Hierarchical ZSM-11 zeolite prepared by alkaline treatment with mixed solution of NaOH and CTAB: characterization and application for alkylation of benzene with dimethyl ether

Abstract

A facile alkaline treatment with the addition of surfactant cetyltrimethylammonium bromide (CTAB) to prepare hierarchical ZSM-11 zeolite (Z-xat-yCTAB) is presented. The textural and structural properties of Z-xat-yCTAB were characterized by XRD and adsorption and desorption of N₂ and benzene. By virtue of CTAB addition, uniform intracrystalline mesopore distribution centered at ca. 4.2 nm was introduced accompanied by well-protected microporosity. Based on characterization results such as TEM images and BJH curves as well as ²⁹Si and ²⁷Al MAS NMR spectra, the composite effects of NaOH and CTAB on mesoporosity production were proposed. Acidity was characterized thoroughly by FTIR of adsorbed pyridine (Py-IR) and pivalonitrile. Accordingly, the ratio and accessibility factor of Brønsted and Lewis acid sites are discussed systematically. In the alkylation of benzene with dimethyl ether, Z-xat-yCTAB series samples exhibited better reaction stability than ZSM-11 samples treated with NaOH solution. The catalytic promotion could be attributed to the dual effects of NaOH and CTAB on the porosity and acidity regulation. Moreover, based on the correlation between the reaction stability and the ratio of Brønsted to Lewis acid concentration with weak–medium strength, measured by Py-IR, it was revealed that the regulation of acidity could play a more important role for better reaction stability. In addition, the physicochemical properties and reaction activity were compared between ZSM-11 samples derived from alkaline treatment with CTAB addition and with tetrapropylammonium bromide (TPABr) addition under the same conditions.