Highly active mesoporous chromium silicate catalysts in side-chain oxidation of alkylaromatics

Article abstract of DOI:10.1039/c2dt31571h

We approach a green method in the production of alkylaromatic ketones over hexagonally ordered mesoporous CrSBA-15 catalysts, which were used, in green routes, in the liquid-phase oxidation of alkylaromatics. A promising chemical treatment method was used with ammonium acetate solution to remove the toxic nature of non-framework chromium oxides deposited on the surface of calcined CrSBA-15(8), and the obtained green mesoporous CrSBA-15(8) catalyst was used to find its catalytic activity while the recyclability of mesoporous CrSBA-15 catalysts was also studied. Particularly, the mesoporous CrSBA-15 catalysts synthesized with a variety of chromium contents were extensively used in the production of acetophenone (APO) with various reaction parameters. On the basis of all catalytic results, the mesoporous CrSBA-15(8) catalyst produced a higher selectivity of alkylaromatic ketones (76-100%) as compared to other CrSBA-15 catalysts and was found to be a highly active, recyclable and promising heterogeneous catalyst for selective synthesis of alkylaromatic ketones. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt31571h

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PAPER
Highly active mesoporous chromium silicate catalysts in side-chain oxidation
of alkylaromatics†
M. Selvaraj,*a D.-W. Park,a I. Kim,b S. Kawic and C. S. Hab
Received 16th July 2012, Accepted 12th September 2012
DOI: 10.1039/c2dt31571h
We approach a green method in the production of alkylaromatic ketones over hexagonally ordered
mesoporous CrSBA-15 catalysts, which were used, in green routes, in the liquid-phase oxidation of
alkylaromatics. A promising chemical treatment method was used with ammonium acetate solution to
remove the toxic nature of non-framework chromium oxides deposited on the surface of calcined
CrSBA-15(8), and the obtained green mesoporous CrSBA-15(8) catalyst was used to find its catalytic
activity while the recyclability of mesoporous CrSBA-15 catalysts was also studied. Particularly, the
mesoporous CrSBA-15 catalysts synthesized with a variety of chromium contents were extensively used
in the production of acetophenone (APvO) with various reaction parameters. On the basis of all catalytic
results, the mesoporous CrSBA-15(8) catalyst produced a higher selectivity of alkylaromatic ketones
(76–100%) as compared to other CrSBA-15 catalysts and was found to be a highly active, recyclable and
promising heterogeneous catalyst for selective synthesis of alkylaromatic ketones.
liquid-phase EB oxidation using molecular oxygen or air as the
1. Introduction
oxidizing agent over homogeneous chromium containing in-
organic/organic catalysts2 and organometallic catalysts.4,5
Selective side chain oxidation of alkylarenes to aromatic ketones
is one of the most important fundamental reactions,1 is mainly
Although homogeneous catalysts exhibit excellent activity and
based on the conventional homogeneous catalysis in chemical
industries and is mostly carried out with acetic acid as the
solvent as well as molecular oxygen as the consumable oxidant.2
In these catalytic oxidations the poisonous Cr(VI) reagents are
also used as the homogeneous catalysts.3 However, the con-
ditions of the conventional method are often harsh, the reagent
mixture is corrosive and highly toxic. The reaction temperature
is high and demands the use of autoclave reactors, which are
made of expensive raw materials due to the corrosive medium.
Therefore, there is a need for the development of new hetero-
geneous catalysts in the production of alkylaromatic ketones
with higher selectivities.
