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The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production
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The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production Shurong Wang a,*, Junhao Chen a, Qinjie Cai b, Fan Zhang a, Yurong Wang a, Bin Ru a, Qi Wang c,** a
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China c College of Metrological Technology and Engineering, China Jiliang University, Hangzhou 310018, China b
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abstract
Article history:
In order to reduce the coke formation and improve catalyst activation in the catalytic
Received 30 October 2015
cracking of bio-oil, ethanol addition and mild hydrogenation pretreatment were introduced
Received in revised form
to improve the cracking stability. According to the analysis on the bio-oil molecular
27 November 2015
distillation, several typical compounds were selected as the representative of bio-oil
Accepted 12 December 2015
distillation. Based on the two-stage fixed-bed reactor, the effect of mild hydrogenation
Available online xxx
on the cracking behavior of bio-oil model compound mixture was studied in detail. In a single-stage cracking process, the selectivity of oil phase was only 28e23 wt%, with the
Keywords:
formation of abundant oxygenated byproducts. When mild hydrogenation was introduced
Bio-oil
before the cracking, the selectivity of oil phase increased up to 40wt% at 8 h, with an ar-
Catalytic cracking
omatic hydrocarbon content of over 90%.
Mild hydrogenation
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Aromatic hydrocarbon
Introduction The concern for the exhaustion of fossil fuel has made biomass energy draw extensive attentions in the world [1,2]. Bio-oil via biomass fast pyrolysis is supposed to be a substitute for conventional transport fuels. However, the inferior properties of crude bio-oil, including high oxygen and water contents, low heating value and strong corrosiveness [3,4], limit its direct utilization. Therefore, upgrading technology is essential for the bio-oil high-grade utilization.
Various upgrading techniques have been studied in recent decades, such as catalytic cracking, catalytic hydrogenation, esterification, steam reforming, emulsification, etc [5,6]. Catalytic cracking is an effective bio-oil upgrading technology, which can transform bio-oil into the liquid fuel abundant in aromatic hydrocarbons by removing oxygen in the form of CO, CO2 and H2O [7]. Various catalysts, such as HZSM-5, HeY, Hmordenite, silicalite and silica-alumina, were investigated in catalytic cracking process, in which HZSM-5 was favorable for aromatic hydrocarbon production [8]. However, the composition of crude bio-oil is quite complicated, which consists of
* Corresponding author. Tel.: þ86 571 87952801; fax: þ86 571 87951616. ** Corresponding author. Tel: þ86 571 86914542; fax: þ86 571 86914542. E-mail addresses: [email protected] (S. Wang), [email protected] (Q. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.12.024 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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ketones, acids, aldehydes, phenols, sugars, etc. Therefore, even the cracking of bio-oil over HZSM-5 could not achieve satisfactory results. Previous studies on the cracking of bio-oil model compounds over HZSM-5 showed that, ketones and acids could be converted into hydrocarbons, whereas largemolecular-weight phenolic oligomers and sugars resulted in the coke formation, which caused rapid catalyst deactivation [9e11]. Consequently, the removal of phenolic oligomers and sugars in bio-oil is of great significance. Molecular distillation is an effective separation technology, which can separate biooil into a distilled fraction with high reactivity and a residual fraction with low reactivity. The distilled fraction mainly enriches ketones and acids, while the phenolic oligomers and sugars are reserved in the residual fraction [12]. Therefore, the study on bio-oil catalytic cracking based on molecular distillation is more feasible and efficient. Due to the high oxygen content and unsaturation degree, the bio-oil components in the distilled fraction still have the propensity for coke formation. The concept of the effective hydrogen to carbon ratio ((H/C)eff) [13] was introduced to evaluate the cracking performance of different compounds. As shown in Eq. (1), H, O and C represent the mole percentages of hydrogen, oxygen and carbon in the corresponding compounds, respectively.
(H/C)eff ¼ (He2O)/C
(1)
It was reported by Mentzel and Holm that a higher (H/C)eff had a positive impact on the catalyst stability in the cracking of bio-oil model compound mixture [13]. In view of the difference between bio-oil ((H/C)eff < 1) and targeted aromatic hydrocarbons (1 < (H/C)eff < 2), it is necessary to deal with the feedstock to increase the integral (H/C)eff. The introduction of aliphatic alcohols as co-cracking reactants is an effective way to increase the overall (H/C)eff of reactants. In a study on the co-cracking of bio-oil and methanol [14], the initial conversion of bio-oil and the selectivity of aromatic hydrocarbons reached 90% and 40%, respectively. It was also reported by Mentzel and Holm that the addition of methanol into bio-oil model compounds could significantly prolong the lifetime of catalysts during cracking process [13]. In our previous study, the co-cracking of bio-oil model compounds (acids and ketones) with ethanol was investigated, and an oil phase selectivity of 31.5wt% was achieved. The oil products entirely consisted of hydrocarbons, 91.5% of which were aromatic hydrocarbons [15]. Subsequently, we carried out the co-cracking of bio-oil distilled fraction and ethanol in the ratio of 2:3. The oil phase selectivity achieved 25.9wt%, and a hydrocarbon content of 98.3% was obtained within 3 h [16]. However, the blend ratio of ethanol is still relatively high. Furthermore, phenolic oligomers remaining in the distilled fraction have low reactivity and easily lead to the coke formation and poor stability via condensation reactions. Consequently, the cracking process still requires further improvement. Catalytic hydrogenation is another conventional bio-oil upgrading technology. The hydrogen for this technology can be supplied by the steam reforming of partial bio-oil fraction using catalysts such as Ni/Al2O3, CoeFe and Ni/Ash [17e19]. Catalytic hydrogenation mainly including two types: (1) hydrodeoxygenation and (2) mild hydrogenation. For the
purpose of effective deoxygenation, the former removes oxygen in bio-oil to form aliphatic hydrocarbons and H2O at a high pressure (10e20 MPa) in general [20e22], which leads to high consumption of H2 and harsh requirement of equipment. The latter is utilized to saturate double bonds and aromatic rings at a relatively low pressure, in order to improve the biooil stability [23e28]. The typical catalysts for hydrogenation can be divided into several groups: noble metals [23,25e27], Mo-based sulfides [29], metal phosphides [30] and other metal catalysts [28], among which Pd showed its high reactivity in hydrogenation [23,26]. Considering the frequent catalyst deactivation in the cracking process, the mild hydrogenation can be applied prior to catalytic cracking. Vispute et al. carried out the catalytic hydrogenation of bio-oil aqueous phase at 10 MPa in the fixed-bed reactor, followed by the catalytic cracking of condensation products in another reactor, and achieved the aromatic hydrocarbons and olefins selectivity of 18.3% and 43.0%, respectively [31]. Different from the separated hydrogenation and cracking reactions, a bio-oil upgrading technology based on a two-stage continuous hydrogenation-cracking was proposed in this study. Namely, the gaseous products from hydrogenation reaction are directly fed into the cracking reactor. This technology exhibits better continuousness and feasibility, thus in favor of industrialization. In the first stage, 3%Pd/nano-SiO2 is adopted for the mild hydrogenation process to increase the integral (H/ C)eff of feedstock. In the second stage, HZSM-5 is used for the cracking process to convert the reactants rich in hydrogen into hydrocarbons. Meanwhile, the coke formation was successfully inhibited by combining the molecular distillation and co-cracking with ethanol [16]. The typical model compounds in distilled fraction, namely, hydroxypropanone, cyclopentanone, acetic acid, guaiacol, phenol and furfural, were mixed with ethanol as reactants.
