Development of a polymer embedded reusable heterogeneous oxovanadium(IV)catalyst for selective oxidation of aromatic alkanes and alkenes using green oxidant

Article abstract of DOI:10.1016/j.ica.2019.04.037

A new heterogeneous polymer supported solid phase oxovanadium(IV)catalyst was synthesized successfully. The designed catalyst furnished excellent results in the oxidation reactions of various aromatic alkanes, e.g. toluene, para-xylene, mesitylene. The polymer supported vanadium complex was proved also an efficient catalyst for the oxidation of aromatic alkenes, like substituted styrenes, trans-stilbene, etc. under mild reaction conditions. The supported catalyst was nicely elucidated by SEM-EDAX, TGA, FT-IR and UV–Vis spectral analysis. The catalytic activity was tested in the presence of an environment-friendly oxidant, 30% aqueous H₂O₂ during the oxidation of broad range of substrates. Another important fact is that the designed oxovanadium(IV)catalyst is heterogeneous in nature. Moreover, the newly synthesized oxovanadium(IV)complex exhibited a notable recoverability and it could be recycled up to six runs devoid of any prominent reduction in catalytic behavior.

Full text of DOI:10.1016/j.ica.2019.04.037

InorganicaChimicaActa492(2019)198–212
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Development of a polymer embedded reusable heterogeneous oxovanadium
(IV) catalyst for selective oxidation of aromatic alkanes and alkenes using
green oxidant
Priyanka Paula,b, Aniruddha Ghosha, Sauvik Chatterjeec, Apurba Berad, Seikh Mafiz Alamb,⁎
,
Sk. Manirul Islama,⁎
a Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, West Bengal, India
b Department of Chemistry, Aliah University, Newtown, Kolkata 700156, West Bengal, India
c Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032 India
d Department of Chemistry, University of Burdwan, Burdwan, West Bengal, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Oxidation
Aromatic alkanes
Oxovanadium(IV) catalyst
Aromatic alkenes
30% aqueous H2O2
A new heterogeneous polymer supported solid phase oxovanadium(IV) catalyst was synthesized successfully.
The designed catalyst furnished excellent results in the oxidation reactions of various aromatic alkanes, e.g.
toluene, para-xylene, mesitylene. The polymer supported vanadium complex was proved also an efficient cat-
alyst for the oxidation of aromatic alkenes, like substituted styrenes, trans-stilbene, etc. under mild reaction
conditions. The supported catalyst was nicely elucidated by SEM-EDAX, TGA, FT-IR and UV–Vis spectral ana-
lysis. The catalytic activity was tested in the presence of an environment-friendly oxidant, 30% aqueous H2O2
during the oxidation of broad range of substrates. Another important fact is that the designed oxovanadium(IV)
catalyst is heterogeneous in nature. Moreover, the newly synthesized oxovanadium(IV) complex exhibited a
notable recoverability and it could be recycled up to six runs devoid of any prominent reduction in catalytic
behavior.
1. Introduction
carboxylic acid compounds have enormous importance in a large
number of organic transformations [4]. Several metal based oxides like
The catalytic oxidation process contributes a major and funda-
mental area in the industry as well as in academic world. Catalytic
oxidation reaction of aromatic alkenes and sp3 CeH bonds of the aro-
matic alkanes to the corresponding oxygenated products is an inter-
esting and challenging part of investigation in the catalytic field [1].
The products isolated from the catalytic oxidation reactions of aromatic
alkanes (hydrocarbons) are the essential intermediates for many agro-
chemicals, chemical feedstocks, and polymers. Such products are uti-
lized as preliminary materials for fragrances and pharmaceuticals.
Usually alkyl (eCH3, eCH2CH3) substituted aromatic hydrocarbon
holds inactive sp3 CeH bonds. These hydrocarbons undergo oxidation
process to deliver some value added oxidized products [2]. The most
promising product is benzaldehyde, due to its amazing consumption in
manufacturing of preservatives, dyes, perfumes, plasticizers and phar-
maceuticals [3]. Aromatic alkenes like styrene, trans-stilbene when
subjected to catalytic oxidation process consequence to their corre-
sponding carbonyl compounds or carboxylic acids. Those carbonyl or
Ru(VIII)/Os(VIII) oxides are reported for this transformation [5]. But
this method encompasses several drawbacks from the viewpoints of
safety and chemical wastes. Hence an alternative and advanced ap-
proach is necessary for the catalytic system to the development of
sustainable industrial organic conversions. Environmentally secure and
greener processes are also desirable for economically feasible catalytic
systems [6]. The previously achieved oxidation reactions were reported
in presence of homogeneous metal catalysts, experiencing a major
drawback. After completion of the reaction the homogeneous catalyst
cannot be isolated from the reaction system. Further activity of the
costly catalysts in subsequent cycle is not possible accordingly. In this
state, one cannot ignore the possibility of resulting product con-
tamination with the catalyst, which limits their extensive purpose in the
industrial process. Thus the oxidation reactions of aromatic alkanes/
alkenes are favored in presence of polymer-supported heterogeneous
catalysts with reasonable thermal stability in contrast to their homo-
geneous analogues. In recent times, various solid supports, like zeolite
⁎ Corresponding authors.
E-mail addresses: mafizchem@gmail.com (S.M. Alam), manir65@rediffmail.com (S.M. Islam).
https://doi.org/10.1016/j.ica.2019.04.037
Received 9 September 2018; Received in revised form 18 April 2019; Accepted 18 April 2019
Availableonline20April2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

P. Paul, et al.
InorganicaChimicaActa492(2019)198–212
[7], alumina [8], amorphous silicates [9], polymers [10] and MCM-41
[11] are reported for the establishment of different heterogeneous
metal complexes. Amino-polystyrene is an effective heterogeneous
solid, is privileged to its good stability and reusuability [12]. Vanadium
(IV) ion is most effective one for the oxidation of hydrocarbons [13,14]
with most of the recognized oxidants: H2O2, tert-butyl hydroperoxide,
molecular oxygen [15–20] under heterogeneous phase. The important
fact for using vanadium instead of other metals is that it is a low cost
metal. Secondly it also carries more than one oxidation states suitable
for various organic electron-transfer reactions [21,22]. Vanadium
complexes having +4 or +5 oxidation states are established com-
pounds for successful employing in synthetic, biological and material
chemistry section [23,24]. Until now, many d-block transition metal
compounds of manganese [25], iron [26], nickel [27], copper [28] and
cobalt [7b] with polymer supported ligands are reported throughout
the heterogeneous catalytic liquid phase selective partial oxidation re-
actions of aromatic hydrocarbon CeH oxidation using different oxi-
dants. Polymer supported transition metal catalyst are proved very ef-
ficient for the partial and selective oxidation of aromatic hydrocarbons
to the corresponding aldehydes under mild reaction conditions
[25–28]. Among these metals, vanadium is preferred over manganese,
iron and copper on account of the corrosive nature of these metal ions.
