Heterogeneous catalysts hold the edge over homogeneous systems: Zeolite-Y encapsulated complexes for Baeyer-Villiger oxidation of cyclohexanone

Article abstract of DOI:10.1016/j.molcata.2016.05.016

A series of heterogeneous catalysts [M(pamp)]-Y (Where, M = V, Mn, Fe or Cu; pamp = 2,2′-((1E,1′E)-(1,2-phenylenebis(azanylylidene))bis(methanylylidene))diphenol) was prepared by encapsulation of metal complexes within zeolite-Y. Synthesized materials were characterized by chemical, BET and thermogravimetric (TG) analysis, X-ray diffraction (XRD), UV-vis and infrared spectroscopies, and scanning electron microscopy (SEM). Zeolite-Y encapsulated complexes were tested in Baeyer-Villiger (B-V) oxidation of cyclohexanone beside neat complexes to check the aptitude of heterogeneous catalysis over the homogeneous system. The effect of experimental variables viz., solvents, catalysts amount, the mole ratio of substrate to an oxidant, temperature and reaction time on substrate conversion was also tested. Under the optimized reaction conditions, [VO(pamp)]-Y was found to be a potential candidate by providing 1500 TONs in cyclohexanone oxidation, and the selectivity towards ?-caprolactone was 85.2%.

Full text of DOI:10.1016/j.molcata.2016.05.016

Accepted Manuscript
Title: Heterogeneous catalysts hold the edge over
homogeneous systems: Zeolite-Y encapsulated complexes for
Baeyer-Villiger oxidation of cyclohexanone
Author: Jignasu P. Mehta Digvijaysinh K. Parmar Dinesh R.
Godhani Haresh D. Nakum Nisheeth C. Desai
PII:
S1381-1169(16)30187-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.05.016
MOLCAA 9891
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
30-1-2016
10-5-2016
13-5-2016
Please cite this article as: Jignasu P.Mehta, Digvijaysinh K.Parmar, Dinesh
R.Godhani, Haresh D.Nakum, Nisheeth C.Desai, Heterogeneous catalysts hold the
edge over homogeneous systems: Zeolite-Y encapsulated complexes for Baeyer-
Villiger oxidation of cyclohexanone, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.05.016
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HIGHLIGHTS:
Heterogenization of M(II/IV) complexes within zeolite-Y by Flexible-Ligand method.
Characterization of synthesized materials with ICP-OES, BET, XRD, IR, UV-Vis, TGA.
B-V oxidation of cyclohexanone over neat and zeolite-Y encapsulated complexes.
The effect of reaction parameters on substrate conversion was also tested.
[VO(pamp)]-Y exhibited eventual performance by providing 1500 TONs.

GRAPHICAL ABSTRACT:

Heterogeneous catalysts hold the edge over homogeneous systems: Zeolite-Y
encapsulated complexes for Baeyer-Villiger oxidation of cyclohexanone
Jignasu P. Mehta*, Digvijaysinh K. Parmar, Dinesh R. Godhani, Haresh D. Nakum 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
ABSTRACT
A series of heterogeneous catalysts [M(pamp)]-Y (Where, M = V, Mn, Fe or Cu; pamp = 2,2'-
((1E,1'E)-(1,2-phenylenebis(azanylylidene))bis(methanylylidene))diphenol) was prepared by
encapsulation of metal complexes within zeolite-Y. Synthesized materials were
characterized by chemical, BET and thermogravimetric (TG) analysis, X-ray diffraction (XRD),
UV-vis and infrared spectroscopies, and scanning electron microscopy (SEM). Zeolite-Y
encapsulated complexes were tested in Baeyer-Villiger (B-V) oxidation of cyclohexanone
beside neat complexes to check the aptitude of heterogeneous catalysis over the
homogeneous system. The effect of experimental variables viz., solvents, catalysts amount,
the mole ratio of substrate to an oxidant, temperature and reaction time on substrate
conversion was also tested. Under the optimized reaction conditions, [VO(pamp)]-Y was
found to be a potential candidate by providing 1500 TONs in cyclohexanone oxidation, and
the selectivity towards ε-caprolactone was 85.2%.
Keywords: Heterogeneous catalysts; Zeolite-Y encapsulated complexes; Baeyer-Villiger
oxidation; Cyclohexanone
*Corresponding author: Jignasu P. Mehta
Corresponding Author’s address
Department of Chemistry, (UGC NON-SAP & DST-FIST sponsored department) Mahatma
Gandhi Campus, Maharaja Krishnakumarsinhji Bhavnagar University, Bhavnagar 364 002,
Gujarat, India
E-mail: jpm1569@yahoo.co.in (M) 09408706147