Experimental section Nano-SiO2 and PdCl2 were purchased from Aladdin Industrial Corporation, and HZSM-5 was obtained from the Catalyst Plant of Nankai University. Similar with that preparation procedure for Pd based catalysts [32], Pd/nano-SiO2 used for hydrogenation was prepared by incipient wetness impregnation and the loading amount of Pd was 3wt%. A requisite amount of PdCl2 was dissolved in the deionized water in the presence of hydrochloric acid at ambient temperature to obtain the aqueous solution (PH ¼ 3), and then pretreated nano-SiO2 was added in the PdCl2 solution. After stabilization of 12 h, the mixture was dried in an oven at 110 C overnight. The final sample was calcined at 550 C for 6 h. As for the catalyst used for catalytic cracking process, the HZSM-5 (Si/ Ai ¼ 25) was calcined at 550 C for 6 h before the experiment. Pd/nano-SiO2 and HZSM-5 were both sieved to 40e60 mesh. The bio-oil model compounds consisted of hydroxypropanone, cyclopentanone, acetic acid, guaiacol, phenol and furfural. Hydroxypropanone, guaiacol and cyclopentanone were purchased from Alfa Aesar and Aladdin Industrial Corporation, respectively, while acetic acid, phenol and furfural were purchased from the Sinopharm Chemical Reagent Corporation. The ratio of model compounds was set
Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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according to the distribution of different chemical families in the distilled fraction of bio-oil [16]. Our previous cracking study indicated that as a method for (H/C)eff adjustment of feedstock, the addition of ethanol could remarkably facilitate the conversion of bio-oil [15,16,33,34]. Therefore, ethanol (Sinopharm Chemical Reagent Corporation) has been still introduced as a co-reactant in this study. The ratios of model compound mixture to ethanol were 1:1 both in the singlestage cracking and two-stage hydrogenation-cracking processes. The composition of feedstock was 20wt% hydroxypropanone, 5wt% cyclopentanone, 15wt% acetic acid, 5wt% guaiacol, 2.5wt% phenol, 2.5wt% furfural and 50wt% ethanol. The reactions were performed in a two-stage continuous fixed-bed system, as shown in Fig. 1. Two reactors were both stainless steel tubes with an inner diameter of 8 mm. The catalysts (3 g) were placed in the reactor and supported on quartz wool. Pd/nano-SiO2 was reduced at 350 C for 2 h at a H2 flow rate of 30 ml/min. The reactants were introduced by a high-performance liquid chromatography (HPLC) pump and then entered into the reactor after nebulization with H2. The reaction pressure (4 MPa) was maintained by H2 in a flow rate of 30 ml/min. The weight hourly space velocity (WHSV) of reactants was 1 h1. The outlet gas was cooled through a condenser and separated into non-condensable gases and liquid products. The reaction temperature of hydrogenation and cracking process was 300 C and 400 C, respectively. The gaseous products were quantified by online gas chromatograph (GC, Huaai GC 9560). The oven temperature was maintained at 50 C for 3 min and then increased to 120 C at a heating rate of 5 C/min, afterwards was held for 13 min. The oil phase was analyzed by a gas chromatograph-mass spectrometer (GCeMS; TraceDSQII) system to detect the structure of compounds, which were quantified by the area normalization method. A DB-WAX polar column was used for GCeMS. The oven temperature was maintained at 40 C for 1 min and then increased to 240 C at a heating rate of 8 C/min, subsequently was held for 10 min. To calculate the conversion of reactants, residual reactants in the liquid phases were quantified by gas
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chromatograph (Agilent 7890A) using external standard method. INNOWAX capillary column was used and the temperature programming was in accordance with GCeMS. The conversion of reactants (Xi) and the selectivity of products (Sj, Sk) are defined by Eq.(2)e(4). When calculating the selectivity of liquid products, the unconverted reactants were excluded. The symbol “m” in the equation represents the mass of corresponding substances.
Xi ¼ [(mi)in (mi)out]/(mi)in 100%
(2)
Sj(j ¼ oil phase, aqueous phase) ¼ mj/ [(mreactants)in (mreactants)out] 100%
(3)
Sk(k ¼ C1~4hydrocarbons, CO, CO2) ¼ mk/ [(mreactants)in (mreactants)out] 100%
(4)
The coke content of spent HZSM-5 was calculated by TGA/ SDTA851e thermogravimetric analyzer. At an air flow of 30 ml/min, the experiments were performed from ambient temperature to 900 C at a heating rate of 20 C/min. The specific surface area and pore volume of fresh and spent HZSM-5 were measured by N2 adsorption-desorption at 196 C using an automated surface area and pore size analyzer (Quantachrome; Autosorb-1).
