Metallic nanoparticles immobilized in magnetic metal-organic frameworks: Preparation and application as highly active, magnetically isolable and reusable catalysts

Article abstract of DOI:10.1039/c4cy00072b

Separation and recycling of catalysts after catalytic reactions are critically required to reduce the cost of catalysts as well as to avoid the generation of waste in industrial applications. In this paper, ultrafine noble metallic nanoparticles are incorporated into cauliflower-like porous magnetic metal-organic frameworks (MOFs). With the restriction effects of the pore/surface structure in the MOFs, "surfactant-free" metallic nanoparticles are successfully obtained on a 2-3 nm scale. In addition, both the thickness of MOFs shell and the content of noble metallic NPs are tunable on the MOFs coating. Moreover, the microspheres exhibit excellent performance for the catalytic reduction of p-nitrophenol with a turnover frequency of 3094 h^-1. The uniform cavities in the MOFs shell provide docking sites for p-nitrophenol and act as confinement nanoreactors, which greatly improves the catalytic performance. Most importantly, the magnetically responsive microspheres can be easily recovered by a magnetic field and show excellent reusability. The as-prepared catalyst also shows good activity for the reduction of other nitrobenzenes. Consequently, this work provides a highly active, magnetically isolable, and recyclable catalyst, which can be used for various catalytic industrial processes. The fundamental model can be further employed in a variety of biomedical fields including drug delivery and biological molecules separation. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00072b

View Article Online
View Journal
Catalysis
Science &
Technology
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: H. Zhang, S. Qi, X.
Niu, J. Hu, C. Ren, H. Chen and X. Chen, Catal. Sci. Technol., 2014, DOI: 10.1039/C4CY00072B.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/catalysis

Page 1 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
Graphical Abstract
Cauliflower-like porous magnetic MOFs embedded with noble metallic nanoparticles
possesses superior catalytic performance and can be easily recycled.

Catalysis Science & Technology
Page 2 of 43
View Article Online
Metallic nanoparticles immobilized in magnetic metal-orgaDnOiI:c10.1039/C4CY00072B
frameworks: preparation and application as highly active,
magnetically isolable and reusable catalysts
1
2
3
a
a
a
a
a
4
5
Hai-juan Zhang , Sheng-da Qi , Xiao-ying Niu , Jing Hu , Cui-ling Ren , Hong-li
Chen a, Xing-guo Chen a, b, *
a
6
7
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China
8
9
b Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
Province, Lanzhou University, Lanzhou, Gansu 730000, China
10
11
12
* Corresponding author
E-mail address: chenxg@lzu.edu.cn
Tel: 86-931-8912763
13
14
Fax: 86-931-8912582
1

Page 3 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
15
16
Abstract
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
Separation and recycling of catalysts after catalytic reactions are critically
required to reduce the cost of catalysts as well as to avoid the generation of waste in
industrial applications. In this paper, ultrafine noble metallic nanoparticles are
incorporated into cauliflower-like porous magnetic metal-organic frameworks
(MOFs). With the restriction effects of the pore/surface structure in the MOFs,
“surfactant-free” metallic nanoparticles are successfully obtained on a 2-3 nm scale.
In addition, both the thickness of MOFs shell and the content of noble metallic NPs
are tunable on the MOFs coating. Moreover, the microspheres exhibit excellent
performance for the catalytic reduction of p-nitrophenol with a turnover frequency of
3094 h-1. The uniform cavities in the MOFs shell provide docking sites for
p-nitrophenol and act as confinement nanoreactors, which greatly improve the
catalytic performance. Most importantly, the magnetically responsive microspheres
can be easily recovered by a magnetic field and show excellent reusability. The
as-prepared catalyst also shows good activity for the reduction of other nitrobenzenes.
Consequently, this work provides a highly active, magnetically isolable, and
recyclable catalyst, which can be used for various catalytic industry processes. The
fundamental model can be further employed in a variety of biomedical fields
including drug delivery and biological molecules separation.
35
2

Catalysis Science & Technology
Page 4 of 43
View Article Online
DOI: 10.1039/C4CY00072B
36
37
1. Introduction
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
Noble metallic nanoparticles (NPs) have attracted growing attention owing to
their unique physical, chemical, and biological properties and great potential in
1-7
various fields. They are expected to be the suitable candidates for design of highly
active and selective catalysts. However, nanoparticles tend to aggregate to form larger
particles due to their large surface area, leading to the decrease of their initial
activities during the catalytic cycles 8-9. Therefore, many attempts have been made to
immobilize noble metallic NPs on solid supports 9-11. Among the wide range of
supports, magnetic nanomaterials exhibit distinct advantages. Magnetic supports not
only enable the catalysts to have magnetic separation property, but also facilitate them
to be recycled. Recent examples demonstrate the successful synthesis of metallic NPs
12-13
on the surface of magnetic substrates.
However, it is fairly difficult to obtain the
metallic NPs on the sub-10 nm scale, especially in the absence of a
stabilizer/surfactant 14. Consequently, the development of producing monodispersed
noble metallic NPs on solid supports within several nanometers is highly desirable.
MOFs are crystalline materials composed of metal ions or metal clusters that are
connected by organic linkers. They have crystalline structures, large internal surface
areas, uniform but tuneable cavities and tailorable chemistry. These distinct
characteristics make them very promising for a variety of applications, including gas
3

