Rhodium-containing hypercross-linked polystyrene as a heterogeneous catalyst for the hydroformylation of olefins in supercritical carbon dioxide

Article abstract of DOI:10.1016/j.tetlet.2012.12.063

A simple procedure for the incorporation of rhodium nanoparticles into a hypercross-linked polystyrene matrix is developed. The prepared heterogeneous catalyst shows high activity in the hydroformylation of olefins in supercritical carbon dioxide, and can be recycled six times without any noticeable decrease in productivity.

Full text of DOI:10.1016/j.tetlet.2012.12.063

Tetrahedron Letters 54 (2013) 1116–1119
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Rhodium-containing hypercross-linked polystyrene as a heterogeneous
catalyst for the hydroformylation of olefins in supercritical carbon dioxide
⇑
Sergey E. Lyubimov , Eugenie A. Rastorguev, Kseniya I. Lubentsova, Alexander A. Korlyukov,
Vadim A. Davankov
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str. 28, 119991 Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple procedure for the incorporation of rhodium nanoparticles into a hypercross-linked polystyrene
matrix is developed. The prepared heterogeneous catalyst shows high activity in the hydroformylation of
olefins in supercritical carbon dioxide, and can be recycled six times without any noticeable decrease in
productivity.
Received 30 October 2012
Revised 23 November 2012
Accepted 13 December 2012
Available online 26 December 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Hypercross-linked polystyrene
Nanoparticles
Heterogeneous catalyst
Hydroformylation
Rhodium
One of the widely studied catalyzed chemical processes, the
hydroformylation reaction, provides a versatile route for the syn-
thesis of industrial solvents, biodegradable detergents, surfactants,
lubricants, and plasticizers.1–3 All current commercial hydrofor-
mylation processes are based on homogeneous catalysts, mostly
using rhodium complexes, which have supplanted cobalt deriva-
tives due to their higher activity.4–6 These processes also use phos-
phine or phosphite ligands to increase the regioselectivity toward
the desired linear or branched aldehyde.1 Homogeneous catalysts,
however, suffer from the problems of separating the catalyst from
the reaction mixture and catalyst recycling. The development of a
successful heterogeneous catalyst would allow a much simpler
commercial process to be implemented. Binding the catalysts to
organic or inorganic solid supports is a widely used procedure
for heterogenising homogeneous catalysts. The attachment of
homogeneous catalysts to dendritic, polymeric organic, inorganic
or hybrid supports are such examples.7–12 Although such catalysts
can be recycled and easily separated from the reaction mixtures,
they are usually significantly less active and selective than their
homogeneous analogues. Recently, it was found that metal nano-
particles immobilized on solid or liquid supports can, in some
cases, give rise to novel and unique catalytic behavior. In particu-
lar, supported metal nanoparticles have been utilized extensively
as catalysts for hydrogenations, hydrosilylations, hydrogenolysis,
oxidation, McMurry, Suzuki and Heck-type couplings and other
synthetic organic reactions.13,14 It should be noted, that the cata-
lytic activity depends on the metal surface area available to the
reacting molecules.13 One advanced polymeric support, which con-
tains high specific surface areas (typically in the range of 1000 m2/
g), is hypercross-linked polystyrene (HPS).15 Moreover, HPS poly-
mers can swell in a wide range of solvents, even thermodynami-
cally poor examples.16 In the presence of Pt, Pd or Ru
nanoparticles formed in a HPS matrix, lactose is oxidized selec-
tively with O2 to lactobionic acid,
-sorbose to 2-keto- -gulonic acid, an intermediate in the produc-
tion of vitamin C.17–20 Industrial MN-270 HPS copolymer impreg-
nated with Pt nanoparticles was recently tested as
D-glucose to gluconic acid and
L
L
a
heterogeneous catalyst for the oxidation of phenol dissolved in
water.21 Finally, it should be mentioned that styrene–divinylben-
zene HPS copolymer containing Pd nanoparticles has shown high
activity in the coupling of aryl bromides and chlorides with phen-
ylboronic acid.22
Another paradigm in organic synthesis is the elimination of
flammable and toxic organic solvents and their replacement with
benign solvents. Studies have shown that many commercially
important reactions that use organic solvents may also be per-
formed in a supercritical fluid.23 One of the most promising of
these fluids is supercritical carbon dioxide (scCO2).24 It is cheap,
not flammable and eco-friendly. Among the many advantages of
scCO2 is the high solubility of reagent gases. Permanent gases such
as H2 and CO are completely miscible in all proportions with
scCO2.25 ScCO2 can also be advantageous for heterogeneous catal-
ysis because of the low solubility of solid species often used as
⇑
Corresponding author. Tel.: +7 499 1352548; fax: +7 499 1355085.
E-mail address: lssp452@mail.ru (S.E. Lyubimov).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.12.063

