Ruthenium supported on MIL-101 as an efficient catalyst for hydrogen generation from hydrolysis of amine boranes

Article abstract of DOI:10.1039/c4nj00739e

Ultrafine Ru nanoparticles were successfully immobilized inside the pores of MIL-101 by using a simple liquid impregnation method. The turnover frequency (TOF) value of the as-synthesized 2.80 wt% Ru@MIL-101 toward catalytic hydrolysis of ammonia borane is 178 mol H₂ min^-1 (mol Ru)^-1, which is among the highest values ever reported. the Partner Organisations 2014.

Full text of DOI:10.1039/c4nj00739e

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Ruthenium supported on MIL-101 as an efficient
catalyst for hydrogen generation from hydrolysis
of amine boranes†
Cite this: New J. Chem., 2014,
38, 4032
Received (in Montpellier, France)
7th May 2014,
Nan Cao,a Teng Liu,a Jun Su,b Xiaojun Wu,a Wei Luo*ac and Gongzhen Chenga
Accepted 24th June 2014
DOI: 10.1039/c4nj00739e
www.rsc.org/njc
Ultrafine Ru nanoparticles were successfully immobilized inside the
On the other hand, hydrogen is a globally accepted clean
pores of MIL-101 by using a simple liquid impregnation method. fuel due to its high efficiency and power density. Over the past
The turnover frequency (TOF) value of the as-synthesized 2.80 wt% decades, numerous condensed-phased hydrogen storage approaches
Ru@MIL-101 toward catalytic hydrolysis of ammonia borane is have been explored, including metal hydrides,10 sorbent materials,11
178 mol H2 minꢀ1 (mol Ru)ꢀ1, which is among the highest values and chemical hydride systems.12 Boron–nitrogen containing
ever reported.
compounds have attracted much attention for use as hydrogen
storage materials because of their high gravimetric hydrogen
The synthesis of metal nanoparticles (NPs) with controllable size densities and favorable kinetics of hydrogen release.13 Among
and distribution has attracted growing attention owing to their them, ammonia borane (NH3–BH3, AB) has recently received
unique properties and potential applications in biological and great interest because of its 19.6 wt% hydrogen content, highly
chemical sensing, optoelectronics, magnetic memory, and hetero- stability, and environmentally benign nature.14 As the derivative
geneous catalysis.1 It is well-known that the catalytic performance of AB, methylamine borane (CH3NH2–BH3, MeAB) has also been
of metal NPs is highly dependent on the dispersion of active metal studied due to its high gravimetric hydrogen content (11.1 wt%).
sites. Therefore, to avoid the aggregation, many techniques have The release of hydrogen from AB and MeAB could be obtained
been designed in their synthesis route.2 Among them, stabilizing via solid phase thermolysis,15 catalytic dehydrogenation in non-
metal NPs in the matrices of solid materials is one of the most aqueous solvents,16 and hydrolysis.17 With appropriate catalysts,
common and efficient way to prevent the aggregation.
hydrolysis of AB or MeAB could release as much as 3 mol of
Until now, a number of porous materials such as zeolite, hydrogen per mol of AB or MeAB, which appears to be the most
metal oxide, polymers, carbon nanotubes, and graphene have convenient one for portable hydrogen storage applications according
been designed for restraining the aggregation of metal NPs. to eqn (1) and (2).18 So far, many catalyst systems have been tested
Metal–organic frameworks (MOFs) have been emerging as a class for hydrogen generation from the hydrolysis of AB and MeAB,
of promising porous functional materials owing to their high however, the optimal compromise between costs, efficiency and
porosity, large surface area, chemical tunability and thus specific recyclability still remains a considerable challenge.
applications, such as gas sorption and storage,3 molecular recog-
catalyst
ƒƒƒ!
nition and separation,4 drug delivery,5 luminescence,6 magnetic
properties,7 and catalytic properties.8 Given the similarity to
zeolites, loading metal NPs into the pores of MOFs is expected
to control the limited growth of metal NPs in the confined
cavities, and produce monodispersed NPs, which could further
increase their catalytic activities.9
NH ꢀBH ðaqÞ þ 2H OðIÞ
NH BO ðaqÞ þ 3H ðgÞ (1)
3
3
2
4
2
2
catalyst
MeNH ꢀBH ðaqÞ þ 2H OðIÞ
ðMeNH ÞBO ðaqÞ þ 3H ðgÞ
ƒƒƒ!
