An unprecedented anionic Ln-MOF with a cage-within-cage motif: Spontaneous reduction and immobilization of ion-exchanged Pd(II) to Pd-NPs in the framework

Article abstract of DOI:10.1039/c5ta04937g

An unprecedented microporous anionic Ln-MOF, [Me₂NH₂]₂₄[Tb₁₂(TATB)₁₆(HCOO)₁₂]·12DMF·48H₂O (1) (H₃TATB = 4,4′,4′′-s-triazine-2,4,6-tribenzoic acid), which is a rare cage-within-cage structure through interpenetration rather than covalent bonding, has been synthesized. Compound 1 contains a 3D net which is constructed using a large and a small Ln-carboxylate cage alternately arranged by sharing faces with each other. Interpenetration of two identical 3D nets occurs in such a way that each small cage of one net is encapsulated within the large cage of the other and vice versa, generating an overall 3D double-walled cage framework. Such interpenetration creates a unique structure of double-shelled hollow space to accommodate Pd nanoparticles (Pd-NPs), which could effectively prevent Pd-NPs from aggregation and leaching. Moreover, the ion-exchanged Pd(ii) embedded in the framework can be readily reduced at room temperature with no requirement of any chemical or thermal treatments, affording Pd-NPs with uniform size and even distribution. As a result, the as-prepared Pd-NPs@1 exhibits excellent activity and cycling stability for the hydrogenation of styrene and its derivatives.

Full text of DOI:10.1039/c5ta04937g

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Materials Chemistry A
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An unprecedented anionic Ln-MOF with a cage-
within-cage motif: spontaneous reduction and
immobilization of ion-exchanged Pd(II) to Pd-NPs in
the framework†
Cite this: DOI: 10.1039/c5ta04937g
ab
a
a
a
Yun-Hu Han, Chong-Bin Tian, Ping Lin and Shao-Wu Du*
An unprecedented microporous anionic Ln-MOF, [Me
2
2 2
NH ]24[Tb12(TATB)16(HCOO)12]$12DMF$48H O (1)
0
00
(H
3
TATB ¼ 4,4 ,4 -s-triazine-2,4,6-tribenzoic acid), which is a rare cage-within-cage structure through
interpenetration rather than covalent bonding, has been synthesized. Compound 1 contains a 3D net
which is constructed using a large and a small Ln-carboxylate cage alternately arranged by sharing faces
with each other. Interpenetration of two identical 3D nets occurs in such a way that each small cage of
one net is encapsulated within the large cage of the other and vice versa, generating an overall 3D
double-walled cage framework. Such interpenetration creates a unique structure of double-shelled
hollow space to accommodate Pd nanoparticles (Pd-NPs), which could effectively prevent Pd-NPs from
aggregation and leaching. Moreover, the ion-exchanged Pd(II) embedded in the framework can be
readily reduced at room temperature with no requirement of any chemical or thermal treatments,
affording Pd-NPs with uniform size and even distribution. As a result, the as-prepared Pd-NPs@1 exhibits
excellent activity and cycling stability for the hydrogenation of styrene and its derivatives.
Received 2nd July 2015
Accepted 9th November 2015
DOI: 10.1039/c5ta04937g
www.rsc.org/MaterialsA
challenges. One of the key challenges that needs to be overcome
is that the preparation of M-NPs@MOFs oen requires the
Introduction
The synthesis of palladium nanoparticles (NPs) immobilized on reduction of noble metal ions by hydrogen at temperatures
ꢀ
porous supports has attracted extensive interest in heteroge- higher than 200 C or by utilizing traditional strong redox
1
5
neous catalysis. The size, distribution and morphologies of regents such as NaBH
NPs as well as the properties of supports have important effects organic porous materials, MOFs are normally less stable espe-
. However, compared to inorganic and
4
2
6
on the catalytic performance. Generally, small particle size may cially under hash reduction conditions. Therefore, it is desir-
result in a high surface-to-volume ratio and provide a large able to either synthesize MOFs with robust porous frameworks
number of available active sites per unit area for reaction or seek a method to produce M-Ps within the pore space under
substrates. Therefore, much effort has been devoted to mild conditions. In addition, MOFs constructed using anionic
embedding palladium NPs into porous materials containing cages are better candidates for making M-NP@MOFs as the
coordination groups that offer great potential for the control of noble metal ions can enter the pore by ion-exchange, avoiding
3
particle size and prevention of NP aggregation. In recent years, the interference of the anions from the metal salts.
