) are derived from this route. van de Moesdijk15

has given an interesting account of the development ofcompeting processes for caprolactam up to the late1970s. Caprolactam came into prominence in the late1930s when I. G. Farben synthesized a spinnablepolymer (Nylon-6 or Perlon) from it. Early processes(1940s) for the manufacture of caprolactam were basedon the hydrogenation of phenol [Allied Signal (U.S.) andDSM are still users of this route]: phenol f cyclohex-anone f cyclohexanone oxime f caprolactam. Capro-lactam production expanded in Europe after the war,when the I. G. Farben process was freed from patentrestrictions. In view of long-range forecasts for capro-lactam, (former) Dutch State Mines (now DSM) startedresearch on the cyclohexane oxidation route in the 1950seven though the company was operating phenol-basedplants. By 1959, the company had the designs of a plantready, and the DSM caprolactam processes have sincebeen licensed widely by Stamicarbon, a subsidiary ofDSM. Other companies in Europe and the U.S. (Inventa,BASF, Snia Viscosa, Du Pont, Rhone Poulenc, NitrogenWorks in Poland, etc.) have also been active in this areaand have developed their own processes.

Even prior to the advent of Perlon in Europe,Carothers at Du Pont had produced the condensationpolymer of adipic acid and hexamethylenediamine,which he called “Nylon”. In the late 1930s, the knowhowfor Perlon and Nylon were exchanged between Du Pontand I. G. Farben, and industrial development of bothwere undertaken. Du Pont’s original development of acommercial adipic acid process dates from 1937. Thebasis for the industrially applied two-step process, withthe oxidation of cyclohexane to cyclohexanol and cyclo-hexanone as the first step, was laid sometime duringthe war.16 A single-step process from cyclohexane toadipic acid makes obvious economic sense, and severalattempts at developing such a process have been made.Asahi17,18 and Gulf research19 seem to have arrived ata feasible process using activated cobalt catalyst in anacetic acid medium, but these processes have not beencommercialized. Patent activity has, however, been rifein the area. PPC ventures have developed a commercialroute directly from cyclohexane to adipic acid up to pilotscale (Fluor Daniel offers this process). Also, Twenty-first century research corporation is very active in thisarea and has filed as many as 17 patents recently (forexample, see Vassiliou et al.20 and Dassel et al.21).Today, practically the entire world output of adipic acidis via the liquid-phase oxidation (LPO) of cyclohexane

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by air, followed by the further oxidation of the productsusing nitric acid. Air oxidation for the second step hasnot found favor because of loss of selectivity andproliferation of secondary products.16

The oxidation of cyclohexane must be one of the leastefficient of all major industrial chemical processes.Methods of improving the selectivity have naturallybeen the focus of research effort over the decades. Adevelopment that led to a technology and several processplants in the U.S., Japan, and Europe in its wake wasthe application, by Scientific Design, of Bashkirov’smethod of using boric acid to convert the alcohol thatforms from the hydroperoxide directly to the boric acidester of the alcohol to prevent the overoxidation of thealcohol.22 Although this process is still in use by Bayerin cyclohexane oxidation, the boric acid variant of theoxidation process seems to have largely given way toother processes in the following decades, mainly becauseof the costs of handling the borate ester adducts andrecycling the boric acid.23 Although the Flixboroughdisaster of 1974 opened up important questions ofsafety24 in cyclohexane oxidation processes, the routecontinues to occupy a dominant position today in themanufacture of the raw materials for nylon (bothNylon-6 and Nylon-6,6),3 and research interest in theprocess continues unabated (see, for example, Schuc-hardt et al.25). Recent developments in the selectivehydrogenation of benzene to cyclohexene, however, holdthe promise of a much more economical route to cyclo-hexanol and would seem to have the potential ofdisplacing the direct oxidation of cyclohexane. Althoughthis new route to adipic acid has been commercializedby Asahi, the process has not yet been applied outsideAsahi.

Yet another illustration of the link between polymerfeedstocks and organic oxidation is the development ofthe oxirane process, in the late 1960s, for the oxidationof propylene to propylene oxide. This process, developedby Atlantic Richfield (ARCO), uses an interesting co-oxidation principle to oxidize propylene in an indirectmanner. Oxygen is incorporated into the propylenemolecule by reaction with a hydroperoxide, which itselfis produced by a liquid-phase air oxidation step from aconveniently oxidized substance such as isobutane orethylbenzene. The features of co-oxidation are a directresult of the free-radical chemistry and had been studiedin the 1950s.26 In fact, most oxidation systems, exceptat very low conversions, ought to be treated as co-oxidation systems because of the participation of theproducts. Deliberate co-oxidation routes have also beenattempted for the oxidation of p-xylene to terephthalicacid by using butane,19,27 acetaldehyde, and paralde-hyde8 as co-oxidizing substances. However, the commer-cial success of such a process also depends on the marketfor the coproduct. The oxirane process accounted formore than half of the production of propylene oxide inthe U.S. in 1991.8 ARCO is a world leader in this field.

The functionalization of alkanes by treatment withoxygen has an obvious attraction for the industrialchemist and has been a recurring theme. The develop-ment of paraffin oxidation began around 1930 in Ger-many.8 A process that yields a small number of productsis obviously desirable, but a multiplicity of products isobtained because of the comparable reactivity of all themethylene groups, and the high reactivity of the pri-mary and secondary reaction products. A substantialselectivity to a mixture of secondary alcohols was

obtained by the use of boric acid in Bashkirov’s method.The process was of importance in Japan, the U.S., andthe U.S.S.R. until the late 1970s8 and continues tooperate in the C.I.S. The desired specificity could,however, be obtained in small molecules such as n- andisobutane. The Celanese LPO process for acetic acidfrom n-butane is still operative. The interest in isobu-tane oxidation in the context of the oxirane process hasbeen mentioned earlier. The pioneering work of Winklerand Hearne11 on this reaction led to a number ofpatents.28,29 The conflicting demands of rate and selec-tivity was again the major problem, and efforts to solvethis saw the application in the 1980s of the emergingarea of reaction in supercritical media.30-32 Interest inisobutane oxidation has been rekindled in recent years33

because of the possibility of its development for theproduction of the fast-growing petrochemical, methyltert-butyl ether (MTBE), apart from tert-butyl alcohol.

The development of industrial processes naturallyrequired that extensive engineering studies be carriedout. Although not much of this was published in theopen literature, mention must be made of the work ofSteeman et al.,34 who reported the first comprehensiveset of data on the oxidation of cyclohexane to thescientific world. These data demonstrated conclusivelythe importance of backmixing in the reactor. Over theyears, reactor and process design aspects have receivedconsiderable attention.35-37 However, the models usedin the early attempts were rather crude and did not payadequate attention to the influence of mass transfer,and hence, process development usually involved ex-tensive experimentation at different scales. Althoughan awareness of the issues involved in mass transportwith chemical reaction is evident in the early Russianwork on organic oxidations (for example, Berezin etal.10), the first attempt to explicitly acknowledge the roleof mass transfer in organic oxidations was probably thework of Hobbs et al.,38 in which the possibility of thereaction becoming mass-tranfer-limited and the bulkbecoming starved of oxygen was recognized. Theseauthors made an attempt to explain certain experimen-tal observations that could not be explained on the basisof considerations of chemistry alone by invoking the roleof mass transfer, albeit in a qualitative manner. Thepossibility of reaction within the diffusion film was,however, not considered. Meanwhile, the theory of gas-liquid reactions had made significant strides, startingwith the work of Higbie in 1935, and hydrocarbonoxidations had been recognized as a major class ofreactions of industrial significance falling within thescope of the theory.39 In the 1960s, van de Vusse40-42

applied the theory to complex reactions with kineticsof the type encountered in organic oxidations. However,although one can find studies delving into the questionof mass-tranfer limitations in organic oxidations (sum-maries are available in Doraiswamy and Sharma43), itmust be said that the subject has received relativelylittle attention in the literature and has remainedlargely inconclusive. One reason for this was probablythat engineering descriptions of the kinetics (which wereinvariably of the “lumped” type) did not often accountfor features such as induction periods and autocatalysis.Another reason was the lack of data on the importantmass-tranfer parameters in systems and under condi-tions of temperature and pressure which were of indus-trial interest. Attempts to bring hydrocarbon oxidationswithin the ambit of the theories of mass transfer with

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chemical reaction therefore met with limited success.In the last two or three decades, several studies, atMonash University,44-50 Twente,51,52 and other places,have attempted to address these gaps, but much re-mains to be done.

The nonlinear kinetics of organic oxidations, espe-cially in combination with mass-transport influences,can, in principle, lead to a rich variety of dynamicbehavior with possible consequences for the safe opera-tion of industrial reactors among other things. Onceagain, while theoretical aspects have received muchattention in this area (see, for example, the review byGray and Scott53), and experimental studies do showevidences of complex behavior, quantitative applicationof the theories to hydrocarbon oxidations is still to come.The observations of Hobbs et al.35 on “critical conditions”in the oxidation of methyl ethyl ketone suggest hyster-esis-type effects. Because such situations can lead tounstable operation in practice, these authors suggestedthat industrial reactors should be operated “with amass-transfer-limited zone present”.

A growing appreciation for the interaction betweenphysical transport phenomena and chemical kinetics inorganic oxidations has led to an examination of thepossibility of using oxygen-rich gases for oxidation.Recent developments in air separation technology havealso contributed to the interest in this area. The issuesof safe design and operation of oxidation reactors,always of paramount importance in this field, are beingreexamined in this context. A quantitative assessmentof the costs and benefits for such a changeover tooxygen-rich gases does not yet seem possible withoutactual experimentation for most hydrocarbon oxidations,although some clues are available from the modeling

and simulation carried out on particular oxidations (see,for example, Suresh et al.,50 Cao et al.,54 and Roby andKingsley55).

4. A Survey of Industrial HydrocarbonOxidations

A broad survey of the commercially important pro-cesses employing liquid-phase hydrocarbon oxidationsis presented in Table 1. The production volumes of thechemicals listed is impressive and serves to underlinethe importance of liquid-phase oxidations in industry.It is interesting to note the different roles of theoxidation step in these processes. In some of theprocesses, such as the Amoco process for the manufac-ture of PTA, the oxidation step leads directly to theproduct of interest. There are others, such as the processfor caprolactam, in which oxidation is the step thatproduces a key intermediate that is then further pro-cessed to the product of interest. In the oxirane process,hydrocarbon oxidation provides a convenient carrier ofoxygen for the selective oxidation of propylene to pro-pylene oxide. A choice of hydrocarbons is thereforeavailable (several have been suggested; see Weissermeland Arpe8), and the market for the coproduct determineswhich hydrocarbon is chosen in a given context.

Although treatises on hydrocarbon oxidations under-line the similarities that exist in the mechanisms thatgovern various organic oxidations, from an engineeringpoint of view, it is the differences that exist betweenhydrocarbons that is often of interest; these differencescall for innovative technological developments. Further-more, depending on the place of the desired product inthe reaction sequence (whether as a primary, secondary,

Table 1. Major Chemical Processes Utilizing Hydrocarbon Oxidation1,8,55

productcapacitya

106 tpy oxidation stepimportantprocesses

mainapplication remarks

purifiedterephthalicacid (PTA)

11.38(1995)

p-xylene toterephthalic acid

AmocoMid-Century

PET(fiber, film, resin)

applies also to otheralkylaromatics such asm-xylene, pseudocumene,2,6-dimethylnaphthalene

dimethylterephthalate(DMT)

4.06(1995)

p-xylene to p-toluic acidand monomethyl esterof pTA to DMT

Witten, BASF,DuPont

PET(fiber, film, resin)

two-step processor one-step process

cumylhydroperoxide(CHPO)

6.5(1998)

cumene to cumylhydroperoxide

Hock(BP, Kellogg, etc.)

phenol coproduct: acetonealso applies to cresols,resorcinol/hydroquinonefrom cymenes,diisopropylbenzenes

benzoic acid 0.28(1995)

toluene tobenzoic acid

DSM, Dow phenol andsalts, estersof benzoic acid

also employed in theSnia-Viscosa route toε-caprolactam andHenkel route to TA

ε-caprolactam 3.7(1995)

cyclohexane to KA BASF, Bayer,DuPont, DSM,StamicarbonScientific Des.

Nylon-6(Perlon)

adipic acid 2.2(1993)

cyclohexane tocyclohexanone/cyclohexanol (KA)and KA to adipic acid

BASF, Bayer,Dupont,StamicarbonScientific Des.

Nylon-6,6 K:A ratio depends onuse of boric acid

propyleneoxide

4(1993)

iso-butane to TBHP oxirane propylene glycol,polyols

coproduct: tert-butyl alcohol(gasoline additive)

ethylbenzene tohydroperoxide

oxirane styrene phenyl methyl carbinol(dehydrated to styrene)

acetic acid 6.0b

(1994)butane/naphtha Celanese,

BP, UCCvinyl acetate,

cellulose acetate,PTA, solvent, etc.

main route is via carbonylationof methanol

a Capacities shown are worldwide figures and refer to the year indicated in parentheses. b About 9% of the total production capacityshown is by the oxidation route.

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or tertiary intermediate), the process needs tend to bevery different. In the manufacture of terephthalic acid,the product of interest forms fairly late in the reactionsequence, and, considering its resistance to oxidation,it may in fact be considered as the end product. In amajority of situations however, the desired product isan intermediate susceptible to further oxidation, andselectivity considerations become extremely important.Table 2 provides some relevant details for the oxidationsinvolved in the processes presented in Table 1. Choiceof reaction conditions, as well as the conversion to whichthe reaction must be allowed to proceed, are oftendetermined by the selectivity-coversion relationships,and significant differences between the various casesstand out.

4.1. p-Xylene and Other MC Oxidations

In terms of tonnages, the conversion of p-xylene toPTA and dimethyl terephthalate (DMT) ranks as thehighest among the oxidation processes. As raw materialfor conversion to PET fiber, the use of terephthalic acidlagged behind that of DMT in the past. Although directesterification of terephthalic acid with ethylene glycolis chemically the simplest method for manufacturingPET, the difficulty of manufacturing terephthalic acidon a large scale at a purity sufficient for polyconden-sation has prevented its widespread use in the past. Thepresence of small amounts of 4-carboxybenzaldehyde,a partial oxidation product, would interfere with thepolycondensation. The problem here is thus the presenceof an intermediate, when the final product is desired.However, with improvements in the purification tech-nology by high-pressure hydrogenation, which convertsthe aldehyde to the easily separable p-toluic acid, thedominance of terephthalic acid has been continuously

increasing. Thus, the share of PTA in the PET rawmaterials went up from 29% in 1976 to 55% in 1988. In1995, the worldwide production of PTA was 11.38million tonnes and that of DMT, 4.06 million tonnes.In 1998, the share of PTA has further increased. Afascinating account of the development of these tech-nologies from early stages has recently been written byLandau.14

In the oxidation of p-xylene, the oxidation proceedsup to the stage of p-toluic acid (oxidation of one methylgroup to -COOH), but for reasons that are still a matterof debate,3 the resulting carboxylic group seems toconfer resistance to further oxidation. Technologically,two approaches have been found to circumvent theproblem. In the older Witten method, the acid group isesterified with methanol, a procedure that allows oxida-tion to proceed. In practice, the esterification and thesecond stage of oxidation are carried out simultaneously.A single-step variant of the process is practiced byBASF, Montecatini, and Du Pont, using countercurrentcontacting of the p-xylene (and oxidation products) witha stream of air and methanol. Whereas the two-stepprocess gives a higher selectivity with respect to metha-nol (80% vs 60-70% in the single-step process8), theselectivity with respect to the hydrocarbon is better inthe single-step process (Table 2). In either case, DMTis the product, which can be hydrolyzed to terephthalicacid or used directly in the manufacture of PET bytransesterification.

In the second method, used in the Amoco (Mid-Century) process, the bromide in the catalyst promotesthe reaction sufficiently to give reasonable rates ofoxidation even beyond the p-toluic acid stage. Thismethod uses less catalyst and provides higher rates12

than the first and is being increasingly preferred. The

Table 2. Some Important Process Parameters in the Commercially Important Oxidations1,8,55

reaction conditions

hydrocarbonliquid gas

desiredproduct

temp°C

pressurebar medium catalyst

conver-sion

select-ivity

reactortype process

p-xylene air PTA 190-205 15-30 acetic acid(90-95%)

MC cata 95% >90% stirredreactors,Ti/hastelloylined

Amocoprocess

p-xylene air p-toluicacid pTA

140-170 4-8 Co and Mnsalts

Wittenprocess

monomethylester of pTA

air DMT 140-240 40 pTSA orother acid

85% Wittenprocess

p-xylene +monomethylester of pTA

air andmethanol

DMT 100-200 5-20 p-xyleneandrxn prod

90% countercurr.column

Du Pontone-stepprocess

cyclohexane air KA oil 125-165 8-15 Co and Mnsalts

5-8% 80-85% stirredreactors

cyclohexane air cyclohexanol 125-165 8-15 Co and Mnsalts withboric acid

12-13% >90% stirredreactors

cumene air cumylhydroperoxide

130-140 5 CHPOinitiator

30% 93-95% bubblecolumn

iso-butane air TBHP 120-140 35 initiator 25% 60% oxiraneethylbenzene air EBHP 120-140 35 initiator 15-17% 87% bubble cap

traysoxirane

110-120 2-3 Co salts 90% Dowtoluene air benzoic

acid165 9 Co acetate 90% Snia viscosa

caprolactamprocess

MC cata 99% 96% Amocobutane air acetic

acid175180

5415-20

product mixacetic acid

Co acetateuncat

10-20 HulsUCC

a MC catalyst consists of Co and Mn acetate, NH4Br, or tetrabromoethane.

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major disadvantages of the method are the need forspecial materials (Table 2) and the loss of acetic acidsolvent through decarboxylation. The former is engen-dered by the use of metal/bromide/acid mixtures. Thelatter problem is present to some extent whenever aceticacid is used as a solvent and is, in fact, less of a problemwith the Mid-Century catalyst than with cobalt alone.The principle of the Amoco process has been successfullyapplied in the case of other aromatics and alkylaromat-ics. Thus, m-xylene can be oxidized to isophthalic acid,pseudocumene to trimellitic anhydride and trimelliticacid, mesitylene to trimesic acid, and 2,6-dimethylnaph-thalene to naphthalene-2,6-dicarboxylic acid. Isoph-thalic acid finds application in polyesters and copoly-esters, and its application in PET resins is rising at animpressive rate. Trimellitic acid is mainly used for themanufacture of esters for plasticizers. Because of workcarried out by Du Pont, trimellitic anhydride is emerg-ing as an important component for the manufacture ofextremely heat-resistant polyimides.8 Naphthalene-2,6-dicarboxylic acid is now emerging as an important rawmaterial in the manufacture of PEN resins as well asco-polyesters with PTA, which have higher temperatureratings and better barrier properties than PET. The MCmethod lends itself to batch as well as continuousprocessing. Amoco has manufacturing capacities of 2 ×105 tons/year of isophthalic acid (continuous process)and about 4.7 × 104 tons/year of trimellitic acid (batchprocess), both based on the MC process with yields ofover 90%.12,56 The catalytic principle of the Mid-Centuryprocess and the process details have been discussedcomprehensively by Partenheimer.12

Co-oxidation of p-xylene with various other sub-stances, such as acetaldehyde and paraldehyde in aceticacid solution (Toray process), has been attempted withgood results; the conditions are milder (100-140 °C),bromide corrosion is not a problem, and a selectivity ofover 97% has been claimed. However, a use must befound for the coproduct, acetic acid. Other processes forterephthalic acid have been discussed by Weissermeland Arpe.8 One route involves the LPO of toluene andis discussed later.

4.2. Oxidation of Cyclohexane, Other SaturatedHydrocarbons, and Terpenes

The oxidation of cyclohexane is notoriously inefficient.It is therefore interesting that this process, which offersselectivities no higher than 80% even at 4% conver-sion,23 has not only become established in industry, butalso stood the test of time. The alternative route tocyclohexanol and cyclohexanone involves the hydroge-nation of phenol, a reaction that can be carried out witha selectivity in excess of 97% at 99% conversion. Theprocess economics still favors cyclohexane oxidation.Practically all of the adipic acid16 and about 63% of thecaprolactam57 produced in the world in 1990 usedcyclohexane oxidation as the first step. This oxidation,as usually practiced, uses cobalt naphthenate or stear-ate as the catalyst, is carried out only to very smallconversions (usually less than 8%), and produces amixture of cyclohexanol and cyclohexanone. For capro-lactam, a ketone-rich mixture is preferred. The ratio ofalcohol to ketone in the oxidation mixture dependsmainly on the use of catalyst (the alcohol-to-ketone ratiobeing about 0.5 in the uncatalyzed oxidation and 1.5 inthe presence of cobalt stearate4) and can be made tostrongly favor the alcohol by the use of boric acid

(alcohol-to-ketone ratio of 9:1). Castellan et al.16 andWeissermel and Arpe57 have recently summarized theinformation available on the oxidation of cyclohexaneand the manufacture of adipic acid and caprolactam.The former of these references also has an extensivelisting of patents on the manufacture of adipic acid andsummarizes the status of alternative routes for adipicacid.

