Tuning chemical compositions of bimetallic AuPd catalysts for selective catalytic hydrogenation of halogenated quinolines

Article abstract of DOI:10.1039/c6ta09916e

Catalytic hydrogenation of halogenated quinolines is a longstanding challenge due to the harsh reaction conditions and disillusionary chemoselectivity owing to dehalogenation. Exploration of novel catalytic materials is still a big challenge. Herein, density functional theory calculations indicate that halogenated quinolines are selectively adsorbed on the Au surface via the nitrogen atom in the tilted orientation and on Pd via the quinoline ring in the flat orientation. In the tilted orientation, the C-Cl bond is away from the surface of catalysts, which can avoid the hydrogenation of the C-Cl bond by the surface activated hydrogen species. A series of Au_1?xPd_x bimetallic catalysts were deposited on CeO₂ nanorods by a facile electroless chemical deposition method. The Au_1?xPd_x catalysts with low Pd content delivered enhanced activity and improved chemoselectivity for the hydrogenation of halogenated quinolines. Highly dispersed Pd in the Au matrix of bimetallic catalysts with low Pd content triggers hydrogen activation on Pd sites and leads to the selective adsorption of halogenated quinolines on Au sites in the tilted orientation. The generated active hydrogen species can diffuse from Pd to Au sites for the hydrogenation of the tilted halogenated quinolines, resulting in suppressed dehalogenation and high chemoselectivity to the expected products.

Full text of DOI:10.1039/c6ta09916e

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Tuning chemical compositions of bimetallic AuPd catalysts for
selective catalytic hydrogenation of halogenated quinolines
Sai Zhang,a Zhaoming Xia,a Ting Ni,a Huan Zhang,a Chao Wua and Yongquan Qu*a
Received 00th January 20xx,
Accepted 00th January 20xx
Catalytic hydrogenation of halogenated quinolines is a longstanding challenge due to the harsh reaction conditions and
disillusionary chemoselectivity owing to dehalogenation. Exploration of novel catalytic materials is still a big challenge.
Herein, density functional theory calculations indicate that halogenated quinolines are selectively adsorbed on Au surface
via nitrogen atom in the tilted orientation and on Pd via quinoline ring in the flat orientation, respectively. In the tilted
orientation, the C-Cl bond is away from the surface of catalysts, which can avoid the hydrogenation of C-Cl bond by the
surface activated hydrogen species. Then, a series of Au1-xPdx bimetallic catalysts were deposited on CeO2 nanorods by a
facile electroless chemical deposition method. The Au1-xPdx catalysts with low Pd entities delivered enhanced activity and
improved chemoselectivity for hydrogenation of halogenated quinolines. Highly dispersed Pd in Au matrix of bimetallic
catalysts with low Pd entities triggers hydrogen activation on Pd sites and leads to the selective adsorption of halogenated
quinolines on Au sites in the tilted orientation. The generated active hydrogen species can diffuse from Pd to Au sites for
DOI: 10.1039/x0xx00000x
www.rsc.org/
hydrogenation of the tilted halogenated quinolines, resulting in the suppressed dehalogenation and
chemoselectivity to the expected products.
