Oxidation Mechanism of Aliphatic Ethers - American Chemical Society

Article pubs.acs.org/JPCA

Oxidation Mechanism of Aliphatic Ethers: Theoretical Insights on the Main Reaction Channels Stefania Di Tommaso,†,‡ Patricia Rotureau,‡ and Carlo Adamo*,†,§ †

Laboratoire d’Electrochimie, Chimie des Interfaces et Modélisation pour l’Energie, CNRS UMR 7575, Chimie ParisTech, 11, rue Pierre et Marie Curie, F-75231 Paris Cedex 05 ‡ INERIS, Parc Technologique Alata - BP 2 - 60550 Verneuil-en-Halatte § Institut Universitaire de France, 103 Boulevard Saint Michel, F-75005 Paris, France S Supporting Information *

ABSTRACT: This paper presents a quantum chemical study on oxidation process of a series of aliphatic ethers. On the basis of a detailed theoretical work on diethyl ether oxidation, the mechanism has been reduced at three competing reactions: the β-scission of the alkyl radical (RIORII•) issued from the initiation step, the isomerization of the peroxy radical (RIORIIOO•) produced by reaction of the alkyl radical with molecular oxygen, and the hydroperoxide production, a bimolecular reaction between the peroxy radical and an ether molecule that also regenerates a RIORII• radical. Results obtained from DFT calculations, including thermochemistry and rate constant evaluations, have been reported and discussed. The influence of the presence of the oxygen atom in the ether skeleton has been evaluated by making a comparison between some ethers and parent hydrocarbons. In particular, it has been found that oxygen increases the reactivity of vicinal sites by lowering activation barriers and favors the stabilization of radicals. Direct proportionality relationships have been searched between activation and reaction enthalpies of each class of competing reactions, but one has been found only for the isomerization reaction.

1. INTRODUCTION With specific attention at the atmospheric pollution a great attention has been given in last years to the use of oxygenated compounds as solvents and, in particular, as additives in fuels and biofuels.1,2 Such compounds, are, in fact, increasingly recognized as a safe and cheap way to reduce the levels of soot and particulate emissions and to improve combustion.3 Due to their large use, oxygenated chemicals are more and more present in atmosphere.4,5 In particular, ethers, of general formula RIORII, are also counted, due to some structural features (α activated hydrogen atom), among the chemicals that can easily react in the process of autoxidation,6 a radicalic and self-propagating process that could generate a large variety of peroxide species. The presence of these last compounds in ethers, often stored in poor conditions and for long periods, is considered responsible of many laboratory accidents.7−9 Nevertheless, few mechanistic studies on ether oxidation are present in literature, often carried out in very different conditions, like combustion10 and pyrolysis,11 or, more often, atmospheric.12,13 Basically, most of these studies have been carried out within the large field of volatile organic compounds (VOC) chemistry related to atmosphere problems. A notably exception is represented by methyl tert-butyl ether (MTBE) whose oxidation processes have been studied recently reviewed, well underlining the complexity of its oxidation and the need of computational studies in the field.14 © 2012 American Chemical Society

Moreover, the chemistry of the oxidation of saturated branched ethers has some similarities with that well-known of alkanes,15−18 as also show recently by some extensive mechanistic studies on dimethyl ether (DME)10 and diethyl ether (DEE).19 In fact, it has been demonstrated that reactions as β-scission of RIORII• radical or isomerization of peroxy radical10,19 play a central role in ether oxidation as well as in the same process involving hydrocarbons. This large variety of experimental studies originated very different propositions for the reaction mechanism of ethers oxidation, depending on experimental conditions of the study, notably in terms of temperature and pressure. Furthermore, few and vague indications are available on the initiation step.12,20,21 However, it is generally accepted that the process of ethers autoxidation is initiated, at least in atmospheric conditions, by the hydroxyl radical. The most explored scenario for the reaction mechanism is that at high temperature (800−1300 K) combustion or pyrolysis.11,22 At this condition, the process seems to proceed via direct decomposition of molecular species (like methanol and isobutene production for methyl tert-butyl ether decomposition). Different hypotheses have been made for low pressure and temperature oxidation. In a theoretical Received: January 11, 2012 Revised: May 25, 2012 Published: July 16, 2012 9010

dx.doi.org/10.1021/jp300356v | J. Phys. Chem. A 2012, 116, 9010−9019

The Journal of Physical Chemistry A

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Figure 1. Global scheme of the proposed mechanism for aliphatic ethers oxidation.

