Nickel nanoparticles supported on MOF-5: Synthesis and catalytic hydrogenation properties

Article abstract of DOI:10.1016/j.inoche.2011.10.040

Nickel nanoparticles (2-6 nm) were successfully deposited on MOF-5 (Zn ₄O(BDC)₃, BDC = 1,4-benzenedicarboxylate) by a facile wet impregnation strategy to prepare Ni@MOF-5 employing Ni(acac)₂ (acac = acetylacetonate) as the precursor in absolute ethanol solvent owing to the sensitivity to the moisture of MOF-5. Ni@MOF-5 exhibited excellent catalytic activity for hydrogenation of C = C bond using crotonaldehyde as a probe molecule under mild reaction conditions (conversion > 90.0%, selectivity > 98.0%, 2.0 MPa, 100 °C, 40 min). In addition, the catalyst could be reused and the structure of MOF-5 framework was still maintained. Therefore, MOF-5 promised a novel candidate of support for hydrogenation catalyst.

Full text of DOI:10.1016/j.inoche.2011.10.040

Inorganic Chemistry Communications 15 (2012) 261–265
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Nickel nanoparticles supported on MOF-5: Synthesis and catalytic
hydrogenation properties
Huahua Zhao a,b, Huanling Song a, Lingjun Chou a,
⁎
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
b
Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel nanoparticles (2–6 nm) were successfully deposited on MOF-5 (Zn4O(BDC)3, BDC=1,4-benzenedi-
carboxylate) by a facile wet impregnation strategy to prepare Ni@MOF-5 employing Ni(acac)2 (acac = acet-
ylacetonate) as the precursor in absolute ethanol solvent owing to the sensitivity to the moisture of MOF-5.
Ni@MOF-5 exhibited excellent catalytic activity for hydrogenation of C=C bond using crotonaldehyde as a
probe molecule under mild reaction conditions (conversion>90.0%, selectivity>98.0%, 2.0 MPa, 100 °C,
40 min). In addition, the catalyst could be reused and the structure of MOF-5 framework was still maintained.
Therefore, MOF-5 promised a novel candidate of support for hydrogenation catalyst.
Received 24 August 2011
Accepted 17 October 2011
Available online 3 November 2011
Keywords:
Wet impregnation strategy
Ni@MOF-5
Crotonaldehyde
Hydrogenation
© 2011 Elsevier B.V. All rights reserved.
Introduction
hydrogenation of C=C bond [21,22]. Fischer et al. deposited non-
precious metal Cu and ZnO particles on MOF-5 by vapor deposition
In recent years, metal–organic frameworks (MOFs) have attracted
considerable attention in the field of catalysis on account for their
large specific surface areas and high porosities unparalleled so far
by other traditional materials [1–3]. Owing to their low thermal sta-
bilities (200–500 °C) [4–6], MOFs can be utilized for the reactions re-
quiring mild conditions, such as knoevenagel condensation,
cyanosilylation of aldehyde, oxidation of olefin, transesterification
and so on [7–11]. Despite many exciting achievements have been
made recently, the area of MOF-based catalysis is still in its infancy
[12,13].
One of the most representative MOFs is MOF-5 (Zn4O(BDC)3) with
BET surface area from 260 m2/g to 4400 m2/g based on the synthetic
method and good thermal stability up to 400 °C [14]. Recently, its po-
tential applications in gas storage (H2 and CO2) and heterogeneous
catalysis were intensive attracted to study [15–17]. In the field of ca-
talysis, as an efficient catalyst, Ravon et al. and Phan et al. investigated
the catalytic activity of MOF-5 over the alkylation reactions [17,18].
As the support for catalysts, MOF-5 was applied into some reactions,
for example, hydrogenation, aerobic oxidation of alcohols and Sono-
gashira coupling reactions [19–21]. Especially, there was a little
focus on MOF-5 for hydrogenation. Sabo et al. and Opelt et al. respec-
tively reported that noble metal palladium loaded on MOF-5 by solu-
tion infiltration strategy and coprecipitation method to prepare the
catalyst Pd@MOF-5, which showed high catalytic activity for
technique to synthesize Cu/ZnO@MOF-5, which displayed a good ini-
tial catalytic productivity in the hydrogenation of CO2 [23]. In addi-
tion, nickel-based catalysts are extensively employed on various
hydrogenations including the hydrogenation for C=C bond, which
exhibit excellent and particular performance [24–26]. However,
making use of MOF-5 as the support of nickel catalyst has not been
well explored so far.
In the following, the wet impregnation strategy was successfully
applied to prepare Ni@MOF-5 employing Ni(acac)2 as the precursor,
absolute ethanol as the solvent, and MOF-5 made-in-house with
large surface area as the support. The catalytic activity of Ni@MOF-5
was explored on the hydrogenation for crotonaldehyde under mild
conditions [27]. Furthermore, the reusability of the catalyst was also
studied.
Experimental
Catalyst preparation and characterization
All chemicals were received and directly used without further puri-
fication except crotonaldehyde purified by distillation. H2BDC was pur-
chased from Merck-Schuchardt (Germany). Zinc nitrate hexahydrate
(Zn(NO3)2·6H2O) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O)
were from Sinopharm Chemical Reagent Co., Ltd (China). N,N-
dimethylformamide (DMF), triethylamine (TEA), trichloromethane
(CHCl3), acetylacetone (Hacac), NaOH and ethanol were all from Tianjin
Chemical Reagent Company (China).
⁎
Corresponding author. Tel.: +86 0931 4968136; fax: +86 0931 4968129.
E-mail address: ljchou@licp.cas.cn (L. Chou).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.10.040

