Highly selective oxidation of tetralin to 1-tetralone over mesoporous CrMCM-41 molecular sieve catalyst using supercritical carbon dioxide

Article abstract of DOI:10.1039/b812137k

Selective oxidation of tetralin by molecular oxygen over mesoporous CrMCM-41 molecular sieve catalyst using supercritical carbon dioxide (scCO ₂) solvent has been investigated. CrMCM-41 catalyst gave high selectivity (96.2%) and good yield (63.4%) of 1-tetralone. The presence of scCO₂ medium improves 1-tetralone selectivity and suppresses leaching of chromium from the CrMCM-41. The activity over recycled CrMCM-41 remains nearly the same under the present experimental conditions. The effect of the reaction parameters on CrMCM-41 was also studied in detail along with comparison of its catalytic activities with other mesoporous catalysts, viz. MnMCM-41, CoMCM-41, microporous CrAPO-5, CoMFI, and macroporous Cr/SiO₂ catalyst, respectively. In addition this catalytic system was also applied for the oxidation of other benzylic compounds such as indane, fluorene, acenaphthene and diphenylmethane. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009.

Full text of DOI:10.1039/b812137k

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www.rsc.org/njc | New Journal of Chemistry
Highly selective oxidation of tetralin to 1-tetralone over mesoporous
CrMCM-41 molecular sieve catalyst using supercritical carbon dioxidewz
Sudhir E. Dapurkar,* Hajime Kawanami,* Toshirou Yokoyama
and Yutaka Ikushima
Received (in Montpellier, France) 25th June 2008, Accepted 3rd October 2008
First published as an Advance Article on the web 14th November 2008
DOI: 10.1039/b812137k
Selective oxidation of tetralin by molecular oxygen over mesoporous CrMCM-41 molecular sieve
2
catalyst using supercritical carbon dioxide (scCO ) solvent has been investigated. CrMCM-41
catalyst gave high selectivity (96.2%) and good yield (63.4%) of 1-tetralone. The presence of
scCO medium improves 1-tetralone selectivity and suppresses leaching of chromium from the
2
CrMCM-41. The activity over recycled CrMCM-41 remains nearly the same under the present
experimental conditions. The effect of the reaction parameters on CrMCM-41 was also studied in
detail along with comparison of its catalytic activities with other mesoporous catalysts, viz.
2
MnMCM-41, CoMCM-41, microporous CrAPO-5, CoMFI, and macroporous Cr/SiO catalyst,
respectively. In addition this catalytic system was also applied for the oxidation of other benzylic
compounds such as indane, fluorene, acenaphthene and diphenylmethane.
acid catalyst technology enables a substantially decreased
production cost owing to reduction in the volume of wastes.
Though zeolites have attracted strong attention as solid acid
Introduction
Tetralones are important intermediates in the production of
many compounds, including sertraline, an antidepressant,
naphthyl carbamate, an insecticide, and 18-methylnorethisterone,
8
catalysts, they present severe limitations due to their smaller
pore size when larger molecules are involved, especially in
9
liquid phase oxidation systems.
1
a contraceptive, respectively. It is estimated that the
worldwide market for tetralone as
a pharmaceutical/
agrochemical intermediate exceeds approximately h3 million
Another well studied process for synthesis of 1-tetralone
is the direct oxidation of tetralin. Numerous reports are
available on oxidation of tetralin in the presence of oxidative
2
per year.
Tetralones have been produced by intramolecular condensation
10–17
catalysts.
From these investigations it is established
3
,4
reactions using metallic reagents, such as aluminium chloride
which generate large volumes of metallic hydroxides as waste.
These processes are also multi-step synthesis processes, so the
4
overall yield of tetralone is very low. A number of studies
that the primarily formed 1-tetralin hydroperoxide (THP)
decomposes catalytically into a mixture of 1-tetralone and
1
-tetralol. Selective transformation of THP is a vital issue to
improve the yield of 1-tetralone. Since hydroperoxide is
explosive, its complete decomposition is also essential. Therefore,
many inorganic reagents such as chromium oxide, manganese
dioxide, pyridinium dichromate, etc., and/or metallic cata-
claiming one-step synthesis of tetralones have been proposed
including those by intramolecular Friedel–Crafts reaction of
5
4
-arylbutyric acids in the presence of Lewis acid catalysts,
alkylation–acylation of aromatics with g-butyrolactone using
supported heteropolyacids and ionic hydrogenation of
lysts, e.g., chromium, manganese, cobalt, copper, iron
10–12
6
salt etc., have been extensively studied.
