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

Multinuclear solid state NMR has been applied to investigate the use of chl oroform as a probe molecule to characterise the basic properties of zeolite s. The characteristics of the H-1 signals of adsorbed chloroform are consis tent with highly mobile molecules leading to an average chemical shift that reflects the interaction of the molecule with the host zeolite through hyd rogen bonding with basic framework oxygen atoms. The H-1 chemical shift of CHCl3 adsorbed on alkali exchanged zeolites X and Y, as well as on other le ss 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 chlorofo rm as an infrared probe molecule. The use of C-13 enriched (CHCl3)-C-13 has allowed us to establish that the C-13 chemical shift position is also a me asure of framework basicity for zeolites X and Y. The changes observed on t he Na-23 magic angle spinning (MAS) NMR spectra with the amount of adsorbed chloroform are consistent with the decrease in the quadrupolar coupling co nstants of sodium at sites SII and SI'. C-13 MAS NMR of (CHCl3)-C-13 shows that this molecule decomposes at room te mperature over alkali exchanged zeolite X to give small amounts of dichloro methoxy species bound to the framework. Further reaction at 423 K gives ris e to the formation of (CO)-C-13 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.