WHAT DETERMINES THE PROCESSINGOF BIOMASS TO FUEL AND CHEMICALS?
Governments of various countries have recentlyprovided substantial material resources for usinglocally available biomass to obtain energy, reduce CO2emissions, and lower their dependence on petroleum[1]. The advantages of using biomass are increasinglyrecognized among industrialists and politicians inFinland and other countries of Europe, due to the dif�ficult situation in the wood and paper industry. Lowprofitability and the enormous production capacity fortransforming wood into pulp (which exceeds currentdemand) force us to consider wood as a source of fueland chemicals, which would of course help to raise thecompetitiveness of the wood industry. In countries thatare not rich in forests, plant biomass can serve as thesource of biomass.
Our natural sources of fuel and chemicals (petro�leum) will, according to today’s estimates, be depletedin another 40–50 years. Petroleum is becoming moreexpensive, and alternative sources of fuel (carbon orbiomass) will be more profitable to use even if it is stillavailable.
Petroleum is distilled by fractions at refineries andis subject to further treatment. Lignocellulose rawmaterials can also undergo fractionation, and chemi�cals and fuel obtained from biomass are neutral withrespect to CO2 emissions.
The first generation of biofuel is being producedfrom plant sources (hydrocarbons and oils) that couldalso be used in the production of food. The dilemmathis poses is quite difficult, since not only is synthesiz�
ing bioethanol from grain immoral in the opinion ofmany potential users and producers, it also makes foodmore expensive. Millions of people around the worldsuffer from hunger, so we can use sources of food forthe first generation of biofuel only temporarily.
It should be noted that the transport sector usesnearly 28% of all energy [1], and the selection of fuelsis not so great. Biofuel may not be needed at all if alter�native automobile engines are introduced. There havebeen attempts to use so�called renewable electricity(solar energy) for cars, and examples of alternative carsalready exist. Our experience in using electric autos isnot yet great, and we will probably not stop using liq�uid fuel made from petroleum any time soon, and it ishardly likely that aviation fuel (also made from petro�leum) will be replaced with alternative fuel.
If we use sources of alternative energy (e.g., solar),there may be enough petroleum not just for aviationfuel, but for petrochemicals as well.
The chemical industry now accounts for only 5 to10% of the total use of oil and gas; even with anincrease in the demand for petrochemicals in thefuture, we therefore may have enough petroleum tomeet our needs. Renewable raw materials could beeven cheaper than petroleum.
Since the first generation of fuel and chemicalsobtained from biomass (e.g., bioethanol from feedgrain) has many shortcomings, much attentionaround the world is being given to developing subse�quent generations of biofuel and chemicals fromlignocellulose wood and plant biomass, which has nonutritional value for humans [1–3].
GENERAL REVIEW
Catalysis in Biomass ProcessingD. Yu. Murzina and I. L. Simakovab
aÅbo Academi University, Turku FI�20500, FinlandbBoreskov Institute of Catalysis, Novosibirsk, 630090 Russia
Abstract—Biomass has in recent years been considered as a raw material for the production of fuels andchemicals. This work discusses the reasons for the increased interest in mainly lignocellulosic biomass. Gas�ification, pyrolysis, and depolymerization by hydrolysis are analyzed as key biomass technology. We also dis�cuss which of the sugars obtained via depolymerization by hydrolysis can be processed into fuel or key inter�mediates of the chemical industry. Lignocellulosic biomass contains such extractants as fatty acids and terpe�nes, and we therefore describe the catalytic reactions of these substances for the synthesis of fuels andchemicals. Some typical reactions of biomass processing (oxidation, hydrogenation, cracking, etc.) are con�ceptually close to the process widely known in the refining and chemical industries. There are, however, otherconsiderations due to, e.g., the large number of functional (hydroxyl and other) groups, and the processingof biomass components therefore requires dehydration, aldol condensation, ketonization, decarboxylation,etc. We cover the fundamentals of the approaches to selecting catalysts for these reactions.
Due to the importance of this topic, hundreds ofarticles and dozens of surveys have appeared that dis�cuss both so�called biorefineries and individualaspects of biomass processing [4–20].
Using biomass for the production of both fuel andchemicals is nothing new. In developing his engine,Rudolf Diesel used oils of vegetable origin to power it;during the Second World War, wood biomass was usedas fuel (Fig. 1); and the chemical industry of, e.g.,Sweden was based on raw materials from wood.
With the discovery of oilfields, petroleum sup�planted not only biomass but coal as well, which in
some countries of Europe (e.g., Germany) remainedthe main fuel down to the end of the 1950s.
Wood Biomass
Lignocellulose is the fibrous material that makes upthe cell walls of plants. It contains cellulose (≈40 wt. %),hemicellulose (≈25 wt. %), and lignin (Fig. 3).
There are 700 billion metric tons of cellulose on ourplanet, making it the most abundant organic materialon Earth; hence the interest in it as a renewable sourceof energy. Each year, nearly 40 billion metric tons ofbiomass is generated, but only 200 million of this isrecycled. Cellulose and hemicellulose are polysaccha�rides. Cellulose is a linear polysaccharide, built fromglycoside units. The units of D glucose in cellulose arelinked by β�1,4�glycoside bonds, resulting in the for�mation of a crystal structure of cellulose with intra�and intermolecular hydrogen bonds.
There is not much space around the glycoside(bridging) oxygen in cellulose (Fig. 3), and the accessof homogeneous or heterogeneous catalyst to thisbond is limited. The number of glucose monomers canbe as high as 15 thousand; the average molecular mass,(3–5) × 105 g/mol. Cellulose is soluble in some ratherunusual solvents, e.g., concentrated ZnCl2 solutionand ionic liquids [5]. The development of economi�cally competitive techniques would allow wider use ofcellulose.
Hemicellulose is composed of several monomers(Figs. 3, 4 and 5). The arabinogalactan of larch (Fig. 4)
Fig. 1. Using biomass as a motor fuel during the SecondWorld War: a car with a suction gas engine (archive of Prof.A. Holmen, Trondheim, Norway).
G
S
H
G
S
H
G
S
H
H
S
H
S
G
G
H
G
H
OH
OH
OH
OH
O
OH
OH
OO
H G S
Cell
LigninCell wall
Hemicellulose 10–20 nm
Pentoses
Hexose
CelluloseGlucose
CyclodextrinHydrogen bond
Lignin
n�3
n�3
n�3
n�3
n�3
Fig. 2. Structure of lignocellulose biomass [4].
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MURZIN, SIMAKOVA
has bonded (1, 3) β�galactopyranose as its basis andunits of D�galactopyranose, L�arabinofuranose, andD�glucuronic acid units in its branching. The averageratio of galactose, arabinose, and glucuronic acid isclose to 5 : 1 : 0.08; the molecular mass is (2–10) ×104 g/mol. The C6 carbohydrates in hemicellulose arecomposed of mannose and galactose in addition toglucose; and C5 carbohydrates contain xylose in addi�tion to arabinose.
Steric hindrance due to the presence of side chainsand axial hydroxyl groups in different sugars of hemi�cellulose prevent the formation of a crystal structure,promoting reactions with different biomass compo�nents, including cellulose.
As soon as the structure of a polysaccharide (cellu�lose or hemicellulose) is disrupted, its sugars can betransformed into valuable chemical substances viahomogeneous, heterogeneous, or enzyme catalysis.
Lignocellulose is composed of both lignin andpolysaccharides (see Fig. 2), forming a three�dimen�sional polymer composed of propylphenol units. As itis included in the structure of the cell network, ligninis not only a binder but also an antibacterial substancethat defends the lignocellulose from microorganisms.
In addition to polymers, lignocellulose containsproteins and so�called extractants (fatty acids, terpe�nes, stilbenes, and lignans) and inorganic compo�nents, including metals.
GlucoseCellulose
Glycosidic bonds
Hydrogen bond
Fig. 3. Structure of cellulose.
O
OHHO
H
HO
H
H
OHH
O
OHO
H
HO
H
H
OHH
O
OHO
H
H
H
OH O
O
O
HO
H
O
H
H
OH O
O
O
HO
H
H
H
OH O
O
O
HO
H
H
H
OH O
OH
O
HO
H
H
H
OHO
H
H
H
H H H H HO O O O
H
HHO H
H HOO
HOH
HHO H
H HOO
HO
Fig. 4. Structure of arabinogalactan.
CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
CATALYSIS IN BIOMASS PROCESSING 221
Technologies for Biomass Processing
The general approach to biomass processing isgiven in Fig. 6. Since the structure of biomass is differ�ent from the composition of petroleum (which is now�adays the basis for the production of fuel and chemi�cals), it should be obvious that the technologies of bio�mass processing differ from those widely used inpetroleum refining. It should be noted that the oxy�gen�to�carbon ratio in lignocellulose biomass is gener�ally close to unity; this is undesirable in the productionof motor fuels, the raw materials for which are hydro�carbons. However, a slight amount of oxygen in thefuel is at the same time good for gasoline engines, andthe strategy for using biomass for motor fuels shouldtherefore include stages with the substantial (but notabsolute) removal of oxygen.
Many intermediates of the chemical industry con�tain oxygen; e.g., the carbon�to�oxygen ratio in meth�anol, acetic acid, and ethylene glycol is close to thatfor biomass, and it can take values of 0.3 to 0.6 forother important chemicals. These substances thus donot require a very high degree of oxygen removal.
Gasification
The main routes of biomass processing can bedivided into two classes. The first is based on gasifica�tion or the pyrolysis of biomass at rather high temper�atures, while the second uses relatively mild condi�tions and involves hydrolysis as the key step. In thecase of gasification with oxygen or air, oxidation takesplace with the formation of CO, H2, and H2O:
C6(H2O)6 + 1.5O2 6CO + 3H2 + 3H2O. (1)
The synthesis gas we obtain is characterized by anunfavorable CO�to�H ratio if the next step of biomassprocessing is the Fischer–Tropsch synthesis at CO :H2 = 0.5. It is therefore advisable to conduct the reac�tion of a water gas shift after the Fischer–Tropsch step:
CO + H2O CO2 + H2 . (2)The Fischer–Tropsch reactions and their practical
application with coal used as the raw material were
considered in [21]; we shall therefore not dwell onthese and note only those gasification processes inwhich biomass can be used as the raw material.
