Yolk-shell nanocrystal@ZIF-8 nanostructures for gas-phase heterogeneous catalysis with selectivity control

Article abstract of DOI:10.1021/ja306869j

A general synthetic strategy for yolk-shell nanocrystal@ZIF-8 nanostructures has been developed. The yolk-shell nanostructures possess the functions of nanoparticle cores, microporous shells, and a cavity in between, which offer great potential in heterogeneous catalysis. The synthetic strategy involved first coating the nanocrystal cores with a layer of Cu₂O as the sacrificial template and then a layer of polycrystalline ZIF-8. The clean Cu₂O surface assists in the formation of the ZIF-8 coating layer and is etched off spontaneously and simultaneously during this process. The yolk-shell nanostructures were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and nitrogen adsorption. To study the catalytic behavior, hydrogenations of ethylene, cyclohexene, and cyclooctene as model reactions were carried out over the Pd@ZIF-8 catalysts. The microporous ZIF-8 shell provides excellent molecular-size selectivity. The results show high activity for the ethylene and cyclohexene hydrogenations but not in the cyclooctene hydrogenation. Different activation energies for cyclohexene hydrogenation were obtained for nanostructures with and without the cavity in between the core and the shell. This demonstrates the importance of controlling the cavity because of its influence on the catalysis.

Full text of DOI:10.1021/ja306869j

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Communication
Yolk-Shell Nanocrystal@ZIF-8 Nanostructures for Gas
Phase Heterogeneous Catalysis with Selectivity Control
Chun-Hong Kuo, Yang Tang, Lien-Yang Chou, Brian T Sneed, Casey N
Brodsky, Zipeng Zhao, Chia-Kuang (Frank) Tsung, and Lien-Yang Chou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja306869j • Publication Date (Web): 17 Aug 2012
Downloaded from http://pubs.acs.org on August 19, 2012
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Yolk‐Shell Nanocrystal@ZIF‐8 Nanostructures for Gas Phase Heter‐
ogeneous Catalysis with Selectivity Control
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Chun‐Hong Kuo, Yang Tang, Lien‐Yang Chou, Brian T. Sneed, Casey N. Brodsky, Zipeng Zhao,
and Chia‐Kuang Tsung*
Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon Street, Chestnut Hill, Massa‐
chusetts 02467, United States.
Supporting Information Placeholder
in most of the previous yolk‐shell catalysts, the shell only
serves one function, a protective layer to prevent aggregation
between particles during the reactions.6‐13 This is mainly be‐
cause in previous works, the shell materials were limited to
non‐ordered porous materials, which restricts the functions
of the shells. Here, we report a general method for the syn‐
thesis of yolk‐shell structures with metal organic framework
(MOF) shells. The uniform and controllable microporous
structure, large internal surface area, and tunable chemical
property of the MOF shell could introduce new functions
into the yolk‐shell nanostructures and make the yolk‐shell
structures more attractive to different applications. We chose
zeolitic imidazolate frameworks‐8 (ZIF‐8) as the shell mate‐
rial to demonstrate our method. ZIF‐8, a subclass of MOFs, is
of high thermal and chemical stability.14 It is one of the few
commercially available MOFs because of its great potential in
gas separation15‐17 and gas storage.18,19
ABSTRACT: A general synthetic strategy for yolk‐shell
nanocrystal@ZIF‐8 nanostructures is developed. The yolk‐
shell nanostructures possess the functions of nanoparticle
cores, microporous shells, and a cavity in‐between, which
offer great potential in heterogeneous catalysis. The synthet‐
ic strategy involved first coating the nanocrystal cores with a
layer of Cu2O as the sacrificial template and then a layer of
polycrystalline ZIF‐8. The clean Cu2O surface assists the ZIF‐
8 layer coating and is etched off spontaneously and simulta‐
neously during the ZIF‐8 coating. The yolk‐shell nanostruc‐
tures were characterized by TEM, SEM, XRD and nitrogen
adsorption. To study the catalytic behavior, model reactions
including ethylene, cyclohexene, and cyclooctene hydrogena‐
tions were carried out over the Pd@ZIF‐8 catalysts. The mi‐
croporous ZIF‐8 shells provide excellent molecular‐size se‐
lectivity. The results show high activity in ethylene and cy‐
clohexene hydrogenations but not in cyclooctene hydrogena‐
tion. Different activation energies for cyclohexene hydro‐
genation were obtained for the nanostructures with and
without the cavity in‐between the core and shell. This
demonstrates the importance of controlling the cavity be‐
cause of its influence on the catalysis.
