In-situ FTIR studies of dimethyl ether adsorption on H-ZSM-5 zeolites

In situ infrared spectra of dimethyl ether adsorbed on two H-ZSM-5 zeolites are reported which are different in both lattice and extraframework aluminium contents. A number of dissociatively adsorbed dimethyl ether species associated respectively with the Bronsted acid sites, the external silanol groups and extraframework aluminium species at different temperatures are identified. In particular, dimethyl ether reacts with non acidic hydroxyl species associated with extraframework aluminium to form the third methoxy species. The distributions of hydrocarbons desorbed from a single pulse of dimethyl ether (1.0 mL) onto these zeolite samples are presented. Propene is the major product, and the yield of hydrocarbons correlates with the concentration of Bronsted acid bound methoxy groups.