Destruction of dioxin and furan pollutants via electrophilic attack of singlet oxygen

Ecotoxicology and Environmental Safety 184 (2019) 109605

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Destruction of dioxin and furan pollutants via electrophilic attack of singlet oxygen

T

Nassim Zeinalia, Ibukun Oluwoyea,∗, Mohammednoor Altarawnehb, Bogdan Z. Dlugogorskic a

Discipline of Chemistry and Physics, College of Science, Health, Engineering and Education, Murdoch University, 90 South Street, Murdoch, WA, 6150, Australia Chemical Engineering Department, UAE University, Al-Ain, 15551, United Arab Emirates c Office of Deputy Vice Chancellor, Research & Innovation, Charles Darwin University, Ellengowan Drive, NT, 0909, Australia b

ARTICLE INFO

ABSTRACT

Keywords: Dioxin Furan PCDDs/PCDFs Singlet oxygen Low-temperature oxidation

Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) remain of particular concern owing to their extensive toxicity towards health and accumulation in the environment. Atmospheric oxidation (by ambient oxygen molecules) of this class of persistent environmental pollutants has little to no kinetic feasibility due to very sizable activation energies in the entrance channel. The current control measures involve energy-intensive source incineration of contaminated materials at high temperatures as high as 850 °C. This study finds an alternative low-energy approach of destroying dioxin-like compounds, proposing that advanced oxidation by highly reactive singlet oxygen (O2 1Δg, originated from chemical, surface-mediated and photochemical processes) can initiate low-temperature remediation of these pollutants. This contribution completes the first milestone in mapping out the mechanisms of the electrophilic addition of singlet oxygen to unsubstituted and chlorinated dibenzo-p-dioxin (DBD) and dibenzofuran (DBF) structures, according to density functional theory DFT-B3LYP method in conjunction with the 6–311+g(d,p) basis set, as well as energy refinements based on the approximate spin-projection scheme. The [2+2]-cycloaddition mechanism appears dominant for singlet oxidation of dibenzo-p-dioxin with a fitted rate constant of k(T) = 5.01 × 10−14 exp(-98000/RT). On the other hand, the addition of singlet oxygen to the aromatic ring of dibenzofuran primarily transpires via [4+2]-cycloaddition channel with a fitted rate constant of k(T) = 2.16 × 10−13 exp(-119000/RT). The results suggest that application of singlet oxygen can reduce the energy cost of recycling halogenated and flame retarded materials.

1. Introduction Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) represent aromatic environmental contaminants displaying severe impacts on ecosystems and human safety. They are generally resistant towards ambient oxidation, reduction and hydrolysis, and thus classified as persistent organic pollutants (POPs) (Altarawneh et al., 2009). PCDDs and PCDFs exist as impurities in chlorinated phenols found in industrial chemicals such as wood preservatives, herbicides and pesticides (Safe et al., 1990). Pulp and paper mill effluents remain another source of dioxin/furan discharge into the surrounding environment (Thacker et al., 2007). Likewise, they materialise in the incineration of halogenated and flame retarded materials via well-characterised homogenous and heterogeneous pathways, as well as constitute the flue gas of the electric arc furnaces (EAFs) and secondary aluminium smelters (ALSs). The amount of PCDD/PCDF emission from

∗

EAFs and secondary ALSs are 20 and 18 g I-TEQ/year, respectively (Lee et al., 2005); nearly 40% of that released from sinter plants. Generally, the PCDD/PCDF are emanated from industrial processes such as power plants (Lin et al., 2007), road transport vehicles (Kumari et al., 2011) and chlorinated waste incinerators (Schuhmacher et al., 2002) that represent the chief source, comprising nearly 95% of the total environmental emission by volume (Vallero, 2014). Natural disasters such as volcanic eruption and forest fires can also give rise to the atmospheric discharge of such chlorinated aromatic hydrocarbons. The stability of PCDDs and PCDFs increases with the total number of chlorine substituents (Safe et al., 1990), contributing to their accumulation in various elements of the global ecosystem and environmental matrices, such as food chain and living tissues. Removal of dioxin/furan-like compounds from fly ash is thermally assisted in tubular treatment furnaces at temperatures above 350 °C (Wu et al., 2011) or through a non-thermal degradation by calcium

Corresponding author. E-mail address: [email protected] (I. Oluwoye).

https://doi.org/10.1016/j.ecoenv.2019.109605 Received 12 June 2019; Received in revised form 12 August 2019; Accepted 22 August 2019 Available online 07 September 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.

