Pseudopolymorphic seeding for the rational synthesis of hybrid membranes with a zeolitic imidazolate framework for enhanced molecular separation performance

This paper reports a novel methodology involving the use of pseudopolymorphic seeding for the rational synthesis of hydrogen-selective hybrid membranes with zeolitic imidazolate frameworks (ZIFs). 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 permeation 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.