From environmental concern vanadium is relatively non-toxic and 20th
most abundant element in the universe [29]. Several works on aromatic
alkane/alkene oxidations are typically available by homogeneous va-
nadium catalysts [30]. But a severe issue was found with these homo-
geneous catalysts since they could not be straightforwardly removed or
recycled. The aromatic alkane/alkene oxidations were performed with
heterogeneous vanadium catalyst by the oxidant, tert-butyl hydroper-
oxide accordingly [17,20]. But tert-butyl hydroperoxide is not an eco-
friendly oxidizing agent, thus hydrogen peroxide can be used as a mild
substitute for the present oxidation. From the perspective of green
amendment, hydrogen peroxide is not only an interesting 'green' and
cheaper oxidant, it is more reactive than molecular O2 [15]. Pertaining
to the liquid phase oxidation using H2O2, the VO(acac)2 catalyst was
very well known to furnish oxidized product benzaldehyde by partial
oxidation of toluene [31]. Rezaei et al. have also investigated the same
oxidized product from toluene by environmentally safe and economic
oxidant H2O2 in presence of vanadium phosphate catalyst [3]. The at-
tractive point of our system is the liquid phase partial oxidation of
substituted toluenes over polymer grafted PS-VO-naph catalyst to pro-
duce benzaldehydes with reasonable selectivity. Parallel report on the
liquid phase oxidation of styrene by H2O2 was also evaluated using Co-
Ag co-doped ZnO catalyst [32]. The Cr-ZSM-5 zeolite catalyzed selec-
tive oxidation of styrene to benzaldehyde was reported using H2O2 as
an efficient oxidant [33]. Benzaldehyde was formed with significant
selectivity by the liquid phase partial oxidation of styrene; as the
styrene oxide produced in the first step converted into benzaldehyde
through a nucleophilic attack of H2O2 on styrene oxide [33,34]. In the
present experiment the PS-VO-naph complex was successfully re-
cognized as heterogeneous and recyclable catalyst for the liquid phase
oxidation of styrene to benzaldehyde with satisfactory yield. The oxi-
dation process experienced with H2O2 liberates water as the only sus-
tainable by-product [27]. Acetonitrile has been utilized instead of the
solvents containing sulphur (e.g. DMSO) producing corrosions, serious
environmental hazards, and health issues [35]. Here, we have prepared
an efficacious heterogeneous catalyst, viz. a polystyrene grafted vana-
dium Schiff base complex, [PS-VO-naph], to oxidize a range of sub-
stituted aromatic hydrocarbons with 30% aqueous H2O2. The general
procedure, characterization and the overall experimental phenomena
have been illustrated briefly.
2. Experimental
2.1. Materials
Poly(styrene-co-divinyl benzene) 2% cross linked, (Art. No. 434442-
50G) was purchased from Aldrich, USA. 30% aqueous hydrogen per-
oxide was supplied by SRL. Other organic reagents were received from
Fluka or Merck. By following the standard procedures, necessary sol-
vents were perfectly dried and distilled before use. All other chemicals
were used for the reactions as taken without additional treatment for
refinement.
2.2. Characterization methods in solid state
FT-IR (Fourier transform infrared) spectra of the materials were
recorded using KBr disc in Perkin-Elmer FT-IR 783 spectrophotometer.
The UV–Vis spectra of the synthesized ligand and oxovanadium(IV)
catalyst were recorded in solid state with the help of a Shimadzu
doubled beam spectrophotometer (UV-2401 PC) attached with an in-
tegrating sphere in the diffuse reflectance mode by taking BaSO4 disk as
internal reference. The thermal analyzer Mettler-Toledo TGA-DTA 851e
under N2 flow was operated for the thermogravimetric (TGA) analysis
of the materials. Scanning electron microscope (SEM) (ZEISS EVO40,
England) having EDAX facility was used to compare the morphological
alteration of the supported ligand and catalyst. Transmission electron
microscopy (TEM) images were recorded with a JEOL JEM 2010
transmission electron microscope operating at 200 kV, and the neces-
sary samples were prepared by mounting an ethanol-dispersed sample
on a lacey carbon Formvar coated Cu grid. Powder X-ray diffraction
(PXRD) pattern of the materials was obtained by using a Bruker D8
Advance
X-ray
diffractometer
using
Ni-filtered
Cu
Kα
(λ = 0.15406 nm) radiation. Elemental analysis of the materials was
attempted with a Perkin Elmer 2400C elemental analyzer. The Varian
AA240 atomic absorption spectrometer was set to measure the vana-
dium metal content in oxovanadium catalyst. The oxidized products
were recognized over a Varian 3400 gas chromatograph (GC) fitted to a
30 m CP-SIL 8CB capillary column with a facility of flame ionization
detector.
2.3. Synthesis of amino polystyrene anchored azo ligand, PS-naph
The para-amino polystyrene (1) was prepared from 2% crosslinked
poly(styrene-co-divinyl-benzene) according to the available literature
sources [36]. Then the ligand was prepared by following the literature
procedure [37]. 1 g of para-amino polystyrene (1) was suspended in
20 mL methanol to which a 20 mL methanolic solution of 0.5 g of 1-
nitroso-2-naphthol was added upon constant stirring and it was re-
fluxed for 24 h. The green colored PS-naph ligand (2) was obtained
after the end of the reaction. The resultant mixture was set to filter and
washing was performed with methanol followed by desiccated under
vacuum.
2.4. Synthesis of polymer anchored oxovanadium(IV) catalyst, PS-VO-
naph
1 g of polymer anchored PS-naph ligand (2) was stirred with 0.5 g of
vanadyl acetylacetonate [38] in absolute ethanol (20 mL) at 70 °C for
24 h. After cooling it to nearly room temperature the complex was fil-
tered and washed systematically with ethanol, dioxane and methanol
successively to remove any unreacted metal ions. It was dried under
vacuum. Finally the polymer bound oxovanadium catalyst (3) was ob-
tained which was brown in color. The scheme for synthesizing polymer
bound oxovanadium catalyst, PS-VO-naph, is represented in Scheme 1.
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InorganicaChimicaActa492(2019)198–212
Table 1
Assignment of the chemical composition of polymer supported ligand and its
oxovanadium complex.
Compound
Colour
C%
H%
N%
Metal%
Nitrogen:Metal
PS-naph
PS-VO-naph
Green
Brown
76.14
66.73
4.31
4.05
11.28
7.43
–
6.58
4.11:1
thermo gravimetric analysis (TGA) together with elemental analysis,
powder XRD, TEM and surface area measurement.
3.1.1. Elemental analysis
The elemental analysis of the polymer grafted ligand and ox-
ovanadium(IV) catalyst are expressed in Table 1. It reveals that the
nitrogen to metal ratio in the oxovanadium(IV) catalyst is nearly
4.11:1. Atomic absorption spectroscopy (AAS) measures the 6.58 wt%
metal loading on the catalyst (PS-VO-naph) positively.
3.1.2. FT-IR spectroscopic studies
The FT-IR bands of polymer grafted ligand (PS-naph) and catalyst
(PS-VO-naph) are depicted in Fig. 1. In the polymer supported ligand,
the broad absorption band at 3412 cm−1 corroborates the vibrational
frequency of the naphtholic eOH existing in the material. The sharp
aromatic vibration of CeH bonds came out at 3024 and 2919 cm−1 for
the azo ligand. In the ligand, PS-naph, the characteristic peak around
1492–1452 cm−1 describes that azo (N]N) group is attached with the
naphthyl moiety [39]. The catalyst shows the N]N stretching at slight
lower frequency in the range 1491–1449 cm−1, which ascribes the
eN]Ne group bonded to vanadium metal site. In the ligand, the peak
at 1369 cm−1 directs the vibrational frequency of CeO stretching
(while not connected to vanadium) is shifted to 1364 cm−1 after
complex formation with vanadium in the PS-VO-naph. The polystyrene
embedded oxovanadium complex revealed a discrete peak near at
537 cm−1 signifying the V-N bond [40]. The corresponding frequencies
appeared at 461 cm−1 for VeO bond [40] and 965 cm−1 due to
stretching vibration of V]O bond respectively [41].
