Study of methane dehydroaromatization on impregnated Mo/ZSM-5 catalysts and characterization of nanostructured molybdenum phases and carbonaceous deposits

Methane dehydroaromatization was studied over a series of impregnated Mo/ZSM-5 catalysts with different molybdenum contents and Si/Al atomic ratios in the parent H-ZSM-5 zeolites. The maximum catalytic activity (similar to 14% CH4 conversion) and benzene formation selectivity (similar to 70%) were observed for the samples with 2%-5% molybdenum. The activity and selectivity are improved when the Si/Al ratio is decreased from 45 to 17. After pretreatment in argon and reaction at 720 degrees C, the catalysts have been characterized by textural methods, X-ray diffractometry (XRD), differential thermal analysis (DTA), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX). As determined by HRTEM, after the pretreatment, the MoOx phase is highly dispersed on the external surface of the zeolite. During the reaction, molybdenum carbide nanoparticles 2-15 nm in size are formed on the external surface, and the molybdenum-containing clusters are deposited in the zeolite channels. The carbonaceous deposits (CDs) are formed as graphite layers on the surface of Mo2C nanoparticles that were > 2 nm in size, and as friable layers with a disordered structure on the external surface of the zeolite. According to EDX, XRD, and DTA studies, the content of the CD and the extent of their condensation (the C/H ratio) increase with the time-on-stream. For all the studied molybdenum contents (1%-10%) and time-on-stream values (0.5-6 h), the CDs formed on the catalysts with Si/Al = 17 are characterized by one maximum of the exothermic burn-out effect in DTA, whereas on the catalysts with Si/Al = 30 and 45, they are characterized by two such maxima. A correlation between the catalyst activity, the selectivity versus nanostructure, and the location of the molybdenum phases and CDs is discussed.