Pd?Zn Alloy Nanoparticles Encapsulated into Mesoporous Silica with Confinement Effect for Highly Selective Semi-Hydrogenation of Phenylacetylene

Article abstract of DOI:10.1002/cctc.202001159

In the process of hydrogenation of phenylacetylene to styrene, it is a crucial step to improve the reaction selectivity of the target product styrene. Herein, we have designed the Pd?Zn alloy nanoparticles encapsulated into mesoporous silica with confinement effect (Pd?Zn@MS) for highly selective semi-hydrogenation of phenylacetylene via annealing the Pd NPs@ZIF-8 in reducing atmosphere. The as-obtained Pd?Zn@MS achieved up to the 95 % of conversion rate and 92 % of selectivity, because the mesoporous silica exhibits a steric hindrance effect for the reactant molecular size, and the Pd?Zn alloy nanoparticles shows the high selectivity for the target product styrene. DFT calculations demonstrates that the PdZn alloy nanoparticles can reduce significantly the desorption energy barrier of styrene from 1.72 eV to 0.95 eV, compared with the Pd nanoparticles. Meanwhile the mesoporous silica in the outer layer greatly guarantees the stability of the catalytic performance of the catalyst, and the catalytic performance did not drop after the five-cycle test. This work provides a new idea for the selective hydrogenation of phenylacetylene and the process of limited-area catalysis.

Full text of DOI:10.1002/cctc.202001159

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Pd-Zn Alloy Nanoparticles Encapsulated into Mesoporous Silica
with Confinement Effect for Highly Selective Semi-Hydrogenation
of Phenylacetylene
Authors: Zhen-Xing Li, Mingliang Hu, Bowen Liu, Jiahao Liu, Ping
Wang, Jiasai Yao, Xin Zhang, Miao He, and Weiyu Song
This manuscript has been accepted after peer review and appears as an
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To be cited as: ChemCatChem 10.1002/cctc.202001159
Link to VoR: https://doi.org/10.1002/cctc.202001159
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1/2020

ChemCatChem
10.1002/cctc.202001159
FULL PAPER
Pd-Zn Alloy Nanoparticles Encapsulated into Mesoporous Silica
with Confinement Effect for Highly Selective Semi-Hydrogenation
of Phenylacetylene
[
a]
[a]
[b]
[a]
[a]
[a]
[a]
Zhenxing Li,* Mingliang Hu, Bowen Liu, Jiahao Liu, Ping Wang, Jiasai Yao, Xin Zhang,
[
a]
[b]
Miao He, and Weiyu Song*
[
a]
Prof. Z. Li[†], M. Hu[†], J. Liu, P. Wang, J. Yao, X. Zhang, M. He
Institution State Key Laboratory of Heavy Oil Processing, College of New Energy and Materials, China University of Petroleum (Beijing), Beijing 102249,
China
E-mail: lizx@cup.edu.cn
[
b]
B. Liu, Prof. W. Song
College of Science, China University of Petroleum (Beijing), Beijing, 102249, China.
E-mail: songwy@cup.edu.cn
[
†]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: In the process of hydrogenation of phenylacetylene to
styrene, it is a crucial step to improve the reaction selectivity of the
target product styrene. Herein, we have designed the Pd-Zn alloy
nanoparticles encapsulated into mesoporous silica with confinement
effect (Pd-Zn@MS) for highly selective semi-hydrogenation of
phenylacetylene via annealing the Pd NPs@ZIF-8 in reducing
atmosphere. The as-obtained Pd-Zn@MS achieved up to the 95% of
conversion rate and 92% of selectivity, because the mesoporous
Recently, various noble metal and non-noble metal catalysts
have been explored, including Pd, Ru, Au, Rh, Ni, Cu, Zn, etc.[
Among these metal catalysts, Pd exhibits an excellent activity in
the semi-hydrogenation of phenylacetylene.[9] More recently, Pd-
based catalysts have become more and more widely applied in
semi-hydrogenation of phenylacetylene.[10] For example, Yu et al.
