Synthesis, characterization and catalytic activity of Fe(Salen) intercalated α-zirconium phosphate for the oxidation of cyclohexene

Article abstract of DOI:10.1016/j.molcata.2011.05.005

Iron(III)-Salen intercalated α-zirconium phosphate, abbreviated as {α-ZrP·Fe(Salen)} was synthesized in situ by the flexible ligand method. The resulting compound was characterized by BET surface area, X-ray diffraction, scanning electron microscopy, ener

Full text of DOI:10.1016/j.molcata.2011.05.005

Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis, characterization and catalytic activity of Fe(Salen) intercalated
␣
-zirconium phosphate for the oxidation of cyclohexene
∗
Savita Khare , Rajendra Chokhare
School of Chemical Sciences, Devi Ahilya University, Takshashila Campus, Khandwa Road, Indore 452017 (M. P.), India
a r t i c l e i n f o
a b s t r a c t
Article history:
Iron(III)-Salen intercalated ␣-zirconium phosphate, abbreviated as {␣-ZrP·Fe(Salen)} was synthesized in
situ by the flexible ligand method. The resulting compound was characterized by BET surface area, X-ray
diffraction, scanning electron microscopy, energy dispersive X-ray analysis, Fourier transform infrared,
Mössbauer and atomic absorption spectroscopy. The catalytic activity of ␣-ZrP·Fe(Salen) with dry tert-
butylhydroperoxide (TBHP) as an oxidant was studied for oxidation of cyclohexene. In the oxidation
reaction, cyclohexene was oxidized to cyclohexene oxide, cyclohexenol and cyclohexenone. A maximum
conversion of 18.04% for the oxidation of cyclohexene was observed after 5 h of reaction when concen-
trations of the catalyst, the substrate and the oxidant were 0.20, 20 and 4 mmol respectively. The catalyst,
Received 7 September 2010
Received in revised form 21 April 2011
Accepted 6 May 2011
Available online 12 May 2011
Keywords:
␣
-Zirconium phosphate
Oxidation
Cyclohexene
tert-Butylhydroperoxide
Ion-exchanger
␣-ZrP·Fe(Salen) was recycled for eight cycles.
© 2011 Elsevier B.V. All rights reserved.
Fe(Salen) complex
1
. Introduction
Transition metal Schiff base complexes are active homogeneous
calation chemistry [9–16], ion exchange properties [10–12,17–19],
and catalytic properties [11–15,20]. Various transition-metal ions,
viz. Mn(II), Co(II), Ni(II), Cu(II), and Zn(II), are substituted on ␣-
zirconium phosphate for catalytic epoxidation of propylene [21].
Fe(Salen) is used as homogenous catalyst for oxidation of hydro-
carbons [5,22–25] and tert-butylhydroperoxide (TBHP) is used as a
oxygen donor for oxidation of alkenes via Salen complexes [8,26].
In the present paper, we have synthesized a heterogeneous cata-
lyst, Fe(Salen) intercalated ␣-zirconium phosphate, abbreviated as
{␣-ZrP·Fe(Salen)} in situ by the flexible ligand method and studied
its catalytic behaviour for oxidation of cyclohexene using dry TBHP
as an oxidant.
catalysts in oxidation reactions of several organic substrates. One of
the most important oxidation reactions is the oxidation of alkenes
[
1–4]. Iron(III) and manganese(III) complexes with salen type lig-
and have been shown to be highly effective, chemoselective and
stereoselective for this oxidation reaction [5–7]. The inherent dif-
ficulty in separating and recycling of homogeneous catalysts has
hampered its industrial utilization. The heterogeneous catalysts
with metal complexes immobilized into solid supports can be eas-
ily separated from the reaction media and reused because they are
quite stable in comparison to their corresponding homogeneous
counterparts, due to hindrance of deactivation pathways by local
site isolation of the complexes inside the solid matrix. The het-
erogeneous catalysts have become very important for ecofriendly
industrial processes because these materials are developed with
the objective to perform reactions under milder conditions and
without hazardous wastes. Immobilization of transition-metal ions
on layered compounds by ion-exchange method to synthesize
catalysts is better because it provides temperature- and solvent-
stable supports of known structure [8]. Crystalline ␣-zirconium
phosphate, Zr(HPO ) ·H O, a cation exchanger, has a structure of
2. Experimental
2.1. Materials
Zirconium oxychloride (ZrOCl ·8H O), phosphoric acid,
2
2
hydrofluoric acid, ferric chloride FeCl ·6H O, salicylaldehyde,
3
2
ethylenediamine and cyclohexene were of reagent grade (E.
Merck). Cyclohexene was checked by gas chromatography (GC) to
ensure that no oxidation products were present in the substrate.
The solution of dry TBHP in benzene (1.12 N) was obtained by
careful azeotropic distillation of the aqueous 70% commercial
solution of TBHP (E. Merck) and its strength was estimated by the
method described by Sharpless and Verhoeven [27]. Benzene (E.
Merck), used as solvent, was purified by a known method [28].
4
2
2
zeolite type cages and has been extensively studied for its inter-
∗ Corresponding author. Tel.: +91 0731 2487315.
E-mail address: kharesavita@rediffmail.com (S. Khare).
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.05.005

8
4
S. Khare, R. Chokhare / Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
The reference sample of products was prepared by the standard
procedure [29].
2.3.4. Preparation of ˛-ZrP·Fe(Salen)
Intercalation of salen into ␣-ZrP·Fe(III) to form ␣-ZrP·Fe(Salen)
was carried out by flexible ligand technique. 70 mmol of salen was
added to 5 g of ␣-ZrP·Fe(III) and stirred at 423 K in an oil bath for
2.2. Physical methods and analysis
4
h under nitrogen gas flow. The resulting material was extracted
with methanol using soxhlet extractor to remove excess ligand that
Powder X-ray diffraction (XRD) patterns of the samples were
remained uncomplexed in the layer ␣-ZrH P as well as located
2
◦
recorded on a Rigaku diffractometer in the 2ꢀ range of 5–40 using
on the surface of the ␣-ZrH P along with neat complexes, if any.