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oxide additive via mechanochemical treatment (Yan et al., 2007) which requires high rotational blending energy. Adsorption of PCDD/PCDF within activated-carbon supported packed-beds is an alternative hightemperature destruction technique (Karademir et al., 2004). A further efficient technology to reduce the emission of PCDD/PCDFs includes their ozone-enhanced oxidation over iron oxide catalyst at a relatively low operating temperature of 120–180 °C (Wang et al., 2008). A detailed radical mechanism for the oxidation of dibenzofuran (DBF), model molecule for dioxin-like toxicants, is presented by Marquaire et al. (Marquaire et al., 2003), in which the initiation step takes place via the following two bimolecular reactions at high temperatures (900 °C) by hydrogen abstraction as expressed in Eqs. (1) and (2). These reactions usually incur a sizable entrance barriers in the order of 350 kJ/mol, hence the high operating temperature. DBF + O2 → DBF'• + HO2 •

•

DBF + R → DBF' + RH •

•

DBF + O2 → DBF′-OO

of organic matters laden with chlorines and transition metals (Tuppurainen et al., 1998). To the best of our knowledge, no previous study has investigated the interaction of singlet oxygen with PCDD/PCDF. Therefore, we selected dibenzodioxin (DBD) and dibenzofuran (DBF) as modelled compounds for PCDD and PCDF, respectively, to report the mechanism and kinetics of their reactions with singlet oxygen. We extended the study to explore the characteristic effect of chlorine substitution on the reactions. These outcomes should have practical implication in photo-induced oxidation and enhanced thermal incineration of halogenated waste products. Regarding the latter, similar technology has been tested for compression engines; wherein minuscule application of singlet oxygen in oxyfuel systems reduces the activation energy of the initiation reactions, accelerating the chain-branching mechanism to decrease the overall operating temperature. Therefore, enhanced thermal destruction of dioxins and furans through the electrophilic attack of singlet oxygen should be instrumental in reducing the risk of exposure of these notorious compounds.

(1) (2) (3)

•

2. Computational details

where R symbolises any radical formed. According to Altarawneh et al. (Altarawneh et al., 2006), dibenzofuranyl radical (DBF'•) could further undergo oxidation by ground state molecular oxygen (O2 3Σg) to generate dibenzofuranylperoxy (DBF′-OO•), according to Eq. (3). Pyrolysis of PCDD/PCDF with excess amounts of hydrogen is another high-temperature treatment deployed for degradation of these lethal compounds leading to the formation of CO, benzene and naphthalene in the temperature range between 600 and 1000 °C (Cieplik et al., 2002). Effective low-temperature decomposition of dioxin/furan derivatives has been proven feasible via catalytic destructions. For instance, ternary-component noble metal catalyst deposited on different metal oxides (Au/Fe2O3-Pt/SnO2-Ir/La2O3) (Okumura et al., 2003) or a combination of transition metal oxides (TiO2-V2O5-WO3) (Ide et al., 1996) and (TiO2-V2O5-CeO2) (Yu et al., 2016) resulted in efficient catalytic oxidation of dioxins. In fact, the V2O5/TiO2 catalysts are the basis for the efficient gas-phase removal of dioxin at temperatures around 100 °C in shell dedioxin systems (SDDS) (Liljelind et al., 2001). Additionally, fly ash has been reported to catalyse the decomposition of PCDD and PCDF in the low-temperature region (at around 300 °C) under lean oxygen condition (Hagenmaier et al., 1987; Behnisch et al., 2002). Nano-sized iron oxides (FexOy) could also catalyse oxidation of gaseous PCDD/Fs by ozone at temperatures as low as 120–180 °C with efficiencies up to 91% (Wang et al., 2008). Singlet oxygen undergoes unique interactions with hydrocarbons (Zeinali et al., 2016, Al-Nu’airat et al., 2017, 2018, 2019a,b), stemming from its electronic configuration of paired electrons in antiparallel spins of its external orbital. Such characteristic tends singlet oxygen to cling to the electron rich double bonds of unsaturated hydrocarbons at reaction rates about 1500 times higher than the ground state O2 (Johnson and Decker, 2015). Thus, singlet oxygen-induced oxidation should also represent an alternative technique for the destruction of PCDD/PCDF at ambient or low temperatures. Singlet oxygen forms either spontaneously by photosensitization (DeRosa and Crutchley, 2002), electrical discharge (DeRosa and Crutchley, 2002) or thermally via surface mediated reactions on particles of transition metal oxides (Al-Nu'airat et al., 2019a,b). Metaloxide surfaces (e.g., V2O5/TiO2 catalyst) transform the electronic features of absorbed molecular oxygen, serving as excellent generators of singlet oxygen that reacts with hydrocarbons in the gas phase. Therefore, singlet oxygen may coexist with PCDD/PCDFs during incineration

