Fast and Selective Semihydrogenation of Alkynes by Palladium Nanoparticles Sandwiched in Metal–Organic Frameworks

Article abstract of DOI:10.1002/anie.201913453

The semihydrogenation of alkynes into alkenes rather than alkanes is of great importance in the chemical industry. Unfortunately, state-of-the-art heterogeneous catalysts hardly achieve high turnover frequencies (TOFs) simultaneously with almost full conversion, excellent selectivity, and good stability. Here, we used metal–organic frameworks (MOFs) containing Zr metal nodes (“UiO”) with tunable wettability and electron-withdrawing ability as activity accelerators for the semihydrogenation of alkynes catalyzed by sandwiched palladium nanoparticles (Pd NPs). Impressively, the porous hydrophobic UiO support not only leads to an enrichment of phenylacetylene around the Pd NPs but also renders the Pd surfaces more electron-deficient, which leads to a remarkable catalysis performance, including an exceptionally high TOF of 13835 h^?1, 100 % phenylacetylene conversion 93.1 % selectivity towards styrene, and no activity decay after successive catalytic cycles. The strategy of using molecularly tailored supports is universal for boosting the selective semihydrogenation of various terminal and internal alkynes.

Full text of DOI:10.1002/anie.201913453

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Accelerating Selective Semihydrogenation by Metal-Organic
Frameworks
Authors: Kwanghak Choe, Fengbin Zheng, Hui Wang, Yi Yuan, Wenshi
Zhao, Guangxin Xue, Xueying Qiu, Myonghak Ri, Xinghua
Shi, Yinglong Wang, Guodong Li, and Zhiyong Tang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913453
Angew. Chem. 10.1002/ange.201913453
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10.1002/anie.201913453
Angewandte Chemie International Edition
RESEARCH ARTICLE
Accelerating Selective Semihydrogenation by Metal-Organic
Frameworks
Kwanghak Choe,[+] Fengbin Zheng,[+] Hui Wang,[+] Yi Yuan, Wenshi Zhao, Guangxin Xue, Xueying Qiu,
Myonghak Ri, Xinghua Shi, Yinglong Wang, Guodong Li* and Zhiyong Tang*
Abstract: Semihydrogenation of alkynes into alkenes against alkanes
is of great importance in the chemical industries. Unfortunately, state-
of-the-art heterogeneous catalysts hardly achieve high turnover
frequency (TOF) simultaneously with near unity conversion, excellent
selectivity and good stability. Here we demonstrate metal-organic
frameworks (MOFs) containing Zr metal nodes (known as UiO) with
tunable wettability and electron-withdrawing ability, which are easily
realized by selecting organic linkers, as activity accelerators for
semihydrogenation of alkynes catalyzed by the sandwiched palladium
nanoparticles (Pd NPs). Impressively, the porous hydrophobic UiO
supports not only enrich phenylacetylene (C6H5C≡CH) around Pd NPs
but also endow Pd surfaces with less electrons, which leads to
remarkable catalysis performance including exceptionally high TOF
value of 13835 h-1, 100% conversion of phenylacetylene, 93.1%
selectivity toward styrene (C6H5CH=CH2), and no activity decay after
successive catalytic cycles. The strategy of molecularly-tailored
supports is universal for boosting selective semihydrogenation of
various terminal and internal alkynes.
must be developed for accelerating catalytic reactions and
breaking the trade-off between high conversion and good
selectivity/stability.
A typical and critical example is selective semihydrogenation
of alkynes, which is an extremely important and valuable
transformation owing to broad application of the target products
(alkenes) in pharmaceuticals, vitamins, agrochemicals,
fragrances, polymerization and olefin metathesis reactions, etc.[5,
6]
To find the hydrogenation catalysts of high selectivity always
remains great challenge, since the adsorption capability of C≡C
and C=C groups over active metal sites is very difficult to be
distinguished and accordingly the overhydrogenation products
(alkanes) are hardly avoided. Many supports including CaCO3,[7,
8]
metal oxides,[4,5g,9,10] carbon materials,[11] molecular sieves[12]
and MOFs[13] have been utilized for enhancing the selectivity[5c]
but the reported catalysts show rather low TOF values.
In this work, we choose MOFs, which are formed by self-
assembly of metal ions or clusters with ditopic or polytopic organic
linkers, as the porous encapsulation supports due to their
advantages including abundant composition, large surface area
and ordered pore structure.[14-16] Indeed, MOFs have exhibited
great potential in catalytic liquid phase reactions, for example,
some strategies have been developed to improve the catalytic
performance by functionalizing the organic linker of MOFs with
different functional group.[17] In addition, Pd NPs are employed as
the active components because of their moderate reactivity to H2
and stronger adsorption of C≡C group than C=C group compared
with other noble metal NPs such as Pt, Au or Ag,[18] which might
alleviate overhydrogenation of alkynes to some extent. We show
that the organic linkers in MOFs supports, which receive much
less attention in catalytic reactions compared with metal nodes,
are the key to accelerate semihydrogenation of multiple alkynes
to alkenes.
