Supported Ultrafine NiCo Bimetallic Alloy Nanoparticles Derived from Bimetal-Organic Frameworks: A Highly Active Catalyst for Furfuryl Alcohol Hydrogenation

Article abstract of DOI:10.1021/acscatal.7b03795

Highly dispersed NiCo bimetallic alloy nanoparticles have been successfully immobilized on the SiO₂ frameworks by using heteronuclear metal-organic frameworks (MOFs) as metal alloy precursors. Catalyst characterizations revealed that the average size of NiCo alloy particles was less than 1 nm, with a total metal loading of about 20 wt %. As compared to individual Ni or Co MOF-derived catalysts and the catalysts prepared by the conventional impregnation method, the ultrafine NiCo/SiO₂-MOF catalyst showed a much better catalytic performance in the catalytic hydrogenation of furfuryl alcohol (FA) to tetrahydrofurfuryl alcohol (THFA) under mild conditions, giving 99.8% conversion of FA and 99.1% selectivity to THFA. It was found that a significant synergistic effect existed between Co and Ni within the subnanometer NiCo/SiO₂-MOF catalyst, which was 2 and 20 times more active than Ni/SiO₂-MOF and Co/SiO₂-MOF, respectively.

Full text of DOI:10.1021/acscatal.7b03795

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Article
Supported ultrafine NiCo bimetallic alloy nanoparticles derived from bimetal-
organic frameworks: a highly active catalyst for furfuryl alcohol hydrogenation
Huanjun Wang, Xiaodan Li, Xiaocheng Lan, and Tiefeng Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03795 • Publication Date (Web): 19 Jan 2018
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Supported ultrafine NiCo bimetallic alloy
nanoparticles derived from bimetalꢀorganic
frameworks: a highly active catalyst for furfuryl
alcohol hydrogenation
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Huanjun Wang, Xiaodan Li, Xiaocheng Lan, Tiefeng Wang*
Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of
Chemical Engineering, Tsinghua University, Beijing 100084, China.
ABSTRACT: Highly dispersed NiCo bimetallic alloy nanoparticles have been successfully
immobilized on the SiO frameworks by using heteroꢀnuclear metalꢀorganic frameworks (MOFs)
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as metal alloy precursors. Catalyst characterizations revealed that the average size of NiCo alloy
particles was less than 1 nm, with a total metal loading of about 20 wt%. Compared with
individual Ni or Co MOFꢀderived catalysts and the catalysts prepared by the conventional
impregnation method, the ultrafine NiCo/SiO ꢀMOF catalyst showed a much better catalytic
2
performance in the catalytic hydrogenation of furfuryl alcohol (FA) to tetrahydrofurfuryl alcohol
(THFA) under mild conditions, giving 99.8% conversion of FA and 99.1% selectivity to THFA.
It was found that a significant synergistic effect existed between Co and Ni within the
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subnanometer NiCo/SiO ꢀMOF catalyst, which was twice and 20 times more active than
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Ni/SiO ꢀMOF and Co/SiO ꢀMOF, respectively.
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KEYWORDS: NiCo bimetallic catalyst, subnanometer, metalꢀorganic framework, furfuryl
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alcohol, hydrogenation, tetrahydrofurfuryl alcohol
1. INTRODUCTION
Furfuryl alcohol (FA) is an important chemical derived from the hydrogenation of furfural, a
promising biomass platform compound produced by hydrolysis and dehydration of
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hemicellulose.
FA can be converted into various valueꢀadded chemicals, such as
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tetrahydrofurfuryl alcohol (THFA),
cyclopentanone,
2ꢀmethylfuran (2ꢀMF),
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pentanediol,
1,5ꢀpentanediol and alkyl tetrahydrofurfuryl ether (ATE), by selective
hydrogenation, hydrogenolysis or etherification reactions. Among these reactions, the
hydrogenation of FA to THFA is of great importance, because THFA is not only a biodegradable
and environmentally benign furanic chemical with many agricultural and industrial applications,
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but also a promising intermediate to prepare diols.
Although a few examples of homogeneous catalysis have been reported in the literature for FA
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hydrogenation, the difficult separation and loss of active components in homogeneous catalysis
make it necessary to develop the THFA production by heterogeneous catalysis. Group VIII (Ni,
Ru, Rh, Pd, Pt) metal catalysts are active for the hydrogenation of FA to THFA. Among nonꢀ
noble metals, Niꢀbased catalysts are most used for this reaction, which generally give a high
yield of THFA but need very harsh reaction conditions. Recently, some research groups reported
that supported noble metal catalysts showed good performance under mild conditions for FA
hydrogenation. For example, Khan et al. reported that metallic Ru nanoparticles intercalated in
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hectorite catalyzed the hydrogenation of FA to THFA with a yield of 99% in methanol solvent at
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0 C and 2.0 MPa H . Yuan et al. showed that Pd supported on TiO nanotubes had 98%
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conversion of FA and 98% selectivity to THFA at room temperature and 0.1 MPa H . However,
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the high price and low reserve of noble metals limit their applications, and it is desirable to
develop nonꢀnoble catalysts for FA hydrogenation under mild reaction conditions.
