Catalysis of Cumyl Hydroperoxide Decomposition by Calcium Salts and Complexes: A Kinetic Experiment and Quantum Chemical Modeling

Article abstract of DOI:10.1134/S0023158419060089

Abstract: The kinetic and thermodynamic parameters of the decomposition of cumyl hydroperoxide in a chlorobenzene medium in the presence of calcium 2-ethylhexanoate and a complex of CaCl₂ with crown ether were experimentally obtained. The forma

Full text of DOI:10.1134/S0023158419060089

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 6, pp. 796–801. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Kinetika i Kataliz, 2019, Vol. 60, No. 6, pp. 776–781.
Catalysis of Cumyl Hydroperoxide Decomposition by Calcium
Salts and Complexes: A Kinetic Experiment
and Quantum Chemical Modeling
a,
a
a
a
a
N. M. Nurullina *, N. N. Batyrshin , Yu. Kh. Usmanova , A. N. Masliy , and Kh. E. Kharlampidi
a
Kazan National Research Technological University, Kazan, Tatarstan, 420015 Russia
*е-mail: nyryllina@mail.ru
Received March 11, 2019; revised July 2, 2019; accepted July 9, 2019
Abstract—The kinetic and thermodynamic parameters of the decomposition of cumyl hydroperoxide in a
chlorobenzene medium in the presence of calcium 2-ethylhexanoate and a complex of CaCl with crown
2
ether were experimentally obtained. The formation of intermediate complexes in the catalyst–hydroperoxide
system was kinetically established. The formation of intermediate complexes was confirmed using quantum
chemical modeling, their structure was optimized, and the thermodynamic characteristics of reactions with
their participation were found. A mechanism of the stepwise decomposition of the intermediate was pro-
posed.
Keywords: cumyl hydroperoxide, catalytic decomposition, organic calcium salts, dibenzo-18-crown-6, com-
plexation, quantum chemical modeling
DOI: 10.1134/S0023158419060089
INTRODUCTION
This allowed us to study in detail the decomposi-
tion of tert-butyl hydroperoxide in the presence of the
chloride complexes of IIa subgroup metals [8].
In a developed process of the oxidation of alkylar-
omatic hydrocarbons by molecular oxygen, transition
metal compounds noticeably accelerate the process by
decomposing a small hydroperoxide portion [1–3]. A
disadvantage of the use of these compounds is their
low solubility in alkylaromatic hydrocarbons and a
tendency to form colloidal solutions [1].
This article presents the results of a study of the cat-
alytic decomposition of cumyl hydroperoxide (CHP)
in the presence of calcium 2-ethylhexanoate
(
Ca(EH) ) and a complex of calcium chloride with
2
dibenzo-18-crown-6 (CaCl ∙ DBC).
2
The need to compare the catalytic activity of vari-
ous metals under identical conditions and to relate it to
the atomic properties of metals has led us to a search
for a solubilizing ligand that is equally suitable for all
EXPERIMENTAL
To avoid solvent entrainment, the catalytic decom-
nontransition metals. The residue of 2-ethylhexanoic position of CHP was studied using an ampoule
acid was found as such a ligand for IIb subgroup metals method [8] at [CHP] = 0.1–1.5 mol/L and [Cat] =
0
0
[
4]. However, it was found that the ethyl hexanoates of
–3
(
0.5–7) × 10 mol/L over a temperature range of
not all metals of interest are soluble in chlorobenzene,
a standard solvent for studying the decomposition of
hydroperoxides. A more universal ligand, crown ether,
was found, the main advantage of which is the ability
to transfer salts initially insoluble in an organic
medium into solution [5–7].
1
00–130°C. The hydroperoxide concentration was
monitored by an iodometric method [9]. Kinetic
curves were approximated by a cubic polynomial and
differentiated with respect to time. The initial rates of
decay were determined as the derivative as τ → 0.
Cumyl hydroperoxide was purified through a
sodium salt [10]. The concentration of CHP was
Abbreviations: CHP, cumyl hydroperoxide; Ca(EH) , calcium
2
6
.98 mol/L (99.8%). The solvent (chlorobenzene) was
2
-ethylhexanoate; CaCl ∙ DBC, a complex of calcium chloride
2
with dibenzo-18-crown-6; PBE, density functional theory purified by an acid–base method [11]. The procedure
method; [CHP] , the initial concentration of CHP; [Cat] , the used for the preparation of 2-ethylhexanoates was
0
0
initial catalyst concentration; n, the number of hydroperoxide
molecules bound in a complex; K, the equilibrium constant of
complex formation; k, the rate constant of decomposition of the
described in detail earlier [12]. Unlike naphthenates
and stearates, ethylhexanoates are not prone to self-
association, and they are homogeneous in organic
complex; w , the overall rate of decay of CHP; w, the rate of
0
CHP decay; w , the maximum rate of CHP decay obtained solutions over wide ranges of concentrations and tem-
max
experimentally; E , the activation energy of decomposition.
peratures.
a
796

