Isothermal activation of Mo2O52+–ZSM-5 precursors during methane reactions: effects of reaction products on structural evolution and catalytic properties

Abstract

The dynamics of carburization of Mo-oxo precursors exchanged onto H–ZSM-5 strongly influence initial induction periods and steady-state rates during catalytic pyrolysis of CH₄ to alkenes and arenes at 900–1000 K. The effects of co-reactants and of activating conditions were examined by on-line time-resolved mass spectrometric analysis of effluent streams using rigorous analyses to account for equilibrium effects on measured rates. Ethene co-reactants and the larger hydrocarbons to which it converts on acid sites in H–ZSM-5 led to much faster carburization of exchanged (Mo₂O₅)⁵⁺ dimers and to shorter induction periods than with pure CH₄ reactants, but steady-state pyrolysis rates were unchanged, indicating that CH₄ and C₂H₄ form similar MoC_x clusters during carburization of exchanged Mo-oxo precursors. H₂ treatment at 973 K before CH₄ reactions led to reduction of Mo⁶⁺ species to Mo⁴⁺, which carburize faster than (Mo₂O₅)⁵⁺ precursors during initial contact with CH₄. This H₂ pretreatment or the use of CH₄–H₂ reactant mixtures did not influence steady-state pyrolysis rates, once contributions from reverse reactions were taken into account. With pure CH₄ streams, (Mo₂O₅)⁵⁺–ZSM-5 converts to active MoC_x clusters within zeolite channels via autocatalytic processes, in which higher hydrocarbons, initially formed during initial conversion of MoO_x to MoC_x structures, lead to faster carburization of downstream catalyst sections. Concurrently, H₂O and CO₂ formed during this incipient carburization of exchanged (Mo₂O₅)⁵⁺ and unexchanged MoO₃ present in trace amounts inhibit and even prevent carburization and lengthen activation periods. Activation protocols with C₂H₄ were also successful in the activation of more refractory high-valent metal-oxo species, such as WO_x and VO_x, exchanged onto H–ZSM-5. The formation of active carbide structures occurred in less than 300 s, instead of 4 ks and 16 ks for VO_x and WO_x samples, respectively, in pure CH₄ reactants. These activation protocols led to VC_x–ZSM5 catalysts about three times more active than those activated in pure CH₄ reactants.