Table 1. Sketches, Acronyms, and Chemical Formula of Studied Ethers

experimental characterization of low temperature oxidation, is the direct decomposition of the alkyl radical. In this complex context, we have recently carried out a detailed study of the oxidation of DEE, which started from the experimental suggestions, leaded to the characterization of all possible reaction paths.19 In particular, at low temperature,

work on DME oxidation it has been suggested that the peroxy radical CH3OCH2OO•, formed by reaction of CH3OCH2• with molecular oxygen, undergoes intramolecular isomerization to give the hydroperoxide radical •CH2OCH2OOH.10 The alternative pathway, proposed for other ethers, namely tertamyl methyl ether23 from a kinetic study and DEE24 from 9011



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Table 2. Sketches of RIORII• Radicals and of the Products of the Three Main Reaction Paths Considered for the Oxidation of Branched Aliphatic Ethers

three different reaction pathways are in competition, namely βscission of RIORII• (eq 1), isomerization of RIORIIOO• (eq 2), and hydroperoxides production from the reaction of the peroxy radical with a solvent molecule (eq 3): CH3CH 2O(CH3)CH 2• → 2CH3CHO + OH•

CH3CH 2O(CH3)CH 2OO• +CH3CH 2OCH 2CH3 → CH3CH 2O(CH3)CH 2OOH + CH3CH 2O(CH3) CH 2•

(1)

(3)

The general oxidation mechanism is also sketched in Figure 1. Starting from this detailed study on DEE, the reactive behavior of some small aliphatic ethers has been analyzed at the

CH3CH 2O(CH3)CH 2OO• → •CH(CH3)OCH 2CH 2OOH (2) 9012



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Table 3. Activation Enthalpies (ΔH⧧, kcal/mol) and Reaction Energies (ΔH, kcal/mol) for the Three Main Reaction Pathways Considered for the Oxidation of Branched Aliphatic Ethers β-scission primary carbon

secondary carbon

tertiary carbon

RIORII•

RIORIIOO•

ΔH⧧

ΔH

ΔH⧧

ΔH

ΔH⧧

ΔH

DME MtBE TAME MEE MnPE MiBE DEE EnPE EiBE EtBE MiPE EiPE DIPE

−16.0 −21.9 −22.1 −22.6 −22.5 −22.4 −22.7 −22.8 −22.8 −24.0 −23.5 −23.6 −24.2

−46.7 −52.0 −52.2 −54.2 −54.4 −51.0 −54.5 −54.5 −54.5 −55.0 −54.5 −54.7 −53.9

17.5 13.6 13.0 20.2 20.1 20.8 19.2 19.5 19.8 13.3 16.9 16.1 14.6

3.5 5.0 4.4 3.5 3.7 3.2 4.0 4.4 4.1 0.1 1.3 −0.8 −2.0

15.3 21.0 20.8 19.4 19.7 17.3 17.2 17.1 17.2 24.4 19.4 17.2 15.8

12.0 18.5 19.8 16.7 15.0 11.1 14.8 14.9 15.2 19.3 16.3 15.1 13.0

10.2 14.3 14.3 13.2 14.2 12.7 13.2 13.2 13.8 13.9 14.2 14.3 13.8

12.8 11.7 12.3 12.7 12.0 10.7 12.8 12.8 12.8 12.6 12.6 12.7 11.2

constant (h), R is the ideal gas constant, and ΔG‡ is the Gibbs free energy of activation for the reaction. The rate constants for the reactions involving intra- or intermolecular hydrogen transfer, namely, isomerization and hydroperoxides production, have been corrected to take into account proton tunneling effect, by multiplying them for a transmission factor, κ, evaluated as suggested by Wigner:37 κ (T ) = 1 +

2 1 ⎡ h Im(ν ⧧) ⎤ ⎢ ⎥ 24 ⎣ kBT ⎦

(5)

where Im is the imaginary frequency associated with the transition state. More robust approximations are available in the literature (see for instance ref 38), but this one allowed for a qualitative agreement with experiments in the case of DEE oxidation.19

2. COMPUTATIONAL DETAILS All the density functional theory (DFT) calculations have been performed using the hybrid B3LYP functional27 and Gaussian 09 program.28 The 6-31+G(d,p) basis set was used to optimize structures and for subsequent frequency calculations to characterize stationary points as minima or first-order saddle points (transition states). Even if the B3LYP functional tends to underestimate some reaction barriers (as hydrogen abstractions),29−31 this level of theory has been used for investigations on related systems and it has been shown to provide energetics close to more sophisticated and time-consuming correlated wave function methods.32−34 Intrinsic reaction coordinate (IRC) calculations were also performed to verify that the identified products and the reactants were correctly connected.35 For radicals (open shell systems), unrestricted DFT calculations were carried out and the obtained results were checked for wave function instability. All the enthalpy values have been obtained as a sum of electronic and thermal correction to electronic total energies. The rate constants for the key reaction steps have been evaluated using the Eyring equation:36 kBT −ΔG⧧ / RT e h

hydroperoxides production

ethers

density functional theory (DFT) level. This has been possible by a systematic analysis, in terms of activation enthalpies and reaction energies (i.e., stabilization energy of products), of the three main pathways mentioned above (eqs 1−3, figure 1) for the group of 13 branched ethers reported in Table 1. Such a mechanistic generalization allows for the prediction of the effect of the chemical and electronic structure on the products of the main reaction paths and a quantitative estimation of activation energies for the reaction channels in competition. Moreover, a relationship between activation and reaction enthalpies5,25,26 has been determined, which could relieve of time-consuming calculation of all transition states involved in the process for molecules of this class.

