Highly regioselective hydroformylation of higher olefins catalysed by rhodium-phosphine complexes in ionic liquid medium

Article abstract of DOI:10.3184/030823407X208247

Hydroformylation of higher olefins was performed efficiently in ionic liquids 1-n-alkyl-3-methylimidazolium p-toluenesulfonate ([Rmim][p-CH ₃C₆H₄SO₃], R = n-butyl, n-octyl, n-dodecyl, n-cetyl) with the rhodium-phosphine complex Rh-BISBIS (Rh = rhodium complex catalyst precursor; BISBIS = sodium salt of sulfonated 2,2′-bis (diphenylphosphinomethyl)-1,1′-biphenyl) as catalyst. The catalytic system offers excellent regioselectivity towards the linear aldehyde with high activity and chemoselectivity for aldehyde. Furthermore, the ionic liquid containing catalyst can be facilely separated and reused three times without a significant decrease in the activity and selectivity. The ionic liquids [Rmim][p-CH₃C₆H₄SO₃] used as the reaction media bring some definitive advantages over the halogen-containing analogues [bmim]BF₄ and [bmim]PF₆.

Full text of DOI:10.3184/030823407X208247

216 RESEARCH PAPER
APRIL, 216–220
JOURNAL OF CHEMICAL RESEARCH 2007
Highly regioselective hydroformylation of higher olefins catalysed by
rhodium-phosphine complexes in ionic liquid medium
Qi Lina,b, Haiyan Fub, Weidong Jiangb, Hua Chenb* and Xianjun Lib
aDepartment of Chemistry and Chemical Engineering, Minjiang University, Fuzhou, 350011, P. R. China
bKey Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University,
Chengdu, 610064. P. R. China
Hydroformylation of higher olefins was performed efficiently in ionic liquids 1-n-alkyl-3-methylimidazolium
p-toluenesulfonate ([Rmim][p-CH3C6H4SO3], R = n-butyl, n-octyl, n-dodecyl, n-cetyl) with the rhodium-phosphine
complex Rh-BISBIS (Rh = rhodium complex catalyst precursor; BISBIS = sodium salt of sulfonated 2,2'-bis
(diphenylphosphinomethyl)-1,1'-biphenyl) as catalyst. The catalytic system offers excellent regioselectivity towards
the linear aldehyde with high activity and chemoselectivity for aldehyde. Furthermore, the ionic liquid containing
catalyst can be facilely separated and reused three times without a significant decrease in the activity and selectivity.
The ionic liquids [Rmim][p-CH3C6H4SO3] used as the reaction media bring some definitive advantages over the
halogen-containing analogues [bmim]BF4 and [bmim]PF6.
Keywords: hydroformylation, higher olefin, ionic liquid, Rh-complex
Hydroformylation of alkenes (Scheme 1) is one of the most
used homogeneous catalysis techniques in industry, producing
more than six million tons of aldehydes annually.1-3 Aqueous-
organic biphasic hydroformylation has attracted a great deal
of attention due to the easy separation of catalyst from product
by simple decantation after reaction. Unfortunately, this
process is limited to C2–C5 olefins due to the poor solubility
of higher olefins in water. As an alternative polar medium and
“green method” for biphasic hydroformylation, ionic liquids
(ILs) have been applied in hydroformylation of higher olefins
in recent years.4-6
However, the typical L/B ratio (molar ratio of linear aldehyde
to branched aldehyde) is lower than 4. From the application
point of view, a highly regioselective catalytic system
for hydroformylation of higher olefins remains desirable.
Moreover, only in a few cases have high regioselectivities
towards the preferred linear aldehyde been obtained with
rhodium catalysts modified by diphosphine or diphosphite
with low catalytic activities7-13 (e.g. turnover frequency
(TOF): 15–58 h-1, conversion of olefin: 10.6–44.3%, reaction
time 8 h7).
as well as successfully application of ionic liquids [Rmim]
[p-CH3C6H4SO3] (Fig.1) in the asymmetric hydrogenation of
aromatic ketones17 and the hydroaminomethylation of long-
chain olefins.18 Encouraged by this progress, herein we report
our recent advances in the Rh-catalysed hydroformylation
of higher olefins using halogen-free ionic liquids [Rmim]
[p-CH3C6H4SO3] as reaction media.
Experimental
Chemicals and instruments
The rhodium complex [Rh(CO)2(acac)] (acac = acetylacetone),19,20
BISBIS,16 TPPTS [TPPTS = trisodium salt of tri(m-sulfonylphenyl)
phosphine],21[Rmim][p-CH3C6H4SO3](R=n-butyl,n-octyl,n-dodecyl,
n-cetyl),22 [bmim]PF6 (bmim = 1-n-butyl-3-methylimidazolium)23
24,25
and [bmim]BF4
were prepared as described in the literature.
