New non-metallic mesoporous SBA-15 catalyst with high selectivity for the gas-phase oxidation of cyclohexylamine to cyclohexanone oxime

Article abstract of DOI:10.1016/j.catcom.2014.06.030

Non-metallic mesoporous SBA-15 is firstly used for the gas-phase selective oxidation of cyclohexylamine to cyclohexanone oxime with air and exhibits the best performance among the investigated silica catalysts. The experimental results show that the catalytic activity is strongly dependent on the pore diameter of SBA-15. By combining catalytic performance experiments with the monitoring of catalyst transformations using themogravimetric analyses, N ₂-physisorption, and solid-state ²⁹Si MAS NMR spectra, we show that the surface silanols play important roles to form a catalytically active carbon species, which is believed to be favorable to the activation of molecular oxygen. The carbon species formed are also studied by IR-spectroscopy, coke analysis by a catalyst dissolution/extraction protocol and elemental analyses.

Full text of DOI:10.1016/j.catcom.2014.06.030

Catalysis Communications 56 (2014) 148–152
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
New non-metallic mesoporous SBA-15 catalyst with high selectivity for
the gas-phase oxidation of cyclohexylamine to cyclohexanone oxime
a,b
b
b
b
b,
c
a,
Wenzhou Zhong , Liqiu Mao , Wenjun Yi , Steven Robert Kirk , Dulin Yin ⁎, Anmin Zheng , Hean Luo ⁎
a
College of Chemical Engineering, Xiangtan University, Xiangtan 411105, PR China
National & Local United Engineering Laboratory for New Petrochemical Materials & Fine Utilization of Resources, Hunan Normal University, Changsha 410081, PR China
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics Chinese Academia of Sciences, Wuhan 430071, 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:
Non-metallic mesoporous SBA-15 is firstly used for the gas-phase selective oxidation of cyclohexylamine to
cyclohexanone oxime with air and exhibits the best performance among the investigated silica catalysts. The
experimental results show that the catalytic activity is strongly dependent on the pore diameter of SBA-15.
By combining catalytic performance experiments with the monitoring of catalyst transformations using
Received 5 February 2014
Received in revised form 21 April 2014
Accepted 30 June 2014
Available online 8 July 2014
themogravimetric analyses, N
2
-physisorption, and solid-state 29Si MAS NMR spectra, we show that the surface
silanols play important roles to form a catalytically active carbon species, which is believed to be favorable to
the activation of molecular oxygen. The carbon species formed are also studied by IR-spectroscopy, coke analysis
by a catalyst dissolution/extraction protocol and elemental analyses.
Keywords:
SBA-15
Cyclohexylamine
Cyclohexanone oxime
Selective oxidation
© 2014 Elsevier B.V. All rights reserved.
1
. Introduction
The selective oxidation of amine is an important chemical transfor-
Silica is generally considered as catalytically inactive solid. Some
silica-based materials have been already proven; however, to be attrac-
tive alternatives to conventional metal-based catalysts for several oxi-
dation reactions, for example, ammoximation of cyclohexanone [10],
oxidation of methane [11], butane [12], carbon oxide [13] or photo-
oxidation processes [14]. They have attracted much interest for their ex-
traordinary catalytic performance, exhibiting an opportunity to develop
new catalysts for more economic and environmentally friendly process-
es. Importantly, the excellent catalytic performances of non-metallic
mation in the synthesis of fine chemicals, medicines, and biologically ac-
tive compounds [1]. Selective oxidation of cyclohexylamine is one such
reaction, one of the products of which viz. cyclohexanone oxime is a
valuable intermediate in the manufacture of nylon-6 [2]. Several oxida-
VI
VII
tion procedures using oxidants such as some salts of Cr or Mn [3],
alkylhydroperoxide [4] and hydrogen peroxide [5] have been reported
in the presence of transition metal compounds as catalysts for this
reaction. Currently, a catalytic system using air/molecular oxygen as a
sole oxidant has been desired in view of green chemistry. For example,
diphenyl-2-picrylhydrazyl and tungstated alumina are found to be good
catalysts for cyclohexylamine selective oxidation [6]. Gold nanoparticles
supported on titania are also known to be effective for oxidation of
cyclohexylamine [7]. The environmental limitation for the usage of sol-
vent and the catalyst separation is, however, still one of the important
remaining problems in the liquid systems. To overcome these problems
and make the processes “greener”, heterogeneously catalyzed reactions
for gas-phase oxidation are under investigation. Most of these solid
2
SiO materials are linked to their manifold structures and surface chem-
istry [15,16]. Mesoporous SBA-15 has attracted a great deal of attention
for applications in the fields of adsorption and heterogeneous catalysis,
due to its well-defined hexagonal structure and high hydrothermal
stability [17]. In catalysis, many efforts have been focused on metal-
supported SBA-15 for heterogeneous catalytic oxidation reactions.