In 2009, Neste and Stora Enso joint companylaunched a pilot plant with a capacity of 656 metrictons a year, with further plans to inaugurate a commer�cial facility in 2016 with a capacity of 100000 metrictons a year. In December of the same year, France’sAgency of Atomic and Alternative Energy announcedplans to build a pilot plant for processing biomass intoliquid fuel (diesel, kerosene, and naphtha) with acapacity of 75000 metric tons a year using wood andagricultural raw materials, and 23000 metric tons ayear using wood and agricultural products.
It should be noted that the composition of theproducts after gasification depends on the type of gas�ification. We present the gas composition for severalreactors in Table 1.
An important step in the technology of gasificationwith subsequent steps of the conversion of water gasand the Fischer–Tropsch reactions is the purificationof gases after gasification, which naturally influencesthe effectiveness of further catalytic transformation.Gases after gasification contain large amounts ofimpurities, both organic and inorganic, that are intrin�sic to biomass; the purification of gases is thereforevery complex, despite available experience in purifyingdifferent gas mixtures of CO2.
Note that not it is only wood biomass that can besubjected to gasification/pyrolysis, but raw materialsobtained in the low�temperature treatment of biomasswith the use of chemical reagents as well (e.g., so�called black liquor, an aqueous solution of a multi�component mixture of organic and mineral substancesformed during the pulping of cellulose). A scheme forthe gasification of black liquor in the production ofsynthesis gas with the subsequent synthesis of dimethylether is given in Fig. 7. A pilot plant was launched inPiteå, Sweden, in September 2010.
The scheme of the CHOREN company’s process isgiven in Fig. 8. It calls for the low�temperature partialoxidation (pyrolysis) of partially dried biomass (up to
15–20% moisture) in the presence of oxygen or air atT = 400–500°C with subsequent high�temperaturegasification.
The pyrolysis of biomass can be performed either asa preliminary step prior to gasification or separately.The facilities of the Ensyn (circulation fluidized bed,4 t/h,) and Dynamotive (fluidized bed, 8 t/h,) compa�nies are characterized by the largest capacities.
Pyrolysis can be performed in a fluidized bed reac�tor with circulation (Fig. 9), and the composition ofthe products depends on the time they spend in thereactor. Prolonged contact results mainly in gas prod�ucts, while so�called rapid pyrolysis with contact times
of one to two seconds makes it possible to obtain up to75% so�called bio oil. A feature of the latter is that theproduct is unstable, subject to transformation duringstorage, and has a low energy value (17 MJ/kg). Thedirect use of bio oil as motor fuel is not possible. Onemethod of bio oil processing is related to its utilizationin generating electrical energy. Further catalytic trans�formations, e.g., with the use of zeolites [25] or thevapor reforming of bio oil [26], can serve as alternativemethods.
Attempts to use catalytic pyrolysis [23, 24] lead toinsubstantial changes in the composition of bio oil (ascompared to thermal pyrolysis) due to the low effi�ciency of the reaction of solid raw material with solidcatalyst. During catalytic pyrolysis with the use of,e.g., zeolites as catalysts, thermal pyrolysis firstoccurs, followed by the catalytic transformation ofvapors. Bio oil obtained during this catalytic pyrolysiscontains nearly 200 products, reflecting the complexcomposition of the raw materials.
Depolymerization
Depolymerization of biomass does not requiresuch rigid conditions (high temperatures) as pyrolysisor gasification. The industrial pulping of wood is per�formed at 150 to 180°C, resulting in delignification,i.e., the decomposition of lignin and hemicellulose.The hydrolysis of hemicellulose in the presence ofmineral acids is possible at even lower temperatures(from 100 to 110°C) [27], but the hydrolysis of crystal�line cellulose is complicated appreciably.
Table 1. Composition of gas, wt. %, obtained after gasification of biomass in flow from 950 to 1200°C (A) and in circulatingfluidized bed: (B) under pressure from 850 to 900°C and (C) Gussing from 550 to 850°C
Component A B C
CH4 < 0.1 8.18 9–11
C2�fraction 0.0 1.61 2–3
C3�fraction 0.0 0.0 0.5–1
CO 50 10.82 22–25
CO2 14 21.05 20–5
H2 27 13.56 38–40
N2 4 8.05 1.2–2.0
H2O (vapor) 35.84 40
H2S 0.12 0.01 ~ 0.15
S organic < 0.1 not determined ~ 0.03
HCl not determined 0.00 not determined
NH3 0.4 0.29 0.1–0.2
HCN 0.3 not determined not determined
C6H6 not determined 0.26 not determined
C10H8 not determined 0.27 not determined
Black
Sulfur�
Gasifier
Green liquor
Fischer–
Absorber–regenerator
liquor
synthesisand synthesis
of methanol/DME
containing gas
O2
Water
Tropsch
Fig. 7. Scheme of gasification of CHEMREC black liquor.
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CATALYSIS IN BIOMASS PROCESSING 223
BiomassPyrolysis gas
Soot
O2 or air
Gas purified
Vapor
Synthesis gas
from resins
Fig. 8. CHOREN technology of biomass processing [22].
Biomass
Pyrolyser
Sand
Hot sand
Carrier gas
BiooilR
ecyc
le g
as
Air afterburner
+char
Gas
Fig. 9. Pyrolysis of biomass in a fluidized bed reactor with circulation.
120
80
40
012008004000
C, mg/g
τ, min
(a) (b)
HCl
Smopex
Amberlist 15
600
400
200
0 1200800400
C, mg/g
τ, min
HCl
Smopex Amberlist 15
800
Fig. 10. Hydrolysis of arabinogalactan with the release of (a) arabinose and (b) galactose in the presence of mineral acids and het�erogeneous catalysts [28].
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The acid hydrolysis of hemicellulose is possible inthe presence of homogeneous and heterogeneous cat�alysts [28]. The acid hydrolysis of arabinogalactan andgalactoglucomannan was studied in the presence ofion�exchange resins that are active at pH = 3, 100°C,and makes it possible to perform the selective elimina�tion (Fig. 10) of arabinose, which is not possible whenusing mineral acids.
Hydrolysis can be performed in the presence ofacids or bases [29, 30], and of enzymes [31], in the lastcase ensuring high selectivity. Drawbacks of enzymehydrolysis include the high cost of the enzymes andlow activity, while the use of mineral acids leads inevi�tably to the corrosion of the unit and to increased costsfor special measures to prevent corrosion. In addition,the use of homogeneous catalysis requires the recoveryof the catalyst.
Enzyme catalysis demands the use of highly spe�cific cellulases and is actually a heterogeneous processthat depends on such physicochemical properties ascrystallinity, the degree of polymerization, surfaceconditions, and the presence of lignin and hemicellu�lose, if cellulose is not used as the raw material. Allthis, as was mentioned above, results in a low reactionrate.
A review of the hydrolysis of cellulose in the pres�ence of mineral acids is given in [9]. Note that dilutedsulfuric acid was used in the hydrolysis of cellulose toglucose as early as the First World War. Other mineralacids were used afterwards, e.g., hydrochloric andhydrofluoric acids. In addition to corrosion and theneed to recover the acids, neutralize the wastes, anddispose of the products of degradation, there is theproblem of the secondary transformation of sugars.This leads to low sugar selectivity due to the subse�quent dehydration reaction, lower quality of the initialraw material for fermentation, and increased costs.Because the rates of hydrolysis of cellulose and degra�
dation of glucose are close, one method to reduce thecontribution from degradation could be combininghydrolysis with other processes, e.g., hydrogenation,leading to the formation of sugar alcohols, which aremore stable than sugars.
Hydrolytic hydrogenation
The concept of combining hydrolysis with hydro�genation was proposed by A.A. Balandin et al., whostudied the hydrolytic hydrogenation of cellulose [32]on applied Ru, Pd, and Pt catalysts in the presence ofphosphoric and sulfuric acids, hemicellulose [33], andeven wood [34]; recently, however, there has been arenaissance of reactions that use only water, withoutthe use of diluted acid solutions.
In 2006, P. Depe and A. Fukuoka [35] mentionedthe possibility of conducting this reaction in water; thiswas the beginning of a series of works with the use ofvarious catalysts [36–42], in which the acid functionof the carrier that accelerates hydrolysis was associatedwith the metallic function that produces hydrogena�tion. In addition to Ru/C and Pt/SiO2–Al2O3, nickel�promoted titanium carbide also exhibited high activityin water at temperatures of 160 to 220°C and at highhydrogen vapor pressures (Fig. 11).
In addition to cellulose, hydrolytic hydrogenationcan take place for the mixtures of cellulose and hemi�cellulose (for example, xylan) [43]. Sorbitol isobtained and, correspondingly, the product of xylosehydrogenation, namely, xylitol (Fig. 12).
Fermentation
The two processes of hydrolysis and fermentationcan be conducted individually to synthesize bioetha�nol from lignocellulose biomass, and it should be espe�cially noted that the side products obtained upon
Cellulose,
Pt/Al2O, 5 MPa
Glucose
Ethylene
Sorbitol
Hydrogen
Alkanes
water,catalyst,hydrogen
glycol
Sorbitol
H2, 463 K Pt/Al2O3, 5 MPa, 463 K
Pt/Ti2O2, H2, 500 K
Pt/C, H2, 500 K
Pt/SiO2�Al2O36 MPa, 518 K
Ni�W2/C, 6 MPaH2, 518 K
H2, 518 KRu/C, 6 MPa
Alkanes
O
OO
HO
OH
OH
O
OH
OHHO
Fig. 11. Examples of combined processes with the hydrolysis of cellulose as the first step.
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CATALYSIS IN BIOMASS PROCESSING 225
hydrolysis do not negatively affect the microorganismsused in fermentation. Some inhibitors of the yeasts offermentation are furfural, hydroxymethylfurfural,organic acids, and the phenols obtained in consider�able amounts when using diluted acids, which requiretemperatures of up to 200°C. The removal of theseinhibitors is possible with the use of activated carbon.
The use of concentrated acids makes it possible tolower the temperature, resulting in a reduction in thenumber of side products, i.e., the inhibitors of sugar’sfermentation into ethyl alcohol. The main drawbackof conducting hydrolysis in the presence of highlyconcentrated acids is the need to remove it from theproducts, e.g., with the use of membrane technologiesor chromatographic separation, naturally leading toincreased costs.
