Pore Size Effects in Fischer Tropsch Synthesis over Cobalt-Supported Mesoporous Silicas

Pore size effects on Fischer Tropsch reaction rates and selectivities over cobalt catalysts were studied at atmospheric pressure using periodic (SBA-15 and MCM-41) and commercial mesoporous silicas as catalytic supports. The catalysts were characterized by nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and oxygen titration. Fischer Tropsch reaction rates were found much higher on cobalt catalysts with the pore diameter exceeding 30 Å than on the narrow pore catalysts. A larger diameter of catalyst pores also led to significantly higher C5+ selectivities. The catalytic effects were interpreted in terms of different cobalt particle size and reducibility in the wide pore and narrow pore silicas. XRD and XPS showed that the size of supported cobalt species strongly depended on the pore size, increasing with increases in catalyst pore diameter. TGA and oxygen titration indicated higher extent of overall reduction of cobalt species in wide pore supports. Lower reducibility of small cobalt particles is likely to be one of the reasons responsible for the lower Fischer Tropsch reaction rates and higher methane selectivities on narrow pore cobalt catalysts.