Introduction
Beyond the conventional supported catalysts, the encapsulated
structures of active noble metal NPs with the porous supports like
molecular sieves[1] and MOFs[2] have been recognized as
emerging heterogeneous catalysts. Such encapsulation not only
maximizes the contact between active NPs and supports but also
secures the NPs to prevent aggregation, so it has been widely
utilized in different types of reactions to enhance the selectivity
and stability of catalysts.[2f,2g] Unfortunately, these encapsulated
structures generally decrease the catalytic activity and the
obtained TOF values are quite low, resulting from partial blockage
of the transfer and diffusion of substrates and products.[3,4]
Therefore, new design strategies on the heterogeneous catalysts
Results and Discussion
[a]
K. Choe, F. Zheng, Dr. H. Wang, Y. Yuan, W. Zhao, Dr. G. Xue, M.
Ri, Prof. X. Shi, Prof. G. Li, Prof. Z. Tang
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication
CAS Center for Excellence in Nanoscience
National Center for Nanoscience and Technology
Beijing 100190, P. R. China
E-mail: zytang@nanoctr.cn, liguodong@nanoctr.cn
F. Zheng, Prof. Y. Wang
Qingdao University of Science and Technology
Qingdao 266042, P. R. China
To achieve highly efficient semihydrogenation reaction, we
suggest that Pd NPs are sandwiched by UiO core and shell (UiO
= University of Oslo) with different pore aperture and wettability
via extension of the frameworks from terephthalic acid (H2BDC)
to
2,2'-bipyridine-5,5'-dicarboxylic
acid
(H2BPYDC),
[b]
[c]
[+]
biphenyldicarboxylic acid (H2BPDC) and diterephthalic acid
(H2TPDC),[15] termed as UiO-66, UiO-67(N), UiO-67 and UiO-68,
respectively (Figure 1a and 1b). The micro-sized UiO-67, which
is synthesized by reacting Zr4+ ion with H2BPDC, is chosen as the
standard core to fabricate the sandwich-like UiO-67@Pd@UiO-X
(X = 66, 67(N), 67 and 68) composites with identical morphology
and shell thickness. The specific surface areas of pure MOFs
K. Choe, M. Ri, Prof. Z. Tang
University of Chinese Academy of Sciences
Beijing 100049, P. R. China
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201913453
Angewandte Chemie International Edition
RESEARCH ARTICLE
microcrystals are 915 m2 g-1 for UiO-66, 2456 m2 g-1 for UiO-67(N),
2409 m2 g-1 for UiO-67 and 2061 m2 g-1 for UiO-68, while the
average pore sizes are 0.85 and 1.18 nm for UiO-66, 1.10 nm for
UiO-67, 1.10 nm for UiO-67(N), 1.71 and 2.36 nm for UiO-68
(Figures 1c, S1-S4 and Table S1),[15,19] exemplifying their tunable
pore structures. More importantly, the wettability property of UiOs
is also modulated by changing their backbones. Density
functional theory (DFT) calculation was performed to assess the
wettability property of UiOs (Figure S5).[20] With increasing the
number of benzene rings in H2BDC, H2BPDC and H2TPDC, the
calculated oil-water partition coefficient (logPoctanol/water) is -33.27,
-15.94 and 1.33, revealing that the hydrophobicity of UiOs is
obviously increased via extension of their backbones with more
benzene rings (Table S2). Note that owing to the presence of
pyridylazo in H2BPYDC, UiO-67(N) is more hydrophilic than UiO-
67 and the corresponding logPoctanol/water value is -330.23. The
order of theoretical calculation is verified by contact angle
experiment: 34.1o for UiO-67(N)  41.2o for UiO-66  74.3o for
UiO-67  90.5o for UiO-68 (Figure 1d). The molecularly-tunable
pore size and wettability property of MOF supports offer the
foundation for high activity catalysis.
then added into the above solution drop by drop to produce UiO-
67@Pd samples characteristic of Pd NPs of ~3.5 nm in diameter
uniformly dispersed on the surface of UiO-67 rather than inside
the pore, which was mainly ascribed to the hydrophobic nature of
UiO-67 and low Pd loading (Figures S6 and S7). Finally, the
sandwich-like UiO-67@Pd@UiO-67 composites were fabricated
by coating with UiO-67 shells (Figure 2), which were well
dispersed in the organic solvents such as DMF and ethylacetate
(Figure S8).
Transmission electron microscopy (TEM) images show that the
obtained sandwich-like structures inherit the octahedral shape of
UiO-67 core, and the average shell thickness is, respectively,
controlled to be 35 and 50 nm via optimizing the precursor
concentration and coating time, which is denoted as UiO-
67@Pd@UiO-67 (35 nm) and UiO-67@Pd@UiO-67 (50 nm)
(Figures 2a-2d, S9 and S10). Powder X-ray diffraction (XRD)
patterns of UiO-67@Pd@UiO-67 (35 nm) and UiO-67@Pd@UiO-
67 (50 nm) display only one set of peaks corresponding to
crystalline UiO-67 (Figure 2e), and the absence of characteristic
XRD peaks of Pd NPs is ascribed to their small size and low
content (0.66% and 0.59% by weight, Table S3). As comparison,
the exposed (111) facets of Pd NPs can be clearly discerned by
high-resolution TEM imaging (Figure 2f). High angle annular dark
field scanning transmission electron microscopy (HAADF-STEM)
of the cross-section of UiO-67@Pd@UiO-67 (50 nm) clearly
indicates that a layer of Pd NPs are uniformly distributed between
the UiO-67 core and shell (Figure 2g and 2h).
