Thermal Recycling of Brominated Flame Retardants with Fe2

Article pubs.acs.org/JPCA

Thermal Recycling of Brominated Flame Retardants with Fe2O3 Mohammednoor Altarawneh,* Oday H. Ahmed, Zhong-Tao Jiang, and Bogdan Z. Dlugogorski School of Engineering & Information Technology, Murdoch University, Murdoch, Western Australia 6150, Australia ABSTRACT: Plastics containing brominated flame retardants (BFRs) constitute the major fraction of nonmetallic content in e-waste. Co-pyrolysis of BFRs with hematite (Fe2O3) represents a viable option for the thermal recycling of BFRs. Consensus of experimental findings confirms the excellent bromine fixation ability of Fe2O3 and the subsequent formation of iron bromides. This contribution provides a comprehensive mechanistic account of the primary reactions between a cluster model of Fe2O3 and major bromine-bearing products from the decomposition of tetrabromobisphenol A (TBBA), the most commonly deployed BFR. We estimate the thermo-kinetic parameters for interactions of Fe2O3 with HBr, brominated alkanes and alkenes, bromobenzene, and bromophenol. Dissociative addition of HBr at a Fe−O bond proceeds through a trivial barrier of 8.2 kcal/mol with fitted parameters in the Arrhenius equation of k(T) = 7.96 × 1011 exp(−6400/RT) s−1. The facile and irreversible nature for HBr addition to Fe2O3 accords with the experimentally reported 90% reduction in HBr emission when Fe2O3 interacts with TBBA pyrolysates. A detailed kinetic analysis indicates that, transformation of Fe2O3 into iron bromides and oxybromides occurs via successive addition of HBr to Fe(Br)−O(H) entities. Elimination of a water molecule proceeds through an intramolecular H transfer. A direct elimination one-step mechanism operates in the dehydrohalogenation of bromoethane into ethene over Fe2O3. Dissociative decomposition and direct elimination channels assume comparable reaction rates in formation of acetylene from vinyl bromide. Results from this study provide an atomic-based insight into a promising thermal recycling route of e-waste. thermal recycling of e-waste.8,9 The coexistence of aromatic brominated precursors with metal oxides induces the catalytic synthesis of PBDD/Fs through prominent intermediate steps.10 Although HCl represents an inactive chlorinating agent for generation of PCDD/Fs,11 conversion of HBr into Br2 opens up a powerful bromination corridor.12 The consensus of opinions in the literature6,13,14 unequivocally illustrates that, introducing hematite (α-Fe2O3) during thermal decomposition of tetrabromobisphenol A (TBBA), the most widely deployed BFR,15 significantly reduces the emission of HBr:Emission of

1. INTRODUCTION Co-pyrolysis of metal oxides with halogen-bearing materials attracts technological interests on two compelling grounds: (i) recycling of bromine and chlorine-containing objects, and (ii) pyro-metallurgical extraction of metals from their oxides. Thermal treatment of the ever increasing electronic and electric waste (e-waste) constitutes a real-world case where the recycling of bromine and extraction of metals overlap.1 The nonmetallic fraction in e-waste bears a significant load of halogenated hydrocarbons, mainly in the form of brominated flame retardants (BFRs).1−5 On the contrary, exposing the metallic constituents in e-waste to oxygen at elevated temperature transforms them readily into metal oxides.3 The interest in studying the co-pyrolysis of BFRs with metal oxides stems from their ability to act as bromine fixation agents,6 the process that ultimately leads to reductive debromination of BFRs. Of particular industrial as well as health importance are ferric oxides that make up most of the ferric fraction in electric arc furnace dust (EAFD). It is estimated that, 4.3−5.7 million tonnes of EAFD arise annually worldwide during crude steel production.7 Thermal degradation of BFRs in the presence of metal oxides achieves a dual benefit, reducing the overall toxicity of the decomposition products of BFRs and forming metal bromides that could be easily leached out. The potential for the conversion of BFRs into hazardous brominated compounds (most notably the notorious polybrominated dibenzo-p-dioxins and furans, PBDD/Fs) often overshadows the environmental and economic benefits of © 2016 American Chemical Society

