Combined GCMC, MD, and DFT Approach for Unlocking the Performances of COFs for Methane Purification

Abstract

Covalent organic frameworks (COFs) are promising materials for gas storage and separation; however, the potential of COFs for separation of CH 4 from industrially relevant gases such as H 2 , N 2 , and C 2 H 6 is yet to be investigated. In this work, we followed a multiscale computational approach to unlock both the adsorption- and membrane-based CH 4 /H 2 , CH 4 /N 2 , and C 2 H 6 /CH 4 separation potentials of 572 COFs by combining grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Adsorbent performance evaluation metrics of COFs, adsorption selectivity, working capacity, regenerability, and adsorbent performance score were calculated for separation of equimolar CH 4 /H 2 , CH 4 /N 2 , and C 2 H 6 /CH 4 mixtures at vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) conditions to identify the best-performing COFs for each mixture. Results showed that COFs could achieve selectivities of 2–85, 1–7, and 2–23 for PSA-based CH 4 /H 2 , CH 4 /N 2 , and C 2 H 6 /CH 4 separations, respectively, outperforming conventional adsorbents such as zeolites and activated carbons for each mixture. Structure–performance relations revealed that COFs with pore sizes <10 Å are promising adsorbents for all mixtures. We identified the gas adsorption sites in the three top-performing COFs commonly identified for each mixture by DFT calculations and computed the binding strength of gases, which were found to be on the order of C 2 H 6 > CH 4 > N 2 > H 2 , supporting the GCMC results. Nucleus-independent chemical shift (NICS) indexes of aromaticity for adsorption sites were calculated, and the results revealed that the degree of linker aromaticity could be a measure for the selection or design of highly alkane-selective COF adsorbents over N 2 and H 2 . Finally, COF membranes were shown to achieve high H 2 permeabilities, 4.57 × 10 3 – 1.25 × 10 6 Barrer, and decent membrane selectivities, as high as 4.3, outperforming polymeric and MOF-based membranes for separation of H 2 from CH 4 .

Computational investigation of multifunctional MOFs for adsorption and membrane-based separation of CF4/CH4, CH4/H2, CH4/N2, and N2/H2 mixtures

“…Dself;2 and S mem,12 = S ads,12 × S diff,12 , successively. 66 . All in all, these results suggest that MTV-MOFs that are not overly porous (i.e., PLDs <7 Å, surface areas <3000 m 2 g −1 ) can attain high adsorption-based CF 4 /CH 4 separation performances in terms of selectivity, working capacity, and regenerability.…”

“…Regenerability of an adsorbent is calculated as . 66 Since all of these metrics, selectivity, working capacity and regenerability, are important to identify the most promising MOF adsorbents, we defined the , where X i and X max denote the value of the individual performance metric X (selectivity/working capacity/regenerability) for material i and the highest value of the individual performance metric X across all materials, respectively. The individual separation performance scores were summed to determine the overall separation performance scores of MOFs and the materials with the highest overall separation performances were assigned the highest adsorbent rankings.…”

High-Throughput Screening of COF Membranes and COF/Polymer MMMs for Helium Separation and Hydrogen Purification

“…The insight gained from the computational screening of COFs for membrane applications could be of substantial importance for deciding on the next COF filler to be used in experimental MMM studies. Recent computational studies predicted gas separation properties of COF membranes. − For example, Yang et al demonstrated that COF-102, -103, -105, and -108 have higher selectivities (11.7, 14, 28.8, and 33.2, respectively) compared to IRMOF-1 and Cu-BTC for membrane-based H 2 /CH 4 separation at 10 bar and 298 K. 295 COF membranes were investigated for CO 2 /N 2 separation at 1 bar and 10 bar, at 298 K, and results showed that CO 2 permeabilities of COF membranes overcome those of MOF membranes, while MOF membranes have higher CO 2 /N 2 selectivities than COF membranes . A computational investigation on the H 2 /CO 2 separation performances of 288 COF membranes at 10 bar and 298 K has shown that COF membranes significantly outperform polymeric membranes due to their H 2 permeabilities ranging from 599 to 1.5 × 10 6 Barrer and H 2 /CO 2 selectivities of up to 4.74 .…”

“…H 2 /CO 2 separation performances of 794 hypoCOF membranes were shown to be even superior, with H 2 permeabilities in the range of 9 × 10 5 –5 × 10 6 Barrer and H 2 /CO 2 selectivities between 2.66 and 6.14 . A total of 572 COFs were studied for membrane-based H 2 /CH 4 separation at 1 bar and 298 K, and several COFs with large porosities showed a good combination of H 2 permeability (>10 5 Barrer) and H 2 /CH 4 selectivities up to 4.6 . In contrast to COF membranes, a very limited number of studies, only two, focused on computational modeling of COF/polymer MMMs.…”

“…36 A total of 572 COFs were studied for membrane-based H 2 /CH 4 separation at 1 bar and 298 K, and several COFs with large porosities showed a good combination of H 2 permeability (>10 5 Barrer) and H 2 / CH 4 selectivities up to 4.6. 37 In contrast to COF membranes, a very limited number of studies, only two, focused on computational modeling of COF/polymer MMMs. Twentynine COFs were computationally evaluated as fillers in three polymers for CO 2 /CH 4 separation, and several MMMs were found to be above the upper bound due to the increase in their CO 2 permeability up to ∼1900 Barrer and CO 2 /CH 4 selectivity up to ∼45.…”