Liquid Phase Solvent-Less Cyclohexane Oxidation Catalyzed by Covalently Anchored Transition-Metal Schiff Base Complex on α-Titanium Phosphate

Article abstract of DOI:10.1007/s10562-015-1647-8

Covalently anchored transition-metal salicylaldimine complexes on α-titanium phosphate {α-TiP.M(salicylaldimine) where M = Co, Cr, Cu and Fe} were synthesized by in situ method and characterized by BET surface area, XRD, SEM, EDX, FTIR, TGA and ICP techni

Full text of DOI:10.1007/s10562-015-1647-8

Catal Lett
DOI 10.1007/s10562-015-1647-8
Liquid Phase Solvent-Less Cyclohexane Oxidation Catalyzed
by Covalently Anchored Transition-Metal Schiff Base Complex
on a-Titanium Phosphate
1
1
Savita Khare
•
Priti Shrivastava
Received: 28 August 2015 / Accepted: 29 October 2015
Ó Springer Science+Business Media New York 2015
Abstract Covalently anchored transition-metal salicy-
laldimine complexes on a-titanium phosphate {a-TiP.
M(salicylaldimine) where M = Co, Cr, Cu and Fe} were
synthesized by in situ method and characterized by BET
surface area, XRD, SEM, EDX, FTIR, TGA and ICP
techniques. Its catalytic activity was tested for the oxida-
tion of cyclohexane under solvent free condition using tert-
butyl hydroperoxide as an oxidant. The oxidation of
cyclohexane gave cyclohexanol, cyclohexanone and some
unidentified products. The activity of heterogeneous cata-
lysts in the oxidation reaction decreased in the order of
a-TiP.Co(salicylaldimine) [ a-TiP.Cr(salicylaldimine) [
a-TiP.Cu(salicylaldimine)[a-TiP.Fe(salicylaldimine). The
a-TiP.Co(salicylaldimine) gave maximum conversion
Keywords Liquid phase Á Cyclohexane oxidation Á
a-Titanium phosphate Á Salicylaldimine complex Á
tert-butylhydroperoxide Á Solvent free
1 Introduction
The liquid-phase oxidation of cyclohexane is an industrially
important process for making cyclohexanol, cyclohexanone.
The oxidation products of cyclohexane which is cyclohex-
anol and cyclohexanone are important intermediates in the
production of adipic acid and caprolactam. Caprolactam and
adipic acid is extensively used in the man-made fiber
industry along with hexamethylenediamine for manufactur-
ing nylon-6 and nylon-66. A mixture of cyclohexanone and
cyclohexanol is known as KA-oil. The industrial commer-
cial process involves oxidation of cyclohexane using
molecular oxygen at temperature 150 °C under pressure of
175 psi in the presence of homogeneous cobalt salts [1]. This
process gave only 4 % conversion of cyclohexane with
85 % selectivity of desired products, cyclohexanol and
cyclohexanone. The drawback of this process is that, 96 %
of cyclohexane must be separated from products and recy-
cled. Keeping this in mind, many heterogeneous catalysts
were developed using various supports such as zeolites,
alumina, silica, clay, polymers, carbon tubes and layered
compounds [2–14]. Salavati-niasari et al. [2–5], synthesized
binuclear transition metal complexes using zeolites Y as a
supported for oxidation of hydrocarbons. Singh et al. [6]
synthesized chromium-containing periodic mesoporous
organosilica and used it as catalyst in the aerial oxidation of
cyclohexane. Anisia et al. [7], have prepared a heteronuclear
bimetallic macrocyclic iron copper complex, FeCuL(NO3)-
1
4.85 % of cyclohexane with 92.13 % selectivity of KA-
oil (cyclohexanol ? cyclohexanone) in 6 h. The catalyst,
a-TiP.Co(salicylaldimine) was reused for five cycles
without significant loss of catalytic activity.
Graphical Abstract A heterogeneous catalytic system,
a-TiP.Co(salicylaldimine)/TBHP gave maximum 14.85 %
conversion and 92.13 % selectivity for KA-oil in oxidation
of cyclohexane. The catalyst can be reused for five cycles.
OH
O
α-TiP.M(salicylaldimine)
+
+
other products
TBHP
Cyclohexane
Cyclohexanol Cyclohexanone
&
Savita Khare
kharesavita@rediffmail.com
1
School of Chemical Sciences, Devi Ahilya University,
Takshashila Campus, Khandwa Road, Indore, MP 452001,
India
24H O supported on zirconium pillared montmorillonite.
2
The oxidation of cyclohexane in the presence of this catalyst
123

