Reaction Mechanism for m-Xylene Oxidation in the Claus Process by

Article pubs.acs.org/JPCA

Reaction Mechanism for m-Xylene Oxidation in the Claus Process by Sulfur Dioxide Sourab Sinha,† Abhijeet Raj,*,† Ahmed S. Al Shoaibi,† and Suk Ho Chung‡ †

Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, UAE Clean Combustion Research Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia

‡

Downloaded by UNIV OF MANITOBA on September 11, 2015 | http://pubs.acs.org Publication Date (Web): September 10, 2015 | doi: 10.1021/acs.jpca.5b06020

S Supporting Information *

ABSTRACT: In the Claus process, the presence of aromatic contaminants such benzene, toluene, and xylenes (BTX), in the H2S feed stream has a detrimental effect on catalytic reactors, where BTX form soot particles and clog and deactivate the catalysts. Among BTX, xylenes are proven to be most damaging contaminant for catalysts. BTX oxidation in the Claus furnace, before they enter catalyst beds, provides a solution to this problem. A reaction kinetics study on m-xylene oxidation by SO2, an oxidant present in Claus furnace, is presented. The density functional theory is used to study the formation of m-xylene radicals (3-methylbenzyl, 2,6-dimethylphenyl, 2,4-dimethylphenyl, and 3,5-dimethylphenyl) through H-abstraction and their oxidation by SO2. The mechanism begins with SO2 addition on the radicals through an O-atom rather than the S-atom with the release of 180.0−183.1 kJ/mol of reaction energies. This exothermic reaction involves energy barriers in the range 3.9− 5.2 kJ/mol for several m-xylene radicals. Thereafter, O−S bond scission takes place to release SO, and the O-atom remaining on aromatics leads to CO formation. Among four m-xylene radicals, the resonantly stabilized 3-methylbenzyl exhibited the lowest SO2 addition and SO elimination rates. The reaction rate constants are provided to facilitate Claus process simulations to find conditions suitable for BTX oxidation.

1. INTRODUCTION The Claus process is widely adopted in oil and gas industries to recover sulfur from H2S gas streams.1−3 This process, involving a system known as sulfur recovery units (SRUs),4,5 primarily consists of two sections: (a) Claus furnace, where noncatalytic partial oxidation of H2S occurs in air at 1000−1200 °C through the reactions, H2S + 1.5O2 → SO2 + H2O and 2H2S + SO2 → 3S + 2H2O, and (b) catalytic units, where the uncombusted H2S and SO2 present in the exhaust gas from the furnace (after heat recovery) undergo the reaction, 2H2S + SO2 → 3S + 2H2O, in the presence of alumina or titania catalysts to form sulfur.6,7 The catalytic units at different temperatures bring this sulfur-recovery reaction close to equilibrium, and an overall recovery of nearly 90−95% of condensed sulfur from H2S is possible in Claus process.8 In practice, the H2S feed gas entering the Claus furnace is never pure and contains various contaminants from the raw field gases such as benzene, toluene, xylenes (collectively known as BTX), small hydrocarbons, NH3, CO2, N2, CS2, and COS.9 The destruction of these contaminants in a Claus furnace is essential to ensure high efficiency of SRUs, prevent catalyst deactivation, and reduce the formation of toxic and corrosive compounds such as H2SO4 and CO that can affect downstream equipment.10,11 Though some contaminants such as NH3, small hydrocarbons, CS2, and COS may get destroyed in the furnace at high temperatures, BTX are able to survive in the high temperature environment of the Claus furnace, as observed in a study by Klint.12 These compounds are able to © XXXX American Chemical Society

reach the catalyst beds, form carbon−sulfur compounds (similar to soot particles), and clog and deactivate the catalyst that lead to an increase in the pressure drop through the reactor beds and production of off-spec black sulfur.10,13 Crevier et al.14 observed that the deactivation potential of the three aromatic hydrocarbons (BTX) differs from each other. Toluene and xylenes result in rapid deactivation of Claus catalysts within hours of operation, with xylenes leading to fastest reduction in catalyst activity among BTX. The temperature inside the furnace is sufficient for BTX oxidation (1050 °C),15 but due to their low concentration, oxidation of H2S by O2 is preferred, and they are able to survive. Several solutions have been proposed to date to remove or destroy BTX through oxidation before they reach the catalytic units. The increase in the temperature in the Claus furnace may help in oxidizing BTX,10,12,14 which can be achieved by the following methods: (a) enriching air with oxygen, (b) cofiring natural gas with feed, and (c) preheating the feed gas. Method a does not raise the furnace temperature high enough for complete BTX oxidation when the H2S concentration in the feed is low. Moreover, enriching air with oxygen is an expensive process. Method b requires burning natural gas along with acid gases in a furnace of high volume for BTX destruction and thus, has a high capital cost. Besides, in the case of incomplete Received: June 23, 2015 Revised: September 2, 2015

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DOI: 10.1021/acs.jpca.5b06020 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

hydrocarbons, DFT is considered to be a standard choice, as it is computationally less expensive than higher level theories and has already been validated and used for aromatic hydrocarbons in previous studies.35−45 The molecular structures were optimized with different spin multiplicities to identify the multiplicity with a minimum energy, reasonable geometry, and low spin contamination. All the calculations were performed using the Gaussian 09 software.46 Spin contamination in open shell systems can significantly affect the energy, geometry, and calculated spin density of the molecules.47 The expectation value, ⟨S2⟩, of the spin operator Ŝ2 gives a measure of the amount of spin contamination introduced by a given level of theory. By observing ⟨S2⟩ for a number of stable species and transition states, it is shown in refs 36 and 48 and in this work that DFT introduces a negligible amount of spin contamination. The values of ⟨S2⟩ for all the chemical species and transition states involved in the proposed processes are presented in the Supporting Information. The rate constants for the reactions involved in the proposed processes were evaluated using the transition state theory. The partition functions for the transition states and reactants were calculated at a range of temperatures (3003000 K) using the vibrational frequencies, moments of inertia, mass and electronic multiplicity, all of which are given by the quantum calculations. Further details are provided in ref 25. A linear least-squares fitting algorithm was used to fit the modified Arrhenius expression (k = ATne(−E/RT)) to the data points of the rate constants to obtain the kinetic parameters of the frequency factor A, the temperature exponent n, and the activation energy E. To assess the role of quantum tunnelling, in the literature, there are four efficient methods to evaluate the tunneling correction factor (or, transmission constant): Wigner correction, Eckart correction, zero-curvature tunneling correction, and small curvature tunneling correction. At temperatures above 500 K, the correction factors from all the methods converge to similar values.49,50 In this paper, the Wigner method was employed to obtain the tunneling correction factors for all the elementary reactions.25

