View Artice Online Review Article Chem Soc Rev cracked/upgraded into hydrocarbons as the biomass is pyro- development for CFP and related fundamental understanding lyzed.s The catalyst could be either directly mixed with biomass of the reaction mechanisms/routes in CFP for the sake of future feedstock or only mixed with the pyrolysis vapors.The process catalyst exploration and design. where the catalyst is mixed directly with the feedstock in the pyrolysis reactor is referred to as in situ catalytic fast pyrolysis (in situ CFP)40 while the process where the catalysts are only 2.Fast pyrolysis chemistry contacted with the pyrolysis vapors is referred to as ex situ catalytic fast pyrolysis(ex situ CFP).CFP has great potential to A fundamental understanding of the chemical properties of produce hydrocarbons directly from biomass or produce higher lignocellulosic biomass and the chemistry of the reactions quality bio-oils with improved stability lending them more taking place during the fast pyrolysis and CFP is essential to amenable for the subsequent upgrading process.The obvious rationally design more effective process and catalyst for fast advantage of CFP is the simplified process and avoided con- pyrolysis and CFP.In this section,we will summarize the recent densation and re-evaporation of the pyrolysis oil,41 since it is advancement in the chemistry of lignocellulosic biomass,the ZO/8 impossible to evaporate the bio-oils completely without degrad- fast pyrolysis of major composition of lignocellulosic biomass, ing once they have been condensed.42 The pyrolysis reaction and the catalytic fast pyrolysis of lignocellulosic biomass. pathways could be the same for both catalytic and non-catalytic Lignocellulosic biomass is a complex material,mainly com- fast pyrolysis of biomass since the bulk physical mixing of the posed of cellulose,hemicellulose,and lignin in addition to biomass and the catalyst will not be able to lead to molecular extractives (tannins,fatty acids,and resins)and inorganic level interaction.However,the presence of the catalyst could salts.34-57 The content of each component varies with the type of promote the secondary reactions of pyrolysis intermediates biomass;the woody biomass typically contains about 40-47 wt% toward certain products,and therefore considerably improve cellulose,25-35 wt%hemicellulose,and about 16-31 wt% the conversion and selectivity to desirable components in the lignin.19.58 Cellulose is a linear polymer of glucose connected produced bio-oil.43 It is known that bio-oil produced by fast by B-1,4-glycoside linkage,which forms the framework of the pyrolysis is a highly oxygenated mixture of carbonyls,carboxyls, biomass cell walls.54Cellulose is the most important element in phenolics,and water.44 In CFP,hydrocarbons are formed by biomass and has both crystalline and amorphous forms.39 removing oxygen from the pyrolysis-vapor intermediates in the Most of them are highly crystalline in nature with the polymeric form of CO2,CO,and H2O.The CFP process will lead to degree frequently in excess of 9000.60.61 Hemicellulose is struc- stabilized products and reduce the hydrogen demand in the turally amorphous and possesses a heterogeneous composition. 410e necessary hydrotreatment process that follows.The removal of It is formed by copolymers of five different Cs and Ce sugars, the most active oxygenates,such as carbonyl-and carboxyl- namely glucose,galactose,mannose,xylose,and arabinose.60 containing components,in CFP could also stabilize primary Unlike cellulose,hemicellulose is soluble in dilute alkali and bio-oils which are less prone to coke deposition and in turn consists of branched structures that vary considerably among improve the carbon yield to the final fuel products and long- biomass resources.62 Lignin is a complex three-dimensional term stability of the upgrading process.3545 CFP also provides polymer of propyl-phenol groups bound together by ether and the possibility of process intensification by means of multi-scale carbon-carbon bonds.The three basic phenol-containing com- integration and coupling of the reactions and reaction heats, ponents of lignin are p-coumaryl/p-hydroxylphenyl,coniferyl/ which reduce processing cost.Many factors affect the performance guaiacyl,and sinapyl/syringyl alcohol units.They are linked and economic feasibility of CFP of biomass.Catalysts,heating rate, with