New Approaches of Metal-Organic Frameworks toward Hydrogen Isotope Separation

Abstract

Metal-organic frameworks (MOFs) have attracted great attention in gas storage and separation over the past decades, owing to their high level of porosity and chemical and structural tunability. Recent increase in global demand for isotopic gases, particularly deuterium, have triggered the researches to separate physico-chemically almost identical isotopic mixture with MOFs by tuning the pore size or introducing open metal sites (OMSs) as strong binding site. Nonetheless, the difficulty of optimizing the pore size to the sub-Angstrom scale and insufficient separation efficiency of OMSs limits the synthesis of MOFs with high deuterium-selectivity. Therefore, there is a critical need for new approaches utilizing MOFs in hydrogen isotope separation. In this talk, we present two novel strategies toward well-designed MOF-based separation systems that can effectively separate deuterium from hydrogen isotopic mixture. The first strategy exploits the dynamic pore change during the breathing of a flexible MOF, MIL-53(Al), for the high selectivity for D2 over H2. During the flexible and reversible transition, the pore apertures are continuously adjusted, thus providing the tremendous opportunity to separate mixtures of similar-sized and similar-shaped molecules that require precise pore tuning. The other strategy is the simple post-modification of introducing imidazole molecules into the highly dense OMS channels of MOF-74 to optimize the aperture size and increase the internal binding energy, simultaneously. It can synergistically maximize the efficiency of deuterium separation, having a high uptake with the highest reported separation factor as high as ∼26 at 77 K. We believe that our results will provide new opportunities for the intelligent design of porous aterials leading to the development of other highly efficient isotope and gas separation systems.