Photooxidation of ethylbenzene with TiO2 and metal coated TiO2 and its kinetics

Article abstract of DOI:10.1007/s00706-004-0215-5

Photooxidation of ethylbenzene with oxygen to give ethylbenzene hydroperoxide has been achieved in a stirred photochemical reactor that was cooled by a water system by irradiation with a 400 W high-pressure mercury lamp and using TiO₂ powder and metal coated TiO₂. The effects of the amount of copper or silver coated on TiO₂ and of the temperature on the rate of oxidation have been investigated. It is suggested that thermal cleavage of the O-O bond and photochemically generated singlet oxygen should be considered as the initiating step in a radical chain mechanism. An optimum loading of 6% Ag or 4-5% Cu was observed for photooxidation of ethylbenzene. Springer-Verlag 2004.

Full text of DOI:10.1007/s00706-004-0215-5

Monatshefte f€ur Chemie 135, 1121–1127 (2004)
DOI 10.1007/s00706-004-0215-5
Photooxidation of Ethylbenzene
with TiO2 and Metal Coated TiO2
and its Kinetics
ꢀ
and M. Montazerozohori
Mohammad H. Habibi , A. Zeini Isfahani, A. Mohammadkhani,
Chemistry Department, Isfahan University, Isfahan 81745-117, Iran
Received September 5, 2003; accepted (revised) April 27, 2004
Published online August 13, 2004 # Springer-Verlag 2004
Summary. Photooxidation of ethylbenzene with oxygen to give ethylbenzene hydroperoxide has been
achieved in a stirred photochemical reactor that was cooled by a water system by irradiation with a
400 W high-pressure mercury lamp and using TiO2 powder and metal coated TiO2. The effects of the
amount of copper or silver coated on TiO2 and of the temperature on the rate of oxidation have been
investigated. It is suggested that thermal cleavage of the O–O bond and photochemically generated
singlet oxygen should be considered as the initiating step in a radical chain mechanism. An optimum
loading of 6% Ag or 4–5% Cu was observed for photooxidation of ethylbenzene.
Keywords. Photooxidation; Ethylbenzene; TiO2; Metal coated; Kinetics.
Introduction
Over the past several years, a large number of examinations on the application,
production, and testing of TiO2 as photocatalyst have been performed [1–7]. Coating
of some metals on the TiO2 surface has been known for the improvement of photo-
catalytic oxidation of organic compounds [8–12]. It has been reported that palla-
dium, gold, and silver are effective for improvement of photocatalytic activity of
TiO2 [13–15]. Propylene oxide is produced by epoxidation of propylene with ethyl-
benzene hydroperoxide (EBHP). The latter is produced by oxidation of ethylbenzene
(EB) with oxygen. The by-product of the above reaction is dehydrated to yield the
monomer styrene (Scheme 1). In this work, we investigated the effect of loaded Cu
or Ag on TiO2 for the photocatalytic oxidation of EB to EBHP with oxygen.
It is expected that loaded Cu or Ag may improve photocatalytic activity of
TiO2, since the work function of Cu is quite similar to the TiO2 conduction band
and to the standard reduction potential of oxygen, which is considered to be a
ꢀ
Corresponding author. E-mail: habibi@sci.ui.ac.ir

