Zeolite-Encaged Single-Atom Rhodium Catalysts: Highly-Efficient Hydrogen Generation and Shape-Selective Tandem Hydrogenation of Nitroarenes

Article abstract of DOI:10.1002/anie.201912367

Single-atom catalysts are emerging as a new frontier in heterogeneous catalysis because of their maximum atom utilization efficiency, but they usually suffer from inferior stability. Herein, we synthesized single-atom Rh catalysts embedded in MFI-type zeolites under hydrothermal conditions and subsequent ligand-protected direct H₂ reduction. C_s-corrected scanning transmission electron microscopy and extended X-ray absorption analyses revealed that single Rh atoms were encapsulated within 5-membered rings and stabilized by zeolite framework oxygen atoms. The resultant catalysts exhibited excellent H₂ generation rates from ammonia borane (AB) hydrolysis, up to 699 min^?1 at 298 K, representing the top level among heterogeneous catalysts for AB hydrolysis. The catalysts also showed superior catalytic performance in shape-selective tandem hydrogenation of various nitroarenes by coupling with AB hydrolysis, giving >99 % yield of corresponding amine products.

Full text of DOI:10.1002/anie.201912367

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Zeolite-Encaged Single-Atom Rh Catalysis: Highly-Efficient
Hydrogen Generation and Shape-Selective Tandem
Hydrogenation of Nitroarenes
Authors: Qiming Sun, Ning Wang, Tianjun Zhang, Risheng Bai, Alvaro
Mayoral, Peng Zhang, Qinghong Zhang, Osamu Terasaki,
and Jihong Yu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912367
Angew. Chem. 10.1002/ange.201912367
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10.1002/anie.201912367
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RESEARCH ARTICLE
Zeolite-Encaged Single-Atom Rh Catalysts: Highly-Efficient
Hydrogen Generation and Shape-Selective Tandem
Hydrogenation of Nitroarenes
Qiming Sun,[a]+ Ning Wang,[a]+ Tianjun Zhang,[a] Risheng Bai,[a] Alvaro Mayoral,*[b] Peng Zhang,[c]
Qinghong Zhang,[d] Osamu Terasaki,[b] and Jihong Yu*[a, e]
Abstract: Single-atom catalysts are emerging as a new frontier in
maximum atom-utilization efficiency and unique catalytic
heterogeneous catalysis for their maximum atom utilization efficiency, properties.[1] However, SACs usually suffer from inferior stability
but usually suffer from inferior stability. Here, we successfully
synthesized single-atom Rh catalysts embedded in MFI-type zeolites
under hydrothermal conditions followed by ligand-protected direct H2
reduction. Cs-corrected scanning transmission electron microscopy
and extended X-ray absorption analyses revealed that single Rh
atoms were encapsulated within 5-membered rings and stabilized by
zeolite framework oxygens. The resultant catalysts exhibited
excellent H2 generation rates from ammonia borane (AB) hydrolysis,
up to 699 min-1 at 298 K, representing the top level among reported
heterogeneous catalysts for AB hydrolysis. Significantly, the
catalysts also showed superior catalytic performance in shape-
selective tandem hydrogenation of various nitroarenes by coupling
with AB hydrolysis, giving > 99% yield of corresponding amine
products. This work demonstrates that the zeolites are ideal
supports for encapsulating single metal atoms and can afford high-
efficiency shape-selective tandem catalysis.
and poor recyclability in the catalytic processes. The isolated
metal atoms are prone to aggregate during synthetic and
catalytic procedures due to their higher surface energy than
corresponding metal clusters and nanoparticles; this significantly
decreases their catalytic performance and limits the practical
applications. The fabrication of SACs with excellent activity and
stability is of great significance from both academic and
industrial points of view. Zeolites are ideal supports to stabilize
metal species for their well-defined microporous channels and
excellent thermal stability.[2] Most importantly, as compared with
other solid supports, the zeolite-encaged metal catalysts can be
also endowed with unique and excellent shape-selective
catalytic performance, which is extremely attractive in the field of
catalysis.[2d, 3] Several subnanometric metal clusters, such as Pd
and Pt, had been encapsulated within the silica zeolite matrix via
ligand-stabilized or precursor-stabilized methods, which
exhibited superior catalytic activity in dehydrogenation and
hydrogenation reactions.[2e, 2g, 2n] Very recently, Liu et al. reported
a ligand-protected strategy for preparing isolated metal atoms
inside low-silica Y zeolite via thermal calcination-reduction
processes.[2j] However, there remains a lack of facile and
generally applicable synthetic methods to fabricate zeolite-
encaged SACs catalysts and until now there is no report for the
synthesis of single metallic atoms confined within purely
siliceous zeolites. Moreover, the determination of precise
locations of single metal atoms within the zeolite matrix remains
a nontrivial task but great challenging.
