(760a) Catalyst Choice Impacts Aromatic Monomer Yields and Selectivity during Hydrogen-Free Reductive Catalytic Fractionation

Lignin is the only large volume feedstock of renewable aromatics, and its efficient conversion into fuels and chemicals is expected to play an important role in future biorefinery viability. Reductive Catalytic Fractionation (RCF) is a promising method to extract and depolymerize lignin from native biomass into aromatic monomers using an alcohol solvent and heterogeneous supported metal catalyst. However, the use of externally supplied hydrogen gas in conventional RCF leads to high capital costs due to the need for catalytic reactors capable of withstanding high operating pressures. Hydrogen-free RCF processes utilize alternative hydrogen sources to stabilize reactive lignin fragments, and thus avoid the use of hydrogen gas directly in the RCF reactor. To understand the role of the catalytic metal in hydrogen-free RCF, we compared four common carbon supported RCF catalysts (Ru, Ni, Pd, Pt) for their ability to stabilize lignin from whole poplar to aromatic monomers with and without externally supplied hydrogen gas. Significant differences in selectivity and total monomer yield were observed: namely, Pd/C and Pt/C exhibited the highest monomer yields during hydrogen-free RCF, indicating that catalyst choice has a significant impact on process performance. Hydrogen gas produced from reforming of the methanol solvent (CH₃OH → CO+ H₂) is often cited as the hydrogen source. However, hydrogen gas production measured during hydrogen-free RCF did not correlate with monomer yield, indicating that the stabilization process may go through hydrogen transfer instead of needing to first make H₂ gas. Hydrogen-free monomer yield and selectivity trends from whole biomass were mirrored in model compound experiments with a monomer substrate, but dimer model compounds gave conflicting trends, potentially indicating that the reactive species during whole biomass reactions is monomeric. Further understanding of hydrogen-free RCF, including identification of the hydrogen donor, will be important for further process improvements.