Selective Catalytic Performances of Noble Metal Nanoparticle@MOF Composites: The Concomitant Effect of Aperture Size and Structural Flexibility of MOF Matrices

Article abstract of DOI:10.1002/chem.201702103

Noble metal nanoparticles embedded in metal–organic frameworks (MOFs) are composite catalysts with enhanced or novel properties compared to the pristine counterparts. In recent years, to determine the role of MOFs during catalytic process, most studies have focussed on the confinement effect of MOFs, but ignored the structural flexibility of MOFs. In this study, we use two composite catalysts, Pt@ZIF-8 [Zn(mIM)₂, mIM=2-methyl imidazole] with flexible structure and Pt@ZIF-71 [Zn(DClIM)₂, DClIM=4,5-dichloroimidazole] with rigid structure, and hydrogenation of cinnamaldehyde as model reaction, to show the confinement effect and the structure flexibility of MOF matrices on the catalytic performance of composite catalysts. Both catalysts showed high selectivity for cinnamic alcohol with the confinement effect of the aperture. But, compared to Pt@ZIF-71, Pt@ZIF-8 exhibited higher conversion but lower selectivity owing to the flexible structure. The above results remind us that we will have to consider both the aperture size of MOFs and structure flexibility to select the proper MOF matrices for the composite materials to achieve the optimized performance.

Full text of DOI:10.1002/chem.201702103

A Journal of
Accepted Article
Title: Selective Catalytic Performances of Noble Metal
Nanoparticle@MOF Composites: the Concomitant Effect of
Aperture Size and Structural Flexibility of MOF Matrices
Authors: Luning Chen, Wenwen Zhan, Huihuang Fang, Zhenmin
Cao, Chaofan Yuan, Zhaoxiong Xie, Qin Kuang, and Lansun
Zheng
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To be cited as: Chem. Eur. J. 10.1002/chem.201702103
Link to VoR: http://dx.doi.org/10.1002/chem.201702103
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Chemistry - A European Journal
10.1002/chem.201702103
FULL PAPER
Selective
Catalytic
Performances
of
Noble
Metal
Nanoparticle@MOF Composites: the Concomitant Effect of
Aperture Size and Structural Flexibility of MOF Matrices
[
a]
[b]
[a]
[a]
[a]
[a]
Luning Chen, Wenwen Zhan, Huihuang Fang, Zhenmin Cao, Chaofan Yuan, Zhaoxiong Xie,
[
a]
[a]
Qin Kuang,* and Lansun Zheng
Abstract: Noble metal nanoparticles embedded in metal organic
frameworks (MOFs) are new composite catalysts with enhanced or
novel properties compared to the pristine counterparts. In the past
years, to determine the role of MOFs during catalytic process, most of
studies focus on the confinement effect of MOFs, but ignore the
structural flexibility of MOFs. In this paper, we use two composite
Recently, composite materials with noble metal NPs (such as
Pt, Pd, Au, and Ag) embedded in metal organic frameworks
(MOFs) have attracted increasing attentions because they
combine the tailorable porosity of MOFs with the versatile
functionality of noble metal NPs, such as catalysis, sensing, and
so on.[4] For this kind of composite materials, the aggregation of
noble metal NPs can be significantly prevented by the MOF
matrices, thereby keeping the surface structure of noble metal
NPs and its related effects well preserved. On the other hand, the
regular and well-defined cavities of MOF matrices can provide a
confined microenvironment for the molecular diffusion/adsorption
onto the NP surface, thereby accelerating reaction rates and/or
2
catalysts, Pt@ZIF-8 [Zn(mIM) , mIM = 2-methyl imidazole] with
flexible structure and Pt@ZIF-71 [Zn(DClIM)
2
,
DClIM 4,5-
=
dichloroimidazole] with rigid structure, and hydrogenation of
cinnamaldehyde as model reaction, to show the confinement effect
and the structure flexibility of MOF matrices on the catalytic
performance of composite catalysts. Both catalysts showed high
selectivity for cinnamic alcohol with the confinement effect of aperture. changing reaction pathways.[4c, 5, 6] Owing to the synergism effect
But, compared to Pt@ZIF-71, Pt@ZIF-8 exhibited higher conversion
but lower selectivity owing to the flexible structure. The above results
remind us that we will have to consider both the aperture size of
MOFs and structure flexibility to select the proper MOF matrices for
the composite materials to achieve the optimized performances.
