A research team led by Professor Jaehoon Kim at Sungkyunkwan University and Dr. Dong Ki Lee at the Korea Institute of Science and Technology (KIST) has developed a highly efficient catalytic process that electrochemically converts lignin, a key component of woody biomass, into value-added aromatic compounds and cyclohexene-based compounds. This study demonstrates that the recalcitrant ether bonds in lignin can be selectively cleaved under relatively mild conditions without the use of external hydrogen gas, while simultaneously upgrading lignin into useful chemical precursors. The research results were published in Applied Catalysis B: Environment and Energy (IF 21.1, top 2% in JCR) in February 2026.
As interest in carbon neutrality and sustainable chemical industries continues to grow, active efforts are being made to replace fossil resource-based aromatic chemicals with biomass-derived materials. Among them, lignin is regarded as a promising source of a wide range of aromatic compounds because it is the most carbon-rich component in woody biomass. However, its selective conversion is extremely difficult due to its complex polymeric structure and strong C–O and C–C bonds. In particular, 4–O–5 and α–O–4 diaryl ether bonds have previously been targeted for cleavage under high-temperature and high-pressure hydrogen atmospheres, but such approaches have been limited by high energy consumption and low selectivity. In addition, previous electrochemical lignin depolymerization studies have also suffered from low monomer yields and insufficient direct identification of actual lignin-derived products.
To overcome these limitations, the research team proposed an electroreductive lignin conversion strategy using a 5 wt% Pd/C catalyst. This process operates by utilizing reactive hydrogen formed on the catalyst surface during water electrolysis to cleave ether bonds in lignin. In other words, it enables simultaneous lignin depolymerization and subsequent hydrogenation using only electrical energy, without any external hydrogen supply, while allowing precise control over the amount of surface-adsorbed hydrogen through current density regulation. The team validated the performance of this approach using both model compounds representing 4–O–5 and α–O–4 bonds and real birch-derived lignin solvolysate.
As a result, the 4–O–5 bond model compounds diphenyl ether (DPE) and phenyl tolyl ether (PTE) were completely converted within 90 minutes at 70°C and 50 mA cm⁻², while the α–O–4 bond model compound benzyl phenyl ether (BPE) was also fully converted at the lower temperature of 30°C. High selectivity was also confirmed in terms of product formation. DPE produced cyclohexanol at 99.8% and cyclohexane at 85.2%; PTE produced 4-methyl cyclohexanol at 99.5% and methyl cyclohexane at 95.6%; and BPE yielded cyclohexanol at 99.2%, toluene at 51.8%, and methyl cyclohexane at 46.3%. These results show that, following ether bond cleavage in lignin, the resulting aromatic intermediates can be selectively hydrogenated into useful upgraded products.
The research team also identified the optimal conditions for improving reaction efficiency. When isopropanol (IPA) was introduced as a co-solvent, both substrate solubility and hydrogen transfer characteristics were enhanced simultaneously. In particular, at 30 wt% IPA, DPE conversion reached 100% and the Faradaic efficiency reached 70.2%. In addition, the best performance was observed at a current density of 50 mA cm⁻², whereas at higher current densities the competing hydrogen evolution reaction increased, which in turn reduced the efficiency of the target reaction. These results experimentally demonstrate that precise control of co-solvent composition and electrochemical conditions is critical for lignin electrochemical conversion.
Important findings were also obtained regarding the catalyst operating mechanism. The research team proposed a bifunctional mechanism in which PdO and metallic Pd in the Pd/C catalyst play different roles. PdO drives the cleavage of C–O bonds in lignin, while the subsequently generated Pd⁰ is responsible for hydrogenating intermediates such as phenol and benzene into cyclohexanol and cyclohexane. In fact, when only Pd foil was used, DPE conversion was limited to 19.3%, and when only PdO was used, it reached only 57.4%; by contrast, Pd/C exhibited the highest activity and selectivity. In addition, Pd/C showed better conversion performance than Pt/C, Ru/C, Ag/C, and Ni/C, together with the highest TOF of 468.0 h⁻¹, and maintained 95.0% DPE conversion even after five cycles, confirming its excellent durability.
The team further demonstrated the scalability of this technology by applying it to real birch biomass. Methanol solvolysis first achieved a delignification yield of 81 wt%, but the yield of lignin-derived phenolic monomers at this stage was only 5.0 C%. When the Pd/C-based electrochemical process was subsequently applied, efficiency was limited under strongly acidic conditions due to rapid repolymerization. However, when the system was switched to a milder 0.5 M acetate buffer (pH ≈ 5), the monomer yield increased to 13.6 C% after 1 hour and 19.6 C% after 4 hours. In particular, a high selectivity of 41.6% was obtained for 4-n-propanol syringol, and GC×GC–TOF/MS analysis confirmed the formation of various monomer products, including 4-n-propyl syringol, 4-n-propyl guaiacol, 4-n-propanol guaiacol, and syringylacetone.
This study is significant in that it presents a new biorefinery platform capable of selectively breaking recalcitrant lignin bonds and simultaneously converting them into value-added chemicals using electricity alone, unlike conventional high-temperature and high-pressure hydrogenation-based lignin upgrading processes. In particular, the study demonstrates mild processing conditions without external hydrogen, applicability to real woody biomass, and the functional division mechanism of PdO/Pd⁰, suggesting strong potential as a key technology for the future production of sustainable chemical materials and biofuel precursors.
Highly efficient electro-reductive conversion of lignin into aromatics and cyclohexenes