Preparation and characterization of organically modified MCM-48 as heterogonous catalyst for oxidation of sulfides and thiols

Article abstract of DOI:10.1007/s10562-015-1572-x

MCM-48 mesoporous synthesized and grafted with aminopropyl triethoxysilane (APTES) and subsequently reacted by isatin to obtain the imine of MCM-48. Addition of ZrOCl₂.8H₂O, leading to form the complex of Zr(IV)/isatin-MCM-48 as new Schiff base complex of zirconium(IV)-modified-MCM-48. The synthesized catalyst was characterized by XRD, TGA, FT-IR and BET. The resulting catalyst was efficient for selective oxidation of sulfides to sulfoxides and oxidation of thiols to disulfides with hydrogen peroxide. In this method isolation and reusability of catalyst have been investigated.

Full text of DOI:10.1007/s10562-015-1572-x

Catal Lett
DOI 10.1007/s10562-015-1572-x
Preparation and Characterization of Organically Modified MCM-
48 as Heterogonous Catalyst for Oxidation of Sulfides and Thiols
1
1
Maryam Hajjami
•
Zakieh Yousofvand
Received: 18 April 2015 / Accepted: 29 June 2015
Ó Springer Science+Business Media New York 2015
Abstract MCM-48 mesoporous synthesized and grafted
with aminopropyl triethoxysilane (APTES) and subse-
quently reacted by isatin to obtain the imine of MCM-48.
Addition of ZrOCl Á8H O, leading to form the complex of
Keywords Zr-Schiff base-MCM-48 Á Mesoporous Á
Sulfides Á Thiols
2
2
Zr(IV)/isatin-MCM-48 as new Schiff base complex of
zirconium(IV)-modified-MCM-48. The synthesized cata-
lyst was characterized by XRD, TGA, FT-IR and BET. The
resulting catalyst was efficient for selective oxidation of
sulfides to sulfoxides and oxidation of thiols to disulfides
with hydrogen peroxide. In this method isolation and
reusability of catalyst have been investigated
1 Introduction
In 1992 mesoporous material was introduced by Mobil Oil
Company researchers. In this class MCM-41 and MCM-48
are outweigh. MCM-41 has a hexagonal arrangement of the
mesopores, space group p6mm and MCM-48 has a cubic
arrangement of the mesopores, space group Ia3d. This
material that characterized by large specific surface areas
and pore sizes between 2 and 50 nm have been obtained
via the coupling of inorganic and organic components by
template synthesis. MCM-48 is highly grace through large
specific surface area, presence of the spread network of
three- dimensional pores and pore volume. Apply modi-
fied-MCM-48 as catalyst has advantages like high stability
under severe condition such as high temperature and
organic solvent and easy separation process [1–6]. The
surfaces of the MCM-48 have a large number of silanol
groups that reaction with various alkoxysilanes by covalent
grafting for manufacture functionalized MCM-48 [7].
