Synthesis of supported ultrafine non-noble subnanometer-scale metal particles derived from metal-organic frameworks as highly efficient heterogeneous catalysts

Article abstract of DOI:10.1002/anie.201508107

The properties of supported non-noble metal particles with a size of less than 1 nm are unknown because their synthesis is a challenge. A strategy has now been created to immobilize ultrafine non-noble metal particles on supports using metal-organic frame

Full text of DOI:10.1002/anie.201508107

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International Edition: DOI: 10.1002/anie.201508107
German Edition: DOI: 10.1002/ange.201508107
Hydrogenation Catalysis
Synthesis of Supported Ultrafine Non-noble Subnanometer-Scale
Metal Particles Derived from Metal–Organic Frameworks as Highly
Efficient Heterogeneous Catalysts
Xinchen Kang, Huizhen Liu, Minqiang Hou, Xiaofu Sun, Hongling Han, Tao Jiang,
Zhaofu Zhang, and Buxing Han*
Abstract: The properties of supported non-noble metal
particles with a size of less than 1 nm are unknown because
their synthesis is a challenge. A strategy has now been created
to immobilize ultrafine non-noble metal particles on supports
using metal–organic frameworks (MOFs) as metal precursors.
Ni/SiO2 and Co/SiO2 catalysts were synthesized with an
average metal particle size of 0.9 nm. The metal nanoparticles
were immobilized uniformly on the support with a metal
loading of about 20 wt%. Interestingly, the ultrafine non-noble
metal particles exhibited very high activity for liquid-phase
hydrogenation of benzene to cyclohexane even at 808C, while
Ni/SiO2 with larger Ni particles fabricated by a conventional
method was not active under the same conditions.
desirable. The supported non-noble metal catalysts such as Ni
and Co with a particle size less of than 1 nm may be efficient
for the reaction at lower temperatures, which is unknown
because synthesis of ultrafine nanocatalysts is a great chal-
lenge.
Metal–organic frameworks (MOFs) have attracted sig-
nificant attention.[9] They are potential materials for gas
storage, separation, catalysis, biomedicine, chromatography,
and sensing.[10] MOFs can also be used as precursors to
synthesize other materials, such as carbon materials and
oxides,[11] as well as composites.[12] However, using MOFs as
precursors to synthesize supported metal catalysts has not
been reported. Herein, we proposed a new strategy to use
MOFs as precursors to synthesize ultrafine non-noble metal
particles immobilized on SiO2 supports. Ni/SiO2 and Co/SiO2
catalysts were synthesized using this method with Ni or Co
average particle size of less than 1 nm, which exhibited very
high activity and stability for the liquid-phase hydrogenation
of benzene below 1008C.
The proposed method to synthesize ultrafine metal
particles on the SiO2 is shown in Figure 1, which is discussed
briefly taking the synthesis of Ni/SiO2 as example, and the
details are given in the experimental section (see the
Supporting Information). In the experiment, the mixed
solvent consisting of ionic liquid (IL) 1-octyl-3-methylimida-
zolium perchlorate (OmimClO4, the structure is shown in the
S
upported nanocatalysts have been widely studied based on
their high activity for different chemical reactions.[1] The size
of metal particles is one of the most important factors that
dictate the catalytic performance.[2] Various routes for syn-
thesizing supported nanocatalysts with metal particle size of
less than 1 nm to 2 nm have been explored.[3] However, most
of the reported ultrafine metal particles were noble metal
particles,[4] and immobilization of ultrafine non-noble metal
particles on supports is challenging.
Catalytic hydrogenation of benzene to cyclohexane is an
important reaction in the chemistry industry. The reaction
usually requires harsh conditions because of the high
stabilization energy of benzene resulting from aromaticity.
The catalytic activity of heterogeneous metal catalysts for the
hydrogenation of benzene decreases in the order of Rh >
Ru > Pt > Ni.[5] Supported Rh, Ru, and Pt metal catalysts
can catalyze liquid-phase benzene hydrogenation with high
activity.[6] However, the high price and low reserve of noble
metals limit their application. Benzene hydrogenation is
usually conducted at above 1508C over supported non-noble
Ni catalysts under gas-phase conditions and at lower turnover
frequency.[7] In many cases, liquid-phase hydrogenation has
economical advantages in terms of capital costs and energy
consumption.[8] Benzene hydrogenation in the liquid phase
using non-noble catalysts at lower temperature is highly
[*] X. C. Kang, Prof. H. Z. Liu, Dr. M. Q. Hou, X. F. Sun, H. L. Han,
Prof. T. Jiang, Dr. Z. F. Zhang, Prof. B. X. Han
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry, Chinese Academy of Sciences (China)
E-mail: hanbx@iccas.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508107.
