Zeolite-Y enslaved manganese(III) complexes as heterogeneous catalysts

Article abstract of DOI:10.1080/00958972.2014.973408

Traditional catalytic procedures for oxidation of phenol produce environmentally undesirable wastes. As a consequence, there is a clear demand for development of an environmentally benign catalytic route for the selective oxidation of phenol. A series of

Full text of DOI:10.1080/00958972.2014.973408

Journal of Coordination Chemistry, 2014
Vol. 67, No. 22, 3678–3688, http://dx.doi.org/10.1080/00958972.2014.973408
Zeolite-Y enslaved manganese(III) complexes as
heterogeneous catalysts
1
CHETAN K. MODI*† and PARTHIV M. TRIVEDI‡
†Department of Applied Chemistry, Faculty of Technology & Engineering, The Maharaja Sayajirao
University of Baroda, Vadodara, India
‡Department of Chemistry, M.K. Bhavnagar University, Bhavnagar, India
(Received 7 January 2014; accepted 17 September 2014)
Traditional catalytic procedures for oxidation of phenol produce environmentally undesirable wastes.
As a consequence, there is a clear demand for development of an environmentally benign catalytic
route for the selective oxidation of phenol. A series of zeolite-Y enslaved Mn(III) complexes with
Schiff bases derived from vanillin furoic-2-carboxylic acid hydrazone (VFCH), vanillin thiophene-2-
carboxylic acid hydrazone (VTCH), ethylvanillin thiophene-2-carboxylic acid hydrazone (EVTCH),
and/or ethylvanillin furoic-2-carboxylic acid hydrazone have been synthesized and characterized by
physico-chemical techniques. Catalytic oxidations of phenol using 30% H O as an oxidant over
2 2
+
+
+
[
2 2 2 2 2 2
Mn(VTCH) ·2H O] -Y, [Mn(VFCH) ·2H O] -Y, and [Mn(EVTCH) ·2H O] -Y under mild
conditions were studied. These zeolite-Y enslaved Mn(III) complexes are stable and recyclable under
current reaction conditions.
Keywords: Zeolite-Y; Mn(III) complexes; Liquid-phase oxidation of phenol
*
Corresponding author. Email: chetanmodi-appchem@msubaroda.ac.in
1
Present address: Reliance Industries, Navi Mumbai, India
©
2014 Taylor & Francis

Zeolite-Y manganese(III) complexes
3679
1. Introduction
Increasingly more restraining policies relating to environmental safety require progress in
technology of chemical processes creating interest in catalysis. Molecular sieves have
expanded into an imposing group of inorganic-crossbreed materials with huge number of
industrial applications, mainly in the field of catalysis. They include zeolites, mesoporous
materials, and metal-organic frameworks. In particular, zeolites are crystalline aluminosili-
cates formed by nanocavities and channels of strictly regular dimensions. In addition, they
have drawn attention for potential applications in ion exchange, magnetism, separation, gas
storage, nonlinear optics, solar energy conversion, heterogeneous catalysis, etc. [1–7]. The
3
-D large internal pore (~13 Å) zeolite-Y has been studied as host lattices. It possesses
properties, such as high surface area, crystalline open structures, tunable pore size and func-
tionality, high adsorption capacity, and partitioning of reactant/products, which allows
considering them as prospective supports for entrapment of catalytically active complexes
[
8–10] or metal complexes [11–14]. The as-prepared inorganic-crossbred material not only
has heterogeneous catalytic characteristics, but also preserves high catalytic efficiency
originating in homogeneous catalysis due to the site secluded effect.
One of the major current challenges in synthetic organic chemistry is selective oxidation
of organic compounds using 30% H O2 in the presence of transition metal catalysts
2
[
15–17]. In particular, oxidation of phenol has been given much attention in industries after
the invention of TS-1 and TS-2 catalyzed commercial process using 30% H O as an oxi-
2
2
dant producing benzenediols [18–20]. Benzenediols viz. catechol (CAT) and hydroquinone
HQ) are important precursors in the production of many valuable products such as
(
pesticides, pharmaceutical product, perfumes, herbicides, and dyestuffs [21, 22].
Manganese is a biologically relevant element and important for health as it helps to
absorb or utilize nutrients such as biotin, thiamin, ascorbic acid, and choline (a B-complex
vitamin). It is important to know that manganese is an antioxidant and thus fights free radi-
cals and prevents our body from damage caused by free radicals. Studies of manganese
compounds derived from various ligands have been developed in recent years, mostly moti-
vated by their intriguing biological features and catalytic applications [23–25]. Manganese
complexes encapsulated in the cavity of zeolite-Y have also been reported for oxidation
III
reactions[26–29]. Ratnasamy et al. encapsulated metallosalen complexes [Mn (salen)] in
zeolite-X and -Y to study their catalytic activity for oxidation of styrene [30, 31]. Maurya
and co-workers studied manganese(III) complexes of pyridoxal-based Schiff base ligands
encapsulated in supercages of zeolite-Y. The as-prepared catalysts have been screened for
oxidation of phenyl sulfide, styrene, and benzoin [32]. The great applicability of manganese
complexes prompted us to prepare manganese(III) complexes enslaved in nanopores of
zeolite-Y. These enslaved complexes have been screened for oxidation of phenol using 30%
H O as oxidant.
2
2
2
2
. Experimental
.1. Materials
Analytical reagent grade thiophene-2-carboxylic acid and furoic-2-carboxylic acid from
Spectrochem, India; vanillin, phenol, and ethylvanillin from E. Merck, India; and 30%
H O from Rankem, India were used as obtained. The Schiff base ligands were synthesized
2
2

