An NMR study on the adsorption and reactivity of chloroform over alkali exchanged zeolites X and Y

Abstract

Multinuclear solid state NMR has been applied to investigate the use of chloroform as a probe molecule to characterise the basic properties of zeolites. The characteristics of the ¹H signals of adsorbed chloroform are consistent with highly mobile molecules leading to an average chemical shift that reflects the interaction of the molecule with the host zeolite through hydrogen bonding with basic framework oxygen atoms. The ¹H chemical shift of CHCl₃ adsorbed on alkali exchanged zeolites X and Y, as well as on other less basic zeolites with different topology and zeotypes, correlates with the mean negative charge over the framework oxygen as calculated by the method of Sanderson, and agrees with the results derived from the use of chloroform as an infrared probe molecule. The use of ¹³C enriched ¹³CHCl₃ has allowed us to establish that the ¹³C chemical shift position is also a measure of framework basicity for zeolites X and Y. The changes observed on the ²³Na magic angle spinning (MAS) NMR spectra with the amount of adsorbed chloroform are consistent with the decrease in the quadrupolar coupling constants of sodium at sites SII and SI′.

¹³C MAS NMR of ¹³CHCl₃ shows that this molecule decomposes at room temperature over alkali exchanged zeolite X to give small amounts of dichloromethoxy species bound to the framework. Further reaction at 423 K gives rise to the formation of ¹³CO accompanied by framework destruction. These processes also occur over alkali exchanged zeolite Y but much more slowly, and the reactivity does not only depend on the zeolite basicity.

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