Carbon-embedded Ni nanocatalysts derived from MOFs by a sacrificial template method for efficient hydrogenation of furfural to tetrahydrofurfuryl alcohol

Article abstract of DOI:10.1039/c7dt00628d

We report a fast and simple method for the synthesis of Ni-based metal-organic-frameworks (Ni-MOFs). Due to the existence of nickel ions and an organic ligand, the MOFs are employed as a sacrificial template for the facile preparation of carbon-embedded Ni (Ni/C) catalysts by a direct thermal decomposition method. The obtained Ni/C catalysts exhibit excellent catalytic activity for selectively transforming furfural (FAL) to tetrahydrofurfuryl alcohol (THFOL) due to the Ni nanoparticles (NPs) embedded uniformly in the ligand-derived carbon. The exemplified results illustrate that the catalytic performance of the Ni/C catalyst is greatly affected by the calcination conditions (temperature and time), composition of the Ni-MOF precursor and the catalysis conditions. The conversion of FAL and selectivity of THFOL both reached 100% under the conditions of 120 °C, 1 MPa H₂ pressure and 120 min of hydrogenation over the Ni/C-500 catalyst, derived from the pyrolysis of Ni-MOFs (Ni:BTC mole ratio of 1.0) at 500 °C for 120 min, which exhibits an average nanoparticle size of ～14 nm and uniform dispersion, and the highest BET surface area (～92 m² g^-1) among all investigated Ni/C catalysts. This facilely prepared heterogeneous catalyst would be very promising for the replacement of noble metal catalysts for the efficient catalytic conversion of biomass-derived feedstocks into value-added chemicals.

Full text of DOI:10.1039/c7dt00628d

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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DOI: 10.1039/C7DT00628D
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Carbon-embedded Ni nanocatalysts derived from
MOFs by sacrificial template method for efficient
hydrogenation of furfural to tetrahydrofurfuryl alcohol
Received 00th January 20xx,
Accepted 00th January 20xx
Yanping Sua,b, Chun Chen*a, Xiaoguang Zhua, Yong Zhanga,b, Wanbing Gonga,b,
Haimin Zhanga, Huijun Zhaoa,c , and Guozhong Wang*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a fast and simple method for synthesis of Ni-based metal-organic-frameworks (Ni-MOFs).
Due to the existence of nickel ions and organic ligand, the MOFs are employed as sacrificial template
for facile preparation of carbon-embedded Ni (Ni/C) catalysts by directly thermal decomposition
method. The obtained Ni/C catalysts exhibits excellent catalytic activity for transforming furfural (FAL)
to tetrahydrofurfuryl alcohol (THFOL) selectively due to Ni nanoparticles (NPs) embedded uniformly in
the ligand-derived carbon. The exemplified results illustrate the catalytic performance of Ni/C catalyst
is greatly manoeuvred by the calcination conditions (temperature and time)，composition of Ni-
MOFs precursor as well as the catalysis conditions. The conversion of FAL and selectivity of THFOL are
both reached 100 % under the conditions of 120 °C and 1 MPa H2 pressure for 120 min of
hydrogenation over the Ni/C-500 catalyst, derived from pyrolysis of Ni-MOFs (Ni:BTC mole ratio of
1.0) at 500 °C for 120 min, which exhibits an average nanoparticle size of ~14 nm and uniform
dispersion, and highest BET surface area (~92 m2/g) among all investigated Ni/C catalysts. This facilely
prepared heterogenous catalyst would be very promising for the replacement of noble metal catalysts
for efficiently catalytic conversion of biomass-derived feedstocks into value-added chemicals.
leaching of the NPs without protecting groups on the supports 11-13
.
