Subnanometer Bimetallic Platinum–Zinc Clusters in Zeolites for Propane Dehydrogenation

Article abstract of DOI:10.1002/anie.202003349

Propane dehydrogenation (PDH) has great potential to meet the increasing global demand for propylene, but the widely used Pt-based catalysts usually suffer from short-term stability and unsatisfactory propylene selectivity. Herein, we develop a ligand-protected direct hydrogen reduction method for encapsulating subnanometer bimetallic Pt–Zn clusters inside silicalite-1 (S-1) zeolite. The introduction of Zn species significantly improved the stability of the Pt clusters and gave a superhigh propylene selectivity of 99.3 % with a weight hourly space velocity (WHSV) of 3.6–54 h^?1 and specific activity of propylene formation of 65.5 mol (Formula presented.) g_Pt^?1 h^?1 (WHSV=108 h^?1) at 550 °C. Moreover, no obvious deactivation was observed over PtZn4?S-1-H catalyst even after 13000 min on stream (WHSV=3.6 h^?1), affording an extremely low deactivation constant of 0.001 h^?1, which is 200 times lower than that of the PtZn4/Al₂O₃ counterpart under the same conditions. We also show that the introduction of Cs⁺ ions into the zeolite can improve the regeneration stability of catalysts, and the catalytic activity kept unchanged after four continuous cycles.

Full text of DOI:10.1002/anie.202003349

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Subnanometer Bimetallic Pt-Zn Clusters in Zeolites for Propane
Dehydrogenation
Authors: Qiming Sun, Ning Wang, Qiyuan Fan, Lei Zeng, Alvaro
Mayoral, Shu Miao, Ruoou Yang, Zheng Jiang, Wei Zhou,
Jichao Zhang, Tianjun Zhang, Jun Xu, Peng Zhang, Jun
Cheng, Dong-Chun Yang, Ran Jia, Lin Li, Qinghong Zhang,
Ye Wang, Osamu Terasaki, and Jihong Yu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202003349
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10.1002/anie.202003349
Angewandte Chemie International Edition
RESEARCH ARTICLE
Subnanometer Bimetallic Pt-Zn Clusters in Zeolites for Propane
Dehydrogenation
Qiming Sun,+[a] Ning Wang,+[a] Qiyuan Fan,[b] Lei Zeng,[b] Alvaro Mayoral,[c] Shu Miao,[d] Ruoou Yang,[e]
Zheng Jiang,[e, f] Wei Zhou,[b] Jichao Zhang,[e] Tianjun Zhang,[a] Jun Xu,[g] Peng Zhang,[h] Jun Cheng,[b]
Dong-Chun Yang,[i] Ran Jia,[i] Lin Li,[k] Qinghong Zhang,[b] Ye Wang,*[b] Osamu Terasaki,[c] and Jihong
Yu*[a, j]
Abstract: Propane dehydrogenation (PDH) has great potential to
meet the increasing global demand for propylene, but the widely-
used Pt-based catalysts usually suffer from short-term stability and
unsatisfactory propylene selectivity. Here, we developed a ligand-
protected direct hydrogen reduction method for encapsulating
subnanometer bimetallic Pt-Zn clusters inside silicalite-1 (S-1)
zeolite. The introduction of Zn species significantly improved the
stability of the Pt clusters and exhibited a superhigh propylene
selectivity of 99.3% with a weight hourly space velocity (WHSV) of
obvious deactivation was observed over PtZn4@S-1-H catalyst even
after 13000 min on stream (WHSV=3.6 h-1), affording an extremely
low deactivation constant of 0.001 h-1, which is 200 times lower than
that of the PtZn4/Al2O3 counterpart under the same conditions.
Significantly, the introduction of Cs+ ions into the zeolite can improve
the regeneration stability of catalysts, and the catalytic activity kept
unchanged after four continuous cycles. These zeolite-encaged Pt-
Zn catalysts represent the best performances to date for PDH
conversions, promising their practical industrial applications.
3.6~54 h-1 and specific activity of propylene formation of 65.5 molC3H6
-1
gPt
h-1 (WHSV=108 h-1) at 550 °C. Moreover, no
Introduction
Propylene is one of the most important feedstocks in the global
economy and is crucial to the production of various valuable
chemicals, commodities, and fuels. Giving the scarcity of fossil
fuels and the discovery of shale gas, propane dehydrogenation
(PDH) is becoming popular and is regarded as a promising way
to meet the ever-increasing demand for propylene across the
globe.[1] Supported Pt-based catalysts are widely used in
industrial PDH processes.[2] Despite high initial propylene yields,
Pt-based catalysts often encounter problems in the term of fast
deactivation and poor propylene selectivity. The size of metal
nanoparticles (MNPs) is a key factor affecting PDH conversion;
a decreased size is favorable for exposing more accessible
active sites on MNPs and thus improves the conversion of
[a]
Dr. Q. Sun, Dr. N. Wang, Mr. T. Zhang, Prof. J. Yu
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, College of Chemistry, Jilin University
Changchun, 130012, P. R. China
Email: jihong@jlu.edu.cn
Ms. Q. Fan, Mr. L. Zeng, Mr. W. Zhou, Prof. J. Cheng, Prof. Q.