k=

isomerization

3. RESULTS In Figure 1 is reported the scheme of the main reaction channels found for DEE oxidation mechanism in our previous work.19 The process, in analogy with the accepted alkanes autoignition mechanism,17 starts with an initiation step, in which a RIORII• radical is produced by the abstraction of a hydrogen atom from neutral ether (RIORII). The produced radical can directly undergo decomposition, via β-scission, or react with molecular oxygen to produce peroxy species (RIORIIOO•). This reaction opens the chain propagating cycle in which RIORIIOO• radicals react with an ether molecule to produce the hydroperoxide RIORIIOOH and another alkyl radical. In the case of DEE oxidation, the alternative fate found for peroxy radicals is the isomerization reaction, in which a hydroperoxide radical (•RIORIIOOH) is produced by intramolecular hydrogen transfer. These three reaction channels have been studied for all the aliphatic ethers reported in Table 2, and the corresponding enthalpies are collected in Table 3. All these species show a similar overall behavior, which is exemplified by the reaction profile of ethyl n-propyl ether (EnPE) reported in Figure 2. Due to this similarity, the oxidation reactions will be discussed step by step in the following. 3.1. Initiation. Oxidation starts with the abstraction of a hydrogen atom from the neutral molecule. In this work, hydroxyl radical has been chosen as initiator of the chain

(4)

where the variables in the pre-exponential factor are the Boltzmann’s constant (kB), temperature (T), and the Planck’s 9013



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the number of substituent on the carbon atom the difference reaches 9.3 kcal/mol (between αI, radical at the secondary carbon and β, radical on a primary center). In Table 3 are collected all the stabilization enthalpy values of RIORII• radicals obtained for the 13 ethers considered in this study. The least stable RIORII• radical is that formed at the methyl group of DME (the simplest existing ether) with a ΔH value of 16 kcal/mol. In fact, the more complex are the functional groups on two different branches of RIORII species, the more stabilized is the correspondent RIORII• radical, due to the possibility of increasing electron delocalization. Indeed, these values depend on the nature of the radical center: the formation of primary radicals requires about 22 kcal/mol, whereas about 23 and 24 kcal/mol are required for secondary and tertiary radicals (Table 3). Nevertheless, in each subgroup, a species that deviates from the typical behavior, due to some structural features of alkyl chains, can be found. This is the case of the DME in the first group (the only not branched ether), EtBE in the second (due to the presence of a tertiary carbon vicinal to oxygen), and DIPE in the third (branched on the two sides of the central oxygen). The effect of the presence of the oxygen atom in ether structure can be evaluated by comparison with the same class of reactions on parent hydrocarbons. Results for butane, pentane, and 2-methylbutane, homologues of MEE, MnPE, and MiPE respectively, are reported in Table 4. The position of the most favored hydrogen abstraction is the same in ether and in hydrocarbons: in the two first cases, the R• radicals in alkanes and the correspondent RIORII• radicals in ethers are formed at a secondary center (in position 2 for butane, 3 for pentane). In the third case, radicals are formed at the methyne group (tertiary carbon). Clearly, the proximity of the oxygen atom makes C−Hα bond weaker in ethers than in hydrocarbons, the difference in enthalpy (ΔΔH) going from a minimum of 2.5 kcal/mol for the couple 2-methylbutane/MiPE to a maximum of 3.5 kcal/mol for the pentane/MnPE one. At the same time, oxygen has an influence on the energy of reaction of the correspondent radicals (e.g., the formation of the MEE radical needs 22.6 kcal/mol whereas 19.2 for the butane radical). This is due to the fact that, when C−Hα of the ether is cleaved the formed α-alkoxyalkyl radical is stabilized by the interaction of the unpaired electron with p-electrons of the oxygen atom.15 3.2. β-Scission. RIORII• radicals formed in the initiation step can directly undergo decomposition by breaking the chemical bond in β position with respect to the radical center. This β-scission reaction gives, in each case, the production of a molecule containing a carbonyl center (an aldehyde, RICHO,