In order to improve the stability of heterogeneous catalysts,
encapsulation of metal NPs into a support to form core-shell
structure material has been widely applied because of protective
effect of shell support 14, especially for designing of intolerable cost
noble metal catalysts 15-17. However, the activity of this type of
material is usually constrained due to the limitation of mass
transfer by the shell. It should take various complicated strategies
to improve the activity of this kind of material, such as synthesis of
Introduction
As the most important renewable resource, biomass and their
derivatives are considered as desirable feedstocks for production of
1
fuels and chemicals . Now, it has been considered as one of the
efficient ways to utilization of biomass resources by catalytic
conversion of biomass-derived unsaturated heterocyclic
compounds, like furfural (FAL), into value-added chemicals over
heterogeneous catalysts 2-5. High active heterogeneous catalysts
with small particle sizes and high dispersion can be obtained by
supporting metal nanoparticles (NPs) on specific carriers or matrix
18
porous shell or hollow nanostructures 19-21, as well as sandwich
22,
structures
23. Thereby, there is an urgent need but still a
challenge to develop highly active and steady catalysts that should
also be facilely prepared for practical use. Based on these
considerations, one-pot embedding non-precious metal NPs into
cheap support may be supposed as one of efficient approaches for
fabrication high performance catalysts for industrial scale
applications.
materials 6-8. Nevertheless, traditional methods for synthesizing
9
these materials contain usually multiple steps
consuming 10, as well as the catalysts with easy agglomeration and
and are time
a.
Key Laboratory of Materials Physics, Centre for Environmental and Energy
Among various feasible strategies to fabricate the suggested
embedded nanostructures, a special attention should be paid to
sacrificial template method in which the precursors of active metal
and support co-exist simultaneously24, 25. Metal organic frameworks
(MOFs) consisted of inorganic metals centers and organic linker
molecules26 can be considered as potential sacrificial templates for
fabrication of carbon-based nanomaterials27 which have drawn the
Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS
Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese
Academy of Sciences, Hefei 230031, P. R. China. E-mail: gzhwang@issp.ac.cn;
chenchun2013@issp.ac.cn
b.University of Science and Technology of China, Hefei, Anhui 230026, P. R. China.
c. Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus,
Queensland 4222, Australia.
Electronic Supplementary Information (ESI) available: Additional TEM images and
XPS spectra of carbon-embedded Ni nanocatalysts, TEM data of acid treated,
influence of other parameters, Ni-based heterogeneous catalysts in recent
published works, and comparison of fresh and spent catalyst. See
DOI: 10.1039/x0xx00000x
attention of researchers in the fields of gas separation, capacitance,
19, 28-30
electrocatalysts as well as heterogeneous catalysts
.
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Enlightened by this superiority, we report a facile and widely
applicable approach to one-pot fabricate carbon-embedded Ni
(Ni/C) nanocatalysts by pyrolysis treatment of Ni-MOFs, and
investigate their catalytic performance in the hydrogenation of FAL,
a potential platform with unsaturated aldehyde group for biofuels
and chemicals into corresponding alcohols 31-33. In convention
process, MOFs are usually synthesized by hydrothermal or
solvothermal methods with high temperature and pressure as well
as long synthesis time (sometimes several days) 34-36. In this work,
Ni-MOFs were simply prepared in an oil-bath at atmospheric
pressure and a short synthesis time (150 min). The as-prepared Ni-
MOFs are used as the Ni precursor for subsequent pyrolysis
treatment, in which Ni/C nanocatalysts are formed by a stepwise
change of organic ligand and Ni ions: ligand decomposition and
carbonization, and then Ni ions gradual interacting with
surrounding carbon, consequently dispersed metallic Ni being Scheme 1 Synthesis of Ni/C catalysts for hydrogenation of FAL to
formed and embedded into the carbon matrix, as shown in Scheme
1. The as-synthesized Ni-MOFs and interrelationship of pyrolysis
treatment condition, physicochemical properties of catalysts
(structure, chemical states, and particle sizes) and catalytic
hydrogenation performance were also investigated by the
characterizations of scanning electron microscope (SEM), powder X-
ray diffraction (XRD), transmission electron microscopy (TEM), and
X-ray photoelectron spectrometer (XPS), as well as the reaction
tests. The high activity and stability of such embedded Ni/C
nanocatalysts may provide the merits of homogeneous dispersion
of active metallic Ni and prevention migration and agglomeration of
Ni NPs by the carbon matrix. The findings in this work would be
helpful to facilely design and develop high performance catalysts
for biomass transformation.
THFOL.
