Synthesis of ethanol perm-selective MFI zeolite membranes by binary structure directing agents

Good quality MFI zeolite membranes were prepared by altering a small amount of tetrapropylammonium (TPA+) with tetrabutylammonium (TBA+) in the synthesis solution. The effects of TBA+/Si ((TBA++TPA+)/Si ratio invariant) on the separation performance, hydrophobicity and compactness of MFI zeolite membranes were explored by pervaporation, water contact angle, scanning electron microscopy (SEM), X-ray diffraction (XRD), single-component gas permeation and permporosimetry. It was found that replacement of a small amount TPA+ with TBA+ could impair the hydrophobicity of membrane surface, but improve the membrane compactness verified by the ideal selectivity of N2/SF6 and permporosimetry. When TBA+/Si ratio increased to 0.02 from 0, the membrane separation factor was increased to 56 from 37 under crystallization time of 24 h, and 71 from 43 under crystallization time of 48 h. With increase of ethanol concentration in the feed from 1 to 50 wt%, the observed ethanol flux increased first and then decreased to that of single component pervaporation of ethanol, resulting in a peak value of the separation factor. The membrane M3 exhibited the highest separation factor of 79 with the flux of 1.40 kg m−2 h−1 for 3 wt% ethanol/water mixture while the highest separation factor of membrane M3-48 was 86 with the flux of 1.52 kg m−2 h−1 for 8 wt% ethanol/water mixture. •A strategy through binary structure directing agents has been used to enhance the separation performance of MFI membrane.•Compactness instead of hydrophobicity contributes to the improvement in separation selectivity of MFI membranes.•With increasing feed concentration, the ethanol flux increases first and then declines sharply.