Ionic liquid immobilized nickel(0) nanoparticles as stable and highly efficient catalysts for selective hydrogenation in the aqueous phase

Article abstract of DOI:10.1002/asia.200900628

Nickel nanoparticles (NPs) well-dispersed in the aqueous phase were conveniently prepared by reducing nickel(II) salt with hydrazine in the presence of the functionalized ionic liquid 1-(3-aminopropyl)-2,3-dimethylimidazolium bromide. UV/Vis spectros-copy, elemental analysis, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) show the presence of a weak interaction of the functionalized ionic liquid with Ni¹¹ and Ni ⁰ complexes. The face-centered cubic structure of the Ni⁰ NPs was confirmed by X-ray diffraction (XRD) characterization. Transmission electron microscopy (TEM) images reveal that smaller Ni⁰ particles of approximately 6-7 nm average diameter assemble to give larger, blackberry-shaped particles with an average diameter of around 35 nm. The Ni NPs were employed as highly efficient catalysts for the selective hydrogenation of C=C double bonds in the aqueous phase under mild reaction conditions (40-90°C at 1.0- 3.0 MPa), and the Ni⁰ nanocatalysts in the aqueous phase are stable enough to be reused at least seven times without significant loss of catalytic activity during subsequent reuse cycles. 2010 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim.

Full text of DOI:10.1002/asia.200900628

FULL PAPERS
DOI: 10.1002/asia.200900628
Ionic Liquid Immobilized Nickel(0) Nanoparticles as Stable and Highly
Efficient Catalysts for Selective Hydrogenation in the Aqueous Phase
[
a]
[a]
[a]
[b]
[a]
[a]
Yu Hu, Yinyin Yu, Zhenshan Hou,* Hanming Yang, Bo Feng, Huan Li,
[a]
[
a]
[a]
[a]
[a]
Yunxiang Qiao, Xiangrui Wang, Li Hua, Zhenyan Pan, and Xiuge Zhao
0
Abstract: Nickel nanoparticles (NPs)
well-dispersed in the aqueous phase
were conveniently prepared by reduc-
ing nickel(II) salt with hydrazine in the
presence of the functionalized ionic
liquid 1-(3-aminopropyl)-2,3-dimethyli-
midazolium bromide. UV/Vis spectros-
copy, elemental analysis, thermogravi-
metric analysis (TGA), and X-ray pho-
toelectron spectroscopy (XPS) show
the presence of a weak interaction of
and Ni complexes. The face-centered
give larger, blackberry-shaped particles
with an average diameter of around
35 nm. The Ni NPs were employed as
highly efficient catalysts for the selec-
tive hydrogenation of C=C double
bonds in the aqueous phase under mild
reaction conditions (40–908C at 1.0–
0
cubic structure of the Ni NPs was con-
firmed by X-ray diffraction (XRD)
characterization. Transmission electron
microscopy (TEM) images reveal that
smaller Ni particles of approximately
6–7 nm average diameter assemble to
0
0
3.0 MPa), and the Ni nanocatalysts in
the aqueous phase are stable enough to
be reused at least seven times without
significant loss of catalytic activity
during subsequent reuse cycles.
Keywords: heterogeneous catalysis ·
hydrogenation · ionic liquids · nano-
particles · nickel
II
the functionalized ionic liquid with Ni
Introduction
cross-coupling reactions, Diels–Alder reactions, and others
have been performed with high reaction rates and selectivi-
[3]
Water is an attractive alternative to traditional organic sol-
vents because it is inexpensive, nonflammable, nontoxic, and
ties in aqueous media. Moreover, the use of water as the
reaction solvent has several benefits, including low cost, im-
proved safety, and abundance. Furthermore, because most
organic compounds are lipophilic, they can be easily extract-
ed from the aqueous phase, avoiding the decomposition and
progressive deactivation during the lifetime of the catalyst
with usual thermal operations. Therefore, in aqueous–organ-
ic biphasic catalysis, the properties of water-soluble catalysts
are very crucial, for example, ionic derivatization, covalent
attachment of hydrophilic auxiliaries, and the addition of co-
[1]
environmentally sustainable. Biphasic catalysis based on
two immiscible liquid phases, water and organic phase, has
[2]
attracted increasing attention. In particular, the immobili-
zation of a catalyst in an aqueous solution immiscible with
the product phase (organic phase) represents an almost
ideal combination of homogeneous and heterogeneous reac-
tion processes. Many of the fundamental and useful reac-
tions such as hydroformylation, hydrogenation, oxidation,
[4]
solvents or surfactants.
