Highly dispersed Pt in MIL-101: An efficient catalyst for the hydrogenation of nitroarenes

Article abstract of DOI:10.1016/j.catcom.2013.06.038

Homogeneously dispersed Pt adatoms were fabricated in ordered mesoporous metal-organic framework (MIL-101) through a simple colloid method. N ₂-adsorpton and time-resolved HRTEM analysis disclosed that Pt dispersed highly in the pores and/or ou

Full text of DOI:10.1016/j.catcom.2013.06.038

Catalysis Communications 41 (2013) 56–59
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Highly dispersed Pt in MIL-101: An efficient catalyst for the
hydrogenation of nitroarenes
a
b
a
b,
a,
Weichen Du , Gongzhou Chen , Renfeng Nie , Yingwei Li ⁎, Zhaoyin Hou ⁎
a
Institute of Catalysis, Department of Chemistry, Key Lab of Applied Chemistry of Zhejiang Province, Zhejiang University, Hangzhou 310028, China
Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Homogeneously dispersed Pt adatoms were fabricated in ordered mesoporous metal-organic framework
Received 13 March 2013
Received in revised form 18 June 2013
Accepted 28 June 2013
2
(MIL-101) through a simple colloid method. N -adsorpton and time-resolved HRTEM analysis disclosed
that Pt dispersed highly in the pores and/or outer surface of MIL-101. Pt@MIL-101 exhibited an extremely
high activity for hydrogenation of nitroarenes under mild conditions. The turnover frequency of each Pt
atom for nitrobenzene hydrogenation reached 1548 min− at 30 °C.
Available online 11 July 2013
1
©
2013 Elsevier B.V. All rights reserved.
Keywords:
Pt adatoms
MIL-101
Thermal stability
Hydrogenation
Nitroarenes
1
. Introduction
stable in water and air atmosphere [16,17]. The large-size pore window
makes it possible for substrates and products to enter and leave the
framework freely [18]. Owing to these properties, the utilization of
MIL-101 as support for metal nanoparticles has attracted much attention
in recent years. A number of MIL-101 supported metal nanoparticles
(e.g. Ni, Pd, Pt, Au) have already been prepared and used as catalysts in
various reactions such as oxidation, C\C coupling, and hydrogenation
[19–26].
In this communication, homogeneously dispersed Pt adatoms were
fabricated in MIL-101 through a simple colloid method. This catalyst
was applied in the hydrogenation of nitroarenes because MIL-101 has
well defined accessible Lewis acidic sites which are proved to be able
to promote the reactivity of aromatic substrate [27].
Recently, metal-organic frameworks (MOFs) have drawn a lot of
attention as a new class of porous polymeric material consisting of
metal ions linked together by organic bridging ligands [1]. Their
remarkable properties such as high surface area and porosity [2,3]
make them promising materials for a variety of applications [4–6].
In the past years, the use of MOFs as a porous material for gas storage
and separation was particularly emphasized. More recently, the
application of MOFs in heterogeneous catalysis is becoming a hot topic
[5–9]. A great deal of related works have revealed the great potential
for such materials to be used as catalyst in a series of reactions, such as
condensation [10], cyanosilylation [11], oxidation [12] and alkylation
[13]. H. Jiang et al. successfully restricted Au@Ag nanoparticles (NPs)
to 2–6 nm with the limitation effect of the pore/surface structure of
2. Experimental
ZIF-8. This catalyst exhibited meritorious activity for the hydrogenation
of 4-nitrophenol using NaBH
4
as reducing agent [14]. Sabo et al. found
2.1. Catalysts preparation
that MOF-5 could be used as support for palladium that showed a higher
activity towards the hydrogenation of styrene than palladium on acti-
vated carbon [8]. Nevertheless, Pd/MOF-5 is instable when contacting
with water or humid air. Férey et al. combined targeted chemistry and
computational design and created a new crystal structure of porous
3 3 2
Cr(NO ) ·9H O (99%) (2.007 g, 5.0 mmol), HF (48 wt.%, 5.0 mmol),
terephthalic acid (98%) (0.823 g, 5.0 mmol), and 24 mL of deionized
water were mixed completely and heated at 220 °C for 8 h. And then,
the mixture was cooled slowly to room temperature, solid powder,
which is formed during the hydrothermal synthesis, was separated by
filtration and washed thoroughly with deionized water and ethanol,
soaked in ethanol (95% EtOH with 5% water) at 80 °C for 24 h. Finally,
the solid was dried overnight at 150 °C under vacuum to obtain MIL-
chromium terephthalate, MIL-101, Cr
15]. MIL-101 has an incredibly large pore size (3.0 to 3.4 nm) and
high specific surface area (Langmuir surface area from N adsorption
reaches 5900 ± 300 m /g). At the same time, MIL-101 is long-term
3 2 2 2 6 4 2 3
F(H O) O[(O C)–C H –(CO )]
[
2
2
1
01. A mixed aqueous solution of H
Mw 85,000–124,000), with H PtCl
metal = 10:1 (molar ratio) was prepared. The mixed solution was
2
PtCl
6
and polyvinyl alcohol (PVA,
200 mg/L and PVA monomer/
⁎
Corresponding authors. Tel./fax: +86 571 88273283.