Figure 1. The route for the synthesis of the Ni/SiO2 catalysts.
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Supporting Information, Figure S1), triethylammonium ni-
trate, and water were prepared. Then tetraethoxysilane
(TEOS, precursor of SiO2), 1,3,5-benzenetricarboxylic acid
(H3BTC) and Ni(NO3)2 (precursors of the Ni-MOF) were
charged into the mixed solvent and a homogeneous solution
was formed because the mixed solvent can dissolve both
organic and inorganic precursors (Figure 1a). Ni-MOF and
SiO2 formed gradually by the coordination reaction and
hydrolysis reaction, respectively. Therefore, the Ni-MOF
crystals were embedded in the SiO2 framework together
with some solvent (Figure 1b). Ni-MOF/SiO2 composites
were obtained after washing and drying to remove the solvent
(Figure 1c). Then, the obtained Ni-MOF/SiO2 composites
were dispersed in 50 wt% DMF aqueous solution and the
Ni2+ ions in the Ni-MOF were reduced by NaBH4 under a N2
atmosphere, and the metallic Ni clusters were immobilized on
the SiO2. Meanwhile the BTC3À ligands were dissolved in the
DMF solution. The Ni/SiO2 catalysts were obtained after
washing and drying (Figure 1d). In this novel route, the
reduction of Ni2+ occurs at Ni-MOF/liquid interphase, and
therefore the rate was slow. Furthermore, the Ni-MOF
particles were trapped in the SiO2 framework, and the
reduction took place close to the surface of SiO2. Moreover,
the porous structure of SiO2 offered sufficient sites for Ni
nanoparticles to attach. These unique features are favorable
to immobilizing ultrafine Ni particles on the supports.
The X-ray diffraction (XRD) pattern of the Ni-MOF/SiO2
composites is shown in the Supporting Information, Fig-
ure S2. All of the peaks in the XRD pattern of the composites
agree well with the reported bulk phase of Ni3(BTC)2·12H2O
(Ni-MOF).[13] The composites had typical micropores of Ni-
MOFs entered at about 0.6 nm as determined by N2
adsorption/desorption method (Supporting Information, Fig-
ure S3). All these indicated that Ni-MOF existed in the Ni-
MOF/SiO2 composites. The N2 adsorption/desorption iso-
therms and mesopore size distribution of the Ni-MOF/SiO2
are shown in the Supporting Information, Figure S4, and the
BET surface area, total pore volume, and average pore size
are listed in Table S1. The mesopore size distribution curve of
the Ni-MOF/SiO2 had a peak centered at about 5 nm, which
resulted from the removal of the entrained solvent from
steps b to c in Figure 1.
The scanning electron microscope (SEM) image of the as-
synthesized Ni/SiO2 is shown in Figure 2a. The size of the Ni/
SiO2 was in the range of 5–10 mm. The transmission electron
microscopy (TEM) image and high-angle annular bright-field
(HAABF) image from scanning transmission electron mi-
croscopy (STEM) of the Ni/SiO2 are shown in Figures 2b and
2c, respectively. The Ni nanoparticles were immobilized
uniformly on the SiO2 supports with an average size of 0.9 nm,
and the Ni loading determined by ICP-AES (VISTA-MPX)
was 21 wt%. The particle size distribution is demonstrated in
Figure 2d, which shows that most of Ni particles have the size
of less than 1 nm. The size of the Ni nanoparticles calculated
by Scherrer equation from XRD pattern shown in Figure 2e is
about 1 nm, which corresponds well with the TEM result. The
X-ray photoelectron spectrum (XPS) for Ni 2p is shown in
Figure 2 f. The characteristic Ni 2p peaks are associated with
Ni(OH)2 and Ni, respectively.[14] On the surface of the
Figure 2. Related characterizations of the Ni/SiO2: a) SEM image,
b) TEM image, c) HAABF-STEM image, d) particle size distribution,
e) XRD pattern, f) XPS spectrum of Ni 2p, g) N2 adsorption/desorption
isotherms and (inset) mesopore size distribution, and h) reusability
for benzene hydrogenation to cyclohexane under the condition given
as entry 1 in Table 1. The Ni loading was 21 wt% as determined by
ICP-AES.
