Synthesis and Characterization of Thin MFI-type Zeolite Layer on Porous Alumina Substrates

The most commonly used and commercially available membranes utilized for water desalination are made of polymeric materials. Due to their limitation and low performance in harsh environment such as high temperature and corrosive medium, ceramic-based membranes constitute a viable alternative. MFI is a particularly interesting inorganic zeolite structure with pores' size close to the kinetic diameter of many industrially important molecules. Controlled thickness, defect-free layer and preferential crystal orientation are still the main challenges with zeolite membrane fabrication.;In this research work, fabricating MFI zeolite membranes was performed using two techniques, in-situ crystallization and secondary growth. With the in-situ crystallization method, MFI zeolite crystals were directly grown on the substrates by subjecting the substrate to a hydrothermal treatment in the synthesis solution. Processing parameters such as hydrothermal treatment temperature and duration were varied. In the secondary growth method, MFI zeolite layer was synthesized by microwave assisted secondary growth, from microwave derived seeds deposited on alpha-Al2O3 substrate by either dipcoating or spin-coating before being exposed to the synthesis solution and subjected to a microwave thermal treatment. The synthesized MFI zeolite layers were analyzed with Xray diffraction (XRD) and scanning electron microscope (SEM) for their surface properties, coverage and the orientation of the zeolite crystals.;It is worth noting that the morphology of MFI zeolite membrane/layer was strongly influenced by several parameters. Albeit the simplicity of the in-situ crystallization technique, the combination of leaching and lack of continuity of the zeolite layer made this technique inadequate. Continuous MFI zeolite thin layer, defect free, with good morphology was grown on porous alumina substrate using secondary growth technique with the assistance of microwave irradiation at 160oC for 2h. Two seeding methods were used; namely dip-coating and spin-coating. The spin-coating technique showed promising results in term of the ability to reach a level of uniform seeds distribution on the top of the alumina substrate leading to formation of a closed packed seeding layer, in addition to a short process time compared to dip-coating technique.;To our knowledge, this is the first published work using spin-coating technique for seeding porous alumina substrate with MFI-zeolite nano-sized crystals. The uniform morphology of MFI zeolite layer was enhanced as the number of seeding runs was increased, and the orientation of the grown crystals was controlled with their crystallographic preferred orientation (CPO) (1 0 1). MFI zeolite layer was calcinated at very low heating and cooling rates of 1°C/min at 500°C for 6 hours. After calcination, the chemical analysis performed using energy dispersive spectroscopy (EDS) confirmed the diffusion of aluminum ions into the zeolite layer. However, the results revealed that using three seeding layers with a thickness of about 120 nm was sufficient to prevent the diffusion of the aluminum ions towards the top surface of MFI zeolite crystals.