Figure 1. Pd NPs sandwiched by UiO core and shell with different pore aperture
and wettability, UiO-67@Pd@UiO-X (X = 66, 67(N), 67 and 68). a) Synthesis
route. b) Structure units of different UiOs via extension of their frameworks from
H2BDC to H2BPYDC, H2BPDC and H2TPDC. c) Pore size distributions of UiO-
66, UiO-67(N), UiO-67 and UiO-68. d) Contact angles of UiO-66, UiO-67(N),
UiO-67 and UiO-68 measured by water adsorption technique at 25oC.
Among the sandwich-like catalysts mentioned above, the UiO-
67@Pd@UiO-67 is used as a representative for preparation
process and structure analysis. In brief, UiO-67 core was
synthesized by coordinating Zr4+ ions with H2BPDC in the N,N-
Figure 2. Morphology of sandwich-like UiO-67@Pd@UiO-67 structures. a, b)
TEM images of UiO-67@Pd@UiO-67 (35 nm). c, d) TEM images of UiO-
67@Pd@UiO-67 (50 nm). e) XRD patterns of UiO-67@Pd@UiO-67 (35 nm)
and UiO-67@Pd@UiO-67 (50 nm). f) HRTEM image of Pd NPs inside UiO-67
matrix in a). g) HAADF-STEM mapping images of the cross-section of UiO-
67@Pd@UiO-67 (50 nm), and h) magnification of the part marked with red
rectangle in g).
dimethylformamide (DMF) solution via
a
solvothermal
method.[15,21] As-obtained products are of uniformly octahedral
shape with a mean diameter of 2.8 μm, and their crystal structure
is indexed to the cubic phase (Figure S3). Subsequently, UiO-67
was impregnated with the aqueous metal precursor, H2PdCl4, to
form a homogeneous solution. The reducing agent, NaBH4, was
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10.1002/anie.201913453
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RESEARCH ARTICLE
Noteworthily, we have found that UiO-67@Pd is a universal
core platform that might be coated with other UiO shells including
UiO-66, UiO-67(N) and UiO-68. The shell thickness is controlled
to be almost same, and the corresponding shell thickness is 44
nm for UiO-66, 50 nm for UiO-67(N), and 48 nm for UiO-68 in
sequence (Figures S11-S13). As-achieved composites are
named as UiO-67@Pd@UiO-66 (44 nm), UiO-67@Pd@UiO-
67(N) (50 nm) and UiO-67@Pd@UiO-68 (48 nm), respectively.
Table 1. Selective semihydrogenation of different alkynes into alkenes by using
various catalysts of this work.a The detailed comparison with relevant literature
works is aummarized in Table S8, and the result in this work is superior to all
the reported heterogeneous catalysts, displaying unprecedented activity and
excellent selectivity.
The catalytic performance of different UiO-67@Pd@UiO-X (X
= 66, 67 and 68) samples is firstly evaluated based on selective
semihydrogenation of PA (9.98 × 6.69 Å) toward styrene (ST)
against ethyl benzene (EB). The typical contrast samples
including pure Pd NPs, carbon sphere@Pd and UiO@Pd
samples are also tested (Figures S6, S14-S18). The average size
of Pd NPs is 8.8 nm for pure Pd NPs, 3.2 nm for carbon
sphere@Pd, 5.0 nm for UiO-66@Pd, 3.5 nm for UiO-67@Pd and
4.5 nm for UiO-68@Pd. All the samples mainly expose the (111)
facets of Pd NPs. To make the performance evaluation more
comparable, the absolute amount of active Pd NPs in each
sample is controlled to be the same (~0.015 mg, Table S3) and
the PA to Pd molar ratio is 6456.
