Enhanced solvent-free selective oxidation of cyclohexene to 1,2-cyclohexanediol by polyaniline@halloysite nanotubes

Article abstract of DOI:10.1039/c7ta02605f

One-dimensional polyaniline@halloysite (PANI@HA) nanotubes with enhanced selective oxidation activity of cyclohexene are fabricated by employing aniline (ANI) chemical polymerization on halloysite nanotubes in situ. By facilely controlling the doping acid, acidity, and ANI/HA weight ratio during the fabrication, PANI with a controllable doping degree, redox state, and content is grown on halloysite nanotubes. The cyclohexene selective oxidation result shows that PANI@HA nanotubes are effective catalysts in a solvent-free reaction system with H₂O₂ as the oxidant, and their catalytic activity relies on the doping acid, acidity, and ANI/HA weight ratio in the fabrication. PANI@HA synthesized with HCl as a doping acid to condition the acidity at 1 M and 2.04 ANI/HA weight ratio (PANI@HA/1 M/2.04-HCl) demonstrates highest catalytic activity (98.17% conversion and 99.50% selectivity to 1,2-cyclohexanediol). The cyclohexene selective catalytic activity matches well with the PANI doping degree in PANI@HA. In addition, the optimal reaction condition is 20 mg catalyst, 2.5 mL H₂O₂, 70 °C, and 24 h. Furthermore, PANI@HA/1 M/2.04-HCl exhibits superior dihydroxylation activity toward 2,3-dimethyl-2-butene and cycling performance with 99.11% conversion and 96.92% selectivity to 1,2-cyclohexanediol after five cycles. The CV of PANI@HA indicates that the cyclohexene selective oxidation is attributed to a reversible redox reaction of PANI in PANI@HA for catalytic decomposition of H₂O₂.

Full text of DOI:10.1039/c7ta02605f

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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ARTICLE
Enhanced solvent-free selective oxidation of cyclohexene to 1,2-
cyclohexanediol by polyaniline@halloysite nanotubes
Received 00th January 20xx,
Accepted 00th January 20xx
Tianzhu Zhou, Yue Zhao, Wenmei Han, Huazhong Xie, Cuiping Li* and Mingquan Yuan*
DOI: 10.1039/x0xx00000x
One dimensional polyaniline@halloysite (PANI@HA) nanotubes with enhanced selective oxidation activity of cyclohexene
are fabricated by employing aniline (ANI) chemical polymerization on halloysite nanotubes in-situ. By facilely controlling the
doping acid, acidity and ANI/HA weight ratio in the fabrication, PANI with controllable doping degree, redox state and
content are growing on halloysite nanotubes. The cyclohexene selective oxidation result shows PANI@HA nanotubes are
effective catalysts in solvent-free reaction system with H2O2 as oxidant and their catalytic activity is relied on the doping
acid, acidity and ANI/HA weight ratio in the fabrication. Wherein, PANI@HA synthesized with HCl as doping acid to condition
the acidity at 1 M and 2.04 ANI/HA weight ratio (PANI@HA/1 M/2.04-HCl) demonstrates the highest catalytic activity
(98.17% conversion and 99.50% selectivity to 1, 2-cyclohexanediol). The cyclohexene selective catalytic activity is with a
well match to the PANI doping degree in PANI@HA. And the optimal reaction condition is 20 mg catalyst, 2.5 mL H2O2, 70 oC
and 24 h. Furthermore, PANI@HA/1 M/2.04-HCl has exhibit superior dihydroxylation activity toward 2,3-dimethyl-2-butene
and cycling performance with 99.11% conversion and 96.92% selectivity to 1, 2-cyclohexanediol after five cycles. CV of
PANI@HA indicates cyclohexene selective oxidation is attributed to reversible redox reaction of PANI in PANI@HA to
catalytic decomposition of H2O2.
www.rsc.org/
recycling performance, has became important for fine chemical, food
and pharmaceutical industry.12-14 For the green oxidants, the typical
ones are air, O2 and hydrogen peroxide (H2O2) ascribed to them or
Introduction
1,2-Cyclohexanediol has played an important role in the synthesis of
pharmaceuticals intermediate, polyester resins, catechol1 and liquid
crystals.2 It can also be acted as a diluter for epoxy resins and a chiral
auxiliary or ligand in asymmetric synthesis.3 Therefore, the synthesis
of 1,2-cyclohexanediol has been extensively studied. Generally,
there are two typical synthetic methodologies for 1,2-
cyclohexanediol: hydrolysis of epoxy cyclohexane4,5 and
dihydroxylation of cyclohexene.5-9 The straightforward and
benchmarked route is cyclohexene dihydroxylation with OsO4 and
RuO4 as catalyst/oxidants.10,11 However, it is costly, toxic or difficult
to control and nonselective, which prevent further application in
industry. On the another aspect, with the increasingly serious
environmental pollution, developing environmentally friendly
synthesis process, such as solvent-free reaction process, using green
oxidants, higher catalytic efficiency to certain product and excellent
their by-product harmless to the environment.5,7-9,15 Although
various dihydroxylation processes of cyclohexene involving a great
number of catalysts and oxidants have been developed, metal-based
catalysts, expensive oxidants or toxic solvents are still used
extensively and low conversion or selectivity to 1,2-cyclohexanediol
are usually accompanied by.16-20 For example, Nowak et al.16 use
mesoporous FDU-1 supported Nb(Co) for cyclohexene oxidation with
H2O2 as oxidant and CH3CN as solvent, resulting in 61% conversion
and 56% selectivity to 1,2-cyclohexanediol. Kang et al.17 adopt a
solvent-free synthesis process for cyclohexene oxidation with Au-
grapheme core-shell nanostructures as catalyst and air as oxidant,
but the conversion and selectivity to 1,2-cyclohexanediol are,
respectively, 55.5% and 61.6%. Recently, Luo5 and Shi9 et al apply Ti-
Beta zeolite and peroxophosphotungstate-ionic liquid brush
assembly to cyclohexene oxidation under solvent-free condition with
H2O2 as oxidant to achieve higher cyclohexene conversion and
selectivity to 1,2-cyclohexanediol (90.2% cyclohexene conversion
and 66.2% selectivity to 1,2-cyclohexanediol for Ti-Beta zeolite, 99%
1,2-cyclohexanediol yield for peroxophosphotungstate-ionic liquid
brush assembly).
a.School of Chemical Science and Technology, Key Laboratory of Medicinal
Chemistry for Natural Resource, Ministry of Education, Yunnan University,
Kunming 650091, China. Fax: +86 871 6503 1567; Tel: +86 871 6503 1567. *E-
mail: licp830@iccas.ac.cn and yuanmingquan@ynu.edu.cn.