Metalꢀorganic frameworks (MOFs) have attracted increasing attention due to their high surface
area, adjustable pore size and periodical skeletal structure. They have been widely investigated in
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catalysis, gas storage and separation, ion exchange and chemical sensors.
More recently,
MOFs have also been demonstrated to be promising precursors and templates to fabricate porous
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carbon,
metal oxides
and other supported metal catalysts. In particular, the
homogeneous component of multivariate MOFs with different metal centers provides an
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opportunity to synthesize multivariate mixedꢀmetal oxides or mixedꢀmetal alloys.
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example, Chen et al. synthesized Ni Co O nanoparticles for supercapacitors by calcination of
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multivariate MOFꢀ74.
Long et al. synthesized multimetalꢀMOFꢀderived NiCo alloy
nanoparticles of 20.6 nm embedded in nitrogenꢀdoped carbon matrix, which showed a high
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transfer hydrogenation performance for nitriles. However, using bimetalꢀMOFs as precursors to
synthesize supported nonꢀnoble subnanometerꢀscale alloy nanoparticles has not been reported.
In this work, we reported successful synthesis of subnanometerꢀscale NiCo nanoparticles
immobilized on SiO support using NiCoꢀMOF as precursor, which showed a very high activity
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and good stability in the FA hydrogenation reaction under mild conditions. It was found that the
prepared subnanometerꢀscale NiCo nanoparticles supported on SiO showed a strong bimetallic
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synergistic effect on the catalytic hydrogenation of FA to THFA. This synergistic effect was
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investigated by comparing with the individual Ni or Co MOFꢀderived catalysts and the
impregnation catalysts, combining various characterization techniques.
2. EXPERIMENTAL SECTION
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.1 Materials
Furfuryl alcohol (FA, 98%) and sodium borohydride (NaBH , purity > 98%) were purchased
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from J&K Scientific Ltd. Ionic liquids (ILs) 1ꢀoctylꢀ3ꢀmethylimidazolium perchlorate
(
OmimClO , purity > 99%) and triethylammonium nitrate (N(C H ) HNO , purity > 99%) were
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provided by the Centre of Green Chemistry and Catalysis, LICP, CAS. Ni(NO ) ·6H O (98%),
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Co(NO ) ·6H O (98%), tetraethoxysilane (TEOS, 99.9%) and silica (SiO , 147 m /g) were
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purchased from Alfa Aesar. 1,3,5ꢀbenzenetricarboxylic acid (H BTC, 98%) was obtained from
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Acros Organics. N,NꢀDimethylformamide (DMF, A. R. grade), acetone (A. R. grade) and ethanol
(A. R. grade) were provided by Modern Oriental (Beijing) Technology Development Co. Ltd.
All agents were used as received except that furfuryl alcohol was purified by vacuum distillation
before use.
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.2 Synthesis of MOF-derived catalysts
The MOFꢀderived catalysts were denoted as M/SiO ꢀMOF (M = Ni, Co and NiCo) and the
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synthetic method for the NiCo/SiO ꢀMOF catalyst was described as follows. In a typical
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synthesis, 70 g OmimClO , 25 g N(C H ) HNO and 30 g water were added to a 250 mL
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conical flask equipped with a condenser to form a homogeneous solution. Then the precursors of
NiCoꢀMOF, 1.5 g Ni(NO ) ·6H O, 1.5 g Co(NO ) ·6H O, 3.0 g H BTC, and the precursor of
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SiO , 6 mL TEOS, were introduced into the above solution at 80 C. The mixture was stirred at
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000 rpm for 48 h, followed by aging at 80 C for 24 h. Afterwards, the precipitate was separated
by centrifugation and washed three times with acetone. Finally, the precipitate was dried in a
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vacuum drying oven at 40 C for 24 h, and the NiCoꢀMOF/SiO composite was obtained.
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The prepared NiCoꢀMOF/SiO was reduced by the liquidꢀphase method to synthesize the
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supported ultrafine bimetal catalyst NiCo/SiO ꢀMOF. In the experiment, 0.5 g NiCoꢀMOF/SiO2
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composite was dispersed into 80 mL 50 wt% DMF aqueous solution in a 250 mL threeꢀnecked
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flask at 25 C. Then 50 mL cold NaBH DMF aqueous solution (10 g·L ) was added into the
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mixture over 60 min under the protection of CO with stirring, followed by further stirring for an
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additional 3 h. After that, the mixture was separated by centrifugation at 4000 rpm and washed
with deionized water and ethanol three times. The obtained sample was dried in a vacuum drying
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oven at 40 C for 24 h to give the NiCo/SiO ꢀMOF catalyst. The procedure to synthesize the
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monometallic MOFꢀderived supported catalysts was similar. The main difference was that only
Ni(NO ) ·6H O or Co(NO ) ·6H O was used as the MOF precursors.
