Ultrasmall and Stable Pd and Pt Nanoparticles Within Zeolite HY Through Impregnated Method with Enhanced Semihydrogenation Selectivity

Article abstract of DOI:10.1007/s10562-020-03523-2

In this study, with zeolite HY as support, ultrasmall Pd and Pt nanoparticles were successfully immobilized into zeolite HY crystals through an optimized impregnation approach. The success of this new approach mainly relied on the selecting appropriate metal precursor to make Pd and Pt element exists with the cation forms, which can facilitate their diffusion into inner channels of zeolite HY through electrostatic attraction and capillary force. Integration of confinement effect of zeolite HY, taking zeolite HY (Si/Al = 3) encapsulation of ultrasmall Pd NPs (Pd@HY-3) as an instance, Pd@HY-3 catalyst exhibited enhanced catalytic selectivity in semihydrogenation of alkynes, in comparison with Pd/HY, Pd/C, Pd/Al₂O₃ and lindlar catalysts. This improved catalytic selectivity can be attributed to the constrained upright adsorption conformation of reactant alkyne and corresponding product alkene on encapsulated Pd surface to make alkyne adsorption on Pd surface with larger adsorption energy than that of alkene, thus achieving the high catalytic selectivity. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03523-2

Catalysis Letters
https://doi.org/10.1007/s10562-020-03523-2
Ultrasmall and Stable Pd and Pt Nanoparticles Within Zeolite HY
Through Impregnated Method with Enhanced Semihydrogenation
Selectivity
1
1
3
1
1
3
1
3
Mengyue Wang · Xuan Liu · Kui Ren · Yiming Zhou · Tianhao Li · Yunfei Bi · Haozhe Kang · Enhui Xing ·
Qiang Chen1
,2
Received: 6 October 2020 / Accepted: 28 December 2020
©
The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
In this study, with zeolite HY as support, ultrasmall Pd and Pt nanoparticles were successfully immobilized into zeolite
HY crystals through an optimized impregnation approach. The success of this new approach mainly relied on the selecting
appropriate metal precursor to make Pd and Pt element exists with the cation forms, which can facilitate their diꢀusion
into inner channels of zeolite HY through electrostatic attraction and capillary force. Integration of conꢁnement eꢀect of
zeolite HY, taking zeolite HY (Si/Al=3) encapsulation of ultrasmall Pd NPs (Pd@HY-3) as an instance, Pd@HY-3 catalyst
exhibited enhanced catalytic selectivity in semihydrogenation of alkynes, in comparison with Pd/HY, Pd/C, Pd/Al O and
2
3
lindlar catalysts. This improved catalytic selectivity can be attributed to the constrained upright adsorption conformation of
reactant alkyne and corresponding product alkene on encapsulated Pd surface to make alkyne adsorption on Pd surface with
larger adsorption energy than that of alkene, thus achieving the high catalytic selectivity.
Supplementary Information The online version contains
supplementary material available at https://doi.org/10.1007/s1056
2
-020-03523-2.
*
Qiang Chen
chenqiang3@mail.sysu.edu.cn
1
2
3
School of Chemical Engineering and Technology, Xi’an
Jiaotong University, Xi’an 710049, Shaanxi, China
School of Chemical Engineering and Technology, Sun
Yat-Sen University, Zhuhai campus, Zhuhai 519082, China
State Key Laboratory of Catalytic Materials and Reaction
Engineering, Research Institute of Petroleum Processing,
Sinopec, Beijing 100083, China
Vol.:(0
1
123456789)
3

M. Wang et al.
Graphic Abstract
Keywords Impregnation · Immobilization of ultrasmall nanoparticles · Zeolite HY · Semihydrogenation · Conꢁnement
eꢀect
1
Introduction
zeolite supported ultrasmall nanoparticles to sustain their
catalytic properties is highly desirable [12].
Zeolite supported ultrasmall noble metal nanoparticles
Z-UNMPs) have attracted much more attention as a sig-
A great number of research eꢀorts have been devoted
to stabilizing ultrasmall nanoparticles onto zeolite sup-
port. Ion-exchange, followed by H2 reduction, is a his-
torical method to introduce noble metal nanoparticles into
zeolite crystals to improve their stability. Although this
method is favoured by its simplicity and economy, the dis-
tribution of these synthesized nanoparticles is relatively
poor on zeolites, and some are unavoidably located at the
outer surface, leading to the occurrence of aggregation
and sintering at some severe occasions [13, 14]. Multi-
step postsynthesis route is another method to enhance
the stability of zeolite supported ultrasmall nanoparticles
[15–17]. For example, Xiao and co-workers developed an
eꢀective two-step seed-directing methodology to encap-
sulate noble metal nanoparticles into zeolite to prompt
their stability [18]. In which, synthesized noble metal
nanoparticles was initially impregnated onto the surface
(
niꢁcant class of nanocatalysts for their excellent catalytic
activities and selectivities in heterogeneous catalytic reac-
tions [1–4]. Ultrasmall nanoparticles possessing large
surface-to-volume ratio and high exposed active sites
marry zeolite support possessing shape-selective char-
acter, thus exhibiting such unique catalytic properties.
