Highly Selective Hydrogenation with Ionic Liquid Stabilized Nickel Nanoparticles

Article abstract of DOI:10.1007/s10562-018-2361-0

Abstract: Nickel nanoparticles (Ni NPs) were conveniently synthesized from the reduction of nickel(II) salt with NaBH₄ or hydrazine in the presence of the ionic liquid 1-butyl-2,3-dimethylimidazolium (S)-2-pyrrolidinecarboxylic acid salt. UV/Vis spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy were employed to characterize the interaction between the metal and the ionic liquid. The face-centered cubic structure of the Ni NPs(0) was confirmed by X-ray diffraction characterization. Transmission electron microscopy images revealed well-dispersed Ni particles of approximately 5.1?nm in average diameter. The ionic liquid immobilized Ni NPs were employed as highly efficient catalysts in chemoselective hydrogenation of quinoline and relevant compounds, as well as aromatic nitro compounds under mild reaction conditions. The Ni NPs can be efficiently recovered and reused. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-018-2361-0

Catalysis Letters
https://doi.org/10.1007/s10562-018-2361-0
Highly Selective Hydrogenation with Ionic Liquid Stabilized Nickel
Nanoparticles
1
1
1
He‑yan Jiang · Si‑shi Zhang · Bin Sun
Received: 20 January 2018 / Accepted: 13 March 2018
Springer Science+Business Media, LLC, part of Springer Nature 2018
©
Abstract
Nickel nanoparticles (Ni NPs) were conveniently synthesized from the reduction of nickel(II) salt with NaBH or hydrazine
4
in the presence of the ionic liquid 1-butyl-2,3-dimethylimidazolium (S)-2-pyrrolidinecarboxylic acid salt. UV/Vis spec-
troscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy were employed to characterize the interaction
between the metal and the ionic liquid. The face-centered cubic structure of the Ni NPs(0) was confirmed by X-ray diffraction
characterization. Transmission electron microscopy images revealed well-dispersed Ni particles of approximately 5.1 nm
in average diameter. The ionic liquid immobilized Ni NPs were employed as highly efficient catalysts in chemoselective
hydrogenation of quinoline and relevant compounds, as well as aromatic nitro compounds under mild reaction conditions.
The Ni NPs can be efficiently recovered and reused.
Graphical Abstract
R1
R2
R1
R2
Ni nanoparticles catalyst
o
N
H
2
, EtOH, 75 C
N
H
high yields
high regioselectivity
high reusability
mild conditions
Keywords Chemoselective hydrogenation · Nanoparticles · Ionic liquids · Quinoline · Nickel
1
Introduction
unless they are stabilized. Stabilizing species, such as poly-
mers, solid supports or ionic liquids with specific stabiliz-
ing groups, can adhere to the metal surface through either
electrostatic, van der Waals or covalent interactions, which
provide stability to the nanoparticles through electrostatic
or steric repulsion between neighbouring particles [6–10].
By now, ionic liquid-stabilized metal nanoparticles were
assessed as catalytic materials, mostly in olefins or arenes
hydrogenation [11–16].
Metal nanoparticles have received boost interest in recent
years, which stems from their application in such fields as
medicine, sensing, and catalysis [1–5]. Metal nanoparticles
are thermodynamically not stable due to their high excess
surface energy, therefore they have a tendency to aggregate
*
He-yan Jiang
1,2,3,4-Tetrahydroquinoline as well as relevant com-
pounds are crucial structure that exists in various pharma-
cologically related therapeutic agents and biologically active
natural products [17–24]. Straightforward hydrogenation of
quinoline and relevant compounds is regarded as the most
economical way to access 1,2,3,4-tetrahydroquinoline and
analogous compounds [17–24]. Both homogeneous and
orgjiang@163.com
1
Key Laboratory of Catalysis Science and Technology
of Chongqing Education Commission, Chongqing Key
Laboratory of Catalysis and New Environmental Materials,
College of Environmental and Resources, Chongqing
Technology and Business University, Chongqing 400067,
China
Vol.:(0
1
123456789)
3

H. Jiang et al.
heterogeneous catalytic reactions have been widely investi-
gated in the chemoselective hydrogenation. However, these
studies were generally focused on nobel-metal catalytic
process on the basis of Pd, Rh, Ru, Pt or Ir etc. [17–24],
and the reaction temperatures were always high (>80 °C).
The development of easily available and relatively abun-
dant-metal catalysts is of high importance from the view-
point of practical use. In this work, we originally report the
highly chemoselective catalytic hydrogenation of quinoline
as well as relevant compounds with ionic liquid (1-butyl-
spectra were recorded on a Varian Cary 500 spectrophotom-
eter. A Perkin-Elmer Pyris Diamond was used in the current
study for the thermogravimetric analysis (TGA). TGA analy-
ses of the separated Ni catalysts were carried out with high-
−
1
purity nitrogen purge (100 mL min ). X-ray diffraction
(XRD) measurement was performed in a D/MAX 2550 VB/
PC with a graphite crystal as monochromator. The transmis-
sion electron microscopy (TEM) analyses were performed in
a JEOL JEM 2010 transmission electron microscope operat-
ing at 200 kV with nominal resolution of 0.25 nm.