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.3 Synthesis of M/SiO catalysts by repeated impregnation method
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For comparison, the M/SiO catalysts were prepared with the repeated incipient wetness
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impregnation method and denoted as M/SiO ꢀIMP (M = Ni, Co, and NiCo). Because the
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solubility of the nitrates and the water absorption capacity of the supports were low, the catalysts
with high loadings could not be prepared in one impregnation. For NiCo/SiO ꢀIMP, the desired
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amount of Ni and Co nitrates were divided into four portions, and used to impregnate the support
four times. The procedures for each impregnation were as follows. First, the precursor solution
was prepared by dissolving a desired amount of Ni(NO ) ·6H O or Co(NO ) ·6H O in deionized
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water just sufficient to fill the pores of 3.0 g SiO support. Then the precursor solution was added
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to the support dropwise and treated with ultrasound for 1 h to facilitate metal dispersion. After
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aging in air at ambient temperature overnight, the catalyst was dried in air at 80 C for 10 h and
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calcined at 450 C for 3 h. The catalyst obtained after four times of impregnation was stored in
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vials and reduced under H flow with a flow rate of 60 mL/min at 450 C for 3 h prior to
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catalytic evaluation. The procedure to synthesize the monometallic catalysts was similar except
that only one kind of nitrate was used.
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.4 Materials characterization
The Xꢀray diffraction (XRD) analysis was carried out on a Bruker D8 Advance powder Xꢀray
diffractometer (40 kV, 40 mA) with Cu Kα (λ = 1.5406 Å) monochromatic radiation source in
the 2θ range of 5–90°, and the scan speed was 2 °/min. The surface area and porous structure
were measured using N adsorptionꢀdesorption isotherms at 77 K with a Quantachrome autosorb
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iQ and AsiQwin instrument. Before measurements, the samples were degassed under vacuum at
o
30 C for 10 h. The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)
methods were used to calculate the specific surface area and pore sizes, respectively. The actual
loadings of Ni and Co on the catalysts were determined by a Spectro Arcos FHX22 inductively
coupled plasma optical emission spectrometer (ICPꢀOES). Scanning electron microscopy (SEM)
images were recorded on a JEOL JSMꢀ7401F instrument. Transmission electron microscopy
(TEM) images and element mapping were collected on a JEOL JEMꢀ2010 instrument operated at
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20 kV with an energy dispersive Xꢀray (EDX) detector. High resolution TEM (HRTEM), highꢀ
angle annular darkꢀfield scanning TEM (HAADFꢀSTEM) and element mapping were conducted
on a JEOL JEMꢀ2100F instrument working at 200 kV. The Xꢀray photoelectron spectroscopy
(XPS) analysis was performed on a Thermal Scientific ESCALAB 250Xi instrument using an Al
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Kα Xꢀray source. A lowꢀresolution survey and highꢀresolution region scans at the binding energy
of interest were collected for each sample.
2.5 Catalytic evaluation
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The selective hydrogenation of FA was carried out in a stainless steel autoclave (Weihai
Chemical Machinery Co., Ltd., 300 mL). In a typical experiment, 4.0 g purified FA, 100 mL
ethanol and 0.150 g catalyst were added to the reactor. After the reactor was sealed, the reaction
system was purged with 180 mL/min H gas flow for 10 min at room temperature. Afterwards,
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the reactor was heated to 80 C under 0.5 MPa. When the temperature reached to 80 C, the
pressure was increased from 0.5 to 3.0 MPa within 5 min. The time when the pressure reached
3.0 MPa was considered as the zero of reaction time. The pressure of the reaction system was
controlled by a backꢀpressure valve. The stirring speed was set at 800 rpm, and the H flow rate
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was set at 180 mL/min, which were found high enough to eliminate the gasꢀliquid mass transfer
resistance. The products were sampled and analyzed by a gas chromatograph (GC 7900 II,
Techcomp Instrument Company) equipped with a superꢀwax capillary column (30 m × 0.25 mm
×
0.5 ꢁm) and an FID detector. The products were identified by comparing with authentic
samples, and the reactant and products were quantified by external calibration.
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. RESULTS AND DISCUSSION
.1 Synthesis and characterization of the NiCo-MOF/SiO precursor
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As shown by XRD patterns in Figure 1, the NiCoꢀMOF/SiO composite had similar diffraction
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peaks to those of the monometallic NiꢀMOF/SiO and CoꢀMOF/SiO composites, indicating that
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these composites had similar crystal structure. Furthermore, all peaks of the three composites
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matched well with those of the reported bulk phase of Ni (BTC) ·12H O (NiꢀMOF) and
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Co (BTC) ·12H O (CoꢀMOF), confirming the successful synthesis of various MOFs.
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NiCo-MOF/SiO2
Ni-MOF/SiO2
Co-MOF/SiO2
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2θ
/degree
Figure 1. Powder XRD patterns of NiCoꢀMOF/SiO , NiꢀMOF/SiO and CoꢀMOF/SiO .