S. E. Lyubimov et al. / Tetrahedron Letters 54 (2013) 1116–1119
1117
recyclable catalysts. On the other hand, most of the solid catalysts
that showed promising hydroformylation results in scCO2 were ob-
tained by the immobilization of rhodium complexes onto a heter-
ogeneous support, or by modification of a solid support containing
anchored phosphine ligands by suitable rhodium precursors, such
as [Rh(acac)(CO)2] or [Rh(COD)Cl]2.26–29 Although, the catalytic
hydroformylation of alkenes with Rh nanoparticle catalysts is
known, the use of such catalysts required activation with phospho-
rus ligands.30–33 Moreover, the catalysts were tested in organic sol-
vents or under solventless hydroformylation conditions. To the
best of our knowledge, only rhodium nanoparticles supported on
activated carbon were tested in hydroformylation reactions in
scCO2 under ligandless conditions, but showed only very moderate
selectivity.34
In the present work, we have developed a simple procedure for
Rh nanoparticle formation in a HPS matrix. The phosphine-free
rhodium heterogeneous catalyst could be reused several times in
hydroformylation reactions in scCO2.
The Rh(0) nanoparticles in Purolite MacronetÒ MN100 resin
were obtained by the adsorption of an ethanolic solution of
RhCl3Á4H2O followed by reduction with excess NaBH4 (see the
experimental section). It should be noted that the resin represents
a biporous hypercross-linked polystyrene/divinylbenzene copoly-
mer bearing non-coordinating tertiary amine moieties and which
possesses a high surface area (900 m2/g). According to X-ray fluo-
rescence analysis, the average rhodium content in the resin was
found to be 4.20%. The powder X-ray diffraction measurement
Figure 2. TEM image of the milled catalyst.
O
H
R
O
H2/CO (1/1, 25 atm), scCO2
Rh/HPS
+
R
R
H
3
1
2
showed a diffraction peak at 2H of 41.21° that corresponded to
the face-centered-cubic crystalline structure of the Rh nanoparti-
cles (Fig. 1).35 Calculation of the diffraction peak according to the
Scherrer equation showed that the average size of the Rh nanopar-
ticles was 3.0 nm.
Scheme 1. Hydroformylation of olefins by Rh/HPS.
same regioselectivity was obtained, but the conversion increased
slightly (Table 1, entry 2). The use of a higher reaction temperature
(60°C) increased significantly the conversion (Table 1, entry 3). To
test the reusability of the catalyst, a series of seven consecutive
runs was carried out at 65°C. Very high conversions were obtained
in the six catalytic cycles (Table 1, entries 4–9), but a significant de-
crease in the catalytic activity was observed on the seventh consec-
utive run (Table 1, entry 10). X-ray fluorescence analysis of the
seven-time recycled resin showed that the rhodium content had
decreased from 4.20% to 2.86% due to leaching of the rhodium me-
tal. It should be noted that the use of toluene as the solvent gave
incomplete conversion of styrene and moderate regioselectivity,
compared to scCO2 (Table 1, entries 4 and 11).
Using the optimized conditions (65°C, 250 atm total pressure)
we studied the hydroformylations of 4-Me-, 4-Br-, and 4-i-Pr-sty-
rene. Complete conversions in 180 min and high (89–93%) regiose-
lectivities for branched aldehydes were observed in all the cases
(Table 1, entries 12–14). We also investigated the hydroformyla-
tion of hept-1-ene and oct-1-ene in scCO2 (Table 1, entries 15
and 16). Despite complete conversion of the substrates at 60°C,
low regoselectivities were obtained, compared to styrenes. The
hydroformylation of 3,3-dimethylbut-1-ene containing a bulky
and electron-donating t-butyl group led to a moderate conversion
(Table 1, entry 17). It should be noted that the analysis of the 1H
NMR spectrum of the reaction mixture revealed the formation of
only linear product 3 (Scheme 1), and no branched product 2
was formed. This may be due to the high steric bulk of the t-butyl
unit and also the higher stability of the linear carbanion intermedi-
ate to that formed during the hydroformylation catalytic cycle.
Trying to improve the reaction rate, the temperature was increased
from 60 to 70°C. In this case, full conversion of 3,3-dimethylbut-1-
ene into the linear isomer 3 was observed (Table 1, entry 18).
The catalyst was also used for the hydroformylation of (R)-
limonene 4 (Scheme 2). The reaction at 65°C and 250 atm total
The size and morphology of the Rh(0) nanoparticles were also
investigated by TEM analysis of the milled MN100 resin (Fig. 2),
the results of which were similar (particle size = 2–4 nm) to those
obtained from the XRD measurements. It should be noted that the
rhodium nanoparticles were unevenly distributed throughout the
polymer matrix.
The catalytic activity of the polymer-supported Rh nanoparti-
cles was initially investigated in the hydroformylation of styrene
(1) at 50°C under 25 atm Syngas pressure (Scheme 1, R = Ph).
The reaction proceeded with high regioselectivity to give the
branched aldehyde 2, but only moderate conversion was observed
after 180 min at 200 atm total pressure (Table 1, entry 1). To eval-
uate the effect of CO2 pressure on the regioselectivity and conver-
sion, we studied the hydroformylation process at higher CO2
pressure (250 atm), but at the same temperature. In this case, the
Figure 1. XRD patterns of the catalyst.

