Thermoregulated phase-transfer rhodium nanoparticle catalyst for hydroaminomethylation of olefins

Article abstract of DOI:10.1016/j.catcom.2013.01.018

Rh nanoparticles used as catalysts for hydroaminomethylation reaction is reported for the first time. An efficient and recyclable Rh nanoparticle catalyst stabilized by thermoregulated ligand Ph₂P(CH ₂CH₂O)_nCH₃ (n = 16) was studied for the hydroaminomethylation of olefins in the aqueous/1-butanol biphasic system through thermoregulated phase-transfer catalysis, which allows not only for a homogeneous catalytic reaction, but also for an easy biphasic separation. Under the optimized conditions, the conversion of 1-octene and the product amine selectivity were as high as 99% and 97%, respectively. After reaction, the Rh nanoparticle catalyst can be separated from products by simple phase separation and recycled directly for the next run.

Full text of DOI:10.1016/j.catcom.2013.01.018

Catalysis Communications 34 (2013) 73–77
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Thermoregulated phase-transfer rhodium nanoparticle catalyst for
hydroaminomethylation of olefins
⁎
Kaoxue Li, Yanhua Wang , Yicheng Xu, Wenjiang Li, Mingming Niu, Jingyang Jiang, Zilin Jin
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Rh nanoparticles used as catalysts for hydroaminomethylation reaction is reported for the first time. An efficient
and recyclable Rh nanoparticle catalyst stabilized by thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n=16)
was studied for the hydroaminomethylation of olefins in the aqueous/1-butanol biphasic system through
thermoregulated phase-transfer catalysis, which allows not only for a homogeneous catalytic reaction, but also
for an easy biphasic separation. Under the optimized conditions, the conversion of 1-octene and the product
amine selectivity were as high as 99% and 97%, respectively. After reaction, the Rh nanoparticle catalyst can be
separated from products by simple phase separation and recycled directly for the next run.
© 2013 Elsevier B.V. All rights reserved.
Received 17 November 2012
Received in revised form 31 December 2012
Accepted 28 January 2013
Available online 4 February 2013
Keywords:
Rhodium nanoparticles
Hydroaminomethylation
Thermoregulated phase-transfer catalysis
Olefin
Catalyst recycling
1. Introduction
(n=16), the thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n=16)-
stabilized Rh nanoparticle catalyst remains in the lower aqueous phase.
Hydroaminomethylation, which was first discovered by Reppe
in 1949 [1], has been used as an important tool in the synthesis
of amines. Hydroaminomethylation is a tandem reaction involving
the olefin hydroformylation to the aldehydes followed by the reaction
of resulting aldehydes with a primary or secondary amine to produce
the corresponding imines or enamines and finally the hydrogenation
of imines or enamines to the desired amines. Among the investigated
catalysts, Rh complexes were most often employed for the hydro-
aminomethylation [2–10].
Over the past decades, soluble transition-metal nanoparticles in catal-
ysis have drawn much attention due to their high efficiency and unique
properties. However, very similar to traditional homogeneous catalysts,
one of the main disadvantages of soluble nanoparticle catalysts is the
problem of separation the catalyst from the products [11,12]. To over-
come these drawbacks, several methods have been reported, with most
studies focusing on the liquid/liquid biphasic system, for example, the
use of aqueous/organic biphasic system [13–17], fluorous/organic biphas-
ic system [18–21], thermoregulated polyethylene glycol (PEG) biphasic
system [22,23] and ionic liquid biphasic system [24–28].
On heating to a temperature higher than the Cp, however, the
thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n=16)-stabilized Rh
nanoparticle catalyst will transfer into the upper 1-butanol phase.
Thus, the catalyst and the substrate will be in the same phase and the
reaction proceeds homogeneously in the 1-butanol phase. As soon as
the reaction is completed and the system is cooled to a temperature
lower than the Cp, the catalyst will return to the aqueous phase. There-
fore, by simple phase separation the catalyst can be separated from
products. Such a novel process provides both the advantages of classic
monophase and biphase system, i.e., highly catalytic efficiency and
good recyclability. Therefore, TRPTC provides a promising approach
for recovery and recycling of soluble transition-metal nanoparticle cat-
alyst, especially for the noble transition-metal nanoparticle catalyst.
Although hydroaminomethylation has been extensively studied
with Rh complexes, to the best of our knowledge, using Rh nano-
particles as catalyst for this reaction is not reported so far. Encouraged
by our previous successful applications of TRPTC for Rh nanoparticle
catalyzed hydrogenation and hydroformylation of olefins, in this
paper, thermoregulated phase-transfer Rh nanoparticle catalyst in the
aqueous/1-butanol biphasic system was applied for the first time to
the hydroaminomethylation of olefins.
Recently, we have reported the Rh nanoparticle catalyzed hydro-
genation and hydroformylation of olefins through thermoregulated
phase-transfer catalysis (TRPTC) [29,30]. The general principle of
TRPTC is illustrated in Scheme 1. At room temperature lower than
the cloud point (Cp) of thermoregulated ligand Ph2P(CH2CH2O)nCH3
2. Experimental
2.1. Materials and analyses
1-Octene was obtained from Acros. 1-Hexene and 1-dodecene were
⁎
Corresponding author. Tel./fax: +86 411 84986033.
E-mail address: yhuawang@dlut.edu.cn (Y. Wang).
purchased from Fluka. Di-n-propylamine was supplied by Sinopharm
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.01.018