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M. H. Habibi et al.
Scheme 1
probable electron acceptor in TiO2 [16]. The oxidation of EB to EBHP is a liquid-
phase radical reaction, carried out at industrial level (Scheme 1). The oxidation of a
C–H bond to a C–O–O–H group (hydroperoxide) occurs with oxygen and is
catalyzed by light [17, 18]. The active oxygen species is an excited singlet state
oxygen and the function of the photosensitizer is to promote the oxygen triplet to
singlet state. Excited singlet oxygen is the reactive intermediate in the photoox-
ygenation of organic compounds [19–21], non-photochemical reaction between
H2O2 and NaOCl, or between ozone and triphenylphosphite [22]. The singlet
oxygen generated by either non-photochemical or photochemical methods is also
the reacting species in the oxidation reaction [23–30].
Results and Discussion
The EB that we used as raw material in our photooxidation experiments was a
Merck product that contained about 0.4%w=w of EBHP. Moreover, any EB without
radical inhibitor added would always contain trace amounts of EBHP due to auto-
oxidation. Thus, a thermal cleavage of the –O–O– bond of EBHP should be taken
into account while working at 120ꢁC, and the radicals formed can act as radical
chain initiators abstracting hydrogen from EB.
This was an important concern at the beginning of our experiments on photo-
oxidation of EB, and we ran two experiments, one with irradiation and photosen-
sitization and the other without irradiation, keeping constant the rest of the
operating variables of the reactor (temperature, stirring speed, oxygen flow, reactor
tank). Figure 1 shows the EBHP (%w=w) versus time (h) for both experiments.
Clearly, the kinetic rate of EBHP formation (wEBHP) is much better for the photo-
chemical experiment (wEBHP ¼ 1.54 ꢂ 10ꢃ5 mol dmꢃ3 sꢃ1) than for the experiment
without photocatalysis (wEBHP ¼ 4.3 ꢂ 10ꢃ6 mol dmꢃ3 sꢃ1).
An experiment was carried out with irradiation in absence of a catalyst. The
results showed that the rate of reaction was very slow. Therefore, in order to get a
good kinetic rate the reaction must be carried out photocatalytically (Fig. 2).
The overall radical chain mechanism of the oxygen photooxidation of EB can
be outlined as shown in Scheme 2.

Photooxidation of Ethylbenzene
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Fig. 1. Kinetic plots of EBHP (%w=w) vs. time (h) for 0.01g of TiO2 catalyst and 10 g of EB at 120ꢁC;
(A) with irradiation; (B) without irradiation
Fig. 2. Kinetic plots of EBHP (%w=w) vs. time (h) at 120ꢁC; 10 g of EB (A) with 0.01 g of TiO2
catalyst and (B) without TiO2 catalyst
Scheme 2
Effect of the Temperature
We tried to keep the thermal initiation process at a minimum level by lowering the
temperature and we ran experiments with irradiation and photosensitization at 120,

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M. H. Habibi et al.
Fig. 3. Kinetic plots of EBHP (%w=w) vs. time (h) for 0.01 g of Ag=TiO2 (2%) catalyst at three
temperatures; (A) 120ꢁC; (B) 95ꢁC; (C) 85ꢁC
Fig. 4. Kinetic plots of EBHP (%w=w) vs. time (h) for 0.01 g of Cu=TiO2 (2%) catalyst at three
temperatures; (A) 120ꢁC; (B) 95ꢁC; (C) 85ꢁC
95, and 85ꢁC. Figures 3 and 4 show the kinetic plots of EBHP (%w=w) versus time
(h) for the three temperatures for two catalysts. The effect of the temperature is
very important, and the EBHP formation kinetic rate drops dramatically at lower
temperatures. The influence of temperature on EBHP formation with the Ag coated
on TiO2 catalyst was less than with Cu coated on TiO2. By plotting the EBHP
formation kinetic rates (mol dmꢃ3 sꢃ1) versus 1=T (Kꢃ1) we have found the ac-
tivation energies (kJ molꢃ1) for the oxidation reaction using different catalysts
(Table 1).
Effect of the Amount of Photosensitizer
The effect of the amount of metal (Ag or Cu) coated on TiO2 on kinetic rate of
EBHP formation is shown in Fig. 5. The three experiments were carried out under
the same conditions, irradiating externally with the mercury lamp and using 0.5, 1,
and 2 g of the catalyst to 1000 g of EB.
The effect of the amount of metals (Ag or Cu) coated on TiO2 on the kinetic
rate of EBHP formation shows that the rate of formation of EBHP is increased by

