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 N-2 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.