It is reported that Z-UNMPs have shown great promise
in various synthetic reactions, including hydrogenation,
oxidation, isomerization, and so on [5–8]. However, due
to the very high surface energy, these supported ultras-
mall noble metal nanoparticles tend to aggregation and
sintering during the practical applications, resulting in the
decreased catalytic reactivity, especially under harsh reac-
tion conditions [9–11]. Consequently, exploring appro-
priate strategy to control the size and dispersion of these
1
3

Ultrasmall and Stable Pd and Pt Nanoparticles Within Zeolite HY Through Impregnated Method…
of zeolite seed, then this prepared sample was covered by
crystalline zeolite shell through a further hydrothermal
crystallization process. Such postsynthesis strategies can
successfully conꢁne nanoparticles within zeolite crystal to
exhibit excellent stability, yet the complex synthesis pro-
cedures and relative strict experimental parameters limit
their practical application in industry. The direct in situ
hydrothermal synthesis method is a newly developed
strategy to encapsulate noble metal nanoparticles within
zeolite crystals to strengthen their thermal and chemical
stability. The key for this method is to select appropriate
noble metal precursor containing ammonia ligand or eth-
ylenediamine ligand, since these precursors can facilitate
the interaction with emerging incipient aluminosilicate
moieties to favor them incorporation into zeolite crystals
during the hydrothermal process, following by the calcina-
Generally, aluminosilicate zeolites bear a negatively
charged framework of micropores, into which cation spe-
cies can be easily attracted and diꢀused into such micropores
to balance the framework charges. In this study, based on
this principle, choosing zeolite HY as support without the
introduction of any other ions into it, the platinum-group
ultrasmall nanoparticles such as Pd and Pt were success-
fully encapsulated into zeolite HY crystals through select-
ing Pd(acac) and Pt(acac) as precursors to ensure Pd and
2
2
Pt elements existing with the cation forms, and the resulted
encapsulated ultrasmall nanoparticles exhibited very high
thermal stability, even when the calcination temperature was
up to 700 °C. The eꢀect of framework charge density in zeo-
lite HY caused by diꢀerent Si/Al ratio on the encapsulation
has also been discussed by experimental results and DFT
calculation. Combined with shape selectivity of zeolite HY,
the aꢀorded zeolite Y encapsulated ultrasmall noble metal
nanoparticles showed enhanced catalytic selectivity in semi-
hydrogenation reaction.
tion and H reduction at high temperature to aꢀord zeolite
2
encapsulated ultrasmall nanoparticles [19–22]. This smart
synthesis strategy is primarily developed by Prof. Iglesia
group, and many ultrasmall noble metal nanoparticles (Pt,
Pd, Au, Rh, Ag, etc.) have been successfully encapsulated
into various zeolites with diꢀerent framework structures
to exhibit good to excellent thermal and chemical stability
2 Experimental Section
[
23, 24]. Notably, in the synthesis system with very high
2.1 Reagents and Materials
basicity, noble metal precursors protected by ligands may
also be precipitated. Therefore, it is still very interesting to
develop facial and eꢂcient alternatives to stabilize zeolite
supported ultrasmall nanoparticles.
Zeolites HY with diꢀerent ratios of Si/Al were supported by
Sinopec in China. Palladium (II) acetylacetonate (Pd 34.7%),
Platinum (II) acetylacetonate (Pt 48%), Didodecyldimeth-
ylammonium bromide (>98%), 1-Ethyl-4-ethynylbenzene
(>98%), and 4-Vinylbiphenyl (97%) were purchased from
Alfa Aesar. Sodium tetrachloropalladate, palladium acetate
(> 99%), 1-dodecanethiol (> 98%), palladium on carbon
(10 wt%), and palladium on alumina (5 wt%) were obtained
from Sigma-Aldrich. 4-ethynyl-1-1′-biphenyl (>97%) were
brought from Ark Pharm. Anhydrous ethanol (≥99.5%) was
purchased from Sigma-Aldrich. Lindlar catalyst (5 wt%)
were purchased from SPC scientiꢁc. And sodium borohy-
dride (>99%) was obtained from Acros Organics.
