Selective hydrogenation of benzoic acid to cyclohexane carboxylic acid over microwave-activated Ni/carbon catalysts

Article abstract of DOI:10.1016/j.mcat.2017.10.015

High yields of cyclohexane carboxylic acids were obtained by direct hydrogenation of aromatic carboxylic acids over different Ni/carbon catalysts having distinctive surface properties. The catalysts were characterized by SEM, TEM, H₂-TPR and N₂ adsorption isotherms for the determination of BET surface area and porosity. The hydrogenation reaction was carried out in batch pressure reactor in gas-liquid phase at 200 °C. High selectivity (100%) of cyclohexane carboxylic acids at 86.2 mol% conversion of benzoic acid was achieved over microwave-activated biochar supported non-precious metal Ni catalyst. The 10%Ni/CSC-b catalyst has been investigated for hydrogenation of benzoic acid to cyclohexane carboxylic acids and shown little deactivation in stability test. The effects of Ni loading, high dispersion of Ni species, appropriate power of microwave heating and strong interaction of Ni species with carbon are of benefit to the reaction.

Full text of DOI:10.1016/j.mcat.2017.10.015

Molecular Catalysis 444 (2018) 53–61
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Research paper
Selective hydrogenation of benzoic acid to cyclohexane carboxylic
acid over microwave-activated Ni/carbon catalysts
X.H. Lua,b,c, Y. Shena,b,c, J. Hea,c, R. Jinga,c, P.P. Taoa,c, A. Hua,c, R.F. Niea,c, D. Zhoua,c
Q.H. Xia
,
a,c,∗
a
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan 430062, PR China
Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang 438000, PR China
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2017
Received in revised form 2 October 2017
Accepted 10 October 2017
High yields of cyclohexane carboxylic acids were obtained by direct hydrogenation of aromatic car-
boxylic acids over different Ni/carbon catalysts having distinctive surface properties. The catalysts were
characterized by SEM, TEM, H2-TPR and N2 adsorption isotherms for the determination of BET surface
area and porosity. The hydrogenation reaction was carried out in batch pressure reactor in gas-liquid
phase at 200 C. High selectivity (100%) of cyclohexane carboxylic acids at 86.2 mol% conversion of ben-
zoic acid was achieved over microwave-activated biochar supported non-precious metal Ni catalyst. The
◦
Keywords:
Selective hydrogenation
Benzoic acid
Cyclohexane carboxylic acids
Ni/carbon catalysts
1
0%Ni/CSC-b catalyst has been investigated for hydrogenation of benzoic acid to cyclohexane carboxylic
acids and shown little deactivation in stability test. The effects of Ni loading, high dispersion of Ni species,
appropriate power of microwave heating and strong interaction of Ni species with carbon are of benefit
to the reaction.
©
2017 Published by Elsevier B.V.
1
. Introduction
nanowire [25–27], have been widely applied in this transformation,
and all exhibit high activities. Due to stability and can be used mul-
tiple times, heterogeneous catalysts are the preferred options for
most processes in chemical industry [28,29]. In principle, in the case
of the supported catalysts, two basic but important issues should be
taken into account in order to design and screen suitable supports
for catalytic applications. One of which is that the support must
show high stability and high surface area; the other is that the sup-
port should contain high density of defect sites. There are a variety
of materials that suitable as supports [30,31], among which carbons
(including activated carbon [32], graphene [33] and carbon nano-
tubes [34]) are the most frequently used. As carbon materials are
mainly composed of carbon and even can be made directly out of
biomass, they are obviously “sustainable” support materials for Pd
nanoparticles (NPs). However, the agglomeration and leaching of
noble metal nanoparticles (such as Pd, Pt, Au) in the catalysts cannot
be effectively avoided by simply treating the support using reagents
such as nitric acid and hydrogen peroxide due to the weak interac-
tion of surface oxygen groups [35], and also the high price of noble
metal catalysts has limited their further application in industry.
Cyclohexanecarboxylic acid is an important organic synthesis
intermediate, which itself is a good light curing agent, but also
for the synthesis of the drug antiemulsion 392 and praziquantel
Catalytic hydrogenation is an ubiquitous reaction and is used for
instance in the synthesis of fine chemicals and fuel upgrading [1–5].
