Continuous gas-phase hydroaminomethylation using supported ionic liquid phase catalysts

Article abstract of DOI:10.1002/anie.201301365

Just SILP-ing through: Hydroaminomethylation of ethylene and diethylamine to diethylpropylamine is demonstrated as a continuous gas-phase reaction (see picture) using a supported ionic liquid phase (SILP) to immobilize the applied homogenous Rh-Xantphos catalyst. Highly selective and long-term stable (18 days) catalyst operation was obtained if the ionic liquid was of low basicity and lipophilicity combined with a porous activated carbon support. Copyright

Full text of DOI:10.1002/anie.201301365

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DOI: 10.1002/anie.201301365
Supported Catalysts
Continuous Gas-Phase Hydroaminomethylation using Supported Ionic
Liquid Phase Catalysts**
Martin Johannes Schneider, Martin Lijewski, Renꢀ Woelfel, Marco Haumann, and
Peter Wasserscheid*
The term hydroaminomethylation refers to a one-pot cascade
reaction of an alkene hydroformylation followed by reductive
amination. Because water is the only byproduct, this reaction
sequence represents an atom-efficient, elegant way to pro-
duce amines. Scheme 1 shows the hydroaminomethylation
sequence for ethylene, hydroformylation followed by reaction
with diethylamine to form N,N-diethylpropan-1-amine
(DEPA) through an enamine intermediate.
TPPTS- and BINAS-modified rhodium complexes in the
hydroaminomethylation using ammonia as the amine sub-
strate. A dual-metal catalyst (Rh, Ir) and a liquid–liquid
biphasic reaction system (water, MTBE) were applied in this
work. Besides the desired primary amine product, secondary
amines, tertiary amines, imines, enamines, and aldol conden-
sation products were formed. Later, the same group obtained
much better selectivities using Rh-IPHOS[8] and Rh-Xant-
phos-systems.[9,10] For the hydroaminomethylation of termi-
nal, aliphatic olefins, very high n/iso ratios of the product
amines could be realized (n/iso > 98:2). Beller and co-workers
also investigated the hydroaminomethylation of 1-octene in
supercritical ammonia and found yields for primary nonyl-
amine of up to 60% under optimized reaction conditions.[11]
Ionic liquids (ILs) were first applied in the hydroamino-
methylation by Eilbracht and co-workers,[12] who patented the
use of ionic liquids to recycle unmodified rhodium and
iridium catalysts. More recently, Vogt and co-workers[13]
reported on the hydroaminomethylation of long-chain (C6–
C12) olefins with piperidine as the amine substrate using
a liquid–liquid biphasic reaction system with Rh/Sulfoxant-
phos in 1-methyl-3-pentylimidazolium tetrafluoroborate
[C5mim][BF4] as the IL phase.[14] The results were compared
with toluene/MeOH and a significant decrease in side-
product formation was found in the IL system.
Scheme 1. Hydroaminomethylation of ethylene using diethylamine as
the amine component.
Despite the fact that the hydroaminomethylation
sequence was first described by Reppe and Vetter already
in 1949,[1,2] extensive research in this field has only taken place
in recent years.[3–5] From this work it is evident that a major
challenge in hydroaminomethylation is to realize high chemo-
selectivity, in particular to suppress aldol condensation of the
aldehyde intermediate (Scheme 2).
Hydroaminomethylation has been shown to work even
with unmodified rhodium catalysts, although with moderate
selectivities.[6] In 1999, Beller and co-workers[7] reported on
Herein, we describe catalytic hydroaminomethylation in
a continuous gas-phase reaction using supported ionic liquid
phase (SILP) materials as microscopically homogeneous but
macroscopically heterogeneous catalysts. Within SILP cata-
lysts, a thin IL film creates a uniform solvent environment for
the dissolved transition-metal complex,[15] while the highly
porous solid on which this ionic catalyst solution is supported
creates a high gas–liquid interfacial area for fast mass
transport (Figure 1).[16,17] Product/catalyst separation takes
place in the system by evaporation of the products from the
Scheme 2. Aldol condensation of propionaldehyde.