Gaussian 09 suite of programs (Frisch et al., 2009) deployed the hybrid density functional theory of B3LYP with the polarised basis set of 6–311+g(d,p) for gas-phase optimisation of the molecules and intermediate species, serving to compute their respective energies and vibrational frequencies. The intrinsic reaction coordinate (IRC) calculation enabled the validation of the geometries of transition states linking reactants and products. We calculated the Fukui functions (Fukui, 1982) for electrophilic attack by employing Hirshfeld population analysis (Hirshfeld, 1977) at the same level of theory according to Eq. (4).

f k = qk (N )

qk (N

1)

(4)

where qk(N) and qk(N-1) is the molecular electronic population on atom k for neutral and cationic electron systems, respectively. The Yamagushi (Yamanaka et al., 1994) model served to correct the energy of biradical systems via the application of a simple approximate spin-projection (AP) scheme. In this scheme, an approximate spinprojected energy (EAP) can be derived from the energies of the brokensymmetry (EBS ) and pure high-spin (EHS ) states:

EAP = f AP EBS

(f

1) EHS

(5)

where f AP denotes the spin-projection factor:

f AP =

S 2 HS s (s + 1) S 2 HS S 2 BS

(6)

where and signify expectation values of spin contamination pertinent to the pure high-spin and broken-symmetry states, respectively. While meta-hybrid DFT methods are generally more accurate than the B3LYP functional, applying the AP approach in meta-hybrid DFT methods resulted in a notable difference in the computed singlettriplet energy gap in the oxygen molecule in reference to the B3LYP functional (Al-Nu'airat et al., 2018). Furthermore, the computation of the Hirshfeld charge and Fukui function, as quantitative measures of molecular electrophilicity and nucleophilicity, were carried out by DMol3 code (Delley, 2000) in Material Studio package, implementing the B3LYP functional through the application of a double numerical plus d-functions (DND) atomic basis set. Due to the strong dependence of Mulliken population analysis (Fonseca Guerra et al., 2004) to the selected basis set, Hirshfeld charge

S 2 HS

2

S 2 BS

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was chosen as a more reliable technique to model atomic charge distribution.

both structures. Furthermore, the relatively smaller difference between LUMO of singlet O2 and HOMO of DBD, as compared in Fig. 2(b), indicates the high feasibility of the reaction of singlet oxygen with dibenzodioxin and justifies the higher reactivity of this compound as compared to DBF.

3. Results and discussion 3.1. Preliminary model validation

3.3. Mechanism of reactions of DBD and DBF with singlet oxygen

The literature lacks both experimental and theoretical studies on the reaction mechanism of PCDD/PCDF with singlet oxygen. Therefore, we considered benzene and naphthalene as prototypical building blocks for such aromatic organic compounds to enable the validation of our proposed reaction kinetic model in reference to the existing literature data. Earlier works have investigated the [4+2]-cycloaddition of singlet oxygen to benzene and naphthalene rings via deploying a corrected ab initio multireference (CAS MCQDPT2) method (Bobrowski et al., 2000), with the 6-31G* basis set (Reddy and Bendikov, 2006). According to their finding, the reaction pathway appears concerted, demanding an energy barrier of 115 kJ/mol and 87 kJ/mol for Diels-Alder addition of singlet oxygen to benzene and naphthalene aromatic rings, respectively. Our computed reaction barriers, i.e., 117 kJ/mol (for benzene) and 88 kJ/mol (for naphthalene) agree well with the literature values. The abovementioned computational approach have been successfully applied for various organic functional groups in our previous studies to elucidate the reaction of singlet oxygen with phenol (AlNu'airat et al., 2019a,b), aniline (Al-Nu’airat et al., 2017), isoprene (Zeinali et al., 2016) and aromatic fuels (Al-Nu'airat et al., 2018).

