The Role of Alkali Metal Exchanged Phosphomolybdic Acid Catalysts in the Solvent Free Oxidation of Styrene to Benzaldehyde at Room Temperature

Article abstract of DOI:10.1007/s10562-018-2352-1

Abstract: A series of alkali metal exchanged phosphomolybdic acid catalysts were synthesized by ion exchange, characterized by various physico-chemical techniques and used in the solvent free oxidation of styrene to benzaldehyde. XRD and infrared results showed that the primary structure of the Keggin ion usually present in phosphomolybdic acid is retained after metal exchange. HR-TEM analysis show a well-constructed spherical morphology of the materials with a lower degree of crystalinity. Type IV isotherms with mesoporous structure are observed from nitrogen adsorption–desorption isotherm studies and ex situ pyridine adsorption experiments reveal that Br?nsted acidic sites increased after metal exchange. The K exchanged phosphomolybdic acid catalysts were most efficient in the conversion of styrene to benzaldehyde and the order of reactivity of the alkali metal exchanged phosphomolybdic acid catalysts was K > Rb > Cs. Insight into the reaction pathway by investigating the oxidation styrene oxide was obtained. The results show that phenyl acetaldehyde together with benzaldehyde are produced, providing some evidence that styrene oxidation proceeds via C=C cleavage to selectively produce benzaldehyde. The catalyst was easily recovered and was reused for up to three cycles showing stable activity. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2352-1

Catalysis Letters
https://doi.org/10.1007/s10562-018-2352-1
The Role of Alkali Metal Exchanged Phosphomolybdic Acid Catalysts
in the Solvent Free Oxidation of Styrene to Benzaldehyde at Room
Temperature
Jhansi Pedada1 · Holger B. Friedrich1 · Sooboo Singh1
Received: 17 October 2017 / Accepted: 7 March 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
A series of alkali metal exchanged phosphomolybdic acid catalysts were synthesized by ion exchange, characterized by
various physico-chemical techniques and used in the solvent free oxidation of styrene to benzaldehyde. XRD and infrared
results showed that the primary structure of the Keggin ion usually present in phosphomolybdic acid is retained after metal
exchange. HR-TEM analysis show a well-constructed spherical morphology of the materials with a lower degree of crystalin-
ity. Type IV isotherms with mesoporous structure are observed from nitrogen adsorption–desorption isotherm studies and
ex situ pyridine adsorption experiments reveal that Brønsted acidic sites increased after metal exchange. The K exchanged
phosphomolybdic acid catalysts were most efficient in the conversion of styrene to benzaldehyde and the order of reactivity
of the alkali metal exchanged phosphomolybdic acid catalysts was K>Rb>Cs. Insight into the reaction pathway by investi-
gating the oxidation styrene oxide was obtained. The results show that phenyl acetaldehyde together with benzaldehyde are
produced, providing some evidence that styrene oxidation proceeds via C=C cleavage to selectively produce benzaldehyde.
The catalyst was easily recovered and was reused for up to three cycles showing stable activity.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-018-2352-1) contains
supplementary material, which is available to authorized users.
* Sooboo Singh
singhso@ukzn.ac.za
1
Catalysis Research Group, School of Chemistry and Physics,
University of KwaZulu-Natal, Durban 4000, South Africa
Vol.:(0123456789)
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J. Pedada et al.
Graphical Abstract
Keywords Oxidation · Styrene · Benzaldehyde · Alkali metal-PMA · Acidity
been undertaken. Among these are the metal oxide systems,
metal complex systems and solid acid catalysts [5–13]. Solid
heteropolyacid (HPA) catalysts are recognized one of the
more efficient catalytic systems for styrene oxidation, due to
their tunable acidic and oxidation functionalities and they are
also environmentally friendly and recyclable [14–29]. It has
also been established that metal containing heteropolyacids
produce excellent results for the oxidation of styrene [6–13].