Increasing sustainability demands are now appealed for chemical
production to improve catalytic selectivity, to minimize byproduct
formation, to circumvent separation/purification operations and to
avoid costly clean-up and disposal. Benzoic acid is an inexpensive
feedstock [1] that can be hydrogenated to benzaldehyde [6–10],
benzyl alcohol [11–13] or cyclohexane carboxylic acid [14–17] as
important commercial products. The chemoselective hydrogena-
tion of benzoic acids (BA) to cyclohexane carboxylic acid (CCA) is
the most important reaction in the synthesis of caprolactam from
toluene, and CCA is also an important organic intermediate for the
synthesis of pharmaceutical industries for example praziquantel
and ansatrienin [18–20]. So chemoselective hydrogenation of BA
to CCA has attracted great interest all over the world [21–24].
The catalytic hydrogenation of carboxylic acids is performed
industrially by using heterogeneous catalysts, such as noble
metal catalysts, including Pd/C, Rh/C, PdxRuy, RuPt and platinum
∗ Corresponding author at: Hubei Collaborative Innovation Center for Advanced
Organic Chemical Materials, Hubei University, Wuhan 430062, PR China.
E-mail addresses: xia1965@yahoo.com, xiaqh518@aliyun.com (Q.H. Xia).
(treatment of schistosomiasis), its derivatives such as cyclohexyl-
http://dx.doi.org/10.1016/j.mcat.2017.10.015
2
468-8231/© 2017 Published by Elsevier B.V.

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X.H. Lu et al. / Molecular Catalysis 444 (2018) 53–61
methyl carbamate and trans-4-isopropylcyclohexyl acid are the
important intermediates for the synthesis of many chemical prod-
ucts and pharmaceutical. Our previous work reported the selective
hydrogenation of aromatic carboxylic acids over basic N-doped
mesoporous carbon supported palladium catalysts [14], these N-
doping materials showed high catalytic activity for the selective
hydrogenation of aromatic ring. The present paper reports a simple
microwave-heating preparation method for biocarbon supported
Ni NPs (nanoparticles) catalyst and its application in the ring hydro-
genation of benzoic acid under mild conditions. The influence of
microwave heating power on the dispersion and morphology of Ni
and the benzoic acid hydrogenation is discussed.
supported catalysts were also applied in the hydrogenation of ben-
zoic acid for comparison. In a typical process, 4 mmol benzoic acid
and a certain amount of catalyst were put into a 100 mL stainless
autoclave, and 10 mL solvent was employed as green solvent. The
hydrogenation of benzoic acid was carried out at desired tempera-
ture with magnetic stirring at a speed of 1000 rpm. After reaction,
the filtrate was analyzed by GC-FID with a 30 m capillary column
(Rtx@-5). The catalyst was recovered by centrifugation and then
washed several times with THF. The separated catalyst was washed
several times with THF and dried overnight in an vacuum oven at
◦
80 C for use in the next run.
2
. Experimental
3. Results and discussion
2.1. Preparation of catalysts
3.1. Structural characterizations
All chemicals were purchased from were purchased from Shang-
Fig. S1 presents the XRD patterns of the samples with dif-
◦
hai Aladdin Chemical Co.The 10%Ni/CSC-a catalyst was prepared
through conventional impregnation method, as described in our
previously published articles [36].
ferent Ni contents. The characteristic peaks of CSC (2␪ = 23.3 )
◦
and Ni metal (2␪ = 45.3, 51.5 and 76.2 ) are observed for all the
Ni/carbon catalysts with the carrier is CSC, AC and G, respectively, in
◦
◦
◦
An aqueous solution of Ni(NO ) ·6H O was impregnated onto
which three peaks at 45.3 , 51.5 and 76.2 are attributed to [111],
[200] and [220] diffraction peaks of Ni. The micropore structure
of the 10%Ni/CSC catalysts were identified by nitrogen adsorp-
tion/desorption isotherms (Fig. 1). The results clearly show BET
3
2
2
various carbon supports like coconut shell charcoal (CSC), activated
carbon (AC) and graphite (G). Typically, 1.0 g carbon was dispersed
in 50 mL deionized water consisted of Ni(NO ) ·6H O (0.493 g).