Introduction
Single-atom catalysts (SACs) with isolated metal atoms
dispersed on solid supports, have been emerging as the most
active new frontier in heterogeneous catalysis because of the
[a]
[b]
Dr. Q. Sun, Dr. N. Wang, Mr. T. Zhang, Mr. R. Bai, Prof. J. Yu
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, College of Chemistry, Jilin University
Changchun, 130012, P. R. China
Hydrogen (H2) has been regarded as the most
environmentally benign energy resource for its high energy
density and carbon-free emissions.[4] Developing efficient and
safe methods for H2 production and storage is of great
significance for the future hydrogen economy.[5] Ammonia-
borane (NH3BH3, AB) is considered as one of the most
outstanding candidates for the chemical H2 storage and
production because of its high hydrogen content, non-toxicity,
Email: jihong@jlu.edu.cn
Dr. A. Mayoral, Prof. O. Terasaki
Center for High-resolution Electron Microscopy (CħEM), School of
Physical Science and Technology, ShanghaiTech University,
Shanghai 201210, P. R. China
Email: amayoral@shanghaitech.edu.cn
[c]
[d]
Prof. P. Zhang
Department of Chemistry, Dalhousie University
Halifax, Nova Scotia B3H 4R2, Canada
6]
and superior stability in aqueous solutions.[5a, So far, many
supported metallic catalysts over various substrates, such as
carbon materials,[7] metal oxides,[8] mesoporous silica[9] and
Prof Q. Zhang
State Key Laboratory of Physical Chemistry of Solid Surfaces,
Collaborative Innovation Center of Chemistry for Energy
Materials, National Engineering Laboratory for Green Chemical
Productions of Alcohols, Ethers and Esters, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen 361005,
P. R. China
6b, 10]
porous metal-organic frameworks/cages,[6a,
have been
employed for the H2 generation from AB hydrolysis reactions;
but these catalysts are usually subjected to poor long-term and
cycling stability. On the other hand, the production of aromatic
amines via the hydrogenation of nitroarenes is a popular and
important industrial process, because the target amine products
are widely used as raw materials or/and intermediates for
[e]
Prof. J. Yu
International Center of Future Science, Jilin University
Qianjin Street 2699, Changchun 130012, P. R. China
11]
manufacturing diverse value-added fine chemicals.[2d,
[+] These authors contributed equally to this work.
Recently, a tandem route that couples the hydrolysis of AB with
hydrogenation of nitro compounds is regarded as an efficient
strategy to significantly boost the productivity of the reduction
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/
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process.[11] However, the stability and efficiency of catalysts
remain key problems to be tackled for achieving high catalytic
performance. Until now, there is no report on the zeolite-
encaged metal catalysts applied for the tandem reduction of
elemental
CHN
analysis
indicated
that
the
[Rh(NH2CH2CH2NH2)3]Cl3 complexes remained intact during the
synthesis process and were encapsulated inside zeolites
(Figures S1, S2 and Table S1). Powder X-ray diffraction
measurements revealed that all Rh-containing samples
possessed the MFI topological structure without detectable
peaks corresponding to Rh species, implying that the lattice
distortion was negligible after encapsulating metal species inside
S-1 zeolites and the absence of large Rh particles in the
obtained samples (Figure S3). The Rh loading amount in all of
the samples was measured as 0.28 wt.% by inductively coupled
plasma atomic emission spectroscopy (ICP-AES).
nitro
compounds
coupled
with
the
highly-efficient
dehydrogenation of AB hydrolysis while keeping attractive
shape-selective activities.
