Ultrathin mesoporous NiCo2O4 nanosheets as an efficient and reusable catalyst for benzylic oxidation

Article abstract of DOI:10.1039/c4ra11972j

Ultrathin mesoporous NiCo₂O₄ nanosheets as a high efficient and reusable catalyst for benzylic oxidation have been prepared. The nanosheets are less than 5 nm in thickness, the pore distribution is in the range of 2-5 nm and the specific surface area of the nanosheets is about 81.0 m² g^-1. The ultrathin and mesoporous features with high specific surface area not only provide large amount of binary active sites of Co and Ni, but also improve the contacting ability between the substrate and these active sites, therefore accelerating the rate of reaction and providing high catalytic efficiency for benzylic oxidation. The NiCo₂O₄ nanosheets are efficient heterogeneous catalyst in the oxidation of 4,4′-difluorodiphenylmethane into 4,4′-difluorobenzophenone with a yield of 99%. Furthermore, the NiCo₂O₄ nanosheets catalyst exhibits general applicability, convenient separability and good stability in the benzylic oxidation.

Full text of DOI:10.1039/c4ra11972j

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Ultrathin mesoporous NiCo2O4 nanosheets have been developed for highly efficient benzylic
oxidation.

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DOI: 10.1039/C4RA11972J
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ARTICLE TYPE
Ultrathin mesoporous NiCo2O4 nanosheets as efficient and reusable
catalyst for benzylic oxidation
Jingjing Wang, a Shuang Fan, a Yi Luan, a Jia Tang, a Zhaokui Jin, a Mu Yanga,* and Yunfeng Lub,*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Ultrathin mesoporous NiCo2O4 nanosheets as high efficient and reusable catalyst for benzylic oxidation
have been prepared. The nanosheets are less than 5 nm in thickness, the pore distribution is in the range of
2ꢀ5 nm and the specific surface area of the nanosheets is about 81.0 m2 gꢀ1. The ultrathin and mesoporous
feature with high specific surface area not only provide large amount of binary active sites of Co and Ni,
10 but also improve the contacting ability between the substrate and these active sites, therefore accelerating
the rate of reaction and providing high catalytic efficiency for benzylic oxidation. The NiCo2O4
nanosheets are efficient heterogeneous catalyst in the oxidation of 4,4'ꢀdifluorodiphenylmethane into 4,4'ꢀ
difluorobenzophenone with a yield of 99%. Furthermore, the NiCo2O4 nanosheets catalyst exhibits
general applicability, convenient separability and good stability in the benzylic oxidation.
benzylic methylenes, heteroaromatic benzylic compound and
secondary benzylic alcohols. This work provided NiCo2O4 based
materials as highly efficient, lowꢀcost, reusable catalyst for
50 benzylic oxidations.
15 Introduction
Benzylic oxidation, as a representative of the oxidation of
activated benzylic CꢀH, is one of the most helpful and significant
transformations in organic chemistry, as generated ketones may
act as valuable building blocks for the manufacture of
20 pharmaceuticals and agrochemicals.1ꢀ3 The development of
catalysts for benzylic oxidation has therefore attracted
considerable interest. In the past decades, tremendous efforts had
been devoted to transitionꢀmetal catalysts involving Cr,4,5 Mn,6,7
Bi,1 Cu,8 V,9,10 Rh,11 Au12, and Fe13 for benzylic oxidation
25 reactions. Bimetallic catalysts such as AuꢀCu,14 AuꢀPd,15,16 and
AgꢀPd,17 particularly, may offer tunable catalytic properties with
synergistic effect between the metallic moieties.18,19 However,
such noble bimetallic catalysts are expensive with limited
applications.
