Hydroxylation of phenol with hydrogen peroxide in protic and aprotic solvents on TS-1 catalyst

Article abstract of DOI:10.1134/S1070427211040148

Hydroxylation of phenol with a 25% aqueous solution of hydrogen peroxide in polar protic and aprotic solvents on TS-1 heterogeneous catalyst under various conditions was studied. The major reaction products (hydroquinone, pyrocatechol) and their ratio wer

Full text of DOI:10.1134/S1070427211040148

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 4, pp. 640−648. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.V. Potekhin, V.A. Kulikova, V.V. Vasil’ev, E.G. Kohina, and V.M. Potekhin, 2011, published in Zhurnal Prikladnoi Khimii, 2011,
Vol. 84, No. 4, pp. 603−611.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Hydroxylation of Phenol with Hydrogen Peroxide
in Protic and Aprotic Solvents on TS-1 Catalyst
V. V. Potekhina, V. A. Kulikovab, V. V. Vasil’evc, E. G. Kohinab, and V. M. Potekhinb
a Himtek Engineering Private Joint-Stock Company, St. Petersburg, Russia
b St. Petersburg State Institute of Technology, St. Petersburg, Russia
c St. Petersburg State Engineering and Economic University, St. Petersburg, Russia
Received June 22, 2010
Abstract—Hydroxylation of phenol with a 25% aqueous solution of hydrogen peroxide in polar protic and aprotic
solvents on TS-1 heterogeneous catalyst under various conditions was studied. The major reaction products
(hydroquinone, pyrocatechol) and their ratio were determined. A kinetic model describing the hydroxylation in
accordance with the Rideal–Eley mechanism was chosen.
DOI: 10.1134/S1070427211040148
Major researchers’ attention is given today to
micro- and mesoporous titanium silicalite catalysts on
which selective oxidation of organic compounds can be
performed [1, 2]. In particular, such microporous catalyst
as TS-1 found use for the synthesis of hydroquinone
and pyrocatechol by hydroxylation of phenol with
hydrogen peroxide in aqueous-acetone solution [2]. The
hydroquinone/pyrocatechol ratio was close to unity. The
reaction is not regioselective.
1,4-dioxane, and their mixtures with water.
EXPERIMENTAL
In our experiments we used methanol [chemically
pure grade, 99.5%, GOST (State Standard) 6995–77],
acetone (analytically pure grade, GOST 2603–79),
acetonitrile [chemically pure grade, 99.85%, TU
(Technical Specification) SOMR2-040–06], 1,4-dioxane
(analytically pure grade, GOST 10455–80), phenol
(analytically pure grade, 99.5%), and 35% aqueous
solution of hydrogen peroxide. The catalyst, TS-1,
was prepared according to [8] and had the following
characteristics: TiO2 content 2.6%, specific micropore
volume 0.140 cm3 g–1, mean particle size 10–100 μm
(determined with a JSM-35CF scanning electron
microscope, JEOL). X-ray diffraction analysis showed
that TS-1 contained no anatase phase.
It is reported in [3–7] that solvents used in
hydroxylation of phenol with hydrogen peroxide affect
the hydroquinone/pyrocatechol ratio. For example,
in aqueous acetone the reaction yields hydroquinone
and pyrocatechol in approximately equivalent ratio,
whereas in aqueous methanol the hydroxylation occurs
with preferential formation of hydroquinone. However,
possible causes of the regioselectivity in hydroxylation
of aromatic compounds on micro- and mesoporous
titanium silicalite catalysts are not discussed. Data on the
influence exerted on the regioselectivity by the solvent
(protic or aprotic, polar or nonpolar) and hydroxylation
conditions are also limited.
Experiments were performed in a temperature-
controlled glass reactor with continuous shaking to
ensure the kinetic control of the reaction. In all the
experiments, the reaction mixture volume was constant:
10 ml. The conditions were varied: initial Н2О2
concentration, from 0.12 to 1.65 M; catalyst amount,
from 0.4 to 1.0 g; initial phenol concentration, from 1.78
to 2.1 M; and temperature, from 40 to 60°С. Experiments
In this study we examined the reaction kinetics
and the solvent effect on the regioselectivity of phenol
hydroxylation with hydrogen peroxide on TS-1. As
solvents we used methanol, acetone, acetonitrile,
640

HYDROXYLATION OF PHENOL WITH HYDROGEN PEROXIDE
641
were performed in aqueous solvents at the water content
c, M
from 3 to 33 vol %.
