Light-Driven Catalytic Upgrading of Butanol in a Biohybrid Photoelectrochemical System

This paper reports the design and preparation of a biohybrid photoelectrochemical cell (PEC) that can drive the tandem enzymatic oxidation and aldol condensation of n-butanol (BuOH) to C₈ 2-ethylhexenal (2-EH). In this work, BuOH was first oxidized to n-butyraldehyde (BA) by the alcohol oxidase enzyme (AOx), concurrently generating hydrogen peroxide (H₂O₂). To preserve enzyme activity and increase kinetics nearly 2-fold, the H₂O₂ was removed by oxidation at a bismuth vanadate (BiVO₄) photoanode. Organocatalyzed aldol condensation of C₄ BA to C₈ 2-EH improved the overall BuOH conversion to 6.2 ± 0.1% in a biased PEC after 16 h. A purely light-driven, unbiased PEC showed 3.1 ± 0.1% BuOH conversion, or ∼50% of that obtained from the biased system. Replacing AOx with the enzyme alcohol dehydrogenase (ADH), which requires the diffusional nicotinamide adenine dinucleotide cofactor (NAD⁺/NADH), resulted in only 0.2% BuOH conversion due to NAD⁺ dimerization at the photoanode. Lastly, the application of more positive biases with the biohybrid AOx PEC led to measurable production of H₂ at the cathode, but at the cost of lower BA and 2-EH yields due to both product overoxidation and decreased enzyme activity.