Scheme 1. Preparation of polystyrene anchored Oxovanadium(IV) catalyst.
2.5. Methods of oxidation reaction catalyzed by PS-VO-naph catalyst
2.5.1. For aromatic alkane oxidation
A mixture of 5.0 mmol of aromatic alkane, 30% aqueous H2O2
(15.0 mmol), acetonitrile (10 mL) and 30 mg of oxovanadium(IV) cat-
alyst was placed in a 100 mL two-necked round-bottomed flask. The
representative reaction mixture was stirred at 65 °C upto 6 h.
2.5.2. For aromatic alkene oxidation
3.1.3. UV–Visible spectroscopic investigations
Additionally another reaction mixture was set up by covering
5.0 mmol of aromatic alkene, 30% aqueous H2O2 (15.0 mmol), acet-
onitrile (10 mL) and 30 mg of oxovanadium(IV) catalyst in a 100 mL
two-necked round-bottomed flask with stirring at 60 °C for 6 h.
In both the experiments the oxidant, 30% aqueous H2O2 was added
dropwise maintaining a definite time interval. The Gas chromato-
graphic analysis was performed for each aromatic alkanes and alkenes
separately. GC technique was used to record the progress of oxidation
reaction by collecting the ultimate reaction mixtures at various time
interims. After the end of reaction, the mixture was cooled and filtered
to separate out the involved catalyst from the medium. Then the filtrate
was passed through anhydrous Na2SO4 to dehydrate the organic layer.
After that the remaining reaction mixture was analyzed through GC
technique using biphenyl for aromatic alkane oxidation and dodecane
for aromatic alkene oxidation reaction as internal standard respectively.
The corresponding oxidized products of aromatic alkanes and alkenes
were detected by comparing the chromatograms with the respective
authentic samples (Figs. S1–S11; supplementary file).
The absorption bands of the PS-naph ligand and its oxovanadium
complex have been recorded (Fig. 2). The characteristic absorption in
the range 295–348 nm is assigned to the π-π* (intramolecular charge
transfer) of the naphthalene ring and n-π* electronic transition for azo
group (eN]Ne) attached to the ligand (PS-naph). The spectral band
observed for azo group in the PS-naph ligand is shifted to higher in-
tensity band of 355 nm in the vanadium catalyst. Such shift in the ab-
sorption value represents an attachment of the vanadium metal to the
eN]Ne group of the polymer anchored ligand. The absorption bands
in the 368–389 nm region substantiates the n-π* electronic transition of
the non-bonding electrons residing on N-atom of the arylazo group
contained in oxovanadium catalyst. The appearance of the broad
shoulder close to 575–580 nm (Fig. 2, inset) corresponds to the weak d-
d electronic transition occurring in the vanadium containing complex.
The band appears in the UV absorption region near 360 nm attributed
to the LMCT from HOMO of PS-naph ligand to the LUMO of vanadium
metal exists in the catalyst (PS-VO-naph) [Fig. 2, inset] [42].
3.1.4. Microscopic analysis
3. Results and discussion
3.1.4.1. SEM analysis. The scanning electron microscopic (SEM)
pictures of the polymer attached azo ligand (PS-naph) (A) and its
oxovanadium complex (PS-VO-naph) (B) are given in Fig. 3. From the
recorded SEM images it was realized that structural alterations occurred
on the polystyrene beads. The SEM image corresponds to the polymer
bound azo ligand depicts the perfectly smoothening of its surface.
However the SEM structure of the metal bound catalyst represents the
attachment of some irregular particles agglomerated at the polymer
3.1. Characterization of polymer attached ligand and oxovanadium
complex
The polymer anchored ligand (PS-naph) and the oxovanadium cat-
alyst (PS-VO-naph) have been systematically analyzed with FT-IR and
UV–Visible spectroscopy, scanning electron microscopy (SEM) and
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InorganicaChimicaActa492(2019)198–212
Fig. 1. FT-IR bands of (A) polymer anchored ligand PS-naph and (B) PS-VO-naph catalyst.
confirmed by the lack of intense XRD peaks in the materials. This could
be attributed due to the non-crystalline or amorphous nature of the
materials. The contrast of the images is mass thickness contrast which
was altered sharply with a absolute change in morphology in the image
of PS-VO-naph catalyst [43]. The UHR-TEM image was taken for the
catalyst providing additional information about the overlapping of
some mass content on the polymeric matrix [Fig. 5C and D]. The EDAX
spectra confirm the presence of vanadium in the studied sample of PS-
VO-naph catalyst. The SAED pattern of polymer grafted PS-VO-naph
catalyst is represented in Fig. 5E.
3.1.5. Thermal analysis
The TGA profile diagram of the polymer attached azo ligand (PS-
naph) and its oxovanadium complex (PS-VO-naph) were marked at
12.00 °C/min temperature rate under N2 flow to recognize the thermal
steadiness of the materials. The TGA plot of the material is represented
with a range of 30–600 °C (Fig. 6). The polymer anchored azo ligand
dropped its weight in 275–435 °C whereas the oxovanadium complex at
280–450 °C. Hence from the comparison of stability of both the mate-
rials, it can be concluded that polymer bound metal catalyst possesses
slightly higher thermal stability than the corresponding ligand only.
From the differential thermal analysis (DTA) of the materials, PS-naph
and PS-VO-naph (Fig. 6), it can be concluded that both the materials
were decomposed through an endothermic reaction approach.
Fig. 2. UV–Vis spectra of (A) polymer anchored ligand PS-naph and (B) PS-VO-
naph catalyst.
surface. This creates a roughening of the smooth polymeric matrix.
Energy dispersive spectroscopic exploration of X-rays (EDAX) for
the polymer anchored catalyst clearly shows the presence of vanadium
metal content as observed in the spectra (Fig. 4). It is quite rational to
realize the progress of the polymer anchored vanadium complex for-
mation with the azo ligand from the following EDAX spectral analysis.
The combination of SEM-EDAX analysis supports to confirm the at-
tachment of vanadium metal into the polymer tagged azo ligand. This
also suggests the loading of vanadium metal on the polymer surface.
3.1.6. Powder X-ray diffraction analysis
The wide angle powder X-ray diffraction patterns of the polymer
bound PS-naph ligand, PS-VO-naph catalyst and the reused catalyst are
represented in Fig. 7. The characteristic XRD patterns of these materials
are almost identical in nature. The XRD pattern displays a broad band
for 2θ in the range of 15-25°. Appearance of no sharp distinct peak in
the wide angle PXRD patterns symbolizes the non crystalline nature of
the PS-VO-naph catalyst as well as reused catalyst. This feature in-
dicates the amorphous character, i.e., there was no crystalline structural
feature visualized in the prepared materials [44].