Scheme 1. The growth procedure of the nanocrys‐
tal@ZIF‐8 yolk‐shell nanostructures.
Yolk‐shell nanostructures have caused a recent interest in
research for their potential in heterogeneous catalysis1, pho‐
tocatalysis2, and biomedicine applications.3,4 Integrating the
functions of the nanocrystal core, nanostructured shell, and
the cavity in‐between provides a tool to optimize the per‐
formance of a nanomaterial. In the yolk‐shell catalysts, the
metal core provides a catalytically active surface for the reac‐
tions and the porous shell serves as a barrier layer to prevent
aggregation of the active surface with neighboring metal
cores during the reactions. Compared to core‐shell
nanostructures,5 in which the shell is directly on the metal
surface, the cavity between the core and shell in the yolk‐
shell structures not only reserves a larger exposed metal sur‐
face for the reactants but also makes the reactants interact
with the surface more homogeneously. It has been proposed
that the shells in the yolk‐shell structures could introduce
multiple functions into the catalysts such as regulation of
diffusion and control of molecular size‐selectivity; however,
Most of the previously reported synthetic strategies of in‐
corporating nanocrystals into MOFs used gas‐phase infiltra‐
tion or grinding, such as Au@ZIF‐820,21 and Cu/ZnO@MOF‐
522. These post‐incorporating methods cannot create the
cavity around the nanocrystal core and the morphology of
the nanocrystal cores is uncontrollable. The morphology is a
very important factor in the function of nanocrystals for cata‐
lytic and optical properties. Furthermore, in these post‐
incorporating methods, the porous structures of MOF could
be damaged during the incorporation and some of the nano‐
crystals were on the external surface of MOFs. A preferred
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strategy, coating MOFs on the preformed nanocrystals, was
final step. The products were then washed with methanol
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reported recently but the synthesis of MOF yolk‐shell
nanostructures is still challenging.23 In this work, we develop
a new strategy. Scheme 1 illustrates the synthetic strategy.
Metal nanocrystals were first coated with a layer of a sacrifi‐
cial template and then coated with ZIF‐8. We specifically
choose Cu2O as the sacrificial template. Not only could the
Cu2O be etched simultaneously and spontaneously by the
protons generated during the formation of ZIF‐8 but also the
capping agent free surface of Cu2O provides a clean surface
for ZIF‐8 coating. Also, methods of Cu2O coating on different
nanocrystals have been well‐developed, which makes our
synthetic strategy general and versatile.24
and collected by centrifugation. The absence of the Cu signal
in the element analysis (ICP‐AES) confirms the removal of
Cu2O in the final products. The SEM and TEM images (Fig‐
ure 1) show that the Pd nanocrystals are well‐separated by
the ZIF‐8 shells. The inset of Figure 1 clearly shows the well‐
preserved morphology of the nanocrystals after the coating
as well as the cavity. The thickness of the ZIF‐8 is around 100
nm and the ZIF‐8 is polycrystalline and crack‐free, which is
further confirmed by the catalysis results later. The solution
colors of these samples at different stages are shown in Fig‐
ure S3. After the Pd nanocrystals were coated with Cu2O, the
color changed from brown to green which indicates the for‐
mation of Cu2O shells.24 During the ZIF‐8 coating, the color
gradually turned back to brown but was turbid, which indi‐
cates the depletion of Cu2O and the simultaneous formation
of ZIF‐8.