2
◦
CuK␣ radiation (ꢁ = 1.5418 A˚ ) at scanning speed 2 per minute
The remaining uncomplexed metal ions in ␣-ZrH P were removed
2
◦
with step size 0.02 . Scanning electron microscopy (SEM) measure-
by stirring with aqueous 0.01 M NaCl solution (200 ml) for 1 h. It
was then washed with double distilled water till no precipitate
of AgCl was observed in the filtrate on treating with AgNO3 and
dried at 383 K. The ␣-ZrP·Fe(Salen), so obtained, was characterized
by BET surface area, XRD, SEM, EDX, FTIR, Mössbauer and atomic
absorption spectroscopy.
ments were performed using a JEOL JSM 6100 electron microscope,
operating at 20 kV. The Fourier transform infrared (FTIR) spectra
were recorded on Perkin Elmer model 1750 in KBr. The Möss-
bauer measurements were carried out using a Standard PC-based
spectrometer equipped with a Weissel velocity drive operating in
the constant acceleration mode at 300 K. The data were analyzed
with the NORMOS-SITE program and the obtained parameters were
fitted with respect to natural iron. Atomic absorption spectrome-
ter, Shimadzu AA-6800 was used for the estimation of iron. The
electronic spectra were recorded on Shimadzu UV-1700 Pharma
Spectrophotometer. The N2 adsorption data, measured at 77 K
by volumetric adsorption set-up (Micromeritics ASAP-2010, USA),
were used to determine BET surface area, pore volume and pore
size. Analytical gas chromatography was carried out on a Shimadzu
Gas Chromatograph GC-14B with dual flame ionization detector
2.4. Catalytic oxidation of cyclohexene
The catalytic oxidation of cyclohexene was carried out using
{
␣-ZrP·Fe(Salen)} catalysts in a three-necked round bottom flask
(100 ml) under nitrogen atmosphere. In a typical reaction, nitro-
gen was flushed for 10 min through the flask, which was loaded
with benzene (10 ml), cyclohexene (1.64 g, 20 mmol) and dry TBHP
(3.57 ml, 4 mmol). After nitrogen flushing, the catalyst (0.066 g,
0
.1 mmol) was added to the contents of the flask, which were
(
3
5
FID) and attached Shimadzu printer having SE-30 ss column at
93 K. The products were identified by GC–MS (Perkin-Elmer Clasus
00 column; 30 m × 60 mm).
heated at 353 K in an oil bath for 6 h with continuous stirring. After
completion of the reaction, the contents of the flask were cooled
in an ice-bath and the catalyst was filtered out and the liquid layer
was analyzed quantitatively by GC using XE-60 ss column at 393 K.
The products were identified by GC–MS. GC–MS analysis revealed
that the main products from the reaction were cyclohexene oxide,
cyclohexenol and cyclohexenone. Selectivity was calculated with
respect to the converted cyclohexene. The filtered catalyst was fur-
ther characterized by BET surface area, XRD, FTIR, Mössbauer and
atomic absorption spectroscopy.
2
2
.3. Preparation of catalyst
.3.1. Preparation of H Salen
2
ꢀ
N,N -bis(salicylidene)-ethylenediamine (H Salen) was synthe-
2
sized by drop wise adding ethylenediamine (10 mol) to a 40 ml
methanolic solution of salicylaldehyde (20 mol). The reaction mix-
ture was heated for 1 h in a water bath with a reflux condenser. After
3
3
3
. Results and discussion
cooling to room temperature, the yellow precipitate of H Salen was
2
filtered off, washed with petroleum ether, dried and characterized.
Yield: 85%. Anal. found C, 71.58%; H, 6.02%; N, 10.34%; O, 12.06%
.1. Characterization of the catalysts
Calcd. for C16H16N O (268): C, 71.64%; H, 5.97%; N, 10.45%; O,
2
2
.1.1. Estimation of metal contents
1
1.94%.
Synthesis of Fe(Salen) intercalated ␣-zirconium phosphate [␣-
ZrP·Fe(Salen)] involved the exchange of Fe(III) ions with hydrogen
of ␣-ZrH P followed by reaction of metal exchanged ␣-ZrH P with
2
.3.2. Preparation of ˛-zirconium phosphate
2
2
H Salen. Here, ligand entered into the layers of ␣-ZrH P due to its
␣-Zirconium phosphate (abbreviated ␣-ZrH P) was prepared
2
2
2
flexible nature and interacted with metal ions. The chemical com-
position, physical and analytical data of compounds obtained at
according to the direct precipitation method reported in a pre-
vious study [30]: 5.5 g of ZrOCl ·8H O was dissolved in 80 ml of
2
2
different stages of synthesis of catalyst, viz. ␣-ZrH P, ␣-ZrP·Fe(III)
distilled water, followed by the addition of 4 ml of 40% hydrofluoric
acid. Then 46 ml of 85% phosphoric acid was added under contin-
uous stirring with a magnetic stirrer. The mixture was kept for 15
days without disturbing for setting. The precipitate obtained was
washed with distilled water until the pH of the supernatant liquid
became ∼5. Then solid was filtered off and dried at 383 K for 24 h.
2
and ␣-ZrP·Fe(Salen) (before and after catalytic reaction) are given
in Table 1. The colours of ␣-ZrH P, ␣-ZrP·Fe(III) and ␣-ZrP·Fe(Salen)
2
(before and after catalytic reaction) compounds were different due
to change in iron content, which was determined by atomic absorp-
tion analysis. There was no iron in ␣-ZrH P and its colour was white.
