Interaction of hydrogen and carbon dioxide with sod-type zeolitic imidazolate frameworks: a periodic DFT-D study

Dispersion-corrected density-functional theory (DFT-D) calculations are used to study the interaction of hydrogen and carbon dioxide with ZIF-8, a prototypical zeolitic imidazolate framework (ZIF) with sodalite topology. Four distinct adsorption sites are identified for each of the two guest species. Two of the sites are associated with the six-ring windows, a third site is located close to one imidazolate moiety, and the fourth site is situated in the close proximity of two methyl substituents of the methylimidazolate linkers. For the case of hydrogen, where experimental data are available, the positions and the energetic ordering obtained in the DFT-D calculations agree well with these data. The investigation is then extended to two groups of isostructural systems. The first group consists of two boron imidazolate frameworks (BIFs), in which the tetrahedrally coordinated atoms (T atoms) differ from those in ZIF-8, while the methylimidazolate linker remains the same. The calculations show that the nature of the T atoms has only a very limited effect on the interaction with the guest molecules. The second group of derivatives comprises four systems that incorporate the same T atom as ZIF-8 (zinc), but linkers with different substituents X, with X = -H, -NO2, -NH2, -CHO. In these cases, the interaction with CO2 and, to a much lesser extent, hydrogen is increased at the adsorption site that is associated with the substituents, most prominently in the nitro-and aldehyde-functionalised systems. A detailed analysis of the adsorption geometries is used to explain the favourable effect of the substituents. Furthermore, it is shown how a reasonable estimate of the average interaction energy can be obtained from a weighted average over the different adsorption sites, accounting for their possible occupancy. In the case of ZIF-8, this averaged value is compared to experimental heats of adsorption, and the deviations are discussed. Finally, possible applications in hydrogen storage and CO2/H-2 separation are discussed. All materials show similar affinities for hydrogen, indicating that their performance in H-2 storage applications is largely independent of the structural modifications considered. Because the CO2/H-2 selectivity is related to the difference in affinity towards the two species, it can be expected from the DFT-D results that the nitro-and aldehyde-functionalised systems will perform considerably better than ZIF-8, especially for the removal of relatively small amounts of carbon dioxide from a hydrogen feed. This finding is particularly encouraging as both systems are synthetically accessible.