Study Of Preparation And Properties Of Iron-Based Fischer-Tropsch Catalysts Derived From Metal-Organic Frameworks

The Fischer-Tropsch synthesis reaction indirectly converts coal,biomass,etc.into clean fuels and high value-added chemicals,which can effectively alleviate the current dependence on petroleum resources and help to diversify energy supply.The research and development of catalysts is the core of this technology,in-depth study of catalyst structure-activity relationship is conducive to improving catalyst performance.The development and utilization of new materials also has good prospects for further improving the performance of Fischer-Tropsch catalysts.Metal-Organic Frameworks（MOF）materials are widely used in the field of catalysis because of their unique structural characteristics,and a series of MOF derivatives are applied to Fischer-Tropsch synthesis reactions and exhibit excellent catalytic properties.Although Fe-based Fischer-Tropsch synthesis catalysts are inexpensive and widely used,they have low catalytic activity and poor stability.In view of the above situation,a series of iron-based Fischer-Tropsch synthesis catalysts were prepared using different MOF materials as precursors,and the structural composition,morphology and valence information of the catalyst were analyzed by N₂physical adsorption,X-ray diffraction（XRD）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,thermogravimetric analysis（TG）,temperature-programmed reduction/desorption（TPR/TPD）and other characterization methods.The effects of different pyrolysis temperatures,promoters and composition on the catalysts were investigated,and the relationship between the structure of the MOF-derived iron-based catalysts and the catalytic performance of the Fischer-Tropsch was studied.The main contents are summarized as follows:1.The effect of different pyrolysis conditions on the structure of MOF-derived porous carbon was studied.It was found that the specific surface area of porous carbon materials increased with the pyrolysis temperature,and the pore size is nearly the same.Fe-based catalyst was prepared by the incipient wetness impregnation（IWI）method using nitrate solution.It was found that the porous carbon support with a large specific surface area and large pore volume is more conducive to the dispersion of active components.The evaluation results of the Fischer-Tropsch catalytic performance of Fe-700,Fe-800 and Fe-900 catalysts show that the catalytic activity of Fe-900 catalysts was the highest.The content of medium and long-chain hydrocarbons of all catalysts is relatively high,which indicates that the porous carbon with a larger specific surface area and pore volume helps improve the catalytic activity and the selectivity of heavy hydrocarbons.At the same time,the research also shows that the product distribution of the catalyst can be effectively controlled by processing the MOF precursor.2.The effects of different types of promoters on MOF-derived Fe@C catalysts were studied.The Fe@C catalyst was prepared by pyrolysis of MIL-100,and the characterization results show that Fe nanoparticles were uniformly dispersed under a high loading of 40 wt%,semi-embedded in a porous carbon matrix,without agglomeration.Under high-temperature Fischer-Tropsch synthesis conditions,the catalyst exhibited good stability.The Fe@C catalyst modified by alkali metal Na,transition metal Cu,and noble metal Ru were prepared by the IWI method.The addition of promoters can reduce the particle size of Fe nanoparticles.Na promoter inhibit the reduction of Fe,while Cu and Ru promoters contribute to the reduction of Fe,increasing the number of catalyst surface active sites.In the Fischer-Tropsch synthesis reaction,the additive significantly increased the catalytic activity of the Fischer-Tropsch synthesis reaction,and the Cu/Fe@C catalyst showed the highest Fe time yield(FTY=154 mmol _COg _Fe^-1h^-1),which is 2 times the Fe time yield of the Fe@C catalyst.The performance of the promoters has a significant effect on the reaction products,the product distribution of Na-promoted catalyst show low methane selectivity and high heavy hydrocarbon selectivity,especially high ratio of olefin to paraffin（O/P）in C_2-4.The Ru/Fe@C catalyst enhanced the formation of methane and light hydrocarbons due to the strong hydrogenation effect of Ru.Cu/Fe@C catalysts show the highest CO₂selectivity.The above results indicate that the promoter has a significant effect in the design and modification of Fe-based catalysts.3.Bimetallic catalysts were developed for regulating the products distribution of Fischer-Tropsch synthesis,and a series of bimetallic catalysts Fe M@C are prepared by high-temperature pyrolysis of bimetallic MOF-74under N₂atmosphere.It was found that high-temperature pyrolysis caused Co,Ni,Mn,Zn and Fe to interact with Fe,forming Fe Co and Fe Ni alloy structures and Fe Mn,Fe Zn solid solution structures.At the same time,the enrichment of Co,Ni,Mn and Zn on the surface of the catalyst to varying degrees changed the distribution of the Fischer-Tropsch synthesis products;Mn promoted the formation of low-carbon olefins in the Fe Mn@C catalyst,the hydrogenation of Co and Ni was obvious,the methane selectivity in the catalyst products of Fe Co@C and Fe Ni@C exceeded 40%,and the influence of Zn on the distribution of the catalyst products was less obvious,similar to the distribution of the Fe-based catalyst products.The catalytic activity of the bimetallic catalyst was significantly higher than that of the monometallic catalyst,of which the highest catalytic activity of the Fe Zn@C was FTY=152mmol _COg _Fe-1 h^-1,which was 1.5 times that of the Fe@C of the monometallic catalyst.The results show that the synergistic effect of the bimetallic catalyst can not only improve the activity of the catalyst but also regulate the selectivity of the product.The method of preparing catalysts by pyrolysis of bimetallic MOFs has good prospects in the design and preparation of new Fischer-Tropsch-Tropsch synthesis catalysts.