11.3 acid or base catalysed processes - treccani · 2018. 6. 30. · its surface area is 0.02 m2/g....

At the end of Nineties of the 20th Century, petroleum,petrochemical, and chemical industry processes usedmore than 100 solid acid, about 10 solid base, andabout 15 solid acid-base bifunctional catalysts (Tanabeand Hölderich, 1999). This chapter summarizes acidand base catalysed processes for the production ofbulk petrochemical intermediates.

11.3.1 Heterogeneous catalysis

The oldest technologies, based on free acid or base catalysts, have several drawbacks. Often suchcatalysts are strong mineral acids or Lewis acids(e.g. HF, H2SO4, AlCl3) or strong bases (e.g. NaOH,KOH). These acids and bases are highly toxic andcorrosive. They are also dangerous to handle and totransport as they corrode storage and disposalcontainers. Moreover, because the reaction productsare mixed with free acids or bases, the separation atthe end of the reaction is often a difficult andenergy-consuming process. Very frequently, at theend of the reaction, these acids or bases areneutralized and therefore the correspondent saltshave to be disposed of.

In order to avoid these problems, much effort hasbeen devoted to the search for heterogeneous catalysts(i.e. solid acids or bases) which are more selective,safer and environmentally friendly.

Acid catalysis Acid catalysis involves carbocation (carbonium or

carbenium ion) intermediates. Removal of a hydrideion (H�) or negatively -charged alkyl group creates apositively-charged carbenium ion. The addition of aproton to an alkane creates a pentacoordinatedcarbonium ion. Tertiary carbenium ions (in which allthree bonds of the positive carbon atom connect with

other carbons) are inherently more stable thansecondary carbenium ions (in which one of the bondsconnects with a hydrogen atom). For this reason, thetransition state leading to a more stable tertiarycarbenium ion intermediate is at a lower energy levelthan that leading to less stable carbenium ions, and theactivation energy of a reaction going through thetertiary carbenium ion is correspondingly lower.Therefore, this reaction will proceed faster than theone involving the secondary carbenium ion. Primarycarbenium ions (in which two of the bonds of thepositive carbon connect with hydrogen atoms) havevery low stability and therefore reactions involvingprimary carbenium ions are extremely unlikely.Enthalpy differences between primary and tertiarycarbenium ions are about 130-140 kJ/mol at 298 K inthe gas phase (Olah and Schleyer, 1968; Corma et al.,1982).

Carbenium ions may be formed by adding a protonto an alkene or an aromatic molecule, or by hydrideabstraction from an alkane.

The most important acid-catalysed reactions ofhydrocarbons are cis-trans isomerization, double-bondshift, skeleton isomerization, olefin alkylation,aromatic alkylation, the isomerization of xylenes andother polyalkylaromatics, certain types of cyclizations,and cracking. Very weak acids can catalyse cis-transisomerization and double-bond shift. The otherreactions require stronger acids.

The most important liquid acids used by theindustry are H2SO4, HF, HCl, and Friedel-Craftcatalysts such as AlCl3 or HF-BF3.

This chapter discusses solid catalysts. Solid acidsinclude oxides such as: silica-alumina, alumina;ZSM-5, mordenite, Y-zeolite, USY, beta, SAPO-11,SAPO-34, and some other zeolites and molecularsieves; phosphates; zirconates; kaoliniteAl2Si2O5(OH)4, montmorillonite Al2Si4O10(OH)2 and

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11.3

Acid or base catalysedprocesses

other clays; pillared clays; and ion-exchange resinssuch as Amberlyst and Nafion.

Nafion is perfluorinated resinsulphonic acid. It ismade by copolymerization of perfluorinated vinylether and tetrafluoroethylene, followed bysulphonation. Even though the number is relativelysmall, it has very strong acid sites (the Hammettacidity function H0 ranges from 11 to 13). Its surfacearea is 0.02 m2/g. It is a non-porous solid, and it isthermally stable up to about 280°C. Although Nafionhas very strong acid sites that can catalyse manyreactions, its low surface area renders it unrealistic forpractical use. DuPont has produced a high surface areaNafion-silica nanocomposite. The external surfacearea of the dispersed Nafion particles ranges from 5 to150 m2/g yet it retains the characteristic strong acidityof Nafion. The Nafion-silica nanocomposite is activefor alcohol dehydration, Friedel-Craft aromaticalkylations, a-methylstyrene dimerization, andacylation of m-xylene with benzoyl chloride (Harmeret al., 1996; Corma and Garcia, 1997).

Relatively recently, Toda and co-workers at theTokyo Institute of Technology developed a new solidacid catalyst that is much less expensive but eighttimes more active than Nafion. Carbonization of sugar,starch, or cellulose generates polycyclic aromaticsheets. Sulphonation with H2SO4 gives sheets ofamorphous carbon with hydroxyl, carboxyl, and�SO3H groups. The black powder produced may bemade into hard pellets or thin films. The catalyst isstable up to 180°C and can be recycled (Sugary [...]2005; Toda et al., 2005).

ZeolitesToday, the majority of solid acid catalysts used by

the petrochemical industry are zeolites. In 1948, theLinde Division of Union Carbide commercializedsynthetic zeolites A and X as adsorbents. Rabo and co-workers saw the potential of zeolites as catalysts byintroducing acid sites. They also realized thatinteractions between these acid sites and the reactantsalso involve adsorption of the reactants inside thezeolite crystals. At about the same time Mobilintroduced a Fluid Catalytic Cracking (FCC) catalystbased on Y zeolite. These events perhaps initiated thebiggest revolution in petroleum and petrochemicaltechnologies.

In 2001, about 81% of the world’s synthetic zeoliteproduced was for detergent additives and only 13% forcatalysts (the remaining 6% was adsorbers anddessiccants). The catalysts represented 55% of theoverall value of all zeolites produced. The majority ofthe zeolite catalysts is used in the FCC process.

Zeolites are porous crystalline aluminosilicatesbuilt from SiO4 and AlO4 tetrahedra. These tetrahedra

are crosslinked to each other through the oxygen ions.In almost all natural zeolites, aluminium or siliconoccupies all the tetrahedra. In some syntheticmolecular sieves boron, gallium, germanium, iron,titanium, phosphorus, or other heteroatoms maysubstitute aluminium or silicon.

Four properties make zeolites applicable asheterogeneous catalysts: • Pore diameters are uniform. In most zeolites the

pores have one or more discrete sizes. • The pore diameters are similar to the dimensions

of simple organic molecules.• Exchangeable cations allow the introduction of

different cations with various catalytic properties. • Cationic sites exchanged to H� have a high number

of strong acid sites. The first two properties account for the molecular

sieving action and the other two for the catalyticactivity.

The Brønsted acid sites are associated withframework aluminium (or other trivalent) ions. Thenumber of the Brønsted acid sites is directlyproportional to the concentration of these frameworktrivalent ions. The strength of the Brønsted acid sitedepends on the environment of the framework Al. Acompletely isolated Al tetrahedron will create thestrongest type of Brønsted acid site. Therefore, thestrength of the acid site is usually inverselyproportional to the concentration of frameworkaluminium, up to about a silica/alumina ratio of 10.Above this ratio the aluminium level does not affectacid strength. Changing the framework Si�Al ratio,either by synthesis or by postchemical synthesis willchange acid strength. Thus, one would prefer zeoliteswith lower framework Si�Al ratios for reactionsdemanding low acidities. In contrast, when strongacidities are required, zeolites with isolated frameworkAl (Si�Al ratios 9-10 or higher) should be chosen.

The strength of the Brønsted acid sites is alsoaffected by isomorphic substitution. The acid strengthdepends on the type of heteroatom; gallium or ironzeolites are much less acidic than aluminium zeolites.Boron-substituted zeolites have very weak acidity.AlPO4-s (molecular sieves containing only Al and P)have no exchangable cations and therefore no inherentacidity.

Lewis sites in zeolites are usually associated withextraframework Al. The simultaneous presence ofBrønsted and Lewis sites could enhance the acidactivity of zeolites. However, extraframework Alaffects different reactions differently and thereforecould affect selectivities. Some processes (e.g.ethylation of toluene, toluene disproportionation)benefit from the removal of extraframework Al. Thenature of Lewis sites, and how they affect catalyst

702 ENCYCLOPAEDIA OF HYDROCARBONS

SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

performance are still not fully understood. Moreresearch would be desirable in this area.

Pore diameters depend on the number of tetrahedrain the ring around the pores. Zeolites with 8 tetrahedraare called small-pore, those with 10 tetrahedramedium-pore, and those with 12 tetrahedra large-poremolecular sieves. Those with more than 12 tetrahedraare ultralarge-pore or extralarge-pore molecular sieves.Maximum free diameters of small-, medium-, andlarge-pore zeolites are 0.43, 0.63 and 0.80 nm,respectively. The orientation of the plane of the ring,and whether the elements forming the ring are or arenot in the same plane, also affect pore diameters. Thisgives a wide range of pore diameters and pore shapes.Furthermore, the pores may be straight or zigzag, andthe pore system may be one-, two-, orthree-dimensional. Thus we may select the mostappropriate acid strength, dimensionality, pore sizeand pore shape from a large variety of molecular sievezeolites to optimize almost any reaction.

Shape selectivity is a very important property ofmany commercial zeolite catalysts. Shape-selectivecatalysis differentiates between reactants, products, orreaction intermediates according to their shape andsize. If almost all of the catalytic sites are confinedwithin the pore structure of a zeolite and if the poresare small, the fate of reactant molecules and theprobability of forming product molecules aredetermined by molecular dimensions andconfigurations as well as by the types of catalyticallyactive sites present. Only molecules whose dimensionsare less than a critical size can enter the pores, haveaccess to internal catalytic sites, and react there.Furthermore, only molecules that can leave appear inthe final product.

Weisz and Frilette were the first who recognizedand described shape-selective catalysis (Weisz et al.,1962). Weisz and his co-workers and many othersresearchers of Mobil Research and Development werethe pioneers of shape-selective catalysis. Subsequentresearch explored and described shape-selectivecatalysis in more detail (Weisz and Frilette, 1960;Venuto and Landis, 1968; Weisz, 1973; Breck, 1974;Csicsery, 1976; Heinemann, 1981; Barrer, 1982; Chenand Garwood, 1986; Dyer, 1988; Chen, et al., 1989;Szostak, 1989).