1. Introduction
The Baeyer-Villiger (B-V) oxidation, which could freely transform ketones to lactones
or esters, is of marked synthetic value and has been extensively used in various syntheses of
fine chemicals, pharmaceuticals and monomers for polymerization [1-7] Usually, B-V
oxidation with peracids, like trifluoroperacetic acid [8], perbenzoic acid [9] and m-chloro-
perbenzoic acid [10] eventually produce large amount of waste. In last few years, a wide
range of homogeneous metal complexes have been used for B-V oxidation [11-13] to
improve reaction rate and selectivity, since their acid-base and redox properties can be
tuned easily by simply changing ligand composition. However, homogeneous systems are
often regarded as ill-defined with many limitations, including the decomposition and
deactivation due to the formation of dimeric μ-oxo- and peroxo- bridged species, and
separation problem as well.
On the other end, heterogeneous systems lights up the catalysis field with sparkling
rewards as catalyst separation from the homogeneous systems is typically stroppy. Plentiful
efforts have been made to incorporate homogeneous Schiff base complexes on solid
supports, such as anchoring metal complexes in a polymeric matrix [14], sulfonic acid-
functionalized mesoporous SBA-15 [15], and encapsulation in zeolite cavities [16-19] have
also been reported. In specific, encapsulation of metal complexes in the nanopores of
zeolite-Y is an eye-catching eco-friendly technique for heterogenization; since no leaching is
perceived when the complex is confined entirely in the nanocavity of zeolite-Y [20-23]. The
as-prepared hybrid inorganic-organic material holds the number of heterogeneous catalysis
feature over homogeneous catalysts in this context viz., shape selectivity, site isolation,
better reactivity, withstand high temperatures, be prepared with the desired texture, be
recycled, and be reused [24-28]. It has been prominent that the space constraints forced by
the host framework can have a weighty impact on the geometry of the guest complex,
which can induce changes in magnetic, spectroscopic and redox properties of the
encapsulated complex, leading to a boosted activity and selectivity rather than its outlook in
homogeneous account [29-31]. Zeolite-Y encapsulated complexes have been shown to be
potential heterogeneous catalysts for numerous reaction processes viz., oxidation of alkanes
[32], alkenes [33,34], alcohols [35,36], olefins [37], and ketones [38], with certain oxidants,

viz., tert-butylhydroperoxide (TBHP) [34], hydrogen peroxide (H2O2) [35], and molecular
oxygen (O2) [39].
So, there is still an attention in catalytic systems able to perform B-V reaction in a
more environmentally friendly way, using innocuous oxidants, solvents, and reagents. The
use of heterogeneous catalyst and hydrogen peroxide allows a clean oxidation as the
catalyst can be easily removed and reused. Hydrogen peroxide is the ideal oxidant in zeolite
systems, as it is extremely mobile in the zeolitic pores due to a smaller size. Additionally, it is
inexpensive and environmentally-friendly to be used on a commercial scale. Although,
aerobic oxidation is likely to proceed at a slower rate due to the incompetence of molecular
oxygen to be initiated as compared to very reactive species like peroxides. However, zeolite-
Y encapsulated complexes have not been mainly focused on catalysts in the B-V oxidation
reaction, thus received only limited attention [38]. Therefore, with the purpose of choosing
a cleaner system for B-V oxidation, here, zeolite-Y encapsulated complexes were used as a
heterogeneous catalyst for the liquid-phase oxidation of cyclohexanone with 30% H2O2 as an
oxidizing agent.
2. Materials and Methods
2.1. Materials and physical measurements
The zeolite-Y (Si/Al
= 2.60) was purchased from Hi-media (India). 2-
hydroxybenzaldehyde, o-phenylenediamine, 30% hydrogen peroxide (H2O2) and
cyclohexanone were purchased from Aldrich. Transition metal salts and solvents were
purchased from Merck and were used as received.
To ensure the encapsulation of complex and preservation of the zeolite framework
after the formation of the metal complex inside the cavity of zeolite-Y, synthesized materials
were characterized by various physico-chemical techniques. ICP-OES (Model: PerkinElmer
optima 2000 DV) was carried out to determine the composition of Si, Al, Na, V, Mn, Fe, and
Cu metal ions. The elemental analysis (C, H, N) of the synthesized materials was carried out
on Perkin Elmer, USA 2400-II CHN analyzer. The specific surface area and pore volume were
measured by a multipoint BET method using ASAP 2010, micrometrics surface area analyzer.
The X-ray diffractograms of the solids were recorded on a Bruker AXS D8 Advance X-ray
powder diffractometer with a Cu Kα radiation as the incident beam to confirm the

crystallinity of zeolite-Y encapsulated complexes. The scanning electron micrographs of the
zeolite-Y encapsulated VO(IV) complex were recorded using a SEM instrument (Model: LEO
1430 VP). The sample materials were coated with a thin film of gold before recording the
SEM to shield the material surface from thermal damage by the electron beam. Infrared
spectra were recorded in the 4000-400 cm1 range on a Thermo Nicolet IR200 FT-IR
spectrometer in KBr. UV-vis spectra were recorded in the range of 200-800 nm on a UV-
1800 Spectrophotometer, Shimadzu. Thermogravimetric (TG) analysis was performed using
a thermal gravimetric analysis instrument (Shimadzu TGA-50H) with a flow rate of 20.0 mL
min1 and a heating rate of 10 °C min1. Products of the catalytic oxidation reactions were
identified using GC-MS having a BP-5 capillary column (30m × 0.25mm × 0.25 μm) 95%
silicoxane surface and FID detector. The reaction products were confirmed by GC-MS model
Shimadzu, QP-2012.
2.2. Synthesis of Schiff base ligand ‘pamp’
Schiff base ligand pamp (2,2'-((1E,1'E)-(1,2-phenylenebis(azanylylidene))bis(methan-
ylylidene))diphenol) was synthesized by stirring a mixture of 2-hydroxybenzaldehyde (2.10
mL, 0.02 mole) with o-phenylenediamine (1.08 g, 0.01 mole) in 50 mL of ethanolic medium.
The solid product was filtered and recrystallized from ethanol [40].
2.3. Synthesis of neat complexes [M(pamp)]
The solution of transition metal salts M(II/IV) (Where, M = VOSO4∙5H2O/ MnSO4∙H2O/
FeSO4∙7H2O/ CuSO4∙5H2O) of 0.01 mole in 25 mL of deionized water was added to a 25 mL of
an ethanolic solution of 0.01 mole ligand pamp, and the resulting solution was heated under
reflux with constant stirring at 80 ˚C for 4-5 h. The pH of the solution was adjusted 5-6 by
drop wise addition of an ethanolic solution of sodium acetate in it. The complexes were
precipitated and was separated by filtration.
2.4. Synthesis of metal exchanged zeolite-Y [M(II/IV)-Y]
An amount of 4.0 g of zeolite-Y was suspended in 200 mL deionized water containing
0.012 M transition metal salt M(II/IV). The mixture was then heated to 100 °C with constant
stirring for 24 h. The solid was filtered, washed with hot deionized water till the filtrate was
free from any metal ion content, and dried for 6 h at 150 °C in a muffle furnace.