Herein, we developed
encapsulate single rhodium (Rh) atoms within MFI-type silicalite-
(S-1) and aluminosilicate ZSM-5 zeolites by using
a
facile synthetic method to
1
[Rh(NH2CH2CH2NH2)3]Cl3 complex as a precursor under one-pot
hydrothermal synthesis conditions followed by ligand-protected
direct hydrogen reduction. The single-atom nature of Rh was
confirmed by detailed Cs-corrected scanning transmission
electron microscopy (STEM), in situ CO-diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS), and
extended X-ray absorption fine structure (EXAFS) analyses.
Significantly, we for the first time revealed the location of the
single Rh atoms within sinusoidal 5-membered rings (MRs) of
MFI which are stabilized by zeolite framework oxygens. The
resultant catalysts exhibited an excellent H2 generation rate from
AB hydrolysis. Most significantly, the zeolite-encaged single-
atom Rh catalysts showed superior shape-selective catalytic
performance in tandem hydrogenation of diverse nitroarenes by
coupling with AB hydrolysis, giving > 99% yield of corresponding
amine products, which is three orders of magnitude higher than
that using the H2 as a reduction agent. This work demonstrates
the great advantages of zeolites in encaging single metal atoms,
improving catalytic performance, affording catalytic shape
selectivity, and promising tandem catalytic reactions.
Cs-corrected high-angle annular dark-field STEM (HAADF-
STEM) and high-resolution TEM (HRTEM) images of samples
are shown in Figures 1 and S4. Initially, it can be observed that
in any type of as-synthesized samples excellent crystallinity is
retained and the 10-membered ring (MR) channels of MFI are
empty along the b-axis of the nanosized S-1 zeolites (~200 nm)
(Figure 1A). As in this mode, the contrast is sensitive to the
atomic number of the elements, and brighter signals can be
directly inferred to the presence of Rh within the zeolite cages.
For Rh@S-1-H, a small number of signals can be observed in
some regions of the zeolite framework due to the presence of
atomically dispersed Rh atoms (Figures 1A and B). However, for
the Rh@S-1-C, very clear homogeneously-distributed Rh
clusters can be observed (Figures 1C and D).
Results and Discussion
Scheme 1. Schematic illustration of the fabrication of the Rh@S-1 catalysts.
Figure 1. Cs-correct STEM images of A, B) Rh@S-1-H and C, D) Rh@S-1-C
viewed along b-axis orientation, E-G) Rh@S-1-H viewed along [011]
orientation as well as the schematic models on the same projection. H)
HAADF-STEM image of Rh@S-1-H and the corresponding EDX mapping
images for O, Si, and Rh elements.
As shown in Scheme 1, the Rh atoms confined within S-1
zeolites were prepared by the in-situ hydrothermal synthesis in
the
reaction
gel
with
a
molar
composition
of
SiO2/TPAOH/H2O/[Rh(NH2CH2CH2NH2)3]Cl3 = 1/0.4/35/2.25×10-
3
followed by ligand-protected direct reduction in pure H2 at 500
Considering the three-dimensional nature of the zeolite
crystals, different crystallographic orientations were taken in
order to achieve a clear location of the Rh species. MFI-type
framework is formed by n-membered rings (n-MRs), n = 5, 6,
and 10, viewed along [010] direction (see Figure 1A). However,
on the [011] orientation the projection is very dense, and it does
°C. The obtained samples were named as Rh@S-1-H. For
comparison, the Rh@S-1-C sample prepared via a conventional
calcination (550 °C in the air)-reduction (400 °C in H2) procedure
and the Rh/S-1-im sample prepared by incipient wetness
impregnation method were also synthesized. Solid 13C MAS
NMR, thermogravimetric-differential thermal (TG-DTA), and
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not allow to discern between atomic columns or even membered
rings; and only “rectangular units” (see Figures 1E-G) can be
discerned, marked by a red circle. Notably, these units are only
formed by the sinusoidal 5-MRs; the contrast observed there
can be only owed to the presence of Rh species confined within
these rings (Figures 1E-G). In the models presented in Figure
1G, a rectangular unit is marked in yellow that would correspond
to the 5 MRs along the [011] and [010], respectively. All above
Cs-corrected STEM analyses confirm that (i) no agglomerated
Rh metal clusters could mostly observe but uniformly dispersed
and encapsulated within sinusoidal 5-MRs of S-1 crystals in
Rh@S-1-H sample; (ii) agglomerations of Rh were rarely
observed at external surface of the crystal in both Rh@S-1-H
and Rh@S-1-C samples; (iii) no clear evidence of break-down of
zeolite framework was observed. Elemental mappings analysis
for the Rh@S-1-H further suggests that the Rh atoms are
homogeneously dispersed throughout zeolite crystals (Figure 1I).
