Nanocomposites of Platinum/Metal-Organic Frameworks Coated with Metal-Organic Frameworks with Remarkably Enhanced Chemoselectivity for Cinnamaldehyde Hydrogenation

Article abstract of DOI:10.1002/cctc.201501256

The encapsulation of metal nanoparticles (MNPs) within metal-organic frameworks (MOFs) to form core-shell structural nanocomposites is one of the most promising methods to enhance the durability and selectivity of MOF-supported metal catalysts. However, it is still a challenge to fully encapsulate tiny MNPs inside MOFs. Herein, we report a facile and general strategy to coat MOFs on the surface of Pt/MOFs by direct homoepitaxial growth. The obtained Pt/MOFs@MOFs nanocomposites retained the intrinsic properties (e.g., crystalline structure, pore texture, and surface area) of Pt/MOFs. Strikingly, the MOF-coated materials exhibited a significantly enhanced chemoselectivity compared to the uncoated materials, for example, in the hydrogenation of cinnamaldehyde, the selectivity to cinnamyl alcohol through C=O hydrogenation was improved from 55 to 96 % at complete conversions of cinnamaldehyde. Moreover, Pt/MOFs@MOFs can be recycled without any remarkable loss in both activity and selectivity. The enhanced catalytic selectivity and stability of MNPs/MOFs could be related to the synergetic effects of the electron donation and nanoconfinement offered by the surrounding MOF networks. In the framework: Tiny metal nanoparticle (MNP) composites encapsulated fully by metal-organic frameworks (MOFs) are fabricated by the direct homoepitaxial growth of a MOF shell on the surface of MNPs/MOFs. These materials exhibit a remarkably enhanced chemoselectivity to the unsaturated alcohol in cinnamaldehyde hydrogenation.

Full text of DOI:10.1002/cctc.201501256

DOI: 10.1002/cctc.201501256
Full Papers
Nanocomposites of Platinum/Metal–Organic Frameworks
Coated with Metal–Organic Frameworks with Remarkably
Enhanced Chemoselectivity for Cinnamaldehyde
Hydrogenation
Hongli Liu, Lina Chang, Liyu Chen, and Yingwei Li*[a]
The encapsulation of metal nanoparticles (MNPs) within metal–
organic frameworks (MOFs) to form core–shell structural nano-
composites is one of the most promising methods to enhance
the durability and selectivity of MOF-supported metal catalysts.
However, it is still a challenge to fully encapsulate tiny MNPs
inside MOFs. Herein, we report a facile and general strategy to
coat MOFs on the surface of Pt/MOFs by direct homoepitaxial
growth. The obtained Pt/MOFs@MOFs nanocomposites re-
tained the intrinsic properties (e.g., crystalline structure, pore
texture, and surface area) of Pt/MOFs. Strikingly, the MOF-
coated materials exhibited a significantly enhanced chemose-
lectivity compared to the uncoated materials, for example, in
the hydrogenation of cinnamaldehyde, the selectivity to cin-
namyl alcohol through C=O hydrogenation was improved
from 55 to 96% at complete conversions of cinnamaldehyde.
Moreover, Pt/MOFs@MOFs can be recycled without any re-
markable loss in both activity and selectivity. The enhanced
catalytic selectivity and stability of MNPs/MOFs could be relat-
ed to the synergetic effects of the electron donation and nano-
confinement offered by the surrounding MOF networks.
Introduction
Supported metal nanoparticles (MNPs), among the most im-
portant classes of heterogeneous catalysts, are widely used in
industrial applications because of their facile recovery, but un-
fortunately they generally exhibit lower selectivities to the de-
sired products compared to homogeneous catalysts.[1] Conse-
quently, great efforts have been devoted to the development
of new strategies to enhance the reaction selectivity of
MNPs.[2]
the amorphous structure of silica and its inherent shortcom-
ings make it difficult to achieve satisfactory confinement ef-
fects (in particular for electronic interactions) to improve the
chemoselectivity. Therefore, the exploration of new types of
shell materials to enable better confinement effects to obtain
an optimized catalytic performance is imperative.
