Design and Remarkable Efficiency of the Robust Sandwich Cluster Composite Nanocatalysts ZIF-8@Au25@ZIF-67

Article abstract of DOI:10.1021/jacs.0c00378

Heterogeneous catalysts with precise surface and interface structures are of great interest to decipher the structure-property relationships and maintain remarkable stability while achieving high activity. Here, we report the design and fabrication of the new sandwich composites ZIF-8@Au25@ZIF-67[tkn] and ZIF-8@Au25@ZIF-8[tkn] [tkn = thickness of shell] by coordination-assisted self-assembly with well-defined structures and interfaces. The composites ZIF-8@Au25@ZIF-67 efficiently catalyzed both 4-nitrophenol reduction and terminal alkyne carboxylation with CO2 under ambient conditions with remarkably improved activity and stability, compared to the simple components Au25/ZIF-8 and Au25@ZIF-8, highlighting the highly useful function of the ultrathin shell. In addition, the performances of these composite sandwich catalysts are conveniently regulated by the shell thickness. This concept and achievements should open a new avenue to the targeted design of well-defined nanocatalysts with enhanced activities and stabilities for challenging reactions.

Full text of DOI:10.1021/jacs.0c00378

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Communication
Design and Remarkable Efficiency of the Robust Sandwich
Cluster Composite Nanocatalysts ZIF-8@Au25@ZIF-67
Yapei Yun, Hongting Sheng, Kang Bao, Li Xu, Yu Zhang, Didier Astruc, and Manzhou Zhu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00378 • Publication Date (Web): 12 Feb 2020
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Design and Remarkable Efficiency of the Robust Sandwich
Cluster Composite Nanocatalysts ZIF-8@Au25@ZIF-67
Yapei Yun,†§ Hongting Sheng,*,†§ Kang Bao,†§ Li Xu,†§ Yu Zhang,†§ Didier Astruc,*,‡ Manzhou Zhu*,†§
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† Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of
Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei, Anhui 230601, China
§ Key Laboratory of Structure and Functional Regulation of Hybrid Materials (Anhui University), Ministry of Education,
Hefei 230601, China
‡ Université de Bordeaux, ISM, UMR CNRS N° 5255, 351 Cours de la Libération, 33405 Talence Cedex, France
Supporting Information Placeholder
stability but poor activity owing to high diffusion resistance that
hinders the access of reactants to the MNPs.6b-d Therefore, this
creates a dilemma regarding the expected target for an ideal MNP-
decorated MOF nanocomposite with enhanced reactivity and
excellent stability for a given reaction. Sandwich MOFs@MNPs
@MOFs composites may be able to work to this end, because the
sandwich structure maximizes the metal-support interface to
regulate the activity, and the controllable shell prevents MNPs from
separating during the reaction process. Tang's group fabricated
ABSTRACT: Heterogeneous catalysts with precise surface and
interface structures are of great interest to decipher the structure–
property relationships and maintain remarkable stability while
achieving high activity. Here, we report the design and fabrication
of the new sandwich composites ZIF-8@Au25@ZIF-67[tkn] and
ZIF-8@Au25@ZIF-8[tkn] [tkn
=
thickness of shell] by
coordination-assisted self-assembly with well-defined structures
and interfaces. The composites ZIF-8@Au25@ZIF-67 efficiently
catalyzed both 4-nitrophenol reduction and terminal alkyne
carboxylation with CO2 under ambient conditions with remarkably
improved activity and stability, compared to the simple
components Au25/ZIF-8 and Au25@ZIF-8, highlighting the highly
useful function of the ultrathin shell. In addition, the performances
of these composite sandwich catalysts are conveniently regulated
by the shell thickness. This concept and achievements should open
a new avenue to the targeted design of well-defined nanocatalysts
with enhanced activities and stabilities for challenging reactions.
