Tailored chemical modification of ZSM-5 zeolite supports for Mo-based catalysts optimized for methane dehydroaromatization: Hydrothermal dealumination was key to improving the selectivity of desired aromatic compounds

Abstract

In this study, we considered Mo-impregnated H-ZSM-5 zeolite catalysts (denoted by Mo/Z) for methane dehydroaromatization (MDA). In particular, we attempted to reduce the amount of Brønsted acid sites (BAS) of the H-ZSM-5 supports via hydrothermal dealumination to disfavor the final product of the MDA (i.e., coke) and, thus, improve the intermediate aromatic compounds (mainly, benzene and toluene: BT). In fact, the hydrothermal treatment of H-ZSM-5 supports at ca. 400 °C prior to Mo-impregnation (the resulting catalyst is denoted by Mo/Z_400) improved the BT formation rates over the 12-h reaction relative to Mo/Z. In particular, coke analyses on the spent catalysts recovered after different reaction times revealed that Mo/Z_400 suppressed the formation of hard coke fractions and, accordingly, increased the intermediate BT products, as compared to Mo/Z. For this, thermogravimetric analysis complemented with micropore analysis clearly indicated that the chemically modified property (here, BAS), not the physical counterpart (here, 10-membered-ring micropores), was key to achieving such activity and, furthermore, their gradual decrease could account for the deactivation behavior as a function of time. In addition, the coke formed outside was likely to contribute to the deactivation as well. However, higher hydrothermal treatment temperatures (500, 600, or 700 °C) rather deteriorated the catalytic activity, suggesting that extensive dealumination of the zeolite framework resulted in the excessive loss of desired BAS. Such improvement based on the optimal modification of BAS allowed for competitive MDA performance to be as good as those of other complex post-processed, high-performance Mo-based ZSM-5 zeolites reported in the literature.