Theoretical study on reaction mechanism of thermal decomposition of dialkyl peroxides

Accepted Manuscript Theoretical study on reaction mechanism of thermal decomposition of dialkyl peroxides Yuqing Ni, Yong Pan, Jiandu Zhang, Xiaochen Gu, Xuhai Pan, Juncheng Jiang, Qingsheng Wang PII: DOI: Reference:

S2210-271X(18)30167-1 https://doi.org/10.1016/j.comptc.2018.05.007 COMPTC 2793

To appear in:

Computational & Theoretical Chemistry

Received Date: Revised Date: Accepted Date:

5 April 2018 7 May 2018 8 May 2018

Please cite this article as: Y. Ni, Y. Pan, J. Zhang, X. Gu, X. Pan, J. Jiang, Q. Wang, Theoretical study on reaction mechanism of thermal decomposition of dialkyl peroxides, Computational & Theoretical Chemistry (2018), doi: https://doi.org/10.1016/j.comptc.2018.05.007

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Theoretical study on reaction mechanism of thermal decomposition of dialkyl peroxides Yuqing Nia, Yong Pana,*, Jiandu Zhanga, Xiaochen Gua, Xuhai Pana, Juncheng Jiangb and Qingsheng Wangc

a

College of Safety Science and Engineering, Nanjing Tech University, Nanjing 210009, China

b

Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control, Nanjing Tech University,

Nanjing 210009, China c

Department of Fire Protection and Safety, Oklahoma State University, OK 74078, USA

* Corresponding author: Yong Pan, Tel: +86-25-83587305, Fax: +86-25-83587411, Email: [email protected]

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ABSTRACT

Dialkyl peroxides are hazardous chemicals that can easily decompose, potentially causing fire or explosion. The thermal decomposition process of dialkyl peroxides were studied in this study. The initial decomposition steps of ten different dialkyl peroxides were investigated by using density functional theory. It is observed that there are similar structures in the transient state of decomposition reactions, indicating that the decomposition mechanisms are similar. A comprehensive decomposition mechanism was proposed, which could be reasonably employed to explain the transformation of dialkyl peroxides in the decomposition. Reduced density gradient (RDG), electron density (ρ), and the sign of the second Hessian eigenvalue (Sign(λ2)) were used to visualize the weak interaction in peroxide molecules, respectively. The results showed that the weak interaction would affect the decomposition pathways of peroxides. Laplacian bond orders (LBO) in the structure of the intrinsic reaction coordinate points were calculated to reflect the variation of bond strength in two initial decomposition pathways. The results showed that the weak O-O bond smoothly breaks during the two essential pathways. This study could provide some guidance for understanding the incident-causing process of the thermal decomposition of dialkyl peroxides.

Keywords: Dialkyl peroxides; Thermal decomposition; Reaction mechanism; Density functional theory

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1. Introduction Organic peroxides are commonly used as curing agents, free radical initiators, oxidants, and catalysts [1]. Examples of the most popular types of organic peroxides are dialkyl peroxides, alkyl hydroperoxide, diacyl peroxide, peroxy ester, and ketone peroxide [2]. The decomposition of organic peroxide has attracted more interest recently [3,6] as along with other unstable chemicals. Dialkyl peroxides are molecules with the characteristic peroxy bonds connected to diverse alkyl groups. For the weakness of peroxy bonds (-O-O-), three types of incidents, including explosion, fire, and release of hazardous materials, are easily caused by dialkyl peroxides mishandled [7]. Consequently, it is important to recognize the mechanism of thermal runaway reactions of dialkyl peroxides for safe storage, transportation, and operation of chemical process units. Thermal or reactive hazards rating for di-tert-butyl peroxide (DTBP), one of the most commonly used industrial organic peroxides, is studied in previous papers [8,11]. Duh, Y.-S et, al. [10] performed an overview in the field of chemical kinetics on the thermal decomposition of gaseous DTBP, neat DTBP, and DTBP solution. They determined the activation energy of DTBP in the alkyl or aromatic solvent by DSC and adiabatic calorimeters, with an averaged value of 157.0 (±4.1) and 159.7 (±3.9) kJ mol-1, respectively. Then, they also proposed mechanisms of thermal decomposition for DTBP in a gaseous state, neat and in organic solvents, shown in scheme 1~3, respectively. The cleavage of O-O bond that will produce two tert-butoxys is considered to be the only initial step of decomposition mechanism for DTBP in the previous work [12,14].

