Liquid-phase ethyl benzene oxidation catalysed by manganese salts

Article abstract of DOI:10.1021/op990031i

The liquid-phase oxidation of ethyl benzene by O₂ catalysed by manganese stearate (MnSt₂) in a stirred batch reactor has been studied. The main reaction products are ethyl benzene hydroperoxide, methyl phenyl carbinole, and acetophenone. It was established that the reaction includes four basic macrostages: an induction period, a period of accelerated ethyl benzene consumption, a period of slowing-down, and an inhibition step. It was shown that the initial rate of the second step is described by the equation r₀ = k[EB]_o√ [Mn³⁺]. The observed experimental data allowed the supposition that the reaction proceeds by the radical-chain mechanism with participation of hydroperoxide radicals. The overall process is possibly initiated by Mn³⁺ ions.

Full text of DOI:10.1021/op990031i

Organic Process Research & Development 1999, 3, 400−403
Liquid-Phase Ethyl Benzene Oxidation Catalysed by Manganese Salts
Tatiana V. Bukharkina,* Olga S. Grechishkina, Nikolai G. Digurov, and Nadezhda V. Krukovskaya
D.I. MendeleeV UniVersity of Chemical Technology of Russia, 9 Miusskaya sq., Moscow, 125047, Russia
Abstract:
of manganese (II) and (III), i.e., MnSt
2
(St ) stearate) and
The liquid-phase oxidation of ethyl benzene by O
2
catalysed
Mn(OAc) . The aim of the study was: (a) to establish the
3
by manganese stearate (MnSt
2
) in a stirred batch reactor has
pathway of the main product formation and (b) to determine
the influence of the reaction products on the activity of the
catalyst.
been studied. The main reaction products are ethyl benzene
hydroperoxide, methyl phenyl carbinole, and acetophenone. It
was established that the reaction includes four basic macro-
stages: an induction period, a period of accelerated ethyl
benzene consumption, a period of slowing-down, and an
inhibition step. It was shown that the initial rate of the second
Results and Discussion
The preliminary experiments showed that the basic
reaction products are ethyl benzene hydroperoxide (EBHP),
methyl phenyl carbinole (MPC), acetophenone (AP), benzoic
acid (BA), and phenol (P). We also determined the main
oxidation dependencies. In particular, the induction time was
observed after the oxidant gas was fed. During this time there
was no product formation, and the oxidation state of the
catalytic metal ions did not change. The duration of this time
depended on reaction conditions. The increase in EB and
Mn(II) concentrations resulted in the prolongation of the
induction time. Then the solution quickly acquired the dark
brown colour typical for Mn(III) salts, and the evolution of
water and accumulation of the reaction products (EBHP,
MPC, and AP) began. The oxidation process itself can be
provisionally divided into two steps. During the first step
the EBHP concentration was low (at least 10-fold lower than
those of the other reaction products) and virtually constant.
During the second step the reaction mixture slowly lost its
colour, the EBHP concentration increased quickly, and the
rate of EB consumption decreased compared with that in
the first step. Also there were observed small amounts of
BA and P. After some time, the oxidation stopped com-
pletely, reagent concentrations did not change, and the
solution acquired pale yellow colour. During the experiment
the EB concentration dropped, and that of MPC passed
through the maximum and other components accumulated.
The rates of EB consumption and products accumulation
were the largest during the first step at the low EBHP
concentration.
Thus, in all of the oxidation experiments the process
included four steps: (I) induction period, (II) period of
“stationary” EBHP concentration and maximum reaction rate,
(III) period of EBHP accumulation, and (IV) retardation. The
typical kinetic curves are shown in Figures 1 and 2.
Table 1 lists the mass balance of EB oxidation by oxygen.
According to the kinetic curves and data in Table 1 the
general oxidation scheme can be presented as shown in
Scheme 1). It is known that P is formed by its acid-catalysed
decomposition. The indirect evidence of this pathway is seen
in Table 1. The accumulation of phenol begins simulta-
neously with the appearance of BA in the reaction mixture.
Catalyst Transformations in the Course of the Reac-
tion. The change of reaction mixture colour during the
3
+
step is described by the equation r
0
) k[EB]
0
x
[Mn ]. The
observed experimental data allowed the supposition that the
reaction proceeds by the radical-chain mechanism with par-
ticipation of hydroperoxide radicals. The overall process is
possibly initiated by Mn3 ions.
+
Introduction
The number of studies devoted to the liquid-phase
1-7
oxidation of ethyl benzene (EB) is enormous. In general,
these studies follow two trends: those devoted to the
industrial process improvement and those aimed at the
reaction mechanism. There are a lot of experimental data in
the both fields, and some of the facts were explained.
Nevertheless, there is no general conception which would
combine both of these trends.
The approach based on the determination of the most
essential reagent interactions and their further isolation from
the multitude of all the possible reactions proved to be the
most fruitful in our previous studies of oxidation of aromatic
hydrocarbons in acetic acid. It allowed us to obtain the
adequate description of the kinetics of the main product
formation and to choose the suitable catalytic system and
reaction conditions. On the other hand, the obtained descrip-
tion is, strictly speaking, not complete from the theory
viewpoint, but quite feasible practically. However, it is based
on the chemistry of the process, and the proposed mechanism
includes experimentally verified steps. The same approach
was applied in the present work devoted to study of the ethyl
benzene oxidation in the apolar media catalysed by the salts
*
Authorforcorrespondence.Fax: +7(095)9733136.E-mail: kra@muctr.edu.ru.
(
1) Emanuel, N. M.; Denisov, E. T.; Maizus, Z. K. Tsepnyje reaktsii okisleniya
ugleVodorodoV V zhidkoi faze [Chain reactions of liquid-phase hydrocarbon
oxidation]; Moscow: Nauka Publ., 1965.
(2) Emanuel, N. M.; Gal, D. Okisleniye etilbenzola [Ethyl benzene oxidation].
Moscow: Nauka Publ., 1984.
(
(
3) Henrici-Olive, G.; Olive, S. Angew. Chem. 1974. B86, 1-7.
4) Yoshino, Y.; Hayashi, Y.; Iwahama, T.; Sakaguchi, S.; Ishii, Y. J. Org.
Chem. 1997, 62, 6810-6813.
(5) Valentine, Y. C. Chem. ReV. 1973, 73, 235-241.
(6) Skibida, I. P. Usp. Khim. [Russ. Chem. ReV.] 1985, 54, 1487.
(7) Bukharkina, T. V.; Digurov, N. G.; Shelud’ko, A. B. Zh. Prikl. Khim. 1989,
N7, 1591-1596.
4
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Vol. 3, No. 6, 1999 / Organic Process Research & Development
10.1021/op990031i CCC: $18.00 © 1999 American Chemical Society and The Royal Society of Chemistry
Published on Web 09/23/1999