sandwich MIL-101@Pt@MIL-101 composite exhibiting
a
considerable improvement in the reactivity, stability and selectivity
for the hydrogenation of α,β-unsaturated aldehydes compared with
the corresponding Pt@MIL-101.7 Despite this success for
MOFs@MNPs@MOFs, the key drawback herein is the ill-defined
MNPs and blurred interaction at the interfaces (the inevitable
presence of a stabilizing polymer (PVP) in the obtained
MOFs@MNPs@MOFs) that impedes the clarification of the
structure–property relationships. To our knowledge, an effective
and general solution to overcome this challenge has not yet
appeared. In light of these requirements, we inferred that the
catalysts MOFs@APNCs@MOFs (APNCs: monodisperse
atomically precise nanoclusters), in which all the components are
well-defined, would work to this end, i.e., allow to better decipher
With the ever-increasing desire toward sustainable energy, a
great deal of efforts have been devoted to the design of
heterogeneous nanocatalysts.1 In some cases, special emphasis is
placed on improving reactivity of these nanomaterials while
ignoring their stability.2 In fact, the stability of heterogeneous
nanocatalysts, especially for ultrasmall metal nanoparticles, is a
key parameter for practical utilizations and should be one of the
most sought properties, because stability guarantees catalytic
recyclability.3 Various strategies have been developed to pursue
nanocatalysts with remarkable activity and stability,4 among which
the combination of MNPs (metal nanoparticles) and MOFs (metal-
organic frameworks) holds great promise.5 Indeed MNP-decorated
MOF nanocomposites are endowed with additional active sites to
work synergistically, and confined effects prevent MNP
aggregation, resulting in a well enhanced catalytic performance.
the structure–property relationships at the molecular level.4c,
Moreover, the comparison of the catalytic properties of three
8
monodisperse APNCs-decorated
MOF
nanomaterials:
APNCs/MOFs, APNCs@MOFs and MOFs@APNCs@MOFs
would be conducive to draw a rigorous conclusion toward precise
customization of
Recently, a growing number of publications have indicated that
the catalytic performance of MNP-decorated MOF nanocomposites
markedly depends on the MNP location relative to the MOF.6 In
most cases, MNPs/MOFs (MNPs deposited on the surface of
MOFs) exhibit high activity but low stability, because MNPs
loaded on the outer surface easily fall off. On the other hand
MNPs@MOFs (MNPs intercalated in MOFs) present a good
Scheme 1. Synthetic route for the sandwich structures of ZIF-8@Au25@ZIF-
67[tkn] and ZIF-8@Au25@ZIF-8[tkn] [tkn = thickness of shell].
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APNCs-decorated MOF nanocatalysts. Our strategy for fabricating
consistent with the interaction between Au25(SG)18 (83.7 eV) and
Au25(SG)18/ZIF-8 (84.1 eV).9,12 BET shows ZIF-8@Au25@ZIF-
67[12] has similar isothermal features (Figure S3) and a pore
diameter ~1.1 nm (Table S1) to ZIF-8 and ZIF-67, indicating that
the porous structures of the ZIF-67 and ZIF-8 matrices remain
intact after embedding Au25(Cys)18. The surface area of ZIF-
8@Au25@ZIF-67[12] increased correspondingly, which is
presumably related to the self-assembly (Table S1). The
morphology and composition of ZIF-8@Au25@ZIF-67[12] were
characterized by TEM and energy dispersive X-ray spectroscopy
(EDS) (Figure 2).
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the sandwich nanomaterial MOF@APNCs@MOF involved a
design by coordination-assisted self-assembly including a three-
step process (Scheme 1). First, the nanocluster Au25(Cys)18 9 with
terminal ligand groups COOH and ZIF-810 were synthesized,
characterized by TEM and UV-vis. spectroscopy (Figure S1), and
evenly dispersed in aqueous solution, respectively. Then a solution
of Au25(Cys)18 was slowly poured into the ZIF-8 solution via
physical impregnation to form the nanomaterial Au25/ZIF-8 that
was characterized by TEM (Figure S2) and IR spectroscopy (Figure
1B). Finally, the composite Au25/ZIF-8 was further uniformly
dispersed in methanol solution, and the solution of Co(NO3)2 at a
specific concentration was slowly added to the solution of
Au25/ZIF-8, resulting in Co2+ coating on the Au25/ZIF-8 surface by
coordination between Co2+ and the carboxylate group. The solution
of 2-MeIm was slowly added to the above system, and the sandwich
composite ZIF-8@Au25@ZIF-67 was obtained by gentle agitation
at room temperature for 24 hours. To the best of our knowledge,
this is the first report on the rational fabrication of APNCs-
decorated MOF sandwich materials. Interestingly, the shell
thickness of ZIF-8@Au25@ZIF-67[tkn] was controlled at will by
precisely adjusting the concentration of 2-MeImA and Co2+
solution. In addition, the nanomaterials ZIF-8@Au25@ZIF-8[tkn]
were also fabricated using the same strategy.
