Catalytic Systems Based on Magnesium and Zinc Compounds in the Oxidation Reactions of Alkylarenes and the Decomposition Reactions of the Corresponding Hydroperoxides

Article abstract of DOI:10.1134/S0023158418030096

The oxidation of aromatic hydrocarbons in the presence of magnesium and zinc 2-ethylhexanoates and a mixed catalyst based on these compounds is studied. It is shown that magnesium and zinc carboxylates are active catalytic systems which catalyze the decomposition of hydroperoxides representing the primary alkylarene oxidation products alongside with activation of oxygen.

Full text of DOI:10.1134/S0023158418030096

ISSN 0023-1584, Kinetics and Catalysis, 2018, Vol. 59, No. 3, pp. 328–332. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © Kh.E. Kharlampidi, N.M. Nurullina, N.N. Batyrshin, Yu.Kh. Usmanova, 2018, published in Kinetika i Kataliz, 2018, Vol. 59, No. 3, pp. 335–339.
Catalytic Systems Based on Magnesium and Zinc Compounds
in the Oxidation Reactions of Alkylarenes and the Decomposition
Reactions of the Corresponding Hydroperoxides
Kh. E. Kharlampidia, N. M. Nurullinaa, *, N. N. Batyrshina, and Yu. Kh. Usmanovaa
aKazan National Research Technological University, Kazan, Tatarstan, 420015 Russia
*e-mail: nyryllina@mail.ru
Received September 29, 2017
Abstract⎯The oxidation of aromatic hydrocarbons in the presence of magnesium and zinc 2-ethylhexa-
noates and a mixed catalyst based on these compounds is studied. It is shown that magnesium and zinc car-
boxylates are active catalytic systems which catalyze the decomposition of hydroperoxides representing the
primary alkylarene oxidation products alongside with activation of oxygen.
Keywords: catalytic oxidation, isopropylbenzene, ethylbenzene, magnesium and zinc 2-ethylhexanoates,
hydroperoxide
DOI: 10.1134/S0023158418030096
INTRODUCTION
opportunities for improving the conditions of oxida-
tive processes. As shown by our studies, 2-ethylhexa-
noates of metals from Subgroup IIa and IIb of the
Periodic System are highly efficient catalysts of the
oxidation reactions [6–9].
In the submitted work, the catalytic activity of
magnesium and zinc 2-ethylhexanoates and a mixed
binary catalyst based on these compounds was studied
in the processes of both liquid-phase oxidation of
alkylaromatic hydrocarbons to hydroperoxides (for
the example of ethylbenzene and isopropylbenzene)
and the decomposition of the formed hydroperoxides.
Liquid-phase oxidation processes play a leading
part in large-capacity petrochemical synthesis, as they
can give a broad spectrum of products and the initial
compounds for the production of polymers. There are
reliable sources of feedstocks for their implementa-
tion, and the oxidizing agent (air oxygen) is readily
available and cheap. Industrially, aromatic hydrocar-
bons are oxidized at a high level of conversion, but
with a low degree of selectivity; thus, the search and
creation of new catalytic systems for their oxidation
under mild conditions remain a topical problem.
An significant contribution to the development of
the theory and practice of the liquid-phase oxidation
of organic compounds has been made by Soviet and
Russian scientists [1–3]. In the 1970s–1980s, it was
proposed to use nontransition metal compounds as
homogeneous catalysts for the gentle initiation of oxi-
dation processes [4, 5]. Salts of organic acids (stea-
rates, naphthenates, resinates, etc.) are most often
used for this purpose. They are cheap, available, and
easily soluble in aromatic hydrocarbons, and the
methods of their synthesis are quite simple.
EXPERIMENTAL
Magnesium (Мg(EG)2) and zinc (Zn(EG)2)
2-ethylhexanoates were synthesized using the method
detailed in [10] by the reaction between 2-ethylhexa-
noic acid (chemically pure grade) and magnesium or
zinc oxide (chemically pure grade) in boiling benzene.
