Crystal Size Control During The Hydrothermal Synthesis Of ZSM-5Zeolite

ZSM-5zeolite has important industrial applications owing to its siliceous framework and three-dimensional intersected channels with10-membered ring openings. However, the catalytic property of ZSM-5zeolite is strongly dependent on its crystal size. This dissertation mainly focuses on the crystal size control synthesis of ZSM-5. The effect of Na+and K+, the polymerization degree of silicon and aluminum sources and the addition of seed on the crystallization processes and products morphology of ZSM-5zeolite were studied systemically. Main results and conclusions obtained are as following:Firstly, in the synthesis study using water glass and Al2(SO4)3as raw materials, the development of the solid and liquid phases of the Na+and K+adding gels during the crystallization of ZSM-5was traced by XRD, SEM, IR, XRF and ICP. Results show that, Na+could facilitate the formation and condensation/rearrangement of the secondary structure building unite of five-membered structure and increase the number of nuclei in the solid phase, and Na+could also decrease the dissolving capacity of the liquid phase. All these together made the crystallization of the Na+adding gel taking place via the solid-phase transformation mechanism and producing fine crystal ZSM-5zeolite. Contrarily, K+could hinder the formation of the five-membered structures and reduce the number of nuclei in the gel, and K+could also enhance the dissolving capacity of the liquid phase. All these together made the crystallization of the K+adding gel occurring by the solution-mediated transport crystallization and producing large ZSM-5crystals. The Addition of suitable amount of Na+and K+proved to be an effective mean for the regulation of the gel crystallization path and the crystal size of ZSM-5zeolite.Secondly, in the synthesis study using silica sol and alumina as raw materials, the regulation of the polymerization degree of silica sol by using NaOH and the regulation of the polymerization degree of Al2O3by using H2SO4were characterized by UV Raman and NMR spectra, and the crystallization processes of the gels were followed by XRD and SEM. Results show that, the gels prepared with slightly depolymerized silica sol (Na2O/SiO2ratio is0.064～0.142) and alumina (H2SO4/Al2O3ratio is0～2.433) were crystallized via the liquid-phase transformation mechanism, which produced large crystal ZSM-5zeolite. Whereas the gels formed by highly depolymerized silica sol (Na2O/SiO2ratio is0.154～0.568) and alumina (H2SO4/Al2O3ratio is2.808～15.725) were crystallized by the solid-phase transformation mechanism, which gave fine crystal ZSM-5zeolite. It was concluded that the regulation of the polymerization degrees of silicon and aluminum sources was also useful for the regulation of the gel crystallization path and the crystal size of ZSM-5zeolite. In addition, the role of both homogeneous and heterogeneous seeds in the synthesis of ZSM-5zeolite was investigated. Results show that, the homogeneous seed like various ZSM-5zeolites (difference in size) and pure silicon Silicalite-1, as well as the heterogeneous seed like zeolite β, all could promote the crystallization of ZSM-5zeolite.Both the dissolved fragments and the residues of the seed showed structure directing effect. Unexpectedly, the dissolved fragments exhibited much stronger structure directing effect than the residues did. However, seeding did not alter the crystallization path of the gels. The role of seed on size regulation seemed strongly gel dependents. That is, the crystal size of ZSM-5zeolite could be regulated by seed if the-gel was crystallized via the liquid phase transportation process, could not if the gel was crystallized via the solid phase transportation process.In this study, twelve well-crystallized ZSM-5zeolite products differing in crystal size and SiO2/Al2O3ratio have been synthesized, and three of them have been commercialized.