Au- and Ag-loaded MnO2 nanostructures supported on nitrogen- and nitrogen–sulfur-doped pyroproteins: Synthesis and catalytic activity in organic transformations

Article abstract of DOI:10.1002/aoc.3624

Four novel Au- and Ag-loaded MnO₂ nanostructures supported on nitrogen-doped pyroprotein of natural silk (Au–MnO₂@PPNS and Ag–MnO₂@PPNS) and nitrogen–sulfur-doped pyroprotein of natural wool (Au–MnO₂@PPNW and Ag–MnO₂@PPNW) have been synthesized. Nitrogen- and nitrogen–sulfur-doped pyroproteins were prepared by carbonization of natural silk and wool proteins, respectively. The catalysts were investigated for the aerobic oxidation of aromatic hydrocarbons of petroleum naphtha and the aerobic oxidative synthesis of 2-phenylbenzo[d]thiazoles, 2-phenyl-1H-benzo[d]imidazoles and 2-phenyl-4-quinazolinones in the absence of any co-promoter and additional oxidizing reagent. The prepared catalytic systems showed higher catalytic activity in comparison to aggregated catalysts.

Full text of DOI:10.1002/aoc.3624

Received: 14 July 2016
DOI 10.1002/aoc.3624
Revised: 3 August 2016
Accepted: 19 August 2016
F U L L PA P E R
Au‐ and Ag‐loaded MnO2 nanostructures supported on nitrogen‐
and nitrogen–sulfur‐doped pyroproteins: Synthesis and catalytic
activity in organic transformations
*
Zeinab Hezarkhani | Ahmad Shaabani
Faculty of Chemistry, Shahid Beheshti University,
Four novel Au‐ and Ag‐loaded MnO2 nanostructures supported on nitrogen‐doped
pyroprotein of natural silk (Au–MnO2@PPNS and Ag–MnO2@PPNS) and
nitrogen–sulfur‐doped pyroprotein of natural wool (Au–MnO2@PPNW and Ag–
MnO2@PPNW) have been synthesized. Nitrogen‐ and nitrogen–sulfur‐doped
pyroproteins were prepared by carbonization of natural silk and wool proteins,
respectively. The catalysts were investigated for the aerobic oxidation of aromatic
hydrocarbons of petroleum naphtha and the aerobic oxidative synthesis of 2‐
phenylbenzo[d]thiazoles, 2‐phenyl‐1H‐benzo[d]imidazoles and 2‐phenyl‐4‐
quinazolinones in the absence of any co‐promoter and additional oxidizing reagent.
The prepared catalytic systems showed higher catalytic activity in comparison to
aggregated catalysts.
GC, PO Box 19396‐4716 Tehran, Iran
Correspondence
Ahmad Shaabani, Faculty of Chemistry, Shahid
Beheshti University, GC, PO Box 19396‐4716,
Tehran, Iran.
Email: a‐shaabani@sbu.ac.ir
KEYWORDS
MnO2 nanostructures, natural silk, natural wool, nitrogen‐ and nitrogen–sulfur‐doped
carbon, pyroprotein
1
|
INTRODUCTION
The β‐sheets are the most stable secondary structures. The
interchain hydrogen bonding between adjacent amino acids
Manganese dioxide (MnO2) as an abundant, low‐toxicity and
environmentally friendly reagent has attracted much attention
because of its physical and chemical properties and wide
range of applications in oxidation catalysis.[1–5] Impurity
doping as an effective method for manipulating the physical
and chemical properties of catalysts could be done by non‐
equilibrium doping with an appropriate metal.[6,7] Recently,
gold‐doped and silver‐doped MnO2 were shown to have a
significantly improved performance compared with undoped
MnO2.[8–10] The specific surface area of catalytic materials
has an obvious influence on the performance of cata-
lysts.[11,12] Consequently, doped MnO2 nanostructures not
only have distinctive physicochemical properties, associated
with a large specific surface area of dimensions on the
nanometre scale, but also appropriate metal doping.[13]
Various amino acids link via peptide bonds with each
other to create proteins. Each peptide has specific characteris-
tics and the linking of specific sequences of amino acids via
the peptide bonds leads to individual secondary structures.
