View Artide Online Review Article Chem Soc Rev temperatures of 300-450 C.173174 All these acids can be phenol derivatives.The dominant pathway depends on the completely converted into ketones and the smaller carboxylic catalyst composition,the properties of both the metal,and its acid showed higher ketonization reactivity than the longer-chain support.194 For instance,with low acidity support,guaiacol first acids.173 Two aldehydes and/or ketones with at least one a-H can undergoes demethylation to form catechol.Catechol is very undergo self-or cross-aldol condensation to form a new unsatu- active and undergoes sequential HDO to benzene via phenol. rated aldehyde or ketone with oxygen rejected as H2O,and both Anisole follows a similar pathway which first demethylated to Bronsted and Lewis acids are efficient catalysts for these kinds of phenol.18 With acidic support,both transalkylation and hydro- reactions.75-1s Vapor phase condensation of propanal over a deoxygenation were achieved at significant extent.190192 For ceria-zirconia catalyst was found to proceed via both aldol carbonyl compounds such as propionic acid,2-methylpentanal. condensation and ketonization pathways (Fig.7).168 Partially furfuryl,decarbonylation is dominant over noble metals and 8S:2t:0912/60 oxidized propanal undergoes ketonization to form 3-pentanone. hydrogenation to corresponding aldehyde and alcohol is domi- 3-Pentanone can react with another propanal to form 4-methyl-3-nant with base metal catalyst.951%6 Vapor phase hydrodeoxy- heptanone via aldol condensation.Self-aldol condensation of genation is very attractive for converting those lignin derived propanal leads to 2-methyl-2-pentenal,which forms 2-methyl-small oxygenates. pentanal by hydrogenation or aromatics via further con- 2.2.5.Steam reforming.Pyrolysis vapor components,espe- densation(Fig.7).The presence of acids inhibits the aldol cially the small oxygenates,can also undergo steam reforming condensation by competitive adsorption on active sites,and to produce renewable HThe general reaction scheme therefore a ketonization step is necessary. for the steam reforming of oxygenates proceeds as eqn(1).200 2.2.4.Hydrodeoxygenation.Hydrodeoxygenation rejects the When combined with a water-gas shift reaction the overall oxygen as water and retains most of the carbon in the products.A reaction will be as shown in eqn(2).200 Acetic acid and hydroxy- wide range of components of bio-oils can readily react with hydro- acetaldehyde undergo rapid thermal decomposition and achieve gen under pressure in the presence of a suitable catalyst.182.183 complete steam reforming over commercial Ni-based catalysts.19 Hydrodeoxygenation of bio oil has been extensively studied and The decomposition of acetic acid leads to the formation of well documented during the past decades.Only coke,which is removed by subsequent steam reforming. those conducted under atmospheric pressure have the potential Hydroxyacetaldehyde is found to be completely decomposed on to be integrated in biomass fast pyrolysis units.Phenol and its the metal catalyst surface without detectable intermediates.197 derivatives are the least active compounds in hydrodeoxygenation Acetone can be completely reformed to hydrogen over Co sup- but the most studied substrates in vapor phase hydrodeoxygena- ported on graphitized active carbon.204 However,acetic acid 410 tion over supported noble metals.18-1 Hydrodeoxygenation undergoes rapid coking at temperatures below 650C.202 Besides and transalkylation are the two competitive reactions of acetic acid,m-cresol and dibenzyl ether are also completely 3-pentanone入 H20 (+)CH,O 4-methyl-3-heptanone (+)CH.O 2-methyl-3-pentanone Light Aldol oxygenates Other C6-C9 condensation coupling Ketonization products ()H20 Hydrocarbons 2-methyl-2-pentenal (t)H2 (+)H20 2-methylpentanal Fig.7 Proposed reaction pathway of propanal over ceria-zirconia catalyst.(Adapted with permission from Gangadharan et al.Appl.Catal.A.2010.385. 80-916Copyright 2010 Elsevier.) This joumnal is The Royal Society of Chemistry 2014 Chem.Soc.Rev,2014.43.7594-7623|7603
