Wacker oxidation of 1-hexene in 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), supercritical (SC) CO2, and SC CO2/[bmim][PF6] mixed solvent

Article abstract of DOI:10.1039/b201997c

Oxidation of 1-hexene by molecular oxygen is conducted in 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF₆], supercritical (SC) CO₂, SC CO₂/[bmim][PF₆] mixed solvent, and in the absence of solvent. T

Full text of DOI:10.1039/b201997c

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Wacker oxidation of 1-hexene in 1-n-butyl-3-methylimidazolium
hexafluorophosphate ([bmim][PF ]), supercritical (SC) CO ,
6
2
and SC CO /[bmim][PF ] mixed solvent
2
6
Zhenshan Hou, Buxing Han,* Liang Gao, Tao Jiang, Zhimin Liu, Yanhong Chang,
Xiaogang Zhang and Jun He
The Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences,
Beijing 100080, P. R. China. E-mail: hanbx@infoc3.icas.ac.cn; Fax: +86-10-62569373;
Tel: +86-10-62562821
Received (in London, UK) 26th February 2002, Accepted 2nd July 2002
First published as an Advance Article on the web 19th July 2002
Oxidation of 1-hexene by molecular oxygen is conducted in 1-n-butyl-3-methylimidazolium
hexafluorophosphate [bmim][PF ], supercritical (SC) CO , SC CO /[bmim][PF ] mixed solvent, and in
6
2
2
6
the absence of solvent. The selectivity to the desired product 2-hexanone is much higher when the reaction is
carried out in the mixed solvent, and the catalysts is more stable in SC CO
in SC CO2 .
2 6
/[bmim][PF ] mixed solvent than
2
according to the procedure in the literature. The IL was dried
1
1
. Introduction
ꢀ
and degassed under vacuum at 70 C for 48 h prior to use. The
experimental apparatus and procedures were similar to those
The use of supercritical (SC) CO as a substitute for toxic and
flammable organic solvents already represents an important
2
1
5
reported in the literature. Briefly, it consisted mainly of
CO and O cylinders, a high-pressure pump, a stainless steel
1
,2
method for waste reduction in green chemistry. SC CO2
changes back to gas after reducing the pressure, which makes
it easy to recover and reuse. Recently, many catalytic reactions
in SC CO
Room temperature ionic liquids (ILs) have recently
2
2
reactor, a constant temperature oil bath, a pressure gauge, a
cold trap, and a flow meter. The procedure to conduct reaction
in CO /IL mixed solvent is described because that in other sol-
3
–5
2
have been performed with unique properties.
2
vents is similar. In each experiment, 1.5 g [bmim][PF ], 0.032 g
6
6
–10
attracted more and more attention.
They can be considered
PdCl , 0.25 g CuCl , 1 ml 1-hexene, and 2 ml methanol which
2
2
as green solvents due to their high thermal stability and negli-
gible vapour pressure under ambient conditions. Very recent
work has demonstrated that SC CO is highly soluble in some
serves as a nucleophilic reagent (the reaction can not
occur without the nucleophilic reagent) were charged into
the 18 ml reactor. The reactor was placed into the oil bath at
333.2 K. O was charged into the reactor up to 21 bar and then
2
CO was added to the desired pressure. After a suitable reac-
2
tion time, the vapour phase was released slowly passing though
2
1
1,12
ILs, while the ILs are insoluble in SC CO
CO /IL systems has been successfully used to separate SC CO
2
.
This feature of
2
2
3–16
1
soluble products from the ILs in some reaction processes.
Biphasic catalytic reaction systems for homogenous catalysis
typically consist of a lower phase solvent that is able to dis-
solve the catalyst and an upper phase solvent that carries the
reactants into the reaction phase. Some of the catalytic reac-
the cold trap containing isopropanol as absorbent. The CO2-
soluble components in the reactor were extracted by SC CO2
at 110 bar. Our experiments showed that 100 g CO2 was
required to extract all extractable substances into the cold trap.
The sample in the cold trap was analysed by GC (GC112,
Shanghai Analytical Instrument Factory). After the extraction
process the ionic liquid and the catalysts were left in the
reactor. The metal concentration (Pd, Cu) in the extracted
sample was analysed by atomic absorbance spectroscopy
(Model AAS 6).
tions in SC CO /IL biphasic systems have been investi-
2
1
4–16
gated.
Oxidation of alkenes to methyl ketones using oxygen has
been developed in synthetic organic chemistry as well as in
industrial processes. A well-known example is the Wacker
1
7
1
8,19
process using PdCl
water or organic solvents are usually used as solvents.
Recently, Wacker oxidation of olefins in SC CO has also been
2
and CuCl
2
as catalysts,
and acidic
2
2
0
reported.
BothSCCO
3. Results and discussion
2
andILscanbeusedassolventsforchemicalreac-
tions, and each of them has their own unique properties. Combi-
nation of the advantages of the two classes of solvents is a new
and interesting topic. In this work, the Wacker oxidation of
Conversion and selectivity in different solvents
The experiments showed that the main products of the reac-
tion were 2-hexanone and 3-hexanone, and 2-hexanone was
the desired product. The data of the conversion of 1-hexene
and the selectivity to 2-hexanone in different solvents and in
the absence of solvent are listed in Table 1. The selectivity is
defined as moles of the desired product divided by the total
moles of all the products. In all the solvents the conversion
approaches 100% after a reaction time of 17 h. It is very
1
mixed solvent, and in the absence of solvent is studied.
-hexene in IL [bmim][PF
6 2 2 6
], SC CO , and SC CO /[bmim][PF ]
2
. Experimental
The room temperature ionic liquid 1-n-butyl-3-methylimidazo-
