Article pubs.acs.org/EF
Theoretical and Experimental Study on the Inhibition of Diethyl Ether Oxidation Stefania Di Tommaso,†,‡ Patricia Rotureau,*,‡ Wassila Benaissa,‡ Peggy Gruez,‡ 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, France ‡ INERIS, Parc Technologique Alata - BP 2 - 60550 Verneuil-en-Halatte, France § Institut Universitaire de France, 103 Boulevard Saint Michel, F-75005 Paris, France S Supporting Information *
ABSTRACT: The inhibition mechanism of diethyl ether oxidation was investigated both at the experimental and theoretical (density functional theory, DFT) level to find the best inhibitor for such a process. First, a detailed theoretical analysis was conducted by DFT calculations on reactions with 12 potentially effective antioxidants from different chemical classes (phenols, amines, and phosphines). Results of this study show that phenolic antioxidants are the best performers for inhibiting the process. In particular, the efficacy of butylated hydroxytoluene, an additive already commonly used in diethyl ether (DEE) storage, was clearly explained from a kinetic (low activation barrier) and a thermodynamic (product stabilization) point of view. The order of effectiveness of the antioxidants obtained from DFT calculations was confirmed by the complementary experimental work on DEE oxidation, which were conducted under accelerated conditions in autoclaves. previous work,19 was chosen as model molecule in this investigation. Several molecules were tested as a potential DEE oxidation inhibitor (phenols; primary, secondary, and tertiary amines; and phosphines) to determine the best antioxidant for DEE and to understand better the different mechanisms which may be involved in the inhibition process. The molecules studied were chosen from the most commonly used antioxidants for the inhibition of oxidation processes of alkanes and ethers.13,20,21
1. INTRODUCTION A great number of organic compounds spontaneously react in a self-propagating process of autoxidation. This process not only degrades the chemical compound (e.g., change in chemical and physical properties) but it may also generate a variety of hazardous compounds, such as peroxides which are considered to be responsible for many laboratory accidents.1−3 Different empirical methods are used to inhibit this process of chemical degradation at the laboratory scale. For instance, use and storage of peroxidizable compounds under an inert atmosphere or storage in opaque containers are helpful precautions that reduce the rate of peroxidation in most cases.4 Nevertheless, the method currently recognized as the most effective in the stabilization of peroxidizable compounds is the addition of trace quantities of oxidation inhibitors.4−6 One of the industrial and research fields in which the efficiency of antioxidants has gained scientific interest is that of fuels7−9 and biofuels10−12 because of their large commercial volume. Several molecules may be used as inhibitors, and their mechanisms of action can be very different.13−18 Indeed, depending on their activity they can react with neutral molecules or with radical species produced during the oxidation process and are gradually depleted.13 Therefore, knowledge of the inhibition mechanism of the antioxidant could be very useful in laboratory activities and industrial processes to give insight into what is the most suitable antioxidant, the concentration needed to stabilize the chemicals during storage, and also to estimate the duration of stabilization. In this work, the inhibition of oxidation was studied both theoretically (using density functional theory, DFT) and experimentally (under accelerated conditions with autoclaves at elevated temperatures and pressures). Diethyl ether (DEE), a well-known peroxide producer whose oxidation mechanism has already been the subject of in-depth theoretical studies in our © 2014 American Chemical Society
2. INHIBITION MECHANISMS AND INHIBITORS FOR DIETHYL ETHER OXIDATION Figure 1 shows the scheme of the self-catalytical cycle of the DEE oxidation mechanism (RH; see Table 1 for the formula of all species involved in the reaction) as characterized in our previous work.19 The process starts with an initiation step in which the radical R· is produced by the abstraction of a
Figure 1. Inhibition mechanisms of hydrogen donors (AH) and of hydroperoxide-decomposer antioxidants (A) (in the box) in the global scheme of oxidation of a generic organic molecule RH. Received: December 19, 2013 Revised: February 25, 2014 Published: February 26, 2014 2821
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emphasized that reaction 3 between the radical A· and a neutral molecule RH, producing a new R· radical, must however be disfavored.
