Green Aromatic Hydrocarbon Production from Cocracking of a Bio-Oil

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Green aromatic hydrocarbon production from co-cracking of a bio-oil model compound mixture and ethanol over Ga2O3/HZSM-5 Shurong Wang, Qinjie Cai, Junhao Chen, Li Zhang, Xiangyu Wang, and Chunjiang Yu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5024029 • Publication Date (Web): 18 Aug 2014 Downloaded from http://pubs.acs.org on August 23, 2014

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Industrial & Engineering Chemistry Research

Green aromatic hydrocarbon production from co-cracking of a bio-oil model compound mixture and ethanol over Ga2O3/HZSM-5 Shurong Wang, Qinjie Cai, Junhao Chen, Li Zhang, Xiangyu Wang, Chunjiang Yu* State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China Email address: [email protected] Abstract: The distilled fraction (DF) from bio-oil molecular distillation can be used for the generation of aromatic hydrocarbons by co-cracking with ethanol over HZSM-5. In this work, typical model compounds were selected to simulate the DF, and a modified Ga2O3/HZSM-5 was adopted to promote the aromatization reaction. Compared with unmodified HZSM-5, 15% Ga2O3/HZSM-5 could increase the yield of the oil phase from 31.5 wt% to 39.2 wt% during co-cracking of a mixture of model compounds and ethanol, with an obvious decrease in the C3-C4 gaseous hydrocarbon yield. Moreover, the total content of aromatic hydrocarbons in the oil phase was around 95%. The production of mono-aromatics, especially xylenes, was favored. The influences of reaction temperature and feedstock weight hourly space velocity (WHSV), as well as the catalyst regeneration, were also investigated. Based on the experimental results, a reaction mechanism is proposed and the promotion of aromatic hydrocarbon generation by Ga2O3 is discussed. Key words: Bio-oil; model compound mixture; co-cracking; Ga2O3/HZSM-5; aromatic hydrocarbons 1

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1. Introduction The generation of bio-based liquid fuels is significant for solving the shortage of traditional fossil liquid fuels. Through fast pyrolysis, solid biomass waste, such as rice husk and straw, can be effectively converted into liquid bio-oil with higher energy density. However, due to the inferior properties of the crude bio-oil, including high water content, high oxygen content, strong corrosiveness, and a low heating value, it cannot be used directly as an engine fuel. Therefore, it is necessary to upgrade bio-oil for its high-grade utilization.1 Catalytic cracking is an efficient bio-oil upgrading technology, which can remove the oxygen therein in the forms of CO, CO2, and H2O, and transform the bio-oil into liquid products rich in aromatic hydrocarbons.2, 3 Aromatic hydrocarbons are important chemical raw materials, and they can also be blended with aliphatic hydrocarbons to produce commercial gasoline. Zeolites were often used for the catalytic cracking of bio-oil because of their acidity and shape selectivity. Adjaye and Bakhshi investigated the cracking of bio-oil over different zeolites, and the highest hydrocarbon yield was found over HZSM-5, followed by HY and silica-alumina, while only a few hydrocarbons were generated over silicalite and H-mordenite.4 In addition, HZSM-5 and H-mordenite were found to favor the formation of aromatics, whereas the other three zeolites were found to favor the production of aliphatics. Similar results were obtained by Vitolo et al., who compared the activities of HZSM-5 and HY for bio-oil cracking and concluded that HZSM-5 was more superior in aromatic oil production.5 Some mesoporous catalysts, such as SAPO-5, SAPO-11 and MgAPO-36, were also 2