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The N adsorptionꢀdesorption isotherms of NiCoꢀMOF/SiO and corresponding pore size
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distribution are shown in Figures 2a and 2b, respectively. The isotherms of NiCoꢀMOF/SiO2
could be categorized as type IV with H hysteresis loop, in accordance with mesoporous
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materials. The pore size distribution had a peak centered at 10.1 nm. The BET surface area and
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pore volume were 190 m /g and 0.62 cm /g, respectively. The N adsorptionꢀdesorption
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isotherms and mesopore size distributions of NiꢀMOF/SiO and CoꢀMOF/SiO are shown in
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Figure S1. The BET surface area, total pore volume and average pore size of these three
composites are listed in Table S1. As revealed by Figure S1 and Table S1, the NiꢀMOF/SiO and
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CoꢀMOF/SiO composites showed mesoporous characteristics similar to the NiCoꢀMOF/SiO2
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composite.
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3.0
NiCo-MOF/SiO Adsorption
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NiCo-MOF/SiO Desorption
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NiCo/SiO -MOF Adsorption
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NiCo/SiO -MOF Desorption
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0
200
0.5
0.0
0
0
.0
0.2
0.4
0.6
0.8
P/P0
1.0
1
10
100
Relative Pressure
(
)
Pore Width (nm)
Figure 2. (a) N adsorptionꢀdesorption isotherms and (b) mesopore size distribution of NiCoꢀ
2
MOF/SiO and NiCo/SiO ꢀMOF.
2
2
The surface morphology of the NiCoꢀMOF/SiO composite was investigated by SEM. As shown
2
in Figure 3a, the asꢀsynthesized NiCoꢀMOF/SiO composite was a mixture of NiCoꢀMOF and
2
amorphous SiO . The NiCoꢀMOF particles, which had smooth surfaces and were made of
2
fragments of rectangular bars of ~2 ꢁm in size, were mainly trapped in the SiO framework. The
2
SiO support had a loose and porous morphology, consistent with the BET results. The SEM
2
images of NiꢀMOF/SiO and CoꢀMOF/SiO are shown in Figure S2, indicating that these two
2
2
composites were also mixtures of MOF and amorphous SiO . The TEM image of the NiCoꢀ
2
MOF/SiO composite and the corresponding elemental mapping results are shown in Figures 3b
2
and 3c, respectively, indicating a highly uniform distribution of Ni, Co and O in NiCoꢀMOF
surrounded by the SiO framework and confirming the successful formation of heteroꢀdinuclear
2
MOFs rather than physical mixing of monometallic MOFs.
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Co
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Si
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O
1s
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d)
(e)
C
Co
Ni 2p
Si
2
p
C
1s
2p
Si
Ni
CoNi
O
Co
Co
Ni
1
400 1200 1000 800 600 400 200
0
0
2
4
6
8
10
Binding Energy/eV
Energy/KeV
Figure 3. Related characterizations of NiCoꢀMOF/SiO composite: (a) SEM image, (b) TEM
2
image and (c) corresponding elemental mapping for Ni, Co, Si, O, (d) EDS spectrum and (e)
XPS survey spectrum.
The EDS results of NiCoꢀMOF/SiO are shown in Figure 3d. The peaks of C, O, Co, Ni and Si
2
agreed well with the composition of NiCoꢀMOF/SiO , which further confirmed the successful
2
formation of bimetalꢀMOF composites. The chemical composition of NiCoꢀMOF/SiO was
2
measured by XPS. As shown in Figure 3e, the fullꢀsurveyꢀscan spectrum has five dominant
peaks at 103.8, 284.8, 533.1, 781.5 and 856.1 eV corresponding to Si 2p, C 1s, O 1s, Co 2p and
Ni 2p, respectively, consistent with the EDS results. The Ni and Co loadings in NiCoꢀMOF/SiO2
determined by ICPꢀOES were 6.6 wt% and 6.9 wt%, respectively, which agreed well with the
dosage of metal salts. All these characterizations indicated the successful formation of NiCoꢀ
MOF in the SiO frameworks.
2
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3
.2 Synthesis and characterization of the NiCo/SiO -MOF catalyst
2
After liquidꢀphase reduction, the organic ligands in the MOF were dissolved in the DMF
aqueous solution and the reduced metal nodes were immobilized on SiO . The actual metal
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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4
4
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5
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5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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8
9
0
loading in the final catalysts were measured by ICPꢀOES, as listed in Table 1. The overall metal
loading of MOFꢀderived catalyst was around 20 wt% (Table 1, entry 1ꢀ3). For comparison, we
also synthesized the SiO supported Ni, Co and NiCo catalysts with a total metal loading of 20
2
wt% using the repeated impregnation method.
Table 1. Metal loading, BET surface area (S), total pore volume (V ), average mesopore
p
diameter (Dpore) and the average particle size (Dparticle) of different catalysts.