It is not surprising that there have been efforts tocombine the hydrolysis of cellulose and fermentation.The Swedish firm Sekab Ab operates a pilot plant forthe production of bioethanol from wood chips accord�ing to the single�cascade principle of combininghydrolysis and fermentation. One of the main advan�tages of this design (in addition to simplification of theprocess) is the low concentration of sugar (whichinhibits the fermentation enzymes) in the reactor.
Catalytic transformations of ethanol weredescribed in detail recently by Tretyakov et al. [44],therefore, we do not consider these reactions here.
Biobutanol, like bioethanol, can be obtained notonly from chemical sources via the hydrogenation ofbutyric aldehyde (a product of propylene hydroformy�lation) but also through processing the sugar andstarch of agricultural plant cultures (first generationbiobutanol) or plant cellulose. In the first half of the20th century, biobutanol was produced from crops orsyrup via fermentation with the use of Clostridiumacetobutylicum bacteria; as a result, acetone, butanol,and ethanol were obtained (the ABE process) [45].Side products of ABE fermentation are hydrogen; iso�propyl alcohol; acetic, lactic, propionic, and butyricaids; and carbon dioxide and lipids. The need to sepa�rate the main products of fermentation and remove theside products raises the cost of biobutanol.
(Bio)butanol can substitute for gasoline as fuel toan even larger degree than ethanol, due to its physicalproperties, economy, and safety; and because its usedoes not require the modification of automobileengines.
Compared to ethanol, butanol can be mixed withgasoline in higher proportions and used in existing carswithout modifying the systems for forming the air–fuel mixture. The calorific capacity of butanol is com�parable to that of gasoline and is 30% higher than eth�anol’s; butanol is also safer to use, since its vaporiza�tion is one�sixth that of ethanol and it is has one�fourth the volatility of gasoline.
O OH
OH OH
HO OH
OH OH
OH
OO
O
HO
O
OH HO
OH
OH
OH
OHHO
OH OH
OHOH
O
O
OH
Xylose XylitolCatalyst
Decarboxylation
Cellulose and xylane Tar oil, resins
Furfural Furfuryl alcohol
Sorbitol
H2O, H2
Glucose
5HMF
Dehydrocyclization
Catalyst
Dehydrocyclization
H2O, H2
H2
H2
H2
Fig. 12. Scheme of catalytic transformations upon the hydrolytic hydrogenation of a cellulose and xylane mixture.
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MURZIN, SIMAKOVA
In 2009, the small Butyl Fuel Co. in the UnitedStates built a pilot plant for the production of butanolusing technology that prevents the synthesis ofunwanted products, including acetic, lactic, and pro�pionic acids; acetone; isopropyl alcohol; and ethanolwith accompanying carbon dioxide, hydrogen, butyricacid, and butanol. This process doubles the yield ofbutanol per unit of raw material.
Among large companies, Du Pont and BritishPetroleum, which have announced their partnershipin the production of biobutanol, should be noted.They plan to invest $400 million in the construction ofa plant that will initially produce ethanol and thenbegin the production of biobutanol.
As was mentioned above, the production of biobu�tanol was almost terminated in the developed Westerncountries in 1960s, while ABE units continued tooperate in the Soviet Union for several years more[45].
Lactic acid has been mentioned as an unwanted sideproduct of fermentation in the synthesis of alcohols, butit can also be target product of the homolactic fermenta�tion of glucose: C6H12O6 2CH3CHOHCOOH.
At present, half of the world’s production of lacticacid is performed microbiologically, based on theattenuation of such valuable sugar�containing plantsubstrates as refinery syrup, molasses, crystal saccha�rose, sugar syrup, etc., reflected substantially in thecost of the target product. In Russia, lactic acid is pro�duced industrially only at the Zadubrovskii factory[46], which operates on sugar�containing raw materi�als. It produces 2000 metric tons of 40% lactic acid ayear, while the industrial demand for lactic acid isnearly 5000 t/year. Russia produces absolutely no lac�tic acid salts for food purposes, the demand for whichis more than 1000 t/year and will grow in the future.Meanwhile, Dutch�produced 60% sodium lactate and
pentahydrous calcium lactate are sold on the Russianmarket, in addition to cheap 80% lactic acid. Russianbiotechnologists are making substantial efforts todesign high�performance and resource�saving tech�nologies for the production of lactic acid with the aimof replacing expensive and scarce carbon resourceswith cheaper and more available raw materials, e.g.,such industrial processing and agricultural wastes aswhey, along with the wastes from apple processing(mash, marc, and nonstandard apples). This will makeit possible to provide industry with edible lactic acid ofhigh quality. In turn, low�cost lactic acid and lactatescan serve as a cheap feedstock for the development ofnew promising technology of propylene glycol synthe�sis instead of current petroleum�based technology.
Processing Sugar into Fuel and Platform Chemicals
The general scheme of cellulose and hemicelluloseprocessing is given in Fig. 13. Sugars obtained afterhydrolysis can be processed further via heterogeneouscatalysis into derivatives (i.e., alcohols) after hydroge�nation (or into acid, according by oxidation reaction).Examples of reactions with arabinose are described indetail in Fig. 14. Alcohol hydrogenation and oxidationreactions are considered in [8], where it is shown thatapplied Ru catalysts are the ones most active in sugarhydrogenation reactions [47], while Au catalystsapplied to sugar oxidation have recently enjoyed greatpopularity [48–50]. The reaction is structure�sensitiveand its rate depends on the size of the gold crystallites(Fig. 15) [50]. The kinetic models describing thesedependences (which are analogous to those given inFig. 15) and the fundamental reasons for the structuralsensitivity were discussed recently using the thermo�dynamic model suggested by V.N. Parmon [51] andD.Yu. Murzin [52, 53].
Hydrolysis
HMF
Dehydration
FurfuralCellulose,
OligomersAldoses
Oxidation Sugar
Isomerization
Hydrogenation Fermentation
Pentoses
Sugar
Chemicals FuelLubricants
Xylose
Aqueous
hemicellulose
acids
Esterification
reforming
alcohols
OOH O
OO
O
OH
OH
OHOH
Fig. 13. Scheme of the transformation of cellulose and hemicellulose.
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CATALYSIS IN BIOMASS PROCESSING 227
Glucose and the product of its hydrogenation (sor�bitol) can be processed into hydrogen, synthesis gas,alkanes, and oxygenates by means of aqueous reform�ing (Fig. 16), which was initially designed for the syn�thesis of hydrogen [11, 54–56]. Other oxygenates canbe used in addition to glucose and sorbitol: glycerol,ethylene glycol, and methanol. The reaction occursmainly on supported Pt�catalysts, while aluminum,titanium, and zircon oxides can be used as carriers.Palladium is less active, while rhodium, ruthenium,and nickel are more selective in preparing alkanes thanhydrogen [11], due to the metals’ different abilities forbreaking down C–O and C–C bonds. The acidity of
the carrier is also important, since the sequence ofdehydration and hydrogenation steps can be consid�ered one of the many reaction routes that result in theformation of alkanes. The overall scheme of transfor�mations is far more complex (Fig. 17) [57], and a greatvariety products are formed in this reaction (Fig. 18).
The yield of hexane can be increased by usinghydrogen, but the number of carbon atoms in alkanestheoretically cannot exceed six in any case. This is notenough to use the product from the aqueous reformingof polyols as motor fuel.
When Pt–Re catalyst is present on a carbon carrier(Fig. 19), oxygen can be partially removed with the
O
H HO
H
H
OH
O
HO
HO O
OH OH
OH OH
HO OH
O
OH
OH
HO OH
OH
OH
OH O
HO
OH
OHHO
O OHO
OH
HO OH
OH
Hydrogenolysis C–C
Arabinolactone
Dehydration
Furfurol
Arabinonic acid
Reforming
Arabinitol
Hydrogenation Isomerization
Araboketose
Propanediol Glyceron
CO2 + H2
+
Fig. 14. Catalytic transformations of arabinose.
Reaction rate, mmol/(g s)
dAu, nm
6
5
4
3
2
1
0 12 201684
Fig. 15. Oxidation rate of arabinose vs. size (d) of goldclusters.
cleavege
Syngas
Fuel
Synthesis of H2
Aqueous reforming Liquid
Synthesis
Light alkanes
of compoundwith onefunctional group
Break
fuel
elements
Pt�Re/CC�C
Pt/Al2O3
Pt�SiO2/Al2O3
Pt�Re/C
C�O
FT
HO OH
OH
O
OH
HO
OH
OH
HO
Fig. 16. Aqueous reforming of sugars and polyols.
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CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
MURZIN, SIMAKOVA
formation of ketones, secondary alcohols, heterocy�cles, and carboxylic acids. The aldol condensation ofketones with the use of MgAlOx, Pd–MgO/ZrO2,La/ZrO2, and Mg/TiO2 basic catalysts and the keton�ization of carboxylic acids (e.g., on CeZrOx with sub�
sequent hydrogenation) can be an efficient strategy forthe synthesis of alkanes, which can be used as liquidfuels (Fig. 19). A detailed analysis of the abilities of thecatalysts of both aqueous reforming, aldol condensa�tion, and subsequent ketonization and is given in the
HO OH
OH
OHOH
OHOHHO
OHHO OH
HO OH
O
HOHO OH
HO
O
OH OHOH
OH
O
OHOH
OH
OH
O
O
O
OH
O O
C3–C5�ketones, aldehydes, and alcohols
Light alkanes
Similarly via the dehydration–hydrogenation step
Aldol condensation,then hydrogenation
Furan derivatives
R
R R
Substituted ethers C4–C6�alcohols
Alkanes
C6�ketones
Retroaldol
condensation –H2O
–H2O
–H2O, –CO2+H2
+H2
H2 H2H2
Fig. 17. Scheme for the transformation of sorbitol upon aqueous reforming.
OH
OH
OHOH
O O O O
O OO O
O O O
O O
OO
O
O
O
OO
O
O
O
O
O
O
OH
O
OH
O
OH
O
OH
O
O
OH O
O
OO
O
OO
O
OOH
KetonesAlcohols Acids
Hydrocarbons
Other products + aromatics
Fig. 18. Main products of the aqueous reforming of sorbitol.