HBr from pyrolysis of phenolic-resin treated with TBBA contributes about 85% of the initial bromine content in TBBA and occurs in two main temperature intervals, below and above 400 °C. Nearly 50% of HBr is released in the first temperature interval whereas 35% is emitted between 400 and 700 °C.16 When TBBA is co-pyrolyzed with hematite, emitted HBr converts hematite into iron bromides and iron oxybromides.13 This is evident from results of TGA-DSC runs that show the Received: May 15, 2016 Revised: June 27, 2016 Published: July 1, 2016 6039

DOI: 10.1021/acs.jpca.6b04910 J. Phys. Chem. A 2016, 120, 6039−6047

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The Journal of Physical Chemistry A

Figure 1. Optimized structure of the (Fe2O3)n=3 cluster (a) and its charge distribution (b). Distances are in Å. Oxygen and iron atoms are represented by red and slate blue spheres, respectively.

reactions underpinning thermal treatment of e-waste with metal oxides.

weight reduction in the TBBA−Fe2O3 system at temperatures above the main weight reduction stage corresponds to evaporation of iron bromides. The efficiency of Fe2O3 in suppressing the formation of HBr is comparable with that of other metal oxides such as ZnO and CaO.13 Addition of αFe2O3 also decreases the yield of brominated phenols and alkanes from degradation of TBBA. During the course of the reductive debromination of TBBA and suppression of the formation of all Br-containing products, Fe2O3 is partially brominated into FeBr2 and subsequently reduced into metallic iron. Co-pyrolysis of PVC with Fe2O3 exhibits a similar behavior, 17 significant reduction in formation of HCl accompanied by production of FeCl2/FeCl3. Iron halides hold very low boiling points and readily vaporise from the cocombusted stream. The exact mechanism of the interaction of HBr (and HCl) with α-Fe2O3 remains unclear. A global one-step reaction could be expressed as18

2. THEORETICAL METHODOLOGY The DMol3 package21 affords all structural optimizations and energy calculations. The theoretical approach comprises the local density approximation (LDA) as the exchange− correlation potential alongside the Perdew and Wang (PAW) functional.22 The total energy converges with a tolerance of 1 × 10−6 hartree. The electronic core treatment includes all electrons and deploys a double numerical plus polarization (DNP) basis set23 and a global cutoff of 3.6 Å. Final calculated energies are corrected by adding a dispersion correction term based on the methodology developed by Tkatchenko and Scheffler24 to account for any nonbonding interactions in the investigated systems. All energetic values are reported at 0 K. We estimate reaction rate constants on the basis of the conventional transition state theory (TST). In the TST calculations, vibrational frequencies yield activation enthalpies and entropies at the temperature of interest (300−1000 K). Afactors for barrierless reactions are estimated on the basis of the difference in entropies between reactants and products. We report electronic charges for the α-Fe2O3 cluster on the basis of the Hirshfeld25 formalism that defines the relative deformation charge density, as the difference between the molecular and unrelaxed atomic charge densities. Unlike the commonly deployed Milliken charges, Hirshfeld charges are insensitive to the utilized basis set. We report Mayer’s bond orders in the α-Fe2O3 clusters.26 This methodology accounts for the multiplicity of chemical bonds based by the off-diagonal matric elements or the so-called Milliken overlap populations.

6HBr + Fe2O3(s) → 2FeBr3(s,l,g) + 3H 2O(g)

Trekado et al.13 suggested that, reduction of the iron oxide occurs through a carbothermal process facilitated by high concentration of CO and CO2 resulting from degradation of TBBA. Thermodynamic computations by Shibata et al.19 demonstrate a spontaneous nature of bromination of metal oxides by HBr. On the contrary, thermodynamic calculations by Al-Harahsheh et al.17 have disclosed significant endothermicity of chlorination of Fe2O3(s) by HCl. To this end, this contribution reports a first-principle investigation into the reaction involving HBr and major products from the decomposition of TBBA on a model cluster of dehydrated α-Fe2O3. Although molecular water and/or hydroxyl groups cover most metal oxide surfaces at ambient temperatures, dehydroxylated surfaces become the dominant facets as the temperature increases and the relatively weakly bounded OH2/OH species are gradually removed.20 The primary objective herein is 2-fold: first, to construct mechanisms for interaction of brominated hydrocarbons and HBr with α-Fe2O3 and, second, to report thermokinetic parameters of the HBr uptake by ferric oxides and dehydrohalogenation of brominated alkanes and alkenes by α-Fe2O3. Results from this study resolve the molecular-level

