Synthesis of aromatic ketones from aromatic compounds using vanadium-containing mesoporous silicates

Article abstract of DOI:10.1002/jccs.201100684

Aromatic ketones were synthesized from aromatic compounds via liquid-phase oxidation at 60 °C and 1 atm over vanadium-containing MCM-41 catalysts using a batch reactor. The catalysts were prepared by direct hydrothermal (4V-MCM-41) and wet impregnation (9V/MCM-41) methods. Their physico-chemical properties were determined with various characterization techniques. For the oxidations of all substrates in this work, 4 V-MCM-41 exhibits superior catalytic performance than 9 V/MCM-41 due to its larger values of unit cell parameter, BET surface area, and vanadium dispersion as well as stronger oxidation ability of vanadium-oxygen species. Apparently, the single site, isolated vanadium centers in 4V-MCM-41 possess much higher activity (based on the turnover number) than those containing more vanadium atoms in 9V/MCM-41. In addition, the substrate activities decrease in the order of diphenylmethane > fluorene > 9,10-dihydroanthracene > ethylbenzene ≥ 4-nitroethylbenzene, which are attributed to their distinct molecular structures.

Full text of DOI:10.1002/jccs.201100684

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Synthesis of Aromatic Ketones from Aromatic Compounds Using
Vanadium-Containing Mesoporous Silicates
Chih-Wei Chen and An-Nan Ko*
Department of Chemistry, Tunghai University, Taichung 40704, Taiwan, R.O.C.
(Received: Nov. 23, 2011; Accepted: Mar. 26, 2012; Published Online: ??; DOI: 10.1002/jccs.201100684)
Aromatic ketones were synthesized from aromatic compounds via liquid-phase oxidation at 60 °C and 1
atm over vanadium-containing MCM-41 catalysts using a batch reactor. The catalysts were prepared by
direct hydrothermal (4V-MCM-41) and wet impregnation (9V/MCM-41) methods. Their physico-chemi-
cal properties were determined with various characterization techniques. For the oxidations of all sub-
strates in this work, 4 V-MCM-41 exhibits superior catalytic performance than 9 V/MCM-41 due to its
larger values of unit cell parameter, BET surface area, and vanadium dispersion as well as stronger oxida-
tion ability of vanadium-oxygen species. Apparently, the single site, isolated vanadium centers in 4V-
MCM-41 possess much higher activity (based on the turnover number) than those containing more vana-
dium atoms in 9V/MCM-41. In addition, the substrate activities decrease in the order of diphenylmethane
> fluorene > 9,10-dihydroanthracene > ethylbenzene ³ 4-nitroethylbenzene, which are attributed to their
distinct molecular structures.
Keywords: Oxidation; (V) MCM-41; Diphenylmethane; Ethylbenzene; 4-Nitroethylbenzene;
Fluorene; 9,10-Dihydroanthracene.
1
0,18,19
12,13
INTRODUCTION
MCM-41,
and Cr-MCM-41
were prepared and
Aromatic ketones are important organic intermedi-
applied to the liquid-phase oxidation of diphenylmethane,
ethylbenzene, or 4-nitroethylbenzene with low to moderate
catalytic activities. Although vanadium-containing MCM-
41 materials have been applied to the oxidation of light al-
ates in the industries of flavors, perfumes, pharmaceuticals,
and agrochemicals. Traditionally, these ketones were pro-
duced by Friedel-Crafts acylation of aromatic compounds
using Lewis acids or strong protonic acids as catalysts. An-
2
5
26,27
27
kanes, benzene,
and styrene, only few studies have
other different route utilized KMnO
4
, K
2
Cr
2
O
7
, or SeO
2
as
been reported for other aromatics so far. In the oxidation of
diphenylmethane over V-MCM-41, low conversion (<16%)
oxidants to oxidize the aromatics. However, both processes
exhibited many disadvantages including highly corrosive
wastes, environmental hazards, and separation difficulty of
5
and benzophenone selectivity (48%) were observed.
In this study, the vanadium-containing MCM-41 cat-
alysts ((V)MCM-41) were synthesized by direct hydrother-
mal (4 V-MCM-41) and wet impregnation (9 V/MCM-41)
methods. These two materials were characterized with var-
ious techniques and were utilized for the liquid-phase oxi-
dation of aromatics such as diphenylmethane, ethylben-
zene, 4-nitroethylbenzene, fluorene, and 9,10-dihydroan-
thracene. The catalytic performance was compared be-
tween the two catalysts and correlated to their properties.
