Direct oxidative esterification of primary alcohols and oxidation of secondary alcohols over mesoporous spherical silica encapsulated MnO2 nanoparticles

Article abstract of DOI:10.1039/c9nj01345h

In this work, a simple and efficient strategy for the fabrication of novel encapsulated MnO₂ nanoparticles inside spherical mesoporous silica hollow-nanoparticles was described. They were synthesized by consecutively anchoring MnO₂ nanoparticles on poly(styrene-co-methacrylic acid) particles, coating with a mesoporous silica shell, and subsequently removing the polymeric core by dissolving in acetone. The catalytic activity of the nanoparticles was examined in the aerobic oxidation of various primary and secondary alcohols, which showed good activity and selectivity for the transformation of primary alcohols to the corresponding esters through the oxidative esterification process and secondary alcohols to ketones in short reaction times under mild reaction conditions. In addition, the catalyst system was utilized for the oxidation of primary alcohols to aldehydes using tert-butyl hydroperoxide (TBHP) as an oxidant under mild conditions and produced an excellent product yield.

Full text of DOI:10.1039/c9nj01345h

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Direct oxidative esterification of primary alcohols and oxidation of secondary
alcohols over mesoporous spherical silica encapsulating MnO2 nanoparticles
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Shahram Zare Hosseinzadeh, Mirzaagha Babazadeh*, Gholam Hossein Shahverdizadeh*,
Rahim Hosseinzadeh-Khanmiri
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Department of Chemistry, Tabriz Branch, Islamic Azad University, Tabriz, Iran
E-mail: Shahverdizadeh@iaut.ac.ir. Fax: +98-491-2231616
Abstract
In this work, a simple and efficient strategy for the fabrication of a novel encapsulated MnO2
nanoparticles inside the spherical mesoporous silica hollow-nanoparticles was described. It was
synthesized by consecutively anchoring the MnO2 nanoparticles on the poly(styrene-co-
methacrylic acid) particles, coating by mesoporous silica shell, and subsequently removing the
polymeric core by resolving in acetone. The catalytic activity of the nanoparticles was examined
in the aerobic oxidation of various primary and secondary alcohols that were shown the good
activity and selectivity for the transformation of primary alcohols to corresponding esters through
the oxidative esterification process and secondary alcohols to ketones in short reaction times under
mild reaction conditions. In addition, the catalyst system was utilized for the oxidation of primary
alcohols to aldehydes using tert-butyl hydroperoxide (TBHP) as oxidant under mild conditions
and produced the excellent product yield.
Introduction
Selective oxidation of organic substrates plays a key role in organic chemistry both at the
laboratory and industry level for direct synthesis of valuable oxygen-containing compounds such
as aldehydes, ketones, carboxylic acids, and esters.1-4 The selective oxidation of alcohols is one of
the most effective approaches to produce these compounds that were abundantly used in recent
decades.5-7 Several conventional approaches are available for the oxidation of alcohols to
corresponding carbonyl groups such as using potassium dichromate, nitric acid, potassium
permanganate, organic peroxides, and sodium hypochlorite.8-11 These methods have any economic

