(257e) Rate and Reversibility of CH4 Dehydroaromatization on Mo/H-ZSM-5

Mo-modified H-ZSM-5 zeolites catalyze methane dehydroaromatization (DHA) with high benzene (>70%) and aromatic (>95%) selectivity at conversions near the thermodynamically-prescribed ~10% equilibrium limit at 973 K. Reversibility of DHA stipulated by stringent thermodynamic barriers complicates kinetic analyses; rigorous kinetic study of highly-reversible reaction networks requires quantitative de-conflation of thermodynamic and kinetic contributors to rate to identify kinetically-relevant 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 DHA reaction network, (ii) develop thermo-kinetic formalisms to ascertain intrinsic kinetic information from integral rate data, and (iii) explicate the distinct manner by which flow reactors carry kinetic and thermodynamic information.

Exploit of “non-selective” Mo/H-ZSM-5 deactivation enables collection of rate data as a function of contact time from which we identify (i) ethane is the sole primary product of methane activation and (ii) acetylene is formed and consumed in kinetically-relevant dehydrogenation and oligomerization steps, respectively.

The thermodynamic and kinetic influences of each step are respectively quantified by the degree of reversibility control, defined for the first time, and degree of rate control – both of which we calculate from the measured reversibility of each overall gas-phase reaction (e.g. C₂H₄ ⇌ C₂H₂ + H₂). Degrees of reversibility and rate control confirm kinetic relevance of intermediate acetylene and inform calculation of the overall forward rate – the intrinsic kinetic rate of CH₄ DHA.

Rigorous calculation of the DHA forward rate requires the measured, effluent reversibility of each step to be length-averaged along the catalyst bed because reversibility, unlike net rate, is a thermodynamic state function and, therefore, does not carry path-dependent information (see scheme). Forward benzene synthesis rates are found to be invariant with contact time and provide a compendious intrinsic kinetic metric by which to appraise and compare DHA catalysts.