There are three types of shape selectivity; pore sizeand shape can limit the entrance of some reactingmolecules, the departure of some product molecules,or the formation of a specific transition state:• In reactant selectivity some of the molecules in a

reactant mixture are too large to diffuse throughthe catalyst pores.

• In product selectivity some of the products formedwithin the pores are too bulky to diffuse out as

observed products. They are either converted toless bulky molecules (e.g. by equilibration orcracking) or coke (which would eventuallydeactivate the catalyst).

• In restricted transition-state selectivity, certainreactions are prevented because thecorresponding transition state would requiremore space than available in the cavities orpores. Neither reactant nor product moleculesare prevented from diffusing through the pores.Reactions requiring smaller transition statesproceed unhindered (Csicsery, 1967, 1969,1970b, 1971, 1986, 1987).In reactant and product selectivities a molecule

will react preferentially and selectively if its diffusivityin the shape-selective catalyst is at least one or twoorders of magnitude higher than that of the competingones.

Reactant and product selectivities differ in animportant way from restricted transition-stateselectivity. The first two types aremass-transfer-related phenomena and therefore dependon particle size. Restricted transition-state selectivityis governed by the intrinsic properties of the crystalstructure and therefore does not depend on diffusionand crystal size. Thus, particle-size effects maydistinguish reactant and product type selectivities fromrestricted transition-state selectivity.

Restricted transition-state selectivity and productselectivity may operate together.

In acid-catalysed reactions, shape selectivityreverses the usual order of the relative reaction rates ofcarbocations. Generally, acid-catalysed reactivities ofprimary, secondary, and tertiary carbons differ. Overnon-shape-selective catalysts, hydrocarbons that havetertiary carbon atoms, and therefore can form tertiarycarbon ions, react much faster than those that can formonly secondary carbon atoms. Only secondarycarbocations can form on n-paraffins, whereas tertiary carbocations can be generated onsingly-branched isoparaffins. Therefore, in most cases,isoparaffins crack and isomerize much faster thann-paraffins. The order of these rates is reversed inmost shape-selective acid catalysis; that is, n-paraffinsreact faster than branched ones, which sometimes donot react at all. This is the essence of reactant orproduct type shape-selective acid catalysis and this isat the base of many industrial applications ofshape-selective catalysts.

The advantages of shape-selective catalysts are thatthey may favour the formation of desirable isomersover less desirable ones, crack undesirable moleculesto smaller fragments which are easily removed bydistillation and avoid undesirable competing reactionssuch as coking or polymerization.

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ACID OR BASE CATALYSED PROCESSES

Many reactions may proceed by both mono- andbimolecular mechanisms. Usually the bimolecularreaction has lower activation energy than themonomolecular one. However, when the spacearound an active site is insufficient to accommodatethe bimolecular transition state, the reactionproceeds via the monomolecular route. One exampleis the cracking of small (i.e. less than C7) paraffins.Both the bimolecular and the monomolecularreactions are allowed over amorphous acid catalystsand large-pore zeolites. The rate-determining step ofthe bimolecular (or ‘classical’ carbenium ion)mechanism involves hydride transfer between acarbocation and a neutral molecule. The transitionstate is composed of these two entities. Highertemperatures (above 720-770 K), low acid sitedensity, and insufficient space to form a bulkybimolecular transition state (for example, in manyreactions proceeding in ZSM-5) favour themonomolecular mechanism. According to Kranilla,Haag, and Gates (Kranilla et al., 1992), thetransition state of this monomolecular reactioninvolves a pentacoordinated carbonium ion. The tworeactions give different products. The end productsof the bimolecular reaction are propane, propylene,isobutane, and isobutene. The predominant lowmolecular weight products of the monomolecularreaction are hydrogen, methane and ethane.

Alkylaromatics could also either isomerize bymonomolecular or by bimolecular mechanisms. Themonomolecular reaction proceeds through consecutive1,2-shifts, whereas in the latter, diphenylmethaneintermediates are involved. The ratio of themonomolecular/bimolecular reaction paths in xyleneisomerization depends on acid site density (i.e. thedegree of dealumination) and adsorptioncharacteristics. Corma and co-workers showed thatover ultrastable HY zeolite, up to 20% ofisomerization may occur via the bimolecular pathway(Corma et al., 1992).

At present, most industrial shape-selective catalyticprocesses use medium-pore zeolites, usually one of theso-called ‘pentasil’ family. ZSM-5 is by far the mostimportant medium-pore zeolite. It has high acidcatalytic activity and it is very stable. Several authorsdescribed the ZSM-5 structure (Kokotailo et al., 1978;Olson et al., 1980). ZSM-5 has two types often-membered ring pores. One is sinusoidal and nearlycircular (0.54·0.56 nm). The other system has ellipticalpores (0.52·0.58 nm). These are straight andperpendicular to the first system. The silica�aluminaratio in ZSM-5 varies from the teens to the thousands.High silica�alumina ratios give hydrophobicity, highacid strength, and thermal, hydrothermal, and acidstabilities.

A very important advantage of ZSM-5 andsimilar molecular sieves is that as a consequence ofrestricted transition state selectivity, coke precursorscannot form in their pores. Therefore these zeolitesdeactivate much more slowly than other crystallineand amorphous catalysts. The difference is nottrivial; whereas large-pore zeolites such as HY andH-mordenite in most acid-catalysed reactions woulddeactivate in minutes or in hours, ZSM-5 lifetimescould range from weeks to more than a year.ZSM-5s resistance to coking makes a number ofindustrial processes not only possible but alsoeconomical.

Base catalysisBase catalysis proceeds through negatively charged

carbanions. The removal of a proton from an alkane orother hydrocarbon creates carbanions. The mechanismstarts with an initiation step that creates a carbanion:

C6H5CH3�R�Na�� C6H5CH2�Na��RH

Addition of an olefin then creates anothercarbanion:

C6H5CH2��CH2�CHCH3�

�

�� C6H5CH2�CH(CH3)CH2�

Proton transfer propagates the chain:

C6H5CH2CH(CH3)CH2��C6H5CH3�

�

�� C6H5CH2CH(CH3)2�C6H5CH2�

Primary carbanions are more stable than secondaryones. Tertiary carbanions are the least stable.

Solid bases are oxides and modified oxides,Al2O3-NaOH-Na, Al2O3-KOH-K, ZrO2-KOH,ZrO2-K2O, MgO, MgO-Al2O3, hydrotalcites,mesoporous silicas modified with amino groups, andCsNaX, CsNaY and other alkali-treated zeolites(exchanging zeolites with a less electronegativecharge-balancing cation such as Cs� and�or occlusionof alkali metal oxide clusters or alkali metal clusters inzeolite cages makes the zeolites basic).

Basic zeolites contain either Lewis base sitesassociated with the framework oxygen ions, orBrønsted base sites linked to basic hydroxyl groups, orboth. The former is influenced by the negative chargeof the oxygen, while the latter depends on the nature ofextraframework cation present in the zeolite. The basestrength of the zeolite rises with the increasingaluminium content of the framework (i.e.MFI�MOR�L�Y�X) and reduces with thedecreasing ionic radius of the exchange cation (e.g.Li�Na�K�Rb�Cs). It is also influenced by theexchange of Al and/or Si ions in the zeolite frameworkfor Ga and/or Ge respectively (Barthomeuf, 1996;Camblor et al., 1992).



Unlike in acid-catalysed toluene methylation,where the aromatic ring is attacked and theproducts are xylenes, basic zeolites alkylate theside chain and form ethylbenzene and styrene.Product yields sharply increase with increasingzeolite basicity. Sodium vapour treatment increasesthe activity of NaX, NaY, NaL, CsX and CsYzeolite catalysts by increasing the negative chargeon adjacent framework oxygen ions.

Base-catalysed side-chain alkylation ofalkylaromatics proceeds under mild conditions.Heteroaromatics such as aniline are alkylated withmethanol with high conversion and selectivity toN-alkylates such as N-methyl- andN,N-dimethylaniline, and N,N-dimethylparatoluidine.Alkylation with dimethylcarbonate is also veryselective.

The first steps of BP-Amoco’s and Optatech’sdimethylnaphthalene processes are base-catalysedreactions (Sikkenga et al., 1990a, 1990b; Amelse,1993; Lillwitz and Karachewski, 1993; Vahteristo etal., 1999; Lillwitz, 2001). The reactions are discussedbelow.

Shape selectivity may contribute to the very highbenzene selectivity of n-hexane dehydrocyclizationover noble metal containing basic L, X, Y zeolites.Chevron’s Pt�BaL catalyst is probably the best of thesesystems.

Basic zeolites also catalyse Knoevenagelcondensations (formation of a C�C bond between acarbonyl and a methylene group) and aldolcondensations (involving two C�O bonds).

Na forms of Al, Ga, Si, and Ge beta zeotypescatalyse the condensations of benzaldehyde with ethylcyanoacetate (pKa�9.0), ethyl acetoacetate(pKa�10.7) and ethyl malonate (pKa�13.3). Na-betazeolite is more basic than NaY and NaX. Replacementof aluminium by gallium and silicon by germaniumfurther increases the basicity.

Substitution of aluminium by titanium creates anew type of basic zeolite. Philippou and Andersondemonstrated that ETS-10 type materials exchangedby Cs-ions are very active and selective inbase-catalysed aldol condensation (Philippou andAnderson, 2000).

11.3.2 Selected catalytic processesbased on acid or basecatalysis

The technologies of many commodity petrochemicalsproduced by using acid (and base) catalysts aredescribed below. The space available in this reviewallows details of only a selected few.

Chemicals from natural gas

Methanol to gasoline and methanol to olefinsMobil’s Methanol to Gasoline (MTG) and

Methanol to Olefins (MTO) processes convertmethanol to gasoline or light olefins. Since methanolcan be made from practically any organic material, theprocess can make synthetic gasoline or light olefinsfrom coal, natural gas, petroleum residua, agriculturalwastes, municipal garbage, wood, etc. Originally,Mobil developed the MTG process to convertmethanol to gasoline in New Zealand because thatcountry had a large amount of natural gas but littlegasoline. MTG converted methanol to a mixture ofparaffins (26-29%), olefins (2.6-3.5%), aromatics(12-14%) and water (56%) over shape-selectiveHZSM-5 catalyst. The Methanol to Olefins processpredominantly produces light olefins. Contact times inthe MTO process are shorter than in the MTG process.Other reaction conditions are also somewhat different.Chang, Stocker, and Keil reviewed the MTG process(Chang, 1983; Keil, 1999; Stocker, 1999).

