Kinetic model of ethyl benzene oxidation catalysed by manganese salts

Article abstract of DOI:10.1021/op9900986

A practical model was developed for liquid-phase Mn³⁺-catalysed oxidation of ethyl benzene by O₂. The model describes time profiles of concentrations of ethyl benzene and two intermediate oxidation products (methyl phenyl carbinol and acetophenone). The kinetic model of acetophenone oxidation to benzoic acid was also obtained. The influence of the main products of the ethyl benzene oxidation on the reaction kinetics is discussed. It was established that the addition of ethyl benzene hydroperoxide does not affect the reaction rate; the addition of methyl phenyl carbinol inhibits the reaction. The addition of acetophenone promotes the reaction and neutralises the inhibiting effect of methyl phenyl carbinol. The proposed reaction mechanism includes free-radical ethyl benzene oxidation. The process is initiated by the acetophenone interaction with Mn³⁺ ion, and the chain is propagated due to the hydroperoxide interaction with ethyl benzene. The chain termination is quadratic. The length of the oxidation chain was calculated. The inhibition is caused by the manganese bonding to form an inactive complex. The obtained system of differential rate equations adequately describes the process.

Full text of DOI:10.1021/op9900986

Organic Process Research & Development 2003, 7, 148−154
Articles
Kinetic Model of Ethyl Benzene Oxidation Catalysed by Manganese Salts
Tatiana V. Bukharkina,* Olga S. Grechishkina, Nikolai G. Digurov, and Nadezhda V. Krukovskaya
Department of Carbon Materials Technology, Faculty of Organic Compounds, D.I. MendeleeV UniVersity of Chemical
Technology of Russia, 9 Miusskaya Square, Moscow 125047, Russia
Abstract:
oxidation in the presence of the main reaction products and
aimed at the building of a mathematical model of the process
that can be used for equipment design. The information on
the reactions occurring in the system is vital for the
construction of the model. Thus, EB can interact either with
Mn(III) or with free radicals. The latter can be formed both
in the decomposition of the first intermediate, that is, ethyl
benzene hydroperoxide (EBHP), and in the oxidation of other
intermediates: methyl phenyl carbinol (MPC) or acetophe-
none (AP) to benzoic acid (BA). It is evident that all these
compounds can influence the rate of ethyl benzene (EB)
consumption and the type of the kinetic equations in the
model. Preliminary experiments1 showed that the initial rate
of EB oxidation is described by the following equation:
A practical model was developed for liquid-phase Mn3+
-
catalysed oxidation of ethyl benzene by O2. The model describes
time profiles of concentrations of ethyl benzene and two
intermediate oxidation products (methyl phenyl carbinol and
acetophenone). The kinetic model of acetophenone oxidation
to benzoic acid was also obtained. The influence of the main
products of the ethyl benzene oxidation on the reaction kinetics
is discussed. It was established that the addition of ethyl benzene
hydroperoxide does not affect the reaction rate; the addition
of methyl phenyl carbinol inhibits the reaction. The addition
of acetophenone promotes the reaction and neutralises the
inhibiting effect of methyl phenyl carbinol. The proposed
reaction mechanism includes free-radical ethyl benzene oxida-
tion. The process is initiated by the acetophenone interaction
with Mn3+ ion, and the chain is propagated due to the
hydroperoxide interaction with ethyl benzene. The chain
termination is quadratic. The length of the oxidation chain was
calculated. The inhibition is caused by the manganese bonding
to form an inactive complex. The obtained system of differential
rate equations adequately describes the process.
r ) k[EB] [Mn3+
]
0x
0
The following reactions occur in the system:
Introduction
The kinetic model of ethyl benzene (EB) oxidation is
relevant for many commercial processes. This model would
become eventually more valuable if it were to include the
reactions of the intermediates. In some cases these intermedi-
ates can become desired products themselves. Also, they can
participate in the oxidation of the starting EB and change
the rate of its consumption. This work is devoted to EB
It is evident that these data are not sufficient for the
construction of the kinetic model. Therefore, we had to
* Author for correspondence. Fax: +7 (095) 978 9569. E-mail: htum@
muctr.edu.ru.
