A highly dispersed and stable Ni/mSiO2-AE nanocatalyst for benzoic acid hydrogenation

Article abstract of DOI:10.1039/c7cy02195j

A Ni/mSiO₂-AE nanocatalyst was successfully prepared via loading the active nickel species on mSiO₂ by an ammonia evaporation (AE) method. It exhibited excellent catalytic performance in the selective hydrogenation of benzoic acid with the conversion of benzoic acid and selectivity to cyclohexane carboxylic acid being 98.9% and 99.1%, respectively. Furthermore, the catalyst can be recycled four times without appreciable loss of its initial activity. As demonstrated by TEM, the active nickel species was highly dispersed with an average particle size of 3.2 nm in this nanocatalyst, which is much smaller than that of Ni/mSiO₂-IMP (～18 nm), prepared by a conventional impregnation method. TPR and XPS results revealed the existence of a stronger interaction between the active nickel species and the mSiO₂ support in Ni/mSiO₂-AE, compared to Ni/mSiO₂-IMP. This strong metal-support interaction in Ni/mSiO₂-AE can effectively suppress the loss of the active nickel species during the reaction, resulting in its good stability under relatively harsh reaction conditions.

Full text of DOI:10.1039/c7cy02195j

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A highly dispersed and stable Ni/mSiO2-AE
nanocatalyst for benzoic acid hydrogenation†
Cite this: DOI: 10.1039/c7cy02195j
Huiling Zhang,‡a Xuejia Gao,‡a Yuanyuan Ma,a Xue Han,ab
a
Libo Niu a and Guoyi Bai
*
*
A Ni/mSiO2-AE nanocatalyst was successfully prepared via loading the active nickel species on mSiO2 by
an ammonia evaporation (AE) method. It exhibited excellent catalytic performance in the selective hydro-
genation of benzoic acid with the conversion of benzoic acid and selectivity to cyclohexane carboxylic acid
being 98.9% and 99.1%, respectively. Furthermore, the catalyst can be recycled four times without appre-
ciable loss of its initial activity. As demonstrated by TEM, the active nickel species was highly dispersed with
an average particle size of 3.2 nm in this nanocatalyst, which is much smaller than that of Ni/mSiO2-IMP
(∼18 nm), prepared by a conventional impregnation method. TPR and XPS results revealed the existence of
a stronger interaction between the active nickel species and the mSiO2 support in Ni/mSiO2-AE, compared
to Ni/mSiO2-IMP. This strong metal–support interaction in Ni/mSiO2-AE can effectively suppress the loss of
the active nickel species during the reaction, resulting in its good stability under relatively harsh reaction
conditions.
Received 26th October 2017,
Accepted 31st October 2017
DOI: 10.1039/c7cy02195j
rsc.li/catalysis
is essential to develop alternative catalysts based on much
cheaper metals such as Ni, Fe and Cu for the hydrogenation
Introduction
Selective hydrogenation of benzoic acid (BA) is arguably the
most efficient method for the synthesis of cyclohexane car-
boxylic acid (CCA),1 which is an important pharmaceutical
intermediate for the synthesis of ansatrienin and
praziquantel.2,3 However, aromatic ring hydrogenation needs
to overcome higher resonance energy compared with other
unsaturated functional groups (such as CC, CO, CN,
etc.)4,5 and generally requires relatively harsh reaction condi-
tions.6 Presently, most of the catalysts used in this transfor-
mation rely on noble metals, such as Ir/γ-Al2O3,7 Pd/AC,8 Pd/
CN,9 PdxRuy,10 Rh/C,11 RuPt,12 and RuPd/CN,13 mainly due to
the less active nature of the aromatic ring. Among these, Pd/
CN9 exhibits excellent catalytic performance with both the
conversion of BA and the selectivity for CCA being 100%. Fur-
thermore, it can be reused four times without appreciable
loss of its initial activity. Nevertheless, the high price of noble
metals has limited their industrial applications. Therefore, it
of BA.