3
680
C.K. Modi and P.M. Trivedi
by condensation of vanillin with thiophene-2-carboxylic acid hydrazide and furoic-2-
carboxylic acid hydrazide according to reported procedure [33]. Mn(CH COO) ·2H O was
3
3
2
prepared by oxidation of Mn(CH COO) ·4H O (E. Merck) using Christensen’s method
3
2
2
[
34]. Sodium form of zeolite-Y (Si/Al = 2.60) was procured from Hi-media, India.
2
.2. Instrumentation
The Si, Al, Na, and Mn(III) ions of the enslaved complexes were determined by ICP-OES
Model: Perkin Elmer Optima 2000 DV). Carbon, hydrogen, and nitrogen were analyzed
(
with a Perkin Elmer, USA 2400-II CHN analyzer. IR spectra of zeolite-Y enslaved Mn(III)
complexes were recorded on a Thermo Nicolet IR200 FT-IR spectrometer in KBr.
Electronic spectra were recorded on a Varian Cary 500 Shimadzu spectrophotometer. The
crystallinity of compounds was ensured by X-ray diffraction (XRD) (Model: Bruker AXS
D Advance X-ray powder diffractometer) with a Cu Kα target. The surface areas of the
8
compounds were measured by multipoint BET method using ASAP 2010, micrometrics
surface area analyzer. The scanning electron micrographs (SEMs) of enslaved Mn(III)
complexes were recorded using a SEM instrument (Model: LEO 1430 VP).
2
.3. Synthesis of Mn(III) based neat complexes
The procedures for preparation of Mn(III) complexes are 0.02 M vanillin thiophene-2-car-
boxylic acid hydrazone (VTCH), VFCH, ethylvanillin thiophene-2-carboxylic acid hydra-
zone (EVTCH), and/or ethylvanillin furoic-2-carboxylic acid hydrazone (EVFCH) ligands
were dissolved in 25 mL ethanol and then heated to boiling. This was followed by dropwise
addition of a solution of 0.01 M Mn(CH COO) ·2H O in 10 mL ethanol. The pH of the
3
3
2
resulting solution was adjusted to 5–6 by dropwise addition of CH COONa solution. The
3
resultant solution was stirred and refluxed for 4 h. After cooling, the solid product was
separated by filtration and dried in vacuum.
2.4. Preparation of Mn(III)-Y (metal exchanged zeolite-Y)
A series of zeolite-Y enslaved Mn(III)-Y complexes have been prepared by Flexible Ligand
Method. 5.0 g of zeolite-Y was suspended in 300 mL of deionized water containing 12 mM
Mn(CH COO) ·2H O with constant stirring. The reaction mixture was then heated at 90 °C
3
3
2
for 24 h. The solid was filtered, washed with hot deionized water until the filtrate was free
from any metal ion content, and dried for 15 h at 150 °C in air.
2
.5. Preparation of zeolite-Y enslaved Mn(III) complexes
One gram of metal-exchanged zeolite-Y was uniformly mixed with an excess of ligand
nligand/nmetal = 3) in ethanol into a sealed round bottom flask. The reaction mixture was
(
refluxed (~24 h) in an oil bath with stirring. The resulting material was Soxhlet extracted
with ethanol, acetone, and finally with acetonitrile (6 h) to remove uncomplexed ligand and
the complex adsorbed on the exterior surface of zeolite-Y. The extracted material was ion-
exchanged with 0.01 M NaCl aqueous solution for 24 h to remove uncoordinated metal
−
ions, followed by washing with deionized water until no Cl ion could be detected with
AgNO solution. The final product was dried at 120 °C.
3

