Continuous rhodium-catalyzed hydroformylation of 1-octene with polyhedral oligomeric silsesquioxanes (POSS) enlarged triphenylphosphine

Article abstract of DOI:10.1002/anie.201001926

Go with the flow: Homogeneous catalyst recycling can be applied to the production of aldehydes in a continuous flow nanofiltration reactor (see picture) that can operate for an unprecedented period of up to two weeks without showing typical deactivation or leaching phenomena. The catalyst is based on a bulky, rigid, and robust silsesquioxane-modified PPh₃ ligand, and combines efficient recovery with high activity and selectivity.

Full text of DOI:10.1002/anie.201001926

Communications
DOI: 10.1002/anie.201001926
Homogeneous Catalysis
Continuous Rhodium-Catalyzed Hydroformylation of 1-Octene with
Polyhedral Oligomeric Silsesquioxanes (POSS) Enlarged
Triphenylphosphine**
Michꢀle Janssen, Jos Wilting, Christian Mꢁller,* and Dieter Vogt*
The efficient recovery and recycling of homogeneous cata-
lysts is one of the major drawbacks of this area, especially
when the high costs of precious metals are taken into
account.[1] Specific solutions have been developed for all
larger-scale applications, however, new generic methods,
preferably with low energy profiles, are highly desirable for
future process intensification and development of more
sustainable and “greener” processes. The various approaches
that have been studied to date each have their own
advantages and disadvantages, and can be divided into two
main classes: biphasic catalysis,[2–4] and immobilization on
either soluble (molecular weight enlargement, MWE) or
insoluble supports (heterogenization).[5–9]
The hydroformylation of alkenes is among the largest-
scale industrial applications of homogeneous catalysis
(Scheme 1).[10] An elegant example is the Ruhrchemie/
Rhꢀne Poulenc (RCH/RP) Process for the biphasic hydro-
formylation of propene. The water-soluble catalyst (TPPTS/
Rh; TPPTS = sulfonated PPh3, triphenylphosphino trisulfo-
nate) is immobilized in the aqueous phase and the product, n-
butanal, forms the organic phase, which can easily be
removed by phase separation. It is important to note,
however, that the applicability of this approach is limited
exclusively to lower alkenes with sufficient solubility in the
aqueous phase (the process is used commercially only up to
butenes). A range of methods that overcome this difficulty
have been investigated for the hydroformylation of higher
alkenes. Table 1 gives a concise overview of results obtained
Table 1: Overview of rhodium-catalyzed alkene hydroformylation com-
bined with catalyst recycling.
Entry Method
Ligand
TPPTS
Substrate TOF TON
[hꢀ1
Ref.
[4]
]
1
2
3
water/
1-octene 150 n.d.[b]
1-decene n.d. 2ꢀ770
1-octene 760 5800
organic+
latex[a]
water/
TPPTS
[11]
[12]
organic+
CD[c]
water/
covalently
organic+
bound to poly-
amphiphilic mer
copolymer
4
5
6
ionic liquid TPPTS
1-hexene 2000 10ꢀ1600 [13]
TPPMS[e]
1-octene 180 n.d.
1-octene 4400 n.d.
[14]
[15]
[d]
scCO2
fluorous
phase
MWE
P(4C6H4-
C6F13)3
POSS-
7
1-octene 1350 120 000 this
work
Scheme 1. The rhodium-catalyzed hydroformylation of n-alkenes.
enlarged
[a] PS-latex used as a phase-transfer agent; [b]n.d.=not determined;
[c] CD=cyclodextrin; [d] supercritical CO2; [e] TPPMS=triphenylphos-
phino monosulfate.
[*] Dr. M. Janssen, Dr. C. Mꢁller, Prof. D. Vogt
Department of Homogeneous Catalysis
Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
Fax: (+31)40-245-5054
in the rhodium-catalyzed hydroformylation of alkenes fol-
lowed by catalyst recycling, either by means of biphasic
catalysis (entries 1–6) or by MWE (entry 7). In all cases, a
modified version of triphenylphosphine was applied as the
ligand.
E-mail: c.mueller@tue.nl
d.vogt@tue.nl
Homepage: http://www.catalysis.nl
Nanofiltration is still a relatively unexploited field for the
recovery of MWE catalysts. Recently, our research group and
others showed the application of nanofiltration for the
recycling and reuse of homogeneous catalysts in a diffusion-
driven process.[16,17] However, traditional systems for MWE
have proven to be rather inefficient in continuous processes,
as low conversions were reached because of rapid catalyst
deactivation, significant metal leaching, or poor retention of
the whole system. Consequently, new concepts have to be
developed in order to circumvent these drawbacks. Herein,
we report the synthesis of molecular-weight-enlarged triphe-
Dr. J. Wilting
Hybrid Catalysis B.V.
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
[**] We thank W. van Herpen and T. Staring for technical assistance, A.
Elemans-Mehring for help with the ICP-OES measurements, and Dr.
