Synergetic effect of FeVO4 and α-Fe2O 3 in Fe-V-O catalysts for liquid phase oxidation of toluene to benzaldehyde

Article abstract of DOI:10.1016/j.cclet.2011.10.015

Synergistic effect of FeVO₄ with α-Fe₂O ₃ was found in Fe-V-O catalyst, which was responsible for the high apparent formation rate (A.F.R.) of benzaldehyde in liquid phase oxidation of toluene by hydrogen peroxide. The syn

Full text of DOI:10.1016/j.cclet.2011.10.015

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 145–148
www.elsevier.com/locate/cclet
Synergetic effect of FeVO4 and a-Fe2O3 in Fe–V–O catalysts
for liquid phase oxidation of toluene to benzaldehyde
*
Gui Quan Zhang, Xin Zhang , Tao Lin, Ting Gong, Min Qi
School of Chemical Engineering, Northwest University, Xi’an 710069, China
Received 14 July 2011
Available online 21 December 2011
Abstract
Synergistic effect of FeVO4 with a-Fe2O3 was found in Fe–V–O catalyst, which was responsible for the high apparent formation
rate (A.F.R.) of benzaldehyde in liquid phase oxidation of toluene by hydrogen peroxide. The synergistic effect might create VOx
species as active sites; moreover, it improved the reducibility and the reactivity of Fe–V–O catalyst. In order to gain the high A.F.R.
of benzaldehyde, the catalyst should have the moderate reducibility.
# 2011 Xin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Liquid oxidation; Synergistic effect; Toluene; Benzaldehyde; FeVO4; a-Fe2O3
Benzaldehyde (BzH) is widely used for producing dyestuffs, perfumery, pharmaceuticals, etc. Liquid phase
oxidation of toluene to BzH by hydrogen peroxide (H2O2) on solid catalyst has attracted great attention, because this
process takes some advantages, such as, (1) the high selectivity of BzH, (2) the mild reaction conditions, (3) the easy
separation of catalysts from products, (4) the preferable industrial application, etc. [1]. Vanadium (V)-based oxide is
considered to be one of the promising catalysts for the reaction [1]. For example, 72.8% selectivity of BzH and 13.6%
conversion of toluene were obtained at 363 K with bis (acetylacetonato) oxovandium (VO(acac)2) as catalyst in acetic
acid [2]. VOx/g-Al2O3 (14 wt.% loading of V2O5) catalyst showed 88.4% selectivity of BzH with 29.8% conversion of
toluene in the reaction and acetonitrile as solvent at 50 8C [3]. Nevertheless, the reactivity of these catalysts should be
further improved for the industrial application. On the other hand, there are some controversies concerning the active
sites for V-based catalyst. Some people considered that VOx species were responsible for the high reactivity of the
catalysts; the others thought that V4+/V5+ species were possible active sites [2–5]. Without deep understanding the
structure–function relationship, it is difficult to develop an effective catalyst by the modification of catalyst
composition and preparation.
In this work, the comparatively high apparent formation rate of BzH was achieved on novel Fe–V–O catalyst with
Fe/(Fe + V) = 0.5 mol ratio in the liquid oxidation of toluene to BzH by H2O2. Synergetic effect between FeVO4 and
a-Fe2O3 in the catalyst was found, and its role on the properties and reactivity of the catalyst were discussed.
Fe–V–O catalysts were prepared by method described as following. The calculated amount of ferric nitrate
(Fe(NO3)3ꢀ9H2O, Shanghai Chem., A.R.), ammonium metavanadate (NH4VO3, Shanghai Chem., A.R.) and citric acid
* Corresponding author.
E-mail address: zhangxinzhangcn@yahoo.com.cn (X. Zhang).
1001-8417/$ – see front matter # 2011 Xin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.10.015

