Kolbe Electrolysis of Biomass‐Derived Fatty Acids Over Pt Nanocrystals in an Electrochemical Cell

Electrochemical decarboxylation of acetic acid on boron-doped diamond and platinum-functionalised electrodes for pyrolysis-oil treatment

Electrochemical decarboxylation of acetic acid on boron-doped-diamond (BDD) electrodes was studied as a possible means to decrease the acidity of pyrolysis oil. It is shown that decarboxylation occurs without the competitive oxygen evolution reaction (OER) on BDD electrodes to form methanol and methyl acetate by consecutive reaction of hydroxyl radicals with acetic acid. The performance is little affected by the applied current density (and associated potential), concentration, and the pH of the solution. At current densities above 50 mA cm-2, faradaic efficiencies (FEs) of 90% towards the decarboxylation products are obtained, confirmed by in situ electrochemical mass spectrometry (ECMS) investigation showing only small amounts of oxygen formed by water oxidation. Using platinum-modified BDD electrodes, it is shown that selectivity to ethane, the Kolbe product, strongly depends on the shape and geometry of the platinum particles. Using nano-thorn-like Pt particles, a faradaic efficiency of approx. 40% towards ethane can be obtained, whereas 3D porous platinum nanoparticles showed high selectivity towards the OER. Using thin platinum layers, a high FE of >70% towards ethane was obtained, which is thickness-independent at layer thicknesses above 20 nm. Comparison with other substrates revealed that BDD is an ideal support for Pt functionalisation, giving advantages of stability and high-value-product formation (ethane and methanol). In short, this work provides guidelines for electrode fabrication in the context of the electrochemical upgrading of biomass feedstocks by acid decarboxylation.

Amorphous RuO2 Catalyst for Medium Size Carboxylic Acid to Alkane Dimer Selective Kolbe Electrolysis in an Aqueous Environment

The catalytic transformation of biomass-derived volatile carboxylic acids in an aqueous environment is crucial to developing a sustainable biorefinery. To date, Kolbe electrolysis remains arguably the most effective means to convert energy-diluted aliphatic carboxylic acids (carboxylate) to alkane for biofuel production. This paper reports the use of a structurally disordered amorphous RuO2 (a-RuO2 ) that is synthesized facilely in a hydrothermal method. The a-RuO2 is highly effective towards electrocatalytic oxidative decarboxylation of hexanoic acid and is able to produce the Kolbe product, decane, with a yield 5.4 times greater than that of commercial RuO2 . A systematic study of the reaction temperature, current intensity, and electrolyte concentration reveals the enhanced Kolbe product yield is attributable to the more efficient oxidation of the carboxylate anions for the alkane dimer formation. Our work showcases a new design idea for establishing an efficient electrocatalysts for decarboxylation coupling reaction, providing a new electrocatalyst candidate for Kolbe electrolysis.

Hetero-Coupling of Bio-Based Medium-Chain Carboxylic Acids by Kolbe Electrolysis Enables High Fuel Yield and Efficiency

Mixtures of n-carboxylic acids (n-CA) as derived from microbial conversion of waste biomass were converted to bio-fuel using Kolbe electrolysis. While providing full carbon and electron balances, key parameters like electrolysis time, chain length of n-CA, and pH were investigated for their influence on reaction efficiency. Electrolysis of n-hexanoic acid showed the highest coulombic efficiency (CE) of 58.9±16.4 % (n=4) for liquid fuel production among individually tested n-CA. Duration of the electrolysis was varied within a range of 0.27 to 1.02 faraday equivalents without loss of efficiency. Noteworthy, CE increased to around 70 % by hetero-coupling when electrolysing n-CA mixtures regardless of the applied pH. Thus, 1 L of fuel could be produced from 12.4 mol of n-CA mixture using 5.02 kWh (<1 € L^-1 ). Thus, a coupling with microbial processes producing n-CA mixtures from different organic substrates and waste is more than promising.

Bio-Electrocatalytic Conversion of Food Waste to Ethylene via Succinic Acid as the Central Intermediate

Ethylene is an important feedstock in the chemical industry, but currently requires production from fossil resources. The electrocatalytic oxidative decarboxylation of succinic acid offers in principle an environmentally friendly route to generate ethylene. Here, a detailed investigation of the role of different carbon electrode materials and characteristics revealed that a flat electrode surface and high ordering of the carbon material are conducive for the reaction. A range of electrochemical and spectroscopic approaches such as Koutecky–Levich analysis, rotating ring-disk electrode (RRDE) studies, and Tafel analysis as well as quantum chemical calculations, electron paramagnetic resonance (EPR), and in situ infrared (IR) spectroscopy generated further insights into the mechanism of the overall process. A distinct reaction intermediate was detected, and the decarboxylation onset potential was determined to be 2.2–2.3 V versus the reversible hydrogen electrode (RHE). Following the mechanistic studies and electrode optimization, a two-step bio-electrochemical process was established for ethylene production using succinic acid sourced from food waste. The initial step of this integrated process involves microbial hydrolysis/fermentation of food waste into aqueous solutions containing succinic acid (0.3 M; 3.75 mmol per g bakery waste). The second step is the electro-oxidation of the obtained intermediate succinic acid to ethylene using a flow setup at room temperature, with a productivity of 0.4–1 μmol ethylene cm_electrode^–2 h^–1. This approach provides an alternative strategy to produce ethylene from food waste under ambient conditions using renewable energy.