This journal is © The Royal Society of Chemistry 2014 Chem. Soc. Rev., 2014, 43, 7594--7623 | 7603 temperatures of 300–450 1C.173,174 All these acids can be completely converted into ketones and the smaller carboxylic acid showed higher ketonization reactivity than the longer-chain acids.173 Two aldehydes and/or ketones with at least one a-H can undergo self- or cross-aldol condensation to form a new unsaturated aldehyde or ketone with oxygen rejected as H2O, and both Brønsted and Lewis acids are efficient catalysts for these kinds of reactions.175–181 Vapor phase condensation of propanal over a ceria–zirconia catalyst was found to proceed via both aldol condensation and ketonization pathways (Fig. 7).168 Partially oxidized propanal undergoes ketonization to form 3-pentanone. 3-Pentanone can react with another propanal to form 4-methyl-3- heptanone via aldol condensation. Self-aldol condensation of propanal leads to 2-methyl-2-pentenal, which forms 2-methylpentanal by hydrogenation or aromatics via further condensation (Fig. 7). The presence of acids inhibits the aldol condensation by competitive adsorption on active sites, and therefore a ketonization step is necessary. 2.2.4. Hydrodeoxygenation. Hydrodeoxygenation rejects the oxygen as water and retains most of the carbon in the products. A wide range of components of bio-oils can readily react with hydrogen under pressure in the presence of a suitable catalyst.182,183 Hydrodeoxygenation of bio oil has been extensively studied and well documented during the past decades.35,36,182–187 Only those conducted under atmospheric pressure have the potential to be integrated in biomass fast pyrolysis units. Phenol and its derivatives are the least active compounds in hydrodeoxygenation but the most studied substrates in vapor phase hydrodeoxygenation over supported noble metals.188–193 Hydrodeoxygenation and transalkylation are the two competitive reactions of phenol derivatives. The dominant pathway depends on the catalyst composition, the properties of both the metal, and its support.194 For instance, with low acidity support, guaiacol first undergoes demethylation to form catechol.188,193 Catechol is very active and undergoes sequential HDO to benzene via phenol.188 Anisole follows a similar pathway which first demethylated to phenol.188 With acidic support, both transalkylation and hydrodeoxygenation were achieved at significant extent.190,192 For carbonyl compounds such as propionic acid, 2-methylpentanal, furfuryl, decarbonylation is dominant over noble metals and hydrogenation to corresponding aldehyde and alcohol is dominant with base metal catalyst.195,196 Vapor phase hydrodeoxygenation is very attractive for converting those lignin derived small oxygenates. 2.2.5. Steam reforming. Pyrolysis vapor components, especially the small oxygenates, can also undergo steam reforming to produce renewable H2. 197–203 The general reaction scheme for the steam reforming of oxygenates proceeds as eqn (1).200 When combined with a water–gas shift reaction the overall reaction will be as shown in eqn (2).200 Acetic acid and hydroxyacetaldehyde undergo rapid thermal decomposition and achieve complete steam reforming over commercial Ni-based catalysts.199 The decomposition of acetic acid leads to the formation of coke, which is removed by subsequent steam reforming.197 Hydroxyacetaldehyde is found to be completely decomposed on the metal catalyst surface without detectable intermediates.197 Acetone can be completely reformed to hydrogen over Co supported on graphitized active carbon.204 However, acetic acid undergoes rapid coking at temperatures below 650 1C.202 Besides acetic acid, m-cresol and dibenzyl ether are also completely Fig. 7 Proposed reaction pathway of propanal over ceria–zirconia catalyst. (Adapted with permission from Gangadharan et al., Appl. Catal., A, 2010, 385, 80–91.168 Copyright 2010 Elsevier.) Review Article Chem Soc Rev Published on 07 May 2014. Downloaded by Shanghai Jiaotong University on 18/02/2016 07:32:58. View Article Online
