Unlocking large-area free-standing MOFglasses for molecular sieving gas separation membranes

Membranes fromMOF glasses hold significant promise for gas separations due to the absence of grain boundary diffusion, liquid processibility, and tunability. The inherent high viscosity of MOF melts renders them prone to cracking and further handling, and their propensity to densify at high temperatures and long time in molten state severely limits their upscaling potential. A solution to overcome these limitations is demonstrated by selecting suitable materials that fit the thermal and mechanical behaviour to MOF-glass, enabling processing and making of large, crack-free MOF-glass sheets. This is demonstrated on the example of the well-known MOF-glass former ZIF-62. By optimizing each step of the process – from melting to performance testing – we successfully fabricate a crack-free, self-supported ZIF-62 glass membrane. The microstructure is investigated using microscopy as well as SEM-EDX analysis, confirming homogeneous boundary-freeMOF-glass, while gas permeation experiments prove the applicability of MOF-glass as gas separation membrane. The membrane exhibits exceptionally sharp methane molecular sieving cut-off with such low permeability that gas chromatography is unable to detect CH4.We conclude this work by giving a brief outlook of the remaining challenges and perspectives for MOF glasses, envisioning transferability of our approach to other glass-forming systems and their scaling perspectives.