Solvent-free oxidation of ethylbenzene over LDH-hosted Co(II) Schiff base of 2-hydroxy-1-naphthaldehyde and 4-amino benzoic acid

Article abstract of DOI:10.1080/24701556.2019.1653320

Co(II) Schiff base complexes derived by condensation of 4-amino benzoic and 2-hydroxy-1-naphthaldehyde was immobilized into layered double hydroxide, {LDH-[NAPABA-Co(II)]} and characterized by various techniques namely ICP-AES, SEM, TEM, EDX, XRD, FTIR, BET surface area, EPR, TGA, and DRUV–vis. spectroscopy. The structures of the Co(II) Schiff base complex are optimized by DFT calculations. The catalytic activity of the heterogenized Co(II) Schiff base complex for the oxidation of ethylbenzene using tert-butylhydroperoxide is studied under a solvent-free condition. Ethylbenzene upon oxidation gave benzaldehyde, acetophenone, and benzoic acid with 67.4% conversion of ethylbenzene and 99.57% selectivity of acetophenone. The catalyst is recyclable up to six cycles without notable loss of their activity.

Full text of DOI:10.1080/24701556.2019.1653320

Inorganic and Nano-Metal Chemistry
ISSN: 2470-1556 (Print) 2470-1564 (Online) Journal homepage: https://www.tandfonline.com/loi/lsrt21
Solvent-free oxidation of ethylbenzene over
LDH-hosted Co(II) Schiff base of 2-hydroxy-1-
naphthaldehyde and 4-amino benzoic acid
Savita Khare, Jagat Singh Kirar & Swati Parashar
To cite this article: Savita Khare, Jagat Singh Kirar & Swati Parashar (2019): Solvent-
free oxidation of ethylbenzene over LDH-hosted Co(II) Schiff base of 2-hydroxy-1-
naphthaldehyde and 4-amino benzoic acid, Inorganic and Nano-Metal Chemistry, DOI:
10.1080/24701556.2019.1653320
To link to this article: https://doi.org/10.1080/24701556.2019.1653320
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INORGANIC AND NANO-METAL CHEMISTRY
https://doi.org/10.1080/24701556.2019.1653320
Solvent-free oxidation of ethylbenzene over LDH-hosted Co(II) Schiff base of
2-hydroxy-1-naphthaldehyde and 4-amino benzoic acid
Savita Khare , Jagat Singh Kirar, and Swati Parashar
School of Chemical Sciences, Devi Ahilya Vishwavidyalaya, Takshashila Campus, Indore, India
ABSTRACT
ARTICLE HISTORY
Received 25 September 2018
Accepted 4 August 2019
Co(II) Schiff base complexes derived by condensation of 4-amino benzoic and 2-hydroxy-1-naph-
thaldehyde was immobilized into layered double hydroxide, fLDH-[NAPABA-Co(II)]g and character-
ized by various techniques namely ICP-AES, SEM, TEM, EDX, XRD, FTIR, BET surface area, EPR, TGA,
and DRUV–vis. spectroscopy. The structures of the Co(II) Schiff base complex are optimized by
DFT calculations. The catalytic activity of the heterogenized Co(II) Schiff base complex for the oxi-
dation of ethylbenzene using tert-butylhydroperoxide is studied under a solvent-free condition.
Ethylbenzene upon oxidation gave benzaldehyde, acetophenone, and benzoic acid with 67.4%
conversion of ethylbenzene and 99.57% selectivity of acetophenone. The catalyst is recyclable up
to six cycles without notable loss of their activity.
KEYWORDS
Layered double hydroxide;
heterogeneous catalyst;
Co(II) Schiff base complex;
ethylbenzene oxidation;
tert-butylhydroperoxide
1. Introduction
immobilized onto SBA-15. The maximum 38% conversion
of ethylbenzene with 82.50% selectivity of acetophenone was
observed after 24 h.
The method of intercalation for the preparation of het-
erogeneous catalyst is very useful because it uses tempera-
ture and solvent stable entities such as layered compounds
as a support.
Layered double hydroxides (LDHs) also known as hydro-
talcite-like compounds are anionic clays of inorganic nature
having a layered structure,[14–16] therefore, extensively inves-
tigated for their ion exchanger properties,[17,18] intercalation
chemistry,[19–21] and as a precursor heterogeneous cata-
lyst.[22,23] LDH-supported catalysts are efficient in the devel-
opment of green and sustainable chemistry,[24] since the
LDHs have well-defined nanometer-scaled layers for inter-
calation of metal complexes.[25,26] The LDH consists of bru-
cite-like layers with positive charge and compensating
Conversion of hydrocarbons to their oxygenated derivatives
is one of the most important reactions in the context of
industrial biology. The conversion of ethylbenzene to aceto-
phenone through oxidation is a great industrially significant
process because acetophenone acts as an intermediate in the
production of many pharmaceuticals, fragrances, chewing
gum, resins, alcohols, esters, aldehydes, etc.[1,2] Earlier
reports demonstrate that acetophenone is conventionally
synthesized by oxidation of ethylbenzene using stoichiomet-
ric amounts of inorganic oxidants (permanganate or dichro-
mate).[3,4] The main drawback of this process is that it
generates a large amount of hazardous and corrosive
wastes.[5] The present industrial process of acetophenone
production utilizes cobalt acetate as a homogeneous catalyst
in acetic acid in the presence of oxygen. However, in this
process, homogeneous transition metal catalyst is used;
hence, the major disadvantage is the recovery of the catalyst
for its reuse, which affects the overall economics of the
process.[6] Overcoming from this problem, concepts of het-
erogenization of homogeneous metal complex have aroused
attention. Various methods have been developed to
synthesize heterogeneous catalysts for the oxidation of
ethylbenzene.[7–13]
anions in the interlayers, which have a general formula as
III
[MII
M
1-x
(OH)2]xþ.(An-x/n).mH2O, where M(II) and
x
M(III) are divalent and trivalent metal cations, respectively,
and An- is the interlayer anion.[27,28] LDHs have a layered
crystal structure with wide variations depending upon the
nature of cations and M(II)/M(III) molar ratios, as well as
on the type of anions. The x is the coefficient which is equal
to the molar ratio [MIII/(MIIþMIII)], and m is the water
molecules located in the interlayer along with the anions.
Earlier, we have reported the oxidation of various alkenes
and alkanes over recyclable using heterogeneous catalysts,
which have been synthesized by intercalation and flexible
ligand method.[29–33] In the present paper, we report the
Xie et al. have synthesized alumina supported Co–Zn–Al-
mixed metal oxides as heterogeneous catalysts for ethylben-
zene oxidation using tert-butylhydroperoxide. The system
gave 72.30% conversion of ethylbenzene with 69.6% selectiv-
ity of acetophenone after 12 h. Brutchey et al. have been
reported oxidation of ethylbenzene over recyclable heteroge-
neous catalysts (4,4’-di-tBu-bipy)Co[OSi(OtBu)3]2 complex synthesis of heterogeneous catalyst, LDH-NAPABA-Co(II),
CONTACT Savita Khare
kharesavita@rediffmail.com
School of Chemical Sciences, Devi Ahilya Vishwavidyalaya, Takshashila Campus, Khandwa Road, Indore,
MP 452001, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsrt.
ß 2019 Taylor & Francis Group, LLC