a high
quinolines are useful reagents in metal-catalyzed cross-coupling
reactions for formation of C-C, C-N or C-S bonds.25-28 However, only
a few heterogeneous cases with low selectivity have been reported
due to dehalogenation under the reductive environment (Table
Introduction
Catalytic hydrogenation of quinoline and their derivatives
represents a promising and challenging approach with the practical
simplicity and high atom economy for the production of 1,2,3,4-
thetrahydroquinoline, which is an important building block of many
biologically active natural products and organic intermediates.1-4
Transition metal and/or metal-free catalysts have been reported to
catalyze the hydrogenation of N-heteroarenes, which require high
reaction temperature ( > 120 oC) and long reaction time ( > 24 h)
due to their low capability for H2 activation.5-8 Noble metal (e.g. Ir,
Ru, Rh, Pd, Pt) catalytic systems exhibit high catalytic activity for
hydrogenation of quinolines.9-20 However, either heterogeneous or
homogeneous catalysts generally show a poor substrate scope in
the presence of other reducible functional groups (-Cl, -OH, -CHO
ect.), leading to a low catalytic chemoselectivity.21-23 Moreover, the
strong adsorption of quinolines and hydrogenated quinolines on the
surface of noble metals may cause the poison of catalysts.24
S1).7, 8, 14, 24, 29 Recently, Au–based catalysts have been reported to
24
realize the hydrogenation of N-heteroarenes.18,
Especially,
Au/TiO2 catalysts featured with very small Au nanoparticles ( < 2 nm)
and large surface area of support ( > 120 m2/g) can deliver a high
chemoselectivity hydrogenation of 6-chloroquinoline into 6-chloro-
1,2,3,4-tetrahydroquinolin.24 Heterolytic H2 cleavage was proposed
at the interface of Au nanoparticles and TiO2 support. However, the
catalytic mechanism is still unclear. Compared to Pd catalysts, the
capability of Au for hydrogen activation is generally 1 ~ 2 orders of
magnitude lower than that of Pd.30, 31 Pd catalysts delivered high
catalytic activity for hydrogenation of quinolines and their
derivatives but very poor chemoselectivity.8, 24
Generally, halogenated quinolines can be adsorbed on the
surface of metal catalysts with a tilted configuration via nitrogen
atom in N-heteroarene or with a flat configuration via quinoline
Among various functionalized derivatives, halogenated
ring.
The tilted orientation of halogenated quinolines can
effectively avoid dehalogenation due to a long distance between
the C-X (X=Cl, Br and I) bond and surface of catalysts. A feasible
approach to enhance the chemoselectivity is to control adsorption
configuration of quinoline molecules in the tilted configuration on
the surface of catalysts. While, hydrogen also can be effectively
activated on the surface of catalysts. For this part, bimetallic
catalysts with controllable surface chemical compositions and
electronic structures may provide a potential solution to improve
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the chemoselectivity of the hydrogenation of halogenated
quinolines.32, 33
Initially, density functional theory (DFT) calculations were
employed to investigate the adsorption configurations of 6-
chloroquinoline on Pd(111) and Au(111) surfaces. Results show
that 6-chloroquinoline is selectively adsorbed on Au(111) with a
tilted configuration and on the Pd(111) with a flat configuration.
When small amount of Pd is alloying with the second metal,
hydrogen activation on surface Pd species and the selective
adsorption of quinolines on the second metal with the tilted
configuration may realize a high chemoselectivity for hydrogenation
of halogenated quinolines. Guided by theoretical predictions, a
series of alloyed AuPd catalysts supported on CeO2 nanorods (Au1-
xPdx/NR-CeO2) were synthesized through
a facile electroless
chemical deposition (ECD) process. To our delight, the AuPd
catalysts with the decreased Pd entities can significantly enhanced
both catalytic activity and chemoselectivity for hydrogenation of 6-
chloroquinoline into 6-chloro-1,2,3,4-tetrahydroquinoline. The
improved catalytic activity and chemoselectivity can be attributed
Figure 1. Adsorption configurations and energy of 6-chloroquinoline
on Pd (111) and Au (111) surfaces.
to the selectively adsorbed quinolines on Au via
a tilted
configuration and efficiently activated hydrogen by surface Pd
species, respectively.
This value is much higher than that of 6-chloroquinoline on Au(111)
(-0.11 eV) and larger than that on Pd(111) (-0.44 eV) both in the flat
orientation. Motivated by DFT calculations and native capability of
metals for H2 activation, we envision that the bimetallic AuPd
catalysts with tunable chemical compositions have potential to
present the satisfactory catalytic activity and improve
chemoselectivity of hydrogenation of halogenated quinolines. In
this strategy, the activated hydrogen species on Pd can diffuse to
Au for hydrogenation of the selective adsorbed halogenated
quinolines in the tilted orientation.
