Impregnating Subnanometer Metallic Nanocatalysts into Self-Pillared Zeolite Nanosheets

Article abstract of DOI:10.1021/jacs.1c00578

Impregnation is the most commonly used approach to prepare supported metal catalysts in industry. However, this method suffers from the formation of large metal particles with uneven dispersion, poor thermal stability, and thus unsatisfied catalytic performance. Here, we demonstrate that the self-pillared MFI zeolite (silicalite-1 and ZSM-5) nanosheets with larger surface area and abundant Si-OH groups are ideal supports to immobilize ultrasmall monometallic (e.g., Rh and Ru) and various bimetallic clusters via simple incipient wetness impregnation method. The loaded subnanometric metal clusters are uniformly dispersed within sinusoidal five-membered rings of MFI and remain stable at high temperatures. The Rh/SP-S-1 is highly efficient in ammonia borane (AB) hydrolysis, showing a TOF value of 430 molH2 molRh-1 min-1 at 298 K, which is more than 6-fold improvement over that of nanosized zeolite-supported Rh catalyst and even comparable with that of zeolite-supported Rh single-atom catalyst. Because of the synergistic effect between bimetallic Rh-Ru clusters and zeolite acidity, the H2 generation rate from AB hydrolysis over Rh0.8Ru0.2/SP-ZSM-5-100 reaches up to 1006 molH2 molmetal-1 min-1 at 298 K, and also shows record activities in cascade hydrogenation of various nitroarenes by coupling with the hydrolysis of AB. This work demonstrates that zeolite nanosheets are excellent supports to anchor diverse ultrasmall metallic species via the simple impregnation method, and the obtained nanocatalysts can be applied in various industrially important catalytic reactions.

Full text of DOI:10.1021/jacs.1c00578

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Impregnating Subnanometer Metallic Nanocatalysts into Self-
Pillared Zeolite Nanosheets
Ning Wang, Qiming Sun,* Tianjun Zhang, Alvaro Mayoral, Lin Li, Xue Zhou, Jun Xu, Peng Zhang,
and Jihong Yu*
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ABSTRACT: Impregnation is the most commonly used approach
to prepare supported metal catalysts in industry. However, this
method suffers from the formation of large metal particles with
uneven dispersion, poor thermal stability, and thus unsatisfied
catalytic performance. Here, we demonstrate that the self-pillared
MFI zeolite (silicalite-1 and ZSM-5) nanosheets with larger surface
area and abundant Si−OH groups are ideal supports to immobilize
ultrasmall monometallic (e.g., Rh and Ru) and various bimetallic
clusters via simple incipient wetness impregnation method. The
loaded subnanometric metal clusters are uniformly dispersed
within sinusoidal five-membered rings of MFI and remain stable at
high temperatures. The Rh/SP-S-1 is highly efficient in ammonia
borane (AB) hydrolysis, showing a TOF value of 430 molH2
molRh min−1 at 298 K, which is more than 6-fold improvement over that of nanosized zeolite-supported Rh catalyst and even
comparable with that of zeolite-supported Rh single-atom catalyst. Because of the synergistic effect between bimetallic Rh−Ru
clusters and zeolite acidity, the H2 generation rate from AB hydrolysis over Rh0.8Ru0.2/SP-ZSM-5-100 reaches up to 1006 molH2
−1
−1
molmetal min−1 at 298 K, and also shows record activities in cascade hydrogenation of various nitroarenes by coupling with the
hydrolysis of AB. This work demonstrates that zeolite nanosheets are excellent supports to anchor diverse ultrasmall metallic species
via the simple impregnation method, and the obtained nanocatalysts can be applied in various industrially important catalytic
reactions.
of reactions.2 In contrast, the impregnation method is one of
the most feasible and commonly used approaches to prepare
INTRODUCTION
■
Zeolite-supported metal nanocatalysts have emerged as an
zeolite-supported metal catalysts in industry. However, the
indispensable class of industrial catalysts because of their
impregnation method often leads to the formation of large
superior catalytic activity and excellent stability, which are
metal particles with uneven dispersions and severe aggrega-
widely used in diverse catalytic conversions, such as hydro-
tions during catalytic reactions, showing inferior catalytic
genations, dehydrogenations, and reforming reactions.1−6 The
activity and stability.2,19 It is highly desired yet challenging to
fabrication of ultrasmall metal species is regarded as an
effective strategy to improve the catalytic activity and metal
fabricate zeolite-supported metal catalysts with small metal
sizes and superior stability via the impregnation method to
utilization, because the small metal species possess high metal
meet the requirements of practical industrial applications.
dispersion and increased exposure of active sites.1−3,7,8
Recently, nanosheet-like zeolites have aroused widespread
Recently, several novel synthetic strategies, such as the
interest due to their high specific surface areas and reduced
ligand-protected method and precursor-stabilization methods
have been employed to fabricate zeolite-supported metal
micropore diffusion length, which exhibit superior catalytic
performance and adsorption properties.20−27 For example,
catalysts, which exhibit excellent thermal stability and catalytic
Ryong Ryoo and co-workers synthesized MFI zeolite nano-
performance.9−18 However, most of these synthetic strategies
either rely on the use of additional organic templates/ligands
or are not universal, which limits their large-scale industrial
production. In addition, the obtained zeolite-supported metal
Received: January 17, 2021
Published: March 4, 2021
catalysts usually suffer from mass transfer limitations in the
narrow micropores and the bulky substrate molecules cannot
enter microporous channels of zeolites and fail to access active
metallic sites, giving unsatisfied universality in different kinds
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Figure 1. Synthetic procedure and Cs-corrected STEM images of representative self-pillared MFI nanosheet-supported metal nanocatalysts. (a)
Schematic illustration of the synthetic procedure of subnanometer Rh−Ru clusters in self-pillared MFI nanosheets. Cs-corrected STEM images of
(b−d) Rh0.8Ru0.2/SP-S-1 and (f−h) Rh0.8Ru0.2/SP-ZSM-5-100 as well as corresponding size distributions of metal particles of (e) Rh0.8Ru0.2/SP-S-1
catalyst and (i) Rh0.8Ru0.2/SP-ZSM-5-100. Inset of (h): the schematic model of MFI zeolites viewed along [001] orientation. Surface weighted
mean cluster diameter dTEM = Σnidi3/Σnidi2, where ni is the number of metallic clusters having diameter di.