2
,3-dimethylimidazolium (S)-2-pyrrolidinecarboxylic acid
salt ([BMIM][Pro])) stabilized nickel nanoparticles (Ni NPs)
under mild conditions.
2.2 Synthesis of Nickel Nanoparticles
Nanocatalysts 1, 3 or 5 (Scheme 1) preparation: typically, Ni
NPs stabilized by ionic liquids were synthesized by dissolv-
ing Ni(OAc) ·4H O (0.132 g, 0.53 mmol) in water (30 mL),
2
Experimental
2
2
then ionic liquids (1.06 mmol) were introduced. The solu-
tion color changed from light green to blue in the period of
ionic liquids addition. The color change is a strong elec-
tron transfer indication between nickel(II) acetate and ionic
2
.1 Materials
All manipulations were performed using standard Schlenk
line techniques under N . Nickel acetate tetrahydrate was
2
from Aldrich. Other reagents were analytical grade. H
2
purity was over 99.99%. Ionic liquids were prepared in
Footnote 1 (continued)
accordance with published procedure [25, 26]. [BMIM]
3
H), 1.53–1.60 (m, 1H), 1.93–1.99 (m, 1H), 2.79–2.86 (m, 2H),
1
[
OAc]: HNMR (300M, CDCl ), δ 0.93–1.29 (m, 5H),
3.39–3.48 (m, 1H), 3.60 (s, 1H), 6.45–6.51 (m, 2H), 6.93–7.00 (m,
3
13
2
H). C NMR, δ 20.5, 26.9, 31.3, 49.5, 115.8, 119.1, 121.6, 126.9,
1
2
3
.69–1.85(m, 5H), 2.73(s, 3H), 3.90(s, 3H), 4.19–4.35 (m,
+
1
3
129.8, 145.3. HRMS: calcd. for C10H13N [M+H] 148.1126, found
H), 7.99–8.27 (m, 2H). C NMR, δ 10.9, 12.9, 18.5, 24.3,
1
1
48.1127. Ea: HNMR (300M, CDCl ), δ 2.99–3.06 (m, 2H), 3.81–
3
1.5, 35.1, 49.5, 120.9, 123.3, 136.8, 175.6. HRMS: calcd.
13
3
.90 (m, 2H), 7.21–7.36 (m, 4H). C NMR, δ 29.9, 70.6, 109.3,
+
+
+
for C H N 153.1392 [M] , found 153.1393; calcd. for
120.8, 124.6, 126.9, 127.3, 160.3. HRMS: calcd. for C H O [M+H]
9
17
2
8
8
1
−
−
1
21.0653, found 121.0648. Fa: HNMR (300M, CDCl ), δ 3.33–3.45
3
13
C H O 59.0139 [M] , found 59.0136. [BMIM][Pro]:
2
3
2
1
(m, 4H), 7.19–7.29 (m, 4H). C NMR, δ 33.9, 36.6, 122.5, 124.3,
HNMR (300M, CDCl ), δ 1.03–1.25 (m, 5H), 1.54–1.62
3
+
1
1
24.8, 127.5, 139.0, 142.2. HRMS: calcd. for C H S [M+H]
8 8
1
(
m, 2H), 1.69–1.85(m, 4H), 2.73(s, 3H), 2.81–2.97 (m,
37.0425, found 137.0421. Aniline: HNMR (300M, CDCl ), δ
3
1
3
2
H), 3.32 (m, 1H), 3.63 (s, 1H), 3.93(s, 3H), 4.22–4.38
3.55 (S, 2H), 6.60–7.02 (m, 5H). C NMR, δ 115.4, 119.7, 119.9,
+
1
3
1
29.6, 130.0, 148.1. HRMS: calcd. for C H N [M+H] 94.0657,
(
m, 2H), 7.79–7.87 (m, 2H). C NMR, δ 12.6, 25.3, 15.1,
6
7
1
found 94.0651. m-Methylaniline: HNMR (300M, CDCl ), δ 2.52
3
2
1
1
1
9.6, 31.5, 35.5, 44.0, 46.4, 54.9, 61.8, 120.5, 122.7, 135.9,
13
(
S, 3H), 3.46 (S, 2H), 6.47–6.56 (m, 2H), 6.97–7.11 (m, 2H).
C
+
+
74.0. HRMS: calcd. for C H N 153.1392 [M] , found
9
17
2
NMR, δ 21.4, 112.2, 115.9, 119.5, 129.1, 140.1, 146.5. HRMS: calcd.