The impregnation method is a simple and useful tech-
nique to load noble metal into zeolite matrix, which can be
easily scaled up for industrial applications. However, with
this method, when noble metal precursors such as H PtCl
2
6
and H PdCl diꢀuse into zeolite channel through capillary
2
6
force, owing to co-existenced diꢀusion resistance between
the internal and external surfaces, some noble metal pre-
cursors could absorb on the external surface of the zeolite,
hence causing the surface aggregation of prepared noble
metal nanoparticles [25]. How to make noble metal precur-
sors diꢀuse into zeolite inner structure maximumly is still a
challenging problem needed to be addressed. Recently, Prof.
2.2 Synthesis of Zeolite HY Encapsulated
Platinum‑Group (Pd and Pt) Ultrasmall
Nanoparticles
+
Wan group reported a Na -pretreatment method to overcome
+
this problem [26]. They disclosed that Na -pretreatment of
HY zeolite (Al/Si =0.36) to form H(Na)Y can change the
surface charge of pristine HY zeolite from negative to posi-
50 mg HY-3 support was suspended in 10 ml ethanol, and
tive, thus facilitating to attract negative gold species such
the ethanol solution (1.12 ml) of Pd(acac) (2.74 mg) was
2
−
as [AuCl ] into positive zeolite internal pores eꢀectively,
added dropwise to the HY-3 gel very slowly under vigorous
magnetic stirring. Then, the prepared mixture was stirred for
12 h at room temperature. Followed with centrifuging and
washing with 10 ml ethanol for three times, the obtained
samples were suspended again in 10 ml ethanol and treated
4
minimizing the deposition of large Au nanoparticle onto the
external surface of zeolite Y. However, the thermal stabil-
ity of this synthesized Au nanoparticles in H(Na)Y zeolite
support was not discussed. It also should be noted that the
+
introduction of another ion, such as Na , into zeolite struc-
at 60 °C under an H atmosphere (1 atm) for 24 h. The solid
2
ture could inꢃuence the intrinsic property of parent zeolite
was collected through centrifuged, washed with 5 ml deion-
[
27, 28].
ized water and 5 ml ethanol for three times before drying
1
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M. Wang et al.
in a vacuum dryer at room temperature overnight. ICP-MS
analysis indicated that the loading amount of Pd in this
encapsulated catalyst was 0.41%. Zeolite HY-3 encapsulated
platinum ultrasmall nanoparticles catalyst was synthesized
with the same procedure.
force, and maximum displacement for geometry optimiza-
−
5
tion are 2×10 Ha, 0.004 Ha/Å, 0.005 Å, respectively. The
adsorption energies, E , are calculated by following:
ads
ꢀ
ꢁ
Eads = Eadsorbate + HY− E
+ EHY
adsorbate
Here, E
, E
, and E represent the total
adsorbate + HY
adsorbate
HY
2
.3 Synthesis of Pd/HY‑3 Catalyst with Traditional
energy of HY cluster model covered with adsorbates, the
Impregnation Method
energy of adsorbate, and the energy of HY cluster model,
respectively. With this deꢁnition, a negative value of E
ads
For Pd/HY-3 catalyst with Pd nanoparticle supported on the
surface (Pd/HY-3), according to the ultrasmall metal nano-
particles synthesized method supported by B. L. V. Prasad
group [29], the 2 nm Pd nanoparticles (Pd NPs) has been
synthesized and suspended in toluene. To load Pd NPs on
HY-3 zeolite, 1 ml Pd NPs gel (0.02 M) was diluted with
means a release of energy or a stable adsorption mode on
the HY model.
2
.6 Characterization
Powder XRD patterns were recorded by using a diꢀractom-
eter (Xray diꢀractometer SmartLab (3), Rigaku) operated at
3 kW, equipped with CuKa radiation source from 6° to 90°
2
ml ethanol, and 40 mg HY-3 zeolite powder was added.
With 30 min ultrasonic dispersion, the sample was centri-
−
1
fuged (5000 rmp min for 5 min) and dried in a vacuum
dryer at room temperature overnight to get the nominal 5.0%
loading of Pd in this synthesized Pd/HY-3 catalyst.
−
1
with a scan rate of 5 deg min . TEM images were recorded
on a Hitachi HT-7700 microscope equipped with a tungsten
ꢁ
lament, operating at 100 kV. HRTEM and HAADF-STEM
images were recorded on a Philips Tecnai F20 FEG-TEM
instrument with an accelerating voltage of 200 kV. Metal
contents were determined by ICP-MS analysis with an Agi-
lent 7500CE instrument.