Page 5 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
17
20
21
56
storage 15-16, chemical separation , catalysis 18-19, sensing and drug delivery . In
the context of catalysis, the uses of MOFs as heterogeneous catalysts have recently
received tremendous attention. The nodes of most MOFs are reasonably good Lewis
acid sites, and terminal ligands can present Lewis basic sites. These acid and basic
sites may well function as organic reactions catalysts 22-23. Recently, by serving as
unique host matrices, the potential applications of MOFs can be extended further by
encapsulating noble metallic (e.g., Pd, Au, Ru, and Pt) or bimetallic alloy NPs within
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
24
the frameworks . MOFs limit the growth of NPs in the confined cavities to produce
monodispersed metallic or bimetallic NPs 25-28. Taking into account the novelty of
MOFs and the recent interest in exploiting their properties as solid catalysts, there is
no doubt that the field of catalysis by embedded NPs in MOFs will continue to attract
much interest in the recent years. However, many unresolved problems still remain a
great challenge. The synthetic procedures require hydrogen and plasma reduction
procedure at high temperature, leading to a high cost and energy wastage. Besides,
efficient separation and recycling of MOFs catalysts after catalytic reactions is
specifically required. Therefore, it is still a challenge to fabricate the type of
multifunctional MOFs materials with a hierarchical structure, which can be applied in
heterogeneous catalysis as well as efficient separation of biological molecules.
Herein, we report a facile and efficient method for the fabrication of
magnetically recyclable nanocatalyst with noble metallic NPs encapsulated into
magnetic MOFs. The cauliflower-like microspheres consist of a magnetite core and an
4

Catalysis Science & Technology
Page 6 of 43
View Article Online
DOI: 10.1039/C4CY00072B
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
ordered shell of MIL-100(Fe) crystals via a layer-by-layer assembly process.
Encapsulating the noble metallic NPs within the frameworks was carried out through
a simple deposition-reduction method. The whole process is shown in Scheme 1.
Compared with other methods of magnetic-Au heterogeneous catalyst formation, no
additional toxic reagent is needed. At the same time, both the thickness of MOFs shell
and the content of noble metallic NPs are tuneable on the MOFs coating. The
synergistic effect between the ultrafine noble metallic NPs (2-3 nm) and MOF shell
significantly improved the catalytic performance for the reduction of nitrobenzenes.
Moreover, due to the high saturation magnetization and super-paramagnetic property,
the as-synthesized nanocatalyst shows high efficiency in magnetic separation and
recovery. Thus a magnetically controllable on-off reaction was achieved in the
catalytic reduction process. Long-life and high reusability of the as-prepared
Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) microspheres are demonstrated without
visible decrease in the catalytic performance after several running cycles. These
facilitate the practical applications of the catalyst in catalysis.
93
2. Experimental section
94
95
96
2.1 Materials
All chemicals were from commercial and used without further purification:
mercaptoacetic acid (MAA, Tianjin Chemical Reagent Factory), ferric chloride
5

Page 7 of 43
Catalysis Science & Technology
View Article Online
97
hexahydrate (FeCl3·6H2O, Sinopharm Chemical Reagent Co., Ltd., analyticaDlOgI:r1a0d.10e3)9,/C4CY00072B
1,3,5-benzenetricarboxylic acid (H3btc, J&K Scientific Ltd., 98%), sodium
borohydride (NaBH4, Sinopharm Chemical Reagent Co., Ltd., 96%), hydrogen
tetrachloroaurate (III) tetrahydrate (HAuCl4·4H2O, Shenyang research institute of
nonferrous metals, analytical grade), hydrogen hexachloroplatinate hexahydrate
(H2PtCl6·6H2O, Shenyang keda reagent factory, analytical grade), palladium chloride
(PdCl2, Shenyang keda reagent factory, analytical grade), o-nitrophenol,
m-nitrophenol, p-nitrophenol, 2,4-dinitrophenol, o-nitroaniline, m-nitroaniline,
98
99
100
101
102
103
104
105
106
107
108
109
p-nitroaniline,
4-Methyl-2-nitroaniline,
4-Methyl-3-nitroaniline
and
p-Nitrophenylhydrazine (Tianjin Chemical Reagent Factory, analytical grade). A
stock solution of tetrachloropalladinic acid (H2PdCl4) was prepared by dissolving 520
mg of PdCl2 into 20 mL of 10% v/v HCl ethanol solution with sonication at room
temperature until complete dissolution.
110
111
112
113
114
115
116
2.2 Preparation of magnetic Fe3O4 microspheres
The magnetic particles were prepared via a solvothermal method 29. Briefly, 2.70
g of FeCl3·6H2O and 7.20 g of sodium acetate were dissolved in 100 mL of ethylene
glycol under vigorous stirring. The resultant homogeneous yellow solution was
transferred to a Teflon-lined stainless-steel autoclave, sealed and heat at 200 °C for 8
h. Then the autoclave was cooled to room temperature. The obtained black magnetite
particles were washed with ethanol for 6 times, and dried in vacuum at 60 °C for 12 h.
6

Catalysis Science & Technology
Page 8 of 43
View Article Online
DOI: 10.1039/C4CY00072B
117
118
119
120
121
122
123
124
125
126
127
128
129
2.3 Preparation of MAA-functionalized Fe3O4 microspheres
100 mg Fe3O4 was dispersed in 20 mL of 0.29 mM MAA ethanol solution and
stirred for 24 h. The solid products were collected using an external magnetic field
and rinsed with ultrapure water and ethanol for three times, then re-dispersed in
ethanol for further use.
2.4 Preparation of core-shell magnetic Fe3O4@MIL-100 (Fe) microspheres
Fe3O4@MIL-100 (Fe) core-shell magnetic microspheres were synthesized
30
according to
a
previous report of Qiu
.
Typically, the as-prepared
MAA-functionalized Fe3O4 microspheres were dispersed into the ethanol solution of
FeCl3 (for 15 min) and H3btc (for 30 min) separately at 70 °C for a given repeated
cycles. Between each step the microspheres were separated with a magnet, and
washed with ethanol. Finally the samples were washed with ethanol for three times
and dried overnight at 150 °C under vacuum.
130
131
132
133
134
135
136
2.5 Synthesis of magnetic Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) microspheres
Prior to metal loading, the as-synthesized Fe3O4@MIL-100 (Fe) microspheres
described above were activated by heating at 150 °C for 10 h under dynamic vacuum.
Supported noble metallic NPs were prepared via a simple deposition-reduction
method. Typically, 20 mg of Fe3O4@MIL-100 (Fe) powder was dispersed in 2 mL of
ethanol and the mixture was sonicated until homogeneous. After stirring for 30 min,
ethanol solution of the precursor (HAuCl4·4H2O, H2PtCl6·6H2O and H2PdCl4, 10.00
7