used phosphorus-doped porous carbon as the carrier and
loaded Pd nanoparticles as the active sites for phenylacetylene
8]
silica exhibits a steric hindrance effect for the reactant molecular size, hydrogenation.[11] This catalyst benefits from the interaction
and the Pd-Zn alloy nanoparticles shows the high selectivity for the
target product styrene. DFT calculations demonstrates that the PdZn
alloy nanoparticles can reduce significantly the desorption energy
barrier of styrene from 1.72 eV to 0.95 eV, compared with the Pd
nanoparticles. Meanwhile the mesoporous silica in the outer layer
greatly guarantees the stability of the catalytic performance of the
catalyst, and the catalytic performance did not drop after the five-
between the carrier and Pd nanoparticles. Compared with the
Lindlar catalyst used in industry, the performance of the catalyst
was obviously improved, the conversion rate reached 86%, and
the selectivity was 73%. Shao et al. loaded Pd nanoparticles on
three-dimensional (3D) porous carbon with phenylacetylene
conversion of as high as 100%,[12] but the styrene selectivity was
only about 60%. Hu and his colleagues used nano-Pd/α-Al
2 3
O
cycle test. This work provides
a new idea for the selective
as the catalyst for phenylacetylene hydrogenation, and the
-1 [13]
hydrogenation of phenylacetylene and the process of limited-area
catalysis.
turnover frequency (TOF) was up to 11.8 s . The conversion
rate of phenylacetylene was also close to 100%, but the
selectivity of styrene still did not reach satisfactory results, lower
than 70%.
The above pure Pd as the catalytic active sites for selective
hydrogenation of phenylacetylene all encounters the problem of
low styrene selectivity.[14] The density functional theory (DFT)
calculation shows that the key reason for low selectivity is that
styrene adsorption on Pd nanoparticles is difficult to desorb in
time, thus causing excessive hydrogenation.[15] Therefore, a
Introduction
In the traditional catalytic system, the catalytic reactions are
carried out in an open and unrestricted space , and the
conversion rate or selectivity of the reaction is uncontrollable.[2]
[
1]
A
classic example is the selective hydrogenation of
large number of researchers began to introduce other metal
phenylacetylene to alkene, and it is well known that styrene is a
very important organic chemical raw material in polystyrene or
rubber industry.[3] At present, the styrene is main from the
dehydrogenation of ethylbenzene. However, in the process of
dehydrogenation of ethylbenzene to styrene, the by-product
phenylacetylene can result in poisoning and deactivation of
dehydrogenation catalyst.[4] Meanwhile the presence of
phenylacetylene greatly reduces the quality of styrene
products.[5] Up to now, the simplest and most direct way to
remove phenylacetylene from styrene is the semi-hydrogenation
of phenylacetylene.[6] It is huge challenge to obtain both the high
selectivity of styrene and the high conversion rate of
[16-22]
elements into Pd nanoparticles for solving this problem.
Wang and his colleagues introduced Au to form Pd-Au bimetallic
catalyst, which was loaded on carbon nanotubes in the semi-
phenylacetylene.[23]
hydrogenation
of
The
selective
hydrogenation performance of phenylacetylene was enhanced,
and the selectivity could reach 80% while the conversion rate
was close to 100%. In addition to the Au element, Luo et al.
used PdIn and PdGa compounds to carry out the semi-
hydrogenation experiments of acetylene, and obtained 80% and
5% ethylene selectivity, respectively. [24] Recent studies have
7
found that Zn is also an element commonly used to form alloys
with Pd, Zn-MOF (such as ZIF-8) is a crystal that easily supports
[
7]
phenylacetylene. Therefore, it is increasingly urgent to find a
suitable catalyst to improve the conversion and selectivity.
[5, 8, 30]
Pd precursors and then forms PdZn alloys.
