Understanding CO2 Adsorption in CuBTC MOF: Comparing Combined DFT-ab Initio Calculations with Microcalorimetry Experiments

A combined experimental and theoretical investigation of CO2 adsorption in the metal organic framework CuBTC is presented. Adsorption enthalpies were measured as a function of coverage up to 13 mmol g(-1) adsorbed amount (corresponding roughly to CO2/Cu = 5:2) by a Tian-Calvet-type microcalorimeter. Experimetal data are interpreted based on accurate calculations employing a combined DFT-ab initio computational scheme. CO2 molecules adsorb preferentially on coordinatively unsaturated sites for coverages below CO2/Cu = 1:1; at higher coverages (up to CO2/Cu = 5:3), CO2 adsorbs in cage window sites; and at higher coverages, the sites in cage centers and in large cages start to be occupied. Experimental adsorption enthalpies are almost constant (-29 kJ mol(-1)) up to the CO2/Cu = 5:3 coverage, suggesting a homogeneity of adsorption sites. However, calculations clearly show that adsorption sites in CuBTC are rather heterogeneous. The experimentally observed independence of adsorption enthalpies with respect to coverage is due to the cancellation of two effects: the decrease in the adsorbate adsorbent interaction is compensated by an increase in the lateral interactions.