Effective solvent-free oxidation of cyclohexene to allylic products with oxygen by mesoporous etched halloysite nanotube supported Co2+

Article abstract of DOI:10.1039/c7ra11245a

One dimensional mesoporous etched halloysite nanotube supported Co²⁺ is achieved by selective etching of Al₂O₃ from halloysite nanotube (HA) and immersing the etched HA (eHA) into the Co(NO₃)₂·6H₂O solution consecutively. By facilely tuning the etching time and the weight ratio of Co(NO₃)₂·6H₂O to eHA, the morphology, specific surface area and the supported Co²⁺ content of the mesoporous material can be tuned. The method for mesoporous material is scaled up and can be extended to other clay minerals. The mesoporous eHA supported Co²⁺ is used as catalyst for the selective catalytic oxidation of cyclohexene in solvent-free reaction system with O₂ as oxidant. The results shows the catalytic activity is dependent on etching time, weight ratio of Co(NO₃)₂·6H₂O to eHA, calcination treatment and reaction time/temperature. Among them, mesoporous eHA supported Co²⁺ prepared with 18 h etching time and 2:1 Co(NO₃)₂·6H₂O/eHA weight ratio without calcination (HA/HCl-18 h/Co²⁺-2:1) demonstrates the highest catalytic activity under 75 °C reaction temperature and 18 h reaction time (58.30% conversion and 94.03% selectivity to allylic products). Furthermore, HA/HCl-18 h/Co²⁺-2:1 has exhibit superior cycling stability with 37.69% conversion and 92.73% selectivity to allylic products after three cycles.

Full text of DOI:10.1039/c7ra11245a

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PAPER
Effective solvent-free oxidation of cyclohexene to
allylic products with oxygen by mesoporous etched
halloysite nanotube supported Co †
Cite this: RSC Adv., 2018, 8, 14870
2
+
Cuiping Li, * Yue Zhao, Tianwen Zhu, Yan'ge Li, Jiajia Ruan and Guanghui Li
2+
One dimensional mesoporous etched halloysite nanotube supported Co is achieved by selective etching
of Al from halloysite nanotube (HA) and immersing the etched HA (eHA) into the Co(NO $6H
solution consecutively. By facilely tuning the etching time and the weight ratio of Co(NO $6H O to
O
2 3
3
)
2
2
O
3
)
2
2
2+
eHA, the morphology, specific surface area and the supported Co content of the mesoporous material
can be tuned. The method for mesoporous material is scaled up and can be extended to other clay
2+
minerals. The mesoporous eHA supported Co is used as catalyst for the selective catalytic oxidation of
cyclohexene in solvent-free reaction system with O as oxidant. The results shows the catalytic activity is
dependent on etching time, weight ratio of Co(NO $6H O to eHA, calcination treatment and reaction
2
3
)
2
2
2+
time/temperature. Among them, mesoporous eHA supported Co prepared with 18 h etching time and
2+
Received 12th October 2017
Accepted 9th April 2018
2
: 1 Co(NO
3
)
2
$6H
2
O/eHA weight ratio without calcination (HA/HCl-18 h/Co -2 : 1) demonstrates the
ꢀ
highest catalytic activity under 75 C reaction temperature and 18 h reaction time (58.30% conversion
2+
DOI: 10.1039/c7ra11245a
and 94.03% selectivity to allylic products). Furthermore, HA/HCl-18 h/Co -2 : 1 has exhibit superior
rsc.li/rsc-advances
cycling stability with 37.69% conversion and 92.73% selectivity to allylic products after three cycles.
12–15
surface area and good stability are extensively investigated.
Through surface modication of the mesoporous material, and
1
Introduction
Cyclohexene oxidation is an important reaction for the synthesis the active center coordination with the homogeneous catalyst,
of oxygen-containing intermediates such as epoxy cyclohexane, the mesoporous material supported catalyst is obtained and
1
,2-cyclohexanediol, adipic acid, 2-cyclohexen-1-one, 2- demonstrates good cyclohexene catalytic oxidation activity. For
cyclohexen-1-ol and cyclohexene peroxide, which widely applies example, Khalili et al. support metal porphyrin (Mn(III)TClPPCl,
1
in medicine, pesticides, spices, surfactant and polymer area.
Fe(III)TClPPCl and Co(III)TClPPCl) on MCM-48, and achieve 80%
Among various oxidation products, allylic oxidation products (2- cyclohexene conversion and 95% selectivity to 2-cyclohexen-1-one
cyclohexen-1-one and 2-cyclohexen-1-ol) arouse particularly with MCM-48 supported Fe(III)TClPPCl as catalyst and TBHP as
ꢀ
attention because of their multifarious application in pharma- oxidant in CH CN at 60 C. Rahiman et al. demonstrate MCM-
3
12
2–4
ceutical, chemical and material industries. However, due to the 41 supported metal porphyrins have various selectivity: the main
presence of multiple oxidation sites-unsaturated C]C double oxidation product is epoxy cyclohexane when with MCM-41
bond and a-H atoms, cyclohexene oxidation usually accompanies supported manganoporphyrin as catalyst; whereas the main
poor selectivity to allylic product, which will result in complex oxidation product is allylic products when with MCM-41 sup-
5
13
oxidation products and difficulty in separation. The commonly ported ferriporphyrin as catalyst. Park et al. prepare meso-
catalysts for the catalytic oxidation of cyclohexene to allylic porous silica with porphyrin structure by microwave co-
product is supported metal based catalyst as they have the condensation, and Fe(III) as the central atom is introduced into
advantage of high stability, high catalytic activity and easy sepa- porphyrin by ion exchange. The mesoporous silica supported
6
–15
ration and recycling
when compared with the homogeneous ferriporphyrin exhibits 41.5% cyclohexene conversion and 68.9%
ꢀ
14
catalyst system. For various supports for metal based catalyst, selectivity to allylic product at 60 C. However, to form uniform
mesoporous materials with tunable pore size, large specic and ordered pore channels, so template agent and high
temperature calcinations are usually involved in, which result in
complicated, time-consuming and energy consumption prepa-
School of Chemical Science and Technology, Key Laboratory of Medicinal Chemistry ration process.