Metal loading/wt%
S
Vp
Dpore
nm
Dparticle
nm
Entry
Catalysts
2
ꢀ1
3
ꢀ1
Ni
10.3
17.0
/
Co
10.5
/
m ·g
cm ·g
1
2
3
4
5
6
NiCo/SiO ꢀMOF
340
347
445
114
118
124
1.07
1.05
1.12
0.40
0.46
0.45
12.6
10.1
10.1
12.6
12.1
12.6
<1
2
Ni/SiO ꢀMOF
<1
2
Co/SiO ꢀMOF
19.9
10.0
/
<1
2
NiCo/SiO ꢀIMP
10.0
20.0
/
14.3
12.0
12.7
2
Ni/SiO ꢀIMP
2
Co/SiO ꢀIMP
20.0
2
The XRD patterns of different catalysts are shown in Figure 4. All the catalysts had a diffraction
peak at 21.98°, which was the characteristic peak of SiO . For the Ni/SiO ꢀIMP catalyst, three
2
2
peaks centered at 44.47°, 51.85° and 76.34° were found, which could be assigned to (111), (200)
and (220) of faceꢀcentered cubic (fcc) nickel, respectively. No signals of nickel hydroxide or
nickel oxide were observed. The XRD pattern of Co/SiO ꢀIMP had three peaks at 44.33°, 51.57°
2
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and 75.89°, corresponding to (111), (200) and (220) of fcc cobalt, respectively. For NiCo/SiO2ꢀ
IMP, the XRD peaks occurred at 44.44°, 51.72° and 76.30°, similar to those of either fcc Ni or
fcc Co. Slight variations in the peak position were observed and all the peaks lied between those
3
4
0
1
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of pure fcc Ni and Co, confirming the formation of NiꢀCo alloy. The characteristic peaks of Ni,
Co and NiCo for the three catalysts synthesized by impregnation are sharp, indicating high
crystallinity and large particle sizes. However, except the peak at 21.98° for SiO , the XRD
2
patterns of the three MOFꢀderived catalysts had only one broad peak at around 44.5°, indicating
very small metal particles supported on SiO2.
Ni/SiO -IMP
2
NiCo/SiO -IMP
2
(111)
Co/SiO -IMP
2
NiCo/SiO -MOF
2
Ni/SiO -MOF
2
SiO (111)
(200)
2
Co/SiO -MOF
2
(220)
Co (PDF # 15-0806)
Ni (PDF # 04-0850)
10
20
30
40
50
60
70
80
90
2θ/degree
Figure 4. Powder XRD patterns of various catalysts.
To investigate the BET surface area and pore size of the NiCo/SiO ꢀMOF catalyst, N
2
2
adsorptionꢀdesorption measurements were performed. As shown in Figure 2a, the N adsorption
2
amount of the NiCo/SiO ꢀMOF catalyst was higher than that of the NiCoꢀMOF/SiO precursor,
2
2
indicating that NiCo/SiO ꢀMOF had a bigger surface area. The pore size distribution remained
2
almost unchanged before and after reduction, but the total pore volume became larger, ascribing
to the organic ligands removal of NiCoꢀMOF particles trapped in the SiO framework, as shown
2
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in Figure 2b. The N adsorptionꢀdesorption isotherms and pore size distribution curves of the
2
monometallic MOFꢀderived catalysts were similar to those of NiCo/SiO ꢀMOF, as shown in
2
Figure S3. The BET surface area, total pore volume and average pore size of all catalysts are
listed in Table 1. Compared with the impregnation catalysts, the MOFꢀderived catalysts had
bigger BET surface area and larger total pore volume (Table 1 and Figure S4), which was
attributed to the removal of organic ligands.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
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0
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8
9
0
1
2
3
4
5
6
7
8
9
0
The microstructure of the NiCo/SiO ꢀMOF catalyst was further characterized by TEM
2
measurements. The brightꢀfield HRTEM image (Figure 5a) showed that the tiny NiCo
nanoparticles were highly dispersed on the SiO support and had a narrow size distribution of 0.6
2
± 0.2 nm, as shown in Figure 5b. The HAADFꢀSTEM and corresponding element mapping
analyses were performed to detect the element distribution in the NiCo/SiO ꢀMOF catalyst. As
2
shown in Figures 5c and 5d, the element distribution of Ni and Co in this catalyst obviously
overlapped, further confirming that NiCo alloy nanoparticles were highly dispersed on the SiO2
support. Because the metal particles of NiCo alloy were very small, it was difficult to identify the
exact composition of an individual particle. We performed EDS analyses of three small regions
enclosed with circles during the HAADFꢀSTEM observation, as shown in Figures 5e and 5f. It
was found that the characteristic peaks of Ni and Co appeared simultaneously in all three
regions, suggesting that bimetallic alloy particles were formed on the SiO support. The HRTEM
2
images and particle size distribution of the monometallic MOFꢀderived catalysts and
impregnation catalysts are shown in Figures S5 and S6, respectively. The particle sizes of both
Ni/SiO ꢀMOF and Co/SiO ꢀMOF were less than 1 nm, whereas the average particle sizes of the
2
2
impregnation catalysts were larger than 10 nm, being 14.3, 12.0 and 12.7 nm for NiCo/SiO2ꢀ
IMP, Ni/SiO ꢀIMP and Co/SiO ꢀIMP, respectively.