2.5. Synthesis of zeolite-Y encapsulated complexes [M(pamp)]-Y
Flexible ligand (FL) method [41] was used to prepare zeolite-Y encapsulated
complexes. Typically a 1.0 g of activated metal exchanged zeolite-Y was added to an
excessive amount of ligand pamp in 1,4-dioxane (50 mL). The mixture was then heated for
24 h with constant stirring under reflux condition on an oil bath. The resulting solid was
treated for Soxhlet extraction with 1,4-dioxane, and acetonitrile to remove unreacted
ligand, as well as metal complexes adsorbed on the peripheral surface of the zeolite-Y. Then
the material was treated with aqueous 0.01 M NaCl to remove uncoordinated metal ions.
Subsequently, it was washed with deionized water to remove any chloride ions present, and
dried at 120 °C for 12 h. Chemical composition and IR stretching frequencies of ligand, neat
complexes and zeolite-Y encapsulated complexes are given in Tables 1 & 2.
2.6. B-V oxidation of cyclohexanone
The B-V oxidation of cyclohexanone was tested to perceive the effect of
heterogeneous catalysts over homogeneous stuff. The catalytic reactions were carried out
in a two-necked 50 mL round bottomed flask. Reaction conditions for the liquid-phase
oxidation of cyclohexanone were optimized as follows: cyclohexanone (1.03 mL, 0.01 mole),
30% H2O2 (2.06 mL, 0.02 mole) and catalyst (30 mg) were mixed in acetonitrile (5 mL). The
resulting mixture was then refluxed at 80 °C in an oil-bath for 12 h with continuous stirring.
After filtration and washing with a solvent, the filtrate was subjected to GC-MS to analyze
and identify the reaction products.
3. Results and Discussion
3.1 Synthesis of catalysts
Synthesis of metal complexes [M(pamp)] (M = VO(IV), Mn(II), Fe(II), and Cu(II))
encapsulated in the nanocavity of zeolite-Y involved the exchange of transition metal(II) ions
with sodium form of zeolite-Y followed by reaction of metal-exchanged zeolite-Y (M(II)-Y)
with ligand pamp, where ligand move into the nanocavity of zeolite-Y due to its flexible
nature and interacted with previously exchanged metal ions. It is expected that the
[M(pamp)] complexes will be formed in the nanocavity as well as on the surface of the
zeolite-Y. As the complex is neutral only a van der Waals interaction is active there to grip

the complex on the surface of the zeolite-Y. The resulted materials were subjected to
Soxhlet extraction in 1,4-dioxane to remove excess ligand that remained uncoordinated in
the nanocavities as well as situated on the exterior surface of the zeolite alongside free
[M(pamp)]. The uncomplexed M(II) ions from the zeolite was removed by exchanging back
[M(pamp)]-Y with aqueous 0.01 M NaCl solution. Therefore, presence of transition metal
assessed by elemental analysis is correspond to the encapsulated complex. The resulted
materials were further characterized by SEM, XRD, IR, UV-vis, and TGA. All these studies
further supported the encapsulation of [M(pamp)] inside the nanocavities of zeolite-Y. Neat
[M(pamp)] complexes have also been prepared for comparing its physico-chemical and
catalytic properties with the encapsulated one. The FL method (scheme 1) leads to the
encapsulation of VO(IV), Mn(II), Fe(II), and Cu(II) complexes of pamp ligand inside the
nanocavities of zeolite-Y.
3.2 Catalysts characterizations
3.2.1 The chemical analysis data
The analytical data of synthesized materials were performed and summarized in
Table 1. Neat complexes derived from ligand pamp are formed by coordination of metal and
ligand in 1:1 molar ratio. The chemical analysis of the zeolite-Y encapsulated complexes
reveals the presence of organic matter with a C/N ratio almost similar to that of neat
complexes. As well as, Si and Al contents in the zeolite-Y encapsulated complexes and
parent zeolite-Y are nearly in the same ratio. This specifies a slight variation in the zeolite
framework is not due to the dealumination during encapsulation. Moreover, the
concentration of metal ion in the encapsulated complexes is found to be reduced than the
metal exchanged zeoliteY, which leads to the fact of metal leaching during the
encapsulation process. Therefore, only small amount of complex could be there inside the
cavities of zeolite-Y.
3.2.2 Specific surface area and pore volume studies
Specific surface area and pore volume values measured by nitrogen adsorption
isotherms at relative pressures (p/p0). As tabulated in Table 3, the results of the surface area
and pore volume data showed a considerable decrease in zeolite-Y encapsulated complexes

