A simple and efficient zeolite catalyst for toluene oxidation in aqueous media

Article abstract of DOI:10.1016/j.apcata.2012.03.021

In this work, it was found that alkali-treated zeolites could efficiently catalyze oxidation of toluene with H₂O₂ in aqueous media. No heavy metals or their oxides were employed in the synthesis of catalyst and no additives were used

Full text of DOI:10.1016/j.apcata.2012.03.021

Applied Catalysis A: General 425–426 (2012) 191–198
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
A simple and efficient zeolite catalyst for toluene oxidation in aqueous media
Bin Du, Song-Il Kim, Lan-Lan Lou, Aizhong Jia, Gaixia Liu, Ben Qi, Shuangxi Liu∗
Institute of New Catalytic Materials Science and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin
300071, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, it was found that alkali-treated zeolites could efficiently catalyze oxidation of toluene with
H2O2 in aqueous media. No heavy metals or their oxides were employed in the synthesis of catalyst
and no additives were used in this catalytic system. Through optimizing the experimental conditions,
alkali-treated HZSM-5 exhibited good catalytic activity for toluene oxidation in only 5 h at a low temper-
ature (below 100 ◦C): 32.0% conversion of toluene with selective formation of 25.0% benzaldehyde, 20.8%
benzyl alcohol and 27.5% benzoic acid. Moreover, the results of reusability studies indicated that alkali-
treated HZSM-5 was a durable and green catalyst for toluene oxidation in aqueous media. The changes in
acidities and textural features of the zeolites induced by alkali-treatment were characterized by ammo-
nia temperature programmed desorption (NH3-TPD) and X-ray powder diffraction (XRD), respectively.
The results indicated the reactivity for toluene oxidation could be related to the amount of species Al•
and AlOH in this system. A probable redox mechanism for toluene oxidation catalyzed by alkali-treated
zeolites in aqueous media was proposed in this work.
Received 15 December 2011
Received in revised form 12 March 2012
Accepted 13 March 2012
Available online 23 March 2012
Keywords:
Aqueous media
Toluene oxidation
Heterogeneous catalysis
Alkali-treatment
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
carbonyl compounds with aqueous media. Besides its advantages
in C–C bond formation reactions, water is also a promising medium
Using water as the desirable solvent in organic reactions has
received a great deal of attentions and shows significant environ-
mental advantages [1–3]. In organic synthesis, it has been found
out that a variety of reactions [4–8], including Claisen rearrange-
ment, Michael addition, Mannich and Diels–Alder reaction, could
be accelerated in aqueous media. However, some reactions proceed
slowly when pure water is employed as the sole medium due to the
hydrophobicity of organic molecules. In order to facilitate these
reactions, modification of the hydrophobic reagents and employ-
ment of organic cosolvents, phase-transfer catalysts, or surfactants
in the reaction processes had been widely investigated [9–13]. In
recent years, the research focusing on the development of efficient
heterogeneous catalysts for the organic reactions in aqueous media
has aroused great interests. Various polymer-supported catalysts
[14–16] with rare earth metals as active sites have been successfully
produced and have exhibited highly catalytic activity in water for
for the selective oxidation of carbonyl compounds [20].
The catalytic oxidation of toluene has been investigated with
great efforts during the past decades. The primary products in
selective oxidation of toluene are important organic intermedi-
ates, including benzaldehyde, benzyl alcohol and benzyl acid. In
deep oxidation of toluene, the formation of carbonaceous deposits
(coke) [21] that are mainly composed of aromatic hydrocarbons
and oxygenated aromatic compounds, leads to the deactivation of
the catalysts in the fix-bed system. The formation and then the
removal of coke resulted in a great increase of costs for the indus-
trial production. What’s more, in the traditional process of using
toluene oxidation to produce benzaldehyde and benzyl alcohol, the
employment of inorganic oxidants (e.g. KMnO4 or CrO3) inevitably
generates toxic waste and hazardous byproducts, which are harm-
ful to the environment [22]. By now, great efforts [23–28] have been
made to develop new efficient catalytic systems in liquid-phase
using non-toxic oxidant (O2 or air) from the environmental view-
point. In these catalytic systems, heavy metals or their oxides were
widely used as active component in the synthesis of catalysts and
additives were also employed to improve the reactivity of toluene
oxidation in some cases. Recently, a major breakthrough in this
field had been made. Single metal or bimetallic catalyst was hired
in O2 or air oxidation of toluene with or without solvent, especially
Hutchings et al. [29] reported that they could selectively oxidize
toluene to give benzyl benzoate by using Au–Pd alloy nanoparticles
with a very high selectivity.
