the role of catalysis for the sustainable production of bio-fuels and bio-chemicals || the role of...

C H A P T E R

7

The Role of Catalytic Pretreatmentin Biomass Valorization Toward

Fuels and ChemicalsChristos K. Nitsos*, Chrysa M. Mihailof†, Konstantinos A.Matis*, Angelos A. Lappas†, Kostas S. Triantafyllidis*

*Department of Chemistry, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece†Chemical Process and Energy Resources Institute, Centre for Research and Technology Hellas,

P.O. Box 60361, Thermi, 570 01 Thessaloniki, Greece

O U T L I N E

7.1 Introduction 217

7.2 Pretreatment with Acid Catalysts 2247.2.1 Inorganic Acids 2247.2.2 Organic Acids 228

7.3 Pretreatment with Basic Catalysts 2297.3.1 Inorganic Bases 2297.3.2 Organic Bases 232

7.4 Self-catalyzed Pretreatment 233

7.5 Combining Chemical Catalysiswith Physical Methods 238

7.6 Oxidation Catalysts 243

7.7 Solid Acid Catalysts 248

7.8 Ionic Liquids 249

7.9 Summary and Outlook 252

7.1 INTRODUCTION

Production and use of energy is the foundation of modern civilization. Fossil fuels (coal andpetroleum-derived fuels such as diesel, gasoline, and kerosene) have been the main source ofenergy in the form of electrical and thermal power or transportation fuels throughout the twen-tieth century. However, the unstable petroleum market, as well as concerns regarding the

217The Role of Catalysis for the Sustainable Production

of Bio-fuels and Bio-chemicals # 2013 Elsevier B.V. All rights reserved.

future availability of fossil fuels, has spurred research on alternative sources of energy that areabundant in nature or can be produced locally so as to reduce the dependence of national econ-omies on imported petroleum oil. In addition, the necessity for renewable and more environ-mentally friendly energy sources has been accentuated by the need to reduce the carbondioxide emissions fromburning of fossil fuels, as the CO2 produced by anthropogenic activitieshas been considered as a major contributor to the “greenhouse” and “global warming” effect.

Lignocellulosic biomass, especially in the form of residues/wastes from forestry andagricultural activities and related industries, represents themost promising renewable sourcefor the production of the so-called second-generation biofuels [1]. The first-generationbiofuels, i.e., biodiesel and bioethanol, originate from edible raw materials (i.e., sunfloweroil and corn, respectively), and they have therefore been accused of contributing to theincrease in food prices and to the alteration of biodiversity. Lignocellulosic biomass is renew-able and can be found in large quantities in nature (trees, grasses) and as agriculturalby-products (straws, corn stover, trimmings, etc.). It is produced during photosynthesis,where carbon found in the atmosphere in the form of carbon dioxide is fixed in the livingplants in the form of the various biomass constituents. The carbon dioxide released fromthe burning of biomass-derived fuels, therefore, makes a smaller contribution to the carboncycle compared to fossil fuels. Biofuels are therefore considered as “carbon neutral.”

Lignocellulosic biomass consists mainly of three natural polymers: cellulose, hemicellulose,and lignin. Cellulose—themost abundant organic substance in nature—is a b-1,4-glucose poly-merwith cellobiose, a b-1,4-glucose disaccharide as the repeatingunit, and typical chain lengthsbetween 500 and 14,000 glucose molecules. These cellulose chains are organized into microfi-brilswith a crystalline structure, held together by hydrogen bonds.Hemicellulose is a branchedheteropolysaccharide, i.e., it consists of many types of C5 sugars (xylose, arabinose), C6 sugars(galactose, mannose, glucose), and sugar acids (glucuronic and galacturonic acids). Dependingon the plant species, hemicelluloses can be xylans (hardwoods, straw, and grasses), mannansand glucomannans (softwoods), xyloglucans, etc. Hemicellulose is connected to both the cellu-losemicrofibrils and the ligninmatrix. Lignin is a phenylpropanepolymer that encases celluloseandhemicellulose and enhances themicrofibrils’ structural rigidity andhydrophobicity, aswellas their resistance to enzymatic hydrolysis by microorganisms (Figure 7-1) [2]. The fraction ofeach of these major biomass components varies according to the type of biomass; it also varies,for the same type of biomass, with climate, as well as seasonal and geographical parameters.Typical compositions for various lignocellulosic biomass feedstocks that have been reportedin the literature are given in Table 7-1.

The existing technologies for the production of biofuels from lignocellulosic biomass can bedivided into thermochemical and biochemical conversion routes [3,4]. The thermochemicalroute includes two main processes: the biomass-to-liquid (BtL) process and the biomass fastpyrolysis (BFP). TheBtLprocess comprises the gasificationof biomass for the production of syn-gas (CO andH2), which can be used to produce liquid fuels, such as “green” diesel and “green”gasoline, through classical Fischer-Tropsch (F-T) synthesis [5] with (orwithout) subsequent cat-alytic upgrading of the primary products, i.e., hydrocracking of highly paraffinic F-T waxes togasoline and diesel [6]. The fast pyrolysis of biomass [7] leads to the production of pyrolysis oil(bio-oil), which contains mainly oxygenated organic compounds, such as carboxylic acids,ketones, and phenols, and can be catalytically upgraded (downstream or in situ during biomasspyrolysis by the use of appropriate catalysts [8]) into fuel precursors (mainly aromatics).

218 7. THE ROLE OF CATALYTIC PRETREATMENT IN BIOMASS VALORIZATION

10-20 nm

Lignin

G

G

G

G

G

G G

S

S

S

OOOH

OOHOH

OH OH OH

GH

p-Coumaryl alcohol Coniferyl alcohol Sinapyl alcohol

S

S

S

H

H H H

HH

Macrofibril

Macrofibril

Lignin

Cellwall

Plant cell

Plant

Hemicellulose

Pentose

Hexose

Crystallinecellulose

CellodextrinHydrogenbond

n-3

n-3

n-3

n-3

n-3Glucose

FIGURE 7-1 The main component of lignocellulose: cellulose, a beta(1-4)-linked chain of glucose molecules.Hydrogen bonds between different layers of the polysaccharides contribute to the resistance of crystalline celluloseto degradation. Hemicellulose, the second most abundant component of lignocellulose, is composed of various five-and six-carbon sugars such as arabinose, galactose, glucose, mannose, and xylose. Lignin is composed of three majorphenolic components, namely, p-coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S). Lignin is syn-thesized by polymerization of these components, and their ratio within the polymer varies between different plants,wood tissues, and cell-wall layers. Cellulose, hemicellulose, and lignin form structures called microfibrils, whichare organized into macrofibrils that mediate structural stability in the plant cell wall. Reproduced from Ref. [2] with

permission from Nature Publishing Group.

2197.1 INTRODUCTION

The biochemical conversion route includes the utilization of enzymes andmicroorganismsfor the conversion of cellulose and hemicellulose into C5 and C6 sugars and their fermentationinto ethanol (usually referred to as second-generation bioethanol). Although ethanol is themain targeted chemical investigated through the biochemical route, production of other fuelsand chemicals such as butanol [9], succinic acid [10–12], and lactic acid [13] through fermen-tation of lignocellulosic sugars, as well as of methane (biogas) via anaerobic digestion [14] andof biohydrogen via fermentation of sugars [15], is also possible. A typical integrated biochem-ical conversion process for the production of ethanol from lignocellulosic biomass is shown inFigure 7-2 [16]. It consists of four main process steps (but may include various other interme-diate steps): (i) a pretreatment step necessary for the opening of the microfibril structure, (ii)an enzymatic hydrolysis step for the conversion of cellulose to glucose, (iii) a fermentationstep for the conversion of glucose and pentoses to ethanol, and (iv) a distillation step forthe separation of ethanol from the fermentation medium. When steps (ii) and (iii) areperformed in tandem, the process is referred to as separate hydrolysis and fermentation(SHF) [17]. When they are performed in one step, the process is known as simultaneoussaccharification and fermentation (SSF) [17]—or simultaneous saccharification andcofermentation (SSCF) when glucose and pentoses are fermented simultaneously [18].Finally, it is referred to as consolidated bioprocessing (CBP) [19,20], where cellulase produc-tion, cellulose hydrolysis, and fermentation of derived sugars are carried out in a single stepand only one microbial community is employed for both the production of cellulases and thefermentation of sugars [16].

TABLE 7-1 Composition (wt% on Dry Biomass) of Typical Lignocellulosic Biomass Sources, IncludingAgricultural Residues, Dedicated Energy Crops, Hardwoods, and Softwoods, as Reported in the Literature

Biomass type

Cellulose

(wt%)

Hemicellulose

(wt%)

Lignin

(wt%) Reference

Agriculturalresidues

Wheat straw 31.0 24.2 25.0 34

Rice straw 31.1 22.3 13.3 43

Corn stover 40.0 29.6 23.0 207

Energy crops Switch grass 32.2 29.4 17.3 53

Alfalfa 27.5 24.1 17.5 53

Reed canarygrass

26.5 23.3 14.8 53

Sorghum bagasse 40.1 22.2 18.0 153

Hardwoods Salix 42.5 25.0 26.0 205

Eucalyptus 44.4 21.9 27.7 180

Poplar 39.2 20.8 26.2 159

Softwoods Lodgepole pine 45.4 22.6 45.4 208

Spruce 44.0 24.6 27.5 207

Douglas fir 47.3 19.5 30.3 208


Oneof themajorhurdles in theefficientproductionofbioethanol fromlignocellulosicbiomassis that the enzymatic digestibility of cellulose by cellulolytic enzymes is hinderedbyanumberoffactors including cellulose crystallinity and degree of polymerization, accessible surface area(porevolume), ligninbarrier,hemicellulosesheathing,andbiomassparticle size [21,22].Thecon-sequence is that high enzyme loadings are required for the efficient conversion of cellulose toglucose if the biomass is not previously treated. The goals of pretreatment are to remove oneormore of these barriers, i.e., removehemicellulose and/or lignin, reduce cellulose crystallinity,and increase biomass accessible surface area in order toproduce biomass fractions that aremorereadily converted to sugars at lower enzyme loadings (Figure 7-3). In addition, biomasspretreatment can affect the type of enzymatic activity required, the yield and type (monomericand oligomeric) of sugars derived from both hemicellulose and cellulose, the concentrationof fermentation inhibitors (i.e., acetic acid, furfural, formic acid) in the pretreatment processliquids, and the quantity/quality of the lignin-rich solids that are left at the end of the overallbiomass-to-ethanol conversion process [23].

Effluent

treatment

Ethanol

dehydration

Pentose

fermentation

Hexose

fermentation

Cellulose

hydorolysis

Detoxification

Pretreatment

Production of

cellulases

AnhydrousethanolConventional

distillation

CBP

CF

SSF SSCF

(EtOH+L) (EtOH)

(P+1)

(Cel)

(C+L) (P+I)

Liquid fractionSolid fraction

Biomass

(C+H+L)

Waste streams

(L)

(G)

FIGURE 7-2 Generic block diagram of fuel ethanol production from lignocellulosic biomass. Possibilities for re-action-reaction integration are shown inside the shaded boxes: CF, cofermentation; SSF, simultaneous saccharifica-tion and fermentation; SSCF, simultaneous saccharification and cofermentation; CBP, consolidated bioprocessing.Main stream components: C, cellulose; H, hemicellulose; L, lignin; Cel, cellulases; G, glucose; P, pentoses; I, inhibitors;EtOH, ethanol. Reproduced from Ref. [16] with permission from Elsevier.

2217.1 INTRODUCTION

In addition to the thermochemical (BtL and BFP) and biochemical (mainly biomass-to-ethanol) processes for the production of second-generation lignocellulosic fuels, otherroutes have been proposed recently, mainly via acid or bifunctional (metal-acid) catalyticconversion of the C5 and C6 sugars derived from hemicellulose and cellulose and of the phe-nolic molecules that originate from lignin. One example of the first case is the production offuel (i.e., gasoline range) alkenes by dehydration of xylose or glucose (via intermediate isom-erization to fructose) to furfural (and furfuryl alcohol via subsequent hydrogenation) andHMF,hydrolysis of HMF or alcoholysis of furfuryl alcohol to levulinic acid (LA) and its subsequentesterification to levulinateesters, andhydrogenationofLAor levulinate esters tog-valerolactonefollowed by decarboxylation to butenes and oligomerization to C8þ alkenes. A typical bifunc-tional catalytic pathway would be the hydrogenation (aided by the metal sites) of carbonylcompounds (i.e., furfural) to the corresponding alcohols, the dehydration of alcohols (on theacid sites) to alkenes, and the hydrogenation (aided by the metal site) of alkenes to alkanes.A good overview of these types of reactions for upgrading biomass carbohydrates to platformchemicals and fuel precursors can be found in Chapter 8 [24]. The pretreatment of biomass intheabove-mentionedupgradingprocesscanalsoplayanimportantrolewithrespect to the initialdestruction of the biomass structure andpartial or complete depolymerization of hemicellulose,cellulose, and lignin to their monomeric sugars or phenolics.

Among the desirable characteristics of pretreatment technologies is the ability to decreaseor avoid the use of toxic, hazardous, corrosive, and costly chemicals; minimization ofby-product formation, and reduction of the overall operation costs, in order to developenvironment friendly and sustainable processes [21,25]. Differences in the type of biomass(i.e., softwood, hardwood, agricultural) are associated with changes in the content of themajor structural components (i.e., hemicellulose, cellulose, lignin). In addition, variationsin the composition of hemicellulose and lignin (type of sugars and degree of acetylationof hemicellulose and type of phenylpropane units of lignin) occur among different typesof biomass. These differentiations have led to the investigation of various methods and tech-nologies for the efficient pretreatment of biomass. These include mechanical, physical, chem-ical, and biological, as well as combined physical-chemical-mechanical methods. In mostcases, however, chemical and/or biological catalysis (using liquid, solid, and gaseous

Cellulose Cellulose

HemicelluloseHemicellulose

Lignin

Pretreatment

Lignin

FIGURE 7-3 Main role of pretreatment for the enzyme-based bioethanol production process is the disruption ofthe recalcitrant lignocellulose structure, enabling easier access of enzymes to cellulose for a more effective hydrolysistoward fermentable sugars. Reproduced from Ref. [25] with permission from ACS Publications.


catalysts, as well as enzymes and microorganisms) is responsible for the major effectsachieved by pretreatment. This chapter focuses on the various catalytic pretreatmentmethodsthat have been reported in the literature and discusses the reaction mechanisms and the in-duced effects on the structural, compositional, and morphological characteristics of biomassas well as on the effectiveness of the overall process (i.e., production of bioethanol, high-value-added chemicals). Table 7-2 summarizes briefly the various pretreatment methods,catalyst types, and induced effects on biomass that are presented and analyzed later.

TABLE 7-2 Overview of the Most Important Pretreatment Methods, Catalyst Types Involved, and AssociatedEffects on Biomass and Derived Products

Pretreatment Catalyst type Effect

Dilute acid Inorganic: H2SO4, HCl, H3PO4, HNO3

Organic: acetic, lactic, maleic, fumaric, formic,oxalic

Hydrolysis of hemicellulose; partial removal/relocation of lignin; little or no hydrolysis ofcellulose; formation of degradation productssuchasfurfural,HMF, levulinicacid, formicacid

Alkaline Inorganic: NaOH, KOH, Ca(OH)2Organic: NH3, soaking in aqueous ammonia,ammonia recycle percolation

Disruption of lignin-hemicellulose bonds;removal of lignin; removal of acetyl/uronicacids from hemicellulose; reduction ofcellulose crystallinity

Hydrothermal/steam

CH3COOH (due to hydrolysis of hemicelluloseacetyl esters contained in the biomass, aceticacid is released and acts as a pretreatmentcatalyst reducing the medium pH)

Deacetylation of hemicellulose/release ofacetic acid; hydrolysis of hemicellulose; partialremoval/relocation of lignin; little or nohydrolysis of cellulose; disruption of fibers(steam explosion); formation of degradationproducts such as furfural, HMF, levulinic acid,formic acid

Combination ofphysical andchemical (acid)

H2SO4-catalyzed steam explosion Improved hemicellulose removal

SO2-catalyzed steam explosion Improved enzymatic hydrolysis of cellulose

Supercritical CO2 explosion Reduction of cellulose crystallinity

Sulfite and mechanical size reduction (SPORL) Hemicellulose solubilization, partialdelignification, chip size reduction

Combination ofphysical andchemical (base)

Ammonia fiber explosion (AFEX) Decrystallization of cellulose; alterations inlignin; deacetylation of hemicellulose; increaseof accessible surface area; increased enzymatichydrolysis at low enzyme loading

Oxidative Ozonation (O3) Selective removal of lignin

Peroxide (H2O2) Removal of lignin and hemicellulose

Wet oxidation (O2) Solubilization of hemicellulose and lignin

Peracetic acid Selective removal of lignin

Enzymes (laccases) Delignification by use of natural or addedmediators

Continued

2237.1 INTRODUCTION

7.2 PRETREATMENT WITH ACID CATALYSTS

7.2.1 Inorganic Acids

7.2.1.1 Sulfuric Acid

Sulfuric acid is among the most extensively studied acids for the pretreatment of lignocel-lulosic biomass, because it combines efficiency and low cost. It has been used in both concen-trated and dilute forms. The main advantage of using concentrated acids [26–28] is theirability to hydrolyze hemicellulose and cellulose at moderate temperatures while keepingthe formation of degradation products small, and the ethanol yields achieved (in the bio-mass-to-ethanol process) are high. Concentrated acid hydrolysis is a two-step process. Thefirst step is performed at atmospheric pressure and relatively low temperatures, and sulfuricacid concentrations of 70-80%. In this stage, hemicellulose is solubilized and cellulosedecrystallization takes place by the breakage of hydrogen bonds in the microfibrils. In thesecond step, the hydrolysis of hemicellulose and cellulose takes place with diluted acid,ca. 20-30% [29]. Despite their good performance, processes that use concentrated acid entailserious disadvantages, which include handling of the corrosive and toxic concentrated acids,the need to recycle the acids in order to reduce the operational cost [30], and neutralization ofthe process liquid stream (usually performed with Ca(OH)2 treatment, termed overliming), aswell as the high capital investment in the corrosion-resistant equipment required. In an effortto improve the fermentation of sugars produced by the concentrated sulfuric acid process, thechromatographic separation of sugars in concentrated sulfuric acid hydrolysates has beenproposed [29,31,32].

