Solubility assessment of lignin monomeric compounds and organosolv lignin in deep eutectic solvents using in situ Fourier-transform infrared spectroscopy

Over the last years, the depletion of fossil resources allied with the growing energy demands and the increasing environmental pollution turned imperative the search for alternative renewable resources. Lignocellulosic biomass has been recognized as an excellent alternative feedstock due to its availability, renewability and low cost. Lignin, one of three major components of lignocellulosic biomass, arises as a by-product from the bioethanol production and the pulp and paper industry with high abundance of polyaromatic chemical structures. Yet, in spite of being the major aromatic resource of the bio-based economy with potential to obtain a wide variety of chemicals, only 2 % is being processed. In this sense, this works aims at shedding some light on the solubility of lignin monomeric compounds in presence of deep eutectic solvents (DES), as these have shown promising results in the processing of lignocellulosic biomass. Herein, we have focused on the development of a FTIR method to infer about the compounds solubility owing to its simplicity, fast determination and low cost. Lignin monomeric compounds (benzene, 1,3-dimethoxybenzene, phenol and syringol) were studied in presence of [ChCl]:Urea and [ChCl]:Glycerol at different concentrations (20, 50, 100, 200 and 300 g.L-1) and at two temperatures (25 and 40 ?C). Results showed that syringol displays a high solubility in both DES: 700 ? 20 g.L-1 in [ChCl]:U and 810 ? 20 g.L-1 in [ChCl]:G at 25 ?C, respectively. Besides, effective solubility was observed for phenol in [ChCl]:U (930 ? 20 g.L-1) at 40 ?C, whereas in [ChCl]:G, solubility was higher at 25 ?C. In contrast, lignin showed very poor solubility in [ChCl]:U and was insoluble in [ChCl]:G, at 40 ?C.