View Artice Online Chem Soc Rev Review Article Table 2 Comparison of three pyrolysis techniques Process conditions Products Pyrolysis technology Residence time Heating rate Temperature (C) Char (% Bio-oil (% Gases (% Conventional 5-30 min <50C min-1 400-600 <35 <30 <40 Fast pyrolysis <5s ~1000℃s-1 400-600 <25 <75 <20 Flash pyrolysis <0.1s ~1000℃s-1 650-900 <20 <20 <70 Those secondary reactions could take place either inside or 2.1.2.Fast pyrolysis of cellulose.Cellulose is the most outside the biomass particles.The intra-particle vapor-solid extensively studied component in lignocellulose due to its 8S:2t:509126/603 interactions are particularly important for large size particles abundance and the simplicity of its structure.The degree of (>0.5 cm).Thus biomass particle size <2 mm has been crystallinity and the dimensions of the crystallites are the most recommended for maximum bio-oil yield.34.64 important properties related to the stability and reactivity of Earlier efforts to understand the fundamentals of thermal cellulose.Each repeating unit of cellulose has three hydroxyl pyrolysis of lignocellulose were mainly focused on the global groups;those hydroxyl groups form either intramolecular or kinetic modeling development using thermogravimetric and intermolecular hydrogen bonds,which are highly relevant to differential scanning calorimetry techniques with products and the single-chain conformation and stiffness.The intermolecular intermediates lumped according to the phase and molecular hydrogen bonding in cellulose is responsible for the sheet-like weight.65-68 The conversion was defined by the weight loss nature of the native polymer.ss while the products were lumped into char,tar,and gases.The Cellulose,together with cellobiose,a-cyclodextrin,glucose, thermal decomposition behavior of the three main components and levoglucosan,are widely employed in mechanism studies of of lignocellulose,namely,cellulose,43.69-77 hemicellulose,77-1 cellulose fast pyrolysis.4Free radical mechanisms,5900 and lignin,were investigated to decouple the complexity in concerted mechanisms,and ionic mechanisms both chemistry and kinetic models.Generally the three main have been proposed for cellulose pyrolysis.Cellulose transforms components were assumed to decompose independently,and to a liquid before its degradation and then decomposes in two volatiles are evolved from cellulose and hemicellulose while char pathways.One directly leads to certain small molecular pro- is mainly from lignin.23 In most of the reports,process para- ducts such as furan,levoglucosan,glycolaldehyde,and hydroxyl meters such as particle size,heating rate,and pyrolysis temperature acetone,.74while the other pathway forms low-degree oligo- 410 were discussed and optimized to achieve high liquid yields.Here,mers.The low-degree oligomers can further break down to we mainly focus on the development of understanding the chem- form furan,light oxygenates,char,permanent gases,and istry and molecular products of pyrolysis.In thermogravimetric levoglucosan (Fig.3).727 compounds including char have analysis(TGA)studies,it was found that pyrolysis of hemicellulose been identified using GC-MS analysis of pyrolyzed cellulose and and cellulose occurred quickly.462,65.7 Hemicellulose mainly its surrogates.74 The major products are levoglucosan,hydroxy- decomposed at 220-315 C,and cellulose decomposed mainly at acetaldehyde,furfural,formic acid,acetic acid,and aldehyde 315-400C(Fig.2).However,lignin is more difficult to decompose compounds.54,74,108,109 and the weight loss occurred in a wide temperature range Initially,levoglucosan is generated in its liquid form in (160-900C)with generation of high solid residue(Fig.2).57 Next, cellulose pyrolysis and then some of it volatilizes to be a primary we will further summarize the chemistry of the reaction occurring volatile product.It can also undergo condensed-phase secondary during pyrolysis of the individual component in lignocellulose. pyrolysis to fomm pyrans and light oxygenates (Fig.) Various small linear oxygenates have been formed from gradual decomposition of levoglucosan.73,94.112 It is interesting that 3.0 100 hemicellulose levoglucosan itself is relatively stable and does not break down cellulose when pyrolyzed alone.37.13 The secondary decomposition of lignin 2.5 80 levoglucosan was found induced by the pyrolysis vapors from 2.0 cellulose and lignin and inhibited by the xylan-derived vapor 60 1.5 Dehydration and isomerization of levoglucosan lead to the 40 formation of other anhydro-monosaccharides.These anhydro- 1.0 monosaccharides may either re-polymerize to form anhydro- 20 0.5 oligomers or further transform to smaller oxygenates by fragmentation/retro-aldol condensation,dehydration,decarbonyl- 0.0 ation,or decarboxylation. 200 400 600 800 Char is obviously an undesired product in CFP.The secondary Temperature (C) reaction of primary pyrolysis products was found to increase Fig.2 Pyrolysis curves of hemicellulose.cellulose,and lignin from TGA the char yield.5.14 Re-polymerization and secondary pyrolysis (Adapted with permission from Yang et al.Fuel.2007.86.1781-1788.77 of levoglucosan was found to be an important pathway for char Copyright 2007 Elsevier.) formation.11113 Increasing the residence time of volatiles 7598|Chem.Soc.Rev.2014.43.7594-7623 This joumal is The Royal Society of Chemistry 2014
