Dealumination of HZSM-5 zeolites .2. Methanol to gasoline conversion

Changes in the structure and properties of HZSM-5 zeolites used for extended periods as catalysts in the conversion of methanol to gasoline have been investigated, HZSM-5 samples were characterized using solid-state NMR spectroscopy (Al-27 and Si-29 MAS), infrared spectroscopy; chemical analysis, water and nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, and Xe-129 NMR spectroscopy of adsorbed xenon. Use of HZSM-5 in methanol conversion was found to cause dealumination of the zeolite lattice. The catalyst lifetime, however, increased after repeated use in MTG cycles. The dealumination is caused by water produced during methanol conversion, although the extralattice aluminum species formed appear to differ from those produced by calcination or mild hydrothermal treatment. The increase in catalyst lifetime is attributed to a decreased rate of coke formation in catalysts containing lower densities of acid sites. (C) 1996 Academic Press, Inc.