《生物质能工程》课程教学资源（阅读材料）1.催化热解 Catalytic fast pyrolysis of lignocellulosic biomass

Volume 43 Number 22 21 November 2014 Pages 7457-7956 Chem Soc Rev Chemical Society Reviews www.rsc.org/chemsocrev Themed issue:Catalysis for production of renewable energy ISSN0306-0012 ROYAL SOCIETY OF CHEMISTRY REVIEW ARTICLE Yong Wang et al. Catalytic fast pyrolysis of lignocellulosic biomass

Chem Soc Rev Chemical Society Reviews www.rsc.org/chemsocrev ISSN 0306-0012 REVIEW ARTICLE Yong Wang et al. Catalytic fast pyrolysis of lignocellulosic biomass Themed issue: Catalysis for production of renewable energy Volume 43 Number 22 21 November 2014 Pages 7457–7956

ROYAL SOCIETY OF CHEMISTRY Chem Soc Rev REVIEW ARTICLE View Article Online View Joumal View Issue Catalytic fast pyrolysis of lignocellulosic biomass Cite this:Chem.Soc.Rev.,2014, Changjun Liu,Huamin Wang,5 Ayman M.Karim,Junming Sun and Yong Wang*ab 43.7594 Increasing energy demand,especially in the transportation sector.and soaring COz emissions necessitate '8S:E:L0910/8I uo KusIan uooer yeyS Kq papeojumod the exploitation of renewable sources of energy.Despite the large variety of new energy carriers,liquid hydrocarbon still appears to be the most attractive and feasible form of transportation fuel taking into account the energy density.stability and existing infrastructure.Biomass is an abundant,renewable source of energy:however.utilizing it in a cost-effective way is still a substantial challenge.Lignocellulose is composed of three major biopolymers,namely cellulose.hemicellulose and lignin.Fast pyrolysis of biomass is recognized as an efficient and feasible process to selectively convert lignocellulose into a liquid fuel-bio-oil.However bio-oil from fast pyrolysis contains a large amount of oxygen,distributed in hundreds of oxygenates.These oxygenates are the cause of many negative properties,such as low heating value,high corrosiveness,high viscosity.and instability:they also greatly limit the application of bio-oil particularly as transportation fueL.Hydrocarbons derived from biomass are most attractive because of their high energy density and compatibility with the existing infrastructure.Thus,converting lignocellulose into transportation fuels via catalytic fast pyrolysis has attracted much attention.Many studies related to catalytic fast pyrolysis of biomass have been published.The main challenge of this process is the development of active and stable catalysts that can deal with a large variety of decomposition Received 15th November 2013 intermediates from lignocellulose.This review starts with the current understanding of the chemistry in 喜话02 D0:10.1039/c3cs60414d fast pyrolysis of lignocellulose and focuses on the development of catalysts in catalytic fast pyrolysis. Recent progress in the experimental studies on catalytic fast pyrolysis of biomass is also summarized www.rsc.org/csr with the emphasis on bio-oil yields and quality. 1.Introduction The Gene and Linda Voiland School of Chemical Engineering and Bioengineering. The world's total primary energy supply and consumption in 2010 was double that in 1971,as was the CO,emission.The Washington State University,Pullman,WA 99164,USA.E-mail:wang42@wsu.edu Institute for Integrated Catalysis,Pacific Northwest National Laboratory. worldwide delivered energy consumption is projected to increase Richland,WA 99352,USA continuously in the next two decades with an average annual Changjun Liu received his PhD in Dr Huamin Wang is currently Chemical Engineering from Sichuan a research engineer in Pacific University in 2010 (supervised by Northwest National Laboratory. Prof.Enze Min and Prof.Bin Liang), He received his PhD from Nankai and then worked as a postdoc University,China,and then did research associate with Prof Yong his postdoctoral research in ETH Wang in the Gene Linda Voiland Zurich and UC Berkeley.He has School of Chemical Engineering experience in heterogeneous cata- and Bioengineering,Washington lysis,inorganic material synthesis, State University,USA.His current hydroprocessing,and biomass con- research interests include biomass version.His current research comversion, bio-oil upgrading, involves thermochemical comer- Changjun Liu selective hydrogenation,acid-base Huamin Wang sion of biomass and fundamental catalysis,and two-phase flow. understanding of catalytic con- version of oxygenates. 7594|Chem.Soc.Rev.2014,43.7594-7623 This joumal is The Royal Society of Chemistry 2014