Results and discussion Single-stage cracking Reactant conversion The reactant conversion in the single-stage cracking of model compound mixture and ethanol is presented in Fig. 2. HPO, CPO, HOAc, G-OH, PhOH, FAL and EtOH represent
Fig. 1 e Scheme of experiment system. Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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Fig. 2 e Reactant conversion of single-stage cracking (400 C/4 MPa; HPO-hydroxypropanone, CPOcyclopentanone, HOAc-acetic acid, G-OH-guaiacol, PhOHphenol, FAL-furfural, EtOH-ethanol).
hydroxypropanone, cyclopentanone, acetic acid, guaiacol, phenol, furfural and ethanol, respectively. At the reaction time of 5 h, hydroxypropanone, cyclopentanone, guaiacol, furfural and ethanol were almost completely converted, while the conversions of acetic acid and phenol were as low as 81.5% and 73.5%, respectively. In our previous study, acetic acid and phenol also showed relatively low conversions at 400 C over HZSM-5 compared with other reactants [35]. Gayubo et al. [10] found that acetic acid was firstly converted into acetone as an intermediate and subsequently transformed according to the mechanism of ketones conversion, leading to its lower conversion efficiency than ketones. The chemical structure of phenol is relatively stable; hence its conversion is relatively difficult. Guaiacol and furfural have low (H/C)eff, whereas the methoxyl side chain in the chemical structure of guaiacol is more active than benzene ring and phenolic hydroxyl in phenol [36,37], and the aldehyde group of furfural is also active. Hence, they both exhibited high conversions. It was noticeable that the conversion of acetic acid sharply decreased from 81.5% to 69.4% and then 47.3% at the reaction time of 6.5 h and 8 h, respectively. In addition, the conversion of hydroxypropanone, cyclopentanone and ethanol also decreased to some degree. This can be ascribed to the insufficient catalytic capacity for high reactant conversions caused by carbon deposition, in conjunction with the competition among the reactants, resulting in the difficult conversion of acetic acid.
Product selectivity Fig. 3 shows the product selectivity in the single-stage cracking of model compound mixture and ethanol. The collected liquid product consisted of two separable layers. The upper layer was a dark brown oil phase and the bottom layer was a light yellow aqueous phase. The selectivity of oil phase was obviously lower than that of aqueous phase, and
displayed a decline in the cracking process. At the reaction time of 5 h, it was 27.7wt%, lower than what were obtained in the cracking of two other model compound mixtures with ethanol over HZSM-5 (31.5wt% and 28.1wt%, respectively) [15,35]. This was reasonable since the (H/C)eff of reactants was reduced and the capacity of oil phase formation was weakened in the presence of phenols and low ethanol blend ratio. Furthermore, the oil phase selectivity was observed to decrease gradually along with reaction time, implying the inhibition of catalyst reactivity. The main gaseous product was C3H8 at 5 h. Subsequently, the selectivity of C2H4 prominently increased and dominated from 3.2wt% to 10wt% at 6.5 h and to 15.6wt% at 8 h, respectively. C2H4 was regarded as an important intermediate during cracking [38], since it was released from the deoxygenation and subsequently participated in the aromatization to produce liquid hydrocarbons that was rich in aromatic hydrocarbons. Therefore, a large amount of C2H4 indicated that the aromatization ability of HZSM-5 was lower and the transformation from intermediate olefins into liquid hydrocarbons was no longer favored, in good agreement with the low selectivity of oil phase. Moreover, the gaseous products also contained other gaseous hydrocarbons and COx. The reduction of COx selectivity suggested that the intensity of decarbonylation and decarboxylation was weakened.
Oil phase composition The oil phase from the single-stage cracking of model compound mixture and ethanol was identified by GC-MS. At the reaction time of 5 h, the relative contents of aromatic hydrocarbons and aliphatic hydrocarbons were 68.1% and 7.6%, respectively, and the content of C6~C9 monoaromatics was only 45.8%. It is generally believed that the oxygenated compounds first undergo deoxygenation to form light olefins, followed by the aromatization to produce aromatic hydrocarbons [9,10,38]. The deoxygenation and subsequent olefins aromatization can be catalyzed by the acid site on HZSM-5. Meanwhile, the shape selectivity due to the unique channel structure of HZSM-5 promotes the formation of monoaromatics and diaromatics [39]. However, because of the low (H/C)eff of bio-oil components, although ethanol was introduced as the co-cracking reactant, some oxygenated byproducts and even coke were generated in the single-stage cracking. The content of oxygenated compounds was up to 24.3%. Phenolic derivatives were the most abundant compounds, which were produced from the alkylation of phenol and guaiacol, implying the low deoxygenation efficiency of phenols in the single-stage cracking. Furthermore, a few ketones, esters and furans were observed in the oil phase. The main ketones were cyclopentenone derivatives, which were derived from dehydrogenation and alkylation of cyclopentanone [35], while a small amount of acetone was also produced from the incomplete deoxygenation of acetic acid [10]. The typical ester was ethyl acetate, which was produced from the esterification of acetic acid and ethanol. Besides, the alkylation of furfural contributed to the formation of furans such as 2,5-dimethyl-furan. Afterwards, the quality of oil phase dramatically declined, with a content of aromatic hydrocarbons as low as 31% at 8 h. In contrast, the contents of aliphatic hydrocarbons and especially oxygenated
Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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Fig. 3 e Product selectivity of single-stage cracking: (a) liquid product; (b) gaseous product (400 C/4 MPa).
compounds increased during the reaction. At 8 h, the content of oxygenated compounds exceeded 50%, suggesting the aromatization capacity of HZSM-5 catalyst was severely suppressed as well as the deoxygenation efficiency, which might be ascribed to the catalyst deactivation.
Two-stage hydrogenation-cracking On the condition that blend ratio of ethanol was 50% and phenols were introduced, low reactivity and stability were observed in the single-stage cracking process, mainly attributing to the poor conversion ability for low (H/C)eff reactants [13], in conjunction with strong propensity of phenols for coke formation [9], and its notable impact was also studied over HZSM-5 [40]. The (H/C)eff of bio-oil can be improved by mild hydrogenation to enhance the stability. Consequently, we carried out the study of a two-stage continuous hydrogenation-cracking to improve single-stage cracking.
low reactivity also had a stable conversion of more than 80% during the 8 h, demonstrating the notable improvement of cracking stability by mild hydrogenation pretreatment.
