Tuning of the reaction parameters to optimize allylic oxidation of cyclohexene catalyzed by zeolite-Y entrapped transition metal complexes

Article abstract of DOI:10.1016/j.molcata.2016.01.024

The synthetic protocols for entrapment of transition metal complexes reported here are to expand the diversity in catalysis made possible by the ability of microporous solid to select reactants, transition states, and products based on their molecular size. Herein, we report a synthetic route for the entrapment of transition metal complexes within the nanopores of zeolite-Y. The complexes of transition metals [M = Fe(II), VO(IV)] with Schiff base ligands that are synthesized by simple condensation of 2-hydroxyacetophenone and/or 2-hydroxy-5-chloroacetophenone with ethylenediamine have been entrapped within nanopores of zeolite-Y by flexible ligand method. These materials have been characterized by various physicochemical and spectroscopic techniques such as ICP-OES, FT-IR, ¹H and ¹³C NMR, elemental analyzes, and UV-vis electronic spectral studies, BET, TGA, scanning electron micrographs (SEMs), X-ray diffraction patterns (XRD), conductivity, magnetic susceptibilities as well as AAS. These synthesized catalysts have been utilized as heterogeneous catalysts for liquid phase oxidation of cyclohexene. The reaction parameters have been tuned to optimize higher cyclohexene conversion (%) along with higher selectivity towards the formation of corresponding allylic products. These catalysts were recovered and reused for three times without remarkable loss of activity. Moreover, the intermediate species involved during the catalytic oxidation reaction was synthesized and identified by FTIR and UV-vis spectroscopy.

Full text of DOI:10.1016/j.molcata.2016.01.024

Accepted Manuscript
Title: Tuning of the reaction parameters to optimize allylic
oxidation of cyclohexene catalyzed by zeolite-Y entrapped
transition metal complexes
Author: Dinesh R. Godhani Haresh D. Nakum Digvijaysinh
K. Parmar Jignasu P. Mehta Nisheeth C. Desai
PII:
S1381-1169(16)30024-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.01.024
MOLCAA 9756
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
11-1-2016
21-1-2016
21-1-2016
Please cite this article as: Dinesh R.Godhani, Haresh D.Nakum, Digvijaysinh
K.Parmar, Jignasu P.Mehta, Nisheeth C.Desai, Tuning of the reaction parameters
to optimize allylic oxidation of cyclohexene catalyzed by zeolite-Y entrapped
transition metal complexes, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.01.024
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Tuning of the reaction parameters to optimize allylic oxidation of
cyclohexene catalyzed by zeolite-Y entrapped transition metal
complexes
Dinesh R.Godhani*, Haresh D. Nakum, Digvijaysinh K.Parmar, Jignasu P. Mehta and Nisheeth
C. Desai
Department of Chemistry, (UGC NON-SAP & DST-FIST sponsored Department) Mahatma
Gandhi Campus, Maharaja Krishnakumarsinhji Bhavnagar University, Bhavnagar 364 002,
Gujarat, India

Graphical abstract
Highlights





We have synthesized VO(IV) and/or Fe(II) based neat and their entrapped complexes into
the nanopores of zeolite-Y.
The comparative spectroscopic, thermal, morphological and crystalline properties of
these complexes were achieved.
Their catalytic activities were tested over oxidation of cyclohexene reaction using 30%
H
2
2
O as oxidizing agent.
The reaction parameters were optimized for higher oxidation with higher selectivity
towards allylic products.
-1
Among all catalyst, [VO(hacen)]-Y catalyst shows higher catalytic (TOF, 2963h )
activity.

ABSTRACT
The synthetic protocols for entrapment of transition metal complexes reported here are to expand the
diversity in catalysis made possible by the ability of microporous solid to select reactants, transition
states, and products based on their molecular size. Herein, we report a synthetic route for the
entrapment of transition metal complexes within the nanopores of zeolite-Y. The complexes of
transition metals [M = Fe (II), VO (IV)] with Schiff base ligands that are synthesized by simple
condensation of 2-hydroxyacetophenone and/or 2-hydroxy-5-chloroacetophenone with ethylenediamine
have been entrapped within nanopores of zeolite-Y by flexible ligand method. These materials have
been characterized by various physicochemical and spectroscopic techniques such as ICP-OES, FT-IR,
1
13
H and C-NMR, elemental analyzes, and UV-Vis electronic spectral studies, BET, TGA, scanning
electron micrographs (SEMs), X-ray diffraction patterns (XRD), conductivity, magnetic susceptibilities
as well as AAS. These synthesized catalysts have been utilized as heterogeneous catalysts for liquid
phase oxidation of cyclohexene. The reaction parameters have been tuned to optimize higher
cyclohexene conversion (%) along with higher selectivity towards the formation of corresponding
allylic products. These catalysts were recovered and reused for three times without remarkable loss of
activity. Moreover, the intermediate species involved during the catalytic oxidation reaction was
synthesized and identified by FTIR and UV-Vis spectroscopy.
Keywords: Zeolite-Y entrapped complexes; Schiff base ligand; oxidation of cyclohexene;
optimization of reaction parameters

1
. Introduction
Allylic oxidation of olefins is a very chief transformation in synthetic organic chemistry
since the products obtained are valuable and resourceful commercial intermediates, and undergo
further reactions [1, 2].
Upon oxidation of cyclohexene, which has activated hydrogen at allylic position, it
undergoes annoying olefinic oxidation at the double bond in addition to allylic oxidation. It is
believed that due to an attack of oxidant at the allylic position as well as at the double bond
simultaneously, it lead to the formation two allylic products (2-cyclohexen-1-ol, 2-cyclohexen-1-
one) and two epoxidation products such as cyclohexene epoxide, and cyclohexane -1,2- diol [3-
5
]. It has been reported that the type of oxidant used in reaction decides the selectivity of the
product. Much better epoxidation occurs at the double bond if hydrogen peroxide, and/or NaOCl,
and/or KHSO and/or PhIO are used rather than using bulky tert-butylhydroperoxide (TBHP) as
5
an oxidant, which hinders the attack of oxidant at the double bond of cyclohexene [6-8].
Epoxidation of cyclohexene mainly leads to the formation of cyclohexene epoxide which further
transformed to cycloheaxane-1,2-diol in aqueous acidic conditions subsequently [9,10]. While
the use of TBHP as an oxidant facilitates the allylic oxidation which leads to the formation of 2-
cyclohexen-1-one and 2-cylohexen-1-ol [11-13]. Normally, allylic oxidation proceeds via a free
radical chain reaction pathway; it generally occurs when the intermediate metallic species are in
a low oxidation state [14]. While oxometallic species with higher oxidation state, such as O =
V
VIII
Mn , O = Ru
are more likely facilitate olefinic oxidation because these compounds are
capable of accepting electron pairs in vacant d-orbital and form stable complexes with organic
peroxides having more electrophilic peroxide oxygen atom which readily attacked by an olefinic
double bond [15, 16]. Most of the time both mechanisms may occur simultaneously, which leads
to the formation of the allylic product along with epoxide products, since tuning in various
reaction parameter, may facilitate either allylic oxidation or olefinic oxidation.
However, the homogeneous metal complexes are more active and selective; the catalyst
separation from the homogeneous liquid medium is usually troublesome. To tackle this problem,
nowadays numerous heterogenization methods of homogeneous catalytic systems are being used
to facilitate easy separation of the catalyst and the product(s) such as polymer anchoring [17],
polymerization of homogeneous catalyst itself [18], entrapment of metal complex on a solid