7594 | Chem. Soc. Rev., 2014, 43, 7594--7623 This journal is © The Royal Society of Chemistry 2014 Cite this: Chem. Soc. Rev., 2014, 43, 7594 Catalytic fast pyrolysis of lignocellulosic biomass Changjun Liu,a Huamin Wang,b Ayman M. Karim,b Junming Suna and Yong Wang*ab Increasing energy demand, especially in the transportation sector, and soaring CO2 emissions necessitate the exploitation of renewable sources of energy. Despite the large variety of new energy carriers, liquid hydrocarbon still appears to be the most attractive and feasible form of transportation fuel taking into account the energy density, stability and existing infrastructure. Biomass is an abundant, renewable source of energy; however, utilizing it in a cost-effective way is still a substantial challenge. Lignocellulose is composed of three major biopolymers, namely cellulose, hemicellulose and lignin. Fast pyrolysis of biomass is recognized as an efficient and feasible process to selectively convert lignocellulose into a liquid fuel—bio-oil. However bio-oil from fast pyrolysis contains a large amount of oxygen, distributed in hundreds of oxygenates. These oxygenates are the cause of many negative properties, such as low heating value, high corrosiveness, high viscosity, and instability; they also greatly limit the application of bio-oil particularly as transportation fuel. Hydrocarbons derived from biomass are most attractive because of their high energy density and compatibility with the existing infrastructure. Thus, converting lignocellulose into transportation fuels via catalytic fast pyrolysis has attracted much attention. Many studies related to catalytic fast pyrolysis of biomass have been published. The main challenge of this process is the development of active and stable catalysts that can deal with a large variety of decomposition intermediates from lignocellulose. This review starts with the current understanding of the chemistry in fast pyrolysis of lignocellulose and focuses on the development of catalysts in catalytic fast pyrolysis. Recent progress in the experimental studies on catalytic fast pyrolysis of biomass is also summarized with the emphasis on bio-oil yields and quality. 1. Introduction The world’s total primary energy supply and consumption in 2010 was double that in 1971, as was the CO2 emission.1 The worldwide delivered energy consumption is projected to increase continuously in the next two decades with an average annual a The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA. E-mail: wang42@wsu.edu b Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA Changjun Liu Changjun Liu received his PhD in Chemical Engineering from Sichuan University in 2010 (supervised by Prof. Enze Min and Prof. Bin Liang), and then worked as a postdoc research associate with Prof. Yong Wang in the Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, USA. His current research interests include biomass conversion, bio-oil upgrading, selective hydrogenation, acid–base catalysis, and two-phase flow. Huamin Wang Dr Huamin Wang is currently a research engineer in Pacific Northwest National Laboratory. He received his PhD from Nankai University, China, and then did his postdoctoral research in ETH Zurich and UC Berkeley. He has experience in heterogeneous cata￾lysis, inorganic material synthesis, hydroprocessing, and biomass con￾version. His current research involves thermochemical conver￾sion of biomass and fundamental understanding of catalytic con￾version of oxygenates. Received 15th November 2013 DOI: 10.1039/c3cs60414d www.rsc.org/csr 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 Journal | View Issue

View Artide Online Review Article Chem Soc Rev growth of about 1.6%(Table D1 in the reference).2Despite the and therefore has been identified as scalable,economically large variety of new energy carriers,liquid hydrocarbon still viable,and potentially carbon neutral feedstock for the production appears to be the most attractive and feasible form of trans- of renewable biofuels via appropriate technologies.Biochemical portation fuel,including aviation fuel.3 The U.S.renewable fuels conversion methodologies proposed for lignocelluloses await cost- standard(RFS2)requires an increase in the domestic supply of effective technologies5 and can only process cellulosic and alternative fuels to 36 billion gallons by 2022,including 15 billion hemicellulosic portions of lignocellulosic biomass.However, gallons from corn-based ethanol and 21 billion gallons of the thermochemical conversion routes are more energy efficient,' advanced biofuels from lignocellulosic biomass.The U.S.Energy and more flexible in terms of feed and products.s Among the Information Administration projects that the production of primary