2
The colour of ␣-ZrP·Fe(III) became yellow due to presence of iron
by 23.03%. The colours of ␣-ZrP·Fe(Salen) was brown before and
after catalytic reaction due to presence of iron by 8.39% and 6.02%
respectively.
Finally the structure of ␣-ZrH P was confirmed by powder XRD.
2
2.3.3. Preparation of ˛-ZrP·Fe(III)
The ␣-ZrP·Fe(III) was prepared by ion exchange procedure.
Hydrogen ions of ␣-ZrH P were exchanged by Fe(III) ions from a
3.1.2. Surface area analysis
2
FeCl3 solution. Stock solution of FeCl3 (0.1 N) was prepared and
The results of surface area analysis of ␣-ZrH P, ␣-ZrP·Fe(III) and
2
1
00 ml of FeCl3 solution per gram of ␣-ZrH P was refluxed for 24 h
␣-ZrP·Fe(Salen) are incorporated in Table 1. The ion exchange of
2
2
at 373 K. Then it was filtered hot through a sintered glass crucible.
The clear filtrate was allowed to cool to 298 K and analyzed for
their metal ion content by atomic absorption analysis. The solid
obtained, was washed with distilled water and dried at 383 K. The
Fe(III) led to a increase of surface area of ␣-ZrP·Fe(III) to 18.56 m /g
2
from 13.58 m /g (␣-ZrH P). This is due to increase in number
2
of extra-framework cations while replacing hydrogen ions with
trivalent Fe(III) cations. The presence of trivalent Fe(III) cations
3
␣
-ZrP·Fe(III) was obtained as a yellow solid.
increased the pore volume of ␣-ZrP·Fe(III) to 0.05 cm /g from

S. Khare, R. Chokhare / Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
85
Table 1
Chemical composition, physical and analytical data.
Catalyst
Colour
Iron content
BET surface
area (m /g)
Pore volume
(cm
Pore size ( A˚ )
d-Spacing ( A˚ )
IR group frequency (cm−1)
2
3
−1
g )
(
wt%)
PO43
−
Zr–O
CH2–N
␣
␣
␣
␣
-ZrH2P
White
Yellow
Brown
Brown
0
13.58
8.56
12.51
8.41
0.04
0.05
0.03
0.02
116.69
116.68
87.36
7.40
7.46
8.98
8.98
1044
991
991
990
599
602
602
603
–
–
-ZrP·Fe(III)
-ZrP·Fe(Salen)
-ZrP·Fe(Salen)
23.03
8.39
6.02
b
a
1700–1300
1700–1300
83.18
a
After catalysis.
Before catalysis.
b
Table 2
EDX measurements of ␣-ZrP, ␣-ZrP·Fe(III) and ␣-ZrP·Fe(Salen).
b
a
Elements
␣-ZrP (%)
␣-ZrP·Fe(III) (%)
␣-ZrP·Fe(Salen) (%)
␣-ZrP·Fe(Salen) (%)
O
Zr
P
Fe
N
81.47
7.09
11.44
–
80.05
5.76
10.3
3.81
–
64.33
2.79
5.03
1.87
25.98
49.32
0.95
1.25
0.34
48.14
–
a
After catalysis.
Before catalysis.
b
3
0
␣
.04 cm /g (␣-ZrH P). However the pore sizes of ␣-ZrP·Fe(III) and
(3.81%). The catalyst, ␣-ZrP·Fe(Salen), contains oxygen (64.33%),
zirconium (2.79%), phosphorous (5.03%), iron (1.87%) and nitrogen
(25.98%), which confirms the presence of Fe(Salen) on ␣-ZrH2P.
The catalyst, ␣-ZrP·Fe(Salen) after eight catalytic cycles was further
analyzed by EDX. A decrease in percentage of all elements, viz. oxy-
gen (49.32%), zirconium (0.95%), phosphorous (1.25%), iron (0.34%)
and nitrogen (48.14%), in comparison of unused catalyst. The loss
of Fe from 1.87 to 0.34% as indicated in the EDX measurement of
used catalyst after eight cycles suggest that leaching of Fe(Salen)
occurred from the surface of ␣-ZrH2P.
2
-ZrH P remained almost same. It is also observed that the inter-
2
calation of Fe(Salen) on ␣-ZrP·Fe(III) led to decrease in the surface
2
area of the catalyst, ␣-ZrP·Fe(Salen), to 12.51 m /g. This is due to
the presence of organic ligand. The pore volume and pore size of ␣-
3
ZrP·Fe(Salen) decreased to 0.03 cm /g and 87.36 A˚ respectively due
to coordination of organic ligand with Fe(III) cation. The decrease in
the surface area, pore volume and pore size may be to the presence
of metal ion near the opening of the layered structure and block
the access of N2 molecules to the whole lamellar structure [31].
2
The surface area of the catalyst reduced from 12.51 to 8.41 m /g
after eight runs. This indicates leaching of the catalyst.
The SEM images of ␣-ZrP, ␣-ZrP·Fe(III), ␣-ZrP·Fe(Salen) (before
catalysis) and ␣-ZrP·Fe(Salen) (after eight catalytic runs) are shown
in Fig. 3. The SEM image of ␣-ZrH P clearly shows a platelike struc-
2
ture and the round edges of the sheets indicate that its crystallinity
3
.1.3. XRD studies
The XRD patterns of ␣-ZrP, ␣-ZrP·Fe(III), ␣-ZrP·Fe(Salen) (before
was not very high [17]. The SEM image of ␣-ZrP·Fe(III) shows that
its structure is much less regular in comparison to that of ␣-ZrH P.