30 During the past few years, lowꢀcost nickelꢀbased and cobaltꢀ
based monoꢀmetallic catalysts had also been explored for
benzylic oxidation with considerable catalytic results.20,21 Herein,
we reported the synthesis of mesoporous bimetallic oxide
NiCo2O4 as an effective catalyst for benzylic oxidation. NiCo2O4
35 generally adapts a spinel structure in which nickel occupies the
octahedral sites and cobalt is distributed over both octahedral and
tetrahedral sites,22ꢀ24 affording multiple oxidation sites and
excellent heat stability, which makes it as potentially excellent
catalyst for benzylic oxidation. Although NiCo2O4 has been
40 extensively studied for water splitting,25 batteries,26,27 and
supercapacitors,28,29 its utilization as catalyst for benzylic
oxidation was investigated scarcely. In this work, NiCo2O4
nanosheets with mesoporous structure and high specific surface
area were synthesized using a facile lowꢀtemperature solution
45 method, and their use as effective benzylic oxidation catalysts
was demonstrated using various benzylic substrates such as
Experimental
Materials
Ni(NO3)2·6H2O, Co(NO3)2·6H2O, hexamethylenetetramine, 70
wt% aqueous tertꢀbutylhydroperoxide (THBP) and pyridine were
55 purchased from Alfa Aesar. Ethyl alcohol and acetonitrile were
provided from Beijing Chemical Reagents Company. NiO and
Co3O4 power were also purchased from Alfa Aesar. All reagents
were used as received without further purification.
Catalyst preparation
60 NiCo2O4 nanosheets were prepared according to previous
research.29 In a typical procedure, Ni(NO3)2·6H2O (0.58 g, 2.0
mmol), Co(NO3)2·6H2O (1.16 g, 4.0 mmol) and
hexamethylenetetramine (1.26 g, 9.0 mmol) were dissolved into a
mixed solution of deionized water (80.0 mL) and ethyl alcohol
65 (40.0 mL) to form a transparent pink solution. The solution was
heated to 90 oC for 4 h, and then cooled down to room
temperature. The product was washed with ethanol and deionized
o
water several times, then dried under vacuum at 50 C for 12 h.
o
The powder annealed at 300 C for 2 h under air with a heating
70 rate of 1 oC minꢀ1 to achieve mesoporous NiCo2O4 nanosheets.
Characterization and measurement
Fieldꢀemission scanning electron microscope (FESEM) images
were observed by a ZEISS SUPRA55 operated at the accelerating
voltages of 10 kV. Highꢀresolution transmission electron
75 microscope (HRTEM) images were obtained by a FEI Tecnai
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F20 operated at the accelerating voltages of 200 kV. Powder Xꢀ
ray diffraction (XRD) patterns were recorded by a M21X
diffractometer (MAC Science Co. Ltd., Japan) using a Cu
Karadiation (k = 1.541 Å) source at 40 kV and 200 mA. Fourier
transform infrared (FTIR) spectra were acquired by a Nicolet
6700 using the potassium bromide (KBr) pellet technique. The
thermal analysis (TG) was used to investigate the thermal
decomposition behavior of the samples using Netzsch STA449F3
instrument at a heating rate of 5 oC minꢀ1 under an air flow.
5
60
10 Nitrogen adsorptionꢀdesorption isotherms were measured at 77 K
with an AUTOSORBꢀ1C analyzer (USA Quantachrome
Instruments). The specific surface area was calculated with the
BrunauerꢀEmmettꢀTeller (BET) method. The pore size
distribution was derived from the adsorption branches of
15 isotherms using the BarrettꢀJoynerꢀHalenda (BJH) model. The
chemical compositions were analysed with an Xꢀray
photoelectron spectrometer (XPS, ESCALAB 250Xi) and
inductively coupled plasmaꢀatomic emission spectrometry (ICPꢀ
AES, Vavian 715ꢀES).
65
70
20 Catalytic procedure
Fig. 1. Typical morphology of NiCo2O4 nanosheets: (a) SEM, (b)
TEM, (c) and (d) HRTEM images. The inset in (c) is a SAED
Oxidation of 4,4’ꢀdifluorodiphenylmethane (Scheme 1):
a
75 pattern.
mixture of 4,4’ꢀdifluorodiphenylmethane (1 mmol), 70 wt%
aqueous TBHP (3.0 mmol), additive and catalyst were added into
acetonitrile (1.0 mL) in a 25 mL flask. The mixture was stirred at
25 90 oC for 24 h. After filtering the mixture, the filtrate was
analyzed by Agilent 7890/5975CꢀGC/MS using nitrobenzene as
internal standard.