Figure 2. Relative enthalpies of the three main pathways of oxidation process: the example of EnPE. Enthalpies are relative to the attack of the hydroxyl radical on the ether.

process, as suggested in some previous studies simulating ether liquid phase oxidation,19 combustion,10 and ether oxidation in atmospheric conditions.12 All possible hydrogen abstractions have been considered for each studied compound. A sketch of the most stable RIORII• radicals, found for all ethers by DFT calculation, is reported in Table 2. As expected,12,19,39 the most favored hydrogen abstraction is the one at carbon atoms in the vicinal (α) position to the oxygen. For asymmetric ethers, where two different α-hydrogen abstractions are possible, the most stable radical is, of course, the one at the most substituted carbon atom. Indeed, it is well-known that a radical at a tertiary carbon is more stable than one at a secondary center and so on. In Figure 3 is reported the example of methyl isopropyl ether (MiPE). In the case of this asymmetric ether, three different possibilities of hydrogen abstraction can be found, two for the different vicinal hydrogens (labeled α, αI) and one for the methyl hydrogens (labeled β). As discussed above, differences between the three radicals are due to two different effects: the nature of the carbon and the oxygen vicinity. In particular, the most stable MiPE radical is the αI one, issued from hydrogen abstraction at the secondary carbon vicinal to oxygen. Then, less stable by about 2 kcal/mol, we find the radical at terminal methyl group next to oxygen atom dubbed α. At last, the radical at the methyl group in the β position, directly linked only to the secondary carbon (Figure 3). The change of position of the methyl group with respect to the oxygen atom produces a difference in stabilization enthalpy of about 7 kcal/mol (between α and β radicals); if we also add the difference in

Figure 3. View of MiPE. The equivalent hydrogens are also shown with correspondent product stabilizations of the reactions: R + •OH → R• + H2O. 9014



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Table 4. Comparison between Some Ethers and Homolog Hydrocarbons in Terms of Reaction Energy (ΔH, kcal/mol) of the Initiation Step and Activation Enthalpies (ΔH⧧, kcal/mol) for the Three Explored Pathways

Table 5. Activation Free Gibbs Energies (ΔG⧧, kcal/mol) and Rate Constants for the Three Main Paths of Oxidation Process β-scission ΔG DME MtBE TAME MEE MnPE MiBE DEE EnPE EiBE EtBE MiPE EiPE DIPE

⧧

16.2 13.6 11.6 18.9 19.0 19.2 17.7 17.9 18.6 11.8 15.1 13.8 12.4

isomerization −1

kdec (s ) 8.27 6.66 1.95 8.67 7.32 5.23 6.57 4.69 1.44 1.39 5.29 4.75 5.05

× × × × × × × × × × × ×

102 104 10−2 10−2 10−2 10−1 10−1 10−1 104 101 102 103

ΔG

⧧

18.7 23.5 23.1 21.5 21.7 19.1 19.3 19.2 19.2 26.6 21.1 18.8 17.0

hydroperoxides production −1

kiso (s ) 4.48 1.32 2.60 3.91 2.80 2.25 1.58 1.86 1.87 6.86 7.53 3.61 7.22

or a ketone, RICOR) and an alkyl radical, the break of the radical, in fact, taking place always at a C−O bond. Moreover, these two products are characterized by different complexities of structure, depending on the initial structures of the parent radicals, as shown in Table 2. The β-scission of the radicals formed at a primary carbon produces formaldehyde (CH2O) in all three cases. The nature of alkyl radicals formed then determines the difference in activation energies of the process for different ethers (Table 3). Indeed, the more stable is the produced alkyl radical, the lower is the energetic barrier. In this subgroup of decomposition reactions, a methyl radical is produced for DME with an activation barrier of 17.5 kcal/mol and tertiary radicals are formed in MtBE and TAME β-scission, with activation energies of 13.6 and 13.0 kcal/mol respectively. For the analysis of data relative to β-scission of the radicals at a secondary and at a tertiary center the same discussion presented above for the decomposition of primary radicals is valid. In the case of the secondary radicals, production of

× × × × × × × × × × × ×

−1

10 10−4 10−4 10−3 10−3 10−1 10−1 10−1 10−1 10−7 10−3 10−1

ΔG⧧

kOOH (s−1)