The content of hydrogen peroxide in the reaction
mixture was determined by iodometric titration. The
phenol hydroxylation products were determined with
a Khromos GKh-1000 chromatograph.
Kinetic studies were performed with TS-1 catalyst
with a particle size of 10–100 μm, with which the
external and internal diffusion did not control the phenol
hydroxylation with hydrogen peroxide [9, 10].
τ, min
Figure 1 shows the kinetic curves of the Н2О2
consumption in phenol hydroxylation in a methanol
solution on TS-1 under various conditions. As can be
seen, at the initial hydrogen peroxide concentration of
0.52 M, initial phenol concentration of 1.78 M, and
temperature of 40–60°С, the decomposition of hydrogen
peroxide appreciably increased. The reaction order with
respect to hydrogen peroxide varied from zero to unity
with the conversion. In the examined temperature range,
at a hydrogen peroxide conversion Х(H2O2) < 50%,
the reaction order was zero, which indicates that the
forming products do not affect the process kinetics. At
the hydrogen peroxide conversion Х(H2O2) > 50%, first
order was observed.
Fig. 1. Kinetic curves of Н2О2 consumption in hydroxylation
of phenol in methanol. Initial phenol concentration 1.78 M. (с)
Hydrogen peroxide concentration and (τ) time; the same for
Figs. 3 and 4. Initial Н2О2 concentration, M: (1) 0.36, (2) 0.70,
(3) 1.30, and (4–6) 0.52. Temperature, °C: (1–3) 60, (4) 40, (5)
50, and (6) 60. Catalyst amount, wt %: (1–3) 1.0 and (4–6) 4.0.
concentrations of hydrogen peroxide and phenol and
elevated temperatures, benzoquinone is also formed
(Table 2), and the catalyst is noticeably deactivated
as Н2О2 is consumed. The catalyst deactivation is due
to formation of tarry products which are sorbed on
the surface and in pores of the catalyst. In particular,
p-benzoquinone is the product of hydroquinone
The zero reaction order with respect to hydrogen
peroxide was also confirmed by the method of initial
concentrations. At the initial hydrogen peroxide
concentration varied from 0.36 to 1.3 M and constant
temperature of 60°С (Fig. 1, curves 1–3), the initial rate
of the Н2О2 consumption r0 remained constant, (2.2 ±
0.1) × 10–4 mol g–1 min–1, irrespective of the hydrogen
peroxide concentration c(H2O2), which corresponded
to the rate constant of a pseudo-zero-order reaction.
The initial rate was also found to be independent of the
catalyst amount.
Table 1. Hydroxylation of phenol in methanol. c0(H2O2) =
0.52, с0(C6H5OH) = 1.78 M; mc = 4.0 wt %; τ = 6 h
Relative content in products, wt %,
at indicated temperature
Compound
40
50
60
Phenol
87.8
10.0
2.2
87.0
10.2
2.8
85.0
11.7
3.3
Hydroquinone
Pyrocatechol
The temperature dependence of the initial rates r0
of Н2О2 consumption is satisfactorily linearized in the
Arrhenius coordinates log r0–1/T and is characterized
Table 2. Hydroxylation of phenol in methanol. c0(H2O2) =
1.65, с0(C6H5OH) = 1.78 M; mc = 10 wt %; τ = 5 h
by the activation energy of the overall process Eeff
=
Relative content in products, wt %,
47 ± 5 kJ mol–1.
at indicated temperature
Compound
The major products of phenol hydroxylation with
hydrogen peroxide in methanol are hydroquinone and
pyrocatechol (Table 1).
