Grain boundary engineering of polycrystalline metal–organic framework membranes for gas separation

Polycrystalline metal–organic framework membranes (PMOFMs) feature well-defined molecular transport pore channels, high porosity, and tailorable frameworks, representing a new opportunity for gas separation membranes. For a few important gas separation pairs (e.g., C₃H₆/C₃H₈), PMOFMs have demonstrated the ability to compete with the existing separation technologies (e.g., cryogenic distillation) by showing their separation performance under the economically attractive criteria. As a further step in the application of PMOFMs to the field of gas separation in industry, the development of advanced fabrication techniques to achieve defect-free PMOFMs in large-area constructions is a key challenge. To this end, this review will outline and summarize the so far established strategies for the fabrication and microstructure manipulation of PMOFMs from the perspective of grain boundary engineering. Discussion and comparison between these strategies in controlling the grain boundary structure of PMOFMs or eliminating the grain boundary defects are presented. The link between the gas separation performance, especially for the selectivity, after these grain boundary engineering strategies is highlighted. Finally, we provide our perspective on future research and development in large-scale engineering applications of PMOFMs.