Electrochemical Oxidation of Bio-derivable Alcohols Using Inorganic Materials and Mediators

Abstract

Upcoming depletion of non-renewable fossil fuels provides an urgent prompt for reshaping the current structure of both direct energy sources and processing for industrial chemicals. Biorefinery hints an alternative pathway especially in processing industrial chemicals through oxidation of bio-derivable alcohols towards aldehydes and carboxylic acids that may serve as precursors to esters and polymers. With development in photovoltaic electrolyzers, renewable-energy compatible electrocatalysis serves as a promising tool to perform the selective oxidation reaction under the principles of green chemistry. Aldehydes are considered relevant industrial chemicals that are usually evolved from fossil-fuel dependent procedures. Alcohol oxidation towards aldehyde products usually suffers from poor selectivity, remains challenging in the field and relies heavily on noble metal complexes or electrocatalysts. Here, this thesis outlines strategies to use inorganic mediators and noble-metal-free inorganic materials to achieve selective ethanol oxidation towards acetaldehyde. Solvent-free electrolysis, with ethanol as both the solvent and the substrate, is heavily implemented, where acetalization allows the generated acetaldehyde to be protected. This scheme is however understudied with ethanol being thought of as a difficult solvent for electrochemistry. Under these circumstances, Chapter 2 investigates the use of chloride as redox mediator for solvent-free ethanol oxidation. On glassy carbon (GC), cyclic voltammetry (CV) shows chloride oxidation originates at lower potential compared to oxidation in chloride-free electrolyte. Constant potential chronoamperometry (CPC) shows 2-electron ethanol oxidation to 1,1-diethoxyethane (DEE) proceeds with >95% faradaic efficiency (FE) with chloride electrolyte. DEE arises from acetalization of acetaldehyde and protects acetaldehyde from overoxidation. UV–vis spectroscopy shows that ethyl hypochlorite (EtOCl) is the sole chloride oxidation product, which is known to decompose unimolecularly to form HCl and acetaldehyde. Finally, kinetic experiments show steady-state formation of EtOCl during electrolysis. Chapter 3 seeks to understand the mechanism of the chloride oxidation reaction (COR) in ethanol. CV carried out at varying scan rate in inert dichloromethane solvent establishes a Volmer step, where solution chloride ion adsorbs as chlorine(0) on GC. At higher applied potential, a second electron transfer from ethanol solvent occurs, forming EtOCl. Rotating ring-disk electrode (RRDE) measurement and in situ spectroelectrochemical measurements corroborate the mechanism by showing no EtOCl is produced in lower potential (<0.8V), despite anodic current being observed. Thus, the 2-electron COR mechanism is confirmed to be Volmer step followed by a subsequent reaction between ethanol and adsorbed chlorine(0). Koutecky-Levich (K-L) analysis quantifies the kinetic rate constant of the COR to be 10–5 to 10–6 s–1, which is 2 – 3 orders of magnitude faster than direct alcohol oxidation. Chapter 4 develops a noble-metal-free electrocatalyst for neat ethanol oxidation. MnOx is deposited on FTO by electrochemical deposition. The material is amorphous by X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) reveals a mixed-valent manganese (between Mn3+ and Mn4+). Similar linear-sweep voltammetry (LSV) current responses are observed when tested with Cl--, NO3¬--, and OTf-- containing electrolytes, indicating direct ethanol oxidation occurs on MnOx. After 16-hour CPC, >90% FE for DEE is observed by gas chromatrography in both Bu4NOTf and HOTf electrolytes. Elemental analysis of the electrolyte solution shows <1% Mn dissolved after long-term electrolysis, highlighting its operational stability. With no potential applied, Mn dissolution occurs, and we propose the chemical oxidation of ethanol by Mn3+ is the rate limiting step, supported by Tafel analyses in electrolytes of varying acidity.