Accelerated catalytic activity of Pd NPs supported on amine-rich silica hollow nanospheres for quinoline hydrogenation

Article abstract of DOI:10.1039/c7cy00394c

Tuning the catalytic performance of metal nanoparticles (NPs) is very important in nanocatalysis. Herein, we report that amine-rich mesoporous silica hollow nanospheres (HS-NH₂) synthesized by one-pot condensation could efficiently stabilize ultra-small Pd NPs and also increase the surface electron density of Pd NPs due to the coordinating and electron-donating effects of the amine group. Pd NPs supported on HS-NH₂ afford TOF as high as 5052 h^-1 in quinoline hydrogenation reaction and are much more active than Pd/C with a TOF of 960 h^-1 as well as most reported solid catalysts. The intrinsic activity of Pd NPs increases as the particle size of Pd decreases, revealing that quinoline hydrogenation is a structure-sensitive reaction. The results of TEM, XPS, CO adsorption and CO stripping voltammetry indicate that the high activity of Pd NPs supported on HS-NH₂ is mainly attributed to their ultra-small particle size and high surface electron density. Our primary results demonstrate that the organo-modified silica nanospheres are promising solid supports for modifying the electronic properties of metal NPs supported and consequently tailoring their catalytic functions.

Full text of DOI:10.1039/c7cy00394c

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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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DOI: 10.1039/C7CY00394C
Journal Name
ARTICLE
Accelerated catalytic activity of Pd NPs supported on Amine-Rich
Silica Hollow Nanospheres for Quinoline Hydrogenation
Received 00th January 20xx,
Accepted 00th January 20xx
ab
a
a
Miao Guo, Can Li* and Qihua Yang*
DOI: 10.1039/x0xx00000x
Tuning the catalytic performance of metal nanoparticles (NPs) is very important in nanocatalysis. Herein, we report that
amine rich mesoporous silica hollow nanospheres (HS-NH ) synthesized by one-pot condensation could efficiently stabilize
ultrasmall Pd NPs and also increase the surface electronic density of Pd NPs due to the coordinating and electron donating
2
www.rsc.org/
-1
2
effect of the amine group. Pd NPs supported on HS-NH afford TOF as high as 5052 h in quinoline hydrogenation reaction
-
1
and is much more active than Pd/C with TOF of 960 h and also most reported solid catalysts. The intrinsic activity of Pd
NPs increases as particle size of Pd decreases, revealing that quinoline hydrogenation is a structure sensitive reaction. The
results of TEM, XPS, CO adsorption and CO stripping voltammetry indicate that the high activity of Pd NPs supported on
2
HS-NH is mainly attributed to its ultrasmall particle size and high surface electronic density. Our primary results
demonstrate that the organo-modified silicas are promising solid supports for modify the electronic property of metal NPs
supported and consequently tailoring their catalytic functions.
supports have been widely used for stabilizing ultrasmall metal
NPs. For example, Li and coꢀworkers demonstrated that Nꢀ
functionalized ordered mesoporous carbons could be used for
Introduction
Metal nanoparticles (NPs) with ultrasmall size (1ꢀ3 nm)
generally exhibit dramatically different electronic and chemical
1
5
stabilizing ultrasmall Pd NPs.
1
ꢀ4
Compared with the Nꢀdoped carbon materials and
functionalized porous polymers, the organoꢀmodified silica
nanomaterials with tunable structure and morphology and easy
functionalization would be more desirable support materials for
obtaining the ultrasmall metal NPs. Considering the
properties. Studies show that downsizing the particle size of
metal NPs could dramatically enhance their catalytic activity in
most cases owing to the increased low coordination sites and
5
, 6
surface vacancies.
Various strategies are used for the synthesis of ultrasmall metal
NPs with precise size control.
1
6,
7
ꢀ10
2
coordination ability and electron donating properties of NH ,
Generally, the ultrasmall NPs
1
7
herein, we choose amine modified mesoporous silica hollow
was prepared by the colloid method and subsequently deposited
on the solid material, followed by removing the stabilizer or
directly impregnated the precursor of the metal on the oxide
support by using the metal support interaction. However, either
the aggregation of NPs or the residual cappers will inevitably
influence their catalytic activity. Another alternative approach
is to encapsulate the metal NPs in the porosity materials, such
nanospheres as support materials to tune the particle size and
surface electronic density of Pd NPs. With quinoline
hydrogenation as model reaction, the relation of the catalytic
performance of Pd NPs with particle size and surface electronic
density was investigated. Our preliminary results suggest that
ultrasmall Pd NPs with electronꢀrich surface afford high
catalytic activity.
