Tuning surface composition of Cs exchanged phosphomolybdic acid catalysts in C[sbnd]H bond activation of toluene to benzaldehyde at room temperature

Article abstract of DOI:10.1016/j.molcata.2016.10.007

A series of Cs exchanged phosphomolybdic (PMA) catalysts were synthesised by the ion exchange method. The prepared materials were characterized by various physico-chemical techniques such as X-ray diffraction, FT-IR, Raman spectroscopy, pyridine adsorbed

Full text of DOI:10.1016/j.molcata.2016.10.007

Accepted Manuscript
Title: Tuning Surface Composition of Cs Exchanged
Phosphomolybdic Acid Catalysts in C-H Bond Activation of
Toluene to Benzaldehyde at Room Temperature
Author: Viswanadham Balaga Jhansi Pedada Holger B.
Friedrich Sooboo Singh
PII:
S1381-1169(16)30428-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.10.007
MOLCAA 10069
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
3-7-2016
3-10-2016
4-10-2016
Please cite this article as: Viswanadham Balaga, Jhansi Pedada, Holger
B.Friedrich, Sooboo Singh, Tuning Surface Composition of Cs Exchanged
Phosphomolybdic Acid Catalysts in C-H Bond Activation of Toluene to
Benzaldehyde at Room Temperature, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.10.007
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Tuning Surface Composition of Cs Exchanged Phosphomolybdic Acid
Catalysts in C-H Bond Activation of Toluene to Benzaldehyde at Room
Temperature
Viswanadham Balaga*, Jhansi Pedada, Holger B. Friedrich and Sooboo Singh
_
__________________________________________________________________________
Catalysis Research Group,
School of Chemistry & Physics,
University of KwaZulu-Natal-Durban,
South Africa.
Email: visubalaga@gmail.com/Balagiav@ukzn.ac.za
Tel.: +27 31 2608648; Fax: +27 31 260 3091
_
__________________________________________________________________________
1

Graphical abstract
Highlights:



Cs exchanged phosphomolybdic materials were synthesised by ion exchange method
CsPMA-2 exhibits higher surface area and Brønsted acidity
CsPMA-2 is highly selective and more active than the PMA and other Cs exchanged
catalysts in C-H bond activation

Regenerated CsPMA- catalyst exhibits stable activity during reaction
Abstract
A series of Cs exchanged phosphomolybdic (PMA) catalysts were synthesised by the ion
exchange method. The prepared materials were characterized by various physico-chemical
techniques such as X-ray diffraction, FT-IR, Raman spectroscopy, pyridine adsorbed FT-IR
3
1
spectroscopy, P NMR, BET surface area, nitrogen isotherms, TEM and STEM analysis.
XRD results show crystallites of the Keggin ion of heteropolyacid catalysts. FT-IR and
Raman spectra data reveal that the Keggin structure of PMA catalyst is maintained even after
2

the exchange of the protons with Cs ions. Pyridine adsorbed FT-IR spectra show that
available Brønsted acidic sites increases after Cs is incorporated in PMA. TEM analysis
shows that the spherical shape remains intact after Cs exchange and STEM analysis shows a
uniform distribution of elements in the Cs exchanged PMA catalyst. Nitrogen isotherms
studies reveal a type IV isotherms for the Cs exchanged catalyst. The Cs exchanged
phosphomolybdic acid (CsPMA) showed better catalytic performance than the
phosphomolybdic acid (PMA) catalyst when evaluated for the C-H bond activation of toluene
to benzaldehyde. The catalyst was easily recovered and reused for five cycles with stable
activity.
Keywords: Cesium, phosphomolybdic acid, ion-exchange, C-H bond activation, Brønsted
Acidity.
1
.
Introduction
Polyoxometalates (POMs) or heteropolyacids (HPAs) are well known solid acid catalysts due
to their tunable bi-functional acid and redox properties for various acid and oxidation
reactions, respectively [1, 2]. Recently, HPAs have been received considerable attention in
many acid catalyzed and oxidation reactions with industrial applications [3-15]. HPAs are
well known strong Brønsted acidity materials, however one of its major drawbacks is the low
surface area which may impact on its catalytic performance. This can be resolved, to a certain
extent, by modifying the textural properties and acidic functionalities of the compound,
simply by exchanging or replacing the protons of HPA, specifically PMA with a large cation
such as cesium Cs. Cs exchanged PMA catalysts are water tolerant solid acid catalysts which
possess a higher surface area than the PMA catalysts and were studied in number of reactions
including carbonylation, dehydration, condensation, alkylation, acylation, isomerization of n-
3