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tested for bio-oil cracking, and the results showed they are not comparable to some zeolites like HZSM-5.6 Therefore, the recent studies mainly focused on the application of HZSM-5 or modified HZSM-5 in the conversion of bio-oil and its model compounds into aromatic hydrocarbons.7, 8 However, serious coke formation and catalyst deactivation would occur during direct cracking of bio-oil over HZSM-5, resulting in rapid termination of the cracking process.4, 5 There are two reasons for the propensity for coke formation. First, there are many sugar and phenol oligomers in bio-oil, which are non-volatile, have low reactivity, and are easily coked in the catalytic bed.7 Second, because the bio-oil components have high oxygen content and a high degree of unsaturation, the corresponding (H/C)eff is low, leading to the formation of cracking products with low H/C ratios, such as coke. The formula for (H/C)eff is shown as Eq. (1).2, 8 A study of the cracking of bio-oil model compounds with different (H/C)eff ratios showed the catalyst to have a higher coking tendency upon the cracking of compounds with low (H/C)eff ratios.8 (H / C) eff =

H − 2 ⋅O − 3⋅ N − 2 ⋅S C

(1)

Therefore, to suppress coke formation, removing high molecular weight compounds from bio-oil and increasing the (H/C)eff are necessary. The introduction of co-cracking reactants with high (H/C)eff ratios is an efficient way to increase the integral (H/C)eff. Two kinds of compounds are considered as suitable co-cracking reactants, namely ﬂuid catalytic cracking (FCC) feed gasoil ((H/C)eff>2) and aliphatic alcohols ((H/C)eff=2).9 Graca et al. found that the introduction of gasoil could facilitate the conversion of bio-oil model compound mixtures into fuel gas, liquefied petroleum 3



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gas (LPG), and gasoline.10 Fogassy et al. used partially hydrodeoxygenated bio-oil for co-cracking with gasoil, and high LPG and gasoline yields of 24% and 46%, respectively, were achieved, while the coke yield was only 5.8%.11 Aliphatic alcohols are also suitable co-cracking reactants because of their good performance during similar processes such as methanol to gasoline (MTG) and ethanol to gasoline (ETG) conversions, and they can be obtained by bio-based processes such as bio-syngas synthesis or biochemical conversion.12, 13 Moreover, since methanol and ethanol have a high (H/C)eff of 2, both aliphatic and aromatic hydrocarbons will be produced in large amounts. When bio-oil with a relatively low (H/C)eff is co-cracked with alcohols, the (H/C)eff of the final cracking products will be shifted towards the range of aromatic hydrocarbons, and hence co-cracking is superior for the selective generation of aromatic hydrocarbons. Mentzel et al. found that the introduction of methanol could prolong the lifetime of the catalyst during cracking of bio-oil model compounds, and the selectivity for aromatic hydrocarbons was higher than that of pure methanol cracking.8 In a co-cracking study of thermally treated bio-oil and methanol by Valle et al., the selectivity for aromatic hydrocarbons was as high as 40% and the total selectivity for C2-C4 gaseous hydrocarbons reached about 30%.7 In general, an increase in (H/C)eff will inhibit coke formation and catalyst deactivation, and the production of gaseous hydrocarbons will also be promoted. In our previous co-cracking study of ketones and alcohols over a HZSM-5 catalyst, a high oil-phase selectivity of over 30 wt% was obtained, of which the aromatic hydrocarbon content was greater than 90%, and the C3H8 content in the vent gas also exceeded 4


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30%.14, 15 In subsequent research, the distilled fraction from the molecular distillation of bio-oil, which was enriched in reactive ketones and acids, was co-cracked with ethanol.16 A high gasoline yield of 25.9 wt% was obtained, the aromatic hydrocarbon content was up to 91.2%, and the C3-C4 hydrocarbon content in the vent gas reached about 40%. Therefore, if the propensity for the aromatization reaction during cracking is enhanced, the formation of light hydrocarbons will be suppressed, and more aromatic hydrocarbons will be generated. As a typical catalyst for the aromatization reaction, Ga2O3/HZSM-5 has been widely studied in the field of C3H8 aromatization, the MTG process, and the catalytic upgrading of bio-based liquid fuels. In a study of C3H8 aromatization, Kitagawa et al. found that Ga2O3 played an important role in the aromatization of olefins.17 The traditional MTG process using HZSM-5 catalyst has a high selectivity for C1-C4 hydrocarbons, which can account for more than 60% of the product. However, by using Ga2O3/HZSM-5, the selectivity for aromatic hydrocarbons is significantly increased. Freeman et al. used a Ga2O3/HZSM-5 catalyst prepared by physical mixing for the MTG process.18 Their results showed that over an unmodified HZSM-5 catalyst, the selectivity for C3-C4 hydrocarbons reached 64.9% and the selectivity for aromatic hydrocarbons was only 8.1%. By using a 25%Ga2O3/HZSM-5 catalyst, the selectivity for C3-C4 hydrocarbons decreased to 43.2% and the selectivity for aromatic hydrocarbons was up to 37.2%. Comparing Ga2O3/HZSM-5 catalysts prepared by different methods for their efficacy in the MTG process, Miao et al. found that an appropriate amount of non-framework Ga species promoted the formation of aromatic 5