Recently, the application of NPs has also attracted much
interest, probably because the small size of the metal NPs
leads to high surface/volume ratios, and consequently high
numbers of potential active sites are available to the sub-
strates, which result in significant enhancement of their cata-
[
a] Y. Hu, Y. Yu, Prof. Dr. Z. Hou, B. Feng, H. Li, Y. Qiao, X. Wang,
L. Hua, Z. Pan, X. Zhao
Key Laboratory for Advanced Materials
Research Institute of Industrial Catalysis
East China University of Science and Technology
Shanghai, 200237 (China)
[5]
Fax : (+86)21-64253372
E-mail: houzhenshan@ecust.edu.cn
lytic activity. However, most studies have focused on the
preparation and application of noble-metal nanocatalysts
(e.g., Pt, Pd), including those water-soluble noble-metal NPs
that were synthesized by using thiol, ether, carboxylic acid,
amino, and hydroxy functionalized ionic liquids as stabiliz-
[
b] Prof. Dr. H. Yang
Key Laboratory of Catalysis and Materials Science of the State
Ethnic Affairs Commission & Ministry of Education
South-Central University for Nationalities
Wuhan, Hubei Province, 430074 (China)
[6–10]
ing reagents.
For practical applications, easily available
and non-precious-metal catalysts such as nickel nanocata-
lysts need to be developed. There have been some reports
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900628.
1178
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1178 – 1184

[
11]
about the synthesis and characterization of the conducting
and magnetic properties of Ni NPs, as well as their applica-
To avoid the formation of surface-coordinating species, hy-
drazine was chosen as a clean reducing agent instead of
[
12]
tions in catalysis, especially in catalytic hydrogenation, de-
NaBH . In addition, the reducing process was much safer
4
hydrogenation, CÀC coupling and oxidative coupling reac-
and convenient than that using molecular hydrogen as the
[13]
tions. However, most of the above reactions were carried
out in ecologically unfavorable organic solvents. Therefore,
the development of much more economic, greener, and sus-
tainable pathways for organic transformations has become
the goal of many scientists. Wangelinꢁs group has utilized
the magnetic properties of Raney–Nickel catalyst for the se-
lective hydrogenation of heteroarenes in ethanol, and the Ni
catalyst could adhere to the magnetic stir bar after reaction
reducing agent. If IL-NH was not protected in the C2 posi-
2
tion, the color of the solution under reduction conditions
changed to purple from green, and even reduction over-
night, the complex showed no reactivity towards hydrogena-
tion in the aqueous phase. We proposed that a stable Ni-N-
heterocyclic carbene species was very likely to be formed
[19]
under the present reduction conditions. Thus, all the imi-
dazolium ionic liquids used herein were intentionally pro-
tected in the C2 position by a methyl group to suppress the
possible formation of Ni-N-heterocyclic carbenes. Aqueous
[14]
and thus facilitate the recycling of the catalyst. Recently,
supercritical carbon dioxide (scCO ) and ionic liquid as al-
2
ternative green media have also been employed in catalytic
solutions of IL-NH -Ni, Ni ACHNUTGTRENNUGN( OAc) , IL-NH , and Ni ACHTUNGTRENNUNG( OAc) -
2 2 2 2
[15,16]
hydrogenation by using a nickel(0) catalyst.