butane, hydration of olefins and biodiesel synthesis [16-22]. In the view of several organic
transformations carried out over Cs containing Keggin HPA catalysts, we proceeded to
investigate the selective C-H bond activation of toluene to to produce benzaldehyde at room
temperature over Cs exchanged phosphomolybdic acid catalysts.
The catalytic selective oxidation of toluene to the benzaldehyde has great importance in both
basic and applied research due to its extensive applications in academic and industrial
chemistry. Benzaldehyde is a versatile aromatic carbonyl compound used in the perfumery,
cosmetics and agrochemical industries. It is currently produced by liquid phase chlorination
of toluene followed by hydrolysis [21,22] and also by the partial oxidation of toluene [23].
Among the two processes, oxidation of toluene is the more preferable route since chlorination
of toluene followed by hydrolysis is a non-eco-friendly method. However, the oxidation of
toluene in the gas phase leads to the formation of carbon oxides instead of benzaldehyde as
the major product. Liquid phase oxidation or activation of toluene to benzaldehyde at room
temperature could address these problems and at the same time, promote the selective
synthesis of benzaldehyde as an eco-friendly process. Various studies involve the synthesis of
benzaldehyde from toluene at room temperature and among them, heteropolyacid (HPA) has
received considerable attention due to its eco-friendly and bi-functional nature [24-28]. Liu et
al. [24] synthesized a tripodal tridentate copper(II) complex, CuImph (Imph = bis(4-
imidazolyl methyl)benzylamine) to mimic the active site of copper enzymes that mediate the
oxidation of aliphatic C–H bonds under mild conditions. Their results show high reactivity
and selectivity toward toluene aliphatic C–H bond oxidation, converting the toluene initially
to benzyl alcohol and subsequently to benzaldehyde as the major product in a kinetic
consecutive reaction. No evidence for benzoic acid is obtained. Wurtele et al. [26], on the
other hand used copper peroxo complexes which they found to be stable at room temperature
for the aliphatic C-H bond oxidation of toluene. To test the potential of these complexes, they
4

reacted four peroxo complexes with toluene at room temperature under air and observed the
formation of benzaldehyde in yields of up to 20%, along with small amounts of benzyl
alcohol.
Our present work is related to effect of Cs exchanged phosphomolybdic acid catalysts for the
selective C-H bond activation of toluene to benzaldehyde at room temperature. These
materials were characterized by various techniques to investigate its surface, structural and
acidic functionalities and its correlation with the catalytic activity.
2
2
.
Experiential
.1
Catalyst synthesis
A series of Cs exchanged phosphomolybdic acid catalysts with various Cs content, ranging
from 1 to 3 wt% loadings were prepared by ion exchanged method. In a typical synthesis, for
a 1 wt% loading, 0.326 g of Cs
distilled water. This solution was added drop wise to a solution of 3.65g of H
Sigma Aldrich, SA) made up in 15 mL of double distilled water. The resulting mixture was
2
CO
3
(Sigma Aldrich, SA) was dissolved in 10 mL of double
3
PMo12
O
40
(
centrifuged to remove the precipitate which was the washed repeatedly with double distilled
water. The solid was dried at 200°C for 10 h. These prepared catalysts which had the
formulae, Cs1.0PMo12O40, Cs2.0PMo12O40 and Cs3.0PMo12O40 were labelled as CsPMA-1,
CsPMA-2 and CsPMA-3, respectively.
.2 Catalyst characterization
2
Powder X-ray diffraction patterns of samples were obtained with a Bruker D8 Advance
diffractometer, using a Cu Ka radiation source (1.5406 A˚) at 40 kV and 30 mA. The
measurements were recorded in steps of 0.045° with a count time of 0.5 s in the range of 2-
o
6
0°. The surface area of catalysts was determined using N
2
adsorption isotherms at -196 C by
2
the multipoint BET method taking 0.162 nm as its cross-sectional area. The pore size
5