The research has been focused on metals such as Mn, Co and
Ni. On further investigation, it has been found that the cata-
lytic properties can be greatly tuned by considering the use
of alkali metals and recently there has been a growing inter-
est in the study of alkali metals such K, Rb and Cs contain-
ing materials as catalysts in various catalytic transformations
[30–44]. Pure heteropolyacid catalysts shows low yields of
benzaldehyde during styrene oxidation, probably due to its
low surface area. However when protons are exchanged with
alkali metal species such as K, Rb and Cs into the Keggin
ion of heteropolyacids, an improvement in the surface area
is achieved, also altering the acidity is accomplished to the
benefit of the oxidation reaction. Recently, the oxidation of
1 Introduction
Solvent free oxidation of styrene to the benzaldehyde and
styrene oxide has shown great importance in basic and
applied research due to its extensive applications in indus-
trial chemical processes and also in academia. Benzaldehyde
is the versatile aromatic carbonyl compound used in perfum-
ery, cosmetics and agrochemical industries, while styrene
oxide is used a starting material in the manufacture of epoxy
resin, absorbents and flavoring agents [1, 2]. The oxidation
of styrene was initially carried out using homogeneous cata-
lysts containing components such as permanganates, chro-
mates and hypochlorites. In addition to these formulations
being toxic, poor selectivity was obtained in all reactions and
the inherent use of solvents is a disadvantage with regards
to environmental concerns [3]. Transition metal complexes
have been used as catalysts. Although reasonable activities
were obtained, a major drawback was recyclability and prob-
lems associated with separation of the product stream [4, 5].
As a result, extensive research into more viable and environ-
mentally friendly catalysts for these type of reactions has
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The Role of Alkali Metal Exchanged Phosphomolybdic Acid Catalysts in the Solvent Free Oxidation…
toluene over Cs exchanged phosphomolybdic acid (PMA)
catalysts was reported. In this study, it was clear that the
acidity and mesoporous nature of catalysts influenced the
activity as well as the selectivity [21]. The oxidation of sty-
rene was studied using oxidants such as air, H2O2 and TBHP
[5–13]. Of the oxidants employed, TBHP showed the highest
selectivity towards the desired product although there was
moderate conversion of the substrate [45].
as its cross-sectional area. The pore size distribution was
measured by N2 adsorption–desorption isotherms using a
Micrometrics ASAP 2020 multi-point BET surface area ana-
lyzer. Prior to N2 gas sorption experiments, the materials
were degassed under helium flow overnight at 200 °C using
a Micrometrics Flow Prep 060 to remove adsorbed mois-
ture or impurities from surface of the catalyst. The experi-
ments were carried over a p/po pressure range of 0.05–0.9
by using nitrogen adsorption–desorption method. The sur-
face area was calculated from adsorption isotherm points
at relative nitrogen pressures (p/po) between 0.05 and 0.3,
while the pore size was calculated by BJH method. Infra-
red (IR) spectra were recorded on a Perkin Elmer Precisely
equipped with a Universal ATR sampling accessory using a
diamond crystal. All spectra generated were analyzed using
Spectrum 100 software. Ex-situ pyridine adsorbed infrared
experiments were carried out by placing a drop of pyridine
on a small amount of the catalyst, followed by evacuation in
air for 1 h to remove the reversibly adsorbed pyridine and the
spectra were recorded on a PerkinElmer ATR spectrometer
at room temperature. The surface morphology of the materi-
als was obtained using scanning electron microscopy (SEM)
utilizing a Leo 1450 Scanning Electron Microscope. Prior
to SEM analysis, the samples were mounted on aluminium
stubs using double sided carbon tape and subsequently gold
spluttered using the Polaron E5100 coating unit. Transmis-
sion electron microscopy (TEM) images were obtained on a
Jeol JEM-1010 electron microscope. The images were cap-
tured and analyzed using iTEM software. Distribution and
chemical analysis of elements in the materials was examined
by STEM analysis.
In the present work, the liquid phase solvent free oxida-
tion of styrene to benzaldehyde was studied over alkali metal
exchanged heteropolyacid catalysts to assess the effect of the
different alkali metals.
2 Experimental
2.1 Catalyst Synthesis
A series of alkali metal exchanged phosphomolybdic acid
catalysts with different metal loadings, ranging from 1 to
3 wt% were synthesized by ion exchange according to a
method reported previously [21]. In a typical synthesis,
for a 1 wt% K loading, 0.138 g of K2CO3 (Sigma Aldrich,
SA) was dissolved in 10 mL of double distilled water. This
solution was added drop wise to a solution of 3.65 g of
H3PMo12O40 (Sigma Aldrich, SA) made up in 15 mL of
double distilled water. The resulting mixture was centrifuged
to remove the precipitate which was then washed repeat-
edly with double distilled water. The solid was dried at
200 °C for 10 h. This catalyst with the theoretical formula,
K1.0PMo12O40, was denoted as 1KPMA. The other catalysts
were prepared using the method described and were denoted
in a similar manner to 1KPMA.