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Figure 1. The nanocrystal@ZIF‐8 yolk‐shell nanostructure.
(a) The scanning electron micrograph (SEM). (b) and (c) The
transition electron micrograph (TEM). The cores are Pd oc‐
tahedrons with edge sizes of 60 nm and the shells are mi‐
croporous ZIF‐8 with thickness of around 100 nm. (d) A
schematic sketch of the yolk‐shell nanostructure.
A typical single shape‐controlled metal nanocrystal incor‐
porated in crack‐free polycrystalline ZIF‐8 crystals is shown
in Figure 1 and Figure S1. The shape and size of nanocrystals
were well maintained during the synthesis. All the nanocrys‐
tals were incorporated in ZIF‐8. A small amount of ZIF‐8
crystals with no nanocrystals were also formed, which could
be removed by centrifugation. In order to demonstrate that
we can preserve the size and shape of the nanocrystals by
using this strategy, relatively large shape controlled metal
nanocrystals were used. The same method could be applied
to differently sized nanocrystals. In a typical synthesis, Pd
octahedrons25 were coated by Cu2O to form a Pd@Cu2O
core‐shell structure (Figure S2a). After Cu2O coating, the
Pd@Cu2O structure was mixed with ZIF‐8 precursors, 2‐
methylimidazole (2‐meIm) and zinc nitrate in methanol.26 As
the pH of the solution decreased from 7 to 5 during the for‐
mation of ZIF‐8 because of the de‐protonation of 2‐meIm
during the formation of ZIF‐8, the Cu2O was etched off sim‐
ultaneously.10 A trace amount of the Cu2O residue was ob‐
served (Figure S2b). A solution of 3% NH4OH in methanol
was used to remove the trace amount of residual Cu2O in the
Figure 2. The crystal and pore structures of Pd@ZIF‐8 yolk‐
shell nanostructure. (a) Powder X‐ray diffraction patterns of
the yolk‐shell nanostructure and pure ZIF‐8 crystals. The
peaks of the ZIF‐8 shells (upper) correspond to those of the
pure ZIF‐8 nanocrystals, which reveals that the ZIF‐8 shells
have the same crystal structure as the pure ZIF‐8 crystals.
The inset shows the higher angle diffraction peaks contribut‐
ed by the Pd octahedrons. (b) Nitrogen adsorption‐
desorption isotherm of the Pd@ZIF‐8 yolk‐shell nanostruc‐
ture shows the type‐I behavior and confirms the microporous
structure of the ZIF‐8 shells. The BET surface area is 1396
m2/g.
The crystal structure and porosity of the shell were studied
by powder X‐ray diffraction (XRD) and nitrogen‐sorption
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measurements. The XRD patterns of pure ZIF‐8 and Pd@ZIF‐
genation kinetics. It also shows that there is no significant
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8 yolk‐shell nanostructure are shown in Figure 2a. Both
patterns generated from the ordered porous structure of ZIF‐
8 shells (2θ, 5°‐35°)27 and the Pd cores (2θ, 35°‐85°) were ob‐
served for Pd@ZIF‐8 yolk‐shell nanostructure. All the promi‐
nent peaks of the ZIF‐8 shells, including 011, 002, 112, 022, 013,
and 222, correspond to those of pure ZIF‐8 crystals. This con‐
firms the sodalite zeolite‐type crystal structure of the ZIF‐8
shell and the well‐defined peaks reveal the high crystallinity.