than that of parent zeolite-Y, which is the sign to prove the internal location of encapsulated
complexes [42]. However, the zeolite framework is not affected by the encapsulation as
verified by the XRD pattern.
3.2.3 Powder X-ray diffraction study
The X-ray diffraction (XRD) patterns of synthesized materials were recorded to
ensure the encapsulation of complexes inside the cavities of zeolite-Y (see supplementary
data). The encapsulated complexes show similar peaks to those of zeolite Y; except for a
minor variation in the intensity of the peaks, no new crystalline pattern appears. These facts
verify that the framework and crystallinity of zeolite were unspoiled during the
encapsulation and that the complexes were well-defined in the cages. The peak intensities
of the 2 2 0 and 3 1 1 reflections have been supposed to be associated with the sites of
cations. In zeolite Y, the order of peak intensity is in the order: I 2 2 0 > I 3 1 1, while in M(II/IV)-
Y and [M(pamp)]-Y, the order of peak intensity became I 3 1 1 > I 2 2 0. This intensity reversal
has previously been identified and empirically associated with the existence of a large
complex within the cavity of zeolite-Y [43,44], which may be associated with the
rearrangement of randomly coordinated free cations during complex formation [45]. The
above observations clearly indicate the unspoiled crystallinity of the zeolite-Y framework
during the metal exchange and encapsulation process as well. These results also state that
the cavities are capable enough to keep these complexes without any loss in crystallinity of
the zeolite-Y.
3.2.4 Scanning electron microscopic (SEM) study
Basically, the formation of metal complexes was targeted inside the cavities of
zeolite-Y during the encapsulation process via FL method. However, formation of some of
the metal complexes can take place on the surface and some ligands molecules may remain
un-coordinated as well. As stated ahead, zeolite-Y encapsulated complexes were Soxhlet
extracted with 1,4-dioxane and acetonitrile to remove undesired molecules from the
external surface. To ensure the surface morphology of the zeolite-Y after encapsulation
process, SEM images of a representative [VO(pamp)-Y] catalyst were recorded. The SEM
image of [VO(pamp)]-Y taken before Soxhlet extraction display the deposition of complexes
on the peripheral surface. However, the absence of extraneous materials in the SEM

photographs after Soxhlet extraction specifies the complete removal of extraneous
complexes from peripheral surface, leading to the presence of well-defined encapsulation in
the cavity (see supplementary data) [46].
3.2.5 FTIR spectral study
To verify the presence of the complex in the zeolitic framework, the IR spectra of the
neat and encapsulated complexes have been compared (Table 2). A sharp band is detected
in zeolite-Y encapsulated complexes at ∼1070cm1, which is missing in the ligand pamp and
their neat complexes (Fig. 1). Therefore, it could be recognized to the asymmetric stretching
vibrations of (SiO4/AlO4) units [47]. The spectra of the neat and encapsulated complexes
were matched with that of the ligand pamp to define the coordination sites which involved
in chelation. The IR spectrum of the neat and encapsulated complexes shows a sharp band
at ∼1615cm−1; which can be recognized by the vibration band of the azomethine group
[48,49]. But the hybrid materials usually tend to absorb moisture, hence, this band in the
case of [M(pamp)]-Y may overlap with the HOH bending band appearing in this region.
The bending band of H2O can be eliminated upon careful drying of the zeolite material. The
[M(pamp)]-Y was heated in an air-oven at 120 ˚C for two hours and was immediately dried
in a vacuum desiccator, so that water cannot be absorbed. No band for zeolite-Y appears in
this region. So, the appearance of a band at ∼1615cm1 can be attributed to the azomethine
vibration bands and it can be predicted that metal complex is really present in the matrix of
zeolite-Y. In IR spectra of ligand, a sharp band is observed at ∼1315cm1 attributed to
aromatic -OH. However, in the case of neat and zeolite-Y encapsulated complexes, this band
is completely missing, which proves the involvement of aromatic OH in coordination with
metal. The absorption band in the ligand at 1350cm1 attributed to the phenolic oxygen
(CO) group was shifted to lower frequencies in all complexes. This indicates the
contribution of the phenolic oxygen and azomethine nitrogen in metal-ligand coordination
[50]. An important feature is the presence of new intense bands are observed in the far-IR
region (415-480cm1/403-414cm1) in zeolite-Y encapsulated complexes assigned to (MO)
and (MN), stretching vibrations, which confirms the coordination of oxygen and nitrogen
atoms of the ligand to the metal atom.

A broad band in the region 3430–3449cm1, and two weaker bands in the region
∼830cm1 and ∼675cm1 in all zeolite-Y encapsulated complexes are attributed to OH
stretching, rocking and wagging vibrations, respectively, of coordinated water molecules
[51]. The symmetric stretching and bending bands of the Al–O–Si framework of the zeolite
appears at ∼732cm−1 and 454cm−1 respectively, shows that the zeolite network remains
intact [52,53]. However, in the spectra of zeolite-Y encapsulated complexes, low
concentration of the complex within zeolite is responsible for the weak peak intensities [54].
IR data showed that the ligand is a tetradentate binegative ligand coordinating to the metal
ion through two azomethine nitrogen and two deprotonated oxygen atoms.
3.2.6 Electronic spectral study
The electronic spectra of ligand pamp and neat complexes were recorded in
methanol, while of zeolite-Y encapsulated complexes were taken in dilute HF solution. As
tabulated in Table 4, electronic spectra of the ligand pamp show three bands at 228, 288
and 331 nm and these are assigned due to ILCT (intra-ligand charge transfer transition),
π→π* transition in phenyl ring and n→π* transition of C=N chromophore, respectively. The
values of the former two bands are comparable to the values for the neat complexes.
Though, n→π* transition has undergone hypsochromic shifts and appeared at higher
wavelength viz., 401 nm in VO(IV), 532 nm in Mn(II), 363 nm in Fe(II) and 413 nm in Cu(II)
neat complexes resulting from the chelation of the ligand with the transition metal ions.
This value is exceptionally comparable to the values for the encapsulated complexes
observed at ∼471 nm.
The electronic spectra of neat VO(IV) complex consist three absorption bands in the
region of 237 nm, 314 nm, and 401 nm confirms square pyramidal structure [55]. Mn(II)
complex show intense absorption at 330 nm assigned to ligand centered transitions. The
weak transition observed at 532 nm may be referred to d–d transition. The spectroscopic
profile supports tetrahedral Mn2+ stereochemistry. The distorted tetrahedral geometry of
Fe(II) is strongly indicated by similarities in the visible spectra with those of known
tetrahedral complexes containing oxygen–nitrogen donor atoms [56]. The electronic spectra
of Cu(II) complex exhibit four bands at 306, 340, 413, and 566 nm. The band at 566 nm in
2
the electronic spectrum of the Cu(II) complex is indicative of a B1g→2A1g which is in
conformity to a square planar geometry [57]. All encapsulated complexes derived from