for Natural Resource, Ministry of Education, Yunnan University, Kunming 650091,
On the other hand, halloysite nanotube is a type of natural
occurring clay minerals with nanotubular structures and
China. E-mail: licp830@iccas.ac.cn; Fax: +86 871 6503 1567
†
Electronic supplementary information (ESI) available: Data of HA, eHA and
2+
consists of two-layered aluminosilicate with
a
chemical
eHA@Co ; effect of reaction condition on cyclohexene oxidation and recycling
16
2+
2 2 5 4 2
composition of Al Si O (OH) $nH O).
Its structure and
stability study with HA/HCl-18 h/Co -R as catalyst. See DOI: 10.1039/c7ra11245a
14870 | RSC Adv., 2018, 8, 14870–14878
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chemical composition is similar to that of kaolinite, dickite or were regulated by the etching time. A typical procedure to
24
nacrite except for the unit clay layers are separated by a mono- prepare mesoporous eHA was as follows: HCl solution (36–
17
layer of water molecules. Compared to other supports, hal- 38%, 350 mL) and halloysite nanotubes (6.25 g) were added into
loysite nanotube has the advantage of abundant reserves, high a 500 mL round ask. Aer dispersion with ultrasonication, the
ꢀ
porosity and excellent chemical stability, which is usually ask was placed in 80 C oil bath, stirred and reuxed to
1
8–27
utilized as an attractive support for catalyst.
2 3
In our previous selectively remove Al O . At certain etching time, some samples
24
studies, it has been demonstrated that mesoporous SiO
SiO /Al
selective etching of the interior Al
2
and were withdrawn, washed repeatedly with water to neutral and
ꢀ
2
2
O
3
can been controlled synthesized in large scale by dried at 75 C. The eHA was marked as HA/HCl-T, wherein HA,
2
O
3
from hydrophobically HCl and T, respectively, represented halloysite nanotube, HCl
modied halloysite nanotube or halloysite nanotubes by HCl. as etching solution and etching time.
As the selective etching by HCl is the positive charged Al O , the
2
3
surface of the etched halloysite nanotubes should be negative 2.3 The preparation of mesoporous etched halloysite
2
+
charged and can induce the adsorption of metal cation to nanotubes supported Co
achieve mesoporous etched halloysite nanotubes supported
2+
In the preparation of mesoporous eHA supported Co
metal ions. Although mesoporous material supported metal
cation has already been used in cyclohexene oxidation, mostly
involves in supported metal compounds and complicated
modication/treatment before supporting. And it is usually
2+
(
eHA@Co ), the above eHA without any modication was
directly used as the support of Co and the concentration of eHA
was xed. A typical process was as follows: eHA (0.50 g),
3 2 2
Co(NO ) $6H O (0.25–2.00 g) and ethanol (100 mL) were added
into a 150 mL conical ask. Aer dispersion with ultrasonication,
the ask was stirred at room temperature for 24 h. Then it was
separated by centrifugation, washed with ethanol to colourless
and dried at 75 C. This eHA supported Co was marked as HA/
HCl-T/Co -R, wherein R represented the weight ratio of
2+
12–15
conducted with toxic, volatile solvent and expensive oxidant.
For all we know, the inuence of etching time, weight ratio of
Co(NO $6H O to etched HA (eHA) in the fabrication on the
cyclohexene oxidation activity of mesoporous eHA supported
3
)
2
2
ꢀ
2+
2
+
2+
Co (eHA@Co ), especially the environment-friendly cyclo-
hexene oxidation process (solvent-free, O as oxidant and only
bubbling with O a few minutes before oxidation reaction) and
2+
2
Co(NO
amplied; when compared with the amount of ethanol (100 mL),
the amounts of eHA and Co(NO $6H O were low for well
dispersion of eHA and Co(NO $6H O to achieve well dispersion
of Co in eHA; and the amount of Co(NO $6H O was overdose
in order to achieve adsorption saturation.
3 2 2
) $6H O to eHA. Noting: the preparation process can be
2
effective catalytic activity (58.30% conversion and 94.03%
)
3 2
2
selectivity to allylic products) have rarely been reported.
)
3 2
2
2
+
In this study, the mesoporous eHA supported Co
2+
)
3 2
2
2
+
(
eHA@Co ) are prepared through HCl selective etching and in
2
+
situ Co adsorption, consecutively. By simply tuning the etching
2
+
time and feeding amount of Co in the preparation, mesoporous
2.4 Characterization
2
+
2+
eHA@Co with different BET specic surface area and Co
content can be achieved. The inuence of catalyst (etching time,
2
+
The morphology of HA/HCl-T and HA/HCl-T/Co -R was char-
acterized by JSM-2100 under an accelerating voltage of 200 kV.
FT-IR was conducted on a Thermo Nicolet iS10 instrument with
KBr pellet as background. The diffuse reectance spectra of UV-
vis were performed on a Hitachi U-4100 PC photometer from
200 to 800 nm. Energy-dispersive X-ray (EDX) analysis was per-
formed with a FEI Quanta-200 scanning electron microscope
2
+
feeding amount of Co and calcination in the preparation) and
reaction condition (reaction time/temperature, solvent and
oxidant) on the cyclohexene selective oxidation of mesoporous
2
+
eHA@Co are investigated in detail. In addition, the cycling
2
+
stability of eHA@Co is also investigated. In line with the results,
a mechanism for cyclohexene oxidation is proposed.