2
S. KHARE ET AL.
cobalt Schiff base complex intercalated into LDH, and its 2.2. Method of synthesis of catalyst
catalytic behavior for the oxidation of ethylbenzene under
milder conditions. Various parameters such as temperature,
molar ratio of ethylbenzene to TBHP, amount of catalyst,
reaction time, and various oxidizing agents are studied for
obtaining maximum conversion of ethylbenzene.
2.2.1. Synthesis of ligand f[NAPABA]g
For the preparation of NAPABA, an equimolar methanolic
solution of 2-hydroxy-1-naphthaldehyde (10 mmol) and 4-
amino benzoic acid (10 mmol) is refluxed for 3 h under
nitrogen atmosphere. The solid yellowish product is
obtained, which is filtered and washed with methanol, then
dried at 333 K for overnight. The elemental percentage of
CHN for C18H13NO3: Anal. Found: C, 74.17%; H, 4.43%; N,
4.75%. Calc. For C18H13NO3: C, 74.22%; H, 4.50%; N,
4.81%. FTIR (KBr pellet: cmꢁ1): 3381(br), 1721(w), 1607(m),
1547(m), 1435(w).
2. Experimental methods
2.1. Materials and methods
Zn(NO3)2.6H2O, Al(NO3)3.9H2O, CoCl2.6H2O, 4-amino
benzoic acid, 2-hydroxy-1-naphthaldehyde and NaOH were
all of analytical grade and purchased from E. Merck
(Kenilworth, NJ). The 70% of the aqueous solution of TBHP
was also purchased from E. Merck (Kenilworth, NJ). The
HPLC grade ethylbenzene is purchased from SDFCL and its
purity is checked by gas chromatography (GC). Anhydrous
TBHP in ethylbenzene (72% in ethylbenzene) is prepared by
extraction of commercial TBHP (55 ml, 70% in water) into
2.2.2. Synthesis of homogeneous complex
fNAPABA-Co(II)g
For synthesizing neat NAPABA-Co(II) complex, initially
methanolic solution of NaOH (20 mmol) is prepared and it
is added with 4-amino benzoic acid (10 mmol) in 2:1 molar
ratio. Now, to this resultant solution, 2-hydroxy-1-naphthal-
ethylbenzene (15 ml). To enhance phase separation, satu- dehyde (10 mmol) is dissolved in methanol that has been
added equimolar to 4-amino benzoic acid. The resultant
mixture turns yellow. After that, methanolic solution of
cobalt salt, i.e. CoCl2.6H2O (5 mmol), is mixed together with
the resultant solution and it is left for 4 h on a magnetic
stirrer at room temperature. The greenish brown precipitate
of NAPABA-Co(II) complex starts forming which is sepa-
rated by filtration followed by washing with petroleum ether.
Then the solid is dried overnight. The characterization has
been performed using FTIR spectroscopy and elemental ana-
lysis. The elemental percentage of CHN for C36H22N2O6Co:
Anal. Found: C, 67.23%; H, 3.27%; N, 4.35%. Anal. Calc. C,
67.83%; H, 3.45%; N, 4.40%; O, 15.07%; Co, 9.25%. FTIR
(KBr pellet: cmꢁ1): 3371(br), 1617 (w) 788(w), 512(w).
rated NaCl solution is used. In an organic layer, anhydrous
MgSO4 was added to remove the water content, then fil-
tered, and kept at 5 ꢀC. This solution has a 1:3 molar ratio
of ethylbenzene to TBHP.
The Fourier transform infrared (FTIR) spectra were
recorded on Perkin Elmer model 1750 in KBr (PerkinElmer,
Waltham, MA). Powder X-ray diffraction (XRD) patterns of
the samples were recorded on a Rigaku diffractometer in the
2h range of 2ꢀ–70ꢀ using CuKa radiation (k ¼ 1.5418 Å) at a
scanning speed of 2ꢀ per minute with a step size of 0.02ꢀ.
Transmission electron microscope (TEM) studies were per-
formed on a Tecnai G2 20 microscope. Scanning electron
microscopy (SEM) measurements were performed using a
JEOL JSM 6100 electron microscope (JEOL Ltd., Tokyo,
Japan), operating at 20 kV. The electronic spectrum of solid
is recorded on Varian, Cary 5000 Spectrophotometer. The
thermo-gravimetric analysis (TGA) was conducted on a
Perkin Elmer USAA diamond system (PerkinElmer,
Waltham, MA) in the range of 298–973 K, at a heating rate
of 10 ꢀC minꢁ1. Inductively coupled plasma atomic emission
spectroscopy, Thermo Electron IRIS INTREPID II XSP
DUO (ICP-AES), is used for the estimation of cobalt. The
N2 adsorption data, measured at 77 K by volumetric adsorp-
tion set-up (Micromeritics ASAP-2010, USA), were used to
determine the BET surface area. The electron paramagnetic
resonance (EPR) spectrum is recorded on a JES-FA200 ESR,
X band spectrometer operated at a microwave frequency of
9.65 GHz at room temperature. The g value is reported rela-
tive to a 2,2-diphenyl-1-picrylhydrazil (dpph) standard with
g ¼ 2.0036. The oxidation products were identified by
GC–MS (Perkin-Elmer Clasus 500 column; 30 m ꢂ 60 mm,
PerkinElmer, Waltham, MA). Analytical gas chromatography
is carried out on a Shimadzu Gas Chromatograph GC-14B
(Shimadzu, Kyoto, Japan) with a dual flame ionization
detector (FID) and attached Shimadzu printer having XE-60
ss column (Shimadzu, Kyoto, Japan).
2.2.3. Synthesis of support fLDH-[NH2-C6H4COO]g
The LDH-[NH2-C6H4COO] was prepared by a co-precipita-
tion method by literature method.[34] The solution having
Zn/Al molar ratio 3 has been prepared by the addition of
Zn(NO3)2.6H2O (14.82 g) and Al(NO3)3.9H2O (6.25 g) in
decarbonized water. It is added with a solution of 4-amino
benzoic acid (10.86 g) and sodium hydroxide (7.8 g) in dec-
arbonized water under continuous stirring. The obtained gel
like mixture has been digested for 48 h at 348 K. The
obtained gel has been cooled and filtered. Such obtained
solid product is washed with water and methanol and kept
for drying at 333 K for overnight.
2.2.4. Synthesis of ligand fLDH-[NAPABA]g
The methanolic solution of 2-hydroxy-1-naphthaldehyde
(2.77 mmol) was mixed with LDH-[NH2-C6H4COO] (1.0 g).
The yellow color of solution indicates the formation of
imine which is refluxed for 3 h under nitrogen atmosphere
with continuous stirring. The obtained solid product has
been filtered and washed with methanol and acetonitrile.
The last step is drying, performed at 333 K temperature
for overnight.