Instead of ZSM-5, newer and probably better MTOprocesses use different catalysts. For example,UOP�Norsk Hydro uses SAPO 34 in fluidized bedoperation. ExxonMobil also has many new MTOpatents.

DimethyletherDimethylether is the feedstock for methylacetate

and dimethylsulphate and an intermediate in the MTOand MTG processes. It is also a candidate for cleantransportation fuels. The preferred catalyst in the pastwas g-allumina. However, competitive adsorption ofthe water produced in the reaction slowed down theetherification reaction. Hydrophobic HZSM-5 (i.e.ZSM-5 with high Si/Al ratios) is much less affected bywater and therefore performs much better thang-allumina. However, secondary reactions to olefinsand other hydrocarbons decrease ether selectivity.Such secondary reactions were absent with ZSM-5partially neutralized with Na� or other alkali ions. Thebest catalyst was ZSM-5 with a Si/Al ratio of 20. Etherselectivity reached 100% (Roh et al., 2004).

Methylamine Methylamines are intermediates of many

N-containing chemicals. They are made fromammonia and methanol over amorphous silica-aluminacatalysts. The products are monomethylamine,dimethylamine, and trimethylamine. Equilibriumfavours the formation of trimethylamine. For example,over silica-alumina at 623 K and at 99.8% methanolconversion the product mixture contains 15%monomethylamine, 23% dimethylamine and 62%



trimethylamine. However, market demand is muchhigher for the mono- and dimethylamines.

Shape-selective catalysts can shift productcomposition to contain much less trimethylamine.Segawa and co-workers found that Na-mordenitetreated with SiCl4, and then ion -exchanged for theH-form, produced less than 1% trimethylamine at 90%methanol conversion (Ilao et al., 1996). Anotherselective catalyst is H-chabazite. At 96% conversion, itproduces a mixture of 48% monomethylamine, 49%dimethylamine, only 3% trimethylamine and nodimethylether. Restricted transition state selectivitydoes not allow trimethylamine formation over thesemedium-pore zeolites because the methylation ofdimethylamine requires a rather bulky transition state.A 40,000 t�yr plant was set up in 1985 (the Nittoprocess).

Isomerization of butene to isobuteneIsobutene, the intermediate of MTBE

(methyl-ter-butyl ether), polybutenes and gasolinealkylate, is more valuable than the n-butenes. In thepast, isobutene was prepared from n-butane in atwo-step reaction. The first step isomerized n-butaneto isobutane over Pt-AlCl3-alumina (UOP’s Butamerand BP’s processes). The isobutane wasdehydrogenated to isobutene after that (Air Products’Catofin, Phillips’ Star, UOP’s Oleflex andSnamprogetti’s Isobutane dehydrogenation processes).

Many acid catalysts can convert light n-olefins tobranched ones. The isobutene��

��n-butenesequilibrium limits conversions and yields. Bothmonomolecular and bimolecular mechanisms arepossible (Xu et al., 1994; Guisnet et al., 1996). Inmedium-pore zeolites, restricted transition-stateshape-selectivity favours the monomolecularpathway and restricts other side reactions, such asthe formation of coke precursors and coking, as wellas hydrogen transfer, which forms alkanes andaromatics. Non-shape-selective catalysts favour thebimolecular isomerization in which the butenes firstdimerize to octenes. The primary octenes thenisomerize to other branched C8H16 isomers. Octeneswith trimethylpentene or dimethylhexene skeletonscrack by b-scission to butenes and isobutene orpropylene plus pentanes or propane and pentanes.Ferrierite (Shell’s process), ZSM-22 and ZSM-23(Mobil’s processes) are the most selective zeolitecatalysts recommended for the reaction. As ZSM-23does not have cavities, it is more resistant to cokingin this reaction than ferrierite/ZSM-35. At 693 Kisobutene selectivities over ZSM-23 are 85-95% at20-30% yields.

Isobutene copolymers require very pure isobutene.Snamprogetti’s process decomposes MTBE to

isobutene and methanol over a zeolite catalyst. Theacidity of the catalyst has to be very low to avoidsecondary reactions of isobutene. Boron-ZSM-5containing some cerium has the required low acidity.

MTBE and other ethersMTBE is manufactured from methanol and

isobutene. MTBE has been used in the United Statesgasoline at low levels since 1979 to replace lead as anoctane enhancer. Since 1992, MTBE has been used athigher concentrations in some gasolines to fulfil theoxygenate requirements set by the US Congress in the1990 Clean Air Act Amendments. By 1999, more than200,000 barrels per day of MTBE were produced inthe Unites States. Almost all was used as gasolineadditive. MTBE and other oxygenated gasolineadditives also optimize oxidation during combustionand thus reduce harmful exhaust emissions.

MTBE, however, is water soluble. Therefore,accidental seepage from underground gasoline storagecan contaminate drinking water supplies and lowlevels of MTBE can make drinking water suppliesundrinkable due to its offensive taste and odour. In1996, two drinking water wells of the city of SantaMonica, in California (Unites States) becamecontaminated with MTBE at levels as high as 610 and86 ppb, respectively. The two wells supplied 50% ofSanta Monica’s drinking water supply. As a result, bothwere shut down. This major water contaminationbrought public attention to the problems that MTBEcauses. Soon after, many other communities detectedMTBE in their drinking water. In addition, MTBE atsomewhat higher doses is a potential humancarcinogen. As a consequence, MTBE demand ismuch lower now than it used to be.

Ethyl-ter-butyl ether (ETBE), methyl-ter-amylether (TAME), and some other ethers share MTBE’sbenefits but fewer of its disadvantages.

MTBE, ETBE, and TAME are prepared by reactingmethanol or ethanol with isobutene or 2-methylbuteneover macroporous sulphonic acid resin catalyst, suchas Amberlyst-15 or Dowex M32.

Benzene, toluene and xylenes bydehydrocyclodimerization

Some of the more important commodities madefrom benzene, toluene and xylenes (BTX) are styreneand technopolymers, terephthalic acid, cumene, phenolderivatives, acetone, nylon, methacrylates, diphenols,polyurethanes, benzyl chloride derivatives and cresols.

Dehydrocyclodimerization is the conversion oflight (C3-C5) paraffins or olefins to aromatics. Thereaction involves consecutivedehydrogenation-dimerization-cyclization andaromatization steps (Csicsery, 1970a, 1979). The



reaction may proceed over many differentdual-function catalysts. ZSM-5 is much more stablethan most other catalysts because it does not formpolycyclic aromatics and it makes much less coke.British Petroleum and UOP developed the Cyclarprocess jointly. The catalyst is ZSM-5 modified withgallium (Davies and Kolombos, 1979a, 1979b). Inthe Cyclar process, propane, butanes, pentanes andeven ethane are converted to aromatics in a series ofstacked reactors. Butane, for example, yields anaromatic product containing about 28% benzene,43% toluene, 20% xylenes and ethylbenzene, andabout 9% higher aromatics. Byproducts arehydrogen, methane, ethane, and C3-C5 aliphatics.Olefin-containing feeds yield more aromatics. Thedeactivated catalyst is regenerated by air. The firstCyclar plant started operation in 1990 at BritishPetroleum’s Grangemouth refinery, in Scotland. An improved catalyst, introduced in 1991, has vastlyimproved performance. Mobil’s M2-Formingprocess (Chen and Yan, 1986) is similar to theCyclar process. A catalyst containing both Pt and Gacan also catalyse the aromatization of ethane.

The Cyclar and M2-Forming processes areattractive where LPG is available and where hydrogenis needed. They are also good alternatives toconventional hydrotreating-plus-extraction to upgradepyrolysis gasoline. The large amount of co-producthydrogen may have a significant positive impact oneconomics.

Considerable research effort was made tounderstand the role of Ga and Zn in thedehydrocyclodimerization reaction. The consensus isthat Ga3� is in a reduced form (perhaps as Ga�) in theworking catalyst, and it catalyses the dehydrogenationof propane and butanes to propylene and butenes, andthe recombination of H� to H2. The balance between thedehydrogenation function of the Ga species and theacid function of the zeolite is the key factor that affectscatalytic activity (Meriaudeau and Naccache, 1992).

Aromatic intermediates

Xylene isomerizationTerephthalic acid, the monomer of many polyester

polymers, is made from p-xylene by oxidation.p-xylene is therefore more valuable than the other twoisomers. The xylene isomerization process convertsthe less valuable m- and o-xylenes to p-xylene.

In the past, dual-functioning catalysts (e.g.Octafining) and various monofunctional acid catalysts(e.g. silica-aluminas) were used to isomerize xylenes.Selectivities are much better and there is much lesscoking over shape-selective ZSM-5, modified ZSM-5,and related other pentasil catalysts than in these older

systems. One reason is that there is not enough spaceinside medium-pore zeolites to accommodate thetransition states leading to trialkylbenzenes and cokeprecursors (this is an example of the benefits ofrestricted transition-state selectivity). Less cokingallows much longer operation between regenerationsthan was ever possible with the old amorphouscatalysts.

Isomerization may proceed either by means of amonomolecular or a bimolecular mechanism.Diphenylmethane intermediates are involved in thebimolecular reaction. The monomolecular reactionproceeds through consecutive 1,2-shifts (Corma,2003). Zeolites with strong acid sites isomerizexylenes by the monomolecular pathway whereasmesoporous molecular sieves with weaker acid sitesmostly catalyse the bimolecular reaction.

In ZSM-5, at 200°C the kinetic reaction controlsthe rate of isomerization. At and above 300°C the rateis diffusion-controlled. The rate of diffusion ofp-xylene is higher than that of o-xylene. m-xylene hasthe lowest diffusion rate.

Xylene isomerization feedstocks usually containmore than 10% ethylbenzene. If not removed,ethylbenzene will accumulate in the recycle streams.Shape-selective zeolite catalysts offer the additionaladvantage that they can convert ethylbenzene to easilyremovable products. For example, ethylbenzene couldbe converted simultaneously by isomerization tobenzene and dialkylbenzenes. Distillation can removeboth. Otherwise, the ethylbenzene may be dealkylated,and the ethylene formed could be hydrogenated over ahydrogenation component (e.g. platinum) undermoderate hydrogen pressure. This makes thedeethylation irreversible.

