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By Feng Wang, Ph.D., Professor of Physical Chemistry, Dalian Institute of Chemical Physics (CAS) and Awardee of 2020 ACS Sustainable Chemistry & Engineering Lectureship, and Ning Li, Ph.D., Postdoctoral Associate, Dalian Institute of Chemical Physics (CAS)
I started to realize the negative environmental impacts instigated by the vast consumption of depleting fossil resources when I was a graduate student. My Ph.D. project was the efficient catalysis of fossil products using metal oxides and metal nanoparticles as catalysts. When I initiated my research group at Dalian Institute of Chemical Physics (DICP), I decided to step forward to the concept of biorefinery (i.e., utilizing the toolbox available in oil refining to solve the issues in renewable biomass valorization for fuels and value-added products). The great abundance and availability of biomass was fascinating, while I soon realized it was also quite challenging due to the complexity of biomass components (e.g., lignin, carbohydrates, bio-lipids), all of which bear distinct chemical structures.
I decided to narrow down my first target to lignin. It was stereotypically recognized as a waste from the pulp industry and was poorly destined for calcination. Instead of using homogenous acid or base catalysts in pulping processes which resulted in value-deficient lignin fractions, I adopted a recyclable catalyst (heterogeneous and non-precious nickel catalysts) to selectively break down the lignin fragments in methanol. Monophenols were harvested in high yields and the residual cellulose pulps (free of lignin) were ready for biochemical processing1. This study opens a new avenue to utilize lignin-based phenols as platform chemicals for fuels, aromatic chemicals, and pharmaceutical molecules, and has come into being a widely used lignin conversion strategy, i.e. lignin first.
While it was exciting to implement my preliminary green idea by recyclable and cheap catalysts, I knew greener improvements would be demanded in the value-addition of lignin fractions. Lignin depolymerization via a thermochemical process inevitably suffers from high-energy consumption. It would be desirable to depolymerize lignin under mild conditions. Since biomass moieties (including lignin) are derived from organic matter photosynthesized at ambient temperature under light illumination, what if catalytic biomass conversion occurs using light as an energy source?
Photocatalytic reactions, which are non-toxic and energy-efficient, fulfill multiple green chemistry principles, with the added benefit of being inexpensive. Incorporating photocatalysis into biomass conversion is a milestone in my research journey toward a sustainable biorefinery. I am lucky because the catalysts I played with in my Ph.D. projects (such as metal oxides and metal sulfides) exhibit certain photocatalytic performance. After screening and tuning the defects and surface structures of heterogeneous catalysts, a couple of efficient candidates (such as ZnIn2S4, CuOx/ceria/TiO2) emerged to be effective in catalytic cleavage of C−C and C−O bonds between lignin units, once exposed to visible or UV lights2-3.
Tackling carbohydrate photocatalysis is on my bucket list for a sustainable biorefinery. Carbohydrates (such as cellulose and hemicellulose) embody the most abundant renewable resources, co-existing with lignin in lignocellulosic biomass. In our journey toward a sustainable biorefinery, it is imperative to validate the efficient conversion of carbohydrates and their derivatives to green fuels and chemicals. Inspired by the solar-driven water-splitting reactions for H2 production, we developed a Ru-doped ZnIn2S4 catalyst for the coproduction of H2 and diesel fuel precursors from carbohydrate-derived methylfurans via acceptorless dehydrogenative C−C coupling4. Subsequent hydrodeoxygenation reactions yielded the diesel fuels comprising straight- and branched-chain alkanes. To integrate the biorefinery processes into the existing petrochemical-fuel chain, bio-methanol is designed as a clean liquid energy carrier and a pivotal chemical for the synthesis of olefins and aromatics. In contrast to thermochemical reforming at harsh conditions (over 300 °C), we discovered the Cu-dispersed titanium oxide nanorods were effective for photo-splitting of sugars and bio-derived polyols to methanol at room temperature5.
As we strive to knock down the complexity in biomass compositions, my interest extends to the photocatalytic bio-lipid conversion for sustainable fuels. Capturing the subtle changes of radical intermediates on the photocatalyst surface, we found a photocatalytic decarboxylation route to efficiently upgrade bio-derived fatty acids into long-chain alkanes by Pt/TiO2 as a robust and recyclable catalyst under mild conditions (ambient temperature with hydrogen gas pressure no more than 2 atm)6. Compared to the traditional hydrodeoxygenation and hydrodecarboxylation processes, the photocatalytic decarboxylation is highlighted for its low energy and H2 consumption.
On my journey towards sustainable valorization of biomass, emphases have been given to validating various renewable feedstocks and digging into the fundamental mechanism of green catalysis. However, new challenges keep emerging as our awareness in this field grows. For example, our photocatalytic decarboxylation process for biodiesel turned out to be less competitive than the current fossil-based process in the life-cycle assessment. Continuous research efforts will be needed for process optimization and catalyst design. To build a sustainable world, block polymers and functional materials must be developed from biomass. This underexplored domain will be of great interest to venture on.
I am an optimist in green chemistry and catalysis. Green is the color of the natural environment, but also signifies the youth and prosperity of this research area. Compared to the traditional chemical (engineering) processes, there are enormous opportunities in this green area. Aiming at a bright future of sustainable biorefinery, I am in. And you are more than welcome to join us.
References
- Qi Song, Feng Wang, Jiaying Cai, Yehong Wang, Junjie Zhang, Weiqiang Yu, Jie Xu. Lignin depolymerization (LDP) in alcohol over nickel-based catalysts via a fragmentation-hydrogenolysis process. Energy Environ. Sci., 2013, 6 (3), 994-1007. https://doi.org/10.1039/c2ee23741e
- Tingting Hou, Nengchao Luo, Hongji Li, Marc Heggen, Jianmin Lu, Yehong Wang, Feng Wang. Yin and yang dual characters of cuox clusters for c–c bond oxidation driven by visible light. ACS Catalysis, 2017, 7 (6), 3850-3859. https://doi.org/10.1021/acscatal.7b00629
- Nengchao Luo, Min Wang, Hongji Li, Jian Zhang, Tingting Hou, Haijun Chen, Xiaochen Zhang, Jianmin Lu, Feng Wang. Visible-light-driven self-hydrogen transfer hydrogenolysis of lignin models and extracts into phenolic products. ACS Catalysis, 2017, 7 (7), 4571-4580. https://doi.org/10.1021/acscatal.7b01043
- Nengchao Luo, Tiziano Montini, Jian Zhang, Paolo Fornasiero, Emiliano Fonda, Tingting Hou, Wei Nie, Jianmin Lu, Junxue Liu, Marc Heggen, Long Lin, Changtong Ma, Min Wang, Fengtao Fan, Shengye Jin, Feng Wang. Visible-light-driven coproduction of diesel precursors and hydrogen from lignocellulose-derived methylfurans. Nat. Energy, 2019, 4 (7), 575-584. https://doi.org/10.1038/s41560-019-0403-5
- Min Wang, Meijiang Liu, Jianmin Lu, Feng Wang, Min Wang, Meijiang Liu. Photo splitting of bio-polyols and sugars to methanol and syngas. Nat Commun, 2020, 11 (1), 1083. https://doi.org/10.1038/s41467-020-14915-8
- Zhipeng Huang, Zhitong Zhao, Chaofeng Zhang, Jianmin Lu, Huifang Liu, Nengchao Luo, Jian Zhang, Feng Wang. Enhanced photocatalytic alkane production from fatty acid decarboxylation via inhibition of radical oligomerization. Nat. Catal., 2020, 3 (2), 170-178. https://doi.org/10.1038/s41929-020-0423-3