View Artice Online Chem Soc Rev Review Article converted to hydrogen and carbon oxides above 650C.202 An Table 3 Composition of switchgrass ash.(Adapted with permission from overall positive effect on hydrogen production was observed for Patwardhan et al.Bioresour.Technol.2010.101.4646-4655.229 Copy- high concentrations of lower molecular-weight oxygenates, right 2010 Elsevier.) such as acetic acid and acetol in steam reforming of water- Compound Content (wt%) soluble components at 500C.205 The presence of steam in the SiOz 72.2 CFP reactor could generate a hydrogen atmosphere in the Al2O3 3.74 reactor by eliminating the detrimental small oxygenates via FeO 1.28 S03 0.39 steam reforming. CaO 6.19 CnHmOp +(n-p)H2OnCO+(n-p +m/2)H2 Mgo 2.33 (1 5.2.50 Na2O 0.77 CnHmOp +(2n -p)H2O nCOz +(2n-p +m/2)H2 K2O 6.17 (2 P,Os 2.63 TiO2 0.18 Sro 0.02 3.Catalysts for CFP of lignocellulosic BaO 0.01 Loss on ignition@750℃ 1.63 8 biomass Catalytic fast pyrolysis combines the fast pyrolysis of ligno- of biomass and promote char formation.Potassium cellulose with the catalytic transformation of the primary was found to catalyze pyrolytic reactions and promote the pyrolysis vapors to more desirable and less oxygenated liquid formation of CO2 and CO from polysaccharides,acetic acid from fuels.Removing oxygen as CO2 is most desirable since it would hemicellulose,formic acid from polysaccharides,and methanol minimize the need for external H,and improve the H/C ratio from lignin.215,222 in the final products.36 Catalytic cracking at 300-500 C can Soluble inorganics affect the pyrolysis temperature,product produce liquid hydrocarbons from oxygenates,with the oxygen distribution,and yields of biomass.A systematic examination removed in the form of H2O,CO,and CO2.Catalytic cracking is of the effect of KCl on the pyrolysis of wheat straw and single generally performed at atmospheric pressure and without a biopolymers (cellulose,xylan,and lignin)showed that the hydrogen requirement.Solid acids such as zeolites,silica-presence of 2 wt%KCl significantly increased the char yield alumina,silicalite,FCC catalysts,alumina,molecular sieves,as and decreased tar yield.The yields to char,tar,and gases from well as metal oxides such as zinc oxide,zirconia,ceria,and cellulose were highly influenced by the presence of KCl,whereas 410 copper chromite have been studied as catalysts in catalytic xylan and lignin were influenced to a less extent.215 MgCl2, cracking of pyrolysis vapors.155206-210 Besides those solid cata-NaCl,FesO,and ZnCl2 also increased the char yield in sugar lysts,other inorganic materials including metal chlorides,cane bagasse pyrolysis.22 Using TGA with mass spectroscopy, phosphates,sulfates,and alkali have also been investigated Varhegyi et al.found that impregnating 1 mol%each of MgCl2, in CFP of biomass.They can be categorized into NaCl,FeSO4,and ZnCl2 in microcrystalline cellulose dramati- several groups including soluble inorganics,metal oxides,micro-cally changed in the product distribution,characteristic decom- porous materials,mesoporous materials,and supported metal position temperature,and weight loss.35,223 MgCl2 does not catalysts. affect the weight loss or decomposition temperature of cellu- As mentioned in Section 1,catalysts can be mixed either lose but suppresses the yield to aldehydes,ketones,furans,and directly with biomass feedstock in the fast pyrolysis reactor or 2-furfuraldehydes.NaCl leads to a lower onset temperature only with the hot pyrolysis vapor after the fast pyrolysis reactor,and higher ending temperature in cellulose decomposition i.e.,in situ and ex situ CFP.40 The processes