Direct calorimetric measurement of enthalpy of adsorption of carbon dioxide on CD-MOF-2, a green metal-organic framework

The enthalpy of adsorption of CO₂ on an environmentally friendly metal-organic framework, CD-MOF-2, has been determined directly for the first time using adsorption calorimetry at 25 C. This calorimetric methodology provides a much more accurate and model-independent measurement of adsorption enthalpy than that obtained by calculation from the adsorption isotherms, especially for systems showing complex and strongly exothermic adsorption behavior. The differential enthalpy of CO₂ adsorption shows enthalpy values in line with chemisorption behavior. At near-zero coverage, an irreversible binding event with an enthalpy of -113.5 kJ/mol CO₂ is observed, which is followed by a reversible -65.4 kJ/mol binding event. These enthalpies are assigned to adsorption on more and less reactive hydroxyl groups, respectively. Further, a second plateau shows an enthalpy of -40.1 kJ/mol and is indicative of physisorbed CO₂. The calorimetric data confirm the presence of at least two energetically distinct binding sites for chemisorbed CO₂ on CD-MOF-2.