FORMATION OF SURFACE-BONDED METHOXY GROUPS IN THE SORPTION OF METHANOL AND METHYL-IODIDE ON ZEOLITES STUDIED BY C-13 MAS NMR-SPECTROSCOPY

Methanol and methyl iodide as methylating agents react chemically with the surface atoms of zeolites and, under favorable conditions, yield surface-bonded methoxy groups. Zeolites with well-defined structures of faujasites, mordenites and ZSM5-containing various cations (Na+, Rb+, Mg2+, Ca2+) and/or acid protons were investigated in chemisorption experiments using CH3I. The observed signals in the 53-59 ppm TMS region were assigned to the methoxy groups, i.e., methyl groups surface-bonded to the lattice oxygens. The chemical shift of these signals depends linearly on the equalized electronegativity of the oxygen of the given zeolite structure and becomes a measure of the lattice oxygen basicity (nucleophility). A new method has been proposed for basicity (nucleophility) detection of different lattice oxygens in zeolites, based on the C-13 MAS NMR signals of surface methoxyls in high magnetic field experiments.