Selective solvent-free oxidation of toluene to benzaldehyde over zeolite supported iron

Article abstract of DOI:10.1016/j.catcom.2013.05.001

Iron-modified HZSM-5 materials were prepared by the impregnation techniques at ambient temperature and characterized by X-ray powder diffraction (XRD) and ICP-MS to determine the phase structure and loadings of iron dioxide. This material was tested for s

Full text of DOI:10.1016/j.catcom.2013.05.001

Catalysis Communications 39 (2013) 115–118
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Selective solvent-free oxidation of toluene to benzaldehyde over zeolite
supported iron
Xin Li, Bin Lu, Jia Sun, Xin Wang, Jingxiang Zhao, Qinghai Cai ⁎
Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, Key Lab for Design and Synthesis of Functionalized Materials and Green Catalysis, School of Chemistry
and Chemical Engineering, Harbin Normal University, No. 1 Shida Road Limin development Zone, Harbin 150025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Iron-modified HZSM-5 materials were prepared by the impregnation techniques at ambient temperature and
characterized by X-ray powder diffraction (XRD) and ICP-MS to determine the phase structure and loadings
of iron dioxide. This material was tested for solvent-free catalytic oxidation of toluene under mild conditions.
The result shows that Fe O /HZSM-5 is an effective catalyst, exhibiting 17.3% conversion of toluene, 51.4% se-
2 3
lectivity to benzaldehyde at 90 °C. Furthermore, the catalyst can be easily recovered and reused for three
Received 14 February 2013
Received in revised form 1 April 2013
Accepted 2 May 2013
Available online 15 May 2013
times without a significant loss in its activity and selectivity.
Keywords:
©
2013 Elsevier B.V. All rights reserved.
Selective oxidation
HZSM-5 supported iron
Benzaldehyde
Toluene
1
. Introduction
a significant mass loss of solvents is inevitable by its overoxidation to
CO or evaporation. Corrosion and environmental pollution are also
2
The oxidation of toluene is one of the most important functional
major industrial challenges. In particular, overoxidation is difficult to
control in the hydrocarbon oxidation process. Du et al. [17] found that
alkali-treated zeolites could efficiently catalyze the oxidation of toluene
group transformations in organic synthesis. Toluene can be oxidized
into the corresponding alcohol, aldehydes and acids. The toluene oxida-
tion includes the gas-phase and liquid-phase oxidation. Compared with
the gas-phase oxidation, liquid-phase oxidation not only produces good
yield and selectivity but also allows the catalytic oxidation to proceed
over the transition metal catalyst under mild conditions [1–3].
2 2
with H O in aqueous media. However, the selectivity to BzH was lower.
Therefore, a new method of selective oxidation of toluene into BzH with
clean oxidants and catalyst has been an attractive field as well as an im-
portant chemical challenge [18,19].
Benzaldehyde (BzH), a typical product of toluene oxidation, is a very
important chemical used as a starting material for the preparation of in-
termediates in perfumery, pharmaceutical, dyestuff and agrochemical
industries. Now, BzH is prepared by hydrolysis of benzal chloride in in-
dustry. However, traces of chlorine inevitably exist in the product, lim-
iting to utilization of the product BzH. Therefore, attempts to overcome
this problem have prompted investigation on the liquid-phase oxida-
tion of toluene to BzH [4]. Many scholars [5,6] tried a variety of different
As our continuous research on environmentally benign catalytic
oxidation of toluene or other hydrocarbons, here we wish to report
an effective heterogeneous catalyst for the solvent-free oxidation of
2 3
toluene. The supported iron catalyst Fe O /HZSM-5 exhibited good
catalytic activity for toluene oxidation in only 4 h at lower tempera-
ture (90 °C). Especially, higher selectivity to BzH was achieved.
These make such a process not only more environmentally acceptable
but also more economically attractive.
catalysts in the oxidation of toluene including Ce(III) [7], MnO
x
[8], VO
x
[
[
9], CrO
11–13].
x
[10] and other transition metal or a noble metal catalysts
2. Experimental
2 2
H O is widely used in the liquid oxidation of organic compounds
due to its clean and environmentally friendly nature. Consequently,
many efforts have been made on the liquid-phase oxidation of toluene
2.1. Materials
into BzH using H
O
2 2
oxidant. Although these systems exhibit good cata-
Chemicals used in this study including toluene (A.R.) were provid-
lytic activity, these reactions are usually allowed to proceed in organic
2 2
ed by Tianjin Fuyu Fine Chemical Co., Ltd. H O (30%) was obtained
solvents such as acetic acid and acetonitrile [14–16]. During the reaction,
from Tianjin Chemical Reagent Co. Iron nitrate (A.R.) and the stan-
dard BzH were purchased from Tianjin Guangfu Institute of Fine
Chemicals. HZSM-5 and HY zeolites were provided by the Tianjin
Camry Manchester Science and Technology Development Co. Ltd.
⁎
Corresponding author. Tel./fax: +86 451 88060580.
E-mail address: qinghaic@yahoo.com.cn (Q. Cai).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.05.001

