Elucidating the Nature of Active Sites in Heterogeneous Catalysts through the Functionalization of Metal-Organic Frameworks

Wang, Qining

doi:https://doi.org/10.21985/n2-9y1k-bp25

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Elucidating the Nature of Active Sites in Heterogeneous Catalysts through the Functionalization of Metal-Organic Frameworks

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Heterogenous catalysis is the pillar of chemical production and a crucial aspect for optimization toward a sustainable future. To improve the current design of heterogeneous catalysts of maximal activity and product selectivity, gaining fundamental understanding of the catalytic active sites is crucial. The nature of active sites has been the center of debate in the scientific community for the past few centuries due to the structural complexity of heterogeneous catalysts. This Thesis compiles a few examples of simplifying active site structures by constructing different heterogeneous catalysts on a structurally well-defined porous support, metal-organic framework (MOF) NU-1000. Those heterogeneous catalysts include metal ion catalysts with tunable ligand environment, nuclearity, and composition. The functionalization of the MOF support was also explored. For the modulation of the ligand environment, NU-1000-supported Ni-thiophenolate complexes were chosen to understand how changing the substituent on the thiophenolate groups can influence the electronics of Ni and how this modulation affects the reactivity of the Ni center for ethylene hydrogenation. The nuclearity of Ni in the same NU-1000-supported Ni system was modified by changing the available deposition sites in the framework. Experimental results showed that di-nuclear Ni2+ species are more reactive than single Ni ions for ethylene hydrogenation. Computational studies corroborate those observations, where ethylene hydrogenation is more enthalpically favorable on di-nuclear Ni sites. The effect of catalyst composition was studied on a bimetallic catalyst, NiCu supported on NU-1000, where the Cu:Ni ratio of the catalytic species was modulated. The catalyst with higher Ni concentration showed significantly lower activation energy than Cu-rich and Cu-only catalysts. Those results were rationalized through spectroscopic studies, which show the beneficial role of Ni in facilitating the rate-limiting βC-H bond scission and suppressing the reduction of reactive Cuδ+ species. Further exploration of the support modification was examined on thermally distorted NU-1000 nodes, where the sulfidation of the nodes was achieved with thiols bearing different functional groups. Overall, this Thesis showcases various possibilities for functionalizing the nodes and the cavity of MOF NU-1000 to alter the properties of supported catalysts and realize the nature of catalytic active sites. The studies compiled in this Thesis would further inspire the utilization of MOF-based catalysts for low-temperature chemical productions and encourage the implementation of novel strategies in catalyst design.

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