combustion of natural gas, aromatic hydrocarbon concentration may get enhanced due to their formation from small hydrocarbons in the high temperature anoxic region in Claus furnace.16 Method c is able to raise the furnace temperature because the adiabatic flame temperature increases with feed preheating, but it is not high enough to completely destroy BTX, especially when the feed is lean in H2S. The drawbacks of these methods are the requirement of huge capital costs and/or high energy consumption but are still not suitable for complete removal of BTX. Some physical solution such as BTX adsorption on activated carbon by passing the feed gas through adsorption beds have also been suggested, but in the presence of moisture in the feed stream, the adsorption process is not very effective.17 All the BTX removal techniques have concentrated on feed pretreatment or on the destruction of BTX in Claus furnace. However, the presence of BTX does not have a detrimental effect on the efficiency of the Claus furnace. The effect is primarily seen in the catalytic section. Thus, a BTX destruction unit between the Claus furnace and the catalytic units may solve the problem, where BTX can be oxidized under suitable process conditions. Because SO2 is present in the exhaust gas from furnace in high concentration, an effective solution to BTX removal could be their oxidation by SO2.18 There is experimental evidence that the oxidation of aromatic hydrocarbons can take place by SO2.18−24 However, no studies exist on BTX destruction by SO2 at high temperatures. To assess the capability of SO2 to oxidize BTX in the Claus process and to find the process conditions suitable for it, a reaction mechanism for the BTX−SO2 interaction is required to carry out simulations at different temperatures, pressures, and feed compositions. In this direction, the mechanisms for the oxidation of benzene and toluene have been developed in previous studies.25,26 The Claus feed gas contains three xylene isomers (o-, m-, and p-xylenes). The meta- and ortho-isomers have similar reactivities, but both of them are more reactive than the para-isomer.27 In ref 28, it was shown that m-xylene exhibits the longest ignition delay time among the three isomers and, hence, has the high possibility of surviving in the furnace conditions. The oxidation of m-xylene by O2 and OH has been recently studied in refs 29−31, where several reaction pathways involved in its oxidation have been reported. However, the detailed reaction mechanism for its oxidation by SO2 has not been examined so far. Our present investigation focuses on the development of a reaction mechanism for the oxidation by SO2 of m-xylene radicals. Different pathways for the abstraction of H-atom from the m-xylene to its radicals are studied. Thereafter, the reactions through which m-xylene radicals and SO2 interact with each other to form small chemical species such as CO, SO, OH, and CHO, as detected in experimental studies,32−34 have been identified. The energetics and molecular parameters for various species involved in their reactions have been determined through the density functional theory (DFT) calculations. The rates of the elementary reactions have also been evaluated using the transition state theory.

3. RESULTS AND DISCUSSION The oxidation of m-xylene can be initiated by H-abstraction either from the methyl group or from the phenyl ring by H-, OH-, or SH-radicals that are present in considerable amounts in a high temperature Claus furnace to form four m-xylyl radicals (as shown in Figure 1): 3-methylbenzyl (M1), 2,6-dimethylphenyl (M2), 2,4-dimethylphenyl (M3), and 3,5-dimethylphenyl (M4). The potential energy diagrams for H-abstraction from mxylene by H-, OH-, and SH-radicals are presented in Figures S1, S2, and S3, respectively. For the H-abstraction from the methyl group by H-atom (i.e., m-xylene + H → M1 + H2 in Figure S1),

2. COMPUTATIONAL DETAILS The ground state molecular structures of the stable chemical species as well as the transition states were determined using DFT with Becke three-parameter exchange and the Lee, Yang, and Parr correlation (B3LYP) functional with the 6-311+ +G(d,p) basis set. For large molecules such as aromatic

Figure 1. Radials of m-xylene formed from H-abstraction reactions. M1: 3-methylbenzyl. M2: 2,6-dimethylphenyl. M3: 2,4-dimethylphenyl. M4: 3,5-dimethylphenyl. B


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The Journal of Physical Chemistry A an activation energy of 13.4 kJ/mol is required, and the reaction is exothermic. However, the H-abstraction reactions from the phenyl ring of m-xylene to form M2, M3, and M4 were endothermic with activation energies of 49.9, 50.8, and 49.3 kJ/ mol, respectively. In a previous study on toluene,26 the activation energy for the H-abstraction from the methyl group (i.e., toluene + H → benzyl + H2) was found to be 13.8 kJ/mol, whereas the activation energies for the formation of o-, m-, and p-methylphenyl radicals through H-abstraction from the phenyl ring were 50.2, 50.1, and 51.6 kJ/mol, respectively. In another study on benzene,25 the activation energy for the reaction benzene + H → phenyl + H2 was calculated as 50.9 kJ/mol. Clearly, the energies involved in Habstraction from m-xylene match very well with analogous reactions on toluene and benzene. A clear difference in the activation energies of H-abstraction from the methyl group and from the phenyl ring can be seen, and the reasons for this difference are explained in refs 26 and 51. Figure S2 presents the potential energy diagram of Habstraction from m-xylene by OH-radical to form M1−M4. All four H-abstraction reactions by OH-radical were exothermic. For M1 formation, the reaction was found to be barrierless with B3LYP/6-311++G(d,p), but with M062X/6-311++G(d,p), a small barrier of 4.8 kJ/mol was found. For such a reaction on toluene, a similar barrier height of 5.6 kJ/mol was found.26 The activation energies required for H-abstraction from the aromatic ring of m-xylene by OH fall in the range 7.8−9.3 kJ/mol, which is marginally lower than the activation energies of 9.4−11.3 kJ/mol for H-abstraction from the aromatic ring of toluene26 and the activation energy of 11.4 kJ/mol for Habstraction from benzene.25 In ref 31, Zhao et al. performed an experimental study on m-xylene oxidation by OH using a fast flow reactor coupled to ion drift-chemical ionization mass spectrometry (ID-CIMS) and observed the oxidation to begin with H-abstraction by OH, but the information on reaction energetics was not provided. Figure S3 presents the potential energy diagram for Habstraction from m-xylene by HS-radicals. Except for 3methylbenzyl (M1) formation, H-abstraction from the rest of the sites was found to be highly endothermic in nature. The transition states for these reactions could not be found. The reaction energies for the formation of M2−M4 were in the range 99.1−102.3 kJ/mol. The high energies involved for these reactions suggest that H-abstraction by HS to form dimethylphenyl radicals is less likely than H-abstraction by Hand OH-radicals. After the formation of m-xylene radicals (M1, M2, M3, and M4), SO2 addition on them takes place to initiate oxidation. Their oxidation reaction mechanisms are described in the forthcoming sections. To facilitate the description of the mechanisms, two terms have been used. (a) Pathways: They describe the elementary reactions involved in the oxidation of methylbenzyl and dimethylphenyl radicals by SO2. (b) Routes: They describe the alternate reaction channels for the decomposition of intermediate species in a pathway. The chemical species involved in the mechanism will be denoted by “CS”, and the transition states by “TS”. Some of the important functional groups that were observed on the intermediate species during oxidation are presented in Figure 2. Interestingly, these functional groups are also observed during the oxidation by O2 of m-xylene.29 The reaction leading to the loss of one or two methyl groups from m-xylene would form

