Acidic and basic amino acid-based novel deep eutectic solvents and their role in depolymerization of lignin
Abstract
Amino acid-based deep eutectic solvents (AADESs) are emerging type of DESs with a wide range of applications. In this study, two novel AADESs were formulated using basic, L-(+) arginine (Arg), and acidic, L-glutamic acid (Glu), amino acids as hydrogen bond acceptors (HBA) and glycerol (Gly) as the hydrogen bond donor (HBD) at a 1:3 M ratio. The eutectic points of the formulated AADESs were found to be −0.14 °C for Glu-Gly and −1.36 °C for Arg-Gly. The successful interactions among the constituents were investigated using FT-IR, 1H NMR spectroscopy, and mass spectrometry. These studies found that Glu-Gly could form ester impurities. However, the mass spectrometry showed that the impurities are negligible. The TGA revealed that both DESs could be applied up to 150–160 °C without losing weight, while Glu-Gly could be used up to 200 °C. Formation of Arg-Gly was strongly dominated by glycerol, while Glu-Gly DES was formed by an equal contribution of both components. Principal component analysis (PCA) helped to visualize the spectroscopic outcomes. Since AADESs are excellent pretreatment media for biomass, lignin was treated as a model biomass in this study with the formulated AADESs to determine their reaction products. It was found that Arg-Gly can isolate only one monomeric compound (4-methyl benzaldehyde), while Glu-Gly can isolate three monomeric compounds. Oxidative depolymerization of the lignin residues validated the outcomes obtained from the AADES-lignin reactions. Glu-Gly was more efficient for lignin depolymerization because of its smaller viscosity, higher stability, acidic environment, and stronger hydrogen bond formation between lignin and glycerol.