Characterization
The as-synthesized products were characterized by scanning
electron microscope (SEM, FEI Sirion 200), transmission electron
microscopy (TEM, JEOL-2010 at 200 kV), X-ray diffraction (XRD,
Philips X-Pert Pro, Cu Kα radiation at 40 keV and 40 mA), and
thermogravimetric analysis (TGA, Pyris 1, heating rate 5 °C/min at
N2 atmosphere). XPS analysis was performed on an ESCALAB 250 X-
ray photoelectron spectrometer (Thermo, USA) equipped with Al
Kα1,2 monochromatized radiation at 1486.6 eV X-ray source. The
surface area and porosity of samples were measured at 77 K by a
Surface Area and Porosity Analyzer (Autosorb iQ Station 2).
Hydrogenation of Furfural
Catalytic hydrogenation of FAL was carried out in a 25 mL of
stainless steel autoclave with a mechanical stirring. In a typical
experiment, the reaction solution containing 30 mg FAL, 30 mg
catalyst and 5.0 mL of 2-propanol was loaded into the reactor. The
reactor was sealed, purged with 1 MPa H2 and then pressurized
with 3 MPa N2. The reaction was held at a certain temperature (60-
170 °C) and time (10-120 min) with 500 rpm stirring. After reaction,
the autoclave was cooled down quickly and the solid-liquid mixture
was filtered by a 0.22 µm microporous membrane filter.
Experimental section
Catalyst preparation
Preparation of Ni-MOFs. The Ni-MOFs were prepared in an oil-bath
under atmospheric pressure. Typically, a calculated amount of
Ni(NO3)2·6H2O were mixed with 1.37 g 1,3,5-benzenetricarboxylic
acid (H3BTC) with different mole ratios (0.5, 1.0, 1.5 and 2.0) of Ni
and BTC in 30 mL of dimethylformamide (DMF) to form a green
transparent solution. The obtained solution was heated at 150 °C in
a preheated oil-bath for 150 min, and then cooled to room
temperature. After that, a green powder was collected by
centrifugation with the speed of 10,000 rpm. Then the product was
washed by DMF and ethanol for several times, and dried in a
vacuum oven at 60 °C for overnight. The yield of Ni-MOFs was
calculated to be 88%, higher than other Ni-MOFs prepared by
hydrothermal or solvothermal method with high temperature and
pressure and long reaction time, as shown in Table S1, ESI†.
Product analysis
The liquid product was identified by gas chromatography–mass
spectrometry (GC-MS, Thermo Fisher Scientific-TXQ Quntum XLS,
column-TG-WAXMS, 30 m × 0.25 mm × 0.25 m), and was
quantitatively analyzed by GC (Shimadzu, GC-2010 Plus), equipped
with FID and a (30 m × 0.25 mm × 0.25 m) KB-WAX capillary column
(Kromat Corporation, USA) using n-octanol as an internal standard.
The carbon balance was checked in every run and it was found to
be higher than 95 %. The conversion of FAL and selectivity of THFOL
and FOL are calculated by the formulae as follows.
Preparation of Ni/C nanocatalysts. To prepare Ni/C nanocatalysts,
the obtained Ni-MOF green powder was placed into a tube furnace
and pyrolytically decomposed at a setting temperature (400-700 °C)
for 120 min with the heating rate of 5 °C/min in a nitrogen
atmosphere. The product is denoted as Ni/C-x，where x represents
as the pyrolysis temperature.
（nFAL） （- nFAL
）
in
out
FAL conversion =
(1)
×100%
（nFAL
）
in
nTHFOL
nTHFOL + nFOL + nOthers
THFOL selectivity =
(2)
×100%
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FOL selectivity =
ARTICLE
nFOL
(3)
×100%
nTHFOL + nFOL + nOthers
where (nFAL)in is moles of FAL before reaction, (nFAL)out is moles of
FAL, nTHFOL is moles of THFOL, nFOL is moles of FOL, and nOthers is
moles of other products in product samples.
Results and discussion
Material Characterization
Fig. 1a shows the SEM image of the prepared Ni-MOFs. It is
obvious that the morphology of Ni-MOFs exhibits bulk structure
consisting of laminar structure with the thickness of ~28 nm. To
determine the structure of Ni-MOFs, we searched the Cambridge
Crystallographic Data Centre (CCDC) standard pattern of Ni-BTC
MOFs. The peaks of the synthesized Ni-MOFs match well with the
CCDC standard No. 636900 (Fig. 1b) by mercury and the structure is
shown in Fig. 1c. The formula of the MOFs should be noted as
NiC15H18N2O8.