116
X. Li et al. / Catalysis Communications 39 (2013) 115–118
2
.2. Preparation and characterization of the catalyst
the supported materials by ICP-MS verified the existence of iron spe-
cies in the sample.
Preparation of Fe
pregnated with 0.5 mol/L Fe(NO
Fe(NO · 9H O in 20 mL ethanol) and stirred at room temperature
for 12 h. The resulting mixture was filtered to remove the filtrate. The
obtained solid was washed with anhydrous ethanol, followed by drying
at 100 °C for 4 h, and then calcined at 550 °C for 2 h in the muffle fur-
2
O
3
/HZSM-5 was as follows: 2.0 g HZSM-5 was im-
)
3 3
solution in ethanol (4.04 g of
3.2. Catalytic performance of various catalysts
The oxidation of toluene for producing BzH over different catalytic
)
3 3
2
materials Fe , Fe
2
O
3
2 3 2 3 2 3 2 3
O /MCM-41, Fe O /HY, Fe O /MCM-22, Fe O /
ZSM-5, Fe O /HZSM-5 HZSM-5 and zeolites supported other metals
2 3
nace to afford Fe
method of preparing Fe
dure; the only difference between them was that 2.0 g HZSM-5 was
substituted by 2.0 g HY.
The XRD of the samples was performed on a Bruker-D8 Advance
X-ray diffractometer with Cu Kα radiation (40 kV and 36 mA). The
2
O
3
/HZSM-5 after cooling to room temperature. The
/HY was very similar with the above proce-
and bimetals such as HZSM-5. Supported Cu, V, Ce, Co, Fe-Cu, Fe-V,
Fe-Ce and Fe-Co materials were tested, and the results were shown
in Table 1. In blank reactions, the pure molecular sieve ZSM-5 and
2 3
O
α-Fe
tivity for the oxidation as compared with ZSM-5 and α-Fe
1–3). The similar results were obtained with respect to Fe
materials Fe /HZSM-5 and Fe /ZSM-5. In contrast, Fe
HZSM-5 was found to be more active than Fe /ZSM-5 with 17.3%
conversion and 51.4% selectivity to BzH. It was also more active
than Fe /MCM-41 and Fe /MCM-22 due to its nature of acidity
entries 4–7). To explore the role of surface acidity, we prepared the
composite material Fe /HY with stronger acidity than Fe
2
O
3
exhibited lower catalytic activity. HZSM-5 showed higher ac-
(entries
-loaded
2 3
O
O
2 3
loading of the Fe
Company).
2
O
3
on HZSM-5 was made on ICP-MS (Agilent
O
2 3
2
O
3
2 3
O /
2 3
O
2
O
3
2 3
O
2
.3. Oxidation of toluene
(
O
2 3
2 3
O /
Five milliliters (45.8 mmol) of toluene and 0.4 g of the catalyst
2 3
Fe O /HZSM-5) were added into a 50-mL three-necked flask fitted
HZSM-5 [20]. This material was unexpected to be low activity for
(
the oxidation of toluene (entry 8). Moreover, the specific surface
with a reflux condenser and a mechanical stirrer. The reactor was
heated to 90 °C in a water bath with electric heater, and then 15 mL
of 30% H O (0.14 mol) was slowly dropped in. The resulting system
2 2
was stirred at 90 °C for 4 h. At the end of the reaction, the mixture so-
lution was filtered to remove the catalyst and the upper organic liquid
of the reaction mixture was analyzed by GC and GC-MS.
2
area of Fe
(
2
O
3
/HY (471.1 m /g) is more than that of Fe
2
O
3
/HZSM-5
2
303.5 m /g). Therefore, the activity difference between the two cat-
alysts was likely ascribed to their acidity. The stronger acid sites
would strongly adsorb benzaldehyde, resulting in its overoxidation