In the past decades, nickel-based catalysts have been
widely applied in the field of hydrogenation, dehydrogenation
and steam reforming due to their good activity and low cost.14
However, the active nickel species in unsupported catalysts
usually tend to aggregate, mainly due to their high surface en-
ergies and small sizes, resulting in poor stability.15 Thus, de-
positing the active nickel on certain supports with large sur-
face areas and channels, including SBA-15, MCM-41 and
mSiO2, is regarded as an efficient method to improve their ac-
tivity and stability. mSiO2 has been widely used as a catalyst
support due to its unique structural features, such as a large
surface area, a suitable channel structure, and low cost.16–18
For example, Williams et al.19 prepared a novel nano-Ni/SiO2
catalyst with good Ni dispersion and high BET surface areas
by a simple sol–gel method, which exhibited high stability in
the steam reforming of ethanol. However, there are few re-
ports on the preparation of Ni/SiO2 catalysts and their applica-
tions in liquid hydrogenation.
Recently, we have prepared a NiZrB amorphous alloy
supported on a γ-Al2O3 catalyst (NiZrB/γ-Al2O3), which can
achieve high selectivity for CCA from BA simply by adding
small amounts of bases in cyclohexane.20 However, its stabil-
ity was still poor, although it was a little higher than that of
the unsupported one,21 mainly due to aggregation and loss of
the active nickel under the harsh reaction conditions. In an
attempt to develop an active and stable nickel-based catalyst
a Key Laboratory of Chemical Biology of Hebei Province, College of Chemistry and
Environmental Science, Hebei University, Baoding 071002, P.R. China.
E-mail: libo_niu@126.com, baiguoyi@hotmail.com; Fax: +86 312 5937102;
Tel: +86 312 5079359
b Hebei Geological and Mineral Central Laboratory, Baoding 071051, People's
Republic of China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7cy02195j
‡ These two authors contributed equally to this work.
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for BA hydrogenation, we have investigated the effects of sup-
ports and preparation methods on the stability of such cata-
lysts. Herein, we report that a Ni/mSiO2-AE nanocatalyst, pre-
pared by an ammonia evaporation (AE) method, exhibited
high catalytic activity and stability in this transformation, su-
perior to the one prepared by an impregnation method. The
high dispersion of the active nickel species on the surface of
mSiO2 and the formation of strong interactions between
nickel and mSiO2 during the preparation process, which can
inhibit the agglomeration and loss of the active nickel spe-
cies, are demonstrated to be the main reasons for the good
catalytic performance of this catalyst.
dispersing uniformly, 25% aqueous ammonia was added
dropwise to adjust the pH value of the suspension to 11–12.
After that, the suspension was stirred at 40 °C for 4 h. Then,
the temperature of the bath was raised to 80 °C to evaporate
ammonia and deposit the nickel species on mSiO2. When the
pH value of the suspension decreased to 6–7, the evaporation
process was stopped. The resulting suspension was filtered
and the solid washed several times with deionized water and
absolute ethanol, and then dried at 80 °C. After the solid was
calcined at 600 °C for 4 h, the resulting metal oxide was re-
duced under a 1 MPa, 99.99% H2 flow (50 mL min−1) at 600
°C for 3 h in a microreactor. The catalyst so obtained was de-
noted as Ni/mSiO2-AE.
Experimental
Materials
Catalyst characterization
The nitrogen adsorption and desorption isotherms were mea-
sured at −196 °C using a Micromeritics Tristar II 3020 surface
area and pore analyzer. Prior to the measurement, all the sam-
ples were dried at 200 °C for 4 h in a vacuum oven. The specific
surface areas were determined according to the Brunauer–
Emmett–Teller (BET) method. The average pore diameters and
pore volumes were calculated by the Barrett–Joyner–Halenda
(BJH) method. The pore size distributions were calculated
using the desorption regimes of the isotherms.
The actual Ni loading on the catalysts were evaluated
using ICP–OES (Varian Vista-MPX) on an Agilent 7700 ×
spectrometer.
X-ray diffraction (XRD) patterns were recorded on a Bruker
D8-Advance X-ray diffractometer equipped with a Cu Kα radi-
ation source at 40 kV and 40 mA with a step size of 0.02° over
the range 10–80° at a scanning rate of 0.1 s per step.
Transmission electron microscopy (TEM) was carried out
using a JEM-2100F microscope operating at an accelerating
voltage of 200 kV. The samples were prepared by dispersing
the catalyst powder in ethanol under ultrasound and then
supported on a carbon film of copper grid.