between two functional materials, the noble metal NPs embedded
in MOFs often exhibit enhanced or even novel properties
compared to their pristine counterparts; thereinto, they are
hopefully endowed with high activity and good selectivity in
heterogeneous catalytic reactions.[7] Determing the role of MOF
matrices in catalysis is one of the central issues for the successful
application of such composite materials.[8] In the past years, most
of studies just focus on the confinement effect of MOF matrices
due to their small apertures, but the influence from the structural
Introduction
flexibility of MOF matricesis, which have been deeply studied in
gas absorption/seperation, are often ignored.[9] However, the
Noble metal nanoparticles (NPs) are high-efficiency catalysts for
many chemical reactions due to their large surface area and
abundant unsaturated coordinated atoms.[1] Among noble metals,
Pt has excellent catalytic activity for hydrogenation reactions with
its superior “H-H” bond breaking capacity.[2] However, just
because of this high catalytic activity, Pt NPs have certain
limitations in the chemoselective hydrogenation of the
compounds with two and more double bonds, which is crucial for
the production of commodity chemicals as well as fine and
special chemicals.[3] Consequently, it is highly desirable to
explore Pt-based catalysts with high activity and selectivity for
selective hydrogenation reactions.
structural flexibility of MOFs likewise has a significant influence
on the diffusion/adsorption of reaction molecules in MOF
cavities.[10] Consequently, flexible frameworks may weaken their
size confinement effect during the catalytic process so as to
produce some exceptional results compared to those composite
catalyst with rigid MOFs as matrices.
Herein, we specifically synthesized two kinds of composite
catalysts with Pt NPs embedded in MOF matrices, Pt@ZIF-8 and
Pt@ZIF-71, to show the concomitant effect of the aperture size
and the structure flexibility of MOF matrices on the catalytic
performance of composite catalysts. The results demonstrated
that the two composite catalysts were highly efficient to the
selective hydrogenation of cinnamaldehyde (CAL) to cinnamic
alcohol under mild conditions due to the confinement effect from
the small apertures of MOF matrices. But the catalyst with Pt NPs
embedded in flexible matrices (i.e., Pt@ZIF-8) was found to be
much higher in conversion but lower in selectivity than the
catalyst with embedded in rigid matrices (i.e., Pt@ZIF-71) due to
the flexible structure of ZIF-8 frameworks.
[a]
L. Chen, H. Fang, Z. Cao, C. Yuan, Prof. Z. Xie, Prof. Q. Kuang,
Prof. L. Zheng
State Key Laboratory for Physical Chemistry of Solid Surfaces
Collaborative Innovation Centre of Chemistry for Energy Materials
Department of Chemistry
College of Chemistry and Chemical Engineering
Xiamen University, Xiamen, 361005 (P.R. China)
E-mail: qkuang@xmu.edu.cn
Results and Discussion
[
b]
Dr. W. Zhan
Department of Chemistry
School of Chemistry and Chemical Engineering
Jiangsu Normal University, Xuzhou, 221116 (P.R. China)
Supporting information for this article is given via a link at the end of
the document.
1
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Scheme 1. The synthetic process of Pt NP embedded MOF composite
catalysts and the crystallographic structures of corresponding ZIF-8 and ZIF-71
matrix.
The Pt@ZIF-8 and Pt@ZIF-71 composite catalysts were
Figure 1. (a, b) TEM images and (c, d) their XRD patterns of Pt@ZIF-8-X% and
Pt@ZIF-71-X% (X% = 1, 3, 5, 10%).
prepared according to
a previously reported encapsulation
method, in which the Pt NPs used need to be modified with a
layer of PVP to improve the compatibility of Pt NP surfaces with
organic ligands (Scheme 1).[11] In both synthetic processes, the
content of Pt NPs in the composite catalysts was tuned through
adding different volumes of the pre-prepared methanol solution of
PVP-modified Pt NPs (5 mg/mL) into the growth solutions of MOF
matrices. For convenience, the as-prepared composite catalysts
were denoted as Pt@ZIF-8-X% or Pt@ZIF-71-X% (X% =1%, 3%,
The composition of composite catalysts was examined by
powder X-ray diffraction (XRD, Figure 1c, d). For Pt@ZIF-8-1%
and Pt@ZIF-71-1%, the XRD patterns match well with the
simulated pattern of ZIF-8 and ZIF-71, respectively, and no
diffraction peaks assigned to Pt are detected due to the small size
of Pt NPs and their low contents in the composites. When the
theoretical Pt content is above 3%, some weak and broad
diffraction peaks corresponding to the characteristic peaks of face
centerd cubic Pt (JCPDS no.04-0802) appear in the XRD patterns
of composites together with the peaks assigned to ZIF-8 or ZIF-
71. With the increase of Pt NPs added, these additional diffraction
peaks become more and more prominent. According to the
Debye-Scherrer formula, the calculated average sizes of Pt NPs
from the (111) diffraction peaks are ca. 4.5 nm, which is
consistent with the TEM observed value (4.3 nm) of the original
5%, and 10%) according to the theoretical contents of Pt NPs.
