Long alkyl chain-containing organosilica/silicalite-1 composite membranes for alcohol recovery

A long alkyl chain-containing organosilica layer obtained from 1, 8-bis(triethoxysilyl)octane (BTESO) was used to modify silicalite-1 membranes via a sol−gel method to obtain bi-layered BTESO/silicalite-1 composite membranes on macroporous α-Al 2 O 3 supports for the recovery of diluted ethanol by pervaporation . Silicalite-1 zeolite membranes were hydrothermally synthesized using two seeding methods: those prepared using wet−rubbing seeding method exhibited larger separation factors than those obtained using the dip−coating method in the pervaporation of ethanol/water due to the thinner seed layer in the former. After modified with BTESO, the surface became smoother and denser with higher hydrophobicity , and the thickness of BTESO layer was approximately 300 nm on both types of membranes. In pervaporation, two types of silicalite-1 membranes unexpectedly showed similar performances after BTESO modification. Ethanol flux increased or was maintained while water flux decreased greatly for silicalite-1 membranes after BTESO coating with increasing sol concentrations, and a higher separation factor was attained without sacrificing ethanol flux. The long alkyl chains in BTESO and silanols reacted with hydroxyl groups in silicalite-1 provided flexibility and hydrophobicity for ethanol transport with minor resistance. The optimized 2.0% BTESO/silicalite-1 membranes synthesized using wet−rubbing method showed a maximum flux of 6.84 kg m −2  h −1 and separation factor of 24.6 with overall pervaporation separation index of 161 in the pervaporation of 5.0 wt% ethanol/water mixture at 60 °C.