Eco-efficiency analysis as a reaction-engineering tool—Case study of a laccase-initiated oxidative C–N coupling

The laccase-mediated heterocoupling of 3-methylcatechol and n-hexylamine was investigated at the milligram to gram scale, and an early eco-efficiency analysis and evaluation revealed bottlenecks in enzymatic synthesis. Eco-efficiency analysis was used as an optimisation tool to create more sustainable solutions and improve subsequent reactions, enabling the scaled-up biocatalytic process to become more economical and competitive compared with the chemocatalysed procedure using sodium iodate. When working with higher substrate concentrations (up to 100 mM), the following key aspects must also be considered: (1) the low solubility of n-hexylamine and the reaction products in aqueous solution, and (2) the poor stability of enzymes in unconventional solvents. The present study includes activity and stability experiments using laccases in methanol, a suitable solvent for the hydrophobic compound n-hexylamine. Contrary to previously published reports, excellent activities were obtained using Novozym 51003 at high methanol contents and substrate concentrations up to 100 mM. In addition, the use of this volatile organic solvent drastically simplified downstream processing.