Pd Nanoparticles and Aminopolymers Confined in Hollow Silica Spheres as Efficient and Reusable Heterogeneous Catalysts for Semihydrogenation of Alkynes

Article abstract of DOI:10.1021/acscatal.8b04653

A yolk-shell nanostructured composite composed of Pd nanoparticles (NPs) and aminopolymers, poly(ethylenimine) (PEI), confined in hollow silica spheres which act as an efficient and stable heterogeneous catalyst for semihydrogenation of alkynes is reported herein. The yolk-shell nanostructured Pd-PEI-silica composite catalysts (Pd+PEI@HSS), consisting of Pd NPs core ca. 5-9 nm in diameter and a porous silica shell ca. 30-50 nm in shell thickness, are fabricated by a facile one-pot method using linear- or branched-type PEI (M_w = 1,800-2,500) as an organic template. On the basis of comprehensive structural analyses by FE-SEM, TEM, N₂ physisorption, IR, TG, and Pd K-edge XAFS, we show that metal Pd NPs and PEI molecules are encapsulated in the hollow silica sphere with a size of ca. 100-160 nm. The Pd+PEI@HSS composite shows an activity at near room temperature in the liquid-phase hydrogenation of diphenylacetylene to selectively produce cis-stilbene with 95% yield, which outperforms those of the previously reported Pd/PEI and Lindlar catalysts. Interestingly, the catalyst encapsulating linear-type PEI provides a markedly high alkene selectivity in the semihydrogenation of phenylacetylene to produce styrene owing to the strong poisoning effect of linear PEI, which is clearly revealed by an isotope study using H₂/D₂/acetylene (or ethylene) gases. The catalyst synthesized with optimum silica shell thickness can be easily recovered and recycled without any loss of palladium species and PEI and retaining high activities and selectivities over multiple cycles owing to the ability of the protective effect of silica shell, rendering this material an efficient and stable catalyst for semihydrogenation of alkynes.

Full text of DOI:10.1021/acscatal.8b04653

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Article
Pd Nanoparticles and Aminopolymers Confined in
Hollow Silica Spheres as Efficient and Reusable
Heterogeneous Catalysts for Semihydrogenation of Alkynes
Yasutaka Kuwahara, Hiroto Kango, and Hiromi Yamashita
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04653 • Publication Date (Web): 22 Jan 2019
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ACS Catalysis
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Pd Nanoparticles and Aminopolymers Confined in Hollow Silica
Spheres as Efficient and Reusable Heterogeneous Catalysts for
Semihydrogenation of Alkynes
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Yasutaka Kuwahara,†,‡ Hiroto Kango,† and Hiromi Yamashita*,†,‡
† Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-
oka, Suita, Osaka 565-0871, Japan
‡ Unit of Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520,
Japan
ABSTRACT: A yolk-shell nanostructured composite composed of Pd nanoparticles (NPs) and aminopolymers, poly(eth-
yleneimine) (PEI), confined in hollow silica spheres which act as an efficient and stable heterogeneous catalyst for semihy-
drogenation of alkynes is reported herein. The yolk–shell nanostructured Pd-PEI-silica composite catalysts (Pd+PEI@HSS),
consisting of Pd NPs core ca. 5−9 nm in diameter and a porous silica shell ca. 30−50 nm in shell thickness, are fabricated by
a facile one-pot method using linear- or branched-type PEI (Mw = 1,800~2,500) as an organic template. On the basis of
comprehensive structural analyses by FE-SEM, TEM, N2 physisorption, IR, TG, and Pd K-edge XAFS, we show that metal
Pd NPs and PEI molecules are encapsulated in the hollow silica sphere with a size of ca. 100−160 nm. The Pd+PEI@HSS
composite shows an activity at near room temperature in the liquid-phase hydrogenation of diphenylacetylene to selectively
produce cis-stilbene with 95% yield, which outperforms those of the previously-reported Pd/PEI and Lindlar catalysts. In-
terestingly, the catalyst encapsulating linear-type PEI provides a markedly high alkene selectivity in the semihydrogenation
of phenylacetylene to produce styrene owing to the strong poisoning effect of linear PEI, which is clearly revealed by an
isotope study using H2/D2/acetylene (or ethylene) gases. The catalyst synthesized with optimum silica shell thickness can
be easily recovered and recycled without ant loss of palladium species and PEI and retaining high activities and selectivities
over multiple cycles owing to the ability of the protective effect of silica shell, rendering this material an efficient and stable
catalyst for semihydrogenation of alkynes.
KEYWORDS: yolk-shell nanostructure, porous materials, Pd nanoparticles, aminopolymer, semihydrogenation
Owing to the strong hydrogenation ability of Pd, alkynes
are readily over-hydrogenated to give the corresponding
1. INTRODUCTION
undesirable alkanes over naked Pd NPs.11 Selectivity can be
improved by controlling the adsorption kinetics of the re-
actants (i.e. alkynes and alkenes) on Pd surface, which can
typically be manipulated by introducing organic modifiers
containing N or S atoms that can strongly coordinate on
Pd NPs.12-15 For example, Pd NPs stabilized with alkylthiol
surfactants show different selectivity in the liquid-phase
hydrogenation of alkynes and allyl alcohols.16-17 Addition of
dimethyl sulfoxide (DMSO) is known to improve the al-
kene selectivity by acting as homogeneous modifiers.18-19
Amine-based dendrimers have been used as effective scaf-
folds to immobilize Pd NPs to demonstrate selective hy-
drogenation of alkynes.20-21 Recently, poly(ethyleneimine)
(PEI) emerged as promising supports for Pd NP catalysts.22-
Selective hydrogenation of alkynes to produce alkenes is
an important and fundamental reaction for the synthesis
of commodity chemicals and fine chemicals.1-3 Although a
large number of homogeneous/heterogeneous catalysts
have been developed for semihydrogenation of alkynes,4-5
overwhelming majority employed for liquid-phase semihy-
drogenation of alkynes is Pd nanoparticle (NP) catalyst.6-8
The Lindlar catalyst (Pd/CaCO3 treated by Pb salts) has
long been utilized as a benchmark heterogeneous cata-
lyst.9-10 Lead is used as a modifier to promote selective re-
duction of alkynes to cis-alkenes, and a basic compound
such as quinoline is often added as an organic modifier in
the reaction solution to retard the subsequent hydrogena-
tion of cis-alkenes to alkanes; however, this catalyst suffers
from some critical drawbacks such as high toxicity of Pb
and the low alkene selectivity toward terminal alkynes. De-
velopment of alternative, greener catalysts while achieving
high alkene selectivity and acceptable reaction rate has
been a grand challenge for this reaction.
24
Abundant amine groups in its polymer chain efficiently
coordinate and immobilize Pd NPs, enabling chemoselec-
tive semihydrogenation of both internal and terminal al-
kynes to afford the corresponding alkenes in the presence
of 1 atm H2.22-23 Although these surfactant-capped and pol-
ymer-stabilized Pd NPs show efficient and tunable cata-
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lytic properties due to their adequate coordination effi-
Page 2 of 15
to strongly coordinate on metal NPs due to the presence of
lone pairs on N atoms of amines.22-23
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ciency, the inherent difficulties in separation and leaching
of modifiers as well as sintering of Pd species during cata-
lytic use limit their practical application.
In this study, we report a synthesis of new type of yolk-
shell nanostructured composite composed of Pd NPs and
aminopolymers, PEI, confined in hollow silica spheres
(Pd+PEI@HSS) for selective hydrogenation of alkynes to
produce alkenes. The yolk–shell nanostructured catalyst
was for the first time fabricated via a facile one-pot synthe-
sis by using linear or branched PEI as a self-template. The
resulting composite was found to act as an efficient and re-
usable catalyst in the semihydrogenation of internal/ter-
minal alkynes in the presence of 1 atm of H2 while retaining
high alkene selectivity and minimizing the unfavorable
over-hydrogenation, which far outperformed those
achieved on the prototype supported Pd NP catalysts. Ki-
netic analysis and isotope experiment were examined to
understand the role of PEI in tuning the alkene selectivity.
