Research Article Cite This: ACS Catal. 2019, 9, 2626−2632
pubs.acs.org/acscatalysis
Selective Production of Aromatics by Catalytic Fast Pyrolysis of Furan with In Situ Dehydrogenation of Propane Xiaoduo Qi and Wei Fan* Department of Chemical Engineering, University of Massachusetts Amherst, 686 North Pleasant Street, Amherst, Massachusetts 01003, United States
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S Supporting Information *
ABSTRACT: Catalytic fast pyrolysis (CFP) of furan was studied with the co-feeding of propane over ZSM-5, CrOx/ Al2O3, and mixed catalysts with both ZSM-5 and CrOx/Al2O3 to combine the dehydrogenation of propane and CFP of furan in the presence of in situ generated propylene. With 15 vol % propane co-feeding, a slight increase in aromatic yield was observed over ZSM-5 catalyst. These enhancement was further improved by using mixed catalysts consisting of CrOx/Al2O3 and ZSM-5 with different weight ratios. Over the best mixed catalyst with CrOx/Al2O3 to ZSM-5 weight ratio of 2, a nearly 2-fold increase in the aromatic yield was obtained. The toluene selectivity was also increased from 37% to 50%. The increase in aromatic yield is directly dependent on the propane to total flow volume ratio. Moreover, the coke formation within ZSM-5 was also reduced, leading to a slower catalyst deactivation. From mechanistic studies, a two-step cascade reaction pathway was observed over the bifunctional mixed catalysts. It was proposed that propane dehydrogenates to propylene over CrOx/Al2O3. The propylene then reacts with furan within ZSM-5 catalyst to selectively form aromatics. This work demonstrates a simple and economical strategy to further improve the CFP technique. KEYWORDS: biomass, catalytic fast pyrolysis, aromatics, co-feeding of propane, in situ dehydrogenation, CrOx/Al2O3
1. INTRODUCTION Because of diminishing fossil fuel reserves and increasing environmental concerns, lignocellulosic biomass has attracted significant attention as an alternative carbon source to produce fuels and chemicals.1−5 Catalytic fast pyrolysis (CFP), which directly converts biomass over solid catalysts in one integrated reactor, is a promising technique for the production of renewable aromatics such as benzene, toluene, and xylenes from lignocellulosic biomass.6−8 In this process, lignocellulosic biomass is converted into pyrolysis gas at an intermediate temperature. The produced pyrolysis gas then diffuses within the micropores of zeolite catalysts where it undergoes various reactions to form final products. While numerous studies have been carried out to improve the efficiency of CFP technique, the fast deactivation of zeolite catalysts and the low yields to aromatics and olefins still remain as challenges of the CFP technique.7,9−11 The fast catalyst deactivation is attributed to the accumulation of coke formed inside the micropores of the zeolite catalysts, which covers the active sites on the micropore surface. To overcome the issue, hierarchical zeolites with short diffusion lengths have been developed to improve the mass transport within zeolite catalysts so that the formation of coke within the micropores of zeolite catalysts can be reduced.12−14 Other challenges of CFP are the low yields to desired products (e.g., aromatics and olefins) and the lack of flexibility to tune the product selectivity.15 Ga/ZSM-5 catalysts have been used © XXXX American Chemical Society
to increase the aromatic yield because of their bifunctional nature.16 In addition to developing new zeolite catalysts, higher aromatic yield can also be achieved by altering the chemistry involved in CFP. The Diels−Alder reaction, which involves the addition of alkenes to conjugated dienes to form cyclohexene derivatives, has been proposed as an effective reaction pathway to produce aromatics.17,18 On the basis of this hypothesis, Cheng and Huber studied the effect of propylene co-feeding on CFP of furan over ZSM-5 catalyst and reported an increase in the aromatic yields and higher selectivity to toluene.19 The enhancement with the co-feeding of propylene was attributed to the higher H/C ratio in the feed and the Diels−Alder cycloaddition between furan and propylene. While the cofeeding of propylene showed promising results, an inevitable drawback of this method is the use of light alkene, which is also a high-demand commodity chemical for producing polymers. Hence, it is desired to use alkane as the co-feeding source and integrate the dehydrogenation of alkane with CFP process. Recently, shale gas has become an increasingly important source of natural gas. Great opportunities have appeared for using shale gas as the co-feeding chemical to produce renewable chemicals from biomass.20,21 While shale gas is Received: December 4, 2018 Revised: February 1, 2019
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DOI: 10.1021/acscatal.8b04859 ACS Catal. 2019, 9, 2626−2632
Research Article
ACS Catalysis
coupled with FID reactor. A plot-Q GC column was used to separate light olefins from alkanes. The standard CFP reaction conditions are as follows: temperature, 550 °C; total flow, 330 sccm; catalyst load, 80 mg for ZSM-5; 100 mg for CrOx/ Al2O3; 120, 150, and 210 mg for mixed catalysts with CrOx/ Al2O3 to ZSM-5 ratio of 0.5, 1, and 2, respectively. The conversion, yield, and selectivity were calculated as described in the Supporting Information.