metal–organic frameworks (MOFs) with attractive architectures,
Self-assembly of metal–organic coordination cages (MOCCs)
large surface areas and tunable functional pores have attracted with high symmetry, well-dened cavities, and various poly-
much attention and may be considered as the next generation hedral geometries has led to a number of 3D cage-based MOFs,
of porous materials to load metal nano-particles (M-NPs) for such as the well-known MILs, ZIFs, PCNs, NOTTs, HKUST-1,
4
7–12
heterogeneous catalysis. According to the investigation, and so on.
These species display fascinating properties that
loading of noble metals on MOFs still faces a number of make them excellent candidates for gas storage and separation,
13
drug delivery, ion exchange, molecular sensor and catalysis.
Until now, most of these cage-based MOFs have been con-
structed by simply sharing vertexes, edges and faces of the
neighbouring cages, or by cage catenation and interlocking. In
very rare cases, a cage-within-cage motif where one cage is
entirely encapsulated by the other cage and the outer and inner
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002,
14
P. R. China. E-mail: swdu@irsm.ac.cn
b
Graduate University of Chinese Academy of Sciences, Beijing 100039, P. R. China
†
Electronic supplementary information (ESI) available: Experimental details,
additional gures, and UV-Vis, TGA, XRD, XPR and FT-IR results of the cages are interconnected by covalent bonds has been
synthesized samples. CCDC 1063756. For ESI and crystallographic data in CIF
15
observed. However, to the best of our knowledge, the cage-
or other electronic format see DOI: 10.1039/c5ta04937g
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within-cage porous framework through interpenetration of two Preparation of [Me NH ] [Tb (TATB) (HCOO) ]$
Paper
2
2 24
12
16
12
identical 3D nets has not been reported. Besides, there have 12DMF$48H O (1)
2
been only a few examples of cage-based MOFs constructed by
A mixture of Tb(NO
4
2
3
)
3
$6H
,4 ,4 -s-triazine-2,4,6-tribenzoic acid (H
20.70 mg) was sealed in a 20 mL Teon-lined stainless steel
2
O (0.50 mmol, 226.47 mg) and
16
lanthanide ions.
It has been reported recently that some MOFs, e.g. MOF-5
and [Zn17(thb)14(m -O) (H O)(Me NH )]$Me NH $xguest, can be
employed to prepare M-NP@MOFs under mild conditions.
These types of MOFs are able to propel the charge transfer
efficiently from the organic linkers to Zn 13 inorganic clusters
0
00
3
TATB, 0.5 mmol,
4
4
2
2
2
2
2
vessel with 6 mL of dimethylformamide (DMF). The mixture was
17
ꢀ
heated to 150 C in 4 hours and this temperature was main-
tained for 3 days. Then the reaction system was cooled slowly to
room temperature during another 4 days. The colorless cubic
crystals of 1 were collected, washed with DMF and acetonitrile
and dried in air (yield 85% based on Tb(NO
Elemental analysis calcd (%) for H N O180Tb12
4
O
+
+
2+
and readily reduce Au , Ag , Pt and other noble metal ions
upon either UV irradiation or heating. The quantum dot (QD)
behaviour of the Zn O inorganic clusters in these compounds
3 3 2
) $6H O).
4
13
C
480 576 72
18
is believed to be responsible for this self-redox ability.
(
12 141.31): C 47.48, H 4.78, N 8.31; found: C 47.19, H 4.73, N
2
+
However, until now, spontaneous reduction of Pd ions to
Pd-NPs in MOFs at room temperature without the help of any
additional agents, which would be advantageous for catalytic
applications, has not been reported yet. In this contribution, we
report a highly rare anionic cage-within-cage microporous
MOF, [Me NH ] [Tb (TATB) (HCOO) ]$12DMF$48H O (1)
ꢁ1
8
1
9
4
.19. IR (KBr, cm ): 3377s, 2978w, 2881w, 1629w, 1589w,
541vs, 1427s, 1367vs, 1307w, 1172w, 1182w, 1089w, 1014w,
33vw, 879s, 831vs, 806s, 779s, 700w, 648w, 604w, 559s,
97w, 428w.
