The effects of silanation of external acid sites on the structure and catalytic behavior of Mo/H-ZSM5

The selective silanation of external acid sites in H-ZSM5 using large organosilane reagents was used to decrease the density of such sites and the number of MoOx species retained at external surfaces during cation exchange using MoO3/H-ZSM5 physical mixtures. On samples prepared using silica-modified H-ZSM5, acid sites, MoOx precursors, and the active MoCx species formed during CH4 reactions at 950 K reside predominately within zeolite channels, where spatial constraints inhibit bimolecular chain-growth pathways. As a result, these samples show higher selectivities to desired C-2-C-6 aromatics and lower deactivation rates than those prepared from untreated H-ZSM5. CH4 conversion rates are also higher on Mo/H-ZSM5 samples prepared from silanated H-ZSM5, because of the more complete exchange and higher dispersion of MoOx precursors. In situ X-ray absorption measurements showed that silanation of external OH groups does not influence the structure of the MoOx precursors formed during synthesis or of active MoCx species formed during CH4 reactions from exchanged (MO2O5)(2+) dimers. The substantial absence of acidic OH groups and of MoCx species sites at unconstrained external surfaces leads to significant improvements in activity, selectivity, and stability for Mo/H-ZSM5 catalysts for CH4 pyrolysis reactions. (C) 2002 Elsevier Science (USA).