74
K. Li et al. / Catalysis Communications 34 (2013) 73–77
Scheme 1. General principle of TRPTC involving Rh nanoparticles.
Chemical Reagent Co. Ltd. N-Methylaniline was obtained from Alfa
Aesar. 1-Butanol was of analytical grade and supplied from Fluka.
n-Decane was purchased from Kermel. RhCl3·3H2O was received from
Beijing Research Institute of Chemical Industry and used without
further purification. Thermoregulated ligand Ph2P(CH2CH2O)nCH3
(n=16) was prepared according to the method reported in the litera-
ture [31]. Gas chromatography analyses were performed on a Tianmei
7890 GC equipped with a 50 m OV-101 column (inner diameter
0.25 mm) and an FID detector (N2 as a carrier gas). GC–MS measure-
ment was performed on a HP 6890 GC/5973 MSD instrument (with a
30 m HP-5MS column, inner diameter 0.25 mm; He as a carrier gas).
ICP-AES analyses of Rh were carried out on Optima 2000 DV (Perkin
Elmer, USA).
hydrogen (4 MPa) at 70 °C for 2 h. Then, the reactor was cooled to
room temperature and depressurized. The color of the aqueous phase
turned from light yellow to brownish black, indicating the formation
of Rh nanoparticle catalyst.
2.3. TEM of the Rh nanoparticle catalyst
The size of Rh nanoparticle catalyst was characterized by trans-
mission electron microscopy (TEM). The solution containing the Rh
nanoparticle catalyst was diluted with ethanol. Then, a drop of the so-
lution was placed onto a carbon-coated copper grid, which was dried
at ambient temperature. The TEM images were taken with a Philips
Tecnai G2 20 TEM at an accelerating voltage of 200 kV.
2.2. Preparation of the Rh nanoparticle catalyst
2.4. Hydroaminomethylation of olefins
In
a
typical experiment,
a
mixture of RhCl3·3H2O (2 mg,
Hydroaminomethylation of olefins was carried out in a 75 mL
standard stainless-steel autoclave immersed in a thermostatic oil
bath. The stirring rate was the same for all experiments performed.
The autoclave was charged with Rh nanoparticle catalyst, 1-butanol,
0.0076 mmol), thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n=16)
(13.9 mg, 0.015 mmol), 1-butanol (4 mL) and water (4 mL) was
added in a 75 mL standard stainless-steel autoclave and stirred under
Table 1
Hydroaminomethylation of 1-octene with di-n-propylamine catalyzed by thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n=16)-stabilized Rh nanoparticles.a
Entry
P (MPa)
T
(°C)
t
1-Octene/Rh
(molar ratio)
Conv.b (%)
Selectivity (%)
n/b
(h)
Isomeric octenes
Aldehydes
Amines
1
2
3
4
5
6
7
8
6
6
6
6
2
3
4
5
6
6
6
6
6
90
100
110
120
120
120
120
120
120
120
120
120
120
4
4
4
4
4
4
4
4
2
3
4
4
4
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1500
2000
1000
97
97
98
99
97
98
98
98
99
99
98
98
99
29
16
5
14
5
5
0
5
3
2
0
8
3
4
14
78
57
79
90
97
57
74
88
95
79
89
71
58
19
93:7
91:9
89:11
79:21
95:5
89:11
84:16
80:20
96:4
88:12
87:13
93:7
3
38
23
10
5
13
8
25
28
3
9
10
11
12
13c
89:11
a
Reaction conditions: 4 mL 1-butanol, 4 mL water containing 7.6×10−3 mmol Rh (Ph2P(CH2CH2O)nCH3 (n=16)/Rh=2 (molar ratio)), di-n-propylamine/1-octene=1.5
(molar ratio), 200 mg of n-decane as internal standard, and n/b: the ratio of normal to branched amine.
b
Conversion of 1-octene.
The mercury poisoning experiment.
c