(
SEM) equipped with an EDX analyzer operated at an acceler-
ating voltage of 10 kV. The quantication of the EDX analysis is
based on the eZAF Smart Quant method calibrated with pure
reference materials-Cu and Al sheet. Nitrogen adsorption–
desorption was performed on a Micromeritics (USA) Tristar II
2
Experimental
2
.1 Materials
Halloysite nanotubes were purchased from Fenghui Minerals
Trade Co., Ltd. Cobalt(II) nitrate hexahydrate (Co(NO $6H O)
3020. The oxidation product was detected by a gas chromato-
3
)
2
2
graph (Agilent 7890A). The gas chromatography was performed
in a Agilent 19091J-413 column with a cross-linked 5% phenyl
methyl siloxan (30 m  320 mm  0.33 mm) and a FID detector
and cyclohexene (99%) were purchased from Shanghai Titan
Scientic Co., Ltd. Hydrochloric acid (HCl), ethanol and tri-
chloromethane (CHCl ) were purchased from Chongqing
Chuandong Chemical (Group) Co., Ltd. O2 (99.99%) was
purchased from Kunming Messer Gases Products Co., Ltd. All
the chemical reagents were analytical grade and used directly.
3
27
À1
under the following conditions: carrier gas (N
2
, 40 mL min );
ꢀ
temperature program-40 C holding for 2 min, then heating to
ꢀ
ꢀ
À1
ꢀ
5
5
0
C at 10 C min , nally further heating to 300 C at
ꢀ
À1
0 C min
.
2
.2 Selective etching of halloysite nanotubes
In the preparation of mesoporous etched halloysite nanotube
eHA), the amounts of halloysite nanotubes and 36–38% HCl Catalyst (HA/HCl-T/Co -R, 40 mg) and cyclohexene (0.8 mL)
were constant and the etching degree and morphology of eHA were added to a 50 mL round-bottom ask. Aer bubbled with
2.5 Selective oxidation of cyclohexene by mesoporous etched
halloysite nanotubes supported Co
2
+
2
+
(
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2 2 3
O for 10 min at appropriate ow rate to avoid the blown away of morphology is lost and cavity disappears when the Al O is
ꢀ
24,28
cyclohexene, the ask was placed in 75 C oil bath, stirred and completely removed.
reuxed (noting: the joint of the snake-shaped condenser and FT-IR is further used to characterize the eHA. Along
round-bottom ask must be carefully sealed by PTFE tape, increasing the etching time, the bands intensity of the Al OH-
2
À1
paralm “M,” and electric insulation tape consecutively to avoid stretching (3696 and 3620 cm ) and the single Al
2
OH-
À1
16,24,28
the evaporation of cyclohexene). Aer 18 h, it was separated by bending (912 cm ) gradually decrease (Fig. 2).
In addi-
centrifugation, then 25 ml supernatant was withdraw and tion, the bands intensity of Si–O–Al in plane bending mode
À1
diluted with
characterization.
1
mL CHCl
3
for gas chromatograph (754, 692 and 534 cm ) also show the same trend with the
increase of etching time (along further etching). Correspond-
À1
ingly, SiO–H vibrations band at 3200–3700 cm appears (the
formation of silica) and the band intensity of symmetric Si–O–Si
À1
3
Results and discussion
stretching (798 cm ) increases with the etching time. The
above result demonstrates the removal of Al
2
O
3
can be
3.1 Characterization of mesoporous etched halloysite
24,28,29
2
+
controlled.
Eventually, all the Al O characteristic bands
2
3
nanotubes supported Co
disappear (trace e, Fig. 2). The Si–O stretching band of halloysite
The halloysite nanotube used for synthesis of mesoporous
support for Co is about 1–2 mm length, 20–30 nm inside
diameter and 40–60 nm outside diameter (Fig. S1 in ESI†). It
consist of 17.5% Si, 17.6% Al and 64.9% O and with 31.7 m g
BET surface area.
À1
nanotubes (1032 cm ) completely disappears accompanied by
2
+
the enhance of the in-plane Si–O–Si deformation peak
1092 cm ) and the nal spectra bear a resemblance to that of
amorphous SiO2. The eHA is further characterized by XRD and
EDX. It demonstrates the single 1 : 1 layer aluminosilicate
À1
(
2
À1
30
19,20,22–27
To demonstrate the Al O in halloysite
2
3
nanotubes can be controlled etched by HCl, the TEM of etched
halloysite nanotubes (eHA) at different etching time is given in
Fig. 1. It shows when the etching time is 6 h, the inside diameter
of eHA increase from the original 20–30 nm to 45–55 nm and
the interior surface is coarse. Furthermore, part of the tubular
structure has been collapsed (Fig. 1a). This sample is labelled as
HA/HCl-6 h. With increase the etching time, for example 7.5 h
˚
˚
thickness (7.30 A), other typical reections of halloysite (4.41 A,
˚
˚
˚
˚
˚
˚
3
.61 A, 2.49 A, 2.36 A, 2.22 A, 1.68 A, 1.48 A) (Fig. S2 in ESI†) and
Al/Si atom ratio (Table S1 in ESI†) decrease with etching time,
and eventually disappear with a broad band peak present which
2
is attributed to amorphous SiO for XRD and to nearly zero for
Al/Si atom ratio, respectively. The above results are consistent
with the result of HCl selective etching of octadecyltri-
chlorosilane silane (C18) modied halloysite nanotube.