catalysis) and ␣-ZrP·Fe(Salen) (after eight catalytic runs) are shown
in Fig. 1 and their d-spacing corresponding to the plane (0 0 2) are
incorporated in Table 1. The d-spacing of the most intense reflec-
2
The structure of ␣-ZrP·Fe(III) consists of the aggregates of both
sheets and spherical particles. The SEM image of ␣-ZrP·Fe(Salen)
tion corresponding to the (0 0 2) plane of ␣-ZrH P was 7.40 A˚ , which
2
was increased to 7.46 A˚ in ␣-ZrP·Fe(III). The shift of 0.06 A˚ in the d-
spacing is due to intercalation of the Fe(III). A shift of 0.05 A˚ in the
d-spacing is observed due to intercalation of the Fe(III) ions in ␣-
ZrP·Fe [14]. The magnitude of shift in d-spacing depends on the
amount of loading of metal ions on the support. Even, no appre-
ciable modification in the d-spacing is observed due to lower Al
loading in ␣-ZrP·Al [32]. The value of d-spacing further increased
to 8.98 A˚ in ␣-ZrP·Fe(Salen). The shift of 1.52 A˚ in the d-spacing of ␣-
ZrP·Fe-(Salen) shows intercalation of Fe(Salen), while presence of
unshifted peak suggests that some traces of the ␣-ZrH P remained
2
without intercalation. Similar results are also reported [33]. The
fully loaded phase is isomorphous [34]. The value of the d-spacing
◦
did not change after eight catalytic runs, but a peak arises at 8.98
(
2ꢀ) due phase change of ␣-ZrH P. Similar observations are also
2
reported [17,33].
3.1.4. EDX and SEM analysis
The formation of ␣-ZrP·Fe(Salen) was confirmed by EDX analy-
sis. The EDX spectra of ␣-ZrH P, ␣-ZrP·Fe(III) and ␣-ZrP·Fe(Salen)
2
(
before catalysis) and ␣-ZrP·Fe(Salen) (after eight catalytic runs) are
shown in Fig. 2 and EDX measurements results are incorporated in
Table 2. The support, ␣-ZrH P contains oxygen (81.47%), zirconium
2
(
7.09%) and phosphorous (11.44%). The ␣-ZrP·Fe(III) contains oxy-
b
Fig. 1. XRD pattern of (A) ␣-ZrP, (B) ␣-ZrP·Fe(III), (C) ␣-ZrP·Fe(Salen) , b: before
a
gen (80.05%), zirconium (5.76%), phosphorous (10.30%) and iron
catalysis and (D) ␣-ZrP·Fe(Salen) , a: after catalysis.

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S. Khare, R. Chokhare / Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
b
Fig. 3. SEM images of ␣-ZrH2P, ␣-ZrFe(III)P, ␣-ZrP·Fe(Salen) , b: before catalysis
a
and ␣-ZrP·Fe(Salen) , a: after catalysis.
shows platelike structure with spherical particles, which clearly
indicates the presence of Salen on its surface. The SEM image of ␣-
ZrP·Fe(Salen) after eight catalytic cycle shows clear change in the
surface structure.
b
Fig. 2. EDX spectra of ␣-ZrP, ␣-ZrP·Fe(III), ␣-ZrP·Fe(Salen) , b: before catalysis and
a
␣
-ZrP·Fe(Salen) , a: after catalysis.
3.1.5. FTIR spectral studies
The FTIR spectra of ␣-ZrH P, ␣-ZrP·Fe(Salen) before cataly-
2
sis and ␣-ZrP·Fe(Salen) after catalysis are shown in Fig. 4. The

S. Khare, R. Chokhare / Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
87
Fig. 4. FTIR spectra of (A) ␣-ZrP, (B) ␣-ZrP·Fe(Salen) before catalysis and (C) ␣-
ZrP·Fe(Salen) after catalysis.
peaks at 3500 and 1625 cm−1 in the spectrum of ␣-ZrH P con-
2
firm the presence of external water in addition to the strongly
b
hydrogen-bonded OH or extremely strongly coordinated H O [35].
Fig. 5. Mössbauer spectra of (A) ␣-ZrP·Fe(Salen) , b: before catalysis and (B) ␣-
ZrP·Fe(Salen)a, a: after catalysis.
2
−
1
The bands appearing at 1044 cm are due to symmetrical stretch-
ing vibration of PO43− and at 599 cm
−1
are due to stretching
Table 3
Mössbauer parameters.
vibration of Zr–O [36]. Detailed band variation data of ␣-ZrH P,
2
␣
-ZrP·Fe(III), ␣-ZrP·Fe(Salen) are listed in Table 1. The band posi-
3−
tions of PO
in ␣-ZrP·Fe(III) and ␣-ZrP·Fe(Salen) were shifted
Sample
Isomer shift (ı)
mm/s)
Quadrupole splitting
(ꢂEQ) (mm/s)
4
(
to lower frequencies while that of Zr–O were shifted to higher
frequencies due to presence of Fe–O interaction and removal of
␣-ZrP·Fe-(Salen)b
0.38
0.33
0.81
0.71
a
Fe(III) mainly by inner-sphere complex formation with ␣-ZrH P.
␣-ZrP·Fe-(Salen)
2
The peaks associated with the CH –N bonds of the Salen com-
a
2
−
After catalysis.
Before catalysis.
1
b
plex are observed at 1700–1300 cm region of the IR spectrum of
-ZrP·Fe(Salen).
␣
3
.1.7. Electronic spectral studies
The electronic spectra of H Salen (ligand) and Fe(Salen) (com-
2
3
.1.6. Mössbauer spectroscopy
The Mössbauer spectroscopy of ␣-ZrP·Fe(Salen) was carried
plex) are shown in Fig. 6. The electronic spectral data of H Salen
2
and Fe(Salen) along with their probable assignments are given in
Table 4. Two bands observed at 255 and 316 nm in the electronic
out at 300 K to evaluate the electronic density and the coordi-
nation environment around the iron center in the catalyst. The
Mössbauer spectra of ␣-ZrP·Fe(Salen), before and after catalytic
reactions are presented in Fig. 5. The presence of only one sym-
metric doublet is clear evidence of the formation of a mononuclear
Fe(III) complex, which is in agreement with the electrochemi-
cal results [22]. The Mössbauer parameters of isomer shift (ı)
spectrum of H Salen are assigned to ␲ → ␲* and n → ␲* transi-
2
tions respectively. These bands were shifted to 231 and 320 nm
in Fe(Salen) indicating the coordination of ligand to metal ions.
