Liquid phase oxidation of acetophenone over rice husk silica vanadium catalyst

Article abstract of DOI:10.1016/S1872-2067(11)60361-6

Rice husk silica catalyst loaded with 10 wt vanadium was synthesized from agricultural biomass via a sol-gel synthetic route at pH = 9. The catalyst was characterized by different physico-chemical methods. The FTIR spectra showed the formation of Si-O-V and V=O stretching bands. The presence of vanadium was confirmed by EDX elemental analysis. RH-10V possessed a high specific surface area of 276 m²/g and pore volume of 0.83 ml/g. The prepared catalyst possessed a narrow pore size distribution centered around 7.9 nm. The catalytic performance of RH-10V was tested in the oxidation of acetophenone at 70°C. RH-10V was found to be an active catalyst in the oxidation of acetophenone, producing 36.28 conversion efficiency. The products identified were benzoic acid, 2-hydroxyacetophenone, phenol, acetic acid, and 3-hydroxyacetophenone.

Full text of DOI:10.1016/S1872-2067(11)60361-6

CHINESE JOURNAL OF CATALYSIS
Volume 33, Issue 3, 2012
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2012, 33: 518–522.
ARTICLE
Liquid Phase Oxidation of Acetophenone over Rice Husk
Silica Vanadium Catalyst
Farook ADAM*, Thiam-Seng CHEW, Jeyashelly ANDAS
School of Chemical Sciences, Universiti Sains Malaysia, Penang 11800, Malaysia
Abstract: Rice husk silica catalyst loaded with 10 wt% vanadium was synthesized from agricultural biomass via a sol-gel synthetic route at
pH = 9. Theꢀcatalystꢀwas characterized by different physico-chemical methods. The FTIR spectra showed the formation of Si–O–V and
V=O stretching bands. The presence of vanadium was confirmed by EDX elemental analysis. RH-10V possessed a high specific surface
2
area of 276 m /g and pore volume of 0.83 ml/g. The prepared catalyst possessed a narrow pore size distribution centered around 7.9 nm. The
catalytic performance of RH-10V was tested in the oxidation of acetophenone at 70 °C. RH-10V was found to be an active catalyst in the
oxidation of acetophenone, producing 36.28% conversion efficiency. The products identified were benzoic acid, 2-hydroxyacetophenone,
phenol, acetic acid, and 3-hydroxyacetophenone.
Key words: vanadium; rice husk silica; oxidation; acetophenone; sol-gel technique
Acetophenone is an asymmetric ketone and its oxidation
results in the formation of a mixture of products such as ben-
zoic acid, benzoylformic acid, dibenzoylfunzan 2-oxide, ben-
zyl alcohol, and phenol which have well-established industrial
applications [1–5]. For example, benzoic acid is used as plas-
ticizer, as a food preservative, in flavoring, in the synthesis of
perfumes and antifungal agents [6]. Phenol is widely used to
produce phenolic resin, solvents, dyes, and pharmaceuticals
resulting in the formation of maleic anhydride, benzyl alcohol,
benzaldehyde, benzoic acid, and phthalic anhydride. They have
also observed trace amounts of toluene, xylene, and phenol as
by-products. Although this method manages to achieve high
selectivity toward benzoic acid, it needs high temperature and
long contact times which are limitations.
Vanadium shows a wide range of oxidation states (+2 to +5)
in aqueous solution. Moreover, heterogeneous vanadium
catalysts have been used in various oxidation reactions [9–11].
To the best of our knowledge, the oxidation of acetophenone
has not been carried out using catalyst synthesized from bio-
mass rice husk (RH) silica. Since RH silica exhibits high sur-
face area and high porosity [12], it is thought to be advanta-
geous to prepare vanadium incorporated RH silica and evaluate
the synthesized catalyst in the oxidation of acetophenone with
aqueous hydrogen peroxide under mild conditions. This work
describes the simplest, cost-effective, and green preparation of
RH-vanadium catalyst for the liquid-phase oxidation of ace-
tophenone.