C
C bond formation reactions. Meanwhile, some inorganic materi-
als [17–19] (e.g. amorphous SiO2) that possess good hydrothermal
and mechanical stabilities are also considered as excellent supports
to develop heterogeneous catalyst. Kim et al. [19] prepared a Lewis
acid solid via immobilization of Lanthanum sulfonate on SBA-15
and used it to catalyze the Diels–Alder and allylation reactions of
∗
Corresponding author. Tel.: +86 22 23509005; fax: +86 22 23509005.
E-mail address: sxliu@nankai.edu.cn (S. Liu).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2012.03.021

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B. Du et al. / Applied Catalysis A: General 425–426 (2012) 191–198
Here we proposed an effective heterogeneous catalytic system
3. Results and discussion
employing aqueous media for the oxidation of toluene. In this
reaction, a series of zeolites and alkali-treated zeolites were used
as heterogeneous catalysts in water with H2O2 as the oxidant.
According to the report on the TS-1/H2O2 system [30], the employ-
ment of water as the solvent led to significant enhancement in the
reaction rates in the hydroxylation of toluene, while the products
(benzaldehyde, benzyl alcohol and benzyl acid) of the toluene oxi-
dation could not be detected. However, the alkali-treated HZSM-5
exhibited good catalytic activity for toluene oxidation in aque-
ous media in only 5 h at a low temperature (below 100 ◦C). The
investigation on the oxidation of benzaldehyde and benzyl alcohol
could be helpful to comprehend the selectivities for benzalde-
hyde, benzyl alcohol, benzoic acid and phenols in this catalytic
system. To the best of our knowledge, alkali-treated zeolites were
firstly used as heterogeneous catalysts for oxidation of toluene
in aqueous media with H2O2. In the absence of heavy met-
als and additives, this catalytic system is simple, efficient and
environment-friendly.
The powder XRD patterns of the samples are shown in Fig. 1.
It was clear that all the samples exhibited typical MFI structure.
Though the NaOH solution would dissolve Si atoms from the zeo-
lite framework, the specific MFI structure had been preserved after
the alkali-treatment. Among the alkali-treated HZSM-5 samples,
HZSM-5-0.05, HZSM-5-0.10 and HZSM-5-0.20 exhibited almost the
same powder XRD patterns as that of the parent zeolite HZSM-5
(Fig. 1A). In the patterns of these samples, no obvious decrease in
the relative crystallinity could be observed. On the contrary, the
employment of NaOH solution with higher concentration (0.3 and
0.4 mol/L) resulted in lower relative crystallinity of the zeolites,
which is attributed to the extraction of Si and Al atoms from frame-
work [31]. The relationship between specific MFI structure of the
microporous zeolite and concentration of alkali-treated solution
would also be displayed when NaZSM-5 zeolite was employed as
the parent sample. As shown in Fig. 1B, the relative crystallinity
of the samples decreased after NaZSM-5 zeolite was treated under
severe conditions. For comparison, HBeta zeolite was also treated
with alkaline solution. Even though mild conditions of alkali-
treatment were employed, the typical structure of HBeta zeolite
was destructed evidently (Fig. 1C). It suggested that HZSM-5 and
NaZSM-5 showed higher stability toward alkali-treatment than
HBeta. In addition, it could be affirmed that the alkali-treatment
under severe conditions could cause the destruction of zeolite
structure.
The NH3-TPD method was used to characterize the amount and
acid strength of acid sites in zeolites and the profiles are shown
in Fig. 2. The profiles of all the HZSM-5-x samples (Fig. 2A) exhib-
ited broad peaks in the temperature region from 100 ◦C to 500 ◦C.
