Mesoporous silica gel as an effective and eco-friendly catalyst for highly selective preparation of cyclohexanone oxime by vapor phase oxidation of cyclohexylamine with air

Article abstract of DOI:10.1016/j.jcat.2016.03.008

A simple and environmentally benign approach to highly selective preparation of cyclohexanone oxime by vapor phase catalytic oxidation of cyclohexylamine with air over mesoporous silica gel under atmospheric pressure has been successfully developed in this work. The results demonstrate that the nonmetallic mesoporous silica gel is an effective and eco-friendly catalyst for the vapor phase selective oxidation of cyclohexylamine to cyclohexanone oxime and the surface silicon hydroxyl groups as active sites are responsible for the excellent catalytic performance of silica gel. The present silica gel catalyst has advantages of low cost, long-time stable reactivity, easy regeneration, and reusability. This method employing inexpensive mesoporous silica gel as catalyst and air as green terminal oxidant under facile conditions is a promising process and has the potential to enable sustainable production of cyclohexanone oxime from the selective oxidation of cyclohexylamine with air in industrial applications.

Full text of DOI:10.1016/j.jcat.2016.03.008

Journal of Catalysis 338 (2016) 239–249
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Journal of Catalysis
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Mesoporous silica gel as an effective and eco-friendly catalyst for highly
selective preparation of cyclohexanone oxime by vapor phase oxidation
of cyclohexylamine with air
a
a
c
c
a,b
Shuilin Liu a, Kuiyi You a,b, , Jian Jian , Fangfang Zhao , Wenzhou Zhong , Dulin Yin , Pingle Liu
,
⇑
Qiuhong Ai a,b, He’an Luo a,b
a School of Chemical Engineering, Xiangtan University, Xiangtan 411105, People’s Republic of China
b National and Local United Engineering Research Center for Chemical Process Simulation and Intensification, Xiangtan University, Xiangtan 411105, People’s Republic of China
c School of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and environmentally benign approach to highly selective preparation of cyclohexanone oxime
by vapor phase catalytic oxidation of cyclohexylamine with air over mesoporous silica gel under atmo-
spheric pressure has been successfully developed in this work. The results demonstrate that the non-
metallic mesoporous silica gel is an effective and eco-friendly catalyst for the vapor phase selective
oxidation of cyclohexylamine to cyclohexanone oxime and the surface silicon hydroxyl groups as active
sites are responsible for the excellent catalytic performance of silica gel. The present silica gel catalyst has
advantages of low cost, long-time stable reactivity, easy regeneration, and reusability. This method
employing inexpensive mesoporous silica gel as catalyst and air as green terminal oxidant under facile
conditions is a promising process and has the potential to enable sustainable production of cyclohex-
anone oxime from the selective oxidation of cyclohexylamine with air in industrial applications.
Ó 2016 Elsevier Inc. All rights reserved.
Received 4 January 2016
Revised 5 March 2016
Accepted 7 March 2016
Keywords:
Mesoporous silica gel
Cyclohexylamine
Cyclohexanone oxime
Selective oxidation
Vapor phase
1. Introduction
Considering the source and cost of reactants (starting materials
and oxidants) and the energy efficiency of processes, as well as
Cyclohexanone oxime (CHO) is an important intermediate in
the production of nylon-6 fibers and resins, which is widely used
in polymer synthesis. At present, the preparation methods for
CHO mainly include the ‘‘cyclohexanone–hydroxylamine” method
[1,2], ammoximation of cyclohexanone, ammonia, and hydrogen
peroxide over TS-1 zeolite [3,4], light-nitrosation of cyclohexane
with nitrosyl chloride [5], aerobic oxidation of cyclohexylamine
originated from nitrobenzene, aniline, cyclohexene, or cyclohex-
anol [6], and one-step synthesis from nitrobenzene, hydrogen,
and hydroxylamine hydrochloride over Au/C and Pd/C catalysts
[7] (Scheme 1). Nowadays, the modern chemical industry demands
the intensification of chemical processes in order to save chemicals
and energy, reduce environmental pollution, and enhance eco-
nomic benefits. Therefore, developing an environmentally benign
process for CHO production has been highly desired for a long time
in the chemical industry.
recent advances in the selective nitration of cyclohexane to nitro-
cyclohexane (NCH) [8,9], one-step hydrogenation of NCH into
CHO is a promising and more economical route [10,11]. However,
this process inevitably generates the main byproduct cyclohexy-
lamine. Therefore, highly selective oxidation of cyclohexylamine
to CHO under mild conditions is still one of the current challenges
that must be met to make this process more economical and envi-
ronmentally friendly. In recent years, some research on the selec-
tive oxidation of cyclohexylamine to CHO has been reported. The
oxidizing agents mainly include molecular oxygen, hydrogen per-
oxide, and alkyl hydroperoxides [12]. Rakottyay et al. [13,14]
report the oxidation of cyclohexylamine with molecular oxygen
over modified alumina. Suzuki and co-workers report 1,1-
diphenyl-2-picrylhydrazyl and WO3/Al2O3 as excellent composite
catalysts for the selective oxidation of cyclohexylamine [15,16]
with a satisfactory yield of CHO. Christensen [17] and Kaszonyi
et al. [18] report bifunctional Au–TiO2 and WO3/c-Al2O3 as effec-
tive catalysts for the selective oxidation of cyclohexylamine with
molecular oxygen, having around 70% of selectivity to CHO with
ca. 20% conversion of cyclohexylamine. Recently, Zhong et al.
reported that CHO could be obtained from the oxidation of
⇑
Corresponding author at: School of Chemical Engineering, Xiangtan University,
Xiangtan 411105, People’s Republic of China. Fax: +86 731 58298267.
E-mail address: youkuiyi@126.com (K. You).
http://dx.doi.org/10.1016/j.jcat.2016.03.008
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

240
S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
investigated, and the durability and reusability of the catalyst
was evaluated. The detailed results obtained from the catalytic
oxidation process will be reported in this paper.
