Silver nanoparticles encapsulated by metal-organic-framework give the highest turnover frequencies of 105 h?1 for three component reaction by microwave-assisted heating

Article abstract of DOI:10.1016/j.jcat.2017.02.013

Microwave (MW)-assisted heating, due to its unique properties has been used for synthesis of metal, oxide, zeolite, and metal-organic framework (MOF), and even to carry out organic reactions. However, less attention has been paid to integration of two or

Full text of DOI:10.1016/j.jcat.2017.02.013

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Journal of Catalysis 348 (2017) 276–281
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Priority Communication
Silver nanoparticles encapsulated by metal-organic-framework give the
highest turnover frequencies of 105 hÀ1 for three component reaction by
microwave-assisted heating
Di Li a,1, Xiaoping Dai a,1, Xin Zhang a, , Hongying Zhuo b, Yan Jiang b, Yan-Bing Yu b, Pengfang Zhang b,
⇑
Xingliang Huang b, Hai Wang b
a State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, Beijing 102249, China
b National Institute of Metrology, Beijing 100013, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Microwave (MW)-assisted heating, due to its unique properties has been used for synthesis of metal,
oxide, zeolite, and metal-organic framework (MOF), and even to carry out organic reactions. However,
less attention has been paid to integration of two or more functional components into advanced materials
by MW-assisted strategy. We report herein for the first time for controllable integration of silver
nanoparticles (NPs) and IRMOF-3 as a core-shell Ag@IRMOF-3 nanostructures into highly efficient cata-
lysts by a facile one-pot MW irradiation method. Impressively, the uniform core-shell Ag@IRMOF-3
nanostructures can be fabricated as short as 5 min and are highly active for three-component coupling
reaction of amine, aldehyde and acetylene (A3-coupling) by MW-assisted heating strategy. Specifically,
the Ag@IRMOF-3 with average size of Ag NPs of 5.2 nm and IRMOF-3 of 91.5 nm exhibited the highest
turnover numbers and turnover frequencies of 3052 and 149,004 hÀ1 reported up to date, respectively,
for the A3-coupling reaction. Thanks to the MOF shell coating, the Ag@IRMOF-3 can be recycled at least
for 8 runs without losing any activity; furthermore, the size-selective catalysis was also successfully
achieved due to the size selective for propargylamines with different sizes.
Received 24 November 2016
Revised 21 January 2017
Accepted 13 February 2017
Keywords:
Core-shell
Ag nanoparticles
IRMOF-3
Microwave
A3-coupling reaction
Ó 2017 Elsevier Inc. All rights reserved.
Controllable integration of two or more functional components
is of significant importance in design and synthesis of advanced
composite materials with collective properties and improved per-
formance [1]. Recently integration of metal/metal oxide nanoparti-
cles (NPs) and metal-organic framework (MOF) into core-shell
NPs@MOF nanostructure has been the subject of intense research.
[1–3] Two approaches including ‘‘ship-in-a-bottle” and ‘‘bottle-ar
ound-ship” for the encapsulation of NPs into MOF have been
reported [2a]. These approaches usually consist of two or multi
steps by using conventional solvothermal synthesis. Tang’s group
has prepared Au@MOF-5 in one-pot solvothermal synthesis. And
the solvothermal synthesis took 3 h and the obtained Au
nanoparticles were larger than 30 nm [2e]. So far most of
reported synthesis was from several hours to multiday and the
core size of NPs or shell thickness of MOF was in large sizes.
straightforward synthetic strategy to prepare metal NPs@MOF
with tunable nano-sizes is highly desired.
Microwave (MW)-assisted heating due to its unique advantages
such as fast crystallization, high dispersion, decreasing reaction
time, and narrow particle size distribution has been used to syn-
thesis of metal and metal oxide, zeolite as well as MOF, respec-
tively [4]. However, there are few reports to integrate two or
more functional components into advanced composite materials
by MW-assisted strategy. We report herein a facile one-pot synthe-
sis to prepare Ag@IRMOF-3 nanostructures with uniform shape
and tunable sizes through a rapid MW-assisted methodology.