1-Hexene (Acros, >97%), 1-octene (Merck, >97%), 1-decene
(Sigma, 96%) and 1-dodecene (Acros, 93–95%) were used as
received without further treatment. Hydrogen (99.99%) and carbon
monoxide (99.9%) gases were purchased and mixed directly in the
ratio 1:1. The compositions of products in olefin hydroformylation
were determined by a 6890/5973N GC-MS (HP-5MS, 0.25 mm ×
0.25 μm × 30 m). The conversion and selectivity were determined
by a GC HP1890 equipped with hydrogen flame ionisation detector
(FID) and a capillary column SE-30 (0.25 mm × 0.25 μm × 30 m).
The rhodium content in the organic phase was determined by
inductively coupled plasm-atomic emission spectrometry (ICP-AES)
on an IRIS Advantage instrument.
Recently, we described the application of the water-
soluble complex Rh-BISBIS (Rh = rhodium complex
catalyst precursor; BISBIS = sodium salt of sulfonated 2,2'-
bis(diphenyl phosphinomethyl)-1,1'-biphenyl, Fig.1) in
aqueous-organic biphasic hydroformylation of olefins,14-16
Rh-catalyst
CO + H2
R
+
+
R
R
CHO
linear aldehyde
CHO
branched aldehyde
Scheme 1 Hydroformylation of higher olefins.
-
[p-CH3C6H4SO3 ]
NaO3S
NaO3S
N
N
PPh2-n(m-C6H4SO3Na)n
CH3
R
PPh2-n(m-C6H4SO3Na)n
R=butyl, octyl, dodecyl, cetyl
Fig. 1 Ligand BISBIS and ionic liquids [Rmim][p-CH3C6H4SO3].
* Correspondent. E-mail: qlin1990@163.com
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JOURNAL OF CHEMICAL RESEARCH 2007 217
Rh-catalysed hydroformylation of higher olefins in IL
Table
1 Effect of the BISBIS/Rh ratio on 1-octene
The hydroformylation of higher olefins was carried out in a 60 ml
stainless steel autoclave with a magnetic stirrer. The desired amounts
of catalyst precursor [Rh(CO)2(acac)], ligand, ionic liquid and olefin
were introduced successively into the autoclave, which was purged
with syngas (CO/H2) several times. The reaction was carried out
under the designed conditions for a desired time. After the reaction,
the autoclave was quickly cooled to ambient temperature. The
organic phase was separated by simple decantation and analysed
by gas chromatography. The ionic liquid layer containing rhodium
catalyst was reused in the successive runs.
hydroformylation
Entry P/Rh
Conv./%a S(ald.)/%b
L/Bc
TOF/h-1d
1
4
6
28.4
64.7
68.8
89.6
79.3
95.6
90.8
85.7
42.9
62.1
81.8
92.2
85.0
84.8
85.4
76.7
7.3
9.3
15.1
15.5
16.6
2.1
156
513
2
3
10
12
14
12
12
12
719
4
1055
861
1035
990
940
5
6e
7f
8h
15.3
17.9
Results and discussion
Reaction conditions: [Rh] = 5.0 mmol/l, 1-octene: 12.8 mmol,
[bmim][p-CH3C6H4SO3]: 1 ml, initial pressure: 3.0MPa, 120°C, 2
h, stirring speed 1400 rpm.
In order to gain a first insight into the influences of reaction
parameters, the hydroformylation of 1-octene catalysed by
water-soluble rhodium-phosphine complex Rh-BISBIS in
[bmim][p-CH3C6H4SO3] was used as a model reaction under
different reaction conditions.
aConversion: percent of converted 1-octene; bS (ald.): selectivity
of aldehydes; cL/B: molecular ratio of linear aldehyde to
branched aldehyde; dTOF: mole (aldehyde) per mol (rhodium)
per hour.; ethe ligand BISBIS was replaced by TPPTS; fcatalyst
was replaced by RhCl2(BISBIS)2; hcatalyst was replaced by
[Rh(COD)Cl]2.
Effect of the molar ratio of phosphine/rhodium (P/Rh)
The P/Rh ratio is one of the important factors to affect the
activity and regioselectivity of the hydroformylation.14
The influence of the P/Rh ratio was investigated on the
1-octene hydroformylation, and the results were summarised
in Table 1.