Scheme 1
Mn(III) [Mn(OAc)
can bind Mn or Mn into inactive complex, thus the
retardation of the reaction is caused by other factors. One
may suppose that retardation is caused by binding of Mn
into some inactive form unable to transfer to Mn or by its
3
]. It means that there are no products that
2
+
3+
Figure 1. EB oxidation at 120 °C. Initial concentrations (in
mol/L): [MnSt
] ) 0.0075; [EB] ) 5.25 (×), 8.17 (b). (1) EB,
2) AP, (3) MPC.
2+
2
3+
(
transfer to higher oxidation state. In all possibility it is caused
2
by the formation of MnO because in some runs, especially
in the presence of large amounts of MPC, the sedimentation
of the dark brown precipitate was observed.
Thus, comparison of the changes in the catalyst oxidation
state with the reaction rate allows us to suppose that the
3
+
2+
active species is Mn . Mn inhibits the reaction but can
3
+
2
be oxidised to Mn . Manganese dioxide (MnO ) formed in
the end of the reaction is the irreversibly “poisoned” catalyst.
Correlation of the Oxidation Rate with the Reactants
Concentration. Preliminary experiments showed that EB
autoxidation rate in the absence of hydroperoxides at 130
°
C is negligible. This is confirmed by the published data.2
First we correlated the initial rate of EB consumption with
the composition of the reaction mixture at the second step
of the process, i.e., at the “stationary” EBHP concentration.
The initial rate was determined at the end of the induction
time. In the majority of the runs (we will discuss the
Figure 2. Concentration profiles of Mn3+ and HPEB during
EB oxidation. Initial EB concentrations (mol/L): 5.25 (×), 8.17
3
+
(b).
exceptions separately) the concentration of Mn reaches at
this moment its maximal value equal to initial MnSt
concentration.
The influence of the reactant concentrations was deter-
mined by varying them in the wide range using chloroben-
zene as a solvent.
2
Table 1. The mass balance of EB oxidation t ) 120 °C,
MnSt ) 0.0075 mol/L
[
2 0
]
concentration, mol/L
t, min
EB
AP
MPC
EBHP
BA
P
The obtained results showed that the EB oxidation rate
0
8.17
7.37
6.59
6.00
5.82
5.43
0
0
0
0
0
0
0
0
0
increases with the growth of EB and MnSt
To determine the type of the kinetic equation we plotted the
initial rate of EB consumption (r ) vs initial concentrations
2
concentrations.
3
6
9
0
0
0
20
80
0.37
0.96
1.34
1.52
1.84
0.41
0.55
0.56
0.53
0.44
0.02
0.07
0.25
0.34
0.37
0.019
0.037
0.083
0.001
0.003
0.007
0
1
1
of the reactants (see Figure 3). Both plots are nonlinear. The
EB concentration at the end of the induction time is virtually
constant, and all the catalyst is in Mn(III) state. Due to this
3
+
fact the initial rate r
rather than vs that of MnSt
curve 2) becomes linear in coordinates r - ([Mn ]) . At
0
was plotted vs Mn concentration
reaction shows that the oxidation state of the catalytic species
changes. Application of spectroscopy together with other
methods allows us to monitor these changes and make
justified assumptions concerning the concentrations of the
different catalytic species. During the induction time the
concentration of Mn3+ increases and then decreases steeply
and disappears completely after the retardation. Introduction
of the fresh portion of the catalyst eliminates the retardation.
2
. This dependence (Figure 3,
0
3+ 1/2
3
+
low EB concentrations [Mn ] at the end of the induction
period is lower than [MnSt ] and direct correlation of r
0
2
0
with [EB] is hardly possible. It is evident that the reaction
3+
2+
mixture at this moment contains Mn , Mn , and EB. Thus,
it is necessary to determine which catalytic species involves
EB into the oxidation process and so determines the overall
reaction rate. The comparison of the kinetic curves of EB
2
The catalyst can be introduced either as Mn(II) (MnSt ) or
Vol. 3, No. 6, 1999 / Organic Process Research & Development
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401