Dilute sulfuric acid pretreatment remains among the most promising available technologiesfor the commercialization of ethanol production from lignocellulosic feedstocks. The main rea-son is that it combines an effective pretreatment with low cost. It has been used as a catalyst forthe pretreatment of a variety of feedstocks that encompass the full range of available sources oflignocellulosic biomass, including agricultural by-products, such as wheat straw [33,34], corncobs [35,36], corn fiber [10,37], corn stover [38,39], sugarcane bagasse [40,41], rice straw [42,43],

TABLE 7-2 Overview of the Most Important Pretreatment Methods, Catalyst Types Involved, and AssociatedEffects on Biomass and Derived Products—Cont’d

Pretreatment Catalyst type Effect

Solid (acid)catalyst

Zeolites, mesoporous materials, active carbons Hydrolysis of cellulose and hemicellulose tosugars; conversion of sugars into platformchemicals/fuel precursors such as furans(furfural, HMF)

Ionic liquids No additional catalyst Dissolution of biomass; separation ofhemicellulose, cellulose, lignin

Inorganic acids, zeolites, resins, metalchlorides

Depolymerization of hemicellulose, celluloseand lignin; conversion of biopolymers tooligomers, monomeric, sugars, furans,phenols, etc.


cassava stems [44], olive cake [45], olive pruning [46,47], and oil palm empty fruit bunch [48];hardwoods such as southern red oak [49], poplar [38,50], and yellow poplar [51]; softwoodssuch as douglas-fir [49], silver wattle [52], spruce [11], and mixed softwood chips [39]; and en-ergy crops such as alfalfa stems [53], reed canarygrass [53], switchgrass [38,53,54], and sorghumstraw [55]; as well as microalgal biomass [56]. Dilute sulfuric acid pretreatment is usuallyperformed at concentrations of <4 wt%, with concentrations as high as 6% [54] and as lowas 0.05% [52] having been reported. A wide temperature range from 90 �C [10] to 250 �C[49], with pretreatment times ranging from a few minutes [44] up to several hours [49], havebeen employed. The effect of these three parameters (temperature, time, and pH) is incorpo-rated into one combined severity factor [57] given by the equation

CSF ¼ log t expT � 100

14:75

� �� pH

which allows the direct comparison of results obtained at different time (t), temperature (T),and acid concentration (pH) combinations. Other important factors that affect the efficiencyof the pretreatment process include the biomass particle size, which affects diffusion of theliquid catalyst and of products out of the particle [58], and the solid loading, which affectsthe viscosity of the biomass/water mixture, concentration of the produced sugars, as wellas the formation and concentration of inhibitors in the process liquids [59].

The main effect of the dilute sulfuric acid pretreatment—and dilute acid pretreatment pro-cesses in general—is the removal of hemicellulose from biomass via acid-catalyzed hydroly-sis. This results in the production of hemicellulosic sugar—mainly xylose, since xylan is thepredominant hemicellulose constituent—oligomers and monomers in the process liquid.Xylose can then be catalytically dehydrated into furfural or catalytically converted to otherproducts. Cellulose remains mostly unaffected by the pretreatment, while lignin can undergosolubilization and recondensation reactions. These can lead to the formation of lignin dropletson the biomass surface with a subsequent adverse effect on the enzymatic hydrolysis [60].Usually, however, the remaining lignin- and cellulose-rich solid can be more efficiently hy-drolyzed into glucose with the use of cellulolytic enzymes. Twomodels have been adopted topredict the hydrolysis of hemicellulose under acidic conditions [61]. According to the Seamanmodel [62–64], hemicellulose is hydrolyzed to xylose, which in turn is converted to degrada-tion products by a subsequent reaction (Figure 7-4A). The biphasic model [38,48,65] acceptsthe existence of two types of hemicellulose (xylan), one that is easy to hydrolyze and one dif-ficult (Figure 7-4B). It was adopted to explain the observed drop in hemicellulose hydrolysisafter conversion of around 70% has been achieved [66]. The remaining difficult-to-removexylan fraction has been attributed to the parts of hemicellulose that have chemical bonds withlignin or cellulose. A modified version of the second (biphasic) model considers the initialformation of xylose oligomers and their subsequent hydrolysis to xylose (Figure 7-4C) [61].

Dilute sulfuric acid pretreatment has been performed in batch (autoclave) reactors, wherebiomass and acid are mixed and reacted for a certain time at the desired temperature[35,38,52], as well as in continuous systems, where the acid and biomass are continuouslymixed/fed and withdrawn from the reactor, followed by separation of solids and liquids[65]. Percolation (flow-through) reactors have also been used, where the acid passes througha stationary bed of biomass and hydrolyzes/removes hemicellulose as well as a largerfraction (compared to the batch reactor) of the solubilized lignin, achieving greater enzymatic


cellulose digestibility compared to batch processes. This system also suppresses sugar decom-position and improves sugar yields. Higher sugar concentrations can also be obtained as it is apacked-bed reactor, which allows a high solid/liquid ratio. Sugar yields as high as 90% and anenzymatic digestibility of 80%were reported [64,67].A variation of the percolation reactor is thetwo-stage shrinking-bed percolation reactor, where the biomass bed shrinks as the hemicellu-lose and lignin are removed. This reactor further improves sugar yields to 95%, compared to the90% of the nonshrinking-bed process, with the fast and slow hydrolyzed fractions of hemicel-lulose, proposed in the biphasic model, being removed at each stage of the operation [68]. Thecountercurrent shrinking-bed reactor allows continuous feeding of biomass in a countercurrentdirection to the acid and offers even more advantages compared to the batch and percolationsystems. It allows very low (0.08%) acid concentrations to be used and can almost achieve thetheoretical yield for hemicellulose sugars (97-99%) and a yield of 80-90% for cellulose sugarswhile maintaining a high sugar concentration in the liquid (2-4 wt%) [69].

Degradation products generated by the pretreatment with dilute sulfuric acid are mainlyfurfural, 5-hydroxymethylfurfural, LA, and formic acid. Sulfuric acid and the degradationproducts inhibit the fermentation of the sugar-rich liquid process streams, and therefore haveto be removed. This detoxification is usually performed with the addition of Ca(OH)2, whichis efficient but leads to sugar losses and the formation of insoluble gypsum that has to beremoved from the process liquid, adding to the process steps and the final cost. The formationof inhibitors, detoxification, removal of gypsum, and the need for corrosion-resistant equip-ment are the main disadvantages of the dilute acid pretreatment [70].

Slow-hydrolyzingxylan

Fast-hydrolyzingxylan

Slow-hydrolyzingxylan

Fast-hydrolyzingxylan

Xylan Xylose

Xylose

Xylose Furfural

Furfural

Furfural

Xyloseoligomers

K1

K1f

K1s

K1f

K1s

K2

K2

K2 K3

Saeman model

A

B

C

Biphasic model

Modified

biphasic model

FIGURE 7-4 Kinetic models for hemicellulose hydrolysis: (A) Seamanmodel, (B) biphasic model, and (C) biphasicmodel with oligomeric sugar intermediate. Reproduced from Ref. [61] with permission from Humana Press, Springer.


7.2.1.2 Phosphoric Acid

Dilute phosphoric acid has also been investigated for the pretreatment of lignocellulosicmaterials but not as extensively as sulfuric acid. The main reason for this is the price ofphosphoric acid, which is approximately 20 times higher than that of sulfuric acid [71].This disadvantage, however, can be somewhat balanced by the fact that, during neutraliza-tion with NaOH, sodium phosphate is formed, a salt that can remain in the fermentationmedia because several microorganisms use it as a nutrient. This can lead to a reductionin the cost of nutrients required for the fermentation and the elimination of a process step,i.e., that of filtration to remove the salt formed in the neutralization with other acids [72].Again, the main goal of the pretreatment is the removal of hemicellulose from the biomass,but lignin is also partially removed. The biphasic kinetic model [66], i.e., the existence oftwo hemicellulose fractions, one of which is hydrolyzed quickly and the other slowly,has also been proposed for hemicellulose removal with dilute phosphoric acid pretreatmentof lignocellulosic biomass [48]. Treatment of ryegrass straw for preparation of animal feedwith dilute (0.5N) H3PO4 showed no hemicellulose removal compared to H2SO4 and HCltreatments [73]. Pretreatment of sugar cane bagasse at 100 �C with 6% phosphoric acid [72]or at 122 �C and 4% phosphoric acid [74] and treatment times of 300 min enabled efficientfractionation of biomass, with high sugar yields and relatively low inhibitor formation. Byincreasing the pretreatment temperature, lower acid concentrations and shorter treatmenttimes could be employed. More specifically, treatment with 1% phosphoric acid for 10 minin the temperature range of 140-190 �C was able to remove all hemicellulosic sugarsand provide good sugar and ethanol yields. Best results were observed at 180 �C, but themaximum concentration of inhibitors was also produced at these conditions [75]. Furandehydration products in the treatment with phosphoric acid were approximately one-thirdcompared to the levels observed with sulfuric acid. LA and formic acid, which were formedin high concentrations in sulfuric acid hydrolyzates, were essentially absent with phosphoricacid treatment, while both acids exhibited similar sugar yields [71]. Pretreatment of olive treepruning at 90 �C and 240 min with 0.5N phosphoric acid resulted in 74.5% of the theoreticalethanol yield, but with incomplete removal of hemicellulose [76]. The pretreatment of olivetree pruning with a very low acid concentration of 0.2% H3PO4 at 160

�C and 10 min yielded91.3% of total xylose, while a synergistic effect was shown by the combined use of sulfuric andphosphoric acid [48].

Concentrated phosphoric acid solutions have also been used for the fractionation of var-ious lignocellulosic materials such as industrial hemp [77], bermuda grass, reed and rapeseed[78], bamboo [79], construction and demolition lignocellulosic wastes [80], and paper tuberesiduals [81]. As in the case of concentrated sulfuric acid, the mode of action of concentratedphosphoric acid is not that of a catalyst, but that of a cellulose solvent that can disrupt thelinkage among cellulose, hemicellulose, and lignin as well as the hydrogen bonds of crystal-line cellulose [77]. Depending on the concentration of the acid and type of biomass, cellulosecan be swollen or dissolved. The introduction of an organic solvent, such as acetone, furtherimproves the fractionation of lignin and separation of partially soluble hemicelluloses fromsolid cellulose. The main drawbacks of this method are the high capital investment due to theneed for corrosive-resistant materials, the large volume of solvent recycling, and the neutral-ization of process streams required.


7.2.1.3 Hydrochloric Acid

Hydrochloric acid pretreatment has been investigated mainly as a method for thepretreatment of lignocellulosic agricultural by-products. A 2% concentration of mineral acidat 120 �C for 90 min was used for the prehydrolysis of rice and bean hulls, with hydrochloricacid beingmore effective than sulfuric acid at the same conditions [82]. Hydrochloric acidwasfound to be a good alternative to sulfuric acid for the pretreatment of sweet sorghum bagassebecause of the relatively low hydrolysis time and the generation of C5 and C6 fermentablesugars [83]. For the pretreatment of wheat straw, the use of hydrochloric acid providedthe highest yield of sugars in the liquid fraction compared to sulfuric and acetic acids, buta two-step hydrochloric acid-sodium hydroxide pretreatment was employed for maximumenzymatic hydrolysis of straw [84], which was attributed to the 68% removal of ligninachieved in the second NaOH pretreatment step. Hydrochloric acid pretreatment also im-proved the enzymatic sugar production of cellulosic municipal wastewater treatment processresiduals from 31% to 54% [85]. The use of concentrated acid for prehydrolysis (32%HCl) andmain hydrolysis (41% HCl) of Lupinus nootkatensis main stems has been reported, with theproposed advantages of the method being the exothermic hydrolysis, the possibility of acidrecovery, and the high yields of fermentable carbohydrates [86]. The use of concentratedhydrochloric acid was proposed for the hydrolysis of cellulose and production of ethanolfrom cellulosic municipal solid waste [26]. The high price of hydrochloric acid, however,necessitates the recycling and reuse of the acid, as in the case of concentrated sulfuric acid,for such processes to be economically viable [28].

7.2.1.4 Nitric Acid

The high cost of nitric acid compared to sulfuric acid has also prevented its extensive use asa catalyst for the pretreatment of lignocellulosic materials. It has been used as a cocatalyst at2% together with 35% acetic acid for the removal of 80% lignin from newsprint [87]. Thehydrolysis of sugar cane bagasse with 6% HNO3 at 122

�C and 9.3 min was more efficientcompared to hydrochloric and sulfuric acid pretreatments [88]. The successful treatment ofcorn stover at even lower acid loadings of 0.6% and processing time of 1 min at 150 �Cwas also demonstrated [89], with high xylose and low furfural concentrations. It was alsoused in the acid-catalyzed ball milling of corn stover [90], where it exhibited similar resultsas other acids such as sulfuric, hydrochloric, acetic, and formic, with sugar yields close to 60%.This yield, however, was inferior to those achieved with alkaline catalysts, which yielded upto 90% sugars. Rice straw pretreated with 5% HCl and used as a substrate for the productionof fungal cellulases yielded lower cellulose activities compared to that pretreated with 2%NaOH or 28% ethylenediamine [91].

7.2.2 Organic Acids

Various organic acids have been investigated as catalysts for the pretreatment of lignocel-lulosic materials. Among them, acetic acid is the only one that can be found in significantconcentrations in various lignocellulosic materials, in the form of acetylated hemicellulose.It is, therefore, examined as a catalyst in the self-catalyzed pretreatment section that followslater. A 24-h soaking of corn stalkwith acetic acid at room temperature has proved less effective


than soaking in sulfuric acid, ammonia, and sodium hydroxide, as well as steam explosion, inhemicellulose removal, as well as in microbial growth and organic acid production [92].

Fumaric and maleic acids were compared with sulfuric acid as catalysts in thepretreatment of wheat straw and were found to produce comparable amounts of xylose fromthe pretreatment and glucose from the enzymatic hydrolysis at lower furfural concentrations,and have therefore been proposed as an alternative to sulfuric acid pretreatment [93,94].These results are attributed to the lower degradation rate constants of arabinose [95], glucose[96,97], and xylose [98] in these organic acids compared to sulfuric acid. A Seaman and abiphasic kinetic model (Figure 7-4) were proposed for corn stover hemicellulose hydrolysiswith maleic acid at high (150-200 �C) and low temperatures (100-140 �C), respectively [99].A xylose yield of 96% of the theoretical value was achieved with 1 M maleic acid concentra-tions at 150 �C in the same study. Maleic acid was also investigated as a catalyst for the de-hydration of xylose to furfural, with a high selectivity of 67% toward furfural observed forxylose derived from corn stover, switch grass, and poplar, and a modest selectivity of 39%for xylose derived from pine wood [100].

The acetic acid-catalyzed pretreatment of corn stover at 195 �C for 15 min led to recoveries ofglucose and xylose of 98% and 82%, respectively, low inhibition during fermentation, and in-creased ethanol production [101]. Recovery of arabinan in these conditions was much lower(50%) [102]. Use of lactic acid or a combination of lactic and acetic acids for the pretreatmentof corn stover exhibited good cellulose recovery, showed no inhibition effects during fermen-tation, and achieved 89% of the theoretical ethanol yield [103]. The combined use of both acids,however, led to a decreased xylose recovery of 59%. Pretreatment with dilute formic acidachieved lower glucose and xylose recoveries of 89% and 55%, respectively, compared touncatalyzed pretreatment, and an ethanol yield of 77%. Formic acid (60%) led to the removalof lignin and hemicellulose from sugarcane bagasse, an increase in crystallinity, and a 50%ethanol yield [104]. Similarly, high concentrations of formic acid (60-90%) were used for theremoval of 80% hemicellulose and lignin from sugarcane bagasse, but cellulose formylationat these conditions was also observed, and an alkaline deformylation step was necessary torecover cellulose digestibility [105]. The hypothesis that glucose degradation at formic acid con-centrations of 5-20% at 180-220 �C proceeds through formation of an unknown intermediatecompound has been proposed [106]. At these very high concentrations, as in the case ofmineralacids, the organic acid actsmore as a solvent than as a catalyst, with delignification and removalof hemicellulose being the main effects of the treatment. Pretreatment of corncobs with oxalicacid showed good hemicellulose removal with low inhibitor formation, as well as 20 g/Lethanol production at both laboratory [107] and pilot scales [108].

Representative data on the performance of various pretreatment methods utilizing acidiccatalysts, with respect to sugars and ethanol yields, are given in Table 7-3.

7.3 PRETREATMENT WITH BASIC CATALYSTS

7.3.1 Inorganic Bases

Pretreatment of lignocellulosic materials with basic catalysts is, in effect, a delignificationprocess, contrary to the use of acidic catalysts, which aim mainly at hemicellulose removal


(Table 7-4). Delignification is achieved by breaking the ester bonds between lignin andhemicellulose, and leads to the production of a biomass rich in cellulose and hemicellulose[119]. The mechanism of delignification is similar to that of soda or Kraft pulping, which usesa solution of sodium hydroxide and sodium sulfide to dissolve and remove the majority oflignin. This goal of pulping to liberate carbohydrate polymers from their complex with ligninis generally consistent with that of the pretreatment focusing on lessening the recalcitrance oflignocellulose for ethanol production [120,121]. It is better suited for the pretreatment ofagricultural by-products, such as straws, and herbaceous biomass [21], which has been attrib-uted to the lower activation energy for delignification of agricultural by-products compared

TABLE 7-3 Performance of Various Pretreatment Methods Utilizing Acidic Catalysts

Pretreatment

method Catalyst Biomass

Temperature

(�C)Time

(min)

Sugars yield (%)Ethanol

yield ReferenceXylose Glucose

Dilute acid H2SO4

(0.08 wt%)Yellowpoplar

170-190 –a 99 97 – [69]

Phosphoricacid(0.5-8N)

Olive treepruning

90 240 89b – 74.5%c [76]

Nitric acid(0.6%, w/w)

Cornstover

150 1 95.9 – – [89]

Maleic acid(1 M)

Cornstover

150 15 96 – – [100]

Acetic acid Cornstover

195 15 82d 98d 33.72 g/L [101]

Hydrothermal Acetic acid Cornstover

190 15 82 91 88% [109]

Steamexplosion

Acetic acid Wheatstraw

195 6-12 – – 89-92% [110]

Steamexplosion

Two-stageH2SO4

(0.7%/0.4%)

Softwoodforestthinnings

190/215 3/3 90 50 74-89% [111]

Steamexplosion

SO2 (4%wt) Lodge-pole pine

200 5 – 92 77% [112]

SCe CO2

explosionCO2

(3100 psi)Aspen 165 30 – 84.7 – [113]

Southernyellowpine

165 30 – 27.3 – [113]

aContinuous process.bHemicellulose sugars at 8N phosphoric acid.cEthanol yield at 0.5N phosphoric acid.dExpressed as maximum xylan and glucan recovery.eSC¼ supercritical.


to wood-derived biomass [122]. Lower temperatures and pressures and larger pretreatmenttimes, which can span from days to even weeks, are employed with bases compared to othertechnologies, such as acid, steam, or hot water treatment, which are performed at higher tem-peratures and process times ranging from a few minutes to a few hours [25]. One of the keyfeatures of pretreatment with basic catalysts is that cellulose and hemicellulose are largelyretained in the biomass, and therefore, can be cofermented in an SSCF (Figure 7-2), leadingto a simplified bioethanol production process.

NaOH. Sodium hydroxide is among the most studied bases for alkaline pretreatment ofbiomass. Treatment of Sorghum bicolor straw with 2% NaOH at 121 �C for 60 min led to80% lignin removal and 95% enzymatic saccharification of the remaining solids [114](Table 7-4). Similar results were reported for the NaOH pretreatment of wheat straw [120].A low-temperature alkali pretreatment of sweet sorghum bagasse removed up to 80% of lig-nin, reduced cellulose crystallinity, and improved glucan saccharification yield, the latterreaching as high as 98%. This performance was better compared to dilute sulfuric acidpretreatment [121]. A mild alkali pretreatment of corn fiber led to higher glucose yields com-pared to untreated biomass during SSF but ethanol yields reported were similar [123]. NaOHpretreatment of six biomass species, including various agricultural by-products and grassesas well as mixed hardwood, achieved up to 80% yields of glucose and xylose, depending onbiomass type and NaOH loading. The mixed hardwood, as expected, exhibited the loweryields among the six biomass species [124]. A 2% sodium hydroxide pretreatment of the an-aerobic digestate (AD) fiber at 130 �C and 3 h showed an increase in cellulose concentration oftheAD fiber from 34% to 48%, a 62.6% cellulose utilization of the raw cellulose in the AD fiber,and an 80.3% ethanol yield [125].