Product selectivity Fig. 5 shows the product selectivity from the two-stage hydrogenation-cracking of model compound mixture and ethanol. There were also two separable phases present in the liquid product, of which the upper was a light yellow oil phase and the bottom was a clear aqueous phase. It was observed that the selectivity of oil phase was much higher than that of aqueous phase and remained about 40wt%, which exceeded what was obtained over HZSM-5 in the presence of phenols [35]. These results indicated that hydrogenation-cracking favored the formation of oil phase and maintained the catalyst reactivity rather than single-stage cracking, reaching 27.7wt% at 5 h. In the hydrogenation-cracking process, the
Reactant conversion The reactant conversion in the hydrogenation-cracking of model compound mixture and ethanol is shown in Fig. 4. Compared with single-stage cracking, the conversion of reactants was largely improved. Especially for acetic acid, at 5 h, its conversion increased from 81.5% to 100%. In addition, an increase of phenol conversion was observed from 73.5% to 84.3%, while other reactants were nearly completely converted, indicating the significance of the hydrogenation stage. For the hydrogenation reaction over Pd/SiO2, hydrogen molecules were adsorbed and dissociated on the Pd active sites, and then the addition reactions of oxygenated functional groups occurred in the presence of activated hydrogen [41,42]. Ketones were converted into alcohols while furans were converted into alcohols [23] or gaseous products [43]; phenols were first transformed into ketones and then alcohols [44,45]; acids were converted into esters via esterification [26,27]. The oxygenated and unsaturated functional groups were converted into the ones with higher reactivity in the hydrogenation stage, which accounted for the higher integral conversion of reactants. Moreover, different from single-stage cracking, the reactant conversion remained stable and the phenol with
Fig. 4 e Reactant conversion of two-stage hydrogenationcracking (300 C/4 MPa-400 C/4 MPa; HPOhydroxypropanone, CPO-cyclopentanone, HOAc-acetic acid, G-OH-guaiacol, PhOH-phenol, FAL-furfural, EtOHethanol).
Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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Fig. 5 e Product selectivity of two-stage hydrogenation-cracking: (a) liquid product; (b) gaseous product (300 C/4 MPa-400 C/ 4 MPa).
gaseous product included COx, C1~C4 gaseous hydrocarbons and roughly remained stable. The dominant product was C3H8, which could be generated from the aromatization [15,33], in which light olefins were converted into aromatic hydrocarbons with a higher degree of unsaturation. When the aromatization was favored, a lot of surplus hydrogen was released and entered the saturated gaseous hydrocarbons, thus a high content of C3H8 was obtained. In our previous study, the better cracking performance was always achieved when C3H8 dominated among the gaseous products [15,16,33]. In addition, the absence of C2H4 in the vent gas also suggested the effective aromatization by the active catalyst.
Oil phase composition The oil phase obtained from the two-stage hydrogenationcracking of model compound mixture and ethanol was also analyzed by GC-MS. The relative content of aromatic hydrocarbons increased significantly from 68.1% to 90.7%, with a low content of oxygenated compounds in the form of phenolic derivatives such as methyl phenol, ethyl phenol and diethyl phenol. The oxygenated compounds were sharply decreased from 24.3% to 2.1%, indicating the high deoxygenation efficiency superior to single-stage cracking. It was noteworthy that the quality of oil phase during the reaction remained quite stable without visible weakening, among which the aromatic hydrocarbons constituted more than 90% within 8 h, and the C6~C9 monoaromatics had a high content of about 70%. Among the monoaromatics, benzene, toluene, xylenes are important chemicals in industry and can be added into commercial gasoline to improve its octane value. The distribution of products is shown in Fig. 6.
Coke analysis of catalysts The formation of coke accounts for the catalyst deactivation in the catalytic cracking process [10]. As the aforementioned results, the two reaction pathways showed great difference in the catalyst stability. Thermogravimetric analysis was performed for the spent catalysts after 8 h, as shown in Fig. 7. The
weight loss of the spent HZSM-5 from single-stage cracking was much more notable and a high coke/catalyst value of 0.33 g/g was acquired. The coke/catalyst value for hydrogenation-cracking had a remarkable decline to only 0.19 g/g. It could be concluded that the application of mild hydrogenation before bio-oil cracking could significantly suppress catalyst coke deposition and largely prolong the HZSM-5 lifetime. The specific surface area and pore volume of the fresh and spent HZSM-5 are listed in Table 1. The fresh HZSM-5 had the largest specific surface area and pore volume, which were 269.4 m2/g and 0.213 cm3/g, respectively. The specific surface area of the spent catalysts dropped dramatically due to pore blockage by coke deposition. Especially for single-stage cracking, its specific surface area was 16.2 m2/g, only 6.0% of the fresh HZSM-5. The pore volume showed a similar variation trend from 0.213 cm3/g to 0.031 cm3/g. In comparison, the decrease in surface area and pore volume was largely eased for the hydrogenationcracking process. The results above further corresponded to the prolongation of catalyst lifetime via mild hydrogenation pretreatment.
Conclusions In this study, the effective conversion from bio-oil model compound mixture and ethanol into aromatic hydrocarbons was realized via the two-stage continuous hydrogenationcracking process. The performance of single-stage cracking and hydrogenation-cracking was investigated and compared. It was found that during single-stage cracking, the insufficient reactant conversion showed a decreasing trend, so were the low selectivity and quality of oil phase. In comparison, a nearly complete and stable reactant conversion was obtained during hydrogenation-cracking. Meanwhile, the selectivity of oil phase was obviously enhanced and remained about 40wt%. In the obtained oil phase, the aromatic hydrocarbons constituted more than 90% within
Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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Fig. 6 e Oil phase composition of two-stage hydrogenation-cracking at 5 h (300 C/4 MPa-400 C/4 MPa).
8 h, mainly in the form of monoaromatics. It could also be confirmed that hydrogenation-cracking could maintain the high catalyst reactivity by the characterization of spent HZSM-5 catalysts. Consequently, the two-stage continuous hydrogenation-cracking technology had a significant advantage in comparison with the conventional single-stage cracking.
Acknowledgments
Fig. 7 e TG curves of spent HZSM-5 after 8 h.
The authors are grateful for the financial support from the National Science and Technology Supporting Plan Through Contract (2015BAD15B06), the National Natural Science Foundation of China (51276166) and the National Basic Research Program of China (2013CB228101).
references Table 1 e Physical properties and coke deposition of the fresh and spent HZSM-5. Catalyst Fresh HZSM5 Spent HZSM5a Spent HZSM5b a b
SBET (m2/ g)
Pore volume (cm3/g)
269.4
0.213
16.2
0.031
0.33
56.7
0.103
0.19
Single-stage cracking. Two-stage hydrogenation-cracking.