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Figure 2. TEM images of (A) ZIF-8@Au25@ZIF-67[12], (B-G) High-angle
annular dark field scanning transmission electron microscopy (HAADF-STEM)
images and corresponding EDS elemental (C, N, Zn, Co, Au, S) mapping of ZIF-
8@Au25@ZIF-67[12]; (H) ZIF-8@Au25@ZIF-67[2], (I) ZIF-8@Au25@ZIF-
67[8], (J) ZIF-8@Au25@ZIF-67[18], and (K) ZIF-8@Au25@ZIF-67[25].
The nanocomposite presents a sandwich-like structure with a
shell of about 12 nm (Figure 2A). It also clearly appears that the
nanoclusters Au25 NCs are evenly distributed in the interlayer of
the ZIF-8@Au25@ZIF-67[12] in the TEM micrograph. The
composition and element distribution of ZIF-8@Au25@ZIF-67[12]
further confirm that the surface of Au25/ZIF-8 is fully covered by
the ZIF-67 shell (Figure 2B-G). The ZIF-8@Au25@ZIF-67[2],
ZIF-8@Au25@ZIF-67[8], ZIF-8@Au25@ZIF-67[18] and ZIF-
8@Au25@ZIF-67[25] with different shell thicknesses were
successfully prepared by regulation of the concentration of Co2+
and characterized by TEM (Figure 2H-K) and EDS energy
spectrum element diagrams, respectively (Figure S4-S6, S8). In the
meantime, the Au25 NCs did not aggregate in these nanomaterials,
and most of their sizes were less than 1.2 nm (Figure 2A-K). The
EDS line scanning of ZIF-8@Au25@ZIF-67[18] verifies the shell
thickness (Figure S7), consistent with the TEM results. Using the
same strategy, ZIF-8@Au25@ZIF-8[12] was successfully
synthesized by replacing Co(NO3)2 by Zn(NO3)2, and related
characterizations were performed (Figure S9). All the above results
indicate that the “coordination-assisted self-assembly” strategy for
the fabrication of MOFs@APNCs@MOFs is effective and
universal. It guarantees the integrity of the Au25 NCs. Importantly
this method avoids the use of PVP and ensures the atomic accuracy
of the whole structure.
Figure 1. (A) XRD patterns of ZIF-8, ZIF-67 and ZIF-8@Au25@ZIF-67[12]; (B)
FTIR characterization of ZIF-8, ZIF-67, Au25/ZIF-8 and ZIF-8@Au25@ZIF-
67[12]; (C) XPS spectrum of ZIF-8@Au25@ZIF-67[12]; (D) XPS spectrum of
Au in Au25(Cys)18 and ZIF-8@Au25@ZIF-67[12].
The composite nanomaterial ZIF-8@Au25@ZIF-67[12] was
characterized by X-ray diffraction (XRD), Fourier-transform
infrared (FT-IR), X-ray photoelectron spectroscopy (XPS) analysis,
and N2 absorption-desorption isothermal (BET). The XRD pattern
of ZIF-8@Au25@ZIF-67[12] revealed that there was no significant
crystallinity loss after embedding Au25 into the ZIF interlayer,
indicating that the addition of Au25 did not affect the ZIF skeleton
(Figure 1A). There is no peak of Au in the XRD pattern, indicating
that the size of Au25 NC is ultra-small. The coordination interaction
between COO- in Au25 NC ligand and Zn2+ or Co2+ ions was
detected by Fourier transform infrared spectroscopy (FT-IR). From
Figure 1B, ZIF-8@Au25@ZIF-67[12] shows a new peak at 520 cm-
1, which is attributed to the Zn-O (or Co-O) stretching vibration,11
indicating that Au-Cys-Co or Au-Cys-Zn linkage is the key for self-
assembling. XPS analysis confirmed the existence of Au25 NC,
ZIF-8, and ZIF-67 in the ZIF-8@Au25@ZIF-67[12] composite.