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hydrocarbons.19 In a catalytic pyrolysis study of the typical bio-based oxygenated compound furan, Cheng et al. found that bifunctional Ga2O3/HZSM-5 increased the selectivity for aromatic hydrocarbons to 43.5% as compared to 31.0% for the unmodified HZSM-5.20 As mentioned above, in our previous study of co-cracking of DF and ethanol over HZSM-5, a large amount of C3-C4 light hydrocarbons was produced, which lowered the selectivity for liquid aromatic hydrocarbons. Therefore, in this work, a bifunctional Ga2O3/HZSM-5 catalyst has been used for the co-cracking process, and the mixture of typical model compounds in DF, namely hydroxypropanone (HPO), cyclopentanone (CPO), acetic acid (HOAc), has been selected to be mixed as the reactants. The effects of Ga2O3 loading and reaction temperature on the promotion of aromatization have been investigated, and the deactivated catalyst has also been regenerated. 2. Experimental section HZSM-5 (Si/Al=25) was purchased from the Catalyst Plant of Nankai University and Ga2O3 was obtained from Aladdin Industrial Corporation. The Ga2O3/HZSM-5 catalyst was prepared by physical mixing. Ga2O3 and HZSM-5 were crushed to fine powders and then mixed uniformly by mechanical agitation. The intimate mixture was molded into pellets (40-60 mesh), and activated at 550 °C for 6 h. This physical mixing method has been proved to promote the aromatization of C3-C4 hydrocarbons in studies by Freeman et al. and Buckles et al.18, 21 The loading amount of Ga2O3 was 0−20%. The reactant was the mixture of typical model compounds in DF, including HPO, CPO and HOAc. HPO and CPO were purchased from Alfa Aesar and Aladdin 6


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Industrial Corporation, respectively, and HOAc and EtOH were purchased from the Sinopharm Chemical Reagent Co. The mixing ratio was set according to the distribution of chemical families in the actual DF. Our previous study showed the main components in the DF to be ketones and acids, accounting for 48.6% and 27.5%, respectively, and the contents of HPO, CPO derivatives, and HOAc reached 27.9%, 5.1%, and 23.9%, respectively.16 Therefore, the mixing ratio was correspondingly set at 6:1:5. Moreover, it was also found that a reactant composition of 30% model compound and 70% alcohol gave a good cracking performance.14, 15 Thus, this weight ratio was also used for the co-cracking of model compound mixture and ethanol in this work. The catalytic experiments were performed in a fixed-bed reactor, detailed information on which has been provided in our previous study 15. The reaction pressure was maintained by the carrier gas N2, at a flow rate of 30 mL/min. Each catalytic run lasted for 3 h. The feedstock WHSV was set within the range of 3 to 12 h−1. The reaction temperature ranged from 300 to 450 °C, and the reaction pressure was 2 MPa. The gaseous products were quantified by online gas chromatography (Agilent 7890A). The temperature of GC oven was maintained at 50 °C for 1 min and then increased to 180 °C at a rate of 10 °C/min. Most liquid products consisted of an oil phase and an aqueous phase. The oil phase was analyzed by a GC-MS system and the identified compounds were quantified by the area normalization method. The temperature of GC oven was maintained at 40 °C for 1 min and then increased to 240 °C at a rate of 8 °C/min. The residual reactants in both the oil and aqueous phases were quantified by gas chromatography, allowing calculation of the conversion of the 7