Another option is to isomerize ethylbenzene toxylenes. Examples are Union Carbide�UOP’sSAPO-11 or the iron-containing pentasil catalyst(Encilite) developed jointly by the National ChemicalLaboratory of India and Indian PetrochemicalsCorporation. With these catalysts, xylene yields can behigher than 100% if the C8 feed contains ethylbenzeneand some, or most, of this ethylbenzene is convertedinto xylenes. The most likely reaction involvesring-contraction�ring-expansion steps. A plant in India,in Vadodara (Baroda), used this process.

The rate of disproportionation relative to that ofisomerization diminishes with the decreasing zeolitepore diameter. Catalyst pore size and pore mouth arespecifically tailored in Mobil’s MPTX (and perhaps inMTPD) process to minimize disproportionation and tooptimize p-xylene selectivities. Today, more thanone-half of the western world’s xylene isomerizationplants use one of Mobil’s xylene isomerizationprocesses.



p-xylene processes from toluene The economic incentive to convert toluene to

p-xylene is that toluene is less expensive than eitherbenzene or xylenes. Toluene may be converted top-xylene by disproportionation or by alkylation withmethanol.

Toluene disproportionation. Mobil’s toluenedisproportionation process converts two mol oftoluene to one mol each of xylenes and benzene. Heretoo, restricted transition-state selectivity minimizescoking and the formation of higher molecular weighthydrocarbons. Additional advantages are thatseparation costs are lower than those of xyleneisomerization and that the benzene produced is verypure (99.99%, pure enough for ethylbenzenemanufacturing). The first plant went on stream in 1975in Naples, Italy (Smith et al., 1980). The catalyst wasZSM-5.

Mobil’s latest and more sophisticated SelectiveToluene Disproportionation process maximizesp-xylene yield.

Pore size regulation can increase p-substituteddialkylbenzene selectivities in toluenedisproportionation (Halgeri and Das, 2002).

Toluene alkylation with methanol. Amorphousacid catalysts and large-pore zeolites produce thexylene isomers at or near equilibriumconcentrations. ZSM-5 treated with Mg, P, etc.,yields p-xylene preferentially. It is possible andprobable that inside the pores, all three xyleneisomers are made at equilibrium concentrations.However, the diffusivity of p-xylene in this system isabout four orders of magnitude higher than those ofthe other two isomers. As a consequence, productp-xylene concentrations may be higher than 90%.This is an example of product-shape selectivity(Chen et al., 1979; Smith et al., 1980).

4,4�-dialkylbiphenyls 4,4�-dicarboxylbiphenyl is the monomer of

thermotropic liquid crystal polymers, highlyheat-resistant and very strong films, and the so-called‘engineering polymers’. The dicarboxylic acids aremade from 4,4�-dialkylbiphenyls by side-chainoxidation.

The dimensions of the twelve dimethylbiphenylisomers are similar. Therefore, selective directnaphthalene methylation to yield mostly4,4�-dimethylbiphenyl is very difficult. Cleverstructural modifications of the zeolite pores to exploiteven subtle size-differences may make methylationselective. Shen, Sun, and Song deposited phosphate orcalcium oxide on the internal and external surfaces ofZSM-5 to improve their selectivity (Shen et al., 2001).Isomorphous post-synthesis substitution of framework

Al with Fe also improved 4,4�-dimethylbiphenylselectivity (Song et al., 1999; Shen et al., 2001).

Transalkylation with polymethylbenzenes isanother way to enhance selectivity. Restrictedtransition-state selectivity favours the linear transitionstate between the biphenyl and the polyalkylbenzenemolecules. Transition states that could yield otherisomers are much more bulky. Transalkylation inultrastable faujasite produces preferentially4�-methylbiphenyl and 4,4�-dimethylbiphenyl.Eliminating some acid sites with tributylphosphite orphosphoric acid reduces isomerization on the externalsurface and thus further improves selectivity (Song,2000).

Isopropyl groups are much bulkier than methylgroups and therefore differences between the kineticdiameters of 4,4�-dialkylbiphenyl and those of theother isomers are much larger. Thus,4,4�-diisopropylbiphenyl may be selectivelysynthesized over shape-selective catalysts because itskinetic diameter is considerably less than those of theother diisopropylbiphenyl isomers. Whereas the4,4�-diisopropylbiphenyl yield in homogeneousphosphoric acid catalyst was only 1.9% (at 48%conversion), 4,4�-diisopropylbiphenyl selectivity was73.5% (at 98% conversion) over three-dimensional-dealuminated mordenite (Lee et al., 1989). Mordenitehas somewhat smaller pore dimensions than otherlarge-pore zeolites. 4,4�-diisopropylbiphenylselectivities in most large-pore zeolites such as HY, Lor offretite seldom surpass 15%.

Lee leached almost all Al from the framework ofhis mordenite catalyst with a mineral acid. Thistreatment introduced mesopores into the structure andconverted the original one-dimensional mordenite poresystem to a three-dimensional one. Restrictedtransition state causes the high selectivity here.Furthermore, diffusion is much less hindered here thanin the original one-dimensional mordenite (Lee et al.,1989).

Note, however, a potential disadvantage of anyprocess involving the diisopropyl intermediate. Theoxidation of the diisopropyl intermediate to thedicarboxylic acid product would generate about threetimes as much heat than that of the dimethylintermediate.

Biphenyl alkylation with ter-butyl alcohol over HYand H-beta zeolites favours 4-ter-butylbiphenyl and4,4�-di-ter-butylbiphenyl over the other isomers. HY ismore stable in this reaction than H-beta (Sivasanker etal., 1992).

2,6-dialkylnaphthalenesOxidation of 2,6-dialkylnaphthalenes yields

naphthalene-2,6-dicarboxylic acid. The



polycondensation of naphthalene-2,6-dicarboxylic acidwith ethylene glycol gives polyethylenenaphthenate.This polymer has a four times higher oxygen barrier, afive times higher CO2 barrier, a 3.5 times highermoisture barrier, a 35% higher tensile strength and a50% higher flexural modulus than polyethyleneterephthalate. It is an intrinsically higher UV barrierand also has better thermal performance. Despite itsexcellent properties, high production costs limitapplications. Costs are high because the difficulties inselectively preparing 2,6-dialkylnaphthalenes parallelthose of the 4,4�-dialkylbiphenyls. In the earlyseventies Teijin Petrochemical Industries made apolyethylene naphthenate-based extruded film, theso-called -Q polymer’. Today the world’s 2,6-dimethyl-naphthenate production is just over 40,000 t�yr.

Direct methylation with methanol or otherconventional methylating agents is not selective andproduces many of the ten possibledimethylnaphthalene (DMN) isomers. Separation bydistillation of 2,6-dialkylnaphthalenes from the isomermixture is extremely difficult and very expensivebecause the boiling points of the isomers are veryclose to each other. Today 2,6-DMN is separated fromthe other isomers by crystallization.

Isomerization between DMN isomers is rather easywhen it involves a to b or reverse methyl shifts. Shiftsbetween the two rings, or b to b-methyl shifts, aremuch more difficult. Only 1,5-DMN and 1,6-DMNcan be easily isomerized to 2,6-DMN. The otherisomers (2,7-DMN; 1,7-DMN; 1,8-DMN; 1,4-DMN;1,3-DMN; 2,3-DMN and 1,2-DMN) must beconverted by disproportionation or transalkylation.

The most troublesome isomer is 2,7-DMN. Thethermodynamic ratio between 2,6-DMN and 2,7-DMNis close to one. 2,7-DMN forms a eutectic with 2,6-DMN. Therefore, separation would bedifficult if 2,7-DMN were present in large quantities.

Several technologies follow different strategies toavoid naphthalene methylation and the separation ofDMN isomers. Sun Oil and Teijin were extremelyactive during the 1970s and the 1980s in developing atechnology to make 2,6-DMN.

In 1995, BP-Amoco started the first large-scalenaphthalene-2,6-dicarboxylic acid plant in Decatur,Alabama. The technology starts by reacting o-xyleneand 1,3-butadiene in the presence of an alkali metalcatalyst to produce o-tolyl-2-pentene. This isconverted into 1,5-dimethyltetraline over a Pt-Yzeolite catalyst. The final steps are dehydrogenationto 1,5-DMN at 420°C over Pt-Re supported ong-alumina and acid-catalysed isomerization to2,6-DMN over beta zeolite. Crystallization purifiesthe 2,6-DMN isomer product. The other isomers arerecycled to the isomerization unit (Sikkenga et al.,

1990a, 1990b; Amelse, 1993; Lillwitz andKarachewski, 1993; Lillwitz, 2001).

The Finnish company, Optatech, purchased asomewhat similar technology from Neste OY. Optatechmakes 2,6-DMN directly, eliminating theisomerization and recycling steps. Rather thano-xylene, the process starts with p-xylene, which isreacted with butadiene in the presence of potassium ongraphite. The product contains about 67%1-p-tolyl-2-methyl-butene. This intermediate is thendehydrocyclized over a supported nonacidic chromiumoxide catalyst at 510°C and 4 s contact time(Vahteristo et al., 1999).

Kobe-Steel and Exxon-Mobil have disclosed amulti-step 2,6-DMN process. A mixture of2-methyl-naphthalene and 1-methyl-naphthalene isalkylated with methanol over MCM-22 catalyst at400°C. Conversion is around 58% but selectivity tothe desired 2,6-DMN isomer is only 14%. Afterseparation in a high-pressure crystallization unitdeveloped by Kobe-Steel, the undesired isomersare transalkylated with naphthalene over MCM-22.This approach is rather interesting because it usesraw purified and therefore inexpensive, startingmaterials.

The technology of Polimeri Europa (formerlyEnichem) is based on a mixture of naphthalenes andalkylnaphthalenes derived from light cycle oil or fromcracking fuel oil. This mixture is added to a solvent ofpseudocumene and other highly methylated benzenesderived from the same raw feed. Alkylation withmethanol over a zeolite catalyst follows. The mainreactions are methylation of benzenes andnaphthalenes and transalkylation between thepolymethylbenzenes and the naphthalene derivatives.The methanol maintains the degree of methylation ofthe alkylbenzene solvent at the desired level. Thedistillate, rich in 1,5-DMN, 1,6-DMN, and 2,6-DMN,is fed into an isomerization reactor. Finalcrystallization yields pure 2,6-DMN. All other sidestreams go first to the dealkylation/transalkylationunit. Finally, the sidestreams are recycled in thealkylation reactor. ZSM-12 zeolite is the most stable,and also the most selective, of the zeolites tested inthis reaction.

