Metal–organic framework (MOF) membranes offer great potential to address the energy penalties of energy-intensive gas separations1,2,3,4,5. Deriving convenient scalable protocols is essential for the successful translation and deployment of MOF-based membranes into practical applications6,7,8. Here we report a new membrane architecture, the quasi-pure MOF membrane (>90 vol% MOF), manufacturable at scale using industry-reliable solution-processing techniques and offering separation performance approaching that of associated pure MOF membranes, fully realizing the MOF intrinsic potential. This advance is enabled by a merged-phase approach fusing MOFs and polymers into a single pseudo-continuous phase and proffering MOFs with polymer-like surface properties, allowing rheologically controlled flocculated networks even at extreme solid concentrations. Representative benchmark MOFs (ZIF-67, CALF-20 and CuBDC) were fabricated into quasi-pure membranes and evaluated for essential separations, including propylene/propane, ethylene/ethane, carbon capture and hydrogen purification. Demonstrating industrial relevance, continuous roll-to-roll fabrication of quasi-pure (110)-oriented ZIF-67 membranes was achieved at an industrial plant. For propylene/propane separation, these membranes achieve a propylene permeability of about 160 barrer and mixed-gas selectivity of about 100—sufficient to produce polymer-grade propylene, offering an 80% purification cost reduction compared with distillation based on techno-economic analysis. This study addresses the long-standing gap between performance and scalability of crystalline membranes for demanding molecular separations.