3.1.4.2. TEM analysis. The ultra high resolution transmission electron
microscopy (UHR-TEM) images of polymer grafted PS-naph ligand and
PS-VO-naph catalyst are demonstrated in Fig. As observed from Fig. 5A
and B for the ligand and Fig. 5C and 5D for the catalyst, the lattice
fringes are absent in the bright field TEM images which suggests that
both of the ligand and catalyst are amorphous in nature. This fact is also
3.1.7. Surface area measurement
3.1.7.1. BET analysis of catalyst. The characteristic and precise N2
sorption analysis was executed to determine the specific surface area
of PS-VO-naph catalyst at 77 K temperature in the presence of nitrogen
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InorganicaChimicaActa492(2019)198–212
Fig. 3. SEM images of (A) polymer anchored ligand PS-naph and (B) PS-VO-naph catalyst.
gas as adsorbate molecule after outgassing the sample for 12 h at 413 K.
The N2 adsorption/desorption isotherm of PS-VO-naph sample is shown
in Fig. 8. The BET (Brunauer-Emmett-Teller) surface area of PS-VO-
naph material was 25.4 m2 g−1, as determined with the help of the plot.
The pore size distribution curve has been drawn with the help of NLDFT
(non-local distribution functional theory) method and is represented in
Fig. B. The De Boer statistical thickness (t-plot) illustrates that the total
the substrate were obtained as 34% and 59% respectively. Whereas the
benzaldehyde selectivity was 55% and 68% respectively (Table 2, en-
tries 2 and 4). Molecular O2 (1 atm.) was inert towards this reaction
(Table 2, entry 3). In presence of 30% aq·H2O2, the highest conversion
of toluene was 79% of and the benzaldehyde selectivity was obtained as
87% (Table 2, entry 5). However without PS-VO-naph catalyst in the
medium, no progress of reaction was monitored (Table 2, entry 6).
Consequently 30% aq·H2O2 may be preferred as suitable oxidizing
agent for oxidation of toluene derivatives.
BET surface area of the PS-VO-naph material is only 25.4 m2 −1, due
g
to the amorphous nature of the catalyst containing the polystyrene
polymer.
3.2.3. Effect of solvents
3.2. Interpretation of catalytic activities
A variety of solvents like acetonitrile, dichloromethane, ethanol,
DMF, and H2O were used to determine the suitable and optimized
solvent for toluene oxidation reaction and the corresponding observa-
tions are depicted in Fig. 9. Using acetonitrile as the solvent, a max-
imum of 79% conversion and 87% of benzaldehyde selectivity were
found. The conversion and selectivity of product were gradually de-
creased on moving from dichloromethane to ethanol and then DMF,
while contribution of water was negligible (Fig. 9).
3.2.1. Oxidation of aromatic alkanes with H2O2 catalyzed by PS-VO-naph
The heterogeneous PS-VO-naph catalyst played an excellent cata-
lytic activity in the oxidation of aromatic alkanes and alkenes with 30%
aqueous H2O2 as a non-hazardous oxidant. The catalytic oxidation of
aromatic alkanes was investigated with methyl benzene as a model
aromatic alkane (Scheme 2). The measurable reaction parameters were
accurately optimized by variation of several oxidants, solvents, toluene
to H2O2 molar ratio, temperature, amount of vanadium loading and
along with the effect of catalytic surface area.
3.2.4. Effect of H2O2 molar ratio
The concentration of H2O2 in the oxidation is a significant para-
meter affecting the toluene oxidation process. In this perspective we
have examined the influence of molar ratio of toluene:H2O2 and results
are shown in Fig. 10. The toluene oxidation was conducted with 1:1,
1:2, 1:3 and 1:4 M ratio of toluene to 30% aq·H2O2 respectively to
monitor whether the oxidation process was influenced by the con-
centration of substrate and oxidant (Fig. 10). The maximum toluene
conversion was recorded as 79% which leads to the 87% of benzalde-
hyde selectivity in presence of 1:3 M ratio of the substrate to oxidant.
During the oxidation process, addition of substrate to oxidant in 1:4 M
3.2.2. Effect of several oxidants
The oxidation of toluene was performed with different oxidants,
such as NaIO4, 30% aq·H2O2, O2 (1 atm.), and TBHP (70% aq.) to op-
timize the suitable oxidant. The corresponding product selectivity and
conversion to benzaldehyde are highlighted in Table 2. In absence of
oxidant the catalytic oxidation reaction did not run, although the cat-
alyst PS-VO-naph was employed in the reaction (Table 2, entry 1).
When NaIO4 and 70% aq. TBHP were used as oxidant the conversion of
Fig. 4. EDAX of (A) polymer anchored ligand PS-naph and (B) PS-VO-naph catalyst.
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Fig. 5. The UHR-TEM images of PS-naph at two different magnifications: 100 nm (A) and 50 nm (B); and of PS-VO-naph at two different magnifications: 100 nm (C)
and 50 nm (D). The SAED pattern of PS-VO-naph catalyst (E).
ratio exhibited an abrupt fall in benzaldehyde selectivity. In some of
earlier reported work, it was observed that with increase in the molar
ratio of toluene:H2O2, the conversion of toluene gradually decreased
while the change in selectivity did not follow a regular trend [3]. In fact
when toluene:H2O2 molar ratio was adjusted to 1:3, the conversion of
toluene was 79% with the benzaldehyde selectivity of 87%. However,
when the toluene:H2O2 molar ratio increased to 1:4, the toluene con-
version increased to just 81% whereas the selectivity of benzaldehyde
was decreased to 65%. In another report, toluene conversion was ob-
served to be increased with rise in the toluene:H2O2 molar ratio but the
selectivity of benzaldehyde was found to be decreased slightly [45].
The enhancement in molar ratio of H2O2, accelerated the oxidation
of the side chain, hence the toluene conversion increased accordingly.
But the selectivity of benzaldehyde was decreased in presence of excess
of H2O2 concentration. Surprisingly, using toluene:H2O2 molar ratio as
1:4, the benzaldehyde selectivity was started to decrease. This suggests
that higher the stoichiometric amount of H2O2 was utilized during
oxidation, the higher degree of thermal decomposition of H2O2 oc-
curred, consequence to the small increment in toluene conversion as
well as reduction in benzaldehyde selectivity. Therefore, the optimal
molar ratio of toluene to H2O2 was 1:3 [13b]. One of the possible
contributions for the decrease in selectivity is probably due to the
formation of the over-oxidized products if any. The conversion factor
did not improve rapidly (from 79% to 81%), as the reactant molecules
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Fig. 6. (a) TGA plot of polymer anchored ligand PS-naph and PS-VO-naph catalyst; (b) DTA plot: (A) polymer anchored ligand PS-naph and (B) PS-VO-naph catalyst.
might be restricted to appear in close vicinity of the catalyst through
water formation in the reaction medium by the decomposition of H2O2
[46].
benzaldehyde may be the another reason for decreased selectivity of
benzaldehyde in the reaction mixture [2].
3.2.6. Effect of amount of catalyst
The quantity of catalyst was varied from 10 mg to 50 mg for the
reaction to optimize the PS-VO-naph catalytic amount and the corre-
sponding outcomes are indicated in Fig. 12. An increment of the cata-
lyst quantity from 10 mg to 30 mg, the toluene conversion improves to
such extent but further rise in catalyst amount the selectivity of ben-
zaldehyde was dropped. In that context the conversion of toluene was
remain almost unaltered (Fig. 12).