Page 9 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
137
µmol) was added dropwise with constant vigorous stirring. The resulting solution was
continuously stirred for 5 h. Then 500 µL ethanol was added into the slurry followed
by adding NaBH4 (100 mM) ethanol solution under vigorous stirring for the complete
reduction of the anionic metal species (700 µL for Fe3O4@Au/MIL-100 (Fe), 325 µL
for Fe3O4@Pt/MIL-100 (Fe) and 93 µL for Fe3O4@Pd/MIL-100 (Fe)). After 30 min
stirring, the resulting Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) microspheres were
recovered by a magnet, rinsed with ethanol to remove Cl-, and dried under vacuum at
25 °C for 12 h.
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
2.6 Catalytic reduction of aqueous solution of p-nitrophenol
Liquid-phase reduction of p-nitrophenol to p-aminophenol by NaBH4 was used
to characterize the catalytic activity of the Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd)
microspheres. It is a well-known model reaction and has been extensively used to
evaluate the catalytic rate of functional composite nanomaterials 31-32. Typically, 2.75
mL ultrapure water and an aqueous solution (200 µL) of freshly prepared NaBH4 (100
mM, 20 µmol) were added to a solution (25 µL) of p-nitrophenol (10 mM, 0.25 µmol),
respectively. Subsequently, 25 µL of aqueous dispersion of the 5.09 wt%
Fe3O4@Au/MIL-100 (Fe) (1.0 mg/mL, 6.46 nmol, Au/p-nitrophenol=2.58 mol%),
4.29 wt% Fe3O4@Pt/MIL-100 (Fe) (1.0 mg/mL, 5.50 nmol, Pt/p-nitrophenol=3.65
mol%) or 3.88 wt% Fe3O4@Pd/MIL-100 (Fe) (1.0 mg/mL, 9.11 nmol,
Pd/p-nitrophenol=2.58 mol%) catalyst was added to start the reaction. After addition
8

Catalysis Science & Technology
Page 10 of 43
View Article Online
DOI: 10.1039/C4CY00072B
157
158
159
160
161
of the as-synthesized microspheres, the bright yellow solution gradually faded as the
reaction proceeded which could be observed by naked eye. The kinetic study of the
reaction was performed by measuring the change in the absorbance at 400 nm with
time. The absorbance spectra of the solution were measured with a UV-Vis
spectrometer in the range 250-550 nm.
162
163
164
165
166
167
168
169
170
The recyclability of the as-prepared catalyst was investigated in the reaction of
p-nitrophenol reduction. The reduction reaction was amplified twenty times than in
previous investigations to facilitate separation and minimize handling errors. After
each run, the solid catalyst was recovered using a magnet (complete separation within
30 s), washed with water and ethanol (twice each), dried under vacuum, and reused in
the next cycle. The catalytic reduction of other nitrobenzenes was conducted under
the same condition of p-nitrophenol. The conversions were determined by the gas
chromatography/mass spectroscopy (GC/MS) analysis of the residual nitrobenzene in
the mixture after reaction.
171
2.7 Characterization
172
173
174
175
176
The powder X-ray diffraction (XRD) patterns of the samples were collected on a
D/MAX-2000 diffractometer (Rigaku, Japan) using Cu Ka radiation (wavelength λ=
1.514178 Å) in the 2θ range 0-80 º. FT-IR spectra were recorded on a Nicolet Nexus
670 fourier transform infrared spectrometer (FT-IR, America) using KBr pellets.
Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449C
9

Page 11 of 43
Catalysis Science & Technology
View Article Online
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
thermal analyzer from room temperature to 800 ºC at a ramp rate of 10 ºC/DmOiI:n10i.1n03a9/C4CY00072B
flowing 100 mL/min air atmosphere. The morphologies and sizes of the as-prepared
samples were characterized by a Hitachi-600 transmission electron micro-scope
(TEM, Hitachi, Japan) equipped with an energy dispersive X-ray analyzer (EDX) and
a JSM-6701F field emission scanning electron microscope (FE-SEM) at an
accelerating voltage of 15 kV, respectively. Elemental analyses for C/H were
performed on a Perkin-Elmer 2400 Series II analyzer and the species of Fe, Au, Pt
and Pd were analyzed by inductively coupled plasma atomic emission spectrometry
(ICP-AES) (Thermo Jarrel Ash, Franklin, MA, USA). X-ray photoelectron spectra
(XPS) were acquired with a PHI 5702 spectrometer equipped with an Al Kα exciting
source. A 2 kV Ar+ sputter beam was used for depth profiling of Fe3O4@M/MIL-100
(Fe) (M=Au, Pt, Pd) samples after the initial data were collected. Nitrogen
adsorption-desorption isotherms were obtained at -196 ºC on a Quantachrome
AS1Win analyzer. The magnetic behavior was investigated using a vibrating sample
magnetometer (VSM, Lake Shore 7304, Lake Shore, USA) with an applied field
between -15 kOe and 15 kOe at room temperature. The UV-Vis spectra were recorded
on a UV-Vis spectrometer (TU-1901, Beijing, China).
194
195
3. Results and discussion
3.1 Structural and Morphological Characterization
10