support like alumina, silica, or zeolites [19-21], and covalently anchoring to multi-wall carbon
nanotubes (MWNTs) [22]. The entrapment of transition metal complex into zeolite-Y is found to
be more suitable and ideal because the complex, once formed inside the nanopores of the zeolite-
Y, is too large to diffuse out and is not lost into the liquid phase during the reaction.
In present research work, afford have been made to maximize allylic oxidation over
olefinic oxidation of cyclohexene using green oxidant hydrogen peroxide (HP), which usually
promotes annoying olefinic oxidation. Using various catalysts of Vanadium (IV) and/or Iron (II)
complexes with H
2
hacen and/or H chacen type ligands and/or by tuning the various reaction
2
parameters such as effect of various solvent, ratio of substrate/oxidant, temperature, amount of
solvent, reaction time, pH, the amount of catalyst, have been studied to manage higher allylic
oxidation. The reaction mechanism was investigated with the all the four neat homogeneous
catalysts using Uv-Vis spectroscopy, which support the formation of dimeric metal peroxide
species intermediate responsible for oxidation. However, the intermediate is a very active species
at reaction temperature and participate in oxidation reactions by transferring one of its oxygen to
the cyclohexene.
2
. Experimental
2
.1 Materials
All reagents and solvents for the synthesis and analysis were commercially available and
used as received without further purifications. The compound 2-hydroxyacetophenone, 5-chloro-
2
7
-hydroxyacetophenone, and ethylenediamine were purchased from sigma Aldrich Ltd. (India).
0% t-butylhydroperoxide (TBHP), 30% H , VOSO4· 5H O and FeSO4· 7H O in extra pure
2
O
2
2
2
form were purchased from Rankem (India). The Sodium form of zeolite-Y (Si/Al = 5.62) was
procured from Hi-media, India.

2
.2 Physical methods and analysis
Many various physicochemical methods have been employed to characterize the structure of
Schiff base ligand and their neat and zeolite-Y entrapped complexes. The quantitative analysis of
Si, Al, Na and transition metal ions of zeolite-Y entrapped complexes was carried out by ICP-
OES using a model Perkin Elmer optima 2000 DV. Electronic spectra of Schiff base ligand, their
neat complex and VO(IV) and/or Fe(II) entrapped complexes were recorded on “SHIMADZU”
3
–3
UV-2450 spectrophotometer using a quartz cell of 1 cm optical path in 10 M methanol, and/or
-1
DMF and/or dilute HF solutions. FTIR (4000-400 cm ) of Schiff base ligands, and they're neat
and entrapped complexes were recorded with KBr on an FTIR-8400S Shimadzu.
Thermogravimetric (TG) analysis of ligand, their neat as well as entrapped complexes was
carried out in an air atmosphere in the temperature range 30-700 °C using Shimadzu TGA-50
Instrument. The crystallinity of compounds were ensured by powder XRD patterns using Bruker
AXS D Advance X-ray powder diffractometer with a CuKα (λ=1.54058) target and movable
8
detector, which scans the intensity of diffracted radiation within the range of 5° to 70° as a
function of the angle 2θ between the incident and diffracted beams. Scanning electron
micrographs (SEMs) of Na-Y, VO(II)-Y, and [VO(hacen)]-Y were carried out using an SEM
instrument (model-JSM-5610LV), JEOL to analyze the morphology of the samples. BET surface
area and pore volume of Na-Y, M-Y (where M= VO(IV), Fe(II)) and their entrapped complexes
were measured by a multipoint BET method using Micromeritics, ASAP 2010 surface area
analyzer. Prior to the BET measurements, the samples were de-gasified at 110 °C for 2 h to
remove any adsorbed gasses. The magnetic properties of the materials were measured with a
magnetic susceptibility balance of models Johnson Matthey and Sherwood. Atomic absorption
spectra (AAS) were recorded on a PerkinElmer 4100-1319 Spectrophotometer using a flame
approach after acid (HF) dissolution of known amounts of the zeolite material and SiO was also
2
determined by gravimetric analysis. The melting points of the both Schiff bases were determined
1
using a Griffin apparatus and were uncorrected. Proton nuclear magnetic ( H NMR) resonance
spectrum of both Schiff bases was carried out Varian- Gemini (200 MHZ) instrument using
CDCl as a solvent. All catalyzed reaction products were analyzed using GC–MS having a BP-5
3
capillary column (30m × 0.25mm × 0.25 m) 95% silicoxane surface and FID detector. The
identities of the products were confirmed by GC–MS model Shimadzu, QP-2012.

2
.3 Synthesis section
The synthesis method of the Schiff base, neat transition metal complex, and their respective
Zeolite-Y entrapped complex are displayed in scheme 1.
2
.3.1 Synthesis of Schiff base ligands
A dropwise addition of a methanolic solution (10 mL) of ethylenediamine (13 mmol, 0.844
mL) into methanolic solution (30 mL) of o-hydroxy acetophenone (26 mmol, 3.85 g) and/or 5-
chloro-2-hydroxyacetophenone (26 mmol, 4.50 g) leads to the formation of Schiff base ligand
instantly. The progress of the reaction was tested by using thin layer chromatography (TLC) with
the suitable solvent system. Once the reaction was completed (3h, 60 °C), the product was
filtered, thoroughly washed with methanol, and then dried well. The purified product, H hacen
2
and H
2
chacen were crystallized with chloroform and dioxane.
1
H
2
hacen: yellow crystalline, yield 90.90 %, M.P. 198 °C. H NMR (200 MHz, CDCl
3
): δ (ppm)
1
5.94 (2H, s, -OH), δ 2.45 (6H, s, -CH
3
), δ 3.94 (4H,s, -H
2
C•N), [6.92 (2H, d), 7.50 (2H, d), 6.81
) δ(ppm) 14.15 (-CH
13
(2H, t),7.35 (2H, t) for aromatic protons. C NMR (200 MHz, CDCl
3
3
group carbon), ~116.54, ~117.27, ~118.85, ~128.92, ~132.92, ~164.92 ppm corresponding to
carbons of the aromatic ring moiety, 175.43 ppm attributed to C=N, 50.27 ppm were assigned
N-CH
2
groups. Mass spectrum: The peak at m/z 296 (base peak), other fragments: m/z 161 and
1
07 were assigned to the loss of C ON and C O.
8
H
9
7
H
7
1
H
2
chacen: yellow amorphous solid, yield 94.93 %, M.P. 233-235 °C. H NMR (200 MHz,
): δ (ppm) 16.04 (2H, s, -OH), δ 2.49 (6H, s, -CH ), δ 4.34 (4H,s, -H C•N), [6.90 (2H, d),
.65 (2H, d), 7.81 (2H, s), for aromatic protons. C NMR (200 MHz, CDCl ) δ(ppm) 15.45 (-
group carbon), ~118.54, ~121.27, ~123.85, ~134.92, ~147.92, ~165.92 ppm corresponding
CDCl
3
3
2
1
3
7
3
CH
3
to carbons of the aromatic ring moiety, ~173.67 ppm attributed to C=N.~57.37 ppm were
assigned N-CH groups.
2