CATALYSIS OF CUMYL HYDROPEROXIDE DECOMPOSITION
797
1
80
50
20
The formal kinetic reaction scheme of hydroperox-
ide decomposition can be represented as follows:
1
К
⎯ ⎯⎯
1
⎯
⎯ ⎯→
nCHP + Cat
[nCHP −Cat],
(I)
←
1
k
[
nCHP−Cat] ⎯ ⎯ ⎯→ Cat + products,
(II)
9
0
0
0
where n is the number of hydroperoxide molecules
bound in a complex; K is the equilibrium constant of
complex formation, and k is the rate constant of
decomposition of the complex.
6
3
2
The processing of the formal kinetic reaction
scheme in a quasi-equilibrium approximation led to
the Michaelis–Menten equation for the overall rate of
0
0.5
1.0
1.5
2.0
[
CHP], mol/L
decay of CHP (w ):
0
Fig. 1. Dependence of the rate of decay of cumyl hydrop-
eroxide on the concentration of CHP in the presence of
n
0
–3
kK[Cat] [CHP]
1 + K[CHP]0
different catalysts ([Cat] = 5 × 10 mol/L; T = 130°C;
0
0
w0 =
.
(1)
n
chlorobenzene): (1) CaCl ⋅ DBC and (2) Ca(EH) .
2
2
Equation (1) shows that the rate of decomposition
is a hyperbolic function of hydroperoxide concentra-
tion, and it tends to a limiting value.
The quantum chemical calculations were per-
formed with the Priroda program suite [13] using the
PBE density functional theory method [14]. The atomic
To determine the stoichiometry of the complexes,
orbitals were described using the 6-31G(d,p) basis set we used the following equation [17]:
[
15]. All of the geometric parameters were optimized;

1
1 

1

thereafter, the frequencies of normal vibrations were cal-
culated. The absence of imaginary modes indicated the
achievement of a minimum energy. The enthalpy,
entropy, and Gibbs free energy values for the test com-
pounds were calculated using a standard thermochemi-
cal procedure in the Priroda program suite.
ln
−
= ln 
[ ]
− n × ln CHP , (2)
0

w


w
wmaxK 
max

where w is the rate of hydroperoxide decay, and w
=
max
k × [Cat] is the maximum rate of decay of CHP mea-
0
sured experimentally.
The numbers of CHP molecules included in the
intermediate complexes were determined from the
slopes of the corresponding straight lines in the coor-
RESULTS AND DISCUSSION
dinates of Eq. (4). It was found that Ca(EH) retains only
2
one CHP molecule. The complex with CaCl ∙ DBC
2
As can be seen in Fig. 1, calcium compounds acceler-
ate the decomposition of the test hydroperoxide. The rate
of decay tends to a limiting value as the hydroperoxide
contains two hydroperoxide molecules (Fig. 2).
In the formation of organic hydroperoxide com-
plexes with group II metal salts, hydrogen bonds at the
oxygen atoms of the acid residue can be formed. Theoret-
ically, up to four hydroperoxide molecules can be
attached to one calcium carboxylate molecule. However,
concentration [CHP] is increased. In the concentration
0
range presented, the curve characterizing the rate of the
process in the presence of CaCl ∙ DBC is S-shaped, and
2
it first reaches a plateau. The flattening out (Fig. 1) is a based on the above kinetic data, we can assume that, on
kinetic sign of the formation of an intermediate hydrop- the formation of organic hydroperoxide complexes with
eroxide complex with the catalyst [16].
group II metal salts, a hydrogen bond of cumyl hydrop-
eroxide with oxygen of the acid residue is initially formed;
The nearly first order of reactions with respect to the then, a six-membered ring is closed with the formation of
catalyst in the test systems was determined from the lin- a metal–oxygen bond. As a result, the 1 : 1 complex of
earity of plots in the ln (w )–ln ([Cat] ) coordinates.
Ca(EH) with CHP is formed (Scheme 1):
2
0
0
RO
O
H
O
O
O
C
O
O
O
O
R1
Ca
R + ROOH
1
R1
Ca
products
O
R1
Scheme 1.
KINETICS AND CATALYSIS Vol. 60 No. 6 2019