The XPS result revealed the presence of Au, S, Co, Zn, and N in
the composite (Figure 1C). As shown in Figure 1D, the Au 4f7/2 of
ZIF-8@Au25@ZIF-67[12](84.2 eV) exhibits a slight negative shift
compared to Au25(Cys)18 (84.6 eV) likely due to the electron
transfer between Au25(Cys)18 and ZIF-8 or ZIF-67. This is
The useful carboxylation using CO2 of terminal alkyne was
selected as a model reaction to explore the influence of the fixation
location of Au25 NC on the catalytic performance and to compare
with Au25/ZIF-8 and Au25@ZIF-812 (Figure 3). As shown in Figure
3A, the superior performance of ZIF-8@Au25@ZIF-67[12], i.e.
remarkable maintenance of 99% yield even after 3 cycles (Figure
3B), is evidently better than that of all the other catalysts, including
Au25/ZIF-8 (40.5%) and Au25@ZIF-8 (38.2%), Au25 NC (10.8%),
ZIF-8 (27.9%) and ZIF-8@ZIF-67 (52.3%). ZIF-8@Au25@ZIF-
67[12] is one of the rare examples in which the Au catalyst exhibits
high performance for the carboxylation reactions of alkynes under
mild conditions (The TON value reached 4433, Table S2). By
contrast, the activity of Au25/ZIF-8 decreased significantly after the
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first cycle. In addition, the reactivity of Au25@ZIF-8 is as low as
deprotonated forming intermediate II under the combined action of
Cs2CO3 and Au25 (confirmed by IR, Raman and NMR, see Figure
S13-14), which is consistent with previous reports.14 Then CO2 is
captured (confirmed by TPD-CO2) and activated by 2-MIem of the
ZIF-67 shell to give intermediate III.13,15 Next, activated CO2
inserts into the C≡C---Au bond to form intermediate IV. Finally,
the products are released with regeneration of catalyst I.
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hypothesized. Then, ZIF-8@Au25@ZIF-67 composites with
different shell thicknesses were tested (Figure 3C). Interestingly,
the shell thickness had a significant effect on the activity. At the
beginning, the activity increased with the thickness. The highest
activity occurred for a thickness of 12 nm. Starting at 12 nm, the
activity began to decline slowly as the shell thickness increased.
This suggests that the shell may have other beneficial effects on the
reaction beyond the mass transfer and the stability of the material.
We then used temperature-programmed desorption (TPD) of CO2
to confirm that the shell does help capture and activate the CO2
molecules (Figure 3D).13 At the same time, the high activity of ZIF-
8@Au25@ZIF-67[12] (99%) compared to that of ZIF-
8@Au25@ZIF-8[12] (69%) also highlights the fact that the ZIF-67
shell is responsible for the enhancement of the catalytic
performance of ZIF-8@Au25@ZIF-67 (Figure 3A). All these
results confirmed that the catalytic performance of sandwich
systems is precisely optimized by controlling the shell thickness.
To further confirm the excellent stability of the sandwich
nanomaterial, we characterized the used ZIF-8@Au25@ZIF-67[12]
after five cycles and compared with the fresh catalyst by XPS, FTIR,
XRD and TEM and tested the UV tracking for Au25 NC (Figure
S10-12). These characterizations showed that all parameters
remained unchanged before and after the reaction, including the
electronic structure and size of the embedded Au25 NC. At the same
time, the mapping showed that the distribution of Au, S, Zn, Co and
N of the recycled catalyst was the same as that of the fresh catalyst,
indicating that ZIF-8@Au25@ZIF-67[12] maintains high stability
during the reaction.
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Figure 4. Proposed catalytic mechanism of the reaction between terminal
alkynes and CO2 over ZIF-8@Au25@ZIF-67[tkn].