However, in the literature there are also some announcements con-
cerning the activity of SBA-15 silica in the oxidation processes [18].
Herein, we report a highly selective and efficient catalytic method for
the oxidation of cyclohexylamine to oxime by employing mesoporous
SBA-15 itself acting as a metal-free catalyst with air as the terminal.
catalysts are Al
Si(W /Al
2
O
3
-based catalysts such as WO
3 2 3 4
/Al O [8] and H
[
3
O
10
)
4
2
O
3
[9], which exhibit moderate deactivation. In particu-
2. Experimental
lar, the sensitivity of cyclohexylamine to metal catalysts leads to rela-
tively low selectivity for oxime.
2.1. Catalyst preparation
The mesoporous SBA-15 materials were synthesized by a tem-
plate method as described in the previous paper [19]. For a typical
synthesis, 4.0 g of P123 [EO20PO70EO20, poly(ethyleneglycol)-
⁎
Corresponding authors. Tel./fax: +86 731 88872531.
E-mail addresses: dulinyin@126.com (D. Yin), hluo@xtu.edu.cn (H. Luo).
http://dx.doi.org/10.1016/j.catcom.2014.06.030
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

W. Zhong et al. / Catalysis Communications 56 (2014) 148–152
149
block-poly(propyleneglycol)-block-poly(ethyleneglycol), average mo-
lecular weight = 5800, Aldrich] was dissolved in 90 ml deionized
water at ambient temperature under violent stirring. Subsequently,
beads. The weight hourly space velocity (WHSV) of cyclohexylamine
was 43.2 h . The oxidation was performed by air at atmospheric
−
1
pressure and at a temperature of 190 °C. The flow rate of gases was
−
1
3
−1
6
0 ml of 4 mol l HCl (CR, Sinophar) was introduced with continuous
70 cm min . The reactant was fed into the reactor using a syringe in-
fusion pump. The products, collected in the receiver flask, were ana-
lyzed in a gas chromatograph (Agilent 6890N) equipped with a DB-5
capillary column (30 m) and a flame ionization detector using naphtha-
lene as an internal standard. The components of the gas phase were an-
alyzed by gas chromatography with a TCD detector. The products were
identified by GC–MS (SHIMADZU, QP2010 PLUS) and by comparison
with commercially pure products. The conversion of cyclohexylamine
and selectivity of cyclohexanone oxime were calculated using the
following formulas (Eqs. (1) and (2)). All raw materials and products
were established carbon mass balances.
stirring. 9.8 ml of tetraethyl orthosilicate (CR, Damao) was added
dropwise into the former solution at 40 °C. After being stirred continu-
ously for 24 h, the mixture was transferred to a Teflon-lined stainless
steel autoclave and placed in an oven at 100 °C (aging temperature)
for 24 h. The precipitate was in turn filtered, washed with deionized
water, and dried at 80 °C overnight. The obtained powders were finally
heated up to 550 °C at a ramp rate of 1 °C/min and calcined at this tem-
perature in air for 8 h. To investigate the effect of the structural proper-
ties on the catalytic performance, the preparation was conducted with
different aging temperatures (60, 80, 100, 110, 120 and 140 °C) and
the addition of a swelling agent (trimethylbenzene) according to [20],
which allow one to change the pore diameter and pore volume. For
comparison, the other mesoporous silicas MCM-41, SBA-3 and HMS
were also prepared according to [21–23], respectively. Grance gel was
supplied by Aldrich.
