Cesium salts of manganese based lacunary phosphotungstate supported mesoporous silica: An efficient catalyst for solvent free oxidation reaction

Article abstract of DOI:10.1016/j.catcom.2014.09.038

Keggin-type Cs salt of Mn (II)-substituted mono-lacunary phosphotungstate supported on mesoporous silica, a novel catalyst exhibited excellent activity towards solvent free oxidation reactions. The catalyst materials were fully characterized by different

Full text of DOI:10.1016/j.catcom.2014.09.038

Catalysis Communications 59 (2015) 73–77
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Cesium salts of manganese based lacunary phosphotungstate supported
mesoporous silica: An efficient catalyst for solvent free oxidation reaction
a,b,1
a
a
b, ,1
a,
Surjyakanta Rana
, Suresh Maddila , Ramakanth Pagadala , K.M. Parida ⁎⁎ , Sreekantha B. Jonnalagadda ⁎
a
School of Chemistry & Physics, University of KwaZulu-Natal, Westville Campus, Chiltern Hills, Durban 4000, South Africa
Colloids & Materials Chemistry Department, CSIR — Institute of Minerals & Materials Technology, Bhubaneswar, 751013 Odisha, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2014
Received in revised form 19 September 2014
Accepted 22 September 2014
Available online 30 September 2014
Keggin-type Cs salt of Mn (II)-substituted mono-lacunary phosphotungstate supported on mesoporous silica, a
novel catalyst exhibited excellent activity towards solvent free oxidation reactions. The catalyst materials were
fully characterized by different techniques including XRD, BET analysis, TG–DTA, Raman spectra, FT-IR and
XPS. While XPS study shown the presence of Mn (II) on the material, the existence of Keggin structure even
after modification was confirmed by XRD and FT-IR data. Excellent conversion and selectivity were achieved
in selective oxidation of benzyl alcohol to benzaldehyde. Optimum reaction conditions were established with re-
spect to various parameters that influence the selective oxidation.
Keywords:
Lacunary
Keggin
© 2014 Published by Elsevier B.V.
Mesoporous silica
Oxidation reaction
1
. Introduction
The oxidation of alcohols to the corresponding carbonyl compounds,
etc.) and M is an addenda atom (W6+, Mo6+, V5+, etc.) [8]. When one
or more addenda atoms from polyoxometalates are replaced they are
known as lacunary polyoxometalates. The removal of one or two
n−
i.e., benzyl alcohol to benzaldehyde is one of the most important and
vital transformations in synthetic organic chemistry. Benzaldehyde is a
valuable chemical in the perfumery, dyestuffs, and agrochemical indus-
tries [1]. Numerous catalyzed methods have been reported for alcohol
oxidations [2,3], but many are either expensive or involving toxic
chemicals or solvents. Solvent free approach is an attractive green
process for selective oxidation of benzyl alcohol and oxidants like
molecular oxygen, hydrogen peroxide and TBHP are in the order as
green options. Molecular oxygen as oxidant generates significant
amount of inorganic waste and is not very active, while hydrogen per-
oxide is a good oxygen donor in catalytic oxygen-transfer reactions.
However, most of the catalyst systems using hydrogen peroxide are
based on noble metals such as Pt, Pd, and Ru and are expensive and dif-
ficult to synthesize [4–7]. Thus, polyoxometalates (POMs) are definitely
an attractive alternative in terms of economic viability and easy-to-
manufacture alternative with a heterogeneous nature.
MO units from the fully occupied polyoxometalates [XM12
O
40
]
,
(
n + 4)−
gives rise to mono-lacunary [XM11
V
1
O
39
]
and di-lacunary
polyoxometalates respectively. The reducing abili-
(
n + 5)−
2 36
[XM10V O ]
ty of these materials can be enhanced by creation of lacuna in them.
Lacunary and di-lacunarypolyoxometalates are gaining more impor-
tance because of their unique structural properties [9]. The reduced
oxidizing ability of these materials can further be boosted by simply
substituting an appropriate transition metal in their skeleton. Hence,
we developed interest in the synthesis of Mn substituted mono-
lacunary polyoxometalates and to investigate their scope as catalysts.