7
98
NURULLINA et al.
The formation of complexes with crown ethers is
7
RT
4.28 179.78
2
2
ln K =
−
(L mol )
(7)
facilitated by the possibility of the formation of
numerous hydrogen bonds with the crown atoms of an
ether and coordination bonds with the central metal
atom.
R
for CaCl ∙ DBC.
2
Note that the complexation of calcium com-
pounds, as well as of other group II metals, is an exo-
thermic reaction [16]; therefore, equilibrium is shifted
in the opposite direction with increasing temperature,
and the concentration of the complex decreases.
Hydroperoxide molecules are bound in the com-
plex with the oxygen atoms of the catalyst by hydrogen
bonds. For all of the test compounds, the heat of com-
plexation significantly exceeds the strength of hydro-
gen bonds in hydroperoxides (8–13 kJ/mol). Conse-
quently, an oxygen atom of the hydroperoxide O–O
group is also coordinated to the central metal atom.
The kinetic parameters of overall decay (k, K) were
determined by the linearization of Eq. (1) according to
the Lineweaver–Burk method [18]:
1
w
1
1
1
=
+
×
.
(3)
n
wmax wmaxK
CHP]0
[
Then, the activation parameters of decay were
determined from the temperature dependence of the
rate constant of decomposition of the complex.
Below are given the Arrhenius expressions obtained
for the rate constants in a chlorobenzene medium:
The formation of hydrogen bonds weakens the O–O
bond in the hydroperoxide and facilitates its homolytic
cleavage. Note that, for all of the catalysts under con-
3
1
0
 81.55 ×10  −1
k = 1.82 ×10 × exp −
(s )

(4)
(5)



RT
sideration, the activation energy E of decomposition
a
for Ca(EH) and
2
is much lower than the O–O bond strength in hydrop-
3
eroxides (167 kJ/mol) [19], and this decrease in E
a
1
5
 125.26 ×10  −1
k = 5.44 ×10 × exp −
(s )



cannot be explained only by the appearance of hydro-
gen bonds. This fact is additional evidence of the coor-
dination of CHP with the metal.

