Uncoordinated carbonyl groups of MOFs as anchoring sites for the preparation of highly active Pd nano-catalysts

Article abstract of DOI:10.1039/c2jm30867c

A novel Tm-containing metal-organic framework, [Tm(μ₂-L) (μ₄-L)_0.5(H₂O)₂]·3H ₂O (Tm-MOF), was prepared from the hydrothermal reaction of Tm(NO₃)₃ with 5-methylpyrazine-2-carboxylic acid in water. The 2,5-pyrazinedicarboxylate ligand (L) in Tm-MOF was formed in situ from 5-methylpyrazine-2-carboxylic acid under the above conditions. Tm-MOF is a three-dimensional (3D) coordination network with 1D open channels running across the coordination layers. Each L in bridging mode μ₂ contains two uncoordinated carbonyl groups which point to the channels. By using the predesigned Tm-MOF as a host, we have successfully incorporated small palladium nanoparticles into the channels by a simple impregnation method. The Pd/Tm-MOF catalyst exhibited a high catalytic activity in the hydrogenation of styrene. Moreover, the catalyst could be reused at least three times without loss of any activity. In contrast, a low dispersion of metal and a low activity in styrene hydrogenation were observed over the Pd catalyst supported on ZIF-8, which has a similar surface area as the Tm-MOF, but no uncoordinated carbonyl groups. The results suggest that the uncoordinated carbonyl groups in the Tm-MOF may play an important role in facilitating the dispersion of Pd nanoparticles through an intermolecular interaction with the Pd cations in the course of immobilization. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm30867c

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PAPER
Uncoordinated carbonyl groups of MOFs as anchoring sites for the
preparation of highly active Pd nano-catalysts†
c
c
c
a
Yingyi Pan,a Deyun Ma,ab Huimin Liu, Hao Wu, Dehua He and Yingwei Li
*
Received 13th February 2012, Accepted 21st March 2012
DOI: 10.1039/c2jm30867c
A novel Tm-containing metal–organic framework, [Tm(m2-L)(m4-L)0.5(H2O)2]$3H2O (Tm-MOF), was
prepared from the hydrothermal reaction of Tm(NO3)3 with 5-methylpyrazine-2-carboxylic acid in
water. The 2,5-pyrazinedicarboxylate ligand (L) in Tm-MOF was formed in situ from 5-
methylpyrazine-2-carboxylic acid under the above conditions. Tm-MOF is a three-dimensional (3D)
coordination network with 1D open channels running across the coordination layers. Each L in
bridging mode m2 contains two uncoordinated carbonyl groups which point to the channels. By using
the predesigned Tm-MOF as a host, we have successfully incorporated small palladium nanoparticles
into the channels by a simple impregnation method. The Pd/Tm-MOF catalyst exhibited a high
catalytic activity in the hydrogenation of styrene. Moreover, the catalyst could be reused at least three
times without loss of any activity. In contrast, a low dispersion of metal and a low activity in styrene
hydrogenation were observed over the Pd catalyst supported on ZIF-8, which has a similar surface area
as the Tm-MOF, but no uncoordinated carbonyl groups. The results suggest that the uncoordinated
carbonyl groups in the Tm-MOF may play an important role in facilitating the dispersion of Pd
nanoparticles through an intermolecular interaction with the Pd cations in the course of
immobilization.
employing a MOF support with a relatively low surface area by
Introduction
using a liquid-phase synthesis method.7 The inefficiency could be
largely related to the weak intermolecular interactions between
the MOFs and metal-salt precursors during the deposition.
Recently, a postsynthetic functionalization approach has been
proposed for the inclusion of metal nanoparticles in MOFs
through the immobilization of organic molecules which after
neutralization have strong interactions with the metal precur-
sors.8 Nevertheless, most of these reports target the uncoordi-
nated amino groups that were selectively grafted on the MOFs
with open metal centers. Thus the scope of this method is
restricted in some degree.