catalyst, some Ni atoms existed as Ni0, and most of them
were oxidized. The fractal structure of the Ni/SiO2 was
analyzed by small-angle X-ray scattering (SAXS). It was
found that mass fractal existed in the Ni/SiO2 and the fractal
dimension Dm was 1.97 (Supporting Information, Figure S5),
indicating the Ni/SiO2 had very loose structure,[15] which
agrees with the porous and loose structure observed from the
TEM image (Figure 2b). Figure 2g shows N2 adsorption/
desorption isotherms and mesopore size distribution of Ni/
SiO2; the BET surface area, total pore volume, and average
pore size are shown in the Supporting Information, Table S1.
The mesopore size distribution curve has a peak centered at
about 8 nm, which was larger than that of Ni-MOF/SiO2
composites. The enlarged pore size derives from the removed
Ni-MOF particles, further confirming that the Ni-MOF
particles in the Ni-MOF/SiO2 composites were trapped in
the SiO2 framework.
We also prepared Co/SiO2 using the same method.
Similarly, the Co/SiO2 was synthesized via Co-MOF/SiO2 as
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precursor. The XRD pattern (Supporting Information, Fig-
ure S6) agrees well with that reported bulk phase of Co3-
(BTC)2·12H2O (Co-MOF),[16] and the micropores centered at
0.6 nm further confirmed the existence of MOF particles in
the composites as well (Supporting Information, Figure S7).
The BET data of Co-MOF/SiO2 is shown in Figure S8 and
Table S1. The related characterizations such as TEM,
HAABF-STEM, particle size distribution, XPS, XRD pat-
tern, and N2 adsorption/desorption isotherms of the as-
synthesized Co/SiO2 are shown in Figure 3 and the Supporting
catalysts with different Ni loadings of 5 wt%, 10 wt%, and
21 wt% prepared by conventional impregnation method are
shown in the Supporting Information, Figure S9. The average
particle size of different catalysts (Table 1) increased with the
increase of the Ni loading. The catalysts with large Ni particle
size exhibited very low catalytic activity for benzene hydro-
genation at 1008C and had no activity at 808C (Table 1,
entries 7 and 8). Actually, benzene hydrogenation using Ni
catalysts is usually carried out at the temperature higher than
1508C under gas-phase conditions,[7] which further confirms
that the Ni and Co catalysts with particle size less than 1 nm
had extremely high activity. The surface active metal atoms of
all the catalysts were determined by chemisorption method as
previously reported,[19] and the results are listed in Table 1.
The number of surface active metal atoms increased with
decreasing size of the nanoparticles. The main reason is that
smaller particles had more oxidized metal atoms as shown by
the XPS spectra (Figure 2 f; Supporting Information, Fig-
ure S10), which are not active compared with the larger
particles. The turnover frequency (TOF) at low benzene
conversion (20%) based on the surface active metal atoms
was calculated.[19] It can be seen from Table 1 that the TOF of
the Ni/SiO2 catalysts with particle size of less than 1 nm was
much larger than that of the Ni/SiO2 catalysts with larger
particle size. Furthermore, the TOF based on active metal of
Ni nanoparticles is larger than that of Co nanoparticles with
similar size, illustrating that Ni is a more active hydrogenation
catalyst than Co. Moreover, the study of reusability of the Ni/
SiO2 catalysts with a particle size of less than 1 nm indicated
that the catalysts can be used at least five times without
obvious change of the activity (Figure 2h), and the TEM
image and XPS spectrum of the Ni/SiO2 catalysts after used
five times are shown in Figure S11. It can be seen that the size
of the Ni nanoparticles were nearly the same as that in the
virgin catalyst. Some Ni atoms existed in Ni0 state and most of
them were oxidized.
There are at least two factors that are favorable to having
high activity of the catalysts. First, the ultrafine metal particles
offer sufficient active centers and have higher surface energy
based on the low-coordination and unsaturated atoms,[20]
which may play an important role for the high activity at
lower temperature. Second, the SiO2 supports were porous
which benefits for reactants adsorption and mass transfer.
Exploration of the detailed mechanism for the phenomenon
that ultrafine Ni and Co particles are very active for the
reaction at lower temperature is very interesting, and needs to
be studied further.
