A. James Clark School of Engineering

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    TWO DIMENSIONAL (2D) LEWIS ACIDIC ZEOLITE NANOSHEETS: SYNTHESIS, CHARACTERIZATION AND APPLICATIONS IN CATALYSIS AND SEPARATION
    (2020) Wu, Wei; Liu, Dongxia; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Zeolites are generally defined as 3D crystalline microporous aluminosilicates, widely used for adsorption, separation, and acid catalysis in chemical and petrochemical industry. Except for aluminum, other elements, such as B, Ti and Sn, also can be incorporated into zeolite frameworks, bringing different Lewis acidities and conferring various chemical properties. Although the zeolites are eco-friendly industrial-scale catalysts, they are challenged in the catalytic processes involving bulky molecules. The inaccessibility of active sites confined in zeolite micropores and slow mass transport lead to inferior catalytic performances. In comparison to 3D zeolite, 2D zeolites, which are endowed intrinsic properties of micropores and enhanced mass transport as well as active site accessibility of mesopores, have gained enormous attention owing to their improved performance. This dissertation introduces several methods to prepare 2D zeolites and explores their applications in catalysis and separation. Firstly, we reported a low-cost method to prepare long-range ordering multilamellar titanium silicalite-1 (TS-1) zeolite. The pillarization on multilamellar TS-1 zeolite has been achieved to preserve the mesoporosity, while the influences of several synthesis parameters have been investigated to optimize the synthesis conditions. Afterwards, aiming to advance the understanding of Lewis acidity in these 2D TS-1 zeolites, we quantitatively described the concentration, strength and local environment of these Lewis acid sites, and evaluated their kinetic behaviors. Besides, a bottom-up method and a top-down method were introduced to prepare delaminated 2D zeolite nanosheets. With the assistance of a special-designed structure-directing agent, several heteroatom-containing 2D zeolite nanosheets have been directly synthesized. Their structural and textural properties have been evaluated by various techniques. In the top-down method, starting from multilamellar silicalite-1 (S-1) zeolites via exfoliation and separation steps, the delaminated S-1 zeolite nanosheets can be obtained and used to fabricate ultra-thin zeolite membrane applied in ethanol/water pervaporation. At last, we have created a series of novel ultra-thin film photocatalytic light absorbers (UFPLAs) to solve the long-standing trade-off between optical absorption and carrier transportation in photocatalyst. The UFPLAs have significantly boosted the activities in photocatalysis, in comparison to the benchmark Aeroxide®P25 catalyst, such as photocatalytic carbon dioxide reduction and photocatalytic chemical war agent simulant detoxification.
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    Two-Dimensional Zeolite Materials: Structural and Acidity Properties
    (MDPI, 2020-04-12) Schulman, Emily; Wu, Wei; Liu, Dongxia
    Zeolites are generally defined as three-dimensional (3D) crystalline microporous aluminosilicates in which silicon (Si4+) and aluminum (Al3+) are coordinated tetrahedrally with oxygen to form large negative lattices and consequent Brønsted acidity. Two-dimensional (2D) zeolite nanosheets with single-unit-cell or near single-unit-cell thickness (~2–3 nm) represent an emerging type of zeolite material. The extremely thin slices of crystals in 2D zeolites produce high external surface areas (up to 50% of total surface area compared to ~2% in micron-sized 3D zeolite) and expose most of their active sites on external surfaces, enabling beneficial effects for the adsorption and reaction performance for processing bulky molecules. This review summarizes the structural properties of 2D layered precursors and 2D zeolite derivatives, as well as the acidity properties of 2D zeolite derivative structures, especially in connection to their 3D conventional zeolite analogues’ structural and compositional properties. The timeline of the synthesis and recognition of 2D zeolites, as well as the structure and composition properties of each 2D zeolite, are discussed initially. The qualitative and quantitative measurements on the acid site type, strength, and accessibility of 2D zeolites are then presented. Future research and development directions to advance understanding of 2D zeolite materials are also discussed.