Switching Xe/Kr adsorption selectivity in modified SBMOF-1: a theoretical study

The separation of Xe/Kr mixtures in used nuclear fuel (UNF) has attracted lots of attention, but no report on the adsorption and separation of Kr from mixed Kr/Xe at room temperature can be found. From grand canonical Monte Carlo (GCMC) simulation, it is found that by replacing the metal center Ca of SBMOF-1 with Mg, due to the appropriate pore size, the adsorption selectivity (S _Kr/Xe) was extremely high (250 000) and the adsorption capacity for Kr on Mg-SBMOF-1 modified with -NH₂ was increased by 300% to 1.020 from 0.248 mmol g^-1. Based on the calculations of density functional theory (DFT), we found that the stronger electron-donating ability of a functional group will increase the polarizability of the ligand, and thus increase the adsorption capacity to Kr. In addition, the analysis of electronic structures with independent gradient model (IGM) and energy decomposition analysis (EDA) indicates that van der Waals forces will be responsible for the interaction of Mg-SBMOF-1 and Kr gas. Among them, the interaction of Mg-SBMOF-1 and Kr gas is mainly an induction force, while that of modifications with -CH₃ and -NH₂ is mainly a dispersion force. The present theoretical study represents the first report of the separation of Kr from Xe with MOF adsorption at room temperature. We hope this work may promote the experimental synthesis of Mg-SBMOF-1 for efficient separation of Kr and Xe.