3. RESULTS AND DISCUSSION 3.1. α-Fe2O3 Cluster. Nanoparticles of Fe2O3 often find applications as structural, catalytic, magnetic, electronic, and optical materials.27,28 Dehydrated single-phase hematite also represents the end-product from the oxidation and hydrolysis of other forms of iron oxides, such as magnetite (Fe3O4)29 and akaganeite (β-FeOOH).30 The phase transition and dehydroxylation of β-FeOOH into α-Fe2O3 starts at temperatures as low as 500 K.30 Herein, we model the α-Fe2O3 structure by utilizing 6040

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The Journal of Physical Chemistry A a model cluster of stoichiometric neutrally charged α-Fe6O9, i.e., (Fe2O3)n=3 obtained from bulk α-Fe2O3. Figure 1 depicts an optimized structure of the cluster that contains 3-fold coordinated iron atoms and 2-fold coordinated oxygen atoms. In a real combustion environment, the exact shape of the αFe2O3 nanoparticle is dictated by the thermodynamic stability of crystal surfaces, at a given oxygen chemical potential. A recent ab initio atomistic thermodynamic study31 illustrated that the three most stable terminations of hematite αFe2O3(0001) surface contains surface Fe−O bonds (i.e., mixed Fe/O termination), which are present in a considered cluster. As will be demonstrated in the next sections, fission of H−Br or C−Br bonds take place on exposed Fe−O linkages in the cluster. All Fe−O bonds are electronically very similar, as follows from the inspection of Hirshfeld charges and Mayer bond orders listed in Tables 1 and 2, respectively. Calculated Mayer’s

Figure 2. Dissociative addition of HBr at the Fe2O3 cluster. Values are in kcal/mol with respect to the initial reactant. The bromine atom is denoted by a large yellow sphere.

Table 1. Electronic Properties of the (Fe2O3)n=3 Cluster

a

atoma

Hirshfeld charges (e)

atom

Hirshfeld charges (e)

Fe1 Fe2 Fe3 Fe4 Fe5 Fe6 O1 O2

0.316 0.330 0.323 0.330 0.327 0.344 −0.255 −0.229

O3 O4 O5 O6 O7 O8 O9

−0.1998 −0.218 −0.205 −0.222 −0.224 −0.223 −0.223

Refer to numbering in Figure 1a.

Table 2. Mayer Bond Orders for the (Fe2O3)n=3 Cluster Fe1−O1 Fe6−O1 Fe2−O2 Fe6−O2 Fe1−O3 Fe3−O3 Fe6−O3 Fe1−O4 Fe4- O4 Fe4−O9

0.5519 0.6086 0.6089 0.6158 0.3174 0.5065 0.4218 0.5777 0.6188 0.6083

Fe1−O5 Fe5−O5 Fe4−O6 Fe5−O6 Fe2−O7 Fe5−O7 Fe2−O8 Fe3−O8 Fe3−O9

0.5443 0.6255 0.6107 0.6135 0.6192 0.6208 0.5964 0.6273 0.626

bond orders in Table 2 concur with the expected value of 2/3. The deviation of Mayer’s bond orders for Fe1−O1 (0.552) and Fe1−O4 (0.578) from the expected value is due to the emergence of a weak bond between Fe1−O3 (2.090 Å) with Mayer’s bond order of 0.317. Furthermore, interatomic Fe−O and Fe−Fe distances in the Fe2O3 cluster (Figure 1a) reasonably match corresponding distances in bulk α-Fe2O3;32 1.92/2.13 and 2.86 Å, respectively. 3.2. Dissociative Adsorption of HBr. Figures 2 and 3 map out reactions between HBr and the α-Fe2O3 cluster. In the first step, the HBr molecule undergoes a physisorption over a Fe−O bond. The formed M1 adduct resides 22.3 kcal/mol below the separated reactants. The catalytic activity of Fe2O3 is evident when the exothermic reaction energy for the formation of the M1 structure is contrasted with the energy required to break the H−Br bond in the gas phase, i.e., 87.1 kcal/mol.33 The H−Br bond in the M1 structure is elongated by 10.2%, when compared with the equilibrium distance in the gaseous HBr molecule (i.e., 1.61 Å). The strong interaction between

Figure 3. Subsequent reactions of HBr with the α-Fe2O3 cluster. Values are in kcal/mol with respect to reactants in each step.