7598 | Chem. Soc. Rev., 2014, 43, 7594--7623 This journal is © The Royal Society of Chemistry 2014 Those secondary reactions could take place either inside or outside the biomass particles. The intra-particle vapor–solid interactions are particularly important for large size particles (40.5 cm). Thus biomass particle size o2 mm has been recommended for maximum bio-oil yield.54,64 Earlier efforts to understand the fundamentals of thermal pyrolysis of lignocellulose were mainly focused on the global kinetic modeling development using thermogravimetric and differential scanning calorimetry techniques with products and intermediates lumped according to the phase and molecular weight.65–68 The conversion was defined by the weight loss while the products were lumped into char, tar, and gases. The thermal decomposition behavior of the three main components of lignocellulose, namely, cellulose,43,69–77 hemicellulose,77–81 and lignin,43,77,82–91 were investigated to decouple the complexity in both chemistry and kinetic models. Generally the three main components were assumed to decompose independently, and volatiles are evolved from cellulose and hemicellulose while char is mainly from lignin.92,93 In most of the reports, process parameters such as particle size, heating rate, and pyrolysis temperature were discussed and optimized to achieve high liquid yields. Here, we mainly focus on the development of understanding the chemistry and molecular products of pyrolysis. In thermogravimetric analysis (TGA) studies, it was found that pyrolysis of hemicellulose and cellulose occurred quickly.14,62,65,77 Hemicellulose mainly decomposed at 220–315 1C, and cellulose decomposed mainly at 315–400 1C (Fig. 2). However, lignin is more difficult to decompose and the weight loss occurred in a wide temperature range (160–900 1C) with generation of high solid residue (Fig. 2).65,77 Next, we will further summarize the chemistry of the reaction occurring during pyrolysis of the individual component in lignocellulose. 2.1.2. Fast pyrolysis of cellulose. Cellulose is the most extensively studied component in lignocellulose due to its abundance and the simplicity of its structure. The degree of crystallinity and the dimensions of the crystallites are the most important properties related to the stability and reactivity of cellulose. Each repeating unit of cellulose has three hydroxyl groups; those hydroxyl groups form either intramolecular or intermolecular hydrogen bonds, which are highly relevant to the single-chain conformation and stiffness. The intermolecular hydrogen bonding in cellulose is responsible for the sheet-like nature of the native polymer.58 Cellulose, together with cellobiose, a-cyclodextrin, glucose, and levoglucosan, are widely employed in mechanism studies of cellulose fast pyrolysis.73,74,94–98 Free radical mechanisms,75,99,100 concerted mechanisms,94,101–104 and ionic mechanisms105,106 have been proposed for cellulose pyrolysis. Cellulose transforms to a liquid before its degradation and then decomposes in two pathways. One directly leads to certain small molecular products such as furan, levoglucosan, glycolaldehyde, and hydroxyl acetone,70,74 while the other pathway forms low-degree oligomers. The low-degree oligomers can further break down to form furan, light oxygenates, char, permanent gases, and levoglucosan (Fig. 3).67,107 27 compounds including char have been identified using GC-MS analysis of pyrolyzed cellulose and its surrogates.74 The major products are levoglucosan, hydroxyacetaldehyde, furfural, formic acid, acetic acid, and aldehyde compounds.54,74,108,109 Initially, levoglucosan is generated in its liquid form in cellulose pyrolysis and then some of it volatilizes to be a primary volatile product. It can also undergo condensed-phase secondary pyrolysis to form pyrans and light oxygenates (Fig. 4).69,74,107,110,111 Various small linear oxygenates have been formed from gradual decomposition of levoglucosan.73,94,112 It is interesting that levoglucosan itself is relatively stable and does not break down when pyrolyzed alone.67,113 The secondary decomposition of levoglucosan was found induced by the pyrolysis vapors from cellulose and lignin and inhibited by the xylan-derived vapor.98 Dehydration and isomerization of levoglucosan lead to the formation of other anhydro-monosaccharides. These