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Figure 1. Scheme 1, 2, and 3 are the decomposition mechanism for DTBP in gas state, liquid state, and organic solvents respectively. In this study, ten specific dialkyl peroxides are investigated to identify similar decomposition pathways among their thermal runaway reactions. Then, thirty transient state structures are proposed, and reaction energy barrier and the peroxy bond dissociation enthalpies are calculated by Gaussian [15]. The homolytic cleavage of peroxy bond, which is always considered the only initial decomposition pathway for the peroxides for decades, is the most favorable step, compared with the other possible initial thermal runaway channels proposed in this paper. However, the other probable initial reactions, such as the pathway producing the hydrogen and aldehydes (or ketones), cannot be neglected especially when there are H adjacent to alpha Cs. The effect of weak interaction on the decomposition reaction and the variable of peroxides in two special pathways are investigated in this work. The motivation of this research is to find a comprehensive reaction mechanism for the dialkyl peroxides, which can be extrapolated to

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understand the hazardous materials incidents inherently caused by peroxides in storage and transportation. 2. Computational Details There are ten various dialkyl peroxides in this study including di-methyl peroxide (DMP), methyl ethyl peroxide (MEP), methyl isopropyl peroxide (MIPP), methyl tert-butyl peroxide (MTBP), di-ethyl peroxide (DEP), ethyl isopropyl peroxide (EIPP), ethyl tert-butyl peroxide (ETBP), di-isopropyl peroxide (DIPP), isopropyl tert-butyl peroxide (IPTBP), and DTBP. All initial decomposition steps are calculated by Gaussian 09 with the B3LYP functional, which is the abbreviation of the functional named Becke 3 Lee, Yang, and Parr [16], and 6-311+G** basis set [17,18]. Structures of peroxides are optimized to be minimal without imaginary frequencies. However, transient state structures are confirmed as saddle points that have only one imaginary frequency apiece. Then, the bond dissociation energy (BDE) of peroxy bonds are measured at B3LYP/6-311+G** (L1), M06-2X [19] /def2-TZVP [20,21] //B3LYP/6-311+G** (L2), and CBS-QB3 [22] (L3) level respectively. RDG function is calculated to distinguish the weak interaction region from other regions in the peroxide molecules and Sign(λ 2)ρ is applied to differentiate different kinds of weak interaction by Multiwfn [23]. LBO, which is defined as a scaled integral of negative parts of the Laplacian of electron density in fuzzy overlap space [24], in intrinsic reaction paths is also calculated by Multiwfn.

5

6

Figure 2. Initial decomposition pathways on the potential energy surface of the ten dialkyl peroxides. Transient structures for the decomposition pathways as optimized at L1 level. 3. Results and Discussion The decomposition pathways are shown in Table 1 with the activation barrier and enthalpy of reactions. Then the transient structures corresponding to the pathways are established in Figure 2.

