Ru nanoparticles immobilized on metal-organic framework nanorods by supercritical CO2-methanol solution: Highly efficient catalyst

Article abstract of DOI:10.1039/c1gc15340d

A simple and efficient route for the immobilization of noble metal nanoparticles onto metal-organic framework (MOF) has been designed in this work. The Ru/MOF nanocomposites were prepared using RuCl₃·3H ₂O as a precursor in supercrit

Full text of DOI:10.1039/c1gc15340d

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PAPER
Ru nanoparticles immobilized on metal–organic framework nanorods by
supercritical CO2-methanol solution: highly efficient catalyst
Yueju Zhao, Jianling Zhang,* Jinliang Song, Jianshen Li, Jinli Liu, Tianbin Wu, Peng Zhang and
Buxing Han*
Received 29th March 2011, Accepted 25th May 2011
DOI: 10.1039/c1gc15340d
A simple and efficient route for the immobilization of noble metal nanoparticles onto
metal–organic framework (MOF) has been designed in this work. The Ru/MOF nanocomposites
were prepared using RuCl3·3H2O as a precursor in supercritical CO2-methanol solution at
200 ◦C. The as-synthesized Ru/MOF composite presented a high catalytic performance for the
hydrogenation of cyclohexene and benzene. There was almost no activity loss after the catalyst
was reused five times. The intermolecular interactions between the MOF and Ru nanoparticles
were investigated by X-ray photoelectron spectra and FT-IR spectra. It was shown that the
carboxylate groups of MOF were coordinated to Ru. The formation mechanism of the Ru/MOF
nanocomposites in supercritical CO2-methanol solution through an inorganic reaction route was
discussed. This method can be easily applied to the synthesis of other MOF-supported metal
nanoparticles.
and stable catalysts is of great importance and still remains
Introduction
challenging.
The metal–organic frameworks (MOFs), which are self-
Supercritical carbon dioxide (SC CO2) has been widely
assembled by the coordination of metal cations/clusters with
utilized in material science because of its nontoxic and non-
organic linkers,1 have potential applications in broad areas, such
flammable nature, low viscosity, high diffusivity, near zero
as catalysis,2 gas storage,3 sensors,4 nonlinear optics,5 and molec-
surface tension, and strong solvent power for some small
ular recognition and separation.6 Particularly, MOFs appear
molecules.11 Recently, a supercritical fluid chemical deposition
to be excellent candidates for catalysis by metal nanoparticles.
technique has been explored to immobilize metal nanoparticles
Because of the very high surface area of MOFs, high metallic
on carbon nanotubes.12 In the process, methanol or ethanol
dispersion can be expected on such materials, and the narrow
served as both the reductant for the precursors and a cosolvent
micropore distribution may lead to monodisperse nanometric
to enhance the dissolution of inorganic precursors in SC CO2.
metallic clusters, which is of high interest for catalytic activity
Dry products can be easily obtained through in situ extraction
and selectivity.2f However, the reports on immobilizing the metal
with SC CO2 after reactions.
nanoparticles onto MOFs as catalysts for heterogeneous cataly-
Herein we report for the first time the synthesis of a
sis are relatively limited. The few examples are the Au,7 Pd,8 Pt9
Ru/MOF catalyst by a SC CO2-methanol method. The small
and Ru10 nanoparticles supported on different MOF structures.
Ru nanoparticles (about 2 nm) were highly dispersed on MOF
Wetness impregnation is usually used for immobilizing metal
nanorod support. The as-synthesized Ru/MOF composite had
nanoparticles on MOFs.8a–8c Chemical vapor deposition (CVD)
a very high catalytic performance for the hydrogenation of
was also shown to be an appropriate method for synthesizing
cyclohexene and benzene. There was almost no activity loss after
MOF-supported metal nanoparticles.7b,8d,9,10 In addition, solid
the catalyst was reused five times.
grinding7a and coprecipitation8e methods were proposed. To
develop simple and efficient route for the immobilization of
noble metal nanoparticles on MOFs to obtain highly efficient
Experimental
Materials
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Colloid and Interface and Thermodynamics, Institute of
Chemistry, Chinese Academy of Sciences, Beijing, China.
E-mail: zhangjl@iccas.ac.cn;hanbx@iccas.ac.cn;
Fax: (+)86-10-62559373
The surfactant cetyltrimethylammonium bromide (CTAB) and
methanol were provided by Beijing Chemical Reagent Com-
pany (A. R. grade). n-Heptane supplied by Beijing Chemi-
cal Factory was A. R. grade. Lanthanum(III) nitrate hydrate
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(La(NO3)3·nH2O) (A. R. grade) was purchased from Sinopharm
Chemical Reagent Co., Ltd. 1,3,5-Benzenetricarboxylic acid
(H3BTC) (purity 95%) was purchased from Aldrich. Ruthe-
nium(III) Chloride Hydrate (RuCl3·3H2O) was purchased from
Shenyang Jinke Reagent Plant. Cyclohexene (AR) and benzene
(AR) were purchased from Beijing Chemical Reagent Company.