1
1
12
13
14
as zeolites, MOFs, ZIFs, polymers, etc. Yu et al. reported
a facile strategy to synthesize Pd NPs with size of 1.8 nm
encapsulated within the intersectional channels of nanosized
zeolite, which shows superior catalytic activities and excellent
Experimental section
1
1
Chemicals and reagents
recyclability of hydrogen generation from formic acid.
However, the very less metal loading and tedious synthesis
procedure restrict the application. Recently, Nꢀmodified solid
All materials were of analytical grade. Tetraethoxysilane
(
TEOS) was obtained from Sinopharm Chemical Reagent Co.,
Ltd (China). 3ꢀAminepropyltriethoxysilane (APTES),
cetyltrimethyl ammonium bromide (CTAB) were purchased
from SigmaꢀAldrich Company Ltd. (USA). Commercial Pd/C
(
5 wt%) was obtained from TCI (Shanghai) Development Co.,
Ltd. Quinoline compounds were purchased from TCI, Admas
or Ark Chemicals. Other reagents were purchased from
Shanghai Chemical Reagent, Inc. of the Chinese Medicine
Group.
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Journal Name
Synthesis of amine-rich silica hollow nanospheres (HS-NH
2
).
in this work and commercial Pd/C (5 wt%) were degassed at
o
o
1
20 C and 250 C for 6 h prior to the measurement,
0
.16 g of CTAB and 0.5 mL of NH •H O (25ꢀ28 %) were
3
2
dissolved in 40 mL of H O and 10 mL of EtOH solution at 50 respectively. Xꢀray photoelectron spectroscopy (XPS) was
2
o
C, followed by the addition of 0.16 ml of TEOS (in 1 mL recorded on VG ESCALAB MK2 apparatus using Al Kα (h =
λ
EtOH). After stirring for 2 min, the clear solution turned milky
white. Then, 0.1 mL of APTES (in 1 mL EtOH) was added and
1
486.6 eV) as the excitation light source. The peaks in the
spectra were fitted by using the shareware program XPSꢀPEAK
with GaussianꢀLorentzian peak shapes and Shirley
background. Highꢀresolution transmission electron microscopy
o
the mixture was stirred for another 2 h at 50 C. Then the
o
a
temperature was raised to 80 C and the mixture was stirred at
this temperature for 2 h. After cooling down to room
temperature, the powder product was isolated by filtration, (HRꢀTEM) was performed on a JEOL JEMꢀ2100 at an
washed with distilled water and airꢀdried. The surfactant was acceleration voltage of 200 kV. Transmission electron
extracted by dispersing asꢀsynthesized materials (1 g) in 120
microscopy (TEM) was performed using a HITACHI HT7700
at an acceleration voltage of 100 kV. Scanning electron
microscopy (SEM) was undertaken by using a FEI Quanta
mL of ethanol (95%) containing 80 mg of ammonium nitrate
o
and the mixture was heated at 60 C for 30 min. The process
was repeated for three times. After filtration and drying, the
2
00F operating at an acceleration voltage of 1ꢀ30 kV. FTꢀIR
spectra were performed on Nicolet Nexus 470 IR
spectrometer. The IR spectra were collected in KBr pellets with
powder product obtained was denoted as HSꢀNH
For comparison, SiO HS with particle size of 110 nm was
prepared with a modified method according to our previous
2
.
a
2
ꢀ1
1
8
the range of 400ꢀ4000 cm . The Pd content was determined by
PLASAMꢀSPECꢀII inductively coupled plasma atomic
emission spectrometry (ICPꢀAES). The solidꢀstate NMR
report (details see SI).
Preparation of Pd/HS-NH
2
(X%).