2
.3.2 Synthesis of VO(IV) and Fe(II) based neat transition metal complexes
The procedure for the preparation of VO(IV) and Fe(II) based neat complexes is as follows:
1
.13 mmol of Schiff base (H
2
hacen or H chacen) was dissolved in 15 mL acetone, and then
2
heated to boiling temperature, followed by the dropwise addition of aqueous solution of 1.13
mmol metal salt (VOSO ·5H O and/or FeSO · 7H O. The pH of the resulting solution was
adjusted to 5-6 by dropwise addition of CH COONa solution in reaction media. The resultant
4
2
4
2
3
reaction medium was stirred and refluxed for 4 hours. After cooling, the solid product was
separated by filtration and dried in vacuum for overnight at 60 °C.
2
.3.3 Synthesis of metal exchanged M-Y [M=Fe(II), VO(IV)]
Metal-exchanged M-Y was prepared by the usual ion-exchange method. As highly
concentrate (1 M) metal salt solution causes dealumination in the zeolite framework [23], lower
concentrate metal salt solution having a pH range between 4-4.5 were used. Herein, the reaction
medium was stirred at 90 °C for 24 h after addition of the 5.O g of pure Na-Y to the lower
concentrated metal salt (0.03 M metal salt, 300 ml, VOSO
4
· 5H
2
O and/or FeSO
4
· 7H O) solution
2
having pH 4-4.5. The slurry was filtered and washed with deionized water until the filtrate was
free from any free metal ion (filtrate were tested by AAS) on the surface of the zeolite, and dried
at 120°C for 12 h.
2
.3.4 Synthesis of zeolite-Y entrapped metal complexes
Zeolite-Y entrapped VO(IV) and Fe(II) complexes were prepared by taking 1.0 g of activated
M-Y [VO(IV), Fe(II)] successively mixed with an excessive amount of Schiff base ligand
hacen or H chacen) in chloroform (25 mL). It was then refluxed for 16 h with stirring in an
(
H
2
2
oil bath. The filtered solid was treated for Soxhlet extraction with chloroform, methanol, acetone,
and acetonitrile to remove uncomplexed ligand and the complex formed on the exterior surface
of metal exchanged zeolite-Y. The material then gently stirred for 6 h in an aqueous solution of
0
.01 M NaCl to allow the exchange of uncomplexed transition metal ions with sodium ions.

Subsequently, it was washed with deionized water until the filtrate was free from any chloride
ions present and then dried overnight at 140 °C.
2
.3.5 Catalytic oxidation of Cyclohexene
The catalytic oxidation of cyclohexene by utilizing synthesized catalyst was undertaken in a
two-necked 25 mL round bottomed flask. Initially, an arbitrary reaction conditions for the liquid-
phase oxidation of cyclohexene were chosen using lesser metal complex loaded entrapped
catalyst [VO(hacen)]-Y as a representative catalyst. Subsequently, various reaction parameters
were tuned to optimize conversion of cyclohexene along with higher selectivity toward the
formation of the allylic product. The progress of the reaction was scrutinized as a function of
time by withdrawing small aliquots after certain time intervals and analyzing them quantitatively
by gas chromatography technique.
3
. Results and Discussion
3
.1 Elemental Analysis
The elemental analysis data of both the Schiff base ligand and their respective neat transition
complexes are presented in Table 1. The results obtained are in good agreement with those
calculated from the suggested formulae and it reveals that each complex is formed in 1:1mole
ratio of metal ion and Schiff base ligand. Moreover, the molar conductance of the all synthesized
2
-1
neat complexes is found in a range of 1.00 – 5.00 S cm mol , which reveals that the all the neat
complexes are non-ionic in nature and hence has non-electrolytic nature [24]. As shown in Table
2
, the Si/Al ratios in VO (IV)-Y, Fe(II)-Y and zeolite- Y entrapped complexes were almost
similar as compared to pure Na-Y. This specifies no destruction in the zeolite framework has
occurred during ion exchange and entrapment of metal complexes into Zeolite-Y. The quantities
of transition metal ion in the all the four entrapped complexes are fewer compared to the metal
exchanged VO(IV)-Y, Fe(II)-Y zeolites. This reduction in the metal ion content in the entrapped
complexes can be attributed to leaching of some transition metal cation during complex
formation inside the nanopores of zeolite-Y. However, the metal to ligand ratio is found to be 1:1

in all neat complexes, the metal ion percentage obtained in zeolite-Y entrapped complexes is
slightly higher than that required for 1:1 complexes. Nevertheless, each entrapped catalyst was
+
treated with 0.01 M NaCl to replace the uncomplexed metal ion by Na , a small percentage of
uncomplexed transition metal ion still remains trapped in the cage of zeolite-Y that is found to be
leached out during (compare entry 5 with 8,9 in table 2) catalytic oxidation of cyclohexene.
3
.2 Brunauer, Emmett and Teller (BET) surface area analysis
The BET surface area, Langmuir Surface area, and pore volume data of pure Na-Y and modified
zeolite-Y are presented in table 3. The possibility of the destruction of the zeolite-Y framework
on ion-exchange can also be ruled out by the fact that only negligible reduction (13-15%) in the
surface area, Langmuir Surface area, and pore volume were found upon ion exchange of sodium
ion by Fe(II) and/or VO (IV) into the zeolite-Y framework. Furthermore, the data divulge that
the surface area, Langmuir surface area, and pore volume are much lower in the case of zeolite-Y
entrapped complexes than those of the corresponding metal exchanged zeolite-Y. This reduction
in surface area, Langmuir surface area, and pore volume can be attributed to the filling of the
zeolite-Y nanopores with transition metal complexes. This adequately indicates the existence of
metal complexes inside the nanopores rather than on the external surface of the zeolite-Y. Some
workers [25, 26] have reported such lowering in surface area on entrapment earlier.
The BET surface analysis data of the one time used catalyst [VO(hacen)]-Y for oxidation of
cyclohexene reveals further lowering in the surface area as compared to fresh catalyst (table 3
entry 5, 8). This might be due to loading of the organic substrate and the product inside the
nanopores of zeolite-Y entrapped complexes during the catalytic study. The BET of one time
used catalyst [VO(hacen)]-Y taken after dehydrated at various temperature, such as 350°C,
4
50°C (table 3 entry 9,10) have shown no significance change in surface area, Langmuir surface
area, and pore volume suggesting stability of entrapped complex up to 450 °C. Moreover, the
unexpected elevation in the surface area, Langmuir surface area, and pore volume of one time
used catalyst [VO(hacen)]-Y, which was dehydrated at 550 °C (table 3 entry 11) prior to BET
analysis were observed. This might be due to minor breakdown of entrapped metal complex at
5
50 °C within nanopores of zeolite-Y, which is further supported by TGA.