Conversion of cyclohexylamine ð%Þ
the amount ðmolÞ of starting cyclohexylamineꢀ
½the amount ðmolÞ of cyclohexylamine recoveredꢀ
ð1Þ
½
−
¼
ꢁ100%
½
the amount ðmolÞ of starting cyclohexylamineꢀ
2
.2. Catalyst characterization
Selectivity of cyclohexanone oximeð%Þ
¼
½
½
the amount ðmolÞ of cyclohexanone oximeꢀ
the amount ðmolÞ of starting cyclohexylamineꢀ
The nitrogen adsorption–desorption measurements were carried
ꢁ 100% ð2Þ
out at −196 °C on a Tristar 3000 sorptometer. Themogravimetric
analyses (TGA) were performed with TA Instruments equipment
from 50 to 800 °C operating at 10 °C/min under airflow. Solid-state
−½the amount ðmol of cyclohexylamine recoveredÞꢀ
2
9
Si MAS NMR spectra were recorded on a Varian Infinitypuls at a
Larmor frequency of 121.35 MHz. The chemical shifts for the 29Si res-
onance were referred to tetramethylsilane. All solid-state NMR
experiments were carried out with a spinning rate of 8 kHz and
silanol concentrations were calculated by dividing the molar fraction
of the respective silicon species by the molar weight of the sample
3. Result and discussion
3.1. Catalytic performance of various silica catalysts
Catalytic performances of various silica materials are summarized in
Table 1. As expected, the blank experiment without any catalyst gives
no cyclohexylamine conversion as a result of the catalytic oxidation re-
action. High activity is obtained over SBA-15 to yield a conversion of
11.2% (with 96.6% cyclohexanone oxime selectivity), which is remark-
ably higher than those on the other silica catalysts. When silica gel is ap-
plied to the reaction, the selectivity for the oxidation is as low as 61.3%,
which is the lower compared with the other mesoporous silica catalysts
(N93%). This great difference between the catalytic activities of SBA-15
and the other silica catalysts is very interesting considering that the var-
ious walls of mesoporous silica and silica gel are all made of amorphous
silica. What is the cause of this great difference? First, we compare the
specific surface area, pore volume size and pore size distribution of var-
(
determined as sum of molar weights and fractions of the different
silicon species). Fourier transformation infrared (FT-IR) spectra
were obtained on an AVATAR 370 Thermo Nicolet spectrophotome-
ter with a resolution of 4 cm . Samples were dehydrated at
2
1
were performed using a Carlo Erba CE Instruments EA 1110.
In order to determine the nature of the carbon species, ca. 15 mg of
the spent SBA-15 was dissolved in 1 cm of a concentrated HF aqueous
−
1
00 °C for 6 h under nitrogen and ground with KBr in the ratio
:150 and pressed into thin wafers. Elemental analyses (C, H, N)
3
3
solution (40%, Sinophar) at room temperature. Subsequently, 4 cm of
CH
its separation from the aqueous phase, was analyzed by HPLC–MS
Agilent 1260/6120).
2 2
Cl was added to extract the soluble carbon material which, after
(
2
ious silica catalysts obtained by using N -physisorption. As shown in
Table 1, the commercial silica gel with the lower specific surface area
has higher catalytic activity than those of mesoporous silica (MCM-41,
HMS, SBA-3). This indicates that the specific surface area is not the
most important factor which determines the oxidation activity of silica.
On the other hand, both SBA-15 and silica gel have relatively high pore
volume with a wide pore size distribution centered above 6 nm and the
higher conversions are achieved, indicating that pore volume size is
more responsible for the formation of oxime. Next, we studied the de-
pendence of catalytic activity on the pore size of the SBA-15 samples,
as shown in Fig. 1. Surprisingly, catalytic activity is strongly dependent
on the pore diameter of the catalyst and is maximized at around
6.1 nm. Smaller or larger pores are not suitable for the catalytic oxida-
tion of cyclohexylamine, which can not be only associated that the
shape selective catalysis may be involved in this process. In the present
reaction, the diameter of cyclohexylamine is 0.79 nm (the distance be-
tween hydrogen and the most distant hydrogen is 0.59 nm; the van
der Waals radius of hydrogen is 0.10 nm) and that of the product should
be 0.86 nm, while the most appropriate pore diameter for the catalyst is
approximately 6.1 nm. The phenomenon may be accounted for by struc-
tural characteristics of SBA-15. First, some synergistic effect may be in-
volved at larger pores, but for pores less than 4 nm in size, diffusion
2
.3. Catalyst testing
The vapor phase oxidation of cyclohexylamine was carried out in a
fixed bed vertical downward flow glass reactor of internal diameter
cm. About 1.2 g of a catalyst was placed at the center of the reactor
2
supported on either side with a thin layer of quartz wool and ceramic
Table 1
a
Physicochemical properties and catalytic performances of various silicas.