[*] M. Sc. M. J. Schneider, M. Lijewski, Dr. R. Woelfel, Dr. M. Haumann,
Prof. Dr. P. Wasserscheid
Department of Chemical and Bioengineering
Friedrich-Alexander-Universitꢀt Erlangen-Nꢁrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
E-mail: wasserscheid@crt.cbi.uni-erlangen.de
Homepage: http: www.crt.cbi.uni-erlangen.de
[**] We gratefully thank the Deutsche Forschungsgemeinschaft within
the Erlangen Excellence Cluster “Engineering of Advanced Materi-
als” and the European Community within its project SYNFLOW
(FP7; grant agreement n8 NMP2-LA-2010-246461) for financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301365.
Figure 1. Schematic view of a supported ionic liquid phase (SILP)
catalyst system.
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ionic catalyst phase making use of the extremely low vapor
pressure of the latter. SILP systems have been successfully
applied in various gas-phase catalytic applications[18] and
recently, even for reactions with complex selectivity require-
ments, such as asymmetric hydrogenations.[19] Herein, we
introduce the possibility of SILP tandem reactions as the
formation of the desired amine requires a sequence of
selective hydroformylation, enamine formation, and enamine
hydrogenation in one gas-contacted catalyst bed.
catalysts with ILs of different hydrophobicities and basicities
(Table 1, for detailed data see Supporting Information). The
selectivity and activity of the hydroaminomethylation was
strongly influenced by the nature of the IL applied in the
Table 1: Hydroaminomethylation of ethylene and diethylamine with SILP
catalysts in a continuous gas-phase reaction—influence of different ionic
liquid coatings.[a]
Ionic
Liquid
X
[%]
TOF
[hÀ1 [c]
Yield [%][b]
propanal
The hydroaminomethylation of ethylene with synthesis
gas (H2/CO = 2:1) and diethylamine (Scheme 1) was inves-
tigated using Rh(acac)(CO)2 and Xantphos (4,5-bis(diphe-
nylphosphino)-9,9-dimethylxanthene) as a phosphine ligand
(L/Rh = 5:1). Silica 100 was applied as a porous solid support
and the first experiments were performed without any ionic
liquid to establish a baseline comparison of the purely
physisorbed catalyst. This purely physisorbed Rh-Xantphos
catalyst on silica showed high hydroformylation activity
(Xethylene = 84%, TOF = 500 hÀ1; for details see the Supporting
Information) owing to the fact that the reaction products
remain as liquids in the pores by capillary forces and provide
the solution phase for the working homogeneous catalyst.
Consequently, an activation period during the first hours of
the experiment is observed, which is attributed to the
development of a high boiling liquid layer of side-products
in the pores. However, the reductive amination step rarely
took place and the yield in the desired DEPA faded out
completely over time (see Supporting Information for
details). The majority of the intermediate propionaldehyde
readily reacted in an aldol side reaction (Scheme 2) to form
the aldol adduct (Yaldol add = 50%) and aldol condensation
product (Yaldol cond = 16%, only traces of hydrogenated aldol
condensation product were found) leading to very low
propionaldehyde concentrations (Ypropanal = 15%) in the mix-
ture analyzed at the reactor outlet. The observed catalytic
activity was only obtained in the presence of the Xantphos
ligand and even Rh/ligand ratios of less than or equal to two
led to inactive systems.
]
aldol prod.
DEPA
None
84
74
60
51
76
500
519
406
351
549
15
21
20
13
7
66
52
24
2
2
0
14
35
28
[EMIM][OAc]
[EMIM][OTf]
[MMMIM][NTf2]
[OMIM][NTf2]
40
[a] Reaction conditions: 1208C, 10 bar, L=Xantphos, L/Rh 5:1, 0.2 wt%
Rh, Silica 100, aIL(pore filling degree)=0.1, 2.5 g SILP catalyst, gas flows
are given in milliliter per minute under standard conditions (NmLminÀ1
)
200 NmLminÀ1 H2, 100 NmLminÀ1 CO, 10 NmLminÀ1 ethylene, 1 ghÀ1
diethylamine, residence time=8.4 s. Values after 20 h time-on-stream.
[b] Determined by GC (enamine, alcohols, and higher boiling aldol
condensation products were less than 1%). [c] Turnover frequency in
[molethylene(molRhÀ1 hÀ1)].