However, these
7
-naphthol by superacid. However, the above processes use
1
methods do not promote complete decomposition of THP.
Additionally, these catalytic systems generate copious
quantities of metal wastes, along with tedious work up,
difficulties in separation/reusability of the catalyst etc. which
makes them environmentally unsuitable.
excess quantities of starting materials and require highly acidic
catalysts. Hence these procedures are hardly compatible
with environmental regulations. Recently, alkyl-substituted
tetralone compounds have been synthesized from p-xylene,
or other benzene derivatives with cyclic lactones using a
Consequently, a large number of heterogeneous catalysts
have been analyzed and proved to be effective for the
2
molecular sieve based catalyst (e.g., zeolite). This new solid
1
3–17
selective oxidation of tetralin.
Usually heterogeneously
catalyzed tetralin oxidations are performed in volatile and/or
toxic organic solvents such as methanol, chlorobenzene,
dichloromethane etc., in combination with tert-butyl
hydroperoxide solution (TBHP, 70 wt% in water) as an
Research Center for Compact Chemical Process, National Institute of
Advanced Industrial Science and Technology (AIST), Tohoku,
4-2-1 Nigatake, Miyagino-ku, Sendai, 983-8551, Japan.
E-mail: s-dapurkar@aist.go.jp. E-mail: h-kawanami@aist.go.jp;
Fax: +81 22-237-5215; Tel: +81 22-237-5214
w Note that the term ‘‘supercritical’’ in this paper is used for a reaction
mixture exceeding the critical point of the solvent, despite several
phases present.
z Electronic supplementary information (ESI) available: Preparation
of catalysts, characterization, and catalytic experimental details. See
DOI: 10.1039/b812137k
1
3–17
oxidant.
These catalytic systems are effective for the
decomposition of THP. In addition, mesoporous CrMCM-41
molecular sieve catalyst has been found to be promising for
the liquid phase oxidation of tetralin with 70 wt% TBHP
1
5,16
oxidant.
5
38 | New J. Chem., 2009, 33, 538–544
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Although hydroperoxide is a satisfactory oxidant, its use
along with toxic solvents during these processes is far from
Product analysis
The reaction products were analyzed by capillary gas
chromatography using a FID (Chromatograph: Varian CP
1
8
being environmentally benign.
For developing a more environmental-friendly process for
the synthesis of 1-tetralone, it is beneficial to use molecular
oxygen/air and non-toxic solvents along with an effective
catalytic system. A new ‘‘greener’’ approach which has
attracted attention is the use of supercritical carbon dioxide
3
800, column: HP-5, 30 m (length) ꢁ 0.32 mm (inner diameter)
ꢁ
0.25 mm (film thickness). The identity of the products was
first confirmed by using authenticated standards and their
individual response factors were determined by using a
suitable internal standard phenylacetonitrile by the calibration
method. Additionally, the identity of the reaction products
was also confirmed by using GC-MS (Varian CP 3800, 1200L
Quadrupole MS/MS, column: HP-1 30 m (length) ꢁ 0.32 mm
(
scCO
oxidation reactions with a clean oxidant (molecular oxygen).
It has been reported that the presence of scCO can improve
yields, reaction rates and/or product selectivity in oxidation
2
) as solvent for various alcohol, alkene, and alkane
1
9
2
(
inner diameter) ꢁ 0.25 mm (film thickness). The THP, TOne
1
2
0
reactions.
and TOl selectivity was also determined by H NMR on
Varian Unity INOVA-500.
Here we report a simple one-step process for the synthesis of
-tetralone by direct oxidation of tetralin over CrMCM-41
catalyst using scCO with molecular oxygen.