Db
S
b
V
b
Conversion
(%)
Selectivity
(%)
Sample
BET
p
2
3
(
nm)
(m /g)
(cm /g)
Silica gel
SBA-15
MCM-41
SBA-3
HMS
Null
9.25
6.13
3.32
2.86
3.01
–
314
783
736
594
785
–
0.86
1.33
0.69
0.33
0.45
–
3.7
11.2
1.6
0.15
0.24
–
61.3
96.6
93.4
98.1
96.4
–
a
Reaction conditions: space velocity 43.2 h−1; air, 70 cm−3/min; temperature, 190 °C;
time, 11 h.
b
p
D: pore size; SBET: surface area calculated by BET method; V : pore volume.

150
W. Zhong et al. / Catalysis Communications 56 (2014) 148–152
1
1
2
0
8
6
4
2
0
traces of cyclohexanone, aniline, phenol, cyclohexanol, cyclohexene,
nitrocyclohexane, and were detected in
the liquid phase products. Some deep oxidation products such as CO
and CO were not detected in the gas phase when the reaction tem-
perature was over 190 °C.
2
The change in the SBA-15 structural characteristics during the reac-
tion is monitored using N
(
2
-physisorption, themogravimetric analyses
TGA) and solid-state 29Si MAS NMR spectra. The catalysts sampled
from the reaction are denoted by SBA-15/xh, where x indicates the reac-
tion time (h). First, we investigate the relationship between the amount
of carbon species present on the catalyst and its catalytic activity. During
the reaction, a weight loss at about 350 °C, which can be observed by
TG-DTA, gradually form, and their number increases, as evidenced by
a color change of the sampled catalysts due to the increase in the surface
amount of carbon species (Fig. S1). Approximately 10.7%, 22.1%, 37.6%
and 40.9% carbon species content are estimated in Table 2, indicating
that carbon species formation occurs very quickly at the beginning
and then slowly increases with time on stream. Meanwhile, the total
3
4
5
6
7
8
9
10
11
Pore Diameter (nm)
Fig. 1. The dependence of catalytic activity on the pore size of the SBA-15 samples.
2
surface area and pore volume dramatically reduce from 783 m /g and
3
2
3
effects may determine the rate. Second, the catayst may be rapidly
deactivated by the deposition of carbon species less than 4 nm in size,
which block the reactants from accessing the active sites. The third pos-
sible explanation is that all of the surface hydroxyl groups on the pore
will point to the center of each pore, and thus they could work as a
group. From these results, we can conclude that SBA-15 served as a cat-
alyst for the vapor-phase oxidation by taking advantage of its specific
structure (like key and lock).
1.33 cm /g to 80 m /g and 0.14 cm /g, respectively, with increasing re-
action time from 1 to 6 h (Table 2). However, a slight decrease in the
total surface area and total pore volume after 6 h reaction is also noticed.
These results are attributed to the change of catalyst surface covered by
the amount of carbon species.
As seen from the catalytic behavior of cyclohexylamine oxidation
(Fig. 2A), N -physisorption and TGA data, at the beginning the forma-
2
tion of a sufficient number of carbon species is accompanied by the ap-
pearance of highly catalytic activity, which indicates that such carbon
species are active in cyclohexylamine oxidation. Moreover, the data do
not display a direct correlation between activity and surface area. We
3
.2. Correlation between catalytic performance and structural characteristics
of SBA-15
2
believe that formed carbon species are responsible for O activation,
Fig. 2A presents the evolution of the catalytic behavior with the time
which is similar to ammoximation of cyclohexanone in the gas phase
[16]. The introduction of small amounts of carbon species in SBA-15
using phenolic resin as carbon precursors serves to activate the original
catalyst. The absence of an induction period for supported carbon spe-
cies in SBA-15 supports this conclusion (Fig. 2B).
on stream over SBA-15 catalyst. We found that the conversion to cyclo-
hexanone oxime was zero in the beginning of the reaction. After 1 h, the
conversion and the oxime selectivity increased progressively, indicating
a process of activation of the catalyst. Along with the reaction time of
Next, the change of the silanols in the samples is assessed by 29Si MAS
NMR technology. As shown in Fig. S2, the spectra of the three samples
display three resonances centered at ca. −110, −100, and −92 ppm
1
1 h, 96.6% cyclohexanone oxime selectivity was achieved at a conver-
sion of 11.2%, and then kept approximately constant for some hours.