distribution was measured by N adsorption-desorption isotherms using a Micrometrics
2
ASAP 2020 multi-point BET surface area analyzer. Prior to these experiments, the materials
were degassed under helium flow overnight at 200°C using a Micrometrics Flow Prep 060
and all experiments were carried out at a relative pressure range (p/p°) of 0.05 to 0.9.
Infrared (IR) spectra were recorded on a Perkin Elmer Precisely equipped with a Universal
ATR sampling accessory using a diamond crystal. The powdered material was placed on the
crystal and a force of 120 psi was applied to ensure proper contact between the material and
the crystal. The spectra were analyzed using Spectrum 100 software. Ex-situ pyridine
adsorbed FT-IR experiments were carried out to investigate the nature of acidity, such as
Brønsted and/or Lewis acid sites present on the catalyst. The spectra were recorded on a
Perkin-Elmer ATR spectrometer at room temperature. Prior to analysis, pyridine adsorption
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. Raman
spectra were collected using a Horbia-Jobin-Yvon LabRam high resolution spectrometer
equipped with a confocal microscope with 2400/900 grooves/mm gratings and a notch filter.
The visible laser excitation at 532 nm (visible/green) was supplied by a Yag double diode
pumped laser (20mW). Transmission electron microscopy (TEM) images were obtained on a
Jeol JEM-1010 electron microscope. The images were captured and analyzed using iTEM
software. Distribution and chemical analysis of elements in the materials was examined by
3
1
STEM analysis. P NMR spectra of catalyst were recorded on a Bruker Advance 400 MHz
spectrometer and chemical shifts were reported relative to 85 % H
3
PO
4
in D O used as an
2
external standard at 298K.
2
.3
Catalytic testing
Solvent free liquid phase oxidation of toluene was carried out in a 10 mL round bottom flask
at room temperature. In a typical run, 10 mmol of toluene, 2 mL of 70% TBHP and 4 mol %
6

of the catalyst were placed in the flask, under stirring (Scheme 1) and the reaction was
constantly monitored by GC. The catalyst was then recycled and used for the same reaction.
3
3
3
.
Results & Discussion
Catalyst characterization
X-ray diffraction
.1
.1.1
X-ray diffraction patterns of pure PMA and synthesised Cs exchanged PMA catalysts are
shown in Figure 1. These profiles indicate that PMA shows the triclinic structure, whereas the
Cs exchanged PMA catalyst possesses the cubic structure of the Keggin ion of a
heteropolyacid. The XRD profile also shows that in the pure PMA, the Keggin ion, usually
found around a 2-thetha value of 9-10° is more pronounced when compared to the Cs
exchanged PMA, consistent with findings in literature [29].
3
.1.2
FT-IR spectroscopy
The FT-IR spectra of PMA and Cs-PMA catalysts shows four characteristic bands in the 500-
-1
-1
1
200 cm region. The bands for the pure PMA at 760, 889, 961, 1058 cm are attributed to
Mo-Oc-Mo, Mo-O -Mo, Mo=O and P-O, respectively [3,4,6,7]. In Cs exchanged catalysts, a
t
slight shift in the bands found at 754-767, 860, 960-962 and 1060-1061 are assigned to Mo-
-Mo, Mo=O and P-O, respectively. The shift of the peaks of about 2 to 7 cm-1
Oc-Mo, Mo-O
t
indicates that Cs successfully exchanges with protons in PMA, maintaining the characteristic
Keggin ion of the heteropolyacid, even at higher loadings of cesium [30].
3
.1.3
Raman spectroscopy
-1
From Figure. 3, the characteristic Keggin ion band is present at 989-992 cm for the PMA
-1
and the CsPMA catalysts. The more intense bands at 239, 600-603, 814 and 876-884 cm are
assigned to Mo-O
a
c b t
, Mo-O -Mo, Mo-O -Mo and Mo-O , respectively [3,4,6,7]. It is also
7