Catalysis Science & Technology
Page 12 of 43
View Article Online
DOI: 10.1039/C4CY00072B
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
The magnetite particles were prepared via a robust solvothermal method as
described above. The TEM image shows that the obtained magnetite particles are
uniform with a mean diameter of ~250 nm (Figure 1A). SEM image of the magnetite
particles further confirms the uniform size of ~250 nm and nearly spherical shape
with rough surface (Figure S1A). The surface roughness is attributed to the fact that
the particles are formed by packing many tiny nanocrystals. After 20 assembly cycles
coating with MIL-100 (Fe), core-shell Fe3O4@MIL-100 (Fe) microspheres with a
MOF layer of about 35 nm in thickness were obtained (Figure 1B). The microspheres
display uniform three-dimensional architecture with cauliflower-like morphology
(Figure 1I). Moreover, with increasing number of assembly cycles, the size of the
whole NPs increases (Figure S1 and Figure 2A-F). In fact, the size of
Fe3O4@MIL-100 (Fe) microspheres and the thickness of shell shows linear
dependence on the number of assembly cycles (Figure 2G). This suggests that the
thickness of the MOF shell can be rationally controlled by varying the number of
growth cycles using the step-by-step assembly strategy. The subsequent in situ growth
process results in a uniform loading of noble metallic NPs (Figure 1C-E). The average
sizes of noble metallic NPs (Figure S2) are about 2.0±0.2 nm (Au NPs), 1.9±0.2 nm
(Pt NPs) and 2.2±0.2 nm (Pd NPs), respectively, which are small enough to be
accommodated in the mesoporous cavities of MIL-100(Fe). However, metallic
particles with bigger sizes were tend to be located on framework surface, as in the
case of metallic NPs loaded into ZIF-8 (diameter of pore window, 11.6 Å), MOF-5
11

Page 13 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
217
218
219
220
221
222
223
224
225
226
(diameter of pore window, 25.7 Å), and other porous materials 28, 33-35. In addition, the
morphology of the magnetic MOFs microspheres remains unchanged after noble
metallic NPs loading (Figure 1J-L). The high-resolution transmission electron
microscopy (HRTEM) images show lattice fringes corresponding to an interfringe
distance of 0.24 nm (for Au NPs) and 0.23 nm (for Pt and Pd NPs), which can be
attributed to the (111) plane of Au 36, Pt 37 and Pd NPs 38, respectively (Figure 1F-H).
The EDX spectra of the microspheres (Figure S3) also show the presence of Fe and
Au/Pt/Pd. In addition, the C, H, Fe and M ratios of the Fe3O4@M/MIL-100 (Fe)
(M=Au, Pt, Pd) microspheres were obtained by elemental analysis and ICP-AES, and
the values were listed in Table 1.
227
228
229
230
231
232
233
234
235
236
To further verify the formation of the Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd)
microspheres, the XRD patterns of Fe3O4, Fe3O4@MIL-100 (Fe) and
Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) samples were carried out, as shown in
Figure 3. The diffraction peaks in curve a can be indexed to the face-centred cubic
39
lattice of Fe3O4 nanospheres according to JCPDS 75-1609 . After coating with the
MOF layer, the diffraction pattern of the resulting material (curve b) shows some new
diffraction peaks (labeled with the symbol ) in addition to the Fe3O4 reflections,
demonstrating the formation of the crystal MIL-100 (Fe) 30. The sharp and strong
40
peaks at 2-4° reveal the presence of both micropores and mesopores . After the in
situ growth process, there is no loss of crystallinity in the XRD patterns for the
12

Catalysis Science & Technology
Page 14 of 43
View Article Online
DOI: 10.1039/C4CY00072B
237
238
239
240
241
242
243
244
resultant Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) microspheres (curves c, d and e).
Compared with the simulated pattern of MIL-100 (Fe) (curve f), we conclude that the
integrity of the MIL-100 (Fe) framework is maintained. The weak diffraction
reflections of typical cubic Au NPs 36 (curves c), face-centred Pt NPs 41 (curve d) and
38
Pd NPs (curve e) are also detected, indicating the formation of very small M NPs
(M=Au, Pt, Pd), which matches well with the TEM observations. No diffraction peaks
from any other impurities are detected. The XRD results indicate that the metal
precursors are successfully reduced to metallic NPs.
245
246
247
248
249
250
251
252
253
254
255
Figure 4A shows the FT-IR spectrum of Fe3O4, Fe3O4@MIL-100 (Fe), and
Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) nanocomposites. The band at 590 and 636
cm-1 are assigned to Fe-O and C-S stretching vibrations of MAA-functionalized Fe3O4,
respectively. The peaks at 1435 and 1575 cm-1 are owing to the stretching vibrations
of C=C in benzene ring, while the peaks at 1376 and 1618 cm-1 are attributed to C=O
stretching vibrations of carboxylic acid, suggesting that MIL-100 (Fe) shell has been
successfully coated on the surface of Fe3O4. In addition, FT-IR and TGA analyses
(Figure 4B) show that the as-synthesized microspheres have similar peaks or curves to
Fe3O4@MIL-100 (Fe), indicating the retention of the MIL-100 (Fe) framework and
the absence of new bond/structure. The results are in good agreement with that
obtained from XRD pattern.
13