In contrast, the Rh/S-1-im displays different situations that many
large and uneven Rh nanoparticles (NPs) (> 5 nm) are located
outside of zeolites (Figure S4). Scanning electron microscopy
(SEM) shows that no Rh species can be detected on the outer
surface of Rh@S-1-H and Rh@S-1-C, but some larger Rh NPs
can be observed outside of Rh/S-1-im (Figure S5).
atoms without clear Rh clusters are observed in HRTEM images
even if the Rh loading is increased to 0.71 wt.% (Figure S9).
Figure 2. A) XPS spectra of various samples. B) Rh K-edge XANES spectra
and C) Fourier transform of k2-weighted EXAFS spectra of Rh foil and various
zeolite-encaged Rh catalysts at Rh K-edge. The peak intensity of the Rh foil is
multiplied by a coefficient. D) In-situ CO-DRIFTS spectra of the samples.
The X-ray absorption near edge structure (XANES) and
extended X-ray absorption fine structure (EXAFS) spectra of the
samples were measured to determine the electronic and
structural information of Rh species. The detailed structural
parameters based on EXAFS fittings are summarized in Table
S3. The Rh K-edge XANES spectra in Figure 2B indicates that
all zeolite-encaged Rh species possess a higher oxidation state
than the Rh foil due to the formation of ultrafine Rh species.[10]
Compared to Rh@S-1-C, the increased intensity of the white
line and a small shift to the higher binding energy are observed
for Rh@S-1-H, suggesting their higher oxidation state. Based on
the EXAFS fitting results (Figure 2C), the average coordination
number (CN) of the Rh-O shell (the bond distance of ca. 2.04 Å)
for Rh@S-1-H is 4.6 ± 0.1, which is higher than that of Rh@S-1-
C (3.3 ± 0.3). No Rh−Rh peak and Rh-O-Rh peak attributed to
the second shell of rhodium oxide can be detected in Rh@S-1-H,
demonstrating that isolated Rh atoms are formed and stabilized
by the oxygen atoms in the zeolite frameworks. In contrast, the
peak at 2.70 Å corresponding to Rh-Rh metallic bond (CN = 1.4
± 0.2) is observable in Rh@S-1-C. It indicates that some
subnanomatric Rh clusters are formed in the Rh@S-1-C, which
is in accordance with the observation of Cs-correct STEM
measurements. Notably, there is still no Rh-Rh and Rh-O-Rh
peaks can be observed in Rh@S-1-H even if the Rh loading
reaches up to 0.71 wt.%, suggesting the Rh species remains
atomically distributed at a high Rh content. In-situ CO-DRIFTS
measurements were further employed to characterize the
structure of Rh species. As shown in Figure 2D, two peaks at
2092 and 2023 cm-1 attributed to the symmetrical and
asymmetrical stretching of CO from the isolated mononuclear
RhI(CO)2 species are observed in Rh@S-1-H;[2h] however, only
the peak at 2048 cm-1 attributed to the linear-adsorbed CO on
These data indicate that the in-situ hydrothermal synthesis
method followed by ligand-protected direct H2 reduction
treatment possesses significant advantages for the fabrication of
small-sized metal species confined within the zeolite matrix than
the incipient wetness impregnation method. Significantly,
compared with the conventional calcination-reduction treatment,
the ligand-protected direct H2 reduction method can further
decrease the sizes of metal species with an atomic-scale
dispersion, in which the organic ligands and templates can serve
as protective agents to inhibit the aggregation of metal species
during the reduction process. TG-DTA analysis revealed that no
exothermic peaks can be observed in both Rh@S-1-H and
Rh@S-1-C, indicating all organic species were completely
removed under the calcination and/or reduction processes
(Figure S6); this was further confirmed by elemental CHN
analyses (Table S1). The encapsulated metal species in zeolite
caused a slight decrease of microporous volume (~0.012 cm3/g),
but large microporous volumes (0.094 and 0.099 cm3/g) and
areas (234 and 245 m2/g) retained in Rh@S-1-H and Rh@S-1-C,
respectively (Figure S7).