The water formed during the reaction was separated
out in a Dean–Stark trap. A small (1–2%) excess of
the oxide was used to ensure the complete involvement
This work is devoted to the development of new of the acid in the reaction. After the separation of the
catalytic systems for the synthesis of hydroperoxides excess oxide and the vacuum evaporation of benzene,
based on mixtures of organic acids of nontransition magnesium 2-ethylhexanoate in the form of a white
metals. Such systems have a significant potential to solid substance or zinc 2-ethylhexanoate in the form
influence the path and rate of a reaction, and this is of a semitransparent light-yellow paste were obtained.
attained by varying the quantitative ratios between the The content of magnesium and zinc 2-ethylhexa-
elements inside a system and replacing these elements noates determined in the resulting preparations by the
by others. This appreciably facilitates the control for complexometric method [9] was 99.44 and 99.62 wt %,
the properties of such synergic systems and opens great respectively.
328

CATALYTIC SYSTEMS BASED ON MAGNESIUM AND ZINC COMPOUNDS
329
The ethylbenzene (EB) and isopropylbenzene
(IPB) was oxidized in a glass bubble reactor at the
atmospheric pressure and a temperature of 110–120°C
in a kinetic regime. The oxidation time was 6 h, and
the air flow rate was 0.3 L/min. The catalyst was intro-
duced into the substrate at concentrations ranging
from 0.001 to 0.01 mol/L. Samples for the iodometric
estimation of the concentration of IBP and EB
hydroperoxides were taken in the course of the exper-
iment. The oxidation products were analyzed by gas-
liquid chromatography on a Crystal chromatograph
(Chromatec-Crystal, Russia; capillary column with a
polar phase; polyethylenglycol was used as a carrier).
[IPBHP], wt %
20
16
12
8
1
2
3
4
4
The catalytic decomposition of hydroperoxides
ROOH was studied by the ampoule method [11] at
[ROOH]0 ranging from 0.1 to 2.5 mol/L within the
temperature range of 100–120°C. The synthesized
samples of hydroperoxides were purified through a
sodium salt [12]. The finished product contained
6.98 mol/L (99.8%) of IPB hydroperoxide (IPBHP)
and 7.70 mol/L (98.9%) of EB hydroperoxide
(EBHP). The solvent used in the decomposition reac-
tion was benzene chloride (chemically pure grade).
0
1
2
3
4
5
6
Reaction time, h
Fig. 1. Kinetic IPBHP accumulation curves at 110°C in pres-
ence of different catalysts: (1) [Мg(EG) ] = 5 × 10 mol/L +
–3
2
–3
[Zn(EG) ] = 5 × 10 mol/L, (2) [Мg(EG) ] = 5 ×
2
2
–3
–3
10 mol/L, (3) [Zn(EG) ] = 5 × 10 mol/L, (4) self-
2
oxidation.
RESULTS AND DISCUSSION
ing 5 × 10–3 mol/L of Mg(EG)2 and 5 × 10–3 mol/L of
Zn(EG)2 (Table 1).
The main contribution to the catalytic activity of
the studied compounds is made by magnesium 2-eth-
The data on the accumulation of IPB hydroperox-
ide during the oxidation of isopropylbenzene with air
oxygen in the presence of magnesium and zinc 2-eth-
ylhexanoates are shown in Fig. 1. It can be seen that
the addition of the magnesium salt appreciably
increases the IPB hydroperoxide accumulation rate in
comparison with self-oxidation (curve 1).
Table 1. Accumulation of ROOH in presence of different
catalysts during reaction for 6 h*
[IPBHP],
wt %
(110°C)
[EBHP],
wt %
(120°C)
[CT],
mol/L
From Table 1 it follows that isopropylbenzene is
oxidized faster than ethylbenzene in the presence of
5 × 10–3 mol/L of Mg(EG)2 even at a lower tempera-
ture. Thus, the reaction performed for 6 h leads to the
accumulation of 19.7% of IPB hydroperoxide (at
110°C) and only 4.8% of EB hydroperoxide (at
120°C). This can be comprehensively explained by the
relatively low strength of the Н–С tertiary bond in the
α-position to the benzene ring. As already established
long ago, the reactivity of the Н–С bond of hydrocar-
bons decreases in the following order: tertiary > sec-
ondary > primary [13].
