Atomic-layer-deposition-formed sacrificial template for the construction of an MIL-53 shell to increase selectivity of hydrogenation reactions

Article abstract of DOI:10.1039/c9cc02727k

We developed a method for using atomic layer deposition (ALD)-formed layers of Al₂O₃ as a sacrificial template to generate MIL-53(Al). The MOF shell in the CeO₂/Pd@MIL-53(Al) configuration not only stabilized the Pd nanoparticles, but also regulated the selectivity of the hydrogenation of unsaturated aldehydes. This work represents the first demonstration of ALD-formed layers of metal oxides serving as sacrificial templates in the design of MOF-shell-based sandwich-type structures.

Full text of DOI:10.1039/c9cc02727k

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DOI: 10.1039/C9CC02727K
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Atomic Layer Deposition-Created Sacrificial Template for the
Construction of MIL-53 Shell to Increase Selectivity in
Hydrogenation Reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Tiantian Xu† [a], Kai Sun† [b], Daowei Gao , Cuncheng Li , Xun Hu*[b], and Guozhu Chen*[a]
[a]
[a]
DOI: 10.1039/x0xx00000x
We develop a method of using the atomic layer deposition (ALD)-
created Al as sacrificial template to generate MIL-53(Al). The
MOF shell in the configuration of CeO /Pd@MIL-53(Al) not only
oxides can provide metal ions through themselves, and then
initiate the growth of MOFs shell without any surface
modification/multiple adsorption.3 Unfortunately, for some
2 3
O
2
stabilizes the Pd nanoparticles, but also regulates the selectivity
in hydrogenation of unsaturated aldehyde. This work represents
the first demonstration of ALD-created metal oxides being
sacrificial templates to design MOF-shell based sandwich
structure.
2
metal oxides, e.g. CeO , one of the most widely studied
4
supports for noble metal NPs in catalytical reactions, it hardly
dissolves metal ion, and thus it seems not feasible to be
sacrificial template at present. Therefore, it remains a great
challenge to develop one general strategy for encapsulation of
noble metal NPs supported metal oxides into MOFs without
any assistance of surfactant or polymer.
Metal-organic frameworks (MOFs) are one class of porous
crystalline materials that are orderly assembled by metal ions
with organic ligands. Owing to their unique features, MOFs
are considered as a promising candidate for shell materials in
Atomic layer deposition (ALD) is a powerful technique that
is capable of depositing a wide range of metal oxides on the
5
substrates surface via sequential self-limiting reactions. In
the configuration of core@shell structured catalysts.1
A
this way, the surface modification of substrates is not
necessary and the deposited metal oxides are expected to
uniformly disperse onto the substrates. By virtue of this
feature, the deposited metal oxides can be “borrowed” as
typical example is the encapsulation of supported noble metal
nanoparticles (NPs) into MOFs, where the cavities and small
pore windows of MOFs shell could prevent the NPs from
aggregation/leaching.2 More importantly, the size-sieving
effect of MOFs is proposed to obstruct the diffusion of large
reactants to active sites, which enables the sandwiched
catalysts to be selectivity regulator in catalytic reactions.
6
sacrificial templates for MOFs formation. In this work, we
2 3 2
demonstrate that the ALD-created Al O on the CeO /Pd
nanospheres surface, is qualified for the use as a sacrificial
Considering that the noble metal NPs supported metal
oxides is one major category of heterogeneous catalysts, how
to efficiently encapsulate them into MOFs and avoid
homogenous nucleation of MOFs shell are both important and
fundamental for the construction of sandwich structures. To
this end, the surface modification of metal oxides and/or
tediously repeated adsorption of organic ligands/metal ions are
adopted for the assistance of MOFs heterogeneous nucleation.
Alternatively, the engagement of metal oxide-based sacrificial
template is demonstrated to be a feasible solution since metal
a. School of chemistry and chemical engineering, University of Jinan, Shandong
province, 255022, China.
b.
School of material science and engineering, University of Jinan, Shandong
province, 255022, China.