in the peptide blocks causes the formation of the β‐sheet
structures. This property is obviously observable in natural
protein fibres such as silk (Scheme 1).[14] The α‐helices are
the usual secondary structure of proteins. In this conforma-
tion the N─H group in the backbone hydrogen bonds to the
C═O group of the backbone, four residues earlier. Such a
structure is observable in the polypeptide chain of wool
(Scheme 1).[14] Due to the secondary structures of various
natural protein‐rich materials, the behaviours of them are
different in the process of pyrolysis.
The nitrogen atoms included in nitrogen‐containing car-
bon compounds improve the properties of the bulk carbons
which increases the applications suggested for these carbon‐
containing materials. Varying the amounts of nitrogen can
be used to alter the properties of the materials, and most of
the time a higher amount of nitrogen improves the specific
properties.[15] The pyrolysis of nitrogen‐rich biopolymers is
one of the most appropriate processes for the synthesis of
the mentioned materials.[16–18]
Appl. Organometal. Chem. 2016; 1–11
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
HEZARKHANI AND SHAABANI
RWK. The obtained RWK was dried under vacuum at 45°C
during three days.
The prepared RSF and RWK were heated to 500°C with a
controlled heating programme in a tubular furnace under a
nitrogen atmosphere. The carbonization process was done
in three steps. The silk fibroin proteins and wool keratin
proteins were rapidly heated to 100°C and maintained at this
temperature for 1 h to remove the absorbed water. The tem-
perature of the samples was increased from 100 to 500°C at
5°C min−1). The samples were maintained at 500°C for 5 h
to induce the formation of the corresponding PPNS and
PPNW. The prepared PPNS and PPNW were thoroughly
ground at room temperature using a pestle.
SCHEME 1 Structure of proteins of silk and wool.
Silk and wool proteins have unique biological functional-
ity and they are of great interest to the scientific community.
Therefore in the study presented here the pyrolysis of silk
and wool proteins was used to obtain nitrogen‐doped
pyroproteins and their usability as catalyst supports for organic
transformations was investigated. Because of our ongoing
interest in developing new MnO2‐containing catalytic systems
for organic reactions,[19–23] herein we report the synthesis of
Au–MnO2 and Ag–MnO2 nanostructures supported on
nitrogen‐ and nitrogen–sulfur‐doped pyroproteins prepared
by pyrolysis of natural silk and wool proteins, respectively.
We investigated their catalytic activity in the one‐pot tandem
aerobic oxidative two‐component synthesis of organic
compounds.
2.2 | Synthesis of Au–MnO2 and Ag–MnO2
nanostructures supported on PPNS (Au–MnO2@PPNS
and Ag–MnO2@PPNS) and PPNW (Au–MnO2@PPNW
and Ag‐MnO2@PPNW)
A mixture of pyroprotein powder (0.5 g) in 50 ml of water
was prepared and sonicated for 30 min. The temperature of
the mixture was raised to 60°C. A solution of MnCl2
(0.02 M, 30 ml) at 70°C and a solution of KMnO4 (0.01 M,
40 ml) at 60°C were prepared, separately. The as‐prepared
solutions of KMnO4 and MnCl2 were added dropwise to
the mixture of pyroprotein powder with sonication during
2
| EXPERIMENTAL
2.1 | Synthesis of N‐doped pyroprotein of natural silk
(PPNS) and N,S‐doped pyroprotein of natural wool
(PPNW)
Regenerated silk fibroin (RSF) was prepared as described
elsewhere.[16,24] Briefly, to extract the silk sericin, wax
and impurities, natural silkworm cocoons were boiled for
30 min in an aqueous solution of 5% (w/v) Na2CO3. The
extracted silk fibroin was washed with deionized water
several times and dried at room temperature for two days.