The main problem in the oxidation of cyclohexane tocyclohexanol and cyclohexanone is one of selectivity andarises from the fact that these products are much morereactive with respect to oxidation than cyclohexane itselfin the oxidation medium and under the conditionsemployed. It is interesting to note, in passing, the widelydivergent process needs in the cases of p-xylene andcyclohexane. The problem in the former case is theinhibiting effect of the intermediate (p-toluic acid),whereas in the latter, it is the high reactivity of theintermediate. Industrially, one of two approaches isfollowed in order to contend with the selectivity problemin the case of cyclohexane oxidation. The first approachis to keep the conversion very low in order to keep theconcentration of the products low in the oxidationmixture and, hence, to prevent their overoxidation. Theconflicting demands of selectivity on one hand andsafety and rate on the other are best reconciled byhaving several stirred reactors in series (reactor selec-tion issues are discussed in section 8.2 and safety insection 11). The disadvantage of the method is the needto recycle large volumes of unconverted cyclohexane.The second approach (Halcon and IFP) allows higherconversions to be used, as in this case, boric acid is usedto direct the reaction of the hydroperoxide towardcyclohexanol and protect the alcohol as its boric acidester to prevent its over-oxidation. Recovery and recy-cling of the boric acid used is the problem in this case.

In the further conversion of the KA (ketone-alcohol)mixture to adipic acid, catalytic air oxidation undermilder conditions (acetic acid medium, Cu or Mn acetateas catalyst, 80-85 °C, 7 bar) has received some atten-tion. Scientific Design has developed a process for a two-stage autoxidation route to adipic acid. However, theroute using oxidation with nitric acid as the secondstage is preferred because of better selectivity, althoughthere are the additional problems of corrosion andrecovery of the nitrogen oxides produced. (Also, in recentyears, formation of N2O, an important greenhouse gas,has become a major concern with this process.) Weis-sermel and Arpe8 report that the Rohm & Haas plantin the U.S., based on autoxidation technology, wasabandoned because of poor product quality.

The low conversion levels in the oxidation of cyclo-hexane with traditional cobalt catalysts have motivateda large literature on alternative catalysts. Two ap-proaches in particular, which have been targeted forindustrial development in cyclohexane oxidation, willbriefly be discussed here, although neither seems tohave yet reached the stage of commercialization. Thefirst of these is to maximize the selectivity to thehydroperoxide in the first oxidation step and achieve aselective conversion of the hydroperoxide in a secondstage by employing milder conditions and special cata-lysts. Developments in the low-temperature, selectivedecompositions of cyclohexyl hydroperoxide using ho-mogeneous as well as heterogeneous catalysts has ledto a rejuvenation of interest in this approach. DSMdeveloped such a process and applies it on a large scale.

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Tolman et al.58 describe a family of active and long-livedcatalysts for selective conversion of the hydroperoxide.Ciborowski23 argues the case for this process alternativeand discusses the possibilities of utilizing the hydro-peroxide. According to this author, overall selectivity inthe oxidation step to (cyclohexyl hydroperoxide + cy-clohexanol + cyclohexanone) can approach 90% underoptimum conditions, with the hydroperoxide constitut-ing up to 80% of these products. Obviously, in theoxidation step, a total exclusion of circumstances thatcould lead to a decomposition of the hydroperoxide isneeded. No transition metal catalyst is used, andpassivation of the wall, choice of noncatalytic wallmaterial,59 homogeneous oxidation,59-61 and use of asecond phase to remove the hydroperoxide by reactionas it is formed62 are some of the methods that have beenclaimed to offer high selectivities. The interest inhomogeneous oxidation, considering the low solubilityof oxygen in cyclohexane, indicates a recognition of thepossibility that the reaction can move into mass-transfer-limited regimes. Recent studies at MonashUniversity on homogeneous oxidation in a variety ofreactor types in aluminum and glass-lined reactors haveproduced encouraging results.59,61,63

In the second approach, which is relevant to adipicacid manufacture, a direct oxidation to adipic acid isattempted. If reasonable selectivity can be achieved,such an approach would obviously have the advantageof simplicity, as an entire process step is eliminated.Asahi17,18 and Gulf64 have made attempts in this direc-tion, and claims of 70-75% selectivity to adipic acid atconversion levels of 50% and above have been made. Theoxidation is carried out in acetic acid medium, with acobalt salt as catalyst. Typical reaction conditionsquoted include temperatures of 70-100 °C and reactiontimes of 2-6 h. According to the Asahi work, conditionsthat favor the formation of Co3+ in the reaction mediumare to be preferred. They use aldehydes and ketones aspromoters for this purpose. Gulf patents claim that thepresence of water improves the adipic acid yieldssubstantially. However, neither process seems to havebeen implemented industrially. In recent years, interestin direct routes to adipic acid would seem to haverevived, judging from the large amount of patent activityin the area. The work of Twenty-first century researchcorporation was cited in section 3; also see Park andGoroff65 and Vassiliou et al.66 The role of water and thesolvent-to-hydrocarbon ratio would seem to be impor-tant from these patents, as also would a careful regula-tion of temperature versus holdup time. Some patents(Dassel and Vassiliou,67 for example) deal with uncon-ventional contacting methods, such as contacting liquidin atomized form with a stream of the oxidizing gas.

While on the whole, it must be admitted that theresults on single-step oxidation to adipic acid have beenless than satisfactory,16 some of the principles recog-nized by the earlier work, such as the need to keep thetransition metal in the higher oxidation state,17 are ofinterest for further development (see, for example,Steinmetz et al.68). Also, some interesting chemistry hasbeen brought out as a result of this research. Inparticular, we mention the work of Iwahama et al.,69

which demonstrates the oxidation of cyclohexane at 100°C and atmospheric pressure using N-hydroxyphthal-imide as a radical catalyst. The reaction is conductedin acetic acid medium with trace amounts of transitionmetal salts as acetylacetonates. The manganese salts

appear to yield the best selectivity to adipic acid.Cyclohexanone and adipic acid are the main products.Surprisingly, cyclohexanol is not formed, and it issuggested that it is further converted under the reactionconditions. In another development, Kulsreshtha et al.70

have reported selectivities of 77% at 85% cyclohexaneconversion using a Co(III) catalyst at 100 °C.

The oxidation of cyclododecane is analogous to theoxidation of cyclohexane and, indeed, has been used bysome researchers as a model system for studying thekinetics of cyclohexane oxidation.36 At the same tem-peratures (150-160 °C) as employed for cyclohexaneoxidation, cyclododecane can be oxidized in the liquidphase at atmospheric pressure to a cyclododecanol/cyclododecanone mixture. Boric acid is used in theindustrial process,8 and selectivities of about 80% areachieved at 25-30% conversion. Alcohol-to-ketone ratiosare similar to those in the analogous cyclohexaneoxidation. Processes are operated by Huls in Germanyand Du Pont in the U.S. Further oxidation with nitricacid is used to convert the alcohol/ketone mixture to 1,-12-dodecanedioic acid, a product of some importance inpolyamides (Nylon-6,12) and polyesters. 1,12-Dode-canedioic acid is the second most important dicarboxylicacid after adipic acid. This dicarboxylic acid has beenattracting additional attention in recent years, as themacrocyclic musk, made via esterification with ethyleneglycol, is preferred over polycyclic musk for environ-mental reasons, and in the new generation of polycar-bonates. Cyclododecanol and cyclododecanone are alsoprecursors to lauryl lactam (monomer of Nylon-12),again in analogy with cyclohexane oxidation products.

A recent claim by Ube industries71 refers to the liquid-phase oxidation of cyclododecane in the presence ofmono(2-ethylhexyl) phosphate and di(2-ethylhexyl) phos-phate at 160 °C, where 73% selectivity to hydroperoxideat 12% conversion has been realized. A novel feature ofthis claim is the use of phosphate ester, whose actualrole is unclear.

The Bashkirov principle of using boric acid to producealcohol esters, and thereby gain alcohol selectivity, wasfirst developed for the oxidation of n-paraffins as one ofthe routes to linear secondary alcohols in the C10-C20range. These alcohols have application in detergentmanufacture. This process still operates in Japan andthe C.I.S.8 Atmospheric operation at 140-190 °C, in thepresence of about 0.1% KMnO4 and 4-5% metaboricacid, to conversion levels of 15-25% is used. The borateesters are later hydrolyzed with NaOH. The oxidationyields sec-alcohols almost exclusively, with a statisticaldistribution of OH groups. Weissermel and Arpe8 dis-cuss some process variants.

Also allied to cyclohexane oxidation is the oxidationof p-menthane and cis-pinane, steps of importance inthe manufacture of several flavor intermediates apartfrom hydroperoxides (which are used in free-radicalpolymerizations as initiators). Thus, pinane, derivedfrom R-pinene (a plant product), is oxidized to a mixtureof cis- and trans-pinane hydroperoxides. This mixtureis in turn hydrogenated to a mixture of cis- and trans-pinanols, which, on pyrolysis, yields linalool.3 Apartfrom being a product of importance in its own right,linalool can also be readily isomerized to nerol andgeraniol using an orthovanadate catalyst. Similarly,limonene can be converted, via air oxidation, to carvoneand other products using a cobalt catalyst.72 Theseauthors have also studied the oxidation of R- and

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â-pinenes. The oxidation of limonene in glacial aceticacid containing lithium chloride in the presence ofPdCl2-CuCl2 gives trans-carveyl acetate as the majorproduct, as has been reported by Gusevskaya andGonsalves.73 Although some of these products can alsobe directly extracted from plant sources, the syntheticroutes mentioned above are far more important. Theseproducts are also important as intermediates in themanufacture of other terpene products such as citral,ionones, citronellol, citronellal, menthol, vitamins A andE, and carotenoids.

4.3. LPO-Based Routes to Phenol and BenzylicAlcohols

Phenol is another major chemical derived from aliquid-phase oxidation route. In the U.S., it ranks 34thin terms of capacity in the overall list of industrialchemicals.74 The direct oxidation of benzene to phenolis highly unselective, as total oxidation of phenol ispreferred to partial oxidation of benzene, and hence,only the indirect manufacturing processes have beenimplemented.8 Of the five routes that have attainedcommercial significance, three involve a liquid-phaseautoxidation. By far the most important process is theHock process, first commercialized in 1953. The processis based on the oxidation of cumene to cumene hydro-peroxide and the subsequent cleavage of the hydroper-oxide in acidic media to phenol and acetone. The BP,Hercules, and Kellogg processes use an air/aqueousemulsion at a pH of 8.5-10.5, and other conditions areshown in Table 2. The table reflects the more recenttrend to reduce or even eliminate water from thereaction medium. Alternatively, air/cumene contactingcan be used at 120 °C (Hercules process). As oxidationcatalysts encourage hydroperoxide decomposition (seediscussion on cyclohexane oxidation above), none isused, although substances such as Cu, Mn, and Co saltscan reduce the induction period at the start.8 Theoxidate is worked up to 65-90% before being subjectedto acid cleavage.

Cumene was originally produced in World War II asan additive for gasoline. Its potential for making phenol,via neat air oxidation to hydroperoxide and subsequentacid-catalyzed cleavage, was recognized in the 1950sand came to fruition thereafter. Today, more than 90%of the phenol produced in the world, at a level ofapproximately 6.5 million tons per annum, is producedby this route. Several single-stream plants producingphenol at 200 000 tons per annum exist, and these havereal large-size oxidizers. The process is, in theory,applicable to other feedstocks such as ethylbenzene(coproduct acetaldehyde) and cyclohexylbenzene (co-product cyclohexanone). Cumene is, however, expectedto remain the dominant feedstock because of the marketfor acetone. Another potential possibility is to use sec-butyl benzene in place of cumene so that the corre-sponding hydroperoxide will, on cleavage, yield phenoland methyl ethyl ketone.

The other LPO-based routes to phenols8 are thoseusing cyclohexane oxidation (Scientific Design) andtoluene oxidation [DSM, Kalama USA (now Goodrich)].In the former case, the oxidation is carried out until thestage of cyclohexanol/cyclohexanone, which is dehydro-genated to phenol at 400 °C. In the latter, toluene isoxidized to benzoic acid, which is oxidatively decarboxy-lated to phenol. The first step of toluene oxidation inthe second process is of some importance in its own

right, because of the various other uses of benzoic acid(as well as its salts), for example, as an intermediate(to caprolactam in the Snia Viscosa process and toterephthalic acid in the Henkel process) and as anadditive in the rubber and food industries. A range ofprocesses are available for the oxidation of toluene,among them the Amoco Mid-Century process discussedearlier. Two other processes are shown in Table 2.

In a manner analogous to the Hock process, cymenecan be oxidized and cleaved to m- and p-cresol (coprod-uct acetone). These processes have been operated forover 25 years by Sumitomo and Mitsui.8 Again, 2-iso-propylnaphthalene has been converted, via the hydro-peroxide, to 2-naphthol (a widely used dye intermediate)and acetone with a yield of about 95%.75 The oxidationtakes place at 110 °C. Modified Hock processes are alsoused in the manufacture, from diisopropylbenzenes, ofthe more important dihydroxybenzenes, hydroquinone,and resorcinol (combined world capacity of about 85 000tons/year in 1994). Thus, m-diisopropylbenzene has beenconverted to resorcinol and p-diisopropylbenzene and tohydroquinone in Japan (Mitsui and Sumitomo) and theU.S. (Goodyear). The presence, in the case of thediisopropylbenzenes, of two hydroperoxide groups afterthe oxidation leads to a greater number of byproductsand lower rates than are attained with cumene hydro-peroxide, requiring modifications to the cumene-phenolprocess.76

4.4. The Oxirane Process: Oxidation ofiso-Butane and Ethylbenzene

The oxirane processes, or the “indirect oxidation”routes to propylene oxide, which are of interest in thecontext of liquid-phase autoxidations, accounted forabout 48% of the worldwide propylene oxide (PO)capacity in 1991.8 The importance of the oxirane routein PO manufacture has been increasing through theyears. Indirect oxidations exploit the ability of hydro-peroxides (or peroxy acids) to selectively transfer theirperoxidic oxygen to olefins to form epoxides, whilethemselves being converted to the alcohol (or the acid).The hydroperoxide or the peroxy acid is produced, eitherin a preliminary step or in situ; the former has estab-lished a dominant position in industry. Although achoice of hydroperoxides/peroxy acids is available (see,for example, Weissermel and Arpe8 and Kahlich et al.77),coproduct economics have meant that the oxiranecapacity is split almost equally between isobutane- andethylbenzene-based plants, the former accounting forabout 56% of the capacity. The conditions of oxidationand typical conversion-selectivity figures are shown inTable 2. The aim in the oxidation is to maximize theselectivity to the hydroperoxide. Under the conditions,the hydroperoxide and the alcohol are substantially theonly products formed, with small amounts of acetonein the case of isobutane and acetophenone in the caseof ethylbenzene. For example, in isobutane oxidation,even at 48% conversion, the total selectivity to tert-butylhydroperoxide (TBHP) and tert-butyl alcohol (TBA) is96%, 50% being the selectivity to TBHP. The coproducttert-butyl alcohol in the isobutane-based process, apartfrom being a gasoline additive, can be dehydrated toisobutene and further converted to methyl tert-butylether (MTBE). In the ethylbenzene-based process, thecoproduct phenyl methyl carbinol is converted to sty-rene. A small amount of primary hydroperoxide ofethylbenzene is also formed, which gives phenyl ethyl

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alcohol (which has a high value as an aroma chemical).In the context of the recent interest in the oxidation ofcyclohexane to cyclohexyl hydroperoxide, the possibilityof using cyclohexyl hydroperoxide to convert propyleneto PO in a step analogous to the existing oxiraneprocesses has also been commented upon (e.g., Ci-borowski23).

Although the interest in isobutane oxidation has beenin the context of the oxirane process, possibilities forits development for MTBE production are being activelyexamined. The spread of unleaded fuels and legislationsuch as the U.S. Clean Air Act have resulted inincreasing demands for MTBE (12th in terms of capacityin the list of industrial chemicals in the U.S. in 199674).According to the projections made in 1993, MTBE wasexpected to continue its growth at about 15% per year.78

If this growth is sustained, the traditional route basedon isobutenesin most crackers, the C4 stream contain-ing isobutene is directly converted to MTBE withmethanolsmay not be able to meet the demand becauseof problems of isobutene availability. It will be necessaryto resort to other routes based on TBA and isobu-tane.79,81 Oxidation of isobutane followed by etherifica-tion of TBA with methanol seems to hold promise.Although oxidation with hydrogen peroxide is a researchfocus (see, for example, Grieken et al.79), H2O2 isexpensive. Furthermore, a common disadvantage of allprocesses based on H2O2 is that, with the currenttechnology, safety considerations require H2O2 facilitiesto be restricted in size.77 Hence, interest in the autoxi-dation of isobutane has resurfaced33,82 and is likely tocontinue. Some of the recent literature (patent andpublished) that focusses on TBA rather than TBHP inthe oxidation of isobutane (for example, Shen et al.83

and Fan et al.84) and patents dealing with the selectiveconversion of TBHP to TBA (for example, Sanderson etal.85,86) are also indicative of this trend.

While one considers the potential of developmentscited in the previous paragraph, some recent develop-ments should be borne in mind. Most recently, MTBEhas come under a cloud, and its usage in motor gasolineis being phased out. (The state of California has bannedthe addition of MTBE to gasoline.) ETBE (ethy tert-butylether) is a possible replacement for MTBE, as there doesnot seem to be any objection to its use. TBA as anadditive also seems to be safe.

TAME (tert-amyl methyl ether) is another gasolineadditive with properties similar to MTBE. As withisobutene in cracker streams, isopentene, which is alsoproduced in small amounts during steam cracking, isused for TAME production. However, of relevance in thepresent context is the possibility of development ofisopentane autoxidation for TAME, along lines similarto isobutane oxidation discussed above. The first plantfor TAME began production in 1987, and since then,other plants have been put into operation. These plantsuse the acid-catalyzed reaction of isopentene withmethanol.8

4.5. Acetic Acid from Paraffin Oxidation

Liquid-phase oxidation routes also account for asignificant proportion of acetic acid produced in theworld, although they have been superseded in recentyears by methanol carbonylation. Acetic acid is one ofthe most important aliphatic intermediates and the firstcarboxylic acid used by man. It ranked 33rd in the listof U.S. industrial chemicals according to a recent survey

and was one of the few organics to register a double-digit growth during 1994-1995.74 In 1994, LPO routeswith acetaldehyde and butane/naphtha as feedstocksaccounted for about 32% of world production, with theacetaldehyde-based process being the more important(23% of world capacity).57 This, therefore, is a case ofautoxidation of an oxygen-containing compound, incontrast to most oxidations discussed so far (with theexception of the oxidation of cyclohexanone). The chem-istry is, however, similar, with the free-radical mecha-nism producing peracetic acid as the primary product.Under mild conditions (no catalyst, ethyl acetate me-dium, -15 to 40 °C, 25-40 bar), peracetic acid can berecovered as the main product in acetaldehyde oxida-tion, as in the commercial processes operated by UCCin the U.S., Daicel in Japan, and British Celanese inthe U.K. for peracetic acid. In the processes for aceticacid however, solutions of Co and Mn acetates at lowconcentrations (0.5 wt %) are used; as in other processes,they accelerate the reaction by helping in radicalgeneration and facilitating the decomposition of theperacetic acid. The Hoechst process8 uses oxygen; typicalreaction conditions are indicated in Table 2. Carefultemperature control is needed to limit degradationreactions and obtain the high selectivities shown. TheRhone-Poulenc/Melle Bezons process uses air insteadof oxygen, and although selectivities are similar, thegreater amount of inert species in the gases requireswashing of the gases to remove acetaldehyde and aceticacid. The byproducts are separated by distillation, andan interesting feature is that they serve as entrainingagents for water, so that anhydrous acetic acid isdirectly obtained.

Various routes to the manufacture acetic acid fromlight paraffin feedstocks have been commercialized, andWeissermel and Arpe8 describe them in some detail.Some of these processes illustrate how, whereas selec-tivity to the product of interest remains a problem whena straight through-process from the alkane to the acidis attempted, byproduct recovery can contribute to theoverall success of the process when reasonable quanti-ties can be recovered. The Huls process (Table 2) is atypical catalyzed LPO that produces, in addition toacetic acid, other byproducts such as acetaldehyde,acetone, MEK, etc. A feature of this process is that,depending on the demand, MEK can be manufacturedat up to 17% of the plant capacity, although at theexpense of acetic acid. Normally, byproducts are recycledto simplify product workup. The Distillers-BP processoperates in Europe, Japan, and the C.I.S. and uses amixed feedstock of crude oil distillates in the 15-95 °Cboiling range (roughly C4-C7). The process uses anuncatalyzed oxidation, at 160-200 °C and 40-50 bar.Byproduct recovery is important to the profitability ofthe process.

We have surveyed some of the more important LPOprocesses so far. The unifying thread of the free-radicalchemistry (modified in some instances by the action ofthe catalysts used) that runs through all these pro-cesses, as well as the individual peculiarities of thespecific processes that lead to highly specific technolo-gies, are both evident in this broad overview. Thecomplex chemistry of these oxidations makes themhighly versatile and makes process optimizations to-ward a number of alternatives from a given feedstockbecome possible.

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5. Mechanism of Hydrocarbon Oxidation

The chemistry of liquid-phase autoxidation of hydro-carbons, both catalyzed and uncatalyzed, has been thesubject of several monographs and reviews (see, forexample, Emanuel et al.,1 Sheldon and Kochi,2 Berezinet al.,10 and Partenheimer12,56). Therefore, only theimportant features and those relevant to the consider-ations of this paper will be summarized in this section.

The generic features exhibited by hydrocarbon oxida-tions in the liquid phase suggest similarities in theunderlying kinetic mechanisms. Thus, the presence ofan induction period, autocatalytic behavior, and zero-order kinetics with respect to oxygen, are all charac-teristics commonly observed, irrespective of the hydro-carbon. Again, the multiplicity of products that formindicates a complexity of chemistry, which is occasionedby the fact that the primary products of oxidation arethemselves amenable to oxidation by mechanisms verysimilar to those that apply to the parent hydrocarbon.