2
2
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.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
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(
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(d
Si
O
2
0 nm
Co
Ni
(e)
C
Spectrum 1
(
f)
Spectrum 2
Spectrum 3
Si
O
Co
Ni
CoNi
CoNi
2
0 nm
0
2
4
6
8
10
Energy/kev
Figure 5. Related morphology characterization of the NiCo/SiO ꢀMOF catalyst: (a) HRTEM
2
image and (b) corresponding particle size distribution; (c) HAADFꢀSTEM image and (d)
corresponding EDS layered image, and element mappings for Si, O, Co, Ni; (e) HAADFꢀSTEM
image and (f) corresponding EDS profiles for different regions enclosed with three circles.
The chemical states of the NiCo/SiO ꢀMOF catalyst were measured by XPS analysis. The green
2
line in Figure 6a shows that the Ni 2p peak splits at 856.28 and 874.03 eV in a 2:1 peak area
2
+
ratio, which are assigned to the characteristics of 2p3/2 and 2p1/2 levels of Ni in NiCoꢀMOF,
35ꢀ36
respectively.
Moreover, two shakeꢀup peaks at the high binding energy side to the main lines
2
+
37
were detected. These satellite peaks are characteristic of Ni oxidation state. After reduction,
the red line in Figure 6a shows that the Ni 2p3/2 of the NiCo/SiO ꢀMOF catalyst is deconvoluted
2
0
into two main peaks at 855.99 and 852.40 eV, which are assigned to Ni(OH) and Ni ,
2
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respectively. The peak area ratio between Ni and Ni(OH) was 27:73, indicating that nickel was
2
mainly present in an oxidation state because the smaller particles were more easily oxidized.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
4
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0
1
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0
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Oxidized Ni
Ni 2p
Oxidized Co Co 2p
shake-up
Co
(
a)
(b)
Oxidized Ni
shake-up
Oxidized Co
shake-up
Ni
(
0
)
Ni
(
0
)
Co(0)
shake-up
(0)
2
p1/2
2p3/2
2p1/2
2p3/2
the NiCo/SiO -MOF catalyst
2
the NiCo/SiO -MOF catalyst
2
the NiCo-MOF/SiO2 precursor
the NiCo-MOF/SiO2 precursor
800 790 780 770
8
90
880
870
860
850
840
820
810
Binding Energy/eV
Binding Energy/eV
Figure 6. XPS spectrum of MOFꢀderived bimetallic NiCo catalyst before and after reduction: (a)
Ni 2p and (b) Co 2p. Reduction conditions: 0.5 g NiCoꢀMOF/SiO composite, 80 mL 50 wt%
2
ꢀ1
DMF aqueous solution as solvent, 50 mL NaBH DMF aqueous solution (10 g·L ) as reductant,
4
o
2
5 C, 0.1 MPa CO and TOS = 4 h.
2
Similarly, the Co 2p spectrum of NiCoꢀMOF/SiO (the green line in Figure 6b) also exhibits two
2
contributions at 781.25 and 797.22 eV with two shakeꢀup peaks at high binding energy, which
2+
are assigned to 2p3/2 and 2p1/2 levels of Co in NiCoꢀMOF, respectively. After reduction, the Co
2
p spectrum of the NiCo/SiO ꢀMOF catalyst (the red line in Figure 6b), which is fitted by
2
considering two resolved doublets, clearly evidences the presence of two chemical environments
for surface Co atoms. The first predominant doublet, present at 781.25 eV (2p ) and 797.03 eV
3
/2
2
+
(
2p ), is characteristic of Co oxidation state. The second one with a lower intensity is located
1/2
at lower binding energies, 777.74 eV (2p ) and 792.87 eV (2p ), corresponding to cobalt in
3
/2
1/2
3
8
0
2+
metallic state. The peak area ratio between Co and Co was 20:80, indicating that most of
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cobalt was in an oxidation state. As shown in Table S2, the ratio of metal state Ni to oxidation
state Ni in the NiCo/SiO ꢀMOF catalyst is higher than that in the Ni/SiO ꢀMOF. The reason was
2
2
that preferential oxidation of Co occurred because the Gibbs free energy of Co oxides formation
39ꢀ40
0
1
2
3
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is lower than that of Ni oxides.
The wide survey XPS spectrum of the NiCo/SiO ꢀMOF
2
catalyst revealed that the surface atomic ratio between Ni and Co was 66:34, which was higher
than the ICP measured value 49:51, implying that the Ni element in the NiCo/SiO ꢀMOF catalyst
2
was mainly located at the surface of the catalyst.