1
2
Results and discussion
10–12
In case of tetralin oxidation, it is established in the literature
Experimental
that the primarily formed 1-tetralin hydroperoxide (THP)
(Scheme 1, Step I) decomposes in presence of the catalyst
into a mixture of 1-tetralone (TOne) and 1-tetralol (TOl)
Catayst preparation
CrMCM-41 catalyst comprising 2.0 wt% chromium was
prepared as described previously with slight modification in
1
molar gel composition. Synthesis and characterization are
(
Scheme 1, Step II).
2
At first, we studied the effect of various reaction parameters,
pressure, oxygen pressure, reaction
for example, CO
2
provided in the ESI.z
temperature, and reaction time on the conversion of tetralin
and the product selectivity over CrMCM-41 catalyst.
Reaction procedure
It is well-known that the properties of scCO
2
are sensitive to
Oxidation of tetralin was carried at 80 1C for 14 h in a stainless
steel high pressure reactor equipped with a specially designed
pressure. Therefore, the effect of CO pressure on the
2
conversion and the product selectivity for the oxidation of
tetralin was investigated over CrMCM-41 at 80 1C for 14 h
and the results are shown in Table 1. As can be seen from
3
Teflon insert (Fi.d.: 38 mm and inner volume: 40 cm ). In a
typical experiment, 2 mmol tetralin and 40 mg CrMCM-41
were placed inside the reactor vessel. The reactor was flushed
Table 1, CO
2
pressure dramatically influences the catalytic
solvent (entry 1), the catalyst
with 2 MPa CO
heated to 80 1C. To start the reaction, the reactor was charged
with 2 MPa O from the O cylinder followed by pumping CO
using JASCO SCF-Get CO delivery pump to the desired
2
followed by 1.0 MPa O
2
(two times) and then
activity. In the absence of scCO
2
showed low conversion of approximately 41% as well as low
selectivity of 1-tetralone (75%). Surprisingly, in the presence
2
2
2
of scCO
of THP, while in the absence of scCO
1-tetralone selectivity was observed in the presence of CO
2
the catalyst revealed complete decomposition
2
pressure. Stirring was achieved by means of a magnetic
stirrer, the motor of which was set at one-half max speed
2
it did not. High
2
.
(
approximately 360 rpm). The total pressure of reaction
A similar observation was reported in the literature that
chromium containing catalysts do promote more decomposition
system was maintained by SCF-BPG/M (JASCO) back
pressure regulator. After the reaction was over, the reactor
was cooled in an ice-bath and the reactor slowly depressurized
1
0,11
of THP in liquid phase oxidation of tetralin. Furthermore,
with increase in CO pressure from 6 to 11 MPa, 1-tetralone
selectivity increases (entries 2–7). However at the moment
2
2
2
through a back pressure regulator. The reaction products
were collected by simple conventional filtration of the reaction
mixtures through a Advantec filter paper. Finally, the catalyst
was separated and calcined at 540 1C for 6 h. The regenerated
catalyst was used for recycling experiments.
Visual observation of the phase behavior in scCO
2
A high-pressure view cell (JASCO; Model STR-458, internal
volume 10 ml) was used to observe the phase behavior of
2
tetralin in scCO . The cell is equipped with windows and
connected to a temperature-controlled bath. A magnetic bar
is used to mix the contents in the viewing cell. The phase
behavior experiment was conducted in the range of 6 MPa to
1
6 MPa CO pressure with 2 mmol of tetralin at 80 1C and
2
2
MPa oxygen pressure.
Scheme 1 Reaction scheme of the oxidation of tetralin.
