A periodic DFT study of N2O4 disproportionation on alkali-exchanged zeolites X

Adsorption and disproportionation of dinitrogen tetraoxide on sodium-, potassium-, and rubidium-exchanged zeolites X with Si/Al ratio of 1.18 were studied using density functional theory calculations with periodic boundary conditions. It is found that the stabilization and activation of most of the N2O4 isomers confined in the zeolitic cage does not follow Lewis acidity difference of the extraframework cations. This is also observed for the energetics of the N2O4 disproportionation reaction resulting in formation of a space-separated NO+center dot center dot center dot NO3- ion pair. The reaction energy increases in the row NaX < RbX < KX. The strength of perturbations and, therefore, the low-frequency shift of the N-O stretching frequency of the adsorbed NO+ cations correlate well with the basicity of the zeolite (RbX > KX > NaX). However, this factor is not the relevant reactivity parameter for the N2O4 disproportionation in the cationic forms of zeolites. The higher activity for the disproportionation as well as the stronger molecular adsorption of N2O4 on RbX and KX zeolites as compared to that on NaX is ascribed to the features analogous to the molecular recognition characteristics of supramolecular systems. The steric properties of the zeolite cage and the mobility of the extraframework cations induced by adsorption are essential to shape the optimum configuration of the active site for N2O4 disproportionation.