3
.3 Catalytic evaluation of NiCo/SiO -MOF
2
A series of experiments were designed to study the catalytic performance of the NiCo/SiO ꢀMOF
2
catalyst for FA hydrogenation to THFA under liquidꢀphase conditions. The results are
o
summarized in Table 2 and Figures 7, 8. Typically, the reactions were performed at 80 C and
3
.0 MPa H atmosphere in a batch reactor. It is worth noting that the solvent plays an essential
2
4
1
role in the hydrogenation activity. Considering that alcohols have a better H solubility and can
2
6
stabilize the reactive intermediates of this reaction, we used ethanol as the solvent.
Table 2. Catalytic performances of different catalysts for furfuryl alcohol hydrogenation to
a
tetrahydrofurfuryl alcohol.
Selectivity (%)
kr
Entry
Catalysts
Conversion (%)
b
ꢀ4
ꢀ1
THFA
Others
(10 min )
124.7
63.8
1
2
3
4
NiCo/SiO ꢀMOF
99.8
96.9
28.1
73.0
99.1
95.3
50.1
92.9
0.9
4.7
2
Ni/SiO ꢀMOF
2
Co/SiO ꢀMOF
49.9
7.1
6.3
2
NiCo/SiO ꢀIMP
27.1
2
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Ni/SiO ꢀIMP
90.6
21.9
97.6
60.1
2.4
47.8
4.9
2
Co/SiO ꢀIMP
39.9
2
a
Reaction conditions: 4.0 g furfuryl alcohol, 100 mL ethanol as solvent, 0.150 g catalyst, 80
C, 3.0 MPa H and TOS = 8 h. Sideꢀproducts were the intermediates of partialꢀhydrogenation
of one C=C bond in the furan ring and traces of 1,5ꢀpentanediol.
For better comparison, we also evaluated the Ni/SiO ꢀMOF and Co/SiO ꢀMOF catalysts with a
o
b
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
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3
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3
4
4
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0
1
2
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1
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2
2
similar total metal loading. As shown in Table 2 and Figure 7a, the NiCo/SiO ꢀMOF catalyst has
2
the best catalytic performance, with 99.8% conversion of FA and 99.1% selectivity to THFA
after 8 h of reaction (Table 2, entry 1). The catalyst activity and selectivity followed a trend of
NiCo/SiO ꢀMOF > Ni/SiO ꢀMOF > Co/SiO ꢀMOF. The plots of –ln(1ꢀX) as a function of
2
2
2
reaction time are straight lines passing through the origin (Figure 7b), indicating that the reaction
is pseudo firstꢀorder with respect to FA. The calculated rate constants for NiCo/SiO ꢀMOF,
2
ꢀ4
ꢀ1
Ni/SiO ꢀMOF and Co/SiO ꢀMOF are 124.7, 63.8 and 6.3×10 min (Table 2, entry 1ꢀ3),
2
2
respectively. The rate constant of NiCo/SiO ꢀMOF is about twice higher than Ni/SiO ꢀMOF, and
2
2
2
0 times higher than Co/SiO ꢀMOF, showing a significant synergistic effect between Co and Ni.
2
As revealed by the XPS results, alloying Co with Ni has a profound effect on the oxidation state
of Ni and the abundance of surface Ni species, which not only suppresses the oxidation of Ni,
but also enhances the content of Ni on the surface of the catalyst.
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Ni/SiO -MOF
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NiCo/SiO -MOF
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Co/SiO -MOF
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200
300
400
500
0
100
200
300
400
500
Reaction time/min
Reaction time/min
Figure 7. Comparison of monometallic and bimetallic Ni and Co MOFꢀderived catalysts: (a)
Conversion vs. reaction time and (b) –ln(1ꢀX) vs. reaction time for hydrogenation of FA.
o
Reaction conditions: 4.0 g furfuryl alcohol, 100 mL ethanol as solvent, 0.150 g catalyst, 80 C,
3.0 MPa H and TOS = 8 h.
2
To further evaluate the catalytic performance of NiCo/SiO ꢀMOF, we also performed the
2
reactions using the catalysts prepared by impregnation. As shown in Table 2 and Figure 8,
among the three impregnation catalysts, the Ni/SiO ꢀIMP catalyst has the best catalytic
2
ꢀ4
ꢀ1
performance, with a rate constant of 47.8×10 min and 97.6% selectivity to THFA (Table 2,
ꢀ4
ꢀ1
entry 5). The Co/SiO ꢀIMP catalyst has the lowest rate constant of 4.9×10 min and only
2
6
0.1% selectivity to THFA (Table 2, entry 6). Different from the MOFꢀderived catalysts, the
ꢀ4
bimetallic NiCo/SiO ꢀIMP catalyst shows a moderate activity, with a rate constant of 27.1×10
2
ꢀ1
min and 92.9% selectivity to THFA (Table 2, entry 4). Among all the catalysts studied in this
work, the NiCo/SiO ꢀMOF catalyst has the best catalytic performance with 99.8% conversion of
2
FA and 99.1% selectivity to THFA (Table 2, entry 1), which was mainly ascribed to the
subnanometer size of metal particles and bimetallic effect.