t
Conv. b
Sel.b (%)
TOFc
Entry
Catalysts
(min)
A (%)
(h-1)
B
C
Substrate I: phenylacetylene
1
2
UiO-67@Pd@UiO-66 (44 nm)
500
25
100
100
100
100
100
100
100
100
100
-
91.1
92.0
93.1
88.6
89.1
88.1
87.4
86.8
81.8
-
8.9
8.0
775
UiO-67@Pd@UiO-67 (35 nm)
UiO-67@Pd@UiO-67 (50 nm)
UiO-67@Pd@UiO-68 (48 nm)
UiO-66@Pd
15495
13835
17608
12912
19368
29798
3689
3
28
6.9
4
22
11.4
10.9
11.9
12.6
13.2
18.2
-
5
30
6
UiO-67@Pd
20
7
UiO-68@Pd
13
8
Carbon sphere@Pd
Pd NPs
105
195
180
9
1987
Table 1 summarizes the catalytic results, in which symbol A, B
and C in the first row represent reactant alkynes, target product
alkenes and byproduct alkanes, respectively (Table S4). No
hydrogenation of PA catalyzed by pure UiO is found (Table 1,
entry 10), while pure Pd NPs in size of 8.8 nm exhibit the
conversion of 100% within 195 min reaction and the
corresponding selectivity of target ST is 81.8% with a TOF value
of 1987 h-1 (Table 1, entry 9, Figure S19). When we used carbon
sphere@Pd sample with Pd NPs of 3.2 nm in diameter as catalyst,
the conversion of 100% and the selectivity of 86.8% with a TOF
value of 3689 h-1 are obtained (Table 1, entry 8, Figure S20). The
higher catalytic activity is mainly caused by the smaller sized Pd
NPs (3.2 nm) in carbon sphere@Pd sample compared with pure
Pd samples (8.8 nm). With respect to bare Pd NPs and carbon
sphere@Pd samples, the supported UiO@Pd samples display
the significantly enhanced catalytic efficiency. UiO-66@Pd
sample shows the conversion of 100% and the selectivity of 89.1%
with a TOF value of 12912 h-1 (Table 1, entry 5, Figure S21), UiO-
67@Pd sample exhibits the conversion of 100% and selectivity of
88.1% with a TOF value of 19368 h-1 (Table 1, entry 6, Figure
S22), and UiO-68@Pd sample manifests the conversion of 100%
and selectivity of 87.4% with a TOF value of 29798 h-1 (Table 1,
entry 7, Figure S23). Evidently, the TOF value is gradually
increased by introducing more benzene rings in the linear
dicarboxylates, which gives rise to the improved hydrophobicity of
UiOs and causes enrichment of more organic substrate PA
around Pd NPs for hydrogenation reaction.
10
UiO-66, UiO-67 or UiO-68
UiO-67@Pd@UiO-67(N)(50
nm)
11
12
380
480
100
100
83.1
75.3
16.9
24.7
1019
807
UiO-67(N)@Pd@UiO-
67(N)(14 nm)
13
14
UiO-67(N)@Pd
135
125
100
100
83.0
88.2
17.0
11.8
2869
3099
MOF-74(Ni)@Pd
Substrate II: 4-ethynyltoluene
15
UiO-67@Pd@UiO-67 (50 nm)
38
32
35
30
14
15
20
100
100
100
100
100
100
100
89.2
89.6
93.2
90.6
88.8
96.7
95.3
10.8
10.4
6.8
10194
12105
11068
12912
27669
25825
19368
Substrate III: cyclohexylethyne
16
UiO-67@Pd@UiO-67 (50 nm)
Substrate IV: 1-octyne
17
UiO-67@Pd@UiO-67 (50 nm)
Substrate V: 4-pentyn-1-ol
18
UiO-67@Pd@UiO-67 (50 nm)
9.4
Substrate VI: 2-methyl-3-butyn-2-ol
19
UiO-67@Pd@UiO-67 (50 nm)
11.2
3.3
Substrate VII: 2-butyne
20
UiO-67@Pd@UiO-67 (50 nm)
Substrate VIII: 3-hexyne
21
UiO-67@Pd@UiO-67 (50 nm)
4.7
a
Reaction condition: each catalyst containing 0.015 mg Pd NPs, 0.91 mmol
alkynes, alkyne to Pd molar ratio of 6456, 10oC and 0.5 MPa H2. b Conversion
c
of alkyne and selectivity of alkene were determined by gas chromatography.
TOF was calculated by the mole number of converted substrate·(mole number
of total Pd)-1·h -1.
Based on the above results from the contrast experiments, UiO-
67@Pd@UiO-X (X = 66, 67 and 68) samples characteristic of
shells with varied pore aperture and wettability are adopted as
catalysts for selective semihydrogenation of PA. UiO-
67@Pd@UiO-66 (44 nm) shows the conversion of 100% and the
selectivity of 91.1% with a very small TOF value of 775 h-1 (Table
1, entry 1, Figure S24). The low activity originates from the small
pore aperture of UiO-66 shell that blocks mass transfer of both PA
and ST during the catalytic process. This deduction is verified by
dimension comparison that the molecule size of PA is 9.98 × 6.69
Å while the pore aperture of UiO-66 is around 7 Å.[15] When UiO-
67 and UiO-68 with bigger pore aperture (around 8 Å and 10 Å)
and better hydrophobic property are employed as the shells, their
catalytic performance is dramatically improved. UiO-
67@Pd@UiO-67 (50 nm) sample displays the conversion of
100% and the selectivity of 93.1% with a TOF value of 13835 h-1
(Table 1, entry 3, Figure S25 and Table S5), and UiO-
67@Pd@UiO-68 (48 nm) sample exhibits the conversion of 100%
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10.1002/anie.201913453
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RESEARCH ARTICLE
and the selectivity of 88.6% with a TOF value of 17608 h-1 (Table
1, entry 4, Figure S26). More importantly, the utilization of UiO-67
and UiO-68 shells results in improvement of the selectivity toward
ST (93.1% for UiO-67@Pd@UiO-67 (50 nm) and 88.6% for UiO-
67@Pd@UiO-68 (48 nm)), likely benefitting from the interface
synergy effect between Pd NPs and porous UiO in the sandwich-
like structure. We also find that the activity and selectivity of these
sandwich-like catalysts are easily manipulated by altering the
shell thickness, for instance, UiO-67@Pd@UiO-67 (35 nm) of
thinner shell shows a larger TOF value of 15495 h-1 but the slightly
decreased selectivity of 92.0% (Table 1, entry 2, Figure S27). In
short, the Pd NPs surrounded with the porous and hydrophobic
MOFs exhibit the extremely high TOF values along with complete
conversion and excellent selectivity in the semihydrogenation of
PA.