3.3.1. Dibenzo-p-dioxin (DBD) Singlet oxygen reacts with aromatic hydrocarbons mainly via [4+2]- and [2+2]-cycloaddition reactions, while the ene type reaction is unattainable presumably due to the lack of allylic carbon and hydrogen atoms. Herein, we first investigated the oxidation of dioxin and furan parent molecules by singlet oxygen, aiming to provide an overview of the possible initial reaction channels. We thereupon substituted chlorine atoms at the reactive molecular sites to explore the reactivity of the chlorinated reagents. As depicted in Fig. 3, oxidation of DBD by O2 (1Δg) branches into four channels; two of which represent [4+2]-cycloaddition and the rest being classified as [2+2]-cycloaddition reactions. The reaction enthalpy barriers are quite close, ranging between 91 and 125 kJ/mol. A stepwise [2+2]-cycloaddition mechanism (TS1) is responsible for binding singlet oxygen to the benzene ring from C1 position forming P1 diradical intermediate. This step appears the most energetically favourable one, leading to the formation of P2 dioxetane that further converts to oxirane-like product (P3) via TS3 saddle point undergoing a relatively high enthalpy barrier of 116 kJ/mol. The P5 diradical arises from an alternative stepwise [2+2]-cycloaddition attack by electrophilic singlet oxygen to in-ring C3 or C4 atoms which incurs an enthalpic penalty of 111 kJ/mol (TS5). The closure of highly unstable P5 diradical to P6 dioxetane occurs via TS6 transformation through almost no enthalpic barrier pathway. Singlet oxygen tackles the DBD aromatic ring diagonally through TS4 and TS7 in a mode labelled as supra-supra C2v symmetry approach (Tonachini et al., 1986) during which one π system of 1O2 is solely involved in the bond formation between carbon and oxygen atoms. These conformations lead to the formation of P4 and P7 endoperoxide intermediates with modest enthalpic levels of 97 and 79 kJ/mol, respectively. In our effort, the produced [4+2]-cycloadducts arise from concerted mechanisms, even though the stepwise mechanisms are generally entitled as energetically more favourable (Dewar, 1984). It is noteworthy to point out that the focus of this section is to explore the mechanism of initiation oxidation of DBD/DBF by singlet oxygen, which is believed to be the more energetically favourable process for destruction of such pollutants at low temperatures, as compared to the ground-state molecular oxygen. The initiation oxidation exclusively involves the formation of endoperoxide or diradical

3.2. Geometry and electronic structures of DBD and DBF Fig. 1 illustrates the geometries of DBD and DBF, tri-cyclic aromatic compounds, as obtained by B3LYP/6–311+g(d,p) level of theory. The C1-C6 and C3-C4 bonds within DBD and DBF structures represent the longest bonds, suggesting that these are the most vulnerable sites for electrophilic attack by singlet oxygen 1O2. The computed Fukui indices (f−1), as the local reactivity descriptors for the electrophilic attack by singlet oxygen, confirms that C3 and C4 atoms possess the largest Fukui function values and are the most susceptible targets for electrophilic attacks. Although C3 and C4 are predicted to have different Fukui indices in DBF, they possess the largest f-values and thus, C3=C4 should remain the most reactive sites. As shown in Fig. 2(a), the LUMO-HOMO energy gap in dibenzodioxin is smaller by 0.3 eV than that in dibenzofuran, perhaps corresponding to the higher reactivity of DBD. The HOMO orbital distribution represents the electron-rich molecular sites that are most likely to donate electrons to the vacant orbital within singlet oxygen to afford the electrophilic interaction. The HOMO electron density surfaces are fairly evenly spread across the carbon atoms of the aromatic rings in

Fig. 1. Optimised geometries of DBD (left) and DBF (right) at the B3LYP/6–311+g(d,p) level of theory. The Fukui indices (f−1) of carbon atoms for electrophilic attack are reported in skeletal sketch below each figure. Bond lengths are in Å.