Metal–organic frameworks (MOFs) are a new class of porous
materials, which have potential applications in a wide range of
areas including catalysis, gas storage, separation, and sensors.1
Owing to their high surface area, porosity, and chemical
tunability, the utilizations of MOFs in heterogeneous catalysis
have recently attracted tremendous attention, especially, the ones
about applying MOFs as a catalyst support for metal nano-
particles.2 Over the past decade, there have been a number of
reports on the immobilization of noble metal nanoparticles (e.g.,
Pd, Au, Ru, and Pt) on MOFs by employing various preparation
methods, such as impregnation,3 colloidal deposition,4 solid
grinding,5 and chemical vapor deposition.6 In spite of these
promising approaches, the development of a facile and efficient
route to incorporate small noble-metal nanoparticles within
MOFs to obtain highly active and stable catalysts is of great
importance and still remains challenging, particularly when
Because of the similar electronic properties between nitrogen
and oxygen atoms, it can be speculated that the uncoordinated
oxygen atoms (with lone pair electrons) in MOFs could also act
as anchoring sites for metal cations through coordination inter-
actions. Actually, the utilization of free oxygen-containing
groups (e.g. carboxylic acid) on MOFs for the chelation of metal
cations has been reported in the literature.9 More recently,
Fischer et al. have described the successful encapsulation of very
small gold nanoparticles inside a zeolitic imidazolate framework
(ZIF-90) bearing aldehyde groups via gas phase infiltration of the
volatile gold complex [Au(CO)Cl].10 However, the Au nano-
particles exhibited a low catalytic activity as well as a low
selectivity in the aerobic oxidation of benzyl alcohol.10 Herein,
we report the synthesis of a novel 3D Tm-containing MOF
aSchool of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China. E-mail: liyw@scut.edu.cn
bSchool of Chemistry and Chemical Engineering, Zhaoqing University,
Zhaoqing 526061, China
cDepartment of Chemistry, Tsinghua University, Beijing 100084, China
† Electronic supplementary information (ESI) available: Crystal data
and powder XRD patterns. CCDC 711852. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2jm30867c
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(Tm-MOF) with carbonyl groups pointing to the channels,
which are not coordinated with any metal ions. By using the
predesigned MOF as a host, we have successfully incorporated
small palladium nanoparticles into the pores by a simple
impregnation method. Moreover, the as-synthesized Pd/Tm-
MOF composite exhibited a high catalytic activity in the
hydrogenation of styrene. To the best of our knowledge, this
work represents the first example of utilizing uncoordinated
carbonyl groups to immobilize metal nanoparticles on MOFs as
a highly active catalyst in heterogeneous catalysis.
Results and discussion
Synthesis and crystal structure
The hydrothermal reaction of Tm(NO3)3 with 5-methylpyrazine-
2-carboxylic acid in water yielded [Tm(m2-L)(m4-L)0.5(H2O)2]$
3H2O (Tm-MOF). The 2,5-pyrazinedicarboxylate ligand (L) in
Tm-MOF was formed in situ by ligand reaction of 5-methyl-
pyrazine-2-carboxylic acid under the above conditions
(Scheme 1). As is well known, the methyl groups can be easily
oxidized to carboxyl under the solvothermal conditions.11 The
structure of Tm-MOF was determined by single crystal X-ray
diffraction at room temperature (Table S1, ESI†). It is a three-
dimensional (3D) coordination network with 1D open channels
running across the coordination layers. In the asymmetric unit of
Tm-MOF, there are one Tm(III) ion, one and a half L ligands,
two coordinated water molecules and three lattice waters
(Fig. 1a). The nine-coordinated Tm(III) center is surrounded by
four oxygen and three nitrogen atoms from four L ligands, and
two water molecules, which exhibits a distorted tricapped
trigonal-prism coordination geometry with Tm–O and Tm–N
distances and O–Tm–O, O–Tm–N, and N–Tm–N bond angles
Fig. 1 View of the structure of Tm-MOF. (a) Perspective view of the
asymmetric unit, symmetry codes: 1+x, y, z. (b) View of the mog-type
(4,4)-connected architecture. Tm cations, turquoise balls; m4 L ligands,
black balls; m2 L ligands, pink sticks. (c) An infinite rod-shaped SBU of
zigzag connection between adjacent Tm3+ centers. (d) View of channels in
the a-axis direction. All H atoms were omitted for clarity.
€
(4,4)-connected framework with the Schalfli symbol of
{4.64.8}2{42.62.82}, in which the Tm(III) cations are considered as
4-connected nodes and the m4 ligands L as 4-connected nodes
(Fig. 1b). Interestingly, each L in bridging mode m2 contains two
uncoordinated carbonyl groups that point to the open channel
without interaction with any metal ions. Moreover, after the
removal of the coordinated and free water molecules, Tm-MOF
contains 42.4% solvent-accessible void volume.13 Therefore,
these readily accessible carbonyl groups could act as anchoring
sites for Pd cations.