2. Experimental
2.1. Reagents and instrument
Cyclohexylamine (AR) was purchased from the Tianjin Kermel
Reagent Development Center, China. Mesoporous silica gel
(60 mesh) was purchased from the Qingdao Micro-Nano Silicon
Technology Co., Ltd., China. The other reagents were purchased
from the Sinopharm Chemical Reagent Co., Ltd., China. Except
where specified, all chemicals were of analytical grade and used
Scheme 1. Currently employed routes for the synthesis of CHO.
as received. Gas chromatography (GC) was performed on
Shimadzu GC-2010 plus equipped with a DB-5 (30 m ꢀ 0.25 mm ꢀ
0.25 m) column for quantitative analysis. Gas chromatography–
a
l
cyclohexylamine with air over nonmetallic mesoporous SBA-15
catalyst [19]. In addition, several reagents, such as PCWP-H2O2
[20], VS-1-H2O2 [21], and TS-1-H2O2 [22,23], have been reported
to catalyze the oxidation of amines into oximes. However, these
methods show poor selectivity in terms of product distribution.
Although the reduction product of hydrogen peroxide as oxidizer
is ecological H2O, its price and utilization efficiency should be con-
sidered cautiously. Likewise, if alkyl hydroperoxides are used as
oxidizers, the separation of their reduction products from the reac-
tion mixture is also necessary. Therefore, molecular oxygen, espe-
cially in air, as a green terminal oxidant for the selective oxidation
of cyclohexylamine over heterogeneous catalysts is of increasing
interest in view of environmental, economical, and synthetic
aspects.
Silica gel is generally considered a catalytically inactive solid.
Up to now, the silica gel directly used as a catalyst in oxidation
reactions is little reported. However, some catalysts with silica
as carrier have been proven to be effective for several oxidation
reactions, for example, oxidation of methane [24,25], oxidation
of carbon oxide [26], and selective oxidation of sulfides [27,28].
Importantly, the excellent catalytic performance of nonmetallic
SiO2 materials is linked to their manifold structures and surface
chemistry [29,30]. Silica gel offers an opportunity to develop
new catalysts for economical and environmentally friendly
processes.
mass spectrometry (GC–MS) was run on a Shimadzu GCMS-
QP2010 PLUS for qualitative analysis of products.
2.2. Experimental procedure
The vapor phase oxidation of cyclohexylamine with air was car-
ried out in a fixed-bed vertical downward-flow quartz glass reactor
of 60 cm length with 10 mm inner diameter (see Fig. 1). A certain
amount of silica gel was placed at the center of the reactor, sup-
ported on either side by a thin layer of quartz wool and ceramic
beads. The oxidation reaction was performed at atmospheric pres-
sure and a preselected temperature. Cyclohexylamine was injected
into the quartz glass reactor using a pulsation-free reciprocating
pump with a calibrated constant flow rate, and the flow of air
was measured by a mass flow controller. The resulting gas flowing
from the outlet of the reactor was condensed with a low-
temperature cooling liquid at 6–8 °C to collect the products. The
liquid phase products was collected at different time intervals
and analyzed by a gas chromatograph with FID detector (quantita-
tive analysis by internal standard methods with toluene as internal
standard sample). The components of the gas phase were analyzed
by gas chromatography with a TCD detector. The conversion of
cyclohexylamine and selectivity of CHO were calculated using
the following formulas:
ꢀ
ꢁ
ꢀ
ꢁ
the amount ðmolÞ of
the amount ðmolÞ of
ꢁ
starting cyclohexylamine
cyclohexylamine recovered
Conversion of amine ¼
ꢀ
ꢁ
ꢀ 100%
ð1Þ
ð2Þ
the amount ðmolÞ of
starting cyclohexylamine
ꢀ
ꢁ
the amount ðmolÞ of
cyclohexanone oxime
Selectivity of oxime ¼ ꢀ
ꢁ
ꢀ
ꢁ ꢀ 100%:
the amount ðmolÞ of
the amount ðmolÞ of
ꢁ
starting cyclohexylamine
cyclohexylamine recovered
Here, we report a simple and effective catalytic approach for
highly selective preparation of CHO from cyclohexylamine
employing mesoporous silica gel as a metal-free catalyst and air
as a terminal oxidant, as is shown in Scheme 2. Several techniques,
including N2 adsorption–desorption, 29Si MAS NMR, XRD, TG/DTG,
FT-IR, and UV–vis DRS, were employed to characterize the struc-
ture of fresh and spent catalysts. Various parameters such as
catalyst type, reaction temperature, residence time, and the molar
ratio of cyclohexylamine to oxygen have been systematically
All raw materials and products were established carbon mass
balances.
2.3. Activation and characterization of catalyst
Silica gel was calcined at 773 K in air for 6 h (with a heating rate
of 5 °C minꢁ1) before used. X-ray diffraction (XRD) analysis of silica
gel samples was performed in a Japan Rigaku D/Max-2550VB+
18 kW X-ray diffractometer with a monochromatic CuKa radiation

S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
241
Scheme 2. Catalytic synthesis of CHO from vapor phase selective oxidation of cyclohexylamine with air.
Cyclohexylamine
Tubular gasifier
Thermocouple
Reciprocating pump
Fixed-bed reactor
T
P
air
Cooling water
r
se
en
nd
o
C
P
Mass flowmeter
Collection bottle
Fig. 1. Experimental equipment for vapor phase catalytic oxidation.
source (k = 1.5418 Å) at a voltage and current of 40 kV and 300 mA.
