M. Yang: Lignin-derived compounds valorization on metal-free carbon catalysts

Content of the thesis: 

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 co-

catalytic 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 metal-

free 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.

Opponent: Prof. Dmitry Murzin, Åbo Akademi University

Supervisor: Prof. Ville Alopaeus, Aalto University School of Chemical Engineering

Link to electronic thesis: LINK