Table 4
UV–visible data for ligand and complex in methanol.
and quadrupole splitting (ꢂE ) are incorporated in Table 3. The
Q
ı and ꢂEQ of ␣-ZrP·Fe(Salen) before catalytic reactions were
Compound
H2Salen
ꢁmax (nm)
Assignment
0
.38 and 0.81 mm/s respectively, indicating the presence of Fe(III)
255
316
231
␲–␲*
n–␲*
␲–␲*
n–␲*
Salen complex on ␣-ZrH P. The ı and ꢂE of ␣-ZrP·Fe(Salen)
2
Q
Fe(Salen)
after catalytic reactions were changed to 0.34 and 0.71 mm/s
respectively.
320

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S. Khare, R. Chokhare / Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
divided by initial area percentage (cyclohexene peak area from GC)
to get the response factor. The unreacted moles of cyclohexene,
remaining in the reaction mixture, were calculated by multiplying
the response factor by the area percentage of the GC peak for cyclo-
hexene, obtained after the reaction. The conversion and selectivity
were calculated according to literature [38] as follows:
Conversion (%) = (
initial mol%) − (final mol%)
× 100
initial mol%
GC peak area of products
GC peak area of all products
Selectivity of products =
× 100
Scheme 1 represents the formation of all these products, which
are well reported in literature [25,26,39,40].
The effects of various oxidants, dry TBHP concentration, sub-
strate concentration, catalyst concentration and the presence of
water were studied in detail to optimize the reaction conditions
for the maximum oxidation of cyclohexene.
3.2.1. Effect of various oxidants
Fig. 6. Electronic spectrum of (A) H2Salen and (B) Fe(Salen).
The effect of various oxidants, viz. H O , PhIO and TBHP on the
2
2
oxidation of cyclohexene, catalysed by ␣-ZrP·Fe(Salen) was studied
to develop an efficient catalytic system and the results are given in
Table 6. In each case, the reaction was carried out at 353 K in an
oil bath with same amount of oxidant (4 mmol) along with fixed
amount of cyclohexene (1.64 g, 20 mmol) and catalyst (0.066 g,
3
.2. Catalytic oxidation of cyclohexene
Oxidation of cyclohexene with ␣-ZrH P, Fe(Salen) and ␣-
2
ZrP·Fe(Salen) using dry TBHP as an oxidant was investigated
0
.1 mmol) in 10 ml of benzene. The water content was 70% in H O .
2 2
separately under similar experimental conditions. It was observed
It was observed that leaching of the catalyst occurred after15 min
in case of 30% H O . The other oxidants, PhIO and TBHP, effec-
that ␣-ZrH P was catalytically inactive in the oxidation of cyclo-
2
2
2
hexene. The catalyst Fe(Salen) was active for the oxidation of
cyclohexene in a homogeneous reaction. In the reaction cyclo-
hexene was oxidized to cyclohexene oxide, cyclohexenol and
cyclohexenone. The results are incorporated in Table 5. The het-
erogeneous catalyst was developed by intercalation of Fe(Salen)
tively oxidized cyclohexene to cyclohexene oxide, cyclohexenol
and cyclohexenone. The major product, in both the cases, was
cyclohexenone. The conversion of cyclohexene was 4.72% in case of
PhIO and the recovery of catalyst was also poor. The TBHP was used
in two ways, one with 30% water (70% TBHP) and the other with no
water (dry TBHP). In both the cases, the conversion of cyclohexene
was more than that of PhIO. The conversion of cyclohexene, after
on ␣-ZrH P. It was observed that ␣-ZrP·Fe(Salen) was active for
2
the oxidation of cyclohexene, which shows that the exchange of
+
[
H ] ions with Fe(III) ions converted the non-catalyst, ␣-ZrH P,
2
5
h of reaction time, was 12.70% and 18.04% in cases of 70% TBHP
into an active catalyst for the oxidation of cyclohexene. The results
of oxidation of cyclohexene ␣-ZrP·Fe(Salen) with TBHP as oxi-
dant are also shown in Table 5. In the cases of both Fe(Salen)
and ␣-ZrP·Fe(Salen), cyclohexene was oxidized to cyclohexene
oxide, cyclohexenol and cyclohexenone. The cyclohexenone was
the major product in both cases. The conversion of cyclohexene
was more in case of Fe(Salen) (25.99%) compared to ␣-ZrP·Fe(Salen)
and dry TBHP respectively. Hence the presence of water has detri-
mental effect on the oxidation of cyclohexene [8]. Thus the best
oxidant for our catalytic system is dry TBHP, which gave 18.04%
cyclohexene conversion.
3.2.2. Effect of dry TBHP concentration
The concentration of dry TBHP was varied to study its effect
(
18.04%) however recovery of the catalyst was not possible in the
on the oxidation of cyclohexene. In 10 ml of benzene, the amounts
of cyclohexene (1.64 g, 20 mmol) and catalyst (0.066 g, 0.1 mmol)
were kept fixed while concentration of dry TBHP was changed and
the reaction was carried out at 353 K in an oil bath. Four differ-
ent concentrations (2, 4, 6 and 8 mmol) of dry TBHP were used
and the results of the conversion of cyclohexene, consumption of
dry TBHP, efficiency of dry TBHP and product selectivity are pre-
sented in Table 7. Initially conversion of cyclohexene first increased
sharply from 8.9% for 2 mmol of the dry TBHP to 18.04% for 4 mmol
of the dry TBHP, which was the maximum conversion of cyclo-
hexene in our experimental conditions. Further increase in the
amount of the dry TBHP to 6 mmol and 8 mmol, the conversion
of cyclohexene decreased to 15.56% and 15.65% respectively. It is
case of Fe(Salen). The consumption of dry TBHP was determined
iodometrically after each catalytic reaction. The amount of dry
TBHP consumed and the efficiency of dry TBHP were calculated
according to reported procedure [37] as follows:
ꢀ
ꢁ
remaining TBHP
initial TBHP
TBHP consumed (%) = 1 −
× 100
× 100
mmol of products
mmol of TBHP consumed
TBHP efficiency (%) =
The conversion was calculated on the basis of molar percentage
of cyclohexene; the initial molar percentage of cyclohexene was
Table 5
Effect of support and various catalysts on the oxidation of cyclohexene and product selectivity.