Sodium hydroxide pretreatment has also been used to increase methane production fromthe anaerobic digestion of lignocellulosic biomass. A 24-fold increase in methane yield was

TABLE 7-4 Pretreatment Methods with Alkaline Catalysts

Pretreatment


Temperature

(�C)Time

(min)

Lignin

removal

(%)

Sugars (%) Ethanol

yield

(%) ReferenceXylose Glucose

Alkaline NaOH(2%)

Sorghumstraw

121 60 >70 – 90 – [114]

Ca(OH)2(7.5%)

Cornstover

120 240 60 47 60 – [115]

Soaking inaqueous ammonia(SAA)

NH3

(15%)Barleyhull

75 48 61.1 63 83 89.4 [116]

Ammoniarecycled percolation(ARP)

NH3

(15%)Cornstover

170 90 84.7 – 92.5 – [117]

Ammoniafiber explosion(AFEX)

NH3 (1:1,w/w)

Cornstover

90 5 – 80 100 96 [118]


achievedwith 3.5%NaOH treatment in a solid-state anaerobic digestion of fallen leaves [126].Methane production from dried grass silage improved by 38.9% when a 7.5% NaOHpretreatment at 100 �Cwas employed [127].Microwave-assisted alkali pretreatment is amod-ification of the process, where the alkali biomass slurry is heated with the use of microwavesfor a short duration. It has been used in the pretreatment of switchgrass at 190 �C for 30 minand NaOH loading of 0.1 g/g biomass, resulting in 80% lignin removal and 99% enzymatichydrolysis sugar yield [128], as well as for the production of lignocellulolytic enzymes frompretreated rice straw and hulls [129]. Photocatalysis-assisted alkali pretreatment of rice strawhas also been studied and found to increase enzymatic hydrolysis by 2.6-fold compared to thenonassisted process [130].

Ca(OH)2. Calcium hydroxide pretreatment is an interesting substitute to that with NaOHbecause of its low cost, safety, and ability to recover lime as calcium carbonate by carbonatingthewater [131,132]. Itwas proposed that the resultingCaCO3 can then be converted to lime in alime kiln [131]. Concentrations of lime used are much lower compared to NaOH due to thelower solubility of the former in water, and usually are 0.1 g lime/g dry biomass [132,133]or lower [115] (Table 7-4), but excess liming has also been used [122,134]. Removal of ligninas well as acetic and uronic acids from hemicellulose seems to be the major effect of limepretreatment. The percentage of lignin removal can be moderate to small compared to thevalues reported inNaOHtreatments, and the resultingyieldofenzymatically convertedsugarsor ethanol canbe lowerdependingonbiomass type andpretreatment conditions. For example,limepretreatmentof bagasse led to a14% lignin solubilization, andenzymatic hydrolysis of thepretreated sample yielded 60% glucose and 80% xylose [131]. Yields of 60% cellulose and 47%xylanwere achieved fromenzymatic hydrolysis of lime-pretreated corn stover, but an increasein the enzyme loading and hydrolysis time increased yields to 88.0% and 87.7%, respectively[115]. TheSSFof lime-treated switchgrass andcorn stover resulted inethanol yieldsof 72%and62%, respectively [135]. In the case of sugarcane bagasse, lime pretreatment resulted in about60% glucose yield [136]. A combination of Ca(OH)2 and O2, i.e., oxidative lime pretreatment,of poplar wood led to 73% ethanol yield by SSF [135]. These values, although relatively high,are quite low compared to the almost theoretical sugar yields reported for the NaOH treat-ments. An interesting variation reported was the combined use of the more effective NaOHwith the less expensive Ca(OH)2 for the pretreatment of switchgrass. The combined use of0.1 g/g biomass of NaOH and 0.02 g/g biomass of Ca(OH)2 for 6 h at ambient temperatureled to an increase in sugar yields compared to biomass treated only with NaOH [137].

7.3.2 Organic Bases

NH3. Ammonia is the most widely utilized organic base for the pretreatment of lignocel-lulosic materials. As in the case of inorganic bases seen earlier, the main effect of ammoniapretreatment is the removal of lignin and, to a lesser extent, of hemicellulose. The mainpretreatment methods utilizing ammonia are termed soaking in aqueous ammonia (SAA)and ammonia recycle percolation (ARP) (Table 7-4). SAA is implemented by soaking the bio-mass in an ammonia solution at room or moderate temperature and atmospheric pressure,without agitation. It is a method that consumes low energy but requires long pretreatmenttimes. It has been used for the pretreatment of corn stover with 29.5 wt% of aqueous ammoniawith reaction times of 1-60 days. This led to the removal of up to 75%of the lignin and retained


100% of the cellulose and 85% of the xylan in the biomass. The SSCF of solids gave a 77% eth-anol yield based on the glucan and xylan content of the substrate [138]. Similar results wereobtained by increasing the treatment temperature to 60 �C, which led to a reduction ofpretreatment time to 12 h [139]. Pretreatment of barley hulls at 75 �C for 48 h with 15 wt%aqueous ammonia and 1:12 of solid/liquid ratio resulted in saccharification yields of 83%for glucan and 63% for xylan, and 89.4% of the maximum theoretical ethanol yield basedon glucan and xylan. It was also reported that the pretreatment via SAA increases the surfacearea and pore size, which in turn increases the enzymatic digestibility of the biomass [116](Table 7-4). Up to 50% lignin removal and 72% of theoretical ethanol yield were achievedin the SAA pretreatment of switchgrass [140]. In the case of rice straw, enzymatic digestibilityof 71.1% and an ethanol yield of 83.1% were obtained with pretreatment at 69 �C for 10 h andammonia concentration of 21% (w/w) [141]. Despite these high yields, however, SAA resultsare somewhat inferior to those of other pretreatment technologies, such as ammonia fiber ex-plosion (AFEX, discussed later), dilute sulfuric acid, liquid hot water, and concentrated phos-phoric acid combined with ethanol extraction [142,143].

In ARP pretreatment, the ammonia solution flows through a packed bed of biomass, whichis treated at high temperatures for short times. The ammonia is recycled and reused. It has beenusedfor thepretreatmentofhybridpoplar,where itwasshownthat, togetherwithdelignification,a significant amount of xylan was also removed. For this reason, a two-stage pretreatmentprocess was applied in which hemicellulose was first removed by a self-hydrolysis step (hydro-thermal treatment), which was followed by ARP treatment for delignification. The enzymatichydrolysis yields achieved were as high as 95% [144,145]. Treatment of corn stover via ARPresulted in 70-85% delignification, 40-60% solubilization of hemicellulose, and as high as 92%enzymatic digestibility of cellulose at moderate enzyme loadings [117] (Table 7-4).

7.4 SELF-CATALYZED PRETREATMENT

This category involves the use of water or steam for the pretreatment of lignocellulosicmaterials and has been referred to as steam pretreatment with or without explosive decom-pression (steam explosion), hydrothermal or liquid (compressed) hot water pretreatment,autohydrolysis, aquasolv, and others [21]. It is an attractive pretreatment method becauseno other chemical or solvent except water is required, rendering it cost-effective and environ-mentally friendly. Neither corrosion-resistant equipment, as in the acid pretreatment, norneutralization of the liquid process streams is needed. Inwater pretreatment, the temperatureregime usually implemented is in the range of 130-230 �C. In these conditions, water is withinthe subcritical range, meaning that it is above its boiling and below its critical point, and hy-drolytic reactions as well as degradation reactions of sugars are favored [146]. As describedearlier, hemicellulose is partially acetylated to a greater or lesser degree depending on thetype of biomass. The acetyl ester bonds formed between acetic acid units and hemicellulosesugars are hydrolyzed in the operating conditions of the steam or liquid hot waterpretreatment and acetic acid is released, lowering the pH of the pretreatment medium andacting as an acid catalyst [147,148]. Therefore, this type of pretreatment can be also consideredas a self-catalyzed pretreatment, along with the various other names reported. Since no externalcatalyst is added, the acetyl content of the biomass plays an important role in the performance


of the pretreatment. Uronic acids contained in the hemicellulose, as well as acids generated inthe process as degradation products, may also play a role.

The acetyl content of lignocellulosic biomass varies among species, but in general it isgreater in hardwoods, with concentrations between 3% and 5%, and is much smaller in soft-woods, in the range of 0.8-2% [149]; it may also vary significantly among different types ofagricultural by-products (1-4%) [150]. Based on this, hardwoods and agricultural by-productsare better suited for the self-catalyzed pretreatment under steam or liquid hot water condi-tions, and have been more extensively studied as feedstocks compared to softwoods. Inaddition, softwood hemicelluloses are mainly hexosans and are more resistant to acid hydro-lysis compared to hardwood hemicelluloses, which are mainly pentosans [151]. This type ofpretreatment has been testedwith a variety of lignocellulosic materials, including agriculturalwastes such as wheat straw [152,153], rice straw [154], corn stover [155], sweet sorghumbagasse [153] and sugar cane bagasse [156], herbaceous plants such as Brassica carinata[153] and prairie cord grass [157]; hardwoods such as yellow poplar [158], Populus nigra[159] and Eucalyptus globulus [153], mixed oak and gum wood chips [160]; and softwoods,such as Loblolly pine [161].

Since the amount of acid catalyst of the process is biomass specific, time and temperatureare themain parameters controlling the effectiveness of hydrolysis. Their combined effect hasbeen expressedwith the introduction of a severity parameter (log Ro) [162] where the reactionordinate (Ro) is given by the equation

Ro ¼ t expT � 100

14:75

� �

Solid loading also plays an important role in the process; high solid loadings are desirable,but they will also affect the concentration of sugars and degradation products in the processliquids [59]. Chip size is essential, since it affects the diffusion of water or steam inside thebiomass particles and the diffusion of hydrolysis products out of the particles [163]. It wasshown, however, that the enzymatic digestion and fermentation of derived sugars ofpretreated poplar biomass were not significantly affected by the chip size (for two size ranges2-5 and 12-15 mm) for both type of treatments, i.e., steam or liquid hot water [159]. It was alsoreported that the effect of chip size is dependent upon the type of biomass and the type(effectiveness) of the pretreatment method [164]. Moisture content is also an importantparameter, as higher moisture content will impede heat transfer from steam to biomass, asenergy is consumed to heat up all the water in the chips [165].

The main effect of the steam or liquid hot water pretreatment is hemicellulose hydrolysis,which is recovered in the process liquid as sugar (mainly xylose)monomers and oligomers. Inthe hydrothermal pretreatment (130-220 �C, 15-180 min, water-to-biomass mass ratio¼15, inan autoclave high-pressure stirred reactor under autogeneous pressure) of beech wood, up to40% of biomass was solubilized at higher process severities (Figure 7-5A, unpublished data).Xylose was recovered in the pretreatment hydrolyzates as monomers and oligomers at highconcentrations for a wide range of treatment severities, while furfural was formed onlyat treatment conditions of relatively high severity, i.e., either high temperature and short re-action time (220 �C, 15 min) or moderate temperature and long reaction time (170 �C,180 min), with the higher temperature more favorable to the dehydration of xylose to furfural(Figure 7-5B; unpublished data). Under these hydrothermal pretreatment conditions, a small


amount of lignin is also solubilized and, depending on the process severity and pH [166], mayrecondense on the biomass surface [167], which in some cases has been shown to have anadverse effect on the enzymatic hydrolysis [60]. A similar behavior has been observed inour studies [168] for the hydrothermal pretreatment of beech wood at high severities(log Ro¼4.7) and pH<3, where lignin was deposited on the biomass particles in the formof droplets (Figure 7-6A; unpublished data). Although at these high-severity (220 �C,

0

0.2

~10mm

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2 2.5 3 3.5 4 4.5 5

LogRo

Sp

ecif

ic s

urf

ace a

rea

(m

2g

-1)

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

To

tal p

ore

vo

lum

e (

cm

3g

-1)

B

ASpecific surface area Total pore volume

FIGURE 7-6 (A) Scanning electron microscopy (SEM) image of hydrothermally pretreated beech wood at log Ro

4.7 (pH<3) showing the deposition of lignin as droplets on the biomass surface. (B) Effect of log Ro on the specificsurface area and total pore volume of hydrothermally pretreated beech wood.

0

1

2

3

4

5

6

7

8

C (

mg

/ml)

LC 1

30 °C

15m

in

LC 1

50 °C

15m

in

LC 1

70 °C

15m

in

LC 1

80 °C

15m

in

LC 1

90 °C

15m

in

LC 2

20 °C

15m

in

LC 1

70 °C

30m

in

LC 1

70 °C

60m

in

LC 1

70 °C

90m

in

LC 1

70 °C

120

min

LC 1

70 °C

180

min

B

0

10

20

30

40

50

1 2 3 4 5

LogRo

% S

olu

biliz

ati

on

A

Xylose Furfural

FIGURE 7-5 (A) Solubilization of hydrothermally pretreated beech wood versus pretreatment severity factorlog Ro, and (B) concentrations of xylose and furfural in the pretreatment hydrolyzate.


15 min) treatment conditions, almost all the hemicellulose was removed and the surface areaof the sample was almost 2.5-fold higher than that of the parent biomass (Figure 7-6B;unpublished data), the deposition of lignin on the external surface of biomass particlesseverely impeded enzymatic digestibility. The percentage of enzymatic conversion of cellu-lose dropped from 67% at log Ro¼4.3 to 12% at log Ro¼4.7, a value very close to the 6.9%conversion measured for the native, parent biomass [168]. The adverse effect of lignin depo-sition on enzymatic saccharification has been attributed to the protection of cellulose fromlignin as well as nonspecific adsorption of enzymes on the lignin [169]. The lignin, however,can be easily removed from the hydrothermally pretreated solids with a mild solvent alkaliextraction, further increasing enzyme accessibility of the biomass [170]. Sugar degradationproducts such as furfural, 5-hydroxymethylfurfural (HMF), levulinic and formic acids[171], as well as lignin degradation products, are also formed, especially at higher processseverities and low pH values, which can inhibit the fermentation of biomass-derived sugarsto ethanol [70,172]. High-severity pretreatment usually yields a solid that can be easilyconverted to glucose by enzymatic hydrolysis, but the higher the severity, the greater theloss of hemicellulose sugars due to formation of degradation products, such as furfural(Figure 7-5B) and HMF. A great loss of sugars will reduce the economic viability of the pro-cess, but a high conversion of cellulose to glucose is also required. A compromise betweenthese two targets is, therefore, required, and the process parameters have to be fine-tunedin order to achieve optimal results. A way to reduce by-product formation at high severitiesis the adjustment of pH to near-neutral values, which can reduce the formation of degradationproducts and increase cellulose recovery in the solids [173].

The hydrolysis of hemicellulose under hydrothermal conditions has been described on thebasis of the biphasic kinetic model, which assumes the existence of two xylan fractions, onehard and one easy to remove as in the case of dilute acid hydrolysis (Figure 7-4), also takinginto account the production of xylose either directly from xylan or from xylose oligomers, aswell as the formation of various degradation products, such as furfural [174–177]. However,at high treatment temperatures (ca. 240 �C), xylan removal was almost complete and a singlereaction kinetic model that does not consider significant differences in the rate of xylanhydrolysis was suggested [175].

In the hydrothermal pretreatment of mixed herbs and sunflower seed shells, around 53%and 63% xylo-oligomers were recovered in the liquid, respectively, with xylose being around10% [178]. Production of oligosaccharides from six agricultural residues was found to be pro-portional not only to the xylan content but also to the acetyl content of the biomass sample[150]. Corncobs provided the highest yield of sugar oligomers (60%) with 4% acetyl groups,while rice husks gave the lowest yield of oligosaccharides (30%) with 1.1% acetyl units. Inthe hydrothermal pretreatment of wheat straw, maximum values for hemicellulose sugar re-covery (71.2%), enzymatic hydrolysis (79.8%), and ethanol yield (90.6%) were achieved atthree different pretreatment conditions [179]. Although hydrothermal pretreatment of poplarproduced better hemicellulosic sugar and enzymatic hydrolysis yields compared to steam ex-plosion, the latter was deemed better overall, giving above 95% cellulose recovery, 60% en-zymatic hydrolysis yield, 60% SSF yield, and 41% xylose recovery in the liquid fraction at210 �C and 4 min [159]. In the pretreatment of sugarcane bagasse in a batch reactor,hot water performed better than steam in terms of xylan recovery (68% versus 25%) forSSF yields above 90% based on residual cellulose [156]. The hydrothermal pretreatment of


E. globulus [180]with a severity factor log Ro 3.94 led to the recovery ofmore than 80% of xylanin the pretreatment liquids, with no degradation of cellulose or lignin observed untillog Ro 4.97. Maximum enzymatic conversion of cellulose was obtained from log Ro of 4.38and above. SSF of solids treated at log Ro 4.38 and 4.67 gave ethanol yields of 68% and86%, respectively. Utilization of a plug-flow coil reactor for the continuous hydrothermal,pH-controlled pretreatment of corn stover has also been proposed [109] (Table 7-3).

Steam can be used for the pretreatment of lignocellulosic materials with or without explo-sive decompression [181]. Steam heating without explosion, as in the case of hydrothermal(i.e., liquid hot water) pretreatment, has the ability to remove hemicellulose and increasethe enzymatic digestibility of the treated biomass. When poplar wood was steam-heated at210 �C for 15 min, a glucose and xylose total yield of 61.9% was reached. The alkaline extrac-tion of lignin from the solids improved saccharification yields only in a narrow range ofpretreatment severities [182]. In the case of steam heating of aspen wood chips, the residualalkali-insoluble lignin content of the biomass seemed to control the digestibility of cellulose,which reached 85% of the theoretical yield [183]. When rapid decompression of steam (steamexplosion) is used, the biomass undergoes physical (defibration) as well as chemical (hydro-lysis) transformation [170,184–186]. Formation of sugar monomers is favored with steamexplosion, whereas sugar oligomers are more predominant in hot-water-pretreated samples.Steam explosion is the preferred method of steam treatment, although the effect of explosionhas been in doubt as the differences in enzymatic saccharification between steam-heated andsteam-exploded samples were not significant [165,185]. The steam-exploded biomass iswashed with water to extract sugars and degradation products. Continuous steam explosion(at log Ro 3.8) of Populus tremuloides in the pilot scale utilizing the STAKEII process [151,176]resulted in up to 65% pentosans and 80% lignin recovery by alkali extraction. A significantreduction in the degree of cellulose polymerization from 1400 to 600 was also observed atthe same severity, but maximum glucose recovery from enzymatic hydrolysis is reachedat log Ro values greater than 4, where the yield of pentosans is reduced [187]. The rapid steamhydrolysis/extraction (RASH) is a variation of steam explosion, where the steam condensateis continuously removed from the pretreatment reactor, thereby removing acetic acid anddecreasing the formation of degradation products, enhancing the enzymatic degradationof cellulose, and leading to the precipitation of a more reactive lignin in both the solidsand the condensate [160,188]. Representative pilot and commercial-scale steam-explosionprocesses are reviewed elsewhere [151,189].