sheets with a 2 nm thickness by using a bifunctional
diquaternary ammonium-type surfactant, which exhibited
remarkably improved activity of the conversion of large
organic molecules and prolonged lifetime in methanol-to-
gasoline conversions as compared with conventional MFI
zeolites.22 Michael Tsapatsis and co-workers reported the
synthesis of self-pillared zeolite nanosheets by using relatively
inexpensive tetrabutylphosphonium hydroxide as a template.20
The tetrabutylphosphonium ions that act as structure-directing
groups are incorporated intact in the zeolite framework, and
contribute to the anisotropic growth and the 90° rotational
intergrowths of the MFI framework, giving rise to self-pillared
zeolites composed of orthogonally connected single−unit cell
lamellae. The zeolite nanosheets show superior catalytic
efficiency for the etherification of bulky substrates. In addition,
compared with conventional zeolite crystals, the ultrathin
zeolite nanosheets possess significantly increased external
surface area and abundant Si−OH defects, which will facilitate
the dispersion of metal precursors in zeolites and the formation
of ultrafine metal species after the reduction process. The
zeolite nanosheet-supported metal catalysts can be expected to
possess excellent catalytic performance by integrating ultra-
small and ultrastable metal species with remarkably enhanced
mass transport efficiency of ultrathin zeolites for reactants and
products during catalytic reactions.
distributed throughout zeolite nanosheets, and most of the
metal clusters are confined within the sinusoidal five-
membered rings of zeolite nanosheets. Compared with
nanosized zeolite-supported metal catalysts, the self-pillared
zeolite nanosheet-immobilized metal catalysts possess remark-
ably improved thermal stability at various redox atmospheres at
600 °C. The obtained zeolite nanosheet-supported metal
catalysts are applied in ammonia borane (AB, NH3BH3)
hydrolysis, which is regarded as one of the most promising
liquid-phase chemical hydrogen storage methods.28−31 The
Rh/SP-S-1 exhibits an excellent H2 generation rate (430
min−1) from AB hydrolysis at 298 K, which is more than 6-fold
improvement than that over nanosized zeolite-supported Rh
catalyst (Rh/Nano S-1, 66 min−1), and even comparable with
that over the single Rh atom catalyst (Rh1@S-1, 499 min−1).
The excellent catalytic activity of Rh/SP-S-1 can be mainly
attributed to the remarkably improved hydrophilicity and
transport efficiency. In addition, thanks to the synergistic effect
between bimetallic Rh−Ru clusters and Brønsted acid sites of
zeolites, the optimized Rh0.8Ru0.2/SP-ZSM-5-100 exhibits a
superior H2 generation rate from AB hydrolysis, affording a
superhigh turnover frequency (TOF) of 1006 min−1 at 298 K,
which represents the top activity among all of the best
heterogeneous catalysts under similar conditions. Isotopic
experiments reveal that both acidities of zeolites and bimetallic
Rh−Ru synergy can promote the activation of water, which
remarkably accelerates the H2 evolution rate from the
hydrolysis of AB. In addition, the Rh0.8Ru0.2/SP-ZSM-5-100
also exhibits record activities in cascade hydrogenation of
various nitroarenes by coupling with the hydrolysis of AB at
room temperature.
Herein, we employed self-pillared MFI zeolite nanosheets
(SP-S-1 and SP-ZSM-5) as efficient supports to immobilize a
series of ultrasmall monometallic (Rh and Ru) and bimetallic
clusters (Rh−Ru, Rh−Au, Rh−Ni, Rh−Co, Rh−Fe, Rh−Mn,
Rh−Cu, Rh−Zn, Ru−Cu, Ru−Fe, and Ru−Ni) via a simple
incipient wetness impregnation method. Extended X-ray
absorption fine structure (EXAFS) analyses and Cs-corrected
scanning transmission electron microscopy (STEM) confirm
that the metal clusters are subnanometer in size and uniformly
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RESULTS AND DISCUSSION
Synthesis and Characterization of Catalysts. Figure 1a
shows the schematic illustration of the synthetic procedure of
■
Figure 4. Catalytic activity of H2 generation from AB hydrolysis over
various catalysts. Volume of the H2 generated from AB (1 M)
hydrolysis versus time at 298 K (nMetal/nAB = 0.001) catalyzed by (a)
Rh/Nano S-1 Rh/SP-S-1, Ru/SP-S-1, and various RhxRu1−x/SP-S-1
catalysts, (b) Rh/SP-S-1, Rh/SP-ZSM-5-200, and Rh/SP-ZSM-5-100
catalysts, and (c) Rh/SP-ZSM-5-100, Ru/SP-ZSM-5-100, and various
RhxRu1−x/SP-ZSM-5-100 catalysts. (d) The recycling tests of AB
hydrolysis over Rh0.8Ru0.2/SP-ZSM-5-100 catalyst at 298 K. (e)
Volume of the H2 generated from AB (1 M) hydrolysis versus time at
various temperatures over Rh0.8Ru0.2/SP-ZSM-5-100 catalyst and (f)
corresponding TOF values (nMetal/nAB = 0.001). Inset of e: Arrhenius
plot (ln TOF versus 1/T).
