Confined Pyrolysis within Metal-Organic Frameworks to Form Uniform Ru3 Clusters for Efficient Oxidation of Alcohols

Article abstract of DOI:10.1021/jacs.7b05018

Here we report a novel approach to synthesize atomically dispersed uniform clusters via a cage-separated precursor preselection and pyrolysis strategy. To illustrate this strategy, well-defined Ru₃(CO)₁₂ was separated as a precursor by suitable molecular-scale cages of zeolitic imidazolate frameworks (ZIFs). After thermal treatment under confinement in the cages, uniform Ru₃ clusters stabilized by nitrogen species (Ru₃/CN) were obtained. Importantly, we found that Ru₃/CN exhibits excellent catalytic activity (100% conversion), high chemoselectivity (100% for 2-aminobenzaldehyde), and significantly high turnover frequency (TOF) for oxidation of 2-aminobenzyl alcohol. The TOF of Ru₃/CN (4320 h^-1) is about 23 times higher than that of small-sized (ca. 2.5 nm) Ru particles (TOF = 184 h^-1). This striking difference is attributed to a disparity in the interaction between Ru species and adsorbed reactants.

Full text of DOI:10.1021/jacs.7b05018

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Communication
Confined Pyrolysis within Metal-Organic Frameworks to
Uniform Ru3 Clusters for Efficient Oxidation of Alcohols
Shufang Ji, Chen Yuanjun, Qiang Fu, Yifeng Chen, Juncai Dong, Wenxing Chen, Zhi Li, Yu Wang, Lin Gu,
Wei He, Chen Chen, Qing Peng, Yu Huang, Xiangfeng Duan, Dingsheng Wang, Claudia Draxl, and Yadong Li
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05018 • Publication Date (Web): 11 Jul 2017
Downloaded from http://pubs.acs.org on July 12, 2017
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Confined Pyrolysis within Metal-Organic Frameworks to Uniform Ru3
Clusters for Efficient Oxidation of Alcohols
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Shufang Ji,1† Yuanjun Chen,1† Qiang Fu,2† Yifeng Chen,3 Juncai Dong,4 Wenxing Chen,1 Zhi Li,1
Yu Wang,5 Lin Gu,6 Wei He,3 Chen Chen,1 Qing Peng,1 Yu Huang,1,7 Xiangfeng Duan,7 Dingsheng
Wang,1* Claudia Draxl,2 and Yadong Li1*
1 Department of Chemistry, Tsinghua University, Beijing 100084, China
2 Institut für Physik and IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin 12489, Germany
3 School of Pharmaceutical Sciences, Tsinghua University, Beijing 100084, China
4
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing
100049, China
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Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Science,
Shanghai 201204, China
6 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Bei-
jing 100190, China
7 Department of Materials Science & Engineering, University of California Los Angeles, California 90095, USA
Supporting Information
species is significantly desirable but still remains a grand
challenge.
ABSTRACT: Here we report a novel approach to synthesize
atomically dispersed uniform clusters via a cage-separated
precursor-preselected pyrolysis strategy. To illustrate this
strategy, well-defined Ru3(CO)12 was separated as a precursor
by suitable molecular-scale cages of zeolite-imidazolate
frameworks (ZIFs). Followed by a thermal treatment under
confinement of cages, uniform Ru3 clusters stabilized by ni-
trogen species (Ru3/CN) were obtained. Importantly, we
found that Ru3/CN exhibits excellent catalytic activity (100%
conversion) ， high chemoselectivity (100% for 2-
aminobenzaldehyde) and significantly high turnover fre-
quency (TOF) for oxidation of 2-aminobenzylalcohol. The
TOF of Ru3/CN (4320 h-1) is about 23 times higher than that
of small-size (ca. 2.5 nm) Ru particles (TOF=184 h-1). This
striking difference is attributed to a disparity in the interac-
tion between Ru species and adsorbed reactants.
