Microstructural optimization of MFI-type zeolite membranes for ethanol–water separation

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Vol. 2; no. 38; pp. 16093 - 16100
• Main Authors: Peng, Yong, Lu, Huibin, Wang, Zhengbao, Yan, Yushan
• Format: Journal Article
• Language: English
• Published: 01.01.2014
• Subjects: Crystal defects; Membranes; Microstructure; Pervaporation; Separation; Silicon; Synthesis; Zeolites
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/C4TA02837F

High quality pure-silica MFI-type zeolite membranes are successfully prepared by simply controlling the amount of structure directing agent (SDA), i.e., tetrapropylammonium (TPA super(+)) in the synthesis solution for seeded growth. The effects of several synthesis parameters such as alkalinity (OH super(-)/Si), TPA super(+) concentration (TPA super(+)/Si), and crystallization time on the membrane pervaporation performance are investigated in detail. The synthesized MFI-type zeolite membranes are thoroughly characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, the water contact angle test, X-ray diffraction (XRD), the pervaporation test and gas permeation. The membrane microstructure is very sensitive to the TPA super(+)/Si ratio of the synthesis solution. At a high TPA super(+)/Si ratio (typically 0.17), parasitic twin crystals are intergrown in the zeolite layer, thus resulting in the formation of membrane defects during SDA removal treatment (calcination at 500 degree C). When the membrane is prepared with a low TPA super(+)/Si ratio ( less than or equal to 0.05), the appearance of twin crystals can be significantly suppressed and the synthesized dense MFI-type zeolite membranes exhibit pervaporation separation factors higher than 85 for 5 wt% ethanol-water mixtures at 60 degree C. We demonstrate for the first time that controlling the SDA concentration in the precursor solution is beneficial to the elimination of membrane microstructural defects, showing a pathway to high quality MFI-type zeolite membranes.