The nitrogen adsorption‐desorption isotherm (Figure 2b) of
the Pd@ZIF‐8 yolk‐shell nanostructure displays type‐I behav‐
ior. The steep step at low relative pressure reveals that the
shells are microporous. The specific surface area estimated
by the BET method is 1396 m2/g. The lower surface area of
the yolk‐shell structure compared with pure ZIF‐8 (1643
m2/g)23 is mainly due to the heavier and nonporous metal
cores. Thermal stability of the yolk‐shell structure was char‐
acterized by thermogravimetric analysis (Figure S4). The
decomposition temperature of Pd@ZIF‐8 yolk‐shell
nanostructure is 425°C which is only marginally lower than
that of pure ZIF‐8 crystals (470°C). This difference can be
contributed by the crystal size difference between the ZIF‐8
shells and few hundred nanometer pure ZIF‐8 crystals.
diffusional influence caused by the ZIF‐8 shells. The activity
difference is mainly due to the size difference of Pd nano‐
crystals. Using the same nanocrystals for different catalysts is
challenging because the core‐shell synthetic strategy is dif‐
ferent from the yolk‐shell strategy; however, after the nor‐
malizing by the Pd surface area, the activities are similar. For
the cyclooctene hydrogenation, both the core‐shell and yolk‐
shell catalysts show no detectable activity but the Pd on ZIF‐
8 catalyst shows good activity. This result clearly exhibits the
molecule‐size‐selective property of the ZIF‐8 shells. Ethylene
molecules are small (2.5 Å) enough to diffuse through the
pore apertures of the ZIF‐8 shells (3.4 Å) without serious
hindrance; however, the size of the cyclooctene molecules
(5.5 Å) are much bigger than the size of the pore aperture.
Therefore, only the catalyst in which the Pd nanocrystals
were directly deposited on the external surface of ZIF‐8 (Pd
on ZIF‐8) shows activity for cyclooctene hydrogenation. This
result also clearly suggests that the ZIF‐8 shells of both yolk‐
shell and core‐shell are devoid of cracks or fractures. No sig‐
nificant deactivation was observed even when the catalysts
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were heated to 150 ̊C. The catalysts were checked after the
reactions by TEM. Most of them still maintain the yolk‐shell
structure.
Table 1. The catalytic behaviors of different ZIF‐8
nanostrucutures.
Scheme 2. Molecule size‐selective catalysis.
Pd@ZIF-8
core-shell
Pd@ZIF-8
yolk-shell
Pd on ZIF-8
Activityb
1.3×10-2
41.7
3.2×10-3
42.7
3.0×10-3
41.0
(mol·gPd-1·s-1)
Ethylenea
Eac
(kJ/mol)
Activityb
1.5×10-4
0.27
3.2×10-5
0.23
2.8×10-5
0.21
(mol·gPd-1·s-1)
Cyclohexenea
TOF (s-1)
Cyclohexene hydrogenation displays very interesting cata‐
lytic phenomena in this series of hydrogenation reactions.
Although the size of cyclohexene molecules (4.2 Å) is compa‐
rable to the aperture size of ZIF‐8, all the catalysts show good
activities for cyclohexene hydrogenation. This indicates the
flexibility of ZIF‐8 frameworks.28 The yolk‐shell Pd@ZIF‐8
catalyst and Pd on ZIF‐8 have similar activation energies. The
core‐shell Pd@ZIF‐8 catalyst has lower activation energy. To
exclude the chance that the activation energy difference is
contributed by capping agents or supports, a series of Pd
nanocrystals were prepared by using different capping
agents, and then were directly deposited on different sup‐
ports. Table S1 shows the activation energies of all the cata‐
lysts. The values of the activation energies are all around 40
kJ/mol; except for the core‐shell Pd@ZIF‐8 catalyst, which is
lower (27 kJ/mol). The observation of lower activation energy
could be generated by different reasons. It has been known
that the internal diffusion influence could lead to a change in
measured activation energy.29,30 Although it is not conclusive
enough to use values of activation energy alone to explain
diffusional influence, the comparable sizes of the pore aper‐
ture and cyclohexene make the transport limitation highly
possible. The diffusion through the core‐shell catalyst is
mainly configurational diffusion and the diffusion through
the yolk‐shell catalyst is a combination of configurational
diffusion (shell) and Knudsen diffusion (cavity). This could
Eac
(kJ/mol)
27.5
40.1
41.1
Activityb
N/A
N/A
2.8×10-6
0.02
(mol·gPd-1·s-1)
Cyclooctenea
TOF (s-1)
N/A
N/A
Eac
(kJ/mol)
N/A
N/A
34.9
a
b
c
In each experiment: 10 torr hydrocarbon (C2H4, C6H4, or C8H14), 100
torr H2, and 650 torr He at 323 K.