Metal-organic frameworks (MOFs) are a unique type of
porous crystalline materials with fine-tunable and uniform
pore structures4. As a subclass of MOFs, zeolitic imidazolate
frameworks (ZIFs) possess interconnected large three-
dimensional molecular-scale cages that are accessible
through small apertures5. They can act as templates to obtain
nitrogen doped porous carbon with abundant nitrogen spe-
cies6. By using striking features of ZIFs, we develop a novel
approach to synthesize atomically dispersed uniform clusters
via a cage-separated precursor-preselected pyrolysis strategy.
Our strategy includes two steps: (1) encapsulating and sepa-
rating preselected metal cluster precursors by molecular-
scale cages of ZIFs, followed by (2) pyrolysis for removing
ligands of metal cluster precursors and forming nitrogen
doped porous carbon to stabilize the clusters.
Scheme 1. Illustration of preparation process of Ru3/CN.
Supported metal cluster and single atom catalysts have at-
tracted considerable research interests because they possess
unique electronic structures and extraordinary catalytic
properties and can bridge heterogeneous and homogeneous
catalysis1. However, atomic dispersion of metal species is not
easy to be controlled under preparation and reaction condi-
tions, which usually results in the coexistence of multiple
forms of species including nanoparticles, clusters even single
atoms2. This phenomenon brings difficulties for identifying
active species. Moreover, atomically dispersed catalyst with
uniform active sites bears great potential to provide funda-
mental insight into a deep understanding of the correlation
between the structure and catalytic mechanisms at molecular
scale3. It appears that synthesis of uniformly dispersed metal
To illustrate this strategy, the preparation procedure is
shown in Scheme 1. ZIF-8, possessing a large sod cage (cage
diameter, dc = 11.6 Å) accessible through a narrow six-ring
pore (pore diameter, dp = 3.4 Å), was selected as a host for
preselected precursor Ru3(CO)12 with an optimal size (mo-
lecular diameter, dm = ca. 8.0 Å)7. Since dm is larger than dp
and smaller than dc, one Ru3(CO)12 molecule can be accom-
modated by one cage and not release. In the synthesis,
Ru3(CO)12 was mixed with the precursors of ZIF-8 in one pot
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to be in-situ encapsulated along with the assembly of Zn ion
Figure 1d). As shown in intensity profiles (Figure 1f), the dis-
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and 2-methylimidazole during ZIF-8 crystallization. Followed
by a thermal treatment, isolated Ru3(CO)12 molecules within
the cages of ZIF-8 (Ru3(CO)12@ZIF-8) were decomposed to
obtain uniform Ru3 clusters stabilized by nitrogen species.
tances between two neighboring Ru atoms are in the range
from 1.9 to 2.5 Å, consistent with the metal bond lengths in
Ru3 triangular structures. Additionally, some Ru3 clusters
were fragmented due to influence of the high-energy or pro-
longed electron beam9,11
.
The powder X-ray diffraction (PXRD) pattern of
Ru3(CO)12@ZIF-8 matched well with that of ZIF-8 in Figure
S1. The FT-IR spectrum of Ru3(CO)12@ZIF-8 in Figure S2 dis-
played the characteristic νco absorption bands at 2060 (s),
2025 (s), 2000 (w) and 1987 cm-1 (s), consistent with that of
Ru3(CO)12, indicating that Ru3(CO)12 had been encapsulated
To further identify the structure of Ru3 clusters, X-ray ab-
sorption near-edge structure (XANES), extended X-ray ab-
sorption fine structure (EXAFS) and a wavelet transform
(WT) were carried out. In Figure 2a, the Ru K-edge XANES
curves show that the energy absorption threshold value of
Ru3/CN is higher than that of Ru foil and lower than that of
RuO2, indicating that Ru3 clusters carry positive charge. The
corresponding FT curve (Figure 2b) exhibits a main peak at
about 1.4 Å (without phase correction), associated with the
first shell of Ru-N scattering. The two higher shell peaks at
distances of 2.3 Å and 3.0 Å in Ru3/CN were observed and
were ascribed to Ru-Ru scattering from Ru3 triangular metal
frame structures and Ru-C scattering, respectively.