Figure 2. Characterizations of various catalysts. (a) XRD patterns of
representative samples as well as standard MFI zeolite and Rh foil. (b)
1H MAS NMR spectra, (c) N2 adsorption/desorption isotherms, (d)
in situ infrared spectra of the adsorbed pyridine at 350 °C, (e) water
adsorption isotherms, and (f) H2O-TPD curves of various catalysts.
silicalite-1 (SP-S-1) and ZSM-5 (SP-ZSM-5) nanosheets are
first synthesized by using tetrabutyl phosphorus hydroxide
(TBPOH) as the template with molar compositions of
SiO2:0.3TBPOH:yAl2O3:zNaOH:10H2O:4EtOH (y = 0,
0.0025, and 0.005; z = 0, 0.005, and 0.0125) under
hydrothermal conditions at 115 °C for 2 days.20 Following
this, the organic templates and Na ions are removed by
calcination in air and ion-exchange with NH4NO3. Finally,
various RhxRu1−x (x = 1.0, 0.8, 0.6, 0.4, 0.2, and 0) clusters are
immobilized in MFI zeolite nanosheets by impregnating
different amounts of the RhCl3 and RuCl3 solution in zeolites
followed by H2 reduction at 400 °C. The obtained catalysts are
named as RhxRu1−x/SP-S-1, RhxRu1−x/SP-ZSM-5-200, and
RhxRu1−x/SP-ZSM-5-100, in which the 200 and 100 represent
the Si/Al ratio in initial synthesis gels. In comparison, the Rh/
Nano S-1 is also prepared by the same incipient wetness
impregnation method as the Rh/SP-S-1 except for replacing
the self-pillared S-1 nanosheets with conventional S-1 nano-
crystals that synthesized by using tetrapropylammonium
hydroxide (TPAOH) as the template with a molar
composition of SiO2:TPAOH:H2O = 1:0.4:35.
Figure 3. XANES and EXAFS measurements of various samples and
corresponding standards. (a) Rh K-edge XANES spectra of Rh foil,
Rh2O3, Rh/SP-ZSM-5-100, and Rh0.8Ru0.2/SP-ZSM-5-100. (b) Ru K-
edge XANES spectra of Ru foil, RuO2, Ru/SP-ZSM-5-100, and
Rh0.8Ru0.2/SP-ZSM-5-100. Fourier transform of k2-weighted EXAFS
spectra of (c) Rh foil, Rh2O3, Rh/SP-ZSM-5-100, and Rh0.8Ru0.2/SP-
ZSM-5-100 at Rh K-edge, and (d) Ru foil, RuO2, Ru/SP-ZSM-5-100,
and Rh0.8Ru0.2/SP-ZSM-5-100 at Ru K-edge.
The powder X-ray diffraction (XRD) patterns of RhxRu1−x
/
SP-S-1, RhxRu1−x/SP-ZSM-5, and Rh/Nano S-1 all exhibit
typical diffraction peaks corresponding to the MFI framework
(Figures S1 and S2 and Figure 2a). The diffraction peaks of
RhxRu1−x/SP-S-1 and RhxRu1−x/SP-ZSM-5 are widened
metallic subnanometer clusters in self-pillared MFI nanosheets
taking Rh−Ru as representative examples. The self-pillared
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Rh0.8Ru0.2/SP-S-1, Rh0.6Ru0.4/SP-S-1, Rh0.4Ru0.6/SP-S-1, and
Rh0.2Ru0.8/SP-S-1 are 0.81/0.19, 0.62/0.38, 0.39/0.61, and
0.19/0.81, respectively. The molar ratios of Rh/Ru in
RhxRu1−x/SP-ZSM-5-100 are also consistent with the prede-
signed values (Table S1).
Figure S3 shows the TEM images of pure SP-S-1, SP-ZSM-
5-200, and SP-ZSM-5-100 zeolites. All of these zeolites exhibit
self-pillared nanosheet-like morphology and the thickness of
each lamellae in SP-S-1 and SP-ZSM-5-200 are only 2−3 nm.
The introduction of more Al atoms into zeolite frameworks
can slightly enlarge the zeolite crystals, but the average
thickness of zeolites in SP-ZSM-5-100 is only about 10 nm. In
comparison, the conventional S-1 nanocrystals exhibit a
morphology of hexagonal prism with dimensions of 100−200
nm. The high-resolution TEM and Cs-corrected high angle
annular dark field STEM (HAADF-STEM) images of
representative samples are shown in Figure S4 and Figure
1b−i. The introduction of metal species does not change the
nanosheet-like morphology of zeolites. Significantly, most of
the metal species are confined within micropores of self-
pillared MFI nanosheets and more than 80% of metal species
are less than 1 nm in size. The average metal sizes of
Rh0.8Ru0.2/SP-S-1 and Rh0.8Ru0.2/SP-ZSM-5-100 are as low as
0.78 and 0.70 nm, respectively, and the ultrasmall metal
clusters are uniformly immobilized throughout the zeolite
nanosheets with superhigh dispersions and narrow size
distributions. By looking at the zeolite along with the [001]
projection, based on the Cs-corrected STEM analyses and the
schematic crystallographic representation along this zone axis
(Figure 1h inset); most of the metal clusters are confined
within one or several “rectangular units” which are formed by
the sinusoidal five-membered rings. In contrast, the metal
nanoparticles (NPs) supported onto S-1 nanocrystals are quite
large (∼4 nm) with uneven distributions and substantially all
of the metal NPs are located on the outside of zeolite
nanocrystals (Figure S4f1−f3). H2-chemisorption measure-
ments reveal that the metal dispersions of Rh/SP-S-1, Rh/SP-
ZSM-5-100, and Rh0.8Ru0.2/SP-ZSM-5-100 are 83.4, 86.1, and
84.4%, respectively, which are much higher than that of Rh/
Nano S-1 (23.0%) (Figure S5). The above results indicate that
compared with conventional zeolite nanocrystals, the ultrathin
zeolite nanosheets are much beneficial for improving the
dispersion of metal precursors and forming ultrafine metal
clusters upon reduction process. Scanning electron microscopy
(SEM) exhibits that many large Rh particles are located on the
outside of zeolites in Rh/Nano S-1, but they are hardly
observed in Rh/SP-S-1, Rh/SP-ZSM-5-100, and Rh0.8Ru0.2/
SP-ZSM-5-100 (Figure S6). The chemical elemental mappings
for Si, O, Al, Rh, and Ru elements of Rh0.8Ru0.2/SP-ZSM-5-100
are shown in Figure S7. The Rh and Ru are uniformly
distributed throughout the SP-ZSM-5 crystals in a similar
position, demonstrating the formation of a bimetallic system.
