Effect of pore topology and accessibility on gas adsorption capacity in zeolitic-imidazolate frameworks: Bringing molecular simulation close to experiment

When all cages are assumed to be accessible, popular force fields such as universal force field (UFF) and DREIDING dramatically overpredicted gas adsorption capacity in two widely studied zeolitic-imidazolate frameworks (ZIFs), ZIF-68 and -69. Instead of adjusting the force-field parameters to match the experiments, herein we show that when the pore topology and accessibility are correctly taken into account, simulations with the standard force fields agree very well with the experiments. Careful inspection shows that ZIF-68 and -69 have two one-dimensional channels, which are not interaccessible to gases. The small channel consists of alternating small (HPR) and medium (GME) cages, while the large channel comprises the large (KNO) cages. Our analysis indicates that the small channel is not accessible to gases such as CO2. So when the cages in the small channel are intentionally blocked in our simulation, the predicted adsorption capacities of CO2, CH4 and N2 at room temperature from standard force-field parameters for the framework show excellent agreement with the experimental results. In the case of H2, all cages are accessible, so simulation results without cage-blocking show excellent agreement with experiment. Due to the promising potential of ZIFs in gas storage and separation, our work here shows that pore topology and accessibility should be carefully examined to understand how gases adsorb in ZIFs.