Figure 2. Some oxygenated functional groups formed during the oxidation by SO2 of 3-methylbenzyl and dimethylphenyl radicals.

toluene or benzene radicals, and their oxidation reactions have been studied before.25,26 3.1. Pathway 1:3-Methylbenzyl (M1)−SO2 Reactions. Figure S4 presents the reactions involved in the oxidation of M1 by SO2. The reaction between them begins with the attack of more electronegative O-atom of SO2 to the free radical sites of M1. In our previous studies on the oxidations of benzene25 and toluene26 by SO2, it was found that, after the addition of SO2 on the radical site, the most likely subsequent reaction is the elimination of SO molecule through the breakage of one of the S−O bonds. The O-atom remaining on the aromatic ring is then able to oxidize the molecule. The mechanisms for the oxidation of m-xylene radicals by SO2 were developed by keeping this information in mind. Figure 3 presents the potential energy diagram for pathway 1 with the energies of the chemical species and transition states

Figure 3. Pathway 1: Potential energy diagram at 0 K showing the formation of toluene (CS4) and 3-methylbenzaldehyde (CS5) as end products during 3-methylbenzyl (M1) oxidation by SO2.

relative to the total energy of the reactants (M1 and SO2). The addition of SO2 on M1 to form CS1 is exothermic with a small activation energy of 3.9 kJ/mol and a reaction energy of 78.5 kJ/mol. For SO2 addition on benzyl,26 a slightly higher activation energy of 5.4 kJ/mol and a lower reaction energy of 76.4 kJ/mol were found. No experimental study on the oxidation of M1 by SO2 could be found for comparison. However, in refs 52 and 53, for the oxidation of small hydrocarbons (methane and ethane) by SO2, reaction energetics were found. The activation energies for SO2 addition on methyl (CH3•)52 and ethyl (C2H5•)53 radicals were found to be 6.3 and 12.9 kJ/mol, respectively, which are higher than that for M1 + SO2 → CS1. The fact that the energy barriers for SO2 addition on methyl and substituted methyl (methylbenzyl, benzyl, ethyl) radicals are present in a narrow range of 3.9−12.9 kJ/mol is noteworthy.25,26,52,53 From CS1, the loss of SO results in the formation of CS2 (3methylbenzyloxy). This barrierless and endothermic reaction involves a reaction energy of 188.6 kJ/mol. In ref 26, for the loss of SO from an adduct formed from SO2 addition on benzyl, a slightly higher reaction energy of 192.7 kJ/mol was found. The species CS2 can then undergo decomposition through various routes, as shown in Figures 3−6, and discussed below as Routes a−e. C


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Figure 4. Pathway 1: Potential energy diagram at 0 K showing the formation of 1-methyl-5-methylenecyclopenta-1,3-diene (CS11) and 2-methyl-5methylenecyclopenta-1,3-diene (CS16) as end products during CS2 decomposition.

Figure 5. Pathway 1: Potential energy diagram at 0 K showing the formation of 1-methyl-5-methylenecyclopenta-1,3-diene (CS11) as end product during CS2 decomposition.

Route a. Represented by solid lines in Figure 3, this route involves H-migration in CS2 from the CH2-group to the ipsoposition to form CS3. This step requires overcoming an energy barrier of 90.8 kJ/mol and forms a species that is marginally more stable than CS2. The species CS3 can undergo CHOgroup desorption to form CS4 (toluene) as one of the end products that lies 9.6 kJ/mol above CS3. The reactions of toluene have already been studied in ref 26. Route b. Represented by dotted lines in Figure 3, this route involves the elimination of H-atom from CS2 to form 3methylbenzaldehyde (CS5) as a byproduct. This H-desorption reaction requires overcoming an activation energy of 100.6 kJ/ mol, and involves a reaction energy of 83.9 kJ/mol. Route c. Figure 4 presents the potential energy diagram of Route c, which involves an attack of the dangling O-atom at the ipso-position in CS2 to form an epoxide ring in CS6 after crossing an energy barrier of 60.4 kJ/mol. The species, CS6 can then follow two similar subroutes, one involving CS7 formation (represented by solid lines) and the other involving CS12 formation (represented by dotted lines). For CS7 formation from CS6, the O-atom forms a bond with the ortho-carbon atom between the epoxide ring and the methyl group, which requires overcoming an energy barrier of 219.6