The leaching tests were performed by separating the catalyst at 8
h from the reaction mixture, the filtrate was stirred for a further
30 16 h. The reaction progress was monitored by GC as previously
described.
respectively. The HRTEM image in Fig. 1d clearly shows the
polycrystalline nature of these nanosheets. The space between
adjacent fringes of 0.28 nm and 0.24 nm (the inset of Fig. 1d)
80 corresponds to the (220) and (311) lattice spaces of spinel
NiCo2O4 respectively, which agrees well with previous reports.30ꢀ
33
Xꢀray diffraction (XRD) pattern of the NiCo2O4 nanosheets is
shown in Fig. 2a. The wellꢀdefined diffraction peaks at 2θ values
85 of 18.9°, 31.1°, 36.6°, 44.6°, 59.1° and 64.9° are indexed to
(111), (220), (311), (400), (511) and (440) plane of the spinel
NiCo2O4 (card no. 20ꢀ0781) with a space group of F*3 (202),
respectively.34,35 No peaks from other crystallized phases are
observed, indicating the formation of pure NiCo2O4 product. ICPꢀ
90 AES shows the molar ratio of Co to Ni is 2.06:1. XPS result
The reusability of the catalyst was investigated. After completion
of the reaction, the catalyst was filtered out and washed with
acetonitrile for three times. Catalyst recovery was tested in the
35 oxidation
of
4,4'ꢀdifluorodiphenylmethane
into
4,4'ꢀ
difluorobenzophenone.
Scheme 1 The conversion of 4,4'ꢀdifluorodiphenylmethane into
40 4,4'ꢀdifluorobenzophenone using the NiCo2O4 nanosheets as
catalyst.
95
Results and discussion
Characterization
Typical morphology of NiCo2O4 nanosheets is shown in Fig. 1. It
45 is obvious that the nanosheets are highly interconnected forming
the threeꢀdimensional nanosheets networks (Fig. 1a). TEM image
clearly reveals that the ultrathin NiCo2O4 nanosheets are less than
5 nm in thickness and 500ꢀ1000 nm in length and width (Fig. 1b).
Interestingly, HRTEM image (Fig. 1c) demonstrates that each
50 ultrathin nanosheet consists of numerous interconnected
nanoparticles and forms a mesoporous structure.28 The size of the
mesopores is estimated to be in the range of 2ꢀ5 nm. The
diffraction shown in the selected area electron diffraction (SAED)
pattern (the inset of Fig. 1c) is indexed to (200), (311), (400),
55 (420), (440) and (110) crystal planes of the spinel NiCo2O4,
100
105
Fig. 2. (a) Typical XRD pattern of NiCo2O4 nanosheets, (b)
Nitrogen adsorptionꢀdesorption isotherms (inset: poreꢀsizeꢀ
distribution) of NiCo2O4 nanosheets, (c) TGA/DTA curves of the
110 NiꢀCo precursor and (d) FTIR spectrum of NiCo2O4 nanosheets.
2
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indicate that both Co to Ni contain divalent and trivalent ions
Catalytic study
(†ESI, Fig S2).
The catalytic potential of NiCo2O4 nanosheets was evaluated in
benzylic oxidation reactions which included the oxidation of
benzylic methylenes, heteroaromatic benzylic compound and
50 secondary benzylic alcohols for the synthesis of keto compounds.