Ethylbenzene (EB) oxidation is one of the alkylaromatic oxi-
dations and is largely used in the production of acetophenone
(APvO). APvO is one of the key products in the manufacture
of pharmaceuticals, resins, alcohols and tear gas (chloroaceto-
phenone) and is also used as a component in perfumery as well
as a solvent for cellulose ethers. APvO is produced by the
selectivity, the technical problems encountered in their appli-
cations, such as the difficulty in product separation and self
aggregation of active sites, have slowed down their industrial
applications. These catalytic systems also create other problems
such as handling, catalyst recovery, recycling, environmentally
unsuitable and highly produced tarry wastes. To overcome these
problems, solid acid catalysts have been successfully used in the
liquid phase oxidation of alkylarenes.6–17 The heterogeneous
chromium reagents are widely used as catalysts for the oxidation
of alkylaromatic compounds in organic chemistry.18 Therefore,
several chromium containing solid materials have been utilized
as the catalysts in liquid-phase oxidation of alkylaromatics with
a variety of oxidizing agents.6–19 For example, the microstruc-
tured CrAPO-5 catalyst produced less EB conversion with
higher APvO selectivity under liquid phase EB oxidation
because its small pore diameter (∼0.73 Å) restricts the diffusion
of reactants/intermediate and products.6,7 In recent years, the
mesoporous CrMCM-41 catalyst was used in the EB oxidation
to improve the EB conversion with higher APvO selectivity
because it has larger uniform pore size than CrAPO-5.6,7,20 In
2007, Selvaraj and co-workers reported the well hexagonally
uniform pore structured CrSBA-15.13–17 The mesoporous
CrSBA-15 catalysts have been successfully used for the syn-
thesis of very useful fine chemicals with higher selectivities, and
have superior catalytic activities than CrMCM-41 due to the
higher Cr species coordinated on the thick silica pore walls.13–17
However, to the best of our knowledge, the CrSBA-15 catalysts
have not been used in oxidation of EB, and the production of
aSchool of Chemical and Biomolecular Engineering, Pusan National
University, Busan 609-735, Korea. E-mail: chems@pnu.edu;
Fax: +82-51-512-8563; Tel: +82-51-510-2397
bDepartment of Polymer Science and Engineering, Pusan National
University, Busan 609-735, Korea
cDepartment of Chemical and Biomolecular Engineering, National
University of Singapore, Singapore 119260, Singapore
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt31571h
14204 | Dalton Trans., 2012, 41, 14204–14210
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alkylaromatic ketones over CrSBA-15 has not been extensively
reported, in the open literature so far.
isotherms, ESR, 29Si MAS NMR, UV-vis DRS, FE-SEM and
TEM according to the published procedure.13,17 The mesoporous
CrMCM-41(40) catalyst was also characterized by ICP-AES,
XRD, N2 sorption isotherms and UV-vis DRS according to a
published procedure.12
In our report, for selective synthesis of alkylaromatic ketones,
the mesoporous CrSBA-15 catalysts have been efficiently used
in the liquid-phase oxidation of alkylaromatics. Particularly, the
selective synthesis of acetophenone (APvO) over mesoporous
CrSBA-15 catalysts has been extensively studied with various
reaction parameters. The catalytic results obtained from all meso-
porous catalysts are correlated and compared for the selective
synthesis of alkylaromatic ketones.
2.4. Oxidation of alkylaromatics
All the chemicals, for side-chain oxidation of alkylaromatics,
were purchased from Aldrich Chemical Inc., and used as
received without further purification. In order to explain the
liquid-phase oxidation of alkylaromatics, oxidation of EB to
APvO was taken as an example and performed under vigorous
stirring in a thermostatted quartz vessel reactor with various
reaction parameters. In a typical experimental procedure, 0.1 g of
CrSBA-15(8) was taken in the reactor with 20 mmol of EB and
20 ml of CB solvent. After that, the reactor was stirred constantly
with slowly raising the temperature to 393 K. Then, the reactant
mixture was continuously refluxed for 10 h after adding
40 mmol of t-butylhydroperoxide (TBHP) through the septum.
After completion of the reaction, the products were collected by
recovering CrSBA-15(8). To find the best catalyst, the diverse
Cr-containing mesoporous catalysts were used for the oxidation
of EB under similar reaction conditions. Various reaction para-
meters such as time, temperature, stoichiometric molar ratios of
reactant (EB-to-TBHP) were studied to find the best reaction
conditions over CrSBA-15(8). For the identification of a better
solvent over CrSBA-15(8), the oxidation of EB was carried out
with different solvents like MeCN, CB and MeOH. The oxi-
dation of EB was also carried out with hydrogen peroxide
(H2O2, 30%) with MeOH. Furthermore, using the mesoporous
CrSBA-15 catalysts the side-chain oxidation of other alkyl-
aromatics, such as 4-ethylanisole, 1-chloro-4-ethylbenzene,
1-ethyl-4-nitrobenzene, 4-ethylphenol, 2-ethylanisole, 1-chloro-
2-ethylbenzene, 1-ethyl-2-nitrobenzene, 2-ethylphenol, was
extensively performed using a vigorously stirred thermostatted
glass vessel reactor under various reaction conditions.