Coke/Catalyst (g/g)
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The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production Shurong Wang a,*, Junhao Chen a, Qinjie Cai b, Fan Zhang a, Yurong Wang a, Bin Ru a, Qi Wang c,** a
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China c College of Metrological Technology and Engineering, China Jiliang University, Hangzhou 310018, China b
article info
abstract
Article history:
In order to reduce the coke formation and improve catalyst activation in the catalytic
Received 30 October 2015
cracking of bio-oil, ethanol addition and mild hydrogenation pretreatment were introduced
Received in revised form
to improve the cracking stability. According to the analysis on the bio-oil molecular
27 November 2015
distillation, several typical compounds were selected as the representative of bio-oil
Accepted 12 December 2015
distillation. Based on the two-stage fixed-bed reactor, the effect of mild hydrogenation
Available online xxx
on the cracking behavior of bio-oil model compound mixture was studied in detail. In a single-stage cracking process, the selectivity of oil phase was only 28e23 wt%, with the
Keywords:
formation of abundant oxygenated byproducts. When mild hydrogenation was introduced
Bio-oil
before the cracking, the selectivity of oil phase increased up to 40wt% at 8 h, with an ar-
Catalytic cracking
omatic hydrocarbon content of over 90%.
Mild hydrogenation
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Aromatic hydrocarbon
Introduction The concern for the exhaustion of fossil fuel has made biomass energy draw extensive attentions in the world [1,2]. Bio-oil via biomass fast pyrolysis is supposed to be a substitute for conventional transport fuels. However, the inferior properties of crude bio-oil, including high oxygen and water contents, low heating value and strong corrosiveness [3,4], limit its direct utilization. Therefore, upgrading technology is essential for the bio-oil high-grade utilization.
Various upgrading techniques have been studied in recent decades, such as catalytic cracking, catalytic hydrogenation, esterification, steam reforming, emulsification, etc [5,6]. Catalytic cracking is an effective bio-oil upgrading technology, which can transform bio-oil into the liquid fuel abundant in aromatic hydrocarbons by removing oxygen in the form of CO, CO2 and H2O [7]. Various catalysts, such as HZSM-5, HeY, Hmordenite, silicalite and silica-alumina, were investigated in catalytic cracking process, in which HZSM-5 was favorable for aromatic hydrocarbon production [8]. However, the composition of crude bio-oil is quite complicated, which consists of
* Corresponding author. Tel.: þ86 571 87952801; fax: þ86 571 87951616. ** Corresponding author. Tel: þ86 571 86914542; fax: þ86 571 86914542. E-mail addresses: [email protected] (S. Wang), [email protected] (Q. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.12.024 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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ketones, acids, aldehydes, phenols, sugars, etc. Therefore, even the cracking of bio-oil over HZSM-5 could not achieve satisfactory results. Previous studies on the cracking of bio-oil model compounds over HZSM-5 showed that, ketones and acids could be converted into hydrocarbons, whereas largemolecular-weight phenolic oligomers and sugars resulted in the coke formation, which caused rapid catalyst deactivation [9e11]. Consequently, the removal of phenolic oligomers and sugars in bio-oil is of great significance. Molecular distillation is an effective separation technology, which can separate biooil into a distilled fraction with high reactivity and a residual fraction with low reactivity. The distilled fraction mainly enriches ketones and acids, while the phenolic oligomers and sugars are reserved in the residual fraction [12]. Therefore, the study on bio-oil catalytic cracking based on molecular distillation is more feasible and efficient. Due to the high oxygen content and unsaturation degree, the bio-oil components in the distilled fraction still have the propensity for coke formation. The concept of the effective hydrogen to carbon ratio ((H/C)eff) [13] was introduced to evaluate the cracking performance of different compounds. As shown in Eq. (1), H, O and C represent the mole percentages of hydrogen, oxygen and carbon in the corresponding compounds, respectively.
(H/C)eff ¼ (He2O)/C
(1)
It was reported by Mentzel and Holm that a higher (H/C)eff had a positive impact on the catalyst stability in the cracking of bio-oil model compound mixture [13]. In view of the difference between bio-oil ((H/C)eff < 1) and targeted aromatic hydrocarbons (1 < (H/C)eff < 2), it is necessary to deal with the feedstock to increase the integral (H/C)eff. The introduction of aliphatic alcohols as co-cracking reactants is an effective way to increase the overall (H/C)eff of reactants. In a study on the co-cracking of bio-oil and methanol [14], the initial conversion of bio-oil and the selectivity of aromatic hydrocarbons reached 90% and 40%, respectively. It was also reported by Mentzel and Holm that the addition of methanol into bio-oil model compounds could significantly prolong the lifetime of catalysts during cracking process [13]. In our previous study, the co-cracking of bio-oil model compounds (acids and ketones) with ethanol was investigated, and an oil phase selectivity of 31.5wt% was achieved. The oil products entirely consisted of hydrocarbons, 91.5% of which were aromatic hydrocarbons [15]. Subsequently, we carried out the co-cracking of bio-oil distilled fraction and ethanol in the ratio of 2:3. The oil phase selectivity achieved 25.9wt%, and a hydrocarbon content of 98.3% was obtained within 3 h [16]. However, the blend ratio of ethanol is still relatively high. Furthermore, phenolic oligomers remaining in the distilled fraction have low reactivity and easily lead to the coke formation and poor stability via condensation reactions. Consequently, the cracking process still requires further improvement. Catalytic hydrogenation is another conventional bio-oil upgrading technology. The hydrogen for this technology can be supplied by the steam reforming of partial bio-oil fraction using catalysts such as Ni/Al2O3, CoeFe and Ni/Ash [17e19]. Catalytic hydrogenation mainly including two types: (1) hydrodeoxygenation and (2) mild hydrogenation. For the
purpose of effective deoxygenation, the former removes oxygen in bio-oil to form aliphatic hydrocarbons and H2O at a high pressure (10e20 MPa) in general [20e22], which leads to high consumption of H2 and harsh requirement of equipment. The latter is utilized to saturate double bonds and aromatic rings at a relatively low pressure, in order to improve the biooil stability [23e28]. The typical catalysts for hydrogenation can be divided into several groups: noble metals [23,25e27], Mo-based sulfides [29], metal phosphides [30] and other metal catalysts [28], among which Pd showed its high reactivity in hydrogenation [23,26]. Considering the frequent catalyst deactivation in the cracking process, the mild hydrogenation can be applied prior to catalytic cracking. Vispute et al. carried out the catalytic hydrogenation of bio-oil aqueous phase at 10 MPa in the fixed-bed reactor, followed by the catalytic cracking of condensation products in another reactor, and achieved the aromatic hydrocarbons and olefins selectivity of 18.3% and 43.0%, respectively [31]. Different from the separated hydrogenation and cracking reactions, a bio-oil upgrading technology based on a two-stage continuous hydrogenation-cracking was proposed in this study. Namely, the gaseous products from hydrogenation reaction are directly fed into the cracking reactor. This technology exhibits better continuousness and feasibility, thus in favor of industrialization. In the first stage, 3%Pd/nano-SiO2 is adopted for the mild hydrogenation process to increase the integral (H/ C)eff of feedstock. In the second stage, HZSM-5 is used for the cracking process to convert the reactants rich in hydrogen into hydrocarbons. Meanwhile, the coke formation was successfully inhibited by combining the molecular distillation and co-cracking with ethanol [16]. The typical model compounds in distilled fraction, namely, hydroxypropanone, cyclopentanone, acetic acid, guaiacol, phenol and furfural, were mixed with ethanol as reactants.