2.3 Catalytic Testing
2.2 Catalyst Characterization
Solvent free styrene oxidation experiments were conducted
in a 50 mL round bottom flask at room temperature. In a
typical run, 10 mmol of styrene, 10 mmol of tertiary butyl
hydrogen peroxide (TBHP) and 0.1 g of the catalyst were
placed in the flask, under stirring at room temperature and
the reaction was constantly monitored by GC. The overall
reaction is shown in Scheme 1. At the end of the reaction,
the catalyst was removed, dried in an oven set at 200 °C and
used for the same reaction.
Powder X-ray diffraction patterns of samples were obtained
with a Bruker D8 Advance diffractometer, using a Cu Ka
radiation source (1.5406 Å) at 40 kV and 30 mA. The meas-
urements were recorded in steps of 0.045° with a count
time of 0.5 s in the range of 5–40°. The surface area of
catalysts was estimated using N2 adsorption isotherms at
−196 °C by the multipoint BET method taking 0.162 nm2
Scheme 1 Oxidation of
styrene over a potassium metal
exchanged heteropolyacid
catalyst
1 3

J. Pedada et al.
The XRD diffractograms of the 2 wt% loadings of K, Rb
and Cs exchanged PMA catalysts, together with the pure
PMA are shown in Fig. 1b. Similar profiles exhibited by
the Rb and Cs exchanged catalysts to the K are observed
and there is every indication that the Keggin ion is present.
3 Results and Discussion
3.1 Catalyst Characterization
3.1.1 X‑ray Diffraction
3.1.2 Infrared Spectroscopy
X-ray diffraction patterns of the pure PMA and the K
exchanged PMA catalysts are shown in Fig. 1a. Pure PMA
shows the triclinic structure, whereas the cubic structure
of the Keggin ion is observed for the K exchanged materi-
als [21]. Sharp diffraction patterns were observed in the K
exchanged PMA catalysts compared to pure PMA, revealing
a more crystalline nature of the Keggin ion [21].
Infrared spectra of pure PMA and the modified KPMA cata-
lysts show four well defined characteristic IR bands in the
500–1200 cm−1 region (Fig. 2). These IR bands, in the case
of pure PMA at 760, 889, 961 and 1058 cm−1 are assigned
to Mo–Oc–Mo, Mo–Ot–Mo, Mo=O and P–O bands, respec-
tively [21]. In the modified catalysts, slight shifts in these
bands are observed, probably implying exchange of the pro-
ton by the metal [21]. This was also true for the Rb and
Cs exchanged catalysts as shown in Fig. S1, supplementary
information.
The nature of acidic sites were examined by ex-situ
pyridine adsorbed IR spectroscopy (1600–1400 cm−1). The
spectra of the parent PMA and K exchanged PMA catalysts
are shown Fig. 3. The bands at 1539, 1487 and 1442 cm−1
are attributed to Brønsted (B), Brønsted and Lewis (B+L)
and Lewis (L) acidic sites, respectively [21]. The parent
PMA contained more L acid sites that B acid sites. The
Brønsted acidity increased when metal was incorporated and
B acidity increase further with an increase in K loadings, but
reached a maximum for the 2KPMA, thereafter showed a
slight decrease for 3KPMA where Lewis acidity was domi-
nant. The acidity of the Rb and Cs exchanged PMA-2 cata-
lysts was also determined and compared with 2KPMA (Fig.
Fig. 1 a X-ray diffraction patterns of a PMA, b 1KPMA, c 2KPMA
and d 3KPMA catalysts. b X-ray diffraction patterns of a PMA, b
2KPMA, c 2RbPMA and d 2CsPMA catalysts
Fig. 2 IR spectra of a PMA, b 1KPMA, c 2KPMA, d 3KPMA and e
regenerated 2KPMA catalysts
1 3

The Role of Alkali Metal Exchanged Phosphomolybdic Acid Catalysts in the Solvent Free Oxidation…
were obtained which indicated presence of mesoporous
materials (Fig. S3, supplementary information). The high
surface area 2KPMA which implies a lower Keggin anion
density and this is reflected by lower crystalinity of the
Keggin ion units obtained compared to other loadings. This
result correlates well with the X-ray diffraction studies.
Comparing the isotherms of PMA and 2KPMA with its
Rb and Cs counterparts (Fig. 4; Table 1), it can be seen
clearly that all the materials possess mesoporous character
in keeping with their type IV isotherms.