HBr and the Fe2O3 cluster is also evident from the Mayer bond order of Br−Fe that amounts to 0.166, i.e., a value that reveals the establishment of a chemical interaction rather than pure 6041

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The Journal of Physical Chemistry A Table 3. Arrhenius Coefficients Fitted in the Temperature Range 300−1000 K species HBr

bromobenzene 2-bromopropane

bromoethane

bromoethene

1-bromo-1-propene

A (s−1 or cm3 molecule−1 s−1)

reaction (Fe2O3)n=3 + HBr → M1 M1 → M2 M2 + HBr → M3 M3 → M5 M3 → M4 M4 → M3 M4 → M6 M9 → M10 M14 → M16 M14 → M15 + CH3CHCH2 direct elimination M14 → M15 + CH3CHCH2 dissociative adsorption M11 → M12 M11 → M13 + C2H4 direct elimination M11 → M13 + C2H4 dissociative adsorption M17 → M19 M17 → M18 + C2H4 direct elimination M17 → M18 + C2H2 dissociative adsorption M20 → M22 M20 → M21 + CH3CCH direct elimination M17 → M21 + CH3CCH dissociative adsorption

5.67 7.96 7.51 2.16 2.64 1.10 6.50 7.91 5.50 9.81

× × × × × × × × × ×

10−12 1011 10−12 1011 1011 1011 1011 1011 1011 1010

Ea (kcal/mol) 0 6.4 0 21.6 8.9 28.0 35.6 10.2 22.9

2.9 × 1011 7.91 × 1011 1.31 × 1011

9.1

6.50 × 1010

13.5

3.23 × 1011 4.80 × 1010

3.2 39.6

1.31 × 1011

40.1

8.86 × 1011 4.20 × 1011

4.8 51.6

7.20 × 1010

38.4

such a process is kinetically less favorable than HBr dissociation over Fe−O bond, viz., 21.9 kcal/mol (TS3) versus 8.2 kcal/mol (TS2). Nonetheless, if all Fe−O bonds convert into Fe(Br)− O(H), at high concentrations of HBr, reaction M3 → M5 becomes very plausible. Table 3 lists fitted Arrhenius parameters for reactions in Figures 2 and 3. Figure 4 provides the Arrhenius plots for these reactions between 300 and 1000 K. Kinetic parameters in Table

physisorption. In the next step, the adsorbed HBr decomposes over Fe−O bond to yield the M2 structure in a considerably exothermic reaction. The dissociative adsorption of HBr occurs via a trivial reaction barrier of only 8.2 kcal/mol characterized by the transition state TS1. The facile and irreversible nature for the uptake of HBr, as Figure 2 portrays, is in line with the experimentally reported ∼90% reduction13 in HBr formation from TBBA in the presence of Fe2O3. As illustrated in Figure 3, subsequent HBr addition to the M2 structure forms the M3 physisorbed adduct that undergoes further rearrangements leading to M4 and M5, in two separate channels. The M4 structure forms via TS2 upon dissociative decomposition of the adsorbed HBr on another Fe−O bond than that associated with the first added HBr. TS2 displays the same barrier height as TS1 (i.e., 8.2 kcal/mol). Formation of the M4 structure is exothermic by 25.0 kcal/mol and accompanied by emergence of the second hydroxyl group. In a subsequent step, an intramolecular hydrogen transfer between the two OH groups produces a water molecule attached to an iron atom. This step demands a sizable activation barrier of 41.2 kcal/mol (TS4) and is marginally endothermic by 2.1 kcal/mol (M6). The Fe−O bond of the ferryl Fe in M6 amounts to 2.0 Å, slightly shorter than the analogous bond formed upon water adsorption on a α-Fe2O3(0001) surface (i.e., 2.17 Å).34 Elimination of water in the barrierless reaction M6 → M7 + H2O marks the formation of the experimentally observed iron oxybromides. Clearly, the subsequent HBr addition and water elimination steps lead to the formation of ferric bromides, the main iron species that arises in the co-pyrolysis of Fe2O3 with TBBA. In the alternate pathway, water elimination is initiated via decomposition of HBr over a Fe(Br)-O(H) bond. However,