However,
IL-NH were examined by UV/Vis spectroscopy, and the as-
2
to the best of our knowledge, there are few examples of cat-
alytic hydrogenation by using green and low-cost aqueous
phase as reaction media over Ni nanocatalysts, probably be-
prepared IL-NH -Ni was also characterized by elemental
2
analysis, X-ray diffraction (XRD), thermogravimetric analy-
sis (TGA), transmission electron microscopy (TEM), and X-
ray photoelectron spectroscopy (XPS) analyses.
0
cause of their instability in the aqueous phase under reac-
[17]
tion conditions.
On the basis of our previous investiga-
The UV/Vis spectra of IL-NH -Ni, Ni ACHTUNRTGENNUNG( OAc) , IL-NH ,
2 2 2
tions on the hydrogenation with ionic liquid-stabilizing
and Ni ACHTUNGTENRUNG( OAc) -IL-NH in water showed remarkable differen-
2 2
[
18]
metal nanocatalysts, we attempted in this work to use the
hydrophilic, easily prepared amino-functionalized ionic
liquid 1-(3-aminopropyl)-2,3-dimethyl-imidazolium bromide
ces in the intensity and wavelength in the region 300–
800 nm (Figure 1). There was no obvious absorption peak
for IL-NH and IL-NH -Ni in this region (Figure 1a,c). The
2
2
(
IL-NH ) to stabilize water-soluble nickel NPs (IL-NH -Ni)
2 2
for selective hydrogenation of various substrates.
Results and Discussion
0
The Preparation and Characterization of Ni Nanocatalysts
The synthetic route of the functionalized ionic liquid IL-
NH stabilized Ni NPs (IL-NH -Ni) is shown in Scheme 1.
2
2
Figure 1. UV/Vis spectra of a) IL-NH
2 2 2
-Ni, b) Ni AHCTUNGERTUNNNG( OAc) , c) IL-NH , and
d) Ni(OAc) -IL-NH complexes in water.
A
H
U
G
R
N
N
2
2
UV/Vis spectrum of Ni
to that of parent Ni(OAc) in terms of the number of bands
and transition energies (Figure 1b,d), although the adsorp-
tion peak of Ni(OAc) at 395 and 722 nm exhibited a hypso-
ACHTUNGTNRENUNG( OAc) -IL-NH complex was similar
2 2
AHCTUNGTRENNUNG
2
AHCTUNGTRENNUNG
2
chromic shift around 24 and 115 nm after the addition of
Scheme 1. Functionalized ionic liquid stabilized nickel NPs (IL-NH
2
-Ni).
functionalized ionic liquid IL-NH , which implies the occur-
2
rence of electron transfer between the amine group of the
[20]
functionalized ionic liquid and Ni ACHTUNGRETNN(UNG OAc) .
2
0
The XRD pattern (Figure 2) of the isolated Ni particles
0
Abstract in Chinese:
(IL-NH -Ni) confirmed the presence of crystalline Ni and
2
the absence of NiO or Ni(OH) . The most representative re-
2
0
flections of Ni were indexed as a face-centered cubic (fcc)
structure with a unit-cell parameter a=0.35251 nm. The
Bragg reflections at 44.488, 51.828, and 76.428 correspond to
0
the indexed planes of the crystals of Ni (111), (200), (220).
0
The mean diameter of Ni NPs was estimated by the Scher-
Chem. Asian J. 2010, 5, 1178 – 1184
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1179

Z. Hou et al.
FULL PAPERS
2
Figure 4. XPS spectrum of the separated nickel nanocatalyst IL-NH -Ni
showing the Ni 2p and N 1s regions (inset) and the fitting results.
0
Figure 2. X-ray diffraction pattern of the isolated Ni NPs before hydro-
genation. The peaks are labeled with the hkl indices of the planes for the
corresponding Bragg angles.