The prepared silk fibroin fibres were dissolved into a
9.3 M LiBr solution at 60°C for 6 h. The obtained silk
fibroin solution was dialyzed in deionized water for three
days and centrifuged at 7000 rpm for about 10 min to
remove insoluble impurities. Finally, the solvent was
evaporated to afford RSF.
SCHEME 2 Aerobic oxidation of aromatic hydrocarbons of petroleum
naphtha (toluene and three isomers of xylene) catalysed by Au–
MnO2@PPNS, Ag–MnO2@PPNS, Au–MnO2@PPNW and Ag–
MnO2@PPNW.
SCHEME
3
Oxidative synthesis of 2‐arylbenzo[d]thiazole, 2‐aryl‐1H‐
To prepare regenerated wool keratin (RWK), a method
similar to that of a previous report was used.[25] Briefly, to
3.0 g of 1‐butyl‐3‐methylimidazolium chloride ([BMIM]Cl)
was added 75 mg of natural wool fibres. The reaction mixture
was heated at 130°C under a nitrogen atmosphere. The reac-
tion mixture was stirred magnetically until the natural wool
fibres were completely dissolved. Addition of water (50 ml)
to the [BMIM]Cl solution was done for precipitation and
regeneration of the dissolved wool keratin. The wool keratin
precipitate was separated via centrifugation (8000 rpm for
20 min). RWK was washed with water (using a centrifuge)
to remove any ionic liquid that may be retained in the
benzo[d]imidazole and 2‐aryl‐4‐quinazolinone derivatives catalysed by Au–
MnO2@PPNS, Ag–MnO2@PPNS, Au–MnO2@PPNW and Ag–
MnO2@PPNW.
TABLE 1 Amounts of carbon, nitrogen and sulfur in prepared PPNS and
PPNW
N‐doped pyroprotein
N,S‐doped pyroprotein
of natural silk
of natural wool
Carbon (%)
Nitrogen (%)
Sulfur (%)
80.2
7.7
—
82.1
8.3
0.9

HEZARKHANI AND SHAABANI
3
3 h at 60°C. Finally, the mixture was filtered and the residue
was washed successively with water (3 × 10 ml) and EtOH
(3 × 10 ml) and dried under vacuum at 60°C to afford
MnO2@PPNS and MnO2@PPNW.
For the synthesis of Au–MnO2 and Ag–MnO2 nanostruc-
tures supported on N‐ and N,S‐doped pyroproteins, the modi-
fied method similar to that of previous reports[9,10] was used.
A mixture of MnO2@PPNS or MnO2@PPNW (0.3 g) in
TABLE 2 Amounts of Au, Ag and MnO2 in prepared catalysts
MnO2@PPNS
Au–MnO2@PPNS
Ag–MnO2@PPNS
MnO2@PPNW
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au (%)
—
7.68
—
0.11
7.66
—
—
—
8.07
—
0.20
8.04
—
—
MnO2 (%)
Ag (%)
7.63
0.34
8.03
0.41
FIGURE 1 SEM–EDS results for MnO2@PPNS (I), MnO2@PPNW (II),
Au–MnO2@PPNS (III), Au–MnO2@PPNW (IV), Ag–MnO2@PPNS (V)
and Ag–MnO2@PPNW (VI).
FIGURE 1 (Continued)

4
HEZARKHANI AND SHAABANI
20 ml of water was sonicated for 30 min. An aqueous solution
of HAuCl4 (1.6 ml, 10 mM) was added to the prepared mixture
and the mixture was stirred magnetically for 48 h. Then, the
reaction mixture was centrifuged and the residue was col-
lected. The prepared solid was redispersed in distilled water
(10 ml). An aqueous solution of NaBH4 (0.8 ml, 0.1 mM)
was added to the mixture and the reaction medium was stirred
magnetically for 4 h. The mixture was centrifuged to obtain the
Au–MnO2@PPNS and Au–MnO2@PPNW catalysts.
medium was stirred under reflux conditions for the required
times. Air was bubbled into the reaction mixture at a rate
of 5 ml min−1. The progress of the reaction was monitored
using TLC. After completion of the reaction, the catalyst
was separated from the reaction medium via filtration and
washed with ethanol. Then the product was isolated using
To prepare the Ag–MnO2@PPNS and Ag–
MnO2@PPNW catalysts, a similar method was used but with
AgNO3 (1.6 ml, 20 mM) instead of HAuCl4.