2
2 24
12
16
12
2
0
00
Single-crystal X-ray diffraction study
(
H TATB ¼ 4,4 ,4 -s-triazine-2,4,6-tribenzoic acid, Me ¼ CH –),
3
3
where each double-walled cage-within-cage motif contains
forty-eight negative charges. This compound has a cage-within-
cage structure formed through interpenetration of two identical
Single-crystal X-ray diffraction data were collected on a Rigaku
Diffractometer with a Mercury CCD area detector (Mo Ka: l ¼
˚
0
.71073 A) at room temperature. Crystal Clear soware was used
3
D cage nets and is the second example of cage-MOFs based on
for data reduction and empirical absorption correction. The
structure was solved by a direct method using SHELXTL and
rened by full-matrix least-squares on F using SHELX-97
2
+
lanthanide ions. Signicantly, the ion-exchanged Pd ions can
be spontaneously reduced to metallic Pd by formate groups of
the framework, affording well-dispersed Pd-NPs with small and
narrow size distribution. Moreover, such a spontaneous
reduction process can be accelerated by slightly heating the
2
program. Metal atoms in the compound were located from the
E-maps, and other non-hydrogen atoms were located in
successive difference Fourier syntheses. All non-hydrogen
atoms were rened anisotropically. The organic hydrogen
atoms were positioned geometrically. Because partial guest
solvent molecules in channels were highly disordered and could
not be modelled properly, the SQUEEZE routine of PLATON was
applied to remove contributions to scattering from solvent
molecules. The reported renements are of guest-free struc-
tures obtained by the SQUEEZE routine. The nal formulae of
compounds were determined by combining the results of the
elemental analysis and thermogravimetric analysis. Crystallo-
graphic data and other pertinent information for 1 are
summarized in Table S1.† The CCDC number for 1 is 1063756.†
2
+
Pd -treated sample of 1 or soaking it in methanol.
Experimental section
General information
All reagents were commercially purchased and used without any
further purication. The purity of all gases is 99.999%. The TGA
was performed on a TGA/NETZSCH STA449C instrument heated
ꢀ
from 40 to 800 C under a nitrogen atmosphere at a heating rate
ꢀ
ꢁ1
of 10 C min . Powder X-ray diffraction patterns were recorded
on a PANalytical X'pert PRO X-ray Diffractometer using Cu-Ka
ꢀ
radiation in the 2q range of 5–50 . The Fourier transform
infrared spectra using KBr pellets were collected on a Spectrum-
ꢁ1
PXRD and TGA measurements
One FT-IR spectrophotometer in the range of 4000–400 cm
.
Elemental analyses (C, H, and N) were carried out with an In order to check the phase purity of the sample, the PXRD
Elemental Vairo EL III Analyser. Gas adsorption measurements patterns of the compound were recorded at room temperature.
were performed in an ASAP (Accelerated Surface Area and The peak positions of the simulated pattern closely match those
Porosimetry) 2020 System. All the gas-chromatography (GC) of the experimental ones, indicating phase purity of the as-
measurements were analysed using a GC-Smart (SHIMADZU) synthesized sample (Fig. S1†). The TGA measurements were
with a DM-1 capillary column (L ¼ 30 m; ID ¼ 0.25 mm) and performed to study the thermal stability of 1 (Fig. S2†). For
a ame ionization detector (FID). Analysis of the noble metal a fresh sample of 1, the TGA curve does not show any platform
ꢀ
content was performed by inductively coupled plasma atomic from 40 to 400 C, which is due to the sustained loss of lattice
emission spectroscopy (ICP-AES) on an Ultima 2 analyser (Jobin solvent molecules, then the compound begins to decompose
ꢀ
Yvon). X-ray photoelectron spectra (XPS) were recorded by using upon further heating up to approximately 500 C. But, when we
1
ꢀ
a Thermo Scientic ESCALAB 250Xi Spectrometer. NMR treat the sample at 150 C under vacuum for 24 hours, we can
ꢀ
spectra were recorded on a Bruker AVANCE III NMR spec- markedly see a platform appearing up to nearly 250 C, indi-
trometer. Transmission electron microscopy (TEM) images were cating that the lattice solvent molecules can be largely removed
ꢀ
obtained with a TECNAI GzF20.
at 150 C under vacuum for 24 hours.
