Date of Award

Spring 2022

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Aaron K. Vannucci

Abstract

The current field of catalysis is notably advanced in terms of both conceptual and technical innovation. There is a constant need to design new catalysts and develop catalytic processes to improve the scope, efficiency, and selectivity of various chemical transformations. It is also imperative to design catalytic systems which are effective, truly-recyclable and energy efficient for a sustainable future. This can be done through comprehensive knowledge and understanding of catalyst behavior, reaction kinetics and process designing.

For most industrial processes, the preferred catalysts are heterogeneous because these solid catalysts can be easily separated from the final product. However, they often lack selectivity and produce waste due to unwanted side reactions. Traditional homogeneous molecular catalysts are highly selective but lack practical industrial utility due to challenges in catalyst separation, final product separation and catalyst degradation in aqueous medium and thus face manufacturing scale-up complexities. To address these challenges and build an active, robust and highly selective catalytic system, designing a hybrid catalyst by immobilizing these molecular catalysts onto a solid support is a viable solution for sustainable catalysis. This could transform the practical utility of molecular catalysts, increase the reaction selectivity with simplified catalyst separation and product recovery, and prevent catalyst decomposition by impeding any bimolecular catalyst interactions. Thus, Chapter 2 details this study based on the well-studied Suzuki carbon– carbon cross-coupling reaction, to demonstrate the ability to achieve catalytic performance using a non-noble nickel-based molecular catalyst in high aqueous content solvents. Furthermore, this study demonstrates a new paradigm in the design of hybrid catalysts in which Atomic Layer Deposition (ALD) is used to improve the attachment and stability of molecular catalysts on solid metal oxide supports. This research has helped achieve a combination of ligand first surface attachment with molecular design and ALD application for new approaches in catalyst discovery. Encouraged by the results from this study, further study on synthesizing nickel bipyridine based hybrid catalysts anchored onto the silica oxide support via carboxylic acid and silatrane linking group was undertaken to analyze if covalent binding of the silatrane linker promotes stronger immobilization of the molecular catalyst onto the oxide support. Comparative analysis on synthesis and characterization of molecular and hybrid versions of carboxylic acid and silatrane linker is discussed in Chapter 3.

Another foremost area which has gained increased momentum is developing hydrocarbon-free, green energy technologies and exploring promising solutions for utilizing and producing sustainable energy to meet our current and future energy needs. In this regard, the aims for Chapter 4, are to study energyefficient photocatalytic hydrogen production and investigate the catalytic pathway as part of the solution to the green energy transition. Natural photosynthesis uses an elaborate array of chemical structures to absorb multiple photons of light and convert that energy into chemical bonds in a “Z-scheme” process. Extensive research has been performed in the field of artificial photosynthesis to mimic this natural Z-scheme. To date, however, an artificial Z-scheme composed only of molecular components has yet to be realized.Here, we have developed a novel Z-scheme based molecular photocatalytic system by utilizing the light absorbing properties of aryl-alcohol photocatalysts for hydrogen production at significantly lower reaction overpotentials (≤40meV). Chemical, electrochemical, and spectroscopic data studies supports a Z-scheme based pathway and provides mechanistic insights into hydrogen production via dual photoexcitation. This research can open new avenues for further study of molecular photocatalytic hydrogen production.

Rights

© 2022, Pooja J. Ayare

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