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and environmental limitations include producing abundant wastes and using the stoichiometric
amount of hazardous reagent.12, 13 Several methods are reported to improve this transformation,
and continuous attention is drawn to newer and more selective ways of alcohols oxidation.
Catalytic aerobic oxidation (oxidation with molecular oxygen or air) is exclusively attractive from
an economic and environmental point of view.14 Recently, V2O5,15 Au@TiO2,16 Au@C,17 Au:
PVP,18 Au-Pd,19 MoV(TeNb),2 MnO2@Cellulose20 and MnO2@GO21 have been designed and
developed for this conversion and await the further discovery of suitable catalytic systems.
Investigation of newer and economical methods for esterification is part of the challenges of
chemistry, due to the importance of ester functional group as an abundant functional group in the
bulk chemicals, fine chemicals, natural products, and polymers.22-24 The conventional and recent
esterification approaches can be classified in four ways consisted of reaction of carboxylic acids
(acyl chlorides, anhydrides, esters, and so on) with alcohols,25 oxidative reactions of aldehydes
with alcohols,26, 27carbonylations of aryl halides,28-30 and direct oxidative esterification of
alcohols.31, 32 Many report of direct oxidative esterification of alcohols are presented as a novel
and straightforward approach for synthesis of esters using transition metals under high temperature
and pressure conditions or long reaction times.33-40
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Manganese dioxide (MnO2) as a cheap, mild, low toxic, and selective reagent has been widely
used for the oxidation of a variety of functional groups, especially for benzylic and allylic alcohols
to their corresponding carbonyl compounds. MnO2 is one of the most attractive inorganic materials
with excellent physical and chemical properties under ambient conditions, an extensive range of
applications in catalysis, molecular adsorption, ion exchange and biosensor, and also its low cost
and environmentally benign nature.41-44
Recently, the synthesis of inorganic hollow nanoparticles (NPs) has attracted much attention in the
chemistry and material communities because of their low density, large specific area, mechanical
and thermal stabilities, and surface permeability.45, 46 The hollow NPs were widely used in organic
reactions as catalyst and reactors.47-49 Immobilization or encapsulation of catalyst species into the
inorganic hollow NPs can be improved their catalytic activity and provided the facile reaction
conditions due to the synergic effects of these structures.50, 51 These effects have become of
effective interaction of catalyst species and more concentration of substrates on the inside reactor
surface, under these conditions, the reaction intermediate can be stabilized by additional
interaction by reactor surface and significantly changed the reaction pathway.52-54

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In continuation of our works in the fields of aerobic oxidation of alcohols and design of
heterogeneous catalyst55, 56, herein, we aimed to design and preparation of a novel encapsulated
MnO2 NPs into the mesoporous spherical silica hollow structure via the simple procedure as a
catalyst for aerobic oxidation of alcohols.
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EXPERIMENTAL
Materials and techniques
All of the chemicals were purchased from Merck and Aldrich and used without further purification.
Melting points were taken on an Electrothermal 9100 apparatus. Fourier transform infrared (FTIR)
spectra were recorded using a Bomem MB-Series FTIR spectrophotometer. Morphological
measurements were performed using a transition electron microscopy (TEM) instrument (Philips
CM10) operated at an 80 kV electron beam accelerating voltage. Scanning electron microscopy
(SEM) and energy-dispersive X-ray spectroscopy (EDX)was performed with a Philips XL-300
instrument. The sample was sputtered by gold to avoid undesirable electron charging. X‐ray
diffraction (XRD) was conducted using a Philips X'pert Pro (PW 3040) X‐ray diffractometer with
monochromatic Cu‐Kαradiation (k = 1.54056 Å, 40 kV, 30 mA). 1 H and 13 C NMR spectra were
recorded on a Bruker DRX‐400 spectrometer at 300MHz. NMR spectra were obtained on solutions
in CDCl3 using tetramethylsilane as an internal standard. The textural structures were measured
by N2 adsorption at −196 °C in a Micromeritics TriStar ASAP 3000 system, and specific surface
areas of the as‐prepared samples were measured using Brunauer–Emmett–Teller (BET) method.
The concentration of manganese was estimated using Shimadzu AA‐680 flame atomic absorption
spectrophotometer (FAAS). Thermogravimetric analysis (TGA) was conducted using a
TG/DTA6300 instrument, at a heating rate of 20 °C min-1 under nitrogen flow. Catalysis products
were analyzed using a Thermo Electron Corporation K0723B GC (GC conversion was obtained
from the ratio of alcohols to the product, using n-decane as an internal standard).
Preparation of encapsulated-MnO2 NPs into the mesoporous silica hollow spheres (En-
MnO2/SiO2)
Potassium permanganate (0.16 g, 1.00 mmol), manganese(II) chloride (0.01 g, 0.08 mmol) and
solution of poly(styrene-co-methacrylic acid) (PSMA, 0.20 g) in 2-propanol (2.00 mL) were added