(1) Bukharkina, T. V.; Grechishkina, O. S.; Digurov, N. G.; Krukovskaya, N.
V. Org. Process Res. DeV. 1999, 3, 400-403.
148
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Vol. 7, No. 2, 2003 / Organic Process Research & Development
10.1021/op9900986 CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/22/2003

investigate the reactions of the intermediate products leading
to the final product of EB oxidation, that is, benzoic acid.
We also investigated the influence of the intermediates on
the ratio of catalyst species in different oxidation states. As
a result, we obtained the system of differential equations that
correctly describes the process.
The mean relative error of the analysis is (5%.
2. GLC Analysis. Analysis of the reaction mixture was
performed on the gas chromatograph “Tsvet” model 500 with
FID and glass 1 m × 3 mm i.d. column packed with
INERTON super AW-DMCS (d ) 0.16-0.2 mm) with 5%
FFAP. Injector temperature 220 °C, column temperature
100-160 °C (3 min isotherm at 100 °C, temperature
programming 10 °C/min up to 160 °C, 15 min isotherm at
160 °C).
The concentrations were determined with the help of the
internal standard (n-octanol for AP and MPC and p-
nitroacetophenone for BA and phenol). Triphenylphosphin
was added to all samples of reaction mixture. It selectively
converted HPEB to MPC, then internal standards were added.
The mean relative error of the analysis is (5%.
Experimental Section
Kinetic Procedure. All the runs were carried out under
intensive mixing at standard pressure and 120 °C. In the
preliminary experiments it was established that the initial
reaction rate became independent of the stirrer speed at 2000
rpm and of oxygen flow above 0.5 L/min. All the kinetic
experiments were run under these conditions, thus ensuring
the absence of mass-transfer effects. The solvent was
pure ethyl benzene or its mixture with chlorobenzene. To
remove all traces of EBHP, EB prior to the reaction
was passed through the column packed with alumina and
distilled. After the purification, no traces of EBHP were
found. The reactor was a glass cylinder vessel of 80-120
mL volume, equipped with turbine mixer, gas inlet pipe,
sampler, contact thermometer, and Dean-Stark head for the
isolation of water from the gaseous reaction products, which
were condensed in the reflux cooler. The reaction temperature
was kept constant within (0.5 °C due to the system control
thermometer-relay.
After assembling the setup the Dean-Stark head was
filled with ethyl benzene. The reactor was loaded with
reaction components. Then the mixer and heating were turned
on. After the reaction temperature was reached, oxidant gas
was fed into the reactor. This moment was assumed as the
beginning of the reaction. The gas flow was controlled with
the flow meter.
The reaction was monitored by taking samples of the
reaction mixture with time.
The setup for the EB oxidation under anaerobic conditions
was eventually the same as described above only it lacked
the Dean-Stark head. To maintain an inert atmosphere argon
or nitrogen were fed into reactor instead of air.
Analysis Procedure. The concentrations of MPC, AP,
BA, and phenol were determined by GLC. The EBHP
concentration was determined by iodometric titration, and
concentration of Mn(III) by UV-spectroscopy. The EB
concentration was determined as the difference between its
initial concentration and the sum of organic reaction product
concentrations.
1. Iodometric Titration. The 1-mL sample of the reaction
mixture was put into a stoppered flask. To the flask was
added 10 mL of AcOH and 5 mL of 10% KI solution. The
flask was kept in the dark for 25 min. The evolved I2 was
titrated with 0.05 N Na2S2O3 solution. The EBHP concentra-
tion was calculated according to the equation:
3. UV Spectroscopy. To 1 mL of the sample was added
3 mL of freshly distilled acetic acid. The optical density of
the obtained solution was measured on “Specord-M40” UV-
vis spectrophotometer at 460 nm. The concentration of Mn3+
was determined by the calibration curve linking [Mn3+] with
the solution optical density.
The mean relative error of the analysis is (5%.
