Uncoordinated Amine Groups of Metal-Organic Frameworks to Anchor Single Ru Sites as Chemoselective Catalysts toward the Hydrogenation of Quinoline

Article abstract of DOI:10.1021/jacs.7b01686

Here we report a precise control of isolated single ruthenium site supported on nitrogen-doped porous carbon (Ru SAs/N-C) through a coordination-assisted strategy. This synthesis is based on the utilization of strong coordination between Ru³⁺ and the free amine groups (-NH₂) at the skeleton of a metal-organic framework, which plays a critical role to access the atomically isolated dispersion of Ru sites. Without the assistance of the amino groups, the Ru precursor is prone to aggregation during the pyrolysis process, resulting in the formation of Ru clusters. The atomic dispersion of Ru on N-doped carbon can be verified by the spherical aberration correction electron microscopy and X-ray absorption fine structure measurements. Most importantly, this single Ru sites with single-mind N coordination can serve as a semihomogeneous catalyst to catalyze effectively chemoselective hydrogenation of functionalized quinolones.

Full text of DOI:10.1021/jacs.7b01686

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Communication
Uncoordinated amine groups of MOFs to anchor single Ru sites as
chemoselective catalysts towards the hydrogenation of quinoline
Xin Wang, Wenxing Chen, Lei Zhang, Tao Yao, Wei Liu, Yue Lin, Huanxin Ju, Juncai
Dong, Lirong Zheng, Wensheng Yan, Xusheng Zheng, Zhijun Li, Xiaoqian Wang,
Jian Yang, Dongsheng He, Yu Wang, Zhaoxiang Deng, Yuen Wu, and Yadong Li
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01686 • Publication Date (Web): 29 Jun 2017
Downloaded from http://pubs.acs.org on June 29, 2017
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Table of contents
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Uncoordinated amine groups of MOFs to anchor single Ru sites as
chemoselective catalysts towards the hydrogenation of quinoline
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Xin Wang,†,Δ Wenxing Chen,‡,Δ Lei Zhang,† Tao Yao,§ Wei Liu,§ Yue Lin,† Huanxin Ju,§ Juncai
Dong,⊥ Lirong Zheng,⊥ Wensheng Yan,§ Xusheng Zheng,§ Zhijun Li,† Xiaoqian Wang,† Jian
Yang,† Dongsheng He, † Yu Wang,# Zhaoxiang Deng,† Yuen Wu,*,† and Yadong Li*,‡
†Department of Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), University of
Science and Technology of China, Hefei 230026, China
‡ Department of Chemistry, Tsinghua University, Beijing 100084, China
§ National Synchrotron Radiation Laboratory (NSRL), University of Science and Technology of China, Hefei, Anhui
230029, China
⊥Institute of High Energy Physics Beijing 100029 (China)
#Shanghai Synchrontron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Science, Shanghai
201800, China
Supporting Information Placeholder
support. The design of the anchoring sites, which are usually
the oxygen or carbon vacancies, is the key to achieve the
atomic dispersion and identical geometric structure for
homogenized catalytic sites.3-5 Therefore, developing rational
and general synthetic methods to construct well-defined
single sites catalysts, that can potentially/ideally inherit the
merits of homogeneous and heterogeneous analogues, are
highly desired but challenging.
ABSTRACT: Here we report a precise control of isolated
single ruthenium site supported on nitrogen-doped porous
carbon (Ru SAs/N-C) through
a coordination-assisted
strategy. This synthesis is based on the utilization of strong
coordination between Ru3+ and the free amine groups (-NH2)
at the skeleton of metal-organic framework (MOF), which
plays
a critical role to access the atomically isolated
dispersion of Ru sites. Without the assistance of the amino
groups, the Ru precursor is prone to aggregation during the
pyrolysis process, resulting in the formation of Ru clusters.
The atomic dispersion of Ru on N-doped carbon can be
verified by the spherical aberration correction electron
microscopy and x-ray absorption fine structure (XAFS)
measurements. Most importantly, this single Ru sites with
Although abundant works employing MOFs as precursors
to access the metal/MOFs hybrids have been reported, few of
them focused on the preparation of single metal atoms
catalysts.6-7 Recently, our group reported that single atom
catalysts can be prepared by converting atomically dispersed
metal nodes in-situ from MOFs.8 However, noble metals (Au,
Ag, Pt, Ru ions, etc.) have been rarely used as the nodes of
MOFs, thus hindering the in-situ evolution from metal nodes
to single atoms catalysts.9-10 Ru-based nanomaterials have
proven to be effective in various catalytic reactions.11 Herein,
we have utilized the uncoordinated -NH2 located at the
terephthalic acid linkers in UiO-66-NH2 as the Lewis base to
stabilize the RuCl3. The strong interaction between lone pair
electrons and d-orbitals would ensure the Ru single atoms to
be confined in the pores of MOFs and prevent their
aggregation during the pyrolysis at high temperature.