In general, the
1
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ChemCatChem
10.1002/cctc.202001159
FULL PAPER
introduced metal can tune the electronic structure of the Pd-
based catalyst, which is more favorable for desorption of the
target product styrene.[25] Meanwhile it is an effective way to
design selective hydrogenation catalyst for phenylacetylene by
space confinement effect.[26] Encapsulating a guest (molecule,
cluster, or nanoparticle) as a catalytic active center in a
nanoporous host is one of the key strategies.[27-29] Metal organic
framework (MOF) materials are ideal candidates for the
nanoporous host.[30-33] The microporous structure of MOF can
provide the confinement effect, and the fixation of the guest in
the MOF can avoid the aggregation or fusion of the active
center.[34] Huo et al. used MOF to encapsulate Pt nanoparticles
for the catalytic reaction of alcohols and olefins, and achieved
successfully encapsulated in ZIF-8 (Pd NPs@ZIF-8) by a static
growth method. Then the Pd NPs@ZIF-8 is coated with a layer
of the ordered mesoporous silica. After calcination in hydrogen
atmosphere, the unique nano-hollow structure encapsulated with
Pd-Zn alloy nanoparticle is obtained, owing to the pyrolysis of
ZIF-8 and high thermal stability of mesoporous silica layer.
9
7% conversion and 42.28 h-1 TOF at 60︒C. However, this type
of catalyst can only be used for reactions with a relatively low
reaction temperature, and cannot be stable under high
temperature conditions.[35]
In this work, we use a typical metal-organic framework, ZIF-
8
as a template, and the prepared Pd nanoparticles were
encapsulated into the ZIF-8 (Pd NPs@ZIF-8). Meanwhile the Pd
NPs@ZIF-8 was coated with the ordered mesoporous silica to
improve the stability of ZIF-8 in the heating treatment. The Pd-
Zn alloy@mesoporous silica (Pd-Zn@MS) was obtained via
annealing in hydrogen atmosphere, which was used as a
Figure 1. (a) TEM image of Pd NPs@ZIF-8, (b) SEM image of Pd-Zn@MS, (c)
TEM image of Pd-Zn@MS, (d) HR-TEM image of red square in (c), the inset of
Figure 1d shows the lattice fringe spacing, (e) and (f) HAADF-STEM image
and corresponding element mapping analysis of Pd-Zn@MS.
phenylacetylene
hydrogenation. The as-obtained Pd-Zn@MS achieved up to the
5% of conversion rate and 92% of selectivity, due to the highly
dispersed Pd-Zn alloy nanoparticles and the mesoporous
structure with confinement effect. In addition, the Pd-Zn@MS
can still maintain high catalytic performance after 5 cycles. The
DFT calculation reveals that the PdZn alloy nanoparticles can
reduce significantly the desorption energy barrier of styrene from
hydrogenation
catalyst
for
selective
9
The transmission electron microscope (TEM) image, particle
size distribution and X-ray diffraction (XRD) pattern of Pd
nanoparticles is shown in Figure S1 and Figure S2, and the
particle size is mainly concentrated in 3 nm. TEM image of Pd
NPs@ZIF-8 (Figure 1a) displays that the Pd nanoparticles are
uniformly distributed in ZIF-8, and the size of Pd NPs@ZIF-8 is
about 200 nm. X-ray diffraction (XRD) pattern of Pd NPs@ZIF-8
is shown in Figure S3, we can find that the XRD diffraction
peaks of Pd@ZIF-8 are completely consistent with those of pure
ZIF-8, indicating that the synthesized Pd@ZIF-8 has perfect
crystal structure, and no obvious diffraction peaks of Pd
nanoparticles are found because of its small size and complete
coverage by ZIF-8. The unique nano-hollow structure of the Pd-
Zn@MS is observed in scanning electron microscope (SEM)
1
.72 eV to 0.95 eV, compared with the Pd nanoparticles. This
work provides a new strategy for designing microenvironment
with confinement effect to improve the selectivity of catalytic
reaction.
Results and Discussion
The schematic illustration of the preparation of Pd-Zn
alloy@mesoporous silica (Pd-Zn@MS) is shown in Scheme 1.