3.1.4 Electron Microscopy
The surface morphology of PMA and metal exchanged PMA
catalysts are shown in Fig. 5. The PMA shows bulk like mor-
phology, whereas the metal exchanged catalysts exhibited a
well-defined spherical morphology.
From HR-TEM images (Fig. 5), the metal exchanged
catalysts display well defined particles with sizes ranging
from 300 Å to 450 Å, whereas for the PMA, particles varied
between 18,000 Å and 20,000 Å.
Fig. 3 Pyridine infrared spectra of a PMA, b 1KPMA, c 2KPMA, d
3KPMA and e regenerated 2KPMA catalysts
S2, supplementary information). 2KPMA contained more B
acidic sites than both the 2RbPMA and 2CsPMA catalysts.
BF-STEM analysis of the metal exchanged PMA cata-
lysts was carried out to map the structure and relative spa-
tial distribution of the elements. Examples of some of these
analyses for the K exchanged catalysts are shown in Fig. 6.
In general, the metal content in the Keggin anion matches
theoretical loadings (Table 1). These results are reinforced
with data obtained by flame photometry and ICP analysis.
3.1.3 Surface Studies
Surface properties of the catalysts such as surface area, pore
volume and pore size of parent PMA and K-PMA catalysts
were determined. The BET surface area of the catalysts
increased with an increase in the metal loadings. How-
ever, this trend changed for all the catalysts with a load-
ing of 3 wt% which showed a decrease in the surface area
(Table 1). From the adsorption isotherms for PMA and the K
exchanged catalysts, type IV isotherms with hysteresis loops
3.2 Catalytic Activity Studies
A detail study was performed on the oxidation of styrene
to benzaldehyde to optimize the conditions at room tem-
perature with various reaction parameters, such as, mole
Table 1 Elemental content and surface study data of PMA and metal exchanged catalysts
Catalyst
Surface area
(m2/g)
Pore volume
(cm3/g)
Pore diameter Metal content
Metal content
expt. (wt%)a
Metal content
expt. (wt%)b
Keggin ion
density (HPA/
nm2)
(Å)
calc. (wt%)
PMA
2.7
66
133
101
28
181
115
25
146
80
0.01
0.02
0.05
0.03
0.04
0.07
0.09
0.03
0.14
0.08
128
81
46
43
89
40
60
80
40
50
0
–
–
0.9
1.8
2.7
0.92
1.89
2.8
0.95
1.92
3.0
1.20
KPMA-1
KPMA-2
KPMA-3
RbPMA-1
RbPMA-2
RbPMA-3
CsPMA-1
CsPMA-2
CsPMA-3
1.0
2.0
3.0
1.0
2.0
3.0
1.0
2.0
3.0
1.1
2.0
3.0
–
1.9
2.8
0.9
2.1
2.9
0.0489
0.0238
0.0307
0.1126
0.0166
0.0252
0.1231
0.0197
0.0338
aMetal content obtained from STEM analysis
bMetal content obtained from Flame photometry and ICP analysis
1 3

J. Pedada et al.
Fig. 4 N2 isotherms of a PMA, b 2KPMA, c 2RbPMA and d 2CsPMA catalysts
ratio of styrene to TBHP, amount of catalyst, various oxi-
dants, reaction temperature, time on stream and recycling.
Initially, blank reactions showed a 2% conversion with a
73 and 18% selectivity towards benzaldehyde and styrene,
respectively, recorded at room temperature for 24 h. The
pure phosphomolybdic acid showed a 4.5% conversion,
with similar selectivity as the blank. Among the potassium
exchanged catalysts, 2KPMA showed highest activity with
an 80% selectivity towards benzaldehyde, probably attrib-
uted availability of active sites because of its large surface
area and the surface acidity of the catalyst (Table 2). The
results correlated well with the nature of acidic sites of the
catalyst, in this case, Brønsted acidic sites [21]. From the
pyridine infrared studies, 2KPMA had the highest surface
Brønsted acidic sites than the other K exchanged PMA and
PMA, itself. It is also possible that the mesoporous nature of
the 2KPMA catalyst influences activity, since it has higher
pore volume and surface area.
of styrene oxidation follows the order, K>Rb>Cs, probably
due to K being more efficient in generating TBHP radicals
than Rb or Cs in the other catalysts.