into CO [12]. In other words, the catalyst with the moderate acidic
x
sites on the surface played a key role for the reaction. Zeolite that sup-
ported other metals such as Cu/HZSM-5, V/HZSM-5, Ce/HZSM-5 and
Co/HZSM-5 exhibited low catalytic activity, so did some supported bi-
metals, e.g., Fe-V/HZSM-5, Fe-Ce/HZSM-5 and Fe-Co/HZSM-5 (entries
9–15). Besides, Fe-Cu/HZSM-5 displayed a higher catalytic activity for
the reaction (entry 16).
It was noted in Table 1 that the main oxidation products, except
BzH, also included benzyl alcohol (BzOH), benzoic acid (BzA) and
phenol (Phen). The total selectivity (Ssum) of these four products,
BzH, BzOH, BzA and Phen, was almost estimated to be more than 96%.
3
. Results and discussion
3
.1. Characterizations of the catalyst
Fig. 1 illustrates the XRD pattern of Fe
2
O
3
/HZSM-5 as compared
/HZSM-5 basi-
2 3
with that of HZSM-5. The materials HZSM-5 and Fe O
cally retain the original structure of HZSM-5, although addition of
iron has a positive effect on the crystalline. Moreover, no obvious dif-
fraction peak corresponding to crystalline iron oxides could be ob-
served in the XRD pattern of Fe
O
2 3
/HZSM-5 (Fe
2
O
3
loading 2.25%).
3.3. Effect of reaction conditions
Possibly, the particle size of iron oxides in the sample was too small
to be detected by X-ray diffraction [20]. When the loading of iron
onto HZSM-5 was increased, two characterization peaks at 35.6°
It has been widely recognized that reaction temperature has a cru-
cial effect on the reaction. In this study, the effect of reaction temper-
2 3
ature on the reaction activity with Fe O /HZSM-5 catalyst (Fe O
loading 2.25%) was investigated, and the results were shown in
Fig. 2. The conversion of toluene was increased with increasing reac-
tion temperature from 60 °C to 90 °C. Beyond 90 °C, the conversion
was slightly decreased. This was possibly caused by the acceleration
and 46.8° corresponding to highest peaks of α-Fe
in the XRD pattern (not listed here), which suggested that iron has
been supported on the materials in the α-Fe style. Furthermore,
the brown of the obtained solid materials and content analysis of
2 3
O were observed
2
3
2 3
O
1
1
1
1
60
40
20
00
Table 1
Catalytic performance over various catalysts.
Entry
Catalysts
Conv.
%)
S
(%)
BzH
S
(%)
BzOH
S
(%)
BzA
S
(%)
Phen
S
sum
(%)
(
1
2
3
4
5
6
7
8
9
Fe O
2
3
4.4
4.6
8.0
5.6
17.3
6.2
6.7
5.7
7.4
6.3
7.1
4.5
4.1
5.3
5.8
15.6
81.5
81.5
61.6
81.5
51.4
69.6
80.0
88.9
77.7
84.5
80.3
84.1
82.5
79.0
78.4
52.2
8.1
9.3
5.1
–
98.9
98.3
98.5
98.6
96.4
97.8
98.6
99.1
96.5
97.9
93.7
98.5
97.5
98.7
99.1
96.3
ZSM-5
HZSM-5
10.4
22.5
10.8
24.7
9.3
12.5
6.7
10.9
3.9
5.1
3.2
5.5
11.3
9.6
1.3
4.2
2.1
8.2
–
8
6
4
2
0
0
0
0
0
Fe O /HZSM-5
10.2
4.2
12.1
18.9
6.1
3.5
5.2
7.2
6.3
8.6
7.9
6.2
8.6
2
3
Fe
Fe
Fe
Fe
Fe
2
2
2
2
2
O
O
O
O
O
3
/ZSM-5
/HZSM-5
3/MCM-41
/MCM-22
/HY
3
3
–
3
–
Cu/HZSM-5
V/HZSM-5
Ce/HZSM-5
Co/HZSM-5
Fe-V/HZSM-5
Fe-Ce/HZSM-5
Fe-Co/HZSM-5
Fe-Cu/HZSM-5
2.7
2.3
2.0
2.6
1.6
2.2
2.5
2.9
HZSM-5
60
1
1
1
1
1
1
1
0
1
2
3
4
5
6
-
20
20
30
40
50
70
2Theta (deg)
15.8
25.4
2 3
Fig. 1. XRD patterns of HZSM-5 and Fe O /HZSM-5 (loading 2.25%).

