Catalytic hydroprocessing of lignin under thermal and ultrasound conditions

Lignin isolated from Miscanthus × giganteus using acidic (FAL) and basic (AL) conditions was thereafter subjected to catalytic depolymerization under thermal or ultrasonic activation. The characterization of lignin samples was achieved by thermogravimetric analysis and FTIR. Three different classes of catalysts containing nickel as the active species have been prepared in this respect: (i) nano-Ni(0) by reduction of NiCl2 with NaBH4 under ultrasonication, (ii) Fe3O4–(NiMgAlO)x and (NiAlO)x by calcination of Mg(Ni)–Al hydrotalcite incorporating Fe3O4, followed by reduction with hydrogen, and (iii) NiO(1 1 1) nanosheets by reduction of Ni(NO3)2 with urea using benzyl alcohol as a structure directing agent. The catalysts were characterized by XRD and XPS techniques. Reduced mixed oxides displayed a moderate activity while a significant increase in conversion (up to 90%) was observed with nano-Ni(0) and NiO(1 1 1) nanosheets catalysts. The conversion and the mass distribution of the reaction products were strongly related to the procedure used for the extraction of lignin. In all the reactions AL samples had a greater extent of depolymerization. The performance from the tested catalysts under ultrasonic conditions was inferior to those tested under conventional heating conditions. The nature of the solvent was also found to be very important to this process, with the ionic liquid, 1-butyl-3-methylimidazolium acetate ([BMIM]OAc) demonstrating the best results in autoclave conditions and methanol under ultrasonic conditions.