3
2
2
◦
2
2
Then, the resulting mixture was heated to 90 C while stirring and
heating in microwave reactor. Afterwards, the composite catalyst
precursors were obtained by evaporating the water under rotary
surface area of 10%Ni/CSC-a is 599.6 m /g, smaller than 678.2 m /g
of 10%Ni/CSC-b (prepared by microwave-heating). The support CSC
still retained its microporsity well after the loading of Ni nanoparti-
cle. Table S1 presents the BET surface areas and total pore volumes
of various catalysts determined by the BET analysis. Very clearly,
the support G and the catalyst 10%Ni/G-b have the smallest BET
surface areas and the pore volumes. Both values of the catalyst
10%Ni/AC-b (activated carbon as the support) are the largest.
Usually, SEM is used to determine the particle size and parti-
cle morphology of the synthesized sample. The SEM backscattered
composition images of the monometallic Ni/carbon samples are
given in Fig. 2, inclusive of 10%Ni/CSC-a, 10%Ni/CSC-b, 10%Ni/AC-
b and 10%Ni/G-b. Apparently, SEM images of four samples exhibit
irregular spherical morphologies, and it is clear that most of the
catalyst particles are in good dispersion. TEM images of metal parti-
cle size distribution of 10%Ni/CSC-a, 10%Ni/CSC-b, 10%Ni/AC-b and
10%Ni/G-b are shown in Fig. 3. For 10%Ni/CSC-a, a wide particle size
distribution is observed and the metal particle size is varied from
6.3 to 39.9 nm with an average value of 13.2 nm (Table 1). The par-
ticle sizes of Ni metal supported on AC and G are determined to be
17.7 and 12.7 nm, respectively. For 10%Ni/CSC-b, highly dispersed
Ni nanoparticles are found to evolve and distribute throughout the
CSC-b. The mean diameter is estimated to be 9.4 nm. This demon-
strates that the microwave-heating plays an important role in the
dispersion of metallic species over the CSC support.
◦
◦
evaporator at 70 C and dried in a vacuum oven at 120 C for 8 h,
◦
◦
followed by reduction with H at 400 C (with a rate of 5 C/min H )
2
2
for 3 h in a tubular reactor. The resultant catalyst was designated as
0%Ni/CSC-b, 10%Ni/AC-b, 10%Ni/G-b. Other two different samples
with the same nominal loading of 10 wt% (weight percent) were
1
prepared by using SiO2 and Al O3 as supports and designated as
2
1
0%Ni/SiO -b and 10%Ni/Al O -b.
2
2
3
2
.2. Characterizations
X-ray diffraction (XRD) measurements were performed on a
Bruker D8A25 diffractometer with Cu K␣ radiation. The tube volt-
age was 30 kV, and the current was 25 mA. The XRD diffraction
◦
patterns were taken in a range of 5−65 2␪. Quantachrome iQ-
MP was used to determine N2 adsorption–desorption properties
of the samples. The specific surface area was calculated by using
the Brunauer−Emmett–Teller (BET) method, and the pore size dis-
tributions were measured by using Barrett−Joyner−Halenda (BJH)
