Gate Opening, Diffusion, and Adsorption of CO₂ and N₂ Mixtures in ZIF-8

Abstract

Diffusion and adsorption of CO 2 /N 2 mixtures in the zeolitic imidazolate framework ZIF-8 are investigated by molecular dynamics (MD) and Gibbs ensemble Monte Carlo (GEMC) simulations. Structural changes called "gate opening" could be found for the adsorbed single-component gases and for the mixture. The gate opening appears for the mixture at a total number of guest molecules per cavity between that for the pure CO 2 and that for the pure N 2 but closer to that of N 2 which is lower. Due to the stronger dependence of CO 2 adsorption upon the temperature in comparison with N 2 , the adsorption selectivity is predicted to be higher at lower temperatures, which is in accordance with experimental findings.

Methane in zeolitic imidazolate framework ZIF-90: Adsorption and diffusion by molecular dynamics and Gibbs ensemble Monte Carlo

“…The results showed a significant effect between rigid and flexible framework in the self-diffusion coefficient (D s ) and the flexible model using GAFF gives better results compared with the experiment. This influence of the flexibility on D s can also be found in other ZIFs [13][14][15] and MOFs [16] while the adsorption isotherm seems to be much less sensitive with respect to the lattice flexibility [17,18] as long as no structural phase transition like the so called gate opening [19][20][21] takes place. On the other hand, Zhang et al.…”

“…But also a possible influence of the adsorbed molecules on the lattice structure e.g. gate opening as found in [16,19,21] for other MOF's and ZIF's and guest molecules can be checked. In addition, a better understanding on the atomic level of the adsorption isotherm, diffusion mechanisms, or adsorption sites can be achieved.…”

“…[17,19]. A combination of GEMC and MD by DL_POLY was also applied in [18] where the different lattice structures for lower and higher pressures as obtained from flexible lattice MD were used for GEMC simulations.…”

“…A combination of GEMC and MD by DL_POLY was also applied in [18] where the different lattice structures for lower and higher pressures as obtained from flexible lattice MD were used for GEMC simulations. In contrary, in [17,19] the GEMC has been carried out only in pressure regions where no structural phase transition could be found in the corresponding MD. Hence, only one structure was of interest.…”

“…This technique has meanwhile been used in many papers e.g. in [19,31]. In the Gibbon software the Coulomb interactions are dropped down smoothly at a cutoff distance of 30Å.…”