S. Khare, P. Shrivastava
with molecular oxygen as an oxidant at 190 °C, the cyclo-
hexane conversion was 14.2 % and cyclohexanone was
formed with 87.4 % selectivity. Gupta et al. [8–10], pre-
pared polymer supported schiff base complexes in oxidation
reaction. Karabach et al. [11], synthesized copper(II) coor-
dination polymers from triethanolamine and pyromellitic
acid for bioinspired mild peroxidative oxidation of cyclo-
hexane by hydrogen peroxide in acidic medium. An overall
yield (based on cyclohexane) of 29 % has been achieved.
Heterogeneous catalysts are stable, can be easily separated
from reaction zone, gave high catalytic activity and can be
reused for several runs without loss of their catalytic activ-
ity. The oxidation of cyclohexane was also studied using
various oxidants such as molecular oxygen, tert-butyl
hydroperoxide (TBHP) and hydrogen peroxide (H O )
covalently bonded transition metal salicylaldimine com-
plexes to a-titanium phosphate, abbreviated as {a-
TiP.M(salicylaldimine) where M = Co, Cr, Cu and Fe}
and their catalytic behavior for oxidation of cyclohexane
using tert-butyl hydroperoxide (TBHP) as an oxidant.
2 Experimental
2.1 Materials
Titan(IV)-isopropylate (97 %, Aldrich), phosphoric acid,
hydrofluoric acid, metals chloride, metals acetate, salicy-
laldehyde, 3-aminopropyltriehtoxysilane (APTES) and
cyclohexane were of reagent grade (E. Merck). Cyclo-
hexane was checked by gas chromatography (GC) to
ensure that the absence of any oxidation products in the
substrate. A stock solution of TBHP (70 % in cyclohexane)
was prepared by extraction of 50 ml of commercial TBHP
(70 % in water) into 15 ml of cyclohexane. Phase separa-
tion was promoted by saturation of the aqueous layer with
2
2
[15–23]. Transition metal plays an important role in oxida-
tion of cyclohexane; in biological system an enzyme cyto-
chrome P-450 catalyzed this reaction. To mimic this
reaction some researchers has been developed heteroge-
nization of various homogeneous transition metal complexes
having various ligands. Salvati-Nisari et al. [24, 25], have
been encapsulated transition metal (M = Mn(II), Co(II),
Ni(II) and Cu(II)) complexes with tetrahydro-salophen (sa-
lophen = N,N-bis-salycilidene)-1,2- phenylenedimine: H2
NaCl [38]. The organic layer was dried over MgSO , fil-
4
tered, and stored at 5 °C. The molar ratio of cyclohexane to
TBHP in this solution was 1:2.18. The reference sample of
catalytic products was prepared by the standard procedure
[39].
[
H salophen] = 2-({2-[(2hydroxybenzyl)amino]anilino}
4
methyl)phenol) and bis(salicyaldehyde) oxaloyldihydra-
zone on zeolite-Y. Gua et al. [26], synthesized iron, man-
ganese and cobalt tetraphenylporphyrins to catalyze
cyclohexane oxidation with air in the absence of additives
and solvents. Parton et al. [27] have been studied cyclo-
hexane oxidation with tert-butylhydroperoxide catalyzed
by iron phthalocyanine complex homogeneously supported
on Y zeolite. Fan et al. [28] and Tembe et al. [29] designed
schiff base metal complexes to make these systems more
practical using zeolites, Cruz et al. [30], synthesized
chromium containing silicates by a simple acid-catalyzed
sol–gel process. Carvalho et al. [31], synthesized meso-
porous Si-MCM-41, which can be easily organofunction-
alised by anchoring [H NC H -Si(OCH ) ] at the surface
2.2 Catalysts Preparation
2.2.1 Preparation of a-Titanium Phosphate
a-Titanium phosphate (abbreviated a-TiP) was prepared by
reported method [40]. A sample of 10 ml Titan(IV)—iso-
propylate (97 %), was mixed with 100 ml 3 M H PO in a
3
4
flask to which 57 ml of HF solution (5 M) was added to
-
?
reach a molar ratio of F /Ti = 6. The mixture was refluxed
for 24 h at 100 °C. After the reaction, the product was
washed with distilled water until the pH of the supernatant
liquid becomes 5, then it was dried at 110 °C for 24 h. The
final product was identified as a-TiP. Finally the structure
of a- TiP was confirmed by powder XRD.
2
3
6
3 3
of the Si-MCM-41 with iron and copper. The use of layered
compound as a support for the synthesis of heterogeneous
catalysts is effective because their structure is known and
they are temperature- and solvent-stable [14]. The a-tita-
nium phosphate is a layered compound with a flexible layer
structure and has been studied for its cation exchange,
proton transport, intercalation chemistry, and catalytic
properties [32–35]. In previous studies we have reported
the use of Fe(Salen) and Cu(Salen) intercalated a-zirco-
nium phosphate as an active, and reusable catalyst for the
oxidation of cyclohexene [36, 37]. Here with, we are
reporting in situ synthesis of heterogeneous catalysts,
2.2.2 Preparation of Ligand (Salicylaldimine)
The salicylaldimine ligand was synthesized by the con-
densation of 3-aminopropyltriethoxysilane (1 mol), and
salicyaldehyde (1 mol) in ethanol (40 ml), the solution
instantly became yellow due to imines formation [41], the
elemental percentage of CHN are following: Anal. Found:
C, 58.8 %; H, 8.1 %; N, 4.3 %. Calc. for C H NO Si: C,
1
6
27
4
59.1 %; H, 8.3 %; N, 4.2 %.
1
23