INORGANIC AND NANO-METAL CHEMISTRY
3
2.2.5. Synthesis of catalyst fLDH-[NAPABA-Co(II)]g
2.3.2. Separation of acetophenone
Heterogeneous catalyst, LDH-[NAPABA-Co(II)], was syn- Isolation of acetophenone from the reaction mixture of
thesized by the intercalation method. The ligand, LDH-
[NAPABA] (1 g) was taken in methanol (50 ml) and then a
methanolic solution of CoCl2.6H2O (0.27 g) is added with
continuous stirring. The mixture is refluxed under nitrogen
atmosphere for 4 h. The solid is filtered and washed with
methanol followed by soxhlet extraction using various
solvents to remove uncomplexed ligand and metal salt
present in the layers of LDH. The catalyst is dried at
333 K overnight.
ethylbenzene oxidation was carried out by the reported
method after appropriate modifications.[35] First of all, the
reaction mixture was concentrated under reduced pressure
and then dissolved it in 15 ml of ethanol. After dissolution,
Zn granules (1 mmol) were added, and the mixture was
stirred at room temperature for 30 min. After filtration, it
was distilled under reduced pressure for removing the solv-
ent. Then it was treated with saturated solution of NaHCO3
for removing benzoic acid. After separation of the aqueous
layer, the organic layer was washed several times with water,
then extraction with ether. The ethereal layer is dried over
anhydrous MgSO4. Ether was removed by evaporation at
room temperature. The obtained yellowish oil contains 66%
acetophenone with 91% purity which was characterized by
FTIR and LC-MS analysis m/z ¼ 21, 57, 77, 105, 121. FTIR
(KBr pellet: cmꢁ1): 2978 (w), 1690 (m), 1263 (w).
2.3. Catalytic activity
2.3.1. Oxidation of ethylbenzene
In a typical experiment, round bottom flask (three necked)
was loaded with ethylbenzene, TBHP, and catalyst at par-
ticular temperature for 7 h in an oil bath on a magnetic hot
plate under continuous stirring. After 7 h, the solid catalyst
was filtered out from the reaction media and washed with
methanol and acetone many times. The reuse of the catalyst
was possible after drying at 333 K for overnight.
Identification of the oxidation products was done by GC-
MS qualitatively. Quantitative analysis of oxidation products
was carried out by GC using XE-60 ss column at 363 K. The
benzaldehyde, acetophenone, and benzoic acid were identi-
fied as main products as suggested by GC-MS. The conver-
sion and selectivity of the products were estimated using the
internal standard (dodecane) method.
3. Results and discussion
3.1. Characterization of catalyst
3.1.1. Estimation of metal contents
Scheme 1 shows the route of synthesis of heterogeneous
catalyst, LDH-[NAPABA-Co(II)]. In Table 1, physical and
b
analytical data of LDH-[NH2-C6H4COO], LDH-[NAPABA-
Co(II)] and aLDH-[NAPABA-Co(II)] (b ¼ before and
a ¼ after catalytic reaction) are included. It was observed
Scheme 1. Synthesis of heterogeneous catalyst, LDH-[NAPABA-Co(II)].
Table 1. Chemical composition and textural properties of support and heterogeneous catalyst.
Metal
BET
Particle
size
(nm)
contentsa
(%)
surface area
d-spacing
(Å)
EDX data (wt%)
Catalyst
(m2gꢁ1
)
C/Co
N/Co
Zn
Al
C
O
N
Co
LDH-[NH2-C6H4COO]
LDH-[NAPABA-Co(II)]b
LDH-[NAPABA-Co(II)]d
–
5.12
5.05
26.39
19.18
19.03
7.73
7.17
7.11
32.49
48.21
48.15
29.12
15.75
15.53
4.27
3.87
3.65
–
5.43
5.07
6.92
9.91
9.58
15.71
22.19
22.19
2.4
2.6
2.6
–
–
46.20 (35.96)c
46.78
3.18 (2.78)c
3.04
aICP-AES.
bBefore catalysis.
cThe theoretical.
dAfter catalysis.