40
50
60
Phenol
96.4
0.5
2.3
0.8
89.7
2.0
7.6
0.7
85.0
3.2
As seen from Table 1, the process is regioselective
with preferential formation of hydroquinone. The
Hydroquinone
Pyrocatechol
Benzoquinone
hydroquinone/pyrocatechol
concentration
ratio
11.0
0.8
decreases from 4.5 to 3.5 as the temperature is increased
from 40 to 60°С. At increased ratio between the initial
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 4 2011

642
c, M
POTEKHIN et al.
r0 × 105, mol l–1 min–1
of the phenol hydroxylation with hydrogen peroxide is
observed in polar aprotic solvents (acetone, acetonitrile)
and their mixtures with water at the ratio c0(C6H5OH)/
c0(H2O2) > 3.
Figure 2 shows the kinetic curves of the hydrogen
peroxide consumption in hydroxylation of phenol
in aqueous acetone at 40°С and various initial
concentrations of hydrogen peroxide. The dependence
of the initial rate of the hydrogen peroxide consumption
on its concentration (Fig. 2, curve 5) is described by
a curve with saturation. The reaction rate is proportional
to the Н2О2 concentration, and at c(H2O2) from 0 to
0.25 M the reaction is first-order with respect to Н2О2.
At c(H2O2) > 0.3 M, the reaction rate is independent of
the Н2О2 concentration, i.e., the kinetics of the hydrogen
peroxide consumption follows a zero-order law.
τ, min
Fig. 2. Kinetic curves of Н2О2 consumption in hydroxylation
of phenol in aqueous acetone. Initial concentrations, M:
phenol 2.1 and acetone 11.3; catalyst content 4.0 wt %; 40°С.
(с) Hydrogen peroxide concentration, (τ) time, and (r0) initial
reaction rate. Initial Н2О2 concentration, M: (1) 0.12, (2) 0.23,
(3) 0.35, and (4) 0.47. (5) Variation of the initial reaction rate
in relation to the initial Н2О2 concentration.
The temperature dependence of the initial rate of
the Н2О2 consumption at c(H2O2) > 0.3 M is linearized
in the coordinates log r0–1/T and corresponds to the
activation energy of the overall process of the phenol
hydroxylation in aqueous acetone of 38 ± 5 kJ mol–1.
oxidation with hydrogen peroxide, and the colored tarry
product is formed by condensation of highly reactive
o-benzoquinone, the product of pyrocatechol oxidation.
Thus, the nature of the solvent (protic methanol or
aprotic acetone) does not affect the range of hydrogen
peroxide concentrations in which the Н2О2 consumption
follows the zero- or first-order law. This fact may be due
to significant difference in the adsorption equilibrium
constants for Н2О2 and the solvent, including water
and reaction products, on active sites of the titanium
silicalite catalyst. However, the concentration ratio of
the components in the reaction system should be taken
into account.
We also found that, when the reaction is performed
at 40°С and c0(C6H5OH)/c0(H2O2) ratio close to unity,
the hydrogen peroxide decomposition follows a zero-
order equation. At the hydrogen peroxide conversion
exceeding 36%, the phenol hydroxylation rapidly
decelerated; the selectivity was about 28%. The reaction
solution acquired a dark cherry color.
The major products of phenol hydroxylation in
aqueous acetone are hydroquinone and pyrocatechol
(Table 3).
At the ratio c0(C6H5OH)/c0(H2O2) > 3, the hydrogen
peroxide decomposition at the start of the reaction also
followed zero order, but the hydroxylation occurred
with practically complete consumption of hydrogen
peroxide (Fig. 1, curves 4–6), and the selectivity of the
hydroquinoneandpyrocatecholformationwithrespectto
Н2О2 exceeded 85%. However, at elevated temperatures
and the ratio c0(C6H5OH)/c0(H2O2) > 3 preserved, the
hydroxylation selectivity decreased (for example, at
60°С it was about 70%). This is apparently due to the
fact that the rate of the hydrogen peroxide consumption
in methanol with the formation of methanol oxidation
products and molecular oxygen (Eeff = 72 ± 5 kJ mol–1)
increases with temperature more rapidly than does the
parallel reaction of the phenol hydroxylation in the same
solvent (Eeff = 47 ± 5 kJ mol–1).