ligand pamp, possess basically similar spectral patterns with their respective neat
complexes, which confirms the presence of complexes in the cavity of zeolite-Y (see
supplementary data) [58].
3.2.7 Thermogravimetric (TG) analysis
The thermal behavior of the neat and zeolite-Y encapsulated complexes has been
characterized (see supplementary data) on the basis of a thermogravimetric (TG) method to
check the presence of water molecules in the coordination sphere of metal complexes and
the existence of metal complexes in the cavity of zeolite-Y as well. As shown in Table 5, TGA
profile of ligand pamp exhibits a mass loss of 36.41% at 140-160 °C, which may be
recognized to the liberation of the first part of the ligand. In the second stage, remaining
part loses within the temperature range 161-250 °C, with an estimated mass loss of 63.59%.
The thermal degradation of neat complexes takes place in two stages. The first
decomposition stage is in the temperature range of ∼30-500 °C, which is attributed to the
loss of ligand pamp (C20H16N2O2), with the observed mass loss of ∼70-90%. The final
residues were estimated as metal oxides. The thermal decomposition of zeolite-Y
encapsulated complexes usually undergoes in two stages. The removal of intrazeolite and
coordinated water molecules follows in the temperature range of ∼30-200 °C. The second
step starts above 400 °C and continues to 700 °C, which involves the weight loss of ∼8% due
to the decomposition of metal complexes. Less fraction of weight loss specifies the
existence of merely a small amount of metal complex within zeolitic voids.
3.3 Catalytic properties
With cyclohexanone as a typical substrate, the B-V oxidation was catalyzed by
zeolite-Y encapsulated complexes in the presence of 30% H2O2 as an oxidant. In a typical
reaction, 1:1 molar ratio of the cyclohexanone (0.01 mole) and 30% H2O2 (0.01 mole) were
taken with 3 mL of acetonitrile, 30 mg of the catalyst was added to it and equilibrated at 70
°C in an oil-bath with continuous stirring for 12 h. The filtrate was collected at different time
intervals and exposed to GC in order to identify the products.
The blank reactions performed over zeolite-Y under the same reaction conditions
showed minor conversion specifying that host alone without the metal complexes is
catalytically inactive. As shown in Fig. 2, neat complexes displayed some activity for B-V

oxidation of cyclohexanone, but both cyclohexanone conversion and ε-caprolactone
selectivity were much less than the respective encapsulated complexes under identical
conditions (Table 6). Additionally, heterogeneous catalyst [VO(pamp)]-Y showed higher
activity by providing 967 TONs, hence, it was chosen as a representative catalyst to check
the effect of experimental variables on cyclohexanone oxidation as described below.
3.3.1 Effect of solvents
In the present study, five different solvents were used with [VO(pamp)]-Y catalyst to
see their effect on B-V oxidation of cyclohexanone (Fig. 3). The conversion percent and
TONs obtained with different solvent decreases in the order, acetonitrile > 1,4-dioxane >
1,2-dichloroethane > methanol > n-butanol > chloroform. Acetonitrile shows higher TON
though it is difficult to describe that which goods of the solvent influenced the
cyclohexanone conversion. It may be possible that due to high polarity, it may readily
dissolve H2O2 beside cyclohexanone and subsequently can direct the reactants in such a way
that appropriately be adsorbed on the catalyst exterior and thereby boost the catalytic
activity [59]. It has also been used in other cases as the most appropriate solvent for similar
B-V oxidation reactions [60,61].
3.3.2 Effect of catalyst amount
The concentration of catalyst was crucial to the conversion of the heterogeneous
catalyzed B-V oxidation of cyclohexanone. Values of TON are given in Fig. 4 for typical
essays. Increasing the amount of [VO(pamp)]-Y catalyst results in higher TON, e.g. the TON
enhances from 961 to 1115 upon changing that amount from 10 to 25 mg. However, the
TON decreased by further raising the catalyst amount up to 30 mg. The increase in catalyst
amount above 10 mg leads to increase in amount of the radical formation, where the rate of
radical formation is higher compared to radical recombination. However, when amount of
catalyst is large (above 25 mg), the rate of radical recombination becomes higher in
comparison to radical formation. In this case, this change is observed when amount of
catalyst is 25 mg. Therefore, 25 mg catalyst was sufficient to carry out the catalytic
reactions.

