Experimental study and theoretical analysis on decomposition mechanism of
benzoyl peroxide
Jiayu Lv, Wanghua Chen *, Liping Chen, Haisu Gao, Yingtao Tian, Xin Sun
Dept. of Safety Eng., Nanjing University of Science and Technology,
Nanjing, Jiangsu, 210094, China
Abstract
Benzoyl peroxide (BPO), a source of radical used as initiator, is an important member in the organic
peroxide family. To master its thermal characters, experimental study and theoretical analysis were
employed on studying its decomposition mechanism. The autocatalytic process, which was deduced
from dynamic experimental data by differential scanning calorimeter (DSC), was testified by
isothermal DSC tests. Thermal scanning unit (TSU) was taken to predict pressure hazard of BPO
decomposition and accelerating rate calorimeter (ARC) tests were conducted to investigate the thermal
behavior of BPO in adiabatic condition. The high pressure and fast pressure release rate indicated a
high possibility of accompanied pressure hazard once the runaway decomposition was trigged.
Combined with experimental results, quantum chemistry method was used to locate the most possible
reaction path and calculate thermodynamic energies of BPO molecule in gas phase. The calculated total
released heat during the whole decomposition was in good accordance with the values tested by
thermal analysis instruments. And bond dissociation enthalpy (BDE) obtained at different theory levels
provided a comparable value in runaway possibility analysis.
Keywords: Benzoyl peroxide; Micro-calorimeter; Autocatalysis; Thermal analysis; Quantum chemistry
1. Introduction
Benzoyl peroxide (BPO) is widely used in fine chemistry industry as initiator because of its special
structure. The free radicals provided by peroxy group (-O-O-) in BPO play a crucial role in chemical
transformations. Nevertheless, like a double-edged sword, thermal instability and explosion property
were also brought [1,2]. Many countries have suffered accidents due to BPO’s high sensitivities to heat,
shock, impact and friction [3].
BPO decomposed with strong exothermic reaction and its kinetics was sensitive to temperature: not
detectable at ambient temperature, but as soon as the temperature rises, the decomposition rate can
quickly become important. Thus, the follow-up of such reactions seems to be practically impossible in
calorimetric reactions because of the energy released during its realization [4]. Although several studies
[5-8] have been done to detect the thermal behavior of BPO, most of them were focused on the usual
kinetic methods which were based on nth reaction, or presented the experimental results without further
mechanism analysis. Besides, papers which applied quantum chemistry method to calculate thermal
ability of organic peroxide were less common. This paper tried to work out a comprehensive thermal
evaluation of BPO. Differential scanning calorimeter (DSC), thermal scanning unit (TSU) and
accelerating rate calorimeter (ARC) were employed to track the temperature histories and violent
pressure during BPO’s decomposition, testing under dynamic, isothermal and adiabatic condition
respectively. Thermal kinetic parameters such as activation energy Ea and frequency factor A were
calculated.
*
Corresponding author. Tel.: 0086-025-84315526-8005
E-mail address: [email protected] (Prof. Chen)
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Based on quantum chemistry, the optimized structures of BPO and its corresponding radicals were
obtained using density-functional method (DFT) B3LYP and B3PW91 with the 6-31G(d) basis set.
After analyzing related energies of the assumptive intermediates, we pointed out the most possible
reaction route and presented overall reaction path to study the reaction mechanism. Then, peroxy
group’s gas-phase bond dissociation enthalpy BDE was calculated.
2. Experimental
2.1. Dynamic and isothermal modes by DSC
BPO, with a purity of 98%, was provided by Shanghai Lingfeng Chemical reagent co., LTD. DSC
(DSC1, Mettler Toledo) conducted tests both in dynamic and isothermal modes with extra pure
nitrogen purging (30mLmin-1). Stainless steel high pressure crucibles were used for the experiment.
Sample masses in this work were around 2mg.