Zhang, Prof. Y. Wang
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
[b]
Email: wangye@xmu.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
[c]
3]
propane.[2c, Al2O3 is the most commonly used substrate for
supporting Pt species, and state-of-the-art Al2O3-supported Pt-
based catalysts have been applied in industrial processes.[4] The
PDH reaction is thermodynamically limited and highly
endothermic, so high reaction temperature (>500 °C) is essential
to achieve a high propane conversion. However, the small Pt
species are subjected to severe sintering during the process of
PDH conversions at high temperatures due to the considerable
increase in surface free energy at smaller sizes and the lack of
the confinement effect of supports or/and strong metal-support
interactions. Exploring better supports to stabilize Pt species is
of great significance yet remains a nontrivial target for PDH
industry.
[d]
[e]
Dr. S. Miao
JEOL(Beijing) Co., Ltd., Beijing 100080, P. R. China
Dr. R. Yang, Prof. Z. Jiang, Dr. J. Zhang
Shanghai Synchrotron Radiation Facility, Zhangjiang Laboratory,
Shanghai Advanced Research Institute, Chinese Academy of
Sciences, Shanghai 201210, P. R. China.
Prof. Z. Jiang
Shanghai Institute of Applied Physics, Chinese Academy of
Sciences, Shanghai 201204, P. R. China
Prof. J. Xu
Wuhan Center for Magnetic Resonance, State Key Laboratory of
Magnetic Resonance and Atomic and Molecular Physics, Wuhan
Institute of Physics and Mathematics, Chinese Academy of
Sciences, Wuhan 430071, P. R. China
[f]
[g]
[h]
[i]
Prof. P. Zhang
Department of Chemistry, Dalhousie University, Halifax, Nova
Scotia B3H 4R2, Canada
Mr. D. Yang, Prof. R. Jia
Institute of Theoretical Chemistry, Jilin University, Changchun, 130
023, P.R. China
Zeolites with ordered microporous structures and outstanding
thermal stability are regarded as ideal supports for anchoring
3, 5]
small MNPs.[2c,
However, supporting metal species on
zeolites by using conventional impregnation methods usually
generates large metal particles with poor dispersion. Recently,
Iglesia and coworkers employed a ligand-stabilized method to
confine various types of small MNPs (1~2 nm) within
[j]
Prof. J. Yu
International Center of Future Science, Jilin University, Qianjin
Street 2699, Changchun 130012, P. R. China
Dr. L. Li
Electron Microscopy Center, Jilin University, Changchun, 130012,
P. R. China
[k]
aluminosilicate zeolites, and the resulting materials showed
6]
excellent resistance to sintering.[5b,
However, for the PDH
[+] These authors contributed equally to this work.
reaction, introducing Al into zeolites has a negative effect on the
selectivity of propylene due to the resultant undesirable
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/
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10.1002/anie.202003349
Angewandte Chemie International Edition
RESEARCH ARTICLE
Brønsted
acidity.[7]
Corma and
coworkers
prepared
performance and stability have been rarely reported for PDH
conversions.
subnanometer Pt clusters within a purely siliceous MWW zeolite
via the transformation of different-dimensional zeolites.[2c]
Pt/MWW catalysts exhibited good shape-selective alkene
hydrogenation and propane dehydrogenation performance but
suffered from severe Pt leaching at high temperatures. Recently,
we encapsulated ultra-small Pd and Pd-Ni(OH)2 clusters within
pure silica zeolites under direct hydrothermal conditions followed
by calcination and reduction processes, and demonstrated that
the synergistic effect of bimetallic catalysts can significantly
enhance the hydrogen generation rates from formic acid
decomposition. Moreover, the resultant catalysts exhibited
excellent stabilities after calcination in various industrial
oxidation-reduction conditions at 600 to 700 °C since the
bimetallic hybrid clusters were well-confined within the zeolite
channels.[5g, 5h] The design of supported bimetallic catalysts by
Here we develop a ligand-protected direct H2 reduction
method for encaging subnanometer Pt-Zn clusters into silicalite-
1 zeolite (S-1, MFI zeotype) subject to direct hydrothermal
crystallization; such an approach could effectively prevent from
sintering of metal nanoclusters during the conventional
calcination-reduction process, thus producing subnanometer
metal clusters. The resulting PtZn@S-1 catalysts are
superefficient for PDH conversions. The introduction of Zn
species could remarkably enhance the stability of the
subnanometer Pt clusters, and the PtZn4@S-1-H catalyst
exhibited extremely high propylene selectivity (99.3%) with a
weight hourly space velocity (WHSV) of 3.6 ~ 54 h-1 and specific
activity of propylene formation (up to 65.5 molC3H6 gPt h-1,
-1
WHSV = 108 h-1) at 550 °C, and 40.4% propane conversion was
alloying Pt with various metallic elements, such as Sn,[8] Ga,[2d, 4c, retained even after 13000 min on stream (WHSV = 3.6 h-1)
9]
2h]
Cu,[2b] Fe,[2f] and Zn,[2g,
is also considered an effective
without co-feeding H2. In contrast, the propane conversion over
mono-metallic Pt@S-1-H and Pt@S-1-C (treated by
conventional calcination/reduction process) sharply declined to
19.8% and 2% within 2900 and 340 min on stream, respectively.
Significantly, the deactivation constant of PtZn4@S-1-H catalyst
was as low as 0.001 h-1, which was 200 times lower than that of
Pt-Zn/Al2O3 counterpart under the same conditions. The
PtZn@zeolite catalysts demonstrate the best performances for
strategy to enhance the stability of Pt species and propylene
selectivity in PDH conversions, but the stability and catalytic
efficiency of these catalysts remain to be improved. Recently,
several silica-supported Pt-Zn alloy catalysts were investigated
for the dehydrogenation of ethane and n-butane, showing
enhanced catalytic activity and the selectivity of corresponding
alkene as compared with monometallic Pt catalysts.[10] However,
these catalysts possessed larger sizes of MNPs and suffered
from the sintering of MNPs during the reaction and/or
regeneration process, which restricted their practical application.