N2 adsorptionꢀdesorption measurements are used to study the
porosity and textural properties of the ultrathin NiCo2O4
nanosheets. As shown in Fig. 2b, a distinct hysteresis loop is
observed in the large range of 0.1ꢀ0.9 P/P0, suggesting that
typical mesoporous and macroporous structures for NiCo2O4
nanosheets.36 These pores come from the interstices of
nanoparticles and also nanosheets. The pore distribution is
10 relatively narrow, and mainly centered in the range of 2ꢀ5 nm (the
inset of Fig. 2b). The specific surface area, average pore size and
mesoporous volume of the ultrathin mesoporous NiCo2O4
nanosheets are 81.0 m2 gꢀ1, 3.7 nm and 0.25 cm3 gꢀ1, respectively.
Such intriguing twoꢀdimensional ultrathin mesoporous NiCo2O4
15 nanosheets feature could be critical to the catalytic performance,
because large specific surface area provides more active sites and
greatly increases the catalystꢀsubstrate contact area in the
catalytic reaction.
5
Optimization of reaction condition
Initially, the conversion of 4,4'ꢀdifluorodiphenylmethane into
4,4'ꢀdifluorobenzophenone was chosen as a model reaction in
o
acetonitrile at 90 C for 24 h. To identify the significance of the
55 catalyst for the oxidation of 4,4'ꢀdifluorodiphenylmethane, a
blank control experiment was performed (Table 1, entry 1), and
no 4,4'ꢀdifluorobenzophenone was detected. An outstanding
catalytic conversion (67%, entry 2) is observed when the
NiCo2O4 nanosheets are used as the catalyst. This conversion is
60 higher than that of NiO (16%, entry 3), Co3O4 (17%, entry 4) and
their mixture (35%, entry 5). The NiCo2O4 nanosheets provide
exposed binary active sites of Co and Ni species, which increase
the contacted probability between active sites and substrates in
liquid benzylic oxidation system and then improve the conversion
65 of 4,4'ꢀdifluorodiphenylmethane. Furthermore, a low conversion
(54%, entry 6) is observed when NiCo2O4 nanorods (Synthesis
and SEM image of NiCo2O4 nanorods were shown in †ESI) are
used as the catalyst for the oxidation of 4,4'ꢀ
difluorodiphenylmethane, indicating the NiCo2O4 nanosheets
70 with ultrathin, mesoporous and high specific surface area feature
can accelerate the reaction and improve the conversion of 4,4'ꢀ
difluorodiphenylmethane. The selectivity of benzylic oxidation is
all 99%, which can be due to that the intermediate could be
immediately converted into 4,4'ꢀdifluorobenzophenone under
75 enough TBHP oxidant and active sites.40
TG curve of the NiꢀCo precursor shows that the total weight loss
20 occurs at a typical stepwise decomposition process (Fig. 2c). The
initial weight loss of 5.6 % accompanied by an endothermic
o
reaction below 150 C is assigned to the loss of adsorbed water
and the evaporation of intercalated water molecules. The
subsequent weight loss of 21.6 % at 150ꢀ300 oC and a broad
25 exothermic peak at 280 oC is due to the loss of water produced by
the decomposition and dehydroxylation of Co(OH)2 and
Ni(OH)2.32,37
FTIR spectrum of the NiCo2O4 nanosheets is presented in Fig. 2d.
Two sharp peaks at 642 cmꢀ1 and 550 cmꢀ1 are ascribed to the
30 stretching vibration of lowꢀspin Co ions at octahedral site and
tetrahedrally coordinated Ni ions respectively, which corresponds
to the formation of spinel NiCo2O4.35,38,39
Solvents play a crucial role in liquid phase catalytic reaction. The
conversion of 4,4'ꢀdifluorobenzophenone is about 52% without
solvent (Table 2, entry 1). The conversion of 4,4'ꢀ
difluorobenzophenone is 0%, 26%, 51%, 58% and 67% when
Table
1
Catalyst screening in the oxidation of 4,4'ꢀ
difluorodiphenylmethanea
Entry
1
Catalyst
no
Conv. (%)b
no
Sel. (%)b
ꢀ
80 Table 2 Oxidation of 4,4'ꢀdifluorodiphenylmethane in different
solvents over NiCo2O4 nanosheets catalysta
NiCo2O4
nanosheets
2c
67
>99
Dielectric
constant
Conv.