kJ/mol. The C−C bond connecting the ipso- and ortho-carbon breaks to form highly stable species, CS8 with a sevenmembered ring. Thereafter, a C−O bond in CS8 breaks to form CS9, which then undergoes molecular rearrangement to form a five-membered ring in CS10 after crossing an energy barrier of 94.4 kJ/mol. The elimination of the CHO-group from CS10 leads to the formation of 1-methyl-5-methylenecyclopenta-1,3-diene, CS11, as one of the end products. For CS12 formation from CS6, the O-atom of the epoxide ring forms a C−O bond with the sixth carbon of the ring after overcoming an energy barrier 218.5 kJ/mol. A C−C bond of the epoxide ring of CS12 breaks to form a seven-membered ring with an ether group in CS13. Thereafter, a C−O bond in CS13 breaks to form CS14, which then undergoes molecular rearrangement to form a fvie-membered ring in CS15. The release of the CHO-group from CS15 forms 2-methyl-5methylenecyclopenta-1,5-diene, CS16. Though the end products in the two subroutes for CS6 were different, the reaction energetics and molecular structures in both the subroutes were nearly identical. Route d. Figure 5 presents the potential energy diagram of Route d, where the dangling O-atom in CS2 bonds with the ortho-carbon atom between methylene and methyl group. The D


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Figure 6. Pathway 1: Potential energy diagram at 0 K showing the formation of 2-methyl-5-methylenecyclopenta-1,3-diene (CS16) as end product during CS2 decomposition.

Figure 7. Pathway 2. Potential energy diagram at 0 K showing the formation of 1,2-dimethylcyclopent-2,4-dienyl radical (CS35), 6-methyl-2Hbenzo[b]oxete (CS36), and 2,4-dimethylcyclopenta-1,2,3-triene (CS42) as end products during 2,6-dimethylphenyl (M2) oxidation by SO2.

Route e. Figure 6 presents the potential energy diagram of this route, where the dangling O-atom of CS2, after crossing an energy barrier of 138.9 kJ/mol, attacks the ortho-carbon on the left to form CS23. A C−O bond then breaks to form CS24 after overcoming an energy barrier of 88.4 kJ/mol. The species CS24 can then follow three subroutes: CS24 → CS25 (represented by solid lines), CS24 → CS12 (represented by dotted lines), and CS24 → CS15 (represented by dashed lines). The end products in this route are CS12, CS15, and CS16, which are discussed in previous routes. The reactions and their energetics presented in this route are very similar to those in Route d. Out of all the routes presented here for CS2, Routes a and b involve lower energy barriers than the other routes and appear to be the likely channels for CS2 decomposition. 3.2. Comparison of Benzyl−SO2 and M1−SO2 Pathways. As discussed above, the most probable routes for M1 oxidation by SO2 are Routes a and b that lead to the formation of CS4 and CS5 as end products, while requiring CS2 as an important intermediate. In ref 26, during benzyl oxidation by SO2, the main products were benzene and benzaldehyde, and their formation was mediated by benzyloxy radical. For benzene formation, H-migration from methylene group to the ipso-position in benzyloxy was required, which involved an energy barrier of 85.9 kJ/mol. Thereafter, the desorption of

breakage of a C−O bond forms CS18 after overcoming an energy barrier of 90.4 kJ/mol. For the further decomposition of CS18, three subroutes are shown in Figure 5: CS18 → CS19 (represented by solid lines), CS18 → CS7 (represented by dotted lines), and CS18 → CS10 (represented by dashed lines). The conversion of CS18 to CS19 with a six-membered and a three-membered epoxide ring requires a very small activation energy of 0.1 kJ/mol placing CS19 57.2 kJ/mol below CS18. Thereafter, the C−C bond of the epoxide ring breaks to form a seven-membered ring after overcoming an energy barrier of 59.3 kJ/mol. The breakage of a C−O bond leads to the formation of CS21, which undergoes molecular rearrangement to form a five-membered ring CS22 after crossing an energy barrier of 15.1 kJ/mol. The elimination of CHO-group from CS22 forms CS11 (1-methyl-5-methylenecyclopenta-1,3-diene) as one of the end products. For CS7 formation from CS18, the ipso-attack of the dangling O-atom of CS18 leads to the formation of CS7 after crossing an energy barrier of 36.7 kJ/mol. The further reactions of CS7 have already been discussed above. The species, CS18 can also undergo molecular rearrangement after crossing an energy barrier of 84.6 kJ/mol to form a five-membered ring in CS10. The further reactions of CS10 has also been discussed above. E


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Route c. Represented by dotted lines, this route involves addition of an O-radical on the C-atom of one of the methyl groups in CS30 with simultaneous release of H-atom from the methyl group to form a stable molecule, CS36 (6-methyl-2Hbenzo[b]oxete). This endothermic reaction requires overcoming an energy barrier of 401.9 kJ/mol. Route d. Represented by short dashed and dashed lines, this route for CS30 involves the formation of a bridged structure in CS37 through the attack of O-atom on the carbon located at the ortho-position on the other side. The high endothermicity of this reaction makes this route less likely than Routes a, b, and c. The decomposition of CS37 can take place through two subroutes: CS37 → CS38 (represented by thin solid lines) and CS37 → CS43 (represented by dashed lines). To form CS38 from CS37, the bridged O-atom attacks the carbon at ipso-position to form an epoxide ring after overcoming an energy barrier of 74.7 kJ/mol. The C−C bond of the epoxide ring then breaks to form a sevenmembered ring with an ether group in CS39. Thereafter, a ringopening reaction takes place in CS39 to form CS40, which requires overcoming an activation energy of 33.8 kJ/mol. The species CS40 undergoes molecular rearrangement to form a five-membered ring in CS41 and an aldehyde group. The elimination of a CHO-group from CS41 forms CS42 (2,4dimethylcyclopenta-1,2,3-triene) as an end product. To form CS43 from CS37, the bridged O-atom attacks the C-atom at the meta-position (with respect to the methyl groups) to form an epoxide ring, CS43, lying 144 kJ/mol below CS37. The C−C bond of the epoxide ring connecting the ortho- and meta-carbons breaks to form a seven-membered ring in CS44 after crossing an energy barrier of 29.6 kJ/mol. The species, CS44 then undergoes molecular rearrangement to form a five-membered ring with aldehyde group in CS45 after overcoming an energy barrier of 263.3 kJ/mol. This is followed by CHO-elimination to form CS42 (2,4-dimethylcyclopenta1,2,3-triene). By comparison of the overall activation energies involved in the three routes, it is clear that Route b provides the most probable reaction channel for CS30 decomposition in Pathway 2. 3.4. Pathway 3:2,4-Dimethylphenyl (M3)−SO2 Reactions. Figure S6 presents the reaction mechanism for Pathway 3 that involves M3 oxidation by SO2. The potential energy diagram for this pathway is provided in Figures 8 and 9. For SO2 addition on M3 to form CS46, a reaction energy of 183.1 kJ/mol is released and requires a very small activation energy of 4.5 kJ/mol. The elimination of the SO-group from CS46 leads to the formation of CS47, which lies 81.3 kJ/mol above CS46. For the further reactions of CS47, five routes were found, as discussed below as Routes a−e: (a) CS47 → CS48, (b) CS47 → CS51, (c) CS47 → CS53, (d) CS47 → CS57, and (e) CS47 → CS61. Route a. Represented by solid lines in Figure 8, this route involves migration of a H-atom from the nearby methyl group to the O-radical in CS47 to form a hydroxyl group in CS48. For this migration reaction, an activation energy of 159.3 kJ/mol is required to be overcome. The methylene radical in CS48 then attacks the C-atom attached to the hydroxyl group to form a six- and a three-membered bicyclic structure in CS49. A reaction energy of 236.2 kJ/mol is involved in this endothermic reaction. The backward reaction for CS48 involving Hmigration back to the methylene group involves a smaller activation energy than the forward reaction, which makes the

