Crystallization of zeolite Beta in the presence of an anionic surfactant AESA

Abstract

Organic molecules are widely used as structure directing agents, mesopore generating agents or zeolite growth modifiers in the synthesis of various zeolites. However, the organic molecules used in zeolite synthesis are mostly positively charged cationic molecules, since the cationic group can balance the negative charge in the zeolite structure. Here we attempt for the first time to apply an anionic surfactant fatty alcohol polyoxyethylene ether ammonium sulfate (AESA) in the crystallization of zeolite Beta. It is found that both the crystallization process and the characteristics of the product were affected by the addition of AESA into the synthesis. Compared with that without AESA, it shows that the induction period was significantly prolonged for the synthesis of zeolite Beta with AESA, and the crystallinity and the completeness of the crystal were improved. The yield of the product was increased when AESA was used in the synthesis. The framework Si/Al ratio was increased when AESA was added into the synthesis gel, which could be due to the fact that the anionic surfactant AESA facilitated the formation of Si–O–Si bonds and hindered the combination of the negatively charged aluminum species to silicon species to form Si–O–Al bonds, thus increasing the incorporation of Si and reducing the incorporation of Al into the framework. Moreover, we demonstrate an ability to enrich chiral polymorph-A in zeolite Beta by using AESA in the crystallization, with the proportion of polymorph-A in the as synthesized zeolite Beta reaching 77.9%. It is reported that AESA did not remain in the structure of zeolite Beta when the crystallization was completed, which implies that AESA plays a role of zeolite growth modifiers during the crystallization. The results of this study suggest that anionic surfactants can be used in zeolite synthesis to optimize the crystallization process and achieve zeolites with the desired properties.