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Results and discussion
Colourless crystals of 1 were prepared under solvothermal
3 3 2
conditions via the combination of Tb(NO ) $6H O salt with the
0
00
organic linker 4,4 ,4 -s-triazine-2,4,6-tribenzoic acid (H
1
3
TATB) at
ꢀ
50 C in DMF. A single-crystal X-ray diffraction study reveals that
ꢀ
1
crystallizes in the cubic space group Im3. The crystal structure is
consisted of an anionic subjectival network and dimethylamine
counter cations. The asymmetry unit of 1 is composed of one
3
+
3ꢁ
fourth Tb ions, one third TATB ligands, one fourth formic
acid and half dimethylamine cations, which are derived from the
decomposition of DMF. In the 3D anionic framework, the crys-
3
+
tallographic independent Tb ion adopts distorted enneahedral
pyramidal coordination geometry, and is coordinated by nine-
oxygen atoms, eight from four different TATB ligands while the
remaining one from formic acid (Fig. S3†). The Tb–O bond
3ꢁ
Fig. 2 (a and b) The large/small cage enriched by eight small/large
cages. (c) Cage-within-cage motif resulting from the interpenetration
of two identical 3D nets. (d and e) The approximate diameters of the
small and large cages.
˚
lengths vary from 2.237(5) to 2.417(6) A, which fall in a normal
range for lanthanide carboxylate compounds.
The most fascinating topological feature of 1 is the highly
rare cage-within-cage motif, in which the small cage is caged by
a large cage (Fig. 1b), a result from the interpenetration of two
independent but identical 3D cage nets (Fig. 1a). The small cage
such a way that every small cage in one net is completely
encapsulated by a large cage belonging to the other net and vice
versa, resulting in a 3D net with double-walled cages (Fig. 2c).
When the lattice solvent molecules, along with the counter
cations in 1 are removed using the SQUEEZE routine of
PLATON, the PLATON/VOID calculation shows that the total
3
+
is constituted by twelve Tb ions interconnected by eight
3
ꢁ
TATB ligands and six formic acids, which denes an irregular
cavity with a diameter of approximately 1.92 nm (Fig. 2d).
3
+
3ꢁ
Another twelve Tb ions and eight TATB ligands form a large
cage with a regular icosahedral geometry, whose diameter is up
to 2.67 nm (Fig. 2e). Each small cage possesses twelve pentag-
3
˚
empty volume of the anionic framework is 12 464.1 A , corre-
3
˚
sponding to 65.3% of the total crystal volume (19 090.7 A ).
However, if only the lattice solvent molecules are removed, it
onal windows, and the aperture of these windows is ca. 11.4 ꢂ
2
˚
3
˚
decreases to 4167.5 A , corresponding to 21.8% of the total
crystal volume. This result suggests that the void space of the
1
6.3 A (Fig. 1c). The large icosahedral cage has six boat-
2
˚
windows, whose window size is about 26.7 ꢂ 19.9 A (Fig. 1c).
The network topology of the structure analysed with the framework is mainly lled with counter cations, which take up
3
6
4
at least 43.5% of the total crystal volume. From the structure of
program package TOPOS is {6 } ${6 $8 } ${6} net, which is
8
6
3
+
1
2
, we can see that every cage requires twenty-four Me NH2
a new topological type (Fig. S4†). Further analysing the structure
of 1 reveals that these two types of cages are alternatively
arranged to form a 3D network in which each small cage is
encircled by eight large cages (Fig. 2a), and each large cage is
surrounded by eight small cages through the sharing of faces
cations to balance the negative charges of the cage. In other
words, each double-walled cage should contain forty-eight
+
1
Me
2 2
NH cations inside the cavity for charge balance. The H
NMR spectrum of the acid-digested sample displays signals of
+
ꢁ
(Fig. 2b). Two identical 3D cage networks are interpenetrated in
2 2
Me NH and HCOO in a ratio of approximately 12 : 1, which
is in agreement with the formula (Fig. S5b†). It is expected that
+
these Me NH cations can be readily exchanged with noble
2
2
metal ions and the anionic cage should be benecial for the
impregnation and immobilization of noble metal ions.