thermochemical conversion routes (i.e.,gasification and liquid fuels from biomass will soar in the next 30 years irrespec- fast pyrolysis),fast pyrolysis is the most economically feasible way tive of whether the oil prices are low or high(Fig.59 in ref.4).to convert biomass into liquid fuels,and has therefore attracted New technologies must be developed for the efficient conversion a great deal of research over the past two decades.A techno- of biomass to fuels that have high energy density and compat- economic analysis of three conversion platforms (ie.,pyrolysis, ibility with the existing energy infrastructure.3 gasification,and biochemical)comparing capital and operating Lignocellulosic biomass(such as wood,grass,and agricul- costs for near-term biomass-to-liquid fuels technology scenarios tural waste)is the most abundant and cheapest carbon source was performed recently.The analysis showed that the stand- alone biomass-to-liquid fuel plants are expected to produce fuels with a product value in the range of $2.00-5.50 per gallon gasoline equivalent,with fast pyrolysis being the lowest,and Ayman M.Karim is currently a bio-chemical conversion the highest.Fast pyrolysis shows the senior research scientist at Pacific highest yield to liquid fuel products and retains most of the Northwest National Laboratory energy from feedstocks in the liquid products.-11 Biomass (PNNL).Prior to joining PNNL conversion via fast pyrolysis is also on the verge of commercia- he did a postdoctoral stay lization.2 For instance,Envergent (a joint venture between (2007-2008)with Prof.Dionisios UOP/Honeywell and Ensyn)has a pilot-scale demonstration G.Vlachos at the University of plant under construction in Hawaii for biomass conversion to Delaware.He obtained his PhD fuels via fast pyrolysis.3 in chemical engineering from the The primary liquid product of fast pyrolysis of biomass is University of New Mexico (2007) generally called bio-oil,which is obtained by immediately under the guidance of Prof.quenching the pyrolysis vapors.Bio-oils are composed of a Abhaya K.Datye.His current large variety of condensable chemicals derived from many Ayman M.Karim research interests include funda- simultaneous and sequential reactions during the pyrolysis of mental studies of colloidal nano-lignocellulosic biomass.Bio-oil is a highly complex mixture of particles synthesis mechanisms,in situ and in operando catalyst more than 300 oxygenated compounds.10.14.5 characterization by X-ray absorption spectroscopy and developing Typical bio-oil from fast pyrolysis of woody biomass has a novel catalytic materials for the synthesis of fuels and chemicals high oxygen content and a low H/C ratio compared to crude oil from biomass. (Table 1).The chemical composition classified by functional Junming Sun is an assistant Yong Wang joined Pacific research major professor in Northwest National Laboratory Prof.Yong Wang's group at (PNNL),USA,in 1994 and was Washington State University, promoted to Laboratory Fellow in USA.He received his PhD from 2005.In 2009,he assumed a joint Dalian Institute of Chemical position at Washington State Physics of Chinese Academy of University (WSU)and PNNL.In Science in 2007 (Prof.Xinhe Bao), this unique position,he continues after which he worked with Prof. to be a Laboratory Fellow at PNNL Bruce C.Gates at UC Davis and is the Voiland Distinguished (2007-2008)and then with Prof. Professor in Chemical Engineering Yong Wang at Pacific Northwest at WSU,a full professorship with Junming Sun National Laboratory(2008-2011) Yong Wang tenure.His research interests as a postdoc researcher.His include the development of novel current research interests include fundamental understanding and catalytic materials and reaction engineering for the conversion of rational design of acid-base/supported metal catalysts for biomass fossil and biomass feedstocks to fuels and chemicals. derived small oxygenates,bimetallic catalysis for hydrodeoxygenation. This joumnal is The Royal Society of Chemistry 2014 Chem.Soc.Rev.2014.43.7594-762317595