Table 1. Names and Formulas of the Different Radicals Produced during the DEE Oxidation Process
A· + RH → AH + R·
(3)
The second class of proposed inhibitors14 has as a principal function to favor the nonradical decomposition of hydroperoxides produced to alcohol. Phosphines (reaction 4), phosphites (reaction 5), and tertiary amines (reaction 6) can be cited in such a class of inhibitors. The reaction occurs by the transfer of a hydroperoxide oxygen atom to the antioxidant resulting in the formation of an oxidized form of the species and the corresponding alcohol. P(OR)3 + ROOH → OP(OR)3 + ROH
(5)
NR3 + ROOH → ONR3 + ROH
(6)
PR3 + ROO· → OPR3 + RO·
(7)
PR3 + RO· → OPR3 + R·
(8)
In the past, few experimental studies have been devoted to the inhibition of DEE oxidation. For instance, Lemay and Ouellet24 experimentally tested the influence of some chemicals (nitric oxide, nitrogen tetroxide, propene, formaldehyde, and hydrobromic acid) on the first step of the process. The authors explain that the inhibition of the process was due to the interaction between tested antioxidants with molecular oxygen or other chain carriers instead of their reaction with DEE or radicals derived from DEE. These inhibitors would therefore change the behavior of DEE by modifying only its reactive environment. Another experimental study by Waddington21 on the inhibition of DEE oxidation considered as antioxidants several primary (pentylamine, n-butylamine, n-propylamine, ethylamine, methylamine, and isopropylamine), secondary (dimethylamine and diethylamine), and tertiary amines (trimethylamine and triethylamine) and tested their effect on experimental parameters such as the induction time or the variation of pressure in a reactor. Waddington identified secondary amines as the most effective antioxidants and attributed this inhibition efficacy to their ability to stabilize free radicals produced during the DEE oxidation in the gas phase. This stabilization would be due to the formation of AH/ R· complexes, coming from the interaction between the nitrogen atom of the amine and the radical center. Even though no other recent studies on the inhibition of DEE oxidation have been reported in the literature to the best of our knowledge, several experimental works are currently
(1)
Reaction 1 is in direct competition with the autocatalytic process involving the peroxy radical and a new neutral molecule to form two very reactive species: hydroperoxide and an alkyl radical. ROO· + RH → R· + ROOH
(4)
This second class of antioxidants was also studied in the field of biology, far removed from fuel storage concerns, by quantum mechanical modeling (DFT), as described in reference 23 for xenobiotics. Considering the reaction of tert-butyl hydroperoxide (CH3)3COOH with two different antioxidants, trimethylphosphine and trimethylamine, calculated activation energies are respectively equal to 4 and 31 kcal/mol. For this last class of antioxidants that decompose hydroperoxides, it was also proposed in the literature13 that they can react with oxygenated radicals (such as ROO· and RO·) produced during the oxidation process. In the case of the phosphine PR3, we could have, for example the following reactions:
hydrogen atom from neutral ether in analogy with the wellknown autoignition mechanism of alkanes.13,22 The reactive species produced can directly undergo decomposition (βscission) or react with molecular oxygen leading to the peroxy radical ROO·. This last reaction opens a chain propagation cycle in which ROO· radical reacts with a new ether molecule to produce the hydroperoxide ROOH and another alkyl radical. The peroxy radical can also isomerize by intramolecular hydrogen transfer, giving a hydroperoxy radical (·ROOH). Therefore, from each DEE molecule that decomposes, one hydroperoxide and one alkyl radical are formed and thus restart the self-catalytic cycle by reacting with molecular oxygen. An effective antioxidant for such a process acts by preventing hydroperoxide production or by decomposing these molecular products. The reactivity of some antioxidants normally used in fuels and biofuels was studied at both theoretical and experimental levels. Zabarnick et al.14 were interested in the autoxidation process of a jet fuel (containing different alkanes and alkylated aromatic ratios9) occurring in hot components of aircraft fuel systems. In particular, the reaction was modeled using a quantum chemistry approach (DFT/B3LYP) and considering only a simple and isolated hydrocarbon. Starting from their computational results, the authors proposed two classes of antioxidants for the inhibition of the oxidation process in the fuel. The first class is composed of chemicals containing a labile hydrogen atom in their structure, such as phenols with an OH enolic group, and the second class comprises primary and secondary amines with their NH moiety. In the first proposed mechanism,14 the antioxidant (AH) reacts with the peroxy radical ROO· (reaction 1) produced by the reaction between the alkyl radical (R·) and molecular oxygen (see Figure 1). ROO· + AH → A· + ROOH
PR3 + ROOH → OPR3 + ROH
(2)
The antioxidant is effective if reaction 1 is favored in terms of the activation barrier with respect to reaction 2. It has to be 2822
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Table 2. Names and Structures of the 12 Antioxidants Considered in the Theoretical Study of the Inhibition of DEE Oxidation
were obtained as a sum of electronic and thermal correction to the total electronic energies. 3.2. Experimental Tools. To study the influence of the addition of inhibitors on DEE oxidation under accelerated conditions, experiments were carried out at high temperature using six 50 mL autoclaves, as described in ref 35. Each stainless steel vessel is equipped with a temperature sensor and a pressure transducer. The six autoclaves are inserted into a heating block controlling the temperature. Diethyl ether (99.7%), diphenylamine, and triphenylphosphine were obtained from VWR, whereas BHT and diethylamine were obtained from Sigma Aldrich. DEE was provided with an initial concentration of 5 to 10 ppm of BHT. DEE (14 mL) with different inhibitor concentrations were introduced into each autoclave and pressurized by synthesized air from gas cylinders to approximately 11 bar gauge. The samples were heated to 100 °C, and the temperature and pressure inside the autoclaves were recorded throughout the experiment. For two experiments, the amount of peroxide produced was measured as a function of time: DEE without additional inhibitors (here considered as a control) and DEE containing 1 ppm of BHT. At set storage times, the samples were recovered and the concentration of peroxide was evaluated using a potentiometric titrator with a solution of potassium permanganate.35 This method gives the global amount of peroxide function (equivalent H2O2).
devoted to the measurement of this oxidation process and to its inhibition in fuels and biofuels during storage.10−12 In general, fuel producers use methods based on vegetable oils and fats and measure peroxide levels and the induction period at elevated temperatures to accelerate the oxidation process. Concerning diesel and biodiesel, measurements are carried out according to several American Society of Testing and Materials (ASTM) methods (like ASTM methods D 227425 and D 462526) or European test methods (such as EN 1411227 and EN 1421428). In particular, calorimetric methods (such as differential scanning calorimetry, DSC) and the Rancimat instrument, which measures the oil stability index, make it possible to perform analyses at elevated temperatures and thus accelerated conditions of oxidation. However, the more common practice currently used to stabilize diethyl ether is to add butylated hydroxytoluene (BHT), a phenolic compound well-known to be a good antioxidant for DEE. Small quantities (1 to 10 ppm of BHT)5 are employed to minimize costs.