1118
S. E. Lyubimov et al. / Tetrahedron Letters 54 (2013) 1116–1119
Table 1
Hydroformylation of olefins using Rh/HPS (180 min)
Entry
Medium
R
Cycle
P Totala (atm)
T (°C)
Conversion (%)
Selectivity 2/3
1
2
3
4
5
6
7
8
scCO2
scCO2
scCO2
scCO2
scCO2
scCO2
scCO2
scCO2
scCO2
scCO2
Toluene
scCO2
scCO2
scCO2
scCO2
scCO2
scCO2
scCO2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
—
—
—
1
2
3
4
5
6
7
—
—
—
—
—
—
—
—
200
250
250
250
250
250
250
250
250
250
—
250
250
250
250
250
250
250
50
50
60
65
65
65
65
65
65
65
65
65
65
65
60
60
60
70
50
67
97
100
100
100
100
100
98
94/6
93/7
91/9
91/9
91/9
91/9
91/9
91/9
91/9
92/8
70/30
91/9
93/7
89/11
40/60
39/61
0/100
0/100
9
Ph
Ph
Ph
10
11
12
13
14
15
16
17
18
49
72
4-MeC6H4
4-BrC6H4
4-iPrC6H4
(CH2)4CH3
(CH2)5CH3
t-Bu
100
100
100
100
100
41
t-Bu
100
a
Syngas partial pressure = 25 atm.
+
H2/CO (1/1, 25 atm), scCO2
Rh/HPS
H
O
O
H
5
6
4
Scheme 2. Hydroformylation of (R)-limonene by Rh/HPS.
pressure provided only the linear product 5 in a very good conver-
sion in 300 min (Table 2, entry 1). Increasing the reaction temper-
ature from 65 to 75°C resulted in complete conversion (Table 2,
entry 2). It should be noted that at the higher temperature, only
the terminal double bond of 4 was hydroformylated and no trace
of the corresponding branched product 6 could be detected.
In conclusion, a simple procedure for the incorporation of Rh
nanoparticles into a hypercross-linked polystyrene matrix was
developed. The prepared catalyst showed high activity in the
hydroformylation of several olefins in scCO2. The rate of aldehyde
formation was improved by increasing the total pressure and the
reaction temperature. The catalyst could be reused six times with-
out any noticeable decrease in the productivity and constant selec-
tivity was obtained.
50 ml flask. The resin was allowed to swell for 1.5 h in the mixture.
Subsequently, two portions of NaBH4 (200 mg each) were added to
the swelled resin. The resulting mixture was heated for 10 min at
50°C. The obtained beads were washed with hot (80°C) water
(150 ml), MeOH (100 ml), and dried in vacuo (1 mm Hg) with heat-
ing (90°C, 1 h).
Hydroformylation in scCO2 (typical procedure)
Hydroformylation experiments were carried out in a 10 cm3
high-pressure stainless steel reactor equipped with a magnetic
stirrer. The reactor was charged with the unsaturated substrate
(1.5 mmol) and the catalyst (71 mg, 2% mol Rh with respect to
the substrate). The reactor was charged with Syngas (25 atm, CO/
H2 = 1/1) followed by the introduction of CO2 to the desired pres-
sure. Next, the reactor was heated to the desired temperature
(10 min). After the appropriate reaction time, the reactor was
cooled to 5°C and slowly depressurized. The liquid reaction mix-
ture was separated from the solid catalyst using a Pasteur pipette
and analyzed by 1H NMR-spectroscopy.
Experimental section
Preparation of Rh-nanoparticles in HPS matrix
Industrial MacronetÒ MN100 resin (6 g) was washed with 10%
NaHCO3 (200 mL), H2O (50 mL), MeOH (10 ml), and dried in vacuo
(1 Torr) with heating (80°C) for 20 min. RhCl3Á4H2O (0.76 g) was
dissolved in EtOH (4 ml) and added to the MN100 resin (6 g) in a
Reuse of the polymer-supported Rh resin
The reaction mixture was diluted with Et2O (1 mL) and the resin
was separated by filtration, washed several times with Et2O, and
dried in vacuo. The resin was reused in subsequent runs.
Table 2
Hydroformylation of (R)-limonene by Rh/HPS in scCO2 (300 min)
Acknowledgment
Entry
P Totala (atm)
T (°C)
Conversion (%)
Selectivity 5/6
Financial support from the Russian Foundation for Basic Re-
search (Grant No 11-03-12010-ofi-m-2011) is gratefully
acknowledged.
1
2
250
250
65
75
72
100
100/0
100/0
a
Syngas partial pressure = 25 atm.

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