mostly made up of methane (∼90%) and ethane (∼10%), the composition of propane can be as high as 5%,22 which guarantees propane availability considering the total reserve of shale gas. In this work, we studied CFP of furan with co-feeding of propane. The idea is to combine dehydrogenation of propane and CFP of furan in the presence of both dehydrogenation catalysts and zeolite catalysts. Improved aromatic yield, higher selectivity to toluene, less coke formation, and slower catalyst deactivation were achieved by using mixed catalysts consisting of CrOx/Al2O3 and ZSM-5. The effects of reaction parameters including catalyst composition and the propane to total flow volume ratio on the aromatic yield were studied. From mechanistic studies, a typical two-step cascade reaction pathway was observed over the bifunctional mixed catalysts. Propane first dehydrogenates to propylene over CrOx/Al2O3, which further reacts with furan within the micropores of ZSM5 to form aromatics. The results observed in this work support a simple and economical method to improve the current CFP technique toward longer catalyst lifetime and higher aromatic production.
3. RESULTS AND DISCUSSION Synthesized CrOx/Al2O3 sample was characterized using X-ray diffraction (XRD) technique and compared to γ-Al2O3. The XRD patterns are shown in Figure 1. γ-Al2O3 shows typical
2. EXPERIMENTAL METHODS 2.1. Catalyst Preparation. Five different catalysts were prepared and studied in this work: CrOx/Al2O3 (10 wt % Cr) as the dehydrogenation catalyst, commercial ZSM-5 (Zeolyst CBV 3024E) as the CFP catalyst, and mixtures of these two phases as co-feeding catalysts. Three co-feeding catalysts with different CrOx/Al2O3 to ZSM-5 weight ratios (0.5, 1, and 2) were prepared, denoted as CrZSM_0.5, CrZSM_1, and CrZSM_2, respectively. CrOx/Al2O3 (10 wt % Cr) was synthesized by incipient wetness impregnation of Cr precursor on γ-Al2O3. Cr(NO3)3· 9H2O (0.4 g) was dissolved into 0.5 mL of deionized water. The solution was then added into 0.5 g of γ-Al2O3 dropwise with vigorous stirring. The product was dried overnight at 80 °C and calcined at 500 °C for 30 h. ZSM-5 zeolite was purchased from Zeolyst (CBV3024E). Three catalysts with different CrOx/Al2O3 to ZSM-5 weight ratios (0.5, 1, and 2) were prepared by mixing CrOx/Al2O3 with ZSM-5. Specifically, 100 mg of ZSM-5 was always used and CrOx/Al2O3 was weighed accordingly (50, 100, and 200 mg for weight ratio of 0.5, 1, and 2, respectively). Thereafter, two phases were placed into a mortar and ground for 20 min for a uniform mixture. 2.2. Catalyst Characterization. Powder X-ray diffraction (XRD) patterns were recorded on X’pert Powder (PANalytical) diffractometer system equipped with a Cu Kα radiation (operation at 40 kV, 30 mA, λ = 0.15418 nm) and a Ni filter. The measurement was carried out from a 2θ range of 10° to 70° with a scanning step size of 0.02°. 2.3. CFP Reaction. CFP reactions were carried out in a continuous flow quartz fixed-bed reactor placed into a proportional integral derivative (PID) controlled furnace. Prior to reaction, the catalyst bed was heated to the reaction temperature and equilibrated for 30 min under He flow (100 sccm). After calcination, the He stream was switched to bypass the reactor. Furan was then fed into He carrier flow using a syringe pump (Fisher KDS100) and allowed to equilibrate for 30 min. For co-feeding reactions, the furan/He flow was mixed with co-feeding stream using a three-way tee. After 30 min bypass, the feed stream was switched into the reactor, and after 1 min, an online gas chromatograph (GC) was started. The products were characterized using an online Agilent 7890A GC
Figure 1. XRD patterns of γ-Al2O3 and CrOx/Al2O3 (10 wt % Cr).
XRD peaks with no indication of impurities. No Cr2O3 peak was observed for the CrOx/Al2O3 sample, suggesting that CrOx is well dispersed on the γ-Al2O3 support. To find optimum reaction conditions, propane dehydrogenation reactions over CrOx/Al2O3 were first carried out at different temperatures ranging from 450 to 600 °C. As shown in Table S1, CrOx/Al2O3 catalyst shows low activity (90% propylene selectivity. This result is expected as CrOx/Al2O3 has been well-known for its superior performance as an industry dehydrogenation catalyst.23 Dehydrogenation of propane over ZSM-5 produced a significant amount of cracking products including 24% selectivity to methane and 45% selectivity to ethylene, suggesting that the ZSM-5 catalyst is not ideal for propane dehydrogenation, possibly because of the strong acidity of ZSM-5 catalyst.24 In the case of mixed catalysts, the samples with higher CrOx/Al2O3 to ZSM-5 weight ratios promote the dehydrogenation reaction, resulting in an increase in propylene selectivity from 73% to 94% (Table S4). In all cases, the aromatic selectivity is negligible (