3. MATERIALS AND METHODS 3.1. Computational Details. All the density functional theory calculations were performed using the hybrid B3LYP functional29 and Gaussian 03 program.30 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. The Cartesian coordinates, the three lower frequencies, and thermochemistry of the transition states characterized for all the reactions studied in the present study are collected in Supporting Information. As discussed in our previous works,19,31 B3LYP has been proven to provide energies close to more sophisticated and timeconsuming methods for related systems.32,33 Intrinsic reaction coordinate (IRC) calculations were also performed to verify that the identified products and the reactants were correctly connected.34 For radicals (open shell systems), unrestricted DFT calculations were carried out and the results obtained were checked for wave function instability. All the enthalpy values (in kilocalories per mole)
4. RESULTS AND DISCUSSION 4.1. Molecular Modeling. The theoretical study was carried out on the 12 inhibitors indicated in Table 2, according to the two main mechanisms identified in the literature on the inhibition process of alkanes and ethers.13,20,21 Most of the antioxidants considered in this work, in particular phenolic compounds and primary and secondary amines, are from the class of hydrogen donor inhibitors (AH). Following the mechanistic propositions found in the literature, it is taken that AH antioxidants react preferentially with peroxy radicals. The mechanism studied for such inhibitors is shown in Figure 1. 2823
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Table 3. Prereactive Complex Stabilizations (PRC), Activation Enthalpies (ΔH⧧), and Product Stabilizations (ΔH) Calculated for the Inhibition of DEE Oxidation Reactions 1 and 3 by Phenolic Compounds ROO· + AH (reaction 1) phenols
A· + RH (reaction 3)
antioxidant
PRC (kcal/mol)
ΔH⧧ (kcal/mol)
ΔH (kcal/mol)
ΔH⧧ (kcal/mol)
ΔH (kcal/mol)
BHT 2,6-di-tert-butylphenol 2,4-di-tert-butylphenol TBMP hydroquinone
0.0 0.1 −5.0 −4.8 −3.0
2.6 3.7 4.7 4.3 3.5
−6.1 −4.2 −2.7 −3.2 −1.6
24.1 23.0 17.2 17.6 16.6
18.5 17.0 15.5 16.0 14.5
Activation enthalpies (ΔH⧧) for the two competing reaction pathways 1 and 2 were calculated at DFT (B3LYP/631+G(d,p)) level of theory for the 10 concerned inhibitors and compared to verify their efficacy in the inhibition process. The activation barrier of reaction 3 regenerating a R· radical was also calculated for each inhibitor. The last two antioxidants studied (namely, triphenylphosphine and triethylamine) belong to the class of the hydroperoxide-decomposer inhibitors (A). Their mechanisms of inhibition according to the literature13,14 are shown in Figure 1. For this class of inhibitors, activation enthalpies were calculated for the following reactions: A + ROOH → AO + ROH
A + ROO· → AO + RO·
(9) (10)
The reactivity of each antioxidant considered in this study was tested on the oxidation of DEE. Structures of the different radicals produced during this process are shown in Table 1. 4.1a. Proton-Donor Antioxidants (AH). As can be seen in Figure 1, inhibitors having in their structure a labile hydrogen atom intervene in the radical process of DEE oxidation when a peroxy radical (ROO·) is produced: AH antioxidants stop the self-catalytical cycle of the DEE oxidation process, by preventing further production of alkyl radicals. Phenolic compounds and primary and secondary amines, having respectively an O−H and an N−H bond in their structures, were chosen to study this class of antioxidants. Results of the DFT calculations are reported in the following. Phenolic compounds are well-known to be effective inhibitors for alkane14 and ether20 oxidation. Among this class of chemicals, the butylated hydroxytoluene (known as BHT), can be considered as the most important antioxidant used for diethyl ether stabilization. Such compounds react with the peroxy radicals (ROO·) by giving the hydrogen atom of the OH group. Activation enthalpies (ΔH⧧) and relative product energies (ΔH) of this process were calculated for the five phenolic compounds considered in this work, with the aim of verifying their efficiency. The results obtained are reported in Table 3 and Figure 2. For this reaction, activation barriers are calculated with respect to the prereactive complexes (PRC) generated by the interaction between the peroxy radical and each antioxidant (see Figure 2). On the other hand, product stabilizations are calculated with respect to the isolated reactants. Considering the reaction with ROO· (reaction 1), all the phenolic compounds studied present activation barriers lower than those calculated for the corresponding reaction with DEE (reaction 2). Indeed, the activation enthalpy, without the presence of antioxidant, is equal to 13.3 kcal/mol19 for the reaction between DEE and the ROO· radical, whereas the highest barrier calculated with antioxidants of this group is about 5 kcal/mol (for 2,4-di-tert-butylphenol). This remarkable
Figure 2. Potential energy profiles of (A) the two reactions 1 and 2 in competition during the inhibition of DEE oxidation using BHT and (B) DEE oxidation inhibited by TBMP. Enthalpy values are relative to the initiation step of the process.
