Natural silk supported manganese dioxide nanostructures: Synthesis and catalytic activity in aerobic oxidation and one-pot tandem oxidative synthesis of organic compounds

Article abstract of DOI:10.1016/j.poly.2016.01.019

Manganese dioxide nanostructures were coated on natural silk (MnO₂@silk) by simple immersion of the silk fibers into a KMnO₄ aqueous solution. The silk was found to act as a in situ reducing agent and substrate in aqueous KMnO₄ solution for the heterogeneous production of MnO₂ nanostructures. The structure of synthesized catalyst was characterized by X-ray diffraction (XRD), flame atomic absorption spectroscopy (FAAS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) analysis. The catalytic activity of the MnO₂@silk was examined in the aerobic oxidation of alkyl arenes, alcohols, and oximes to their corresponding carbonyl compounds and a one-pot tandem oxidative two-component reaction of aromatic hydrocarbons of petroleum naphtha in the absence of any other oxidizing reagent or initiator.

Full text of DOI:10.1016/j.poly.2016.01.019

Accepted Manuscript
Natural Silk Supported Manganese Dioxide Nanostructures: Synthesis and Cat-
alytic Activity in Aerobic Oxidation and One-Pot Tandem Oxidative Synthesis
of Organic Compounds
Ahmad Shaabani, Zeinab Hezarkhani, Elham Badali
PII:
S0277-5387(16)00033-4
DOI:
http://dx.doi.org/10.1016/j.poly.2016.01.019
POLY 11781
Reference:
Polyhedron
To appear in:
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Accepted Date:
17 December 2015
15 January 2016
Please cite this article as: A. Shaabani, Z. Hezarkhani, E. Badali, Natural Silk Supported Manganese Dioxide
Nanostructures: Synthesis and Catalytic Activity in Aerobic Oxidation and One-Pot Tandem Oxidative Synthesis
of Organic Compounds, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.01.019
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Natural
Silk
Supported
Manganese
Dioxide
Nanostructures: Synthesis and Catalytic Activity in
Aerobic Oxidation and One-Pot Tandem Oxidative
Synthesis of Organic Compounds
Ahmad Shaabani,* Zeinab Hezarkhani, and Elham Badali
Faculty of Chemistry, Shahid Beheshti University, G. C., P. O. Box 19396-4716, Tehran, Iran
Email: a-shaabani@sbu.ac.ir
Abstract
Manganese dioxide nanostructures were coated on natural silk (MnO2@silk) by simple immersion of the silk
fibers into a KMnO4 aqueous solution. The silk was found to act as a in situ reducing agent and substrate in
aqueous KMnO4 solution for the heterogeneous production of MnO2 nanostructures. The structure of synthesized
catalyst was characterized by x-ray diffraction (XRD), flame atomic absorption spectroscopy (FAAS),
thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy dispersive spectroscopy
(EDS) analysis. The catalytic activity of the MnO2@silk was examined in the aerobic oxidation of alkyl arenes,
alcohols, and oximes to their corresponding carbonyl compounds and a one-pot tandem oxidative two-
component reaction of aromatic hydrocarbons of petroleum naphtha in the absence of any other oxidizing
reagent or initiator.
Keywords
Silk; Manganese dioxide nanostructures; Aerobic oxidation; Tandem oxidative synthesis