Catalyst
Self-oxidation
Mg(EG)2
Mg(EG)2
Mg(EG)2
Mg(EG)2
Zn(EG)2
Absent
0.001
0.003
0.005
0.010
0.005
0.010
0.54
3.14
0.84
2.36
3.38
4.83
6.80
–
10.85
19.71
24.29
5.05
Zn(EG)2
6.48
–
Mg(EG)2
Zn(EG)2
0.001
0.005
It is known [1] that the noncatalytic oxidation of
aromatic hydrocarbons occurs with self-acceleration
and is characterized by a certain induction period.
Magnesium and zinc compounds reduce this period
for the IPB and EB oxidation reaction. The curves of
ROOH accumulation in the presence of Mg-, Zn-,
and binary catalysts have an S-like shape typical for
chain degenerate branching oxidation reactions. The
hydroperoxide accumulation rate grows with an
increase in the catalyst concentration. The maximum
hydroperoxide yield and accumulation rate are
observed in the presence of a mixed catalyst contain-
7.05
5.71
–
–
–
–
Mg(EG)2
Zn(EG)2
0.001
0.010
Mg(EG)2
Zn(EG)2
0.005
0.005
18.44
18.43
Mg(EG)2
Zn(EG)2
0.005
0.010
* Dashes mean that EBHP is not formed in presence of corre-
sponding catalysts.
KINETICS AND CATALYSIS Vol. 59 No. 3 2018

330
KHARLAMPIDI et al.
Table 2. Kinetic characteristics of catalytic IPBHP decomposition*
wmax × 104,
–ΔH,
kJ/mol
–ΔS,
J/(mol K)
k0, s–1
k × 102, s–1
Ea, kJ/mol
Catalyst Т, °C
K, L/mol
mol/(L s)
(1.82 0.21) × 108
(5.37 0.27) × 1015
(1.08 0.06) × 109
Zn(EG)2
Mg(EG)2
100
110
1.13 0.02 3.43 0.04 0.56 0.01 77.8 3.9
4.17 0.02 1.09 0.02 2.09 0.02 122.8 6.1
74.2 3.7 191.5 8.7
96.8 4.8 258.8 13.0
78.5 3.9 188.3 9.4
Mg(EG)2 +
Zn(EG)2
120 2.79 0.05 0.72 0.02 1.40 0.02 73.8 3.7
–3
* Reaction was performed in benzene chloride medium. Initial catalyst and IPBHP concentrations were 5 × 10 mol/L and 0 to
2.5 mol/L, respectively.
ylhexanoate. It is also interesting that the hydroperox- value of ΔН, the ROOH–Mg(EG)2 intermediate is
ide yield decreases with an increase in the Zn(EG)2
the strongest one. The comparison of the formation
concentration from 0.005 to 0.01 mol/L at fixed con- enthalpies for the complexes with the participation of
tents [Мg(EG)2] = 0.001 mol/L and [Мg(EG)2] = Mg(EG)2, Zn(EG)2 and a mixed catalyst (Table 2) also
0.005 mol/L (Table 1). This may be provoked by the indicates the appearance of a more ordered structure.
inhibiting effect of phenol formed during the decom-
position of IPB hydroperoxide.
The enthalpy of complexation with the participa-
tion of all the studied metals appreciably exceeds the
strength of the hydrogen bonds in the self- and het-
eroassociates of hydroperoxides (8–13 kJ/mol) [15].
Hence, one of the oxygen atoms of the hydroperoxide
O–O group is also coordinated to the central metal
atom. Let us note that the decomposition activation
energy (Ea) for all the considered catalysts is much
lower than the hydroperoxide О–О bond strength
equal to 163–200 kJ/mol [1]. This is another sign,
alongside with the dependence of the decomposition
rate on the nature of the metal, in support of the coor-
dination of IPB hydroperoxide with the metal.
