Effects of support and dispersion on the CO hydrogenation activity/selectivity properties of cobalt

Specific activities and selectivities of unsupported cobalt and cobalt supported on alumina, silica, titania, carbon, and magnesia carriers for CO hydrogenation were measured in a single-pass differential reactor at low conversions, 1 atm, and 175–350 °C. The results indicate that specific activity and selectivity of cobalt vary with support, dispersion, metal loading, and preparation method. The order of decreasing CO hydrogenation activity at 1 atm and 225 °C for catalysts containing 3 wt% cobalt is CoTiO2, CoSiO2, CoAl2O3, CoC, and CoMgO. The specific activity of cobalt decreases significantly with increasing dispersion. Product selectivity is best correlated with dispersion and extent of reduction i.e., the molecular weight of hydrocarbon products is lower and the CO2H2O ratio is higher for catalysts having higher dispersions and lower extents of reduction. This effect may be due to stable oxides in the well-dispersed, poorly reduced catalysts, which catalyze the water-gas-shift reaction thereby increasing the H2CO ratio at the surface. In the CoAl2O3 system, activity and selectivity for high molecular weight hydrocarbons increase very significantly with increasing cobalt loading. A 15% CoAl2O3 is 20 times more active than 3% CoAl2O3; moreover, 86 wt% of its hydrocarbon fraction is in the C5–C12 (gasoline) range compared to 18 wt% for 3% CoAl2O3.