Influence of alkali promoters on synthetic diesel production over Co catalyst

The effects of the addition of alkali metal and alkali-earth metal promoters over cobalt based catalyst on Fischer–Tropsch activity and on hydrocarbon product distribution were studied. The influence of promoters on the reducibility and cobalt particle size was studied by different techniques, including N2 adsorption, X-ray diffraction, temperature-programmed reduction, temperature-programmed desorption and acid–base titrations. Experiments were carried out on a bench scale fixed bed reactor and catalysts were prepared by incipient wetness impregnation. It was observed that the addition of small amounts of these promoters (0.5 wt%) improved the cobalt oxide reducibility by reducing the formation of cobalt aluminate species. The interaction between cobalt oxide and promoted support was reduced and larger cobalt oxide particles were formed on the surface. A positive correlation between basicity and particle size was observed with the exception of K promoted sample. Furthermore, CO conversion and C5+ selectivity were found to be also influenced by the addition of promoters. An enhanced dispersion that provides abundant catalytically active sites was observed on promoted catalyst. The addition of Ca to the cobalt based catalyst greatly improved the diesel selectivity. However Ca promoted catalyst yield a higher amount of methane at 242 °C than the Na sample.