Selective oxidation of ethylbenzene to acetophenone over Cr(III) Schiff base complex intercalated into layered double hydroxide

Article abstract of DOI:10.1002/aoc.4408

A new heterogeneous catalyst, Cr(III) Schiff base-containing layered double hydroxide, was synthesized using the intercalation method. The Cr(III) Schiff base complex derived from 2-hydroxy-1-naphthaldehyde and 4-aminobenzoic acid was intercalated into th

Full text of DOI:10.1002/aoc.4408

Received: 22 February 2018
DOI: 10.1002/aoc.4408
Revised: 6 April 2018
Accepted: 10 April 2018
F U L L P A P E R
Selective oxidation of ethylbenzene to acetophenone over
Cr(III) Schiff base complex intercalated into layered double
hydroxide
Jagat Singh Kirar | Savita Khare
School of Chemical Sciences, Devi Ahilya
A new heterogeneous catalyst, Cr(III) Schiff base‐containing layered double
University, Takshashila Campus,
Khandwa Road, Indore, MP 452001, India
hydroxide, was synthesized using the intercalation method. The Cr(III) Schiff
base complex derived from 2‐hydroxy‐1‐naphthaldehyde and 4‐aminobenzoic
Correspondence
Savita Khare, School of Chemical
Sciences, Devi Ahilya University,
Takshashila Campus, Khandwa Road,
Indore (MP) – 452001, India.
acid was intercalated into the layered double hydroxide. The synthesized mate-
rials were characterized using inductively coupled plasma atomic emission
spectrometry, energy‐dispersive X‐ray analysis, scanning electron microscopy,
X‐ray diffraction, Brunauer–Emmett–Teller surface area measurement, Fourier
transform infrared spectroscopy, thermogravimetric analysis, diffuse reflec-
tance UV–visible spectroscopy and electron paramagnetic resonance spectros-
copy. The catalytic activity was investigated for the oxidation of ethylbenzene
with tert‐butylhydroperoxide as an oxidant under solvent‐free conditions as
well as with lower chromium concentrations. In the oxidation reaction, ethyl-
benzene was oxidized to acetophenone and benzaldehyde. The catalyst was
recycled ten times without significant loss of catalytic activity. Leaching studies
performed with hot filtration experiments showed that the chromium catalyst
was heterogeneous in nature and stable under the reaction conditions.
Email: kharesavita@rediffmail.com
Funding information
University Grant Commission New Delhi,
India, Grant/Award Number: F1‐17.1/
2
016‐17/RGNF‐2015‐17‐SC‐MAD‐19994/
SAIII/WeUGC‐SAP: No. F.540/13/DRS‐I/
016 (SAP‐I)
2
KEYWORDS
chromium(III) Schiff base complex, ethylbenzene, heterogeneous catalyst, layered double hydroxide,
tert‐butylhydroperoxide
1
| INTRODUCTION
reagents also produce huge amounts of corrosive and
[
5]
hazardous wastes.
In general, chromium‐containing
Side‐chain oxidation of saturated hydrocarbons to the
catalysts are used in the oxidation of most organic com-
[
6,7]
corresponding oxygenates is among the most important
pounds in homogeneous media.
Current industrial
[
1]
and challenging reactions in the chemical industry.
processes for the production of acetophenone are based
on the liquid‐phase oxidation of ethylbenzene by oxygen
using homogeneous cobalt‐based compounds as catalysts
in acetic acid media. This method suffers from its corro-
Recently, the selective oxidation of ethylbenzene to
acetophenone has drawn immense attention, because
acetophenone serves as a crucial platform as an
intermediate for the manufacturing of some perfumes,
[
8]
sive and environmentally unfriendly nature. Hence,
there has been an interest in the development of
ecofriendly heterogeneous catalysts for the oxidation of
ethylbenzene. Heterogeneous catalysts have become very
important for ecofriendly industrial processes because
[2]
pharmaceuticals, resins, alcohols, aldehydes, etc. Con-
ventionally, acetophenone is synthesized by oxidation of
ethylbenzene with stoichiometric inorganic oxidants such
[3,4]
as permanganate or dichromate.
However, these
Appl Organometal Chem. 2018;e4408.
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1 of 12
https://doi.org/10.1002/aoc.4408

2
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KIRAR AND KHARE
these materials are developed with the objective of
performing reactions under milder conditions and without
hazardous wastes. Many researchers have used Cr(III)‐
based heterogeneous catalysts for the oxidation of ethyl-
as well as selectivity for desired product were optimized
by varying different parameters such as oxidizing agent,
molar ratio of ethylbenzene to TBHP, amount of catalyst
and temperature.
[9–13]
[9]
benzene using various oxidants.
Dapurkar et al.
have reported catalytic activity of (Cr)MCM‐48 for oxida-
tion of ethylbenzene with tert‐butylhydroperoxide (TBHP)
and achieved 76.3% conversion of ethylbenzene after
2
2
| EXPERIMENTAL
.1 | Materials and Methods
[10]
1
2 h. Miao et al.
have prepared mesoporous transition
metal–zirconium oxophosphate composites (M–Cr–ZrPO)
Zinc(II)
nitrate
hexahydrate
(Zn(NO ) ⋅6H O),
3
3
2
for the oxidation of ethylbenzene with TBHP which gave
aluminium(III) nitrate nonahydrate (Al(NO ) ⋅9H O),
3
3
2
9
0% conversion of ethylbenzene with 90% selectivity to
chromic chloride (CrCl ⋅6H O), 4‐aminobenzoic acid, 2‐
3
2
[11]
acetophenone after 24 h. Selvaraj et al.
have synthe-
hydroxy‐1‐naphthaldehyde and sodium hydroxide were
all of analytical grade and purchased from E. Merck. A
70% commercial aqueous solution of TBHP was also pur-
chased from E. Merck. HPLC‐grade ethylbenzene was
purchased from SDFCL and its purity was checked using
GC to ensure that no oxidation products were present.
