Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Charles B. Musgrave

Second Advisor

Christopher N. Bowman

Third Advisor

Will J. Medlin

Fourth Advisor

Andrew B. Bocarsly

Fifth Advisor

James T. Hynes

Abstract

The efficient chemical reduction of CO2 to fuels has been of interest to scientists for decades with growing concerns about the impact of CO2 on climate and future global energy demands motivating increasing efforts to meet this challenge. One conversion of specific interest — the reduction of CO2 to methanol (CH3OH) — is the focus of my thesis. Arguments here involve CH3OH’s utility as a practical C1 source for chemical synthesis and its attractive properties as a fuel not demanding the massive changes to the transportation fuels infrastructure required for a hydrogen economy.

My thesis focuses on understanding the role of pyridine in catalyzing the conversion of CO2 to CH3OH. In particular, I employed quantum chemical simulations as an invaluable tool to probe the redox properties of a number of pyridine-derived intermediates involved in the catalytic cycle of CO2 reduction. Accurate determination of redox properties, e.g. reduction potentials and hydricity is important to paint a detailed picture of energetics involved in the transformation of transient species during the course of CO2 reduction, and thus the role of the catalytic species is revealed. One central aspect is the determination of the driving force to effect hydride transfer. 1,2-dihydropyridine (PyH2) is a potent recyclable organo-hydride donor because it is driven by its proclivity to regain aromaticity; this mimics important aspects of the role of NADPH in the formation of C-H bonds in the photosynthetic CO2 reduction process.

The aspect of controlling redox properties of molecules was applied to organic photocatalyst that affects photo-polymerization. In collaboration with the Stansbury’s group, we elucidated the mechanism of polymer synthesis involving methylene blue chromophore with a sacrificial sterically-hindered amine reductant and an onium salt oxidant. The combination of these components yield interesting results: light-initiated free-radical polymerization continues over extended time intervals (hours) in the dark after brief (seconds) low-intensity illumination. We proposed that these observations are due to the latent production of free radicals from energy stored in a redox potential through a 2e-/1H+ transfer process, which transforms the methylene blue chromophore to its high energy closed-shell intermediate of leuco methylene.

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