4
S. KHARE ET AL.
that before catalytic reaction, 3.12% cobalt was present in
the fresh catalyst, LDH-[NAPABA-Co(II)], while after six
catalytic cycles, it was reduced to 3.05%, which was deter-
mined by ICP-AES.
The elemental composition of the catalyst was deter-
mined by EDX analysis. The results are incorporated in
Table 1. The presence of various elements namely C, N, O,
Zn, Al, and Co in LDH-[NAPABA-Co(II)] confirms the for-
mation of heterogeneous catalyst. The ratio of C/Co and N/
Co was calculated on the basis of elemental analysis. The
experimental results indicate that C/Co and N/Co ratio in
LDH-[NAPABA-Co(II)] was lesser than the theoretical value
which concludes that some metal ions were not coordinated
with the ligand in LDH-[NAPABA-Co(II)]. The ideal for-
mula for LDH-[NAPABA-Co(II)], according to elemental
content and Zn/Al ratio, was [Zn0.72Al0.28(OH)2].[NAPABA-
Co(II)]0.14[NH2-C6H4COO]0.06.1.44H2O.
3.1.2. SEM and EDX analysis
The morphology of the support, LDH-[NH2-C6H4COO] and
catalyst, LDH-[NAPABA-Co(II)] was investigated by SEM
analysis. The SEM images (Figure 1) of LDH-[NH2-
C6H4COO] and LDH-NAPABA-Co(II) both have similar
flake-like aggregates with unusual flak sizes[33] which shows
that the morphology of the support, LDH-[NH2-C6H4COO],
was not changed by the intercalation of the cobalt complex.
3.1.3. XRD analysis
The XRD patterns of LDH-[NH2-C6H4COO], bLDH-
a
[NAPABA-Co(II)] and LDH-[NAPABA-Co(II)] are shown
in Figure 2. The d-spacing corresponding to the plane (003)
is incorporated in Table 1. In XRD pattern of LDH-[NH2-
C6H4COO], the most intense basal reflection appears at
ꢀ
15.71 Å which corresponds to the (003) plane along with its
characteristic reflections corresponding to (110) plane that
was also present which indicates atomic distribution density.
This reflection depends on the molar ratio of Zn/Al in
b
LDH. In case of LDH-[NAPABA-Co(II)], the basal spacing
ꢀ
of (003) plane of appears at 22.19 Å on intercalation cobalt
Schiff base complex, whereas no change was observed in
characteristic reflections corresponding to the (110) plane.
This confirms that the basic structure of LDH[36] was not
changed after intercalation of the cobalt Schiff base complex.
For the calculation of gallery height of the LDH-[NH2-
C6H4COO] and heterogeneous catalyst, bLDH-[NAPABA-
Co(II)] thickness of the brucite layers (4.7 Å) was subtracted
from the basal spacing 15.71 and 22.19, respectively. Thus,
the gallery height of the LDH-[NH2-C6H4COO] and hetero-
geneous catalyst were found to be 11.01 and 17.49 Å,
respectively. The increment in the gallery of height of the
heterogeneous catalyst from the support clearly indicates
that intercalation of Co(II) Schiff base complex in the LDH
host layers was successful. Furthermore, Debye–Scherrer
equation was used for calculating average particle size for
Figure 1. SEM image of (A) LDH-[NH2-C6H4COO] and (B) LDH-[NAPABA-Co(II)].
Figure 2. XRD pattern of (A) LDH-[NH2-C6H4COO], (B) LDH-[NAPABA-Co(II]b and (C) LDH-[NAPABA-Co(II]a fa ¼ after and b ¼ before catalytic reactionsg.

INORGANIC AND NANO-METAL CHEMISTRY
5
Figure 5. TGA curve of (A) LDH-[NH2-C6H4COO] and (B) LDH-[NAPABA-Co(II)].
good agreement with the data calculated from the XRD pat-
tern of LDH-[NH2-C6H4COO] and LDH-[NAPABA-Co(II)].
3.1.5. BET surface area analysis
The surface area measurement is an important property, in
context of heterogeneous catalysis. The surface area, pore
size, and pore volume of support of LDH-[NH2-C6H4COO]
and heterogeneous catalyst LDH-[NAPABA-Co(II)] are
determined by the BET method of analysis. The results are
shown in Table 1. The support LDH-[NH2-C6H4COO] and
heterogeneous catalyst LDH-[NAPABA-Co(II)] have
a
surface area of 6.92 and 9.91 m2 ꢁ1, respectively. The
g
LDH-[NAPABA-Co(II)] shows a higher surface area than
LDH-[NH2-C6H4COO], which is due to intercalation of
cobalt Schiff base complex into the layers which causes the
expansion of LDH layers. These results have been supported
by the XRD pattern.[26]
Figure 3. TEM image of (A) LDH-[NH2-C6H4COO] and (B) LDH-[NAPABA-Co(II)].
3.1.6. FT-IR analysis
Figure 4 represents the typical FTIR spectra of support,
LDH-[NH2-C6H4COO], homogeneous catalyst, NAPABA-
Co(II), and heterogeneous catalyst, LDH-[NAPABA-Co(II)].
The FTIR spectrum of LDH-[NH2-C6H4COO] in
Figure 4(A) shows the broad band’s at 3381 and 3265 cmꢁ1
,
which are assigned for stretching modes of interlayer water
molecules and amino groups, respectively. A band at
1549 cmꢁ1 is assigned due to stretching vibration of
the > C ¼ O bond while the bands at 1438 and 1399 cmꢁ1
are assigned due -C ¼ C- stretching of aromatic ring and
Figure 4. FTIR spectra of (A) LDH-[NH2-C6H4COO], (B) NAPABA-Co(II) and (C)
LDH-[NAPABA-Co(II)].
NO3 ions, respectively.[37] The bands due translation
–
stretching vibrations of M–O and O–M–O (M ¼ Zn, Al)[38]
appear in the range of 428–784 cmꢁ1. These are representa-
tive bands for this class of compounds.[39,40] Figure 4(B)
shows the spectrum of a homogeneous catalyst, NAPABA-
Co(II) which shows a broad band at 3371 cmꢁ1 due to
stretching modes of the amino group. The band due C ¼ N
stretching of imines appears at 1614 cmꢁ1 whereas the bands
due to Co-N and Co-O frequency appear at 788 and
512 cmꢁ1, respectively. Figure 4(C) shows the FTIR spec-
trum of a heterogeneous catalyst, LDH-[NAPABA-Co(II)].
most intense peak corresponding to the (003) plane of
LDH-[NH2-C6H4COO] and LDH-[NAPABA-Co(II)], which
was found to be 2.4 and 2.6 Å, respectively (Table 1).
3.1.4. TEM analysis
Figure 3 shows the TEM images of LDH-[NH2-C6H4COO]
and LDH-[NAPABA-Co(II)]. It is found that both have
almost a similar type of aggregated hexagonal flake-like
structure. In addition, the particle size which is calculated
using TEM analysis varied from 2 to 20 nm, which are in This spectrum consists of all absorption bands comparable