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G.Q. Zhang et al. / Chinese Chemical Letters 23 (2012) 145–148
Table 1
Catalytic reactivity and properties of Fe–V–O catalysts in the liquid-phase oxidation of toluene by hydrogen peroxide.a
b
Catalysts
SBET (m2/g)
Vpore (cm3/g)
Phase composition (mol%)
Apparent formation rate of product (ꢃ10ꢁ2, g mꢁ2 hꢁ1
)
BzH
BzOH
BzA
Phenol
Cresol
Blank
–
–
–
–
–
–
12.5
0
–
34.5
6.0
0
–
28.0
0
V2O5
3.0
2.8
0.6
0.6
35.2
0.20
0.26
0.26
0.26
0.07
–
20.5
71.5
210.0
180.0
–
9.5
12.5
35.0
0
Fe0.5V0.5
Fe0.7V0.3
Fe0.8V0.2
a-Fe2O3
FeVO4 (ꢄ100)
FeVO4 (50.0), a-Fe2O3 (50.0)
FeVO4 (20.0), a-Fe2O3 (80.0)
a-Fe2O3 (100)
0
0
0
0
0
–
–
–
–
a
Reaction condition: 5 mL toluene, 20 mL H2O2 (30 wt.%), 20 mL acetic acid, 0.2 g catalyst, 60 8C, 3 h.
The date in the parentheses is the relative content of the phase.
b
(C6H8O7ꢀH2O, Tianjin Chem. A.R.) were solved thoroughly by deionized water to form solution. In this solution,
Fe:V:critic acid:H2O was about x:(1 ꢁ x):2:30 (molar ratio, 0 ꢂ x ꢂ 1). The solution was slowly evaporated at 80 8C
for 12 h, and then calcined at 800 8C for 6 h. The as-prepared Fe–V–O catalyst is denoted as FexV(1 ꢁ x), in which x
expresses Fe/(Fe + V) molar ratio in the catalyst. a-Fe2O3 was prepared by the decomposition of powder ferric nitrate
(Fe(NO3)3ꢀ9H2O, Shanghai Chem., A.R.) at 600 8C for 8 h, and V2O5 was prepared by the pyrolyzation of powder
ammonium metavanadate (NH4VO3, Shanghai Chem., A.R.) at 600 8C for 8 h.
N2 isothermal adsorption–desorption of Fe–V–O catalyst was performed at the temperature of liquid nitrogen by
Micromeritics ASAP400 adsorptionmeter. The specific surface area (SBET) was calculated according to BET method,
and the volume of pores (Vpore) was evaluated by t-plot analysis of adsorption isotherm. As shown in Table 1, the SBET
of Fe–V–O catalyst was much low, possibly due to the high calcination temperature of the catalyst. The addition of Fe
into the catalyst decreased the SBET and hardly changed the Vpore
.
XRD patterns of Fe–V–O catalysts were collected by Rogaku Rotflex D/Max-C powder X-ray diffractometer with
Cu Ka radiation (l = 0.15046 nm) operated at 40 kV and 30 mA (Fig. 1). FeVO4 (2u = 16.58, 24.88, 27.08, 31.28 and
42.08) and a-Fe2O3 (2u = 24.18, 33.18, 35.68, 40.88, 49.48, 54.08, 62.48 and 63.98) were detected from Fe0.7V0.3 and
Fe0.8V0.2 catalyst, indicating that these two catalysts were mainly composed of a-Fe2O3 and FeVO4. Fe0.5V0.5 was
FeVO4 with trace a-Fe2O3 impurity. Relative content of each phase is estimated as the formula Ii/SIi ꢃ 100%, where Ii
is the strongest diffraction intensity of FeVO4 (I2u = 27.08) and a-Fe2O3 (I2u = 33.18). With the increase of Fe/V ratio, the
relative content of FeVO4 decreases while that of a-Fe2O3 increases in Fe–V–O catalysts (Table 1).
Raman spectra of Fe–V–O catalysts were recorded by Renishaw Raman system 1000 spectrometer equipped with
Ar+ laser at l = 532 nm and the power of 5 mW (Fig. 2). For Fe0.5V0.5 (FeVO4), Raman bands attributed to stretches of
V O in VO4 unit (950 cmꢁ1), asymmetric stretches of isolated VO4 unit (913, 883, 819 and 709 cmꢁ1), asymmetric
stretching vibration of V–O–Fe bond (648 cmꢁ1), asymmetric and symmetric bending vibration of VO4 unit (363 and
FeVO4
(a)
(b)
(c)
α-Fe2O3
(d)
10
20
30
40
50
60
70
2θo
Fig. 1. XRD patterns of (a) Fe0.5V0.5, (b) Fe0.7V0.3, (c) Fe0.8V0.2 and (d) a-Fe2O3.

