Thermoregulated phase-separable rhodium nanoparticle catalyst for selective hydrogenation of α,β-unsaturated aldehydes and ketones

Article abstract of DOI:10.1039/c7ra10351d

Thermoregulated phase-separable Rh nanoparticles (abbreviated as TPS-Rh_nano) were found to be efficient catalysts for selective hydrogenation of CC bonds in α,β-unsaturated aldehydes and ketones with a selectivity of >99% at >99% conversion. More importantly, the catalyst could be easily separated by simple phase separation and reused directly eight times without evident loss in activity and selectivity.

Full text of DOI:10.1039/c7ra10351d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Thermoregulated phase-separable rhodium
nanoparticle catalyst for selective hydrogenation
Cite this: RSC Adv., 2017, 7, 50343
of a,b-unsaturated aldehydes and ketones†
Xiuru Xue,
Mingming Niu, Yicheng Xu and Yanhua Wang
*
Received 18th September 2017
Accepted 23rd October 2017
Thermoregulated phase-separable Rh nanoparticles (abbreviated as TPS-Rhnano) were found to be efficient
catalysts for selective hydrogenation of C]C bonds in a,b-unsaturated aldehydes and ketones with
a selectivity of >99% at >99% conversion. More importantly, the catalyst could be easily separated by
simple phase separation and reused directly eight times without evident loss in activity and selectivity.
DOI: 10.1039/c7ra10351d
rsc.li/rsc-advances
Soluble transition-metal nanoparticles have drawn much
The selective hydrogenation of C]C or C]O bond in
attention because of their high catalytic activity and excellent a,b-unsaturated aldehydes and ketones to the corresponding
selectivity. However, the direct separation and recovery of these saturated aldehydes/ketones or unsaturated alcohols has broad
nanoparticles from the products in homogeneous catalytic applications in the synthesis of many drugs, medical interme-
9
–11
reactions have always been technical problems that have diates and ne chemicals.
Although various catalyst systems
1–4
needed to be solved for the past few decades. Recently, based for this selective hydrogenation reaction have been extensively
on our discovery that the polyether chain-based quaternary reported, until now only a few catalytic systems affording the
+
ꢀ
ammonium salt ionic liquids [CH
ILPEG, n ¼ 12, 16, 22) exhibit electrosteric stabilization for recyclability were reported.
nanoparticles and a distinctive critical solution temperature exciting results of TPSCS and with the aim to expand the
CST) property in toluene and n-heptane mixtures, we have application scopes of this system, in this paper, TPS-Rhnano was
established thermoregulated phase-separable catalytic system explored rstly for selective hydrogenation of a,b-unsaturated
3
(OCH
2
CH
2
)
n
N Et
3
][CH
3
SO
3
]
C]C bond hydrogenation product with good selectivity and
12–19
(
Encouraged by our pervious
(
5
(
TPSCS) involving soluble transition-metal nanoparticles. The aldehydes and ketones.
basic principle of TPSCS was described in Fig. 1. When
temperature is lower than the CST, the lower ILPEG stabilized RhCl
nanoparticle catalyst phase is immiscible with the upper mixed details). Fig. 2 shows the UV-visible absorption spectra of the
solvents phase containing substrate. Excitingly, once the reac- solution of RhCl
tion temperature is increased beyond the CST, the two-phase Apparently, aer reduction the characteristic absorption peak
TPS-Rhnano catalyst was prepared by hydrogen reduction of
$3H
O using ILPEG (n ¼ 22) as a stabilizer (see ESI† for
3
2
3
and ILPEG (n ¼ 22) before and aer reduction.
system becomes transparent and homogeneous which allows of Rh (III) at 520 nm disappeared completely, which indicated
0
20
the reaction runs smoothly. Aer reaction, the homogeneous the formation of Rh . Further analysis by TEM revealed that
system returns into two-phase again at the temperature lower the Rh nanoparticles stabilized by ILPEG (n ¼ 22) dispersed very
than CST. Therefore, the lower nanoparticle catalyst phase can well with an average particle size of 1.7 ꢁ 0.2 nm (Fig. 3a). For
be separated easily from the upper product phase by simple
phase separation, and the catalyst can be reused in the next
cycle. Obviously, such a catalytic system takes advantages of
both classical homogeneous and heterogeneous systems. Till
now, the TPSCS has been applied successfully for the hydroge-
6
nation of olens, selective hydrogenation of 1,5-cyclo-
5
7
octadiene, Heck reaction and hydroaminomethylation of
1
8
-octene with high catalytic efficiency and good recyclability.