The integrated biomass utilization system (IBUS) is a continuous pilot scale process devel-oped for converting lignocellulosic waste biomass to bioethanol using steam and enzymesonly, at very high solid loadings [190]. A three-reactor hydrothermal pretreatment wasemployed, where presoaked wheat straw was heated at 80 �C for 20 min in a screw conveyor(reactor 1), then heated at 170 or 180 �C for 7.5 and 15 min (reactor 2), and then passed tothe final reactor (reactor 3) where the biomass was steam-heated at 195 �C for 3 min withorwithout addition ofwater (leaching step) [191].With highwater addition (600 kg/h) in step3, a high hemicellulose recovery of 83% was achieved as a result of low degradation of hemi-cellulose sugars, but the enzymatic conversion of cellulose to glucose was low (55%). With nowater addition, enzymatic conversion of 73% was obtained but at the cost of 33% hemicellu-lose recovery due to increased severity, leading to enhanced formation of degradationproducts. Ethanol yield was calculated taking into account all available sugars, i.e., glucose,


xylose, and arabinose. At high water addition, 60% of the theoretical yield was achievedbecause of the recovery of hemicellulosic sugars, whereas at low water addition, a similarethanol yield (58.5%) was obtained as a result of the higher enzymatic digestibility of cellu-lose. In an effort to reducewater consumption in the IBUS process, the third stepwas omitted,and the optimization of pretreatment parameters in the second step enabled recovery of 70%hemicellulose, 93-94% cellulose, and more than 90% lignin. Addition of xylanase enzymes tothe cellulose mixture to facilitate removal of residual xylan from the fibers increased ethanolyields from 89% to 92% by SSF [110] (Table 7-3).

7.5 COMBINING CHEMICAL CATALYSIS WITHPHYSICAL METHODS

Sulfuric acid-catalyzed steam explosion. The introduction of a catalyst in a physical or phys-icochemical pretreatment process is done to combine the physical effect of the basic methodwith the chemical effect of the added catalyst, aiming at optimization and tuning of theprocess outcome. Steam explosion, as described above, is a physicochemical pretreatmentprocess that combines the chemical effect of water or steam at elevated temperatures andof the in situ released acetic acid present as acetyl groups in hemicellulose, with the physicaleffect of the steam decompression that disrupts biomass fibers and enhances the biomass ac-cessibility to enzymes. Its performance can be enhanced by impregnation of biomass with anacid catalyst, such as dilute sulfuric acid. In the case of aspenwood chips, impregnation with0.2% sulfuric acid increased pentosan solubilization, decreased pentosan destruction, im-proved enzymatic saccharification, and allowed the use of lower pretreatment temperatures[165]. Mixed hardwood treated with 1% sulfuric acid in a continuous plug-flow reactor led tothe complete removal of hemicellulose and near-quantitative cellulose hydrolysis for samplestreated at the highest severities [192]. Acid-catalyzed steam explosion of corn stover affordedsoluble xylose yields as high as 96.3% and ethanol yields as high as 92%with SSF [193]. In thecase of barley straw, another agricultural by-product, two different methods of introducingthe acid catalyst, namely, soaking and spraying, were evaluated. Soaking was found to givebetter results, with higher overall xylose and glucose yields, in combined pretreatment andenzymatic hydrolysis [194]. In the case of softwoods, aswas shown earlier, the acetic acid con-tent is relatively low and the addition of an acid catalyst is necessary for effectivepretreatment [151]. A two-stage sulfuric acid biomass loaded steam explosion was employedfor the pretreatment of softwood from forest thinnings, with hemicellulose and cellulose be-ing hydrolyzed at the first and second stage, respectively [111] (Table 7-3). The yieldsobtained were 50% for glucose and 70-98% for other sugars, and the ethanol yield was inthe range of 74-89% based on available hexose sugars. The combined two-stage processwas beneficial compared to the single-stage one in terms of total sugar yields, enzymaticdigestibility of fibers, and ethanol yield [195]. Acid-catalyzed steam explosion has also beenused to improve the production of methane from biofibers. A 15-min treatment of biofibers at155 �C with the addition of 2.1% (w/w) H2SO4 increased methane production by as high as67% compared to untreated biofibers [196].

SO2-catalyzed steam explosion.Another acidic catalyst used for the steam explosion of ligno-cellulosic materials is sulfur dioxide (SO2). Comparing the uncatalyzed to catalyzed (in the


presence of SO2) steam explosion of spruce, it was shown that bothmethods provided similarhigh glucose yields of more than 90% and xylose yields of more than 80%. SO2-catalyzedsteam explosion, however, managed to achieve these high yields of glucose (which is moredifficult to produce because of cellulose recalcitrance) and xylose at similar severities, mean-ing that a greater total sugar yield could be achieved, since further conversion of sugars todegradation products was limited [197]. Several studies compared the performance ofSO2-catalyzed steam explosion with that of H2SO4-catalyzed steam explosion. In the caseof spruce softwood, it was found that the total sugar yields were approximately 66% for bothmethods. The main difference was that sulfuric acid released more hemicellulosic sugars inthe pretreatment stage, while SO2 induced higher glucose yields during the subsequentenzymatic hydrolysis [198,199]. However, enzymatic hydrolysis of the SO2-pretreated bio-mass in the presence of the prehydrolyzate (pretreatment liquid) led to a reduction of upto 36% in cellulose conversion due to inhibition from degradation products. The separate fer-mentation of the prehydrolyzate followed by enzymatic hydrolysis of the treated biomassalleviated the problem but at the expense of introducing a further step in the process[200]. Similar effectswere observed in the steam-explosion pretreatment ofwillow,where sul-furic acid addition led to a high xylose recovery of 80% compared to 62%when SO2was used.Enzymatic hydrolysis results, however, were reversed, with 67% and 95% glucose yieldsachieved, respectively [201]. In SO2-steam-explosion pretreatments, therefore, a significantamount of hemicellulosic sugars remains in the biomass, while enzymatic hydrolysis ofthe remaining solid is enhanced. In the case of SO2-steam-explosion-pretreated corn stover,the addition of xylanases in the enzymatic hydrolysis step led to increased removal of hemi-cellulose, as well as increased enzymatic digestibility of cellulose reaching almost 100%conversion to glucose. Partial delignification of the solids, following the SO2-steam-explosionpretreatment, increased the glucose yield slightly, but at the expense of xylose [202].

In order to improve both hemicellulose removal and enzymatic digestibility, a two-stagepretreatment of spruce was investigated. In the first stage, SO2-steam explosion was appliedat relativelymild conditions to optimize hemicellulose removal, while in the second stage, thesolid received from the first step was treated again with SO2-steam but at more severe con-ditions, aiming at increased enzymatic digestibility [203]. This approach resulted in 80% totalsugar yield at 25 FPU of cellulose/g cellulose and ethanol yields of 69% and 72%with SSF andSHF, respectively, which, however, were comparable to the yields achieved by the one-stageprocess. With the same reasoning, a two-step process comprising H2SO4-steam explosion tooptimize hemicellulose removal followed by SO2-steam explosion to optimize enzymatichydrolysis of softwoods (spruce) was investigated. The two-step approach with H2SO4 inthe first step and SO2 in the second step resulted in lower sugar yields after enzymatic hydro-lysis compared to the two-step pretreatments using either SO2 or H2SO4 in both steps[195,203,204]. Ethanol yields were 66% and 71% for SSF and SHF, with no obvious advantageobserved for the two-stage two-catalyst pretreatment. In a one-step pretreatment in which avariation of time-temperature profile was applied, the recovery of glucose and mannose wasover 90% and ethanol yield reached 75% [205].

The SO2-steam explosion of healthy and beetle-killed lodgepole pine resulted in totalethanol yields of 63% and 75% with SSF [112] (Table 7-3). When sugarcane bagasse was used,total sugar yields ofmore than 87%were achieved,withmore than 60% ethanol yield from thexylose-rich pretreatment liquids and 80% from the enzymatic hydrolyzates [206].

2397.5 COMBINING CHEMICAL CATALYSIS WITH PHYSICAL METHODS

Comparison of SO2-steam explosion for three different feedstocks, namely, a hardwood(Salix), a softwood (spruce), and an agricultural by-product (corn stover), showed higher pen-tose losses for the softwood due to degradation products and higher ethanol yield for cornstover (85%). However, the general trends observed in the process were similar among thethree types of biomass tested [207]. Similar behavior between feedstocks was also reportedin the SO2-steam-explosion pretreatment of six different Douglas fir trees of varying ageand of lodgepole pine treated at the same conditions, where enzymatic hydrolysis conversionranged from 60% to 72% at high enzyme loadings [208]. At low enzyme loadings, the glucoseyield was reduced considerably. After a delignification step, high glucose yields wereobtained irrespective of the enzyme loading, indicating the necessity of a post-treatment step(for removing lignin) to optimize process performance.

The use of lactic acid as an alternative to SO2 for the steam pretreatment of sugarcanebagasse and spruce was also evaluated [209]. The total glucose yield for the bagasse treatedwith lactic acid was 79%, which was similar to that with SO2, but performance of lactic acidwas inferior to that of SO2 in the case of spruce [209].When acetic acid impregnationwas usedfor the steam-explosion pretreatment of wheat straw, enzymatic conversion increased consid-erably compared to steam explosion alone [210].

Ammonia freeze/fiber explosion. The AFEX pretreatment [211] is based on the treatmentof lignocellulosic materials with liquid ammonia under high pressure (10-52 atm), moderatetemperatures (25-90 �C), and short times (10-60 min), combined with explosive decompres-sion at the end of the reaction time. Ammonia can be recycled and reused. A combinationof chemical effects—decrystallization of cellulose, alterations in lignin, and acetate removalfrom hemicellulose—and physical effects—increase in accessible surface area—led to in-creased reactivity of cellulose [212]. The chemical composition of biomass is not greatly af-fected by AFEX because only small amounts of hemicellulose or lignin are removed,compared to other pretreatments such as dilute acid and acid-catalyzed steam explosionthat remove large portions of hemicellulose [42]. Generation of inhibitors is moderate,and the remaining ammonia in the biomass increases the nitrogen content, which is a neces-sary nutrient for the fermentation [213]. Nearly quantitative sugar yields were demonstratedfor AFEX-pretreated coastal bermuda grass and bagasse at a low enzyme loading (5 IU/g),while newspaper was more resistant to enzymatic hydrolysis [211]. When AFEX wasapplied to lignocellulosic municipal solid waste (paper), the digestibility was only slightlyincreased because of the delignification of biomass during the papermaking process [214].Low-temperature AFEX of coastal bermuda grass at 32 �C, even after three consecutive AFEXpretreatments, gave only 53% of total sugar yields, whereas temperatures of 90 �C were ableto convert over 90% of cellulose and hemicellulose to sugars [215]. AFEX-pretreated rice strawwas not able to yield more than 45% of total sugars after enzymatic hydrolysis even at exces-sive enzyme loadings of 67-100 FPU/g, compared to complete glucose conversions achievedwith acid and steam pretreatment of rice straw [42]. In another study, however, theAFEX pretreatment of rice straw achieved 80% yields for both xylose and glucose [216]. Suchinconsistency in results indicates that careful selection and optimization of process parame-ters, such as ammonia loading, moisture content of biomass, temperature, and residencetime, are required for maximum enzymatic digestibility of AFEX-treated biomass [217].By optimization of these parameters at 90 �C, 60% biomass moisture content, 1:1 (w/w) am-monia:dry biomass loading, and 5 min treatment time, led to 85% enzymatic conversion of


corn stoverwith an enzyme loading of 7 FPU/g,while 96% ethanol yieldwas obtained duringSSF with 15 FPU/g enzyme loading [118] (Table 7-4). This ability to reduce enzymerequirements represents one of the main advantages of the AFEX process, since reducingthe cost/loading of enzymes represents one of the main bottlenecks in commercial second-generation bioethanol production [23]. Near-theoretical enzymatic yields were achievedfor ryegrass straw, corn fiber, and switchgrass at very low enzyme loadings (1-5 IU/g),highlighting the high potential of AFEX as a pretreatment method [218].

Pretreatment conditions of 1:1 kg ammonia:kg biomass loading, for 80% moisture contentand 100 �C for 5 min, were used in the pretreatment of switch grass with almost 93% celluloseconversion and 70% xylan conversion at 15 FPU/g enzyme loading [219]. Similar results werereported in a comparative study of various technologies for the pretreatment of switchgrass,with about 80% and 70% of glucose and xylose yields, respectively, but AFEX performancewas inferior to that of dilute acid and steam explosion [143]. Harvesting seasons and locationwere found to have an effect on sugar release but not on the enzyme requirements ofAFEX-pretreated switchgrass, suggesting the need for an integrated approach comprisingagricultural production and process development for optimal productivity [220]. In suchan integrated approach to addressing supply chain challenges of bioethanol production,AFEX was assessed as a profitable pretreatment method of a local biomass processing depot[221]. In the same context, a low-temperature and long-residence-time AFEX has been pro-posed as a decentralized low-cost pretreatment process. It was shown that pretreatment ofcorn stover at 40 �C for 8 h could release over 90% of the glucose and nearly 60% of xyloseafter 72 h of hydrolysis, yields that are comparable to those obtained with AFEX at high tem-peratures [222].

Supercritical CO2 explosion. Another method combining a physical effect—the explosivepressure release—but in the presence of a chemical agent/catalyst is the supercritical CO2

explosion operated at lower temperatures than steam explosion and lower cost than AFEX[223]. The main advantages of CO2 are that it is environmentally friendly, is less expensive,and is easily recovered after use [113] (Table 7-3). The explosive CO2 pretreatment of purecellulose in the presence of an aqueous phosphate buffer induced a reduction in cellulosecrystallinity and an increase in enzymatic digestibility [223]. The structure of lignocellulosicbiomass is altered extensively, with anomalous porosity lamellar structures being reportedfor rice straw [224]. Supercritical states of CO2 and high pressure are beneficial, probably be-cause of better and faster diffusivity of the gas in the biomass. Similar increases in enzymatichydrolysis and ethanol productionwere obtained in the case of recycled papermix, repulpingwaste of recycled paper, and sugarcane bagasse. No inhibitors were formed because ofthe low pretreatment temperature [225]. Explosive pretreatment of wheat straw, bagasse,and Eucalyptus regnans woodchips at 200 �C and CO2 pressures of 3.45-13.8 MPa led to cellu-lose digestibilities of 81%, 78%, and 75%, respectively [226]. A high moisture content of thebiomass is beneficial to the effectiveness of the process. When the moisture content of aspenwood (hardwood) was increased from 0% to 75%, the glucose yield increased from around12%—which was similar to the yield of the untreated sample—to 79.4%. The maximumglucose yield of 36.6% for southern yellow pine (softwood) was achieved at 57% moisture.Increasing the pressure beyond a point had a negative effect on enzymatic digestibilityand this was attributed to an increased dissolution of water from the biomass in the super-critical CO2, leading to a reduction of biomass moisture content [113]. Pretreatment of rice

2417.5 COMBINING CHEMICAL CATALYSIS WITH PHYSICAL METHODS

straw at varying pressure (10-30 MPa), temperature (40-110 �C), and time (15-45 min) showedonly a marginal increase in enzymatic digestibility from 28% to 32% [224]. In the supercriticalCO2 explosion of corn stover and switchgrass, a positive impact of increased pretreatmenttime, pressure, and temperature on glucose yield was observed, but the highest glucose yieldachieved was only 30% compared to 12% for the untreated material [227].

Sulfite pretreatment. Sulfite pretreatment to overcome the recalcitrance of lignocellulose(SPORL) is a pretreatment method that combines the chemical treatment of lignocelluloseat high temperatures in the presence of sulfite and sulfuric acid, with the mechanical size re-duction effect of disk refining. Besides sulfite, the chemical agent/catalyst used can be bisul-fite, sulfur dioxide, or combinations of the three according to the desirable process pH. Sulfitepretreatment of lignocellulosic biomass is based on the sulfite pulping process used inthe pulp and paper industry. SPORL is applied mainly in softwoods because of the moderateperformance of existing pretreatment technologies, such as alkaline, dilute acid, hot water/steam (explosion), ammonia, and organosolv, for this type of lignocellulosic biomass [228].Optimal results for the treatment of spruce and red pine were obtained with 8-10% bisulfiteand 1.8-3.7% sulfuric acid (based on untreated dry wood weight) at 180 �C for 30 min. Thispretreatment achieved near-complete hemicellulose solubilization, partial delignification,and sulfonation of lignin, which makes it more hydrophilic. The chemical pretreatmentsoftened the wood chips and enabled size reduction with low energy consumption, whileenzymatic hydrolysis achieved higher than 90% cellulose-to-glucose conversion. Fermenta-tion inhibitors, such as HMF and furfural, were formed in relatively small amounts of about5 and 1 mg/g of untreated dry wood weight, respectively [228]. SPORL exhibited a betterperformance compared to dilute sulfuric acid pretreatment of spruce at similar conditions,in terms of total sugar recovery (87.9% for SPORL and 56.7% for dilute acid), cellulosesugar recovery (92.5% from SPORL and 77.7% from dilute acid), and enzymatic hydrolysis(over 90% for SPORL and 55% for dilute acid) [229]. Fermentation inhibitors such as formicacid, acetic acid, furfural, HMF, and LA were all much lower in the case of SPORL,while the lignin portion removed (32%) was sulfonated and is considered as a potentialhigh-value coproduct. An ethanol yield of 72% was achieved with SPORL pretreatmentof lodgepole pine [230].

Although developed for softwoods, SPORL has been successfully applied for thepretreatment of hardwoods such as aspen, maple, and eucalyptus. Due to the large acetylcontent of hardwood species, the pretreatment could be performed without the addition ofsulfuric acid, which alleviates the need for a corrosion-resistant reactor or product neutrali-zation, while achieving a near-complete enzymatic conversion of cellulose to glucose and lowfermentation inhibitor concentration, as in the case of softwoods [231]. High tier ethanolproduction (59 g/L) at high solid loading (18%) was achieved by SSF of sulfite-pretreatedaspenwood, with 72% of the theoretical ethanol yield [232]. The SPORLmethod has also beensuccessfully used for the pretreatment of agricultural residues (corn stalk), with over 90%removal of hemicellulose and half dissolution of lignin achieved at pH 2.2-4.7, 7% sodiumbisulfite, 180 �C, and 30 min. Increasing the bisulfite loading can further increase hemicellu-lose and lignin removal, but at the expense of hemicellulose loss in the form of sugar degra-dation products, such as furfural and HMF. A 70% enzymatic conversion was achieved atthese conditions with low enzyme loadings of 5 FPU. Increasing the temperature to 190 �Cimproved the enzymatic hydrolysis to 81% [233].