X-ray photoelectron spectroscopy (XPS) analysis was car-
ried out using a PHI 1600 spectrometer using a Mg Kα X-ray
source for excitation. The binding energies were calibrated
based on the adventitious C1s peak at 284.8 eV.
H2-chemisorption was performed using a Micromeritics
Autochem II 2920 instrument to measure the active nickel
surface areas and metal dispersion. Prior to the tests, all the
samples were reduced under a 1.0 MPa, 99.99% H2 flow (50
mL min−1) at 600 °C for 3 h. Then chemisorption was carried
out by the pulsing of a mixture of 10% H2/Ar (50 mL min−1).
The nickel surface areas and metal dispersion were calcu-
lated from the volume of H2 adsorbed, assuming the stoi-
chiometric ratio Hadsorbed/Nisurface = 1 and a density of active
sites on the surface of 1.54 × 1019 atoms per m2.23
A temperature-programmed reduction of hydrogen (H2-TPR)
test was also performed using the Micromeritics Autochem II
2920 instrument. In a typical experiment, the sample (100 mg)
was first pretreated at 200 °C for 1 h in an Ar flow (50 mL
min−1) to remove moisture and impurities. After cooling to 50
°C, a 10% H2/Ar (50 mL min−1) gas mixture was introduced.
Unless otherwise stated, all reagents are of analytical grade.
Benzoic acid and nanosilica were purchased from Aladdin
Reagent (Shanghai) Co., Ltd. Tetraethyl orthosilicate (TEOS)
and cyclohexane were purchased from Tianjin Kemiou Chem-
ical Reagent Factory. Nickel nitrate hexahydrate was pur-
chased from Sinopharm Group Chemical Reagent Co., Ltd.
Ammonia and anhydrous ethanol were purchased from Tian-
jin East China Reagent Factory. Ultrapure Ar (99.999%) and
10 vol% H2/Ar were purchased from Baoding Zhuoda Gas
Co., Ltd.
Material preparation
Preparation of mSiO2. Mesoporous silica was obtained by
the hydrolysis of TEOS with the aid of dodecylamine.22
Firstly, 1.0 g dodecylamine was dissolved in a mixed solvent
(20 mL ethanol and 50 mL H2O) under mild stirring; then 4
mL of TEOS was added dropwise, followed by stirring at
room temperature for 8 h. The resulting white precipitate
was filtered and washed several times using deionized water
and dried at 100 °C. Finally, the precipitate was calcined by
slowly increasing the temperature from room temperature to
600 °C with a ramping rate of 1 °C min−1 and then kept at
600 °C for 4 h.
Preparation of Ni/mSiO2-IMP and Ni/SiO2-IMP. Ni/mSiO2-
IMP was prepared by an incipient wetness impregnation
method. The above obtained mSiO2 (0.5 g) support was
added to an aqueous solution containing 0.62 g of NiIJNO3)2
·6H2O under ultrasound for 20 min. The resulting paste was
dried at 80 °C for 4 h and then calcined at 600 °C for 4 h to
obtain a supported metal oxide. It was then reduced under a
1 MPa, 99.99% H2 flow (50 mL min−1) at 600 °C (the selected
temperature based on the experimental and characterization
results: Fig. S1, S2 and Table S1†) for 3 h in a microreactor.
The catalyst so obtained was denoted as Ni/mSiO2-IMP. Fur-
thermore, Ni/SiO2-IMP was prepared by the same method as
described above using nano-silica instead of mSiO2.
Preparation of Ni/mSiO2-AE. Ni/mSiO2-AE was prepared by
an AE method using mSiO2 as the support. Firstly, 0.62 g of
NiIJNO3)2·6H2O was dissolved in 21 mL of deionized water un-
der stirring, followed by the addition of 0.5 g of mSiO2. After
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The catalyst was heated to 900 °C at a rate of 10 °C min−1 and
the TCD signal was recorded continuously.