may have different and increases the total amount of low molecular-weight pro- advantages and disadvantages although the catalysts play a ducts by a factor of about three.In addition,due to the strong similar role in each.The effects of various catalysts on both inhibition of transglycolylation reactions by sodium,the presence in situ and ex situ CFP of lignocellulosic biomass in terms of Nacl dramatically suppresses the formation of levoglucosan of bio-oil yields and selectivity to desirable products will be (0.4 wt%)compared to that of the pyrolysis of untreated cellulose included in the current discussion. (36 wt%).95,224225 Feso catalyzed both the formation of levo- glucosan and levoglucosenone from wood and decreased the 3.1.Soluble inorganics decomposition temperature of cellulose by 50 C.95,223.226 Biomass contains certain amounts of inorganic species such as Chlorides including CuCl2 and FeCl2 also promoted the yield K,Na,Ca,and P(see Table 3).214-216 The presence of inorganic to levoglucosan.226 ZnCl led to two decomposition peaks of materials influences the thermal properties of each component cellulose with the lower one associated with dehydration reac- of lignocellulose.217.218 It is well known that small amounts of tions and the higher one resembling that of the untreated mineral matter naturally present in whole biomass samples sample.Pyrolysis of corncob impregnated with 15 wt%ZnCl2 strongly catalyze the decomposition of the cellulose component.5 in a lab-scale downflow reactor at about 340C vielded more Generally the presence of inorganics increases the yield to solid than 8 wt%of furfural and 4 wt%of acetic acid,which accounts products and decreases the yield to gaseous products for over 50 wt%and 25 wt%of pyrolytic liquids on a water-free These inorganics,especially K and Ca,catalyze the decomposition basis,respectively.227 More recently,the alkali and alkaline 7604|Chem.Soc.Rey,2014.43.7594-7623 This joumal is The Royal Society of Chemistry 2014
7604 | Chem. Soc. Rev., 2014, 43, 7594--7623 This journal is © The Royal Society of Chemistry 2014 converted to hydrogen and carbon oxides above 650 1C.202 An overall positive effect on hydrogen production was observed for high concentrations of lower molecular-weight oxygenates, such as acetic acid and acetol in steam reforming of watersoluble components at 500 1C.205 The presence of steam in the CFP reactor could generate a hydrogen atmosphere in the reactor by eliminating the detrimental small oxygenates via steam reforming. CnHmOp + (n p)H2O - nCO + (n p + m/2)H2 (1) CnHmOp + (2n p)H2O - nCO2 + (2n p + m/2)H2 (2) 3. Catalysts for CFP of lignocellulosic biomass Catalytic fast pyrolysis combines the fast pyrolysis of lignocellulose with the catalytic transformation of the primary pyrolysis vapors to more desirable and less oxygenated liquid fuels. Removing oxygen as CO2 is most desirable since it would minimize the need for external H2 and improve the H/C ratio in the final products.56 Catalytic cracking at 300–500 1C can produce liquid hydrocarbons from oxygenates, with the oxygen removed in the form of H2O, CO, and CO2. Catalytic cracking is generally performed at atmospheric pressure and without a hydrogen requirement. Solid acids such as zeolites, silica– alumina, silicalite, FCC catalysts, alumina, molecular sieves, as well as metal oxides such as zinc oxide, zirconia, ceria, and copper chromite have been studied as catalysts in catalytic cracking of pyrolysis vapors.155,206–210 Besides those solid catalysts, other inorganic materials including metal chlorides, phosphates, sulfates, and alkali have also been investigated in CFP of biomass.43,56,95,211–213 They can be categorized into several groups including soluble inorganics, metal oxides, microporous materials, mesoporous materials, and supported metal catalysts. As mentioned in Section 1, catalysts can be mixed either directly with biomass feedstock in the fast pyrolysis reactor or only with the hot pyrolysis vapor after the fast pyrolysis reactor, i.e., in situ and ex situ CFP.40 The processes may have different advantages and disadvantages although the catalysts play a similar role in each. The effects of various catalysts on both in situ and ex situ CFP of lignocellulosic biomass in terms of bio-oil yields and selectivity to desirable products will be included in the current discussion. 