Immobilized Sandwich-Type Polyoxometalates [Mn4(XW9O34)2]n? on Tb-Doped TiO2 Nanoparticles as Efficient and Selective Catalysts in the Oxidation of Sulfides and Alcohols

Article abstract of DOI:10.1007/s10562-016-1896-1

Abstract: Sandwich-type polyoxometalates of [M₄(XW₉O₃₄)₂]^n?(M = Mn²⁺, Cu²⁺, Zn²⁺, X = Ge⁴⁺, P⁵⁺, As⁵⁺) supported on Ln-doped TiO₂(Ln = Nd³⁺, Sm³⁺, Tb³⁺and Dy³⁺) nanoparticles were prepared and their catalytic activities examined for oxidation of sulfides and alcohols. Results show that the catalytic activities of new nanocomposites were significantly higher than homogeneous conditions with corresponding sandwich-type polyoxometalates. The best results obtained by [Mn₄(XW₉O₃₄)₂]^10?(X = P⁵⁺, As⁵⁺) supported on Tb/TiO₂which were characterized by various techniques. The catalysts can be easily recovered and reused for at least five times without obvious decrease of catalytic activities and the structures of the catalysts remain unchanged after recycling and the yields for catalyst recovery are all above 95%. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1896-1

Catal Lett
DOI 10.1007/s10562-016-1896-1
Immobilized Sandwich-Type Polyoxometalates [Mn4(XW9O34)2]n−
on Tb-Doped TiO2 Nanoparticles as Eicient and Selective
Catalysts in the Oxidation of Sulides and Alcohols
Sahar Gholamyan1 · Roushan Khoshnavazi1 · Amin Rostami1 · Leila Bahrami1
Received: 11 June 2016 / Accepted: 25 October 2016
© Springer Science+Business Media New York 2016
Abstract Sandwich-type
polyoxometalates
of
The best results obtained by [Mn4(XW9O34)2]10− (X=P5+,
As5+) supported on Tb/TiO2 which were characterized by
various techniques. The catalysts can be easily recovered
and reused for at least ive times without obvious decrease
of catalytic activities and the structures of the catalysts
remain unchanged after recycling and the yields for catalyst
recovery are all above 95%.
[M4(XW9O34)2]n− (M=Mn2+, Cu2+, Zn2+, X=Ge4+, P5+,
As5+) supported on Ln-doped TiO2 (Ln=Nd3+, Sm3+, Tb3+
and Dy3+) nanoparticles were prepared and their catalytic
activities examined for oxidation of sulides and alcohols.
Results show that the catalytic activities of new nanocom-
posites were signiicantly higher than homogeneous condi-
tions with corresponding sandwich-type polyoxometalates.
Graphical Abstract
Keywords Sandwich-type polyoxometalates · Support ·
Ln-doped TiO2 · Nanocomposite · Homogeneous catalyst ·
Hydrogen peroxide · Oxidation · Sulides · Alcohols
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1896-1) contains supplementary
material, which is available to authorized users.
* Roushan Khoshnavazi
r.khoshnavazi@uok.ac.ir; khoshnavazi@yahoo.com
1 Introduction
Sahar Gholamyan
gholamyansahar@yahoo.com
Polyoxometalates (POMs) are a class of nano-sized inor-
ganic clusters presenting abundant potential in the ields
of chemistry [1]. Over the last decades catalytic properties
of Keggin and Dawson-type POMs were one of the most
important researches topics [2, 3]. Sandwich-type POMs
Amin Rostami
A.rostami@uok.ac.ir
1
Department of Chemistry, University of Kurdistan, P.O.
Box 66135-416, Sanandaj, Iran
1 3