3.3.3 Effect of molar ratio of substrate to oxidant
A series of reactions were carried out to investigate the consequences of the
substrate to oxidant molar ratios on the B-V oxidation of cyclohexanone. Four different
ratios of cyclohexanone to 30% H2O2 were used to investigate the corresponding variations
of the oxidation results (Fig. 5). We steadied the substrate value and changed the amount of
oxidant. Conversion of cyclohexanone was increased up to 37% with 1423 TONs upon
increasing molar ratio up to 1:2. Further, rising in molar ratio results in low conversion and
ε-caprolactone selectivity as well. It may be owing to the breakdown of H2O2 produces the
additional water, which dilutes the reaction mixture thus dropping the conversion rate.
Moreover, the presence of excess water may have also deactivated the catalyst. Hence, 1:2
molar ratio was chosen to be as optimized for B-V oxidation of cyclohexanone.
3.3.4 Effect of temperature with reaction time
To adjust the prime reaction temperature, catalytic B-V oxidation was examined at
different temperature (Fig. 6). It can be seen that cyclohexanone conversion increased with
a temperature rising and reached the top until 80 ˚C with 1500 TONs. However, the
conversion was decreased with a temperature above 80 ˚C, owing to the decomposition of
H2O2. Additionally, more than 12 h reaction time results in low selectivity of ε-caprolactone,
viz., 90, 89, 89, 85.2, 84, 79, 71, and 64 % for 3, 6, 9, 12, 15, 18, 21, and 24 h reaction time,
respectively (at 80 ˚C). It was due to the further hydrolysis of ε-caprolactone to hexanoic
acid [62,63]. It meant that the reaction temperature was one of influencing factor for B-V
oxidation of cyclohexanone, and the optimum temperature was 80 ˚C with 12 h reaction
time.
3.4 Stability of catalysts
Homogeneous metal complexes as catalysts are more prone due to the formation of
µ-oxo and µ-peroxo dimeric and other polymeric species, which is expected to be reduced
by encapsulating them in zeolites. After optimization of reaction conditions, a series of
reactions were performed to check the efficiency of [VO(pamp)]-Y catalyst on B-V oxidation
of cyclohexanone (Table 7). Fresh [VO(pamp)]-Y catalyst has shown 39% conversion of
cyclohexanone with 85.2% selectivity towards ɛ-caprolactone by providing 1500 TONs.

Furthermore, it was recycled and washed with acetonitrile after each run and dried at 120
˚C for the B-V oxidation of cyclohexanone with a view to establishing the effect of
encapsulation on thermal stability, catalytic activity, and reusability. Under the same
reaction conditions, the catalytic activity in successive runs has no apparent reduction as
compared to the fresh catalyst, providing 1466, 1458, and 1451 TONs for three consecutive
runs, respectively. Moreover, the atomic absorption spectroscopic analysis did not show any
vanadium metal content in the filtrate, which validates that vanadium is not leaching out
from the catalyst during B-V oxidation reaction. Hence, it is flawless to say that the porous
support prevents the complex from dimerization and decomposition during the reaction,
and makes it recyclable and reusable.
Catalytic B-V oxidation of cyclohexanone by an assortment of groups through varied
homogeneous and/or heterogeneous systems has been discussed. Q. Ma et al. (Table 8,
entry 1) reported B-V oxidation of cyclic ketones catalyzed by transition metal oxides viz.,
MoO3, WO3, TiO2, Fe2O3, Co3O4, ZnO and ZrO2 using aqueous hydrogen peroxide. About 92%
conversion of cyclohexanone was observed with MoO3, however, the selectivity of ε-
caprolactone was only 22% [64]. S. Baj et al. (Table 8, entry 2) carried out oxidation of
cyclohexanone over acidic ionic liquid [hmim][AlxCly] using (Me3SiO2)2 as oxidant results in
60% conversion with 55% yield of ε-caprolactone [11]. Fe(III) picolinate and Fe(III)
dipicolinate complexes were immobilized on to kaolinite furnished heterogeneous catalysts
by E.H. de Faria et al. (Table 8, entry 3). As prepared catalysts were effective in
cyclohexanone oxidation with 60% substrate conversion and 100% selectivity for ε-
caprolactone [65]. A. Corma (Table 8, entry 4) described a new heterogeneous catalyst, Sn-
MCM-41, providing 97% selectivity of ε-caprolactone. Though, conversion and TONs were
low [66]. B-V oxidation of cyclohexanone have been done by Z. Lei et al. (Table 8, entry 5)
with Sn-palygorskite catalyst in the presence of H2O2, affording 90-100% lactones or esters
selectivity. However, they also faced low conversion and TONs as well [67]. A.J.L. Pombeiro
and coworkers (Table 8, entry 6) tested [ReO3{ᴋ2HC(pz)3}(PTA)][ReO4] catalyst in the
oxidation of cyclohexanone in the presence of aqueous H2O2, allowing to achieve 38%
conversion and 26% ε-caprolactone selectivity [12]. Sn-doped hydrotalcite (Sn/HT) (Table 8,
entry 7) have been found to be an efficient catalyst for the liquid phase B-V oxidation of
cyclohexanone to the corresponding lactone by providing 119 TONs in the presence of H2O2