2
.4 Catalytic Hydrogenation
of 4‑Ethylphenylacetylene
In a typical hydrogenation reaction, Pd@HY-3 catalyst
(
50 mg), ethanol (2.5 ml), and 4-Ethylphenylacetylene (14
uL; 99%, Alfa Aesar) were held in an autoclave. After being
ꢃushed with H three times, the reaction pressure in the
3 Results and Discussion
2
autoclave was set to 1 MPa H while the reaction tempera-
2
ture was 50 °C. The catalytic conversion and selectivity were
determined by GC–MS (Agilent 5975C instrument with a
thermal conductivity detector) and GC analysis.
Drawing lessons from Prof wan group’s interesting result,
encapsulation of ultrasmall platinum-group (Pd and Pt)
nanoparticles into zeolite HY were successfully prepared
through a modiꢁed impregnation method. Taking zeolite
HY (Si/Al=3) encapsulation of ultrasmall Pd nanoparticles
2.5 DFT Calculations
(
denoted as Pd@HY-3) as an example, for this zeolite HY
All of the calculations based on density functional theory
plus dispersion (DFT-D) were performed with the program
package DMol3 in Materials Studio (version 8.0) of Accel-
rys Inc [30, 31]. The generalized gradient approximation
possessing negative zeta potential, choosing Pd(acac) as
2
2
+
metal precursor, ethanol as solvent, the positive charge Pd
species in Pd(acac) precursor can smoothly diꢀuse into the
2
inner micropores of zeolite HY through electrostatic attrac-
(
GGA) with the Perdew−Burke−Ernzerh (PBE) formulation
tion and capillary force, then obtained zeolite HY encapsu-
of the exchange–correlation functional was employed [32,
lated Pd species was reduced with H balloon at 60 °C to
2
3
3]. The valence electron wave functions were expanded into
get desired Pd@HY-3 catalyst. Noteworthily, the reduction
a set of atomic orbitals composed of the double numerical
plus polarization (DNP) basis set, and the cutoꢀ radius is
of Pd(acac) to Pd nanoparticles by H at very mild condi-
2
2
tion has also been reported by another authors, such as the
4
Å [34]. A cluster of 12 T model included a 12-member-
disclosed reaction temperature at 75 °C with 5 min to aꢀord
ring window was extracted from the crystal structure of Y
zeolites to represent the HY zeolite [35]. H atoms were used
to saturate the boundary of the zeolite cluster model, and
thus Si–H dangling bonds were formed. Furthermore, all
the dangling Si–H bonds are with the same direction of the
faujasite framework, for keeping the intrinsic features of the
zeolite. What is more, the Si–H bond lengths were ꢁxed at
Pd nanoparticles from Pd(acac) [36, 37].
2
XRD characterization in Fig. 1a showed that the synthe-
sized Pd@HY-3 catalysts exhibited the identical XRD peaks
as the zeolite HY catalyst, which implied that inclusion of
palladium species into zeolite HY did not interfere with par-
ent zeolite HY structure. Besides, no observation of lines
corresponding to palladium metals indicated the absence of
large palladium NPs in zeolite HY support. HAADF-STEM
0
.147 nm. The convergence tolerance of energy, maximum
1
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Ultrasmall and Stable Pd and Pt Nanoparticles Within Zeolite HY Through Impregnated Method…
Fig. 1 a Powder XRD patterns of synthesized Pd@HY-3 catalysts
and zeolite HY catalysts. b HAADF-STEM image of Pd@HY-3 cata-
lysts. c High-resolution (HR) TEM images of the Pd@HY-3 catalyst.
And d size distribution of palladium NPs in Pd@HY-3 catalysts. Typ-
ical encapsulated palladium NPs are highlighted by red circles
image in Fig. 1b suggested that the brighter Pd nanoparticles
for the heavier atomic weight than the elements of parent
zeolite HY were distributed uniformly throughout the zeolite
HY crystal. The statistical average size of formed Pd NPs is
much larger than the diameter size of SOD cage (0.22 nm)
in zeolite Y, thus preventing such diꢀusion. In addition, as
shown in Fig. 1c, the reduced palladium NPs can be directly
observed to be within zeolite HY micropore instead of on
the surface, demonstrating that these Pd NPs are true to be
encapsulated into zeolite structure with a manner similar to
the formation of a core–shell structure.
1
.57 nm, which was belonging to the ultrasmall nanoparticle
size (Fig. 1d). Notably, the larger size of synthesize Pd NPs
than the diameter size of supercage (1.3 nm) in zeolite Y
could reꢃect the local disruption of zeolite Y crystal struc-
tures around the position of the formed Pd NPs, yet this
disruption was not detectable in diꢀractograms based on the
XRD characterization. This local disruption phenomenon
has also been observed in the MFI structure encapsulation of
Pt NPs [21]. Presumably, the formed Pd NPs should reside
in the supercage instead of SOD cage in zeolite Y crys-
tal, due to the molecular size of metal precursor Pd(acac)2
X-ray photoelectron spectroscopy (XPS) of the synthe-
sized Pd@HY-3 catalyst in Fig. 2a showed that there was
no obvious XPS signals appeared, indicating the ultrasmall
Pd NPs indeed conꢁned into zeolite HY structure (Fig. 2b).
