The framework basicity of zeolites

Zeolites exchanged with metal cations can be considered as solid Lewis acid–base materials. Lewis acidity is associated with the exchangeable cations and Lewis basicity with the lattice oxygens. This Lewis basicity can be probed by following the spectral changes of adsorbed molecules such as CO2, pyrrole, methanol, methyl iodide and N2O4 and by DFT modeling. The best molecules are those which disproportionate into cations and anions. The anions immobilize the exchangeable cations. The cations adsorb onto the lattice oxygens of the zeolitic framework and probe the basicity of these oxygens. Basicity of zeolites can be described in the frame of conceptual DFT. Local hardness is not sensitive enough to describe basicity trends in zeolites. Local softness, approximated by the ratio of the local and global density of states, is found to be a good descriptor of framework basicity, but it has only been calculated on a cluster model. Also the Fukui function of the framework oxygens is a good basicity descriptor, but it is only calculated at the semi-empirical EEM level. Of the other parameters that have been proposed in the literature the minimum of the molecular electrostatic potential at the oxygen atoms describes the basicity trends correctly, whereas the charge on the oxygens is not a good basicity descriptor.