2. Experimental
2.1. Materials
For the preparation of mesoporous Cr-containing silica catalysts,
all chemicals viz. triblock copolymer poly(ethylene glycol)-block-
poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic
P123, molecular weight = 5800, EO20PO70EO20), tetraethylortho-
silicate (TEOS, 98%), hydrochloric acid (HCl, 37%), and Cr(III)
nitrate nonahydrate (CN, 99%) were purchased from Aldrich
Chemical Inc. and used as received without further purification.
For the selective synthesis of alkylaromatic ketones, all chemi-
cals viz. ethylbenzene (EB, 99.8%), electron donating (ED)
group substituted EBs, such as 1-ethyl-4-methoxybenzene
(4-ethylanisole, 97%) and 1-ethyl-2-methoxybenzene (2-ethyl-
anisole, 99%), and electron withdrawing (EW) group substituted
EBs, such as 1-chloro-4-ethylbenzene (97%), 1-ethyl-4-nitro-
benzene, 4-ethylphenol (99%), 1-chloro-2-ethylbenzene, 1-ethyl-
2-nitrobenzene (96%) and 2-ethylphenol (99%), t-butylhydro-
gen-peroxide (TBHP, 70%), hydrogen peroxide (30% H2O2),
acetonitrile (MeCN, 99.8%), chlorobenzene (CB, 99.8%) and
methanol (MeOH, 99.8%), were also purchased from Sigma-
Aldrich (USA) and TCI (Japan) Chemical Inc. and used as
received without further purification.
2.2. Synthesis and characterization of CrSBA-15 and
CrMCM-41
The collected organic products after completion of the reaction
were extracted using ether from the resulting mixture cooled at
room temperature and analyzed with authentic samples by gas
chromatography (GC) employing a ZB-5 capillary column.
Additionally, the products were further confirmed using com-
bined gas chromatography-mass spectrometry (GC-MS, Hewlett
G1800A).
As-synthesized mesostructured CrSBA-15 catalysts with nSi/
nCr = 8, 16, 20, and 50 were synthesized with a molar gel com-
position 1 TEOS/0.02–0.125 Cr2O3/0.016 P123/0.43 HCl/127
H2O using the pH-adjusting direct hydrothermal method accord-
ing to a previously published procedure.13,17 Finally, the samples
were calcined at 813 K in air for 6 h for complete removal of the
template. The calcined mesoporous samples with their corres-
ponding nSi/nCr ratios of 8, 16, 20, and 50 are denoted as
CrSBA-15(8), CrSBA-15(16), CrSBA-15(20), and CrSBA-15
(50). The mesoporous CrMCM-41(40) catalyst with nSi/nCr = 40
was synthesized using cetyltrimethylammonium bromide as the
structuring agent with a gel molar ratio 1 SiO2/0.025 Cr2O3/0.25
CTMABr/100 H2O under hydrothermal method according to a
previously published procedure.12
2.5. Experimental procedures for stability of catalyst
The used Cr-containing mesoporous silica catalysts, such as
CrSBA-15(8) and CrSBA-15(50), were regenerated by washing
and calcination, as reported in our published method,15 and
reused (0.1 g of catalyst) in the side-chain oxidation of alkyl-
aromatics to determine their catalytic stabilities. The washed
CrSBA-15(8) catalyst was also prepared using our published
method15 and used (0.1 g of catalyst) in this reaction to find its
catalytic activity. After completion of each catalytic reaction, the
catalyst was filtered and analyzed by ICP-AES to determine the
percentage of Cr, and the conversion of alkylaromatics and
2.3. Characterization
The calcined, washed and recyclable CrSBA-15 mesoporous
catalysts were characterized by ICP-AES, XRD, N2 sorption
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selectivity of ketones were calculated with the standard formulas
followed by analysis of the results of GC and GC-MS.
CrSBA-15 catalysts are listed in Table 1S.† Additionally, com-
paring the results of similar chromium content of mesoporous
catalysts (Table 1S†), CrSBA-15(20) has significantly higher
catalytic activity as compared to CrMCM-41(40), as shown in
Table 1. This reaction is also carried out using SiSBA-15 syn-
thesized by the pH-aDH method.13 In this case, ∼15% EB con-
version as well as ∼82% EHP is obtained (Table 1), thus
indicating that the major activity is only due to chromium
species incorporated on the silica pore walls of SBA-15. On the
basis of catalytic activities, it is noted that the well-ordered
mesoporous catalyst plays an important catalytic role in the pro-
duction of APvO with higher selectivity.