Fourier transform infrared (FT-IR) spectra of the samples were col-
lected using a Nicolet-380 instrument in a KBr matrix in the range
400–4000 cmꢁ1. The ultraviolet–visible diffuse reflectance spectra
(UV–vis DRS) of the samples were recorded in a UV-2550 spec-
trophotometer from 200 to 800 nm in the BaSO4 phase. Thermo-
gravimetric/derivative thermogravimetric (TG/DTG) analysis was
conducted using a Mettler TGA/DSC1/1600HT thermogravimetric
analyzer under nitrogen at a gas flow rate of 40 ml/min and a heat-
ing rate of 10 °C/min from 30 to 800 °C. The textural properties of
the samples were characterized by N2 physisorption on a NOVA-
2200e (Quantachrome, USA) at 77 K after outgassing at 150 °C
and 1 mmHg for 6 h. Elemental analyses (C, H, N) were performed
using a Carlo Erba CE Instruments EA 1110. The solid state 29Si
NMR spectra were recorded on a Varian Infinity-plus400 NMR
spectrometer with a commercial double-resonance MAS probe at
a Larmor frequency of 79.47 MHz for 29Si. The MAS speed was
3 kHz in all 29Si DP/MAS (direct-polarization magic angle spinning)
NMR measurements. The 29Si chemical shifts were determined
using solid kaolin as an external reference resonating at
ꢁ91.5 ppm relative to tetramethylsilane (TMS).
famous as a radical catalyst for the oxidation of alkane to the cor-
responding oxygen-containing compounds [33–35]. In addition,
WO3, c-Al2O3, and sepiolite were also used as typical catalysts in
this oxidation reaction. Unfortunately, all these attempts failed to
obtain acceptable conversion and selectivity to the desired pro-
duct; especially, VPO and WO3 catalysts exhibited no catalytic
activity in the oxidation reaction of cyclohexylamine with air. Then
we tried some catalysts containing hydroxyl groups on the surface,
such as Hb, MCM-41, and silica gel, to catalyze this oxidation reac-
tion. The catalytic activity of various catalysts for the oxidation of
cyclohexylamine with air is summarized in Table 1. Obviously,
CHO was not obtained in the absence of any catalysts, which indi-
cates that the catalyst plays an extraordinarily important role in
the formation of the desired product. In this catalytic system, the
catalytic performance of the catalysts was different. CHO could
Table 1
Comparison of catalytic performance of various catalysts in the oxidation reaction.a
Catalyst
Conversion (%)
Selectivity (%)
CHO
NCH
CCA
None
VPO
–
–
–
–
–
–
–
–
3. Results and discussion
WO3
NHPI
–
–
–
–
–
0.9
1.1
1.2
4.7
8.3
10.2
63.9
81.3
14.9
63.6
80.3
88.6
36.1
12.85
85.1
33.8
12.5
6.7
3.1. Comparison of catalytic performance of various catalysts in the
oxidation reaction
c
-Al2O3
5.85
–
2.6
7.2
4.7
Sepiolite
Hb
MCM-41
Silica gel
To seek an effective catalyst for the vapor phase oxidation of
cyclohexylamine with air, we screened several typical oxidation
catalysts. For example, vanadium phosphorus oxide (VPO) compos-
ite catalyst is well known in the vapor phase oxidation of n-butane
to maleic anhydride (MA) [31,32]. N-Hydroxyphthalimide (NHPI) is
a
Reaction conditions: the molar ratio of cyclohexylamine to O2 is 1:5, the flow
rate of air is 100 ml/min, time on stream is 6 h, reaction temperature is 140 °C, the
residence time is 4.6 s, and GHSV is 784.8 hꢁ1
.

242
S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
be formed from the aerobic oxidation of cyclohexylamine cat-
alyzed by NHPI, -Al2O3, and sepiolite catalyst; however, the con-
NO2
c
version was below 2% and the selectivity to oxime was also not
considerable. The cyclohexylamine conversion was slightly
improved as Hb was introduced into this oxidation reaction, and
4.7% conversion with 63.6% selectivity to oxime was obtained.
The cyclohexylamine conversion and selectivity to CHO were evi-
dently enhanced as the catalysts MCM-41 and silica gel containing
hydroxyl groups on the surface were introduced into the oxidation
reaction. Silica gel catalyst exhibited excellent catalytic perfor-
mance. Under our present conditions, 10.2% cyclohexylamine con-
version with 88.6% selectivity to CHO was obtained. This result
showed that catalysts containing hydroxyl groups on the surface
were very suitable for the oxidation of cyclohexylamine with air
to CHO.
O2
OH
O
NH2
N
O2
O2
N
(main reaction)
3.2. Effects of reaction temperature on the oxidation reaction
side reaction
Fig. 2 shows the effects of reaction temperature on the oxida-
tion reaction of cyclohexylamine in air. It is clearly found that there
is significant influence of reaction temperature on the conversion
and selectivity. The conversion of cyclohexylamine increased from
10.2% to 22.8% and the selectivity to CHO decreased gradually from
88.6% to 66.1% as the reaction temperature was elevated from 140
to 180 °C. Obviously, the selectivity of byproducts were found to
increase with elevated temperatures; especially the selectivity to
cyclohexyl–cyclohexylidene–amine (CCA) increased rapidly with
reaction temperature from 6.5% at 140 °C to 17.2% at 180 °C. In this
oxidation process, the main reaction to generate CHO and the side
reaction to product NCH and CCA may be competitive. Cyclohexy-
lamine can be partly oxidized into CHO. However, it can be
completely oxidized into NCH and cyclohexanone, while cyclohex-
anone can easily react with cyclohexylamine to generate CCA
[10,11,36]. The reaction network is drawn in Scheme 3. The side
reaction may be very sensitive to reaction temperature. To explore
the activation energy level of the main reaction and side reactions,
we investigated the variation of selectivity to CHO and CCA with
altering reaction conditions (e.g., temperature, residence time,
and GHSV) for the same conversion of cyclohexylamine, and the
results are depicted in Fig. 3. It can be seen that selectivity of
CHO gradually decreases with elevated temperatures, while selec-
tivity to CCA gradually increases with temperature. This indicates
Scheme 3. The schematic reaction network for the oxidation of cyclohexylamine
with air.