Figure 3. The four-center structure in transition states

Figure 4. The six-cyclic structure in transition states

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Table 1. Initial decomposition steps of the ten dialkyl peroxides with thermodynamic parameters calculated at L2 level. Pathways 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Reactions CH3O-OCH3  CH3OH + CH2O CH3O-OCH3  CH3OCH2OH CH3O-OCH3  H2 + 2 CH2O CH3CH2O-OCH3  CH3CH2OH + CH2O CH3CH2O-OCH3  CH3OH + CH3CHO CH3CH2O-OCH3  CH3CH2OCH2OH CH3CH2O-OCH3  H2 + CH2O + CH3CHO (CH3)2CHO-OCH3  CH3OH + (CH3)2CO (CH3)2CHO-OCH3  (CH3)2CHOCH2OH (CH3)2CHO-OCH3  (CH3)2CHOH + CH2O (CH3)2CHO-OCH3  H2 + CH2O +(CH3)2CO (CH3)3CO-OCH3  (CH3)3COH + CH2O (CH3)3CO-OCH3 (CH3)3COCH2OH (CH3)3CO-OCH3  CH3OH + CH3COCH2CH3 (CH3)3CO-OCH3  CH3OCH3 + (CH3)2CO (CH3)3CO-OCH3  (CH3)2C(OCH3)2 CH3CH2O-OCH2CH3  CH3CH2OH + CH3CHO CH3CH2O-OCH2CH3  H2 + 2 CH3CHO (CH3)2CHO-OCH2CH3  (CH3)2CHOH + CH3CHO (CH3)2CHO-OCH2CH3  CH3CH2OH + (CH3)2CO (CH3)2CHO-OCH2CH3  H2 + CH3CHO + (CH3)2CO (CH3)3CO-OCH2CH3  (CH3)3COH + CH3CHO (CH3)3CO-OCH2CH3  CH3CH2OH + CH3COCH2CH3 (CH3)3CO-OCH2CH3  CH3CH2OCH3 + (CH3)2CO (CH3)2CHO-OCH(CH3)2  (CH3)2CO + (CH3)2CHOH (CH3)2CHO-OC(CH3)3  (CH3)3COH + (CH3)2CO (CH3)2CHO-OC(CH3)3  (CH3)2CHOH + CH3COCH2CH3 (CH3)2CHO-OC(CH3)3  (CH3)2C(OCH3)OCH(CH3)2 (CH3)3CO-OC(CH3)3  (CH3)3COCH3 + (CH3)2CO (CH3)3CO-OC(CH3)3  (CH3)3COC(CH3)2OCH3

a

Total enthalpy change of reaction (kcal mol-1).

b

Energy barrier in the forward direction (kcal mol-1).

ΔH298a -54.2 -58.1 -39.9 -54.6 -60.7 -60.2 -46.3 -65.2 -57.5 -54.8 -50.8 -56.8 -59.2 -63.8 -56.7 -48.6 -61.3 -53.0 -60.8 -65.1 -56.8 -61.5 -62.4 -55.1 -65.5 -66.6 -63.2 -46.4 -53.9 -40.8

ΔE298b 57.1 78.2 45.4 57.3 56.7 78.1 44.6 56.7 78.5 57.3 45.0 55.7 76.3 79.5 84.3 74.5 56.4 43.8 56.0 57.4 44.8 57.0 81.3 86.3 57.6 56.9 86.0 75.3 85.9 76.2

3.1 Initial decomposition steps As shown in Figure 2, it can be found that there is a four-center transition state structure (S1) illustrated in Figure 3 and a six-cyclic transition state structure (S2) as shown in Figure 4 in the transient structures for the pyrogenic decomposition of peroxides. The four-center structure can be found in twelve transient structures, while the six-cyclic structure can be found in five transient structures. The S1 and S2 are almost planar, and their structural information is given in

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Table 2 and Table 3. The S1 and S2 structures in different transient structures are so similar that these two specific structures are considered essential in the pyrolysis of dialkyl peroxides. Table 2. Structure information of the four-center transition states (bond lengths are in Å, angles are in °)