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to one-necked flask containing of deionized water (20 mL) and stirred for 30 minutes at room
temperature. After this time, the brown suspension was produced (PSMA@MnO2). Next, the
reaction flask was charged by 2-propanol (30 mL) and ammonia solution (2 mL, 25%). To the
reaction mixture tetraethyl orthosilicate (TEOS, 0.4 mL) was added and stirring continued at room
temperature for 24 h. The resulting NPs (PSMA@MnO2@SiO2) were collected by centrifugation
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(6000 per minute), washed several times by deionized water and ethanol, and dried at 80 C for
2h. At the end of this process, the PSMA was removed from the PSMA@MnO2@SiO2 NPs by
dispersing into the acetone (50 mL), sonicated for 1 h, collected same of the previous step (this
process was repeated three times) and dried at 100 oC for 5 h. The loading of MnO2 was obtained
4.0 mmol g−1 by FAAS.
General procedure for direct oxidative esterification of primary alcohols in the presence of En-
MnO2/SiO2
Primary alcohol (1.00 mmol), KOH (0.006 g, 10.00 mmol %) and En-MnO2/SiO2 (0.03 g, 12.00
mmol%) were added to a two-necked flask equipped with a gas bubbling tube containing
acetonitrile (5 mL). Air was bubbled into the reaction mixture and stirred at 70 oC. The reaction
progress was followed by tin layer chromatography (TLC). After completion of the reaction, the
catalyst was separated by filtering and washed by acetonitrile (2 mL). The filtrate solvent was
evaporated under vacuum and product was purified with flash chromatography.
General procedure for aerobic oxidation of secondary alcohols in the presence of En-MnO2/SiO2
Secondary alcohol (1.00 mmol) and En-MnO2/SiO2 (0.05 g, 20.00 mmol%) were added to a two-
necked flask equipped with a gas bubbling tube containing acetonitrile (5 mL) and stirred under
air blowing at 70 oC. The reaction progress was followed by TLC. At the end of the reaction, the
reaction mixture was filtered and washed by acetonitrile (2 mL). The resulting solution was
purified by flash chromatography.
General procedure for oxidation of alcohols using TBHP in the presence of En-MnO2/SiO2
Primary alcohol (1.00 mmol), En-MnO2/SiO2 (0.05 g, 20.00 mmol%) and TBHP (2.00 mmol) were
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added to a two-necked flask containing acetonitrile (5 mL) and stirred at 70 C. The reaction

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progress was followed by TLC. At the end of the reaction, the reaction mixture was filtered and
washed by acetonitrile (2 mL). The resulting solution was purified by flash chromatography.
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Results and discussion
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As illustrated in Fig. 1, MnO2 NPs were firstly fabricated using KMnO4 and MnCl2 in presence of
PSMA in the aqueous medium (the mixture of water: 2-propanol) under vigorously stirring that
was formed MnO2 NPs immobilized on PSMA spheres. These spheres were coated by silica layers
through the sol-gel process using TEOS. During the sol-gel process, TEOS is hydrolyzed to
Si(OH)4, adsorbed on the PSMA/MnO2 NPs via hydrogen bond interactions with the carboxylic
acid functions and electrostatic interaction with MnO2 particles with a negative surface potential
and polymerized to make a layer of SiO2. Finally, the PSMA spheres, which have templates role
in this system were removed by resolving in acetone (Fig 1). The resulting NPs were characterized
by FTIR, XRD, TEM, SEM, EDX, FAAS, TGA and BET.
O
Ph
OH
n
m
PSMA
SiO2
MnO2
MnCl2
2-propanole
TEOS, NH3
2-propanole
acetone
KMnO4
Water
Figure 1. Preparation of encapsulated MnO2 NPs into the mesoporous silica hollow sphere.
Each step of the catalyst preparation procedure was tracked by FTIR spectroscopy. The FTIR
spectra of MnO2, PSMA@MnO2@SiO2, and En-MnO2/SiO2 were shown in Fig. 2. The FTIR of
MnO2 NPs shows the broad peak at 540-600 cm-1 attributed to the Mn-O bond. The peaks at 1384,
1649 and 3435 cm-1 were observed due to the stretching vibrations of hydrogen-bonded surface
water molecules.57 The FTIR of PSMA@MnO2@SiO2 consisted of (i) the Mn-O bond at 547 cm-
1, (ii) the stretching and bending vibrations of Si–O–Si bonds at 1086 and 467 cm-1,(iii) the
stretching, asymmetric stretching, and bending of silanol groups (Si–OH) of the silica shell at

