MFI Zeolite Membranes for Desalination

In the last years, zeolite membranes have attracted increasing interest in industrial and academic fields owing to their crystalline structure and to their uniform pore diameters close to molecular size of different species [1]. Nowadays, the industrial applications are only represented by LTA and T membranes for organic solvent dehydration owing to different problems as reproducibility during the synthesis and costs [2]. Different preparation techniques were developed to obtain almost defect free zeolite membranes. However, the most used method is the secondary growth. It involves two steps: seeding of zeolite crystals on the support and crystals growth by hydrothermal treatment. Decoupling zeolite nucleation from crystals growth, it is possible to control the conditions of each step independently and thus to obtain a higher reproducibility compared to the other methods. Different seeding procedures are used to cover the support with crystals. More controllable seeding involves the cross-flow filtration of a zeolite water suspension through a support [3]. However, coverage uniformity problem can happen using tubular supports. This problem can be overcame using a seeding procedure combining the support tilting and rotation with the cross-flow filtration [4, 5]. In the present work, supported MFI zeolite membranes were synthesized by secondary growth method. For the first time the cross-flow seeding procedure was employed for the preparation of membranes having this topology. The influence of different seeding parameters like support pore size and zeolite slurry concentration was investigated to optimize the membrane quality. Morphology of the zeolite layer and crystals topology were observed by scanning electron microscopy and X-ray diffractometry, respectively. Transport properties like flux and selectivity were investigated before and after calcination by permeation tests of single gases and pure water. The vacuum membrane distillation performance of the prepared membranes was also explored using both distilled water and NaCl solution.