Among the other Cr-containing 2D mesoporous catalysts,
CrSBA-15(8) and CrSBA-15(50) produce the highest and lowest
APvO selectivity, accordingly. From this information, we have
used the catalysts viz. CrSBA-15(8) and CrSBA-15(50) as re-
usable catalysts, under the same reaction conditions noted in
Table 1, to find their catalytic stabilities. Initially, the used
CrSBA-15 catalysts usually produce less product selectivity
because most of the active sites can be screened by unreacted
organics that make one of the sintering effects on the surface of
catalysts, and hence the catalysts, before reuse in this catalytic
reaction, need to be regenerated by washing and calcination.
The catalysts, such as CrSBA-15(8) and CrSBA-15(50), were
washed four times with acetone and dried at 393 K overnight.
Finally the catalysts were calcined at 773 K for 6 h in air for the
removal of the organics and unreacted DPM molecules, comple-
tely. The treated catalysts, such as CrSBA-15(8) and CrSBA-15
(50), were reused for this reaction (Table 1). The EB conversion
as well as APvO selectivity decreases in the first two runs (not
shown in Table 1). On the basis of the first two runs, the leaching
of non-framework chromium species, such as toxic chromium
3. Results and discussion
The liquid-phase oxidation of alkylaromatics over the CrSBA-15
catalyst (Scheme 1), which proceeds via a radical-chain mecha-
nism.15,21 Initially, for the side-chain oxidation of alkylaromatics,
selective synthesis of APvO was investigated using the reaction
conditions noted in Table 1. This reaction also produces minor
byproducts such as 1-phenylethanol (PE–OH) and ethylbenzene
hydroperoxide (EHP). In order to increase the APvO selectivity,
the catalytic activity increases in the following order, CrSBA-15
(8) > CrSBA-15(16) > CrSBA-15(20) > CrMCM-41(40) >
CrSBA-15(50). CrSBA-15(8) exhibits the best performance with
95.6% EB conversion as well as 98.2% APvO selectivity, and
its catalytic activity is higher than other CrSBA-15 catalysts due
to the higher loading of tetrahedral Cr5+/Cr6+ species on the
inner pore walls of SBA-15, resulting in a huge number of
accessible active sites because the tetrahedral Cr5+/Cr6+ species
create a large number of Lewis acid sites on the inner surface of
the pore walls. The characteristic results of ICP-AES, XRD, N2
sorption isotherms, ESR, UV-vis DRS strongly support that the
CrSBA-15(8) catalyst has higher catalytic activity than other
Cr-containing mesoporous catalysts; ICP-AES studies show that
the amounts of Cr-ions incorporated on the silica pore walls of
CrSBA-15(8) are higher than those of other CrSBA-15 cata-
lysts.13 Furthermore, the results of 29SiMAS NMR, as shown in
Fig. 1S,† prove that the signal intensity of CrSBA-15(8) is much
reduced as compared to that of siliceous SBA-15;13,15,17 this
observation clearly supports the stabilization of chromium ions
via silanol groups (defect sites). As shown in Fig. 2S and 3S,†
the results of ESR and UV-vis DRS prove that the amounts of
tetrahedral Cr5+/Cr6+-ions coordinated to Si4+ on the silica walls
of the calcined CrSBA-15(8) catalyst are higher than those of
other CrSBA-15 catalysts.13,15,17 Even though the huge amounts
of Cr-ions are incorporated into the silica pore walls, the results
of N2 sorption isotherms and XRD reveal that the textural (pore
size, pore volume and wall thickness) and structural (unit cell
size) parameters of the CrSBA-15(8) catalyst are higher as com-
pared to other CrSBA-15 catalysts.13,15,17 The results of
elemental composition, structural and textural properties of
Scheme 1 Oxidation of EB over CrSBA-15.
Table 1 Oxidation of ethylbenzene over Cr-containing mesoporous silica catalystsa
nSi/nCr ratio
Product selectivity (%)
Catalysts
Gel
Productb
EB conversion (%)
APvO
PE–OH
EHP
CrSBA-15(8)
CrSBA-15(16)
CrSBA-15(20)
CrSBA-15(50)
CrMCM-41(40)
CrSBA-15(8)c
CrSBA-15(50)c
CrSBA-15(8)d
SiSBA-15
8
9.9
17.3
45.0
99.8
45.0
12.4
99.9
12.3
—
95.6
82.3
75.6
54.3
69.5
90.4
45.1
90.5
15.2
98.2
84.5
78.7
56.7
71.3
95.5
54.5
95.9
17.3
1.8
10.3
15.6
12.5
12.5
4.5
5.2
4.1
1.0
—
5.2
5.7
30.8
16.2
—
40.3
—
16
20
50
40
—
—
—
—
81.7
a Reaction conditions: 0.1 g of catalyst, 1 : 2 mmol ratio of EB to TBHP (20 mmol of EB : 40 mmol of TBHP), reaction time = 10 h, 20 ml of CB, T =
393 K. b nSi/nCr ratios of products are determined by ICP-AES. c The results obtained by regenerated catalysts used in the 4th run. d Washed catalyst.