NPs even when the isolation procedure was repeated several
times and implied that the electron clouds tended to transfer
from the Ni surface to IL-NH . The presence of IL-NH on
2
2
rer equation to be 6.0 nm, which is close to that determined
by TEM analysis (see below).
the surface of Ni nanoparticles was also verified by the ele-
mental analysis (C,10.27, H 1.95, N 4.40, Ni 72.53%), as
An aqueous dispersion of fresh IL-NH -Ni was placed as
well as from the TGA curve of IL-NH -Ni. As shown in Fig-
2
2
a thin film on a carbon-coated copper grid and characterized
by TEM analysis. The micrographs displayed a blackberry-
like morphology with good dispersion in aqueous phase, and
each particle with an average size of 35 nm was an aggregate
of smaller particles, which might result from the magnetic
ure 2S (see the Supporting Information), the Ni NPs began
to degrade (5% TGA loss) with elevation of temperature at
1878C and finally lost approximately 24.9% weight within
the temperature range studied (40–7008C).
As also shown in Figure 4, the 840–890 eV region that
comprised the doublet of Ni 2p1/2 and 2p3/2 transitions was
analyzed. The binding energies corresponded to 873.2 and
855.5 eV, respectively. The Ni 2pl/2 emission lines were found
at 17.7 eV higher binding energy than the Ni 2p3/2 line. For
both Ni 2pl/2 and Ni 2p3/2 emission lines, two shake-up satel-
lites can be seen at approximately 5.8 eV higher binding
[12d]
interaction between Ni NPs.
The mean diameter of the
smaller particles was estimated to be 6–7 nm from ensem-
bles of 200 particles found in an arbitrarily chosen area of
0
the enlarged micrograph (Figure 3). However, when the Ni
catalyst was prepared in the absence of IL-NH (blank-Ni),
2
the bulky Ni particles adhered completely to the magnetic
stir bar in aqueous phase without stirring and they could not
be redispersed in water even after sonicating for a long
time.
energy. The presence of NiO or Ni(OH) was highly unlikely
2
because the position of the Ni 2p line in the oxidic cata-
3
/2
[23]
lysts was found at 856.1–856.4 eV. However, a significant
percentage of metallic Ni species (BE(2p ) 852.5 eV) was
AHCTUNGTRENNUNG
3
/2
not present on the NP surface. Both Zarbin’s and Zanchet’s
groups observed similar results in colloidal Ni NPs produced
[24,25]
in a different way.
As the XRD data and TEM results
(
Figures 2 and 3) did not indicate the presence of nickel
oxide or hydroxide particles or a core–shell structure (metal
oxide), the XPS results indicated the existence of a few
2
+
atomic layers of Ni species on surface of the NPs.
Catalytic Hydrogenation in Water and Ionic Liquid Phase
Figure 3. a) TEM image and b) histogram showing the particle size distri-
bution of IL-NH -Ni in water.
2
over IL-NH -Ni Catalyst
2
No hydrogenation reaction occurred when Ni ACHTUNGETRNN(UNG OAc) was
2
used as the catalytic species with or without IL-NH2 in
II
The surface characteristics of IL-NH -Ni were investigat-
water, which indicated the reduction of Ni species is very
2
ed by XPS (Figure 4). It has been reported that the N 1s
peaks of the nitrogen atom in the amino group and imida-
zole ring appear at about 398.8 and 401.65 eV, respective-
crucial for the catalytic hydrogenation. From the UV/Vis
0
and XPS results, we propose that the Ni surface composi-
2
+
tion is probably dominated by adsorbed Ni -IL-NH com-
2
II
[
21,22]
ly.
As a result, the spectrum of N 1s of IL-NH -Ni fitted
plexes in the present system. The surface layer of Ni com-
plexes could hamper further bulk oxidation of Ni particles
2
0
with two curves at about 398 and 400 eV indicated that a
[16,26]
residue of the IL-NH remained on the surface of the Ni
by air,
and possibly could also protect the NPs from ag-
2
1180
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1178 – 1184

Ionic Liquid Immobilized Nickel(0) Nanoparticles
gregation under the reaction conditions, which leads to good
dispersion and high stability of the nanocatalysts in the
aqueous phase. The hydrogenation of 4-phenyl-3-buten-2-
one was chosen as a model reaction because both of the C=
C and C=O double bonds are present in the molecule,
which allow us to investigate the catalytic activity and the
selectivity of hydrogenation conveniently over IL-NH -Ni
2
catalyst in different media (Table 1). Under the present con-
Table 1. Hydrogenation of 4-phenyl-3-buten-2-one in water and ionic liq-
[
a]
uids.