3.2.5. Effect of reaction temperature
The activity of the catalyst for toluene oxidation with varying
temperature has been outlined in Fig. 11. The selectivity of benzalde-
hyde was gradually decreased with rise in the temperature upto 85 °C,
on account of the decomposition of H2O2 and probably due to the
formation of uncontrolled oxidized product e.g. benzoic acid. We be-
lieve that due to the presence of vanadium species in the catalytic
system, the self decomposition of H2O2 was restricted to higher tem-
perature [46]. At lower temperature, a slight conversion of toluene was
achieved while the selectivity of benzaldehyde was observed high but
with increasing temperature, the toluene conversion increases sig-
nificantly (Fig. 11). The selectivity of benzaldehyde was decreased to
some extent because of formation of benzoic acid with increasing
temperature correspondingly. Some earlier report reveals that in pre-
sence of CuCr2O4 nanoparticles catalyst, the conversion of toluene was
found to be increased with rise in reaction temperature [2]. Another
reported work shows that with increment of reaction temperature, the
conversion of toluene was found to be increased whereas the selectivity
of benzaldehyde was found to follow a decreasing order [45]. Oxidation
leads to the highest toluene conversion (79%) and a slight lower se-
lectivity (87%) of benzaldehyde at 65 °C temperature compared to the
lower temperature region. Now, if the reaction temperature was raised
from 65 °C, the toluene conversion remains almost unchanged; on the
contrary the benzaldehyde selectivity was decreased at a greater extent.
While the reaction was subjected to the higher temperature greater than
65 °C, the presence of other oxidized products in minute amounts with
3.2.7. Effect of vanadium metal loading
The influence of the vanadium metal loading on the catalytic oxi-
dation reaction of toluene has been studied by varying the vanadium
metal loading (5.0–8.0 wt% of V) on the polymer grafted catalyst and
the corresponding results are shown in Table 3. It is observed that with
increasing the metal loading, the conversion of toluene was also in-
creased. From this standpoint, the observed higher catalytic activity in
the relatively lower vanadium loading (5.0–6.58 wt% of V) could be
attributed to the fact that vanadium species comprising the active sites
in the catalyst responsible for toluene oxidation. The initial increase of
catalytic activity with the vanadium content, upto 6.58 wt% may be-
cause of the monolayer loading. But after a certain higher range of
vanadium loading (6.58 wt% of V), the conversion of toluene in terms
of TON was dropped slightly. The rise in the vanadium loading above
monolayer coverage switches the formation of multilayer segment as a
consequence to the decrease in toluene conversion. Further enrichment
with vanadium wt% reduces the conversion of toluene gradually which
may be attributed to the appearing of reasonably less active sites on top
Fig. 7. The wide angle powder XRD pattern of (A) polymer anchored ligand PS-naph, (B) PS-VO-naph catalyst and (C) reused catalyst.
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InorganicaChimicaActa492(2019)198–212
Fig. 8. N2 adsorption/desorption isotherms of PS-VO-naph catalyst (A); Pore size distribution Plot (B).
Scheme 2. Oxidation of toluene in presence of PS-VO-naph catalyst.
Table 2
Optimization of oxidant and catalyst for the oxidation of toluene.a
Entry Catalyst
Oxidant
Conversion(%)b Selectivity of
benzaldehyde (%)b
1
2
3
4
PS-VO-naph
No
NaIO4
O2 (1 atm.)
70% aq.
TBHP
No reaction
34
No reaction
59
No reaction
55
No reaction
68
PS-VO-naph
PS-VO-naph
PS-VO-naph
5c
PS-VO-naph 30%
aq·H2O2
30%
aq·H2O2
79
87
Fig. 9. Dependence of solvent on PS-VO-naph catalyzed toluene oxidation.
Reaction conditions: toluene (5.0 mmol), solvent (10 mL), 30% aq·H2O2
(15.0 mmol), catalyst (30 mg, 6.58 wt% Vanadium loading), temperature 65 °C,
time 6 h.
6c
Nil
No reaction
No reaction
a
Reaction conditions: toluene (5.0 mmol), acetonitrile (10 mL), oxidant
(15.0 mmol), catalyst (30 mg, 6.58 wt% Vanadium loading), temperature 65 °C,
time 6 h.
b
Determined by GC.
H2O2 (30% aq.) was added maintaining a definite time interval.
c
of the multilayer portions and their lower surface concentration [47].
3.2.8. Effect of catalytic surface area
The catalytic efficiency of the polymer supported vanadium catalyst
was compared with the different reported heterogeneous catalyst to
realize the influence of surface area of PS-VO-naph on the selectivity of
product throughout the toluene oxidation process. It is clearly seen
from the Table 4 that the polymer supported catalyst exhibited sig-
nificantly higher toluene conversion (79%) in contrast to that reported
other heterogeneous materials [3,25b]. In line with the conversion
factor a higher selectivity (87%) of the oxidized product of toluene was
observed when polystyrene supported VO-naph catalyst was applied.
The result signifies that the surface of the polystyrene anchored vana-
dium complex has definite contribution to the toluene conversion as
well as selectivity of the benzaldehyde. From the comparative analysis
of BET surface area on Table 4, it was observed that the surface area of
PS-VO-naph catalyst contributed the activity of the catalyst as well as
selectivity during toluene oxidation.
Fig. 10. Effect of toluene:H2O2 molar ratio during PS-VO-naph catalyzed oxi-
dation of toluene. Reaction conditions: toluene (5.0 mmol), acetonitrile
(10 mL), 30% aq·H2O2, catalyst (30 mg, 6.58 wt% Vanadium loading), tem-
perature 65 °C, time 6 h.
3.2.9. Catalytic oxidation of different aromatic alkanes
The exact optimized condition for toluene oxidation was examined
using 30% aq·H2O2 (as the best oxidant) in presence of 30 mg of PS-VO-
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InorganicaChimicaActa492(2019)198–212
Additionally, the electron-donating effect of eOCH3 in toluene mole-
cule brought about up to 92% conversion and 64% selectivity of 4-
methoxy benaldehyde (Table 5, entry 8). Ethyl benzene contributed
almost 96% conversion and the selectivity of the resultant acet-
ophenone was 95% (Table 5, entry 9). The highest H2O2 efficiency of
30.4% was observed for ethyl benzene (Table 5, entry 9) and the lowest
efficiency i.e. 7% was observed for 2-nitro toluene (Table 5, entry 7)
[46].
3.2.10. PS-VO-naph catalyzed oxidation of aromatic alkenes by H2O2
For the typical catalytic oxidation of aromatic alkenes we choose
styrene for model substrate (Scheme 3). The different reaction variables
such as: oxidant, solvents, styrene to H2O2 molar concentration ratio,
temperature, amount of vanadium metal loading and the effect of cat-
alytic surface area were varied to optimize the process.
Fig. 11. Temperature effect on oxidation of toluene catalyzed by PS-VO-naph
complex. Reaction conditions: toluene (5.0 mmol), acetonitrile (10 mL), 30%
aq·H2O2 (15.0 mmol), catalyst (30 mg, 6.58 wt% Vanadium loading), time 6 h.
3.2.11. Effect of several oxidants
We have investigated the suitable oxidant competent for the oxi-
dation of styrene, out of the different oxidants like NaIO4, 30% aq·H2O2,
O2 (1 atm.), 70% aq. TBHP. Throughout the optimization, it was
identified that in absence of any oxidant no conversion of styrene was
experienced (Table 6, entry 1) irrespective of the presence of PS-VO-
naph catalyst in the medium. The greater conversion of styrene i.e. 97%
with 88% selectivity of benzaldehyde was detected in presence of 30%
aq·H2O2 (Table 6, entry 2). When 70% aq. TBHP and NaIO4 were used
as oxidant (Table 6, entries 3–4), the conversion as well as selectivity
was found smaller than that of 30% aq·H2O2. In presence of O2 (1 atm.)
very low conversion was observed (Table 6, entry 5). Conversely, when
30% aqueous H2O2 was treated with the reaction in absence of catalyst,
a remarkably poor conversion was detected (Table 6, entry 6).
