AJ.Foster et al.Applied Catalysis A:General 423-424(2012)154-161 157 Table 1 a田Unidentified Microporous and mesoporous volumes of samples used for SiOz/Al2O3 study as 100 Coke measured by N2 adsorption. 90 Sample Vmicro (cm3/g) Vmeso(cm3/g) ZSM-5.SAR=23 0.115 0.029 ZSM-5,SAR=30 0.107 0.056 ZSM-5.SAR=50 60 0.124 0.059 ZSM-5.SAR=80 0.119 0.077 of occluded organic structure-directing molecules after zeolite synthesis,as well as experimental error in the quantification of coke 20 and product gases likely contributed to any mass balances above 10- 100%. 0 23 30 50 0 3.Results and discussion Bulk Sio,/Al,O Fig.1.Yield of aromatic hydrocarbons,CO2.and CO from catalytic fast pyrolysis Experiments were conducted to determine the relationship of glucose over ZSM-5 with varying SiOz/Al2O composition.Reaction conditions: between different properties of ZSM-5 catalysts on the yield and 600C.19mg catalyst/mg glucose,and 240s reaction time. distribution of aromatic hydrocarbons obtained through glucose and maple wood catalytic fast pyrolysis.The key parameters necessary to limit bimolecular coke-forming reactions.As the investigated were bulk silica-to-alumina ratio,mesoporosity and silica-to-alumina ratio of ZSM-5 is decreased.the concentration of removal of external surface acid sites. acid sites in close proximity to one another will increase.The incor- poration of these additional acid sites in closer proximity to each 3.1.Optimizing aromatic yield by tuning Al content of ZSM-5 other may promote secondary reactions responsible for converting aromatic species to coke within the micropores.This effect of acid- Changing the alumina content of the zeolite particles will impact site density enhances the rate of coke formation in the conversion both the hydrophilicity of the catalyst [31]and the number density of methanol to olefins over ZSM-5 catalysts with similar SAR [39]. of Bronsted acid sites and hence may impact the aromatic yield [35]. Similar effects of the silica-to-alumina ratio on reactivity have also ZSM-5 can be synthesized over a wide range of silica-to-alumina been observed in the esterification of acetic acid and butanol over ratios,and this has an effect on its activity for biomass catalytic USY [40]and dodecane cracking over ZSM-5 [41]. fast pyrolysis.Four ZSM-5 samples (obtained from Zeolyst)with The amount of CO2 produced during the reaction depends different silica-to-alumina ratios(23,30,50,80)were tested for weakly on the Al content of the ZSM-5 samples.The sample with catalytic conversion of glucose to study the effect of changing the Al the highest aluminum content produced the highest amount of CO2. content in the catalyst.X-ray diffraction of the samples confirmed suggesting that the decarboxylation is enhanced by Bronsted acid that all were highly crystalline MFI-type framework materials(see catalysis but is less sensitive to acid site density than decarbonyla- supporting information). tion. Nitrogen adsorption measurements confirmed that all samples The distribution of aromatic hydrocarbon products from glu- had microporous volume of approximately 0.12 cm3/g and meso- cose pyrolysis changed slightly between samples as shown in Fig.2. porous volume between 0.03 and 0.08 cm3/g (see Table 1).SEM The ZSM-5 sample with the highest aluminum content showed images revealed that there were no visible morphological differ- the highest selectivity towards smaller aromatic products (ben- ences between the samples with different SAR (see supporting zene and toluene),and samples with a lower amount of aluminum information).The size of primary particles in the samples slightly were slightly more selective towards larger products (Cs*aro- decreased with decreasing aluminum content.The decrease in pri- matics and polyaromatics).The larger aromatic products included mary particle size led to an increase in the number of particle boundaries and voids within the samples,which likely accounts for the apparent increase in mesopore volume with SAR.However,the 45- Si0/AN,0,=23 observed mesoporous volume of these samples is modest,and is not 国Si0/AN,O2=30 expected to contribute significantly to the activity and selectivity of 40 ZZ☑Si0JA,0,=50 these samples during the CFP reaction.The effects of mesoporosity 图Si0/A,0,=80 is discussed in more detail in the following section. 35- The yield of aromatic products from glucose as a function of bulk 里 30- silica-to-alumina ratio is shown in Fig.1.The maximum aromatic yield occurred at SiO/Al2O3=30,with a concurrent minimum in 25 the amount of coke produced.The CO and CO2 produced during 20 pyrolysis are considered to be the products of decarbonylation and decarboxylation reactions,respectively [11].The strong Bronsted 15 acid sites in ZSM-5 have been shown to be active for the decar- bonylation of benzaldehydes [36]and furfurals [37].two types of compounds produced during biomass fast pyrolysis [38].The amount of CO produced is at a maximum for the SiO2/Al203=30 sample,suggesting that there may be a relationship between the rate of oxygen removal via decarbonylation and the formation C6 C7 C8 C9 Polyarom. of aromatic species.A silica-to-alumina ratio of 30 represents an Fig.2.Distribution of aromatic products from fast pyrolysis of glucose over ZSM-5 optimal composition for the high availability of Bronsted sites with varying SiOz/Al2O3 composition.Reaction conditions:600C.19mg cata- while simultaneously maintaining the distance between acid sites lyst/mg glucose,and 240s reaction time.A.J. Foster et al. / Applied Catalysis A: General 423–424 (2012) 154–161 157 Table 1 Microporous and mesoporous volumes of samples used for SiO2/Al2O3 study as measured by N2 adsorption. Sample Vmicro (cm3/g) Vmeso (cm3/g) ZSM-5, SAR = 23 0.115 0.029 ZSM-5, SAR = 30 0.107 0.056 ZSM-5, SAR = 50 0.124 0.059 ZSM-5, SAR = 80 0.119 0.077 of occluded organic structure-directing molecules after zeolite synthesis, as well as experimental error in the quantification of coke and product gases likely contributed to any mass balances above 100%. 