Contrastly, when the structure of Pd@HY-3 was destroyed
with a high concentrated NaOH solution (Fig. 2c), the typi-
cal peaks at 337.1 and 342.3 eV could be observed clearly,
II
corresponding to the Pd species (Fig. 2d). This XPS result
1
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M. Wang et al.
Fig. 2 Pd@HY-3 catalyst for the XPS characterization (a) and TEM image (b). Concentrated NaOH solution treated Pd@HY-3 catalyst for the
XPS characterization (c) and TEM image (d)
clearly indicated that the synthesized Pd nanoparticles were
conꢁned into the zeolite Y crystals.
to the conꢁning eꢀect of the zeolite to block this large reac-
tant to enter into zeolite micropore. These results demon-
strated that prepared ultrasmall Pd NPs were encapsulated
into zeolite HY crystal successfully instead of locating on
the surface.
To further certify that the ultrasmall Pd NPs are encap-
sulated into zeolite HY crystal rather than on the external
surface, shape-selective reduction of nitrobenzene and
2
,4,6-tri-tetra-butylnitrobenzene were conducted over the
Signiꢁcantly, the synthesized Pd@HY-3 catalyst exhib-
ited excellent thermal stability, which would be very valu-
able in view of industrial utilization. As shown in Fig. 4, the
calcination at 500 °C for 2 h under air atmosphere, palladium
NPs were still distributed uniformly throughout the zeolite
crystal without observed aggregation and sintering. The
calculated average size of Pd NPs after treatment at 500 °C
became 1.68 nm, compared to that of fresh Pd@HY-3 cata-
lyst (1.57 nm). Furthermore, increasing the treatment tem-
perature to 600 °C and 700 °C, as shown by HAADF-STEM,
Pd@HY-3 catalysts and Pd/C catalyst. As shown in Fig. 3,
for the reduction of small molecule nitrobenzene, the Pd@
HY-3 and Pd/C catalysts both exhibited very high catalytic
activity, and the conversion was up to 100%. Conversely,
for the reduction of large molecule 2,4,6-tri-tetra-butylnitro-
benzene, Pd/C catalyst showed modest catalytic activity to
give the hydrogenation product 2,4,6-tri-tetra-butylaniline,
but not any catalytic activity was observed over Pd@HY-3
catalyst even at extended reaction time, which was attributed
1
3

Ultrasmall and Stable Pd and Pt Nanoparticles Within Zeolite HY Through Impregnated Method…
to resist sintering and aggregation. Notably, compared with
calcination at 500 °C, the decreased average size of Pd NPs
calcination at 600 °C and 700 °C could be due to the dissolu-
tion of Pd atoms from the ultrasmall nanoparticles, which is
consistent with the reported literature [38].
On the basis of above results, it could be sure that posi-
tive charge Pd species from Pd(acac) precursor diꢀused
2
into the micropores of zeolite HY-3 successfully through
eletrostatistic attraction and capillary force, and formation of
ultrasmall Pd NPs within zeolite HY-3 crystal via the reduc-
tion of balloon H atmosphere at mild temperature without
2
high-temperature treatment. To further study the eꢀect of
negative charge property of zeolites HY framework on the
preparation of ultrasmall Pd NPs within zeolite crystals, zeo-
lites HY with diꢀerent ratios of Si/Al (zeolite HY-4, zeolite
HY-5, zeolite HY-10) have been employed. Clearly, Pd NPs
with large size located on the surface of zeolite HY were
formed when the zeolite HY-5, zeolite HY-10 was chosen as
parent supports (Fig. 5a, b). As the ratio of Si/Al decreased
to 3 and 4 respectively, the formation of ultrasmall Pd NPs
within zeolites was observed distinctly, and their dispersions
on corresponding supports were very uniformly (Fig. 5c and
d). Hence, zeolite HY with a smaller ratio of Si/Al to give
Fig. 3 Catalytic properties of Pd@HY-3 and Pd/C catalysts in hydro-
genation of nitrobenzene and 2,4,6-tri-tert-butylnitrobenzene respec-
tively. Reaction condition in hydrogenation of nitrobenzene Reaction
condition: (T=120 °C, 1 MPa H ); Reaction condition in hydro-
2
genation of 2,4,6-tri-tert-butylnitrobenzene: Reaction condition:
(
T=120 °C, 1 MPa H )
2
palladium NPs were also dispersed evenly over the zeolite
crystal, and the average size of encapsulated Pd NPs was
1
.43 nm and 1.61 nm, respectively, indicating the extremely
good thermal stability of immobilized Pd NPs in zeolite HY
during such harsh treatment, thus possessing great ability
Fig. 4 HAADF-STEM images of Pd@HY-3 catalyst after treatment at 500 °C (a), 600 °C (b) and 700 °C (c). Corresponding size distribution of
Pd NPs after treatment at 500 °C (d), 600 °C (e) and 700 °C (f). Typical encapsulated palladium NPs are highlighted by red circles
1
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M. Wang et al.