−
−
+
53.1399; calcd. for C H NO 114.0561 [M] , found
for C H N [M+H] 108.0813, found 108.0810. p-Methylaniline:
5
8
2
7
9
1
HNMR (300M, CDCl ), δ 2.23 (S, 3H), 3.51 (S, 2H), 6.49–6.58
14.0554. Raney Ni was synthesized in terms of literature
3
1
3
(
m, 2H), 6.95–7.05 (m, 2H). C NMR, δ 20.7, 115.2, 115.3, 127.5,
[
27]. Products were analyzed by GC with FID detector and
+
1
29.6, 129.9, 144.9. HRMS: calcd. for C H N [M+H] 108.0813,
7
9
HP-5 column (30 m × 0.25 mm). Hydrogenation products
1
found 108.0818. o-Methoxyaniline: HNMR (300M, CDCl ), δ 3.69
3
1
13
were identified by GC-MS, NMR and HRMS. UV/Vis
(S, 3H), 3.75 (S, 2H), 6.65–6.76 (m, 4H). C NMR, δ 55.2, 110.7,
1
14.9, 118.3, 121.1, 136.6, 147.6. HRMS: calcd. for C H NO
7 9
+ 1
1
1
Aa: HNMR (300M, CDCl ), δ 1.90–1.95 (m, 2H), 2.70–2.76 (m,
[M+H] 124.0762, found 124.0763. p-Methoxyaniline: HNMR
(300M, CDCl3), δ 3.45 (S, 3H), 3.60 (S, 2H), 6.60–6.69 (m, 4H).
3
1
3
2
H), 3.20–3.25 (m, 2H), 3.69 (s, 1H), 6.38–7.10 (m, 4H). C NMR,
1
3
δ 22.2, 26.8, 41.5, 114.9, 117.0, 121.6, 126.0, 129.6, 144.9. HRMS:
C NMR, δ 55.5, 110.6, 115.0, 118.8, 121.1, 136.5, 147.5. HRMS:
+
1
+
calcd. for C H N [M+H] 134.0970, found 134.0966. Ba: HNMR
calcd. for C7H9NO [M+H] 124.0762, found 124.0766. p-Fluoro-
9
11
1
(
300M, CDCl ), δ 1.26 (s, 3H), 1.54–1.65 (m, 1H), 1.90–1.93 (m,
aniline: HNMR (300M, CDCl3), δ 3.69 (S, 2H), 6.45–6.96 (m, 4H).
3
1
3
1
H), 2.75–2.86 (m, 2H), 3.36–3.45 (m, 1H), 3.73 (s, 1H), 6.49–6.58
C NMR, δ 116.8, 117.0, 117.2, 118.0, 146.7, 154.2. HRMS: calcd.
1
3
+
(
m, 2H), 6.96–7.02 (m, 2H). C NMR, δ 22.6, 26.7, 30.3, 47.2,
14.1, 117.0, 121.2, 126.7, 129.2, 144.9. HRMS: calcd. for C H N
for C6H6FN [M+H] 112.0563, found 112.0569. o-Chloroaniline:
1
1
HNMR (300M, CDCl3), δ 3.93 (S, 2H), 6.66–6.72 (m, 2H), 7.02–
1
0
13
3
+
1
13
[
M+H] 148.1126, found 148.1125. Ca: HNMR (300M, CDCl ), δ
7.20 (m, 2H). C NMR, δ 115.8, 115.9, 118.9, 127.5, 129.3, 142.2.
+
1
2
.23 (s, 3H), 1.53–1.63 (m, 1H), 1.87–1.91 (m, 1H), 2.74–2.85 (m,
H), 3.37–3.45 (m, 1H), 3.69 (s, 1H), 6.47–6.54 (m, 2H), 6.95–7.03
HRMS: calcd. for C6H6FN [M+H] 128.0267, found 128.0261.
1
p-Chloroaniline: HNMR (300M, CDCl3), δ 3.60 (S, 2H), 6.55–6.70
13
13
(
m, 2H). C NMR, δ 21.5, 26.8, 30.1, 47.0, 114.3, 117.9, 121.9,
(m, 2H), 7.00–7.16 (m, 2H). C NMR, δ 116.2, 118.0, 122.4, 128.7,
+
+
1
26.6, 129.6, 143.8. HRMS: calcd. for C H N [M+H] 148.1126,
129.0, 145.2. HRMS: calcd. for C6H6FN [M+H] 128.0267, found
1
0
13
1
found 148.1118. Da: HNMR (300M, CDCl ), δ 1.23–1.27 (m,
128.0270.
3
1
3

Highly Selective Hydrogenation with Ionic Liquid Stabilized Nickel Nanoparticles
separated by centrifugation and decantation of the nanopar-
ticles after reaction completion.
