Author's personal copy L Zhang et al.Renewable and Sustainable Energy Reviews 24 (2013)66-72 traditional hydrogenation process for OHE.Yu et al.[21]screened characteristic analysis of obtained bio-oils using elemental out 5%Pd/Al2(SiO3)3 with the best catalytic performance among GC-MS and FTIR technologies [31].Both in a fixed bed reactor tested bifunctional catalysts,and demonstrated that it is viable to [31,32]and in fluidized bed [13],the investigation indicated that convert these unstable constituents of bio-oil to esters and catalytic pyrolysis lowered the oxygen content of the bio-oils and alcohols through this simple and effective OHE reaction.Besides, aggrandized the calorific values compared to the direct pyrolysis for the OHE reaction,tests by Tang et al.[22]demonstrated the without catalysts.This conclusion can be drawn in many experi- effectiveness of the bifunctional catalyst system for combined ments with different biomass,including green microalga [31]. hydrogenation/esterification and a synergistic effect between corncob [13].herb residue [32]and waste woody biomass [33]. metal sites and acid sites over respective catalysts.Moreover. Besides,catalytic pyrolysis can lead to higher content of aromatic some measures were taken to improve the catalytic performance hydrocarbons in bio-oils with HZSM-5 [31],alumina [32]or HZSM- of the bifunctional catalyst [23].Obviously,the new hydrogenation 5/y-Al2O3 [35]as catalyst while direct pyrolysis promoted the method is much better than the traditional method due to the use increase of carbon chain compounds [31].However,Zhang et al of bifunctional catalysts. [13]reported that the addition of HZSM-5 zeolite catalyst in the experiments caused a significant decrease of heavy oil fraction and 3.2.Hydrodeoxygenation an increase of the coke,water and non-condensable gas yields This was because the HZSM-5 zeolite catalyst could promote the Hydrodeoxygenation(HDO).a variant of catalytic pyrolysis,is a conversion of oxygen element in heavy oil into CO,CO2 and H2O. bio-oil upgrading process which removes the oxygen under high So.choosing proper catalysts is crucial to catalytic pyrolysis. pressure of hydrogen with a catalyst.It can reduce oxygen content Besides,some new reports on the catalytic pyrolysis were of many kinds of oxygenated chemical groups,such as acids, reported recently in China.During biomass pyrolysis,Cao addition aldehydes,esters,ketones and phenols,etc.Hydrodeoxygenation could catalyze dehydration reactions [36].CaCl2 was reported to has been considered to be one of the most promising methods for have an apparent catalytic effect on elephant grass pyrolysis [37] bio-oil upgrading [24].Recently in China,a 500 ml autoclave and polluting heavy metal Cu showed effective catalytic activity in reactor [24]with diameter of 10 mm and length of 420 mm is the thermo-decomposition of biomass [33].This research can the largest experimental facility for hydrodeoxygenation.Addi- contribute to the development of catalysts applied on the more tionally,the largest dosage of the catalyst for this kind of research efficient catalytic pyrolysis process. is1.5g[241 Therefore,catalytic pyrolysis can promote the production and Most of the previous researches on hydrodeoxygenation of bio-oil quality of bio-oils through using the appropriate catalysts.But,it focused on industrial NiMo or CoMo sulfide/supported hydrotreating also encounters some problems,such as catalyst deactivation, catalysts.For instance,Wang et al.[24]demonstrated Pt supported reactor clogging,coke production and high water content in bio- on mesoporous ZSM-5 showed better performance than Pt/ZSM-5 oils,etc. and Pt/Al2O3 in dibenzofuran hydrodeoxygenation.However,these catalysts have several inherent shortcomings in hydrodeoxygenation, 3.4.Catalytic cracking such as product contamination and catalyst deactivation.The noble metal catalyst exhibits high catalytic activity in the HDO reactions Recently in China,catalytic cracking for upgrading pyrolysis [24-26].but with high cost.So,novel and economical catalysts that bio-oil can be divided into two patterns,the traditional catalytic can be used for hydrodeoxygenation of bio-oil with high oxygen cracking and the combination of catalytic pyrolysis and catalytic content should be developed. cracking.Traditionally,the catalytic cracking referred to a thermal Due to excellent hydrodeoxygenation activity and selectivity in conversion process of bio-oil under certain conditions,including the catalytic reactions [27,281,amorphous catalysts have aroused a hydrogen flow,proper catalysts (e.g.,HZSM-5 [38])and a specific great attention in China recently.Wang et al.