Furthermore, different catalytic activities were observed
for various aromatic compounds due to the variation of
their molecular structures.
1
catalyst from the reaction mixture. As a consequence, for
the past decade much efforts have been made to develop
more economic and clean processes for the synthesis of ar-
omatic ketones by the oxidation of aromatics over a variety
of heterogeneous catalysts. Up to now, the majority of stud-
ies reported the oxidation of diphenylmethane to benzo-
2
-12
3-6,12-19
phenone
and ethylbenzene to acetophenone.
3
Other aromatics like 4-nitroethylbenzene and fluo-
,6
1
2,20-22
rene
have received much less attention. To our
knowledge, so far there is no report on the oxidation of
9
,10-dihydroanthracene over solid catalyst.
MCMs type materials possess characteristic mesopo-
rous structure, large surface area, and high thermal stabil-
EXPERIMENTAL
ity, which allow their potential applications in catalysis,
2
Chemical reagents
3,24
adsorbants, and energy conversion.
5
1 (M = Ti, V, Cr), cobalt containing MCM-41,
Recently, M-MCM-
8,11
Mn-
Fluorene (99%), benzophenone (99%), and anthra-
cene (99%) were purchased from Fluka Corp. tert-Butyl-
4
*
Corresponding author. Tel: +886-4-23590248; Fax: +886-4-23590426; E-mail: anko@thu.edu.tw
J. Chin. Chem. Soc. 2012, 59, 000-000
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1

Article
Chen and Ko
hydroperoxide (70% in H
2
O) and 4-nitroethylbenzene
nadium in the sample) × 100%, where the value of 2 was
3
1
(
99%) were supplied by Lancaster and Avocado Corp., re-
2
the adsorption coefficient of O gas on vanadium.
spectively. Ethylbenzene (99%) and acetophenone (98%)
were obtained from Riedel-de Haen Corp. Diphenylmeth-
ane (99%), 9,10-dihydroanthracene (97%), 9-fluorenone
Catalytic oxidation
Liquid-phase oxidation of aromatics with tert-butyl-
hydroperoxide as oxidant was performed in a stirred, batch
reactor (250 mL) connected with a condenser at 1 atm and
60 °C. Typically, 0.6 mL of ethylbenzene (5 mmol), 2.8 mL
of tert-butylhydroperoxide (20 mmol), and 50 mL of aceto-
nitrile (1000 mmol) were added into the glass reactor which
was immersed in a water-bath. After the reaction tempera-
ture was reached, 0.05 g of catalyst was added into the reac-
tor to start the reaction. The products were collected peri-
odically and analyzed on a gas chromatograph (China G. C.
9800) equipped with a HP 5 column (30 m × 0.32 mm ×
0.25 mm) and a flame-ionization detector. Further, they
were confirmed with authentic samples as well as a com-
bined gas chromatograph-mass spectrometer (Micromass
Trio 2000).
(
98%), anthrone (97%), anthraquinone (97%) and other
chemicals were purchased from Aldrich Corp.
Catalyst preparation
4
V-MCM-41 was prepared by direct hydrothermal
synthesis from vanadyl sulfate hydrate (VOSO ×5H O,
9%), sodium silicate, n-hexadecyltrimethylammonium
bromide (95%), and sulfuric acid, following the procedures
4
2
9
2
8
described in the literature. The Si/V mol ratio in the gel
was 12.5 while such ratio was 16.0 and the vanadium con-
tent was 4 wt.% in the calcined sample. Si-MCM-41 was
synthesized similarly except no vanadyl sulfate hydrate
was added into the starting mixture. To prepare 9 V/MCM-
4
1
1, 4 g Si-MCM-41 was added into a solution containing
.704 g vanadyl sulfate dissolved in 20 mL deionized wa-
ter. The mixture was stirred, dried at 110 °C for 12 h, and fi-
nally calcined at 540 °C for 6 h. The vanadium content in
this sample is 9.1 wt.%.
RESULTS AND DISCUSSION
Catalyst properties
The low angle XRD patterns of various calcined sam-
ples are shown in Fig. 1. All samples exhibit four distinct
peaks, corresponding to (100), (110), (200), and (210) re-
flections. This clearly indicates that these samples possess
Catalyst characterization
The vanadium content of 4 V-MCM-41 was analyzed
by inductively coupled plasma-atomic emission spectros-
copy (ICP-AES, Tobin Yvon JY 38 Plus). The powder
X-ray diffraction was determined by a diffractometer
3
2
highly ordered mesoporous structure. Apparently, both
vanadium-containing MCM-41 samples display smaller 2q
angles than Si-MCM-41. Furthermore, the (100) peak in-
tensity of 4 V-MCM-41 is lower than that of 9 V/MCM-41,
which reveals the diminished crystallinity for the 4 V-
MCM-41 sample prepared by direct hydrothermal method.