Liquid Phase Solvent-Less Cyclohexane Oxidation Catalyzed by Covalently Anchored…
2
.2.3 Preparation of Neat M(salicylaldimine) Complex
Where M = Co, Cr, Cu and Fe}
volume and pore size. The electronic spectra were recorded
on Shimadzu UV-1800 Pharma Spectrophotometer. Ana-
lytical gas chromatography was carried out on a Shimadzu
Gas Chromatograph GC-14B with dual flame ionization
detector (FID) and attached with Shimadzu printer having
SE-30 ss column at 393 K. The oxidation products were
identified by GC–MS (Hewlett Packard, GCD-1800A) with
an electron ionization detector using (Perkin-Elmer Clasus
500 column; 30 m 9 60 mm). Thermal analysis measure-
ments were performed using (Perkin-Elmer USA model
diamond TGA) at a heating rate of 10 °C/min.
{
The metal salicylaldimine complexes was synthesized by
adding 3-aminopropyltriethoxysilane (1 mol), and sali-
cyaldehyde (1 mol) in ethanol (40 ml), the solution
instantly became yellow due to imines formation then
added metals chloride (0.5 mol) to the solution and mixture
was stirred for 30 min at room temperature. The precipitate
of M(salicylaldimine) complexes was filtered off, washed
with petroleum ether and dried. The complexes formed
were characterized by elemental analysis and FTIR spec-
troscopy. In Co(salicylaldimine) complexes, the elemental
percentage of CHN is following: Anal. Found: C, 53.7 %;
H, 8.0 %; N, 3.8 %. Calc. for C H N O Si Co: C,
2.4 Catalytic Performance
3
2
54
2
8
2
The catalytic oxidation of cyclohexane was carried out
using {a-TiP.M(salicylaldimine)} catalysts in a three-
necked round bottom flask (100 ml) equipped with a
refluxed condenser. In a typical experiment, the flask was
loaded with catalyst (0.05 g) and 5 ml mixture of cyclo-
hexane (17.86 mmol%) and TBHP (38.89 mmol %). An
additional amount of cyclohexane (1.59 mmol%) was
added to maintain 1:2 molar ratio of cyclohexane to TBHP.
The mixture was stirred at the desired temperature for 6 h.
After completion of the reaction, the contents of the flask
was cooled in an ice-bath and the catalyst was filtered out.
The liquid layer was analyzed quantitatively by GC using
XE-60 ss column at 70 °C. The products were identified by
GC–MS. The GC–MS analysis revealed that the cyclo-
hexanol, cyclohexanone and some unidentified products
are formed in the reaction.
5
3.86 %; H, 8.14 %; N, 3.93 %.
2
.2.4 Preparation of a-Titanium Phosphate
M(salicylaldimine) Complex
The metal salicylaldimine complexes were synthesized by
insitu method, 3-aminoproplytriethoxysilane (1 mol) was
added to 40 ml ethanolic solution of salicyaldehyde
(
1 mol), the solution instantly became yellow due to imines
formation then ethanolic solution of metal chloride
0.5 mol) was added to the mixture and stirred for 30 min
(
at room temperature. After 30 min, a-TiP (1 g) was added
and mixture was further stirred overnight at room tem-
perature. The final product was washed with distilled water
and ethanol. Finally the product was soxhlet extracted
using methanol followed by dichloromethane and diethyl-
ether until the washing was colourless. The solid was dried
in an oven at 110 °C for 12 h. The catalysts were charac-
terized by FTIR, XRD, BET surface area analysis, SEM,
EDX, TGA and ICP-AES analysis.
3
Result and Discussion
3.1 Characterization of the Catalyst
3.1.1 Estimation of Metal Contents
2
.3 Catalysts Characterization
Covalently anchored catalysts, a-TiP.M(salicylaldimine)
were prepared by covalent bonding of M(salicylaldimine)
complex to a-titanium phosphate. Scheme 1 shows the
synthetic route for formation of a-TiP.M(salicylaldimine).
Physical, analytical and surface properties of all com-
pounds are given in Table 1. The colours of a-TiP,
Inductively coupled plasma atomic emission spectroscopy
ICP-AES) was used for estimation of metals. Energy
dispersive X-ray (EDX) and scanning electron micrographs
SEM) were performed using a JEOL JSM 6100 electron
microscope, operating at 20 kV. Powder X-ray diffraction
XRD) measurements were carried out on a Rigaku
(
(
b
a
(
a-TiP.Co(salicylaldimine) ,
a-TiP.Co(salicylaldimine)
diffractometer in the 2h range of 5°–40° using CuKa
radiation (k = 1.5418 A) at a scanning speed of 28/min
with step size 0.02°. Fourier transform infrared (FTIR)
spectra were recorded on Perkin Elmer model 1750 FTIR
(b = before and a = after five catalytic reaction) and
a-TiP.M(salicylaldimine) compounds were different due to
change in metal content, which was determined by ICP-
AES analysis. There were no metals in a-TiP and its colour
was white. The colours and metal contents in
a-TiP.M(salicylaldimine) depend on the presence of metals
loading.
˚
in KBr. The N adsorption data, measured at 77 K by
2
volumetric adsorption set-up (Micromeritics ASAP-2010,
USA), were used to determine BET surface area, pore
123

S. Khare, P. Shrivastava
Scheme 1 Synthetic route for formation of a-TiP.M(salicylaldimine)
Table 1 Chemical composition and physical data of a-TiP.M(salicylaldimine)
2
Colour Metal contents (%) BET surface area (m /g) Pore volume (cm g
3 -1
˚ ˚
Pore size (A) d-spacing (A)
Catalysts
)
a-TiP
White
–
5.46
5.38
4.41
5.32
5.34
5.25
0.04
0.03
0.02
0.02
0.03
0.02
23.51
22.00
18.05
20.49
22.89
21.32
7.56
7.56
7.54
7.56
7.56
7.56
b
a-TiP.Co(salicylaldimine) Green 2.93
a
a-TiP.Co(salicylaldimine) Brown 1.45
a-TiP.Cr(salicylaldimine)
Brown 2.52
a-TiP.Cu(salicylaldimine) Green 2.48
a-TiP.Fe(salicylaldimine)
Green 2.30
a
After catalysis
b
Before catalysis
3
.1.2 EDX and SEM Analysis
a-TiP.Co(salicylaldimine) (Fig. 1b) are less ordered than
that of a-TiP, contains both sheets and spheres of different
shapes and sizes which indicate the presence of homoge-
neous complex on the surface. The SEM images of a-
TiP.Co(salicylaldimine) (Fig. 1c) after five catalytic cycles
shows clear change on the surface structure.
The formation of a-TiP.M(salicylaldimine) was confirmed
by EDX analysis. The EDX measurements results of a-TiP
and a-TiP.M(salicylaldimine) are incorporated in Table 2.
The presence of metals ions, nitrogen and silicon in
a-TiP.M(salicylaldimine) confirms the presence of M(sal-
icylaldimine) complex on the a-TiP surface. The SEM
images, which allow us to monitor how the morphology of
the sample evolves during the above commented process,
3.1.3 XRD Studies
The powder XRD patterns of a-TiP, a-TiP.Co(salicy-
b
b,
a
are shown in Fig. 1 {a-TiP, a-TiP.Co(salicylaldimine)
and a-TiP.Co(salicylaldimine) }. The SEM image of a-TiP
laldimine) a-TiP.Co(salicylaldimine) and a-TiP.M(sali-
cylaldimine) catalysts are shown in Fig. 2 and their
d-spacing incorporated in Table 1. The XRD pattern of a-
TiP shows most intense peak corresponding to the (002)
a
(
Fig. 1a) revealed the presence of well-defined hexagonal
plates with smooth surface. The SEM images of
1
23