The phase purity of 1 was checked using PXRD patterns
recorded at room temperature. As illustrated in Fig. S3,† the
peak positions of the simulated pattern closely match those of
the experimental ones, indicating phase purity of the
as-synthesized sample. The TGA measurement was performed
to study the thermal stability of 1 (Fig. S4†). The TGA curve of 1
shows a continuous weight loss without an obvious plateau
ꢀ
from 40 to 500 C. Aer removing the lattice solvents by
ꢀ
treating 1 under vacuum at 150 C for 24 hours, no obvious
change was observed for XRD (Fig. S3†), indicating that 1
retains its structural integrity and crystallinity upon solvent
removal. The TGA curve of the desolvated sample shows no
ꢀ
weight loss from 0 to ca. 250 C. Aer that the framework starts
to decompose.
Fig. 1 (a) Interpenetration of two identical 3D cage nets. (b–d) The
cage-within-cage motif of 1.
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+
2+
The Me
2
NH
2
cations in solvent-free sample 1a can perform the complete reduction of Pd ions and afforded bigger Pd-NPs
2
+
ion-exchange with Pd cations by soaking 500 mg colourless 1a in 1 (denoted as Pd-NPs@1) with good uniform size and even
in a 20 mL 0.01 M THF solution of Pd(NO ) . When the soaking distribution (Fig. 3a and b). We found that it was also possible
3
2
2
+
experiment was conducted at room temperature, we found that to speed up the reduction of Pd ions by soaking 1b in meth-
some brown-black solid of metallic Pd was formed as conrmed anol. Aer 24 hours, Pd-NPs with relatively poor dispersion and
by XPS (Fig. S6†), which sticked to the surface of 1a and distribution compared to those obtained at 80 C were formed
ꢀ
0
conned the further ion-exchange process. This result sug- in the framework of 1 (denoted as Pd-NPs@1 ) (Fig. 3c and d).
2
+
gested that the Pd ions can be facilely reduced to metallic Pd The generation of metallic Pd in the Pd-NP loading sample
in the anionic framework of 1. It can be speculated that the Pd-NPs@1 is visible with a colour change from orange-yellow to
formate group in the framework may be responsible for the dark brown-black (Fig. S9†). By extending the soaking time
spontaneous reduction of the Pd ion to Pd. Aer reduction, using solution with a high concentration of Pd(NO ) , the
the intensity of the proton resonance attributed to the formate content of Pd can reach to a maximum of 12.67 wt%, affording
group is dramatically reduced in the H NMR spectrum of the ion-exchange sample 1c. The H NMR spectrum of 1c shows
2 2
sample (Fig. S5d†). The loss of the formate anion did not result almost no residual signal for the methyl proton of Me NH ,
in the collapse of the subjectival framework, which was indicating complete replacement of Me
conrmed by XRD (Fig. S1†). One way to avoid spontaneous (Fig. S5c†). As expected, reduction of Pd in 1c afforded
reduction was to perform ion-exchange at low temperature. Pd-NPs@1c with bigger and more densely packed Pd-NPs
2
+
19
3
2
2
+
1
1
+
+
2+
2
+
NH
2
by Pd
2
2
+
0
Thus, the orange-yellow Pd embedded product 1b could be compared to Pd-NPs@1 and Pd-NPs@1 (Fig. S10†).
successfully obtained by repeatedly soaking the sample in
The N2 adsorption measurements of 1a and 1c are also
ꢀ
+
2
0 mL of fresh THF solution at ꢁ18 C every 24 h for one week. recorded. Because the cavities of 1a are lled with Me NH
2
2
2
+
No metallic Pd was found in 1b as evidenced by the XPS result cations, when 1a is ion-exchanged with Pd , the N
2
adsorption
2
+
(Fig. S6†). The content of Pd in 1b determined by inductively capacity would be expected to increase. The N adsorption
2
coupled plasma atomic emission spectroscopy (ICP-AES) was measurements veried this point, giving Langmuir and BET
2
ꢁ1
2
.52 wt%. In comparison with other cases where the formation surface areas of 284 and 200 m g for 1a and 627 and 447
2
ꢁ1
of Pd-NPs@MOFs usually required additional reducing agents,
m
g
for 1c (Fig. 4a and c). The permanent porosity of
photoactivation or thermal energy, the reduction of Pd to Pd-NPs@1c is also proved by N adsorption measurements,
2
+
2
2
ꢁ1
Pd-NPs in 1b took place spontaneously and smoothly.