The 1H and 29Si MAS NMR spectra of samples are shown in
Figure 5. Kinetic isotope measurements and catalytic activity of H2
generation from AB hydrolysis over various self-pillared ZSM-5-100-
supported Rh-based nanocatalysts. The volume of the H2 generated
from AB (1 M) hydrolysis versus time using H2O and D2O as the
reactants at 298 K over (a) Rh/SP-S-1, (b) Rh0.8Ru0.2/SP-S-1, (c)
Rh/SP-ZSM-5-100, and (d) Rh0.8Ru0.2/SP-ZSM-5-100 catalysts. (e)
TOF values of various self-pillared ZSM-5-100-supported Rh-based
nanocatalysts.
Figure 6. Catalytic activity of the cascade hydrogenation of
nitrobenzene and 4-nitroxylene coupling with the hydrolysis of AB
over various catalysts. (a) Cascade hydrogenation of nitrobenzene (b)
Cascade hydrogenation of 4-nitroxylene coupling with AB hydrolysis
over various catalysts. Reaction condition: 0.2 mmol of nitrobenzene
or 4-nitroxylene, 1 mmol of NH3BH3, the ratio of Rh/substrate is
0.0025, MeOH/H2O = 8 mL/12 mL, 298 K. The Rh0.8Ru0.2/SP-
ZSM-5-100 (1 bar H2) are tested under the same condition with
others just replacing NH3BH3 by 1 bar of H2.
compared to that of Rh/Nano S-1, which is attributed to the
formation of smaller zeolite crystals. No diffraction peaks of
metallic Rh and Ru (foils) are detected in samples because of
the relatively low metal loadings, small size, and uniform metal
dispersions. On the basis of the inductively coupled plasma
(ICP) atomic emission spectroscopy measurements, the molar
ratios of Si/Al of SP-ZSM-5-200 and SP-ZSM-5-100 are 253
and 75, respectively. The loading amount of P element in SP-S-
1, SP-ZSM-5-200, and SP-ZSM-5-100 are 1.26, 1.22, and 1.12
wt %, respectively, because of the formation of a small amount
of P2O5 after calcination of templates in air. All metal-
containing samples possess similar metal loading amounts of
0.42−0.44 wt % (Table S1) and the molar ratios of Rh/Ru in
1
Figure 2b and Figures S8 and S9. In the H MAS NMR
spectra, the signals at 1.8−2.1 and 3.7 ppm can be attributed to
isolated Si−OH and H-bonded Si−OH, respectively.32
Notably, compared with the Nano S-1, the SP-S-1, SP-ZSM-
5-100, and Rh/SP-ZSM-5-100 possess more isolated Si−OH
defects, and the introduction of metal species leads to some
decreases in isolated Si−OH sites of zeolites. On the basis of
the above results, we can deduce that the isolated Si−OH
defects are probably responsible for immobilizing metal
clusters, and the metal dispersions and sizes become better
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Table 1. Catalytic Results for the Cascade Hydrogenation of Nitroarenes by Coupling with the Hydrolysis of AB over Various
Catalysts
a
Reaction condition: 0.2 mmol of nitro compound, 1 mmol of NH3BH3, the Rh/substrate ratio is 0.0025, MeOH/H2O = 8 mL/12 mL at 298 K.
b
c
d
Replacing NH3BH3 by 1 bar of H2. All products were analyzed by gas chromatography−mass spectrometry. The activity was calculated based on
the total amount of metal atoms in catalysts when the conversion of nitroarenes reached 100%.
and smaller with the increase of isolated Si−OH defects in
zeolites. The peak at 4.2 ppm ascribed to bridging OH groups
(SiOHAl) is observable in SP-ZSM-5-100 and Rh/SP-ZSM-5-
100, indicating the presence of Brønsted acid sites.33 In the 29Si
MAS NMR spectra, the peaks at −110 to −115 ppm are the
predominant signal in all samples corresponding to Si(4Si)
species, and the SP-S-1 possesses more Si(3Si1OH) species
than the Nano S-1, indicating the formation of more defect
sites.
To identify the atomic structure and electronic property of
metal species, we measured the X-ray absorption near edge
structure (XANES) and extended X-ray absorption fine
structure (EXAFS) of Rh/SP-ZSM-5-100, Ru/SP-ZSM-5-
100, and Rh0.8Ru0.2/SP-ZSM-5-100 (Figure 3 and Figure
S10, Table S2). Notably, Rh/SP-ZSM-5-100 and Rh0.8Ru0.2/
SP-ZSM-5-100 possess a more intense white line (i.e., the first
prominent peak following the absorption edge) in Rh K-edge
XANES spectra than that of Rh foil, suggesting the oxidized
state of Rh in these two samples. Such phenomenon can be
attributed to the formation of ultrasmall metal clusters and the
interaction between metal clusters and skeleton oxygen atoms
of zeolites. Similarly, the XANES spectra of Ru/SP-ZSM-5-100
and Rh0.8Ru0.2/SP-ZSM-5-100 at Ru K-edge (Figure 3b) also
exhibit some differences from the Ru foil. Notably, at the Ru
K-edge XANES spectra, the intensity of the white line at 22140
eV of Rh0.8Ru0.2/SP-ZSM-5-100 is lower than that of Ru/SP-
ZSM-5-100, and the Rh0.8Ru0.2/SP-ZSM-5-100 also exhibits a
shift to lower binding energy as compared with Ru/SP-ZSM-5-
100. However, compared with Rh/SP-ZSM-5-100, the
opposite trend in the intensity of the white line at 23235 eV
and binding energy appeared in Rh0.8Ru0.2/SP-ZSM-5-100 at
the Rh K-edge XANES spectra. All the above changes in the
XANES demonstrate that electrons are transferred from Rh to
Ru in Rh0.8Ru0.2/SP-ZSM-5-100, indicating the formation of a
bimetallic Rh−Ru structure.34 According to the EXAFS results,
the average coordination numbers (CNs) of Rh−O with a
bond length of about 2.02 Å for Rh0.8Ru0.2/SP-ZSM-5-100 is
3.6, which is slightly higher than that of Rh/SP-ZSM-5-100
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(CN = 3.4), Figure 3c. In contrast, the CN of Ru−O with a
bond length of about 2.00 Å for Rh0.8Ru0.2/SP-ZSM-5-100 is
lower than that of Ru/SP-ZSM-5-100 (3.4 vs 4.1), Figure 3d.