Furthermore, leaching and reusability tests were per-
formed to verify the ability of the silica shell to protect the
encapsulated components.
Recently, much effort has been directed toward develop-
ment of recoverable and reusable heterogeneous catalytic
systems with good activity and selectivity.3 Silica has typi-
cally been utilized as an inert oxide support to immobilize
25-26
Pd NPs with good dispersion and stability.21,
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Mitsudome et al. synthesized silica-supported Pd NPs cov-
ered with alkyl sulfoxide groups-containing silica shell for
selective semihydrogenation of internal/terminal al-
kynes.27 Amine-functionalized silicas have been used to
immobilize Pd NPs due to a strong metal-support interac-
tion; however, high alkene selectivity was unachievable
without the addition of homogeneous organic modifiers.19,
28 Jones et al. synthesized Pd-PEI-silica composite catalyst
by immobilizing Pd NPs into a SBA-15 mesoporous silica
functionalized with PEI polymer, which showed excellent
activity and selectivity in the liquid- phase hydrogenation
of diphenylacetylene. 29 However, leaching of amine-func-
tional groups and Pd species during catalytic use through
open-ended pores and the associated catalyst deactivation
still remain as a major concern.
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2. EXPERIMENTAL SECTION
2.1. Chemicals and Materials. Tetraethoxy orthosilicate
(TEOS, 95%), disodium tetrachloropalladate(II) (Na2PdCl4,
>95%), palladium(II) chloride (PdCl2, 99%), palladium(II)
acetate (Pd(OAc)2, >97%), sodium borohydride (NaBH4,
>95%), aqueous ammonia solution (28%), methanol
(99.8%), ethanol (99.5%), 1,4-dioxane (99.5%) and 8-hexa-
decyne (>99%) were purchased from Nacalai Tesque Inc.
Poly(ethyleneimine) (linear, Mw = 2,500, 99%) and diphe-
nylacetylene (98%) was purchased from Sigma-Aldrich.
Poly(ethyleneimine) (branched, Mw = 600, 1,800, and
10,000, 99%) was purchased from Alfa Aesar. Phenylacety-
lene (>97%), methyl phenylpropiolate (>98%), ethyl phe-
nylpropiolate (>97%), phenylpropiolic acid (>98%) and
Lindlar catalyst (5% Pd on CaCO3 poisoned with 2.4-3.0%
Pb) as a reference catalyst were purchased from Tokyo
Chemical Industry Co., Ltd. 20% H2/Ar, 20% D2/Ar, 10%
C2H2/Ar, and 10% C2H4/Ar gas used for gas-phase hydro-
genation experiment were purchased from Taiyo Nippon
Sanso Corp.
Herein, we envisioned that hollow silica spheres having
enclosed cavity spaces could be a reasonable option to im-
mobilize Pd NPs together with PEI, since the silica shell is
expected to act as a protective barrier to circumvent the
sintering of Pd NPs and leaching of PEI during the cataly-
sis, leading to an improved stability and reusability of cat-
alyst. Hollow silica encapsulating catalytic components in
their internal void spaces (so-called “yolk–shell”
nanostructures) is emerging category of nanostructured
materials.30-37 In most studies on yolk-shell nanostructured
catalysts, the enclosed inner void spaces are used for en-
capsulation of active catalytic components (such as metal
NPs,38-43 metal oxides,44-46 and metal complexes47-48); how-
ever, encapsulation of bulky functional polymers for im-
provement of catalytic performances has rarely been exam-
ined. Although a number of strategies have been developed
for the fabrication of yolk-shell silica nanostructures, most
of them requires complex multiple synthetic procedures,
such as ship-in-bottle,49 hard templating,50-51 and selective
etching approaches.38, 44, 52-53 Furthermore, selective intro-
duction of PEI polymer with large molecular weight within
the confined nanospace of hollow silica via a post-synthetic
approach is quite difficult.39, 54 In this context, He et al. re-
cently developed a facile one-pot method to encapsulate
metal NPs in hollow silica spheres with a size of about 100
nm using poly(acrylic acid) (PAA) as a sole organic tem-
plate.55 PAA acted as a core template and as a stabilizing
agent of the metal NPs through the coordination interac-
tion between the carboxylate groups on the chain ends and
the empty orbital of the metals, and a silica shell was sub-
sequently created around the PAA-metal NP aggregate to
form yolk-shell nanostructured silica particle. This prece-
dent work inspired us to develop a new method to synthe-
size hollow silica spheres encapsulating Pd NPs and PEI us-
ing PEI as an organic template, since PEI also has an ability
2.2. Characterization. Field-emission scanning electron
microscopy (FE-SEM) images were taken with a JEOL JSM-
6500F operating at 12.0 kV. Transmission electron micros-
copy (TEM) images were taken with a Hitachi H-800 FE-
TEM operating at 200 kV. Scanning transmission electron
microscope (STEM) images and elemental maps were col-
lected on a FEI TITAN80-300 instrument operated with an
accelerating voltage of 200 kV and quipped with an EDAX
r-TEM/SuperUTW energy-dispersive X-ray detector. The
Pd loadings of the samples were determined by inductively
coupled plasma atomic emission spectrometry (ICP-AES).
The PEI content of the samples were determined from the
thermogravimetric (TG) analysis data measured on a
Rigaku Thermo plus EVO2 under a flow of air (30 mL/min)
with a ramping rate of 10 °C/min. Infrared (IR) spectra were
collected on a JASCO FT/IR-6300 instrument under vac-
uum using samples diluted with KBr. Nitrogen adsorption–
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ACS Catalysis
desorption isotherms were measured at –196 °C with a Mi- by adding varied amounts of TEOS (0.9, 1.35, 1.8, and 2,7
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crotracBEL BELSORP-max system. Prior to measurements,
samples were degassed at 120 °C under vacuum for 3 h to
vaporize the physisorbed water. The surface area was cal-
culated from the adsorption data in the range p/p0 = 0.05-
0.25 using the Brunauer–Emmett–Teller (BET) method,
and the pore size distributions were calculated by using the
Barrett–Joyner–Halenda (BJH) method. Powder X-ray dif-
fraction (XRD) patterns were recorded on a Rigaku Ultima
IV diffractometer with CuKα radiation (λ = 1.54056 Å).
mL) using linear PEI as a template polymer. The thus ob-
tained samples were denoted as Pd+PEI(L)@HSS_X, where
X represents the volume of TEOS added in mL.
2.4. Preparation of Pd/PEI. PEI-supported Pd(0) NPs
were synthesized according to the method previously re-
ported in literature.22 Typically, 0.12 g of linear PEI (Mw =
2,500) was completely dissolved in EtOH (90 mL) with
magnetic stirring at room temperature, and 0.8 mL of
aqueous solution of Na2PdCl4 (20 mg/mL) was added in Ar
atmosphere, followed by ultrasonication and stirring for 10
min. The resulting solution was stirred in a continuous
flow of H2 (10 mL/min) at room temperature for 24 h to
yield a black colloidal solution. After removal of EtOH by
evaporation, a black gummy Pd/PEI was obtained. The Pd
loading is 4.6 wt% based on the incipient ratio of Na2PdCl4
and PEI.
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The X-ray absorption fine structure (XAFS) measure-
ments for Pd K-edge were performed in fluorescence mode
at the BL01B1 beamline at the SPring-8 (JASRI), Hyogo, Ja-
pan. A double Si(111) crystal monochromator was used for
energy selection and a Pd foil was used for energy calibra-
tion. Fluorescence Pd K-edge XAFS spectra were collected
at room temperature with a Lytle detector. The extended
X-ray absorption fine structure (EXAFS) data were normal-
ized by fitting the background absorption coefficient. Fou-
rier transformation of k3-weighted normalized EXAFS data
(FT-EXAFS) from k space to r space was performed over
the range of 3.0 ≤ k (Å−1) ≤ 11.0 to obtain the radial structure
functions (RDFs).