Double-distilled water was used. The commercial Ru/C catalyst
was purchased from Baoji Ruike Corporation, China.
experiment, a suitable amount of cyclohexene (or benzene)
and the required amount of catalyst were placed in the 20 mL
stainless steel autoclave with magnetic stirrer at 300 rpm, and
the air in the reactor was then replaced by H2 for five times. The
reactor was placed in an oil bath at 60 ◦C. The H2 pressure was
kept at the desired pressure. The pressure of the reaction system
was monitored using a pressure transducer (Foxboro/ICT
model 930). The product was collected by centrifugation (1200
rpm) and analyzed by a gas chromatograph (Agilent 6820)
equipped with a flame ionization detector (FID) and a PEG-
20M capillary column (0.25 mm in diameter, 30 m in length).
Synthesis of MOF
La(NO3)3·6H2O (0.2 mmol) and H3BTC (0.2 mmol) were added
into a cell containing 10 mL of the heptane/CTAB/H2O
emulsion ([CTAB] = 0.02 g mL-1, water : oil (v/v) = 1 : 1). The
emulsion was stirred at 30 ◦C for 24 h. After the reaction,
the precipitate was collected by centrifugation, washed for
several times with ethanol and water, and dried under vacuum
at room temperature.
Results and discussion
Synthesis of MOF
Emulsions are widely utilized to synthesize nanoparticles.13
The lanthanum-BTC (BTC = benzenetricarboxylate) MOF was
prepared in heptane/CTAB/H2O emulsion, where CTAB was
a normally used emulsifier to oil/water systems. From the SEM
image shown in Fig. 1a, MOF nanorods were synthesized on
a large scale with the length of 2–3 mm. Fig. 1b is a magnified
SEM image, from which the width and thickness of the nanorod
can be discerned as 200 nm 50 nm, respectively. The powder
X-ray diffraction (XRD) pattern shows that the sample was
crystalline (Fig. 1d), in spite of the moderate reaction conditions
for its formation. All diffraction peaks can be well indexed to a
known bulk phase of La(1,3,5-BTC)·6H2O.14 The experimental
contents of C, H, N and La were determined to be 24.56, 3.54,
ꢀ0.3 and 30.28% by element analysis, which are basically in
agreement with theoretical value of C (23.80%), H (3.31%)
and La (30.59%), confirming the molecular formula La(1,3,5-
BTC) ·6(H2O).
Synthesis of Ru/MOF composites in SC CO2-methanol solution
and aqueous solution
In a typical experiment, a suitable amount of MOF (0.15 g)
dispersed in 5 mL of RuCl3·3H2O methanol solution with a
concentration of 0.8 mg mL-1 was added into a high-pressure
reactor of 20 mL. The reactor was placed in a constant water
bath of 35 ◦C, followed by charging CO2 up to 5.0 MPa. It
was then moved to an oven of 200 ◦C and maintained at this
temperature for about 5 h. After being cooled to 35 ◦C, the liquid
chemicals in the vessel were extracted with CO2 at 35 ◦C and 9.0
MPa. Dry products were obtained after releasing CO2.
For comparison, the Ru/MOF composites were prepared in
aqueous solution.15 RuCl3·3H2O (1 mmol) was dissolved in water
(100 mL) with stirring for 24 h. MOF (0.1 g) was added to the
aqueous solution of RuCl3 (10 mM, 10 mL) in a 50 mL flask with
vigorous stirring. The mixture was stirred for 24 h. The solvent
was then removed from the solution by evaporation and the
Ru/MOF catalyst was obtained after hydrogenation at 220 ◦C
for 2 h.
Characterization
The morphology of the sample obtained was characterized
by scanning electronic microscope (SEM) (HITACHI S-4800)
and transmission electronic microscope (TEM) (JeoL-1010 and
JeoL-2010). Powder XRD analysis of the sample was performed
on the X-ray diffractometer (Model D/MAX2500, Rigaka) with
Cu Ka radiation. FT-IR spectra were obtained using a Bruker
Tensor 27 spectrometer, and the samples were prepared by
the KBr pellet method. The X-ray photoelectron spectroscopy
(XPS) measurement was performed on the VG Scientific
ESCALab220i-XL spectrometer using Al Ka radiation. The
loading content of Ru in the catalysts was determined by ICP-
AES (VISTA-MPX).