Catalyst activity tests
Selective hydrogenation of BA was performed as follows; BA
(0.5 g), the catalyst (0.1 g) and cyclohexane (40 mL) were se-
quentially added into a 100 mL Parr 4848 stainless steel auto-
clave equipped with a mechanical stirrer and an electric
heating system. The autoclave was first filled with H2 three
times, followed by evacuation, to exclude residual air and
then pressurized with H2 to 5.0 MPa. After it was heated to
150 °C, the reaction was started by stirring the mixture and
allowed to proceed for 3 h. After the reaction was complete,
the mixture was cooled to room temperature and separated
by filtration. The solution was analyzed by Agilent 7820A gas
chromatography (GC) and the product structures were identi-
fied by gas chromatography-mass spectrometry (GC-MS)
using an Agilent 5975C spectrometer.
Results and discussion
Catalyst selection
Firstly, a series of supported nickel-based catalysts were pre-
pared using an impregnation method and evaluated in the hy-
drogenation of BA to investigate the effect of the support. As
shown in Table 1, a Ni/SiO2-IMP catalyst with nanosilica (SiO2)
as the support exhibited low catalytic activity with the conver-
sion of BA being only 60.1%. The conversion and selectivity
only slightly increased using MgO as the support. In contrast,
the conversion of BA reached 79.8% over a Ni/Al2O3-IMP cata-
lyst, but the selectivity was reduced. According to our previous
research results,20 we speculated that the decrease of the selec-
tivity for CCA can be ascribed to the acidic sites on the surface
of the Al2O3 support. Notably, the conversion of BA increased
to 95.2% and the selectivity for CCA reached 97.8% over a Ni/
mSiO2-IMP catalyst, which is much better than other
supported catalysts. Considering that good stability is an im-
portant feature for an industrial catalyst, the preliminary sta-
bility of Ni/mSiO2-IMP was subsequently investigated (Fig. 1).
It was found that the activity of Ni/mSiO2-IMP decreased rap-
idly during recycling and the conversion of BA was reduced to
74.0% after only three cycles. Then, to improve the stability of
this catalyst, different methods were applied for the prepara-
tion of Ni/mSiO2 and the catalyst prepared by the AE method
was found to exhibit good catalytic performance in the hydro-
Fig. 1 Stabilities of (a) Ni/mSiO2-IMP and (b) Ni/mSiO2-AE.
genation of BA. The conversion of BA and the selectivity for
CCA reached 98.9% and 99.1%, respectively, in the first run
over Ni/mSiO2-AE, and significantly, both values remained
above 95% after recycling the catalyst four times (Fig. 1b),
probably due to the strong interactions between nickel and
mSiO2 coming from the ammonia evaporation process.18,19
Notably, Ni/mSiO2-AE also showed comparable activity with
most noble metal catalysts (Table S2†). Thus, Ni/mSiO2-AE was
selected as the catalyst of choice for further studies.
Characterization of catalysts
The physicochemical properties of different SiO2-supported
catalysts are summarized in Table 2. As can be seen, Ni/
mSiO2-IMP exhibits the largest Ni loading and specific sur-
face area (745 m2 g−1) among the catalysts studied, mainly
due to the large surface area of the mSiO2 support and the ef-
fect of the preparation method. The pore sizes of the catalysts
are all in the range of 2–50 nm, indicating their mesoporous
structure. Notably, Ni/mSiO2-AE has the largest H2-
chemisorption value, metal dispersion and metallic surface
area, although it shows a relatively smaller surface area and
pore volume than Ni/mSiO2-IMP. These results indicate that
the AE method is more conducive to the dispersion of the ac-
tive nickel species than the IMP method. In addition, the de-
crease of its surface area may result from the partial etching
of silica by ammonia during preparation.24
Table 1 Catalytic performance of nickel-based catalysts over different
supports
Catalyst
Conversion (%)
Selectivity (%)
Ni/SiO2-IMP
Ni/MgO-IMP
Ni/Al2O3-IMP
Ni/mSiO2-IMP
60.1
60.5
79.8
95.2
94.8
99.1
91.6
97.8
Reaction conditions: BA (0.5 g), catalyst (0.1 g), cyclohexane (40 mL),
T = 150 °C, initial PIJH2) = 5 MPa, reaction time = 3 h.
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Table 2 Physicochemical properties of different SiO2-supported catalysts
Ni loadinga
(wt%)
Surface area
Pore size
(nm)
Pore volume
H2-chemisorption
Metal dispersion
(%)
Metallic surface
Catalyst
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
area (m2 g−1
)
Ni/SiO2-IMP
Ni/mSiO2-IMP
Ni/mSiO2-AE
20.2
23.0
18.0
244
745
506
10.8
2.4
5.1
0.66
0.44
0.65
0.74
0.87
1.1
1.94
2.27
2.98
2.59
3.02
3.97
a
Based on ICP results.