2.3 | Oxidation of toluene or xylene: general procedure
To obtain the desired product (Scheme 2), toluene or xylene
(8.0 ml) and the catalyst prepared as described in Section 2.2
were added to a two‐necked flask. The reaction medium was
stirred magnetically under reflux conditions for the required
times. Air was bubbled into the reaction mixture at a rate of
5 ml min−1. The progress of the reaction was monitored using
GC and TLC. After completion of the reaction, the catalyst
was separated using filtration and washed with ethanol. The
product was identified using NMR analysis and quantified
using GC with 1‐dodecane as internal standard.
2.4 | General procedure for preparation of products
3a–d, 5a–d and 7a–d
FIGURE 3 XRD patterns of natural silk (I), PPNS (II), MnO2@PPNS (III),
Au–MnO2@PPNS (IV), Ag–MnO2@PPNS (V), natural wool (VI), PPNW
(VII), MnO2@PPNW (VIII), Au–MnO2@PPNW (IX) and Ag–
MnO2@PPNW (X).
To obtain the desired product (Scheme 3), in a two‐necked
flask containing toluene or xylene (10 ml), an amine source
(1.0 mmol) and the catalyst were added. The reaction
FIGURE 2 UV–visible spectra of natural silk, PPNS, MnO2@PPNS, Au–
MnO2@PPNS, Ag–MnO2@PPNS, natural wool, PPNW, MnO2@PPNW,
Au–MnO2@PPNW and Ag–MnO2@PPNW in water–aqua regia (10:1).
FIGURE 4 TGA curves of natural silk, PPNS, Au–MnO2@PPNS, Ag–
MnO2@PPNS, natural wool, PPNW, Au–MnO2@PPNW and Ag–
MnO2@PPNW in air.

HEZARKHANI AND SHAABANI
5
column chromatography (ethyl acetate–n‐hexane; 2:1). The
3 | RESULTS AND DISCUSSION
1
product was identified using IR, H NMR and 13C NMR
The carbon, nitrogen, and sulfur contents of the silk protein‐
and wool protein‐derived carbons were determined using
elemental analysis (Table 1). The amounts of Au, Ag and
Mn in the catalysts were determined using the flame atomic
spectral analyses and quantified by isolated yield. The char-
acterization data are provided in the supporting information.
FIGURE 5 SEM images of PPNS (a, b), PPNW (c, d), Au–MnO2@PPNS (e,
f), Ag–MnO2@PPNS (g, h), Au–MnO2@PPNW (i, j) and Ag–
MnO2@PPNW (k, l).
FIGURE 5 (Continued)

6
HEZARKHANI AND SHAABANI
absorption spectroscopy and inductively coupled plasma
optical emission spectrometry methods. The results of the
analysis are presented in Table 2.
For the determination of the chemical composition of
MnO2@PPNS, MnO2@PPNW, Au–MnO2@PPNS, Ag–
MnO2@PPNS, Au–MnO2@PPNW and Ag–MnO2@PPNW,
the energy dispersive spectroscopy (EDS) method was
applied (Figure 1).
UV–visible spectral analysis of natural silk, PPNS,
MnO2@PPNS, Au–MnO2@PPNS, Ag–MnO2@PPNS, natu-
ral wool, PPNW, MnO2@PPNW, Au–MnO2@PPNW and
Ag–MnO2@PPNW was also conducted (Figure 2). For this
FIGURE 5 (Continued)
FIGURE 5 (Continued)

HEZARKHANI AND SHAABANI
7
analysis the prepared catalysts were dissolved in aqua regia
(concentrated HCl–concentrated HNO3 at 3:1) and then
diluted with deionized water.
