Lignin-derived compounds valorization on metal-free carbon catalysts

Biomass is a promising alternative to fossil fuels, addressing rising energy demands and supporting carbon neutrality. Among biomass components, lignin is abundant but challenging to utilize fully, making its valorization an important focus. Common methods like oxidative dehydrogenation (ODH) and alkylation-hydrodeoxygenation often require toxic agents or noble metal catalysts, which present environmental concerns. Metal-free, sustainable routes are needed, and carbon catalysts show potential as eco-friendly substitutes. This thesis investigated lignin valorization pathways using carbon catalysts, discussing the mechanisms and comparing their performance with traditional metal-based methods. The biaryl structural unit was synthesized using an air-oxidized activated carbon (oACair) catalyst in an ODH reaction from lignin-derived ketones. The oACair catalyst demonstrated a 74% biphenyl yield with a 9.1×10-2 h-1 reaction rate constant, showing excellent recyclability over six runs and a broad substrate scope across 15 substituted compounds. The quinoidic carbonyl active site and positively charged intermediated were proposed based on surface oxygen functional group analysis, model compound, functional group blocking, and Hammett plot. Similarly, the diaryl amine N-phenyl-1-naphthylamine (P1NA) was produced from lignin-derived aniline and 1-tetralone via an oACair-catalyzed tandem ODH (TODH) reaction, achieving a 71% yield of P1NA with a 0.23 h-1 Max. TOF. The reaction’s robustness was confirmed by its five-run recyclability and compatibility with 10 substrates, with the carboxylic acid group exhibiting cocatalytic effects. Free radical scavenger tests and simulations suggest a single-electron transfer free-radical mechanism for the TODH reaction. The alkylation of alcohols and phenolic compounds was another pathway explored in this thesis. Lignin-derived acidic carbon (SLC400) displayed a high acid density of 2.92 mmol·g-1 with dominated Bronsted acid sites. SLC400 exhibited good catalytic performance in the alkylation with a Max. TOF of 14.2 h⁻¹ in the dehydration step and a Max. TOF of 0.5 h⁻¹ in the alkylation step. Additionally, zeolite-supported tungsten oxide (WO₃/HY500) was applied for guaiacol ethanol alkylation (GEA), confirming pentaethylphenol as the main product and suggesting an alkylation-demethylation mechanism based on product structure and reaction monitoring. Surface acid analysis identified weak and strong Lewis acid sites as the primary active sites for this reaction. These routes offer practical methods to valorize lignin-derived compounds in an environmentally friendly and sustainable way, emphasizing the importance of metalfree carbon catalysts. The investigation of the kinetics, active site, and mechanism enhances the understanding of carbon catalysts and contributes to the further optimization of these routes.