difference (at least 10 kcal/mol) is due to the fact that the O− H bond present in the structure of phenolic compounds is weaker than the C−H bond, typical of the DEE carbon chain. Dissociation enthalpies for these two bonds were calculated (as enthalpy difference between the initial reagent and radical products by the heterolytic cleavage of the bond) for BHT and DEE, respectively. The bond dissociation enthalpies were evaluated to 73.7 kcal/mol for the O−H bond (in BHT) and to 92.6 kcal/mol for the C−H bond (in DEE). On a thermodynamic level, reaction 1 is exothermic for the five phenolic compounds considered in this work with enthalpies of reaction spanning the range from 1.6 kcal/mol (calculated for hydroquinone) to 6.1 kcal/mol (for BHT). In contrast, the corresponding reaction for DEE (reaction 2) is endothermic with an enthalpy of reaction of 13 kcal/mol.19 Because the hydroperoxide ROOH is produced in both reactions 1 and 2, the difference in the stabilization of the products is essentially due to the different stabilizations of R· and A· radicals. A· species are indeed more stable than the R· radicals because of the possible delocalization of electrons in aromatic cycles. Considering BHT as an antioxidant, potential energy profiles of these two competing pathways are represented in Figure 2A. 2824
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Table 4. Activation Enthalpies (ΔH⧧) and Product Stabilizations (ΔH) Calculated for the Inhibition of DEE Oxidation (Reactions 1 and 3) by Primary and Secondary Amines ROO· + AH (reaction 1) NH2-R NH-R2
A· + RH (reaction 3)
antioxidant
ΔH⧧ (kcal/mol)
ΔH (kcal/mol)
ΔH⧧ (kcal/mol)
ΔH (kcal/mol)
2-methylbutylamine 3-methylbutylamine dimethylamine diethylamine diphenylamine
11.3 11.4 7.0 7.8 3.6
15.6 16.4 10.6 10.2 1.8
9.7 9.0 12.7 14.4 21.0
−2.8 −3.6 2.2 2.7 11.0
efficiency as an antioxidant compared to the other phenols considered in this work. If we look at the inverse reaction of 3 (AH + R· → A· + RH), enthalpies presented in Table 3 show that the activation barriers of this process (calculated as the difference between the values in the fifth and fourth columns) are between 1.6 kcal/ mol (for TBMP) and 2.8 kcal/mol (for 2,4 tert-butyl phenol) and are quite similar to those calculated for reaction 1. Nevertheless, for the phenols that have been considered here, activation enthalpies calculated for the reaction between AH and alkyl radicals are all higher than those obtained for the reaction with peroxy radicals (reaction 1) with a difference of about 2 kcal/mol. For BHT, for instance, this difference is equal to 2.8 kcal/mol. Amines too have been proposed and studied in the literature as inhibitors for DEE oxidation.20,21 Concerning reaction mechanisms, primary and secondary amines can be counted among the proton-donor antioxidants because of the presence of NH2 and NH groups, respectively. They stop the oxidation chain reaction by giving up hydrogen atoms to peroxy radicals. In this work, among the amines studied in the literature,21 two primary and three secondary amines were considered. Activation enthalpies (ΔH⧧) and enthalpies of stabilization (ΔH) calculated for the reaction of each amine with the DEE peroxy radical (reaction 1) are presented in Table 4. These results show that amines are less effective antioxidants than phenols for DEE. The activation enthalpies are in fact quite similar to the energetic barrier calculated for the reaction between the peroxy radical and DEE (reaction 2, with a ΔH⧧ of about 13 kcal/mol). Moreover, for the five amines considered, reaction 1 is endothermic with an enthalpy of reaction up to 16.4 kcal/mol. As previously suggested in an experimental study,21 secondary amines are more effective than primary amines as antioxidants, both at a kinetic (activation barrier) and at thermodynamic (product stabilization) level. Indeed, if we consider only aliphatic amines, the lowest energy obtained for primary amines is 11.3 kcal/mol (for 2-methyl butylamine) and goes down to 7.0 kcal/mol for secondary amines (dimethylamine). Concerning enthalpies of reaction (Table 4), the difference between the two subgroups reaches a maximum of 6.2 kcal/mol. This can be easily explained from the greater stabilization of A· radicals produced from secondary amines because of the effect of the double substitution on the nitrogen atom. The difference is even more significant if we consider the enthalpy values obtained for diphenylamine, the only aromatic amine considered in this work. For this species, indeed, results (ΔH⧧ = 3.6 kcal/mol and ΔH = 1.8 kcal/mol) are very similar to those calculated previously for phenolic ortho-disubstituted compounds. In this case, the aromatic rings bonded to the nitrogen atom allow an electronic delocalization greater than that of an amine with aliphatic substitution, as shown in Figure
Concerning activation and stabilization enthalpies calculated for this class of antioxidants, the differences in values obtained from our calculations (Table 3) are essentially due to structural characteristic features of each species. Indeed, some common behaviors can be underlined. Focusing on the first two compounds of this class (Table 2), namely, BHT and 2,6-ditert-butylphenol, the activation enthalpies of 2.6 and 3.7 kcal/ mol were obtained (Table 3) for reaction 1, and the process in both cases is exothermic with enthalpies of reaction of −6.1 and −4.2 kcal/mol, respectively. These two compounds contain in their structures two tert-butyl substituents in ortho position with respect to the phenolic OH group. This double substitution makes the hydroxyl group less accessible and also explains these higher calculated activation barriers. Moreover, the presence of the two tert-butyl groups allows an important electronic delocalization in the A· radicals produced in reaction 1 and, as a consequence, a major stabilization of products. The comparison between the two ortho-disubstituted compounds underlines that the simple presence of a methyl group in the para position to the enolic OH in BHT produces a