Pseudopolymorphisms of Zeolitic Imidazolate Frameworks and Their Application in Gas Separation

This work reports on the synthesis and stability of a polymorphic system of a metal-organic framework (MOF) composed of zinc and 2-methylimidazole, as well as its potential applicability in gas storage/separation. Three polymorphs, ZIF-8, ZIF-L, and dia(Zn) are discussed in this work. It was found that the synthesis of dia(Zn) with a crystal morphology of hexagonal nanosheets requires a catalyst (NH4OH, CH3COOH, or HCOONa), and a synthesis temperature of 60 °C. In contrast, the synthesis of ZIF-8 and ZIF-L can be conducted in the absence of a catalyst and at room temperature. This suggests that the activation energy of dia(Zn) exceeds that of ZIF-8 and ZIF-L. The three crystals were subjected to hydrothermal treatment at 100 °C to evaluate their stability. The ZIF-8 presented the highest hydrothermal stability, whereas ZIF-L presented the lowest. Nitrogen physisorption tests performed at 77K suggests that the microporosity of ZIF-8 exceeds that of ZIF-L and dia(Zn), which were nearly nonporous. Interestingly, CO2 thermogravimetric analysis revealed that the CO2 adsorption of ZIF-L and dia(Zn) at 323.15K is on par with that of ZIF-8, which implies that the flexibility of ZIF-L and dia(Zn) framework increased considerably with temperature. Tuning the flexibility of the framework is another effective approach to the design of new MOF materials for molecular separation. A novel methodology involving the use of pseudopolymorphic seeding for the rational synthesis of highly hydrogen-selective hybrid membranes with a zeolitic imidazolate framework (ZIF) was reported. A proof-of-concept was demonstrated using two-dimensional layered ZIF-L as seed crystals for the growth of its pseudopolymorph ZIF-8 in the formation of ZIF-L@ZIF-8 hybrid membranes. This approach enables the incorporation of ZIF-L (with high hydrogen diffusivity) within the ZIF-8 matrix with a volume fraction of ZIF-L of approximately 28%. Compared with conventional secondary growth methods used in the synthesis of pure ZIF-8 membranes, we employed leaf-like ZIF-L with a high aspect ratio as seed crystals for the growth of ZIF-8 membranes with a preferred orientation along the <100> direction. Compared to pure ZIF-8 membranes, the ZIF-L@ZIF-8 hybrid membranes enable a three-fold enhancement in hydrogen permeability and increase the permeative selectivity of hydrogen-over-carbon dioxide from 2.3 to 4.7. Simulation of mass transfer at the microscopic level was used to elucidate the reasons for the enhanced performance of the membrane in gas separation. We determined that the interlayer spacing among ZIF-L crystals, which allows for the rapid diffusion of hydrogen, is probably the key reason for the high separation performance of the ZIF-L@ZIF-8 hybrid membranes.