Grand-Canonical Monte Carlo and Molecular-Dynamics Simulations of Carbon-Dioxide and Carbon-Monoxide Adsorption in Zeolitic Imidazolate Framework Materials

The nature of guest binding sites and selectivity in the zeolitic imidazolate framework (ZIF) 68 and 69 structures are studied by grand-canonical Monte Carlo and molecular-dynamics simulations. We have used the force field of Liu and co-workers to simulate CO2 and CO adsorption isotherms in the ZIF 68 and 69 structures at 273 K for pressures up to 1 atm. The experimental CO2 adsorption isotherms are reproduced with good accuracy, but the same force-field protocol overestimates the CO adsorption isotherms. In addition to the total guest adsorption in the unit cell of the ZIF structures from the adsorption isotherms, we determined the relative distribution of the guests in the pores and channels. The nature and number of the binding sites in the channels and pores are determined from the guest probability distributions in the ZIF structures. We find that the CO2 molecules associate strongly with the benzene rings of the benzimidazolate anion. The binding energies in the pores and channels for different numbers Of CO2 and CO guests have been calculated from molecular-dynamics simulations. As expected, the binding energies of the CO guests in the lattice are much smaller than those of CO2 We have also studied the diffusion of the CO2 and CO guests in the pores and channels of the two ZIF structures. The size of the guest and the aperture size in lattice affect the diffusion rates considerably, and the guests do not diffuse through the pores and channels at equal rates.