established that Keggin ion remains intact, even at higher loading of Cs. The findings in
Raman spectroscopy complements the observations made in FT-IR spectroscopy.
3
.1.4
Pyridine adsorbed FT-IR spectroscopy
The nature of the acidity in the catalysts was studied by ex situ pyridine FT-IR spectroscopy
-1
and the results are shown in Figure 4. The IR band at 1538 cm is assigned to Brønsted
-1
acidic sites. The peak observed at 1486 cm is due to both Brønsted and Lewis acidic sites,
-1
whereas the band is observed at 1441 cm is attributed to Lewis acidic sites of the catalysts.
From Figure 4, it is also apparent that Brønsted acidic sites increased slightly with an increase
in the Cs content in PMA catalysts, but decreased considerably at higher Cs content. Lewis
acidity is predominant at higher Cs content. A combination of both Brønsted and Lewis
acidic sites decreased at higher Cs loadings.
Brønsted acidity is observed more in the case of CsPMA-2 catalyst rather than other loadings,
which might be due to a large surface area exhibited by the CsPMA-2 catalyst, resulting in
the availability of more protons on the surface area of catalyst.
3
.1.5 Nitrogen adsorption-desorption isotherms
Textural properties of PMA and the synthesized Cs exchanged PMA catalysts are tabulated in
Table 1. The surface area of the catalysts increases with an increase in Cs content up to the
CsPMA-2 catalyst. All the Cs exchanged catalysts exhibits a type IV isotherm and a
hysteresis loop associated with mesoporous materials [31]. At higher Cs loadings, the BET
surface area decreased substantiantly with a slight increase in pore size, probably due to
agglomeration. The high BET surface area of the CsPMA-2 catalyst results in a lower Keggin
anion density as reflected by the lower crystallinity of Keggin ion units when compared to
other Cs loadings. These results correlate well with X-ray diffraction studies which shows
8

that pure PMA is more crystalline than the Cs exchanged PMA catalysts. The particle size
obtained from sorption experiments shows large particle sizes for the PMA catalyst compared
to the Cs exchanged PMA catalysts.
3
1
3
.1.6 P NMR spectroscopy
31P NMR spectra of PMA and the Cs exchanged PMA catalysts were recorded in D
O solvent
2
(
Figure 6). The pure PMA catalyst showed a chemical shift at -3.9 ppm, whereas in the case
of Cs exchanged PMA catalysts, this peak disappears indicating that the Cs exchanged PMA
is not soluble in the D O solvent, providing evidence that the Cs exchanged catalysts have no
impurities such as the starting material which is PMA.
2
3
.1.7 TEM analysis
The internal morphology of pure PMA and the CsPMA catalysts was studied by TEM
analysis and the results are shown in Figure 7. There is every indication that the pure PMA
has a spherical structure and this structure was maintained with the Cs exchanged
compounds.
3
.1.8 STEM analysis
BF-STEM imaging with chemical analysis of fresh and regenerated CsPMA-2 catalyst was
conducted to map the structure and relative spatial distribution of the elements present in the
catalysts (Figure 8) and to obtain the Cs content in the Keggin anion (Table 1).
3
.2
Catalytic activity studies
The catalytic activation of C-H bond in toluene was studied over Cs exchanged PMA
catalysts under nitrogen atmosphere and the results are tabulated in Table 2.
9