Distinguished with the earlier metal NPs@MOF, the well-defined
Ag@IRMOF-3 can be fabricated as short as 5 min. The core size of
Ag NPs and the shell size of IRMOF-3 can be tuned in the range
of 5.2–45.9 nm and 24.0–174.9 nm, respectively.
Therefore the development of
a
rapid, efficient and
We propose that the uniform core-shell Ag@IRMOF-3 could be
used as the high-efficient catalyst for three-component coupling
reaction. The coupling reaction of amine, aldehyde and acetylene
(A3-coupling) is designed as the model reaction [5a]. The reason
to study this A3-coupling is that the target products of
propargylamines are versatile synthetic intermediates organic
⇑
Corresponding author.
E-mail address: zhangxin@cup.edu.cn (X. Zhang).
1
These authors contributed equally.
http://dx.doi.org/10.1016/j.jcat.2017.02.013
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

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D. Li et al. / Journal of Catalysis 348 (2017) 276–281
277
synthesis and are also important structural elements in natural
products and therapeutic drug molecules [5b]. These compounds
were usually synthesized by lithium acetylides or Grignard
reagents in stoichiometric amounts, or recently by homogenous
gold, silver, and copper salt [5c–5g]. But these reagents and
homogeneous catalysts are difficult to recycle and cannot be
reused. Recent progress in this area has been reported using
gold, silver nanoparticles and heterogeneous gold and copper
catalysts [5a,6]. However the catalytic performance especially
stability of these heterogeneous gold catalysts is still not
satisfactory. Encouraged by the novel synthesis of the core-shell
Ag@IRMOF-3 nanoparticles, the three-component coupling
reaction of acetylene (benzacetylene), aldehyde (formaldehyde or
benzaldehyde), and amine (piperidine) (A3-coupling) was carried
out by MW and conventional heating by oil-bath, respectively
decreased from 18.6 to 11.9 and 5.2 nm, respectively, while the
shell sizes of IRMOF-3 remained almost intact (92–97 nm)
(Fig. 1B, D, and E). Analysis of the cores by high-resolution trans-
mission electron microscopy (HRTEM) imaging of 2.06%
Ag@IRMOF-3 indicated that the spacing between two adjacent lat-
tice planes was about 0.24 nm (Fig. 1F). This value was in agree-
ment with the spacing of (111) planes of cubic Ag,
demonstrating that the cores were Ag NPs. The high-angle annular
dark field scanning transmission electron microscopy (HAADF-
STEM) image (Fig. 1G) further clearly demonstrated the core–shell
structure. And the energy-dispersive X-ray (EDX) elemental map-
ping showed that the Ag element was distributed only in the core
and that the elements of C, N, O, and Zn of IRMOF-3 were homoge-
nously distributed throughout the whole shell, suggesting that the
Ag NP core was surrounded with a uniform IRMOF-3 shell (Fig. 1H–
L).
(Scheme 1). Particularly, such
a novel type of core-shell
Ag@IRMOF-3 catalyst exhibited the highest turnover numbers
and turnover frequencies of 3052 and 149,004 hÀ1 up to date,
respectively, for the A3-coupling reaction. To the best of our
knowledge, this is the first example to engineer silver
nanoparticles (NPs) and IRMOF-3 as core-shell Ag@IRMOF-3
nanostructures into highly efficient catalysts by a facile one-pot
MW irradiation method.