S. Gholamyan et al.
due to the presence of multi metal ions in their structures
are of most interest. Recently, catalytic application stud-
ies of sandwich-type POMs in which three or four metal
ions located between two Keggin or Dawson moieties have
been more attractive [4–7]. Although, these eforts have led
to signiicant progress in the ield of catalyst, the reported
based POMs catalytic systems show the following disad-
vantages: (1) the recovery and reuse of the used catalysts
are diicult due to their homogeneous nature; (2) the cata-
lytic eiciency and the yields are not satisfactory in most
cases for the heterogeneous catalytic systems [8]; (3) the
bulk of POMs display no characteristic porosity and have
low surface area. One of the most promising solutions to
these problems seems to be immobilization of POMs onto
solid support such as silica [9], titanium dioxide [10], zeo-
lite [11], alumina [12]. Nanometal oxides have drawn the
researcher’s attention due to their unusual physical and
chemical catalytic properties [13–15] and are proved to
be promising catalysts because of their high activity, non-
toxicity, ease of availability, reusability, strong oxidizing
power, and long-term stability [16–19].
In this work, new nanocomposites consist of sandwich-
type polyoxometalates [M4(XW9O34)2]n− (M=Mn2+, Cu2+,
Zn2+, X=Ge4+, P5+, As5+) on Ln-doped TiO2 (Ln=Nd3+,
Sm3+, Tb3+ and Dy3+) nanoparticles (POMs/Ln/TiO2) were
prepared and their catalytic behaviors were examined in
oxidation of sulide and alcohols using H2O2 as oxidant.
The [M4(XW9O34)2]n− POMs are belonged to the B-type
sandwich polyoxometalates which their structure consist of
two laconary B–α-[(XW9O34)2]n− Keggin moieties linked
via a rhomblike M4Ol6 group leading to a C2h symmetry
sandwich-type complex (Fig. S1) [4, 5].
2 Experimental
2.1 General
All reagents were commercially obtained and used as
received. Titanium(IV) isopropoxide (TTIP) and absolute
ethanol were obtained from Acrōs Organics. The lantha-
nides salts of LnCl3.6H2O were obtained from Alfa Aesar.
Potassium salts of [M4(XW9O34)2]n− (M=Mn2+, Cu2+,
Zn2+, X=Ge4+, P5+, As5+) were prepared according to
the previously reported method [56, 57]. X-ray difraction
(XRD) patterns were recorded with an X’PertPro Analyti-
cal, Holland difractometer (Model PW1840) in 40 kV and
30 mA with a CuKα radiation (λ=1.5418 A). FT-IR spec-
tra were obtained using a Bruker Vector 22 FT-IR spec-
trometer using KBr pellet. Scanning electron micrographs
(SEM) and energy-dispersive X-ray (EDX) analysis of the
samples were taken with FESEM-TESCAN MIRA3. The
X-ray photoelectron (XPS) spectra of the supported cata-
lyst was recorded on a VG Microtech Twin anode XR3E2
X-ray source and aconcentric hemispherical analyzer
operated at a base pressure of 5×10−10 mbar using AlKa
(hm=1486.6 eV). Peak itting of all spectra were per-
formed using the Shirley background correction and Gauss-
ian–Lorentzian peak shapes. Binding energies (BEs) were
referenced to C 1 s peak at 284.5 eV. Low-resolution sur-
vey spectra, as well as higher-resolution spectra for W, Mn,
O, As and Ti were collected.
Oxidation catalysis by POMs has received much atten-
tion because their chemical properties can inely be tuned
by choosing constituent elements and counter cations, and
are thermally and oxidatively stable in comparison with
organometallic complexes, organocatalysts, and enzymes
[20–24]. Oxidation of sulides to the corresponding sul-
foxides or sulfones is an important type of organic reaction
since it is closely related to the petroleum industry, which is
known as oxidative desulfurization [25–31]. Sulfones and
sulfoxides are important intermediates in the synthesis of
many interesting biologically active compounds [32–36].
Therefore the oxidation of sulides to sulfoxides or sulfones
has been the subject of many studies and a large number
of synthetic procedures are now available [16, 37–45]. The
selective oxidation of primary and secondary alcohols to
the corresponding carbonyl compounds is a fundamental
preparative reaction, which has been widely investigated
[46, 47]. A variety of diferent catalytic systems for hydro-
gen peroxide promoted oxidation of alcohols have been
developed e.g. tungsten based polyoxometalate catalysts
[48–53].
The use of ‘green oxidants’ such as hydrogen perox-
ide is attractive, since it is readily available, low cost,
and eco-friendly, with formation of water as the only
by-product [54, 55]. However, its poor oxidizing power
is a main drawback, which needs catalyst for the activa-
tion of H2O2. To date, various kinds of POMs have been
developed for the H2O2 and O2 based green oxidations
[20–24].
2.2 Methods
2.2.1 Preparation of Ln/TiO2 Nano-Sized
The support was synthesized using reported procedure
[58]. 10 mL of Ti{OCH(CH3)2}4 is added at room tem-
perature to 30 mL of EtOH. The pH of mixture is adjusted
to 1.5 by concentrated hydrochloric acid (solution 1).
LnCl3.6H2O (0.2 mmol), the rare earth precursor, was
dissolved into the solution containing H2O (2 mL) and
EtOH (4 mL) (solution 2). Then, solution 2 was added
In comparison with the Keggin and Dawson-type POMs,
the catalytic properties of sandwich-type POMs as hetero-
geneous catalyst have not been extensively studied [4–7].
1 3