E-mail: chm_chengz@ujn.edu.cn; mse_hux@ujn.edu.cn
These authors equally contributed to this work.
Scheme 1. Schematic illustration for the formation of
CeO /Pd@MIL-53(Al) sandwich structured catalyst.
2
†
Electronic Supplementary Information (ESI) available: Experimental procedures,
XRD, TEM, HR-TEM, SAED, SEM, FT-IR and the tables of catalyst tests. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C9CC02727K
Figure 1. (a) XRD patterns of CeO
2
/Pd@MIL-53(Al), MIL-
sorption
@MIL-53(Al).
5
3(Al) and MIL-53(Al)-simulated, (b) The N
2
isotherms of Pd/CeO @Al and Pd/CeO
2
2
O
3
2
Figure 2. (a) Low magnification SEM image, (b) enlarged
SEM image, (c) TEM image, and EDX mapping analysis of
template for the growth of MIL-53(Al) shell. Via this
approach, the interface between CeO /Pd and MOFs is free of
2
(d) Ce, (e) Pd, (f) Al, and (g) O of CeO /Pd@MIL-53(Al).
2
surfactants and other surface modifications, which is
advantageous for synergistic effects at the interface. As
expected, the resulting CeO /Pd@MIL-53(Al) sandwich
2
structured catalyst exhibits high catalytic activity and good
selectivity in the hydrogenation of unsaturated aldehyde.
Although the focus herein is on the construction of
CeO /Pd@MIL-53(Al), we also demonstrate that ALD-
2
created metal oxides can be extended to fabricate other
sandwich structured materials.
2
The CeO /Pd@MIL-53(Al) sandwich structured catalyst
was fabricated by an ALD-created sacrificial template method,
as schematically illustrated in Scheme 1. Specifically, the
After etching, the peaks centered at 336.2 and 341.4 eV can be
clearly identified and indexed to 3d5/2 and 3d3/2 of Pd0,
respectively (Figure S4).
SEM images show uniform and monodisperse particles with
around ~150 nm in size (Figure 2a and Figure S5). The
enlarged view in Figure 2(b) suggests the surface of the
CeO /Pd@MIL-53(Al) catalyst holds irregular and rough
2
feature. From the TEM images (Figure 2c and Figure S5-S6),
it shows the characteristic core@shell structure in which the
2
contrast difference between CeO /Pd and ragged MOFs shell
is clearly distinguished. No free-standing MIL-53(Al) crystals
are observed in both SEM and TEM images, suggesting that
the nucleation and growth of MIL-53(Al) crystals are
monodispersed CeO
hydrothermal method, followed by the loading of Pd NPs
using NaBH reduction. The CeO /Pd NPs were then drop-
cast on a silicon substrate and Al was deposited at 125°C
2
spheres were prepared from
localized on the surfaces of the CeO
verify whether the MIL-53(Al) coats the CeO
2
nanospheres. To further
/Pd surface or
4
2
2
2 3
O
not, high resolution TEM (Figure S7) and elemental maps
were acquired on an individual NP. As shown in Figure 2d-g
and Figure S8, the Ce and Pd elements are mainly located in
using trimethylaluminum and water as precursors at a
deposition rate of 0.1 nm/cycle (see the experimental details in
Electronic Supplementary Information). The ALD-created
the core region while the elements Al and
O are
Al
terephthalic acid to synthesize MIL-53(Al) shell onto the
CeO /Pd surface.
Figure 1(a) shows the XRD pattern of the as-synthesized
CeO /Pd@MIL-53(Al). It exhibits two sets of diffraction
peaks, one is corresponding to fluorite-phase CeO (JCPDS
5-5923), and the diffraction peaks centered at 9.2° and 18.1°
are indexed to MIL-53(Al) (Deposition No. 1523999) (Figure
S1). Figure 1b shows the N sorption isotherms of unwrapped
/Pd@MIL-53(Al). Compared with
/Pd, the introduction of MIL-53(Al) shell produces
2
O
3
,
being sacrificial template, was reacted with
homogeneously distributed throughout the whole particle,
further confirming the successful encapsulation of CeO
2
/Pd
2
nanosphere into MIL-53(Al).
