Development of an eco-friendly acetosolv protocol for tuning the acetylation of coconut shell lignin: Structural, antioxidant, solubility and UV-blocking properties

The optimization of the parameters involved in lignin extraction is crucial for obtaining a lignin with specific structural features for its further valorization. The aim of this work was to develop an eco-friendly organosolv protocol for tuning the acetylation degree of coconut shell lignins (CSLs) by using MgCl2 and HCl as catalyst and co-catalyst, respectively. CSLs were obtained by mixing coconut shell powder with 90% v/v acetic acid combined to no catalyst, 2% v/v HCl and 2% w/v MgCl2 (1, 2 and 3 h) and 2% w/v MgCl2 combined to 0.1, 0.25 and 0.5% v/v HCl (2 h) at 110 °C. CSLs were characterized by FTIR, 1H NMR, GPC and TGA. The effects of the acetylation degree were evaluated on their antioxidant activity (DPPH assay) and UV-blocking capacity in sunscreen formulations. The results have shown that the use of HCl as co-catalyst increased the lignin yield (from 21.4 to 48.8%) and the acetylation degree (from 0.81 to 1.58 mmol g-1), which positively affected thermal (200 < Tonset < 226 °C), antioxidant (46.6 < IC50 < 67.5 μg mL-1) and UV-blocking capacities of CSLs. It can be concluded that the design of the organosolv process was capable of generating lignins with peculiar functionalities and properties through an eco-friendly protocol.