2
6
E-mail address: zyhou@zju.edu.cn (Z. Hou).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.06.038

W. Du et al. / Catalysis Communications 41 (2013) 56–59
57
placed in an ice bath and vigorously stirred for 1 h. After that, freshly
prepared aqueous solution of NaBH (0.01 M, NaBH /metal = 5:1
molar ratio)) was added under vigorous stirring, and then the activated
surfactant (PVA) could get into the porous windows and occupy partial
pores. The calculated BET surface areas of MIL-101 and Pt@MIL-101 are
2869 and 2500 m /g, respectively (Fig. 1). The reduction in surface area
4
4
2
(
MIL-101 was added immediately. The suspension was stirred at 0 °C for
another 4 h, followed by washing thoroughly with deionized water. The
sample was finally dried under vacuum at 100 °C for 2 h to obtain the
Pt@MIL-101 catalyst. The loading amount of Pt in the fresh prepared
sample was 0.994 wt.%, based on AAS analysis.
is also attributed to the incorporation of Pt and/or surfactant in the
pores of MIL-101.
Fig. 2 shows the time-resolved TEM images of Pt@MIL-101 catalyst.
The fresh Pt@MIL-101 particles were sized in 150–350 nm. Pt cannot be
identified in the frameworks of fresh Pt@MIL-101 (Fig. 2(a)–(c)). The
oriented and ordered pore channels can be seen when the microscope
was focused on a single particle (Fig. 2(b)). There exist two kinds of
fringes in the particle which correspond to interchannel spacing of
2
.2. Characterizations
Powder X-ray diffraction (XRD) patterns of the samples were
obtained by a Rigaku diffractormeter (D/MAX-IIIA, 3 kW) using Cu Kα
radiation (40 kV, 40 mA, λ = 0.1542 nm). The surface area and pore
4.323 nm (d
and 2.714 nm (d ,
1
, wall thickness = 1.636 nm, pore diameter = 2.687 nm)
wall thickness = 0.985 nm, pore diameter =
2
1.729 nm). These bright spots (in Fig. 2(c)) arranged in orderly rows
would be the cross-sections of the microporous channels with a uniform
pore-size (1.7–1.9 nm), which agrees well with the XRD pattern and
pore size distribution curves (Fig. 1). The thermal stability of Pt@
MIL-101 is low when it was exposed under the electron beam in TEM
analysis. The external layer of the framework melted when the micro-
scope was focused on a single particle for a time longer than 10 min.
Fig. 2(c) and (d) are images of a same corner of a Pt@MIL-101 particle
acquired at 2 and 10 min under the electron beam. A clear image and
stable framework could be identified in Fig. 2(c). After 10 min under
the electron beam in TEM analysis (Fig. 2(d)), the Pt@MIL-101 particle
shrank. Pt nanoparticles (NPs) agglomerated at the edge and surface of
the particle (Fig. 2(d)). The Pt NPs distribution diagram shown as an
insert in Fig. 2(d) indicated that most Pt adatoms agglomerated to
2.0 ± 0.5 nm particles when Pt@MIL-101 was exposed for 10 min
under the electron beam in TEM analysis. Fig. 2(e) is the image of a
Pt@MIL-101 particle (10 min under the electron beam) from another
orientation. XPS analysis confirmed that all Pt(IV) were reduced to
Pt(0) in the prepared Pt@MIL-101 (see Fig. S2 in Supplementary data).
distribution measurements were performed with N
2
adsorption/
desorption isotherms at 77 K on a Micromeritics ASAP 2020 instrument.