lium hexafluorophosphate, [bmim][PF ], was synthesized
6
2 6
interesting that the selectivity in SC CO /[bmim][PF ] mixed
1
246
New J. Chem., 2002, 26, 1246–1248
DOI: 10.1039/b201997c
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Table 1 Conversion and selectivity of the reaction in different sol-
vents at 333.2 K with a reaction time of 17 h
was used, there were two fluid phases in the mixed solvent
system, a CO -rich phase and an IL-rich phase, and the reac-
tants, products, and the nucleophilic reagent were distributed
2
Solvents
P/bar
Conversion (%)
Selectivity (%)
between the CO
Fig. 1 illustrates that the difference in conversion at various
reaction times in SC CO and in the CO /IL mixture is not
2
-rich phase and the IL-rich phase.
SC CO
SC CO
IL
2
125
125
98.9
98.2
97.0
99.9
70.5
91.9
64.2
63.2
+ IL
2
2
2
considerable. One of the main reasons may be that the cata-
lysts are well dispersed in the IL-rich phase in the presence
of the IL, which favors enhancement of the reaction rate in
Without solvent
the CO
face exists in the CO
ing the reaction rate due to the interface mass transfer. The
two opposite factors compensate each other, and thus the con-
version is similar. The effect of reaction time on the selectivity
in the two solvents is not significant, as can be seen from the
2
/IL system. On the other hand, the liquid/vapor inter-
/IL system, which does not favor increas-
2
solvent to the desired product 2-hexanone is much higher than
those in SC CO and in the IL, or in the absence of solvent.
2
This is discussed in the following.
3
-Hexanone is generated from 2-hexene which is produced
2
2
from the isomerization of the reactant (1-hexene). The cata-
lysts PdCl and CuCl are not soluble in SC CO , and they
2
figure. However, in the CO /IL mixed solvent the selectivity
2
2
2
to the desired product is much higher than that in SC CO2 .
Moreover, the selectivity in the mixed solvent increases slightly
with reaction time, while the selectivity decreases slowly with
exist in the IL-rich phase. SC CO
2
is soluble in the IL, and
1
1,12
the IL is insoluble in SC CO2 .
1-Hexene is soluble in both
phase and the IL phase, and thus the reactant is dis-
tributed in the two phases. Considering these facts, it can be
deduced that SC CO in the reaction system has at least two
functions. One is that the SC CO -rich phase serves as a
SC CO
2
2
reaction time in SC CO .
2
2
Dependence of pressure on the conversion and selectivity
‘
‘reservoir’’ for the reactant, which supplies the reactant for
the IL-rich phase continuously and keeps lower concentration
of the reactant in the IL-rich phase that contains the catalysts.
In other words, less amount of the reactant contacts with the
catalysts, and the degree of isomerization of the reactant is
reduced. As a result, the amount of 3-hexanone produced is
It is known that the properties of supercritical fluids (SCFs)
are sensitive to pressure, and thus pressure may influence the
conversion and selectivity of the reaction. In this work, we also
explore the effect of the pressure on the conversion and the
selectivity in CO /IL mixed solvent, and the results are given
2
reduced. Second, the dissolution of CO
2
in the IL can reduce
in Fig. 2. In the pressure range studied, the conversion is very
high and is nearly independent of pressure. However, the selec-
tivity increases with pressure significantly, especially in the
low-pressure range. A very rough explanation is that the sol-
the viscosity and enhance the mass transfer of the IL, which
may also reduce the isomerization reaction. In addition, the
catalyst–reactant, solvent–reactant, and catalyst–solvent inter-
actions in CO
or IL, which may favor increasing selectivity for 2-hexanone.
2
/IL mixed are different from those in pure CO
2
2
vent power of CO increases with increasing pressure. There-
fore, less reactant exists in the IL-rich phase at higher
pressure, which favors reduction of the isomerization of the
reactant. Meanwhile, the solubility of CO in the IL increases
2
1
1,12
with pressure,
and so the diffusivity of the solvent is
Effect of reaction time on the conversion and selectivity
improved more significantly at the higher pressures, which
may also enhance the selectivity. This explanation is consistent
with the discussion above.
Fig. 1 shows the dependence of the conversion and selectivity
on the reaction time in SC CO and the SC CO /IL mixed sol-
vent at 125 bar. Our phase behavior experiments using an opti-
2
2
2
3
cal cell showed that only one fluid phase existed in the
reaction system when SC CO was used as the solvent, i.e.,
the reactants, products, and the nucleophilic reagent (metha-
nol) were dissolved in SC CO . When CO /IL mixed solvent
2
Stability of the catalysts
In order to study the stability of the catalysts in SC CO
CO /IL, we repeated the experiment in each solvent six times
2
and
2
2
2
at 125 bar with a reaction time of 17 h. The results are shown
in Fig. 3, which indicates that the catalysts are fairly stable in
both solvents. Obviously, the catalysts are more stable in SC
CO /IL mixed solvent than in CO . In other words, the mixed
2 2
solvent can not only enhance the selectivity to the desired
Fig. 2 The dependence of conversion and selectivity on reaction pres-
sure in SC CO
17 h.
Fig. 1 The dependence of conversion and selectivity on reaction time
at 333.2 K and 125 bar; IL: ionic liquid [bmim][PF ].
2
/IL mixed solvent at 333.2 K with a reaction time of
6
New J. Chem., 2002, 26, 1246–1248
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