Fig. 2 TGA curves of Ni-MOF
temperature of 470-605 °C. Continuous raising the temperature to
700 °C, the weight of sample hold steady at the step, indicating no
thermal event.
The decomposition of Ni-MOFs by pyrolysis approach was first
investigated by TG analysis from room temperature to 700 °C in N2
atmosphere and the result is shown in Fig. 2. A weight loss of 4.3
wt% in the first-stage corresponds to the removal of lattice water
appearing from room temperature to 110 °C 37, while the weight
loss of 31.1 wt% in second-stage can be ascribed to the
decomposition of residual solvent DMF from the surface and pores
of Ni-MOFs with the pyrolysis temperature increased to 390 °C 38. A
significant weight loss of 39.6 wt% appears with subsequently
increasing the pyrolysis temperature to 470 °C. The dramatic weight
loss of Ni-MOFs at this stage can be attributed to the
decomposition of organic ligand from Ni-MOFs precursor and
partial reducing of Ni ions. Further elevating the pyrolysis
temperature, the weight loss of sample, most likely, originates with
the reducing of Ni ions or its intermediates into Ni NPs by the
interaction of pyrolytic carbon and Ni ions at the pyrolysis
As previous description, the Ni/C catalysts are derived from
pyrolysis treatment of Ni-MOFs by the sacrificial template method.
The crystallographic structures and morphology of the Ni catalysts
are strongly relied on the condition of thermal treatment. The TG
analysis has confirmed the decomposition of Ni-MOFs taking place
at the pyrolysis temperature of 400-700 °C, and the corresponding
XRD patterns of the samples are shown in Fig. 3. The diffraction
peaks of 2θ = 44.5°, 51.8°, 76.4° can be indexed to the Ni of (111),
(200), (220) crystal planes (Fig 3a), which agree well with the
standard face-centered cubic (fcc) Ni phase (JCPDS No. 04-0850).
The weak diffraction peaks of 2θ = 41.9°, 48.2° for the Ni catalysts
obtained at 400 °C, 500 °C and 600 °C can be indexed as (006) and
(202) planes of nickel carbide (JCPDS No. 01-072-1467). This result
illustrates the existence form of nickel consists mostly of metallic Ni
as well as a small amount of nickel carbide, which are derived from
the interaction of Ni ions and pyrolytic carbon during the process of
Ni-MOFs decomposition, and the verdict is further proved by TEM
analysis (Fig. S1, ESI†). As shown in Fig. S1d (ESI†), lattice spacing of
2.03 Å in high-resolution TEM (HRTEM) image manifests the Ni
nanoparticle and the corresponding TEM-EDX line scan of the
particle (Fig. S1b, ESI†) shows that the content of carbon atoms
decreases with the increase of Ni atoms, further illustrating the
existing of metallic Ni on the carbon substrate. In addition, there is
also handful of nanoparticles with the lattice spacing of 2.15 Å (Fig.
S1e, ESI†) belonging to nickel carbide, which is further proved by its
Fig. 1 (a) SEM image, (b) XRD pattern and (c) schematic
structure of Ni-MOFs (Ni: light blue; N: blue; O: red; C: grey).
Fig. 3 XRD patterns of Ni/C catalysts. (a) from 400 °C to 700 °C for
120 min and (b) from 90 min to 150 min at 500 °C.
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Fig. 5 XPS spectra of Ni/C-500 nanocatalysts. (a) Ni 2p and (b) C1s
regions.
that the existence of Ni in these samples are mainly metallic Ni and
little nickel carbide as well. The pyrolysis time has a great influence
on the final product, and the product of pyrolysis is more and more
complete with the pyrolysis time prolong，further influences the
activity of the catalysts.