3

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Fig. 2. LUMO-HOMO electron density surfaces and their energy gaps (a) of DBD and DBF and (b) between singlet O2 and DBD/DBF obtained at B3LYP/6–311+g (d,p) level of theory.

Fig. 3. Initial reaction routes for DBD and singlet oxygen interaction by the calculation level of B3LYP/6–311+g(d,p). The values in bold and italic signify reaction and activation enthalpies computed at 298.15 K, respectively (in kJ/mol).

products. Subsequent decomposition into permanent oxidation products will follow a similar mechanism described in Altarawneh et al. (Altarawneh, Dlugogorski et al. 2006, 2009).

TS4 transformation depicts a [2+2]-cycloaddition mode, and the remaining four addition modes are [4+2]-cycloaddition. The [2+2]addition follows a two-step (non-concerted) reaction forming a hypothetical and highly unstable diradical intermediate (P4); closure of which is readily attainable via a low lying TS5 step which yields to the formation of P5 dioxetane as the four-membered ring product. The mode of 1O2 approach towards the benzylic ring in DBF structure is a concerted supra-supra approach which is divided into three

3.3.2. Dibenzofuran (DBF) As illustrated in Fig. 4, the reaction of DBF and singlet oxygen follows [4+2]- and [2+2]-cycloaddition mechanisms. The addition of singlet oxygen to dibenzofuran consists of five opening channels; the

4

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Fig. 4. Initial reaction routes for DBF and singlet oxygen interaction by the calculation level of B3LYP/6–311+g(d,p). The values in bold and italic signify reaction and activation enthalpies computed at 298.15 K, respectively (in kJ/mol).

possible conformations: TS3, TS6 and TS7. Although according to Fukui indices (Fig. 3) C3 and C4 atoms are the most electron rich and vulnerable molecular spots for 1O2 attack through [4+2]-cycloaddition mode, TS6 embeds the lowest enthalpy barrier corresponding to the higher tendency of singlet oxygen to simultaneously bind with C1 and C4 atoms to produce P6 endoperoxide. This proves the fact that Fukui function is not necessarily a reliable indicator in localising the most reactive molecular site. The reaction of 1O2 with the furan moiety of DBF substrate via TS1 would initially result in the formation of highly unstable endoperoxide P1 which later would collapse via TS2 to form dione P2. This transformation is energetically the least likely to occur, leaving benzene aromatic ring as the most favourable site for electrophilic interactions.

aromatic rings of DBD and DBF and corroborate the leading role of [2+2]- and [4+2]-cycloaddition of O2 1Δg to DBD and DBF structures, respectively. The branching ratios for bimolecular reaction channels are also computed based on the Arrhenius parameters for ki(T) itemised in Table 1. We plotted the ki/Σ ki values in Fig. 5, attesting to the leading role of channels TS1 and TS6 in the reaction networks of DBD and DBF with singlet oxygen, respectively. The role of TS4 in both of the aromatic hydrocycles’ reaction pathways become more momentous as the system temperature rises, while the leading reaction pathways (TS1 for DBD and TS6 for DBF) are inversely affected by temperature rise.

3.4. Kinetic considerations

This section focuses on the effect of substitution of chlorine (for hydrogen atoms) on the reactivity of DBD/DBF aromatic moieties with singlet oxygen. Our calculation indicates that the susceptibility of benzylic ring towards singlet oxygen electrophilic attack remains mostly intact once hydrogen atoms are replaced by chlorine at reaction sites. As listed in Tables 2 and 3, the chlorine substitution on DielsAlder reactive sites has approximately no effect on altering the activation energy of singlet oxidation of PCDD/PCDF compounds, and both pre-exponential factor (A) and activation energies (Ea) stay consistent in reference to unsubstituted parent compounds. It should be noted that due to the great instability of diradical intermediates produced during singlet oxidation of both DBD and DBF, chlorine substitution inhibits optimisation and convergence to equilibrium through repeated interactions and thus, [2+2]-cycloaddition is neglected from analysis of chlorination effects on title reactions. Due to the trivial enthalpic requirements of the entrance reaction channels, relatively small at around 100 kJ/mol, the molecular interactions of the selected hydrocarbon with singlet oxygen should be termed as “spontaneous reactions”, contrasting those initiated at higher temperatures by the ground state molecular oxygen that typically proceed via very sizable entrance barriers in the window of 200–350 kJ/mol.