For the profile of HZSM-5, the peak corresponding to low tem-
perature, which centered at around 200 ◦C, could be attributed
to the NH3 chemisorbed on the weak acid sites. These acid sites
were related to the interaction between ammonia molecules and
surface oxide or hydroxyl groups by non-specific hydrogen bond-
ing [32]. In the high temperature region, the peak which centered
at 450 ◦C could be ascribed to the strong acid sites. For the pro-
file of HZSM-5-0.20, the strong acid sites disappeared with the
reinforcement of medium acid sites. A comparison between HZSM-
5-0.40 and HZSM-5-0.20 indicated a decrease in medium acidity
for the reason that the framework Al atoms was seriously dis-
solved in the alkaline solution under severe condition. Two distinct
NH3 desorption peaks were observed for the profiles of NaZSM-
5 and alkali-treated NaZSM-5 (Fig. 2B), indicating that two types
of acid sites with different acid strengths existed on the zeolite.
For the sample alkali-treated under severe conditions, the amount
of medium acid sites decreased obviously. The results of NH3-TPD
suggested the medium and strong acid sites are more sensitive to
alkaline solution than the weak acid sites [31].
2. Experimental
2.1. Catalyst preparation
The zeolites (HZSM-5, NaZSM-5 and HBeta, SiO2/Al2O3 = 25,
obtained from Catalyst Factory of NanKai University) were cal-
cined in air at 550 ◦C for 6 h before the alkali-treatment. After the
precalcination, 4.0 g of zeolite was added into the NaOH solution.
The mixture was stirred at 80 ◦C for 2 h. The resulting zeolite was
recovered by filtration and washed with distilled water until a neu-
tral filtrate was received. The obtained white powder was dried at
110 ◦C for 12 h. The concentration of NaOH solution varied from 0 to
0.40 mol/L. The alkali-treated HZSM-5 and NaZSM-5 were denoted
as HZSM-5-x and NaZSM-5-x, respectively, where x represented the
concentration of NaOH solution.
The structure of the zeolites was characterized by X-ray
powder diffraction (XRD). The XRD patterns of samples were
recorded on a Rigaku D/max-2500 diffractometer through a 2ꢀ
range from 5◦ to 80◦ using CuK˛ radiation at 40 kV and 100 mA.
Temperature-programmed desorption of ammonia (NH3-TPD) was
performed on a chemisorption analyzer (Quanta-chrome Chem-
Bet 300). 0.1 g of sample was preheated at 450 ◦C under flowing
He for 1 h to remove physisorbed species, then cooled to 100 ◦C.
The sample was saturated with 5% NH3/Ar and then purged
by He gas to remove excessive physisorbed ammonia. Desorp-
tion of ammonia was performed in the temperature range of
100–600 ◦C at a rate of 10 ◦C/min. The 27Al MAS NMR experi-
ments were carried out by a Varian Infinityplus 400 MHz NMR
spectrometer.
Series of zeolites, amorphous SiO2 and Al2O3 were employed
as catalysts for the oxidation of toluene. The results are listed in
Table 1. All of the parent zeolites showed certain oxidation activity
in contrast to amorphous SiO2 and Al2O3. The oxidation by NaZSM-
5 gave 16.2% conversion of toluene which was higher than those of
HZSM-5 and HBeta zeolite. The alkali-treated HBeta zeolite pre-
sented no catalytic activity for the oxidation of toluene. So did the
amorphous SiO2 and Al2O3. As it had been described by powder
XRD, the specific structure of HBeta zeolite was seriously destroyed
during the alkali-treatment. Therefore, it could be deduced that the
catalytic active sites should locate in the framework of zeolites.
According to the results of NH3-TPD, medium acid sites were gen-
erated in zeolites via the alkali-treatment that caused the variation
of acidity for HZSM-5 and NaZSM-5. Hence, the alkali-treated zeo-
lites were tested in the oxidation of toluene with H2O2 (Table 1).
