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

Citation
Ej. Munson et al., AN IN-SITU SOLID-STATE NMR-STUDY OF THE FORMATION AND REACTIVITY OF TRIALKYLONIUM IONS IN ZEOLITES, Journal of physical chemistry, 97(28), 1993, pp. 7321-7327
Citations number
34
Categorie Soggetti
Chemistry Physical
ISSN journal
00223654
Volume
97
Issue
28
Year of publication
1993
Pages
7321 - 7327
Database
ISI
SICI code
0022-3654(1993)97:28<7321:AISNOT>2.0.ZU;2-J
Abstract
In situ C-13 and Se-77 magic angle spinning (MAS) NMR was used to inve stigate the formation and reactivity of trialkylonium species on zeoli te catalysts HZSM-5 and HY. Trimethyloxonium. trimethylsulfonium, and trimethylselenonium ions were all formed on HZSM-5 by adsorption of th e corresponding dimethyl chalcogenide. The identity of these species w as confirmed by comparison to solution-state chemical shifts as well a s to an authentic sample of trimethylsulfonium-ZSM-5. Trimethyloxonium formed only on the strongly acidic zeolite HZSM-5 whereas trimethylsu lfonium formed on both HZSM-5 and the less acidic HY. This result sugg ests that onium ions may be useful for measuring the strengths of Bron sted acid sites in catalysts. Unlike trialkyloxonium ions, trialkylsul fonium ions were stable at high temperatures and in the presence of al cohols The adsorption of more than 1 equiv per acid site of dimethyl s ulfide poisoned the catalyst by titration of the Bronsted acid sites. With less than 1 equiv per acid site of dimethyl sulfide, the remainin g sites were active for MTG chemistry, but the trimethylsulfonium was not consumed in the formation of hydrocarbons. These results are inter preted in terms of onium-ylide mechanisms proposed for methanol-to-gas oline chemistry on HZSM-5. In situ C-13 MAS NMR studies of dimethyl et her at various loadings reconcile these studies with a previous in sit u 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 observ ed as the sample was heated. Trimethyloxonium formed readily at higher dimethyl ether loadings (e.g., 2 equiv per acid site).