Fig. 1 SEM images (a, b), TEM image (c) and powder X-ray diffraction
pattern (d) of the MOF synthesized in the heptane/CTAB/H2O
emulsion system ([CTAB] = 0.02 g mL-1, water : heptane (v/v) = 1 : 1
at room temperature for 24 h.
Catalytic activity test
To evaluate the catalytic characteristics of the as-prepared
Ru/MOF, hydrogenation of cyclohexene and benzene was
investigated. The apparatus and processes of the hydrogenation
reaction were similar to a previous report.19 In the typical
Ru/MOF composite synthesized in SC CO2-methanol solution
The Ru/MOF nanocomposites were prepared using
RuCl3·3H2O as a precursor in SC CO2-methanol solution
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at 200 ◦C. The morphology of the resultant Ru/MOF
composites was examined by TEM. As shown in Fig. 2a, the
MOF nanorod was decorated with nanoparticles with a mean
diameter of about 2 nm, and almost all nanoparticles deposited
on the MOFs even after thorough washing and prolonged
sonication. The HRTEM inspection (Fig. 2b) demonstrates
the crystalline quality of Ru nanoparticles with distinct lattice
fringes (lattice spacing is 0.23 nm). The energy-dispersive X-ray
spectroscopy (EDS) analysis (Fig. 2c) confirms the existence of
Ru element. The XRD pattern of Ru/MOF shown in Fig. 2d is
identical to that of pure MOF (Fig. 1d), indicating the structure
of the MOF was not changed after immobilizing Ru on it.
There is no diffraction peak for Ru nanoparticles in the XRD
pattern in the wide-angle range, due to the fact that the Ru
particles are very small. The loading content of the Ru on the
MOF was 0.98 wt%, determined by ICP-AES method.
Table 1 The hydrogenations catalyzed by Ru/MOF nanocomposite
Entry Substrate
PH /MPa
Time/h
Yielda (%)
TOFb
2
1
Cyclohexene
2
2
2
2
2
2
6
6
6
6
6
0.5
0.5
0.5
0.5
0.5
1
1
1.5
2
>99
>99
>99
>99
>99
67
82
96
>99
93
18
>9900
>9900
>9900
>9900
>9900
3350
4100
3200
2475
664
2c
3c
4c
5c
6
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
7
8
9
10d
11e
7
2
450
Reaction conditions: Substrate/Ru (mol/mol) = 5000, reaction tem-
perature: 60 ◦C. Analysis: GC (Agilent 6820) equipped with a flame
ionization detector (FID) and a PEG-20M capillary column (0.25 mm
in diameter, 30 m in length).a Yield of cyclohexane by the hydrogenation
of cyclohexene and benzene. b Turnover number (TON) = mol of product
(cyclohexane)/mol of Ru, turnover frequency (TOF) = TON·h-1.
c Ru/MOF was reused for the further four runs. d The commercial Ru/C
catalyst. e The Ru/MOF catalyst synthesized in aqueous solution.
it is clear that the as-synthesized Ru/MOF catalyst has a much
higher activity than the commercial Ru/C.
The stability of the Ru/MOF catalyst was investigated, taking
the cyclohexene hydrogenation as an example. After the catalyst
was reused five times for this reaction, there was almost no
activity loss (Table 1, entries 1–5), indicating that the as-
prepared catalyst was rather stable. The morphologies of the
Ru/MOF composite after reused for five times were observed
by TEM and the results are shown in Fig. 3. Clearly, the
Ru nanoparticles were still firmly decorated on the surface of
the MOF and no remarkable aggregation of Ru nanoparticles
occurred. Therefore, there was almost no activity loss after the
catalyst was reused for five times for this reaction.
Fig. 2 TEM image (a), HRTEM image (b), EDS analysis (c) and XRD
pattern of the Ru/MOF nanocomposite.
Catalytic activity test
The as-prepared Ru/MOF nanocomposite was used to catalyze
the hydrogenation of cyclohexene and benzene at different con-
ditions, and the results are listed in Table 1. Cyclohexene could
almost completely convert into cyclohexane within 0.5 h at 60 ◦C
and H2 pressure of 2 MPa with a substrate/Ru molar ratio of
5000 (Table 1, entry 1). The turnover frequencies (TOFs), defined
as moles of substrate consumed per mole of ruthenium per hour,
could reach about 10 000 h-1. The hydrogenation of benzene
catalyzed by the Ru/MOF nanocomposite was also investigated
(Table 1, entries 6–9). The benzene could be transformed into
cyclohexane completely at 60 ◦C and H2 pressure of 6 MPa with
a reaction time of 2 h at the substrate/Ru molar ratio of 5000.
For comparison, we performed the benzene hydrogenation with
the commercial Ru/C catalyst under the same conditions and
the result is given in Table 1 (entry 10). Comparing with entry 9,
Fig.
hydrogenation.
