Catalytic Depolymerization of Enzymatic Hydrolysis Lignin into Monomers over an Unsupported Nickel Catalyst in Supercritical Ethanol

Lignin is the largest renewable source of aromatic compounds in nature, and the production of value-added aromatic compounds from lignin can reduce the dependence on fossil fuel feedstocks. Here, a number of unsupported nickel-based catalysts were prepared from the decomposition of nickel formate and were employed for the depolymerization of enzymatic hydrolysis lignin (EHL) in a batch reactor at 280 °C for 6 h with 2 MPa hydrogen in ethanol. Among the samples, Ni (220H) achieved the complete liquefaction of EHL and the highest monomer yield of 28.5%. The results of 1H-nuclear magnetic resonance (1H-NMR) and gel permeation chromatography (GPC) studies indicate that the ether linkages were cleaved and the molecular weight of EHL significantly decreased in ethanol without the catalyst, and the Ni (220H) catalyst promoted the depolymerization of EHL and hindered the repolymerization reaction. The para-alkyl side chains of phenolic products are independent of the solvent used, while the structures of aromatic esters are closely related to the solvent used. This work provided a novel and efficient strategy for lignin depolymerization, which is promising for the industrial application of lignin.