3.1. Soluble inorganics Biomass contains certain amounts of inorganic species such as K, Na, Ca, and P (see Table 3).214–216 The presence of inorganic materials influences the thermal properties of each component of lignocellulose.217,218 It is well known that small amounts of mineral matter naturally present in whole biomass samples strongly catalyze the decomposition of the cellulose component.65 Generally the presence of inorganics increases the yield to solid products and decreases the yield to gaseous products.211,215,216,219 These inorganics, especially K and Ca, catalyze the decomposition of biomass and promote char formation.214,215,220,221 Potassium was found to catalyze pyrolytic reactions and promote the formation of CO2 and CO from polysaccharides, acetic acid from hemicellulose, formic acid from polysaccharides, and methanol from lignin.215,222 Soluble inorganics affect the pyrolysis temperature, product distribution, and yields of biomass. A systematic examination of the effect of KCl on the pyrolysis of wheat straw and single biopolymers (cellulose, xylan, and lignin) showed that the presence of 2 wt% KCl significantly increased the char yield and decreased tar yield. The yields to char, tar, and gases from cellulose were highly influenced by the presence of KCl, whereas xylan and lignin were influenced to a less extent.215 MgCl2, NaCl, FeSO4, and ZnCl2 also increased the char yield in sugar cane bagasse pyrolysis.223 Using TGA with mass spectroscopy, Varhegyi et al. found that impregnating 1 mol% each of MgCl2, NaCl, FeSO4, and ZnCl2 in microcrystalline cellulose dramatically changed in the product distribution, characteristic decomposition temperature, and weight loss.95,223 MgCl2 does not affect the weight loss or decomposition temperature of cellulose but suppresses the yield to aldehydes, ketones, furans, and 2-furfuraldehydes. NaCl leads to a lower onset temperature and higher ending temperature in cellulose decomposition and increases the total amount of low molecular-weight products by a factor of about three. In addition, due to the strong inhibition of transglycolylation reactions by sodium, the presence of NaCl dramatically suppresses the formation of levoglucosan (0.4 wt%) compared to that of the pyrolysis of untreated cellulose (36 wt%).95,224,225 FeSO4 catalyzed both the formation of levoglucosan and levoglucosenone from wood and decreased the decomposition temperature of cellulose by 50 1C.95,223,226 Chlorides including CuCl2 and FeCl2 also promoted the yield to levoglucosan.226 ZnCl2 led to two decomposition peaks of cellulose with the lower one associated with dehydration reactions and the higher one resembling that of the untreated sample. Pyrolysis of corncob impregnated with 15 wt% ZnCl2 in a lab-scale downflow reactor at about 340 1C yielded more than 8 wt% of furfural and 4 wt% of acetic acid, which accounts for over 50 wt% and 25 wt% of pyrolytic liquids on a water-free basis, respectively.227 More recently, the alkali and alkaline Table 3 Composition of switchgrass ash. (Adapted with permission from Patwardhan et al., Bioresour. Technol., 2010, 101, 4646–4655.229 Copyright 2010 Elsevier.) Compound Content (wt%) SiO2 72.2 Al2O3 3.74 Fe2O3 1.28 SO3 0.39 CaO 6.19 MgO 2.33 Na2O 0.77 K2O 6.17 P2O5 2.63 TiO2 0.18 SrO 0.02 BaO 0.01 Loss on ignition @750 1C 1.63 Chem Soc Rev Review Article Published on 07 May 2014. Downloaded by Shanghai Jiaotong University on 18/02/2016 07:32:58. View Article Online