O
O O
OH
O
CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
CATALYSIS IN BIOMASS PROCESSING 229
H2, light alkanesLight
Alcohol/polyol
Aromatic
AlkanesKetones Alcohols
AcidsHeterocyclic
Water
hydrocarbons
isoparaffins
compounds
Pt�Re/C503 K
H2, COx,
C4�C12
H2O2H2
CuMg12Al7Ox
573 K
CO2, H2, light
Water Water Water
Pd/CeZrOxCeZrOx623 K
ZSM�5673 K
alkanes
OR1 R1OR1
R3 OH
O
R3
O
R3
O
R3
OH
R3
OHR4
OH
Fig. 19. Synthesis of alkanes from sugars and polyols [11].
HO OH
malic acidfumaric acid
3�hydroxypropionic acid
fermentation
glucose
itaconic acid
HO
O
OH
OH
O
HO
O
O
HO
O
OH
O
O
OH
HO OH
OHOH
amination
HO
O
OH
ONH2
asparaginic acid
HO
NH2O
OH
O
succinic acid
OHHO
OO
asparaginic acid
HOOH
OO
HO
O
NH2
OH
O
glutaminic acidO
HO
O
3�hydroxybutyrolactone
fermen
tation
fermentationfermentation
ferm
enta
tionferm
entation
hyd
roge
�
oxidation
dehydration
and
oxid
atio
n
HO
OH
OH
OH
OH
OH
5�hydroxymethylfurfural
sorbitol
HO
OH
OH
OH
OH
OH
Ogluconic acid
OHOoxidation
dehydration
HO
O
Olevulinic acid
oxidation
HO
OH
OH
OH
OH
O
OHO
glucaric acid
Fig. 20. Catalytic transformations of glucose.
nat
ion
230
CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
MURZIN, SIMAKOVA
works of J.A. Dumesic et al. [11]. An example of theretro�aldol condensation of fructose would be
Most of the works in this field are devoted to thetransformation of sorbitol and lower polyalcohols, par�ticularly ethylene glycol. There are fewer works on theuse of pentoses and C5�polyalcohols [58]; nonetheless,
OH
OH
O
HO
O
OH
HOOH
OH
OH
OH
O
these substrates (which are obtained from hemicellulo�ses) can be used in aqueous reforming [11, 58].
Both sugars and polyols obtained from biomass(and their numerous derivatives) can serve as platformchemicals. These compounds include levulinic, suc�cinic, fumaric, lactic, and 3�hydroxypropionic acids;glycerol; sorbitol; and xylitol. A detailed analysis of theuse of these substances and some of their transforma�tions is given in [10, 27]. It is impossible to discuss hereall of the catalytic transformations of these platformmolecules, therefore we present only some catalytictransformations with glucose used as the initial sub�strate (Fig. 20) and levulinic acid (Fig. 21). Figure 20obviously does not cover all possible transformations;for example, levulinic acid can be transformed tomethyltetrahydrofuran, which can be mixed with gas�oline to 70% without having to modify the engine.Though there is no direct route for transforminglevulinic acid into methyltetrahydrofuran, its synthe�
OH
O
O O OH
Oγ�valerolactone
OH
O
OH
O O
OH
O
O
O
γ�valerolactone
Ringdehydration/isomerization
C9 alkanes,
Hydrogenation
Ketonization
Dehydration,
Oligomerization
alkenes
C9–C18 alkanes
Lactic acid Propionic acid
Coke, tarProducts
opening/
GVL
Pd/Nb2O5
H2
Ce0.5Zr0.5O2 CO2, H2O
5–nonanone
Ru/C
H�ZSM�5
H2O
C9–C18
Ru/C
hydrogenation
H2 amberlist
gasoline
H2Hydrogenation
CO2, H2O
H2 H2O
H2H2O
H2
H2
H2OCO2
5�nonanone
OO
OH
OH
O
Fig. 21. Synthesizing components of engine fuel from levulinic acid.
CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
CATALYSIS IN BIOMASS PROCESSING 231
sis using γ�valerolactone with subsequent hydrogena�tion to 1,4�pentanediol and dehydration is consideredmore economical [11]. The synthesis of γ�valerolac�tone is performed by dehydration–reduction orreduction–dehydration at relatively low temperatures(373 to 543 K) and pressures of 5 to 15 MPa using bothhomogeneous and heterogeneous catalysts [59], e.g.,Ru/C. In the literature we find works on the one�potsynthesis of γ�valerolactone, which involves thehydrolysis/dehydration of carbohydrates with subse�quent hydrogenation [60]. The opening of the ring inγ�valerolactone with the formation of unsaturatedacids and subsequent hydrogenation to saturated pen�tanoic acid is interesting. The catalyst used wasPd/Nb2O5, which is stable in aqueous media andmakes it possible to conduct the two reactions men�tioned above [11]. The ketonization of pentanoic acidresults in the formation of 5�nonanone and CO2; itcan be performed, for example, on CeZrOx catalyst at698 K and pressures of 0.1 to 2.0 MPa [61]. An alter�native to this use of C5�acids is decarboxylation onalumosilicate catalyst [62] to butenes and subsequntisomerization on more acidic catalysts, i.e., ion�exchange resins [62] or zeolites [63]. Some catalytictransformations of levulinic acid are reflected in Fig.
22; other transformations that yield valuable chemicalcompounds are possible [10].
Among the key compounds obtained from biom�ass, we should emphasize lactic (2�hydroxypropionic)acid, which has great potential for use in the produc�tion of chemicals, including monomers. The reactionwith alcohols results in esters of lactic acid, which canbe used as so�called “green” solvents. The dehydrationof lactic acid to acrylic opens a lot of opportunities forthe synthesis not only of esters of the latter, but ofpolymers as well. Unfortunately, the yield at the dehy�dration step is low and acidic catalysts deactivate [62].One of the most promising techniques involves biode�gradable plastics (polylactate, produced by the NatureWorks Co.). The hydrogenation of lactic acid leads topropylene glycol, which is widely used as a solvent anda valuable intermediate in, e.g., the synthesis of propy�lene oxide. Due to the importance of this reaction, letus consider the hydrogenation process in detail.
The industrial production of 1,2�propanediol istraditionally based on the hydration of propyleneoxide in acidic media. Until 1970, propylene oxidewas obtained from propylene chlorohydrin only:
CH3 CH CH2 CH3 CH CH2
O
Cl2, H2O Ca(OH)2, H2O
–CaCl2. CH3 CH CH2
OH Cl
OHOH
OH
HOHO
N
Ar
O
COOH
O
N O
O O
O OOO
OH
COOR
O
N O
humines
5�hydroxymethylfurfural
formic acid
levulinic acid
vacuum
HCOOH
H+
–3H2O
ArNH2, H25% Pt/C
150°C, 3.5 MPa
ArNH2, H25% Rh/C
150°C, 3.5 MPa
1) H2, Ru,150°C, 3.5 MPa2) formaldehyde
H2 Ir/SiO2PhCN
1) glycerol,2) NaOMe, MeOH
ROH, acidic/alkalinecatalysts
Fig. 22. Synthesis and transformations of levulinic acid [10].
O CHOCH2OHHOH2C
232
CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
MURZIN, SIMAKOVA
All attempts to synthesize propylene oxide via thedirect oxidation of propylene with oxygen on a silvercatalyst were unsuccessful, since the C–H bonds ofthe methyl group in the allylic position toward thedouble bond were subject to direct oxidation.
An alternative industrial method (the Halcon pro�cess) for the synthesis of propylene oxide was devisedsomewhat later [64]. In the Halcon process, propyleneoxidizes into oxide under the action of tert�butyl hydro�peroxide or ethylbenzene hydroperoxide (Fig. 23). Therequired hydroperoxides are synthesized through theoxidation of isobutene or ethylbenzene in the liquidphase with oxygen at temperatures of 120 to 150°C andpressures of 3.0 MPa. A nearly 25% transformation ofhydrocarbon into hydroperoxide is attained. Hydroper�oxide then reacts with propylene in the liquid phase at120 to 140°C and 3.5 MPa in the presence of catalyst(molybdenum naphthenate). Two variants of the Hal�con process are illustrated in Fig. 23. The yield of pro�pylene oxide on propylene is 90%. In both variants,nearly 3t of tertiary butyl alcohol and 3.5 t of 1�phenyle�thanol is formed for each metric ton of propylene oxide[64]. Even though tertiary butyl alcohol is used to raisethe octane number of gasoline and to synthesize isobu�tylene, while 1�phenylethanol is used in the synthesis ofstyrene, the problem of completely utilizing these sideproducts remains actual.
There are several possible routes for the synthesis ofpropylene glycol. One possibility is the fermentativetransformation of sugars to dihydroxyacetone phos�phate (which is further reduced to 1,2�propanediol).Different 6�deoxyhexoses (L�rhamnose and L�fruc�tose) can be transformed into S�propylene glycol withthe use of several bacteria, but this process is unprofit�able, since 6�deoxyhexoses are very expensive.
Another variant of this process allows the use of cheapglucose and xylose in the presence of several Clostrid�ium sphenoides and Thermoanaerobacterium thermo�saccharolyticum [65] or natural strains of recombinantEscherichia coli [66].
Another variant involves the catalytic hydrogena�tion of sugars and sugar alcohols (saccharose, xylitol,and sorbitol) on such catalysts as Ni/SiO2 [67],Ni/SiO2/Al2O3 [68], Ru/C [69], Ni–Re/C, and Pd–Re/C [70]. A common drawback of these processes istheir low selectivity. As a result, we obtain mixtures ofmonoatomic (methanol and ethanol) and diatomic(ethylene glycol, propylene glycol, and butandiols)alcohols, glycerol, and lactic acid and lactates.
Ultimately, we can combine the enzyme fermenta�tion of sugars to lactic acid [71] with the subsequentcatalytic hydrogenation of lactic aid to 1,2�propandiolon various metallic catalysts in the flow and stationarymodes [72, 73]:
The synthesizing of propylene glycol from lacticacid (and especially its lactates) by selective reductionof the carboxylic group of lactic acid to hydroxyl ismost promising. From an economic point of view, theprospect of synthesizing propylene glycol from lacticacid is due mainly to the cost of the process, i.e., thecost of the initial lactic acid. As has been noted, lacticacid is attractive raw material for the synthesis of vari�ous chemicals: propylene glycol, acrylic acid and acry�lates [74–76], acetaldehyde [77], 2,3�pentanedione[78–81], and biodegradable polymers based on poly�lactide [82, 83]. Promising catalytic processes for pro�cessing lactic acid are described in Fig. 24.
