Surface properties of molybdenum-impregnated ZSM-5 catalysts

Abstract

Molybdena-containing H-ZSM5 zeolites have been prepared and used in the hydrodesulfurization of thiophene and dibenzothiophene model compounds. The catalysts containing 3 and 6% MoO₃ were prepared by aqueous impregnation with ammonium heptamolybdate, followed by calcination at 383, 623 and 773 K. They have been characterized by X-ray diffraction (XRD), infrared spectroscopy (FTIR), temperature-programmed reduction (TPR), and scanning electron microscopy (SEM). The set of data indicated that the zeolite crystallinity is mostly preserved for the high Mo-containing catalysts, and that most of the molybdena appears as MoO₃ crystals on the external zeolite surface. The appearance of MoS₂ structures produced by sulfidation has already been observed by X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy of an NO probe molecule. These also showed a better dispersion for the low Mo-containing catalysts. All these data and activity data are jointly discussed.