Structurally, ZIF-8 possesses a sodalite zeolitic-type structure
featuring large cavities (11.6 Å) and small apertures (3.4 Å), and
thus it only allows in principle for intrusion and diffusion of small
molecules into the frameworks.[12] As counterpart, ZIF-71 is a rigid
MOF with a RHO topology and has larger cavities (16.8 Å) and
pore apertures (4.8 Å).[13] But recent studies have demonstrated
that the sodalite cell of ZIF-8 can shrink or expand upon the
pressure stimulus, thereby allowing the intrusion of larger
molecules than the aperture size of ZIF-8 into cavities.[14] Besides, PVP-modified Pt NPs (Figure S6). The inductively coupled
plasma mass spectrometry (ICP-MS) analysis indicates that the
actual contents of Pt in the composites increase with the addition
of PVP-modified Pt NPs in growth solution, but all are lower than
their corresponding nominal values. This deviation is probably
caused by the loss of unsuccessfully embedded Pt NPs in the
washing treatment.
it is also proposed that a short-lived “open“ state happens to ZIF-
due to the linker dissociation, which allows larger substrate
molecules easily pass through the aperture of ZIF-8.[15]
Predictably, this unique flexible feature of ZIF-8 has a significant
impact on the confinement effect of MOF matrices in the practical
applications including catalysis.
8
Nitrogen adsorption/desorption isotherms were measured to
evaluate the influence of embedding of Pt NPs on the internal
surface areas and pore size distributions of MOF matrices. The
two series of composites both display type-I isotherms similar
with the blank ZIF-8 or ZIF-71 matrices, in which the steep
The morphologies of blank MOF matrices (i.e., ZIF-8 and ZIF-
1) and their composites with Pt NPs embedded were first
7
revealed by scanning electron microscopy (SEM) (Figure S1). All
the products, whether with Pt NPs embedded or not, are rhombic
dodecahedra with uniform shape and size. This indicates that the
crystallization habits of ZIF-8 or ZIF-71 cannot be changed by the
introduction of Pt NPs. Compared to the microsized ZIF-71 series,
the sizes of ZIF-8 series are only 300‒400 nm. The transmission
electron microscopy (TEM) images (Figure 1a, b) and associated
elemental mappings (Figure S2, S3) further reveal that the most
of Pt NPs are randomly embedded in ZIF-8 or ZIF-71 matrices,
without obvious aggregation, and the embedding densities
increase with the increase of feeding ratio of the Pt NPs.
Accordingly, the colours of both two series are gradually
deepened, changing from white to dark grey (Figure S4). Of
particular note, high-magnification TEM images (Figure S5)
clearly reveal that a few Pt NPs are embedded on the external
surface of MOFs, especially in the cases of high Pt content (5%
and 10%).