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
1
+
†
1
16024, PR China. E-mail: yhuawang@dlut.edu.cn; Fax: +86 411 84986033; Tel:
86 411 84986033
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra10351d
Fig.
1 Schematic diagram of thermoregulated phase-separable
catalytic system.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 50343–50346 | 50343

View Article Online
RSC Advances
Paper
steric stabilizations. Furthermore, all HRTEM images of Rh
nanoparticles prepared by different stabilizers showed that the
lattice fringes spacing of 0.20 nm were close to the (111) facet of
Rh nanoparticles.
For the TPS-Rhnano-catalyzed a,b-unsaturated aldehydes and
ketones, cinnamaldehyde was chosen as a representative
substrate to evaluate the catalyst performance and by varying
temperature, hydrogen pressure, reaction time and molar ratio
of cinnamaldehyde to Rh to identify the optimum reaction
conditions (Table 1). The effect of reaction temperature was
ꢂ
explored rstly in the range of 30–70 C. When temperature was
3
Fig. 2 UV-visible absorption spectra of the solution of RhCl and ILPEG
ꢂ
increased from 30 to 60 C, the conversion of cinnamaldehyde
increased from 12% to 99%, while the selectivity of the hydro-
(n ¼ 22) before and after reduction.
genation product of C]C bond remained >99% all the time
ꢂ
(
entries 1–4). With a further increase in temperature to 70 C,
comparison, TOAB (tetraoctyl ammonium bromide) with elec-
the selectivity towards the hydrogenation product of C]C bond
dropped to 94% although almost total conversion was observed
entry 5). And the by-product was proved to be the hydrogena-
tion product of both C]C and C]O bonds. Further optimisa-
tion was the role of hydrogen pressure on the reaction and the
results indicated that the conversion of cinnamaldehyde
improved obviously with increasing hydrogen pressure from 0.1
to 1.0 MPa and 99% conversion was achieved at 1.0 MPa (entries
21–24
trostatic stabilization,
monomethylether 1000) with steric stabilization
MPEG-1000 (polyethylene glycol
25–27
and the
(
mixture of TOAB and MPEG-1000 with electrosteric stabilization
were used separately as a stabilizer for Rh nanoparticles under
the same conditions. The results showed that Rh black could be
seen by the naked eye when TOAB acted independently as
stabilizer. Meanwhile, the nonuniform dispersion or even
obvious aggregation was observed for nanoparticles prepared
from MPEG-1000 or the mixture of TOAB and MPEG-1000
4
, 6 and 7). When the hydrogen pressure was further increased
to 1.5 MPa, the conversion reached >99% (entries 4 vs. 8). It was
clear that prolonging the reaction time was in favour of
improving the conversion of cinnamaldehyde (entries 4 and
(Fig. 3b and c). Therefore, the formation of homogeneous-
dispersed Rh nanoparticle catalyst stabilized by ILPEG (n ¼ 22)
was attributed to the perfect combination of electrostatic and
9
–11). In addition, we found that the conversion of cinna-
maldehyde declined apparently when the molar ratio raised
from 500 to 2000 (entries 4 and 12–14). For the sake of product
separation and catalyst recycling, the best molar ratio of cin-
namaldehyde to Rh was chosen to be 500 in our studies. It's
worth mentioning that the selectivity towards the hydrogena-
tion product of C]C bond in cinnamaldehyde was totally
unaffected by hydrogen pressure, reaction time and the molar
ratio of cinnamaldehyde to Rh. Under the optimized reaction
conditions, the conversion of cinnamaldehyde and the selec-
tivity of the C]C bond hydrogenation product were all >99%.
ꢀ
1
The TOF was 245 h
.
The reusability is a signicant parameter in evaluation of
a catalyst, especially for transition metal nanoparticle catalysts.