The free-radical chain theory of autoxidation,1,2 as ageneral framework within which to interpret the kinet-ics of hydrocarbon oxidations in the liquid phase, is nowgenerally accepted. According to this theory, oxidationis mediated by free-radical intermediates, and is char-acterized by the elementary processes of initiation,propagation, and termination. Also, because the hydro-peroxide molecules, which are the “links” in this chainmechanism, are often unstable with respect to theformation of free radicals, the degenerate chain-branch-ing reaction is usually important. Often, therefore, thebranching mechanism soon overtakes the primary ini-tiation mechanisms in the generation of free radicals.The degenerate branching reactions have been shownto be the cause of several features observed in hydro-carbon oxidations. In what follows, we shall brieflyconsider the elementary processes in the free-radicalchain mechanism.

5.1. Initiation

The reaction may be initiated either by the hydro-carbon itself, e.g.

or by the decomposition of an initiator such as aperoxide, e.g.

When no initiator is used, either or both reactions 1 and2 may be important. For example, reaction 1 has beenshown to be prominent in cyclohexane, cumene, ando-xylene, and reaction 2 in the case of tetralin. Cyclo-hexanol shows evidence of both mechanisms.1,87 Thesereactions are endothermic and quite slow. Therefore,induction periods are often observed in oxidationswithout added initiators. Indeed, Sheldon and Kochi2

opine that initiation from the hydrocarbon is kineticallyand thermodynamically unfavorable and that initiationin the absence of added initiators is due to the decom-position of adventitious peroxidic impurities. Initiationfrom the hydrocarbon can be facilitated by the partici-pation of metal ions, as described later, and this factor

could be important in laboratory reactors in whichconsiderable surface-to-volume ratios are present, eventhough no metal ions may be intentionally added. Useof initiators is favored in fundamental studies where itcircumvents problems such as poor reproducibility ofinduction periods in batch reactors. When used, theseinitiators are usually peroxides or hydroperoxides withreasonable rates of decomposition in the temperaturerange 50-150 °C. Rate constant information on someof the initiators commonly used in autoxidations isavailable from Sheldon and Kochi.2 In industrial pro-cesses, if initiators are used, they are usually chosen soas not to complicate the chemistry. Thus, TBHP mightbe used as an initiator in the oxidation of isobutane;11

being itself a product, it would not lead to products thatwould otherwise be absent. Similarly, acetaldehyde hasbeen used in studies on the oxidation of cyclohexane toadipic acid, as its oxidation product is acetic acid, whichis used as a solvent in these attempts. Whether aninitiator is used or not, the mechanisms shown aboveare of importance only in the initial stages of thereaction and are overtaken even at small conversionsby the chain-branching reactions to be discussed later.This explains why reproducible kinetic behavior isobserved in the post-induction period even when induc-tion periods themselves may be irreproducible (see, forexample, Suresh et al.50 and Suresh88).

5.2. Propagation

The following two reactions represent the generalchain-propagation mechanism:

At oxygen partial pressures greater than about 100Torr, reaction 5 (rate constants > 109 mol L-1 s-1,Sheldon and Kochi2) is usually much faster than reac-tion 6, so that the R* radicals are effectively scavenged,and the overall reaction shows a zero-order behavior inoxygen. Each occurrence of reaction 6 represents theformation of a “link” in the chain reaction, and thechains are said to be long when the number of occur-rences of reaction 6 per initiation event is large. A“kinetic chain length”, which represents the averagenumber of links per chain, can be calculated as the ratioof the rate of the propagation reaction to the rate of theinitiation reaction (assuming that each initiation eventstarts one chain). When the chains are long, hydrocar-bon consumption occurs essentially by reaction 6. Thekinetic chain length can be related to the efficiency ofhydroperoxide formation.3 The rate of reaction 6 de-pends on the nature of the hydrocarbon as well as onthe nature of the radical. The peroxy radicals arerelatively stable, and abstract preferentially only themost weakly bound hydrogen atom. Thus, the facilityof hydroperoxide formation decreases in the order:2tertiary C > secondary C > primary C. Thus, forexample, in the oxidation of cumene, attack alwaysoccurs on the tertiary C in the isopropyl group, withnegligible attack on the ring or the methyl carbons. Onthe other hand, the reactivity of the alkylperoxy radicalstrongly depends on its structure, being influenced bysteric as well as polar effects, in general increasing asthe electron-withdrawing capacity of the R-substituentincreases. For example, acylperoxy radicals are much

RH + O2 f R* + HO2* (1)

2RH + O2 f 2R* + H2O2 (2)

I f I* (3)

RH + I* f IH + R* (4)

R* + O2 f RO2* (5)

RO2* + RH f ROOH + R* (6)

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more reactive than alkylperoxy radicals, thus explainingthe high rates and long chain lengths observed in theautoxidation of aldehydes. Thus, autoxidation rates (andselectivity, through chain length) depend not only onthe nature of the hydrocarbon itself, but also on thestructure of the peroxy radical derived from it. Howard89

has some compilations of propagation rate constants atroom temperature.

At moderate and high conversions, there are severalhydrocarbons and several types of peroxy radicalsparticipating in reactions of the same type as 6 above.Similar is the situation with co-oxidations. In suchcases, the C-H bond energies of the hydrocarbons andthe relative reactivities of the peroxy radicals togetherdetermine the relative rates of oxidation of the differentcompounds in the oxidizing mixture and the selectivitiesthat result.

Propagation (and hence the overall reaction) kineticsat very low oxygen partial pressures, when reaction 5begins to control the overall propagation rate, areexpected to become first-order in oxygen. These situa-tions are not as well studied as the zero-order behaviorat higher oxygen partial pressures. It is possible thatsuch a situation arises in industrial reactors, which areoften operated with extremely low exit oxygen partialpressures in the interest of safety. The free-radicalpopulation would be expected to become more complexwith several types (alkyl, alkyloxy, alkylperoxy) of freeradicals, and this situation is believed to lead to a muchgreater number of products than at higher oxygenconcentrations, thereby lowering selectivities.2 Theconcentration of these radicals can vary both spatiallyand temporally, depending on local oxygen concentra-tions. Some of the reactions of the other radical classesdo, however, lead to similar products as would otherwiseform (for example,3 ROH can form from the attack ofthe primary alkoxy radical on RH, while secondary andtertiary alkoxy radicals can give, through â-scissionreactions, aldehydes and ketones respectively). Somedata available on the oxidation of isobutane31 suggesthigher selectivities to TBHP under low oxygen partialpressures. The isobutane data have been discussedrecently by Suresh;90 a summary is given in section6.4.

5.3. Termination

Given the relative abundance of the peroxy type offree radicals under normal circumstances, the dominantmode of termination would be by (“Russell mechanism”)reactions of the type2,3

It is possible that mutual termination between differenttypes of radicals becomes important under conditionsof extremely low oxygen partial pressures, when typesof radicals other than peroxy radicals can also ac-cumulate in solution. The tetroxide that forms in thereaction shown above undergoes decomposition in amanner that depends on its structure. Thus, the tetrox-ides derived from secondary and primary alkylperoxyradicals decompose by disproportionation to the corre-ponding alcohol and carbonyl compound. However, whenchains are long, most of the observed concentrations ofalcohols and ketones in the reaction mixture derives

from the decomposition of the hydroperoxide. Thetermination mechanisms associated with t-alkylperoxyradicals lead to dialkyl peroxides.11,91

In general, the rate constant for termination ofprimary alkylperoxy radicals is higher than those ofsecondary alkylperoxy radicals, which are themselvesmuch higher than those of tertiary alkylperoxy radicals.Howard89 presents some data on these rate constants.Under low oxygen concentrations, when alkyl andalkoxy radicals can accumulate to some extent, termi-nation mechanisms involving these species would beexpected to become important.

5.4. Degenerate Chain Branching

As mentioned earlier, except in the very early stagesof the reaction, the main source of free radicals in mostliquid-phase organic oxidations are the so-called degen-erate branching reactions in which the primary productsof the chain mechanism participate. These reactionslead to a number of consequences in the conversionrange of industrial interest, such as autocatalysis, aprogressive decrease in the kinetic chain length withconversion, and a multiplicity of secondary and tertiaryproducts.

The decomposition of hydroperoxides, the primaryoxidation products, to radical species can itself be acomplex process, as both nonchain and chain mecha-nisms have been implicated. Three mechanisms havebeen identified as being important87 in the breakdownof hydroperoxides to radical species. These are a uni-molecular homolysis with -O-O- bond breakage, viz.,

a bimolecular interaction of a hydroperoxide moleculewith the original hydrocarbon, viz.,

and a bimolecular mechanism involving associationbetween two hydroperoxide molecules, viz.,

Alkenic hydrocarbons show a linear relationship be-tween the rate and the concentration of the hydroper-oxide, suggesting the predominance of mechanism 10.The oxidations of cyclohexane and isobutane show aproportional relationship between the rate and totalproduct concentration, which again can perhaps beexplained on the basis of such mechanisms.

As an example of the importance of the chain-branching process vis-a-vis initiation as a source ofradicals under oxidation conditions, it can be calculatedthat, for cyclohexane oxidation at 1500 °C, the conver-sion at which the two mechanisms become competitiveis as low as 0.01% even if reaction 8 alone is consideredto operate among the branching mechanisms cited. Thesituation is not much different in general at othertemperatures, as the activation energies of the initiationand chain-branching reactions are usually comparable,being in the range 25-35 kcal/mol.

The heat effects associated with the branching reac-tions are usually small and could be of either sign,depending on the bond energies of the participatingbonds. The relative importance of the three types ofmechanisms described influences the overall kinetic

RO2* + RO2

* f RO4R f

O2 + non-radical products (7)

ROOH f RO* + *OH (8)

ROOH + RH f RO* + R* + H2O (9)

2ROOH f RO* + RO2* + H2O (10)

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form. Mechanism 9 has been observed, for example, inthe case of tetralin and cumene and can often takeprecedence over the mechanism 8 because of bondenergy considerations.87 Furthermore, the hydrocarbonsparticipating in these reactions at higher conversionscould be the reactants as well as the intermediateproducts formed during the oxidation. The bimolecularmechanism 10 has been observed with, for example, tert-butyl hydroperoxide and several alkenic hydrocarbonsand is generally more important in cases where thehydroperoxides occur in reasonable concentrations. Theobserved linear dependence of the oxidation rate on theproduct concentrations with species such as cyclohex-ane49,92 and isobutane90 is perhaps indicative of thismechanism being important in these cases as well.

Apart from the mechanisms discussed, there areothers that often play a substantial role in radicalgeneration after the induction period. For example, inthe case of isopropyl alcohol, H2O2, which is producedduring the oxidation, decomposes to free-radical species.Again, the peroxy radical can itself isomerize anddecompose into a smaller radical and a molecularproduct, a mechanism that explains the occurrence oflow-molecular-weight compounds even at early stagesin some oxidations.87 Such unimolecular decompositionof the hydroperoxide is often catalyzed by metal sur-faces, and this explains, in part, the differences inbehavior that are often observed between laboratoryreactors made from different materials (for example, thedifferent peroxide levels in cyclohexane oxidation inglass and stainless steel reactors; see Berezin et al.10).It is therefore clear that, to obtain scale-independentkinetics, the influence of wall catalysis is to be recog-nized and appropriate precautions taken. These aspectsare further elaborated in section 7.

5.5. Overall Kinetic Features Based on theMechanism

The elementary processes involved in hydrocarbonoxidations have been established from extensive experi-mentation. It is therefore possible, in principle, to treatoxidation kinetics directly from the mechanisms pre-sented above. For example, Berezin et al.10 presentvarious kinetic models for different combinations of theelementary processes discussed above. However, thereare some difficulties with this approach in treatingtechnical kinetics. First, whereas consensus has beenreached on the broad features of the free-radical chainmechanism, sharp controversies remain on variouspoints of detail, which are sometimes important fromthe industrial viewpoint. Second, sometimes, particu-larly when catalysts are involved, nonradical pathwaysbecome important in intermediate product conversionsand add to the complexity of the chemistry. Finally, thechemistry gets complicated anyway, even for simplematerials, at the conversions of industrial interest,because of the participation of the primary products inthe various elementary processes. Thus, tractable mod-els can be formulated with confidence only for the veryinitial stages of industrial oxidations.

At moderate to high oxygen concentrations and whenbranching is unimportant, it can be shown2,87 that therate of hydrocarbon consumption is given by

where rin is the rate of initiation and kp and kt are therate constants for propagation and termination (eq 7),respectively. The kinetic features thus depend on theinitiation mechanism. Some of the features anticipatedfrom eq 11 have been experimentally observed. Forexample, the rate of hydroperoxide formation in theLPO of methylpentanes, with hydroperoxides being usedas initiators, was observed to be proportional to thesquare root of the hydroperoxide concentration byFarkas and co-workers (cited by Farkas6), consistentwith the above equation when the initiation is byhomolytic decomposition of the hydroperoxide. If initia-tion occurs according to reaction 1, one would expectthe reaction to show a half-order dependence on oxygenand a 3/2-order dependence on the hydrocarbon. Inpractice however, the order with respect to the hydro-carbon is difficult to discern at the low conversions atwhich eq 11 applies, and the order with respect tooxygen is more often seen to be zero, showing perhapsthe dominance of other initiation mechanisms even ata fairly early stage. Equation 11 does not explicitlyaccount for the autocatalysis and accounts for it onlyindirectly if the products influence the rate of initiation,as in the work of Farkas mentioned above.

Despite the shortcomings and limited scope of equa-tions of this type, they are useful in comparing thesusceptibility to oxidation of various substances. Giventhat the rate of initiation can often be manipulated, sayby adding initiators, the ratio kp/(2kt)1/2 is a measure ofthe intrinsic tendency of the compound to becomeoxidized and has been called the oxidizability.2,4 It isthus seen that the ease of oxidation of an organicdepends not only on the value of kp, but on the value ofkt as well. The significant rate of termination of primaryand secondary alkylperoxy radicals is the main reasonan otherwise reactive hydrocarbon such as toluene hasa rather slow rate of autoxidation.2 Oxidizability valuescan also be used to rationalize, for example, the highreactivity of benzaldehyde, the moderate reactivity ofcumene, and the low reactivity of p-xylene89 undercomparable conditions.

For a given oxidizability, whereas the rate of reactionis seen to vary directly on rin, the chain length variesinversely as its square root. Because short chains areassociated with a proliferation of products and hencewith low selectivities, this implies that addition ofinitiators to increase the rate can result in poor selec-tivities, a feature often observed.

At moderate conversions, when hydroperoxide is themajor product, a self-sustaining chain reaction becomespossible with ROOH as the radical source. The followingequation has been derived2,3 to describe the rate ofhydrocarbon conversion under these circumstances:

where n is the number of radicals produced by decom-position of one molecule of the hydroperoxide and f isthe fraction of RH consumed, which disappears throughalkylperoxy radical attack.

5.6. Catalysis of Organic Oxidations

It has been pointed out that the most importantcatalysts used in hydrocarbon oxidations involve transi-tion metals. Cobalt has been the most prominent among

rRH ) kp[RH]x( rin

2kt) (11)

rRH ) nkp2 [RH]2

2fkt(12)

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the transition metals used. According to Partenheimer,12

cobalt performs at least three functions in the oxidationmedium: (a) It quickly reacts with the primary perox-ides via the Haber-Weiss cycle

(b) It acts as a radical initiating species when in thehigher (+3) oxidation state (i.e., it generates R* radicalsfrom RH), thereby enhancing the rate by participatingin the initiation step (see eq 11). (c) It reacts rapidlyand selectively with peracids, which are formed in theoxidation of aldehydes, and thus facilitates intermediateproduct conversion at advanced stages of oxidation.

The first two mechanisms can operate in all cobalt-catalyzed reactions. The ease of reactions 13 and 14depends on the redox potential of the Co(III)/Co(II)couple; the nature of the ligand (acetate or bromide, etc.)and the solvent also have some influence on this. Thereason for the effectiveness of cobalt (and manganese)is the fact that the two oxidation states in its case areof comparable stability, so that reactions 13 and 14 canoccur concurrently, and a catalytic process results.2 Withother metals (such as copper), alternative routes (to 14)for reducing the metal, such as hydrogen transfer bythe solvent, sometimes operate to close the catalyticcycle. The possibilities are discussed in Sheldon andKochi.2

In other words, the presence of cobalt leads to a higherrate of initiation and a higher rate of decomposition ofthe hydroperoxide, the primary product in any autoxi-dation. Both of these features are observed in practice;induction periods and hydroperoxide levels are bothsmaller in cobalt-catalyzed oxidations.

The last property seems to play a particularly impor-tant role in oxidations in acetic acid medium as, underthese conditions, Co(II) can exist as a dimeric speciesthat provides a nonradical, fast, selective route from theperacid to the acid.12

It is clear from the discussion above that the solventhas some role in catalytic oxidations. It has been pointedout by several authors12,17,68 that the ratio of Co(III) toCo(II) (which is influenced by, among other things, thepolarity of the solvent) in the oxidation medium isimportant. Careful studies, quoted by Sheldon andKochi,2 have shown that the commencement of theautoxidation virtually coincides with the conversion ofcobalt to the III species. [During oxidation, the Co(III)level increases to a maximum and then decreases. Thedecrease is attributed to the reduction to the II speciesby the aldehydic species generated in the oxidation. Theoxidation state of cobalt is readily recognized by thecolor of the solution.] The role of the solvent in MCoxidations is described in section 6. The role of promot-ers such as aldehydes, ketones, etc. in reducing induc-tion periods (Castellan et al.16 give a summary of theuse of promoters in cyclohexane oxidation to adipic acid)is also explained by their role in promoting the IIIspecies.

A phenomenon of some importance associated withthe cobalt-catalyzed autoxidations is the “catalyst-inhibitor conversion”.2 In media of low polarity, cobaltcomplexes are catalysts at low concentrations butinhibitors at high concentrations, and the transitionseems to occur abruptly at some concentrations. This

is why solvents such as acetic acid are used when highcatalyst loading is needed.

Metal catalysts are usually added in the form ofhydrocarbon soluble salts such as stearates, naphthen-ates, etc. Normally, the ligand only has a marginalinfluence on the course of the autoxidations. Whenreactions are carried out in acetic acid, the metal acetateis commonly employed. In nonpolar media, the phenom-enon of catalyst deactivation is sometimes observed, asthe catalyst is extremely sensitive to polar substancesformed during the reaction.2 For example, in the oxida-tion of cyclohexane, an insoluble precipitate of cobaltadipate is sometimes formed, leading to a decrease inrate.2,10 A striking observation is reported by Suresh etal.,93 who observed the autocatalytic reaction to movefrom the bulk liquid into the film, and then again fromthe film back into the bulk, as a result possibly of suchcatalyst deactivation. These problems are much lesssignificant in a polar protic medium, such as acetic acid.

Finally, at high catalyst loadings, the mechanism ofoxidation is believed to be different, with the autoxida-tion being suppressed.17,94 It is possible that the directattack on the substrate by the metal complex, with theregeneration of the catalyst by reaction with oxygen orperoxidic intermediates (or manganese and bromide, seesection 6.1), becomes important in this case. Sheldonand Kochi2 discuss the mechanisms by which the directinteraction of strong metal oxidants with organic sub-strates can lead to the production of radical intermedi-ates. These oxidations therefore show features notcommonly expected from the free-radical mechanism.Typical rate expressions for such mechanisms are givenby Carra and Santacesaria4 and Tanaka;17 these rateexpressions explicitly involve the concentrations of thecatalyst in its different oxidation states.

5.7. Co-oxidations

Consideration of co-oxidations, in which a mixture oftwo or more substrates is autoxidized, is important forseveral reasons. First, they form the basis for someindustrial production processes (for example, p-xyleneand paraldehyde or acetaldehyde; see section 4). Sec-ondly, even other oxidation systems, at moderate to highconversions should, strictly speaking, be considered asco-oxidation systems because of the participation of theproducts. Third, they provide the chemist with a toolfor obtaining quantitative information on the relativereactivities of peroxy radicals on different hydrocarbons.

Sheldon and Kochi2 cite several instances wheredramatic effects have been observed by the addition ofsmall amounts of a second substrate. The drasticreduction in the rate of oxidation of cumene in thepresence of readily oxidizable impurities has been welldocumented,7,26 and explained on the basis of the ratesof the cross-propagation (i.e., between one hydrocarbonand the alkylperoxy radical derived from the otherhydrocarbon) and the cross-termination (between alky-lperoxy radicals derived from different hydrocarbons)reactions. It is also often observed that a large increasein the rate of oxidation of an unreactive component isobtained in the presence of a small amount of asubstance easily attacked by alkylperoxy radicals. In-deed, this is one of the reasons for the autocatalysiscommonly observed in organic oxidations. When amaterial with a low oxidizability is co-oxidized with oneof high oxidizability, the former may oxidize faster thanthe latter if kp ratios favor the former. Methods of

ROOH + Co(II) f RO* + Co(III) + -OH (13)

ROOH + Co(III) f RO2* + Co(II) + H+ (14)

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estimating such ratios are available, and Hobbs et al.38

have developed procedures based on co-oxidation con-siderations to predict product distributions in hydro-carbon autoxidations.

6. Chemistry of Selected Oxidations

Having given an overview of the general mechanismsthat operate in autoxidations in the previous section,in this section, we will summarize the available infor-mation on some oxidations of commercial significanceas well as some emerging oxidation systems, to showhow the general mechanisms provide a backdrop againstwhich the similarities as well as the differences amongspecific oxidation reactions can be understood. Onceagain, where extensive treatises are available, thediscussion here is kept brief.