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3460, 1100 and 960 cm-1, and (iv) the peaks of aromatic rings, carbonyl groups and C-H bonds of
PSMA cores at 1664, 1722 and 2926-2960 cm-1, respectively.58 Finally, the FTIR of En-
MnO2/SiO2 confirms the removal of PSMA core due to the not existence of its characteristic peaks.
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Si-O-Si
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Mn-O
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Figure 2. The FTIR spectra of MnO2 (a), PSMA@MnO2@SiO2 (b) and En-MnO2/SiO2 (c)
The XRD patterns of PSMA@MnO2@SiO2 and En-MnO2/SiO2 were depicted in Fig. 3. The
PSMA@MnO2@SiO2 pattern shows the three characteristic diffractions of MnO2 tetragonal
phases at 2Ө = 26.11o, 36.94o, 52.02o and 67.78o (JCPDS 44-0141) and the broad diffractions
below the 2Ө = 25o related to the amorphous silica layer.59, 60 After removal of PSMA cores, the
XRD pattern did not change that was confirmed the retaining of MnO2 into the silica hollow
structure during the reaction procedure.

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Figure 3. The XRD patterns of PSMA@MnO2@SiO2 (green) and En-MnO2/SiO2 (red)
The SEM and TEM analysis were used for morphological study and validation of the structure of
the catalyst. The SEM images of En-MnO2/SiO2 show the quasi-spherical particles with diameters
of 87-125 nm into uniform shapes. The TEM images of PSMA@MnO2@SiO2 displayed the core-
shell structure NPs with a thickness of silica about 15 nm and each particle has PSMA@MnO2
cores as aggregated spherical particles. After extraction of PSMA structure from the NPs, the
aggregated cores of the structure were removed and the MnO2 NPs were dispersed into the silica
hollow sphere and its porous walls. Furthermore, the absence of PSMA (the existence of C, H)
presence of MnO2 into the structure is confirmed by EDX analysis of En-MnO2/SiO2, indicating
the Mn, S, and O.

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h
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Porous structure
Silica shell

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j
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Figure 4. The SEM images of PSMA@MnO2@SiO2 (a-c), TEM of PSMA@MnO2@SiO2 (d and e) and
En-MnO2/SiO2 (f-i), EDX analysis of En-MnO2/SiO2 (j) and SEM mapping of En-MnO2/SiO2 (k)
The TGA analysis of before and after removal of PSMA was shown in the fig. 5. The TGA curve
of the PSMA@MnO2@SiO2 involves four stages of weight losses. The first stage below of 150 oC
is related to escaping of adsorbed water and volatiles, the next stage that occurred about 320-385
oC can be assigned by PSMA decomposition. The third stage of weight losses was occurred about
o
590-610 C due to the dehydroxylation of SiO2 shell and the last stage attributed to the
decomposition of the crystalline structure of MnO2 at above 700 oC. In the case of En-MnO2/SiO2,
the decomposition stage of PSMA did not observe and dehydroxylation and decomposition
temperatures of SiO2 and MnO2 have occurred in the same region with PSMA@MnO2@SiO2. The
comparison of the remaining weights of both cases at 800 oC, the weight percentage of PSMA in
PSMA@MnO2@SiO2 was determined 22 %.

