Pushing the Limits on Metal–Organic Frameworks as Catalyst Supports: Small Hydrocarbons Reactions over NU-1000-Supported CatalystsPublic
Heterogeneous catalysts based on metal oxides are of significant interest for many industrial chemical reactions. These catalysts, however, often suffer from ill-defined structures that preclude better understanding of the surface phenomena. Thus, structurally well-defined catalysts have received growing attention by making it feasible to understand the kinetics and reaction mechanisms. Structurally precise catalysts can be achieved by utilization of atomically controlled synthesis technologies and/or structurally well-defined catalyst supports. Zr based metal–organic framework (MOF), NU-1000 (NU stands for Northwestern University) is a suitable candidate for the well-defined catalyst support as grafting sites are regularly ordered and isolated from each other. This dissertation seeks to develop a more fundamental understanding of how NU-1000-supported metal oxide catalysts serve as well-defined analogs to conventional metal oxide catalysts. Chapter 1 introduces the reason why NU-1000 is one good candidate as a catalyst support and presents a synthesis protocol, post-modification methods, physicochemical properties (given by N2 isotherm, powder x-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, scanning electron microscopy), and additional characterization methods (x-ray absorption spectroscopy and difference envelope density measurements) to investigate structures upon modification of NU-1000. Lastly, this part enumerates examples of application of NU-1000-supported metal oxides as heterogeneous catalysts. Chapter 2 through Chapter 4 focus on my own contribution to develop synthesis-structure-activity relationships of NU-1000-supported metal oxide catalysts in small hydrocarbon reactions. To be specific, Chapter 2 presents NU-1000-supported Nb oxide as Lewis acid catalysts to study H2O2 activation in the condensed-phase cyclohexene epoxidation. Since NU-1000 provides isolated grafting sites, almost all Nb oxide sites are accessible despite high Nb loading. In Chapter 3, motivated to further understand H2O2 activation in the vapor-phase, our team has built a vaporphase hydrogen peroxide reactor, and we study kinetics and selectivity trends of the same epoxidation reaction over supported metal oxide catalysts, which are Ti or Nb grafted on SiO2 and NU-1000 supports. Moreover, Chapter 4 discusses pushing the limit of NU-1000 as a catalyst support under harsh reaction conditions to investigate the applicability as a solid acid catalyst through installing isolated W oxide or clusters of W oxide in NU-1000, and then by testing isomerization and disproportionation of o-xylene as a model reaction. Separately, the Appendix summarizes my collaborating works with other research groups to develop understanding of MOFs as catalysts in oxidation and oxidative dehydrogenation reactions. The synthesis-structure-function relationships developed here have important implications for the rational design of MOFs as catalyst supports in the industrially relevant reactions.