Metal–organic frameworks (MOFs), which are emerging as
a particular class of hybrid porous materials with many exciting
characteristics,[4] might open new opportunities to achieve
confinement. In addition to high specific surface areas and tun-
able pore sizes and volumes, MOFs possess abundant intercon-
nected 3D cavities accessible through small pore windows.[5]
This characteristic could simultaneously prevent the encapsu-
lated MNPs from escaping from cavities or agglomerating and
guarantee the easy diffusion of the reactants. Interestingly,
MOFs constructed by metal nodes and aryl (hetero) linkers can
certainly establish charge transfer interactions by coordination
or p–p interactions, which may offer an additional electron
donation effect to the encapsulated MNPs as compared to tra-
ditional porous materials such as silicas.[6] The combination of
electron donation and steric restriction effects would enable
MOFs to be efficient shell materials for the confinement of
MNPs in terms of stability and selectivity.
Among the already developed strategies, a promising ap-
proach is to encapsulate MNPs within appropriate porous ma-
terials. Steric restrictions might be imposed on the confined
MNPs, and their electronic configuration would be modified by
the surrounding porous shell, which would create a different
microenvironment around the MNPs compared with that of
the nonencapsulated analogues. This microenvironment can
not only protect the MNPs from leaching and aggregation/sin-
tering during reactions but also, importantly, may induce im-
proved selectivity toward target molecules because of the
electronic interactions and steric restrictions as well as the ori-
entation of the reactive groups. Mesoporous silicas, which pos-
sess good stability and high surface areas and pore volumes,
are currently utilized widely as shells for MNPs.[2a–d,3] However,
Consequently, many endeavors have been devoted to the
development of effective approaches to introduce MNPs into
MOFs.[5a,b,6,7] Currently, the most widely adopted strategy is
based on the incorporation of metal precursors as guests and
followed by reduction or decomposition of the embedded pre-
cursors to form MNPs/MOFs.[5a,b,6a,7d–j] However, with these syn-
thetic methods it is extremely difficult to encapsulate MNPs
[a] Dr. H. Liu, L. Chang, L. Chen, Prof. Y. Li
School of Chemistry and Chemical Engineering
Key Lab for Fuel Cell Technology of Guangdong Province
South China University of Technology
Guangzhou 510640 (P.R. China)
E-mail: liyw@scut.edu.cn
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201501256.
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fully inside MOFs because of the diffusion resistance between
the external and internal MOF surfaces, which results in a lot
of MNPs distributed undesirably on the external surface of the
MOF crystals. Such MNPs outside the MOF lack the beneficial
microenvironment offered by the MOF networks, which would
make the catalyst system suffer from poor catalytic selectivity.
To enhance the chemoselectivity of the MNPs/MOFs, it could
be feasible to coat the MNPs/MOFs fully with MOFs to obtain
MNPs/MOFs@MOFs nanocomposites.
Herein, we report a facile and general synthetic strategy to
obtain designable MNPs/MOFs@MOFs nanocomposites by the
direct homoepitaxial growth of a MOF shell on the surface of
MNPs/MOFs (Scheme 1). To enable a perfect lattice match, the
Figure 1. PXRD patterns of a) MIL-100(Fe), b) Pt/MIL-100(Fe), Pt/MIL-
100(Fe)@MIL-100(Fe) prepared by c) two, d) four, and e) six assembly cycles,
and f) Pt/MIL-100(Fe)@MIL-100(Fe) prepared by four assembly cycles after
the catalytic reaction.
affect the framework structure of MIL-100. The PXRD patterns
of the Pt/MIL-100@MIL-100 nanocomposites with different
shell thickness were identical to that of the parent MIL-100,
which suggests a successful epitaxial growth of the outer MIL-
100 shell with the crystallographic direction of the Pt/MIL-100
core. Moreover, it is difficult for us to observe the XRD patterns
of Pt, which could presumably be related to the low Pt content
and the small particle size in these materials.