Liquid Phase Solvent-Less Cyclohexane Oxidation Catalyzed by Covalently Anchored…
Table 2 EDX measurements
for elemental composition
Catalyst*
Atomic % of elements
Ti
P
O
Si
N
Metals
a-TiP
11.14
9.83
8.42
9.46
9.76
9.57
14.48
12.78
10.95
12.30
12.70
12.45
74.38
65.71
65.26
66.68
65.28
66.42
–
–
–
b
a
a-TiP. Co(salicylaldimine)
a-TiP. Co(salicylaldimine)
a-TiP. Cr(salicylaldimine)
a-TiP. Cu(salicylaldimine)
a-TiP. Fe(salicylaldimine)
a
6.94
6.04
6.85
6.95
6.83
3.13
8.09
3.61
3.61
3.50
1.61
1.24
1.10
1.70
1.23
*
After catalytic reaction
b
Before catalytic reaction
˚
plane at 7.56 A (d value), this shows material is crystalline.
In all a-TiP.M(salicylaldimine) catalysts the d-spacing is
virtually constant which indicates that metal complexes are
not present in interlayer of a-TiP. Thus, XRD pattern con-
firms that the supports essentially retain their crystallinity
and structure after covalent bonded of the metal complex.
data are in good agreement with reported data [44]. In the
-
1
spectrum of a-TiP, the bands at 3400 and 1635 cm are
assigned due to the presence of external water in addition
to the strongly hydrogen-bonded OH or extremely strong
-
1
coordinated H O [45]. A band in the region *1038 cm
2
is attributed to the presence of P=O stretching. A medium
-
1
intensity band at 1400 cm is attributed to the presence of
d(POH). These bands indicate the presence of structural
hydroxyl groups/protonic sites in the material. The band at
3
.1.4 Surface Area Analysis
-
1
The results of BET surface area, pore volume and pore size
of all the compounds are incorporated in Table 1. The sur-
599 cm is due to stretching vibration of Ti–O. In the
spectrum of covalently bonded heterogeneous catalyst,
a-TiP.Co(salicylaldimine) Fig. 3c, the band positions of
2
face area of a-TiP is 5.46 m /g, whereas the surface area of
a-TiP.M(salicylaldimine) ranges from 5.25 to 5.38 m /g was
2
3-
PO4 was shifted to lower frequencies while that of Ti–O
lower than the supports, which indicates that covalent
bonding of M(salicylaldimine) complexes lead to marginal
decrease in surface area. Similar observation is also reported
for silica immobilized salicylaldimine Cu(II) and Co(II)
complexes [42]. The pore size and pore volume of the a-TiP
was shifted to higher frequencies due to presence of Co–O
interaction. The C=N stretching frequency of the imines
group metal complexes was much weaker and shifted to
-
1
higher frequency 1619–1622 cm
in a-TiP.M(salicy-
laldimine). The Si–O vibration of metal complexes was
3
3-
slightly merged with of PO4 stretching vibration of a-TiP
sample were 23.51 nm and 0.04 cm /g respectively which
was slightly decreases in the a-TiP.M(salicylaldimine). The
decrease in pore volume and pore diameter due to the
presence of metal ion near the opening of the layered
structure, block the access of nitrogen molecules to the
whole lamellar structure [43]. The surface area, pore volume
and pore size of the a-TiP is retained after covalent bonding
of the metal complexes on its surface which indicate the
formation of a-TiP.M(salicylaldimine).
in a-TiP.M(salicylaldimine). Figure 3 shows comparison
of FTIR spectra of free ligand, Co(salicylaldimine),
a-TiP.Co(salicylaldimine) and a-TiP which provides evi-
dence of coordination of the metal complexes on a-TiP.
3.1.6 Electronic Spectral Studies
Typical electronic spectra of ligand and Co(salicy-
laldimine) complex in methanol are shown in Fig. 4. The
probable electronic assignments of ligand and M(salicy-
laldimine) complexes are given in Table 4. The electronic
spectrum of the free ligand exhibit three bands at 216, 251
and 320 nm are assigned as due to u ? u, p ? p* and
n ? p* transitions, respectively. The band at 251 nm is
assigned due to p-p* transition of phenol ring, this energy
is raised by *19 nm in the complexes. The band at
320 nm is assignable to n ? p* transition of the (C=N)
moiety in the salicylaldimine ligand. This band is shifted to
higher energies 372–375 nm in metal-complexes due to
ligand to metal charge transfer (LMCT) transition, which
indicates the coordination of phenolate oxygen of ligand to
3
.1.5 FTIR Spectral Studies
The FTIR spectra of all compounds were recorded in the
-
1
000 to 400 cm range. FTIR spectral frequencies of free
4
ligand, neat M(salicylaldimine) complexes, a-TiP.M(sali-
cylaldimine) and a-TiP are given in Table 3. The spectrum
-
1
of the free ligand shows bands at 1627 and 1280 cm due
to C=N and C–O stretching frequency respectively. These
bands are shifted to lower frequency upon coordination in
metal complexes. The Si–O vibration of ligand was
-
1
observed in the range 1192 cm was also shifted to lower
frequency in metal complexes due to coordination. These
123

S. Khare, P. Shrivastava
Fig. 2 XRD pattern of (A) a-TiP, (B) a-TiP.Co(salicylaldimine),
(
(
C) recycled a-TiP.Co(salicylaldimine), (D) a-TiP.Cr(salicylaldimine),
E) a-TiP.Cu(salicylaldimine) and (F) a-TiP.Fe(salicylaldimine)
and a-TiP.Co(salicylaldimine) are shown in Fig. 5. The
TGA-curve of the a-TiP (Fig. 5a) showed first weight loss
between 120-248 °C which is attributed to loss of mois-
ture/hydrated water. Second weight loss was observed
between 255 and 755 °C which is attributed to the con-
densation of structural-OH groups. The TGA-curve of the
a-TiP.Co(salicylaldimine) (Fig. 5b) complex shows weight
loss in the temperature range of 120–135 °C which is
attributed to loss of moisture/hydrated water. Second
weight loss was observed between 187 and 558 °C which is
due to loss of water molecule followed by decomposition
of the imine group in the complex.
3.2 Catalytic Experiments
Catalytic oxidation of cyclohexane was investigated using
various a-TiP.M(salicylaldimine) catalysts with TBHP
under solvent free condition. Under similar experimental
conditions support, a-TiP, homogeneous catalyst, M(sali-
cylaldimine) and reaction without catalyst (blank reaction)
were also investigated. The results are incorporated in
Table 5. It was observed that either no catalyst (as in case
of blank reaction) or support was used as catalyst, the
oxidation reaction did not proceed. This shows support is
catalytically inactive. The homogeneous M(salicy-
laldimine) catalyst as well as heterogeneous catalysts,
a-TiP.M(salicylaldimine), oxidized cyclohexane to cyclo-
hexanol, cyclohexanone and some other products. The
major products are cyclohexanol and cyclohexanone in all
cases. Further, it was observed that the homogeneous
complexes, M(salicylaldimine), are partially soluble in
cyclohexane, therefore the homogeneous complexes does
not show high catalytic activity. Whereas heterogeneous
catalysts, a-TiP.M(salicylaldimine) showed high catalytic
activity. The catalytic oxidation of cyclohexane catalyzed
Fig. 1 SEM image of (a) a-TiP, (b) a-TiP.Co(salicylaldimine), and
c) recycled a-TiP.Co(salicylaldimine)
(
an empty d orbital of metal ions. An additional weak and
broad band at 570-572 nm in metal complexes is due to the
d–d transition as reported in the literature [46].
3
.1.7 TGA Analysis
Thermal behavior of a-TiP and a-TiP.Co(salicylaldimine)
were studied by TGA analysis. The TGA-curve of a-TiP
1
23