3
.3 X-ray Diffraction (XRD) Study
The powder XRD pattern of the Pure Na-Y, Fe(II)-Y, VO(IV)-Y and their respective entrapped
complexes are shown in Figure 1. The XRD patterns of zeolite-Y entrapped complexes are
essentially similar to those of pure Na-Y. Nevertheless, there are little changes observed in the
relative peak intensities of the 220 and 311 reflections at 10° and 12°, respectively. For pure Na-
Y, Fe(II)-Y, and VO(IV)-Y I220> I311, however for the entrapped complexes I311> I220. This
retraction in intensities has been empirically associated with the existence of a large metal
complex within the nanopores of zeolite-Y [27-29]. Moreover, no new crystalline pattern appears
in the XRD pattern of entrapped complexes, which confirm the preservation of crystalline and
the framework of zeolite-Y upon entrapment of the transition metal complexes inside nanopores
of zeolite-Y. The XRD pattern of one-time used (for oxidation of cyclohexene) entrapped
catalyst [VO(hacen)]-Y were also found to be exactly similar (Figure 1h) as XRD pattern of
fresh catalyst [VO(hacen)]-Y. This further signpost that framework of the zeolite-Y does not
suffer from any significant structural changes during catalytic study too.
3
.4 Scanning Electron Microscopy (SEM) Analysis
Generally, entrapment of metal complex into zeolite-Y is achieved using a flexible ligand (FL)
method. In this FL method, the excess amount of Schiff base ligand H hacen and/or H chacen,
2
2
which are sufficiently flexible to diffuse into the nanopores of zeolite-Y, were reacted with the
pre-exchanged metal ions (Fe(II)-Y and/or VO(IV)-Y) in the nanopores of zeolite-Y to achieve
the entrapped complexes. The filtered product may hold the unreacted ligand and the annoying
complex formed on the exterior surface of zeolite-Y as impurities. Consequently, these
impurities were removed by Soxhlet extraction using various solvents such as chloroform,
methanol, acetone, and acetonitrile in order to preclude an intrusion of impurities during the
catalytic study. SEMs of Na-Y, VO(IV)-Y, [VO(hacen)]-Y taken before Soxhlet extraction and
that of [VO(hacen)]-Y taken after Soxhlet extraction are shown in Figure 2a−d, respectively. As
shown in SEM of [VO(hacen)]-Y taken before Soxhlet extraction (Figure 2c), the unreacted
ligands and complexes formed on the exterior surface of zeolite-Y are discernable. In the SEM of
[VO(hacen)]-Y after soxhlet extraction (Figure 2d), no unreacted ligand and surface complexes
could be seen, which indicates the removal of the surface species during soxhlet extraction. The
particle boundaries on the exterior surface of zeolite-Y are clearly distinguishable that further

reveals a complete removal of extraneous complexes and unreacted ligand. Furthermore, no new
crystalline patterns were observed for the entrapped complexes due to their good distribution in
the lattice [30].
3
.5 Fourier Transform Infrared (FTIR) Spectroscopy
Infrared spectroscopy is a finer analytical tool to confirm the complexation of the transition
metal ions with Schiff base ligand [31, 32]. As shown in figure 3, while comparing the spectra of
-1
neat complexes with a respective ligand, ʋ(C=N) band for Schiff base found at ~ 1616 cm , which
-1
is shifted towards lower wave number (1602- 1580 cm ) in neat complexes. This provides an
evidence of complex formation via coordination of azomethyne nitrogen to a metal center. The
-1
weak (due to hydrogen bonding) broadband of ʋ(O-H) present at 3400-3300cm in both Schiff
base ligands disappeared in all the neat complexes suggesting coordination of hydroxyl oxygen
to a metal center via deprotonation. Moreover, the ʋ(C–O) band for all the neat complexes slightly
shifted towards lower wave number compared to that for the free Schiff base ligand, which may
be due to the coordination of phenolic oxygen to metal center [33]. A strong broadband in the
−1
−1
region 3440–3040 cm and a weaker band in the region 620–600 cm in neat complexes of iron
are ascribed to ʋ(O–H) stretching and wagging vibrations, respectively, which signposted the
existence of coordinated water molecules in neat complexes of iron [34]. Binding of metal ions
to the ligand through the nitrogen and oxygen atoms is further reinforced by the existence of new
bands in the region 570-520cm−1 and 460-420 cm due to the ʋ(M-O) and ʋ(M-N) modes,
respectively [35]. Both the neat complexes of vanadium exhibit a medium-sharp band around ~
−1
−
1
9
75 cm due to ʋ(V=O) stretch [36-38].
The FTIR spectra of Na-Y and the modified zeolite-Y (as shown in figure 3) are mainly
−1
−1
dominated by the strong zeolitic bands: a broadband in the range 3700–3300 cm and 1635 cm
due to hydroxyl groups on the surface and lattice water molecules present in zeolite-Y,
respectively. As shown in figure 3d-k, the pure Na–Y and modified zeolite-Y material exhibited
-1
characteristic bands nearly about ~461, ~680, ~1080 cm attributed to T-O (T= Si, Al) bending
mode, symmetric stretching, and antisymmetric vibrations respectively [39]. No shift or
broadening of these zeolite-Y vibrations were observed upon entrapment of the complexes,
which further divulges that the zeolite-Y framework left over unpretentious upon entrapment of
metal complex inside zeolite-Y [40-42]. The bands due to the entrapped metal complexes are

weaker (due to a low concentration of the complexes) and, therefore, could only be observed in
the region where the zeolite-Y framework does not absorb (as shown in figure 3f-k) i.e. from
−
1
1
4
600–1250 cm . The IR bands of entrapped complexes occur at frequencies shifted within 20–
0 cm−1 from those of their respective neat complex (see figure 3 inset). Furthermore, some
vicissitudes in band intensities not only confirm the presence of metal complexes in the zeolite-Y
but also propose that its structure is almost identical to that of the neat complex with slight
distortion in the metal complexes upon entrapment.
As shown in FTIR of entrapped complex, the bands appeared due to of water molecules
present in zeolite-Y veneer the weak bands of entrapped metal complex. To ensure this, the FTIR
spectrum of the solid samples under vacuum after dehydration is shown in Figure 4. The FTIR
spectra of the dehydrated material are found to be much extra strong and no additional peak
−1
appeared in the region of 1200−1600 cm (Figure 4). These results confirm the formation of
metal complexes inside the zeolite-Y.
3
.6 Electronic spectra and magnetic moments
As shown in figure 5a, the electronic spectrum of H
20, 284, 272, 256 nm. The first two lower-energy transition are due to n → π*, and the
remaining three high energy bands are due π → π* transitions occurring in aromatic rings. The
electronic spectra of H chacen show two bands at 401 and 332 nm due to n → π* transition that
occurs in azomethyne group. Moreover, H chacen ligand display four higher energy bands at
87, 277,263 and 258 nm attributed π → π* transitions.
2
hacen ligand shows five bands nearly at 390,
3
2
2
2
It is observed that the vanadium neat complexes of both the ligand give nearly analogous
patterns of electronic spectra in methanol. As shown in figure 5c and 5e, both the complex show
higher energy bands between 297 – 255 nm, attributed to ligand based π → π* transition. A blue
shift in the one of the higher energy n → π* transition of Schiff base was observed in their
corresponding vanadium neat complex, which indicate the chelation of the azo-methyne nitrogen
to vanadium. The appearance of the new intense LMCT transitions in both the vanadium
complex at 363-352 nm veiled the second less intense n → π* transition, that was clearly seen in
the uncomplexed ligand (390 in H
2
hacen, 401 in H chacen). Moreover, the electronic spectra of
2

