AN IN-SITU SOLID-STATE NMR-STUDY OF THE FORMATION AND REACTIVITY OF TRIALKYLONIUM IONS IN ZEOLITES

In situ C-13 and Se-77 magic angle spinning (MAS) NMR was used to investigate the formation and reactivity of trialkylonium species on zeolite catalysts HZSM-5 and HY. Trimethyloxonium. trimethylsulfonium, and trimethylselenonium ions were all formed on HZSM-5 by adsorption of the corresponding dimethyl chalcogenide. The identity of these species was confirmed by comparison to solution-state chemical shifts as well as to an authentic sample of trimethylsulfonium-ZSM-5. Trimethyloxonium formed only on the strongly acidic zeolite HZSM-5 whereas trimethylsulfonium formed on both HZSM-5 and the less acidic HY. This result suggests that onium ions may be useful for measuring the strengths of Bronsted acid sites in catalysts. Unlike trialkyloxonium ions, trialkylsulfonium ions were stable at high temperatures and in the presence of alcohols The adsorption of more than 1 equiv per acid site of dimethyl sulfide poisoned the catalyst by titration of the Bronsted acid sites. With less than 1 equiv per acid site of dimethyl sulfide, the remaining sites were active for MTG chemistry, but the trimethylsulfonium was not consumed in the formation of hydrocarbons. These results are interpreted in terms of onium-ylide mechanisms proposed for methanol-to-gasoline chemistry on HZSM-5. In situ C-13 MAS NMR studies of dimethyl ether at various loadings reconcile these studies with a previous in situ FTIR investigation by Forester and Howe (J. Am. Chem. Soc. 1987, 109, 5076). Trimethyloxonium ion formation on HZSM-5 is strongly loading dependent. At low loadings of dimethyl ether (e.g., 0.2 equiv per acid site), no trimethyloxonium formed, and protonation shifts were observed as the sample was heated. Trimethyloxonium formed readily at higher dimethyl ether loadings (e.g., 2 equiv per acid site).