6
S. KHARE ET AL.
to LDH-[NH2-C6H4COO] as well as the band due to C ¼ N 3.1.8. EPR analysis
stretching of imine, which is shifted to the lower frequen-
The spectrum of heterogeneous catalyst, LDH-[NAPABA-
cies. Whereas the bands due to the Co-N and Co-O are Co(II)], shown in Figure 6 shows recorded at room tem-
perature. The coordination and symmetry of cobalt(II) in
shifted to higher frequencies 789 cmꢁ1 and 518 cmꢁ1
.
LDH-[NAPABA-Co(II)] were investigated to distinguish the
valence and the spin state of cobalt ion (d7, S ¼ 1/2 and 3/
2). The cobalt ion may exhibit two variable oxidation states
3.1.7. TGA analysis
Coþ2 and Coþ3, where Coþ3 have t6 configuration and
The TGA has been studied to find out thermal stability of
LDH-[NH2-C6H4COO] and LDH-[NAPABA-Co(II)]. In
Figure 5(A), TGA curves of LDH-[NH2-C6H4COO] show
the first weight loss in between 30 and 170 ꢀC, it is due to
the removal of adsorbed water. In the next stage, the first
2g
diamagnetic in nature, therefore, does not give EPR signals,
whereas Coþ2 is paramagnetic and exists in both low-spin
(S ¼ 1/2) and high-spin (S ¼ 3/2) states, hence, studied by
EPR spectroscopy. The LDH-[NAPABA-Co(II)] complex
weight loss is observed in between 200 and 500 ꢀC, which is exhibits g and g values as 2.16 and 2.05, respectively.
jj
?
These values suggest a low-spin square planar geometry for
the Co(II) complex. Similar results have been reported by
Suja et al.[41]
due the degradation of the brucite-like layer along with the
removal of benzoate anions from the interlayer. Similarly, in
Figure 5(B), in the TGA curve of LDH-[NAPABA-Co(II)],
the first weight loss is observed in between 30 and 175 ꢀC
whereas the second step is observed in the range of
213–330 ꢀC. In the third stage, the weight loss is observed in
between 359 and 550 ꢀC which corresponds to complete
degradation of the organic part.
3.1.9. DRUV–vis analysis
Figure 7 represents the DRUV–vis spectra of LDH-[NH2-
C6H4COO], NAPABA-Co(II), and LDH-[NAPABA-Co(II)].
In the electronic spectrum of NAPABA- Co(II), three bands
appeared at 264, 323, and 425 nm. The first band appeared
ꢃ
at 264 nm was assigned to p ! p of aromatic rings. The
ꢃ
second band is due to n ! p transitions of the imines
group (C ¼ N) which appeared at 323 nm. The third band is
assigned due to the ligand to metal charge transfer (LMCT)
transition in the metal complex, which appears in the range
of 415–490 nm. The band due to d–d transition appears as
weak and broad in between 550 and 725 nm in metal com-
plexes. In the case of heterogeneous catalyst, LDH-
[NAPABA-Co(II)], the intensity of all bands which appear
in the homogeneous catalyst was reduced and but appears at
almost in a similar position. The d–d transition band also
appears at a very low intensity.
3.1.10. Computational details
Geometry optimization of ligand and Co(II) complex was
calculating using the Gaussian 09 W program,[42] with
B3LYP functional[43,44] and LanL2DZ basis set for C, H, O,
and Co metal.[45–48] Figure 8 represents fully optimized geo-
metries of the ligand and Co(II) complex and their ball and
stick model while Scheme 2 represents the numbering
scheme for ligand (A), and Co(II) complex (B). The results
of calculation of bond lengths and bond angles are given in
Table 2. On the basis of calculating bond lengths and bond
angles, square planar geometry around the Co(II) center was
suggested. The C1–O13 and C24–O23 bond lengths become
shorter in complex as compared to the ligand, whereas
C14–N12 and C36–N35 bond lengths were elongated in
complex as compared to the ligand on coordination through
azomethine nitrogen, phenolic hydroxyl moiety. As a result
of elongation in bond lengths, a change in the bond angle
around the coordination sphere was observed. In the com-
plex, cobalt atom is coordinated with two azomethine nitro-
gen and two oxygen atoms of phenolic hydroxyl moiety in
the Schiff base ligand. The angles among O13–Co69–N12
Figure 6. ESR spectra of LDH-[NAPABA-Co(II)].
Figure 7. DRUV–vis spectra of (A) LDH-[NH2-C6H4COO], (B) NAPABA-Co(II), and
(C) LDH-[NAPABA-Co(II)].
(90.94ꢀ),
O13–Co69–O23
(146.91ꢀ),
O13–Co69–N35

INORGANIC AND NANO-METAL CHEMISTRY
7
Figure 8. Optimized structure of (A) NAPABA ligand and (B) NAPABA-Co(II) complex.
Scheme 2. Oxidation of ethylbenzene.
(100.25ꢀ),
N12–Co69–O23
(92.87ꢀ),
N12–Co69–N35 hydroxy-1-nephtheldehyde ring toward the oxygen atoms
(153.11ꢀ), and O23–Co69–N35 (90.91ꢀ) also support the low- O13 and O23, respectively. The four independent Co–N and
spin square planar geometry. The values of bond lengths of Co–O distances are in the range of 1.93–2.0 Å which suggest
C1–O13 and C24–O23 are in between double (1.33 Å) and that the geometry of Co(II) complex is closer to the square
single (1.43 Å) bonds in the coordinated ligand, which was planar.[49] Furthermore, EPR analysis also supports the
due a significant delocalization of the p electrons of the 2- square planar geometry.