anhydromonosaccharides may either re-polymerize to form anhydrooligomers or further transform to smaller oxygenates by fragmentation/retro-aldol condensation, dehydration, decarbonylation, or decarboxylation.69 Char is obviously an undesired product in CFP. The secondary reaction of primary pyrolysis products was found to increase the char yield.95,114 Re-polymerization and secondary pyrolysis of levoglucosan was found to be an important pathway for char formation.111,113 Increasing the residence time of volatiles Table 2 Comparison of three pyrolysis techniques Pyrolysis technology Process conditions Products Residence time Heating rate Temperature (1C) Char (%) Bio-oil (%) Gases (%) Conventional 5–30 min o50 1C min1 400–600 o35 o30 o40 Fast pyrolysis o5 s B1000 1C s1 400–600 o25 o75 o20 Flash pyrolysis o0.1 s B1000 1C s1 650–900 o20 o20 o70 Fig. 2 Pyrolysis curves of hemicellulose, cellulose, and lignin from TGA. (Adapted with permission from Yang et al., Fuel, 2007, 86, 1781–1788.77 Copyright 2007 Elsevier.) 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
View Artice Online Review Article Chem Soc Rev Other anhydrous hexose 8S:E:L0910/0/8I uo AusIan uooe yueyS Kq papeojumodIO KeW LO uo poys!iqnd Fig.3 Reaction pathways for the direct decomposition of cellulose molecules.(Adapted with permission from Shen et al,Bioresour.Technol.,2009. 100,6496-6504.Copyright 2009 Elsevier.) CH OH CO CH3OH g→ OH CO 。·c,o Fig.4 Reaction pathways for secondary decomposition of anhydrosugars (especially levoglucosan).(Adapted with permission from Shen et al. Bioresour.TechnoL.2009.100.6496-6504.5 Copyright 2009 Elsevier.) This joumnal is The Royal Society of Chemistry 2014 Chem.Soc.Rev,.2014.43.7594-7623|7599
This journal is © The Royal Society of Chemistry 2014 Chem. Soc. Rev., 2014, 43, 7594--7623 | 7599 Fig. 3 Reaction pathways for the direct decomposition of cellulose molecules. (Adapted with permission from Shen et al., Bioresour. Technol., 2009, 100, 6496–6504.75 Copyright 2009 Elsevier.) Fig. 4 Reaction pathways for secondary decomposition of anhydrosugars (especially levoglucosan). (Adapted with permission from Shen et al., Bioresour. Technol., 2009, 100, 6496–6504.75 Copyright 2009 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 Article Online Chem Soc Rev Review Article results in a higher char yield due to the higher degree of Xylose could be formed while the xylosyl cations react with H' secondary reaction of the primary pyrolysis products.14 and OH in its vicinity. 2.1.3.Fast pyrolysis of hemicellulose.Hemicellulose is 2.1.4.Fast pyrolysis of lignin.Lignin is an important cell- a complex polysaccharide usually with the general formula wall component of biomass,especially the woody species. (CsHsO)m and polymerization degree of 50-200.52 Xylan is Recently Laskar et al.and Saidi et al.reviewed the pathway the most abundant hemicellulose;it widely exists in woody for lignin conversion with the focus on lignin isolation and biomass.Commercially available xylan has often been used as catalytic hydrodeoxygenation of lignin-derived biooils.The a surrogate for hemicellulose.?s Hemicellulose is more readily three basic structural units of lignin are p-coumaryl alcohol, decomposed than cellulose in thermal pyrolysis(Fig.2).77 Fast coniferyl alcohol,and sinapyl alcohol.The relative abundances pyrolysis of hemicellulose is also speculated to proceed by a of p-coumaryl alcohol,coniferyl alcohol,and sinapyl alcohol :25:5911/7s radical mechanism.Similar to the pyrolysis of cellulose,small units vary with the sources of biomass but the linkages(Fig.6) oxygenates are formed either competitively or consequentially are similar.2.118-120 Among all the interphenylpropane linkages from fast pyrolysis of hemicellulose(Fig.5).78 Water,methanol, involved in lignin substructures,the guaiacylglycerol B-aryl formic,acetic,propionic acids,hydroxyl-1-propanone,hydroxyl- ether substructure is the most abundant (40-60%).The abun- 1-butanone,2-methylfuran,2-furfuraldehyde,dianhydroxylo- dances of other substructures found in lignin macromolecules pyranose,and anhydroxylopyranose are identified as the main are phenyl coumarone(10%),diarylpropane(5-10%),pinoresinol products.The production of dianhydro xylopyranosea (5%or less),biphenyl (5-10%),and diphenyl ether(5%).2 double dehydration product of xylose was explained by the lack Lignin has a high resistance to microbial and chemical of a sixth carbon and a substituted oxygen at the fourth position attacks due to its complex three-dimensional network formed which helps to stabilize the primary pyrolysis product by forming by different non-phenolic phenylpropanoid units linked with a a single dehydration product.78,117 Thus the xylosyl cation variety of ether and C-C bonds,62.123 and is the most recalcitrant formed from pyrolysis undergoes subsequent glycosidic bond component of lignocellulose.Thermal pyrolysis can break cleavage and dehydration,which forms dianhydro xylopyranose. down these phenyl-propane units of the macromolecule lattice. Aq papeojunod't10Z ABW LO uo poys!jqnd HO OH HO H.C Xy Formic acid acetaldehyde ring seission. e-polymeriza rizatio CH C0C0, 10 2-methyl furan DG depolvmerization. 