Redox-active and Brønsted basic dual sites for photocatalytic activation of benzylic C–H bonds based on pyridinium derivatives

Guan

et al. 2022

Environ. Sci. Technol.

Radicals in advanced oxidation processes (AOPs) degrade micropollutants during water and wastewater treatment, but the transformation of dissolved organic matter (DOM) may be equally important. Ketone moieties in DOM are known disinfection byproduct precursors, but ketones themselves are intermediates produced during AOPs. We found that aromatic alcohols in DOM underwent transformation to ketones by one-electron oxidants (using SO 4•− as a representative), and the formed ketones significantly increased trichloromethane (CHCl 3 ) formation potential (FP) upon subsequent chlorination. CHCl 3 -FPs from aromatic ketones (Ar−CO−CH 3 , average of 22 mol/mol) were 6−24 times of CHCl 3 -FPs from aromatic alcohols (Ar−CH(OH)−CH 3 , average of 0.85 mol/mol). At a typical SO 4•− exposure of 7.0 × 10 −12 M•s, CHCl 3 -FPs from aromatic alcohol transformation increased by 24.8%−112% with an average increase of 53.4%. Notably, SO 4•− oxidation of aliphatic alcohols resulted in minute changes in CHCl 3 -FPs due to their low reactivities with SO 4•− (∼10 7 M −1 s −1 ). Other one-electron oxidants (Cl 2 •−, Br 2•− ,and CO 3 •− ) are present in AOPs and also lead to aromatic alcohol−ketone transformations similar to SO 4•− . This study highlights that subtle changes in DOM physicochemical properties due to one-electron oxidants can greatly affect the reactivity with free chlorine and the formation of chlorinated byproducts.

Section: Resultsmentioning

confidence: 97%

“…50,51 Two other DOM isolates, PPFA and ESHA with high aromaticities, also showed slight increases in carbonyl groups, as indicated by FTIR spectra (Figure S16 and Text S3). Meanwhile, the intensities of 1 H NMR spectra at 2.1 ppm, (i.e., the characteristic peak of R− CO−CH 3 ) 52,53 also increased in SO 4…”

Section: •−mentioning

confidence: 97%

One-Electron Oxidant-Induced Transformations of Aromatic Alcohol to Ketone Moieties in Dissolved Organic Matter Increase Trichloromethane Formation

Guan

et al. 2022

Environ. Sci. Technol.

“…More importantly, surface hydroxyl could get replenished by forming 1 O 2 ( • OH + • O 2 – → 1 O 2 + OH), which explained the stable performance of ZnSnAl-50 in the absolute dry condition. It was reported that 1 O 2 can effectively activate the C–H bond and had a relatively longer lifetime, contributing to the degradation of aromatic VOCs. , Therefore, the rapid ring-opening of aromatic VOCs over ZnSnAl-50 can be attributed to the synergy of rich surface hydroxyl and oxygen vacancies, enhancing reactant adsorption-activation and reactive oxygen species formation.…”

Section: Results and Discussionmentioning

confidence: 99%

Surface Hydroxyl and Oxygen Vacancies Engineering in ZnSnAl LDH: Synergistic Promotion of Photocatalytic Oxidation of Aromatic VOCs

Liu,

Zhang,

Liu

et al. 2024

Environ. Sci. Technol.

Photocatalytic oxidation has gained great interest in environmental remediation, but it is still limited by its low efficiency and catalytic deactivation in the degradation of aromatic VOCs. In this study, we concurrently regulated the surface hydroxyl and oxygen vacancies by introducing Al into ZnSn layered double hydroxide (LDH). The presence of distorted Al species induced local charge redistribution, leading to the remarkable formation of oxygen vacancies. These oxygen vacancies subsequently increased the amount of surface hydroxyl and elongated its bond length. The synergistic effects of surface hydroxyl and oxygen vacancies greatly enhanced reactant adsorption-activation and facilitated charge transfer to generate • OH, • O 2 − , and 1 O 2 , resulting in highly efficient oxidation and ring-opening of various aromatic VOCs. Compared with commercial TiO 2 , the optimized ZnSnAl-50 catalyst exhibited about 2-fold activity for the toluene and styrene degradation and 10-fold activity for the chlorobenzene degradation. Moreover, ZnSnAl-50 demonstrated exceptional stability in the photocatalytic oxidation of toluene under a wide humidity range of 0−75%. This work marvelously improves the photocatalytic efficiency, stability, and adaptability through a novel strategy of surface hydroxyl and oxygen vacancies engineering.

ACS Appl. Mater. Interfaces

“…The direct and selective oxidation of C–H bonds of organic compounds to corresponding high value-added chemicals, such as alcohols, ketones, and acids, holds great theoretical and practical significance. , Among them, the direct oxidation of benzyl C–H bonds is particularly important in heterogeneous catalysis, , such as the oxidation of ethylbenzene (EB) to produce acetophenone (AP), which is a key intermediate in the production of resins, fragrances, chewing gum, and drugs . However, the activation and cleavage of C–H bonds of EB typically require high-temperature initiation and catalyst excitation activation .…”

Section: Introductionmentioning

confidence: 99%

Bridging Effect of Carbon Nitride with More Negative Conduction Potential and Halogens Promotes the Liquid-Phase Oxidation of Aromatic C–H Bonds

Zhu,

Zhao,

et al. 2023

The selective oxidation of benzyl C−H bonds of alkyl aromatic hydrocarbons under solvent-free conditions by using heterogeneous catalysis is a challenging task. In this work, we designed a carbon nitride photocatalyst with a high charge separation efficiency and a directed charge transfer path, which was doped with Ni and Br in the carbon nitride skeleton. Br was deposited directionally onto the electron-rich Ni surface traps to form a bond with Ni, which acted as a charge transfer bridge connecting CN and Br, resulting in a bridging effect. Photogenerated electrons were transferred from Ni target to Br, and electrons were aggregated to form a directional charge transfer path, thereby enhancing the photocatalytic performance of CN. The photocatalyst was utilized for the selective oxidation of ethylbenzene at room temperature, atmospheric pressure, and solvent-free conditions. Under batch conditions simulating solar irradiation, the conversion of ethylbenzene was 43.3% and the selectivity of the product acetophenone was up to 92.0%. With the continuous flow strategy, the conversion of ethylbenzene was increased to 52.4 and 48.1%, respectively, while the selectivity reached 92.7 and 91.0%, and the reaction time was reduced from 24 to 2.1 h. The catalyst was also found to be broadly applicable for the selective oxidation of C−H bonds in the benzyl position of alkyl aromatic hydrocarbons.