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Chemistry Letters Vol.37, No.2 (2008)
Effective Aerobic Oxidation of Alcohols over Chromium Containing Mesoporous
Molecular Sieve Catalyst with Supercritical Carbon Dioxide
and Polyethylene Glycol Biphasic Reaction System
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Sudhir E. Dapurkar, Hajime Kawanami, Toshishige M. Suzuki, Toshirou Yokoyama, and Yutaka Ikushima
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST),
4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551
(Received November 7, 2007; CL-071231; E-mail: s-dapurkar@aist.go.jp; h-kawanami@aist.go.jp)
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Aldehyde and ketone were selectively obtained by aerobic
for CrMCM-41 which was found to be 1250 m ꢁg , 1.06
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ꢂ1
oxidation of alcohols over mesoporous CrMCM-41 catalyst with
supercritical carbon dioxide and polyethylene glycol (scCO2/
PEG) as a biphasic reaction system.
cm ꢁg , and 2.4 nm, respectively. Other characterizations of
10
the catalyst are discussed in detail in previous work. Figure 1
shows the results of benzyl alcohol oxidation over CrMCM-41
with various reaction systems. It can be seen from the figure that
the catalyst shows poor activity in scCO2 (at 16 MPa total pres-
sure), further over wide range of CO2 pressure from 10 to
Oxidation of alcohols to the corresponding oxo-compounds
1
are of primary industrial importance. Traditionally, these trans-
formations are carried out in organic solvents using stoichiomet-
18 MPa, the conversion remains less than 10%. The addition
of co-solvent (0.5 mL of acetone) does not improve the catalytic
2
ric amounts of toxic chromium(VI) reagents. Even though these
performance. Recently, scCO2/PEG biphasic system has been
3d,12
reagents show good oxidative performance, problems related to
corrosion, handling, recovery, and reuse of the reagent set up
limitations of their use at industrial scale. Therefore, the intro-
duction of heterogeneous catalytic processes with alternative
solvents is one of the main objectives of current industrial and
academic research. In this context, using supercritical carbon di-
oxide as reaction medium to perform the oxidation of alcohols
by O2 with many heterogeneous catalysts, based on noble and
proved to be effective for oxidation reactions.
So we tested
this biphasic system for alcohol oxidation. The addition of
PEG dramatically promotes the reaction and gives 91% conver-
sion with 84% benzaldehyde yield. The reaction was also per-
formed using only PEG as well as acetonitrile (3-mL ACN) as
a solvent. However, the conversion was low compared to the
reaction carried out in scCO2/PEG.
To determine the optimum reaction conditions, we studied
the effect of reaction time, temperature, and oxygen concentra-
tion on conversion of benzyl alcohol. Figure 2 shows the effect
of reaction temperature on conversion of benzyl alcohol. As ex-
pected, with increasing temperature from 333 to 373 K, the con-
version increases but the selectivity of benzaldehyde decreases.
This is due to the further auto-oxidation of benzaldehyde into
benzoic acid. This was also confirmed by increasing the O2 pres-
sure from 0.5 to 2 MPa at 353 K, where remarkable decrease
from 92 to 75% in benzaldehyde selectivity was noticed. Similar
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rare metals have been reported. However, these catalysts are
expensive and some of them are deactivated easily. Recently,
supported chromium trioxide (Cr/SiO2) has been used for the
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catalytic oxidation of alcohols using scCO2. In this case,
chromic oxide is used as an oxidizing agent but the handling
of the catalyst could be a problem and also Cr-species can detach
from the support. In view of the concerns related to the toxicity
VI
of Cr compounds, Cr-based (heterogeneous) molecular sieve
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catalysts, such as CrSilicalite-1, CrAPO-5, CrBeta, CrHMS,
CrMCM-41,9 CrMCM-48, etc., can be some of the alterna-
tive catalysts. Among the various chromium catalysts mentioned
above, it was found that the mesoporous molecular sieve can
stabilize the active chromium species and thus facilitate the cat-
,10a
10b
3a
observation was also made in earlier study.
Conversion of benzyl alcohol
Selectivity of benzaldehyde
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alyst reusability/separation from the product.
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In this investigation, we studied the oxidation of alcohols by
molecular oxygen over mesoporous CrMCM-41 catalyst with
scCO2/PEG biphasic reaction system. CrMCM-41 catalyst hav-
ing 2.5 wt % chromium was prepared as described previously
with slight modification in molar gel composition. Oxidation
of alcohols were carried out in a stainless steel high pressure
reactor equipped with a Teflon insert. In a typical experiment,
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0a
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1-mmol alcohol, 0.1-g CrMCM-41, and 0.5-mL PEG-400 were
placed inside the reactor vessel. To start the reaction, the reactor
was charged with 0.5-MPa O2 and CO2 upto 16 MPa (The detail
catalyst synthesis and reaction procedure is given in Supporting
Information).11
scCO2 scCO2 / scCO2 / PEG ACN
Acetone PEG
Powder XRD patterns of catalyst CrMCM-41 shows four
well-resolved reflections, (100), (110), (200), and (210), charac-
teristics of hexagonal MCM-41 structure (Figure S1 in Support-
ing Information).11 The specific surface area, pore volume, and
pore diameter were determined from N2 adsorption isotherm
Figure 1. Oxidation of benzyl alcohol by molecular oxygen
over CrMCM-41 with various solvent system (1-mmol benzyl
alcohol; 0.1-g CrMCM-41; T ¼ 353 K; t ¼ 14 h).
Copyright Ó 2008 The Chemical Society of Japan
Dapurkar, Sudhir E.
