Characterization of a Mo/ZSM-5 catalyst for the conversion of methane to benzene

The dehydroaromatization of methane to benzene has been investigated over a 2 wt% Mo/ZSM-5 catalyst in the absence of an added oxidant. The reaction is characterized by an induction period, prior to the initiation of benzene production, during which Mo2C is formed and coke deposition occurs. The formation of the carbide was confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Pretreatment of the catalyst in a CH4/H-2 gas mixture at 700 degrees C reduces Mo6+ ions in the calcined catalyst into Mo2C and almost eliminates the induction period, confirming that Mo2C is the active species in the activation of methane. Under typical CH4 reaction conditions at 700 degrees C, 60-80% of the original Mo6+ ions are reduced to form Mo2C, with the remaining Mo occurring primarily as Mo4+ and traces of Mo5+ ions. These nonreducible Mo ions are most likely within the channels of the zeolite. XPS, ion-scattering spectroscopy, and FT-IR measurements indicate that Mo species in a Mo/ZSM-5 sample dried at 130 degrees C are present as small (30 to 50 Angstrom) crystallites of the original ammonium heptamolybdate impregnated salt on the external surface of the zeolite. After calcination at higher temperatures (500-700 degrees C), Mo becomes more highly dispersed, but not uniformly distributed, on the external surface of the zeolite. During preparation and/or pretreatment of the catalyst, a portion of the Mo ions diffuses into the channels of the zeolite. The amount of Mo ions within the channels depends on the temperature, time, and atmosphere of calcination The roles of Mo2C, partially reduced Mo ions, and the origin of the induction period are discussed on the basis of kinetic results and physical/chemical characterization measurements of the catalyst. (C) 1997 Academic Press.