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a
Table 1 The effect of CO
2
pressure on the oxidation of tetralin over CrMCM-41
d
Selectivity (%)
Conversion
of tetralin (%)
Yield
of TOne (%)
b
c
e
Entry
CO
2
pressure/MPa
Phases
THP
TOl
TOne
1
2
3
4
5
6
7
0
6
7
8
9
s/l
40.8
47.4
48.7
60.0
65.9
42.4
20.5
18.0
0.0
0.0
0.0
0.0
0.0
0.0
7.0
9.2
8.0
4.5
3.8
2.9
1.6
75.0
90.8
92.0
95.5
96.2
97.1
98.4
30.6
43.0
44.8
57.3
63.4
41.2
20.2
s/l/g
s/l/g
s/l/g
s/l/g
s/l/g
s/sc
10
11
a
b
Reaction conditions: 2 mmol tetralin; m(CrMCM-41) = 40 mg; PO2 = 2 MPa; T = 80 1C; t = 14 h. s—solid phase; l—liquid phase; g—gas
c
d
phase; sc—supercritical phase. Conversion of tetralin analysed by GC using phenylacetonitrile as an internal standard (see ESIw). Product
selectivity analysed by GC and also determined by H-NMR. Selectivity of other by-product was left out because it was less than 1%. Yield of
1
e
TOne = [(conversion of tetralin ꢁ selectivity of TOne)/100].
reason for the increase in selectivity of 1-tetralone is not
understood.
As CO pressure increases (entries 2 to 4), tetralin conversion
2
increases and reaches a maximum at 9 MPa CO2 pressure
(entry 5). Under this liquid/gas (l/g) biphasic system, CO2
allows substantial quantities of oxygen to solubilize, providing
2
3
for high rate of reaction. Recent work in the literature shows
that one can achieve high gas solubility and hence high reaction
rate in such a moderate pressure system. With further increase
in CO
from 65.9 to 20.5% (entries 5 to 7). This suggests that the
drop in activity with increasing CO pressure is due to the
2
pressure from 9 to 11 MPa, conversion decreases
2
transformation of the two-phase reaction system to a one-phase
reaction system (sc phase). Therefore, we think that at high CO2
pressure the reaction system is diluted as a consequence there is
less interaction between the reactant/intermediate and the
catalyst leading to a decrease in reaction rate. Similar trend of
decrease in activity and an increase in product selectivity has
been observed in other oxidation reactions using heterogeneous
Fig. 1 Effect of reaction temperature on tetralin conversion and
product selectivity over CrMCM-41: (’) conversion of tetralin, (m)
selectivity of 1-tetralone, (.) selectivity of 1-tetralol. Reaction condi-
O
tions: 2 mmol tetralin; m(CrMCM-41) = 40 mg; P 2 = 2 MPa;
2
4
P
CO2 = 9 MPa; t = 14 h.
catalysts in scCO
the range of CO
2
/O systems. It is worth mentioning that in
2
pressure from 6 to 11 MPa, undecomposed
2
THP was negligible. This clearly indicates that CrMCM-41
catalyst is highly effective for the generation and decomposition
attempt to study the kinetics of the reaction in batch mode.
However, as expected from the kinetic data, the reaction
system is complex and does not follow the Arrhenius equation.
We will be looking into getting a deeper insight on the complex
behavior of reaction system by high pressure in situ infrared
spectroscopy (FT-IR). A high 1-tetralone selectivity was
observed even at lower reaction temperature and which
increased as the temperature increased. On the other hand,
as the reaction temperature increases generation of 1-tetralol is
suppressed. This indicates that high reaction temperature
promotes selective decomposition of THP into 1-tetralone
and additional oxidation of 1-tetralol into 1-tetralone. The
latter observation was confirmed by carrying out a separate
catalytic experiment on 1-tetralol oxidation using scCO2 at
different reaction temperatures (see also Table 3, entry 2).
The catalytic performance of CrMCM-41 as a function of
reaction time was examined at 80 1C using scCO2 and the
results are shown in Fig. 2. At 4 h, we obtained only 32%
tetralin conversion. The highest conversion of approximately
66% was obtained in the range of 14 to 20 h and with further
increase in reaction time, conversion did not improve. The
possible reason could be that after an initially fast reaction, the
2
of THP in the presence of CO media. In addition, we obtained
less than 1% other by-products, viz. 1,4-naphthoquinone under
present reaction conditions, which could have been formed due
1
0,12
to the further oxidation of 1-tetralone.