Scheme 1. Illustration of the preparation of MNPs/MOFs@MOFs nanocompo-
sites.
same MOF is selected for the shell as that in the core to sup-
port the MNPs. Such a lattice match ensures pore connections
at the interface, which may facilitate the diffusion and accessi-
bility of reactants to the active MNPs without deteriorating the
intrinsic properties of MNPs/MOFs. To demonstrate the univer-
sality of the present approach, two representative MOFs, MIL-
100(Fe) and NH2-MIL-101(Al), are investigated here. Significant-
ly, the thickness of the outer MOF shell can be tuned in the
composites, which exhibit a remarkably enhanced chemoselec-
tivity in the hydrogenation of cinnamaldehyde. Remarkably,
the selectivity to cinnamyl alcohol through C=O hydrogenation
is improved from 55 to 96% at a complete conversion of cin-
namaldehyde over the MNPs/MOFs@MOFs.
The specific surface areas and porosities of the samples
were determined by N2 physisorption at 77 K. All the samples
displayed similar isotherms with secondary uptakes at P/P0 ~
0.1 and ~0.2 (Figure 2), which suggests the presence of two
Results and Discussion
Initially, one of the representative mesoporous MOFs, FeIII-
based MIL-100 (Fe3O(H2O)2F·[C6H3(CO2)]3·H2O) with two types of
large pores (25 and 29 Š) accessible through microporous win-
dows (ꢀ5.5 and 8.6 Š) and a high specific surface area as well
as excellent stability,[8] was selected as a host matrix to incor-
porate Pt nanoparticles (NPs) by a simple colloidal deposition
method.[6c] Given the lattice match, the resultant Pt/MIL-100
was used subsequently as the core for the epitaxial growth of
an outer MIL-100 shell to produce a fully encapsulated Pt ma-
terial (denoted as Pt/MIL-100@MIL-100) through a stepwise
liquid-phase epitaxy method without any surface modification.
The thickness of the outer MIL-100 shell can be controlled ef-
fectively by altering the number of assembly cycles.
Figure 2. N2 adsorption isotherms of MIL-100(Fe), Pt/MIL-100(Fe), and Pt/
MIL-100(Fe)@MIL-100(Fe) prepared by different numbers of assembly cycles.
types of microporous windows. The BET surface area and total
pore volume of MIL-100 were calculated to be 2334 m2 gÀ1 and
0.98 cm3 gÀ1, respectively. Compared to parent MIL-100, Pt/MIL-
100 showed a decreased BET surface area and pore volume,
mainly because of the blockage of the cavities of MIL-100 by
the highly dispersed Pt NPs. After the coating of MIL-100, the
N2 adsorption amounts for the formed Pt/MIL-100@MIL-100
composites increased slightly with up to six assembly cycles,
probably because of the contribution of the porous MIL-100
shell.
Powder X-ray diffraction (PXRD) patterns (Figure 1) revealed
no apparent loss of crystallinity upon the incorporation of Pt
on MIL-100, which implies that the addition of Pt did not
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Figure 4. Pt4f XPS spectra of Pt/MIL-100 and Pt/MIL-100@MIL-100 prepared
by four assembly cycles.
actions to verify the chemoselectivity of a catalyst because of
its two distinct competing sites for hydrogenation: a C=O
bond and a conjugated C=C group,[9c,d] which generally leads
to a complex reaction network that involves a parallel and con-
secutive reduction at different functional groups (Scheme 2).
Figure 3. TEM images of a) Pt/MIL-100 and Pt/MIL-100@MIL-100 prepared by
b) two, c) four, and d) six assembly cycles. Scale bars: 100 nm.
