(370e) Metal-Mediated Transient Hydrogen Scavenging for Enhanced Aromatics Yield during Non-Oxidative Methane Aromatization on Mo/H-ZSM-5 Catalysts

We report a polyfunctional catalyst formulation that comprises a hydrogen-selective zirconium adsorbent with a Mo/H-ZSM-5 methane dehydroaromatization catalyst that results in enhanced methane conversion, accelerated rates, and increased selectivity to desired products. Chemical transient experiments show that 2.4 ± 0.1 O:Mo are lost as CO, CO₂ and H₂O during initial CH₄ reactions with MoO_x precursors in a stoichiometric reaction that leads to MoC_x evincing the formation of (Mo₂O₅)²⁺ dimers on air treatment of MoO₃/H-ZSM-5 physical mixtures at 973 K. This synthetic protocol gives a Mo/H-ZSM-5 formulation with a steady-state net benzene formation rate of 2.5x10^-4 mol mol_Mo^-1 s^-1. Mo/H-ZSM-5 produces ethene, benzene, and naphthalene with high selectivity via bifunctional catalysis where MoC_x species catalyze the C-H bond activation of methane to ethene/ethane, residual zeolitic Brønsted acid sites catalyze olefin coupling for chain growth, and MoC_x species catalyze aromatic formation through aliphatic dehydrogenation. Ethylene, benzene, and naphthalene yields on Mo/H-ZSM-5 approach values prescribed by equilibrium (6CH₄ ↔ C₆H₆ + 9H₂; equilibrium conversion ~10% at 973 K). Addition of zirconium (Zr) particles to the MoC_x/ZSM-5 formulation results in methane conversion of ~30% demonstrating that the limitation of equilibrium conversion of ~10% at 973 K is circumvented due to hydrogen scavenging by formation of a hydride species on the adsorbent Zr. A 3.1 times increase in methane converted with a 1.4, 2.1, 2.6, and 5.6 times increase in benzene, naphthalene, toluene, and C₁₀⁺ yields respectively (after 8.52 ks time-on-stream) are achieved via interpellet mixtures of Zr and MoC_x/ZSM-5 as compared to conventional MoC_x/ZSM-5 catalyst. Furthermore, ~8.5 mol % of methane converted is deposited as coke at the end of 8.52 ks time-on-stream. Subsequent thermal treatment of the catalyst results in desorption of hydrogen and in regeneration of the Zr adsorbent leading to partial regeneration of the polyfunctional catalyst formulation.

We discuss the transients in rates and selectivity for the Zr + MoC_x/ZSM-5 formulation as a result of spatial gradients in hydrogen concentration across the reactor length due to the gradual, stoichiometric reduction of the metal adsorbent to form a hydride as well as loss of acid sites due to coke deposition. Detailed catalyst characterization, hydrogen uptake experiments on Zr adsorbent, and investigations of relevant length scales will also be presented.