2.5. Liquid-phase Semihydrogenation of Alkynes.
Catalytic reactions were performed at 30 °C in a schlenk
flask connected to a water-cooling condenser under a con-
tinuous flow of 1 atm H2. In a typical reaction procedure,
as-prepared Pd+PEI@HSS (ca. 0.05 g: Pd 0.005 mmol) was
placed in the flask, and the reactor was purged with H2 gas
for 10 min. Next, alkyne (1.0 mmol) dissolved in MeOH (5
mL) and 1,4-dioxane (5 mL) was injected into the flask via
a syringe to initiate the reaction. During the reaction, the
reaction mixture was magnetically stirred (600 rpm) at 30±
0.5 °C under a continuous flow of 1 atm H2 (10 mL/min). A
small aliquot of reaction solution was withdrawn and the
amounts of reactant and products were quantified by a gas
chromatograph (GC; Shimadzu, GC-14B) equipped with a
flame ionization detector (FID) and a Zebron ZB-WAX
plus capillary column using biphenyl as an internal stand-
ard. To examine the catalyst reusability, the spent catalyst
was recovered by centrifugation, washed with MeOH,
dried under vacuum at room temperature, and then sub-
jected to multiple catalytic cycles.
2.3. Preparation of Pd-PEI-Silica Composites
(Pd+PEI@HSS). Yolk-shell nanostructured Pd+PEI@HSS
composites were synthesized by an one-pot method devel-
oped based on Stöber method55-56 using linear- or branched
PEI as an organic template, as schematically shown in
Scheme 1. In a typical procedure, 0.12 g of PEI was dissolved
in aqueous ammonia (2.8 mL, 28%) with magnetic stirring
at room temperature, and 0.8 mL of aqueous solution of
Na2PdCl4 (20 mg/mL) and 90 mL of EtOH were subse-
quently added, followed by vigorous stirring for 30 min to
obtain a transparent solution (Scheme 1, step 1; see also the
Supporting Information, Figure S1a). Next, 2.0 mL of
freshly-prepared aqueous NaBH4 solution (20 mg/mL) (ap-
proximately 20 equiv. of Pd) was slowly added to obtain
brownish suspension (step 2; see also Figure S1b). To this
solution was rapidly added 1.8 mL of TEOS as a Si source
under vigorous stirring (step 3), followed by continuous
stirring for 6 h at room temperature to ensure silica shell
formation (step 4; see also Figure S1c). The resulting pre-
cipitate was recovered by centrifugation, washed with DI
water (50 mL) and EtOH (50 mL), and dried under vacuum
overnight to afford Pd+PEI@HSS as a greyish powder.
2.6. Gas-phase Hydrogenation Experiment. Gas-
phase hydrogenation of acetylene and ethylene in the flow
of H2/D2 gases was carried out at 120 °C in a temperature-
programmed desorption (TPD) system (BELCAT-II, Micro-
tracBEL Corp.). About 0.05 g of catalyst (ca. Pd 0.005
mmol) was mounted in a pyrex reactor and pre-reduced at
12o °C under H2 flow (20 mL/min) for 30 min. In the initial
step, mixed gas of H2 and D2 balanced with Ar (H2:D2 = 1:1
volume ratio, 4.3 kPa H2, 4.3 kPa D2, 91.4 kPa Ar) was
flowed (total flow: 50 mL/min) under the same tempera-
ture condition to check H2-D2 isotope exchange reaction.
After approximately 10 min reaction, either C2H2 or C2H4
gas was flowed simultaneously with H2/D2/Ar
((H2+D2)/C2H2 (or C2H4) = 1.5 volume ratio, 4.3 kPa H2, 4.3
kPa D2, 2.9 kPa C2H2 (or C2H4), 88.5 kPa Ar) (total flow: 50
mL/min). The generated species (m/z = 3 for HD, m/z = 28-
30 for ethylene isotopes, m/z = 30-34 for ethane isotopes)
were monitored using an online mass spectrometer (Mi-
crotracBEL BELMass)
To investigate the impacts of PEI type on the structure
and the catalytic performance of the final solid, synthesis
was performed by using linear PEI (Mw = 2,500) and
branched PEI (Mw = 1,800), which were denoted as
Pd+PEI(L)@HSS and Pd+PEI(B)@HSS, respectively. Fur-
thermore, these samples were calcined at 550 °C in air for
5 h to remove organic components, which were then re-
duced at 200 °C in a flow of H2 (20 mL/min) to give Pd(0)
NPs encapsulated in hollow silica spheres (denoted as
Pd+PEI(L/B)@HSS_cal). To obtain Pd+PEI@HSS with dif-
ferent silica shell thickness, synthesis was also performed
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Step 1
Step 2
Step 3
Step 4
Pd NPs
porous SiO2
PEI
1
2
3
PEI
Pd NPs
Pd2+
Pd2+
+ NaBH4
+ TEOS
Aging
Pd2+
4
5
Pd2+
6
Pd ions + PEI
Pd NPs-PEI aggregates
Pd+PEI(L/B)@HSS
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8
9
Template
Si source
Et
H
N
NH2
O
H
N
n
N
N
H2N
N
N
H
N
H
O
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NH2
N
Et
=
Et
Si
H
O
O
NH2
NH2
n
Et
Poly(ethyleneimine)
(Linear)
Poly(ethyleneimine)
(Branched)
Tetraethoxy orthosilicate
(TEOS)
Scheme 1. Schematic representation of the synthetic procedure of hollow silica spheres encapsulating Pd NPs and poly(ethylene-
imine) (Pd+PEI@HSS).
and 107 nm for Pd+PEI(L)@HSS and Pd+PEI(B)@HSS,
respectively, which are interconnected with each other
3. RESULTS AND DISCUSSION
3.1.
Preparation
and
Characterization
of
(Figure 1a and d). TEM images clearly show that the silica
spheres have hollow nanostructure with an silica shell
thickness of ca. 49 nm and 36 nm for Pd+PEI(L)@HSS and
Pd+PEI(B)@HSS, respectively, and Pd NPs are observed in
their hollow spaces (Figure 1b,c,e and f) (for the
distribution diagrams of silica particle size, silica shell
thickness, and Pd NP size, see Figure S2). Hollow silica
particles without encapsulating Pd NPs were scarcely
observed. This indicates that silica formation is directed by
the preformed Pd-PEI aggregates as nuclei as well as the
electrostatic interaction between amine groups on the pol-
ymer chain and TEOS.55 Branched-type PEI afforded
smaller silica particles with thinner silica shells, which
might be due to the lower molecular weight of PEI than
that of linear PEI used for synthesis. The average diameter
of Pd NPs was determined to be ca. 8.9 nm and 6.0 nm for
Pd+PEI(L)@HSS and Pd+PEI(B)@HSS, respectively;
however, no clear characteristic peaks for Pd species were
observed in XRD patterns, indicating that Pd NPs are
highly dispersed (for XRD patterns, see Figure S3).
Pd+PEI@HSS composites.
Scheme 1 schematically depicts the synthetic procedure
of Pd+PEI@HSS composites using linear- or branched PEI
as an organic template. In the first step, a transparent so-
lution was obtained after mixing of PEI and Pd precursor
in aqueous ammonia solution (Figure S1a), which immedi-
ately turned brown after the addition of NaBH4 (Figure
S1b), indicating that PdCl42- ions entrapped by the nitrogen
ligands on the polymer chain are reduced to form Pd NP-
PEI aggregates. In a following step, a brownish milky sus-
pension was obtained in a few minutes after the addition
of TEOS as a Si precursor (Figure S1c), representing the
condensation of TEOS to form silicate network around the
Pd-PEI aggregates as nuclei, which is catalyzed by NH3
added in the solution as a base. The supernatant solution
collected after the recovery of the solid was colorless, sug-
gesting that nearly all of Pd had been incorporated in the
product.