both VO(IV) complex displayed low intensities d-d bands in the range of 869-840, 660-662, and
62-556 nm due to2b2g→2e , 2b2g→2a1gtransition, respectively, characteristic of a
5
g
, 2b2g→2b1
g
square pyramidal geometry around the vanadium ion [43].
In the Fe(II) complexes, four bands are observed virtually at 290, 284, 274, and 261nm due to π
→
π* transitions. In addition to these bands, more three bands are observed in both complexes.
The bands at 634, 362 and 314 nm in the case of [Fe(hacen)(OH
2
)
2
]· H
2
O and bands at 600, 316,
O are attributable to 5T
g → Eg transitions (d-
5
and 296nm in the case of [Fe(chacen)(OH ]· H
2
)
2
2
2
d transition), ligand to metal charge transfer transition (LMCT) and n → π* transition
respectively. The d-d transition is characteristic of the octahedral environment around the iron
[44]. The magnetic moment of neat complexes (table 1) of vanadium (IV) and iron (II)
complexes were found nearly at 1.61–1.72 and 5.5-5.7 B.M, respectively, which are the
characteristic values for mononuclear oxovanadium complex with square pyramidal geometry
and mononuclear iron complexes with octahedral geometry surrounded to the central metal ion,
respectively [45].
As shown in the figure 5g, pure dehydrated Na-Y shows one intense band at 306 nm and another
weak band at 345 nm. Both the transition occurs due to charge transfer for oxygen to aluminum
atoms of two different Al–O units present in zeolite-Y [44, 46]. The electronic spectrum of
dehydrated Fe(II)–Y shows (Figure 5i) a two bands at 321 and 490nm assigned to ligand to metal
g→π*)and weak d-d transition (5T
g → Eg), respectively,
5
charge transfer transition (t
2
2
characteristic of an octahedral configuration around to Fe(II) ions [44]. Additional two bands
observed at 341 nm and 306 nm are allied to the zeolite-Y structure. The electronic absorption
spectrum (figure 5h) of the dehydrated VO(IV)-Y exhibits six bands at778, 675, 554, 321, 306
and 342 nm. The last two higher energy bands attributed to charge transfer transitions (Al←O) in
zeolite-Y [46]. The bands observed at 324 is due to charge transfer transition instigated by the
transition from oxygen to VO(IV) ion. The first three lower energy bands are assigned to
2b2g→2a1g , b2g→2b1g, 2b2g→2e
2
, and are related to Penta-coordinated VO (IV) ion.
g
The UV-Vis spectrum of all dehydrated entrapped complex exhibits two bands closely at 306
and 343 nm assigned charge transfer transitions (Al←O) in zeolite-Y [44, 46].The absence of
dealumination during synthesis of each entrapped complex was amply confirmed the existence of
intense bands at 306 nm due charge transfer transition (Al←O) in framework units of zeolite-Y.

As shown in the figure 5k and 5l, the UV-Vis spectrum of both dehydrated VO(IV) entrapped
complex shows four higher energy bands closely at 299, 286, 271 and 258nm assigned to π →
π* transitions. The bands at 330-313, 554-550, 709-649,890-883 nm in both VO(IV) entrapped
complexes attributed to charge transfer transition (LMCT), 2b2g→2a1g
,
2b2g→2b1
,and
g
2b2g→2e
transitions (d-d), characteristic of the square pyramidal geometry of both the complexes.
g
The electronic spectrum of both the Fe(II) entrapped (Figure 5j, 5m) complex exhibits three
higher energy bands below 286 nm due to π → π* transitions. Moreover, both the entrapped
complex displays charge transfer transition (LMCT) nearly at 325-322 nm. The two d-d transition
instead of one above 575 nm was observed, this might be due to the distortion in octahedral
geometry around the iron (II) ion in zeolite-Y. While comparing electronic spectra of entrapped
complex with corresponding neat complex, shifting in the peaks to the lower energy side was
observed due to extra splitting of energy levels from the bary center under the influence of the
zeolite-Y framework.
3
.7 Thermal analysis
The TGA pattern of both the Schiff base H
ligands decompose in two steps. In the first step, estimated mass loss of 80.11and 76.56%were
obtained within the temperature range of 30-300 °C for H hacen and H chacen, respectively.
This first step weight loss may be attributed to major breakdown of the Schiff base ligand. In the
secondstep, remaining part of both Schiff base H hacen and H chacen with an estimated mass
loss of 19.89 and 23.44 % were lost due to complete decomposition of Schiff bases as CO, CO
NO, NO , etc. gasses, respectively.
2
hacen and H chacen (Figure 6a, 6d) reveals that these
2
2
2
2
2
2
,
2
The TG curve of [VO(hacen)] neat complex found to be (Figure 6b) thermally decomposed in
two steps. In [VO(hacen)], first step weight loss about 25.32 % corresponds to the initial
breakdown of the neat complex in the range of 27- 400 °C. The second step weight loss of 37.02
%
within the temperature range 401-700 °C is credited to the loss of the organic moiety from
[VO(hacen)] neat complex. No further weight loss beyond 550 °C were found in the complex,
which might be a due fraction of vanadium oxide and carbon residues(37.66 %) remained at the
end. Another neat complex [VO(chacen)] decomposes in only one step (Figure 6e) by mass loss
of 68.8 % between 27-700 °C owing to lose of the organic moiety and then a leftover fraction of

vanadium oxide and carbon residues (31.20 %).The thermogram of both the Fe(II) complex
signifies four stages of decomposition within the temperature range of 27-700 °C. In
[
Fe(hacen)(OH
2
)
2
]· H O neat complex, the first stage of decomposition(Figure 6c)at 30-120 °C
2
with weight loss of 4.99 %(cal., 4.45%) is corresponding to the loss of one water molecule of
hydration. The second step with a weight loss of 6.89 % (cal., 8.90%) at 121-220 °C is attributed
to the loss of two coordinated water molecules. The third step at 221- 520°C, with a weight loss
of 19.78% owing to the initial breakdown of the complex. The fourth step at 521 -700 °C with a
weight loss of 29.85% is referring to the elimination of organic moiety from complex and
leaving behind ferric oxide (39.52%) as residue. Neat complex [Fe(chacen)(OH
2
)
2
]· H O shows
2
an almost similar decomposition pattern (Figure 6f) as a former complex of Fe(II). The First and
second decomposition steps with a weight loss of 5.11 % (cal. = 3.18%) and 8.40 % (cal.,
7
.62%) attributed to the loss of one water molecule of hydration and two coordinated water
molecules at 27–120 °C, respectively. The third and fourth steps with a weight loss of 51.32%
at221-500°C is attributed to the elimination of organic moiety from complex and leaving behind
Fe
2
3
O as residue (35.01%).
The TGA profile of Na-Y, Fe(II)-Y, and VO(IV)-Y divulges (figure 6g, 6h, 6k) that the thermal
decomposition about 11.73, 9.87, and 10.06% were found in the first step at 27-120 °C,
respectively. These chief weight losses in these materials may be due the loss of free Intra zeolite
water molecules. Furthermore, on increasing the temperature from 120 to 500 °C, considerable
weight loss about5-6 percentage was also observed in each material due to loss of intra zeolite
water molecules hydrogen bonded to the oxygen of the zeolite-Y framework. No further weight
loss was observed in each material on increasing temperature from 500- 700 °C, leaving back
completely dehydrated zeolite-Y material without any damage in its framework. The TGA
pattern of zeolite-Y entrapped metal complexes are shown in the figure 6i, 6j, 6l, and 6m, for
[
Fe(hacen)(OH
2
)
2
]-Y, [Fe(chacen)(OH
2
2
) ]-Y, [VO(hacen)]-Y, and [VO(chacen)]-Y, respectively.
In addition, to the first and the second step weight loss in each zeolite-Y entrapped metal
complex due to the removal of intra zeolite water molecules, each zeolite-Y entrapped complex
shows one more decomposition step of about 2-4 % at 500-700 °C. The TG curve of dehydrated
[VO(hacen)]-Y shows (Figure 6n) negligible weight loss up to 500 °C and then shows
considerable weight loss due to loss of metal complex at 500-700 °C. A lesser weight percentage
loss in the third step can be attributed the presence of a small amount of metal complex