X-ray photoelectron spectroscopy (XPS) analysis shows that
the peaks at 307.5 and 312.2 eV ascribed to Rh 3d7/2 and Rh
3d5/2 of Rh (0) are observable in Rh/S-1-im sample, but no XPS
signals are detectable in Rh@S-1-H and Rh@S-1-C, further
indicating that all Rh atoms are completely encaged inside
zeolites (Figure 2A).[12] The Rh@S-1-H samples with higher
Rh/Si ratios of 3.4 ~ 6.75 × 10-3 in the initial synthesis gels,
giving Rh loadings in zeolites of 0.45 ~ 0.71 wt.% (determined
by ICP-AES, Table S2), were also prepared via ligand-protected
direct H2 reduction method (Figure S8). Uniformly-dispersed Rh
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Rh NPs is observable in Rh/S-1-im, but this peak barely exists in
Rh@S-1-H, further demonstrating that the Rh species in Rh@S-
1-H are atomically dispersed. Note that the peaks at 2087, 2045,
2018 cm-1 can be observed in Rh@S-1-C, indicating both of Rh
clusters and atoms exist in Rh@S-1-C, in accordance with the
observation of Cs-corrected STEM images. Significantly, the
Rh@S-1-H exhibited excellent high-temperature stability under
various atmospheres. No clear Rh clusters were observed even
after calcination in H2 at 700 °C and the Rh sizes were only
increased to 1.5 and 1.0 nm after calcination in O2 at 700 °C and
water-vapor treatment at 600 °C, respectively (Figures S10-13).
Moreover, after calcination treatments in various atmospheres,
no metal species can be observed on the outside of zeolite
crystals. In contrast, the particle size of Rh NPs in Rh/S-1-im
dramatically increased to 5.9 and 11.5 nm and located on the
outer surface of zeolite crystals after calcination in H2 and O2
700 °C, respectively (Figure S14). Significantly, the structure of
zeolites and Rh contents of Rh@S-1-H kept unchanged after
thermal/hydrothermal treatments (Figure S15 and Table S2).
amounts of Rh@ZSM-5-H (Si/Al = 245) and Rh@ZSM-5-H (Si/Al
= 105) are 0.30 and 0.32 wt.%, respectively. To get the clear
location of Rh species inside ZSM-5 crystals, the Cs-corrected
STEM images were recorded on the Rh@ZSM-5-H (Si/Al = 105).
As shown in Figure 3, the introduction of metal species does not
break the zeolite structure and excellent crystallinity is retained.
10 MR channels are empty as observed along the b-axis of the
crystals. Notably, single Rh atoms are clearly observed inside
the zeolites (inside the red circles in Figure 3C) and located
within the sinusoidal 5-MRs (Figure 3D), which is the same as
the case of Rh@S-1-H.
Figure 4. A) Volume of generated H2 versus reaction time for the hydrolysis of
AB (1 M, 1 mL) catalyzed by various catalysts at 298 K (nRh/nAB = 0.0011); B)
Volume of generated H2 versus reaction time for the hydrolysis of AB (1 M, 1
mL) catalyzed by Rh@S-1-H at different temperatures (nRh/nAB = 0.0011),
insert of B) Arrhenius plot (ln TOF versus 1000/T). C) Recycling stability tests
over Rh@S-1-H. D) Volume of generated H2 versus reaction time for the
hydrolysis of AB in the mixed solution of water/methanol with or without
nitroarenes catalyzed by Rh@S-1-H at 298 K (nRh/nAB = 0.0011, volume of
water/methanol = 10 mL/8 mL, nAB = 1 mmol, nnitroarene = 0.1 mmol).
Figure 4A shows the catalytic performance for H2 generation
from the hydrolysis of AB at 298 K catalyzed by various catalysts.
