AJ.Foster et al./Applied Catalysis A:General 423-424(2012)154-161 155 hydrocarbons over an HZSM-5 catalyst with aromatic yields up of different conditions,giving rise to different crystal sizes,mor- to 27 wt%.They also showed that HZSM-5 was much more active phologies,and elemental compositions.This flexibility allows for an than silicalite-1,suggesting that Bronsted acid sites have a criti- effort to study some of the factors affecting the aromatic yield from cal role in the upgrading reactions.Other investigators obtained biomass pyrolysis in detail to develop a better understanding of similar aromatic yields from the catalytic upgrading of pyrolysis biomass catalytic fast pyrolysis and to create a better ZSM-5-based oils from maple wood [26]and Canadian oak [28]over HZSM-5 pyrolysis catalyst. catalysts.ZSM-5 has also been shown to be the most active and One simple method to increase the yield towards aromatics may selective catalyst for the conversion of aqueous sugar solutions to be to increase the density of available catalytic sites.Zeolites with hydrocarbons [29].Gayubo et al.[24]found that oxygenates such higher aluminum content will have more acid sites able to catalyze as furfural and guaiacol contribute to rapid coke formation and the series of reactions necessary to form aromatics.However,as catalyst deactivation on HZSM-5 during pyrolysis oil upgrading. more aluminum is incorporated into the zeolite framework,the Model biomass compounds can be used to develop an under- zeolite will become more hydrophilic [31]and the appearance of standing ofthe catalytic factors which influence the overall reaction closely located Bronsted acid sites may have an effect on the cat- and aid in the design of CFP catalysts.We have previously shown alytic chemistry within the zeolite.This suggests that an optimum that furan and glucose are good model compounds for studying cel- silica-to-alumina ratio (SAR)may exist for this reaction.Since the lulose catalytic fast pyrolysis as CFP of all these compounds gave glucose derivatives in the proposed reaction scheme are compara- similar product selectivities [9,10,13,14].The proposed reaction ble in size to the micropore openings of zeolites [15].one strategy chemistry for glucose CFP is shown in Scheme 1.Glucose pyrol- to improve the conversion of these molecules is to improve the dif- ysis begins with a dehydration reaction to form anhydrosugars fusion characteristics of the catalysts.This may be accomplished by such as levoglucosan [11].Glucose can also undergo retro-aldol either decreasing the size of the zeolite particles or by creating hier- condensation to form dihydroxyacetone and glyceraldehydes.The archical mesopores within the zeolite framework [32].Because the anhydrosugars undergo further dehydration to form furanic species catalytic conversion of these molecules is likely limited by diffu- and water.These furans then undergo a series of decarbonylation sion into the micropores,any improvement in access to micropore and oligomerization reactions inside the zeolite to form a carbo- openings will have a positive effect on the ability of the zeolites to cation hydrocarbon pool and carbon monoxide.This hydrocarbon catalyze the desired reactions.As a side effect,the increased surface pool can then be converted into non-oxygenated olefins,monocylic area will also lead to an increase in the number of external surface aromatics and polycyclic aromatics.Coke can form on the catalyst acid sites. surface through parallel reactions of anhydrosugars,furans and the Because reactions confined within the zeolite micropores ben- hydrocarbon pool. efit from shape-selectivity,they are likely the key sites for the It has been shown that the micropore openings in ZSM-5 have formation of monoaromatic species.Acid sites on external particle a size near to the optimum for conversion of glucose towards surfaces and mesopore walls may have different activity and selec- aromatic species [15].This is due to the similarity between the tivity than the micropore sites [33].Selectively deactivating these diameter of benzene and the size of the largest micropore open- sites makes it possible to study their role during catalytic fast pyrol- ings in the zeolite.We have previously studied the effect of zeolite ysis.Deactivation can be accomplished by using a silylating agent micropore dimensions on glucose CFP using a range of small-, to make the sites inaccessible or by selective leaching from the zeo- medium-,and large-pore zeolites [15.The estimated kinetic diam- lite surface using an acid treatment.If the silylating/dealuminating eters of the reactants and products were used to determine whether agent used is larger than the micropores of the zeolite,the acid sites the reactions are able to occur inside the micropores or are limited on exterior surfaces can be selectively deactivated. to external surface sites for different zeolite catalysts.This analy- We have investigated the effects of modifying ZSM-5 bulk sis showed that the majority of aromatics and oxygenated species silica-to-alumina ratio,incorporating hierarchical mesopores,and present during reaction can fit inside the pores of most medium- selectively removing external surface acid sites on the CFP of glu- and large-pore zeolites but are excluded from entering small pores. cose,furan,and maple wood.Each of these aspects of the catalyst We showed that the aromatic