5-hydroxy-2H-pyran- dehydration, 4(3H)-one depolvmerization. rearrangement depolymerization, rearrangemenl rearranger用enL H0 dchvdration HO HO OH furfural 4-hydroxy-5,6-dihydro- 1.4-anhydro 2H-pyran-2-one xylopyranose 1.5-anhydro-beta-D- xylofuranose 0 OH OH 2.5-anhydroxylos (1R,4R)-2,7-dioxabicyclo (1R,4S)-2.7-dioxabicyelo 12.2.1Jheptan-5-one [2.2.1]heptan-6-one Fig.5 Proposed reaction scheme of hemicellulose pyrolysis.(Adapted with permission from Patwardhan et aL.ChemSusChem.2011,4.636-643.78 Copyright 2011 John Wiley and Sons.) 7600|Chem.Soc.Rev.2014.43.7594-7623 This joumal is The Royal Society of Chemistry 2014
7600 | Chem. Soc. Rev., 2014, 43, 7594--7623 This journal is © The Royal Society of Chemistry 2014 results in a higher char yield due to the higher degree of secondary reaction of the primary pyrolysis products.114 2.1.3. Fast pyrolysis of hemicellulose. Hemicellulose is a complex polysaccharide usually with the general formula (C5H8O4)m and polymerization degree of 50–200.62 Xylan is the most abundant hemicellulose; it widely exists in woody biomass.115 Commercially available xylan has often been used as a surrogate for hemicellulose.78 Hemicellulose is more readily decomposed than cellulose in thermal pyrolysis (Fig. 2).77 Fast pyrolysis of hemicellulose is also speculated to proceed by a radical mechanism.80 Similar to the pyrolysis of cellulose, small oxygenates are formed either competitively or consequentially from fast pyrolysis of hemicellulose (Fig. 5).78 Water, methanol, formic, acetic, propionic acids, hydroxyl-1-propanone, hydroxyl- 1-butanone, 2-methylfuran, 2-furfuraldehyde, dianhydroxylopyranose, and anhydroxylopyranose are identified as the main products.78,116 The production of dianhydro xylopyranose—a double dehydration product of xylose was explained by the lack of a sixth carbon and a substituted oxygen at the fourth position which helps to stabilize the primary pyrolysis product by forming a single dehydration product.78,117 Thus the xylosyl cation formed from pyrolysis undergoes subsequent glycosidic bond cleavage and dehydration, which forms dianhydro xylopyranose. Xylose could be formed while the xylosyl cations react with H+ and OH in its vicinity. 2.1.4. Fast pyrolysis of lignin. Lignin is an important cellwall component of biomass, especially the woody species. Recently Laskar et al. and Saidi et al. reviewed the pathway for lignin conversion with the focus on lignin isolation and catalytic hydrodeoxygenation of lignin-derived bio-oils.37,38 The three basic structural units of lignin are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The relative abundances of p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol units vary with the sources of biomass but the linkages (Fig. 6) are similar.62,118–120 Among all the interphenylpropane linkages involved in lignin substructures, the guaiacylglycerol b-aryl ether substructure is the most abundant (40–60%). The abundances of other substructures found in lignin macromolecules are phenyl coumarone (10%), diarylpropane (5–10%), pinoresinol (5% or less), biphenyl (5–10%), and diphenyl ether (5%).62 Lignin has a high resistance to microbial and chemical attacks due to its complex three-dimensional network formed by different non-phenolic phenylpropanoid units linked with a variety of ether and C–C bonds,62,123 and is the most recalcitrant component of lignocellulose. Thermal pyrolysis can break down these phenyl–propane units of the macromolecule lattice. Fig. 5 Proposed reaction scheme of hemicellulose pyrolysis. (Adapted with permission from Patwardhan et al., ChemSusChem, 2011, 4, 636–643.78 Copyright 2011 John Wiley and Sons.) 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
View Artice Online Review Article Chem Soc Rev 8S:ZE:L0910Z/Z0/8I uo Aulsianlun uoloeif meyaueys Beta- Beta-Beta 人 5-5 Dibenzodioxocin Alpha-0-4 Beta-1 Fig.6 Common phenylpropane linkages in lignin.(Adapted with permission from Chakar et al.Ind.Crops Prod.2004.20(2).131-141121 Copyright 2004 Elsevier.) 