Schiff bases are condensation products of primary amines
and carbonyl compounds (also known as imine or azome-
thine). Schiff bases and their metal complexes are used as
catalysts in various organic synthesis such as oxidation,
epoxidation and polymerization of alkenes. Also they are
play important role in pharmaceutical, agriculture and
industrial chemistry [8, 9]. Choosing a catalytic system for
oxidation of organic compounds that is heterogeneous,
salubrious with environment and recyclable is very
important [10]. Many methods have been developed for the
heterogenization of homogeneous catalysts. But
Graphical Abstract
H O
2
2
H
N
H
N
O
O
O
S
Zr
O
R
R
2
1
R -S-R2
1
N
N
R-SH
R-S-S-R
Si
Si
EtO
OEt
O
O
O
O
MCM-48
&
Maryam Hajjami
mhajjami@yahoo.com; m.hajjami@ilam.ac.ir
1
Department of Chemistry, Faculty of Science,
Ilam University, P.O. Box 69315516, Ilam, Iran
123

M. Hajjami, Z. Yousofvand
immobilization on silica supports offers several advantages
in terms of thermal stability, high surface areas and con-
venient porosities. That’s why todays metal-grafted func-
tionalized mesoporous silica materials used as catalyst in
many oxidation reactions. For example, Co(II) Schiff base
complex on SBA-15 was used for the oxidation of alcohols
to carbonyl compounds using molecular oxygen as oxidant
However, many of these methods have drawbacks, such
as harsh acidic conditions, long reaction times, need for
expensive catalyst, unsatisfactory yields, over oxidation to
sulfones and use of not reusable catalysts. Therefore, to
improve the mentioned limitations, we decided to design a
new system for oxidation of sulfides into sulfoxides as well
as for oxidative coupling of thiols into their corresponding
disulfides using H O at room temperature in the presence
[
11]. Also oxo-vanadium Schiff base complex supported
2
2
on mesoporous silica was found to be highly active and
selective for the oxidation of a variety of alcohols [12].The
Cu(II) and Ni(II) grafted mesoporous materials have been
reported as catalysts for the oxidation of olefins in the
presence of tert-butyl hydroperoxide [13]. Co(II) and
Mn(II)-amine-functionalized mesoporous SBA-15 was
produced and tested for hydroxylation of benzene using
H O as oxidant in the presence of O atmosphere [14]. In
of Zr complex functionalized MCM-48 as mesoporous
catalyst.
2 Experimental
2.1 Materials and Instrumentation
2
2
2
another report Ti-containing MCM-41 showed the highest
catalytic activity for the asymmetric oxidation of sulfide
with hydrogen peroxide in the presence of optically active
tartaric acid [15]. Also Mn(II) complex immobilized SBA-
All reagents and used materials were buying from Sigma-
Aldrich and Merck, and were used any additional purifi-
cation. All the melting points were recorded by open
capillary method and are uncorrected. IR spectra were
recorded in KBr on a BOMEN Infra-Red Spectropho-
tometer FT-IR. Thermogravimetric analysis (TGA) was
used with a heating rate of 10 °C/min in air and the sam-
ples were heated from room temperature to 850 °C. X-ray
diffraction (XRD) patterns were recorded using a Cu Ka
1
5 was used for the asymmetric oxidation of thioanisole
16].
Sulfoxides are important intermediates in the synthesis
[
of many natural and pharmalogical products [17, 18], such
as antibacterial, antifungal, anti-atherosclerotic, antihyper-
tensive [19], cardiotonic agents, psychotonics and
vasodilators [20]. Many reagents are used for the oxidation
˚
radiation source with wave length 1.54 A. N adsorption/
2
desorption measurements analysis (BET) were recorded by
BEL sorp-mini II (volumetric adsorption analyzer, Japan).
All of the samples were degassed at 100 °C for 5 h before
of sulfides such as nitric acid, KMnO , m-chloroperbenzoic
4
acid, sodium metaperiodate, hydrogen peroxide, sulfinyl
peroxy compounds, 4-methylmorpholine n-oxide/osmium
analysis and range of relative pressures (p/p ) was
o
tetroxide, peroxotungstate complexes [21], MeNO solu-
2
0.028–0.989. The specific surface area of the synthesized
materials was evaluated using the BET method, and the
pore size distribution was calculated by the BJH method.
tion in dilute HNO /H SO [22] and etc. In recent years, a
3
2
4
considerable number of methods have been reported for the
oxidation of sulfides to sulfoxides using H O in combi-
2
2
nation with, V/TiO2 [23], Cerium(IV) triflate [24],
molybdate-based catalyst [25], metalloporphyrins immo-
bilized into montmorillonite [26], Zn complex [27], Cu
Salen-Fe O [28], Copper(II) Schiff base complex [29],
2.2 Synthesis of MCM-48 at Room Temperature
(Sol Gel)
2.6 g of CTAB was added to 120 mL of deionized H O
2
3
4
pre-formed manganese complex [30], tantalum(V) chloride
and 50 mL of ethanol under stirring. When CTAB was
[
31] and zirconium tetrachloride [32].