(
2
Figure 1b), and the Pd-Zn@MS is regular hexagon with size of
00-250 nm. As shown in Figure 1c, the Pd-Zn@MS is hollow
structure, and the ordered mesoporous structure is on the outer
layer. The thickness of mesoporous silica is 20 nm and the
mesopore diameter is about 4 nm. These mesopores can be
used as channels for transporting reactants and products, and
the mesopores and hollow structure in Pd-Zn@MS also offers
shape selection effect via the space confinement effect. The
inside of the Pd-Zn@MS (red square in Figure 1c) is further
observed under
a
high-resolution transmission electron
microscope (HRTEM). As shown in Figure 1d, the Pd-Zn alloy
nanoparticles are confined in the hollow structure, and the lattice
fringes with lattice spacing of 0.219 nm is correspond to (111)
crystal planes of Pd-Zn alloy, indicating that the Pd
2
+
nanoparticles and Zn on ZIF-8 framework are converted
successfully to the Pd-Zn alloy nanoparticles after calcination in
hydrogen atmosphere. The size of Pd-Zn alloy nanoparticles is 4
nm, which is larger than Pd nanoparticles, because the alloying
of Pd nanoparticles with Zn2+ will lead to a slight increase in
particle size. A high-angle annular dark field scanning TEM
Scheme 1. Schematic of the synthesis process of Pd-Zn@MS.
Pd nanoparticles, dimethylimidazole solution and zinc nitrate
solution are mixed, and the Pd nanoparticles can be
(
HAADF-STEM)
image
and
energy-dispersive
X-ray
2
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spectroscopy (EDS) mapping analysis of the as-obtained Pd-
Zn@MS is shown in Figure 1e and f, and the Si and O element
is mainly distributed in the outer layer and the Pd and Zn
element is uniformly distributed inside the Pd-Zn@MS, which
further confirms that the Pd-Zn@MS is hollow structure.
The X-ray diffraction (XRD) pattern of Pd-Zn@MS was shown
in Figure 2a, and there is an obvious wide diffraction peak in the
range from 15° to 30°, which is the characteristic "steamed
bread peak" of silica on the outer layer of Pd-Zn@MS.[36]
Meanwhile, the diffraction peaks corresponding to the (111) and
The hydrogenation path of phenylacetylene is shown in
Scheme 2. The phenylacetylene reacts with one mole of
hydrogen to obtain the target product styrene.[
43, 44]
The second
hydrogenation process is carried out when the styrene is failure
to desorb in time, and the styrene will continue to react with
hydrogen to generate the by-product ethylbenzene. Therefore, it
is the key for the phenylacetylene hydrogenation process that
the main product styrene can be desorbed from the active site of
the catalyst in time and avoid the excessive hydrogenation. In
order to study the catalytical performance of Pd-Zn@MS with
space confinement effect in selective hydrogenation of
phenylacetylene, the as-prepared Pd-Zn@MS is used as
catalyst with 0.1 MPa hydrogen for hydrogenation of
phenylacetylene in ethanol, and the reaction is carried out at
100 °C.
(
4
200) crystal planes of PdZn alloy can be found at 41.2° and
4.1° (Pd Zn
, PDF#06-0620).[37] The weak diffraction peak of
1
1
PdZn alloy is attributed to the ultra-small particle size (4 nm).
The X-ray photoelectron spectroscopy (XPS) full spectrum of
PdZn@MS is shown in Figure S4, including Zn, O, Si, Pd, N, C.
Among them,
N
element mainly comes from the
dimethylimidazole in the structure of ZIF-8. Figure 2c is the XPS
analysis of Pd, in which the binding energies of Pd are mainly
distributed at 335.6 eV and 340.8 eV, corresponding to Pd 3d5/2
and Pd 3d3/2 peaks.[38] Compared with pure metal Pd, the
binding energy shifts by 0.6 eV, suggesting that the alloy is
formed by the combination of Pd and Zn. Similarly, the binding
energy of Zn is mainly distributed in 1021.6 eV and 1044.6 eV[39]
in Figure 2d, which also shifted 0.5 eV relative to metal Zn.[40]
Therefore, the formation of PdZn alloy is further proved. The
XPS analysis of O element and Si element is shown in Figure
S5 and S6, and the peak of O is concentrated at 532.1 eV, while
the peak of Si is mainly located at 103.0 eV.[41] This result is
completely consistent with the mesoporous silica reported by the
previous researchers.[42] In order to further explore the porous
Scheme 2. Hydrogenation of PA and possible products.