(%)b
Sel.
Entry
Solvent
(%)b
3d
4e
5f
NiO
Co3O4
16
17
35
>99
>99
>99
1
2
3
4
5
6
7
Neat
nꢀHexane
Isopropanol
Acetone
Toluene
Acetonitrile
H2O
ꢀ
52
no
26
51
58
67
no
>99
ꢀ
1.89
15.7
20.7
2.4
NiO+Co3O4
>99
>99
>99
>99
ꢀ
NiCo2O4
nanorods
6g
54
>99
a
35 Reaction conditions: 4,4'ꢀdifluorodiphenylmethane (1.0 mmol),
catalyst, aqueous TBHP (70 wt%) (3.0 mmol) and acetonitrile
(1.0 mL) at 90 oC stirring for 24 h.
37.5
80.4
b
Conversion and selectivity were determined by GCꢀMS using
nitrobenzene as internal standard.
40 c NiCo2O4 nanosheets (5.0×10ꢀ2 mmol).
d NiO (5.0×10ꢀ2 mmol).
a
Reaction conditions: 4,4'ꢀdifluorodiphenylmethane (1.0 mmol),
12.0 mg (5.0×10ꢀ2 mmol) NiCo2O4 nanosheets catalyst, aqueous
TBHP (70 wt%) (3.0 mmol) and solvent (1.0 mL) at 90 oC
85 stirring for 24 h.
e Co3O4 (3.3×10ꢀ2 mmol).
f NiO (5.0×10ꢀ2 mmol) + Co3O4 (3.3×10ꢀ2 mmol).
g NiCo2O4 nanorods (5.0×10ꢀ2 mmol).
45
b
Conversion and selectivity were determined by GCꢀMS using
nitrobenzene as internal standard.
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Table 3 Oxidation of 4,4'ꢀdifluorodiphenylmethane under a
function of additivea
45
50
55
Entry
Additive (mol%)
Conv. (%)b
Sel. (%)b
1
2
3
4
5
6
7
Pyrrole (100)
Imidazole (100)
Pyridine (100)
Pyridine (50)
Pyridine (10)
Pyridine (5)
8
43
88
92
>99
73
67
>99
>99
>99
>99
>99
>99
Pyridine (0)
>99
Fig. 4. The reusability of the NiCo2O4 nanosheets catalyst for the
oxidation of 4,4'ꢀdifluorodiphenylmethane.
a
Reaction conditions: 4,4'ꢀdifluorodiphenylmethane (1.0 mmol),
12.0 mg (5.0×10ꢀ2 mmol) NiCo2O4 nanosheets catalyst, aqueous
TBHP (70 wt%) (3.0 mmol), additive and solvent (1.0 mL) at 90
oC stirring for 24 h.
60 The influence of reaction temperature on the conversion of 4,4'ꢀ
difluorodiphenylmethane using NiCo2O4 nanosheets catalyst is
5
o
b
also investigated. When the reaction temperature keeps at 70 C,
Conversion and selectivity were determined by GCꢀMS using
80 oC and 90 oC, the conversion is 50%, 58% and 99%,
respectively. This indicates that the increasing of the temperature
65 can improve the conversion of 4,4'ꢀdifluorobenzophenone.
Increased the temperature to 100 oC, the conversion maintains
99%, however the selectivity decreases slightly (95%), which is
attributed to the formation of the byproduct (bis(4ꢀ
fluorophenyl)methanol). Therefore, the optimized temperature for
70 the conversion of 4,4'ꢀdifluorodiphenylmethane is 90 oC.
Fig. 3 shows the evolution of the conversion and selectivity as
reaction time using the NiCo2O4 nanosheets catalyst. The
conversion of 4,4'ꢀdifluorodiphenylmethane increases linearly as
the reaction time and reach 88% at 12 h. Then, 4,4'ꢀ
nitrobenzene as internal standard.