Experimental section Nano-SiO2 and PdCl2 were purchased from Aladdin Industrial Corporation, and HZSM-5 was obtained from the Catalyst Plant of Nankai University. Similar with that preparation procedure for Pd based catalysts [32], Pd/nano-SiO2 used for hydrogenation was prepared by incipient wetness impregnation and the loading amount of Pd was 3wt%. A requisite amount of PdCl2 was dissolved in the deionized water in the presence of hydrochloric acid at ambient temperature to obtain the aqueous solution (PH ¼ 3), and then pretreated nano-SiO2 was added in the PdCl2 solution. After stabilization of 12 h, the mixture was dried in an oven at 110 C overnight. The final sample was calcined at 550 C for 6 h. As for the catalyst used for catalytic cracking process, the HZSM-5 (Si/ Ai ¼ 25) was calcined at 550 C for 6 h before the experiment. Pd/nano-SiO2 and HZSM-5 were both sieved to 40e60 mesh. The bio-oil model compounds consisted of hydroxypropanone, cyclopentanone, acetic acid, guaiacol, phenol and furfural. Hydroxypropanone, guaiacol and cyclopentanone were purchased from Alfa Aesar and Aladdin Industrial Corporation, respectively, while acetic acid, phenol and furfural were purchased from the Sinopharm Chemical Reagent Corporation. The ratio of model compounds was set
Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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according to the distribution of different chemical families in the distilled fraction of bio-oil [16]. Our previous cracking study indicated that as a method for (H/C)eff adjustment of feedstock, the addition of ethanol could remarkably facilitate the conversion of bio-oil [15,16,33,34]. Therefore, ethanol (Sinopharm Chemical Reagent Corporation) has been still introduced as a co-reactant in this study. The ratios of model compound mixture to ethanol were 1:1 both in the singlestage cracking and two-stage hydrogenation-cracking processes. The composition of feedstock was 20wt% hydroxypropanone, 5wt% cyclopentanone, 15wt% acetic acid, 5wt% guaiacol, 2.5wt% phenol, 2.5wt% furfural and 50wt% ethanol. The reactions were performed in a two-stage continuous fixed-bed system, as shown in Fig. 1. Two reactors were both stainless steel tubes with an inner diameter of 8 mm. The catalysts (3 g) were placed in the reactor and supported on quartz wool. Pd/nano-SiO2 was reduced at 350 C for 2 h at a H2 flow rate of 30 ml/min. The reactants were introduced by a high-performance liquid chromatography (HPLC) pump and then entered into the reactor after nebulization with H2. The reaction pressure (4 MPa) was maintained by H2 in a flow rate of 30 ml/min. The weight hourly space velocity (WHSV) of reactants was 1 h1. The outlet gas was cooled through a condenser and separated into non-condensable gases and liquid products. The reaction temperature of hydrogenation and cracking process was 300 C and 400 C, respectively. The gaseous products were quantified by online gas chromatograph (GC, Huaai GC 9560). The oven temperature was maintained at 50 C for 3 min and then increased to 120 C at a heating rate of 5 C/min, afterwards was held for 13 min. The oil phase was analyzed by a gas chromatograph-mass spectrometer (GCeMS; TraceDSQII) system to detect the structure of compounds, which were quantified by the area normalization method. A DB-WAX polar column was used for GCeMS. The oven temperature was maintained at 40 C for 1 min and then increased to 240 C at a heating rate of 8 C/min, subsequently was held for 10 min. To calculate the conversion of reactants, residual reactants in the liquid phases were quantified by gas
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chromatograph (Agilent 7890A) using external standard method. INNOWAX capillary column was used and the temperature programming was in accordance with GCeMS. The conversion of reactants (Xi) and the selectivity of products (Sj, Sk) are defined by Eq.(2)e(4). When calculating the selectivity of liquid products, the unconverted reactants were excluded. The symbol “m” in the equation represents the mass of corresponding substances.
Xi ¼ [(mi)in (mi)out]/(mi)in 100%
(2)
Sj(j ¼ oil phase, aqueous phase) ¼ mj/ [(mreactants)in (mreactants)out] 100%
(3)
Sk(k ¼ C1~4hydrocarbons, CO, CO2) ¼ mk/ [(mreactants)in (mreactants)out] 100%
(4)
The coke content of spent HZSM-5 was calculated by TGA/ SDTA851e thermogravimetric analyzer. At an air flow of 30 ml/min, the experiments were performed from ambient temperature to 900 C at a heating rate of 20 C/min. The specific surface area and pore volume of fresh and spent HZSM-5 were measured by N2 adsorption-desorption at 196 C using an automated surface area and pore size analyzer (Quantachrome; Autosorb-1).