ꢀ
ranging from 2.298(6) to 2.620(6) A and from 62.69(19) to
144.1(2)ꢀ, respectively, all of which are within the range of those
found in other nine-coordinated Tm(III) complexes with oxygen
and nitrogen donating ligands.12
In the polymeric structure of Tm-MOF, the L ligands adopt
two different bridging modes: one acts as bridging mode m2 to
link two Tm(III) ions, whereas the other as bridging mode m4
connecting four Tm(III) ions (Scheme 2). Based on above-
mentioned connections of ligands L, Tm(III) centers are
Characterization of catalysts
Tm-MOF is stable in air. The thermogravimetric analysis (TGA)
curve of Tm-MOF reveals that the structure of Tm-MOF is
thermally stable up to 400 ꢀC (Fig. 2). The surface area and pore
volume of the Tm-MOF were determined by N2 physisorption
at 77 K, which was calculated to be ca. 1000 m2 gꢂ1 and
0.44 cm3 gꢂ1, respectively (Table 1).
connected together to form
a thulium–carboxylate chain
(Fig. 1c). This 1D infinite rod-shaped thulium–carboxylate chain
can be regarded as a second building unit (SBU). Every Tm(III)
ion in this rod-shaped chain further coordinates to ligands L in
bridging mode m2. Adjacent rod-shaped chains connect to form
ꢀ
a 3D framework with channels about 4 ꢁ 9 A (excluding van der
Waals radii of the atoms) along the a-axis (Fig. 1d).
For a better understanding of the framework of Tm-MOF, the
overall network topology can be described as a mog-type
Scheme 1 In situ formation of L ligand from 5-methylpyrazine-2-
Scheme 2 The coordination modes of L ligand in the structure of
carboxylic acid under the hydrothermal conditions.
Tm-MOF.
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Fig. 2 TGA curve of Tm-MOF.
Table 1 Structural characterizations of Tm-MOF and ZIF-8
BET surface
area (m2 gꢂ1
Pore size
ꢀ
(A)
Pore volume
(cm3 gꢂ1
)
a
Sample
)
Fig. 4 TEM images of Tm-MOF (A) and Pd/Tm-MOF (B); XRD
patterns of Tm-MOF (C-a) and Pd/Tm-MOF (C-b); and the EDS pattern
of Pd/Tm-MOF (D).
Tm-MOF
ZIF-8
1000
1005
6.5
7.1
0.44
0.45
a
Mean pore size determined by the Horvath–Kawazoe mode.
Fig. 5 TEM image (A) and EDS pattern (B) of Pd/ZIF-8.
Fig. 3 XPS spectra of the Pd 3d region for (a) Pd(II)–Tm-MOF and (b)
PdCl2(CH3CN)2.
After removing the guest molecules, the activated Tm-MOF
was impregnated with PdCl2(CH3CN)2 that was diluted in
acetone. The intermolecular interactions between the MOF and
Pd ions were investigated by X-ray photoelectron spectroscopy
(XPS). Fig. 3 shows the Pd 3d spectrum of PdCl2(CH3CN)2
loaded on the MOF (Pd(II)–Tm-MOF). Accordingly, the binding
energy (BE) of the Pd 3d5/2 (Pd 3d3/2) peak is 338.2 eV (343.5 eV),
with a measured spin–orbit splitting of 5.3 eV. For comparison,
the XPS spectrum of pristine PdCl2(CH3CN)2 was also examined
(Fig. 3), and the binding energies are consistent with the reported
values for Pd(II).14 It should be noted that the 3d peaks of Pd(II)–
Tm-MOF are shifted by ca. 0.40 eV toward lower binding
energies, compared to those of the pristine PdCl2(CH3CN)2.
Fig. 6 Kinetics of styrene hydrogenation catalyzed by Pd/Tm-MOF (:)
and Pd/ZIF-8 (-).
Such shifts were previously reported on palladium supported
zeolites15 and reflected an increase in the electron density of Pd
via metal–support interactions. The XPS results imply that there
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Table 2 Hydrogenation of styrenea
ZIF-8 (Pd/ZIF-8) are all the same as for the preparation of
Pd/Tm-MOF. The PXRD patterns (Fig. S2, see ESI†) show that
the structure of ZIF-8 remained intact after loading the Pd
nanoparticles. The absence of a Pd diffraction pattern could be
related to the low Pd content (ca. 1.0 wt%) in the sample. As can
be seen from the TEM image of Pd/ZIF-8 (Fig. 5), most of the Pd
was aggregated in various degrees. These results indicate that the
Pd nanoparticles tended to aggregate on ZIF-8. Therefore, the
high dispersion of Pd nanoparticles on the Pd/Tm-MOF might
not be attributed to the surface area and pore size of the host.