410 The pyrolysis of lignin starts with dehydration at about 200 C lignocellulose biomass pyrolysis.Steam was found to enhance the followed by breakdown of the B-o-4 linkage,leading to the thermal degradation of wood and lower the activation energy.126 formation of guaiacol,dimethoxyphenol,dimethoxyacetophenone The decomposition temperature of cellulose was lowered in the (DMAP),and trimethoxyacetophenone(TMAP).122 The B-0-4 bond presence of steam.127 Steam also enhances the heat transfer and scission occurs at temperatures between 250C and 350C.124a,favors the fast desorption of low molecular weight products,which and B-aryl-alkyl-ether linkages break down between 150 and leads to a higher bio-oil yield and dominant water-soluble polar 300C.2 The aliphatic side chains also start splitting off from products.74Another major effect of steam is a 30-45 wt% the aromatic ring at about 300 C.An even higher temperature decrease in coke formation.138140 (370-400 C)is required to break the C-C bond between lignin In the presence of hydrogen,char formation was suppressed structural units.2 More generally,there are three kinds of bond but the gas yields and liquid product composition were not cleavage including two C-O bond cleavages and one side chain significantly affected.The product distribution and bio-oil C-C bond cleavage.The cleavage of a methyl C-O bond to form composition were quite different when a hydroprocessing cata- two-oxygen-atom products is the first reaction to occur in the lyst such as supported Mo-sulfide was introduced.Catalytic thermolysis of 4-alkylguaiacol at 327-377C.Then the cleavage hydropyrolysis led to a higher bio-oil yield with a simpler of the aromatic C-o bond leads to the formation of one-oxygen-composition and reduced oxygen content.128.129 In hydro- atom products.The side chain C-C bond cleavage occurs pyrolysis of rice husk,the presence of a catalyst led to about between the aromatic ring and an a-carbon atom.However, 16%less oxygen than that without the catalyst.130 Hydrogen the product distribution varies with the source of biomass. pressure is a significant parameter in this process.128 Increase Guaiacol is the main product from coniferous wood while in hydrogen pressure decreased both the oxygen content and guaiacol and pyrogallol dimethyl ether are dominant from the extent of overall aromatization of bio-oil Increasing the deciduous woods.62.125 Lignin produces more char and tar than pyrolysis temperature from 500 to 650 C further decreased wood despite the higher methoxyl content of lignin. the oxygen content.However,at low pyrolysis temperatures 2.1.5.Fast pyrolysis with reactive gas.Fast pyrolysis is (375-400 C),hydrogen has only a minimal effect on product usually performed in the absence of oxygen using nitrogen as distribution and bio-oil composition,particularly at low hydro- the carrier gas.However,other carriers such as H2,CO2,CO,CHa, gen pressure(2 MPa).131 steam,and even oxidative atmospheres have also been investigated Generally CO,CO2,CH,and H2 are present in recycled pyrolysis to different extents.Water is one of the major products of gas.Those gases are also tested as biomass pyrolysis media. This joumnal is The Royal Society of Chemistry 2014 Chem.Soc.Rev,2014.43.7594-7623|7601
This journal is © The Royal Society of Chemistry 2014 Chem. Soc. Rev., 2014, 43, 7594--7623 | 7601 The pyrolysis of lignin starts with dehydration at about 200 1C followed by breakdown of the b-O-4 linkage, leading to the formation of guaiacol, dimethoxyphenol, dimethoxyacetophenone (DMAP), and trimethoxyacetophenone (TMAP).122 The b-O-4 bond scission occurs at temperatures between 250 1C and 350 1C.124 a-, and b-aryl–alkyl–ether linkages break down between 150 and 300 1C.62 The aliphatic side chains also start splitting off from the aromatic ring at about 300 1C. An even higher temperature (370–400 1C) is required to break the C–C bond between lignin structural units.62 More generally, there are three kinds of bond cleavage including two C–O bond cleavages and one side chain C–C bond cleavage. The cleavage of a methyl C–O bond to form two-oxygen-atom products is the first reaction to occur in the thermolysis of 4-alkylguaiacol at 327–377 1C. Then the cleavage of the aromatic C–O bond leads to the formation of one-oxygenatom products. The side chain C–C bond cleavage occurs between the aromatic ring and an a-carbon atom. However, the product distribution varies with the source of biomass. Guaiacol is the main product from coniferous wood while guaiacol and pyrogallol dimethyl ether are dominant from deciduous woods.62,125 Lignin produces more char and tar than wood despite the higher methoxyl content of lignin. 