Liquid Phase Solvent-Less Cyclohexane Oxidation Catalyzed by Covalently Anchored…
Table 3 FTIR frequency
-1
IR group frequency (cm )
Catalysts
4-
PO
Ti–O
C=N
Si–O
C–O
M–O (M = metal)
a-TiP
1038
–
599
–
–
1627
1618
1619
1619
1619
1622
1620
1621
1614
1618
–
1192
1042
1040
1039
1143
1132
1144
1040
1142
1131
–
1280
1277
1297
1280
1251
1279
1196
1234
1248
1317
–
–
Ligand (salicylaldimine)
Co(salicylaldimine)
a-TiP.Co(salicylaldimine)
a-TiP.Co(salicylaldimine)
Cr(salicylaldimine)
–
–
505
512
510
501
517
502
513
503
522
b
a
1037
1034
–
611
603
–
a-TiP.Cr(salicylaldimine)
Cu(salicylaldimine)
a-TiP.Cu(salicylaldimine)
Fe(salicylaldimine)
1031
–
621
–
1032
–
614
–
a-TiP.Fe(salicylaldimine)
1035
614
a
After catalysis
b
Before catalysis
Fig. 3 FTIR spectrum of (A) Free ligand, (B) Co(salicylaldimine),
C) a-TiP.Co(salicylaldimine), and (D) a-TiP
(
-5
Fig. 4 UV–Vis spectrum of (A) and (B) Ligand-salicylaldimine 10
and 10
complex in methanol
-4
M solution in methanol and (C) Co(salicylaldimine)
by a-TiP.M(salicylaldimine)/TBHP system can be explain
by Scheme 2.
The selectivity of the products is the measure of the
reactivity of the catalyst {a-TiP.M(salicylaldimine)}. The
percentage-conversion of the substrate and the percentage-
selectivity of the products in the oxidation reaction are
calculated as [14]. Turn over number (TON) was calculated
according to the following formula [47]:
The consumption of TBHP was determined iodometri-
cally after each catalytic reaction. The amount of TBHP
consumed and the efficiency of TBHP were calculated
according to reported formula [37, 48] as follows:
ꢀ
ꢁ
remaining TBHP
initial TBHP
TBHP consumed ð%Þ ¼ 1 À
 100
 100
Substrate conversion ð%Þ¼½Substrate converted ðmolesÞ
mmol of products
=
substrate used ðmolesފ  100
TBHP efficiency ð%Þ ¼
mmol of TBHP consumed
Product selectivity ð%Þ¼½Product formed ðmolesÞ
substrate converted ðmolesފ  100
Turn over number = mmol of products/mmol of catalyst:
All the four heterogeneous catalysts, TiP.Fe(salicy-
laldimine), TiP.Cu(salicylaldimine), TiP.Cr(salicy-
laldimine) and TiP.Co(salicylaldimine) were studied for
=
123

S. Khare, P. Shrivastava
Table 4 UV–Vis data of ligand
and complexes in methanol
Compounds
salicylaldimine) ligand
k
max (nm)
Assignment
(
216, 251, 320
u ? u, p ? p* and n ? p*
Co(salicylaldimine)
Cr(salicylaldimine)
Cu(salicylaldimine)
Fe(salicylaldimine)
225, 269, 372, 570
218, 268, 371, 571
220, 267, 372, 572
221, 268, 373, 572
u ? u, p ? p*, n ? p* and d–d transition
u ? u, p ? p*, n ? p* and d–d transition
u ? u, p ? p*, n ? p* and d–d transition
u ? u, p ? p*, n ? p* and d–d transition
Fig. 5 TGA analysis of
(a) a-TiP and (b) a-
TiP.Co(salicylaldimine)
the oxidation of cyclohexane were found to be active as the
shown in Table 5. It is observed that the conversion of
cyclohexane was varied from 3.59 to 14.85 %, whereas the
selectivity of KA-oil (cyclohexanol ? cyclohexanone)
varied from 92.13 to 98.54 %. A maximum conversion
The catalyst, a-TiP.Co(salicylaldimine) shows best
activity for the oxidation of cyclohexane in comparison
with other catalysts. Therefore, the optimization of reaction
conditions for oxidation of cyclohexane to obtain maxi-
mum conversion of cyclohexane using various parameters
were considered such as, effects of various solvents, oxi-
dants, cyclohexane: TBHP molar ratios, amount of catalyst
and the reaction temperature for detail study.
1
4.85 % was obtained with a-TiP.Co(salicylaldimine),
however the maximum selectivity 98.54 % of KA-oil, was
obtained with a-TiP.Cr(salicylaldimine). In the case of
a-TiP.Cr(salicylaldimine), TiP.Cu(salicylaldimine) and
a-TiP.Fe(salicylaldimine) conversion of cyclohexane is
less in comparison with a-TiP.Co(salicylaldimine). Con-
version of substrate plays an important role in a catalytic
reaction, Therefore the orders of catalytic activity of the
a-TiP.M(salicylaldimine) catalysts on the basis of cyclo-
hexane conversion are as follows:
The effect of various oxidants, viz. 30 % H O , 70 %
2
2
TBHP and TBHP (70 % in cyclohexane) on the oxidation
of cyclohexane, catalyzed by a-TiP.Co(salicylaldimine)
was studied to develop an efficient catalytic system and the
results are given in Table 6. In each case, the reaction was
carried out by taking oxidant and cyclohexane in 1:2 molar
ratio at 353 K in an oil bath for 6 h. It was observed that
leaching of the catalyst occurred after 15 min in case of
30 % H O . In case of 70 % TBHP, 3.59 % conversion of
a-TiP.Co(salicylaldimine)[a-TiP.Cr(salicylaldimine)[
a-TiP.Cu(salicylaldimine)[a-TiP.Fe(salicylaldimine).
2
2
1
23

