A Molecular Mechanism of Fenton Oxidation Degradation of m-Xylene①

ZHAO Jia-Min YUNA Ying ZHOU Ru-Jin

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A Molecular Mechanism of Fenton Oxidation Degradation of-Xylene①

ZHAO Jia-Min YUNA Ying ZHOU Ru-Jin②

(525000)

Density functional theory (DFT) quantum chemical method was used to study the process of meta-xylene Fenton oxidation. TheHOMOenergies of meta-xylene molecules are higher, so they have better ability to provide electron and prone to the nucleophilic reaction. M-xylene mainly reacts with OH free radical in addition reactions. And the position of C(6) is most likely to react with the OH free radical, rather than the C(3), which is the most difficult to occur. According to all the above results, the most likely reaction mechanism of advanced oxidation degradation of meta-xylene is determined.

molecular simulation, Fenton oxidation, meta-xylene, reaction mechanism;

1 INTRODUCTION

In recent years, with the development of industry and agriculture, people increasingly rely on organic compounds. As a consequence, a significant amount of wastewater containing organic pollutants that are difficult to biodegrade have also increased accor- dingly. For example, the wastewater discharged from chemical, pesticide, and other industries often inclu- des chlorinated benzenes and polycyclic aromatic hydrocarbons (PAHs)[1-3]. These pollutants have high toxicity and poor biodegradability; moreover, they are the major sources of pollution of the water environment and endanger human health. Conven- tional treatment techniques (physical, chemical and biological methods) do not lead to satisfactory results in the presence of these pollutants. Therefore, the processing technology of benzene pollutants in industrial and agricultural organic wastewater has gradually become a challenging and hot topic in the water treatment scientific community[4-6].

Advanced oxidation technology (AOT) is an emer- ging water treatment strategy with the advantage of high-processing speed, through oxidation, and no secondary pollution[7-9]. AOTs mainly utilize highly active ·OH to oxidize and decompose organic pollutants in water and eventually mineralize into CO2, H2O, and inorganic salts[10, 11]. Among them, the Fenton oxidation technology, as a commonly used AOT, has the advantages of high efficiency, flocculation, simple equipment, and low technical requirements compared with other advanced oxida- tion technologies[12-14]. In recent years, a series of studies were carried out to investigate benzene com- pounds utilizing Fenton oxidation technology. Kwo[15]and Yao[16]studied the chlorophenol charac- teristics of the Fenton reagent; Sedlak[17]studied the degradation of chlorobenzene using the Fenton reagent; Zhao[18]explored nitrobenzene wastewater via the US-Fenton method. Beltran[19]used Fenton reagent to study three PAHs (fluorene, phenanthrene, and acenaphthene) in water. At the same time, Fenton advanced oxidation was also employed to study bentazon in wastewater[20], bisphenol in electronic wastewater[21], and wastewater derived from amino- phenol production[22], azobenzene, and anthraqui- none[23].

Most studies show that the advanced oxidative degradation of aromatic hydrocarbons in wastewater is complicated. At present, the experimental research mostly focuses on the generation mechanism of ·OH. Due to the low concentration of free radical interme- diates, the secondary reactions take place, and the instability of PAHs during the oxidative degradation process, their detection and quantification is incredi- bly challenging. Indeed, the oxidation-degradation reaction mechanism is mostly unknown. Theoretical calculations can overcome some of the difficulties encountered during the experimental studies and become more and more prominent in the study of reaction mechanisms[24, 25]. Taking advantage of the computational method of density functional theory (DFT), we systematically studied the reactants of ·OH-induced-xylene Fenton reaction system, including its intermediate transitional ions and reaction products. Combined with the theoretical calculations and experimental results, a more detailed-xylene Fenton advanced oxidation mechanism was presented.

2 COMPUTATIONAL METHODS

Using the Material Studio, Version 8.0 software from the Accelrys company[26], we carried out quantum chemical calculations on the ·OH-induced-xylene reaction system. Structural models of reactants, intermediates, and products were construc- ted via the Builder module. Structural optimization was performed using molecular mechanics module Discover; additional structural optimization was carried out by using semi-empirical quantum che- mistry module VAMP. The final structural optimiza- tion was performed using quantum chemicalcalculation module Dmol3, which is based on the DFT method. PW91 functional and the DNP basis set were selected for all the DFT calculations. The processing method for a central electron is full-electrons. Frequency analysis on the resulting configurations confirmed the absence of any virtual frequency. The standard total force criterion for the convergence of atomic relaxation was less than 0.15 eV/?, whereas the corresponding maximum stress for an atom was lower than 0.05 eV/?. The Dmol3 module was used to search and optimize the transition states of the elementary reactions. The NEB method was used to determine the energy barriers of the reactions.

