The effect of the nanofibrous Al2O3 aspect ratio on Fischer-Tropsch synthesis over cobalt catalysts

A series of nanofibrous alumina materials with diameters of 4-6 nm and with different aspect ratios ranging from 3 to 16 were prepared. Cobalt impregnated catalysts were prepared by means of incipient wetness impregnation on alumina nanofibers while the 'rearranged' catalysts were prepared by using ultrasonication assistance to mix the fibers with the Co3O4 nanoparticles. The effects of the alumina nanofiber aspect ratios on the Co catalyst structure and performance for Fischer-Tropsch synthesis were studied. The pore size of the two series of catalysts increased as the aspect ratio of the alumina nanofiber increased. For impregnated catalysts, large Co3O4 particles were formed on the external surface of the alumina support when the aspect ratio was 3 and 5, while the crystallite sizes of Co3O4 increased from 13.3 nm to 15.6 nm with the increase of the aspect ratio from 7 to 16. The four 'rearranged' catalysts possessed similar and homogeneously dispersed Co3O4 crystallites of 9.5 nm. As expected the reduction behavior of the two series of catalysts was primarily influenced by the Co3O4 crystallite size and structure. The FT data of the two series of catalysts indicate that dispersed Co catalysts on alumina nanofibers with large aspect ratios having large inter-crystallite pores significantly improve the catalyst activity and C5+ selectivity. The FT data of the 'rearranged' catalysts strongly demonstrated that the internal mass transfer of reactants and products increased with a decrease in inter-crystallite pore size, resulting in a decrease of C5+ selectivity and C-3 olefin/paraffin ratio, and an increase of CH4 selectively, while the CO consumption rate was little altered. Furthermore, catalytic stability tests showed that the alumina nanofibers with larger aspect ratios inhibited Co migration and coalescence in the matrices of the nanofibrous alumina, and this significantly enhanced the stability of the catalyst. The Co-p/Al2O3-16 catalyst possessing uniformly distributed cobalt, improved reducibility and large pores is the preferred choice to generate high catalytic activity, stability and C5+ selectivity.