R. Franke of the Evonik Oxeno company for helpful discussions. M.J.
thanks The Netherlands Research School Combination Catalysis
(NRSCC) and C.M. The Netherlands Organization for Scientific
Research (NWO-CW) for financial support. Hybrid Catalysis B.V. is
kindly acknowledged for POSS starting compounds.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001926.
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Chemie
nylphosphine, its application as a ligand in the rhodium-
catalyzed hydroformylation of 1-octene, and its long-term
behavior in a continuous flow nanofiltration reactor setup.
POSS has received relatively little attention for use in
MWE.[18–22] These compounds are commercially available at
low cost, have a highly defined structure, and are kinetically
and thermally stable, thus making them ideal candidates for
use in MWE. Moreover, the rigid, cubical shape of the POSS
cages is beneficial in nanofiltration, since rigid 3D structures
are known to display better retention and less membrane
fouling than more flexible macromolecules.[23]
fulfilled: straightforward synthesis, high activity, high total
turnover number (TTN), and high retention. Therefore, the
activity of POSS-enlarged PPh3 in the hydroformylation of 1-
octene was compared with PPh3 in an autoclave (batch)
experiment. Plots of conversion versus time, based on the
uptake of CO/H2, are shown in Figure 2a. The turnover
frequencies (TOF) were determined at 20% conversion, and
were found to be high: 1350 hꢀ1 and 3150 hꢀ1 for POSS-
enlarged PPh3 and unmodified PPh3, respectively. No degra-
dation of the POSS ligand was observed by 31P NMR
spectroscopy after the reaction was finished.
We initially chose to immobilize PPh3 as it has been the
ligand of choice in the industrial rhodium-catalyzed hydro-
formylation of alkenes. The synthesis of POSS-enlarged PPh3
turned out to be straightforward (Figure 1). Firstly, 4-bro-
mostyrene (1) was hydrosilylated with HSiCl3 in the presence
Since our catalytic system based on POSS-enlarged PPh3/
Rh fulfilled all the requirements discussed above, it was
applied in a novel, continuous-flow nanofiltration reactor that
was designed and developed in-house. This reactor (Figure 3)
is filled continuously with substrate solution by an HPLC
pump, while a capacitive level sensor controls the liquid level
in the reaction vessel by operating the HPLC pump. The
product-containing solution is continuously collected at the
backside of the membrane module. The reactor consists of
two loops: a gas-saturation/reaction loop (A), and a mem-
brane filtration loop (B). Loop A contains the reaction vessel
and the gas mixer, in which the reaction mixture is injected
into the gas phase for gas saturation. Both loops meet in the
crossflow chamber. The reaction mixture subsequently flows
along the ceramic nanofiltration membrane in which the
product-containing phase is separated from the MWE catalyst
and is continuously collected. We chose a ceramic membrane
because of its good solvent, pressure, and temperature
resistance. The flow in the membrane loop is kept higher
than in the reaction loop to guarantee turbulent flow along
the membrane (Reynolds number Re ꢁ 4400), thus prevent-
ing formation of a polarization layer. The reaction pressure
forms the driving force for the nanofiltration in this setup. The
flux through the membrane is controlled with a Rheodyne
two-position six-port fluid processor (PR700-100-01)
equipped with a 500 mL sample loop controlled by a flip-
flop relay, and is kept constant during the course of the
reaction. The whole reactor, including the membrane unit, is
kept at the operation temperature of 808C. The molecular-
weight cut-off (MWCO) of this membrane is 450 Da.[24]
Further reactor and reaction specifications are given in the
Supporting Information.
From our batch experiments (Figure 2a), we can conclude
that under the reaction conditions applied (T, P, [1-octene],
[Rh]; see the Supporting Information), the reaction is pseudo-
zero-order in 1-octene up to about 90% conversion. For a
zero-order reaction we can derive an equation for the
conversion X in a CSTR, where [oct]in and [oct]out are the
concentration of 1-octene flowing into and out of the reactor,
respectively, t is the space time, and k is the rate constant
[Eq. (1)].
Figure 1. a) Synthesis of POSS-enlarged PPh3. b) 3D model (MM+) of
POSS-enlarged PPh3 showing the steric bulk induced by the POSS
moieties.
of a Pt catalyst. The product (2) was subsequently used in a
corner-capping reaction with trisilanol iBu-POSS 3 in the
presence of Et3N to produce 4-bromophenylethyl-POSS 4.
After lithiation of 4 with nBuLi and reaction with PCl3, POSS-
enlarged PPh3 5 was obtained in 80% yield (Figure 1). The
ligand was fully characterized by NMR spectroscopy, ele-
mental analysis, and MALDI-TOF mass spectrometry, and
has a molecular mass of 2791 gmolꢀ1. In order to apply MWE
to homogeneous catalysts, several requirements have to be
½octꢂin ꢀ ½octꢂout
½octꢂin
tk
½octꢂin
X ¼
¼
The results of a variable-flux experiment are shown in
Figure 2b, and compared with the expected values according
to [Eq. (1)]. In this experiment, the conversion was deter-
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Communications
rationalized by for example, nonideal mixing,
premature catalyst decomposition, slight differ-
ences in temperature between the batch and
continuous setup that result in a different value
for k.