Zeolite-Y manganese(III) complexes
3681
Table 1. Catalytic performances over oxidation of phenol to CAT and HQ.
Selectivity (%)
−
1
Sr. no.
Compound
Conversion (%)
TOF (h ) for 6 h
Catechol
Hydroquinone
1
2
3
4
5
6
7
8
9
Na-Y
3.1
16.3
37.4
18.6
43.1
24.6
19.8
16.5
20.2
17.4
41.9
39.4
–
–
75
–
85
–
37
–
44.3
48.3
69.9
36.5
74.6
46.4
52.8
30.7
54.9
39.4
72.6
69.8
55.7
51.7
30.1
63.5
25.4
53.6
47.2
69.3
45.1
60.6
27.4
30.2
Mn(III)-Y
[Mn(VFCH)
[Mn(VFCH)
[Mn(VTCH)
[Mn(VTCH)
[Mn(EVFCH)
[Mn(EVFCH)
[Mn(EVTCH)
[Mn(EVTCH)
+
2
2
·2H
(L)H
2
O] -Y
O]
2
+
2
·2H
2
O] -Y
O]
2
(L)H
2
+
2
·2H
2
O] -Y
O]
O] -Y
2
(L)H
2
+
2
2
·2H
·(L)H
2
38
–
–
1
0
2
O]
+
a
1
1
[Mn(VTCH)
[Mn(VTCH)
2
·2H
·2H
2
O] -Y
+
b
12
2
2
O] -Y
–
−
Note: L = CH
3
COO ion.
a
First reused catalyst.
Second reused catalyst.
b
2.6. Catalytic oxidation of phenol
The catalytic oxidation of phenol over zeolite-Y enslaved Mn(III) complexes using 30%
H O as oxidant were optimized as follows: phenol (30 mM), 45 mM of 30% H O ,
2
2
2 2
catalyst (60 mg), acetonitrile (2 mL) at 80 °C for 6 h. The products were collected at
different time intervals and were identified and quantified by GC. Blank experiments were
performed over Na-Y, Mn(III)-Y and neat complexes under identical conditions with only
negligible conversion (see table 1).
3
3
. Results and discussion
.1. Morphological and textural properties of materials
The results of chemical analyses of neat zeolite-Y and their enslaved Mn(III) complexes are
given in table 2, revealing that the Si/Al ratio (i.e. 2.60) is constant even after metal
exchange and formation of complexes, confirming the retention of the zeolite framework
during complexation. Furthermore, the chemical analyses confirm the presence of Mn(III)
complexes in the zeolite matrix. The surface area and pore volume values estimated by
nitrogen adsorption isotherms are given in table 3. The results show considerable decrease
in the surface area and pore volume of zeolite-Y on entrapment of Mn(III) complexes, con-
firming the presence of complexes inside the nanopores of zeolite-Y.
Table 2. Analytical data of compounds.
Elements %found
Sr. No.
Compound
%C
%H
%N
%M
%Si
%Al
%Na
Si/Al
1
2
3
4
5
6
Na-Y
–
–
4.65
4.79
4.93
4.85
–
–
1.46
1.53
1.61
1.57
–
–
0.93
0.97
1.02
0.98
–
17.16
16.86
16.75
16.67
16.59
16.70
6.60
6.48
6.44
6.41
6.38
6.42
9.86
7.73
7.11
6.90
6.38
7.20
2.60
2.60
2.60
2.60
2.60
2.60
Mn(III)-Y
[Mn(VFCH)
3.69
2.29
2.33
2.41
2.37
+
2
·2H
·2H
[Mn(EVTCH) ·2H
[Mn(EVFCH) ·2H
2
O] -Y
+
[Mn(VTCH)
2
2
O] -Y
+
2
2
O] -Y
O] -Y
+
2
2