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22 wt%
Figure 5. The TGA analysis of PSMA@MnO2@SiO2 (a) and En-MnO2/SiO2 (b)
The nitrogen adsorption isotherm obtained, and pore size distribution graphs for
PSMA@MnO2@SiO2 and En-MnO2/SiO2 are shown in Fig. 6. The hysteresis loop appeared in the
range of 0.5 < P/Po < 1.0 is related to capillary condensation. According to IUPAC classification,
these isotherms was classified in type IV that shown an H1 hysteresis loop. This is characteristic
of the highly ordered mesoporous material.61, 62
The BET analysis showed that the specific surface area of PSMA@MnO2@SiO2 and En-
MnO2/SiO2 was 261.8 m2 g-1 and 439.15 m2 g-1, respectively. Thus, the higher specific surface
area of En-MnO2/SiO2 can be assumed to be due to the absence of PSMA cores and produced the
hollow structure. The increase in the surface area may also be due to the availability porous internal
surface of hollow spheres that was occupied by PSMA and thus preventing access to the pores. As
seen in BJH plot, after the PSMA removing the pore size distribution was changed, but the average
pore sizes not significantly changed due to the elimination of PSMA from the porous structure that
appeared the smaller pores than average.

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Figure 6. The N2 adsorption/desorption isotherms of En-MnO2/SiO2 adsorption-desorption (black),
PSMA@MnO2@SiO2 adsorption-desorption (red). The inset shows the corresponding pore size distribution
En-MnO2/SiO2 (black) and PSMA@MnO2@SiO2 (red).
The activity of this catalyst system was evaluated in the oxidation of various alcohols. At first, the
oxidation of benzyl alcohol was chosen as the model reaction and air was used as the oxidant
(Table 1). The various solvents such as water, n-hexane, toluene, DMSO, ethyl acetate and
acetonitrile were examined in the model reaction in the presence of KOH as a base at reflux
temperature. In the case of water, the reaction was failed. However, in the cases of organic

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solvents, the reaction was done and produced the mixture of products consisting of benzaldehyde,
benzoic acid, and benzyl benzoate. In the non-polar solvent such as n-hexane and toluene, a
mixture of benzaldehyde and benzoic acid was formed. However, benzoic acid and benzyl
benzoate were synthesized in the polar solvent. Next, the various mixture of DMSO: water were
examined and afforded the moderate yield of benzoic acid as a single product due to the
participation of water in reaction. The best solvent for this transformation is acetonitrile, which
just produced the good yield of benzyl benzoate at short reaction time. According to the results, it
appears to be found that the solvents with high dipole moments can be stabilized the polar
intermediate of the oxidative-esterification reaction in contrast of the nonpolar solvents, and the
bad result of water become from its inability to dissolve the organic substrate and competition of
water with the substrate to interaction with MnO2. In addition, the various organic and inorganic
base was used in this reaction in acetonitrile and the best results obtained by KOH. Finally, the
catalyst amount and reaction temperature were optimized, and acetonitrile, KOH (10 mol %), 0.03
gram of catalyst and 70 oC were chosen as the best solvent, base, catalyst amount and temperature,
respectively. It is prominent to note that the model reaction was repeated using 10 mol % of MnO2
with same reaction condition, and this reaction just provided a low amount of benzoic acid (less
than 10%).
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
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Optimization the reaction conditionsa
O
O
O
OH
OH
O
Entry
Catalyst (g, mol %)
0.03, 12.0
0.03, 12.0
0.03, 12.0
0.03, 12.0
0.03, 12.0
0.03, 12.0
0.03, 12.0
0.03, 12.0
0.03, 12.0
0.03, 12.0
0.03, 12.0
Base (mol %)
KOH (10)
KOH (10)
KOH (10)
KOH (10)
KOH (10)
KOH (10)
KOH (10)
KOH (10)
KOH (10)
NaOH (10)
K2CO3 (20)
Solvent
Water
Time (h)
12.0
12.0
6.0
Temp. (oC)
Conversion
Yieldb
-
1
2
100
40
-
n-Hexane
Toluene
DMSO
31
57
73
65
30
<5
65
68
56
51
14:15:0
38:19:0
0:54:27
0:65:0
0:30:0
0:4:0
3
100
100
70
4
6.0
5
DMSO: Water (2: 1)
DMSO: Water (1: 1)
DMSO: Water (1: 2)
Ethyl acetate
2.0
6
2.0
70
7
2.0
70
8
3.0
70
0:25:40
0:0:68
0:0:56
0:0:51
9
Acetonitrile
2.0
70
10
11
Acetonitrile
1.0
70
Acetonitrile
1.0
70

