Fischer-Tropsch synthesis catalysed by cobalt-rhodium and cobalt-ruthenium carbonyl clusters on silica:Dissertation

A systematic study was made of the use of tetranuclear Co4-nRunHn(CO)12 (n = 0-4), Co2Ru2(CO)13 and Co4-nRhn(CO)12 (n = 0,1,2 or 4) clusters as catalyst precursors. Catalysts were characterized by temperature-programmed reduction and Fourier-transform infrared spectroscopy techniques, and their activity and selectivity in Fischer-Tropsch synthesis were determined, as well as the effect on their performance of reaction temperature, pretreatment atmosphere and method of preparation. Monometallic Co4/SiO2 and Ru4/SiO2 catalysts were very active and all rhodium-containing Co4-nRhn(CO)12/SiO2 catalysts were fairly weakly active in Fischer-Tropsch synthesis. Deactivation was associated with the bimetallic sites on cobalt-rhodium catalysts and with ruthenium on cobalt-ruthenium catalysts. The effect of temperature on the catalysts followed a characteristic trend: when the temperature increased, product selectivity shifted to lighter molecular mass compounds and selectivity for oxygenates mainly decreased. Per metal atom, refluxed catalysts tended to show higher activity in Fischer-Tropsch synthesis than did the corresponding impregnated catalysts. However, since the metal content of the refluxed catalysts was significantly lower than that of impregnated ones, the overall reaction rates over refluxed catalysts were slower. In the bimetallic catalysts, the interaction between cluster and support probably occurred between the rhodium species and silica, and between ruthenium species and silica, not between cobalt and silica. Evidently rhodium and ruthenium interact more strongly with silica than cobalt does