Easy and efficient processes for catalyst recycling and product recovery in organic biphase systems tested in the hydrogenation of hex-1-ene

Article abstract of DOI:10.1039/a909101g

Two organic liquid biphase systems containg poly(ethylene oxide), heptane and either CH₂Cl₂ or methanol have been tested in the catalytic hydrogenation of hex-1-ene using, respectively, Wilkinson's catalyst and a cationic rhodium complex, both leading to high yields and selectivity, with the latter showing a better performance and enabling easy and efficient product separation and catalyst recycling.

Full text of DOI:10.1039/a909101g

Easy and efficient processes for catalyst recycling and product recovery in
organic biphase systems tested in the hydrogenation of hex-1-ene
a
a
b
b
Ricardo G. da Rosa, Laura Martinelli, Luís H. M. da Silva and Watson Loh*
a
Instituto de Química, Universidade Federal do Rio Grande do Sul, Caixa Postal 15003, Porto Alegre, RS,
1501-970, Brazil
Instituto de Química, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970, Campinas, SP, Brazil.
E-mail: wloh@iqm.unicamp.br
9
b
Received (in Cambridge, UK) 17th November 1999, Accepted 26th November 1999
Two organic liquid biphase systems containg poly(ethylene
oxide), heptane and either CH Cl or methanol have been
tested in the catalytic hydrogenation of hex-1-ene using,
respectively, Wilkinson’s catalyst and a cationic rhodium
complex, both leading to high yields and selectivity, with the
latter showing a better performance and enabling easy and
efficient product separation and catalyst recycling.
for the ternary system PEO 3350–MeOH–heptane (not shown)
2
2
is similar, though with a larger biphase area. Additional
experiments have shown that in both systems there are only
small changes when the polymer molecular weight is varied
from 200 to 10 000, provided the polymer content is expressed
on a monomer basis. There are, however, some important
changes of the phase boundaries with temperature, with the
6
biphase region increasing as the temperature decreases.
Liquid biphase systems have been the focus of great attention in
catalysis recently as media for alternative less polluting and
more efficient catalytic processes, providing an easier separa-
tion between products and catalyst. Among the strategies
studied so far, examples include the use of aqueous/organic
Of relevance to this investigation, there are some composi-
tions at which the system may be moved from a biphasic to a
homogeneous region by temperature increase, for instance, with
2 2
19.5% of PEO 3350, 51.6% of CH Cl and 28.9% heptane
[compositions expressed on the basis of mol% (for PEO this
refers to mol of monomer units)], this transition occurs at 9 °C.
However, a significant rhodium loss was spectrophotomet-
rically detected to the upper phase (ca. 15%) for this and other
compositions which presented temperature-driven phase sepa-
ration, causing poor recycling properties.
1
biphase systems with water soluble catalysts, fluorous biphase
systems, where the use of fluorinated ligands leads to catalyst
2
solubility in the perfluorinated phase, functionalised thermo-
sensitive polymers as supports for traditional organometallic
3
homogeneous catalysts and the application of molten salts as
4
ionic liquids. All these procedures present some advantages
For this reason, we developed another procedure to improve
the selectivity of catalyst partitioning. As temperature de-
creases, these polymer solutions show phase separation of
UCST (upper critical solution temperature) type. For PEO
and disadvantages. Among the latter, is the need for specially
prepared catalysts, occurrence of catalyst leaching, or the
somewhat difficult handling of the biphase system.
We have recently described5 an organic biphase system
2 2
solutions in CH Cl , these phase separation temperatures range
