J . Phys. Chem. 1993,97, 10732-10737
10732
Formation of Surface-Bonded Methoxy Groups in the Sorption of Methanol and Methyl Iodide on Zeolites Studied by I3C MAS NMR Spectroscopy V . BoshEek J. Heyrouskj Institute of Physical Chemistry and Electrochemistry, Czech Academy of Sciences, DolejSkoua 3, 18223 Prague 8, Czech Republic Received: December 2, 1992; In Final Form: June 1, 1993"
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 CHd. 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 13CMAS N M R signals of surface methoxyls in high magnetic field experiments.
surface-bonded methyl on zeolites of various composition and structure, under conditions excluding the formation of the protonated forms and dimethyl ether as interfering species. A promising system was found in catalytic reactions of methyl i0dide.I' The experiments leading to surface methylation were carried out using the substitution reaction of alkaline forms of zeolites and methyl iodide, according to the expected mechanism of the substitution reaction
Introduction
In the study of reactions involving simple alcohols using I3C MAS NMR, it was necessary to resolve signals in the spectra of adsorbed methanol in the relatively narrow region from +50 to +70 ppm TMS, which apparently contains the signals of both physically adsorbed methanol and its protonated forms, of surface methoxyls as well as of products of chemical reactions (e.g., dimethyl ether) and of molecules chemisorbed on various lattice sites. The literature contains a considerable amount of inforCHJ Na6+.0