Page 15 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
The BET surface area measurements were performed with N2
adsorption/desorption isotherms at -196 ºC. Before measurements, all the samples
were evacuated at 150 °C for 12 h. The pore volume was calculated by a single point
method at P/P0=0.992. As determined by N2 sorption isotherms (Figure 5), the BET
surface areas of Fe3O4@MIL-100 (Fe), Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd)
microspheres are 319, 211, 173 and 186 m2/g, respectively. And their pore volumes
are 0.45, 0.41, 0.39 and 0.40 cm3/g, respectively. Furthermore, estimation of the pore
size distribution by the density functional theory (DFT) shows two maxima at 1.5 and
2.2 nm for Fe3O4@MIL-100 (Fe), which is consistent with the previously reported
42
data . The pore sizes are big enough for the metallic precursors to diffuse into the
-
2-
2-
pores (the diameters of the metallic precursors (AuCl4 , PtCl6 , and PdCl4 ) were
calculated by Chemdraw to be 3.8 Å, 4.6 Å, and 4.2 Å, respectively). The nucleation
can take place within it to form the noble metallic NPs. As compared with
Fe3O4@MIL-100 (Fe), the surface areas and the pore volumes of Fe3O4@M/MIL-100
(Fe) (M=Au, Pt, Pd) are appreciably decreased. The results indicate that the pores of
the host frameworks are occupied by dispersed M NPs (M=Au, Pt, Pd) and/or blocked
by the M NPs located on framework surface, as in the case of M NPs loaded into
ZIF-8, MOF-5, and other porous materials 28, 33-35
.
274
275
The magnetic properties of the resultant multifunctional microspheres were
investigated at room temperature by VSM in the field range from -20 to +20 kOe, as
14

Catalysis Science & Technology
Page 16 of 43
View Article Online
DOI: 10.1039/C4CY00072B
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
shown in Figure 6. They exhibit super-paramagnetic behaviour, little hysteresis,
remanence and coercivity due to the fact that the particles are composed of ultrafine
magnetite nanocrystals. A reduction of magnetization saturation (Ms) value is
observed for Fe3O4@MIL-100 (Fe) microspheres after growing assembly cycles
(Figure S4). This could be attributed to the decrease in the density of Fe3O4, which is
corresponding to the increase of outer-layer thickness as shown in SEM (Figure S1)
and TEM (Figure 2). After loading with Au, Pt and Pd NPs, the Ms values of
Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) are slightly decreased to 44.47, 43.90 and
45.07 emu/g, respectively (Figure 6A). It should be noted that the microspheres can
be dispersed in water by sonication or vigorous shaking, resulting in a
brown-coloured suspension. One can observe fast aggregation of the microspheres
from their homogeneous dispersion in the presence of an external magnetic field, and
quick redispersion with a slight shaking once the magnetic field is removed (Figure
6B). These results directly demonstrate the excellent magnetic responsivity and
redispersibility, which is important in terms of their practical manipulation.
291
292
293
294
295
In order to further demonstrate the composition details of the resulting hybrid
magnetic microspheres, XPS was performed. The sensitivity of XPS in chemical
analysis stems mostly from its ability to resolve the chemical identity of the atoms
from the measured binding energies. Figure 7A exhibits the overall surveys of the
as-synthesized Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) microspheres, which clearly
15

Page 17 of 43
Catalysis Science & Technology
View Article Online
296
297
298
299
300
301
302
shows the signals of C, O and Fe elements. Well-defined peaks correspoDnOdIi:n10g.10t3o9/C4CY00072B
metallic Au, Pt and Pd species can also be detected (Figure 7B-D). The position of the
Au 4f, Pt 4f and Pd 3d peaks are corresponding to Au0, Pt0 and Pd0, respectively,
which are in good agreement with the previous literature 43-44. From these spectra, it
can be observed that all the metal ions (Au3+, Pt4+ and Pd2+) impregnated in
Fe3O4@MIL-100 (Fe) are reduced to noble metallic NPs after the in situ growth
process by NaBH4 reduction.
303
3.2 Application in catalytic reduction of p-nitrophenol
304
305
306
307
308
309
310
311
312
313
314
315
In this investigation, the catalytic performance of Fe3O4@M/MIL-100 (Fe)
(M=Au, Pt, Pd) microspheres for efficient reduction of p-nitrophenol to
p-aminophenol has been intensively investigated 32, 45. P-nitrophenol aqueous solution
exhibits a distinct spectral profile with an absorption maximum at 317 nm. After the
addition of NaBH4, a red-shift of the absorption peak to 400 nm is observed (Figure
8A). A colour change from light yellow to bright yellow indicates the formation of
p-nitorphenolate 46. Moreover, there is little change for the maximum absorption at
400 nm over time even after adding excess NaBH4 solution, confirming that the
reduction couldn’t proceed without catalyst. As a comparison, the catalytic ability of
an equal amount of Fe3O4 and Fe3O4@MIL-100 (Fe) is also studied. As shown in
Figure 8B, there is little change in absorbance at 400 nm within 30 min over Fe3O4
and Fe3O4@MIL-100 (Fe), while complete reduction of p-nitrophenol is achieved
16

Catalysis Science & Technology
Page 18 of 43
View Article Online
316
317
318
within a short time over the as-synthesized catalysts. Superior catalytic perfDoOrIm: 10a.n10c3e9/C4CY00072B
of the Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) catalysts confirm that the applied
magnetic MOFs can serve as an excellent support for noble metallic NPs.
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
Figure 9A-C unambiguously exhibit the time-dependent UV-Vis absorption
spectra for a typical reduction process using Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd)
microspheres as catalysts. After addition of the catalyst, the absorption intensity at
400 nm significantly decreases and the characteristic absorption of p-aminophenol
appears at 300 nm. The appearance of this new peak was verified by superimposing
the UV-Vis spectrum of pure p-aminophenol under identical conditions (Figure 8A, d).
The formation of p-aminophenol was further confirmed from gas
chromatography-mass (GC-MS) analytic technique (Figure S5). As can be seen from
Figure S5, after the reduction reaction, the characteristic peak of p-nitrophenol
(Figure S5A, M-1: 138.06) disappeared, accompanied by the appearance of
characteristic peak of p-aminophenol (Figure S5B, M+1: 110.06). The data provide a
supportive evidence for the efficient reduction of p-nitrophenol to p-aminophenol.
The UV-Vis spectra show an isosbestic point at 313 nm (Figure 8A), illustrating that
the catalytic reduction of p-nitrophenol yields p-aminophenol only, without
32
by-product . Considering that the concentration of NaBH4 is much higher than that
of p-nitrophenol (CNaBH4/Cp-nitrophenol=80) in the reaction system, a pseudo first-order
kinetic equation with regard to p-nitrophenol only can be applied to evaluate the
17