Results and Discussion
The Effect of the Hydroperoxide Addition. In the
noncatalysed oxidation free radicals are produced by the
thermal destruction of hydroperoxides.2,3 This destruction
which can proceed both by radical and molecular pathways
is also catalysed by metal ions.4,5 If the molecular mechanism
is prevailing, then the hydrocarbon oxidation rate would not
be increased by the addition of hydroperoxide despite the
increase in its decomposition rate as compared with non-
catalysed process. Air oxidation of EB in the presence of
Mn(II) salts in the absence of organic additives at 140 °C
begins after fairly long induction time. During that time
reaction mixture is only slightly coloured, and no reaction
products are accumulated. The oxidation begins after rapid
and complete transfer Mn2+ f Mn3+. Oxidation proceeds
rapidly at first, later slows, and eventually stops. Transforma-
tion of EB to MPC and AP via EBHP starts after induction
time. In all possibilities, inhibition is the result of some kind
of catalyst deactivation. However, when the oxidation is
carried out in the absence of inhibiting products of EBHP
decomposition (i.e., phenol and MPC), the reaction gives
benzoic acid in nearly quantitative yield.
On addition of EBHP to the starting reaction mixture, all
manganese ions were immediately oxidised to Mn3+. The
consumption of EB occurred without an induction time. All
kinetic curves were congruent with those observed in the
runs without added EBHP after the induction time. The same
results were obtained with addition of cumene hydroperoxide
(2) Emanuel, N. M.; Denisov, E. T.; Maizus, Z. K. Tsepnye Reakcii Okisleniya
UgleVodorodoV V Zhidkoi Phaze [Liquid-Phase Chain Hydrocarbon Oxida-
tion]; Nauka: Moscow, 1965.
(3) Emanuel, N. M.; Gal, D. Okislenie Etilbenzola [Ethyl Benzene Oxidation];
Nauka: Moscow, 1984.
VNa S O - V0
2
2 3
c )
NNa S O
2 2 3
2V
where c is EBHP concentration,VNa S O is the volume of
2
2 3
(4) Bal’kov, B. G.; Skibida, I. P.; Maizus, Z. K. IzV. AN SSSR, Ser. Khim.
titrant used, mL, V0 is the volume of titrant is for the titration
of the “blank” sample, mL; V is the sample volume, mL;
NNa S O is the normality of the titrant in mol/L.
1970, 1780-1785.
(5) Skibida, I. P.; Brodskii, M. S.; Gervits, M. Ya.; Goldina. L. A.; Maizus, Z.
K. Kinet. Katal. 1973, 14, 885-890.
2
2 3
Vol. 7, No. 2, 2003 / Organic Process Research & Development
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149

Figure 2. Influence of added AP on the ethyl benzene
oxidation. Initial concentrations (in mol/L): [EB] ) 8.17.
[MnSt2] ) 0.0075. (Curve 1) [AP] ) 0. (Curve 2) [AP] ) 0.14.
(Curve 3) [AP] ) 0.32. (Curve 4) [AP] ) 1.06.
Figure 1. Influence of added MPC on the ethyl benzene
oxidation. Initial concentrations (in mol/L): [EB] ) 8.17.
[MnSt2] ) 0.0075. (Curve 1) [MPC] ) 0. (Curve 2) [MPC] )
0.10. (Curve 3) [MPC] ) 0.33. (Curve 4) [MPC] ) 0.44.
step can be estimated by studying AP oxidation under the
conditions of ethyl benzene reaction.
The addition of benzoic acid eliminated the induction
time. A bell-like dependence of the initial rate on benzoic
acid concentration was obtained.
to the reaction mixture. The continuous feed of hydro-
peroxides after induction time also did not change the rate
of EB consumption.
The decomposition of EBHP under Ar catalysed by
manganese(II) stearate (MnSt2) is extremely fast, and at 120
°C it yields MPC, AP, and all manganese(II) is oxidised to
Mn3+. Thus, it can be supposed that main role of EBHP
during the oxidation is the generation of Mn3+ rather than
abstracting hydrogen atoms from EB. This hypothesis also
corresponds with the published4 data that EBHP decomposi-
tion catalysed by Mn proceeds mainly by the molecular
pathway. The low steady-state concentration of EBHP during
the reaction is explained by its high decomposition rate. This
concentration stays low until the moment of the catalyst
poisoning when the rate of the process significantly drops.