6.1. p-Xylene and Other MC Oxidations

The particular kinetic features of these processes areessentially due to the catalyst used, and the relevantchemistry has been comprehensively reviewed recentlyby Partenheimer.12 Acetic acid solvent is essential to themethod. Among other things, Co(II) exists as a dimerin acetic acid/water mixtures, which provides for anonradical, highly selective, and very fast generationof acids from the peracids that form in these oxidationsen route to the final products. A typical solvent compo-sition might be 10% water/90% acetic acid. In theoxidation of p-xylene, a ratio of hydrocarbon to solventof 1:3 is typically used, and the yield of the final productis about 90% (Table 2). The catalyst contains cobalt,manganese, and bromide (for example, cobalt acetate,manganese acetate, and hydrogen bromide) in solutionin acetic acid. This combination offers a high activityas compared to other autoxidation catalysts; it alsooffers a high selectivity over a wide temperature range.These features make the catalyst suitable for a varietyof oxidations. An advantage incidentally arising out ofthe use of acetic acid solvent is that the carboxylic acidend products are usually insoluble in acetic acid at roomtemperature and precipitate out, thus simplifying thedownstream separation of the reaction product.

It is instructive to observe the changes when bromineand manganese are added into a co-catalyzed oxidationof a methylaromatic. Observable effects include a drasticincrease in the rate and an equally dramatic decreasein the vent carbon monoxide and carbon dioxide (anindication of the higher selectivity). Induction times aregenerally reduced, and higher temperatures than ap-plicable to co-catalyzed oxidations become available. Theconcentration of the higher-valency form of cobalt, Co-(III), decreases dramatically, indicating the availabilityof new catalytic pathways that recycle cobalt to the +2state very efficiently. According to a model describedby Partenheimrer,12 the efficacy of the MC catalyst isdue to the fact that the catalytic cycles of cobalt,manganese, and bromide become coupled to producesynergistic results. Thus, the Co(III), which is producedby the oxidation of Co(II) by the hydroperoxide, isrecycled to the +2 state by redox reaction with Mn,which, in the process, is oxidized from the +2 to the +3state. The manganese is, in turn, recycled to the loweroxidation state by reaction with bromide, and finally,the bromide is regenerated with the generation of ahydrocarbon radical of the R* type. Bromine atom beinga much better H abstractor from the aromatic methyl

group than Co3+, the net result is a much higher activitydue to a higher rate of initiation) and selectivity(presumably due to increased chain length).

In general, it can be surmised that the reactiveagents, alkylperoxy radicals and bromine atoms, selec-tively abstract the most weakly bound H. Thus, ringcarbons are left untouched; on the other hand, any Catom attached directly is susceptible to attack. Thus,benzylic methyl group is attacked preferentially toaliphatic methyl group (e.g., in acetic acid). The tert-butyl group will be oxidized, but at a much lower ratethan the benzylic methyl group.

The relative rates of oxidation of methylaromaticcompounds in MC oxidations (relative to toluene) havebeen correlated by the Hammett function

where k and k0 are the rate constants for the disap-pearances of the hydrocarbon and toluene, respectively,σ is the Hammett constant characteristic of the sub-stituent on the ring, and F, a constant that depends onthe conditions of oxidation (such as solvent, catalystcomposition, etc.), has a value of -0.95 for the solventand catalyst typical of industrial applications. Thenegative sign indicates that the electron-withdrawingsubstituents are less active. In the industrially impor-tant case of polymethylbenzenes, this correlation an-ticipates that, once the first methyl group is oxidized,the second methyl group, in a para- position withrespect to first, will be much less reactive because ofthe reduction in the ring electron density. Thus, in theoxidation of p-xylene, p-toluic acid is about 10 times lessreactive than p-xylene, and high yields of p-toluic acidcan be obtained before oxidation of the second methylgroup commences. Alternative explanations have alsobeen advanced for the low reactivity of this intermedi-ate, for example, by Hobbs.3 In any case, the effect isseen to be much more pronounced in cobalt-catalyzedsystems (as indicated by the higher value of F for thesesystems). Thus, if one wants to stop the reaction afterthe oxidation of a single methyl group, one prefers acobalt-catalyst system, and if one wants the oxidationof all the methyl groups, one prefers an MC system.12

Although several byproducts have been detected inMC oxidations, most form in less than 0.1% yield andinclude CO2 and methyl acetate intermediates from theoxidation of subsequent CH3 groups after the first (forexample, the troublesome 4-carboxy benzaldehyde in theoxidation of p-xylene). Despite the features mentionedabove, some combustion of the solvent acetic acidresults, and this is one of the areas in which researchefforts are active. With naphthalene derivatives, bro-mination of the ring to give undesired products is aproblem. Another problem that is intrinsic to themethod is that water (which is a reaction product)deactivates the MC catalyst. Continuous addition ofacetic anhydride has been suggested12 as a way toreduce the problem. However, another intrinsic problem,that of precipitation of the catalyst metal by the productcarboxylic acid, is less serious if water is allowed toaccumulate. This latter problem can also be containedby a decrease in pH and an increase in bromideconcentration. Anything that affects the steady-stateconcentrations of phenolic radicals, of Co(III) and Mn-(III), and of the chemical form of the catalyst is apotential deactivating agent. Thus, phenols, sulfides,primary amines, etc. in trace amounts can deactivate

log(k/k0) ) σF (15)

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the catalyst (phenolic radicals are more long-lived thanphenyl). Some metals are strong inhibitors. The re-agents that are themselves antioxidants must be maskedfor oxidation to proceed (acetylation of phenols andamines, etc.). The aromatic acids also directly deactivatethe catalyst, but precipitation of acids from the aceticacid contains this problem. It is therefore advisable tochoose the minimum possible temperature for theoxidation so that the solubility of acid is kept small. Thepurity of the precipitated acid also depends on thetemperature. For example, 96% terephthalic acid pre-cipitates from pure acetic acid at 200 °C, whereas 99.4%precipitates at 100 °C.

6.2. Oxidation of Cyclohexane

Because of the selectivity problems in cyclohexaneoxidation, the uncatalyzed oxidation to increase thehydroperoxide is a relevant process option, as discussedin section 4. The role of wall catalysis must be dulyconsidered in such process designs. Berezin et al.10 showthat the product spectrum in glass reactors is quitedifferent to that in steady-state reactors. This leads toobvious problems in translating the results from labora-tory reactors to commercial-size reactors. Use of alu-minum for the wetted surfaces in laboratory reactorsis an innovation that has been tried recently with goodresults.59

Oxidation in glass reactors follows the general radicalmechanism outlined above rather closely. The charac-teristic feature of cyclohexane oxidation is that theconversions are kept deliberately low. It is therefore thechemistry under these low-conversion conditions thatis of interest. The kinetic chains are usually reasonablylong under such conditions. Cyclohexyl hydroperoxideis the sole primary product and its radical decompositionis responsible for most of the degenerate chain branch-ing. At extremely small conversions, the overall rateshows a square-root dependence on the hydroperoxideconcentration, as is to be expected from unimolecularbranching and quadratic termination. Some strikingdifferences are observed when oxidation is carried outin a steady-state reactor. The hydroperoxide is no longerthe sole primary product; cyclohexanol forms from thestart. It has been shown10 that up to 30% of thecyclohexanol forms directly from the cyclohexane, therest coming via the hydroperoxide. Furthermore, ketoneand not the hydroperoxide has been implicated as theproduct mainly responsible for chain branching. In fact,the hydroperoxide levels in oxidation in steel vessels (asin the case of catalyzed oxidation) are much smallerthan those in the case of oxidation in glass vessels.

The loss of selectivity with conversion in the case ofcyclohexane oxidation is because of the high reactivityof the oxidation products. Both cyclohexanol and cyclo-hexanone are oxidized many times faster than cyclo-hexane itself. At equal rates of initiation, the rates ofoxidation of pure compounds follow the ratio95 cyclo-hexane, 1, to cyclohexanol, 40, to cyclohexanone, 27. Thecase of cyclohexanol is somewhat curious, as its additionat the start of the reaction actually retards the oxida-tion, whereas if it is added during the oxidation, it hasan accelerating effect. This result has been rational-ized10 as being due to the fact that, whereas cyclohex-anol is more reactive than cyclohexane, the radicals thatform from it are less reactive than the cyclohexylperoxyradicals. In the medium of oxidizing cyclohexane, cy-clohexanol is converted almost quantitatively to cyclo-

hexanone; hence the beneficial effects of boric acid,which traps cyclohexanol, thereby preventing its furtherconversion and all of the side reactions that therebyresult. Cyclohexanone is also a highly reactive com-pound in the oxidizing medium; the ratio of the rateconstants for its formation and consumption has beenestimated as 1:30. Although Berezin et al.10 claim fromestimated values of activation energies of these reac-tions that higher temperatures should favor selectivitiesto the ketone, no clear evidence of this nature is seenin the available data. Water, which forms late in thereaction, seems to have an inhibiting effect on thereaction and can, under certain conditions, cause asecond phase to form.

When oxidation is carried out in a solvent mediumsuch as acetic acid, decarboxylation of the solvent is aproblem as in the case of MC oxidations. There are fewreports of MC catalysts having been tried for cyclohex-ane oxidation, say in efforts to develop a single-stepprocess to adipic acid.

6.3. Oxidation of Cumene

The liquid-phase oxidation of cumene is a uniqueexample of “uncatalyzed” reaction, taken to levels ofconversion around 30%, that is commercially practiced.

To avoid problems of leakage and the attendant prob-lems of safety, such oxidations are carried out in large-size bubble-column reactors (sparged reactors) that arejacketed, and in the recent past, even provided with coilsinside the reactor. The liquid circulation velocities insuch bubble columns are remarkably high, and fairlyhigh values of the heat-transfer coefficient are realizedeven through the jacket, facilitating the heat removalproblem.

Typically, oxidation is carried out at around 135 °C(see Table 2) and a pressure at the top of the reactor onthe order of 3-4 atm; liquid heights are typically 10-15 m. The oxidation is carried out in 4-6-stage oxidizersto improve the selectivity. The bubble-column reactorsare staged, as the extent of liquid-phase backmixing inbubble columns is very high. An optimized overall levelof conversion is about 30%; above this level, the yielddecreases because of side reactions associated withdecomposition of the hydroperoxide to the carbinol,cleavage to acetophenone and methanol, etc.

At overall levels of conversion approaching 30%,inevitably some oxidation at -CH3 occurs along with

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cleavage, and some formic acid is formed. This formicacid catalyses the decomposition of the hydroperoxideto phenol and acetone, and this, in turn, inhibits theoxidation. To obviate this problem, a small amount ofwater, containing buffered carbonate-bicarbonate, isused so that formic acid is instantaneously neutralized.Originally, the aqueous-phase volume used was high(even 5%), but in subsequent years, because of a betterunderstanding of the process, this volume has beenreduced to only 1-2%, and of late even “dry” oxidationshave been adopted, i.e., use of water is renderedunnecessary because of better control of operatingconditions.

Because the level of conversion of cumene is only upto 30%, the remaining cumene, separated after thehydroperoxide is concentrated to about 80% (and of lateeven up to 90%), must be recycled and constitutes themajor part of the feed to the reactor. This cumene, aswell as cumene obtained from cleavage of cumylphenoland R-methyl styrene (AMS) dimers, obtained duringacid-catalyzed cleavage of the hydroperoxide to phenol,has R-methylstyrene as an impurity, which gives a“bromine number” to cumene and is detrimental to theoxidation. This side-product must be suitably removed,e.g., via clay-catalyzed reaction to convert AMS tohigher-boiling alkylated products, which can be easilyremoved by fractionation.

The cumene-based phenol technology has been adoptedfor making m- and p- cresols and follows similar plantpractices. However, in large plants, additional problemsarise, as there is some oxidation at the methyl groupas well.

The hydroperoxide on the CH3 group, upon cleavage,will give -CH2OH, and this further complicates theseparation train.

The oxidation of m- and p- diisopropylbenzene, whichcan be separated by distillation because of a significantdifference in boiling points, eventually leads to dihy-droperoxides, which, on cleavage, give m-dihydroxyben-zene (resorcinol) and p-dihydroxybenzene (hydroqui-none). The array of products that can be formed hasparallels with that encountered in manufacturing phe-nol.However, since the switch over to the second isopropylgroup starts when the first group is nearly fullyconverted to the hydroperoxide, the level of carbinolsthat are formed will be relatively much higher thanthose obtained in the case of phenol. This will also bethe case for acetophenone-type products. The byproductsdo have some commercial value. In the recent past, thedehydrated products of dicarbinols giving R-methylstyrenes have gained some importance, as they have

some unique properties, in some respect like divinylbenzenes, and as they can be converted to some valueadded substances, via reaction with phenol to make aspecial bisphenol, via reaction with HCN and conversionto the corresponding amines, or even direct reactionwith ammonia to give the corresponding amines, asshown below.

The deliberate desire to make the above R-methylstyrenes would benefit from this route in view ofmildness of conditions and high yields. Catalytic dehy-drogenation of two isopropyl groups is most unlikely togive a high yield of R-methyl styrenes.

6.4. Oxidation of Isobutane

The oxidation of isobutane in the liquid phase to tert-butyl hydroperoxide (TBHP) and tert-butyl alcohol(TBA) is widely practiced. Although products are im-portant themselves (e.g., TBHP as an oxidizing agentand a free-radical initiator and TBA as a gasolineadditive), a major application of the process in the pasthas been in the indirect oxidation process for themanufacture of propylene oxide, wherein TBHP is usedto epoxidize propylene.8 The recent interest in thepossibility of developing isobutane oxidation for theproduction of MTBE and some related issues have beendiscussed in section 4. MTBE can be produced conve-niently by etherification of TBA with methanol. The

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production of isobutene from isobutane via oxidation toTBHP, decomposition of TBHP to TBA, and finaldehydration, may be cheaper than catalytic dehydro-genation. However, if the TBHP is used for epoxidation,then the economics of producing isobutene from TBAimproves dramatically.96

The first comprehensive work on isobutane oxidationwas that of Winkler and Hearne,11 who carried out theliquid- and vapor-phase oxidation in a semibatch reac-tor. The authors do not recommend oxidation in thevapor phase because of the formation of unacceptablylarge quantities of side products. Hence, it has beensuggested that a reaction product consisting essentiallyof TBHP and TBA can be obtained in high yield byreacting isobutane with molecular oxygen in the liquidphase of a vapor-liquid mixture at a temperature inthe range of 373-423 K and a pressure of at least 25atm. The prominent features of hydrocarbon oxidation,such as induction period, autocatalysis, and catalysisby the metal wall of the reactor, are evident in theirwork. Their results indicate that, with proper precautionto prevent wall catalysis, the total yield of TBHP andTBA is fairly constant between 90 and 96% at temper-atures of 398 K and below, over a fairly wide range ofconversion.

Winkler and Hearne11 used di-tert-butyl peroxide asan initiator in most of their experiments. Under thesecircumstances, radical initiation in the initial stages islikely to be by the decomposition of the peroxide

with R* taking part in the normal propagation reactions5 and 6 (see section 5). If TBHP is the desired product,use of the initiator can have the undesirable conse-quence of reducing its concentration because of thereaction of the RO* radical with the hydroperoxide toproduce the alcohol in a new propagation step.

In any case, the RO* radical may decompose to acetoneand a methyl radical, a possibility that accounts for thepresence of acetone in the reaction mixture. The methylradical so produced leads to other products such asmethanol, formic acid, CO, and CO2. The levels of mostof these are quite small, as indicated by the highselectivity figures quoted above.

If cobalt is used to catalyze the reaction, the conse-quences of catalyzed decomposition of tert-butyl hydro-peroxide ensue, which in this case include, in additionto a faster rate of oxidation and more TBA, higher levelsof RO* decomposition products.

Winkler and Hearne11 proposed that the rate of theoxidation reaction in the liquid phase can be enhancedby carrying out the oxidation above the critical temper-ature of isobutane (407.9 K). However, it is necessarythat the reaction be conducted in the presence of arelatively high boiling solvent. The use of externallysupplied reaction solvents, e.g, organic acids, is discour-aged by Winkler and Hearne11 because of the increasedcomplexity of the oxidation reaction and subsequentproduct separation and recovery. The idea of usingadditives such as 2-propanol or water for improving theyield, which appears in several patents subsequent tothe work of Winkler and Hearne,11 does not seem to

have been established in industrial practice, probablyfor the same reason.

The conditions under which the oxidation of isobutaneis normally carried out in industry (temperature of 393-408 K and pressure of around 35 bar) are quite close toits critical properties (Tc ) 406.9 K and Pc ) 35 bar). Inview of recent interest in studying chemical reactionsunder supercritical conditions, it can be expected thatphenomena such as the clustering effect and the specialionization effects of the supercritical fluid would havea strong influence on the reaction mechanism.84,97

Surprisingly, limited literature is available on isobutaneoxidation under supercritical conditions. However, theinfluence of supercritical conditions was explicitly ad-dressed by researchers from the Shell oil companythrough patents.30,32 They carried out detailed studieson batch and continuous reactors to identify the effectof the important operating variables on productivity andselectivity under supercritical conditions and concludedthat operation in the supercritical phase gives TBHPproductivities several times higher than those obtainedwith the conventional liquid-phase process.30 A temper-ature range of 423-433 K has been claimed to be thebest for high productivities. Baumgartner31 carried outa reaction of isobutane and molecular oxygen in acontinuous stirred tank reactor (CSTR) with a residencetime from 15 to 70 min, and concluded that operationin supercritical phase at less than 0.04 mol % oxygenconcentration leads to TBHP selectivity better than thatobtained at a high concentration of oxygen (1% mol/mol)in the feed. This is somewhat counterintuitive, ashydrocarbon oxidations are usually zero-order in oxygen,and if oxygen concentrations are too low for the zero-order behavior to be observed, one only expects poorerselectivity because of the increased heterogeneity of theradical population. However, as has been pointed out,data in the low-oxygen regime is far too minimal in theliterature to permit firm conclusions. Building on thework of Baumgartner,31 Foster32 employed a reactorconfiguration that keeps isobutane concentration highwhile keeping product and oxygen concentration low,which also results in better productivities. The proposedreaction system envisages a cascade of several CSTRswith the supercritical isobutane moving from one CSTRto the next in series and the oxygen being fed in a cross-current manner.

Shah et al.98 have examined the kinetics of isobutanein the liquid phase and under supercritical conditions.The influence of supercritical conditions on the rate andselectivity of the reaction was investigated, and thispermits a comparison of liquid-phase and supercritical-phase oxidation. The reactions were also studied in aglass-lined continuous reactor using predissolved oxy-gen. This mode of operation allows kinetic studieswithout an intervention of gas-liquid mass transferand, in addition, eliminates the catalytic effect, if any,of the stainless steel walls. The reaction is autocatalytic,and the selectivity toward the hydroperoxide decreaseswith an increase in overall conversion. Under super-critical conditions, the rates and selectivity were sig-nificantly higher than those obtained in liquid-phaseoxidation. In both subcritical and supercritical oxida-tions, temperature has an adverse effect on selectivitytoward tert-butyl hydroperoxide. The temperature andwall material exhibit a strong impact on the reactionkinetics. TBHP has a tendency to decompose and reactwith isobutane to form TBA. The decomposition is

ROOR f 2RO* (16)

RO* + RH f ROH + R* (17)

RO* + ROOH f ROH + ROO* (18)

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highly influenced by the wall material, and it is at itsmaximum in the unpassivated stainless steel reactor.

Fan et al.84 have studied the oxidation of supercriticalisobutane to TBA and have investigated the influenceof silica-titania and palladium-carbon as heteroge-neous catalysts. The authors compare the performanceof the reaction under gas-, liquid-, and supercritical-phase conditions and conclude that the rate as well asselectivity to TBA are superior in the supercritical case.Some dehydration of TBA to isobutene was also ob-served in their work, which the authors attribute to theaction of acid sites on the catalyst surface. Althoughtheir data show the superior rate and selectivity ofsupercritical oxidation over gas-phase oxidation (as isto be expected), comparison with liquid-phase oxidationis less clear, as the latter was conducted at a lowertemperature. Furthermore, contrary to the claims of theauthors, their results, in fact, show a smooth transitionin rate and selectivity as the temperature is increasedat a constant (supercritical) pressure to go from liquidto supercritical state. The promotional effect of the het-erogeneous catalyst is, however, clearly demonstrated.

Suresh,90 in his analysis of isobutane oxidation undersupercritical conditions, has concluded that reactionkinetics in the liquid phase can be extrapolated conve-niently in order to predict the kinetics under supercriti-cal conditions. An enhancement in the reaction rate isrealized under supercritical conditions. However, themodeling work indicates that this increase arises onlybecause of the increase in temperature employed tobring the reaction mixture to the supercritical state. Thefact that the rate expression used for predicting theliquid-phase kinetics works reasonably well under su-percritical conditions indicates that the “supercritical”state of the substrate, as such, does not induce anyremarkable effect on either the rate of the reaction orthe selectivity toward the desired products.

6.5. Oxidation of Cycloalkenes

The interesting and diverse chemistry exhibited bythe oxidation of saturated hydrocarbons is also seen inthe oxidation of unsaturated hydrocarbons. Early stud-ies (see Kamneva and Panfilova99) were concerned withelucidating the structure of the hydroperoxide. Criegeeet al.100 demonstrated that cyclohexene hydroperoxidehas an open structure with the double bond retained.Van Sickle et al.101 discuss some of the early work.Among the cycloalkenes, cyclopentene is the most reac-tive, and cyclooctene is the least reactive. The authorshave emphasized that many olefins react partly or fullythrough addition of peroxy radicals to double bondsrather than through hydrogen abstraction (or transfer).The products formed from these oxidations depend onwhether the hydrogen abstraction or the addition reac-tions dominate.