21.0 25.1 25.5 23.6 24.3 23.3 23.7 24.1 24.4 24.7 25.3 25.7 24.6

9.16 9.02 4.59 1.13 3.44 1.91 9.45 4.80 2.89 2.95 6.42 3.28 2.09

× × × × × × × × × × × × ×

10−3 10−6 10−6 10−4 10−5 10−4 10−5 10−5 10−5 10−5 10−6 10−6 10−5

kdec/kiso 1.84 × 101 5.04 × 106 7.48 × 107 2.21 × 101 2.62 × 101 2.3 × 10−1 4.15 2.53 7.7 × 10−1 2.03 × 1010 7.03 × 103 1.31 × 103 6.99 × 102

acetaldehyde (CH3CHO, except for MnPE and MiBE) and different alkyl radicals occurs. The lowest activation enthalpy has been found for EtBE decomposition (13.3 kcal/mol), which gives acetaldehyde and the tertiary radical •C(CH3)3. This barrier is clearly lower than the other ones of the subgroup, having values between 19.2 and 20.8 kcal/mol. For what concerns tertiary radicals, their decomposition allows the production of acetone (CH3COCH3) and various alkyl radicals. In this subgroup of ethers, the activation energy varies between 14.6 kcal/mol for DIPE and 16.9 kcal/mol for MiPE decomposition (with production of a tertiary and of a primary radical respectively). The presence of the oxygen in the chain has a fundamental role in the case of RIORII• β-scission. This can be seen more clearly in the comparison with activation energy values of homologue hydrocarbons (Table 4). In all three cases, as expected,17 the oxygen lowers the reaction barrier of at least 9 9015



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Figure 4. View of DEEoo radical and optimized structures of transition states involved in all the possible intramolecular hydrogen abstractions with activation energies.

it is 15.8 kcal/mol for β-scission and 20.7 kcal/mol for isomerization. These last results well illustrate the relevant role of entropic effects on this reaction channel. Concerning the effect of oxygen in the carbon chain on this class of reaction, the comparison between activation enthalpy values obtained for ethers and their homologue hydrocarbons (Table 4) demonstrate that, for the intramolecular hydrogen transfer, the influence of the heteroatom is not so important as in β-scission reaction. Indeed, the difference between activation barriers evaluated for ethers and hydrocarbons is about 4 kcal/ mol, on average, whereas a larger value is found for β-scission reactions. 3.4. Isomerization. Data collected in this section concern the reaction of internal displacement of a hydrogen atom within peroxy radicals.10,19,40 Between the different possibilities for intramolecular hydrogen abstraction found for ethers, it has been commented only in the following the one that, in each case, has the lower activation energy. The behavior of all the ethers seems to be very similar in the case of isomerization reaction, independently from the position of the peroxy functional group (linked on a primary, secondary, or tertiary carbon). Where different intramolecular hydrogen abstractions are possible, the lower activation energy has been found for the one that have the lower ring strain in the cyclic transition state. In Figure 4 are reported the three transition states found in the case of DEE peroxy radical with correspondent activation energies. For this radical, the isomerization path with the lowest activation energy (17.2 kcal/mol) begins with the abstraction of a hydrogen atom from the methylene adjacent to the ether oxygen (in α position). The correspondent transition state (dubbed DEEiso_α) features a six-membered ring arrangement with pseudo chair conformation, in which the strain is lower than in the five-membered (DEEiso_βI) and seven-membered (DEEiso_β) rings of the alternative transition states.

kcal/mol (for the butane/MEE pair) and up to a maximum of 11 kcal/mol (for 2-methylbutane/MiPE pair). As an alternative to decomposition, RIORII• radicals can react with molecular oxygen to produce peroxy species, RIORIIOO•. In analogy with literature data,10,19,39 this reaction is exothermic by about 30 kcal/mol, from a minimum of 28.6 kcal/mol for MnPE, to a maximum of 31.9 kcal/mol for MiBE (see Table 3 where the exothermicity of reaction can be evaluated as the difference between the third (RIORII•) and the fourth (RIORIIOO•) column of the table). This reaction is also barrierless for all considered ethers. The produced RIORIIOO• radicals can then react with a new molecule of ether, to produce hydroperoxide (RIORIIOOH) and to regenerate an RIORII• species or can undergo to isomerization by intramolecular hydrogen transfer to give hydroperoxy radicals (•RIORIIOOH). 3.3. Hydroperoxides Production. Hydroperoxides are produced by the reaction of intermolecular hydrogen transfer between a peroxy radical and an ether molecule. If different groups of equivalent hydrogen atoms are present in the ether molecule, the abstraction that gives the regeneration of the most stable RIORII• radical has been only considered. For this reaction path, values of activation enthalpies (Table 3) are close for all the ethers studied, independently of the nature of the radicalic carbon and of the peroxy group positions on the radical structures. Values range from a minimum of 13 kcal/mol (MiBE) to a maximum of 14 kcal/mol (MtBE, TAME, and EiPE). The only exception is represented by DME, with its activation enthalpy of about 10 kcal/mol. Among the three reaction pathways studied here, this bimolecular reaction is the one presenting the lowest enthalpy of activation (in average 13.5 kcal/mol, Table 3). The opposite behavior is, instead, observed for the corresponding Gibbs free energies of activation (Table 5). Indeed, the average ΔG‡ is 24.3 kcal/mol for hydroperoxides production reaction, whereas 9016