Results and discussion
Bimetallic AuPd catalysts have been widely explored for many
reactions in both theoretically and experimentally.34-38 Adsorption
of 6-chloroquinoline molecule on Pd(111) and Au(111) as a model
system were investigated by DFT calculations. As shown in Fig. 1
and Fig. S1, 6-chloroquinoline molecule preferentially adsorbs on Pd
via flat orientation with high adsorption energy of -0.44 eV. While,
the adsorption energy of +0.23 eV suggests that the tilted
orientation is energetically and thermodynamically unstable
configuration. In the flat orientation, the reductive hydrogen
species activated by Pd can attack both N-heteroarene and C-Cl
bond simultaneously, implying the concurrent hydrogenation and
dehalogenation. Moreover, the bond length of C-Cl in the adsorbed
6-chloroquinoline on Pd via the flat orientation is extended from
the intrinsic 1.73 Å to 1.75 Å, indicating the activated C-Cl bond on
Bimetallic AuPd catalysts with controllable atomic ratios were
obtained by a three-step wet chemical approach: (1) synthesis of
CeO2 nanorods (CNR) as supports;39 (2) chemical reduction of CNR
by ascorbic acid (AA)40 and (3) metal ECD in the presence of the
desired amount of metal precursors. CNR was selected as the
functional support due to their excellent catalytic performance in
various hydrogenation or oxidation reactions.41-45 X-ray diffraction
(XRD) pattern and dark field transmission electron microscope
(TEM) image demonstrated that as-synthesized CeO2 was cubic
fluorite phase and rod-like morphology with a length of ~ 90 nm
and diameter of ~ 6.1 nm (Fig. S3). The apparent blue-shift of the
ultraviolet-visible (UV-vis) absorption spectra induced by AA
Pd for dehalogenation.
Thus, the flat orientation of 6-
chloroquinoline on Pd leads to a poor chemoselectivity. In contrast,
the tilted orientation of 6-chloroquinoline can effectively avoid
dehalogenation due to a long distance between the C-Cl bond and
activated hydrogen species. Thus, the strategy to control the
adsorption of halogenated quinolines shows the potential to
improve the chemoselectivity of catalytic hydrogenation.
treatments indicated the successful surface reduction with
a
decreased surface Ce4+ fraction (Fig. S4). The reduced CNR
preserved their structural features as that of the freshly synthesized
nanorods (Fig. S5).
Despite very low capability of Au for H2 activation, the selective
hydrogenation of halogenated quinolines by Au was reported.24
Heterolytic H2 cleavage was proposed at the interface of very small
Au nanoparticles and TiO2 support with a very large surface area.
However, the catalytic mechanism is still unclear. This unusual
catalytic activity and selectivity of Au for hydrogenation of
halogenated quinolines may indicate their alternated adsorption
structures on Au. DFT results show 6-chloroquinoline molecule
tends to bind with Au(111) via nitrogen atom in the tilted
orientation with an adsorption energy of -0.54 eV (Fig. 1 and Fig.
S2).
A series of Au1-xPdx/CNR with a similar metal weight loading
(0.9%~1%) were synthesized by ECD process between the reduced
CNR and aqueous Na2PdCl4 and/or HAuCl4 precursors in the
absence of any reductant and additives. Typical TEM images of Au1-
xPdx/CNR catalysts showed the rod-like morphology of as-
synthesized catalysts decorated with uniform metal particles (Fig.
2a and Fig. S6). The actual Au and Pd loadings were determined by
inductively coupled plasma optical emission spectroscopy (ICP-OES,
Table 1). The ICP-OES characterization is capable of bulk elemental
analysis but is not able to distinguish the surface elemental
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Table 1. Summary of various Au1-xPdx/CNR catalysts
Pd:Au atomic ratio
Adjacent lattice
fringes (nm)
Average
diameter (nm)
Au loading
(wt %)
Pd loading
(wt %)
Catalyst
ICP
XPSa
Au/CNR
Au0.93Pd0.07/CNR
0.238
0.235
0.231
0.23
0.228
0.227
0.223
6.7 ± 2.1
4.1 ± 0.9
4.5 ± 0.8
4.2 ± 1.2
5.0 ± 1.5
4.7 ± 1.3
4.7 ± 0.8
0.92
0.96
0.85
0.81
0.55
0.35
0
0
0
0.075
0.11
0.33
1.5
0
0.039
0.047
0.146
0.43
0.56
0.96
0.091
0.099
0.24
1.98
3.4
Au0.9Pd0.1/CNR
Au0.75/Pd0.25/CNR
Au0.4Pd0.6/CNR
Au0.25Pd0.75/CNR
Pd/CNR
3
∞
∞
a The atomic ratios of Pd:Au were calculated from the area ratios of Pd:Au in XPS spectrum (Fig. S8).