containing poly(ethylene oxide) (PEO), CH
2
Cl
2
and heptane
from 280 to 240 °C, for PEO 3350 concentrations between 1
and 60% (w/w). As the polymer phase separates, we have
verified that most of the catalyst is also removed from the
solution phase. Therefore, by cooling the reaction system with
which shows a strong segregation between the polar polymer
and the hydrocarbon solvent, the polymer being concentrated in
6
the bottom phase. Further studies revealed that this phenome-
non is also observed in mixtures containing other polar solvents
such as methanol, chloroform and acetonitrile. Because of its
characteristics, this ternary mixture may be interchanged
between homogeneous and biphasic regions by either composi-
tion or temperature changes. These features suggest a variety of
applications in different separation processes where the absence
of water is desirable including catalysis.
2
liquid N , it is possible to selectively separate the catalyst and
the reaction products. The striking feature is that, probably
owing to large density differences, this biphase system is
kinetically stable for a long time (even hours) even at
temperatures close to room temperature, allowing easy separa-
tion of the liquid phases. Although this system is not in
thermodynamic equilibrium, this procedure has provided an
easy way of catalyst recycling.
This procedure was employed using Wilkinson’s catalyst in
the hydrogenation of hex-1-ene,‡ allowing product recovery
with no apparent catalyst leaching to the apolar phase (verified
spectrophotometrically) and efficient and selective substrate
conversion. However, a marked decrease in the catalytic
activity was observed after the third recycle. This could be
related to the recycling process, which may induce a continuous
loss of the triphenylphosphine free ligand in equilibrium with
the rhodium complex, as revealed by 31P NMR spectra,
generating inactive species.
This communication describes studies on the application of
these biphase systems in the hydrogenation of hex-1-ene
catalysed by Wilkinson’s complex, RhCl(PPh
cationic complex [Rh(cod)(dppe)]PF , cod
,5-diene, dppe = 1,2-bis(diphenylphosphine)ethane].† Hydro-
3
)
3
, and by the
6
= cycloocta-
1
genation was chosen as a test reaction because of its
technological importance and hex-1-ene was used as substrate
owing to its easy product characterisation. The rhodium
complexes used were selected considering their well established
hydrogenation catalytic behaviour and poor solubility in apolar
solvents. Therefore, ideally, this reaction may be conducted in
a homogenous system and, after induced phase separation, the
catalyst may be selectively separated from the products,
allowing for their removal and recycling. In a similar procedure
In order to overcome this problem, a cationic rhodium
complex containing a chelating phosphine, [Rh(cod)(dp-
pe)]PF
showed poor catalytic activity using the ternary mixture
containing CH Cl . On the other hand, as suggested by other
6
, was tested as a catalytic precursor. This complex
7
Bianchini et al. performed two catalytic reactions in a mixture
of methanol and hydrocarbons that was homogeneous at high
temperatures and formed a biphase system at room temperature,
using a rhodium complex with a chosen ligand that ensured the
complex solubility only in the alcohol-rich phase.
2
2
8
catalytic hydrogenation studies, its performance using metha-
nol as the polar component of the ternary mixture was clearly
enhanced. With this second ternary system, the procedure of
inducing phase separation by cooling was not effective since the
volume of the upper phase was too small thus impeding the
The phase diagram for the ternary system PEO
5
3
350–CH
2
Cl
2
–heptane has been previously reported and that
Chem. Commun., 2000, 33–34
This journal is © The Royal Society of Chemistry 2000
33