A stock solution of TBHP (72% TBHP in ethylben-
zene) was prepared by extraction of 55 ml of commercial
TBHP (70% in water) into 15 ml of ethylbenzene. Phase
separation was promoted by saturation of the aqueous
layer with NaCl. The organic layer was dried over MgSO4,
filtered and stored at 5 °C. The molar ratio of ethylben-
zene to TBHP in this solution was 1:3.
sized a recyclable hexagonally ordered mesoporous
CrSBA‐15 catalyst for the oxidation of ethylbenzene to
acetophenone using TBHP which gave 95.6% conver-
sion of ethylbenzene with selectivity of 98.2% to
acetophenone after 10 h of reaction.
Layered double hydroxides (LDHs) are some of the
most important inorganic materials with a layered struc-
ture, which are anionic clays or hydrotalcite‐like com-
[
14–16]
pounds.
The structures of LDHs consist of brucite‐
II
like (M (OH) ) layers with some of the divalent cations
2
II
III
(M ) substituted by trivalent ions (M ). Both divalent
and trivalent ions are located at the centre of octahedral
−
(OH ) units of the brucite layers. The resulting layers
Powder X‐ray diffraction (XRD) patterns of samples
were recorded with a Rigaku diffractometer in the 2θ range
2–70° using Cu Kα radiation (λ = 1.5418 Å) at a scanning
speed of 2° per minute with step size 0.02°. Scanning elec-
tron microscopy (SEM) measurements were performed
using a JEOL JSM 6100 electron microscope, operating at
20 kV. Fourier transform infrared (FT‐IR) spectra were
recorded with a PerkinElmer model 1750 in KBr. Thermo-
gravimetric analysis (TGA) was conducted with a
PerkinElmer USAA Diamond system in the range 298–
are in an excess of positive charge acquired to balance
the intercalated anions. The general formula of LDHs is
II
III
x+
n−
n−
[
M
− xM (OH) ] ⋅(A
)⋅mH O, where A is the
1
x
2
17,18]
x/n
2
[
interlayer anion.
molar ratio M /(M + M ), and m is the number of
water molecules located in the interlayer region together
with the anions.
anion‐exchange capacity and expansion properties which
make them promising materials for the intercalation of
anionic metallic complexes.
sively studied for their intercalation chemistry,
heterogeneous catalyst precursors and for their ion
exchange properties.
interest for green and sustainable chemistry.
cases, LDHs have been prepared using co‐precipitation
The coefficient x is equal to the
III
II
III
[19]
LDHs have large surface area, good
[20,21]
−1
They have been exten-
973 K, at a heating rate of 10 °C min . Inductively coupled
[
22–24]
as
plasma atomic emission spectrometry (ICP‐AES; Thermo
Electron Iris Intrepid II XSP DUO) was used for estimation
of chromium. The electronic spectrum of the solid was
recorded with a Varian Cary 5000 spectrophotometer.
The electron paramagnetic resonance (EPR) spectrum
was recorded with a JES‐FA200 ESR X‐band spectrometer
operated at a microwave frequency of 9.65 GHz at room
temperature. The g value is reported relative to a 2,2‐
diphenyl‐1‐picrylhydrazyl standard with g = 2.0036. N2
adsorption data, measured at 77 K with a volumetric
adsorption set‐up (Micromeritics ASAP‐2010, USA), were
used to determine Brunauer–Emmett–Teller (BET) surface
area. Analytical gas chromatography was carried out using
a Shimadzu GC‐14B with dual flame ionization detector
and attached Shimadzu printer having XE‐60 ss column.
The products were identified by GC–MS (PerkinElmer
Clasus 500 column; 30 m × 60 mm).
[25–28]
LDH‐based catalysis is of high
[29]
In most
[25,26]
methods.
Because of our interest in the oxidation of hydrocar-
bons, in previous studies we have reported layered mate-
rial‐supported recyclable heterogeneous catalysts for the
[30–35]
oxidation of hydrocarbons.
In continuation of our
work, herein we report the synthesis of a Cr(III) Schiff
base complex derived from 2‐hydroxy‐1‐naphthaldehyde
and 4‐aminobenzoic acid (NAPABA‐Cr(III)), which was
intercalated into a LDH to afford LDH‐[NAPABA‐
Cr(III)]. Its catalytic behaviour was investigated for the
oxidation of ethylbenzene using TBHP as an oxidant.
The conditions for maximum conversion of ethylbenzene

KIRAR AND KHARE
3 of 12
2
.2 | Preparation of Catalyst
mixture was refluxed for 4 h under nitrogen atmosphere.
After cooling, the solid was filtered and wash with meth-
anol. The resulting solid was extracted with methanol
followed by acetone and acetonitrile using a Soxhlet
extractor to remove excess ligand and metal salt remain-
ing uncomplexed in the host layer as well as on the sur-
face, if any, and dried at 333 K overnight. Scheme 1
illustrates the preparation of the heterogeneous LDH‐
The Cr(III) complex was synthesized by dissolving 4‐
aminobenzoic acid (10 mmol) into a 50 ml methanolic
solution of NaOH (20 mmol) followed by a methanolic
solution of 2‐hydroxy‐1‐naphthaldehyde (10 mmol), then
immediately CrCl ⋅6H O (5 mmol) was added and the
mixture was kept under continuous stirring for 4 h at
room temperature. After 4 h, a greenish brown precipi-
tate of NAPABA‐Cr(III) complex was formed which was
filtered, washed with petroleum ether and dried in air.