Figure 5. Conversion of 4-phenyl-3-buten-2-one conversion (solid) and se-
lectivity (open) with blank-Ni (circles) and IL-NH
time. Reaction conditions: Ni (27 mmolL atomic nickel), substrate/Ni=
2
-Ni (squares) with
À1
[
b]
Entry
Catalysts
Solvents
Conv. [%]
Sel. [%]
5
2 2
0:1 (molar ratio), H O (2 mL), H (3.0 MPa), 908C.
1
2
3
4
5
6
IL-NH
IL-NH
IL-NH
IL-NH
blank-Ni
blank-Ni
2
2
2
2
-Ni
-Ni
-Ni
-Ni
H
2
O
92.3
<5
<5
<5
53.3
<5
93.4
–
–
–
93.0
–
A
H
U
T
N
U
G
6
A
T
N
R
N
U
G
4
tionalized ionic liquid are explained mainly by the effects of
the functionalized ionic liquid in controlling the size and
high dispersion of Ni NPs in aqueous phase.
A
H
N
T
E
U
3
SO
3
H
2
O
AHCTUNGTNERUNG[ BMMIM]PF
6
À1
[
5
a] Reaction conditions: Ni (27 mmolL atomic nickel), substrate/Ni=
0:1 (molar ratio), solvent (2 mL), H (3.0 MPa), 908C, 10 min. [b] Selec-
2
Hydrogenation of C=C Double Bonds of Various
tivity. The by-product was mainly 4-phenyl-2-butanol.
Functionalized Alkenes using IL-NH -Ni
2
The scope of the substrates was established for a wide range
of functionalized alkenes and in most cases the reaction was
completed within 9 h under 1.0–3.0 MPa hydrogen pressure
at 908C with excellent conversion and selectivity for the hy-
drogenation of C=C double bonds. As shown in Table 2, cy-
clohexene was totally converted into cyclohexane over IL-
ditions, the main product was 4-phenyl-2-butanone, and a
minor by-product 4-phenyl-2-butanol was also identified. To
our surprise, when water media was replaced by ionic liq-
uids, both IL-NH -Ni and blank-Ni catalysts showed almost
2
no activity for hydrogenation of 4-phenyl-3-buten-2-one
under the same reaction conditions (Table 1, entries 1–6).
When citral was chosen as the testing substrate, a similar in-
fluence was also observed (result not shown). We still
cannot give a confirmed explanation for this result. Some of
NH -Ni catalyst under 1.0 MPa hydrogen pressure in 9 h,
2
which is much higher activity than that of the control experi-
ment with blank-Ni as a catalyst (Table 2, entry 1). The IL-
NH -Ni catalyst was also highly active even in the presence
2
0
the reasons might be the lower solubility of the Ni catalyst
of hydroxy group in the functionalized alkenes (Table 2, en-
tries 2 and 3). Substituting the two H groups in allyl alcohol
(1b) by two methyl groups as in 3-methyl-2-buten-1-ol (1c)
near the C=C double bond resulted in lower conversion be-
cause of steric hindrance, which inhabited access of the mo-
lecular substrate to the catalytic center. However, increasing
the pressure of hydrogen from 1.0 to 3.0 MPa improved the
or slower mass-transfer process in ionic liquids compared
[27]
with that in the water phase. This was shown by the obser-
vation that IL-NH -Ni catalyst is highly hydrophilic and thus
2
well-dispersed in aqueous phase under stirring during cata-
lytic recycles, but could not be dispersed homogeneously in
the ionic liquids used herein even if the mixture was stirred
or sonicated for a long time.
reactivity significantly (Table 2, entry 3). The IL-NH -Ni cat-
2
Encouraged by the high activity of hydrogenation over
alyst also showed excellent activity and selectivity towards
hydrogenation of styrene (1d) or ethyl acrylate (1e), and no
products of hydrogenation of benzene ring or hydrogenoly-
sis by-products were detected (Table 2, entries 4 and 5).