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The excellent catalytic performance of NiCo/SiO ꢀMOF can be attributed to the following
2
aspects. First, the ultrafine metal alloy nanoparticles offer more active sites than the large NiCo
particles prepared by impregnation, thus giving a higher activity for FA hydrogenation. Second,
the synergistic effect between Ni and Co in ultrafine NiCo alloy particles suppresses the
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oxidation of Ni, contributing to a higher activity of NiCo/SiO ꢀMOF than its monometallic
2
counterparts. Third, the removal of organic ligands during the liquidꢀphase reduction increases
the total pore volume and surface area, which benefits for reactants adsorption and internal
diffusion. This highly active catalyst has great potential applications in conversion of other
biomassꢀderived platform compounds. In addition, the significant bimetallic synergistic effect of
the subnanometerꢀscale NiCo alloy catalyst reveals that nanoclusters exhibit novel properties that
differ greatly from those predicted by simple scaling laws, which provides new opportunity for
developing highꢀperformance catalysts.
100
(a)
6
5
4
3
2
1
0
(b)
80
NiCo/SiO -MOF
2
Ni/SiO -IMP
2
NiCo/SiO -IMP
60
2
Co/SiO -IMP
NiCo/SiO -MOF
2
2
Ni/SiO -IMP
40
20
0
2
NiCo/SiO -IMP
2
Co/SiO -IMP
2
0
100
200
300
400
500
0
100
200
300
400
500
Reaction time/min
Reaction time/min
Figure 8. Comparison of monometallic and bimetallic Ni and Co impregnation catalysts: (a)
Conversion vs. reaction time and (b) –ln(1ꢀX) vs. reaction time for hydrogenation of FA.
Reaction conditions are the same as in Figure 7.
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The reusability of NiCo/SiO ꢀMOF was investigated by recycling experiments. After reactions,
2
the reaction solution was centrifuged to separate the catalyst. The collected catalyst was washed
with ethanol, dried under vacuum, and then reused in the next reaction cycle. Five consecutive
runs were conducted. The conversion of FA and the selectivity to THFA after 8 h are shown in
0
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Figure S8. The NiCo/SiO ꢀMOF catalyst maintains high conversion and selectivity in the five
2
consecutive runs with only slight decrease in activity. The XRD pattern and TEM images of the
spent catalyst are shown in Figure S9 and S10, respectively. It can be seen that the crystal
structure is maintained as determined by XRD analysis, and the size of NiCo nanoparticles on
the spent catalyst is nearly the same as that of the fresh catalyst as shown by TEM images. ICPꢀ
OES analysis further reveals that the leaching loss in the NiCo/SiO ꢀMOF catalyst after five runs
2
is only 1.0 wt% for Ni and 1.3 wt% for Co, indicating that the NiCo/SiO ꢀMOF catalyst has a
2
reasonable stability in FA hydrogenation to THFA.
4. CONCLUSIONS
We have developed a novel and facile method to prepare ultrafine NiCo bimetallic alloy
nanoparticles supported on porous SiO2 by reducing heteroꢀdinuclear MOFs. Catalyst
characterizations reveal that bimetallic MOFꢀderived NiCo alloy NPs less than 1 nm are highly
dispersed on the porous SiO support with a total metal loading of about 20 wt%. The
2
NiCo/SiO ꢀMOF catalyst exhibits the best catalytic performance for the hydrogenation of FA to
2
THFA, giving 99.8% conversion of FA and 99.1% selectivity to THFA. Significant synergistic
effect exists between Co and Ni within the subnanometer NiCo/SiO ꢀMOF catalyst, which is
2
twice and 20 times more active than Ni/SiO ꢀMOF and Co/SiO ꢀMOF, respectively. In contrast,
2
2
such a synergistic effect is not observed within the NiCo/SiO ꢀIMP catalyst prepared by
2
conventional impregnation. This interesting finding of sizeꢀdependent bimetallic synergetic
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effect provides guidance for enhancing the activity of bimetallic catalysts. Considering the
diversity of MOFs, the method developed in this work can be easily extended to prepare other
highꢀperformance subnanometer alloyꢀbased catalysts, such as NiFe, NiZn and CoNiCu, with a
wide range of catalytic applications.
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AUTHOR INFORMATION
Corresponding Author
*
Eꢀmail for T.W.: wangtf@tsinghua.edu.cn
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.
SEM, N adsorptionꢀdesorption isotherms and mesopore size distributions of the NiꢀMOF/SiO2
2
and CoꢀMOF/SiO composites; TEM, XPS, N adsorptionꢀdesorption isotherms and mesopore
2
2
size distribution of the Ni/SiO ꢀMOF, Co/SiO ꢀMOF and impregnation catalysts; Reusability of
2
2
the NiCo/SiO ꢀMOF catalyst; XRD、HRTEM、HAADFꢀSTEM and element mapping of the
2
used NiCo/SiO ꢀMOF catalyst (PDF)
2
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ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science Foundation of China (No.
21676155) and the PetroChina Innovation Foundation (No. 2016Dꢀ5007ꢀ0507).
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