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transition states are close to the ones of homologue ethers, the difference in activation barriers values of approximately 4 kcal/ mol for the three pairs of ether/hydrocarbon is only due to the absence of the oxygen in hydrocarbons chains. Furthermore, isomerization reactions are all endothermic by about 15 kcal/ mol. 3.5. Simple Relationship between Activation and Reaction Enthalpies. Data relative to the three main reaction pathways (β-scission, hydroperoxides production, and isomerization) studied for the ether oxidation process have been examined with the aim to verify if a simple (notably linear) relationship exist between activation and reaction energies for each class. A similar approach has been already employed in literature for other types of radicals. Atkinson25 and Ferenac et al.,5 for instance, have used a linear relationship to study the decomposition reaction of alkoxy radicals (RO•) in atmospheric conditions, involving activation and reaction enthalpies (ΔH‡ and ΔH, respectively) of RO• decomposition process:

The most part of peroxy radicals studied isomerizes, as DEE, in a six-membered transition state. Values collected in Table 3 show that, if a hydrogen atom is abstracted from a methylene group (as in the case of EnPE and EiPE), the activation enthalpies for ethers are comparable to the one calculated for DEE equal to 17.2 kcal/mol.19 On the other hand, if the abstraction of the proton happens on a methyl group (MEE and MnPE, for instance), activation enthalpies are slightly higher (about 2 kcal/mol) if compared to the one obtained for DEE. Some ethers deviate from the described behavior. In particular, DME and DIPE both isomerize in six-membered transition states but they have to overcome activation barriers (15.3 and 15.8 kcal/mol, respectively) lower than the value discussed above. That is due to some characteristic structural features of the two ethers, notably DME has two nonbranched alkyl groups and in DIPE, the abstraction is made on a tertiary carbon (see optimized structures of both transition states in Figure 5).

ΔH ⧧ = a + bΔH

(6)

In this equation the parameters a and b are fixed a priori and depend on the reaction class and on the nature of radicals formed during the decomposition. In particular in the work of Ferenac et al.,5 this relationship has been validated for the decomposition process on a group of seven alkoxy radicals. A similar equation has also been used in the work of Pfaendtner and Broadbelt26 to evaluate the correlation between activation and reaction enthalpies for some reactions involved in hydrocarbons oxidation. In our work, a simple linear relation, obtained without any parametrization, has been found between activation and reaction enthalpies only for peroxy radical isomerization reactions of the thirteen considered ethers (Figure 7). In this model stabilization enthalpies (ΔH) increase linearly with activation energies (ΔH‡). This is not surprising, due to the similarity of structures of transition states and •RIORIIOOH radicals that are in agreement with Hammond’s postulate.41 The point that deviates more from the linear trend is that relative to MiBE (ΔH‡ = 17.3 kcal/mol and ΔH = 11.1 kcal/ mol), the ether having the greater difference between activation and reaction enthalpies for the isomerization process (Table 3, this difference is equal to 6.2 kcal/mol for MiBE against 3.1 kcal/mol for other ethers). Even though not perfect (R2 = 0.71), this relation between activation and reaction enthalpies, allows for a first significant prediction of activation barriers without a full computational characterization of the transition state. In comparison, Ferenac5 obtained a good correlation for the prediction of the activation barriers of the decomposition process based on six alkoxy radicals with a rms deviation of 5.8% (and a maximum error of 8.9%) whereas Pfaendtner26 obtained R2 = 0.96 for a first model describing the decomposition of six hydroperoxides and

Figure 5. View of optimized structures of the transition states involved in the isomerization of DME and DIPE peroxy radicals.

Other peroxy radicals can only isomerize by forming in transition states seven-membered cycle. This is the case of MtBE, TAME, and EtBE due to the presence in their structures (Figure 6) of a quaternary carbon [−C(CH3)3] linked to oxygen. The abstraction of the proton is therefore possible on a carbon atom in β-position with respect to oxygen. Activation enthalpies calculated for the isomerization of these ethers' peroxy radicals are higher (between 20.8 and 24.4 kcal/mol, Table 3) than the one evaluated for DEE. This is due to two different effects: first of all the ring strain, higher in a sevenmembered ring than in a six-membered one. A second contribution can be attributed to the fact that a hydrogen atom is abstracted from a methyl group not directly linked to the ether oxygen: the C−H bond cannot be easily broken as the one in the α position to oxygen. The effect of the oxygen in the carbon chain can also be evaluated by considering the isomerization of hydrocarbons peroxy radicals. For these species, activation enthalpy values (Table 4) are very close to the ones found for the last analyzed subgroup of ethers (average value is 23.3 kcal/mol). Because the structures of

Figure 6. View of optimized structures of the transition states involved in the isomerization of MtBE, TAME, and EtBE peroxy radicals. 9017



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as already reported in literature for alkoxy radicals.5 On the other hand, a strong competition between the two paths can be found for ethers with energetic features close to those of DEE (MiBE, EnPE, and EiBE), which present values of the ratio kdec/kiso between 0.23 and 2.53 (kdec/kiso = 4.15 for DEE).