compositions.
X-ray photoelectron spectroscopy (XPS) was absorption spectra of various catalysts, (d) High resolution
performed to track the surface chemical compositions of the HAADF-STEM image. EDX elemental mapping of (e) Au and (f)
alloyed AuPd catalysts. The ratio of integral area of Au peak to Pd, respectively.
integral area of Pd peak was calculated as a descriptor to reveal the on CNR support (Fig. 2e and 2f).
Initially, the catalytic performance of various catalysts was
evaluated using a model hydrogenation of 6-chloroquinoline (1).
The optimized reaction conditions were under 100 °C and 2.0 MPa
H2 in toluene (Table S2). The Au/CNR catalysts delivered no
hydrogenation activity under the optimized conditions (Fig. S9).
Introduction of small amount of Pd (x=0.07) in Au particles
triggered the hydrogenation of 1 with a conversion of 26.4% for the
initial 1 h (Fig. 3a). Increase of x to 0.1, 0.25 and 0.6 resulted in
improved conversions of 1 to 34.6%, 65.9% and 94.7% in the same
duration (1 h), respectively. However, the conversions of 1
inversely decreased to 88.5% and 30.5% on the further increase of
Pd contents at x = 0.75 and 1.0, respectively. Introduction of Pd in
Au can lower the activation barrier to H2 dissociation and the bound
surface atomic ratios of Au:Pd for various Au1-xPdx/CNR catalysts, as
shown in Fig. S8. With the increase of bulk Pd, the surface Pd
fractions were also increased (Table 1). Fig. 2 showed the TEM
images of AuPd bimetallic catalysts with a chemical composition of
Au0.9Pd0.1, in which the weight loadings of Au and Pd were 0.85%
and 0.047%, respectively. The Au/CNR and Pd/CNR catalysts were
also synthesized with a similar total weight loading (Table 1 and Fig.
S7). Derived from TEM images, the average sizes of Au0.9Pd0.1, Au
and Pd particles were 4.5 ± 0.8, 6.7 ± 2.1 and 4.7 ± 0.8 nm,
respectively. Compared with the Au/CNR, the characteristic
absorption peak of Au nanoparticles at 540 nm was absent in the
UV-vis spectrum of the Au0.9Pd0.1/CNR (Fig. 2c), indicating their
bimetallic structure. The measured planar spacing of 0.231 nm of
Au0.9Pd0.1 in the aberration-corrected high-angle annular dark field
scanning transmission electron microscopy (HAADF-STEM) image
was slightly smaller than 0.238 nm of the Au(111) crystalline plane
but larger than 0.223 nm of Pd(111) plane (Fig. 2d and Fig. S7),
which can be attributed to the large Au atoms replaced by the small
Pd atoms.46 Energy dispersive X-ray (EDX) elemental mapping of
metal particles further confirmed the alloyed structure of Au0.9Pd0.1
activated hydrogen is weakly enough to allow hydrogen spillover
48
onto Au surface.47,
Thus, high catalytic activity of the Au1-
xPdx/CNR over the Au/CNR or Pd/CNR can be understood by this
synergistic effect, similar to previous reports.30, 31, 49, 50 Compared
to the Pd/CNR, the relative low activity of the Au0.93Pd0.07/CNR can
be attributed to the limited amount of the surface Pd species for H2
activation.