262
H. Zhao et al. / Inorganic Chemistry Communications 15 (2012) 261–265
Fig. 1. The XRD patterns of MOF-5: (a) simulated; (b) as-synthesized.
Ni(acac)2 was prepared by grinding Ni(NO3)2·6H2O, NaOH and
Results and discussion
Hacac in ratio of 1:2:4 (mole ratio) for 15 min at room temperature,
then drying under vacuum at 100 °C for 10 h to obtain the green
powder in our experiment.
Characterization of the catalysts
The support MOF-5 was prepared by direct mixing strategy de-
scribed by Huang et al. with slight modifications [28]. Prior to prepare
the catalyst, as-synthesized MOF-5 was calcinated in N2 flow at
250 °C for 6 h to remove the guest molecules from the pores. Ni
(acac)2 (1.0 g) was dissolved in ethanol (150.0 mL) to form a dark
green solution at 60 °C. Subsequently, guest-free MOF-5 (2.0 g) was
added into the solution under continuous agitation. The mixture
was stirred for 12–15 h in air, and then filtered, dried under vacuum
at 60 °C for 2 h to yield a green powder Ni(acac)2@MOF-5. Finally,
the powder was directly reduced under N2/H2 (4:1, v/v) flow at
340 °C for 3 h to obtain Ni@MOF-5 catalyst.
XRD patterns were measured on an X'Pert Pro multipurpose
diffractometer (PANalytical) with Ni-filtered Cu Kα radiation
(λ=0.15046 nm). Measurements were performed with a voltage of
40 kV, current setting of 40 mA and scan step size of 0.02°.
TEM was done on a JEM-2010 transmission electron microscope.
The nickel loadings of the catalyst were determined by atomic ad-
sorption spectroscopy (AAS) 18080 from Hitachi Company of Japan.
The N2 adsorption–desorption isotherms were performed with a
Quantachrome autosorb iQ gas-sorption apparatus at −196 °C. All
samples were outgassed under vacuum at 250 °C for 4 h prior to the
measurements.
Fig. 1 shows the XRD pattern of MOF-5. It was clearly seen that the
four main diffraction peaks (6.8°, 9.7°, 13.7° and 15.4°) of the as-
synthesized product were in good agreement with that of the simu-
lated one [29], suggesting that the as-synthesized product had the
MOF-5 structure. Furthermore, the sharp peaks indicated that MOF-
5 had good cystallinity. Additionally, the small peaks (31.5°, 34.6°
and 36.1°) suggested that MOF-5 framework was entrapped by a
trace amount of ZnO [16].
The XRD patterns of the catalysts at different steps are presented
in Fig. 2. It is clearly seen that two extra small peaks (10.4° and
12.4°) appeared in the XRD patterns of the catalyst after loading
with Ni(acac)2, which was probably owing to the symmetry of
MOF-5 structure changing from cubic to trigonal [30] during the
preparation of the catalyst. However, the four main peaks of MOF-5
were basically maintained. The two extra peaks disappeared after
the catalyst reduced, suggesting that the MOF-5 structure was recov-
ered, although the peak intensity of ZnO became a little stronger. Fur-
thermore, a small diffraction peak (44.5°) was attributable to metallic
Ni in the XRD patterns of the catalyst after reduction indicating that
Ni particles were probably well dispersed on MOF-5. Fig. 2d–f
shows the XRD patterns of the catalyst during the retest runs. The
four main peaks were maintained implying that the MOF-5 structure
was persevered well after the retest runs.
The TEM images of the reduced and reacted catalysts are shown in
Fig. 3. It could be seen from Fig. 3b that the crystal size of MOF-5 was
uniformly about 60–80 nm, which was consisitent with the results
Huang et al. reported [28]. The grain size of Ni was 2–6 nm, which
was larger than the pore diameter (1.02 nm) of MOF-5. Thus, nickel
nanoparticles were distributed on the outer surface of MOF-5 sup-
port. Due to the small grain size, the XRD diffraction peak intensity
of Ni was very small. It was also seen that the Ni nanoparticles had
a little agglomeration on the support.
The BET specific surface areas and pore volumes of the catalysts
are presented in Table 1. The BET surface areas of MOF-5 could attain
822.0 m2/g based on the N2 adsorption–desorption measurements.
The BET surface area greatly decreased from 821.6 m2/g to
619.9 m2/g and the micropore volume decreased from 0.28 cm3/g to
0.22 cm3/g after reduction, probably due to the destructive influence
Hydrogenation
The liquid phase hydrogenation reaction was carried out in a
300 mL stainless steel autoclave at 100 °C under 2.0 MPa H2, and
with a stirring speed of 500 rmp. For a typical experiment, 0.85 g of
the catalyst, 90 mL of ethanol, and 10 mL of the purified crotonalde-
hyde were loaded. The reaction products were identified by a GC-
MS 5973 equipment from Agilent Technology Company and analyzed
by a gas chromatograph with a SE54 capillary column and a flame
ionization detector.
The recycle of the catalyst: After the first run, the catalyst was col-
lected by configuration, dried under vacuum at 60 °C, then reduced
under N2/H2 (4:1, v/v) flow at 340 °C for 30 min and utilized for the
next run.