Initially, blank reactions were studied at same reaction conditions, however there was no
activity recorded. The phosphomolybdic acid showed a 0.6% conversion and 53% towards
benzaldehyde. Among the various Cs exchanged PMA catalysts, the CsPMA-2 catalyst
showed the highest conversion with almost maximum selectivity towards benzaldehyde
compared to the other Cs/P ratios, probably due to the larger number of available active sites
as reflected by the high surface area of the catalyst. Surface acidity of the catalyst also plays a
role. At a higher Cs loading, i.e. for CsPMA-3, the catalyst is more crystalline leading to a
decrease in the surface acidity resulting in lower activity, similar to that observed for the
CsPMA-1 and pure PMA catalyst. It is also interesting to note that the higher turnover
number of CsPMA-2 leads to better activity compared to the other loadings and the PMA
itself. The catalytic results are strongly associated with the nature of acidity of the catalyst.
From pyridine adsorbed FT-IR studies, more surface Brønsted acidic sites are observed in the
case of CsPMA-2 catalyst than the other Cs containing materials including the pure PMA
catalyst. The mesoporous nature of CsPMA-2 catalyst with its high pore volume and surface
area also favours formation of benzaldehyde compared other catalysts.
The oxidation of toluene over the CsPMA-2 catalyst was studied with various volumes of the
oxidant, TBHP and the results are presented in Table 3. These results are show that the
activity increases significantly when the volume of the oxidant is doubled from 1 mL to 2
mL. Thereafter, there is a slight increase in the activity as the volume of the oxidant
increased. An increase in the concentration of TBHP results in the formation of benzoic acid
at the expense of benzaldehyde.
The effect of amount of catalyst used on the activity was investigated and the results are
given in Table 4. The conversion of toluene increased with an increase in the amount of the
catalyst. Although the conversion increased with an increase of catalyst loadings, the
selectivity towards benzaldehyde selectivity decreased substantiantly, probably be due to
1
0

further oxidation of benzaldehyde to benzoic acid at higher concentration of the
heteropolyacid at room temperature. The above findings also show that 0.1 g of catalyst is the
optimum amount required.
We also studied effect of toluene concentration over the CsPMA-2 catalyst under similar
reaction conditions (Table 5). These findings show that an increase in the concentration of
reactant results in a decrease in the conversion but selectivity towards benzaldehyde increases
significantly. The effect of various oxidants was studied and results are shown in Figure 9.
TBHP is the most efficient oxidant that can activate the C-H bond in the presence of the
heteropolyacid catalyst.
Time on stream experiments were conducted to investigate the influence of time on the
activity of the CsPMA-2 catalyst. From Figure 10, at the outset, conversion of toluene and
selectivity towards benzaldehyde is low, but increases significantly over time.
The catalyst was subjected to a number of cycles to study its stability (Figure 11). Before
regeneration, the catalyst was separated, filtered and washed several times with organic
solvent to remove any organic moiety on the surface of the catalyst and dried in an oven over
night. The catalyst was characterized by FT-IR (Figure 2) to observe any structural changes
and pyridine adsorbed FT-IR (Figure 4) to investigate any change in the surface acidity.
STEM analysis was also conducted to observe any change in the Cs content of the
regenerated catalyst. All these analyses showed no change in terms of structure, surface
acidity and concentration in the regenerated CsPMA-2 catalyst as a result consistent results
were obtained over the five cycles investigated.
1
1

The reaction was also studied a different temperatures various reaction temperatures and the
results indicate that the formation of benzaldehyde increased with an increase in reaction
temperature. However, as the temperature was increased to 120°C, the benzaldehyde
selectivity decreased sharply and benzoic acid was observed as the major product. Brutchey
et al. [32] studied toluene oxidation over Co supported SBA-15 using TBHP as oxidant
showed a 8% conversion with a 64% selectivity towards benzaldehyde at 80°C. However, in
our study, the Cs exchanged PMA catalyst achieved a 86% selectivity towards benzaldehyde
for a 3.1 % conversion at 80°C, showing that the CsPMA-2 catalyst is highly selective
towards benzaldehyde compared to Co-SBA-15 catalyst at a lower reaction temperature.
3
.3
Proposed reaction mechanism
The selective synthesis of benzaldehyde over Cs-PMA catalysts could be due to the proposed
pathways shown in Scheme 2 [33]. Initially, in path 1, the Cs-PMA catalyst reacts with
TBHP to form tert-butyl peroxy and tert-butyloxy radicals. In path 2, these radicals react with
toluene and produce benzaldehyde as the major product. However, the benzyl radical reacts
with the catalyst to from the benzyl cation by a transfer of an electron to the catalyst which
later reacts with the hydroxyl ion to form benzyl alcohol [33]. The formation of the
benzaldehyde from benzyl alcohol over phosphomolybdic acid catalysts occurs through the
transfer of two electrons and two protons from the benzyl alcohol to the catalyst to produce
benzaldehyde [34].
4
.
Conclusion
In the present investigation, we have demonstrated an eco-friendly heterogeneous Cs
exchanged phosphomolybdic acid catalysts exhibiting remarkable benzaldehyde selectivity
1
2