The mesoporous structure of eHA is characterized by
nitrogen adsorption–desorption. It exhibits clearly halloysite
nanotube itself is a mesoporous material (Fig. 3). In addition,
according to the International Union of Pure and Applied
Chemistry (IUPAC) classication (Fig. 3, Table S2 in ESI†), the
adsorption–desorption isotherm of halloysite nanotubes is
ascribed to type IV with H3 hysteresis loops as majority of the
(HA/HCl-7.5 h) and 11 h (HA/HCl-11 h), nearly all of the interior
24
surface is le exposed accompanied by the disappearance of the
inner cavity (Fig. 1b and c). When the etching time is 18 h,
porous nanorods with totally lost of the tubular structure are
achieved (HA/HCl-18 h) (Fig. 1d). Based on the TEM results, the
process for selective etching of Al
as follows: rstly, nanotubes with varying enlargement of the
cavity are achieved when a handful of Al is removed;
2 3
O from HA can be explained
2 3
O
secondly, pores in the HA walls begin to appear and grow with
31
pore diameter were in the range of 2–8 nm and 15–35 nm. The
average pore diameter, pore volume and BET surface area of
the etching time when more Al O is removed; nally, tubular
2
3
Fig. 2 FT-IR of halloysite nanotubes (a) and halloysite nanotubes after
ꢀ
Fig. 1 TEM images of halloysite nanotubes after being etched by HCl being etched by HCl at 80 C for (b) 6 h, (c) 7.5 h, (d) 11 h and (e) 18 h,
ꢀ
at 80 C for (a) 6 h, (b) 7.5 h, (c) 11 h and (d) 18 h, respectively.
respectively.
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porosity (Fig. 3). It also can be seen with the increase of etching
time, the average pore diameter and pore volume of the eHA
also increase, implying they can be conditioned by the etching
time. As an example, with the increase of etching time from 6 h
to 18 h, the average pore diameter and pore volume, respec-
3
À1
tively, increase from 6.8 nm to 12.0 nm and 0.56 cm g to 0.87
3
À1
cm g
.
2
+
The above mesoporous eHA is used to support Co by an
2
+
impregnation method and marked as HA/HCl-T/Co -R,
wherein R represent the weight ratio of Co(NO ) $6H O to eHA.
3
2
2
Fig. 4 shows the TEM of HA/HCl-18 h aer immerging in
different concentration of Co(NO ) $6H O solution. It is
3
2
2
demonstrable that the rough/porous structure of HA/HCl-18 h is
not affected. Moreover, there are 2.5 nm black spots on eHA and
the amount increases with the increase of weight ratio of
Co(NO
3 2 2
) $6H O to HA/HCl-18 h (Fig. 4, Table S3 in ESI†). We
2
+
ascribe them to the Co ionic clusters. The Co content of the
HA/HCl-18 h/Co -R is roughly determined by EDX. The FT-IR of
eHA@Co has no signicant difference from that of eHA
2
+
2
+
2
+
(Fig. S3 in ESI†). The above results indicate Co can be sup-
ported on the eHA by the impregnation method and the sup-
2
+
ported amount of Co can be tuned by the feeding amount of
weight ratio of Co(NO $6H O to HA/HCl-18 h in the prepara-
tion. It's hard to ignore that the Al/Si atom ratio and Co content
wt%) determined by EDX are not accurate as there are many
3
)
2
2
(
factors affecting the EDX test result, for example the atness/
concentrations of sample, the distribution of elements, accel-
erating voltage of SEM and so on. However, EDX can be used to
present the change trend of the Al/Si atom ratio and Co content
with the increase of etching time and the weight ratio of
3 2 2
Co(NO ) $6H O to eHA, respectively.
Fig. 3 Nitrogen adsorption–desorption isotherms (A) and pore size
distribution curves (B) of halloysite (HA) and HA/HCl-T.
3
.2 Solvent-free oxidation of cyclohexene to allylic products
2
+
with oxygen by mesoporous eHA@Co
3
À1
halloysite nanotubes are respectively 11.5 nm, 0.16 cm g and
2
+
2+
2
À1
Mesoporous eHA@Co (HA/HCl-T/Co -R) is directly used as
3
1.7 cm g . Compared with those of halloysite nanotubes, the
pore volume of eHA is higher and the pore diameter is lower
Fig. 3B, Table S2 in ESI†) when the etching time is lower than
8 h and an opposite situation is observed when the etching
ꢀ
catalyst for cyclohexene oxidation at 75 C with O
2
as oxidant
(
1
time is 18 h (Fig. 3B, Table S2 in ESI†). Aer selective etching,
mesopores in the sizes of 3.0 nm observed in the parent hal-
loysite nanotubes has shied to larger size, especially for the
HA/HCl-18 h sample (has shied to 5.4 nm), while the pores
about 14 nm has shied to higher position or has disappeared.
This indicates that the cavity in the eHA has been partly
destroyed or completely disappeared, which is in good agree-
ment with the results of the TEM images. What is deserved to
mention that the BET surface area is increased evidently from
2
À1
2
À1
3
1.7 m g for the halloysite nanotube to 176.5 m g for the
HA/HCl-6 h sample. However, with further increasing the
etching time, the increase of BET surface area is not signicant,
for example, with the etching time increase from 6 h to 18 h, the
2
À1
2
BET surface area just increases to 228.4 m g from 176.5 m
g
À1
(Table S2 in ESI†). The raise of the BET surface area of eHA
from hal-
could be resulted from the selective etching of Al
2
O
3
2+
2+
Fig. 4 TEM images of eHA@Co : (a) HA/HCl-18 h/Co -0.5 : 1, (b)
2+
2+
loysite nanotubes, which will increase surface roughness and HA/HCl-18 h/Co -1 : 1, (c) HA/HCl-18 h/Co -2 : 1 and (d) HA/HCl-
2+
1
8 h/Co -4 : 1.
