Synthesis Of ZSM-5 Zeolite By Template-free Crystallization Method

ZSM-5 zeolite has been widely applied to catalytic cracking, aromatic hydrocarbon alkylate, methyl alcohol system gasoline and the light hydrocarbon aromatization and such like various industry, due its unique miropore structure, the well skeleton thermostability and water thermostability, relative strong acidity as well as outstanding shape-selecting catalytic performance. Since Mobil Corporation first synthesized the ZSM-5 zeolite using TPA+as template in 1972, organic amines and ammonia as well as the many kinds of low-carbon alcohol have been used as structure-directing reagents towards to ZSM-5 zeolite. Moreover, ZSM-5 zeolite even can be crystallized without the template of amine (ammonia) and alcohol.Template-free methods towards to ZSM-5 zeolite can not only reduce the production cost but also reduce environment pollution of the volatile amines (ammonia). So, this paper mainly discussed the synthesis of ZSM-5 zeolite by template-free hydrothermal method.(1) In the template-free system, it is very difficult to obtain the ZSM-5 without seeds, but there will be a new mixed crystal formed with excessive seeds. Not only the ratio of Na2O/SiO2 decides the alkalinity of the gelatin, but also Na+cation plays a structure directing role, so it is an important factor. Therefore it is not propitious for synthesis ZSM-5 with too low or too high ratio of Na2O/SiO2. Highly pure ZSM-5 can be synthesized with inexpensive silicon resource such as water glass and the silicon sol, and its appearance is influenced by the different silicon source; Adding Tween-20 can obviously optimize the gel state, simultaneously enhances the crystallinity, and synthesis ZSM-5 with special appearance.(2) ZSM-5 with high silicon is difficult to prepare, because of along with the rise of SiO2/Al2O3, the silicon is more difficult to enter the skeleton, and the weak guidance of Na+ causes this kind of phenomenon to be more obvious in the template free system. Mordenite will be easily formed if the SiO2/Al2O3 quite is low and kenyaite if the SiO2/Al2O3 quite is high. Crystallization temperature and time should be strictly controlled, otherwise mixed crystal forms. We can see that agitation can obviously reduce the size of ZSM-5.(3) Characterizations carried out with SiO2/Al2O3 range of 30 to 300 samples indicate that template-free ZSM-5 possess higher thermal and hydrothermal stability, good micropore diffusivity, and stronger acidity, as well as displays a good catalyzed performance in the n-hexane cracking reaction. Because of the low cost and environment friendly, the template-free ZSM-5 means a promising prospect of industrial application.