Until now, supported Pt-Zn catalysts with excellent catalytic
PDH conversion, which are superior to the state-of-the-art
2f, 3]
supported Pt-based catalysts.[2b-d,
Significantly, the
introduction of Cs+ ions into the zeolite can improve the
regeneration stability, and the catalytic activity of the catalyst
kept unchanged after four continuous cycles.
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10.1002/anie.202003349
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RESEARCH ARTICLE
Figure 1. a) Schematic of the synthesis of zeolite-encaged subnanometer Pt-Zn clusters catalysts. Cs-corrected STEM-HAADF images of b-d) Pt@S-1-C, e-g)
Pt@S-1-H, h-j) PtZn4@S-1-H. k) Cs-corrected STEM-ABF image of along PtZn4@S-1-H the [010] projection with the model superimposed. (l, m) High-resolution
Cs-corrected STEM-HAADF images of PtZn4@S-1-H sample. n) Cs-corrected STEM-HAADF image and elemental mappings for Si, O, Pt, and Zn of PtZn4@S-1-
H sample.
method can effectively inhibit the aggregation of Pt species. In
Results and Discussion
addition, during the direct H2 reduction process, the organic
ligands and templates can act as metal protective agents that
are gradually decomposed along with the reduction of metal
species. Most significantly, the direct reduction method also
makes reduction steps more convenient and energy-effective.
Scanning electron microscopy (SEM) reveals some large Pt
particles located outside of the zeolite crystals in Pt@S-1-C and
Pt/S-1-im, but such large particles are indiscernible in the cases
of Pt@S-1-H and PtZn4@S-1-H (Figure S6). The element
distributions and chemical composition of a representative
PtZn4@S-1-H sample were determined by energy-dispersive X-
ray (EDX) spectral mapping. As shown in Figure 1n, Pt and Zn
are distributed homogeneously throughout the zeolite and
located at almost the same position, indicating the establishment
of a bimetallic system. Inductively coupled plasma atomic
emission spectroscopy (ICP–AES) analysis reveals that all the
Pt-containing samples possess a similar Pt loading of 0.77–0.72
wt.%, and the mole ratios of Pt/Zn of PtZnx@S-1-H are 1/1.1,
1/1.9, 1/2.9 and 1/4.2 for x= 1, 2, 3, and 4, respectively, in
accordance with the preset ratios and EDX analysis (Table S1
and Figure S7). Based on raw materials in the synthesis gels,
the product yields of all zeolite samples are more than 95%, and
almost all of Pt species are encapsulated inside the zeolites. N2
As shown in Figure 1a, nanosized S-1 zeolites encapsulating
Pt–Zn species were prepared in situ in gel systems with molar
compositions
of
TEOS
(tetraethoxysilane):TPAOH:H2O:
a[Pt(NH2CH2CH2NH2)2]Cl2:b[Zn(NH2CH2CH2NH2)3](OAc)2
=
1:0.4:35:a:b (a = 2.25×10-3, a/b = 1/0, 1/1, 1/2, 1/3, 1/4, and 0/4)
under hydrothermal conditions at 170 °C for 3 days. Solid 13C
MAS NMR measurements and thermogravimetric-differential
thermal analysis confirmed that the metal-ethylenediamine
complexes maintained integrity inside the zeolites (Figures S1
and S2). Next, the as-synthesized nanosized samples were
treated in two different ways: one was via calcination in air at
550 °C followed by reduction under H2 at 400 °C (these samples
are identified as PtZnx@S-1-C, where x represents the molar
ratio of Zn/Pt in the synthesis gel); another approach involved
direct reduction in pure H2 at 400 °C (identified as PtZnx@S-1-
H). For comparison, a Pt/S-1-im sample was prepared with the
incipient wetness impregnation method. Powder X-ray diffraction
measurements reveal that all obtained samples possess the
MFI-type framework structure (Figure S3). Transmission electron
microscopy (TEM) and Cs-corrected high-angle annular dark-
field scanning TEM (HAADF-STEM) images of the samples are
shown in Figures S4, 1b-j. For the Pt@S-1-H and PtZnx@S-1-H
samples, metal nanoclusters are uniformly observed inner part
of S-1 crystals (~200 nm), but none is observed at “outer-shell”
and all nanoclusters are located within the intersectional
channels between the straight and sinusoidal 10-ring channels
of MFI. In contrast, the sizes of metal species in the Pt@S-1-C
samples are considerably larger, showing an average MNP size
of 3.1 nm, but are still much smaller than that of Pt/S-1-im (6.4
nm). Cs-corrected STEM-annular bright-field (ABF) images of
PtZn4@S-1-H confirm that the MFI-type framework structure is
perfectly retained after metal loading (Figure 1k). High-resolution
Cs-corrected STEM-HAADF images of the PtZn4@S-1-H
sample further confirm that the Pt nanoclusters are encaged
around the 5- and 6-membered rings (MRs) of zeolites (Figures
1l-m). The Pt@S-1 catalyst was also calcinated at 300~500 °C
in air before the reduction in H2 at 400 °C. STEM images reveal
that the particle sizes of Pt species increase successively from
1.4 to 2.7 nm with the increase of calcination temperatures in air
(Figure S5). This indicates that the low calcination process in air
favored the formation of small Pt clusters. Compared with the
conventional incipient wetness impregnation method and
calcination-reduction process, apparently, the ligand-protected
direct H2 reduction method can significantly lead to smaller metal
species. Pt species are prone to sintering at high temperatures,
especially in an oxidizing atmosphere;[11] hence, avoiding the
conventional calcination process and employing direct reduction
adsorption-desorption analyses show
a slight decrease in
microporous volume (~0.015 cm3/g) due to the partially occupied
metal species in some void spaces of the zeolites, but sufficient
microporous volumes (> 0.09 cm3/g) and areas (> 229 m2/g) are
observed in metal-containing samples (Figure S8 and Table S1).
