Efficient Transfer Hydrogenolysis of a Lignin Model Compound Using Non-Noble Metal Catalysts

Significance 

Lignin is a complex and abundant component of lignocellulosic biomass characterized by its heterogeneous phenolic polymer structure that can be great potential as a renewable source for valuable aromatic chemicals. The annual global production of lignin is estimated at around 100 million tonnes which highlights the significant opportunity for its valorization and if lignin can be converted in an efficient way into aromatic compounds it will be an important sustainable supply source of these chemicals. However, the depolymerization of lignin remains a significant technical challenge due to its heterogeneous and recalcitrant nature. There are several methods for lignin depolymerization such as pyrolysis, base or acid-catalyzed depolymerization, hydrolysis, and oxidation, however, hydrogenolysis stands out as a promising approach because it facilitates the cleavage of aryl ether bonds (e.g., β-O-4, α-O-4, 4-O-5’) in lignin through the introduction of hydrogen and therefore promotes the breakdown of lignin into valuable monomers. However, traditional hydrogenolysis methods have challenges to overcome such as the need to use noble metal catalysts which are expensive and limited. Additionally, the use of high-pressure hydrogen gas requires safety and logistical regulations in industrial operations. There have been some recent studies to improve the process where non-noble metal catalysts such as nickel and cobalt are used in combination with hydrogen-donating solvents like 2-propanol (IPA), as an alternative to compressed hydrogen gas, to achieve efficient lignin depolymerization. Despite these advancements, the hydrogen transfer pathways involved in transfer hydrogenolysis require further investigation to optimize the process for industrial applications. Specifically, to understand whether hydrogen is directly transferred from the solvent to the substrate or through the formation of hydrogen gas is important for the development of more efficient and effective depolymerization methods. To this end, new study published in the Journal Industrial & Engineering Chemistry Research and conducted by PhD student Taishi Dowaki, Assistant Professor Osamu Sawai, and led by Professor Teppei Nunoura from The University of Tokyo, investigated the reaction kinetics and hydrogen transfer mechanisms in the transfer hydrogenolysis of diphenyl ether (DPE) as a lignin model compound. They employed a non-noble metal catalyst (NiCo/C) and IPA as a hydrogen-donating solvent to determine the dominant hydrogen transfer pathway and the associated reaction kinetics.

The team first examined the effects of the catalyst and solvent on the transfer hydrogenolysis of DPE and found that without a catalyst, DPE conversion was less than 5% and no aromatic or cyclic products were detected and the acetone yield was negligible. On the other hand, with non-reduced NiCo/C catalyst, the conversion remained below 5%, and minimal acetone was produced, which indicates that the surface of activated carbon in the catalyst played a minor role in IPA dehydrogenation. However, with reduced NiCo/C catalyst, the DPE conversion soared to nearly 90% with significant yields of aromatic and cyclic products such as benzene, cyclohexane, phenol, and cyclohexanol and additionally acetone yield was substantial which shows that the NiCo/C catalyst was essential for both hydrogenolysis and IPA dehydrogenation. Moreover, the researchers monitored DPE conversion and product yields over time at 235 °C to try to gain deeper knowledge into the reaction behavior and found that in the initial stages of up to 2 hours, DPE conversion reached nearly 90% and benzene yield increased to 40% but then decreased as it was further hydrogenated to cyclohexane. Phenol yield peaked rapidly but remained below 10%, while cyclohexanol yield exceeded 70% which indicates rapid phenol hydrogenation. In contrast, at later stages beyond 2 hours the yield of cyclohexanol decreased slightly which suggested to the authors further dehydrogenation to cyclohexanone with acetone yield plateaued at 13% on an IPA basis. Furthermore, to determine the dominant hydrogen transfer pathway the researchers developed two kinetic models with one that assumes direct hydrogen transfer from IPA to DPE and the other involves H2 gas formation as an intermediate step. They compared the experimental data with simulations from the direct hydrogen transfer model and found excellent agreement with the experimental results for DPE conversion, product yields, and acetone formation. In contrast, indirect hydrogen transfer model failed to predict acetone yield. Consequently, the model comparison results indicated that hydrogen was directly transferred from IPA to DPE without involving H2 gas as an intermediate and H2 gas formation played a minor role in the hydrogenolysis process. The researchers further investigated the effect of temperature on the reaction kinetics and conducted experiments at temperatures from 218 °C to 253 °C and observed that increased reaction rates with higher temperatures led to faster DPE conversion and product formation. However, the reaction rates at 245 °C were unexpectedly lower than those at 235 °C probably because of the phase transition of IPA from liquid to supercritical state. Moreover, calculated activation energies for DPE hydrogenolysis and IPA dehydrogenation confirmed that the latter occurred more readily and the corrected rate constants was in good agreement with the Arrhenius equation.

In conclusion, Professor Teppei Nunoura and colleagues developed a new transfer hydrogenolysis of DPE using IPA as a hydrogen-donating solvent and a non-noble metal NiCo/C catalyst. Their experiments confirmed the efficiency of the catalyst and solvent, elucidated the dominant direct hydrogen transfer pathway, and provided essential kinetic parameters for further optimization. It is noteworthy to mention, the method reduces dependency on expensive and scarce noble metals which makes the process more economically viable and can lead to the development of scalable industrial processes for lignin valorization, and support the use of renewable biomass resources. It also mitigates the safety risks associated with high-pressure hydrogen and simplifies the operational requirements and facilitates safer industrial applications.

Efficient Transfer Hydrogenolysis of a Lignin Model Compound Using Non-Noble Metal Catalysts - Advances in Engineering

About the author

Teppei Nunoura is a Professor of the Environment, Health and Safety Office in the Graduate School of Frontier Sciences, the University of Tokyo. He focuses on developing treatment technologies for chemically hazardous waste and transformation techniques that convert waste into resources and energy. Recent research includes decomposition of hazardous waste using hot compressed water, conversion of lignin from herbaceous biomass into valuable chemicals, toxicity assessment of waste treatment water using bioassays, and the synthesis of nanomaterials using supercritical fluids. He is also in charge of the management of chemical substances and hazardous waste in his university.

About the author

Osamu Sawai is an Assistant Professor affiliated to the Environmental Science Center, the University of Tokyo. He received his Ph.D. from the Graduate School of Engineering, University of Tokyo under the supervision of Yoshito Oshima in 2009. He started his academic career from 2010 at the Environmental Science Center serving to Professor Kazuo Yamamoto as a postdoctoral fellow and became an assistant professor from 2013 for Professor Teppei’s research group. His work focuses on topics such as supercritical fluids, nanomaterials, and wastewater treatment.

About the author

Taishi Dowaki is a doctoral student in the Department of Environment Systems, Graduate School of Frontier Sciences, the University of Tokyo. He obtained his bachelor’s degree in Chemical Engineering from Tohoku University under the supervision of Professor Richard Lee Smith, Jr. in 2022. He received his master’s degree in Environmental Studies from the University of Tokyo under the supervision of Professor Teppei Nunoura in 2024. He is currently pursuing a PhD in lignin conversion technologies, with a focus on transfer hydrogenolysis under the supervision of Professor Nunoura.

Reference

Taishi Dowaki, Osamu Sawai, and Teppei Nunoura*. Kinetic Analysis of Hydrogen Transfer Route from Hot-Compressed 2-Propanol to Diphenyl Ether over NiCo/C Catalyst. Ind. Eng. Chem. Res. 2024, 63, 7, 2991–3002 .

Go to Ind. Eng. Chem. Res.

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