An increase in the initial concentration of hydrogen
peroxide leads to an increase in the Н2О2 conversion
and selectivity of formation of hydroquinone and
pyrocatechol. The hydroquinone/pyrocatechol ratio
varies insignificantly and is in the range (1.1–1.2) : 1. At
the initial concentration of hydrogen peroxide exceeding
0.4 M, tarry products start to form, the reaction solution
becomes colored, and the catalyst activity decreases,
which was also noted above for the hydroxylation of
phenol in methanol.
It should be noted that, in phenol hydroxylation in
aqueous acetone, in contrast to the oxidation in methanol,
the regioselectivity with respect to hydroquinone
decreased by a factor of more than 3.
Similar temperature dependence of the selectivity
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HYDROXYLATION OF PHENOL WITH HYDROGEN PEROXIDE
643
Table 3. Hydroxylation of phenol in aqueous acetone. 40°С, с0(C6H5OH) = 2.1 M, c0(H2O) = 13 vol %, τ = 6 h
Relative content in products, wt %
Initial H2O2 concentration,
M
Selectivity with
respect to H2O2, %
H2O2 conversion, %
phenol
hydroquinone
0.7
pyrocatechol
0.12
0.23
0.35
0.47
98.7
93.5
89.5
88.5
0.6
2.8
4.7
4.9
95.0
93.0
85.0
72.0
74.0
75.0
82.0
84.0
3.7
5.8
6.6
Table 4. Hydroxylation of phenol in aqueous acetone. 40°С, c0(H2O2) = 0.47, с0(C6H5OH) = 2.1 M; mc = 4 wt %; τ = 6 h
Relative content in products
Selectivity with
respect to H2O2,
%
H2O2
conversion, %
Water, vol %
phenol
hydroquinone pyrocatechol
wt %
hydroquinone/pyrocatechol
ratio
3.0
8.0
93.7
91.3
89.4
87.2
82.7
3.3
4.9
5.3
6.9
9.1
3.0
3.9
4.9
5.9
8.2
1.1
1.3
1.1
1.2
1.1
49.4
58.2
71.9
85.0
95.5
55.0
65.0
68.0
69.0
83.0
13.0
23.0
33.0
Introduction of water into the reaction solution
noticeably affects the rate of the hydrogen peroxide
consumption and the selectivity of formation of
dihydric phenols (Table 4). The major products of
phenol hydroxylation in aqueous acetone at the water
concentration from 3 to 33 vol % are hydroquinone and
pyrocatechol.
The phenol oxidation at temperatures exceeding
50°С and high Н2О2 conversions is accompanied by
noticeable formation of tars which are deposited on the
surface and in pores of the catalyst. The reaction solution
c, M
As seen from Fig. 3 and Table 4, an increase in
the water content in the reaction mixture leads to an
increase not only in the Н2О2 conversion, but also in
the reaction selectivity with respect to hydroquinone
and pyrocatechol. At the same time, the hydroquinone/
pyrocatechol ratio remains practically constant, (1.1–
1.2) : 1, i.e., the regioselectivity does not change.
Thus, introduction of water into the reaction zone
favors acceleration of phenol hydroxylation to dihydric
phenols and decreases the contribution of the concurrent
reaction of the hydrogen peroxide decomposition with
the formation of molecular oxygen.
τ, min
Fig. 3. Kinetic curves of Н2О2 consumption in hydroxylation
of phenol in aqueous acetone. Initial concentrations, M:
phenol 2.1 and Н2О2 0.47; catalyst content 4.0 wt %; 40°С.
Initial Н2О concentration, M: (1) 1.6, (2) 4.4, (3) 7.2, (4) 12.7,
and (5) 18.3. Initial acetone concentration, M: (1) 12.7, (2)
12.0, (3) 11.3, (4) 9.9, and (5) 8.6.
In phenol hydroxylation, with an increase in the
temperature from 40 to 60°С, the yields of hydroquinone
andpyrocatechol ncrease, but the selectivityofformation
of hydroquinone and pyrocatechol decreases (Table 5).
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644
POTEKHIN et al.