2
increases in N uptake at a low relative pressure (<0.01) present
the characteristic of microporous materials (Figure S7). Of note,
in contrast to the ZIF-71 series with larger size, the ZIF-8 series
2
of 300‒400 nm display an obvious N uptake at high relative
pressure, which indicates the existence of textural
meso/macroporosity formed by packing of Pt@ZIF composite
particles.[16] And with increasing of Pt feeding, more PVP exist in
the solution, which will somewhat influence the size and
morphology of ZIF-8. Table S1 lists the calculated specific
surface areas of the two series according to the Brunaurer-
Emmett-Teller (BET) model. Because of the smaller sizes, the
2
surface areas of the ZIF-8 series (1060‒1400 m /g) are higher
2
than those of the ZIF-71 series (720‒820 m /g). Compared with
the pure ZIF-8 or ZIF-71, the composite catalysts with low Pt
loading amounts possess higher specific surface areas (e.g.,
2
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2
2
1
402 m /g for Pt@ZIF-8-1% and 821 m /g for ZIF@ZIF-71-1%),
shown in Figure 2a, the selectivity of cinnamic alcohol over
Pt@ZIF-8-1% and Pt@ZIF-71-1% is raised to 88.7% and 96.9%,
respectively, although the conversion of CAL significantly
decreases to 58.6% and 11.2%, respectively. Furthermore, the
excessive hydrogenation of CAL is almost completely inhibited in
the two Pt embedded cases (Figure 2b). However, the pure ZIF-8
and ZIF-71 without Pt NPs showed no activity, which indicates
owing to the presence of structure defects in the MOF matrices
during the incorporation of Pt NPs. And with more Pt NPs
embedded, the surface area of these composite catalysts
consistently decrease, since more and more cavities of MOF
matrices are blocked by Pt NPs.[17]
4
that organic links (mIM, DClIM), ZnN tetrahedra, and their
defective sites do not contribute to the catalytic reaction. It should
be noted that all the composite catalysts possess very good
structural and chemical stability during the catalysis tests (Figure
S8, S9), and the size of Pt NPs can be well kept without
agglomeration (Figure S10). Besides that, the ICP-MS analysis
reveals that there is no loss of Pt NPs after catalysis (Table S3),
which indirectly proves that most of Pt NPs are embedded in the
MOF matrices.
Scheme 2. Schematic diagram of the cinnamaldehyde hydrogenation.
In this study, we chose cinnamaldehyde (CAL) as a model
molecule, which is a common chain compound with a “C=C” bond
and a “C=O” bond, to show how the aperture size and structural
flexibility of MOFs influence the catalytic performance in
chemoselective hydrogenation of α,β-unsaturated aldehydes.[18]
Principally, CAL can be converted to various hydrogenated
products such as (a) cinnamic alcohol, (b) phenylpropyl aldehyde,
(
c) phenpylpropanol, and (d) phenylpropane as illustrated in
Figure 3. (a
composites. (a
TEM images and (a
after encapsulated in the corresponding ZIFs.
1
, b
1
) The formation process of sandwich-structured Pt@ZIF@ZIF
) TEM images of Pt@ZIF-8-1% and Pt@ZIF-71-1%. (a , b
, b ) STEM images of Pt@ZIF-8-1% and Pt@ZIF-71-1%
Scheme 2, of which, cinnamic alcohol is a common essence and
medical intermediate to synthesize hypotensor and anti-
carcinogen. However, the formation of saturated aldehydes for α,
β-unsaturated aldehydes like CAL is thermodynamically favoured
over the unsaturated alcohol. Therefore, the chemoselective
hydrogenation of α,β-unsaturated aldehydes to the corresponding
unsaturated alcohol remains a scientific challenge to date.[19]
2
, b
2
3
3
)
4
4
As observed in TEM images, some Pt NPs fail to be
embedded into the MOF matrices in the two series of Pt@ZIF
composites with the amount of Pt loading increasing. Admittedly,
the presence of these unencapsulated Pt NPs may influence the
determination of the intrinsic role of MOF matrices, because they
exhibit distinct catalytic properties from those in the MOF
matrices due to the lack of confined microenvironment. In order to
exclude the effect of the unencapsulated Pt NPs located on the
surface, two series of composites with a unique sandwich-like
structure, Pt@ZIF-8@ZIF-8 and Pt@ZIF-71@ZIF-71, were
specially synthesized in our study (Figure S11). As illustrated in
1 2
Figure 3a and 3b , another ZIF-8 or ZIF-71 shell epitaxially
Figure 2. (a) Comparison of the conversions of CAL hydrogenation and the
selectivities for cinnamic alcohol over Pt/C-20%, Pt-PVP, Pt@ZIF-8-1% and
Pt@ZIF-71-1%. (b) Comparison of the selectivity for four products over four
catalysts.