Therefore, the reusability of TPS-Rhnano catalyst for selective
hydrogenation of cinnamaldehyde was examined under the
above-mentioned optimum conditions. Aer reaction, the lower
TPS-Rhnano catalyst phase was simply separated from the
product phase by simple phase separation and reused directly
in the next catalytic cycle. As shown in Fig. 4, the TPS-Rhnano
catalyst could be used eight times without evident loss in
conversion and selectivity. Comparison of the TEM images of
Rh nanoparticles between newly prepared and aer eight cycles
revealed that the particle size increased slightly from original
1.7 to current 2.2 nm (Fig. 3a vs. Fig. 5). In addition, the
oxidation state and recyclability of the TPS-Rhnano catalyst
basically remained before and aer the catalytic reactions
judging from XRD and XPS (Fig. S1 and S2, see the ESI† for
details). Moreover, ICP-AES analysis of the upper organic phase
Fig. 3 TEM images and particle size histograms of Rh nanoparticles
prepared by different stabilizers: (a) electrosteric stabilization by ILPEG
(n ¼ 22); (b) steric stabilization by MPEG-1000; (c) electrosteric
stabilization by the mixture of TOAB and MPEG-1000 (200 particles). aer the rst run indicated that the leaching of Rh was only
50344 | RSC Adv., 2017, 7, 50343–50346
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
a
Table 1 Selective hydrogenation of cinnamaldehyde using TPS-Rhnano as catalyst
ꢂ
ꢀ1
b
c
d
ꢀ1
Entry
T ( C)
PH2 (MPa)
t (h)
S/C (mol mol
)
Conv. (%)
Sel. (%)
TOF (h
)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
30
40
50
60
70
60
60
60
60
60
60
60
60
60
1.0
1.0
1.0
1.0
1.0
0.1
0.5
1.5
1.0
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
1.5
2.0
2.0
2.0
500
500
500
500
500
500
500
500
500
500
500
1000
1500
2000
12
48
72
99
>99
9
38
>99
22
65
91
87
65
54
>99
>99
>99
>99
94
>99
>99
>99
>99
>99
>99
>99
>99
>99
30
119
178
245
233
22
94
245
218
322
300
431
483
535
0
1
2
3
4
a
ꢀ3
Reaction conditions: ILPEG 0.3 g (containing 2.6 ꢃ 10 mmol rhodium), toluene (3.5 g), n-heptane (0.7 g). The miscibility temperature of the system is
ꢂ
0 C. Determined by GC with cyclohexane (0.2 g) as internal standard. Selectivity of the C]C bond hydrogenation product, the only by-product was
d
b
c
6
the hydrogenation product of both C]C and C]O bonds. Turnover frequency (TOF): moles of main product per mole of rhodium per hour.
0
.05 wt%, and from the second run on the leaching of Rh was Table 2 Selective hydrogenation of different a,b-unsaturated alde-
a
hydes and ketones catalyzed by the TPS-Rhnano
lower than the detection limit. These results again conrmed
that the TPS-Rhnano catalyst had good stability, catalytic activity,
selectivity and recyclability.
ꢀ1
Entry Substrate
1
Product
Conv. (%) Sel. (%) TOF (h )
>99
>99
>99
>99
245
245
2
3
>99
>99
245
a
ꢀ3
Reaction conditions: ILPEG 0.3 g (n ¼ 22, containing 2.6 ꢃ 10 mmol
ꢂ
Rh), toluene 3.5 g, n-heptane 0.7 g; T ¼ 60 C, P
H
¼ 1.0 MPa, t ¼ 2.0 h,
2
ꢂ
substrate/Rh ¼ 500, the miscibility temperature of the system is 60 C.
The conversion was determined by GC with cyclohexane (0.2 g) as
internal standard; the selectivity was the corresponding
hydrogenation product of C]C bond.
Fig. 4 Recyclability of the TPS-Rhnano catalyst for selective hydroge-
nation of cinnamaldehyde.
In order to evaluate the generality of the above TPS-Rhnano
catalyst, the selective hydrogenation of different a,b-unsatu-
rated aldehydes and ketones, such as crotonaldehyde, benzyli-
deneacetone and 2-cyclohexen-1-one, were also explored. Under
the optimized reaction conditions, all substrates afforded their
corresponding hydrogenation products of C]C bond with
>99% conversion and >99% selectivity (Table 2).
Conclusions
In summary, the TPS-Rhnano catalyst was employed rstly for
the selective hydrogenation of a,b-unsaturated aldehydes and
ketones. Excellent selectivity (>99%) at the site of C]C bond
Fig. 5 TEM image and particle size histogram of Rh nanoparticles after
eight catalytic cycles (200 particles).
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 50343–50346 | 50345

View Article Online
RSC Advances
Paper
with conversion of >99% was achieved. More excitingly, the 11 M. Tamura, D. Yonezawa, T. Oshino, Y. Nakagawa and
catalyst could be easily separated from the product by simple K. Tomishige, ACS Catal., 2017, 7, 5103.
phase separation and reused directly for eight times without 12 N. M. Callis, E. Thiery, J. L. Bras and J. Muzart, Tetrahedron
evident loss in activity and selectivity.
