Valorization of industrial lignin as lubricating additives by C–C Bond Cleavage and doping heteroelement-rich groups

In this study, heteroelement-rich groups (N and B elements, tris (2-hydrazinylethyl) borate) were successfully synthesised on lignin by microwave amination. The reaction between lignin and heteroelement-rich groups was confirmed using several characterization techniques, including fourier transform infrared (FT-IR) spectroscopy, thermogravimetric (TG) analysis, and X-ray photoelectron spectroscopy (XPS). A lignin model compound (2-phenoxyacetophenone (L-M)) was used to study the bond-breaking mechanism of lignin depolymerization, and the amine group, which acted as a nucleophile, attacked the C–C bond to form an amide bond with the lignin. As an effective additive, modified lignin can increase the viscosity of poly (ethylene glycol) 200 (PEG200), reducing the wear volume and friction coefficient in the friction test. Theoretical calculations showed that the polarity of the modified lignin increased, which allowed the modified lignin to form a strong physical adsorption on the metal surface. In addition, the reciprocal hydrogen bonding between the modified lignin and PEG200 contributes synergistically to the enhanced lubrication film strength, and thus, the better tribological properties of these new lubricants.