Fig. 4 The TEM images of samples (a), (b) Ni/C-400, (c) Ni/C-500,
Fig. 5 shows the Ni2p and C1s XPS spectra of Ni/C-500. For the
Ni2p (Fig. 5a), the binding energy of the Ni2p3/2 peak at 852.8 eV
corresponds to literature data for metallic fcc-Ni 39. And the peaks
at 870.5 and 853.3 eV in the Ni2p XPS spectrum suggest the
existence of the carbide phase in the carbon composite 40. The small
peak at 283.8 eV in the C1s spectrum (Fig. 5b) can also be related to
the carbon in the carbide, assigned to nickel carbide 39, 41, 42. The C1s
peak at 284.8 eV corresponds to graphite 43. The XPS spectra of Ni
2p and C1s regions further proves that the interaction of pyrolytic
carbon and Ni ions can form metallic Ni and nickel carbide during
the process of Ni-MOFs pyrolysis, which is consistent with the
results of TG, TEM, line-scan of TEM-EDX, and XRD analysis. The XPS
full spectrum, N1s and O1s spectra are provided in Fig. S2 (ESI†).
Fig. 6 shows the N2 adsorption and desorption isotherms of
Ni/C-500. It exhibits type IV isotherms curves, and the uptake at
pressure of P/P0 higher than 0.8 implies existence of macro pores
generated by packing Ni/C-500. Table 1 summarizes the surface
properties of samples obtained at different pyrolysis temperature,
which show 69-92 m2/g of specific surface area (SBET) and 0.25-0.34
cc/g of pore volume (Vpore). Ni/C-500 expresses the higher specific
surface area than that of Ni/C-400 and Ni/C-600, as well as its
catalytic activity.
and (d) Ni/C-700 and corresponding size distribution (the insets).
corresponding TEM-EDX line scan (Fig. S1c, ESI†). As it shows the
content of carbon atoms increases with the increase of Ni atoms
which is antiparallel with the line scan of metallic Ni NPs. When the
pyrolysis temperature elevating to 700 °C (Fig. 3a), in contrast, the
diffraction peaks of nickel carbide disappear indicating the forming
nickel carbide can transform into metallic Ni by the sequential
interaction of nickel carbide and pyrolytic carbon, meaning that the
nickel carbide is the intermediate product of the pyrolysis process.
One can also find the dispersion and size of the sample obtained at
700 °C are deteriorated due to significant agglomeration of NPs (Fig.
4d). The existence of nickel carbides may play a role in preventing
the agglomeration of Ni NPs. When the nickel carbides do not exist,
which indicates that the process is complete and all nickel carbides
are converted into Ni nanoparticles, finally the Ni nanoparticles will
grow up. In addition, the particle sizes of metallic Ni have also been
provided: the average crystallite sizes of Ni/C nanocatalysts derived
from pyrolyzing at 400 °C, 500 °C and 700 °C are found to be 13 nm,
18 nm and 37 nm from the Scherrer’s formula, respectively, and
these corresponding values are 25 nm, 14 nm and 33 nm from the
TEM images (Fig. 4a, c and d), respectively. The crystallite sizes from
XRD characterization is essentially consistent with TEM data for the
samples obtained at 500 °C and 700 °C. But there is a deviation for
the sample derived at 400 °C. The corresponding HRTEM image for
3.2 Liquid phase hydrogenation of FAL over Ni/C nanocatalysts
Screen for catalytic performance
It is known that FAL is a hetero aromatic aldehyde composed
of a furan ring and a carbonyl group. Both of the two functional
groups can be hydrogenated to the corresponding alcohols of
furfuryl alcohol (FOL) and tetrahydrofurfuryl alcohol (THFOL), as
shown in Fig S3, ESI†. But the forming of THFOL is usually more
resistant due to the high stability of the furan ring. To investigate
their catalytic performance, theNi/C nanocatalysts have been
screened for the hydrogenation of FAL in a batch reactor at
temperature 120 °C under H2 pressure 1 MPa (Fig. 7). It shows that
the FAL conversion increases from 33.8 % to 100 % with the
pyrolysis temperature for Ni-MOFs being elevated from 400 °C to
500 °C. Meanwhile, the distribution of products is also influenced
by the pyrolysis temperature: The FAL is almost completely
the Ni/C-400 shows
a whole particle with several lattice
orientations (Fig. 4b), which indicates a large particle consisting of
several small crystal particles. But the Scherrer’s formula is used to
calculate the average grain size rather than the average particle size.