The Pd/HSꢀNH (X%) was synthesized by conventional
impregnationꢀreduction method. Typically, 400 mg of HSꢀNH
was dispersed in 5 mL of deionized water in a centrifuge tube
under ultrasound. Then desired amount of Na PdCl aqueous
solution (0.01 g mL ) was added into the tube. After ultrasound
treatment for 10 min, a freshly prepared NaBH aqueous
2
spectra were obtained on a Bruker DRX 400 spectrometer
2
13
equipped with a MAS probe by using a 4 mm ZrO
2
rotor.
C
29
and Si signals were referenced to tetramethylsilane (TMS).
The experimental parameters were as follows: 8 kHz spin rate,
2
4
ꢀ1
3
s pulse delay, 4 min contact time, and 1000 scans. The
4
thermogravimetric analysis (TGA) was performed using a
ꢀ1
solution (7.5 mg mL ) was added slowly. The brownish red
colour transformed into gray black. After ultrasound treatment
for another 30 min, the mixture was filtered and the black
powder product was washed with deionized water and EtOH
o
NETZSCH STA 449F3 analyzer from 30 to 900 C with a
o
ꢀ1
heating rate of 10 C min under air atmosphere. CO
o
chemisorption measurement was performed at 40 C on a
Quantachrome Autosorbꢀ1/C chemisorb apparatus. Samples
o
for several times. After drying in an oven at 60 C overnight,
o
were treated in H
2
atmosphere at 200 C for 2 h before
2
Pd/HSꢀNH (X%) was obtained, where X refers to the weight
percent of Pd.
We also tried to prepare Pd/HS (5 wt%) in a similar method to
measurement. The metal dispersion and particle size were
estimated on the basis of the assumption of a spherical
geometry of Pd particles, assuming
stoichiometry of Pd/CO = 2. The average Pd particle size was
calculated by using the classical equation
), where dCO is the Pd particle size, S is
a
chemisorption
Pd/HSꢀNH
cannot disperse Pd NPs well on HS possibly due to the weak
interaction of Pd and SiO . Thus, the reduction was performed
2 4
(5 wt%). However, reduction with NaBH solution
d
CO•S
=
2
(
1/QCO)•(6ρsite/Nρ
m
o
2
at 200 C with H .
average ratio between Pd surface atoms and chemisorbed CO
Hydrogenation of quinoline.
ꢀ1
molecules (S=2), QCO is the CO monolayer uptake (mmol g ),
ꢀ3
ꢀ2
In a typically process, a desired amount of solid catalyst (5×10
ρsite is the Pd surface site density (for Pd: 12.7 atoms nm ), ρ
m
ꢀ3
mmol Pd) was added in an ampoule tube, followed by the is the metal density (for Pd: 11.9 g cm ), and N is the
addition of quinoline (1 mmol) (S/C = 200) and 2 mL of Avogadro number (6.022
solvent. The ampoule tube was loaded into a stainless steel Electrochemical measurements.
ꢀ1
19, 20
╳ 1023 mol ).
2
1
To prepare
a working
autoclave (300 mL) with a thermocouple probed detector. After electrode, an ethanol dispersion of catalysts (7.5 µg) was
purging with hydrogen for six times, the final pressure was deposited on a glassy carbon electrode (3 mm). After the
o
adjusted to 1 MPa and the reactor was heated to 50 C with ethanol was dried, 3 µL of 2.5 V% Nafion solution was
vigorous stirring. After reaction, the solid catalyst was dropped on the electrode surface to stabilize the catalysts on the
separated by centrifugation and the filtrate was collected, electrode surface. A saturated calomel electrode (SCE) and a
diluted with ethanol and analyzed by an Agilent 7890B GC platinum wire were used as the reference and counter electrode,
equipped with an Agilent J&W GC HPꢀ5 capillary column (30 respectively. For CO stripping, pure CO (99.999%) gas was
m × 0.32 mm × 0.25 ꢁm) and nꢀdecane as internal standard.
purged through the catalyst surface in the cell filled with 0.5 M
For recycling the catalyst, the liquid was decanted after H SO4 electrolyte for 30 min while holding the working
2
centrifugation, and the catalyst was thoroughly washed with electrode at 0.2 V (versus SCE). After transferring the
EtOH and dried under vacuum at room temperature and used electrodes to another cell filled with fresh H SO electrolyte
2
4
directly for the next run.
(without CO) and purging the solution with N for 15 min, the
2
Characterization.