3 RESULTS AND DISCUSSION

The Fenton reagent comprises of Fe2+and H2O2. In an acidic solution, Fe2+catalyzes the decompo- sition of H2O2and produces the strong oxidizing agent ·OH, which further induces the generation of more other free radicals. The strong oxidation capability of ·OH decomposes the organic com- pounds in water, leading to a wastewater purification.

3.1 Initial addition of ·OH

The redox reaction of-xylene mainly occurs with ·OH. In 1997, Kramp[27]measured the rate constants of the reaction between-xylene and ·OH through experimental measurements in combination with the previously reported experimental results. Mehta[28]further discovered the reaction mechanism. Indeed, there are two possible reaction routes between-xylene and ·OH: (1) hydrogen extraction reaction, where the hydrogen of methyl group is captured, producing-methyl benzaldehyde and ester compounds[29, 30], (2) ·OH is added to the benzene ring, forming an-xylene-hydroxy adduct (mX-OH)[31].

It is generally believed that Fenton catalyzed the elementary reaction of xylene in accordance with the radical reactions. Therefore, in the quantum chemical study and the construction of elementary reaction network for the reaction system, we assumed that the reaction had been carried out in accordance with the free radical reaction mechanism. The reactant of elementary reaction system is-xylene, whose molecular structure is shown in Fig. 1.

Fig. 1. Molecular structure and atom number of-xylene

In the Fenton oxidation, the reaction of-xylene and ·OH may generate eight transition states with different structures, whereas five ·OH addition reaction processes can be inferred, as shown in Table 1. According to the Dmol3 module search and the transition states of the so-optimized elementary reaction, we determined the energy barrier of the reaction process via the NEB method. The calcu- lation results are reported in Table 2.

According to our computational results, the hy- drogen-extraction reaction between-xylene and ·OH has an energy barrier of about 61～83 kcal/mol, which is significantly higher than that in the formation of the-xylene-hydroxyl adduct (mx-OH) by adding ·OH to the benzene ring. Therefore, we can infer that the addition reaction of-xylene with ·OH mainly occurs under the Fenton reagent; the-xylene is unlikely to undergo the hydrogen extraction reaction in the presence of ·OH.

Table 1. Initial Addition Reaction of ·OH

Table 2. Initial Addition Reaction Energy of ·OH

Through the comparison of energy barriers in the addition reaction between-xylene and ·OH, it can be found that the energy barrier of Rea002 reaction is the lowest one (3.066 kcal/mol), suggesting that the C(6) of-xylene is the most prone atom for the addition reaction; moreover, the energy barrier of Rea005 is the highest (9.306 kcal/mol), suggesting that the C(3) of-xylene is the most difficult atom to undergo addition reaction.

To further investigate the reaction characteristics between-xylene and ·OH, the structure-activity relationship of-xylene was investigated via Dmol3. The nature of the reaction between-xylene and ·OH was further explored according to the Fukui index and frontier orbital theory of-xylene.

According to the frontier orbital theory, any reaction occurs at the position and direction of the maximum effective overlap between the HOMO of one reactant and the LUMO of the other reactant. The highest occupied molecular orbital (HOMO) with the highest energy and the lowest unoccupied molecular orbital (LUMO) with the lowest energy electron space orbital play an essential role[32]. During the reaction, the electron is transferred from the HUMO of one reactant to the LOMO of the other reactant. Since the LUMO energy of ·OH (0.3735 Ha) is higher than the HOMO energy of-xylene (–0.20337Ha), ·OH can only react with the LUMO of-xylene. The composition of the LUMO in-xylene (see Table 3) shows that the electron density of the benzene ring has a significant contri- bution; therefore, ·OH should preferentially attack the benzene ring. Our calculations show that theHOMOof-xylene is –0.20337 Ha, whereas theLUMOis –0.02451 Ha. The Δof the two orbitals is 0.101731 Ha and meets the symmetry matching requirements. Although it is low, the Δis in the range of forming a stable chemical bond[33, 34], and thus this reaction can occur.

The Fukui index is an effective method to study the characteristics of nucleophilic/electrophilic reac- tions and the molecular active sites of organic molecules[35]. In the Fukui index method,(-) and(+) indicate the ability of an atom to give and receive electrons, respectively. Because the outer electron layer of ·OH has unpaired electrons, it always tends to receive electrons. Therefore, the higher(-) is, the easier this atom is to react with ·OH, while the higher the(+) index is, the more unlikely this atom is to react with ·OH.

