Fig. 8: Effects of synthesis temperature/time on N2/SF6 ideal selectivity of ZSM-5 zeolite membranes
Based on the above-mentioned discussion, proper intergrowth of small crystals penetrating into the defects/gaps of the large seed layer is the main factor for a successful different-sized seeding method. This requirement depends on both nucleation and crystal grow reactions with the priority of nucleation reactions [28]. Therefore, an optimum variable temperature/time method with a short-term low-temperature initial stage (4 h at 140˚C) facilitates the production of zeolite nuclei which can transfer into the defects of the seed layer. The faster growth of these nuclei in every direction during the high-temperature final stage (8 h at 180˚C) effectively fills the inter-crystalline gaps resulting in the formation of a high-quality zeolite layer.
It is known that the small seeds at the top layer intend to grow faster than those penetrated into the defects at the bottom layer, wherein, zeolite crystals with the highest vertical growth rate with respect to the support surface have the highest probability of survival [5]. Therefore, controlling the rate of crystal growth reactions by adjusting the synthesis temperature/time can effectively increase the likelihood of small internal seeds growing. The slower growth rate of zeolite crystals at the initial stage of the variable temperature method facilitates the nutrition of nuclei/small crystals penetrating into the defects providing them the opportunity to grow enough to fill defects and gaps in the bottom layer [24]. The growth rate of the small seeds on the top of the seed layer then overcome during the high-temperature final stage accelerating crystal growth rate on top, improving the crystal intergrowth and overgrowing by adjacent crystals which dominate the surface of the film. It would be responsible for the higher selectivity [24]. Contrariwise, starting the synthesis at high temperatures, similar to the M9 membrane, favors the faster intergrowth of small seeds on the top which eliminates the nutrition of internal seeds and consequently increases the inter-crystalline gaps and membrane thickness simultaneously [5, 29]. However, understanding the effect of synthesis conditions (particularly synthesis temperature and time) on growth competition between small crystals on the top of the seed layer and those stuck into the defects/gaps still need to be studied in detail.