The morphology and structure of the Pt/MIL-100@MIL-100
samples were investigated by TEM. The as-prepared Pt/MIL-
100 exhibited a uniform distribution of Pt NPs in the MIL-100
framework with a mean size of 3.0 nm (Figure 3a). Some Pt
particles could be observed bulging out from the MOF edges
(Figure 3a), which suggests that some of the Pt nanoparticles
were doped on the outer surface of the MOF. After it was
coated with MIL-100, the surface of Pt/MIL-100 was well cov-
ered with a MIL-100 shell layer and clearly exhibited a core–
shell structure (Figure 3b–d, Figure S1). The average thickness
of the MIL-100 shell was ꢀ11, 24, and 35 nm for Pt/MIL-
100@MIL-100 with two, four, and six assembly cycles, respec-
tively. This result demonstrated that the thickness of the MIL-
100 shell could be controlled effectively by varying the assem-
bly cycle numbers. Moreover, the coating of the MIL-100 shell
did not affect the distribution and size of Pt NPs in the Pt/MIL-
100 significantly.
Scheme 2. Reaction network of cinnamaldehyde hydrogenation.
One product of this reaction, the unsaturated alcohol through
only C=O bond hydrogenation, is more valuable industrially
compared to the other products. However, it is quite difficult
to achieve a high selectivity to the unsaturated alcohol prod-
uct at a high conversion because the hydrogenation of the
C=C bond is more thermodynamically favorable than C=O hy-
drogenation.[10] Thus, the development of highly chemoselec-
tive and active catalysts for the selective hydrogenation of un-
saturated aldehydes to unsaturated alcohols under mild condi-
tions is challenging but of great importance.
The XPS spectra of the Pt/MIL-100 and Pt/MIL-100@MIL-100
with four assembly cycles are shown in Figure 4. The binding
energies of the Pt4f7/2 and Pt4f5/2 peaks for Pt/MIL-100 were
observed at ꢀ71.3 and 74.8 eV, respectively, typical of metallic
Pt, which suggests that the Pt species were mostly in the re-
duced state on Pt/MIL-100. After the coating with MIL-100, the
Pt4f peaks for Pt/MIL-100@MIL-100 were shifted to lower bind-
ing energies by approximately 0.2 eV compared to that of Pt/
MIL-100. Such shifts may be attributed to electron donation to
the encapsulated Pt NPs from the surrounding MOF net-
works.[6]
Here, cinnamaldehyde, a representative a,b-unsaturated al-
dehyde, was chosen as a model substrate to investigate the
effect of the MOF coating on the chemoselectivity of Pt/MOF
materials. Pt/MIL-100 provided only 55% selectivity to the de-
sired cinnamyl alcohol (COL) at >99% cinnamaldehyde con-
version within 2 h under the investigated conditions (Figure 5).
To investigate the effect of residual polyvinylpyrrolidone (PVP)
on the reactivity of Pt NPs, Pt/MIL-100 was heated at 2208C for
2 h under N2. The reaction results demonstrated that the treat-
ed Pt/MIL-100 showed negligible changes in both activity and
selectivity compared to the untreated Pt/MIL-100 (Table S2),
which implies that there only traces of PVP molecules re-
mained in the material after the washing procedure that did
not affect the reactivity significantly.
The selective hydrogenation of a,b-unsaturated aldehydes
to their corresponding unsaturated alcohols is an important
process in the industrial synthesis of fine chemicals, particularly
of pharmaceuticals, perfumes, and cosmetics.[9] This is also con-
sidered as one of the most extensively investigated model re-
If Pt/MIL-100@MIL-100 prepared by two assembly cycles was
employed, the selectivity to cinnamyl alcohol was enhanced to
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mum shell thickness to achieve the highest selectivity because
the shell thickness influences the diffusion rate and enrichment
of the reactants in the materials.
For the Pt/MIL-100@MIL-100 catalysts, the access of reac-
tants to the fully encapsulated Pt NPs by MIL-100 would be re-
stricted strongly by the microporous windows (ꢀ5.5 and 8.6 Š
in size) as well as the aromatic p–p stacking between the
phenyl groups of cinnamaldehyde and aryl ligands of MIL-100.