grows on the surface of the original Pt@ZIF catalysts with the
assistance of cetyltrimethyl ammonium bromide (CTAB), leading
to the formation of the sandwich-like structure.[8a,
21]
By
comparison of the TEM and scanning transmission electron
microscopy (STEM) images of the Pt@ZIFs before and after the
second growth, it is clearly seen that all the Pt NPs have been
absolutely encapulated within MOF matrices in the sandwich-like
The results of CAL hydrogenation over the blank MOF
matrices and their composites with different Pt loading amounts
are listed in Table S2. And 20% carbon matrixed platinum (Pt/C-
structures (Figure
a2‒4 and Figure b2‒4). Interestingly, after
growing a ZIF shell, the morphologies of all Pt@ZIF-8 composites
change from rhombic dodecahedra to cube due to the specific
adsorption of CTAB on the ZIF-8 {100} faces.[22] By contrast, the
morpholgies of all the as-formed Pt@ZIF-71@ZIF-71 still keeps
rhombic dodecahedral, the same as Pt@ZIF-71. The nitrogen
adsorption/desorption isotherms of the sandwich-structured
samples is simlar to the original Pt@ZIF samples (Figure S12).
2
0%) and PVP-stabilized Pt NPs (Pt-PVP) were also tested as
the reference catalysts. It should be stressed here that the Pt NPs
embedded in all the composite catalysts added are normalized to
the same weight (5 mg) in the tests according to their ICP-MS
results. Over Pt/C-20%, the conversion of CAL was very high
(95.4%), but all possible hydrogenation forms are produced, with
only 24.4% selectivity for cinnamic alcohol. By contrast, the PVP-
modified Pt NPs give higher cinnamic alcohol selectivity (44.3%)
and lower conversion of CAL (55.1%), which is attributed to the
steric effect of PVP on the adsorption of CAL molecules on the
surface of Pt NPs.[20] However, after Pt NPs are embedded in
MOF matrices, the resulting composite catalysts show distinct
catalytic performances from those unencapsulated Pt NPs. Take
Pt@ZIF composites with loading amount of 1% for example. As
2
The surface areas of ZIF-8 series (1000‒1200 m /g) are higher
2
than ZIF-71 series (700-800 m /g) like the original composites
(Table S4). The ICP-MS analysis reveals that the actual Pt
contents in the sandwich-structured samples almost remain the
same with the values in the original Pt@ZIF samples, which
shows that Pt NPs embedded on the external surface of MOF
matrices didn’t fall off during the growth of ZIF outer shells.
3
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Figure 5. (a) The conversion of CAL and (b) the selectivity of cinnamic alcohol
over different catalysts (Pt/C-20%, Pt-PVP, Pt@ZIF-8@ZIF-8-1% and Pt@ZIF-
71@ZIF-71-1%) at different times.
It is widely acknowledged that the diffusion/absorption of
guest molecules in MOFs is greatly confined to the sizes of
apertures and cavities of MOFs, which usually leads to the
decreased conversion of reactants. In view of small aperture
sizes of ZIF-8 and ZIF-71, the CAL molecule with a benzene ring
prefers to vertically adsorb on the Pt surface with aldehyde
groups (“C=O”). Consequently, the H atoms produced due to the
“H-H” bond breaking on the Pt surface would be selectively added
into the “C=O” instead of “C=C”, thereby improving the selectivity
of cinnamic alcohol in the products.[
20, 23]
However, in terms of this
Figure 4. Comparison of the conversions of CAL hydrogenation and the
selectivities for cinnamic alcohol over Pt@ZIF-8-1%, Pt@ZIF-71-1%, and
Pt@ZIF-8-1%, Pt@ZIF-71-1% embedded in corresponding ZIF shell.
confinement effect of MOF matrices, there is a distinct difference
between ZIF-8 and ZIF-71. Compared with Pt@ZIF-71 catalysts,
Pt@ZIF-8 catalysts show higher conversion but lower selectivity
for cinnamic alcohol. Typically, the conversion of Pt@ZIF-8-1%
(55.1%) is five times of Pt@ZIF-71-1% (11.2%), while the
selectivity (88.7%) of the former is lower than that (96.9%) of the
latter. This result is not normal, since ZIF-8 is supposed to display
a stronger confinement effect in the catalytic conversion of CAL
due to its smaller apertures than ZIF-71. Clearly, there are other
factors that significantly influence the confinement role of MOF
matrices in the catalysis reaction.