Most of the cycloalkenes give the hydroperoxide asthe primary product. However, cyclooctene differs fromits analogues in that it gives high yields of the epoxide,presumably through the addition of the peroxy radical.

Furthermore, in the case of cyclooctene, phase separa-tion occurs even at conversions as low as 0.4%. Figure1 provides a schematic diagram of the various chemicalpathways for cycloalkenes.

Murphy et al.,102 in their lucid review of allylicoxofunctionalization of cyclic olefins, point out thatallylic oxidation (which preserves the unsaturation andresults in R,â-unsaturated ketones and alcohols of cyclicolefins) and epoxidation are two competing processesboth in vivo and in vitro. Allylic oxidation (involving freeradicals) is most likely in the presence of low-oxidation-state transition metal species, and epoxidation is to beexpected in the presence of species such as Ru(VIII), Cr-(VI), etc., but the two are often competitive processesin practice. The process that dominates depends, amongother things, on the nature of the olefin and the relativestability of the allylic radical formed. With the morecomplex cyclic olefins, competitive isomerization andstructural rearrangements often result in poor selectiv-ity to the desired R,â-unsaturated ketone. The authorsillustrate these points with the aid of examples takenfrom the industrially relevant cases of cyclohexene,isophorone (to ketoisophorone, a key intermediate in thesynthesis of carotenoids and flavoring substances), andR-pinene (to verbenone, considered a suitable precursorto taxol). Developments in both homogeneous andheterogeneous catalysis are reviewed.

Mahajani et al.103 have examined the kinetics of theoxidation of cyclohexene. Cyclohexene is emerging asan important raw material for cyclohexanol, cyclohex-anone, cyclohexene epoxide, cyclohexenol, cyclohex-enone, 1,2-cyclohexanediol, and cyclohexadiene.104,105

The successful commercialization of the selective hy-drogenation of benzene by Asahi chemicals provides acost-effective route to this raw material. The oxidationof cyclohexene is, in some respects, similar to that ofcyclohexane. The various chemical pathways for cyclo-hexene are shown schematically in Figure 1. The uncat-alyzed oxidation of cyclohexene yields large quantitiesof the hydroperoxide, and its decomposition yields thealcohol and ketone. The uncatalyzed reaction also yieldscyclohexene oxide. Moreover, the oxide appears to be aprimary product, but the exact mechanism for itsformation from the hydrocarbon is not clear. The kineticmodel proposed by Suresh et al.49 for cyclohexane oxida-tion applies equally well for this reaction. The oxidationof cyclohexene can be taken to much higher levels,approaching 20% conversion, than in the case of cyclo-hexane, for which conversions of 3-5% are needed toensure high selectivity. The products obtained fromcyclohexene allow a wider range of speciality chemicalsto be produced.

The hydrogenation of benzene is the only viable routeto cyclohexene, and selectivity is only on the order of50%. In the case of cyclododecene, almost quantitativeyields can be obtained by the selective hydrogenationof cyclododecatriene, which, in turn, can be convenientlymade by the selective trimerization of butadiene. Simi-larly, the selective hydrogenation of cyclooctadieneyields cyclooctene. Thus, there may be a distinct ad-vantage in obtaining cyclododecanone and other prod-ucts, similar to those realized in the oxidation ofcyclohexene via the oxidation of cyclododecene ratherthan from cyclododecane. Interest in this area can bediscerned from some patents for the oxidation of cy-clododecene106,107 using a ruthenium/cerium system asthe catalyst. In the first of these,106 a two-phase system

RO2* + R ) RO2H + R* (19)

RO2* + R ) RO2R (20)

RO2* + R ) RO* + epoxide (21)

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is used with ruthenium tetroxide in the organic phaseas the oxidizing agent. The tetroxide is regenerated inthe second (aqueous) phase, which contains cerium ionsin the +4 state. The latter, it is suggested, can beregenerated electrolytically in a separate step. Thesecond patent107 provides a way of combining these stepsin order to achieve simultaneous regeneration of thecerium (+4) species.

Van Sickle et al.101 have reported preliminary dataon the noncatalytic oxidation of cyclododecene, wheresubstantially high yields of epoxide have been indicated.Recent unpublished results from Monash University,108

carried out in a way to obtain data of commercialimportance, clearly establish the formation of the ep-oxide, in addition to cyclododecenone; there are clear

differences in the rate of oxidation of the cis- and trans-cyclododecene.

The epoxidation of alkenes (cyclic as well as noncyclic)using molecular oxygen in a single step has remaineda challenge in the field of oxidation chemistry. Indirectoxidations in two steps (that of propylene to propyleneoxide is an example; see section 4) are the rule inindustry. Recently, Iwahama et al.109 reported a one-pot epoxidation of alkenes (such as octene-1-cyclohexeneand cis- and trans-2-octene) using molecular oxygen,using hydrocarbons such as ethylbenzene and tetralinas the hydrocarbon source under mild conditions. Theautoxidation of the hydrocarbon was assisted by N-hydroxypthalimide (NHPI), and the epoxidation of thealkene with the resulting hydroperoxide was catalyzed

Figure 1. Prominent chemical pathways in cycloalkene oxidations.

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by Mo(CO)6. Although the rates were generally lowunder the conditions employed (6-12 h at 60-70 °C),the results are interesting in view of the excellentselectivities reported. With cyclohexene and cyclooctene,selectivities of 74% and 83%, at conversion levels of 80%and 89%, respectively, were obtained. The use of NHPIseems to overcome the mismatch between the muchhigher rate of oxidation of the hydrocarbon to hydrop-eroxide as compared to the rate of epoxidation inconventional systems. In the case of adamantane, in thepresence of carbon monoxide and oxygen, 1-adamantanecarboxylic acid was formed.

6.6. Oxidation of Vinyl Cyclohexene and VinylCyclohexane

These reactions are potentially attractive both scien-tifically and commercially, as these versatile olefiniccompounds are available in high yield from butadiene.

In the case of vinyl cyclohexene, the oxidation cantake place at the olefinic double bond in the ring, as inthe case of cyclohexene, or at other allylic positions.There is really scanty information on autoxidation ofthis important olefinic compound, although some infor-mation is available on Wacker-type oxidation, withcupric chloride-PdCl2 catalyst, to a ketonic compound.Biela et al.110 have studied the autoxidation of this olefinas well as vinyl cyclohexane, which can be obtained byselective ring hydrogenation of vinyl cyclohexene. In thecase of vinyl cyclohexane, about 40% of the absorbedoxygen goes to the peroxides. In vinyl cyclohexene, theC-H bonds in position 6 are preferentially attacked,although other allylic positions also undergo oxidationto give the products illustrated below.

The relative product distribution with respect to the cis-and trans- isomers is not obvious (see also section 12.8).

In the case of vinylcyclohexane, the following domi-nant products are obtained in equimolar amounts:

The other products are shown below.

6.7. Miscellaneous Oxidations

Interest in the epoxidation of alkenes has beenmentioned earlier (see section 6.5) because of theimportance of such a process from the synthetic andindustrial points of view. The work of Iwahama et al.109

was discussed earlier and involves some interestingchemistry. These authors have demonstrated the Mo-catalyzed epoxidations of a wide variety of alkenes withhydroperoxides generated in situ by the NHPI-catalyzedaerobic oxidation of hydrocarbons. The selectivity to theepoxide depended, among other things, on the hydro-carbon used, ethylbenzene and tetralin giving goodresults whereas toluene gave poor results with oct-2-ene, probably as a result of competing reactions in thecase of toluene. Although Mo(CO)6 was employed as theepoxidation catalyst in most of their work, the epoxi-dation of allylic alcohol, trans-hex-2-en-1-ol, to epoxyalcohol was achieved in high yield when VO(acac)2 wasemployed instead, even with very small amounts. It ispossible that VO(acac)2 not only catalyzes the epoxida-tion, but also serves to activate NHPI, a functionperformed by cobalt acetate in the other oxidations.

Another recent development of interest in this areais the direct epoxidation of linear aliphatic olefins bymolecular oxygen using Schiff base complexes.111 Pos-sibilities of heterogenizing the catalyst by encapsulationin modified zeolite cages have also been demonstrated,and good selectivities in both configurations have beenrealized at reasonable conversions.

Mayo et al.112 have reported some interesting resultson the oxidation of R-methylstyrene (AMS) at 110-160°C. For temperatures up to 100 °C and adequate oxygen,the principal product is the alternating polyperoxide.This peroxide is reasonably stable at the temperatureof oxidation but cleaves cleanly to acetophenone andformaldehyde at higher temperatures and reducedpressure. Between 100 and 120 °C and at atmospherictotal pressure, the principal products are AMS oxide andacetophenone; it appears that there was mass-tranferlimitation and that the liquid phase was not saturatedwith oxygen.

7. Kinetics of Hydrocarbon Oxidation

The determination of kinetics of liquid-phase organicoxidations presents a nontrivial problem. The quantita-tive analysis of the various products from a reactor is,by itself, a significant challenge. In addition, there areother issues to consider. First, as is obvious from thepreceding discussion on the chemistry and kineticmechanism of these reactions, a detailed treatment ofkinetics is usually not feasible; fortunately, it is alsoprobably unnecessary for engineering applications. Anengineering approach to kinetics, involving lumpedmodels, is thus usually followed. One postulates aseries-parallel reaction network that attempts to cap-ture the essential features of the reaction depending onthe purpose for which the kinetic model is intended.Second, because the most convenient way for conductingthe reaction is through contact of a liquid, hydrocarbonunder the appropriate conditions with a gaseous stream

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containing oxygen, the possible influence of mass-tranfer factors on the observed behavior must beconsidered carefully in interpretations of rate data.Finally, although one would like laboratory reactors andtheir operation to be flexible in order to explore all thefeatures of the reaction, considerations of safety imposerestrictions on reactor design and operation. Theseissues have resulted in several innovative approachesto the determination and interpretation of technicalkinetics in the field of organic oxidations.

7.1. Laboratory Reactors

Early studies considered the reaction to be generallyslow enough for intrinsic kinetics to control in anylaboratory reactor that provides reasonable mass-tran-fer rates. Several arguments can be advanced againstsuch an assumption. First, the reactions are known toexhibit autocatalytic features, and the reaction rateincreases as the products accumulate. Therefore, theinfluence of mass transfer can become important duringthe course of the reaction, even if the reaction is initiallyslow and kinetically controlled. Second, at the elevatedtemperatures and pressures necessary for these reac-tions, the knowledge of mass-tranfer coefficients, evenin laboratory reactors, is far from perfect, and extrapo-lation from data and correlations under ambient condi-tions is hazardous.48 For many systems, even propertiessuch as solubility and diffusivity are not documentedin the open literature and must be estimated for theconditions of reaction. (The available information onthese aspects will be reviewed in section 9.) Thus, onemust necessarily consider the implications of masstransfer with chemical reaction in designing kineticexperiments and interpreting data from them.

The theories of mass transfer with chemical reactionindicate that gas-liquid reactions can occur in one ofseveral “regimes”, depending on the relative rates ofmass transfer and chemical reaction. To determine thekinetics with reasonable accuracy, the reaction musttake place in the slow-reaction regime with substantialkinetic control (in which case the overall rate is deter-mined by the reaction rate in the bulk liquid) or in thefast-reaction regime (in which the reaction is completein a very small region close to the interface). Several“model” reactors suitable for kinetic studies on gas-liquid reactions have been described in the literature(such as laminar jet, wetted wall column, stirred con-tactor, etc), in which the mass-tranfer rates can becharacterized with a high degree of confidence (see, forexample, Doraiswamy and Sharma43). An appropriatechoice of the model reactor must be made, dependingon the velocity of the reaction being studied, so that theright regime can be engineered and the reaction kineticsdetermined. The conditions of organic oxidations (el-evated temperature and pressure) usually necessitateconsiderable design modifications if such model contac-tors are to be used. Most of the kinetic studies describedin the literature have therefore been conducted inminiature versions of industrial equipment, such assparged and mechanically agitated reactors, bubblecolumns, etc. Because the mass-tranfer parametersneeded to properly interpret reaction data from thesereactors are usually not known with confidence (espe-cially under the reaction conditions employed), it ishazardous to infer the regime on the basis of conven-tional tests such as the effect of agitation speed, etc.For example, the effect of agitation speed on interfacial

area is quite different at low and high speeds inpressurized contactors46,113 and not taking cognizanceof such differences can result in erroneous conclusionsbeing drawn. It is therefore preferable in such studiesto rely on some direct indicator of the reaction regime.In their studies on cyclohexane oxidation, Suresh et al.49

developed a technique for measuring dissolved oxygenlevels during reaction. The presence of measurablelevels of dissolved gas shows that the reaction is in theslow-reaction regime with some degree of kinetic control.These authors observed that the reaction starts off beingkinetically controlled (with the liquid being saturatedwith oxygen at the prevailing partial pressure) butgradually moves to other regimes because of autoca-talysis. This transition could be followed directly withthe help of the observed variation in dissolved oxygenlevels, and the part of the experiment in which kineticshad a role in determining the observed absorption ratescould be identified. Even at the rates of mass transferthat could be achieved in small mechanically agitatedcontactors with intense agitation, measurable concen-trations of dissolved oxygen could be detected only atconversions less than 5-7%. Therefore, it seems incor-rect to assume the absence of mass-tranfer limitationsin such reactions in general.

An alternative strategy for circumventing the problemof mass-tranfer interferences in kinetic measurementsis to eliminate the mass-tranfer step altogether bypredissolving enough oxygen in the liquid before thestart of the reaction and then to conduct the reactionhomogeneously. The zero-order dependence of oxidationrates on oxygen is an advantage here, as the oxygenconcentration need not be followed as long as it doesnot become so small during the reaction as to call intoquestion the zero-order assumption. However, if reason-able conversions must be achieved, acievement of theoxygen requirement necessitates high pressures at thedissolution stage. Suresh et al.49 used this principle tostudy cyclohexane oxidation in small batch reactors(“microautoclaves”). Oxygen was dissolved by equilibra-tion at high pressures at room temperature, and thetemperature was rapidly raised by plunging the reactorin a fluidized bed maintained at the desired reactiontemperature. Reaction was stopped by plunging thereactor in a cold-water bath, the oxygen conversion wasmeasured by slow depressurization and measurementof the volume of the oxygen released, and the liquidcontents of the reactor were analyzed for hydrocarbonconversion and product profile. In such work, becauseeach experiment gives a single conversion point, it isimportant to reproduce induction periods exactly in aseries of experiments by thorough cleaning betweenruns. Passivation of the walls by procedures similar tothose used by Winkler and Hearne11 may also benecessary to prevent wall catalysis. With sufficient care,however, reliable kinetic information can be obtained.Wen et al.63 and Guo61 have used a similar principle tostudy cyclohexane oxidation in homogeneous continu-ous-flow reactors. The cyclohexane was saturated withoxygen at room temperature at the required pressure(depending on the conversion desired), and the satu-rated liquid was pumped through the reactor. At theoutlet of the reactor, the pressure was allowed todecrease, and the flow rates of the gas evolved and theliquid were separately measured. Wen et al.63 used aCSTR constructed from a length of tube and stirred bya magnetic ball moved to and fro by the action of an

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external magnet, whereas Guo61 used a glass-lined plug-flow reactor. The usefulness of these reactors is dem-onstrated by the internal consistency of the data ob-tained on the oxidation of cyclohexane from severaltypes of reactors, including the mechanically agitatedreactor, batch microautoclaves, CSTR, and PFR. Takentogether, these studies49,61,63 represent the most com-prehensive studies on a single oxidation reaction ofcommercial importance.

Attention was drawn earlier to the presence of aninduction period in organic oxidations and some of theconsequences that follow from it. Induction periods canbe quite long in uncatalyzed oxidations and may not bequite eliminated even in the presence of some reactionproducts or dissolved catalysts.49,93,114 The length of aninduction period can be influenced by the presence ofsmall levels of impurities, wall catalysis (in steady-statereactors), etc. Variability of induction periods is par-ticularly important in laboratory studies, where thesmall volumes of reactors employed result in an exag-gerated effect of such factors. Although the use of glass-lined reactors, or passivation techniques to render thewall noncatalytic if steady-state reactors must be used,is recommended for complete reproducibility of batchdata, Suresh et al.49 have shown that good reproduc-ibilty of post-induction-period behavior in batch experi-ments can be achieved with thorough cleaning of thereactor between experiments, even if induction periodvariability is not totally eliminated. This also meansthat, in flow reactors, any differences in inductionbehavior do not influence steady-state behavior. How-ever, in batch microautoclave studies, in which severalexperiments are necessary to construct a single timecourse of the reaction, absolute reproducibility of induc-tion periods is a must. Some authors (for example,Winkler and Hearne11 and Morbidelli et al.115) attemptto eliminate induction periods by the use of free-radicalinitiators, but (depending on the concentrations em-ployed) such initiators could affect more than just theinduction behavior, and it is difficult to be certain thatthe kinetics one obtains is not itself influenced. Thus,initiators could reduce the chain length (see discussionin section 5) to a point where initiation and terminationmechanisms influence the overall behavior more thanthe propagation reactions. Indeed, the data and modelof Morbidelli et al.115 for ethylbenzene oxidation showno autocatalysis, perhaps as a consequence of the useof fairly high levels of azobisisobutyronitrile (AIBN) asan initiator.

The presence of an induction period can often be usedto advantage in the measurement of various physicalparameters in gas-liquid contact under the conditionsof reaction. The reaction is slow enough to be neglectedunder these conditions, and one sees essentially aphysical absorption behavior. This was first demon-strated by Suresh et al.,49 who determined oxygensolubilities and mass-tranfer coefficients at reactiontemperatures and pressures for the oxygen-cyclohexanesystem by taking advantage of the induction period.More recently, Tekie et al.116 have also used the samemethod to determine the mass-tranfer characteristicsfor the oxygen-cyclohexane system with two types ofcontactors.

Aspects of safety deserve the utmost attention in thedesign and operation of laboratory equipment for hy-drocarbon oxidation studies. Apart from the usualprecautions to be taken in the design and operation of

pressure vessels (provision of high pressure and tem-perature alarms, pressure-relief systems etc.), the dan-gers inherent in hydrocarbon-air contact must beconsidered in detail. Furthermore, to serve their purposeadequately, experimental reactors must be built so asto be flexible and allow operation over a wide and variedrange of conditions. It is to be ensured that safety isnot compromised in such designs. Some of the importantconsiderations are summarized in section 11.

7.2. Kinetic Models from Laboratory Studies

It is clear from the discussion of the mechanisms thatoperate in hydrocarbon oxidations that any attempt toadequately reflect all aspects of the mechanism in akinetic model is bound to fail, not only because of theinherent detail and complexity of these mechanisms, butalso because of the fact that not all pieces of themechanistic puzzle are available yet. Under such cir-cumstances, therefore, one must be satisfied withempirically derived models of technical kinetics that aremeant to address specific modeling or simulation needs,with the understanding that the model or the parameteror both may require modification as the needs change.In literature, one sees several such models. In manysuch cases, an attempt is made to show that the kineticmodel is consistent, or at least not inconsistent, withthe basic free-radical mechanism. Lumped kinetics,involving a series-parallel network of reactions havebeen written for a number of oxidations, in which thelumping follows the logic of process objectives. Thus, incyclohexane oxidation, if the emphasis is on producinga mixture of cyclohexanol and cyclohexanone in the firststage of air oxidation, a series scheme such as A f B fC might be considered (for example, see Spielman117),where A stands for cyclohexane; B, a mixture of alcoholand ketone; and C, the secondary oxidation productssuch as acids and other undesired compounds. If theratio of alcohol to ketone is important, the lump B canbe further split up and the kinetics elaborated to includethe formation of the ketone from the alcohol. If, on theother hand, the process objective is to stop at thehydroperoxide, then the kinetic model would reflect thisby introducing the hydroperoxide as the primary prod-uct, the alcohol and ketone (either separately or as alump) as the secondary product, and acids as thetertiary product. Clearly, the experimental plan toestablish such kinetics could also be dictated by themodeling approach. The kinetic models of Morbidelliand co-workers54 for the oxidation of p-xylene and theearly models for cyclohexane oxidation34,36,117 and isobu-tane28 are of this type.

Although such models usually incorporate explicitlythe observed zero-order behavior of the kinetics withrespect to oxygen, autocatalysis, another commonlyobserved characteristic of most hydrocarbon oxidations,is only accounted for indirectly, if at all. For example,whereas the early models for cyclohexane oxidation donot mention autocatalysis at all, the model for isobutaneoxidation proposed by Brejc et al.33 consists of a networkof reactions whose kinetics are such that autocatalysisis indirectly the result. Botton et al.,92 looking to includeexplicitly a term in the concentration of reaction prod-ucts to explain autocatalysis in cyclohexane oxidation,found that an expression that was first-order withrespect to (lumped) products was able to correlate thedata well. Suresh et al.49 further elaborated thesekinetic expressions by invoking the free-radical mech-

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anism and obtained the following expressions for therates of consumption of cyclohexane (c) and oxygen (o)and the rate of formation of the intermediates i (cyclo-hexyl hydroperoxide + cyclohexanol + cyclohexanone),respectively, in the uncatalyzed oxidation of cyclohex-ane:

Here, cp, cL, and cI are, respectively, the concentrationsof (lumped) products, oxygen, and intermediates. Al-though the rates are expected (from the mechanism) tobe first-order in cyclohexane, because of the low conver-sions to which cyclohexane oxidation is usually run, theconcentration of cyclohexane is essentially constant anddoes not figure in the rate expressions. The equationswere tested by fitting them to data obtained from twotypes of reactors in the kinetic regime and were foundto provide a consistent explanation across both reactortypes and a range of oxygen and product concentrations.Recently, Suresh90 has shown that similar principles canbe used to obtain rate expressions for the oxidation ofisobutane in the liquid and supercritical phases. Ma-hajani et al.103 have also shown the applicability of thecyclohexane oxidation model above for the oxidation ofcyclohexene. It thus appears that some generalizationof rate forms for hydrocarbon oxidation can be at-tempted along these lines.