In addition to the evolution of the catalytic activity of bimetallic
AuPd as a function of chemical compositions, the chemoselectivity
into 6-chloro-1,2,3,4-thetrahydroquinoline (2) was found to be
significantly affected by the Pd entities within the initial 1 h (Fig.
3a). The Pd/CNR catalysts delivered a very poor selectivity of 51.7%
to the desired product 2. Decreasing the Pd percentage in the
bimetallic catalysts, the chemoselectivity to 2 was significantly
Figure 2. Structural characterizations of the Au0.9Pd0.1/CNR
catalysts. (a) TEM image, (b) size distribution (c) UV-vis
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species (Ha) on Pd can spill over onto the surface Au sites for
hydrogenation. The selective adsorbed halogenated quinolines on
Au sites via tilted orientation allow the selective hydrogenation of
N-heteroarenes and block the dehalogenation, resulting in the
improved chemoselectivity to the desired products.
High
percentages of Pd in the bimetallic particles result in more surface
Pd atoms and formation of the large Pd clusters on the surface of
the bimetallic catalyst. It induces a primary change in the
adsorption configurations of reactants from the selective tilted
orientation on Au at low Pd entity to both the tilted orientation on
Au and flat orientation on Pd at high Pd entity. Thus, the
dehalogenation and poor chemoselectivity were indeed observed
when more Pd was introduced into the bimetallic catalysts.
Among various bimetallic catalysts, the Au0.9Pd0.1/CNR exhibited
much better selectivity and higher catalytic activity over the
Pd/CNR. As shown in Fig. 4a, the yield of 86.5% for product 2 was
obtained with the 96.4% conversion of 1 after 5 h reaction. In
contrast, the Pd/CNR catalysts led to a much lower yield (47.3%) of
Figure 3. Catalytic activity and chemoselectivity of hydrogenation of
6-chloroquinoline over a series of Au1-xPdx/CNR catalysts. Reaction
conditions: 0.2 mmol of 6-chloroquinoline, 5 mg of catalysts, 2 mL
of toluene, 100 °C and 2.0 MPa H2. (a) Catalytic activity and
selectivity of 2 for 1 h. (b) Product distributions when the
conversions of 6-chloroquinoline were over 90% for all catalysts.
product
2 under the same reaction conditions, where the
conversion of 1 was
Schematic 1. Proposed catalytic process for hydrogenation of 6-
chloroquinoline catalyzed by the Au1-xPdx/CNR at low Pd entity.
improved to 56.3%, 64.9%, 75.7%, 89.7% and 92.3% for the Au1-
xPdx/CNR catalysts with x=0.75, 0.6, 0.25, 0.1 and 0.07, respectively.
The similar trend was also observed in the final yields of product 2.
As shown in Fig. 3b, the yield of 2 reached 91.6% for the
Au0.93Pd0.07/CNR catalysts with low yields of by-product 3 (5.4%)
and 4 (1.4%), respectively. With the increase of the surface Pd
fractions in the bimetallic catalysts, the yields of 2 decreased to
86.8%, 73.6%, 61.7%, 51.9% for the Au1-xPdx/CNR catalysts with
x=0.1, 0.25, 0.6 and 0.75, respectively and only 47.9% for the
Pd/CNR catalysts.
The similar sizes of bimetallic Au1-xPdx and monometallic Au and
Pd particles suggest that their difference in catalytic hydrogenation
is originated from the alloying effect. At low Pd entities, Pd metal is
highly dispersed in the Au host matrix. As shown in Schematic 1,
one Pd metal atom can be supported on or sandwiched between
the Au host atoms, as well as the formation of very small Pd clusters
on the surface of bimetallic catalysts. In this situation, 6-
chloroquinoline molecules tend to be selectively adsorbed on the
surface abundant Au sites via the tilted orientation (Schematic 1),
according to DFT calculations (Fig.1). It has been well documented
experimentally and theoretically that these highly dispersed Pd can
Figure 4. (a) Time course of conversion and (c) catalytic stability of
Au0.9Pd0.1/CNR for hydrogenation of 6-chloroquinoline. (b) Time
course of conversion and (d) catalytic stability of Pd/CNR for
hydrogenation of 6-chloroquinoline.