Conversion
90
80
Selectivity to CHO
Selectivity to CCA
10
5
0
c
a
b
Fig. 3. Comparison of catalytic performance of silica gel at different reaction
conditions in the oxidation reaction. Reaction conditions: (a) reaction temperature
is 140 °C, residence time is 4.6 s, and GHSV is 784.8 hꢁ1; (b) reaction temperature is
160 °C, residence time is 2.9 s, and GHSV is 1259.3 hꢁ1; (c) reaction temperature is
180 °C, residence time is 2.1 s, and GHSV is 1750.2 hꢁ1
.
90
80
70
that the activation energy of side reactions is higher than that of
the main reaction.
In order to further understand the dynamic properties of this
oxidation reaction, the reaction rates of the main reaction and side
reaction are measured in the temperature range 413–453 K, and
the obtained reaction data are summarized in Table 2. The Arrhe-
nius fittings of the main reaction to form the desired product
60
Conversion
Selectivity of CHO
50
Selectivity of NCH
Selectivity of CCA
Table 2
20
The reaction data for the main reaction (Rmain) to form CHO and the side reaction
(Rside) to form CCA at different temperatures.
Temperature (K)
413
423
0.00236
10.89
1.24
0.00218
0.00025
433
0.00231
12.9
1.97
0.00258
0.00039
443
0.00226
15.0
3.27
0.00030
0.00065
453
0.00221
17.2
4.89
0.00344
0.00098
10
0
1/T (Kꢁ1
)
0.00242
Yielda (%)
Rmain 9.04
Rside 0.68
Rmain 0.00181
Rside 0.00014
140
150
160
170
180
Yb (mol/
min)
oC)
(
Temperature
a
Reaction conditions: the molar ratio of cyclohexylamine to O2 is 1:5, the flow
rate of air is 100 ml/min, the flow rate of cyclohexylamine is 0.02 ml/min, residence
Fig. 2. Effects of reaction temperature on oxidation reaction. Reaction conditions:
time is 4.6 s, time on stream is 6 h, and GHSV is 784.8 hꢁ1
.
the molar ratio of cyclohexylamine to O2 is 1:5, the flow rate of air is 100 ml/min,
b
residence time is 4.6 s, time on stream is 6 h, and GHSV is 784.8 hꢁ1
.
Y (mol/min) = Yield ꢀ flow rate of cyclohexylamine (mol/min).

S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
243
-5.6
-5.7
-5.8
-5.9
-6.0
-6.1
-6.2
-6.3
B
A
-7.0
-7.5
-8.0
Y=13.7041-9325.6172X
Y=1.1568-3085.4732X
(R=0.9992)
(R=0.9994)
-8.5
-9.0
0.00220
0.00225
0.00230
0.00235
0.00240
0.00245
0.00220
0.00225
0.00230
0.00235
0.00240
0.00245
1/T (K-1
1/T (K-1)
)
Fig. 4. Arrhenius fitting of ln Y versus 1/T for the main reaction to form CHO (A) and the side reaction to form CCA (B).
CHO and the side reaction to form the byproduct CCA are depicted
in Fig. 4. Using the Arrhenius formula, ln k = lnA ꢁ Ea/RT, we can
find the apparent activation energy for the main reaction and side
reaction. The calculated results show the apparent activation
energy to form the desired product, CHO, and to form the bypro-
duct, CCA, are 25.65 and 77.53 kJ/mol, respectively. Obviously,
the apparent activation energy of the side reaction is much higher
than that of the main reaction. Therefore, elevating the tempera-
ture was not beneficial for the formation of CHO. These results
indicate that the reaction temperature was a significant factor in
cyclohexylamine conversion and CHO selectivity. From the point
of view of CHO selectivity, the favorable reaction temperature
was 140 °C.
selectivity to CHO showed a declining trend, and the selectivity
to NCH and CCA slowly increased with prolonged residence time.
This reason may be that it is difficult for the formed products to
desorb quickly from the catalyst surface, which results in a consec-
utive side reaction to form by-products, so that the selectivity to
CHO decreases with residence time. Therefore, selecting a suitable
residence time is very important for the selective oxidation of
cyclohexylamine in air to CHO. From the point of view of conver-
sion and selectivity, a suitable residence time would be 4.6 s under
the present reaction conditions.
3.4. Effects of the molar ratio of cyclohexylamine to oxygen on the
oxidation reaction
3.3. Effects of residence time on the oxidation reaction
As the molar ratio of reactants has a strong influence on the
conversion and selectivity of products, the reactivity of the silica
gel at different molar ratios of cyclohexylamine to oxygen was
evaluated at a reaction temperature of 140 °C and a residence time
of 4.6 s, and the results are listed in Table 3. It can be seen that the
conversion of cyclohexylamine increases with decreasing molar
ratio or rising concentration of oxygen. However, the increased
conversion is accompanied by a rapid decline in CHO selectivity,
and 88.6% selectivity to CHO with 10.2% of cyclohexylamine con-
version was achieved at molar ratio 1:5. When the molar ratio
reached 1:9, cyclohexylamine conversion increased significantly
to 17.6% and CHO selectivity slowly decreased to 74.9%. However,
the selectivity to NCH and CCA increased gradually with reduced
molar ratio. This phenomenon indicates that a high concentration
of oxygen facilitates the formation of NCH and CCA instead of
As we know, the residence time is an important factor in mate-
rial conversion and products selectivity in the vapor phase oxida-
tion reaction. To investigate the effects of residence time on
cyclohexylamine conversion and selectivity to products, the vapor
phase oxidation reaction was performed at different bed heights of
the catalyst, and the results are shown in Fig. 5. It can be seen that
the conversion of cyclohexylamine gradually increased with
elevated residence time, and 10.2% conversion with 88.6% selectiv-
ity to CHO was achieved at 4.6 s of residence time. However, the
90
85
Table 3
Effects of the molar ratio of cyclohexylamine to oxygen on the oxidation reaction.a
Conversion
Selectivity of CHO
Selecitvity of NCH
Selectivity of CCA
Molar ratiob
Conversion (%)
Selectivity (%)
CHO
NCH
CCA
Othersc
80
1:1
1:2
1:5
1:7
1:9
4.3
6.8
10.2
12.6
17.6
90.3
89.5
88.6
85.3
74.9
3.8
3.9
4.1
5.8
6.4
5.9
6.6
7.3
7.8
13.8
–
–
–
1.1
4.9
10
5
a
Reaction conditions: the flow rate of cyclohexylamine is 0.02 ml/min, the flow
rate of feed gas is 100 ml/min, temperature is 140 °C, the residence time is 4.6 s,
time on stream is 6 and GHSV is 784.8 hꢁ1
.
