Probing the basicity of zeolite frameworks with N2O4: A DFT approach

The basicity of alkali exchanged faujasite type zeolites is estimated on the basis of DFT calculations of the NO+ stretching frequencies of adsorbed N2O4. N2O4 is allowed to interact with the alkali cations and a range of molecules covering a wide basicity scale. An alkali nitrate salt is formed and NO+ interacts with the basic molecules. The resulting NO+ stretching frequencies decrease with increasing proton affinity and basicity of the basic molecules for each alkali cation. If experimental proton affinities and basicities of the model molecules are used, the data fall on two straight lines, which are almost parallel. The zeolitic basicities and proton affinities are obtained by interpolation of the experimental NO+ frequencies of alkali exchanged faujasites on the linear plots of NO+ frequencies against the experimental proton affinities and basicities. With theoretical proton affinities and basicities two non-parallel straight lines are obtained and three molecules are offset. The reason for this is unclear and needs more investigation. The NO+ stretching frequency is linearly correlated with the hardness of the alkali cations in zeolite X. For zeolite Y the same trend is observed, but the scatter of the data is too large to define it as linear.