Abstract
Lignin is a highly abundant polyphenolic polymer that imparts mechanical strength to plant biomass. Transition-metal complexes can catalyze lignin oxidation to produce value-added products, but low catalytic efficiency has hampered their use in industry. Identifying the chemical and structural factors that govern catalytic activity is a prerequisite to rational design of catalysts with improved activity. Here, we combine computational and experimental approaches to investigate the mechanism of Co(salen)catalyzed oxidation of the monomeric lignin models syringyl (S), vanillyl (G), and 4-hydroxybenzyl alcohol (H) to produce benzoquinone and benzaldehyde products. Experimentally, S oxidation to form dimethoxybenzoquinone proceeded efficiently with a Co(salen) catalyst coordinated by a pyridine ligand, but G and H did not undergo oxidation. Density functional theory calculations reveal that catalyst regeneration is energetically unfavorable in the presence of H, which prevents oxidation. In contrast, S readily facilitates catalyst regeneration. Formation of methoxybenzoquinone from G was achieved experimentally by adding bulky, noncoordinating bases. These findings provide a fundamental baseline for enhancing the activity of Co-Schiff base catalysts toward lignin-like molecules by adding sterically hindered nitrogenous bases or potentially by including a cocatalyst that promotes catalyst regeneration.
Original language | English |
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Pages (from-to) | 7225-7234 |
Number of pages | 10 |
Journal | ACS Sustainable Chemistry and Engineering |
Volume | 8 |
Issue number | 18 |
DOIs | |
State | Published - 2020 |
Funding
This research was supported by the High-Performance Computing for Manufacturing Project Program (HPC4Mfg), which is managed by the U.S. Department of Energy (DOE) Advanced Manufacturing Office within the Energy Efficiency and Renewable Energy Office. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. DOE Office of Science User Facility operated under Contract no. DE-AC02-05CH11231, and resources of the Compute and Data Environment for Science (CADES) at ORNL, which is managed by UT-Battelle, LLC for the U.S. DOE under Contract no. DE-AC05-00OR22725. C.J.C. was supported by a National Science Foundation Graduate Research Fellowship under Grant no. 2017219379.
Keywords
- Biomass
- Catalysis
- Density functional theory
- Schiff base
- Transition metal
- Valorization