3.5. Analysis of chlorination effects

To explore the reaction kinetic models and rate constants for the interaction of singlet oxygen with DBD/DBF, the ChemRate code (Mokrushin et al., 2006) afforded the estimation of Arrhenius parameters. Table 1 summarises the reaction rate coefficient for the bimolecular reactions corresponding to the interactions of dibenzodioxin and dibenzofuran with singlet oxygen. The pre-exponential factors (A) and activation energies (Ea) are fitted to the first order Arrhenius equation Ea

RT at high-pressure limit in the temperature range of of k (T ) = Ae 300–600 K. As reported in Table 1, the rate constant values (k) at 298.15 K enable a comparison of singlet oxygen cycloaddition to the

Table 1 Kinetic parameters of DBD and DBF interactions with O2 1Δg, and reaction rate constants at 298.15 K and 1 atm. Reaction

A (cm3 molecule−1 s−1)

Ea (kJ/ mol)

k (cm3 molecule−1 s−1)

DBD + O2 1Δg → P1 DBD + O2 1Δg → P4 DBD + O2 1Δg → P5 DBD + O2 1Δg → P7 DBF + O2 1Δg → P1 DBF + O2 1Δg → P3 DBF + O2 1Δg → P4 DBF + O2 1Δg → P6 DBF + O2 1Δg → P7

5.01 × 10−14 4.42 × 10−14 4.95 × 10−13 8.87 × 10−14 1.73 × 10−13 9.79 × 10−14 8.52 × 10−13 2.16 × 10−13 1.22 × 10−13

98 104 129 132 153 133 126 119 128

3.39 × 10−31 2.66 × 10−32 1.24 × 10−35 6.63 × 10−37 2.70 × 10−40 4.89 × 10−37 7.16 × 10−35 3.06 × 10−34 4.58 × 10−36

4. Conclusion This study has elucidated the thermodynamic and kinetic feasibility of singlet oxygen-assisted oxidation of polycyclic and halogenated aromatic hydrocarbons. In cases of DBD, the non-concerted 1,25

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Fig. 5. Plot of branching ratios (%) of dibenzo-p-dioxin + O2 1Δg reaction (left) and dibenzofuran + O2 1Δg reaction (right) at different temperatures (K). Table 2 Effect of chlorination in changing Arrhenius parameters of dioxin titled reactions. Dioxins + O2 1Δg

Products

A (cm3 molecule−1 s−1)

Ea (kJ/ mol)

4.42 × 10−14

104

3.02 × 10−14

106

3.26 × 10−14

105

8.87 × 10−14

−14

4.34 × 10

−14

4.41 × 10

Table 3 Effect of chlorination in changing Arrhenius parameters of furan titled reactions. Furans + O2 1Δg

Products

A (cm3 molecule−1 s−1)

Ea (kJ/ mol)

1.22 × 10−13

128

7.14 × 10−14

132

6.94 × 10−14

130

2.16 × 10−13

119

1.33 × 10−13

121

1.44 × 10−13

121

9.79 × 10−14

133

6.94 × 10−14

137

8.60 × 10−14

138

132

128

131

cycloaddition of O2 1Δg displays favourable energy barrier yielding dioxetane product via diradical intermediate channels. However, singlet oxygen binds with the benzylic ring of DBF diagonally via a concerted [4+2]-cycloaddition mechanism forming an endoperoxide product. The enthalpic requirements of the initial reactions in both cases are relatively small at around 100 kJ/mol, as compared to those overcame in high-temperature incineration by ground-state molecular oxygen. Moreover, the supplementary Diels-Alder reaction corridor appears inferior as the so-called ene reaction is impractical due to the absence of allylic hydrogen in the selected aromatic systems. Eventually, the chlorine substitution at reactive sites is shown to impose negligible effect on the reactivity of parent compound towards singlet oxygen. Overall, the data provides single-step kinetic information which would enable to present a reaction formalism to report real-time profiles of species as a function of operational conditions in singlet oxygen-led reactions. 6