Table 5. Hydroxylation of phenol in aqueous acetone. 40–60°С, c0(H2O2) = 0.47, с0(C6H5OH) = 2.1 M; mc = 4 wt %; c0(H2O) =
13 vol %; τ = 6 h
Relative content in products, wt %
Selectivity with
Т, °С
H2O2 conversion, %
respect to H2O2, %
phenol
hydroquinone
pyrocatechol
40
50
89.6
84.7
5.4
8.3
5.0
7.0
72.0
87.0
84.0
80.0
55
60
84.3
83.0
8.6
9.0
7.1
8.0
93.0
97.0
80.0
77.0
becomes cherry-colored, and the catalyst becomes
inactive. To enhance the catalyst activity, it is necessary
to repeatedly (3–5 times) wash the spent catalyst with
the solvent.
hydrogen peroxide varies from 81 (at 40°С) to 70%
(at 60°С), with the regioselectivity remaining constant.
Irrespective of temperature, hydrogen peroxide at its
conversion no higher than 50% is mainly consumed
(to more than 80%) for phenol hydroxylation. Then
the process noticeably decelerates (as in the case of
hydroxylation in methanol and aqueous acetone), which
is due to formation of tarry products which are sorbed
on the surface and in pores of TS-1 catalyst.
The hydrogen peroxide consumption in phenol
hydroxylation in an acetonitrile solution on TS-1 catalyst
at 40–60°С occurs faster than in aqueous acetone (water
concentration 3 vol %). As seen from Fig. 4, the reaction
order with respect to Н2О2 varies with the hydrogen
peroxide conversion. At the Н2О2 conversion of up to
50%, zero order is observed. At higher conversions, the
hydrogen peroxide consumption follows the first-order
law. From the initial rates of the hydrogen peroxide
consumption at different temperatures in the examined
range, we determined the effective activation energy of
the process, Еeff = 34 ± 5 kJ mol–1.
The experiments on phenol hydroxylation with
hydrogen peroxide in a low-polarity aprotic solvent,
1,4-dioxane, at 40–60°С [с0(H2O2) = 0.52, с0(C6H5OH)
= 1.78 M; mc = 4 wt %] gave negative results. Only at
60°С hydroquinone and pyrocatechol formed in a small
amount (less than 2%), although the hydrogen peroxide
consumption was noticeable.
As in the case of phenol hydroxylation in aqueous
acetone, decomposition of hydrogen peroxide in
acetonitrile is mainly accompanied by hydroxylation
of phenol into hydroquinone and pyrocatechol in (1.2–
1.3) : 1 ratio (Table 6).
The hydrogen peroxide consumption in a 1,4-dioxane
solution containing phenol is mainly accompanied by the
solvent oxidation with the effective activation energy of
51 ± 5 kJ mol–1, calculated from the initial rates of the
c, M
In the process, the selectivity of the formation
of hydroquinone and pyrocatechol with respect to
Table 6. Hydroxylation of phenol with hydrogen peroxide in
acetonitrile. c0(H2O2) = 0.52, с0(C6H5OH) = 1.78 M; mc =
4 wt %; τ = 6 h
Relative content in products, wt %, at
indicated temperature, °C
Compound
40
50
60
τ, min
Phenol
83.2
9.2
86.2
7.8
86.2
7.4
Fig. 4. Kinetic curves of Н2О2 consumption in hydroxylation
of phenol in acetonitrile. Initial concentrations, M: phenol
1.78 and Н2О2 0.52; catalyst content 4.0 wt %. Temperature,
°С: (1) 40, (2) 50, and (3) 60.
Hydroquinone
Pyrocatechol
7.6
6.0
6.4
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HYDROXYLATION OF PHENOL WITH HYDROGEN PEROXIDE
645
Н2О2 transformation. Low degree of hydroxylation is
peroxide complexes [1, 11, 12].
due to low concentration of phenol in the pore volume
of TS-1 catalyst.
It is suggested in the literature [1, 11] that, in a protic
solvent, in particular, in alcohol, the hydroperoxy group
is stabilized by the alcohol molecule sorbed on the cata-
lytic site with the formation of five-coordinate Ti(IV):
Thus, the following conclusions can be made. In
polar protic and aprotic solvents (methanol, acetone,
acetonitrile) causing decomposition of Н2О2 with
the formation of О2 and oxidative decomposition of
the solvents to 10–15%, phenol undergoes selective
hydroxylation to dihydric phenols at 40–60°С with
an aqueous solution of hydrogen peroxide on the
surface and in pores of TS-1 microporous catalyst.