Electronic Supplementary Information (ESI) available: [Data of HA; FT-IR and TEM of
PANI@HA/1 M/W-H; chemical composition, mass ratio, doping degree and Q/B of
PANI in PANI@HA/A/W-H; C1s deconvoluted spectra of PANI@HA/1 M/2.04-H;
effect of ANI/HA weight ratio in the fabrication, reaction time, reaction
temperature, solvent, oxidant and catalyst amount on selective oxidation of
cyclohexene with PANI@HA/1 M/W-HCl as catalyst; recycling study with
PANI@HA/1M/2.04-HCl as catalyst; GC, GC-MS and 1H NMR spectra of the reaction
products for olefin oxidation]. See DOI: 10.1039/x0xx00000x
On the other side, metal-free catalysts gradually become the
research hotspot, because they have the advantage of low-cost,
facilely fabrication, corrosion resistance and conveniently
modification.21-24 Some pioneering studies have demonstrated
metal-free catalyst can present comparable or superior catalytic
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activity to the metal-based catalyst. For example, Afonso et al have the question whether halloysite nanotube supported PANI has
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DOI: 10.1039/C7TA02605F
reported p-toluenesulfonic acid can promote cyclohexene oxidation enhanced or comparative cyclohexene oxidation activity than that of
with H2O2 as oxidant in a biphasic system to achieve 97.9% nano-PANI and the doping acid’ concentration/variety can give an
conversion and 80.7% selectivity to 1,2-cyclohexanediol, and p- impact on the cyclohexene oxidation activity. Although nano-PANI
toluenesulfonic acid is also efficient for the dihydroxylation of prepared by microemulsion polymerization has recently been
substituted acyclic olefins and alkyl cyclic olefins.7,8 Legros et al have applied in cyclohexene oxidation, it is conducted with toxic and
shown supramolecular fluorous catalyst can used as recyclable volatile CHCl3 as solvent and the selectivity to1,2-cyclohexanediol is
catalyst for the Morita-Baylis-Hillman reaction.22 And 1,2-bis[3,5- lower than 70%.25 For all we know, the influence of doping acid with
bis(trifluoromethyl) phenyl]diselane can also be used to catalyze the same or different acidity in the fabrication on the cyclohexene
cyclohexene oxidation to 1,2-cyclohexanediol in acetonitrile.23 oxidation activity of halloysite nanotubes supported PANI, especially
Nitrogen-doped CNTs exhibit 59.0% cyclohexene conversion and its green cyclohexene oxidation process (solvent-free and H2O2 as
52.3% selectivity to allylic products using molecular oxygen as oxidant) and superior catalytic activity (>98% conversion and
oxidant in CH3CN.24 So it proposes whether or not polymer material selectivity to 1,2-cyclohexanediol) have rarely been reported.
can present enhanced or comparable catalytic oxidation activity for
In this study, the heterogeneous polyaniline@halloysite
cyclohexene oxidation. Recently, nano-polyaniline (PANI) with nanotubes (PANI@HA) are fabricated in different doping acid
different redox state has been used as catalyst directly to selective through aniline chemical polymerization in-situ on the parent
oxidation of cyclohexene to 1,2-cyclohexanediol with H2O2 as oxidant halloysite nanotubes (HA) at room temperature. By conveniently
in CHCl3 and exhibit excellent catalytic activity.25 The study shows the controlling the variety and concentration of doping acid in the
catalytic activity of nano-PANI is correlated with the size and redox fabrication, heterogeneous PANI@HA nanotubes with various PANI
state. Among them, 10-20 nm emeraldine base PANI (NEB) possesses redox state and doping degree can be achieved. The impact of
highest catalytic activity (92.63% conversion and 64.22% selectivity catalyst (the variety and concentration of doping acid and ANI/HA
to 1,2-cyclohexanediol), whereas bulk leucoemeraldine base PANI weight ratio in the fabrication) and reaction condition (reaction
shows lowest catalytic activity (1.46% conversion and 72.74% time/temperature, solvent, oxidant and catalyst amount) on the
selectivity to 1,2-cyclohexanediol). However, in this study, to cyclohexene selective oxidation properties of PANI@HA are
synthesize nano-PANI and avoid the nano-PANI from aggregation, investigated in detail. Furthermore, the cycling performance of
microemulsion polymerization is adopted, which will lead to the PANI@HA is also investigated. According to the results of CV, a
residual emulsifier on the surface of nano-PANI and thereby interfere mechanism of reversible redox reaction of PANI in PANI@HA to
with the catalytic activity. Furthermore, toxic and volatile CHCl3 is catalytic decomposition of H2O2 is suggested for cyclohexene
chosen as the solvent, and the selectivity to 1,2-cyclohexanediol is selective oxidation by PANI@HA.
still lower than 70% under the highest conversion.
The nanostructure can also be synthesized by supporting active
species on nano-support, which not only can avoid the use of
Experimental
stabilizer or emulsifier, but also sometimes can enhance the
properties of active species owing to the synergistic effect of nano-
support and active species.16,17,19 In our previous studies, it has
shown that nano-Ag, crystalline TiO2, a-Ni(OH)2 and PANI can be
induced in-situ to grow on the exterior surface of halloysite
nanotubes to synthesize halloysite nanotubes supported
nanostructure in virtue of the negative charged exterior surface and
massive surface hydroxyl groups of halloysite nanotubes.26-30
Compared with carbon nanotubes, halloysite nanotube is an
abundantly widespread natural clay with different outside/inside
surface chemical properties, which endow them facilitating
modification and growing functional materials.31,32 So using
halloysite nanotube as the support for growing nano-PANI will have
the economic and resource advantage. Furthermore, the redox state
and doping degree of PANI associated with the chain nitrogens can
Materials
Parent halloysite (HA) nanotubes were obtained from Fenghui
Minerals Trade Co., Ltd. Aniline (ANI) and ammonium persulfate
((NH4)2S2O8) were obtained from Sinopharm Chemical Reagent Co.,
Ltd. Trichloromethane (CHCl3) was obtained from Guangzhou
Jinhuada Chemical Reagent Co., Ltd. Cyclohexene (99%) was
obtained from J&K Scientific Ltd. Hydrogen peroxide (H2O2, 30%)
were purchased from Tianjin Fengchuan Chemical Reagent
Technology Co., Ltd. All the chemical reagents were analytical grade
and used without any further purification.
One-pot fabrication of PANI@HA nanotubes
In the fabrication of PANI@HA nanotubes, the amount of halloysite
nanotube in various system was constant and the acidity of the
reaction system was controlled by the feeding amount of 2 M
inorganic acid. A typical procedure with HCl as doping acid to control
the acidity at 1 M and 2.04 ANI/HA weight ratio was as follows:33 HCl
solution (2 M, 200 mL) was added into a PP container with
continuously stirring at room temperature. Then deionized water
(200 mL) was added to achieve 1 M HCl solution. After 1 h, HA (0.8
g), ANI (1.63 g) and (NH4)2S2O8 (4 g) (molar ratio of ANI to (NH4)2S2O8
was 1:1) were added to the above 1 M HCl solution with continuously
stirring 24 h at room temperature to form PANI@HA nanotubes.
Then the PANI@HA heteroarchitectures were separated by
centrifugation, washed with water several cycles to neutral and dry
at 65 oC. This sample was labelled as PANI@HA/1 M/2.04-HCl, in
be
controlled
by
the
synthesis/redoping
conditions
(concentration/variety of oxidants/doping acid).30,31 It has
demonstrated that the visible light photocatalytic dye degradation
activity of PANI/TiO2/halloysite nanotubes and Cr(VI)
adsorption/reduction properties of halloysite nanotube@PANI are
dependent on doping acid’ concentration/variety in the
synthesis/redoping process.30,33 And compared with nano-TiO2 and
bulk PANI prepared under the comparative condition, they all exhibit
enhanced visible light photocatalytic dye degradation activity and
Cr(VI) adsorption/reduction properties, respectively. So it proposes
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which 1 M, 2.04 and HCl, respectively, represent acidity, ANI/HA
weight ratio and doping acid. The weight ratio of PANI in PANI@HA
can be adjusted by the ANI/HA weight ratio in the fabrication. By the
similar process, PANI@HA nanotubes with 2.04 ANI/HA weight ratio
in the presence of H3PO4, HNO3 and H2SO4 were prepared and
labelled as PANI@HA/1 M/2.04-H (“H” standards for doping acid).
For comparison, bulk PANI and PANI@HA with various acidity and
ANI/HA weight ratio in the presence of HCl (PANI@HA/A/W-HCl, in
which A and W, respectively, standard for acidity and ANI/HA weight
ratio) were also prepared in the similar way.
After 24 h, the reaction system was extracted by CHCl3 or CH2Cl2 (5
mL) three times and the extract was collected together and analyzed
by gas chromatograph.