This journal is © The Royal Society of Chemistry 2014 Chem. Soc. Rev., 2014, 43, 7594--7623 | 7595 growth of about 1.6% (Table D1 in the reference).2 Despite the large variety of new energy carriers, liquid hydrocarbon still appears to be the most attractive and feasible form of trans￾portation fuel, including aviation fuel.3 The U.S. renewable fuels standard (RFS2) requires an increase in the domestic supply of alternative fuels to 36 billion gallons by 2022, including 15 billion gallons from corn-based ethanol and 21 billion gallons of advanced biofuels from lignocellulosic biomass. The U.S. Energy Information Administration projects that the production of liquid fuels from biomass will soar in the next 30 years irrespec￾tive of whether the oil prices are low or high (Fig. 59 in ref. 4). New technologies must be developed for the efficient conversion of biomass to fuels that have high energy density and compat￾ibility with the existing energy infrastructure.5 Lignocellulosic biomass (such as wood, grass, and agricul￾tural waste) is the most abundant and cheapest carbon source and therefore has been identified as scalable, economically viable, and potentially carbon neutral feedstock for the production of renewable biofuels via appropriate technologies. Biochemical conversion methodologies proposed for lignocelluloses await cost￾effective technologies6 and can only process cellulosic and hemicellulosic portions of lignocellulosic biomass. However, the thermochemical conversion routes are more energy efficient,7 and more flexible in terms of feed and products.8 Among the primary thermochemical conversion routes (i.e., gasification and fast pyrolysis), fast pyrolysis is the most economically feasible way to convert biomass into liquid fuels,6 and has therefore attracted a great deal of research over the past two decades. A techno￾economic analysis of three conversion platforms (i.e., pyrolysis, gasification, and biochemical) comparing capital and operating costs for near-term biomass-to-liquid fuels technology scenarios was performed recently. The analysis showed that the stand￾alone biomass-to-liquid fuel plants are expected to produce fuels with a product value in the range of $2.00–5.50 per gallon gasoline equivalent, with fast pyrolysis being the lowest, and bio-chemical conversion the highest.6 Fast pyrolysis shows the highest yield to liquid fuel products and retains most of the energy from feedstocks in the liquid products.9–11 Biomass conversion via fast pyrolysis is also on the verge of commercia￾lization.12 For instance, Envergent (a joint venture between UOP/Honeywell and Ensyn) has a pilot-scale demonstration plant under construction in Hawaii for biomass conversion to fuels via fast pyrolysis.13 The primary liquid product of fast pyrolysis of biomass is generally called bio-oil, which is obtained by immediately quenching the pyrolysis vapors. Bio-oils are composed of a large variety of condensable chemicals derived from many simultaneous and sequential reactions during the pyrolysis of lignocellulosic biomass. Bio-oil is a highly complex mixture of more than 300 oxygenated compounds.10,14,15 Typical bio-oil from fast pyrolysis of woody biomass has a high oxygen content and a low H/C ratio compared to crude oil (Table 1). The chemical composition classified by functional Junming Sun Junming Sun is an assistant research & major professor in Prof. Yong Wang’s group at Washington State University, USA. He received his PhD from Dalian Institute of Chemical Physics of Chinese Academy of Science in 2007 (Prof. Xinhe Bao), after which he worked with Prof. Bruce C. Gates at UC Davis (2007–2008) and then with Prof. Yong Wang at Pacific Northwest National Laboratory (2008–2011) as a postdoc researcher. His current research interests include fundamental understanding and rational design of acid–base/supported metal catalysts for biomass derived small oxygenates, bimetallic catalysis for hydrodeoxygenation. Yong Wang Yong Wang joined Pacific Northwest National Laboratory (PNNL), USA, in 1994 and was promoted to Laboratory Fellow in 2005. In 2009, he assumed a joint position at Washington State University (WSU) and PNNL. In this unique position, he continues to be a Laboratory Fellow at PNNL and is the Voiland Distinguished Professor in Chemical Engineering at WSU, a full professorship with tenure. His research interests include the development of novel catalytic materials and reaction engineering for the conversion of fossil and biomass feedstocks to fuels and chemicals. Ayman M. Karim Ayman M. Karim is currently a senior research scientist at Pacific Northwest National Laboratory (PNNL). Prior to joining PNNL he did a postdoctoral stay (2007–2008) with Prof. Dionisios G. Vlachos at the University of Delaware. He obtained his PhD in chemical engineering from the University of New Mexico (2007) under the guidance of Prof. Abhaya K. Datye. His current research interests include funda￾mental studies of colloidal nano￾particles synthesis mechanisms, in situ and in operando catalyst characterization by X-ray absorption spectroscopy and developing novel catalytic materials for the synthesis of fuels and chemicals from biomass. 