As a result of the steric hindrance, cinnamaldehyde diffused
into the MOF cavities through the windows most probably
with the orientation that the C=O bond pointed inward. Mean-
while, the aromatic p electronic interaction repelled the phenyl
group of cinnamaldehyde to the pore wall of MIL-100.[1b] Thus,
the terminal C=O bond was able to approach the Pt surface
with priority compared to the C=C bond in the middle of the
cinnamaldehyde molecule. However, the Pt NPs nestled within
the MIL-100 frameworks were surrounded by numerous aryl li-
gands, which would donate electrons to the Pt surface
through the p-bond interaction or coordination (as indicated
by the XPS data; Figure 4). A higher electron density of the Pt
NPs is more favorable for the activation of the C=O in the cin-
namaldehyde.[11] Nevertheless, the MIL-100-coated Pt/MIL-100
materials also showed slightly lower reaction rates than the
uncoated Pt/MIL-100 catalyst because of the plausible steric
hindrance to the reactants caused by the MIL-100 shell.
Figure 5. Catalytic results of the hydrogenation of cinnamaldehyde. Reaction
conditions: cinnamaldehyde (0.5 mmol), catalyst (Pt 0.2 mol%), isopropyl
alcohol (5 mL), 258C, 1 atm H2.
78% at a complete conversion of cinnamaldehyde in 3 h. Pt/
MIL-100@MIL-100 prepared by four and six assembly cycles
also worked well in the hydrogenation reaction and displayed
a 95% cinnamaldehyde conversion with 96% selectivity to cin-
namyl alcohol in 4 h for the catalyst prepared by four assembly
cycles and 85% conversion with 90% selectivity in 5 h for the
material prepared by six assembly cycles. The above results
showed that the MIL-100-coated Pt/MIL-100 materials all exhib-
ited a higher selectivity to cinnamyl alcohol than the uncoated
Pt/MIL-100 catalyst. To account for the increased Fe content
after MIL-100 coating, we added FeCl3·6H2O to a reaction run
to investigate the effect of Fe on the selectivity of COL. The
result suggested that the addition of Fe3+ did not affect the se-
lectivity of COL significantly (Table S2). Additionally, after the
MOF coating, the Pt loading decreased (Table S1). As the Pt
content may influence the catalytic performance of the sup-
ported catalysts, the effects of Pt loading on the hydrogena-
tion were also investigated. Pt/MIL-100 materials with similar
Pt contents to the coated samples were used under typical hy-
drogenation conditions. The reaction results indicated showed
no apparent differences in activity and selectivity to COL over
the Pt/MIL-100 materials in the range of Pt loadings investigat-
ed (Table S2).
It is known that supported metal catalysts can deactivate
upon reuse because of the leaching and aggregation of the
metal active sites. Therefore, it is crucial that a highly active
and selective heterogeneous catalyst is stable and could be
reused. In view of this, we performed a recycling experiment
and a heterogeneity test on Pt/MIL-100@MIL-100 prepared
with four assembly cycles. No appreciable loss in activity and
selectivity was observed in the cinnamaldehyde hydrogenation
in up to five runs (Figure 6). Subsequently, a heterogeneity test
was performed to examine if there was any leaching of active
species into the reaction solution. After the removal of the
solid catalyst, the reaction solution did not exhibit any further
reactivity under similar conditions (Figure S2). Furthermore, the
Furthermore, hydrogenation using the product COL as the
substrate was performed over Pt/MIL-100 and Pt/MIL-100@MIL-
100 prepared with four assembly cycles under the investigated
conditions. As expected, the rate of C=C bond hydrogenation
on the Pt/MIL-100@MIL-100 was clearly slower than that on Pt/
MIL-100 (Table S3). Moreover, the hydrogenation rate of COL
was slower than that of cinnamaldehyde over Pt/MIL-100@MIL-
100. These results indicated that C=C hydrogenation was effec-
tively suppressed over the Pt/MIL-101@MIL-100 catalyst. Based
on these results, the remarkably enhanced selectivity of the
MIL-100-coated Pt/MIL-100 materials compared with the un-
coated materials should be related to both the steric and elec-
tron donation effects offered by the MOF shell. Notably, the
slight decrease of selectivity to cinnamyl alcohol over the
coated sample prepared with six assembly cycles compared to
that prepared with four cycles suggested that there is an opti-
Figure 6. Reuse of the Pt/MIL-100@MIL-100 prepared by four assembly
cycles in the hydrogenation of cinnamaldehyde. Reaction conditions:
cinnamaldehyde (0.5 mmol), catalyst (Pt 0.2 mol%), isopropyl alcohol (5 mL),
258C, 1 atm H2, 3 h.