Among all these samples, HZSM-5-0.20 performed the highest
2.2. Catalytic reactions
The oxidation of toluene was carried out in a three-necked flask
equipped with a refluxing condenser. In such a typical experiment,
toluene (20.0 mmol) and catalyst were added to distilled water
(24.0 ml) under vigorous stirring. The mixture was heated to reflux-
ing followed by the dropping of a certain amount of H2O2 into
the mixture. The mixture was refluxed for a certain time under
atmosphere, and then cooled to room temperature. The recov-
ery of the product was extracted by 20 ml of ethyl acetate for
three times. Conversions and selectivities were determined by gas
chromatography with a SGE AC20 capillary column (polyethylene
glycol, 30 m × 0.32 mm × 0.50 ␮m), and the product was identified
by 1H NMR spectroscopy.

B. Du et al. / Applied Catalysis A: General 425–426 (2012) 191–198
193
Fig. 1. (A) Powder XRD patterns of HZSM-5 and alkali-treated HZSM-5: (a) HZSM-5, (b) HZSM-5-0.05, (c) HZSM-5-0.10, (d) HZSM-5-0.20, (e) HZSM-5-0.30 and (f) HZSM-
5-0.40; (B) Powder XRD patterns of NaZSM-5 and alkali-treated NaZSM-5: (g) NaZSM-5, (h) NaZSM-5-0.05, (i) NaZSM-5-0.10, (j) NaZSM-5-0.20 and (k) NaZSM-5-0.30; (C)
Powder XRD patterns of (l) HBeta and (m) HBeta-0.20.
Table 1
The performance of amorphous SiO2, Al2O3, parent zeolites and alkali-treated zeolites for the oxidation of toluene.a
Catalyst
Conversion (%)
Selectivity (%)
Benzaldehyde
Benzyl alcohol
Benzoic acid
Phenols
HZSM-5
5.9
20.2
23.2
32.0
18.1
9.5
16.2
20.5
24.9
17.0
10.2
2.1
52.4
40.8
26.9
25.0
40.2
49.2
38.1
23.9
21.7
31.2
39.1
37.6
–
22.9
26.0
27.3
20.8
22.1
21.4
22.8
17.8
18.7
19.6
20.1
19.7
–
14.6
14.8
22.2
27.5
17.0
8.8
3.3
14.5
18.6
22.3
17.9
14.3
26.7
41.2
37.7
31.2
27.6
6.0
HZSM-5-0.05
HZSM-5-0.10
HZSM-5-0.20
HZSM-5-0.30
HZSM-5-0.40
NaZSM-5
NaZSM-5-0.05
NaZSM-5-0.10
NaZSM-5-0.20
NaZSM-5-0.30
HBeta
9.0
12.6
17.7
13.5
8.9
2.4
–
HBeta-0.20
Al2O3 or SiO2
Blank
<1
<1
<1
–
–
–
–
–
–
–
–
–
a
Reaction conditions: toluene (20.0 mmol), catalyst (0.125 g), H2O (24.0 ml), H2O2 (8.0 ml), refluxing for 5 h.

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B. Du et al. / Applied Catalysis A: General 425–426 (2012) 191–198
Fig. 2. NH3-TPD profiles of (A) HZSM-5, HZSM-5-0.20 and HZSM-5-0.40; (B) NaZSM-
5, NaZSM-5-0.10 and NaZSM-5-0.30.
conversion of toluene, suggesting that the amount of medium acid
sites should be related to the catalytic oxidation activity. On the
contrary, the HZSM-5 zeolites alkali-treated under severe con-
dition gave relatively low conversion (9.5% by HZSM-5-0.40 and
18.1% by HZSM-5-0.30). The powder XRD profiles of HZSM-5-0.30
and HZSM-5-0.40 presented lower relative crystallinity than that
of HZSM-5-0.20, implying that the reactivity for the oxidation of
toluene depended on the amount of active sites in the framework.
Either the production of benzaldehyde via the oxidation of benzyl
alcohol or benzoic acid from benzaldehyde could be catalyzed by
zeolites. Considering this fact, it was worthy to notice that the cat-
alysts exhibited low conversion of toluene while high selectivity
for benzaldehyde would be received. It suggested that the cat-
alytic activity of alkali-treated zeolites for the oxidation of carbonyl
compounds was weakened as the concentration of NaOH solution
rose up from 0.20 to 0.40 mol/L. Among the alkali-treated NaZSM-
5 zeolites, the highest conversion of toluene could be obtained by
NaZSM-5-0.10. Compared with the relevant HZSM-5 zeolites, all of
the alkali-treated NaZSM-5 samples exhibited lower selectivity for
benzaldehyde and benzoic acid, but higher selectivity for phenols.
