Unlocking Large-Area Free-Standing MOF-Glasses for Molecular Sieving Gas Separation Membranes

Membranes prepared of MOF glasses holds significant promise for gas separations due to the absence of grain boundary diffusion, liquid processibility, and tunability. However, the inherent high viscosity of MOF melts renders them prone to cracking upon rapid cooling and further handling, and their propensity to densify at high temperatures and long time in molten state severely limits their upscaling possibilities. Here, we present a solution to overcome these limitations and demonstrate that - by selecting suitable materials particularly fitting thermal and mechanical behavior to those of MOF-glass – it is possible to process large, crack-free MOF-glass sheets. By carefully optimizing each step of the process – from melting to performance testing – we successfully fabricate a crack-free, self-supported ZIF-62 glass membrane from pristine ZIF-62 crystals. The microstructure is investigated using microscopy as well as SEM-EDX analysis, confirming homogeneous boundary-free MOF-glass, while gas permeation experiments are carried out to 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 in a view of gas separation membranes.