2
+
Mn . The same mechanism of oxidation of alkylaromatics
is described in ref 1. To test this hypothesis we oxidised EB
by Mn(III) in Ar atmosphere. This reaction was run twice.
In the first case the feed of O
2
was stopped when Mn3
+
reached maximum and the reactor was flushed with Ar. In
the second run EB was oxidised by Mn(OAc) under Ar
3
atmosphere. In both cases no products even in trace amounts
were observed after 6 h at 120 °C. Thus, the first pathway
is out of question. In all possibility the reaction proceeds by
3
+
the radical chain pathway initiated with Mn without EB
participation and with the predominant square chain termina-
tion. In this case one can expect the linear dependence of
3
+ 1/2
reaction rate on [EB] and [Mn ] . The hydrocarbon is
involved in the oxidation chain by the hydroperoxy radical,
possibly by the EBHP radical.
Figure 3. Plot of the nitial rate of EB oxidation r
0
vs (EB) (1)
Thus, it is evident that oxidation of EB proceeds by the
radical chain mechanism and Mn participates in the
and Mn3 (2) concentrations at the end of the induction time.
+
3+
initiation step. The processes affecting the change of the
catalyst composition in the course of the reaction still remain
unclear. Also it is not known which factors cause catalyst
activation and inactivation and whether these transformation
are linked with the transformations of the oxidation products.
These problems can be solved by studying the product
influence on reaction kinetics and oxidation of the products
themselves in the presence of the manganese catalyst.
Experimental Section
Kinetic Procedure. All of the runs were carried out under
intensive mixing at the standard pressure and 120 °C. In the
preliminary experiments it was established that the initial
reaction rate became independent of the stirrer rate at 2000
rpm and of oxygen flow above 0.5 L/min. All of the kinetic
experiments were run under these conditions, thus securing
the kinetic region. The solvent was pure ethyl benzene or
its mixture with chlorobenzene. To remove all traces of
EBHP, prior to the reaction EB was passed through the
column packed with alumina and distilled. After the purifica-
tion no traces of EBHP were found. The experimental setup
is shown in Figure 5.The reactor was a glass cylinder vessel
(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.
+
Figure 4. Linearization of r
2) concentrations.
0
dependencies vs EB (1) and Mn3
(
3+
consumption with the plots of [Mn ] vs time shows that
the oxidation does not start until a significant amount of
3+
3+
[
Mn ] is formed. It can mean that [Mn ] is the active
catalytic species. To prove this hypothesis we ran the EB
oxidation using Mn(OAc) ‚2H O as the catalyst. The induc-
tion period disappeared. All of the kinetic curves were
congruent to those obtained at the same MnSt concentration
0.0075 M) excluding induction time. It confirms our
hypothesis that Mn is the active catalytic species. There-
0 0
fore, we plotted r /([Mn ]) vs [EB] , thus taking into
account the change in the catalyst composition (Figure 4,
curve 1). The obtained plot is virtually linear. Then the initial
oxidation rate is described by the equation
3
2
2
(
3+
3+
1/2
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.
3
+
r0 ) k[EB]0
x
[Mn ]
0
where [EB] is the initial EB concentration.
There are possible two pathways of EB involvement into
oxidation: either
1) by the reaction Mn3+ + RH f Mn + R + H or
2+
•
+
The reaction was monitored by taking samples of the
reaction mixture with time.
(
(
•
•
2) by the radical pathway ROO + RH f ROOH + R
In the first case the reaction proceeds by the ion-radical
mechanism and its rate is determined by the rate of one-
electron transfer. The nonlinear rate dependence on Mn
concentration may be explained by its complexing with
Analysis Procedure
3
+
The concentrations of MPC, AP, BA, and phenol were
determined by GLC. The EBHP concentration was deter-
402
•
Vol. 3, No. 6, 1999 / Organic Process Research & Development