Catalyst
Cyclohexene conversion (%)
Product selectivity (%)
Cyclohexene oxide
Cyclohexenol
Cyclohexenone
␣
-ZrH2P
Fe(Salen)
-ZrP·Fe(Salen)
–
–
6.38
6.18
–
–
25.99
18.04
19.87
19.20
73.75
74.62
␣

S. Khare, R. Chokhare / Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
89
Scheme 1.
Table 6
Effect of various oxidants on the oxidation of cyclohexene and product selectivity.
Oxidant
Time (h)
Cyclohexene conversion (%)
Product selectivity (%)
Cyclohexene
Cyclohexenol oxide
Cyclohexenone
PhIO
TBHP (70%)
TBHP (dry)
1
5
5
4.72
12.7
18.04
2.87
18.21
6.18
19.78
15.44
19.20
77.35
66.34
74.62
found that when the amount of dry TBHP was changed from 2 to
mmol, its efficiency increased from 9.78 to 22.01% while its con-
hexene was increased from 4 to 20 mmol, the efficiency of dry TBHP
increased from 5.13 to 22.01% while its consumption decreased
from 95.12 to 82.00%. Further increase in the amount of cyclohex-
ene to 25 mmol resulted in decreasing the efficiency of dry TBHP to
21.92% whereas its consumption decreased to 82.02%. The decrease
of the percentage conversion of cyclohexene and the efficiency of
dry TBHP at higher concentration of cyclohexene is possibly due to
non-availability of oxidant to the substrate. Thus, a large concentra-
tion of substrate with respect of oxidant may reduce the percentage
conversion of cyclohexene. The major product at all concentrations
of cyclohexene was cyclohexenone. Thus the best concentration of
cyclohexene to obtain the maximum conversion of cyclohexene
(18.04%) in 5 h reaction time is 20 mmol.
4
sumption decreased from 91.10 to 82.00%. When the amount of
dry TBHP was further increased to 6 and 8 mmol, then its efficiency
decreased to 18.43 and 18.55%, while its consumption decreased
to 84.44 and 84.35% respectively. This was probably due to higher
oxygen concentration produced by more oxidant inhibit the pro-
ceeding of reaction [41]. Thus, a higher concentration of oxidant
is not an essential condition to maximize oxidation of cyclohex-
ene. At all concentrations, cyclohexenone was the major product.
The selectivity of cyclohexenone increased from 40.60% (2 mmol)
to 74.62% (4 mmol), then decreased to 38.41–68.80% for 6–8 mmol
of dry TBHP. The selectivity of cyclohexenol does not show reg-
ular pattern first decreased from 41.16% for 2 mmol to 19.20% for
4
mmol of the dry TBHP, then increased to 41.37% for 6 mmol, again
3
.2.4. Effect of catalyst concentration
To study the effect of amount of catalyst on the oxidation of
decreased to 23.60% for 8 mmol of the dry TBHP. However, the
selectivity of cyclohexene oxide was maximum 20.22% at 6 mmol of
cyclohexene, the amount of ␣-ZrP·Fe(Salen) was varied from 0.05 to
0.30 mmol while the amounts of the cyclohexene (1.64 g, 20 mmol)
and dry TBHP (4 mmol) were fixed in 10 ml of benzene and the
reaction was carried out at 353 K in an oil bath. The results of the
study for four different concentrations of ␣-ZrP·Fe(Salen), viz. 0.05,
the dry TBHP. Similar observation is reported in case of [–CH {VO
2
(
sal-dach)·DMF}–]n/TBHP system [39]. Thus the best concentration
of dry TBHP to obtain the maximum conversion of cyclohexene
18.04%) in 5 h reaction time is 4 mmol.
(
0
.10, 0.20 and 0.30 mmol as a function of time are illustrated in
3
.2.3. Effect of substrate (cyclohexene) concentration
To study the effect of amount of cyclohexene on the oxidation
Fig. 7. It is evident in Fig. 7 that the conversion of cyclohexene
first increased sharply from 2.53% for 0.05 mmol of the catalyst
to 18.04% for 0.066 g (0.10 mmol) of the catalyst, which was the
maximum conversion of cyclohexene in our experimental condi-
tions. Further increase in the amount of the catalyst to 0.20 and
0.30 mmol, the conversion of cyclohexene decreased to 10.2 and
11.98% respectively. The conversions of cyclohexene, consump-
tion of dry TBHP, efficiency of dry TBHP and product selectivity
of various products are presented in Table 9. The increase in the
amount of catalyst from 0.05 to 0.10 mmol resulted in increase of
the efficiency of dry TBHP from 2.60 to 22.01% whereas its con-
sumption decreased from 97.47 to 82.00%. Further increase in the
amount of catalyst to 0.20 and 0.30 mmol resulted in decreas-
ing the efficiency of dry TBHP to 11.36 and 13.61% respectively,
of cyclohexene, the amount of cyclohexene (substrate) was var-
ied from 4 to 25 mmol while the amounts of the oxidant (4 mmol)
and catalyst (0.066 g, 0.1 mmol) were fixed in 10 ml of benzene and
the reaction was carried out at 353 K in an oil bath. The conver-
sion of cyclohexene, consumption of dry TBHP, efficiency of dry
TBHP and product selectivity for five different concentrations of
cyclohexene (4, 10, 15, 20 and 25 mmol) are presented in Table 8.
