Metal–Organic Frameworks for CO2 Storage and CO2/CH4 Separation
Ravichandar Babarao, National University of Singapore, Singapore, 117576, Singapore and Jianwen Jiang, Chemical land Biological Engineering, National University of Singapore, Singapore, Singapore.
By varying the metal center, organic linker, functional group and framework topology, a series of MOFs (IRMOF1, Mg-IRMOF1, Be-IRMOF1, IRMOF3-(NH2)4, IRMOF10, IRMOF13, IRMOF14) are systematically examined for CO2 storage using Monte Carlo simulations. The affinity with CO2 is enhanced by adding functional group, the constricted pore is formed by interpenetrating framework; both lead to a larger isosteric heat and Henry constant and subsequently a stronger adsorption at low pressures. The organic linker plays a critical role in tuning the free volume and accessible surface area, and largely determines CO2 adsorption at high pressures. As a combination of open framework and low density, IRMOF10 and IRMOF14 exhibit higher capacity than other MOFs and even surpass the experimentally reported highest capacity in MOF-177. The gravimetric and volumetric capacities at high pressures correlate well with the framework density, free volume, porosity and accessible surface area of MOFs. These molecular-based structure-function correlations are useful for a priori prediction of CO2 capacity and for rational screening of MOFs toward the high-efficacy CO2 storage. Using Monte Carlo and molecular dynamics simulations, separation of CO2 and CH4 mixture in three different nanomaterials, namely, silicalite, C168 schwarzite and IRMOF-1 is compared. The permselectivity based on adsorption and self-diffusivity in the mixture is marginal in IRMOF-1 and greatest in C168 schwarzite. Although IRMOF-1 has the largest storage capacity for CO2, its selectivity is not satisfactory.