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reactants. The conversion of the reactants (Xi) and the yields (Yi) of the liquid and gaseous products are defined by Eqs. (2)−(5). In the calculation of yield, the unconverted reactants were excluded from the liquid products. The symbol “m” in the following equations denotes the mass of the corresponding substances. Xi (i = HPO,CPO,HOAc,EtOH) =

YOil phase =

(mi )in − (mi )out ×100% (mi )in

mOil phase (mReactants )in − (mReactants )out

YAqueousphase =

(mReactants )in − (mReactant )out

Yi (i = C1−4 hydrocarbons,COx ) =

(3)

×100%

mAqueousphase

(2)

(4)

×100%

mi ×100% (mReactants )in − (mReactant )out

(5)

Each reaction condition was repeated for three times, and error of the yields of gaseous and liquid produces was within ±5%. 3. Results and discussion 3.1 Influence of Ga2O3 loading The influence of Ga2O3 loading on the co-cracking of model compound mixture (MCM) and Ethanol was studied at 400 °C and 2 MPa, which was considered to be the optimum reaction condition for cracking over unmodified HZSM-5.14,15 3.1.1 Reactant conversion and liquid product yield When Ga2O3/HZSM-5 with different Ga2O3 loadings (0−20%) was adopted for the co-cracking of MCM and EtOH, the reactant conversion consistently reached 100%, showing the high conversion efficiency by co-cracking. In our previous study, we found that the cracking of pure CPO gave a very low conversion of 23.8%, but when 70% 8


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methanol was introduced, the conversion of CPO reached 100%.14 This confirmed that the presence of methanol promoted the conversion of CPO, and similar promotion was also found during the co-cracking of HPO and EtOH.15 Moreover, in a study of MTG over Ga2O3/HZSM-5 by Freeman et al., it was observed that when the Ga2O3 loading was lower than 50%, methanol could be completely converted.18 Therefore, although the loading of Ga2O3 decreased the number of Brønsted acid sites due to the lower amount of HZSM-5, the results showed that an appropriate loading would not affect the conversion efficiency during cracking. Although the reactants were completely converted over all the catalysts, the promoting effect of Ga2O3 on the generation of the oil phase was significant, as shown in Fig. 1. On increasing the Ga2O3 loading from 0 to 15%, the yield of the oil phase increased from 31.5 wt% to 39.2 wt%, suggesting that the Ga2O3 loading of 15% was very effective for the promotion of liquid hydrocarbon production. Increasing the Ga2O3 loading to 20% did not further increase the yield of the oil phase, which indicated that the promotion of oil phase generation by Ga2O3 incorporation got nearly saturated under the Ga2O3 loading of 15%. Meanwhile, the yield of the aqueous phase also increased slightly. A similar observation was made in the MTG study by Freeman et al., who found that 25%Ga2O3/HZSM-5 could increase the selectivity for aromatic hydrocarbons from 8.1% to 37.2%, proving the important role of Ga2O3 in promoting the aromatization reaction.18 3.1.2 Oil phase composition GC-MS chromatograms of the oil phases are presented in Fig. 2. The retention 9



