Substituent effects on the oxidation reactions of 4-nitrophenol, phenol, 4-methylpheol, and 4-methoxyphenol mediated by reduced graphene oxide in water

Abstract

Substituent effects of 4-nitrophenol (4-NP), phenol (PE), 4-methylpheol (4-MP), and 4-methoxyphenol (4-MOP) on their oxidative coupling reactions mediated by reduced graphene oxide (rGO) were examined using a batch method. The reaction products were identified as the oligomers of their individual parent compound, and CC and CO coupling between phenolic radicals were the main reaction pathways. The oxidative kinetics were well described with pseudo-first order model. The rate constants (kobs) of 4-NP, PE, 4-MP and 4-MOP were 0.47 × 10−3, 1.00 × 10−3, 2.03 × 10−3, and 3.53 × 10−3 h−1, respectively, following the order 4-NP < PE < 4-MP < 4-MOP. The kobs of four phenolic compounds was negative linear correlation with their homolytic bond dissociation energies (BDE(OH)), suggesting that BDE(OH) was an effective indicator to predict the oxidation susceptibility of phenolic compounds in rGO system. Dissolved oxygen played a critical role in the oxidation reactions through the formation of superoxide anion (O2−) on rGO. These O2− further cleaved the OH bond of phenolic compounds to produce phenolic radicals, which interacted each other and led to the following oxidative coupling reactions. The finding of this study is important for predicting the environmental fate of rGO and phenolic compounds.