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solution was analyzed by atomic absorption spectroscopy
(AAS), which demonstrated that no metal had leached into the
liquid phase during reaction. These findings were in accord-
ance with the results of XRD, N2 physisorption, and TEM char-
acterization of the reused catalyst. PXRD patterns and TEM
images of the reused catalyst were very similar to those of the
fresh material (Figure 1, Figure S3), which suggests that both
the MOF structure and metal dispersion were mostly preserved
after the hydrogenation reaction. Furthermore, the N2 adsorp-
tion isotherm of the recycled catalyst revealed that the porous
structure of the catalyst was also maintained (Figure 2,
Table S1). These results demonstrated the excellent stability
and recyclability of the Pt/MIL-100@MIL-100 catalyst under the
investigated conditions, which is believed to be related to the
protection effects of the MIL-100 shell to prevent the encapsu-
lated Pt NPs from aggregation and leaching during the reac-
tion.
Experimental Section
Catalyst preparation
All chemicals were purchased from commercial sources and used
without further purification. H2PtCl6·6H2O (A.R.), FeCl3·6H2O (A.R.),
AlCl3·6H2O (A.R.), ethanol (A.R.), methanol (A.R.), and isopropyl alco-
hol (A.R.) were obtained from Sinopharm Chemical Reagent Co.,
Ltd. 1,4-Benzenedicarboxylic acid (98%), 2-aminoterephthalic acid
(99%), PVP (M.W.=30000), NaBH4 (98%), and cinnamaldehyde
(98%) were purchased from Alfa Aesar.
Synthesis of Pt/MIL-100(Fe)
MIL-100(Fe) was synthesized by a solvothermal method reported
previously.[8] Pt/MIL-100(Fe) samples were prepared by using
a simple colloidal deposition method.[6c] Briefly, the required
amount of PVP (PVP monomer/Pt=10:1, molar ratio) was added to
an appropriate volume of H2PtCl6 methanol solution (1”10À3 m),
and the resulting mixture was stirred for 1 h. Then, a freshly pre-
pared methanol solution of NaBH4 (0.1m, NaBH4/Pt=5:1, molar
ratio) was added rapidly to the mixture under vigorous stirring.
After sol formation, which took a few minutes, the activated MIL-
100(Fe) was added immediately, and the solution was further
stirred for 8 h. Subsequently, the solids were suspended in metha-
nol, stirred for 10 min, and centrifuged. This washing procedure
was repeated four times to remove residual ClÀ, Na+, and PVP as
well as Pt species that do not interact with the support. Finally, the
sample was dried under vacuum at 1008C for 2 h to obtain Pt/MIL-
100(Fe).
To demonstrate the general applicability of this MOFs-coat-
ing approach to the enhancement of reaction selectivity, we
attempted to extend this method to other types of MOFs, for
example, NH2-MIL-101(Al), another representative MOF. This
MOF has a rigid zeotype crystal structure and possesses two
quasispherical cages (ꢀ2.9 and 3.4 nm) accessible through
windows of ꢀ1.2 and 1.6 nm, respectively. As expected, NH2-
MIL-101(Al) could also be coated successfully onto the Pt/NH2-
MIL-101(Al) surface, and the MOF structure was retained (Fig-
ures S4–S6). Furthermore, Pt/NH2-MIL-101(Al)@NH2-MIL-101(Al)
also exhibited a remarkably enhanced selectivity to cinnamyl
alcohol compared to Pt/NH2-MIL-101(Al) under identical reac-
tion conditions (Figure S7).