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times of mono-aromatics were shorter than 10 min, while the peaks of the heterocyclic compounds and di-aromatic hydrocarbons were mainly located in the range of 11−24 min. The peak intensities of the heterocyclic compounds and di-aromatic hydrocarbons clearly decreased with increasing Ga2O3 loading. The compositions of the oil phases varied correspondingly. It was found that loading of Ga2O3 at an appropriate level (5−20%) did not affect the deoxygenation efficiency, and the hydrocarbon contents in the oil phases were 100%, of which aromatic hydrocarbons constituted around 95%. This was mainly due to the low (H/C)eff of the bio-oil model compounds, which placed the (H/C)eff of the feed between 1 and 2 when co-cracking with EtOH, the range favoring the formation of aromatic hydrocarbons. In the case of cracking over unmodified HZSM-5, significant amounts of indene, naphthalene, and their derivatives were formed. With increasing Ga2O3 loading, the total content of C6-C9 aromatic hydrocarbons increased, reaching 70.6% in the case of 15%Ga2O3/HZSM-5, while the contents of indene, naphthalene, and their derivatives decreased. The increases in the toluene content from 9.9% to 14.2% and in the xylene content from 26.3% to 32.6% were notable. These results showed when the aromatization was promoted by the incorporation of Ga2O3, the formation of C6-C9 mono-aromatics was more favored than that of other aromatics. Among the mono-aromatics, benzene, toluene, and xylenes (BTX) are important chemical raw materials, and C7-C9 aromatics are also typical components in commercial gasoline, while the heterocyclic compounds and di-aromatic hydrocarbons are more difficult to be completely burnt. Therefore, the promotion of mono-aromatics production by the 10


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incorporation of Ga2O3 not only benefits the further processing of the BTX in the oil phase, but also makes the oil phase superior when it is blended with other aliphatic hydrocarbons to produce commercial gasoline. In a study of the catalytic pyrolysis of furan and methylfuran, Cheng et al. also found that the loading of Ga2O3 on commercial HZSM-5 promoted the formation of xylenes.22 The study by Park et al. confirmed that incorporation of an appropriate amount of Ga species increased the yields of BTX and decreased the yields of C10+ PAHs during the upgrading of biomass pyrolytic vapor. 23 Cracking over 20%Ga2O3/HZSM-5 gave a similar oil phase composition to that with 15% Ga2O3/HZSM-5. Based on the yield and composition of the oil phase, it can be concluded that a Ga2O3 loading of 15% is suitable for the production of aromatic hydrocarbons. 3.1.3 Gaseous product yield The vent gas was analyzed by online GC, and the yields of gaseous products are shown in Fig. 3. C3H8, C4H8, CO, and CO2 were the main gaseous products. Among the hydrocarbon gases, the decrease in the yield of C3H8 was the most prominent, from 13.4 wt% over unmodified HZSM-5 to 8.2 wt% over 15%Ga2O3/HZSM-5. Additionally, the yield of C4H8 decreased from 3.8 wt% to 1.6 wt% when the Ga2O3 loading was increased from 0 to 15%, while the yields of other hydrocarbon gases remained almost the same. The decreases in the yields of C3H8 and C4H8 showed the higher contribution from the aromatization reaction, which lowered the yields of gaseous hydrocarbons and increased the yields of aromatic hydrocarbons. Meanwhile, compared with the MTG process, much more CO and CO2 were also produced, which 11



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could be attributed to the decarbonylation and decarboxylation reactions of the carbonyl and carboxyl groups in the model compounds. The yield of CO decreased from 3.0 wt% to 2.1 wt% and that of CO2 decreased from 4.2 wt% to 3.8 wt% when the Ga2O3 loading was increased from 0 to 15%. The slight decrease in the yields of COx corresponded to an increase in the yield of the aqueous phase, indicating that more oxygen was removed in the form of water. 3.2 Influence of reaction temperature The experiments described above were conducted at the optimum reaction temperature (400 °C) for unmodified HZSM-5. Considering the possible influence of reaction temperature on the activity of Ga2O3, the catalytic activity of bifunctional Ga2O3/HZSM-5 with an optimum loading of 15% was investigated at various temperatures. 3.2.1 Reactant conversion The conversions of the reactants at different temperatures are shown in Fig. 4. At 300 °C, the conversion of ketones was almost 100%, while the conversions of acetic acid and ethanol were 77.0% and 49.0%, respectively. When the reaction temperature increased to 350 °C, the conversion of ethanol reached 85.5%, whereas the conversion of acetic acid decreased to 63.2%. At 400 and 450 °C, all of the reactants were completely converted. 3.2.2 Liquid product yield Fig. 5 presents the yields of liquid products obtained at different reaction temperatures. At 300 °C, no obvious oil phase was generated, and a mixed liquid was 12


