The Study Of Iron-Based Catalysts For Fischer-Tropsch Synthesis

Depletion of crude oil resources and environmental concerns have driven a worldwide research on alternative processes for the production of commodity chemicals.Fischer-Tropsch synthesis(FTS)is currently a process of converting natural coals,gas,and biomass via syngas(CO and H2)into super clean liquid fuels and basic building blocks in the chemical industry such as lower olefins to alleviate the oil crisis and reduce environment pollution.Iron and cobalt are used industrially in FTS.Iron-based catalysts are cheaper and more widely available compared with cobalt-based catalysts.The fundamental understanding of the control of catalytic activity,selectivity and deactivation in iron-based FTS is of prime scientific and industrial importance.In this work,the inter-effects of various supports and promoters(K,Na and S)in FTS have been studied intensively to elucidate and draw the key points of a better reaction performance.In this paper,iron-based catalyst was prepared by incipient wetness impregnation method to reach high stability,high activity and desired selectivity to lower olefins.The iron-based catalysts supported on SiC were also developed,and the effect of the alkali metal promoter on the iron-catalyzed FTS reaction was investigated.Meanwhile,the activated gases,reaction temperature and reaction pressure were further investigated.By changing reaction conditions,it is found that the catalyst activated by H2 realizes the best catalytic performance and CO conversion markedly increases with an increase in the reaction temperature.However,the selectivity to paraffins(methane and C2-C4)increases but the selectivity to C2=-C4= in the hydrocarbons does not change with increasing reaction temperature.With an increase in the reaction pressure,the CO conversion substantially increases and the selectivity to C2=-C4= decreases rapidly.The changing reaction pressure does not affect the selectivity to CO2 and C2-C4 paraffins obviously.The effect of supports on K promotion for the iron-based FTS reaction was investigated by using different supports.Combining the abundant experimental work with various characterization measurements,including BET,XRD,TEM,XPS,TMS,TG,FTIR,and H2-TPR,it is found that the SiC-supported iron catalyst shows a pronounced improvement in the CO conversion and O/(O + P).Iron catalysts supported on ?-Al2O3 and SiO2 exhibit no difference in the catalytic activity when K is added.In contrast,the AC and TiO2 supports bring about a negative effect on the catalytic activity becauce the K promoter suppress the reducibility of the catalysts.The K-promoted unsupported catalyst shows the highest initial CO conversion with a rapid increase to maximum followed by a fast deactivation with TOS.In this study,the catalyst deactivation is mainly ascribed to K-induced carbon deposition while the reversible transformation of ?-Fe5C2 into Fe3O4 is another reason to some extent.A series of iron catalysts supported over carbon-silica composite materials were prepared using fructose via a mild hydrothermal approach.Different from conventional silica-supported catalysts,no interaction between iron and silica was observed.The Fischer-Tropsch tests showed enhanced performance of the iron catalysts supported on the carbon-silica composite materials compared to the silica-supported counterparts.In the study,the use of K or Na alone is insufficient due to its favorable chain growth to form higher hydrocarbons.It is well confirmed that S can shift the selectivity toward short-chain C2-C4 hydrocarbons without an increased selectivity to methane.Combining this distinctive function of S with the favoring of ?-hydride abstraction termination of K and a suitable support to have a weak interaction with iron particles,a substantially higher selectivity to lower olefins could be obtained at a milder reaction temperature and elevated reaction pressure.