0
b
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
This denotes the molar ratio of cyclohexylamine to O2.
Residence time (s)
c
Other byproducts are N,N0-dicyclohexyl-hydrazine (
), N-cyclo-
hexyl-N0-cyclohexylidene-hydrazine
).
(
), and N-cyclohexylaniline
Fig. 5. Effects of the residence time on the oxidation reaction. Reaction conditions:
the molar ratio of cyclohexylamine to O2 is 1:5, the flow rate of air is 100 ml/min,
reaction temperature is 140 °C, time on stream is 6 h, and the catalyst is silica gel.
(

244
S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
CHO. Therefore, from the point of view of CHO selectivity and
cyclohexylamine conversion, the favorable molar ratio of cyclo-
hexylamine to oxygen is about 1:5. In order to gain simpler and
more convenient operation in industrial production, we use air
directly as oxidizer (the molar ratio of cyclohexylamine to oxygen
is ca. 1:5) to obtain CHO in the present reaction system.
In summary, from the single-factor-experiment results, the
optimal reaction conditions obtained were that the molar ratio of
cyclohexylamine to oxygen was 1:5, the reaction temperature
was 140 °C, the residence time was 4.6 s, and the GHSV was
784.8 hꢁ1 in the oxidation reaction of cyclohexylamine with air;
88.6% selectivity to CHO with 10.2% cyclohexylamine conversion
was achieved.
3.6. Effects of solvent methanol addition on the oxidation reaction
To verify the promotion effect of solvent methanol in the cyclo-
hexylamine oxidation reaction, the selectivity to CHO in the
absence and presence of MeOH solvent was compared at the same
cyclohexylamine conversion, and the results are depicted in Fig. 7.
In the range of the examined time on stream (4–34 h), the curve of
selectivity in a pure cyclohexylamine medium increased gradually
from 85.2% to 88.6% and then decreased with time on stream. The
selectivity to CHO was ultimately maintained at around 81.2% with
8.1% cyclohexylamine conversion at 34 h (curve a). When an added
10 wt.% of methanol solvent was introduced into the oxidation
reaction system, an increase in CHO selectivity was observed
(curve b, finally achieving ca. 8.2% conversion with 82.5% oxime
selectivity at 34 h). When 30 wt.% of methanol solvent was added
to the oxidation reaction system, the selectivity to CHO was further
improved, and could reach 92.0% with 9.9% of cyclohexylamine
conversion at 6 h time on stream. Finally, 89.2% selectivity to
oxime with 7.8% conversion was achieved (curve c). It is noted that
when 50 wt.% of methanol solvent was introduced into this oxida-
tion reaction system, the selectivity to oxime was further
improved; 93.4% selectivity to CHO with 9.0% cyclohexylamine
conversion was achieved at 6 h time on stream, and could be sta-
bilized at around 91% with ca. 8.0% conversion (curve d), finally.
These results show that the addition of methanol solvent can effec-
tively improve the selectivity of oxime. This likely implies that
methyl alcohol can effectively remove the organic products depos-
ited on the surface of silica gel, which prevents side reactions and
improves the selectivity to oxime.
3.5. Durability test of silica gel in the oxidation of cyclohexylamine
with air
Catalyst durability is one of the key issues in the development
of efficient catalysts for the oxidation of cyclohexylamine to CHO.
To investigate the stability of the silica gel catalyst, long-term per-
formance tests were performed at 413 K. The effect of time on
stream on cyclohexylamine conversion and CHO selectivity over
silica gel was studied, and the results are depicted in Fig. 6. It is
clearly seen that cyclohexylamine conversion as well as CHO selec-
tivity increased mildly in the first 6 h, and 10.2% cyclohexylamine
conversion with 88.6% selectivity to CHO was achieved. After that,
the conversion and selectivity to oxime slightly decreased with
time on stream. A possible reason was that more and more organic
species were formed and deposited on the catalyst’s surface with
time on stream, covering some active sites, which resulted in
blocking part of the pores, keeping the reactants from accessing
the active sites. Furthermore, along with time on stream, the
formed CHO can be hydrolyzed to cyclohexanone, which is easily
converted to CCA in the presence of cyclohexylamine. In addition,
3.7. Characterization of silica gel samples
The structural characteristics of silica gel and spent samples are
characterized using N2-physisorption, 29Si MAS NMR, XRD, TG/
DTG, FT-IR, and UV–vis DRS. Nitrogen adsorption–desorption iso-
therms of samples are shown in Fig. 8 and the textural properties
(surface area, pore volume, and pore diameter) are summarized
in Table 4. Obviously, all samples are similar to a type IV isotherm
with an H1 hysteresis loop, which is characteristic of mesoporous
materials. From Table 4, it is clearly seen that the BET surface area
traces
of
), N-cyclohexylaniline
N-cyclohexyl-N0-cyclohexylidene-hydrazine
), and N,N0-
) are detected under the pre-
(
(
dicyclohexyl-hydrazine (
sent reaction conditions, while some deep oxidation byproducts
such as CO2 and CO are not detected. It is worth noting that CHO
selectivity can be stabilized at around 81% with 8.5% cyclohexy-
lamine conversion till 58 h. These results indicate that the silica
gel catalyst exhibits better stability and catalytic activity in the
oxidation of cyclohexylamine to CHO.