Lett., 2007, 48, 8128.
1
1
3 S. Morrissey, I. Beadham and N. Gathergood, Green Chem.,
2009, 11, 466.
Conflicts of interest
4 V. R. Landaeta, F. L ´o pez-Linares, R. S ´a nchez-Delgado,
C. Bianchini, F. Zanobini and M. Peruzzini, J. Mol. Catal.
A: Chem., 2009, 301, 1.
There are no conicts to declare.
1
1
1
5 X. Ni, B. Zhang, C. Li, M. Pang, D. Su, C. T. Williams and
C. Liang, Catal. Commun., 2012, 24, 65.
6 M. Niu, Y. Wang, W. Li, J. Jiang and Z. Jin, Chin. J. Catal.,
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21173031, 21373039).
2013, 34, 674.
7 Y. Zhang, X. Yang, Y. Zhou, G. Li, Z. Li, C. Liu, M. Bao and
W. Shen, Nanoscale, 2016, 8, 18626.
Notes and references
18 W. Li, Y. Wang, P. Chen, M. Zeng, J. Jiang and Z. Jin, Catal.
Sci. Technol., 2016, 6, 7386.
1
2
3
A. Roucoux, J. Schulz and H. Patin, Chem. Rev., 2002, 102,
757.
S. Shylesh, V. Sch u¨ nemann and W. R. Thiel, Angew. Chem.,
Int. Ed., 2010, 49, 3428.
N. Koukabi, E. Kolvari, M. A. Zolgol, A. Khazaei,
B. S. Shaghasemi and B. Fasahati, Adv. Synth. Catal., 2012,
1
9 S. Doherty, J. G. Knight, T. Backhouse, E. Abood,
H. Alshaikh, I. J. S. Fairlamb, R. A. Bourne,
T. W. Chamberlain and R. Stones, Green Chem., 2017, 19,
3
1
635.
0 W. Tu, H. Liu and K. Y. Liew, J. Colloid Interface Sci., 2000,
29, 453.
1 V. Cimpeanu, M. Ko ˇc evar, V. I. Parvulescu and W. Leitner,
Angew. Chem., Int. Ed., 2009, 48, 1085.
2 N. Podoliak, D. Bartczak, O. Buchnev, A. G. Kanaras and
M. Kaczmarek, J. Phys. Chem. C, 2012, 116, 12934.
3 L. M. Rossi, L. L. R. Vono, M. A. S. Garcia, T. L. T. Faria and
J. A. Lopez-Sanchez, Top. Catal., 2013, 56, 1228.
2
2
2
2
2
2
2
2
2
3
54, 2001.
B. Karimi, F. Mansouri and H. M. Mirzaei, ChemCatChem,
015, 7, 1736.
Y. Xu, Y. Wang, Y. Zeng, Y. Song, J. Zhao, J. Jiang and Z. Jin,
Chin. J. Catal., 2012, 33, 1871.
4
5
6
7
8
9
2
Y. Zeng, Y. Wang, J. Jiang and Z. Jin, Catal. Commun., 2012,
19, 70.
4 Y. Yu, B. W. Goodfellow, M. R. Rasch, C. Bosoy,
D.-M. Smilgies and B. A. Korgel, Langmuir, 2015, 31, 6924.
5 H. B ¨o nnemann and R. M. Richards, Eur. J. Inorg. Chem.,
Y. Zeng, Y. Wang, Y. Xu, Y. Song, J. Jiang and Z. Jin, Catal.
Lett., 2013, 143, 200.
Y. Xu, Y. Wang, K. Li, M. Niu, Y. Zeng, J. Jiang and Z. Jin, J.
Nanosci. Nanotechnol., 2013, 13, 5048.
M. Bartok and A. Molnar, The Chemistry of Double-bonded
Functional Groups, Wiley, New York, 1997.
2001, 2001, 2455.
6 Z. Sun, Y. Wang, M. Niu, H. Yi, J. Jiang and Z. Jin, Catal.
Commun., 2012, 27, 78.
7 M. Niu, Y. Wang, P. Chen, D. Du, J. Jiang and Z. Jin, Catal.
Sci. Technol., 2015, 5, 4746.
10 D. D. Falcone, J. H. Hack and R. J. Davis, ChemCatChem,
2016, 8, 1074.
50346 | RSC Adv., 2017, 7, 50343–50346
This journal is © The Royal Society of Chemistry 2017