CO stripping was performed in the potential range of 0.2ꢀ1.2 V
ꢀ1
(
versus SCE) at a scan rate of 5 mV s .
N
2
sorption isotherms were carried out on a Micromeritics
ASAP2020 volumetric adsorption analyzer. Samples prepared
2
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Results and discussion
Synthesis and characterization of amine-rich silica.
Scheme 1. General procedure for the preparation of HS-NH
X%).
2 2
and Pd/HS-NH
(
The synthesis procedure for amineꢀrich silica hollow
nanospheres is outlined in Scheme 1. The process for the
synthesis of hollow nanospheres involves the hydrolysis of
3
ꢀ1
2
TEOS in basic waterꢀethanol solution with CTAB as the pore volume (0.54 cm g ) and high BET surface area (301 m
ꢀ
1
surfactant, and then APTES was added to the above mixture. g ), which will benefit the diffusion of guest molecules during
o
2
The hydrothermal treatment of the mixture at 80 C gives final the catalytic process. The FTꢀIR spectrum of HSꢀNH clearly
ꢀ1
ꢀ1
hollow nanospheres. Based on the results of morphology displayed the vibrations at 2942 cm (υ CꢀH ), 1385 cm (δ CꢀH),
transformation with reaction time (Figure S1), the formation of 3441 cm (υ NꢀH) and 1558 cm (δ NꢀH) (Figure 2B). The solidꢀ
ꢀ1
ꢀ1
1
3
hollow nanospheres possibly related with the etching effect of state C CP/MAS NMR spectrum displays the chemical shifts
2
2ꢀ24
APTES.
at 9.3 ppm, 21 ppm and 42.3 ppm, which could be assigned to
1
2
3
3
2
1
The morphology and structure of HSꢀNH were characterized C , C and C of ꢀC H
2
C H
2 2 2
C H NH (Figure 2C). The results
2
1
3
with SEM and TEM (Figure 1). The SEM shows that HSꢀNH2 of FTꢀIR and C CP/MAS NMR suggest the successful
2
9
is mainly composed of nanospheres with particle size of ~ 110 incorporation of amine groups. Si NMR spectrum exhibits
nm though some oval spheres coꢀexist. Oval spheres have both T site (ꢀ66.7 ppm for T , T = RꢀSi(OSi) (OH)3ꢀn) and Q
smaller diameter of ca. 110 nm and larger diameter varying in site (ꢀ102.6 ppm for Q and ꢀ110.7 ppm for Q , Q
the range of 80 to 150 nm. The TEM image clearly shows that Si(OSi) (OH)4ꢀn) with T / (Q + T) ratio of 31 %, showing the
all spheres have hollow structure with shell thickness of 20 nm. integration of high content of amine groups in HSꢀNH (Figure
3
n
n
3
4
n
=
n
2
Based on the TEM image, hollow spheres have mesoporous 2D). Based on TG analysis (Figure S2), the weight loss of 21.7
o
channels aligning perpendicular to the cores in the shell, which wt% in the range of 240 to 700 C could be assigned to the
would benefit the diffusion of reactants and products during the content of amine groups, which corresponds to amine amount
ꢀ1
catalytic process. The SEM and TEM characterizations confirm of 3.74 mmol g . The C, H, N elemental analysis affords N
that HSꢀNH2 has hollow spherical morphology with content of 5.18 wt%, corresponding to amine content of 3.70
ꢀ1
mesoporous structure in the shell.
mmol g (Table S1). The above results confirm that HSꢀNH
2
N adsorptionꢀdesorption isotherm of HSꢀNH is of type IV,
2
2
with a sharp capillary condensation step and an H hysteresis
3
loop starting from the relative pressure P/P of 0.40 (Figure
0
2
A), showing that HSꢀNH has mesoporous structure. The H3
2
hysteresis loop is from the inner void space. HSꢀNH has large
2
Figure 3. (A) TEM images, (B) HAADF-STEM images and (C) Pd particle size
distribution estimated by STEM of (a) Pd/HS-NH
(c) Pd/HS-NH (5 wt%), (d) Pd/HS-NH (10 wt%) and (e) Pd/HS-NH
scale bar: 20 nm).