Results and discussion Single-stage cracking Reactant conversion The reactant conversion in the single-stage cracking of model compound mixture and ethanol is presented in Fig. 2. HPO, CPO, HOAc, G-OH, PhOH, FAL and EtOH represent
Fig. 1 e Scheme of experiment system. Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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Fig. 2 e Reactant conversion of single-stage cracking (400 C/4 MPa; HPO-hydroxypropanone, CPOcyclopentanone, HOAc-acetic acid, G-OH-guaiacol, PhOHphenol, FAL-furfural, EtOH-ethanol).
hydroxypropanone, cyclopentanone, acetic acid, guaiacol, phenol, furfural and ethanol, respectively. At the reaction time of 5 h, hydroxypropanone, cyclopentanone, guaiacol, furfural and ethanol were almost completely converted, while the conversions of acetic acid and phenol were as low as 81.5% and 73.5%, respectively. In our previous study, acetic acid and phenol also showed relatively low conversions at 400 C over HZSM-5 compared with other reactants [35]. Gayubo et al. [10] found that acetic acid was firstly converted into acetone as an intermediate and subsequently transformed according to the mechanism of ketones conversion, leading to its lower conversion efficiency than ketones. The chemical structure of phenol is relatively stable; hence its conversion is relatively difficult. Guaiacol and furfural have low (H/C)eff, whereas the methoxyl side chain in the chemical structure of guaiacol is more active than benzene ring and phenolic hydroxyl in phenol [36,37], and the aldehyde group of furfural is also active. Hence, they both exhibited high conversions. It was noticeable that the conversion of acetic acid sharply decreased from 81.5% to 69.4% and then 47.3% at the reaction time of 6.5 h and 8 h, respectively. In addition, the conversion of hydroxypropanone, cyclopentanone and ethanol also decreased to some degree. This can be ascribed to the insufficient catalytic capacity for high reactant conversions caused by carbon deposition, in conjunction with the competition among the reactants, resulting in the difficult conversion of acetic acid.
Product selectivity Fig. 3 shows the product selectivity in the single-stage cracking of model compound mixture and ethanol. The collected liquid product consisted of two separable layers. The upper layer was a dark brown oil phase and the bottom layer was a light yellow aqueous phase. The selectivity of oil phase was obviously lower than that of aqueous phase, and
displayed a decline in the cracking process. At the reaction time of 5 h, it was 27.7wt%, lower than what were obtained in the cracking of two other model compound mixtures with ethanol over HZSM-5 (31.5wt% and 28.1wt%, respectively) [15,35]. This was reasonable since the (H/C)eff of reactants was reduced and the capacity of oil phase formation was weakened in the presence of phenols and low ethanol blend ratio. Furthermore, the oil phase selectivity was observed to decrease gradually along with reaction time, implying the inhibition of catalyst reactivity. The main gaseous product was C3H8 at 5 h. Subsequently, the selectivity of C2H4 prominently increased and dominated from 3.2wt% to 10wt% at 6.5 h and to 15.6wt% at 8 h, respectively. C2H4 was regarded as an important intermediate during cracking [38], since it was released from the deoxygenation and subsequently participated in the aromatization to produce liquid hydrocarbons that was rich in aromatic hydrocarbons. Therefore, a large amount of C2H4 indicated that the aromatization ability of HZSM-5 was lower and the transformation from intermediate olefins into liquid hydrocarbons was no longer favored, in good agreement with the low selectivity of oil phase. Moreover, the gaseous products also contained other gaseous hydrocarbons and COx. The reduction of COx selectivity suggested that the intensity of decarbonylation and decarboxylation was weakened.
Oil phase composition The oil phase from the single-stage cracking of model compound mixture and ethanol was identified by GC-MS. At the reaction time of 5 h, the relative contents of aromatic hydrocarbons and aliphatic hydrocarbons were 68.1% and 7.6%, respectively, and the content of C6~C9 monoaromatics was only 45.8%. It is generally believed that the oxygenated compounds first undergo deoxygenation to form light olefins, followed by the aromatization to produce aromatic hydrocarbons [9,10,38]. The deoxygenation and subsequent olefins aromatization can be catalyzed by the acid site on HZSM-5. Meanwhile, the shape selectivity due to the unique channel structure of HZSM-5 promotes the formation of monoaromatics and diaromatics [39]. However, because of the low (H/C)eff of bio-oil components, although ethanol was introduced as the co-cracking reactant, some oxygenated byproducts and even coke were generated in the single-stage cracking. The content of oxygenated compounds was up to 24.3%. Phenolic derivatives were the most abundant compounds, which were produced from the alkylation of phenol and guaiacol, implying the low deoxygenation efficiency of phenols in the single-stage cracking. Furthermore, a few ketones, esters and furans were observed in the oil phase. The main ketones were cyclopentenone derivatives, which were derived from dehydrogenation and alkylation of cyclopentanone [35], while a small amount of acetone was also produced from the incomplete deoxygenation of acetic acid [10]. The typical ester was ethyl acetate, which was produced from the esterification of acetic acid and ethanol. Besides, the alkylation of furfural contributed to the formation of furans such as 2,5-dimethyl-furan. Afterwards, the quality of oil phase dramatically declined, with a content of aromatic hydrocarbons as low as 31% at 8 h. In contrast, the contents of aliphatic hydrocarbons and especially oxygenated
Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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Fig. 3 e Product selectivity of single-stage cracking: (a) liquid product; (b) gaseous product (400 C/4 MPa).
compounds increased during the reaction. At 8 h, the content of oxygenated compounds exceeded 50%, suggesting the aromatization capacity of HZSM-5 catalyst was severely suppressed as well as the deoxygenation efficiency, which might be ascribed to the catalyst deactivation.
Two-stage hydrogenation-cracking On the condition that blend ratio of ethanol was 50% and phenols were introduced, low reactivity and stability were observed in the single-stage cracking process, mainly attributing to the poor conversion ability for low (H/C)eff reactants [13], in conjunction with strong propensity of phenols for coke formation [9], and its notable impact was also studied over HZSM-5 [40]. The (H/C)eff of bio-oil can be improved by mild hydrogenation to enhance the stability. Consequently, we carried out the study of a two-stage continuous hydrogenation-cracking to improve single-stage cracking.
low reactivity also had a stable conversion of more than 80% during the 8 h, demonstrating the notable improvement of cracking stability by mild hydrogenation pretreatment.
