Tuning the adsorption and separation properties of noble gases and N2 in CuBTC by ligand functionalization

Abstract

The Grand Canonical Monte Carlo method was used to investigate the adsorption and separation properties of noble gas and N₂ mixtures on a MOF material, namely, CuBTC, functionalized with different groups, including amino, hydroxyl and fluorine groups, in order to understand the potential applications of the materials in noble gas separation. Binary equimolar mixtures of Xe/Ar, Xe/Kr and Kr/Ar as well as nonequimolar mixtures of Xe/N₂ and Kr/N₂ containing 0.01% (molar fraction) noble gas were examined. Amino-functionalized CuBTC displayed an attractive interaction with all the noble gases due to the high polarity of amino-functionalized benzene and its large van der Waals interaction with all the gases, whereas the hydroxyl groups exhibited a lower impact on the adsorption properties of CuBTC. On the other hand, functionalization with fluorine groups largely decreased the adsorption capacity for all the noble gases compared to the parent Cu-BTC. The functional groups also exhibited different effects on the adsorption selectivity to the noble gases over N₂. The amino-functionalized CuBTC showed enhanced selectivities towards specific noble gases over N₂ or the other noble gases. Hydroxyl and fluorine groups exhibited similar selectivities with the original CuBTC. The difference in the adsorption selectivity is mainly attributed to the enhanced electron delocalization and the polarizability of the aromatic ring due to the presence of amino groups, which comprise the best binding sites for the noble gases and N₂. According to the simulation results, it can be concluded that amino-functionalized CuBTC can enhance both the adsorption capacity and the selectivity of specific noble gases over N₂ or other noble gases at low pressure from 0 to 100 kPa at 292 K.