As shown in Table 1, the activity and chemoselectivity for
aldehyde increased with P/Rh ratio from 4 to 12. But further
increase of the P/Rh ratio was unfavourable for the reaction.
The optimum activity and chemoselectivity for aldehyde
were obtained at P/Rh = 12. The regioselectivity towards the
linear aldehyde increased with increasing the P/Rh ratio, and
a sudden jump in L/B ratio appeared at P/Rh = 10 (entry 3
in Table 1). The same trend was also observed in aqueous-
organic biphasic systems.14
Similar to the mechanism of aqueous-organic biphasic
hydroformylation, the mechanism of ionic liquid biphasic
hydroformylation studied by Leeuwen26 confirmed the
presence of ee configuration and ea configuration of the
active species [RhH(CO)2(diphosphine)] complexes, both of
them are responsible for the production of aldehydes,27 and
there is an ee–ea equilibrium between ee configuration and ea
configuration during the reaction (Scheme 2).
According to the above mechanism, there is a ligand
exchange between acetylacetone in [Rh(CO)2(acac)] and
BISBIS under reaction conditions before the hydroformylation
reaction (Eq.1), and the mechanism also followed the generally
accepted mechanism proposed by Wilkinson.28
The concentration of [RhH(CO)2(BISBIS)] increased
with the increase of P/Rh ratio, leading to high activity
and regioselectivity towards the linear aldehyde. Since
most hydroformylation catalysts exhibited a first order
dependency in the concentration of this rhodium-hydride
species,29 when the P/Rh ratio was 12, the rhodium catalyst
was completely converted into water-soluble active species
[RhH(CO)2(BISBIS)], and therefore the highest reaction
activity was observed. Further increasing the P/Rh ratio
would be disadvantageous for the dissociation of BISBIS
from the centre metal in catalytic recycle, which resulted in
lower activity.
The data (entries 4, 7 and 8 in Table 1) illustrated that the
nature of the rhodium catalyst precursor did not influence
reaction rates and regioselectivities towards the linear
aldehyde. Compared with the Rh-BISBIS complex, the Rh-
TPPTS complex exhibited lower regioselectivity towards the
linear aldehyde in the same conditions (entry 6 in Table 1).
It further suggests that the ligand play an important role in
the regioselectivity-determining step and the diphosphine
BISBIS is advantageous for highly selective catalysts, which
is due to their natural large P–Rh–P bite angle.30 According
30
to the results reported by Casey et al. and van Leeuwen
et al.,27,31,32 it is possible that the rhodium active species still
keep the highly selective structure with ee configuration in
the case of BISBIS, while the less selective structure with ea
configuration could be the main active species in the case of
TPPTS (as shown in Scheme 3).
H2/CO
(1)
[Rh(CO)2(acac)] + BISBIS
[RH(CO)2(BISBIS)]
Effect of the syngas pressure
When the P/Rh ratio was low (entries 1–2 in Table 1),
the concentration of BISBIS in the ionic liquid was too
low to convert entirely [Rh(CO)2(acac)] into the active
species [RhH(CO)2(BISBIS)] complexes.26 The catalyst
precursor [Rh(CO)2(acac)] was still the main active species
and responsible for the low activity and regioselectivity.
The effect of total initial pressure on the hydroformylation
of 1-octene is listed in Table 2. The results indicate that the
pressure strongly influenced the regioselectivity towards
the linear aldehyde. The regioselectivity towards the linear
aldehyde gradually decreased with increasing pressure.
Higher pressure caused the increase of syngas concentration
H
H
H
OC
P
P
P
P
BISBIS
P
P
Rh
P
CO
Rh
CO
ee
CO
Rh
CO
A
CO
P
ea
P
P
: BISBIS
Scheme 2 The ee–ea equilibrium of [RhH (CO)2(diphosphine)] active species.
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218 JOURNAL OF CHEMICAL RESEARCH 2007
As shown in Table 3, the L/B ratio increased from 20.9 to
28.6 while the reaction temperature was raised from 110 to
120°C. The L/B ratio decreased from 28.6 to 15.9 while the
reaction temperature elevated from 120 to 130°C, which is due
to the increase of ea species. Furthermore, according to the
literature16, at 130°C, [RhH (CO)2(BISBIS)] would become
unstable towards reversible dissociation of a phosphorus atom
in BISBIS and produce a monodenate trigonal bipyramidal
complex A (Scheme 2). Both the formation of less selective
species ee and complex A were responsible for the decrease in
the regioselectivity.
H
H
OC
Ar3P
Ar3P
Rh
CO
Rh CO
Ar3P
PAr3
CO
ee
ea
PAr3: TPPTS
Scheme 3 The ee–ea equilibrium of [RhH (CO)2(TPPTS)2]
active species.