In summary, we proposed a novel strategy for synthesizing
non-noble metal catalysts supported on porous SiO2 using
MOFs as metal precursors, and Ni/SiO2 and Co/SiO2 catalysts
were prepared by this method. In the catalysts, the metal
nanoparticles with average size of less than 1 nm were
immobilized uniformly on the porous supports with metal
loading of about 20 wt%. The catalysts prepared exhibited
excellent activity and stability for benzene hydrogenation to
cyclohexane in liquid phase below 1008C. In contrast, the Ni/
SiO2 with larger Ni particles fabricated by conventional
method was not active at the same condition. We believe that
the cheap and highly active catalysts have great potential of
Figure 3. Related characterizations of Co/SiO2: a) TEM image,
b) HAABF-STEM image, c) particle size distribution, d) XPS spectrum
of Co 2p, e) XRD pattern, and f) N2 adsorption/desorption isotherms
and mesopore size distribution (inset).
Information, Table S1. The characteristics of the Co/SiO2
were similar to those of Ni/SiO2. In particular, the Co
nanoparticles with average particle size of < 1 nm were
immobilized uniformly on the SiO2 supports with the Co
loading of 19 wt% determined by ICP-AES. The two peaks in
XPS spectrum are associated with Co3O4 and Co0, respec-
tively.[17]
Herein, we studied the catalytic performances of the Ni/
SiO2 and Co/SiO2 catalysts for benzene hydrogenation to
cyclohexane in the liquid phase under solvent-free conditions,
and the results are listed in Table 1. It can be known from
Table 1 that the Ni/SiO2 and Co/SiO2 showed excellent
activity for the reaction at 808C and 1008C. For comparison,
we carried out the reaction catalyzed by Ni/SiO2 catalysts
prepared by conventional impregnation method using Ni-
(NO3)2 as precursor and SiO2 as support.[18] The detailed
procedures to prepare the catalysts are discussed in the
experimental section. The TEM images of the Ni/SiO2
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Table 1: Catalytic performances of Ni/SiO2 and Co/SiO2 catalysts for benzene hydrogenation to cyclohexane.[a]
Entry
Catalyst
Average particle size
[nm]
Metal loading
[%][b]
Surface-active metal atoms
[%][c]
T
P
t
[h]
Yield
[%]
TOF1
[hÀ1 [d]
TOF2
[hÀ1 [e]
[oC]
[MPa]
]
]
[f]
[f]
[f]
[f]
1
2
3
4
5
6
7
8
9
10
Ni/SiO2
Ni/SiO2
Ni/SiO2
Ni/SiO2
<1 m
<1 m
<1 m
<1 m
<1 m
<1 m
7.9
7.9
4.7
6.1
21
21
21
21
19
19
21
21
5
0.1511
0.1511
0.1511
0.1511
0.4778
0.4778
3.7292
3.7292
0.2921
0.7026
100
100
80
8
6
8
6
8
6
8
8
8
8
1.8
3
9
15
3.0
6.5
5
5
5
5
>99
>99
>99
>99
>99
>99
31
0
20
28
132.1
90.1
28.6
18.5
84.3
40.4
15.8
0
18359.4
12522.2
3974.9
2571.1
3352.2
1606.5
89.0
80
Co/SiO2[g]
Co/SiO2[g]
100
100
100
80
100
100
[h]
Ni/SiO2
Ni/SiO2
Ni/SiO2
Ni/SiO2
[h]
0
[h]
36.3
24.0
621.4
341.6
[h]
10
[a] Reaction conditions: 3.0 g benzene, 0.05 g catalysts, solvent free. [b] The metal loading was determined by ICP-AES (VISTA-MPX). [c] The surface
active metal atoms was determined by chemisorption method. [d] TOF1 was calculated as moles of converted benzene per mole of metal per hour after
20% benzene conversion.[19] [e] TOF2 was calculated as moles of converted benzene per mole of surface active metal atoms per hour after 20%
benzene conversion.[19] [f] Ni/SiO2 catalysts synthesized by the route proposed herein (Figure 2). [g] Co/SiO2 catalysts synthesized by the route
proposed herein (Figure 3). [h] Ni/SiO2 catalysts synthesized by conventional impregnation method using Ni(NO3)2 as precursor (Supporting
Information, Figure S9).
A. Q. Wang, X. F. Yang, L. F. Allard, Z. Jiang, Y. T. Cui, J. Y.
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Keywords: benzene hydrogenation · metal–
organic frameworks · non-noble metals · supported catalysts ·
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