A first principles study of gas adsorption on charged CuBTC

We examined the adsorption of selected guest molecules on the open Cu sites with different charge states in CuBTC by means of density functional methods. Our calculations revealed that CO and O2 are chemisorbed on the open Cu site, whereas CO2, H2 and CH4 are mainly physisorbed. As an exception, the adsorption nature of N2 is dependent on the charge state of the open Cu ion. The reduced positively charged Cu ion can significantly enhance binding strength of CO, O2 and N2 because of the chemisorptive nature. On the other hand, losing electrons from the open Cu ion can slightly improve the physisorption strength through electrostatic interactions. Therefore, it may be possible to enhance gas adsorption and control selective separation for specific gas mixtures by appropriately tuning charge state of the exposed metal in metal organic frameworks.