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noting: the reaction system is only bubbled with O
Paper
(
2
for 10 min NaOH. The sample etched by NaOH for 18 h (HA/NaOH-18 h) is
2
+
before reaction; during the reaction, no O2 is bubbled or O2 used to support Co with 1 : 1, 2 : 1 and 4 : 1 weight ratio of
balloon is added). And for facilitating comparison, the cyclo- Co(NO ) $6H O to HA/NaOH-18 h. It still shows the optimal
3
2
2
hexene oxidation efficiency of HA, HA/HCl-18 h (etched with HCl weight ratio is 2 : 1, which achieves 29.06% cyclohexene
2
+
2+
2+
for 18 h, but without supporting Co ), HA/Co -2 : 1 (HA directly conversion. For the oxidation product of HA/NaOH-18 h/Co -R,
2
+
supporting Co at 2 : 1 weight ratio of Co(NO
and bulk Co(NO $6H O are also studied. It is demonstrable that However, the catalytic activity of HA/NaOH-18 h/Co -R is lower
cyclohexene oxidation with HA, HA/HCl-18 h and HA/Co -2 : 1 as than that of HA/HCl-18 h/Co -R at the same weight ratio of
catalyst and catalyst-free demonstrates extremely low cyclo- Co(NO $6H O to eHA, indicating HA/HCl-T can support more
hexene conversion (1.63%, 9.30%, 9.58% and 8.13%, respec- Co owing to the negative charged of SiO in HA/HCl-T.
tively). Nevertheless with HA/HCl-T/Co -R as catalyst,
cyclohexene conversion increase signicantly, for example, with oxidation catalyzed by HA/HCl-T/Co -R exceeds 90%, inde-
HA/HCl-18 h/Co -2 : 1 as catalyst, the cyclohexene conversion pendence on the etching time and Co(NO
3 2 2
) $6H O to HA) it is still allylic product (the selectivity is higher than 79%).
2
+
)
3 2
2
2
+
2+
3
)
2
2
2
+
2
2
+
Notably, the selectivity to allylic product of cyclohexene
2
+
2
+
3 2 2
) $6H O/eHA weight
increases to 58.30%. In combination of 9.58% and 9.30% cyclo- ratio in the preparation, which will simplify the later purica-
2
+
hexene conversion with HA/Co -2 : 1 and HA/HCl-18 h as cata- tion and separation. The cyclohexene conversion and selectivity
lyst, respectively, the above results signify both of the porous to allylic products (2-cyclohexen-1-one and 2-cyclohexen-1-ol)
2
+
2+
structure of eHA and supported Co are contributed to the with HA/HCl-18 h/Co -2 : 1 as catalyst are 58.30% and
2
+
enhanced cyclohexene oxidation activity of HA/HCl-T/Co -R. 94.03% respectively, indicating it can selectively oxidize cyclo-
However, the cyclohexene oxidation efficiency of HA/HCl-T/Co - hexene to allylic products. Both of the cyclohexene conversion
R is varied along the change of etching time (T) and weight ratio and selectivity to allylic products are higher than that of Co(II)-
2
+
3 2 2
of Co(NO ) $6H O to eHA (R) in the preparation. For the inu- containing MOFs (Co-MOF, 32.80% conversion and 90.80%
ence of etching time, at 2 : 1 weight ratio of Co(NO
eHA, cyclohexene conversion increase with the etching time. For ture (75 C) and time (18 h) are lower than that of Co(II)-con-
example, when the etching time increases from 0 h to 18 h, the taining MOFs (80 C and 20 h) and the simplication of
3
)
2
$6H
2
O to selectivity to allylic products), even both the reaction tempera-
ꢀ
ꢀ
cyclohexene conversion increases from 9.58% to 58.30% and an oxidation process (only bubbled with O
2
for 10 min before
optimal etching time is 18 h (Entry 2–6, Table 1). The tendency is reaction; during the reaction, no O is bubbled or O balloon is
2
2
6
consistent with the BET surface area of eHA (longer etching time added). Based on the above results, the etching time and
will result in higher BET surface area, which nally lead to higher Co(NO ) $6H O/eHA weight ratio in the preparation will exert
3
2
2
2
+
Co supporting content). Evidently, the etching time in the a signicant impact on the cyclohexene oxidation efficiency of
preparation will affect the cyclohexene oxidation efficiency of HA/ HA/HCl-T/Co -R. Thereinto, HA/HCl-18 h/Co -2 : 1 performs
2
+
2+
2
+
HCl-T/Co -R.
the highest catalytic efficiency, corresponding to a 1209.98 TON
O to eHA in the and 1140.69 mmol g
À1
À1
For the effect of weight ratio of Co(NO
3
)
2
$6H
2
h
mass-normalized activity. This
preparation, when the weight ratio of Co(NO
3
)
2
$6H
2
O to eHA is activity is signicantly higher than that of halloysite nanotubes
between 0.5 : 1–2 : 1 (Table 1, Fig. S4 in ESI†), cyclohexene supported PANI (PANI@HA/1 M/2.04-HCl, 148.66 TON and
À1
À1 27
conversion increases with the increase of Co(NO ) $6H O/eHA 67.33 mmol g
h
)
and Co(II)-containing metal–organic
3
2
2
À1
weight ratio, and then when the Co(NO ) $6H O/eHA weight frameworks (MOFs) (Co-MOF, 80.22 TON and 68.07 mmol g
3
2
2
À1
6
2+
ratio is in the range of 2 : 1–4 : 1, the cyclohexene conversion
decreases with the increase of Co(NO $6H O/eHA weight oxidation of cyclohexene and chosen for further studies.