Instead, the free carbonyl groups in the Tm-MOF could play an
important role in facilitating the dispersion of Pd nanoparticles
through an intermolecular interaction with the Pd cations in the
course of immobilization.
Catalyst
Pd (wt%) Time (h) Conv. (%) Sel (%) TOF (hꢂ1
)
Tm-MOF —
Pd/Tm-MOF 1.0
1.5
1.5
1.5
1.5
—
10.3
4.5
ꢃ10.0
—
—
>99.0
>99.0
>99.0
703
307
682
Pd/ZIF-8
Pd/MOF-5b
1.0
1.0
a
Reaction conditions: styrene (2.0 g), catalyst (0.02 g), 800 rpm stirring
b
speed, 35 ꢀC, p ¼ 1 atm. Reaction results of ref. 17.
Catalytic testing
exist certain interactions between the oxygen atoms of the
uncoordinated carbonyl groups and the Pd cations loaded on the
Tm-MOF.
The as-prepared Pd/Tm-MOF was used to catalyze the hydro-
genation of styrene at 35 ꢀC and H2 pressure of 0.1 MPa. As
shown in Fig. 6, the conversion of styrene was almost completed
with >99% ethylbenzene selectivity after 12 h. The turnover
frequency (TOF: mole of product per mole of metal catalyst per
hour) was calculated to be 703 hꢂ1 at ca. 10% conversion of
styrene within 1.5 h (Table 2). For comparison, pristine Tm-
MOF and Pd/ZIF-8 were also tested under the same reaction
conditions. The reaction with Tm-MOF gave no activity in the
system. Only 55% yield of ethylbenzene was obtained in 24 h
when Pd/ZIF-8 was used as a catalyst (Fig. 6), and the reaction
appeared to slow down after 12 h, which suggests a possible
catalyst deactivation during the reaction. The TOF was 307 hꢂ1
at 1.5 h for Pd/ZIF-8 (Table 2). The much higher activity (TOF)
of Pd/Tm-MOF, as compared to Pd/ZIF-8, could be mainly
attributed to the high dispersion of palladium active sites because
at 1.5 h of reaction time the deactivation of Pd/ZIF-8 (if any) is
believed to be insignificant (Fig. 6). Also, it should be noted that
the catalytic performance of Pd/Tm-MOF is on a par with the
highly active Pd nano-catalysts (Table 2) supported on MOF-5
with a much larger surface area (BET, 2885 m2 gꢂ1).17 These
results confirm that the uncoordinated carbonyl groups of Tm-
MOF have played an important role in achieving a high
dispersion as well as a high activity of the Pd nanoparticles
loaded on the MOF.
Supported palladium nanoparticles (Pd/Tm-MOF) were
prepared by reducing the Pd(II)–Tm-MOF with an aqueous
solution of NaBH4. After the reduction of Pd(II), there was no
apparent loss of crystallinity in PXRD patterns (Fig. 4C), indi-
cating that the integrity of Tm-MOF was mostly maintained
after the immobilization of Pd nanoparticles. Moreover, no
diffraction peak for Pd nanoparticles in the PXRD pattern was
detected. This could be attributed to the low content of palla-
dium and the high dispersion of Pd nanoparticles in the sample.
The Pd content for all the samples is about 1.0 wt%, as measured
by atomic absorption spectroscopy (AAS).
Fig. 4 shows the typical TEM images of the Pd/Tm-MOF. The
ꢀ
lattice pattern with a distance of ca. 4 A is close to the diameter of
Tm-MOF framework channels along the a axis. No apparent
aggregates could be observed in the TEM image of Pd/Tm-MOF,
indicating a high dispersion of Pd (Fig. 4B). The EDS analysis
confirms the existence of Pd element in the Pd/Tm-MOF sample.
For comparison, we have also deposited Pd nanoparticles on
a representative MOF, ZIF-8 (Zn(2-methylimidazole)2),16 which
has a similar surface area (ca. 1000 m2 gꢂ1) and pore size (ca. 7 A)
ꢀ
as Tm-MOF, but no uncoordinated carbonyl groups (Table 1).
