Zeolites with multi-level pore hierarchy: synthesis, characterization of diffusion property and catalytic investigation

Abstract

Zeolites are a family of crystalline aluminosilicates whose structure is composed of a regular array of uniform micropores of a molecular dimension (micropores are defined as pores with diameters of less than 2 nm). Zeolite is hydrothermally synthesized using a micropore-generating agent (i.e., microporogen) such as metal cations (e.g., Na+, K+) or quaternary ammonium compounds. This material has been widely used in industry as a solid acid catalyst. However, due to the sole presence of small micropores, it has serious diffusion limitations in various applications, especially in catalytic applications. To resolve this problem, introducing larger pores (diameter > 2 nm) in hierarchical order has been suggested. The hierarchical structure of mesopores and micropores can be generated using a mixture of two different porogens, typical ammonium-type micro-porogen and organosilane surfactants which are self-assembled into micelles which act as meso-porogen. Even where zeolite crystal was obtained with a very large particle size (~ 10μm), the organosilane surfactant could generate uniform mesopores throughout the crystal. In addition, a hierarchical pore structure can be generated using a multi-ammonium surfactant which can generate both micropores and mesopores simultaneously. With this method, if the zeolite can be synthesized while retaining its initial morphology, a higher-level pore hierarchy containing an additional larger pore (i.e., larger mesopore or macropore) could be generated where a macroporous (diameter > 50 nm) solid zeolite precursor or a gel-type precursor which contained a larger mesopore (diameter ~ 30 nm) is typically used. Hierarchical pore structures have been generated from the beginning of the zeolite-crystal growth phase using an organosilane surfactant or a multi-ammonium surfactant, which was investigated in detail via a structural investigation using electron microscopy and x-ray diffraction analysis. In addition, the effect of the hierar...