V
- V0
Na S O
2
2
3
c )
N
Na S O
2
2 3
2
V
where c is EBHP concentration, VNa S O is the volume of
2
2 3
titrant used, mL, V
0
is the volume of titrant for the titration
of the “blank” sample, V is the sample volume in mL, NNa S O
2
2 3
is the normality of the titrant in mol/L.
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
1
00-160 °C (3 min isotherm at 100 °C, temperature
programming 10 °C/min up to 160 °C, 15 min isotherm at
1
60 °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%.
2
Figure 5. Experimental setup: (1) reactor, (2) O inlet, (3)
Dean-Stark head, (4) -turbine mixer and drive, (5) sampler,
3
. UV-spectrophotometry. To 1 mL of the sample was
(6) contact thermometer, (7) relay. (I) O
2
flow after flow meter,
(
II) flow of effluent gases and vapours to condenser.
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 concen-
mined by iodometric titration, and concentration of Mn(III)
by UV-spectrophotometry. The EB concentration was de-
termined as the difference between its initial concetration
and the sum of organic reaction product concentrations.
3
+
tration of Mn was determined by the calibration curve
3
+
linking [Mn ] with the solution optical density.
The mean relative error of the analysis is (5%.
1
. Iodometric titration. The 1 mL sample of the reaction
mixture was put into stoppered flask. To the flask was added
0 mL of AcOH and 5 mL of 10% KI solution. The flask
was kept in the dark for 25 min. The evolved I was titrated
with 0.05 N Na solution. The EBHP concentration was
calculated according to the equation
Acknowledgment
Authors wish to express their thanks to Dr. Felix Sirovski
for his help in the preparation of the paper.
1
2
Received for review April 8, 1999.
OP990031I
2 2 3
S O
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