Adsorption of Propane and Propylene on CuBTC Metal-Organic Framework: Combined Theoretical and Experimental Investigation

A combined experimental and theoretical investigation of propane and propylene adsorption in the metal-organic framework CuBTC is presented. The dependence of adsorption enthalpies on the adsorbed amount was determined by microcalorimetry up to 8 mmol g(-1) coverage (roughly C3Hn/Cu2+ ratio of 1.5). Trends observed experimentally were interpreted on the basis of accurate calculations carried out at the hybrid DFT-ab initio level. Three types of adsorption sites were identified; however, qualitatively different results were obtained for propane and propylene. Propane preferentially adsorbs at the cage center sites (-Delta H degrees = 43 kJ mol(-1)), followed by adsorption at the cage window sites (31 kJ mol(-1)), while the interaction with the coordinatively unsaturated sites (CUS) is relatively weak (24 kJ mol(-1)). On the contrary, propylene preferentially interacts with the CUS (56 kJ mol(-1)), while the adsorption at the cage center and cage window sites was found to be only 45 and 34 kJ mol(-1), respectively. Due to the topology of CuBTC, lateral interactions are significantly more important among the adsorbates located at the cage center and cage window sites (populated in the case of propane) than among adsorbates at the CUS and cage center sites (populated in the case of propylene). Therefore, adsorption energies obtained for coverages above 6 mmol g(-1) of adsorbed amount were larger for propane than for propylene. Consequently, the presence of small cages makes the CuBTC MOF less suitable for propane/propylene separation than MOFs having the Cu2+ CUS but without small cages (e.g., CPO-27).