Fig. 1 illustrated the heat flow versus temperature of BPO decomposition at various heating rate β (1,
2, 4 and 8 ºCmin-1) at the range of 30-200ºC. A sudden temperature rise appeared just following with a
sharp endothermic peak due to fusion of BPO, which meant the decomposition of BPO overlapped
with its fusion in dynamic mode. Because of this, the exothermic onset temperature T0 was unable to
accurately determine. However, the maximum temperature Tp could be obtained directly by DSC
dynamic scanning tests [9] and the heat of decomposition ΔH was calculated by DSC analysis software
STARe, showed in Fig.1. And with the increase of β, the value of ΔH added except 8ºCmin-1, which
might due to stronger endothermic reaction during BPO’s fusion at a quick forced heating.
Fig. 1 Heat flow during DSC experiments on BPO at various heating rates
To estimate kinetic parameters, Kissinger method [10], presented as the following equation, was
chosen.
ln =
( βi Tpi2 ) ln AR Ea − Ea RTpi (i=1,2,…,6)
(1)
Where β is the heating rate, Tp is the temperature maximum, A is the frequency factor, Ea is the
activation energy of the reaction, and R is the gas constant.
By plotting of ln(βi/T2pi) vs. 1/T pi (β=1, 2, 4 ºCmin-1 were chosen), which was expected to be a
straight line, Ea and lnA were calculated as 637kJmol-1 and 204s-1 with a correlation coefficient value
of 0.9995 (Fig. 2).
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Fig. 2 Estimated Ea for BPO by Kissinger’s method
The Ea mentioned above was apparent activation energy. For Ea between 220 and 1000kJmol-1, the
decompositions were of autocatalytic nature [11]. These types of reactions required our special
attention and should be clearly distinguished from nth order reactions because of their high hazards.
The high values of steepness of the exothermal peak in Fig. 1 also indicated the great probability that
this decomposition reaction was autocatalytic. Considering isothermal measurements were the most
reliable way to detect and characterize an autocatalytic decomposition [12], we showed the thermal
profiles of BPO in isothermal mode DSC in Fig. 3. Temperatures were set at 96ºC, 94ºC, 92ºC, 90ºC
and 88ºC.
Fig. 3 BPO decomposition in the isothermal condition
In Fig. 3, the heat flow passed through a maximum exothermic peak after an induction period θ and
then decreased again. And the higher the isothermal temperature was, the shorter the θ was gained. The
corresponding θ were 17min, 30min, 55min, 100min and 180min respectively.
Fig. 3 also indicated that as the temperature increased, the exothermic peak became sharper and had
a poorer symmetry. At 88 and 90ºC, the DSC curves were symmetrical and identified as bell-shaped
curves, which confirmed that BPO decomposes in autocatalytic mechanism. From 92ºC, the curve
symmetry decreased gradually because heat released in exothermic reaction was covered by
decalescence in the beginning of reaction. From this phenomenon, we deduced that the fusion point of
BPO was approximately from 90 to 92ºC.
To estimate isothermal kinetic parameters of BPO, we employed the Prout-Tompkins model [13]:
k
A + B 
→ 2B
with −rA =
−dC A dt =⋅
k C A ⋅ CB
(2)
Where r is reaction rate, k is constant of reaction rate, t is time and C is concentration.
In isothermal condition, r is proportional to C of the product. So the rate equation can express as a
function of conversion α:
dα dt =kC A0α（）
1- α
(3)
After integral, function below can be get:
ln α (1-α ) = kt + c
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(4)
Where c is constant.
By inserting the k values calculated in different temperatures into Arrhenius’ equation, Ea and A was
revealed as 191kJmol-1 and 4.18E25s-1 (Fig. 4).
Fig. 4 Kinetic parameters calculated with P-T model
2.2. Thermal scanning by TSU
DSC tests have presented a heat release details of BPO’s decomposition and pointed out its
autocatalytic property. To understand both temperature and pressure histories during decomposition,
0.4g sample was added into a 316ss test bomb (8mL) for TSU (HEL in UK) scanning, and the ramp
rate was 0.5°Cmin-1. Temperature and pressure recorded was drawn in Fig. 5.
Fig. 5 Temperature and pressure histories of BPO by TSU
After an initial delay due to “thermal lagging” effects, the sample temperature was found to follow
the oven ramp at the same rate with a slight offset. To get the exact T0 value, we used iQ software to
reduce the raw data and calculate dT/dt value, presented in Fig. 6. At 90°C, dT/dt started to drift from 0.