micropores through electrostatic interactions to overcome
the diꢀusion resistance to aꢀord desired ultrasmall nanopar-
ticles lastly. Instead, relatively weaker electrostatic interac-
tions between Pd(acac) precursor and zeolites HY-5 and
2
HY-10 could cause some Pd(acac) species located at outer
2
surface of zeolite due to the existed internal diꢀusion resist-
ance. Clearly, these DFT calculations were consistent with
the above experimental results in Fig. 5 very well.
Combined with the shape selectivity of zeolite, cata-
lytic selectivity of zeolite encapsulated ultrasmall Pd NPs
should be improved [41, 42]. Herein, with prepared Pd@
HY-3 catalyst as an instance, the catalytic property of this
encapsulated catalyst was evaluated and compared with Pd/
HY-3 catalyst (Pd NPs supported on the external surface of
zeolite HY, Figure S3), classical lindlar catalysts, Pd/C and
Pd/Al O catalysts using semihydrogenation of 4-ethylphe-
2
3
nylacetylene as a probe reaction.Time dependent catalytic
property of Pd@HY-3 catalyst in this hydrogenation reac-
tion was summarized in Fig. 6a. Clearly, Pd@HY-3 catalyst
showed good catalytic selectivity throughout the reaction
and could give desired 4-ethylphenylene product in more
than 95% yield, indicating that the undesired over-hydro-
genation byproduct 1,4-diethylbenzene could be circum-
vented well. As shown, Pd@HY-3 catalyst exhibited 100%
catalytic selectivity at the initial of the reaction and slightly
decreased to 95% selectivity at the end of the reaction. The
performed catalytic selectivity of Pd@HY-3 catalyst was
much better than other compared catalysts. As can be seen
in Fig. 6b, for supported Pd/HY-3 catalyst with similar Pd
size (1.8 nm in Figure S3) to Pd@HY-3, at 100% conversion
of the reactant, the selectivity was around 30%, indicating
Pd NPs located at the surface of zeolite HY unfavourable
for this reaction. Commercial Pd/C and Pd/Al O catalysts
Fig. 5 TEM image of synthesized Pd/HY-5 (a) and Pd/HY-10 (b)
samples. HAADF-STEM images of synthesized Pd@HY-3 (c) and
Pd@HY-4 (d) samples. Typical palladium atoms are highlighted by
red circle
higher negative charge value on zeolite framework was very
necessary to aꢀord ultrasmall Pd NPs within zeolite crystal.
As proposed, NaPdCl as a precursor would cause the for-
4
mation of large nanoparticles at the surface of zeolite HY-3
(
Figure S1).
DFT calculation was also conducted to investigate the
inꢃuence of the Si/Al ratio on the formation of ultrasmall Pd
2+
NPs within zeolite HY crystals. In consideration of the Pd
2
3
species in Pd(acac) diꢀusing into micropores of zeolite HY
were also exhibited much lower catalytic selectivity than the
encapsulated Pd@HY-3 catalyst at the end of this reaction.
