Revisiting organosolv strategies for sustainable extraction of valuable lignin: the CoffeeCat process

Abstract

An innovative and sustainable strategy for the selective extraction of lignin from lignocellulosic biomass has been designed, namely the CoffeeCat process, in which only green solvents and reagents are required: (i) water, (ii) 2-methyltetrahydrofuran-3-one (coffee furanone) recognized as a food grade ingredient and readily biodegradable and (iii) glutamic acid. Two fractions have been isolated from Miscanthus x giganteus, the delignified fraction (DL-glu) and the enriched-lignin fraction (L-glu). Competitive extraction yields of 27% and 43% of enriched-lignin fractions were respectively obtained at 140 °C (L-glu-140) and 180 °C (L-glu-180), based on the lignin content of the original biomass. The structural properties of these lignins were characterized by spectroscopic (FTIR and NMR), microscopic (SEM) and separative (SEC-MALLS) methods. Compared to other processes described in the literature, our strategy involved the isolation of lignin fractions with high purity (up to 84%). Both fractions have been valorized: (i) the DL-glu fractions have been subjected to ionic liquid pretreatment and subsequent enzymatic hydrolysis leading to a total depolymerization of the constitutive cellulose (99%) and to an efficient conversion of hemicellulose into xylose (70%); (ii) the L-glu fractions have been used to produce lignin nanoparticles (LNPs) in a mixture of 2-methyltetrahydrofuran-3-one/water (1/110 v/v). The size distribution (272 ± 9 nm and 472 ± 6 nm), charge (−29.2 ± 0.8 mV and −20.8 ± 0.4 mV) and regular spherical shape of these LNPs have been determined using Zetasizer-DLS measurements and SEM images of L-glu-140 and L-glu-180, respectively. In addition, the possibility of easy incorporation of the L-glu fraction into polylactic acid without requiring previous lignin modification has been preliminarily explored. The CoffeeCat process was thus demonstrated as a relevant eco-solution for an integrated lignocellulosic biorefinery.