The 4-nitrophenol (4-NP) reduction is an excellent gauge of the
performance of catalysts.16 To verify the importance of the ultrathin
shell, we further evaluated the catalytic properties of ZIF-
8@Au25@ZIF-67[12] for 4-NP reduction (Figure 5). Compared
with Au25@ZIF-8 and Au25/ZIF-8, ZIF-8@Au25@ZIF-67[12]
exhibited superior catalytic performance. The TOF value is up to
588 min-1, better than most reported catalysts in Table S4, due to
the dual interface effect between Au25 NC and the MOF. In the
meantime, the stability of ZIF-8@Au25@ZIF-67[12] is particularly
remarkable, maintaining excellent activity for at least 10 cycles.
These results further demonstrate that the sandwich system not only
facilitates stability but also accelerates activity.
Figure 3. (A) Catalytic activity of various catalysts for the carboxylation of
phenylacetylene. Reaction conditions: catalyst (1.12×10-4 mmol of Au25), alkyne
(0.5 mmol), Cs2CO3 (0.24 mmol), CO2 (1.0 bar), 50 °C, 12 h. (B) Column
diagram of three different catalysts. (C) Broken line diagram of catalytic
performance of ZIF-8@Au25@ZIF-67 with various shell thickness. (D) TPD-
CO2 by various catalysts.
Based on the stability of ZIF-8@Au25@ZIF-67[12], we explored
its tolerance to different functional groups (Table S3). Under
optimal conditions, ZIF-8@Au25@ZIF-67[12] exhibits good to
excellent activity (83~99%) for carboxylation of substituted-
phenylacetylene regardless of the electron-releasing or electron-
withdrawing nature of the attached group. Besides, 2-ethylpyridine
Figure 5. UV-Vis absorption spectra for 4-NP reduction by A) ZIF-
8@Au25@ZIF-67[12]; B) Au25/ZIF-8. C) Au25@ZIF-8. D) TOFs of recycled
ZIF-8@Au25@ZIF-67[12].
In summary, we have developed for the first time a coordination-
assisted self-assembly method to control the fabrication of
sandwiches ZIF-8@Au25@ZIF-8[tkn] and ZIF-8@Au25@ZIF-
67[tkn]. These well-defined composites feature simultaneously
improved activity and stability for terminal alkyne carboxylation
using CO2 and 4-NP hydrogenation. Interestingly, the catalytic
performance of the sandwich system is easily optimized by precise
adjustment of the shell thickness. Therefore, this work should
acetylene,
propargylamine,
cyclopropyl
acetylene
and
trimethylsilylacetylene were also probed with ZIF-8@Au25@ZIF-
67[12] providing yields of 89.4%, 82.9%, 84.4% and 88.8%,
respectively. Generally speaking, ZIF-8@Au25@ZIF-67[12] was
an excellent catalyst for the carboxylation reaction. Based on the
experimental data and literature reports,
a carboxylation
mechanism is proposed in Figure 4. First, phenylene is
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Nanoparticles. Adv. Mater. 2018, 30, 1800202. (c) Wang, N.; Sun, Q.;
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
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Y.; Wang, X.; Han, S.; Liu, X.; DuChene, J. S.; Zhang, H.; Zhang, Q.;
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distribution of metal nanoparticles within metal-organic frameworks
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Wan, M.; Zhang, X.; Li, M.; Chen, B.; Yin, J.; Jin, H.; Lin, L.; Chen,
C.; Zhang, N. Hollow Pd/MOF Nanosphere with Double Shells as
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Experimental procedures, crystallographic data, TEM, BET, UV-
vis., Raman, XPS, XRD, IR, and absorption spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
shenght@ahu.edu.cn; didier.astruc@u-bordeaux.fr
zmz@ahu.edu.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
(7) Zhao, M.; Yuan, K.; Wang, Y.; Li, G.; Guo, J.; Gu, L.; Hu, W.; Zhao,
H.; Tang, Z. Metal–organic frameworks as selectivity regulators for
hydrogenation reactions. Nature 2016, 3, 76-80.
We acknowledge financial support by the National Natural Science
Foundation of China ((21972001, 21871001), Anhui University,
University of Bordeaux and CNRS.
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