The effects of lignin source and extraction on the composition and properties of biorefined depolymerization products

We explore the efficacy of depolymerization as a function of lignin source and extraction method under the conditions of our previously described biorefinery. Biomass source (i.e. herbaceous, softwood, and hardwood) influences the total available lignin and structural features such as the ratio of monomers (S/G) and the percentage of β-O-4 linkage. The method of extraction (i.e. Milled Wood (mild), Organosolv (medium), and Klason (harsh)) determines levels of solubilization, preservation of the intrinsic lignin structure, and distribution and properties of products. Herbaceous lignin extracted by the Organosolv process is best suited for depolymerization in our biorefinery and shows the greatest extent of solubilization (∼100% over 7 days), the highest yields of phenolics and flavonoids, and the most and smallest lignin nanoparticles. Additionally, this depolymerized product mixture has the highest antioxidant capacity. Only the harsh, Klason method successfully isolates lignin, through carbohydrate conversion, from all three sources, but the herbaceous and hardwood sources are depolymerized to a lesser extent than by the Organosolv method. With low density of labile bonds, softwood lignin is not effectively depolymerized under these conditions. Although modifying intrinsic lignin structure the least, Milled Wood lignin does not efficiently extract lignin from any of the biomass, and the extracted lignin is low in purity inhibiting further processing. These results further detail the promise of herbaceous Organosolv lignin as a renewable feedstock to high value products in a distributed-scale biorefinery.