entrapped inside the nanopores of zeolite-Y [47]. The decomposition of zeolite-Y entrapped
complexes occurred at high temperature as compared to their respective neat complex, which
indicates the entrapment metal complex inside the nanopores of zeolite-Y increases their thermal
stability due to the shielding effect of zeolite-Y.
4
. Catalytic oxidation of cyclohexene
Generally, the oxidation of cyclohexene leads to the formation of 2-cyclohexen-1-ol, 2-
cyclohexen-1-one, cyclohexene epoxide, cyclohexane-1, 2-diol [11-13, 48-65]. Herein, reaction
conditions for the liquid-phase oxidation of cyclohexene were optimized to manage allylic
oxidation over olefinic oxidation of cyclohexene. To afford this, initially, lower metal
complexes loaded entrapped catalyst [VO(hacen)]-Y were taken as representative catalyst and
reaction condition were optimized one by one as below.
4
.1 Influences of various oxidants on cyclohexene oxidation
The effect of different oxidants such as 70% TBHP and/or 30% H
2
2
O on cyclohexene oxidation
was carried out by choosing an arbitrary reaction parameter as shown in Table 5. The data
presented in table 5 shows that use of hydrogen peroxide as an oxidant gives a higher conversion
of cyclohexene (TOF, 716.5) and less number of products than TBHP. Moreover, hydrogen
peroxide is a strong oxidant, often more effective, quite cheap, easy to handle and yield water as
a byproduct [66]. Hence, hydrogen peroxide is taken as an oxidant throughout for further the
catalytic study.
4
.2 Influences of various solvent on cyclohexene oxidation
In order to investigate suitable solvent with selectivity towards allylic oxidation products along
with the higher conversion of cyclohexene, we studied the oxidation of cyclohexene using
[
VO(hacen)]-Y as a catalyst and 30 % H
in table 6, the solvent with no ability to coordinate with the metal center and lower dielectric
constant such as CCl , chloroform, and n-hexane, lower conversion (1-7%) of cyclohexene with
2
O as an oxidant in eleven various solvents. As shown
2
4

higher yield (86-91%) of epoxidation products were obtained. In the polar solvents such as
acetonitrile, acetone, dimethyl sulfoxide, ethanol, methanol, dichloromethane-water and non-
polar dioxane with the ability to coordinate with metal center high allylic products were
obtained. Moreover, coordinating properties of the above solvents with metal center may have
produced some short of steric hindrance in the forming metalperoxo intermediate species that are
responsible for the epoxidation of cyclohexene, thus mainly allylic products were obtained.
Among all these solvent, acetonitrile was found to more appropriate solvent due to their higher
ability to yield allylic products (70%) along with considerable cyclohexene conversion (28.66%).
Therefore, acetonitrile were chosen for optimization of other reaction parameters in the further
catalytic study.
4
.3 Influences of various substrate/ oxidant ratios on cyclohexene oxidation
As shown in figure 7, the ratio of cyclohexene/H
cyclohexene conversion as well as on the selectivity of the products. The conversion of
cyclohexene increase as the cyclohexene/H ratio decrease from 1 to 0.5 with a reduction in
the selectivity of allylic products. A further decrease in cyclohexene/H ratio beyond 0.5
reduced conversion of the cyclohexene as well as the selectivity of allylic products. This may be
due to excess chemisorption of H rather than cyclohexene on the active sites of catalyst [67].
However, the decreasing of cyclohexene/H ratio from 1 to 0.5 upsurges the cyclohexene
conversion from 28.66 to 34.21%, selectivity of allylic products was reduced from 69.9 to
2.7%.Therefore, the cyclohexene/H ratio 1 has been kept as an optimal ratio for further
reaction parameter optimization.
2
O
2
had a shown remarkable influence on the
2
O
2
2
O
2
2
O
2
2
O
2
5
2
O
2
4
.4 Influences of reaction temperature on cyclohexene oxidation
To find out the effect of reaction temperature, oxidation of cyclohexene was carried out at
various temperatures such as 303, 313, 323,333,343, and 353 K. As shown in figure 8, elevation
in temperature from 303 to 353 K upsurges the conversion of cyclohexene from 5.6 to 44.38%.
Furthermore, the data reveals that lower temperature favors the formation of the cyclohexene
oxide (CyOx) and cyclohexene-1,2- diol (Cydiol) whereas higher temperature greatly forces the

reaction to proceed through allylic oxidation resulting in 2-cyclohexen-1-ol (CyOl), 2-
cyclohexen-1-one (CyOne) as major products. This may be due to less quantity metal peroxo
species formation at a higher temperature. Consequently, 353 K has been kept as an optimum
reaction temperature for further optimization of reaction parameters.
4
.5 Influences of reaction time on oxidation of cyclohexene
As shown in table 7, four reaction times 6, 12, 18, and 24 h were taken into consideration. The
presented data reveal that elevation in the reaction time from 6 to 18 h, drastically increase
-1
conversion of cyclohexene from 19 to 81.63 % and TOF/h from 92 to 1316 h , simultaneously.
However, raise in reaction time beyond 18 h upsurge the conversion of cyclohexene, extreme
lowering in the TOF was obtained. Thus, reaction time 18 h was taken as an optimal reaction
time for optimization of remaining reaction parameters.
4
.6 Influences of the solvent amount on oxidation of cyclohexene
The reactions carried out with various quantities of acetonitrile such as 3, 6, 9, and 12 mL
divulges that the solvent amount extremely affects the cyclohexene conversion and selectivity.
As displayed in figure 9, an increase in acetonitrile amount from 3 to 6 mL slightly increases the
cyclohexene conversion from 81.63 to 84.22%. A further increase for acetonitrile causes a large
decrease in cyclohexene conversion and selectivity of CyOne. The increase in the amount of
acetonitrile prevents the oxidation of formed CyOl and cyclohexene. This may be due to lower
concentration of cyclohexene and catalyst at the interface, which results in a decrease in
cyclohexene conversion and CyOne selectivity. Consequently, 6 ml of acetonitrile were taken as
an optimal amount for optimization of remaining reaction parameters.
4
.7 Influence of catalyst amount on oxidation of cyclohexene.
As shown in table 8, the oxidation of cyclohexene was carried out by using six amounts of
catalyst (12.5, 25, 37.5, 50, 62.5, and 75 mg) with reaction conditions optimized so far.
According to data presented in table 8, it was clear that the 12.5mg of catalyst the led highest