14206 | Dalton Trans., 2012, 41, 14204–14210
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CrSBA-15(8), as shown in Table 1. From this evidence, it is
interesting to note that the catalysts used in the 4th run show
nearly the same conversion, indicating that the leaching of active
chromium is not observed on the surface of the SBA-15 matrix.
Additionally, ICP-AES analysis confirms the absence of Cr
species after analyzing the reaction mixture. After finding the
results of the four runs, the hot-catalyst filtration experiments
were also carried out twice at 393 K during oxidation of EB over
CrSBA-15(8). In this case, the filtrate solution gives a very low
EB conversion (10.5%) with trace amounts of APvO selectivity
(<5.3%) due to leaching of non-framework toxic-chromium
oxides on the surface of the catalyst, indicating that the oxidation
of EB takes place on the surface of CrSBA-15(8) and is found to
be a true heterogeneous process.22 As an important point, the
UV-vis DRS results, as shown in Fig. 3S,† clearly confirm that
the non-framework toxic-chromium oxides, such as polychro-
mate and Cr(VI) oxide species, are completely removed by
washing treatments, and similar results are almost observed from
the hot-catalyst filtrate solution of CrSBA-15(8). Furthermore,
the characteristic results of ICP-AES, XRD, N2 sorption iso-
therms and UV-vis DRS strongly prove that the used catalysts,
such as washed CrSBA-15(8) and recyclable CrSBA-15(8),
show that their structural and textural parameters (Table 1S and
Fig. 4S†), including the environments of Cr species coordinated
on the surface of SBA-15, are still maintained.15,17 It is observed
on the basis of recycling results that CrSBA-15(8) is exclusively
found to be the unprecedented catalyst among the other Cr-
containing mesoporous catalysts.
Fig. 1 Oxidation of EB with various reaction temperatures. Reaction
conditions: 0.1 g of CrSBA-15(8) catalyst, 1 : 2 mmol ratio of EB to
TBHP (20 mmol of EB : 40 mmol of TBHP), reaction time = 10 h,
20 ml of CB.
Since CrSBA-15(8) is the best active catalyst among the other
CrSBA-15 catalysts, it is further used in this reaction with differ-
ent reaction parameters such as temperature, time and ratios of
reactants. The oxidation of EB is conducted at different reaction
temperatures as well as times using the reaction conditions noted
in Fig. 1 and 2. When the reaction temperature and time are
decreased from 393 K to 313 K and 10 to 1 h, respectively, the
rate of both ethylbenzene consumption and APvO formation
decreases. The feasible EB conversion decreases with decreasing
temperature or time. This may be due to the lower activity of
Lewis active sites on the surface of the catalyst at a lower temp-
erature or time, which cannot be promptly supported for the
decomposition of EHP into PE–OH whereas the PE–OH con-
verted into APvO is low. Moreover, the EB conversion as well
as APvO selectivity is not significantly increased when the
reaction temperature and time are increased to 403 K and 12 h,
respectively, under similar reaction conditions. However, at
above 403 K, the EB conversion as well as APvO selectivity
slightly decreases because the self decomposition of TBHP pro-
ceeds faster and it does not participate effectively in the process
of side-chain oxidation of EB. In contrast the PE–OH selectivity
increases from 390 to 310 K but decreases after 390 K because
of increasing APvO selectivity. When this catalytic reaction is
carried out with a 1 : 2 ratio of EB to TBHP using the reaction
conditions noted in Fig. 3, higher EB conversion as well as
APvO selectivity is obtained. A reason may be that the mixture
of reactants in the ratio of 1 : 2 reaches an equilibrium state and
the reactants react mutually with each other on the catalytic
surface without deactivation. The conversion as well as selec-
tivity decreases in other EB to TBHP ratios of 1 : 1, 1 : 3 and
2 : 3. Note that overoxidation products, such as benzoic acid and
Fig. 2 Oxidation of EB with different reaction times. Reaction con-
ditions: 0.1 g of CrSBA-15(8) catalyst, 1 : 2 mmol ratio of EB to TBHP
(20 mmol of EB : 40 mmol of TBHP), reaction temperature = 393 K,
20 ml of CB.