d
90
c
90
80
b
80
Conversion of cyclohexylamine
Selectivity to CHO
a
a: in pure cyclohexylamine
b: add 10wt% MeOH
c: add 30wt% MeOH
d: add 50wt% MeOH
20
70
Conversion of cyclohexylamine
Selectivity of cyclohexanone oxime
10
20
5
10
15
20
25
30
35
10
0
Time on stream (h)
Fig. 7. Comparison of the selectivity to CHO in the oxidation reaction in the absence
and presence of MeOH solvent. Reaction conditions: the flow rate of feed liquid is
0.02 ml/min and the flow rate of air is 100 ml/min; (a) reaction temperature is
140 °C, residence time is 4.6 s, and GHSV is 784.8 hꢁ1; (b) reaction temperature
is 142 °C, residence time is 4.6 s, and GHSV is 784.8 hꢁ1; (c) reaction temperature is
145 °C, residence time is 4.6 s, and GHSV is 784.8 hꢁ1; (d) reaction temperature is
0
5
10
15
20
25
30
35
40
45
50
55
60
Time on stream (h)
Fig. 6. Durability test of silica gel in the oxidation reaction. Reaction conditions: the
flow rate of air is 100 ml/min, the molar ratio of cyclohexylamine to O2 is 1:5,
temperature is 140 °C, the residence time is 4.6 s, and GHSV is 784.8 hꢁ1
.
145 °C, residence time is 5.2 s, and GHSV is 687 hꢁ1
.

S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
245
1400
1200
1000
800
600
400
200
0
adsorption
desorption
fresh
2h
4h
12h
58h
fresh
2h
4h
12h
58h
20
40
60
80
2-theta (degree)
0.2
0.4
0.6
0.8
1.0
Fig. 9. XRD patterns of silica gel samples.
P/P0
Fig. 8. N2 adsorption–desorption isotherms of silica gel samples.
surface area and pore volume could be recovered from 62.7 m2/g
and 0.25 cm3/g to 278.6 m2/g and 0.85 cm3/g, respectively. When
the silica gel sample run for 12 h was soaked with methanol at
60 °C for 10 h, the BET surface area were recovered from 157.0 to
391.2 m2/g again, and the catalytic activity still reached the same
conversion and CHO selectivity as with the fresh catalyst. In addi-
tion, the BET surface area and pore volume of the spent silica sam-
ple were basically recovered to those of fresh silica gel by calcining
in air at 600 °C for 4 h. These results demonstrate that the polar
organic species formed in the reaction process block the porous
structure of the catalyst, which results in a decrease of BET surface
area and pore volume. The textural properties and catalytic perfor-
mance of spent silica gel catalyst can be restored by calcination or
an organic solvent soaking method.
The XRD patterns of silica gel samples are shown in Fig. 9. It is
clearly seen that the patterns of all samples exhibited a broad
diffraction peak at 22°, indicating the typical amorphous structure
of silica gel. Furthermore, the characteristic peaks of silica gel were
not destroyed and silica gel still kept its unordered mesoporous
structure in the reaction process.
Table 4
Textural properties of fresh and spent silica gel.
Samples
Surface area
(m2/g)
Pore volume
(cm3/g)
Pore diameter
(nm)
Silica gel – fresh
Silica gel – 2 h
Silica gel – 4 h
Silica gel – 12 h
Silica gel – 12 ha
Silica gel – 58 hb
Silica gel – regeneratedc
395.3
280.0
218.7
157.0
391.2
278.6
393.6
1.02
0.88
0.76
0.53
1.0
7.93
7.91
7.89
7.89
7.92
7.91
7.92
0.85
1.01
a
b
c
Spent silica gel sample soaked in methanol at 60 °C for 10 h.
Spent silica gel sample soaked in methanol for 4 h.
Spent silica gel sample calcined at 600 °C in air for 4 h.
of silica gel samples sharply decreases from 395.3 to 62.7 m2/g at
58 h time on stream, and the pore volume of samples also gradu-
ally decreases from 1.02 to 0.25 cm3/g at 58 h. However, the pore
size distribution obtained from the desorption branches of the N2
isotherms using the BJH method reveals that the pore diameter
of the samples remain substantially unchanged. When the silica
gel sample run for 58 h was soaked in methanol for 4 h, the BET
The TG/DTG curves of the samples are shown in Fig. 10. Obvi-
ously, for the fresh and spent samples, a mass loss process occurred
below 100 °C, which was assigned to the loss of adsorbed water,
and there was only a mass loss stage (ca. 5%) in the fresh silica
0.05
B
A
100
95
90
85
80
75
70
65
fresh
0.00
2h
4h
-0.05
-0.10
-0.15
% (fresh)
% (2h)
12h
58h
% (4h)
% (12h)
% (58h)
100
200
300
400
500
600
700
800
100
200
300
400
500
600
700
800
Temperature (oC)
Temperature (oC)
Fig. 10. TG/DTG curves of fresh and spent silica gel samples.

246
S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
gel. This demonstrates that silica gel is thermally stable at temper-
atures below 800 °C. However, the mass loss of the spent catalyst
increased with time on stream. From the TGA curves in Fig. 10A,
it can be seen that the total mass loss percentage for silica gel sam-
ples at time on stream of 2, 4, 12, and 58 h are about 10%, 13%, 20%,
and 30%, respectively. This shows that the accumulated amount of
organic species deposited on silica gel surface increases gradually
with time on stream.