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17
0.03, 12.0
0.03, 12.0
0.05, 20.0
0.02, 8.0
Na2CO3 (20)
Et3N (10)
KOH (10)
KOH (10)
KOH (10)
KOH (10)
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
1.0
1.0
1.0
2.0
2.0
2.0
70
70
70
70
85
25
45
70
68
40
68
12
0:0:45
0:28:42
0:0:68
0:0:40
0:0:68
0:0:12
0.03, 12.0
0.03, 12.0
aBenzyl alcohols (1 mmol), solvent (5 mL), catalyst and base under air blowing. measured by GC
(benzaldehyde: benzoic acid: benzyl benzoate).
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
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
b
In order to investigate the limitation and comprehensiveness of this approach, five derivatives of
benzyl alcohols were used. The oxidation reactions were selectively carried out for both of
electron-donating and electron-withdrawing groups with good yields, and the conversion of
electron-donating groups was lower than others. In addition, this process was applied to some
aliphatic primary alcohols that led to producing their corresponding carboxylic acid with good
yields.
Table 2. Derivatives of direct esterification of primary alcoholsa
O
O
O
O
O
O
F
F
Yieldb:
57%
0.89
68 %
66 %
1.08
TOF (min-1): 1.38
O
O
O
O
O
O
OMe
NO2
Cl
MeO
O2N
Cl
Yieldb:
51 %
62%
2.08
25 %
0.94
TOF (min-1): 1.14
O
O
OH
OH
Yieldb:
TOF (min-1):
75 %
0.91
73 %
1.00
O
OH
Yieldb:
TOF (min-1):
80 %
1.08

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aReaction conditions: primary alcohols (1 mmol), catalyst (0.03 g, 12.0 mol%), KOH (0.006 g, 10.0 mol
%) in acetonitrile (5 mL) under air blowing at 70 oC. bIsolated yield.
6
7
As a complementary investigation, the model reaction was examined under non-aerobic conditions
8
9
using TBHP as oxidant. When the reaction was ran using 2 equivalent of TBHP in acetonitrile
with the same amount of the catalyst and KOH at 70 oC, a mixture of benzaldehyde and benzoic
acid was formed (70% 48:22). Next, the reaction was repeated in the absence of KOH.
Unexpectedly, the benzaldehyde was selectively formed after 30 min (scheme 1). Accordingly,
some benzyl alcohol derivatives were used and provided their benzaldehydes in good yields (Table
3).
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
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 3. Derivatives of oxidation of primary alcoholsa
O
cat (12.0 mol%), TBHP
O
OH
KOH (10.0 mol%), acetonitrile, 70 oC
OH
cat (12.0 mol%), TBHP
acetonitrile, 70 oC
O
O
O
O
F
Yieldb:
57%
0.89
68 %
66 %
1.08
TOF (min-1): 1.38
O
O
O
Cl
O2N
MeO
Yieldb:
51 %
62%
2.08
25 %
0.94
TOF (min-1): 1.14