TS 1 TS 4 TS 5 TS 8 TS 10 TS 12 TS 17 TS 19 TS 20 TS 22 TS 25 TS 26

B1 1.990 2.001 1.993 1.996 2.006 2.006 2.003 2.004 2.002 2.009 2.006 2.010

B2 1.304 1.303 1.307 1.311 1.303 1.303 1.306 1.311 1.307 1.306 1.311 1.311

B3 1.228 1.229 1.221 1.219 1.231 1.232 1.222 1.215 1.219 1.224 1.217 1.218

A1 86.49 86.30 87.25 87.89 86.07 86.11 87.08 87.87 87.23 86.86 87.81 87.61

A2 96.56 96.53 95.84 94.61 96.40 96.36 95.84 94.93 96.03 95.69 94.80 94.80

Table 3. Structure information of the six-cyclic transition state (bond lengths are in Å, angles are in °) B1 TS3 1.283 TS7 1.287 TS11 1.296 TS18 1.293 TS21 1.299

B2 2.003 1.995 1.987 1.986 1.982

B3 1.283 1.287 1.291 1.293 1.294

B4 1.263 1.252 1.239 1.238 1.235

B5 1.263 1.255 1.238 1.238 1.231

A1 112.3 111.8 113.6 112.7 112.6

A2 112.3 111.6 112.2 112.7 111.8

A3 107.5 107.6 107.6 106.0 106.0

A4 107.5 106.0 104.5 106.0 104.6

The S1 and S2 structures suggest what the production would be. As shown in Figure 2, the S1 implies that the outcome would be the alcohol and aldehyde (or ketone) while the S2 produces the hydrogen and aldehyde (or ketone). As expected, the similar S1 structure also indicates similar energy barrier. However, the activation barrier in Table 1 reveals that the lighter alkoxy fragment can more easily lose an H forming the corresponding alcohol than the heavier one by

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comparing the activation energy of TS4 and TS5, TS8 and TS10, or TS19 and TS20. Analogously, the analogical six-cyclic structure suggests the same level reaction energy barrier. It is also confirmed that the symmetrical structures are more stable by comparing the activation energy barrier of TS3, TS7, and TS11, or TS18 and TS21. Compared with the reaction barrier of the S1 and S2 for the same peroxide, the S2 implies lower activation energy and a greater risk of fire and explosion because of the hydrogen. There is also another similar special pathway, the isomerization of O-OCH3 to an O-CH2OH fragment, as shown in pathways 2, 6, 9, and 13, However, the energy barrier of the isomerization reaction is much higher than the other thermal decomposition pathways for DMP, MEP, MIPP, and MTBP, respectively. It also means that the isomerization reaction is unlikely to occur, although the outcome has a low flash point. Congruously, MTBP, ETBP, IPTBP, and DTBP have similar thermal decomposition pathways, the isomerization of O-OC(CH3)3 as shown in path 15, 24, 28, and 30. The isomerization of O-OC(CH3)3 can be considered the same as isomerization of O-OCH3 where the H is substituted by CH3. However, it is also the replacement leading to another reaction on MTBP, ETBP, and IPTBP, as shown in reaction (1). R-O-O-C(CH3)3  ROH + CH3COCH2CH3

(1)

3.2 Mechanism According to all thermal decomposition pathways observed above, there are 2 kinds of pathways for the dialkyl peroxides, where the S1 and S2 can be found in its transient structure respectively while the two alpha Cs are adjacent to the H. Reactions are shown as (2) and (3). R1-O-O-R2  R1`=O + R2OH

(2)

R1-O-O-R2  R1`=O +R2`=O + H2

(3)

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Moreover, while CH3 fragment is connected with O-O group, reaction (4), an isomerization of O-O-CH3 to O-CH2OH, occurs. R1-O-O-CH3  R1-O-CH2OH

(4)

While only one alpha C is connected with H, the reaction (2) with the S1 is still one possible pathway for the dialkyl peroxides. Besides, there are also 3 pathways for the peroxides including reaction (1) and the two following. Whereas, if there is no H linked to the alpha C, reactions (5) and (6) could occur on the peroxides, exactly as it did on DTBP in this research. R-O-O-C(CH3)3  R-O-C(CH3)2OCH3