2.1.5. Fast pyrolysis with reactive gas. Fast pyrolysis is usually performed in the absence of oxygen using nitrogen as the carrier gas. However, other carriers such as H2, CO2, CO, CH4, steam, and even oxidative atmospheres have also been investigated to different extents.126–133 Water is one of the major products of lignocellulose biomass pyrolysis. Steam was found to enhance the thermal degradation of wood and lower the activation energy.126 The decomposition temperature of cellulose was lowered in the presence of steam.127 Steam also enhances the heat transfer and favors the fast desorption of low molecular weight products, which leads to a higher bio-oil yield and dominant water-soluble polar products.98,107,134–139 Another major effect of steam is a 30–45 wt% decrease in coke formation.138,140 In the presence of hydrogen, char formation was suppressed but the gas yields and liquid product composition were not significantly affected. The product distribution and bio-oil composition were quite different when a hydroprocessing catalyst such as supported Mo-sulfide was introduced. Catalytic hydropyrolysis led to a higher bio-oil yield with a simpler composition and reduced oxygen content.128,129 In hydropyrolysis of rice husk, the presence of a catalyst led to about 16% less oxygen than that without the catalyst.130 Hydrogen pressure is a significant parameter in this process.128 Increase in hydrogen pressure decreased both the oxygen content and the extent of overall aromatization of bio-oil.129 Increasing the pyrolysis temperature from 500 to 650 1C further decreased the oxygen content. However, at low pyrolysis temperatures (375–400 1C), hydrogen has only a minimal effect on product distribution and bio-oil composition, particularly at low hydrogen pressure (2 MPa).131 Generally CO, CO2, CH4, and H2 are present in recycled pyrolysis gas. Those gases are also tested as biomass pyrolysis media.132,133 Fig. 6 Common phenylpropane linkages in lignin. (Adapted with permission from Chakar et al., Ind. Crops Prod., 2004, 20(2), 131–141.121 Copyright 2004 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 It was found that a Co atmosphere gave the lowest liquid yield Hydrocarbons are formed in catalytic cracking.C-C4 hydro- (49.6%)while a CH atmosphere gave the highest(58.7%).133 carbons are found to be the main cracking products over More oxygen was converted into CO,and H2O under CO and H, HZSM-5 from the conversion of model compounds such as atmospheres,respectively.The higher heating value(HHV)of acetic acid,propanoic acid,cyclopentanone,methylcyclopenta- the resulting bio-oil is increased compared to that obtained none,and alcohols like methanol,t-butanol,and 1-heptanol.138,152 under an inert atmosphere.Fewer methoxyl-containing com- Thermally stable oxygenates like sorbitol and glycerol can be pounds and more monofunctional phenols were found when converted into olefins(ethylene,propylene,and butenes),aro- using CO and CO2 as carrier gases.133 Syngas was found to be matics,or light paraffins(methane,ethane,and propane)while an economical alternative to pure hydrogen in hydropyrolysis of oxygen is removed as H2O,CO,or CO2.151 Lignin-derived coal.141.142 The weight loss profiles of biomass under hydrogen phenolics can undergo oxygen-aromatic carbon bond cleavage and syngas were found to be almost the same.This indicates to form phenol/aromatic hydrocarbons or undergo oxygen- that syngas has the potential to replace hydrogen as the alkyl carbon bond cleavage to form benzenediols or benzene- pyrolysis medium.132 Mante et al.studied the influence of triols.These benzenediols or benzenetriols then undergo recycling non-condensable gases such as CO/N2,COz/N2,CO/HDO to phenol.153 The cracking of guaiacol can be initiated CO2/N2,and H2/N2,in CFP of hybrid poplar and found that it by hemolytic cleavages of CH3-O or O-H bonds and results in potentially increased the bio-oil yield and decreased the char/the production of 1,2-dihydroxybenzene,methane,o-cresol, coke yield.46 2-hydroxybenzaldehyde,and coke.54 Thus cracking of fast pyrolysis vapors could lead to significant removal of oxygen 2.2.Chemistry in catalytic fast pyrolysis and improvement of bio-oil quality. For bio-oil upgrading,many chemical routes including cracking, 2.2.2.Aromatization.The abundant small-molecule oxygenates aromatization,ketonization/aldol condensation,and hydro- and olefins in fast pyrolysis vapor could be converted into treating have been extensively used to improve the quality of valuable aromatics via aromatization with the oxygen rejected biooilsCFP could integrate fast pyrolysis and these as CO,COz,and H In the presence of HZSM-5, chemical process for vapor upgrading into a simple process acids,aldehydes,esters,and furans are completely converted at that could produce bio-oils with improved quality and reduced temperatures above 370C and alcohols,ethers,ketones,and cost.47.48.147 A high oxygen content and the active oxygenates phenols are also largely reduced with aromatic hydrocarbons as such as acids,ketones,and aldehydes in bio-oil are mainly the main products.52.15556A high aromatic hydrocarbon yield responsible for the adverse attributes of bio-oils.148 Thus the was observed from propanal.152 The hydrocarbon product dis- 410 role of a catalyst in CFP is to promote the removal of most of tributions from methanol,ethanol,t-butanol,and 1-heptanol the oxygen in selective ways and convert the active species to are strikingly similar,which suggests a common reaction path- stable