1
29Si and H-29Si CP-MAS NMR analyses reveal that the zeolite
framework remains intact after the introduction of metal species
and that the metal species replace the H atoms of Si-OH groups
and are anchored on the defect sites of the zeolites (Figure S9).
X-ray photoelectron spectroscopy (XPS) analyses reveal peaks
at 74.7 eV and 71.3 eV, attributed to Pt4f5/2 and Pt4f7/2 of Pt(0),
in the Pt/S-1-im sample,[12] but no XPS signals of Pt and Zn
species can be detected in Pt@S-1-H, PtZn2@S-1-H, and
PtZn4@S-1-H, further indicating that metal clusters are confined
within the zeolite crystals (Figure S10). To further demonstrate
that all metal species are confined within the zeolite matrix
rather than on the outer surface of crystals, shape-selective
hydrogenation of nitrobenzene and 4-nitro-o-xylene reactions
were performed by using the ammonia borane as a reductant at
room temperature over Pt@S-1-H and commercial Pt/C
catalysts. Both Pt@S-1-H and Pt/C catalyst exhibited 100%
conversion for the hydrogenation of nitrobenzene to aniline.
However, different with the Pt/C catalyst showing a 100%
conversion for the reduction of 4-nitro-o-xylene, the Pt@S-1-H
catalyst exhibited no activity for the conversion of 4-nitro-o-
xylene; this is because the molecular size of 4-nitro-o-xylene is
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10.1002/anie.202003349
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RESEARCH ARTICLE
larger than that of microporous channels of S-1 zeolites, and
thus the molecules cannot enter into zeolite crystals and contact
with Pt clusters (Figure S11). The above results further prove
that the Pt clusters are completely encaged within the zeolite
crystals rather than supported on the crystal surfaces of zeolites.
Figure 2. a) Pt L3-edge XANES spectra of Pt@S-1-C, Pt@S-1-H, PtZn2@S-1-H, and PtZn4@S-1-H samples. b) Zn K-edge XANES spectra of PtZn2@S-1-H and
PtZn4@S-1-H samples. c) The Fourier transform of k3-weighted EXAFS spectra of Pt@S-1-C, Pt@S-1-H, PtZn2@S-1-H, and PtZn4@S-1-H samples at Pt L3-
edge. d) The Fourier transform of k3-weighted EXAFS spectra of PtZn2@S-1-H and PtZn4@S-1-H samples at Zn K-edge. e) In situ CO-DRIFTS spectra of
various samples.
To obtain detailed structural information about the metal
species in zeolites, X-ray absorption near edge structure
(XANES) and extended X-ray absorption fine structure (EXAFS)
spectra of the as-prepared samples were measured (Figures 2
and S12, Table S2). The Pt L3-edge XANES spectrum of Pt@S-
1-C is quite similar to that of Pt foil and the average coordination
number (CN) of Pt-Pt bonds is 10.9±0.8 based on EXAFS
analysis, indicating the zero-valence metal state and larger size
of Pt nanoparticles (NPs). In contrast, the Pt L3-edge XANES
spectra of Pt@S-1-H, PtZn2@S-1-H, and PtZn4@S-1-H showed
an increased oxidation state for these Pt atoms, and in the Pt L3-
edge EXAFS, the average CN of Pt–O (peak at ~2.02 Å) of
PtZn4@S-1-H, PtZn2@S-1-H, and Pt@S-1-H are 3.9 ~ 3.3. The
above results can be attributed to the fact that the small Pt
clusters tend to be oxidized in air and/or be stabilized by
bonding with oxygen atoms of the zeolite framework.[5c, 13] Note
that the average CNs of Pt-Pt bonds in Pt@S-1-H, PtZn2@S-1-
H, and PtZn4@S-1-H are only 1.5~2.1, which reveals that the Pt
species predominantly consist of atomically dispersed Pt along
with a small number of ultrafine nanoclusters, with an average of
only three Pt atoms in each nanocluster (regular triangle Pt3
structure, CN = 2). The Zn K-edge XANES energies of
PtZn2@S-1-H and PtZn4@S-1-H resemble that of reference
ZnO, suggesting that the Zn species consist of Zn2+ ions. Only a
significant peak at ~ 1.95 Å, attributed to Zn–O with a CN of ~3.0
at the Zn K-edge, can be observed in PtZn2@S-1-H and
PtZn4@S-1-H. No peaks at 2.66 and 3.32 Å, corresponding to
Zn–Zn bonds in Zn foil and ZnO, can be observed in PtZn2@S-
1-H and PtZn4@S-1-H, respectively; above results reveal that
the Zn species in the PtZnx@S-1-H are single-site Zn2+ ions
connected with Pt and Si atoms in the zeolite frameworks via
three bridging O atoms. It should be pointed out that the XANES
and EXAFS spectra of the as-prepared samples were measured
under atmosphere conditions; the subnanometer metal species
in the samples might be partially oxidized in air, which could not
reflect the nature of the catalytic active sites of catalysts under
PDH reaction conditions (550 °C under reduction atmosphere).
