(376bk) Mixed-Metal Influence on Purity of Process Performance for Metal-Organic Frameworks with Open Metal Sites for Olefin/Paraffin Separation

Metal-organic frameworks (MOFs) with open metal sites (OMS) are known to be selective for ethylene relative to ethane. Our previous work demonstrated that the species of metal played a more important role than the topology and linker in determining the binding energies of small molecules on these open dimer metal sites. It suggested mixed-metal strategy is promising to tune the binding affinity of guest molecule in bimetallic systems. Additionally, the combination of molecular simulation and process modeling is very attractive and powerful to identify the performance of adsorbents in ethylene/ethane separation. First, we developed a Langmuir model to predict the isotherms of both mixed-metal MOFs and pure MOFs with OMS, then used them to estimate their process performance. The widely-used force field based-GCMC simulations is inaccurate to predict the adsorption isotherm in OMS MOFs because it doesn’t account for the more localized interactions that can exist with OMS. In this work, we developed a dual-site Langmuir adsorption model to predict isotherms in OMS MOFs by dividing the total adsorption into chemisorption at OMS and physisorption at other sites. All model parameters were obtained by the combination of accurate density functional theory calculation and GCMC simulation. The prediction of a material process performance in practice is appealing. We developed an idealized PSA process to estimate their separation purity and further combined the isotherm model with the PSA process to identify whether mixed-metal MOFs with OMS is better than the pure MOFs with OMS. After a careful search, three pairs of mixed-metal MOFs have been demonstrated to improve the process purity. Additionally, the influence of the linker functionalization on the purity of mixed-metal MOFs with OMS were investigated through tuning binding energy. This work paves the way for more robust material design for ethylene/ethane separation.