According to the TG analysis (Fig. 2), organic ligands of MOFs just
start to decompose at 400 ºC, leading to uncomplete decomposition
of the particle at this temperature. When the temperature
increases to 500 ºC, the pyrolysis carbon could shrink completely
and reduce the Ni ions to form small nanoparticles. The undivided
particles (25nm) stemming from incomplete decomposition at 400
ºC, which will grow to smaller particles (13 nm) at 500 ºC. We also
explored the influence of pyrolysis time on phase and activity of
Ni/C catalysts. The XRD patterns of the samples calcined at 90-
150min by fixing pyrolysis temperature at 500 ºC (Fig. 3b) indicate
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Fig. 6 N2 adsorption/desorption isotherms and pore size distribution
Fig. 7 Effect of calcination conditions on the hydrogenation of FAL
over Ni/C-500 nanocatalyst. Reaction conditions: 120 °C, 1 MPa H2
pressure, 120 min.
of Ni/C-500 nanocatalyst
Table
1 The BET surface area and pore volume of Ni/C
nanocatalysts.
Pyrolysis
temperature (°C)
400
does not show any activity suggesting that the active site is the
available metallic Ni on the Ni/C catalyst. We estimated the content
of Ni and nickel carbide in Ni/C-500 catalyst from ICP after
microwave digestion of Ni/C-500 and acid-treated sample. As
shown in Table S2, ESI†, The content of the metallic Ni and nickel
carbide in Ni/C-500 is 51.10% and 2.53% respectively. By measuring
the content of Ni in Ni/C-500 catalyst after catalytic reaction from
ICP after microwave digestion of after-reacted catalyst, the content
of total Ni does not change, indicating no Ni nanoparticles coming
into the solvent in the catalytic process.
Surface area (m2/g)
Pore Volume (cc/g)
74.1
92.3
68.9
0.34
0.28
0.25
500
600
converted into THFOL over Ni/C-500 but the selectivity of THFOL is
only 20.6 % over Ni/C-400. It indicates that the Ni/C-500 catalyst
has a higher catalytic performance than Ni/C-400. Also, one can
find that the activity and product distribution of Ni/C catalyst are
deteriorated as continuously elevating the pyrolysis temperature to
600 °C and 700 °C. As previous description, the Ni/C catalysts are
derived from thermal decomposition of Ni-MOFs, in which the
organic ligands are gradually pyrolyzed and shrunk, and then the Ni
ions are progressively interacted with the surrounded pyrolytic
carbon and ultimately reduced to metallic Ni NPs. Although the Ni/C
catalysts can be formed at 400 °C, the activity of this catalyst is
unsatisfactory due to insufficient active sites and large particle size
originating in the incomplete decomposition of Ni-MOFs at 400 °C,
as illustrating in TG analysis (Fig. 2) and TEM images (Fig. 4a-b). The
obviously upgrading catalytic performance at Ni/C-500 can be
obtained because of improvement on the dispersion, particle size
and active site during pyrolysis process at this temperature, as
shown in TEM image of Fig. 4c. We have shown that the interaction
of pyrolytic carbon and Ni ions during pyrolysis process leading to
the generation of metallic Ni and nickel carbide by using TEM and
XRD analysis.
At the pyrolysis temperature of 500 °C, the adequate
interaction of pyrolytic carbon and Ni ions gives plentiful active sites
as well as superior dispersion of Ni NPs for high activity on the
hydrogenation of FAL. Although the nickel carbide can transform
into metallic Ni as the pyrolysis temperature elevating over 500 °C,
the activity of the Ni/C-500 catalyst declines due to significant
agglomeration of small Ni NPs into large particles (Fig. 4d). A similar
reaction behavior can also be found for the influence of pyrolysis
time of the embedded Ni catalyst on the hydrogenation of FAL (Fig.
S5, ESI†). The mole ratio of Ni to organic ligand (Ni:H3BTC) in the
precursor of Ni-MOFs will ultimately influence the Ni content in the
Ni/C nanocatalysts. Therefore, the influence of Ni:H3BTC mole ratio
in the Ni-MOFs on the hydrogenation of FAL is also investigated
over the samples. As shown in Fig. S6, ESI†, the FAL conversion
increases from 46.5 % to 100 % with Ni:H3BTC mole ratio increase
from 0.5 to 1.0 and 1.5. Moreover, the THFOL selectivity is also
upgraded from 40.6 % to 100 % and 66.4 %, respectively. The
enhanced performance can be attributed to more active sites
forming at a higher Ni content. However, the FAL conversion as well
as THFOL selectivity gradually reduces when further increasing the
Ni:H3BTC mole ratio to 2.0, which probably gives rise to little
agglomeration of Ni NPs. These results are agreed well with the
analysis of XRD (Fig. S6b, ESI†) and TEM (Fig. S6c and d, ESI†).