analysis from the desorption branch of the isotherms. The Scan-
ning Electron Microscope (SEM) has been used a JEOL JSM-6510A
to detect the morphology and size of crystals. Transmission elec-
tron microscope (TEM) (JEOL-135 2010F, operating at 200 kV) was
used to investigate morphology and size of the particles. The sam-
ples for TEM were prepared by dispersing the material in ethanol
and drop-drying onto a Formvar resin coated copper grid. The X-
ray photoelectron spectroscopy (XPS) analysis was performed on
a Perkin-Elmer PHI ESCA system with 0.1 eV per step for detail
scan, and the X-ray source was standard Mg anode at 12 kV and
Fig. 4 shows TEM images of 10%Ni/CSC-b, in which particle
size and dispersion on catalysts are affected by microwave-heating
power. When the microwave heating power is 400 W, the mean
particle size of Ni on 10%Ni/CSC-b is 14.7 nm. As the microwave
power increases to 500 W, the 10%Ni/CSC-b shows the presence
of uniformly well-dispersed Ni on CSC, and the mean diameter is
estimated to be 9.4 nm. The mean particle size of Ni is decreased
to 7.9 nm on 10%Ni/CSC-b (600 W). When the catalyst 10%Ni/CSC-b
(700 W) is prepared by 700 W microwave-heating power, the mean
particle size of Ni nanoparticles on the CSC support is reduced to
7.7 nm with a wide distribution. However, the hydrogenation activ-
ity of the catalysts shows that the microwave-heating power of
500 W is optimal for the preparation of the catalyst.
3
00 W. The TPR (temperature programmed H2 reduction) experi-
ments were performed with 10 vol% hydrogen in N (50 mL/min) at
2
◦
a heating rate of 10 C/min, the hydrogen consumption being mea-
sured by a TCD detector. All TPR profiles were normalized for the
same catalyst mass.
2
.3. Catalytic tests
X-ray photoelectron spectroscopy is a convenient method of
studying supported nickel catalysts. The reducibility and interac-
Hydrogenation of benzoic acid and derivatives was carried out
using the as-prepared 10%Ni/CSC-a as the catalyst. Other nickel
tions between metal and support are further studied by H -TPR, as
2

X.H. Lu et al. / Molecular Catalysis 444 (2018) 53–61
55
Fig. 1. Nitrogen sorption isotherms and pore size distribution of 10%Ni/CSC catalysts.
Fig. 2. SEM images of carbon supported Ni catalysts.
Table 1
Physical properties of the supported Ni/carbon materials.
Catalysts
Preparation mode
Particle size (nm)
XRD
Relative atomic percentage (%)
TEM
Ni0
NiO
Ni(OH)2
1
1
0%Ni/CSC-a
0%Ni/CSC-b
No microwave-heating
Microwave-heating
15.5
8.9
13.2
9.4
9.7
12.4
6.9
20.0
83.4
67.6
Ni is mainly in the state of NiO and Ni2+ (Ni(OH) ). These results
shown in Fig. 5. The metal nickel is characterized by a peak at about
2
2+
8
53 eV, and peaks above this energy correspond to Ni [37]. The
show that part of the metallic nickel has been oxidized after the
contact of the sample with air [39]. In the case of 10%Ni/CSC-a and
10%Ni/CSC-b, a minor contribution is recorded at a lower binding
energy of 853 eV, which can be assigned to Ni in the metallic state.
0
peak at 853 eV corresponding to the Ni is observed for the cata-
lysts, as shown in Fig. 5. The main Ni 2p3/2 peaks for all samples
are found to be at 854.2 and 856.0 eV [38], revealing that surface

5
6
X.H. Lu et al. / Molecular Catalysis 444 (2018) 53–61
Fig. 3. TEM images of the supported catalysts and size distribution of Ni particles on x%Ni/carbon.
For example, the percentage of metallic Ni (Table 1) in 10%Ni/CSC-b
is 12.4%, which is higher than that of 10%Ni/CSC-a (9.7%).
TPR is a favorable technique for studying supported nickel cata-
of CCA (entry 1). As compared with 10%Ni/CSC-a, the catalytic
activity of 10%Ni/CSC-b increases to 86.2 mol% conversion of BA
with 100% selectivity of CCA (entry 2). The 10%Ni/AC-b catalyst
shows a poor activity with only 40.8 mol% conversion of BA (entry
3). Other heterogeneous Ni-based catalysts inclusive of 10%Ni/G-
b, 10%Ni/SiO -b and 10%Ni/Al O -b (entries 4–6) achieve a poor
lysts. Fig. 6 shows the H -TPR profiles of the supported Ni catalysts,
2
a “NiO”-like defect phase, produced by oxidation of supported
◦
metal nickel, is reduced in the temperature interval of 250 − 450 C.
