Abstract Liquid‐phase chemical separations from complex mixtures of hydrocarbon molecules into singular components are large‐scale and energy‐intensive processes. Membranes with molecular specificity that efficiently separate molecules of similar size and shape can avoid phase changes, thereby reducing the energy intensity of the process. Here, forward osmosis molecular differentiation of hexane isomers through a combination of size‐ and shape‐based separation of molecules is demonstrated. An ultramicroporous carbon membrane produced with 6FDA‐polyimides realized the separation of isomers for different shapes of di‐branched, mono‐branched, and linear molecules. The draw solvents provide the driving force for fractionation of hexane isomers with a sub‐0.1 nm size difference at room temperature without liquid‐phase pressurization. Such membranes could perform bulk chemical separations of organic liquids to achieve major reductions in the energy intensity of the separation processes.
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