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Conflicts of interest

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The authors declare no conflict of interest. Acknowledgement This study has been supported by grants of computing time from the National Computational Infrastructure (NCI) and from the Pawsey Supercomputing Centre, Australia as well as funds from the Australian Research Council (ARC). N.Z. thanks Murdoch University for the award of postgraduate scholarships. References Al-Nu'airat, J., Dlugogorski, B.Z., Oluwoye, I., Gao, X., Altarawneh, M., 2019a. Effect of Fe2O3 nanoparticles on combustion of coal surrogate (Anisole): enhanced ignition and formation of persistent free radicals. Proc. Combust. Inst. 37 (3), 3091–3099. Al-Nu'airat, J., Dlugogorski, B.Z., Gao, X., Zeinali, N., Skut, J., Westmoreland, P.R., Oluwoye, I., Altarawneh, M., 2019b. Reaction of phenol with singlet oxygen. Phys. Chem. Chem. Phys. 21 (1), 171–183. Al-Nu’airat, J., Altarawneh, M., Gao, X., Westmoreland, P.R., Dlugogorski, B.Z., 2017. Reaction of aniline with singlet oxygen (O2 1Δg). J. Phys. Chem. A 121 (17), 3199–3206. Al-Nu'airat, J., Altarawneh, M., Oluwoye, I., Gao, X., Dlugogorski, B.Z., 2018. Role of singlet oxygen in combustion initiation of aromatic fuels. Energy Fuels 32 (12), 12851–12860. Altarawneh, M., Dlugogorski, B.Z., Kennedy, E.M., Mackie, J.C., 2006. Quantum chemical study of low temperature oxidation mechanism of dibenzofuran. J. Phys. Chem. A 110 (50), 13560–13567. Altarawneh, M., Dlugogorski, B.Z., Kennedy, E.M., Mackie, J.C., 2009. Mechanisms for formation, chlorination, dechlorination and destruction of polychlorinated dibenzop-dioxins and dibenzofurans (PCDD/Fs). Prog. Energy Combust. Sci. 35 (3), 245–274. Behnisch, P.A., Hosoe, K., Shiozaki, K., Ozaki, H., Nakamura, K., Sakai, S.-I., 2002. Lowtemperature thermal decomposition of dioxin-like compounds in fly ash: combination of chemical analysis with in vitro bioassays (EROD and DR-CALUX). Environ. Sci. Technol. 36 (23), 5211–5217. Bobrowski, M., Liwo, A., Ołdziej, S., Jeziorek, D., Ossowski, T., 2000. CAS MCSCF/CAS MCQDPT2 Study of the mechanism of singlet oxygen addition to 1,3-butadiene and benzene. J. Am. Chem. Soc. 122 (34), 8112–8119. Cieplik, M.K., Epema, O.J., Louw, R., 2002. Thermal hydrogenolysis of dibenzo‐p‐dioxin and dibenzofuran. Eur. J. Org. Chem. (16), 2792–2799 2002. Delley, B., 2000. From molecules to solids with the DMol3 approach. J. Chem. Phys. 113 (18), 7756–7764. DeRosa, M.C., Crutchley, R.J., 2002. Photosensitized singlet oxygen and its applications. Coord. Chem. Rev. 233, 351–371. Dewar, M.J., 1984. Multibond reactions cannot normally be synchronous. J. Am. Chem. Soc. 106 (1), 209–219. Fonseca Guerra, C., Handgraaf, J.W., Baerends, E.J., Bickelhaupt, F.M., 2004. Voronoi deformation density (VDD) charges: assessment of the Mulliken, Bader, Hirshfeld, Weinhold, and VDD methods for charge analysis. J. Comput. Chem. 25 (2), 189–210. Frisch, M., Trucks, G., Schlegel, H., Scuseria, G., Robb, M., Cheeseman, J., Scalmani, G., Barone, V., Mennucci, B., Petersson, G., 2009. GAUSSIAN 09 Revision A. 2, 2009. Gaussian Inc., Wallingford, CT. Fukui, K., 1982. Role of frontier orbitals in chemical reactions. Science 218 (4574), 747–754. Hagenmaier, H., Kraft, M., Brunner, H., Haag, R., 1987. Catalytic effects of fly ash from waste incineration facilities on the formation and decomposition of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans. Environ. Sci. Technol. 21 (11), 1080–1084.

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