oxide, polychromate and pentavalent chromium oxide, is
observed on the catalytic surface while the catalytic activity is
slightly decreased, as shown in Table 1. But, the EB conversion
as well as APvO selectivity remains constant in the 4th run,
indicating that the active tetrahedral chromium species could not
be further leached on the mesoporous matrix, which is in good
agreement with the ICP-AES results of filtrate solutions where
no active chromium species was detected in the solution. We
suspect that in this reaction the aqueous t-butanol generated as a
byproduct is responsible for the extraction of non-framework
pentavalent chromium species in CrSBA-15 catalysts. Moreover,
the washed CrSBA-15(8) is also used as a green mesoporous
catalyst in this reaction to find its catalytic stability and its effect
is nearly similar to the catalytic results obtained in the 4th run of
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conversion (75%) as well as APvO selectivity (86%) is much
reduced because the high quantity of EB may not be completely
dissolved and cannot be completely reacted with TBHP. At
353 K, MeOH gives 88.3% EB conversion with 100% APvO
selectivity. But the EB conversion is still higher as compared to
that obtained in TBHP when the reaction is carried out with
H2O2 instead of TBHP in the presence of MeOH under similar
reaction conditions. Additionally, the EB conversion over
washed CrSBA-15(8) is almost similar in both solvents, such as
CB and MeOH, as shown in Tables 1 and 2. Even though
MeCN is a green solvent, it creates low catalytic activity on the
surface of the catalyst and produces low EB conversion. When
the liquid-phase oxidation of EB is carried out with H2O2
instead of TBHP in the presence of MeCN at 343 K under
similar reaction conditions, the EB conversion as well as APvO
selectivity is very low because the EHP could not be completely
oxidized whereas the EHP converted into PE–OH is much less.
For the confirmation of the catalytic activity decrease by the sol-
vents, this reaction was carried out without any solvent at 393 K
with a higher reaction time (24 h). In this case, the EB conver-
sion (53%) and APvO selectivity (67%) are obtained. From the
above experiments, the unsuitable solvents could not be pro-
moted to higher catalytic activity and may have blocked most of
the active sites on the surface of catalyst.23 Comparing the cata-
lytic activity obtained with different solvents, it is found that CB
with TBHP at higher temperature (393 K), and MeOH with
H2O2 at lower temperature (353 K), are the best solvents for the
highly selective synthesis of APvO.
Fig. 3 Variation of EB to TBHP ratios over CrSBA-15(8). Reaction
conditions: 0.1 g of CrSBA-15(8) catalyst, reaction temperature =
393 K, reaction time = 10 h, 20 ml of CB.
Table 2 Oxidation of ethylbenzene with different solvents over
CrSBA-15(8)a
Product selectivity (%)
Temp.
(K)
Solvent
(20 ml)
EB conversion
(%)
APvO
PE–OH
EHP
To explore the scope and limitations of this catalytic system
over the CrSBA-15(8) catalyst, the side-chain oxidation of
p-substituted EBs, such as 4-ethylanisole, 1-chloro-4-ethylbenzene,
1-ethyl-4-nitrobenzene and 4-ethylphenol, and o-substituted
EBs, such as 2-ethylanisole, 1-chloro-2-ethylbenzene, 1-ethyl-
2-nitrobenzene and 2-ethylphenol, has been studied using the
reaction conditions noted in Table 3. As shown in Table 3, the
mesoporous CrSBA-15(8) catalyst exhibits outstanding perform-
ance in oxidation of the ED group substituted at the p-position
of EB and produces higher ketone selectivity (entry 1). But in
the case of the ED group substituted at the o-position of EB, the
ketone selectivity is much reduced because of steric effects
(entry 5), a similar trend has already been reported in the
literature.24 Among all the EW groups (e.g., OH and NO2)
substituted at the p-position of EBs, the oxidation of p-Cl-substi-
tuted EB over CrSBA-15(8) is excellent and produces 97%
ketone selectivity (entries 2–4), as shown in Table 3. However,
in the case of EW groups (e.g., Cl, OH and NO2) substituted at
the o-position of EBs the ketone selectivity decreases because
of steric effects (entries 6–8). Finally, the oxidation of p-substi-
tuted EBs (ED and EW groups) has positive effects on the
ketone selectivity whereas the oxidation of o-substituted EBs
(ED and EW groups) has somewhat negative effects on the
ketone selectivity.