O
N
nBu
3
N n
N
N
Bu
N
H
O
CH COO
3
Results and Discussion
[
BMIM] [Pro]
[BMIM] [OAc]
3.1 Characterization of Ni NPs
The synthetic route of the ionic liquids [BMIM][Pro] and
1
-butyl-2,3-dimethylimidazolium acetic acid salt ([BMIM]
Reduction
Ionic liquid
Ni(OAc)2
Ni
[OAc]) stabilized Ni NPs 1–5 is displayed in Scheme 1. For
comparison, both NaBH and hydrazine were utilized as
4
[
BMIM] [Pro]
NaBH4
1
2
3
4
5
reducing agents herein. Ni NPs were prepared through the
[
BMIM] [Pro]
BMIM] [OAc]
NH NH .H O
reduction of Ni(OAc) in the presence of 2.0 equivalents of
2
2
2
2
[
NaBH4
ionic liquids. Additionally, Ni catalyst was also prepared in
[
BMIM] [OAc]
NH NH .H O
the absence of ionic liquid for the control experiment.
2 2 2
no ligand
NaBH4
Ethanol solutions of [BMIM][Pro]-Ni 1, Ni(OAc) ,
2
[
BMIM][Pro], and Ni(OAc) -[BMIM][Pro] were measured
2
by UV/Vis spectroscopy. Ionic liquid-Ni 1–5 were also char-
Scheme 1 Synthesis of ionic liquid stabilized Ni NPs
acterized by TEM, XPS, XRD and TGA.
The UV/Vis spectra of [BMIM][Pro]-Ni 1, Ni(OAc) ,
2
liquids, which was verified by UV/Vis spectra. The solution
was stirred for another 5 h at 25 °C. Sodium borohydride
[BMIM][Pro], and Ni(OAc) -[BMIM][Pro] in ethanol
2
exhibited surprising differences in both intensity and wave-
length in 300–800 nm region (Fig. 1). There has no evident
absorption for [BMIM][Pro]-Ni 1 in this region (Fig. 1a).
(
0.018 g) addition led to the reduction of Ni ions. The color
of the solution immediately turned black after the addition
of sodium borohydride. The solutions were kept at 25 °C
for 1 h to ensure the completion of the reaction. Ni NPs
isolation for TEM, XRD, X-ray photoelectron spectroscopy
The absorption peaks of Ni(OAc) were 394 and 720 nm
2
(Fig. 1b), and the absorption peaks of [BMIM][Pro] were
322 and 631 nm (Fig. 1c). UV/Vis spectra of Ni(OAc) -
2
(
XPS), TGA analysis and catalytic experiments was attained
[BMIM][Pro] complex was comparable to the parent
[BMIM][Pro] according to the transition energy. How-
ever, the absorption peaks of [BMIM][Pro] at 631 nm and
by centrifuging (5000 rpm for 15 min), washing with etha-
nol (3×20 mL) and drying under vacuum. Furthermore, Ni
catalyst was also prepared in the absence of ionic liquid for
the control experiment.
Ni(OAc) at 394 and 720 nm disappeared (Fig. 1, inset), and
2
the absorption peak of [BMIM][Pro] at 322 nm showed an
Nanocatalysts 2 or 4 (Scheme 1) preparation: typically, Ni
NPs stabilized by ionic liquids were synthesized by dissolv-
ing Ni(OAc) ·4H O (0.132 g, 0.53 mmol) in water (30 mL),
obvious hypsochromic shift about 17 nm with the introduc-
tion of Ni(OAc) (Fig. 1d). The disappearance of absorption
2
peaks and the hypsochromic shift imply the occurrence of
2
2
then ionic liquids (1.06 mmol) were introduced. The solu-
tion was stirred for another 5 h at 25 °C, NaOH (0.106 g)
and N H ·H O (1 mL) were introduced into the solution at
electron transfer between [BMIM][Pro] and Ni(OAc) .
2
The Ni NPs were placed on a carbon-coated copper grid
for TEM analyses, which was utilized to characterise the
nanoparticles obtained and check the mean diameter (Fig. 2).
2
4
2
9
0 °C. A dark-brown mixture was obtained in 10 min. Ni
NPs isolation for TEM, XRD, XPS, TGA analysis and cata-
lytic experiments was attained by centrifuging (5000 rpm
for 15 min), washing with ethanol (3×20 mL) and drying
under vacuum.
Generally, Ni NPs reduced by NaBH showed better disper-
4
sion than nanoparticles reduced by hydrazine (Fig. 2, images
1, 2 versus 3, 4). The TEM image of 5 exhibited a trend
toward agglomeration of the particles in the absence of ionic
liquids (Fig. 2, image 5). The TEM image of Ni NPs 1 exhib-
ited regular spherical shape and narrow size distribution,
with the average diameter of 5.1 nm.
2
.3 General Procedure for the Heterogeneous
Chemoselective Hydrogenation
The surface characteristics of Ni NPs 1 were examined
by XPS (Fig. 3). Long XPS spectra of Ni NPs 1 confirmed
the existense of nickel, nitrogen and carbon, which sug-
gested the existense of the [BMIM][Pro] on the surface
of Ni NPs 1. The existense of [BMIM][Pro] in the ligand
sphere of nanoparticles was additionally supported by the
In the stainless steel autoclave, ionic liquid-stabilized Ni
0.053 mmol) and appropriate substrate were dispersed in
(
ethanol (2 mL). Autoclave was sealed and purged with H
2
five times. Reaction timing began after the system heated to
predetermined temperature. Hydrogenation products were
1
3

H. Jiang et al.
TGA curve of [BMIM][Pro]-Ni 1. The degradation of Ni
NPs started with elevation of temperature at about 200 °C
and lost around 36.0% weight at last during the temperature
range 40–800 °C. Nanoparticles prepared by boron hydride
reduction are usually contaminated by B, and herein the B
contamination was also detected in long XPS.