Also, a, b-unsaturated ketones were comparatively easier to
be reduced with almost quantitative conversion and good se-
lectivity with short reaction times (Table 2, entries 6 and 7).
Even citral (1h), which is generally difficult to hydrogenate
selectively, could be efficiently reduced under 3.0 MPa hy-
drogen pressure at 708C with satisfactory selectivity of
94.4% (Table 2, entry 8).
the present IL-NH -Ni catalyst, we investigated the reaction
2
kinetic in the aqueous phase. The kinetic curves of blank-Ni
and IL-NH -Ni catalysts for selective hydrogenation of 4-
2
phenyl-3-buten-2-one were compared in the aqueous phase
Figure 5). With IL-NH -Ni catalyst, after only 20 min,
2
quantitative conversion with 88.5% selectivity towards 4-
phenyl-2-butanone was observed. However, the reaction
needed 1 h for completion with blank-Ni catalyst, which dis-
(
played a similar selectivity to that of IL-NH -Ni. Thus,
2
modification of the functionalized ionic liquid (IL-NH )
2
0
with Ni particles played a very important role in improving
the catalytic activity. On the basis of the TEM results
(
Figure 5), we propose that the promoting roles of the func-
Chem. Asian J. 2010, 5, 1178 – 1184
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1181

Z. Hou et al.
FULL PAPERS
[
a]
Table 2. Hydrogenation with IL-NH
2
-Ni in water.
after the catalyst was recycled
seven times. The mean diame-
ter of Ni NPs was estimated by
0
the Scherrer equation to be
[
b]
Entry
1
Substrate
t [h]
Product
Conv. [%]
100
Sel. [%]
6
.5 nm, which is slight larger
cyclohexene (1a)
9
cyclohexane (2a)
-
-
than that of the freshly pre-
pared catalyst, which might be
the cause of the slight decrease
in activity. When Raney Ni was
used as a catalyst for the selec-
tive hydrogenation, it was very
active in the first run; however,
it suffered 59.9% deactivation
in the aqueous phase through-
out only three continuous hy-
drogenation recycling steps, and
was almost inactive after five
recycling steps, possibly because
the micropores became jammed
by the resulting reaction mix-
ture during subsequent reuse
[c]
5
0
2
3
2-propen-1-ol (1b)
3-methyl-2-buten-1-ol (1c)
9
9
1-propanol (2b)
3-methyl-1-butanol (2c)
100
100
100
100
100
100
99.0
1.0
97.0
3.0
94.4
5.4
52.3
[
d]
9
2.8
4
5
6
styrene (1d)
ethyl acrylate (1e)
cyclohex-2-enone (1 f)
9
9
4
ethylbenzene (2d)
100
100
100
ethyl propionate (2e)
cyclohexanone (2 f)
cyclohexanol (3 f)
[
[
e]
f]
7
8
4-phenyl-3-buten-2-one (1g)
citral (1h)
4
3
4-phenyl-2-butanone (2g)
96.0
89.6
4-phenyl-2-butanol (3g)
citronellal (2h)
citronellol (3h)
À1
[
a] Reaction conditions: Ni (27 mmolL atomic nickel), substrate/Ni=200:1 (molar ratio), H
water (2 mL), 908C; unless otherwise stated. [b] Selectivity. [c] Blank-Ni as catalyst. [d] H (3.0 MPa). [e] Sub-
strate/Ni=50:1, H (3.0MPa), 408C. [f] Substrate/Ni=50:1, H (3.0 MPa), 708C, 0.2% of the substrate convert-
ed into an unidentified product.