in acetonitrile solvent [68]. Z. Lei, et al. (Table 8, entry 8) have reported B-V oxidation of
cyclohexanone by Montmorillonite supported SnCl2 catalyst resulted in 100% conversion
with 100% ε-caprolactone selectivity though TONs was only 166 [69]. E.C.B.A. Alegria et al.
2
tested [ReCl2{Ƞ N,OC(O)Ph}(PPh3)2] as a homogeneous catalyst for the oxidation of
cyclohexanone (Table 8, entry 9) resulted in 57% conversion and 30% ε-caprolactone
selectivity [13]. B. Dutta et al. (Table 8, entry 10) obtained a good selectivity of ε-
caprolactone by using zeolite-Y immobilized complex Sn(salen)-NaY (1210 TONs) as a
heterogeneous catalyst [38].
3.5 Catalytic mechanism
As shown in Scheme 2, the probable mechanism for B-V oxidation of cyclohexanone
using [VO(pamp)]-Y catalyst in the presence of H2O2 oxidant is proposed. From UV-Vis
spectra and TGA data, it can be decided that [VO(pamp)]-Y (A in Scheme 2) catalyst has a
square pyramidal geometry in the zeolite framework. The ligand pamp may perhaps stay on
the coordination sites of the equatorial positions (parallel with respect to metal) while
oxygen atom settle in the axial positions (perpendicular with respect to metal). The first step
involves the activation of carbonyl group of the ketone by the coordination to the
Lewisacid vanadium (B in Scheme 1) [70,71], thereby increasing the nucleophilic attack of
H2O2 on the carbonyl carbon to give an intermediate (C in Scheme 2) alike to the Criegee
adduct, which rearranges to the ε-caprolactone (D in Scheme 2), and desorbed from the
zeolite-Y by a fresh cyclohexanone moiety. Even though the conversion was not too upright
and the selectivity for ε-caprolactone was bit poor as well, which was ascribed to zeolitic
acid sites favoring opening of the lactone ring, led to the formation of 6-
hydroxyhexanoicacid [72,73].
4. Conclusions
Encapsulation of metal complexes inside the zeolite-Y framework was successfully
done by the FL method as verified by various characterization techniques viz.,
chemical analysis, BET, UV-Vis, IR, powder XRD, SEM, and TGA.
B-V oxidation of cyclohexanone was catalyzed in both the homogeneous and
heterogeneous systems by using neat and zeolite-Y encapsulated complexes,
respectively.

Factors that affect the B-V oxidation (solvents, catalyst amount, the molar ratio of
substrate to oxidant, temperature, and reaction time) were also thoroughly checked
and the reaction conditions were improved as 0.01 mole cyclohexanone, 0.02 mole
30% H2O2, 25 mg [VO(pamp)]-Y, 3 mL acetonitrile, 80 °C, 12 h.
[VO(pamp)]-Y showed concluding performance by providing 1500 TONs (39%
cyclohexanone conversion; 85.2% ε-caprolactone selectivity) that will go up in the
innovative research.
Therefore, heterogeneous catalysis will be more than pleased with the plenty of
advantages over homogeneous as it is looked-for the handling, separation and
recycling abilities from an economical, technical and environmental point of view.
Acknowledgements
One of the authors Digvijaysinh K. Parmar is thankful to UGC, New Delhi for
providing partial financial assistant (JRF) for this work.

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Table 1.
Compound
Elements found (%)
C
-
H
-
N
-
M
-
C/N
Si
Al
Si/Al
Na-Y
-
17.16 6.60 2.60
16.84 6.47 2.60
16.30 6.25 2.60
16.62 6.39 2.60
16.13 6.21 2.60
VO(IV)-Y
-
-
-
5.86
5.74
6.85
6.24
-
Mn(II)-Y
-
-
-
-
Fe(II)-Y
-
-
-
-
Cu(II)-Y
-
-
-
-
pamp
75.91 (75.93) 5.03 (5.10) 8.85 (8.86)
-
8.57 (8.56)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
[VO(pamp)]
[Mn(pamp)]
[Fe(pamp)]
[Cu(pamp)]
[VO(pamp)]-Y
[Mn(pamp)]-Y
[Fe(pamp)]-Y
[Cu(pamp)]-Y
60.04 (63.00) 3.89 (3.70) 6.99 (7.35) 12.42 (13.36) 8.58 (8.57)
59.20 (65.05) 4.21 (3.82) 6.89 (7.59) 13.11 (14.88) 8.56 (8.57)
59.01 (64.89) 4.36 (3.81) 6.87 (7.57) 13.32 (15.09) 8.58 (8.57)
63.60 (63.57) 4.18 (3.73) 7.40 (7.41) 16.26 (16.82) 8.59 (8.57)
3.00
2.05
2.63
2.79
0.85
0.97
0.83
0.87
0.37
0.24
0.32
0.34
0.53
0.82
0.72
0.67
8.24
8.33
8.40
8.32
15.02 5.75 2.61
15.60 6.01 2.59
15.71 6.00 2.61
15.69 6.05 2.59
*% found (Calculated).

Table 2.
Compound
Internal vibrations External vibrations
(CN) (OH) (CO)
asym sym bend
sym
asym
D–R
T–O T–O T–O
T–O
T–O
pamp
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1635 3405 1350
1608 3056 1317
1614 3053 1276
1608 3059 1319
1608 3072 1328
[VO(pamp)]
[Mn(pamp)]
[Fe(pamp)]
[Cu(pamp)]
[VO(pamp)]-Y
[Mn(pamp)]-Y
[Fe(pamp)]-Y
[Cu(pamp)]-Y
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1072 678
1074 675
1072 680
1070 666
459
607 831
607 831
607 831
607 829
1174 1614 3471 1335
1176 1627 3439 1323
1172 1618 3445 1340
1174 1610 3500 1317
459
459
457