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by cages of ZIF-88. Followed by pyrolysis at 800 C under 5%
H2/Ar atmosphere, ZIF-8 transformed into nitrogen doped
porous carbon, and simultaneously the Ru3(CO)12 within the
cage was decomposed and decarbonylated, leading to the
formation of Ru3 clusters anchored by nitrogen species. The
broad shoulder peak at about 26o in the XRD pattern of
Ru3/CN corresponds to the C (002) plane (Figure S1). The X-
ray photoelectron spectroscopy (XPS) analysis of C and N of
Ru3/CN were shown in Figure S3. BET surface area analysis
(Figure S4) reveals that Ru3/CN maintains porous structure,
as supported by obvious voids in transmission electron mi-
croscopy (TEM) images (Figure 1a and S5). In Figures 1b and
1c, the HAADF-STEM images and the corresponding element
maps indicate uniform distributions of Ru, C, and N
throughout Ru3/CN.
To further investigate the nature of Ru3/CN, the wavelet
transform (WT) analysis of Ru K-edge EXAFS oscillations
was performed. In Figure 2c, the WT contour plot of Ru3/CN
in the first coordination shell shows one intensity maximum
at ~4.2 Å-1, associated with Ru-N contribution. However, for
the high coordination shell, the WT contour plot shows two
additional maxima at ~2.3 and 7.0 Å-1, besides the intensity
maximum at ~4.2 Å-1, owing to Ru-C contribution. As the WT
maximum corresponds to the same location of the maximum
in the q-space magnitude, complementary comparison of the
q-space magnitudes for FEFF-calculated k2-weighted EXAFS
paths was constructed (Figure 2d). The Ru-N(C) path shows
one maximum near 4.2 Å-1, while the Ru-Ru path shows one
maximum near 8.7 Å-1 associated with a shoulder around 3.8
Å-1, which is well compatible with the WT of Ru foil (Figure
S6). Moreover, with the Debye–Waller factors σ2 increased
from 0.005 to 0.015 Å2, an obvious shift of the Ru-Ru path
maximum to 7.0 Å-1 is clearly exhibited, suggesting that the
WT maximum at 7.0 Å-1 in Figure 2c can be ascribed to Ru-
Ru scattering. Taken together, the result further well sug-
gests the existence of Ru-Ru bonds. So, the Ru-N coordina-
tion and Ru-Ru metallic bond from Ru3 triangular structures
were detected, indicating that uniform Ru3 clusters were
atomically dispersed and were stabilized by nitrogen species,
as further confirmed by Time-of-Flight Secondary Ion Mass
Spectrometry (TOF-SIMS) analysis (Figure S7). The quantita-
tive EXAFS curve fitting was carried out, and the result of
Ru3/CN is displayed in Figure 2e and Figure S8. For compari-
sons, the fitting results of Ru foil and RuO2 are shown in Fig-
ure S9 and S10. Moreover, the corresponding fitting parame-
ters are listed in Table S1. The coordination number for the
first shell of Ru-N (Ru-Ru) is 2.2±0.7 (1.6±0.5) and the corre-
sponding mean bond length is 1.99±0.02 Å (2.53±0.02 Å).
Figure 1. (a) TEM, (b, c) HAADF-STEM images and corre-
sponding element maps showing distributions of Ru (red), N
(blue), C (green), respectively, (d) Aberration-corrected
HAADF-STEM images of Ru3/CN, (d) Corresponding intensi-
ty maps (left), models (right) (e), and (f) intensity profiles
obtained in area 1,2 and 3.
To discern the atomic structure of Ru3/CN, aberration-
corrected high-angle annular dark-field scanning transmis-
sion electron microscopy (HAADF-STEM) was used. In Fig-
ure 1d, several uniformly dispersed bright dots, correspond-
ing to Ru atoms, can be clearly distinguished from the nitro-
gen doped carbon matrix owing to the Z-contrast9. As the
HADDF images represent two-dimensional projections of
samples along the incident beam direction, the configura-
tions are different from that in the three-dimensional per-
spective10. The observation of a group of three bright dots
(marked in yellow and green cycles) is consistent with trian-
gular Ru3 structures, whereas that of two bright dots (marked
in red cycles) corresponds to Ru3 that were not aligned in
projections. As shown in Figure 1e, the corresponding models
were constructed for three different regions (area 1, 2 and 3 in
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substituted benzyl alcohols (2c, 2f) were also converted into
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corresponding aldehydes in 96% yields. Furthermore, allylic
alcohol (2h) and N-heterocyclic benzyl alcohols (2i) also re-
acted smoothly to give the desired aldehydes in 93-94%
yields with a longer reaction time.
