The metal-organic framework (MOF) membrane has been widely studied in gas separation applications. However, less attention was paid to organic liquids separation, particularly for the separation of straight and branched alkane isomers which is critical to producing high-octane gasoline. Our work is the first example of a dense and defect-free Al-bttotb membrane synthesized on α-Al₂O₃ support surface through solvothermal in-situ synthesis. The microstructure of the membrane surface was adjusted by controlling the growth rate of the crystal facet through the addition of formic acid during synthesis. The resultant membrane showed great thermal stability, long-term stability, and chemical stability due to the bonding reaction between the ligands and the support surface. The pervaporation test showed that Al-bttotb membrane preferentially permeated straight and mono-branched alkane. The mass fraction of n-hexane (nHEX, straight) and 3-methylpentane (3MP, mono-branched) was accumulated from 66.66% on the feed side to 88.53% on the permeate side after 20 h of the test, while a mass fraction of 2,2-dimethylbutane (22DMB) decreased from 33.33% to 11.47%. This work sheds new light on the separation of organic liquids through MOF polycrystalline membrane.