The X‐ray diffraction (XRD) patterns of the initial natural
silk and wool, of PPNS and PPNW and of the prepared
catalysts were employed to study the structure of the
catalysts. The two broad peaks at 2θ of around 9.5° and
19.5° are related to the silk fibres (Figure 3‐I). The pattern
of wool has two characteristic broad peaks at 2θ of 9° and
21° (Figure 3‐VI). PPNS and PPNW show specific peaks dif-
ferent from those of the original parent natural biopolymers
(Figure 3‐II and VII). For PPNS the characteristic broad
peaks at 2θ of 10° and 21° and for PPNW at 2θ of 10.5°
and 24° are observable. In the case of MnO2@PPNS and
MnO2@PPNW, PPNS and PPNW show very strong peaks
which means that the peaks related to MnO2 cannot be
observed because of very weak peaks of MnO2 moieties. In
addition, the observations imply that the nature of the PPNS
and PPNW supports is not altered during the process of the
preparation of the MnO2@PPNS and MnO2@PPNW cata-
lytic systems (Figure 3‐III and VIII).
The Au‐loaded structure of MnO2 nanostructures in Au–
MnO2@PPNS and Au–MnO2@PPNW nanocatalysts was
confirmed through powder XRD according to a previous
report.[8] Also, the XRD patterns of Ag–MnO2@PPNS and
Ag–MnO2@PPNW nanocatalysts show the characteristics
of an Ag‐loaded structure of MnO2 nanostructures.[9,10]
The thermal behaviour of the prepared catalytic systems
was studied using thermogravimetric analysis (TGA). From
the TGA data (Figure 4) it is clear that both natural silk and
wool show two decomposition stages. The results of TGA
of PPNS and PPNW show that they have very high thermal
stability in comparison with the parent materials. Also, the
prepared Au–MnO2@PPNS, Ag–MnO2@PPNS, Au–
MnO2@PPNW and Ag–MnO2@PPNW show high thermal
stability as can be seen from Figure 4.
Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) were used to study the morphol-
ogy and the structure of the prepared catalytic systems
(Figures 5 and 6). From the SEM and TEM results it is
observed that the Au–MnO2 and Ag–MnO2 nanostructures
are dispersed uniformly on the surface of the prepared PPNS
and PPNW supports.
Since selective oxidation of primary C─H bonds with air
is of fundamental importance for the sustainable exploitation
of available feedstocks, the catalytic activity of the prepared
catalysts was studied in the aerobic oxidation of aromatic
hydrocarbons of petroleum naphtha (toluene and three iso-
mers of xylene) (Scheme 2).
To achieve a suitable catalytic system for the mentioned
reactions,
we
examined
Au–MnO2@PPNS,
Ag–
MnO2@PPNS, Au–MnO2@PPNW and Ag–MnO2@PPNW
catalysts in the reaction media. Aerobic oxidation of toluene
was selected as a model reaction (Table 3). From the results
presented in Table 3 (entries 1 and 11) it can be seen that
FIGURE 6 TEM images of Au–MnO2@PPNS (a), Ag–MnO2@PPNS (b),
Au–MnO2@PPNW (c) and Ag–MnO2@PPNW (d).