To understand the high activity of HZSM-5-0.20, it was reason-
able to suppose that the oxidation process of toluene could also be
Fig. 3. (A) Effect of reaction time on conversion and selectivity of toluene oxidation
catalyzed by HZSM-5-0.20. Reaction conditions: toluene (20.0 mmol), HZSM-5-
0.20 (0.125 g), H2O (24.0 ml), H2O2 (8.0 ml), refluxing; (B) Effect of HZSM-5-0.20
amount on conversion and selectivity of toluene oxidation. Reaction conditions:
toluene (20.0 mmol), H2O (24.0 ml), H2O2 (8.0 ml), refluxing for 5 h; (C) Effect of
H2O2 amount on conversion and selectivity of toluene oxidation catalyzed by
HZSM-5-0.20. Reaction conditions: toluene (20.0 mmol), HZSM-5-0.20 (0.125 g),
H2O (24.0 ml), refluxing for 5 h.
promoted by the consumption of benzyl alcohol and benzaldehyde
in the formation of benzoic acid.
Fig. 3A displays the catalytic performance of HZSM-5-0.20 for
toluene oxidation with different reaction times. In the first hour, the
conversion of toluene could rapidly reach 12.1%, and finally 37.5%
was obtained when the reaction time was prolonged to 6 h. The

B. Du et al. / Applied Catalysis A: General 425–426 (2012) 191–198
195
Table 2
The performance of HZSM-5-0.20 for the oxidation of benzaldehyde, benzyl alcohol and esterification of benzyl alcohol with benzoic acid.
Reaction
Conversion (%)
Selectivity (%)
Benzaldehyde
Benzyl alcohol
Benzoic acid
Phenols
Benzaldehyde oxidationa
Benzyl alcohol oxidationb
Esterificationc
95.6
68.1
<1
–
36.3
–
–
–
–
92.9
49.5
–
3.2
2.9
–
a
Benzaldehyde (20.0 mmol), HZSM-5-0.20 (0.125 g), H2O2 (8.0 ml), distilled water (24.0 ml), refluxing for 3 h.
Benzyl alcohol (20.0 mmol), HZSM-5-0.20 (0.125 g), H2O2 (8.0 ml), distilled water (24.0 ml), refluxing for 3 h.
HZSM-5-0.20 (0.125 g), benzyl alcohol (20.0 mmol), benzoic acid (20.0 mmol), distilled water (24.0 ml), refluxing for 5 h.
b
c
effect of reaction time on the selectivity for the product showed
different amounts of HZSM-5-0.20 used in the reaction. Thus, ben-
zoic acid could be the final products of oxidation in this system and
be difficult to be further oxidized. Without hydrogen peroxide the
oxidation of toluene could not occur in aqueous media under the
presence of catalyst. The amount of oxidant used in the reaction
was a key factor for the reactivity of toluene oxidation. The effect
of H2O2 amount on the oxidation of toluene was investigated and
the results are showed in Fig. 3C. Though higher toluene conver-
sion could be obtained when the amount of H2O2 increased up
to 10 ml, lower selectivities for benzaldehyde and benzyl alcohol
were received. Thus, higher amount of H2O2 used in the reaction
system would lead to higher reactivity of the oxidation of toluene,
benzaldehyde and benzyl alcohol.
Besides the oxidation reactions, the esterification of benzyl alco-
hol with benzoic acid could also be catalyzed by the zeolites [33,34].