These results further indicate that compared to monometallic
counterparts, the bimetal Rh−Ru clusters in Rh0.8Ru0.2/SP-
ZSM-5-100 are electron-rich (Ru) and electron-poor (Rh)
metal sites, respectively. Notably, all of the CNs of metal−
metal (e.g., Rh−Rh, Ru−Rh, and Ru−Ru) bonds in Rh/SP-
ZSM-5-100, Ru/SP-ZSM-5-100, and Rh0.8Ru0.2/SP-ZSM-5-
100 are less than 2. These low CNs show that the obtained
metal clusters are composed of a small number of atoms and
possess subnanometer dimensions, which is consistent with the
observation of Cs-corrected STEM images. The X-ray
photoelectron spectroscopy (XPS) measurements were also
performed to determine the electronic states of metal species
in samples. The peaks at 307.8 and 312.5 eV corresponding to
the Rh 3d5/2 and Rh 3d3/2 of Rh(0) can be observed in Rh/SP-
ZSM-5-100 (Figure S11).35 By comparison, the corresponding
peaks slightly shift to the higher binding energy of 308.1 and
312.8 eV in Rh0.8Ru0.2/SP-ZSM-5-100, which indicates that the
Rh species in Rh0.8Ru0.2/SP-ZSM-5-100 is more electron-poor
than that in Rh/SP-ZSM-5-100, in accordance with XANES
results.
N2 adsorption/desorption measurements show that the
specific surface areas of the SP-S-1 and SP-ZSM-5-100 are 608
and 567 m2/g, which is much higher than that of Nano S-1
(375 m2/g) (Figure 2 c and Figure S12, Table S1). It can be
mainly attributed to the formation of ultrathin zeolite
nanosheets in the SP-S-1 and SP-ZSM-5-100, possessing
significantly increased external surface areas. The large specific
surface area of self-pillared zeolite nanosheets is also a very
important factor in the formation of ultrafine metal clusters
with high dispersions. Compared to the pure SP-S-1 and SP-
ZSM-5-100 zeolites, a decrease in micropore volume (0.007−
0.008 cm3/g) can be observed for metal/SP-S-1 and metal/SP-
ZSM-5-100, attributed to the partial occupation of zeolite
micropores by the metal clusters. However, more than 90% of
void spaces (>0.075 cm3/g microporous volumes and >507
m2/g surface areas) are reserved in self-pillared MFI nano-
sheet-supported metal samples. In contrast, the microporous
volumes of Rh/Nano S-1 and pure Nano S-1 are identical,
which indicates that almost all of the metal nanoparticles are
likely located on the outer surface of Nano S-1 rather than the
inside of the zeolite matrix, in accordance with the observation
of TEM images.
The ammonia temperature-programmed desorption (NH3-
TPD) measurements are employed to evaluate the acidity of
samples. As shown in Figure S13, the Rh/Nano S-1 shows no
acidity, whereas a peak at about 160 °C can be observed in
Rh/SP-S-1, which can be mainly attributed to the existence of
the P2O5 in the sample. With the introduction of Al into zeolite
frameworks, a peak at 350−400 °C can be observed in the Rh/
SP-ZSM-5-200 and Rh/SP-ZSM-5-100, indicating the gen-
eration of strong acid sites. Among all of the samples, the Rh/
SP-ZSM-5-100 exhibited the highest acid concentration and
strength because of its lowest Si/Al ratio. In situ infrared (IR)
spectroscopy of the adsorbed pyridine at different temper-
atures was also used to probe the acidity of samples. As shown
in Figure 2d and Figure S14, the bands at 1455 and 1545 cm−1
are assigned to the Lewis and Brønsted acid sites,
respectively.36,37 The numbers of Lewis acid sites of Rh/SP-
S-1, Rh/SP-ZSM-5-200, and Rh/SP-ZSM-5-100 are 6.1, 17.6,
and 17.8 μmol/g at 350 °C, respectively. Among all samples,
the Rh/SP-ZSM-5-100 possesses the highest number of
Brønsted acid sites up to 56.0 μmol/g at 350 °C, which is
over 2-fold higher than that of Rh/SP-ZSM-5-200 (24.8 μmol/
g). In contrast, there are no Brønsted acid sites in Rh/SP-S-1
(Table S3). The hydrophilicity of samples is determined by
water absorption measurement. As shown in Figure 2e, the
water absorption of the Rh/SP-S-1 sample is as high as 43.8 wt
%, more than 8 times higher than that of Rh/Nano S-1 (5.1 wt
%), suggesting Rh/SP-S-1 is much more hydrophilic than Rh/
Nano S-1, which can be attributed to its ultrathin nanosheet-
like morphology and the existence of more defects in the SP-S-
1 nanosheets. The H2O-TPD measurements further reveal that
the Rh/SP-S-1 and Rh/SP-ZSM-5 possess stronger chem-
isorption for water than does Rh/Nano S-1, and among all of
the samples, the Rh/SP-ZSM-5-100 exhibits the strongest
chemisorption for water (Figure 2f).
The thermal stability of representative Rh0.8Ru0.2/SP-ZSM-
5-100 is also investigated in various atmospheres. The average
sizes of Rh−Ru species are only 1.1, 0.95, and 1.2 nm after
thermal treatment at 600 °C under H2, N2, and O2−H2 cycle
atmospheres, respectively (Figures S15−S17). In contrast, the
metal sizes of Rh/Nano S-1 are dramatically increased to 4.5−
8.5 nm after calcination under these atmospheres. It indicates
that the self-pillared zeolite nanosheet-immobilized subnan-
ometer metallic clusters possess significantly improved thermal
stability than conventional nanosized zeolite-supported metal
catalysts. Most significantly, besides Rh−Ru clusters, the SP-
ZSM-5-100 can be also used as powerful supports to
immobilize various ultrafine metal clusters. As shown in
Figures S18−S27, a series of metal clusters, such as Rh−Au,
Rh−Ni, Rh−Co, Rh−Fe, Rh−Mn, Rh−Cu, Rh−Zn, Ru−Cu,
Ru−Fe, and Ru−Ni with subnanometric sizes and superhigh
dispersions are successfully immobilized into SP-ZSM-5-100
nanosheets via the incipient wetness impregnation method.