The nitrogen adsorption–desorption isotherms and pore
size distributions of mSiO2, Ni/mSiO2-IMP and Ni/mSiO2-AE
are shown in Fig. 2. It should be noted that all the materials
exhibit typical type IV isotherms with H3-type hysteresis
loops (P/P0 > 0.4), demonstrating the formation of meso-
pores.25 Notably, Ni/mSiO2-AE has a much larger pore size
than the mSiO2 support and Ni/mSiO2-IMP, probably due to
the etching effect of ammonia on the mSiO2 support during
the preparation process. The increase of the pore size can
promote the mass transfer process in catalytic reactions.
The XRD patterns of the calcined and reduced Ni/SiO2-
IMP, Ni/mSiO2-IMP and Ni/mSiO2-AE catalysts are shown in
Fig. 3. As can be seen, all samples exhibit a broad diffraction
peak at about 2θ = 23°, which can be attributed to the amor-
phous silica.26 In the calcined Ni/SiO2-IMP and Ni/mSiO2-
IMP samples (Fig. 3A), the diffraction peaks at about 37.3°,
43.3°, 62.9°, 75.4° and 79.4° can be assigned to the character-
istic peaks of NiO.27 In contrast, the NiO peaks are very weak
in the Ni/mSiO2-AE sample, implying the existence of small
NiO particles. Similar results were also observed in the XRD
patterns of the reduced samples (Fig. 3B). Ni/SiO2-IMP and
Ni/mSiO2-IMP exhibit sharp characteristic peaks of Ni at
about 44.5°, 51.8° and 76.4°, together with the disappearance
of the peaks of NiO, demonstrating the full reduction of
these samples.28,29 However, the diffraction peaks related to
Ni are much weaker in Ni/mSiO2-AE, indicating the small size
of Ni in this sample.
spectra of Ni/mSiO2-IMP and Ni/mSiO2-AE present two states
of nickel. Generally, the binding energies of Ni2p3/2 in
The TEM images of Ni/SiO2-IMP, Ni/mSiO2-IMP and Ni/
mSiO2-AE are shown in Fig. 4. It is obvious that Ni/SiO2-IMP
has a large nickel particle size of about 18 nm. The particle
size of nickel in Ni/mSiO2-IMP slightly decreases to 14 nm
but with a broad particle distribution and poor dispersibility.
In contrast, the nickel particles are well dispersed in the
range of 2.4–4.0 nm in Ni/mSiO2-AE. The mean size of nickel
nanoparticles is about 3.2 nm, which is much smaller than
that of Ni/SiO2-IMP and Ni/mSiO2-IMP. The lattice fringes
can be clearly observed from the HRTEM image of this sam-
ple (Fig. 4d), with a crystal plane spacing of 0.201 nm, which
is consistent with the 111 crystal plane in cubic (fcc) Ni
(JCPDS 87-0712).30 The above observations are in good accor-
dance with the XRD results. The presence of small and highly
dispersed nickel species in Ni/mSiO2-AE is suggested to ac-
count for its high catalytic activity.
To further explore the surface chemical states of nickel
and the interaction between nickel and the mSiO2 support,
the reduced Ni/mSiO2-IMP and Ni/mSiO2-AE catalysts were
subjected to XPS analyses (Fig. 5). As can be seen, the Ni2p3/2
Fig. 2 N2 adsorption–desorption isotherms and pore size distributions
(inset) of (a) mSiO2, (b) Ni/mSiO2-IMP, and (c) Ni/mSiO2-AE.