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 Artide Online Chem Soc Rev Review Article Table 1 Typical elementary composition of bio-oil and crude oil. bio-oil,which is considered to be a primary issue among the (Adapted with permission from Dickerson et al,Energies,2013,6, 514-538.20 Copyright 2013 MDPL) differences between bio-oils and hydrocarbon fuels.18.24 The low heating value and flame temperature,greater ignition Composition Bio-oil Crude oil delay,and lower combustion rate of bio-oil are largely due to Water (wt%) 15-30 0.1 the high water content (15-30 wt%),although water could PH 2.8-3.8 reduce the viscosity and enhance the fluidity.25 The low pH Density (kg L-1) 1.05-1.25 0.86-0.94 of 2-3 of bio-oil is due to the significant amount of carboxylic Viscosity 50 C(cP) 40-100 180 HHV (MJ kg) 16-19 44 acids,mainly formic and acetic acids(Fig.1),and leads to its C (wt%) 55-65 83.86 corrosiveness to common construction materials such as carbon O(wt%) 28-40 1 steel and aluminum as well as some sealing materials.'The high H (wt%) 5-7 11-14 S(wt%) <0.05 <4 content of acidic components also makes the bio-oils extremely N(wt%) <0.4 <1 unstable.The physicochemical properties of bio-oils change as a Ash (wt%) <0.2 0.1 function of time under ordinary storage conditions.26 The visco- H/C 0.9-1.5 1.5-2.0 o/c 0.3-0.5 0 sity of bio-oil increases due to secondary condensation and polymerization of the high concentration of reactive compo- nents like aldehydes,ketones,and phenols.2 In distillation, 0 35-50 wt%of the primary bio-oil is left as a residue due to the Hemicellulose and Cellulose ☑Low wt% High wt% polymerization of reactive components and the substantial Acids Lignin amounts of nonvolatile sugars and oligomeric phenols.Highly Mise Oxy: 30 Acetic Glycolaldehyde oxygenated bio-oils are immiscible with hydrocarbon fuels, Propanoic Acctol DiOH-benzene which hinder their use as fuel additives. Dimeth-pnenol It is desirable and necessary to improve the quality of bio-oil Alcohols ydroglucose nol 20 toward properties similar to those of hydrocarbon fuel by certain Methanol Ethanol Furans Methyl guaiacol Ethylene Glycol upgrading techniques.Oxygen must be removed before the bio-oil can be used as a replacement for diesel and gasoline.10.23 Methyl formate Butyrolactone Ketor Bio-oil can be upgraded either off-line or during the fast pyrolysis Angelicalactone assisted by a catalyst,the so-called catalytic fast pyrolysis(CFP) process.Both cases require catalysts to efficiently remove oxygen. To date,catalytic deoxygenation has been extensively investigated for more than three decades and generally includes two approaches:catalytic cracking and hydrotreating.Catalytic Fig.1 Chemical composition of bio-oil from wood biomass and the most cracking creates products of lower oxygen content than the feed abundant molecules of each of the components.(Adapted with permission from Huber et al.Chem.Rev..2006.106.4044-4098.23 Copyright 2006 by solid acid catalysts,such as zeolites at atmospheric pressure American Chemical Society.) without the requirement of hydrogen.However,the process produces low grade products (benzene,toluene,and small chain alkanes),which require further refining,and has a low groups with relative abundance is shown in Fig.1.The main carbon yield because of significant coke formation,which components include three major families of compounds: results in a very short catalyst lifetime.34 Hydrotreating of bio- (i)small carbonyl compounds such as acetic acid,acetaldehyde, oil adopts the conventional fuel hydrotreating technologies and acetone,hydroxyaldehydes,hydroxyketones,and carboxylic acids;gives desired products by removing oxygen by hydrodeoxygena- (ii)sugar-derived compounds such as furfural,levoglucosan,tion and breaking the larger molecules in the presence of a anhydrosugars,furan/pyran ring-containing compounds;and pressurized hydrogen atmosphere and a catalyst such as sup- (iii)lignin-derived compounds,which are mainly phenols and ported molybdenum sulfide.3 Bio-oil hydrotreating has been guaiacols;oligomers of a molecular weight ranging from 900 to well developed and produces high grade products.There are 2500 are also found in significant amounts.The distribution excellent reviews that have summarized the historical develop- of these compounds mostly depends on the type of biomass ments,35 recent advances,36 and new focus on hydrogeoxygena- used and the process severity.4-2 Such a distribution also tion of lignin-derived bio-oils.However,because of bio-oil