The reaction results of hydrogenation of phenylacetylene
catalyzed by Pd-Zn@MS catalyst are shown in Figure 3a. In the
period of 20-60 min, the conversion rate of the reaction is
obviously improved, from 40% to 90%. After 60 min, the
increase of conversion rate slows down, and finally reaches 95%
conversion rate at 100 min, while the selectivity of styrene
decreased slightly from 98% to 92% in the whole process. The
ideal result is to obtain 95% conversion while maintaining 92%
selectivity. We also investigated the catalytic performance of Pd
nanoparticles under the same reaction conditions (Figure 3b).
Although the conversion rate of phenylacetylene can still exceed
95%, the selectivity of styrene is less than 50%, which suggests
that Pd-Zn alloy nanoparticles can significantly promote the
selectivity of styrene. For comparison, the silica-coated pure
ZIF-8(ZIF-8@MS) is also used as a catalyst to carry out this
reaction, and there was no obvious conversion rate and
selectivity. The stability of catalytic performance is also one of
the important factors to evaluate the performance of catalysts.
After five cycles, the conversion rate of phenylacetylene and the
selectivity of styrene have not decreased obviously (Figure 3c),
illuminating that the outer layer of mesoporous silica can avoid
the aggregation of the Pt-Zn alloy particles and keep the Pd-
Zn@MS stable in the catalytical reaction process. In addition,
the TEM and XRD of Pd-Zn@MS after the semi-hydrogenation
of phenylacetylene are shown in Figures S7 and S8, and there is
no obvious change, which further proves the stability of the
catalyst. For the purpose of investigation of confinement effect in
Pd-Zn@MS, the different molecular sizes, phenylacetylene (PA,
properties and large specific surface area of Pd-Zn@MS, N
adsorption-desorption was measured in Figure 2b. The N
2
2
adsorption-desorption curve has a clear hysteresis loop of a
typical IV-type line, which is consistent with the mesoporous
characteristics of the outer silica. The Brunauer-Emmett-Teller
2
(
BET) specific surface areas of the Pd-Zn@MS is 321 m /g. The
pore size is further evidenced by corresponding Barrett-Joyner-
Halenda (BJH) pore size distribution analysis, and the aperture
of Pd-Zn@MS is mainly concentrated at 4 nm, which agreed
well with TEM result. These uniform pore sizes are beneficial to
the reaction process with the corresponding steric effects.
0
.74 nm), 4-nitrophenylacetylene (4-NPA, 0.84 nm) and
diphenylacetylene (DPA, 1.14 nm) are used as reaction
substrates for hydrogenation reaction (Figure 3d). The
conversion rate of 4-NPA (92%) decreases slightly, because the
size of 4-NPA is larger than phenylacetylene. The selectivity of
4
-nitrostyrene (4-NST) achieves 90%. Since the size of DPA is
much larger than phenylacetylene, the conversion rate of DPA
50%) drops dramatically, and the selectivity of cis-stilbene and
trans-stilbene reaches nearly 100%. Based on above the
Figure 2. (a) XRD patterns of Pd-Zn@MS, (b) N
2
adsorption-desorption
isotherm of Pd-Zn@MS (The inset in Figure 2b is the pore size distribution of
PdZn@MS), (c) Pd 3d XPS spectra for Pd-Zn@MS, (d) Zn 2p XPS spectra for
Pd-Zn@MS.
(
3
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experimental results, the outer layer of mesoporous silica also
exhibits a confinement effect for the molecular size.
alloyed PdZn alloy during the hydrogenation of phenylacetylene.
Similarly, the structural state of each hydrogenation step is
shown in Figure 4b.
Figure 3. (a) Conversion curve of phenylacetylene hydrogenation and styrene
selectivity, (b) Conversion rate and selectivity of Pd, Pd-Zn@MS and ZIF-
8@MS, (c) Catalytic performance of Pd-Zn@MS for several recycles of
repeated reactions., (d) Conversion of PA; 4-NPA; DPA and selectivity of ST;
4-NST and Cis-stilbene and Trans-stilbene; correspondingly upon Pd-Zn@MS.
