Preparation and Characterization of SSZ-13 Molecular Sieve

SSZ-13 molecular sieve is prepared by the traditional hydrothermal method with silica sol, aluminium sulphate, sodium hydroxide and deionized water as raw material, and N, N, N-trimethyl-l-adamant ammonium hydroxide as a template. It is investigated in terms of the influencing factors and optimal reaction conditions, and characterized with XRD, BET specific surface area meter and FT-IR. The experimental results indicate that the obtained SSZ-13 molecular sieve has advantages of shorter reaction time, higher crystallinity and larger specific surface area. The reaction time is further shortened to 2 days by introducing crystal seed or accelerant, which lowers the total preparation cost of SSZ-13 and makes its industrialization highly feasible.

In addition, SSZ-13 works well in the methanol-to-olefin (MTO) catalysis (Zhu Q J et al, 2007, p. 5409-5415), and can increase the yield of low carbon olefin. However, it hasn't been reported so far in China as to this type of SSZ-13 molecular sieve because the template for it is very difficult to obtain and expensive, and its reaction period is too long (about 7 days), which leads to the increase of the cost of SSZ-13 molecular sieve. Therefore, it is necessary to shorten the reaction time and lower the cost by optimizing the formulation and reaction condition of SSZ-13, as well as introducing crystal seed or accelerant in SSZ-13.

Preparation of SSZ-13 molecular sieve
SSZ-13 molecular sieve is prepared by the traditional hydrothermal method with silica sol, aluminium sulphate, sodium hydroxide and deionized water as raw material, and N, N, N-trimethyl-l-adamant ammonium hydroxide as a template. Mix SiO 2 , A1 2 O 3 , Na 2 O, R 2 O and H 2 O uniformly in the ratio of 40:1:16:5:900, age the mixture at room temperature for 0.5 h, then pour the mixture into a PTFE-lined high-pressure reactor, and crystallize at 155 for 2~5 days. SSZ-13 molecular sieve is then prepared by pouring the obtained crystal into beaker, heating it to 70~80 , drying the separated crystal at 120 after three cycles of ion exchange with a certain amount of ammonium chloride for 2 hours followed by vacuum filtration, and removing the template and water from the crystal by temperature-programmed calcinations.

Result and discussion
Figure 1 exhibits the standard XRD patterns of SSZ-13 molecular sieve reported by International Zeolite Association (IZA). Figure 2 exhibits the XRD spectra of SSZ-13 molecular sieve sample. Both figures exhibit the same features in peak position and peak shape. It indicates that the prepared crystal is a SSZ-13 molecular sieve without any mixed crystal.

Effects of the amount of sodium hydroxide on the preparation of SSZ-13 molecular sieve
Sodium hydroxide acts as a pH regulator in the preparation of SSZ-13 molecular sieve. The effects of alkalinity on the preparation of SSZ-13 molecular sieve are observed by altering the amount of sodium hydroxide. Figure 3 exhibits the XRD spectra of SSZ-13 molecular sieve prepared at different amount of sodium hydroxide (the ratio of n Na2O to n Al2O3 is 4, 8, 12, 16 and 20 respectively). From figure 3, we can see that the crystallinity increases with the rise of the alkalinity at the beginning, and reaches the maximum when the ratio of n Na2O to n Al2O3 is 16, however decreases rapidly when the alkalinity continues to increase, and forms a compact amorphous structure when the ratio of n Na2O to n Al2O3 is 20. Generally, the increase of alkalinity can promote the reaction, improve the solubility of silicon and aluminum, and change the polymerization state and distribution of raw materials in synthetic system, as well as accelerate the crystallizing speed, in turn shorten the induction period and nucleation period of crystallization. However, overhigh alkalinity leads to the overhigh solubility of silicon and aluminum, and the decline of crystallinity and specific surface area, in turn the formation of a compact amorphous structure. Therefore, either overhigh or overlow alkalinity has an adverse effect on the formation of crystal. In other words, SSZ-13 molecular sieve of high crystallinity can be obtained only in the proper alkalinity range.

Effects of the amount of water on the preparation of SSZ-13 molecular sieve
Water acts as a reaction medium and regulates the concentration of raw materials in the preparation of SSZ-13 molecular sieve. The effects of water on the preparation of SSZ-13 molecular sieve are observed by altering the amount of water. Figure 4 exhibits the XRD spectra of SSZ-13 molecular sieve prepared at different amount of water (the ratio of n H2O to n Al2O3 is 700, 900, 1100, 1350 and 1500 respectively). From figure 4, we can see that the crystallinity increases with the rise of the amount of water at the beginning, reaches the maximum when the ratio of n H2O to n Al2O3 is 900, and changes little until the ratio of n H2O to n Al2O3 is 1100, however decreases when the amount of water continues to increase. It is because the amount of water is associated with the concentrations of raw materials; in turn has an effect on the polymerization state and distribution of raw materials in synthetic system. In addition, the amount of water directly affects the change in alkalinity, in turn affects the preparation of SSZ-13 molecular sieve. Therefore, properly lowering the amount of water can increase the crystallinity of SSZ-13.

Effects of crystallization period on the preparation of SSZ-13 molecular sieve
Crystallization period has a great effect on the crystallinity of SSZ-13. The effects of crystallization period on the preparation of SSZ-13 molecular sieve are observed by altering the reaction time. Figure 5 exhibits the XRD spectra of SSZ-13 molecular sieve prepared within different periods of crystallization (from 1 day to 5 days). From figure 5, we can see that the crystallinity increases with the reaction going on at the beginning, and reaches the maximum on the 3 rd day, then levels off and even decreases a little. It is because a too short period of crystallization leads to the formation of amorphous structure or mischcrystal, whereas a too long period of crystallization changes the originally formed pure crystal into other crystal. Therefore, either too long or too short period of crystallization has an adverse effect on the crystallinity of SSZ-13. Generally, without addition of any crystal seed or accelerant, the best crystallization period of SSZ-13 is 3 days.

Effects of accelerant on the preparation of SSZ-13 molecular sieve
The effects of accelerant on the preparation of SSZ-13 molecular sieve are observed by introducing a certain amount of accelerant into the reaction system. Figure 6 exhibits the XRD spectra of accelerant-contained SSZ-13 prepared within different periods of crystallization (from 1 day to 4 days). By comparing figure 6 with figure 5, we can see that the accelerant-contained SSZ-13 reveals an increase in crystallinity. It is probably because the accelerant speeds up the formation of silicon-oxygen tetrahedron oligomer and aluminum-oxygen tetrahedron oligomer, in turn accelerates the nucleation and growth of crystal (Kumar R et al, 1998, p. 23-31;Kumar R et al, 1996, p. 298-300). As a result, it rapidly forms the desired crystal on the 2 nd day.

Characterization of SSZ-13 molecular sieve
Figure 7 exhibits a SEM image of SSZ-13 molecular sieve sample. We can see a highly overlapped and irregular crystalline structure of SSZ-13 from it.

Conclusion
We have inclusively investigated the preparation conditions and influencing factors of SSZ-13 to achieve the best formulation: SiO 2 /A1 2 O 3 =40, Na 2 O /A1 2 O 3 =12~16, R 2 O/A1 2 O 3 =5, H 2 O/A1 2 O 3 =900~1100. The SSZ-13 molecular sieve is then prepared by crystallizing the above mixture at 155 for either 3 days without addition of crystal seed and accelerant or 2 days in the presence of crystal seed and accelerant. This study not only shortens the crystallization period and lowers the preparation cost, but also increases the specific surface area and improves the catalysis property of the crystal. It indicates that the method studied here is valuable for broader applications and feasible for industrialization.