and significant conversion than the pure PMA catalyst during C-H bond activation of toluene.
BET surface area and pore size distribution studies shows that the mesoporous nature of
CsPMA-2 catalyst results in higher selectivity towards desired product. FT-IR and Raman
spectroscopy reveal that Keggin structure is maintained even after the exchange of Cs in
3
1
PMA. P NMR spectra strongly indicate the water tolerant nature of HPA catalysts, while
pyridine FT-IR shows that surface acidity is related to the catalytic activity of the Cs
exchanged catalysts. STEM images show an even spatial distribution of metals in both the
fresh and regenerated CsPMA-2 catalyst. The catalyst maintains its structure and acidity even
after recycling up to 5 times.
Acknowledgments: Authors thank to the University of KwaZulu-Natal for financial support.
1
3

References
[
[
[
1]
2]
3]
I.V. Kozhevnikov, Chem. Rev. 98 (1998) 171.
N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199.
B. Viswanadham, P. Jhansi, K.V.R. Chary, H.B. Friedrich, S. Singh, Catal. Lett. 146
(
2016) 364.
[
[
[
[
4]
5]
6]
7]
B. Viswanadham, J. Pedada, H.B. Friedrich, S. Singh, Catal. Lett. 146 (2016) 1470.
B. Viswanadham, V.P. Kumar, K.V.R. Chary, Catal. Lett. 144 (2014) 744.
B. Viswanadham, A. Srikanth, K.V.R. Chary, J. Chem. Sci. 126 (2014) 445 .
B. Viswanadham, A. Srikanth, V.P. Kumar, K.V.R. Chary, J. Nanosci. Nanotech. 15
(
2015) 5391.
[
[
8]
9]
S. Yamaguchi, S. Sumimoto, Y. Ichihashi, S. Nishiyama, S. Tsuruya, Ind. Eng. Chem.
Res. 44 (2005) 1.
G. Wang, Y. Han, F. Wang, Y. Chu, X. Chen, Reac. Kinet. Mech. Cat. 115 (2015)
6
79.
[
[
[
[
[
[
[
10] L. Marosi, G. Cox, A. Tenten, H. Hibst, J. Catal. 194 (2000) 140.
11] Z. Long, Y. Zhou, G. Chen, W. Ge, J. Wang, Sci. Report. 4 (2014) 3651.
12] Z. Fumin, G. Maiping, G. Hanqing, W. Jun, Chin. J. Chem. Eng. 15 (2007) 895.
13] P. Sharma, A. Patel, Ind. J. Chem. 48A (2009) 964.
14] I.K. Song, J.K. Lee, G.I. Park, W.Y. Lee, Stu. Sur. Sci. Catal. 110 (1997) 1183.
15] M. Misono, Chem. Comm. 13 (2001) 1141.
16] A. Srikanth, B. Viswanadham, V.P. Kumar, N.R. Anipindi, K.V.R. Chary, Appl.
Petrochem. Res. 6 (2016) 145.
[
[
[
17] P. Guyraud, N. Essyem, J.C. Vedrine, Catal. Lett. 56 (1999) 35.
18] A. Corma, M. Martinez, C. Martinez, J. Catal. 164 (1996) 422.
19] N. Essayem, G. Coudurier, M. Fournier, J.C. Vedrine, Catal. Lett. 34 (1995) 223.
1
4