7.6 OXIDATION CATALYSTS

Ozonolysis. All oxidative pretreatment methods aim at lignin removal from lignocellulosicbiomass (Table 7-5). Oxidative techniques were traditionally used for the bleaching of pulpsby removal and/or modification of lignin. Ozone has been shown to attack and subsequentlydegrade the aromatic ring structure of lignin [240]. Its advantages over other lignin removalmethods are that degradation is essentially limited to lignin (hemicelluloses are slightlyattacked and cellulose is hardly affected), it is usually carried out at room temperatureand pressure, and ozone can be easily decomposed using a catalyst or by increasing the tem-perature [234] (Table 7-5). Wheat straw was treated with 1 g ozone per 5 g of biomass untilbleached, which led up to 70% organic matter digestibility [241]. Increasing the concentra-tions of ozone and biomass was shown to have a positive effect on the enzymatic digestibilityof pretreated bagasse, wheat straw, eucalyptus, and pine. The samples were partiallybleached at 3% ozone and turned white at 15%, at which concentration, low pH values frombiomass-derived organic acids and sugars were detected in the biomass water extracts. Highenzymatic digestibilities were achieved depending on feedstock type with optimum valuesfor biomass water content (25-35%), ozone concentration (2-6%), and process times (1-2 h)[242]. In the pretreatment of cotton stalks, ozonation led to a very small (11.9-16.6%) reduction

TABLE 7-5 Pretreatment Methods with Oxidation Catalysts

Pretreatment


Temperature

(�C)Time

(min)

Lignin

removal (%)

Sugars (%) Ethanol

yield

(%) ReferenceXylose Glucose

Ozonolysis O3 (65 ppm) Poplar R.T. 180 66 – 100 – [234]

Alkalinehydrogenperoxide

H2O2 (1%)/NaOH(pH�11.5)

Corn cobs,stalks, andhusks

R.T. 24 h >50 – >95 >90 [235]

Wetoxidation

O2 (12 bar) Commonreed(Phragmites

australis)

195 12 58.3 (plus51.7% ofhemicellulose)

– 82.4 73.2 [236]

Peraceticacid

20% Sugarcane R.T. 7days

66 – 93.1 91.9 [237]

20% Bagasse/hybridpoplar

R.T. 7days

61 – 98.3 92.8 [238]

Enzymaticoxidationþalkalineextraction

Trametesvillosa

laccase50 U g�1,2.5% HBT

Eucalyptusglobulus

50 24 h 48 10.5 47.3 12.3 [239]

Pennisetumpurpureum

50 24 h 32 16.3 54.4 16.2 [239]


of lignin. This was attributed to insufficient ozonation under the experimental conditionsused, which were 10% (w/v) mixture of cotton stalks and deionized water for 30, 60, and90 min at 4 �C [243]. A fixed-bed reactor was shown to be more effective than a stirredsemibatch reactor for the ozonation of poplar sawdust. Again, water content was shown tobe the most important parameter of the pretreatment, with 75%moisture content performingbetter than 30%. Delignification reached 66%, and maximum theoretical glucose yield wasachieved with the enzymatic hydrolysis of pretreated biomass. Ozonolysis of wheat andrye straw in a fixed-bed reactor led to enzymatic hydrolysis yields of 88.6% and 57%, respec-tively, although similar amounts of residual lignin were measured in both pretreated feed-stocks. Moisture content was found to be significant, up to 30%, and addition of alkalireduced delignification and increased cellulose and hemicellulose degradation [244].

Alkaline hydrogen peroxide.Agricultural residues such as wheat straw and corn stover weretreated at 25 �C in an alkaline solution of hydrogen peroxide, resulting in 50% removal of lig-nin and 100% enzymatic saccharification. Optimal conditions were 0.25 g H2O2/g substrateand pH of 11.5 adjusted by NaOH [245]. A mechanism was suggested in which H2O2 decom-position products such as �OH and O2

�� are the primary lignin oxidizing species [246]. Theethanol efficiency was over 90% based on cellulose for corn cobs, stalks, and husks butwas much less for kenaf and oak feedstocks. Lack of inhibition in the pretreatment superna-tants was also demonstrated [235] (Table 7-5). Disintegration of particles was observedafter the completion of the alkaline hydrogen peroxide pretreatment, accompanied by fiberbundle disruption and increase in water absorbance from the pretreatment pulp by 300%[246]. By increasing the pretreatment temperature of rye straw up to 70 �C, removal of ligninand hemicellulose as high as 88% and 72%, respectively, was achieved, and the pretreatmentat the employed conditions did not affect the overall structure of the isolated pure ligninfractions [247]. Hydrogen peroxide treatment of hydrothermally pretreated rice husks ledto significant delignification and enzymatic conversions of cellulose to glucose in the rangeof 60-80% [248]. The fractionation of the three lignocellulose components of the corn cobs/corn stover mixture was achieved with ammonia-hydrogen peroxide pretreatment in apacked-bed flow-through-type reactor, with recycling of ammonia. The extent ofdelignification was 90-94% and that of hemicellulose removal was 80%, with little decompo-sition of sugars [249]. A two-step pretreatment process combining alkaline peroxide in thefirst step and electrolyzed water—which also exhibits an oxidizing effect—in the second stepwas used for the pretreatment of Miscanthus biomass. Around 63% hemicellulose and 64%lignin were removed by the alkaline peroxide treatment, while the subsequent use of electro-lyzed water led to the removal of residual hemicellulose and lignin, further exposing fibersand making them more susceptible to enzymatic hydrolysis. Cellulose digestibility ofthe two-stage pretreated biomass reached 95% [250]. In another combined method, alkalineperoxide was used following a biological pretreatment with the white rot fungusEchinodontium taxodi, improving delignification and cellulose desorption from biomass andincreasing the initial enzymatic hydrolysis rate and sugar yield [251].

Wet oxidation.Wet oxidation is the process of treating lignocellulosicmaterial withwater andair or oxygen at temperatures above 120 �C. At low temperatures—up to 160 �C—hydrolyticreactions are more prominent compared to oxidation reactions and lead to the solubilizationof hemicellulose in the form of oligosaccharides and the partial solubilization of lignin. Thehydrolytic reactions are catalyzed by the formation of small amounts of organic acid, mainly


acetic acid, from the deacetylation of hemicellulose [252]. At higher temperatures, oxidationbecomes more prevalent, and destruction of hemicellulosic sugars as well as hydrolysis ofthe more recalcitrant cellulose also takes place. Under high temperatures, the biomass isconverted to acetic and formic acids; a mixture of organic acids such as erythronic, threonic,succinic, glyceric, and glycolic; methanol; gases such as carbon dioxide; and an insoluble solidmade up of inorganic ash and organic material [253]. Hardwood lignin is more readily solubi-lized even at lower temperatures, whereas softwood lignin is more difficult to remove even athigher temperatures. As much as 75% of hemicellulose in the form of sugars was recovered inthe treatment of loblolly pine and black oak, and the acid digestibility of the cellulose-richpretreated biomass was increased. Increasing the oxygen pressure increases the rate of acidformation and the rate of lignin breakdown, and yields can be increased with longerpretreatment times. Addition of ferric sulfate catalyst also increased acid formation and woodsolubilization [252].

Wheat straw was treated with the wet oxidation method with the addition of Na2CO3.At 170 �C, 65% of lignin and most of the hemicellulose were removed from the biomass (with45% hemicellulose recovered as sugars in the liquid) and 85% of the cellulose was enzymat-ically converted to glucose. At higher temperatures, cellulose removal also became significant(>20%). Acetic, formic, glycolic, isobutyric, and oxalic acids detected in the liquids wereattributed to the oxidation of lignin and hemicellulose, and 10% loss of biomass was attrib-uted to complete oxidation to CO2 and H2O. Furfural and HMF were not detected, and thesugar-rich filtrates could be used as carbon source for A. niger cultures without the need fordetoxification [254]. Addition of Na2CO3 was found to decrease the yield of solubilized hemi-cellulose sugars, but it also decreased the formation of inhibitors such as furfural bymaintaining a near-neutral pH of the filtrates. Other inhibitors such as lignin degradationproducts were, however, produced and were attributed to the oxidative action of oxygen, be-cause, when oxygen was removed from the process, ethanol yields were increased as a resultof the lack of inhibition, but at the expense of lower sugar yields [152]. In a further study ofwheat strawwet oxidation, process temperature was found to be amore important parameterthan time and oxygen pressure. At the optimal temperature of 185 �C, 95-100% of celluloseand 60% of hemicellulose were recovered, and 55% of lignin and 80% of hemicellulose weresolubilized, with cellulose being practically insoluble. A pseudo-first-order kineticmodelwasemployed to describe the reactions of the three biomass components, with cellulose exhibitingthe lowest rate constant and hemicellulose the highest [255].

Wet oxidation was also evaluated for other agricultural residues such as sugarcanebagasse, rice hulls, cassava stalks, and peanut shells at 195 �C for 10 min, with 2 g/kg ofNa2CO3 and an oxygen pressure of 3 or 12 bar. At these conditions, the pretreatment resultswere influenced by the type of biomass. Sugarcane bagasse showed the best resultsexhibiting large xylan solubilization, with 45.2% recovered as xylose and xylo-oligosaccha-rides in the liquid fraction, and 56.5% of the cellulose contained in the raw bagasseconverted to glucose by enzymatic hydrolysis. For the rest of the materials, wet oxidationdid not perform well under the employed conditions; rice hulls, for example, exhibited only16% enzymatic cellulose conversion [256]. Wet oxidation was successfully applied for thepretreatment of maize silage with 70% of xylan and a significant amount of lignin solubi-lized, and cellulose enzymatic conversion increasing from 60% to 90% for the pretreated ma-terial. When the pretreated maize silage was fermented together with anaerobically digested


and wet oxidized manure (which was added as a nutrient, nitrogen, and water supply), anethanol yield of 82% was reached [257]. Common reed (Phragmites australis) was investi-gated as a lignocellulosic feedstock for the production of ethanol. Wet oxidation at optimalconditions led to the solubilization of 51.7% of hemicellulose and 58.3% of lignin as well asan enzymatic conversion of cellulose to glucose of 82%. SSF resulted in 73% of the theoreticalethanol yield [236] (Table 7-5).

Peracetic acid. Peracetic acid is produced by the reaction of hydrogen peroxide and aceticacid. It has been used for the pretreatment of sugarcane bagasse with concentrations as highas 60% (w/w) peracetic acid/biomass for a period of 1 week at room temperature [237](Table 7-5). Xylanwasmostly unaffected by the pretreatment, showing that peracetic acid actsspecifically on lignin. With the 60% peracetic acid treatment, 100% enzymatic conversion ofcellulose to sugars was possible in 24 h. With 20% peracetic acid, 93.1% of cellulose wasconverted to glucose during a 120-h enzymatic hydrolysis. A pre-pretreatment with theaddition of alkali (NaOH) could decrease the peracetic acid loading to 15% and 9% and stillobtain sugar yields in the range of 80-90%. Similar results were obtained in the peracetic acidpretreatment of hybrid poplar [237]. The enzymatic hydrolysis of cellulose was improved bythe addition of xylanases to the enzyme mixture, because they facilitate xylan removal fromthe biomass, exposing cellulose fibers to the action of cellulolytic enzymes [237]. SSCF ofperacetic acid-treated hybrid poplar and sugarcane bagasse yielded an average of 92.8%and 91.9% ethanol, respectively, based on glucan and xylan content [238] (Table 7-5). Itwas also shown that the alkaline pre-pretreatment step removes acetyl units from the biomassand has a beneficial effect on enzymatic saccharification yields. The reduction of peraceticacid loading is also beneficial since, at peracetic acid concentrations higher than 20%, the pos-itive increase in enzymatic conversion is balanced by the observed growth inhibition of thefermentative microorganism due probably to lignin degradation products [238]. Peraceticacid treatment gave superior results compared to NaOH and dilute sulfuric acid pretreat-ments of sugarcane bagasse. It was also found that parameters such as peracetic acid charge,liquid/solid ratio, temperature, and time had a significant effect on cellulose conversionby peracetic acid pretreatment [258]. Despite its very good performance as a pretreatmentreagent, peracetic acid is expensive and can be explosive in high concentrations. To reducestorage cost and safety risks, the in situ generation of peracetic acid catalyzed by hydrolaseenzymes was reported [259]. With this method, 60-70 mM peracetic acid was produced, avery low concentration that necessitated multiple pretreatment cycles (up to eight cycles),leading, however, to up to 61.7% removal of lignin and 90% sugar yield in the enzymatic sac-charification [259]. Improvements of this method include a 10-fold reduction in the amount ofenzyme used by the use of amore efficient variant of the enzyme and the reuse of the enzyme.Also, increasing the reaction temperature and time, as well as the volume of peracetic acidreduced the number of pretreatment cycles from eight to one [260].

Enzymatic oxidation. The ligninolytic enzyme system of white rot fungi has attracted muchattention because of its ability to selectively remove lignin. The most important componentsinclude oxidative enzyme families such as lignin peroxidases (LiPs), manganese peroxidases,and laccases, as well as ancillary components such as glyoxal oxidase and veratryl alcohol[261,262]. Since these enzymes are large molecules that cannot penetrate the plant cell walleasily, the oxidation is achieved through oxidized mediator molecules, which in turn oxidizelignin and are regenerated/reoxidized by the enzymes in redox cycles (Figure 7-7).


Lignin peroxidases are heme-containing glucoproteins with iron in the Fe(III) state, and arestrong oxidizers that can oxidize phenols, aromatic amines, aromatic ethers, and polycyclicaromatic hydrocarbons [261]. LiPs are oxidized by H2O2, and in the oxidized state the iron ispresent as Fe(IV), which in turn oxidizes lignin and returns to the Fe(III) state. The H2O2 isproduced by the ancillary glyoxal oxidase enzyme. The role of veratryl alcohol, a fungal met-abolic product, is thought to be the protection of the enzyme from further oxidation to aninactivated form. Mn peroxidases (MnP), like LiPs, are heme-containing glycoproteins thatrequire H2O2 in order to function. They oxidize manganese (Mn2þ) ions into manganese(Mn3þ), which are stabilized by fungal chelators such as oxalic acid. Chelated Mn3þ in turnacts as a low-molecular weight, diffusible redox-mediator that attacks phenolic lignin struc-tures [263]. These stabilized Mn3þ mediators, however, are not very strong oxidants, and areunable to achieve extensive ligninolysis [264]. Laccases are multi-copper-containing enzymesthat oxidize phenolic compounds [261]. They reduce molecular oxygen to water by oxidationof an aromatic substrate. They can oxidize lignin directly or act through an oxidizedmediator, which can be phenolic compounds found in lignocellulosic materials such assyringaldehyde, acetosyringone, vanillin, acetovanillone, methyl vanillate, and p-coumaricacid [265]. Laccases are among the most promising enzymes for industrial applications, such

Mediator

MediatorOX LaccaseLaccase O2

H2O

H2O

H2O2

H2O2O2Veratrylalcohol

Glyoxal oxidaseGlyoxilic acidGlyoxal

O2

H2O LaccaseOXLaccaseOX LigninOX

LigninOX

Lignin

Mn2+ MnPOX

MnPMn3+

LigninOX

LigninOX

C

A

B

D

LiP LiPOX

Lignin

Lignin

Lignin

FIGURE 7-7 Mechanism of lignin oxidation by fungal oxidases: (A) Oxidation by lignin peroxidases (LiP), (B)oxidation by manganese peroxidases (MnP), (C) direct oxidation by laccases, and (D) oxidation by laccases througha mediator. Reproduced from Ref. [261] with permission from Elsevier.


as forest industry. These applications include biopulping, biobleaching of lignocellulosic pulp,enhancement of paper strength, pitch control, deinking of recycled paper, mill process waterand effluent treatment, biografting, fiber/lignin cross-linking, and delignification of lignocellu-lose feedstocks for ethanol production [265–267]. The use of laccase-mediated delignification oflignocellulosic substrates to enhance enzymatic digestibility and ethanol production has beeninvestigated.A laccase fromTrametes villosawith 1-hydroxybenzotriazole asmediatorwas usedtogether with alkaline extraction for the delignification of wood (E. globulus) and nonwood(Pennisetum purpureum) feedstocks, with 48% and 32% lignin removed, respectively, whileenzymatic hydrolysis reached 58% and 71% of sample weight, respectively. Improved ethanolproduction was also reported [239] (Table 7-5). Laccase from Trametes hirsute, withN-hydroxy-N-phenylacetamide and its acetylated precursor used as mediators, was used for thedelignification of steam-pretreated softwood, and a 20% increase of enzymatic saccharificationwas achieved [268]. Sclerotium sp. laccase was used for the delignification of steam-explodedwheat straw. The highest cellulose conversion was 84.2%, which was achieved by the use ofsteam explosion followed by laccase treatment. This cellulose conversion level correspondedto an increase of 16.8% compared to the straw onwhich no laccase treatment was applied [269].

7.7 SOLID ACID CATALYSTS

The replacement of homogeneous catalysts in the liquid phase with solid catalysts hascertain advantages. The solid catalysts are usually nontoxic and noncorrosive, compared,for example, to strong inorganic acids and bases, thus eliminating the need for expensive cor-rosive-resistant equipment and special handling of hazardous chemicals. Furthermore, thesolid catalysts can be easily separated from the liquid phase and reused for several catalyticcycles, thus reducing the cost of the process. The use of solid catalysts, therefore, contributesto the development of safer and environmentally friendly processes according to the princi-ples of Green Chemistry and Green Technology [270,271]. Their performance, however, maybe inferior to that of homogeneous catalysts in terms of conversion, selectivity, and yield.

In the case of the pretreatment of lignocellulosic materials, the main obstacle to the use ofsolid catalysts with acidic, basic, or oxidative properties is the difficulty of separation of thesolid catalyst from the solid pretreated biomass. For this reason, little or no work has beenpublished in this area, to date. A possible combined approach comprises the hydrothermalpretreatment of lignocellulosic biomass for the selective removal of hemicellulose in the pres-ence of various acidic catalysts (i.e., zeolites) and the subsequent fast pyrolysis of the pretreatedbiomass containing the acidic catalyst. In this context, it was shown that the catalytic ZSM-5zeolite fast pyrolysis of hydrothermally pretreated beech wood provided a pyrolysis oil(bio-oil) with improved composition/properties (i.e., less carboxylic acids andmore aromatics)[8]. Solid catalysts are being used, however, instead of acids or enzymes for the conversion ofthe lignocellulosic carbohydrates, i.e., cellulose and hemicellulose, or the corresponding oligo-saccharides, into monomeric sugars [272–276]. Solid catalysts have also been used for the con-version of lignocellulosic sugars to platform chemicals and fuel precursors, such as furfuralfrom xylose or xylan [277–279], and 5-hydroxymethyl furfural and other furans from biomassor glucose [280–283]. A detailed review of this type of reaction for converting biomass sugarsinto platform chemicals and fuel precursors is provided in Chapter 8 [24].


7.8 IONIC LIQUIDS

Ionic liquids (ILs) are organic salts with a low melting point, below 100 �C, which areusually liquid at room temperature and have very low volatility, high thermal, and chemicalstability. These are features that have led to their classification as “green solvents.” Their maindrawbacks are their high density and viscosity, both higher than those of normal organic sol-vents. The cationic component of an IL is organic and contains heteroatoms, while the anioniccomponent is either inorganic or organic (Figure 7-8). Their aforementioned properties renderILs attractive solvents, as well as media for catalytic or noncatalytic reactions [284]. Lately,there has been an increase in the use of ILs for the pretreatment of lignocellulosic biomasstoward the production of chemicals and/or fuels, where the ILs act either as delignificationagents or as solvents with simultaneous catalytic function for the degradation of biomasscomponents, e.g., hydrolysis of cellulose to oligomers and sugars and/or subsequent dehy-dration of sugars to furans [285,286]. In effect, the use of ILs aims at the fractionation of bio-mass into its structural components with the simultaneous reduction of cellulose crystallinityand increased porosity of the retrieved treated solid [287]. The effect of ILs on biomassdepends on (a) the IL’s structure and properties; (b) the biomass’s structure (recalcitrance),composition, and particle size; and (c) the pretreatment conditions employed, e.g., tempera-ture, time, and use of catalysts.