Figure 4. Arrhenius plots for reactions involving HBr and the Fe2O3 cluster. 6042

DOI: 10.1021/acs.jpca.6b04910 J. Phys. Chem. A 2016, 120, 6039−6047

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The Journal of Physical Chemistry A 2 indicate that, the dissociative adsorption of HBr proceeds very fast in agreement with the reported fast conversion rate of the bromine content in TBBA into iron bromide.13 For instance, at the onset temperature for the decomposition of TBBA (i.e., 550 K), a rate constant of 7.0 × 106 s−1 accounts for the uptake of HBr by Fe2O3. Two reactions compete for the fate of the M4 structure, the reverse formation of HBr (M4 → M3) and the intramolecular H transfer (M4 → M6). Figure 5

Dehydrohalogenation of alkyl halogens is a common method for the industrial production of olefins. This process is often mediated by strong acid sites on metals or metal oxides, most notably alumina39 and Pt-based catalysts.40 The β-hydride elimination constitutes the rate-determining step in which the acid Lewis site activates not only the C−halogen bond but also C−H bonds.41 In this section, we report thermokinetic parameters for Fe2O3-mediated dehydrohalogenation of selected alkanes and alkenes. This scenario may operate in conversion of bromine-containing hydrocarbons during thermal recycling of the nonmetallic fraction in e-waste into useful products.

Figure 5. Branching ratios for the two competing channels M4 → M3/M6.

plots the branching ratios for these channels. The reverse formation of HBr predominates at all considered temperature with a rather negligible contribution from reaction leading to water production (i.e., 3.0 to 12.0% between 700 and 1000 K). This finding implies that, the formation of O−FeBr2 occurs following a subsequent addition of HBr to Fe(Br)−O(H) bonds. Overall, mechanistic pathways in Figures 2 and 3 and rate coefficients in Table 3 show that Fe2O3 reacts with HBr at temperature as low as the evaporative decomposition temperature of BFRs (i.e., 500−600 K),15 i.e., when HBr starts to form. 3.3. Decomposition of Brominated Alkanes and Alkenes. In addition to HBr, thermal decomposition of TBBA produces several brominated compounds ranging from small alkanes and alkenes to polyaromatic hydrocarbons.35−37 Herein, we investigate interaction of four bromine-bearing compounds with the Fe2O3 cluster, namely, bromobenzene, bromoethane, 2-bromopropane, and 1-bromo-1-propene. We selected these compounds because of their distinct C−Br bond dissociation enthalpies (BDHs): aromatic (83.5 kcal/mol in bromobenzene), ethyl (70.7 kcal/mol in bromoethane), secondary (71.5 kcal/mol in 2-bromopropane), and vinylic (80.2 kcal/mol in 1-bromo-1-propene and 79.4 in vinyl bromide).38 In dehydrated Fe2O3 structures, a Lewis-acid/base considerations dictate reactions between the cluster and a brominated hydrocarbon. In principle, two channels operate in the interaction of the brominated hydrocarbons with the Fe2O3 cluster: dissociative addition and direct elimination. Following the dissociative addition, cleavage of the β C−H bond affords an alkene. In the case of H3C−CH2Br, these two routes can be presented as

Figure 6. Reaction of bromoethane with the α-Fe2O3 cluster. Values are in kcal/mol in reference to the physisorbed bromoethane (M11). In Figures 6−9, double/triple C−C bonds are denoted by thicker line segments.

Figures 6−9 illustrate the potential energy surfaces for these two channels for bromoethane, bromoethene, 2-bromopropane, and 1-bromo-1-propene, respectively. Calculated reaction barriers for the direct elimination step correlate with the gasphase BDHs for the target compounds. For instance, formation of ethene and propene via TS6A (Figure 6) and TS7A (Figure 8) require modest activation energies of 13.6 and 23.0 kcal/ mol, respectively. Nearly 10.0 kcal/mol higher BDHs in C−Br bonds in bromoethene and 1-bromo-1-propene translate into significantly higher reaction barriers for the direct elimination pathway at 40.0 kcal/mol (TS in Figure 7) and 53.1 kcal/mol (TS1 in Figure 9). In a recent study,42 we obtained a sizable barrier of 53.1 kcal/mol for the HBr elimination from bromoethene in the gas phase in a reaction that is endothermic by 16.5 kcal/mol. The catalytic effect of Fe2O3 in mediating the 6043