The maximum dT/dt was above 220°Cmin-1, and high pressure presented by TSU indicated that
failures during manufacture or transportation might give rise to a pressure hazard and more attention
should be paid.
Fig.6 Temperature rate vs. temperature of BPO by TSU
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2.3. Adiabatic test in ARC
Considering the case of an adiabatic runaway, test was further conducted by esARC (THT. UK). The
start temperature was 70°C based on results above and temperature step was 3°C. Temperature rate
sensitivity was set 0.02°C·min-1. Sample mass was 0.32g. Relationship between temperature, pressure
and time was displayed in Fig. 7. For better analyzing, the temperature and pressure history during
exothermal was zoomed in into Fig. 8.
Fig. 7 BPO decomposition in adiabatic condition
Fig. 8 Temperature and pressure of BPO’s decomposition during exothermal
T0 of this decomposition was obtained as 94.3°C, and the rises of temperature and pressure were
38°C and 7bar respectively. The maximum value of dT/dt and dT/dt tested by ARC were above
150°C·min-1 and 16bar·min-1 (Fig. 9). Released heat after thermal inertia correction was 1120J·g-1.
Fig. 9 Temperature and pressure rate vs. temperature of BPO by ARC
Within the tested temperature range, BPO had only one exothermic curve. The temperature remained
stable during the induction period and suddenly increased sharply, which was different from the case of
nth order reactions (dash line in Fig. 8). This was due to the fact that BPO’s autocatalytic reaction
initially shows no or only a small heat release (with an onset temperature rate of 0.089°C·min-1). As
stated by Manfred A. Bohn [14], the steeper the slope of the self-heat rate is the greater the part of
autocatalytic decomposition, decomposition process of BPO showed a high autocatalytic feature from
ARC test. Whilst, the delay of temperature increase leaded a later detection of runaway reaction until
the reaction rate became sufficiently fast. If a temperature alarm was set as a design of emergency
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measure [15], it was not effective because there was no time left to take measures due to the sharp
temperature increase.
3. Theoretical study
3.1. Computational details
Density-functional quantum mechanical method, B3LYP [16] and B3PW91 [17,18] were employed
to calculate the thermal energies for hemolytic cleavage of bonds which may break during BPO’s
decomposition, with a basis set 6-31G(d) [19]. After that, two more basis sets and one more quantum
chemical method were designated to calculate the O-O bond dissociation energy BDE(O-O) based on
the molecular structures optimized with B3LYP/6-31G(d). The calculations were performed with the
Gaussian 03 program package [20].
3.2. Design of reaction path
Although for most organic peroxides, the decomposition reactions were regarded as taking up from
the break of peroxy bond, the existence of benzoxy radical was controversial [21,22]. In this study, the
reaction mechanism of BPO decomposition in gas phase is described below:
1
→ 2(C6 H 5CO)O ⋅
(C6 H 5CO)OO(COC6 H 5 ) 
IM1
2
→ (C6 H 5CO)OOCO ⋅+ ⋅ C6 H 5
(C6 H 5CO)OO(COC6 H 5 ) 
IM2
IM5
3
→ (C6 H 5CO)OO ⋅+ C6 H 5 CO ⋅
(C6 H 5CO)OO(COC6 H 5 ) 
IM3
IM4
3.3. Results and discussion
Geometries of the parent molecules and the corresponding radical (noted as IM1, IM2, Im3, IM4 and
IM5) have been optimized at the theory levels, followed by a frequency calculation. To confirm that
stationary points were minimum energy structures, vibration frequency was analyzed. Table 1 showed
calculated energies E of the five structures, and the lower E values obtained by B3LYP illustrated that
this method level gave more stable structures, presented in Fig. 10.
Table 1
Calculated energies of the five structures (Hartree)
BPO
IM1
IM2
IM3
IM4
IM5
B3LYP/6-31G(d)
-840.3435
-420.1517
-608.7362
-495.2962
-344.9206
-231.5613
B3PW91/6-31G(d)
-840.0171
-419.9878
-608.4948
-495.1032
-344.7850
-231.4719
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Fig. 10 B3LYP/6-31G(d) optimized structures (Bond distances in Å, bond angle in °)
In Fig. 10, the bond distances R and bond angle A were partly marked. For BPO, the experimental
R(4,5) and R(1,2) were 1.45Å and 1.54Å [23], which proved the creditability of our calculated results.