In order to fully exploit the potential of biomass as a source of chemicals and fuels, itsthree component biopolymers (hemicellulose, cellulose, and lignin) have to be separatedfrom one another and their intramolecular bonds have to be disrupted, leading to theproduction of monomeric sugars and phenolic molecules. The chemical structure of ILsin combination with their hydrogen-bonding capacity and ionic strength allows them tointeract with the biomass biopolymers via physical and chemical interactions, as isevidenced by the structural modification of the retrieved biopolymers [284,286]. The phys-ical interaction of ILs and biomass is dictated by the extent of diffusion of IL moleculeswithin the biomass; therefore, the smaller the biomass particles, the better [288,289].The chemical effect is the actual breaking of the covalent bonds and the hydrogen bonds

Imidazolium 1,8-diazabicyclo[5.4.0]undec-7-eniumPyridinium Pyridinium

Cholinium Phosphonium Ammonium

Cl-, Br-, I-

Al2Cl7-

PO43-, HSO4

-, SO4-

PF6-, SbF6-, BF4

-

COOH-, CH3CH(OH)COO-

(CF3SO2)2N-, N(CN)2

-, (CF3SO2)3C-

CF3SO3-, ROSO3

-, CF3CO2-, C6H5SO3

-

N+ NR1

R4

R3

R2

N+

R1 R2 N+

R2

R1

N

NH+

N+

R4

R3R1

R2

P+

R4

R3R1

R2

N+R1

R4

R3

OH

FIGURE 7-8 Main cations and anions of ILs that have been used for the treatment of lignocellulosic biomass andits constituents.

2497.8 IONIC LIQUIDS

not only between the three biopolymers but also within their structure. Recently, thedissolution of pine wood and wheat straw in three imidazolium-based ILs was tested[290], and it was concluded that lignocellulosic biomass can actually be hydrolyzed byan IL releasing its biopolymers, which in turn are dissolved in the IL phase. In addition,the released acetic acid (from the acetyl units of hemicellulose) promotes further thehydrolysis reactions.

The solvolytic properties of ILs depend on the structure and physicochemical properties oftheir anion and cation. This allows the ILs to be tuned and tailored to each specific application.The effect of the cation on the cellulose dissolution has been extensively reviewed[284,286,291] and it was concluded that the smaller the cation, the highest its ability to dis-solve cellulose. Long alkyl chains, especially if they have an odd number of C atoms attached,e.g, to the imidazolium ring, result in decreasing ILs’ ability to form hydrogen bonds withcellulose. Accordingly, the presence of substituents on the ring with the ability to formhydrogen bonds (i.e., alkyl chains with functional groups) may enhance the interaction withcellulose, leading to increased dissolution [292], or it may have an opposite effect due to in-creased hydrogen bonding with the anions of the IL [293]. The organic cations also interactwith the biopolymers via p-p interactions, especially with the aromatic rings of lignin. How-ever, the anion affects the ILs’ properties and is actually considered to have the most activerole in lignocellulosic biomass dissolution. For example, in the case of cellulose, theanion acts as an H-bond acceptor and interacts with the dOH groups of cellulose, formingan electron-donor acceptor complex, effectively dissociating the molecular chains [293](Figure 7-9).

The less crystalline and cross-linked the biopolymer, the more easily it is dissolved in theIL. Out of the three biopolymers, hemicellulose has the most amorphous regions and lesscross-linking and, consequently, it is more easily dissolved in the ILs. It is easily dissolvedin temperatures below 100 �C and may be recovered almost quantitatively [294]. At highertemperatures and longer residence times, hemicellulose can be easily hydrolyzed to its mo-nomeric sugars [295]. The disruptive effect that ILs have on the hydrogen bonds keeping the

Cellulose

Cellulose

Cellulose

Cellulose

Cellulose

[C4mim]d+CId

-

[C4mim]d+

[C4mim]d+

[C4mim]d+

O

O

O

H

H

H

Cellulose

Solvent

+HO OH

OH

CId-

CId-

CId-

FIGURE 7-9 Dissolutionmechanism of cellulose in ILs.Reproduced from Ref. [293] with permission from Elsevier B.V.


cellulose fibrils together results eventually in the decrease of cellulose’s crystallinity, turningit into an amorphous biopolymer [296,297]. In this form, cellulose is more amenable to furthertreatment, either enzymatic or catalytic, toward the production of chemicals. The dissolutionof cellulose is accompanied by partial hydrolysis, which may be negligible when mild con-ditions are employed. The aromatic nature of lignin renders it very insoluble unless oxidativeconditions are employed. Studies based on the selective dissolution of lignin have concludedthat sulfonate anions are more effective [298] as well as fairly selective, since the cellulose andhemicellulose are only slightly affected. An analogous effect was also demonstrated by animidazolium-based IL bearing CH3COO� as an anion (1-ethyl-3-methylimidazolium acetate,EmimAc), whose effect on untreatedmaple wood flour was the dissolution of more than 40%of lignin after treatment for 24 h at 80 �C [299], while on rice hulls, the lignin removal wasalmost 100% at above 100 �C without important loss of cellulose [300]. The effect of theemployed conditions on biomass pretreatment with ILs is highlighted by the fact that, whenEmim Ac was used on yellow poplar powder at 60 �C, even after 72 h of treatment, the massloss observed was minimal (less than 5%), but its crystallinity was reduced and, whenthe recovered biomass was enzymatically hydrolyzed, it had a conversion yield >85%[301]. Emim cation with different anions has also been tested for the dissolution of cassavapulp residue and rice husks at temperatures ranging between 25 and 180 �C. Up to 120 �C,the percentage of recovered biomass was more than 70%, with very high yields of extractedlignin and a significant reduction in cellulose crystallinity. At higher temperatures,however, adverse effects were observed, probably because of biomass char formation[302]. The changes in the crystallinity and composition of the recovered biomass after dis-solution in ILs have been also shown to affect the fast pyrolysis process for the production ofbio-oil [303].

In addition to the role of ILs as solvents of biomass aiming at the separation of its maincomponents, their simultaneous catalytic function toward depolymerization of the bio-mass biopolymers or subsequent degradation of monomeric sugars can also be important.Such a catalytic effect is usually observed for functionalized ILs, such as the SO3H-functionalized ILs. COOH� and SO3H

� moieties were attached to the imidazolium ringand the resulting ILs were used for the dissolution of bagasse as catalysts, acting syner-gistically with hot compressed water, to promote the decomposition of bagasse to small,water-soluble compounds (alcohols, ketones, acids, etc.) [304]. Analogous functionalizedILs were employed for corn stalk dissolution and it was concluded that the depolymeri-zation reaction was possibly further promoted to sugar formation and subsequent degra-dation [305]. It has been recently demonstrated that the IL 1-H-3-methylimidazoliumchloride may act simultaneously as a solvent and catalyst since it can efficiently dissolveand depolymerize lignin at temperatures between 110 and 150 �C. The IL attacks alkyl-arylether linkages, via a hydrolysis reaction, resulting in lignin oligomers with significantlyreduced MWs [306].

In ILs, the addition of a solid catalyst promotes the dissolution and hydrolysis reactionseven further, leading to the production of furans, such as HMF, and acids, such as LA, whichare very valuable chemicals. It has been suggested that the above-mentioned moieties, espe-cially the sulfonic, can not only disrupt the H-bonds and glycosidic bonds in cellulose but,in the presence of a metal ion, can also form complexes that promote the conversion ofa-anomers of glucose to b-anomers, resulting eventually in the increased production of

2517.8 IONIC LIQUIDS

valuable HMF [307]. Some types of catalysts employed together with ILs are sulfonic resins[308], metal chlorides [309,310], inorganic acids such asHCl, H2SO4, HNO3, H3PO4 [311], and,more recently, H-type zeolites [312].

The pretreated/dissolved biomass is retrieved from the ILmedium usually by the additionof an antisolvent. Water, alcohols (e.g., methanol or ethanol), acetone, or a mixture of waterand alcohol/acetone is often used. The treated biomass precipitates and is removed by filtra-tion, while the IL is distilled and dried to remove the water [286,313,314]. The addition of akosmotropic salt such as K3PO4 has also been proposed, as it induces phase separation be-tween the IL and water [315]. In cases of lignin dissolution, its retrieval might even requirean additional evaporation step. At the end of the process, ILs require extensive washing anddrying prior to reuse, in order to maintain their properties. Furthermore, the number of timesthey can be recycled is limited.

The interest in using ILs in biomass pretreatment has been constantly increasing over thelast few years. Their tunable properties, along with the fact that they are environmentally be-nign and have the ability to effectively dissolve biomass or even depolymerize its compo-nents, i.e., biopolymers, to smaller molecules, offer new opportunities in biomassvalorization toward fuels and chemicals. However, the problems related to their cost andrecycling efficiency need to be addressed if they are to be used in an industrial-scale processfor biomass pretreatment.

7.9 SUMMARY AND OUTLOOK

The physicochemical or biological pretreatment of lignocellulosic biomass is the first impor-tant process step in the conversion of biomass to fuels and chemicals via the biochemical route,which is based on the enzymatic hydrolysis of biomass carbohydrates tomonomeric sugars andtheir fermentation by appropriate microorganisms to ethanol or butanol. The pretreatmentaims mainly at the destruction of the recalcitrant microfibril structure and the controlled sep-aration of the three main individual component biopolymers of biomass, i.e., hemicellulose,cellulose, and lignin.Depending on the targeted application and considering the best utilizationof the whole biomass, these biopolymers can be recovered as solids with altered composition,structure, and morphology, or as solubilized oligomers, or as monomeric C5 and C6 sugars(from hemicellulose and cellulose) and phenolics (from lignin). As such, the derivedpretreatment liquids can also serve as feed for a process scheme of converting xylose and glu-cose to platform chemicals and fuel precursors via a cascade type of catalytic reactions withacidic and/or metal-acid bifunctional catalysts.

In most pretreatment methods, a chemical or biochemical catalytic reaction or multiplereactions take place, depending on the nature of the agent that “catalyzes” the breakage ofthe bonds between the three biopolymers as well as of those within the biopolymers. Thesecatalytic agents can be in the liquid, solid, or gaseous state and can exhibit acidic, basic, oroxidative function. It is obvious that the selection of the most appropriate catalyticpretreatment method depends not only on the targeted application but also on the propertiesof the feed (i.e., biomass type) as well as on the ability to control the properties of the respec-tive catalyst in order to optimize product yields and selectivity. The most important biomass


pretreatment methods and catalyst types involved have been discussed in this chapter andare as follows: (1) inorganic and organic acids, (2) inorganic and organic bases, (3) self-catalyzed pretreatment (utilizing the in situ released acetic acid present in hemicellulose),(4) combined chemical catalysis with physical methods (mainly explosion), (5) oxidativecatalysts (including enzymes), (6) solid acid catalysts, and (7) ILs.

As already discussed extensively, every pretreatment method has its positive and negativeeffects with respect to conversion, selectivity, and yield issues, always in relation to the spe-cific product targeted. A stoichiometric conversion of cellulose to glucose and a high ethanolyield, potential utilization of hemicellulose sugars for the production of platform chemicals,and selective removal and isolation of lignin are, of course, of primary scientific and techno-logical importance, but future exploitation of the newmethods should always consider otherissues related to the environment, water and energy consumption, operational safety, andcosts. In this context, the hydrothermal pretreatment of biomass with pure water, either inthe form of steam or as hot compressedwater, which can utilize the inherently available aceticacid (especially in hardwoods), can be considered as a method that respects many ofthe “green chemistry principles.” Although this method has certain limitations, as it can onlyselectively remove hemicellulose in a more or less uncontrollable manner toward sugars anddegradation products, it can, nevertheless, serve as a basis for developing hybrid/combinedmethods where the “external” chemical or biochemical agent that is needed to fine-tuneactivity and selectivity would be minimum.

Combination of pretreatment techniques or even pretreatment catalysts may allow amore efficient fractionation and selective removal of biomass structural components,but this approach must also consider economic viability issues due to the addition of extracapital and operational costs. In the case of both single and combined pretreatmentmethods, the maximization of fermentable sugar yields and minimization of “losses”due to formation of undesirable degradation products that act as inhibitors in the fermen-tation step are very critical in the biomass-to-ethanol conversion process. A detoxificationstep prior to fermentation would increase the cost of the process. Minimization of enzymeloading necessary to achieve complete enzymatic saccharification of carbohydratesis greatly dependent on the outcome of the pretreatment method and is also very impor-tant, as enzyme prices represent an important fraction of the overall process cost.A pretreatment method that allows the SSCF of all available hemicellulose and cellulosesugars in one step and at low enzyme loadings, or the CBP that will additionally allow thein situ production of cellulolytic/hemicellulolytic enzymes, further reducing the processcost, is highly desirable.

The latest developments in the catalytic upgrading of monomeric sugars (i.e., xyloseand glucose) to platform chemicals (furfural, furfuryl alcohol, HMF, LA, etc.) and fuel rangehydrocarbons (i.e., C8þ alkenes and alkanes) via dehydration, hydrogenation, and condensa-tion reactions, as well as the catalytic conversion of lignin oligomers to phenolic molecules viapyrolysis or to hydrocarbonmolecules via hydrodeoxygenation, offermore alternatives in theselection of pretreatment methods since the biochemical production of ethanol is not theonly targeted route any more. This approach is in line with the “biorefinery” concept, whichrequires the utilization of the whole biomass and can benefit from any sustainable processthat produces high-added-value chemicals/fuels.


References

[1] P.S. Nigam, S. Anoop, Prog. Energy Combust. Sci. 37 (2011) 52–68.[2] E.M. Rubin, Nature 454 (2008) 841–845.[3] R.E.H. Sims, W. Mabee, J.N. Saddler, M. Taylor, Bioresour. Technol. 101 (2010) 1570–1580.[4] J.C. Serrano-Ruiz, J.A. Dumesic, Energy Environ. Sci. 4 (2011) 83–99.[5] R. Luque, A.R. de la Osa, J.M. Campelo, A.A. Romero, J.L. Valverde, P. Sanchez, Energy Environ. Sci. 5 (2012)

5186–5202.[6] V. Calemma, C. Gambaro Jr., W.O. Parker, R. Carbone, R. Giardino, P. Scorletti, Catal. Today 149 (2008) 40–46.[7] A.V. Bridgwater, Biomass Bioenergy 38 (2012) 68–94.[8] S. Stephanidis, C. Nitsos, K. Kalogiannis, E.F. Iliopoulou, A.A. Lappas, K.S. Triantafyllidis, Catal. Today 167

(2011) 37–45.[9] C. Weber, A. Farwick, F. Benisch, D. Brat, H. Dietz, T. Subtil, E. Boles, Appl. Microbiol. Biotechnol. 87 (2010)

1303–1315.[10] K.Q. Chen, M. Jiang, P. Wei, J.M. Yao, H. Wu, Appl. Biochem. Biotechnol. 160 (2010) 477–485.[11] D.B. Hodge, C. Andersson, K.A. Berglund, U. Rova, Enzyme Microb. Technol. 44 (2009) 309–316.[12] M. Jiang, D. Lei, K.Q. Chen, J. Li, J.M. Yao, P. Wei, Chin. J. Anal. Chem. 37 (2009) 605–608.[13] M.Al. Abdel-Rahman, Y. Tashiro, K. Sonomoto, J. Biotechnol. 156 (2011) 286–301.[14] E. Bruni, A.P. Jensen, I. Angelidaki, Bioresour. Technol. 101 (2010) 8713–8717.[15] D.B. Levin, C.R. Carere, N. Cicek, R. Sparling, Int. J. Hydrogen Energy 34 (2009) 7390–7403.[16] C.A. Cardona, O.J. Sanchez, Bioresour. Technol. 98 (2007) 2415–2457.[17] E. Tomas-Pejo, J.M. Oliva, M. Ballesteros, L. Olsson, Biotechnol. Bioeng. 100 (2008) 1122–1131.[18] K. Ojeda, E. Sanchez, M. El-Halwagi, V. Kafarov, Chem. Eng. J. 176-177 (2011) 195–201.[19] Q. Xu, A. Singh, M.E. Himmel, Curr. Opin. Biotechnol. 20 (2009) 364–371.[20] W.H. Van Zyl, L.R. Lynd, R. Den Haan, J.E. McBride, Adv. Biochem. Eng. Biotechnol. 108 (2007) 205–235.[21] T.A. Hsu, C.E. Wyman (Ed.), Handbook on Bioethanol: Production and Utilization, Taylor & Francis, Wash-

ington DC, 1996, pp. 179–212.[22] P. Alvira, E. Tomas-Pejo, M. Ballesteros, M.J. Negro, Bioresour. Technol. 101 (2010) 4851–4861.[23] C.E. Wyman, Trends Biotechnol. 25 (2007) 153–157.[24] E. Gurbuz, J.Q. Bond, J.A. Dumesic, Y. Roman-Leshkov, in: K.S. Triantafyllidis, A.A. Lappas, M. Stocker (Eds.),

The Role of Catalysis for the Production of Bio-fuels and Bio-chemicals, Elsevier, 2013, pp. 263–290.[25] P. Kumar, D.M. Barrett, M.J. Delwiche, P. Stroeve, Ind. Eng. Chem. Res. 48 (2009) 3713–3729.[26] I.S. Goldstein, J.M. Easter, Tappi J. 75 (1992) 135–140.[27] R.O. Lambert, M.R. Moorebulls, J.W. Barrier, Appl. Biochem. Biotechnol. 24-25 (1990) 773–783.[28] C.E. Wyman, Bioresour. Technol. 50 (1994) 3–16.[29] J. Heinonen, A. Tamminen, J. Uusitalo, T. Sainio, J. Chem. Technol. Biotechnol. 87 (2012) 689–696.[30] M. Sivers, G. Zacchi, Bioresour. Technol. 51 (1995) 43–52.[31] M. Laatikainen, J. Heinonen, T. Sainio, Sep. Purif. Technol. 80 (2011) 610–619.[32] J. Heinonen, T. Sainio, Ind. Eng. Chem. Res. 49 (2010) 2907–2915.[33] O. Yemiz, G. Mazza, Bioresour. Technol. 102 (2011) 7371–7378.[34] M.A. Kabel, G. Bos, J. Zeevalking, A.G.J. Voragen, H.A. Schols, Bioresour. Technol. 98 (2007) 2034–2042.[35] B.Y. Cai, J.P. Ge, H.Z. Ling, K.K. Cheng, W.X. Ping, Biomass Bioenergy 36 (2012) 250–257.[36] J.W. Lee, T.W. Jeffries, Bioresour. Technol. 102 (2011) 5884–5890.[37] H. Noureddini, J. Byun, Bioresour. Technol. 101 (2010) 1060–1067.[38] A. Esteghlalian, A.G. Hashimoto, J.J. Fenske, M.H. Penner, Bioresour. Technol. 59 (1997) 129–136.[39] K.H. Kim, M.P. Tucker, Q.A. Nguyen, Biotechnol. Prog. 18 (2002) 489–494.[40] L. Canilha, V.T.O. Santos, G.J.M. Rocha, J. Silva, M. Giulietti, S.S. Silva, M.G.A. Felipe, A. Ferraz,

A.M.F. Milagres, W. Carvalho, J. Ind. Microbiol. Biotechnol. 38 (2011) 1467–1475.[41] W.H. Chen, S.C. Ye, H.K. Sheen, Appl. Energy 93 (2012) 237–244.[42] E.Y. Vlasenko, H. Ding, J.M. Labavitch, S.P. Shoemaker, Bioresour. Technol. 59 (1997) 109–119.[43] W.H. Chen, B.L. Pen, C.T. Yu, W.S. Hwang, Bioresour. Technol. 102 (2011) 2916–2924.[44] M. Han, Y. Kim, Y. Kim, B. Chung, G.W. Choi, Korean J. Chem. Eng. 28 (2011) 119–125.[45] A. Asli, A.I. Qatibi, Biotechnol. Bioprocess Eng. 14 (2009) 118–122.[46] M. Cuevas, S. Sanchez, V. Bravo, J.F. Garcia, J. Baeza, C. Parra, J. Freer, Fuel 89 (2010) 2891–2896.