Reaction conditions: 6-
chloroquinoline (0.2 mmol), Au0.9Pd0.1/CNR or Pd/CNR catalyst (5 mg)
(15 mg for stability test), toluene (2 mL), 100 °C and 2 MPa H2.
91.6% (Fig. 4b). Also, large amount of the undesired dehalogenated
by-products 3 (8.6%) and 4 (39.7%) were yielded. The commercial
Lindlar catalysts (5 wt% Pd) only delivered a 24.1% conversion of 1
and 64.8% selectivity of 2 under the identical reaction conditions
(Fig. S10). Thus, the Au0.9Pd0.1/CNR catalysts exhibited a satisfactory
activity as well as the significantly improved chemoselectivity.
Catalytic stability is also a very important criterion to evaluate
the performance of catalysts.
As shown in Fig. 4c, the
Au0.9Pd0.1/CNR catalysts delivered stabilized activity with slight
deactivation during the cyclic hydrogenation. Importantly, the
chemoselectivity to 2 was well maintained at the level of 89% for
the initial consecutive four cycles. The unchanged morphology of
the Au0.9Pd0.1/CNR also indicated their structural robustness for
dissociate H2 more readily and bind it more weakly than that on the
48, 51
surface of pure metals.47,
The generated active hydrogen
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hydrogenation (Fig. S11). Very low metal bleaching of the by surface Pd can spill over from Pd to Au sites and selectively
Au0.9Pd0.1/CNR with concentrations of 1.45 ppm (Au) and 0.35 ppm hydrogenate the halogenated quinolines to the expected products.
(Pd) in the reaction solution after 4 cycles suggested the stability of Such cooperative effects between two different metals associated
the catalysts. While, for the Pd/CNR, the conversion of 1 decreased with different adsorption configurations of reactants on each metal
continuously from initial 85.6% to 31.5% after four cycles (Fig. 4d). may benefit the design of novel catalysts with improved
Very high Pd concentration of 30.5 ppm in the liquid reaction chemoselectivity for many important reactions.
solution was 87 times higher than that of the Au0.9Pd0.1/CNR
catalysts, suggesting a serious metal bleaching of the Pd/CNR
Experimental
catalysts during the hydrogenation process.
Next, the scope of Au0.9Pd0.1/CNR was extended to various
All chemicals (AR grade) were used as received. Water with a
substituted quinolines containing halogen and reducible groups. As
resistivity of 18.2 MΩ · cm was used for all experiments. All
shown in Table 2, the >99.9% chemoselectivity of 1,2,3,4-
glassware were thoroughly washed by aqua regia (a volume ratio of
tetrahydroquinlone was obtained at both 5 h (46.9% conversion,
1: 3 of concentrated nitric acid and hydrochloric acid) to avoid any
Entry 1) and 10 h (92.6% conversion, Entry 2). For 6-bromo- and 6-
possible contamination. (Caution: Aqua regia is highly corrosive and
iodo-quinoline, the high selectivity (>93.2%) of 6-halo-1,2,3,4-
extremely dangerous and must be handled with the proper
tetrahydroquinoline was yielded under either low or high
protective equipments)
conversions of substrates (Entries 3, 4, 5 and 6). Introducing the
4.1 Theoretical calculation. The density functional calculation
reducible groups, the hydrogenation selectivity of 6-methoxy-
(VASP).52-57 Frozen-core all-electron projector augmented wave
quinoline to 6-methoxy-1,2,3,4-tetrahydroquinolin reached 98.3%
(DFT) was carried out by Vienna Ab-initio Simulation Package
(PAW) method and plane wave basis were set. The generalized
Table 2. Hydrogenation of various substituted quinolines by the
gradient approximation (GGA) in the form of the Perdew-Burke-
Au0.9Pd0.1/CNR*
55
Ernzerhof functional (PBE) was chosen for electron exchange and
Time
(h)
5
Con.
(%)
Sel.