2
3
2
3
2
2
(2)
Herein, we report the facile synthesis of MIL-101 supported Ru NPs
by a simple liquid impregnation method. MIL-101, a chromium-
based MOF, Cr3F(H2O)2O[(O2C)C6H4(CO2)]3ꢁnH2O (n E 25), with
two cavities of ca. 2.9 and 3.4 nm free diameters accessible through
two pore windows of ca. 1.2 and 1.6 nm, was chosen because of its
extra high specific surface area, high thermal stability (up to 300 1C)
and high stability in water.19 The as-synthesized Ru@MIL-101
NPs exhibit excellent catalytic activity and durability toward
a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan,
Hubei 430072, P. R. China. E-mail: wluo@whu.edu.cn; Tel: +86 2768752366
b Wuhan National Laboratory for Optoelectronics, Huazhong University of Science
and Technology, Wuhan, Hubei, 430074, P. R. China
c Suzhou Institute of Wuhan University, Suzhou, Jiangsu, 215123, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4nj00739e
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Fig. 1 Powder X-ray diffraction patterns of samples.
Fig. 2 N2 sorption isotherms of activated MIL-101, 1.62 wt% Ru@MIL-101,
2.31 wt% Ru@MIL-101 and 2.80 wt% Ru@MIL-101. Filled and open symbols
represent adsorption and desorption branches, respectively.
hydrogen generation from hydrolysis of ammonia borane and
methylamine borane.
MIL-101 was synthesized according to the literature,19 followed
by treatment with EtOH and aqueous NH4F solution to remove
the unreacted terephthalic acid. The supported Ru@MIL-101
catalyst was prepared through solution infiltration of activated
MIL-101 with RuCl3, followed by treatment with NaBH4. The low-
angle powder X-ray diffractions (PXRD) of the as-synthesized
MIL-101, activated MIL-101, Ru@MIL-101 with different Ru
loadings exhibit no loss of crystallinity (Fig. 1), indicating that
the integrity of the MIL-101 framework was maintained well
during the catalyst preparation and catalytic process. Further-
more, no significant diffraction peaks of Ru were detected from
the wide-angle PXRD (Fig. S1, ESI†), which might be caused by
the immobilization of Ru NPs into the pores of MIL-101 and the
ultrafine Ru NPs (vide infra). The N2 adsorption–desorption
isotherms of MIL-101 and Ru@MIL-101 are shown in Fig. 2.
The specific areas of MIL-101, 1.62 wt% Ru@MIL-101, 2.31 wt%
Ru@MIL-101 and 2.80 wt% Ru@MIL-101 were 2233, 2089. 2034
and 1942 m2 gꢀ1 respectively. The decrease in the amount of N2
adsorption and the pore volume (Fig. 2 and Fig. S2, ESI†) of
Ru@MIL-101 indicates that the pores of MIL-101 were either
occupied by the well dispersed Ru NPs or blocked by the Ru NPs.
The morphologies of MIL-101 immobilized Ru NPs were further
characterized by transmission electron microscopic (TEM) and
energy-dispersive X-ray spectroscopic (EDX) measurements. TEM
images of 1.62 wt% and 2.31 wt% Ru@MIL-101 (Fig. 3a and b;
Fig. 3 (a and b) TEM images of 1.62 wt% Ru@MIL-101. (c) EDX of 1.62 wt%
Ru@MIL-101. (d) TEM images of 1.62 wt% Ru@MIL-101 after five runs of
catalytic hydrolysis of AB.
Fig. S3a and b, ESI†) indicate that the Ru NPs are well dispersed, 1.62 wt%, 2.31 wt%, 2.80 wt% determined by ICP-AES), the
and encapsulated in the cages of the MIL-101. The EDX spectra 2.80 wt% Ru@MIL-101 has the highest activity, with a TOF
(Fig. 3c) confirm the presence of Ru. The mean diameter of Ru NPs value of 178 mol H2 minꢀ1 (mol Ru)ꢀ1. However, considering
in Ru@MIL-101 was 1.6 nm (Fig. 3a, inset), which is small enough the reaction time, the 1.62 wt% Ru@MIL-101 is more suitable
for them to be accommodated in the two mesoporous cavities of for studying the activation energy and stability, with a TOF
MIL-101 (2.9 and 3.4 nm), and big enough to be limited in the value of 144 mol H2 minꢀ1 (mol Ru)ꢀ1
pores of the framework by the windows of MIL-101 (1.2 and
Moreover, as can be seen from Table S1 (ESI†), the 1.62 wt%
.