2
(1 wt%), (b) Pd/HS-NH
2
(3 wt%),
Figure 1. (A) SEM and (B) TEM image of HS-NH
2
.
2
2
2
(15 wt%)
(
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2
HRꢀSEM image further shows that Pd/HSꢀNH (5 wt%) is
composed of uniformly distributed nanoparticles with particle
size of 80ꢀ150 nm (Figure 4B).
has amine rich character.
Characterization of Pd/HS-NH (X%).
2
Taken 1 wt%, 5 wt% and 15 wt% samples as examples, the
results of BET surface area and pore size distribution are shown
in Figure S3. With Pd content increasing from 1 wt% to 15
Pd/HSꢀNH
reduction method. The HSꢀNH
2
(X%) was prepared by a simple impregnationꢀ
with amine rich character could
easily adsorb Pd because nitrogen atom has a high affinity for
2
2
+
wt%, the BET surface area of Pd/HSꢀNH
2
decreases from 289
with BET surface area
of 301 m g . The pore size of all Pd/HSꢀNH samples is
almost the same and distributed at ca. 2.2 nm. The FTꢀIR
spectra show that the NꢀH vibration of HSꢀNH shifts from
558 cm to 1543 cm after Pd loading, suggesting the
2
+
binding metal cations. Pd was successfully reduced to Pd NPs
with NaBH . Figure 3 shows the TEM images of Pd/HSꢀNH
X%) with different Pd loading (1ꢀ15 wt%). Based on the TEM
results, Pd NPs with particle size of 1.6 nm, 1.9 nm and 2.1 nm
are uniformly distributed in the shell region of HSꢀNH with Pd
loading of 1 wt%, 3 wt% and 5 wt%, respectively. For Pd/HSꢀ
NH (10 wt%) and Pd/HSꢀNH (15 wt%), the particle size
2
ꢀ1
to 241 m g , slight lower than HSꢀNH
2
4
2
2
ꢀ1
2
(
2
2
ꢀ1
ꢀ1
1
formation of interactions between Pd and amine groups (Figure
2
2
28, 29
S5).
distribution of Pd is very broad. It should be noted that the
ultrasmall Pd NPs of 2.1 nm could be maintained nicely when
the metal loading as high as 5 wt%. This could be ascribed to
the amineꢀrich character of HSꢀNH
that metal NPs would be highly dispersed inside the confined
The CO chemisorption was used to give further information of
the particle size and dispersion of Pd NPs (Table 1). The results
clearly show that average particle size of Pd increases gradually
from 3.0 to 8.2 nm along with the metal content from 1 wt% to
2
. Recent work suggested
1
5 wt%. It shows a similar trend with the TEM results although
2
5ꢀ27
mesoporous,
thus, the “confinement effect” of the
larger value was obtained. Through increasing Pd loading, the
size of Pd NPs could be adjusted. It is observed that the metal
dispersion of Pd/HSꢀNH (5 wt%) is better than Pd/HS (5
2
wt%), suggesting the amine group has good coordination and
dispersion ability with metal cations.
Figure 5 shows the XPS spectra in the Pd 3d binding energy
mesoporous structure to prevent the growth of Pd NPs also
contributes to the formation of ultrasmall Pd NPs, especially for
the low Pd loading catalyst (
Pd/HSꢀNH (5 wt%) is shown in Figure 4A. It is notably
observed that Pd NPs were uniformly dispersed in the shell of
HSꢀNH , and the plane spacing measured is about 0.228 nm,
which is in agreement with the (111) crystal plane of Pd. The
≤5 wt%). The HRꢀTEM image of
2
2
region of Pd/HSꢀNH
2
(5 wt%) and Pd/C (5 wt%). The peaks
that appear at 335.1 eV and 336.7 eV could be attributed to the
0
2+
0
2+
Pd and Pd , respectively. Pd /Pd ratio calculated from the
relative peak areas for Pd/HSꢀNH (5 wt%) and Pd/C (5 wt%)
2
Pd
wt%)
Particle
size (nm)
Dispersion
(%)
Samples
a
b
b
(
Pd/HSꢀNH
2
2
2
(1 wt%)
1.0
3.0
4.7
9.4
14.1
4.5
5.0
3.0
3.4
4.0
5.6
8.2
5.8
3.2
36.2
32.5
27.7
20.1
13.6
19.3
35.1
Pd/HSꢀNH
Pd/HSꢀNH
(3 wt%)
(5 wt%)
Pd/HSꢀNH (10 wt%)
2
2
Pd/HSꢀNH (15 wt%)
Pd/HS (5 wt%)
Pd/C (5 wt%)
a
b
Pd content determined by ICP-AES analysis; data calculated from CO
2
Figure 6. CO stripping voltammetry of Pd/HS-NH (5 wt%), Pd/HS (5 wt%) and
chemisorption results.