Page 19 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
47
336
337
338
339
340
341
342
343
catalytic rate . The kinetic equation for the reduction reaction of p-nitrophenol can
be expressed as ln (Ct/C0) = −Kappt, where Kapp is the kinetic rate constant, C0 is the
initial concentration of p-nitrophenol, and Ct is the concentration of p-nitrophenol at
time t. Linear relationships between ln (Ct/C0) and reaction time are displayed in
Figure 9D, which match well with the first-order reaction kinetics. The kinetic rate
constant Kapp is calculated to be 2.58 min-1 for Fe3O4@Pt/MIL-100 (Fe) (Figure S6),
which is superior to other noble metallic-based catalysts 33, 48-49. This means that the
nanocomposites show excellent catalytic activity for the reduction of p-nitrophenol.
344
345
346
347
348
349
350
351
352
353
354
355
The catalytic activity of the as-synthesized Fe3O4@Pt/MIL-100 (Fe) catalyst for
the reduction of other nitrobenzene analogs including nitrophenol, nitroaniline and
nitrophenylhydrazine were also investigated (Table 2). Figure S9 clearly shows the
typical changes of UV-Vis absorption spectra of these nitrobenzene compounds
during the reduction processes, demonstrating that the reduction of these
nitrobenzenes indeed occur in the presence of a small amount of catalyst. Additionally,
as shown in Table 2, the catalyst exhibits good catalytic activity (high TOF values)
with excellent yields toward a series of nitrobenzene compounds regardless of the
types and position of the substituents (the turnover frequency (TOF) is defined as
moles (or numbers) of the product molecules generated per moles (or numbers) of
12
catalyst surface atoms per hour ). Moreover, the catalysts Fe3O4@Au/MIL-100 (Fe)
and Fe3O4@Pd/MIL-100 (Fe) also show high catalytic activity toward the reduction
18

Catalysis Science & Technology
Page 20 of 43
View Article Online
DOI: 10.1039/C4CY00072B
356
357
358
of these nitrobenzene compounds, as illustrated in Table S2 and Table S3. These
results indicate the generality and the efficacy of the new catalysts toward the
reduction of the series of nitrobenzenes.
359
360
361
362
363
364
365
366
It will be significant to compare the catalytic performance of
Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) with previous reported analogous catalysts.
As shown in Table 3, the TOF value can reach 3094 h-1 for Fe3O4@Pt/MIL-100 (Fe),
which is significantly higher than those reported in previous studies 49-53. From Table
3 we can conclude that the as-synthesized catalysts in the present work have a higher
catalytic activity than those of most other catalysts. Thus, a simple and efficient
preparation of a Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) catalyst with high catalytic
efficiency was presented in our work.
367
368
369
370
371
372
373
374
It should be pointed out that the catalytic performance of the as-prepared
Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) microspheres with different assembly cycles
are also studied (Figure S7). Although more Pt NPs are embedded in the porous as the
MOF shell grows (Figure S8), the percentages of the elements in MIL-100(Fe) (C, H
and Fe) increase significantly, leading to a decrease of the metallic content (Table S1).
Based on the relative changes in the contents of the C, H, Fe, Pt observed in Table S1
and the corresponding different catalytic performance, we conclude that the intensities
of the noble metallic NPs plays a key role in the catalysis.
19

Page 21 of 43
Catalysis Science & Technology
View Article Online
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
In addition to catalytic activity, the stability is a very important propertDyOfI:o1r0.1t0h3e9/C4CY00072B
reusability of catalysts. Recyclability of the as-prepared catalyst is tested. The
catalytic activity remains stable even after running for 10 cycles for
Fe3O4@M/MIL-100 (Fe) (M= Pt, Pd) and the conversion of each cycle remains nearly
100%. After reused for 8 cycles, the catalyst Fe3O4@Au/MIL-100 (Fe) gradually
loses its catalytic activity. However, the conversion of upon 85% is maintained
(Figure 10). The high stability can be attributed to the core-shell porous architecture
of the as-prepared microspheres, which provides both docking sites for p-nitrophenol
(the MOF shell) and a super-paramagnetic core allowing easy manipulation by a
magnet. In addition, there’s no change for adsorption spectra of the solutions after
given reduction cycles (Figure S10), which further confirms the long-term stability of
the catalysts. Leaching test with the catalyst Fe3O4@Pt/MIL-100 (Fe) was also carried
out under reaction conditions. The solid catalyst was magnetically separated from the
reactant mixture after each catalytic cycle. The residual solution was collected and
measured by ICP-AES to determine the lost amount of Pt and Fe. No Fe element was
found in the residual solution, suggesting that the Fe3O4 NPs were well protected by
the MOFs shell. Besides, only trace amount of Pt element was detected in the residual
solution. As shown in Figure S11, nearly 80% of Pt content was maintained on the
catalyst even after 10 runs, which proved the good stability of the catalyst. Together
with the high recyclability of the catalyst (Figure 10), these results clearly
demonstrate the stability of the noble metallic NPs in Fe3O4@MIL-100 (Fe).
20