Catalytic activity in an alkylation pilot plant wasmaintained constant for 800 h. Faujasite and betazeolites deactivated fast, and MCM-22, EUO, mazziteand mordenite even faster.

Transmethylation with 1,2,4-trimethylbenzeneselectively methylates naphthalene or2-methylnaphthalene over MTW acidic zeolite. Theproduct contains up to 35% 2,6-DMN, much morethan the equilibrium value of 12%.2,6-DMN�2,7-DMN ratios vary from 2 to 2.5 (the



equilibrium value is 1). This high 2,6-DMN yield isdue to restricted transition-state type selectivity insidethe MTW pores. Selectivities, activities, and catalystlife with other zeolites are lower. Methanol may beadded to the spent solvent before it is recycled torestore its 1,2,4-trimethylbenzene content (Pazzuconiet al., 2001).

Computer graphics were used to translate androtate diffusing molecules within the pores of knownzeolites, and count the number and magnitude ofoverlaps between the organic molecule and the zeoliteframework. Cross Polarization Magic Angle SpinningNuclear Magnetic Resonance (CP-MAS-NMR)measurements and other experimental work confirmedthe results of the computer graphics study; a mildlydealuminated (steamed and acid-washed) mordenitemay be the catalyst of choice (Aguilar et al., 2000).Song reviewed the alkylation of biphenyl andnaphthalene (Song et al., 1999; Song, 2000).

Aromatic building blocks

EthylbenzeneIn 2001 about 70% of the world’s benzene

production was alkylated with ethylene or propylene tomake ethylbenzene or cumene. Ethylbenzene is theintermediate of styrene, the monomer of polystyreneand styrene copolymers. In 2001 world production ofstyrene was more than 20 million metric t. In 2004, theUnited States and Europe produced 5,779,000 and858,000 metric t of ethylbenzene and 5,394,000 and1,666,000 metric t of styrene, respectively (Production[...] 2005). United States ethylbenzene production hasincreased by about 1.7% over the past ten years. 61%of this has been used to produce polystyrene and 39%to make so-called technopolymers. Table 1 showsdetails of the production of ethylbenzene and otheraromatic hydrocarbons.

Since the 1930s ethylbenzene was produced byalkylating benzene with ethylene or ethanol withhighly toxic and corrosive strong mineral acids orFriedel-Craft catalysts (e.g. HF, H2SO4,H3PO4-silica, AlCl3). The separation of the productsfrom the acid is a difficult and energy-consumingprocess. Furthermore, at the end of the reaction, theacids have to be neutralized and the salts producedhave to be disposed of in anenvironmentally-conscious way. Polyalkylation todi- and other polyethylbenzenes is another problem.An additional disadvantage of processes using AlCl3is the contamination of the product with chlorinatedcompounds. Nevertheless, until the implementationof zeolite-based technologies, the H3PO4-silica andAlCl3 processes were the only commercial ways tomake ethylbenzene (and cumene).

The Badger-Mobil gas-phase ethylbenzene processused ZSM-5 catalyst in the vapour phase (Dwyer etal., 1976). Reaction conditions of the Badger-Mobilprocess were 673 K, about 20 bar, a benzene�ethylenemole ratio between 6 and 7, and a benzene WeightHourly Space Velocity (WHSV) between 300 and 400h�1. Because coking is very slow in ZSM-5, cycletimes between regenerations were 40-60 days. Movingfrom a single step gas-phase process to a two-stepliquid-phase process, in which alkylation andtransalkylation are separated, improves the economicsof the process. The polyethylbenzenes are recycledback to the reactor to undergo transalkylation.Temperature control is better and catalyst life is longerin the liquid-phase process than in that of thegas-phase. The application of large-pore zeolites athigher pressure and lower temperature (to keep thereactive system in liquid phase condition) broughtfurther improvements.

The Albene process, jointly developed by theNational Chemical Laboratory of India and byHindustan Polymers, alkylates benzene withethanol in a single-step process over ‘Encilite’, aZSM-5-like molecular sieve which contains ironinstead of aluminium in its framework (Ratnasamyand Kumar, 1991). The Albene process is operatedat 623-673 K, with a benzene�ethanol ratio of 4and a WHSV of 6 h�1. Any diethylbenzene formed,was reacted with benzene in a separate reactor toform more ethylbenzene. The Albene ethylbenzeneprocess used dilute ethanol instead of ethylenebecause India had little ethylene but plenty ethanolfrom molasses.

Newer liquid-phase processes use USY (Unocaland Lummus), beta (Enichem), and MCM-22(ExxonMobil’s EBMax process) catalysts. Theseprocesses have better thermal control and longercatalyst life than gas-phase operations. Technologiesusing USY catalysts produce considerable amounts ofdiethylbenzenes which can be controlled by increasingthe feed benzene/ethylene ratio (Bellussi, 2004). Theadvantages of beta zeolite over USY are betterdiethylbenzene transalkylation activity and higherselectivity.

Mobil-Raytheon’s EBMax ethylbenzene processuses MCM-22 for the liquid-phase alkylation andZSM-5-based catalyst for the gas-phasepolyethylbenzene transalkylation. The alkylationactivity of MCM-22 is comparable to that of USY. Itdeactivates about 2.4 times slower than beta zeoliteand makes fewer diethylbenzenes than either of thosetwo. MCM-22 has two two-dimensional medium-poresystems. One has sinusoidal 0.55�0.4 nm pores. Theother system has large (1.81�0.71 nm) supercageswhich are accessible only through narrow pores. Thus,



diffusional limitations in MCM-22 are more severethan in ZSM-5, or in other medium-pore zeolites, butin its large supercages, transition state selectivity isless important. The result is better monoalkylationselectivity than is possible in large-pore HY or H-betazeolites. The first commercial plant, Chiba StyreneMonomer, started up in Chiba, Japan, in 1995.Ethylbenzene selectivity was 95.5% with only 4.1%diethylbenzene and 0.2% triethylbenzenes and otherproducts.

More recent TRANS�4 technology also performstransalkylation in the liquid phase. The nature of thezeolite has not been disclosed.

Lummus/UOP’s process uses EBZ-500 andEBZ-100 zeolite catalysts in the alkylation and in thetransalkylation liquid-phase units, respectively.Although details were not disclosed, both catalysts aremost likely modified beta zeolites.

CDTECH’s catalytic distillation process furtherimproved ethylbenzene technology (CDTECH is apartnership between ABB Lummus Global andChemical Research and Licensing). In theircatalytic distillation, the alkylation reaction anddistillation occur simultaneously in a single

column, with the catalyst contained in thedistillation column. Distillation continuouslyremoves the reaction products from the catalyticzone. This minimizes polyethylbenzene production.Nevertheless, a transalkylation reactor is stillrequired. Although an earlier patent suggests aY·type zeolite catalyst, recent evidence favoursbeta zeolite.

Advantages of the zeolite-based processes over theearlier AlCl3-based processes are: • They use non-corrosive catalysts. • They use regenerable catalysts.• They are environmentally very friendly because

they generate much less pollution.• Unlike the AlCl3-based processes, they don’t have

spent catalyst disposal problems.• They can use dilute or concentrated ethylene or

ethanol.• They recover most of the exothermic reaction heat.• They have much lower capital and operating costs.

Whereas the H3PO4-silica process generates 0.12 tsolid waste and 0.2 t liquid waste (benzene in water)per 100 t of ethylbenzene produced, the gas-phaseprocess using ZSM-5 catalyst makes less than 0.009 t



1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004Change %2003-2004

Benzene Europe 3,257 3,617 3,561 3,345 3,705 4,565 6,670 6,817 6,535 4,265 –34.7

Japan 3,620 4,013 4,177 4,502 4,203 4,459 4,425 4,261 4,313 4,551 4,758 4.5

Taiwan 422 489 511 506 415 605 690 1,070 931 998 1,088 9.0

China 1,358 1,341 1,535 1,850 1,988 2,131 2,408 2,556 5.1

Toluene Europe 1,329 1,161 209 1,130 1,172 1,155 886 919 848 853 0.6

Japan* 1,219 1,374 1,370 1,419 1,349 1,488 1,489 1,423 1,548 1,584 1,634 3.2

Taiwan 39 20 13 43 23 18 26 54 42 64 140 118.8

p-xylene Japan 2,199 2,476 2,329 2,921 2,754 2,969 2,920 2,814 2,920 3,097 3,164 3.7

Xylenes Europe 240 129 1,368 2,514 2,497 2,602 579 1,122 626 594 –5.1

Japan* 3,627 4,154 3,931 4,634 4,340 4,641 4,681 4,798 4,900 5,213 5,395 3.5

Ethylbenzene USA 4,880 6,194 4,699 5,432 5,743 5,945 5,967 4,642 5,412 5,578 5,779 3.6

Europe 679 684 937 149 1,180 769 911 858 –5.8

Styrene USA 5,123 5,165 5,386 5,156 5,166 5,397 5,405 4,214 4,899 5,167 5,394 4.4

Europe 3,025 3,152 2,989 3,215 958 3,078 3,215 1,666 –48.2

Japan 2,598 2,939 3,085 3,035 2,770 3,055 2,968 3,004 3,016 3,201 3,345 2.6

Taiwan 386 425 411 464 386 806 956 1,146 1,249 1,274 1,247 –2.1

Cumene USA 2,366 2,551 2,667 2,776 3,045 3,162 3,741 3,186 3,503 3,397 3,736 10.0

Table 1. Yearly production of selected aromatic petrochemicals (thousands of metric tons)(Production [...] 2005: partially modified data)

*Petroleum and non-petroleum sources.

solid and less than 0.072 t liquid waste (Davis, 2005).In 2001 about 17 million t of benzene was used to

make ethylbenzene worldwide. 76% of theethylbenzene was made in processes using solid acidcatalysts (40% in gas-phase, 36% in liquid phase). Theremaining 24% was made with AlCl3 catalyst. Peregoreviewed ethylbenzene production technology (Peregoand Ingallina, 2002, 2004). Table 2 summarizes someof the most important commercial ethylbenzeneprocesses.

p-ethyltoluene and p-diethylbenzeneAnother important monomer, p-methylstyrene,

may be produced from 1-methyl-4-ethylbenzene(p-ethyltoluene) by dehydrogenation. The process tomake 1-methyl-4-ethylbenzene is similar to theethylbenzene process discussed above, except the feedis toluene instead of benzene (Kaeding et al., 1982).The shape-selective phosphorous-doped ZSM-5catalyst forms the desired p-ethyltoluene preferentiallybecause the para isomer can diffuse faster than theortho and meta isomers.