The Influence of Added Methyl Phenyl Carbinol,
Acetophenone, Benzoic Acid, and Phenol. The kinetic
curves obtained in the presence of MPC added at the start
of the oxidation are shown in Figure 1. It is seen that the
increase in MPC concentration results in the elongation of
the induction time, and the reaction rate drops. The oxidation
is halted completely at the MPC concentration of 0.44 mol/
L. Moreover, the maximum obtained Mn3+ concentration is
lower than the initial concentration of manganese stearate.
This was observed only in the presence of added MPC. The
samples of the reaction mixture contained the brown MnO2
precipitate. The oxidation of MPC did not proceed in the
absence of EB, and begins only after the addition of AP. In
the presence of AP no inhibition was observed. It may be
supposed that the catalyst poisoning is due to the MPC
oxidation and competes with manganese-AP interaction.
This interaction prevents the irreversible inactivation of the
catalyst.
The addition of phenol inhibited EB oxidation. Its addition
at the beginning resulted in virtually infinite induction time.
When it was added after the induction, the reaction was
drastically slowed.
The results show that AP and Mn3+ participate in the
initiation step. The acting concentration of Mn3+ is the result
of its reactions with oxygen-containing reaction products.
These may be both red-ox processes and simple binding of
manganese into a complex. The experimental data show that
the accumulation of Mn(III) in high concentration is the result
of reactions with participation of acetophenone and benzoic
acid. Thus, the next step was the study of their possible
transformations.
Oxidation of Acetophenone in the Presence of the
Manganese Catalyst. It is known6 that AP oxidation in an
acetic acid solution is a nonchain process rate-limited by the
Mn3+-AP interaction. The same reaction can proceed when
ethyl benzene, chloro-, or dichlorobenzene is used as the
solvent. Thus, it was necessary to study both the anaerobic
oxidation of AP by Mn3+ ions using Mn(OAc)3‚2H2O under
Ar atmosphere and its oxidation to benzoic acid by oxygen
in the presence of Mn(OAc)2‚4H2O catalyst. The formation
of complexes in the system Mn3+-Mn2+-AP-BA has been
studied earler,7 and the results obtained were used in the
construction of the kinetic model. The results of UV
spectroscopy showed that in the chloro- and dichlorobenzene
solutions exist free ions Mn3+, Mn2+ together with associates
Mn2+‚‚‚Mn2+, Mn3+‚‚‚Mn3+, and Mn3+‚‚‚Mn2+. All these
catalytic species can interact with AP and BA, thus influenc-
ing the reaction kinetics.
The addition of AP at the beginning of the reaction
resulted both in the reduction of the induction time and in
the increase in the initial rate of ethyl benzene oxidation (see
Figure 2). It should be noted that the rate of EB consumption
grows with the increase in the initial AP concentration. This
implies that the kinetic equation for the EB oxidation should
include the AP concentration. It is reasonable to suppose
that AP participates in the initiation step. The rate of this
Anaerobic Oxidation of Acetophenone by Manganese
(III) Acetate. The reaction kinetics was studied in the
presence of benzoic acid that was necessary to ensure a
(6) Mil’ko, S. B.; Rzhevskaya, N. N. Digurov, N. G. IzV. VUZOV, Ser. Khim.
1990, 33,
(7) Bukharkina, T. V; Digurov, N. G.; Mil’ko, S. B.; Shelud’ko, A. B. Org.
Process Res. DeV. 2000, 4, 404-408
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These relationships are valid only for the beginning of the
reaction when no Mn2+ species are present.