The preparation conditions for the Pd nanoparticles loaded on
Furthermore, the Pd/Tm-MOF could be recycled and reused
at least three times without any loss of efficiency for the hydro-
genation of styrene. PXRD experiments were conducted to check
the structure stability of Pd/Tm-MOF after hydrogenation
(Fig. 7). It can be seen that the crystalline structure of the Tm-
MOF host matrix mostly remained unchanged after three runs of
catalysis.
Conclusions
In summary, a novel Tm-containing MOF with uncoordinated
carbonyl groups was synthesized and employed as a support for
the deposition of Pd nanoparticles by a simple impregnation
method, which exhibited a high catalytic activity in the hydro-
genation of styrene. The presence of interactions between the
oxygen atoms of the uncoordinated carbonyl groups and the Pd
cations loaded on the Tm-MOF could be responsible for the high
Fig. 7 Powder X-ray diffraction patterns of Pd/Tm-MOF: (a) as-
synthesized and (b) after 3 runs of catalysis.
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dispersion of Pd nanoparticles on the MOF. The reported new
synthesis strategy by using uncoordinated carbonyl groups as
anchoring sites might bring new opportunities in the develop-
ment of highly active metal catalysts by employing MOFs as
a support. Work is underway to further optimize the preparation
method for Pd or other noble-metal nanoparticles, as well as
expand the Tm-MOF support to other MOFs bearing uncoor-
dinated carbonyl groups and their catalysis.
Synthesis of Pd/ZIF-8. Pd/ZIF-8 was synthesized by the same
method as that for the preparation of Pd/Tm-MOF. The Pd
loading on the sample was 1.0 wt% based on AAS analysis.
Characterization
Powder X-ray diffraction patterns of the Tm-MOF, Pd/Tm-
MOF, and ZIF-8 samples were obtained by a Rigaku diffrac-
tometer (D/MAX-IIIA, 3 kW) using Cu Ka radiation (40 kV,
30 mA, l ¼ 0.1543 nm). The BET surface area measurements
were performed with N2 adsorption/desorption isotherms at
77 K on a Micromeritics ASAP 2020 instrument. Before the
analysis, the samples were evacuated at 150 ꢀC for 12 h. The size
and morphology of Pd/Tm-MOF and Pd/ZIF-8 were investi-
gated by high-resolution transmission electron microscopy (HR-
TEM, JEM-2010 of JEOL) equipped with an energy dispersive
X-ray detector (EDX). The accelerating voltage was 120 kV.
X-ray photoelectron spectroscopy (XPS) measurements were
performed on a Kratos Axis Ultra DLD system with a base
pressure of 10ꢂ9 Torr. Thermogravimetric analysis (TGA) of
samples was performed on a NETZSCH STA449C instrument
using a constant heating rate of 10 ꢀC minꢂ1 under N2 atmo-
sphere. The palladium contents of the samples were determined
quantitatively by atomic absorption spectroscopy (AAS) on
a HITACHI Z-2300 instrument.
Experimental
All starting materials and solvents were obtained from
commercial suppliers (Sigma-Aldrich, Alfa Aesar, and others)
and used without further purification.
Catalyst preparation
Synthesis of Tm-MOF. A mixture of 5-methylpyrazine-2-
carboxylic acid (0.1381 g, 1 mmol), Tm(NO3)3$6H2O (99.99%,
0.4630 g, 1 mmol), and H2O (20 mL) was heated to 150 ꢀC for 24
h in a 50 mL Teflon-lined stainlessꢀ-steel autoclave, followed by
cooling to room temperature at 5 C hꢂ1 to yield yellow block
single crystals of Tm-MOF (yield: 55%, based on Tm).
Synthesis of Pd(II)–Tm-MOF. Supported palladium was
prepared via a simple impregnation method. Typically, 15 mg of
PdCl2(CH3CN)2 was dispersed in 3 mL of acetone and was
stirred for 0.5 h at room temperature. 100 mg of activated Tm-
MOF was slowly added to the above solution. The reaction
mixture was allowed to stand unperturbed for 24 hours. The
impregnated Tm-MOF sample was washed with acetone till the
filtrate became colorless and then immersed in fresh acetone for
12 h. Finally, these dark yellow crystals were slowly dried in air at
room temperature for 1 h to obtain Pd(II)–Tm-MOF.