[
7]. Nevertheless, only few researchers [1–5] have studied the
oxidation of acetophenone. This is presumably due to aceto-
phenone being a terminal ketone which has no hydrogen atoms
on the carbinol carbon. Thus, oxidation of acetophenone can
only be possible by breaking the ꢀ carbon-carbon bond [8].
Perkin [1] has reported the conversion of acetophenone to
benzoic acid using sodium hypochlorite, which is the haloform
reaction. However, from an environmental point of view, this
method is not suitable for industrial use due to the formation of
chloroform as a side product. Homogeneous catalystV such as
1,3-dinitrobenzene [2] and nitric acid [3] have been used to
oxidize acetophenone. These homogeneous systems create
problems in the separation process and catalyst recovery.
Hence, Chumbahale and co-workers [4] have reported the
vapor phase oxidation of acetophenone by V O -MoO catalyst
1 Experimental
2
5
3
1.1 Preparation of sodium silicate
Received 27 September 2011. Accepted 31 December 2011.
Corresponding author. Tel: +60-46533567; Fax: +60-46574854; E-mail: farook@usm.my, farook_dr@yahoo.com
*
This work was supported by the Malaysian Government (RU Grant 1001/PKIMIA/811092)
Copyright © 2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(11)60361-6

Farook ADAM et al. / Chinese Journal of Catalysis, 2012, 33: 518–522
Preparation of sodium silicate was carried out by a modifi-
cation of the technique reported by Adam and Chew [13]. The
used: initial oven temperature = 383 K, final oven temperature
= 503 K, ramp 1 = 20 K/min to 403 K, ramp 2 = 10 K/min to
503 K, total run time = 11 min.
dried RH (30.0 g) was stirred in 600 ml of 1.0 mol/L HNO for
3
2
4 h to remove unwanted metal. The acid treated RH was
filtered and washed with copious amount of distilled water
until it reached a constant pH. It was oven dried at 383 K
2 Results and discussion
overnight. On cooling, the rice husk (RH-HNO ) was weighed.
3
2.1 Characterization of catalyst
The RH-HNO was stirred in 500 ml of 1.0 mol/L NaOH for 24
3
h. The mixture was suction filtered to obtain sodium silicate to
be used as the silica source. The recovered residue (RH-NaOH)
was washed with distilled water and dried in an oven at 383 K
and the mass of RH-NaOH weighed. The difference in the mass
between RH-HNO3 and RH-NaOH was assumed to be
equivalent to the mass of silica extracted.
The infrared spectra of RH-10V before and after reaction are
illustrated in Fig. 1. As shown in Fig. 1(1), the band at 3462
–
1
cm was either due to the O–H stretching vibration of the
silanol or adsorbed water molecules on the silica surface. The
–
1
band at 1639 cm was due to the bending vibration of the water
–
1
molecules. The strong band at 1096 cm corresponded to the
asymmetric vibration of the siloxane bond Si–O–Si, and the
–
1
1
.2 Preparation of RH-10V catalyst
band at 805 cm was assigned to the bending vibration of
–
1
Si–O–Si bond. The transmission band observed at 472 cm
was due to the stretching vibration of the Si–O–Si bond. The
The resulting sodium silicate was titrated slowly (ca. 1.0
–
1
ml/min) to pH = 9 with 150 ml of 3.0 mol/L HNO
3
containing
presence of a band at 974 cm indicated the Si–OH stretching
vibration in the parent silica reported in our previous study
2
.03 g of NaVO 4H O to get 10 wt% of vanadium in the cata-
3
2
lyst. A brown gel was obtained and aged for 48 h. The gel was
suction filtered, washed thoroughly with distilled water, and
[
15]. When H was exchanged with metal, in this case vana-
dium, an obvious shift of this band could be observed. There-
fore, in RH-10V, the observed Si–OV vibration was shifted to
+
followed by hot water several times in order to remove Na ,
–
1
and oven dried at 383 K for 24 h. The powder was allowed to
cool in a desiccator and ground into fine powder and the sam-
ple denoted as RH-10V was kept for further use.