Pd/C (5 wt%).
4
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2
are 68/32 and 43/57, showing that Pd/HSꢀNH (5 wt%) has
0
0
2+
more Pd than Pd/C (5 wt%). Pd /Pd ratio (73/27) of Pd/HS (5
wt%) is a little higher than Pd/HSꢀNH (5 wt%), suggesting
amine group or reduction method has a little influence on the
oxidation state of Pd NPs. HSꢀNH with a high nitrogen content
2
ꢀ
1
Catal.
Conv. (%) Sel. (%) TOF (h )
2
ꢀ1
Pd/HSꢀNH (1 wt%)
95
94
96
63
29
98
90
59
36
42
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
121
128
135
85
2
of 3.70 mmol g (Table S1) could be suitable for stabilizing
0
highly dispersed Pd particles and preventing their reꢀ
2
Pd/HSꢀNH (3 wt%)
3
0, 31
0
oxidation.
NH (5 wt%). The XPS result also shows that Pd 3d binding
energy of Pd/HSꢀNH has a downshift of ~ 0.1 eV and ~ 0.2 eV
in comparison with Pd/HS and Pd/C, suggesting that Pd on HSꢀ
NH has more electronꢀrich character than that on SiO and C
Figure S6). This is possibly due to the electron denoting
This could explain higher Pd ratio for Pd/HSꢀ
2
Pd/HSꢀNH (5 wt%)
2
2
Pd/HSꢀNH
2
2
(10 wt%)
2
2
Pd/HSꢀNH
(15 wt%)
28
(
b
property of amine group. The N 1s core level for Pd/HSꢀNH (5
2
Pd/HSꢀNH (5 wt%)
1084
5052
81
2
wt%) and HSꢀNH2 could also verify the electron donating
+
c
character of nitrogen atom although small remnant ꢀNH3 coꢀ
2
Pd/HSꢀNH (5 wt%)
3
2, 33
exist in the amineꢀfunctionalized silicas (Figure S7).
The
Pd/HS (5 wt%)
Pd/C (5 wt%)
surface Pd contents of Pd/HSꢀNH (5 wt%) calculated from the
2
XPS results is 4.0 wt%, similar to the ICP results (4.7 wt%),
suggesting that Pd NPs are uniformly distributed on the shell of
HSꢀNH2.
58
c
Pd/C (5 wt%)
960
The CO stripping voltammetry is an effective technique to
2
1, 34
measure the surface electronic structure of noble metals.
As
Reaction conditions: quinoline 1 mmol, S/C = 200, solvent 2 mL cyclohexane,
a
shown in Figure 6, the CO stripping potential for Pd/HSꢀNH (5
H2 pressure (1 MPa), 50 °C; Apparent TOF is calculated as moles of
2
wt%), Pd/HS (5 wt%) and Pd/C (5 wt%) is 0.84 V, 0.77 V and
converted quinoline per mol of Pd per hour based on the slope of the
b
conversion with the conversion less than 30%; S/C=1000, solvent 1 mL
0
.72 V, respectively. Amine in HSꢀNH acts as an electronꢀ
2
o
c
2
cyclohexane, 3 MPa H , 80 C; S/C = 4000, solvent 1 mL cyclohexane, 3 MPa
donating ligand that increases the electronic density of Pd NPs,
thereby favouring the backꢀdonation of electrons to the 2
antibonding orbitals of CO, which accounts for the C O bond
o
2
H , 100 C
π
*
also optimized the H pressure using cyclohexane as solvent
≡
2
3
5
(
Figure S8). It was found that Pd/HSꢀNH was very active and
2
weakening (high potential to oxide CO). Therefore, higher
CO stripping potential indicates higher electronic density on
could catalyse the reaction even with H pressure as low as 0.1
2
MPa. In the following reaction, H2 pressure of 1 MPa was
employed.