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DOI: 10.1039/C7TA02605F
Results and discussion
Characterization of PANI@HA nanotubes
As the parent support of PANI, halloysite consists of cylindrical
nanotubes with about 1-2 μm length, 40-60 nm diameter and 20-30
nm lumen diameter (Fig. S1). The chemical composition of halloysite
is 17.5% Si, 17.6% Al and 64.9% O as detected by XPS and EDX (Fig.
S2, S3) and with 38.6 m2/g BET surface area. According to our
previous reports, the outer surface of the halloysite nanotube is
negatively charged under pH<2 and can induced the growing of PANI
and inorganic TiO2 nanocrystals under various inorganic acid with
(NH4)2S2O8 as oxidant of ANI at low temperature.30 In the present
work, the PANI@HA nanotubes were prepared with H3PO4, HNO3,
HCl, and H2SO4 as the doping acid/acidity tuning agent and
Characterization
The morphology of the fabricated PANI@HA nanotubes
(PANI@HA/A/W-H) was characterized by TEM (JSM-2100) under an
accelerating voltage of 200 kV. FT-IR was conducted on a Thermo
Nicolet iS10 instrument with KBr pellet as background and 2 cm-1
spectral resolution. The diffuse reflectance spectra of UV-vis were
performed on a Hitachi U-4100 PC photometer in the range from 200
to 800 nm. The X-ray photoelectron spectroscopy (XPS) was
conducted on a PHI5000 Versaprobe-II electron spectrometer with
50 W Al Kα radiations and C 1s line at 284.6 eV as reference. Cyclic (NH4)2S2O8 as the oxidant. The various redox state and doping degree
voltammetry measurements were conducted on
a CHI760E
of PANI can be achieved by changing the doping acid and acidity in
the fabrication. The PANI@HA nanotubes prepared in 1 M acidity and
with 2.04 ANI/HA weight ratio are firstly synthesized and
characterized.
electrochemical equipment (CHI Inc., China) with modified carbon
paste electrode, platinum wire and Ag/AgCl, respectively, as working
electrode, auxiliary electrode and reference electrode. Cyclohexene
and 2,3-dimethyl-2-butene oxidation reaction product was detected
by a gas chromatograph (Agilent 7890A). The gas chromatography
was performed in a Agilent 19091J-413 column with a cross-linked
5% Phenyl Methyl Siloxan (30 m×320 um×0.33 mm) and a FID
detector under the following conditions: carrier gas (N2, 40mL/min);
temperature program-40 oC holding for 2 min, then heating to 50 oC
o
at 10 oC /min, finally further heating to 300 C at 50 oC /min. Other
olefin oxidation reaction product was identified by GC-MS (TRACE
DSQ). GC-MS was performed in a Agilent DB-17 column (30 m×0.25
um×0.25 mm) and a EI ion source under the following conditions:
carrier gas (He, 1.0 mL/min); temperature program-40 oC holding for
3 min, then heating to 250 oC at 20 oC /min, finally further heating to
o
280 oC at 5 C /min. 1H-NMR spectra were recorded on a Bruker
AVANCE300-MHz spectrometer with CDCl3 as solvent and
tetramethylsilane as an internal standard.
Selective oxidation of cyclohexene by PANI@HA nanotubes
Method A: in a typical experiment, catalyst (PANI@HA, 20 mg),
oxidant (30% H2O2, 2.5 mL ) and cyclohexene (1.23 mL) are added to
a 20 mL dry round-bottom flask equipped with a magnetic stirrer.
After the flask was sealed with rubber turnover stopper, parafilm
“M” and electric insulation tape consecutively, the flask is placed in
Fig. 1 TEM images of PANI@HA nanotubes fabricated in 1 M acidity with 2.04
ANI/HA weight ratio (PANI@HA/1 M/2.04-H): the doping acid/acidity tuning
agents are respectively (a) H3PO4, (b) HNO3, (c) H2SO4 and (d) HCl.
Fig. 1 shows the TEM images of PANI@HA nanotubes fabricated in
1 M acidity with 2.04 ANI/HA weight ratio (PANI@HA/1 M/2.04-H).
Obviously, the external surface of PANI@HA/1 M/2.04-H is rough
and the outside diameter increases significantly when compared
with that of the parent halloysite nanotube. By a closer observation,
PANI@HA synthesized in different doping acid also has similarity in
morphology without blocking the lumen of halloysite nanotube. TEM
demonstrates that the PANI has been successfully germinated on the
surface of the halloysite nanotube in-situ. The polymerization
mechanism of aniline on the surface of halloysite nanotube can be
explained as follows:33,34 at the beginning of polymerization, the
surface of halloysite nanotube is mostly occupied by the oligomer of
ANI; the new nucleation sites are arising on the external surface of
the halloysite nanotubes due to the faster polymerization activity of
o
an oil bath thermostated at 70 C. After 24 h, the reaction system
was extracted by CHCl3 or CH2Cl2 (5 mL) three times and the extract
was collected together and analyzed by gas chromatograph. For the
GC-MS characterization, the extract was rotary evaporated at room
temperature to remove CHCl3 or CH2Cl2, then 15 mL ethyl acetate
was added. During the range of reaction experiments, explosion and
leakage of reaction medium did not occur even the oxidation
reaction was conducted at 90 ºC as the cushioning/sealing effect of
rubber turnover stopper. The oxidation reaction also can be
conducted in screw glass tube.
Method B: catalyst (PANI@HA, 20 mg), oxidant (30% H2O2, 2.5 mL),
cyclohexene (1.23 mL) and solvent are added to a 20 mL round-
bottom flask with a condenser and were continually stirred at 70 oC.
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oligomer species than that in the solution. Finally, the PANI chains intensity of PANI in PANI@HA/1 M/W-HCl fortifies with the
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grow vertically to the axis of the halloysite nanotube due to the steric increasement on ANI/HA weight ratio, whichDsOtaI:te10s.1t0h3a9t/tCh7eTAc0o2n6t0e5nFt
hindrance effect.
of PANI can be adjusted by ANI/HA weight ratio in the fabrication.
FT-IR spectra of the synthesized PANI@HA nanotubes From FT-IR (Fig. 2 and Fig. S4), the characteristic peaks of halloysite
(PANI@HA/1 M/2.04-H) are present in Fig. 2. When compared with nanotubes (3799 cm-1, 3622 cm-1, 910 cm-1, 535 cm-1 and 470 cm-1)
the spectrum of halloysite nanotubes (line a in Fig. 2), new still maintain in PANI@HA/1 M/W-H,26-33 illustrating the physical
characteristic bands attributed to PANI (3446 cm-1-N–H stretching structure of halloysite nonutubes remain stable by the PANI growing
mode, 1566 cm-1-C=C stretching vibration of quinoid ring, 1492 cm-1- under different doping acid. Furthermore, the tunable PANI content
C=C stretching vibration of benzenoid ring, 1300 cm-1-C–N stretching by adjusting the ANI/HA weight ratio in the preparation is further
vibration of benzenoid ring, 1244 cm-1-C–N+ stretching vibration of proved by the increase of PANI layer thickness with the ANI/HA
benzenoid ring, 1140 cm-1-=N+–stretching vibration in quinonoid weight ratio increasing (Fig. S5).
unit) are observed in PANI@HA/1 M/2.04-HCl. The existing
The redox state of PANI in the heterogeneous PANI@HA
characteristic peaks at 1566, 1492 and 1300 cm-1 signify there are nanotubes is further studied by UV-vis absorption spectra (Fig. 3).