ratio. An optimal Co(NO $6H O/eHA weight ratio in the Furthermore, the effect of calcinations on the cyclohexene
fabrication is 2 : 1. The effect of Co(NO $6H O/eHA weight catalytic oxidation activity of HA/HCl-18 h/Co -2 : 1 and HA/
ratio on the conversion is attributed to a cooperative effect of Co NaOH-18 h/Co -2 : 1 is investigated. HA/HCl-18 h/Co -2 : 1
h ). So HA/HCl-18 h/Co -2 : 1 is an effective catalyst for allylic
3
)
2
2
3
)
2
2
2
+
3
)
2
2
2
+
2+
2
+
2+
ꢀ
content and Co dispersion on the eHA. Combined with the and HA/NaOH-18 h/Co -2 : 1 are treated at 550 C for 6 h, cited
EDX result that the Co content increases with the increase of as HA/HCl-18 h/Co -2 : 1/550 C-6 h and HA/NaOH-18 h/Co -
Co(NO ) $6H O/eHA weight ratio (Table S3 in ESI†), the cyclo- 2 : 1/550 C-6 h, respectively (Entry 2 and 4, Table S4 in ESI†).
hexene oxidation efficiency of HA/HCl-T/Co -R will be affected And they are used as catalyst and compared with those without
2
+
ꢀ
2+
ꢀ
3
2
2
2
+
2
+
by the Co content in the mesoporous eHA@Co (Co(NO
6H O/eHA weight ratio in the preparation). It is deserved to calcinations will decrease the catalytic activity of HA/HCl-18 h/
mention that the cyclohexene conversion, TON and r
3
)
2
-
calcinations (Entry 1 and 3, Table S4 in ESI†). It is demonstrable
$
2
2
+
w
of HA/ Co -2 : 1, whereas it has a positive effect on HA/NaOH-18 h/
2
+
2+
HCl-18 h/Co -2 : 1 are higher than those of the bulk Co -2 : 1. It can be explained by the fact calcinations will lead
Co(NO ) $6H O (Entry 6, 14, Table 1) at the equivalent amount to the aggregation of Co or cobalt oxide for HA/HCl-18 h/Co -
of Co, indicating the porous structure of HA/HCl-18 h can well 2 : 1 owing to the adsorption saturation of Co on HA/HCl-18 h/
disperse Co . About the selectivity, the main oxidation prod- Co -2 : 1, whereas it will solid the adsorbed Co on HA/NaOH-
2
+
2+
3
2
2
2
+
2
+
2+
2+
2
+
ucts are allylic products (2-cyclohexen-1-one and 2-cyclohexen- 18 h/Co -2 : 1 because NaOH etching will lead to partly positive
1
2 3
-ol) and the selectivity to allylic products accounts for more charged Al O , which will produce electrostatic repulsion to
2
+
2+
than 90%.
Co . Therefore, HA/HCl-18 h/Co -2 : 1 is selected for optimi-
zation of reaction condition.
Furthermore, as the chemical composition of HA is Al
2 2
Si -
O
5
(OH) $nH O), the SiO in HA can be selectively etched by
4
2
2
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2+
2+
Table 1 Cyclohexene conversion and selectivity of HA/HCl-T/Co -R and HA/NaOH-T/Co -R
Selectivity (%)
a
b
À1
Catalyst
HA
HA/Co -2 : 1
HA/HCl-6 h/Co -2 : 1
Conversion (%)
TON
w
r (mmol g h)
0
0
0
0
5.52
10.76
5.10
8.97
5.14
5.70
0
1.10
4.88
4.70
8.76
17.95
7.48
3.52
50.34
99.50
59.74
45.86
69.02
53.36
65.57
54.60
59.17
51.73
49.16
60.68
49.22
54.22
60.02
60.87
14.68
0.05
34.74
43.38
25.88
37.66
28.84
39.43
39.31
43.83
42.46
33.53
40.93
25.77
31.68
30.20
29.78
0.45
1.63
9.58
—
—
—
—
—
—
—
—
—
—
1140.69
—
1004.20
607.88
478.67
—
—
—
265.90
—
67.33
2
+
2
+
10.49
16.82
23.92
58.30
9.30
18.33
26.63
29.27
19.21
29.06
22.51
13.59
8.13
2
+
HA/HCl-7.5 h/Co2 -2 : 1
+
HA/HCl-11 h/Co2 -2 : 1
0.45
0.27
1.51
3.34
3.50
1.09
1.10
2.05
0.82
5.40
5.20
0
+
HA/HCl-18 h/Co -2 : 1
1209.98
—
1065.20
644.80
507.74
—
HA/HCl-18 h
2
+
HA/HCl-18 h/Co2 -0.5 : 1
+
HA/HCl-18 h/Co -1 : 1
2
+
HA/HCl-18 h/Co -4 : 1
2
+
HA/NaOH-18 h/Co2 -1 : 1
+
HA/NaOH-18 h/Co -2 : 1
—
—
2
+
HA/NaOH-18 h/Co -4 : 1
c
Co(NO
Catalyst-free
Halloysite nanotubes supported
3
)
2
$6H
2
O
282.05
—
148.66
98.17
d
PANI (PANI@HA/1 M/2.04-HCl)
Co(II)-containing
metal–organic frameworks
3.05
0
51.26
39.34
32.80
80.22
68.07
e
ꢀ
a
Reaction conditions: 40 mg catalyst, 0.8 mL cyclohexene, O
2
(10 min), 75 C, 18 h. TON (turnover number) is dened as total mol of cyclohexene
b
c
molecules converted per mol of catalyst. Initial reaction rate of cyclohexene consumption normalized by catalyst mass. The added amount of
2+
d
Co(NO
cyclohexene, 2.5 mL H
3 2 2
) $6H O is 1.1 mg, comparable to the amount of Co in 40 mg HA/HCl-18 h/Co -2 : 1. Reaction conditions: 20 mg catalyst, 1.23 mL
e
ꢀ ꢀ
2 2
O , 70 C, 24 h (ref. 27). Reaction conditions: 50 mg catalyst, 5 mL cyclohexene, oxygen balloon, 80 C, 20 h (ref. 6).