K. Li et al. / Catalysis Communications 34 (2013) 73–77
75
2.4 0.3 nm
70
60
50
40
30
20
10
0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Diameter (nm)
Fig. 1. TEM image and particle size histogram of thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n=16)-stabilized Rh nanoparticles (freshly prepared).
R'2NH
H
CO/
2
H
C
O
+
+
NR'2
R
R
R
H
R
NR'2
C
O
R
-
H
2O
Rh nanoparticles
H2
Rh nanoparticles
+
NR'2
R
NR'2
R
R= n-hexyl, R' =n-propyl
Scheme 2. Hydroaminomethylation of 1-octene with di-n-propylamine catalyzed by Rh nanoparticles.
water, olefin, di-n-propylamine (or N-methylaniline) and n-decane
(as internal standard) and flushed three times with 2 MPa CO. The re-
actor was pressurized with syngas (CO/H2 =1) up to the required
pressure and held at the scheduled temperature with magnetic stir-
ring for a fixed length of time. Then, the reactor was cooled to room
temperature and depressurized. The upper organic phase was care-
fully removed from the lower aqueous phase by syringe and immedi-
ately analyzed by GC and GC–MS.
6 MPa. As shown in Table 1, with increasing of reaction temperature,
the conversion of 1-octene at around 97–99% was nearly independent
of temperature. The amine selectivity increased, whereas the n/b ratio
dropped from 93:7 to 79:21 with increasing of temperature (Table 1, en-
tries 1–4). The effect of syngas pressure on the reaction was studied
(Table 1, entries 4–8). In the range from 2 to 6 MPa, the conversion of
1-octene was almost independent of pressure. Although the isomeriza-
tion products of 1-octene were formed preferentially rather than amines
at lower pressure, the amine selectivity increases with increasing pres-
sure and 97% amine selectivity was achieved at 6 MPa. It was also
clear from the data in Table 1 (entries 4, 9, 10) that prolonging reaction
time led to an increase in amine selectivity. In addition, we investigated
the effect of 1-octene to Rh molar ratio on the hydroaminomethylation
and observed that increasing the molar ratio from 1000 to 2000 de-
creased the amine selectivity (Table 1, entries 4, 11, 12), which was in
agreement with the reported result [36]. It is important to note that
2.5. Mercury poisoning test experiment
0.51 g of Hg (0) (500 equiv. of Rh) was added to the freshly pre-
pared Rh nanoparticle catalyst and stirred at room temperature for
1 h. Then, the hydroformylation was performed as described above
under the same reaction conditions as entry 4 in Table 1.
3. Results and discussion
Table 2
The preparation of Rh nanoparticles was reported by using different
reducing agents and Rh precursors [32–35]. For the studies reported
here, Rh nanoparticles were prepared by the reduction of RhCl3·3H2O
with hydrogen using thermoregulated ligand Ph2P(CH2CH2O)nCH3
(n=16) as stabilizing agent. As depicted in Fig. 1, TEM image showed
that the Rh nanoparticles thus obtained display a homogeneous distribu-
tion with an average particle size of 2.4 0.3 nm in diameter. This obser-
vation suggested that thermoregulated ligand Ph2P(CH2CH2O)nCH3
(n=16) was a good stabilizer for the preparation of Rh nanoparticles.
1-Octene was chosen as the standard substrate for the hydro-
aminomethylation with di-n-propylamine employing thermoregulated
ligand Ph2P(CH2CH2O)nCH3 (n=16)-stabilized Rh nanoparticles as cat-
alyst (Scheme 2). Various reaction conditions for this transformation
were studied and the results were summarized in Table 1. The effect of
reaction temperature was investigated in the range of 90–120 °C at
Recycling efficiency of thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n=16)-stabilized
Rh nanoparticle catalyst for the hydroaminomethylation of 1-octene.a
Entry Recycle number
t
Conv.b Selectivity (%)
(h) (%)
Isomeric octenes Aldehydes Amines
1
2
3
4
5
6
1
2
3
4
5
6
4
4
99
99
99
99
98
98
3
4
0
6
97
90
85
86
80
85
4
6
9
5
6
9
8
10
10
10
5
12
a
Reaction conditions: 4 mL 1-butanol, 4 mL water containing 7.6×10−3 mmol Rh
(Ph2P(CH2CH2O)nCH3 (n=16)/Rh=2 (molar ratio)), di-n-propylamine/1-octene=
1.5 (molar ratio), 1-octene/Rh=1000 (molar ratio), 200 mg of n-decane as internal
standard, T=120 °C and P=6 MPa (CO/H2=1:1).
b
Conversion of 1-octene.