two form PANI unit-pernigraniline (PB) and leucoemeraldine (LB) Compared with the parent spectra of halloysite nanotube,
concomitant in PANI@HA/1 M/2.04-HCl. Furthermore, the Q/B PANI@HA/1 M/2.04-H appear new three peaks (352 nm assigning to
(indicating the redox state of PANI) and the doping degree of PANI π-π* transition of benzenod ring, 432 nm attributing to π-polaron of
can be, respectively, estimated roughly by the peak intensity of C=C benzenod ring, and the broad peak of 470-800 nm ascribing to the
stretching vibration of quinonoid ring at 1566 cm-1 to that of the C=C polaron-π*transitions of quinonoid ring), demonstrating the growing
stretching vibration of benzenoid ring at 1492 cm-1, and the peak of PANI on the halloysite nanoubes.30,35-37 The intensity ratio of peak
intensity of doped PANI at 1140 cm-1 and 1244 cm-1 to that of C=C at 470-800 nm and 350 nm in PANI@HA/1 M/2.04-H is much close to
stretching vibration of quinonoid ring and benzenoid ring at 1566 one, indicating the PANI on HA is in the form of emeraldine salt.37
cm-1 and 1492 cm-1. It can be seen from Fig. 2 that PANI @HA with However, they still have slight differences in the starting of broad
H3PO4, HNO3 and H2SO4 as doping acid tuning the acidity at 1 M in band and the band intensity ratio of 470-800 nm/350 nm even if they
the synthesis take on analogous spectra
are synthesized in the comparative acidity, which is in accordance
with the results of FT-IR. The above data indicates that the PANI@HA
with various doping acids adjusting the acidity in the synthesis
process has varied redox state and doping degree.
Fig. 2 FT-IR spectra of the fabricated PANI@HA/1 M/2.04-H: (a) HA;
the acidity of the starting solutions are respectively adjusted by (b)
H3PO4, (c) HNO3, (d) H2SO4 and (e) HCl.
Fig. 3 UV-vis spectra of the fabricated PANI@HA/1 M/2.04-H: (a) HA; the
acidity of the starting solutions are respectively adjusted by (b) H3PO4, (c)
HNO3, (d) H2SO4 and (e) HCl.
as that of PANI@HA/1 M/2.04-HCl (Fig. 2b-e). However, they have
differences in doping degree and redox state (Q/B): the doping
degree of PANI in the PANI@HA/1 M/2.04-H follows the order of
PANI@HA/1 M/2.04-HCl > PANI@HA/1 M/2.04-H2SO4 > PANI@HA/1
In order to deeply analyze the redox state and doping degree of
PANI in PANI@HA/1 M/2.04-H, the XPS spectra is further used to
characterize PANI@HA/1 M/2.04-H (Fig. 4, Table S1). It shows that
the fabricated PANI@HA/1 M/2.04-H is mainly constituted of Si, Al,
O, C and N and the corresponding doping counter-ion element, such
as P, N, S and Cl also can be observed for PANI@HA/1 M/2.04-H3PO4,
PANI@HA/1 M/2.04-HNO3, PANI@HA/1 M/2.04-H2SO4 and
PANI@HA/1 M/2.04-HCl, respectively. The high-resolution N 1s
spectra of PANI@HA/1 M/2.04-H demonstrate three peaks at
M/2.04-HNO3
>
PANI@HA/1 M/2.04-H3PO4, whereas Q/B
demonstrates the reverse order. Higher doping degree indicates
higher electrical conductivity of the fabricated PANI@HA/1 M/2.04-
HCl.30,33 The different redox state and doping degree of PANI in the
PANI@HA/1 M/2.04-H may exert an impact on the catalytic activity
of cyclohexene oxidation.
The FT-IR spectra of PANI@HA nanotubes fabricated in 1 M HCl
with different ANI/HA weight ratio (PANI@HA/1 M/W-HCl) has also
been presented (Fig. S4). It manifests the characteristic peak
398.9/398.7/398.9/398.8,
399.9/399.7/399.8/399.7
and
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400.8/400.5/400.8/400.5 eV, which are respectively attributed to Meanwhile, the PANI content in PANI@HA/A/2.04-HCl prepared in
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nitrogen atoms in quinonoid imine (-N=), benzenoid amine (-NH-) different acidity with HCl as doping acid and 2D.0O4I:A1N0.I1/0H3A9/wCe7TigAh0t2r6a0t5ioF
and doped imine/amine group (-N+=/-NH+).33,35,36,38 Moreover, the is fairly near in 35-37%, meaning acidity in the fabrication has little
doping degree and redox state of PANI in PANI@HA/1 M/2.04-H can effect on the PANI content. The above results demonstrate the redox
be quantitatively calculated according to the deconvoluted N 1s XPS state and doping degree of PANI in the fabrication can be regulated
spectra. The doping degree and Q/B, respectively, follow
a
by the variety of doping acid.
descendant order and elevatory order of PANI@HA/1 M/2.04-HCl, Selective oxidation of cyclohexene to 1,2-cyclohexanediol by the
PANI@HA/1 M/2.04-H2SO4, PANI@HA/1 M/2.04-HNO3 and PANI@HA nanotubes
PANI@HA/1 M/2.04-H3PO4 (Table S2), which tie greatly in with the
The above PANI@HA nanotubes with H3PO4, HNO3, H2SO4, and HCl
FT-IR spectra data. Whereas PANI@HA/1 M/2.04-H has similar high-
as doping acid/acidity adjusting agent in the fabrication are directly
resolution C 1s spectra (Fig. S6).
applied to selective oxidation of cyclohexene in solvent-free reaction
system at 70 oC with H2O2 as oxidant. And for comparison, the
cyclohexene oxidation activities of the parent HA and the bulk PANI
fabricated in 1 M doping acid solution without adding HA are also
investigated. The reaction products are analyzed by gas
chromatograph (GC). The GC spectra of the reaction products with
PANI@HA/1 M/2.04-H3PO4, PANI@HA/1 M/2.04-HNO3, PANI@HA/1
M/2.04-H2SO4 and PANI@HA/1 M/2.04-HCl as catalyst are given in
Fig. S8. And to facilitate the analysis, the GC spectra of 99%
cyclohexene, epoxy cyclohexane, 2-cyclohexen-1-ol, 2-cyclohexen-1-
one and 1,2-cyclohexandiol are also given (Fig. S7). It is demonstrable
the reaction products of cyclohexene oxidation with PANI@HA as
catalyst mainly appear two peaks around the retention time of 1.25
and 4.60 min, which are ascribed to cyclohexene and 1,2-
cyclohexandiol, respectively, indicating the main oxidation product is
1,2-cyclohexandiol. And the peak intensity of 1,2-cyclohexandiol
increases with the order of PANI@HA/1 M/2.04-HCl, PANI@HA/1
M/2.04-H2SO4, PANI@HA/1 M/2.04-HNO3 and PANI@HA/1 M/2.04-
H3PO4.
The reaction products are further characterized by 1H NMR spectra
(Fig. S10). The signal assignment of cyclohexene, epoxy cyclohexane,
2-cyclohexen-1-ol, 2-cyclohexen-1-one and 1,2-cyclohexandiol is
given in Fig. S9. The signals at 5.74-5.58 ppm, 5.91-5.67 ppm and
6.11-5.96 ppm are respectively corresponded to the two vinyl
protons (-CH=CH-) of cyclohexene, 2-cyclohexen-1-ol and 2-
cyclohexen-1-one. The signal at 3.20-3.02 ppm is assigned to the two
methylidyne protons (-CH-O-CH-) of epoxy cyclohexane.39 The signals
at 3.85-3.65 ppm and 3.44-3.26 ppm are ascribed to the two
methylidyne protons (-CHOH-CHOH-) of cis-1,2-cyclohexanediol and
trans-1,2-cyclohexanediol,
respectively.7,40
So
cyclohexene
conversion can be calculated by the sum of the integral values of the
signals at 5.91-5.67, 6.11-5.96, 3.20-3.02, 3.85-3.65 and 3.44-3.26
ppm to that at 5.74-5.58, 5.91-5.67, 6.11-5.96, 3.20-3.02, 3.85-3.65
and 3.44-3.26 ppm. The selectivity to 1,2-cyclohexanediol can be
estimated by the sum of the integral values of the signals at 3.85-3.65
and 3.44-3.26 ppm to that at 5.91-5.67, 6.11-5.96, 3.20-3.02, 3.85-
3.65 and 3.44-3.26 ppm. Based on the 1H NMR spectra, the
cyclohexene conversion/selectivity to 1,2-cyclohexanediol for
PANI@HA/1 M/2.04-H3PO4, PANI@HA/1 M/2.04-HNO3, PANI@HA/1
M/2.04-H2SO4 and PANI@HA/1 M/2.04-HCl are respectively
Fig. 4 XPS survey spectra (A) and the corresponding N 1s deconvoluted
spectra (B) of PANI@HA/1 M/2.04-H: a) HA, the acidity of the starting
solutions are adjusted by b) H3PO4, c) HNO3, d) H2SO4 and e) HCl, respectively.