In the next, optimization of reaction condition (such as solvent-free reaction system. Noting: the volume of solvent is six
oxidant, solvent, reaction time and reaction temperature) for times (4.8 mL) the volume of cyclohexene (0.8 mL) to avoid the
2
+
cyclohexene oxidation by HA/HCl-18 h/Co -2 : 1 is further evaporation of cyclohexene in the oxidation process. The result
undergoing. At rst, the effect of oxidant on cyclohexene is in Table S6.† It is obvious the catalytic efficiency of the
oxidation is investigated. Considering environmentally-friendly solvent-free reaction system is higher when compared with the
aspect, O
2
and 30% H
2
O
2
are chosen as oxidant. The detailed solvent reaction system (58.30% cyclohexene conversion and
for 94.03% allylic products selectivity). It can also be observed that
0 min before oxidation, cyclohexene conversion achieves high polar solvents (acetic anhydride and DMF) can result in the
8.30%; whereas without bubbling O , the conversion is only selectivity to allylic products (57.70% and 76.70% for acetic
0.41%. Combined with 10.41% cyclohexene conversion of HA/ anhydride and DMF, respectively) decreasing and accompany
result is shown on Table S5.† It is clearly when bubbling O
2
1
5
1
2
2
+
HCl-18 h/Co -2 : 1 without bubbling O (Entry 2, Table S5 in the selectivity to epoxy cyclohexane (20.00% for DMF) and 1,2-
2
ESI†) and 9.30% cyclohexene conversion with HA/HCl-18 h as cyclohexanediol (41.80% for acetic anhydride) increasing (Table
catalyst and O
the role of Co in HA/HCl-18 h/Co -2 : 1 should be catalysized cyclohexene and 1,2-cyclohexanediol can easily desorb from the
to achieve active oxygen species. Whereas, with H as catalyst, thus facilitating the formation of epoxy cyclohexene
oxidant, the cyclohexene conversion is very low, only 25.70% and 1,2-cyclohexanediol.
2
as oxidant (Entry 7, Table 1), we conclude that S6 in ESI†). It could be under the high polar solvent, the epoxy
2
+
O
2
2 2
O
27,32
and the main oxidation product is 1,2-cyclohexandiol (98.70%
selectivity), demonstrating under O and 30% H
hexene oxidation catalyzed by HA/HCl-18 h/Co -2 : 1 investigated. It shows that the cyclohexene conversion increases
Other reaction condition, for example, the effect of reaction
, cyclo- temperature (20–80 C) and reaction time (0–24 h) is further
ꢀ
2
2
O
2
2
+
undergoes different pathway. Considering the facilitation of signicantly with the increase of reaction temperature when the
ꢀ
later purication and allylic products as the main oxidation reaction temperature is in the range of 20–75 C and the
with O
catalytic reaction system.
In the following, the effect of solvent (CH
2
as oxidant, O
2
is selected as oxidant in the current conversion achieves 58.30% with 94.03% allylic products
27
ꢀ
selectivity when the reaction temperature is 75 C (Fig. S5 and
3
CN, CH
2
Cl
2
, THF, Table S7 in ESI†). The increase of catalytic efficiency with the
DMF, n-heptane and acetic anhydride) on the selective oxida- reaction temperature may be ascribed to the fact high temper-
tion of cyclohexene is investigated and compared with the ature can afford sufficient energy for selective oxidation of
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Fig. 5 The effect of reaction time on the cyclohexene conversion and
2+
selectivity of HA/HCl-18 h/Co -2 : 1.
Fig. 6 The cycling stability of cyclohexene oxidation with HA/HCl-18
h/Co -2 : 1 as catalyst.
2+
27
cyclohexene. Then the cyclohexene conversion increases
indistinctly when the cyclohexene oxidation is conducted at
ꢀ ꢀ
8
0 C. So 75 C is selected as reaction temperature in the
following studies. It is worth mentioning the selectivity to allylic
products stays at a high level (>90%) when the reaction
ꢀ
temperature is at 20–80 C. For the effect of reaction time, when
the reaction time is 6 h, the conversion is only 18.84%; never-
theless when the reaction time is 18 h, the conversion quickly
increases to 58.30%; with further increasing the reaction time,
the conversion increases slightly (Fig. 5, Table S8 in ESI†). And
the selectivity to allylic products is higher than 89% within 6–
24 h reaction time, indicating during the oxidation process, the
main oxidation products are allylic products. So the reaction
time is xed at 18 h for cyclohexene oxidation.
Scheme 1 Proposed mechanism of cyclohexene selective oxidation
to allylic product (2-cyclohexen-1-one and 2-cyclohexen-1-ol) with
2+
mesoporous eHA@Co as catalyst.
3
.3 Cycling stability of mesoporous etched halloysite
2
+
nanotubes supported Co
6–15,27
Stability and recycling is an important index for catalyst.