Under the optimized reaction conditions, the catalytic
performance of Pd/HSꢀNH2 with different Pd loading was
metal NPs surface. This suggests that Pd/HSꢀNH (5 wt%) is
2
more electron rich than Pd/C (5 wt%), which is consistent with
XPS results. The CO stripping potential of Pd/HS (5 wt%) is
higher than Pd/C (5 wt%) because of the ꢀOH group in silica
3
4
tested in quinoline hydrogenation (Table 2). All Pd/HSꢀNH2
electron donation effect to the Pd NPs.
The strong,
shows excellent selectivity to 1, 2, 3, 4ꢀtetrahydroquinoline,
background oxidized electric current of commercial Pd/C (5
wt%) is possibly due to high conductivity of carbon support.
Catalytic performance of Pd NPs catalysts.
3
8
which is consistent with the literature results. The kinetic
curves for quinoline hydrogenation of Pd/HSꢀNH with
2
different Pd loadings are displayed in Figure S9. The quinoline
conversion increases almost linearly with reaction time. It
seems that all the catalysts afford zeroꢀorder reaction curves
when the conversion is less than 80 %, indicating a near
saturation coverage of active sites by quinoline species at the
1
, 2, 3, 4ꢀTetrahydroquinoline framework is a common
structural motif and is found in numerous biologically active
natural products and pharmacologically relevant therapeutic
agents.
one of the efficient method for the production of 1, 2, 3, 4ꢀ
tetrahydroquinoline (Scheme 2). The catalytic performance of
2
Pd/HSꢀNH was tested in quinolines hydrogenation to produce
tetrahydroquinolines. Considering the important role of solvent
plays in catalytic activity and selectivity, solvent effect was
investigated firstly (Table S2). The product selectivity is > 99
3
6, 37
The quinoline hydrogenation reaction represents
3
9
initial stage. The apparent TOF increases slightly from 121 to
ꢀ1
1
35 h as the increment of Pd content from 1 wt% to 5 wt%.
Further increasing Pd loading causes sharp decrease in TOF.
The influence of Pd loading on the catalytic activity of Pd/HSꢀ
NH
2
is related with the particle size of Pd. Due to the broad
with high Pd loading,
particle size distribution for Pd/HSꢀNH
2
%
for all solvents investigated. However, the type of solvent
greatly affects the catalytic activity of Pd/HSꢀNH . The
2
quinoline conversion in aprotic solvents is much higher than
that in protic solvents. This is possibility relative to the
hydrogen bonding effect between protic solvent and quinoline
2
or amine in Pd/HSꢀNH . Considering the volatility, toxicity and
activity, cyclohexane was selected as the reaction solvent. We
Scheme 2. Possible products of the quinoline hydrogenation.
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the particle size measured with CO chemisorption method was
used for the following discussion. Pd NPs with average particle
size of in the range of 3.4 to 4.0 nm affords highest TOF and
with particle size larger than 4.0 nm give low activity,
suggesting that quinoline hydrogenation is a size sensitive
reaction (Figure S10). Recently, Yin and coꢀworkers also
reported the size dependent property of quinoline
hydrogenation with Pt NPs stabilized by resorcinolꢀ
4
0
formaldehyde resin as catalysts.
Under similar reaction conditions, the apparent TOF of Pd/HSꢀ
NH (5 wt%) is almost 1.6 times and 2.3 times as high as the
control sample, Pd/HS (5 wt%, TOF of 81 h ) and the
2
ꢀ1
ꢀ1
benchmark catalyst, commercial Pd/C (5 wt%, TOF of 58 h ).
This shows superior catalytic performance of Pd/HSꢀNH (5
2
wt%). The adsorption and desorption of substrates and products
play an important role in a catalytic reaction. Generally, the
electronꢀrich metal surface can result in a strong repulsive
interaction with electronꢀrich reactants during reaction, which
may enhance the catalytic activity. Based on the above
discussion, the high activity of Pd/HSꢀNH
Pd/C is possibly due to the electron rich surface of Pd NPs on
HSꢀNH
The catalytic performance of model Pd/HSꢀNH
tested under different conditions (Table 2). The selectivity to 1,
, 3, 4ꢀtetrahydroquinoline is >99% under all conditions
employed. The apparent TOF of Pd/HSꢀNH (5 wt%) could
2
than Pd/HS and
3
4
may enhance the activity or selectivity. Recently, Zheng and
coꢀworkers have revealed that the activity and selectivity of
nitrobenzene hydrogenation to Nꢀhydroxyaniline is improved
2
.