The hydrogenation of lactic acid on Re blackobtained by the reduction of commercial rheniumheptoxide results in quite large yields of propylene gly�col [84]. On Ru�catalysts, the hydrogenation of opti�cally active L�(+)�lactic acid takes place at T ≤ 353 Kwithout racemization [85]. Similar results wereobtained in [86], where the process was performed athigher temperatures (403 to 443 K) but with hydrogenpressure reduced to 3.3 MPa. Propylene glycol isformed on Ru�catalysts at high hydrogen pressures (upto 10 MPa) and moderate temperatures (≈423 K), andthe main side product is methane.
Copper chromite, used since the 1940s and ‘50s toobtain different alcohols through the hydrogenolysisof esters (including butyl and ethyl lactate) [87–89],exhibits low activity during the hydrogenation of lacticacid in the liquid phase [90]. Other catalysts (e.g.,Pd/C and Ni/Al2O3) proposed for the hydrogenolysisof esters turned out to be inactive in the hydrogenationof lactic acid, while Raney nickel partially dissolved inthe acid reaction mixture [86].
Under the mildest conditions, lactic acid isreduced to propylene glycol in the presence of Cu–Si
H3C C CH O
OHOH
H3C C CH2H
OHOH
+H2, –H2O
Cat .
HC CH2H3C
HC CH2H3C
O
O
OH
OH
HC CH2H3C
HC CH2H3C
O
OHO HO
O2
150°C, 3.0 MPa
(a)
(b)
120°C, 3.5 MPa
120°C, 3.5 MPa
O2
150°C, 3.0 MPa
Fig. 23. Variants (a, b) of the industrial synthesis of propy�lene oxide with the use of organic hydroperoxides.
CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
CATALYSIS IN BIOMASS PROCESSING 233
catalysts [73, 91–94]. The catalytic properties of cop�per�containing catalysts (copper chromite, copper–zinc hydroxysillicate, and copper hydroxysillicate)were studied in [91–94]. The highest activity wasexhibited by copper–silicon catalysts that had a com�pound precursor with a chrysocolla�type structure.Their high activity was presumably due to the forma�tion of stable, highly dispersed particles of metalliccopper during preliminary reduction. In the presenceof this catalyst, high selectivity (~75%) was attained inthe formation of propylene glycol at a close to 100%degree of conversion of lactic acid (Fig. 25) [91–94]under optimum conditions (200°C and =
0.1 MPa), substantially exceeding the 7.3% conver�sion of lactic acid attained under the same conditionsin [73]. During the hydrogenation reaction, however,we observe the slow deactivation of copper–silica cat�alyst due to the rather high acidity of the medium. It isknown that carboxylic acid esters are reduced to alco�hols under milder conditions (at lower temperaturesand hydrogen pressures) than do unsubstituted acids.The advantages of using lactic acid esters in the gas�phase process of hydrogenolysis are their chemicalnonaggressiveness and relatively low boiling point, thepossibility of increasing the concentration of substratewhile retaining a high degree of conversion, and the
pH2
capability of raising the selectivity and stability of thecatalyst due to the inhibition of side reactions.
The formation of methanol and n�butanol, respec�tively, and of hydroxyacetone and propylene glycol,was detected during the hydrogenolysis of methyl andbutyl lactate in [94]. The dehydrogenation of butanolto butanal was not observed under the reaction condi�tions (Fig. 26). As follows from Fig. 27 [94], the degreeof conversion increases for alkyl lactates upon a rise intemperature, and the selectivity of the formation of
propylene glycol
acrylic acid
acetaldehydelactic acid
2,3�pentanedione
lactide polylactic acid
OH
OH
O
OH
OH
OH
OH
O
O
H
O
O
O
OO
O
+ H2O
+ H2O+ 2H2
+ CO + H2O
+ CO2 + H2
O
O
+ H2O**
+ CO + 2H2O
n
Fig. 24. Promising catalytic processes of lactic acid processing.
Selectivity, %
τcontact, 10–3 min
80
70
30
20
101.00.80.60.40.2
1
2
~~
Fig. 25. Hydrogenation of lactic acid with the formation of(1) propylene glycol and (2) propionic acid in the presenceof Cu/SiO2.
234
CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
MURZIN, SIMAKOVA
propylene glycol during hydrogenolysis is somewhathigher than that for methyl lactate.
A comparison of the literature data and the dataobtained as a result of our studies of the hydrogenolysisof lactic acid esters is presented in Table 2.
During experiments on the hydrogenolysis of lac�tates, it was discovered that a more profound transfor�mation of both methyl and butyl lactate occurs whenthe copper content is raised to 45.5 wt % (Figs. 27 and28), and the use of butyl lactate is preferential due tothe higher degree of conversion at T > 180°C. In addi�tion, methanol undergoes undesirable transforma�tions in the presence of copper�containing catalysts,
limiting the extent of its repeated use in the esterifica�tion of lactic acid. Under the conditions investigatedin [94], the amount of methanol was somewhat lowerthan the amount of transformed methyl lactate, dueprimarily to the formation of methyl formiate, dime�thyl ether, and carbon monoxide. Butanol, unlikemethanol, probably does not have side reactions ofdehydrogenation. Cu–Si catalyst with a content of45.5 wt. % Cu is the most active. A comparison of thereactivity of methyl and butyl lactate shows that thenature of the alkoxyl group does not substantiallyaffect the ratio of the products. Any of the volatile lac�tic acid esters can therefore be used to synthesize pro�
O
ORHO O
OR
OH
ORHO
H3C
OH
ORHO
H3C O
HHO
H3C OH
HHO
H3C
OH
HHO
H3C OH
HO
H3C
+H+
+H2
+
–H+
R = CH3, C4H9
alkyl lactate
hydroxyacetone
hemiacetal 2�hydroxypropane propylene glycol
2�hydroxypropanal
+H+
–ROH
+H2
hydroxyacetone
+H2 –H2
Fig. 26. Scheme for the transformation of alkyl lactates in the presence of Cu/SiO2.
Con
vers
ion
, %
Sel
ecti
vity
to
prop
ylen
e
Methyl lactateButyl lactate
100
90
80
70
60
50210200190180
100
90
80
70
60
glyc
ol,
%
T, °C
Fig. 27. Comparison of the degree of conversion and selec�tivity of formation of propylene glycol under comparableconditions. Injection rate of methyl lactate is 0.4 ml/h;butyl lactate, 0.6 ml/h; and hydrogen, 10 l/h; the amountof catalyst is 0.5 g.
Conversion, %
90
70
50
30
10190180170160 T, °C
45.5
22.0 14.2
Fig. 28. Effect of copper content wt. % (on curves) on theactivity of Cu/SiO2 catalyst upon hydrogenolysis. Condi�tions of reaction are as follow: hydrogen flow rate, 10 l/h;injection rate of methyl lactate, 0.4 ml/h.
CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
CATALYSIS IN BIOMASS PROCESSING 235
pylene glycol, but the use of methyl lactate is undesir�able due to its side catalytic transformations. Note thehigh stability of Cu–Si catalyst during the hydrogenol�ysis of lactates to propylene glycol, as compared to thehydrogenation of initial nonesterificated lactic acid.
Lignin
Lignin, one of the main components of lignocellu�lose biomass, can theoretically be processed into valu�able chemicals catalytically. In practice, only vanillinis now obtained from lignin, even with all the potentialof the latter, a source of phenols and aromatic hydro�carbons. Among the reactions of the transformation oflignin, we should emphasize hydrogenation, oxida�tion, cracking, and hydrolysis. A detailed and verycomplete review of the catalytic transformations oflignin was published recently [95].
The cracking or hydrocracking of lignin can be per�formed on the same catalysts used at oil refineries: theacidic centers of zeolites are required for cracking,while the hydrogenating function is achieved, e.g., byadding cobalt, nickel, or palladium to the catalyst.
HZSM�type zeolites have been used for cracking inthe range of 613 to 683 K with the formation of consis�tently heavy low�volatile substances, volatile alkylaro�matic compounds, and coke and gases [96]. Ther�mocatalytic pyrolysis includes cracking, cyclization,aromatization, and iso� and polymerization. Its activ�ity, selectivity, and stability depend on the type of zeo�lites and their acidity [95].
Hydrolysis in the presence of bases has been stud�ied in detail, while delignification [97, 98] is one of themain processes in the manufacture of cellulose andpaper. It should be noted that selecting effective cata�lytic processes of lignin transformation into valuablechemical source remains a problem.
Biodiesel Fuel
The use of plant raw materials for the preparationof biofuel (particularly biodiesel) has a long history.The first engine designed by Rudolf Diesel in 1900operated on vegetable oils, including pure peanut oil.The drawbacks intrinsic to vegetable oils (e.g., high
viscosity and low stability) led to his rejection of natu�ral oils in favor of more practical petroleum fuel.
The biodiesel concept was recently applied to fattyacid methyl esters, a product of trans�esterification[99–101]. The term “biodiesel” is now used to desig�nate every diesel fuel made from renewable raw mate�rials.
Most biodiesel is produced by the trans�esterifica�tion (alcoholysis) of plants and fats as a result of thereaction of alcohol with triglycerides in the presence ofhomogeneous catalyst (usually acids, alkali, orenzymes) with the formation of glycerol and fatty acidesters [99]. In addition to mainly industrial methanol,there are works on the use of other alcohols up to C5[99, 102–104]. The results from studies of the trans�esterification of natural oils in alcohols in the super�critical state were also described in [105, 106].
The drawbacks of fuels based on fatty acid esters arerelated mainly to their chemical instability andreduced performance characteristics at low tempera�tures, diminishing their attractiveness for regions witha cold climate, such as Russia. The need to use homo�geneous catalyst complicates the technological pro�cess due to the bulky equipment required to separatehomogeneous catalyst from the products of reaction;this in turn is reflected in costs, and ultimately in theprice of biodiesel. It is not surprising that considerableattention has recently been given to developing newmethods for the trans�esterification of triglycerides in thepresence of heterogeneous catalysts [101, 107–109].
Note that not only rapeseed, palm, or soybean oilcan be used as raw materials in the preparation of fattyacid methyl esters, but tall oil obtained from theextractants of lignocellulose biomass as well. In addi�tion to fatty acids, tall oil includes rosin acids and ste�rols. The products of Sweden’s Sunpine Co. (Piteå)with its capacity of 100000 t/year should be noted asan example of wood biomass being used in the manu�facture of biodiesel.