9
74 cm due to the exchange of the H with V. Thus, it was
–
1
possible to conclude that the band at 974 cm in the vanadium
containing catalyst was due to the presence of Si–OV bonds
14]. Moreover, the shoulder at ~1024 cm was assigned to the
–
1
[
1
.3 Catalyst characterization
V=O stretching vibration as reported by Gomez et al. [16] for
vanadia- zirconia catalyst.
RH-10V was characterized by FTIR spectroscopy (Perkin
The N adsorption-desorption isotherms of RH-10V (Fig. 2)
2
Elmer System 2000), N adsorption-desorption analysis (Mi-
2
shows type IV isotherms and H1 hysteresis loops. H1 hystere-
sis loop is a characteristic of mesoporous material consisting
of agglomerates or spherical particles having a relatively uni-
form pore size [16]. The specific surface area of RH-10V was
cromeritics Instrument Corporation model ASAP 2000, Nor-
cross), transmission electron microscopy (TEM, Phillips
CM12), energy dispersion spectroscopy (EDS, Edax Falcon
System) and DR/UV-Vis analysis (Lambda 35, Perkin Elmer).
2
found to be 276 m /g, and the pore volume was 0.83 ml/g. The
catalyst showed a narrow pore size distribution in the range of
1
.4 Catalytic activity
5–8 nm which is in accordance with the characteristics of H1
As a common practice, prior to the reaction, RH-10V (50
mg) was activated for 24 h in an oven at 383 K. The catalyst
was removed from the oven and cooled in a desiccator prior to
use in the catalysis. The round-bottom flask equipped with a
condenser was set up in a temperature control oil bath at 343 K.
Acetophenone (2.40 g) in 10 ml of acetonitrile was placed into
the flask. H O (4.53 g, 30%) was added drop wise into the
(
2)
2979
805
1
096
9
74
(1)
2
2
reaction vessel within 5 min. The reaction was allowed to run
for 3 h. During the reaction, 0.5 ml of the sample was with-
drawn and filtered, then 10 ꢁl of cyclohexane was added as
internal standard and analyzed by gas chromatography (Perkin
Elmer Clarus 500) equipped with Elite Wax (30 m × 0.2 mm
ID) and further confirmed by GCMS (Perkin Elmer, Clarus
1639
3462
472
1
226
4
000 3500 3000 2500 2000 1500 1000 500
ꢀ1
Wavenumber (cm )
600) analysis. The following are the GC and GCMS condition
Fig. 1. FT-IR spectra of RH-10V before (1) and after (2) the reaction.

Farook ADAM et al. / Chinese Journal of Catalysis, 2012, 33: 518–522
Table 1 The elemental composition of RH-10V
6
5
4
3
2
1
00
00
00
00
00
00
0
6
5
4
3
2
1
0
Elemental
Average weight (%)
C
O
Si
2.1
69.1
23.4
5.4
V
Total
100.0
1
10
100
Pore diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
p/p0
2
Fig. 2. The N adsorption-desorption isotherms and narrow pore size
distribution (inset) of RH-10V.
200
300
400
Wavelength (nm)
Fig. 4. The DR/UV-Vis spectrum ofꢀRH-10V.
500
600
700
800
The band around 330–500 nm region was due to the octahedral
5
+
coordinated V [18]. In this case, the d–d transitions were
also observed between 650 and 700 nm [17]. Thus, it could be
5
+
concluded that vanadium was present solely as V in the
framework of silica.
5
0 nm
2
.2 Catalyst evaluation
Fig. 3. TEM image of RH-10V catalyst showing the distribution of
particle sizes.
The prepared catalyst was tested in the liquid phase oxida-
tion of acetophenone. Table 2 shows the acetophenone con-
version with respect to the mass of the catalyst at 343 K. The
acetophenone conversion was found to increase sharply from
11.4% to 36.3% when the catalyst mass used was increased
from 30 to 50 mg. However, further increase in the catalyst
mass to 100 mg resulted in a decrease in acetophenone con-
version to 12.6%. This was due to the rapid decomposition of
H O over the large surface area available with a higher amount
hysteresis loop.