The direct preparation of Pd-NPs was achieved by simply areas (Fig. 4b). The total pore volumes of 1a, 1c and Pd-NPs@1c
treating 1b under vacuum for four weeks at room temperature. obtained from the N adsorption isotherms recorded at 77 K are
However, the size of Pd-NPs obtained by this procedure was too 0.13, 0.33 and 0.27 cm
which gives 565 and 398 m g for Langmuir and BET surface
2
3
ꢁ1
g
, respectively. The experimental
2
+
3
ꢁ1
small and incomplete reduction of Pd ions was observed value of 1a is less than 0.23 cm g calculated by using the
2
+
(
Fig. S7 and S8†). In order to accelerate the reduction of Pd , we accessible volume from PLATON and the unit cell mass. This is
carried out the experiment at 80 C for one hour, which led to either because of the incomplete removal of solvent molecules
from the sample, or the slight porosity loss during the activation
ꢀ
process. No characteristic peaks of Pd metal appeared in the
XRD pattern of Pd-NPs@1c due to the little amount of Pd-NPs
compared to that of the host solid (Fig. S1†). It also indicated
Fig. 4 The nitrogen adsorption isotherms of (a) 1c, (b) Pd-NPs@1c,
Fig. 3 The TEM images of Pd-NPs@1 (a and b) and Pd-NPs@1 (c and d). and (c) 1a.
0
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Table 1 Hydrogenation of styrene and its derivatives
a
Entry
Substrates
Products
Conv. (%)
1
2
100
100
3
4
5
98
100
100
6
7
99
98
Fig. 5 The TEM images of the catalysts after six runs for Pd-NPs@1 (a
0
and b) and Pd-NPs@1 (c and d).
8
9
87
0
that small-sized Pd-NPs were well dispersed throughout the
MOF support.
The existing state of the surface palladium was investigated
1
1
0
1
12
0
2
+
by XPS. In the XPS spectrum of the Pd -embedded sample
prepared at room temperature, the Pd 3d region is divided into
two spin–orbital pairs, indicating the presence of two types of
surface-bound palladium species (Fig. S6†). The binding energy
peaks at 343.38 (Pd 3d5/2) and 338.18 eV (Pd 3d3/2) are assigned
2
+
to Pd species, while the small peaks at 340.98 (Pd 3d5/2) and
35.88 eV (Pd 3d3/2) correspond to metallic Pd species, which 12
b
3
0
2
+
probably resulted from the spontaneous reduction of Pd by
the anionic framework. When the sample was prepared at
a
Reaction conditions: olen (2 mmol),
H
2
1
atm and [Pd]
ꢀ
2+
ꢁ
18 C, only peaks related to Pd were observed (Fig. S6†). The (0.06 mmol%), in 2.5 mL THF at room temperature for 7 hours.
b
Conversion was determined by GC. Hydrogenation of styrene with 1
as the catalyst.
XPS spectrum of Pd-NPs@1 exhibits only two peaks for metallic
2
+
Pd, indicating complete reduction of Pd to Pd-NPs (Fig. S8†).
2
+
In contrast, a considerable amount of Pd remains unreduced
0
in Pd-NPs@1 , as can be seen from Fig. S11.† Transmission
electron microscopy (TEM) images reveal that the average Pd-NPs@MIL-101 and Pd-NPs@MOF-5, which require 7 and
2
1
diameter of Pd-NPs is 4.28 ꢃ 0.21 nm for Pd-NPs@1 and 9.12 ꢃ 12 hours, respectively, to complete the hydrogenation.
0
0
.32 nm for Pd-NPs@1 , showing a narrow distribution. These Notably, a 100% conversion of styrene to ethylbenzene can be
Pd-NPs are a little larger than some other immobilized Pd-NPs achieved within one hour when the temperature of hydroge-
2
0
ꢀ
in organic–inorganic hybrid materials.