Ni 2p XPS is also displayed in Fig. 3, The region between
8
2
8
40 and 890 eV was analyzed, the doublet transitions of Ni
p
and 2p were observed, and the binding energies are
3/2
1
/2
74.0 and 856.1 eV correspondingly. The binding energy
of Ni 2p was found to be 17.9 eV higher than the binding
l/2
energy of Ni 2p . For emission lines of both Ni 2p and
3
/2
l/2
Ni 2p , two shake-up satellites could be observed at about
3
/2
5
.8 eV higher in binding energy. For the Ni NPs 1 surface, a
significant ratio of metal Ni(0) species (2p binding energy:
3/2
Fig. 1 UV/Vis spectra of (a) 1, (b) Ni(OAc) , (c) [BMIM][Pro], and
852.5 eV [15]) was not detected. However, the XRD data in
2
(
d) Ni(OAc) -[BMIM][Pro] complexes in EtOH
2
Fig. 4 did not demonstrate the existence of nickel oxide or
Fig. 2 TEM images of Ni NPs
, 2, 3, 4 and 5
1
1
3

Highly Selective Hydrogenation with Ionic Liquid Stabilized Nickel Nanoparticles
Fig. 3 XPS spectrum of Ni NPs
1
and Ni 2p in 1
hydroxide particles. Similiar to some previous report [28],
we speculate that Ni nanoparticles herein probably com-
posed of a small cap layer of NiO around a core of Ni metal.
In Fig. 4, the existence of crystalline Ni(0) and the
Raney Ni was employed as the catalyst for the chemoselec-
tive hydrogenation, low efficiency toward 1,2,3,4-tetrahyd-
roquinoline were obtained (Table 1, entry 6). Considering
the presence of ionic liquid on some classical heterogeneous
catalysts metal surface might have a profound effect on both
activity and selectivity [29], 0.106 mmol [BMIM][Pro] was
introduced to Raney Ni during the hydrogenation, however,
no obxious change in activity or selectivity was observed
(Table 1, entry 7). TOFs were also calculated based on both
the overall Ni atoms and the surface exposed Ni atoms [30],
and the TOF on the basis of surface exposed Ni atoms could
absence of NiO or Ni(OH) in Ni particles 1, 2 and 3 were
2
demonstrated by the XRD patterns. And the representa-
tive Ni(0) reflections were indexed as a face-centered cubic
(
fcc) structure. The Bragg reflections at 44.48°, 51.82°, and
7
6.52° match the indexed planes of the crystals of Ni(0)
(
111), (200) and (220).
−
1
3.2 Catalytic Hydrogenation
reach as high as 28.8 h in the quinoline hydrogenation.
In heterogeneous chemoselective catalytic reactions,
catalytic performance are generally rather sensitive to the
solvent employed [31]. The influence of different solvents
was studied in Table 2. Briefly, the results illustrated that
protic solvent was more effective than aprotic one. However,
no clear interrelationship between catalytic performance
and solvent polarity was observed in Table 2. EtOH was the
most suitable solvent for the reaction, in which the quinoline
hydrogenation activity and chemoselectivity were the high-
est (Table 2, entry 2).
Hydrogenation of quinoline and related compounds is of
extensive industrial concern in the manufacture of pet-
rochemicals, fine chemicals as well as pharmaceuticals
[
17–24]. The chemoselective hydrogenation of quinoline
under various reaction conditions is shown in Table 1. In
comparison to the Ni NPs prepared with NaBH , nanopar-
4
ticles prepared with hydrazine exhibited rather poor hydro-
genation activity (Table 1, entries 1, 3 versus 2, 4). With
the TEM in Fig. 2, the difference in hydrogenation activity
might reasonably owing to the obviously difference in Ni
NPs sizes accompany with the amount of surface exposed Ni
atoms between nanoparticles prepared with hydrazine and
In Table 3, quinoline could be conveniently hydrogen-
ated to 1,2,3,4-tetrahydroquinoline alongwith 99.1% selec-
tivity with Ni NPs 1 in ethanol (Table 3, entry 1). Catalytic
nanoparticles reduced by NaBH . The ionic liquid stabilizer
4
is fairly important in the catalytic system. The hydrogena-
tion brought about 64.0% 1,2,3,4-tetrahydroquinoline yield
in the absence of any stabilizer (Table 1, entry 5). In com-
bination with the UV/Vis spectra in Fig. 1, the variation of
ionic liquid stabilizers exhibited obvious influence on the
electron density as well as the catalytic performance of Ni
NPs. Generally, Ni NPs which have better dispersion exhib-
ited higher catalytic activity and chemoselectivity toward
1
,2,3,4-tetrahydroquinoline product. Ni NPs 1 exhibited
the best performance toward 1,2,3,4-tetrahydroquinoline
(
Table 1, entry 1). Above results demonstrated that ionic
liquid act as both the stabilizer during the preparation of
the metal particles and the modifier in the catalytic chem-
oselective hydrogenation. Furthermore, when the traditional
Fig. 4 X-ray diffraction pattern of (a) 1, (b) 2, and (c) 3
1
3