Recently, Nagai et al. reported Fe inclusion in the Keggin anion of
heteropoly acid catalysts for selective oxidation of isobutene [10].
Patel et al. have reported the detailed synthesis and characterization
of Keggin-type manganese (II)-substituted phosphotungstate and its
activity towards liquid phase oxidation of styrene [11]. Excellent activity
of single-site iron catalysts towards epoxidation as well as oxidation re-
action was reported by Thomas and Raja [12]. So far there is little or no
literature available on the catalytic aspects of supported Cs salt of Mn
substituted lacunary anions. Earlier, Parida et al. have reported the
synthesis of Pd and Fe substituted lacunary polyoxometalates support-
ed on mesoporous silica [13,14]. Choice of the support material is vital
for activity, as support materials can provide high surface area and
active sites, which are crucial in enhancing conversion and selectivity
efficiencies. Mesoporous silica (MCM-41) contains high surface area,
uniform and controllable pore sizes and the periodic orders of their
pore packing which provides a tremendous dispersion of the active
The general formula of Keggin type heteropoly acids is
Xn+
M
O
]
(8 − n)−
, where X
n+
is a central hetero atom (P , Si
5+
4+
[
12
40
⁎
Correspondence to: S.B. Jonnalagadda, School of Chemistry & Physics University of
KwaZulu-Natal, Durban-4000, South Africa.Tel.: +27 31 260 7325/3090; fax: +27 31
60 3091.
Correspondence to: K.M. Parida, Centre for Nano Science and nanotechnology, ITER,
2
⁎
⁎
SOA University, Bhubaneswar-751030, Odisha, India. Fax: 91674 2581637.
E-mail addresses: kulamaniparida@soauniversity.ac.in (K.M. Parida),
jonnalagaddas@ukzn.ac.za (S.B. Jonnalagadda).
1
Tel.: +91 6742581635x9425.
http://dx.doi.org/10.1016/j.catcom.2014.09.038
1566-7367/© 2014 Published by Elsevier B.V.

74
S. Rana et al. / Catalysis Communications 59 (2015) 73–77
species over it. In this communication, we report the synthesis of novel
materials containing varied wt% of manganese substituted mono-
lacunary phosphotungstates supported on mesoporous silica and their
catalytic activity and selectivity towards partial oxidation of benzyl
alcohol.
3.1.2. BET
The nitrogen adsorption–desorption isotherm was carried for MCM-
41 and 50 wt.% loaded Mn(II)-substituted mono-lacunary phospho-
tungstate on MCM-41 is shown in Fig. 2. The isotherm of the parent
shows a H4 type hysteresis loop (according to IUPAC nomenclature)
with well-developed step in the relative pressure range ≈ 0.9. Modifica-
tion of MCM-41 framework with LMn, it seems to lower the P/P0 for
capillary condensation step, indicating the shift in pore size to lower
value due to incorporation of manganese lacunary acid in the pore of
that support and also pore volume is found to decrease with increasing
LMn content over the MCM-41 surface (Fig. 2). The BET surface area of
2
. Experimental section
In the preparation of catalyst, physico-chemical characterization and
procedure of oxidation reaction are provided in supplementary content.
2
parent MCM-41 sample was 1380 m /g. After loading different wt % of
3
3
3
. Results and discussion
LMn, the surface area and pore volume gradually decrease as shown
in Table 1. This may be due to the fact that the Mn(II)-substituted a
mono-lacunary phosphotungstate molecules that block the pores on
the silica matrix.
.1. Physicochemical characterization of the catalyst
.1.1. XRD
The SXRD patterns of MCM-41 and 50LMn@MCM-41 are shown in
Fig. 1a. Both the materials exhibit a strong peak at 2θ = 2.2° due to
100) plane and also small peaks due to higher order (110), (200) and
210) plane reflections within 5° indicate the formation of well-
ordered mesoporous materials. Thus, the mesoporosity remains intact
after the modification of the silica network with manganese lacunary
polyoxometalates. Marginal reduction and broadening of the (100)
peak of 50LMn@MCM-41 after modification with manganese lacunary
on the support surface observed, suggest a slight disturbance in hexag-
onal symmetry. The high angle XRD spectra of LMn and 50LMn@MCM-
3
.1.3. FT-IR
The FT-IR spectra of LMn and 50LMn@MCM-41 are shown in Fig. 3.