(a)
(b)
(c)
The morphology of final solid was strongly dependent on
the molecular weight of PEI and the kind of Pd precursor
employed; branched PEI with smaller molecular weight
(Mw = 600) gave solid silica spheres without hollow cavities
(Figure S4a). Use of branched PEI with larger molecular
weight (Mw = 10,000) resulted in the formation of
amorphous silica without any defined nanostructure
(Figure S4c). When other Pd(II) precursors, such as
H2PdCl4 or Pd(OAc)2 were used, aggregates of small silica
particles with ununiformlly-distributed Pd NPs were
obtained (see Figure S5). Thus, appropriate choice of PEI
and Pd precursor is needed to synthesize yolk-shell
nanostructured Pd-PEI-silica composite, since the
coordination interaction between Pd species and amine
sites on PEI polymer differs depending on the kind of Pd
precursor, and the size of Pd-PEI aggregate can be varied
depending on the molecular weight of PEI.
500 nm
100 nm
100 nm
(d)
(e)
(f)
500 nm
100 nm
100 nm
Figure 1. (a, d) FE-SEM images, (b, e) TEM images and (c, f)
magnified TEM images of Pd+PEI@HSS synthesized with (a–
c) linear PEI (Mw = 2,500) and (d–f) branched PEI (Mw = 1,800).
FE-SEM images show that Pd+PEI@HSS is composed of
spherical silicas with an average particle size of ca. 155 nm
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ACS Catalysis
(a)
(d)
(b)
(e)
(c)
(g)
Pd NPs
PEI
Pd
1
2
3
4
5
6
7
8
direction
Si
O
C
N
Pd
(f)
9
Si
O
N
10
11
12
13
14
15
16
17
0
20 40 60 80 100 120 140 160
Distance / nm
Figure 2. (a) STEM image, (b) High-angle annular dark-field STEM (HAADF-STEM) image, (c-f) the corresponding STEM ele-
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mental maps of (c) Pd, (d) Si, (e) O, and (f) N of Pd+PEI(L)@HSS. (g) STEM elemental line scan across the Pd+PEI(L)@HSS particle
in (b).
Table 1. Compositional and Textural Properties of Pd+PEI@HSS Composites
Pd contenta
(wt%)
PEI contentb
(wt%)
N2 physisorption
Vtotald(cm3/g)
Samples
PEI type
SBETc (m2/g)
De (nm)
2.7
Pd+PEI(L)@HSS
Pd+PEI(L)@HSS_cal
Pd+PEI(B)@HSS
Pd+PEI(B)@HSS_cal
Lindlar catalystf
Linear, Mw = 2,500
1.1
1.5
1.1
24.0
0
34
19
0.12
-
0.09
2.8
Branched, Mw = 1,800
21.9
0
35
28
30
0.16
2.8
-
-
1.4
5.0
0.15
2.6
0
0.09
2.7
a Determined by ICP-AES. b Determined by TG analysis. c Surface area calculated by BET method. d Total pore volume reported at
p/p0=0.99. e Peak pore size determined by BJH method. f 5% Pd on CaCO3 (poisoned with 3% Pb).
In the HAADF-STEM micrograph of Pd+PEI(L)@HSS
(Figure 2), Pd NPs with more electron density located
inside the hollow cavities, mostly on the inside silica wall,
are clearly observed (Figure 2b). Whole area mapping
analysis visualizes that the distributions of Si atoms with
blue dots (Figure 2d) and O atoms with pink dots (Figure
2e) overlap with the shell region, and Pd atoms with yellow
dots (Figure 2c) are seen in the hollow region, again
evidencing the formation of yolk-shell nanostructure
consisting of Pd NPs as a core and silica as a shell. N atoms
with white dots (Figure 2f) are uniformly distributed
throughout the particles, indicating that the PEI is
incorporated in both shell and hollow regions. The line
mapping of a selected silica particle confirms the inclusion
of N, C and Pd atoms in the same axial region (Figure 2g),
suggesting a close proximity betwen Pd NPs and PEI.
21.9 wt% for Pd+PEI(L)@HSS and Pd+PEI(B)@HSS,
respectively, from thermogravimetric (TG) data (Figure
3B), and the Pd contents were determined to be 1.1 wt% in
both cases (see Table 1) by elemental analysis, confirming
the majority of Pd and PEI added during the synthsis was
incorporated in the final solid.
The porous structure of Pd+PEI@HSS materials was
investigated by N2 physisorption measurement. A clear
hysteresis observed in the range of 0.4 < p/p0 < 1.0 in N2
adsorption-desorption isotherms indicates the existence of
hollow cavities surrounded by mesopore systems (Figure
3C).57 The pore size distributions calculated by the BJH
method confirm the existence of broadly-distributed
mesopores with average pore size of ca. 2.7 nm (Figure 3D)
probably due to the formation of hydroxylated amorphous
silica network. The textural properties obtained from the
N2 physisorption analysis are listed in Table 1. BET surface
area (SBET) and total pore volume (Vtotal) were determined
to be 34 m2/g and 0.12 cm3/g for Pd+PEI(L)@HSS and 35
m2/g and 0.16 cm3/g for Pd+PEI(L)@HSS, showing that the
type of PEI had little influence on the porous structures of
the final solid. Nitrogen adsorption data of calcined
In FT-IR spectra of Pd+PEI@HSS (Figure 3A), the IR
bands at 2962 and 2858 cm−1 assigned to the stretching
vibration of aliphatic C−H bond, the band at 1467 cm−1
assigned to the bending vibration of C−H bond, and the
peak at 1560 cm−1 assignable to the bending vibration of
N−H bond are observed, which completely disappear after
thermal treatment at 550 °C in air, further ascertaining the
successful incorporation of PEI in the as-synthesized
material. The PEI contents were estimated to be 24.0 and
samples
(i.e.
Pd+PEI(L)@HSS_cal
and
Pd+PEI(L)@HSS_cal) shows substantial decreases in
surface area and pore volume (Table 1), which might be due
ACS Paragon Plus Environment

ACS Catalysis
to the thermal shrinkage of silica network to create denser (A)
silica shells (for the comparison of N2 physisorption
isotherms of as-synthesized and calcined samples, see
Figure S6).
Page 6 of 15
(B)
1
2
3
4
5
6
7
8
Pd-Pd
Pd foil
(A)
(B)
5
Pd-N
Pd-O
Pd+PEI(L)@HSS
Pd+PEI(B)@HSS
Pd+PEI(L)@HSS
Pd+PEI(B)@HSS
0
(a)
(b)
-5
Pd foil
-10
-15
-20
-25
-30
Pd+PEI(L)@HSS
Pd+PEI(B)@HSS
PdO
PdO
(c)
9
Pd(NH3)2Cl2
Pd(NH3)2Cl2
10
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13
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24300
24350
24400
Photon energy / eV
24450
0
1
2
3
4
5
6
Interatomic distance / Å
4000
3000
2000
1000
0
100 200 300 400 500 600
-1
Temperature / ℃
Wavenumber / cm
Figure 4. (A) Pd K-edge XANES spectra and (B) radial distri-
bution functions (RDFs) obtained from Pd K-edge EXAFS os-
cillations for Pd+PEI@HSS composites and reference Pd sam-
ples.
(C) 150
(D) 0.025
0.020
Pd+PEI(L)@HSS
Pd+PEI(B)@HSS
Pd+PEI(L)@HSS
Pd+PEI(B)@HSS
2.7 nm
100
50
0
0.015
3.2. Catalytic Performance of Pd+PEI@HSS compo-
sites.