To further investigate the semihydrogenation of PA under mild
condition, the H2 pressure is reduced from 0.5 MPa to the ambient
pressure by bubbling hydrogen in the reaction solution. UiO-
67@Pd@UiO-67 (50 nm) sample still exhibits the high selectivity
of 97.4% to ST, but the conversion of PA is dramatically
decreased to 17.6% at the reaction time of 28 min, indicating that
semihydrogenation of PA prefers occurring at a higher H2
pressure (Figure S28 and Table S6). In addition, due to the high
surface area of sandwich catalysts, the physical adsorption of
substrates and products needs to be considered. To evaluate the
potential of sandwich UiO-67@Pd@UiO-67 (50 nm) as efficient
catalyst, a higher PA to Pd molar ratio of 19368 is selected (Figure
S29 and Table S7). The experimental result shows that at 18.5%
conversion of PA, the selectivity of ST is 97.0% with the TOF
value of 21499 h-1, suggesting that UiO-67@Pd@UiO-67 (50 nm)
catalyst exhibits the higher catalytic activity compared with the PA
to Pd molar ratio of 6456. With extending the reaction time, the
TOF value gradually decreases. When the conversion of PA
reaches 100%, the selectivity of ST is 91.7% and corresponding
TOF value is 9684 h-1. It is noted that sandwich UiO-
67@Pd@UiO-67 (50 nm) catalyst still possesses the excellent
catalytic activity and selectivity of ST at a higher PA to Pd molar
ratio.
Beside the activity and selectivity, stability is known to be
another key parameter to evaluate the performance of catalysts.
No obvious deactivation occurs on UiO-67@Pd@UiO-67 (50 nm)
sample at the conversion of either 52.6% or 97.5% after five
successive recycle tests, respectively, resulting from the firm
immobilization of active Pd NPs inside the sandwich-like
structures (Figure 3a, 3b, 3d and 3e). On the contrary, the
conversion using the supported UiO-67@Pd is considerably
decreased from 97.2% to 53.8% after five successive recycle
tests. TEM images indicate severe aggregation of Pd NPs on the
surface of UiO-67 (Figure 3c, 3f and 3g). These indicate that
coating the supported catalyst with porous MOF shells is an
effective way to achieve the heterogeneous catalysis with high
stability.
Figure 3. Stability test of UiO-67@Pd@UiO-67 (50 nm) along with UiO-67@Pd
samples. Catalytic selective semihydrogenation of PA by UiO-67@Pd@UiO-67
(50 nm) for five successive runs at the conversion of a) 97.5% and b) 52.6%. c)
Catalytic selective semihydrogenation of PA by UiO-67@Pd sample for five
successive runs. d, e) TEM images of UiO-67@Pd@UiO-67 (50 nm) sample
before and after catalytic reactions in a). f, g) TEM images of UiO-67@Pd
sample before and after catalytic reactions in c).
To clarify the importance of shell wettability on the catalytic
performance, more hydrophilic UiO-67(N) is selected (as
evidenced in Figure 1d), and three samples including UiO-
67(N)@Pd, UiO-67@Pd@UiO-67(N) (50 nm) and UiO-
67(N)@Pd@UiO-67(N) (14 nm) are prepared (Figures S12, S30
and S31). The average sizes of Pd NPs are 3.0 nm for UiO-
67(N)@Pd, 4.0 nm for UiO-67@Pd@UiO-67(N) (50 nm), and 3.4
nm for UiO-67(N)@Pd@UiO-67(N) (14 nm). When used as
catalysts for selective semihydrogenation of PA, UiO-67(N)@Pd
sample shows the conversion of 100% and the selectivity of
83.0% with a considerably low TOF value of 2869 h-1 (Table 1,
entry 13, Figure S32), demonstrating that the hydrophilic supports
are unfavorable for hydrogenation of PA. As for the sandwich-like
UiO-67@Pd@UiO-67(N) (50 nm) and UiO-67(N)@Pd@UiO-
67(N) (14 nm) samples, the TOF value is only 1019 h-1 and 807
h-1 (Table 1, entries 11 and 12, Figures S33 and S34),
respectively. In addition, the Pd NPs supported by UiO-67(N)
sample show the dramatic decrease in the selectivity toward ST,
e. g. 75.3% for UiO-67(N)@Pd@UiO-67(N) (14 nm) vs. 93.1% for
UiO-67@Pd@UiO-67 (50 nm) (Table 1, entries 3 and 12). As for
the decreased activity, it is mainly caused by the hydrophilic
property of UiO-67(N); while in regard of the decreased selectivity,
it is associated with the electronic structure of active Pd NPs
modulated by UiO-67(N) as will be discussed below.