Therefore, the Ni/C nanocatalysts with optimal catalytic
performance can be obtained from the precursor of Ni-MOFs with
the Ni:H3BTC mole ratio of 1.0. These results indicate that the
In order to determine the active sites, metallic Ni or nickel
carbide, we design an experiment for removing the available
metallic Ni on the external surface of Ni/C-500 catalyst by acid
treatment method (concentrated hydrochloric acid, three times). As
the TEM image shows (Fig. S4, ESI†), the available metallic Ni NPs
are almost completely removed remaining nickel carbide particles
(non reaction with acid) and carbon substrate. When it is applied to
the hydrogenation of FAL, the acid treatment Ni/C-500 catalyst
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Fig. 8 The influence of reaction conditions on the conversion of FAL
and product distribution over Ni/C-500 catalyst. (a) reaction
temperature from 60-170 °C for 120 min, (b) reaction time from 10-
120 min at 120 °C.
Fig. 9 The cyclic stability of Ni/C-500catalyst. Reaction conditions:
activity and distribution of products are highly related to the
pretreatment condition and the ratio of component in Ni-MOFs for
preparation of Ni/C nanocatalysts. Therefore, the Ni/C nanocatalyst
derived from pyrolyzing at 500 °C for 120 min and with the
Ni:H3BTC mole ratio of 1.0 (Ni/C-500) is selected for studying the
influence of reaction condition and its stability.
120 °C, 1 MPa H2 pressure, 60 min
extension, the furan ring can be activated by the Ni/C-500 catalyst
resulting in high selectivity toward THFOL. These results further
indicate the converting of FAL generating FOL first, and then THFOL
over the Ni/C-500 catalyst, as shown in Fig S3, ESI†. Compared with
other catalysts for hydrogenation of FAL to THFAL reported
recently(Table S3, ESI†), the activity of Ni/C-500catalyst is generally
speaking higher, even higher than some supported noble metals
catalysts.
Effect of reaction conditions
Based on previous study, we further investigated the catalytic
performance of Ni/C-500 nanocatalysts under different reaction
temperature and time to give an optimal reaction condition for
hydrogenation of FAL, and the results are summarized in Fig. 8. It is
obviously point out the reaction temperature has a great impact on
FAL conversion and product distribution (Fig. 8a). FAL conversion is
gradually increased from 5.8 % at 60 °C to 36.0 % and 78.1 % at 90
°C and 100 °C, respectively. With the reaction temperature further
elevating to 120 °C, complete conversion of FAL is obtained, and
then remains at this level. It indicates that catalytic hydrogenation
of FAL over the catalyst is temperature-sensitive. Besides, the
product distribution is also related to the reaction temperature. At
low temperature of 60 °C, FAL is selectively converted into FOL.
With the temperature elevating, the selectivity for the desired
product of THFOL gradually increases, and complete conversion of
FAL into THFOL is obtained when reaction temperature increases to
120 °C. The selectivity of THFOL decreases and that of others
(pentanediol) increases with increasing reaction temperature to
170 °C due to further hydrogenation or hydrogenolysis (C-C or C-O
cleavage). These results proved the activities (conversion and
selectivity) of FAL hydrogenation over the Ni/C-500 nanocatalysts
are temperature sensitive and 120 °C can be considered as
optimum reaction temperature for maximum selectivity of THFOL.
Besides reaction temperature, the reaction time is also a main
factor to influence FAL conversion and product distribution, as
shown in Fig. 8b. With the reaction time increases from 10 min to
120 min, the conversion of FAL is increased from 22.3 % to 100 %,
with the selectivity of THFOL increasing from 7.4 % to 100 %. It is
known FAL is composed of a furan ring and a branched carbonyl
group. The hydrogenation of carbonyl group is usually preferred to
furan ring due to high stability of the latter. Therefore, the reaction
is concentrated on hydrogenation of carbonyl group at low
temperature or short period leading to high selectivity of FOL. With
the temperature increasing or time
Recycling test
To evaluated the reusability of the Ni/C-500 catalyst, it is
separated from the reaction mixture by magnetic separation after
catalytic reaction, washed with 2-propanol for several times. The
catalyst is tested again under identical conditions. As shown in Fig.