Considering that the effect of the unencapsulated Pt NPs has
been excluded in the as-synthesized sandwich-like structures, we
futher investigated their catalytic performances in the CAL
hydrogenation reaction (Table S5. In comparison with the original
Pt@ZIF catalysts, the conversion of CAL over the two series of
sandwich-structured
Pt@ZIF@ZIF
catalysts
significantly
decreases, but the selectivity of cinnamic alcohol is raised to
some extent. Taking Pt@ZIF-8-1% and Pt@ZIF-71-1% as
examples, the performances of the catalysts before and after
growing the ZIF shell are directly compared in Figure 4. The
coversion of CAL over Pt@ZIF-8@ZIF-8-1% and Pt@ZIF-
7
1@ZIF-71-1% is reduced to 34.5% and 7.5% from 58.6% and
1.2%, respectively, while the selectivity of cinnamic alcohol is
1
raised to 90.4% and nearly 100% from 88.7% and 96.9%,
respectively (Figure 4). Clearly, these changes are caused by the
sandwich-structure that comfirms there are no Pt NPs exposed
on the surface of ZIFs. This result indicates that the outer ZIF
shell not only depresses the diffusion of CAL onto the Pt NPs, but
also provides a completely confined microenvironment for the Pt
NPs. Of particular note, the Pt@ZIF-8@ZIF-8 catalyts always
exhibit higher conversion of CAL and lower selectivity of cinnamic
alcohol than the Pt@ZIF-71@ZIF-71 catalysts, which well agrees
with the results in the cases of Pt@ZIF catalysts. The same law
for the catalytic performances of these composites was further
confirmed by the time-dependent catalytic experiments. As shown
in Figure 5a, the conversion of CAL over all the measured
catalysts gradually increases with the extended reaction time at
the initial stage, and reaches a plateau after 6 hours due to the
state of chemical equilibrium. By contrast, the selectivity of
cinnamic alcohol over the catalysts basically remain unchanged
during the whole catalytic process (Figure 5b). However, no
matter the reaction time, the Pt@ZIF-8@ZIF-8 exhibit higher
conversion of CAL and lower selectivity of cinnamic alcohol
compared to the Pt@ZIF-71@ZIF-71. When the above results are
combined together, we could reasonably conclude that the
catalytic performance differences of the composites with Pt NPs
embedded in different MOFs are intrinsically caused by the
structural differences of MOFs.
Figure 6. Schematic illustration for the hydrogenation of CAL on the Pt NPs
embedded within (a) flexible ZIF-8 and (b) rigid ZIF-71, respectively.
ZIF-8 is structurally a flexible structure due to the swinging of
3
the –CH groups and imidazolate linkers and dissociation of 2-
methylimidazole to formation the short-lived “open“ states without
disrupting the underlying MOF crystal structure and morphology,
but ZIF-71 has no similar structural deformation.[14, 15] This flexible
feature of ZIF-8 make CAL molecules more easily pass through
the MOF pores and contact Pt NPs compared to in the case of
ZIF-71, thereby leading to a higher conversion of Pt@ZIF-8
(Figure 6). After passing through ZIF-8’s pores, CAL molecules
have the possibility to absorb on the Pt NPs flatly, which makes
“C=C” bonds interact with the Pt surface and be hydrogenated,
and therefore more by-products phenylpropyl aldehyde (9.6%)
produce for Pt@ZIF-8. No doubt, the abnormal catalytic
performances of Pt@ZIF-8 catalyst should result from the
structural deformation of flexible ZIF-8, which to some extent
weakens the size confinement effect of MOF matrices.
4
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Chemistry - A European Journal
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Conclusions
another five minutes, the resulting solution was stirred for 5 hours
under room temperature. Finally, the as-formed Pt@ZIF-8@ZIF-8
catalyst was collected by centrifugation, followed by washing with
ethanol several times and drying in the vacuum oven. The
resulting composite catalysts were denoted as Pt@ZIF-8@ZIF-8-
X% (X% =1%, 3%, 5%, 10%) according to the Pt@ZIF-8-X%
dispersed.
In summary, two kinds of Pt embedded in MOF composite
catalysts, Pt@ZIF-8 and Pt@ZIF-71, were specifically
synthesized by the encapsulation method. The results revealed
that the two composite catalysts were highly efficient to the
selective hydrogenation of CAL to cinnamic alcohol under mild
conditions due to the confinement effect from small apertures of
MOFs matrices. But the Pt@ZIF-8 catalysts were found to exhibit
higher catalytic efficiency and lower selectivity than Pt@ZIF-71,
although ZIF-8 is smaller in the aperture size than ZIF-71. This is
the first time to demonstrate that the structure flexibility of MOF
matrices does make a significant influence on the confinement
effect of MOF matrices, thereby leading to some exceptional
catalytic performances of composite catalysts. Our present study
provides some important insights into the structure-performance
relationship of NP@MOF composites from a new perspective,
which will help us to construct the composite materials with
desired performances.