H. Zhao et al. / Inorganic Chemistry Communications 15 (2012) 261–265
263
Fig. 2. The XRD patterns of the catalysts: (a) MOF-5; (b) Ni(acac)2@MOF-5; (c) fresh Ni@MOF-5; (d) Ni@MOF-5 1st run; (e) Ni@MOF-5 2nd run; (f) Ni@MOF-5 3rd run.
Fig. 3. TEM images of the catalyst after: (a) reduced; (b) reaction.
of the trace water in ethanol during the catalyst preparation and the
nickel nanoparticles which may take up partial pore channels of the
framework. After reaction, the BET surface area of MOF-5 decreased
largely from 619.9 m2/g to 479.3 m2/g and the volume of micropore
reduced from 0.22 cm3/g to 0.17 cm3/g, whereas the total pore vol-
ume slightly decreased (0.34→0.32 cm3/g), indicating that partial
micropores were probably took up by some small reaction or product
molecules. The BET surface area and pore volume of MOF-5 had no
further decrease during the retest runs, which suggested that the
MOF-5 structure was not significantly influenced by the reaction con-
ditions. In addition, the pore diameter of the support was 1.02 nm,
smaller than the size of Ni nanoparticles, implying that nickel nano-
particles probably bond on the outer surface of MOF-5 support as de-
scribed above.
Catalytic activity
The hydrogenation of cronoaldehyde (Scheme 1) was chosen to test
the activity of Ni@MOF-5, because hydrogenation of unsaturated alde-
hydes was a typical model reaction to establish the relations between
selectivity and catalyst structures [31]. For the hydrogenation of croto-
naldehyde on the catalyst Ni@MOF-5 in our study, butyraldehyde was
Table 1
The pore properties of the catalysts at different steps.
Catalysts
SBET
Vp-SFa
Vp-DFTb
(cm3/g)
Dp-DFTc
(nm)
(m2/g)
(cm3/g)
MOF-5
821.6
619.9
479.3
353.3
441.5
172.2
0.28
0.22
0.17
0.12
0.15
–
0.48
0.34
0.32
0.23
0.29
0.49
1.02
1.02
1.02
1.02
1.02
11.28
Ni@MOF-5 reduced
Ni@MOF-5 1st run
Ni@MOF-5 2nd run
Ni@MOF-5 3rd run
Ni@SiO2
a
Pore volumes of the products calculated with the Saito–Foley (SF) method.
Pore volumes of the products calculated with non local density functional theory
b
(NLDFT).
c
Pore diameters of the products calculated with non local density functional theory
(NLDFT).
Scheme 1. The products of selective hydrogenation for crotonaldehyde.

