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

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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′. 13 C 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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