+0.01
2.8 nm
+50
0.010
0.005
The synthesized Pd+PEI@HSS catalysts were initially as-
sessed in the semihydrogenation of diphenylacetylene (1)
with 0.5 mol% Pd in a flow of atmospheric pressure of H2
at 30 °C. The reaction profiles over the Pd+PEI@HSS cata-
lysts, the calcined analogues and some supported Pd cata-
lysts are shown in Figure 5. The hydrogenation of
diphenylacetylene readily took place without any
induction period, indicating that Pd species encapsulated
in Pd+PEI@HSS samles are predominantly present as
metallic Pd(o) NPs as confirmed by Pd K-edge XAFS. Sur-
prisingly, the as-synthesized Pd+PEI(L/B)@HSS catalysts
both selectively afforded cis-stilbene (2) with >90% selec-
tivities due to the syn-addition of H atoms over Pd(o) NPs
surface, and trans-stilbene (3) and bibenzyl (4) were hardly
produced even after the complete consumption of diphe-
nylacetylene (Figure 5A and C). On the other hand, cis-stil-
bene was quickly transformed into bibenzyl after the com-
plete conversion of diphenylacetylene over the
Pd+PEI(L/B)@HSS calcined in air, PEI-free analogues (Fig-
ure 5B and D). This result clearly demonstrates that PEI is
capable of efficiently suppressing the over-hydrogenation
of stilbene to bibenzyl owing to the strong coordination
ability onto Pd NP, which is confirmed by XAFS analysis. It
is worth mentioning that the calcined catalysts both
showed obviously faster hydrogenation rates compared
with the as-synthesized catalysts, due to the removal of PEI
covering the surface of Pd NPs. Considering the fact that
Pd NPs are the sole active component existing in the pre-
sent catalytic systems, Pd NPs inside the hollow silica
spheres are accessible to reactants through ill-defined mes-
opores created in the silica shell, and PEI protects the Pd
NPs without fully preventing the access to the reactants.
Pd+PEI(B)@HSS provided a slightly faster reaction rate
compared with Pd+PEI(L)@HSS, which might be ex-
plained by the formation of thinner silica shell (36 nm vs.
49 nm) and smaller Pd NP size (6.0 nm vs. 8.9 nm).
0.000
0.0
0.2
0.4
0.6
0.8
1.0
0
5
10 15 20 25 30
Pore diameter / nm
p/p0
Figure 3. (A) FT-IR spectra of (a) PEI(L), (b) Pd+PEI(L)@HSS,
and (c) Pd+PEI(L)@HSS_cal. (B) Weight loss curves, (C) N2
adsorption-desorption isotherms, and (D) the corresponding
pore size distribution curves determined by BJH method for
as-synthesized Pd+PEI@HSS composites.
Figure 4A and 4B show X-ray absorption near-edge
spectra (XANES) and RDFs obtained from the k3-weighted
Pd K-edge EXAFS oscillations of the prepared samples,
respectively, together with those of reference Pd samples.
The X-ray absorption edges of the Pd+PEI@HSS samples
(E0 = 24355.5 eV) are positioned between the absorption
edges of Pd(0) foil (E0 = 24352.1 eV) and Pd(II)O (E0 =
24358.3 eV), and the height of white line intensity increases
in the order of Pd(0) foil < Pd+PEI(L/B)@HSS < Pd(II)O
(Figure 4A), indicating the coexistence of Pd(0) and Pd(II)
species in the Pd+PEI@HSS samples. In the RDFs of Pd K-
edge EXAFS spectra, both Pd+PEI@HSS samples exhibit
two distinct peaks at around r = 1.6 and 2.5 Å (the phase
shift uncorrected); the first shell at around r = 1.6 Å is
associated with the backscattering due to the neighboring
nitrogen or oxygen atoms and the second shell at around r
= 2.5 Å is attributable to the Pd–Pd bond (Figure 4B). Since
the peak ascribed to Pd–O–Pd bond, which typically
appear at around r = 3.0 Å, is scarcely observed, the former
peak (at r = 1.6 Å) can dominantly be ascribed to the Pd(II)
species ligated to the nitrogen moieties of the polymer
framework.58 These results, combined with elemental
mapping analysis, indicate that the Pd species
encapsulated inside hollow silica are primarily present as
Pd(0) NPs of which surface is closely surrounded by PEI
polymers. The XAFS result also suggests that the the
oxidation state of Pd species does not markedly vary
depending on the type of PEI used.
Table 2. Comparison of Catalytic Performances of Pd+PEI@HSS Catalysts and Lindlar Catalyst
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1
ACS Catalysis
Hydrogenation of diphenylacetylene
Hydrogenation of phenylacetylene
a
b
NPd
Nsurf
c
d
c
d
Samples
RY
RE
RY
RE
(mmol/g)
(mmol/g)
RY/RE
RY/RE
(h-1)
4693
770
(h-1)
< 1
(h-1)
4111
(h-1)
260
2
3
4
5
6
7
8
9
Pd+PEI(L)@HSS
Pd+PEI(L)@HSS_cal
Pd+PEI(B)@HSS
Pd+PEI(B)@HSS_cal
Lindlar catalyst
a
0.103
0.141
0.103
0.132
0.470
0.0016
0.0110
0.0020
0.0135
0.0566
N/A
2.25
N/A
0.63
18.4
15.8
0.65
0.88
0.80
0.96
343
< 1
594
910
3065
1758
1510
4016
6751
1866
4555
8408
1952
2809
82
b
Number of Pd atoms contained per gram of catalyst. Number of surface-exposed Pd atoms contained per gram of catalyst.
Determined by CO pulse measurement with MicrotracBEL BEL-METAL-1 system at 50 °C using 1% CO/He gas. c Hydrogenation
rate of alkyne calculated from the slope of the initial stage of reaction (conversion < 50%); RY = (mol of alkyne consumed)/(mole
of surface Pd atoms × time). d Hydrogenation rate of alkene calculated from the slope of the initial stage of reaction (conversion <
50%); RE = (mol of alkene consumed)/(mol of surface Pd atoms × time).
10
11
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59
60
silica-free analogue provided an excellent alkene selectivity
(96% (cis : trans = 97 : 3)), but the reaction rate was
apparently lower than that of Pd+PEI(L)@HSS, probably
1
due to the larger size of Pd NPs (Figure 5E).
(A) 100
(B)100
89.8%
Hydrogenation rates per surface Pd atoms were
calculated based on the number of surface-exposed Pd
atoms determined by CO pulse measurement, which are
summarized in Table 2. The hydrogenation rate of
diphenylacetylene per surface Pd (RY) was calculated to be
4693 and 3065 h-1 for Pd+PEI(L)@HSS and
Pd+PEI(B)@HSS, respectively, which were higher than
that of Lindlar catalyst (1510 h-1) and previously-reported
Pd-PEI-silica composite catalyst (3027 h-1).29 In
semihydrogenation of alkyne, hydrogenation rate of alkyne
relative to that of alkene (RY/RE) can be a good indicator of
alkene selectivity. Hydrogenation of cis-stilbene was
hardly observed over Pd+PEI(L/B)@HSS catalysts (RE < 1 h-
1), which thereby resulted in significantly higher RY/RE
values than others. Thus, the yolk-shell nanostructured
Pd-PEI-silica composites developed in this study provide
outstanding activity and alkene selectivity in the
hydrogenation of internal alkyne, which outperform those
of the conventional supported Pd catalysts.
Pd+PEI(L)@HSS
Pd+PEI(L)@HSS calcined
75.0%
80
60
40
20
0
80
60
40
20
0
1
2
3
4
1
2
3
4
0
100 200 300 400 500
0
100 200 300 400 500
Time / min
Pd+PEI(B)@HSS
Time / min
(C) 100
(D)100
94.7%
calcined
86.7%
Pd+PEI(B)@HSS
80
60
40
20
0
80
60
40
20
0
1
2
3
4
1
2
3
4
0
100 200 300 400 500
Time / min
0
20
94.1%
40
60
80
100
Time / min
(E) 100
(F) 100
Pd/PEI(L)
Lindlar catalyst
78.6%
80
60
40
20
0
80
60
40
20
0
1
2
3
4
The yolk-shell nanostructured composite catalysts were
also applicable to the selective hydrogenation of
phenylacetylene (5) as a terminal alkyne (Figure 6).