Single crystal X-ray diffraction analysis of Tm-MOF was
performed on a Rigaku Mercury CCD diffractometer operated
ꢀ
at 90 kV and 50 mA using Mo Ka radiation (l ¼ 0.71073 A) at
room temperature. The empirical absorption corrections were
performed using the CrystalClear program.19 The structure was
solved by direct methods and refined on F2 by full-matrix least
squares technique using the SHELX-97 program package.20 All
non-hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atoms attached to carbon were placed in
geometrically idealized positions and refined using a riding
model. Hydrogen atoms on water molecules were located from
difference Fourier maps and refined with distance restraints of
Synthesis of Pd/Tm-MOF. The as-synthesized Pd(II)–Tm-
MOF was further dried at 100 ꢀC for 4 h. 50 mg of activated
Pd(II)–Tm-MOF was dispersed in 1 mL of DMF and was stirred
for 0.5 h in an ice bath. A freshly prepared aqueous solution of
NaBH4 (0.3 M, 1.5 mL) was added under vigorous stirring to
obtain a dark brown colloidal dispersion. The suspension was
stirred at 0 ꢀC for 2 h, followed by washing thoroughly with
deionized water.18 The sample was dried under vacuum at 100 ꢀC
for 2 h to obtain Pd/Tm-MOF. The Pd loading on the sample was
1.0 wt% based on AAS analysis.
ꢀ
ꢀ
O–H ¼ 0.84(2) A and H/H ¼ 1.39(2) A. The crystallographic
data for Tm-MOF are given in Table S1†.
Crystallographic data for Tm-MOF: C9H13TmN3O11, yellow
block-shaped, Mr ¼ 508.15, crystal size 0.30 ꢁ 0.26 ꢁ 0.21 mm,
ꢁ
ꢀ
ꢀ
triclinic, space group P1, a ¼ 6.020(5) A, b ¼ 12.772(11) A,
3
ꢀ
ꢀ
c ¼ 13.161(11) A, V ¼ 885.79(13) A , Z ¼ 2, m(Mo Ka) ¼
5.064 mmꢂ1, r ¼ 1.905 g cmꢂ3, T ¼ 296 K, reflection numbers
collected ¼ 4623, unique reflections (Rint) ¼ 3147 (0.0279),
R1 [I > 2s(I)] ¼ 0.0407 and wR2 (all data) ¼ 0.0466, GOF ¼
1.047, CCDC 711852.
Synthesis of ZIF-8. ZIF-8 was prepared by the reaction of zinc
nitrate tetrahydrate Zn(NO3)2$4H2O (0.840 g, 3.21 mmol), 2-
methylimidazole (H-MeIM) (0.240 g, 2.92 mmol), and 72 mL
DMF in a 100 mL autoclave. The mixture was heated at a rateꢀof
Typical procedures for the hydrogenation of styrene
Hydrogenation reactions were carried out in solution in a static
hydrogen atmosphere (1 atm) at 35 ꢀC. Typically, 2.0 g of styrene
and 0.02 g of catalyst were added to the Schlenk tubes. The
mixture was stirred at a constant stirring speed of 800 rpm in the
tube under a H2 atmosphere, and then loaded in an oil bath,
which was preheated to the target temperature. At the end of the
reaction, the tube was cooled to 0 ꢀC by placing it in an ice bath.
The reaction products were identified with a GC-MS (Shimadzu
QP5050A) and analyzed by a GC (Agilent 6820) equipped with
5 C minꢂ1 to 140 C for 24 h, then cooled at a rate of 0.4 C
minꢂ1 to room temperature.16 After removal of mother liquor
from the mixture, chloroform (80 mL) was added to the auto-
clave. Colorless polyhedral crystals of the product were collected
from the upper layer, washed with DMF (40 mL ꢁ 3) and then
immersed in methanol at ambient temperature for 48 h and
evacuated at ambient temperature for 5 h, then at an elevated
ꢀ
ꢀ
ꢀ
temperature of 300 C for 2 h.
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This work was supported by the NSF of China (20803024,
20936001, and 21073065), Doctoral Fund of Ministry of
Education of China (200805611045), Guangdong Natural
Science Foundation (S2011020002397 and 10351064101000000),
the Fundamental Research Funds for the Central Universities
(2011ZG0009), and the program for New Century Excellent
Talents in Universities (NCET-08-0203).
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