and useful components in bio-oil.Next,we will sum- way.152.157 A hydrocarbon pool mechanism is widely accepted marize the understanding of the chemistry of the major cata- for conversion of methanol and ethanol to hydrocarbons over lytic reactions that can be used in the CFP process.It is notable HZSM-5.157-160 Johansson et al found significant amounts of that the undergoing reactions during CFP are very complicated ethyl-substituted aromatics in the hydrocarbon pool when and the below reactions might occur simultaneously.Further ethanol was used as feedstock,although only methyl-substituted efforts are still required to understand the reaction network aromatics remained in the product.57 Comparing the conversion mechanisms,and kinetics of these reactions under the condi-of methanol,ethanol,and 2-propanol shows the high carbon tion relevant to CFP. number alcohol leads to quicker deactivation of aromatization 2.2.1.Cracking.Aromatics and olefins can be generated activity.Extensive aromatization was also found from catalytic cracking of oxygenates.The heavy organics that for cyclopentanone,methylcyclopentanone,acetic acid,and formed from re-polymerization or fragmentation can also be propanoic acid.138 The alkylation or trans-alkylation can lead converted to low molecular-weight products by cracking.The to substituted aromatic hydrocarbons.Using an aromatization catalytic cracking chemistry of pyrolysis vapors involves con- catalyst those highly active detrimental small oxygenates in fast ventional FCC reactions,such as protolytic cracking(cleavage pyrolysis vapors could be converted into desired valuable aro- of C-C bonds),hydrogen transfer,isomerization,and aromatic matic hydrocarbons. side-chain scission,as well as deoxygenation reactions,such as 2.2.3.Ketonization/aldol condensation.Pyrolysis vapors dehydration,decarboxylation,and decarbonylation.10.149,150 contain significant carboxylic and carbonyl components such Dehydration occurs on acid sites and leads to the forma- as acetic acid and furfural.Ketonization of carboxylic acids and tion of water and a dehydrated product.Decarboxylation and aldol condensation of ketones and aldehydes can result in the decarbonylation result in the formation of COz and CO.conversion of the carboxylic and carbonyl components into Repeated dehydration and hydrogen transfer of polyols allows longer-chain intermediates that can be converted to gasoline/ the production ofolefins,paraffins,and coke.Aromatics are diesel-range products via subsequent HDO.-Esters can formed by Diels-Alder and condensation reactions of olefins also undergo ketonization to form ketones.71172 Ketonization and dehydrated species.51 The conceptually complete deoxy-yields a new ketone via C-C coupling and oxygen is rejected as genation reaction of pyrolysis vapors predicts a maximum oil CO2 and H2O.Acetic acid,propionic acid,hexanoic acid,and yield of 42 wt%.10 heptanoic acid were tested on a series of solid oxide catalysts at 7602|Chem.Soc.Rev.2014.43.7594-7623 This joumal is The Royal Society of Chemistry 2014
7602 | Chem. Soc. Rev., 2014, 43, 7594--7623 This journal is © The Royal Society of Chemistry 2014 It was found that a CO atmosphere gave the lowest liquid yield (49.6%) while a CH4 atmosphere gave the highest (58.7%).133 More oxygen was converted into CO2 and H2O under CO and H2 atmospheres, respectively. The higher heating value (HHV) of the resulting bio-oil is increased compared to that obtained under an inert atmosphere. Fewer methoxyl-containing compounds and more monofunctional phenols were found when using CO and CO2 as carrier gases.133 Syngas was found to be an economical alternative to pure hydrogen in hydropyrolysis of coal.141,142 The weight loss profiles of biomass under hydrogen and syngas were found to be almost the same. This indicates that syngas has the potential to replace hydrogen as the pyrolysis medium.132 Mante et al. studied the influence of recycling non-condensable gases such as CO/N2, CO2/N2, CO/ CO2/N2, and H2/N2, in CFP of hybrid poplar and found that it potentially increased the bio-oil yield and decreased the char/ coke yield.46 2.2. Chemistry in catalytic fast pyrolysis For bio-oil upgrading, many chemical routes including cracking, aromatization, ketonization/aldol condensation, and hydrotreating have been extensively used to improve the quality of bio-oils.23,28,143–146 CFP could integrate fast pyrolysis and these chemical process for vapor upgrading into a simple process that could produce bio-oils with improved quality and reduced cost.47,48,147 A high oxygen content and the active oxygenates such as acids, ketones, and aldehydes in bio-oil are mainly responsible for the adverse attributes of bio-oils.148 Thus the role of a catalyst in CFP is to promote the removal of most of the oxygen in selective ways and convert the active species to stable