SILP material. The IL-dissolved Rh-Xantphos catalyst
switched from the production of only aldol condensation
products for the basic IL [EMIM][OAc] (SDEPA < 0.5%) to
high combined selectivities for hydroformylation and reduc-
tive amination in the case of [MMMIM][NTf2] (Spropanal+DEPA
>
94%). With the latter IL coating the catalyst activity was
somewhat lower than with [OMIM][NTf2] indicating a bene-
ficial effect of higher IL lipophilicity on the reaction rate.
However, with [OMIM][NTf2] the formation of aldol prod-
ucts increased over time-on-stream from 7% after two hours
to 40% after 20 hours. This can be attributed to the
accumulation of DEPA and high-boiling basic side products
in the IL film, leading to an increase in basicity of the ionic
catalyst solution and thus to enhanced aldol formation. All
catalytic results confirm that the formation of the desired
DEPA benefits from ILs carrying anions of low basicity and
cations of low lipophilicity.
Further experiments were done to optimize the support
material of the SILP hydroaminomethylation catalyst.
Because a detrimental influence of IL basicity on DEPA
selectivity was clear from the IL variation experiments, we
expected similar effects for the support (Table 2). Indeed,
variation of the support confirmed that SILP materials based
on neutral oxidic supports showed less aldol formation than
using the slightly basic support Pural TH 60. The highest
DEPA selectivity was achieved on the highly porous carbon
support PBSAC (PBSAC = polymer-based spherical acti-
vated carbon; Figure 3, SDEPA > 99%). The fact that the
PBSAC support shows high basicity in the PZC measurement
indicates that the simple relation between support basicity
and aldol formation seems to only hold true for a comparison
of different oxidic supports.
We further investigated the influence of different ionic
liquid coatings (Figure 2) on the catalytic performance of the
Rh-Xantphos catalyst. Because ILs can be tuned through the
combination of cations and anions,[20] we examined SILP
To demonstrate the long-term stability of the developed
hydroaminomethylation catalyst we operated the Rh-Xant-
phos-SILP system with the IL [MMMIM][NTf2] on the
Figure 2. Order of lipophilicity and basicity of the cations and anions
used in this study.
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Table 2: Continuous gas-phase hydroaminomethylation of ethylene and
diethylamine using SILP catalysts (Rh-Xantphos in [MMMIM][NTf2])—
influence of different support materials.[a]
tandem reaction of hydroformylation, amination, and enam-
ine hydrogenation using Rh-Xantphos complexes as the
immobilized homogeneous catalyst. Interestingly, the main
effect of the ionic liquid solvent (compared to reaction of the
Rh-complex dissolved in supported aldol products in the
absence of the IL; Table 1) is on the competition between the
desired amination/hydrogenation and the undesired aldol
condensation. While the IL-free system and the SILP systems
with a basic acetate coating form almost exclusively aldol
condensation products from the initial hydroformylation
product propanal, SILP catalysts based on neutral oxide and
porous carbon supports and ILs of low basicity and lip-
ophilicity result in very selective hydroaminomethylation
catalysis. The long-term stability of the developed systems is
remarkably high and this highlights the potential of these
SILP catalysts for future applications in industrial amine
production.
Support (PZC)[b]
X
[%]
TOF
Yield [%][c]
propanal
[hÀ1
]
aldol prod.
DEPA
Silica 100 (6.38)
Pural TH 60 (7.95)
AlOx neutral (7.69)
PBSAC[d] (11.3)
51
32
22
72
351
219
144
520
13
13
5
2
8
1
0
35
10
15
70
2
[a] Reaction conditions: 1208C, 10 bar, L=Xantphos, L/Rh 5:1, 0.2 wt%
Rh, IL=[MMMIM][NTf2], aIL =0.1, 2.5 g SILP catalyst, 200 NmLminÀ1
H2, 100 NmLminÀ1 CO, 10 NmLminÀ1 ethylene, 1 ghÀ1 diethylamine,
residence time=8.4 s. Values after 20 h time-on-stream. [b] PZC=point
of zero charge; PZC values were measured to compare the acidity of the
support (see Supporting Information for details). [c] determined by GC
(enamine, alcohols, and higher boiling aldol condensation products were
less than 1%). [d] 2 ghÀ1 diethylamine.