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To understand the catalytic behavior of Ru3 clusters, other
Ru species containing Ru single atoms (Ru1/CN) and small-
size Ru nanoparticles (Ru NPs/CN) as comparisons were
prepared and characterized (Figure S16-27 and Table S4). The
activities of Ru1/CN, Ru NPs/CN and commercial Ru/C were
shown in Figure 3a. Notice that lower conversions of 2-
aminobenzylalcohol for Ru1/CN (21%) and Ru NPs/CN (15%)
compared to Ru3/CN (100%) were obtained after 0.5 hour.
The TOFs of Ru1/CN (416 h-1) and Ru NPs (184 h-1) are re-
markably lower than that of Ru3/CN at the 20% conversion
(Figure 3b). Similarly, commercial Ru/C also showed low
activity for this reaction, whose TOF is 103 h-1. These experi-
mental results confirm that uniform Ru3 clusters are major
active species for the oxidation of 2-aminobenzylalcohol,
demonstrating that control over the formation of uniform
sites is really an efficient way of enhancing performance.
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Figure 2. (a) Ru K-edge XANES spectra, (b) EXAFS Fourier
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transformed (FT) k2-weightedχ(k)-function spectra of Ru3
/CN and reference. (c) Wavelet transforms for the k2-
weighted Ru K-edge EXAFS signals for the high coordination
shells in Ru3/CN. The colour in the contour figures indicate
the moduli of the Morlet wavelet transform. (d) Comparison
of the q-space magnitudes for FEFF-calculated k2-weighted
EXAFS paths. (e) Corresponding EXAFS R space fitting curve
of Ru3/CN. (f) Schematic model of Ru3/CN, Ru (teal), N
(blue) and C (gray)
Figure 3. (a) Conversion (%) of 2-aminobenzylalcohol versus
time for Ru3/CN (red), Ru1/CN (blue), Ru NPs/CN (yellow)
and commercial Ru/C (green). Inset in (a) shows scheme of
oxidation of 2-aminobenzylalcohol to 2-aminobenzaldehyde.
(b) Corresponding TOFs obtained at 20% conversion of 2-
aminobenzylalcohol.
First-principles calculations were performed to elucidate
the possible structure of Ru3/CN. The CN framework was
simulated using a simplified model of a nitrogen-doped gra-
phene monolayer, where the triangular Ru3 cluster was em-
bedded within a hole that is surrounded by four pyrrole sub-
units (Figure 2f). The Ru-N bond lengths are in the range
from 1.95 to 2.04 Å, consistent with the EXAFS measurement.
Bader charge analysis reveals that the in-plane and the out-
of-plane Ru atoms exhibit a positive charge of 0.62 and 0.46,
respectively, consistent with the XANES results (Table S2).
Considering the striking difference in reactivity between
Ru3/CN, Ru1/CN, and small-size Ru NPs/CN, first-principles
calculations were performed to understand the mechanism.