3.2.12. Effect of solvents
The styrene oxidation was treated with a number of polar solvents
and the corresponding conversion of substrate and selectivity of ben-
zaldehyde are shown in Fig. 13. Acetonitrile as a solvent accomplished
a maximum of 97% styrene conversion with 88% selectivity of ben-
zaldehyde.
Fig. 12. Effect of PS-VO-naph catalyst amount on oxidation of toluene by H2O2.
Reaction conditions: toluene (5.0 mmol), acetonitrile (10 mL), 30% aq·H2O2
(15.0 mmol), temperature 65 °C, time 6 h.
3.2.13. Effect of styrene:H2O2 molar ratio
Table 3
The oxidation of the model substrate styrene was performed suc-
cessively with 1:1, 1:2, 1:3 and 1:4 M concentration ratio of styrene to
oxidant (30% aq·H2O2) for defining the best substrate to oxidant molar
ratio (Fig. 14). It indicates that in presence of styrene:oxidant in 1:3 M
ratio, 97% styrene conversion were obtained, providing 88% selectivity
of benzaldehyde after reaction completion. Later increase in oxidant
concentration, the selectivity of aromatic aldehyde was considerably
reduced.
Optimization of the amount of vanadium metal loading for the oxidation of
toluene.a
Entry
Vanadium metal loading (wt%)
Conversion(%)b
TON
1
2
3
4
5
6
5.0
5.5
6.5
6.58
7.5
8.0
69.5
72.3
75.3
79
65
61.2
90
93
97
102
84
79
3.2.14. Effect of reaction temperature
a
Reaction conditions: toluene (5.0 mmol), acetonitrile (10 mL), 30%
aq·H2O2 (15.0 mmol), catalyst, temperature 65 °C, time 6 h.
Variable temperature (from 40 to 70 °C) experiments were con-
ducted for the progress of styrene oxidation to optimize the exact
temperature required as shown in Fig. 15. The diagram clearly shows
that the conversion and selectivity factor is increased gradually with
temperature enhancement. At 60 °C temperature, a maximum of 97%
styrene conversion was reached however the selectivity of benzalde-
hyde was found 88%. Further raise in temperature the benzaldehyde
selectivity was dropped at a larger extent.
b
Determined by GC.
naph catalyst in acetonitrile for 6 h at 65 °C temperature. These opti-
mized reaction conditions for selective oxidation of toluene to benzal-
dehyde were extended to the other substituted aromatic alkanes
(Table 5). As obvious from Table 5, the electron donating eCH3 group
attached to o-xylene, p-xylene and m-xylene experienced the higher viz
88%, 85% and 82% conversion with respect to the substrate commit-
ting up to 91%, 93% and 94% selectivity of the corresponding ben-
zaldehydes (entries 2–4, Table 5). Mesitylene contributed 86% con-
version and 92% selectivity of 3,5-dimethyl benzaldehyde (Table 5,
entry 5). The electron-withdrawing factor was realized for eCl and
eNO2 substituted toluenes (Table 5, entries 6–7) with 56% and 23%
conversion respectively while the selectivity of 4-chloro benzaldehyde
and 2-nitro benzaldehyde were 85% and 91% respectively.
3.2.15. Effect of amount of catalyst
The studied oxidation was executed with different quantity of cat-
alyst to optimize exact amount of catalyst compulsory for the procedure
(Fig. 16). The styrene conversion was improved progressively by con-
suming up to 30 mg of catalyst, even though the selectivity of benzal-
dehyde was remained almost same. Subsequent increasing the amount
of PS-VO-naph catalyst, both the conversion and selectivity were re-
duced gradually.
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InorganicaChimicaActa492(2019)198–212
Table 4
Effect of surface area on the activity as well as selectivity of the catalyst for the oxidation of toluene.
Entry
Complex
Surface area (m2 g−1
)
Conversion (%)e
Selectivity of benzaldehyde (%)e
[Ref.]
1a
2a
3b
4b
5c
6c
7d
8d
PS-naph
PS-VO-naph
KIT-6
KIT-6-VPO
Cu-NZ
Cu-MZ
20.2
25.4
493
150
512.64
432.89
303.5
471.1
Trace
79
–
87
Present work
Present work
[3a]
[3a]
[3b]
[3b]
[26a]
[26a]
0.14
17.76
95.4
85.7
17.3
5.7
˃99.9
69.24
34.07
47.61
51.4
88.9
Fe2O3/HZSM-5
Fe2O3/HY
Reaction conditions:
a
Toluene (5.0 mmol), acetonitrile (10 mL), 30% aq·H2O2 (15.0 mmol), complex (30 mg), temperature 65 °C, time 6 h.
Toluene (3.5 mL), acetonitrile (10 mL), toluene: H2O2 mole ratio = 1:1.4, complex (0.1 g), temperature 75 °C, time 10 h.
Toluene (25 mL), catalyst (1.0 g), 25 mL of 30 wt% H2O2 with 25 mL deionized water, pressure 5.0 bar (with N2 gas), temperature 180 °C, time 4 h.
Toluene (5 mL), molar ratio of H2O2 to toluene = 3:1, solvent-free condition, catalyst (0.4 g), temperature 90 °C, time 4 h.
Determined by GC.
b
c
d
e
Table 5
Catalytic oxidation of aromatic alkanes by 30% aq·H2O2 using PS-VO-naph catalyst.a
Entry
1
Substrate
Product
Conversion (%)b
Selectivity (%)b
87
TON
102
H2O2 efficiency (%)
79
23.0
2
88
91
114
26.7
3
4
85
82
93
94
110
106
26.4
25.7
5
86
92
111
26.4
6
7
56
23
85
91
72
30
15.9
7.0
8
9
92
96
64
95
119
124
19.6
30.4
a
Reaction conditions: substrate (5.0 mmol), acetonitrile (10 mL), 30% aq·H2O2 (15.0 mmol), catalyst (30 mg, 6.58 wt% Vanadium loading), temperature 65 °C,
time 6 h.
b
Determined by GC·H2O2 efficiency = [moles of selective product formed/total moles of H2O2 added] × 100.
3.2.16. Effect of vanadium metal loading
The catalytic oxidation reaction of styrene to benzaldehyde was also
influenced by the wt % of vanadium loading in the polymer grafted
catalyst. In order to analyze the effect of metal loading, the conversion
of styrene was recorded by varying the vanadium wt% from 5.0 to 8.0
and the results are shown in Table 7. It was detected that the conversion
of styrene was increased with increasing the vanadium loading. In
Scheme 3. Oxidation of styrene in presence of PS-VO-naph catalyst.
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P. Paul, et al.
InorganicaChimicaActa492(2019)198–212
Table 6
Optimization of oxidant and catalyst for the oxidation of styrene.a
Entry Catalyst
Oxidant
Conversion(%)b Selectivity of
benzaldehyde(%)b
1
PS-VO-naph
PS-VO-naph 30%
No
No reaction
97
No reaction
88
2c
aq·H2O2
70% aq.
TBHP
3
PS-VO-naph
56
81
4
PS-VO-naph
PS-VO-naph
No
NaIO4
O2 (1 atm.)