The obtained complex was characterized using elemental
analysis and FT‐IR and diffuse reflectance UV (DRUV)
3
2
[NAPABA‐Cr(III)] catalyst.
2.3 | Oxidation of Ethylbenzene
Oxidation of ethylbenzene over the LDH‐[NAPABA‐
Cr(III)] catalyst using TBHP as oxidant was carried out
under solvent‐free conditions in a three‐necked round‐
bottom flask. In a typical run, the flask was loaded with
spectroscopies. Anal. Found for C H N O Cr (%): C,
36
22 2 6
6
4
1
8.46; H, 3.31; N, 4.38. Calcd (%): C, 68.57; H, 3.52; N,
.44. FT‐IR (KBr pellet, cm ): 3373(br), 1608(m),
581(m), 1476(w), 1409(m), 657(w), 523(w).
−
1
13 mmol of ethylbenzene, 39 mmol of TBHP (72% in eth-
LDH‐[NH ‐C H COO] was prepared by a co‐precipi-
2
6
4
ylbenzene) and catalyst at the required temperature in an
oil bath for 7 h with continuous stirring using a Teflon
needle on a magnetic hot plate. After completion of the
reaction, the catalyst was filtered off. The catalyst was
dried at 333 K for further use. The oxidation products
were qualitatively as well as quantitatively analysed using
GC–MS and GC, respectively. The GC–MS analysis
revealed that the main products were acetophenone and
benzaldehyde. Selectivity of products was calculated with
respect to the converted ethylbenzene using the internal
standard method. The internal standard was dodecane.
tation method according to a previously reported proce-
dure with certain modifications.
[36]
Solutions of zinc(II)
nitrate hexahydrate (14.82 g) and aluminium(III) nitrate
nonahydrate (6.25 g) in decarbonized water were mixed
having a Zn/Al molar ratio of 3. To that was added a solu-
tion of 4‐aminobenzoic acid (10.86 g) and NaOH (7.8 g) in
decarbonized water with continuous stirring. Immedi-
ately a gel‐like mixture was obtained which was digested
at 348 K for 48 h. Upon cooling, the product was isolated
by filtration, washed with water followed by methanol,
and the solid was dried at 333 K overnight.
The ligand was synthesized by mixing LDH‐[NH2‐
C H COO] (1.0 g) into a methanolic solution of 2‐
2.4 | Separation of Acetophenone
6
4
hydroxy‐1‐naphthaldehyde (0.48 g, 2.77 mmol), the solu-
tion instantly becoming yellow due to imine formation.
The resulting mixture was refluxed for 3 h with continu-
ous stirring under nitrogen atmosphere. The yellow prod-
uct was filtered off, washed with methanol followed by
acetonitrile and then dried at 333 K overnight.
Separation of acetophenone was done on the basis of a
[
37]
previously reported method
after slight modification.
The reaction mixture of ethylbenzene oxidation was col-
lected and concentrated under reduced pressure. The
concentrated mixture was dissolved in 15 ml of ethanol.
Then 1 mmol of Zn granules was added, and the mixture
was stirred for 30 min at room temperature. The mixture
was filtered and the solvent removed under reduced pres-
sure. This reduced mixture was extracted with saturated
The LDH‐[NAPABA‐Cr(III)] catalyst was prepared by
incorporation of Cr(III) into the LDH‐[NAPABA] ligand.
LDH‐[NAPABA] (1 g) was suspended in 50 ml of metha-
nol and then a solution of CrCl ⋅6H O (0.26 g) in 50 ml of
3
2
NaHCO solution to remove benzoic acid, if any. Then
3
methanol was added to it with continuous stirring. The
the mixture was washed with water, followed by
SCHEME 1 Synthesis of heterogeneous
LDH‐[NAPABA‐Cr(III)] catalyst

4
of 12
KIRAR AND KHARE
extraction with ether. After removing the aqueous layer,
C H COO] shows the most intense basal reflection at
6
4
the ethereal layer was dried over anhydrous MgSO . Yel-
15.71 Ǻ (003). The characteristic reflections correspond-
ing to (110) plane show atomic distribution density
depending on Zn/Al molar ratio. The results are incorpo-
rated in Table 2. The basal spacing of (003) plane
increases from 15.71 to 22.21 Ǻ in LDH‐[NAPABA‐
4
lowish oil was obtained which was 90.67% acetophenone
with 92% purity. It was characterized using LC–MS and
FT‐IR analyses. LC–MS: m/z = 21, 57, 77, 105, 121. FT‐
−
1
IR (KBr pellet, cm ): 2978(w), 1690(m), 1263(w).
b
Cr(III)] . The gallery height of the catalyst is 17.51 Å
when the thickness of the brucite layers (4.7 Å) is
subtracted. There was no change in characteristic reflec-
tions corresponding to (110) plane after intercalation of
the Cr(III) Schiff base complex, which indicates that there
3
3
| RESULTS AND DISCUSSION
.1 | Characterization of Catalyst
[39]
The analytical and physical data including chemical
composition of compounds at various stages of the syn-
thesis of the catalyst, namely LDH‐[NH ‐C H COO],
was no change in the basic structure of LDH.