influences the physical properties of bio-oil. instability and the high oxygen content,hydrotreating suffers Some properties of bio-oil from fast pyrolysis of ligno-from high operating cost associated with significant catalyst cellulosic biomass significantly limit its direct utilization as deactivation,expensive catalysts used,and substantial hydro- transportation fuel in current systems.Generally,bio-oils are gen consumption.39 characterized by low vapor pressure,low heating value,high An alternative way is to use a catalyst to directly upgrade the acidity,high viscosity,and high reactivity.1819,23 Bio-oils show a pyrolysis vapors prior to quenching to produce bio-oil with wide range of boiling temperatures due to their complex com- improved quality,a process that is called catalytic fast pyrolysis positions.These adverse characteristics,particularly the instability (CFP).By instantly treating the hot pyrolysis vapor with a of bio-oil,are associated with the high oxygen content in the suitable catalyst,the pyrolysis intermediates are simultaneously 7596|Chem.Soc.Rev,2014.43.7594-7623 This joumal is The Royal Society of Chemistry 2014

7596 | Chem. Soc. Rev., 2014, 43, 7594--7623 This journal is © The Royal Society of Chemistry 2014 groups with relative abundance is shown in Fig. 1. The main components include three major families of compounds: (i) small carbonyl compounds such as acetic acid, acetaldehyde, acetone, hydroxyaldehydes, hydroxyketones, and carboxylic acids; (ii) sugar-derived compounds such as furfural, levoglucosan, anhydrosugars, furan/pyran ring-containing compounds; and (iii) lignin-derived compounds, which are mainly phenols and guaiacols; oligomers of a molecular weight ranging from 900 to 2500 are also found in significant amounts.16–18 The distribution of these compounds mostly depends on the type of biomass used and the process severity.14,18–22 Such a distribution also influences the physical properties of bio-oil. Some properties of bio-oil from fast pyrolysis of ligno￾cellulosic biomass significantly limit its direct utilization as transportation fuel in current systems. Generally, bio-oils are characterized by low vapor pressure, low heating value, high acidity, high viscosity, and high reactivity.18,19,23 Bio-oils show a wide range of boiling temperatures due to their complex com￾positions. These adverse characteristics, particularly the instability of bio-oil, are associated with the high oxygen content in the bio-oil, which is considered to be a primary issue among the differences between bio-oils and hydrocarbon fuels.18,24 The low heating value and flame temperature, greater ignition delay, and lower combustion rate of bio-oil are largely due to the high water content (15–30 wt%), although water could reduce the viscosity and enhance the fluidity.18,25 The low pH of 2–3 of bio-oil is due to the significant amount of carboxylic acids, mainly formic and acetic acids (Fig. 1), and leads to its corrosiveness to common construction materials such as carbon steel and aluminum as well as some sealing materials.18 The high content of acidic components also makes the bio-oils extremely unstable. The physicochemical properties of bio-oils change as a function of time under ordinary storage conditions.26 The visco￾sity of bio-oil increases due to secondary condensation and polymerization of the high concentration of reactive compo￾nents like aldehydes, ketones, and phenols.27 In distillation, 35–50 wt% of the primary bio-oil is left as a residue due to the polymerization of reactive components and the substantial amounts of nonvolatile sugars and oligomeric phenols. Highly oxygenated bio-oils are immiscible with hydrocarbon fuels, which hinder their use as fuel additives. It is desirable and necessary to improve the quality of bio-oil toward properties similar to those of hydrocarbon fuel by certain upgrading techniques.28–30 Oxygen must be removed before the bio-oil can be used as a replacement for diesel and gasoline.10,23 Bio-oil can be upgraded either off-line or during the fast pyrolysis assisted by a catalyst, the so-called catalytic fast pyrolysis (CFP) process. Both cases require catalysts to efficiently remove oxygen. To date, catalytic deoxygenation has been extensively investigated for more than three decades and generally includes two approaches: catalytic cracking and hydrotreating.31–33 Catalytic cracking creates products of lower oxygen content than the feed by solid acid catalysts, such as zeolites at atmospheric pressure without the requirement of hydrogen. However, the process produces low grade