Transmission electron microscopy (TEM) images were obtained using
an accelerating voltage of 200 kV (TEM, JEOL-2020F). X-ray photoelec-
tron spectroscopy (XPS) measurements were performed on a Kratos
−
9
Axis Ultra DLD system with a base pressure of 10 Torr.
2
.3. Hydrogenation of nitroarenes
In a typical procedure [28], catalyst (containing 0.1 mg Pt) was
dispersed in 30 mL ethanol, and then a certain amount of substrate
was added. The reactor was sealed, purged with hydrogen and pressur-
ized to 1.0 MPa, followed by stirring with a magnetic stirrer at a rate of
1
000 rpm at 0–30 °C. After reaction, the mixture was separated by cen-
trifugation. The reactants were analyzed by GC (HP 5890, USA), and all
products were confirmed by GC–MS (Agilent 6890–5973 N).
3
. Results and discussion
3
.1. Characterizations of the catalysts
3.2. Catalytic activity
The XRD pattern of the synthesized MIL-101 and Pt@MIL-101 is
Table 1 summarized the reaction results of the hydrogenation of
nitrobenzene (NB) over Pt@MIL-101 at different ratios of NB molecule
to Pt atom. The catalyst exhibited inconceivable activity even at a
high ratio between NB molecule and Pt atom (72000:1). It was found
that NB was converted completely within 60 min at a NB/Pt ratio of
36000:1, and the selectivity of aniline (AN) reached 68.0%. Beside AN,
intermediates of the consecutive hydrogenation process such as
nitrosobenzene (NSB) and azoxybenzene (AOB) were also detected in
the reaction mixture. The conversion of NB reached 78.6% even when
the NB/Pt ratio increased to 72000:1, and the calculated turnover
frequency (TOF) of each Pt atom for NB conversion reached
similar to the simulated pattern of MIL-101 reported by Férey et al.
see Fig. S1 in supplementary data) [15]. Both of them are all of high
crystallinity and the sharp and strong peaks at 2–4° reveal the presence
of both micropores and mesopores. N sorption isotherms of both sam-
ples are of type I model with two uptake peaks at 0.0–0.02 and 0.21–
.24 (of P/P ), which is the characteristic of two kinds of microporous
(
2
0
0
windows [15]. Both pore-size distribution curves of MIL-101 and Pt@
MIL-101 that calculated from their adsorption isotherms show two
peaks at 2.2, 3.0 nm and 1.8, 2.7 nm, respectively (Fig. 1). This decrease
in pore diameter (about 0.3–0.4 nm) infers that Pt and/or some
−
1
9
43.2 min . When the reaction temperature was raised to 30 °C, the
−
1
calculated TOF of Pt atom further increased to 1548 min . And this
value is higher than that of multiwall carbon nanotube supported
ultrafine Pt NPs (1115 min ) [28,29], Pt/SiO
−
1
−1
2
(37 min ) [30], Pt
−
1
−1
NPs (53 min
for AN) [31], Pt/MCM-41 (1 min ) [32], Pt/C and
−
1
Pt/PMO-SBA-15 (around 10–400 min ) [33] in the similar or more
critical reaction condition. Besides, the deactivation of the catalyst was
inconspicuous even after five cycles (see Fig. S3 in supplementary data).
The predominant activity of Pt@MIL-101 could be attributed to the
highly dispersed Pt adatoms in the framework of MIL-101, and the
good distribution of Pt@MIL-101 in the reaction mixture. In addition,
Pt@MIL-101 has a mesoporous zeotype architecture with mesoporous
cages (1.8 and 2.7 nm) and large free apertures (1.2 nm from pentag-
onal windows and 1.6 × 1.45 nm from hexagonal windows) [34],
providing free access for NB and AN molecules [35].
Taking the excellent performance of Pt@MIL-101 in NB hydrogena-
tion reaction into consideration, it is possible that the catalyst would
also exhibit high activity for other nitroarene derivatives. As shown in
Table 2, these reactions proceed very well with Pt@MIL-101 despite of
the group substituting on aryl nitro including electron-donating groups
Fig. 1. Nitrogen adsorption isotherms at 77 K: (a) MIL-101; (b) Pt@MIL-101. The inset
image is pore-size distribution curves for MIL-101 (a) and Pt@MIL-101 (b).