To further identify the structure and electronic state of metal
species in the samples under reduction atmosphere, in situ
diffuse reflectance infrared Fourier-transform spectroscopy
(DRIFTS) measurements were performed. The samples were
first pre-reduced in H2 at 300 °C for 1 h and then the
temperature was decreased to room temperature. Finally, the
CO gas was in situ pumped into the reaction cell and the
DRIFTS spectra were recorded over the samples. As shown in
Figure 2e, the peak at 2086 cm-1 can be observed in Pt/S-1-im,
which is attributed to the linear-adsorbed CO on terrace Pt
sites.[14] In contrast, the peak at 2071 cm-1 ascribed to linearly
bonded CO on unsaturated Pt sites was observed in Pt@S-1-C
and this peak further shifted to a lower wavenumber of 2064 cm-
1
in Pt@S-1-H. It indicated that the particle sizes of Pt species
decreased in the order of Pt/S-1-im, Pt@S-1-C, and Pt@S-1-H,
in accordance with the observation of TEM images and EXAFS
results.[15] Moreover, compared with the Pt@S-1-H, all
PtZnx@S-1-H samples exhibited obvious red-shifts to lower
wavenumbers. Considering the similar Pt sizes of Pt@S-1 and
PtZnx@S-1-H, the decrease of CO stretching frequency
suggests the formation of Pt-Zn alloys resulting in a higher
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10.1002/anie.202003349
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RESEARCH ARTICLE
surface electron density of Pt clusters in PtZnx@S-1-H.[16] In
addition, H2 temperature-programmed reduction (TPR)
measurements can also provide evidence for the formation of
Pt-Zn alloy structure in PtZn4@S-1-H. As shown in Figure S13,
the hydrogen consumption profile of Zn4@S-1-H is almost the
same as that of pure S-1, indicating the Zn2+ ions anchoring on
zeolite frameworks keep stable during the reduction process
until 750 °C. In contrast, although the PtZn4@S-1-H catalyst
possessed relatively lower metal contents, a more obvious peak
at 300-450 °C attributed to the reduction of Zn species can be
observed in PtZn4@S-1-H, which indicates that the Pt species
are conducive to the reduction of Zn and some Pt-Zn alloy
structure can be formed during the reduction process.[17]
[a] 0.3 of catalyst mixed with 1.0 g of quartz sand, atmospheric pressure,
C3H8/N2 = 10/30 mL min-1, WHSV = 3.6 h-1. [b] 0.1 g of catalyst mixed with 1.2
g of quartz sand, atmospheric pressure, C3H8/N2 = 25/75 mL min-1, WHSV =
27 h-1. [c] 0.05 g of catalyst mixed with 1.25 g of quartz sand, atmospheric
pressure, C3H8/N2 = 25/75 mL min-1, WHSV = 54 h-1. [d] 0.025 g of catalyst
mixed with 1.275 g of quartz sand, atmospheric pressure, C3H8/N2 = 25/75 mL
min-1, WHSV = 108 h-1. [d] ꢀꢁ = [ln �1−ꢂꢃꢄꢅꢆꢇꢈꢉ� − ln �1−ꢂꢃꢄꢊꢇꢆꢋꢆꢈꢉ�] ꢌ, Wherein,
⁄
ꢂꢃꢄꢅꢆꢇꢈꢉ
ꢂꢃꢄꢊꢇꢆꢋꢆꢈꢉ
the ConFinal and ConInitial are the final and initial conversions of the PDH
reaction, respectively, t is the duration of a PDH reaction in hour, kd is the
deactivation constant (h-1).
The catalytic performances of various catalysts for PDH
conversion were investigated at 550 °C with a WHSV of 3.6 h-1
without co-feeding H2. As shown in Figure 3a, b, S21 and Table
1, a high initial propylene yield (42.7%) was achieved over
Pt@S-1-H; this value was much higher than that of Pt@S-1-C
(29.4%) and Pt/S-1-im (22.7%). After 1200 min on stream, the
propane conversion of Pt@S-1-H decreased to 34.2% with
95.4% of selectivity for propylene. In contrast, the propane
conversion over Pt@S-1-C and Pt/S-1-im dropped rapidly to 2%
within 340 min and 160 min on stream, respectively. Compared
with Pt@S-1-C and Pt/S-1-im, the enhanced catalytic activity of
Pt@S-1-H can be attributed to the remarkably reduced Pt size
and greater numbers of available active sites for propane
conversion. Significantly, the introduction of Zn species coupled
with subnanometer Pt clusters remarkably improved the
propylene selectivity and catalytic stability. The initial propylene
selectivity was improved with increasing Zn content due to the
formation of more bimetallic Pt-Zn alloys, and the initial
propylene selectivity of PtZn4@S-1-H reached 93.2%, much
higher than that of Pt@S-1-H (79.0%). The PtZn4@S-1-H
catalyst retained high propane conversion (44.8%) and high
propylene selectivity (98.9%) after 1200 min on stream. To
further illustrate the influence of the Zn content in PtZn@S-1-H
sample on catalytic performance, PtZn8@S-1-H with a Zn/Pt
ratio of 7.56 (determined by ICP measurement) was prepared
and the TEM images of PtZn8@S-1-H are shown in Figure S22.