CHO took place after overcoming an energy barrier of 55.5 kJ/ mol, and placed benzene 111.5 kJ/mol above the reacting species on the potential energy surface. For a similar Hmigration reaction in CS2 in Route a, the required activation energy was higher by 4.9 kJ/mol as compared to that for benzyl. For CHO-desorption, a similar activation energy of 55.6 kJ/mol was involved that placed the end product, CS4, 113.4 kJ/mol above the reacting species. For benzaldehyde formation from benzyloxy through the desorption of H-atom from CH2O-group, an energy barrier of 95.4 kJ/mol was required to be overcome.26 The products were found to be 194.9 kJ/mol above the reacting species on the potential energy surface. For a similar reaction in CS2 (i.e., Route b) that led to CS5 formation, the energy barrier required for H-desorption was higher by 5.2 kJ/mol, and on the potential energy surface, the products were 194.0 kJ/mol above the reacting species. On the basis of comparison of the energies for the two important routes for benzyl and M1 oxidation by SO2, it can be inferred that, though the reaction energies were similar, the Hmigration and H-desorption reactions during M1 oxidation required slightly higher activation energies than those during benzyl oxidation. This small increase in activation energy could be attributed to the induction (+I) effect of electron-donating methyl group that increased the stability of the CS2-radical and made the C−H bond breakage difficult. 3.3. Pathway 2:2,6-Dimethylphenyl (M2)−SO2 Reactions. Figure S5 presents the reaction mechanism for Pathway 2, for which the potential energy diagram is shown in Figure 7. The addition of SO2 on the radical site of M2 leads to the formation of CS29 with O−S−O chain between two methyl groups and requires overcoming an activation energy of 5.2 kJ/ mol. This addition reaction is highly exothermic with a reaction energy of 180.0 kJ/mol and is comparable to the reaction energies of 177 kJ/mol for phenyl + O2,54 182 kJ/mol for phenyl + SO2,25 and 181.6 kJ/mol for methylphenyl + SO226 addition reactions. The elimination of the SO-group from CS29 takes place to form CS30 with an O-radical between two methyl groups. The decomposition of CS30 can occur via four routes, discussed below as Routes a−d in Figure 7. Route a. Represented by thick solid lines in Figure 7, this route involves the migration of H-atom from the nearby methyl group to the O-radical to form CS31 and requires an activation energy of 159.1 kJ/mol to be overcome. Thereafter, the methylene group attacks the ortho-carbon (carrying the OHgroup) of the phenyl ring to form a bicyclic structure in CS32. The desorption of OH-group from CS32 leads to 2-methyl-1Hcyclopropabenzene (CS33) formation as the end product. Route b. Represented by solid lines, this route involves molecular rearrangement of CS30 to form a five-membered ring with a carbonyl group in CS34. This reaction involved an activation energy of 228.8 kJ/mol. A similar reaction on the methylphenoxy radical in ref 26 required an activation energy of 224.7 kJ/mol. From CS34, after overcoming a small energy barrier of 15.2 kJ/mol, CS35 (1,2-dimethylcyclopent-2,4dienyl) is formed. Note that, in this pathway, from CS30, no intermediate species could be found with fused threemembered and five-membered rings (as observed during phenyl and benzyl oxidation in refs 25 and 26) possibly due to the presence of two methyl groups on adjacent C-atoms in the resulting structure that destabilize such an intermediate species. F


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mol. The breakage of C−O bond in CS58 requires an activation energy of 74.1 kJ/mol to form CS59, which subsequently releases the CHO-group to form CS60 (1,4dimethylcyclopenta-1,2,4-triene) as the end product. Route e. Represented by dashed lines, this route involves the attack of the dangling O-atom in CS47 to the nearby methyl group with simultaneous release of the H-atom to give 4methyl-2H-benzo[b]oxete, CS61, as one of the end product. On the basis of the reaction energetics involved in Routes a− e for the decomposition of CS47, Route c involved lowest energy barriers and, thus, appears to be the most probable route for it. 3.5. Pathway 4:3,5-Dimethylphenyl (M4)−SO2 Reactions. Figure S7 presents the reaction mechanism for M4 oxidation by SO2, and the potential energy diagram for the reaction involved in it is shown in Figure 10. For SO2 addition

Figure 8. Pathway 3: Potential energy diagram at 0 K showing the formation of 3-methyl-1H-cyclopropabenzene (CS50) and 3,5dimethylcyclohexa-2,4-diene-1-yne (CS52) as end products during 2,4-dimethylphenyl (M3) oxidation by SO2.