Experimental Section
SILP catalysts were prepared by impregnation under argon atmos-
phere using standard Schlenk techniques.
A solution of Rh-
(acac)(CO)2 (0.0144 g, 0.056 mmol) and Xantphos (4,5-bis(diphenyl-
phosphino)-9,9-dimethylxanthene; 0.162 g, 0.28 mmol) in dichloro-
methane was stirred for 20 min and the respective amount of ionic
liquid (0.36–0.42 g) equal to aIL = 0.1 (aIL = VIL/Vpore) was added.
Finally, 2.86 g of the support calcinated Silica 100 (pore volume
V
pore = 0.98 mLgÀ1, BET surface area = 358.9 m2gÀ1), Pural TH 60
(Vpore = 0.60 mLgÀ1, BET surface area = 325.63 m2 gÀ1), Alox neutral
(Vpore = 0.28 mLgÀ1, BET surface area = 1358 m2 gÀ1), or activated
carbon (PBSAC from Blꢀcher; Vpore = 1.18 mLgÀ1, BET surface
area = 2005 m2 gÀ1) was added and the suspension was stirred for
another 10 min before the solvent was slowly removed under reduced
pressure at 408C. The continuous gas-phase hydroaminomethylation
experiments were conducted at 1208C and 10 bar pressure using
Figure 3. Hydroaminomethylation of ethylene and diethylamine with
SILP catalyst (Rh-Xantphos in [MMMIM][NTf2] on PBSAC support).
^
Conversion of ethylene ( ), yield of aldol adduct (&), yield of aldol
a
fixed bed reactor set-up. The gaseous feedstocks (H2
condensation product (*), yield of propionaldehyde (*), yield of
DEPA (&)—Reaction conditions: 1208C, 10 bar, L=Xantphos, L/Rh
5:1, 0.2 wt% Rh, PBSAC, aIL =0.1, 2.5 g SILP catalyst, 200 NmLminÀ1
H2, 100 NmLminÀ1 CO, 10 NmLminÀ1 ethylene, 0–17 h; 1 ghÀ1 diethyl-
amine, 17–40 h; 2 ghÀ1 diethylamine, residence time=8.4 s.
200 NmLminÀ1, CO 100 NmLminÀ1) were fed by mass flow con-
trollers (MFC) and ethylene (10 NmLminÀ1) through a mini coriolis
flow (mini CORI) from Bronkhorst. Liquid diethylamine (1–2 ghÀ1
)
was pumped through an HPLC pump (Techlab) and fed into
a controlled evaporator and mixer (CEM) from Bronkhorst, where
it was evaporated. The gaseous feed stream led to a reactor of 30 cm3
volume where a fixed bed of SILP material (2.5 g) catalyzed the
reaction with a residence time of 8.4 seconds. The product feed stream
was continuously analyzed by a DANI Master GC equipped with an
FID detector. All gas flow meters and the GC were calibrated prior to
use. Additional experimental details can be found in the Supporting
Information.
PBSAC support for more than 18 days time-on-stream reach-
ing a total turnover number of 115000. [NTf2]À based ionic
liquids are particularly suitable for such long-term testing,
because these hydrophobic ILs are characterized by high
stability against thermal decomposition and hydrolysis.[21]
After a slight initial deactivation (by approximately 20%),
the catalyst activity remained almost stable during this
extended period at a level of TOF = 450 hÀ1 with an average
DEPA selectivity of more than 99% (for details see the
Supporting Information). No formation of aldol side products
and enamines was observed during the entire time of
operation. During the long-term experiment, the product
condensate was analyzed for traces of Rh, phosphine, and
ionic liquid by inductively coupled plasma atomic emission
spectroscopy (ICP-AES). Gas-phase leaching was found to be
negligible, with all relevant elements remaining below the
detection limit of 2 ppm.
Received: February 15, 2013
Revised: March 29, 2013
Published online: May 16, 2013
Keywords: amine formation · hydroaminomethylation ·
.
hydroformylation · ionic liquids · supported catalysts
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