Altering Framework Topology and Heteroatom Distributions of Molecular Sieves by Designed Organic Structure-Directing Agents

Citation

Park, Youngkyu (2024) Altering Framework Topology and Heteroatom Distributions of Molecular Sieves by Designed Organic Structure-Directing Agents. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/d1xj-kn25. https://resolver.caltech.edu/CaltechTHESIS:04292024-055507772

Abstract

The growing demand for chemical production combined with the urgent need to mitigate the accelerated climate and environmental changes motivates efforts to create highly efficient and selective catalysts and adsorbents. Zeolites and molecular sieves are a key class of materials for addressing these needs because of their high activity and selectivity with catalytic reactions. Additionally, they can show superior adsorption properties because of their structure and surface polarity that can also give shape selectivity with molecules smaller than ca. 1 nanometer. Further advancements in molecular sieve properties will rely on advancements in preparation methods. To this end, the research results presented here explore synthetic approaches for controlling the framework topology and the heteroatom incorporation within silicate-based molecular sieves by means of the strategic design of their organic structure-directing agents (OSDAs).

Part I presents the synthesis of STW-type germanosilicate molecular sieves with high-silica framework compositions and the enrichment of chirality. A chiral OSDA is computationally designed based on the predicted stabilization energy toward the pure-silica STW framework. An improved synthesis route for both enantiomers of the OSDA is developed. The enantiopure OSDA is capable of crystallizing a high-silica STW-type germanosilicate molecular sieve that shows distinct framework compositions from previously reported germanium-rich STW. The enantiomeric enrichment of powdered samples without occluded enantiopure OSDAs is characterized by the dynamical refinement of microcrystal electron diffraction data. The high-silica, enantiomerically enriched STW exhibits the framework stability upon thermal treatment and the enantioselective adsorption of 2-butanol. The results in Part I demonstrate the design strategy of OSDAs for crystallizing stable, enantio-enriched molecular sieves for enantioselective chemical separations and catalysis.

In Part II, the distribution of heteroatoms incorporated within borosilicate molecular sieves is studied with regard to its control by cationic OSDAs. To aid in the characterization of the heteroatom sites within borosilicate molecular sieves, the relationship between the ¹¹B NMR chemical shift and the local geometry of boron within tetrahedrally coordinated silicate frameworks is first investigated. From crystalline borosilicate minerals with highly ordered, tetrahedrally coordinated boron atoms, it is revealed that the chemical shifts from ¹¹B NMR linearly correlate with the local geometric parameters. Further studies on the borosilicate molecular sieves that possess more open space and wider angles suggest that the correlation between the average bond angles and ¹¹B NMR chemical shifts can be employed for the entire class of three-dimensional, crystalline borosilicates. Two structurally similar quaternary ammonium OSDAs with different locations of positive charge are designed and synthesized. MWW-type borosilicate molecular sieves are crystallized by both OSDAs, and the quaternary ammonium moieties in the two OSDAs are found to interact with boron species with significantly different ¹¹B NMR chemical shifts. Using the correlation developed here, the characterization results demonstrate that the heteroatom siting within the molecular sieve framework can be selectively altered by tailoring the OSDA structure in terms of the position of positive charge.