Crystal structure analysis from powder X-ray diffraction (XRD)
patterns revealed a typical phase of IRMOF-3 for all Ag@IRMOF-3
nanocomposites. Only a very weak diffraction pattern centered at
38.1° ascribed to face-centered cubic (fcc) phase of Ag was found
on 2.06% and 2.63%Ag@IRMOF-3 samples while no diffraction pat-
tern of fcc phase of Ag was found on 0.09%Ag@IRMOF-3. The rela-
tive crystalline degree in terms of the intensity of classic peak
centered at 2h = 6.8° of IRMOF-3 was significantly increased with
the reaction time increased from 5, 15 to 30 min (Fig. 2A). Fourier
transform infrared (FTIR) spectra (Fig. S1) indicated that the char-
acteristic AOH stretching frequency in the carboxylic acid group of
the NH2-H2BDC linker centered at 2670 cmÀ1 disappeared after
formation of the Ag@IRMOF-3 nanocomposites, suggesting occur-
rence of the coordination interaction between Zn2+ ions and car-
boxylic acid group of NH2-H2BDC to form IRMOF-3. The
characterization by X-ray photoelectron spectroscopy (XPS)
showed that there were not any Ag signals, indicating that Ag
nanoparticles were completely covered with IRMOF-3 shells that
shielded the underlying Ag surface (Fig. 2B). The UV/Vis spectra
showed that the vibration of Ag NPs at 400 nm wavelength became
weaken when the reaction time increased from 5 to 15 min, and it
was completely disappeared at 30 min due to the shield of the lar-
ger length of the shell (Fig. 2C). These results further demonstrated
the core-shell nanostructure of Ag@IRMOF-3. Hydrogen-
temperature-programmed reduction (H2-TPR) experiments
showed that only one peak centered at ca. 455 °C which was
assigned to the decomposition of IRMOF-3, and no hydrogen con-
sumption peak ascribed to the reduction of cationic silver species.
The result indicated that the silver species in Ag@IRMOF-3 samples
were in metallic state (Fig. 2D). The features of the MOF shells,
especially the porosity, determine the properties of the core–shell
NPs. The representative of Ag@IRMOF-3 nanostructures deter-
mined by nitrogen adsorption-desorption experiments featured a
classic Type 1 nitrogen sorption isotherms with the steep increase
in N2 uptake at low relative pressure (<0.01), indicating
Ag@IRMOF-3 possesses microporous pore. The average pore size
was ca. 1.9 nm calculated according to Saito-Foley method based
on molecular modulation (Fig. S2). And the BET surface areas were
Different from the conventional two-step method to synthesize
NP@MOF composites by adding the pre-synthesized NPs into the
MOF precursors [2d,7], we prepared the core–shell Ag@IRMOF-3
nanostructures by directly mixing the Ag with MOF precursors of
AgNO3, Zn(NO3)2Á6H2O, and 2-aminoterephthalic acid (NH2-
H2BDC)
in
the
reaction
solution
containing
N,N-
dimethylformamide (DMF), and polyvinylpyrrolidone (PVP) under
MW irradiation method. By controlling the reaction conditions,
the formation rates of Ag NPs and IRMOF-3 in the solution could
be adjusted effectively and thus a series of core-shell Ag@IRMOF-
3
crystals with different nano-sizes were obtained (see
Experimental sections of the Supporting Information).
The morphology and structure of as-prepared Ag@IRMOF-3
nanocomposites were in-depth investigated by different character-
ization techniques. Clearly the well-defined core-shell structure of
Ag@IRMOF-3 can be obtained at 5 min by using the one-pot syn-
thesis assisted by MW heating at 120 °C (Fig. 1A). By prolonging
the reaction time from 5 to 15, and 30 min, the average sizes of
core were increased from 7.4 to 18.6, and 45.9 nm, respectively,
and the shell size of IRMOF-3 also increased from 24.0 nm to
97.4 and 174.9 nm, respectively (Fig. 1A–C). It is evident that the
feature of core-shell structure was well preserved, and the core
size of Ag NPs and the shell size of IRMOF-3 are significantly
increased by increasing the reaction time. The Ag loadings mea-
sured by ICP-AES also increased from 2.06% to 2.10% and 2.63%,
respectively, with increasing the reaction time from 5 to 15, and
30 min. By decreasing the AgNO3 amount from 0.014 mmol to
0.0014, and 0.0005 mmol at 15 min with MW at 120 °C, the corre-
sponding Ag content decreased from 2.10% to 0.25%, and 0.09%,
respectively. The well-defined core-shell Ag@IRMOF-3 nanostruc-
tures were still remained. The core size of Ag nanoparticles was
1
2
Scheme 1. The A3 coupling reaction of phenylacetylene, formaldehyde or benzaldehyde, and piperidine on Ag@IRMOF-3 nanostructures.

