Understanding Gas Adsorption Selectivity in IRMOF-8 Using Molecular Simulation

Abstract

Grand canonical Monte Carlo simulations were used to explore the adsorption behavior of methane, ethane, ethylene, and carbon dioxide in isoreticular metal-organic frameworks, IRMOF-1, noninterpenetrated IRMOF-8, and interpenetrated IRMOF-8. The simulated isotherms are compared with experimentally measured isotherms, when available, and a good agreement is observed. In the case of IRMOF-8, the agreement is much better for the interpenetrated model than for the noninterpenetrated model, suggesting that the experimental data was obtained on an essentially interpenetrated structure. Simulations show that carbon dioxide is preferentially adsorbed over methane, and a selective adsorption at low pressures of ethane over ethylene, especially in the case of IRMOF-8, confirm recent experimental results. Analysis of simulation results on both the interpenetrated and the noninterpenetrated structures shows that interpenetration is responsible for the higher adsorbed amounts of ethane at low pressures (<100 kPa) and for the interesting selectivity for ethane in ethane/ethylene binary mixtures. Van der Waals interactions seem to be enhanced in the interpenetrated structure, favoring ethane adsorption. This indicates that interpenetrated MOF structures may be of interest for the separation of small gas molecules.

The right isotherms for the right reasons? Validation of generic force fields for prediction of methane adsorption in metal-organic frameworks

“…Although DREIDING has been shown to significantly over-estimate methane adsorption when compared to experiment in ZIF-8 [23], very good agreement was observed in the case of UiO-66(Zr) [22] (~9-20% over-estimation for θ < 0.5) and in the present work, while simulations in IRMOF-1 and IRMOF-6 [12] and in IRMOF-8 [48] found excellent quantitative agreement with experiment (5-10% difference when averaged over the full isotherm). While there are too many variables to fully rationalise these apparent differences in quantitative agreement (e.g.…”

“…1) are representative of the trends observed in all seven systems. As has been noted previously in the case of covalent organic frameworks [20] and IRMOF-8 [48], UFF tends to significantly over-estimate adsorbed amounts compared to DREIDING, by between 15 and 50%. The OPLS-AA force field performs very similarly to UFF, over-estimating compared to DREIDING by between 8% and 40%.…”

“… While OPLS-AA performed well in predictions of gas-ligand binding, it is likely to over-estimate gas adsorption in MOFs which use carboxylic acid as a coordinating group and did not offer a significant improvement over UFF or DREIDING in the prediction of adsorption isotherms.  DREIDING offered the best performance of the three tested force fields in the prediction of adsorption isotherms in all the systems considered in this study, and in literature studies in which more than one force field was evaluated [23,48,57]. It also performed very well in the prediction of gas-ligand interactions and, as it considers each element to have only one set of LJ parameters, has the advantage of being easily implemented in simulations.…”

“…Of these, a similar force field evaluation has been previously undertaken for IRMOF-8 [48]. For each MOF, methane adsorption isotherms were simulated using UFF, OPLS-AA and DREIDING force fields (see SI).…”

Computational screening of MOFs for C 2 H 6 /C 2 H 4 and C 2 H 6 /CH 4 separations

“…Pillai et al (Pillai et al, 2015) recently used GCMC simulations to explore C 2 H 6 /C 2 H 4 separation in two isoreticular MOFs, non-interpenetrated IRMOF-8 and interpenetrated IRMOF-8. Their results showed that C 2 H 6 selectivities are less than 2 at 10 bar, 298 K. (Chung et al, 2014) was used in addition to 45 well-known MOFs taken from our previous study (Sezginel et al, 2015) to cover widely studied subfamilies such as ZIFs, covalent organic frameworks (COFs), bioMOFs.…”