To promote the esterification, water as the main byproduct need
to be removed from the reaction system. The ester had been fre-
quently reported to be the main byproduct in liquid oxidation of
toluene [35]. In order to restrict the production of esters, it was
necessary to employ water as the sole medium in the process of
oxidation. According to the results of esterification of benzyl alco-
hol with benzoic acid shown in Table 2, no esters could be detected
when the reaction carried out in aqueous media for 5 h. The amount
of water as the function of reactivity for the oxidation of toluene
was checked and the results are listed in Table 3. When the H2O2
was directly added into the system, low toluene conversion and
75.2% of selectivity for the primary products (benzaldehyde, ben-
zyl alcohol, benzoic acid and phenols) were obtained, whereas the
byproducts took a great part of the products. The decomposition of
H2O2 and the oxidation of toluene dramatically occurred without
the employment of aqueous media. When water was used as sol-
vent for the reaction, the selectivity for primary products obviously
increased. The increment of water added into the system efficiently
relaxed the decomposition of H2O2, thus resulting in the increase
of conversion. But much more water would lead to the decrease
of oxidative activity of H2O2, due to the diluted H2O2. The effect
of reaction temperature on the oxidation of toluene was also stud-
ied. As Table 3 displays, when the temperature increased in the
range from 20 to reflux temperature, the conversion of toluene
the inherent feature of the oxidation in aqueous media. During
the oxidative process of toluene, the selectivity for benzaldehyde
continuously went down as the reaction time prolonging. Accord-
ing to these facts, benzaldehyde should be the primary product of
toluene oxidation. Due to the oxidation of benzaldehyde with H2O2,
benzaldehyde was transformed into benzoic acid. As a result, the
selectivity for benzoic acid increased steadily as the reaction pro-
ceeded. If the yield of benzaldehyde was given the top priority,
the reaction time should be no longer than 2 h. In this case, the
oxidation by HZSM-5-0.20 gave about 45% selectivity for benzalde-
hyde and 20% toluene conversion. On the other hand, the oxidation
of benzyl alcohol into benzaldehyde could be also carried out by
alkali-treated zeolites with H2O2 (Table 2). Taking all of these facts
into account, benzyl alcohol should be consumed as the reaction
proceeds. In Fig. 3A, no obvious decrease in the selectivity for benzyl
alcohol could be observed. It indicated that the rate for the produc-
tion of benzyl alcohol via oxidation of toluene corresponded to the
rate of consumption of benzyl alcohol in the oxidation. The ben-
zyl alcohol was another primary product of toluene oxidation with
H2O2 in aqueous media. The oxidation of toluene, benzaldehyde
and benzyl alcohol should be catalyzed by zeolites in this system.
Due to the further oxidation of primary products, the attempts
intended to improve the conversion of toluene usually resulted in
the high selectivity for benzoic acid along with the low selectivi-
ties for benzaldehyde and benzyl alcohol. As it has been discussed
above, the enhancement of selectivities for benzaldehyde and ben-
zyl alcohol with moderate conversion of toluene could be achieved
through the choice of experiment conditions or different alkali-
treated zeolites.
The amount of the catalyst used in the reaction was also checked
as a function of the catalytic activity. The conversion of toluene
continuously increased as the catalyst amount rose up, whereas
the selectivity for benzaldehyde continuously decreased (Fig. 3B)
and the selectivity for benzyl alcohol slightly decreased. This phe-
nomenon suggested that the oxidation of toluene, benzaldehyde
and benzyl alcohol might benefit from the increase of catalyst
amount. Interestingly, the selectivity for benzoic acid varied syn-
chronously with the toluene conversion which was induced by
Table 3
The effect of reaction temperature and H2O amount on the oxidation of toluene.a
Reaction temperature (◦C)
H2O (ml)
Conversion (%)
Selectivity (%)
Benzaldehyde
Benzyl alcohol
Benzoic acid
Phenols
20
40
60
70
24.0
24.0
24.0
24.0
24.0
–
12.0
36.0
48.0
<1
7.3
–
–
–
6.6
–
3.5
4.3
11.3
22.5
25.4
24.9
20.5
18.4
61.9
57.5
47.4
29.1
24.6
18.0
19.7
26.2
23.6
23.0
23.3
20.9
12.4
17.8
21.5
25.0
10.2
17.4
27.1
14.4
21.8
35.5
34.7
10.9
11.7
19.4
12.8
29.5
28.1
22.3
80
Refluxedb
Refluxed
Refluxed
Refluxed
a
Reaction conditions: HZSM-5-0.20 (0.125 g), toluene (20.0 mmol), H2O2 (8.0 ml), reaction time 5 h.