The thickness of MIL-53(Al) shell can be easily controlled
by changing the ALD cycles. As shown in Figure 3a, the
sandwich structured catalyst with ~10 nm thickness shell can
be obtained by decreasing the ALD cycles from 400 to 50,
while the shell with ~21 nm is achieved with 200 ALD cycles
2
2
6
2
(
Figure 3b and Figure S9). To explore the universality of this
CeO
CeO
2
/Pd and CeO
2
developed method, the trimethylaluminum precursor for
2
micropores in the sandwiched catalyst. The BET surface area
2
-1
of CeO
larger than that of CeO
2
/Pd@MIL-53(Al) is around 162.9 m •g , which is far
2
-1
2
2 3
/Pd@Al O (~35.9 m • g ). The
creation of micropores (Figure S2) and the increase in surface
area obviously originates from the high porosity of MIL-
-
1
5
(
(
3(Al). The appearance of characteristic peaks at 1514 cm
-1
stretching vibrations Vassy (–COO–) ) and 1411 cm (Vsym
–COO–)) in FT-IR spectrum of sandwich structured
Figure 3. TEM images of (a, b) CeO
2
/Pd@MIL-53(Al) with
catalyst further indicates the introduction of MIL-53(Al) onto
CeO /Pd surface (Figure S3). Due to the coverage of MIL-
2
different shell thickness, (c) CeO /Pd@ZIF-8.
2
7
5
3(Al), the signal of Pd is difficult to be detected by XPS.
2
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DOI: 10.1039/C9CC02727K
Table1. Selective hydrogenation of different α,β-unsaturated aldehydes by different catalysts.
Selectivity (%)
Entry
Substrate: furfural[a]
Catalysts
Conversion (%)
B
C
D
1
CeO
CeO
2
/Pd@MIL-53(Al)
/Pd
100.0
100.0
85.3
0.2
0.8
15.6
13.9
84.2
2
2
Substrate: cuminaldehyde[
b]
3
4
5
CeO
Pd/MIL-53(Al)
2
/Pd
93.3
21.1
24.6
0
100
100
0
0
0
96.8
0
0
CeO
2
/Pd@MIL-53(Al)
Substrate: cinnamaldehyde[
c]
6
7
8
CeO
2
/Pd@MIL-53(Al)
98.4
51.8
90.8
-
-
-
99.9
91.4
54.7
0
0
28.8
Pd/MIL-53(Al)
CeO /Pd
2
[a]Reaction conditions: furfural: 0.04 g, H
[b]Reaction conditions: cuminaldehyde: 0.04 g, ethanol: 4.00 g, catalyst: 20 mg, T = 25°C, reaction time: 120 min, H
2
2
O: 4.00 g, catalyst: 20 mg, T = 80°C, reaction time: 120 min, H : 6 MPa.
2
: 6 MPa.
[c]Reaction conditions: cinnamaldehyde: 0.04 g, octanol: 4.00 g, catalyst: 20 mg, T = 25°C, reaction time: 240 min, H
2
: 6 MPa.
The content of Pd is ~2.02 wt% (determined by ICP-MS) in the as-prepared catalysts.
Al
2
O
3
is replaced with diethylzinc for ZnO, the ZIF-8 shell
2
alcohol dominates over the unwrapped CeO /Pd catalyst
can be easily distinguished (Figure 3c), suggesting that the
strategy of ALD-created metal oxides as sacrificial templates
has a certain generality for the construction of MOFs shell. In
(Entry 2) (Figure S11). The hydrogenation of the conjugated π
bonds in the furan ring, requires the involvement of multiple
Pd sites and/or the multiple contact of the furan ring with the
Pd sites. Both the carbonyl group and the furan ring can
2 2 3
contrast, when the CeO /Pd@Al O was prepared from an
impregnation method, followed by the reaction with
terephthalic acid, the MIL-53(Al) is not perfectly coated on
the CeO /Pd surface (Figure S10), further indicating the
2
2
access the Pd sites in CeO /Pd from various orientations, as
majority of Pd sites are accessible easily without MOFs
encapsulation. Nevertheless, when the Pd sites are confined
advantage of ALD-created templates for the MOFs shell
growth in the construction of sandwich structured materials.