An interesting alternative to biodiesel is “green”diesel, obtained according to the NExBTL technologyof the Neste Oil Co. [110]. The process includes thecatalytic hydrogenation of vegetable oils into theircorresponding alkanes. The formation of glycerol is
Table 2. Conversion of lactic acid esters and selectivity to propylene glycol on various catalysts ((A) conversion and (B)selectivity)
impossible, since the glycerol fragment of triglyceridehydrogenates into propane if triglycerides (and notonly tall oil) are used. Hydrodeoxygenation takesplace during this reaction; i.e., the diesel fuel obtainedcontains no oxygen, as in the case of biodiesel synthe�sized by trans�esterification. In contrast to trans�ester�ification biodiesel, which is yellow, the final product isa transparent, colorless paraffin with a high cetanenumber (from 85 to 99) and fuel properties that areeven better than those of petroleum�based diesel fuel.The chemical product is identical to conventional die�sel fuel, thus no changes need be made in the structureof the fuel. The low�temperature properties (the cloudpoint) of NExBTL fuel can be controlled according toclimatic conditions at temperatures of –5 to –30°C.
The first unit based on this technology was con�structed in the summer of 2007 at a Neste Oil Co.refinery in Porvo, Finland. A second unit was put intooperation in 2009. The yearly production of each unitis 190000 metric tons. In November 2010, Neste Oilbegan the production of renewable NExBTL dieselfuel in Singapore with a capacity of 800000 t/year,making it the largest company in the world in terms ofthe production of diesel fuel from renewable sources.A similar plant is being built in Rotterdam with ascheduled launch date in 2011.
Decarboxylation
To ensure the competitiveness of biodiesel relativeto conventional diesel fuel, its production mustbecome less expensive and the fuel properties of thefinal product must be improved. n�Paraffins obtainedfrom biological sources are a good basis for dieselfuels, due to their high cetane number and ecologicaladvantages. Paraffin diesel fuels can be produced fromnatural oils and fats via the selective deoxygenation ofnatural oils and fats, along with fatty acids and theiresters. Oxygen is removed from the molecules of trig�lycerides by decarboxylation in the liquid phase
and/or decarbonylation, resulting in the formation oflinear hydrocarbons (Fig. 29) [111–127].
There are several possible reaction paths for pro�ducing linear hydrocarbons from fatty acids. Fattyacids can be directly decarboxylated or decarbony�lated. Direct decarboxylation results in the elimina�tion of carboxylic groups with the formation of carbondioxide and the obtaining of paraffin hydrocarbon,while direct decarbonylation leads to the formation ofolefin hydrocarbon through the elimination of carbox�ylic groups with the formation of CO and H2O. Fattyacids can also be decarboxylated according to theirreaction with hydrogen; direct hydrogenation or indi�rect decarbonylation in this case lead to the formationof linear hydrocarbons.
The catalytic decarboxylation of renewable prod�ucts [111–127] has been performed in batch, semi�batch, and continuous modes. Semi�batch experi�ments were performed in an autoclave with stirring,reducing the effect of external mass transfer. Compar�ative experiments were performed in a batch reactor todetermine the optimum mode of the process. In con�trast to the semi�batch mode, there is no constantremoval of the gas phase in the batch mode. The pres�sure therefore does not remain constant and increasessomewhat during the reaction, due to the formation ofgaseous products.
Several catalysts were studied in [111]. The decar�boxylation of stearic acid was studied on Pd, Pt, Ru,Mo, Ni, Rh, Ir, Ru, and Os. Tests of bulky catalysts(Rhaney nickel) were performed. Industrial Pd cata�lyst on a carbon carrier was found to be the most activeand selective. Pd/C catalysts were also tested withSibunite (a mesoporous carbon carrier) in particularduring decarboxylation [127].
The heterogeneous catalytic deoxygenation ofstearic acid results in the formation of n�heptadecane(n�C17). In small quantities, however, the formation of1�heptadecene (1�C17) and other unsaturated C17products was observed.
In comparing catalytic activity during the deoxy�genation of stearic acid on various catalysts, it has beenfound that the activity of catalyst on a carbon carrier isthe highest. The high activity of such catalysts is prob�ably due to their texture, highly developed porous sys�tem, and specific surface, which is by far greater forcarbon carriers than that for silica or alumina carriers,minimizing the deactivation of catalyst as a result ofagglomeration and/or coking. Another possible expla�nation is that the properties of carbon carriers dependon the origin (amphotericity) of the carbon and thepresence of surface functional groups that allow the cat�alytic reaction to be conducted effectively.
As in the semi�batch mode, the deoxygenation ofethyl stearate was successfully performed in the con�tinuous mode on a 5% Pd/C powder catalyst (AldrichCo.). Hydrocarbons, intermediate (fatty acids), andsome side compounds were found among the products
n, s–1
r, nm
160
120
80
40
03.02.51.51.00.5 2.0
H–O–C–R' CO2 + R'–H
O
Fig. 29. Decarboxylation effect of the dispersion of Pd par�ticles on n, the turnover frequency.
CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
CATALYSIS IN BIOMASS PROCESSING 237
of deoxygenation. During the deoxygenation of ethylstearate (5 mol % in hexadecane), a high yield of hep�tadecane was attained at 330°C that was maintainedover time (80% after 5 h). The catalyst remained quitestable under these conditions and provided constantselectivity of the formation of heptadecane of ~90% atminimum deactivation. Among the side products wereproducts of the isomerization and dehydrogenation ofethyl stearate.
The effect of the chain length in fatty acids wasstudied on the decarboxylation rate, and it was shownthat the reaction rates for stearic and palmitinic acidare comparable [124].
Note that decarboxylation is a structure�sensitivereaction [120]. In [120], a series of catalysts of 1 wt. %Pd on mesoporous carrier Sibunite with various dis�persion was studied. The use of this carrier allows theinitial organic substrates free access to the active metalparticles in the pores of the carrier. The results given inFig. 29 demonstrate that the curve of specific catalyticactivity has a maximum whose position depends onthe size of the Pd particles. In [120], TPD and IR dataare given to explain the observed patterns.
The decarboxylation of a mixture of stearic andpalmitic acids in a flow reactor [121] showed that5 wt. % Pd on Sibunite with a core�shall distributionof palladium is quite stable at 360°C in an argon flowor a 5% H2/Ar mixture. The main products of deoxy�genation in the liquid phase were hepta� and pentade�cane, indicating that this technology can be used in theflow mode as well.
Hydrogenation
The decarboxylation of fatty acids depends on thepresence of unsaturated bonds and leads to deactiva�tion; catalytic hydrogenation should therefore be amandatory step prior to decarboxylation.
There are many works (e.g., [128–131]) on studiesof the hydrogenation of the vegetable oils used toincrease the resistance of natural oils and fats to oxida�tion upon storage and processing. Hydrogenationleads to a change in their consistency and raises theirmelting point to the required level upon minimalchanges in the ratio of unsaturated and saturated acidsin the fatty acid composition of the initial raw material(edible hydrogenated oils) and changes in the consis�tency, melting point, and content of the saturated fattyacids in hydrogenated raw materials (multipurposehigh–melting point hydrogenated lipids). Duringhydrogenation, the migration and cis/trans isomeriza�tion of the double bond take place simultaneously,thus complicating the process (Fig. 30).
The properties of hydrogenated oils are determinedby the chemical composition of the hydrogenated rawmaterials, the ratio of the rates of chemical transforma�tions upon hydrogenation, and the degree of complete�ness of the chemical transformations in the raw materi�als. During the production of confectionery fats, prod�ucts of the partial hydrogenation of fatty acidtriglycerides with a certain ratio of cis/trans isomers arethe target products. The traditional technonogy for thehydrogenation of vegetable oils in the presence of Nicatalysts is characterized by a number of major short�comings: the strict mode of the process (from 190 to230°C) with the formation of products of the thermal
isomerizationC 18:1 isomers(10) cis� or (10) trans�or (11) cis� or (11) trans�
Fig. 30. Scheme for the hydrogenation of C18 fatty acids of vegetable oils.
(CH2)10CH3(CH2)4 CO
OEt
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MURZIN, SIMAKOVA
decomposition of oils (including carcinogenic prod�ucts), a large number of side trans isomers, and impos�sibility of completely separating toxic nickel from thefinal products of hydrogenation. The use of catalystsbased on Pt�group metals makes it possible to performhydrogenation under milder conditions than with Nisystems and to obtain products for both edible andindustrial use. A clear advantage of using catalysts basedon VIII�group metals is the possibility of hydrogenatingfree fatty acids, while Ni systems are not stable in acidicmedia and form soluble salts with acids.
Controlling the process of hydrogenation to pro�duce hydrogenated oils with specified propertiesinvolves selecting the raw materials for hydrogenationthe catalyst, and the technical conditions of the pro�cess (temperature; hydrogen pressure; length of con�tact between the raw material, hydrogen, and catalyst;and the ratio of the rates of adding the raw material,hydrogen, and catalyst to the reaction zone). Hydro�genated oils with various properties are produced(along with hydrogenated fatty acids) by combiningthe controlled parameters of the process on an indus�trial scale.
There are several methods for the hydrogenation ofvegetable oils and fats in the presence of catalystsbased on transition metals (Mo, W, Rh, Ir, Ru, Os, Re,Fe, Co, Ni, Pd, Cu, Ag, Au, Zn, Ga, Gd, etc.) [132].At present, however, the catalysts used for hydrogena�tion in the fats�and�oils industry are based mainly onnickel (Nyssosel N�222 (Engelhard, now BASF Cata�lysts) and Pricat 9908 (Johnson Matthey)). The pro�cess is conducted at 120 to 220°C and hydrogen pres�sures of 0.1 to 0.9 MPa. However, since nickel and itscompounds produce allergic and carcinogenic effects,and products of hydrogenation in the presence of Nicatalyst are characterized by a high content of undesir�able trans isomers, much attention is now being givento finding and developing new catalysts based of VIII�group noble metals instead of conventional nickel forthe hydrogenation of vegetable oils.
