Carbon dioxide induced phase switching for homogeneous-catalyst recycling

Article abstract of DOI:10.1002/anie.200804729

SwitchPhos: Rhodium complexes formed from PPh₃ ligands functionalized with weakly basic amidine groups are highly active catalysts for the hydroformylation of alkenes. On bubbling with CO₂ in the presence of water, the yellow rhodium complexes move into the water phase, whereas bubbling with N₂ at 60 °C causes them to switch back into the organic phase. The catalysts can be used for reactions in water or an organic phase.

Full text of DOI:10.1002/anie.200804729

Communications
DOI: 10.1002/anie.200804729
Homogeneous Catalysis
Carbon Dioxide Induced Phase Switching for Homogeneous-Catalyst
Recycling**
Simon. L. Desset and David J. Cole-Hamilton*
Homogeneous catalysts typically display high activity, high
selectivity, and can be tailored through ligand modification.
However, because they are dissolved in the reaction medium,
their separation from the products and subsequent reuse can
be complicated, hindering their widespread use for commer-
cial applications. Herein we report a new approach to solve
the product–catalyst separation problem involving carrying
out the reaction in a single phase and switching the catalyst
into a separate phase by bubbling with or removal of CO2
(Scheme 1).
Although the introduction of a second phase ensures
efficient catalyst separation and recycling, it induces mass-
transfer limitations that reduce the reaction rate. For higher
alkenes (> C5), which display poor solubility in water, mass-
transport limitations render the reaction rate too low to be
economically viable, greatly limiting the scope of this elegant
technology.
An interesting approach for a general solution to this
limitation is to carry out the reaction in a single phase and to
transfer the catalyst into the second phase once the reaction is
complete. Heat has been used to trigger such phase transi-
tions.[4] At the reaction temperature (ꢀ 708C) a rhodium
complex catalyst bearing polyoxoethylene moieties is soluble
in the organic phase, whereas upon cooling hydrogen bonds
form between the polyether chains and water rendering the
catalyst soluble in water. The phenomenon is reversible and
this strategy has been used for the hydroformylation of higher
alkenes, allowing the catalyst to be recycled eight times
without noticeable loss in activity.
Variation of pH value has also been used to alter catalyst
solubility.[5] By incorporating amine groups onto the ligand,
the catalyst can be extracted from an organic phase into an
acidic aqueous solution. Deprotonation of the ammonium
group by addition of base allows re-extraction into a fresh
organic phase for catalyst reuse. This approach, however,
results in the formation of significant quantities of salt by-
products. CO2 has also been described for such recycling,
when using P{(CH2)3NEt2}3, with the CO2 being removed by
boiling to reverse the extraction. Rhodium leaching in the
organic phase was found to be < 1 ppm.[6,7] Herein we report
catalysts for which the solubility in organic and aqueous phase
can be reversibly switched upon addition or removal of
carbon dioxide (1 bar).
Jessop and co-workers have described CO2 phase-switch-
able surfactants.[8] They demonstrated that hydrophobic long-
chain alkyl amidines could be protonated and become water-
soluble surfactants upon addition of CO2. The amidinium ion
could then be deprotonated and returned to the organic phase
by removing the CO2 using N2, a process similar to the catalyst
recycling shown in Scheme 1 but with the phenyl phosphine
being replaced by an alkyl chain. This approach allowed the
reversible formation and breaking of water–oil emulsions
stabilized by the amidine-based surfactants. We reasoned that
by introducing amidine moieties onto phosphorus ligands we
could make catalysts for which the solubility could be
switched using CO2 as a trigger.
Scheme 1. Hydroformylation of alkenes (R=C6H13 (1-octene) or
CH2OH (allyl alcohol)) using a catalyst that can be switched between
an organic phase and water by bubbling with CO2 or N2.
Several methods have been developed, aimed at efficient
ways of recycling homogeneous catalysts.[1,2] An aqueous
biphasic system is currently used in industry for the hydro-
formylation of propene and butene.[3] In this process, the
catalyst is dissolved in an aqueous phase by the use of
sulfonated ligands, whereas the starting material and products
form a separate phase. This method allows fast and efficient
separation of the product from the catalyst by simple
decantation. The aqueous phase containing the catalyst can
then be reused for further operation.
[*] S. L. Desset, Prof. D. J. Cole-Hamilton
EaStCHEM, School of Chemistry
University of St. Andrews
St. Andrews, Fife, KY16 9ST (Scotland)
Fax: (+44)1334-463-808
E-mail: djc@st-and.ac.uk
Homepage: http://ch-www.st-and.ac.uk/staff/djc/
[**] We thank Sasol Technology (UK) Ltd. for financial support (S.L.D.),
Sylvia Williamson, Dr. David Henderson, and Dr. Lorna Eades for
ICP measurements, and Dr. Douglas Foster for helpful discussions.
We prepared and characterized ligand 1 based on PPh3
bearing three amidine groups and tested it for the hydro-
formylation of 1-octene (Scheme 1, R = C6H13). The catalyst
was prepared in situ by heating 1 together with [Rh-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804729.
1472
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 1472 –1474