C-O(B-0-4,a-0-4,4-0-5 linking style)and C-C(B-5,5-5,B-1, residence time,and reaction temperature are the four pivotal B-B linking style)bonds.53 factors.The atmosphere in the reactor is also critical.43.46 Lin and Huber pointed out how critical the catalysis is in ligno- 2.1.Chemistry of non-catalytic fast pyrolysis of lignocellulosic cellulosic biomass conversion;47 a suitable catalyst is the key to biomass a successful CFP process.48 For instance,aromatic carbon yield 2.1.1.Fast pyrolysis process.Fast pyrolysis of ligno- as high as 30%was achieved by catalytic fast pyrolysis of cellulose proceeds by rapid heating of biomass to moderate glucose on ZSM-5,49 and this number can be further increased temperature in the absence of oxygen and immediate quench- to 40%on Ga/ZSM-5.30 Recently,Rezaei et al.reviewed the ing of the emerging pyrolysis vapors.Pyrolysis products are catalytic cracking of oxygenate compounds derived from bio- separated into char,gases,and bio-oil.Table 2 compares the mass pyrolysis with the emphasis on aromatic selectivity and process conditions and product distributions of three different olefin selectivity using zeolite catalysts.1 pyrolysis techniques.Fast pyrolysis gives the highest yield to CFP has attracted increasing attention in recent years,and bio-oil.Pyrolysis temperature,heating rate,residence time,and numerous studies have been reported over a variety of catalysts particle size all are important operation parameters affecting regarding the fundamental and practical aspects of CFP.Few bio-oil production.The optimum pyrolysis temperature was recent reviews have focused on CFp20.52 and other more general found to be about 500 C.64 Residence time greatly affects reviews have also highlighted the importance of CFP.12.43.53 the secondary reactions of pyrolysis vapors.Increasing the This review will start with the pyrolysis mechanism of ligno- residence time could either increase the gas phase cracking or the cellulose and mainly focus on the recent advances on catalyst secondary decomposition of pyrolysis vapors on the char surface. This joumnal is The Royal Society of Chemistry 2014 Chem.Soc.Rev.2014.43.7594-7623|7597This journal is © The Royal Society of Chemistry 2014 Chem. Soc. Rev., 2014, 43, 7594--7623 | 7597 cracked/upgraded into hydrocarbons as the biomass is pyro￾lyzed.5 The catalyst could be either directly mixed with biomass feedstock or only mixed with the pyrolysis vapors. The process where the catalyst is mixed directly with the feedstock in the pyrolysis reactor is referred to as in situ catalytic fast pyrolysis (in situ CFP)40 while the process where the catalysts are only contacted with the pyrolysis vapors is referred to as ex situ catalytic fast pyrolysis (ex situ CFP).40 CFP has great potential to produce hydrocarbons directly from biomass or produce higher quality bio-oils with improved stability lending them more amenable for the subsequent upgrading process. The obvious advantage of CFP is the simplified process and avoided con￾densation and re-evaporation of the pyrolysis oil,41 since it is impossible to evaporate the bio-oils completely without degrad￾ing once they have been condensed.18,42 The pyrolysis reaction pathways could be the same for both catalytic and non-catalytic fast pyrolysis of biomass since the bulk physical mixing of the biomass and the catalyst will not be able to lead to molecular level interaction. However, the presence of the catalyst could promote the secondary reactions of pyrolysis intermediates toward certain products, and therefore considerably improve the conversion and selectivity to desirable components in the produced bio-oil.43 It is known that bio-oil produced by fast pyrolysis is a highly oxygenated mixture of carbonyls, carboxyls, phenolics, and water.44 In CFP, hydrocarbons are formed by removing oxygen from the pyrolysis-vapor intermediates in the form of CO2, CO, and H2O. The CFP process will lead to stabilized products and reduce the hydrogen demand in the necessary hydrotreatment process that follows. The removal of the most active oxygenates, such as carbonyl- and carboxyl￾containing components, in CFP could also stabilize primary bio-oils which are less prone to coke deposition and in turn improve the carbon yield to the final fuel products and long￾term stability of the upgrading process.35,45 CFP also provides the possibility of process intensification by