Figure 4. (a) Potential energy profiles of hydrogenation of PA on Pd (111)
and PdZn (111), (b) Schematic diagram of the structure of
phenylacetylene and its products on PdZn (111), (c) Schematic diagram
of the structure of phenylacetylene and its products on Pd (111).
In order to understand deeply the enhancement mechanism
of the styrene selectivity on Pd-Zn@MS, the density functional
theory (DFT) are conducted to calculate the energy change of
Pd (111) and PdZn alloy (111) facet in the hydrogenation of
phenylacetylene. The free gaseous phenylacetylene is adsorbed
on (111) facet, forming a stable adsorption state, which is the
first transition state (TS1). As shown in Figure 4, the adsorption
energy of phenylacetylene on Pd (111) is -2.08 eV. The original
triple bond of phenylacetylene and two H atoms forms a double
bond. At this time, the target product styrene in the adsorbed
state is obtained, and the energy in this state is -2.68 eV. The
next process is the desorption of adsorbed styrene on Pd (111)
or the process of excessive hydrogenation to produce
ethylbenzene, which are competing reactions. The energy
barrier to be overcome by styrene desorption on Pd (111) is 1.72
eV, and the energy barrier to continue the process of
hydrogenation to ethylbenzene is 1.04 eV. Finally, ethylbenzene
is produced with an energy of -2.53 eV. Therefore, when the Pd
NPs are used as the active site, the styrene tends to continue
hydrogenation. Even though the lower hydrogen pressure is in
favor for improving the yield of styrene, it is still difficult to obtain
the ideal styrene selectivity. The structure state corresponding to
each DFT calculation step on Pd (111) is shown in Figure 4c. As
a comparison, when the reaction was carried out on PdZn (111),
the phenylacetylene firstly reaches a stable adsorption state with
an energy of -0.59 eV. After the first step of hydrogenation, the
energy reaches -2.92 eV to form adsorbed styrene. Different
Conclusion
In summary, we successfully stabilized PdZn alloy nanoparticles
in mesoporous silica via annealing the Pd NPs@ZIF-8 in
reducing atmosphere. The obtained Pd-Zn@MS showed high
conversion rate (95%) and selectivity (92%) for semi-
hydrogenation of phenylacetylene, owing to the highly dispersed
Pd-Zn alloy nanoparticles and the mesoporous structure with
confinement effect. DFT calculations demonstrates that the
PdZn alloy nanoparticles can reduce significantly the desorption
energy barrier of styrene from 1.72 eV to 0.95 eV, compared
with the Pd nanoparticles. In addition, the mesoporous silica on
the outer layer of Pd-Zn@MS can provide stable support for the
PdZn alloy nanoparticles and avoid the agglomeration resulting
in the reduction of catalytic performance in the reaction process.
After five-cycle experiments, the conversion of phenylacetylene
and the selectivity of styrene did not decrease obviously. This
finding provides a new idea for designing a catalyst with space
confinement effect, which has practical significance to improve
the selectivity of styrene in the hydrogenation of phenylacetylene
catalyzed by traditional Pd-based catalysts.
from Pd (111), the desorption energy barrier of styrene on PdZn Experimental Section
111) is 0.95 eV, and the energy barrier of the excessive
(
hydrogenation process is 0.56 eV. Both of energy barriers are
very close, and the final energy reaches -2.91 eV while
generating ethylbenzene. Therefore, compared with Pd (111),
the tendency of styrene to desorb on PdZn (111) is greatly
increased, which is the key to the high styrene selectivity of the
Experimental Details are provided in supporting information.
4
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ChemCatChem
10.1002/cctc.202001159
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Entry for the Table of Contents
The Pd-Zn alloy nanoparticles are encapsulated into mesoporous silica (Pd-Zn@MS) for highly selective semi-hydrogenation of
phenylacetylene via annealing the Pd NPs@ZIF-8 in reducing atmosphere, which achieves up to the 95% of conversion rate and
9
2% of selectivity, because the mesoporous silica exhibits a steric hindrance effect for the reactant, and the Pd-Zn nanoparticles
shows the high selectivity for the target product styrene.
6
This article is protected by copyright. All rights reserved.