Manipulation Of Morphology And Size Of Silicalite-1 Crystals And Synthesis Of MFI Membranes

MFI-type zeolites are widely used due to their unique pore structure and good hydrothermal stability. The silicalite-1 framework is completely free from aluminium atoms, which is hydrophobic. The control of the particle morphology and size of a given zeolite structure is important not only for applications in catalysis and adsorption, but also for a variety of uses of these zeolite particles as seeds for the growth of zeolite membranes. So the manipulating of morphology and size for MFI zeolite has always been a crucial aspect in the research of zeolite. But so far, the manipulation for MFI zeolite was focused on nano-sized particles and need some expensive materials such as trimer-TPA, TPAOH and TEOS. To solve these problems, we try to synthesize micro-sized siliclaite-1 crystals using a cheap and simple way.In this thesis, in order to study the effects of morphology and size of silicalite-1 zeolite on the preparation of MFI zeolite membranes, silicalite-1 zeolite crystals with different morphologies and sizes are prepared by hydrothermal synthesis. And different MFI zeolite crystals are used as seeds to synthesize MFI membranes. The main experimental results and conclusions are summarized as follows:1. Manipulation of morphology and size of silicalite-1 crystalsSilicalite-1 crystals with different sizes and morphologies are successfully synthesized by hydrothermal synthesis using cheap fumed silica as silica source and TPABr as a template. The morphology and particle size of the crystals can be characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and laser light scattering (LLS). Two different methods(One-stage Method and Two-stage Method) were used to control these features in this paper.In One-stage Method, the key points to synthesize silicalite-1 crystal include adjusting the contents of water, alkaline (Na2O), tetrapropyl ammonium (TPA), crystallization conditions and stirring methods. It is found that the shape of silicalite-1 crystals changed from sphere to coffin with the increase of water content. On the contrary, increasing the alkalinity of the synthesis solution can change the crystal shape back to sphere, however, the size of crystals decreased while the alkalinity keeps growing. We can get small size crystals through increasing the TPA content or shorten the crystallization time with no change in the yield of zeolites. The stirring methods also have influences on the crystals size and distribution. Silicalite-1 crystals synthesized in a magnetically stirred reactor have smaller size and wider size distribution than those synthesized in a rotation reactor.In two-stage method, we studied the affect of temperature in STAGE 1 (Pre-Crystallization) and STAGE 2 (Crystallization). We found that the temperature in STAGE 1 (Pre-Crystallization) has significant effects on the size of silcialite-1 particle. We can get big particle size crystals in 40℃ or 80℃, while small particle size in the range from 50℃-60℃. The particle size also decreased with increase of time in STAGE 1. However, the particle size of silicalite-1 did not affected by STAGE 2 (Crystallization).0.5 μm silicalite-1 zeolites are synthesized at 60℃ for 24 h in STAGE 1 and 130℃ for 3 h in STAGE 2 with yield of 70%. Compared with one-stage method, two-stage method can synthesize smaller silicalite-1 particle with less crystallization time and higher yield.2. Preparation of MFI zeolite membranes by different seeds and their separation performanceMFI zeolite membranes were prepared by using seeds with different morphology(coffin, coffin with twin crystal, sphere), Si/Al(∞,500,140,40), particle size distribution and content of Na+. MFI zeolite membranes were synthesized on the surface of porous alumina tube under certain conditions, and their morphologies are observed by SEM. Pervaporation performances are tested for 5 wt.% ethanol feed at 60℃. It is found that membranes synthesized by mult-crystal morphology seeds(coffin with twin crystal, sphere) have more twin crystals on the surface than mono-crystal morphology seeds(coffin). It is much easier for mult-crystal morphology seeds to synthesize dense membranes than mono-crystal morphology seeds. Membranes synthesized by seeds(coffin with twin crystals) still have slightly higher separation performance. The separation factor can reach 114 when TPAOH/TEOS=0.005 in the synthesis solution. MFI membranes prepared by seeds with different Si/Al ratios were also investigated. The results show that small amount of Al would not affect the performance of membrane, on the contrary, the separation factor can reach 103 when TPAOH/TEOS=0.05 in the synthesis solution using ZSM-5(140) as seeds. We also found that membranes synthesized with seeds after ball milling have higher separation factors(10 higher). Meanwhile, the prevaporation performance decreased with the increase of Na+ content.