Similarly, the regioselectivity also depended strongly on
the stirring speed. As shown in Table 3 (entries 2, 4 and 5),
the lower stirring speed was favourable for forming linear
aldehyde, because low stirring speed was favourable for the
formation of the active species ee. Meanwhile, the results
listed in Table 3 also indicate that the reaction suffered from
mass transfer limitations as the activity increased when the
reaction mixture was vigorously stirred.
Table 2 Effect of pressure on 1-octene hydroformylation
Pressure/
MPa
Conversion
S(aldehyde)
/%
L/B
/%
TOF/h-1
1.5
2.0
2.5
3.0
90.8
89.8
85.6
89.6
85.6
86.9
87.3
92.2
28.6
25.4
23.6
15.5
992
996
954
1055
Effect of different ionic liquid
Reaction conditions are the same as in Table 1 except BISBIS/
Rh = 6.
Several kinds of ionic liquids with different cations or anions
were used as solvents in the hydroformylation using Rh-
BISBIS as catalyst. The results summarised in Table 4 indicate
the anion and cation of ionic liquid played important roles in
the Rh-catalysed hydroformylation. The n-octyl-substituted
imidazolium ionic liquid showed the better activity. The ionic
liquids [Rmim][p-CH3C6H4SO3] with shorter or longer chain-
length alkyl were disadvantageous for reaction rate. This
suggests that there is a relationship between the lengths of the
olefin chain and alkyl substituents in the ionic liquid. However,
the regioselectivity towards the linear aldehyde increased
when the chain-length of the alkyl substituent increased. The
role of ionic liquids in the regioselectivity towards the linear
aldehyde remains to be further clarified. But this phenomenon
may be attributed to the influence of chain-length alkyl on
the solubilities of water-soluble catalyst and 1-octene in ionic
liquid media.33
For comparison, the halogen-containing analogue
[bmim][BF4] and [bmim][PF6] were used as media under
the same conditions. The experimental results showed lower
activity and regioselectivity towards the linear aldehyde while
the hydrogenation and isomerisation of olefin became the
main reactions. The results could be attributed to the lower
solubility of water-soluble rhodium-phosphine complexes
and the relative ratio of syngas/phosphine concentration in
the ionic liquid. This variation could create a more unsuitable
reaction microcricumstance producing linear aldehyde with a
smaller L/B ratio. On the other hand, according to reported26
results, there is an ee–ea equilibrium of active species in the
catalytic system (Scheme 2), and the equilibrium between
two complexes is strongly dependent on the syngas pressure;
complex ee can readily be converted into complex ea at high
syngas pressure. The complex ea is disadvantageous for
producing linear aldehyde, which is in accordance with the
observation at lower L/B ratio.
Effect of reaction temperature and stirring speed
The effect of temperature on the 1-octene hydroformylation
follows two ways. Firstly, high temperature is favourable for the
formation of the stable chelated species [RhH (CO)2(BISBIS)],
leading to higher reaction activity and regioselectivity.
Secondly, high temperature can shift the equilibrium of ee
and ea species to ea (Scheme 2), which is favourable for the
formation of branched aldehyde. It results in the decrease of the
regioselectivity towards the linear aldehyde.
Table 3 Effect of the reaction temperature on 1-octene hydro-formylation
Entry
Temp/°C
Stirring speed/rpm/)
Conv./%
S(ald.)/%
L/B
TOF/h-1
1
2
3
4
5
110
120
130
120
120
1400
1400
1400
1000
600
66.2
90.8
97.4
88.5
81.9
84.0
85.6
81.3
81.5
83.6
20.9
28.6
15.9
31.1
34.4
710
992
1011
921
874
Reaction conditions are the same as in Table 1 except BISBIS/Rh = 6, 1.5 MPa.
Table 4 Effect of reaction medium on 1-octene hydroformylation
Reaction medium
Conversion/%
S(aldehyde)/%
L/B
TOF/h-1
[bmim][p-CH3C6H4SO3]a
[omim][p-CH3C6H4SO3]b
[dmim][p-CH3C6H4SO3]c
[cmim][p-CH3C6H4SO3]d
[bmim]BF4
81.9
84.2
81.6
75.0
82.0
31.8
83.6
84.6
85.0
82.8
16.5
20.2
34.4
35.5
43.1
43.5
8.2
874
909
886
793
61
[bmim]PF6
1.5
29
Reaction conditions: [Rh] = 0.5 mmol/l, 1-octene: 12.8 mmol, BISBIS/Rh = 6, 1.5 MPa, 120°C, 2 h, 600 rpm. a[bmim]: 1-n-butyl-
c
3-methylimidazolium; b[omim]: 1-n-octyl-3-methylimidazolium; [dmim]: 1-n-dodecyl-3-methylimidazolium; d[cmim]: 1-n-cetyl–3-
methylimidazolium.