Catalytic test revealed that as compared PtZn4@S-1-H, the
PtZn8@S-1-H possessed higher initial propylene selectivity up
to 97.7% at 550 °C with a WHSV of 3.6 h-1 (Figure S23). After
1200 min on stream, the propylene selectivity of PtZn8@S-1-H
increased to 99.2%, but the propylene yield was 2% lower than
that of PtZn4@S-1-H, which was attributed to the excessive Zn
may partially cover the active Pt surface, resulting in a negative
effect on the interaction between the propane and the active Pt
sites and reduced the reactivity. Moreover, PDH conversion was
also tested over PtZn4@S-1-H at 550 °C under different WHSVs
(Figures 3c and S24). With an increase in WHSV from 3.6 to
108 h-1, the initial propane conversions decreased from 47.4% to
21.2%, but the initial propylene selectivity increased from 93.2%
to 98.3%. The specific activity of propylene formation under a
The high-temperature stability of a representative sample
PtZn4@S-1-H was also investigated. The sizes of bimetallic
species remained unchanged after calcination at 700 °C under
H2 and N2 atmosphere (Figure S14). After steam aging treatment
under a water vapor flow of 3 and 18 mL/min at 600 °C for 5 h,
the average metal sizes only increased to 1.8 and 2.8 nm,
respectively (Figures S15-16). Moreover, even after five
consecutive oxidation-reduction cycles at 550 and 650 °C, more
than 60% of the metal particles remained less than 4 nm in size
(Figures S17-18). In contrast, the MNPs of Pt/S-1-im aggregated
dramatically, reaching a size of 19.5 nm, and the large Pt NPs
were completely located outside of the zeolite crystals after only
three oxidation-reduction cycles at 550 °C (Figure S19).
Significantly, the crystallinity of the zeolites and the Pt loading
amount remained unchanged after treatment at high
temperatures (Figure S20).
Table 1: Catalytic performance for propane dehydrogenation over various
catalysts.
Catalyst
T
[°C]
WHSV Time
ConC3H8
[%]
SelC3H6 Kd
[%]
[h-1]
[min]
[h-1][e]
PtZn4@S
-1-H
550
3.6[a]
40
47.4
40.4
40.0
21.8
34.7
29.6
21.2
11.8
66.7
43.2
47.8
40.4
54.1
34.2
80.3
9.0
93.2
99.2
99.1
99.3
99.1
99.3
98.3
98.6
90.8
95.5
88.5
99.0
79.0
95.4
51.1
78.4
77.3
88.9
92.9
91.8
0.001
0.008
0.023
0.072
0.017
0.016
0.042
0.57
13000
40
27[b]
6300
40
54[c]
620
40
108[d]
3.6[a]
3.6[a]
3.6[a]
3.6[a]
3.6[a]
3.6[a]
620
40
600
550
3500
40
PtZn2@S
-1-H
1200
40
Pt@S-1-H 550
600
1200
40
400
40
Pt@S-1-C 550
38.0
2.0
0.68
340
40
high WHSV of 108 h-1 reached 65.5 molC3H6 gPt h-1; such value
-1
Pt/S-1-im
550
24.4
2.0
1.38
represented the highest catalytic activity for propylene
generation from PDH conversion and is much better than those
of all reported state-of-the-art Pt-based catalysts (see Table
160
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S3).[2b-d, 2f, 3] Strikingly, after 13000 min on stream (WHSV = 3.6
h-1), the propane conversion remained at 40.4% with 99.2%
selectivity for propylene, affording an extremely low deactivation
constant of 0.001 h-1 (Figures 3d and S25). In contrast, the
propane conversion over Pt@S-1-H sharply decreased to 19.8%
after 2900 min on stream. Moreover, under high WHSV of 27 h-1,
a propane conversion of 21.8% with 99.3% selectivity for
propylene over PtZn4@S-1-H was achieved after 6300 min on
stream (Figures S26 and S27). Furthermore, PDH conversion
was also performed at 600 °C (WHSV=3.6 h-1). The propane
conversion of Pt@S-1-H sharply decreased from 80.3% to 9.0%
within 400 min on stream, while two-thirds of the initial propane
conversion over PtZn4@S-1-H (up to 43.2%) with a propylene
selectivity of 95.5% was retained even after 3500 min on stream
(Figures S28-30). All of the above results demonstrate that
bimetallic PtZn4@S-1-H possesses extremely high long-term
stability, far superior to that of monometallic Pt@S-1-H.
Industrially, the Al2O3 has been used to support Pt species for
the PDH conversions. For comparison, the Pt/Al2O3 and
PtZn4/Al2O3 catalysts with the same metal loadings as Pt@S-1-
H and PtZn4@S-1-H, respectively, were prepared by incipient
wetness impregnation method and then reduce in H2 at 400 °C.
High-resolution STEM measurements revealed that the Pt and
Pt–Zn NPs are dispersed onto the Al2O3 supports, giving
average particle sizes of 2.8 and 2.6 nm, respectively (Figure
S31). The catalytic performances of Pt/Al2O3 and PtZn4/Al2O3 for
PDH conversions were also investigated at 550 °C with a WHSV
of 3.6 h-1 (Figure S32-34). The initial propane conversion over
Pt/Al2O3 was only 15.8%, and it decreased to 3.5% with a low
propylene selectivity of 67.9% after 400 min on stream. In
contrast, PtZn4/Al2O3 exhibited enhanced catalytic performance
and the highest propylene selectivity can reach up to 97.6 %, but
this value remained about 2% lower than that of PtZn4@S-1-H.
