Natural eumelanin-based porous N-doped carbon as an active bio-catalyst for base- and initiator-free aerobic oxidation of olefins and alkyl aromatic hydrocarbons

Abstract

Eumelanin is an indolic biopolymer with catechol and heteroatom groups that possess highly redox active sites and a single source of nitrogen and carbon and could be an excellent candidate for the preparation of metal-free biocatalysts for the transformation of various organic molecules. Herein, we synthesized porous N-doped carbon materials utilizing microbial eumelanin and used them for the aerobic oxidation of olefins and alkyl aromatic hydrocarbons. The synthesized catalyst exhibits higher catalytic activity for the oxidation of styrene than the commercial activated carbon. The porous N-doped carbon materials showed 66% conversion of styrene with 58% selectivity of benzaldehyde under base- and initiator-free milder reaction conditions. In this study, an N-doped biocatalyst was utilized for the oxidation of olefins/alkyl aromatic hydrocarbons to carbonyl compounds (conversion: 10–100% and selectivity: 30–>99%). Furthermore, control experiments indicated that the N atom present on the carbon material was responsible for oxygen activation and ESR spectroscopy studies showed that the reaction proceeded via radical formation. Furthermore, the synthesized catalyst was reused up to five times with a slight drop in conversion. This may be the first report on the preparation of metal-free porous N-doped carbon materials employing microbial eumelanin and their use in the activation of molecular oxygen.