Even for the industrially used Lindlar catalyst, it can only
give 80% selectivity to desired alkene products, which was
inferior to the prepared encapsulated Pd catalyst. The reac-
tion was further conducted at high stirring speed (700 rpm)
to exclude external diꢀusion eꢀect, and the selectivity to
4-ethylphenylene was evaluated at the 10%± 3% conversion
of 4-ethylphenylacetylene to minimize the further conver-
sion of the alkene intermediate. As shown in Figure S4, for
Pd/HY-3 catalyst, the selectivity to product was only 55%,
yet for the encapsulated Pd@HY-3 catalyst, the selectivity
was nearly 100%, indicating that encapsulation played a
key role in the enhancement of catalytic selectivity. In addi-
tion, Pd@HY-3 also showed a higher catalytic selectivity
than that of Pd/C and Pd/Al O catalysts. Although Lindlar
2
through their twelve-membered ring, the change of Si/Al
ratio in the zeolite HY was reꢃected in their twelve-mem-
bered ring structure to simplify the DFT calculation process
[
39, 40]. As shown in Figure S2, after the introduction of
one aluminum atom into a twelve-membered ring structure,
the calculated energy based on the electrostatic interactions
between this twelve-membered ring structure and Pd(acac)
2
precursor was 72.1 kJ/mol. When further introducing two
and six aluminum atoms into a twelve-membered ring
structure, the calculated energy was increased to 95.8 and
1
09.2 kJ/mol respectively, indicating that smaller Si/Al ratio
in zeolite HY could enhance the electrostatic interactions
between the Pd(acac) and micropores of zeolite HY to facil-
2
itate their diꢀusion into zeolite inner structure. Reasonably,
compared with zeolites HY-5 and HY-10 (Si/Al=5 and Si/
Al=10 respectively), zeolites HY-3 and HY-4 with smaller
Si/Al ratio (Si/Al=3 and Si/Al=4 respectively) possessed
stronger negatively charged frameworks, which can make
2
3
catalyst exhibited comparable catalytic selectivity (96%) to
Pd@HY-3, the main problem for the Lindlar catalyst is its
toxicity in practical usage. Therefore, Pd@HY-3catalyst is
the optimal choice in this semihydrogenation reaction. The
2
+
Pd in Pd(acac) more easily to be diꢀused into zeolite
2
1
3

Ultrasmall and Stable Pd and Pt Nanoparticles Within Zeolite HY Through Impregnated Method…
Fig. 6 Time dependent catalytic property of Pd@HY-3 catalyst dur-
ing the semihydrogenation of 4-ethylphenylacetylene (a). Reaction
catalysts during the semihydrogenation of 4-ethylphenylacetylene at
approaching 100% conversion (b)
condition: (T=65 °C, 1 MPa H ). Compared with diꢀerent Pd-based
2
leaching test was further conducted to elucidate the chemi-
cal stability of Pd@HY-3 catalyst (Scheme S1). Meanwhile,
ICP-MS analysis of the supernatant of reaction mixture was
introduced. Both results suggested that no Pd species was
leached from encapsulated Pd NPs to the reaction solution,
demonstrated good chemical stability of Pd@HY-3 catalyst
during this semihydrogenation reaction.
The origin of the high catalytic selectivity of Pd@HY-3
catalyst in the semihydrogenation reaction was studied
with FT-IR analysis to illustrate the adsorption behavior
of alkyne substrates on the encapsulated catalyst surface.
Because it has been proven that the adsorption step would
be very critical to determine the catalytic selectivity in semi-
hydrogenation process. Due to the undetectable FT-IR sig-
nal of 4-ethylphenylacetylene on Pd@HY-3 catalyst surface
(
Figure S5), conjugated ethynyl-1,1′-biphenyl (EB-H) was
chosen as a model molecule. As shown in Fig. 7a, EB-H
adsorbed on encapsulated PdNPs surface exhibited aromatic
−
1
C-H vibrational stretch from 3100 to 2900 cm and char-
acteristic aromatic C–H deformation vibration from 840 to
−1
7
00 cm , which was similar to the corresponding stretching
band of pure EB-H, indicating the adsorption of the sub-
strate on the encapsulated Pd NPs surface deꢁnitely [43,
−
1
4
4]. Notably, the C≡C vibrational stretch at 2120 cm was
disappeared after the adsorption of EB-H on encapsulated
Pd surface, which could be due to the interparticle charge
delocalization, further certifying the interaction between
the alkynyl group and the Pd@HY-3 catalyst [45]. Notably,
compared with pure EB-H, the disappearance of an intense
−
1
characteristic band at 3200 cm assigned to a vibrational
stretch of terminal ≡C–H moiety was observed, indicating
the adsorption of this substrate molecule on encapsulated
Pd NPs surface through the process of dissociation of the
alkynyl C–H bond, thus leading to the upright adsorption
conformation rather than common flat-lying adsorption
Fig. 7 FT-IR spectras of pure 4-ethynyl-1-1′-biphenyl (EB-H) and
Pd@HY-3 catalyst treated with EB-H molecule (a). FT-IR spectras of
pure 4-ethynyl-1-1′-biphenyl (EB-H), 4-vinylbiphenyl molecule, and
Pd@HY-3 catalyst treated with mixture EB-H and 4-vinylbiphenyl
molecules (b)
1
3

M. Wang et al.
geometry [46]. Such interesting upright adsorption mode
was consistent with the conꢁnement eꢀect of zeolite HY.