The Indian Petrochemical producesp-diethylbenzene over tetraethylortosilicate-treatedHZSM-5 zeolite catalyst. Tetraethylortosilicatecannot penetrate the small pores of ZSM-5 andtherefore only affects the surface. It narrows poreopenings, coats the external surface and eliminatesexternal acid sites without affecting internal ones.This treatment is called Chemical VapourDeposition (CVD). Thus, the primary para productcan not isomerize on the non-shape-selective outersurface to the other isomers. CVD treatmentincreased p-diethylbenzene selectivity from 55% to99.5% (Halgeri, 2001).

Cumene Cumene is the intermediate for the production of

phenol and acetone. In 2004, the United Statesproduced 3,736,000 metric t of cumene (see againTable 1). During the past ten years, cumene productionin the USA has increased by about 4.7%�yr. In theyears 2000-2001 worldwide cumene capacity wasaround 8 million t�yr, distributed over about 40 plants.Today more than a dozen of these plants use zeolitecatalysts. Cumene is produced by alkylating benzenewith propylene. Side-products are diisopropylbenzenesand n-propylbenzene. Diisopropylbenzenes (and eventriisopropylbenzenes) may be transalkylated later withbenzene to give more cumene.

n-propylbenzene is the product of secondaryisomerization of cumene. Experiments with13C-containing cumene showed that in zeolites,cumene isomerizes to n-propylbenzene in abimolecular reaction (Ivanova et al., 1993, 1995).

Side reactions of propylene may also produceolefins and other alkylbenzenes.

For many years, the only catalysts used for thealkylation of benzene with propylene were aluminiumthrichloride (Monsanto-Lummus’ technology) orphosphoric acid supported on silica (UOP’stechnology in the 1940s). The operating conditions ofUOP’s technology are 180-240°C, and between 30 and40 bar pressure. As the phosphoric acid could notreconvert the diisopropylbenzenes by transalkylationwith benzene, the benzene�propylene ratio had to bekept high, between 5 and 10, to minimize theformation of propylene oligomers and polypropylatedcompounds. Operating conditions in Monsanto-Lummus’AlCl3-based technology (1970s) are 110°C,benzene�propylene ratios between 2 and 5, and 100 barpressure. The AlCl3-based process suffers from highdiisopropylbenzene and propylene oligomerformation.

In the 1990s, zeolites substituted the mineral acidcatalysts. The advantages of zeolite-based processesare similar to those shown above for the zeolite-basedethylbenzene process. Different companies developeddifferent zeolites. The zeolite catalysts used aredealuminated mordenite (Dow-Kellog), MCM-22(Mobil), beta (Enichem), and Y (UOP and CDTECH).Dow-Kellog’s dealuminated mordenite has apseudo-three-dimensional structure that improvesperformance and stability. Y and MCM-22 zeolitesmake much diisopropylbenzene. ZSM-5 and ZSM-12 make less diisopropylbenzene than the othersolid acid catalysts but more n-propylbenzene. Betaand MCM-22 are probably the most selective amongthese zeolites.

If process conditions are carefully set,isomerization of the product cumene ton-propylbenzene can be minimized. The small poresize of MCM-22 catalyst suggests that with this zeolitecatalyst, cumene can only form on the externalsurface. Thus, there is no diffusion limitation.

The CDTECH technology is based on a catalyticdistillation column-reactor and operates as describedabove for ethylbenzene.

Mobil-Raytheon, EniChem and UOPindependently started up separate industrialzeolite-based cumene plants in 1996. Propylene purityaffects catalyst lifetime, which in Enichem’s plant isover four years.

Cumene yields are above 99.5% in allzeolite-based processes. Product purity is as high as99.95%. Cumene selectivity in the Mobil/Badgercumene process may even reach 99.92-99.97%. Then-propylbenzene level is between 200-250 ppm. After1996 the capacity of the plant that started operation inPasadena, Texas; was about 0.7 million t/yr.



Cymenes (methylisopropylbenzenes)m-cymene and p-cymene are intermediates for the

production of m- and p-cresols. Oxidation of thecymenes followed by acid cleavage yields the cresols.It is difficult to oxidize o-cymene. In addition,o-cymene also inhibits the oxidation of the other twoisomers. Therefore, o-cymene formation should bekept as low as possible.

n-propyltoluene is an unwanted byproduct.Cejka and Wichterlova elucidated the mechanismof n-propyltoluene formation. In a bimolecularreaction, the primary products, cymene andtoluene, convert to n-propyltoluene (Cejka andWichterlova, 2002).

The demand for cymene is much lower than thatfor cumene. Nevertheless, several commercial unitsare operating now at capacities of around 40,000 t�yr.Cymene is commercially produced by alkylatingtoluene with propylene. The alkylation produces amixture of o-, m-, and p-cymenes.

Toluene isopropylation is an electrophilicsubstitution in which the methyl group directssubstitution to the ortho and para positions. Sterichindrance of the methyl group favours the paraposition over the ortho. Secondary isomerization overthe acidic alkylation catalyst converts the primaryproducts to the thermodynamically more stablem-cymene. Relative alkylation-isomerization ratesdetermine final isomer distributions.

Production technologies are based either on AlCl3-HCl or phosphoric acid. In the liquid-phaseAlCl3-HCl process, the isomer ratio is close to thethermodynamic equilibrium of 3% o-, 64% m-, and33% p-cymenes. Oxidation to cymene converts most

of the m- and p-cymenes to the correspondinghydroperoxides, but leaves much of the o-isomerunconverted. After oxidation all unconverted cymenes,now rich in o-cymene, are recycled in the alkylationreactor. Here the o-cymene is isomerized to the othertwo isomers.

The phosphoric acid process makes much moreo-cymene (40%) than the AlCl3 process. A separateunit (Cymex), based on a 13X molecular sieve,separates the m- and p-cymenes. These aresubsequently oxidized to the corresponding purecresols. The o-isomer is isomerized.

At 250°C, ZSM-5 is more p-selective thanmordenite, Y, and other large-pore zeolites. Theproduct made with ZSM-5 contains 97.2%p-cymene, versus 46.0% and 37.6% made overmordenite and Y, respectively. Unfortunately, ZSM-5also makes almost 10% n-propyltoluene, whilemordenite and Y produce none, only cymenes. Morerecent results suggest that, depending on the desiredisomer composition, beta zeolite or mesoporousalumina may be the best solid acid catalyst forcymene production.

Linear alkylbenzenesIn surfactant production, linear alkylbenzene

technology has almost completely replaced olderbranched alkylbenzene technologies because linearalkylbenzene sulphonates are biodegradable. Thetechnology of choice today is dehydrogenation ofn-paraffins to n-olefins, followed by benzenealkylation, to produce the linear alkylbenzenes. Solid acid catalyst systems are slowly replacinghydrofluoric acid units, not only to ensure



Monsanto-Mobil-

Lummus-Mobil-

CDTECH EBMobil- Lummus-

LummusBadger

Unocal-Badger

1994Raytheon UOP

1975(2nd gen.)

UOP 1989(3rd gen.) EBMax EBOne

1980 1990 1995 1996

ALKYLATION

Temperature, °C 160 390-440 240-270 390-440 N.A. N.A. N.A.

Catalyst AlCl3 ZSM-5 Y ZSM-5 Y MCM-22 EBZ-500

Phase Liquid Vapour Liquid Vapour N.A. Liquid Liquid

Feed ratio 2.5 7.6 7.2 7.6 N.A. 4 4-6

Life, yr N.A. 0.25 1 1 5 3 2

Yeld, % 99.7 98.1 98.2 99.2 99.7 99.5 99.6

TRANSALKYLATION

Catalyst No transalkylation No transalkylation Y ZSM-5 Y ZSM-5 EBZ-100

Phase unit unit Liquid Vapour Liquid Vapour Liquid

Table 2. Ethylbenzene processes

environmental safety but also to improve economics.Materials evaluated as solid acid catalysts includezeolites, clays, various metal oxides, and supportedaluminium chloride. At present, only UOP’s Detalprocess uses a solid acid catalyst. Because of ongoingfundamental studies on reaction mechanism andcatalyst properties, significant progress is being madeto improve the selectivity, catalytic stability, and long-term stability of the above solid acids undercommercial operating conditions.

Chemicals from aromatic hydrocarbons

Bisphenols Bisphenols are intermediates for resol and novolac

resins, components of glues, paints, and mouldingmaterials. Bisphenols are made by the acidic or, lessfrequently, basic condensation of 1 mol ketone(or aldehyde) with 2 mol phenols. Aldehydes, andparticularly formaldehyde, react with phenols andmonosubstituted phenols under acid catalysis toproduce a mixture of isomers and polynuclearbisphenols (Fiege et al., 1991; Hesse, 1991; de Angeliset al., 2005).

The petrochemical industry used strong mineralacids (HCl, H2SO4, H3PO4) to catalyse suchhydroxyalkylation processes. These acids are highlycorrosive, dangerous to handle, and the reactors andcontainers have to be constructed from specialexpensive steels. In addition, these acids have to beneutralized before disposal. Aromatics, usually presentin the byproduct salts, create additional problems.Here, too, environmentally-friendly solid acid catalystswould alleviate most problems. Furthermore, just as inmany other processes already described, solid acidcatalysts can be regenerated and reused. They also areusually more selective than their mineral acidcounterparts.

The most important bisphenol is bisphenol A (or BPA) ( p,p�-isopropylidenediphenol). The worldcapacity of bisphenol A in 2001 was 2.67 millionmetric t. General Electric (25% of world-installedcapacity), Bayer (16%), Shell (12%), and DowChemical Corporation (10%) are the four main BPAproducers. The average annual growth rate is between6.5 and 7%. Bisphenol A is mostly used for theproduction of polycarbonates (65% of the BPAproduced) and epoxy resins (about 25%). Theremaining 10% is used to produce some finechemicals.

