Catalytic function of acid sites located on the external surface of hierarchically nanoporous zeolites

Abstract

The work presented in this thesis addresses the catalytic performance of hierarchically nanoporous zeolites with focus on the acid sites located on the large external surfaces of the nanoporous zeolites. Zeolites are crystalline aluminosilicate materials that have uniform micropores (0.3 < diameter < 1.2 nm) exhibiting the molecular-sieving effect. Zeolites also exhibit very strong acidity and structure stability. Due to these advantages as catalysts, zeolites account for 40% of the total heterogeneous catalysts in industrial applications. Typically, synthetic zeolites are obtained in the form of micrometer-sized crystals, so that the external surface area of zeolite crystal is much smaller than the micropore wall. In this case, since most of acid sites are located at the micropore wall, it is generally regarded that the catalytic reaction occurs mostly inside the micropores. In the micrometer-sized crystals, the length of micropore channels is quite long and diffusion of molecules is limited inside the channels. Due to this limitation, there would be an in-efficient usage of the deep part of the zeolite catalyst and the catalyst deactivation attributed to the blockage of pore channel by coke deposition. In order to overcome these problems, there have been many efforts to enhance the molecular diffusion by the addition of mesopores as a highway for molecules and reduction of crystal size to shorten the micropore channel length. Recently, the Ryoo and co-workers developed a surfactant-directed synthesis method for the preparation of hierarchically nanoporous zeolite with sub-nanometer crystal thickness. The method can be applied to the synthesis of various structure types of zeolites with MFI, BEA, MTW, and MRE structures. The nanoporous zeolites are composed of nanometer-sized crystals and have extremely short diffusion path length in the micropore channel. In addition to the short pore channel, the nanoporous zeolites have a considerable amount of acid sites ...