Hierarchical Hexagonal Zeolites: Intrinsic Transformation Directed by Precise Control of Synthetic Conditions

Hierarchical hexagonal zeolites with narrow pore size distribution were obtained by a simple temperature-gradient method using a multiammonium surfactant as micromesopore generating agent. The pore textures of the as-synthesized samples were characterized by electron tomography and nitrogen adsorption–desorption. The samples possessed a disordered and interconnected micro- and meso-pore structure, in which the mesoporous structure was introduced at lower temperature and then partially converted to microporous zeolite at higher temperature. The acid sites were characterized by FT-IR pyridine adsorption experiments under different temperatures. A relative higher amount of Brønsted and Lewis acid sites (0.074 and 0.240 mmol/g, respectively) were found in the HAM(9) sample, which also had a bigger external surface. Catalytic performance of the samples was evaluated by the Claisen-Schmidt condensation of benzaldehyde and 2′-hydroxyacetophenone. An outstanding conversion as high as 86% by sample HAM(9) demonstrated the synergistic effect of micropores with acid sites of high activities and mesopores for effectively transporting bulky reactants. The stability of catalyst was also illustrated, where less than 5% change in conversion was found after four cycles.