(5)

R-O-O-C(CH3)3  R-O-CH3 + CH3COCH3

(6)

At last, the thermal disintegration reaction forming two alkoxy radicals is the initial step in most cases for the weak bond of O-O, which has no obvious transient structure. The BDE of peroxy bonds in the 10 peroxides researched here are calculated at L1, L2, and L3, respectively, as shown in Table 4. It indicates that the calculated values at L3 level are much more fitted to the experimental ones than L1 and L2. The BDE values of the ten dialkyl peroxides are approximated and the average value is 41.9 Kcal mol-1 at L3 level. The low BDEs also verify that the weakness of O-O bond is the unstable source for the thermolysis of peroxides. Table 4. The energies for O-O bond fission on theory and experiment.

Calculated (kcal mol-1) DMP MEP MIPP MTBP DEP

L1 27.0 26.2 27.5 27.5 25.4

L2 35.9 35.6 36.9 37.5 35.3

L3 40.2 40.8 41.6 41.8 40.8

Experimental (kcal mol-1) 40±1.5a 39.7±1.3a

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EIPP ETBP DIPP IPTBP DTBP a

26.7 26.0 27.9 27.3 26.3

36.7 36.8 38.0 38.4 38.0

41.6 42.0 42.9 43.6 43.2

37.7[25] 38.0±1[26]

These data are available from internet Bond-energy Datbank (iBOND).

3.3 RDG and isosurface analysis Based on the mechanism illustrated above, it reveals that the fragment adjacent to O-O group is the essential fact for the thermal decomposition of dialkyl peroxides. Therefore, only DEP, DMP, DIPP, and DTBP are studied in this section. For the rules proposed above, there should be a six-cyclic transition state in the decomposition of DIPP. Then the steric hindrance may be a reasonable excuse for this phenomenon. Visual study of weak interaction by reduced density gradient (RDG) function, which is also known as the noncovalent interaction method [27], is applied to investigate the weak interaction in these four peroxides. The RDG function is defined as

r

ρ r (

)

ρ(r)

(7)

Sign(λ )ρ and RDG function are calculated as the X-ray and Y-ray respectively, and the result is established via scatter graph in Figure 5 using GNUPLOT [28]. Obviously, there is a special weak interaction region in DIPP and DTBP that cannot be found in DMP and DEP. In Figure 5, the low-density, low-gradient spike for the DIPP and DTBP is approaching zero, indicative of weak interaction. Then, the color-filled graph method is applied to visualize the isosurface of RDG=0.5 inner peroxide molecules using VMD [29]. As shown in Figure 6, there is Van der Waals interaction and the steric effect in DIPP and DTBP molecules, which is different from DMP and DEP. Consequently, it is the Van der Waals interaction and steric effect involving the

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decomposition pathways of peroxide. In other words, the Van der Waals interaction and steric effect prevent the reaction (3) on DIPP and reaction (1) on DTBP.

Figure 5. Plots of the

versus the ρ multiplied by sign(λ2). The a, b, c, and d is for DMP,

DEP, DIPP, and DTBP, respectively. The data were obtained at the L1 level.

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Figure 6. Visual study of the isosurface of RDG=0.5. The a, b, c, and d is for DMP, DEP, DIPP, and DTBP, respectively. 3.4 Laplacian bond order As shown above, except for the rupture of O-O bond, thermal decomposition pathways with S1 and S2 are the ones we are most interested in for the lower reaction barrier and their universality. In this study, Multiwfn is utilized to calculate LBOs in S1 and S2, which are formed in the decomposition pathways of DMP for all IRC points. Because of the symmetry of the S2 structure, only part of its LBOs are established in Figure 7. There is some similarity between the two pathways. Clearly, the C1O5 becomes stronger and stronger as the reaction proceeds. The value of LBO gradually increases from 0.37 to 1.22, which means a new double bond forms. However, the C1H3 diminution from 0.83 to 0.0 indicates that there is no chemical bond for the production. The O5O6 decreases from 0.09 to