For performance comparison, we also summarize the
outstanding results from recent literature works (Table S8).
Superior to all the reported heterogeneous catalysts,
semihydrogenation of PA on UiO-67@Pd@UiO-67 (50 nm)
displays the extraordinary activity, excellent selectivity and good
stability.
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10.1002/anie.201913453
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N 1s shifts to a higher value of 399.76 eV with respect to pure
UiO-67(N) (Figure 4d-4f). The interfacial electron transfer from
UiO-67(N) to Pd NPs occurs in UiO-67(N)@Pd, because the lone-
pair electrons of N in UiO-67(N) frameworks prefer moving to low-
lying bonding states (Figure S38b). Altogether, the electronic
structure of Pd NPs could be effectively manipulated by grafting
different functional organic linkers into UiO frameworks.
To elucidate the effect of electronic structure of Pd NPs on the
selectivity of ST, DFT calculations were done on the neutral and
charged Pd(111) surfaces, which present the most exposed
facets of Pd NPs (Figure 2f), using a four-layered (3 × 3) supercell
composed of 36 Pd atoms (see the detailed calculation method in
the Supporting Information). The most stable adsorption of H2, PA,
ST and EB on the neutral Pd(111) surface is compared. H2
adsorbs on the neutral Pd surface in a dissociation mode and H
atom prefers attaching at the hollow site with an adsorption
energy of -2.63 eV (Figure S39a). The aromatic rings of
adsorbates are strongly bonded to the Pd surface with a distance
of 2.3 Å, while the H atoms on benzene rings lift away from the
surface, which makes C-H bonds a hybridization transition from
sp2 to sp3 (Figure S39b). Interestingly enough, the alkynyl group
of PA and the alkenyl group of ST exhibit different adsorption
configuration on the Pd surface. The alkynyl group of PA is
bonded tightly to the Pd surface with a distance of 2.1 Å, which is
even closer than that of the aromatic ring to the surface;
meanwhile, the hydrogen atom of the alkynyl group is 50° away
from the surface, suggesting the C-H hybridization transition from
sp to sp2. As for semihydrogenation product ST (Figure S39c), the
distance between the alkenyl group of the ST and Pd surface is
2.2 Å, more away from the surface than the alkynyl group of PA,
implying that weaker bonding of the alkenyl group than the alkynyl
group on the Pd surface. When ST is hydrogenated to EB, the
alkyl group has no interaction with the Pd surface, suggesting its
facile desorption (Figure S39d).
Figure 4. XPS profiles of UiO-67@Pd and UiO-67(N)@Pd samples along with
UiO-67, UiO-67(N) and pure Pd NPs. a) Pd 3d level for pure Pd NPs and UiO-
67@Pd. b) Zr 3d level and c) O 1s level for pure UiO-67 and UiO-67@Pd. d) Pd
3d level for pure Pd NPs and UiO-67(N)@Pd. e) N 1s level and f) O 1s level for
pure UiO-67(N) and UiO-67(N)@Pd. Inserted numbers represent the peak
positions of different elements.
The reaction pathway is calculated to understand the selective
semihydrogenation of PA to ST (Figures 5 and S40). As plotted,
PA firstly adsorbs on Pd(111) surface to form the adsorbed state
(named as PA) and then converts to ST, followed by competitive
process of its desorption or hydrogenation to EB. Clearly, the key
for enhancing the selectivity of ST is to promote desorption of ST
against its hydrogenation on the Pd surface. With respect to
neutral Pd(111) surface, PA is stabilized with an adsorption
energy of -1.64 eV, and then is converted to ST with an
exothermicity of -1.34 eV. Subsequently, desorption of ST is
endothermic by 1.49 eV, whereas hydrogenation of ST to EB is
an exothermic process that needs to overcome two energy
X-ray photoelectron spectroscopy (XPS) was performed to
inspect the interfacial interaction between Pd NPs and UiO
supports. As for pure Pd NPs, characteristic Pd 3d5/2 binding
energy at 335.66 eV is assigned to zero-valent Pd in the metallic
state.[22] With respect to UiO-67@Pd sample, Pd 3d5/2 binding
energy shifts toward a higher value of 335.93 eV, and the
corresponding binding energy of Zr 3d5/2 shifts toward a lower
value of 182.74 eV compared with that of pure UiO-67 sample
(Figure 4a and 4b, and Figure S35a). Similarly, Pd 3d5/2 binding