(
(
−
1
The CsPTA shows prominent bands at 1080, 985, 890 and 800 cm
which are characteristic of Keggin structure and are assigned to corner-
sharing and edge-sharing respectively [15]. In the case of lacunary
−
1
structure LW, the 1080 cm
band is split into two components
1084–1044 cm ), due to the symmetry decrease of the PO4 tetrahe-
dron. Other bands found are 953 (nas(W\O )), 860 (nas(W\O \W)),
\W)), and differ from those of PW12
15]. After modification with Mn, the spectra for LMn showed character-
−
1
(
d
b
−
1
8
[
09 and 742 cm
(nas(W\O
c
4
1 are shown in Fig. 1b. In that figure shows very high dispersion of LMn
in a non-crystalline form on the surface of MCM-41.
−
1
istic splitting for the P\O bond frequency at 1073 and 1052 cm , which
was slightly shifted towards lower frequency as compared to LW
confirming the exchange of W atom by Mn. Both LMn and LMn modified
MCM-41 support displayed similar bands.
a
2
1
1
0000
5000
0000
(
a)
(
100)
3.1.4. TG–DTA
The TG–DTA of the MCM-41 and 50LMn@MCM-41 are shown in
Fig. 4. The TGA of MCM-41 shows weight loss in the temperature
range 100–200 °C due to the loss of crystalline water. It also shows
weight loss at 500 °C. This may be due to desorption of several organic
species. TGA of LMn@MCM-41 shows weight loss in the temperature
range 70–200 °C due to loss of adsorbed water. It does not show any
weight loss up to 400 °C, indicating the synthesized catalyst is stable
up to 400 °C. The DTA of the MCM-41 shows an endothermic peak
around 100 °C due to water loss. The exothermic peak above 500 °C is
due to surfactant removal. DTA of the 50LMn@MCM-41 sample
(
b)
5000
(110)
(200)
(Fig. 4b) shows an endothermic peak with maximum at 80 °C ascribed
to loss of adsorbed water. A wide exothermic peak with maximum at
0
2
4
6
8
10
Degree (2)
b
(a)
(b)
20
40
60
80
Degree (2)
Fig. 1. a: Low angle XRD spectra of MCM-41 (a) and 50LMn@MCM-41. b: High angle XRD
spectra of 50LMn@MCM-41 (a) and MCM-41 (b).
Fig. 2. N2-adsorption & desorption study of MCM-41 (a) and 50LMn@MCM-41 (b).

S. Rana et al. / Catalysis Communications 59 (2015) 73–77
75
Table 1
a
Surface properties of LMn@MCM-41.
100
Catalyst
Surface area (m2 g−1
)
Pore volume (cm /g)
3
MCM-41
LMn
1380
3
820
740
625
585
1.28
0.041
0.71
0.68
0.56
0.51
98
3
4
5
6
0 LMn@MCM-41
0 LMn@MCM-41
0 LMn@MCM-41
0 LMn@MCM-41
96
94
9
9
2
0
around 520 °C was observed, which is assigned to decomposition of
metal-modified lacunary Keggin anion.
1
00 200 300 400 500 600 700 800
3.1.5. Raman spectra
o
Temperature ( C)
Fig. 5 indicates the Raman scattering spectra of LMn (a) and 50
LMn@MCM-41 (b). The bulk LMn gives peaks at 966 for P\O, 921 for
−
1
W\O
brations. The band at about 220 cm may be attributed to symmetric
stretching vibration of W\O band [16]. The 50 wt.% of LMn loaded
b
\W, 885 for W\O
c
\W and 824 cm for W\O
t
stretching vi-
b
−
1
100
a
on MCM-41 shows all the above described peaks of LMn, but the inten-
sity of the peaks is low, compared to the bulk LMn and slightly shifted
towards higher wave number values due to strong interactions between
the MCM-41 support and lacunary Keggin unit. These results confirm
the incorporation of un-degraded LMn on MCM-41 surface.
