Humin, endowed with abundant redox functional groups, can be reduced anaerobically under dark. When reduced humin encounters O2, the possibility of ·OH formation arises. However, the exploration of ·OH generation mediated by humin has not been comprehensively conducted. The study found that O2 oxidized the reduced humin, generating 8.61 μmol/g of ·OH. After isolating humin using the methyl isobutyl ketone (MIBK) method, the lipid component was identified as the primary contributor to ·OH generation. Subsequent polar separation revealed that the lipid fraction extracted from the ethanol–water mixture with a volume ratio of 7:3 (LFEW7:3) played the most significant role in ·OH production. Further characterization confirmed that the simultaneous presence of aromatic C═C and C═O were the predominant features contributing to the ·OH generation. The ·OH generation experiments with humin-pyridine analogue compound demonstrated that polycyclic pyridine N (≥3 rings) played a significant role in promoting the ·OH generation. Most importantly, the study compared the ·OH production by humin and homologous humic acid, indicating that ·OH generated by humin was higher than that of humic acid. Overall, these affirmative findings manifested the overlooked role of humin in ·OH production and offered valuable insights into the mechanism of ·OH generation by humin in the dark.
The production of hydroxyl radicals (·OH) via reduced humin oxygenation is well established. As biochar is widely used in environmental restoration, its coexistence with humin in restored soils may affect ·OH generation due to its electron transfer ability. However, the role of biochar in ·OH generation by humin remains largely unexplored. Herein, we investigated the ·OH generation by humin-biochar at different mass ratios. Our results indicated that biochar could significantly enhance the ·OH production by humin when the mass of humin exceeded that of biochar. Notably, when the mass ratio of humin to biochar was 50, the ·OH produced reached 75.1 μM, which is nearly 1.7 times the amount generated by humin alone (43.1 μM). Both single-electron and two-electron transfer pathways coexisted in ·OH generation by humin-biochar. C = O in humin were necessary conditions for the production of ·OH, while the presence of C = C in biochar promoted the generation of ·OH. Using graphene and carbon nanotubes both rich in C = C groups as the model compounds, humin-graphene and humin-carbon nanotubes produced ·OH at 122 μM and 98.8 μM, respectively, significantly higher than those generated by humin alone (43.1 μM). This result confirmed the promoting effect of aromatic C = C on ·OH generation. By contrast, when the biochar mass was greater than or equal to that of humin, ·OH generation was inhibited due to the significant reduction of C = O. In this case, environmentally persistent free radicals (EPFRs) in biochar were the primary structures responsible for ·OH production. This study provides comprehensive insights for the first time into the influence of biochar on ·OH generation by humin and offers a new potential method for the green and efficient removal of pollutants.
Lin Dong,Jie Xia,Yong Guo,Xiaohui Liu,Haifeng Wang,Yanqin Wang
Efficient and selective cleavage of C aromatic -C linkages in lignin to enhance the productivity of monocyclic aromatic hydrocarbons is an extremely challenging and attractive task. Previously, we reported a Ru/NbOPO 4 catalyst that realized the catalytic cleavage of both interunit C-C and C-O bonds in one-pot lignin conversions [ Chem , 2019, 5, 1521–1536]. However, the roles of NbO x species in the selective C aromatic -C cleavage are not clear and need to be extensively studied. Herein, we focus on the exploration of the catalytic mechanisms of C aromatic -C bond cleavage over NbO x -supported Ru catalyst, by comparing with those of Ru/TiO 2 , Ru/ZrO 2 , and Ru/Al 2 O 3 catalysts. It shows that the Ru/Nb 2 O 5 catalyst exhibits high activity and remarkable selectivity to monocyclic hydrocarbons. A combined diffuse reflectance infrared Fourier Transform spectra (DRIFTS) and density functional theory (DFT) calculation analysis unambiguously confirmed that (i) NbO x not only can strongly adsorb phenylcyclohexane via benzene ring, but also selectively activate C aromatic -C bond between benzene ring and cyclohexane ; (ii) NbO x can promote the protonation of phenylcyclohexane with the assistance of H 3 PO 4 ; and (iii) NbO x can significantly reduce the barrier of C aromatic -C bonds cleavage of the carbocation intermediate (C 6 H 5 (H)C 6 H 11 + ). A detail underlying mechanism has been established, which would have further implications to promote the design of new, high efficient catalysts.
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