Without the assistance of free -NH2, Ru clusters were found
to be the dominant moieties dispersing on the final porous
carbon. These differently gathered Ru catalysts exhibit size-
dependent catalytic properties towards the hydrogenation of
quinolones, revealing a robust route to tailor and optimize
the activity and selectivity by atomic-scale design.
single-mind
N
coordination can serve as
a
semi-
homogeneous catalyst to effectively catalyze chemoselective
hydrogenation of functionalized quinolones.
Considering the global motif of sustainable development
in energy and environment area, isolated single site catalysts
have attracted extensive attention due to their maximum
atom utilization and superior reactivity and selectivity.1 As an
ideal model bridging between heterogeneous and
homogeneous catalysis, the performance of catalysts based
on single atoms is highly related to their structures, such as
coordination ligands, dispersion gesture and interactions
with supports.2 To date, traditional synthetic protocol to
access single atom catalysts are commonly based on an initial
adsorption of metal precursor with a subsequent reduction
process. Therefore, the final properties of single site catalysts
are highly dependent upon the binding and stabilization of
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The MOFs chosen for hosting the Ru atoms was UiO-66,
precursor and further prevent their assembling during
pyrolysis. By contrast, without the assistance of -NH2 groups,
a large proportion of Ru atoms would aggregate at high
temperature, resulting in the formation of clusters on the
support (Figure 1e and f).
[Zr6O4(OH)4(BDC)6 (BDC=1,4-benzenedicarboxylate)], which
features three-dimensional structure, uniform porosity, high
surface area, and robust thermal and chemical stability.12 The
derivative UiO-66-NH2, during which the BDC was
functionalized with ammonium, could largely inherit the
symmetry and morphology of UiO-66 (Figure S1).13 Merging
ZrCl4 and BDC with the RuCl3 during the synthesis process,
the Ru ions were adsorbed within the evacuated UiO-66 or
UiO-66-NH2 to obtain the corresponding complexes. As
shown in Figure S2, these adsorption processes do not
change the octahedral shape of inital polyhedrons and size
distribution. The powder X-ray diffraction (XRD) pattern of
the sample collected after the adsorption of RuCl3 displayed
characteristic peaks of UiO-66, demonstrating the retention
of UiO-66 crystallinity and structure (Figure S3). The
interaction of RuCl3 precursors with the -NH2 groups of UiO-
66-NH2 was further confirmed by FT-IR spectroscopy. The
peaks at 1620 and 1060 cm−1 (Figure S4) can be ascribed to
the -NH2 scissoring and rocking vibrations, respectively.14
The intensity changes and position shifts of the peaks
strongly support the interaction between Ru ions and -NH2
groups. Following a pyrolysis process at 700 °C, the building
skeleton of UiO-66 was transferred in-situ into porous
carbon, which was decorated by small ZrO2 crystals and Ru
species. As revealed by the XRD pattern in Figure S3c, the
addition of HF solution could readily etch the inert ZrO2
nanoparticles, resulting in the formation of Ru single atoms
(0.30 wt% Ru loading) or Ru clusters (0.35 wt% Ru loading)
within the carbon architecture (Ru SAs/N-C or Ru NCs/C).
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Figure 2. (a) XPS spectra for the N 1s region of Ru SAs/N-C.
(b) N K-edge NEXAFS spectra of Ru SAs/N-C. (c) XANES
spectra of Ru SAs/N-C and Ru NCs/C for Ru K-edge. (d)
Fourier transformed (FT) k2-weighted χ(k)-function of the
EXAFS spectra for Ru K-edge. (e) WT of the Ru-K edge and (f)
Fitting results of the EXAFS spectra of Ru SAs/N-C.
The X-ray diffraction (XRD) patterns for the Ru SAs/N-C
and Ru NCs/C showed no characteristic peaks of Ru crystals,
excluding the generation of large particles and aggregates
(Figure S3c). X-ray photoelectron spectroscopy (XPS) was
used to characterize the valence state of N (Figure 2a) and C
(Figure S6). The peak at 401.3 eV and its shakeup satellites
could be assigned to graphitic N. The pyridinic N (398.7 eV)
and pyrrolic N (398.7 eV) are dominant in the Ru SAs/NC.