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In the next step, the oxidation of the encumbered secondary alcohols to related carbonyl
compounds was studied. Accordingly, aerobic oxidation of 1-phenylethanol to acetophenone was
investigated under base-free conditions, the reaction was completed in the significantly short
reaction time (20 min) with good yields. In addition, the effect of TBHP (2 equivalent) was
examined in the 1-phenylethanol oxidation that showed almost the same result. Next, for
investigating the scope and limitations of this catalyst in the aerobic oxidation of secondary
alcohols, a variety of encumbered secondary alcohols were used in the optimized reaction
conditions. All products were obtained in high yields. The structure of the products is presented in
Table 4.
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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
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 4. Aerobic oxidation of secondary alcoholsa
OH
R2
O
R1
R1
R2
O
O
O
O
O
O
90 %
0.94
91 %
84 %
0.99
97 %
0.89
Yieldsb:
TOF (min-1)
92 %
1.02
1.07
O
O
O
O
O
Yieldsb:
TOF (min-1)
75 %
0.82
92 %
1.21
71 %
0.79
86 %
0.95
94 %
1.32
o
b
aalcohol (5.0 mmol) and catalyst (0.05 g, 20 mol %) in acetonitrile (5 mL) with air blowing at 70 C, Isolated
yield.
Given these promising results, it seems that the mesoporous silica hollow sphere acts as a reactor in
these processes and provided the high-pressure medium for the catalyst and substrate. In this medium,
more concentration of substrate and oxidant interacted with catalyst.

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Recyclability of En-MnO2/SiO2 was examined in both of the oxidative esterification and non-
aerobic oxidation of benzyl alcohol. The recycling was carried out by filtering the reaction mixture.
The catalyst was washed three times by acetonitrile and water, dried at 100 oC and reused in the
reaction. This procedure was carried out for five repeated cycles. The results show that the activity
of the catalyst was retained after recycling without any significant loss of its activity in both cases
(Fig. 7). The amount of MnO2 in the recovered catalyst after using in five cycles was measured by
FAAS that was the same as the fresh catalyst. The high stability and recyclability of the catalyst
may be raised from its rigid structure that not resolved or decomposed under the reaction
conditions and during the catalyst recovery process and also the stability of the MnO2 NPs.
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
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
110
105
100
95
90
85
80
75
70
1
2
3
4
5
1-phenylethanol
benzyl alcohol
Figure 7. Recyclability and reusability of the catalyst in both of the oxidative esterification of benzyl
alcohol and oxidation of 1-phenylethanol.
The catalytic results of the En-MnO2/SiO2 in aerobic oxidative esterification of benzyl alcohol
were comprised of recent similar reports. The results were summarized in Table 5.
Table 5. The comparison of this work and recent reports for oxidative esterification of benzyl alcohol
Entry Cat. (mol%)
T(oC)
70
Time (h)
Yield
68
Oxidant
Air
Product
Ref
1
2
En-MnO2/SiO2 (12.0)
PdBiCo/C (10.0)
2
Benzyl benzoate This work
Methyl benzoate [63]
60
14
88
O2

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2
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5
6
3
4
5
6
Pd(OAc)2 (5.0)/AgPF6 (120.0)
Pl-Au (1.0)
90
25
20
24
12
48
85
97
62
97
O2
O2
O2
O2
Benzyl benzoate [64]
Methyl benzoate [65]
Methyl benzoate [66]
Methyl benzoate [67]
PdCl2(CH3CN)2 (5.0)/AgBF4 (10.0) 45
Co@NOSC (21.0) 60
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Conclusion
Is summary, MnO2 NPs was stabilized into the mesoporous silica hollow spheres by the sol-gel
process using poly(styrene-co-acrylic acid) as a soft template that was removed at the end of the
process. These structures were characterized by different methods such as FTIR, XRD, TEM,
SEM, EDX, FAAS, TGA and BET and shown that the catalyst successfully synthesized. These
NPs have spherical morphology with uniform shapes and size, high surface area, and high thermal
stability and chemical stability in the presence of organic solvents. When these NPs were applied
in the oxidation of primary alcohols as a catalyst, the oxidative esterification occurred with high
selectivity. Furthermore, this catalyst successfully did the aerobic oxidation of secondary alcohols
and non-aerobic oxidation of benzyl alcohols with TBHP as oxidant. This catalyst system was
recoverable and reusable for several times.
Acknowledgments
The authors would like to thank Tabriz Branch, Islamic Azad University for the financial support
of this research, which is based on a research project contract.
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