58
W. Du et al. / Catalysis Communications 41 (2013) 56–59
Fig. 2. TEM images of Pt@MIL-101. (a) overview of Pt@MIL-101 particles; (b) an enlarged view of a single Pt@MIL-101 particle with two kinds of porous channels: d1 = 4.323 nm,
d2 = 2.714 nm (wall thickness + pore diameter); (c), (d) a same corner of a particle exposed under microscope for 2 min and 10 min respectively; (e) another orientation of a
particle under electron beam for 10 min.
and electron-withdrawing groups. However, different substrates
exhibited varied reaction activity because of the conjugative effect,
inductive effect, and steric constraint of different substitution groups.
For example, the hydrogenation of chloronitrobenzene exhibited a
relatively modest activity due to the electron-withdrawing effect of \Cl.
4. Conclusion
To sum up, it was demonstrated that Pt adatoms could be fabricated
homogenously in MIL-101 via a simple colloid method. Pt@MIL-101
exhibits extremely high catalytic activity in the hydrogenation of
nitroarenes under relatively mild conditions. The high efficiency was
attributed to the Pt homogeneous deposition in MIL-101. On the other
hand, the large pore size and surface area of MIL-101 support make it
much easier for the substrates to reach the active sites.
Table 1
Hydrogenation of NB over Pt@MIL-101 at different NB/Pt ratio a.
Mole ratio
Conversion (%)
TOF (min−1)
Selectivity (%)
Acknowledgment
AN
NSB
PHA
AOB
b
9
000:1
100
100
78.6
64.5
91.6
68.0
57.8
64.1
8.4
28.3
31.0
28.2
0
0
1.5
0.4
0
c
We acknowledge that this research work was supported by the
National Natural Science Foundation of China (Contracts 21273198,
21073159), the Zhejiang Provincial Natural Science Foundation
(Grant No. LZ12B03001) and the Pre-research Ocean Foundation of
Zhejiang University (2012HY025B).
3
6000:1
7
2000:1
3.7
9.7
7.3
d
e
2000:1
943.2
1548.0
7
a
e
Reaction condition: NB ( 19.6 mmol, c,d,e39.2 mmol), ethanol (30 mL), time (b,c,d60 min,
b
b,c,d
e
3
0 min), H
2
(1.0 MPa),
20 °C, 30 °C, 1000 rpm.

W. Du et al. / Catalysis Communications 41 (2013) 56–59
59
[
[
8] M. Sabo, A. Henschel, H. Fröde, E. Klemm, S. Kaskel, Journal of Materials Chemistry
17 (2007) 3827–3832.
9] J. Lee, O. Farha, J. Roberts, K. Scheidt, S. Nguyen, J. Hupp, Chemical Society Reviews
38 (2009) 1450–1459.
Table 2
Hydrogenation of nitroarenes to amines over Pt@MIL-101 .
a
Entry
Substrate
Time
Conversion
(%)
Yield
(%)
Activity
(mol-amines/(mol-Pt·min))
[
10] S. Hasegawa, S. Horike, R. Matsuda, S. Furukawa, K. Mochizuki, Y. Kinoshita, S.
(
min)
Kitagawa, Journal of the American Chemical Society 129 (2007) 2607–2614.
1
75
100
92.5
100
99.1
92.5
98.3
118.9
13.9
[11] M. Fujita, Y. Kwon, S. Washizu, K. Ogura, Journal of the American Chemical Society
16 (1994) 1151–1152.
12] R. Zou, H. Sakurai, Q. Xu, Angewandte Chemie International Edition 45 (2006)
542–2546.
13] C. Wu, A. Hu, L. Zhang, W. Lin, Journal of the American Chemical Society 127
2005) 8940–8941.
14] H. Jiang, T. Akita, T. Ishida, M. Haruta, Q. Xu, Journal of the American Chemical
Society 133 (2011) 1304–1306.
15] G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé, I.
Margiolaki, Science 309 (2005) 2040–2042.
1
2b
3
600
75
[
[
[
[
[
[
2
118.0
(
4
5
60
100
100
98.3
100
147.4
18.8
480
16] A. Henschel, K. Gedrich, R. Kraehnert, S. Kaskel, Chemical Communications (2008)
4
192–4194.
17] Y. Li, R. Yang, AICHE Journal 54 (2008) 269–279.