Catalysis Science & Technology
Page 22 of 43
View Article Online
DOI: 10.1039/C4CY00072B
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
The interactions between noble metallic NPs and the support directly affect the
catalytic activity of the catalyst, which could mostly explain the different catalytic
activities of noble metallic NPs in different reports. In this work, a magnetic MOF
was used as support for Au, Pt and Pd NPs with superior catalytic activity. Scheme 2
depicts the performance of the Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) microspheres
as catalysts in the reduction of p-nitrophenol to p-aminophenol by NaBH4. Possible
mechanism of excellent catalytic activity may be explained as follows: given that
p-nitrophenol is π-rich in nature, it is expected that p-nitrophenol can be adsorbed
onto the porous MOFs shell by means of π–π stacking interactions between the
aromatic rings of p-nitrophenol and the organic units of the framework, and
coordination effects between phenolic hydroxy of p-nitrophenol and metal center of
the framework 48, 54. The mesoporous of MOFs serve as millions of nanoreactors,
-
where the reduction actions take place via relaying electrons from the donor BH4 to
-
the acceptor p-nitrophenol. In aqueous medium BH4 is adsorbed onto the surface of
catalyst. The hydrogen atom, which is formed from the hydride, after electron transfer
to the noble metallic NPs, attacks p-nitrophenol molecules to reduce it 55. The
adsorption over the porous MOFs shell provides a high concentration of p-nitrophenol
near to the noble metallic NPs embedded in the MOFs cavities, leading to highly
efficient contact between them. Thus enhancement of the catalytic efficiency for
p-nitrophenol reduction is achieved. It is worthwhile to note that more experimental
21

Page 23 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
416
417
or theoretical evidences are still needed to get the exact mechanism of the enhanced
activity.
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
4. Conclusions
In summary, we have developed an environmentally friendly method to prepare
cauliflower-like magnetic Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) microspheres and
investigated their superior catalytic properties in the effective reduction of
nitrobenzenes compounds. The microspheres show unique structural features such as
magnetic core, porous MOFs shell and monodispersed small noble metallic NPs. The
contents of noble metallic NPs and the thickness of MOFs shell can be easily
controlled by varying the assembly cycles on MOFs coating. The magnetic MOFs
support not only enhances the catalytic activity, but lends the catalyst easiness of
separation and recovery. The Fe3O4@M/MIL-100 (Fe) (M=Au, Pt, Pd) microspheres
maintain high catalytic activity over several reduction cycles. Therefore, this
functional nanostructure holds great promise as novel noble metallic-based catalysts
for various catalytic reactions. Additionally, the fundamental concept for the
multifunctional magnetic MOFs can be extended to the fabrication of multicomponent
nanosystem with integrated and enhanced properties for various advanced
applications, such as chemical/biosensor, biological molecule separation, drug
delivery, and so on.
22

Catalysis Science & Technology
Page 24 of 43
View Article Online
DOI: 10.1039/C4CY00072B
436
Acknowledgements
437
438
This work was supported by the National Natural Science Foundation of China
(Nos. 21375053, 21207057).
439
23

Page 25 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
440
References
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
1. J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao and R. P. Van Duyne,
Nat. Mater., 2008, 7, 442.
2. C. Langhammer, E. M. Larsson, B. Kasemo and I. Zoric
3529.
́, Nano Lett., 2010, 10,
3. J. Huang, T. Jiang, H. Gao, B. Han, Z. Liu, W. Wu, Y. Chang and G. Zhao,
Angew. Chem. Int. Ed., 2004, 116, 1421.
4. A. O. Govorov and H. H. Richardson, Nano Today, 2007, 2, 30.
5. B. D. Chithrani, A. A. Ghazani and W. C. W. Chan, Nano Lett., 2006, 6, 662.
6. T. K. Sau, A. L. Rogach, F. Jäckel, T. A. Klar and J. Feldmann, Adv. Mater.,
2010, 22, 1805.
7. T. S. Hauck, A. A. Ghazani and W. C. W. Chan, Small, 2008, 4, 153.
8. M. Zhu, Y. Wang, C. Wang, W. Li and G. Diao, Catal. Sci. Technol., 2013, 3,
952.
9. C. Zhu, L. Han, P. Hu and S. Dong, Nanoscale, 2012, 4, 1641.
10. Z. Sun, B. Sun, M. Qiao, J. Wei, Q. Yue, C. Wang, Y. Deng, S. Kaliaguine and
D. Zhao, J. Am. Chem. Soc., 2012, 134, 17653.
11. T. Yu, J. Zeng, B. Lim and Y. Xia, Adv. Mater., 2010, 22, 5188.
12. Q. M. Kainz, R. Linhardt, R. N. Grass, G. Vilé, J. Pérez‐Ramírez, W. J. Stark
and O. Reiser, Adv. Funct. Mater., doi: 10.1002/adfm.201303277.
24

Catalysis Science & Technology
Page 26 of 43
View Article Online
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
13. R. Linhardt, Q. M. Kainz, R. N. Grass, W. J. Stark and O. Reiser, RSC ADdOvIa: 1n0c.1e03s9,/C4CY00072B
2014, 4, 8541.
14. Y.-T. Kim, H. Lee, H.-J. Kim and T.-H. Lim, Chem. Commun., 2010, 46, 2085.
15. N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O'Keeffe and O. M.
Yaghi, Science, 2003, 300, 1127.
16. O. K. Farha, A. Özgür Yazaydın, I. Eryazici, C. D. Malliakas, B. G. Hauser, M.
G. Kanatzidis, S. T. Nguyen, R. Q. Snurr and J. T. Hupp, Nat. Chem., 2010, 2,
944.
17. Y. Hu, Z. Huang, J. Liao and G. Li, Anal. Chem., 2013, 85, 6885.
18. J. Hermannsdörfer, M. Friedrich, N. Miyajima, R. Q. Albuquerque, S. Kümmel
and R. Kempe, Angew. Chem. Int. Ed., 2012, 51, 11473.
19. T. Toyao, M. Saito, Y. Horiuchi, K. Mochizuki, M. Iwata, H. Higashimura and
M. Matsuoka, Catal. Sci. Technol., 2013.
20. C.-X. Yang, H.-B. Ren and X.-P. Yan, Anal. Chem., 2013, 85, 7441.
21. P. Horcajada, T. Chalati, C. Serre, B. Gillet, C. Sebrie, T. Baati, J. F. Eubank, D.
Heurtaux, P. Clayette, C. Kreuz, J.-S. Chang, Y. K. Hwang, V. Marsaud, P.-N.
Bories, L. Cynober, S. Gil, G. Ferey, P. Couvreur and R. Gref, Nat. Mater., 2010,
9, 172.
22. A. Dhakshinamoorthy, M. Opanasenko, J. Čejka and H. Garcia, Adv. Synth.
Catal., 2013, 355, 247.
25

