Mesoporous HMS molecular sieves supported cobalt catalysts for Fischer–Tropsch synthesis

Abstract

Hexagonal mesoporous silica (HMS), Al-HMS, MCM-41, as well as ZrO 2 modified HMS supported cobalt and small amount of MnO promoted cobalt catalysts are investigated by using X-ray diffraction, N 2 adsorption, transmission electron microscopy, and temperature programmed reduction techniques, and by testing their catalytic properties in Fischer–Tropsch synthesis. Cobalt oxides are dispersed highly on the surface of these mesoporous supports. However, their BET surface area, pore volume, and pore size decrease, and pore wall thickness increases a little. The main hydrocarbon products obtained over these catalysts are the long chain hydrocarbons (wax). Comparing with MCM-41 and Al-HMS, the HMS supported Co catalyst shows better catalytic activity and C 5 + selectivity, which is due to its smaller domain size with shorter channels and larger textural mesoporosity. With increasing Co loading on the HMS support, the reduction temperature of cobalt oxides increases a little, but the activity apparently increases, which is caused by more active sites in the channels of HMS. When Al-HMS is used as a support, the interaction between Co species and Al-HMS support caused by the acid properties on the surface of Al-HMS, leads to a decrease of available Co metal sites and a decrease of activity and C 5 + selectivity. The pre-impregnated ZrO 2 on the surface of HMS support favors the reduction of cobalt oxides and increases the activity and C 5 + selectivity. The addition of small amount of Mn (⩽2 wt.%) to Co/HMS catalyst, high yield of wax (36.2%) is obtained, which might be ascribed to that MnO segregates partially the active Co sites and prevents the formation of methane at high reaction temperature.