energy of UiO-66@Pd and UiO-68@Pd shifts toward higher value
of 336.06 eV and 335.86 eV, and the corresponding binding
energy of Zr 3d5/2 shifts toward lower value of 182.81 eV and
182.96 eV compared with pure UiO-66 and UiO-68, respectively
(Figures S36 and S37). These suggest that there is electron
transfer from Pd NPs to UiO-66, UiO-67 or UiO-68. Moreover, O
1s binding energy shifts toward a lower value for UiO-67@Pd with
respect to that in pure UiO counterparts due to the strong
interfacial interaction between Pd NPs and Zr nodes in UiO-67
(Figure 4c, and Figure S38a).[23]

barriers of 0.55 eV for transition state (TS1) of forming PhCHCH3

and 1.12 eV for transition state (TS2) of forming PhCH2CH3 . Thus,
the energy cost on desorption of ST and hydrogenation of ST to
EB is comparable on the neutral Pd(111) surface. As for Pd(111)
surface with a positive charge of 0.6 in UiO-67@Pd samples, the
adsorbates show weak adsorption, and the desorption energy of
ST is only 0.33 eV, which is much lower than the energy barriers
of TS1 (0.55 eV) and TS2 (1.12 eV) for hydrogenation of ST to
EB on neutral Pd(111) surface. Evidently, ST prefers desorbing
on the positive Pd surface, and correspondingly the selectivity of
ST product is enhanced. As comparison, the desorption energy of
ST (2.24 eV) on the Pd(111) surface with a negative charge of
0.6 in UiO-67(N)@Pd samples is greatly higher than energy
In sharp contrast, Pd 3d5/2 binding energy of UiO-67(N)@Pd
sample shifts to a lower value of 334.60 eV compared with pure
Pd NPs (Figure S35b), while the corresponding binding energy of
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10.1002/anie.201913453
Angewandte Chemie International Edition
RESEARCH ARTICLE
barriers of TS1 (0.55 eV) and TS2 (1.12 eV) for hydrogenation of
ST to EB on neutral Pd(111) surface, thus leading to decrease
in selectivity of ST.
(88.2% vs. 88.1%), ascribing to wettability mismatch between
hydrophilic MOF-74 pores and hydrophobic substrates. Moreover,
compared with UiO-67(N)@Pd, MOF-74@Pd shows similar TOF
value (3099 h-1 vs. 2869 h-1) but considerably enhanced selectivity
(88.2% vs. 83.0%), attributing to interfacial electron transfer from
Pd NPs and MOF-74 supports (Figure S45). These results
validate the experimental observations on catalytic performance
of UiO-67@Pd@UiO-X toward selective semihydrogenation of
PA (Table 1).
Other common terminal alkynes including 4-ethynyltoluene
(10.84 × 6.69 Å), cyclohexylethyne (9.98 × 6.72 Å) and 1-octyne
(11.63 × 5.03 Å) are employed as substrates for selective
semihydrogenation reactions on UiO-67@Pd@UiO-67 (50 nm)
(Figure S47). Remarkably, when the conversion is 100% for 4-
ethynyltoluene, cyclohexylethyne and 1-octyne, respectively, the
corresponding selectivity is 89.2% for 4-methylstyrene with a TOF
value of 10194 h-1, 89.6% for cyclohexylethene with a TOF value
of 12105 h-1, and 93.2% for 1-octene with a TOF value of 11068
h-1 (Table 1, entries 15-17, Figures S48-S50). When two typical
alkynols including 4-pentyn-1-ol (8.89 × 4.80 Å) and 2-methyl-3-
butyn-2-ol (7.74 × 6.71 Å) are used as substrates and completely
converted, the corresponding selectivity is 90.6% for 4-penten-1-
ol with a TOF value of 12912 h-1, and 88.8% for 2-methyl-3-buten-
2-ol with a TOF value of 27669 h-1 (Table 1, entries 18 and 19,
Figures S51 and S52). Last but not least, when two internal
alkynes including 2-butyne (7.31 × 4.17 Å) and 3-hexyne (10.40
× 5.03 Å) are utilized as substrates and completely converted, the
corresponding selectivity is 96.7% for 2-butylene with a TOF value
of 25825 h-1, and 95.3% for 3-hexene with a TOF value of 19368
h-1 (Table 1, entries 20 and 21, Figures S53 and S54). Again, Pd
NPs encapsulated by hydrophobic UiO shells show the
unprecedented catalytic performances in semihydrogenation of
different alkynes compared with the reported heterogeneous
catalysts (Table S8).
Figure 5. Potential energy profiles of hydrogenation of PA on Pd(111) by using
the (3 × 3) supercell composed of 36 Pd atoms. a) PhCCH*, b) PhCHCH2*, c)
TS1, d) PhCHCH3*, e) TS2, f) PhCH2CH3*. Black, red and blue short lines stand
for the neutral, positive (δ = + 0.6) and negative (δ = - 0.6) Pd(111) surfaces in
turn. The values refer to the experimental temperature (T = 283.15 K).