[27,28]prepared and temperature higher than 350C as well as rather high pressure. tested hydrodeoxygenation activity of lots of amorphous catalysts, Hydrogenation with simultaneous cracking occurred during the and demonstrated that Co-W-B had higher thermal stability than catalytic cracking process.The products of the catalytic cracking Ni-Co-W-B and Ni-W-B catalyst.And,the catalyst activity could process consist of solid,liquid and gases.The solid is called coke, be increased with the Co/Mo ratio increase of surface composition, and the liquid can be divided into two phases:aqueous phase and which means the catalyst activity could be further improved at organic phase.The gas is combustible.In China,the traditional proper conditions.Therefore,this new kind of amorphous catalyst catalytic cracking had been carried out in a tubular fixed bed will be a potential candidate for the HDO process due to its many reactor [38]and micro-fixed bed reactor [391.The advantage of advantages,such as simple preparation,high thermal stability and this technique was the probability of obtaining a good deal of light high HDO activity as well as low cost [28,29]. product,but catalyst coke deposition was a bottleneck for sustain- able application of catalysts(e.g.,HZSM-5 [39]). 3.3.Catalytic pyrolysis Compared to the traditional catalytic cracking.scholars in China made the combination of catalytic pyrolysis and catalytic cracking to Recently,catalytic pyrolysis has aroused a great interest for the upgrade pyrolysis bio-oil.It adopted a sequential biomass pyrolysis advantages of operating at atmospheric pressure and the lack of reactor which consisted of a traditional pyrolysis reactor followed by need for hydrogen [30],which has been demonstrated by many the subsequent apparatus that supported decomposition of gaseous researches.The experiments on catalytic pyrolysis of biomass were intermediate [40].The largest reactor for this kind of investigation generally carried out in a fixed bed reactor [31]or fluidized bed was made of 316 stainless tube,with a length of 1000 mm and an [13]under nitrogen flow with some catalysts,such as HZSM-5 inner diameter of 20 mm [38].For example,Xiwei et al.[40]applied [13,311.ZSM-5 [32].Al-SBA-15 [321.alumina [32]and Cu [33].etc this method and found that biomass could be fully converted into So far in China,the largest scale of catalytic pyrolysis reactor is a gaseous products,such as H2,CH4 and CO,etc.Catalyst(Fe/y-Al2O3) tubular reactor with a length of 250 mm and an inner diameter of activities were affected by several factors,including calcination 38 mm.Many aspects of catalytic pyrolysis have been studied. temperature,temperature of catalytic pyrolysis and Fe/Al mass ratio. including screening of feasible catalysts with high deoxygenating Hong-yu et al.[41]demonstrated that when the cracking tempera- activities [32,34]or preferred selectivities [35].influence of tem ture was 500C,with a Weight Hourly Space Velocity of 3 h-,the perature and catalyst-to-material ratio on product yields [13]. liquid yield reached the maximum and the oxygenic compounds alsoAuthor's personal copy traditional hydrogenation process for OHE. Yu et al. [21] screened out 5% Pd/Al2(SiO3)3 with the best catalytic performance among tested bifunctional catalysts, and demonstrated that it is viable to convert these unstable constituents of bio-oil to esters and alcohols through this simple and effective OHE reaction. Besides, for the OHE reaction, tests by Tang et al. [22] demonstrated the effectiveness of the bifunctional catalyst system for combined hydrogenation/esterification and a synergistic effect between metal sites and acid sites over respective catalysts. Moreover, some measures were taken to improve the catalytic performance of the bifunctional catalyst [23]. Obviously, the new hydrogenation method is much better than the traditional method due to the use of bifunctional catalysts. 3.2. Hydrodeoxygenation Hydrodeoxygenation (HDO), a variant of catalytic pyrolysis, is a bio-oil upgrading process which removes the oxygen under high pressure of hydrogen with a catalyst. It can reduce oxygen content of many kinds of oxygenated chemical groups, such as acids, aldehydes, esters, ketones and phenols, etc. Hydrodeoxygenation has been considered to be one of the most promising methods for bio-oil upgrading [24]. Recently in China, a 500 ml autoclave reactor [24] with diameter of 10 mm and length of 420 mm is the largest experimental facility for hydrodeoxygenation. Addi￾tionally, the largest dosage of the catalyst for this kind of research is 1.5 g [24]. Most of the previous researches on hydrodeoxygenation of bio-oil focused on industrial NiMo or CoMo sulfide/supported hydrotreating catalysts. For instance, Wang et al. [24] demonstrated Pt supported on mesoporous ZSM-5 showed better performance than Pt/ZSM-5 and Pt/Al2O3 in dibenzofuran hydrodeoxygenation. However, these catalysts have several inherent shortcomings in hydrodeoxygenation, such as product contamination and catalyst deactivation. The noble metal catalyst exhibits high catalytic activity in the HDO reactions [24–26], but with high cost. So, novel and economical catalysts that can be used for hydrodeoxygenation of bio-oil with high oxygen content should be developed. Due to excellent hydrodeoxygenation activity and selectivity in the catalytic reactions [27,28], amorphous catalysts have aroused a great attention in China recently. Wang et al. [27,28] prepared and tested hydrodeoxygenation activity of lots of amorphous catalysts, and demonstrated that Co–W–B had higher thermal stability than Ni–Co–W–B and Ni–W–B catalyst. And, the catalyst activity could be increased with the Co/Mo ratio increase of surface composition, which means the catalyst activity could be further improved at proper conditions. Therefore, this new kind of amorphous catalyst will be a potential candidate for the HDO process due to its many advantages, such as simple preparation, high thermal stability and high HDO activity as well as low cost [28,29]. 