8
HEZARKHANI AND SHAABANI
TABLE 3 Optimization of reaction conditions for aerobic oxidation of aromatic hydrocarbons of petroleum naphtha catalysed by Au–MnO2@PPNS, Ag–
MnO2@PPNS, Au–MnO2@PPNW and Ag–MnO2@PPNWa
Selectivity (%)b
Entry
Catalyst
PPNS
Amount of catalyst (g)
Time (h)
Conversion (%)b
Benzaldehyde
Others
1
0.10
0.01
0.11
0.11
0.09
0.07
0.01
0.11
0.09
0.07
0.10
0.11
0.11
0.09
0.07
0.11
0.09
0.07
12
8
Trace
8
—
99
99
87
89
88
99
84
85
87
—
99
85
84
90
89
91
92
—
—
—
13
11
12
—
16
15
13
—
—
15
16
10
11
9
2
Au–MnO2
3
MnO2@PPNS
Au–MnO2@PPNS
8
17
33
15
7
4
5
5
5
6
5
7
Ag–MnO2
8
6
8
Ag–MnO2@PPNS
6
29
18
10
Trace
20
36
18
12
31
16
9
9
6
10
11
12
13
14
15
16
17
18
6
PPNW
12
8
MnO2@PPNW
Au–MnO2@PPNW
5
5
5
Ag–MnO2@PPNW
6
6
6
8
aReaction conditions: toluene (8.0 ml), air as oxidant, reflux.
bConversion and selectivity determined by GC analysis.
eachsupport itself, in the absence of MnO2, Au–MnO2
and Ag–MnO2, cannot catalyse the reaction. Entries 2, 4, 7,
8, 13 and 16 show that the aggregated catalyst can catalyse
the oxidation of the starting material to the corresponding
aldehyde but the conversions are very low. Thus by
dispersing the catalysts on the surface of the supports the
activity of the catalytic systems is increased by a considerable
amount. Also, when the catalytic systems in which only
MnO2 nanostructures supported on PPNS and PPNW were
used (entries 3 and 12), the activity is lower than that of the
Au‐ and Ag‐loaded systems (entries 4, 8, 13 and 16).
To extend the scope of the prepared catalysts, the
aerobic oxidation of the three isomers of xylene to the
corresponding aldehydes was carried out under reflux
TABLE 4 Aerobic oxidation of aromatic hydrocarbons of petroleum naphtha catalysed by Au–MnO2@PPNS, Ag–MnO2@PPNS, Au–MnO2@PPNW and
Ag–MnO2@PPNWa
Selectivity (%)b
Time
(h)
Conversion
(%)b
Entry
Substrate
Catalyst
Others
1
o‐Xylene
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
5
6
5
6
5
6
5
6
5
6
5
6
30
29
31
27
21
22
24
19
31
30
31
28
88
91
87
99
96
93
95
90
91
92
82
79
12
9
2
3
13
–
4
5
m‐Xylene
p‐Xylene
4
6
7
7
5
8
10
9
9
10
11
12
8
18
21
aReaction conditions: arene (8.0 ml), Au–MnO2@PPNS (0.11 g), Ag–MnO2@PPNS (0.11 g), Au–MnO2@PPNW (0.11 g), Ag–MnO2@PPNW (0.11 g), air as oxidant,
reflux.
bConversion and selectivity determined by GC analysis.

HEZARKHANI AND SHAABANI
9
TABLE 5 Oxidative synthesis of 2‐arylbenzo[d]thiazole, 2‐aryl‐1H‐benzo[d]imidazole and 2‐aryl‐4‐quinazolinone derivativesa
M.p. (°C)
Reported
Time
(h)
Yield
(%)b
Entry
Y
Arene
Catalyst
Product
3a
Found
1
S
Toluene
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
2
3
2
3
2
3
2
3
5
7
5
7
2
3
2
3
2
3
2
3
5
7
5
7
2
3
2
3
2
3
2
3
5
7
5
7
2
3
2
3
2
3
2
3
69
62
67
59
61
60
59
55
76
72
79
68
62
61
66
67
53
50
52
48
68
59
67
65
46
37
42
40
40
34
39
37
49
44
51
40
74
71
79
73
70
64
67
68
108–109
110–112[26]
292–293[27]
232–235[28]
55–57[29]
2
3
4
5
NH
CONH
S
4a
5a
3b
4b
5b
3c
4c
5c
3d
4d
289–290
231–233
55–56
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
o‐Xylene
m‐Xylene
p‐Xylene
NH
223–225
211–212
66–68
223–224[27]
212–215[28]
67–68[26]
CONH
S
NH
232–234
209–210
85–86
231–232[27]
210–211[30]
86[26]
CONH
S
NH
275–277
275–276[27]
(Continues)

10
HEZARKHANI AND SHAABANI
TABLE 5 (Continued)
M.p. (°C)
Time
(h)
Yield
(%)b
Entry
Y
Arene
Catalyst
Product
5d
Found
Reported
45
46
47
48
CONH
Au–MnO2@PPNS
Ag–MnO2@PPNS
Au–MnO2@PPNW
Ag–MnO2@PPNW
5
7
5
7
87
76
83
79
229–230
231–234[28]
aReaction conditions: arene (10 ml), amine source (1.0 mmol), catalyst (0.11 g), air as oxidant, reflux.