Catalytic Activity of Hydrogen Generation from AB
Hydrolysis. We demonstrate the catalytic efficiency of various
self-pillared zeolite nanosheet-supported metal nanocatalysts
for the H2 generation from AB hydrolysis. As in Figure 4a, the
H2 generation rate from AB hydrolysis over Rh/SP-S-1 reaches
up to 430 molH2 molRh min−1 at 298 K, which is 6-fold
−1
higher than that of Rh/Nano S-1 (66 molH2 molRh min−1).
−1
Notably, such a value is also comparable with that of Nano S-1-
−1
encaged single Rh atom (Rh1@S-1) (499 molH2 molmetal
min−1) under the same catalytic condition,13 although the Rh
clusters in Rh/SP-S-1 are larger than single Rh atoms. The
excellent catalytic activity can be mainly attributed to the
superior hydrophilicity and enhanced mass transport efficiency
of reactants and products among zeolite nanosheets.
Significantly, the introduction of a suitable amount of Ru
into Rh species to form Rh−Ru bimetallic clusters could
further enhance the catalytic performance for AB hydrolysis.
Among all of the bimetallic catalysts, the optimized Rh0.8Ru0.2/
SP-S-1 exhibits the highest H2 generation rate from AB
−1
hydrolysis, affording a TOF value of 620 molH2 molmetal
min−1. In addition, the H2 generation rates over the Rh/SP-
ZSM-5 are further improved than that over the Rh/SP-S-1, and
the H2 generation rates are improved gradually with the
increase in zeolite acidity (Figure 4b and Figure S28, Table
S4). The catalytic activities of AB hydrolysis are further tested
over Rh/SP-SZM-5-100, Ru/SP-SZM-5-100, and various
RhxRu1−x/SP-ZSM-5-100 catalysts at 298 K (Figure 4c).
Among all of the catalysts, the Rh0.8Ru0.2/SP-SZM-5-100
catalyst exhibits the best catalytic performance: 73.5 mL of
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stoichiometric H2 (H2/AB = 3) can be released within 3.0 min
The kinetic studies of AB hydrolysis are also investigated
over the Rh0.8Ru0.2/SP-SZM-5-100. The H2 generation rates
over the Rh0.8Ru0.2/SP-SZM-5-100 almost keep unchanged
with the increase of AB concentrations from 0.5 to 2 M, but
gradually accelerate with the increase of catalyst concentrations
(Figures S36 and S37a). The logarithmic plots of H2
generation rates versus catalyst concentrations exhibited a
slope of 1.274 (Figure S37b), indicating the first-order kinetics
with respect to the catalyst concentration. To further
investigate the mechanistic insights of AB hydrolysis, the
isotopic experiments using D2O instead of H2O as the reactant
are also carried out over various catalysts. As shown in Figures
5a−d, compared with H2O as the reactant, all of the catalysts
show much slower H2 generation rates when the D2O is used
as the reactant. Based on previous literature, the kinetic isotope
effect (KIE) value is calculated as the ratio of rate constants
between H2O and D2O as the reactants, which is regarded as
an effective method to determine whether the cleavage of the
O−H bond is the rate-determining step during the AB
hydrolysis.42−44 The KIE values of Rh0.8Ru0.2/SP-S-1 and Rh/
SP-ZSM-5-100 are 3.62 and 3.40, respectively, which are lower
than that of Rh/SP-S-1 (KIE = 4.61). The above results
demonstrate that the activation of the water molecule or the
cleavage of the O−H bond is the rate-determining step for AB
hydrolysis and both of the bimetallic Rh−Ru synergy and
acidic sites of zeolites can promote the activation of the water
molecule. Notably, among all of the catalysts, the Rh0.8Ru0.2/
SP-ZSM-5-100 catalyst exhibits the lowest KIE value of 2.88,
suggesting its strongest ability to active water molecules.
The above results further indicate that the establishment of
efficient synergy by coupling bimetallic clusters with Bronsted
acid sites of zeolites can remarkably contribute to the
activation of water molecules and boost the efficiency of H2
generation from AB hydrolysis. The catalytic activities for H2
generation from AB hydrolysis are also investigated over
various SP-ZSM-5-100 nanosheet-supported Rh-based bimetal
catalysts at 298 K (Figure S38). On the basis of the Rh atoms,
the TOF values of Rh0.8M0.2/SP-ZSM-5-100 (M = Cu, Mn, Ni,
Fe, Co, Au, and Ru) are increased in turn, which are higher
than that of monometallic Rh/SP-ZSM-5-100 (Figure 5e).
Among all bimetallic catalysts, the Rh0.8Ru0.2/SP-ZSM-5-100
(nmetal/nAB = 0.001) toward 100% decomposition of AB,
−1
affording a superhigh TOF value of 1006 molH2 molmetal
min−1 at 298 K. Such a TOF value is more than 15-fold
improvement compared to that of the Rh/Nano S-1, which is
also much higher than that of most state-of-the-art
heterogeneous catalysts (Table S5).13,38−40 The activities of
AB hydrolysis over bare SP-S-1 and SP-ZSM-5 zeolites are also
investigated at 298 K. H2 (0.55 mL) is released in 10 min over
bare SP-ZSM-5 zeolite, which is about 2-fold higher than that
over bare SP-S-1 (0.28 mL), indicating that the Brønsted acid
site of zeolite benefits for the hydrolysis of AB (Figure S29).
