Genesis of methane activation sites in Mo-exchanged H–ZSM-5 catalysts

Abstract

Exchanged (Mo 2O 7) 2− dimers form during treatment in air of MoO 3/H–ZSM-5 (Si/Al=14.3) physical mixtures at 773–973 K. The amount of water desorbed during exchange and the number of residual protons (measured by D 2–OH exchange) showed that each Mo 6+ replaces 1.1 (±0.1) protons in H–ZSM-5 (for Mo/Al<0.37). 27Al NMR, X-ray absorption, and Raman spectra confirmed the proposed (Mo 2O 7) 2− structure and its location at zeolite exchange sites. When the Mo content exceeds that required to form a MoO x bilayer on the external zeolite surface (Mo/Al≈0.5), MoO x sublimes or forms Al 2(MoO 4) 3 by extracting framework Al; Al 2(MoO 4) 3 domains reduce slowly and lead to low CH 4 reaction rates. The rate of hydrocarbon synthesis from CH 4 at 930–973 K increased as exchanged MoO x dimers reduce and carburize during CH 4 reactions. About 2.5 O atoms per Mo (±0.1) are removed as CO, CO 2, and H 2O during activation, suggesting that all but the zeolite framework oxygen atoms are removed during activation. Reduction and carburization rates depend on Mo content and on the rate of removal of CO, CO 2, and H 2O, which inhibit reduction/carburization steps. Hydrocarbons were not detected during the initial removal of about one O per Mo, but ethylene, benzene, naphthalene, and H 2 formation rates increased as larger amounts of O were removed during CH 4 reactions. CH 4 reactions require initial activation of CH 4 on MoO x , but with the retention of C-atoms to form MoC x . These MoC x then activate CH bonds and desorb the hydrocarbons formed, which then oligomerize and cyclize via rapid bifunctional pathways on H + and MoC x sites within constrained ZSM-5 channels.