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formed in 81.2 wt% yield. At 350 °C, a separable oil phase and an aqueous phase were produced, and the yield of the oil phase was 14.8 wt%. On further increasing the temperature to 400 °C, the yield of the oil phase reached 39.2 wt%. However, cracking at 450 °C gave a lower oil phase yield of 36.3 wt%, indicating that this temperature induced some secondary decomposition reactions of oil phase products. 3.2.3 Oil phase composition The oil phases from cracking at different temperatures (including the mixed liquid obtained from the cracking at 300 °C) were analyzed by GC-MS, and the components (excluding unconverted reactants) were classified into six groups, namely aromatic hydrocarbons, aliphatic hydrocarbons, ethers, esters, ketones, and others. The results are shown in Fig. 6. The mixed liquid from the cracking at 300 °C mainly contained esters and ethers, with their contents up to 55.0% and 38.5%, respectively. The typical compounds were ethyl acetate and diethyl ether, indicating that esterification of acetic acid and ethanol, and etherification of ethanol dominated at a low reaction temperature. These partial deoxygenation reactions converted the reactants into oxygenated liquid by-products, principally esters and ethers, which are good solvents. Therefore, the mixed liquid was obtained in high yield and appeared homogeneous. When the reaction temperature was increased to 350 °C, separable oil and aqueous phases were formed. The main components in the oil phase were still esters and ethers, but their contents decreased to 31.5% and 23.5%, respectively, while the content of hydrocarbons (mainly aromatics) increased to 25.0%. In a similar cracking study of bio-oil model compounds, a reaction temperature of 350 °C was found to be more favorable for the conversion of 13



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methanol into hydrocarbons than 300 °C, while decarbonylation and decarboxylation reactions of acetic acid did not proceed easily at temperatures below 400 °C.18,

24

Therefore, the decrease in acetic acid conversion between 300 and 350 °C might be attributable to the higher conversion of ethanol into hydrocarbons, leaving less of it to participate in the esterification reaction with acetic acid. In general, the hydrocarbon yields at 300 and 350 °C were very low, and a lot of oxygenated by-products were generated. This is very similar to the results of our previous co-cracking study using unmodified HZSM-5, which showed that at these two reaction temperatures the catalytic activity of Ga2O3 is low.15, 16 In the MTG study by Freeman et al., production of aromatics was also only found to be promoted by Ga2O3 addition at temperatures higher than 400 °C 18. When the reaction temperature was increased to 400 or 450 °C, the hydrocarbon contents reached 100%, of which aromatic hydrocarbons accounted for around 95%. 3.2.4 Gaseous product yield The yields of gaseous products obtained at different reaction temperatures are shown in Fig. 7. At 300 and 350 °C, C2H4 was the main gaseous hydrocarbon, obtained in yields of 13.4 wt% and 22.8 wt%, respectively. The higher C2H4 yield at 350 °C also showed a greater contribution from the intramolecular dehydration of ethanol. It also indicated that the aromatization reaction was not favored, and the intermediate C2H4 was released instead of participating in the subsequent aromatization. When the reaction temperature was 400 or 450 °C, C3H8 became the dominant gaseous hydrocarbon, with a yield of about 8 wt%. The yields of CO and CO2 increased 14