9, the FAL conversion and product selectivity remained almost the
same after five recycles, indicating the good stability of the catalyst.
The TEM image and XRD patterns (Fig. S7, ESI†) can provide clues
concerning the good stability of Ni/C-500 catalyst. As it displaying,
the TEM images and XRD patterns of the fresh and spent catalyst
are similar with each other. Generally, the superior stability of Ni/C-
500catalyst possibly stems from the embedded structure metallic Ni
NPs are anchored in the carbon matrix to prevent it migration and
agglomeration into large particle. The substrate carbon is
responsible as support, stabilizer and dispersant of metallic Ni NPs
to improve its catalytic properties.
Conclusion
In summary, we proposed a simple method for synthesis of Ni-
MOFs in an oil-bath at atmospheric pressure and a shorter synthesis
time (150min). A Ni/C nanocatalyst can be structurally fabricated by
directly pyrolysis of the Ni-MOFs, which are employed as the
precursor of active metal (Ni) and support (carbon) simultaneously.
The component, NPs size and dispersion of the obtained Ni/C
nanocatalyst are subject to pyrolysis condition and Ni-MOFs
precursor, as well as the BET surface area, which will further
influence the activity of the catalyst for transformation of FAL to
THFOL. The Ni/C-500 nanocatalyst, delivered a complete conversion
of FAL to THFOL at the condition of 120 °C, 1MPa H2 pressure and
120 min. The superior catalytic performance of the Ni/C-500
nanocatalyst can be attributed to the Ni NPs embedded in the
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7DT00628D
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ARTICLE
ligand-derived carbon substrates resulting in uniform and small size, 19. R. Radhakrishnan, K. Kannan, S. Kumaravel and S.
as well as the highest BET surface area combined with other Thiripuranthagan, RSC Adv., 2016, 6, 45907-45922.
temperature. The finding based on the embedded metal catalyst 20. C. Wang, L. Wang, J. Zhang, H. Wang, J. P. Lewis and F. S. Xiao, J
derived from MOFs precursor and its application may also be Am Chem Soc, 2016, 138, 7880-7883.
expanded to develop other heterogeneous catalyst for efficiently 21. Z. Dong, X. Lai, J. E. Halpert, N. Yang, L. Yi, J. Zhai, D. Wang, Z.
catalytic transformation of biomass.
Tang and L. Jiang, Advanced Materials, 2012, 24, 1046-1049.
22. J. Chen, D. Wang, J. Qi, G. Li, F. Zheng, S. Li, H. Zhao and Z. Tang,
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Acknowledgements
23. M. Zhao, K. Yuan, Y. Wang, G. Li, J. Guo, L. Gu, W. Hu, H. Zhao
and Z. Tang, Nature, 2016, 539, 76-80.
This work was financially supported by the Natural Science
Foundation of China (Grant No. 51502297, 51372248, and
51432009); Instrument Developing Project of the Chinese Academy
of Sciences (Grant No.yz201421); the CAS/SAFEA International
Partnership Program for Creative Research Teams of Chinese
Academy of Sciences, China; the CAS Pioneer Hundred Talents
Program.
24. Y. Wang, S. Sang, W. Zhu, L. Gao and G. Xiao, Chemical
Engineering Journal, 2016, 299, 104-111.
25. A. Primo, I. Esteve-Adell, J. F. Blandez, A. Dhakshinamoorthy, M.
Alvaro, N. Candu, S. M. Coman, V. I. Parvulescu and H. Garcia, Nat
Commun, 2015, 6, 8561.
26. S. Zhao, Y. Wang, J. Dong, C.-T. He, H. Yin, P. An, K. Zhao, X.
Zhang, C. Gao, L. Zhang, J. Lv, J. Wang, J. Zhang, A. M. Khattak, N. A.
Khan, Z. Wei, J. Zhang, S. Liu, H. Zhao and Z. Tang, Nature Energy,
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Table of Contents Entry
Ni/C catalyst prepared by pyrolysis of Ni-MOFs, synthesized with a facile oil-bath method,
showing high activity of hydrogenation of FAL.