Since the catalytic effect of the studied compounds
is associated with the partial decomposition of
hydroperoxide and the appearance of free radicals,
which participate in the continuation of the chains, it
seemed necessary to estimate the activity of different
catalysts in the hydroperoxide decomposition reac-
tion, and this was accomplished using IPB hydroper-
oxide as an example. When studying the catalytic
properties of magnesium and zinc 2-ethylhexanoates
and the mixed catalyst in the IPB decomposition reac-
tions by the kinetic method, we established that this
results in an intermediate activated hydroperoxide–
catalyst complex with a molar composition of 1 : 1.
The concentration dependences of the initial rate of
the catalytic decomposition of hydroperoxide are
characterized by tending to a limit. The formation of
the IPB hydroperoxide–Mg(EG)2 and IPB hydroper-
oxide–Zn(EG)2 intermediate activated complexes is
confirmed by the results of the quantum-mechanical
calculations performed with the Priroda 11 and
Gaussian 09 software suites. Thus, the intermediate
complex formation enthalpy is −114.2 kJ/mol for
ROOH–Mg(EG)2 and −22.6 kJ/mol for ROOH–
Zn(EG)2.
Hence, it may be considered proven that individual
zinc and magnesium compounds and their mixtures
can catalyze the decomposition of organic hydroper-
oxides.
Since the hydroperoxide accumulation rate wacc
grows during the oxidation of isopropylbenzene, the
rate of its brutto-decomposition wbr must also simulta-
neously grow. However, the experimental data show
that wacc > wbr during the entire period of the process at
catalyst concentrations of ≤5 × 10–3 mol/L. The
brutto-decomposition rate was calculated by the
Michaelis–Menten equation using the parameters
listed in Table 2 as
The kinetic and thermodynamic characteristics
and activation parameters obtained by us for the
decomposition of a ROOH–catalyst complex are
given in Table 2. The kinetic characteristics of brutto-
decomposition were determined by the Lineweave–
Burk method [14] (linearization of the Michaelis–
Menten equation). The algorithm for processing the
experimental data is given in detail in [11]. A compar-
ison of the IPB hydroperoxide brutto-decomposition
rates shows that magnesium carboxylate decomposes
faster than Zn(EG)2. The formation of an intermedi-
kK[CT]0[IPBHP]0
1+ K[IPBHP]0
where [CT] is the catalyst concentration, k is the com-
plex decomposition rate constant, and K is the com-
plex formation equilibrium constant.
wbr =
,
At [Mg(EG)2] = 1 × 10–2 mol/L, the decomposi-
tion rate exceeds the IPBHP accumulation rate within
2 h of oxidation. Nevertheless, accumulation is con-
tinued, and the kinetic curve has no maximum. A sim-
ilar picture is also observed in the presence of zinc-
ate complex represents an exothermic process (ΔН < 0) containing and mixed catalysts. Hence, the initiating
in all the catalytic systems. As can be judged from the catalytic effect caused by the hemolytic decomposi-
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CATALYTIC SYSTEMS BASED ON MAGNESIUM AND ZINC COMPOUNDS
331
Table 3. Oxidation of isopropylbenzene in presence of magnesium and zinc 2-ethylhexanoates*
Contents of products, wt %
IPB
[CT],
mol/L
Catalyst
S, % conversion,
IPBHP
AP
DMPC phenol
α-MS
IPB
%
Мg(EG)2
0.005
0.005
19.71
5.04
0.39
–
4.38
0.02
0.29
0.09
0.04
70.02
94.62
82.31
93.85
29.98
5.38
Zn(EG)2
Traces
0.005
0.005
0.005
0.010
0.001
0.005
Мg(EG)2 + Zn(EG)2
18.44
18.44
7.05
0.07
0.09
0.02
0.97
1.34
0.42
0.19
0.29
0.07
–
–
–
75.63
79.22
92.63
93.70
91.47
93.25
24.37
20.78
7.37
Мg(EG)2 + Zn(EG)2
Мg(EG)2 + Zn(EG)2
* Reaction temperature, 110°C; time, 6 h. Dashes mean that reaction products do not contain corresponding compounds.
tion of hydroperoxide is most pronounced at the initial catalyst is also rather high (91.47–93.70%), but the
stage of the process. The chain process can inde-
pendently develop, and the decomposition of hydrop-
eroxide is further completely compensated by other
branching reactions.