When the concentration of cyclohexene was increased from 4 to
2
0 mmol, the percentage conversion of cyclohexene also increased
from 4.88% to 18.04%. However a further increase of concentration
of cyclohexene to 25 mmol reduced the percentage conversion of
cyclohexene to 17.98%. It is found that when the amount of cyclo-
Table 7
Effect of TBHP concentration on the oxidation of cyclohexene and product selectivity.
Oxidant conc
mmol)
Cyclohexene
conversion (%)
Dry TBHP
consumed (%)
Dry TBHP
efficiency (%)
Product selectivity
(
Cyclohexene
oxide
Cyclohexenol
Cyclohexenone
2
4
6
8
8.9
91.10
82.00
84.44
84.35
9.77
22.01
18.43
18.55
18.24
6.18
20.22
13.60
41.16
19.20
41.37
23.60
40.60
74.62
38.41
62.80
18.04
15.56
15.65

9
0
S. Khare, R. Chokhare / Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
Table 8
Effect of substrate (cyclohexene) concentration on the oxidation of cyclohexene and product selectivity.
Substrate conc.
Cyclohexene
conversion (%)
Dry TBHP
consumption (%)
Dry TBHP
efficiency
Product selectivity (%)
Cyclohexene oxide
(
mmol)
Cyclohexenol
Cyclohexenone
4
4.88
11.10
16.24
18.04
17.98
95.12
88.90
83.76
82.00
82.02
5.13
12.49
19.39
22.01
21.92
6.53
19.94
24.28
6.18
72.04
31.57
33.36
19.20
19.19
21.43
48.56
42.36
74.62
74.27
1
1
2
2
0
5
0
5
6.19
Table 9
Effect of catalyst concentration on the oxidation of cyclohexene and product selectivity.
Catalyst conc
mmol)
Cyclohexene
conversion (%)
Dry TBHP
consumed (%)
Dry TBHP
efficiency (%)
Product selectivity (%)
Cyclohexene oxide
(
Cyclohexenol
Cyclohexenone
0.05
0.10
0.20
0.30
2.53
18.04
10.20
11.98
97.47
82.00
89.80
88.02
2.6
23.45
6.18
15.99
18.02
27.99
19.20
42.84
47.53
48.56
74.62
41.17
34.45
22.01
11.36
13.61
whereas its consumption changed to 89.80 and 88.02% respec-
tively. The reason for reduced activities at higher catalyst dose is
possibly due to adsorption/chemisorptions of two reactants on sep-
arate catalyst particles, thereby reducing the chance to interact
ferent temperatures, viz. 343, 353 and 363 K. The results are given
in Table 10 and Fig. 8. A comparison of cyclohexene conversion
(open points) with tert-butylhydroperoxide decomposition (filled
points) is shown in Fig. 8. The results demonstrate a fast non-
productive decomposition of dry TBHP. It must be noted that the
initial amount of dry TBHP is equal to olefin. It was observed that
initially both cyclohexene conversion and the decomposition of dry
TBHP were low at 343 K whereas at 353 K both reaction increases.
This non-productive decomposition of the dry TBHP is the rea-
son why the oxidation process is limited to 18.04% conversion of
cyclohexene. Further increase in temperature (363 K) causes addi-
tional thermal decomposition of dry TBHP and consequently leads
to lower cyclohexene conversion 4.36%. It is found that when the
temperature increased from 343 K to 353 K, the efficiency of dry
TBHP increased from 10.27% to 22.01% whereas its consumption
changed from 90.169% to 82.00%. Further increase in temperature
to 363 K resulted in decreasing the efficiency of dry TBHP to 4.56%,
while its consumption increased to 95.64% which also confirmed
the thermal decomposition of dry TBHP at higher temperatures.
Cyclohexenone was the major product at different reaction temper-
atures. The selectivity of cyclohexenone increased from 58.39% at
[
42]. Cyclohexenone was the major product at different concen-
trations of catalyst. The selectivity of cyclohexenone increased
from 48.56% (0.05 mmol) to 74.62% (0.10 mmol), then decreased
to 41.17–34.45% for 0.2–0.3 mmol of the catalyst. The selectivity
of cyclohexenol showed gradual increase from 27.99% (0.05 mmol)
to 47.53% (0.3 mmol) of the catalyst. However, the selectivity of
cyclohexene oxide was maximum 23.45% at 0.05 mmol of the cata-
lyst, and then reduced to 6.13% (0.1 mmol), to 15.99% (0.20 mmol)
and to 18.02% (0.30 mmol) of the catalyst. Similar observation is
reported in case of [VO (sal-dach)]-Y/H O2 system [43]. Thus the
2
best concentration of catalyst, ␣-ZrP·Fe(Salen), to obtain the max-
imum conversion of cyclohexene (18.04%) in 5 h reaction time is
0.1 mmol (0.066 g).
3.2.5. Effect of temperature
The effect of temperature on the oxidation of cyclohexene was
also studied. The reaction mixture, consisting of optimum values
of the catalyst (0.066 g, 0.1 mmol), cyclohexene (1.64 g, 20 mmol)
and dry TBHP (4mmol) in 10 ml benzene, was stirred at three dif-
3
43 K to 74.62% at 353 K, and then decreased to 19.77% at 363 K. The
selectivity of cyclohexenol showed gradual increase from 15.41%
343 K) to 38.41% (363 K). However, the selectivity of cyclohex-
(
Fig. 8. Effect of temperature on the conversion of cyclohexene and decomposition
of dry TBHP.
Fig. 7. Effect of catalyst concentration on the conversion of cyclohexene.