Table 3 lists the Fukui index map and frontier orbitals of the molecular electrophilic ucleophilic reaction of-xylene. It can be seen that the C(3), C(5), and C(6) are the most active positions: as a consequence, they are the attack positions of electrophilic reaction. It is revealed from the Fukui index of nucleophilic reaction that the nucleophilic attack positions are mainly C(2), C(3), and C(5) atoms. Therefore, according to the Fukui index theory, it can be inferred that C(6) is the most prone atom to react with ·OH.

Above all, after comparing the energy barriers of different pathways of the reaction between-xylene and ·OH, it can be concluded that the addition between-xylene and ·OH mainly occurs at the primary reaction stage. The C(6) of-xylene is the most prone atom for addition reaction, whereas the C(3) of-xylene is the most difficult atom for addition reaction. Therefore, we mainly investigate the subsequent reactions of R2.

Table 3. Fukui Functions and Frontier Orbitals of m-Xylene

3.2 Reaction between the R2 radical and ·OH

In the presence of ·OH, R2 is either stabilized by collision or reacts with ·OH. Similar to the benzene and toluene reacting with ·OH, in the presence of ·OH, R2 can undergo either the addition of ·OH to its benzene ring to form a peroxyl group, or the direct extraction of H to produce 2,6-dimethylphenol. The addition of ·OH can occur at the positions of C1/C3 and C5, forming peroxyl group R2-OO (= 1/3/5). R2-OO can be further linked to produce a double loop radical R2-OO, whereandrepresent the carbon number of O–O bond. The resulting possible reaction path between R2 radical and ·OH is shown in Fig. 2.

Fig. 2. R2 free radicals possible reaction pathway with ·OH

To determine the most probable reaction process between R2 and ·OH, the relative energy and energy barrier of transition states with different structures were calculated by Dmol3 (Table 4). It is clear that the reaction pathways with high-energy barriers can be removed from the mechanism. For example, the reaction barriers of Rea007 and Rea008 are signifi- cantly higher than that of Rea006; therefore, the formation reaction of R2-1OO is mainly considered in the reaction pathway of R2 and ·OH. Similarly, the pathway of R2-1OO by closed loop reaction to generate R2-15OO can also be neglected because its energy barrier is 11.51 kcal/mol higher than that of the reaction generating R2-13OO.

The transition states of the elementary reactions were searched and optimized via the Dmol3 module, whereas the energy barrier of the reaction process was determined via the NEB method. It can be seen in Fig. 3 that, according to the value of the relative energy barrier, the addition reaction of ·OH occurs at the position of C1/C3, whereas the process of generating peroxyl radical R2-1OO is the most kinetically favorable, with the lowest energy barrier of reaction (9.64 kcal/mol). In the next reaction, the formation of R2–13OO by cyclization of R2-1OO is considered to be thermodynamically irreversible because of the higher level of released energy (–11.00 kcal/mol). For R2-13OO, the energy barriers of the O–O bond breaking isomerization to generate R2-1O-34O and R2-13OO-4OO are 19.93 and 11.67 kcal/mol, respectively. Then, the reaction products are ultimately oxidized to CO2, H2O, and inorganic compounds. From these findings, we can deduce the lowest energy reaction path of the reaction between R2 radical and ·OH, as shown in Fig. 4.

Table 4. Reaction Process between R2 and ·OH

Fig. 3. Energy bar of the reaction between R2 and ·OH

Fig. 4. Reaction path of the lowest energy between R2 and ·OH

4 CONCLUSION

In this paper, based on the DFT and transition state theory, we have performed detailed theoretical researches on the Fenton advanced oxidation reaction between-xylene and ·OH. The conclusions are as follows:

(1) The addition of ·OH mainly occurs on C(6), producing mX-OH adduct R2, which is in good agreement with the structure-activity relationship of meta-xylene.

(2) The addition reaction continues to occur between R2 and ·OH, followed by isomerization to form a double-ring free radical intermediate. In the Fenton reagent, the double-ring radical intermediate reacts with ·OH, forming a bicyclic peroxyl radical.

(3) The pathway of the reaction between-xylene and ·OH with the lowest energy barrier has been determined via the NEB method, so we have descri- bed the advanced oxidation degradation process of-xylene.

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2 May 2018;

6 July 2018

the research and practice project of scientific and technological innovation of school enterprise cooperation in Guangdong University of Petrochemical Technology (660648)

. Professor, Ph.D. E-mail: 1075081491@qq.com

10.14102/j.cnki.0254-5861.2011-1705