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Fig. 5 XPS spectra of Ni 2p3/2 in (a) Ni/mSiO2-IMP and (b) Ni/mSiO2-
AE.
and 852.57 eV can be attributed to the metallic Ni,33 and the
peaks at 855.79 and 856.07 eV as well as the broad satellites
centred at 861.29 and 861.32 eV indicate the presence of NiO
in these samples.34,35 The NiO formation may be caused by
the oxidation of metallic Ni during the sample transfer and
settlement before the XPS analysis.17,36 Notably, the binding
energies of metallic Ni (852.57 eV) and NiO (856.07 eV) in Ni/
mSiO2-AE are both higher than those in Ni/mSiO2-IMP
(852.26 and 855.79 for Ni and NiO, respectively). These re-
sults indicate the existence of a strong interaction between
nickel and mSiO2 in Ni/mSiO2-AE,17,37 which might be the
main reason for its good catalytic stability under relatively
harsh reaction conditions.16
Fig. 3 XRD patterns of calcined (A) and reduced (B) catalysts (a. Ni/
SiO2-IMP, b. Ni/mSiO2-IMP, c. Ni/mSiO2-AE).
metallic Ni and NiO are around 852 and 855 eV, respec-
tively.31,32 Based on this phenomenon, the peaks at 852.26
The TPR profiles of the Ni/SiO2-IMP, Ni/mSiO2-IMP and
Ni/mSiO2-AE catalysts are shown in Fig. 6. According to previ-
ous studies, the reduction peaks at different temperatures
can be attributed to the reduction of different nickel spe-
cies.38 Generally, a reduction peak at a low temperature
(<400 °C) is considered to be due to the reduction of bulk
NiO or free NiO,39,40 whereas a reduction peak at a higher
Fig. 4 TEM images of (a) Ni/SiO2-IMP, (b) Ni/mSiO2-IMP, and (c) Ni/
Fig. 6 H2-TPR profiles of (a) Ni/SiO2-IMP, (b) Ni/mSiO2-IMP and (c) Ni/
mSiO2-AE and (d) HRTEM image of Ni/mSiO2-AE.
mSiO2-AE.
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temperature is mainly ascribed to the reduction of NiO spe-
cies with interaction between supports.40,41 As can be seen,
Ni/SiO2-IMP has three reduction peaks at 233, 310, and 481
°C, and Ni/mSiO2-IMP exhibits four reduction peaks at 263,
310, 481, and 584 °C, respectively. The peak at 481 °C is as-
cribed to the reduction of the larger crystallites of NiO, which
interacts weakly with the support.42 The high temperature
peak at 584 °C is attributed to the reduction of NiO that in-
teracts relatively strongly with the mSiO2 support.43 These re-
sults indicate that there should be a stronger interaction be-
tween NiO and mSiO2. In contrast, Ni/mSiO2-AE exhibits only
one large reduction peak at 703 °C, which can be attributed
to the reduction of highly dispersed NiO species with strong
interaction with the mSiO2 support.44 This observation was
consistent with the XPS results. Thus, the preparation
methods are believed to play an important role in the struc-
ture and catalytic performance of these nickel-based cata-
lysts. Compared with the traditional impregnation method,
the ammonia evaporation method could endow the Ni/
mSiO2-AE catalyst highly-dispersed nickel with strong interac-
tion between the mSiO2 support. This can not only disperse
the active nickel species but also inhibit their aggregation
and loss during the reaction, resulting in the good activity
and stability of this catalyst.
(B2014201024 and B2016201167) and the hundred outstand-
ing innovative personnel support plan of Hebei Universities
(SLRC2017020) is gratefully acknowledged.
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Conclusions
In summary, a highly dispersed Ni/mSiO2-AE nanocatalyst
was prepared by an AE method, which showed good cata-
lytic activity and stability in the selective hydrogenation of
BA to CCA under relatively harsh reaction conditions. The
AE method can not only benefit the dispersion of the ac-
tive nickel species on the surface of the mSiO2 support but
also induce the formation of a strong metal–support inter-
action between nickel and mSiO2. As demonstrated by
TEM, the nickel particles are well-dispersed in Ni/mSiO2-AE
with a mean particle size of 3.2 nm, which is beneficial for
its high activity. The strong metal–support interactions be-
tween nickel and mSiO2 in Ni/mSiO2-AE markedly inhibit
the aggregation and loss of the active nickel species during
the reaction, accounting for its good stability. Owing to the
simple preparation process and excellent catalytic perfor-
mance of this catalyst, it could provide a potential method
for the fabrication of supported catalysts for industrial
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Conflicts of interest
There are no conflicts to declare.
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The authors thank Professor David W. Knight (Cardiff Univer-
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Natural Science Foundation of China (21376060 and
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