2
(1.0 MPa),
2
2
2
[28]
cycles. These results indicated
Recyclability of IL-NH -Ni
2
that the IL-NH -Ni catalyst was very stable and active in the
2
The lifetime of the catalyst and its level of reusability are
very important factors for practical applications of catalytic
systems. To clarify this issue, we also investigated the recy-
aqueous phase, and kept their higher hydrogenation reactiv-
ity and selectivity during catalytic reuse cycles than with the
conventional Raney Ni system.
clability of the IL-NH -Ni catalyst and compared it with
2
that of Raney Ni in the hydrogenation of 4-phenyl-3-buten-
2-one in aqueous phase (Figure 6). The products were ex-
Conclusions
tracted three times by liquid–liquid extraction with diethyl
ether after the reaction. Diethyl ether in the catalytic aque-
ous phase was evaporated under vacuum, and the substrate
was charged into the autoclave for the next recycling step. It
was found that 85% conversion of 4-phenyl-3-buten-2-one
could be obtained over seven reuse cycles, and the selectivi-
ty remained almost constant at around 96%. Furthermore,
In summary, very stable and highly dispersed colloidal Ni
NPs were prepared in the aqueous phase by using amino-
functionalized ionic liquid as a stabilizer. Elemental analysis,
UV/Vis spectroscopy, XRD analysis, TGA, and XPS analy-
ses verified that the NH functionalized ionic liquid was ad-
2
0
sorbed on the surface of the Ni NPs and protected them
0
the Ni NPs were found to be stable—no formation of NiO
from being oxidized. TEM analysis revealed that the as-pre-
0
or Ni(OH) was observed by XRD spectral analysis (Fig-
pared Ni colloidal NPs were aggregates of 6–7 nm nanopar-
2
ure 1S)—and still well-dispersed in the aqueous phase even
ticles, which were well-dispersed in the aqueous phase,
which led to high reactivity and selectivity for catalytic hy-
drogenation of C=C bonds of various functionalized alkenes.
Furthermore, our designed catalytic system could also be
reused several times with little loss of catalytic activity and
showed much better catalytic performance than the conven-
tional Raney Ni catalyst.
Experimental Section
All manipulations involving air-sensitive materials were carried out using
standard Schlenk line techniques under an atmosphere of nitrogen. All
solvents were reagent grade and were distilled from appropriate drying
agents under a nitrogen atmosphere prior to use. Nickel acetate tetrahy-
drate (ꢀ98.0%) and aqueous hydrazine (80 wt%) were purchased from
Sinopharm Chemical Reagent Co. Ltd and used as received. The ionic
Figure 6. Recyclability of IL-NH
2
-Ni and Raney Ni catalysts for selective
liquids [BMMIM]
A
H
U
G
R
N
N
[PF
6
], [BMMIM]BF
4
,
3 3
and [BMMIM]CH SO
hydrogenation of 4-phenyl-3-buten-2-one. Reaction conditions: Ni
(BMMIM=1-butyl-2,3-dimethylimidazolium) were synthesized according
À1
[29]
(
27 mmolL
2 mL), H
atomic nickel), substrate/Ni=50:1 (molar ratio),
(3.0 MPa), 408C, 4 h.
H
2
O
the previous procedures.
X-ray diffraction (XRD) analysis was per-
(
2
formed on a D/MAX 2550 VB/PC instrument using a graphite crystal as
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Chem. Asian J. 2010, 5, 1178 – 1184

Ionic Liquid Immobilized Nickel(0) Nanoparticles
monochromator. TEM analysis was performed on a JEOL JEM 2010
transmission electron microscope operating at 200 kV with nominal reso-
lution of 0.25 nm. Elemental analyses for C, H, and N were performed
on an Elementar Vario EI III Elementa analyzer, and analysis of the Ni
Acknowledgements
We are grateful for support from the National Natural Science Founda-
tion of China (No. 20773037), ECUST (No. YJ0142136), and the Com-
mission of Science and Technology of Shanghai Municipality (No.
2
content of IL-NH -Ni was performed on a Varian ICP-710ES ICP–AES
instrument. A Perkin–Elmer Pyris Diamond instrument was used for
thermogravimetric analysis. TGA measurement of the nickel NPs was
0
7J14023), China. We would also like to thank Prof. Walter Leitner for
helpful suggestions and Dr. Nils Theyssen for assistance with the experi-
ments.