30%
36
∼4
∼7
48
69
71
5
6c
aq·H2O2
a
Reaction conditions: styrene (5.0 mmol), acetonitrile (10 mL), oxidant
(15.0 mmol), catalyst (30 mg, 6.58 wt% Vanadium loading), temperature 60 °C,
time 6 h.
Fig. 15. Temperature effect on PS-VO-naph catalyzed oxidation of styrene.
Reaction conditions: styrene (5.0 mmol), acetonitrile (10 mL), 30% aq·H2O2
(15.0 mmol), catalyst (30 mg, 6.58 wt% Vanadium loading), time 6 h.
b
Determined by GC.
c
H2O2 (30% aq.) was added maintaining a definite time interval.
Fig. 13. Effect of solvent on PS-VO-naph catalyzed oxidation of styrene.
Reaction conditions: styrene (5.0 mmol), solvent (10 mL), 30% aq·H2O2
(15.0 mmol), catalyst (30 mg, 6.58 wt% Vanadium loading), temperature 60 °C,
time 6 h.
Fig. 16. Effect of catalyst (PS-VO-naph) amount on hydrogen peroxide oxida-
tion of styrene. Reaction conditions: styrene (5.0 mmol), acetonitrile (10 mL),
30% aq·H2O2 (15.0 mmol), temperature 60 °C, time 6 h.
Table 7
Optimization of the amount of vanadium metal loading for the oxidation of
styrene.a
Entry
Vanadium metal loading (wt%)
Conversion (%)b
TON
1
2
3
4
5
6
5.0
5.5
6.5
6.58
7.5
8.0
81
105
112
120
125
103
99
86.4
93.2
97
80
77
a
Reaction conditions: styrene (5.0 mmol), acetonitrile (10 mL), 30% aq·H2O2
(15.0 mmol), catalyst, temperature 60 °C, time 6 h.
b
Determined by GC.
of TON was dropped slightly. This may be attributed to the fact that
vanadium loading upto 6.58 wt%, the monolayer formation (acts as
active sites) is responsible for styrene oxidation. Indeed loading of
higher wt% of vanadium in the PS-VO-naph, the monolayer formation
then converts into the less active site on top of the multilayer portion, as
a result of which conversion of styrene gets reduced [47].
Fig. 14. Effect of molar ratio (styrene: H2O2) on PS-VO-naph catalyzed oxida-
tion of styrene. Reaction conditions: styrene (5.0 mmol), acetonitrile (10 mL),
30% aq·H2O2, catalyst (30 mg, 6.58 wt% Vanadium loading), temperature
60 °C, time 6 h.
3.2.17. Effect of catalytic surface area
The similar experiment was executed to detect the catalytic activity
and the influence of surface area of PS-VO-naph catalyst during the
styrene oxidation process. Herein we have reported (Table 8) that the
polymer supported vanadium catalyst exhibited quite higher styrene
relatively lower vanadium content (5.0–6.58 wt% of V), the higher
catalytic activity was observed as depicted in Table 7. But after a cer-
tain range of loading (6.58 wt% of V) the conversion of styrene in terms
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InorganicaChimicaActa492(2019)198–212
Table 8
Effect of surface area on the activity as well as selectivity of different catalyst for the oxidation of styrene.
Entry
Complex
Surface area
Conversion (%)e
Selectivity of benzaldehyde (%)e
[Ref.]
1a
2a
3b
4c
5c
6d
PS-naph
PS-VO-naph
Co3O4@HZSM-5
PS-[Cu(Hfsal-aepy)Cl]
PS-[Mn(Hfsal-aepy)Cl]
Mn-MIL-100
20.2 m2 g−1
25.4 m2 g−1
329 cm2 g−1
16.47 m2 g−1
15.68 m2 g−1
1133 m2 g−1
11
97
96.8
87.3
75.4
88.99
63
88
81.5
15.7
21.3
84.97
Present work
Present work
[7b]
[10c]
[10c]
[25b]
Reaction conditions:
a
Styrene (5.0 mmol), acetonitrile (10 mL), 30% aq·H2O2 (15.0 mmol), complex (30 mg), temperature 60 °C, time 6 h.
Styrene (15 mmol), acetonitrile (16 mL), Co3O4@HZSM-5 (0.1 g), H2O2 (75 mmol), 75 °C, 10 h.
Styrene to TBHP mole ratio of 1:3, catalyst (25 mg), temperature 70 °C.
Mn-MIL-100 (5 wt%), 3:1 M ratio of 70% TBHP/styrene, acetonitrile (1 mL), temperature 70 °C, time 10 h.
Determined by GC.
b
c
d
e
conversion (97%) in comparison to that of other reported hetero-
temperature for 6 h. By maintaining the optimized factors the similar
conditions were applied for the oxidation reactions of aromatic alkenes
(Table 9). Oxidation of styrene conversion up to 97% produced the
benzaldehyde with 88% selectivity (Table 9, entry 1). On the other
hand the catalytic oxidation of aliphatic alkene e.g. cyclohexene (84%
conversion) undergoes oxidation to 2-cyclohexene-1-one with 66% se-
lectivity (Table 9, entry 2). The substrate trans-stilbene was oxidized
upto a conversion of 67% with 79% benzaldehyde selectivity (Table 9,
entry 3). 4-methyl benzaldehyde was produced with 85% conversion
and 86% selectivity from the substrate 4-methyl styrene having elec-
tron-releasing eCH3 group (Table 9, entry 4). Styrene containing
electron-attracting eCl and eNO2 groups, (Table 9, entry 5–6) provided
the respective benzaldehyde products with 87% and 82% selectivity.
The efficiency of H2O2 was highest (28.5%) for styrene oxidation
(Table 9, entry 1) while it was examined as lowest 17.6%, in case of
geneous catalyst. Again the selectivity of the styrene oxidized product
was moderately enhanced (88%) in presence of PS-VO-naph catalyst in
comparison to the previously reported catalyst as represented in Table 8
[7b,10c,25b]. The result implies that the surface of the polystyrene
supported vanadium complex has active contribution to the styrene
conversion. From the similar type of comparative analysis with BET
surface area (Table 8), it was noticed that the surface area of PS-VO-
naph catalyst has potential impact on the activity of the catalyst as well
as selectivity throughout styrene oxidation.
3.2.18. Catalytic oxidation of different aromatic alkenes
The best reaction condition was monitored in acetonitrile solvent
after examining the optimization process during the course of oxidation
with 30% aq·H2O2 in presence of 30 mg PS-VO-naph catalyst at 60 °C
Table 9
Catalytic oxidation of aromatic alkenes by 30% aq·H2O2 using PS-VO-naph catalyst.a
Entry
1
Substrate
Product
Conversion (%)b
97
Selectivity (%)b
88
TON
125
H2O2 efficiency (%)
28.5
2*
84
66
108
18.5
3
4
67
85
79
86
86
17.6
24.4
110
5
6
88
83
87
82
114
107
25.5
22.7
* Note: Cyclohexene is an aliphatic alkene.
a
Reaction conditions: substrate (5.0 mmol), acetonitrile (10 mL), 30% aq·H2O2 (15.0 mmol), catalyst (30 mg, 6.58 wt% Vanadium loading), temperature 60 °C,
time 6 h.
b
Determined by GC·H2O2 efficiency = [moles of selective product formed/total moles of H2O2 added] × 100.
