Room temperature depolymerization of lignin using a protic and metal based ionic liquid system: an efficient method of catalytic conversion and value addition

Abstract

Lignin is one of the most abundant biopolymer which can be utilized to synthesize various chemicals via its depolymerization. However, depolymerization of lignin generally occurs under very harsh conditions. Herein, we report the efficient depolymerization of lignin via dissolution in a mixed ionic liquid system: ethyl ammonium nitrate (EAN) + prolinium tetrachloromanganate(II) [Pro]₂[MnCl₄] at 35 °C and under atmospheric pressure conditions. The high dissolution of lignin in ethyl ammonium nitrate provided a large number of H-bonding sites leading to the cracking of lignin and subsequent oxidative conversion by [Pro]₂[MnCl₄] via the formation of metal-oxo bonding between Mn and lignin molecules. The extracted yield of vanillin was found to be 18–20% on lignin weight basis via GC-MS analysis. The depolymerization of lignin was confirmed by SEM, FT-IR and PXRD analysis. Since lignin contains UV-absorbing functional groups, the regenerated biomass after the recovery of the depolymerized products was further utilized to synthesize a UV-shielding material. The constructed films from such a material exhibited a high SPF value of 22 and were found to be very effective by limiting the UV degradation of rhodamine B thus making the lignin valorization process economically viable and environmentally sustainable.