Precisely designing bimodal catalyst structure to trap cobalt nanoparticles inside mesopores and its application in Fischer-Tropsch synthesis

Bimodal catalyst support is a widely applied concept in solid catalysts but until now the supported metal on the bimodal support has no satisfied dispersion and the average metal particle size is generally larger than the pore size of small pores (mesopores) of the bimodal structure, lowering the power and significance of the bimodal structure. To overcome this problem, a novel ZrO2SiO2 bimodal pore support with enlarged depth of small pores, was successfully fabricated though self-assembly of nanosized ZrSiO4 inside pore of silica gel. Cobalt was finely dispersed on bimodal support by incipient-wetness impregnation. The bimodal support and catalyst were characterized by various techniques, such as BET, TEM, XRD, and TPR and H2-chemisorpion. The bimodal catalyst was applied in slurry-phase Fischer-Tropsch synthesis. Characterization results suggested that bimodal pore support processed larger BET surface than pristine silica gel, due to the newly formed small pores organized by packing of the ZrSiO4 nanoparticles. The enlarged BET surface area leaded to good dispersion of cobalt species. More importantly, it was observed that the deep small pores could serve as traps for Co nanoparticles, preventing sintering and agglomeration due to confinement effect. Co/30 wt% ZrO2-Q-50 and Co/20 wt% ZrO2-Q-100 bimodal catalyst with large BET surface and deep small pores exhibited excellent catalytic activity, and selectivity to heavy hydrocarbons due to high dispersion of cobalt and unique pore structure, while coexisting ZrO2 promoted the reduction of finely-dispersed cobalt oxide nanoparticles. It is interesting to find that different from the conventional theory where small metallic particle lowers chain-growth probability, the trapped smaller Co particles inside deep small pores achieved higher chain-growth probability due to the facilitated re-adsorption of olefinic intermediates happening inside deep small pores.