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278
D. Li et al. / Journal of Catalysis 348 (2017) 276–281
25
20
15
10
5
100
80
60
40
20
0
A
B
0
Core size Shell size
Core sirze Shell size
Core
size
Shell size=2r
100 nm
50 nm
200
160
120
80
100
D
C
80
60
40
20
0
40
0
Core size Shell size
Core size Shell size
50 nm
500 nm
100
E
F
80
60
40
20
0
Core size Shell size
50 nm
H
I
G
J
K
L
Fig. 1. Represented TEM images and average sizes of core and shell (inset) of Ag@IRMOF-3 nanostructures under MW heating at 120 °C with 0.014 mmol AgNO3 at
crystallization time of (A) 5 min, (B) 15 min, (C) 30 min, and with (D) 0.0014 mmol, (E) 0.0005 mmol AgNO3 at crystallization time of 15 min, (F) HRTEM image of the
represented core, (G) HAADF-STEM image and (H–L) corresponding EDX elemental mapping of 2.06% Ag@IRMOF-3.

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D. Li et al. / Journal of Catalysis 348 (2017) 276–281
279
shell nanostructure was obtained (Fig. S3). The MW irradiation
could facilitate the rapid crystallization of Ag NPs and change the
coordination environment of metal ions such that IRMOF-3
grows preferentially around the Ag NPs instead of self-nucleating
in solvothermal heating, which can be supported by formation of
the core of Ag NPs at 1 min under the MW irradiation (Fig. S4).
The core-shell structure of Ag@IRMOF-3 can be formed at 5 min
(Fig. 1A). The concentration of AgNO3 was found to be another
key to generate core-shell Ag@IRMOF-3 nanostructures. When no
AgNO3 was added, there was pure IRMOF-3 with an average
diameter of 100 nm to be formed (Fig. S5), while core-shell
Ag@IRMOF-3 crystals were produced with the addition of AgNO3.
The more Ag precursors were added, the larger Ag NP cores were
produced. The appropriate amount (1.09 mg) of PVP also played
B
Zn 2P3/2
O1s
N1s
A
C1s
a
b
a
c
c
384 376 368 360
Binding energy (eV)
d
510 20 30 40 50 60
1200 900 600 300
0
2-Theta (degree)
Binding energy (eV)
A
C
a
D
a
c
c
b
d
d
a
vital role in formation of well-defined Ag@IRMOF-3
200300400500600700800
0
140 280 420 560 700
Temperature
oC
nanostructures. In the absence of PVP, Ag NPs of 2.0%Ag/IRMOF-3
tended to aggregate heavily with average size of 25 nm and wide
size distribution of 9–50 nm. At much less (0.55 mg) or higher
amount (2.19 mg) of PVP, a shell with multiply Ag NPs or
discrete Ag NPs has been observed (Fig. S6). It is concluded that
PVP acts as stabilizer of Ag NPs as well as provides the affinity to
adsorb IRMOF-3 precursors onto the Ag NPs surfaces via
coordination interaction between the pyrrolidone rings (C@O)
and Zn2+ [8].