All the above results disclose that the positive Pd(111) surface
in UiO-67@Pd samples, which are caused by interfacial electron
transfer from Pd NPs and UiO supports, contributes to highly
selective formation of ST rather than EB. To further validate this
deduction, the interfacial electron transfer between Pd(111) and
PA/ST is firstly analyzed. Bader charge analysis discloses that the
electron transfer is from neutral Pd (111) surfaces to PA and ST
in the adsorption structures, and correspondingly, the calculated
electron transfer value is 0.43 e for Pd to PA and 0.29 e for Pd to
ST (Figure S41). Then, the charges on the Pd surface ranging
from -0.6 to 0.6 are systematically calculated for the
adsorption/desorption of PA and ST (Figure S42). With increase
in the negative charge on Pd(111) surface, the adsorption of PA
and ST becomes stronger; whereas it turns weaker with increase
in the positive charge, meaning that the higher selectivity of ST is
obtained. This analysis is consistent with recent observation
about the adsorption of PA and ST on the surfaces of Ni-O(111)
and Ni(111).[24] Note that that the order for selectivity of ST on
different UiO@Pd samples follows UiO-66@Pd (Sel.: 89.1%) >
UiO-67@Pd (Sel.: 88.1%) > UiO-68@Pd (Sel.: 87.4%) (Table 1,
entries 5-7), which is in good agreement with the binding energy
shift order from Pd to UiO with respect to pure Pd NPs, i.e. 0.4 eV
for UiO-66@Pd > 0.27 eV for UiO-67@Pd > 0.2 eV for UiO-
68@Pd (Figures 4, S36 and S37).
Conclusion
In summary, semihydrogenation of different alkynes to alkenes
is greatly accelerated via decoupling the functions of porous MOF
supports at the molecular level. The hydrophobic UiO supports,
via the incorporation of more benzene rings into the organic
linkers, facilitate the enrichment of the organic alkynes around the
active Pd NPs so as to promote the activity, while the electron
transfer from Pd NPs to the electron-withdrawing UiO nodes leads
to the weak adsorption of target alkene products over Pd NPs and
consequently the enhanced selectivity. Thus, the unprecedented
catalytic performances on selective semihydrogenation of
multiple alkynes are acquired with UiO-67@Pd@UiO-67 (50 nm)
sample, superior to all the reported heterogeneous catalysts
(Table S8). The concept of on-demand molecular architecture of
the supports will offer new venues for designing high-performance
heterogeneous catalysts toward high value-added but challenging
reactions.
To further distinguish the key factors influencing catalytic
activity and selectivity, an electron-withdrawing but hydrophilic
MOF-74 (contact angle of 24o),[2e] which is formed via coordination
of Ni2+ ion with 2,5-dihydroxyterephthalic acid and characteristic
with pore size of 1.47 nm, is selected as the support (Figures S43-
S45). The average size of Pd NPs in as-synthesized MOF-
74@Pd is 4.4 nm. When MOF-74@Pd is used as hydrogenation
catalyst, the conversion of PA and selectivity of ST are 100% and
88.2% with a TOF value of 3099 h-1 (Table 1, entry 14, Figure
S46). With respect to UiO-67@Pd, MOF-74@Pd displays much
lower TOF value (3099 h-1 vs. 19368 h-1) but identical selectivity
Acknowledgements
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10.1002/anie.201913453
Angewandte Chemie International Edition
RESEARCH ARTICLE
Yamasaki, K. Gohara, Z. Maeno, T. Mizugaki, K. Jitsukawa, K. Kaneda,
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This work was supported financially by National Key Basic
Research
Program
of
China
(2014CB931801
and
2016YFA0200700, Z.Y.T.), National Natural Science Foundation
of China (21721002, 21475029 and 91427302, Z.Y.T.; 21722102,
51672053 and 21303029, G.D.L.), Beijing Natural Science
Foundation (2182087, G.D.L.), Frontier Science Key Project
of Chinese Academy of Sciences (QYZDJ-SSW-SLH038, Z.Y.T.),
CAS-CSIRO Cooperative Research Program (GJHZ1503, Z.Y.T.),
“Strategic Priority Research Program” of the Chinese Academy of
Sciences (XDA09040100, Z.Y.T.), the project of “K.C. Wong
Education Foundation” (Z.Y.T.), and Youth Innovation Promotion
Association CAS (2016036, G.D.L.). We thank Dr. Peng Xu for
testing and ananlyzing the XPS data.
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J. G. Ulan, E. Kuo, W. F. Maier, R. S. Rai, G. Thomas, J. Org. Chem.
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nanoparticle • selective semihydrogenation • wettability
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10.1002/anie.201913453
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RESEARCH ARTICLE
RESEARCH ARTICLE
Kwanghak Choe,[†] Fengbin Zheng,[†]
Hui Wang,[†] Yi Yuan, Wenshi Zhao,
Guangxin Xue, Xueying Qiu, Myonghak
Ri, Xinghua Shi, Yinglong Wang,
Guodong Li* and Zhiyong Tang*
Metal-organic frameworks (MOFs)
containing Zr metal nodes (known as
UiO) with tunable wettability and
electron-withdrawing ability, which are
easily realized by selecting organic
linkers,
are
used
as
activity
Page No. – Page No.
accelerators for semihydrogenation of
alkynes catalyzed by the sandwiched
Accelerating Selective
Semihydrogenation by Metal-Organic
Frameworks
palladium
nanoparticles
simultaneously with unity conversion,
excellent selectivity and good stability.
This article is protected by copyright. All rights reserved.