À1
performed under high-purity nitrogen purge (100 mLmin ) by heating
À1
the sample to 7008C at a rate of 108C min . The standard material used
for calibrating the equipment was a-sapphire powder (Perkin–Elmer).
The reference and sample pans were made of platinum. UV/Vis spectra
were recorded on a Varian Cary 500 spectrophotometer. X-ray photo-
[
[
[
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a
Thermo ESCALAB 250 spectrometer. Nonmonochromatic AlKa radia-
tion was used as a primary excitation. The binding energies were calibrat-
ed with the C1s level of adventitious carbon (284.8 eV) as the internal
standard reference.
Synthesis of the functionalized ionic liquid IL-NH
following a previous report.
0
2
: IL-NH
Briefly, 1,2-dimethylimidazolium (5.28 g,
.055 mol) and dry ethanol (30 mL) were added into a two-necked flask
2
was prepared
[
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4 h. After this time the liquid was separated and the remaining solid
was washed three times with ethanol. Drying under vacuum gave 1-ami-
nopropyl-2,3-dimethylimidazolium bromide. NaOH (1 equiv) was then
added to the solution of 1-aminopropyl-2,3-dimethylimidazolium bro-
mide in dry methanol (25 mL), and the mixture was stirred for 1 h. After
this time, dry dichloromethane (15 mL) was added, and the mixture was
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the solvents were removed from the filtrate under reduced pressure, and
1
636; e) C. A. Fleckenstein, H. Plenio, Green Chem. 2007, 9, 1287;
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7
[
[
1
the resulting compound was washed with dry ethyl acetate several times
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1
and then dried at 508C for 4 h to give the product IL-NH
2
.
H NMR
(
D
2
O, 500 MHz): d=2.13 (m), 2.58 (m), 3.77 (s), 4.20 (t), 7.32 (s),
.39 ppm (s).
Preparation of Ni catalyst: Ionic liquid stabilized nickel NPs were typi-
cally prepared by dissolving Ni(OAc) ·4H O (0.132 g, 0.53 mmol) in etha-
nol (20 mL), followed by the addition of IL-NH in methanol (2 mL,
.496 g, 2.12 mmol). During addition, the color of the resulting solution
instantly changed from light green to dark blue, indicating electron inter-
action between nickel(II) acetate and IL-NH ligand, as verified by the
UV/Vis spectra. The mixture was stirred for an additional 4 h at room
temperature, and then NaOH (0.106 g) and N ·H O (1 mL) were
7
0
A
H
U
G
R
N
N
2
2
2
0
2
2
H
4
2
added into the mixture successively at 908C. A dark brownish solution
was afforded after 10 min, from which a black powder was isolated by
centrifugation (4000 rpm, 10 min, three times), washed with methanol
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(
3ꢂ10 mL), and dried under reduced pressure. The structure, morpholo-
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gy, and surface state of the as-prepared isolated Ni powder (IL-NH -Ni)
2
were analyzed by XRD analysis, TGA, TEM, and XPS. In a control ex-
periment, unmodified Ni catalyst (blank-Ni) was prepared in a similar
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0
2
way except for the absence of IL-NH . Raney Ni catalyst was prepared
[
31]
according to a published procedure.
General procedure of hydrogenation: A stainless steel autoclave contain-
ing previously prepared catalyst IL-NH -Ni (0.053 mmol) redispersed in
2
water (2 mL) was charged with the appropriate amount of substrate (50
or 200 equiv), and then hydrogen was admitted into the system to the de-
sired pressure. The reaction was conducted by stirring the mixture for a
desired time at the set temperature. After the reaction was complete, the
autoclave was cooled with an ice bath, and then hydrogen was carefully
vented. The product was separated from the catalyst by extraction with
diethyl ether and identified by GC–MS and quantitatively analyzed with
a GC 112A with a FID detector and a HP-5 column (30 m, 0.25 mm in-
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Received: November 10, 2009
Published online: March 25, 2010
1184
www.chemasianj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1178 – 1184