Because of the molecule size of EB-H larger than the diam-
eter of twelve membered ring (7.4 Å), micropores of zeo-
lite HY would constrain EB-H adsorbed on encapsulated
Pd surface with end-on adsorption mode instead of ther-
modynamically favorable flat-lying mode. Fortunately,
such imposed upright adsorption geometry of substrate on
encapsulated Pd surface discriminated the adsorption energy
between EB-H molecule and semihydrogenation product
Fig. 8 HAADF-STEM image of Pt@HY-3 catalysts (left), and size
4
-vinylbiphenyl molecule. As indicated in Fig. 7B, when
distribution of Pt NPs inPt@HY-3 catalysts (right)
Pd@HY-3 catalyst was exposed to the mixture of EB-H
and 4-vinylbiphenyl molecules, the FT-IR curve of Pd@
HY-3-mixture was similar to that of pure EB-H molecular
rather than 4-vinylbiphenyl in view of the lack of aromatic
cationic rather than anionic Pd and Pt precursors were
selected to successfully achieve easy diffusion into the
micropores of zeolite HY (Si/Al=3) with higher negative
charge density via electrostatic attraction and capillary force,
−
1
−1
C–H vibration band at 3000 cm and 690 cm assigned
to pure 4-vinylbiphenyl, suggesting that EB-H preferring to
adsorb on the encapsulated Pd surface for a larger adsorp-
tion energy in the presence of EB-H and 4-vinylbiphenyl.
Reasonably, attributed to the stronger adsorption energy of
EB-H on encapsulated Pd surface through the deprotonation
process with upright geometry than that of constrained end-
on adsorbed 4-vinylbiphenyl, Pd@HY-3 catalyst achieved a
high and enhanced catalytic selectivity in semihydrogena-
tion. In contrast, for the supported catalyst such as Pd/C,
when it was treated with the mixture of EB-H and 4-vinylbi-
phenyl molecules, the similar FT-IR signal of 4-vinylbiphe-
nyl was appeared on the Pd/C-mixture sample, in view of the
observance of C–H stretching vibration band of –C=C–H
then following the reduction with H balloon at mild tem-
2
perature to obtain desired metal@zeolite HY samples to
achieve the objective of stabilization of ultrasmall nanoparti-
cles, without any high temperature treatment step. Combined
with shape selectivity of zeolite HY, taking Pd@zeolite cata-
lyst as an example, this catalyst exhibited enhanced catalytic
selectivity in semihydrogenation of alkynes, in comparison
with Pd/HY catalyst and other most widely used commercial
catalysts such as Pd/C, Pd/Al O and lindlar catalysts. The
2
3,
improvement in catalytic selectivity can be well attributed
to the conꢁnement eꢀect of zeolite HY, which discriminated
the adsorption energy through constrained upright adsorp-
tion conformation to facilitate alkyne preferred to adsorp-
tion on encapsulated Pd surface than corresponding alkene
substrate, thus exhibiting the enhanced catalytic selectivity.
The modiꢁed impregnation approach proposed in this work
not only develops a new method to stabilize ultrasmall nano-
particles for practical chemical industries, but also improves
our understanding of the conꢁnement catalysis.
−1
moiety at 3100–2800 cm and aromatic C–H vibration band
−1
at 800–750 cm both assigned to pure 4-vinylbiphenyl (Fig-
ure S6), indicating that 4-vinylbiphenyl can adsorb on the
supported Pd surface for the comparable adsorption energy
in the presence of EB-H and 4-vinylbiphenyl, thus resulting
the decreased catalytic selectivity.
Signiꢁcantly, this optimized impregnation approach can
be extended to stabilize other ultrasmall platinum-group
nanoparticles, such as Pt. Selecting Pt(acac) as a metal pre-
2
cursor, ethanol as solvent, the positive charge Pt species in
Acknowledgements This work was supported by the National Natu-
ral Science Foundation of China (21706204) and China Petroleum &
Chemical Corporation (No.119004-2). All calculations were performed
with the Material Studios (MS) 2016, a software developed by Biovio
company.
Pt(acac) can enter into the inner structure of zeolite HY -3
2
through electrostatic attraction and capillary force, following
by the reduction process with H balloon at mild tempera-
2
ture to obtain Pt@HY-3 catalyst. Figure 8 showed ultrasmall
PtNPs distributed uniformly throughout the zeolite crystal,
and the calculated average size was 1.9 nm.
Compliance with Ethical Standards
Conflict of interest The authors declare no conꢃict of interest.
4
Conclusion
In conclusion, a facial and versatile approach to stabilize
ultrasmall Pd and Pt nanoparticles within zeolite HY was
developed through an optimized impregnation process. Due
to the essence on the negative charge of zeolite framework,
1
3

Ultrasmall and Stable Pd and Pt Nanoparticles Within Zeolite HY Through Impregnated Method…
2
2
0. Li Y, Li L, Yu J (2017) Applications of zeolites in sustainable
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