completely dissolved, 12 mL of NH OH were added. After
4
The conversion of thiols to disulfides very usage both in
that, 3.6 mL of TEOS was poured into the solution
immediately under vigorous stirring during 16 h at room
temperature. The solid product was recovered by filtration
and dried at room temperature overnight. The CTAB was
removed from the in organic material by calcination of the
sample at 540 °C for 9 h [45].
biological and chemical processes. For example in chem-
ical processes disulfides can be used to prepare sulfinyl and
sulfenyl compounds and in biological systems they control
the cellular redox potential and prevent oxidative damage
and formation disulfide bond is important in peptides [33–
3
6]. Furthermore, disulfides are relatively more stable to
organic reactions such as oxidation, alkylation and acyla-
tion compared to the corresponding free thiols, also the
thiol group can conveniently be protected as a disulfide
2.3 Synthesis of Zirconium(IV) Modified-MCM-48
For grafting of MCM-48 by 3-aminopropyltriethoxysilane,
first 4.8 g 3-aminopropyltriethoxysilane added to 4.8 g of
MCM-48 in 96 mL n-hexane and the reaction mixture was
stirred under reflux condition for 24 h at nitrogen
[
37]. For the conversion of thiols to disulfides many
methods have been developed over the years to find effi-
cient these organic transformations [38–44].
1
23

Preparation and Characterization of Organically Modified MCM-48 as Heterogonous Catalyst…
atmosphere. Then the resulting white solid was filtered,
3 Result and Discussion
washed with n-hexane and dried at room temperature for
2
4 h to obtain aminofunctionalized MCM-48 (MCM-48-
3.1 Catalyst Synthesis and Characterization
nPr-NH ) [46]. Then isatin and MCM-48-nPr-NH with
2
2
ratio 2:1 were added to absolute ethanol and 0.5 mL of
acetic acide and refluxed for 3 h. Then the mixture was
filtered washed with ethanol and dried at room temperature,
resulting is a yellow sediment. Finally ZrOCl Á8H O was
In this work the Zirconium(IV)-modified-MCM-48 were
synthesis using Zirconium(IV) oxide chloride octahydrate
and the Schiff base supported MCM-48 in CH CN. The
3
2
2
Zirconium(IV)-modified-MCM-48 were then characterized
using a variety of different techniques. The TGA curves of
added to this sediment (with ratio 2:1) in acetonytrile and
mixture was stirred for 6 h at room temperature. Then the
resulting product was washed with water to remove excess
of Zirconium(IV) (Sheme 1).
the MCM-48, MCM-48- nPr-NH , MCM-48-nPr-N-isa-
2
tin(Schiff base) and MCM-48-nPr-N-isatin- Zirconium
(Schiff base Complex) show the mass loss of organic
material as they decompose upon heating (Fig. 1). The
initial weight loss from the MCM-48 (\100 °C) is due to
the removal of physically adsorbed water and surface
hydroxyl group. The weight loss of about 2.55 %. The
weight loss (4.75 %) between 100 and 840 °C is attributed
mainly to the condensation of silanol groups and removal
2
.4 General Procedure for the Oxidation of sulfides
to sulfoxide
A mixture of sulfide (1 mmol), H O (0.4 mL) and catalyst
2
2
(
0.01 g) at room temperature, was stirred under solvent-
free condition for the certain period of time. The reaction
was monitored by thin-layer chromatography (TLC). Upon
completion, the reaction mixture was decanted and
extracted with dicoloromethan. The organic layer was dried
over anhydrous Na SO (1.5 g). Finally, the organic sol-
of chemically adsorbed water. MCM-48-nPr-NH shows
2
four-step weight loss behavior. Weight loss of MCM-48-
nPr-NH appears about 10.32 % at 107–572 °C, which is
2
contributed to the thermal decomposition of the 3-amino-
propyl trietoxysilan groups (equivalent 0.466 mmol). For
MCM-48- nPr-N-isatin(Schiff base), the weight loss of
19.05 % between 115 and 684 °C that related to the
breakdown of the isatin (equivalent 0.59 mmol). For
MCM-48-nPr-N-isatin-Zirconium (Schiff base Complex)
the weight loss about 16.03 % between 102 and 657 °C
that is due to decompose of ZrOCl2 (equivalent
0.17 mmol). On the basis of these results, the well grafting
of aminopropyltriethoxysilane, isatin and Zirconium (IV)
groups on the MCM-48 is verified.