difference in the activation barrier of 1.1 kcal/mol and of about 2 kcal/mol in the stabilization of radicals (Table 3). In addition, ortho-monosubstituted species, such as 2,4-ditert-butylphenol and TBMP (see Table 2 for the corresponding formulas), can be considered as a second subgroup of phenolic antioxidants studied in this work. In both cases, the loss of a tert-butyl group in the ortho position to the OH group allows the formation of prereactive complexes better stabilized than the isolated reactants for 5 and 4.8 kcal/mol, respectively (Figure 2B). Considering activation barriers, they are very similar (about 4.5 kcal/mol), proving that the nature of the functional group in the para position to the OH group does not influence the amount of energy needed for the reaction. A· radicals produced from this subgroup of antioxidants are instead more poorly stabilized compared to those of the subgroup discussed above (a difference of about 2.5 kcal/mol). It is noteworthy that hydroquinone does not have any functional group in the ortho position to the hydroxyl group and that its reactivity is very similar to that of orthomonosubstituted compounds, with a difference of about 1 kcal/mol both in activation barrier and product stabilization. As already mentioned, the efficiency of an antioxidant can also be verified from the low aptitude of its corresponding A· radical to react with RH: reaction 3 must be disfavored. The results relative to reaction 3 obtained for the five phenols are reported in Table 3 and indicate that the calculated activation enthalpies are between 24.1 kcal/mol (for BHT) and 16.6 kcal/mol (for hydroquinone). In all cases, this process is endothermic. We can also note that the more stable the A· radical is, the higher the activation barrier for the reaction is and the more endothermic is reaction 3. In the case of BHT, the greater stabilization of the A· radical explains its greater 2825
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Within this class of antioxidants, we studied triethylamine, a tertiary amine previously tested experimentally9 to prevent DEE oxidation and triphenylphosphine (TPP; see Table 2 for its structure), considered to be an antioxidant for some fuels.14 In the experimental study on inhibition of aliphatic amines on the DEE oxidation, Waddington21 also tested two tertiary amines: trimethylamine and triethylamine. Results obtained in its study show that tertiary amines are nearly as effective as secondary amines. However, because of the triple substitution on the nitrogen atom and thus absence of a labile hydrogen atom bonded to N, the inhibition mechanism must be different from other amines. A mechanism A → AO has been proposed in the literature for trimethylamine.23 The mechanism proposed for hydroperoxide-decomposer antioxidants was tested on triethylamine. The results obtained, in terms of activation enthalpies and product stabilizations, are reported in Table 5. Activation enthalpies calculated for
3, based on the comparison between density spin maps of the A· radicals produced from diethylamine and diphenylamine.
Figure 3. Density spin maps of A· radicals produced from the abstraction of an hydrogen atom from diethylamine (left) and diphenylamine (right).
Enthalpies calculated for reaction 3 (Table 4) confirm that aliphatic amines are less effective antioxidants than phenols for the inhibition of DEE oxidation. Activation barriers obtained for this process producing alkyl radicals are in fact, in the case of aliphatic amines, between 9.0 kcal/mol (3-methyl butylamine) and 14.4 kcal/mol (diethylamine). Moreover, for primary amines, this reaction is exothermic up to a maximum of about 4 kcal/mol considering the enthalpy of stabilization, while the process is endothermic and disfavored for all the considered phenols. The only exception is again with diphenylamine: for this inhibitor, enthalpy values comparable to those obtained for the most effective phenolic compounds were calculated. Activation enthalpies obtained for the inverse of reaction 3 for this group of antioxidants (Table 4) are between 10.0 kcal/mol (diphenylamine) and 12.6 kcal/mol (3 methyl butylamine). As seen for the case of phenols, activation enthalpies for the inverse of reaction 3 are higher than for those for reaction 1 with an average difference of 3.2 kcal/mol. For diphenylamine, this difference is 6.4 kcal/mol. For AH antioxidants, above all phenols, mechanism alternatives to the direct hydrogen atom transfer (HAT) have been proposed in the literature,36 like the single electron transfer-proton transfer (SET-PT) or the sequential proton loss electron transfer (SPLET) mechanisms. The net result, in terms of products obtained, for this kind of process is the same as in the HAT mechanism. Moreover, SET-PT and SPLET mechanisms both imply (but in different fashion) a nonconcerted electron−proton transfer; in this case, the comparison between antioxidants should be done on the basis of the ionization potentials (IP) instead of O−H bond dissociation energies (BDE). DFT (B3LYP) data already published in the literature on a series of para substituted phenols36 ensure that the difference between BDE and IP for each compound is, on average, 5 kcal/mol and that this value does not strictly depend on the para substituent. This suggests that the order of effectiveness of the antioxidants here considered would not change if a nonconcerted mechanism was considered. 4.1b. Hydroperoxide-Decomposer Antioxidants (A). The second class of antioxidants considered in this work consists of inhibitors that can decompose hydroperoxides ROOH without producing radical compounds (such as ·OH and RO·).13,14 Such antioxidants work by removing an oxygen atom from OOH group of the hydroperoxide (reaction 9) to transform the hydroperoxide into an alcohol and the inhibitor into its corresponding oxide (the phosphine PR3, for instance, will give the phosphine oxide O=PR3). Some authors13 hypothesized that such inhibitors can also break the reaction chain by reacting with ROO· radicals (reaction 10).
