MFI zeolite membranes: preparation and characterization

In the last years, zeolite membranes have attracted a lot of interest in industrial and academic fields owing to their crystalline structure and to their uniform pore diameters close to molecular size of different species. Considering these characteristics, they can be used in many liquid and gas separation processes. Besides, for their high thermal and chemical stability, they can also replace polymeric membranes in applications that require strong operating conditions like high temperature and presence of aggressive organic solvents [1]. The high costs and poor reproducibility in the synthesis step represent a limiting factor for their development at industrial level. Until now, only LTA and T zeolite membranes were commercialized for organic solvent dehydration by pervaporation and permeation processes [2]. Self-standing zeolite layers are difficult to synthesize, therefore zeolite membranes are usually prepared on porous supports of alumina or stainless steel [1]. The most common methods for their preparation are the in situ and the secondary growth. The last one consists of two different steps: seeding and growth of zeolite crystals on the support. 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 in situ method. Different seeding procedures are used to cover the support with crystals and the more controllable one is represented by the filtration of a zeolite aqueous suspension through the support [3]. In particular, the cross-flow mode, combining support tilting and rotation with the filtration, allows a more uniform and homogeneous coverage with zeolite seeds when membranes are prepared on tubular supports [4, 5]. In the present work, supported MFI zeolite membranes were synthesized by secondary growth method. For the first time the new cross-flow seeding procedure was employed for the preparation of membranes having this topology. The influence of several seeding parameters (support pore size, crystals pore size and zeolite slurry concentration) was studied to optimize the membranes quality. After the synthesis, the membranes are gas tight. The synthesis was carried out in presence of the organic template, therefore, calcination was required in order to remove it. Membrane post treatments like new cross-flow and dip coating with polydimethylsiloxane [6] were also performed with the aim to seal the membrane defects due to the calcination step. Morphology of the zeolite layer was evaluated by scanning electron microscopy, while, crystal topology was examined by X-ray diffractometry. Furthermore, transport properties like flux and selectivity were also investigated before and after calcination by permeation tests of single gases and pure water and by direct contact membrane distillation in order to evaluate the membranes performance and their potential applications in different membrane processes.