The regioselectivity (about 80% with respect to
hydroquinone) was observed when the reaction was
performed in methanol or aqueous methanol, whereas in
aqueous acetone and acetonitrile no regioselectivity of
the hydroxylation was observed.
H
OH
O
O
R
H
O
Ti
O
O
O
The possibility of formation of radical species IIR
from complex II is also noted:
Water in a definite concentration interval exerts
a noticeable positive effect on the rate and selectivity of
phenol hydroxylation.
O
O
O
O
O
O
+3
Ti
+3
Ti
O
O
O
O
When the reaction of phenol with hydrogen peroxide
is performed in 1,4-dioxane at 40–60°С, practically
no hydroxylation of phenol is observed, and hydrogen
peroxide is mainly consumed for the oxygen formation
and solvent oxidation.
IIR
II
This species, along with complexes I, IS, and II,
actively participates in oxidation reactions with oxygen
transfer to the substrate.
The phenol hydroxylation on active four-coordinate
titanium sites is preceded by fast reversible processes
of adsorption of solvent S (water, methanol, acetone,
acetonitrile) and dissociative chemisorption of hydrogen
peroxide (Scheme 1).
Presumably, in phenol hydroxylation, complexes II
and IIR are more active than I and IS, because phenol
hydroxylation with hydrogen peroxide in methanol
occurs with a higher activation energy than in aqueous
acetone and acetonitrile solutions. Excess water favors
the formation of titanium peroxy complex [12].
Hydrogen peroxide is actively chemisorbed on
the catalytic center of titanium, possibly containing
the solvent, with the formation of hydroperoxide and
Scheme 1. Chemisorption of hydrogen peroxide on titanium sites of TS-1 catalyst.
OH
O
S
S
H
O
O
O
Ti
O
Ti
O
+ S
Ti
O
+ H2O2
O
O
O
O
O
O
IS
OH
O
O
O
O
H
O
O
H
O
+n H2O
_n H2O
+3
Ti
O
+ H3O+ (n _ 1)H2O
Ti
O
Ti
O
+ H2O2
O
O
O
O
O
II
I
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 4 2011

646
POTEKHIN et al.
Scheme 2.
H
O
O
H
O
O
H
O
H
O
H
O
O
O
+δ
H
O
CH3
CH3
Ti
O
+
Ti
O
O
H
O
O
H
O
O
H
H3C OH
Ti
+ H2O +
O
O
O
O
O
O
O
H
H
O
H3C OH
Ti
+ H2O +
O
O
O
O
H
H
H
O
H
O
O
O
O
+3
+ H3O+
+
Ti
O
Ti
O
O
O
O
O
O
H
O
O
H
H
Ti
+ 2H2O +
O
O
O
O
O
Ti
+ 2H2O +
OH
O
O
O
O
O
H
It is known that the regioselectivity is influenced by
is accompanied by the reaction of the oxygen atom of
hydrogen peroxide, activated with Ti(IV) (structures I,
IS or II, IIR), with the incident phenol molecule from the
bulk of solution (Scheme 2).
the activity of the species attacking the substrate and by
steric and polar factors. In particular, the regioselectivity,
as a rule, increases with a decrease in the activity of the
attacking species. Apparently, the phenol hydroxylation
with hydrogen peroxide in protic and aprotic solvents
involves peroxy species of different activities.
Increased regioselectivity of phenol hydroxylation in
methanol, compared to aqueous acetone and acetonitrile,
is associated with lower reactivity of the oxygen atom in
complexes I and IS, compared to II and IIR, and with
Thelimitingstepoftheprocess,phenolhydroxylation,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 4 2011

HYDROXYLATION OF PHENOL WITH HYDROGEN PEROXIDE
647
solvation of the OH group of phenol with the solvent
to low conversion of hydrogen peroxide and its high
initial concentration, the reaction product with respect
to Н2О2 should be negative, equal to –1, which is not
the case. Apparently, because of steric hindrance in
catalyst micropores, phenol cannot be sorbed on the
same active site of titanium together with hydrogen
peroxide, with the subsequent hydroxylation of the
aromatic ring. Hydroxylation is not observed, either,
if hydrogen peroxide and phenol are sorbed on TS-1
active catalytic sites remote from each other. Therefore,
the most probable pathway is the reaction of complexes
I, IS and II, IIR with an incident phenol molecule from
the bulk of solution, which corresponds to the Rideal–
Eley kinetics.