However, compared with metal-containing zeolite catalysts, the
H2 generation over pure zeolites is almost negligible, indicating
that the H2 generation from AB hydrolysis over zeolite-
supported metal catalysts mainly relies on the introduction of
metal species. Notably, the TOF ratio of Rh0.8Ru0.2/SP-ZSM-
5-100 to Rh/SP-ZSM-5-100 is 1.96, which is higher than that
of Rh0.8Ru0.2/SP-S-1 to Rh/SP-S-1 (1.44). Considering the
similar metal size of these catalysts, the above results indicate
that the synergistic effect between bimetallic clusters and acidic
zeolites is more conducive to accelerate the H2 generation rate
from AB hydrolysis. According to the gas chromatography
analysis and 1H NMR and 11B NMR measurements, the
+
−
gaseous H2, as well as NH4 and BO2 ions, are the final
products after AB hydrolysis over catalysts (Figures S30−32).
The AB hydrolysis follows the equation as below:
catalyst
NH3BH3 + 2H2O ⎯⎯⎯⎯⎯⎯→ NH+4 + BO−2 + 3H2↑
The Rh0.8Ru0.2/SP-SZM-5-100 exhibits excellent recycling
stabilities during AB hydrolysis. After the completion of the
previous run, the catalyst is isolated from the reaction solution
and washed by dilute HCl solution (0.1 M). After successive
five cycles, the H2 generation rates over Rh0.8Ru0.2/SP-SZM-5-
100 keep unchanged (Figure 4d). Significantly, the size of
metal clusters, morphology and crystallinity of zeolites, and
metal loadings of spent catalysts remain identical with the fresh
one based on the TEM, XRD, and ICP measurements (Figures
S33 and S34, Table S1). Moreover, the durability tests are also
investigated over the Rh0.8Ru0.2/SP-SZM-5-100. Upon com-
pletion of the previous run of the AB hydrolysis, another
equivalent of AB (2 M, 0.5 mL) is directly added into the
reaction system to examine the activity of AB hydrolysis. As
shown in Figure S35a, 73.5 mL of stoichiometric H2 (H2/AB =
3) can be released in each run, and a high TOF value of 812
min−1 is achieved even after consecutive five injections of AB
solution. The slightly decreased catalytic activity after several
consecutive injections of AB solution is mainly attributed to
the increased viscosity of the solution and the deactivation
effect of the increasing metaborate concentration during the
hydrolysis reaction.41 Note that the activity of the spent
catalyst after consecutive five injections of AB solution can be
recovered by isolated from the reaction solution and washed by
dilute HCl solution (Figure S35b). The above results further
reveal excellent durable and recycling stability of self-pillared
zeolite-supported metal catalysts in the AB hydrolysis
reactions. Moreover, the H2 generation rates accelerate along
with the reaction temperatures, and the TOF value of
−1
exhibits the highest TOF value of 1291 molH2 molRh min−1,
indicating the synergistic effect of Rh−Ru is more conducive to
improve the H2 evolution rate from the hydrolysis of AB than
that of other Rh-based bimetal species. Notably, the TOF value
of Rh0.8Zn0.2/SP-ZSM-5-100 is slightly lower than that of Rh/
SP-ZSM-5-100, indicating that the introduction of Zn cannot
enhance the activity of Rh for AB hydrolysis.
Catalytic Activity of Cascade Hydrogenation of
Nitroarenes by Coupling with the Hydrolysis of AB.
The catalytic activity of Rh/Nano S-1, Rh/SP-S-1, Rh1@S-1,
and Rh0.8Ru0.2/SP-ZSM-5-100 catalysts for the cascade
reactions that couples the hydrolysis of AB with the
hydrogenation of nitrobenzene (0.2 mmol) and 4-nitroxylene
(0.2 mmol) at 298 K are investigated. The catalytic results are
shown in Figure 6 and Figures S39−S42 and Table 1. The
nitrobenzene can be 100% converted to aniline (>99%
selectivity) within 3 min over the Rh/SP-S-1, giving a high
aniline generation rate of 133 molnitrobenzene molmetal min−1,
−1
which is 5- and 2-fold higher than that of Rh/Nano S-1 and
Rh1@S-1, respectively. The superior catalytic activity of Rh/
SP-S-1 can be mainly attributed to the remarkably enhanced
transport efficiency of reactants and products across the
−1
Rh0.8Ru0.2/SP-SZM-5-100 is as high as 3001 molH2 molmetal
min−1 at 313 K, with an apparent activation energy of 56.5 kJ
mol−1 (Figure 4e, f).
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ultrathin zeolite nanosheets. Significantly, thanks to the
synergistic effect between bimetal components and Brønsted
acid sites of zeolites, the aniline generation rate over
Rh0.8Ru0.2/SP-ZSM-5-100 reaches up to 267 molnitrobenzene
100 also exhibited record catalytic activities in cascade
hydrogenation of various nitroarenes by coupling with the
hydrolysis of AB, which is even 4-fold higher than zeolite-
encaged single-atom Rh catalysts due to its great superiority in
fast transport of reactants and products during reactions. This
work opens a new perspective for the utilization of ultrathin
zeolite nanosheets as efficient supports to anchor ultrasmall
metal nanocatalysts via simple and industrially available
impregnation method. Most importantly, the zeolite nano-
sheet-supported metal catalysts exhibit excellent thermal
stability under various atmospheres and significantly improved
mass transfer efficiency across narrow micropores of zeolites,
which can be expected to exhibit high performance in other
important catalytic reactions. It should be pointed out that
more kinds of small and stable metal species can also be
expected to be immobilized in zeolite nanosheets via the
impregnation method by taking into account solubilities,
molecular sizes, and electrical properties of metal precursors.
−1
molmetal min−1, which represents the best activity among all
previously reported heterogeneous catalysts under similar
conditions.13,45,46 By comparison, the nitrobenzene conversion
is less than 1% after 20 min when the AB solution is replaced
by the atmospheric H2, and no product can be obtained
without adding catalysts. The above results indicate that the in
situ generated H2 from AB hydrolysis in the catalytic system is
much more efficient than the conventional gaseous H2 for the
reduction of the nitroarenes. The faster the H2 generation rate
from AB hydrolysis is, the higher the cascade nitrobenzene
hydrogenation activity becomes. The catalytic activity of
Rh0.8Ru0.2/SP-ZSM-5-100 for the cascade hydrogenation of
nitrobenzene (1 mmol) in a higher concentration at 298 K is
also investigated. As shown in Figure S43, the nitrobenzene in
higher concentration can be 20% converted to aniline within 1
min over the Rh0.8Ru0.2/SP-ZSM-5-100 catalyst, giving a
ASSOCIATED CONTENT
* Supporting Information
■
−1
superhigh initial TOF value of 400 molnitrobenzene molmetal
sı
min−1. Significantly, the nitrobenzene can be 100% converted
to aniline (>99% selectivity) within 8 min over the Rh0.8Ru0.2/
SP-ZSM-5-100 catalyst, giving an aniline generation rate of 251
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c00578.
molnitrobenzene molmetal min−1.