yield is a function of the pore size of can alter both the activity for conversion of biomass derivatives and the zeolite catalyst.Zeolites with smaller micropores than ZSM- the selectivity for desirable hydrocarbon products.Understanding 5 severely hinder the diffusion of both reactants (i.e.furans)and the impact of these factors can be used to design more effective products(i.e.xylenes)in glucose pyrolysis.Small-pore zeolites did catalysts for the conversion of biomass to aromatics through CFP. not produce any aromatics from glucose,instead producing a mix- ture ofoxygenates,CO,CO2 and coke.In these cases,most reactions are limited to sites on the exterior particle surfaces [30]and coke is 2.Materials and methods the primary product.Zeolites with large pores allow for faster reac- tant diffusion,but the formation of larger polyaromatics within the 2.1.Zeolite synthesis zeolite micropores becomes more prevalent due to the lack of reac- tant confinement.Large-pore zeolites also primarily produce coke Mesoporous ZSM-5 (MesZSM-5)was synthesized using the during fast pyrolysis.Aromatic yields were highest in the medium- surfactant-mediated method reported by Ryoo et al.[34].3- pore zeolites with pore sizes in the range of 5.2-5.9A.In addition (Trimethoxysilyl)propyl dimethyl octadecyl ammonium chloride to micropore diameter,internal pore space and steric hindrance (TMPDOA)was used as a mesoporogen,and sodium aluminate was played a determining role for aromatic production.Medium-pore used as aluminum source.The synthesis gel had a molar oxide zeolites with moderate internal pore space and steric hindrance composition of40Na20:95 SiO2:3.3Al203:5TPA2O:26H2SO4:9000 (ZSM-5 and ZSM-11)gave the highest aromatic yield and the least H2O:5 TMPDOA.Samples were then crystallized in Parr Acid Diges coke formation.This study suggested that the ZSM-5 structure was tion Vessels under autogenous pressure at 150C for 4 days. the optimal zeolite structure for biomass conversion into aromatics. Non-mesoporous samples of ZSM-5 (MicZSM-5)were also The objective of this paper is to study how ZSM-5 can be further synthesized using tetrapropylammonium (TPA)as a structure- modified to increase the aromatic selectivity for CFP of biomass directing agent.The synthesis gel had a molar oxide composition The effects of ZSM-5 composition and mesoporosity on the yield of 5 Na20:100 SiO2:3.3 Al203:8 TPA2O:3000 H2O.Samples were and distribution of aromatics from glucose and maple wood CFP then loaded into Parr Acid Digestion Vessels and hydrothermally are studied in detail.ZSM-5 can be synthesized under a wide range synthesized under autogenous pressure at 150C for 5 days.A.J. Foster et al. / Applied Catalysis A: General 423–424 (2012) 154–161 155 hydrocarbons over an HZSM-5 catalyst with aromatic yields up to 27 wt%. They also showed that HZSM-5 was much more active than silicalite-1, suggesting that Brønsted acid sites have a criti￾cal role in the upgrading reactions. Other investigators obtained similar aromatic yields from the catalytic upgrading of pyrolysis oils from maple wood [26] and Canadian oak [28] over HZSM-5 catalysts. ZSM-5 has also been shown to be the most active and selective catalyst for the conversion of aqueous sugar solutions to hydrocarbons [29]. Gayubo et al. [24] found that oxygenates such as furfural and guaiacol contribute to rapid coke formation and catalyst deactivation on HZSM-5 during pyrolysis oil upgrading. Model biomass compounds can be used to develop an under￾standing ofthe catalytic factors which influence the overall reaction and aid in the design of CFP catalysts. We have previously shown thatfuran and glucose are good model compounds for studying cel￾lulose catalytic fast pyrolysis as CFP of all these compounds gave similar product selectivities [9,10,13,14]. The proposed reaction chemistry for glucose CFP is shown in Scheme 1. Glucose pyrol￾ysis begins with a dehydration reaction to form anhydrosugars such as levoglucosan [11]. Glucose can also undergo retro-aldol condensation to form dihydroxyacetone and glyceraldehydes. The anhydrosugarsundergo furtherdehydrationto formfuranic species and water. These furans then undergo a series of decarbonylation and oligomerization reactions inside the zeolite to form a carbo￾cation hydrocarbon pool and carbon monoxide. This hydrocarbon pool can then be converted into non-oxygenated olefins, monocylic aromatics and polycyclic aromatics. Coke can form on the catalyst surface through parallel reactions of anhydrosugars, furans and the hydrocarbon pool. It has been shown that the micropore openings in ZSM-5 have a size near to the optimum for conversion of glucose towards aromatic species [15]. This is due to the similarity between the diameter of benzene and the size of the largest micropore open￾ings in the zeolite. We have previously studied the effect of zeolite micropore dimensions on glucose CFP using a range of small-, medium-, and large-pore zeolites [15]. The estimated kinetic diam￾eters ofthe reactants and products wereused to determine whether the reactions are able to occur inside the micropores or are limited to external surface sites for different zeolite catalysts. This analy￾sis showed that the majority of aromatics and oxygenated species present during reaction can fit inside the pores of most medium￾and large-pore zeolites but are excluded from entering small pores. We showed that