products (benzene, toluene, and small chain alkanes), which require further refining, and has a low carbon yield because of significant coke formation, which results in a very short catalyst lifetime.34 Hydrotreating of bio￾oil adopts the conventional fuel hydrotreating technologies and gives desired products by removing oxygen by hydrodeoxygena￾tion and breaking the larger molecules in the presence of a pressurized hydrogen atmosphere and a catalyst such as sup￾ported molybdenum sulfide.35,36 Bio-oil hydrotreating has been well developed and produces high grade products. There are excellent reviews that have summarized the historical develop￾ments,35 recent advances,36 and new focus on hydrogeoxygena￾tion of lignin-derived bio-oils.37,38 However, because of bio-oil instability and the high oxygen content, hydrotreating suffers from high operating cost associated with significant catalyst deactivation, expensive catalysts used, and substantial hydro￾gen consumption.39 An alternative way is to use a catalyst to directly upgrade the pyrolysis vapors prior to quenching to produce bio-oil with improved quality, a process that is called catalytic fast pyrolysis (CFP). By instantly treating the hot pyrolysis vapor with a suitable catalyst, the pyrolysis intermediates are simultaneously Table 1 Typical elementary composition of bio-oil and crude oil. (Adapted with permission from Dickerson et al., Energies, 2013, 6, 514–538.20 Copyright 2013 MDPI.) Composition Bio-oil Crude oil Water (wt%) 15–30 0.1 pH 2.8–3.8 — Density (kg L1 ) 1.05–1.25 0.86–0.94 Viscosity 50 1C (cP) 40–100 180 HHV (MJ kg1 ) 16–19 44 C (wt%) 55–65 83.86 O (wt%) 28–40 o1 H (wt%) 5–7 11–14 S (wt%) o0.05 o4 N (wt%) o0.4 o1 Ash (wt%) o0.2 0.1 H/C 0.9–1.5 1.5–2.0 O/C 0.3–0.5 B0 Fig. 1 Chemical composition of bio-oil from wood biomass and the most abundant molecules of each of the components. (Adapted with permission from Huber et al., Chem. Rev., 2006, 106, 4044–4098.23 Copyright 2006 American Chemical Society.) 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 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|7597

This 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

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 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 para￾meters 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 chem￾istry 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 pro￾ducts such as furan, levoglucosan, glycolaldehyde, and hydroxyl acetone,70,74 while the other pathway forms low-degree oligo￾mers. 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, hydroxy￾acetaldehyde, 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 anhydro￾monosaccharides may either re-polymerize to form anhydro￾oligomers or further transform to smaller oxygenates by fragmentation/retro-aldol condensation, dehydration, decarbonyl￾ation, 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, dianhydroxylo￾pyranose, 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 cell￾wall 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 abun￾dances 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-oxygen￾atom 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 cata￾lyst 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 hydro￾pyrolysis 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 hydro￾gen 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 com￾pounds 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 hydro￾treating 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 sum￾marize the understanding of the chemistry of the major cata￾lytic 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 condi￾tion 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 con￾ventional 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 forma￾tion 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 deoxy￾genation reaction of pyrolysis vapors predicts a maximum oil yield of 42 wt%.10 Hydrocarbons are formed in catalytic cracking. C1–C4 hydro￾carbons are found to be the main cracking products over HZSM-5 from the conversion of model compounds such as acetic acid, propanoic acid, cyclopentanone, methylcyclopenta￾none, 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), aro￾matics, 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 benzene￾triols. 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 dis￾tributions from methanol, ethanol, t-butanol, and 1-heptanol are strikingly similar, which suggests a common reaction path￾way.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 aro￾matic 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