Optimization of the synthesis and performance of bifunctional Co-HZSM-5 catalyst for fischer tropsch synthesis

Abstract

To date, the production of liquid fuels is mainly from crude oil via conventional refining processes. The depleting and increasing demand in crude oil motivated the world in discovering an alternative technology for energy production, Fischer-Tropsch synthesis (FTS). FTS process involves the use of synthetic gas (gasified from coal or carbonaceous materials) to produce liquid fuel (hydrocarbons-HCs). Amongst other benefits, FTS can be operated at low temperature Fischer-Tropsch (LTFT) process where the most preferred FT catalyst to be applied is cobalt-based catalyst. The other advantages associated with the use of Co-based catalyst are its activity, stability, lifespan and hindrance of the formation of water and carbon dioxide during FT process. The main challenge associated with Co-based catalyst on FT synthesis has been the product selectivity control. Usually, the FT catalysts obey the Anderson-Schulz-Flory (ASF) product distribution model, which limits the selectivity of the most desired products, known as gasoline-range hydrocarbons (GRHs) (C5 - C11). Researchers have discovered that combining FT cobalt catalyst with zeolite into bi-functional catalyst improve the selectivity to GRHs. Zeolite assists in hydrocracking of heavier hydrocarbons due to its microporous nature. However, mesoporous zeolite improves the catalytic performance in FTS. Promotion of the FT cobalt catalyst is also believed to bring some changes in catalytic performance. But there is no report on the promotion of Co-based catalyst on mesoporous zeolite (H-ZSM-5) support during Fischer-Tropsch synthesis (FTS). The aim of this study was to optimize the performance of bi-functional catalyst (Co/H-ZSM-5) via desilication of H-ZSM-5 zeolite support and calcium Co-based catalyst promotion for Fischer-Tropsch synthesis. A desilication method was performed via alkaline treatment using 0.2M NaOH aqueous solution at 40°C, 55°C and 70°C for 2 h to remove silica from the original ZSM-5 zeolite (Si/Al = 40). The desilicated and un-desilicated H-ZSM-5 supports were employed in 10 wt.% of Co in the preparation of the bi-functional catalyst (Co/H-ZSM-5) via incipient wetness impregnation (IWI) method. Four bi-functional catalysts were synthesized individually with active cobalt metal supported on H-ZSM-5 zeolites treated at three temperature values (40°C, 55°C & 70°C) and one on un-treated H-ZSM-5. The prepared bi-functional catalysts were dried at 120 °C for 12 h and then calcined at 550°C for 2 h to achieve the final products, Co/H-ZSM-5 catalysts. These catalysts were characterized and tested on FT process for the production of hydrocarbons. The characterization was done using the following xiv techniques; scanning electron microscopy (SEM) coupled with Energy Dispersion Spectroscopy (EDS) to study the surface morphology and elemental composition of the catalysts, Nitrogen Physisorption at 77K to determine surface area, pore size, and pore volume and adsorptive properties of catalysts, Fourier Transform Infrared Spectroscopy (FTIR) study chemistry of the catalysts, X-Ray Diffraction (XRD) for crystallinity and structure of catalysts, Transmission Electron Microscopy (TEM) for dispersion of the cobalt metal and Temperature-Programmed Reduction(TPR) for reducibility of the catalysts. The best performing catalyst (found to be Co/H-ZSM-5(55)) during FT process was further promoted by 2.0 wt.% calcium metal. Co/H-ZSM-5 catalyst was also calcium promoted for comparison’s sake. Calcium promoted catalysts were also characterized and evaluated on FTS. For characterization, XRD and EDS were used to check whether Ca was successfully deposited and Ca content on Co-cased catalysts. In total, six catalysts were synthesized and named as follows; Co/H-ZSM-5, Co/H-ZSM-5(40), Co/H-ZSM-5(55), Co/H-ZSM-5(70), Ca-Co/H-ZSM-5 and Ca-Co/H-ZSM-5(55) catalysts, where the value inside the brackets indicates the temperature during desilication of H-ZSM-5. The textural property of the desilicated H-ZSM-5 obtained through N2 Physisorption experiments at 77 K shows an enhancement textural property of the catalyst. Desilication method increased the surface area of the original support from 391.81 m2/g to 419.75 m2/g, pore size from 5.27 nm to 8.09 nm and the pore volume slightly increased by 20.35%. EDS showed a decrease in silicon content from 50.78 % to 43.13 % after the desilication of H-ZSM-5 zeolite, which led to Si/Al molar ratio of 26. For the Co/H-ZSM-5 catalysts, the XRD patterns indicated that cobalt metal was successfully impregnated on H-ZSM-5. Promotion of the Co/H-ZSM-5 catalyst increased the cobalt crystallite sizes from 13.7 nm to 17.0 nm based on XRD results. The effect of desilication of H-ZSM-5 on the catalytic performance of the Co-catalysts was evaluated in FTS under the same process conditions; temperature (250°C), pressure (15 bar), flow rate (1200 gas hourly space velocity), H2/CO syngas ratio (2.5) and catalyst weight (0.5g). The analysis of FT results was based on the CO conversion and product selectivity (olefins and paraffins). Prior to the effect of desilication, Co/H-ZSM-5 catalyst resulted in 49.7 % CO conversion, 47.8 % selectivity to methane and 8.8 % selectivity to C5+. Therefore, the use of mesoporous-H-ZSM-5 as a support to Co-based catalyst decreased methane selectivity, enhanced the CO conversion and C5+ selectivity. The CO conversion and C5+ selectivity xv depended on the temperature used during desilication method by NaOH aqueous solution for mesoporous-H-ZSM-5 preparation. Based on the optimum NaOH temperature of 55°C, the mesoporous-H-ZSM-5 on Co-catalyst increased the rate of CO consumption which led to CO conversion of 84.2 %. In addition, C5+ selectivity increased to 47.9%, being more selective to olefin than paraffin (obtained C5 olefin ratio of 7.7). The influence of calcium promotion to Co-based catalysts maintained its activity, high CO conversion and C5+ selectivity. Regarding Ca-Co/H-ZSM-5 and Ca-Co/H-ZSM-5(55) catalysts, CO conversion increased to 50.2% and 90.9 % respectively. On Ca-Co/H-ZSM-5(55) catalyst, selectivity to C6+ hydrocarbons were 8.6 % olefin and 34.5 % paraffin compared to 19.9 % olefin and 7.5 % paraffin of Co/H-ZSM-5(55) catalyst. This explains that calcium promoter was more favorable to longer hydrocarbons of paraffins than olefins due to the second reaction of shorter alkenes into saturated HCs. At low pressure condition (8 bar and 2 bar) but similar other process conditions, CO conversion decreased to 47.7 and 17.4 % caused by decrease in partial pressures of CO. But lowering the amount of pressure kept selectivity to paraffins more than olefins due to calcium promoter, Ca/H-ZSM-5(55). It was concluded that the mesoporosity of the H-ZSM-5 and calcium promotion on the Co-based catalyst played a significant role in overcoming the over-hydrocracking of hydrocarbons and promote the re-adsorption of smaller olefin hydrocarbons into gasoline-range hydrocarbons, respectively.