The dependence of the initial rate on [Mn3+] is linear as
shown in Figure 4. The addition of Mn2+ to the reaction
mixture at the start resulted in reaction inhibition. It seems
that mixed associates of Mn(III) and Mn(II) do not participate
in the oxidation. Thus, the inhibiting effect of Mn2+ is caused
by its binding with Mn3+ into an inactive complex. When
the acetophenone is in a large excess ([AP] . [Mn3+]0), the
reaction kinetics can be described by the following system:
dCMn(3+)
-
) k1[Mn3+
]
Figure 3. Dependence of the initial rate of the anaerobic AP
oxidation by Mn(III) (r0) on its initial concentration CMn(3+) at
110 °C.
dt
CMn(2+) ) C
- CMn(3+)
Mn(3+)0
CMn(2+) ) [Mn2+] + 2K2[Mn2+]2 + K3[Mn2+][Mn3+
CMn(3+) ) [Mn3+] + 2K1[Mn3+]2 + K3[Mn2+][Mn3+
]
]
where CMn(2+) is the overall (analytical) concentration of
Mn(II), mol/L; CMn(3+) is the overall (analytical) concentration
of Mn(III), mol/L; [Mn2+] is the concentration of the
monomer Mn(II), mol/L; [Mn3+] is the concentration of the
monomer Mn(III), mol/L; K1 ) 16 L/mol is the equilibrium
constant of the reaction:7
2Mn3+ a Mn3+‚‚‚Mn3+
K2 ) 70 L/mol is the equilibrium constant of the reaction:7
Figure 4. Dependence of the initial rate of the anaerobic AP
oxidation by Mn(III) (r0) on the concentration of monomeric
[Mn3+].
2Mn2+ a Mn2+‚‚‚Mn2+
K3 ) 370 L/mol is the equilibrium constant of the reaction:7
homogeneous reaction mixture. The only oxidation product
was 1,4-diphenylbutane-1,4-dione:
Mn2+ + 2Mn3+ a Mn3+‚‚‚Mn3+
The dependences of the rate constant k1 on the initial
acetophenone and benzoic acid concentrations are bell-like
curves. The studies of the complex formation in the system
containing Mn2+, Mn3+, BA, and AP showed that organic
species compete for a position in the metal coordination
sphere. Then k1 can be expressed in terms of the ratio [AP]0/
[BA]0 (both are in a large excess to the metal). This
dependence is also bell-like.
The mass balance experiments showed that one mole of
Mn(OAc)3 is consumed per one mole of acetophenone, thus
making it possible to monitor the reaction by Mn3+ con-
sumption. It was established that the reaction does not
proceed to completion and is inhibited by Mn2+ ions. That
was confirmed by experiments with the addition of Mn(OAc)2
to the reaction mixture. The dependence of the initial rate
of Mn3+ consumption on the initial overall analytical Mn3+
concentration is nonlinear, as shown in Figure 3. This
allowed us to suppose that only free manganese(III) ion is
the oxidant. Its concentration can be calculated according
to the mass balance equation:
a[AP]0/[BA]0
1 + b[AP]0/[BA]0 + c([AP]0/[BA]0)2
k1 )
(1)
The obtained equation allows us to propose the following
scheme of transformations:
CMn(3+) ) [Mn3+] + 2K1[Mn3+ 2
]
where CMn(3+) is the overall (analytical) manganese(III)
concentration, mol/L; [Mn3+] is the concentration of the
monomer Mn(III), mol/L; K1 ) 16 L/mol is the equilibrium
constant of the following reaction:7
If the complex II is not able to transfer an electron from AP
molecule to Mn3+ ion, then the rate constant is described by
2Mn3+ a Mn3+‚‚‚Mn3+
Vol. 7, No. 2, 2003 / Organic Process Research & Development
•
151

Table 1. Constants of eq 2
run
t, °C
k2, min-1
K4
K5 × 103
1
2
3
4
100
110
120
130
0.09
0.17
0.30
0.36
2.6
2.4
2.4
2.2
14
12
11
11
Table 2. Mass balance of the AP oxidation
time, min 30 60 120 180 240 300
0
[AP], mol/L
[BA], mol/L
2.40 2.20 1.68 1.22 0.54 0.19 0.06
0.50 0.70 1.20 1.65 2.32 2.64 2.76
[AP] + [BA], mol/L 2.90 2.90 2.88 2.88 2.87 2.83 2.82
Figure 5. Oxidation of AP by oxygen in the presence of
Mn(OAc)2 at 110 °C. (Curve 1) AP. (Curve 2) BA. (Curve 3)
Mn3+. [Mn(OAc)2]0 ) 0.05 mol/L.