Immobilized Sandwich-Type Polyoxometalates [Mn4(XW9O34)2]n− on Tb-Doped TiO2…
dropwise into solution 1 during 10 min approximately.
The resulting acidic mixture was stirred constantly for
about 3 h until sol was obtained. The sol was maintained
for 24 h till gel formation. The gel was dried at 373 K
for 4 h and then calcined at 823 K for 3 h. The rare earth
doping in the support is 1.0%. Pure TiO2 powder was also
prepared by the same process without doping [58].
2.2.5 General Procedure for the Oxidation of Benzylic
Alcohols
A mixture of [Mn4(PW9O34)2]10−/Tb/TiO2 (40 mg), alco-
hols (1 mmol), 30% H2O2 aqueous solution (6 mmol) and
H2O (1 mL) was stirred at 80°C for the time speciied
(Table 4). After the completion of the reaction, which was
monitored by TLC (EtOAc/n-Hexane, 4/10), the catalyst
was centrifuged. The product was extracted with CH2Cl2
(3×5 mL) and the combined organic extractions washed
with brine (10 mL) and dried over anhydrous Na2SO4. The
solvent was removed under reduced pressure to give the
corresponding pure carbonyl compound in most cases.
2.2.2 Preparations of Catalyst Nanocomposites
The impregnation process is as following: 1.00 g of
Ln/TiO2 support was dispersed in water solution con-
taining a given amount of (10, 20 and 30% w/w) of
[M4(XW9O34)2]n− (M = Mn2+, Cu2+, Zn2+, X = Ge4+,
P5+, As5+). The suspension was stirred continuously
under room temperature. After 24 h, water was removed
by evaporation at 373 K. The inal product was obtained
by drying at 373 K for 2 h. The loading levels of
[M4(XW9O34)2]n− in prepared composites by mass are 10,
20 and 30%.
3 Results and Discussion
3.1 Preparation of POMs/Ln/TiO2 Nanocomposites
TiO2 and Ln/TiO2 support, was synthesized via sol–gel
technique and nanocomposites were prepared by impreg-
nation method [58]. In next step, [M4(XW9O34)2]n−
(M=Mn2+, Cu2+, Zn2+, X=Ge4+, P5+, As5+) were sup-
ported on Ln/TiO2 (Ln=Nd3+, Sm3+, Tb3+ and Dy3+)
nanoparticles and their catalytic behaviors were evaluated.
The surface of Ln/TiO2 particles is positively charged in
acidic media, whereas the POMs are negatively charged, thus
facilitating the adsorption of the POMs species on the surface
of TiO2 particles by simple coulombic interactions. Yoon and
co-workers have demonstrated that there are three diferent
hydrogen bonds between POMs and TiO2 colloidal particles
in aqueous solution, i.e., =Ti–OH...Ot=W, =Ti–OH...Ob=W,
and =Ti–OH...Oc=W (Ot terminal oxygen, Ob,c bridge oxy-
gen), on the basis of infrared spectral analysis. Almost all the
added POMs were completely adsorbed on TiO2 [59].
2.2.3 General Procedure for the Oxidation of Sulides
to Sulfoxides
A mixture of [Mn4(AsW9O34)2]10−/Tb/TiO2 (50 mg),
sulide (1 mmol), 30% H2O2 aqueous solution (6 mmol),
acetonitrile (3 mL) was stirred at room temperature for
the time speciied (Table 2). After the completion of
the reaction, which was monitored by TLC (EtOAc/n-
Hexane, 4/10), the water (5 mL) was added and catalyst
was centrifuged. The product was extracted with CH2Cl2
(3 × 5 mL) and the combined organic extractions washed
with brine (10 mL) and dried over anhydrous Na2SO4.
The solvent was evaporated under reduced pressure to
give the corresponding pure sulfoxide in most cases.
3.2 Characterization of POMs/Tb/TiO2
Nanocomposites
2.2.4 General Procedure for the Oxidation of Sulides
3.2.1 FT-IR Analysis
to Sulfones
Figure
1
shows the FT-IR spectra of Tb/TiO2,
A mixture of [Mn4(PW9O34)2]10−/Tb/TiO2 (50 mg),
sulide (1 mmol), 30% H2O2 aqueous solution (6 mmol)
and acetonitrile (3 mL) was stirred at room temperature
for the time speciied (Table 2). After the completion of
the reaction, which was monitored by TLC (EtOAc/n-
Hexane, 4/10), the water (5 mL) was added and catalyst
was centrifuged. The product was extracted with CH2Cl2
(3 × 5 mL) and the combined organic extractions washed
with brine (10 mL) and dried over anhydrous Na2SO4.
Evaporation of the solvent under reduced pressure gives
the pure corresponding sulfone in most cases.
[Mn4(XW9O34)2]10− (X=P5+ and As5+) and their nano-
composites with diferent loadings of [Mn4(PW9O34)2]10−
on Tb/TiO2. The characteristic peaks of at 1030, 937, 880,
742 cm−1 in [Mn4(PW9O34)2]10− spectrum are attributed
to νas(P–Oa), νas(W=Od), νas(W–Ob–W), νas(W–Oc–W),
respectively (Fig. 1A), and at 951, 868, 838, 730 cm−1
in that of [Mn4(AsW9O34)2]10− are attributed to νas(As–
Oa), νas(W=Od), νas(W–Ob–W), νas(W–Oc–W), respec-
tively (Fig. 1B) [60]. From Fig. 1, it can be seen that the
IR spectrum of Tb/TiO2 illustrates an intense, broad,
indistinct region between 1100 and 400 cm−1. The FT-IR
1 3

