Supported ionic liquid catalysis - A new concept for homogeneous hydroformylation catalysis

Article abstract of DOI:10.1021/ja0279242

The new concept of supported ionic liquid catalysis involves the surface of a support material that is modified with a monolayer of covalently attached ionic liquid fragments. Treatment of this surface with additional ionic liquid results in the formation of a multiple layer of free ionic liquid on the support. These layers serve as the reaction phase in which a homogeneous hydroformylation catalyst was dissolved. Supported ionic liquid catalysis combines the advantages of ionic liquid media with solid support materials which enables the application of fixed-bed technology and the usage of significantly reduced amounts of the ionic liquid. The concept of supported ionic liquid catalysis has successfully been used for hydroformylation reactions and can be further expanded into other areas of catalysis. Copyright

Full text of DOI:10.1021/ja0279242

Published on Web 10/09/2002
Supported Ionic Liquid Catalysis - A New Concept for Homogeneous
Hydroformylation Catalysis
Christian P. Mehnert,*,† Raymond A. Cook, Nicholas C. Dispenziere, and Mobae Afeworki
†
‡
†
ExxonMobil Research and Engineering Company, Corporate Strategic Research, and
ExxonMobil Chemical Company, Basic Chemicals & Intermediates Technology, 1545 Route 22 East,
Annandale, New Jersey 08801
Received July 30, 2002
Homogeneous hydroformylation catalysis is one of the largest
volume processes in the chemical industry with a worldwide oxo-
6
1
aldehyde production of 7.8 × 10 t/a (1997). Although the catalysis
is well established, catalyst separation still remains a big challenge
and continues to be the focus of intense research. Over the last
three decades, several elegant approaches have been explored to
overcome these limitations, for example, aqueous and fluorous
biphase catalysis,2 reactions in supercritical media, and catalyst
a-c
2d
immobilization onto solid supports.2
e-g
More recently, ionic liquids have attracted significant attention
3
as an alternative reaction medium for homogeneous catalysis. On
the basis of their highly charged nature, ionic liquids are well suited
for biphasic reactions with organic substrates. In this respect,
Chauvin et al.4 utilized water-soluble phosphine ligands [tri(m-
sulfonyl)triphenylphosphine trisodium salt (tppts)] to retain active
rhodium complexes in ionic liquid phases and used them success-
fully in biphasic hydroformylation reactions. Following this novel
approach, a variety of metal complexes have been recently
investigated for hydroformylation catalysis in ionic liquid media.4
Although the ability of biphasic hydroformylation catalysis in
ionic liquid media has successfully been demonstrated, the chemical
industry still prefers heterogeneous catalyst systems. Because of
the ease of separation and the possibility to use a fixed-bed reactor,
a solid catalyst is highly advantageous for the production of
aldehyde. Furthermore, the use of a biphasic reaction system
requires a large amount of ionic liquid. On the basis of economic
criteria and possible toxicological concerns, it is desirable to
minimize the amount of utilized ionic liquid in a potential process.
Inspired by the early work of Davis et al.2b on the support of
aqueous catalyst systems, we became interested in the immobiliza-
Figure 1. Supported ionic liquid catalysis used in the hydroformylation
reaction of 1-hexene to form n,i-heptanal. The ionic liquid phase [bmim]-
[PF6] containing the active catalyst HRh(CO)(tppti)3 and excess of the free
ligand (tppti) is immobilized on the surface of the modified support materials
a
6.
Scheme 1. Preparation of Surface Anchored Ionic Liquid Phases
b-h
5
tion of ionic liquid phases onto solid supports. Herein, we report
a new concept for the immobilization of homogeneous catalyst
systems and our initial findings for hydroformylation reactions.
The new concept of supported ionic liquid catalysis (silc) (Figure
1) involves the surface of a support material that is modified with
a monolayer of covalently attached ionic liquid fragments. Treat-
ment of this surface with additional ionic liquid results in the
formation of a multiple layer of free ionic liquid on the support.
These layers serve as the reaction phase in which the homogeneous
catalyst is dissolved. Although the resulting material is a solid, the
active species is dissolved in the ionic liquid phase and performs
as a homogeneous catalyst.
N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazol 1 was reacted with
1-chlorobutane to give the complex 1-butyl-3-(3-triethoxysilylpro-
pyl)-4,5-dihydroimidazolium chloride 2. The resulting compound
was further treated with either sodium tetrafluoroborate or sodium
hexafluorophosphate in acetonitrile to form the corresponding
derivatives 3 and 4. In the immobilization step, pretreated silica
gel was refluxed with a chloroform solution of complex 3 and 4 to
undergo a condensation reaction giving the modified support
materials 5 and 6, respectively. Analysis of the surface coverage
In the preparation of a surface modified silica gel with a
covalently anchored ionic liquid fragment (Scheme 1), the complex
2
*
To whom correspondence should be addressed. E-mail: christian.p.mehnert@