76
K. Li et al. / Catalysis Communications 34 (2013) 73–77
a
2.4 0.4 nm
50
40
30
20
10
0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Diameter (nm)
40
35
30
25
20
15
10
5
b
2.5 0.5 nm
0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Diameter (nm)
Fig. 2. TEM images and particle size histograms of thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n=16)-stabilized Rh nanoparticles (after the first run (a) and the third run (b)).
under these conditions side reactions such as the hydrogenation of
octenes to octane and aldehydes to alcohols, respectively, were not ob-
served. Similar result has been reported in the literature [37]. Moreover,
no intermediate enamine was observed, which underlines the fact that
hydrogenation of the enamine is very fast. Similar phenomenon has
been observed by Vogt [5]. With the aim to give more information
about the nature of the Rh nanoparticle catalyzed hydroformylation,
the first step of the hydroaminomethylation, the mercury poisoning
test probing for heterogeneous catalysis was carried out (Table 1,
entry 13). The aldehyde selectivity of 78% implied that the catalytic
species for Rh nanoparticle catalyzed hydroformylation is perhaps the
homogeneous Rh complex, which Rh nanoparticles only act as the reser-
voir of the catalytic species [38–40].
The lifetime is an important aspect concerning the use of catalysts,
therefore the reusability of the Rh nanoparticles was examined. After
reaction, the upper organic phase was separated from the lower
catalyst-containing phase by simple phase separation. And the lower
catalyst-containing phase was directly used in the next reaction run.
Under the identical reaction conditions to entry 4 in Table 1, the recov-
ered Rh nanoparticles were used for three times with a slight decrease
of amine selectivity from 97% to 85% (Table 2, entries 1–3). TEM images
showed that the particle size of the Rh nanoparticles remained nearly
the same after the first run and the third run (Fig. 2, (a) and (b),
respectively).
10
8
Further studies on the Rh leaching detected in the upper 1-butanol
phase at the end of reaction by ICP-AES indicated that the average
leaching of Rh was 2.7 wt.% (Fig. 3). Therefore, the reason for the de-
crease of amine selectivity during the period of Rh nanoparticle cata-
lyst recycling was mainly due to the loss of Rh rather than the size
change of Rh nanoparticles. Nevertheless, by prolonging the reaction
time the Rh nanoparticle catalyst can be recycled for six times with
the amine selectivity ≥80% (Table 2).
6
4
2
Apart from the hydroaminomethylation of 1-octene and di-n-
propylamine, other olefins (1-hexene, 1-dodecene) and amine
(N-methylaniline) also reacted well in the presence of Rh nano-
particles, giving the corresponding amines in moderate to good
yields (Table 3).
0
1
2
3
4
5
6
Recycle number
Fig. 3. The Rh leaching in the upper organic phase.

K. Li et al. / Catalysis Communications 34 (2013) 73–77
77
Table 3
Hydroaminomethylation of different olefins with di-n-propylamine or N-methylaniline.a
Entry
Olefin
Amine
Conv.b (%)
Selectivity (%)
n/b
Isomeric olefins
Aldehydes
Amines
1
2
1-Hexene
1-Octene
1-Dodecene
1-Octene
Di-n-propylamine
Di-n-propylamine
Di-n-propylamine
N-methylaniline
99
99
99
99
2
3
6
1
0
0
21
14
98
97
73
85
75:25
79:21
88:12
93:7
3
4c
a
Reaction conditions: 4 mL 1-butanol, 4 mL water containing 7.6×10−3 mmol Rh (Ph2P(CH2CH2O)nCH3 (n=16)/Rh=2 (molar ratio)), amine/olefin=1.5 (molar ratio), olefin/
Rh=1000 (molar ratio), 200 mg of n-decane as internal standard, T=120 °C, P=6 MPa (CO/H2=1:1), and t=4 h, n/b: the ratio of normal to branched amine.
b
Conversion of olefin.
t=12 h.
c
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4. Conclusions
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In conclusion, the first hydroaminomethylation reaction catalyzed
by Rh nanoparticles was developed. And we have demonstrated
that thermoregulated phase-transfer Rh nanoparticles stabilized by
thermoregulated ligand Ph2P(CH2CH2O)nCH3 (n=16) were active
and recyclable catalysts for the hydroaminomethylation of olefins in
the aqueous/1-butanol biphasic system.
Acknowledgments
We gratefully acknowledge the financial support provided by the
National Natural Science Foundation of China (21173031, 20573015)
and the Fundamental Research Funds for the Central Universities.
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