Chemoselectivity is an essential parameter for determining
applications of catalysts. Low selectivity to targeting product
would largely decrease the utilization efficiency of the
feedstock and cause the difficulty in separation of targeting
products from reaction mixture. Benefiting from the structural
confinement and size-sieving effects, the MOFs shell would
create the steric configuration for access of reactants/reaction
intermediates to metal sites, and thus is expected to regulate
the chemoselectivity. Herein, using the as-prepared
2
upon the wrapping of CeO /Pd with MIL-Al(53) shell, the
reactant might only be able to access the active sites from a
certain orientation as the space between Pd sites and the MOF
shell is rather limited, leading to the difficulty in further
hydrogenation of the furan ring in FA.
The confinement effect is also observed for the
2
hydrogenation of cuminaldehyde. Over CeO /Pd catalyst,
conversion of cuminaldehyde is relatively higher while the
benzene ring could also be fully hydrogenated (Entry 3). In
contrast, for the Pd/MIL-53(Al) (Figure S12) and
2
CeO /Pd@MIL-53(Al) catalysts, although the conversion of
CeO
2
/Pd@MIL-53(Al) catalyst, the selective hydrogenation of
cuminaldehyde is lower, the selectivity to cumic alcohol
reaches 100% (Entry 4 and 5). Similar to conjugated π bonds
in the furan ring, the hydrogenation of the benzene ring needs
the involvement of multiple metal sites. The structural
confinement of MOFs shell limits the access the benzene ring
to the Pd sites, retaining benzene ring in the product and
achieving the chemoselectivity of cumic alcohol.
furfural to furfuryl alcohol (FA), an important feedstock for
manufacturing resin, is investigated. As shown in Table 1,
being an initial product from the hydrogenation of the C=O in
furfural, FA is the main product over the CeO
8
2
/Pd@MIL-
5
3(Al) catalyst (Entry 1). In comparison, the full
hydrogenation of furfural or the subsequent hydrogenation of
FA to tetra-hydro-furfuryl
The hydrogenation of cinnamaldehyde further confirms the
impacts of the steric selectivity. Over whether
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CeO /Pd@MIL-53(Al) or Pd/MIL-53(Al) catalysts, the C=C
bond in cinnamaldehyde could be selectively hydrogenated
while the C=O bond is retained (Entry 6 and 7) (Figure S13).
The formed hydrocinnamaldehyde (HCMA) is of industrial
importance since it can be used as the inhibitor of the potato
Journal Name
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products such as HIV protease inhibitors. Over CeO
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1
36, 1738; (e) J. Yang, F. Zhang, H. Lu, X. Hong, H. Jiang, Y.
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the inherent character of CeO , e.g. its Lewis acidic and basic
sites, which is proposed to be favorable for the dissociation of
H–H bond.2g Additionally, CeO
/Pd@MIL-53(Al) could still
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maintain sandwich structure (Figure S14), and keep good
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By virtue of ALD-created sacrificial template, MIL-53(Al)
can be heterogeneously nucleated on the CeO /Pd surface in a
2
/Pd@MIL-53(Al) sandwich structured catalysts.
2
4
5
6
controllable manner, without any surfactant modification and
tedious adsorption procedure. Owing to the existence of
MOFs shell, the sandwiched Pd NPs are not only well
stabilized, and more importantly, the MOFs shell can act as
regulator to tune the selectivity in hydrogenation reactions.
The ALD-created sacrificial template may provide an
alternative to generate MOFs shell, and is of great importance
to those support, which are quite incompetent to be sacrificial
templates.
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4
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