revealed an average of 0.4 ionic liquid fragments per nm , which
exxonmobil.com.
†
corresponds to approximately 35% of all the hydroxyl groups based
ExxonMobil Research and Engineering Co.
ExxonMobil Chemical Co.
‡
2
on a hydroxyl content of 2.4 per nm for the pretreated silica gel.
12932
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J. AM. CHEM. SOC. 2002, 124, 12932-12933
10.1021/ja0279242 CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
Table 1. Evaluation of the Hydroformylation Reaction of 1-Hexene
To Form n,i-Heptanal Using Supported Ionic Liquid Catalysis,
Biphasic Catalysis, and Homogeneous Catalysis
[HRh(CO)(tppts)
organic medium, the ionic liquid [bmim][BF
] and [HRh(CO)(tppti)
], are insoluble in the
] partially dissolved
3
3
4
in the organic phase at high aldehyde concentrations and facilitated
the leaching of the rhodium complex. In addition, high aldehyde
concentrations resulted in the depletion of the supported ionic liquid
layer and shortened the catalyst lifetime. To reduce the metal loss
and the depletion of the ionic liquid layer to an acceptable level,
the aldehyde concentration was usually kept below 50 wt % in the
reaction mixtures. In addition, the loss of rhodium complex can be
even further suppressed through an increase of the phosphine ligand
concentration.
conditiona/
ligand
time,
min
yield,
%
tof,b
min-
1
entry
solvent
n/i
1
2
3
4
5
6
7
8
silc/tppti
silc/tppts
silc/tppti
silc/no ligand
biphasic/tppti
biphasic/tppti
biphasic/tppts
homog/PPh3
[bmim][BF4]
[bmim][BF4]
[bmim][PF6]
[bmim][PF6]
[bmim][BF4]
[bmim][PF6]
H2O
300
240
270
180
230
180
360
120
33
40
46
85
58
70
11
95
2.4
2.4
2.4
0.4
2.2
2.5
65
56
60
190
23
22
23
2.6
2.4
400
toluene
Supported ionic liquid catalysis combines the advantages of ionic
liquid media with solid support materials which enables the
application of fixed-bed technology and the usage of significantly
reduced amounts of the ionic liquid. In summary, the concept of
supported ionic liquid catalysis has successfully been used for
hydroformylation reactions and can be further expanded into other
catalysis areas.
a
Reaction conditions: All runs were conducted at 100 °C with a Rh/P
ratio of 1:10, silc runs were evaluated in a 70 mL autoclave at 1500 psi,
and biphasic and homogeneous catalyst systems were evaluated in a 300
mL autoclave at 600 psi. tof defined as mol(aldehyde) per mol(rhodium)
b
per min (full reaction time).
In the next step of the catalyst preparation, an acetonitrile solution
of the precursor, dicarbonylacetylacetonate rhodium, was treated
with either the ligand tri(m-sulfonyl)triphenyl phosphine trisodium
salt (tppts) or the ligand tri(m-sulfonyl)triphenyl phosphine tris(1-
butyl-3-methyl-imidazolium) salt (tppti) (Rh/P ratio of 1:10). The
ligand tppti dissolved in both ionic media, [bmim][BF ] and [bmim]-
4
PF
BF
ionic liquid phases (25 wt % loading) and added to the corre-
sponding support materials 5 and 6. After the solvent was removed
under reduced pressure, a slightly yellow-colored, free-flowing
powder was obtained.
In our catalyst investigation, the substrate 1-hexene was reacted
with syngas (CO/H
Acknowledgment. We thank Dr. H. Freund (TGA), Dr. R. Kolb
(SAXS), and Dr. K. Qian (MS) for their help and assistance.
Supporting Information Available: Experimental details and
analysis data (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
[
[
6
], while the ligand tppts only exhibited solubility in [bmim]-
6
4
]. The resulting acetonitrile solutions were combined with the
7
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(
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). The comparison study between the supported ionic liquid catalyst
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liquid [bmim][BF
5 min , while the biphasic ionic liquid system (entry 5) showed
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] (entry 1) produced n,i-heptanal with a tof of
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-1
a tof of 400 min was recorded. Although the homogeneous system
is clearly favored due to its high activity, the supported ionic liquid
system is attractive on the basis of its convenient product separation.
Both the supported and the biphasic ionic liquid systems exhibit
similar metal leaching behavior. On the basis of the comparable
aldehyde concentration in the reactor (∼60 wt %), the ionic liquid
(6) On the basis of high-pressure 31
P NMR investigation under elevated
reaction conditions (up to 2000 psi CO/H and 150 °C), neither free nor
coordinated tppts ligand was observed in the ionic liquid [bmim][PF ].
2
6
To increase solubility of the ligand tppts, the sodium cation was exchanged
with 1-butyl-3-methyl-imidazolium chloride to form tppti.
(
7) Small-angle X-ray scattering (SAXS) investigations of supported ionic
liquid phases (loading 25 wt %) on defined silica spheres with a diameter
2
of 0.1 µm and a surface area of 30 m /g showed a layer thickness of 80
Å. Accordingly, an average ionic liquid layer thickness of 6 Å can be
[
bmim][BF
4
] and [bmim][PF
6
] exhibited a rhodium metal loss
2
approximated for silica gel with a surface area of 400 m /g.
(metal concentration in the reactor c(Rh) ) 100 µmol) of 2.10 µmol
and 0.07 µmol, respectively. Although the active rhodium species,
JA0279242
J. AM. CHEM. SOC.
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