RT
for CaCl ∙ DBC.
2
The values of the thermodynamic characteristics of
Hydroperoxide decomposition in the presence of a
the complexation reaction are presented in the forms crown ether catalyst proceeds sequentially; a complex
of the corresponding van’t Hoff equations:
chain of decomposition reactions and the formation of
complexes of different stoichiometry occurs in the sys-
(6) tem, and it can be expressed by the following reaction
scheme (Scheme 2) (the formation of products is not
shown for simplicity):
6
RT
5.09 172.37
ln K =
−
(L mol)
R
for Ca(EH) and
2
+
ROOH
k1
k2
2ROOH Cat
ROOH Cat
Cat
+
ROOH
k is the rate constant of decay of the 2ROOH‒Cat complex; k is the rate constant of decay of the ROOH‒Cat complex.
1
2
Scheme 2.
It can be seen in Scheme 2 that, in the case of using catalyst complexes with hydroperoxide (intermedi-
the crown ether catalyst, two complexes of different ates).
stoichiometry, ROOH–CaCl ⋅ DBC and 2ROOH–
2
Figure 3 shows the optimized geometric structure
CaCl ⋅ DBC, can decompose simultaneously to lead
2
of the intermediate complex CHP–Ca(EH) . As
2
to higher observed values of the rate constants of decay
expected, a six-membered ring is formed with the par-
ticipation of a calcium ion. The correspondence of this
(
Fig. 1) than those in the catalysis by Ca(EH) .
2
In the course of quantum chemical calculations, intermediate to reaction (I) was verified by calculating
we primarily optimized the structures of the reagents: the internal reaction coordinate (IRC). The calcula-
the molecules of CHP, Ca(EH) , and CaCl ∙ DBC. tions in both descent directions led to the structures of
2
2
Based on an analysis of the structures of these com- reagents and reaction products, respectively, which
pounds, we proposed and modeled the structures of were identical to those optimized previously.
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CATALYSIS OF CUMYL HYDROPEROXIDE DECOMPOSITION
799
ln(w–1 – w–1
)
max
1
2.2
2
y = –2.20x + 7.38 11.2
2
R = 0.98
1
10.2
9.2
y = –1.22x + 6.76
8.2
2
R = 0.96
1.499
1.305
7
.2
2
.425
1
.152
2
.380
6.5
2
.378
2.322
–
3.0 –2.5 –2.0 –1.5 –1.0 –0.5
0
0.5
ln[CHP]
Fig. 2. Determination of the stoichiometry of the nCHP–
Cat complexes: (1) Ca(EH) and (2) CaCl ⋅ DBC.
2
2
Figure 4 shows an optimal conformation of the
intermediate complex formed with the participation of
CHP and a catalyst containing crown ether. It is inter-
esting to note that, here, the hydroperoxide molecules
are directly bound to the metal atom through their
hydroxyl oxygen atoms without forming additional
hydrogen bonds and cyclic structures. In this case, one
CHP molecule is attached on the side of crown oxygen
atoms and the other molecule, on the back side of the
crown ether, where hydrogen bonds cannot be formed.
Fig. 3. Optimized geometric structure of the intermediate
complex CHP–Ca(EH) .
2
Using special calculations, we found that the crown
ether itself retains two CHP molecules on the side of
crown oxygen atoms in a hydrogen-bound complex.
Thus, the introduction of an ion of calcium (or, prob-
ably, any other metal) into the internal cavity of the
crown ether creates steric hindrances to the formation
of hydrogen bonds between the hydroperoxide and the
oxygen atoms of the crown ether due to a large size of
the central ion and the branched molecular structure
of CHP.
According to the quantum chemical calculations,
changes in the enthalpy of reaction of the formation of
an intermediate complex (reaction (I)) were negative
for all of the catalysts, and this is consistent with the
experimental data. Thus, the calculated enthalpies of
formation were –117.88 kJ/mol for the CHP–
Fig. 4. Optimized structure of the complex 2CHP–CaCl ⋅
2
DBC.
Ca(EH) complex and –103.40 kJ/mol for the
2
2
CHP–CaCl · DBC complex.
2
decomposition process. The discrepancy between the
thermodynamic potentials of the main stages of the
process under study in a gas phase and in chloroben-
zene was 3–7%.
Taking into account the effect of solvation, in the
framework of this study, we obtained the values of ΔG
s
for the solvation of the reaction mixture components.
Based on these values, we found a change in the Gibbs
free energy taking into account the solvation of mole-
cules in chlorobenzene. Table 1 shows the values of ΔG
for complex formation and decomposition reactions
The decomposition of the intermediate complex
nROOH–Cat proceeded with the formation of radi-
cals and then the molecules of CHP decomposition
products and the restoration of the initial catalyst
(
for calcium 2-ethylhexanoate).
Thus, the solvent (chlorobenzene) did not signifi- structure. The lifetime of the intermediate complex,
cantly affect the course of the main stages of the the reaction paths of its decomposition, and the acti-
KINETICS AND CATALYSIS Vol. 60 No. 6 2019