As for Pt@ZIF-71@ZIF-71, the synthetic process was the
same as Pt@ZIF-8@ZIF-8, except that methanol was used as
solvent, 4,5-dichloroimidazole as organic linkers and zinc acetate
as zinc sources.
Catalysis test: The catalysts, which contained about 5 mg Pt
(0.5% molar ratio), were dispersed in 10 mL n-butyl alcohol, and
then transferred into a pressure vessel. 0.4 mL cinnamaldehyde
was added into the above solution. The vessel was flushed with
2
H flow for several minutes to remove the oxygen, and then
pressurized to 3 atm. After reaction at room temperature for 6 h,
the solution was analyzed by gas chromatography-mass
spectrometry (GC-MS, QP2010 Plus).
Acknowledgements
Experimental Section
This work was supported by the National Basic Research
Program of China (2015CB932301), the National Natural Science
Foundation of China (2171163 and 21333008) and the
Fundamental Research Funds for the Central Universities
Sample synthesis:
Synthesis of 3-5 nm PVP-modified Pt nanoparticles: the
PVP-modified Pt NPs were prepared by refluxing a mixture of 533
(20720160026). C. Yuan is grateful for the support of NFFTBS
No. J1310024).
2 6
mg PVP, aqueous solution of H PtCl (15 mL, 8 mM), and
(
methanol (180 mL) in a flask (500 mL) for 3 hours under air. And
then methanol was removed by using rotary evaporator and the
Pt NPs were cleaned with acetone and hexane to remove excess
free PVP. After dried up, Pt NPs were finally dispersed in
methanol to give a solution with Pt concentration of 5 mg / mL.
Synthesis of Pt@ZIF-8 and Pt@ZIF-71 composites: To
make Pt NPs embedded in ZIF-8 matrices, a 1.38 mL pre-
prepared methanol solution of Pt NPs was added into a 40 mL
aqueous solution containing 5.5 g (67.1 mmol) 2-methylimidazole.
Keywords: metal-organic framework • composite catalyst•
heterogeneous catalysis • structural flexibility • selective
hydrogenation
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[2]
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Cremer, X. C. Su, Y. R. Shen, G. A. Somorjai, J. Am. Chem. Soc., 1996,
3 2
After ultrasonic dispersion, a 3.0 mL Zn(NO ) methanol solution
(0.5 mol/L) was added and the resulting solution was stirred for 6
1
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theoretical loading amount of Pt in the composite was 1 wt%, and
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In the synthesis of Pt@ZIF-71 with 1 wt% Pt loading (i.e.,
Pt@ZIF-71-1%), a 0.134 mL prepared methanol solution of Pt
NPs was added into a 15 mL methanol solution containing 0.1046
g (0.76 mmol) 4,5-dichloroimidazole. After ultrasonic dispersion, a
5
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0 mg prepared Pt@ZIF-8 particles as seeds were dispersed in a
solution prepared by mixing 5 mL deionized water and 750 L
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3
0
1
HmIM aqueous solution mixed with 750 L 0.01 mol/L CTAB
aqueous solution was added into the Pt@ZIF-8 solution. After
1
ultrasounding for five minutes, 5 mL 48 mmol/L Zn(NO
3 2
)
aqueous solution was added into the solution. After ultrasound for
5
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Chemistry - A European Journal
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Chemistry - A European Journal
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FULL PAPER
Two composite catalysts (i.e. Pt@ZIF-
Luning Chen, Wenwen Zhan, Huihuang
Fang, Zhenmin Cao, Chaofan Yuan,
Zhaoxiong Xie, Qin Kuang,* and Lansun
Zheng
8
and Pt@ZIF-71) were designedly
synthesized to investigate the
concomitant effect of aperture size
and structural flexibility of
metal−organic framework matrices in
the catalysis application. Compared to
the Pt NPs sheathed with the rigid MOFs
Page No. – Page No.
Selective Catalytic Performances of
Noble Metal Nanoparticle@MOF
Composites: the Concomitant Effect
of Aperture Size and Structural
Flexibility of MOF Matrices
(Pt@ZIF-71), Pt@ZIF-8 composite
catalyst was found to exhibit higher
conversion but lower selectivity owing to
the flexible structure of ZIF-8 matrices.
7
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