PNNL

Abstract

Peracetic acid (PAA) is an effective oxidant capable of solubilising lignin and efficiently depolymerising it to selective phenolic compounds; however, the specific role by which PAA initiates the depolymerisation is still elusive. Herein, interaction between PAA and the lignin macromolecule and the consequent structural changes of the latter were studied by characterising the lignin packing structure and its associated changes during the oxidation process. While the lignin packing structure and its changes associated with the PAA-mediated oxidation was probed by X-ray diffraction, impact of the PAA on different chemical functionalities present in the lignin structure was established by 13C and 13C/1H heteronuclear single-quantum coherence nuclear magnetic resonance (NMR) spectroscopy. Combining the NMR spectroscopy results and the product distribution obtained upon depolymerisation, we conclude that oxidation of the benzylic hydroxyl group, resulting in the formation of a ketone in the alpha-position that then undergoes the Bayer-Villiger oxidation is the predominant reaction pathway for the oxidative depolymerisation of lignin with PAA. The experimental evidence provided herein corroborated that PAA instigates oxygen insertion to the lignin macromolecule resulting in disruption of its packing structures and facilitates depolymerisation. We also investigated various metal oxide and mixed metal oxide catalysts to identify effective catalysts that further enhance the efficiency of PAA mediated depolymerisation of lignin and produce selective monomeric phenolic compounds. Techno-economic analysis was also conducted to identify the key parameters associated with this oxidative process that need to be considered for possible commercial application.