Hence, the experimental data indicate that the cat-
alytic activity of salt catalysts in the alkylarene oxida-
tion reaction is determined by their effect on the
decomposition of hydroperoxide.
The analysis of the kinetic data on the initial
IPBHP accumulation rate allows us to arrange the
studied catalytic systems in the following order
according to their catalytic activity:
isopropylbenzene conversion is 20.78–24.37%.
The nature of the metal in the composition of a cat-
alyst has an effect on the ratio of the products, thus
exhibiting its control function. The alcohol/ketone
ratio decreases in the following order: {Мg(EG)2 +
Zn(EG)2} > Мg(EG)2 > Zn(EG)2. Note that the pres-
ence of zinc in the binary catalyst strongly reduces the
yield of acetophenone, which is not formed at all in
the presence of individual Zn(EG)2.
As can be judged from the numerical value of the
equilibrium constant, the concentration of slowly
decomposing intermediate hydroperoxide complexes
with zinc is much higher than the concentration of
magnesium complexes; thus, the synergic effect of the
binary catalytic system is not exhibited in either the
oxidation or the decomposition reactions.
Мg(EG)2 > {Мg(EG)2 + Zn(EG)2} > Zn(EG)2.
Since oxidation is initiated by the free radicals
resulting from the decomposition of the ROOH–
M(EG)2 complex, the distinctions between the cata-
lytic activities of 2-ethylhexanoates should be related
with the complexing properties of the metal incorpo-
rated into a catalyst. The catalytic activity of magne-
sium compounds is governed by electrostatic factors.
Covalency plays an a significant part in the case of zinc
2-ethylhexanoate. The degree of covalency in zinc
compounds is high and it is less active, while this bond
has a more ionic character for active magnesium. Hence,
the higher the bond ionicity the more active the catalyst.
In fact, purely ionic magnesium complexes are more
active than the zinc subgroup of metals [8].
CONCLUSIONS
Individual zinc and magnesium 2-ethylhexanoates
and their mixture can catalyze both the oxidation of
hydrocarbons and the decomposition of hydroperox-
ides. Hydroperoxide is decomposed through the for-
mation of an intermediate hydroperoxide–catalyst
complex.
The composition of the reaction mixture after IPB
oxidation for 6 h is given in Table 3. Most products
result from the consecutive reactions of hydroperox-
ide. The reaction mass was revealed to contain
dimethylphenylcarbinol (DMPC), acetophenone
(AP), α-methylstyrene (α-MS), and phenol. The
analysis of the data given in Table 3 shows that the
maximum hydroperoxide formation selectivity
(93.85%) is attained in the presence of zinc 2-ethyl-
hexanoate, but the isopropylbenzene conversion is
The optimal catalyst concentration for the purpose
of attaining the maximum hydroperoxide yield and a
high rate of its accumulation should be selected taking
into account the effect produced by the catalyst’s con-
centration on the ratio between the maing hydroper-
oxide formation and decomposition reactions. The
optimal magnesium 2-ethylhexanoate concentration
for attaining the maximum hydroperoxide yield ranges
only 5.38% in this case. The selectivity on the binary from 1 × 10–3 to 5 × 10–3 mol/L.
KINETICS AND CATALYSIS Vol. 59 No. 3 2018

332
KHARLAMPIDI et al.
7. Nurullina, N.M., Batyrshin, N.N., and Kharlam-
ACKNOWLEDGMENTS
pidi, Kh.E., Kinet. Catal., 2007, vol. 48, no. 5, p. 649.
This work was financially supported by the Minis-
try of Education and Science of the Russian Federa-
tion, state task no. 10.956.2017 (PNIL 25.17).
8. Nurullina, N.M., Batyrshin, N.N., and Kharlam-
pidi, Kh.E., Kinet. Catal., 2013, vol. 54, no. 6, p. 662.
9. Nurullina, N.M., Batyrshin, N.N., and Kharlam-
pidi, Kh.E., Pet. Chem., 2014, vol. 54, no. 1, p. 65.
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