The
increase in gallery height clearly indicates successful
intercalation of the NAPABA‐Cr(III) Schiff base complex
in the LDH layers.
2
6
4
b
a
LDH‐[NAPABA‐Cr(III)] and LDH‐[NAPABA‐Cr(III)]
b = before catalytic reaction; a = after catalytic reaction),
(
Furthermore, particle size of LDH‐[NH ‐C H COO]
2
6
4
are given in Table 1. The fresh catalyst, LDH‐[NAPABA‐
support and heterogeneous LDH‐[NAPABA‐Cr(III)] cata-
lyst was calculated using the Debye–Scherrer equation.
The average particle size was calculated for the most
intense peak corresponding to the (003) plane of LDH‐
[NH ‐C H COO] and LDH‐[NAPABA‐Cr(III)] and it was
b
Cr(III)] , contains 4.43% chromium, whereas LDH‐
a
[
NAPABA‐Cr(III)] contains 4.38% chromium after ten
catalytic cycles, which was determined using ICP‐AES.
The reduction in the chromium contents may be due to
the washing of the catalyst after each cycle.
Figure 1 shows the elemental composition of the
catalyst, determined using energy‐dispersive X‐ray
2
6
4
found to be 2.4 and 2.6 nm, respectively. The results are
incorporated in Table 2. The increase in gallery height
and average particle size of the LDH‐[NAPABA‐Cr(III)]
catalyst clearly indicate the successful intercalation of
NAPABA‐Cr(III) Schiff base complex into the LDH layers.
The results of BET surface area, pore volume and pore
size of LDH‐[NH ‐C H COO] support and heterogeneous
(
[
[
EDX) analysis. The EDX measurement results for LDH‐
b
NH ‐C H COO], LDH‐[NAPABA‐Cr(III)] and LDH‐
2
6
4
a
NAPABA‐Cr(III)] are presented in Table 1. The pres-
ence of carbon, nitrogen, oxygen, zinc, aluminium along
with chromium metal in LDH‐[NAPABA‐Cr(III)] con-
firms the formation of the heterogeneous catalyst. The
ideal formula for LDH‐[NAPABA‐Cr(III)], based on the
elemental content and Zn/Al ratio, is [Zn0.76Al0.24(OH)2]
2
6
4
LDH‐[NAPABA‐Cr(III)] catalyst are incorporated in
Table 2. The surface area of LDH‐[NH ‐C H COO] sup-
2
6
4
2
−1
port is 6.915 m g whereas surface area of heteroge-
2
−1
neous LDH‐[NAPABA‐Cr(III)] catalyst is 29.93 m g .
The reason for the increase in surface area of the catalyst
in comparison to LDH‐[NH ‐C H COO] was due to the
[
NAPABA‐Cr(III)(H O) ] [NH2‐
2
2 0.12
C H COO] ⋅0.47H O. The SEM images in Figure 1 of
6
4
0.07
2
2
6
4
LDH‐[NH ‐C H COO] and LDH‐[NAPABA‐Cr(III)]
intercalation of larger complex molecules into the LDH
layers which led to an expansion of the layers. This is fur-
ther supported by the XRD data (the basal spacing
2
6
4
clearly show that both have similar structures, having
[38]
flake‐like aggregates,
but with different sizes of the
[
26]
flakes. This clearly indicates the successful intercalation
of the chromium Schiff base complex into the LDH host.
increases from 15.71 to 22.21 Ǻ).
The pore volume of
3
−1
3
−1
LDH is 0.017 cm g which increases to 0.121 cm g
Figure 2 shows the XRD patterns of LDH‐[NH ‐
in the catalyst while pore size decreases from 100.90 to
36.51 Å in the catalyst. The increase in surface area and
pore volume indicates successful intercalation of the
Cr(III) Schiff base complex into the LDH layers.
2
b
C H COO],
LDH‐[NAPABA‐Cr(III)]
and
LDH‐
6
4
a
[
NAPABA‐Cr(III)] with the d‐spacing corresponding to
the plane (003). The XRD pattern of LDH‐[NH2‐
TABLE 1 Chemical composition and physical data of various compounds
EDX data (wt%)
Metal
Catalyst
Colour
content (%)*
Zn
Al
C
O
N
Cr
LDH‐[NH
2
‐C
6
H
5
COO]
White
Brown
Brown
—
26.39
16.41
16.30
7.73
6.24
6.12
32.49
45.05
44.95
29.12
23.17
23.14
4.27
3.67
4.05
—
b
a
LDH‐[NAPABA‐Cr(III)]
LDH‐[NAPABA‐Cr(III)]
4.43
4.38
5.46
5.44
*
From ICP‐AES analysis.

KIRAR AND KHARE
5 of 12
FIGURE 2 XRD patterns of (a) LDH‐[NH
2
‐C
6
H
4
a
COO], (b) LDH‐
b
[NAPABA‐Cr(III] and (c) LDH‐[NAPABA‐Cr(III]
The FT‐IR spectra of LDH‐[NH ‐C H COO],
2
6
4
NAPABA‐Cr(III) and LDH‐[NAPABA‐Cr(III)] are shown
in Figure 3. In the spectrum of LDH‐[NH ‐C H COO]
2
6
4
(
3
Figure 3a), the broad absorption bands at 3399 and
255 cm are attributed to stretching modes of NH2
−
1
and OH groups, respectively. The strong band at
−
1
1
584 cm is attributed to C═O stretching vibration. A
−
1
band at 1445 cm is due to C═C stretching of aromatic
[
40,41]
−1
ring.