Styrene oxidation was then conducted over the 2KPMA
catalyst by varying the TBHP concentration. The results are
tabulated in Table S1, supplementary information. These
findings show that the activity increased initially with oxi-
dant concentration. Thereafter, only a slight increase in the
activity was observed, but the selectivity decreased mar-
ginally leading to the undesired diol. The catalytic reaction
was also investigated using different amounts of the 2KPMA
catalyst (Table S2, supplementary information). The styrene
conversion increases with an increase in the catalyst amount,
however, the selectivity towards benzaldehyde decreased
significantly due to over oxidation of benzaldehyde to ben-
zoic acid. It was also found that a lower concentration of
reactant used gave higher activity and selectivity (Table S3,
supplementary information). Of the various oxidants stud-
ied, TBHP is the only active oxidant at room temperature,
producing a conversion of 8.5% and an 80% selectivity
towards benzaldehyde. H2O2 and air showed no activity at
room temperature (Fig. S4, supplementary information).
Styrene oxidation over the Rb and Cs exchanged PMA
catalysts under similar reaction conditions indicate that
the 2 wt% catalysts showed a similar activity trend to the
2KPMA catalyst, but not as efficient. The order of reactivity
1 3

The Role of Alkali Metal Exchanged Phosphomolybdic Acid Catalysts in the Solvent Free Oxidation…
Fig. 5 SEM and HR-TEM images of a PMA, b 2KPMA, c 2RbPMA and d 2CsPMA, respectively
1 3

J. Pedada et al.
Fig. 6 BF-STEM analysis of a 1KPMA, b 2KPMA and c 3KPMA catalysts
Table 2 Catalytic results over alkali metal exchanged phosphomolyb-
Table 3 Catalytic results over KPMA-2 catalysts at room temperature
dic acid catalysts
Substrate
Conversion
(%)
Selectivity (%)
Catalyst Conver- Selectivity (%)
sion
TONb
Benzalde-
hyde
Styrene
oxide
Others
Benzaldehyde Styrene oxide Othersa
(%)
Toluene
Styrene
1.2
8.5
19.7
99
80
97
0
16
0
1
4
3
Blank
PMA
2.0
4.5
73
79
79
80
80
79
82
80
79
80
78
18
16
16
16
16
15
14
15
14
15
16
9
5
5
4
4
6
4
5
7
5
6
–
7.6
Benzyl
1KPMA 6.7
2KPMA 8.5
3KPMA 7.0
1RbPMA 6.2
2RbPMA 7.4
3RbPMA 6.3
1CsPMA 5.8
2CsPMA 6.5
3CsPMA 5.9
11.3
14.5
12.0
10.5
13.0
10.8
9.9
alcohol
Reaction conditions: substrate (10 mmol), TBHP (10 mmol), cata-
lyst=0.1 g, reaction time=24 h
Others 1-phenylethane -1,2-diol, phenyl acetaldehyde, benzoic acid;
reactions done in triplicate
11.1
9.8
conducted at room temperature over the 2KPMA catalyst
(Table 3). Benzyl alcohol is more reactive than styrene and
toluene. It is also highly selective towards benzaldehyde,
probably due to the carbinol group being more reactive
towards carbonyl compound than olefins and alkanes. A
similar activity pattern was obtained with the 2RbPMA and
2CsPMA catalysts.
Reaction conditions: styrene (10 mmol), TBHP (10 mmol), cata-
lyst=0.1 g, RT reaction temperature, reaction time=24 h
aOthers: 1-phenylethane -1,2-diol, phenyl acetaldehyde;
bTON number of moles of product/number of moles of catalyst; reac-
tions done in triplicate
A comparative study of the oxidation of styrene to other
aromatic compounds such as benzyl alcohol and toluene was
Time on stream studies was carried out to determine the
influence of reaction time on conversion and selectivity
1 3

The Role of Alkali Metal Exchanged Phosphomolybdic Acid Catalysts in the Solvent Free Oxidation…
supplementary information). The catalyst was separated
from aqueous layer, filtered and washed several times with
organic solvent dichloromethane to remove any organic
moiety on the surface of the catalyst and dried in an oven
over night. It was then characterized by infrared spectros-
copy (Figs. 2, 3) to observe any structural changes and
any changes in the surface acidity. There was minimal
change in structural and acidic properties of the catalysts.
Although there was minimal change in the conversion,
changes in the selectivity were evident. This could be due
to the loss of metal after each cycle as corroborated by
ICP analysis.
