Non-oxidative methane valorization to aromatic products is characterized by stringent thermodynamic limitations which prescribe ~10% equilibrium methane conversion to benzene at ~950K over Mo-modified H-ZSM-5 catalysts. Reversibility of methane dehydroaromatization (DHA) reactions manifest from reaction endothermicity complicate traditional analyses of reaction kinetics requiring rigorous, quantitative deconvolution of thermodynamic and kinetic contributors to rate for elucidation of rate-control of elementary steps and reaction intermediates. In this work, we (i) leverage the “non-selective” deactivation of Mo/H-ZSM-5 catalysts to discern the connectivity of the methane to benzene reaction network, (ii) evoke and develop existing formalisms to extract intrinsic kinetic information in highly-reversible reaction systems, and (iii) highlight and address the inherent nature of flow reactors to carry kinetic and thermodynamic information differently. Variation of catalyst contact time in plug-flow reactions by extent of “non-selective” deactivation permits probe of conversion-selectivity profiles from 0.01-8% methane conversion and reveals (i) ethane is the sole primary product of methane DHA and (ii) acetylene, formed from sequential dehydrogenation of ethane and ethylene, is an intermediate to aromatic formation. The thermodynamic and kinetic influence of C2 hydrocarbons on the rate of the overall methane to benzene reaction is evaluated by the degree of reversibility control, introduced here for the first time, and the degree of rate control. Acetylene is shown to be involved in both rate- and reversibility-controlling elementary steps and is therefore a key intermediate in CH4 DHA. The approach to equilibrium of each intervening elementary step in the DHA reaction network is calculated from concentration of gas-phase intermediates and determines the relationship between the measured net rate and the forward rate – the intrinsic kinetic rate of the overall reaction sequence. The nature of approach to equilibrium as a state function requires measured, effluent approach to equilibrium of each step be length-averaged along the catalyst bed for accurate appraisal of forward rate by De Donder relations. The forward rate of benzene formation is found by to be invariant with contact time and demonstrates the role of hydrogen to inhibit measured net rates is solely thermodynamic in nature.

All that most maddens and torments in CH4 dehydroaromatization on Mo/H-ZSM-5 catalysts

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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