(%)
correlation. The energy cut off is 400 eV. The lattice parameters of
bulk metals were 0.4173 nm (for Au), and 0.3949 nm (for Pd), which
are approach to the experiment data (0.408 nm and 0.389 nm). A
p(3x3) supercell with 4 atom layers of Pd (111) was choose as the
slab model, including a relaxation of the top two layers. A vacuum
thickness of 20 Å was used to avoid the interaction from the top
supercells. The Brillouin-zone integration has been performed with
a 2 x 2 x 1 Monkhorst-Pack grid. For the 6-chloroquinoline
molecular, a supercell of 20 Å x 20 Å x 20 Å is selected, and the
Brillouin-zone integration has been performed with a 1 x 1 x 1
Monkhorst-Pack grid. For the geometry relaxation, a criterion of
0.02 eV/Å on forces is used. And the convergence criterion for the
energy was 10−5 eV.
Entry
Substrate
Product
1
2
46.9
92.6
36.2
>99.9
>99.9
98.5
N
H
N
10
5
Br
Br
3
N
4
8
5
98.2
10.2
76.5
33.6
96.3
35.6
86.3
30.1
96.4
95.1
93.2
98.3
95.6
98.1
96.1
94.1
N
H
I
I
5
N
6
20
3
N
H
H3CO
OHC
H3CO
7
N
8
10
3
N
H
OHC
9
N
10
11
10
3
N
H
Cl
Cl
N
The adsorption energy was defined as the follow equation:
Eadsorption=Etot-Emetal- Emolecule
12
10
89.6
92.3
N
H
* Reaction conditions: 5 mg of Au0.9Pd0.1/CNR, 0.2 mmol of substrate,
2.0 mL of toluene, 100 °C and 2.0 MPa H2.
(Etot: The total Energy of the metal surface and an absorbed
molecule, Emetal: the energy of metal surface model, Emolecule: the
energy of molecule.)
and 95.6% at the conversions of 33.6% and 96.3%, respectively
(Entries 7 and 8). In the presence of the reducible –CHO group, the
high chemoselectivity to 6-formyl-1,2,3,4-tetrahydroquinolin
(>96.1%) was also obtained (Entries 9 and 10). When chlorine
located at 4-positions of quinoline ring, the chemoselectivity to the
desired products was higher than 92.3% (Entries 11 and 12).
4.2 Synthesis of CeO2 nanorods (CNR). Ce(NO3)3·6H2O (1.736 g)
and NaOH (19.2 g) were dissolved in 10 and 70 mL of MQ water,
respectively. After aging for 30 min, the mixture was transferred
into a stainless steel autoclave for hydrothermal treatment at 100
°C for 24 h. The solid products were collected by centrifugation,
washed with ethanol and water, and dried at 60 °C overnight.
4.3 Synthesis of Au1-xPdx/CNR catalyst. Firstly, 100 mg of CNR
support was dispersed in 50 mL of ascorbic acid (0.1 mmol)
solution. The mixture was stirred at room temperature for 1 h. The
solid was obtained by centrifugation and washed immediately with
water for 3 times. Then, the solid was re-dispersed in another 100
mL water and desired amount of HAuCl4 and/or NaPdCl4 was added
subsequently. The mixture was stirred for another 12 h. After
centrifugation, the products were reduced by 5% H2/Ar at 200 °C to
obtain the metallic catalysts.
Conclusions
In summary, we have designed and synthesized the Au1-xPdx
bimetallic catalysts with tunable chemical compositions on CeO2
nanorods synthesized by a facile ECD method, which deliver high
activity and chemoselectivity for hydrogenation of halogenated
quinolines. Improved chemoselectivity could be attributed to the
selective adsorption configurations of halogenated quinolines on Au
via tilted orientation rather than on Pd via flat orientation, when
the Pd entities were low. The reductive hydrogen species activated
4.4 Catalytic hydrogenation of quinolines. Hydrogenation of
quinolines was carried out in a stainless steel autoclave equipped
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We acknowledge the financial support from Natural Science
Foundation of China 21401148 and National 1000-Plan
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