(
Shimadzu XRD-6000) using Ni filtered CuK
0.154 nm) with 2q in the range of 2-10° at a scanning
speed of 1°/min. The BET surface area was measured with
a N sorption analyzer (Quantasorb). The IR spectra of cat-
a
radiation (l
=
2
alyst samples were recorded at room temperature on a FT-
IR spectrometer (Perkin-Elmer 2000) with resolution of 2
-
cm .
1
The catalyst acidity was evaluated by temperature-
programmed desorption (TPD) of ammonia with a self de-
2
9
signed apparatus. The catalyst reducibility was deter-
mined by the temperature-programmed reduction (TPR) of
2 2
H gas. During the run the H uptake was monitored as de-
3
0
scribed elsewhere. The amount of chemisorbed oxygen
and the vanadium dispersion were determined with the
chemisorption apparatus (Micromeretics Pulse ChemiSorb
3
1
2
705) as reported in the literature. The temperature was
kept the same at 370 °C for both sample reduction by hy-
drogen gas and subsequent oxygen chemisorption on the
sample. The vanadium dispersion was estimated as fol-
Fig. 1. The low angle XRD patterns of various cal-
cined samples. (a) Si-MCM-41; (b) 4 V-MCM-
41; (c) 9 V/MCM-41.
2
lows: Dispersion (%) = (2x mol of O adsorbed/mol of va-
2
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CHEMICAL SOCIETY
Synthesis of Aromatic Ketones from Aromatics
Table 1. Physicochemical properties of various samples
a
d100
ao
A
Acid amount
(mmol/g)
H uptake O adsorbed Dispersion
2
BET
2
(m /g)
2
Sample
(
nm)
(nm)
(mmol/g)
(mmol/g)
(%)
Si-MCM-41
3.59
4.11
3.92
4.15
4.75
4.53
900
785
613
0.10
0.24
0.31
¾
¾
¾
4
9
a
V-MCM-41
V/MCM-41
1.92
4.13
280
493
71.4
55.2
Unit-cell length a = 2d100/ 3
0
Table 1 lists physico-chemical properties of various cal-
cined samples. Both 4 V-MCM-41 and 9 V/MCM-41 have
showing the substitution of Si by V in the MCM-41 frame-
-
work. Moreover, Si-MCM-41 exhibits a band at 960 cm ,
1
o
larger d100 basal spacing and unit cell parameter a than
which is attributed to Si-O stretching vibration of Si-OH
8,35
2
Si-MCM-41, which is attributed to the incorporation of va-
nadium atoms in the MCM-41 framework. As the V-O
bond distance is longer than the Si-O bond distance, the
crystal lattices of vanadium-containing MCM-41 samples
are thus expanded. In addition, the 9 V/MCM-41 sample
group.
Although the assignment of this band is still
controversial, the enhanced intensity of this band in both 4
V-MCM-41 and 9 V/MCM-41 samples is generally attrib-
3
6,37
uted to the presence of V-O-Si linkage.
The above re-
sults of vanadium incorporation in the MCM-41 frame-
work are in agreement with the XRD studies.
o
has smaller a value in spite of its larger vanadium content
as compared to 4 V-MCM-41, which implies only a certain
degree of vanadium insertion in the framework of 9 V/
The TPD of ammonia profiles from various samples
are depicted in Fig. 3. A very broad and small peak is found
for Si-MCM-41, showing the location of lesser weak acid
2
7
MCM-41.
3
8
It is clear that the BET surface area is reduced after
vanadium introduction into Si-MCM-41 due to surface
coverage by vanadium species. Fig. 2 shows the FT-IR
spectra of different calcined samples. The broad band at
sites in this sample. Introduction of vanadium in the sam-
ples causes the appearance of a major peak centered at
about 168 °C. Consequently, both vanadium-containing
samples own weak acidity and their acid amounts are larger
than that of Si-MCM-41 (Table 1). Such result demon-
strates that the vanadium sites in the structure should be the
main sources for the acidity of vanadium-containing MCM-
-
1
-1
1
076 cm with a shoulder at 1237 cm and the band at 806
-
cm are ascribed to Si-O stretching and Si-O-Si bending,
1
3
3,34
respectively.