3.3. Catalytic pyrolysis Recently, catalytic pyrolysis has aroused a great interest for the advantages of operating at atmospheric pressure and the lack of need for hydrogen [30], which has been demonstrated by many researches. The experiments on catalytic pyrolysis of biomass were generally carried out in a fixed bed reactor [31] or fluidized bed [13] under nitrogen flow with some catalysts, such as HZSM-5 [13,31], ZSM-5 [32], Al-SBA-15 [32], alumina [32] and Cu [33], etc. So far in China, the largest scale of catalytic pyrolysis reactor is a tubular reactor with a length of 250 mm and an inner diameter of 38 mm. Many aspects of catalytic pyrolysis have been studied, including screening of feasible catalysts with high deoxygenating activities [32,34] or preferred selectivities [35], influence of tem￾perature and catalyst-to-material ratio on product yields [13], characteristic analysis of obtained bio-oils using elemental, GC–MS and FTIR technologies [31]. Both in a fixed bed reactor [31,32] and in fluidized bed [13], the investigation indicated that catalytic pyrolysis lowered the oxygen content of the bio-oils and aggrandized the calorific values compared to the direct pyrolysis without catalysts. This conclusion can be drawn in many experi￾ments with different biomass, including green microalga [31], corncob [13], herb residue [32] and waste woody biomass [33]. Besides, catalytic pyrolysis can lead to higher content of aromatic hydrocarbons in bio-oils with HZSM-5 [31], alumina [32] or HZSM- 5/γ-Al2O3 [35] as catalyst while direct pyrolysis promoted the increase of carbon chain compounds [31]. However, Zhang et al. [13] reported that the addition of HZSM-5 zeolite catalyst in the experiments caused a significant decrease of heavy oil fraction and an increase of the coke, water and non-condensable gas yields. This was because the HZSM-5 zeolite catalyst could promote the conversion of oxygen element in heavy oil into CO, CO2 and H2O. So, choosing proper catalysts is crucial to catalytic pyrolysis. Besides, some new reports on the catalytic pyrolysis were reported recently in China. During biomass pyrolysis, CaO addition could catalyze dehydration reactions [36]. CaCl2 was reported to have an apparent catalytic effect on elephant grass pyrolysis [37] and polluting heavy metal Cu showed effective catalytic activity in the thermo-decomposition of biomass [33]. This research can contribute to the development of catalysts applied on the more efficient catalytic pyrolysis process. Therefore, catalytic pyrolysis can promote the production and quality of bio-oils through using the appropriate catalysts. But, it also encounters some problems, such as catalyst deactivation, reactor clogging, coke production and high water content in bio￾oils, etc. 3.4. Catalytic cracking Recently in China, catalytic cracking for upgrading pyrolysis bio-oil can be divided into two patterns, the traditional catalytic cracking and the combination of catalytic pyrolysis and catalytic cracking. Traditionally, the catalytic cracking referred to a thermal conversion process of bio-oil under certain conditions, including hydrogen flow, proper catalysts (e.g., HZSM-5 [38]) and a specific temperature higher than 350 1C as well as rather high pressure. Hydrogenation with simultaneous cracking occurred during the catalytic cracking process. The products of the catalytic cracking process consist of solid, liquid and gases. The solid is called coke, and the liquid can be divided into two phases: aqueous phase and organic phase. The gas is combustible. In China, the traditional catalytic cracking had been carried out in a tubular fixed bed reactor [38] and micro-fixed bed reactor [39]. The advantage of this technique was the probability of obtaining a good deal of light product, but catalyst coke deposition was a bottleneck for sustain￾able application of catalysts (e.g., HZSM-5 [39]). Compared to the traditional catalytic cracking, scholars in China made the combination of catalytic pyrolysis and catalytic cracking to upgrade pyrolysis bio-oil. It adopted a sequential biomass pyrolysis reactor which consisted of a traditional pyrolysis reactor followed by the subsequent apparatus that supported decomposition of gaseous intermediate [40]. The largest reactor for this kind of investigation was made of 316 stainless tube, with a length of 1000 mm and an inner diameter of 20 mm [38]. For example, Xiwei et al. [40] applied this method and found that biomass could be fully converted into gaseous products, such as H2, CH4 and CO, etc. Catalyst (Fe/γ-Al2O3) activities were affected by several factors, including calcination temperature, temperature of catalytic pyrolysis and Fe/Al mass ratio. Hong-yu et al. [41] demonstrated that when the cracking tempera￾ture was 500 1C, with a Weight Hourly Space Velocity of 3 h−1 , the liquid yield reached the maximum and the oxygenic compounds also 68 L. Zhang et al. / Renewable and Sustainable Energy Reviews 24 (2013) 66–72