The Manipulation of Hydrophobicity in Catalyst Design for Applications of Aerobic Alcohols Oxidation and Electrocatalytic Water Oxidation

  • Batian Chen

Student thesis: Doctoral Thesis

Abstract

Hydrophobicity is the generalized characteristic of non-polar substances that brings about their exclusion from aqueous phases. This property, entropic in its nature, drives key self-assembly and phase separation processes in water. Protein folding, the formation of DNA double helix, the existence of lipid bilayers and the wetting properties of leaf surfaces are all due to hydrophobic interactions. Inspired by Nature, we aimed to use hydrophobicity for creating novel and improved catalytic systems. (I) A number of fluorous amphiphilic star block-copolymers containing a tris(benzyltriazolylmethyl)amine motif have been prepared. These polymers assembled into well-defined nanostructures in water, and their mode of assembly could be controlled by changing the composition of the polymer. The polymers were used for enzyme-inspired catalysis of alcohol oxidation. (II) An enzyme-inspired catalytic system based on a rationally designed multifunctional surfactant was developed. The resulting micelles feature metal-binding sites and stable free radical moieties as well as fluorous pockets that attract and preconcentrate molecular oxygen. In the presence of copper ions, the micelles effect chemoselective aerobic alcohol oxidation under ambient conditions in water, a transformation that is challenging to achieve nonenzymatically. (III) Development of a facile means of photo/electrocatalytic water splitting is one of the main barriers to establishing of a solar hydrogen economy. Of the two half-reactions involved in splitting water into O2 and H2, water oxidation presents the most challenge due to its mechanistic complexity. A practical water oxidation catalyst must be highly active, yet inexpensive and indefinitely stable under harsh oxidative conditions. Here, I shall describe the synthesis of a library of molecular water oxidation catalysts based on the Co complex of tris(2-benzimidazolylmethyl)amine, (BimH)3. A wide range of catalysts differing in their electronic properties, surface affinity, and steric bulk was explored. We identified hydrophobicity as the key variable in mediating the catalytic competence of Co-(BimH)3 complexes. The change in this parameter correlates both with the conformational mobility of the ligand core and the structural changes in the local solvent environment around the catalytic metal site. The optimal ligand identified is superhydrophobic due to three fluorinated side chains. The corresponding Co complex catalyzes water electrooxidation efficiently, with an onset potential equal to that for the well-established CoPi heterogeneous system, albeit with a dramatically higher turnover frequency (TOF) and in the absence of soluble Co salts. As an added benefit, the hydrophobic catalyst can be immobilized through physisorption, and remains stable after prolonged controlled-potential electrolysis. A DFT calculation was also performed to understand the catalytic pathway.
Date of AwardMay 17 2016
Original languageEnglish (US)
Awarding Institution
  • Physical Science and Engineering
SupervisorValentin Rodionov (Supervisor)

Keywords

  • Hydrophobicity
  • Fluorous
  • Aerobic Oxidation
  • Water Oxidation

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