Study of Methane Dehydroaromatization on Impregnated Mo/ZSM-5 Catalysts and Characterization of Nanostructured Molybdenum Phases and Carbonaceous Deposits

Abstract

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 (∼14% CH4 conversion) and benzene formation selectivity (∼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 °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 MoO x 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.

Recent progress in methane dehydroaromatization: From laboratory curiosities to promising technology

“…The accumulation of coke deposits increases with both time-on-stream [92,107] and reaction temperature [63].…”

“…Differing from widely-held opinion about the active species for MDA being partially reduced molybdenum carbides, Zaikovskii et al [91,92] used HRTEM, EDXA, and EPR techniques, and concluded that methane is mainly acti-vated on oxidized molybdenum clusters located in the zeolite channels. They also concluded that Mo 2 C particles present on the external surface are deactivated in the early stages of the reaction due to graphite deposition.…”

“…Similar results were obtained using UV-Raman spectroscopy [33,111], which showed that the deposited carbon is largely in hydrogen-deficient type. Matus et al [92] found that, during the course of the reaction, carbonaceous deposits are formed both on the surface of Mo 2 C particles in the form of graphite layers with a thickness of ca. 2 nm, and on the zeolite surface in the form of a friable disordered layer with a thickness up to 3 nm.…”

Non-oxidative dehydroaromatization of methane: an effective reaction–regeneration cyclic operation for catalyst life extension

“…First, a temperatureprogrammed oxidation (TPO) was carried out on a deactivated Mo/ZSM-5 catalyst to estimate the minimum time and temperature required to remove the formed coke. The conditions for complete coke removal will depend on the amount and type of coke, and this, in turn will depend on catalyst composition and on MDA experimental parameters [67][68][69] . Here, we have limited our study to a single set of reaction conditions, and only TOS will be varied and will have an effect on the coking.…”

Peculiarities of dehydroaromatization of CH4–C2H6 and CH4 over Mo/ZSM-5 catalysts

“…Methane dehydroaromatization (DHA) over Mo/ZSM-5 catalysts under oxygen-free conditions to obtain aromatics and hydrogen has been widely studied in the last two decades [1][2][3][4][5][6][7][8][9][10]. There is also a distinctive interest in DHA of other hydrocarbons, namely ethane, over Mo/ZSM-5 catalysts from the fundamental point of view focusing on the mechanism of DHA reactions [11].…”