We examined the oxygen pressure effect at 9 MPa CO2
pressure at 80 1C for 14 h. At o0.5 MPa oxygen pressure,
conversion was approximately 45%. Above 0.5 MPa oxygen
pressure, conversion increases and reaches a maximum in the
range of 1 to 1.5 MPa pressure. We did notice a drop in the
total pressure of reaction system but with the present reaction
setup, oxygen consumption cannot be precisely measured.
Thus, assuming the stoichiometry of reaction, we used excess
amount of oxygen (32 mmol) equivalent to the oxygen
pressure, i.e. 2 MPa for all catalytic experiments.
Fig. 1 shows the effect of reaction temperature on
conversion of tetralin over CrMCM-41 using scCO2.
It can be seen from the figure that tetralin conversion
increases linearly as the temperature increases from 50 to
8
0 1C. With further increase in reaction temperature above
0 1C, conversion does not increase. Therefore we made an
8
5
40 | New J. Chem., 2009, 33, 538–544
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on CrMCM-41. It can be seen from entry 3, that a slight
decrease in conversion was observed as compared to entry 2,
but, over recycled catalyst, conversion remains almost the
same (entry 3 and 4). The analysis of chromium on recycled
catalyst (entry 4) gave 1.7–1.8 wt% chromium as compared to
fresh catalyst content of 2.0 wt% chromium. This shows that
more than 85% chromium still remains even after the third
2 2
run, indicating that under scCO /O system there is no
further leaching of chromium in the reaction mixture. We also
analyzed the reaction solution obtained from recycled catalyst
by ICP-AES, where no chromium was detected. On further
recycling studies, carried out after the third run, conversion
was found to be almost the same (E60%) and no chromium
leaching was detected. We suspect that the water generated as
a by-product is responsible for the extraction of a small
amount of loosely bound chromium in CrMCM-41 catalyst.
Although minimal leaching occurred during the initial run in
the subsequent runs no leaching was observed. Upon recycling
Fig. 2 Effect of reaction time on tetralin conversion and product
selectivity over CrMCM-41: (’) conversion of tetralin, (m) selectivity
of 1-tetralone, (.) selectivity of 1-tetralol. Reaction conditions: 2 mmol
tetralin; m(CrMCM-41) = 40 mg; PO2 =2 MPa; PCO2 =9 MPa;
T = 80 1C.
1
-tetralone selectivity remains nearly the same (entry 3 and 4).
For comparison, we carried out the liquid phase reaction in
acetonitrile solvent over CrMCM-41 (entry 5) under similar
reaction conditions, although conversion was low (36.3%),
CO within the pores of the catalyst may slow the extraction of
2
the products out of the catalyst. This phenomenon may be
2
1-tetralone selectivity was higher than in the scCO solvent.
2
5
more complex in porous catalysts under scCO /O system.
2
Even at such low conversion, we noticed an approximately
2
In addition we found that the reaction was dependent on
tetralin concentration. Also, a longer reaction period, for
example 40 h, led to formation of approximately 2–3%
by-products, mainly 1,4-naphthoquinone indicating that
longer reaction time allows further oxidation of 1-tetralone.
However, still over a wide range of reaction time from 4 to
20% decrease in conversion between the first and second run
in acetonitrile solvent. This observation was further confirmed
by chromium analysis where around 40% chromium
leached out in the reaction mixture, indicating that organic
solvent and/or water as by-product induces more leaching. A
similar trend has often been observed using chromium
containing silicate or aluminophosphate in liquid phase
2
4 h, 1-tetralone selectivity remains 495%.
The influence of various transition metal containing
1
6,17,21,26
oxidations.
The reaction over macroporous Cr/SiO2
catalysts on the oxidation of tetralin by molecular oxygen
using scCO was studied and the results are shown in Table 2.
catalyst (entry 6) showed higher conversion of approximately
70% and good 1-tetralone selectivity. However, upon
recycling, this chromium catalyst (entry 7) showed a dramatic
decrease in conversion from 70.2% to 30.3%. The decrease in
conversion is again due to the leaching of chromium from
silica support. This clearly indicates that the water generated
(by-product) under the reaction conditions, extracts the
loosely bound chromium species from the silica support. It is
2
It can be seen from entry 1 that in the absence of catalyst,
conversion was only 9% with THP as a major product.