3
682
C.K. Modi and P.M. Trivedi
Table 3. Surface area and pore volume data of Na-Y, Mn(III)-Y and their enslaved
complexes.
2
−1
−1 a
Compound
Surface area (m g
)
Pore volume (cc g )
Na-Y
Mn(III)-Y
548
510
293
219
291
271
0.32
0.27
0.21
0.09
0.20
0.16
+
[Mn(VFCH)
[Mn(VTCH)
[Mn(EVTCH)
[Mn(EVFCH)
2
·2H
·2H
·2H
·2H
2
O] -Y
+
2
2
O] -Y
+
2
2
O] -Y
O] -Y
+
2
2
a
Calculated by the BJH-method.
+
Figure 1 represents SEMs of [Mn(EVFCH) ·2H O] -Y recorded before and after Soxhlet
2
2
extraction. It is clear from the well-defined crystals after Soxhlet extraction that enslaved
Mn(III) complexes have no surface complexes and the particle boundaries on the external
surface of zeolite-Y are clearly distinguishable. These micrographs reveal the efficiency of
purification procedure to effect complete removal of extraneous complexes, leading to the
presence of well-defined entrapment in the nanocavity.
The powder XRD patterns of Na-Y and enslaved Mn(III) complexes recorded at 2θ
degree between 2° and 70° are presented in figure 2. The XRD patterns of zeolite-Y
enslaved Mn(III) complexes show no appreciable change in peak positions of the diffraction
lines of the neat zeolite-Y, indicating that the crystallinity of the zeolitic matrix remained
intact upon entrapment of Mn(III) complexes. However, the relative intensities of some of
the diffraction lines may undergo slight changes because of entrapment.
3
.2. Spectroscopic characterization
Table 4 shows a list of FT-IR spectral data of zeolite-Y enslaved Mn(III) complexes. By
comparison with VTCH, VFCH, EVTCH, and EVFCH, the IR bands of all the enslaved
complexes are weak due to their low contents in the nanocages of zeolite-Y.
The zeolitic bands dominate the FT-IR spectra of the enslaved Mn(III) complexes.
However, the presence of characteristic bands of Mn(III) complexes could be distinguished.
−
1
Mn(III) complexes showed bands at ~3440, ~1620, and ~1280 cm , attributable to ν(O–H),
ν(C=N), and ν(C–O) stretches of the ligand. The framework vibration bands of
−
1
zeolite-Y show below 1200 cm for all samples. The bands in the 572–584, 718–793, and
−1
1
007–1131 cm region are attributed to double ring, symmetric stretching, and antisymmetric
stretching vibrations, respectively [35]. No shift is observed upon introduction of metal ions
and inclusion of metal complexes, further substantiating that the zeolite-Y framework
remains unchanged.
Electronic spectral data of Schiff base ligands and their zeolite-Y enslaved Mn(III) com-
plexes are tabulated in table 5. In the exploited wavelength domain from 200 to 900 nm, the
Schiff base ligands show an intense band at ~324 nm due to intra-ligand charge transfer tran-
sition (π → π*). However, this band undergoes hypsochromic shifts in enslaved Mn(III)
complexes resulting from the chelation of the ligand with Mn(III). In particular, the elec-
+
tronic spectra of [Mn(EVTCH) ·2H O] -Y showed a band at 759 nm corresponding to
2
2
5
5
B → E transition [36]. The present Mn(III) complexes are all six coordinate, therefore,
1
g
g
either octahedral or distorted octahedral geometry around the metal ion is expected. The
appearance of the said band suggests distorted octahedral structures. Such distortion is most
probably due to the Jahn–Teller effect as well as the steric effect of the bulky ligands.