Liquid Phase Solvent-Less Cyclohexane Oxidation Catalyzed by Covalently Anchored…
Table 5 Effect of support, homogeneous and heterogeneous catalysts on the oxidation of cyclohexane
Catalysts
Cyclohexane
conversion (%)
Product selectivity (%)
Selectivity (%)
(ol ? none = *KA-oil)
TON
Cyclohexanol Cyclohexanone Other
products
a-TiP
–
–
–
–
–
–
Co(salicylaldimine)
9.17
14.85
49.55
32.83
50.34
59.30
0.1
99.89
92.13
137.92
184.01
a-
7.87
TiP.Co(salicylaldimine)
Cr(salicylaldimine)
1.86
5.03
35.35
12.58
52.88
85.96
11.77
1.46
88.23
98.54
11.44
85.49
a-
TiP.Cr(salicylaldimine)
Cu(salicylaldimine)
4.35
3.75
54.27
51.57
43.98
45.34
1.75
3.09
98.25
96.91
70.79
55.63
a-
TiP.Cu(salicylaldimine)
Fe(salicylaldimine)
1.52
3.59
54.57
38.53
45.43
54.43
–
100
18.96
41.16
a-
7.04
92.96
TiP.Fe(salicylaldimine)
*
*
Reaction conditions: catalyst (0.05 g); TBHP (38.89 mmol); cyclohexane (19.45 mmol); Time (6 h); 353 K in oil bath
KA-oil: cyclohexanone (K): cyclohexanol (A)
best oxidant for our catalytic system is TBHP in
cyclohexane.
The effect of various solvents, viz. acetonitrile, acetone
and dichloromethane on the oxidation of cyclohexane,
catalyzed by a-TiP.Co(salicylaldimine) was studied to
develop an efficient catalytic system. In each case, the
reaction was carried out by taking solvent (5 ml) at 353 K
in an oil bath. The results are given in Table 6. It was
observed that when solvents were used in the reaction, the
cyclohexane conversion decreases in the following order:
solvent-less (14.85 %) [ acetonitrile (9.45 %) [ acetone
Scheme 2 Oxidation of cyclohexane catalyzed by a-TiP.M(salicy-
laldimine)/TBHP system
cyclohexane was obtained, however, after 1 h the leaching
of catalyst was observed. A maximum, 14.85 % conversion
of cyclohexane with 92.13 % selectivity of KA-oil was
observed with TBHP in cyclohexane. The order of selec-
tivity of products are as follows: cyclohexanone
(8.96 %) [ dichloromethane (8.78 %). The selectivity of
KA-oil decreased in the following ordered:
(
59.30 %) [ cyclohexanol (32.83 %) [ other products
7.87 %). Cyclohexanone was the major product. Thus the
Solvent-less ð92:13 %Þ [ Acetonitrile ð85:79 %Þ
[ acetone ð79:37 %Þ [ dichloromethane ð78:66 %Þ:
(
Table 6 Effect of oxidants and solvents on the oxidation of cyclohexane catalyzed by a-TiP.Co(salicylaldimine)/TBHP
Solvents/oxidants
Time
h)
Cyclohexane conversion Product selectivity (%)
(%)
Selectivity (%)
(ol ? none = *KA-oil)
(
Cyclohexanol Cyclohexanone Other
products
H
2
O
2
/no solvent
1
1
–
–
–
–
–
TBHP (70 %)/no
solvent
3.95
33.26
58.14
8.60
90.40
*
TBHP/no solvent
6
6
6
6
14.85
9.45
8.96
8.78
32.83
32.89
31.78
30.60
59.30
52.90
46.88
48.77
7.87
14.21
21.34
20.63
92.13
85.79
79.37
78.66
Acetonitrile/*TBHP
Acetone/*TBHP
Dichloromethane/
*
TBHP
*
TBHP (70 % in cyclohexane); *KA-oil: cyclohexanone (K): cyclohexanol (A)
123