Element analysis shows the mass ratio of PANI in PANI@HA/1
M/2.04-H is in the range of 37-52% (Table S2). Among them,
PANI@HA/1 M/2.04-HCl has the lowest PANI content, whereas
PANI@HA/1 M/2.04-H3PO4 has the highest PANI content. It is worthy
to be mentioned that the PANI content is lower than 67.1% (the PANI
content considering all the feeding ANI has been grown on HA). As
there is no PANI in the bulk (Fig. 1) and it has been proved the
PANI@HA synthesized by the purified or unpurified ANI has similar
PANI content,33 so the lower PANI content than 67.1% may caused
by the removal of the oligomer in the purification process.
43.42%/96.71%,
48.44%/94.40%,
52.61%/98.35%
and
97.64%/98.34%, which are in accordance with the results of GC
(entry 1-3 and 7, Table 1) . Considering the efficiency of GC
characterization, the reaction products are characterized by GC in
the following.
The detailed cyclohexene conversion and selectivity for various
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products based on GC are summarized in Table 1. It is obvious the doping acid in the fabrication will produce a significant effect on
View Article Online
DOI: 10.1039/C7TA02605F
cyclohexene oxidation by HA and catalyst-free is extremely remote the cyclohexene oxidation activity of PANI@HA/1 M/2.04-H.
(3.66% and 0.1% cyclohexene conversion, respectively). Whereas
Furthermore, the acidity in the fabrication will also affect the
with PANI@HA/1 M/2.04-H as catalyst, cyclohexene oxidation is cyclehexene oxidation activity, for example, when the acidity is in the
noteworthy, especially with PANI@HA/1 M/2.04-HCl as catalyst, the range of 0.03-1 M, the cyclohexene conversion increases with the
cyclohexene conversion is 98.17% within 24 h, indicating the growing acidity increasing and then when the acidity is between 1 and 2 M,
PANI on HA can enhance the cyclohexene oxidation activity of HA. the cyclohexene conversion decreases with the acidity increasing. An
However, the cyclohexene oxidation activity of PANI@HA/1 M/2.04- optimal acidity in the fabrication is 1 M, which is coincided with the
H fabricated under various doping acids is different. PANI@HA/1 Cr(VI) adsorption/reduction result.33 And the effect of ANI/HA weight
M/2.04-HCl performs the highest conversion (98.17%) and high ratio in the fabrication (PANI@HA/1 M/W-HCl) on the cyclohexene
selectivity toward 1,2-cyclohexanediol (99.50%) when compared oxidation activity are further studied (Table S3). It shows that the
with those with H2SO4, HNO3 and H3PO4 as doping acid (entry 1-3, conversion increases with the ANI/HA weight ratio significantly when
Table 1). The reason may be due to the highest doping degree of the ANI/HA weight ratio lower than 2.04, then it increases
PANI in PANI@HA/1 M/2.04-HCl. Cyclohexene conversions of the indistinctively when the ANI/HA weight ratio higher than 2.04.
PANI@HA nanotubes are keeping the following order as PANI@HA/1 Although the cyclohexene conversion (98.32%) with a 2.24 ANI/HA
M/2.04-HCl > PANI@HA/1 M/2.04-H2SO4 > PANI@HA/1 M/2.04- weight ratio (PANI@HA/1 M/2.24-HCl) is slight higher than that
HNO3 > PANI@HA/1 M/2.04-H3PO4, accordance with the redox state (98.17%) with a 2.04 ANI/HA weight ratio (PANI@HA/1 M/2.04-HCl),
and doping degree of PANI obtained by FT-IR and XPS. It is a 2.04 ANI/HA weight ratio is still chosen for further studies when
demonstrable that cyclohexene conversion of PANI@HA/1 M/2.04- considering economy and effectiveness. The effect of ANI/HA weight
HCl is higher than that of the bulk PANI prepared under 1 M HCl ratio on the conversion is owing to a synergy of PANI content and
solution (entry 13, Table 1) due to the induction of HA to achieve PANI dispersion on the HA. From the point of selectivity to 1,2-
nanostructure PANI, which is consist with the reported work that cyclohexanediol, it is on the slight increase with the ANI/HA weight
nano-PANI has improved cyclohexene oxidation activity.25 Obviously, ratio increasing and finally stable with a
Table 1 Cyclohexene conversion and selectivity of PANI@HA, PANI and HA.
Selectivity (%)
Catalyst
Conversion (%)
TONa
rwb(mmol/g/h)
PANI@HA/1 M/2.04-H3PO4
PANI@HA/1 M/2.04-HNO3
PANI@HA/1 M/2.04-H2SO4
PANI@HA/0.03 M/2.04-HCl
PANI@HA/0.3 M /2.04-HCl
PANI@HA/0.5 M/2.04-HCl
PANI@HA/1 M/2.04-HCl
PANI@HA/1.5 M/2.04-HCl
PANI@HA/2 M/2.04-HCl
PANI/1 M-H3PO4
PANI/1 M-HNO3
PANI/1 M-H2SO4
PANI/1 M-HCl
HA
0
0.43
0
0.12
0.12
0.16
0
99.43
96.47
99.61
99.51
99.42
99.57
99.50
97.95
99.52
99.31
99.75
99.60
99.59
94.61
0
0
0.46
0
0
0.05
0
0.05
0.80
0.10
0.17
0
0.57
2.64
0.39
0.37
0.41
0.28
0.45
1.25
0.27
0.18
0.12
0.20
0.28
4.24
33.58
38.70
43.42
54.42
41.69
56.96
66.10
80.32
82.43
-
148.66
-
-
30.68
28.68
28.86
35.05
-
18.88
25.80
29.94
36.38
37.33
-
67.33
-
-
13.90
12.99
13.07
15.87
-
49.89
52.07
53.53
98.17
88.61
58.86
55.00
51.40
51.74
62.82
0
0.13
0.40
0.12
0.20
0.12
1.15
54.68
0
0
0
3.66
0.10
-
-
Catalyst-free
11.74
10-20 nm emeraldine base PANI
21.67
64.22
1.75
10.56
92.63
16.81
7.61
(NEB)c
Au-graphene
core-shell
0.69
61.61
7.24
30.23
55.50
107.80
22.80
nanostructured
aTON (turnover number) is defined as total mol of cyclohexene molecules converted per mol of catalyst.
bInitial reaction rate of cyclohexene consumption normalized by catalyst mass.
c Reaction conditions: 0.1g catalyst, 2 mL cyclohexene, 10 mL CHCl3, 5 mL H2O2, 60 oC, 24 h (ref. 25).
d Reaction conditions: 50 mg catalyst, 5 mL cyclohexene, O2, 60 oC, 24 h (ref. 17).
Reaction conditions: 20 mg catalyst, 1.23 mL cyclohexene, 2.5 mL H2O2, 70 oC, 24 h.