An embedded RuN4 model13 (Figure S26) and a four-layer
Ru(0001) slab were employed to simulate Ru1/CN and Ru
NPs, respectively. It is known that catalytic activity of transi-
tion-metal catalysts is determined by the strength of their
interactions with reactant molecules14. In Figure 4, we display
adsorption energies of the 2-aminobenzylalcohol reactants
on the three systems. On Ru(0001), the interaction is so
strong that the reactants are expected to block active sites at
the surface, which impairs the catalytic activity. On Ru3/CN
and Ru1/CN, the limited number of Ru atoms results in a
significant reduction in the adsorption energies, which
brings about improved TOFs compared with Ru NPs. Com-
paring Ru3/CN and Ru1/CN, the difference in their TOFs is
attributed to a variation of bonding pattern between the re-
actants and Ru species. Among the various adsorption con-
figurations in Figure 4, those merely bound via an amino
group are not expected to be effective, since in aerobic oxida-
tion, amino behaves as a spectator and needs to be far from
the active sites. On Ru1/CN, such structure is 0.58 eV more
stable than that via a hydroxyl group, meaning that the reac-
tants prefer to adopt the ineffective configuration and block
the Ru1 active sites. On Ru3/CN, by contrast, the reactants
can adopt another geometry where one Ru atom connects
The selective oxidation of primary alcohols to aldehyde is
one of the most fundamental processes in organic synthesis12.
The oxidation of 2-aminobenzylalcohol was chosen as a
model reaction to evaluate the catalytic performance of
Ru3/CN. In Figure 3a, the Ru3/CN catalyst exhibited high
conversion of 2-aminobenzylalcohol (up to 100%) after 0.5
hour and products of excessive oxidation (2-aminobenzoic
acid or 2-nitrobenzaldehyde) were not detected even when
extending the reaction time, certificating a high chemoselec-
tivity. The calculated turnover frequency (TOF) of Ru3/CN is
as high as 4320 h-1 (Figure 3b). When the reaction was over,
the used Ru3/CN catalyst was characterized again (Figure S11-
15), confirming that uniformly dispersed structure of Ru3
clusters remained unchanged. To further test the scope of
reaction with Ru3/CN, in Table S3, substituted benzyl alco-
hols with both electron-donating groups (2a, 2b) and elec-
tron-with-drawing groups (2d, 2e, 2g) were converted to cor-
responding aldehydes in excellent yields (95-99%). Halogen-
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with both hydroxyl and amino groups. Such configuration,
S. Ji, Y. Chen and Q. Fu contributed equally to this work.
1
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whose adsorption energy is close to that of the ineffective
structure, is also effective for the reactions, thus explaining
why Ru3/CN behaves better than Ru1/CN.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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This work was supported by China Ministry of Science and
Technology under Contract of 2016YFA (0202801), and the
National Natural Science Foundation of China (21521091,
21390393, U1463202, 21471089, 21671117). We thank Hefei
Light Source, Beijing Light Source and Shanghai Light Source
for use of the instruments. Q. Fu and C. Draxl acknowledge
support from the German Research Foundation (DFG)
through the Collaborative Research Center SFB-658.
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Figure 4. Geometries of 2-aminobenzylalcohol on Ru3/CN
(middle), Ru1/CN (left) and Ru NPs (right). The blue and red
triangles and the black circles mark the adsorption energies
of the molecule bound via amino, hydroxyl, and both the
groups, respectively. A positive value represents an exother-
mic adsorption. The gray, blue, red, white, and teal spheres
represent C, N, O, H, and Ru atoms, respectively.
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In summary, we have prepared uniform Ru3 clusters by a
novel
cage-separated
precursor-preselected
pyrolysis
strategy. The structure of Ru3/CN catalyst was characterized
by HAADF-STEM and XAFS, and the catalytic performance
was evaluated using oxidation of 2-aminobenzylalcohol as a
model reaction. This catalyst shows 100% conversion, 100%
selectivity, and an unexpected high TOF (=4320 h-1), which
are superior to Ru single atoms and small-size Ru particles
catalysts. First-principles calculations reveal that the dra-
matic difference comes from a disparity in the interaction
between Ru species and adsorbed reactants, further confirm-
ing that uniform Ru3 cluster are efficient sites of enhancing
performance. This work would provide a new strategy for
the generation of atomically dispersed uniform cluster
materials and pave a new avenue for further research in the
application of cluster catalysts.
ASSOCIATED CONTENT
Supporting Information
Detailed experimental sections; figures; tables. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* wangdingsheng@mail.tsinghua.edu.cn;
* ydli@mail.tsinghua.edu.cn
Author Contributions
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55
56
57
58
59
60
5
ACS Paragon Plus Environment