Table 5. Activation Enthalpies (ΔH≠) and Product Stabilizations (ΔH) Calculated for the Possible Inhibitions of DEE Oxidation by Triethylamine (TEA) and Triphenylphosphine (TPP)
TEA
A AH
TPP
A
reaction
ΔH⧧ (kcal/mol)
ΔH (kcal/mol)
A + ROOH (reaction 9) A + ROO· (reaction 10) ROO· + AH (reaction 1) A· + RH (reaction 3) A + ROOH (reaction 9) A + ROO· (reaction 10)
27.8 24.8 8.2 15.4 10.6 6.1
3.5 −14.5 8.1 4.7 −76.8 −58.8
reactions 9 and 10 are quite higher than those obtained for AH antioxidants (reaching a maximum value of 11.4 kcal/mol). They are consistent with the activation barrier of about 30 kcal/ mol reported in the literature23 for an analogous reaction between trimethylamine and tert-butyl hydroperoxide. N(CH3)3 + (CH3)3 COOH → ON(CH3)3 + (CH3)3 COH
For the decomposition of DEE hydroperoxide (reaction 9), the reaction shows an activation enthalpy of 27.8 kcal/mol, and the reaction is globally endothermic by 3.5 kcal/mol (Table 5). These values discourage the utilization of triethylamine as hydroperoxide-decomposer, especially because the unimolecular decomposition of DEE hydroperoxide (ROOH → RO· + · OH) does not have a transition state and the reaction is endothermic for approximately 35 kcal/mol (value quite comparable to the activation barrier found for the bimolecular reaction 9). Although the reaction of triethylamine with the peroxy radical (reaction 10) is exothermic (ΔH = −15 kcal/mol), the activation barrier is once again too high (24.8 kcal/mol) to explain the good observed performances21 for the triethylamine as inhibitor of DEE oxidation. Therefore, an alternative mechanism (type AH) was tested. In particular, we studied the hydrogen abstraction by the DEE peroxy radical from one of the triethylamine alkyl chains. The results obtained for reaction 1, reported in Table 5, are quite similar to those calculated for the analogous reaction of secondary amines. If we compare with enthalpies related to diethylamine, we obtain a difference of 0.4 kcal/mol for the activation barrier (ΔH ⧧ is equal to 8.2 kcal/mol for triethylamine and to 7.8 kcal/mol for diethylamine) and of 2826
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about 2 kcal/mol in product stabilization, to the advantage of triethylamine. The hydrogen abstraction on triethylamine alkyl chains is favored on a carbon atom adjacent to nitrogen. The corresponding radical at a vicinal carbon is in fact more stable than the radical at a terminal carbon by about 13 kcal/mol. Optimized structures of triethylamine (A), of its most stable A· radical, and of its oxide (AO) are shown in Figure 4.
Figure 5. Peroxide concentration and pressure evolution at 100 °C.
consumption) and then increases when the concentration of peroxides decreases (associated with the formation of volatile byproducts). We also confirmed experimentally the effectiveness of BHT. The induction time needed to form peroxide is longer (about 3 h) when the inhibitor is added, and the maximum of the peroxide concentration is reached approximately one hour later. This maximum value is lower in the case of inhibited DEE, about 4000 ppm compared to 5300 ppm measured without BHT. Given that pressure decrease is directly linked to DEE oxidation, we considered only the evolution of pressure to study qualitatively the inhibition process. The evolutions of pressure in the autoclaves are shown in Figure 6A for four different concentrations of BHT (1, 10, 50, 100 ppm) at 100 °C. As expected, the higher the concentration of BHT, the longer the time needed to reach the minimum pressure (corresponding to the maximum peroxide production): approximately 5 h for noninhibited DEE compared to about 15 h for DEE containing 100 ppm of BHT. The theoretical study showed that diphenylamine is similar to BHT in terms of activation barriers and product stabilizations. It was demonstrated that secondary and primary aliphatic amines were less effective on the inhibition process and that diethylamine seems to have the best performance in this class of compounds. The performances of these three inhibitors belonging to the class of hydrogen donor antioxidants (AH) were tested under the same operating conditions for a concentration of 100 ppm at 100 °C (Figure 6B). The trends observed are in a good agreement with theoretical results (Tables 3 and 4): BHT and diphenylamine have very similar performances (the pressure minimum is reached after 17 h in both cases), and they are better inhibitors than diethylamine (7 h). For the other class of inhibitors (A, hydroperoxidedecomposer antioxidants), experiments were carried out with triphenylphosphine. Results are reported in Figure 6C. When the results are compared with the three antioxidants previously tested, a much higher concentration of TPP is needed to observe a significant oxidation inhibition, as no influence was detected with less than 100 ppm of this inhibitor. Our experimental results indicate clearly that the concentration of TPP must be 100 times greater than that of BHT to obtain the same effectiveness to inhibit DEE oxidation (e.g., 1000 ppm of
Figure 4. Optimized structures of triethylamine (A), its corresponding oxide (AO), and A· radical. Optimized structures of TPP and of the transition state of the reaction between triphenylphosphine and the DEE peroxy radical (TSTPP+ROO) are also represented.