conversion (91.74%) of cyclohexene with a high yield of desired product CyOl and CyOne
97.75%).A gradual drop in cyclohexene conversion was observed with rise in the catalyst
amount from 12.5 to 75 mg. This may be attributed to the faster decomposition of H in the
(
2
O
2
presence of an excess catalyst [67]. Additionally, increasing the amount of catalyst decreased the
TOF of catalyst and the yield of desired product CyOl and CyOne with more Cydiol. Therefore,
1
2.5 mg of the catalysts was taken as optimal.
4
.8 Cyclohexene oxidation using various catalysts at optimized reaction condition
After optimization of the all reaction parameters (4.1 to 4.7), the catalytic oxidation of
cyclohexene was carried out using the neat complexes and their corresponding entrapped
complexes as catalysts and 30 % H
2
2
O as an oxidant to assess catalytic efficiency of each
catalyst. As presented in table 9, the oxidation of cyclohexene was found to be negligible in the
presence of alone Schiff base ligands, pure zeolite-Y, and M(II/IV)-Y, whereas the presence of
metal complexes potentially acquired higher conversion of cyclohexene, suggesting itself the
chief cause of higher catalytic activity.
As presented in table 9, the neat complexes of VO(IV) and Fe(II) have also shown good activity
and higher conversion of cyclohexene as compared to corresponding entrapped complexes. This
may be due to the presence of easy accessible catalytic site of the neat complexes for the
substrate and oxidant comparative to the deeply buried catalytic site (metal complexes) in the
corresponding zeolite-Y entrapped complexes. However, the neat complexes have shown the
higher conversion of cyclohexene in the first cycle, were destroyed during the first run due to the
continuous degradation of the catalyst, and could not have separated from the reaction medium.
The conversion (%) of cyclohexene catalyzed by neat complexes increases in the order:
[
VO(chacen)] < [Fe(chacen)(OH
2
)
2
]· H
2
O< [VO(hacen)] < [Fe(hacen)(OH
2
)
2
]·H O. As shown in
2
table 9, the same trend was observed for the decomposition rate of hydrogen peroxide utilizing
various neat complexes, which suggest a directly proportional relationship of cyclohexene
conversion with the decomposition rate of hydrogen peroxide.
Zeolite-Y entrapped metal complexes efficiently catalyzed the oxidation of cyclohexene with
-1
higher conversion, TOF (h ) and selectivity towards allylic products. The conversion (%) of
cyclohexene catalyzed by zeolite-Y entrapped complexes increases in the order: Na-Y< Fe(II)-Y

<
VO(IV)-Y <[Fe(chacen)(OH
2
)
2
]-Y < [Fe(hacen)(OH
2
2
) ]-Y <[VO(chacen)]-Y < [VO(hacen)]-
Y. However, zeolite-Y based materials containing higher metal ion (%) showed higher
decomposition rate of hydrogen peroxide, lower conversion of cyclohexene was obtained. This
may be due to higher metal ion concentration in zeolite-Y based material lowersthe effective
collision between cyclohexene, oxidant,and metal complex within the zeolite-Y framework,
whereas lower concentrations of metal complex effectively react with cyclohexene and oxidant,
resulting higher conversion.
It also can be concluded from the results presented in table 9 that the complex having electron
withdrawing group (chlorine) substituted at the para position to phenyl ring has shown lower
catalytic activity comparable to the complex having no substituent on the ligand. The presence of
two electron withdrawing chlorine substituents at para positions of the phenyl group in a
complex of H chacen Schiff base reduces the electron densities on the ligating atoms of the
2
ligand. It should be noted that the chlorine substituent reduces the electron density at the ortho
and para positions with respect to the substituent. Therefore, the ‘‘O” atoms in complexes of the
H
2
chacen Schiff base are likely to be more affected by chlorine groups, whereas in the case of a
complex of H hacen Schiff base the ‘‘O” atoms are the least affected one. Consequently, the
metal center of complex bearing H
bearing H
2
2
hacen is expected to be electron-richer than complexes
chacen and it may due to decrease in redox potentials of M(n+1)/M , which can easily
(n)
2
oxidize complex.
Moreover, in the case of the entrapped catalysts, detection of transition metal ion in the reaction
products by AAS indicates leaching of metal ion from the zeolite-Y framework in the first
catalytic cycle. It was startling to find that no metal ion leached after the second catalytic cycle.
Additionally, upon dropwise addition of H
2
2
O to the decanted liquid of reaction media, no
catalytic effect was observed. These facts signpost that the catalytic activities associated with
metal complex center entrapped within nanoporoes of zeolite-Y. Catalytic oxidation of
cyclohexene using varying homogeneous and/or heterogeneous systems by utilizing various
oxidants has been studied earlier by some other groups. As presented in table 10, our catalyst is
also being compared with the literate catalysts.

4
.9 Influences of acid and base as additives on conversion and selectivity at optimized condition
As shown in figure 10, change in the pH by the addition of 2.5 mmol CH COONH , HCl and
CH COOH in the reaction medium using [VO(hacen)]-Y as catalyst alters the selectivity and
3
4
3
conversion of cyclohexene. The addition of 2.5 mmol HCl or acetic acid to the reaction medium
decrease pH from 2 to 1, which obviously increase the conversion of cyclohexene from 91.74 to
9
7.72 %, but HCl lowers the selectivity of the allylic product whereas acetic acid sustained the
selectivity of allylic products. Likewise, the addition of 2.5mmol of ammonium acetate in the
reaction medium increases the pH from 2 to 3-6which accelerate decomposition of hydrogen
peroxide and increase conversion of cyclohexene up to 100%.
4
.10 Possible reaction pathway of catalysts
In order to comprehend the reaction pathway and intermediate species formed during oxidation
of the substrates, we examined the progress of the reaction using electronic absorption
−3
spectroscopy by treating the 10 M neat Fe(II) complexes in DMSO with a methanolic solution
of H . As shown in figure 11a and 11b slight increases in the intensities for the bands appeared
below 290 (omitted in the figure) and nearly at 306 nm while the intensity of the band closely at
55 and 340 nm decreased without changing their positions. The intensity of d–d band was
decreased and the band was gradually vanishing with the further addition of H . An isosbestic
2
O
2
7
2
O
2
point was found nearly at 308 nm in both Fe(II) complexes clearly indicate the transformation of
Fe(II) to Fe(III).
−3
As shown in Fig. 12a, the addition of a methanolic solution of H
2
O dropwise to a 10 M
2
methanolic solution of [VO(hacen)] ensued in a slight increase in the intensities for the bands at
appeared below 290(omitted in the figure) and at 308nm. While the intensity of the band at 840,
6
60, 556, and 352nm decreased without changing their positions. The d–d bands in [VO(hacen)]
at 840, 660, and 556, nm were decreased in intensity and the bands were vanishing with the
further addition of H . As shown in the figure 12b, the dropwise addition of methanolic H
to 10 M methanol solution of [VO(chacen)] increase the intensity of bands present below 290
omitted in the figure) and band at 310 nm. The d–d band in [VO(chacen)] at 869, 662 and 562
nm were decreased in intensity and the bands were wiped out upon excess addition of H .The
2
O
2
2
O
2
−3
(
2
O
2
vanishing of the d–d band and the appearance of the isosbestic points suggests the conversion of
V(IV) complex into aV(V) species [68].

