A study of zeolite film synthesis on porous substrates

Synthetic zeolites are attractive for chemical processing applications due to their high thermal and chemical stability. Potential new applications include zeolite membranes for gas separation, catalysis, sensor application, etc. The use of zeolite ZSM-5 for gas separation has been widely used due to the intermediate pore size. The formation of a zeolite layer on the heterogeneous surface depends on several factors. This study was conducted in order to investigate the synthesis techniques and the conditions for zeolite layer formation for zeolite ZSM-5 and zeolite LTA. The synthesis of the zeolite ZSM-5 layer was performed using an electrophoretic deposition technique on α-alumina substrates. The effects of chemical composition and applied electrical potential were evaluated using SEM, gas permeation, and the Maxwell-Stefan model. The results show that the thickness of the layers is controlled by the synthesis concentration and electrical potential across two electrodes. This indicates that the surface charge has a strong influence on zeolite layer formation on a substrate. The evaluation of the zeolite layers was performed using gas permeation analysis and the modeling using the Maxwell-Stefan diffusion approach. This analysis reveals that small defects in less than 0.1% of the zeolite layer can give a difference of about two orders of magnitude between the membrane diffusivity and zeolite particle diffusivity, which resulted in the high gas permeation rates observed.; The zeolite LTA framework is composed of an equal amount of silica and alumina arranged in a three-dimension tetrahedral structure. This high alumina content zeolite possesses a strong negative surface charge in a basic solution due to the substitution of aluminum atoms into a SiO₄ tetrahedral structure making it difficult to form a continuous layer in solution. This study is conducted in order to understand the differences in the formation of the zeolite in a solution and on a surface. Synthesis parameters such as synthesis duration, chemical composition, and synthesis temperature were varied. The crystallization kinetics was carried out using the particle size measurement, percent crystallinity from XRD analysis, IR absorption of tetrahedra using Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), and the exponential growth model. The relationship of the substrate variation to the zeolite layer quality was examined using SEM and gas permeation tests. The results show that the formation of the zeolite layer depends on the surface nucleation site, the mass transfer rate of the nutrient onto the surface, and the relative surface charge of the zeolite particles and substrates. The evaluation of the electrical double layer effect using the colloid particle interaction theory shows a strong repulsion between zeolite LTA particles and the substrates. Increasing the crystal size results in a stronger repulsion between particles, causing the particles to grow individually and precipitate out of the solution.