Similar equations have been used to describe catalyticoxidation as well, as at small catalyst concentrations,the qualitative features are similar in catalyzed anduncatalyzed oxidations. An interesting observation madeby Suresh et al.93 was that the (Co-) catalyzed oxidationretained all of the features of the uncatalyzed oxidationas described by the equations given above, the effect ofthe catalyst (1.5 ppm Co added as cobalt naphthenate)being the same as that of a 10 °C increase in temper-ature. However, at late stages of oxidation (perhapsoutside the range of industrial interest), the rate wasobserved to fall in the catalyzed case, perhaps becausethe catalyst was precipitated out as the adipate. Itwould be useful to look for similar correlations in otheroxidations as well.

8. Processing Options

8.1. The Case for Liquid-Phase Air Oxidation

Although this review concerns liquid-phase air oxida-tions exclusively, in view of the fact that other types ofoxidations (e.g., the use of stoichiometric oxidizingagents and oxidations in the vapor phase) also enjoy asignificant presence in the chemical industry, it is notout of place to consider briefly the relative merits anddemerits of the various types of oxidations.

Let us first consider the use of air (or oxygen) as anoxidizing agent as opposed to stoichiometric reagentssuch as permanganate, dichromate, etc. The mainproblem with the latter reagents is the large amountsof byproduct salts that must be disposed of.12 Nitric acid

oxidations usually give good yields of aromatic acids,16,17

but except in the case of adipic acid (where nitric acidoxidation of cyclohexanone has been preferred over theair oxidation in industry), the need to dispose of nitratedbyproducts and handle nitrous oxide emissions has ledto its displacement by autoxidation methods. In mostcases other than adipic acid, autoxidation yields (forexample, using the Mid-Century processes) are compa-rable to or exceed those obtained by using stoichiometricoxidizing agents. There are no reports of the MC methodhaving been tried for adipic acid, as noted in section 4.

The next thing to consider is the case for liquid-phaseoxidation (LPO) as opposed to gas or vapor-phaseoxidation (VPO). VPOs are amenable to solid-phasecatalysis; the large surface areas that can be packed intoa small mass of the catalyst make for good economy.Any diffusion limitations would also be expected to bemuch less severe in gas-solid systems as compared toliquid-solid systems. In general, under otherwise com-parable conditions, VPO is more economical than LPO.Often, however, poor selectivity prevents VPO frombeing economical. For example, whereas VPO of o-xylene to phthalic anhydride is much preferable to LPO(to phthalic acid followed by dehydration), VPO ofcumene is invariably accompanied by cracking (tobenzene and propylene). LPO gives the hydroperoxidein the latter case. The poor selectivities of VPOs areoften due to the fact that the conditions required aremore severe than those required for LPOs (except in thecase of lighter hydrocarbons such as isobutane). Thetemperature required for gas-phase oxidation to takeplace at appreciable rates (300-500 °C) is usually muchhigher than that for LPO (140-170 °C).10,12 The otherfactor that makes for significant differences betweenLPO and VPO is the large difference in density betweenthe liquid and vapor phases. The higher density of theliquid phase usually makes for better productivities inthe case of liquid-phase oxidations. Thus, space timeyields are often an order of magnitude higher forLPOs.12 The higher temperatures, low concentrationsof hydrocarbon normally employed (typically less than3% in air12), and low rates also mean that gas-phaseprocesses tend to be more energy intensive.

As pointed out above, the question of selectivity is animportant one in the consideration of LPO and VPO fora given substance. In general, it is uncommon to findcomparisons being made for oxidations in gas and liquidphases at comparable temperatures. Mayo et al.112 havereported radical-initiated oxidations of isobutylene inbenzene solution at 80 and 147 °C and several atmo-spheres of total pressure and compared the results withgas-phase oxidations at 147 and 197 °C and 0.1-0.5 atmtotal pressure. It is interesting to find that the oxidationreaction occurs mostly by the addition mechanism andthat all oxidations give mostly acetone, isobutyleneoxide, and a high-boiling residue. Similarly, radical-initiated oxidations of cyclopentene were studied at 100°C, at concentrations from 9 M in the neat hydrocarbonto 0.025 M in chlorobenzene and from 0.027 M to 0.056M in the gas phase. Mayo et al.112 found the rates andproducts of reaction to be similar in the two phases. Themain product is cyclopentenyl hydroperoxide, and itcauses autocatalysis and gives secondary products. Onthe other hand, Bulygin et al.118 find that selectivitiesare usually much better in LPO. Although, in somecases, the observed differences in selectivity can beattributed once again to density differences, there are

rc )k01k3cpcL

(k01 + k02cp + k3cL)(22)

ro )k3cpcL(k01 + k02cp)

(k01 + k02cp + k3cL)(23)

ri )k3cpcL(k01 - k02cI)

(k01 + k02cp + k3cL)(24)

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others in which differences in chemistry also play a part.Thus, in the case of isobutane, the differences in theproduct spectra from gas-phase and liquid-phase pro-cesses can be narrowed down by diluting the liquidphase with CCl4 in the latter case and increasing theconcentration of the reactant in the former.2 However,Berezin et al.10 have shown that the observed differencesin the case of oxidation between LPO and VPO cannotbe explained from considerations of density alone andthat changes in reaction mechanism must be taken intoaccount. The higher temperature leads to more C-Cbond scission than in the LPO case, and hence, moresmaller molecules are produced; furthermore, productsof combustion, namely, CO and CO2, form in higheramounts. Thus, one loses out on selectivity to thedesired compounds. This would probably also explainwhy gas-phase oxidation has a greater chance of successwith short-chain hydrocarbons. In particular cases,spectacular selectivity can be obtained in VPO throughthe use of highly specific solid catalysts; an example isthe conversion of butane to maleic anhydride.12

From the point of view of engineering, LPOs aresimpler.12 They are usually carried out in CSTRs orbubble columns, which possess the advantage of supe-rior heat-transfer characteristics for the exothermicLPO reactions. Solid catalytic reactors for VPOs tendto be of the fixed-bed tubular type, and multitubularreactors are required in the interest of efficient heatremoval. These are expensive to fabricate. Fluidizedbeds are used in the oxidation of butane (as well as inthe ammoxidation of propene to acrylonitrile) andexhibit excellent heat-transfer characteristics. Catalystdeactivation and replacement can lead to additionalconstraints on the reactor design. Furthermore, controlof LPOs is easier with little possibility of runaways.VPOs exhibit a much more complex behavior than LPOswith cool flames, hot flames, etc. On the other hand, onthe issue of safe operation, it must be considered thatthe hydrocarbon inventory in VPO processes is muchsmaller than that in LPO processes.

LPOs are almost invariably homogeneously catalyzed.Although catalyst recovery presents problems and cata-lyst cost must be be borne in mind (especially wherecomplex mixtures such as the MC catalyst are involved),relative proportions are easily adjusted in the liquid-phase process. Catalyst composition can even be variedwithin limits during the oxidation. Catalyst concentra-tion is also easily varied.

In view of the above, it is not surprising that gas-phase oxidations have not been studied in the samedetail as LPOs. Of late, reactions in supercritical mediahave been attracting some attention in the context oforganic oxidations.90,97,98,119 The case of isobutane in theliquid vs supercritical phase is referred to in sections6.4 and 7.2. It was shown that the rate of oxidation inthe supercritical phase could be predicted from that inthe liquid phase on the basis of the temperature effect.

Having considered the case for liquid-phase oxida-tions, we must next examine the choice of oxidizing gas.In the past, air has invariably been chosen for reasonsof cost; in some cases, enriched air (up to 28% oxygen120)has been used also. Because most organic oxidations arezero-order in oxygen down to very low oxygen concen-trations, if conditions are such that the kinetic regimeprevails, rate and selectivity characteristics would notbe seriously affected by the choice of gas. However,because oxidations are autocatalytic, it is possible for

one of the mass-tranfer-limited regimes to arise as thereaction proceeds, and indeed, the relevance of mass-tranfer factors has been demonstrated in several cases(this is discussed further in section 9). If the bulk liquidcan become starved of oxygen, one would, in general,expect that higher oxygen partial pressures would bebeneficial. There are other factors also to consider inchosing the oxidizing gas. If substantially pure oxygencan be used under conditions such that most of theoxygen is absorbed, then the volume of gases to betreated before venting decreases. Condensation of thehydrocarbon from the reactor exhaust gases is alsofacilitated if the concentration of noncondensables isreduced. Thus, there would appear to be several advan-tages in going to enriched air or even pure oxygen.However, the safety issues must be examined verycarefully.

Praxair Technology Inc.55,120 has patented what itcalls the “Liquid Oxidation Reactor” (LOR), which usespure oxygen. Through careful design of the reactorinternals (which include a draft tube to promote liquidcirculation) and mixing devices, it is claimed that theoxygen absorption efficiencies are improved to a pointwhere 90-99% of the oxygen entering is absorbed.Several advantages are claimed, such as operation atlower temperatures and pressures, lower solvent com-bustion rates (such as of acetic acid in p-xylene oxida-tion), and lower levels of partially oxidized impurities.It is also claimed that the good mixing conditions insidethe reactor and high extents of absorption before thegases leave the reactor ensure that safety is notcompromised. Furthermore, appropriate modificationsin the reactor hydrodynamics120 make it possible thatthe heat of reaction is removed by evaporation of thehydrocarbon and that heat-transfer surfaces inside thereactor are rendered unnecessary. In MC oxidations, thecarboxylic acid product is often insoluble in the reactionmedium and therefore precipitates out on cold surfaces,necessitating a frequent cleaning of such surfaces insidethe reactor. Elimination of such surfaces therefore hasan advantage.

8.2. Reactor Configurations and Materials

Most oxidations are performed under pressure, andit would be useful to compare the benefits and short-comings of sparged reactors versus mechanically agi-tated contactors. When the demand on the volumetricmass-tranfer coefficient (kLa) is not very high and heatloads are reasonable, such as in the oxidation of cumene,large-size sparged reactors, with diameters up to 4-5m, are widely used. In view of the high heat-transfercoefficient, even jacket cooling may prove to be ad-equate. In any case, coils can be inserted in spargedreactors. Here, the liquid phase is essentially back-mixed, but the gas phase is largely in plug flow and,should the rate be dependent on the partial pressure ofoxygen, it will provide additional advantage, as inmechanically agitated reactors, backmixing in the gasphase is pronounced. Should there be a large loading ofsolids, e.g., a product, as in the case of oxidation ofp-xylene to terephthalic acid (PTA) in acetic acidmedium, unusually high gas velocities will be requiredfor uniform suspension of particles, and this, quite apartfrom excessive power consumption, may even jeopardizethe safety as the partial pressure of oxygen (pO2) at theoutlet may not be in the desirable range. Thus, me-chanically agitated reactors are used in PTA production

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even though problems of leakage through shaft sealingand shaft mechanical soundness are possible problems.

Krishna and co-workers121-123 have recently reviewedthe strategies for multiphase reactor selection. Theyrecommend a three-level strategy for reactor selection.At the first level, in the case of gas-liquid systems, theysuggest a decision on the dispersion mode (gas-in-liquid,liquid-in-gas, or bicontinuous), based on the reactionlocale (as determined from Hatta number calculations).Different dispersion modes are associated with differentranges of the parameter aDA/kL, which determines thevolume ratio between the film (i.e., the near-interfaceregion) and the bulk. Some additional considerations arewarranted in the case of liquid-phase oxidations becauseof the peculiarity of the kinetics involved. Thus, a lowvalue of the Hatta number (say, as occurs at lowconversions in most cases) would suggest a gas-dispersed-in-liquid mode of operation, which is, indeed, the typeof operation used in industrial practice (for example, incyclohexane oxidation). However, one must be carefulin extending the argument too far. Given that mass-tranfer coefficients in such equipment do not vary agreat deal, the film-to-bulk volume ratio can be de-creased by decreasing the gas holdup and the interfacialarea. However, such devices would push the reactiontoward the diffusional end of the slow-reaction regime,with the result that, although the reaction occurs in thebulk liquid, it does so at negligible oxygen concentra-tions with the possible consequence that the chemistryshifts to first-order mechanisms with an attendant lossof selectivity. Furthermore, because of the autocatalyticnature of the kinetics, the reaction could, as conversionincreases, take place to a significant extent in the film,and determinations of the reaction locale with noaccounting for the autocatalytic kinetics could lead tothe wrong conclusions. Aspects of mass transfer withchemical reaction in organic oxidations are discussedfurther in section 9.2.

Liquid-phase backmixing is another considerationthat merits attention in the context of hydrocarbonoxidation in view of the kinetic mechanisms involved,particularly when the desired product is an intermedi-ate prone to further oxidation. Selectivity considerationswould normally dictate a reactor with minimum back-mixing, but the autocatalytic kinetics (particularly inuncatalyzed oxidation) would call for a minimum back-mixing for the reaction to get started. Considerationsof safety would also require good mixing conditions inthe reactor. These diverse requirements can be recon-ciled by having several mixed reactors (or bubblecolumns) in series, with the hydrocarbon moving fromone stage to the next while the gas is fed to all stagesin parallel in a crosscurrent manner.

The above considerations revolve mostly around therate of oxidation. Often, a more important issue is theselectivity of oxidation. If the desired products are theintermediates, as in most oxidations, it becomes impor-tant to choose reactors with high mass-tranfer rates sothat the reaction can be made to occur in the bulk upto reasonable conversions. This is because the selectivityto the intermediates is known to suffer in consecutivereaction schemes in the event of film reaction. Theseconsiderations thus help to rationalize the preponder-ance of bubbling- and agitated-type contactors forindustrial oxidations.

9. Role of Mass Transfer in Liquid-PhaseOxidations

The analysis of gas-liquid oxidation reactors requiresseparation of mass-transfer and reaction-kinetics effectsand is complicated by the need to consider the couplingbetween mass transfer and chemical reaction. Thetheory of simultaneous gas-liquid mass transfer andchemical reaction has been the subject of numeroustheoretical and experimental investigations (a compre-hensive review is available in Doraiswamy and Shar-ma43). The behavior of such reactors is dominated bythe relative rates of mass transfer and chemical reac-tion. The location of the reaction and the impact ofoperating variables on the reaction are also dictated bythese rates. Thus, a rational design of gas-liquidreactors is dependent upon an understanding of thiscomplex interaction. Although technical kinetics areusually determined under the (temperature and pres-sure) conditions of reaction, difficulties arise in predic-tion of mass-tranfer rates. The oxidation reactionsdiscussed in this review take place at high temperaturesand elevated pressures. Our knowledge base on masstransfer almost entirely rests upon experiments con-ducted under ambient conditions. It is not at all clearthat extrapolation of such data to high temperaturesand pressures is a valid exercise. In this section, we firstreview the status of data on mass-tranfer-related pa-rameters and then consider theoretical advances inbringing organic oxidations within the ambit of thetheories of mass tranfer with chemical reaction.

9.1. Mass-Transfer Rates at ElevatedTemperatures and Pressures and under ActualOxidation Conditions

The mass-tranfer rate is determined by the productof the mass-tranfer coefficient (kL) and the interfacialarea per unit volume (a). Experimental techniques havebeen developed to measure both the product and theinterfacial areas.43 The measurement of interfacial areauses either a light-transmission technique46,124 or amodel chemical reaction.125,126 The latter technique givesan average interfacial area. Some discrepancies betweenthese techniques have been noticed for bubbling reac-tors.46 In addition, there are only a few model reactionsavailable for use at high temperatures and pres-sures.51,52 Despite these difficulties, some preliminaryassessment of the effect of temperature and pressureon mass-tranfer parameters is now possible.

The mass-tranfer coefficient depends on the diffusioncoefficient and on a hydrodynamic parameter (variouslydescribed by the theories of mass transfer as a filmthickness or a surface renewal rate), which, among otherthings, would be influenced by properties such as sur-face tension. Most theories of mass transfer predict that

where the exponent m varies between 0.5 and 1.Temperature has a significant effect on the diffusioncoefficient.45,127 Hence, one anticipates that the mass-tranfer coefficient will increase with temperature. Sureshet al.48 present a detailed investigation of the effect oftemperature on the mass-tranfer coefficient. For a flat-interface reactor, the exponent m is found to be around0.6, which is not dissimilar from the surface renewaltheory prediction of m ) 0.5. For a bubbling reactor, kL

kL ) Dm (25)

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is found to be nearly independent of temperature. Thissurprising result is ascribed to the overlap of thediffusion boundary layers around bubbles, which couldbe significant at large dispersed-phase volume fractionand at high diffusivities. The lowering of surface tensionat high temperatures and the presence of any adventi-tious surface-active impurities may also contribute bysuppressing surface renewal rates. Other literaturedata128,130 suggest various values, with Kishinevsky andSerebryansky131 even suggesting zero. This contradic-tory state of affairs results at least partly from themethod used to change diffusion coefficients. The useof different gases does not give adequate variation in Dand could lead to erroneous conclusions. On the otherhand, if temperature is varied, concurrent changes inphysical properties, especially surface tension, alsoresult. The reduction in surface tension leads to areduction in the surface renewal rates, and hence, themass-tranfer coefficient decreases. These counteractingeffects, along with the differences between bubbling andflat-interface reactors mentioned earlier, are responsiblefor the contradictory results in the literature. Under theconditions for organic oxidations, the typical Schmidtnumber varies from 10 to 300. These are extremely lowvalues and are well out of the range of Schmidt numbersfound in the reported studies on mass transfer. It istherefore unwise to rely on such data in the analysis ofoxidation reactors.

The picture is much clearer on the effect of pressureon mass-tranfer coefficients. Because pressure, at leastup to 5 MPa, has only a minor effect on physicalproperties in the liquid state, one would not expect theliquid-phase mass-tranfer coefficients to depend onpressure. The experimental data130,132 are in agreementwith this reasoning. However, the early results ofYoshida and Arakawa133 show a small decrease in themass-tranfer coefficient with increasing pressure. Onthe other hand, the gas-phase mass-tranfer coefficientdecreases with pressure because of the decrease in thegas-phase diffusion coefficient.134

The situation is very similar with interfacial area.There is a paucity of data on pressure and temperatureeffects. There is general agreement135,136 that bubbleformation at a single orifice is affected by pressure.Higher pressures lead to smaller bubble volumes andhigher frequency of formation. A recent review byOyevaar and Westerterp51 discusses the relevant issues;also see Oyevaar and Westerterp.52

The effect of pressure on interfacial area depends onthe type of reactor used. For bubble-column reactors,the bubble sizes formed at the distributor have a majorinfluence. Smaller bubble sizes lead to a reduction inrise velocity, and hence, the gas holdup increases.However, this effect is somewhat modified by thecoalescence rate, which depends on physical propertiesand the hydrodynamic conditions. As a result, a groupof investigators137,138 claims that pressure has a signifi-cant effect on the gas holdup, whereas Deckwer ct al.139

show that pressure has no effect on gas holdup. Oyevaarand Westerterp51 carefully discuss these contradictoryresults and show that at least some of these differencescould result from the sparger design and from thesuperficial gas velocity employed. Oyevaar et al.140 showlarge increases in holdup with pressure, especially if thesuperficial velocity is high. This paper also presents dataon interfacial areas in bubble columns. The resultsindicate that the area increases with pressure, but the

influence of pressure is not as large as on gas holdup.This implies that the average bubble diameter increaseswith pressure, which is contrary to the single-orificedata cited earlier. The chemical method used to measureinterfacial area gives a mass-tranfer-weighted area.This implies that bubbles, which do not contribute tothe mass transfer, are not accounted for. Hence, thegeometric area could well be quite different from themass-tranfer effective area. Thus, different measure-ment techniques could easily lead to different results.46

Pressure effects in bubble columns are also discussedby Letzel et al.,141 Krishna et al.,142 and Wilkinson etal.143,144

In gas-liquid stirred vessels, the impact of hydrody-namics is even greater. The earliest study of pressureeffects on interfacial area in stirred contactors is dueto Sridhar and Potter.46 Using the light-transmissiontechnique, their paper demonstrates significant pres-sure effects under conditions relevant to the oxidationof hydrocarbons. A tentative correlation developed bythese authors ascribes the effect of pressure to twosources. One is the relative contribution of the energyinput because of the gas (kinetic energy), and the otheris an empirical factor depending on gas density. Oyevaaret al.,145 Oyevaar and Westerterp,51 and Oyevaar et al.52

used the chemical method to probe pressure effects instirred contactors at room temperature. The earlierpapers found no pressure effects at low pressures up to2 MPa. However, the more recent study at higherpressures of 8 MPa52 found a significant pressure effect.These papers also identified the gas density and velocityat the orifice as the dominant parameters. However, thespecific form suggested by Sridhar and Potter was notfound to be supported by these experiments. Pressureeffects were found to be uniquely dependent upon theproduct of the gas density and the gas velocity at theorifice. When this product exceeds 200 kg/m2s, a sig-nificant increase in area is observed. Note, however, thatin organic oxidations, the large vapor pressures cancause the gas density to be large even at much smallerpressures. Recently, Tekie et al.116 have conductedexperimental investigations on mass transfer underconditions of high temperatures and pressures in thecyclohexane oxidation process in agitated reactors.However, their reactors employ gas-inducing impellersor surface aeration. Such reactors are not common inindustry, for reasons discussed under section 8.

In summary, mass transfer under conditions obtainedin oxidation reactors is determined by several factors.The available data, although insufficient for designpurposes, nonetheless indicate the need to adequatelyallow for these effects in data analysis.