BPA is made by condensing 2 mol phenol with1 mol acetone. The two most important bisphenol Aprocesses use either HCl or ion-exchange resins(mainly sulphonic resins; SRI Consulting, 1972,1988).

In the industrial process, the bisphenol A producedalways contains a few percent of trisphenols, some(usually up to 1% of the amount of BPA) of chromans,and significant quantities of o,p�-bisphenol Aimpurities. In the resin-based process o,p�-bisphenol Amay approach one-third of the total product.

The temperatures of the HCl-based and theresin-based reactors are 40-50 and 90°C, respectively.The processes use six units: a) a reactor for thecondensation of phenol and acetone; b) a unit for thecrystallization of the bisphenol-bisphenol-phenoladduct; c) the stripping tower, where the adduct iscracked and phenol is recycled; d) the recrystallizationunit; e) a cracker for o,p�-bisphenol A; f ) wastewatertreatment facility (ChemSystems, 1998).

The total cost of the HCl-catalysed plant is muchhigher than the resin-based one because the formerrequires Monel alloy heat exchangers and glass-linedreactors whereas the reactor for the heterogeneous,resin-based process may be made from ordinary steel.A 100,000 metric t/yr bisphenol A plant, usingion-exchange-resin catalyst, requires a total projectinvestment of about $101,000,000. A similar plant,using HCl, costs about $244,000,000 (ChemSystems,2003). Furthermore, the resin-based process generatesmuch less wastewater than the HCl-based one.However, in the HCl-based process the concentrationof o,p�-BPA byproduct is so small that it does notrequire a cracker. The HCl-based process converts allthe acetone. The ion-exchange-resin processes (exceptthe Sinopec-Lummus process) reach only 90-97%acetone conversion.

Mobil Oil Corporation’s more than 30-year-oldpatent uses X and Y zeolites to synthesize BPA(Hamilton and Venuto, 1970). Conversions claimedwere only between 1% and 6%. The process was neverimplemented. Non-zeolitic molecular sieves, such asSAPO, ELAPSO, AlPO, MeAPO, FeAPO, and TAPOalso gave poor results.

Texaco’s hydrogen fluoride- orfluorosulphonic-treated montmorillonite clay gavemuch higher acetone conversions (80%) and BPAselectivities (56%) (Knifton, 1993). Leaching of theliquid acid could be a problem in this process.

Heteropolyacids, i.e. the salts of Cs2HPW12O40 or(NH4)2HPW12O40, encapsulated into mesoporousmolecular sieves, such as MCM-41, are also activeBPA synthesis catalysts. Regeneration of suchcatalysts is problematic (Rocchiccioli-Deltcheff et al.,1996).

Zeolite beta gave complete acetone conversion and49% BPA selectivity. This zeolite also produced 27%byproduct o,p�-BPA, but no chromans.

All of these zeolite catalysts can be rejuvenatedwith phenol or similar compounds at about 280°C.



Other bisphenols produced and used industriallyare bisphenol F, bisphenol Z, and4,4�-bis(4-hydroxyphenyl)pentanoic acid.

Bisphenol F, bis(4-hydroxyphenyl)-methane, is aprecursor of liquid bisphenol F resins. In the years2000-2001 about 3,000 metric t/yr was produced,most of it in Japan. The catalyst is HCl. However,ZSM-5 and related zeolites can produce bisphenol Fwith higher selectivity and complete formaldehydeconversion. The catalyst in a more recent potentialprocess is beta zeolite.

Bisphenol Z is the monomer of polycarbonatefilms. Worldwide production is about 1,360 metrict/yr. Bisphenol Z is made by condensing 1 molcyclohexanone with 2 mol phenols.

4,4�-bis(4-hydroxyphenyl)pentanoic acid isproduced by condensing 1 mol levulinic acid(4-oxopentanoic acid) with 2 mol phenol. It is used tolink polyester resins with phenolic resins.

Methylendianiline (diaminodiphenylmethane)Methylendianiline (MDA)-derived polyurethanes

are used as elastic foams (mattresses, cushions, andcar seats), rigid foams (insulation materials), rigid andflexible moldings with compact skins (window frames,housings, skis, etc.), and engineering moldings withhigh hardness and elasticity, elastomers, coatings,adhesives, binders, etc. The most common isocyanatemonomers of polyurethanes are toluendiisocyanate(TDI) and methylene diphenyl diisocyanate (MDI).Condensation of these monomers with polyols, formedby condensation of alkenyl oxide (e.g. propyleneoxide), and glycols or other polyalcohols forms thepolyurethanes. In 2001, the world capacity was 1.5million t of TDI and close to 3 million t of MDI. Themajor producers are the United States and WesternEurope.

MDA, a product of the condensation of 2 mol ofaniline with 1 mol of formaldehyde, is a precursor,through phosgenation, of methylene diisocianate. Thecatalyst is HCl. The formaldehyde and stoichiometricamounts of HCl and aniline are mixed at 60-80°C,then heated to 100-160°C for about 1 h to completethe condensation. At the end of the reaction, thereaction mixture is neutralized with excess NaOH.This produces almost the stoichiometric amount ofNaCl. The reaction mixture separates into organic andaqueous phases. The aqueous phase contains the NaClmade in the neutralization step, and traces of aromaticamines. These contaminants have to be removedbefore discharging the salt water. o,p�- and o,o�-MDA(3-5%), and oligomeric isocianates (20-25%) are thebyproducts. The amount of the o,p�-MDA�o,o�-MDAimpurity rises with the increasing reactiontemperature. The main unwanted (and very noxious)

product in MDA synthesis is N-methylated MDA (De Angelis et al., 2004).

As in all previous systems, substituting HCl with asolid acid catalyst would have important advantages:regenerability, lack of brines always contaminated witharomatic amines, etc. Some of the solid acid catalyststested were intermetallic compounds, ion-exchangeresins, silica-alumina, clays, and zeolites.

Texaco investigated the intermetallic compoundsmolybdenum aluminide (Mo3Al), tungsten borides(WB, W2B, WB5, W2B5), tungsten sulphide (WS2) andtungsten silicide (W5Si3; Marquis et al., 1981a,1981b). Although very stable, all of these compoundsare very weak acids and therefore their activities aretoo low for serious consideration.

Styrene-divinylbenzene containing sulphonicgroups, and tetrafluoroethylene-perfluorovinyl ethercontaining sulphonic groups (Nafion) were theion-exchange resins tested for this process (Mergerand Nestler, 1982; Saischek et al., 1982; Nafziger etal., 1985). When water was removed throughdistillation from the system, formaldehyde conversionwas complete at 100-110°C. Although MDAselectivity was high, MDA productivities were too lowfor industrial application (Saischek et al., 1983;Nafziger et al., 1985).

Clays and silica-aluminas gave poor results alsobecause they have low acidity. Acceptable conversionswould require higher operating temperatures. Productselectivities would be low under those conditions(Perego et al., 2002).

Better results were obtained with zeolites becausesthey possess higher acidity. In addition, shapeselectivity may increase the yield of the 4,4-MDA(para) isomer with respect to the o-2,2�-MDA andortho-para (2,4�-MDA) isomers. Thus, Y zeolitepretreated with a fluorinated agent or exchanged withvarious metal ions, zeolite beta (large-pore zeolite with12 ring openings) and ERB-1 (medium-pore zeolitewith 10 ring channel systems and 12 ring pockets onthe [001] crystal face) were promising candidates. Inthis case, aminal (the product of the neutralcondensation of aniline with formaldehyde)conversion was almost complete and 4,4�-MDAselectivity was 89%. However, the concentration ofpartially rearranged intermediate in the reactionmixture remained too high (2-3%). In addition, thewater content had to be kept low. ZMS-5 andisomorphously Ti-, B-, or Fe-substituted silicalites arealso active MDA catalysts. The zeolites here catalysethe rearrangement of the preformed aminal. Batch andfixed-bed reactors performed in a similar way.Octamethylcyclotetrasiloxane (OMTS) treatment ofthe zeolite surface significantly decreased theo,p�-MDA byproduct without lowering aminal



conversion or catalyst life. Such treatment usuallyeliminates nonselective acid sites on the externalsurface and narrows pore mouth. These changediffusion. Treatments with phosporous and boroncompounds may also modify zeolite shape selectivity.The advantages of boric acid or phosphoric acidtreated zeolites are: the low price of the reagents, andthat the catalysts produced can operate in the presenceof water. They also give better shape selectivity thanthe other zeolites. However, the reaction in all of thesezeolites is diffusion-controlled, and therefore only afraction of the acid sites is accessible to the aminal.

Eni scientists compared Nu-88 zeolite with betazeolite (the structure of Nu-88 is yet unknown). Nu-88gave better 4,4�-MDA selectivity than beta zeolite (73%vs. 58.5%). 4,4�-MDA productivities were 176 g and260 g 4,4�-MDA/g catalyst with Nu-88 and beta,respectively. Nu-88 may be rejuvenated with aniline at200°C. Nu-88 also catalyses the production ofbisphenol A (De Angelis et al., 2005). Beta with smallercrystallite size was more active than larger crystallitesize beta. This suggested that even with the large-porebeta zeolite (pore diameter 0.67 nm), the process iscontrolled by diffusion. The reaction wasdiffusion-controlled even in the small-crystal beta zeolite.

Delamination of zeolites can substantially increasethe accessibility of acid sites without eliminating ordecreasing their acidity. In addition, water resistancemay also improve. For example, the laminar precursorsof the MWW structure ERB-1, MCM-22, SSZ-25,ferrierite, or Nu-614 can be expanded anddelaminated. This produces structured single layerswith large external surface areas (4,600 m2/g) and verylittle, if any, microporous surface area. The acid sitesin the resulting materials (ITQ-2, ITQ-6 and ITQ-18respectively) are accessible through the externalsurface (Corma et al., 2000, 2001). As a consequence,these delaminated catalysts are more active than thecorresponding zeolites. On the other hand, very littleN-methylated and quinazoline products are formed.Furthermore, the yield of trimers plus tetramers isbelow 30%. It is remarkable that the 4,4�-MDA(para)�[2,2�-MDA (ortho)�2,4�-MDA (ortho-para)]isomer ratio depends on the external surface structureof the delaminated zeolite. In ITQ-18 this ratio is closeto 4. The delaminated zeolites also deactivate moreslowly than large-pore zeolites, probably becausediffusion and desorption are faster. These materialscan tolerate at least 5% water, almost twice as much asbeta. ITQ-2 has a long catalyst life. This is attributedto improved diffusion and faster product desorption.The ITQ-2 catalyst produces fewer N-methylatedbyproducts (Corma et al., 2004).