In addition, from the DTG curves in Fig. 10B, for silica gel sam-
ples at time on stream of 2 and 4 h, it is clearly seen that a distinct
two-step mass loss process occurred. The first peak at lower tem-
peratures (ca. 55 °C) is generally attributed to the loss of adsorbed
water, while the second peak at higher temperatures (ca. 170 °C)
can be attributed to the decomposition of low-boiling organic spe-
cies adsorbed on the silica gel surface. However, silica gel samples
at time on stream of 12 h and 58 h underwent a three-step mass
loss, with two peaks at lower temperatures (ca. 70 and 210 °C)
and a third peak at higher temperature (ca. 350 °C) attributed to
the decomposition of high-boiling organic species formed in the
reaction process. These results indicate that the organic species
formed in the reaction process are gradually accumulated on the
catalyst surface with time on stream.
To obtain further insights into the organic species deposited on
the silica gel framework, FT-IR spectroscopy was performed on the
fresh and spent silica gel samples, and the results are depicted in
Fig. 11. The weak band at 1630 cmꢁ1 and the broad band at
3432 cmꢁ1 can be attributed to a combination of the stretching
vibration of silanol groups or silanol ‘‘nests” with cross
hydrogen-bonding interactions and the HAOAH stretching mode
of adsorbed water [37]. The bands at 809 and 468 cmꢁ1 are attrib-
uted to the SiAOASi symmetric stretching and bending vibration,
respectively, and the strong band at 1102 cmꢁ1 is attributed to
the asymmetric stretching vibration of framework SiAOASi
bridges for silica gel [38]. It is evident that the CAH stretching
bands in the methylene group (CH2) are found at 2938 and
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
fresh
2h
4h
12h
58h
254
330
500
500
200
300
400
600
700
800
Wavelength (nm)
Fig. 12. UV–vis spectra of fresh and spent silica gel samples.
100
Conversion of cyclohexylamine
Selectivity to CHO
80
60
40
20
0
1
2
3
4
5
6
7
8
fresh
2h
Runs
Fig. 13. Results of recycling test of silica gel catalyst. Reaction conditions: the flow
rate of air is 100 ml/min, the molar ratio of cyclohexylamine to O2 is 1:5, reaction
temperature is 140 °C, residence time is 4.6 s, time on stream is 6 h, and GHSV is
4h
784.8 hꢁ1
.
12h
58h
a
2341
2363
2500
2857
2938
1650
1630
809
b
c
3432
3500
1580
1520
1450
468
500
1380
1102
1000
4000
3000
2000
1500
-1
Wavenumber (cm )
1630
Fig. 11. FT-IR spectra of fresh and spent silica gel samples.
809
3432
Table 5
The elemental analysis data of organic species in the spent samples.
468
1102
1000
Sample
C (%)
H (%)
N (%)
Atom ratio (C:N)
4000
3500
3000
2500
2000
1500
-1
500
Silica gel/2 h
Silica gel/4 h
Silica gel/12 h
Silica gel/58 h
7.82
9.13
16.24
26.4
1.72
1.84
2.4
1.48
1.7
2.79
4.61
6.15:1
6.3:1
6.77:1
6.65:1
Wavenumber (cm )
Fig. 14. FT-IR spectra of samples: (a) fresh silica gel, (b) recycling one time, (c)
3.32
recycling eight times.

S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
247
2857 cmꢁ1, and these bands increase in intensity with time on
stream. The 2363 and 2341 cmꢁ1 bands are assigned to the vibra-
tion of the NAN band in the N species adsorbed on active sites, in
accordance with previous work [39,40]. In addition, the bands at
1450, 1520, and 1580 cmꢁ1 are assigned to the C@C stretching
vibration in the aromatic ring. Furthermore, the band at
1650 cmꢁ1 can be associated with the band containing C@N
species [41]. These results show that some organic species contain-
ing CH2 and C@N or NAN groups were deposited on the catalyst
surface in the reaction process. In addition, the elemental compo-
nents for organic species deposited on the silica gel surface were
analyzed, and the results are summarized in Table 5. It is obviously
seen that the organic species are mainly composed of C, H, N, and O
elements, and the percentage of elements gradually increased with
time on stream.
200–800 nm regions. In the UV–vis spectra of spent silica gel sam-
ples, there are obvious UV–vis absorption peaks at 254 and
330 nm, which are assigned to the delocalized electrons p–
p⁄ tran-
sition of conjugated organic species in the aromatic ring [42], and
the absorption intensity increases with time on stream. In addition,
there is a certain absorption peak at ca. 500 nm that shows that the
large conjugated organic species is deposited on the catalyst sur-
face. These results indicate that some conjugated organic species
were gradually formed with time on stream in the reaction
process.
3.8. The recycling of the silica gel catalyst in the oxidation reaction of
cyclohexylamine in air
In order to evaluate the recycling of the catalyst in this oxida-
tion reaction, silica gel was reused for eight runs. After each reac-
tion, the catalyst was calcined at 600 °C in air for 4 h. The
The UV–vis spectra of samples are shown in Fig. 12. It is clearly
seen that the fresh silica gel has no evident absorption peak in the
NH
-
H
-
δ
N
O O H
OH
δ
NH
H2O
O2
OH
OH
OH
+
+
δ
δ
O-Si-O-Si-O
O-Si-O-Si-O
O-Si-O-Si-O
(a)
Silica gel
Silica gel
Silica gel
H2O
Adsorption
(by hydregon bond)
OH
N
H2O
N
O
H
O H
Rearrangement
H
N
OH
OH
O
O-Si-O-Si-O
Silica gel
N
NH2
NH2OH
NH2
OOH
O
N
OH
H
O
N
H
NH2
O2
O
(b)
OOH
t
n
N
silica gel
e
HN
m
e
g
n
a
r
r
a
e
R
H2O
O2
O
O
N
Fig. 15. Possible reaction path of cyclohexylamine with air over silica gel.

