Microwave-assisted depolymerization of lignin and a biphasic extraction method for the recovery of bio-oil and phenolic monomers

Alkaline lignin and de-alkaline lignin were depolymerized under microwave irradiation in the presence of formic acid and ethanol to produce bio-oil and phenolic monomers. The amount of formic acid was identified as the most critical factor influencing product yields among variables of temperature, retention time, feedstock concentration, and formic acid to feedstock mass ratio. Formic acid acted as both catalyst and in-situ hydrogen donor and it is economically and environmentally advantageous over metal catalysts. An ethyl acetate-water extraction method was developed, which was suitable to recover bio-oil from the depolymerization of lignin with high ash content. Acidic extraction condition favored the product recovery than neutral or basic conditions, giving bio-oil product with much improved quality. This is attributed to the fact that pH adjustment induced ionization of solutes and thus altered the affinity between the solutes and the extraction solvents. This work also revealed that alkaline and de-alkaline lignin feedstocks gave comparable bio-oil yield (ash-free basis) and chemical composition. The bio-oil derived from the de-alkaline lignin has a lower average molecular mass than the alkaline lignin-based bio-oil, likely caused by the presence of alkaline compounds in the alkaline lignin, which could neutralize some formic acid (FA) catalyst, leading to a reduced degree of de-polymerization with the alkaline lignin. © 2021 Elsevier B.V.