3
TEM image of Ru/MOF composite after five runs of
Ru/MOF composite synthesized in aqueous solution
For comparison, Ru nanoparticles were immobilized onto the
MOF in aqueous solution.15 Its catalytic property for the
hydrogenation of benzene was studied, and the results are given
in Table 1 (entry 11). It is evident that the Ru/MOF catalyst
prepared in aqueous solution had a much lower activity than
that prepared in SC CO2-methanol solution. Fig. 4 shows the
TEM images of the Ru/MOF composite synthesized in aqueous
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280.3 (Ru 3d5/2) and 284.5 eV (Ru 3d3/2), which confirms the
presence of Ru0 in catalyst.15,16 The peak at 281.1 eV suggests
the presence of a Ru–O component, probably resulting from
the bond between carboxylate group of BTC and Ru. This was
further confirmed by the FTIR spectra. As shown in Fig. 5B,
the characteristic bands appearing in the regions of 1613–1558
and 1436–1373 cm-1, correspond to the antisymmetric and
symmetric stretching vibration of the –COO- groups of the
ionized ligand, respectively. By comparing the FT-IR spectra
of the MOF and the Ru/MOF, it is clear that the wavenumber
difference of the bands for the Ru/MOF is smaller than that of
the pure MOF. This indicates that carboxylate groups of BTC
are coordinated to Ru,17 which may be induced by their electron-
donor ability.18 Therefore, the high stability of the Ru/MOF
composites as catalyst can be well understood.
Fig. 4 TEM images of Ru/MOF synthesized in aqueous solution.
solution. The Ru particles aggregated into large particles in
the composite. This control experiment confirms that SC CO2-
methanol solution excels the conventional liquid solvents for
preparing Ru/MOF composites.
Possible mechanism
The formation mechanism of the Ru/MOF nanocomposites in
SC CO2-methanol solution through an inorganic reaction route
is shown in Scheme 1. The reactions can be expressed by the
equation:12
Intermolecular interactions of Ru/MOF synthesized in SC
CO2-methanol solution
The intermolecular interactions between the MOF and Ru
nanoparticles were investigated by X-ray photoelectron spec-
trum (XPS) and FT-IR spectra. Fig. 5A shows the Ru 3d
spectrum of the composite. It can be seen that although the Ru
3d signal is obscured by the C 1s signal of carbon contaminant
at 284.6 eV, the deconvoluted spectrum shows a doublet for two
chemically different Ru entities with peak binding energies of
2RuCl3+3CH3OH→2Ru+3HCHO+6HCl
Scheme 1 Schematic illustration for the formation of Ru/MOF in SC
CO2-methanol solution.
The inorganic precursor RuCl3 was not soluble in SC CO2, but
was soluble in SC CO2-methanol solution. Therefore, methanol
served as the reductant as well as cosolvent for enhancing
the solubility of RuCl3 in the solution. Owing to the unique
properties of SC CO2-methanol fluid – low viscosity, rapid
diffusivity and absence of surface tension – the transfer of
reactant into small spaces was allowed and consequently the
complete wetting of the MOF with RuCl3 was facilitated
(Scheme 1a). As the temperature was raised to 200 ◦C, the reduc-
tion reaction proceeded. The Ru nanoparticles produced from
the decomposition of RuCl3 preferably deposited on the surface
of the MOF, which was capable of stabilizing Ru nanoparticles
through the interactions with its carboxylate groups, as well as
preventing the particle aggregation. Therefore, MOF-supported
monodisperse metal nanoparticles were obtained (Scheme 1b).
Due to the small size of Ru nanoparticles (about 2 nm) and
their high dispersion on MOF support, this catalyst presents a
much higher activity than the commercial Ru/C composite and
Ru/MOF composite prepared in aqueous solution.
Fig. 5 A: XPS spectra at the Ru 3d edge of the Ru/MOF composites.
The three vertical lines indicate the peak positions of binding energies
of C 1 s, Ru–O, and Ru, respectively. B: FT-IR spectra of the MOF (a)
and the Ru/MOF (b).
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Conclusions
In conclusion, monodisperse Ru nanoparticles of small size
were immobilized onto the MOF nanorods in SC CO2-methanol
solution. The Ru/MOF composite were highly active and stable
for the catalytic hydrogenation of cyclohexene and benzene.
There was almost no activity loss after the catalyst was reused
for five times. The intermolecular interaction between the
carboxylate groups of BTC and Ru was responsible for the
high stability of the Ru/MOF composites. We believe that this
method can also be used to support other transition metals, such
as Rh, Pd, and Ir, on MOFs and highly efficient nano-catalysts
can be prepared.
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Acknowledgements
The authors thank the National Natural Science Foundation of
China (20873164, 20973177), Ministry of Science and Technol-
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