Angewandte
Chemie
(acac)(CO)2] (1/Rh = 50, acac = 2,4-pentanedione; acetylace-
[
]
*
tone)) in toluene under a syngas (CO/H2) atmosphere
yielding a bright yellow solution. Coordination of the ligand
to the rhodium center was confirmed by 31P{1H}-NMR
spectroscopy. This catalytic solution was used for the homo-
geneous hydroformylation of 1-octene under a constant
pressure of syngas (20 bar) at 1008C. The progress of the
reaction was monitored at constant pressure by recording the
pressure drop from a ballast vessel from which gas was
continuously fed to the reactor (Figure 1a, cycle 1a). Another
Figure 2. Photographs obtained during the catalyst recycling process.
a) the crude product from cycle n (number on left of row) with added
water; b) after 10 min of stirring and CO2 bubbling; c) after 1.5 h of
stirring and CO2 bubbling, separation of the organic phase and
addition of fresh toluene; d) after 30 min of stirring at 608C and N2
bubbling.
gently through the solution, the upper organic phase had
completely decolorized while the aqueous phase turned
yellow indicating transfer of the catalyst to the aqueous
phase (Figure 2, image 1b). Bubbling of CO2 was continued
for a total of 1.5 h to ensure complete transfer of the catalyst
to the aqueous phase. The upper colorless organic phase was
separated and analyzed by GC-FID for organics (92.8%
aldehyde yield) and by ICP-MS for rhodium (1.9 ppm, 0.5%
of initial rhodium charged; Table 1, entry 1).
After separation of the two layers, fresh toluene was
added to the remaining aqueous phase (Figure 2, image 1c)
and the biphasic system was stirred for 10 min. There was no
change in the color of either phase. Stirring was continued for
10 min with N2 bubbling gently through the system. A pale
yellow color developed in the upper organic phase indicating
some transfer of the metal complex from the aqueous phase.
The system was heated to 608C, and stirring and N2 bubbling
was continued for 30 min, during which time the aqueous
phase completely decolorized and the organic phase turned
yellow (Figure 2, image 1d). Stirring and N2 bubbling at 608C
were continued for a total of 1.5 h. The upper organic yellow
phase was separated and fresh toluene was added to
Figure 1. Gas uptake curves from a ballast vessel obtained for the
hydroformylation of a) 1-octene and b) allyl alcohol.
reaction was carried out with a freshly prepared catalytic
solution to ensure reproducibility (Figure 1a, cycle 1b). The
catalytic system was found to be very active; impressive initial
turnover frequencies (TOF0) of 11046 hÀ1 and 9996 hÀ1 were
observed. The gas uptake profiles recorded were very similar
(Figure 1a, cycles 1a and b).
The crude product solutions from the two cycle 1 reac-
tions were combined and water was added (Figure 2,
image 1a). This biphasic system was stirred for 10 min and
left to settle. Visual observation did not show any change in
coloration of either phase. This result indicates that the
catalyst does not transfer significantly to the aqueous phase in
the absence of CO2. After 10 min stirring with CO2 bubbling
[*] Although the catalyst conserves the same level of activity upon
recycling when using 1:Rh=10, the rhodium leaching into the
organic phase was found to be higher (18 ppm) than with 1:Rh=50.
Angew. Chem. Int. Ed. 2009, 48, 1472 –1474
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1473