−1
Materials, methods, supporting figures, and tables
(PDF)
Significantly, the 4-nitro-o-xylene with the larger molecular
size can be also 100% converted to 3,4-dimethylaniline within
3 min over the Rh0.8Ru0.2/SP-ZSM-5-100, affording a high
catalytic activity of 133 min−1. However, no product can be
obtained over Rh1@S-1 because the molecular size of 4-nitro-
o-xylene is larger than microporous sizes of MFI zeolite and
fails to access the metal active sites inside the zeolites matrix.
Most importantly, diverse nitroarenes, such as p-chloroni-
trobenzene, p-bromonitrobenzene, p-fluoronitrobenzene, 4-
nitrotoluene, 3-nitrotoluene, and even bulky 1-nitronaphtha-
lene can be totally converted to the corresponding amines
(>99% selectivity) within 1.5−3 min catalyzed by Rh0.8Ru0.2/
SP-ZSM-5-100 (Table 1 and Figures S44−S49). The above
results indicate that the self-pillared zeolite nanosheet
supported metal catalysts possess more extensive practicability
and superior performance for cascade hydrogenation of
different kinds of nitroarenes than previously reported
nanosized zeolite-encaged metal catalysts and other heteroge-
neous catalysts.
AUTHOR INFORMATION
Corresponding Authors
■
Jihong Yu − State Key Laboratory of Inorganic Synthesis and
Preparative Chemistry, College of Chemistry and
International Center of Future Science, Jilin University,
Changchun 130012, People’s Republic of China;
orcid.org/0000-0003-1615-5034; Email: jihong@
jlu.edu.cn
Qiming Sun − Innovation Center for Chemical Sciences,
College of Chemistry, Chemical Engineering and Materials
Science, Soochow University, Suzhou 215123, People’s
Republic of China; orcid.org/0000-0002-8895-5703;
Email: sunqiming@suda.edu.cn
Authors
Ning Wang − State Key Laboratory of Inorganic Synthesis and
Preparative Chemistry, College of Chemistry, Jilin University,
Changchun 130012, People’s Republic of China
CONCLUSION
Tianjun Zhang − State Key Laboratory of Inorganic Synthesis
and Preparative Chemistry, College of Chemistry, Jilin
University, Changchun 130012, People’s Republic of China
Alvaro Mayoral − Center for High-resolution Electron
Microscopy (CℏEM), School of Physical Science and
Technology, ShanghaiTech University, Shanghai 201210,
People’s Republic of China; Institute of Nanoscience and
Materials of Aragon (INMA), Spanish National Research
Council (CSIC), Advanced Microscopy Laboratory (LMA),
University of Zaragoza, 50009 Zaragoza, Spain
■
In summary, we demonstrate that the self-pillared MFI zeolite
nanosheets are powerful supports to immobilize various
ultrasmall metallic clusters via a simple and industrially
available impregnation method. As compared with conven-
tional nanosized zeolite-supported metal catalysts, the self-
pillared zeolite nanosheet-immobilized metal catalysts pos-
sessed remarkably decreased sizes of metal species, improved
hydrophilicity and transport efficiency, and excellent thermal
stability under diverse redox atmospheres at high temperatures.
The as-prepared self-pillared zeolite nanosheet-supported
metal catalysts are highly efficient in AB hydrolysis.
Significantly, the optimized Rh0.8Ru0.2/SP-ZSM-5-100 ex-
hibited a superhigh activity of H2 generation rate from AB
hydrolysis at 298 K, up to 1006 min−1. Isotopic experiments
reveal that both zeolite acidity and Rh−Ru synergy can
promote the activation of water and significantly accelerate the
H2 generation from AB hydrolysis. The Rh0.8Ru0.2/SP-ZSM-5-
Lin Li − Electron Microscopy Center, Jilin University,
Changchun 130012, People’s Republic of China
Xue Zhou − National Centre for Magnetic Resonance in
Wuhan, State Key Laboratory of Magnetic Resonance and
Atomic and Molecular Physics, Innovation Academy for
Precision Measurement Science and Technology, Chinese
Academy of Sciences, Wuhan 430071, People’s Republic of
China; orcid.org/0000-0002-6971-594X
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Jun Xu − National Centre for Magnetic Resonance in Wuhan,
State Key Laboratory of Magnetic Resonance and Atomic and
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Measurement Science and Technology, Chinese Academy of
Sciences, Wuhan 430071, People’s Republic of China
Peng Zhang − Department of Chemistry, Dalhousie
University, Halifax, Nova Scotia B3H 4R2, Canada
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c00578
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Notes
The authors declare no competing financial interest.
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We thank the National Natural Science Foundation of China
(Grants 21835002, 21920102005, and 21621001) and the 111
Project (B17020) for financial support. Q.S. acknowledges the
Academic Start-up Funds of Soochow University
(Q410901120). A.M. acknowledges The Centre for High-
resolution Electron Microscopy (CℏEM), supported by SPST
of ShanghaiTech University under contract. EM02161943;
NFSC-21850410448 and to Spanish Ministry of Science under
the Ramon y Cajal program (RYC2018-024561-I). This
research used resources of the Advanced Photon Source, an
Office of Science User Facility operated for the U.S.
Department of Energy (DOE) Office of Science by Argonne
National Laboratory, and was supported by the U.S. DOE
under Contract No. DE-AC02-06CH11357, and the Canadian
Light Source and its funding partners. We acknowledge Prof.
Osamu Terasaki from ShanghaiTech University for his
guidance and help in the Cs-corrected STEM measurements.
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