The reaction temperature was at a range of 90–95 ◦C.
b

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B. Du et al. / Applied Catalysis A: General 425–426 (2012) 191–198
Table 4
Reusage of HZSM-5-0.20 for the oxidation of toluene.a
RUN Conversion
(%)
Selectivity (%)
Benzaldehyde Benzyl alcohol Benzoic acid Phenols
1
2
3
4
5
6
32.0
32.5
32.5
30.0
29.9
27.4
25.0
22.9
24.1
25.2
26.0
24.1
20.8
23.0
25.0
21.1
24.1
22.0
27.5
26.1
24.2
23.4
22.3
23.7
22.3
20.7
20.4
22.6
20.8
20.7
a
Reaction conditions: HZSM-5-0.20 (0.125 g), toluene (20.0 mmol), H2O
(24.0 ml), H2O2 (8.0 ml), refluxing for 5 h.
went up and the selectivity for the benzaldehyde decreased. The
selectivities for benzoic acid and phenols varied synchronously
with the toluene conversion. Thus, the oxidation of toluene and
benzaldehyde could benefit from the increase of reaction tempera-
ture. Reusability studies with HZSM-5-0.20 resulted in about 5%
loss in catalytic activity after 5 times’ reusing (Table 4). Due to
the fact that the catalytic activity of HZSM-5-0.20 became weaker
in the recycling experiment, the selectivities for benzoic acid and
phenols slightly decreased. The experiments indicated that HZSM-
5-0.20 was a durable heterogeneous catalyst for toluene oxidation
in aqueous media.
TS-1/H2O2 system had been reported to exhibit significant
enhancement in the reaction rates for the hydroxylation of toluene
in aqueous media comparing with anisole as solvent [30]. Though
TS-1 showed well catalytic activity for the ring hydroxylation,
the products which related to the side chain oxyfunctionalization
had not been observed. In our work, zeolites (HZSM-5, NaZSM-5
and HBeta) could catalyze the side chain oxyfunctionalization of
toluene forming benzaldehyde, benzyl alcohol and benzoic acid in
aqueous media. Moreover, the catalytic performance of the zeo-
lite could be significantly enhanced via the alkali-treatment. The
alkali-treated zeolites exhibited high conversion of toluene with
good selectivities toward benzaldehyde, benzyl alcohol and ben-
zoic acid in only 5 h at a low temperature. And the alkali-treated
zeolites were firstly used as heterogeneous catalysts for oxidation
of toluene in aqueous media with H2O2.
Fig. 4. 27Al MAS NMR spectra of (A) HZSM-5-0.20 and (B) NaZSM-5-0.10.
Radical initiator had been proved to be necessary in some
toluene oxidation system which was carried out in liquid phase
under mild conditions [29]. Without any additive, in this work,
toluene oxidation could be directly catalyzed by alkali-treated zeo-
lites in aqueous media. Furthermore, no heavy metals or their
oxides were considered in the synthesis of catalyst. This design of
the catalyst avoids the release of heavy metal ions into the water
and the consequent pollution. It is worthy to notice that the reac-
tion was carried out in atmosphere at a low temperature. The mild
reaction conditions avoid the formation of coke which usually leads
to the inactivity of the catalyst. This is a key factor to determine
the performance of heterogeneous catalyst in the recycling experi-
ment. As it has been discussed above, the alkali-treated zeolites are
able to be reused for 5 times in the oxidation of toluene without
obvious decrease in the catalytic activity.
framework Al atoms formed by splitting Si–OH–Al groups. Except
for the structural features, the acidity properties of MFI were deter-
mined by the Al atoms in the framework. As it has been supported
by the results of NH3-TPD, the amount of medium acid sites in
zeolites would obviously increase via alkali-treatment of parent
zeolites with alkaline solution of appropriate concentration. On the
other hand, more Al atoms would be separated from the zeolite
framework during the formation of oxides as the concentration of
alkaline solution went up, leading to the weak catalytic activity (e.g.