H. Jiang et al.
Table 1 Optimization of reaction conditions for the chemoselective hydrogenation of quinoline
Entry
Catalyst
Yield (%)
TOF (h− ) /TOF (h
1 a
−1 b
)
Selectivity (%)
Aa
1
Ab
0.9
25.8
4.4
1.0
4.5
Ac
1
2
3
4
5
6
7
1
99.0
0.6
97.1
7.4
64.0
75.0
73.6
11.5/28.8c
–/–
9.6/24.0
0.6/–
5.1/–
6.0/–
5.9/–
99.1
74.2
95.6
99.0
95.5
90.0
90.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2
c
3
4
5
Raney Ni
Raney Ni
10.0
9.5
d
Reaction was carried out at 75 °C. Substrate: in a 2 mL EtOH at [2.1M], PH : 3.0 MPa. Substrate/Ni=80:1, Raney Ni (0.053 mmol), reaction
2
time: 10 h
a
TOFs were calculated based on the overall Ni atoms
TOFs were calculated based on the surface exposed Ni atoms
TOFs were calculated based on the 5 h conversions
b
c
d
0
.106 mmol [BMIM][Pro] was introduced
Table 2 Different solvents on
chemoselective hydrogenation
of quinoline with Ni NPs 1
−1 a
Entry
Solvent
Dielectric
constant
Yield (%)
TOF (h ) /TOF1 Selectivity (%)
−1 b
(h
)
Aa
Ab
Ac
1
2
3
4
5
MeOH
EtOH
iPrOH
33.6
24.3
19.9
80.4
7.6
67.2
99.0
30.0
96.3
6.9
5.4/13.4
11.5/28.8c
2.4/6.0
98.5
99.1
97.3
98.4
99.0
1.5
0.9
2.7
1.6
1.0
0.0
0.0
0.0
0.0
0.0
H O
10.6/26.4c
0.5/1.3
2
THF
The reaction conditions are the same as in Table 1 except the solvent
a
TOFs were calculated based on the overall Ni atoms
TOFs were calculated based on the surface exposed Ni atoms
TOFs were calculated based on the 5 h conversions
b
c
reaction activity of 2-methylquinoline, 3-methylquinoline
or 8-methylquinoline was comparable to quinoline. The
hydrogenation chemoselectivity decline in the order of
Interestingly, 8-methylquinoline could be hydrogenated to
5,6,7,8-tetrahydroquinoline with chemoselectivity as high
as 22.0% (Table 3, entry 4). For comparison, 2,3-benzo-
furan and 1-benzothiophene were also examined. 2,3-ben-
zofuran and 1-benzothiophene displayed decreased catalytic
activity. 2,3-benzofuran and 1-benzothiophene could be
2
-methylquinoline > 3-methylquinoline > 8-methylqui-
noline (Table 3, entries 2–4). Introducing a methyl group
on a quinoline molecule, especially on 3-position of qui-
noline, clearly improved the decahydroquinoline yield.
1
3

Highly Selective Hydrogenation with Ionic Liquid Stabilized Nickel Nanoparticles
Table 3 Hydrogenation of quinoline and relevant compounds with Ni NPs 1
Yield (%)a
TOF (h ) /TOF (h
−1 b
−1 c
)
Selectivity (%)
A-Fa
Entry
Substrate
1
A-Fb
A-Fc
d
1
2
3
4
5
6
A
B
C
D
E
F
99.0 (96.0)
91.0 (81.5)
80.9 (72.3)
97.7 (71.1)
59.7 (54.0)
10.8 (9.0)
11.5/28.8
9.3/23.2
6.5/16.2
99.1
96.0
90.6
78.0
100.0
100.0
0.9
4.0
0.0
22.0
0.0
0.0
0.0
0.0
9.4
0.0
0.0
0.0
d
d
10.1/25.2
4.6/11.5
0.9/2.2
The reaction conditions are the same as in Table 1
a
Isolated yield for A-Ea in brackets
TOFs were calculated based on the overall Ni atoms
TOFs were calculated based on the surface exposed Ni atoms
TOFs were calculated based on the 5 h conversions
b
c
d
hydrogenated toward 2,3-dihydrobenzofuran and 1-thiain-
dan with 100.0% chemoselectivity (Table 3, entries 5–6).
Chemoselective hydrogenation of aromatic nitro com-
pounds, as a major challenge in modern synthetic chemistry,
is widely employed to produce aniline and its analogues [32,
based on the overall Ni atoms and the surface exposed Ni
atoms were also calculated. The TOF on the basis of surface
−
1
exposed Ni atoms could reach as high as 32.0 h in the
hydrogenation of aromatic nitro compounds.