eq 1 and the kinetic equation is as follows:
dCMn(3+)
dτ
d[AP]
dτ
)
)
-k2k3K4CMn(3+)[AP]0/[BA]0
1 + K4[AP]0/[BA]0 + K4K5([AP]0/[BA]0)2
(2)
The constants of eq 2 are listed in Table 1. Strictly speaking,
in eq 2 current concentrations of BA and AP should be used
instead of the initial ones. Nevertheless, the use of the initial
concentrations is justified as the concentration of benzoic
acid during the reaction is constant and consumption of AP
is negligible due to its large excess over Mn3+.
Figure 6. Dependence of monomeric [Mn3+] (curve 1) and k4
rate constant (curve 2) on overall CMn(3+)
Thus, we established that the rate of anaerobic AP
oxidation by Mn3+ is determined by the concentration of the
Mn3+ monomer species, and Mn(II) inhibits the reaction
binding Mn(III) into an inactive complex. The complicated
dependencies of the reaction rate upon BA and BA concen-
trations are the result of the complex formation.
Oxidation of Acetophenone by Oxygen in the Presence
of the Manganese Catalyst. The reaction was run in the
absence of mass-transfer effects. The main reaction product
(97-99% yield) was benzoic acid.
The mass balance of the oxidation is listed in Table 2.
The typical profiles of AP consumption and BA and
Mn(III) accumulation are shown in Figure 5. It is seen that
after 20 min all manganese is present in the Mn(III) form,
and its concentration does not change any more.
The primary treatment of the kinetic data was performed
by the differential method. The concentration dependence
of the AP consumption rate at the constant manganese
concentration in coordinates 1/r - [AP]/[BA] is linear. Thus,
the rate of the AP consumption can be expressed by the
following equation:
points from the theoretical curve falls within the limits of
the experimental error.
The dependence of the rate constant k4 on the stationary
analytical concentration Mn3+ is nonlinear as shown in
Figure 6. This constant is linear-dependent on the calculated
monomer species [Mn3+] concentration, that is, k3
)
k4[Mn3+]. At 110°C k4 ) 0.21 min-1. Thus, the kinetics of
the AP oxidation is described by the following equation:
K6[AP]/[BA]
d[AP]
dτ
) -k4[Mn3+
]
1 + K6[AP]/[BA]
The comparison of kinetics of AP oxygen and anaerobic
oxidation (by Mn(OAc)3 in Ar atmosphere) shows that they
are described by the same equation with virtually identical
numerical values of constants. Thus, at 110 °C k4 ) 0.21 (
0.03 min-1, k2 ) 0.17 ( 0.02 min-1, K4 ) 2.4 ( 0.9, K6 )
3.1 ( 0.8. Consequently, in our opinion, the oxidation of
AP proceeds by the nonchain pathway. The rate-limiting step
is the electron transfer from the AP molecule to Mn(III).
Nevertheless, the produced radical intermediates also can
participate in the EB oxidation. The feasibility of this
pathway is confirmed by the promoting effect of AP addition
in the EB oxidation.
k3K6[AP]/[BA]
d[AP]
dτ
) -
1 + K6[AP]/[BA]
The constants k3 and K6 were determined by the numerical
integration of this equation. The fitting was performed using
nonlinear least-squares regression minimising the sum of
squares of residuals of measured and simulated data. The
values at 110° C are k3 ) 57 × 104 L/(mol‚min), K6 ) 3.2.