Communications
Table 1: Hydroformylation of 1-octene using amidine derivatized cata-
and bubbling N2 for 1.5 h at 608C. The results of two initial
reactions and one reaction using recycled catalyst are shown
in Figure 1b, with photographs of the process provided in the
Supporting Information. Once again, the gas uptake profile
using the recycled catalyst is very similar to those obtained
with the initial fresh catalyst solutions. There is slightly higher
rhodium loss (12.9 ppm in the combined first two runs and
9.9 pp in the run using recycled catalyst) in the recovered
aqueous solutions of the product (mainly 2-hydroxytrihydro-
furan, the cyclic hemiacetal from 4-hydroxybutanal, see
Scheme 1) which may be due to the lower excess of phosphine
used (P/Rh = 50 for hydroformylation of octene and P/Rh =
10 for hydroformylation of allyl alcohol). Rhodium leaching
in the organic phase is again low (0.8 and 0.3 ppm).
In conclusion, we report a new method for the separation
of the products from the catalyst in homogeneous catalytic
reactions. It involves using ligands which can be protonated
by carbonic acid (aqueous solutions of CO2), thereby switch-
ing the catalyst from being organic soluble to water soluble.
The catalyst, which has a very high activity for alkene
hydroformylation, can be switched back into the organic
phase by flushing the CO2 from the solution with N2 at 608C.
No residues accumulate during the process. The system can be
used effectively with low catalyst losses for substrates and
products that are hydrophobic or hydrophilic.
lysts.[a]
Entry Catalyst Aldehydes l/b
[%]
TOF0 [Rh]org
[Rh]aq
[ppm]
[hÀ1
]
[ppm]
1
2
3
4
cycle 1a
cycle 1b
cycle 2 94.4
cycle 3 90.9
11046
9996
9660 0.7
9555 0.4
92.8
2.86
1.9
0.4
2.78
2.81
0.3
0.5
[a] The crude product arising from cycles 1a and 1b were combined and
subjected together to the recycling procedure.
compensate for that lost by evaporation. The aqueous phase
was analyzed for the rhodium content by ICP-OES (OES =
optical emission spectroscopy; 0.4 ppm; Table 1, entry 1). A
sample of the remaining organic phase (same volume as that
used for each initial run) was used to carry out another
hydroformylation reaction with fresh 1-octene, whereas the
remainder of the organic phase (solution A) was kept to one
side. The gas uptake profile of the reaction was found to be
almost identical to the ones obtained with fresh catalyst
(Figure 1a and Table 1, cycle 2).
The crude product from cycle 2 was combined with
solution A and the mixture subjected to the same treatment
with water and CO2, phase separation, and treatment of the
aqueous phase with fresh toluene and N2 at 608C (Figure 2,
images 2a–2d). After addition of toluene to compensate for Experimental Section
Full details of the synthesis of the ligand, for carrying out the catalytic
reactions, and for the recycling appear in the Supporting Information.
evaporation, a sample of the toluene phase (same volume as
for the initial run) was again used for a catalytic reaction with
fresh alkene. Once again, the gas uptake curve was very
similar to those of the other runs (Figure 1a).
The phase transfer was carried out to ensure that the
rhodium was not being leached into the organic phase.
(Figure 2, images 3a–3d). Very low levels of rhodium were
detected in the organic and in the aqueous phases (Table 1,
cycle 3).
Received: September 27, 2008
Revised: October 30, 2008
Published online: January 14, 2009
Keywords: homogeneous catalysis · hydroformylation ·
.
phase switching · rhodium · sustainable chemistry
31P{1H}-NMR analysis of the catalyst solution at the end of
the process described above showed the presence of free and
coordinated ligand (d = 39.98 ppm, JRh-P = 153.5 Hz.) as well
as a small amount of phosphine oxide (< 5%). This result
indicates that the integrity of the catalyst is conserved
throughout the recycling procedure and that oxidation of
the ligand is not a serious issue.
[1] D. J. Cole-Hamilton, Science 2003, 299, 1702 – 1706.
[2] (Eds.: D. J. Cole-Hamilton, R. P. Tooze), Catalyst Separation,
Recovery and Recycling; Chemistry and Process Design, Springer,
Dordrecht, 2006.
[3] E. Wiebus, B. Cornils in Catalyst separation, recovery and
recycling: Chemistry and process design (Ed.: R. P. Tooze),
Springer, Dordrecht, 2006, pp. 105 – 143.
To test whether the catalyst recycling could be carried out
for a reaction in water and extracting the product into an
organic solvent, the hydroformylation of allyl alcohol was
carried out in water using the same catalyst system (Rh/1) in
the presence of CO2 (1 bar; Scheme 1, R = CH2OH). Gas
uptake curves, shown in Figure 1b, indicate that the reactions
are slower than for 1-octene and do not proceed to
completion in 60 min. The curves also have an unusual
shape, but are reproducible. The recycling of the catalyst was
carried out as described above, but starting by adding toluene
[4] Y. C. Yang, L. Wei, Z. L. Jin, Chin. J. Org. Chem. 2000, 24, 579 –
584.
[5] A. Buhling, P. C. J. Kamer, P. W. N. M. van Leeuwen, J. W.
Elgersma, K. Goubitz, J. Fraanje, Organometallics 1997, 16,
3027 – 3037.
[6] A. Andreetta, G. Barberis, G. Gregorio, Chim. Ind. 1978, 60, 887 –
891.
[7] A. Andreetta, G. Gregorio, Can. Patent, 1023729, 1978.
[8] Y. X. Liu, P. G. Jessop, M. Cunningham, C. A. Eckert, C. L.
Liotta, Science 2006, 313, 958 – 960.
1474
www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 1472 –1474