[
[
20] K. Narasimharao, D.R. Brown, A.F. Lee, A.D. Newman, P.F. Siril, S.J. Tavener, K.
Wilson, J. Catal. 248 (2007) 226.
21] T. Okuhara, H. Watanabe, T. Nishimura, K. Inumaru, M. Misono, Chem. Mater. 12
(
2000) 2230.
[
[
22] M. Kimura, T. Nakato, T. Okuhara, Appl. Catal. 165 (1997) 227.
23] B. Friedrich, W. Elaine “Benzaldehyde” in Ullmann's Encyclopedia of Industrial
Chemistry, 2002, Wiley-VCH.
[
[
[
24] C.C. Liu, T.S. Lin, S.I. Chan, C.Y. Mou, J. Catal. 322 (2015) 139.
25] M. Mahyari, M.S. Laeini, A. Shaabani, Chem. Comm. 50 (2014) 7855.
26] C. Wurtele, O. Sander, V. Lutz, T. Waitz, F. Tuczek, S. Schindler, J. Am. Chem. Soc.
1
31 (2009) 7544.
[
(
27] S. Bose, A. Pariyar, A.N. Biswas, P. Das, P. Bandyopadhyay, Catal. Comm. 12
2001)
1
193.
[
[
28] K.T.V. Rao, P.S.N. Rao, P. Nagaraju, P.S.S.Prasad, N. Lingaiah, J. Mol. Catal. A:
Chem. 303 (2009) 84.
29] H.R. Ghalebi, S. Aber, A. Karimi, J. Mol. Catal A: Chem. 415 (2016) 96.
[
(
[
30] M. Sun, J. Zhang, C. Cao, Q. Zhang, Y. Wang, H. Wan, Appl. Catal. A: Gen. 349
2008) 212.
31] J.B. McMonagle, J.B. Moffat, J. Colloid. Interface. Sci 101 (1984) 479.
[
[
32] R.L. Brutchey, I.J. Drake, A.T. Bell, T. Don Tilley, Chem. Commun. (2005) 3736.
33] A.M. Khenkin, R. Neumann, J. Am. Chem. Soc. 123 (2001) 6437.
[
34] A. Patel, S. Pathan, Ind. Eng. Chem. Res. 51 (2012) 732.
1
5

Figure 1. XRD profile of (a) PMA, (b) CsPMA-1, (c) CsPMA-2, (d) CsPMA-3 catalysts
1
6

Figure 2: FT-IR spectra of (a) PMA, (b) CsPMA-1, (c) CsPMA-2, (d) Reg. CsPMA-2,
e) CsPMA-3 catalysts
(
1
7

Figure 3: Raman spectra of (a) PMA, (b) CsPMA-1, (c) CsPMA-2, (d) CsPMA-3 catalysts
1
8

Figure 4: Pyridine adsorbed FT-IR spectra of (a) CsPMA-3, (b) CsPMA-2, (c) regenerated
CsPMA-2, (d) CsPMA-1, (e) PMA catalysts
1
9

80
70
60
50
40
30
20
10
0
0
0
0
.25
.2
.15
.1
(a)
(b)
0
0
.05
0
0
0.5
1
1.5
0
50
100
Relative pressure (p/po)
Pore diameter (Å)
Figure 5 Nitrogen adsorption-desorption isotherm (a) and BJH pore size distribution (b) of
the CsPMA-2 catalyst
2
0

(
(
d)
c)
(
(
b)
a)
1
0
0
-10
-20
-30
Chemical shift (ppm)
3
1
Figure 6: P NMR of (a) PMA, (b) CsPMA-1, (c) CsPMA-2, (d) CsPMA-3 catalysts
2
1

(
a)
(b)
(d)
(
c)
Figure 7 TEM images of (a) PMA (b) CsPMA-1, (c) CsPMA-2 and (d) CsPMA-3 catalysts
2
2

A
Cs La 1
Mo Ka 1
P Ka 1
B
Cs La 1
P Ka 1
Mo Ka 1
Figure 8 STEM images of (A) fresh and (B) regenerated CsPMA-2 catalyst
These findings show that theoretical Cs loadings are well correlated with experimental atomic
%
data obtained from STEM chemical analysis. The fresh and regenerated CsPMA-2 shows
uniform spatial distribution of all the elements and shows a 2.1 Cs atomic % of the catalyst.
2
3

