(216g) Reductive Mechanocatalytic Depolymerization of Lignin

Authors: 
Tricker, A., Georgia Institute of Technology
Sievers, C., Georgia Institute of Technology
As society shifts away from fossil based fuels and chemicals, the necessity for complete utilization of lignocellulosic biomass becomes more pressing. Lignin, an amorphous heteropolymer, is an ideal feedstock for many renewable chemicals because it is the largest natural source of aromatic carbon. Currently, nearly all extracted lignin is burned as fuel at pulp mills, but for many mills, lignin production exceeds energy requirements and reboiler capacity, bottlenecking pulp production. Lignin will provide abundant economic value in both the short and long term if viable depolymerization strategies can be realized. Thermal, enzymatic, and catalytic degradation methods have been extensively studied, but none have presented a pathway to economical depolymerization. Catalytic depolymerization approaches have suffered from low product selectivities, intensive separations, or the need for exotic catalysts and solvents. Mechanocatalysis offers an appealing approach to solvent-free lignin depolymerization that avoids both the issues of lignin solubility and of product-solvent separation. Mechanocatalysis uses an input of mechanical energy, such as collisions inside a ball mill, to drive catalytic reactions. The little research on mechanocatalytic lignin depolymerization predominantly focuses on acid and base hydrolysis.

This contribution is focused on the use of reduction catalysts and hydrogen for mechanocatalytic depolymerization of lignin. The use of reduction catalysts allows for integrated depolymerization and product upgrading through hydrodeoxygenation (HDO). In the present process, an acid supported noble or transition metal catalyst and lignin are combined in a vibratory ball mill, while gaseous reagents are flown through the vessel. Reactivity studies are conducted for reductive cleavage of model dimers (e.g. benzyl phenyl ether) and HDO of model monomers (e.g. guaiacol). Acids sites on solid catalysts promote both the depolymerization and HDO reactions. In depolymerization reactions, acid sites catalyze the hydrolysis of ether bonds. For HDO, monomers can adsorb onto acid sites adjacent to metal particles. Activated hydrogen on the metal sites then reacts with the monomer, cleaving the carbon oxygen bond. The depolymerized lignin is characterized with gel permeation chromatography, nuclear magnetic resonance and infrared spectroscopy, as well as acid number and carbonyl titrations. Low molecular weight compounds are identified and quantified with gas chromatography with a mass spectrometer and a flame ionizing detector. Mechanocatalytic hydrogen-processing of lignin can overcome many hurdles for economic lignin valorization (e.g. product separations and upgrading) by simultaneously depolymerizing lignin and refining products using industrially relevant catalysts, all without solvents.