Introduction
Biopolymers, as common structural elements of biological systems, have highly specialized
properties because of their complex three-dimensional structures. Natural polyamino acid
biopolymers (polypeptides or proteins) in organisms are synthesized from combinations of
amino acid (–NH–CHR–CO–) and imino acid (–NR1–CHR2–CO–) monomers linked via
amide bonds (peptide bonds) [1]. Natural silk is a biopolymer that composed mainly of fibroin
and is produced by certain insect larvae to form cocoons. Primary structure of fibroin protein
mainly consists of the recurrent amino acid sequence (Gly-Ser-Gly-Ala-Gly-Ala)n (Scheme 1)
[2]. Due to the structurally ordered amino acid chains in the fibroin, silk proteins can form
functional complexes with metals; therefore, silk protein-based materials have found
application as solid supports for the immobilization of catalysts.
Catalytic oxidation processes play an important role in green organic transformations. The
development of new catalytic materials based on green principles is of significant importance
in organic chemistry. From both industrially and environmentally viewpoints, there is an
important demand for greener approaches that employs aerobic catalytic oxidation processes
with minimum amount of waste [3]. For this reason, various natural biopolymers have been
employed as the solid support to produce green heterogeneous catalytic systems, such as
cellulose [4], wool [5], alginate [6], gelatin [7], starch [8], and chitosan [9] derivatives.
Among several renewable bio-supports, silk have unique properties and can be used as
support for catalytic applications.
Manganese dioxide (MnO2), as a cheap, renewable, mild, easily handled, and low toxic
reagent, is one of the attractive materials due to its selective catalytic properties. MnO2 has
been extensively used as a selective catalyst for the oxidation of a variety of functional groups
[3,10,11]. Unfortunately, MnO2 usually has a relatively low surface area (10–80 m2 g-1)
because of its aggregation into less active large particles during the reaction. Since
aggregation of the heterogeneous catalysts substantially affected and decreased their catalytic
activity, to settle the matter, nanostructured manganese dioxide was prepared for providing a
catalyst with a large surface area. Moreover, aggregation of MnO2 nanostructures is prevented
by its distribution on the supports and high catalytic activity is accessible [12]. In addition,
industrial applications of MnO2 have become more attractive environmentally by the

recycling of MnO2. For this reason, in our previous works it was suggested that it possible to
separate the MnO2 from a cellulose and wool biosupports by burning or chemical
decomposition of biosupport [13,14].
As part of our ongoing program related to the developing new heterogeneous catalysts for
organic transformations [15-18], and based on our previous investigations on MnO2 [13, 14,
19-21], herein we report the synthesis of MnO2 nanostructures on silk (MnO2@silk) and
investigated its catalytic activities in the selective aerobic oxidation of various of benzylic
hydrocarbons, alcohols, and oximes into the corresponding aldehydes and ketones, and one-
pot oxidative two-component reaction of aromatic hydrocarbons of petroleum naphtha in the
absence of any other oxidizing reagent or initiator. We found the MnO2 nanostructures are
effectively immobilized on the surface of silk protein and prevented from its aggregation. In
addition, there is the possibility of separation MnO2 from silk support at the end of the
catalytic process (Scheme 1).
Scheme 1 Structure of fibroin protein of silk and the possible structure of MnO2@silk
Materials and methods
1
General
Natural white silk (silkworm cocoons of Gilan/Iran) was washed with NaOH solution,
distilled water, and acetone, then cut with scissors to very short pieces (about 200 µm-1
mm, based on SEM images). All reagents were obtained from Aldrich or Merck and used
without further purification. The elemental analyses were performed with an Elementar
Analysensysteme GmbH VarioEL. X-ray diffraction (XRD) pattern of catalyst was recorded
on a STOE STADI P with scintillation detector, secondary monochromator using Cu Kα

radiation (λ = 0.1540 nm). Mn(IV) determination was carried out on an FAAS (Shimadzu
model AA-680 flame atomic absorption spectrometer) with a Mn hollow cathode lamp at
279.5 nm, using an air-acetylene flame. Thermogravimetric analysis (TGA) was carried
out using STA 1500 instrument at a heating rate of 10 ºC min−1 in air. Scanning electron
microscopy (SEM) observations were carried out on an electron microscopy Philips XL-30
ESEM. All samples were sputtered with gold before observation. Products were analyzed
using a Varian 3900 GC. Melting points were measured on an Electrothermal 9200
apparatus.
2
Experimental
2.1 Synthesis of MnO2@silk
Natural white silk (silkworm cocoons of Gilan/Iran) was washed with NaOH solution (0.01
M), distilled water and acetone and dried at 60 °C, then cut with scissors to very short
pieces (about 200 µm-1 mm, based on SEM images). For the synthesis of MnO2@silk, a
solution of KMnO4 (0.01 M, 150 mL) was added dropwise to a magnetically stirred
suspension of the natural silk pieces (1.0 g) in 200 mL of H2O during 12 h at room
temperature. The mixture was filtered to give brown MnO2@silk. The prepared catalyst
washed with H2O (3× 10 mL) and EtOH (3 × 10 mL), successively and dried under vacuum
at 80 °C for 12 h.
2.2 Oxidation of alkyl arenes; General procedure
In a typical reaction, an alkyl arene (1.0 mmol), MnO2@silk (0.09 g, 10 mol% of MnO2), and
o-xylene (5.0 mL) were added to a two-necked flask. The mixture was stirred under reflux
conditions and air blowing. The thin layer chromatography (TLC) method was used to
investigate the progress of the reaction. Upon completion, analysis of the crude product has
been done by GC method.
2.3 Oxidation of alcohols; General procedure
In a typical reaction, an alcohol (1.0 mmol) was added to a two-necked flask containing
MnO2@silk (0.09 g, 10 mol% of MnO2), K2CO3 (0.50 mmol), and o-xylene (5.0 mL) and

