| Summary: | The development of alternatives to petroleum derived fuels and chemicals will be essential to curb greenhouse gas emissions and ensure a sustainable future. Lignocellulosic biomass is a promising alternative source for producing drop-in substitutes for fuels and chemicals. It is nonedible, carbon neutral, and renewable by nature. While the carboyhydate fraction of lignocellulosic biomass is currently used industrially, the third fraction, lignin is severely underutilized. Lignin is a complex, irregular, recalcitrant biopolymer, making upgrading challenging. Recent research led to the development of reductive catalytic fractionation (RCF) a lignin-first approach to lignin extraction and depolymerization. While this technology is promising, little is understood about the fundamental chemistry of the process. This work aims to provide insight into the kinetics and mechanisms of the catalytic hydrogenolysis step of the RCF process to enable optimization of the process as a whole. We used synthetic archetype lignin model compounds to investigate the interactions between lignin substrates and the catalyst surface. Our study suggests lignin substrates bind to the catalyst surface through coordination of multiple oxygen groups on the substrate and through the formation of Pd–O bonds. We found two parallel reaction pathways during lignin RCF. The desired pathway is the β–O–4 cleavage reaction, with an activation barrier of 75-85 kJ/mol. The second pathway is reduction of the α–OH group, an activation barrier of 60-70 kJ/mol. Reduction of the α–OH group shuts down the potential for β–O–4 cleavage reaction. However, alkoxylation at the a position, a common reaction during organosolv lignin extraction, allows the β–O–4 reaction to proceed, indicating the presence of an α–O functional group in the substrate is essential to facilitate β–O–4 cleavage. While not essential, the phenolic group in the lignin substrate enhances the β–O–4 cleavage turnover frequency (TOF) by approximately 10x. By measuring the kinetics of a lignin trimer 4 model compound, we found the β–O–4 cleavage in a lignin polymer to be a sequential reaction, starting at the polymers phenolic end and moving inwards. This work highlights the critical functional groups of a lignin polymer and their interactions with catalyst surface that enable hydrogenolysis reactions during lignin RCF.
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