An analysis of the literature data shows that Pd�containing catalysts on carbon carriers are the most
active in the hydrogenation of vegetable oils [133–136]. The effectiveness of the process (the hydrogena�tion rate, selectivity, and degree of formation of unde�sirable trans isomers) in the presence of Pd catalysts isdetermined to a large degree by the nature of the car�rier used, i.e., its liophilicity to the components of thereaction and its inertness to side reactions of iso� andoligomerization, along with the porous system thatemerges to determine the specific surface of the cata�lyst. If the catalyst pores are sufficiently wide, thecomposition of liquid phase will differ insubstantiallyfrom the composition of the hydrogenated raw mate�rial on the external surfaces of the catalyst particles. Ifthe pore diameter is comparable to the sizes of thehydrogenated molecules, the mass transfer in thepores is complicated considerably. There are situationsin which the hydrogenation rate of glycerides of diun�saturated fatty acids on the pore surfaces greatlyexceeds the diffusion of these glycerides into the poresand the selectivity of hydrogenation is diminished.Catalysts with pore sizes of more than 2.5 nm are cre�ated to increase the selectivity of hydrogenation.
There are various methods for obtaining ediblehydrogenated fat, in which the hydrogenation catalystis palladium:
• The hydrogenation of vegetable oils in the pres�ence of Pd/Al2O3 catalyst with particle sizes of 10 to100 μm at a total concentration of palladium in oil of0.001 wt. % [137]. Drawbacks of this method are thelow rate of hydrogenation and the rapid deactivationof catalysts, due to the inhibition of active centersthrough the adsorption of impurities of sodium soap inoil and polar phopshatides on the polar surface of thealuminum oxide.
• The hydrogenation of vegetable oils at 42 to285°C and hydrogen pressures of 0.1 to 0.4 MPa in thepresence of catalysts containing 0.5 to 5.0% Pd onactivated carbon, with a high specific surface at a totalpalladium concentration of 0.0005 to 0.02 wt % of theoil mass [138].
• The liquid�phase hydrogenation of vegetableoils, in which the hydrogenation of vegetable oils and
Table 3. Hydrogenation of rapeseed oil in the presence of 1% Pd/C. Effect of metal cluster size (ds) and concentration pro�file of palladium on the ratio of trans�/cis�isomers
Substance Pd/C* XPS, at % ds, nm trans/cis
C=C
Unsaturated acids, molar
fractionsIodine number
Initial rapeseed oil 0.174 0.007 0.932 108.2
Catalyst:
A(Ground) 0.28
A 0.55 1.8 2.7 0.32 0.758 64.9
B 1.21 1.6 1.7 0.65 0.604 51.8
C 0.97 3.5 3.0 0.54 0.649 55.6
* 0.114 is the theoretical value for the uniform distribution of Pd through the support grain in 1 wt. %.
C–C( )
C=C( )��������������
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CATALYSIS IN BIOMASS PROCESSING 239
fats is conducted in the presence of a Pd catalyst ofmetallic palladium (0.1 to 4.0 wt. %) on 2–20 nmpores. In the catalyst as carrier, carbon material with aspecific surface of transport pores ranging from 50 to500 m2/g, a volume of 0.2 to 1.7 cm3/g, and an addi�tional maximum in the range of 2 to 20 nm, is used.Hydrogenation is performed at T = 80–200°C, =
0.1–0.2 MPa, and a concentration of catalyst on themetallic palladium of 10–4 × (1.5–5)% of the oil mass[139]. A drawback of this high�temperature method isthe high content (nearly 48%) of trans isomers in theproducts of hydrogenation, seriously degrading thequality of the final products (margarine, mayonnaise,cosmetic fats, etc.), and the insufficient conversion ofhydrogenation (the iodine number falls by only 30–40 units).
The authors of [140] studied the effect of the dis�persion of palladium and the distribution of Pd alongthe grain on the content of trans isomers in hydrogena�tion products. It was shown that the lower the disper�sion of palladium and the higher its concentration onthe edge of a grain of carbon carrier, the lower the con�tent of trans isomers in the hydrogenation products(Table 3). In other words, the cis/trans ratio dependson the stability of the adsorption of C=C fragments offatty acid on an active center of the metal and theduration of contact between a molecule and the metalsurface. As was discovered in studying the kinetic prin�ciples of the hydrogenation and isomerization of cism�ethyl oleate on Pd/C, Rh/C, Pt/C, Ru/C, and Ir/C,the activity of VIII�group metals increases in the fol�lowing reactions: aIr < aRu < aPd < aRh < aPt hydrogena�tion (Fig. 31) and aIr < aPt < aRu < aRh < aPd isomeriza�tion [141]. The ratio of isomers during the reactionsdiffers from the equilibrium, due possibly to the differ�ence in the strength of adsorption of substrate on thesurface of VIII�group metals and the relative ability ofdifferent metals to ensure the β elimination of hydro�gen atoms from adsorbed alkyl intermediates.
While not the most active catalyst in the hydroge�nation of unsaturated bonds of fatty acids, Pd/C cata�lyst is nevertheless quite active and, according toexpert estimates, its cost is most reasonable, making itespecially attractive (as compared to conventional Nicatalyst) for industrial use. It is important that underindustrial conditions, the hydrogenation of fatty acidsand their triglycerides is generally conducted underconditions of controlled diffusion of hydrogen. Inpractice, this neutralizes the differences in the hydro�genating ability of VIII�group metals, emphasizingproblems of stability, the high technology involved insynthesizing the catalyst, the optimizing of costs, andthe ease of regenerating an expensive noble metal.
A series of interesting studies was devoted to simu�lating the hydrogenation of vegetable oils and studyingthe problems of mass transfer [142, 143]. A kineticanalysis of the hydrogenation of cis�methyl oleate inthe presence of Pd catalyst [144] showed that, accord�
pH2
W 0H2
, mol/(l g s)
1.6
1.2
0.8
0.4
0Ir/CRu/CPd/CRh/CPt/C
Fig. 31. Effect of the nature of active metal of catalyst on
initial hydrogenation rate of methyl oleate, =
0.1 MPa, = 0.1 MPa, T = 100°C, = 0.16 M,
and mcat = 20–100 mg.
WH2
0: pH2
pN2CC18 : 1
Tr
T
Z
L
LH
LproTk+5
LproCZ
CZ
Ci
k+4
Fig. 32. Scheme for the mechanism of hydrogenation ofcis�methyl oleate with the formation of semi�hydroge�nated pro�cis�intermediate preceding isomerization; L ismethyl linoleate, T is trans, and C is cis�methyl oleate.
Cis/trans ratio
Hydrogen pressure, MPa
5
4
3
21.00.80.60.40.20
Fig. 33. Effect of hydrogen pressure on the ratio ofcis/trans isomers.
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MURZIN, SIMAKOVA
ing to the scheme suggested for the mechanism ofreaction (Fig. 32), stereoselectivity depends to a con�siderable degree on the hydrogen pressure and isdescribed by the following equation:
It was determined that the contribution fromisomerization falls substantially with an increase inhydrogen pressure, approaching a constant value after0.5–0.6 MPa (Fig. 33). This indicates to the inadvis�ability of conducting the reaction under hydrogenpressures above 0.9–1.0 MPa, and is of great techno�logical importance in industrial hydrogenation.
The use of grainy Pd catalyst applied on a carbonsubstrate (effective when processing vegetable oils anddistilled fatty acids in the continuous mode) was dis�cussed in [145]. The hydrogenation of rapeseed andsunflower oils was performed in the presence of 1 wt. %Pd on Sibunit with core�shell Pd distribution in a flowof 0.5 to 0.9 t/h and temperatures of 180 to 230°C,yielding conversion of 80 to 90%. Semi�industrial testsunder various conditions (temperature, hydrogen pres�sure, and flow rates of raw material and hydrogen) over500 h resulted in the production of more than 320 t ofcommercial quality hydrogenated fat.
Fine Chemistry
The scope of this review does not allow us to con�sider in detail other catalytic processes of biomass pro�cessing. Catalysis in the transformation of terpenes,carbohydrates, steroids, and lignans for the processesof fine chemistry was discussed in [3, 8, 146, 147]. Wewould like to emphasize only the fragrance substances(FSes) obtained from natural sources (e.g., essentialoils). These substances are widely used as componentsof perfumes, disinfectants, and scents for plastics, res�ins, and synthetic cleaning agents. In addition tosmelling good, FSes have antiseptic properties and areused in medicine for the production of various drugsand vitamins. The growing interest in FSes by farexceeds the volume of natural essential oils produc�tion. There is thus a need to develop new methods forsynthesizing FSes that would make it possible toincrease the number and variety of new aromas.
Monoterpenes (i.e., α� and β�pinenes), the maincomponents of turpentine oil (there is 60–72% of α�pinene and not more than 5.0% β�pinene in turpen�tine oil), the multi�tonnage byproduct in the produc�tion of cellulose and rosin are of special interest ascheap and safe raw materials for the production of anumber of FSes, vitamins, and drugs. According to[148], more than 25% of the world’s production of α�and β�pinene is used in the manufacture of syntheticfragrances.
Two valuable FSes obtained on the basis of α�pineneare isopiperitinol and citral. The latter has an intrinsic
rcis
rtrans
��������k+4θLprocisθH'
k+6θLprocis
�������������������������k+4 KHpH2
k+6 1 KHpH2+( )
���������������������������������� .= =
lemony smell and is widely used in perfumes and scentsfor cleaning agents; the production of edible essences inthe confectionery industry; and in antiseptic, anti�inflammatory, and analgesic drugs [149, 150]. Citral isalso used in the synthesis of a number of fragrance com�pounds (citronellol, geraniol, nerol, ionone, meth�ylionone, etc.), and in the production of A, B, and K1vitamins [149, 151]. Isopiperitenol can be used to syn�thesize p�menthol. Development an effective method ofsynthesizing isopiperitenol and citral from α�pinene isof current interest in catalytic organic synthesis.
In works by one of the authors of this review, a com�plex scheme for the processing of α�pinene was pro�posed that involved the formation of isopiperitenoland citral with the autooxidation of α�pinene bymolecular oxygen and the hydrogenation of α�pinenehydroperoxide formed in the presence of a Pd/C cata�lyst with the formation of verbenol, the pyrolysis ofwhich yields the target product (Fig. 34, horizontalchain). A feature of this scheme is that, on the onehand, valuable substances are formed as final productsand, on the other hand, intermediate (verbenol), final(citral and isopiperitenol), and side (α�pinene epoxideand verbenone) products are also key compounds inthe chains of synthesis of other valuable fragrance sub�stances [152–154]. Note that the further transforma�tion of citral by hydrogenation in the presence of cop�per�containing catalysts was studied in [155].