A slight decrease in the wavenumber of
3
4
these bands is observed in both samples (Fig. 2b & 2c),
41 catalysts.
In general, the availability and reactivity of active
species in the oxidation catalyst are characterized by the
TPR technique. Fig. 4 shows the TPR of hydrogen profiles
Fig. 2. FT-IR spectra of various calcined samples. (a)
Si-MCM-41; (b) 4 V-MCM-41; (c) 9 V/MCM-
Fig. 3. TPD of ammonia profiles from various calcined
samples. (a) Si-MCM-41; (b) 4 V-MCM-41; (c)
9 V/MCM-41.
4
1.
J. Chin. Chem. Soc. 2012, 59, 000-000
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3

Article
Chen and Ko
from the two calcined vanadium-containing samples. A
broad peak centered at 540 °C and 562 °C is observed for 4
V-MCM-41 and 9 V/MCM-41, respectively. 4 V-MCM-41
reveals lower reduction temperature than 9 V/MCM-41
since it has better vanadium dispersion (see below). As a
result, the vanadium-oxygen species in 4 V-MCM-41 can
be reduced more easily and thus have higher oxidation abil-
ity than those in 9 V/MCM-41. It is noted that the hydrogen
uptake is proportional to the vanadium content, indicating
complete reduction of all vanadium-oxygen species in the
samples during the TPR run.
taining MCM-41 catalysts are illustrated in Scheme I. The
solvent of acetonitrile was selected because it has been
shown as an effective solvent in the related oxidation reac-
9
,32
tions. In case of 9,10-dihydroanthracene, a combination
of acetonitrile and benzene was used as solvents to increase
the mutual solubility among different constituents in the re-
action mixture. The reaction temperature of 60 °C was cho-
sen to avoid faster decomposition of tert-butylhydroperox-
ide at higher temperatures. The conversion and product se-
lectivity are calculated with respect to the moles of aro-
matic substrate converted. According to preliminary runs
at variant stirring speeds of 50, 100, 200, and 300 rpm, it
was found that the reaction system was free of diffusional
limitation at a speed of 200 rpm. Consequently, this stirring
speed was utilized for all further reaction runs.
The metal dispersion of supported V
2 5
O after reduc-
tion in hydrogen has been determined by oxygen chemi-
3
1,39
sorption.
It is important to control the same temperature
of 367 °C for both the reduction and subsequent oxygen
chemisorption of catalyst surface without involving the
3
1
bulk phase. The same operating temperature of 370 °C
was applied in this study. Table 1 indicates the amount of
chemisorbed oxygen per gram of catalyst and the calcu-
lated vanadium dispersion. 4 V-MCM-41 has markedly
higher dispersion than 9 V/MCM-41 in accordance with its
Scheme I
lower reduction temperature in the H
and Table 1).
2
-TPR diagram (Fig. 4
Catalytic oxidation
To compare the effect of different substrates on cata-
lytic activity, two types of aromatics were chosen. One was
alkylaromatic compounds including diphenylmethane, eth-
ylbenzene, and 4-nitroethylbenzene. The other was ben-
zylic cyclics such as fluorene and 9,10-dihydroanthracene.
The products in the liquid-phase oxidation of these aro-
matics with tert-butylhydroperoxide over vanadium-con-
In the oxidation of diphenylmethane, the main and
side products are benzophenone and benzhydryl-tert-bu-
tylperoxide, respectively. With ethylbenzene and 4-nitro-
ethylbenzene, complete selectivity to acetophenone and
4
-nitroacetophenone are observed, respectively. For fluo-
rene oxidation, 9-fluorenone is the only product. In case of
,10-dihydroanthracene, the product anthracene is formed
9
via oxidative dehydrogenation whereas anthrone and an-
thraquinone are produced by successive oxidation on the
methylene groups of the substrate. Fig. 5 shows the conver-
sion and product yields as a function of reaction time in the
oxidation of 9,10-dihydroanthracene over 4 V-MCM-41.
Fig. 4. TPR of hydrogen profiles from various cal-
cined samples. (a) Si-MCM-41; (b) 4 V-MCM-
4
1; (c) 9V/MCM-41.
4
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CHEMICAL SOCIETY
Synthesis of Aromatic Ketones from Aromatics
over number, TON) than those containing more vanadium
atoms in 9 V/MCM-41. Similar results were reported in the
2
6,27
27
oxidation of benzene
and styrene.
