A surfactant-directed sol-gel evolution in the formation of hierarchical zeolite beta.

Even though cetyltrimethylammonium bromide (CTAB) has been successfully used in the surfactant-templating system to create the mesostructured zeolites, the formation mechanism of the pore structure still remains elusive. Here, we investigated the sol-gel and structure evolution in the hierarchical zeolite Beta synthesis focusing on the role of CTAB in a dual templating system. It is found that the induction period was prolonged and the Ostwald ripening was retarded by the presence of CTAB in crystallization. CTAB was involved in the synthesis and incorporated within the solid phase at the first stage when mesopores were formed. However, the incorporated CTAB was released later into the solution when the crystallized microporous structure was developed. The thermogravimetric analysis revealed that CTA+ cation worked both as structure directing agent (SDA) for micropores and mesoporogen for the secondary lager pores in the matrix of hierarchical zeolite Beta. Moreover, CTA+ competed with tetraethylammonium cation (TEA+) to work as the SDA for micropores, with a decrease of the total amount of CTA+ and TEA+ when the concentration of CTAB in the synthesis solution was increased. It is anticipated that the understanding of the role of CTAB in the sol-gel and structure evolution in the hierarchical zeolite Beta synthesis process can provide a basis for designing the synthesis of hierarchical zeolites. [Display omitted] [ABSTRACT FROM AUTHOR]