Prior to use, the reactor was purged five times
with syngas (CO/H2 1:1; 20 bar). The reactor was
then filled with substrate solution by HPLC pump,
and again purged five times with syngas (20 bar).
The reactor was heated to the reaction temper-
ature (808C) and pressurized to the reaction
pressure of syngas (20 bar). At time t = 0, the
catalyst was added to the reaction mixture by
means of a dropping funnel connected to the
reactor, therefore allowing the catalyst solution to
be added to the reactor under the reaction
conditions. Samples were taken at regular inter-
vals, and analyzed by GC to monitor the develop-
ment of conversion and regioselectivity (linear/
branched (l/b) ratio) over time (Figure 2c).
After a gradual increase, the highest conver-
sion of 99% was obtained after 17 hours. The
conversion remained greater than 90% for almost
two weeks, which is unprecedented. In this period,
the regioselectivity remained constant at an l/b
ratio of 2.5, which is identical to the regioselectiv-
ity for unsupported PPh3.
Figure 2. POSS-enlarged PPh3/Rh catalyzed hydroformylation of 1-octene. a) Com-
parison of catalytic activity between PPh3/Rh (5 equiv; c) and POSS-enlarged
PPh3/Rh (5 equiv; a) in a batch reactor. b) Comparison between experimentally
&
obtaꢀin1ed conversion ( ) and conversion predicted by the kinetic model (X=kbatch t
[oct]in ; a); kbatch refers to the value of k calculated from the batch setup; f=flux
[Lhꢀ1]. c) Conversion ( ) and regioselectivity ( ) versus time for the continuous
hydroformylation of 1-octene in the continuous flow membrane reactor. d) Accumu-
lated turnover number obtained in the continuous-flow membrane reactor.
!
&
The accumulated turnover number (TTN)
versus time is shown in Figure 2d, which shows
that the TTN steadily increases to a value of about
120000 Rhꢀ1 after 13 days. This result demonstrates the
stability and robustness of our POSS-enlarged PPh3/Rh
catalyst.
The significant reduction in activity after 13 days still
requires further investigation. The reduction could be caused
by ligand decomposition, for example, by cleavage of the P–C
bond in the ligand, as has been previously reported.[25,26]
However, the 31P NMR spectrum of the reaction mixture at
the end of the experiment showed signals of only the free and
rhodium-coordinated ligands. Another possibility could be
the formation of Rh clusters. The color of the retentate at the
end of the reaction was still light yellow, whereas Rh clusters
usually cause a brown coloration. Furthermore, very small
amounts of impurities present in for example, the solvent or
substrate could eventually accumulate in the reactor and lead
to catalyst deactivation. Clearly, the observed deactivation is
not caused by a substantial loss of Rh, since ICP-OES
(induced coupled-plasma–optical emission spectroscopy)
analysis of the total product solution revealed a total Rh
leaching of only 0.045% and a total P leaching of 0.74% of
the total amount of Rh and P added, respectively. This result
means that 99.96% of the Rh is effectively retained by the
nanofiltration membrane, thus tremendously simplifying the
workup as the Rh concentration in the product is extremely
low. Moreover, after removal of the retentate, the reactor
does not show catalytic conversion of 1-octene (< 1%), thus
indicating that no active Rh species are adsorbed on the
membrane or the reactor wall.
Figure 3. Schematic representation of the continuous-flow nanofiltra-
tion reactor developed in-house.
mined at a certain space time t, after the system had reached
steady-state conditions. Figure 2b shows that the conversion
that is reached in the CSTR is systematically lower than the
expected conversion based on [Eq. (1)]. This behavior can be
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In summary, we have introduced POSS as a versatile unit
for molecular weight enlargement of homogeneous catalysts,
facilitating catalyst recycling, which results in continuous
operation. A Rh catalyst based on the bulky, rigid, and robust
POSS-modified PPh3 ligand was applied in the hydroformy-
lation of 1-octene in an in-house engineered continuous flow
nanofiltration reactor. Unprecedented results in terms of
activity, stability, and retention of the POSS-enlarged catalyst
system were achieved as the hydroformylation setup was
continuously operated for almost two weeks without any
significant deactivation or leaching of the catalyst. These
results highlight a major breakthrough in the sustainable
conversion of alkenes under homogeneous catalytic condi-
tions without the usual drawback of tedious and often
expensive catalyst recovery and recycling. The full scope of
the concept of MWE using POSS units is presently under
investigation for a range of ligands and metal-catalyzed
transformations.
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Received: March 31, 2010
Published online: September 8, 2010
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[23] R. W. Baker, Membrane Technology and Applications, Wiley,
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Keywords: catalyst recycling · homogeneous catalysis ·
hydroformylation · rhodium · silsesquioxanes
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