393
353
353
353
343
343
393d
CB
98.6
88.3
98.7
90.5
58.4
17.6
53.0
100
100
100
100
80.4
56.0
67.0
—
—
—
—
10.9
5.3
2.0
—
—
—
—
8.7
38.7
31.0
MeOH
MeOHb
MeOHc
MeCN
MeCNb
—
a Reaction conditions: 0.1 g of CrSBA-15(8) catalyst, 1 : 2 mmol ratio of
EB to TBHP (20 mmol of EB : 40 mmol of TBHP), reaction time =
10 h. b 20 mmol of 30% H2O2 was used instead of TBHP. c Washed
CrSBA-15(8) catalyst. d Reaction time = 24 h.
benzaldehyde, form when TBHP is increased (1 : 3), and the
diffusion rate of CrSBA-15(8) and the rate of EB conversion
decrease in the ratios of 1 : 1 and 1 : 3 which could not be used
to improve the higher APvO selectivity because of the
unreacted organics deposited on the inner pore walls. Comparing
the results of all catalytic parameters, it is noteworthy that the
EB oxidation at 393 K for 10 h with a 1 : 2 ratio of EB to TBHP
over CrSBA-15(8) gives higher EB conversion as well as
APvO selectivity.
The oxidation of EB was carried out with different solvents
like MeCN, CB and MeOH over CrSBA-15(8) using the reaction
conditions noted in Table 2, for obtaining a higher selectivity of
APvO. CB is a common and high-boiling solvent that has the
ability to form complexes on the catalytic surface for a long
time, and it preferentially attacks the active sites to complete the
maximum conversion of EB. When this reaction is carried out
with 10 ml of CB under similar reaction conditions, the EB
The recyclable CrSBA-15(8) catalyst has also been reused in
the side-chain oxidation of p- and o-substituted EBs under
similar reaction conditions, and the conversion of p- and o-sub-
stituted EBs as well as the selectivity of ketones is obtained as
shown in Table 3. Based on the catalytic activity in the oxidation
of each p- and o-substituted EB, it is noteworthy that the cata-
lytic activity of this catalyst remains constant in the 4th run, and
14208 | Dalton Trans., 2012, 41, 14204–14210
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Table 3 Selective oxidation of ethylbenzene compounds over CrSBA-15(8)a
Entry no.
1
Substrate
Ketone product
Conversion (%)
Selectivity of ketone (%)
Selectivity of alcohol (%)
98
100
98b
98c
—
2b
2c
95b
96c
2
3
90
97
3
86b
87c
94b
94c
6b
6c
73
91
9
67b
68c
85b
85c
15b
15c
4
5
80
94
6
76b
75c
89b
88c
11b
12c
87
93
7
84b
85c
89b
88c
11b
12c
6
7
8
67
88
12
63b
63c
83b
83c
17b
17c
61
84
16
59b
58c
79b
78c
21b
22c
68
81
19
65b
64c
76b
77c
24b
23c
a Reaction conditions: 0.1 g of CrSBA-15(8) catalyst, 20 ml of CB, reaction time = 10 h, temperature = 373 K, 1 : 3 ratio of substrate-to-TBHP
(20 mmol of substrate, 60 mmol of TBHP). b The results obtained by regenerated CrSBA-15(8) catalyst used in the 4th run. c Washed CrSBA-15(8)
catalyst.
the percentage of Cr-content remains constant. Furthermore,
washed CrSBA-15(8) was also used in these reactions to find its
catalytic activity. This catalytic activity is nearly similar to that
of used CrSBA-15(8) in the 4th run, as shown in Table 3.
Overall, on the basis of the catalytic results, it is noted that
CrSBA-15(8) is a highly active, recyclable and unprecedented
heterogeneous catalyst for the highly selective synthesis of
ketones in the oxidation of p- and o-substituted EBs. Comparing
the results with the literature, the CrSBA-15(8) catalyst has
exceptional catalytic activity in the oxidation of p- and o-substi-
tuted EBs among the other solid catalysts.18,20,25,26
4. Conclusions
The mesoporous CrSBA-15 catalysts have been successfully used
in the oxidation of alkylaromatics and produced the higher selec-
tivity of alkylaromatic ketones. From the washing studies, the
green CrSBA-15(8) catalyst has been completely recovered from
the toxic nature of non-framework chromium oxide and has been
successfully used in these oxidations to find its catalytic activity.