The TEM image of RH-10V is shown in Fig. 3. The incor-
poration of V resulted in smaller particles. The elemental
composition of RH-10V was determined by EDX analysis as
represented in Table 1 which confirmed the presence of carbon,
oxygen, silicon, and vanadium. Si and O were detected due to
the formation of siloxane bond (Si–O–Si) and silanol group
2
2
(
Si–OH). These Si and O could form a Si–O–V bond, whereas
of catalyst [19]. At a higher dose of catalyst, the availability of
more active sites resulted in rapid decomposition of H O ,
the presence of small amount of carbon might arise from rice
husk.
The DR/UV-Vis spectrum of the vanadium containing cata-
lyst (Fig. 4) showed a strong band centered at 278 nm. This
2
2
leading to lower yields. As a result, 2-hydroxyacetophenone
was observed as a by-product and decreased the selectivity of
benzoic acid.
5
+
band could be assigned as V in tetrahedral coordination [17].
The reaction temperature was found to have a great influ-
Table 2 Oxidation of acetophenone using different amounts of catalyst
Catalyst mass
mg)
Acetophenone conversion
Selectivity (%)
(
(%)
11.4
36.3
16.4
12.6
Acetic acid
21.1
2-Hydroxyacetophenone
Benzoic acid
28.5
3-Hydroxyacetophenone
3
5
7
0
0
0
50.4
49.1
50.0
—
10.6
—
20.4
25.8
17.3
19.9
24.2
19.8
1
00
62.9
—
Reaction conditions: catalyst = RH-10V, 343 K, molar ratio of substrate:H
2
O
2
3
= 1:2, 10 ml CH CN, 3 h.

Farook ADAM et al. / Chinese Journal of Catalysis, 2012, 33: 518–522
Table 3 The oxidation of acetophenone as a function of temperature
Temperature
K)
Conversion
(%)
Selectivity (%)
Benzoic acid
25.3
(
Acetic acid
26.5
2-Hydroxyacetophenone
3-Hydroxyacetophenone
Ph
—
—
—
11.0
3
3
3
3
03
23
43
53
24.3
30.4
36.3
39.2
48.2
44.6
49.2
58.2
—
12.4
10.6
—
22.8
20.4
14.5
20.2
19.8
16.3
2 2 3
Reaction conditions: 50 mg RH-10V, molar ratio of substrate: H O = 1:2, 10 ml CH CN Solvent, 3 h.
ence on the conversion of the reaction. Thus, in the oxidation of
acetophenone, the effect of temperature on the reaction was
studied in the range of 303–353 K (Table 3). With an increase
in the reaction temperature from 303 to 343 K, there was a
significant increase in the conversion from 24.3% to 36.3%.
Raising the reaction temperature higher to 353 K resulted in a
slight increase of the conversion to 39.2%. This was due to the
rapid decomposition of H O at 353 K [12]. Moreover, the
CH3
O
HO
OH
OH
H O
2
2
+
3
3
C
H C
O
O
Scheme 1. The formation of phenol.
2
2
After optimization of the reaction conditions it resulted in
selectivity toward 2-acetophenone increased with increasing
temperature, this might be due to the formation of water from
H O to hydrolyze acetophenone. However, decomposition of
36.28% conversion of acetophenone yielding 20.4% acetic
acid, 19.8% benzoic acid, 49.2% 2-hydroxyacetophenone, and
10.6% 3-hydroxyacetophenone. In previous studies,
1,3-dinitrobenzene [2] had been used as the solvent for this
reaction and by adding t-BuOK, a conversion of 91.7% ace-
tophenone and 58.2% yield of benzoic acid were achieved.