10 hexane, isopropylalcohol, acetone, toluene and acetonitrile are
employed as the solvent, respectively (entry 2ꢀ6). This suggests
that the aprotic solvents with larger dielectric constant are
beneficial to the enhancing of the conversion of 4,4'ꢀ
difluorobenzophenone. These solvents promote the dispersion
15 and collisions of the oxygen source, the substrate and the catalyst.
Besides, acetonitrile could form peroxyimidic acid, which further
improve
the
conversion.41
However,
no
4,4'ꢀ
difluorobenzophenone is detected when H2O are employed as
solvent (entry 7) owing to the limited dispersion of the oxygen
20 source and the substrate.
75 difluorodiphenylmethane completely converts to
4,4'ꢀ
difluorobenzophenone at 24 h (curve a in Fig. 3). The selectivity
maintains 99% during the whole reaction process. Therefore, the
optimized catalytic procedure involves 1 mmol substrate, 5 mol%
NiCo2O4 nanosheets catalyst and 10 mol% pyridine as an additive
80 and 3.0 equivalents of aqueous TBHP as an oxidant in
acetonitrile (1.0 mL) at 90 oC stirring for 24 hours. This
procedure is more ecoꢀfriendly than previous researches. Bolm
and his coworkers observed only 93% yield of 4,4'ꢀ
difluorobenzophenone when used homogeneous FeCl3·6H2O as
85 catalyst and pyridine (1.0 mL) as solvent.13 Barrett and his
coworkers reported bismuth catalysed benzylic oxidation
involving both pyridine and corrosive acid at 100 oC.1
Previous results confirmed that the role of base was pivotal in this
11
reaction.
The conversion of 4,4'ꢀdifluorodiphenylmethane is
8%, 43% and 88% when additive pyrrole, imidazole and pyridine
is added, respectively (Table 3, entry 1ꢀ3). This is because that
25 pyridine could behave as a hydrogen bond acceptor with TBHP to
accelerate the formation of tꢀbutoxyl radical.1 The radical abstract
hydrogen atom at the benzylic carbon atom from the
substrate.42,43 Moreover, the conversion shows a maximum when
the content of pyridine is 10 mol% (Table 3, entry 4ꢀ7).
30
Heterogeneity and reusability Tests
As we know, a crucial issue for solid catalyst is the migration of
90 active site from the solid catalyst to the liquid phase.44 In order to
verify the stability of Ni and Co active species of NiCo2O4
nanosheets, a leaching experiment was performed to estimate the
contribution of leached active species to the catalytic activity.
The solid catalyst was separated by centrifugation at 8 h. The
95 residual liquid phase was transferred to a new reactor vessel for
another 16 h reaction. GC determinations show that no more 4,4'ꢀ
difluorobenzophenone is observed (curve b in Fig. 3), which
indicates no active sites migrate from the solid catalyst to the
liquid phase during the reaction.
35
40 Fig. 3. The leaching experiment, curve a: change of conversion
and selectivity over the reaction time for model reaction, curve b:
leaching test.
4
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Table 4 Oxidative activity of different benzylic substrates using
recovered and washed with acetonitrile to remove physical
60 absorbed reagents. The NiCo2O4 nanosheets retain 85% of
conversion, 93% of selectivity even after 5 recycles (Fig. 4). The
XRD and FTIR results (†ESI, Fig S3) show that the composition
of the catalyst has no obvious change before and after reaction.
While XPS results (†ESI, Fig S4) show an increasing of the Ni3+
65 moiety in the recovered nanosheets, and SEM images (†ESI, Fig
S3) suggest the assembled structure of the nanosheets is slightly
collapsed. These may lead to the slightly decreasing in the
conversion and selectivity for the oxidation of 4,4'ꢀ
difluorodiphenylmethane during the cycling experiment.