HZSM-5-0.40). It was reasonable to assume that the species Al•
could be related to the medium acid sites in zeolites, as well as
the catalytic activity for the oxidation of toluene. Alkali-treatment
also induced the formation of hydroxyl groups bonded to the dis-
torted framework Al atoms ( AlOH) via the splitting of Si–O–Al.
And the amount of these hydroxyl groups was found to be related to
the reactivity for the oxidation of benzaldehyde and benzyl alcohol
[20,43,44]. Thus, the mechanism for toluene oxidation with H2O2
over the Al-containing alkali-treated zeolites could be proposed as
shown in Fig. 5. The framework aluminum directly catalyzed the
oxidation of toluene into benzaldehyde, benzyl alcohol and ben-
zoic acid. As shown in formula 1, the distorted framework Al atoms
could activate H2O2 molecule, that is, the species Al• reacts with
H2O2 proceeding with the formation of Al(H2O2). The methyl
group in toluene molecule was oxidized by Al(H2O2), resulting
in the formation of benzyl alcohol. The activated H2O2 could also
The 27Al MAS NMR spectra of HZSM-5-0.20 and NaZSM-5-0.10
are shown in Fig. 4. Strong peaks at 54 ppm assigned to tetrahe-
drally coordinated framework aluminum (AlIV) could be observed
in the spectra of samples. No signals could be detected at 0 ppm
assigned to octahedrally coordinated extra-framework aluminum
(AlVI) [36–38]. Both of Si and Al atoms in MFI zeolites could be dis-
solved in the alkaline solution. The alkali-treatment of MFI zeolite
resulted in the increase of the distorted Al atoms on the framework
for the reason that more Si atoms than Al atoms were dissolved in
the alkaline solution [39–42]. The species Al• in the alkali-treated
HZSM-5 and NaZSM-5 were mainly assigned to the distorted

B. Du et al. / Applied Catalysis A: General 425–426 (2012) 191–198
197
Fig. 5. Schematic illustration of the mechanism for the toluene oxidation with H2O2.
oxidize benzyl alcohol to yield benzaldehyde. The further oxidation
of benzaldehyde resulted in the formation of benzoic acid. Further-
more, the oxidation of benzyl alcohol and benzaldehyde could be
also catalyzed by the species AlOH (Fig. 5B), which can react with
H2O2 to yield the aluminum complex AlOOH. In the following pro-
cess, AlOOH would react with benzaldehyde and benzyl alcohol
to produce benzoic acid and benzaldehyde, respectively. As it has
been mentioned above, the oxidation process of toluene could be
promoted by the consumption of benzyl alcohol and benzaldehyde.
Thus, the species AlOH played an important role in the oxidation
of toluene. The difference in the catalytic performance of alkali-
treated HZSM-5 and NaZSM-5 might be related to the amount of
species AlOH and Al• in the zeolites. The catalytic reactivity of
alkali-treated HZSM-5 and NaZSM-5 zeolites for the oxidation of
toluene could be related to the structural defects in zeolites.
results of recycling experiment demonstrated that alkali-treated
HZSM-5 zeolite was a durable heterogeneous catalyst for the liquid-
phase oxidation of toluene. In conclusion, this catalytic system
enriches the common processes of toluene oxidation and further
investigation based on these preliminary results might introduce
new insight into the catalytic mechanism of toluene oxidation.
Moreover, alkali-treated zeolites are promising catalysts for the
oxidation of toluene in the industrial application.
Acknowledgements
This research is supported by the National Natural Science Foun-
dation of China (No. 20773069), the Specialized Research Fund for
the Doctoral Program of Higher Education (No. 20100031120029),
the Fundamental Research Funds for the Central Universities, and
MOE (IRT-0927).
4. Conclusions
References
In this work, it was found that alkali-treated zeolites could effi-
ciently catalyze oxidation of toluene with H2O2 in aqueous media.
Here series of alkali-treated zeolites were firstly used as hetero-
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The liquid-phase oxidation of toluene with 8.0 ml hydrogen perox-
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27.5% benzoic acid and 22.3% phenols. The absence of additive
and heavy metals determined that the catalytic system is simple
and environmental-friendly. Furthermore, the mild reaction con-
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