Synthesis of Pt/MIL-100(Fe)@MIL-100(Fe) composites
MIL-100(Fe)-coated Pt/MIL-100(Fe) was prepared according to the
following process. Typically, Pt/MIL-100(Fe) (0.1 g) was dispersed in
FeCl3·6H2O ethanol solution (8 mL, 10 mm). The mixture was stirred
for 1 h at RT. Subsequently, benzenetricarboxylic acid ethanol solu-
tion (8 mL, 10 mm) was added, and the resulting mixture was
stirred for 30 min at 708C. The MIL-100(Fe)-coated Pt/MIL-100(Fe)
composite was collected by centrifugation and washed with etha-
nol. After the required number of cycles, the sample was washed
thoroughly with ethanol and dried under vacuum at 1008C. AAS
indicated that the ratio of the leached Pt to the loading on Pt/MIL-
100 was below 1.0% after MOF coating, which indicates that the
leaching of Pt during the coating was not significant.
Conclusions
We have demonstrated a facile and general strategy for the
preparation of new Pt catalysts encapsulated fully in metal–or-
ganic frameworks (MOFs) by the coating of MOFs onto the ex-
ternal surface of Pt/MOFs by direct homoepitaxial growth.
Such a coating approach can avoid any modification of the
core with organic groups to facilitate the formation of the
MOF shell over the core. This strategy could also allow the
control of the outer MOF shell thicknesses, and the intrinsic
properties of the Pt/MOF core were retained. If the obtained
Pt/MOFs@MOFs were used as catalysts in the hydrogenation of
cinnamaldehyde under atmospheric H2 pressure and room
temperature, the selectivity to the desired cinnamyl alcohol
could be enhanced up to 96%, which shows a significant in-
crease compared to that of the uncoated Pt/MOFs (55%). The
improved selectivity is believed to be related to the electron
donation and confinement effects on the encapsulated Pt
nanoparticles offered by the surrounding MOF networks. In ad-
dition, the Pt/MOFs@MOFs catalysts were highly stable and re-
usable, without any metal agglomeration and leaching during
a number of recycles. This work might offer a promising new
strategy for the synthesis of metal nanocomposites encapsulat-
ed fully by MOFs and potentially pave the way to new oppor-
tunities for the development of highly selective heterogeneous
catalysts.
Synthesis of Pt/NH2-MIL-101(Al)@NH2-MIL-101(Al) composites
NH2-MIL-101(Al) was synthesized by a solvothermal method report-
ed previously.[12] Pt/NH2-MIL-101(Al) was prepared by using the
same procedure as described above for the synthesis of Pt/MIL-
100(Fe). NH2-MIL-101(Al)-coated Pt/NH2-MIL-101(Al) was prepared
according to the following process. Typically, Pt/NH2-MIL-101(Al)
(0.1 g) was dispersed in AlCl3·6H2O ethanol solution (8 mL, 10 mm).
The mixture was stirred for 1 h at RT. Subsequently, 2-aminotereph-
thalic acid methanol solution (8 mL, 10 mm) was added, and the
resulting mixture was stirred for 30 min at 708C. The NH2-MIL-
101(Al)-coated composite was collected by centrifugation and
washed with ethanol. After the required number of cycles, the
sample was washed thoroughly with ethanol and dried under
vacuum at 1008C.
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PXRD patterns of the samples were obtained by using a Rigaku dif-
fractometer (D/MAX-IIIA, 3 kW) that employed CuKa radiation (l=
0.1543 nm) at 40 kV and 40 mA at RT.
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General procedure for the hydrogenation of cinnamaldehyde
Typically, cinnamaldehyde (0.5 mmol) and an appropriate amount
of catalyst (Pt 0.2 mol%) were added to isopropyl alcohol (5 mL).
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Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (21322606, 21436005, 21406075, and 21576095),
the Fundamental Research Funds for the Central Universities
(2014ZB0004, 2015ZP002, and 2015PT004), Guangdong Natural
Science Foundation (2013B090500027), and China Postdoctoral
Science Foundation (2014M550437, 2015T80908).
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Keywords: heterogeneous catalysis · hydrogenation · metal–
organic frameworks · nanoparticles · platinum
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Received: November 15, 2015
Revised: December 15, 2015
Published online on January 25, 2016
ChemCatChem 2016, 8, 946 – 951
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