2
4
vents were evaporated, and products were obtained in
8
2–99 % yield.
2
.5 General Procedure for the Oxidation of Thiols
to Disulfides
A mixture of thiol (1 mmol), H O (0.4 mL) and catalyst
2
2
(
0.01 g) was stirred in ethanol at room temperature for the
certain period of time. The reaction was monitored by
TLC. After completion of the reaction, the catalyst was
separated by simple filtration and the mixture was washed
with CH Cl (5 9 2 mL), and next, the product was
Figure 2 shows XRD patterns for MCM-48, MCM-48-
nPr-NH , MCM-48-nPr-N-isatin, MCM-48-nPr-N-isatin-
2
2
2
Zirconium and used Catalyst. XRD patterns illustrate typ-
ical peaks corresponding to diffraction at (211), (220), (420)
and (332) that can be indexed to Ia3d cubic structure. Some
loss in the intensities of the peaks was observed upon
modification with 3-aminopropyltriethoxysilane, isatin and
Zirconium which shows that though there is some reduction
in the crystallinity of Si-MCM-48. Also the mesoporosity of
Si-MCM-48 is retained for the catalyst after one used.
These results provide further evidence that functionaliza-
tion occurring mainly inside the mesopore channels.
extracted with CH Cl (5 mL 9 4). The organic layer was
2
2
dried over anhydrous Na SO (1.5 g). After the evapora-
2
4
tion of dicoloromethan gave the pure products in 85–98 %
yields.
OEt
Si
O
8
(
EtO) Si(CH ) NH
4
NH2 Isatin, EtOH, AcOH
NH2
Reflux, N , 3h
3
2
3
2
O
MCM-48
M
n-Hexane, reflux
4 h, N
C
O
M
Si
OEt
2
2
2
O
Additionally, the unit parameter cell (the center-to-cen-
˚
ter pore distance) (a ) of 82.1 A was calculated by using
NH
OEt
Si
NH
O
O
Et
Si
0
8
O
8
4
N
N
-
ZrOCl .8H2O
4
N
N
O
O
2
-
d211-spacing values according with the following Eq. [25].
O
O
O
M
O
M
O
Zr
C
O
O
C
CH CN, rt
M
Si
OEt
3
M
Si
1=2
O
6h
O
a0 ¼ d211ð6Þ
Figure 3 shows the FT-IR spectra of the MCM-48,
ð1Þ
O
Et
NH
NH
MCM-48-nPr-NH , MCM-48-nPr-N-isatin, MCM-48-nPr-
2
Scheme 1 Zirconium(IV)-modified-MCM-48 synthesis
N-isatin-Zirconium that exhibited a significant difference
123

M. Hajjami, Z. Yousofvand
Fig. 1 TGA curve of the
MCM-48, MCM-48-nPr-NH
MCM-48-nPr-N-isatin and
MCM-48-nPr-N-isatin-
Zirconium (Catalyst)
2
,
2
2
1
1
500
000
500
000
2
11
MCM-48
Amine-MCM-48
Imine-MCM-48
Zr-Imine-MCM-48
Zr-Imine-MCM-48- After use
220
5
00
0
420
3
32
2
4
6
8
Diffraction angle, 2 theta
Fig. 2 XRD patterns of the MCM-48, Amine-MCM-48, Imine-
MCM-48, Zr-imine-MCM-48 and used Catalyst
as a compared. In the all spectra, the peaks at 3445 and
-
1
7
56 cm , which are assigned to O–H and Si–O stretching
of the surface silanols of the MCM-48, the sharp peak at
-
1
080 cm can be attributed to the asymmetric stretching
1
-
1
vibration of the Si–O–Si band and 800 cm
can be
assigned to the symmetric stretching vibration of Si–O. In
the spectra of MCM-48-nPr-NH the peak at 1524 and
2
,
Fig. 3 FT-IR spectra of the a MCM-48, b MCM-48-nPr-NH2,
c MCM-48-nPr-N-isatin, d MCM-48-nPr-N-isatin-Zirconium
-
638 cm can be assigned to the amine functionality (N–
1
1
H bending vibration). In the spectra of MCM-48-nPr-N-
-
1
isatin (Schiff base), a certain band in the 1624 cm that
due to C=N in the Schiff base. Also in the spectra of cat-
IV according to IUPAC nomenclature with H1 hysteresis
loops which are representing mesoporous for the expanded
materials. Also, steep condensation step in the relative
pressure range of 0.1–0.3 observed which is associated
with capillary condensation in the channels of MCM-48
and indicating a good structural order. These results are in
agreement with those previously reported in the literature
[47, 48].