G.Q. Zhang et al. / Chinese Chemical Letters 23 (2012) 145–148
147
(a)
(b)
(c)
200
400
600
800
1000
Raman Shift (cm-1)
Fig. 2. Raman spectra of (a) Fe0.5V0.5, (b) Fe0.7V0.3 and (c) Fe0.8V0.2
.
327 cmꢁ1) were detected [6,7]. Raman bands at 264 and 212 cmꢁ1 belonging to a-Fe2O3 were found in Fe0.7V0.3 and
Fe0.8V0.2. These results further indicate that Fe0.7V0.3 and Fe0.8V0.2 contained a-Fe2O3 and FeVO4.
H2-TPR plot of Fe–V–O was obtained by a home-built fixed bed TPR apparatus (Fig. 3). The onset temperature of
reduction (To) and the maximal temperature of reduction peak (Tm) were measured to study the reducibility of these
catalysts. Three reduction peaks of V2O5 occurred at ca. 699, 745 and 795 8C, corresponding to the reduction of
V2O5 ! V2O4 ! V6O11 ! V2O3. a-Fe2O3 displayed two reduction peaks at 426 and 690 8C for the reduction of
Fe2O3 ! FeO ! Fe. The H2-TPR plots of Fe–V–O catalysts presented two reduction processes, due to the
overlapping reduction stages of V and Fe ions. Fe0.7V0.3 and Fe0.8V0.2 catalysts showed the different H2-TPR plots
from V2O5, a-Fe2O3 and Fe0.5V0.5 (FeVO4). To decreased as the sequence of V2O5 (605 8C) > Fe0.5V0.5
(536 8C) > Fe0.8V0.2 (529 8C) > Fe0.7V0.3 (519 8C) > a-Fe2O3 (346 8C). In addition, Tm increased as the order of
Fe0.7V0.3 (555, 703 8C) < Fe0.8V0.2 (615, 718 8C) < Fe0.5V0.5 (741, 784 8C). Thus, the interaction of FeVO4 and a-
Fe2O3 occurred in Fe0.8V0.2 and Fe0.7V0.3. The reducibility of Fe–V–O catalysts became strong as the order
Fe0.7V0.3 > Fe0.8V0.2 > Fe0.5V0.5
.
Liquid phase oxidation of toluene by H2O2 on Fe–V–O catalysts was carried out in a three necked round-bottomed
flask (250 mL) equipped with a reflux condenser as well as water bath and magnetic stirring. Reactants and products
were analyzed by gas chromatography (Beifen Ruili GC-3420) with FID and KB-1 capillary column. In order to
evaluate the intrinsic reactivity of Fe–V–O catalysts, the apparent formation rate of product (A.F.R.) on these catalysts
was tested (Table 1). A.F.R. of product (g mꢁ2 hꢁ1) is defined as the mass of product formed on the unit specific-
surface-area catalyst and in an hour. BzH did not formed on a-Fe2O3 in the reaction. BzH, benzyl alcohol (BzOH),
benzoic acid (BzA), phenol and cresol produced on V2O5 and Fe–V–O catalysts. A.F.R. of BzH decreased as the order
of Fe0.7V0.3 > Fe0.8V0.2 > Fe0.5V0.5 (FeVO4) > V2O5 > a-Fe2O3. Fe0.7V0.3 showed the higher A.F.R. of BzH than the
other reported V-based catalysts [2,3]. These results indicate that vanadium oxygen species were close relative to the
reactivity of the catalysts. The synergetic effect of FeVO4 and a-Fe2O3 occurred and significantly improved the A.F.R.
of BzH on Fe0.7V0.3 and Fe0.8V0.2 catalysts. For achieving the high A.F.R. of BzH, Fe–V–O catalyst should possess the
proper content of FeVO4 and a-Fe2O3.
(e)
(d)
(c)
(b)
(a)
300
450
600
750
900
Temperature (oC)
Fig. 3. H2-TPR plots of (a) V2O5, (b) Fe0.5V0.5, (c) Fe0.7V0.3, (d) Fe0.8V0.2 and (e) a-Fe2O3.

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G.Q. Zhang et al. / Chinese Chemical Letters 23 (2012) 145–148
Fig. 4. Relationship of relative content of FeVO4 with specific activity of BzH and the reducibility (To) of these catalysts.
a-Fe2O3 is trigonal system with six-fold coordination. FeVO4 has a triclinic structure with eight-fold coordination
and consists of three distinct isolated VO4 units [6,7]. During the preparation of Fe–V–O catalysts, the interaction of a-
Fe2O3 and FeVO4 occurred because of their structural difference. The interaction could create lattice defect VOx
species with the weak V–O bond in the interface of FeVO4 and a-Fe2O3. The VOx species were relative high mobility
of V5+ and the driving force to decrease the surface free energy [6,7]. Hence, Fe–V–O catalyst containing FeVO4 and
a-Fe2O3 showed the stronger reducibility than individual V2O5 and FeVO4.
It can be seen in Fig. 4, To is close relative to FeVO4 content in these catalysts, suggesting that FeVO4 content has
great influences on the reducibility of these catalysts. FeVO4 content might affect the number of VOx species formed in
the interface of FeVO4 and a-Fe2O3. Hence, Fe–V–O catalyst with different FeVO4 content showed the different
reducibility. On the other hand, it is well accepted that the liquid oxidation of toluene by H2O2 on oxide catalyst
follows Mars–van-Krevelen mechanism [1–5]. Therefore, the reducibility of Fe–V–O catalysts determined their
catalytic reactivity. The proper increase in reducibility of the catalyst enhanced the A.R.F. of BzH, but the much high
reducibility caused the further oxidation of BzH and the oxidation of phenyl group to phenol and cresol. Fe0.7V0.3
exhibited the higher A.F.R. of BzH than any other catalysts, due to its moderate reducibility.
In conclusion, the occurrence of synergetic effect between FeVO4 and a-Fe2O3 led to the formation of VOx species
as active sites and modified the reducibility of Fe–V–O catalyst, which in turn improved the reactivity of the catalyst. It
is necessary that the catalyst had the proper reducibility for achieving the high A.F.R. of BzH.
Acknowledgments
This work was financially supported by Ministry of Education (No. NCET-10-878, 20096101120018, 2009-37th of
SRFROCS), Shaanxi Province (No. 2009ZDKG-70, 09JK793), Northwest University (No. PR09005, 10YSY08) and
State Key Lab for SSPC (2009).
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