After that, the activity progressively decreased due to a deactivation
phenomenon, which might be attributed to the blocking of part of the
mesopore, blocking the reactants from accessing the active sites.
However, the observed oxime selectivity slightly decreased with
the time on stream. This could be due to the fact that the formed
oxime was hydrolyzed by steam that was produced during oxidation
of cyclohexylamine and converted to cyclohexanone. Moreover,
that can be assigned, respectively, to Si(OSi)
(HO) Si(OSi) [Q ] environments around silicon. As such, the measured
contents of silanols are 6.2, 1.8, and 2.9 mmol/g of SiO for SBA-15/0 h,
4 4 3 3
[Q ], HOSi(OSi) [Q ], and
2
2
2
2
SBA-15/6 h, and SBA-15/11 h, respectively. Interestingly, the content of
silanols for SBA-15/6 h is significantly lower than the corresponding
value based on the fresh SBA-15, indicating that the silanol groups in
(
A)1
(B)
00
100
95
16
14
12
10
8
6
4
2
0
9
9
8
5
0
5
90
85
80
75
8
7
0
5
Conversion (%)
Selectivity (%)
1
1
5
0
5
0
15
10
5
Carbon species/SBA-15
SBA-15
Dehydroxylated SBA-15
0
0
5
10
15
20
25
30
35
40
0
2
4
6
8
10
12
Time (h)
Time (h)
Fig. 2. SBA-15 samples (A) and modified SBA-15 samples (B) reaction time in hours. Reaction conditions: space velocity 43.2 h−1; air, 70 cm−3/min; temperature, 190 °C.

W. Zhong et al. / Catalysis Communications 56 (2014) 148–152
151
Table 2
Main characteristics of the SBA-15/xh.
a
a
Total carbonb
species (wt.%)
Total silanolc
(mmol/g)
29Si chemical environment (%)c
Sample
Da
S
BET
V
p
2
3
(
nm)
(m /g)
(cm /g)
Q
2
Q
3
Q
4
SBA-15/0 h
SBA-15/1 h
SBA-15/2 h
SBA-15/6 h
SBA-15/11 h
6.13
6.26
6.00
6.73
6.91
783
608
324
80.1
65.8
1.33
1.15
0.64
0.14
0.15
–
6.2
–
4
–
–
1
2
32
–
64
–
10.7
22.1
37.6
40.9
–
–
–
1.8
2.9
13
19
86
79
a
D: pore size; SBET: surface area calculated by BET method; V
p
: pore volume.
b
c
Estimated from weight loss in TG curve.
29
Quantified by Si MAS NMR.
the samples take part in reacting with the reactants to form carbon
species. At the same time, the reaction with silanol groups (Q and Q ),
produces weak signals in the spectrum and a signal at 101 ppm is signif-
icantly intense due to the formation of Si-O-Si in the SBA-15/6 h. As was
noted in Table 2, a different trend is observed for the SBA-15/11 h. One
with the Si\N bond-containing species [24], from which we can hy-
pothesize that the silica surface OH shows a weak dehydrogenating
power during the oxidation process as confirmed by the detected prod-
ucts such as aniline and phenol. In addition, the band at 1627 cm can
be due to the bending mode of physisorbed water [25].
2
3
−
1
can see that the intensities for the Q
2
and Q
3
group significantly increase
The spent catalysts were dissolved in hydrofluoric acid, liberating
and for the Q significantly decrease, respectively. Due to the hydrolysis of
4
retained hydrocarbons for extraction with CH
tract analysis by HPLC-MS. The composition and abundance of the
retained hydrocarbon soluble in CH Cl from the two spent catalysts
are shown in Fig. S3. Based on the molecular mass of a given (molecular)
ion, the most plausible structure diCHs (dimerized cyclohexylamine de-
2 2
Cl and subsequent ex-
Si\O\Si groups in the surface structure of SBA-15 with the steam pro-
duced by oxidation of cyclohexylamine, these groups are converted to
new silanol groups as new active sites. Moreover, in order to investigate
the silanol groups as active sites, the surface-OHs of SBA-15 are
dehydroxylated using a trimethylsilylation agent and the formation of
oxime decreases, which supports this conclusion (Fig. 2B).
