Zeolite Synthesis Using Hierarchical Structure-Directing Surfactants: Retaining Porous Structure of Initial Synthesis Gel and Precursors

Abstract

Zeolite beta with a mesopore-micropore hierarchy is hydrothermally synthesized here using various surfactants that can function as a hierarchical structure-directing agent at both the meso- and microstructural levels. Tetraethoxysilane and diatomaceous earth were tested as the silica sources. The pore size distribution of the zeolite was analyzed by N2 adsorption and electron microscopy. The result was related to the surfactant structure, silica source, and Na+ concentration. The zeolite showed quite uniform mesopores corresponding to the surfactant micelles, in addition to zeolitic micropores generated by the surfactant head groups. Furthermore, an additional level of large pores (≥20 nm) could be obtained as a result of the retaining of pores from the initial synthesis gel or the zeolite precursor. The large pores were fully retained when the silica source was transformed into a crystalline zeolite via pseudomorphic transformation without migration into the solution phase. The transformation could be controlled by the choice of the synthesis conditions and surfactants. The resultant zeolite with a macropore-mesopore-micropore hierarchy shows potential applicability where facile diffusion is required.