Linear and Branched Alkanes Separation Through Conjugated-Polymer-Framework Membranes.

The separation of linear and branched alkanes is critically important for improving the research octane number (RON) and advancing the sustainable development of the petrochemical industry. However, owing to their nearly identical kinetic diameters and closely related physicochemical properties (e.g., solubility parameters and boiling points), membrane-based liquid-phase separation of such linear and branched alkanes remains highly challenging. An example is the n-heptane/iso-octane pair, which exhibits minimal differences in molecular size and physicochemical properties but a dramatic difference in RON. Herein, we report a conjugated-polymer-framework (CPF) membrane that enables molecular-level separation of linear and branched alkanes. The crystallized selective layer of CPF membrane features intrinsic sub-nanometer pores of ∼5.5 and 8.4 Å. With ethanol as an assisting agent, the membrane achieves a separation factor of 5.24 for n-heptane/iso-octane mixture, together with a total mixture permeance of 0.020 kg m-2 h-1 bar-1. This pronounced selectivity stems from ethanol-induced modulation of transmembrane transport, which selectively mitigates the transport barrier of linear alkanes while exerting negligible influence on their branched counterparts, thereby enhancing kinetic selectivity. Furthermore, the CPF membrane exhibits robust separation performance over extended operation and across a wide range of feed compositions and pressures, highlighting its robustness for linear/branched alkane separation.