NiCo2O4 nanosheets catalysta
5
10
15
20
25
30
35
40
70 Oxidation of different benzylic substrates
To determine the general applicability of NiCo2O4 nanosheets
catalyst, benzylic oxidation of benzylic methylenes,
heteroaromatic benzylic compounds and secondary benzylic
alcohols are also studied (Table 4). The conversion of ethyl
75 benzene, 1ꢀethylꢀ4ꢀfluorobenzene and 1ꢀethylꢀ4ꢀmethoxybenzene
was 95%, 85% and 99%, respectively (Table 4, entry 1ꢀ3),
indicating that the presence of electronꢀdonating group favor the
oxidation. Similar results are also observed for benzyl methyl
ether and methyl 2ꢀphenylacetate, whose conversion is 42%
80 (entry 4) and 99% (entry 5), respectively. The relative conversion
of the products suggests that the CꢀH bond energy of the benzyl
positions is one main parameter to control the activity. To expand
the scope of the catalytic system, heteroaromatic compounds,
such as 2ꢀethylthiophene (entry 6) is chosen as the substrate. 2ꢀ
85 acetylthiophene is produced in good conversion (91%) due to the
fact that the sulfur in a five membered aromatic ring may be
regarded as an electron donating substituent.21 Interestingly, the
reaction is also efficient for the ring 9Hꢀfluorene (99%, entry 7)
and various secondary benzylic alcohols (99%, entry 8ꢀ10). These
90 exciting results confirm that NiCo2O4 nanosheets are efficient
heterogeneous catalyst for the benzylic oxidation with high yield.
It can be attributed to following critical merits. During the
catalytic reaction, Co3+/Co2+ or Ni3+/Ni2+ redox couples would act
as the active sites circularly and produce the carbonꢀcentered
95 radicals (†ESI, Scheme S1). The ultrathin NiCo2O4 nanosheets
with high specific surface area have an increased portion of
exposed surfaces and ensure high utilization of Co and Ni active
sites. On the other hand, the mesoporous channels can ensure
efficient contact between the active sites and the substrate.
a
Reaction conditions for a 1.0 mmol scale. Reactions were
100 Conclusions
45 conducted using substrates (1.0 mmol), pyridine (8.0 ꢁL, 0.1
mmol), 12.0 mg (5.0×10ꢀ2 mmol) NiCo2O4 nanosheets catalyst,
aqueous TBHP (70 wt%) (3.0 mmol) and acetonitrile (1.0 mL) at
90 oC stirring for 24 h.
In summary, a novel, highly effective and recyclable NiCo2O4
nanosheet catalyst have been successfully synthesized and
applied for benzylic oxidation. The ultrathin and high specific
surface area feature of the catalyst increases the available Co and
105 Ni binary active sites, and the mesopores ensure enough contact
between these highly exposed active sites and the substrate, and
then improve the conversion of the substrates. The NiCo2O4
nanosheets show excellent performance towards the oxidation of
a wide range of benzylic substrates, such as benzylic methylenes,
110 heteroaromatic benzylic compound and secondary benzylic
alcohols. This provides a highly efficient, lowꢀcost and reusable
heterogeneous catalyst for benzylic oxidations and can be
extended to the structure design for other nanohybrid catalysts
with doubleꢀ or multiꢀ metal active sites.
b
Conversion and selectivity were determined by GCꢀMS with
50 internal standard method and 1HNMR analysis. NMR samples
were prepared by careful removal of solvent and dissolving the
residue in CDCl3.
c The reaction was performed 90 oC for 2 h.
d The reaction was performed at ambient temperature for 30 min.
55
In order to further evaluate the stability of NiCo2O4 nanosheets
catalyst, the catalyst was used in the cycled reaction under the
same reaction conditions. After each reaction, the catalyst was
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DOI: 10.1039/C4TA00107A.
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Acknowledgment
70
This work was supported by the Coꢀbuilding Special Project of
Beijing Municipal Education and Natural Science Foundation of
China (NSFC no. 51328202).
5
Notes and references
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Eꢀmail address: yangmu@ustb.edu.cn (Mu Yang); luucla@ucla.edu
(Yunfeng Lu);
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