Fig. 3A compares the formaldehyde conversions over 0.09%
Ag@IRMOF-3, 0.01%Ag/IRMOF-3, and 2.0%Ag/IRMOF-3 catalysts
for the A3 coupling reaction of formaldehyde, piperidine and
phenylacetylene at 120 °C heated by MW irradiation and oil bath,
respectively. Clearly, 2.0%Ag/IRMOF-3 with large size distributions
of Ag NPs showed very low and similar conversions, e.g., 2.6% ver-
sus 1.9% at 5 min and 4.9% versus 4.6% at 20 min by MW irradiation
and conventional heating, respectively. 0.01%Ag/IRMOF-3 with
small Ag NPs exhibited the conversion of 6.3% at 5 min and the
maximum of 12.0% at 60 min with oil bath heating. These conver-
sions can be significantly enhanced to 12.4% and 41.3% at 5, and
60 min, respectively, with MW irradiation. Notably, 0.09%
Ag@IRMOF-3 with core-shell structure showed 97.9% of conversion
Wave length (nm)
(
)
Fig. 2. (A) XRD patterns, (B) XPS, (C) UV-vis, and (D) TPR profiles of Ag@IRMOF-3
nanostructures under MW heating at 120 °C with 0.014 mmol AgNO3 at crystal-
lization time of (a) 30 min, (b) 15 min, (c) 5 min, and (d) pure IRMOF-3.
found to be increased from 290 to 305, and 370 m2/g with increas-
ing the reaction time from 5 to 15 and 30 min, respectively.
The detailed formation mechanism of the core-shell
Ag@IRMOF-3 nanostructures is elucidated by a series of control-
lable experiments including heating method, variation of AgNO3
concentration, PVP amount and crystallization time. The MW irra-
diation heating is vital for the formation of core-shell Ag@IRMOF-3
nanostructures by using the sole solvent of DMF. By using conven-
tional solvothermal heating, the mixed solvent such as DMF and
ethanol was indispensable to prepare the core–shell NPs@MOF
nanostructure in previous works [2d,2e,8]. Indeed, at the
conventional solvothermal heating conditions of 18 h at 100 °C
while keeping other conditions unchanged with Ag@IRMOF-3, a
truncated octahedron of IRMOF-3 with depositing small sizes of
Ag NPs averaged 2.1 nm of 0.01%Ag/IRMOF-3 rather than a core-
150000
3500
3000
2500
2000
1500
1000
500
100
B
Yield,%
TON
a
145000
140000
135000
80
60
40
20
0
A
TOF, h-1
130000
c
100
80
60
40
20
0
b
e
d
f
0
0
10 20 30 40 50 60
Reaction time (minutes)
Ag@IRMOF-3
Au/CeO2
AuBr3
100
100
80
60
40
20
0
C
a
D
80
60
40
20
0
b
c
d
0
10 20 30 40 50 60
Reaction time (minutes)
0
1
2
3
4
5
6
7
8
Reaction cycle numbers
Fig. 3. (A) Formaldehyde conversion versus reaction time over 0.09%Ag@IRMOF-3 (a and b), 0.01%Ag/IRMOF-3 (c and d), and 2.0%Ag/IRMOF-3 (e and f) catalysts for the A3
coupling reaction of formaldehyde, piperidine, and phenylacetylene with MW-irradiation (☐, s, Δ) and conventional heating (j, d, ▲) at 120 °C. (B) The comparison of yield,
TON, and TOF between 0.09%Ag@IRMOF-3 and the previous heterogeneous Au/CeO2 and homogeneous AuBr3 for the A3 coupling reaction. The TON and TOF were calculated
based on the exposed Ag surface atoms for 0.09%Ag@IRMOF-3 catalyst, and total gold species for Au/CeO2 and AuBr3. (C) Catalytic stability of 0.09%Ag@IRMOF-3 catalyst for
the A3 coupling reaction at reaction time of 10 min and 120 °C with MW heating. (D) The A3 coupling reaction of piperidine, benzacetylene and formaldehyde (a and b) or
benzaldehyde (c and d) with MW (☐ and s) and conventional heating (j and d) on 0.09%Ag@IRMOF-3 at 120 °C respectively.