Biomass is a promising alternative to fossil resources, offering the potential to reduce our dependence on fossil fuels while mitigating adverse environmental impacts. In a new study published in the journal of Green Chemistry by Xiaohong Ren, Dr. Zhuohua Sun, Dr. Jiqing Lu, Dr. Jinling Cheng, Professor Panwang Zhou, Xiaoqiang Yu, Professor Zeming Rong, and Professor Changzhi Li, the scientists from Dalian University of Technology, Beijing Forestry University and Dalian Institute of Chemical Physics, etc., developed a novel method that eliminates the need for external hydrogen in biomass conversion processes, thereby significantly improving the efficiency, sustainability, and economic viability of biorefineries.
Hydrogen is a renewable and zero-emission energy resource, but it does not occur naturally and must be produced from hydrogen-containing compounds, such as water or organic materials. Traditional methods for hydrogen production involve fossil fuels, which counteract the objective of achieving a sustainable and green biorefinery industry. Additionally, alternatives like water electrolysis, although environmentally friendly, face economic challenges that limit large-scale adoption. Various biomass conversion methods, such as hydrodeoxygenation (HDO), hydrogenation, and reductive depolymerization, are currently employed in biorefineries. These processes typically require a hydrogen-rich atmosphere to facilitate the transformation of biomass-derived feedstocks into valuable products. However, the conventional sources of hydrogen, primarily derived from fossil fuels like natural gas through steam methane reforming, are not aligned with the sustainability goals of the biorefinery industry.
The authors introduced an innovative concept of hydrogen-free biomass conversion. This new approach seeks to replace the need for externally supplied hydrogen with an in-situ generation mechanism, thus enhancing the atom economy, reducing operational costs, and improving overall process safety. In their previous work, the researchers demonstrated the exceptional performance of a nano-porous Ni catalyst in selectively converting guaiacol to cyclohexanol under a hydrogen atmosphere. This achievement laid the foundation for the current study, where they aimed to use the same catalyst to selectively convert guaiacol to phenol in the absence of external hydrogen. The results were excellent, with guaiacol conversion rates of up to 41.5% and phenol selectivity of 100% achieved at 160°C. These remarkable outcomes were further improved at 190°C, with a guaiacol conversion rate of 90.5% while maintaining a 90.3% phenol selectivity.
The researchers began by comparing the catalytic activity of different heterogeneous catalysts. Nano-porous Ni exhibited superior performance with high guaiacol conversion rates and exclusive selectivity for phenol. Other transition metals, including Fe, Co, and Cu, displayed limited activity, emphasizing the unique catalytic properties of nano-porous Ni. They conducted detailed analysis, including Density Functional Theory (DFT) calculations, confirmed the critical role of the nano-porous Ni catalyst in water activation and subsequent hydrodeoxygenation processes. Water splitting on the catalyst’s surface initiated the reaction, leading to the cleavage of the aryl ether bond in guaiacol.
Moreover, the researchers investigated the source of hydrogen crucial for the reaction. Control experiments using different solvents revealed that water played a significant role in supplying hydrogen for the in-situ hydrodeoxygenation of guaiacol. Isopropanol was also identified as a potential hydrogen donor, but water’s hydrogen supply capacity was comparatively weaker. Furthermore, they proposed reaction mechanism which involves the generation of hydrogen from water by nano-porous Ni. Hydrogen radicals facilitate the cleavage of the aryl ether bond in guaiacol, resulting in the production of methanol and phenol. Subsequently, aqueous-phase reforming of in-situ generated methanol generates additional hydrogen, promoting further hydrodeoxygenation of guaiacol to phenol.
In conclusion, the study by Professor Changzhi Li and the team at the Dalian University of Technology represents an important milestone in the field of sustainable energy production. They successfully demonstrated a “hydrogen-free” HDO strategy using nano-porous Ni catalysts, exhibiting high phenol selectivity from guaiacol. The system involves water chemo-splitting, selective C–O bond cleavage, and aqueous-phase methanol reformation. Characterization and DFT calculations confirmed the catalyst’s crucial role in these processes. Their innovative approach to hydrogen-free biomass conversion offers a promising alternative to conventional biorefinery processes, addressing the challenges of hydrogen sourcing and economic feasibility. By leveraging the unique catalytic properties of nano-porous Ni, the researchers have demonstrated the feasibility of an “H2-free” upgrading process, achieving partial deoxygenation and molecular upgradation without the need for external hydrogen. This research has far-reaching implications, not only for the biorefinery industry but also for the broader renewable energy sector. It opens doors to more sustainable and economically viable methods of biomass conversion, aligning with global efforts to transition toward greener and cleaner energy sources.
Xiaohong Ren, Zhuohua Sun, Jiqing Lu, Jinling Cheng, Panwang Zhou, Xiaoqiang Yu, Zeming Rong and Changzhi Li. Hydrodeoxygenation of guaiacol to phenol using endogenous hydrogen induced by chemo-splitting of water over a versatile nano-porous Ni catalyst. Green Chem., 2023, 25, 1955–1969