[47] P. Manzanares, M.J. Negro, J.M. Oliva, F. Saez, I. Ballesteros, M. Ballesteros, C. Cara, E. Castro, E. Ruiz, J. Chem.Technol. Biotechnol. 86 (2011) 881–887.

[48] D. Zhang, Y.L. Ong, Z. Li, J.C. Wu, Chem. Eng. J. 181-182 (2012) 636–642.[49] A.H. Conner, B.F. Wood, C.G. Hill, J.F. Harris, J. Wood Chem. Technol. 5 (1985) 461–489.[50] B.Y. Chung, J.T. Lee, H.W. Bai, U.J. Kim, H.J. Bae, S.G. Wi, J.Y. Cho, Radiat. Phys. Chem. 81 (2012) 1003–1007.[51] R.W. Torget, J.S. Kim, Y.Y. Lee, Ind. Eng. Chem. Res. 39 (2000) 2817–2825.[52] S. Ferreira, N. Gil, J.A. Queiroz, A.P. Duarte, F.C. Domingues, Bioresour. Technol. 102 (2011) 4766–4773.[53] B.S. Dien, H.J.G. Jung, K.P. Vogel, M.D. Casler, J.F.S. Lamb, L. Iten, R.B. Mitchell, G. Sarath, Biomass Bioenergy

30 (2006) 880–891.[54] K.C. Nlewem, M.E. Thrash, Bioresour. Technol. 101 (2010) 5426–5430.[55] T. Vancov, S. McIntosh, Appl. Energy 92 (2012) 421–428.[56] R. Harun, M.K. Danquah, Process Biochem. 46 (2011) 304–309.[57] H.L. Chum, D.K. Johnson, S.K. Black, R.P. Overend, Appl. Biochem. Biotechnol. 24-25 (1990) 1–14.[58] S.B. Kim, Y.Y. Lee, Bioresour. Technol. 83 (2002) 165–171.[59] A.A. Modenbach, S.E. Nokes, Biotechnol. Bioeng. 109 (2012) 1430–1442.[60] M.J. Selig,S.Viamajala,S.R.Decker,M.P.Tucker,M.E.Himmel,T.B.Vinzant,Biotechnol.Prog.23(2007)1333–1339.[61] S.E. Jacobsen, C.E. Wyman, Appl. Biochem. Biotechnol. 84-86 (2000) 81–96.[62] J.F. Saeman, Ind. Eng. Chem. 37 (1945) 43–52.[63] A.O. Converse, I.K. Kwarteng, H.E. Grethlein, H. Ooshima, Appl. Biochem. Biotechnol. 20-21 (1989) 63–78.[64] D.R. Cahela, Y.Y. Lee, R.P. Chambers, Biotechnol. Bioeng. 25 (1983) 3–17.[65] D. Schell, J. Farmer, M. Newman, J. McMillan, Appl. Biochem. Biotechnol. 105-108 (2003) 69–85.[66] T. Kobayashi, Y. Sakai, Bull. Agric. Chem. Soc. Jpn. 20 (1956) 1–7.[67] Y. Zhu, Y.Y. Lee, R.T. Elander, Appl. Biochem. Biotechnol. 117 (2004) 103–114.[68] R. Chen, W. Zhangwen, Y.Y. Lee, Appl. Biochem. Biotechnol. 70-72 (1998) 37–49.[69] Y.Y. Lee, Z. Wu, R.W. Torget, Bioresour. Technol. 71 (2000) 29–39.[70] P.T. Pienkos, M. Zhang, Cellulose 16 (2009) 743–762.[71] C.C. Geddes, J.J. Peterson, C. Roslander, G. Zacchi, M.T. Mullinnix, K.T. Shanmugam, L.O. Ingram, Bioresour.

Technol. 101 (2010) 1851–1857.[72] S. Gamez, J.A. Ramirez, G. Garrote, M. Vazquez, J. Agric. Food Chem. 52 (2004) 4172–4177.[73] C.J. Israilides, G.A. Grant, Y.W. Han, Appl. Environ. Microbiol. 36 (1978) 43–46.[74] S. Gamez, J.J. Gonzalez-Cabriales, J.A. Ramirez, G. Garrote, M. Vazquez, J. Food Eng. 74 (2006) 78–88.[75] C.C. Geddes, M.T. Mullinnix, I.U. Nieves, J.J. Peterson, R.W. Hoffman, S.W. York, L.P. Yomano, E.N. Miller,

K.T. Shanmugam, L.O. Ingram, Bioresour. Technol. 102 (2011) 2702–2711.[76] I. Romero, M. Moya, S. Sanchez, E. Ruiz, E. Castro, V. Bravo, Ind. Crops Prod. 25 (2007) 160–168.[77] G. Moxley, Z. Zhu, Y.H.P. Zhang, J. Agric. Food Chem. 56 (2008) 7885–7890.[78] H. Li, N.J. Kim, M. Jiang, J.W. Kang, H.N. Chang, Bioresour. Technol. 100 (2009) 3245–3251.[79] N. Sathitsuksanoh, Z. Zhu, T.J. Ho, M.D. Bai, Y.H.P. Zhang, Bioresour. Technol. 101 (2010) 4926–4929.[80] V. Jafari, S.R. Labafzadeh, A. Jeihanipour, K. Karimi, M.J. Taherzadeh, Renew. Energy 36 (2011) 2771–2775.[81] F. Talebnia, M.J. Taherzadeh, Carbohydr. Polym. 87 (2012) 2149–2153.[82] A.M. Adel, Z.H.A. El-Wahab, A.A. Ibrahim, M.T. Al-Shemy, Bioresour. Technol. 101 (2010) 4446–4455.[83] E. Heredia-Olea, E. Perez-Carrillo, S.O. Serna-Saldivar, Bioresour. Technol. 119 (2012) 216–223.[84] M. Pedersen, A. Vikso-Nielsen, A.S. Meyer, Process Biochem. 45 (2010) 1181–1186.[85] P. Champagne, C. Li, Bioresour. Technol. 100 (2009) 5700–5706.[86] B. Kamm, M. Kamm, M. Schmidt, I. Starke, E. Kleinpeter, Chemosphere 62 (2006) 97–105.[87] W. Xiao, W.W. Clarkson, Biodegradation 8 (1997) 61–66.[88] A. Rodriguez-Chong, J.A. Ramirez, G. Garrote, M. Vazquez, J. Food Eng. 61 (2004) 143–152.[89] R. Zhang, X.B. Lu, Y.S. Sun, X.Y. Wang, S.T. Zhang, J. Chem. Technol. Biotechnol. 86 (2010) 306–314.[90] Z.X. Lin, H. Huang, H. Zhang, L. Zhang, L. Yan, J. Chen, Appl. Biochem. Biotechnol. 162 (2010) 1872–1880.[91] A. Singh, N. Singh, N.R. Bishnoi, J. Sci. Ind. Res. 69 (2010) 232–237.[92] P.Guo,K.Mochidzuki,W.Cheng,M.Zhou,H.Gao,D.Zheng,X.Wang,Z.Cui,Bioresour.Technol.102(2011)7526–7531.[93] N.S. Mosier, Ind. Bioprocess 28 (2006) 2.[94] A.M.J. Kootstra, H.H. Beeftink, E.L. Scott, J.P.M. Sanders, Biochem. Eng. J. 46 (2009) 126–131.[95] A.M.J. Kootstra, N.S. Mosier, E.L. Scott, H.H. Beeftink, J.P.M. Sanders, Biochem. Eng. J. 43 (2009) 92–97.[96] N.S. Mosier, C.M. Ladisch, M.R. Ladisch, Biotechnol. Bioeng. 79 (2002) 610–618.


[97] N.S. Mosier, A. Sarikaya, C.M. Ladisch, M.R. Ladisch, Biotechnol. Prog. 17 (2001) 474–480.[98] Y. Lu, N.S. Mosier, Biotechnol. Prog. 23 (2007) 116–123.[99] Y. Lu, N.S. Mosier, Biotechnol. Bioeng. 101 (2008) 1170–1181.[100] E.S. Kim, S. Liu, M.M. Abu-Omar, N.S. Mosier, Energy Fuel 26 (2012) 1298–1304.[101] J. Xu, M.H. Thomsen, A.B. Thomsen, Appl. Microbiol. Biotechnol. 86 (2010) 509–516.[102] J. Xu, M.H. Thomsen, A.B. Thomsen, Biomass Bioenergy 33 (2009) 1660–1663.[103] J. Xu, M.H. Thomsen, A.B. Thomsen, J. Biotechnol. 139 (2009) 300–305.[104] R. Sindhu, P. Binod, K. Satyanagalakshmi, K.U. Janu, K.V. Sajna, N. Kurien, R.K. Sukumaran, A. Pandey, Appl.

Biochem. Biotechnol. 162 (2010) 2313–2323.[105] X. Zhao, D. Liu, Bioresour. Technol. 117 (2012) 25–32.[106] L. Kupiainen, J. Ahola, J. Tanskanen, Chem. Eng. Res. Des. 89 (2011) 2706–2713.[107] J.W. Lee, R.C.L.B. Rodrigues, T.W. Jeffries, Bioresour. Technol. 100 (2009) 6307–6311.[108] J.W. Lee, C.J. Houtman, H.Y. Kim, I.G. Choi, T.W. Jeffries, Bioresour. Technol. 102 (2011) 7451–7456.[109] N. Mosier, R. Hendrickson, N. Ho, M. Sedlak, M.R. Ladisch, Bioresour. Technol. 96 (2005) 1986–1993.[110] M.�. Petersen, J. Larsen, M.H. Thomsen, Biomass Bioenergy 33 (2009) 834–840.[111] Q. Nguyen, M. Tucker, F. Keller, D. Beaty, K. Connors, F. Eddy, Appl. Biochem. Biotechnol. 77 (1999) 133–142.[112] S.M. Ewanick, R. Bura, J.N. Saddler, Biotechnol. Bioeng. 98 (2007) 737–746.[113] K.H. Kim, J. Hong, Bioresour. Technol. 77 (2001) 139–144.[114] S. McIntosh, T. Vancov, Bioresour. Technol. 101 (2010) 6718–6727.[115] W.E. Kaar, M.T. Holtzapple, Biomass Bioenergy 18 (2000) 189–199.[116] T.H. Kim, F. Taylor, K.B. Hicks, Bioresour. Technol. 99 (2008) 5694–5702.[117] T.H. Kim, J.S. Kim, C. Sunwoo, Y.Y. Lee, Bioresour. Technol. 90 (2003) 39–47.[118] F. Teymouri, L. Laureano-Perez, H. Alizadeh, B.E. Dale, Bioresour. Technol. 96 (2005) 2014–2018.[119] Y. Sun, J.Y. Cheng, Bioresour. Technol. 83 (2002) 1–11.[120] S. McIntosh, T. Vancov, Biomass Bioenergy 35 (2011) 3094–3103.[121] L. Wu,M. Arakane, M. Ike, M.Wada, T. Takai, M. Gau, K. Tokuyasu, Bioresour. Technol. 102 (2011) 4793–4799.[122] S. Kim, M.T. Holtzapple, Bioresour. Technol. 97 (2006) 778–785.[123] P. Shrestha, S.K. Khanal, A.L. Pometto, J. Hans van Leeuwen, Bioresour. Technol. 101 (2010) 8698–8705.[124] D.L. Sills, J.M. Gossett, Biotechnol. Bioeng. 109 (2012) 894–903.[125] C. Teater, Z. Yue, J. MacLellan, Y. Liu, W. Liao, Bioresour. Technol. 102 (2011) 1856–1862.[126] L.N. Liew, J. Shi, Y. Li, Bioresour. Technol. 102 (2011) 8828–8834.[127] S. Xie, J.P. Frost, P.G. Lawlor, G. Wu, X. Zhan, Bioresour. Technol. 102 (2011) 8748–8755.[128] Z. Hu, Z. Wen, Biochem. Eng. J. 38 (2008) 369–378.[129] A. Singh, S. Tuteja, N. Singh, N.R. Bishnoi, Bioresour. Technol. 102 (2011) 1773–1782.[130] K. Niu, P. Chen, X. Zhang, W.S. Tan, J. Chem. Technol. Biotechnol. 84 (2009) 1240–1245.[131] V.S. Chang, M. Nagwani, M.T. Holtzapple, Appl. Biochem. Biotechnol. 74 (1998) 135–159.[132] J. Gandi, M.T. Holtzapple, A. Ferrer, F.M. Byers, N.D. Turner, M. Nagwani, S. Chang, Anim. Feed Sci. Technol.

68 (1997) 195–211.[133] V.S. Chang, B. Burr, M.T. Holtzapple, Appl. Biochem. Biotechnol. 63-65 (1997) 3–19.[134] N. Beukes, B.I. Pletschke, Bioresour. Technol. 101 (2010) 4472–4478.[135] V.S. Chang, W.E. Kaar, B. Burr, M.T. Holtzapple, Biotechnol. Lett. 23 (2001) 1327–1333.[136] S.C. Rabelo, R.M. Filho, A.C. Costa, Appl. Biochem. Biotechnol. 153 (2009) 139–150.[137] J. Xu, J.J. Cheng, Bioresour. Technol. 102 (2011) 3861–3868.[138] T.H. Kim, Y.Y. Lee, Appl. Biochem. Biotechnol. 124 (2005) 1119–1131.[139] T.H. Kim, Y.Y. Lee, Appl. Biochem. Biotechnol. 137-140 (2007) 81–92.[140] A. Isci, J.N.Himmelsbach,A.L. Pometto Iii, D.R.Raman,R.P.Anex,Appl. Biochem.Biotechnol. 144 (2008) 69–77.[141] J.K. Ko, J.S. Bak, M.W. Jung, H.J. Lee, I.G. Choi, T.H. Kim, K.H. Kim, Bioresour. Technol. 100 (2009) 4374–4380.[142] J.A. Rollin, Z. Zhu, N. Sathitsuksanoh, Y.H.P. Zhang, Biotechnol. Bioeng. 108 (2011) 22–30.[143] Y. Kim, N.S. Mosier, M.R. Ladisch, V.R. Pallapolu, Y.Y. Lee, R. Garlock, V. Balan, B.E. Dale, B.S. Donohoe,

T.B. Vinzant, R.T. Elander, M. Falls, R. Sierra, M.T. Holtzapple, J. Shi, M.A. Ebrik, T. Redmond, B. Yang,C.E. Wyman, R.E. Warner, Bioresour. Technol. 102 (2011) 11089–11096.

[144] H.H. Yoon, Korean J. Chem. Eng. 15 (1998) 631–636.[145] H.H. Yoon, Z.W. Wu, Y.Y. Lee, Appl. Biochem. Biotechnol. 51-52 (1995) 5–19.


[146] M. Moller, P. Nilges, F. Harnisch, U. Schroder, ChemSusChem 4 (2011) 566–579.[147] G. Garrote, H. Dominguez, J.C. Parajo, Holz Roh Werkst. 59 (2001) 53–59.[148] G. Garrote, H. Dominguez, J.C. Paraj, Process Biochem. 37 (2002) 1067–1073.[149] R.C.Pettersen,R.M.Rowel (Ed.),Thechemistryof solidwood,AmericanChemicalSociety,Washington,DC., 1984,

57–126.[150] D. Nabarlatz, A. Ebringerova, D. Montane, Carbohydr. Polym. 69 (2007) 20–28.[151] L.P. Ramos, Quim. Nova 26 (2003) 863–871.[152] B.K. Ahring, K. Jensen, P. Nielsen, A.B. Bjerre, A.S. Schmidt, Bioresour. Technol. 58 (1996) 107–113.[153] M. Ballesteros, J.M. Oliva, M.J. Negro, P. Manzanares, I. Ballesteros, Process Biochem. 39 (2004) 1843–1848.[154] R. Chandra, H. Takeuchi, T. Hasegawa, Appl. Energy 94 (2012) 129–140.[155] J. Lamptey, C.W. Robinson, M. Moo-Young, Biotechnol. Lett. 7 (1985) 531–536.[156] M. Laser, D. Schulman, S.G. Allen, J. Lichwa, M.J. Antal, L.R. Lynd, Bioresour. Technol. 81 (2002) 33–44.[157] I. Cybulska, H. Lei, J. Julson, Energy Fuel 24 (2010) 718–727.[158] S.G. Allen, D. Schulman, J. Lichwa, M.J. Antal, E. Jennings, R. Elander, Ind. Eng. Chem. Res. 40 (2001)

2352–2361.[159] M.J. Negro, P.Manzanares, I. Ballesteros, J.M. Oliva,A. Cabanas,M. Ballesteros, Appl. Biochem. Biotechnol. 108

(2003) 87–100.[160] C.J. Biermann, T.P. Schultz, G.D. McGinnia, J. Wood Chem. Technol. 4 (1984) 111–128.[161] S.H. Yoon, H.T. Cullinan, G.A. Krishnagopalan, Ind. Eng. Chem. Res. 49 (2010) 5969–5976.[162] R.P. Overend, E. Chornet, Philos. Trans. R. Soc. Lond. A 321 (1987) 523–536.[163] S.A. Hosseini, N. Shah, Bioresour. Technol. 100 (2009) 2621–2628.[164] B.C. Vidal, B.S. Dien, K.C. Ting, V. Singh, Appl. Biochem. Biotechnol. 164 (2011) 1405–1421.[165] H.H. Brownell, E.K.C. Yu, J.N. Saddler, Biotechnol. Bioeng. 28 (1986) 792–801.[166] S. Kumar, U. Kothari, L.Z. Kong, Y.Y. Lee, R.B. Gupta, Biomass Bioenergy 35 (2011) 956–968.[167] J.B. Kristensen, L.G. Thygesen, C. Felby, H. Jorgensen, T. Elder, Biotechnology for Biofuels 1 (2008) 5.[168] C.K. Nitsos, K.A. Matis, K.S. Triantafyllidis, Optimization of hydrothermal pretreatment of lignocellulosic bio-

mass in the bioethanol production process. Submitted for publication, ChemSusChem (2012).[169] J.B. Kristensen, J. Borjesson,M.H. Bruun, F. Tjerneld, H. Jorgensen, EnzymeMicrob. Technol. 40 (2007) 888–895.[170] K. Wang, J.X. Jiang, F. Xu, R.C. Sun, Polym. Degrad. Stab. 94 (2009) 1379–1388.[171] M.H. Thomsen, A. Thygesen, A.B. Thomsen, Appl. Microbiol. Biotechnol. 83 (2009) 447–455.[172] E. Palmqvist, B. Hahn-Hagerdal, Bioresour. Technol. 74 (2000) 25–33.[173] J. Weil, M. Brewer, R. Hendrickson, A. Sarikaya, M.R. Ladisch, Appl. Biochem. Biotechnol. 70-72 (1998) 99–111.[174] A.H. Conner, Wood Fiber Sci. 16 (1984) 268–277.[175] A.H. Conner, L.F. Lorenz, Wood Fiber Sci. 18 (1986) 248–263.[176] G. Garrote, H. Dominguez, J.C. Parajo, Holz Roh Werkst. 57 (1999) 191–202.[177] A. Mittal, S.G. Chatterjee, G.M. Scott, T.E. Amidon, Holzforschung 63 (2009) 307–314.[178] P. Gullon, G. Pereiro, J.L. Alonso, J.C. Parajo, Bioresour. Technol. 100 (2009) 5840–5845.[179] J.A. Perez, I. Ballesteros, M. Ballesteros, F. Saez, M.J. Negro, P. Manzanares, Fuel 87 (2008) 3640–3647.[180] A. Romani, G. Garrote, J.L. Alonso, J.C. Parajo, Bioresour. Technol. 101 (2010) 8706–8712.[181] A.T.W.M. Hendriks, G. Zeeman, Bioresour. Technol. 100 (2008) 10–18.[182] F. Schutt, J. Puls, B. Saake, Holzforschung 65 (2011) 453–459.[183] W. Schwald, C. Breuil, H.H. Brownell, M. Chan, J.M. Saddler, Appl. Biochem. Biotechnol. 20-21 (1989) 29–44.[184] M.N. Angles, F. Ferrando, X. Farriol, J. Salvado, Biomass Bioenergy 21 (2001) 211–224.[185] F. Schutt, B. Westereng, S.J. Horn, J. Puls, B. Saake, Bioresour. Technol. 111 (2012) 476–481.[186] R.F.H. Dekker, A.F.A. Wallis, Biotechnol. Bioeng. 25 (1983) 3027–3048.[187] M. Heitz, E. Capek-Minard, P.G. Koeberle, J. Gagni, E. Chornet, R.P. Overend, J.D. Taylor, E. Yu, Bioresour.