Product selectivity Fig. 5 shows the product selectivity from the two-stage hydrogenation-cracking of model compound mixture and ethanol. There were also two separable phases present in the liquid product, of which the upper was a light yellow oil phase and the bottom was a clear aqueous phase. It was observed that the selectivity of oil phase was much higher than that of aqueous phase and remained about 40wt%, which exceeded what was obtained over HZSM-5 in the presence of phenols [35]. These results indicated that hydrogenation-cracking favored the formation of oil phase and maintained the catalyst reactivity rather than single-stage cracking, reaching 27.7wt% at 5 h. In the hydrogenation-cracking process, the
Reactant conversion The reactant conversion in the hydrogenation-cracking of model compound mixture and ethanol is shown in Fig. 4. Compared with single-stage cracking, the conversion of reactants was largely improved. Especially for acetic acid, at 5 h, its conversion increased from 81.5% to 100%. In addition, an increase of phenol conversion was observed from 73.5% to 84.3%, while other reactants were nearly completely converted, indicating the significance of the hydrogenation stage. For the hydrogenation reaction over Pd/SiO2, hydrogen molecules were adsorbed and dissociated on the Pd active sites, and then the addition reactions of oxygenated functional groups occurred in the presence of activated hydrogen [41,42]. Ketones were converted into alcohols while furans were converted into alcohols [23] or gaseous products [43]; phenols were first transformed into ketones and then alcohols [44,45]; acids were converted into esters via esterification [26,27]. The oxygenated and unsaturated functional groups were converted into the ones with higher reactivity in the hydrogenation stage, which accounted for the higher integral conversion of reactants. Moreover, different from single-stage cracking, the reactant conversion remained stable and the phenol with
Fig. 4 e Reactant conversion of two-stage hydrogenationcracking (300 C/4 MPa-400 C/4 MPa; HPOhydroxypropanone, CPO-cyclopentanone, HOAc-acetic acid, G-OH-guaiacol, PhOH-phenol, FAL-furfural, EtOHethanol).
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Fig. 5 e Product selectivity of two-stage hydrogenation-cracking: (a) liquid product; (b) gaseous product (300 C/4 MPa-400 C/ 4 MPa).
gaseous product included COx, C1~C4 gaseous hydrocarbons and roughly remained stable. The dominant product was C3H8, which could be generated from the aromatization [15,33], in which light olefins were converted into aromatic hydrocarbons with a higher degree of unsaturation. When the aromatization was favored, a lot of surplus hydrogen was released and entered the saturated gaseous hydrocarbons, thus a high content of C3H8 was obtained. In our previous study, the better cracking performance was always achieved when C3H8 dominated among the gaseous products [15,16,33]. In addition, the absence of C2H4 in the vent gas also suggested the effective aromatization by the active catalyst.
Oil phase composition The oil phase obtained from the two-stage hydrogenationcracking of model compound mixture and ethanol was also analyzed by GC-MS. The relative content of aromatic hydrocarbons increased significantly from 68.1% to 90.7%, with a low content of oxygenated compounds in the form of phenolic derivatives such as methyl phenol, ethyl phenol and diethyl phenol. The oxygenated compounds were sharply decreased from 24.3% to 2.1%, indicating the high deoxygenation efficiency superior to single-stage cracking. It was noteworthy that the quality of oil phase during the reaction remained quite stable without visible weakening, among which the aromatic hydrocarbons constituted more than 90% within 8 h, and the C6~C9 monoaromatics had a high content of about 70%. Among the monoaromatics, benzene, toluene, xylenes are important chemicals in industry and can be added into commercial gasoline to improve its octane value. The distribution of products is shown in Fig. 6.
Coke analysis of catalysts The formation of coke accounts for the catalyst deactivation in the catalytic cracking process [10]. As the aforementioned results, the two reaction pathways showed great difference in the catalyst stability. Thermogravimetric analysis was performed for the spent catalysts after 8 h, as shown in Fig. 7. The
weight loss of the spent HZSM-5 from single-stage cracking was much more notable and a high coke/catalyst value of 0.33 g/g was acquired. The coke/catalyst value for hydrogenation-cracking had a remarkable decline to only 0.19 g/g. It could be concluded that the application of mild hydrogenation before bio-oil cracking could significantly suppress catalyst coke deposition and largely prolong the HZSM-5 lifetime. The specific surface area and pore volume of the fresh and spent HZSM-5 are listed in Table 1. The fresh HZSM-5 had the largest specific surface area and pore volume, which were 269.4 m2/g and 0.213 cm3/g, respectively. The specific surface area of the spent catalysts dropped dramatically due to pore blockage by coke deposition. Especially for single-stage cracking, its specific surface area was 16.2 m2/g, only 6.0% of the fresh HZSM-5. The pore volume showed a similar variation trend from 0.213 cm3/g to 0.031 cm3/g. In comparison, the decrease in surface area and pore volume was largely eased for the hydrogenationcracking process. The results above further corresponded to the prolongation of catalyst lifetime via mild hydrogenation pretreatment.
Conclusions In this study, the effective conversion from bio-oil model compound mixture and ethanol into aromatic hydrocarbons was realized via the two-stage continuous hydrogenationcracking process. The performance of single-stage cracking and hydrogenation-cracking was investigated and compared. It was found that during single-stage cracking, the insufficient reactant conversion showed a decreasing trend, so were the low selectivity and quality of oil phase. In comparison, a nearly complete and stable reactant conversion was obtained during hydrogenation-cracking. Meanwhile, the selectivity of oil phase was obviously enhanced and remained about 40wt%. In the obtained oil phase, the aromatic hydrocarbons constituted more than 90% within
Please cite this article in press as: Wang S, et al., The effect of mild hydrogenation on the catalytic cracking of bio-oil for aromatic hydrocarbon production, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.024
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Fig. 6 e Oil phase composition of two-stage hydrogenation-cracking at 5 h (300 C/4 MPa-400 C/4 MPa).
8 h, mainly in the form of monoaromatics. It could also be confirmed that hydrogenation-cracking could maintain the high catalyst reactivity by the characterization of spent HZSM-5 catalysts. Consequently, the two-stage continuous hydrogenation-cracking technology had a significant advantage in comparison with the conventional single-stage cracking.
Acknowledgments
Fig. 7 e TG curves of spent HZSM-5 after 8 h.
The authors are grateful for the financial support from the National Science and Technology Supporting Plan Through Contract (2015BAD15B06), the National Natural Science Foundation of China (51276166) and the National Basic Research Program of China (2013CB228101).
references Table 1 e Physical properties and coke deposition of the fresh and spent HZSM-5. Catalyst Fresh HZSM5 Spent HZSM5a Spent HZSM5b a b
SBET (m2/ g)
Pore volume (cm3/g)
269.4
0.213
16.2
0.031
0.33
56.7
0.103
0.19
Single-stage cracking. Two-stage hydrogenation-cracking.
Coke/Catalyst (g/g)
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