Reductive conversion of lignin to aromatic chemicals with earth abundant catalysts

Abstract

The viability of lignocellulosic biomass as a feedstock for chemicals hinges on the successful utilization of the lignin. Lignin, which comprises 15-30 wt% of biomass, is an amorphous polymer composed of multiple phenolic monomers. Lignin polymerization occurs through an uncontrolled radical coupling of monomers leading to an array of C-O and C-C bonds in the polymer. As such, lignin is highly recalcitrant and is responsible for the poor utilization of biomass. Many harsh thermochemical processes exist to extract lignin, but typically result in the destruction and condensation of the lignin rendering it as process waste. Alternative fractionation techniques focused on preserving lignin have recently been developed. These methods utilize reduction catalysts to depolymerize and stabilize reactive lignin fragments to produce stable phenols.

This work focuses on the design of flow reactors to understand this process, evaluate catalysts and elucidate how structural changes in biomass impact lignin depolymerization and upgrading. Our lignin conversion process operates by adding a heterogeneous reduction catalyst with whole biomass, a solvent and a reducing agent. Two key steps were discovered in the reductive conversion of lignin. First, lignin oligomers are liberated from the biomass by solvolytic cleavage of lignin-carbohydrate bonds. Next, lignin oligomers are reductively fragmented at the catalyst surface to produce stable phenolic compounds. Lignin solvolysis and reduction were physically separated in a dual-bed flow-through reactor, which allows for independent control of each step. Direct control of lignin solvolysis and reduction allowed for limiting conditions to be isolated. Reduction limited conditions were used to study catalyst activity and stability.

Solvolysis limiting conditions were used to probe how lignin structure influences product selectivity and yield. Additionally, molybdenum-based catalysts were developed for the conversion of lignin-derived phenols into aromatics. Gas phase reactions with n-propyl guaiacol demonstrated that molybdenum polyoxometalates are effective catalysts to perform simultaneous alkylationƯhydrodeoxygenation to produce alkylated aromatics from phenolics. Finally, the dual-bed reactor was used to combine reductive fractionation and deoxygenation chemistry to directly convert lignin into aromatics over a molybdenum carbide catalyst. Overall, a versatile reactor system was developed to facilitate fundamental studies to understand biomass structure and catalyst performance.