2
2
3
Thus, in the TPR profile of 10%Ni/CSC-a three TPR signals at 287,
hydrogenation activity <34 mol% under the same reaction condi-
tions.
◦
3
16 and 336 C are ascribable to the reduction of NiO particles to
0
◦
metal Ni without interaction with the support (at 287 and 316 C),
Nickel loading changes the dispersive state of Ni species across
carbon carrier, which will further affect the reaction network in
steam reforming, since the Ni species is the active site for those
reactions [45,46]. Benzoic acid conversion and product distribu-
tion are presented in Fig. 7. As shown in Fig. 7, nickel loading
significantly affects the catalytic activity of the catalyst. The blank
catalyst shows no hydrogenation activity. The presence of 5 or
7 wt.% nickel loading on CSC-b promotes the catalytic activity, but
the promotion is limited, due to the lack of enough active sites
for hydrogenation. Further increasing the nickel loading to 10 or
15 wt.% remarkably improves the hydrogenation activity, the con-
version of BA is also improved from 67.0 to 86.2 mol% or 86.5 mol%.
As for Ni (≥20 wt.%)/CSC catalysts, the conversion of BA is decreased
to 70.2 mol%. This result indicates that the amount of Ni loading lies
in an appropriate range, more and less metal loading is inappropri-
ate. Encouragingly, the selectivity of CCA has been maintained at
the level of 100%.
and to the reduction of Ni2+ with medium interaction (at 336 C)
◦
[
40,41]. This evidences some inhomogeneity of supported nickel.
According to data from the literature [42–44], the TPR peaks are
due to reduction of the supported “NiO”-like phase. There are dif-
ferent reduction peaks for the resulting 10%Ni/CSC-b in four stages
◦
with the main signals at 267, 293, 321 and 393 C, in which the first
reduction peak appears at lower temperature than the other three
◦
samples. The appearance of the peak at high temperature of 393 C,
◦
with other two peaks at 293 and 321 C, is attributable to the reduc-
tion of Ni with strong interaction. When the carrier is replaced
2
+
by AC, the H -TPR profile of 10%Ni/AC-b shows that the reduction
2
of nickel species on AC occurs in five stages with maximal signals
◦
at 276, 288, 313, 330 and 409 C. However, the H -TPR profile of
1
2
◦
0%Ni/G-b has only two maximal signals at 301 and 325 C.
3.2. Catalytic hydrogenation of BA to CCA
The microwave-heating power used in the preparation of
catalyst also has some impact on the catalytic activity of the hydro-
genation catalyst, as shown in Fig. 8. When microwave power
is 400 W, there is 67.9 mol% conversion of BA, which is signifi-
For the hydrogenation of BA to CCA, the activity of different Ni
catalysts is compared in Table 2. Over 10%Ni/CSC-a, the hydrogena-
tion affords only 35.9 mol% conversion of BA with 100% selectivity

X.H. Lu et al. / Molecular Catalysis 444 (2018) 53–61
57
Fig. 4. TEM images of the 10%Ni/CSC-b catalysts prepared using different microwave-heating powers.
Fig. 5. Ni2p XPS spectra of 10%Ni/CSC-a and 10%Ni/CSC-b.
cantly lower than 86.2 mol% conversion on the catalyst prepared
by the microwave power of 500 W. With a further increase of
the microwave power to 700 W, the conversion of BA is slowly
decreased to 78.5 mol%. This shows that suitable microwave power
is very conducive for improving the catalytic activity of the cata-
lyst. TEM images (Fig. 4) and XPS spectra (Fig. S2) also show the
distribution of different catalyst components and particles treated
by different microwave powers, in agreement with different hydro-
genation activities.
The selective hydrogenation of aromatic compounds with both
aromatic rings and other reducible groups usually requires severe
conditions over non-noble metal catalysts. The chemoselective

5
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X.H. Lu et al. / Molecular Catalysis 444 (2018) 53–61
Table 2
a
Hydrogenation of BA over different catalysts.