This AH-like mechanism studied for triethylamine can explain the experimental results of Waddington21 in which secondary and tertiary amines have very similar performances on DEE oxidation inhibition. Phosphorus compounds13 such as phosphines and phosphates are considered to promote the organic hydroperoxide decomposition. In particular, triphenylphosphine (TPP)14 is commonly used as an antioxidant for hydrocarbons and kerosene-based fuels. Results obtained (Table 5) for the reaction of triphenylphosphine with the DEE hydroperoxide (reaction 9) and with the DEE peroxy radical (reaction 10) are very encouraging as both reactions present activation barriers of 10.6 kcal/mol for reaction 9 and 6.1 kcal/mol for reaction 10 (this last reaction proceeds via the transition state named TSTPP+ROO in Figure 4), comparable to the values obtained for reaction 1 in aliphatic amines (Table 5). Both reactions are very exothermic. 4.2. Experimental Results. The theoretical study suggested that among the different classes of inhibitors considered, phenolic antioxidants, and BHT in particular, are the most effective in preventing DEE oxidation. On the basis of this evidence, a first set of experiments was carried out to measure the amount of peroxides produced for a sample of DEE with and without added BHT in autoclaves described previously. Figure 5 shows the pressure evolution inside the autoclaves and peroxide concentration versus time when 1 ppm of BHT was added to the sample. In agreement with preliminary tests35 and literature,37 two trends were observed for the two cases considered: peroxide concentration increases initially to a maximum and then decreases to reach very low values, and the pressure decreases at first when the peroxides are formed (related to oxygen 2827
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Figure 7. Time corresponding to minimum pressure versus antioxidant concentration at 100 °C for (A) different concentrations of BHT in DEE and (B) different concentrations of TPP in DEE. Note that the scales on the two graphics are not the same.
Figure 6. Pressure evolution at 100 °C for (A) different concentrations of BHT in DEE; (B) the addition in DEE of the same concentration (100 ppm) of BHT, diethylamine, and diphenylamine; and (C) different concentrations of TPP in DEE.
inhibitors is BHT, both from a thermodynamic and kinetics viewpoint. Moreover, among the primary and secondary amines considered, only diphenylamine gives values of activation barriers and reaction enthalpies comparable with those of phenols. For the other class of inhibitors, the activation barrier for the reaction of TPP with DEE hydroperoxide is comparable with the one calculated for the reaction of the aliphatic amines with peroxy radical. On the basis of these results, an experimental study was conducted consisting of following the evolution of the pressure under accelerated conditions of DEE samples in autoclaves at a constant temperature with and without inhibitors. Several comments can be made from the experiments: • for all antioxidants studied, the higher the concentration of antioxidant the longer the time needed to reach the minimum pressure corresponding to a maximum peroxide concentration; • at the same concentration, BHT is just as effective as diphenylamine, and both are more effective than diethylamine; • under the same operating conditions, TPP can be considered to be a less effective inhibitor compared to BHT because of the much higher concentration needed to obtain the same effect on the rate of DEE oxidation. Experiments thus confirm the predictions made by theory concerning the order of efficiency of the inhibitors studied for the DEE oxidation process.
TPP is necessary to reach the minimum in the pressure evolution after 10 h whereas 10 ppm of BHT is sufficient). In addition, the time corresponding to minimum pressure versus the antioxidant concentration is represented in Figure 7 from data extracted from Figures 6A,C. As can be observed in this figure, a first correlation consisting of a simple linear relation exists between time until the minimal pressure (corresponding to the maximum peroxide production) and the antioxidant concentration for both BHT and TPP. Even if the objective of these experiments was to qualitatively observe the influence of concentration of antioxidants on the inhibition process, these linear relations perform quite well (with R2 = 0.97 and R2 = 0.99 for BHT and TPP, respectively). They allow for a first significant prediction of concentration of each antioxidant needed to increase the induction time of DEE (before maximum production of peroxide) at 100 °C.
