Solvent Polarity Engineering in Low-DMF ZIF-7 Membrane Growth: Crystallization Behavior, Heterogeneous Intergrowth, and Microstructural Evolution

Molecular transport membranes are promising alternatives to conventional cryogenic separation processes. Here, solvent polarity effects were investigated by varying the DMF/MeOH ratio during the solvothermal synthesis of supported ZIF-7 membranes. A DMF:MeOH ratio of 1:3 preserved the characteristic sodalite topology while suppressing dense-phase formation. Methanol incorporation modified heterogeneous crystallization behavior, intercrystalline organization, membrane morphology, and film densification on α-alumina supports while reducing DMF consumption by approximately 75%. These effects are associated with solvent-mediated precursor solvation, Zn2+–benzimidazole coordination equilibria, and heterogeneous nucleation at the support–solution interface. Although low BET surface areas were obtained from N2 adsorption at 77 K, these values were interpreted cautiously considering the known limitations of nitrogen physisorption in flexible ultramicroporous frameworks. Overall, the results support solvent polarity engineering as a physicochemical strategy for regulating membrane microstructural evolution under reduced DMF conditions. Accordingly, the transport behavior discussed herein is interpreted primarily from a solvent-mediated microstructural perspective rather than as a direct quantitative descriptor of accessible porosity.