Figure 10. Pathway 4: Potential energy diagram at 0 K showing the formation of 1,5-dimethylcyclohexa-1,2,3,4-tetraene (CS65) and 1,4dimethylcyclopenta-1,2,3-triene (CS69) as end products during 3,5dimethylphenyl (M4) oxidation by SO2. (a) H-abstraction from mxylene by H-atom to form methylbenzyl (M1) radical and H2. Experimental data are taken from ref 56. (b) H-abstraction from mxylene by OH to form methylbenzyl (M1) radical and H2O. Experimental data are taken from ref 57.

Figure 9. Pathway 3: Potential energy diagram at 0 K showing the formation of CS56, CS60, and CS61 as end products during CS44 decomposition.

on the free radical site of M4, a small energy barrier of 5.2 kJ/ mol is required to be overcome to form CS62 that lies 182.8 kJ/mol below the reacting species. The elimination of the SOgroup from CS62 forms a symmetrical species with the Oradical, CS63. Similar to other pathways, the decomposition of CS63 can occur through several routes, as discussed below as Routes a−d: (a) CS63 → CS54, (b) CS63 → CS64, (c) CS63 → CS66, and (d) CS63 → CS70. Route a. This route, represented by thick solid lines in Figure 10, involves molecular rearrangement of CS63 to form a five-membered ring with carbonyl group in CS54 after overcoming an energy barrier of 209.6 kJ/mol. The further reactions of CS54 have already been shown in Figure 9. Route b. This route, represented by thin solid lines in Figure 10, involves H-transfer from the nearby carbon to the O-atom in CS63 to form CS64 with the hydroxyl group. This reaction requires an activation energy of 278.4 kJ/mol to be overcome. The loss of hydroxyl group leads to the formation of CS65 (1,5dimethylcyclohexa-1,2,3,4-tetraene) lying 348.9 kJ/mol above CS64. Route c. This route, represented by dotted lines, involves the attack of the O-atom on the nearby C-atom in CS63 to form a seven-membered ring with an ether group in CS66 that lies

forward reaction less likely to occur. The release of the OHgroup from CS49 can form CS50 (3-methyl-1H-cyclopropabenzene) as an end product. Route b. Shown by dotted lines in Figure 8, this route involves the migration of a H-atom in CS47 from the phenyl ring to the O-radical after overcoming an energy barrier of 283.0 kJ/mol to form CS51 with hydroxyl group. The elimination of the hydroxyl group leads to the formation of CS52 (3,5-dimethylcyclohexa-2,4-diene-1-yne) as the end product. Route c. Shown by solid lines in Figure 9, this route involves the conversion of a six-membered ring in CS47 into a bicyclic ring structure with a carbonyl group in CS53 after overcoming an energy barrier of 228.6 kJ/mol. The species CS53 is unstable, and its conversions to CS54 and CS55 are observed to be barrierless. From these species, CO is released to form CS56. Route d. Represented by dotted lines in Figure 9, this route involves the formation of a seven-membered ring in CS57 through the attack of the O-radical to the nearby C-atom in CS47. The molecular rearrangement of CS57 forms a bicyclic structure in CS58 after crossing an energy barrier of 201.9 kJ/ G


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influence the oxidation process. Though the methyl group supported the release of SO molecule, it enhanced the activation energy involved in the release of CO by stabilizing the species with oxy group. 3.7. Reaction Rate Constants. Table S1 in the Supporting Information provides the high pressure limit rate constants for the elementary reactions involved in the H-abstraction from mxylene, and in the oxidation of M1−M4 by SO2, as discussed in the previous section. For some barrierless reactions (CS1 → CS2 + SO, CS29 → CS30 + SO, CS46 → CS47 + SO, and CS62 → CS63 + SO), the variational transition state theory was used to calculate their rate constants by determining their potential energy surfaces through partial geometry optimizations at different O−S bond lengths. The method is described in our previous works.25,26 The pressure in the Claus furnace is about 2−3 atm,16 and the high pressure limits for the aromatic reactions provide a good estimate of their rate constants. All the rate constants obtained in this work were lower than the upper limit of collisional frequency, which is about 1013 s−1.55 Figures 11 and 12 provide comparisons of the experimentally observed rate constants for some reactions with their calculated

283.9 kJ/mol above CS63. The seven-membered ring can rearrange itself to form a fused five- and four-membered ring structure in CS67 after overcoming an energy barrier of 208.5 kJ/mol. A C−O bond can then break apart to form CS68. From CS68, a CHO-group can desorb to form CS69 (1,4dimethylcyclopenta-1,2,3-triene) as the end product. Route d. This route, represented by a dashed line, involves CS70 formation from CS63, but the high energy barrier required to be overcome to form the bridged ether group makes this route unfavorable. The further reactions of CS70 have been shown in Figure S8 in the Supporting Information. Out of the four routes studied here for CS63, Route a, involved the lowest barriers for reactions and, thus, is the most likely route for its decomposition. 3.6. Comparison of M2, M3, M4, and Phenyl Oxidation Pathways. Sections 3.3−3.5 presented different routes for M2−M4 oxidation by SO2. On the basis of the reaction energetics, for all of them, the route involving CO loss was found to be the most favorable one. In this section, the energies involved in this route during M2, M3, and M4 oxidation are compared to understand the influence of the position of the methyl groups on phenyl ring on the oxidation reactions. During the oxidation by SO2 of phenyl25 and omethylphenyl26 radicals as well, a similar route was found to dominate over others. Therefore, the energies associated with M2−M4 oxidation reactions are also compared to those involved in the oxidation of o-methylphenyl and phenyl radicals. For the addition of SO2 on M2, M3, M4, o-methylphenyl, and phenyl, the energy barriers and reaction energies lay within narrow ranges were 78.7, 81.3, 93.5, 87.7, and 97 kJ/mol, respectively. This difference is a result of the presence of one or two methyl groups adjacent to the oxy group in CS30, CS47, and o-methylphenoxy that stabilized them through the +I effect and reduced the reaction energy. In CS63, the methyl groups were far from the oxy group, whereas they were absent in phenoxy radical. The +I effect can also be seen in the activation energies of 228.8, 228.6, 209.6, 224.7, and 220 kJ/mol required for the conversion of six-membered rings in CS30, CS47, CS63, o-methylphenoxy, and phenoxy to five-membered ring intermediates. The species CS63 and phenoxy required lower energy barriers for ring rearrangement due to their lower stability as compared to others. The loss of CO from the fivemembered ring intermediates led to formation of the end products, CS35 (from M2), CS56 (from M3 and M4), omethylcyclopentadienyl (from o-methylphenyl), and cyclopentadienyl (from phenyl) radicals. For the overall endothermic reactions, M2 + SO2 → CS35 + CO + SO, M3 + SO2 → CS56 + CO + SO, M4 + SO2 → CS56 + CO + SO, omethylphenyl + SO2 → o-methylcyclopentadienyl + CO + SO, and phenyl + SO2 → cyclopentadienyl + CO + SO, the reaction energies of 25.6, 15.6, 16.6, 29.2, and 41 kJ/mol, respectively, were found. Among M2, M3, and M4, M2 showed higher endothermicity for the overall reaction. This is possibly a result of the presence of two methyl groups on adjacent C-atoms in its end-product, CS35, which made the molecule unstable due to steric effects. However, M2, M3, and M4 showed lower reaction energies than o-methylphenyl and phenyl due to the +I effect from two methyl groups in CS35 and CS56 that made them relatively more stable than o-methylcyclopentadienyl and cyclopentadienyl radicals. From the comparisons shown above, it can be inferred that the presence of one or more methyl groups on phenyl ring can