264
H. Zhao et al. / Inorganic Chemistry Communications 15 (2012) 261–265
Table 2
describes the activity and selectivity of the catalyst for three retest
runs with the reaction time 60 min. In the first run, the conversion
of crotonaldehyde was up to 99.45% with high butyraldehyde selec-
tivity of 90.11%. In the second run, the conversion of crotonaldehyde
maintained at 91.49%, whereas the selectivity of butyraldehyde in-
creased to 93.39%. In the third hydrogenation, the conversion of cro-
tonaldehyde decreased to 88.73% with the selectivity almost
constant at 93.40%. Fortunately, the structure of MOF-5 was pre-
served well throughout the reaction according to the XRD patterns
of the catalysts. Thus, the catalyst could be reused for several times
with some degree of reduced catalytic performance.
The activity and selectivity of different catalysts over the hydrogenation of
crotonaldehyde.
a
b
Catalysts
Ni loadings Reaction time CCROL
SBRAD
(%)
TOFCROL TOFBRAD
(min)
(%)
(h−1
)
(h−1
)
MOF-5
Ni@MOF-5 7.4
–
60
40
60
40
11.88
91.59
99.45
83.06
1.18
98.30
90.11
94.38
–
–
154.6
111.9
73.9
152.0
100.8
69.8
Ni@SiO2
28.0
a
CROL for crotonaldehyde.
BRAD for butyraldehyde.
b
the main product with a small amount of butanol and other by-products
whereas, no crotyl alcohol was detected.
Conclusions
The results of hydrogenation for crotonaldehyde are summarized
in Table 2. The yield of butyraldehyde was only 0.14% when the reac-
tion was conducted in the presence of MOF-5 at 100 °C for 60 min, il-
lustrating that MOF-5 itself had little catalytic performance for
hydrogenation. Using the same amount of the catalyst Ni@MOF-5,
the conversion of crotonaldehyde increased to 91.59% with high se-
lectivity of butyraldehyde (98.30%) at 100 °C for 40 min. However,
crotonaldehyde was almost completely converted (99.45%) with
slight decrease of selectivity (90.11%) after 60 min.
The catalytic activity of Ni@MOF-5 was also compared with that of
the industrial catalyst Ni/SiO2 containing 28.0 wt.% Ni with BET sur-
face area of 172.2 m2/g. Ni/SiO2 was treated under the same condi-
tions as Ni(acac)2@MOF-5 prior to the hydrogenation reaction. By
contrast, Ni/SiO2 with the catalytic amount (0.43 g) exhibited only
83.06% conversion with 94.38% selectivity of butyraldehyde at
100 °C for 40 min. In terms of the turnover of frequency (TOF,
denoted as the mole content of crotonaldehyde converted over per
mole Ni per hour), Ni@MOF-5 (154.6 h−1) showed a remarkably
higher activity than that of Ni/SiO2 (73.9 h−1). Consequently,
Ni@MOF-5 had a high catalytic activity and selectivity for C=C
bond hydrogenation. The superior activity of Ni@MOF-5 might be
mainly attributed to the larger nickel surface area resulted from the
small nickel nanoparticles supported on the support MOF-5.
In summary, Ni@MOF-5 (7.4 wt.% Ni) was successfully obtained by
wet impregnation strategy to deposit nickel on MOF-5 framework.
The nickel nanoparticles (2–6 nm) were probably located on the
outer surface of the MOF-5 contributing to its small pore diameter
(1.02 nm). Ni@MOF-5 displayed much higher catalytic activity in
comparison with that of the industrial catalyst Ni/SiO2. Furthermore,
the catalyst can be reused for several times with the preservation of
MOF-5 structure. Thus, MOF-5 promised a novel candidate of support
for hydrogenation catalyst. Current research may be well transferable
to load some active components (Pt, Ag and Au) on other robust
MOFs in this reaction, such as MIL series.
Acknowledgement
We gratefully acknowledge the financial support of the State Key
Laboratory for Oxo Synthesis and Selective Oxidation of China.
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