8
S. KHARE ET AL.
3.2. Catalytic activity studies
geneous catalyst.[51] In catalytic oxidation of ethylbenzene,
various aspects and investigation under similar experimental
conditions were considered. The results are given in Table 3.
Under certain conditions, oxidation did not take place: first,
when support LDH-[NH2-C6H4COO] and TBHP used;
second, when heterogeneous catalyst LDH-[NAPABA-
Co(II)] in the absence of without TBHP was used; and third,
when only TBHP without a catalyst was used. It means cata-
lyst and oxidant are must for the oxidation of ethylbenzene
in our experimental conditions. When oxidation of ethyl-
benzene carried out in the presence of either LDH-
[NAPABA-Co(II)] or NAPABA-Co(II) and TBHP, it pro-
ceeds and gave acetophenone, benzaldehyde, and benzoic
acid (Scheme 3). The catalytic activity of catalysts is meas-
ured based on substrate conversion and products selectivity.
It has been calculated using earlier reported formula.[52,53] It
is observed that homogeneous catalyst, NAPABA-Co(II),
gave 64.4% conversion of ethylbenzene, while heterogeneous
catalyst, LDH-[NAPABA-Co(II)], gave 67.4% conversion of
ethylbenzene. The homogeneous catalyst, NAPABA-Co(II),
shows low catalytic activity due to partial solubility in reac-
tion mixture, which consists of ethylbenzene and TBHP, as
we carried out reaction under a solvent-free condition,
ethylbenzene itself act as a solvent. Therefore, it shows lesser
activity as fewer sites are available in solution. The major
product in both cases is acetophenone. For obtaining max-
imum conversion of ethylbenzene and selectivity of aceto-
phenone in the oxidation reaction, various parameters
namely effects of oxidants, solvents, concentration of TBHP,
catalyst concentration, and reaction temperature have been
studied in detail using LDH-[NAPABA-Co(II)] as a catalyst.
3.2.1. Ethylbenzene oxidation
To study the catalytic activity of LDH-[NAPABA-M] cata-
lysts, oxidation of ethylbenzene using tert-butylhydroperox-
ide as an oxidant is investigated. The LDH was used as a
support for the intercalation NAPABA-Co(II) because it has
high adsorption capacity which makes them effective sup-
ports for the preparation of heterogeneous catalyst.[50]
Another important property of LDH is that it has uniform
dispersion of M(II) and M(III) cations in the layers, and
well-defined orientation of anions in the interlayer, there-
fore, it can be used as precursors for the synthesis of hetero-
Table 2. Selected bond lengths (Å) and bond angles (degree) of NAPABA lig-
and and NAPABA-Co(II) complex.
NAPABA-Co(II) complex
Parameters
NAPABA ligand
Bond length
(Å)
Bond length
(Å)
Bond angle
(degree)
C1-O13
C14-N12
C11-N12
C24-O23
C36-N35
C34-N35
O13-Co69
N12-Co69
O23-Co69
N35-Co69
ffO13CoN12
ffO13CoO23
ffO13CoN35
ffN12CoO23
ffN12CoN35
ffO23CoN35
1.43
1.34
1.34
1.43
1.34
1.34
–
–
–
–
–
1.33
1.43
1.33
1.32
1.44
1.33
1.94
1.99
1.93
2.01
–
–
–
–
–
–
–
–
–
–
–
90.94
146.91
100.25
92.87
153.11
90.91
–
–
–
–
–
–
–
–
–
–
Table 3. Effect of oxidants, solvents, support, homogeneous, and heterogeneous catalysts on the oxidation of ethylbenzene.
Products selectivity (%)
Catalysts
Oxidant/solvent
Ethylbenzene
Conversion
(%)
TON
BZ
AP
BA
ꢃ
ꢃ
ꢃ
No catalyst
LDH-[NH2-C6H4COO]
NAPABA-Co(II)
TBHP/solvent-less
TBHP/solvent-less
TBHP/solvent-less
H2O2/solvent-less
O2/solvent-less
–
–
64.4
–
21.2
38.5
67.4
37.7
15.9
32.4
7.9
–
–
0.23
–
0.76
0.44
0.17
6.74
17.57
0.48
0.18
–
–
–
–
0.62
–
2.67
1.68
0.26
26.15
53.66
0.40
46.55
–
–
585
–
192
350
612
342
144
294
72
99.15
–
LDH-[NAPABA-Co(II)]
LDH-[NAPABA-Co(II)]
LDH-[NAPABA-Co(II)]
LDH-[NAPABA-Co(II)]
LDH-[NAPABA-Co(II)]
LDH-[NAPABA-Co(II)]
LDH-[NAPABA-Co(II)]
LDH-[NAPABA-Co(II)]
96.57
97.88
99.57
67.51
28.77
99.12
53.27
70% TBHP/solvent-less
ꢃ
TBHP/solvent-less
TBHP/acetonitrile
ꢃ
ꢃ
TBHP/methanol
TBHP/benzene
ꢃ
ꢃ
TBHP/acetone
ꢃ
AP: acetophenone; BZ: benzaldehyde; BA: benzoic acid; 72% TBHP in ethylbenzene; reaction conditions: ethylbenzene (13 mmol); TBHP
(39 mmol), temperature (120 ꢀC) and time (7 h).
Scheme 3. Numbering scheme of atoms of (A) NAPABA ligand and (B) NAPABA-Co(II) complex.