Zeolite-Y manganese(III) complexes
3683
+
Figure 1. SEM images of [Mn(EVFCH)
2
·2H
2
O] -Y (A) before and (B) after Soxhlet extraction.
However, we could not observe d–d transitions in the remaining Mn(III) enslaved
complexes, perhaps due to lower concentration of complex inside the zeolite-Y cavity.
Furthermore, the highly intense band observed near 225 nm may be due to MLCT transition.
3
.3. Catalytic activity studies
The catalytic oxidation of phenol was carried out under optimized reaction conditions. As
hydroxyl group on phenol is ortho and para directing, the oxidation of phenol is expected
to give two major products viz. CAT and HQ.

3
684
C.K. Modi and P.M. Trivedi
+
+
Figure 2. XRD patterns of (a) Na-Y, (b) [Mn(EVFCH)
2
·2H
2
O] -Y, (c) [Mn(EVTCH)
2
·2H
2
O] -Y, (d) [Mn
+
+
(
2 2 2 2
VFCH) ·2H O] -Y, and (e) [Mn(VTCH) ·2H O] -Y.
−1
Table 4. FT-IR assignments of Schiff base ligands with their zeolite-Y enslaved Mn(III) complexes in cm
Internal vibrations External vibrations
symT–O
.
Compound
νantisymT–O
ν
symT–O
ν
ν
antisymT–O D–R
ν
(C=N)
ν
(O–H)
ν
(C–O)
VTCH
VFCH
EVTCH
EVFCH
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1635
1639
1633
1626
1619
1620
1624
1617
3422
3432
3432
3430
3440
3433
3443
3443
1286
1291
1315
1312
1274
1283
1271
1277
–
–
–
-
–
+
[Mn(VFCH)
[Mn(VTCH)
[Mn(EVTCH)
[Mn(EVFCH)
2
·2H
·2H
·2H
·2H
2
O] -Y
1010
1015
1019
1007
719
718
724
720
785
787
780
793
1118
1125
1120
1131
572
577
584
580
+
2
2
O] -Y
+
2
2
O] -Y
O] -Y
+
2
2
3.3.1. Effect of reaction parameters. Figure 3 summarizes the percent conversion of phe-
nol along with CAT and HQ formation. Selectivity of CAT formation varied (44–74%) from
catalyst to catalyst. All these catalysts are highly selective toward the CAT formation and
the selectivity is maintained even after 24 h reaction time. The effect of amount of catalyst

Zeolite-Y manganese(III) complexes
3685
Table 5. Electronic spectral data of Schiff base ligands
with their zeolite-Y enslaved Mn(III) complexes.
Compound
Wavelength (nm)
VTCH
329
VFCH
324
EVTCH
EVFCH
324
346, 205
306, 222
302, 225
759, 305, 220
303, 249
+
[Mn(VFCH)
[Mn(VTCH)
[Mn(EVTCH)
[Mn(EVFCH)
2
·2H
·2H
·2H
·2H
2
O] -Y
+
2
2
O] -Y
+
2
2
O] -Y
O] -Y
+
2
2
+
1
2
. Na-Y
. Mn(III)-Y
5. [Mn(EVFCH)2.2H O] -Y
2
Conversion (%)
Catechol
+
6
7
. [Mn(EVTCH)2.2H O] -Y
+
2
Hydroquinone
3
. [Mn(VFCH)2.2H O] -Y
2
+
a
. [Mn(VTCH)2.2H O] -Y
2
+
8
6
4
2
0
0
0
0
0
4. [Mn(VTCH)2.2H O] -Y
+
b
2
8
. [Mn(VTCH)2.2H O] -Y
2
1
2
3
4
5
6
7
8
Figure 3. Catalytic conversion (%) of oxidation of phenol (standard deviation = ±0.5%).
and temperature on the oxidation of phenol along with their possible explanations is
summarized below:
Influences of the amount of catalysts used for phenol conversion and product selectivity
+
are shown in figure 4. Four different amounts of [Mn(VTCH) ·2H O] -Y as a representative
2
2
catalyst viz. 40, 50, 60, and 65 mg were used, keeping all other reaction parameters fixed,
namely temperature (80 °C), phenol (30 mM), 30% H O (45 mM) in acetonitrile (2 mL),
2
2
and reaction time (6 h). Increase in phenol conversion with increasing catalyst concentration
shows that the reaction is catalytic. The conversion (%) decreases beyond 60 mg of catalyst
which may be due to fewer catalytic sites and rapid H O decomposition was observed at
2
2
higher catalyst concentration. Therefore, 60 mg of catalyst was taken to be optimal for
phenol conversion.
The effect of reaction temperature on phenol conversion is illustrated in figure 5. As
expected phenol conversion increased with increasing reaction temperature up to 80 °C and