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InorganicaChimicaActa492(2019)198–212
trans-stilbene oxidation (Table 9, entry 3) [46].
3.2.19. Comparative investigation on catalytic activity with other reported
systems
The efficiency of the polystyrene incorporated oxovanadium(IV)
catalyst, (PS-VO-naph) towards the aromatic alkane/alkene oxidation
was compared (Table 10) with the formerly reported works. The cata-
lytic oxidation reaction of aromatic alkane (toluene) furnished 22%
conversion of toluene with 76% selectivity of benzaldehyde in the
presence of MPAV2/Nb2O5 [48]. Another work represented the 17.76%
toluene conversion with 69.24% selectivity of benzaldehyde in the
presence of vanadium phosphorus oxide functionalized KIT-6 catalyst
[3]. CoSBA-15 effectively performed 7.97% conversion of toluene oxi-
dation with 63.8% benzaldehyde selectivity [49]. Using our ox-
ovanadium(IV) catalyst 79% conversion of toluene with 87% selectivity
of the desired product benzaldehyde was obtained. Maurya et al. es-
tablished that catalytic oxidation reaction of aromatic alkene (styrene)
in association with PS-K[VO2(fsal-ohyba)] catalyst produced the 79.7%
conversion of styrene and 60.65% benzaldehyde selectivity [34]. In
another work reports about 30.4% conversion of styrene with 43%
benzaldehyde selectivity was achieved in presence of MOF-74(Cu/Co)
catalyst, [28b]. Conversely the Zeolite Y encaged Ru(III) and Fe(III)
complex were relatively much effective for the 76.1% of styrene con-
version with 93.6% benzaldehyde selectivity [4]. Some more relevant
works are included in the given Table 10. Therefore using our ox-
ovanadium(IV) catalyst 97% conversion of styrene with 88% selectivity
of the desired product benzaldehyde was obtained.
3.3. Test for heterogeneity
During the oxidation reaction the controlled reaction of toluene was
performed to investigate the leaching of the active vanadium metal
from the metal immobilized complex. The same test was also executed
with styrene under controlled reaction condition. A typical hot filtra-
tion test was conducted for oxidation reactions of both toluene and
styrene individually by maintaining suitable optimized condition. After
3 h from the commencement of the title oxidation reaction, the reaction
mixture was settled for cooling for the two cases. Afterwards the cat-
alyst was extracted from the medium by centrifugation process. Under
similar reaction condition without catalyst the subsequent prolonged
oxidation process was executed with toluene and styrene separately for
additional 3 h. The conversion of toluene was almost the same after the
first 3 h and the next 3 h of the reaction. Similar effect was observed for
styrene oxidation. Hence oxidation reaction did not run in absence of
the PS-VO-naph catalyst for both the substrate toluene and styrene re-
spectively. After removing the catalyst from the reaction mixture no
leaching or decomposition of the catalyst was visualized in the filtrate.
Additionally, no trace of vanadium metal content in the filtrate was
appeared from the AAS experiment, within the detection limit
(< 5 ppm). This fact approves no significant leaching of metal ion from
PS-VO-naph catalyst during aromatic alkane/alkene oxidations. From
the above test we can certainly elucidate that the toluene and styrene
oxidation took place with the PS-VO-naph catalyst which is hetero-
geneous by nature.
3.4. Recyclability of catalyst
A catalyst with heterogeneous environment is constantly pre-
dominating over a homogeneous catalyst due to its recyclability and
separation from reaction system. Accordingly the recyclability of the
PS-VO-naph catalyst was checked for the oxidation of both toluene and
styrene by 30% aq·H2O2 oxidant (Fig. 17a). After the first run, the
catalyst PS-VO-naph was easily withdrawn from the reaction system by
centrifugation process. Subsequent washings with ethyl acetate and
methanol were conducted to remove any unreacted materials present in
the catalyst. Further it was desiccated under vacuum for the subsequent
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Fig. 17. (a) Recycling efficiency of oxovanadium(IV) catalyst; (b) FT-IR spectra of reused oxovanadium(IV) catalyst.
reaction run. Fig. 17a shows that the present catalyst can be recycled up
to six runs under the optimized reaction condition. There observed no
leaching of metal and no abrupt change in the conversion of the sub-
strates and selectivity of the oxygenated products in recycling experi-
ments. From FT-IR spectroscopic studies it was observed that the FT-IR
spectrum (Fig. 17b) of the reused vanadium catalyst was almost similar
with the freshly prepared catalyst.
contributed the maximum conversion of 97% and 88% selectivity of
benzaldehyde under optimized conditions. The corresponding oxyge-
nated products were identified using gas chromatograph (GC). Our
catalyst accomplished the excellent catalytic efficiency and activity for
both aromatic alkanes and alkenes oxidation reactions. Furthermore,
the complex did not exhibit any leaching of vanadium, which proves
the heterogeneous activity of the catalyst. Additionally the complex is
reusable up to maximum of six reaction cycles without substantial and
abrupt modification in catalytic property during conversion of both
toluene and styrene to the product benzaldehyde exclusively.
4. Conclusion
In summary, we have successfully outlined a polymer grafted het-
erogeneous oxovanadium naphthyl-azo catalyst (PS-VO-naph) for se-
lective oxidation of aromatic alkanes and alkenes by an environment-
friendly oxidant, 30% aq·H2O2 under ambient conditions. Hydrogen
peroxide is an excellent oxidant throughout the oxidation process, since
it delivers water as the only byproduct. The oxidation of the hydro-
carbons deals with some industrially significant oxidized products
among which benzaldehydes are the most promising owing to its large
industrial importance. Out of the several aromatic alkanes investigated,
ethyl benzene provided the maximum conversion of 96% with respect
to substrate and maximum of 95% selectivity of the resulting carbonyl
compound under optimized reaction conditions. The aromatic alkenes
produced their corresponding carbonyl compounds which could un-
dergo further catalytic oxidation process to provide the carboxylic acids
at definite condition. These resulting compounds have greater sig-
nificance in various industrial organic transformations. Styrene
Acknowledgements
S.M.I. acknowledges Board of Research in Nuclear Sciences (BRNS),
Project reference No: 37(2)/14/03/2018-BRNS/37003, Govt. of India
and the Department of Science and Technology (DST-SERB, Project No.
EMR/2016/004956), New Delhi, Govt. of India, for funding. PP ac-
knowledges Aliah University, Kolkata for financial support. AG ex-
presses his sincere gratitude to the SERB-NPDF, DST, New Delhi, India
(File No. PDF/2017/000478) for the National Post-Doctoral Fellowship.
SMI is also thankful to Council of Scientific and Industrial and Research,
CSIR (project reference no 02(0284)2016/EMR-II dated 06/12/2016)
New Delhi, Govt. of India and Department of Science and Technology,
West Bengal (DST-WB, Sanction No. 811(sanc.)/ST/P/S&T/4G-8/2014
Dated: 04.01.2016) for providing financial support. SMI acknowledges
University of Kalyani, India for providing personal research grant. We
211

P. Paul, et al.
InorganicaChimicaActa492(2019)198–212
sincerely acknowledge the DST and UGC, New Delhi, Govt. of India, for
providing grant under DST-FIST, DST-PURSE and UGC-SAP program to
the Department of Chemistry, University of Kalyani. SMI acknowledges
Dr. Ankur Bordoloi and Manideepa Sengupta of IIP Dehradun for their
various supports for this research work.
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