Fischer-Tropsch Synthesis Study On ZRO₂Cobalt-Based Catalysts With Ecological Environment Strategy

Heretofore, global climate warming is the one of the most serious environmental problem all around the world. It was caused by green house gas (GHG) emissions which were let out when burning the fossil-fuels directly. Global climate warming not only does harm to human’s daily life but has negative influence on the modernization and sustainable development. Due to such severe situation, the international negotiations are aimed to alleviate the harm of global warming. The Fischer-Tropsch synthesis (FTS) is one of the most promising synthetic routes to produce ultra-clean automotive fuels and valuable organic compounds that are originally from coal, natural gas and biomass. The quality of the fuels from FTS can offer significant environmental and efficient benefits over those derived from crude oil. FTS technique is not only a complementarity to the shortage of liquid fuels, but also a measure to reduce effectively the consumptions of non-renewable fuel thus producing lower pollution and controlling the emission of GHG. However, the continued development of FTS technique is limited because of the emissions of GHG (CH4and CO2) in tail gases and the high energy consumptions for FTS reaction. One of the key strategies solves the problem by designing and developing the effective catalysts with excellent performance for FTS. In terms of the practical demand for chemical industry and environment protection, an important topic that attracts much interest in carbon dioxide reforming of methane. Both methane and carbon dioxide are GHG from FTS that can be transformed synthesis gas with low H2/CO ratio, which is proper feed gas for FTS processes. Both FTS and carbon dioxide reforming methane have the same catalyst system. Additionally, FTS is an exothermic reaction and carbon dioxide reforming methane to synthesis gas is an endothermic reaction. Combining both reactions leading to thermal coupling can reduce the emission of GHG, which is actually important and extensive practical prospect in chemical industry.In present paper, a series of cobalt bases catalysts were prepared such as the catalysts supported on ZrO2with different pore structure and mesoporous CeO2-ZrO2solid solution. All cobalt catalysts were prepared by incipient wetness impregnation. X-ray diffraction (XRD), N2adsorption/desorption, transmission electron microscopy (TEM), scanning electron microscopy (SEM), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and hydrogen chemisorption were used to characterize these catalysts. The activity of FTS was measured in a fixed bed reactor. The effects of component, promoter, pore size and structure on FTS catalytic performance have been investigated. Combining both the FTS processes and carbon dioxide reforming methane are investigated on Co-based catalyst supported on CeO2-ZrO2solid solution. It is comprehensively evaluated that catalytic performance for FTS, carbon dioxide reforming methane and the decrease emissions for CH4and CO2are via single cycle. The following conclusions were obtained.1. The3DOM Co/Zr02catalyst was prepared and its catalytic performance was investigated for the FTS. Due to the unique3DOM structure with the interconnected networks of spherical voids for the ZrO2-3support, the Co/ZrO2-3catalyst showed less aggregation, better dispersion and higher reducibility of Co3O4particles, which led to more cobalt surface active sites, further displayed high CO conversion and high FTS reaction rate. The high dispersion of the active components was one of the key factors for improving CO conversion and FTS reaction rate of the Co/ZrO2catalyst with weak interaction between cobalt species and support. The hydrocarbon selectivity of the catalyst greatly depended on the pore structure of the3DOM ZrO2support. The macropores providing channels for rapid molecular transportation were in favor of the enhancement of C5+selectivity, the small pores within the wall of3DOM entities limited the diffusion efficiency of the reactants/products. Thus, the catalyst with bimodal pore structure showed the interesting result of higher C5+selectivity and higher methane selectivity. For cobalt catalyst supported on3DOM structure, the best reaction activity and the best C5+selectivity might be obtained in the FTS by optimizing both the mesopores within the walls and the macropores of the3DOM structure.2. Mesoporous CexZr1-xO2(x=0,0.02,0.05,0.08,0.10) solid solutions with different Ce/Zr molar ratios were prepared by the hydrothermal method. All cobalt catalysts were prepared by incipient wetness impregnation. X-ray diffraction (XRD), N2adsorption/desorption, transmission electron microscopy (TEM), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and hydrogen chemisorptions were applied to characterize these catalysts. The activities of the catalysts were measured in a fixed bed reactor for FTS. The smaller Zr4+ions entered into the lattice of CeO2with increasing Ce/Zr ratios. XRD patterns demonstrated that the diffraction of ceria-zirconia solid solutions shifted to right and got broadening compared with that of CeO2. The support of ceria-zirconia solid solutions exhibited the bimodal pore structure, which the small pores provide a large area for active surface, improving the dispersion of supported metal and enhancing the catalytic activity of the catalysts for FTS reactions. And the large pores of the bimodal pore structure provide pathways or rapid molecular transportation which is beneficial to higher chain growth probability and higher C5+selectivity. The results obtained from different measures were consistent, and the catalytic activity of the catalysts followed the order of Co/Ce0.08Zro.9202> Co/Ce0.02Zro.9802> Co/ZrO2> Co/Ce0.10Zr0.9002. The catalysts supported on ceria-zirconia solid solutions can inhibit the sinter of cobalt active species during the reaction process and display an excellent performance of resistance against deactivation for FTS.3. For cerium promoted Co/ZrO2catalysts, addition of small amount of CeO2to the Co/ZrO2catalyst (Co/ZrO2-5CeO2) can increase Co3O4particles size, improve the reducibility, and enhance the numbers of cobalt active sites on the catalyst surface. Thus the Co/ZrO2-5CeO2catalyst exhibited high CO conversion and the FTS reaction rate. Meantime, it was also possessed of high C5+and low CH4selectivity. However, excess of CeO2resulted in the agglomeration of active metal on the catalyst (Co/ZrO2-10CeO2) surface, which decreased the catalyst reducibility and cobalt active sites, further decreased the catalytic activity. The CeO2-promoted catalysts (Co/ZrO2-5CeO2and Co/ZrO2-10CeO2) could inhibit the agglomeration of cobalt particles during the reaction process so that they displayed an excellent performance of resistance against deactivation for FTS.4. The as-prepared of Co/Ce0.6Zr0.4O2and Co/Ce0.33Zr0.6702retained cubic fluorite structure of CeO2support. The high BET surface area and unique redox property of ceria-zirconia solid solution with cubic fluorite structure improved the dispersion of active components and enhanced the reduction degree of active components. For Co/Ce0.6Zr0.4O2catalyst, the amount of activated carbon was far higher than that of inert carbon. The small cobalt particle could accelerate the elimination of activated carbon in the reaction. The products have low H2/CO ratio from combining both the reactions of FTS and carbon dioxide reforming of methane, which suit to the feed gases for FTS directly. After combining both the reactions via single cycle, the decrease emissions of CH4and CO2were81.0%and82.4%respectively.