It is seen from Figure 5 that the deviation of experimental
Oxidation of Ethyl Benzene after Induction Time. The
plausible scheme of the EB oxidation after induction time
should explain the following phenomena:
1. The linear dependence of the oxidation initial rate on
[Mn3+]1/2 together with the absence of EB interaction with
[Mn3+] ions,
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Vol. 7, No. 2, 2003 / Organic Process Research & Development

2. The absence of reaction acceleration by hydroperoxide
addition,
In this case, the steady-state concentration of peroxide
radicals is equal to:
3. The acceleration of the process by AP addition and its
inhibition by MPC addition.
k4CMn(3+)[AP]
[ROO•] )
Ethyl benzene hydroperoxide decomposes to MPC and
AP with nearly 50% ratio. Assuming that the consumption
of EB and MPC proceeds by the chain mechanism, the
kinetics would be described by the following system of
equations:
k ([AP] + K [MPC])
x
7
7
After that system 3 would became (not taking into account
the rate of consumption of AP to benzoic acid)
k4CMn(3+)[AP]
d[EB]
dτ
d[EB]
dτ
-
) k5[ROO•][EB]
-
) k5[EB]
k ([AP] + K [MPC])
x
7
7
k5
d[MPC]
)
)
[ROO•][EB] - k6[ROO•][MPC]
k4CMn(3+)[AP]
k
d[AP]
dτ
dτ
2
k5
2
)
)
5[EB] + k [MPC]
(4)
(
)
6
2
k ([AP] + K [MPC])
x
7
7
d[AP]
dτ
[ROO•][EB] + k6[ROO•][MPC]
k4CMn(3+)[AP]
k
d[MPC]
5[EB] - k [MPC]
(
)
6
dτ
2
where k5 is the rate constant of the reaction of the hydro-
peroxide radical with ethyl benzene, and k6 is the rate
constant of the reaction of the hydroperoxide radical with
MPC.
If we assume that EBHP and ROO• concentrations are
steady-state and the chain termination is a quadratic one,
then we obtain:
k ([AP] + K [MPC])
x
7
7
It should be noted that the rate of AP consumption (and BA
accumulation) is the rate of initiation of chain EB oxidation.
Let us compare these rates in the case of oxidation of pure
EB at 30 min after the start of the reaction. At this moment
reagent concentrations (mol‚L-1) are: [EB] ) 6.80, [AP] )
0.66, [MPC] ) 0.50. Concentration of Mn3+ was 2 × 10-3
mol‚L-1. Then the rate of EB consumption rEB ) 0.018
d[EBHP]
) 0
dτ
L‚(mol‚min)-1, the rate of benzoic acid accumulation rBA
)
0.57 × 10-4 L‚(mol‚min)-1, and oxidation chain length is
0.018/(0.57 × 10-4) ≈ 300.
d[ROO•]
) k4[AP‚‚‚Mn3+] - k7[ROO•]2 ) 0
dτ
In principle, the rate of AP consumption to benzoic acid
should be added to the rate of its accumulation. However,
under the conditions of EB oxidation its contribution is
negligibly small, as the oxidation of AP is the initiation of
EB oxidation. When the chain length of EB oxidation is
about 300, the rate of BA accumulation would be about 0.3%
of the rate of EB consumption, that is, well within the limits
of an experimental error. Nevertheless, when the concentra-
tions of AP and EB are equal, this reaction should not be
neglected.
k4[AP‚‚‚Mn3+
]
[ROO•] )
k7
x
where k4 is the rate constant of the AP oxidation, and k7 is
the rate constant of the quadratic chain termination.
Spectroscopic investigation of the reaction mixture of EB
oxidation revealed the presence of Mn(III) complexes with
MPC (λ ) 402 nm), and AP or benzoic acid (λ ) 490 nm).
The same complexes were observed in AP oxidation. In all
possibility these complexes are in equilibria:
For the rates of AP accumulation and consumption we
obtain:
Mn3++ AP a
Mn3+‚‚‚AP
(III)
k4CMn(3+)[AP]
d[AP]
dτ
d[EB]
2dτ
Mn3++ MPR a
Mn3+‚‚‚MPC
) -
+ k6
[MPC] -
k ([AP] + K [MPC])
x
7
7
(IV)
CMn(3+)[AP]
4[AP] + K7[MPC]
(III) + MPC {K
\
7} (IV) + AP
k
d[BA]
dτ
d[AP]
dτ
To simplify the equations, molecules of benzoic acid
incorporated into the coordination shell of Mn3+ are omitted.