Technol. 35 (1991) 23–32.[188] T.P. Schultz, J.R. Rughani, G.D. McGinnis, Appl. Biochem. Biotechnol. 20-21 (1989) 9–27.[189] G. Garrote, H. Dominguez, J.C. Parajo, Holz Roh Werkst. 57 (1999) 191–202.[190] J. Larsen, M. Atergaard Petersen, L. Thirup, H.W. Li, F.K. Iversen, Chem. Eng. Technol. 31 (2008) 765–772.[191] M.H. Thomsen, A. Thygesen, A.B. Thomsen, Bioresour. Technol. 99 (2008) 4221–4228.[192] M. Tanaka, R. Matsuno, A.O. Converse, Enzyme Microb. Technol. 12 (1990) 190–195.[193] M. Tucker, K. Kim, M. Newman, Q. Nguyen, Appl. Biochem. Biotechnol. 105 (2003) 165–177.


[194] M. Linde, M. Galb, G. Zacchi, Appl. Biochem. Biotechnol. 130 (2006) 546–562.[195] Q. Nguyen, M. Tucker, F. Keller, F. Eddy, Appl. Biochem. Biotechnol. 84-86 (2000) 561–576.[196] E. Bruni, A.P. Jensen, I. Angelidaki, Bioresour. Technol. 101 (2010) 7668–7671.[197] P. Sassner, M. Galbe, G. Zacchi, Appl. Biochem. Biotechnol. 124 (2005) 1101–1117.[198] C. Tengborg, K. Stenberg, M. Galbe, G. Zacchi, S. Larsson, E. Palmqvist, B. Hahn-Hagerdal, Appl. Biochem.

Biotechnol. 70-72 (1998) 3–15.[199] K. Stenberg, C. Tengborg, M. Galbe, G. Zacchi, J. Chem. Technol. Biotechnol. 71 (1998) 299–308.[200] C. Tengborg, M. Galbe, G. Zacchi, Enzyme Microb. Technol. 28 (2001) 835–844.[201] R. Eklund, M. Galbe, G. Zacchi, Bioresour. Technol. 52 (1995) 225–229.[202] K. Ohgren, M. Galbe, G. Zacchi, Appl. Biochem. Biotechnol. 121 (2005) 1055–1067.[203] J. Soderstrom, L. Pilcher, M. Galbe, G. Zacchi, Appl. Biochem. Biotechnol. 98-100 (2002) 5–21.[204] J. Soderstrom, L. Pilcher, M. Galbe, G. Zacchi, Appl. Biochem. Biotechnol. 105 (2003) 127–140.[205] S. Monavari, A. Bennato, M. Galbe, G. Zacchi, Biotechnol. Prog. 26 (2010) 1054–1060.[206] C. Carrasco, H.M. Baudel, J. Sendelius, T. Modig, C. Roslander, M. Galbe, B. Hahn-Hagerdal, G. Zacchi,

G. Liden, Enzyme Microb. Technol. 46 (2010) 64–73.[207] P. Sassner, M. Galbe, G. Zacchi, Biomass Bioenergy 32 (2008) 422–430.[208] L. Kumar, R. Chandra, P.A. Chung, J. Saddler, Bioresour. Technol. 101 (2010) 7827–7833.[209] S. Monavari, M. Galbe, G. Zacchi, Biomass Bioenergy 35 (2011) 3115–3122.[210] S. Zabihi, R. Alinia, F. Esmaeilzadeh, J.F. Kalajahi, Biosystems Eng. 105 (2010) 288–297.[211] M. Holtzapple, J.-H. Jun, G. Ashok, S. Patibandla, B. Dale, Appl. Biochem. Biotechnol. 28-29 (1991) 59–74.[212] M.T. Holtzapple, E.P. Ripley, M. Nikolaou, Biotechnol. Bioeng. 44 (1994) 1122–1131.[213] M.W. Lau, B.E. Dale, Proc. Natl. Acad. Sci. USA 106 (2009) 1368–1373.[214] M.T. Holtzapple, J.E. Lundeen, R. Sturgis, J.E. Lewis, B.E. Dale, Appl. Biochem. Biotechnol. 34-35 (1992) 5–21.[215] L.B. De La Rosa, S. Reshamwala, V.M. Latimer, B.T. Shawky, B.E. Dale, E.D. Stuart, Appl. Biochem. Biotechnol.

45-46 (1994) 483–497.[216] L.E. Gollapalli, B.E. Dale, D.M. Rivers, Appl. Biochem. Biotechnol. 98-100 (2002) 23–35.[217] F. Teymouri, L. Laureano-Perez, H. Alizadeh, B.E. Dale, Appl. Biochem. Biotechnol. 115 (2004) 951–963.[218] B.E. Dale, C.K. Leong, T.K. Pham, V.M. Esquivel, I. Rios, V.M. Latimer, Bioresour. Technol. 56 (1996) 111–116.[219] H. Alizadeh, F. Teymouri, T.I. Gilbert, B.E. Dale, Appl. Biochem. Biotechnol. 124 (2005) 1133–1141.[220] B. Bals, C. Rogers, M. Jin, V. Balan, B. Dale, Biotechnology for Biofuels 3 (2010) 1.[221] B.D. Bals, B.E. Dale, Bioresour. Technol. 106 (2012) 161–169.[222] B.D. Bals, F. Teymouri, T. Campbell, M. Jin, B.E. Dale, Bioenergy Res. 5 (2012) 372–379.[223] Y. Zheng, H.-M. Lin, J. Wen, N. Cao, X. Yu, G.T. Tsao, Biotechnol. Lett. 17 (1995) 845–850.[224] M. Gao, F. Xu, S. Li, X. Ji, S. Chen, D. Zhang, Biosystems Eng. 106 (2010) 470–475.[225] Y. Zheng, H.-M. Lin, G.T. Tsao, Biotechnol. Prog. 14 (1998) 890–896.[226] V.P. Puri, H. Mamers, Biotechnol. Bioeng. 25 (1983) 3149–3161.[227] N. Narayanaswamy, A. Faik, D.J. Goetz, T. Gu, Bioresour. Technol. 102 (2011) 6995–7000.[228] J.Y. Zhu, X.J. Pan, G.S. Wang, R. Gleisner, Bioresour. Technol. 100 (2009) 2411–2418.[229] L. Shuai, Q. Yang, J.Y. Zhu, F.C. Lu, P.J. Weimer, J. Ralph, X.J. Pan, Bioresour. Technol. 101 (2010) 3106–3114.[230] J. Zhu,W. Zhu, P. Obryan, B. Dien, S. Tian, R. Gleisner, X. Pan, Appl.Microbiol. Biotechnol. 86 (2010) 1355–1365.[231] G.S. Wang, X.J. Pan, J.Y. Zhu, R. Gleisner, D. Rockwood, Biotechnol. Prog. 25 (2009) 1086–1093.[232] J.Y. Zhu, R. Gleisner, C.T. Scott, X.L. Luo, S. Tian, Bioresour. Technol. 102 (2011) 8921–8929.[233] Y. Liu, G. Wang, J. Xu, Y. Zhang, C. Liu, Z. Yuan, BioResources 6 (2011) 5001–5011.[234] P.F. Vidal, J. Molinier, Biomass 16 (1988) 1–17.[235] J.M. Gould, S.N. Freer, Biotechnol. Bioeng. 26 (1984) 628–631.[236] N. Szijarto, Z. Kadar, E. Varga, A.B. Thomsen, M. Costa-Ferreira, K. Reczey, Appl. Biochem. Biotechnol. 155

(2009) 386–396.[237] L.C. Teixeira, J.C. Linden, H.A. Schroeder, Renew. Energy 16 (1999) 1070–1073.[238] L.C. Teixeira, J.C. Linden, H.A. Schroeder, Appl. Biochem. Biotechnol. 84-86 (2000) 111–127.[239] A. Gutierrez, J. Rencoret, E.M. Cadena, A. Rico, D. Barth, J.C. del Rio, T.T. Martinez, Bioresour. Technol. 119

(2012) 114–122.[240] V.P. Puri, Biotechnol. Lett. 5 (1983) 773–776.[241] D. Ben-Ghedalia, J. Miron, Biotechnol. Bioeng. 23 (1981) 823–831.


[242] W.C. Neely, Biotechnol. Bioeng. 26 (1984) 59–65.[243] R.A.Silverstein,Y.Chen,R.R.Sharma-Shivappa,M.D.Boyette, J.Osborne,Bioresour.Technol. 98 (2007)3000–3011.[244] M.T. Garcia-Cubero, G. Gonzalez-Benito, I. Indacoechea, M. Coca, S. Bolado, Bioresour. Technol. 100 (2009)

1608–1613.[245] J.M. Gould, Biotechnol. Bioeng. 26 (1984) 46–52.[246] J.M. Gould, Biotechnol. Bioeng. 27 (1985) 225–231.[247] R.C. Sun, J.M. Fang, J. Tomkinson, Ind. Crops Prod. 12 (2000) 71–83.[248] R. Yanez, J.L. Alonso, J.C. Parajo, Process Biochem. 41 (2006) 1244–1252.[249] S. Kim, Y. Lee, Appl. Biochem. Biotechnol. 57-58 (1996) 147–156.[250] B. Wang, X. Wang, H. Feng, Bioresour. Technol. 101 (2010) 752–760.[251] H. Yu, X. Zhang, L. Song, J. Ke, C. Xu, W. Du, J. Zhang, J. Biosci. Bioeng. 110 (2010) 660–664.[252] G.D. McGinnis, W.W. Wilson, C.E. Mullen, Ind. Eng. Chem. Prod. Res. Dev. 22 (1983) 352–357.[253] G.D. McGinnis, W.W. Wilson, S.E. Prince, C.C. Chen, Ind. Eng. Chem. Prod. Res. Dev. 22 (1983) 633–636.[254] A.B. Bjerre, A.B. Olesen, T. Fernqvist, A. Ploger, A.S. Schmidt, Biotechnol. Bioeng. 49 (1996) 568–577.[255] A.S. Schmidt, A.B. Thomsen, Bioresour. Technol. 64 (1998) 139–151.[256] C. Martin, A.B. Thomsen, J. Chem. Technol. Biotechnol. 82 (2007) 174–181.[257] P. Oleskowicz-Popiel, P. Lisiecki, J.B. Holm-Nielsen, A.B. Thomsen, M.H. Thomsen, Bioresour. Technol. 99

(2008) 5327–5334.[258] X.B. Zhao, L. Wang, D.H. Liu, J. Chem. Technol. Biotechnol. 82 (2007) 1115–1121.[259] S. Duncan, Q. Jing, A. Katona, R. Kazlauskas, J. Schilling, U. Tschirner, W. Wafa AlDajani, Appl. Biochem.

Biotechnol. 160 (2010) 1637–1652.[260] D. Yin, Q. Jing, W.W. AlDajani, S. Duncan, U. Tschirner, J. Schilling, R.J. Kazlauskas, Bioresour. Technol. 102

(2011) 5183–5192.[261] A. Breen, F.L. Singleton, Curr. Opin. Biotechnol. 10 (1999) 252–258.[262] R. ten Have, P.J.M. Teunissen, Chem. Rev. 101 (2001) 3397–3414.[263] M. Hofrichter, Enzyme Microb. Technol. 30 (2002) 454–466.[264] K.E. Hammel, D. Cullen, Curr. Opin. Plant Biol. 11 (2008) 349–355.[265] A.I. Canas, S. Camarero, Biotechnol. Adv. 28 (2010) 694–705.[266] J. Perez, J. Munoz-Dorado, T. De La Rubia, J. Martinez, Int. Microbiol. 5 (2002) 53–63.[267] P. Widsten, A. Kandelbauer, Enzyme Microb. Technol. 42 (2008) 293–307.[268] H. Palonen, L. Viikari, Biotechnol. Bioeng. 86 (2004) 550–557.[269] W. Qiu, H. Chen, Bioresour. Technol. 118 (2012) 8–12.[270] P.T. Anastas, J.C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, New York, 1998.[271] P.T. Anastas, J.B. Zimmerman, IEEE Eng. Manage. Rev. 35 (2007) 16, 16.[272] F. Guo, Z. Fang, C.C. Xu, R.L. Smith Jr, Progress in Energy and Combustion Science 38 (2012) 672–690.[273] R. Ormsby, J.R. Kastner, J. Miller, Catal. Today 190 (2012) 89–97.[274] J.A. Bootsma, B.H. Shanks, Appl. Catal. Gen. 327 (2007) 44–51.[275] A.B. Boraston, D.N. Bolam, H.J. Gilbert, G.J. Davies, Biochem. J. 382 (2004) 769–781.[276] H. Kobayashi, T. Komanoya, K. Hara, A. Fukuoka, ChemSusChem 3 (2010) 440–443.[277] J.B. Binder, J.J. Blank, A.V. Cefali, R.T. Raines, ChemSusChem 3 (2010) 1268–1272.[278] T. vom Stein, P.M. Grande, W. Leitner, P. Domınguez de Marıa, ChemSusChem 4 (2011) 1592–1594.[279] V. Choudhary, A.B. Pinar, S.I. Sandler, D.G. Vlachos, R.F. Lobo, ACS Catal. 1 (2011) 1724–1728.[280] F. Yang, Q. Liu, X. Bai, Y. Du, Bioresour. Technol. 102 (2011) 3424–3429.[281] A. Takagaki, M. Ohara, S. Nishimura, K. Ebitani, Chem. Commun. (2009) 6276–6278.[282] X. Tong, Y. Ma, Y. Li, Appl. Catal. Gen. 385 (2010) 1–13.[283] Y. Zhang, V. Degirmenci, C. Li, E.J.M. Hensen, ChemSusChem 4 (2011) 59–64.[284] H. Olivier-Bourbigou, L. Magna, D. Morvan, Appl. Catal. Gen. 373 (2010) 1–56.[285] C.-Z. Liu, F. Wang, A.R. Stiles, C. Guo, Appl. Energy 92 (2012) 406–414.[286] P. Maki-Arvela, I. Anugwom, P. Virtanen, R. Sjoholm, J.P. Mikkola, Ind. Crops Prod. 32 (2010) 175–201.[287] V.B. Agbor, Biotechnol. Adv. 29 (2011) 675–685.[288] T. Leskinen, A.W.T. King, I. Kilpelainen, D.S. Argyropoulos, Ind. Eng. Chem. Res. 50 (2011) 12349–12357.[289] I. Kilpelainen, H. Xie, A. King, M. Granstrom, S. Heikkinen, D.S. Argyropoulos, J. Agric. Food Chem. 55 (2007)

9142–9148.


[290] J. van Spronsen, M.A.T. Cardoso, G.-J. Witkamp, W. de Jong, M.C. Kroon, Chem. Eng. Process 50 (2011)196–199.

[291] M. Gericke, P. Fardim, T. Heinze, Molecules 17 (2012) 7458–7502.[292] H. Zhao, G.A. Baker, Z. Song, O. Olubajo, T. Crittle, D. Peters, Green Chem. 10 (2008) 696–705.[293] L. Feng, Z.l. Chen, J. Mol. Liquids 142 (2008) 1–5.[294] I. Anugwom, P. Maki-Arvela, P. Virtanen, S. Willfor, R. Sjoholm, J.P. Mikkola, Carbohydr. Polym. 87 (2012)

2005–2011.[295] F. Xu, Y.C. Shi, D. Wang, Bioresour. Technol. 114 (2012) 720–724.[296] N. Muhammad, Z. Man, M.A.B. Khalil, Eur. J. Wood Wood Prod. 70 (2012) 125–133.[297] C. Li, B. Knierim, C. Manisseri, R. Arora, V.H. Scheller, M. Auer, P.K. Vogel, A.B. Simmons, S. Singh, Bioresour.

Technol. 101 (2010) 4900–4906.[298] A. Brandt, M.J. Ray, T.Q. To, D.J. Leak, R.J. Murphy, T. Welton, Green Chem. 13 (2011) 2489–2499.[299] S.H. Lee, T.V. Doherty, R.J. Linhardt, J.S. Dordick, Biotechnol. Bioeng. 102 (2009) 1368–1376.[300] J.G. Lynam, M. Toufiq Reza, V.R. Vasquez, C.J. Coronella, Bioresour. Technol. 114 (2012) 629–636.[301] N. Labbe, L.M. Kline, L. Moens, K. Kim, P.C. Kim, D.G. Hayes, Bioresour. Technol. 104 (2012) 701–707.[302] P. Weerachanchai, S.S.J. Leong, M.W. Chang, C.B. Ching, J.M. Lee, Bioresour. Technol. 111 (2012) 453–459.[303] N.Muhammad,W.N.Omar, Z.Man,M.A. Bustam, S. Rafiq, Y. Uemura, Ind EngChemRes 51 (2012) 2280–2289.[304] J. Long, B. Guo, J. Teng, Y. Yu, L. Wang, X. Li, Bioresour. Technol. 102 (2011) 10114–10123.[305] C. Li, Q. Wang, Z.K. Zhao, Green Chem. 10 (2008) 177–182.[306] B.J. Cox, J.G. Ekerdt, Bioresour. Technol. 118 (2012) 584–588.[307] F. Tao, H. Song, L. Chou, J. Mol. Catal. A 357 (2012) 11–18.[308] H. Watanabe, Carbohydr. Polym. 80 (2010) 1168–1171.[309] H. Yu, J. Hu, J. Fan, J. Chang, Ind. Eng. Chem. Res. 51 (2012) 3452–3457.[310] F. Tao, H. Song, J. Yang, L. Chou, Carbohydr. Polym. 85 (2011) 363–368.[311] C. Li, Z.K. Zhao, Adv. Synth. Catal. 349 (2007) 1847–1850.[312] H. Cai, C. Li, A. Wang, G. Xu, T. Zhang, Appl. Catal. Environ. 123-124 (2012) 333–338.[313] X. Geng, W.A. Henderson, Biotechnol. Bioeng. 109 (2012) 84–91.[314] T.D. Nguyen, K.R. Kim, S.J. Han, H.Y. Cho, J.W. Kim, S.M. Park, J.C. Park, S.J. Sim, Bioresour. Technol. 101

(2010) 7432–7438.[315] K. Shill, S. Padmanabhan, Q. Xin, J.M. Prausnitz, D.S. Clark, H.W. Blanch, Biotechnol. Bioeng. 108 (2011)

511–520.


the role of catalysis for the sustainable production of bio-fuels and bio-chemicals || the role of...

Documents