9.2. Influence of Mass Transfer on Liquid-PhaseOxidations

The need to account for possible mass-tranfer influ-ences in organic oxidations (considering that thesereactions are usually carried out by bubbling an oxygen-containing gas through liquid hydrocarbon) has beengenerally appreciated even in early literature; the earlyRussian work10 or the work of Hobbs35 are examples.However, a combination of factors, such as the lack ofan adequate theoretical analysis and the lack of ad-equate information on the magnitudes of mass-tranferparameters under the conditions of oxidation, has forcedresearchers to correlate observed rate data in terms of“overall” kinetics in many instances. It is therefore not

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surprising that the “reaction rate parameters” in severalpublished accounts of kinetics are functions of suchphysical variables as the gas flow rate (see, for example,Steeman et al.34). Even in work where the mass-tranferstep is included in the rate model, the importance ofthe regime, the locale of reaction, and its influence onthe selectivity to the intermediates is often not ap-propriately brought out. This has been because ofcertain difficulties with treating liquid-phase oxidationreactions within the well-established framework of gas-liquid reactions.39,43 There are several factors that themodeling must address, such as the consecutive-parallel nature of the overall reaction network; the zero-order behavior with respect to the dissolving species(oxygen), which can change to first-order behavior atlow concentrations; and the free-radical chemistry andconsequent autocatalysis. In their review of the litera-ture on the selectivity of gas-liquid reactions, Dardeet al.146 concluded that there were only a few experi-mental investigations of selectivity, and even these haveled only to partial results. The theoretical analyses, onthe other hand, these authors found, involve approxi-mations and often do not consider the concentrationprofile of the dissolved gas. Although early theoreticalwork on hydrocarbon oxidations tended to classify thesereactions as slow and hence occurring in the slow-reaction regime, the autocatalytic nature of the reactionmakes such a conclusion hazardous when applied overa conversion range. Mann and co-workers147,148 were thefirst to consider the effect of autocatalysis within theframework of the film and penetration theories andshowed that much larger enhancements result throughthe accumulation of the product in the diffusion filmand consequent acceleration of the reaction there.

The most systematic effort to bring organic oxidationswithin the ambit of the theories of mass transfer andchemical reaction is probably to be found in the case ofcyclohexane oxidation. The experimental work of Sureshet al.50 on cyclohexane oxidation clearly showed that thereaction is slow enough at the beginning that dissolvedoxygen concentrations in the bulk attain saturation, butthat it accelerates as products accumulate, as shown bythe declining oxygen levels in the liquid. Ultimately,depending on conditions such as the oxygen partialpressure and conversion level, the reaction can get fastenough to take place partly or wholly in the film,producing enhancement of mass-tranfer coefficients.These authors developed a film theoretic approach tothe treatment of cyclohexane oxidation. However, theenhancement factors predicted by the theory (usingindependently determined kinetics and mass-tranferparameters) were much smaller than those observedexperimentally. The authors speculated that the ob-served discrepancies could, at least in part, be due tointerfering concentration fields around bubbles, causedby the high diffusivities and large gas holdup. Theytherefore developed50,149 a bubble-swarm model thataccounts in an approximate manner for these phenom-ena, which appeared to provide a better explanation.However, Suresh150 has subsequently shown, takingfirst-order reactions in general, that swarm effectsshould not be expected to lead to predictions differentfrom the traditional local-rate models (such as film andpenetration theories), provided that the physical mass-tranfer coefficient, on the basis of which enhancementsare calculated, is also determined under similar swarmconditions. Because this is the way in which physical

mass-tranfer coefficients were determined in the workof Suresh et al.,48,50 the whole question would still seemto be somewhat open. It is possible that the mass-tranferparameters (especially interfacial area) could undergochanges during oxidation because of the changingcomposition of the liquid (especially, production ofproducts which lead to foaming and so on). Suresh88

does report some visual observations that suggest thatsuch may have been the case.

In the case of other oxidations, where some effort hasbeen made to apply film and penetration theories todelineate regimes (Doraiswamy and Sharma43 providea summary), the role of reaction regime in modifyingselectivities is usually inadequately appreciated (seeCao et al.,54 for example). Confusion also results froman insufficient knowledge of mass-tranfer parametersunder conditions of reaction.

In summary, therefore, much remains to be done inthis industrially important and intellectually challeng-ing area. One aspect that has hardly received anyattention at all is the change in chemistry that oneobtains under oxygen-deficient conditions. For example,Partenheimer12 points out that, with easily oxidizedfeedstocks such as p-methoxytoluene, insufficient oxy-gen diffusion rates may result in depletion of dissolvedoxygen, leading to dimerization of R* radicals. Lowyields and dark-colored products are often the result(there might be a clue here to understand the inferioroptical properties of the product in batch as comparedto continuous operation in the oxidations of p-xylene,o-xylene, and mesitylene). The way commercial reactorsare operated (with negligible oxygen concentrations inthe reactor off-gases), such conditions could well prevailin a number of cases, at least in a part of the reactionvolume. The kind of selectivities for which the kineticsinherently provide, and the ways in which they aremodified by mass-tranfer limitations, are questions ofsome importance that still await answers.

10. Rate Oscillations and Other NonlinearPhenomena

Considering the complexity of the chemistry thatgoverns organic oxidations, rate laws are invariablynonlinear, and one should not be surprised to see someexotic phenomena associated with nonlinear kinetics.Hobbs et al.35 described some hysteresis-type effects inthe oxidation of MEK, in which the reaction came to astandstill as the temperature was lowered to a certainvalue and could not be re-started until the temperaturewas raised much beyond this value. This behavior isreminiscent of the “ignition-extinction” behavior thathas been theoretically predicted and experimentallyobserved in such nonlinear chemical systems as exo-thermic CSTRs (see, for example, Froment and Bis-choff151), where they are attributed to temperaturefeedback effects. Hronec and Ilavsky152 reported oscil-lations in the isothermal catalyzed oxidation of amixture of p-xylene and p-toluic acid and of n-dodecanein an air sparged reactor. The oscillations were observedin the concentration of oxygen in the exit gases and werereported as being aperiodic. The authors assumed mass-tranfer limitations to be absent, as high stirring rateswere used, and attributed the oscillations to kineticphenomena. Jensen153 and Roelofs et al.154 describeoscillations in the air oxidation of benzaldehyde in aceticacid medium, catalyzed by cobalt/bromide. The oscilla-tions were observed as coincident variations in the color

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of the solution and the oxidation potential. Oscillationsin the dissolved oxygen concentration were also re-ported, in which the oxygen levels went all the way tozero before coming up to about 25% of saturation. Inthe work of Jensen,153 the air flow rate was found to bean important parameter affecting the oscillations. Sureshet al.49,114 observed oscillations in dissolved oxygen intheir studies of the oxidation of cyclohexane, under bothcatalyzed and uncatalyzed conditions. As in the case ofbenzaldehyde oxidation, these oscillations also occurredas the dissolved oxygen declined in the batch oxidations.

The evidence that a variety of nonlinear phenomenaoccur in organic oxidation is thus compelling. However,the explanations for these phenomena are still lacking.Although early analyses of such exotic behavior weremore or less restricted to the exothermic CSTRs, in lateryears, isothermal reactors have also been shown toexhibit a rich array of nonlinear behavior, provided thatthere is an autocatalytic step (with certain kineticfeatures) in the reaction mechanism (the extensive workof Gray and co-workers in this area is summarized byGray and Scott53). Again, many bromate-driven oscil-lators have been reported, the most celebrated being theBelousov-Zhabotinsky reaction.155 Although it is thuspossible that the nonlinearity of the kinetics is solelyresponsible in the case of organic oxidations also, therole of mass-tranfer limitations also must be keenlyexamined, particularly as many of the reports seem tosuggest that these phenomena are coincidental with lowoxygen levels in the liquid. Again, possible changes inreaction chemistry at low oxygen levels also could playa part. Although mechanisms implicating the Co2+ fCo3+ transition have been proposed in the benzaldehydework cited earlier, such mechanisms cannot explain theobserved oscillations in uncatalyzed oxidations.

Suresh et al.88,114 investigated the effect of masstransfer through simulations of the kinetic model theyhad developed earlier, which incorporates the zero-to-first-order transition at low oxygen concentrations.Their simulations did not show any oscillations underthe conditions employed, and they concluded, therefore,that the explanation for the oscillations had to be soughtin the kinetics. They proposed a mechanism in which,under the low dissolved oxygen concentrations thatoccur when some product concentration has been ac-cumulated, the active free radicals are inactivated(either through termination or through conversion toless-active free radicals), thereby lowering oxygen con-sumption rates and allowing oxygen levels to increase.The reaction then accelerates, consuming oxygen, andthe whole cycle repeats. To experimentally test such ahypothesis, they conducted an experiment in which, asthe concentration of dissolved oxygen neared zero in anormal oxidation, the oxygen supply was periodicallyswitched with nitrogen and back every 5 min. In theseruns, they found that, on switching back to oxygen everytime, the dissolved oxygen levels rose nearly to satura-tion again, before immediately decreasing to zero. Itthus appears that, although the reactivity of the liquidis high during oxidation because of the products insolution, a short interruption in oxygen supply wouldsomehow “inactivate” the liquid, thus necessitating ashort “induction” period before the original reactivitycould be restored.

Although the possibility of these and similar nonlin-ear phenomena is of obvious academic interest, implica-tions for industrial operations are no less significant.

For one thing, aspects of reactor control must considerthe variety of dynamic behavior that is possible. It wouldbe interesting to see whether the selectivity of theoxidation is any different under these conditions, be-cause of the change in chemistry associated with lowoxygen levels in the liquid. Suresh et al.,114 for example,observed the alcohol-to-ketone ratio to be different underconditions of oscillation. More work, both experimentaland theoretical, is clearly needed to clarify these aspects.

11. Safety Issues in Organic Oxidations

Oxidations being mediated by highly reactive speciessuch as free radicals, safety considerations are of utmostimportance. The dangers of explosion are very real inhydrocarbon-air contact. Although this has long beenrecognized, there sometimes has been a tendency toregard the dangers involved as being somewhat exag-gerated (see Berezin et al.,10 for example). The wholesubject was brought into sharp focus by the Flixboroughexplosion of June 1974 involving a cyclohexane oxida-tion plant. The evidence presented in that case to thecourt of enquiry24 and the post-enquiry discussion (see,for example, Mecklenburgh156) contains much valuableinformation to guide future designs. Kletz157 discussesthis and other case histories of major industrial disas-ters in order to glean the lessons learned. Some of thestatistics provided (based on an analysis of about 500incidents) are of relevance in the present context. Ofthe incidents in storage and blending areas, about 10%were due to the formation of flammable mixtures in thevapor space. After storage vessels, the equipment mostoften involved was pressure vessels. In 23% of the caseswhere the cause was ascribed to ignition, the source wasunknown; in about one-third of the cases where thesource was known, this was autoignition. Hot surfaces,sparks, and static electricity were among the othercommon causes. Among primary causes, Kletz157 liststhe use of the wrong materials of construction asaccounting for 7% of the incidents.

In the case of most of the hydrocarbons of relevanceto this review, the autoignition temperatures are muchhigher than the operating temperatures. In the case ofcyclohexane, for example, the autoignition temperatureis 523 K, whereas oxidation temperatures are on theorder of 423 K. Hence, one can assume that the mixturesof air and hydrocarbon resulting under normal condi-tions are incapable of spontaneous combustion, providedheat transfer is good enough to prevent the formationof local hot spots. However, dangers of ignition from anexternal source remain. Here, the main difficulty is thatthe flammability limits for mixtures of hydrocarbon inair are hardly ever available under the conditions oftemperature and pressure employed in the oxidations.The range of flammable compositions expands withelevation and temperature, and hence, caution is neces-sary in using data available under ambient conditions.Designs are usually to be based on very conservativeestimates. Exhaust oxygen levels below 8% are gener-ally considered safe. When dry air is used as theoxidizing gas, the usual situation is that the gas-vapormixture passes through the flammable region as itpasses through the reactor and the hydrocarbon vapor-izes into the gas bubbles. At the exit of the reactor, thegas-vapor mixture is usually above the upper flam-mability limit. Good mixing conditions should be pro-vided in the reactor so that good heat-transfer conditionsprevail, and this is usually not a problem in agitated

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vessels and bubble columns, although every care mustbe taken to eliminate possibilities of buildup of staticelectricity. The possibility of liquid entrainment in theleaving gases must also be considered. Because suchliquid is likely to contain reactive hydroperoxides, thedesign of gas lines should ensure that no dead pocketsare possible. Downstream of the reactor, the hydrocar-bon must be condensed out of the gas and returned tothe reactor, and this is another place where the mixturecould pass through the flammability range. Althoughthe reactor designs are usually such that the oxygenlevel by this stage is very small (in steady-state opera-tion), the design of the condenser is important. For onething, situations could occur under which the exitoxygen levels are higher than those found in steadyoperation. Furthermore, in the condenser, there is a caseof condensation of the hydrocarbon from its mixturewith a noncondensable (nitrogen). It is possible for thecondensation to become mass-transfer-controlled and amist to form. One must then provide for efficientdemisting because, apart from the need for loss preven-tion, this mist can prove to be a hazard. Fires havesometimes been observed in the gas lines, especiallyduring start-up. Fluid backing up into the gas lines cancause this.157 Alexander158 cautions that bad dispersioncould lead to a buildup of free radicals, resulting in fireswell below auto-ignition temperatures. Gas inlet andsparger designs should therefore ensure that stagnantpools of liquid do not collect at any point.

The above are general considerations that applyequally well to industrial and laboratory reactor sys-tems. Although safety issues are addressed in theindustrial scenario through methodologies such asHAZOP, the necessity for adequately addressing theseissues is no less in laboratory reactors. Furthermore,laboratory reactors sometimes have features in theinterest of acquiring useful and comprehensive data thatcall for additional considerations. Thus, whereas theindustrial reactor operates in a narrow window, thelaboratory reactor, by its very purpose, must be operatedunder a variety of conditions. Therefore, safety consid-erations not only are important in the design, but alsomust be an essential part of experiment planning.

In general, it is good practice to keep the volume ofthe liquid hydrocarbon handled to a minimum inlaboratory reactors. The microautoclaves49 or the flowreactors of the type employed in the studies of Wen,59

Wen et al.,63 and Guo61 (see section 7.1) are ideal in thisregard, as the reactor holds only about 30-35 cm3 ofliquid. Although the liquid holdup in any experimentis just the reactor volume in the case of microautoclaves,even with flow reactors of similar volumes, an experi-ment for 10-20 residence times can be carried out withfairly small volumes of liquid. There is the furtheradvantage in such equipment that the oxygen supplycan be isolated before the actual experiment, once theliquid is saturated with oxygen at the desired pressure.On the other hand, because pure oxygen is used,adequate precautions are to be taken during the satura-tion step, although this step is carried out in the cold.The presence of moving parts such as mechanicalagitators calls for precautions in preventing an ac-cumulation of static electricity or the generation of hotspots. Despite all of the advantages of small liquidholdup, Wen59 and Guo61 housed their equipment in anenclosure that could be flooded with carbon dioxide incase of an emergency.

The design of gas-liquid contactors, on the otherhand, calls for a much greater consideration of the safetyissues, as the liquid holdup in these equipment isusually much larger. Thus, the semibatch equipmentused in the studics of Suresh et al.49 had a holdup ofabout 20 dm3 of cyclohexane, about half of which wouldbe at reaction temperature and pressure during anexperiment. Furthermore, oxygen or an oxygen-contain-ing gas was continuously passed through the liquid.Sridhar159 and Suresh88 have discussed in detail thesafety considerations in the design of such equipment.Although the remarks made above in regard to the goodheat- and mass-tranfer conditions inside the reactor(where flammable compositions could occur) are evenmore likely to be valid in laboratory reactors than inindustrial reactors, Suresh et al.49 worked with a systemthat avoids the formation of flammable mixtures in thereactor altogether by using a nitrogen stream that waspresaturated with the hydrocarbon upstream of thereactor and mixing the desired (small) percentage ofoxygen into this stream at the reactor inlet. Thisapproach has the additional advantage that vaporiza-tion inside the reactor can be neglected in interpretingthe data using the theories of mass transfer withchemical reaction. However, it also increases the hy-drocarbon holdup in the experimental rig. Suresh et al.48

housed all equipment containing hydrocarbon at or nearreaction temperature inside a cabinet filled with carbondioxide as a precaution.

Although industrial reactors usually operate withnegligible oxygen levels in the gases leaving the reactor,laboratory reactors must often operate with significantoutlet oxygen levels so that absorption rates can bedetermined independently from gas-phase measure-ments. Consequently, the comments made earlier inconnection with the design of condensers to recycle thehydrocarbon are of even greater importance here. Ademister should, therefore, be provided downstream ofthe condenser. In uncatalyzed oxidations, hydroperox-ides can form in nonnegligible concentrations in thereactor and can be carried downstream as entrainment.Care must be taken to avoid dead spots and regionswhere a buildup of such compounds can take place.

12. New Developments in Organic Oxidations

The commercial importance of organic oxidations andthe intellectual challenges they pose remain a potentdriver for new research. In this section, we illustratesome of the new processing options that have beenconsidered recently.

12.1. Biphasic Mode of Operation

Several authors have studied oxidation in two-phase(biphasic) systems. When the second phase is water, thecatalyst can be drawn to the organic-water interfaceby a surface-active complexing agent. An example isprovided by Chung et al.160 who oxidized tetralin toR-tetralone with a nickel complex of a surface-activeligand in the presence of an emulsifier. High selectivitiesare reported, and the reaction can be carried out at 60°C. The products remain in the organic phase, and thefluorous phase containing the catalyst can be recycled.Parenthetically, we note that R-tetralone can be ef-ficiently dehydrogenated to R-naphthol. Similarly,Launay et al.161 used ruthenium trichloride for theoxidation of cycloalkanes with tert-butyl hydroperoxide

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(TBHP). Under reaction conditions, colloidal rutheniumis formed and remains at the interface. Several cycloal-kanes were studied, and the corresponding ketone wasobtained as the predominant product. ten Brink et al.162

have adopted the biphasic system, using a water-solublepalladium complex of bidentate amine ligands such asbathophenanthroline disulfonate for the Wacker-typeconversion of terminal olefins (e.g., hexene-1, octene-1,etc.) to the corresponding ketones. The catalysts arestable and can be recycled. This is an improvement overthe conventional Wacker process. Ito et al.163 carried outthe Wacker-type air oxidation of styrene and its deriva-tives to the corresponding acetophenone using (en)Pd-(NO3). This complex facilitates the transfer of organicmaterial into the aqueous phase in a reverse phase-transfer catalysis. The structure of the cage is reportedin this paper. Dobler et al.164 show that osmium-catalyzed dihydroxylations are facilitated by operatingin a two-phase system. The method is an improvementon the well-known Sharpless dihydroxylations. The useof a two-phase system prevents nonselective oxidationof the diols and permits the recycle of the catalyst.

Fluorous biphasic systems have also been developedwherein a perfluorinated solvent replaces water.165 Anadditional advantage of this system is that, at temper-atures around 60 °C, the reaction mixture becomeshomogeneous, thereby facilitating rapid reaction. Theseauthors report the oxidation of various olefins to thecorresponding ketones in the presence of a palladiumcatalyst with TBHP as an oxidant. On cooling, thereaction mixture again forms two phases: the organicphase contains the products, and the aqueous phasecontaining the catalyst can be recycled. Pozzi et al.166

studied the oxidation of alkenes and obtained highconversions and selectivity to the epoxide using a cobaltcomplex of tetraarylporphyrin. The inertness of thefluorocarbon and the high solubility of oxygen in thisphase are additional advantages, and as such, weanticipate that this technique will continue to attractattention.

Environmental considerations have created a largeinterest in aqueous biphasic catalysis.167 However, thesesystems can generate some aqueous solutions as a wastestream. This has motivated the search for the use of adense-phase CO2-based system. Homogeneous ruthe-nium complexes with fluorous phosphines are beingdeveloped as catalysts. Pesiri168 demonstrates the ep-oxidation of alkenes in dense-phase CO2 using oxovanadiumtriisopropoxide. The rates in supercriticalmedium are three times faster than those in hexane (seesection 12.4).

Harada et al.169 have shown how, in the Wackerprocess for the conversion of R-olefins to the correspond-ing carbonyl compounds, the use of R-cyclodextrin canmake a remarkable change. Thus, C8-C10 olefins givethe corresponding methyl ketones, but higher olefins(C12-C14) do not react. It is surprising that oct-2-enegives poor results. The fundamental aspects pertainingto the prediction of rate and selectivity remain to bestudied.

Microemulsions are thermodynamically stable and,for sparingly soluble hydrocarbons, may well offer someadvantages. There is very little information in theliterature, but a recent claim from Bayer170,171 revealsa Wacker process in a microemulsion medium witholefins such as cyclopentene, cyclohexene, etc.

The use of phase-transfer catalysis in enhancing air

oxidation of hydrocarbons has received attention. Ma-tienko and co-workers172-175 have studied a number ofrelated issues, in connection with the oxidation ofethylbenzene. These authors carried out the liquid-phase oxidation of ethyl benzene with Ni(acac)2 as acatalyst. Macrocyclic polyether 18-crown-6 changes theselectivity, and this can also be realized using quater-nary salts such as Me4NBr and n-C16H38Me3NBr. Thereis also a claim176 for the liquid-phase air oxidation oftoluene to benzoic acid using cobalt catalyst and aphase-transfer catalyst like [Me(CH2)-9]2N+Me2Br- at135-160 °C and 12-15 atm pressure.