9
8
7
6
5
0
0
0
0
0
3
.1.6. XPS
The Mn 2p XPS spectrum of 50 LMn@MCM-41 is shown in Fig. 6. The
catalyst shows two peaks at 641.03 and 652.64 eV. In the reported liter-
ature [17], the binding energy of about 640.6 eV was attributed for Mn
100 200 300 400 500 600 700 800
3/2
3/2
2
p
. The Mn 2p peaks are shifted towards a higher binding energy
o
Temperature ( C)
i.e., 0.4 eV, which indicated the strong interaction of Mn within the
lacunary structure and with the support.
Fig. 4. TG–DTA spectra of 50LMn@MCM-41 (a) and MCM-41 (b).
3
.2. Catalytic activity
Table 2. Out of those catalysts, 50 wt.% LMn@MCM-41 gave better
results relative to that of others. So, a detailed study was carried out
on the oxidation of benzyl alcohol using 50 wt.% LMn@MCM-41 as
catalysts, and reactions were carried out under optimized conditions.
Phosphomolybdic acid supported vanadium–alumina mixed oxide
as catalyst for the oxidation of alcohol with toluene as the solvent
93% conversion) was reported by Manyar et al. [18]. Parida et al.
(
reported oxidation of benzyl alcohol over PMA/ZC catalyst with 85%
conversion and 96% selectivity at 80 °C in 5 h reaction time [19]. Sawant
et al. [20] reported that oxidation of benzyl alcohol with hydrogen per-
oxide over ammonium molybdate and tungstic acid catalyst gives b78%
conversion of benzyl alcohol, at 90 °C in 5 h. Both the methods have
inherent disadvantage of organic solvent. In this communication, we
report a new catalyst LMn@MCM-41, which displays excellent activity
in terms of conversion and selectivity to benzaldehyde (89% conversion,
3.2.1. Effect of temperature
Reaction was investigated at four temperatures, 70, 80, 90 and
100 °C, while other parameters are kept fixed (Fig. 7). The conversion
% increased with increasing temperature from 70 to 100 °C, but the
selectivity towards benzaldehyde drastically decreased. This might be
due to the self-decomposition of H
2 2
O at higher temperatures or further
9
8% selectivity) under solvent free conditions. Reaction was investigat-
oxidation of benzaldehyde to benzoic acid at elevated temperatures.
ed under different wt% (30–60) LMn@MCM-41 and LMn as catalyst
conditions and the results of the oxidation reaction are summarized in
7
6
5
4
3
2
1
000
000
000
000
000
000
000
0
(
b)
9
6
3
0
0
0
(a)
(b)
a)
(
0
2
000 1800 1600 1400 1200 1000 800
0
500
1000
1500
2000
Wavenumber cm-1
Wave number (cm-1
)
Fig. 3. FT-IR spectra of LMn (a) and 50LMn@MCM-41 (b).
Fig. 5. Raman spectra of LMn (a) and 50LMn@MCM-41 (b).

76
S. Rana et al. / Catalysis Communications 59 (2015) 73–77
00
1
Mn 2p
1/2
100
Mn 2p
3/2
9
8
7
6
0
0
0
0
9
5
90
8
5
0
8
635
640
645
650
655
660
70
80
90
100
o
Binding Energy (eV)
Temperature ( C)
Fig. 6. XPS spectra of Mn 2p of 50LMn@MCM-41.
Fig. 7. Effect of temperature on the conversion of benzyl alcohol using 50 LMn@MCM-41
as the catalyst.
3
.2.2. Effect of catalyst amount
The effect of 50 wt.% LMn@MCM-41 catalyst loading on the progress
3.3. Recyclability of the catalyst
of benzyl alcohol oxidation is illustrated in Fig. 8. With the increase of
the catalyst amount from 0.05 g to 0.20 g, the benzyl alcohol conversion
increased from 67% to 94%, but the selectivity towards benzaldehyde re-
duced slightly from 99% to 89%. This may be due to the increase in active
sites resulting from higher amount of catalyst which facilitate the
further oxidation of benzaldehyde to benzoic acid.