M. Yadav, Q. Xu, Chemical Communications 49 (2013) 3327–3329.
19] X. Si, L. Sun, F. Xu, C. Jiao, F. Li, S. Liu, J. Zhang, L. Song, C. Jiang, S. Wang, Y. Liu, Y.
6
150
90
100
98.8
100
98.1
98.8
98.1
58.9
98.8
73.6
7b
[18]
[
[
[
8b
120
Sawada, International Journal of Hydrogen Energy 36 (2011) 6698–6704.
20] B. Yuan, Y. Pan, Y. Li, B. Yin, H. Jiang, Angewandte Chemie International Edition 49
(2010) 4054–4058.
21] E.V. Ramos-Fernandez, C. Pieters, B. Linden, J. Juan-Alcañiz, P. Serra-Crespo,
M.W.G.M. Verhoeven, H. Niemantsverdriet, J. Gascon, F. Kapteijn, Journal of Catalysis
9c
600
180
98.4
98.9
98.4
98.9
14.8
49.4
0c
1
289 (2012) 42–52.
[
22] A. Aijaz, A. Karkamkar, Y.J. Choi, N. Tsumori, E. Ronnebro, T. Autrey, H. Shioyama,
a
Reaction conditions: substrate (19.6 mmol), Pt@MIL-101 (42 mg), ethanol (30 mL),
Q. Xu, Journal of the American Chemical Society 134 (2012) 13926–13929.
b
c
H
2
(1.0 MPa), 20 °C, 30 °C, 40 °C, 1000 rpm.
[23] F.G. Cirujano, A. Leyva-Pérez, A. Corma, F. Xamena, ChemCatChem 5 (2013) 538–549.
[
[
24] Y. Pan, B. Yuan, Y. Li, D. He, Chemical Communications 46 (2010) 2280–2282.
25] H. Liu, Y. Li, R. Luque, H. Jiang, Advanced Synthesis and Catalysis 353 (2011)
3107–3113.
Appendix A. Supplementary data
[
26] J. Hermannsdörfer, M. Friedrich, N. Miyajima, R. Albuquerque, S. Kümmel, R.
Kempe, Angewandte Chemie International Edition 51 (2012) 11473–11477.
27] H. Liu, Y. Li, H. Jiang, C. Vargas, R. Luque, Chemical Communications 48 (2012)
8431–8433.
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.06.038.
[
[
[
28] R. Nie, J. Wang, L. Wang, Y. Qin, P. Chen, Z. Hou, Carbon 50 (2012) 586–596.
29] Z. Sun, Y. Zhao, Y. Xie, R. Tao, H. Zhang, C. Huang, Z. Liu, Green Chemistry 12
(
2010) 1007–1011.
References
[30] M. Tamura, K. Kon, A. Satsuma, K. Shimizu, ACS Catalysis 2 (2012) 1904–1909.
31] S. Kataoka, Y. Takeuchi, A. Harada, T. Takagi, Y. Takenaka, Applied Catalysis A:
[
[
[
1] S. James, Chemical Society Reviews 32 (2003) 276–288.
2] F. Schüth, K. Sing, J. Weitkamp, Handbook of Porous Solids, Wiley-VCH, Weinheim,
General 427–428 (2012) 119–124.
[32] S. Basu, M. Mapa, C. Gopinath, M. Doble, S. Bhaduri, G. Lahiri, Journal of Catalysis
239 (2006) 154–161.
2002.
[
[
[
[
[
3] J. Rowsell, O. Yaghi, Microporous and Mesoporous Materials 73 (2004) 3–14.
4] A. Corma, H. García, F. Xamena, Chemical Reviews 110 (2010) 4606–4655.
5] H. Jiang, Q. Xu, Chemical Communications 47 (2011) 3351–3370.
6] S. Li, Q. Xu, Energy & Environmental Science 6 (2013) 1656–1683.
7] A. Dhakshinamoorthy, H. García, Chemical Society Reviews 41 (2012) 5262–5284.
[33] G. Torres, E. Jablonski, G. Baronetti, A. Castro, S. de Miguel, O. Scelza, M. Blanco, M.
Jiménez, J. Fierro, Applied Catalysis A: General 161 (1997) 213–226.
[34] S. Jhung, J. Lee, J. Yoon, C. Serre, G. Férey, J. Chang, Advanced Materials 19 (2007)
121–124.
[35] G. Liu, Y. Li, J. Jonas, Journal of Chemical Physics 95 (1991) 6892.