Note that the propane conversion over PtZn4/Al2O3 decreased
remarkably from 32.3% to 8.8% within 400 min on stream, giving
a deactivation constant of 0.2 h-1, which indicated that the
deactivation rate of PtZn4/Al2O3 is 200 times faster than that of
PtZn4@S-1-H under the same condition. TG-DTA analyses
demonstrated that after 400 min on stream, the weight losses of
coke deposition over spent Pt/Al2O3 and PtZn4/Al2O3 were 7.2%
and 6.6%, respectively, which were much higher than those of
Pt@S-1-H (2.5%) and PtZn4@S-1-H (1.5%) even after 1200 min
on stream (Figure S35). The above results indicate that
encaging metal species inside the zeolites can significantly
inhibit the generation of coke deposits and prolong the lifetime of
catalysts.
Figure 3. a) The conversion of propane and b) selectivity of propylene over various catalysts. reaction condition: 0.3 g of catalyst mixed with 1.0 g of quartz sand,
atmospheric pressure, C3H8/N2 = 10/30 mL min-1, WHSV = 3.6 h-1, 550 °C. c) The conversion of propane and selectivity of propylene over PtZn4@S-1-H under
various WHSVs (3.6 ~ 108 h-1) at 550 °C. d) Long-term stability test for PDH conversion over PtZn4@S-1-H and Pt@S-1-H catalysts (WHSV = 3.6 h-1, 550 °C).
Temperature-programmed
desorption/mass
reaction on Pt nanoclusters. Notably, the EXAFS and XANES
results reveal that the average CN of Pt–Pt bonds is 2.1 after
1200 min on stream (WHSV=3.6 h-1), indicating no obvious
aggregation of Pt nanoclusters (Figures S37 and S38, Table S2).
spectrometry (TPD/MS) analysis of propane indicated that the
introduction of Zn species can enhance the adsorption of
propane (Figure S36), which may benefit the dehydrogenation
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In contrast, the average CN of Pt-Pt bonds in spent Pt@S-1-H
(1200 min, WHSV=3.6 h-1) increased to 6.6. Note that the Pt-Zn
bond with a CN of 0.6 can be observed in spent PtZn4@S-1-H
sample after 1200 min on stream, indicating the formation of
bond is lower than that of Pt-Pt bond, indicating that the Zn
atoms are most likely located on the outer surface of on Pt core.
High-resolution TEM (HRTEM) measurements demonstrated
that the sizes of the metal species of all spent PtZnx@S-1-H
more Pt-Zn alloys during the reactions. Similar to previous works, catalysts increased slightly compared with those of the fresh
the generation of Pt–Zn alloys will lead to an electron-enrich Pt
surfaces, which benefit for the desorption of olefin and suppress
the formation and polymerization of unsaturated coke precursors
on the metal and enhance the selectivity and stability of
dehydrogenation.[10a, 16] Significantly, the average CNs of Pt-Pt
bonds of spent PtZn4@S-1-H were only increased to 6.0 and
7.4 even after 6300 min (WHSV=27 h-1) and 13000 min
(WHSV=3.6 h-1) on stream, respectively; these values are
comparable with that of spent Pt@S-1-H after 1200 min on
stream (Figures S37-39). These results reveal that the
introduction of Zn species can remarkably improve the stability of
Pt nanoclusters and suppress their aggregation during PDH
conversion. Moreover, the CNs of the Pt–Zn bond further
increased to 1.0 after 13000 min on stream, indicating that more
Pt–Zn alloys were generated gradually as the reaction
proceeded. Notably, the Zn-Zn and Zn-O-Zn peaks could not be
observed in EXAFS for the spent catalyst and the CN of Pt-Zn
counterparts but were still less than 1.5 nm during 1200–13000
min on stream (Figures S40-43). Cs-corrected STEM
measurements of PtZn4@S-1-H revealed that most of the Pt
species remained in the form of subnanometer Pt clusters even
after 13000 min on stream, although a fraction of larger Pt
clusters (~1.5 nm in size) due to the aggregation of smaller Pt
nanoclusters were also observed (Figure S44). After PDH
reactions, the crystallinity of the zeolite remained unchanged
(Figure 45). The coke content of spent PtZn4@S-1-H reached
14.4% after 13000 min on stream; however, a microporous
volume decrease of less than 10% was observed for spent
PtZn4@S-1-H (Figure S46 and Table S4). TEM images showed
that most of the cokes were located on the outer surfaces of the
zeolites (Figures S42 and S43). Based on the above results, the
gradual decrease in propane conversion during the reaction over
PtZn4@S-1-H can be mainly attributed to the increased size of
Pt nanoclusters and the formation of cokes that may partially
block paths to the active sites.
Figure 4. a) The conversion of propane and selectivity of propylene over PtZn4@S-1-Cs catalyst after several regeneration cycles, reaction condition: 0.3 g of
catalyst mixed with 1.0 g of quartz sand, atmospheric pressure, C3H8/N2 = 10/30 mL min-1, WHSV = 3.6 h-1, 550 °C. After the completion of the initial run, the
catalyst was calcinated in air (10 mL/min) with linear heating to 550 °C for 1 h and holding for 1 h, and then the temperature was decreased to 200 °C under N2
atmosphere. Following, the catalyst was reduced in H2 (10 mL/min) with linear heating to 550 °C for 1 h and holding for 1 h then finally fed with propane to start
the PDH conversion. b-e) Cs-corrected STEM images of spent PtZn4@S-1-H-Cs catalyst after two successive regeneration cycles. f-i) Cs-corrected STEM images
of spent PtZn4@S-1-H-Cs catalyst after four successive regeneration cycles.