However, the utilization of t-BuOK as an additive is expensive
for the industrial production of benzoic acid and secondly, the
homogeneous system creates problem for separation. Some
researchers [3,5] used acid catalyst in a two-step oxidation of
acetophenone by forming ꢀ-nitroso-acetophenone as the in-
termediate. However, the intermediate formed is unstable in
acidic condition and isomerizes to form benzoylformaldehyde
oxime as a by-product. Hence, the two-step oxidation of ace-
tophenone is not practical in the industry due to the difficulty in
isolating the products and the lengthy time involved. Chum-
bahale et al. [4] used vapor phase oxidation of acetophenone
over a long contact time (58.5 h) to obtain a high conversion
(74.8%) and high selectivity (96.9%) towards benzoic acid.
However, this method yielded side-products such as maleic
anhydride, benzyl alcohol, benzaldehyde, phthalic anhydride,
and other unidentified products. It is quite clear that RH-10V is
by far a better catalyst for the oxidation of acetophenone and to
break the C–C single bond ꢀ to the carbonyl group.
2
2
H O caused lack of oxidant to convert acetophenone into
2
2
benzoic acid and acetic acid. Even though there was increase in
conversion with temperature, the optimum reaction tempera-
ture was set at 343 K, because at high temperatures, there are
moderate conversion and rapid decomposition of H O .
2
2
The effect of H O was studied by varying acetophenone:
2
2
H O molar ratio from 1:1 to 1:3 (Table 4). The conversions
2
2
were 4.1% and 36.3% for 1:1 and 1:2 molar ratios, respectively.
However, at a molar ratio of 1:3, the acetophenone conversion
decreased. Generally, the decomposition of H O produced
2
2
water as the by-product.The water might poison the catalyst by
adsorbing strongly to the vanadium centre and thus inhibiting
the reaction [18]. Furthermore, decomposition of H O (oxi-
2
2
dant) reduced the formation of acetic acid and benzoic acid,
while the hydroxylation product, 2-acetophenone was easily
formed in the presence of water. It was thus concluded that the
optimum molar ratio was 1:2.
From the data in Tables 3 and 4, phenol was only detected at
3
53 K when the acetophenone:H O molar ratio was 1:3. The
2
2
formation of phenol must originate from the breaking of the
bond as shown in Scheme 1. While acetic acid was detected
under all the experimental conditions studied, it was not clear
why phenol was only detected under certain conditions as
noted above. It was concluded that this anomaly required fur-
ther investigation.
To test the catalyst reusability, RH-10V was used consecu-
tively for 3 cycles as shown in Table 5. The used catalyst was
filtered and regenerated by heating at 383 K for 24 h. After 3
runs, there was a decrease in conversion (17.7%). The re-used
Table 4 The oxidation of acetophenone using different acetophenone:H
2
O
2
molar ratios
Acetophenone:H
molar ratio
O
2 2
Conversion
(%)
Selectivity (%)
Benzoic acid
Acetic acid
56.3
2-Hydroxyacetophenone
3-Hydroxyacetophenone
Ph
—
—
1
1
1
:1
:2
:3
4.1
36.3
17.2
43.7
49.2
54.3
—
19.8
21.1
—
10.6
—
20.4
14.4
10.2
3
Reaction conditions: 50 mg RH-10V, 343 K, CH CN solvent, 3 h.

Farook ADAM et al. / Chinese Journal of Catalysis, 2012, 33: 518–522
Table 5 The reusability of RH-10V
the ꢁ-carbon connected to the carbonyl carbon in ketones.
Further studies are in progress in our group to improve the
reusability of RH-10V by removing the organics adsorbed on
the catalyst.
Cycle
Conversion (%)
1
2
3
36.3
21.5
17.7
Reaction conditions: 50 mg RH-10V, 343 K, molar ratio of substrate:
= 1:2, 10 ml CH CN solvent, 3 h.
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Conclusions
1
1
0
1
In summary, a template-free vanadium-silica catalyst was
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simple, cost-effective, and green procedure to investigate the
liquid phase oxidation of acetophenone. RH-10V was found to
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group compared with the catalysts published in the literature.
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synthesis. This study have benefits to both academic and in-
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