Pd+PEI(L)@HSS and Pd+PEI(B)@HSS catalysts showed
phenylacetylene hydrogenation rates (RY) of 4111 and 4016
h-1, respectively, which were similar to those obtained in
diphenylacetylene hydrogenation, indicating similar reac-
tivity of Pd atoms toward internal and terminal alkynes. A
marked difference was observed in selectivity of alkene.
When Pd+PEI(L)@HSS catalyst synthesized with linear
PEI was used, styrene (6) was selectively produced with
excellent selectivity (87%), affording 84.3% yield of styrene
after 240 min of reaction, and subsequent hydrogenation
into ethylbenzene (7) was significantly suppressed even af-
ter the complete consumption of phenylacetylene (Figure
6A). Such a retention of high styrene selectivity was not
observed for the PEI-removed samples (Figure 6B, D),
demonstrating that the surface poisoning of Pd NPs with
PEI(L) is the main cause for the improved alkene selectiv-
ity. On the other hand, Pd+PEI(B)@HSS synthesized with
branched PEI showed a significant decrease in styrene
1
2
3
4
0
100 200 300 400 500
Time / min
0
100 200 300 400 500
Time / min
Figure 5. Reaction kinetics in the semihydrogenation of di-
phenylacetylene over (A) Pd+PEI(L)@HSS, (B)
Pd+PEI(L)@HSS calcined in air, (C) Pd+PEI(B)@HSS, (D)
Pd+PEI(B)@HSS calcined in air, (E) Pd/PEI(L), and (F) Lindlar
catalyst (+5 mol% quinoline). Reaction conditions: catalyst (Pd
0.5 mol%), 1 (1 mmol), MeOH:1,4-dioxane = 1:1 (10 mL), 30 °C,
under a flow of 1 atm H2 (10 mL/min). 5 mol% quinoline was
added for Lindlar catalyst.
The Lindlar catalyst (with 5 mol% quinoline) as a
benchmark catalyst provided 94.1% selectivity toward cis-
stilbene in 70 min of reaction, but afterward showed a
gradual decrease of selectivity due to the over-
hydrogenation into bibenzyl (Figure 5F). Pd/PEI(L) as a
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ACS Catalysis
of
Page 8 of 15
selectivity
after
the
complete
conversion
NPs more effectively than branched PEI does. In order to
clarify the difference of PEI polymers in controlling alkene
1
2
3
4
5
6
7
8
phenylacetylene, affording ethylbenzene as a main product
(cf. Figure 6C). A silica-free analogue, Pd/PEI(L), showed a
similar reaction trend, but showed a lower styrene yield
(75.9% at a maximum) and a faster reaction rate in the sec-
ond hydrogenation step to produce ethylbenzene com-
pared with Pd+PEI(L)@HSS (Figure 6E). Lindlar catalyst
was totally ineffective for the selective production of sty-
rene under the identical conditions implemented in this
study, giving 100% yield of ethylbenzene after the extended
reaction time (120 min) (Figure 6F).
selectivity, we carried out gas-phase hydrogenation of acet-
ylene and ethylene in a co-flow of H2 and D2 under accel-
erated condition (120 °C), which provided more pro-
nounced product distributions with sufficient signal inten-
sities.12 Time courses of the generated species produced
during the H2-D2 exchange reaction and the subsequent
gas-phase hydrogenation reaction of acetylene and eth-
ylene in the co-flow of H2 and D2 over Pd+PEI(L)@HSS,
Pd+PEI(B)@HSS, and calcined Pd+PEI(L)@HSS are sum-
marized in Figure 7. Under the continuous flow of H2 and
D2, H2-D2 exchange reaction to produce HD (m/z = 3) took
place over all three types of catalysts, evidencing that dis-
sociative adsorption of H2 (D2) on Pd NPs are possible even
in the presence of PEI. Production of a larger quantity of
HD over calcined Pd+PEI@HSS is due to the removal of
protective PEI ligand. After the introduction of acetylene
(Figure 7A), substantial formations of ethylene isotopes
(nondeuterated and deuterated, m/z = 28-30) and ethane
isotopes (nondeuterated and deuterated, m/z = 30-34)
were observed along with the decreased HD formation
over calcined Pd+PEI(L)@HSS catalyst. Pd+PEI(B)@HSS
catalyst showed an apparently decreased formation of
ethane, which however was not fully prevented. On the
other hand, ethane was hardly produced over
Pd+PEI(L)@HSS catalyst, representing a discriminative ad-
sorption of acetylene over ethylene. Furthermore, in the
co-flowing of ethylene with H2/D2 (Figure 7B), substantial
amount of ethane isotopes (m/z = 30-32) were produced
over calcined Pd+PEI(L)@HSS catalyst; however, ethane
species were scarcely detected over Pd+PEI(L/B)@HSS cat-
alysts, showing that adsorption of ethylene on these cata-
lysts are negligible. These results clearly evidence that lin-
ear PEI more strongly coordinates on Pd NPs than
branched PEI does, thereby more effectively prohibiting
the adsorption of ethylene with weaker binding energy on
Pd NPs (ethylene binding energies on Pd(111): −82 kJ/mol),
while allowing the adsorption of acetylene with stronger
binding energy on Pd NPs (acetylene binding energy on
Pd(111): −136 kJ/mol).59 We believe that the different coor-
dination ability of PEI is derived from molecular structure
of PEI, i.e. the steric hindrance effect on the amine sites;60
Linear PEI, almost exclusively composed of secondary
amines with less steric hindrance, can coordinate on Pd
NPs more effectively, whereas branched PEI, containing
primary, secondary, and tertiary amines, has a reduced
ability to coordinate on Pd NPs due to the considerable ste-
ric hindrance around the tertiary amine sites.61 Thereby
linear PEI allows discriminative adsorption of alkynes over
alkenes and prevents the over-hydrogenation, thus leading
to an increased alkene selectivity. It is also worth mention-
ing that Pd+PEI(L/B)@HSS catalysts are thermally stable
up to at least 120 °C with retaining high alkene selectivity.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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40
41
42
43
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(A)100
(B)100
86.7%
84.3%
80
60
40
20
0
80
60
40
20
0
5
6
7
5
6
7
0
100 200 300 400 500
Time / min
0
100 200 300 400 500
Time / min
(C)100
(D)100
89.7%
85.9%
80
60
40
20
0
80
60
40
20
0
5
6
7
5
6
7
0
100 200 300 400 500
Time / min
0
10
20
30
40
50
Time / min
(E)100
(F) 100
88.4%
75.9%
80
60
40
20
0
80
60
40
20
0
5
6
7
5
6
7
0
50
100 150 200 250
Time / min
0
50
100 150 200 250
Time / min
Figure 6. Reaction kinetics in the semihydrogenation of phe-
nylacetylene over (A) Pd+PEI(L)@HSS, (B) Pd+PEI(L)@HSS
calcined in air, (C) Pd+PEI(B)@HSS, (D) Pd+PEI(B)@HSS cal-
cined in air, (E) Pd/PEI(L), and (F) Lindlar catalyst (+5 mol%
quinoline). Reaction conditions: catalyst (Pd 0.5 mol%), 5 (1
mmol), MeOH:1,4-dioxane = 1:1 (10 mL), 30 °C, under a flow of
1 atm H2 (10 mL/min). 5 mol% quinoline was added for Lindlar
catalyst.