INORGANIC AND NANO-METAL CHEMISTRY
9
3.2.2. Effect of oxidants
active sites. Thus, the reaction under solvent-free conditions
is the best one which gave higher conversion and selectivity.
Various oxidants such as H2O2, O2, 70% TBHP, and 72%
TBHP in ethylbenzene were used to study the effect of oxi-
dants on the oxidation of ethylbenzene in the presence of
LDH-[NAPABA-Co(II)] as a catalyst (Table 3). It is
observed that when an oxidant was H2O2, no oxidation
products were formed, due to its high exothermic nature. It
was decomposed rapidly with evolved oxygen.[54] With
molecular oxygen (O2), 21.2% conversion of ethylbenzene
was observed. The easy expulsion of O2 from the reaction
mixture leads to lesser conversion of ethylbenzene. When
70% TBHP was used, only 38.5% conversion of ethylbenzene
was observed. The reason behind this observation is the
interference of water which is present in 70% TBHP.
3.2.4. Effect of TBHP concentration
Oxidant plays an important role in any catalytic reaction.
Thus, the effect of ethylbenzene to TBHP molar ratio under
solvent-free condition was studied by considering three dif-
ferent molar ratios namely 1:1, 1:2, and 1:3 (Table 4). It is
found that when we increase ethylbenzene to TBHP molar
ratio from 1:1 to 1:2, the conversion of ethylbenzene
increases steadily from 21.2 to 52.0%. When the molar ratio
increased from 1:2 to 1:3, the conversion of ethylbenzene
also increases from 52.0 to 67.4%. Acetophenone was the
major product at all molar ratios. Thus, 1:3 molar ratio of
ethylbenzene to TBHP was considered as the optimum.
Furthermore,
a maximum conversion of ethylbenzene
(67.4%) was observed with 72% TBHP in ethylbenzene
which did not contain water. Thus, 72% TBHP in ethylben-
zene is the best oxidant in our experimental conditions.
3.2.5. Effect of catalyst amount
To assess the effect of catalyst concentration on the oxida-
tion of ethylbenzene under solvent-free condition, we have
considered three different amounts of catalysts such as 50,
75, and 100 mg (Table 4). Initially, by increasing the catalyst
amount from 50 to 100 mg, the conversion of ethylbenzene
gradually increases from 32.0 to 67.4%. The selectivity of
acetophenone increases with increasing catalyst concentra-
tion, while benzaldehyde selectivity decreases. This indicates
that higher catalyst concentrations have more active sites,
therefore, oxidation reaction proceeds vigorously. At all
catalyst concentration, acetophenone is the major product.
The turn over number (TON) is calculated based on the
mmol of ethylbenzene converted per mmol of cobalt.
However, with increasing catalyst concentration from 50 to
100 mg, the TON of the catalyst is decreased. Similar obser-
vations are reported by various researchers.[7] Therefore,
100 mg catalyst concentration is considered as optimum for
ethylbenzene oxidation.
3.2.3. Effect of solvents
The efficiency of several organic solvents like acetonitrile,
acetone, methanol, and benzene was studied to find out the
best solvent for the oxidation of ethylbenzene in the pres-
ence of LDH-[NAPABA-Co(II)] and 72% TBHP in ethyl-
benzene (Table 3). The conversion of ethylbenzene was
different with different solvents. The order of conversion
was decreased as solvent-less (67.4%) > 37.7% acetonitrile >
32.4% benzene > 15.9% methanol > 7.9% acetone. It was
observed that the aprotic solvents favored the conversion of
ethylbenzene. Whereas the order of selectivity of acetophe-
none was decreased in the following order: benzene
(99.12%) > acetonitrile (67.51%) > acetone (53.27%) >
methanol (28.77%). The results shows that aprotic solvents
favor the selectivity of acetophenone, while protic solvent
favors the selectivity of benzaldehyde and benzoic acid.[9]
While the acetophenone selectivity on different solvents
decreased in the following order: benzene (99.12%) > aceto-
nitrile (67.51%) > acetone (53.27%) > methanol (28.77%). 3.2.6. Effect of temperature
The selectivity of benzaldehyde and benzoic acid was
favored in protic solvents, while the selectivity of acetophe-
none was favored in aprotic solvents.[9] The maximum con-
The effect of temperature on the oxidation of ethylbenzene
was studied using three different temperatures namely 80,
100, and 120 ꢀC (Table 4). The conversion of ethylbenzene
version of ethylbenzene was obtained in solvent-less increases from 17.6 to 42.8% with increasing temperature
condition. This indicates that the solvent molecules lead to from 80 to 100 ꢀC. Further conversion increases to 67.4%
the blocking of active sites or may be due to diffusion com- with increasing temperature up to 120 ꢀC. The selectivity of
petition between solvent and substrate molecules for the acetophenone increases, whereas the selectivity of
Table 4. Effect of various parameters on the oxidation of ethylbenzene over LDH-[NAPABA-Co(II)].
Selectivity of products
ꢃ
Entry
EB:TBHP
ratio
Catalyst
Amount
(mg)
Temperature
(ꢀC)
Time (h)
Conversion
(%)
TON
BZ
AP
BA
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
50
100
100
120
120
120
120
120
80
7
7
7
7
7
7
7
21.2
52.0
67.4
48.4
32.0
42.8
17.6
0.47
0.40
0.17
0.58
2.25
0.88
1.11
97.79
98.25
99.57
99.13
84.50
96.55
95.42
1.75
1.35
0.26
0.29
13.25
2.57
3.47
192
472
612
583
582
389
159
100
ꢃ
AP: acetophenone, BZ: benzaldehyde, BA: benzoic acid; 72% TBHP in ethylbenzene.