Photooxidation of Ethylbenzene
1125
Table 1. Activation energies of EBHP formation in presence of catalysts
Type of catalyst
Ea=kJ molꢃ1
Anatase
22.055
28.664
43.912
31.604
30.837
22.759
41.701
53.698
26.117
16.792
12.763
12.179
17.095
Rutile
TiO2
Cu=TiO2 (2%)
Cu=TiO2 (4%)
Cu=TiO2 (5%)
Cu=TiO2 (6%)
Cu=TiO2 (8%)
Ag=TiO2 (2%)
Ag=TiO2 (4%)
Ag=TiO2 (5%)
Ag=TiO2 (6%)
Ag=TiO2 (8%)
Fig. 5. Kinetic plot of EBHP (%w=w) vs. time (h) for 10 g of EB and three amounts of Rutile
catalyst; (A) 0.01 g; (B) 0.005 g; (C) 0.02 g
increasing the amount of Cu coated on TiO2 up to 4–5% Cu. By increasing the
amount of Ag up to 6% the rate of formation of EBHP is increased.
Experimental
Ethylbenzene was oxidized with oxygen at 120, 95, and 85ꢁC in a stirred reactor at a pressure of 2 bar
in the presence of TiO2 powder and metal coated on TiO2 as photosensitizer irradiating with a 400 W
high-pressure mercury lamp.
Materials
Ethylbenzene obtained from Merck contains about 0.4%w=w of EBHP. Sodium iodide, glacial acetic
acid, sodium thiosulphate, ethanol, and isopropanol were of analytical grade from Merck. TiO2
anatase and rutile, with surface areas of 50 and 47m2 gꢃ1, respectively, were purchased from Aldrich
and they had a mean particle size of 30 nm. The Ag=TiO2 2, 4, 5, 6, and 8% catalysts were prepared
according to the following experimental procedure: 0.5 g of TiO2 in 9, 8, 7.5, 7, and 6 cm3 of ethanol
were added to 1, 2, 2.5, 3, and 4 cm3 of a 0.0927 M AgNO3 solution, N2 was bubbled through the

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M. H. Habibi et al.
mixtures for 15min, it was irradiated with a mercury lamp for 45–60h, then filtered, and finally heated
to 90ꢁC for 15 h. The Cu=TiO2 2, 4, 5, 6, and 8% catalysts were prepared according to the following
experimental procedure: 0.5 g of TiO2 in 9, 8, 7.5, 7, and 6 cm3 of ethanol, were added to 1, 2, 2.5, 3,
and 4 cm3 of a 0.1574 M CuSO4 solution, N2 was bubbled through the mixtures for 15 min, it was
irradiated with a mercury lamp for 45–60h, then filtered, and finally heated to 90ꢁC for 15 h.
Photoreaction Equipment
The reactor was a round-bottomed glass tank connected to a SH-12 type stirrer, a water thermostat
(Haake model F-122), an oxygen gas inlet system, and a 400 W high-pressure mercury lamp. The set
up of the photooxidation experiments is shown in Fig. 6.
General Procedure for Photooxidation of EB
EB (10 g) was charged into the reactor with 0.010 g of catalyst. When the temperature stabilized at the
value previously set for the experiment, the oxygen flow (0.4 cm3=s) was set and kept constant during
the experiment, and the mercury lamp was switched on. Samples (about 0.7 cm3) were taken from the
reactor at regular intervals (typically 1 h) during the experiment that lasted for about 8 h.
Analysis
For determination of EBHP in EB we chose a volumetric determination method. The weight percent
(%w=w) of EBHP in the samples taken from the reactor was determined by iodometric analysis, in
the following way: 0.5 cm3 of glacial acetic acid was added to 5 cm3 of isopropanol in a 100cm3
Erlenmeyer flask. Later, 2 cm3 of sodium iodide–isopropanol reagent (prepared by refluxing 8.8g of
sodium iodide in 40cm3 of isopropanol, cooling, and filtering) and about 0.5g of the sample were
added. The mixture was refluxed gently for 5 min, and later 20 cm3 of distilled cold water were added
through the reflux condenser. The samples, and the blank, were later titrated using 0.05N sodium
thiosulphate (previously standardized against potassium iodide) until the yellow reddish colour of the
Fig. 6. Photooxidation set up for ethylbenzene; (a) oxygen cylinder; (b) power supply; (c) 400 W
high-pressure Hg lamp; (d) photoreactor with aluminum foil as reflector for complete irradiation of
catalyst; (e) fan; (f) magnetic stirrer; (g) 2 dm3 pyrex beaker; (h) photooxidation cell; (i) water
thermostat Hake model F.122

Photooxidation of Ethylbenzene
1127
iodine disappears [17]. The EBHP (%w=w was calculated from Eq. (1), where S is the volume of
sodium thiosulphate for the titration of the sample, B the volume of sodium thiosulphate for the
titration of the blank, N the normality of sodium thiosulphate, F the factor of the sodium thiosulphate
solution, MW the molecular weight of EBHP, and G the sample weight.
ðS ꢃ BÞ ꢂ N ꢂ F ꢂ MW ꢂ 100
EBHP %w=w ¼
ð1Þ
G ꢂ 2000
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