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
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).