8
00
NURULLINA et al.
Table 1. Comparison of the Gibbs free energies of the main reactions in a gas phase and in a solvent
Parameter
Reaction conditions
Reaction
ΔG of the system in a gas
ΔG of the system in
phase, kJ/mol
chlorobenzene, kJ/mol
i
i
No catalyst
Ca(EH)2
160.41
158.40
–60.60
219.00
ROOH
→ RO + ОН
ROOH + Cat ꢀ ROOH–Cat
–64.79
i
i
225.20
ROOH–Cat → RO + ОН + Cat
Table 2. Thermodynamic characteristics of the main stages of CHP decomposition
Parameter
Reaction
ΔН, kJ/mol
ΔG, kJ/mol
–57.87
ROOH + Cat ꢀ [ROOH−Cat]
–117.88
i
i
3
27.71
31.10
6.61
02.55
25.16
225.20
[
ROOH−Cat] → RO + OH + Cat
i
i
2
185.11
40.09
135.70
89.50
[
[
ROOH−Cat] → [RO ⋅Cat] + OH
i
i
9
RO ⋅ Cat] → RO + Cat
i
i
2
[
ROOH−Cat] → [OH ⋅ Cat] + RO
i
i
1
[
OH ⋅ Cat] → OH + Cat
vation energy of hydroperoxide decomposition diate complex with catalyst regeneration requires
depend on many factors: the nature of the catalyst, the much more energy than the sequential one (Table 2).
concentrations of components, the nature of the sol-
Based on the enthalpies calculated for each partic-
vent, the conditions of the experiment, etc.
ular stage in the studied catalytic systems, we found
We have optimized various structures of decay
that reaction (VI) is a rate-limiting stage. The subse-
i
i
products, including the catalyst and RO and OH rad- quent stage of decay requires less energy. The values of
icals.
ΔH for the limiting stages of decomposition in the
presence of Ca(EH) and CaCl ∙ DBC are 202.50 and
As a test reaction, we considered the primary act of
hydroperoxide decomposition
2
2
117.88 kJ/mol, respectively.
i
i
nROOH−Cat → nRO + nOH + Cat.
(III)
CONCLUSIONS
Obviously, the decay proceeded stepwise because
the process requires high energy consumption. More-
Thus, based on an analysis of the data obtained in
over, this mechanism is valid for both of the catalysts two different ways (experimental and theoretical), we
(ethylhexanoate and crown ether–metal complex):
can note that the formation of activated intermediate
complexes is possible and energetically favorable.
Both methods confirm the catalytic properties of the
test catalytic systems: calcium compounds accelerate
the decomposition of CHP by reducing the activation
energy of O–O bond cleavage in the hydroperoxide.
ROOH + Cat ꢀ [ROOH−Cat],
(IV)
i
i
[
ROOH−Cat] → [OH ⋅Cat] + RO ,
(V)
i
i
[
OH ⋅Cat] → OH + Cat.
(VI)
The results of the quantum chemical calculations
confirm the proposed mechanism of intermediate for-
mation.
In this reaction scheme, the first stage is complex
formation, the second is O–O bond cleavage with the
•
release of an RO radical, and the third is catalyst
regeneration with the release of the hydroxyl radical
In the course of a theoretical study, we determined
into the volume. According to the results of the quan- a stepwise mechanism of the formation and decay of
tum chemical calculations, the simultaneous elimina- the intermediate and a rate-limiting stage of CHP
•
•
tion of both RO and OH radicals from the interme- decomposition.
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CATALYSIS OF CUMYL HYDROPEROXIDE DECOMPOSITION
801
FUNDING
10. Belyaev, V.A. and Nemtsov, M.S., Zh. Org. Khim.,
1
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1. Tsepalov, V.F. and Shlyapintokh, V.Ya., Dokl. Akad.
Nauk SSSR, 1959, vol. 124, no. 4, p. 883.
This work was supported by the Ministry of Education
and Science of the Russian Federation in the framework of
a state contract no. 10.956.2017 (PNIL 25.17).
1
1
2. Nurullina, N.M., Batyrshin, N.N., and Kharlampi-
di, Kh.E., Pet. Chem., 2004, vol. 44, no. 6, p. 441.
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Translated by Valentin Makhlyarchuk
KINETICS AND CATALYSIS Vol. 60 No. 6 2019