As indicated in Table 2, the ratios of catalyst (g) : sub-
strate (mmol) : oxidant (mmol) : solvent (mmol) are not the
same for all substrates. However, it is known that the sub-
strate conversion enhances with the ratio of catalyst/sub-
strate as well as oxidant/substrate. Hence, the conversions
of diphenylmethane and 9,10-dihydroanthracene should
become larger and smaller, respectively, under the same ra-
tio as that for ethylbenzene, 4-nitroethylbenzene, or fluo-
rene. As a result, the activity based on the turnover number
decreases in the order of diphenylmethane > fluorene >
Fig. 5. The influence of reaction time on the conver-
sion and product yields in the oxidation of
9
,10-dihydroanthracene over 4 V-MCM-41.
(
l) conversion; yield: (¡) anthrone; (D) an-
9
,10-dihydroanthracene > ethylbenzene ³ 4-nitroethylben-
thraqinuone; (o) anthracene.
zene. This activity difference can be attributed to the dis-
similarity of molecular structures of various substrates. A
comparison of the oxidation of diphenylmethane and that
Clearly, anthrone is the reaction intermediate and anthra-
quinone is the final product. Table 2 presents the catalytic
results for the oxidation of these aromatics over vana-
dium-containing MCM-41 catalysts. It is noticeable that
for all substrates except ethylbenzene and 4-nitroethylben-
zene both catalysts exhibit good catalytic performance due
to their characteristic mesoporous structure, large specific
surface area and accessible vanadium-oxygen species (Fig.
2
of ethylbenzene reveals that the reactivity of the -CH -
group attached to the benzene ring depends strongly on the
other group like phenyl and methyl. Electronic resonance is
supposed to occur in phenyl group of diphenylmethane, re-
sulting in favorable activity whereas the electron donating
methyl group of ethylbenzene exhibits adverse effect on
the oxidation activity. Therefore, diphenylmethane shows
remarkably higher activity than ethylbenzene. It is interest-
ing to note that the presence of the electron withdrawing
1
and Table 1). Further, 4 V-MCM-41 shows superior activ-
ity than 9 V/MCM-41, which is attributed mainly to the
better dispersion and higher oxidation ability of vanadium-
oxygen species as active sites in the MCM-41 framework.
The role of catalyst acidity should cause little effect on cat-
alytic activity because the catalytic activity does not corre-
late with the catalyst acid amount (Table 1 & 2). It is note-
worthy that the single-site, isolated vanadium centers in 4
V-MCM-41 show much higher activity (based on the turn-
2
-NO group on the 4-postion of benzene ring in ethylben-
3
,6
zene decreases only slightly the reactivity. For the oxida-
tion of fluorene, the fused penta carbon cyclic between two
benzene rings diminishes the electronic resonance on these
rings, which lowers the oxidation reactivity of the methy-
lene group. This effect is even more pronounced for the ox-
Table 2. Catalytic results in the oxidation of various aromatics
Conversion (%)
h/3 h
Selectivity (%)
1 h/3 h
TON
Substrate
Ketone product
Benzophenone
Catalyst
1
1 h/3 h
Diphenylmethane
Ethylbenzene
4 V-MCM-41
43.0/58.0
37.7/55.3
7.8/18.0
8.7/16.7
6.8/14.1
6.1/13.7
52.9/97.3
22.9/83.8
54.6/89.2
28.1/54.6
84.7/80.7
74.6/71.0
100/100
100/100
100/100
100/100
100/100
100/100
35.5/53.5
50.4/63.6
91.2/123
35.2/51.6
9.7/22.4
4.8/9.2
9
V/MCM-41
4 V-MCM-41
V/MCM-41
4 V-MCM-41
V/MCM-41
4 V-MCM-41
V/MCM-41
4 V-MCM-41
V/MCM-41
Acetophenone
9
4
-Nitroethylbenzene
Fluorene
,10-Dihydroanthracene
4-Nitroacetophenone
9-Fluorenone
8.7/18.0
3.4/7.6
9
67.4/124
12.8/46.9
34.8/56.8
7.9/15.3
9
9
Anthrone + Anthraquinone
9
Reaction conditions: temperature = 60 °C, catalyst (g): substrate (mmol): oxidant (mmol): solvent (mmol) = 0.3: 50: 100: 800 for di-
phenylmethane; 0.05: 2.5: 10: 770 (acetonitrile) + 560 (benzene) for 9,10-dihydroanthracene; 0.05: 5: 20: 1000 for all other substrates.
J. Chin. Chem. Soc. 2012, 59, 000-000
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5

Article
Chen and Ko
1
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7
7, 155.
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MCM-41) and wet impregnation methods (9 V/MCM-41).
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