8
5
3
8.0
6.0
2.0
%
%
%
Conversion
Benzaldehyde selectivity
Benzyl alcohol selectivity
8
.0
3.0
2.0
1.0
0
Air
H
O
2 2
TBHP
Oxidant (2 mL)
Figure 9: Catalytic results with various oxidants over CsPMA-2 catalyst
2
5

%
%
Conversion
% Benzaldehyde selectivity
Benzyl alcohol selectivity
100
80
60
40
20
1
1
0
0
.6
.2
.8
.4
0
4
8
12
16
20
24
Reaction time (h)
Figure 10 Time on stream studies over the CsPMA-2 catalyst
2
6

%
%
Conversion
% Benzaldehyde selectivity
Benzyl alcohol selectivity
100
80
60
40
20
2
.0
.6
1
1.2
0
.8
.4
0
0
1
2
3
4
5
Cycle
Figure 11 Conversion of toluene and selectivity towards benzaldehyde and benzyl alcohol
over the CsPMA-2 catalyst for 5 cycles.
2
7

O
H
CH3
Cs-PMA
75% TBHP
Scheme 1. Oxidation of toluene to benzaldehyde over Cs exchanged phosphomolybdic acid
catalyst
Path 1
-
H
O
H
H
O
O
CsPMA (VI) +
CsPMA (V) +
CsPMA (VI) +
O
O
O
O
-
OH
CsPMA (V)
Path 2
+
HO
HO
O
O
O
+
O
H
O
H
H
H
H
O
H
H
H
CsPMA (VI)
CsPMA (VI)
OH
CsPMA (V) +
H
O
H
H
H
OH
H
Scheme 2 Proposed reaction mechanism of C-H bond activation of toluene to benzaldehyde
over Cs exchanged phosphomolybdic acid (Cs-PMA) catalysts.
2
8

Table 1 Textural properties and Cs content of Cs-PMA catalysts
a
Catalyst
BET
Pore
Pore
Particle size Cs content
Cs content
Keggin anion
surface
area (m /g) (cm /g)
volume
diameter
(Å)
(Å)
theoretical per practical per density
2
3
2
Keggin ion
atomic %)
Keggin ion
(atomic %)
(HPA/nm )
(
PMA
2.7
0.01
128
21662
--
--
1.2
CsPMA-1
CsPMA-2
25
0.03
0.14
80
40
1950
409
1.0
2.0
0.9
2.1
0.12
146
0.019
CsPMA-3
80
0.08
50
756
3.0
2.9
0.033
aCs content obtained from STEM analysis
Table 2: Catalytic results over Cs exchanged phosphomolybdic acid catalystsa
Catalyst
Conversion (%) Selectivity (%)
Benzaldehyde Benzyl alcohol
bTON
Blank
--
--
--
--
PMA
0.6
0.7
0.9
0.5
53
85
98
78
47
15
2
1.09
1.37
1.88
1.10
CsPMA-1
CsPMA-2
CsPMA-3
22
a
Reaction conditions: Toluene (10 mmol), TBHP (2 mL), Catalyst = 0.1 g, Reaction temperature =
b
RT, Reaction time = 24 h, TON = Number of moles of products/Number of moles of catalysts
2
9

Table 3 Catalytic results over CsPMA-2 catalysts with various amounts of TBHPa
TBHP (mL) Conversion (%) Selectivity (%)
Benzaldehyde Benzyl alcohol Benzoic acid
1
2
4
6
0.5
0.9
1.0
1.1
96
98
99
97
4
--
--
1
2
--
--
3
a
Reaction conditions: Toluene (10 mmol), TBHP (1-6 mL), Catalyst = 0.1 g, Reaction temperature =
RT, Reaction time = 24 h.
Table 4 Catalytic results over various loadings of CsPMA-2 catalystsa
Catalyst (g) Conversion (%) Selectivity (%)
Benzaldehyde Benzyl alcohol
Benzoic acid
0
0
0
0
.05
.1
0.4
0.9
1.1
1.1
96
98
90
85
4
2
1
-
--
--
9
.2
.3
15
a
Reaction conditions: Toluene (10 mmol), TBHP (2 mL), Catalyst = 0.05 – 0.3 g, Reaction
temperature = RT, Reaction time = 24 h.
3
0