The bands in the range 428–784 cm are due
to stretching vibrations of M─O and O─M─O (M = Zn,
[
40,41]
Al),
which include translation vibrations of Zn─OH
−
1
−1
at 637 cm and Al─OH at around 517 cm and defor-
mation vibration of HO─Zn─Al─OH at 428 cm ,
which are typical of this class of materials.
−
1 [42]
[
43]
In the spec-
trum of homogeneous catalyst, NAPABA‐Cr(III)
−
1
(
Figure 3b), the broad absorption band at 3373 cm is
attributed to stretching modes of ─NH group. The band
2
−
1
at 1608 cm is due to C═N stretching of imines while
bands at 663 and 499 cm are due to Cr─N and Cr─O,
respectively.
−
1
The spectrum of the heterogeneous catalyst, LDH‐
[NAPABA‐Cr(III)] (Figure 3c), shows all the absorption
bands corresponding to LDH‐[NH ‐C H COO] along
2
6
4
−
1
with a band at 1606 cm due to C═N stretching of
imines which was shifted to lower frequency while the
bands arising due to Cr─N and Cr─O stretching were
−
1 [39]
shifted to 640 and 426 cm .
The presence of these
bands clearly indicates the intercalation of NAPABA‐
Cr(III) into LDH.
The thermal decomposition behaviour of LDH‐[NH2‐
C H COO] and LDH‐[NAPABA‐Cr(III)] was evaluated
6
4
using TGA and the results are shown in Figure 4. The
TGA curve of LDH‐[NH ‐C H COO] shows a first weight
2
6
4
loss from 50 to 170 °C due to the removal of adsorbed
water. The second stage corresponds to the degradation
of the brucite‐like layer and removal of interlayer benzo-
ate anions in the range 200–500 °C. The TGA curve of
FIGURE 1 EDX spectra of (a) LDH‐[NAPABA‐Cr(III)] and (b)
LDH‐[NH
2
‐C
6
H
4
COO]. SEM images of (c) LDH‐[NAPABA‐Cr(III)]
‐C COO]
and (d) LDH‐[NH
2
6 4
H

6
of 12
KIRAR AND KHARE
TABLE 2 Textural properties of support and heterogeneous catalyst
BET surface
area (m g
Pore volume
Pore
size (Å)
d‐spacing
(Å)
Particle
size (nm)
2
−1
3
−1
Catalyst
)
(cm g
)
LDH‐[NH
2
‐C
6
H
4
COO]
6.915
29.93
23.73
0.017
0.121
0.097
100.90
36.51
29.09
15.71
22.21
22.20
2.35
2.59
2.58
b
a
LDH‐[NAPABA‐Cr(III)]
LDH‐[NAPABA‐Cr(III)]
in Figure 5. The spectrum of homogeneous complex,
NAPABA‐Cr(III), shows three bands at 255, 321 and
463 nm. The first band at 255 nm may be assigned to
π → π* of aromatic rings. The second band at 321 nm is
due to n → π* transitions of imine group (C═N). The
third band in the range 395–548 nm is assigned to
ligand‐to‐metal charge transfer transition in the metal
complex. The absorption above 300 nm also indicates a
large conjugated system of the complex. An additional
weak and broad band in the range 548–730 nm is due
[39]
to the d–d transition as reported in the literature.
All
these bands appear in the spectrum of the heterogeneous
LDH‐[NAPABA‐Cr(III)] catalyst with lower intensity due
to smaller amount of the homogeneous complex in the
layers of LDH.
FIGURE 3 FT‐IR spectra of (a) LDH‐[NH
NAPABA‐Cr(III) and (c) LDH‐[LDH‐NAPABA‐Cr(III)]
2
‐C
6 4
H COO], (b)
The EPR spectrum of the heterogeneous LDH‐
[NAPABA‐Cr(III)] catalyst was recorded at room temper-
ature (Figure 6). The EPR spectrum shows a single peak
at 320 mT. Also the spectrum gives a g value of 1.947 with
giso of 2.07. This clearly indicates the presence of chro-
mium metal ion in +3 oxidation state and three unpaired
electrons in the d‐orbital. The EPR parameters suggest an
[
44]
octahedral geometry for the Cr(III) complex.
On the
basis of EPR analysis an imaginary structure of the cata-
lyst is proposed (Scheme 2).
FIGURE 4 TGA curves of (a) LDH‐[NH
LDH‐[NAPABA‐Cr(III)]
2 6 4
‐C H COO] and (b)
LDH‐[NAPABA‐Cr(III)] shows a first weight loss from 50
to 174 °C. A second step in the range 179–340 °C is
assigned to partial dehydroxylation of the double hydrox-
[38]
ide layers.
The decomposition of the organic part
occurs in the range 343–525 °C.
