Rapid Syntheses Of MFI Zeolites At Low Temperature And TS-1 Zeolite With High Framework Titanium Content Under Low Water Conditions

Due to unique shape selectivity,adjustable acidity,high thermal stability and hydrothermal stability,the zeolites with MFI structure are one of the most widely used zeolites.However,the synthesis of MFI zeolites often requires a large amount of solvent and to be carried out in a closed autoclave at a relatively high temperature（～170°C）,which has disadvantages of low space utilization efficiency of autoclaves,large discharge of waste liquid,high energy consumption,and high requirements for the equipment.To overcome these shortcomings,the syntheses of MFI zeolites under low water conditions and at low temperature（≤100°C）have become important research directions.In this thesis,rapid syntheses of the three kinds of MFI zeolites,namely,pure silica zeolite siliconalite-1（S-1）,titanosilicate zeolite TS-1 and aluminosilicate zeolite ZSM-5,under low water conditions at 90°C and atmospheric pressure,as well as that of the TS-1 zeolite with high framework titanium content at 190°C were investigated.The main research contents and results are described as follows:Rapid synthesis of S-1 zeolite at the conditions of atmospheric pressure,90℃and applying a small amount of water was studied.It is found that complete crystallization of S-1zeolite and 98%zeolite yield can be achieved by crystallizing the sol prepared by grinding S-1suspension with the mixture composed of organic template tetrapropylammonium hydroxide（TPAOH）and fumed silica（Si O₂）at 90℃,atmospheric pressure and a stirring condition for 9h.The crystallization time is even shorter than the shortest time（30 h）for synthesizing S-1zeolite at 100°C being reported in literature.For the synthesis process,the amount of water applied is only 1/6～1/3 of the conventional hydrothermal synthesis under the same amount of silicon,the mother liquor can be recycled,and no waste is discharged.For the S-1 zeolite product,it has uniform morphology with an average crystal size of 150 nm,single crystal characteristics,intracrystalline mesopores in diameters of 3～30 nm and abundant silanol nests,and exhibited good catalytic performance in the gas-phase Beckmann rearrangement of cyclohexanone oxime.A new method for rapid synthesis of MFI zeolites by using a liquid seed was studied and proposed,in which the liquid seed is prepared by dissolving S-1 crystals in TPAOH solution at80℃and enriched in silicon species with four-and five-membered ring.In this method,completely crystallized nano-ZSM-5 zeolite with Si/Al ratio of 36 could be obtained by crystallizing the solid mixture prepared by grinding the liquid seed and solid raw material at90℃for 30 h.The crystallization time is shorter than half of the shortest time（72 h）reported in the literature for synthesizing ZSM-5 zeolite at the same temperature.Moreover,it is found that the ZSM-5 zeolite with Si/Al ratio of 28 could even be synthesized at 90℃by using this method,whereas no ZSM-5 zeolite with Si/Al ratio less than 50 being synthesized at the same temperature reported in literature.The mechanism accounting for rapid synthesis of ZSM-5zeolite with low Si/Al ratio at low temperature by this method was studied.It is found that the silicon species with four-and five-membered ring in the liquid seed can not only lead to the rapid formation of a large number of crystal nuclei thereby accelerate zeolite crystallization,but also combine with Al so as to promote Al being incorporated into the zeolite framework.For the synthesis by using this method,the amount of water applied is only 1/9 of the conventional hydrothermal synthesis under the same amount of silicon,the crystallization product presents as solid state,and no waste water is discharged.The ZSM-5 zeolite product is rich in mesopores,exists as crystals with an average size of about 25 nm,and its catalytic lifetime（36 h）for the methanol-to-olefin reaction is 1.95 times longer than the corresponding ZSM-5 zeolite being synthesized by conventional hydrothermal method.Rapid synthesis of nano-sized TS-1 zeolite under the conditions of atmospheric pressure,90℃and applying a small amount of water was also studied.It is found that recycling the crystallization mother liquor in the synthesis not only benefits nano-sized zeolite formation,but also greatly shortens the required crystallization time.By using this method,the completely crystallized TS-1 zeolite could be obtained by crystallizing the sol at 90°C for 30 h.The crystallization time is even much shorter than the shortest time（50 h）being reported in literature for the TS-1 zeolite synthesis at 100°C.Rapid synthesis of TS-1 zeolite with high framework titanium content by using the liquid seed method proposed in this thesis under less amount of water conditions was studied.The TS-1 zeolite with Si/Ti ratio of 41 and without Ti O₂ impurity was obtained by crystallizing the sol containing the liquid seed at 190℃for 24 h.The crystallization time of this synthesis is only 1/2 of the shortest time（48 h）for synthesizing the same TS-1 zeolite reported in recent literature.On the other hand,the framework Si/Ti ratio of the TS-1 zeolite obtained by this synthesis is closed to the theoretical minimum value（39）given in literature.The realization of the synthesis also lies in the silicon species with four-and five-membered ring existing in the liquid seed,which contributes to the formation of large number of crystal nuclei as well as the incorporation of Ti into the zeolite framework.The amount of water applied in the synthesis is only 1/8 of the conventional hydrothermal synthesis under the same amount of silicon,and almost no waste water is discharged in the synthesis process.The TS-1 zeolite product exists as crystals with an average size of 240 nm and contains mesopores ranging from 3 to 16 nm,and exhibited higher catalytic activity than that of hydrothermally synthesized TS-1 zeolite for the ammoximation of cyclohexanone.