Microwave-assisted hydrothermal synthesis of nanozeolites with controllable size

In the present work, a microwave-assisted two-step hydrothermal procedure was proposed successfully to prepare various zeolite nanocrystals (e.g., silicalite-1, ZSM-5, LTL, BEA and LTA) with controllable size, morphology and SiO2/Al2O3 ratio. The effects of the starting gel composition (such as water content, alkalinity and ratio of SiO2/Al2O3) as well as the hydrothermal synthesis condition (temperature and time) on the size of the zeolite nanocrystals have been studied in detail. It is found that high synthesis temperature and long reaction time benefit the growth of all the referred zeolite nanocrystals. However, the effects of the gel composition are very complex to different kinds of nanozeolites. For the nanozeolites crystallized in low alkalinity systems (e.g., ZSM-5, BEA and LTA), both increasing the alkalinity and decreasing the water content accelerate their nucleation process and thereby result in the decrease of their crystal size. On the contrary, for those prepared in high alkalinity systems (e.g., LTL and silicalite-1), an inversed trend could be observed. Moreover, the change of SiO2/Al2O3 ratio in the starting gel greatly affects the size of nanozeolite ZSM-5 and LTL, but slightly influences that of nanozeolite BEA. These results would provide a solid foundation to fully exploit the traditional and untraditional applications of various nanozeolites.