2.04 ANI/HA weight ratio. In combination of the FT-IR result that the cyclohexanediol of cyclohexene oxidation by PANI@HA/A/W-H
intensity of PANI characteristic peaks increases with the ANI/HA exceeds 95%, regardless of the doping acid, acidity and ANI/HA
weight ratio increasing (Fig. S4), the cyclohexene oxidation activity of weight ratio in the fabrication. The high selectivity to 1,2-
PANI@HA/1 M/W-HCl will be influenced by the PANI content in cyclohexanediol will facilitate the later separation and purification. It
PANI@HA (ANI/HA weight ratio in the fabrication).
is interesting that the cyclohexene conversion and selectivity to 1,2-
It is deserved to be mentioned that the selectivity to 1,2- cyclohexanediol of PANI@HA/1 M/2.04-HCl are respectively 98.17%
6 | J. Name., 2012, 00, 1-3
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ARTICLE
and 99.50% within 24 h under the above reaction condition, activity, so in the following, cyclohexene selectiveVioewxiAdratictlieoOnnlbinye
DOI: 10.1039/C7TA02605F
indicating PANI@HA/1 M/2.04-HCl is an effective catalyst and can PANI@HA/1 M/2.04-HCl is also investigated with the reaction
selectively oxidize cyclohexene to 1,2-cyclohexanediol. Both of the temperature in the range of 30-80 oC (Table S5). Obviously,
cyclohexene conversion and selectivity to 1,2-cyclohexanediol are cyclohexene conversion is dependent on the reaction temperature:
higher than that of nanosized 10-20 nm PANI (emeraldine base, when cyclohexene oxidation is conducted on below 50 oC, the
92.63% conversion and 64.22% selectivity to 1,2-cyclohexanediol),25 conversion is lower than 1.2%; whereas when cyclohexene oxidation
even the dosage of PANI@HA/1 M/2.04-HCl to cyclohexene (16.3 is underway at 60 oC, the conversion is fleetly raised to 46.78%; when
o
g/L) is lower than that of nanosized PANI to cyclohexene (50.0 g/L). the reaction temperature is 70 C, the conversion is 98.17%. The
Concluded on the above results, cyclohexene oxidation performance increase of catalytic activity with the reaction temperature can be
of PANI@HA/A/W-H is dependent on the acidity, variety of doping explained by the fact that high temperature can provide sufficient
acid and ANI/HA weight ratio in the fabrication. Among them, energy to deeply catalyze oxidation of cyclohexene.41 However, no
PANI@HA/1 M/2.04-HCl shows the highest catalytic activity for obvious conversion promotion of cyclohexene is observed with the
cyclohexene oxidation, corresponding to a 67.33 mmol g-1h-1 mass- increase of reaction temperature to 80 oC (Table S5). The relationship
normalized activity and 148.66 TON. This activity is significantly between selectivity to 1,2-cyclohexandiol and reaction temperature
higher than that of 10-20 nm emeraldine base PANI (NEB, 7.61 mmol is similar with that between conversion and reaction temperature,
g-1h-1 and 16.81 TON) (entry 17, Table 1)25 and comparable to that of but the selectivity to 1,2-cyclohexandiol remains at high level (>90%)
o
supported nano-metal catalyst (entry 18, Table 1).17 Therefore, in the range of 30-80 C. So the reaction temperature is fixed at 70
PANI@HA/1 M/2.04-HCl is an effective catalyst for cyclohexene oC for cyclohexene oxidation.
oxidation and selected for further studies.
Fig. 6 Effect of solvent on cyclohexene selective oxidation with PANI@HA/1
Fig. 5 Effect of reaction time on cyclohexene oxidation with PANI@HA/1
M/2.04-HCl as catalyst.
M/2.04-HCl as catalyst.
Besides, a series of solvents such as n-heptane, tetrahydrofuran
(THF), trichloromethane (CHCl3), acetonitrile (CH3CN) and dimethyl
sulfoxide (DMSO) have been used as solvents for the selective
oxidation of cyclohexene and compared with that of solvent-free
system. As shown in Fig. 6 and Table S6, the catalytic activity has
reliance on the solvent. Maximum cyclohexene conversion (98.17%)
and high selectivity to 1,2-cyclohexandiol (99.50%) is achieved in the
solvent-free reaction system. The low conversion of the reaction
system with solvent could be attributed to segmental decomposition
of H2O2 with solvent in the present condition.42 In solvent-free
reaction system, as the main product can dissolve in the water, thus
accelerate cyclohexene conversion and the water in H2O2 can
promote decompose of O-O bonds of peroxo-species, resulting in the
high selectivity to 1,2-cyclohexanediol. It can also be observed that
high polar solvent (for example, DMSO) can lead to the decrease of
selectivity to 1,2-cyclohexanediol (65.63%) accompanying the
increasing of selectivity to epoxy cyclohexane (24.35%) and 2-
cyclohexen-1-one (10.01%) (Table S6). It is probably due to the
enhanced active oxygen species formation under the high polar
solvent and thus facilitates allylic oxidation and epoxidation.41,43,44
It is well known that oxidants play an important role in the
In the following, we will further explore the effect of reaction
condition (including reaction time/temperature, solvent and oxidant
and catalyst amount) on the cyclohexene selective oxidation
performance of PANI@HA/1 M/2.04-HCl. Firstly, the effect of
reaction time (0-48 h) on the catalytic activity of cyclohexene
oxidation is investigated while keep other reaction conditions
unchanged. It shows that the cyclohexene conversion increases
markedly with the reaction time increasing and it achieves 98.17%
conversion with 99.50% selectivity to 1,2-cyclohexandiol when the
reaction time is 24 h (Fig. 5, Table S4). Then the cyclohexene
conversion increases in an obscure when the reaction time is longer
than 24 h, for example, the conversion only slightly increases to
99.69%, while with a drop off of the selectivity for 1,2-cyclohexandiol
to 94.07%. It is deserved to mention that the selectivity to 1,2-
cyclohexandiol is over 83% throughout the whole reaction stage,
indicating 1,2- cyclohexandiol is the main oxidation product even at
a short reaction time, for example, 0.5 h (83.90% selectivity to 1,2-
cyclohexandiol). So considering the practical application and the
catalytic activity, the reaction time is chosen as 24 h in the
subsequent studies.
Reaction temperature is an important factor affecting the catalytic
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cyclohexene oxidation. The detailed results of conversion of S13). For other olefin oxidation, as the complicaVteiedw Aortixcilde aOtniloinne
cyclohexene and selectivity to various products are concluded in Fig. product and the high boiling point of olefin,DtOhIe: 1e0x.1t0ra3c9t/Cw7TaAs0r2o6t0a5ryF
7 and Table S7. It is demonstrable there is a maximum conversion of evaporated at room temperature to remove CHCl3 or CH2Cl2, then 15
98.17% when the added volume of 30% H2O2 is 2.5 mL. While the mL ethyl acetate was added for the GC-MS characterization. The GC-
selectivity to 1,2-cyclohexanediol is up to 99% with 1.25 mL 30% H2O2 MS spectra are given as Fig. S14, Fig. S16, Fig. S18 and Fig. S20.