-
1
alyst observed this same peak shift to 1634.29 cm
.
Based on inductively coupled plasma atomic emission
spectroscopy (ICP-AES), the amounts of Zr(IV) in the
complex Schiff-base-MCM-48 was 0.695 mmol/g.
The nitrogen adsorption–desorption isotherms of MCM-
4
8 silicas is presented in Fig. 4. The obtained results
showed nitrogen adsorption–desorption isotherms of type
1
23

Preparation and Characterization of Organically Modified MCM-48 as Heterogonous Catalyst…
Physicochemical and structural parameters of the
samples obtained by N2 isotherms are summarized in
Table 1. MCM-48 showed high surface areas and pore
volume, which is typical of mesoporous materials; nev-
ertheless, decrease in the SBET and DBJH average pore
diameter was observed with functionalization in Amine-
MCM-48, Schiff base-MCM-48 and Zr-Schiff base-
MCM-48. This result can be easily interpreted due to the
fact that the presence of pendant group on the surface that
partially blocks the adsorption of nitrogen molecules.
Such significant decrease of textural properties of porous
materials on grafting was already reported [49].The wall
3.2 Catalytic Studies
The prepared catalyst was assessed for its activity in the
oxidation of sulfides and thiols (both aliphatic and aro-
matic) to form sulfoxides and disulfides as the principal
products (Scheme 2). Thus, after screening different
amounts of the synthesized catalyst (Table 2), the results
1
.2
1.0
.8
0.6
˚
thickness of MCM-48 was 14.3 A where calculated by the
0
following equation [50]:
MCM-48
ꢀ
ꢁ
À
Á
Amine-MCM-48
Imine-MCM-48
Zr-Imine-MCM-48
a0
DBJH
Wall thickness ¼
=
À
=
2
ð2Þ
3
:092
0
0
.4
.2
Pore size distribution (ADS) of MCM-48 according to
BJH methods is presented in Fig. 5. After functionaliza-
tion, a decrease in the DBJH and SBET average pore
diameter was observed that can be easily interpreted due
to the fact that the presence of pendant group on the
surface that partially blocks the adsorption of nitrogen
molecules.
