Mo-impregnated H-ZSM-5 zeolites catalyze CH4 dehydroaromatization (DHA) with &gt;95% aromatic selectivity at conversions near the ~10% equilibrium limit at 973 K. The reversibility of CH4 DHA prescribed by stringent thermodynamic barriers renders ineffective differential kinetic analyses; rigorous mechanistic study requires quantitative deconvolution of thermodynamic and kinetic contributors to rate for elucidation of kinetically-relevant active site(s) and reaction intermediate(s). In this work, we identify the key reactive intermediate to benzene formation (i.e. acetylene) and the catalytic active center responsible for its kinetically-relevant dehydrogenative production and subsequent aromatization (i.e. carbidic Mo centers). Overlay of DHA conversion-selectivity profiles on a suite of deactivating MoCx/H-ZSM-5 catalysts demonstrates deactivation is “non-selective”, or occurs without change to chemical nature of active sites. We leverage “non-selective” nature of catalyst deactivation to collect selectivity, rate, and reversibility data as a function of contact time, thereby permitting (i) elucidation of the DHA reaction network, (ii) determination of instantaneous active Mo content based on contemporaneous methane conversion, and (iii) calculation of forward rates by De Donder relations. Product selectivities as a function of contact time, or methane conversion, reveal ethane is the sole primary product of methane activation and dehydrogenates to acetylene which is involved in rate-determining aromatization steps. The “non-selective” nature of catalyst deactivation indicates time-decay of methane conversion is attributable to the loss of a single kinetically-relevant active site. We measure DHA rates with systematically-varied initial contact time (τinitial = 8 – 45 molMo s mol-1) and Mo-to-Al ratio (Mo/Al = 0.10 – 0.35) to appraise the catalytic roles of MoCx centers and framework Al-associated zeolitic Brønsted-acid sites. Methane aromatization rates measured at steady state and during deactivation normalize by active Mo content varied by Mo-to-Al ratio and/or extent of catalyst deactivation – providing unequivocal evidence that carbidic Mo species catalyze all kinetically-relevant steps to aromatic formation on MoCx/H-ZSM-5 materials. The kinetic irrelevance of zeolitic Brønsted acid sites is confirmed by dimethyl ether chemical titration which demonstrates DHA rate and Brønsted-acid site count show no quantitative correlation.

Reaction pathways, site requirements, and thermodynamic barriers of CH4 dehydroaromatization on MoCx/H-ZSM-5 catalysts

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.