Fig.11 showed that the R, A and symmetry of IM1 to IM4 differed with these of BPO, which had a high
symmetrical characteristic. Actually, advantages brought by optimized structures to study the properties
of materials were more than this. For instance, dihedral angles D of atoms illustrated the positional
relation. In BPO, the calculated values of D(3,2,4,5), D(9,1,2,4) and D(2,4,5,6) were 0.0001, 179.9994
and -179.9503°, which meant these atoms were almost coplane.
To confirm the most possible reaction mechanism of BPO, the changed thermodynamic values
between resultants and reactants were listed in Table 2. Equations used were showed as follows [24]:
∆E0 = ∆Eelec + ∆ZPE
∆E = ∆E0 + ∆Evib + ∆Erot + ∆Etrans
∆H = ∆E + ∆RT
Where E0 is original energy, Eelec is electronic energy, ZPE is zero-point energy, E is thermal energy,
Evib is vibrational energy, Erot is rotational energy, Etrans is translation energy and H is thermal enthalpy.
Table 2
Thermodynamic calculations of the three reaction paths (298K, kJ/mol)
Path
B3LYP/6-31G(d)
B3PW91/6-31G(d)
1
2
3
4
5
6
ΔE0
91.3359
100.4438
318.3944
17.2653
-455.4087
-329.5423
ΔE
89.1357
103.1244
317.2260
17.1340
-457.2124
-333.8087
ΔH
91.6142
105.6029
319.7045
19.6125
-459.6909
-328.8518
ΔE0
94.1084
110.3261
323.8423
16.1600
-457.0287
-330.6003
ΔE
92.0422
109.2024
322.7396
25.9788
-458.7903
-334.7906
ΔH
94.5206
111.6809
325.2181
18.4573
-461.2688
-329.8337
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Table 2 displayed that the change of thermodynamics parameters were in good consistency by both
methods, and values of ΔE0, ΔE and ΔH were sequenced as path1 < path2 < path3. Thinking of this in
thermodynamic way, path1 was the easiest reaction path for BPO’s decomposition. H. Fischer [25]
described the NMR diagram when BPO decomposed in cyclohexanone, and the absorption spectrum of
benzene was obtained. According to this, the reaction mechanism followed with path1 could be
composed as:
4
(C6 H 5CO)O ⋅ 
→⋅C6 H 5 +CO 2
5
2 ⋅ C6 H 5 
→ C6 H 5 C6 H 5
Therefore, the overall reaction path of decomposition reaction of BPO was concluded below:
6
(C6 H 5CO)OO(COC6 H 5 ) 
→ C6 H 5C6 H 5 +2CO 2
The related energies were superadded to Table 2. Negative values listed in Table 2 illustrated that
reaction path5, generation of biphenyl, was heat generating reaction. And the total heat released during
the decomposition was calculated to be 333.8 and 334.8 kJ·mol-1 using the two different methods, equal
to 1378 and 1382J·g-1, which provided good convenience in dealing with hazardous evaluation of
dangerous substances due to its approximate heat data compared to calorimetry tests.
Table 3 were BDE(O-O) values under different methods and basis sets. It was shown that with the
increase of basis set, the BDE had a tendency of diminution. Limited by the hardware, higher basis sets
were abandoned. However, due to the dipole and Π-Π affection in BPO molecule, polarization should
be included. If considering the same basis set, B3LYP theory level was more accurate than MP2
method.
Table 3
BDE of O-O calculated with different methods (298K, kJ/mol)
BDE
B3LYP/6-31G(d)
B3LYP/6-31+G(d)
B3LYP/6-311G(d,p)
MP2/6-31+G(d)
105.2826
94.5180
89.7921
119.1977
4. Conclusions
The hazards of heat release and pressure generation were predicted by three micro-calorimeters
under dynamic, isothermal and adiabatic modes. In dynamic DSC measurements, an endothermic
process precedes the exothermic decomposition of BPO, which hindered the T0 capture. By analyzing
kinetic parameters using Kissinger method, we found this conversion affected method was not suitable
for BPO’s autocatalytic decomposition. Therefore, based on isothermal results, Ea and A was revealed
as 191kJmol-1 and 4.18E25s-1 by P-T model and the fusion point of BPO was deduced approximately
from 90 to 92ºC. T0 obtained by TSU and ARC was 90 and 94.3°C, meanwhile, heat release rates and
pressure generation observed in these experiments showed high possibility of thermal runaway.