0.0
2
4
6
8
10
12
14
rp, nm
Fig. 5 Pore size distribution (ADS) of synthesized materials
6
4
2
00
00
00
adsorption (MCM-48)
desorption
adsorption (Amine-MCM-48)
desorption
adsorption (Imine-MCM-48)
desorption
adsorption (Zr-Imine-MCM-48)
desorption
Scheme 2 Oxidation of sulfides to sulfoxide and coupling of thiols to
disulfides
0
.0
0.2
0.4
0.6
0.8
1.0
Relative pressure, (p/p0)
Table 2 Results of varying the amount of catalyst
a
Entry
Amount of catalyst (g)
Time (min)
Yield (%)
Fig. 4 Nitrogen adsorption–desorption isotherms of synthesized
materials
b
1
2
3
4
5
6
7
0.005
0.008
0.01
40
40
40
40
180
10
25
25
b
76
99
94
0.025
0.006
0.01
Table 1 Textural properties of synthesized materials obtained by
nitrogen adsorption–desorption analysis
b
55
96
90
2
-1
)
3
-1
)
Sample name
S
BET (m g
D
BJH (nm)
V
Total (cm g
0.02
MCM-48
1326
554
2.44
1.70
1.67
1.21
1.12
0.40
0.41
0.21
Reaction conditions dibenzilsulfide (1 mmol), H
vent-free and room temperature (Entry 1–4), 2-naphthalene thiol
2
O
2
(0.4 mL), sol-
Amine-MCM-48
Imine- MCM-48
(
2 2
1 mmol), H O (0.4 mL), ethanol and room temperature
369
a
Isolated yields
Zr-Imine- MCM-48 248
b
Purification by preparative TLC
123

M. Hajjami, Z. Yousofvand
2 2
Table 3 Oxidation of sulfides and coupling of thiols with H O in the presence of Zr(IV)/isatin-MCM-48 at room temperature
a
4
TON (910 )
Entry
1
Substrate
Product
Time (min)
30
Yield (%)
99
M.p (°C)
15.66
123–125/128–132 [28]
2
3
30
35
82
97
12.97
15.35
Oil/oil [28]
118–122/116–121 [28]
4
10
90
14.24
Oil/oil [28]
5
6
15
90
96
82
15.19
12.97
84–86/87–89 [28]
Oil/oil [28]
7
65
90
14.24
111–113/110–114 [51]
8
9
5
82
96
12.97
15.19
Oil/oil [28]
10
129–133/134–136 [28]
1
1
0
1
2
10
60
5
98
98
85
15.51
15.51
13.45
169–173/174–175 [28]
87–91/94–96 [28]
36–38/38–40 [28]
1
1
3
4
10
15
90
98
14.24
15.51
Oil/oil [28]
1
a
101–104/103 [28]
Reaction conditions: sulfide or thiol (1 mmol), catalyst (0.01 g) and H
Isolated yields
2 2
O (0.4 mL)
b
show that 0.01 g of catalyst was the best amounts for both
oxidation reaction.
and results are summarized in Table 3. It should be noted,
in this optimized reaction conditions, selective oxidation of
sulfides to sulfoxides have been observed without any over-
oxidation to sulfones. Interestingly, either upon the use of
an excess amount of H O or much more time stirring the
According to the obtained results, the optimized reaction
conditions were selected to determine the scope of func-
tionalized MCM-48 catalyzed the oxidation reactions. In
this light a wide range of aliphatic and aromatic sulfides
and thiols were subjected to oxidize to corresponding sul-
foxides and disulfides in the presence of 0.01 g modified
MCM-48 catalyst. The turn over numbers were calculated
2
2
reaction mixture, the formation of sulfone as by product
did not observed.
A plausible oxidation reaction mechanism is shown in
Scheme 3 [52]. The role of Zr (IV) in modified MCM-48 as
1
23

Preparation and Characterization of Organically Modified MCM-48 as Heterogonous Catalyst…
Scheme 3 Proposed
mechanism for the oxidation of
sulfides
recovered catalyst after one used was investigated by XRD
at Fig. 2.
4
Conclusion
In summary, zirconium-modified-MCM-48 was prepared
and found as an efficient heterogeneous catalyst for oxi-
dation of sulfides to sulfoxides and thiols to disulfides. This
method has many privilege including, high product in
present of small amount of catalyst, excellent isolated and
simple work-up, stand heterogeneous, short reaction time
and accordance with green chemistry principal.
Fig. 6 The recycling experiment of catalyst for oxidation of dibenzyl
sulfide to dibenzyl sulfoxide at room temperature under solvent-free
condition after 30 min
Acknowledgments The authors acknowledge Ilam University,
Ilam, Iran for financial support of this work.
catalyst is to form the active oxidant–sulfide complex.
Finally, transfer of oxygen to sulfur leads to the formation
of sulfoxide.
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