(712e) Critical Role of Dispersive Transport during Absorptive Hydrogen Removal for Enhanced Aromatics Yield in Methane Dehydroaromatization on Mo/H-ZSM-5 Catalysts

Carbidic forms of Mo encapsulated in medium-pore MFI zeolites catalyze high-temperature (~950 K) methane dehydroaromatization (DHA) with high aromatic selectivity (~95%) near the ~10% equilibrium limit set by reaction endothermicity (6CH₄ → C₆H₆ + 9H₂, ΔH⁰_973K = 597 kJ mol^-1). We circumvent thermodynamic limits to CH₄ DHA by addition of Zr metal, a known H₂ absorbent, to pre-carburized MoC_x/H-ZSM-5 catalyst in the form of interpellet physical mixtures and staged packed-bed configurations which increases single-pass methane conversion and cumulative aromatic yield to supra-equilibrium values (see attached figure). Introduction of H₂-absorptive function enhances methane conversion rates without change to prevalent DHA reaction pathways; all polyfunctional formulations maintain similar product distribution with ~95% aromatic selectivity throughout the lifetime of Zr (i.e. before saturated, bulk ZrH_1.75 formation, as confirmed by XRD).

The efficacy of staged MoC_x/H-ZSM-5 and Zr beds to circumvent DHA thermodynamic limits is attributed to the dispersive conveyance of gas-phase H₂ to spatially-distinct catalytic and absorptive functions. Measurement of dimensionless Péclet (Pe = convection rate/dispersion rate ~ 1.3) and Damköhler (Da = reaction rate/convection rate ~ 0.2) numbers by H₂-pulse tracer experiments and kinetic studies, respectively, confirm presence of significant dispersive transport under reaction conditions and motivate development of a reaction-transport model to quantitatively examine the effects of catalyst-absorbent proximity through consideration of kinetic, diffusive, and convective length scales.

Simulation of zero-parameter reaction-transport models demonstrate introduction of zirconium metal rapidly consumes H₂ via absorptive metal hydride formation, generating large axial H₂ partial pressure gradients which are maintained by dispersive H₂ transport. The synergy between ZrH_x formation and H₂ dispersion suppresses thermodynamic limits to DHA catalysis and enhances net methane pyrolysis rates throughout the catalyst bed. Simulation results quantitatively predict effluent aromatic yield and H₂ partial pressure for all studied reactor configurations (i.e. staged catalyst-absorbent beds and interpellet physical mixtures).