10
S. KHARE ET AL.
benzaldehyde decreases with increasing temperature from 80 dried at 60 ꢀC overnight then used for successive cycles. It
to 120 ꢀC.[55] The conversion of ethylbenzene was higher at
is evident from Figure 9 that the conversion of ethylben-
zene gradually decreases from 67.4 to 60.6% after six
repeated cycles. The recycled catalyst is further character-
ized by BET surface area, ICP-AES and XRD (data are
given in Table 1 and Figure 2). The decrease in conversion
of ethylbenzene and loss of cobalt contents of catalyst are
due to the loss of catalyst during several washing after
each successive cycle. Therefore, it can be concluded that
the catalyst, LDH-[NAPABA-Co(II)], is suitable and stable
during recycling.
higher temperature because of an increase in effective colli-
sion of the substrate molecules. Hence, 120 ꢀC is considered
as the optimum temperature for the oxidation reaction.
Acetophenone was the major product at all reaction
temperatures.
Therefore, the optimum conditions at which the max-
imum conversion of ethylbenzene was achieved are 1:3
molar ratio of ethylbenzene to TBHP, 100 mg catalyst, and
120 ꢀC temperature.
3.4. Probable mechanism of the ethylbenzene oxidation
3.3. Recycling of catalyst
The quenching experiment is performed for establishing the
mechanism of oxidation of ethylbenzene, a radical scavenger
2,6-di-tert-butyl-4-methylphenol (BHT) is used as a quench-
ing reagent. Simultaneously, we have carried out two sets of
reactions. The first set of reactions is carried out without
BHT where conversion of ethylbenzene continuously
increases up to 5 h. The second set of the reaction is initially
carried out without BHT, but after 2 h of reaction, BHT is
added and the reaction is continued for the next 5 h. It is
observed that after adding BHT, there is no further increase
in the conversion of ethylbenzene, which is confirmed by
the gas chromatographic analysis. The radical scavenger
inhibited the further formation of free radicals in the oxida-
tion reaction. This indicates that oxidation of ethylbenzene
using tert-butylhydroperoxide that appears to be a rad-
ical process.
Recycling of catalyst is very important in catalysis; there-
fore, we have studied the reusability of LDH-[NAPABA-
Co(II)] for the oxidation of ethylbenzene in optimum reac-
tion conditions. This catalyst is reused after the separation
from the reaction mixture by filtration, washed several
times with methanol, acetonitrile followed by acetone, and
In the oxidation of ethylbenzene, the first step is the
coordination of t-BuOOH to the Co(II) metal center and
produces free alkoxy (t-BuOꢄ) and alkylperoxy radicals (t-
BuOOꢄ) by homolytic cleavage of t-BuOOH. In the second
step, alkylperoxy radicals react with ethylbenzene to form a
1-tert-butylperoxyethylbenzene. Subsequently, 1-tert-butyl-
peroxyethylbenzene leads to the formation of acetophenone,
benzaldehyde, and benzoic acid. The 1-tert-butylperoxyethyl-
benzene is converted into acetophenone by dehydration
while elimination of methanol from 1-tert-butylperoxyethyl-
benzene leads to the formation of benzaldehyde (Scheme 4).
Benzoic acid is formed by further oxidation of benzaldehyde
(Figure 10). In Scheme 4, lower energy pathway that is
dehydration reaction is favored over elimination reaction;
hence, the major product was acetophenone.
Figure 9. Recycling of catalyst.
3.5. Hot filter experiment
The hot filter experiment was performed under optimized
reaction condition to check the heterogeneity of the hetero-
geneous catalyst, LDH-[NAPABA-Co(II)] for the oxidation
of ethylbenzene. During the experiment, in the first cycle,
after completion 1 h of reaction, the catalyst is filtered out
for avoiding re-adsorption of leached cobalt onto the cata-
lyst surface. After 1 h, the catalyst is filtered out and the fil-
trate collected is again placed into the reaction flask and the
reaction is continued for the next 7 h. It was observed that
the reaction mixture did not exhibit any color, which
Figure 10. Quenching experiment.

INORGANIC AND NANO-METAL CHEMISTRY
11
Scheme 4. The probable free radical mechanism for oxidation of ethylbenzene with TBHP catalyzed by the LDH-[NAPABA-Co(II)] catalyst.
indicates the absence of cobalt (analyses atomic absorption experiments. It was found that ethylbenzene on oxidization
spectroscopy) and also the conversion of ethylbenzene does gave benzaldehyde, acetophenone, and benzoic acid. The
not increase (analyses by gas chromatography). The hot fil- acetophenone was the major product. The 67.4% conversion
ter experiment result clearly indicates that no cobalt leaching of ethylbenzene and 99.57% selectivity of acetophenone were
occurred during the catalytic reaction. Thus, it can be con- obtained under optimized reaction conditions. At the end of
cluded that the catalyst is heterogeneous in nature.
reaction, acetophenone was isolated from the reaction mix-
ture with 92% purity. The catalyst was recycled up to six
cycles without loss of catalytic activity. It was quite stable.
The applied method and catalyst have the advantages in
terms of heterogeneous nature, good conversion, selectivity
and recyclability and the separation of major oxida-
tion product.
4. Conclusions
The intercalation method has been used for the synthesis of
heterogeneous catalyst, [LDH-NAPABA-Co(II)]. It was char-
acterized by various techniques namely ICP-AES, SEM EDX,
FTIR, XRD, EPR, BET surface area, TGA, and DRUV–vis
techniques. Its catalytic activity was tested for selective oxi-
dation of ethylbenzene using TBHP under solvent-free con-
dition in the liquid phase. The heterogeneous catalyst,
[LDH-NAPABA-Co(II)], was more active in comparison of
Acknowledgements
Further, the authors are thankful to Head, School of Chemical
Sciences, UGC-DAE Consortium of Scientific Research, Devi Ahilya
its homogeneous, NAPABA-Co(II) counter parts in our University Indore, STIC Cochin and Central Salt and Marines

12
S. KHARE ET AL.
Chemical Research Institute (CSMCRI), Bhavnagar, Gujarat, for pro-
viding various analytical facilities.
and nickel(II) heterobimetallic complex [CuNi(l-OH)(l-OH 2
)(l-OAc)(Bpy) 2 ](ClO 4 ) 2 in aqueous medium in the absence
of a base and co-catalyst. J. Coord. Chem. 2017, 70, 2722–2735.
DOI: 10.1080/00958972.2017.1358812.
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Disclosure statement
There are no conflicts to declare.
Funding
15. Reichle, W. T. Synthesis of anionic clay minerals (mixed metal
hydroxides, hydrotalcite). Solid State Ionics 1986, 22, 135–141.
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The authors are thankful to University Grant Commission New Delhi,
India, for financial support (UGC-RGNF: F1-17.1/2016-17/RGNF-
2015-17-SC-MAD-19994/SAIII/Website)
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16. Carriazo, D.; del Arco, M.; Martın, C.; Rives, V. A comparative
study between chloride and calcined carbonate hydrotalcites as
adsorbents for Cr(VI). Appl. Clay Sci. 2007, 37, 231–239. DOI:
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ORCID
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clays: preparation, properties and applications. Catal. Today
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Savita Khare
http://orcid.org/0000-0002-2236-2948
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