The Rh@S-1-H catalyst completed the hydrolysis of AB (1
mmol) with an H2/AB ratio of 3 within 6.33 min, affording a high
TOF value of 432 molH2 mol−ꢀℎ1 min−1, being more than 2- and 6-
fold higher than that of Rh@S-1-C (195 molH2 mol−ꢀℎ1 min−1) and
Rh/S-1-im (65 molH2 molꢀ−ℎ1 min−1 ), respectively. The improved
catalytic activity can be attributed to the reduced size of Rh
species that can expose more active sites for the hydrolysis of
AB. Notably, the H2 generation rate can be further accelerated
over Rh@ZSM-5-H catalysts. It is attributed to the synergistic
effect between Rh species and Brønsted acidic sites of ZSM-5
zeolite, which can activate both AB and water molecules,
respectively, and enhance the H2 production efficiency in
accordance with our previous work.[13] The AB hydrolysis rates
were further improved in line with the increased acid
concentration of zeolites (Figure S19). The TOF value of
Rh@ZSM-5-H (Si/Al = 105) increased to 699 min-1 at 298 K,
representing the top level among all of the state-of-the-art
Figure 3. Cs-correct STEM images of Rh@ZSM-5-H (Si/Al = 105). A) Low
magnification, B, C) Medium- and High-magnification images taken with [010]
incidence. Fourier Diffractogram and the model superimposed are inset into
Figure B. D) High magnification images taken with [001].
The direct H2-reduction strategy can be also extended to
encapsulate single Rh atoms into aluminosilicate ZSM-5 zeolite
with various Si/Al ratios (Figure S16). TEM measurements
reveal that the introduction of Al atoms into the zeolite
frameworks leads to an irregular crystal morphology and the
Rh@ZSM-5-H (Si/Al = 105) shows an aggregated morphology
consisting of many small nanocrystals (Figures S17-18). All Rh
species are encapsulated within ZSM-5 crystals, and no Rh
clusters are clearly observable both inside and outside of
crystals. ICP-AES measurements reveal that the Rh loading
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10.1002/anie.201912367
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RESEARCH ARTICLE
heterogeneous catalysts for the hydrolysis of AB.[6a, 7a, 8b, 10a, 14]
All products were determined by gas chromatography analysis
and 1H NMR and 11B NMR measurements (Figures S20-22).
The hydrolysis of AB follows the equation as follow:
p-nitroaniline, p-dinitrobenzene, and p-nitrotoluene (entries 2-7)
can be completely converted into the corresponding amines with
excellent selectivity (> 99%) within 2~10 min over Rh@S-1-H.
Note that the electron-withdrawing substituents and the steric
hindrance effect will make the nitroarene reduction a little slower
as compared with the nitrobenzene reduction. No conversion
can be observable when the 3-nitrotoluene and 4-nitroxylene
were used as substrates over Rh@S-1-H and Rh@ZSM-5-H
(Si/Al = 105) even after 60 min (entries 8 and 9), while the 4-
nitroxylene gave a complete conversion over Rh/S-1-im within 5
min. The Rh@S-1-C also exhibited good shape-selective
catalytic activity for the hydrogenation of nitroarenes, and only
less than 3% conversion of 4-nitroxylene can be observed after
60 min due to the hinderance of the zeolite channels. The
above results show that an excellent shape-selective system
can be efficiently established by using Rh@zeolite as a catalyst,
but failed to succeed over the Rh/S-1-im catalyst. Moreover,
compared with conventional calcination-reduction procedure,
which may produce a slight amount of Rh clusters outside of the
zeolite crystals, the direct H2 reduction method is more favorable
for complete confinement of the metal species within zeolite
channels. To better understand the process of the tandem
reaction, the AB hydrolysis over Rh@S-1-H under the nitroarene
hydrogenation condition were also investigated. 73.5 mL of H2
(AB/H2 = 3) was generated within 14 min without nitroarenes;
however, the volume of generated H2 decreased to 66 mL when
the nitrobenzene was introduced into the reaction solution
(Figure 4D). It indicated that a part of generated H2 from AB
hydrolysis was consumed by the reduction of nitrobenzene,
suggesting the tandem catalytic process of AB hydrolysis and
nitroarene hydrogenation. In contrast, the H2 production was not
affected with the introduction of 4-nitroxylene into the reaction
system, further indicating the 4-nitroxylene could not contact
with the Rh species within S-1 zeolites for the tandem reaction.
