Conversion of phenol into resourceful chemicals through selectively electrochemical activation of peroxymonosulfate with single-atom iron electrodes

<sec><p indent="0mm">Phenol pollutants, as a kind of high concentration and high toxicity organic pollutants commonly found in wastewater, have various degrees of harm to human health and the ecological environment. Compared with direct degradation, the selective conversion of phenol into p-benzoquinone, which is more than ten times more valuable than phenol, is more cost-effective and in line with the call for sustainable development and energy recycling. Electrocatalytic oxidation technology has been widely used for the deep treatment of phenolic organic pollutants in sewage wastewater. Still, the selective electrocatalytic conversion of phenols to high-value-added chemicals has rarely been reported, because in the conventional electrochemical oxidation technology, the oxidation process is mainly mediated by hydroxyl radicals. However, advanced oxidation techniques dominated by free radicals such as hydroxyl radicals are susceptible to the influence of water background (e.g., natural organic matter, inorganic salt ions, etc.) in practical applications, resulting in excessive consumption of chemicals and energy. The non-radical pathways of high-valent iron oxidation, singlet oxygen oxidation, and electron transfer processes are highly resistant to interference, highly selective for some pollutants, and widely applicable, and have been used in studies of phenol coupling or polymerization and the resourceful conversion of phenol to benzoquinone. </sec><sec> In this study, monatomic Fe-loaded oxygen-doped graphitic phase carbon nitride (Fe-OCN) cathode materials were prepared by thermal polymerization for the selective oxidation of phenol to benzoquinone via electrocatalytic activation of persulfate (PMS) to produce high-valent iron species and singlet oxygen. The optimal material Fe-OCN-03 obtained by optimizing the Fe loading achieved 91.93% conversion of phenol, and 69.21% and 63.63% selectivity and yield of benzoquinone, respectively. Analysis of the results of quenching and electron paramagnetic resonance (EPR) experiments indicated that the OCN-00 /PMS electrocatalytic system produced singlet oxygen as the main active species. In contrast, the FeOCN-03 /PMS system after introducing monatomic Fe generated singlet oxygen and high-valent iron as the two main oxidatively active species involved in the phenol resourcing process. The involvement of high-valent iron reactive species was instrumental in improving the selectivity of the conversion of phenol to p-benzoquinone. These active species offered a more controlled and targeted oxidation pathway, which minimized the formation of unwanted by-products and ensured preferential conversion of phenol molecules to the desired product p-benzoquinone. Notably, this system involved no generation of free radical oxidation reactive species, which was favorable for controlling the selectivity of the reaction. </sec><sec> In addition, the Fe-OCN-03/PMS electrocatalytic system exhibited excellent catalytic performance in the pH (3–7) range, indicating its adaptability under different environmental conditions. Also, the system exhibited strong resistance to a wide range of inorganic ions, which is an essential advantage for practical application in a complex wastewater matrix. This system also showed excellent stability, which maintained a relatively good performance over multiple catalytic cycles, thus highlighting its potential for long-term use. In real coking wastewater, the Fe-OCN-03/PMS electrocatalytic system achieved efficient and selective conversion of phenol to p-benzoquinone under realistic conditions. It enhanced its applicability as a robust electrocatalyst for resource-oriented pollutant transformation. Compared with other works, the Fe-OCN-03/PMS electrocatalytic system showed a relatively high removal rate of phenol with less time required, which has the potential for application and space for improvement. The results of this research provided a theoretical and experimental basis for the rational design and optimization of electrocatalytic materials for the resource-oriented conversion of phenol to benzoquinone. </sec>