S. Khare, R. Chokhare / Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
91
Table 10
Effect of temperature on the oxidation of cyclohexene and product selectivity.
Temperature
K)
Cyclohexene
conversion (%)
Dry TBHP
consumed (%)
Dry TBHP
efficiency (%)
Product selectivity (%)
Cyclohexene oxide
(
Cyclohexenol
Cyclohexenone
343
353
363
9.31
18.04
4.36
90.69
82.00
95.64
10.27
22.01
4.56
26.20
6.18
41.83
15.41
19.20
38.40
58.39
74.62
19.77
Table 11
Recycling of the catalyst, ␣-ZrP·Fe(Salen) for the oxidation of cyclohexene.
ene oxide was maximum 41.83% at 363 K. Similar observation is
reported in case of Mn(III) and Mo(IV)-Salen complexes immobi-
lized on mesoporous silica gel in oxidation of cyclohexene with
TBHP system [44]. Thus the best reaction temperature to obtain
the maximum conversion of cyclohexene (18.04%) in 5 h reaction
time is 353 K.
No. of cycles
Conversion of cyclohexene (%)
1
2
3
4
5
6
7
8
(fresh)
12.55
12.55
12.40
11.96
11.84
11.72
11.66
10.04
3.2.6. Absorption spectral studies
The methanolic solution of [Fe(Salen)] complex was treated
with TBHP at room temperature and the reaction was monitored
by electronic absorption spectroscopy. Fig. 9 shows the spectral
changes upon addition of TBHP in the methanolic solution of
Fe(Salen). It is evident in Fig. 8 that the intensity of the 320 nm
the reaction mixture by simple filtration, washed with acetone and
dried at 383 K. Now, the catalyst was subjected to further catalytic
reaction under similar conditions. The catalyst, ␣-ZrP·Fe(Salen) was
recycled for eight cycles. Cyclohexene conversion dropped from
−
4
band increases on drop wise addition of TBHP to 10 M methano-
lic solution of Fe(Salen). At the same time the band at 259 nm
marginally shifts towards longer wavelength along with broad-
ening and increasing in band maximum. A significant change is
observed at the 230 nm band, where a large change in the width
of the band has taken place along with a gain in intensity. A weak
broad band appearing at 510 nm (inset) slowly decreases in inten-
sity. The band at 510 nm occurs due to d–d transition. The observed
spectral changes are similar to that observed for iron(III) Salen
complexes due to the generation of oxo(Salen)iron intermediate at
room temperature [45]. Distinct UV–visible spectral changes were
1
2.55 to 10.04% after eight (Table 11 and Fig. 10) and remained
identical in second cycle (12.55%). After second cycle cyclohex-
ene conversion gradually decreased within eight repeated cycles
with regeneration of catalyst. Conversion of cyclohexene reduced
by 0.7% from first to eighth cycle. This suggests that the catalyst was
stable during catalytic reaction and suitable for recycling. It was
observed that after eighth cycle the colour of the catalyst changed
from dark brown to light brown indicating the leaching of Fe(Salen).
The characterization of recycled catalyst by BET surface area, XRD,
EDX, SEM, FTIR, Mössbauer spectroscopy and atomic absorption
spectroscopy also suggest partial degradation of ␣-ZrP·Fe(Salen).
The results are given in respective tables and figures along with
fresh catalyst.
However, metal-leaching is observed in several catalytic
systems on the basis of the conversion/yield. Reduction in con-
version is observed in epoxidation of unfunctionalized olefins
catalysed by chiral Mn(III) Salen immobilized on zirconium
oligostyreneylphosphonate-phosphate due to metal leaching [47].
Loss in conversion is observed chiral Mn(III) Salen complexes
not observed in olefin epoxidation reaction by H O , t-BuOOH and
2
2
MCPBA at room temperature [46].
3.2.7. Recycle ability and heterogeneity of the reactions
The recycle ability of ␣-ZrP·Fe(Salen) was also tested for the oxi-
dation of cyclohexene. The reaction was carried out using catalyst
(
5
1 g), dry TBHP (3.57 g, 4 mmol) and benzene (10 ml) at 353 K for
h. After completion of reaction the catalyst was separated from
Fig. 9. Titration of Fe(Salen) with TBHP; the spectra were recorded after the succes-
−4
sive addition of one drop portion of TBHP to 10 ml of 10 M solution of Fe(Salen) in
methanol.
Fig. 10. Recycling of catalyst {␣-ZrP·Fe(Salen)} for the oxidation of cyclohexene.

9
2
S. Khare, R. Chokhare / Journal of Molecular Catalysis A: Chemical 344 (2011) 83–92
covalently bonded on modified MCM-41 and SBA-15 for enantio
selective epoxidation of nonfuntionalized alkenes [48]. Reduc-
tion in conversion is observed in oxidation of cyclohexane
over Fe(II)-Schiff base in a Zn–Al LDH host [49]. In aerobic
epoxidation of cyclohexene using encapsulated and anchored
alanine-salicylaldehyde Schiff base complexes by sol–gel method
reduction in conversion is observed [50].
In order to check leaching of metal ions from catalyst {␣-
ZrP·Fe(Salen)}into the reaction medium, a blank reaction was
carried out using catalyst (66 mg), dry TBHP (3.57 g,) and ben-
zene (10 ml) at 353 K for 5 h. The absence of iron (estimated using
atomic absorption spectroscopy) in the filtrate suggests no leaching
occurred during the catalytic reaction.
The heterogeneity of the reaction was tested by estimating the
iron contents in the filtrate after first cycle. The filtrate collected
after first cycle of cyclohexene oxidation was placed into the reac-
tion flask and the reaction was continued for next 5 h after adding
fresh oxidant. The gas chromatographic analysis showed no further
increase in the oxidation of cyclohexene. This observation suggests
that the catalyst is heterogeneous in nature.
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