Synthesis And Mechanism Of Zeolites From Silicate Mineral Materials

Zeolites have attracted great interest in industry field because of their unique structures and properties. Efficient synthesis methods of zeolite are always a concern in this field. Synthesizing zeolites from silicates, which little waster water is produced and products are obtained with high yield, is considered to be an important efficient synthesis method. What’s more, parts of secondary building units could be preserved in the course of crystallization of zeolites from silicates. However, few studies have been performed on the mechanisms of zeolites from silicates, leading to insufficient understanding of the crystallization process. At present there is great blindness in the synthesis of zeolites, caused some waste of manpower and materials. To exploit efficient routes for zeolites, it is very important to understand formation mechanisms of zeolites. In this dissertation, four zeolites were obtained from diatomite and magadiite, respectively. The effects of various parameters were discussed and the optimal parameter was presented. Characterizations of the as-synthesized samples were investigated. The changes of medium-range structure during the crystallizations were studied by X-ray diffraction, scanning electron micrograph, vibrational spectroscopy, high-energy X-ray diffraction and solid state nuclear magnetic resonace. According to the above investigations, reasonable formation pathways were proposed. The main results are as follows.1. Mordenite was synthesized from diatomite. Pure mordenite could be obtained at 165℃for 16 h with the reactants molar composition of 25.3 SiO2:1.0 Al2O3:4.0 Na2O: 164.0 H2O. A formation pathway for mordenite from diatomite could be proposed that five-membered rings formed initially are undergoing accommodation of four-membered rings around them to form mordenite.2. Zeolite omega was synthesized from magadiite in a glycerol-water system. Water content plays an important role in this conversion. Pure zeolite omega could be obtained at 120℃ for 10 d with the reactants molar composition of 14 SiO2:Al2O3:10 Na2O:169 H2O: 200 glycerol. Comparing to the synthesis of zeolite omega from pure precipitated silica, the reaction time of this synthesis from magadiite could be substantially decreased. A preferential location of the Al atoms in the six-membered rings of the gmelinites in zeolite omega was observed by 27Al MAS NMR. It is proposed that parts of five-membered rings and six-membered rings in magadiite are preserved as secondary building units although the long-range order of magadiite is collapsed in the initial stage, and the fraction of five-membered rings and six-membered rings increases subsequently. The four-membered rings chains are formed at a stage later than that of five-membered rings and six-membered rings.3. Ferrierite was synthesized from magadiite in the present of ethylene glycol. Pure ferrierite could be obtained at 140 °C for 10 d with the reactants molar composition of 30 SiO2:A12O3:1.2 Na20:1.5 K20:285 H20:210 ethylene glycol. The effect of ethylene glycol is to promote the form of ferrierite and decrease the induction time. It is proposed that the magadiite layers were reconfigured into precursors with five-membered rings and six-membered rings in the initial stage, and then the precursors interconnect via eight-membered rings and ten-membered rings to form the framework of ferrierite.4. ZSM-5 was synthesized from magadiite in the present of tetrapropylammonium bromide. Pure ferrierite could be obtained in a wide range of reactants molar composition (SiO2/Al2O3= 80～580, H20/Na20= 125～700, Na2O/SiO2= 0.02～0.18, TPA+/SiO2= 0.02～0.19) and a wide range of temperature (140～180 °C). A transformation mechanism is proposed, which involves magadiite layers reconfiguring into ten-membered ring zig-zag channels with the structure direction of TPA+cations in the initial stage, and then zig-zag channels interconnect via ten-membered ring linear channels to form the framework of ZSM-5.In this dissertation, several zeolites were obtained from silicate mineral materials. As-synthesized products exhibited high crystallinity and thermal stability. Types and synthesis routes of zeolite were enriched. The changes of medium-range structure during the crystallizations were studied, and crystallization mechanisms were proposed. It is clear that some secondary building units present in the silicate mineral materials are used directly during the crystallization. This finding not only provides a new strong proof for the issue that the formation pathway for zeolites could be affected by the composition and structure of raw materials, but also provide theoretical basis for choice of raw materials for synthesis of zeolites.