4. CONCLUSIONS In this work we reported a study on oxidation of a series of aliphatic ethers. As suggested by our preceding detailed study of DEE oxidation, three reaction pathways have been considered: the direct β-scission of RIORII• radicals, the isomerization of RIORIIOO• radicals and the reaction of hydroperoxides production. For all the studied ethers our results suggest that the direct decomposition of the radical issued of the initiation step is energetically possible. The scenario could be very different in presence of molecular oxygen, when the exothermic and barrierless reaction of the radical with molecular oxygen takes place. A predominance of peroxy radicals production would favor the isomerization pathway. A real competition between the two reaction channels has been found for few types of ethers. The mechanistic generalization made in this work on the oxidation of a class of compounds, from results obtained for a model molecule, allows the identification of the energetic values of the key steps of the process and avoids from a detailed mechanistic study of each considered species. Moreover, thermodynamic data collected in this work show a linear relationship between activation and reaction enthalpies obtained for the isomerization reaction of the peroxy radicals of the thirteen considered ethers. This relationship allows the estimation of activation barriers of isomerization reactions of aliphatic ethers by avoiding of transition states characterization. A qualitative prediction is also possible on the main end products of ethers oxidation process. Starting from the considerations made on the kinetics of competing steps, we can assume that predominant products of reaction of oxidation will be issued from the decomposition pathway (aldehydes or ketones and alkyl radicals) and from the decomposition of produced radicals (alkyl radicals issued from β-scission reaction and hydroperoxide radicals formed in isomerization process.).

Figure 7. Activation enthalpies of isomerization reactions (ΔH‡) as function of the reaction enthalpies (ΔH). The linear relation has been evaluated with the energies of all the thirteen aliphatic ethers.

R2 = 0.93 for another model developed on 11 pairs of values (7 obtained by DFT calculation and 4 experimentally) relative to the β-scission reaction of alkoxy radicals. The notable difference between the performances of the model elaborated in our study and the ones described above is probably due to the parametrization of Ferenac and Pfaendtner’s equations. 3.6. Kinetics of the Process. In our work on DEE oxidation,19 from a detailed mechanistic study of the process and the analysis of rate constants of the key competition steps, the decomposition of RIORII• radicals and the isomerization of peroxy radicals were identified as the most relevant reactions in solution. The same (qualitative) kinetic approach has been used here to identify if a similar behavior can be observed for the 13 considered aliphatic ethers. Values of rate constants calculated for the three main reactions pathways analyzed in this paper are collected in Table 5. Rate constants of isomerization and hydroperoxides production channels have been corrected to take into account the effect of proton tunneling, as detailed in the Computational Details. As in the case of DEE oxidation, the two pathways characterized by higher rate constants for all ethers are the decomposition of the RIORII• radical and the isomerization of the peroxy radical. Nevertheless, if we look at their ratio (kdec/ kiso, Table 5), there is no clear evidence of a common behavior, the values ranging between 1010 and 10−1. In all the cases, direct decomposition seems to be the favorite pathway, because it is unimolecular and involves the augmentation of the molecularity of the process (one reactive gives two products). Notable exceptions are represented by MiBE and EiBE. Nevertheless, this reaction is not directly in competition with isomerization, but with the exothermic and barrierless reaction of the RIORII• radical with molecular oxygen. So this step of the oxidation process will be strongly affected by oxygen concentration in solution, as experimentally observed (see for instance ref 22). A more detailed analysis of the data reported in Table 5 evidence, however, some subgroups with common trends. In particular, for the ethers that cannot isomerize by passing in a six-membered transition state (MtBE, TAME and EtBE), direct decomposition of the radical is the only possibility of reaction,

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ASSOCIATED CONTENT

* Supporting Information S

Convergence of the computed energies with the basis set and complete ref 28. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.A. and S.D.T. thank INERIS for funding part of this work. REFERENCES

(1) Ashok, M. P.; Saravanan, C. G. Energy Fuels 2007, 21, 1878− 1882. (2) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Int. J. Global Ener. Issues 2008, 29, 329−336. (3) Zannis, T. C.; Hountalas, D. T. Energy Fuels 2004, 18, 659−666. (4) Orlando, J. J. Phys. Chem. Chem. Phys. 2007, 9, 4189−4199.