◦
Entry
Catalyst
T ( C)
Conversion (mol%)
Selectivity (%)
CCA
Others
1
2
3
4
5
6
10%Ni/CSC-a
10%Ni/CSC-b
10%Ni/AC-b
10%Ni/G-b
10%Ni/SiO2-b
10%Ni/Al2O3-b
200
200
200
200
200
200
35.9
86.2
40.8
33.8
32.5
18.0
100
100
100
100
100
100
0
0
0
0
0
0
a
Reaction conditions: 4.0 mmol BA, 100 mg catalyst, 10 mL THF, 3.0 MPa H2, 8 h.
Table 3
Hydrogenation of BA in different solvents.
a
◦
Entry
Catalyst
Solvent
T ( C)
Conversion (mol%)
Selectivity (%)
CCA
Other
1
2
3
4
5
6
10%Ni/CSC-b
THF
200
200
200
200
200
200
86.2
35.6
16.2
10.1
13.9
54.2
100
100
100
100
100
100
0
0
0
0
0
0
Cyclohexane
Dioxane
Ethanol
Cyclohexanol
Water
a
Reaction conditions: 4.0 mmol BA, 100 mg catalyst, 10 mL solvent, 3.0 MPa H2, 8 h.
Fig. 7. Catalytic activity of x%Ni/CSC-b catalysts. (Conditions: 4.0 mmol BA, 100 mg
◦
catalyst, 10 mL THF, 3.0 MPa H2, 200 C, 8 h.).
The effect of catalyst amount on the conversion and selectivity is
illustrated in Fig. 9. Reactions were carried out to find the minimum
weight of the catalyst required to achieve the necessary conversion.
The BA conversion increased with catalyst concentration from 25
to 100 mg, the conversion of BA is rapidly increased from 1.9 to
Fig. 6. Effect of Ni content on H2-TPR profiles of NiO/CSC.
8
6.2 mol%, while the selectivity of CCA is maintained at 100%. As
the amount of the catalyst increases, more surface area is available
for the hydrogenation reaction to take place. The initial conversion
rate of BA has a proportional increase with the catalyst amount. As
the catalyst amount increased from 100 to 150 mg, the conversion
of BA is slightly increased from 86.2 to 86.8 mol%.
hydrogenation of BA was therefore studied in a series of solvents
over 10%Ni/CSC-b to investigate the effect of solvents on the activ-
ity, and the results are shown in Table 3. As summarized in Table 3,
among these solvents, cyclohexane, dioxane, ethanol and cyclo-
hexanol are unfavorable for this reaction (entries 2–5). Although
a certain amount of CCA is obtained in ethanol (entry 4), the BA
conversion is only 10.1 mol%. When water is employed as the sol-
The temperature has a significant effect on the conversion of BA.
◦
Reactions were carried out at 160, 180, 190, 200, and 220 C and at
a pressure of 3.0 MPa of hydrogen in the presence of 10%Ni/CSC-b
catalyst. Fig.S3 shows the effect of temperature on the hydrogena-
tion of BA. As the reaction temperature is increased from 160 to
◦
vent (entry 6), the conversion of BA reaches 54.2 mol% at 200 C
for 8 h under 3.0 MPa of H . When THF is employed as the solvent
2
◦
(
entry 1), the conversion of BA is rapidly increased to 86.2%.
180 C, the conversion of BA is increased from 11.5 to 35.8 mol%. As

X.H. Lu et al. / Molecular Catalysis 444 (2018) 53–61
59
Fig. 8. Effect of the microwave power on the hydrogenation. (Conditions: 4.0 mmol
◦
BA, catalyst 10%Ni/CSC-b, 10 mL THF, 3.0 MPa H2, 200 C, 8 h.).
Fig. 10. Effect of the reaction time on the hydrogenation. (Conditions: 4.0 mmol BA,
◦
100 mg catalyst (10%Ni/CSC-b), 10 mL THF, 3.0 MPa H2, 200 C.).
Fig. 9. Effect of the catalyst amount on the hydrogenation. (Conditions: 4.0 mmol
◦
BA, catalyst 10%Ni/CSC-b, 10 mL THF, 3.0 MPa H2, 200 C, 8 h.).