The recyclability of catalyst is rather critical aspects for
practical utilization of catalytic reactions. We studied the
reusability of the [BMIM][Pro]-Ni 1 catalyst and compared
[BMIM][Pro]-Ni 1 with Raney Ni in the nitrobenzene hydro-
genation in EtOH (Fig. 5). The product was separated by cen-
trifugation and decantation of the nanoparticles; EtOH and
nitrobenzene were introduced to the reactor for the next recy-
cle. Only aniline product was detected during all the catalytic
cycles. ICP-AES analysis demonstrated that the Ni leaching
from catalyst 1 was negligible during successive catalyst reuse.
[BMIM][Pro]-Ni 1 could be recycled six times with no obvi-
ous decrease in catalytic activity (Fig. 5). However, Raney
Ni showed poor activity in the nitrobenzene chemoselective
hydrogenation. Moreover, Raney Ni encountered 51.1% activ-
ity loss through only three subsequent reuse, and was almost
inactive after three catalyst reuse. This phenomenon may be
attributed to the Raney Ni micropores became blocked by the
resulting reaction mixture in successive catalyst reuse [34, 35].
Above results suggested that the [BMIM][Pro]-Ni 1 catalyst
was rather stable and active during successive catalyst reuse.
33]. The hydrogenation could be performed employing vari-
ous noble metal catalysts, such as Rh, Pd, Ir or Pt etc., which
exhibited good catalytic activity even under mild reaction
conditions. However, most of the reported hydrogenation
reactions of nitrobenzene or substituted nitrobenzene with
nonprecious Ni nano-metal catalysts were not so efficient.
As shown in Table 4, when traditional Raney Ni was utilized
in the chemoselective catalytic hydrogenation of nitroben-
zene, Raney Ni showed poor activity (Table 4, entry 6). With
the high efficient [BMIM][Pro]-Ni 1, all the aromatic nitro
compounds except substrates with a methoxy substituent
could be converted to the corresponding substituted anilines
efficiently with mild reaction conditions (Table 3, entries
9
–10). The methylnitrobenzenes and chloronitrobenzenes
were hydrogenated to the corresponding substituted anilines
with comparable reactivity and chemoselectivity to nitroben-
zene. Briefly, [BMIM][Pro]-Ni 1 catalyst exhibited excel-
lent activity for chemoselective hydrogenation of aromatic
nitro compounds, which was not obviously influenced by
the steric effect, but probably by the electronic effect. TOFs
1
3

H. Jiang et al.
Table 4 Aromatic nitro
compounds hydrogenation with
Ni NPs
Yield (%)a
−1 b
Entry
Substrate
Catalyst
Product
TOF (h ) /
−1 c
TOF (h
)
1
1
2
3
4
5
6
7
8
9
0
1
2
3
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
m-Methylnitrobenzene
p-Methylnitrobenzene
o-Methoxylnitrobenzene
p-Methoxylnitrobenzene
p-Fluoronitrobenzene
o-Chloronitrobenzene
p-Chloronitrobenzene
1
Aniline
Aniline
Aniline
Aniline
Aniline
Aniline
m-Methylaniline
p-Methylaniline
o-Methoxyaniline
p-Methoxyaniline
p-Fluoroaniline
o-Chloroaniline
p-Chloroaniline
100.0 (95.4)
1.7
100.0
1.0
9.8
12.0/30.0d
0.1/–
2
3
11.2/28.0d
0.1/–
4
5
0.8/–
4.5/–
Raney Ni
56.0
d
1
1
1
1
1
1
1
97.0 (93.6)
100.0 (99.0)
11.8 (9.9)
26.3 (20.0)
100.0 (95.5)
100.0 (98.1)
100.0 (97.5)
9.9/24.8
10.6/26.4d
0.9/2.4
1
1
1
1
2.1/5.3
12.2/30.4d
11.7/29.2d
12.8/32.0d
Reaction was carried out at 75 °C. Substrate: in a 2 mL EtOH at [2.1M], PH : 3.0 MPa. Substrate/
2
Ni=80:1, Raney Ni (0.053 mmol), reaction time: 10 h. Just corresponding amino aromatic products were
detected
a
Isolated yield in brackets
TOFs were calculated based on the overall Ni atoms
TOFs were calculated based on the surface exposed Ni atoms
TOFs were calculated based on the 5 h conversions
b
c
d
Technology and Business University (1751039) and Chongqing
Key Laboratory of Catalysis and New Environmental Materials
(
CQCM-2016-02).
References
1
.
Philippot K (2013) In: Serp P (ed) Concepts in nanocatalysis.
Wiley, Weinheim
2
3
4
5
.
.
.
.
