Atmospheric Low-Temperature Plasma-Induced Changes in the Structure of the Lignin Macromolecule: An Experimental and Theoretical Investigation

Atmospheric low-temperature plasma has emerged as a promising pretreatment for lignocellulose to improve bio-refining. Herein, we investigated plasma-induced changes in the chemical structure of lignin to obtain a fundamental understanding of the plasma–lignocellulose interaction. Based on the results, plasma possesses a strong capacity to cleave C–C covalent bonds in the aliphatic region of lignin, accompanied by oxidation. Plasma treatment leads to the degradation and fragmentation of lignin. Pronounced deconstruction of β-O-4 aryl ether is observed in plasma. The relative content of β-O-4 aryl ether was reduced from the initial value of 65.1/100Ar to 58.7/100Ar for lignin from corncob and from the initial value of 72.5/100Ar to 63.8/100Ar for lignin from poplar after plasma treatment, respectively. According to the density functional theory analysis, the oxygen atom of β-O-4 aryl ether is the most likely potential reaction site and the C β –O covalent bond exhibits the lowest decomposition free energy (50.5 kcal mol –1 ), which will easily be cleaved in plasma. The dominant reaction pathway of lignin degradation is the cleavage of the C β –O covalent bond followed by the cleavage of the C β –C α bond. We propose that this investigation is beneficial to optimize and expand the applications of plasma treatment in pretreatment of lignocellulose.