These two types of N atoms are located in a π conjugated
system,15 whose p-electrons synergistically facilitated the
stabilization of Ru SAs. Studies have shown that N doping in
carbon support could effectively introduce a large amount of
topological defects, and the corresponding density is related
to the ID/IG value in Raman spectra.16 The intensity ratios of
ID/IG for Ru SAs/N-C and Ru NCs/C are calculated to be 1.01
and 0.97, respectively (Figure S7a). This supports our
hypothesis that there would be more dispersive defects in the
Ru SA/N-C than Ru NCs/C.17
Figure 1. (a) TEM of Ru SAs/N-C, (b,c) magnified HAADF-
STEM images of Ru SAs/N-C. (d) TEM of Ru NCs/C, (e, f)
magnified HAADF-STEM images of Ru NCs/C.
Transmission electron microscopy (TEM) and scanning
electron microscope (SEM) images (Figure 1a, d and Figure
S5) revealed the as-prepared Ru SAs/N-C and Ru NCs/C
structure remained the starting octahedral shape of UiO-66-
NH2 and UiO-66, respectively. The image of aberration
corrected
high-angle
annular
dark-field
scanning
transmission electron microscope (HAADF-STEM) revealed
the brighter spots assigned to the heavier Ru atoms exhibited
atomic dispersion on the N-doped carbon support (Figure
1b,c). Some Ru single atoms were circled in red for better
obseravation (Figure 1 c). This strongly supports that the
uncoordinated -NH2 groups could stabilize the RuCl3
To further investigate the electronic structure of the
catalysts, near edge X-ray absorption fine structure (NEXAFS)
measurements were performed (Figure 2b and Figure S8a).
The observed C K-edge spectra of Ru SAs/N-C and Ru NCs/C
are quite similar, suggesting similar environments of carbon
skeletons present. The peak A (ca. 285.6 eV) was derived
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from C-C π* (ring) excitations, and the peak C (ca. 293.2 eV)
Figure 3 shows the proposed formation mechanism of Ru
was originated from C-C σ* (ring) transitions.18 The
occurrence of both peaks implies a graphitization process
during the pyrolysis. Interestingly, sharp peaks at 288.4 eV
(peak B) were observed for both Ru SAs/N-C and Ru NCs/C,
whereas no peaks associated with flaky graphite powder were
found (Figure S8b).19 Since the peak B was assigned to C-O-C
or C-N-C, the above difference indicates that there are a
plenty of defect sites in the carbon lattice for Ru SAs/N-C.20-21
For the N K-edge spectrum (Figure 2b), the peak A (at 399.5
eV) and peak B (at 402.4 eV) result from pyridinic π* and
graphitic π* transitions, respectively. The peak C (at 408.5 eV)
suggests the formation of C-N-C or C-N σ* bond.
SA/N-C. Since UiO-66-NH2 possess abundant dangling -NH2
groups, Ru metal ions can be adsorbed within the MOFs
channels by the strong coordination interaction between the
electron lone pair of nitrogen and d-orbital of Ru atoms.
After the pyrolysis process, the ligand and Zr-based anodes of
UiO-66-NH2 would form porous C and ZrO2 nanoparticles,
respectively. Meanwhile, the adsorbed Ru ions can be
reduced by the surrounding carbon. It is worth pointing out
that the dangling -NH2 played a critical role in the formation
of Ru SAs, as they can largely limit the diffusion of Ru atoms
at high temperature and bond with adjacent Ru atoms.
Without the dangling -NH2, the Ru ions tended to aggregate
into nanoclusters and small nanoparticles at high-
temperature even in confined pores of MOFs.
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To verify the formation of Ru sub-nano species and explore
their inherent structures at atomic level, we performed the
XAFS measurements of Ru K-edge. The XANES spectra
(Figure 2c and Figure S9a) show that the absorption
threshold for the Ru SAs/N-C is close to that of RuCl3,
indicating the valence state of Ru is close to 3+ (Figure S9b).
The curves for both Ru SAs/N-C and Ru NCs/C have a main
peak around 1.5 Å, corresponding to the Ru-N/C scattering
pair (Figure 2d). Ru-Cl coordination is absent and no Ru
oxide species can be observed. We also notice that there is
another coordination peak at 2.25 Å for Ru NCs/C, which can
be ascribed to a metallic Ru-Ru scattering peak. This suggests
that the Ru present in the form of clusters in Ru NCs/C,
consistent with the TEM results (Figure 1e and 1f). In contrast,
the metallic Ru-Ru peak for the Ru SAs/N-C is negligible,
which supports the hypothesis that almost all the Ru atoms
in Ru SAs/N-C are atomically dispersed.22-23 To further
explore the atomic dispersion of Ru in Ru SAs/N-C, wavelet
transform (WT) of Ru K-edge EXAFS was carried out.24 In
Figure 2e, the WT contour plots of Ru SAs/N-C show only
one intensity maximum at 4.0 Å-1, corresponding to Ru-N/C
coordination. Additionally, no intensity maximum related to
Ru-Ru contribution can be found at high coordination shells,
compared with the WT plot of Ru foil in Figure S10.