Dupont J, Scholten JD (2010) Chem Soc Rev 39:1780
Scholten JD, Leal BC, Dupont J (2012) ACS Catal 2:184
Sarvi I, Gholizadeh M, Izadyar M (2017) Catal Lett 147:1162
Ghosh BK, Moitra D, Chandel M, Patra MK, Vadera SR, Ghosh
NN (2017) Catal Lett 147:1061
Fig. 5 Recyclability of [BMIM][Pro]-Ni 1 and Raney Ni catalysts for
selective hydrogenation of nitrobenzene. Reaction conditions are the
same as in Table 4. Reaction time: 10 h for each run
6
.
Yamada YMA, Arakawa T, Hocke H, Uozumi Y (2007) Angew
Chem Int Ed 46:704
7
8
.
.
Astruc D, Lu F, Aranzaes JR (2005) Angew Chem Int Ed 44:7852
Jansat S, Gomez M, Philippot K, Muller G, Guiu E, Claver C,
Castillon S, Chaudret B (2004) J Am Chem Soc 126:1592
4
Conclusion
9. Zhang C, Lu D, Jiang P, Li J, Leng Y (2017) Catal Lett 147:2534
1
0. Nowicki A, Boulaire VL, Roucoux A (2007) Adv Synth Catal
3
49:2326
In conclusion, we have demonstrated that the ionic liquid
stabilized Ni NPs could catalyze the highly chemoselective
hydrogenation of quinoline and relevant compounds under
mild conditions. UV/Vis spectroscopy, TGA, XPS and XRD
analyses clarified that the ionic liquid was adsorbed on the
surface of the Ni NPs. The Ni NPs could be easily recovered
and reused 6 times without obvious loss of catalytic activity
and chemoselectivity.
1
1. Migowski P, Machado G, Texeira SR, Alves MCM, Morais J,
Traversec A, Dupont J (2007) Phys Chem Chem Phys 9:4814
12. Dupont J, Fonseca GS, Umpierre AP, Fichtner PFP, Teixeira SR
(
2002) J Am Chem Soc 124:4228
3. Rossi LM, Machado G, Fichtner PFP, Teixeira SR, Dupont J
2004) Catal Lett 92:149
1
1
(
4. Rossi LM, Machado G (2009) J Mol Catal A 298:69
15. Hu Y, Yu Y, Hou Z, Yang H, Feng B, Li H, Qiao Y, Wang X, Hua
L, Pan Z, Zhao X (2010) Chem Asian J 5:1178
1
6. Denicourt-Nowicki A, Leger B, Roucoux A (2011) Phys Chem
Chem Phys 13:13510
Acknowledgements This work was financially supported by National
1
7. Jiang H, Zheng X (2015) Catal Sci Technol 5:3728
Natural Science Foundation of China (No. 21201184), Chongqing
1
3

Highly Selective Hydrogenation with Ionic Liquid Stabilized Nickel Nanoparticles
1
1
2
8. Zhu D, Jiang H, Zhang L, Zheng X, Fu H, Yuan M, Chen H, Li R
27. Devred F, Hoffer BW, van Langeveld AD, Kooyman PJ, Zandber-
gen HW (2003) App Catal A 244:291
(
2014) ChemCatChem 6:2954
9. Zhang L, Wang X, Xue Y, Zeng X, Chen H, Li R, Wang S (2014)
Catal Sci Technol 4:1939
28. Migowski P, Machado G, Texeira SR, Alves MCM, Morais J,
Traverse A, Dupont J (2007) Phys Chem Chem Phys 9:4814
29. Umpierre AP, de Jesύs E, Dupont J (2011) ChemCatChem 3:1413
30. Schwab F, Lucas M, Claus P (2011) Angew Chem Int Ed 50:10453
31. Blaser HU, Steiner H, Studer M (2009) ChemCatChem 1:210
32. Rathore PS, Patidar R, Rathore S, Thakore S (2014) Catal Lett
144:439
0. Qiao X, Bao Z, Xing H, Yang Y, Ren Q, Zhang Z (2017) Catal
Lett 147:1673
2
2
2
1. Fache F (2004) Synlett 15:2827
2. Jiang H, Zheng X (2015) App Catal A 499:118
3. Fang M, Machalaba N, Sánchez-Delgado RA (2011) Dalton Trans
4
0:10621
33. Mahata N, Cunha AF, Orfao JJM, Figueiredo JL (2008) App Catal
A 351:204
2
2
2
4. Rueping M, Koenigs RM, Borrmann R, Zoller J, Weirich TE,
Mayer J (2011) Chem Mater 23:2008
34. Du Y, Chen HL, Chen R, Xu NP (2004) App Catal A 277:259
35. Hoffer BW, Crezee E, Devred F, Mooijmana PRM, Sloof WG,
Kooyman PJ, van Langeveld AD, Kapteijn F, Moulijn JA (2003)
App Catal A 253:437
5. Fukumoto K, Yoshizawa M, Ohno H (2005) J Am Chem Soc
1
27:2398
6. Liu Q, Wu KK, Tang F, Yao LH, Yang F, Nie Z, Yao SZ (2009)
Chem Eur J 15:9889
1
3