Novel Routes For Synthesis Of SSZ-13 Zeolites

Zeolites are widely used in petrochemical fields due to their excellent properties including structural diversity, acidity adjustability and ion exchange. However, the hydrothermal synthesis of zeolites still have such actual disadvantagesasthe expensive template, air pollution and the low single autoclave yield, etc. Therefore developing some low cost and environmental friendly synthesis routes have received increasing attention.SSZ-13 is a kind of zeolite with CHA topology,which exhibits good performances in the methanol to olefins(MTO) reaction, the treatment of nitrogen oxide in automobile exhaust and the separation of N2/CO2. Currently, the three template susually need to be used in the synthesis of SSZ-13 zeolites, which are N, N, N-trimethy l- 1- 1- adamantammonium hydroxide, benzyltrimethylammonium(BTMA+) and coppercomplex(Cu2+ and tetraethylenepentamine, Cu-TEPA). However, all these templates have their respective undesirable characteristics, which are very expensive, toxic or difficult to exchange into SSZ-13 from Cu-SSZ-13, respectively. These strongly limit the wide industrial application of the SSZ-13 zeolite. Besides, low coking resistance of SSZ-13 zeolites in MTO reaction also restrict its application in this region.Based on the above synthesis and application situation of SSZ-13, developing some economical and green synthesis routes and improving the catalytic performance of SSZ-13 zeolites are of important theoretical significance and application value. The research presented in this thesis is aimed at developing some novel routes for preparing SSZ-13 zeolites with good performance and it basically consists of the following four sections:1. Firstly, the synthesis of SSZ-13 zeolites using single-template methodunder the low H2O/SiO2 ratios condition were investigated. The advantages of syhthesizing zeolites under this condition are higher single autoclave yield and less waste discharge than the traditional hydrothermal method. The SSZ-13 zeolite with pure phase and high crystallinity can be prepared with H2O/SiO2 ratio of 3.35, calcined seed amount of 1%, and TMADaOH/SiO2 ratio of 0.051, respectively. The comparative study of the obtained products under the low H2O/SiO2 ratios and conventional hydrothermal conditon was made using XRD, FTIR, SEM, TG-DTG and NH3-TPD techniques. The results showed that the relative crystallinity, the framework structure, the template amount and the template removal temperature of two kinds of products were very similar. It is worth noting that, compared with the hydrothermal method, the solid yield under the low H2O/SiO2 ratios can increase by 64% when the same template amount were used in the synthesis of SSZ-13. However, the product obtained under low H2O/SiO2 ratios exhibited higer deactivation rate and slightly lower selectivity of ethylene and propylene in the MTO reaction.2. Secondly, the synthesis of SSZ-13 zeolites by combining single-template method with SAPO-34 zeolites as heterogeneous seeds were investigated. SAPO-34 zeolite possess the same CHA topology structure as the SSZ-13. The difference is that SAPO-34 contains three tetrahedron elements(silicon, aluminum and phosphorus) and has more complex properties due to the existence of silicon island than SSZ-13. The results showthat the addition of SAPO-34 zeolites can also assist the synthesis of SSZ-13 zeolites, effectively inhibit the formation of impure crystal and improve product crystallinity. In addition, the calcined and low-crystalline SAPO-34 zeolites can play more effective assisting crystallization role. The SSZ-13 zeolites with pure phase and high crystallinity could be prepared when the heterogeneity-seed/SiO2 mass ratio is 5% and the Na2O/SiO2 molar ratio is 0.18. The synthetic conditions have been studied in detail and the comparative study on the products obtained from SAPO-34 and SSZ-13 seeds are also done by using XRD、SEM、FT-IR、TG-DTG、NH3-TPD techniques. Compared with the SSZ-13 obtained from homogeneity seeds, the products from SAPO-34 heterogeneous seeds exhibit similar physico-chemical properties, acid properties and catalytic performance, which suggested that SAPO-34 zeolite in place of SSZ-13 can be used as the seed for the synthesis of SSZ-13 zeolites.3. Thirdly, the seed assisted synthesis of SSZ-13 zeolites without organic structure directing agent(OSDA)were explored. This method is based on “composite building unit(CBU) hypothesis”. According to this hypothesis, only when both frame works of the target zeolite and zeolite obtained from the seed-free gel contain atleast one common CBU, it is possible to prepare the target zeolite with addition of the target zeolite seeds to the gel without an OSDA. In this work, it has been designed that SSZ-13 zeolite were synthesized by adding SSZ-13 seed into the synthetic systemof LTL zeolite without an OSDA due to the common CBU existing in both SSZ-13 and LTL. The imfluence of diffent synthesis parameters on crystal structure were investigated. The results showed that the SSZ-13 zeolites with low crystallinity could be obtained with Na2O/SiO2 mole ratio of 0.19, SiO2/Al2O3 mole ratio of 33, pre-crystallization at 170℃ for 5h and crystallizationtime for 12 h. Further work is in progress in order to optimise the synthesis condition for the pure SSZ-13 zeolites with high crystallinity.4. Finally, the synthesis hierarchical SSZ-13 zeolite using dual-template method were explored. This route aims at improving the coking resistance of H-SSZ-13, therefore making the catalyst more suitable for industrial applications. Hierarchical nanoporous SSZ-13 zeolites have been obtained by direct synthesis using N, N, N-trimethyl-1-adamantanammoniumhydroxide as a microporous structure directing agent and C18H37N+(CH3)2C6H12N+(CH3)2C6H13(Br-)2 as a mesoporogen. The effect of the amount of mesoporogen on the properties of the hierarchical nanoporous zeolites are studied. The hierarchical zeolites are characterized by XRD, FT-IR, SEM, HRTEM, N2 physisorption, TG-DTG, 27 Al and 29 Si MAS NMR, NH3-TPD techniques. The results showed hierarchical SSZ-13 synthesized herein is a loose aggregate of small primary zeolite nanocrystalsand contains the well-developed mesopores. The best hierarchical sample is obtained with a C18-6-6Br2/SiO2 molar ratio of 0.05 and this sample has high volumes of both micropores and mesopores. The catalytic performance of the hierarchical SSZ-13 zeolites is evaluated using the methanol to olefins reaction. Compared to its conventional counterpart, the hierarchical SSZ-13 zeolites have longer catalytic lifetimes and higher selectivities for ethylene and propylene, which can be attributed to the increased external surface area and the improved mass transport within the hierarchical porous system.