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Article

(5) Ferenac, M. A.; Davis, A. J.; Holloway, A. S.; Dibble, T. S. J. Phys. Chem. A 2003, 107, 63−72. (6) Kelly, R. J. Chem. Health Saf. 1997, 3, 28−36. (7) Robertson, R. Chem. Ind. 1933, 52, 274. (8) Morgan, G. T.; Pickard, R. H. Chem. Ind. 1936, 55, 421. (9) Davies, A. G. J. R. Inst. Chem. 1956, 386−389. (10) Andersen, A.; Carter, E. A. J. Phys. Chem. A 2003, 107, 9463− 9478. (11) Zhang, T. C.; Zhang, L. D.; Wang, J.; Yuan, T.; Hong, X.; Qi, F. J. Phys. Chem. A 2008, 112, 10495−10501. (12) Zavala-Oseguera, C.; Alvarez-Idaboy, J. R.; Merino, G.; Galano, A. J. Phys. Chem. A 2009, 113, 13913−13920. (13) Melloki, A.; Le Bras, G.; Sidebottom, H. Chem. Rev. 2003, 103, 5077−5096. (14) Cooper, W. J.; Cramer, C. J.; Martin, N. H.; Mezyk, S. P.; O’Shea, K. E.; von Sonntag, C. Chem. Rev. 2009, 109, 1302−1345. (15) Semenov, N. Chemical Kinetics and Chain Reactions; Oxford University Press: London, 1935. (16) Benson, S. W. Prog. Energy Combust. Sci. 1981, 7, 125−134. (17) Denisov, E. T.; Afanas’ev, I. B. Oxidation and Antioxidants in Organic Chemistry and Biology; Taylor & Francis: Boca Ratom, FL, 2005. (18) Battin-Leclerc, F. Prog. Energy Combust. Sci. 2008, 34, 440−498. (19) Di Tommaso, S.; Rotureau, P.; Crescenzi, O.; Adamo, C. Phys. Chem. Chem. Phys. 2011, 13, 14636−14645. (20) Tully, F. P.; Droege, A. T. Int. J. Chem. Kinet. 1987, 19, 251− 259. (21) Mellouki, A.; Teton, S.; Lebras, G. Int. J. Chem. Kinet. 1995, 27, 791−805. (22) Goldaniga, A.; Faravelli, T.; Ranzi, E.; Dagaut, P.; Cathonnet, M. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; pp 353−360. (23) Böhm, H.; Baronnet, F.; El Kadi, B. Phys. Chem. Chem. Phys. 2000, 2, 1929−1933. (24) Waddington, D. J. Proc. R. Soc. London, Ser. A 1959, 252, 260− 272. (25) Atkinson, R. Chem. Rev. 1986, 86, 86−201. (26) Pfaendtner, J.; Broadbelt, L. Ind. Eng. Chem. Res. 2008, 47, 2886−2896. (27) Becke, A. D. J. Chem. Phys. 1993, 98, 1372−1377. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian, Inc.: Wallingford, CT, 2009. (29) Tognetti, V.; Le Floch, P.; Adamo, C. J. Comput. Chem. 2010, 31, 1053−1058. (30) Coote, M. L. J. Phys. Chem. A 2004, 108, 3865−3872. (31) Izgorodina, E. I.; Coote, M. L.; Radom, L. J. Phys. Chem. A 2005, 109, 7558−7566. (32) Andersen, A.; Carter, E. A. Isr. J. Chem. 2002, 42, 245−260. (33) Andersen, A.; Carter, E. A. J. Phys. Chem. A 2003, 107, 9463− 9478. (34) Wu, J. Y.; Liu, J. Y.; Li, Z. S.; Sun, C. C. J. Chem. Phys. 2003, 118, 10986−10995. (35) Gonzalez, C.; McDouall, J. J. W.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 7467−7471. (36) Eyring, E. J. Chem. Phys. 1934, 3, 107−115. (37) Wigner, E. Z. Phys. Chem. B 1932, 19, 203. (38) Skodje, R. T.; Truhlar, D. G. J. Phys. Chem. 1981, 85, 624−629. (39) Backtorp, C.; Borje, A.; Nilsson, J. L. G.; Karlberg, A. T.; Norrby, P. O.; Nyman, G. Chem.Eur. J. 2008, 14, 9549−9554. (40) Salooja, K. C. Combust. Flame 1965, 9, 33−41. (41) Hammond, G. S. J. Am. Chem. Soc. 1955, 77, 334−338.

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Oxidation Mechanism of Aliphatic Ethers - American Chemical Society

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