stirred under air blowing at room temperature. The progress of the reaction was followed by
TLC and GC method. After completion of reaction and filtration of mixture, the filtrate was
analyzed by GC method.
2.4 Oxidation of oximes; General procedure
An oxime (1.0 mmol) and MnO2@silk (0.09 g, 10 mol% of MnO2) were well ground using a
pestle at room temperature. The progress of the reaction was monitored by TLC and GC.
After completion of the reaction, the solid was washed with dichloromethane. After the
filtration of the reaction mixture, analysis of the filtrate has been done by GC method.
2.5 General procedure for the preparation of products 3a-d, 5a-d, and 7a-d
In a two-necked flask, to a solution of amine source (1.0 mmol) in toluene or xylene (5.0
mL), MnO2@silk (the amount of catalyst was listed in Table 4) was added and the mixture
was stirred magnetically under reflux conditions and air blowing for the indicated time in
Table 4. The progress of the reaction was monitored by TLC. After completion, the
reaction mixture was filtered and washed with ethanol. After removing of the organic
solvents, the crude product was separated on a silica gel column to give final product.
Analysis and results
The synthesized catalyst was characterized by FAAS, XRD, TGA, SEM, and EDS. The
Mn(IV) contents of the MnO2@silk catalyst was determined using FAAS method. The
amount of MnO2 in this catalyst was determined 9.67%.
The XRD patterns of silk fibers and MnO2@silk catalyst was employed to investigate the
structure of the synthetic catalyst. The diffraction peaks of silk fibers appear as two broad
peaks at 2θ values of around 9.5° and 19.5°, which are its characteristic (Fig. 1-I). Because of
the diffraction peaks of silk fibers are stronger than the diffraction peaks of MnO2, the peaks
related to the MnO2 were not observed. Clearly, the characteristic peaks of silk were observed
in the XRD pattern MnO2@silk catalyst, meaning that the morphology of silk after
immobilization of MnO2 was retained (Fig. 1-II).

Fig. 1 XRD patterns of silk (I) and MnO2@silk (II).
TGA demonstrated that silk was decomposed above 268 °C, which approved relatively high
thermal stability of the silk fibers in air (Fig. 2-I). High thermal stability was observed for
MnO2@silk catalyst (Fig. 2-II), too. In the TGA curves of silk fibers and MnO2@silk catalyst,
three stages could be observed. In the low temperature range (up to 268 °C), the mass slowly
decreased by 4.2% and 4.3% for silk and MnO2@silk, respectively; it should be related to the
removal of adsorbed water on the surface in the silk fibers and MnO2@silk catalyst. In the
range of 268-380 °C and 380-696 °C, a large weight loss for silk was observed in two stages,
which is attributed to the decomposition of silk. Three stages in the TGA curve of MnO2@silk
catalyst should be related to: the removal of adsorbed water (stage 1), decomposition of silk
(stage 2 and 3).
Fig. 2 TG curves of silk fibers (I) and MnO2@silk (II) in air

The morphological characterization of silk and the obtained MnO2@silk catalyst was
conducted by scanning electron microscope (SEM). According to Fig. 3a, the silk fibers have
a length of about 200 µm-1 mm. Comparing Fig. 3b that related to silk fibers, SEM image of
MnO2@silk (Fig. 3c) confirmed that the catalyst material is formed without a large change in
its morphology. The structure and morphology of the obtained MnO2@silk nanocomposite
was illustrated by SEM (Fig. 3-c and d) and MnO2 nanostructures could be observed on the
silk fiber surface which nanostructures are well dispersed on the silk fiber surface (Fig. 3d).
Fig. 3 SEM images of silk (a,b) and MnO2@silk (c,d)
The EDS analysis was employed for determination of the chemical composition of
MnO2@silk nanocomposite (Fig. 4).