CrMCM-41 catalyst showed good conversion (65.9%) and
high 1-tetralone selectivity (96.2%) (entry 2).
Since we found that CrMCM-41 catalyst is highly effective
for the decomposition of THP, we performed a recycling study
a
2
Table 2 Results of the oxidation of tetralin by molecular oxygen over various transition metal containing catalysts using scCO
Selectivity (%)
Conversion
Yield of
TOne (%)
Entry
Catalyst
of tetralin (%)
THP
TOl
TOne
b
1
Blank
9.0
65.9
60.5
59.6
36.3
70.2
30.3
24.6
43.6
70.8
67.0
33.3
78.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
9.5
10.8
22.7
2.4
3.8
4.3
5.8
1.0
7.7
4.1
10.0
4.4
9.5
18.9
13.6
19.6
96.2
95.7
94.2
99.0
92.3
95.9
90.0
95.6
81.0
70.3
63.7
1.8
63.4
57.9
56.1
35.9
64.8
29.0
22.1
41.7
57.3
47.1
21.2
2
3
4e
5
6f
7
CrMCM-41
CrMCM-41
CrMCM-41
CrMCM-41
Cr/SiO
Cr/SiO
c
d
2
2
8
9
CrAPO-5
CrO
g
3
1
1
1
0
1
2
MnMCM-41
CoMCM-41
CoMFI
a
b
Reaction conditions: 2 mmol tetralin; m(catalyst) = 40 mg; PO2 = 2 MPa; PCO2 = 9 MPa; T = 80 1C; t = 14 h. Without catalyst.
c
d
nd run. 3rd run. V(acetonitrile solvent) = 3 ml. 2nd run. Homogeneous run (the amount of chromium was same as that of the
e
f
g
2
amount of chromium (0.02 g Cr) in CrMCM-41.
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known that the Cr/SiO
leaching due the loosely bound chromium to silica. This
2
catalytic system is susceptible to
mesoporous catalysts. The lower conversion could be due to
the smaller pore diameter of MFI structure (pore diameter
0.55 nm) which restricts the diffusion of reactant/intermediate
implies that the CrMCM-41/scCO reaction system suppresses
2
9
the leaching of chromium, thereby recycled catalyst behaves as
a true heterogeneous catalyst under the present experimental
conditions.
and products. It is important to note here that MnMCM-41,
CoMCM-41 and CoMFI catalysts do not promote complete
decomposition of THP even in the presence of scCO . This
2
We also checked the activity over medium pore diameter
chromium aluminophosphate (CrAPO-5) catalyst (pore
diameter 0.73 nm). This chromium catalyst was effective for
the decomposition of THP, thus giving good 1-tetralone
selectivity, but showed low conversion (entry 8).
observation is in the line of study on the tetralin oxidation
1
1
using various metal acetates in acetic acid.
Finally, we studied the selective oxidation of other benzylic
compounds over CrMCM-41 using scCO under similar
2
experimental conditions. The results are summarized in
Table 3. For comparison, oxidation of tetralin (1a) to
1-tetralone (2a) is also included in Table 3. It can be seen
from entry 2 that the catalyst is highly active for the direct
oxidation of corresponding benzylic alcohol viz., 1-tetralol
(1b) and gave very good conversion of 80.1% with 1-tetralone
(2a) selectivity of 95%. It was also highly active for indane (1c)
oxidation and gave high 76.6% conversion and 1-indanone
(2c) selectivity of 95% (entry 3). Fluorene (1d) was likewise
oxidized with 52.4% conversion and 9-fluorenone (2d)
selectivity of 499% (entry 4). When acenaphthene (1e)
A homogeneous run was also carried out with CrO
scCO (entry 9). This chromium reagent gave approximately
4% conversion and high 1-tetralone selectivity, most
3
using
2
4
interestingly no THP was detected.