By applying quantum chemistry method on further mechanism study, the most possible reaction path
of BPO’s decomposition was O-O bond dissociated into benzoxy radical according to calculation by
Gaussian 03 software package. The total released heat was in good accordance between experimental
calorimetry and theoretical results. BDE(O-O) was calculated at B3LYP theory level and MP2 level
with three different basis sets, and B3LYP method had a more accurate values.
References
[1] L. Jiayu, W. Shuiai, C. Wanghua, C. Gufeng, C. Liping, Adv. Materials Research, 550-553 (2012) 2782-2785.
[2] L. Jiayu, C. Wanghua, C. Liping, T. Yingtao, S. Xin, Procedia Engineering, 43 (2012) 312-317.
[3] W. Guanwei, S. Qimin, Thermal hazard evaluation of benzoyl peroxide, National Yunlin University of Science
8/9
and Technology, Taiwan, 2005.
[4] I. Ben Talouba, L. Balland, N. Mouhab, M.A. Abdelghani-Idrissi, J. Loss Prevent. Process Ind. 24 (2011)
391-396.
[5] Z. Fan, X. Chuanxin, G. Jing, Chem. Engineer, 1 (2010) 23-26.
[6] Lu K T, Chen T C, Hu K H, J. Hazardous Materials, 161 (1) (2009) 246-256.
[7] L. Xinrui, H. Koseki, Thermochimica Acta, 403 (2005) 113–116.
[8] F. Zaman, Beezer A E, Mitchell J C, International J. Phamaceutics, 227 (1-2) (2001) 133-137.
[9] Z. Hai, X. Zhiming, G. Pengjiang, H. Rongzu, G. Shengli, N. Binke, F. Yan, S. Qizhen, L. Rong, J. Hazardous
Materials, A94 (2002) 205-210.
[10] Kissinger H E. Anal, Chem., 29 (11) (1957) 1702-1706.
[11] Leila Bou-Diab, Hans Fierz, J. Hazardous Materials, 93 (2002) 137-146.
[12] Francis Stoessel. C.Wanghua, P. Jinhua, C. Liping, tran, Thermal Safety of Chemical Process: Risk
Assessment and Process Design, Science Press, Beijing, China, 2009, 270-282.
[13] Michael E. Brown, Beverley D. Glass, J. Pharma., 190 (1999) 129-137.
[14] Manfred A. Bohn, Heike Pontius. ICT. 57 (2012) 1-40.
[15] J. M. Dien, H. Fierz, F. Stoessel, G. Kille, Chimia, 48 (1994) 542.
[16] A.D. Becke, J. Chem. Phys.98 (1993) 5648.
[17] J.P. Perdew, K.Burke, Y. Wang, Phys. Rev. B 54 (1996) 16533.
[18] Z. Xiulin, C. Wanghua, L. Jiacong, K. Jinlin, J. Molecular Structure, 810 (2007) 47-51.
[19] A.D. McLean, G.S. Chandler, J. Chem. Phys. 72 (1980) 5639.
[20] M. J. Frisch, G. W. Trucks, H. B. Schlegel, et.al, Gaussian, Inc., Pittsburgh PA, 2003.
[21] W. Qihua, S. Yuxiang, Z. Yaoxing, Introductory theory of organic reaction mechanism, Higher Education
Press, Beijing, China, 1991.
[22] Q. Muge, Explosion reaction mechanism and identification study of organic peroxides, Beijing institute of
technology, 2004.
[23] Y. Yunbin, X. Tao, G. Yingmin, Handbook of Chemistry and physics, Shanghai Scientific and Technical
Publishers, Shanghai, China, 1985, 200-202.
[24] E. Frisch, Michael J. Frisch, Gary W. Trucks, Gaussian 03 help, 2003.
[25] H. Lin, Reactive intermediates, Higher Education Press, Beijing, China, 1990, 115-125.
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