Controlling kinetic and diffusive length-scales during absorptive hydrogen removal in methane dehydroaromatization on MoC_x/H-ZSM-5 catalysts

Addition of Zr metal absorbent to MoC_x/H-ZSM-5 in the form of staged-bed, stratified-bed, and interpellet physical mixtures effectively scavenges H₂ from catalyst proximity, enhancing maximum single-pass benzene + naphthalene yield during methane dehydroaromatization (DHA) reactions to 14–16% compared to 8% in formulations without zirconium. The coupling of spatially-distinct catalytic and absorptive functions is achieved by dispersive/diffusive transport which conveys H₂ to staged Zr both co- and counter-current to bulk advection, thereby suppressing axial H₂ partial pressure profiles along the catalyst bed and enhancing net aromatization rates. We evince hitherto unreported significance of dispersive hydrogen transport during methane DHA by measurement of Péclet number, Pe = 1.32, in H₂ tracer studies with step-change or impulse input to inert catalyst proxies. Kinetic limits to methane pyrolysis are quantified by Damköhler number, Da, for synthesis of benzene, Da_B = 0.15, and naphthalene, Da_N = 0.03, determined from kinetic studies which rigorously account for reversibility of DHA reactions. Detailed reaction-transport models synthesize interplay of kinetic, diffusive, and convective length-scales captured by Péclet and Damköhler number to predict influence of catalyst-absorbent proximity and process flow-conditions on aromatization rates. Systematic control of catalyst bed-length, L, or linear flow velocity, u, predictably alters Pe and Da to effect improvements in methane conversion with and without Zr metal, corroborating results from simulation of the reaction-transport model.