Recovery and reutilization of catalyst are the main advantages of
heterogeneous catalysis. The activity of the recovered catalyst provides
useful information about its stability during the catalytic cycle. In order
to regenerate, catalyst was separated by filtration after reaction and
washed several times with distilled water. It was further dried at
100 °C for 12 h followed by calcination at 200 °C for 3 h. The regenerat-
ed catalyst was then used in the reaction with a fresh reaction mixture
and products were analyzed after the reaction. When the regenerated
catalyst was used in three consecutive cycles, selectivity remained unal-
tered at 98%, but conversion decreased by marginal 2% in the third cycle.
3
.2.3. Effect of time
When the reaction time was increased from 2 h to 6 h, the % conver-
sion of benzyl alcohol increased too, but the selectivity towards benzal-
dehyde declined. Fig. S1 shows the influence of reaction time on the
conversion of benzyl alcohol and product selectivity over the 50 wt.%
LMn@MCM-41 catalyst (0.1 g). Thus, optimum reaction time was
found to be 5 h, when catalyst gave the highest conversion and selectiv-
ity, i.e. 89% conversion of benzyl alcohol and 98% selectivity towards
benzaldehyde. With an increase of the reaction time from 5 h to 6 h,
the conversion marginally increased from 89% to 91%, but the selectivity
dropped from 98% to 87%, due to further oxidation of benzaldehyde to
benzoic acid.
4. Conclusions
We have developed a Cs salt of Mn substituted mono-lacunary phos-
photungstate material and supported it on MCM-41 to obtain a new and
robust heterogeneous catalyst. The Keggin structure does not break
after modification which is confirmed by XRD and FT-IR. From the XPS
studies Mn (II) was present on the materials. The 50 wt.% LMn@MCM-
4
1 was found to be an excellent catalyst towards oxidation reaction
under solvent free conditions. The N adsorption–desorption studies
2
3
.2.4. Effect of oxidant
The efficiencies of three different oxidants H
O , TBHP and O were
2 2 2
revealed that the modified samples retained the mesoporosity. The ad-
vantage new catalyst is selective conversion of benzyl alcohol to benzyl
alcohol under solvent free conditions with excellent yield. Furthermore,
the removal of the catalysts consists of a single filtration step and the
catalyst can be re-used for up to two cycles without any significant
loss in conversion or selectivity.
compared under otherwise similar conditions (Fig. S2). The results indi-
cate that after 5 h reaction, the % conversion of benzyl alcohol with the
oxidants was in the order of H
2 2 2
O › TBHP › O , while the selectivity was
in the order of H › O › TBHP. Relative to molecular oxygen, selectiv-
O
2 2
2
ity was poor in presence of TBHP. The LMn supported on mesoporous
silica reacts with TBHP to form LMn-OOH peroxo surface species. In
peroxo surface species, the O\O bond is polarized, which facilitates
the attack by hydroxyl group on the nucleophilic center of aldehyde
again, resulting in further oxidation and formation of benzoic acid.
Thus, optimized conditions for oxidation of benzyl alcohol (89% conver-
sion, 98% selectivity of benzaldehyde) are as follows: amount of the
1
00
100
98
96
94
92
90
9
8
7
6
0
0
0
0
2 2
catalyst (0.1 g), temperature (80 °C), oxidant (H O ) and time (5 h).
Table 2
Conversion and selectivity of various catalysts towards the oxidation of benzyl alcohol.
Catalyst
Conversion of benzyl alcohol Selectivity (%)
Benzaldehyde Benzoic acid
LMn
58
88.2
90
92
98
94.6
11.8
10
8
2
5.4
0.05
0.10
0.15
0.20
3
4
5
6
0 LMn@MCM-41 62
Catalyst
0 LMn@MCM-41 78
0 LMn@MCM-41 89
0 LMn@MCM-41 81
Fig. 8. Effect of catalyst on the conversion of benzyl alcohol using 50 LMn@MCM-41 as the
Catalyst.

S. Rana et al. / Catalysis Communications 59 (2015) 73–77
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Appendix A. Supplementary data
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