The DRUV–visible spectra of LDH‐[NH ‐C H COO],
FIGURE 5 DRUV–visible spectra of (a) LDH‐[NH ‐C H COO],
2
6
4
2
6
4
NAPABA‐Cr(III) and LDH‐[NAPABA‐Cr(III)] are shown
(b) NAPABA‐Cr(III) and (c) LDH‐[NAPABA‐Cr(III)]

KIRAR AND KHARE
7 of 12
oxidation of ethylbenzene. It was also observed that no
products were formed in blank reactions, when the reac-
tion was carried out in the absence of either catalyst or
oxidant. Hence auto‐oxidation was ruled out. The hetero-
geneous LDH‐[NAPABA‐Cr(III)] catalyst and homoge-
neous NAPABA‐Cr(III) catalyst were found to be active
for the oxidation of ethylbenzene and gave acetophenone
and benzaldehyde as oxidation products. The catalytic
activity of catalyst was assessed on the basis of conversion
of substrate and selectivity of products. The percentage
conversion of substrate, percentage selectivity of products
and turnover number (TON) of the catalysts in the oxida-
[
34,45]
tion reaction are calculated as
Substrate conversion ð%Þ ¼ Substrate converted ðmolÞ=
substrate used ðmolÞ×100
FIGURE 6 EPR spectrum of LDH‐[NAPABA‐Cr(III)]
Product selectivity ð%Þ ¼ Product formed ðmolÞ=
substrate used ðmolÞ×100
Turnover number ¼ mmol of products=mmol of catalyst
In order to optimize ethylbenzene oxidation, various
parameters, namely effects of oxidant, solvent, concentra-
tion of TBHP, concentration of catalyst and reaction tem-
perature, have been studied in detail.
The effect of various oxidants, namely H O , O , 70%
2
2
2
TBHP and 72% TBHP in ethylbenzene, was investigated
for the oxidation of ethylbenzene. The reaction with
H O leads to no conversion. The other oxidants effec-
SCHEME 2 Imaginary structure of LDH‐[NAPABA‐Cr(III)]
catalyst
2
2
tively oxidize ethylbenzene to acetophenone, benzalde-
hyde and benzoic acid. The results are presented in
3.2 | Catalytic Activity
Table 3. The conversions of ethylbenzene with O , 70%
2
The oxidation of ethylbenzene was investigated using
LDH‐[NAPABA‐Cr(III)] under solvent‐free conditions
with TBHP as an oxidant. The products are mainly
acetophenone and benzaldehyde and no oxidation prod-
ucts of the aromatic ring are found. Scheme 3 presents
the catalytic oxidation of ethylbenzene.
TBHP and 72% TBHP in ethylbenzene were 23.1, 34.1
and 90.7%, respectively. The oxidant 72% TBHP in ethyl-
benzene gave maximum conversion of ethylbenzene.
Hence, it was considered as the best oxidant for our cata-
lytic system.
The effect of various solvents (acetonitrile, acetone,
methanol and benzene) on the oxidation of ethylbenzene
catalysed by LDH‐[NAPABA‐Cr(III)] was studied to
develop an efficient solvent system. It was observed that
the conversion of ethylbenzene decreased in the follow-
ing order: 52.9% with acetonitrile > 45.2% with benzene
The catalytic activities of LDH‐[NH ‐C H COO],
2
6
4
LDH‐[NAPABA‐Cr(III)] and NAPABA‐Cr(III) for the oxi-
dation of ethylbenzene were investigated under similar
reaction conditions along with blank reactions. The
results are presented in Table 3. It was observed that
the support, LDH‐[NH ‐C H COO], was inactive for the
2
6
4
>
20.7% with methanol > 16.3% with acetone. The results
are given in Table 3. The results indicated that the aprotic
solvents favoured the conversion of ethylbenzene. The
major product in the oxidation of ethylbenzene in all
solvents except methanol was acetophenone. The
acetophenone selectivity for the various solvents
decreased in the following order: benzene (99.70%) > ace-
tonitrile (74.55%) > acetone (52.40%) > methanol
SCHEME 3 Oxidation of ethylbenzene (EB, ethylbenzene; BZ,
benzaldehyde; AP, acetophenone; BA, benzoic acid)

8
of 12
KIRAR AND KHARE
a
TABLE 3 Oxidation of ethylbenzene: effect of oxidant, solvent, support, homogeneous and heterogeneous catalyst
b
Product selectivity (%)
Ethylbenzene
Catalyst
Oxidant/solvent
conversion (%)
BZ
AP
BA
TON
c
No catalyst
TBHP /Solvent‐free
—
—
—
—
—
—
—
—
—
—
—
563
—
c
LDH‐[NH
2
‐C
6
H
4
COO]
TBHP /Solvent‐free
—
—
—
LDH‐[NAPABA‐Cr(III)]
NAPABA‐Cr(III)
Ethylbenzene
—
—
—
c
TBHP /Solvent‐free
62.0
—
0.23
—
99.77
—
LDH‐[NAPABA‐Cr(III)]
LDH‐[NAPABA‐Cr(III)]
LDH‐[NAPABA‐Cr(III)]
LDH‐[NAPABA‐Cr(III)]
LDH‐[NAPABA‐Cr(III)]
LDH‐[NAPABA‐Cr(III)]
LDH‐[NAPABA‐Cr(III)]
LDH‐[NAPABA‐Cr(III)]
H
2
O
2
/Solvent‐free
70% TBHP/Solvent‐free
34.1
23.1
90.7
52.9
16.3
45.2
20.7
0.14
0.63
0.07
0.26
0.68
0.30
0.34
99.75
95.60
99.93
74.55
52.40
99.70
47.78
0.11
3.77
—
310
209
825
480
148
410
188
O
2
/Solvent‐free
c
TBHP /Solvent‐ free
c
TBHP /Acetonitrile
25.19
46.92
—
c
TBHP /Acetone
c
TBHP /Benzene
c
TBHP /Methanol
51.88
a
Reaction conditions: ethylbenzene (13 mmol); TBHP (39 mmol); catalyst (100 mg); 393 K; 7 h.
b
AP, acetophenone; BZ, benzaldehyde; BA, benzoic acid.