Fig. 4 SEM-EDS results for MnO2@silk catalyst.
Results and discussion
To achieve a suitable catalytic system for the aerobic oxidation, we examined MnO2@silk
catalyst in the reaction media. Aerobic oxidation of indane, 1-indanol, and 2,3-dihydro-
1H-inden-1-one oxime were selected as model reactions for transformation of alkyl arenes,
alcohols, and oximes, respectively. The results show that the entries 5, 12, and 15 have
provided the best conditions, respectively (Table 1).
Table 1 Optimization of the reaction conditions for the aerobic oxidation of alkyl arenes, alcohols, and oximes
by MnO2@silk catalysta,b,c
Amount of catalyst
(MnO2 mol%)
0.09 g (10)
Base
(mmol)
Temperature
(°C)
Conversion
Substrate
Solvent
Time
Entry
(%)d
7
n-Hexane
CH2Cl2
CH3CN
-
-
-
-
9 h
9 h
9 h
9 h
Reflux
Reflux
Reflux
Reflux
1
2
3
4
0.09 g (10)
0.09 g (10)
14
35
57
Indane
0.09 g (10)
Toluene
o-Xylene
-
Reflux
5
0.09 g (10)
9 h
89

o-Xylene
o-Xylene
-
-
Reflux
0.07 g (8)
0.05 g (6)
0.09 g (10)
0.09 g (10)
0.09 g (10)
0.09 g (10)
0.09 g (10)
0.07 g (8)
0.05 g (6)
9 h
57
48
16
14
23
58
92
53
40
6
7
9 h
8 h
8 h
Reflux
r.t.
n-Hexane
CH2Cl2
K2CO3 (0.5)
K2CO3 (0.5)
8
r.t.
9
K2CO3 (0.5)
K2CO3 (0.5)
K2CO3 (0.5)
K2CO3 (0.5)
K2CO3 (0.5)
8 h
8 h
8 h
8 h
8 h
CH3CN
r.t.
10
11
12
13
14
Toluene
o-Xylene
o-Xylene
o-Xylene
r.t.
1-Indanol
r.t.
r.t.
r.t.
2,3-Dihydro-
1H-inden-1-
one oxime
-
-
-
-
-
-
15
16
17
0.09 g (10)
0.07 g (8)
0.05 g (6)
5 min
5 min
5 min
r.t.
r.t.
r.t.
98
71
57
a Reaction conditions for oxidation of alkyl arenes: indane (1.0 mmol), solvent (5.0 mL), air as oxidant.
b
Reaction conditions for oxidation of alcohols: 1-indanol (1.0 mmol), solvent (5.0 mL), air as oxidant.
c Reaction conditions for oxidation of oximes: 2,3-Dihydro-1H-inden-1-one oxime (1.0 mmol).
Conversion determined by GC analysis.
d
Finally, the aerobic oxidation of the various alkyl arenes and alcohols to related aldehydes
and ketones was carried out in o-xylene as the preferred solvent. Also, oxidation of
oximes to the related carbonyl compounds has been done under solvent-free conditions at
room temperature (Table 2).
Table 2 Aerobic oxidation of various alkyl arenes, alcohols, and oximes to corresponding aldehydes and
ketonesa,b,c
Conversion
(%)d
Conversion
(%)d
Entry
1
Substrate
Product
Time
10 h
Entry
20
Substrate
Product
Time
14 h
85
84
89
86
10 h
12 h
16 h
16 h
85
89
2
3
21
22
9 h
9 h
91
89
16 h
16 h
87
87
4
5
23
24
10 h
12 h
96
94
16 h
18 h
85
79
6
7
25
26