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0.0, reflecting the instability of O-O bond, because the LBO has a good correlation with bonding strength [24]. However, there is also some distinction between path 1 and path 3 that S1 will produce alcohol, while S2 structure will form H2. As shown in Figure 7, A new H3O6 single bond is

-10 1C3H 1C5O 3H6O 5O6O Relative Energy

1.0

LBO

0.8

-20 -30 -40 -50

0.6

-60 0.4

-70 -80

0.2

-90

0.0

-100 -80

-60

-40

-20

0

20

IRC points

40

60

80

0

1.2

-10

1.0 0.8

1C3H 1C5O 3H8H 5O6O Relative Energy

0.6

-20 -30

0.4

-40

0.2

-50

0.0 80

60

40

20

0

-20

-40

-60

Relative Energy (Kcal mol-1)

0 1.2

Relative Energy (Kcal mol-1) LBO

formed in path 1. It is unlikely that a single H3H8 bond is generated in path 3.

-60 -80

IRC points

Figure 7. Variation of the LBO values for all IRC points in path 1 and path 3. The data were calculated at the L1 level. 4. Conclusions

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A new comprehensive mechanism is established for the thermal decomposition of dialkyl peroxides. In this paper, we propose that the initial thermal decomposition steps of dialkyl peroxides are not limited to the fission of O-O bond. The reaction energy barrier illustrates that the pathways with the S1 and S2 should not be neglected when there is H adjacent to the alpha C. It is also found that similar pathways have similar activation energy barrier, and the symmetric peroxides are more stable than the asymmetric. The BDE of O-O bond at L2 is more fitted to the previous experimental work. By comparing the comprehensive mechanism proposed in this paper with the results of the visual study, it is found that the existence of weak interaction in molecules would affect the decomposition pathways of dialkyl peroxides. Also, the visual study reveals that the weak interaction is the cause for the reaction (3) not occurring on DIPP. The calculated LBOs for all IRC points in path 1 and 3 elucidate the variation and the distinction between these two crucial steps. The distinction shows that path 3 has a lower reaction energy barrier than path 1, and this could be verified in other pathways with S2 and S1 in different dialkyl peroxides. Acknowledgement This research was supported by National Program on Key Basic Research Project of China (No. 2017YFC0804801, 2016YFC0800100), National Natural Science Fund of China (No. 21576136, 21436006), and the Graduate Education Innovation Project of Jiangsu Province (No. SJCX17_0272). We are grateful to the High-Performance Computing Center of Nanjing Tech University for supporting the computational resources. Declarations of interest: none

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Figure captions Figure 1. Scheme 1, 2, and 3 are the decomposition mechanism for DTBP in gas state, liquid state, and organic solvents respectively. Figure 2. Initial decomposition pathways on the potential energy surface of the ten dialkyl peroxides. Transient structures for the decomposition pathways as optimized at L1 level. Figure 3. The four-center structure in transition states Figure 4. The six-cyclic structure in transition states Figure 5. Plots of the

versus the ρ multiplied by sign(λ2). The a, b, c, and d is for DMP,

DEP, DIPP, and DTBP, respectively. The data were obtained at the L1 level. Figure 6. Visual study of the isosurface of RDG=0.5. The a, b, c, and d is for DMP, DEP, DIPP, and DTBP, respectively. Figure 7. Variation of the LBO values for all IRC points in path 1 and path 3. The data were calculated at the L1 level.

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HIGHLIGHTS 

New themolysis steps for dialkyl peroxides are proposed, and two of them are vital.



A comprehensive mechanism is established for the pyrolysis of dialkyl peroxides.



The stronger the weak interaction is, the fewer pyrolysis paths the peroxides have.



The activation energy of similar pyrolysis paths is similar for dialkyl peroxides.

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Graphical Abstract

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