Adsorption and diffusion of carbon dioxide on ZIF-68

• ZIF-68 crystals of hexagonal rod shape were synthesized.
• The molecular simulation results agree well with the experimental CO2 adsorption isotherms and heats of adsorption.
• The molecular simulation has identified a new site for CO2 adsorption on ZIF-68.
• Diffusion of CO2 on ZIF-68 crystals was studied by gravimetrical methods.

Zeolitic imidazolate framework ZIF-68 crystals of hexagonal rod shape were synthesized and used to study adsorption and diffusion of CO2 on ZIF-68. Grand canonical Monte Carlo (GCMC) calculations were performed to understand the CO2 adsorption sites on ZIF-68. Experimentally measured CO2 adsorption isotherms on ZIF-68 can be described by Langmuir adsorption isotherm. The molecular simulation results agree well with the experimental CO2 adsorption isotherms and heats of adsorption. The molecular simulation, coupled with the experimental isotherm data, has identified a new site between the faces of two converse nIM linkers, in addition to the two previously known sites for CO2 adsorption on ZIF-68. Diffusivity of CO2 on ZIF-68, obtained with a one-dimensional diffusion model, is in the range 1.1–1.6×10−7 cm2/s in 298–328 K, with an activation energy of 8.9 kJ/mol. The experimentally measured CO2 diffusivity data of ZIF-68 is about one order of magnitude smaller than that obtained by molecular dynamic simulation.