3
686
C.K. Modi and P.M. Trivedi
4
0 mg
5
4
3
2
1
0
0
0
0
0
0
50 mg
60 mg
5 mg
6
0
60
120
180
240
300
360
420
Reaction/min
Figure 4. Effect of amount of catalyst on the oxidation of phenol (standard deviation = ±0.5%).
5
4
3
2
1
0
0
0
0
0
0
6
7
7
0 °C
0 °C
5 °C
80 °C
0
60
120
180
240
300
360
Reaction time/min
Figure 5. Effect of temperature on the oxidation of phenol (standard deviation = ±0.5%).
higher reaction temperature leads to a dramatic decrease in the selectivity which may be
due to rapid decomposition of H O , as well as significant loss in the conversion. The opti-
2
2
mum temperature was 80 °C.
Figure 6 shows that phenol conversion increased with increase in reaction time up to 6 h.
An attempt to further increase the phenol conversion by extending the reaction time led to
no further conversion; about 90% of H O was consumed when the reaction was carried
2
2
out for 6 h. Decomposition of H O is slow initially but increases with time. This indicates
2
2
that enslaved Mn(III) complex requires longer time to exhibit maximum catalytic activity
37].
[

Zeolite-Y manganese(III) complexes
3687
+
[
[
[
[
Mn(VFCH) .2H O] -Y
2
2
4
3
2
1
0
0
0
0
0
+
Mn(VTCH) .2H O] -Y
2
2
+
Mn(EVFCH) .2H O] -Y
2
2
+
Mn(EVTCH) .2H O] -Y
2
2
0
50
100
150
200
250
300
350
400
Reaction time/min
Figure 6. Effect of reaction time on the oxidation of phenol (standard deviation = ±0.5%).
3.3.2. Recyclability of the catalyst. In order to check leaching of metal ions (under opti-
mized reaction conditions), recycling experiment was carried out on the washed catalyst.
+
The representative catalyst [Mn(VTCH) ·2H O] -Y was recycled for oxidation of phenol to
2
2
establish the effect of encapsulation on stability. The filtrate obtained from [Mn
+
(
VTCH) ·2H O] -Y showed further catalytic activity to some extent in the transformation
2 2
of phenol, indicating slow leaching of metal ions in solution from the cavity of zeolite dur-
ing reaction. The initial run shows a conversion of 43.1% and it was just marginally
reduced to 41.9 and 39.4% after first and second recycling of the catalyst, respectively.
+
These results indicate that [Mn(VTCH) ·2H O] -Y catalyst can be recycled for oxidation of
2
2
phenol without much loss in activity. Consequently, the encapsulation of metal complexes
inside the nanocavity of zeolite-Y increases the life of catalyst by reducing dimerization due
to restriction of internal framework structure.
4
. Conclusion
ꢀ
ꢀ
Four manganese(III) complexes were enslaved into the nanopores of zeolite-Y.
The synthesized heterogeneous nanocatalysts depicted in this article are environmen-
tally benign in liquid phase oxidation of phenol using H O as a catalyst.
2
2
+
ꢀ
Among these enslaved nanocatalysts, [Mn(VTCH) ·2H O] -Y showed the highest
2 2
catalytic activity (43.1%).
Acknowledgements
We express our gratitude to the Head, Department of Chemistry, M.K. Bhavnagar Univer-
sity, Bhavnagar, Gujarat, India, for providing the necessary laboratory facilities. We also
acknowledge the CSMCRI, Bhavnagar, India, for providing the analytical facility.

3
688
C.K. Modi and P.M. Trivedi
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