1.6 nm). These results indicate that the ultra small Ru NPs have Ru@MIL-101 catalyst exhibits a remarkable TOF value for
been effectively immobilized by MIL-101, which results in the high catalytic hydrolysis of AB among the Ru-based nano-catalysts
catalytic activity and durability for hydrolysis of AB (vide infra).
ever reported. Furthermore, as the control experiment, the
Fig. 4 shows the H2 generation from aqueous AB under ambient same amount of Ru NPs reduced by NaBH4 without MIL-101,
conditions in the presence of Ru@MIL-101. Among the Ru@MIL- MIL-101 without Ru loading and the physical mixture of Ru NPs
101 catalysts with different Ru loadings (0.45 wt%, 0.65 wt%, and MIL-101 were synthesized and applied to hydrolysis of AB,
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Fig. 5 Durability test of 1.62 wt% Ru@MIL-101 for decomposition of (a) AB
and (b) MeAB at 25 1C (catalyst = 0.0500 g; AB or MeAB = 1 mmol).
Additional aliquots of AB or MeAB were subsequently introduced into the
reaction vessel after the completion of the last run.
exhibit high catalytic activity and durability for catalytic hydrolysis
of ammonia borane and methylamine borane under ambient
conditions. Compared with Ru nanoparticles without a support,
MIL-101, and the physical mixture of Ru with MIL-101, the Ru
nanoparticles supported on MIL-101 exhibit remarkable increased
catalytic activity toward the catalytic hydrolysis of ammonia
borane, indicating the synergetic effect of Ru nanoparticles
and the framework of MIL-101. Furthermore, this simple liquid
impregnation synthetic method can be extended to other MOF
supported metal nanoparticles for more applications.
Fig. 4 Hydrogen generation from aqueous AB in the presence of
Ru@MIL-101 catalysts at room temperature. Ru/AB (molar ratio)
=
0.0022, 0.0032, 0.0080, 0.011 and 0.014 at Ru loadings of 0.45, 0.65,
1.62, 2.31 and 2.80 wt%.
as shown in Fig. S4 (ESI†). The release of H2 was completed for
more than 7 min for Ru NPs and the physical mixture of Ru and
MIL-101. There was almost no reactivity for MIL-101 toward
hydrolysis of AB. In addition, as shown in the TEM images of
the physical mixture of Ru and MIL-101 (Fig. S3c and d, ESI†),
the Ru NPs have already been aggregated, indicating that there
was no interaction between MIL-101 and Ru NPs, which is in
agreement with their catalytic activity results. These results
confirm the cooperative effect between MIL-101 and Ru NPs
after loading Ru into the pores of MIL-101.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (21201134), the Natural Science Foundation of
Jiangsu Province (BK20130370), the Natural Science Foundation of
Hubei Province (2013CFB288), Ministry of Science and Technology
of China (2011YQ12003504) and Large-scale Instrument and Equip-
ment Sharing Foundation of Wuhan University.
In order to get the activation energy (Ea) of the AB and MeAB
hydrolysis catalyzed by 1.62 wt% Ru@MIL-101 NPs, the hydrolytic
reaction at a different temperature range of 25–40 1C was carried
out. The values of rate constant k at different temperatures were
calculated from the slope of the linear part of each plot from Fig. S5a
and S6a (ESI†). The Arrhenius plot of ln k vs. 1/T for the catalyst is
plotted in Fig. S5 and S6 (ESI†), from which the apparent activation
energy was determined to be approximately 51.12 kJ molꢀ1 and
65.92 kJ molꢀ1 for AB and MeAB, respectively. Moreover, the
as-synthesized Ru@MIL-101 catalysts exhibit the highest catalytic
activity toward catalytic hydrolysis of MeAB ever reported, with a
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