12.2. The Role of Ultrasound in OxidationReactions

The role of free radicals and their impact on reactionpathways has been detailed in section 5. Ultrasonica-tion, including through cavitation,177,178 is known tocreate free radicals. Thus, we expect that the airoxidation of dissolved organics in wastewater would befavorably influenced by ultrasonic treatment duringoxidation. There is scant literature on the oxidation ofhydrocarbons under conditions of industrial importance.This is particularly relevant as cavitation can beemployed in large-scale operations. Sulman179 has citedthe effect of ultrasound on the liquid-phase oxidationof n-tetradecane at 385 K in the presence of 0.3% cobaltstearate as a catalyst. Ultrasound appears to increasethe production of oxygenated compounds by 15-30%.The maximum acceleration of the oxidation is reachedat a frequency of 300 kHz. The oxidation rate underthese conditions does not vary with time, whereas itshows a maximum in the absence of ultrasound. Theoxidation of alkenes in the presence of Mo(CO)6 to formenols and epoxides is also cited in this work.

12.3. Oxidation in Supercritical Media

A thesis from Clemson University has covered oxida-tion of alkyl aromatics to aldehydes and acids insupercritical (SC) water (Tc ) 647.3 K, Pc ) 217.6 atm)and has found it to be promising (cited by Haas andKolis180). The MnBr2-catalyzed reaction of p-xylenegiving terephthalic acid has been reported. The oxida-tion of stilbene with O2 and MoO2(acac)2 or VO(acac)2gave benzaldehyde and benzoic acid (i.e., cleavageoccurred). In SC, water only thermally stable olefinsundergo this type of reaction; simple nonconjugatedolefins such as 1-octene and cyclohexene decompose inthe harsh environment of SC water.

In contrast to SC water, conditions in SC CO2 (Tc )30.4 K, Pc ) 72.8 atm) are milder and benign. Thesolvent is environmentally friendly. SC CO2 is anattractive solvent for catalytic oxidations because it isnoncoordinating and inert toward any further oxidation.Some reports have appeared on oxidations in super-critical media in the literature. Haas and Kolis180 havereported that Mo catalyst can oxidize toluene to ben-zaldehyde in 25% yield in SC CO2. There is a claim181

that, when propylene is oxidized under supercriticalconditions (T > 91.6 °C, P > 46.1 atm) with silvercatalyst, propylene oxide (PO) is obtained. At 3.6%conversion, a 36 mol % selectivity to PO was obtained.However, recently, Dow182,183 has claimed that propyl-ene can be converted to PO by oxygen in a cascade ofreactors using dichlorobenzene as a reaction medium.

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These reports have significance in the pursuit of a directoxidation process for PO from propylene.

12.4. Photochemical Activation of OxidationReactions

The role of photocatalysis in realizing the selectivityfor the desired product in the oxidation of hydrocarbonshas attracted attention in recent years. The photocata-lytic oxidation of hydrocarbons in the aqueous phase inthe presence of finely divided TiO2 has been reportedby Gonzalez et al.184 Thus, toluene was oxidized tobenzyl alcohol, and at 11.6%, conversion about 90%selectivity to benzaldehyde was obtained. Frei et al.185

have given an overview of photocatalyzed oxidation ofhydrocarbons in zeolite cages with examples of tolueneto benzaldehyde. The liquid-phase oxidation of isobu-tane was referred to in section 6.4, for which typically,at about 8% conversion, we get 75% selectivity withrespect to the hydroperoxide. Blatter et al.186 have donesome ingenious experiments in which isobutane and O2gas were loaded in a single-photon process (monitoredin situ by FTIR). It is noteworthy that the selectivity tothe hydroperoxide (HPO) was 98% even when more than50% of the loaded reactants had reacted. This synthesisin zeolite environments offers an opportunity for in situuse of the HPO as an oxidizing agent without isolationand storage. Optically translucent zeolite membraneswill have to be used.

Sanjuan et al.187 describe a photocatalytic systemconsisting of an organic dye trapped in Ti-zeolite pores.The dye absorbs energy from light and generateshydroxyl radicals from water. These radicals react witholefins in the presence of molecular oxygen to formallylic hydroperoxides, which, in turn, facilitate theepoxidation of the alkene at the Ti sites. The photosen-sitizer is protected from attack by hydroxyl radicalsbecause of its location within the pores of the zeolite.Highly dispersed titanium oxide on silica has beenreported to catalyze the photooxidation of propene topropylene oxide using molecular oxygen.188

The catalyzed photooxidation of cyclopentene189 givesthe following products:

Bloodworth and Eggelte190 have reported photooxida-tion, with O2, of cyclopentene, cyclohexene, cyclohep-tene, and cyclooctene in dichloromethane medium con-taining tetraphenylporphyrin, using a 400-W sodiumlamp, to the corresponding hydroperoxides. Liquid-phase air oxidation of saturated hydrocarbons such asdecane, cyclooctane, Decalin, etc. under photochemicalactivation has been reported by Nekhaev et al.191

The catalytic asymmetric dihydroxylation of R-meth-ylstyrene (AMS) by air under visible irradiation is afascinating example of oxidation of a relatively cheaphydrocarbon, AMS, that is obtained as a byproduct ofcumene oxidation (see section 6.3). Although Sharpless’method of asymmetric dihydroxylation is known, thework of Krief and Colaux-Castillo192 yields a newperspective by using air as an oxidant. Under visibleirradiation, this reaction can be conducted in the pres-ence of catalytic amounts of Os(IV), phthalazine dihy-

droquinidine chiral ligands. The optically active dihy-droxy compound can be selectively hydrogenated to theoptically active monohydroxy (primary) species, which,on carbonylation, should give the optically active 2-arylpropanoic acid. Thus, a new route, which has potentialto be commercial, to optically active 2-aryl propanoicacid like S-ibuprofen and S-naproxen, may emerge.

Li et al.193 have used vesicles to direct the photosen-sitized oxidation of olefins either toward the singlet-oxygen-mediated or the superoxide-radical-anion-me-diated products by controlling the status and locationof the substrate and sensitizer molecules in the reactionmedia.

12.5. Enzyme-Catalyzed Reactions

In living organisms, enzymes such as cytochromeP450 catalyze the oxidation of various organic com-pounds. Cytochrome P450 functions to activate molec-ular oxygen by iron porphyrin to generate an oxo ironporphyrin while transferring oxygen atom to the sub-strate. The simulation of these enzymatic functions withtransition metals has received much attention. In suchcases, the oxo metal complex is generated withoutporphyrins. For example, low-valent ruthenium complexcan react with a hydroperoxide, and subsequent cleav-age yields an oxo ruthenium (IV) species, which can thenbe used to oxidize hydrocarbons.194 Adam et al.195 havereported biocatalytic asymmetric hydroxylation of hy-drocarbons with the microorganism Bacillus megate-rium. This strain carried out the hydroxylation chemose-lectively and enantioselectively in the benzylic andnonbenzylic positions of a variety of unfunctionalizedaryl alkanes (the phenyl ring was unaffected). Salycilatephenobarbitols, which are potent inducers of cytochromeP450 activity, changed the regioselectivity of the mi-crobial CH insertion without an effect on enantioselec-tivity. Other enzyme catalysts have also been developed.Alkane hydroxylase from Pseudomonas oleovorans hasbeen used to produce optically pure epoxides fromterminal alkenes. The same enzyme also assisted in theregiospecific introduction of oxygen into alkanes. Detailsof these approaches are contained in a report on the 4thJapanese-Swiss Meeting on Bioprocess Development.196

Dicarboxylic acids such as adipic acid, suberic acid,sebacic acid, azelaic acid, dodecanedioic acid, and bras-sylic acid are commercially important.197 An importantcommercial route, for some of these dicarboxylic acids,is through the oxidation of cyclic compounds (adipic acidfrom cyclohexane, dodecanedioic acid from cyclodode-cane etc.; see section 4). These acids are useful inter-mediates in the production of polyamides. For example,the reaction of 1,12-dodecanedioic acid with hexameth-ylene diamine gives Nylon-6,12, and brassylic acid (1,-13-tridecanedioic acid) is used to manufacture thecorresponding nylon. Brassylic acid and 1,12-dodec-anedioic acid are preferred raw materials for makingenvironmentally friendly musks as aroma compoundsthrough esterification with ethylene glycol. These di-carboxylic acids are useful for making polyesters, andlately, the use of 1,12-dodecanedioic acid for makingmodified polycarbonates has been reported. It is knownthat brassylic acid is commercially manufactured (inJapan and China) by the aerobic fermentation route.189,197

Similarly, fermentation of n-dodecane can give 1,12-dodecanedioic acid. The engineering aspects of suchaerobic fermentation reactions require further attention.

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12.6. Use of O2 + H2 and O2 + CO as Oxidants

Several unusual oxidations have been reported re-cently. Researchers at Tosoh in Japan198 have demon-strated the oxidation of benzene to phenol using mix-tures of oxygen/hydrogen and oxygen/CO. A palladium-ruthenium catalyst was used, and the oxidation wascarried out in an acetic acid medium. The rates appearto be promising, and the method has the added advan-tages that no coproduct acetone is formed, unlike in thecase of the process based on the oxidation of cumene.Meiers and Holderich199 have used PddPt in Ti-Silicalite for a “one-pot” conversion of propylene topropylene oxide using H2 and O2. This probably impliesin situ production of hydrogen peroxide, which subse-quently epoxidizes propylene.

12.7. Catalyst Developments

As in the other areas reviewed in this section, one ofthe prime motivating forces for catalysis research hasbeen environmental concerns: increased efficiencieslead to reduced effluent volumes, heterogenizationfacilitates catalyst recycle and reuse. Langhendries etal.200 have recently reviewed developments in providingclean catalytic technology for liquid-phase hydrocarbonoxidations. Low-efficiency processes such as cyclohexaneoxidation have naturally been at the forefront of re-search attention. A development of some interest is thedemonstration of catalysis by nanostructured amor-phous metals such as cobalt and iron and alloys.201 Withcyclohexane oxidation, a selectivity of 80% (to cyclohex-anol-cyclohexanone) at a conversion as high as 40%was realized with amorphous cobalt. Isobutyraldehydewas used as a co-reductant, and a catalytic amount ofacetic acid was also added. A notable feature was thehigh ratio of alcohol to ketone (as high as 5:1). Thereaction was carried out at room temperature with anoxygen partial pressure of 40 atm. The rates are,however, too low under these conditions for commercialexploitation, and additional studies are needed. Al-though rates were better at 70 °C, selectivities are notreported at the higher temperature. The catalyst wasprepared by a sonochemical method.

Attempts to mimic enzyme action using iron catalysts,either in zeolite-type cages or in membranes, haveyielded some significant improvements in rate andselectivity for cyclohexane oxidation with tert-butylhydroperoxide. Vanoppen et al.202 have attempted het-erogenization of the conventional cobalt catalyst byincorporating it into the framework of aluminophos-phate molecular sieves. This has the effect of keepingthe cobalt dispersed so that there is no deactivation dueto either precipitation or clustering. The authors ob-tained selectivities to hydroperoxide + alcohol + ketonethat are better than those obtained with homogeneouscatalysts at similar conversion levels. Other studies inwhich similar principles have been applied to othermetal catalysts with encouraging results are reviewedby Langhendries et al.200

Functionalized heterogeneous catalysts for the oxida-tion reaction have also been developed. Chisem et al.203

have developed chemically modified mesoporous silicawith metal ions immobilized on a hydrophobic chain.These catalysts are shown to facilitate the epoxidationof cyclohexene with excellent selectivity and also toassist in the oxidation of ethylbenzene to acetophenone.Efficient removal of water is necessary to prevent

catalyst deactivation in the case of oxidation of ethyl-benzene. Das and Clark204 have reported a complex formof Co(III) immobilized on a chemically modified silicasubstrate, which has proved to be useful for conversionof ethylbenzene to acetophenone.

Raja and Thomas205 replaced a portion of the alumi-num ions in molecular sieves with Mn(III) ions anddemonstrated regioselectivity in the air oxidation oflinear dodecane. The selectivity arises from the poredimensions that govern the access of the hydrocarbon.The terminal methyl group in dodecane is able to accessthe active catalyst sites, whereas the carbon atoms alongthe dodecane backbone are protected when the porediameter is small enough. Tailored molecular sieveshave been put forward as oxidation catalysts. Recently,Dugal et al.206 designed a heterogeneous catalyst for airoxidation of cyclohexane to adipic acid based on Fe-AlPO-5 and FeAlPO-31. The latter catalyst showedselectivity to adipic acid as high as 65%, whereas theformer gave a maximum selectivity of 31%. This differ-ence is apparently due to cyclohexane being much moreconfined in the case of FeAlPO-31. Thus, the diffusionof cyclohexane and cyclic intermediates is limited, andhence, further oxidation to linear products such asadipic acid is facilitated. In an earlier work from thesame school, high selectivity for cyclohexanol fromcyclohexane is obtained over an FeIIIAlPO-5 catalyst.It is even more remarkable that Raja et al.207 havedesigned a molecular sieve catalyst for the extremelydifficult air oxidation of n-hexane to adipic acid. Thecatalyst is based on a CoIII framework-substituted AlPO.The catalyst has also been found to be effective for theepoxidation of alkenes with air. This addresses theproblem of selective oxidation of linear alkanes, par-ticularly in the terminal position, to alcohols andcarboxylic acids and can, indeed, change the fundamen-tals of the business of producing primary alcohols anddicarboxylic acids such as adipic acid. However, prob-lems associated with leaching, long-term stability, thestructure of the acid site, and selectivity require furtherinvestigation.

Chromium-, cobalt-, or vanadium-substituted alumi-nophosphates have been shown to be active (for ex-ample, Kraushaar-Czarnetski208). Vanoppen et al.209

used zeolite Y ion exchange with alkali metal ions tostudy the oxidation of cyclohexane. Soluble heteropoly-acids (HPA) and palladium acetate have been used forthe direct oxidation of benzene to phenol, but catalystdeactivation through the irreversible reduction of HPAremains a problem. Passoni et al.210 report on attemptsto heterogenize these catalysts by encapsulating themin MCM-41 or microporous AlPO4-VPI-5 molecularsieves. The HPA leaches out of the former support,whereas the molecular sieves retain the catalyst. Thepoor accessibility of the reactants to the catalyst resultsin low reaction rates. Along similar lines, Khenkin etal.211 report a vanadomolybdophosphate polyoxometa-late supported on mesoporous MCM-41 with which theywere able to get product selectivities in the oxidation ofalkanes and alkenes similar to those obtained viahomogeneous oxidation, although the catalytic activitywas somewhat reduced. Lempers and Sheldon (1998)question whether some of these catalysts truly functionas heterogeneous catalysts. These authors show thatsmall amounts of leached metal could homogeneouslycatalyze the reaction and suggest ways of testing thesecatalysts. However, in the work of Khenkin et al.211 cited

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above, no leakage was observed and the solid catalystcould be recovered and reused without loss in activity.A number of claims have been made in which porphyrincomplexes are used as catalysts to convert olefins suchas propylene, 1 and 2-butene, butylene, cyclopentene,cyclohexene, cyclooctene, etc. Mitsui Toatsu claimsconversion of cyclooctene to cyclooctene epoxide, evenat 25 °C.

A recent patent from China212 refers to the epoxida-tion of olefins with oxygen in the presence of a singleoxygen acceptor H2A, selected from diazo compounds,quinones, and phenazines, and a transition metalselected from Ti, V, Cr, Mo, W, etc. The oxidized H2A isreduced by hydrogen to regenerate the single oxygenacceptor. Another development of significance in thequest for direct air oxidation routes for olefins toepoxides is the use of N-hydroxyphthalimide (NHPI),on which the work of Iwahama and co-workers (section6.6) has been discussed earlier.

Higashijima213 has reported a novel water-solubleoxidation catalyst that is a ruthenium-substituted het-eropolyanion, [SiW11O39Ru(III)(H2O)]5-, which is shownto be effective for the oxidation of, for instance, p-xyleneat 200 °C in water as a solvent. At 99% conversion, theproducts consisted of terephthalic acid (58.8%), p-toluicacid (17.6%), p-toluic aldehyde (0.2%), and carbondioxide (20%). This development is of interest in thecontext of the current search for environment-friendlyprocesses and is an example of a “green” oxidationcatalyst, in a nonhalogen-containing water solventsystem. Methods of synthesis of these heteropolyanioncatalysts are also simultaneously attracting a great dealof interest. Higashijima213 claims that their method ofhydrothermal synthesis is more effective and practicalin comparison with the earlier methods. The anion wassalted out as the cesium salt from the hydrothermalsolution.

Zeolites are interesting supports for oxidation withmetal complexes, because the entrapped complexes areprevented from dimerizing and degrading. However, thesmall pore sizes usually make them unavailable forlarge complexes. Wang et al.111 have used new zeolitesin which the cage size had been expanded as supportsfor encapsulating their Schiff base complexes that showgood activity in oxidizing linear aliphatic olefins to theirepoxides (see section 6.7) and have obtained goodactivity and selectivity with many olefins. As the sizeof the olefin increases, the catalyst activity decreases,possibly as a result of access limitations in the zeolitecage. The authors have investigated the effect of severalparameters such as zeolite type, metal ion, and reactiontemperature. In particular, the influence of temperaturewas found to be complex, indicating interactions amongseveral phenomena.

A number of modifications have been claimed forconversion of olefins to ketones with Pd-based catalysts.Idemitsu and Kosan have claimed conversions of 1-buteneto methyl ethyl ketone in 1,4-dioxane-water in thepresence of PdSO4, H6PV3Mo9O40, and Fe2(SO4)3.

Woltinger et al.214 have studied cobalt salophencatalyst encapsulated in zeolites, in the presence of thepalladium-quinone system for the air oxidation of 1,3-dienes. In the case of 1,3-cyclohexadiene, 1,4-diacetoxy-2-cyclohexene in high yield was obtained at roomtemperature.

Chauvet et al.215 have shown that catalysts such as(CH3CN)2PdCl(NO2), unlike the common Wacker cata-

lysts based on PdCl2 and CuCl/CuCl2, convert strainednorbornenes to the epoxide. This is an important resultas it demonstrates that molecular oxygen can be usedfor epoxidation with low-valent transition metals fromthe Pt group and in the complete absence of anyperoxidic structure element, albeit for restricted olefins.Olefins such as 1-octene, vinylcyclohexene, styrene, etc.are converted to the corresponding ketone, as would bethe case with the usual Wacker catalyst.

12.8. Stereospecificity in Organic Oxidations

In the case of several hydrocarbons of industrialimportance, such as pinane, menthane, cyclooctene,cyclododecene, etc., there are cis and trans configura-tions. In the case of cyclooctene for instance, the cisisomer can be obtained predominantly, if not exclu-sively, by manipulating the catalyst and the conditionsof the hydrogenation of cyclooctadiene. It is expectedthat rates of oxidation of cis and trans isomers, undersimilar conditions, will be different. There is very littleinformation in the literature on this aspect, which isboth scientifically interesting and commercially impor-tant. For examrple, in section 12.3, reference was madeto the selectivity of oxidation of cis- versus trans-stilbene, where only the cis isomer reacts. Similarly,Eggersdorfer216 shows that the air oxidation of pinaneat 95 °C gives 2-pinane hydroperoxide, which is usedas a radical initiator for polymerization reactions. In thisreaction the cis isomer reacts more rapidly than thetrans isomer.

Haas and Kolis180 have reported oxidation of cyclo-hexene, cyclooctene, 1-octene, vinylcyclohexane, cis- andtrans-stilbene, etc., in SC CO2 and Mo(CO)6 catalystprecursor with t-BuOOH. The product can be epoxideor alcohol when 70% aqueous t-BuOOH is used. Thehighest yields and fastest rates of diol and epoxideformation were observed with cis-alkenes, whereastrans-alkenes were considerably less reactive. trans-Stilbene does not undergo the oxidation reaction, prob-ably because of steric effects. Cyclic olefins such ascyclohexene react relatively very rapidly. cis-Cyclooctenewas oxidized in 100% yield to its epoxide with no signof hydrolysis in contrast to, say, the oxidation ofcyclohexene where epoxide is converted to diol as well.

The work of Iwahama et al.109 on the epoxidation ofalkenes with a hydroperoxide generated in situ wasdiscussed earlier (sections 6.5 and 6.7). A notable featureof the results reported is the excellent stereospecificityof the epoxidation. Thus, trans-oct-2-ene was epoxidizedalmost exclusively to trans-2,3-epoxyoctane with aselectivity of 88% at 78% conversion. A small amountof the diol was formed (about 4%). Similar results wereobtained with the cis isomer as the starting material.These results are in contrast to the metal-catalyzedepoxidations of cis-olefins using an aldehyde and oxy-gen, in which a mixture of cis- and trans-epoxides isusually obtained.

13. Conclusions

Liquid-phase air oxidation of important bulk rawmaterials like p-xylene, cumene, ethylbenzene/isobu-tane, cyclohexane, n-butane, etc., to industrially impor-tant products, can be analyzed on a fairly rational basis,taking into consideration aspects of chemistry, engi-neering kinetics, type of reactor, etc.

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There is potential for improving industrially impor-tant reactions/reactors. Strategies such as oxidation ofisobutane under supercritical conditions, oxidation un-der biphasic conditions, photooxidation, etc., suggestnew directions.

Oxidation of cyclohexene may well open up new,efficient processes for making cyclohexanone and spe-ciality chemicals through cyclohexene epoxide, cyclo-hexenone, cyclohexenol, etc. This is also the case forcyclooctene and cyclododecene where uncatalyzed oxida-tion gives high yields of epoxide.

There is considerable scope for further work.

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Received for review February 21, 2000Revised manuscript received August 4, 2000

Accepted August 5, 2000

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