During catalytic reaction using neat VO(IV) complex, at least, two types of intermediates having
vanadium-oxygen interaction viz. side-on OV(V)-O-O- VO (V) and OV–O–O–H (vanadium–
hydroperoxide) are possible. A separate reaction of 30 % hydrogen peroxide with stoichiometry
amount (1:2) of neat [VO(hacen)] (green) at 50 °C for 6 h leads to the formation of the solid
brown product. In the FTIR of brown (Figure 13) complex, the absence of ʋ(O-H) band clearly
disobeying the formation of the OV–O–O–H species during the reaction, whereas the presence
-1
of a strong band at 970 cm attributed to ʋ(V=O), which suggested no involvement of V=O bond
-1
in dimerization. Moreover, the appearance of the new band at 565 cm which can be attributed to
(V-O), clearly suggesting the dimerization of complex through -O-O- (peroxide), resulted in
ʋ
O=V-O-O-V=O intermediate species. The absence of d-d transitions in Uv-Vis spectrum of
intermediate species also supports the formation of peroxode species. In entrapped complexes,
due to restriction in dimerization executed by zeolite-Y nanopores, the formation of monomeric
metal–hydroperoxide in nanopores of zeolite-Y could not be excluded.
The merely tentative mechanism for the oxidation of cyclohexene catalyzed by [VO(hacen)]
complexes is shown in Scheme 2. The oxidation of cyclohexene could hypothetically happen via
two possible pathways, one is olefinic and other is allylic oxidation. In oxidation, the catalyst
interacts with H
2
2
O to form an active dimer O=V-O-O-V=O (II). The activated species II may
undergo homolytic cleavage at the reaction temperature (353 K) and undergo oxidative addition
to the C–C double bond of cyclohexene to give an intermediate (V). The intermediate (V)
undergoes migratory insertion to give cyclohexene epoxide (CyOx) formation through
intermediate (VI) and recover I. Furthermore, CyOx species react with H+/H
2
O to give Cydiol
(V). In the second pathway species(II) attacks at the allylic position of the cyclohexene results in
the formation of species (III) which finally form CyOl product. The further attack of species (II)
on the CyOl form species (IV) which undergoes further oxidation of CyOl and forms a CyOne
with removal of water [69] and regenerates the catalyst I.
Conclusion
The entrapment Fe (II) and VO (IV) Schiff-base complexes within the nanopores of zeolite-Y
using the flexible ligand method have been successfully demonstrated. Entrapment of complexes
was supported by various physicochemical (XRD, BET, SEM, TGA, ICP-OES) and spectral
studies (FTIR, UV) for the well-defined entrapment and distribution of complexes in the

nanopores of the zeolite framework. The catalytic activity of neat and entrapped VO(IV) and
Fe(II) complexes were performed over the oxidation of cyclohexene using 30% H as a clean
oxidant affording allylic products. The reaction parameters were optimized (25 mmol
cyclohexene, 25 mmol H , 12.5mg [VO(hacen)]-Y, 6 mL of acetonitrile, 353 K, 18 h) to get
2
O
2
2
O
2
maximum allylic oxidation products. It was found that the neat complexes are effective catalysts
comparable to their heterogeneous counterpart for oxidation of cyclohexene with
H
2
2
O .Moreover, effectivity of neat catalyst were found to be related to its capability to
decompose hydrogen peroxide. Entrapped complexes, nevertheless, can be recovered and reused
without loss of much catalytic activity, making them superior to their homogeneous counterpart.

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Figure Captions
Figure 1 XRD pattern of the fresh [a] Na-Y, [b] Fe(II)-Y, [c] VO(IV)-Y, [d] [Fe(hacen)(OH ) ]-
2
2
Y, [e] [Fe(chacen)(OH
2
2
) ]-Y, [f] [VO(hacen)]-Y, [g] [VO(chacen)]-Y, and [h] XRD of one time
used catalyst [VO(hacen)]-Y.
Figure 2 SE micrograph of (a) Na-Y, (b) VO(IV)-Y, (c) [VO(hacen)]-Y taken before Soxhlet
extraction and (d) [VO(hacen)]-Y taken after Soxhlet extraction.
Figure 3 FTIR spectra of [a] Schiff base H
2
hacen, Neat complexes [b] [Fe(hacen)(OH
c] [VO(hacen)], Zeolite-Y based material [d] Na-Y, [e] Fe(II)-Y, [f] VO(IV)-Y, [g]
Fe(hacen)(OH ]-Y, [h] [VO(hacen)]-Y, [i] one time used catalyst [VO(hacen)]-Y, [j]
2
)
2
]· H O,
2
[
[
[
[
2 2
)
Fe(chacen)(OH
2
)
)
2
2
]-Y, (k) [VO(chacen)]-Y, (l) Schiff base H
2
chacen, neat complexes (m)
Fe(chacen)(OH
2
]· H O, and (n) [VO(chacen)]. The inset of FTIR shows the peak values for
2
−1
Zeolite-Y based material in the region of 1800− 1000 cm .
Figure 4 FTIR spectra of dehydrated zeolite-Y based materials (a) Na-Y, (b) Fe(II)-Y, (c)
VO(IV)-Y, (d) [Fe(hacen)(OH ]-Y, (e) [VO(hacen)]-Y, (f) [Fe(chacen)(OH ]-Y, and (g)
VO(chacen)]-Y. The inset of FTIR shows the peak values for Zeolite-Y based material in the
2
)
2
)
2 2
[
−1
region of 1700− 1100 cm .
Figure 5 Electronic spectrum of the [a] H
Fe(hacen)(OH ]· H O, [e] [VO(chacen)], [f] [Fe(chacen)(OH
VO(IV)-Y, [i] Fe(II)-Y, [j] [Fe(hacen)(OH ]-Y, [k] [VO(chacen)]-Y, [l] [VO(hacen)]-Y, [m]
Fe(chacen)(OH ]-Y.
2
hacen, [b] H
2
chacen, [c] [VO(hacen)], [d]
[
2
)
2
2
2
)
2
]· H O, [g] Na-Y, [h]
2
)
2 2
[
)
2 2
Figure 6 TGA pattern of the [a] H
chacen, [e] [VO(chacen)], [f] [Fe(chacen)(OH
Fe(hacen)(OH ]-Y, [j] [Fe(chacen)(OH ]-Y, [k] VO(IV)-Y, [l] [VO(hacen)]-Y, [m]
VO(chacen)]-Y, [n] dehydrated [VO(hacen)]-Y.
2
hacen, [b] [VO(hacen)], [c] [Fe(hacen)(OH
2
)
2
]· H
2
O, [d]
H
2
2
)
2
]· H O, [g] Na-Y, [h] Fe(II)-Y, [i]
2
[
[
2
)
2
)
2 2
Figure 7 Influences of cyclohexene/H
2
2
O ratio on cyclohexene oxidation and selectivity of
products.