and useful components in bio-oil. Next, we will summarize the understanding of the chemistry of the major catalytic reactions that can be used in the CFP process. It is notable that the undergoing reactions during CFP are very complicated and the below reactions might occur simultaneously. Further efforts are still required to understand the reaction network, mechanisms, and kinetics of these reactions under the condition relevant to CFP. 2.2.1. Cracking. Aromatics and olefins can be generated from catalytic cracking of oxygenates.51 The heavy organics that formed from re-polymerization or fragmentation can also be converted to low molecular-weight products by cracking. The catalytic cracking chemistry of pyrolysis vapors involves conventional FCC reactions, such as protolytic cracking (cleavage of C–C bonds), hydrogen transfer, isomerization, and aromatic side-chain scission, as well as deoxygenation reactions, such as dehydration, decarboxylation, and decarbonylation.10,149,150 Dehydration occurs on acid sites and leads to the formation of water and a dehydrated product. Decarboxylation and decarbonylation result in the formation of CO2 and CO. Repeated dehydration and hydrogen transfer of polyols allows the production of olefins, paraffins, and coke.151 Aromatics are formed by Diels–Alder and condensation reactions of olefins and dehydrated species.151 The conceptually complete deoxygenation reaction of pyrolysis vapors predicts a maximum oil yield of 42 wt%.10 Hydrocarbons are formed in catalytic cracking. C1–C4 hydrocarbons are found to be the main cracking products over HZSM-5 from the conversion of model compounds such as acetic acid, propanoic acid, cyclopentanone, methylcyclopentanone, and alcohols like methanol, t-butanol, and 1-heptanol.138,152 Thermally stable oxygenates like sorbitol and glycerol can be converted into olefins (ethylene, propylene, and butenes), aromatics, or light paraffins (methane, ethane, and propane) while oxygen is removed as H2O, CO, or CO2. 151 Lignin-derived phenolics can undergo oxygen-aromatic carbon bond cleavage to form phenol/aromatic hydrocarbons or undergo oxygen– alkyl carbon bond cleavage to form benzenediols or benzenetriols. These benzenediols or benzenetriols then undergo HDO to phenol.153 The cracking of guaiacol can be initiated by hemolytic cleavages of CH3–O or O–H bonds and results in the production of 1,2-dihydroxybenzene, methane, o-cresol, 2-hydroxybenzaldehyde, and coke.154 Thus cracking of fast pyrolysis vapors could lead to significant removal of oxygen and improvement of bio-oil quality. 2.2.2. Aromatization. The abundant small-molecule oxygenates and olefins in fast pyrolysis vapor could be converted into valuable aromatics via aromatization with the oxygen rejected as CO, CO2, and H2O.151,152,155 In the presence of HZSM-5, acids, aldehydes, esters, and furans are completely converted at temperatures above 370 1C and alcohols, ethers, ketones, and phenols are also largely reduced with aromatic hydrocarbons as the main products.152,155,156 A high aromatic hydrocarbon yield was observed from propanal.152 The hydrocarbon product distributions from methanol, ethanol, t-butanol, and 1-heptanol are strikingly similar, which suggests a common reaction pathway.152,157 A hydrocarbon pool mechanism is widely accepted for conversion of methanol and ethanol to hydrocarbons over HZSM-5.157–160 Johansson et al. found significant amounts of ethyl-substituted aromatics in the hydrocarbon pool when ethanol was used as feedstock, although only methyl-substituted aromatics remained in the product.157 Comparing the conversion of methanol, ethanol, and 2-propanol shows the high carbon number alcohol leads to quicker deactivation of aromatization activity.8,111,113,161,162 Extensive aromatization was also found for cyclopentanone, methylcyclopentanone, acetic acid, and propanoic acid.138 The alkylation or trans-alkylation can lead to substituted aromatic hydrocarbons. Using an aromatization catalyst those highly active detrimental small oxygenates in fast pyrolysis vapors could be converted into desired valuable aromatic hydrocarbons. 2.2.3. Ketonization/aldol condensation. Pyrolysis vapors contain significant carboxylic and carbonyl components such as acetic acid and furfural. Ketonization of carboxylic acids and aldol condensation of ketones and aldehydes can result in the conversion of the carboxylic and carbonyl components into longer-chain intermediates that can be converted to gasoline/ diesel-range products via subsequent HDO.163–170 Esters can also undergo ketonization to form ketones.171,172 Ketonization yields a new ketone via C–C coupling and oxygen is rejected as CO2 and H2O. Acetic acid, propionic acid, hexanoic acid, and heptanoic acid were tested on a series of solid oxide catalysts at 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