bIsolated yield.
conditions and blowing air, using Au–MnO2@PPNS,
Ag–MnO2@PPNS, Au–MnO2@PPNW and Ag–
MnO2@PPNW catalysts (Table 4).
was used in the reaction as before. This process was done
four successive times to evaluate the recyclability of the
catalysts. The results (Figure 7) imply that the catalysts
are usable for the reaction after recycling and no consider-
able decrease in the conversion is observed.
Because the aerobic oxidation of the aromatic
hydrocarbons of petroleum naphtha (toluene and xylenes)
can be carried out effectively using the prepared Au–
MnO2@PPNS, Ag–MnO2@PPNS, Au–MnO2@PPNW and
Ag–MnO2@PPNW catalysts, the transformation of inert
substances (toluene and xylenes) into reactive substrates as
precursors in one‐pot tandem synthetic processes was exam-
ined using the four catalytic systems (Scheme 3).
Consequently, Au–MnO2@PPNS, Ag–MnO2@PPNS, Au–
MnO2@PPNW and Ag–MnO2@PPNW as effective aerobic
oxidation catalysts were used in the tandem oxidative
synthesis of 2‐arylbenzo[d]thiazole, 2‐aryl‐1H‐benzo[d]
imidazole and 2‐aryl‐4‐quinazolinone compounds using the
aromatic hydrocarbons of petroleum naphtha as precursors
(Table 5).
4
| CONCLUSIONS
Four novel catalytic systems (Au–MnO2@PPNS, Ag–
MnO2@PPNS, Au–MnO2@PPNW and Ag–MnO2@PPNW)
for the conversion of the inert aromatic hydrocarbons of
petroleum naphtha to active materials in the tandem oxidative
synthesis of organic compounds were reported. The Au–
MnO2@PPNS, Ag–MnO2@PPNS, Au–MnO2@PPNW and
Ag–MnO2@PPNW catalytic systems were used successfully
in the synthesis of 2‐arylbenzo[d]thiazole, 2‐aryl‐1H‐benzo
[d]imidazole and 2‐aryl‐4‐quinazolinone derivatives. The
distribution of Au–MnO2 and Ag–MnO2 nanostructures on
the surface of PPNS and PPNW gave more active catalytic
systems in comparison to the aggregated catalysts. In the
presented procedures, air was used as an inexpensive oxidant
which can be used without the need for additional oxidants.
The renewability, biodegradability and recyclability are other
advantages of the Au–MnO2@PPNS, Ag–MnO2@PPNS,
Au–MnO2@PPNW and Ag–MnO2@PPNW catalytic
systems.
The
recyclability
of
Au–MnO2@PPNS,
Ag–
MnO2@PPNS, Au–MnO2@PPNW and Ag–MnO2@PPNW
was studied in the aerobic oxidation of toluene. The recov-
ery of the catalysts from the reaction media was done using
simple filtration. After washing the catalyst with ethanol
(3 × 10 ml) it was dried in an oven. The recovered catalyst
ACKNOWLEDGEMENTS
We gratefully acknowledge the financial support from the
Iran National Science Foundation (INSF) and Research
Council of Shahid Beheshti University.
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