DOI: 10.1021/acs.jpca.6b04910 J. Phys. Chem. A 2016, 120, 6039−6047

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Figure 7. Reaction of bromoethene (M17) with the α-Fe2O3 cluster. Values are in kcal/mol with respect to physisorbed bromoethene (M17).

the very narrow range 35.0−37.0 kcal/mol.38 This indicates that, the significant variation in the activation energies pertinent to the β-H elimination step stems from the difference in the O−C bond strength in the intermediates M12, M16, M19, and M22. Inspection of Figures 6−9 suggests that the reactions of brominated alkanes and alkenes take place preferentially via the largely irreversible dissociative adsorption rather than through direct elimination. However, by considering reaction barriers in reference to the initial physisorbed states, the overall barriers for the dissociative adsorption channels (16.7 kcal/mol for bromoethane, 42.7 for bromoethene, and 44.7 kcal/mol for 1bromo-1-propene) match analogous barriers for the direct elimination channel (13.6 kcal/mol for bromoethane, 40.0 for bromoethene, and 53.1 kcal/mol for 1-bromo-1-propene). The rate-determining step in the dissociative adsorption channel corresponds to the β-H elimination step. An exception is the 2bromopropane, in which the overall barrier for the dissociative adsorption channel resides 33.3 kcal/mol below the separated reactants (TS7B in Figure 8). Table 3 assembles the Arrhenius parameters for all channels in Figures 6−9. Pre-exponential A factors for all barrierless reactions are estimated on the basis of ΔS(T) values between products and reactants. On the basis of the kinetic parameters in Table 3, the direct elimination channel represents the sole corridor for the formation of ethene from dehydrohalogenation of bromoethane over Fe2O3. On the contrary, the dissociative addition channel predominates the direct elimination mechanism in 2-bromopropane and 1bromo-1-propene. The two operating routes assume competing importance in formation of acetylene from vinyl bromide. As iron can switch oxidation states, it will be insightful to assess the kinetic feasibility of iron oxides to act as catalysts in hydrogenation reactions of olefins, i.e., similarly to the wellestablished functionality of transitional metal oxides43 and rare earth metal oxides.44 3.4. Decomposition of Brominated Benzene and Phenol. Brominated aromatic rings constitute major structural blocks in nearly all BFRs, including the emerging BFRs or the so-called novel brominated flame reactants (NBFRs).45 Thus, it is of importance to investigate interaction of bromobenzene with Fe2O3. In this regard, iron oxide nanoparticles were shown to facilitate effectively the sequential debromination reactions of brominated diphenyl ethers (PBDEs).46 In Figure 10a, we illustrate that, the reaction of bromobenzene with Fe2O3

Figure 8. Reaction of 2-bromopropane with the α-Fe2O3 cluster. Values are in kcal/mol in reference to the physisorbed 2bromopropane (M14).

dehydrohalogenation of bromoethane becomes evident when the barriers of TS6A (13.6 kcal/mol in Figure 6) are compared with the analogous barrier in a gas-phase process. On the basis of the energies reported in Figures 6−9, the dissociative addition channel either requires a very low barrier (bromoethene and 1-bromopropene) or procceds without encountering a barrier (bromoethane and 2-bromopropane). Products from the dissociative addition channel are significantly lower in energy than their parent reactants. As shown in Figures 6−9, elimination of the β-H atom and the subsequent migration of the H atom to the O site afford the same products as for the direct elimination step, i.e., corresponding alkenes and alkynes. The activation barriers for the β-H elimination step significantly increase from only 20.0 kcal/mol (TS7B) in the case of the adsorbed i-propyl radical (M16 in Figure 8) to 89.8 kcal/mol (TS11B) in the case of the adsorbed 1-propynyl radical (M22 in Figure 9). The values of BDHs for the β C−H bonds in the adsorbed CxHy radicals (ethyl in M12, i-propyl in M16, ethynl in M19 and 1-propynyl in M22) fall in 6044

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Figure 9. Reaction of 1-bromo-1-propene with the α-Fe2O3 cluster. Values are in kcal/mol in reference to the physisorbed 1-bromo-1-propene (M20).