3. Results and discussion Experiments were conducted to determine the relationship between different properties of ZSM-5 catalysts on the yield and distribution of aromatic hydrocarbons obtained through glucose and maple wood catalytic fast pyrolysis. The key parameters investigated were bulk silica-to-alumina ratio, mesoporosity and removal of external surface acid sites. 3.1. Optimizing aromatic yield by tuning Al content of ZSM-5 Changing the alumina content ofthe zeolite particles will impact both the hydrophilicity of the catalyst [31] and the number density of Brønsted acid sites and hence may impactthe aromatic yield [35]. ZSM-5 can be synthesized over a wide range of silica-to-alumina ratios, and this has an effect on its activity for biomass catalytic fast pyrolysis. Four ZSM-5 samples (obtained from Zeolyst) with different silica-to-alumina ratios (23, 30, 50, 80) were tested for catalytic conversion of glucose to study the effect of changing the Al content in the catalyst. X-ray diffraction of the samples confirmed that all were highly crystalline MFI-type framework materials (see supporting information). Nitrogen adsorption measurements confirmed that all samples had microporous volume of approximately 0.12 cm3/g and meso￾porous volume between 0.03 and 0.08 cm3/g (see Table 1). SEM images revealed that there were no visible morphological differ￾ences between the samples with different SAR (see supporting information). The size of primary particles in the samples slightly decreased with decreasing aluminum content. The decrease in pri￾mary particle size led to an increase in the number of particle boundaries and voids within the samples, which likely accounts for the apparent increase in mesopore volume with SAR. However, the observedmesoporous volume ofthese samples ismodest, andisnot expected to contribute significantly to the activity and selectivity of these samples during the CFP reaction. The effects of mesoporosity is discussed in more detail in the following section. The yield of aromatic products from glucose as a function of bulk silica-to-alumina ratio is shown in Fig. 1. The maximum aromatic yield occurred at SiO2/Al2O3 = 30, with a concurrent minimum in the amount of coke produced. The CO and CO2 produced during pyrolysis are considered to be the products of decarbonylation and decarboxylation reactions, respectively [11]. The strong Brønsted acid sites in ZSM-5 have been shown to be active for the decar￾bonylation of benzaldehydes [36] and furfurals [37], two types of compounds produced during biomass fast pyrolysis [38]. The amount of CO produced is at a maximum for the SiO2/Al2O3 = 30 sample, suggesting that there may be a relationship between the rate of oxygen removal via decarbonylation and the formation of aromatic species. A silica-to-alumina ratio of 30 represents an optimal composition for the high availability of Brønsted sites while simultaneously maintaining the distance between acid sites 23 30 50 80 0 10 20 30 40 50 60 70 80 90 100 Carbon Yield (%) Bulk SiO2 / Al2 O3 Unidentified Coke CO CO Aromatics Fig. 1. Yield of aromatic hydrocarbons, CO2, and CO from catalytic fast pyrolysis of glucose over ZSM-5 with varying SiO2/Al2O3 composition. Reaction conditions: 600 ◦C, 19 mg catalyst/mg glucose, and 240 s reaction time. necessary to limit bimolecular coke-forming reactions. As the silica-to-alumina ratio of ZSM-5 is decreased, the concentration of acid sites in close proximity to one another will increase. The incor￾poration of these additional acid sites in closer proximity to each other may promote secondary reactions responsible for converting aromatic species to coke within the micropores. This effect of acid￾site density enhances the rate of coke formation in the conversion of methanol to olefins over ZSM-5 catalysts with similar SAR [39]. Similar effects of the silica-to-alumina ratio on reactivity have also been observed in the esterification of acetic acid and butanol over USY [40] and dodecane cracking over ZSM-5 [41]. The amount of CO2 produced during the reaction depends weakly on the Al content of the ZSM-5 samples. The sample with thehighest aluminumcontentproducedthehighest amount ofCO2, suggesting that the decarboxylation is enhanced by Brønsted acid catalysis but is less sensitive to acid site density than decarbonyla￾tion. The distribution of aromatic hydrocarbon products from glu￾cose pyrolysis changed slightly between samples as shown in Fig. 2. The ZSM-5 sample with the highest aluminum content showed the highest selectivity towards smaller aromatic products (ben￾zene and toluene), and samples with a lower amount of aluminum were slightly more selective towards larger products (C8 + aro￾matics and polyaromatics). The larger aromatic products included C6 C7 C8 C9 Polyarom. 0 5 10 15 20 25 30 35 40 45 Aromatic Selectivity (%) SiO2 /Al2 O3 = 23 SiO2 /Al2 O3 = 30 SiO2 /Al2 O3 = 50 SiO2 /Al2 O3 = 80 Fig. 2. Distribution of aromatic products from fast pyrolysis of glucose over ZSM-5 with varying SiO2/Al2O3 composition. Reaction conditions: 600 ◦C, 19 mg cata￾lyst/mg glucose, and 240 s reaction time