2
(5 wt%) was
2
1
greatly on electronꢀrich platinum nanowires. Earlier work
4
1
demonstrated that quinoline is electronꢀdeficient in nature.
2
The nature charge of nitrogen atom is ꢀ0.494 and ꢀ0.681 for
quinoline and 1, 2, 3, 4ꢀtetrahydroquinoline, respectively
2
ꢀ
1
o
ꢀ1
o
reach 1084 h at 80 C and 3 MPa H
and 3 MPa H . Pd/C (5 wt%) shows apparent TOF of 960 h at
MPa H and 100 C. Pd/MgO affords TOF of 300 h at 4
MPa H and 150 C. Rh/PEG gives TOF of 762 h at 3 MPa
and 100 C. As far as we know, Pd/HSꢀNH
2
and 5052 h at 100 C
ꢀ1
(Figure S11). Consequently, Pd NPs with electronꢀrich surface
2
o
ꢀ1
would prefer the adsorption of substrates over products, which
3
2
o
42
ꢀ1
2
o
43
H
2
2
(5 wt%) is the
most active solid catalyst ever reported for quinoline
hydrogenation (Table S3).
Subtrates
Products S/C Conv. (%)
Sel. (%)
>99
The applicability of Pd/HSꢀNH
compounds were investigated, and the results are summarized
in Table 3. Pd/HSꢀNH (5 wt%) could efficiently catalyse the
2
(5 wt%) to other heterocyclic
2
00
97
2
hydrogenation of 8ꢀmethylquinoline, 4ꢀmethylquinoline, 2ꢀ
methylquinoline, quinoxaline, pyridine, furan, and benzofuran
to corresponding hydrogenated products with conversion
ranging from 84% to >99% and selectivity ranging from 96 %
to >99%. For methyl substituted quinoline, 8ꢀmethylquinoline
shows the highest reactivity and 4ꢀmethylquinoline affords the
lowest reactivity. This is possibly due to the fact that 4ꢀ
methylquinoline with substitution at pyridine ring is difficult to
be adsorbed on Pd NPs. The wide substrate scope denmostrates
2
5
99
96
5
0
0
99
84
98
99
6
the excellent catalytic performance of Pd/HSꢀNH
2
(5 wt%) for
the hydrogenation of heterocyclic compounds.
We tested the recycle stability of Pd/HSꢀNH
2
(5 wt%) with
4
2
00
00
92
>99
a
quinoline hydrogenation as model reaction. After the first
reaction cycle, the catalyst could be recovered by
centrifugation. After being washed with ethanol thoroughly and
dried under vacuum, the recovered catalyst was directly used
>99
>99
>99
>99
2
for the next run. Pd/HSꢀNH (5 wt%) could be stably recycled
2
5
for 6 cycles without obvious loss in catalytic activity and
selectivity. After the sixth cycle, the catalyst was characterized
by TEM as shown in Figure S12. No aggregation of Pd NPs
could be observed in the TEM image. This also demonstrate the
Reaction conditions: subtrate 1 mmol, solvent 2 mL cyclohexane, H
2
a
pressure 1 MPa, 50 °C, time 2h; S/C=1000, solvent 1 mL cyclohexane, 1 bar
o
2
stability of Pd/HSꢀNH (5 wt%).
2
H , 25 C, time 1 min.
6
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ARTICLE
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Acknowledgements
This work was financially supported by Natural Science
Foundation of China (21325313, 21321002) and by the
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An amine-rich silica hollow nanospheres were prepared by using a one-pot condensation
-1
method. The ultralsmall Pd NPs stabilized by the material with TOF as high as 5052 h is among
the most active solid catalyst for quinoline hydrogenation. The high catalytic activity could be
mainly attributed to the ultrasmall particle size and high surface electronic density of Pd NPs.
1