NH3BH3+2H2O �ca⎯t⎯a⎯ly�st NH4++BO2- +3H2 ↑
The H2 generation rates can be improved with the increase of
reaction temperatures. The TOF value over Rh@S-1-H reached
up to 1829 min-1 at 313 K and the apparent activation energy of
Rh@S-1-H was calculated as 75.5 kJ mol-1 (Figure 4B and S23).
Kinetic studies of AB hydrolysis over Rh@S-1-H were also
investigated. As shown in Figures S24 and S25, the H2
generation rates were gradually improved with the increase of
the catalyst concentrations, but they were hardly dependent on
the AB concentrations. The logarithmic plots of reaction rates
versus catalyst concentrations gave a slope of 1.22 (Figure S26),
suggesting the first-order kinetics with respect to the catalyst
concentration. Significantly, the Rh@S-1-H possessed excellent
recycling stability during the hydrolysis of AB, the H2 generation
rate, the crystallinity of zeolites and the size of Rh species
remained unchanged after five consecutive cycles (Figures 4C,
S27-S30).
The catalytic activity of Rh@S-1-H and Rh@ZSM-5-H (Si/Al =
105) catalysts for the tandem reactions that coupled the
hydrolysis of AB with nitroarene hydrogenation at 298 K were
also investigated. The results are shown in Table 1 and Figures
S31-49. The nitrobenzene can be completely converted to
aniline with > 99% selectivity of aniline within 2, 1.5, and 1 min
over Rh@S-1-C, Rh@S-1-H and Rh@ZSM-5-H (Si/Al = 105),
affording a high TOF value to 40.5, 60.6 and 90.9 molnitrobenzene
molRh min-1, respectively, being 1.5 ~ 3-fold higher than that of
-1
commercial Rh/C catalyst. It reveals that the efficiency of
tandem nitrobenzene hydrogenation can be improved in line with
the increase of hydrogen generation rates from the hydrolysis of
AB. In contrast, the conversion of nitrobenzene was as low as
5% after 900 min, when the AB was replaced by 1 bar of H2.
Moreover, no product can be detected without adding catalysts
although the AB was introduced as a reduction agent. All above
results indicate that (1) the hydrogenation of nitrobenzene is
most likely via the intermediate H2 generated from AB hydrolysis
instead of the direct transfer of H species from the AB and (2)
the tandem hydrogenation reaction is much efficient than the
conventional reduction process by using H2 as a hydrogen
source; this is because the H2 can be in-situ generated inside
zeolite upon hydrolysis of AB which can further reduce the
nitroarenes. Moreover, after five consecutive runs of
nitrobenzene hydrogenation over Rh@S-1-H, the catalytic
activity kept unchanged, indicating their excellent durability
during the tandem reaction. Most significantly, the Rh@S-1-H
and Rh@ZSM-5-H (Si/Al = 105) also exhibited superior shape-
selective catalytic activity for the hydrogenation of various
Table 1. Tandem reactions of the hydrolysis of AB and hydrogenation of
nitroarenes over different Rh-based catalysts.
NH2
NO2
Catalyst, NH3BH3
H2O/MeOH, 298 K
R
R
Time
/min
Con./Sel [i]
%/%
Entry
Substrate
Product
1[a]
1[b]
1[c]
1[d]
1[e]
1[f]
1.5
3
100/>99
100/>99
5/>99
900
120
1
NO2
NH2
Trace
100/>99
100/>99
100/>99
2
1[g]
1.5
nitroarenes
with
different
molecule
sizes.
The
p-
fluoronitrobenzene, p-chloronitrobenzene, p-bromonitrobenzene,
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10.1002/anie.201912367
Angewandte Chemie International Edition
RESEARCH ARTICLE
Acknowledgements
2[a]
3[a]
4[a]
5
5
5
100/>99
100/>99
100/>99
F
NO2
NO2
F
NH2
NH2
NH2
We thank the final supports by the National Key Research and
Development Program of China (Grant No. 2016YFB0701100),
National Natural Science Foundation of China (Grant Nos.