2
2
rivatives
, m/z = 190.1) are the main species, with
amounts of triCHs (m/z = 279.3) and tetraCHs (m/z = 354.3), the cor-
responding trimer and tetramer cyclohexylamine derivatives observed
in all spent samples, with the specific distributions of cyclohexyl-
formed rings being different. Note that an increase in the extent of reac-
tion is accompanied by redistribution between diCHs and triCHs: the
fraction of diCHs in the products slightly drops, while the fraction of
triCHs and tetraCHs increases. This redistribution is obviously caused
by further polymerization reactions of the possible intermediate
cyclohexylimine to triCHs, tetraCHs or pentaCHs on the SBA-15 surface,
which also agrees with the results of the following elemental analyses.
The analytical data of C, H and N percentages for the spent samples
are shown in Table S1. The results indicated that carbon species forma-
tion increases with the time on stream. However, the composition of
carbon species presented as an atom ratio C:H:N = 6:9:1 is almost
3
.3. Analysis of carbon species formed as a function of the time on stream
To obtain further information on the retained carbon species and
their interaction with the SBA-15 framework, FTIR spectroscopy was
performed on the fresh and spent samples. The results are reported in
Fig. 3. The band related to H-bonds in silanols decreases greatly in inten-
sity and shifts to 3357 cm (NH stretchings), indicating some hetero-
geneity in the H-bonded complexes and the band at 1542, 1513, 1457
and 1430 cm
CH groups [24]. The band at 1702 cm can be associated with vibra-
tions of C_C in olefinic compounds and the CH stretching bands are
found at 2932 and 2856 cm [25]. The 2363 and 2341 cm band is
assigned to the v (N\N) band of the containing N-species adsorbed
on active sites are found, in accordance with the previous work [26,
−
1
−
1
are associated with different deformation modes of
−
1
2
−
1
−1
6 9 1 x n
constant, which corresponds to an empirical formula of (C H N O ) .
It was too difficult to get an accurate value for the percentage of O
present.
2
7]. The new bands increase in intensity with time on stream. These re-
sults show that the silanol groups on the surface of spent samples take
part in interactions with reactants, and some N-carbon containing spe-
cies such as olefinic/aromatic compounds are formed in the structure of
the catalytic sample. Further, the band at 1654 cm− can be associated
The “formation of carbon species” pathway may therefore be opera-
tional on silica surfaces. Cyclohexylimine species may be formed via re-
action of cyclohexamine with the surface OH groups, as was proposed
by Dreoni et al. for silica under cyclohexamine atmosphere [24].
Cyclohexylimine species easily undergo oligomerization at reaction
conditions. The silica catalyst was also active in dehydrogenation,
resulting in formation of unsaturated/aromatic hydrocarbon deposits,
in accordance with studies performed with other similar studies [28,29].
1
0
h
1
1
20
00
1
513
542
1
h
3.4. Proposed reaction pathway
1
2
h
On the basis of this observation, a reaction pathway of oxidation of
the amine is proposed by following the next set of processes. First, cy-
clohexylamine should be adsorbed on the catalyst surface before oxida-
tion (cyclohexylamine is not oxidized over dehydroxylated SBA-15).
Subsequently, the autooxidation of cyclohexylamine adsorbed is initiat-
8
6
4
2
0
0
0
0
6
h
1
1h
1
702
3357
ed on the N\H group and forms carbon species, the carbon species can
2
856
−
2
363
1
654
1627
activate oxygen to produce the superoxide radial ion (O
2
) on the sur-
2
341
1457
−
2
932
face of silica [16,30]. The adsorbed amine is rapidly activated by O
to
1
430
2
form the hydroperoxide as shown in the reaction (Scheme 1). Such hy-
droperoxide species in our system are decomposed rapidly to produce
intermediate nitrosocyclohexane. Moreover, the nitroso derivative is
isomerized to oxime.
3
500
3000
2500
2000
1500
Transmittance (%)
Fig. 3. The IR spectra of the fresh and spent samples.

152
W. Zhong et al. / Catalysis Communications 56 (2014) 148–152
Scheme 1. Mechanism of oxidation cyclohexylamine over the SBA-15 catalyst.
[
[
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. Conclusions
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