Lignin is an amorphous, aromatic polymer and the component in lignocellulose responsible for binding the cellulose crystal together. Currently, nearly all of the lignin produced in pulp mills is burned as a low value fuel. However, as the largest natural source of aromatic carbons, lignin has a tremendous potential as a resource for renewable chemicals. To reach this potential, cost-effective processes for isolation and depolymerization of lignin are required. Many efforts have been undertaken, but the separation of products and catalysts from solvents is usually prohibitively expensive. Mechanocatalysis offers a promising solvent-free alternative to the traditional catalytic approaches. Direct solid-solid and solid-gas reactions can be catalyzed at nominally ambient conditions via mechanical impact, and the products and catalysts are obtained in a more concentrated form. When organosolv lignin was milled in the presence of NaOH, the average molecular mass was reduced to less than 500 g/mol within 30 min. A small amount of methanol increased the rate and extent of depolymerization of organosolv lignin by inhibiting repolymerization reactions under depolymerization conditions, so that a reduction of the mean molecular mass to less than 400 g/mol was observed in 5 minutes. The effectiveness of different scavenger and the mechanism by which they affect this process will be discussed. While the use of sodium hydroxide allows for rapid depolymerization, base hydrolysis is generally less effective at depolymerizing lignin compared to reductive cleavage with hydrogen. For this reason, the ball mill reactor was modified to allow for a continuous supply of hydrogen and additional volatile reagents, like water. Hydrolysis, hydrogenolysis, and hydrogenation of diphenyl ether and benzyl phenyl ether occur over supported metal catalysts. Based on the systematic variation of catalysts and experimental conditions, reaction pathways are elucidated.

Mechanocatalytic depolymerization of lignin

2019 Spring National Meeting

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