Figure 11. Comparison of calculated and experimentally observed rate constants for H-abstraction from m-xylene to form m-methylbenzyl (M1) radical by (a) H- and (b) OH-radicals.

values in this work with temperature variation. The experimental data in the literature were available for very few aromatic reactions relevant to this study and over narrow temperature ranges. This restricted the number of validation cases for m-xylene reaction kinetics. In Figure 11, a good agreement between the computed and experimental data for Habstraction from M1 by H and OH could be found, which is known to be an initiation step for oxidation.56,57 Figure 12 presents the rate constants for CO-desorption from phenoxy, methylphenoxy, and dimethylphenoxy (CS30, CS47, and CS63) radicals during the oxidation of benzene, H


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within 1.3 kJ/mol). This is possibly a result of the high stability of resonantly stabilized benzyl and methylbenzyl (M1) radicals, where the free electron is delocalized and is not completely available for reactions. The same trend was seen during the oxidation by O2 of benzyl and o-methylphenyl radicals, where a difference of more than an order of magnitude in their rate constants was observed.51 The low rate constant for M2 compared to those of M3 and M4 could possibly be a result of steric hindrance near the radical site due to the presence of two methyl groups on either side of it that prohibits SO2 (a relatively large molecule) to reach the radical site. The rate constants for the addition of O2 on o-methylbenzyl to form methylbenzylperoxy, obtained from ref 51, is also provided in the figure. The difference between the rate constants for O2 and SO2 addition on M1 diminish with increasing temperature, and beyond 700 K, the two rate constants are within an order of magnitude. It may appear that SO2 addition on M1 may only be competitive at very high temperatures. However, given that SO2 is present in much higher concentration than O2 in the high temperature (1400− 1600 K) zone of Claus furnace, the oxidation of xylenes by SO2 may be possible. This will be validated in the future by carrying out Claus furnace simulations. Figure 14 provides the rate constants for the elimination of SO through the breakage of CO−SO bond in the species CS1,

Figure 12. Comparison of the rate constants for the removal of CO from different molecules (phenoxy,25 methylphenoxy,26 CS30, CS47, and CS63). The experimental profile is from ref 58. The species CS34, CS53, and CS54 are unstable, and they quickly decompose either to release CO barrierlessly or to release CO by crossing very small energy barriers.

toluene, and m-xylene, respectively. This reaction has been highlighted above as a preferred route for m-xylene oxidation. The experimentally observed rate constant for this reaction for phenoxy radical was found in ref 58 and is presented in Figure 12. It matches well with the computed rate constant for phenoxy radical. Though it may not suitably represent the COdesorption rate for methylphenoxy, CS30, CS47, and CS63 due to the presence of one or more methyl groups on them, but it is noteworthy that the rate constants for some reactions are very close to the experimental values. For CS30, CS47, and CS63, the rate constants at temperatures higher than 1000 K differ appreciably (with rates for CS47 and CS63 differing by a factor of 10 or more) due to different locations of methyl groups with respect to carbonyl site that affect the molecular parameters of species and transition states and the rate constants. Figure 13 presents the rate constants for SO2 addition on M1−M4, phenyl, benzyl, and methylphenyl radicals. The rate

Figure 14. Comparison of the rate constants for the breakage of the O−S bond to release SO from different molecules carrying the SO2group (CS1, CS29, CS46, and CS62 from this work, phenyl−SO2 from ref 25, and benzyl−SO2 and o-, m-, and p-methylphenyl−SO2 from ref 26.

CS29, CS46, CS62, phenyl−SO2, benzyl−SO2, o-methylphenyl−SO2, m-methylphenyl−SO2, and p-methylphenyl−SO2 (that were formed from SO2 addition to M1, M2, M3, M4, phenyl, benzyl, and o-, m- and p-methylphenyl radicals, respectively). The elimination of SO after SO2 addition is a preferred route for all the species. This step is similar to the loss of O-radical through the breakage of O−O bond after O2 addition to aromatic species during their oxidation. As shown in refs 25 and 26, the breakage of weak CO−SO bond is kinetically much faster than the breakage of the O−O bond. Although the rate constants for CS1 and benzyl−SO2 were nearly overlapping, they were significantly lower than the rate constants for other species (CS29, CS46, CS62, phenyl−SO2, o-methylphenyl− SO2, m-methylphenyl−SO2, and p-methylphenyl−SO2). This is in line with the higher endothermicity of SO elimination reaction for CS1 and benzyl−SO226 than the other species. For some elementary reactions studied in this work, the rate constants may be pressure-dependent. Thus, the high pressure

Figure 13. Comparison of the rate constants for the addition of SO2 on different radical sites (M1−M4) studied in this work, the phenyl radical studied in ref 25, and benzyl and o-, m-, and p-methylphenyl radicals studied in ref 26. The rate for O2 addition on o-methylbenzyl was obtanied from ref 51.

