Synthesis And Applications Of Two-Dimensional Zeolites To Catalysis And Membrane Separations

Zeolites have proven themselves as attractive materials for catalysis and separation applications due to their ordered microporous structure and presence of strong acid sites. These materials can crystallize in several topologies and compositions that can be used to tune their properties to suit a particular application. However, due to their microporous nature, these materials impose mass transfer limitations, especially on large molecules, that reduce their catalytic activity. Two-dimensional zeolite nanosheets with single unit-cell thickness and high aspect ratio help to overcome this limitation by reducing the diffusion path length and increasing the external surface area that provides better access to catalytic sites in when using large molecules. These nanosheets can also be exfoliated and processed into highly oriented, ultra-thin membranes that can retain the molecular sieving ability of the corresponding bulk 3D zeolite. However, only 10 or so frameworks have been synthesized so far in the 2D morphology and even for the existing 2D zeolite nanosheets, the structure-property relationships have not been fully studied. The work described here aims to develop structure-property relationships for some of the existing zeolite nanosheets by using them in catalytic and membrane separation applications while developing generalized synthesis methodologies to expand the library of zeolites crystallized with a single unit-cell thickness. The first objective of the thesis was to study the effect of Si/Al ratio on the catalytic activity of 2D MFI nanosheets. The nanosheets with varying Si/Al ratios were characterized in detail and their catalytic performance was compared using the liquid phase Friedel-Crafts alkylation of mesitylene with benzyl alcohol & the self-etherification of xvi benzyl alcohol that occurs in parallel, both of which are catalyzed by Brønsted acid sites. The turnover frequency (TOF) of the catalysts is found to decrease with decreasing Si/Al ratio, with the etherification reaction being the main contributor to this trend. When the same reaction is carried out in the presence of a bulky poison, 2,6 di-tert-butylpyridine (DTBP), to selectively deactivate the external acid sites, only the etherification reaction of benzyl alcohol takes place in the micropores of the 2D MFI catalyst and the effectiveness factor is found to decrease with decreasing Si/Al ratio. Thus, increasing the density of acid sites in the micropores by decreasing the Si/Al ratio makes it more difficult for the reactant molecules to access them, as demonstrated by the decrease in TOF and the effectiveness factor. In the next objective, exfoliated AEL nanosheets, with one-dimensional pores running through the plane of the nanosheets, were used to form an intermediate layer on the shell side of α-alumina hollow fiber for forming a poly(amide) (PA) film via interfacial polymerization (IP) between a diamine in an aqueous phase and an acid chloride in an organic phase. PA membranes have been widely used for the desalination of brackish water and seawater. The thin AEL molecular sieve nanosheet layer likely acts as a reservoir to store the diamine molecules, while allowing for its controlled release during the IP reaction. The composite AEL nanosheet-poly(amide) membrane exhibited a high water flux and NaCl rejection as well as displayed long-term stability in pervaporative desalination. Lastly, new organic structure directing agents (OSDAs) were synthesized and screened to crystallize new zeolites with single unit-cell crystal thickness in a one-pot, bottom-up manner. This has resulted in the crystallization of a novel microporous zeolitic nanotube, using a bolaform OSDA with a biphenyl group in the hydrophobic center and xvii hydrophilic quinuclidinium groups at the ends, separated by a C10 carbon chain. The growth mechanism of the nanotubes was studied through time-resolved crystallization of the material that revealed the formation of a mesostructure very early on in the synthesis due to the micellar assembly of the OSDA. The SDA most likely forms a cylindrical or rod-like micellar assembly, with the biphenyl groups of the SDA molecules interacting with each through π-π interactions, while the quinuclidinum groups direct the crystallization of the microporous zeolitic walls of the nanotube. In summary, this thesis will help to advance the field of separations and catalysis by uncovering structure property relations of existing 2D zeolites while developing a new zeolite morphology. All these materials may be advantageous over their conventional 3D zeolite counterparts.Ph.D