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JOURNAL OF CHEMICAL RESEARCH 2007 219
Table 5 The hydroformylation of higher olefins in [bmim][p-CH3C6H4SO3]
Entry
Substrate
Conversion/%
S(aldehde)/%
L/B
TOF/h1
Rh leaching/wt%a
1
2
3
4
5
6
7
8
1-Hexene
94.5
81.9
79.6
47.2
59.1
48.2
29.7
39.8
75.7
83.6
85.9
82.3
75.9
69.7
57.9
75.2
40.4
34.4
33.9
22.1
18.0
18.2
9.5
1155
874
724
351
405
303
155
270
–
–
1-Octene
1-Decene
–
1-Deceneb
Recycling 2
Recycling 3
Recycling 4
Recycling 5b
0.063
0.075
0.062
0.047
-
19.9
Reaction conditions: olefin 2 ml, the others are the same as in Table 4. aPercentage of leached rhodium of initial intake; b0.01 mmol
of additional ligand was added.
Table 6 1-Octene hydroformylation reaction using different catalytic systems
Ionic liquid
Ligand backbone
Conv./%
TOF/h-1
L/B
Ref.
Xanthene
10.6–44.3
15–58
240
552
810
32
18.0–21.3
12.6
1.1
7
8
Phenol
96
–
–
77
30
63
2-Imidazolium
Cobaltocenium
Xantphos
Xantphos
Xantphos
9
[bmim]PF6
16.2
13.1
44
10
11
12
26
65
382
7
[bmim][p-CH3C6H4SO3]
Biphenyl
81.9
874
34.4
This work
in [bmim]BF4 or [bmim]PF6. The phenomenon is in good
agreement with the results reported in the literature.34
[Rmim][p-CH3C6H4SO3] (R = n-butyl, n-octyl, n-dodecyl,
n-cetyl). The catalytic system offers good activity and high
regioselectivity towards the linear aldehyde with a good
retention of the catalyst in the ionic liquid phase, which is
economical and environmentally friendly. The activity and
regioselectivity towards linear aldehyde catalysed in ionic
liquid [Rmim][p-CH3C6H4SO3] are higher than those in the
halogen-containing analogues [bmim]BF4 and [bmim]PF6.
The easy separation and recycling of the catalyst make the
reaction system promising and attractive.
Effect of chain-length of olefin
The hydroformylation of other olefins catalysed by water-
soluble complex Rh-BISBIS in [bmim][p-CH3C6H4SO3]
was investigated under the optimum reaction conditions.
The results are summarised in Table 5.
Upon increasing the chain-length of olefin from 1-hexene
to 1-dodecene, the reaction rates and L/B ratio decreased from
1155 h-1 and 40.4 to 350 h-1 and 22.1, respectively. This further
confirms that the activities depend strongly on the solubility
of olefin. In order to investigate the possibility of catalyst
recycling, the organic products in reactions were carefully
separated by decantation without special precautions, and
the ionic liquid-containing complex Rh-BISBIS was reused
directly in the next run after adding fresh olefin (entries 4–8 in
Table 5). In successive catalytic reaction cycles carried out in
identical reaction conditions, relatively stable reaction rate and
regioselectivity were observed after three recycles. However,
a significant decrease in the reaction rate and regioselectivity
was observed in the fourth cycle (entry 7 in Table 5).
However, the result showed the rhodium catalyst was nearly
completely retained in the ionic liquid phase (<0.08%
rhodium leaching detected by ICP-AES). It indicated the
decrease of activity and regioselectivity in recycles was
mainly due to the partial oxidation of ligand BISBIS during
the separation of catalyst. A significant recovery in activity
and regioselectivity was observed (entry 8 in Table 5) when
0.01 mmol of additional ligand was added in the fifth cycle. It
further confirmed the lower activity and regioselectivity result
from the phosphine oxidation rather than rhodium leaching
into organic phase.
We thank the Young Creative Foundation of Fujian Province
of China (2006F3099) and the Science Research Foundation
of Education Office of Fujian Province of China (JA06054)
for the financial support.
Received 1 March 2007; accepted 4 April 2007
Paper 07/4494 doi: 10.3184/030823407X208247
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PAPER: 07/4494