Figure 10. Reaction of bromobenzene (a) and 2-bromophenol (b) with the α-Fe2O3 cluster. Values are in kcal/mol in reference to the initial reactants.

characterized by fissions of its hydroxyl O−H (M23 → M24) and C−Br bonds (M23 → M25). Formation of an adsorbed bromophenolate via rupture of the hydroxyl bond signifies an elementary step in the well-documented heterogeneous formation of halogenated dioxins catalyzed by metal oxides. Over Fe2O3, this process necessitates 33.1 kcal/mol of activation energy and the formed bromophenolate is in a 25.1 kcal/mol well depth in reference to M23. In an alternative pathway, fission of the C−Br bond along the reaction M23 → M25 entails only 8.6 kcal/mol activation energy and forms 1hydroxyphenyl. Adsorbed bromophenolate and 1-hydroxyphenyl can, in principle, act as building blocks for the formation of PBDD/Fs via the so-called LH and RH mechanisms.11 Thus, it follows that, although Fe2O3 acts to capture the bromine load from thermal decomposition of BFRs, it can also serve as a

proceeds solely via the dissociative adsorption channel through activation energy of 11.3 kcal/mol (TS5). The adsorbed phenyl in the M9 structure forms bridges with two adjacent iron atoms. The very low barrier for the decomposition of bromobenzene indicates that, TBBA may react with Fe2O3 prior to its decomposition or evaporation from the condensed phase. In an agreement with our findings, monitoring the weight loss of a TBBA−Fe2O3 mixture reveals that, the main weight loss episode reflects the evaporation of iron bromide.13 Finally, we investigate the interaction between a 2bromophenol molecule and the Fe2O3 clusters. Bromophenols represent major products from the decomposition of TBBA (and other BFRs), as TBBA contains two 2,6-dibromophenol moieties. Figure 10b demonstrates that, the physisorbed 2bromophenol (M23) undergoes two decomposition pathways 6045

DOI: 10.1021/acs.jpca.6b04910 J. Phys. Chem. A 2016, 120, 6039−6047

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The Journal of Physical Chemistry A

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potent catalyst for the formation of the notorious PBDD/Fs. It is recommended to carry out co-pyrolysis of BFRs with Fe2O3 at temperatures higher than the typical temperatures windows for the formation of PBDD/Fs, i.e., in excess of 1000 K. Undoubtedly, the plausible parallel role of Fe2O3, in mediating formation of PBDD/Fs during thermal recycling of e-waste, warrants further investigation.

4. CONCLUSIONS By utilizing a cluster model of Fe2O3, we have mapped out the initial reactions between hematite (predominant oxide in electric arc furnace dust) and HBr, bromoethane, vinyl bromide, 2-bromopropane, 1-bromo-1-propene, bromobenzene, and 2-bromophenol. The calculated kinetic parameters concur with the experimentally reported ∼90% conversion rate of the bromine content in TBBA into iron bromide. We have assessed the kinetic feasibility of dehydrohalogenation of selected brominated alkanes and alkenes, finding very low or no activation energy barriers for the dissociative adsorption channel. Nonetheless, sizable energy barriers for the β-H elimination step in the dissociative addition corridor render the direct elimination channel to be competitive with the dehydrohalogenation route for some of the considered hydrocarbons. Fe2O3 mediates rupture of O−H and C−Br bonds in the 2-bromophenol molecule, an important step in catalytically assisted condensation of bromophenols into PBDD/Fs.

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AUTHOR INFORMATION

Corresponding Author

*M. Altarawneh. E-mail: [email protected]. Tel: (+61) 893607507. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study has been supported by the National Computational Infrastructure (NCI), Australia, and the Pawsey Supercomputing Centre in Perth, as well as funds from the Australian Research Council (ARC). O.A thanks the Iraqi government for the award of a postgraduate scholarship.

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REFERENCES

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DOI: 10.1021/acs.jpca.6b04910 J. Phys. Chem. A 2016, 120, 6039−6047

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DOI: 10.1021/acs.jpca.6b04910 J. Phys. Chem. A 2016, 120, 6039−6047