2% TBHP in ethylbenzene.
c
7
(
47.78%). The selectivities of benzaldehyde and benzoic
The results are presented in Table 4. It was observed that
the conversion of ethylbenzene gradually increases with
increasing ethylbenzene‐to‐TBHP molar ratio from 1:1
to 1:2, i.e. from 48.3 to 78.5%. On further increasing the
molar ratio to 1:3, the conversion increases to 90.7%.
The conversion of ethylbenzene was increased by 12.3%
when ethylbenzene‐to‐TBHP molar ratio changed from
1:2 to 1:3. The selectivity of benzaldehyde decreased from
4.34 to 0.07% with increasing molar ratio of ethylbenzene
to TBHP from 1:1 to 1:3 while the selectivity of
acetophenone increased from 95.67 to 99.93% with
increasing molar ratio of ethylbenzene to TBHP from
1:1 to 1:3. At all molar ratios, acetophenone was the
major product. The TON of the catalyst also increased
from 438 to 825 with increasing molar ratio of ethylben-
zene to TBHP from 1:1 to 1:3 because at high TBHP
acid were favoured in protic solvents while the selectivity
of acetophenone was favoured in aprotic solvents.
[
46]
These results indicate that dipole moments of the solvent
may play an essential role in the oxidation of ethylben-
zene. However, the reaction under solvent‐free conditions
gave higher selectivity and more catalytic activity than in
the presence of a solvent (protic and aprotic). This was
probably due to competition for the active sites of the
metal centre of the catalyst between substrate and solvent
molecules, resulting in reduction of effective collision
between two substrate molecules.
The effect of concentration of TBHP was studied by
considering three different molar ratios (1:1, 1:2 and
1
:3) of ethylbenzene to TBHP in the oxidation of ethyl-
benzene under solvent‐free condition at 393 K for 7 h.
TABLE 4 Ethylbenzene oxidation of over LDH‐[NAPABA‐Cr(III)] under various conditions
a
Selectivity of products (%)
Substrate:
oxidant ratio
Catalyst
amount (mg)
Temperature
(K)
Conversion
(%)
Entry
Time (h)
BZ
AP
TON
1
2
3
4
5
6
7
1:1
1:2
1:3
1:3
1:3
1:3
1:3
100
100
100
75
393
393
393
393
393
373
353
7
7
7
7
7
7
7
48.3
78.5
90.7
76.7
69.1
77.9
66.6
4.34
0.22
0.07
0.14
0.17
0.12
0.14
95.7
99.8
99.3
99.9
99.8
99.9
99.9
438
713
825
924
1256
708
605
50
100
100
a
BZ, benzaldehyde; AP, acetophenone.

KIRAR AND KHARE
9 of 12
concentration a greater amount of nascent oxygen is pro-
vided by the oxidant. Thus, a molar ratio of ethylbenzene
to TBHP of 1:3 was considered as optimum.
The effect of catalyst concentration on the oxidation of
ethylbenzene was studied by considering three different
concentration of catalyst (50, 75 and 100 mg) at 393 K for
catalyst and selective for oxidation of ethylbenzene to
acetophenone; therefore the subsequent research was
devoted to this catalyst. Thus the optimum conditions
for obtaining maximum conversion of ethylbenzene are
a molar ratio of ethylbenzene to TBHP of 1:3, 100 mg of
catalyst and temperature of 393 K.
7
h. The results are presented in Table 4. It was observed
Table 5 compares the results of the present work with
[
9–13]
that when the concentration of catalyst increases from 50
to 100 mg, the conversion of ethylbenzene increases from
6
from 0.17 to 0.07%, while the selectivity of acetophenone
increases from 99.83 to 99.93% with increasing catalyst
concentration from 50 to 100 mg. The conversion and
selectivity were low at lower catalyst concentration due
those for previously reported catalysts.
The catalyst
and method applied in the present case have advantages
in terms of heterogeneous nature and better conversions,
selectivity and reusability of the catalyst and separation of
acetophenone.
9.1 to 90.7%. The selectivity of benzaldehyde decreases
3
.3 | Recycling of Catalyst
[47]
to the insufficient active sites of the catalyst,
while the
conversion and selectivity increased when the catalyst con-
centration reached to 100 mg. This was probably due to the
active sites releasing a greater amount of nascent oxygen,
which ultimately led to a vigorous oxidation reaction.
Acetophenone was the major product at all catalyst con-
centrations. However, the TON of the catalyst decreased
from 1256 to 825 with increasing catalyst concentration
from 50 to 100 mg. Similar observations have been
The reusability of a heterogeneous catalyst is an important
factor from commercial and economic points of view. In
order to check the reusability of the catalyst, a recycling
experiment was performed under the optimized reaction
conditions. After completion of the reaction, the catalyst
was separated from the reaction mixture by filtration and
[4,48]
reported by various researchers.
lyst is considered as optimum.
Thus, 100 mg of cata-
The temperature plays an important role in catalytic
reactions; hence we studied the oxidation of ethylbenzene
at three different temperatures (353, 373 and 393 K). The
results are presented in Table 4. It was observed that
when temperature increases from 353 to 393 K conver-
sion of ethylbenzene increases from 66.5 to 90.7%. The
selectivity of benzaldehyde decreases from 0.14 to 0.07%,
while the selectivity of acetophenone increases from
9
3
9.86 to 99.93% with increasing temperature from 353 to
93 K. Acetophenone was the major product at all reac-
tion temperatures. The TON of the catalyst also increases
from 605 to 825 with increasing temperature from 353 to
3
93 K. Thus 393 K is considered optimum. Clearly, the
[
LDH‐NAPABA‐Cr(III)] catalyst was a highly active
FIGURE 7 Recycling of catalyst
TABLE 5 Comparisons of present work with previous studies for oxidation of ethylbenzene
a
Catalytic system
Cr)MCM‐48
T (°C)/solvent
120/chlorobenzene
80/acetonitrile
Oxidant
TBHP
Time (h)
Conversion (%)
76.30
Product
AP
Selectivity (%)
82.90
Ref.
