Characterization of pore structure of zeolite nanosponges using 3D electron tomography

Abstract

Zeolites are crystalline microporous (< 2nm in diameter) aluminosilicate materials that are widely used as catalysts and adsorbents in petrochemical processes. However, solely microporous zeolites often suffer from a diffusion limitation of molecules which results in low accessibility of active sites and fast deactivation in catalytic applications. Many strategies have been investigated to obtain hierarchical zeolite that contains mesopores (2 – 50 nm in diameter) in addition to intrinsic zeolite micropores in order to overcome this problem. In 2009, Ryoo and his co-workers developed a successful strategy to synthesize hierarchical zeolite using dual-porogenic surfactants that are specially designed to have a zeolite-structure-directing head group and a mesopore-generating hydrophobic tail. A series of hierarchical zeolites synthesized using dual-porogenic surfactants with various zeolite structure types (MFI, MTW, *MRE, and beta) have been subsequently reported, and they are referred to as ‘zeolite nanosponges’. The zeolite nanosponges have a highly mesoporous texture with a narrow distribution of mesopore diameters, comparable to that of MCM-41, despite the disordered pore arrangement. The mesopore walls have strong Brønsted acid sites, allowing the zeolites to catalyze various reactions involving small or bulky organic molecules. However, because of the disordered nanosponge-like structure, it is difficult to determine the detailed structures of the pores, including the mesopore shapes, openings, interconnection, and connectivity to micropores which affect the catalytic and adsorption. The object of this study is to analyze and visualize the hierarchical pore architecture of the zeolite nanosponges. Mesopore shape, mesopore-mesopore connectivity, mesopore-micropore interconnection, and openness of the mesopores to the exterior are characterized by a combination of conventional characterization tools and 3D electron tomography. For high-contrast tomography, the zeolites were supported with platinum. The resultant tomograms visualized disordered and interconnected networks of Pt nanowires and nanosheets, which corresponded to the shape of the surfactant-directed mesopores. In the calcined zeolites, both the mesopores and zeolitic micropores were fully accessible for argon adsorption. Before calcination, however, no micropores were accessible even after thorough washing with a solvent. This indicated that the surfactant head could be tightly encased within the micropore after guiding the formation of the zeolite framework as a part of the mesopore wall, and consequently the surfactant was difficult to remove by solvent washing. After calcination, the micropore and mesopore could be connected through an aperture in which the neck of the surfactant molecule is located. Neohexane was adsorbed very rapidly due to the micropore-mesopore connectivity. The hierarchical pore connectivity is an important feature of the surfactant-directed zeolites for application in high performance adsorption and catalysis. Also, the electron tomography technique using supported platinum networks can be used to visualized various mesoporous materials.