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

Hexagonal mesoporous. silica (HMS), AI-HMS, MCM-41, as well as ZrO2 modifie d HMS supported cobalt and small amount of MnO promoted cobalt catalysts ar e investigated by using X-ray diffraction, N-2 adsorption, transmission ele ctron microscopy, and temperature programmed reduction techniques, and by t esting their catalytic properties in Fischer-Tropsch synthesis. Cobalt oxid es are dispersed highly on the surface of these mesoporous supports. Howeve r, their BET surface area, pore volume, and pore size decrease, and pore wa ll thickness increases a little. The main hydrocarbon products obtained ove r these catalysts are the long chain hydrocarbons (wax). Comparing with MCM -41 and Al-HMS, the HMS supported Co catalyst shows better catalytic activi ty and C-5(+) selectivity, which is due to its smaller domain size with sho rter 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 acti ve sites in the channels of HMS. When AI-HMS is used as a support, the inte raction between Co species and AI-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 ZrO2 on the surface of HMS support favors the reduction of cobalt oxides and in creases the activity and C-5(+) selectivity. The addition of small amount o f Mn (less than or equal to2 wt.%) to Co/HMS catalyst, high yield of wax (3 6.2%) 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. (C) 2001 Elsevier Science B.V. All rights reserved.