and changes little with the increase of 30% H2O2 volume. Furthermore, 1H NMR spectra of extract (note: in order to reflect the
Furthermore, other oxidants, such as O2 and air are also used to authenticity of the oxidation reaction, the extract is directly
1
cyclohexene oxidation. However, the conversion of cyclohexene is characterized by H NMR, so some spectra will appear strong peak
poor (only 5.20% and 2.91% conversion for O2 and air, respectively, around 5.3 ppm when using CH2Cl2 as extractant) are also given as
Table S7), signifying PANI@HA/1 M/2.04-HCl is unable to activate O2 Fig. S15, Fig. S17, Fig. S19 and Fig. S21. And to facilitate the analysis,
under the current reaction conditions. In combination of 2.91% 1H NMR spectra of olefin and some main reaction product are also
1
cyclohexene conversion with PANI@HA/1 M/2.04-HCl as catalyst but given. The analysis of the spectra (including MS and H NMR) was
without H2O2 (Entry 1, Table S7) and 3.66% cyclohexene conversion based on the data from Spectral Database for Organic Compounds
with HA as catalyst and 30% H2O2 as oxidant (Entry 15, Table 1), we (SDBS). The oxidation result is summarized in Table 2. The values
conclude that the role of different redox state/doping degree of PANI outside and inside the bracket are respectively calculated based on
1
in PANI@HA should be catalytic decomposition of H2O2 to achieve the GC/GC-MS and H NMR data. It is clearly the olefin conversion
active oxygen species. Therefore, the optimized oxidant is H2O2 and and selectivity to 1,2-diol calculated based on the GC/GC-MS and 1H
the effective volume of H2O2 is 2.5 mL for selective oxidation of NMR data is consistent. Generally, higher olefin conversion and 1,2-
cyclohexene in the present catalytic reaction system.
diol selectivity are achieved for substituted alkyl acyclic olefin than
for terminal alkyl acyclic olefin (entry 1-2, Table 2), for example,
98.25% 2,3-dimethyl-2-butene conversion and 100% selectivity to
pinacol for 2,3-dimethyl-2-butene catalytic oxidation (Fig. S12, Fig.
S13), whereas 0.24% 1-hexene conversion and 30.68% selectivity to
1,2-hexanediol for 1-hexene catalytic oxidation. Compared with
cyclohexene oxidation, the efficiency of cyclooctene oxidation (12.90%
conversion) is much decreased and the main oxidation product is 1,2-
epoxycyclooctane (100% selectivity), which may be in virtue of its
poor water solubility and higher activation energy (entry 4, Table
2).8,9 Furthermore, olefin conversion and selectivity to 1,2-diol will be
influenced by the steric hindrance of olefin,8,9 for example, the (R)-
(+)-limonene conversion is 0.55% with 39.22% selectivity to 1-
methyl-4-(1-methylethenyl)-1,2-cyclohexanediol (entry 3, Table 2),
whereas the cyclohexene conversion is 98.17% with 99.50%
selectivity to 1,2-cyclohexanediol (entry 7, Table 1). Aryl olefins,
styrene and 1,1- diphenylethylene, also demonstrate steric effect will
exert influence on the olefin conversion and the selectivity (entry 5,6,
Table 2). For styrene oxidation, the main product is 1-phenyl-1,2-
ethanediol (45.89% selectivity), whereas for 1,2-diphenylethene
oxidation, the main product are benzophenone (47.24% selectivity)
and 2,2-diphenyl-2-hydroxyacetic acid (43.17% selectivity). Although
not all the olefins are oxidized into their corresponding 1,2-diols and
certain olefin oxidation achieve poor conversion catalyzed by
PANI@HA under the optimized conditions for cyclohexene oxidation,
the conversion and selectivity to certain product can be tuned by the
reaction conditions, for example the reaction temperature (entry 3-
6, Table 2). Further optimization of the reaction conditions for
various olefins is undergoing.
Fig.
Fig. 7 Effect of H2O2 volumes on cyclohexene oxidation with PANI@HA/1
M/2.04-HCl as catalyst.
And also, the influence of catalyst amount on the cyclohexene
oxidation activity of PANI@HA/1 M/2.04-HCl is investigated. There
are strong signs that a demonstrable increase in the cyclohexene
conversion is observed when the used catalyst amount is lower than
20 mg (Fig. S11). Exceeding this amount, cyclohexene conversion
decreases distinctly. This could be explained by the possibility that
competent interaction of PANI in PANI@HA/1 M/2.04-HCl with
cyclohexene will inhibit the catalytic reaction and overdose catalyst
will afford excess active sites to accelerate the occurrence of side
reaction.45 Therefore, the optimum catalyst amount is 20 mg in the
present reaction system. What is worthy to be mentioned that
cyclohexene oxidation is usually accompanied by many side
products, but in this study 1,2-cyclohexanediol is the main product
while other product can be ignored.
Table 2 Olefin conversion and selectivity to 1,2-diol with PANI@HA/1 M/2.04-
HCl as catalyst.
Moreover, the dihydroxylation scope of PANI@HA toward a range
of olefins such as 2,3-dimethyl-2-butene (tetrasubstituted alkyl
acyclic olefin), 1-hexene (monosubstituted alkyl acyclic olefin), (R)-
(+)-limonene (trisubstituted cyclic olefin), cyclooctene (disubstituted
alkyl cyclic olefin), styrene and 1,1-diphenylethylene is investigated
under the optimized conditions (20 mg PANI@HA/1 M/2.04-HCl,
1.23 mL olefin, 2.5 mL H2O2, 70 oC, 24 h). The 2,3-dimethyl-2-butene
oxidation product was characterized by GC and 1H NMR (Fig. S12, Fig.
Entry Substrate
1,2-Diol
Selectivity to 1,2-
diol (%)
Conversion (%)
1
2
100.00 (100.00) 98.25 (99.35)
30.68 0.24
8 | J. Name., 2012, 00, 1-3
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3
M/2.04-HCl, cyclic voltammetry (CV) measurements of PANI@HA/1
View Article Online
39.22
0.55
M/2.04-H are made to investigate electronDtOraI:n1s0p.o10r3t9a/nCd7TpAo02ss6i0b5leF
cyclohexene selective oxidation mechanism. It is notable three redox
peaks of 0.23/-0.43 V, 0.53/0.07 V, 0.80/0.76V are appeared in the
CV of PANI@HA/1 M/2.04-HCl and no anodic peak (black line) exists
in HA (Fig. 9A). Among them, the 0.23/-0.43V redox couple is
ascribed to the redox process between LB and EB, and the rest ones
are respectively owing to quinone intermediates and the redox
43.72a
2.14a
11.82 (17.65)b 24.20 (34.00)b
4c
5
0
0a
12.90
22.30a
0 (0)b
39.70 (40.48)b
45.89
50.39
49.81 (44.14)a 69.18 (64.16)a
80.00 (80.00)b 92.06 (92.31)b
6c
0
0a
79.61
94.33a
0 (0)b
98.67 (99.49)b
aReaction conditions: 20 mg catalyst, 1.23 mL olefin, 2.5 mL H2O2, 80 oC, 24 h.
bReaction conditions: 20 mg catalyst, 1.23 mL olefin, 2.5 mL H2O2, 90 oC, 24 h.
cFor cyclooctene oxidation, 100% selectivity for 1,2-epoxycyclooctane;
whereas for 1,1-diphenylethylene oxidation, >37% selectivity for
benzophenone and 2,2-diphenyl-2-hydroxyacetic acid at the present reaction
condition.
Reaction conditions: 20 mg catalyst, 1.23 mL olefin, 2.5 mL H2O2, 70 oC, 24 h.
Recycling study of PANI@HA
As an excellent catalyst, it should be stable and recyclable in the
repeated use.16-24 To illustrate the recycling performance of
PANI@HA, five times recycling experiments are conducted on
PANI@HA/1 M/2.04-HCl and the reaction time of each cycle is 24 h.