The cycling stability experiments are conducted with HA/HCl-18
3
.4 Cyclohexene selective oxidation mechanism of
mesoporous etched halloysite nanotubes supported Co
Based on the above results, a mechanism for the allylic oxida-
2+
2
+
h/Co -2 : 1 as catalyst and each cycle of the reaction time is
2
+
1
8 h. Aer each cycle, HA/HCl-18 h/Co -2 : 1 is separated by
2
+
tion of cyclohexene over mesoporous eHA supported Co is
proposed as follows: rstly, O will bind to the Co in meso-
2
porous eHA supported Co (eHA@Co ), leading to the form of
O
H atom from cyclohexene to form cyclohexenyl radicals and
hydroperoxo complex (eHA@Co –OOH) synchronously. Then
peroxy radical (HOOc) will be produced by cracking the Co –
OOH bond, accompanied by the recovery of eHA@Co . The
peroxy radical will react with cyclohexenyl radicals to achieve 2-
centrifugation, washed by CH Cl , dried consecutively and used
2
2
2
+
as catalyst for the next cycle of cyclohexene oxidation. The
results are concluded in Fig. 6 and Table S9.† There are strong
signs that cyclohexene conversion decreases slightly aer four
cycles, whereas allylic product selectivity demonstrates different
trend, which maintains higher value (>92%) in the rst three
cycles, but decrease to 35.21% in the fourth cycle. Considering
the sharp declination to allylic product selectivity, we attribute
2
+
2+
2
+
2+
2
-eHA@Co adduct; secondly, O
2
-eHA@Co adduct abstracts
3
+
3
+
2
+
2
+
it to the dissolution of Co in the cycling experiment. However,
2
+
cyclohexene-1-hydroperoxide
(reaction
intermediate)
it still indicates HA/HCl-18 h/Co -2 : 1 can be recycled three
times effectively for cyclohexene oxidation. Considering the
green catalytic reaction system (solvent-free, green oxidant and
6,7
(Scheme 1).
Finally, the formed 2-cyclohexene-1-
hydroperoxide will be decomposed into allylic oxidation prod-
ucts (2-cyclohexen-1-one and 2-cyclohexen-1-ol) according to the
Haber–Weiss mechanism, which involved in the Co cycling of
simple and convenient style of adding O ), high allylic product
2
selectivity and good cycling stability, it will promote to puri-
cation and has potential application prospect in the ne
chemical industry.
3
+
2+ 33,34
Co and Co .
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´
´
M. Sipiczki, A. A. Adam, T. Anitics, Z. Csendes, G. Peintler,
3
4
Conclusions
´
´
´
´
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+
Mesoporous eHA@Co nanotubes or nanorods have been
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2 3
O from halloysite nanotubes and the
2
+
impregnation method. Mesoporous eHA@Co
endowing
2
+
different BET surface area and Co content can be achieved by
simply tuning the etching time and the weight ratio of
Co(NO ) $6H O to eHA in the preparation. The mesoporous
eHA@Co
Co(NO
shows the highest BET surface area and high Co content. The
inuence of etching time and Co(NO $6H O/eHA weight ratio
3
2
2
2
+
prepared with 18 h etching time and 2 : 1
2
+
3
)
2
$6H
2
O/eHA weight ratio (HA/HCl-18 h/Co -2 : 1)
2
+
3
)
2
2
2
+
in the fabrication of eHA@Co on the cyclohexene selective 10 Y. Chang, Y. Lv, F. Lu, F. Zha and Z. Lei, J. Mol. Catal. A:
2
+
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97.
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Chem., 2010, 320, 56.
2
Co(NO
3
)
2
$6H
2
O to eHA in the preparation. The optimal reaction
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as oxidant for 14 E. Y. Jeong, A. Burri, S. Y. Lee and S. E. Park, J. Mater. Chem.,
ꢀ
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2
cyclohexene oxidation to allylic product. According to the
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3
+
2+
attributed to reversible Co cycling between Co and Co in
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2
+
eHA@Co to catalytic formation of active [O] is suggested. This
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2
+
synthesis of mesoporous support to achieve eHA@Co nano-
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this work may highlight the prospect of developing mesoporous
2
+
support for Co as green and recyclable heterogeneous cata-
lysts in allylic oxidation.
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Sci., 2014, 436, 70.
2
0 C. Li, J. Wang, S. Feng, Z. Yang and S. Ding, J. Mater. Chem. A,
Conflicts of interest
2013, 1, 8045.
There are no conicts to declare.
21 E. Abdullayev, K. Sakakibara, K. Okamoto, W. Wey, K. Ariga
and Y. Lvov, ACS Appl. Mater. Interfaces, 2011, 3, 4040.
2
2 J. Liang, B. Dong, S. Ding, C. Li, B. Q. Li, J. Li and G. Yang, J.
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Acknowledgements
The authors gratefully acknowledge the nancial support 23 C. Li, T. Zhou, T. Zhu and X. Li, RSC Adv., 2015, 5, 98482.
provided by the National Natural Science Foundation of China 24 C. Li, J. Wang, X. Luo and S. Ding, J. Colloid Interface Sci.,
(
No. 51563023 and 51003091), the Natural Science Foundation
of Yunnan Province (No. 2013FB002), the Education Research 25 C. Li, J. Wang, H. Guo and S. Ding, J. Colloid Interface Sci.,
Foundation of Yunnan Province (No. 2013Y361), the Program
2015, 458, 1.
for Excellent Young Talents, Yunnan University (No. WX069051) 26 T. Zhou, C. Li, H. Jin, Y. Lian and W. Han, ACS Appl. Mater.
2014, 420, 1.
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14878 | RSC Adv., 2018, 8, 14870–14878
This journal is © The Royal Society of Chemistry 2018