Quantitative EXAFS analyses were performed and the fitting
results are shown in Figure 2f, S11, S12 and Table S1. The first
shell of the Ru atom in Ru SAs/N-C has a coordination
number of three and a mean bond length of 2.08 Å (insert of
Figure 2f).
Brunauer-Emmett-Teller (BET) adsorption–desorption
isotherms revealed that the Ru SAs/N-C and Ru NCs/C
exhibited large pore volume and surface area (Figure S13).
This feature contributes to the exposure of active sites and
benefits rapid transportation of hydrogenation-relevant
substrates.
Quinoline
hydrogenation
involves
the
regioselective hydrogenation of the nitrogen-containing ring
to form the (py-THQ, 1a).25 However, by-products such as
5,6,7,8-
tetrahydroquinoline
(bz-THQ,
1b)
and
decahydroquinoline (DHQ, 1c) may inevitably form during
the reaction. For example, commercial Ru/C catalyst has only
9.0% selectivity toward py-THQ.26
Figure 4. (a) Catalytic performance of Ru SAs/N-C and Ru
NCs/C catalysts for the regioselective hydrogenation of
quinolin. Catalytic activity and chemoselectivity of (b) Ru
SAs/N-C and (c) Ru NCs/C in the hydrogenation of quinoline.
As shown in Figure 4a, quinoline can be converted
completely by employing either Ru SAs/N-C or Ru NCs/C,
however, the corresponding selectivity for the desired
products was observed to be different. The Ru SAs/N-C
obtained more than 99% selectivity towards py-THQ (1a),
which is much higher than that of Ru NCs/C (79%). It can be
inferred the multiple reactive sites on the Ru clusters should
be accounted for the diversity of products. Due to the
uniform structure of isolated active Ru sites, the Ru SAs/N-C
achieved almost exclusive selectivity for py-THQ (1a), while
maintaining an excellent conversion efficiency of quinoline
(>99%). We further investigated the durability of Ru SAs/N-
C and Ru NCs/C toward this hydrogenation reaction. As
shown in Figure 4b, the activity and selectivity of Ru SAs/N-
Figure 3. Scheme of the proposed formation mechanisms for
Ru SAs/N-C (top) and Ru NCs/C (bottom).
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C catalysts were retained after 5 recycles of hydrogenation
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reactions. TEM images of the Ru SAs/N-C after the durability
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durability test, indicating the single atom species was
unstable during the catalysis without the protection of free -
NH2 groups. This evolution process was analogous to the
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single Ru atoms were inclined to bond with the more stable
Ru cluster, resulting in the disappearance of single Ru sites.
For Ru NCs/C catalyst, the selectivity to py-THQ and its yield
nearly did not deteriorate, demonstrating the Ru clusters
were dominant reactive sites during the catalytic cycles.
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Having established active and selective Ru SAs/NC catalyst,
we further studied the hydrogenation of various structural
different substituted quinolines. As depicted in Table S2, the
hydrogenation of 6- and 8-hydroxyquinolines afforded the
corresponding biologically active 1,2,3,4-tetrahydro-6-
hydroxyquinolines
and
1,2,3,4-tetrahydro-8-
hydroxyquinolines (Table S2, entries 1 and 2), respectively.
Remarkably, the hydrogenation of quinolines with electron-
donating groups −NH2 at the 6- and 8-position on the
benzene ring was also highly regioselective (Table S2, entries
3 and 4). In a more challenging reaction, where quinolines
bearing other halogens groups, the corresponding valuable
functional tetrahydroquinolines were obtained in excellent
yield (Table S2, entries 5-7). Moreover, this highly selective
Ru SAs/N-C could be also applied to effect the generation of
1,2,3,4-Tetrahydroisoquinoline (Table S2, entry 8). Catalyzed
by the Ru NCs/C, diverse products were also detected for the
hydrogenation of functional quinolones (Table S3).
In summary, we report the preparation of single Ru sites
by employing uncoordinated functional groups on skeletons
of MOFs. Our findings opened new opportunities to
synthesize single metal atoms anchored on N-doped carbon
support for various applications.
ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures; SEM images; FT-IR spectra;
EELS mapping; XRD and XPS spectra. This material is
available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
yuenwu@ustc.edu.cn; ydli@mail.tsinghua.edu.cn
ACKNOWLEDGMENT
This work was supported by China Ministry of Science and
Technology under Contract of 2016YFA (0202801), and the
National Natural Science Foundation of China (21522107,
21671180, 21521091, 21390393, U1463202). This work made use
of the resources and finance support of National Synchrotron
Radiation Laboratory in Beijing and Shanghai.
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