constants for SO2 addition to M3, M4, phenyl, and o-, m-, and p-methylphenyl radicals were similar to each other, and varied within an order of magnitude. The rates for SO2 addition to M1, M2 and benzyl radicals were lower than those for the other radicals even though the activation energies for SO2 addition on all the radicals were very close to each other (varying only I


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The Journal of Physical Chemistry A limit rate constants obtained in this work should be suitable only for high pressure studies. 3.8. Kinetic Modeling. To identify the major products formed during the oxidation of m-xylyl radicals (M1−M4) by SO2 and their pathways, kinetic modeling study was conducted using LOGEsoft software16,59 by including all the reactions identified in this work. The oxidation process was studied in a closed homogeneous zero-dimensional reactor maintained at constant temperature and pressure, because they do not vary appreciably in a Claus furnace.16 In the feed stream to the Claus process, there are several components such as H2S, CO2, O2, N2, and small and large hydrocarbons (at ppm level),16 but H2S, SO2, H2, S 2, and H2 O with approximate molar concentration of about 5−10% each, and N2 with molar concentration of about 40−60% dominate in the entire furnace. For simulation purpose, a simplified reactor feed containing 0.005% m-xylyl, 10% SO2, and 89.995% N2 was assumed. The simulations were performed at temperatures of 1000 and 2000 K. Typically, the residence time in a Claus furnace is about 1−2 s. The simulations were carried out for 2 s. Figure 15 presents the computed profiles of the reactants and products during the oxidation of M1, M2, M3, and M4. The profiles of SO2 are not shown as it varied negligibly due to very low concentration of xylyls. During M1 oxidation, SO and CHO were found to be the main byproducts, and CS4 and CS5 were the main products. Although CS5 formation only dominated at low residence times (less than 3 ms), CS4 (toluene) was the main product of M1 oxidation by SO2 at higher residence times at both the temperatures that formed through the route: CS2 → CS3 → CS4 + CHO. For M2−M4 oxidation by SO2, similar results were obtained. At all the residence times and temperatures studied in this work, the routes involving SO and CO formation as byproducts dominated. In the case of M2, CS35 was the main product that formed through the route CS30 → CS34 → CS35 + CO. Similarly, during M3 oxidation, CS56 formation dominated, which took place through the route CS47 → CS53 → CS54/ CS55 → CS56 + CO. During M4 oxidation as well, CS56 was the major product formed through the following route CS63 → CS54 → CS56 + CO. As evident, the preferred oxidation routes identified from kinetic modeling in his section are in line with those predicted by comparing reaction energetics of different routes in sections 3.1−3.6. Note that, while carrying out Claus furnace simulations by including m-xylene reactions, it is important to consider the interconversion reactions of M2 and M3 to M1 through H-transfer. Such reactions were studied in ref 60, and they were found to have relatively high rate constants at combustion-relevant temperatures. Thus, their oxidation may compete with the unimolecular ring interconversion reactions. This can alter the relative concentrations of m-xylyl radicals and affect their oxidation time scales. With the detailed reaction mechanism and kinetics for the oxidation of benzene, toluene, and xylenes by SO2, it is possible to determine, through simulations, the optimal process conditions for BTX destruction by SO2 in the Claus furnace. Note that the other gas-phase species present in Claus furnace such as H, OH, H2O, O2, and sulfur may affect BTX−SO2 interactions and should also be investigated.

Figure 15. Computed species profiles during M1−M4 oxidation by SO2 at temperatures of 1000 and 2000 K. (a) Profiles of reactant, M1, and major byproducts (SO and CHO). (b) Profiles of M1 oxidation products (CS4, CS5, CS11, and CS16). (c) Profiles of reactant, M2, and major byproducts (SO and CO). (d) Profiles of M2 oxidation products (CS33, CS35, CS36, and CS42). (e) Profiles of reactant, M3, and major byproducts (SO and CO). (f) Profiles of M3 oxidation products (CS50, CS52, CS56, CS60, and CS61). (g) Profiles of reactant, M4, and major byproducts (SO and CO). (h) Profiles of M4 oxidation products (CS56, CS65, CS69, and CS74).

4. CONCLUSIONS The reaction mechanisms for H-abstraction from m-xylene to form 3-methylbenzyl and dimethylphenyl radicals and for the oxidation of these radicals by SO2 were developed using the B3LYP/6-311++G(d,p) level of theory. The addition of SO2 on these radicals was found to be highly exothermic with a reaction energy of 78.5 kJ/mol for 3-methylbenzyl and between 180.0 J


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and 183.1 kJ/mol for dimethylphenyl radicals. This addition reaction required very small activation energy barriers between 3.9 and 5.2 kJ/mol to be overcome. This indicates that their interaction is energetically favored. The rate constants for SO2 addition reactions suggest that it is more favorable for the dimethylphenyl radicals than for the 3-methylbenzyl radical. Several possible routes for the further decomposition of the adducts formed from SO2 addition reactions were studied, and the preferred routes were highlighted. Upon SO2 addition, the breakage of the CO−SO bond leading to SO elimination takes place. The rate constant for this reaction was faster for dimethylphenyl radicals than for the methylbenzyl radical. This oxidation of aromatics by O2 at high temperatures takes place through similar reactions, where the O−O bond breaks after O2 addition on the radical site to facilitate the progress of their oxidation. Some routes for the formation of commonly observed species in the Claus furnace such as CO, OH, H2, CHO, SO, and H2O were identified. The potential energy diagrams and the kinetic modeling studies indicate that CO, SO, and CHO would be forming in higher concentrations as a result of oxidation than the other species mentioned above because their elimination requires lower overall activation energy than the other products.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b06020. Rate constants of the elementary reactions, the energies, coordinates, vibrational frequencies, moments of inertia, and spin multiplicity for all the chemical species and transition states, energy diagrams for less important reaction pathways, and reaction mechanisms (PDF)

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AUTHOR INFORMATION

Corresponding Author

*A. Raj. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Petroleum Institute Gas Processing and Materials Science Research Centre, UAE, through project ID GRC014. S.H.C. was supported by the King Abdullah University of Science and Technology, Saudi Arabia.

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REFERENCES

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