Delaminated zeolite catalysts could replace HCl inMDA production and in most other ketone and

aldehyde condensations because they are very activeand they have long catalyst life. Nevertheless, solidacid catalysts have not gone much beyond laboratorystudies in this field. Only the bisphenol A process usesa solid catalyst (sulphonic resins). Zeolites, especiallydelaminated zeolites, are promising HCl alternatives.

e-caprolattame In 2002, 52% of the 33 million metric t of benzene

produced in the world were used to makeethylbenzene. Cumene production consumed 19% ofbenzene production and 13% of cyclohexaneproduction. These two are the main raw materials forthe production of nylon (Bellussi, 2004).

One of several ways to produce nylon starts withbenzene and propylene. The first step is alkylation toproduce cumene. The cumene is then oxidized tocumylhydroperoxide, which is then decomposed inthe presence of sulphuric acid to phenol andacetone. The phenol is then reduced to cyclohexanoland cyclohexanone. The latter is converted withammonia in the presence of sulphuric acid tocyclohexanone oxime. Finally, the cyclohexanoneoxime is converted, via Beckmann rearrangement,into e-caprolattame. e-caprolattame is the monomerof Nylon 6. This process generates 1.3 kg wasteammonium sulphate per kg of e-caprolattameproduced.

Table 3 shows yearly caprolactam production inEast Asia.

Another important technology starts with thereduction of benzene to cyclohexane. The latter isoxidized to cyclohexanone and cyclohexanol.Cyclohexanol is then oxidized with nitric acid toadipic acid. Half of the adipic acid is then converted to1,6-hexamethylenediamine. The latter is used togetherwith adipic acid to produce Nylon 6,6.

New innovations completely avoid producing theammonium sulphate byproduct in the production ofe-caprolattame by modifying the ammoximation andthe vapour phase Beckmann rearrangement steps.Titanium silicalite (TS-1) is a molecular sieve withpentasil structure. In the presence of ammonia andhydrogen peroxide TS-1 can effectively catalyse theammoximation reaction. The reaction is performed inter-butanol solvent in very mild conditions. Thereaction temperature is 80-95°C and theH2O2/cyclohexanone molar ratio is between 0.8 and1.0. The selectivity based on the oxime is 96-100%and that based on the hydrogen peroxide is 89-95%.There are no SOx and NOx emissions. The onlybyproduct is water. Enichem has built a 12,000 t/yrplant in Italy (Taramasso et al., 1983; Petrini, 1993). In2003, Sumitomo built another plant in Japan using thesame technology.



Although better known for its unique oxidationselectivity, TS-1 and other similar titanium silicalitepreparations, also catalyse olefin epoxidation. Whetherany acid catalysis is involved here is debatable.

Recently, Thomas and Raja designed a newe-caprolattame catalyst. Instead of hydrogen peroxide,the process uses air. The new catalyst is a nanoporousaluminophoshate containing cobalt, silicon,magnesium, and zinc promoters. The air oxidizes theammonia, in situ, over the cobalt sites tohydroxylamine. When these laboratory results arereproduced on an industrial scale, any process basedon this catalyst will be a great improvement overexisting technologies (Halford, 2005).

The catalyst of the traditional Beckmannrearrangement is concentrated sulphuric acid. After rearrangement, the sulphuric acid is neutralizedwith ammonia. This makes about 1.3 kg ammoniumsulphate per kg e-caprolactam. ZSM-5, SAPO-11, andsome other mildly acidic zeolites can also catalyse therearrangement. The feed contains only cyclohexanoneoxime, methanol and water (methanol increasesselectivity from 80% to 95%). The catalytic sites areextremely weak acidic silanol groups such as vicinalgroups or the defective sites known as ‘silanol nests’(Ichihashi, 2001). In 2003 Sumitomo started-up thefirst industrial plant to make e-caprolattame usingsilicalite catalyst.

11.3.3 Conclusions and challenges

During the last three decades, research scientists havedeveloped a number of solid acids in which the aciditycan go from mild to superacid levels. In addition, wecan also control the catalyst pore size. Thus, it ispossible to tune the acidity and select the best poresize to maximize selectivities and minimize byproductformation and coking, and thus optimize a process.One may select from more than ten dozen syntheticzeolites i.e. the one with the pore size and otherstructural parameters best suited for a reaction.However, large-scale production of many of the newlydiscovered high-silica zeolites has some obstacles.The industry at the present time uses only eleven of

the 125 new zeolite structures (covered by over 23,000patents) discovered between 1950 and 2000. Theproblem is high production cost. This is especiallytrue for the otherwise very attractive high-silicazeolites, metallophosphates, metalloorganicframeworks, and silicogermanates. Much of thisexpense is the cost of the synthesis of thestructure-directing agents, i.e. the guestorgano-cations and amines. Production costs have tobe decreased to benefit from these new high-silicazeolites. Below are listed possible ways to decreasesynthesis costs (Zones et al., 2005): • Borosilicate structures are frequently easier to

prepare than their Al-counterparts. By preparingthe appropriate borosilicate structures andsubsequently removing the gueststructure-directing agent, and exchanging B for Al,synthesis costs could be significantly reduced.

• The fluoride route could be less expensive than theusual basic methods to prepare some highlymicroporous, low framework densityall-silicaceous zeolites.

• Substitution of very complex, and therefore veryexpensive organic surface directing agents withless expensive and non-selective amines wouldgreatly reduce costs.

• Using recyclable surface directing agents wouldalso reduce costs (Zones et al., 2005). Some of the other challenges the industry will face

during the next decades are diminishing raw materialsupplies, changing feedstocks and increasingly morestringent governmental regulations. In addition, theindustry needs to convert more plants to use moreenvironmentally-friendly processes, reducewaste-products and at the same time improve processeconomics.

Although zeolites have strong acidities, can beshape-selective, and are quite stable, in many reactionsmass transfer limitations restrict catalysis to theoutermost layers of the zeolite crystals. This isespecially true with bulkier reactants. Mesoporousmolecular sieves, in addition to micropores, also havelarger (i.e. 2-30 nm) pores. They also have largesurface areas, up to 1,000 m2�g. These stablemesoporous or layered or pillared zeolites benefit



1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004Change, %2003-2004

Japan 519 545 555 556 519 581 599 531 508 530 503 –5.1

Taiwan 104 108 104 114 123 119 171 184 186 216 216 0

China 100 120 109 164 152 170 201 228 13.4

tab. 3. Yearly caprolactam production in East Asia (thousands of metric tons) (Production [...] 2005)

from the strong acidity and shape selectivity ofzeolites, and at the same time provide access to thecrystals’ inner parts by decreasing or eliminating masstransfer limitations. Reduced diffusion in themesopores makes active acid or basic sites much moreaccessible than they are in present-day industrialzeolite catalysts.

These days, much research is directed towardsdeveloping and optimizing mesoporous catalystsbecause it is obvious that stable mesoporous materialswould help us to face many of our future challenges.In addition to the production of petrochemicals, thesematerials may also find use as catalysts in petroleumrefining as well as in fine chemical andpharmaceutical processes.

One may create mesopores either during or afterthe synthesis of a zeolite. Some new materials,developed in the last two or three decades, in whichthe mesopores are created after zeolite synthesis are:• Delaminated and pillared structures. Their more

open structures (e.g. ITQ-2, MCM-22) could bevery important because diffusion occurs much lessin these than in crystalline zeolites, yet they retainthe zeolites’ acidity and shape selectivity.

• Variations of known zeolites (e.g. ITQ-17 and thepolymorph C of beta zeolite) synthesized withinorganic structure-directing agents (e.g. GeO2).

• Mesoporous non-siliceous materials, such asmesoporous alumina. These sieves may besynthesized with either anions (carboxylic acids,where pore diameters are related to the chain lengthof the carboxylic acid) or with neutral structure-directing agents such as polyethylene oxide.

• Micro/meso composite structures.Some recent approaches to create mesopores

during zeolite synthesis are:• Carbon nanotubes were combined before synthesis

with the structure-directing agent, NaOH, and theAl and Si reactants. The carbon components wereremoved by controlled burning at 550°C. Catalystsmade with carbon nanotubes had 80% porosity.The mesopores ranged from 30 to 130 nm. Thecatalyst was very active for benzene ethylation,alkane isomerization, and xylene isomerization(Schmidt et al., 2005).

• Partial destruction of mordenite with aqueousNaOH, followed by recrystallization, gavecrystalline mordenite with uniform (3 nmdiameter) mesopores. Treatment with 0.4 M NaOHsolution increased the acidity; treatment withstronger NaOH solutions decreased it. Themodified material had significantly higherbiphenyl-diisopropylbiphenyl transalkylationactivity than the parent mordenite (Ivanova et al.,2005).

• Mesoporous biological materials (i.e. wood, plants,natural sponges, diatoms, etc.) may serve assacrificial temporary templates to prepareMFI-type catalysts containing both micro- andmesopores. The biotemplates recommended areinexpensive, abundant, environmentally benign,and renewable. Before synthesis, the surface of thesacrificial templates is covered with a uniformlayer of 200-400 nm Silicalite-1 zeolite nanoseeds.The nano-crystals are grown into continuousSilicalite-1 or ZSM-5 films by hydrothermaltreatment. Thermal decomposition of thetemporary biological support yields self-supportingpure zeolite structures. These closely mimic themicro- and macro-structure of the biotemplate(Zampieri et al., 2005).Other interesting materials are zeolites with extra

large (i.e. larger than 12-member) rings. It is too early to comment on the practical

applications of these materials as industrial catalysts.

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Sigmund M. CsicseryScientific Consultant

The author thanks Giuseppe Bellussi and Carlo Perego of EniTecnologie



11.3 acid or base catalysed processes - treccani · 2018. 6. 30. · its surface area is 0.02 m2/g....

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