S. Khare, P. Shrivastava
The selectivity of KA-oil increases with increasing in
dipole moment of polar aprotic solvents. Thus, dipole
moment of the solvents probably is an essential factor in
oxidation reaction. Similar observations were reported by
various authors [49–51]. A maximum 14.85 % conversion
of cyclohexane with 92.13 % selectivity of KA-oil (cy-
clohexanol, 32.83 % ? cyclohexanone, 59.30 %) was
obtained under solvent-less condition because according to
molecular collision theory any reaction takes place when
there is effective collision in between molecules of sub-
strate but when we add some solvent in it, the collision
between the two substrate decreases as collision between
substrate and solvent molecules also takes place which
results as lowering of effective collision between two
substrate molecules hence rate of reaction decreases. This
observation also demonstrates that substrate and solvent
molecules are in competition for the active sites of metal
center in catalyst. Similar type of observation is reported in
the oxidation of ethylbenzene [52]. However, the use of
solvent leads to many environmental problems. Therefore,
activation of cyclohexane under solvent free reaction
conditions is favorable and environmental friendly for
developing new catalytic system. Thus, we have chosen
solvent free oxidation of cyclohexane which gave maxi-
mum (14.85 %) conversion of cyclohexane in 6 h of
reaction time.
Fig. 6 Effect of oxidant on the oxidation of cyclohexane catalyzed
by a-TiP.Co(salicylaldimine)
molar ratio of cyclohexane to TBHP best due to economic
point of view.
To study the effect of amount of catalyst on the oxida-
tion of cyclohexane, the amount of a-TiP.Co(salicy-
laldimine) was varied as 0.025, 0.050 and 0.075 g, while
keeping the other parameters fixed. In Fig. 7, the conver-
sion of cyclohexane is plotted against time at different
amount of catalyst. It is evident from Fig. 7 that conversion
increases sharply from 6.97 to 14.85 % with increasing
catalyst amount from 0.025 g to 0.05 g. Further increase in
catalyst amount to 0.075 g conversion of cyclohexane also
increased to 14.98 %. Only little increase in conversion
was observed at higher catalyst amount. Similar results
were observed with Co O -I/O system also [57]. The
The three different molar ratio of cyclohexane to TBHP
(
1:1, 1:2 and 1:2.18) were considered to study the effect of
cyclohexane to TBHP molar ratio on the oxidation of
cyclohexane at 353 K in an oil bath for 6 h. In Fig. 6, the
conversion of cyclohexane as a function of time is plotted.
As shown in Fig. 6, when cyclohexane to TBHP molar
ratio increases from 1:1 to 1:2, the conversion of cyclo-
hexane increases from 12.67 to 14.85 %. Further increase
in cyclohexane to TBHP molar ratio to 1:2.18, the con-
version of cyclohexane increased slightly to 14.96 %. The
results of the conversion of cyclohexane, TBHP efficiency,
consumption, product selectivity and TON are presented in
Table 7. The selectivity of KA-oil decreased from 99 to
3
4
2
results of the conversion of cyclohexane, selectivity of
products, TON, efficiency and consumption of TBHP at
different amount of catalyst are summarized in Table 7.
The selectivity of KA-oil decreased from 99.21 to 91.24 %
with increasing amount of catalyst from 0.025 to 0.075 g.
The reduction of selectivity of KA-oil at higher catalyst
amount is due to the increase in the formation of other
products. It was also observed that the TBHP consumption
decreases, while TBHP efficiency increases with increase
in amount of catalyst from 0.025 to 0.075 g. Only little
increase in conversion and selectivity of KA-oil was
observed at 0.075 g of catalyst. Therefore, less amount of
catalyst, a-TiP.Co(salicylaldimine), 0.05 g was considered
optimum.
9
1.90 % with increasing molar ratio of cyclohexane to
TBHP from 1:1 to 1:2.18. It was also observed that the
TBHP consumption decreases, while TBHP efficiency
increases with increase in cyclohexane to TBHP molar
ratio from 1:1 to 1:2.18. It was observed that higher con-
centration of oxidant showed little increase in conversion
and selectivity of the products [53–55]. This is probably
due to the non-productive decomposition of TBHP at
higher concentration. Similar observation is reported in
case of TS-1/ionic liquid/TBHP system [56]. At all molar
ratio of cyclohexane to TBHP, cyclohexanone was the
major product. Moreover, higher concentration of oxidant
showed little increases in conversion, we have chosen 1:2
Temperature plays an important role in a catalytic
reaction therefore, the effect of temperature on the oxida-
tion of cyclohexane was studied. The temperature of
reaction was varied as 343 K, 353 K and 363 K keeping
the other parameters fixed. In Fig. 8 the conversion of
1
23

Liquid Phase Solvent-Less Cyclohexane Oxidation Catalyzed by Covalently Anchored…
Table 7 Effect of cyclohexane: TBHP molar ratio, catalyst amount and temperature for oxidation of cyclohexane catalyzed by a-
TiP.Co(salicylaldimine)/TBHP
Cyclohexane/TBHP
molar ratio)
Catalyst
amount (g)
Temperature Conversion TBHP
(K)
TBHP
efficiency
(%)
Product selectivity
a
(%)
KA- TON
b
oil
(
(%)
consumption
%)
(
ol
None Others
1
1
1
1
1
1
1
:1
0.050
0.050
0.050
0.025
0.075
0.050
0.050
353
353
353
353
353
343
363
12.67
14.85
14.96
6.97
87.33
85.15
85.05
93.03
85.02
88.02
85.09
14.51
17.44
17.51
11.20
38.40
20.41
38.19
42.00 57.00 1.00
32.83 59.30 7.87
32.75 59.15 8.10
46.89 52.32 0.79
32.21 59.03 8.76
49.00 50.10 0.9
32.81 59.12 8.07
99.00 155.50
92.13 184.01
91.90 185.23
:2
:2.18
:2
99.21
83.49
:2
14.98
11.98
14.90
91.24 184.19
99.10 149.84
91.93 184.09
:2
:2
ol cyclohexanol; none cyclohexanone
a
Reaction conditions: catalyst (0.05 g); oxidant: cyclohexane molar ratio (1:2); 353 K; 6 h; without solvent
b
KA-oil = cyclohexanol (A) and cyclohexanone (K) mixture
Fig. 8 Effect of temperature on the oxidation of cyclohexane
catalyzed by a-TiP.Co(salicylaldimine)
Fig. 7 Effect of amount of catalyst on the oxidation of cyclohexane
catalyzed by a-TiP.Co(salicylaldimine)
increased while its consumption decreased which also
confirmed the thermal decomposition of TBHP at higher
temperatures. The selectivity of KA-oil decreased from
99.10 to 91.93 % with increasing temperature from 343 to
363 K. Cyclohexanone was the major product in the oxi-
dation of cyclohexane. Thus the maximum conversion
(14.85 %) was obtained at 353 K in 6 h of reaction time.
Thus the optimum condition for obtaining maximum con-
version of cyclohexane are 1:2 molar ratio of cyclohexane
to TBHP, 0.05 g catalyst and 353 K temperature.
cyclohexane is plotted against time at different tempera-
ture. It is evident from Fig. 8, the conversion of cyclo-
hexane increases from 11.98 to 14.85 % with increase in
reaction temperature from 343 to 353 K. Further increase
in temperature to 363 K the conversion of cyclohexane,
1
4.90 %, remains almost same. This is due to decomposi-
tion of TBHP at higher temperature which results in lower
conversion of cyclohexane. Similar type of observation is
reported by many researchers [37, 56]. The results of the
conversion of cyclohexane, TBHP consumption, TBHP
efficiency, selectivity of the products and TON are sum-
marized in Table 7. It is found that when the temperature
increased from 343 to 363 K, the efficiency of TBHP
In Table 8, we have compared our catalytic system with
the other catalytic system reported in literature. It was
found that in most of cases conversion of cyclohexane and
selectivity of KA-oil are lesser in comparison with the
present work. Here, we are reporting the oxidation of
123