The catalytic performance of Pd-NPs@1 and Pd-NPs@1 was
evaluated via the hydrogenation of styrene and its derivatives at H–H bond in H
room temperature. In a typical reaction, 5 mg of Pd-NPs@1 hydrogen atoms are added across the double bond of an alkene,
2.52 wt%) was used for the conversion of 2 mmol of styrene, resulting in a saturated alkane (Scheme S1†).
and the reaction was carried out under 1 atm of hydrogen in
In addition to high catalytic activity, the reusability and
nation was increased to 60 C. During the catalytic reaction, the
and olen are attached to the surface of Pd-NPs. First, the
cleaves to generate two Pd–H bonds. Then both
0
H
2
2
(
THF at room temperature. The conversion of styrene was stability are also very important for heterogeneous catalytic
determined by gas chromatography (GC). The conversion of systems. The recycling experiments were conducted using
0
styrene to ethylbenzene is completed in 7 hours with NPsPd@1, Pd-NPs@1 and Pd-NPs@1 as catalysts in the hydrogenation of
0
and 8.5 hours with NPsPd@1 , giving 100% yield of the hydro- styrene without the addition of extra palladium. The results
genation product. The catalytic activity of both catalysts is better indicated that no obvious efficiency loss was detected aer six
0
than other Pd-NPs@MOFs but is comparable to that of runs. However, in the case of Pd-NPs@1 , aer six runs,
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increasing catalytic activity was observed. We speculated that particle size and narrow size distribution at room temperature
2
+
0
the unreduced Pd ions in Pd-NPs@1 were continuingly without the help of any additional chemical or thermal treat-
reduced during the hydrogenation reaction to afford more ments. Furthermore, such a reduction process can be acceler-
2
+
Pd-NPs. This is in agreement with the XPS results where two ated via slightly heating or soaking the Pd -treated sample in
2
+
peaks belonging to Pd ions aer six runs are noticeably methanol. Owing to the efficient stabilization of the double-
reduced compared to those before the reaction (Fig. S11†). As walled cages, the Pd-NPs with controlled size can be dispersed
shown Fig. 5, the TEM image reveals that the granular particles well onto 1. The resultant Pd-NPs@1 has exhibited high activity
of both catalysts are well maintained aer six consecutive runs, in the hydrogenation of olens and it can be used up to six
with the average of diameter being slightly increased to 4.29 ꢃ times without signicant loss of the catalytic activity, showing
0
0
.31 nm for Pd-NPs@1 and 9.4 ꢃ 0.6 nm for Pd-NPs@1 , and no a satisfactory reusability.
obvious aggregation of Pd-NPs was observed. However, in the
case of Pd-NPs@1 , the density of Pd-NPs was increased, indi-
cating again that additional Pd ions were reduced aer cata-
0
Acknowledgements
2
+
lytic reactions.
We thank the National Basic Research Program of China
Leaching experiments were performed to examine the (973 Program, 2012CB821702), the National Natural Science
stability of the Pd-NPs. Aer the rst run, ICP analysis showed Foundation of China (21233009, 21173221 and 21571175) and
that the amount of palladium leached out into the solvent was the State Key Laboratory of Structural Chemistry, Fujian Insti-
0
only 12 ppb for Pd-NPs@1 and 16 ppb for Pd-NPs@1 . Besides, tute of Research on the Structure of Matter, Chinese Academy of
aer three hours when the conversion yield reached 57.3%, the Sciences for nancial support.
catalyst was removed from the solution and the reaction was
continued for three hours, the conversion of styrene to ethyl-
benzene remained unchanged as checked by GC (Fig. S12†),
Notes and references
implying that there was no contribution from palladium
leached into reaction solution. We believe that the unique
structure of 1 can effectively prevent the aggregation and
leaching of Pd-NPs and increase their catalytic active area, and
hence enhance their stability and catalytic performance.
Moreover, the interaction of nitrogen atoms from the TATB
ligand with Pd-NPs may also help stabilize the dispersion of the
nanoparticles.
The excellent catalytic activity of Pd-NPs@1 encouraged us to
further explore the generality of this catalytic system. Further
tests using some styrene derivatives with different electronic
and steric characters were performed (Table 1). For easy
comparison, all the experiments were performed under the
same conditions. The substrates carrying electron-donating
groups in the para position, such as methyl, methoxy or
tert-butyl, resulted in total conversion to the corresponding
hydrogenated products (Entry 1–3). Halogenated styrenes
provided a full conversion, and the substituted position (meta or
para) had no inuence on the activity (Entry 4–7). The substrate
with a methylvinyl group gave 87% conversion whereas that
with a nitrovinyl group led to no hydrogenated product (Entry 8,
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5
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unprecedented double-walled cage motif. It is a rare cage-
within-cage structure generated from interpenetration of two
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