H
|
(–O–H···O–CH3),
which sterically hinders the hydroxylation in the
o-position of the aromatic ring.
The observed mechanism, taking into account
the kinetics of phenol hydroxylation with hydrogen
peroxide, corresponds to the kinetic model of the
Rideal–Eley mechanism, described by the rate equation
kbH2O cH2O (1 + ΣbScS)cC2H OH
2
2
5
r = ————————————————————————,
1 + bH2O cH2O (1 + ΣbScS) + ΣbScS + bC2H OHcC2H OH
2
2
5
5
where k is the rate constant of the hydroxylation on
the active four-coordinate titanium site; b(H2O2),
Н2О2 adsorption equilibrium constant; bS, adsorption
equilibrium constant of solvent S (including Н2О);
с(H2O2), c(C6H5OH), and cS are the concentrations
of Н2О2, С6Н5ОН, and solvent S (including Н2О),
respectively.
CONCLUSIONS
(1) Selective hydroxylation of phenol with an
aqueous Н2О2 solution at 40–60°С and molar ratio
с0(C6H5OH)/c0(H2O2) > 3 on TS-1 into hydroquinone
and pyrocatechol occurs in polar protic and aprotic
solvents resistant to oxidation (methanol, acetone,
acetonitrile), with practically complete conversion
of Н2О2. In a protic solvent (methanol and aqueous
methanol), phenol undergoes regioselective oxidation
with preferential formation of hydroquinone (about
80%). At a temperature exceeding 60°С and initial
Н2О2 concentration in the solution exceeding 0.4 M,
the process selectivity appreciably decreases, with the
catalyst deactivation.
At low hydrogen peroxide conversion, when
b(H2O2)c(H2O2) >> 1 + ΣbScS (c(H2O2) > 0.3 M), the
phenol hydroxylation rate is independent of the Н2О2
concentration (zero reaction order with respect to
hydrogen peroxide).
At a high conversion of hydrogen peroxide,
corresponding to c(H2O2) < 0.2 M at 1 + ΣbScS >
b(H2O2)c(H2O2)(1 + bScS), the hydroxylation rate
becomes proportional to the Н2О2 concentration (first
reaction order with respect to hydrogen peroxide):
(2) An increase in the water concentration in the
reaction solution favors an increase in the rate of phenol
hydroxylation with hydrogen peroxide and in the
reaction selectivity.
kbH2O сH2O cC2H OH
2
2
5
r = ———–—————— .
(3) The kinetics of phenol hydroxylation with
hydrogen peroxide at 40–60°С in protic and aprotic
organic solvents on TS-1 is described by the Rideal–
Eley kinetic model. At the hydrogen peroxide
concentration in the reaction solution below 0.3 M, the
phenol hydroxylation rate is proportional to the Н2О2
concentration, and at c(H2O2) > 0.3 M the reaction rate
reaches a maximum.
1 + ΣbScS
Assuming that the hydroxylation on TS-1 follows
the Langmuir–Hinshelwood mechanism, i.e., that the
reaction of the adsorbed hydrogen peroxide and phenol
molecules takes place, the rate equation will be given in
the form
kbH2O cH2O (1 + ΣbScS)bC2H OHcC2H OH
r = —–—————————————————————— .
2
2
5
5
(4) In a dioxane solution, the phenol hydroxylation
with hydrogen peroxide at 40–60°С does not occur.
Hydrogen peroxide is consumed in redox reactions
with the formation of molecular oxygen and products of
1,4-dioxane oxidation.
2
[1 + bH2O cH2O (1 + ΣbScS) + ΣbScS + bC2H OHcC2H OH
]
2
2
5
5
In this case, at b(H2O2)c(H2O2)(1 + ΣbScS) >> 1 +
b(C6H5OH)c(C6H5OH) + ΣbScS, which corresponds
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648
POTEKHIN et al.
1998, vol. 21, pp. 497–504.
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