Preparation and characterization of Mo catalysts over AlMCM-41/γ-Al2O3 extruded supports

Molybdena catalysts, prepared by incipient wetness impregnation of an aqueous solution of ammonium heptamolybdate on a series of AlMCM-41/gamma-Al2O3 extruded supports, have been investigated by nitrogen adsorption, X-ray diffraction (XRD), diffuse reflectance UV-visible spectroscopy and model compound reactions with thiophene and nickel tetraphenylporphyrin (Ni-TPP), The extruded supports have a bimodal pore structure: a uniform mesoporous system with smaller pore size originating from AlMCM-41 and a mesoporous system with larger pore size originating from gamma-Al2O3. The loading of molybdena had no significant influence on the pore structure of AlMCM-41 regardless of SiO2/Al2O3 ratios and molybdenum contents, XRD results demonstrate that highly dispersed molybdena catalysts have been obtained on both gamma-Al2O3 and AlMCM-41. UV-visible spectra show that molybdena exists as octahedrally coordinated species on AlMCM-41 and mainly as tetrahedrally coordinated species on gamma-Al2O3, which reflects the different kinds of molybdenum oxide entities on AlMCM-41 and gamma-Al2O3, The catalytic activities of the sulfided catalysts show no dependence on the acid sites of AlMCM-41, but they depend on the molybdenum sulfide species. Mo sulfide species inside mesoporous channels have a high activity for smaller reactant thiophene conversion, while Mo sulfide species on gamma-Al2O3 have a high activity for larger reactant Ni-TPP conversion. Therefore, it is proposed that molybdena species inside mesoporous channels of AlMCM-41 have a particular structure and accessibility, which is responsible for the particular catalytic properties of the Mo sulfide species. (C) 1999 Elsevier Science B.V. All rights reserved.