S. Khare, P. Shrivastava
cyclohexane catalyzed by a-TiP.Co(salicylaldimine) using
TBHP under solvent free condition, under the optimum
condition, a maximum 14.85 % conversion of cyclohexane
with 92.13 % was obtained after 6 h of reaction time.
filtration technique. The catalyst was filtered out at 353 K
after 1 h in order to avoid re-adsorption of leached cobalt
onto the catalyst surface. The filtrate collected after 1 h of
first cycle of cyclohexane oxidation was placed again into
the reaction flask and the reaction was continued for next
6 h. The gas chromatographic analysis showed no further
increase in conversion in the oxidation of cyclohexane. The
solution did not exhibit any colour due to the absence of
cobalt, which was estimated using ICP-AES. The result
suggested that no cobalt-leaching occurred during the cat-
alytic reaction. This observation indicates that the catalyst
is heterogeneous in nature.
3
.3 Recyclability and Heterogeneity of the Catalyst
The stability of the heterogeneous catalyst plays an
important role in a catalytic activity study. It is verified by
the use of catalyst, a-TiP.Co(salicyaldimine) in successive
cycles. To examine the recyclability of the catalyst, the
used a-TiP.Co(salicyaldimine) was recovered from the
reaction media by simple filtration and washing with
methanol followed by dichloromethane before being dried
at 110 °C for overnight, then subjected to further catalytic
reaction under similar reaction conditions. The catalyst,
4 Conclusions
{a-TiP.Co(salicylaldimine)} was recycled for the five
cycles. The conversion and selectivity of products obtained
with spent catalyst were similar to those obtained with the
fresh catalyst up to the second cycle (Table 9). The con-
version of cyclohexane was reduced by 0.87 % from first to
fifth cycle. The cobalt content was reduced by 2.93–1.45 %
from first to fifth cycle. The reduction in metal contents
after fifth cycle suggests that metal-leaching has occurred,
which is observed in several catalytic systems [50, 58]. The
regenerated catalyst was further characterized by ICP-AES,
XRD, BET surface area and FTIR analysis after five cycles
A new catalyst, a-TiP.Co(salicylaldimine) was synthesized
and characterized by ICP-AES, SEM, EDX, XRD, BET
surface area, FTIR, UV–Vis spectroscopy and TGA anal-
ysis. The catalytic activity of a-TiP.Co(salicylaldimine)
with TBHP as an oxidant for oxidation of cyclohexane was
studied. The catalyst, a-TiP.Co(salicylaldimine) was found
to have potential catalytic activity for oxidation of cyclo-
hexane. Reaction conditions have been optimized consid-
ering various parameters viz. effects of various oxidants,
solvents, cyclohexane to TBHP molar ratio, catalyst
amount and reaction temperature to get maximum con-
version of cyclohexane. The maximum, 14.85 % conver-
sion of cyclohexane with 92.13 % selectivity of
KA-oil was obtained. The major oxidation products follow
the order: cyclohexanone (59.30 %) [ cyclohexanol
(32.83 %) [ other products (7.87 %). The catalyst,
a-TiP.Co(salicylaldimine) was found to be stable enough
for oxidation of cyclohexane, it can be recycled five times.
(
data are given in corresponding tables). No significant
changes in metal content, composition and morphology
were observed in regenerated catalyst after fifth cycle. This
suggests that the catalyst was stable during catalytic reac-
tion and suitable for recycling. After fifth cycle only small
loss in metal content was observed.
Further experiments on the stability of the catalyst
during oxidation of cyclohexane were performed by hot
Table 8 Comparison of literature catalysts and our catalyst system for oxidation of cyclohexane
Entry Catalytic system
Reaction condition
Conversion
%)
Selectivity (%) (*KA-
oil)
(
1
Ebadi and Nikbakht
18]
CoPc/c-alumina (1.0 g), TBHP, DMF/CH
temperature, 8 h
2
Cl
2
(3:7), reflux
12.96
100
[
2.
3.
4.
5.
6.
7.
8.
9.
Anand et al. [19]
Xia et al. [15]
Co-TUD-1 (0.1 mmol), TBHP, solvent free, 70 °C, 18 h
Cosalen-NaY(0.1 g), O
, solvent free, 150 °C, 3 h
, 130 °C, 8 h
10.3
13.40
1.60
7.5
90.52
23.5
56.7
92.4
90.7
100
2
Pingping et al. [16]
Carneiro et al. [17]
Yuan et al. [21]
Jin et al. [22]
CoAlPO-5 (200 mg), O
2
Co/ZSM-5 (100 mg), O
2
, 100 °C, 4 h
Co-ZSM-5(200 mg), O
Cu–salpn/Y (0.1 g), H
CoPh-HMS (0.12 g), O
a-TiP.Co(salicylaldimine), TBHP, 80 °C, solvent free, 6 h
2
, solvent free, 100 °C, 4 h
7.22
2.3
2 2
O , Acetonitrile, 60 °C, 2 h
Chen et al. [23]
Present work
2
, solvent free, 115 °C, 6 h
6.7
80.5
92.13
14.85
*
KA-oil: cyclohexanone (K): cyclohexanol (A)
1
23

Liquid Phase Solvent-Less Cyclohexane Oxidation Catalyzed by Covalently Anchored…
Table 9 Recycling experiment
No. of
cycle
Cyclohexane
conversion (%)
Product selectivity (%)
Selectivity (%)
(ol ? none = *KA-oil)
TON
Metal content
(%)
Cyclohexanol Cyclohexanone Other
products
Fresh
14.85
14.85
14.60
14.40
14.19
13.98
32.83
32.83
32.46
32.13
31.87
31.25
59.30
59.30
59.11
57.69
57.31
56.90
7.87
92.13
92.13
92.05
91.93
91.85
90.10
184.01 2.93
184.01 2.93
182.76 2.90
180.64 2.88
178.43 2.25
172.98 1.45
1
2
3
4
5
7.87
7.43
0.80
0.67
0.37
*
KA-oil: cyclohexanone (K): cyclohexanol (A)
Acknowledgments The authors are thankful to UGC-DAE Con-
sortium of scientific Research, Central Salt and Marines Chemical
Research Institute Bhavnagar, Gujarat and Sophisticated Analytical
Instrumental Facilities (SAIF), IIT, Powai, Mumbai (Maharashtra),
and Punjab University Chandigarh for providing analytical facilities.
24. Salavati-Niasari M, Shakouri-Arani M, Davar F (2008) Microp-
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8. Fan B, Li H, Fan W, Jin C, Li R (2008) Appl Catal A 340:67
9. Tembe GL, Ganeshpure PA, Satish S (1997) J Mol Catal A
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