A theoretical study on toluene oxidization by OH radical

Abstract

Toluene, a prominent member of volatile organic compounds (VOCs), exerts a substantial adverse influence on both human life and the environment. In the context of advanced oxidation processes, the ·OH radical emerges as a highly efficient oxidant, pivotal in the elimination of VOCs. This study employs computational quantum chemistry methods (G4MP2//B3LYP/6-311++G(d,p)) to systematically investigate the degradation of toluene by ·OH radicals in an implicit solvent model, and validates the rationale of choosing a single-reference method using T1 diagnostics. Our results suggest three possible reaction mechanisms for the oxidation of toluene by ·OH: firstly, the phenyl ring undergoes a hydrogen abstraction reaction followed by direct combination with ·OH to form cresol; secondly, ·OH directly adds to the phenyl ring, leading to ring opening; thirdly, oxidation of sidechain to benzoic acid followed by further addition and ring opening. The last two oxidation pathways involve the ring opening of toluene via the addition of ·OH, significantly facilitating the process. Therefore, both pathways are considered feasible for the degradation of toluene. Subsequently, the UV-H₂O₂ system was designed to induce the formation of ·OH for toluene degradation and to identify the optimal reaction conditions. It was demonstrated that ·OH and ¹O₂ are the primary active species for degrading toluene, with their contribution ranking as ·OH > ¹O₂. The intermediates in the mixture solution after reactions were characterized using GC-MS, demonstrating the validity of theoretical predictions. A comparative study of the toluene consumption rate revealed an experimental comprehensive activation energy of 10.33 kJ/mol, which is consistent with the preliminary activation energies obtained via theoretical analysis of these three mechanisms (0.56 kJ/mol to 13.66 kJ/mol), indicating that this theoretical method can provide a theoretical basis for experimental studies on the oxidation of toluene by ·OH.

Keywords: Advanced oxidation; Quantum chemical calculations; Reaction mechanism; ·OH.

Fig. 1

a Reaction process diagram at the B3LYP/6-311++G(d,p) level of accuracy. b Reaction process diagram at the G4MP2 level of accuracy. c The energy profile under G4MP2 precision conditions

Fig. 1

a Reaction process diagram at the B3LYP/6-311++G(d,p) level of accuracy. b Reaction process diagram at the G4MP2 level of accuracy. c The energy profile under G4MP2 precision conditions

Fig. 1

a Reaction process diagram at the B3LYP/6-311++G(d,p) level of accuracy. b Reaction process diagram at the G4MP2 level of accuracy. c The energy profile under G4MP2 precision conditions

Fig. 2

Structural Diagrams of Toluene (IS) and Benzoic Acid (FS8) with Attack Sites

Fig. 3

Performance of typical UV-AOP systems on the removal and mineralization of toluene (a). Effect of the H₂O₂ solution pH (b), the H₂O₂ concentration (c) and the reaction temperature (d) on the toluene removal

Fig. 4

EPR analysis of UV-H₂O₂ system (a). Radical quenching tests (b)

Fig. 5

GC–MS analyses on key intermediates generated in toluene degradation by UV-H₂O₂ system

Fig. 6

Variation of Reaction rate constants for the initial steps of each reaction in the 303–340 K temperature range