Page 27 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
23. A. Dhakshinamoorthy, M. Opanasenko, J. Čejka and H. Garcia, Catal. Sci.
Technol., 2013, 3, 2509.
24. V. Pascanu, Q. Yao, A. Bermejo Gómez, M. Gustafsson, Y. Yun, W. Wan, L.
Samain, X. Zou and B. Martín‐Matute, Chemistry-A European Journal, 2013,
19, 17483.
25. H. Liu, Y. Li, H. Jiang, C. Vargas and R. Luque, Chem. Commun., 2012, 48,
8431.
26. G. Chen, S. Wu, H. Liu, H. Jiang and Y. Li, Green Chem., 2013, 15, 230.
27. H. R. Moon, D.-W. Lim and M. P. Suh, Chem. Soc. Rev., 2013, 42, 1807.
28. Q.-L. Zhu, J. Li and Q. Xu, J. Am. Chem. Soc., 2013, 135, 10210.
29. Y. Deng, D. Qi, C. Deng, X. Zhang and D. Zhao, J. Am. Chem. Soc., 2007, 130,
28.
30. F. Ke, L.-G. Qiu, Y.-P. Yuan, X. Jiang and J.-F. Zhu, J. Mater. Chem., 2012, 22,
9497.
31. Y. Fang and E. Wang, Nanoscale, 2013, 5, 1843.
32. Y. Deng, Y. Cai, Z. Sun, J. Liu, C. Liu, J. Wei, W. Li, C. Liu, Y. Wang and D.
Zhao, J. Am. Chem. Soc., 2010, 132, 8466.
33. H.-L. Jiang, T. Akita, T. Ishida, M. Haruta and Q. Xu, J. Am. Chem. Soc., 2011,
133, 1304.
26

Catalysis Science & Technology
Page 28 of 43
View Article Online
DOI: 10.1039/C4CY00072B
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
34. Y. K. Hwang, D.-Y. Hong, J.-S. Chang, S. H. Jhung, Y.-K. Seo, J. Kim, A.
Vimont, M. Daturi, C. Serre and G. Férey, Angew. Chem. Int. Ed., 2008, 47,
4144.
35. Y. E. Cheon and M. P. Suh, Angew. Chem. Int. Ed., 2009, 48, 2899.
36. J. Zheng, Y. Dong, W. Wang, Y. Ma, J. Hu, X. Chen and X. Chen, Nanoscale,
2013, 5, 4894.
37. S. Lin, C. Shen, D. Lu, C. Wang and H.-J. Gao, Carbon, 2013, 53, 112.
38. T. Yao, T. Cui, X. Fang, F. Cui and J. Wu, Nanoscale, 2013, 5, 5896.
39. H. Deng, X. Li, Q. Peng, X. Wang, J. Chen and Y. Li, Angew. Chem. Int. Ed.,
2005, 117, 2842.
40. G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé and I.
Margiolaki, Science, 2005, 309, 2040.
41. V.-D. Dao and H.-S. Choi, Chem. Commun., 2013, 49, 8910.
42. R. Canioni, C. Roch-Marchal, F. Secheresse, P. Horcajada, C. Serre, M.
Hardi-Dan, G. Ferey, J.-M. Greneche, F. Lefebvre, J.-S. Chang, Y.-K. Hwang, O.
Lebedev, S. Turner and G. Van Tendeloo, J. Mater. Chem., 2011, 21, 1226.
43. X. Liu, S. Ye, H.-Q. Li, Y.-M. Liu, Y. Cao and K.-N. Fan, Catal. Sci. Technol.,
2013, 3, 3200.
44. X. Gu, Z.-H. Lu, H.-L. Jiang, T. Akita and Q. Xu, J. Am. Chem. Soc., 2011, 133,
11822.
27

Page 29 of 43
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C4CY00072B
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
45. Y. Lu, Y. Mei, M. Drechsler and M. Ballauff, Angew. Chem. Int. Ed., 2006, 45,
813.
46. J.-M. Song, S.-S. Zhang and S.-H. Yu, Small, doi: 10.1002/smll.201301386.
47. Y. Mei, Y. Lu, F. Polzer, M. Ballauff and M. Drechsler, Chem. Mater., 2007, 19,
1062.
48. F. Ke, J. Zhu, L.-G. Qiu and X. Jiang, Chem. Commun., 2013, 49, 1267.
49. W. Hu, B. Liu, Q. Wang, Y. Liu, Y. Liu, P. Jing, S. Yu, L. Liu and J. Zhang,
Chem. Commun., 2013, 49, 7596.
50. B. Liu, S. Yu, Q. Wang, W. Hu, P. Jing, Y. Liu, W. Jia, Y. Liu, L. Liu and J.
Zhang, Chem. Commun., 2013, 49, 3757.
51. X. Wang, D. Liu, S. Song and H. Zhang, J. Am. Chem. Soc., 2013, 135, 15864.
52. E. Lam, S. Hrapovic, E. Majid, J. H. Chong and J. H. T. Luong, Nanoscale, 2012,
4, 997.
53. X. Wu, C. Lu, W. Zhang, G. Yuan, R. Xiong and X. Zhang, J. Mater. Chem. A,
2013, 1, 8645.
54. F. Millange, C. Serre, N. Guillou, G. Férey and R. I. Walton, Angew. Chem. Int.
Ed., 2008, 47, 4100.
55. J.-P. Deng, W.-C. Shih and C.-Y. Mou, J. Phys. Chem. C, 2007, 111, 9723.
28