The mechanistic pathway for the synthesis of benzalde-
hyde from styrene is well known [21, 45]. For the selective
synthesis of benzaldehyde from styrene over KPMA cata-
lysts, the proposed path ways are shown in Scheme 2. Ini-
tially, the catalyst reacts with TBHP to produce tert-butyl
peroxy and tert-butyloxy radicals. These radicals then react
with styrene to produce benzaldehyde as major product and
styrene oxide. Styrene oxide can undergo isomerization
and hydrolysis to produce phenyl acetaldehyde and 1,2-
diol, respectively. Further oxidation of the diol produces
benzaldehyde. Styrene also forms dioxitane and further
cleavage results in the formation of benzaldehyde and
formaldehyde.
Fig. 7 Time on stream results for the oxidation of styrene over
2KPMA at room temperature
Table 4 Styrene oxide oxidation over 2KPMA catalyst
Reaction Conversion (%) Selectivity (%)
time (h)
Phenyl
acetalde-
hyde
Benzaldehyde Others
1
2
3
4
5
97
71
69
68
67
66
22
23
24
25
26
7
8
8
8
8
97.3
97.9
98
98.1
4 Conclusion
Reaction conditions: styrene oxide (10 mmol), TBHP (10 mmol), cat-
Eco-friendly heterogeneous metal exchanged phospho-
molybdic acid catalysts were efficient for the solvent free
oxidation of styrene to benzaldehyde. The 2KPMA catalyst
performed much better than its Rb and Cs counterparts in
terms of selectivity towards benzaldehyde. The catalytic
activity correlated well with the physical and chemical prop-
erties of the catalyst. The metal exchanged catalysts with
a 2 wt% loading showed the highest surface areas ranging
between 130 and 180 m2 g−1, all of them exhibiting Type IV
isotherms, clearly demonstrating their mesoporous nature.
XRD patterns and infrared spectroscopy showed that the pri-
mary structure of the Keggin ion remains intact after incor-
poration of the metal, while pyridine adsorbed spectroscopy
shows the existence of Brønsted as well as Lewis acid sites
on the surface. The surface of all the catalysts possessed a
uniform spherical morphology and from BF STEM analysis,
an even distribution of metals in the catalysts were observed.
The regenerated catalyst showed minimal change in struc-
tural and acidic properties, reflected in the catalytic results
done up to three cycles.
alyst=0.1 g, RT reaction temperature
Others: 1-phenylethane -1,2-diol, benzoic acid; reactions done in trip-
licate
over the 2KPMA catalyst (Fig. 7). Initially, the selectiv-
ity towards benzaldehyde and styrene oxide was 77 and
28%, respectively. Over time, the selectivity towards ben-
zaldehyde increased while there was a decrease in styrene
oxide selectivity, indicating that the formation of benzal-
dehyde also occurs via styrene oxide. This result supports
the assumption that there are most probably two pathways
for benzaldehyde formation, one is C=C cleavage and the
other is the cleavage of styrene oxide to produce benzalde-
hyde. To further gather evidence, the oxidation of styrene
oxide was studied under present conditions. These results
show that phenyl acetaldehyde together with benzalde-
hyde are produced (Table 4), providing some evidence that
styrene oxidation proceeds via C=C cleavage to produce
benzaldehyde.
Recycling experiments were carried out to investi-
gate the stability of the 2KPMA catalyst (Figure S5,
1 3

J. Pedada et al.
Scheme 2 Proposed reaction mechanism of oxidation of styrene to benzaldehyde over KPMA catalysts
14. Kozhevnikov IV (1998) Chem Rev 98:171–198
Acknowledgements Authors thank to the University of KwaZulu-Natal
15. Mizuno N, Misono M (1998) Chem Rev 98:199–218
for providing research support.
16. Viswanadham B, Jhansi P, Chary KVR et al (2016) Catal Lett
146:364–372
Compliance with Ethical Standards
17. Viswanadham B, Pedada J, Friedrich HB et al (2016) Catal Lett
146:1470–1477
Conflict of interest There is no conflict of interest by Jhansi Pedada,
18. Viswanadham B, Kumar VP, Chary KVR (2014) Catal Lett
144:744–755
Holger B. Friedrich and Sooboo Singh.
19. Viswanadham B, Srikanth A, Chary KVR (2014) J Chem Sci
126:445–454
20. Viswanadham B, Srikanth A, Kumar VP et al (2015) J Nanosci
Nanotechnol 15:5391–5402
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The Role of Alkali Metal Exchanged Phosphomolybdic Acid Catalysts in the Solvent Free Oxidation…
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