13 h
12 h
89
90
7 h
66
59
8
9
27e
28e
13 h
9 h
9 h
9 h
9 h
9 h
9 h
91
92
87
90
89
91
15 h
5 min
5 min
5 min
5 min
5 min
70
92
93
90
96
97
10
11
12
13
14
15
29
30
31
32
33
34
8 h
8 h
92
96
5 min
5 min
97
98
16
17
35
36
8 h
94
88
5 min
98
18
19
37
14 h
a
Reaction conditions for oxidation of alkyl arenes: alkyl arene (1.0 mmol), MnO2@silk (0.09 g), o-xylene (5.0 mL), air as oxidant,
reflux.
b
Reaction conditions for oxidation of alcohols: alcohol (1.0 mmol), MnO2@silk (0.09 g), o-xylene (5.0 mL), K2CO3 (0.5 mmol), air
as oxidant, r.t.
c Reaction conditions for oxidation of oximes: oxime (1.0 mmol), MnO2@silk (0.09 g), r.t.
d Conversion determined by GC analysis.
e
Mixed primary and secondary benzyl alcohols (Entry 27) and alkyl alcohols (Entry 28) as the reactants was used to investigate the
selectivity of the MnO2@silk catalytic system.

Because of the oxidation of the alkyl arenes has been carried out successfully using the
prepared catalyst, therefore the usability of this catalytic system has been examined for the
one-pot oxidative two-component reaction of aromatic hydrocarbons of petroleum naphtha
(Scheme 2).
Scheme 2 The oxidative synthesis of 2-aryl-1H-benzo[d]imidazole, 2-arylbenzo[d]thiazole, and 2-aryl-4-
quinazolinone derivatives.
Between the BTX family members (the benzene, toluene, and three isomers of xylene), as
very important materials in the petroleum refining and petrochemical industries, toluene and
xylenes usually are modified by chemical procedures. The products of the mentioned
chemical reactions can be used as precursors in synthetic processes.
One of the important reactants in synthetic chemistry is carbonyl compounds which
unfortunately this limits the diversity of these reactions. To solving this issue, one-pot
oxidative protocol can be done by transforming an inert substance (such as toluene and
xylenes) into a reactive substrate of the synthetic processes.
At first the MnO2@silk catalytic system has been investigated in the oxidative synthesis of
2-phenyl-1H-benzo[d]imidazole (3a), 2-phenylbenzo[d]thiazole (5a), and 2-phenyl-4-
quinazolinone (7a) as model reactions. The results of optimization studies were listed in
Table 3. The results show that the entries 1, 4, and 7 have provided the best results for the
synthesis of 3a, 5a, and 7a, respectively.

Table 3 Optimization of the reaction conditions for the oxidative synthesis of 2-phenyl-1H-benzo[d]imidazole
(3a), 2-phenylbenzo[d]thiazole (5a), 2-phenyl-4-quinazolinone (7a) compounds by silk supported MnO2
nanocatalysta,b,c
Entry
Amount of catalyst (mol% of MnO2)
0.09 g (10)
t (h)
10
Yield (%)d
1
2
3
69
50
39
0.08 g (9)
10
0.07 g (8)
10
Entry
Amount of catalyst (mol% of MnO2)
0.09 g (10)
0.08 g (9)
t (h)
9
Yield (%)d
4
5
6
75
52
40
9
0.07 g (8)
9
Entry
Amount of catalyst (mol% of MnO2)
0.11 g (12)
t (h)
16
Yield (%)d
7
8
9
86
59
38
0.09 g (10)
16
0.07 g (8)
16
a
b
Reaction conditions for the synthesis of 3a: toluene (5.0 mL), o-phenylenediamine (1.0 mmol), air as oxidant, reflux.
Reaction conditions for the synthesis of 5a: toluene (5.0 mL), 2-aminobenzenethiol (1.0 mmol), air as oxidant, reflux.
c Reaction conditions for the synthesis of 7a: toluene (5.0 mL), 2-aminobenzamide (1.0 mmol), air as oxidant, reflux.
d
Isolated yield.
The MnO2@silk catalyst has showed high capability for the oxidation of the toluene and
then the synthesis of 3a, 5a, and 7a. To extend the scope of the prepared catalyst, the
oxidative synthesis of the 3, 5, and 7 compounds with xylenes have been studied. The
reactions were done under air blowing and reflux conditions. The obtained results of the
mentioned reactions have been presented in the Table 4.