248
S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
regenerated catalyst was used for a recycling study under the same
reaction conditions. The results for the reusability of the catalyst
are shown in Fig. 13. It can be seen that the present catalyst was
easily to regenerate and the catalytic performance of the regener-
ated catalyst was stable in the eight runs, and the cyclohexylamine
conversion and selectivity to CHO were basically maintained at
10.2% and 88.6%, respectively.
15
12
9
Silica gel
Silylated silica gel
In addition, in order to gain further insight into the change in
physical structure of the catalyst in reaction and regeneration pro-
cesses, the regenerated catalysts after recycling one time and eight
times were characterized by FT-IR, as shown in Fig. 14. Compared
with the FT-IR characteristic peaks of the fresh catalyst, it is clearly
seen that the characteristic peaks of the regenerated catalyst were
unchanged, which shows that the physical structure of regenerated
silica gel is still maintained.
6
3
0
2
4
6
8
10
3.9. Possible reaction pathway of cyclohexylamine with air catalyzed
by silica gel
Time on stream (h)
Fig. 16. Comparison of the catalytic performance of silylated silica gel and silica gel.
Reaction conditions: the molar ratio of cyclohexylamine to O2 is 1:5, the flow rate of
air is 100 ml/min, reaction temperature is 140 °C, residence time is 4.6 s, and GHSV
On the basis of these results in this work, the obtained products
were mainly CHO, CCA, NCH, and a small amount of cyclohexanone
in the present reaction. We suggested a possible formation path-
way of products in the catalytic oxidation reaction of cyclohexy-
lamine with air over silica gel, as shown in Fig. 15. First,
cyclohexylamine should be adsorbed onto the surface of silica gel
by hydrogen bonds before oxidation. Then, the adsorbed cyclo-
hexylamine will be rapidly oxidized by dioxygen to hydroperoxide
species (Fig. 15a). Finally, the formed hydroperoxide species leave
the catalyst surface under the action of water. Meanwhile, such
hydroperoxide species in our system are decomposed rapidly to
form intermediate nitrosocyclohexane, which is easy to isomerize
or rearrange to CHO. However, the resulting nitrosocyclohexane
can be also overoxidized to NCH. The previous literature reported
is 784.8 hꢁ1
.
-110.2
-101.0
-98.0
fresh
regenerated
that the autooxidation of amines was initiated on the a-CAH group
or NAH group [19,43]. Likewise, we assume that the oxidation of
58h
cyclohexylamine adsorbed with oxygen also can form two
silylated
hydroperoxide species at the
a-CAH and NAH sites (Fig. 15b).
The formed hydroperoxide species at the
a
-CAH site may be
0
-20
-40
-60
-80
-100
-120
-140
-160
decomposed to cyclohexanone and hydroxylamine [14]. The cyclo-
hexanone can react with hydroxylamine to form CHO, and can also
react with cyclohexylamine to generate CCA. In this process, the
surface silicon hydroxyl groups in silica gel play important roles
in initiating the oxidation reaction of cyclohexylamine with
oxygen.
29Si Chemical Shift (ppm)
Fig. 17. 29Si MAS NMR spectra of samples.
These results demonstrate that the surface silicon hydroxyl groups
in silica gel play a crucial role in initiating this oxidation reaction.
In order to further verify the surface silicon hydroxyl groups as
catalytic active sites, we examined the catalytic performance of
silylated silica gel (the silica gel was silylated using
a
trimethylchlorosilane agent), as shown in Fig. 16. This results indi-
cate that the catalytic activity (cyclohexylamine conversion) of
silylated silica gel was decreased remarkably compared with unsi-
lylated silica gel. In order to explore the change of the silicon
hydroxyl groups in the samples, 29Si MAS NMR characterizations
of fresh silica gel, silylated silica gel, spent silica gel, and regener-
ated silica gel were performed, as shown in Fig. 17. Obviously,
the spectra of fresh silica gel, spent silica gel, and regenerated silica
gel samples displayed three resonances centered at ca ꢁ98.0,
ꢁ101.0, and ꢁ110.2 ppm, which can be assigned to (HO)2Si(OSi)2
(Q2), HOSi(OSi)3 (Q3), and Si(OSi)4 (Q4) environments around sili-
con, respectively. Interestingly, the peaks of silylated silica gel at
ꢁ98.0 and ꢁ101.0 ppm significantly disappeared, indicating that
the silicon hydroxyl groups in the samples took part in reacting
with the silylation agent. Furthermore, the spectrum of the regen-
erated silica gel was similar to that of fresh silica gel, which
showed that the surface silicon hydroxyl groups could be recov-
ered and the structure was not damaged in the oxidation process.
4. Conclusions
In conclusion, a simple and environmentally friendly approach
to highly selective preparation of CHO from vapor phase catalytic
oxidation of cyclohexylamine using air as a green terminal oxidant
under atmospheric pressure has been successfully developed in
this work. We have demonstrated that nonmetallic mesoporous
silica gel is an effective catalyst for the vapor phase selective oxida-
tion of cyclohexylamine to CHO. The surface silicon hydroxyl group
as active site is responsible for the excellent catalytic performance
of silica gel, and 88.6% selectivity to CHO with 10.2% cyclohexy-
lamine conversion was obtained under optimal reaction condi-
tions. Moreover, the addition of methanol solvent in
cyclohexylamine can effectively improve the selectivity of CHO. It
is noted that silica gel catalyst has long-term stable activity, and
the spent catalyst can be easy to regenerate and reuse in the pre-
sent oxidation reaction. Finally, a possible reaction pathway for
the oxidation reaction of cyclohexylamine in air over silica gel

S. Liu et al. / Journal of Catalysis 338 (2016) 239–249
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Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (21276216 and 21576078), the Spe-
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tion (20124301130001), the Project of Technological Innovation
and Entrepreneurship Platform for Hunan Youth (2014), and the
Hunan Provincial Innovation Foundation for Postgraduate
(CX2015B228). We also thank the National Center for Magnetic
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