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.

Platinized Titanium as Alternative Cost-Effective Anode for Efficient Kolbe Electrolysis in Aqueous Electrolyte Solutions

Five commercial materials were assessed for electrochemical conversion of n-hexanoic acid by Kolbe electrolysis. Platinized titanium performed best, achieving a coulombic efficiency (CE) of 93.1±6.7 % (n=6) for the degradation of n-hexanoic acid and 48.3±3.2 % (n=6) for the production of n-decane, which is close to the performance of pure platinum (89.7±14.4 and 55.5±3.5 %; n=6). 56.7 mL liquid fuel was produced per mole n-hexanoic acid, converting to an energy demand of 6.66 kWh and 1.22 € per L. Using optical profilometry and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, it was shown that the degree of coverage of the titanium surface with platinum played the most important role. An uncovered surface of as little as 1-3 % already led to a deterioration of the CE of approximately 50 %. Using platinized titanium requires >36 times less capital expenditure at only <10 % increased operational expenditure; an electrode lifetime of 10000 h can be expected.

Hydrogen from Water is more than a Fuel: Hydrogenations and Hydrodeoxygenations for a Biobased Economy

Worldwide a hydrogen-based economy is on the political agenda. Its centre forms molecular hydrogen (H₂ ) that should serve mainly as energy carrier and fuel. However, currently and foreseeable in the future H₂ is playing its main role as reactant in the chemical industry. Electrolytic generation and storage of H₂ gas is energy demanding and may hardly become economically at the large scale. We argue that in the overall transition towards an economy that is based on biomolecules and CO₂ as carbon feedstock electrochemical hydrogenations and hydrodeoxygenations in aqueous solutions need to be moved in the centre. Departing from the well-known fact that electrochemistry allows creating reactive hydrogen species from water, i. e. hydrogen in statu nascendi (H^. ), at ambient temperature and pressure we illustrate the existing diversity of reactions based thereon. We focus on examples of model compounds from thermal biomass pretreatment and products from real thermal biomass pretreatment (bio-oil). Consequently, we advocate that electrochemical hydrogenations and hydrodeoxygenations have to be further explored and interweaved into existing process lines.

Biocatalytic Reduction Reactions from a Chemist's Perspective

Reductions play a key role in organic synthesis, producing chiral products with new functionalities. Enzymes can catalyse such reactions with exquisite stereo-, regio- and chemoselectivity, leading the way to alternative shorter classical synthetic routes towards not only high-added-value compounds but also bulk chemicals. In this review we describe the synthetic state-of-the-art and potential of enzymes that catalyse reductions, ranging from carbonyl, enone and aromatic reductions to reductive aminations.

Coupled Electrochemical and Microbial Catalysis for the Production of Polymer Bricks

Power-to-X technologies have the potential to pave the way towards a future resource-secure bioeconomy as they enable the exploitation of renewable resources and CO2 . Herein, the coupled electrocatalytic and microbial catalysis of the C5 -polymer precursors mesaconate and 2S-methylsuccinate from CO2 and electric energy by in situ coupling electrochemical and microbial catalysis at 1 L-scale was developed. In the first phase, 6.1±2.5 mm formate was produced by electrochemical CO2 reduction. In the second phase, formate served as the substrate for microbial catalysis by an engineered strain of Methylobacterium extorquens AM-1 producing 7±2 μm and 10±5 μm of mesaconate and 2S-methylsuccinate, respectively. The proof of concept showed an overall conversion efficiency of 0.2 % being 0.4 % of the theoretical maximum.

Resting Escherichia coli as Chassis for Microbial Electrosynthesis: Production of Chiral Alcohols

Chiral alcohols constitute important building blocks that can be produced enantioselectively by using nicotinamide adenine dinucleotide (phosphate) [NAD(P)H]-dependent oxidoreductases. For NAD(P)H regeneration, electricity delivers the cheapest reduction equivalents. Enzymatic electrosynthesis suffers from cofactor and enzyme instability, whereas microbial electrosynthesis (MES) exploits whole cells. Here, we demonstrate MES by using resting Escherichia coli as biocatalytic chassis for a production platform towards fine chemicals through electric power. This chassis was exemplified for the synthesis of chiral alcohols by using a NADPH-dependent alcohol dehydrogenase from Lactobacillus brevis for synthesis of (R)-1-phenylethanol from acetophenone. The E. coli strain and growth conditions affected the performance. Maximum yields of (39.4±5.7) % at a coulombic efficiency of (50.5±6.0) % with enantiomeric excess >99 % was demonstrated at a rate of (83.5±13.9) μm h^-1 , confirming the potential of MES for synthesis of high-value compounds.