Fig. 11. Recycling results of the catalyst 10%Ni/CSC-b. (Conditions: 4.0 mmol BA,
the reaction temperatures is increased from 180 to 200, the BA is
rapidly increased from 35.8 to 86.2 mol%. With a continuous lift of
reaction temperature to 220 C, the conversion of BA is changed a
◦
100 mg catalyst (10%Ni/CSC-b), 10 mL THF, 3.0 MPa H2, 200 C.).
◦
little (86.8 mol%), but the selectivity of the product CCA is slightly
reduced to 98.2%.
conducted for 4 h, the conversion of BA reaches 19.5 mol%. Then,
the solid catalyst is filtered off and the reaction is continued for
another 6 h. In the absence of metal catalyst, however, a small incre-
ment of 2.6 mol% conversion of BA could be observed, which further
confirms the characteristics of heterogeneous catalysis.
The 10%Ni/CSC-b catalyst can be easily separated by centrifu-
gation for five recycles without obvious loss of activity or CCA
selectivity (Fig. 11). For example, the 10%Ni/CSC-b catalyst at the
fifth recycle has 85.7 mol% conversion of BA, still comparable to the
fresh catalyst (86.2 mol%). For the first recycle, ICP analyses of the
reaction mixture and the used catalyst did not detect the leaching
of Ni from CSC. The estimated Ni content remained just about the
same as the fresh catalyst. For the latter four recycles, still no leach-
ing of Ni was detected. This study further reveals the recyclable
stability of the active catalyst 10%Ni/CSC-b for the titled reaction.
Fig. 10 gives the effect of reaction time concerning the change of
conversion in selective hydrogenation of BA as a function of time.
In the case of the 10%Ni/CSC-b, within 2 h of reaction time, the con-
version increased slowly (the conversion of BA is only 10.8 mol%.).
At the reaction time of 8 h, the conversion increased sharply to
8
6.2 mol%, while the selectivity maintained at 100%. As prolong the
reaction time to 10 h, the conversion increased slowly (87.5 mol%).
Comparatively, the selectivity of product CCA is slightly decreased
from 100 to 98.2%, probably due to the increase of side product
cyclohexyl-methanol. The yield of CCA reaches the maximum of
8
6.2 mol% in 8 h, which is the optimal time. On the basis of these
data, control experiments have been carried out under identical
reaction conditions, but in the absence of any metal catalysts [47].
As shown in Fig. 10, after the hydrogenation reaction is initially

6
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X.H. Lu et al. / Molecular Catalysis 444 (2018) 53–61
Table 4
a
Hydrogenation of benzoic acid derivatives over 10%Ni/CSC.
Entry
1
Catalyst
Substrate
Product
Conversion (mol%)
86.2
Selectivity(%)
100
10%Ni/CSC-b
2
3
72.8
71.3
100
100
4
5
52.3
55.3
100
100
6
38.6
100
a
◦
Reaction conditions: 4.0 mmol substrate, 100 mg catalyst, 10 mL THF, 3.0 MPa H2, 200 C, 8 h.
Also, 10%Ni/CSC-b catalyst showed good activity in the hydro-
genation of benzoic acid derivatives (Table 4). Benzoic acids with
an electron-donating group on the ring of (meta-methylbenzoic
acid and para-methylbenzoic acid, entry 2 and 3) afford slightly
lower conversions (72.8 and 71.3 mol%) compared to benzoic acid.
For the example of anthranilic acid (entry 4), a relatively low con-
version (52.3 mol%) is obtained. The hydrogenation of phenylacetic
acid reaches 55.3 mol% conversion in 8 h (entry 5). Furthermore,
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Metallic Ni nanoparticles have been highly dispersed over CSC,
AC, G, Al O and SiO2 by an efficient strategy of microwave-
2
3
assisted mode. By microwave-assisted mode, a narrow distribution
of Ni nanoparticle size centred at around 9.4 nm can be obtained.
The robust 10%Ni/CSC-b catalyst shows high catalytic activity and
excellent reusability in the hydrogenation of aromatic acid com-
pounds, such as benzoic acid or other aromatic acids. The excellent
catalytic activity is attributed to the co-effect of the support and the
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