the aromatic yield is a function of the pore size of the zeolite catalyst. Zeolites with smaller micropores than ZSM- 5 severely hinder the diffusion of both reactants (i.e. furans) and products (i.e. xylenes) in glucose pyrolysis. Small-pore zeolites did not produce any aromatics from glucose, instead producing a mix￾ture of oxygenates, CO, CO2 and coke. In these cases, most reactions are limited to sites on the exterior particle surfaces [30] and coke is the primary product. Zeolites with large pores allow for faster reac￾tant diffusion, but the formation of larger polyaromatics within the zeolite micropores becomes more prevalent due to the lack of reac￾tant confinement. Large-pore zeolites also primarily produce coke during fast pyrolysis. Aromatic yields were highest in the medium￾pore zeolites with pore sizes in the range of 5.2–5.9A. ˚ In addition to micropore diameter, internal pore space and steric hindrance played a determining role for aromatic production. Medium-pore zeolites with moderate internal pore space and steric hindrance (ZSM-5 and ZSM-11) gave the highest aromatic yield and the least coke formation. This study suggested that the ZSM-5 structure was the optimal zeolite structure for biomass conversioninto aromatics. The objective of this paper is to study how ZSM-5 can be further modified to increase the aromatic selectivity for CFP of biomass. The effects of ZSM-5 composition and mesoporosity on the yield and distribution of aromatics from glucose and maple wood CFP are studied in detail. ZSM-5 can be synthesized under a wide range of different conditions, giving rise to different crystal sizes, mor￾phologies, and elemental compositions. Thisflexibility allows for an effort to study some of the factors affecting the aromatic yield from biomass pyrolysis in detail to develop a better understanding of biomass catalytic fast pyrolysis and to create a better ZSM-5-based pyrolysis catalyst. One simple method to increase the yield towards aromatics may be to increase the density of available catalytic sites. Zeolites with higher aluminum content will have more acid sites able to catalyze the series of reactions necessary to form aromatics. However, as more aluminum is incorporated into the zeolite framework, the zeolite will become more hydrophilic [31] and the appearance of closely located Brønsted acid sites may have an effect on the cat￾alytic chemistry within the zeolite. This suggests that an optimum silica-to-alumina ratio (SAR) may exist for this reaction. Since the glucose derivatives in the proposed reaction scheme are compara￾ble in size to the micropore openings of zeolites [15], one strategy to improve the conversion of these molecules is to improve the dif￾fusion characteristics of the catalysts. This may be accomplished by either decreasing the size ofthe zeolite particles or by creating hier￾archical mesopores within the zeolite framework [32]. Because the catalytic conversion of these molecules is likely limited by diffu￾sion into the micropores, any improvement in access to micropore openings will have a positive effect on the ability of the zeolites to catalyze the desired reactions. As a side effect,the increased surface area will also lead to an increase in the number of external surface acid sites. Because reactions confined within the zeolite micropores ben￾efit from shape-selectivity, they are likely the key sites for the formation of monoaromatic species. Acid sites on external particle surfaces and mesopore walls may have different activity and selec￾tivity than the micropore sites [33]. Selectively deactivating these sites makes it possible to study their role during catalytic fast pyrol￾ysis. Deactivation can be accomplished by using a silylating agent to make the sites inaccessible or by selective leaching from the zeo￾lite surface using an acid treatment. If the silylating/dealuminating agent used is larger than the micropores ofthe zeolite,the acid sites on exterior surfaces can be selectively deactivated. We have investigated the effects of modifying ZSM-5 bulk silica-to-alumina ratio, incorporating hierarchical mesopores, and selectively removing external surface acid sites on the CFP of glu￾cose, furan, and maple wood. Each of these aspects of the catalyst can alter both the activity for conversion of biomass derivatives and the selectivity for desirable hydrocarbon products. Understanding the impact of these factors can be used to design more effective catalysts for the conversion of biomass to aromatics through CFP. 2. Materials and methods 2.1. Zeolite synthesis Mesoporous ZSM-5 (MesZSM-5) was synthesized using the surfactant-mediated method reported by Ryoo et al. [34]. 3- (Trimethoxysilyl)propyl dimethyl octadecyl ammonium chloride (TMPDOA) was used as a mesoporogen, and sodium aluminate was used as aluminum source. The synthesis gel had a molar oxide composition of 40 Na2O:95 SiO2:3.3Al2O3:5 TPA2O:26 H2SO4:9000 H2O:5 TMPDOA. Samples were then crystallized in Parr Acid Diges￾tion Vessels under autogenous pressure at 150 ◦C for 4 days. Non-mesoporous samples of ZSM-5 (MicZSM-5) were also synthesized using tetrapropylammonium (TPA) as a structure￾directing agent. The synthesis gel had a molar oxide composition of 5 Na2O:100 SiO2:3.3 Al2O3:8 TPA2O:3000 H2O. Samples were then loaded into Parr Acid Digestion Vessels and hydrothermally synthesized under autogenous pressure at 150 ◦C for 5 days