Maximization of kraft lignin depolymerization using synthetic mixed oxide catalysts under microwave exposure

Authors: KOHLI, KIRTIKA - Indian Institute Of Petroleum; PRAJAPATI, RAVINDRA - University Of Illinois; KATUWAL, SARMILA - University Of Illinois; KIM, JAEMIN - University Of Illinois; Mullen, Charles; Strahan, Gary; Biswas, Atanu; Sharma, Brajendra

Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2025
Publication Date: 3/10/2025
Citation: Kohli, K., Prajapati, R., Katuwal, S., Kim, J., Mullen, C.A., Strahan, G.D., Biswas, A., Sharma, B.K. 2025. Maximization of kraft lignin depolymerization using synthetic mixed oxide catalysts under microwave exposure. Industrial Crops and Products. 227/120787. https://doi.org/10.1016/j.indcrop.2025.120787.
DOI: https://doi.org/10.1016/j.indcrop.2025.120787

Interpretive Summary: Efficient use of biomass-derived waste, lignin, is important to improve the economics of biorefineries. Lignin, which makes up 15-30% of biomass has the potential to be converted to value-added chemicals like phenolics. In this study, we have demonstrated the depolymerization of lignin using a catalytic microwave-assisted process to phenolic monomers with conversion as high as 89% after studying various reaction parameters including catalysts. This study provides an efficient approach to producing value-added phenolic compounds from lignin without adding external hydrogen. This will improve the economic viability of biofuel production and thereby help farmers by increasing the value of waste biomass generated on the farms.

Technical Abstract: The efficient valorization of biomass-derived wastes, lignin, is important for the economic viability of biorefineries. Lignin, which accounts for 15–30 % of the available renewable carbon, can be processed to produce value-added phenolic chemicals. In this study, the microwave-assisted depolymerization of a Kraft lignin in a methanol/formic acid (FA) solvent to produce high-value-added phenolic monomers was investigated, in which methanol acts as a solvent and FA acts as an acid catalyst and an in-situ hydrogen donor. The highest lignin conversion of 78 % was achieved at a temperature of 140°C and an FA-to-lignin mass ratio of 4 after a 20 mins reaction time. Acid-catalyzed cleavage of the linkages in lignin occurs in the presence of FA, which results in the formation of a liquid product. Further, the catalytic depolymerization of Kraft lignin with an added methanol/FA solvent system was conducted in the presence of mixed-metal oxides to maximize the liquid product yield and to minimize the solid residue formation. Three different mixed metal oxide catalysts were prepared using hydrotalcite-structured materials, i.e., Mg3AlCO3, Mg3AlSO4, and Mg3AlCl. A mixed-metal oxide-Mg3AlCl catalyst with a particle size of 50 nm was found to be effective for Kraft lignin depolymerization. The effects of catalyst composition, catalyst amount, and reaction conditions on the lignin conversion and product yields were explored. The highest lignin conversion of 89 % (with 81 % bio-oil yield) was obtained at an optimized reaction temperature of 100°C with a catalyst-to-lignin ratio of 0.25, FA-to-lignin mass ratio of 4, and at a 20 mins reaction time. The depolymerized liquid product contains mainly G-type phenolic monomers (~60 %) in the presence of mixed-metal oxide-Mg3AlCl. This work provides an efficient approach to produce value-added phenolic compounds from lignin by microwave heating in the presence of mixed-metal oxides and without added external hydrogen.