Another interesting area in the processing of α�pinene proposed by one of the authors of this reviewbegins at the hydrogenation step with the formation ofcis� and trans�pinanes (Fig. 35) [156, 157] andinvolves the oxidation of cis� and trans�pinanes bymolecular oxygen into their corresponding peroxides[158], and hydrogenation with the formation ofpinanol [159], the isomerization of which results in theformation of linalool, a valuable fragrance substanceand the main component of bergamot [160, 161] (seeFig. 34, vertical chain).
Even though α�pinene is more popular, the use ofβ�pinene is better for the synthesis of valuable FSes(lyral, anethole, citral, and menthol [162]) (Fig. 36),as it makes it possible to reduce the number of steps insynthesis. The amount of β�pinene used in productionexceeds substantially the production of β�pinene pro�duced from turpentine oil, so large amounts of β�pinene are produced by the isomerization of α�pinene.
The isomerization of β�pinene into α�pinene iswell known and performed with the use of a number ofcatalysts, but the isomerization of α�pinene into β�pinene remains insufficiently studied. It was longthought that the reaction is irreversible [163–165], butin 1957 Wystrach suggested the possibility of a reversereaction and determined that the complexity of thetransformation of α�pinene to β�pinene was due to theunfavorable thermodynamics of the reaction. Themost promising and safest method of synthesizing β�pinene is the catalytic isomerization of α�pinene intoβ�pinene, which is done in the presence of noble met�
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CATALYSIS IN BIOMASS PROCESSING 241
als. Consideration of isomerization mechanisms basedon the example of olefins indicates that the migrationof the double bond occurs through semihydrogenatedintermediates. During the migration of the doublebond in hydrogen, the selectivity relative to isomeriza�tion falls considerably as a result of the hydrogenationof the double bond. Noble metals have different cata�lytic activities in hydrogenation and isomerization,but the contribution from isomerization can be quitesubstantial under optimum conditions. In reactionswith Pd�containing catalysts at low hydrogen pres�sures and elevated temperatures, the migration rate ofthe double bond in hydrocarbons is higher than thehydrogenation rate.
Due to the high reactivity of terpenes under theconditions of the catalytic hydroisomerization of α�
pinene, the formation of a large number of products ofhydrogenation, dehydrogenation, double bond migra�tion, skeleton isomerization, and the opening of thepinene cycle is possible. Almost all of the products ofthese reactions are key compounds in one chain of FSsynthesis or another and are of commercial value. Theselectivity of the formation of these products dependson the nature of the active metal and the conditions ofthe reaction. It is obvious that when the isomerizationreaction rate is substantially higher than the rate ofother reactions, β�pinene is predominantly formed.
The noble metals Pd, Rh, and Ru are widely knownas catalysts of the hydrogenation of olefins, but theirisomerization ability remains little studied [166, 167].The use of gold�containing catalysts in double bondmigration is a new and wide�open field of study. The
OOH
OH
OH
O
O OHOHCHCl
OH
α�pinene verbenyl verbenone verbenol
citral isopiperitenol p�menthol
verbanol
cis/trans�pinane
cis/trans�pinane�2�ol
linalool
H2
O2
Pd/C
O2
cis/trans�pinane�2�hydroperoxide
H2 Pd/C
Δ
hydroperoxide
H2, Pd/C
α�pineneepoxide
+ +
OHOOH
DT + H2, Pd/C
H2 Pd/C
Fig. 34. Scheme for the catalytic transformations of α�pinene.
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CATALYSIS IN INDUSTRY Vol. 3 No. 3 2011
MURZIN, SIMAKOVA
activity of nanosized gold particles during isomerizationand hydrogenation was shown recently for allylben�zenes and 1�alkenes. One work was devoted to studyingfeatures of the catalytic transformations of α�pineneunder reduction and inert media in the presence of goldsupported on metal oxides and carbon carriers. Theisomerization of α�pinene in the presence of Au/Al2O3catalyst modified by alkali results in the selective forma�tion of camphene [168,169] and makes it possible tostudy the conditions of the selective formation of oneterpene or another, and to compare the properties ofcatalysts during the isomerization of α�pinene (Fig. 37).
Some processes for manufacturing products of finechemistry from biomass were considered in [170],where not only terpenes but also alkaloids, steroids,and products from lignin were discussed.
Demands on Catalysts of Biomass Processing
We have examined some typical reactions of biom�ass processing; many of these (oxidation, hydrogena�tion, cracking, etc.) are close in concept to the pro�cesses widely known in the petroleum refining andchemical industries. There are definite featuresrelated, e.g., to a large number of functional groups(hydroxyl and otherwise). Dehydration, aldol con�densation, ketonization, and decarboxylation, whichare required for the processing of biomass compo�nents, are less well known in the chemical and petro�chemical industries. In addition, the reactions inpetroleum refining and petrochemistry are often con�ducted in the gas or liquid phase with the use oforganic solvents; the catalytic reactions of sugars andpolyalcohols occur in aqueous solutions and have theirown features. In conducting reactions with acidic cat�alysts based on alumosilicates, there are problems withregard to their stability and the adequacy of methodsfor studying their physicochemical properties, whichwould reflect the state of surfaces in liquid media. Theactivity and selectivity of catalysts can be determinedby the impurities and quality of the lignocellulose rawmaterial, which depends substantially on the type ofwood biomass and its place of origin. There is thus aproblem with respect to the representativeness ofmodel studies, to the amount and choice of biomass,and to analytical approaches on the whole.
Pb
cis� trans�
Pb
Pb
Pb
Pb
Pb
Pb
Pb
H+H2 –H2
HH
H
HH
HH
Fig. 35. Scheme for the hydrogenation of α�pinene to cis�and trans�pinanes.
Fig. 36. Scheme for the transformation of α�pinene into FS.
OH
OHOH
OH
CH
α�Pinene β�Pinene
Nerol
+
Geraniol
OH
Menthol MyrcenolLyral
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CATALYSIS IN BIOMASS PROCESSING 243
Despite these difficulties related to heterogeneityof the raw material, an understanding of catalyticchemistry on a molecular level is in any case requiredfor further progress in using catalysis for biomass pro�cessing. IR, UV, and Raman spectroscopy are amongthe methods widely used in studying catalytic transfor�mations in gas phase reactions. The frequently usedmethods of surface science also require the removal ofmoisture. During studies of the catalytic transforma�tions of biomass under so�called in situ conditions,water can dissociate into H+ and OH–, which can ini�tiate undesirable side reactions during the analysis[16]. One method used in, e.g., asymmetric catalysis isspectroscopy of the attenuated total IR reflectanceand can be applied to reactions of carbohydrates andtheir derivatives.
Recall that the properties of a surface can dependon the pH of the solution, the presence of surfactants,the type of solvent, and the ionic strength of the solu�tion. Problems of the adequacy of surface propertymeasurements by means of TPD or the adsorption ofprobe molecules from the gas phase and the ratio ofthese properties to catalytic measurements in the liq�uid phase are of great interest.
Let us consider the choosing of catalysts moreclosely, starting basically with the arguments in [12].The selection of carriers for the catalytic reactions of thetransformation of alkanes is dictated by the need toconduct these processes at high temperatures. Inor�ganic oxides (Al2O3 or SiO2) undoubtedly meet these
requirements. However, a number of the processes ofbiomass treatment (alcohol oxidation or the hydroge�nation of aldehydes, hydrolysis, and even hydrogenoly�sis) can take place at T < 10°C in polar solvents. Carriersmust be stable at different pH in highly polar liquids.Polymer carriers, which are only of limited use in clas�sical catalysis due to their low temperature stability (120to 140°C) meet these requirements [170]. An interest�ing feature of polymer carriers is the possibility of theiroccasionally great swelling in organic solvents [171],while swelling can be insubstantial in aqueous solutions.Cellulose was used in some reactions [172, 173] in addi�tion to the polymer carriers used most often (polysty�rene [170] and polyethylene [171]).
The pore sizes of micro� and mesoporous carriersare most likely sufficient for the transformation of car�bohydrates, while the penetration of microporousmaterials by polymers or oligomers of biomass can beproblematic. We may therefore expect some inhibitionof diffusion if such penetration does not occur due toterminal groups. In the opposite case, catalysis takesplace on the material’s external surface. It seems thatmacroporous materials are optimal, but their pores aretoo large to provide stearic hindrances that can be nec�essary for some selective transformation of organicmolecules.
An important feature of carriers is their stability(hydrothermal in particular), which limits the use ofsome metal oxides, mesoporous materials, and zeo�lites. For example, alumia can be subject to precipita�
+H+
–H+
–H+
–H+–H+
α�pinene pinanyl–carbo� bornyl–carbo�
camphene
tricyclene
terpenyl–carbonium ion
limonene α�terpinene p�cymene
nium ion nium ion
Fig. 37. Scheme for the isomerization of α�pinene in the presence of Au catalyst [167, 168].
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MURZIN, SIMAKOVA
tion and dissolution, depending on the medium. Acti�vated carbons seem to be stable. Unfortunately, how�ever, the presence of a large number of microporesdoes not allow reagents to attain active centers. Theuse of such materials as mesoporous Sibunite and car�bon nanotubes and nanofibers is thus attracting specialattention. The acidity of these carriers (catalysts)could be insufficient for some reactions of biomasstransformation, therefore, the search for new catalystsis of great interest. Of special note among these newmaterials are those mentioned in the hydrolytic hydro�genation of cellulose [174]: tungsten oxides and car�bides, sulfated silica–carbon nanocomposites, nio�bium molibdate, tantalum–tungsten oxide, het�eropolyacids, and nickel phosphide.
CONCLUSIONS
It should be noted that biomass processing with theuse of catalysts has recently received additional impe�tus due not only to possible limitation of petroleum’savailability in the near future: with the rising prices onpetroleum and petroleum products, the search foralternative sources of energy is of great economicimportance, since the possibility that some valuablechemicals can be produced more profitably from bio�mass than from petroleum cannot be excluded.
Biomass processing is of great interest to specialistsin the field of catalysis, since it is related to the devel�opment of catalytic processes with special chemicalbehaviors different from conventional methods oforganic synthesis and promotes the search for and syn�thesis of catalysts with unique properties that are nec�essary for new processes.
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