On the basis of difference in reaction kinetics observed,
as well as structural and compositional similarity between
Pd+PEI(L)@HSS and Pd+PEI(B)@HSS catalysts, it can be
hypothesized that linear PEI coordinates and passivates Pd
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ACS Catalysis
Pd+PEI(
1
2
A)
6
5
H2 + D2
H2
3
4
5
6
7
8
9
0
500 1000 1500 2000 2500
Time / sec
0
500 1000 1500 2000 2500
Time / sec
0
500 1000 1500 2000 2500
Time / sec
10
11
12
13
14
15
16
17
18
19
0
500 1000 1500 2000 2500
Time / sec
0
500 1000 1500 2000 2500
Time / sec
0
500 1000 1500 2000 2500
Time / sec
Figure 7. Time courses in H2-D2 exchange reaction and the subsequent gas-phase hydrogenation of (A) acetylene (C2H2) and (B)
ethylene (C2H4) in a c0-flow of H2 and D2 over as-synthesized Pd+PEI(L)@HSS, as-synthesized Pd+PEI(B)@HSS and calcined
Pd+PEI(L)@HSS.
20
21
22
23
24
25
26
27
28
29
30
31
32
33
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35
36
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38
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57
58
59
60
3.3. Impacts of Silica Shell Thickness on Catalytic
Performance.
composites were obtained when more than 1.35 mL of
TEOS was added per 0.12 g of PEI, while addition of less
amount of TEOS (0.9 mL) resulted in the formation of solid
silica particles without defined hollow structure (Figure
8Aa). As increasing the TEOS volume from 1.35 mL to 2.7
mL, larger silica particles with a thicker silica shell and a
narrower inner space were obtained; the average silica
shell thickness increased from 29 nm (TEOS: 1.35 mL), 49
nm (TEOS: 1.8 mL) to 74 nm (TEOS: 2.7 mL) (Figure 8A(b-
d)). The Pd NPs were found to be in the range of 5.0-9.0
nm in diameter. The BET surface area and total volume
decreased along with the increased volume of TEOS added,
which can be interpreted by the increased volume fraction
of silica shell relative to that of inner void space.
In order to investigate the impacts of the silica shell
thickness on catalytic performances, an array of
Pd+PEI(L)@HSS catalysts with different silica shell thick-
ness were prepared simply by changing the volume of
TEOS added during the synthesis, and examined in the
semihydrogenation
reactions.
Figure
8A
shows
representative TEM images of Pd+PEI(L)@HSS
synthesized with varied volumes of TEOS (0.9 - 2.7 mL).
The textural properties determined from TEM images are
tabulated in Table 3 (for the distribution diagrams of silica
particle size, silica shell thickness, and Pd NP size, see
Figure S7). Similar yolk-shell nanostructured Pd-PEI-silica
Table 3. Textural Properties of Pd+PEI(L)@HSS Composites Synthesized with Varied TEOS Volumes
N2 physisorption
Average silica Average silica
Average Pd
TEOS added PEI contenta
Vtotal
De
(nm)
c
d
Samples
particle sizeb shell thicknessb particle sizeb
SBET
(mL)
(wt%)
(m2/g)
41
(cm3/g)
0.16
0.17
0.12
0.08
d
(nm)
(nm)
(nm)
Pd+PEI(L)@HSS_0.9
Pd+PEI(L)@HSS_1.35
Pd+PEI(L)@HSS_1.8
Pd+PEI(L)@HSS_2.7
a
0.9
1.35
1.8
27.8
24.8
24.0
23.2
95
98
155
173
N.D.
29
5.6
5.7
8.9
7.5
2.6
2.8
2.7
2.6
42
49
34
2.7
74
24
b
c
Determined by TG analysis. Determined by TEM observation. Surface area calculated by BET method. Total pore volume
reported at p/p0=0.99. e Peak pore size determined by BJH method.
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Page 10 of 15
(A) (a) 0.9 mL
(b) 1.35 mL
(c) 1.8 mL
(d) 2.7 mL
1
2
3
4
5
6
7
100 nm
100 nm
100 nm
100 nm
8
9
(B)
Catalyst (0.5 mol% Pd)
+
+
10
11
12
13
14
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59
60
MeOH/1,4-dioxane
30 ℃, 1 atm H2
1
2
3
4
(a) 100
1(b)100
(c)100
89.8%(d)100
91.6%
2
3
4
76.3%
80
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
1
2
3
4
1
2
3
4
60
40
20
0
1
2
3
4
0
100 200 300 400 500
Time / min
0
100 200 300 400 500
Time / min
0
100 200 300 400 500
Time / min
0
100 200 300 400 500
Time / min
Catalyst (0.5 mol% Pd)
(C)
+
MeOH/1,4-dioxane
30 ℃, 1 atm H2
5
6
7
(a)100
(b)100
(c)100
(d)100
91.3%
5
6
7
84.3%
77.4%
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
71.5%
5
6
7
5
6
7
5
6
7
0
100 200 300 400 500
Time / min
0
100 200 300 400 500
Time / min
0
100 200 300 400 500
Time / min
0
100 200 300 400 500
Time / min
Figure 8. (A) TEM images and reaction kinetics in the semihydrogenation of (B) diphenylacetylene and (C) phenylacetylene over
a series of Pd+PEI(L)@HSS synthesized with different volume of TEOS; (a) 0.9 mL, (b) 1.35 mL, (c) 1.8 mL, and (d) 2.7 mL. Reaction
conditions: catalyst (Pd 0.5 mol%), alkyne (1 mmol), MeOH:1,4-dioxane = 1:1 (10 mL), 30 °C, under a flow of 1 atm H2 (10 mL/min).
The catalytic performance of the series of
Pd+PEI(L)@HSS catalysts were assessed in the semihydro-
genation of diphenylacetylene (Figure 8B) and phenyla-
cetylene (Figure 8C) under the identical conditions
demonstrated above. In the semihydrogenation of diphe-
nylacetylene, Pd+PEI(L)@HSS_1.35~2.7 with moderate
shell thickness (29~74 nm) selectively afforded cis-stilbene
with suppressing the over-hydrogenation into bibenzyl
(Figure 8B(b-d)), while hydrogenation rates decreased as
the shell thickness increased, suggesting that mass transfer
in the silica shell region dominates the reaction rate. In the
2), PEI is not only in the inner cavity spaces but also infil-
trated into the silica shell. In the case of
Pd+PEI(L)@HSS_2.7 with a thicker silica shell, large frac-
tion of PEI is incorporated into the silica shell and the PEI
concentration in the hollow cavity space decreases, there-
fore the PEI coverage on Pd NPs decreases, thus leading to
a
lower alkene selectivity. In the case of
Pd+PEI(L)@HSS_1.35~1.8 with moderate shell thickness,
the proximity between Pd NPs and PEI remains un-
changed, thereby results in higher alkene selectivities.
In contrast, Pd+PEI(L)@HSS_0.9 without defined hollow
structure gave considerable amounts of alkanes from the
initial stage of reaction, and showed a faster reaction rate
in the second hydrogenation step than yolk-shell
nanostructured analogues (Figure 8B(a), 8C(a)). This is
probably due to the segregation of Pd NPs and PEI and
leaching of PEI during the reaction. These results again
verify that the alkene selectivity in these catalysts is deeply
associated with the morphology of silica particles and the
configuration between Pd NPs and PEI.
semihydrogenation
of
phenylacetylene,
Pd+PEI(L)@HSS_1.35~1.8 catalysts having moderate shell
thickness (29, 49 nm) produced styrene as a main product
with 84-91% yield and showed much slower rates in the
second hydrogenation step than Pd+PEI(L)@HSS_2.7 cat-
alyst having a thicker silica shell (74 nm) (cf. Figure 8C(b-
d)). Considering that PEI contents in these samples are al-
most the same (see Table 3), the most probable cause for
the different alkene selectivities observed is the proximity
of Pd NPs and PEI confined in the hollow silica particles.
As confirmed by STEM elemental mapping images (Figure
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of Pd+PEI(L)@HSS catalyst before and after five repeated cat-
alytic runs in the semihydrogenation reaction of 1 (the reac-
tion conditions are the same as those described in Figure 9).