Table 4 The oxidative synthesis of 2-aryl-1H-benzo[d]imidazole (3), 2-arylbenzo[d]thiazole (5), and 2-aryl-4-
quinazolinone (7) derivatives by silk supported MnO2 nanocatalysta
Mp (°C)
Entry
Arene
Product
t (h)
Yield (%)b
Found
289-292
221-223
Reported
3a
3b
3c
3d
toluene
o-xylene
m-xylene
p-xylene
10
10
10
10
69
64
38
78
292-293 [22]
223-224 [22]
1
2
3
4
229-232
273-275
231-232 [22]
275-276 [22]
Mp (°C)
Entry
Arenes
Product
t (h)
Yield (%)b
Found
111-113
56-58
Reported
110-112 [23]
55-57 [24]
67-68 [23]
86 [23]
toluene
o-xylene
m-xylene
p-xylene
9
9
9
9
75
68
36
82
5
6
7
8
5a
5b
5c
5d
68-70
87-89
Mp (°C)
Entry
Arenes
Product
t (h)
Yield (%)b
Found
230-231
210-212
211-212
230-232
Reported
232-235 [25]
212-215 [25]
210-211 [26]
231-234 [25]
toluene
o-xylene
m-xylene
p-xylene
16
16
16
16
86
78
41
90
9
7a
7b
7c
7d
10
11
12
a
Reaction conditions: arene (5.0 mL), o-phenylenediamine (Entries 1-4, 1.00 mmol), 2-aminobenzenethiol (Entries 5-8, 1.0 mmol), 2-
aminobenzamide (Entries 9-12, 1.0 mmol), MnO2@silk (Entries 1-8, 0.09 g), MnO2@silk (Entries 9-12, 0.11 g).
Isolated yield.
b
Recyclability of the MnO2@silk was examined in the oxidation of indane, 1-indanol, and
2,3-dihydro-1H-inden-1-one oxime and oxidative synthesis of 3a compound. Catalyst was
recovered from reaction media by filtration, washed with EtOH, dried and then reused for
five repetitive cycles in the reaction. The results showed that the activity of the catalyst
was saved during successive uses in a considerable amount (Fig. 5).

Fig. 5 Successive use of the MnO2@silk catalyst for the oxidation of indane, 1-indanol, and 2,3-dihydro-1H-
inden-1-one oxime and oxidative synthesis of 3a compound.
Conclusion
In conclusion, we have developed a straightforward procedure for the aerobic oxidation and
tandem oxidative synthesis of organic compounds using new biopolymer supported MnO2
nanostructures catalyst, MnO2@silk. The distribution of MnO2 on the surface of silk fibers
gave a biopolymer based catalytic system, which has high catalytic activity and selectivity in
the aerobic oxidation of the alkyl arenes, alcohols, and oximes to the corresponding aldehydes
and ketones without any overoxidation. As well, the one-pot tandem aerobic oxidative two-
component reactions (O-2CRs) of aromatic hydrocarbons of petroleum naphtha, for the
synthesis of 2-aryl-1H-benzo[d]imidazole, 2-arylbenzo[d]thiazole, and 2-aryl-4-quinazolinone
derivatives, was done using MnO2@silk. Use of air as oxidant (inexpensive oxidant) in the
absence of additional oxidizing reagent, renewability, biodegradability, good yield, easy
workup, the recyclability of catalyst (with no loss in its activity) are some of advantages of the
MnO2@silk catalyst.
Acknowledgements
We gratefully acknowledge financial support from the Iran National Science Foundation
(INSF) and Research Council of Shahid Beheshti University.

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Natural
Silk
Supported
Manganese
Dioxide
Nanostructures: Synthesis and Catalytic Activity in
Aerobic Oxidation and One-Pot Tandem Oxidative
Synthesis of Organic Compounds
Ahmad Shaabani,* Zeinab Hezarkhani, and Elham Badali
Faculty of Chemistry, Shahid Beheshti University, G. C., P. O. Box 19396-4716, Tehran, Iran
Email: a-shaabani@sbu.ac.ir
Silk supported MnO2 nanostructures as a heterogeneous catalyst was synthesized and used in the
aerobic oxidation of alkyl arenes, alcohols, and oximes to their corresponding carbonyl compounds
and a one-pot tandem oxidative two-component reaction of aromatic hydrocarbons of petroleum
naphtha in the absence of any other oxidizing reagent or initiator.