(
12
24
[
[
[
[
[
9]
M–Cr–ZrPO
CrSBA‐15
Cr‐MPO1
TBHP
91.60
AP
87.0
10]
11]
12]
13]
120/chlorobenzene
100/acetonitrile
70/acetonitrile
TBHP
10
6
95.6
8.5
AP
AP
AP
AP
98.20
69.40
78.0
O
2
CuCr
2
O
4
H
2
O
2
6
68.5
90.74
Present work
120/solvent‐free
TBHP
7
99.93
a
AP, acetophenone.

10 of 12
KIRAR AND KHARE
washed several times with methanol, acetonitrile and ace-
tone. The washed catalyst was dried at 333 K overnight to
remove the adsorbed solvent molecules from the catalyst
surface and then it was further subjected to consecutive
catalytic runs. The LDH‐[NAPABA‐Cr(III)] catalyst was
recycled ten times for the oxidation of ethylbenzene. The
conversion of ethylbenzene decreased from 90.7 to 87.1%
after the tenth cycle (Figure 7).
The conversion of ethylbenzene was reduced by 3.6%
from first to tenth cycle. The recycling results indicate
that the catalyst was stable during the catalytic reaction.
There was no significant loss of catalytic activity, showing
that the LDH‐[NAPABA‐Cr(III)] catalyst could be
recycled ten times in the liquid‐phase oxidation of ethyl-
benzene. The recycled catalyst was further characterized
using ICP‐AES, BET surface area, XRD and EDX. It was
observed that no significant changes occur in metal con-
tent, composition and surface area of regenerated catalyst
after the tenth cycle, which also confirm the stability of
the catalyst. Only a small loss in metal content (0.05%)
was observed after the tenth cycle. However, metal
leaching is observed in several catalytic systems on the
[
10,11]
basis of conversion.
3
.4 | Probable Mechanism for
Ethylbenzene Oxidation
The mechanism for ethylbenzene oxidation was
investigated using a quenching experiment with radical
scavenger 2,6‐di‐tert‐butyl‐4‐methylphenol (BHT) as a
quenching reagent. The reaction was carried out under
optimized reaction conditions. After completion of 1 h of
reaction, BHT was added and the reaction was continued
for a further 7 h (Figure 8). GC analysis showed no further
increase in conversion of ethylbenzene after the addition of
BHT. This clearly indicates that oxidation of ethylbenzene
by TBHP appears to be a radical process because addition
of BHT, a radical scavenger, inhibited the formation of
any product in the oxidation reaction. Thus, oxidation of
ethylbenzene with TBHP catalysed by LDH‐[NAPABA‐
Cr(III)] probably proceeds via a free radical mechanism
(Scheme 4). In the first step, the metal centre of LDH‐
FIGURE 8 Results of quenching experiment
SCHEME 4 Probable free radical
mechanism for oxidation of ethylbenzene
with TBHP catalysed by LDH‐[NAPABA‐
Cr(III)]

KIRAR AND KHARE
11 of 12
[
NAPABA‐Cr(III)] catalyst acts as an initiator in the
ACKNOWLEDGEMENTS
•
homolytic cleavage of TBHP into free (t‐BuO ) alkoxy
and (t‐BuOO ) alkylperoxy radicals (initiation step). In
The authors are grateful to the University Grant Com-
mission, New Delhi, India, for financial support
•
the second step, alkylperoxy radicals attack ethylbenzene
to convert it into 1‐tert‐butylperoxyethylbenzene. In the
next step, 1‐tert‐butylperoxyethylbenzene leads to the
formation of acetophenone, benzaldehyde and benzoic
acid. Acetophenone is formed by dehydration while
benzaldehyde by loss of methanol from 1‐tert‐
butylperoxyethylbenzene. Benzaldehyde on further oxi-
dation gives benzoic acid.
(
UGC‐SAP: no. F.540/13/DRS‐I/2016 (SAP‐I) and
UGC‐RGNF: F1‐17.1/2016‐17/RGNF‐2015‐17‐SC‐MAD‐
9994/SAIII/Website). Further, the authors are grateful
1
to Head, School of Chemical Sciences, UGC‐DAE Consor-
tium of Scientific Research, Devi Ahilya University Indore
for providing SEM, XRD and EDX facilities, Central Salt
and Marines Chemical Research Institute (CSMCRI),
Bhavnagar, Gujarat for providing BET surface area and
GC–MS facilities and STIC Cochin for providing DRUV–
visible facility.
3.5 | Hot Filtration Experiment
ORCID
The heterogeneity of the catalyst was investigated for the
oxidation of ethylbenzene using a hot filtration experi-
ment. The catalytic reaction was carried out under opti-
mized reaction conditions, and during the reaction the
catalyst was filtered off after 1 h in the first cycle at
Savita Khare http://orcid.org/0000-0002-2236-2948
REFERENCES
3
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A heterogeneous catalyst, LDH‐[NAPABA‐Cr(III)], was
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