means of multi-scale integration and coupling of the reactions and reaction heats, which reduce processing cost. Many factors affect the performance and economic feasibility of CFP of biomass. Catalysts, heating rate, residence time, and reaction temperature are the four pivotal factors. The atmosphere in the reactor is also critical.43,46 Lin and Huber pointed out how critical the catalysis is in ligno￾cellulosic biomass conversion;47 a suitable catalyst is the key to a successful CFP process.48 For instance, aromatic carbon yield as high as 30% was achieved by catalytic fast pyrolysis of glucose on ZSM-5,49 and this number can be further increased to 40% on Ga/ZSM-5.50 Recently, Rezaei et al. reviewed the catalytic cracking of oxygenate compounds derived from bio￾mass pyrolysis with the emphasis on aromatic selectivity and olefin selectivity using zeolite catalysts.51 CFP has attracted increasing attention in recent years, and numerous studies have been reported over a variety of catalysts regarding the fundamental and practical aspects of CFP. Few recent reviews have focused on CFP20,52 and other more general reviews have also highlighted the importance of CFP.12,43,53 This review will start with the pyrolysis mechanism of ligno￾cellulose and mainly focus on the recent advances on catalyst development for CFP and related fundamental understanding of the reaction mechanisms/routes in CFP for the sake of future catalyst exploration and design. 2. Fast pyrolysis chemistry A fundamental understanding of the chemical properties of lignocellulosic biomass and the chemistry of the reactions taking place during the fast pyrolysis and CFP is essential to rationally design more effective process and catalyst for fast pyrolysis and CFP. In this section, we will summarize the recent advancement in the chemistry of lignocellulosic biomass, the fast pyrolysis of major composition of lignocellulosic biomass, and the catalytic fast pyrolysis of lignocellulosic biomass. Lignocellulosic biomass is a complex material, mainly com￾posed of cellulose, hemicellulose, and lignin in addition to extractives (tannins, fatty acids, and resins) and inorganic salts.54–57 The content of each component varies with the type of biomass; the woody biomass typically contains about 40–47 wt% cellulose, 25–35 wt% hemicellulose, and about 16–31 wt% lignin.19,58 Cellulose is a linear polymer of glucose connected by b-1,4-glycoside linkage, which forms the framework of the biomass cell walls.54 Cellulose is the most important element in biomass and has both crystalline and amorphous forms.59 Most of them are highly crystalline in nature with the polymeric degree frequently in excess of 9000.60,61 Hemicellulose is struc￾turally amorphous and possesses a heterogeneous composition. It is formed by copolymers of five different C5 and C6 sugars, namely glucose, galactose, mannose, xylose, and arabinose.60 Unlike cellulose, hemicellulose is soluble in dilute alkali and consists of branched structures that vary considerably among biomass resources.62 Lignin is a complex three-dimensional polymer of propyl-phenol groups bound together by ether and carbon–carbon bonds. The three basic phenol-containing com￾ponents of lignin are p-coumaryl/p-hydroxylphenyl, coniferyl/ guaiacyl, and sinapyl/syringyl alcohol units. They are linked with C–O (b-O-4, a-O-4, 4-O-5 linking style) and C–C (b-5, 5-5, b-1, b–b linking style) bonds.63 2.1. Chemistry of non-catalytic fast pyrolysis of lignocellulosic biomass 2.1.1. Fast pyrolysis process. Fast pyrolysis of ligno￾cellulose proceeds by rapid heating of biomass to moderate temperature in the absence of oxygen and immediate quench￾ing of the emerging pyrolysis vapors. Pyrolysis products are separated into char, gases, and bio-oil. Table 2 compares the process conditions and product distributions of three different pyrolysis techniques. Fast pyrolysis gives the highest yield to bio-oil. Pyrolysis temperature, heating rate, residence time, and particle size all are important operation parameters affecting bio-oil production. The optimum pyrolysis temperature was found to be about 500 1C.64 Residence time greatly affects the secondary reactions of pyrolysis vapors. Increasing the residence time could either increase the gas phase cracking or the secondary decomposition of pyrolysis vapors on the char surface. 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