5. CONCLUSIONS In this work, the study of the inhibition of DEE oxidation was performed by a combined theoretical and experimental approach. In the theoretical part, 12 antioxidants were studied either reacting with DEE-derived radicals as hydrogen donors (AH)phenols and aminesor decomposing the hydroperoxides produced (A)triethylamine and triphenylphosphine. Results show that, in general, phenolic compounds are more effective AH inhibitors than amines because of their more efficient stabilization of their A· radicals. The most effective antioxidant resulting from the theoretical study of AH type
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ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates, three lower frequencies, and thermochemistry of the transition states characterized for all the 2828
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Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ispida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strani, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, V.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian, Inc: Wallingford, CT, 2004. (31) Di Tommaso, S.; Rotureau, P.; Adamo, C. J. Phys. Chem. A 2012, 116, 9010−9019. (32) Andersen, A.; Carter, E. A. J. Phys. Chem. A 2003, 107, 9463− 9478. (33) Wu, J. Y.; Liu, J. Y.; Li, Z. S.; Sun, C. C. J. Chem. Phys. 2003, 118, 10986−10995. (34) Gonzalez, C.; McDouall, J. J. W.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 7467−7471. (35) Di Tommaso, S.; Rotureau, P.; Sirjean, B.; Fournet, R.; Benaissa, W.; Gruez, P.; Adamo, C. Process Saf. Prog. 2014, 33, 64−69. (36) Klein, E.; Lukeš, V. J. Phys. Chem. A 2006, 110, 12312−12320. (37) Naito, M.; Radcliffe, C.; Wada, Y.; Hoshino, T.; Xiongmin, L.; Arai, M.; Tamura, M. J. Loss Prev. Process Ind. 2005, 18, 469−473.
studied reactions. This material 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) Robertson, R. J. Soc. Chem. Ind., London 1933, 52, 274. (2) Morgan, G. T.; Pickard, R. H. J. Soc. Chem. Ind., London 1936, 55, 421. (3) Davies, A. G. J. R. Inst. Chem. 1956, 386−389. (4) Kelly, R. J. Chem. Health Saf. 1997, 3, 28−36. (5) Clark, D. E. Chem. Health Saf. 2001, 8, 12−22. (6) Lemarquand, J.; Triolet, J. Cahier de note documentaires − Hygiène et sécurité du travail 2002, 186, 17−27. (7) Zabarnick, S. Ind. Eng. Chem. Res. 1993, 32, 1012−1017. (8) Zabarnick, S. Energy Fuels 1998, 12, 547−553. (9) Kuprowicz, N. J.; Zabarnick, S.; West, Z. J.; Ervin, J. S. Energy Fuels 2007, 21, 530−544. (10) Dunn, R. O. Fuel Process. Technol. 2005, 86, 1071−1085. (11) Knothe, G. Fuel Process. Technol. 2007, 88, 669−677. (12) Dunn, R. O. Biofuels, Bioprod. Biorefin. 2008, 2, 304−318. (13) Denisov, E. T., Afanas’ev, I. B. Oxidation and Antioxidants in Organic Chemistry and Biology; Taylor & Francis: Boca Raton, FL, 2005. (14) Zabarnick, S.; Phelps, D. K. Energy Fuels 2006, 20, 488−497. (15) Chiodo, S. G.; Leopoldini, M.; Russo, N.; Toscano, M. Phys. Chem. Chem. Phys. 2010, 12, 7662−7670. (16) Leopoldini, M.; Chiodo, S. G.; Russo, N.; Toscano, M. J. Chem. Theory Comput. 2011, 7, 4218−4233. (17) Leopoldini, M.; Russo, N.; Toscano, M. Food Chem. 2011, 125, 288−306. (18) Iuga, C.; Alvarez-Idaboy, J. R.; Russo, N. J. Org. Chem. 2012, 77, 3868−3877. (19) Di Tommaso, S.; Rotureau, P.; Crescenzi, O.; Adamo, C. Phys. Chem. Chem. Phys. 2011, 13, 14636−14645. (20) Hamstead, A. C.; VanDelinder, L. S. J. Chem. Eng. Data 1960, 5, 383−386. (21) Waddington, D. J. Proc. R. Soc. London, Ser. A 1962, 265, 436− 446. (22) Ingold, K. U. Chem. Rev. 1961, 61, 563−589. (23) Bach, R. D.; Dmitrenko, O. J. Phys. Chem. B 2003, 107, 12851− 12861. (24) Lemay, A.; Ouellet, C. Can. J. Chem. 1957, 35, 124−130. (25) ASTM D 2274: Oxidation Stability of Distillate Fuel Oil (Accelerated Method); American Society for Testing and Materials: West Conshohocken, PA, 2003. (26) ASTM D 4625: Test Method for Distillate Fuel Storage Stability at 43°C (110°F); American Society for Testing and Materials: West Conshohocken, PA, 2009. (27) EN 14112: Fat and oil derivatives - Fatty Acid Methyl Esters (FAME) - Determination of oxidation stability (accelerated oxidation test); European Committee for standardization: Brussels, Belgium, 2003. (28) EN 14214: Automotive fuels - Fatty acid methyl esters (FAME) for diesel engines - Requirements and test methods; European Committee for standardization: Brussels, Belgium, 2008. (29) Becke, A. D. J. Chem. Phys. 1993, 98, 1372−1377. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr..; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; 2829
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