The Preparation Of Bimodal Mesoporous Alumina And Its Application In F-T Synthesis

Bimodal mesoporous alumina is a type of ideal multifunctional inorganic porousmaterial, which can improve both the diffusion efficiency of materials and metaldispersion on the support. It is a rewarding work to research on controlling its poresize and extending its range of application. For Fischer-Tropsch synthesis （FTS）, thecatalyst pore size affects not only the catalytic activity but also the product selectivity（the small pores mainly produce light hydrocarbons while large pores have a wideproduct distribution with high selectivity for heavier hydrocarbons）. The bimodal porecatalyst where both large pores and small pores coexist can guarantee high diffusionefficiency and large surface area of supported metal meantime, for these reasons muchattention has been drew.First, the bimodal mesoporous alumina was prepared by mixed templates （blockcopolymer P123and cationic surfactant CTAB）. TG, FT-IR, XRD, Nitrogenadsorption-desorption isotherms, SEM and HRTEM were used to characterize theeffects on the pore structure of the mass ratio of the P123to CTAB, pH,pore-expanding agent and calcination temperature. The results showed that whenincreasing the content of P123, the small pores were basically unchanged while thediameter of large pores increased to¹8.1nm, and the pore volume also increasedto1.196cm3/g. Only in alkaline conditions, bimodal mesoporous appeared. Theaddition of different pore-expanding agents could expand the diameter of large pores;meanwhile, its structure became less orderly. The specific surface area and volume ofbimodal mesoporous alumina decreased with the increase of calcination temperature;however the diameter of small pores increased to9.7nm. The pore structure andγ-Al₂O₃crystal could be maintained when the calcination temperature was no higherthan800℃.Then, a series of supported Co/γ-Al₂O₃catalysts were prepared by the impregnationmethod, using the prepared bimodal mesoporous alumina as support. The FTSactivities of catalysts prepared under different conditions were also investigated. Theresults showed that, compared with two single-pore catalysts of9.7nm and33nm,double pore catalysts had good performance for FTS, including high CO conversionof84.8%, and C5+selectivity of53.8%. Catalysts using the bimodal mesoporous alumina support prepared by the template method had good FTS performance; whilecatalysts using mechanically blended supports didn’t show much advantage. Theexperimental results showed that the catalyst calcined at550℃had good FTSperformance.