It presents the cyclohexene conversion and selectivity to 1,2-
cyclohexandiol still can achieve 99.11% and 96.92%, respectively,
after five cycles (Fig. 8), which indicates that the PANI@HA/1
M/2.04-HCl is very stable and can be recycled effectively in the
Fig. 9 CV of the fabricated PANI@HA (20 mg) with 50 mV•s-1 scan rate in 2.5
mL 1.0 M H2SO4 solution (A) and in 2.5 mL 30% H2O2 solution (B): a) HA, b)
PANI@HA/1 M/2.04-H3PO4, c) PANI@HA/1 M/2.04-HNO3, d) PANI@HA/1
M/2.04-H2SO4 and e) PANI@HA/1 M/2.04-HCl.
potential of PANI@HA/1 M/2.04-HCl has shifted respectively 56, 5,
reaction of EB and PB.46 The CV of PANI@HA/1 M/2.04-H3PO4,
PANI@HA/1 M/2.04-HNO3 and PANI@HA/1 M/2.04-H2SO4 also
demonstrate three redox peaks, but differ in the potential and
current. By a closer comparison of the curves, the first anodic peak
29 mV from that of PANI@HA/1 M/2.04-H3PO4, PANI@HA/1 M/2.04-
HNO3 and PANI@HA/1 M/2.04-H2SO4, and while the second and the
third peak potential have at least, respectively, shifted 26 mV and 15
mV from those of PANI@HA/1 M/2.04-H (H=H3PO4, HNO3 and
H2SO4). Besides, the anodic peak current of PANI@HA/1 M/2.04-HCl
is maximum among the PANI@HA/1 M/2.04-H samples and the
current order is PANI@HA/1 M/2.04-HCl > PANI@HA/1 M/2.04-
H2SO4 > PANI@HA/1 M/2.04-HNO3 > PANI@HA/1 M/2.04-H3PO4,
which is in agreement on the cyclohexene catalytic activity/doping
degree order of PANI@HA/1 M/2.04-H.
Fig. 8 Cyclohexene selective oxidation recycling study with PANI@HA/1
M/2.04-HCl as catalyst. Reaction condition: 20 mg catalyst, 1.23 mL
cyclohexene, 2.5 mL H2O2, 70 oC, 24 h.
repeated cyclohexene oxidation. With these unique advantages of
green catalytic reaction system (solvent-free and green oxidant),
high catalytic performance and excellent cyclic stability, it will
facilitate to the purification and separation of oxidation products and
have potential application in the fine chemical industry.
Cyclohexene selective oxidation mechanism of PANI@HA
In order to study the enhanced catalytic activity of PANI@HA/1
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The shifts of anodic peak potential and the highest peak currents
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DOI: 10.1039/C7TA02605F
indicate that PANI@HA/1 M/2.04-HCl has a favourable electrical
conductivity, which can be ascribed to high doping degree, surface
area and electron transfer of the nanocomposite.30,47,48 The CV of
PANI@HA/1 M/2.04-H confirms the transition of emeraldine salt (EB)
to pernigraniline (PB) and leucoemeraldine (LB). As halloysite
nanotube shows no demonstrable redox reactivity under the same
testing potential range and PANI@HA/1 M/2.04-HCl shows the
maximum anodic peak current, whereas PANI@HA/1 M/2.04-H3PO4
shows the minimum anodic peak current, the above results also
manifest the doping acid in the fabrication can exert a demonstrable
effect on the cyclohexene catalytic activity of PANI@HA/1 M/2.04-H.
Furthermore, electrocatalysis behaviors of PANI@HA/1 M/2.04-H
for H2O2 decomposition by three-electrode electrochemical cell
measurements are also investigated. The CV of PANI@HA/1 M/2.04-
Scheme 1 Proposed mechanism of selective oxidation of cyclohexene to 1,2-
cyclohexandiol with PANI@HA nanotubes as catalyst.
Conclusions
H
fabricated under different doping acid with the same
The PANI@HA nanotube has been successfully synthesized by the in-
situ chemical polymerization of aniline on the surface of halloysite
nanotubes. PANI with various redox state and doping degree in
PANI@HA nanotube can be achieved by merely adjusting the acidity
and the doping acid in the fabrication. The PANI@HA fabricated with
2 M HCl as doping acid tuning the acidity at 1 M and 2.04 ANI/HA
weight ratio (PANI@HA/1 M/2.04-HCl) demonstrates the maximum
doping degree. The effect of doping acid, acidity and ANI/HA weight
ratio in the synthesis of PANI@HA on the selective oxidation of
cyclohexene are investigated in detail. The result shows PANI@HA/1
M/2.04-HCl exhibits the highest catalytic activity for the selective
oxidation of cyclohexene. And the main oxidized product is 1,2-
cyclohexanediol (99.50% selectivity) with 98.17% conversion in
solvent-free reaction system with H2O2 as oxidants. It turns out that
cyclohexene catalytic oxidation activity is dependent on varieties of
doping acid, acidity and ANI/HA weight ratio in the fabrication. And
the optimal reaction condition is 20 mg catalyst, 2.5 mL H2O2, 70 oC
and 24 h. Furthermore, PANI@HA/1 M/2.04-HCl demonstrates
higher catalytic activity for the selective oxidation of 2,3-dimethyl-2-
butene and styrene to the corresponding 1,2-diol. In light of the CV
result, a mechanism of cyclohexene selective oxidation attributed to
reversible redox reaction of PANI in PANI@HA to catalytic
decomposition of H2O2 is proposed. This findings not only indicates
HA can act as an powerful substrate support to achieve
nanostructure PANI@HA, which avoids the use of surfactant and
simplifies the synthesis process of nanostructure PANI, but also
PANI@HA can enhance cyclohexene oxidation and the selectivity to
1,2-cyclohexanediol. In combination of the solvent-free reaction
system and environmentally friendly oxidant, this work may open a
new path for the fabrication of polymer based green catalyst system
with high performance.
concentration of H2O2 is presented in Fig. 9B. We can observe
obviously that the anodic peak current/reduction potential of
PANI@HA/1 M/2.04-HCl are the highest among the PANI@HA/1
M/2.04-H samples, implying the electron transfer from EB to LB is
more active than others.25 In combination of no anodic peak in HA,
we can conclude the catalytic decomposition of H2O2 is to a great
extent dependent on the PANI in PANI@HA.
In light of the above results, the probable catalytic mechanism of
cyclohexene by PANI@HA is proposed in Scheme 1: at first, H2O2 is
chemical adsorbed onto the surface of PANI in PANI@HA, and then
owing to the catalytic decomposition of PANI, molecular H2O2 is
rapidly decomposed into water and the oxygen atom species to form
active oxygen [O]. Finally, the active oxygen [O] would fleetly attack
the double bond of cyclohexene to form epoxy cyclohexane (which
will be further oxidated into 1,2-cyclohexanediol or other higher
oxidation state products). From the relationship between reaction
time and the selectivity to various oxidation product, it can be seen
that the selectivity to 1,2-cyclohexandiol is over 83% throughout the
whole reaction stage, indicating 1,2-cyclohexandiol is the main
oxidation product even at a short reaction time, for example, 0.5 h,
83.90% selectivity to 1,2-cyclohexandiol (Fig. 5, Table S4). It also can
be seen the selectivity to epoxy cyclohexane decreases with the
reaction time increasing in the range of 0-12 h and finally stable to
0%, whereas the selectivity to 1,2-cyclohexandiol increases
correspondingly. Considering the low selectivity to epoxy
cyclohexane (5.54%) and high selectivity to 1,2-cyclohexandiol
(83.9%) at 0.5 h reaction time, the oxidation process is that when the
active oxygen [O] attack the double bond of cyclohexene, epoxy
cyclohexane is firstly formed as the reaction intermediate, and then
the reaction intermediate is quickly oxidated into 1,2-
cyclohexanediol.
Conflicts of interest
There are no conflicts to declare
Acknowledgements
The authors gratefully acknowledge the financial support of the
National Natural Science Foundation of China (No. 51563023 and
51003091), the Natural Science Foundation of Yunnan Province (No.
10 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7TA02605F
Graphical Abstract
Enhanced solvent-free selective oxidation of cyclohexene to 1,2-cyclohexanediol was achieved by
polyaniline@halloysite nanotubes with H2O2 as oxidant at 70oC.