In addition to finding the catalytic stabilities of CrSBA-15 cata-
lysts, the recyclable and hot-catalytic experiments have been suc-
cessfully studied. Based on the recyclable studies in the oxidation
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7 H. E. B. Lempers and R. A. Sheldon, J. Catal., 1998, 175, 62.
reactions, it reveals that a huge number of tetrahedral chromium
species have successfully participated in the Lewis active sites on
the silica surface of the CrSBA-15(8) catalyst. From the results of
hot-catalyst filtration, it is found that CrSBA-15(8) is a true hetero-
geneous catalyst in the oxidation reactions. Based on all the cata-
lytic results, CrSBA-15(8) is found to be a highly active,
recyclable and environmentally friendly solid catalyst.
8 T. K. Das, K. Chaudhary, E. Nandanan, A. J. Chandwadker,
A. Sudalai, T. Ravindranathan and S. Sivasanker, Tetrahedron Lett.,
1997, 38, 3631.
9 W. Zhang, J. Wang, P. T. Tanev and T. J. Pinnavaia, Chem. Commun.,
1996, 979.
10 A. Corma and H. Garcia, Chem. Rev., 2003, 103, 4307.
11 N. Ulagappan and C. N. R. Rao, Chem. Commun., 1996, 1047–1048.
12 M. Selvaraj, B. H. Kim and T. G. Lee, Chem. Lett., 2005, 34, 1290.
13 M. Selvaraj and S. Kawi, Chem. Mater., 2007, 19, 509.
14 M. Selvaraj and S. Kawi, Microporous Mesoporous Mater., 2007, 101,
240.
15 M. Selvaraj, M. Kandaswamy, D. W. Park and C. S. Ha, Catal. Today,
2010, 158, 286.
16 M. Selvaraj, D. W. Park, S. Kawi and I. Kim, Appl. Catal., A, 2012,
415–416, 17.
17 M. Selvaraj and C. S. Ha, Catal. Today, 2011, 175, 546.
18 R. A. Sheldon and J. K. Kochi, Adv. Catal., 1976, 25, 272.
19 W. A. Carvalho, P. B. Varaldo, M. Wallau and U. Schuchardt, Zeolites,
1997, 18, 408.
Acknowledgements
This research work was supported by Basic Science Research
Program through the National Research Foundation (NRF) of
Korea funded by the Ministry of Education, Science and Tech-
nology (2012-0007172).
20 A. Sakthivel, S. K. Badamali and P. Selvam, Catal. Lett., 2002, 80,
73.
References
1 M. Hudlicky, Oxidations in Organic Chemistry, American Chemical
Society, Washington, DC, 1990, p. 252.
21 S. Boitsov, J. Songstad and J. Muzart, J. Chem. Soc., Perkin Trans. 2,
2001, 2318.
2 K. Weissermel and H.-J. Arpe, Industrial Organic Chemistry, VCH,
Weinheim, 2nd edn, 1993.
22 R. A. Sheldon, M. Wallau, I. W. C. E. Arends and U. Schuchardt, Acc.
Chem. Res., 1998, 31, 485.
3 J. Muzart, Chem. Rev., 1992, 92, 113.
4 R. Alcantara, L. Canoiva, P. Guilharme-Joao, J. Maria Santos and
I. Vazquaz, Appl. Catal., A, 2000, 203, 259.
5 C. Gue, Q. Peng, Q. Liu and G. Jiang, J. Mol. Catal. A: Chem., 2003,
192, 295.
6 B. M. Choudary, A. D. Prasad, V. Bhuma and V. Swapna, J. Org. Chem.,
1992, 57, 5841.
23 S. S. Bhoware, S. Shylesh, K. R. Kamble and A. P. Singh, J. Mol. Catal.
A: Chem., 2006, 255, 123.
24 Y. Bonvin, E. Callens, I. Larrosa, D. A. Henderson, J. Oldham,
A. J. Burton and A. G. M. Barrett, Org. Lett., 2005, 7, 4549.
25 N. Anand, K. H. P. Reddy, G. V. S. Prasad, K. S. R. Rao and D. R. Burri,
Catal. Commun., 2012, 23, 5.
26 S. K. Mohapatra and P. Selvam, J. Catal., 2007, 249, 394.
14210 | Dalton Trans., 2012, 41, 14204–14210
This journal is © The Royal Society of Chemistry 2012