3.4. Catalyst Reusability.
(A)100 (B)
1
2
3
4
5
6
7
8
100
100
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
The catalyst recovered after five repeated cycles was fur-
ther characterized by TEM, Pd K-edge XAFS, N2 physisorp-
tion and TG measurements. TEM image clearly shows that
the yolk-shell nanostrcture remained intact even after five
repeated cycles, and the size of the Pd NPs (dave = 8.0 nm)
was similar to that of the fresh catalyst (dave = 8.9 nm)
(Figure 10A). Pd K-edge FT-EXAFS spectrum of the used
catalyst exhibited two distinct peaks assignable to Pd-N (r
= 1.6 Å) and Pd-Pd bonds (r = 2.5 Å). This result indicates
that the coordination between Pd(0) NPs and nitrogen lig-
ands on PEI polymer remains intact. No significant struc-
tural change was identified from N2 adsorption isotherms
(surface area was 34 m2/g for fresh catalyst, and 32 m2/g for
used catalyst, Figure 10C). In addition, PEI content re-
mained unchanged even after the catalytic reactions (24.0
wt% for fresh catalyst, 23.5 wt% for used catalyst, Figure
1oD), indicating the leaching of PEI polymer during the
catalytic reaction is negligible as well.
Removal of catalyst
1st
2nd
Catalytic run
3rd
4th
5th
0
100 200 300 400 500
Time / min
9
10
11
Figure 9. (A) Leaching test in the semihydrogenation of 1 us-
ing Pd+PEI(L)@HSS_1.8 catalyst. The red square symbols
show the reaction kinetics after the removal of catalyst at 90
min. (B) Reusability test in the semihydrogenation of 1 using
Pd+PEI(L)@HSS_1.8 catalyst. Reaction conditions: catalyst (Pd
0.5 mol%), 1 (1 mmol), MeOH:1,4-dioxane = 1:1 (10 mL), 30 °C,
under a flow of 1 atm H2 (10 mL/min), t = 3 h.
12
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30
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42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The major reasons for deactivation of supported Pd
catalysts in liquid-phase hydrogenation is sintering and
leaching of active Pd species. In the semihydrogenation of
diphenylacetylene, the reaction was immediately
quenched after the removal of the catalyst by centrifuga-
tion (Figure 9A), ensuring that the catalyst acts a hetero-
geneous catalyst. To assess the stability and reusability, the
catalyst was reused up to five repeated catalytic cycles.
Pd+PEI@PEI(L) with an optimum silica shell thickness (ca.
49 nm) exhibited excellent recyclability without any loss of
activity and selectivity as well as reaction rate (Figure 9B,
for reaction profiles, see Figure S8). ICP analysis also con-
firmed that Pd content was maintained at 1.1 wt%, proving
a negligible leaching of Pd species during the catalytic re-
actions.
Table 4. Scope of Substrates in the Semihydrogena-
tion of Alkynes with 1 atm H2 over Pd+PEI(L)@HSS
Catalysta
Time Conv.b Alkene Sel.b Z:Eb
Entry
1
Substrate
(min) (%)
(%)
270
>99
84
-
2
3
95 : 5
94 : 6
360
480
>99
>99
95
81
O
OH
O
4
100 : 0
100 : 0
100 : 0
360
>99
94
OMe
O
(A)
5c
300
180
>99
>99
76
OEt
6
100
fresh
aReaction conditions: catalyst (Pd+PEI(L)@HSS, Pd 0.5 mol%),
alkyne (1 mmol), MeOH:1,4-dioxane = 1:1 (10 mL), 30 °C, under
b
c
a flow of 1 atm H2 (10 mL/min). Determined by GC-FID. E-
tOH:1,4-dioxane = 1:1 (10 mL) was used as solvent.
after 5th use
0
1
2
3
4
5
6
Furthermore, Pd+PEI(L)@HSS_1.8 catalyst, the best per-
formance catalyst among examined in this study, provided
good selectivity for the semihydrogenation of other al-
kynes (Table 4). Several kinds of aromatic and aliphatic in-
ternal alkynes were efficiently hydrogenated to the corre-
sponding alkenes with good to excellent yields (entries 2–
6) using atmospheric pressure of H2 as a reductant, while
suppressing over-hydrogenation to corresponding alkanes.
These results led us to a conclusion that Pd+PEI(L)@HSS
catalyst having an optimum silica thickness can act as a
stable, reusable, and efficient heterogeneous catalyst in the
liquid-phase semihydrogenation of alkynes.
Interatomic distance / Å
150
100
50
5
fresh
after 5th use
fresh
0
-5
after 5th use
-10
-15
-20
-25
-30
+50
0
0.0 0.2 0.4 0.6 0.8 1.0
0
100 200 300 400 500 600
p/p0
Temperature / ℃
Figure 10. (A) TEM image of Pd+PEI(L)@HSS catalyst after
five catalytic runs. (B) Pd K-edge FT-EXAFS spectra, (C) N2
adsorption-desorption isotherms, and (D) weight loss curves
4. CONCLUSIONS
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The synchrotron radiation experiments for XAFS measure-
ments were performed at the BL01B1 beam line in SPring-8
with the approval from JASRI (no. 2018A1089).
A new type of Pd-PEI-silica composite with yolk–shell
nanostructure was developed for the selective hydrogena-
tion of alkynes to produce alkenes. The synthesized com-
posite was composed of Pd NPs core less than 9 nm in di-
ameter and linear/branched PEI which were confined in
hollow silica spheres having ill-defined mesoporous struc-
tures with a particle size of 100-160 nm. The present
synthetic protocol provides a facile one-pot synthetic
process for the fabrication of yolk-shell silica
nanostructure with tunable size and shell thickness
without severe control of synthetic conditions. Such a
yolk-shell nanostructured composite could act as an effi-
cient and reusable heterogeneous catalyst in the semihy-
drogenation of alkynes in the presence of 1 atm H2, giving
a markedly improved alkene selectivity compared with the
previously-reported prototype Pd catalysts. In particular,
the catalyst encapsulating linear PEI afforded a high alkene
selectivity in the semihydrogenation of phenylacetylene to
produce styrene, in which silica shell thickness had a
strong influence on alkene selectivity. Comprehensive
analysis together with isotope experiment revealed that Pd
NPs and PEI polymers are encapsulated within the hollow
silica particles in close proximity, in which the PEI acts as
a macro-ligand to strongly coordinate on Pd NP surface,
enabling discriminative adsorption of alkynes over
alkenes. This leads to highly selective hydrogenation of
alkynes to alkenes with suppressing over-hydrogenation
into alkanes. Owing to the rigidity of the protective silica
shell, the catalyst was easily recoverable and reusable with-
out leaching of Pd species and PEI, with retaining high ac-
tivities and selectivities over multiple cycles, rendering this
material an efficient and stable catalyst for semihydrogena-
tion of alkynes.
1
2
3
4
5
6
7
8
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ASSOCIATED CONTENT
Supporting Information.
This Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
Photographs of synthetic solutions (Figure S1), statistical
structural data of samples (Figure S2), XRD patterns (Fig-
ure S3), TEM images of samples synthesized with different
polymers (Figure S4) or different Pd precursors (Figure
S5), N2 physisorption data upon calcination (Figure S6),
statistical structural data of samples synthesized with var-
ied TEOS amounts (Figure S7), and reaction profiles dur-
ing catalyst reusability test (Figure S8) (PDF)
AUTHOR INFORMATION
Corresponding Author
*yamashita@mat.eng.osaka-u.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid from the Frontier
Research Base for Global Young Researchers, Osaka Univer-
sity, and the Grant-in-Aid for Young Scientists (KAKENHI)
from JSPS (no. 18K14056). A part of this work was supported
by the “Grant for Research” of The japan Petroleum Institute.
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