If we assume that all Mn3+ is bound in the complexes
(IV) and (V), then taking into account the equilibrium 3,
the concentration of the active catalytic complex is:
) -
The rate and equilibrium constants k5 and K7 were determined
by the least-squares fit after numerical integration of the
system of differential equations. The fitting was performed
using nonlinear least-squares regression minimising the sum
of squares of residuals of measured and simulated data. The
concentration of Mn(III) in the each run was approximated
by the polynomial function. The values of the constants are
as follows:
CMn(3+)[AP]
[AP‚‚‚Mn3+] )
[AP] + K7[MPC]
where K7 is the equilibrium constant of the reaction 3, and
CMn(3+) is the analytical Mn(III) concentration.
Vol. 7, No. 2, 2003 / Organic Process Research & Development
•
153

Table 3. Experimental and calculated values of the reagent
k5 ) 12600 ( 1000 L/(mol‚min)
k6 ) 4‚k6 L/(mol‚min)
concentrationsa
CEB, mol/L CMPC, mol/L CAP, mol/L
time,
k7 ) 1.2 × 109 L/(mol‚min)
k4 ) 0.3 min-1
min exptl calcd exptl calcd exptl calcd
[EB]0 )
0
7.96 7.96 0.13 0.13 0.08 0.08
7.54 7.55 0.29 0.34 0.29 0.32
7.15 7.10 0.41 0.50 0.48 0.52
6.80 6.83 0.50 0.51 0.66 0.70
6.50 6.64 0.55 0.52 0.81 0.83
8.17 mol/L 10
K7 ) 12.5 ( 2.4
20
30
40
The rate constants k6 and k7 were found in the literature3, k4
is the rate constant of the AP oxidation.
[EB]0 )
0
6.00 6.00 0.07 0.07 0.05 0.05
5.73 5.80 0.18 0.18 0.18 0.15
5.48 5.45 0.27 0.29 0.31 0.32
5.26 5.30 0.33 0.35 0.41 0.43
6.12 mol/L 10
The obtained system of differential rate eqs 4 adequately
describes experimental dependencies of reaction rates on
concentrations. It reflects all the effects that accompany the
change in the composition of the reaction mixture during
the second step of the EB oxidation in the presence of
significant amounts of Mn3+. The calculated and experi-
mental values of current concentrations are listed in Table
3. It is seen that the discrepancies do not exceed the limits
of the experimental error.
It is possible that accumulating phenol plays an important
role in the process inhibition, but the present data do not
allow us to make justified assumptions on the significant
interactions during this stage of the process.
20
30
[EB]0 )
0
5.19 5.19 0.03 0.03 0.03 0.03
5.00 5.01 0.12 0.13 0.13 0.10
4.85 4.87 0.17 0.20 0.20 0.17
4.69 4.71 0.23 0.27 0.28 0.26
4.50 4.50 0.28 0.35 0.37 0.39
4.38 4.40 0.31 0.36 0.43 0.47
5.25 mol/L 10
20
30
40
50
a Temperature 120 °C, [MnSt2]0 ) 0.0075 mol/L. The end of the induction
time was taken as the start of the reaction.
This process can be defined as the chain co-oxidation of ethyl
benzene with the nonchain acetophenone oxidation.
Conclusions
The results of the investigation of the EB oxidation in
aprotic solvents (or without a solvent) allow us to conclude
that this process is a radical-chain one. Ethyl benzene is
included in the chain by interaction with ROO• radicals. The
radicals are not produced as the result of EBHP decomposi-
tion. The reaction is initiated by Mn3+ + AP interaction.
Acetophenone itself is oxidised in the nonchain catalytic
reaction in which the active catalytic species are regenerated.
Acknowledgment
We express our thanks to Dr. Felix Sirovski for his help
in the preparation of the paper.
Received for review December 20, 1999.
OP9900986
154
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Vol. 7, No. 2, 2003 / Organic Process Research & Development