21835002 and 21621001) and the 111 Project (B17020). AM
and OT acknowledge to The Centre for High-resolution Electron
Microscopy (CħEM), supported by SPST of ShanghaiTech
University under contract No. EM02161943; NSFC NFSC-
21850410448. The APS was operated for the U.S. DOE Office
of Science by Argonne National Laboratory, and the CLS@APS
facilities (Sector 20) were supported by the U.S. DOE under
Contract No. DE-AC02-06CH11357, and the Canadian Light
Source and its funding partners.
Cl
Br
Cl
Br
NO2
NO2
H2N
NH2
NH2
NH2
5[a]
2
2
100/>99
100/>99
H2N
O2N
H3C
NO2 H2N
6[a]
NO2
7[a]
8[a]
10
60
100/>99
Trace
H3C
NO2
NH2
Conflict of interest
8[e]
60
Trace
CH3
CH3
The authors declare no conflict of interest.
9[a]
9[e]
9[f]
NH2
CH3
60
60
60
5
Trace
Trace
NO2
CH3
Keywords: heterogeneous catalysis • zeolites • hydrogen
generation • single-atom catalyst • tandem reaction
CH3
CH3
<3/>99
100/>99
9[h]
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[a] Catalyst: Rh@S-1-H; [b] Catalyst: commercial Rh/C; Reaction condition of
a and b: 0.1 mmol of nitroarene, 1 mmol of NH3BH3, the ratio of Rh/substrate
is 0.011, MeOH/H2O = 8mL/10mL, 298 K; [c] Replacing NH3BH3 by 1 bar of
H2; [d] Without adding any catalyst; [e] Catalyst: Rh@ZSM-5-H (Si/Al = 105);
[f] Catalyst: Rh@S-1-C; [g] by using Rh@S-1-H as a catalyst after 5th cycles.
[h]
Catalyst: Rh/S-1-im; [i] All products were identified by gas
chromatography-mass spectrometry measurements.
Conclusion
In summary, single Rh atoms were successfully encapsulated
within MFI zeolites via a facile ligand-protected direct H2-
reduction method. Detailed Cs-corrected STEM, CO-DRIFTS,
and EXAFS measurements revealed the single-atom nature of
the embedded Rh atoms, which were located within sinusoidal
5-MRs and stabilized by zeolite framework oxygens of zeolite.
The Rh@S-1-H and Rh@ZSM-5-H (Si/Al =105) catalysts
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affording TOF values of 432 and 699 molH2 mol−Rh1 min−1 at 298
K, respectively, representing the top level among all of the state-
of-the-art catalysts ever reported. Most significantly, the zeolite-
encaged single-atom metallic catalyst exhibited superior
efficiency in tandem hydrogenation of nitroarenes by coupling
with AB hydrolysis; meanwhile, these catalysts afforded shape
selectivity for various nitroarenes and showed excellent
recycling stability. This work not only demonstrates the zeolites
are an ideal supports for encaging single metal atoms with the
excellent stability and catalytic activity, but also open up new
perspectives for the application of such catalysts in shape-
selective tandem catalytic reactions.
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Angewandte Chemie International Edition
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10.1002/anie.201912367
Angewandte Chemie International Edition
RESEARCH ARTICLE
Table of Contents
RESEARCH ARTICLE
Single Rh atoms were encapsulated
within MFI zeolites under in-situ
hydrothermal conditions followed by
ligand-protected direct H2-reduction.
The catalyst gave a high turnover
Qiming Sun, Ning Wang, Tianjun Zhang,
Risheng Bai, Alvaro Mayoral,* Peng
Zhang, Qinghong Zhang, Osamu
Terasaki,and Jihong Yu*
Page No. – Page No.
frequency
of
699
−1
molH molꢀℎ
min−1 at 298 K for
(
)
2
Zeolite-Encaged
Single-Atom
Rh
ammonia borane (AB) hydrolysis and
exhibited superior catalytic efficiency
Catalysts: Highly-Efficient Hydrogen
Generation
Tandem
and
Hydrogenation
Shape-Selective
of
in
shape-selective
tandem
hydrogenation of nitroarenes by
coupling with the hydrolysis of AB.
Nitroarenes
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