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remarkably with increasing reaction temperature, suggesting that higher temperature favored the decarbonylation and decarboxylation reactions. This is in good agreement with the COx variation trend with reaction temperature in a study of the cracking of ketones and acids by Gayubo et al. 24. 3.3 Influence of feedstock WHSV Under the optimum reaction temperature (400 °C) and Ga2O3 loading (15%), the influence of feedstock WHSV was further studied. 3.3.1 Reactant conversion The conversions of reactants at different feedstock WHSV are presented in Fig. 8. At the WHSV of 3 h-1 and 6 h-1, all reactants were completely converted. When the WHSV was increased to 9 h-1, although the conversions of HPO and CPO maintained 100%, the conversions of HOAc and EtOH decreased to 70.9% and 91.5%, respectively. Further increasing the WHSV to 12 h-1 would have even lower conversions of HOAc and EtOH, which were 54.0% and 87.1%, respectively. As the increase of feedstock WHSV, the active sites in the catalyst are not enough for the complete transformation of reactants to the desirable products. At the WHSV beyond 9 h-1, the HOAc and EtOH were more difficult to be converted completely than ketones. 3.3.2 Liquid product yield Fig. 9 shows the yields of liquid products obtained at different feedstock WHSV. Although reactants were completely converted at the WHSV of 6 h-1, the yield of oil phase was only 32.0 wt%, much lower than that obtained at the WHSV of 3 h-1, showing the WHSV of 3 h-1 is more favorable for the generation of liquid products. 15



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When the WHSV increased to 9 h-1 and 12 h-1, the yields of oil phase further decreased to 26.3 wt% and 27.5 wt%, respectively. 3.3.3 Oil phase composition The compositions of oil phases obtained at different feedstock WHSV are shown in Fig. 10. At the WHSV of 3 h-1 and 6 h-1, oil phases were completely composed of hydrocarbons, and the majority was aromatics. However, the yield of oil phase obtained at 6 h-1 was lower than that obtained at 3 h-1. Hence, it can be inferred that at the WHSV of 6 h-1, the amount of active sites in the catalyst could achieve the primary conversion of reactants, but it was not sufficient for the subsequent aromatization reaction. When the WHSV reached 9 h-1 and 12 h-1, a lot of oxygenated byproducts from partial deoxygenation were generated, including esters, ethers and ketones. This result indicated that when the amount of active sites was limited, partial deoxygenation reactions involving intermolecular dehydration were favored. It was also found that the total content of esters and ethers in the oil phase obtained at 12 h-1 was obviously higher than that obtained at 9 h-1, showing higher WHSV decreased the deoxygenation efficiency. 3.3.4 Gaseous product yield The yields of gaseous products obtained at different feedstock WHSV are shown in Fig. 11. As the increase of WHSV, the yields of olefins (C2H4 and C3H6) increased notably. The yield of C2H4 was 0.1 wt% at the WHSV of 3 h-1, and it rose to 8.7 wt% as the WHSV reached 6 h-1. Because olefins are important intermediates during cracking, more olefin release matched the above inference that the intensity of aromatization was 16


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lower at the WHSV of 6 h-1. When the WHSV were 9 h-1 and 12 h-1, the yields of C2H4 reached 16.1 wt% and 19.3 wt%, respectively, corresponding to the low yields of hydrocarbons. Moreover, the yield of COx also decreased as the WHSV increased, which was in accordance with the fact that more oxygenated byproducts were generated. 3.4 Reaction mechanism over Ga2O3/HZSM-5 The above experimental results showed that aromatic hydrocarbons could be generated by co-cracking of model compounds and ethanol over HZSM-5, and loading of Ga2O3 further promoted this generation. Based on this result, the reaction mechanism is proposed as shown in Fig. 12. Since the model compounds have low (H/C)eff ratios, their cracking without ethanol addition would favor the formation of products with low H/C such as coke, which would lead to deactivation of the catalyst. Therefore, supplementation with additional hydrogen is necessary to suppress coke formation. During ethanol cracking, the deoxygenation reaction, mainly intramolecular dehydration, first occurs to produce the light olefin C2H4, which can further undergo aromatization to form aromatics. A few aliphatics are also formed by polymerization and isomerization. It is noticeable that during the conversion of olefins (H/C=2) into aromatics (1≤H/C

Green Aromatic Hydrocarbon Production from Cocracking of a Bio-Oil

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