Other mesoporous catalysts containing manganese and
cobalt, viz., MnMCM-41 and CoMCM-41 (entries 10 and 11)
showed comparable and/or little higher conversion than
CrMCM-41, but 1-tetralone selectivities were low as compared
to the CrMCM-41. Small pore cobalt microporous CoMFI
catalyst (entry 12) showed low conversion compared to the
a
Table 3 Selective oxidation of benzylic compounds over CrMCM-41 using scCO
2
Entry Substrate Product
Conversion (%)
Selectivity (%)
1
65.9
96.2
2
80.1
95
3
4
76.6
52.4
95
499
5
35.0
29.6
85
96
6
a
Reaction conditions: 2 mmol substrate; m(CrMCM-41) = 40 mg; P 2
00 1C; t = 14 h, respectively.
O
= 2 MPa; PCO2 = 9 MPa; entries 1–3 and entries 4–6, T = 80 1C and
1
5
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2
1
was oxidized, 35% conversion and 85% selectivity of
-acenaphthenone (2e) was obtained (entry 5). Meanwhile,
species (1b) in the CrMCM-41 catalyst may interact with
molecular oxygen to produce catalytically active chromium(VI)
peroxo-species (1c) (5). Similar type of chromium(VI)
1
diphenylmethane (1f) was oxidized with 29.6% conversion and
benzophenone (2f) selectivity of 96% was obtained (entry 6).
Thus above observations suggest that the CrMCM-41/scCO2
reaction system is also promising for the oxidation of other
benzylic compounds.
1
2
peroxo-species have been proposed as the active species.
The latter reacts with the intermediate (1a) and forms alkyl
peroxochromium(VI) intermediate (1d) (6). The resulting
0
species is then oxidized to the corresponding ketone (RR CO)
with the elimination of water (7).
Scheme 2 shows the hypothetical reaction pathway of
oxidation of benzylic compounds by molecular oxygen
catalyzed with chromium containing mesoporous catalyst. It
is generally recognized that this type of oxidation proceeds via
Conclusions
2
7
In summary, CrMCM-41 catalyst is found to be highly
efficient for the generation and decomposition of THP
to 1-tetralone in the presence of scCO . A high yield of
a radical-chain mechanism. After initiation reaction (1),
0
substrate (benzylic compound of formula: RR CH
2
where
0
R = Ph and R = cycloalkyl, Ph, etc.,) forms corresponding
alkyl radicals. The alkyl hydroperoxide (1a) intermediate is
produced in the sequence of propagation reactions (2) and
2
1
-tetralone is obtained over the CrMCM-41 compared to
the other metal containing nanoporous catalysts. Another
important deduction is that the CrMCM-41 is resistant to
leaching of chromium in the presence of scCO2 medium
under the present experimental conditions. Furthermore this
catalytic system has been found to be suitable for the
oxidation of other benzylic compounds. Therefore, mesoporous
chromium catalyst is promising for various selective oxidation
reactions including bulkier molecules. This approach is
interesting as it ultimately could prove a useful engineering
(
3), while the mutual reaction of two peroxyl radicals (4)
constitutes the major chain-termination step. It has been
assumed that ketone is produced at the termination step (4).
In our study, for all reactions with CrMCM-41/scCO
2
catalytic system, ketone was the major product. Hence, the
observed reaction results with CrMCM-41 are indicative of the
reaction occurring predominantly via a heterolytic pathway,
because a homolytic pathway (Haber–Weiss mechanism)
would afford alcohols as the major product. This type of
mechanism is operative at propagation step. A similar
solution to the problem of solubilizing substrates in scCO
moderate operating pressures.
2
at
1
4
mechanism is reported by Sheldon et al. for the oxidation
of alkylaromatics with molecular oxygen using CrAPO-5
Acknowledgements
2
8
catalyst. Recently Hermans et al. also reported a comparable
mechanism with silica-immobilized chromium colloids for
cyclohexane autooxidation. Accordingly, we have proposed
a hypothetical reaction pathway. Concurrently, chromium(VI)
Financial support for this work was provided by the Japan
Society for the Promotion of Science (JSPS). S. E. Dapurkar is
grateful to the JSPS. We wish to thank Dr Maya Chatterjee
for fruitful discussion.
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