Exploiting Deep Eutectic Solvents and Ionic Liquids for the Valorization of Chestnut Shell Waste

The full utilization of agricultural waste and its recycle into a new chain of value are of primary importance for the development of a sustainable and profitable agricultural industry. Chestnut shell waste (CSW) is an interesting case of study, whose valorization has been though partially investigated to date. This work aims at exploring the complete utilization of CSW, in terms of obtaining both value-added compounds and enriched cellulose and lignin fractions. The results were obtained via the unreported combined use of two classes of nonconventional organic solvents, namely natural deep eutectic solvents and bio-based ionic liquids (bio-ILs). At first, combinations of choline chloride (ChCl)-based DESs with an acid, a polyol, or a sugar as hydrogen bond donors were employed for the extraction of polyphenols from the CSW. The best performing system was found to be ChCl:oxalic acid dihydrate (ChCl:Oax2H2O). The extraction efficiencies of the DESs tested correlate well with the measured Kamlet–Taft α parameters. After polyphenol removal, the residual solid material was treated with a bio-IL [cholinium glycinate (ChGly)] for further separation of lignin and cellulose. The products obtained by the fractionation process were characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis, which confirmed the separation of the residue into a lignin-rich material and a cellulose-rich material. The results obtained were further corroborated by a three parallel reaction model combined with the distributed activation energy model, which allowed for predicting the composition of the pristine CSW and of the ChCl:Oax2H2O-treated CSW as well as the two fractions obtained after ChGly treatment. The recyclability of the best performing DES and the recovery of the bio-IL have also been proven, which make the whole process viable and amenable for large-scale applications.