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10.1002/anie.202003349
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The regeneration activity of PtZn4@S-1-H catalysts was also
examined. The PtZn4@S-1-H was first treated with air flow (10
mL/min) at 550 °C for 1 h followed by a reduction process under
an H2 flow. After two regeneration cycles, ~72% of an initial
propane conversion can be kept while maintaining high
propylene selectivity (99.4%) (Figures S47 and S48). The
decline of catalytic activity after regeneration can be mainly
attributed to the increase in metal particle sizes. After two
regeneration cycles, the average metal particle size increased to
3.9 nm and some metal species migrated to the outer surface of
zeolites during the regeneration process (Figure S49). To meet
the demand for practical application, it is necessary to further
improve the regeneration stability of catalysts. Significantly, we
for the first time found that the introduction of Cs+ ions can
remarkably enhance the regeneration stability of PtZn4@S-1-H
catalyst. The Cs+ ions (1.5 wt.%) were introduced inside the
PtZn4@S-1-H catalysts via the incipient wetness impregnation
method. Elemental mapping measurements indicated that the
Cs+ ions were successfully introduced into the catalysts and
evenly distributed inside the entire zeolite crystals (Figure S50).
Strikingly, the propane conversion and propylene selectivity of
PtZn4@S-1-H-Cs catalyst remained unchanged as compared
with the initial run even after four successive regeneration cycles
significantly reduce the size of metal species confined within the
zeolites. The introduction of Zn species can remarkably promote
the catalytic activity and stability of Pt clusters due to the
formation of ultrasmall Pt-Zn alloy clusters. The PtZn4@S-1-H
catalyst exhibited an extremely high specific activity of propylene
formation (65.5 molC3H6 gPt h-1) with a WHSV of 108 h-1 and
-1
propylene selectivity (99.3%, WHSV = 3.6~54 h-1) at 550 °C.
Significantly, no obvious deactivation was observed even after
13000 on stream without co-feeding H2 (WHSV = 3.6 h-1),
representing the highest stability to date among all state-of-the-
art Pt-based heterogeneous catalysts for PDH reaction. We also
found that the introduction of Cs+ ions can significantly improve
the regeneration performance and the catalytic activity kept
stability after four successive regeneration cycles. This work
introduces a new strategy for the fabrication of the high-
performance catalysts for PDH reaction by encapsulating
subnanometer Pt-Zn clusters inside purely siliceous zeolites.
The superior catalytic performance and excellent long-term and
regeneration stability of PtZn@zeolite catalysts for PDH
conversion suggest their significant promise for future industrial
applications.
(Figure 4a). Cs-corrected STEM measurement revealed that Acknowledgements
large metal NPs can be rarely observed on the outer surface of
zeolite and almost all of the metal species are with
a
The authors thank beamline BL14W1 and BL15U1 of the
Shanghai Synchrotron Radiation Facility (SSRF, China) for
providing the beamtime; The authors 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, 21920102005,
21621001, 21433008, 91545203 and 21850410448), and the
111 Project of China (B17020), CℏEM, SPST of Shanghaies
University (#EM02161943, A.M. & O.T.), Foreign 1000 Talents
plan, China (O.T.).
subnanometric scale and encapsulated inside zeolites after two
successive regeneration cycles (Figures 4b-e). Only a fraction of
large metal NPs can be detected in spent PtZn4@S-1-H catalyst
after four successive regeneration cycles, but most of the
subnanometer metal clusters are still encaged around the 5- and
6- MRs of zeolites (Figures 4f-i). Elemental mapping
measurement indicated that the Pt, Zn, and Cs elements were
distributed homogeneously throughout the zeolite crystals in
spent after PtZn4@S-1-H-Cs catalyst after four successive
regeneration cycles (Figure S51). ICP measurements
demonstrated that the loading amounts of Pt and Zn kept
unchanged after four successive regeneration cycles. These
results indicated that the introduction of Cs+ ions located inside
the zeolite channels can effectively inhibit metal migrations to
the outer surface of zeolite crystals, avoid the severe
aggregation of metal species due to the loss of confinement
effect of zeolites, and improve the regeneration stability of
catalysts. In contrast, the metal NPs of Pt/Al2O3 and PtZn4/Al2O3
increased dramatically to 9.5 and 11.6 nm after four
regeneration cycles (Figures S52 and 53), further indicating the
excellent regeneration stability of PtZn4@S-1-H-Cs.
Conflict of interest
The authors declare no conflict of interest.
Keywords: heterogeneous catalysis
• zeolites • propane
dehydrogenation • confinement synthesis • Pt-Zn clusters
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Table of Contents
RESEARCH ARTICLE
Subnanometer
bimetallic
Pt–Zn
Qiming Sun, Ning Wang, Qiyuan Fan,
Lei Zeng, Alvaro Mayoral, Shu Miao,
Ruoou Yang, Zheng Jiang, Wei Zhou,
Jichao Zhang, Tianjun Zhang, Jun Xu,
Peng Zhang, Jun Cheng, Dong-Chun
Yang, Ran Jia, Lin Li, Qinghong Zhang,
Ye Wang,* Osamu Terasaki, and Jihong
Yu*
clusters are encapsulated inside
silicalite-1 (S-1) zeolite via a ligand-
protected direct hydrogen reduction
method. The PtZn4@S-1-H catalyst
exhibited
a
superhigh propylene
selectivity of 99.3% and specific
activity of propylene formation of 65.5
molC3H6 gPt h-1 at 550 °C. Moreover,
-1
Page No. – Page No.
no obvious deactivation was observed
over catalyst even after 13000 min on
stream.
Subnanometer
Bimetallic
Pt-Zn
Clusters in Zeolites for Propane
Dehydrogenation
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