Ranking of MOF adsorbents for CO2 separations: a molecular simulation study

Abstract

Identifying materials that can efficiently separate CO2 from natural gas (CO2/CH4), power-plant flue gas (CO2/N-2), and petroleum refinery gas streams (CO2/H-2) is crucial. We used molecular simulations to examine the adsorption-based separation performances of MOFs in the separations of CO2/CH4, CO2/N-2, and CO2/H-2 mixtures under different operating conditions. We first compared the results of our molecular simulations with the experimentally available data for the CO2 adsorption and separation performances of various MOFs. Motivated by the good agreement between simulations and experiments, we extended our simulations to 100 different MOF materials. Several adsorbent evaluation metrics including selectivity, working capacity, adsorption figure of merit, sorbent selection parameter, and percentage regenerability were computed for each MOF and for each gas separation. The rankings of the MOFs based on these metrics were examined in detail to understand which parameters play key roles in assessing the gas separation potential of MOF adsorbents. The results showed that regenerability is a very important metric for screening materials in the first step of the adsorbent search and MOFs can then be ranked according to selectivity. We also examined the relationships between easily computable structural properties of MOFs, such as pore size, surface area, and porosity, and adsorbent evaluation metrics to provide structure property relationships that can serve as a guide for experimental studies. Materials with pore sizes of 4-7 A, surface areas of 200-800 m(2)/g, and porosities of 0.18-0.50 were found to be the best adsorbent candidates for CO2/CH4, CO2/N-2, and CO2/H2 separations. Finally, the kinetic-based separation potentials of the MOFs that were identified as the top-performing materials for adsorption-based separations were analyzed. Both the membrane selectivities and the permeabilities of the MOFs were computed for three gas separation processes. Several MOFs were identified to outperform polymers and zeolites in membrane-based CO2 separations.