The metal loading was determined by elemental analysis and used to
normalize the conversion to mass of Pd.
Arrhenius activation energy.
Gas‐phase hydrogenation of ethylene, cyclohexene, and
cyclooctene were carried out to study the molecule size‐
selective catalysis. (Scheme 2) The catalysts of 60 nm Pd
nanocrystal@ZIF‐8 yolk‐shell nanostructure (yolk‐shell
Pd@ZIF‐8), 60 nm Pd nanocrystals directly deposited on ZIF‐
8 crystal surfaces (Pd on ZIF‐8), and 20 nm Pd nanocrystals
coated by ZIF‐8 layers without the cavity (core‐shell Pd@ZIF‐
8) were prepared for comparison. Table 1 shows the activities
and activation energies of the reactions. For ethylene hydro‐
genation, all the catalysts show high activity and similar acti‐
vation energies, which indicates the same ethylene hydro‐
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the kinetics difference due to the conformation of the mole‐
cules on the metal surface. The size of a cyclohexene mole‐
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comparable to the size of the aperture (3.4 Å). Thus, the cy‐
clohexene molecules could only interact with the Pd surface
by the successive variation in their conformations in the
core‐shell structure, in which the ZIF‐8 is directly on the Pd
surface. It has been shown that the kinetics of six‐
membered‐carbon‐ring hydrogenations could be affected by
the conformation of the molecules on the metal surface.31
Detailed studies of the reactions for further understanding
the activation energy change are currently under investiga‐
tion but the study here clearly indicates the importance of
the ability to control the cavity for reactions. The ability to
create the yolk‐shell catalysts provides a strategy to perform
selective catalysis without changing the kinetics or generat‐
ing the diffusional influence.
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In conclusion, we have developed a general strategy for the
synthesis of nanocrystal@ZIF‐8 yolk‐shell nanostructures.
Cu2O is used as a sacrificial template because it could be
etched simultaneously and spontaneously during the for‐
mation of ZIF‐8. The nanostructures were applied to the gas‐
phase hydrogenation of ethylene, cyclohexene, and cy‐
clooctene as catalysts. The ZIF‐8 shell shows interesting size‐
selectivity in ethylene hydrogenation versus cyclohexene
hydrogenation. For cyclohexene hydrogenation, the meas‐
ured activation energy of yolk‐shell nanostructure is different
from that of the core‐shell nanostructure, which demon‐
strates the influence of the cavity structure in the yolk‐shell
structures. Integrating the functions of the nanocrystal core,
microporous shell, and the cavity in‐between provides a new
tool to create selective heterogeneous catalysts.
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SUPPORTING INFORMATION AVAILABLE
Detailed experimental procedures, TEM images of Pd Cu‐
be@ZIF‐8, Pt small particle@ZIF‐8, and Au octahedron@ZIF‐
8 yolk‐shell structures, XRD patterns, photographs, and TEM
images of the samples at different synthetic stages, thermo‐
gravimetric analysis, and the table of activation energies for
cyclohexene hydrogenation. This material is available free of
charge via the internet at http://pubs.acs.org.
ACKNOWLEDGMENT
The research is funded by Boston College. Dr. C. H. Kuo
thanks National Science Council in Taiwan for offering the
scholarship.
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5
ACS Paragon Plus Environment