Table 3.
Compound
Specific surface Specific pore
area (m2/g)
volume (cm3/g)
Na-Y
630
0.320
V(IV)-Y
Mn(II)-Y
Fe(II)-Y
Cu(II)-Y
559
0.282
552
0.267
562
0.254
548
0.240
[VO(pamp)]-Y 358
[Mn(pamp)]-Y 365
0.162
0.170
[Fe(pamp)]-Y
342
0.135
[Cu(pamp)]-Y 332
0.131
Table 4.
Compound
pamp
Wavelength (nm)
228, 288, 331
[VO(pamp)]
[Mn(pamp)]
[Fe(pamp)]
[Cu(pamp)]
[VO(pamp)]-Y
[Mn(pamp)]-Y
[Fe(pamp)]-Y
[Cu(pamp)]-Y
237, 314, 401
288, 330, 532, 588, 623, 975
238, 263, 289, 363
306, 340, 413, 566
306, 395, 471, 501, 542, 968
276, 306, 473, 503, 540
270, 306, 314, 502, 542
256, 306, 471, 498, 543

Table 5.
Compound
TG range (°C) Observed mass Assignment
loss (%) (calc.)
pamp
140-160
161-250
30-520
521-700
30-630
631-70
30-480
481-700
30-550
551-700
50-200
401-700
50-200
401-700
50-200
401-700
50-200
401-700
37.10
62.90
83.86
3.22
Removal of C7H6ON part
Removal of C13H11NO part
Removal of ligand
[VO(pamp)]
[Mn(pamp)]
[Fe(pamp)]
[Cu(pamp)]
[VO(pamp)]-Y
[Mn(pamp)]-Y
[Fe(pamp)]-Y
[Cu(pamp)]-Y
V2O3 as residue
97.62
1.67
Removal of ligand
MnO2 as residue
79.65
19.61
78.21
11.58
11.29
8.07
Removal of ligand
Fe2O3 as residue
Removal of ligand
CuO as residue
Loss of crystallization H2O
Decomposition of complex
Loss of intrazeolite + coordinated H2O
Decomposition of complex
Loss of intrazeolite + coordinated H2O
Decomposition of complex
Loss of intrazeolite H2O
Decomposition of complex
14.57
6.91
14.26
7.24
13.81
8.26

Table 6.
Compound
Conversion TONb TOF
ɛ-caprolactone
(h−1)c selectivity (%)
(%)
Na-Y
1.3
-
-
-
VO(IV)-Y
4.3
-
-
25.0
17.3
24.5
27.0
60.2
Mn(II)-Y
3.5
-
-
Fe(II)-Y
3.9
-
-
Cu(II)-Y
4.0
-
-
[VO(pamp)]
[Mn(pamp)]
[Fe(pamp)]
[Cu(pamp)]
[VO(pamp)]-Y
[Mn(pamp)]-Y
[Fe(pamp)]-Y
[Cu(pamp)]-Y
17.2
9.1
24
13
14
20
967
424
500
693
2
1.08 41.3
1.16 58.0
1.66 63.0
80.58 88.0
35.33 73.0
41.66 85.0
57.75 90.0
10.0
15.4
30.2
19.0
19.3
21.9
a
Reaction conditions: 0.01 mole cyclohexanone, 0.01 mole 30% H2O2, 30 mg catalyst, 3 mL
acetonitrile, 70 °C, 12 h.
b TON: Turnover number (moles of cyclohexanone converted / mole of metal).
c TOF: Turnover frequency (moles of cyclohexanone converted / mole of metal  hour).

Table 7.
Catalyst
Conversion TONb TOF
(%)
(h−1)c selectivity (%)
Fresh [VO(pamp)]-Y 39.0
ɛ-caprolactone
1500 125
85.2d
1466 122.2 85.0
1458 121.5 84.7
1451 120.9 84.7
1st run
2nd run
3rd run
38.1
37.9
37.7
a Reaction conditions: 0.01 mole cyclohexanone, 0.02 mole 30% H2O2, 25 mg [VO(pamp)]-Y,
3 mL acetonitrile, 80 °C, 12 h.
b TON: Turnover number (moles of cyclohexanone converted / mole of metal).
c TOF: Turnover frequency (moles of cyclohexanone converted / mole of metal  hour).
d Remaining product is 6-hydroxyhexanoic acid.

Table 8.
Entry
Catalyst
Type of
catalytic system conversion (%)
Cyclohexanone TONa TOF ε-caprolactone
(h−1)b selectivity (%)
Reference
1
2
MoO3
Homogeneous
Homogeneous
92
60
-
-
22
60
[hmim][AlxCly]
-
-
55
11
3
Fe(Ka-pa)-3
Heterogeneous 60
Heterogeneous 36
Heterogeneous 16
-
-
100
97
62
4
Sn-MCM-41(9% SnO2)
Sn-palygorskite
12
7
63
5
26
1
90
64
[ReO3{ᴋ2HC(pz)3}(PTA)][ReO4]
6
Homogeneous
38
102
119
166
172
1210
1500
17
30
7
26
12
7
Sn-doped hydrotalcite
Heterogeneous 30
>95
100
30
66
8
Montmorillonite supported SnCl2 Heterogeneous 100
67
2
9
Homogeneous
57
29
50
125
13
[ReCl2{Ƞ N,OC(O)Ph}(PPh3)2]
Sn(salen)-NaY
10
11
Heterogeneous 75
Heterogeneous 39
90
38
[VO(pamp)]-Y
85.20
Present work
a TON: Turnover number (moles of cyclohexanone converted / mole of metal).
b TOF: Turnover frequency (moles of cyclohexanone converted / mole of metal  hour).

Fig. 1.