Carbon-13 NMR study of pyridinium ion formation in zeolites

Abstract

By means of highly resolved carbon-13 nmr spectroscopy, Broensted acid sites in zeolites were studied via the formation of pyridinium ions. This method allows a direct determination of the number of accessible acidic OH groups from the carbon-13 resonance shifts in those systems where a fast exchange occurs between pyridine molecules and pyridinium ions. As a basis for this analysis, samples with pyridinium ions were produced by HCl addition to normally non-acidic zeolites. The number of acidic OH groups obtained for HNaY and CaNaY type zeolites are in good agreement with results from the application of ir spectroscopy. Additionally, by means of nmr methods, dynamic properties of the pyridine protonation can be studied. If the number of pyridine molecules is less than the number of acidic OH groups, the thermal mobility of the pyridinium ions is strongly reduced. As already well-known from infra-red spectroscopy, from this behaviour it follows that there is a strong interaction between Broensted acid sites and pyridine molecules. This conclusion is also in accordance with results of proton nmr measurements concerning the mobility of hydroxyl protons. Their increased mobility under the influence of adsorbed pyridine molecules has been interpreted by the action of pyridine molecules as a “transport vehicle” for the hydroxyl protons.