product separation. This reaction was then conducted in a
homogenous system containing 14 mL of MeOH, 3.6 g of PEO,
tions (changing solvents, composition or temperature) that may
suit a wide range of catalytic reactions and products.
0
.0335 mmol of Rh complex and 8.01 mmol of hex-1-ene, at
R. G. da Rosa and L. Martinelli thank FAPERGS for
financial support, L. H. M. da S. thanks CAPES-PICDT for a
doctoral scholarship, and Universidade Federal de Viçosa for a
leave of absence. The authors are grateful to R. F. de Souza and
the Laboratório de Reatividade e Catálise - UFRGS for access
to their facilities, and to M. Spitzer for providing phase
equilibria data for some of the systems used.
room temperature and under H flow. This system requires an
2
activation period of ca. 2.5 h to reach its maximum activity. The
reaction products were analysed by GC§ 30 min after this period
and during eight cycles, producing a 54 ± 5% conversion
associated with 90 ± 10% selectivity to hydrogenation. Under
these conditions, the estimated turnover frequency was 4 min21
.
Maintaining the reaction for 1 h, the substrate was completely
and selectively converted to n-hexane, suggesting that the
isomerisation reaction is faster than hydrogenation and that the
isomers produced are reactive towards the catalyst being
slowly, but continuously, converted to n-hexane. The product
was then extracted by heptane addition, with three extractions
with 4 mL heptane aliquots yielding 75% of product recovery.
In the biphase system then formed, the extent of rhodium
leaching was determined by ICP-atomic emission analyses of
the upper phase,¶ revealing losses of only 0.083% of metal.
Such a low rhodium loss qualifies this procedure as an efficient
catalyst recycling.
Notes and references
†
Wilkinson’s catalyst was prepared according to the procedure described:
J. A. Osborn and G. Wilkinson, Inorg. Synth., 1967, 10, 68 while
Rh(cod)(dppe)]PF was prepared and characterised following R. R.
Schrock and J. A. Osborn, J. Am. Chem. Soc., 1971, 93, 2397.
In all the catalytic tests, the reactor, a 100 mL stainless steel autoclave,
was previously purged by vacuum–argon cycles, then pressurised with
hydrogen (99.999%). CH Cl , methanol and n-heptane were distilled prior
to use, under argon from P , activated molecular sieves (3 Å) and Na/
[
6
‡
2
2
2 5
O
benzophenone respectively. Hex-1-ene (Alpha, 99.9%) was stored over
activated molecular sieves (3 Å) under argon. PEO 3350 (Sigma), was dried
This reaction is efficient even when performed under biphase
conditions as, for instance, using a system composed of MeOH
over P
magnetic stirring.
The chromatographic analyses were performed using a Varian Star 3400
CX Chromatograph, equipped with a 30 m 3 0.25 mm LM-1 capillary
2 5
O . All reactions were conducted at room temperature under
(
14 mL), PEO (3.6 g) and n-heptane (14 mL), the same rhodium
and olefin content as above, with 10 bar of H . Under these
2
§
conditions five runs were made with complete conversion of the
substrate to the hydrogenation product and no loss of catalytic
activity or selectivity was noted. A kinetic study of this system
allowed the determination of an activation time of 50 min. After
this time, GC analysis revealed a complete conversion of hex-
21
2
column, at 50 °C and 1 mL min (10 psi) of H as carrier gas.
¶ Rhodium analyses were performed by ICP-atomic emission spectrometry
with a segmented-array charge coupled device detector (Optima 3000,
Perkin Elmer, Newark, CT). Operating conditions: auxiliary argon flow 0.5
2
1
21
21
L min , plasma argon flow 15 L min , nebulizer argon flow 0.8 L min
,
1
-ene to n-hexane in 15 min, allowing determination of a
2
1
variable incident power (0.75–1.5 kW), cross-flow nebulizer and axial
observation. Duplicate aliquots of the upper phase were digested with aqua
regia for 12 h and then adjusted to produce concentrations close to 0.1 ppm.
The final rhodium loss was determined as (0.083 ± 0.001)%.
turnover frequency of ca. 16 min . This system can be
recycled, at least eight times, with no changes in activity or
selectivity.
In conclusion, this set of results, comprising experiments
performed with two catalysts under different conditions, have
proved the great potential for use of these organic biphase
systems as reaction media that enable an easy product
separation and efficient catalyst recycling. Furthermore, this
procedure allows the use of common catalysts in homogeneous
catalysis, requiring, therefore, no special reagents. These
biphase systems also display advantages over binary solvent
mixtures in that, owing to the presence of the polar polymer, the
phase compositions are more different, increasing the partition-
ing selectivity, without the need of special ligands for the
catalyst. The only requirement for their application is the
selective partitioning of catalyst and of the desired product,
however, this procedure is flexible enough to allow modifica-
1
2
O. Wachsen, K. Himmler and B. Cornils, Catal. Today, 1998, 42, 373.
I. T. Horvath, Acc. Chem. Res., 1998, 31, 641.
3 D. E. Bergbreiter, Catal. Today, 1998, 42, 389.
4 P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and J. Dupont,
Inorg. Chim. Acta, 1997, 255, 207; A. L. Monteiro, F. K. Zinn, R. F. de
Souza and J. Dupont, Tetrahedron: Asymmetry, 1997, 8, 177.
5
6
7
L. H. M. da Silva and W. Loh, Chem. Commun., 1998, 787.
M. Spitzer, L. H. M. da Silva and W. Loh, unpublished work.
C. Bianchini, P. Frediani and V. Sernau, Organometallics, 1995, 14,
5
458.
8
B. D. Vineyard, W. S. Knowles, M. J. Sabacky, G. L. Bachman and D. J.
Weinkauff, J. Am. Chem. Soc., 1977, 99, 5946.
Communication a909101g
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Chem. Commun., 2000, 33–34