X. Li et al. / Catalysis Communications 39 (2013) 115–118
00
117
1
2 2
of H O decomposition at higher temperature. As a main product of
toluene oxidation, the selectivity to BzH was rapidly declined, while
the selectivity to other products such as BzOH, BzA and Phen was in-
creased with the temperature rising. Beyond 90 °C, the selectivity to
these four products almost approached to being unchanged. These
findings suggested that high temperature promoted the side reaction
for Phen generation. As the temperature was being raised, the con-
sumed rate of BzH exceeded its generating rate, and the generating
rate of BzOH was faster than that of its conversion into BzH at higher
temperature. However, the BzH consuming led to the increase in
amount of BzA.
The conversion of toluene and the selectivity of BzH, BzOH, BzA and
Phen with respect to amount of catalyst were displayed in Fig. 3. There
was also a sharply decrease in the selectivity to BzH and increase in the
conversion of toluene as the catalyst amount increased up to 0.4 g. Be-
yond 0.4 g catalyst, the conversion was obviously declined, which indi-
cated that the excess catalyst can accelerate rapid decomposition of
8
6
4
2
0
0
0
0
0
Conversion
BzH Selectivity
BzOH Selectivity
BzA Selectivity
Phen Selectivity
0.1
0.2
0.3
0.4
0.5
Amount of the catalyst (g)
2 2
H O , limiting toluene further oxidized. In order to validate this conclu-
sion, 7.5 mL of H was added into the reaction system after the reac-
2 2
O
Fig. 3. Dependence of the conversion and selectivity on amount of the catalyst. Reaction
tion proceeding 4 h with 0.5 g catalyst, making the reaction further
carried out for additional 2 h under the same conditions. The conver-
sion was surely increased up to 19.2% with reduced selectivity to BzH
2 2
conditions: reaction temperature 90 °C, time 4 h, molar ratio of H O to toluene 3: 1.
(
37.0%), which completely confirmed the above conclusion on promo-
3.4. Reusability of the catalyst
tion of the excess catalyst to decomposition of H . In addition, the se-
2 2
O
lectivity to BzOH, BzA and Phen was increased smoothly, which was the
same as the above situations.
The catalyst was precipitated out at the bottom of the reactor
when the extraction of organic phase to separate product from the bi-
phasic system was carried out at the end of the reaction. The catalyst
was separated and dried at 100 °C for 4 h. After being calcined at
550 °C for 2 h, it was reused in the next run under the same condi-
tions. The results shown in Fig. 5 indicated that the activity of the cat-
alyst was almost unaffected even at the third run. The toluene
conversion was declined at the fourth run, likely attributing to the de-
crease in amount of recovery catalyst, which was validated by the
amount measurement of the reused catalyst in catalyst reusability
The amount of H
fecting the oxidation of toluene, and the results were shown in Fig. 4.
Except the excess amount of H caused the low selectivity to BzH
2 2
O in the system is another important factor af-
2 2
O
and high selectivity to BzA, the other effects were almost similar to
the effect of the catalyst. The optimal molar ratio was determined to
be 3:1.
In short, the effect of various reaction conditions on the catalytic
oxidation of toluene was investigated, and the optimal operational
conditions were obtained: reaction temperature 90 °C, reaction time
2 3
test listed in Fig. 5. The XRD patterns of fresh and reused Fe O /
4
2
h, H
2
O
2
/toluene molar ratio 3/1, amount of the catalyst 0.4 g with
HZSM-5 (not list here) were not changed, which suggested that the
structure and the composition of the reused catalyst were unchanged.
This finding implied that the catalyst can be efficiently recovered and
recycled by simple method of the separation.
.25% loading of Fe. 17.3% conversion of toluene and 51.4% selectivity
to BzH were achieved, giving 8.9% yield of BzH under the optimal con-
ditions. It is higher selectivity to BzH in this case as compared with
the reference [17], although the conversion is higher in this report.
In particular, the Fe
2
3
O /HZSM-5 catalyst catalyzed the oxidation of
4. Conclusions
BzOH so fast that once BzOH generated, it was immediately oxidized
into BzH under the above conditions. This was also confirmed by the
catalytic performance-time profile of the oxidation.
Fe
2 3
O /HZSM-5 was prepared and used as catalyst for the oxidation
of toluene with hydrogen peroxide as oxidant under solvent-free
8
7
0
0
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
60
50
40
30
Conversion
Conversion
BzH Selectivity
BzOH Selectivity
BzA Selectivity
Phen Selectivity
BzH Selectivity
BzOH Selectivity
BzA Selectivity
Phen Selectivity
2
1
0
0
0
0
1: 1
2: 1
3: 1
4: 1
5: 1
60
70
80
90
100
Reaction temperature (deg)
Molar ratio of H2O2 to toluene
Fig. 2. The conversion and selectivity at various reaction temperatures. Reaction condi-
tions: reaction time 4 h, molar ratio of H to toluene 3:1, catalyst 0.4 g.
Fig. 4. The effect of various molar of H
temperature 90 °C, time 4 h and catalyst 0.4 g.
2 2
O to Toluene. Reaction conditions: reaction
2 2
O

118
X. Li et al. / Catalysis Communications 39 (2013) 115–118
Conversion
Science Foundation of Heilongjiang Province, China (nos. 12511143,
Selectivity to BzH
B201119 and ZD200820-01).
5
4
3
2
1
0
0
0
0
0
0
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.05.001.
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We make a great acknowledgement for the financial support of
305–309.
this work by Foundation of Educational Department and the Natural