S. Gholamyan et al.
spectra of POMs/Tb/TiO2 show characteristic peaks of
Tb/TiO2 and POMs which intensities of the later increase
by increasing the amounts of loading polyoxometalate
on the Tb/TiO2. As a result, some characteristic peaks of
[Mn4(XW9O34)2]10− (X=P5+, As5+) with that of Tb/TiO2
in nanocomposites are overlapped in this area (Fig. 1).
However, some vibration peaks of [Mn4(XW9O34)2]10−
(X=P5+, As5+) unit still can be seen clearly, which indi-
cates that the [Mn4(XW9O34)2]10− (X=P5+, As5+) structure
has not been destroyed upon immobilization on Tb/TiO2
nano particle [58].
Fig. 3. It can be observed that the XRD patterns of Tb/TiO2
and [Mn4(PW9O34)2]10−/Tb/TiO2 and [Mn4(AsW9O34)2]10−
are similar, in which all of them crystallized in the anatase
structure. Moreover, there are no characteristic peaks of
POMs and Tb-TiO2 in the XRD spectrum of nanocompos-
ites. This indicates that Tb3+ and POM species are well-
dispersed in the surface of the TiO2 [61, 62].
3.2.3 BET Analysis
The surface area of Tb/TiO2, K10[Mn4(AsW9O34)2]·xH2O
and [Mn4(AsW9O34)2]10−/Tb/TiO2 with 20% loading of
[Mn4(AsW9O34)2]10− was analyzed by BET. The BET analy-
sis showed that Tb/TiO2 has a 74.91 m2/g surface area. This
surface area is larger than that of naked TiO2 [58]. The high
surface area can be explained by small crystallite size and
high dispersion of terbium ion in the Tb/TiO2. As expected,
surface area of the solid K10[Mn4(PW9O34)2]·xH2O is very
small (7.55 m2/g). The BET analysis illustrates that surface
area of the [Mn4(PW9O34)2]10−/Tb/TiO2 with 20% loading of
[Mn4(AsW9O34)2]10− is 68.63 m2/g. The results conirm well
that the efective surface area of K10[Mn4(PW9O34)2]·xH2O
markedly increased by immobilization on Tb/TiO2 surface.
For
surface
analysis
of
nanocomposite
of
[Mn4(PW9O34)2]10−/Tb/TiO2 with 10 and 20% loading of
[Mn4(PW9O34)2]10−, Attenuated Total Relectance Fourier
Transform IR spectroscopy (ATR FT-IR) was used. The
ATR FT-IR spectra of [Mn4(PW9O34)2]10−/Tb/TiO2 with
10 and 20% loading of [Mn4(PW9O34)2]10− and Tb/TiO2
is given in Fig. 2. As expected, the appearance of POMs
component on the surface of the prepared nanocomposites
must be greater than that expected from the mass ratio. The
ATR FT-IR spectrum of POMs/Tb/TiO2 show characteris-
tic peaks of Tb/TiO2 and POMs which it also is an evidence
for loading desired POM.
3.2.2 XRD Analysis
3.2.4 XPS Analysis
The
X-ray
difraction
patterns
of
Tb/TiO2,
X-ray Photoelectron Spectroscopy (XPS) is widely used
to investigate the chemical composition of surfaces. XPS
survey spectra of [Mn4(AsW9O34)2]10−/Tb/TiO2 with 20%
[Mn4(PW9O34)2]10−/Tb/TiO2 and [Mn4(AsW9O34)2]10−
/
Tb/TiO2 with 20% loading levels of POMs are shown in
Fig. 1 The FT-IR spectra of the prepared nanocopmposites A a Tb/
TiO2, b–d [Mn4(PW9O34)2]10−/Tb/TiO2 with 10–30% loading of
[Mn4(PW9O34)2]10− respectively and e [Mn4(PW9O34)2]10−. B a Tb/
TiO2, b–d [Mn4(AsW9O34)2]10−/Tb/TiO2 with 10–30% loading of
[Mn4(AsW9O34)2]10− respectively and e [Mn4(AsW9O34)2]10−
1 3

Immobilized Sandwich-Type Polyoxometalates [Mn4(XW9O34)2]n− on Tb-Doped TiO2…
content intensity of the corresponding peaks are low. The
low intensity peaks at 640 and 650 eV may be attributed
to the Mn 2p3/2 and Mn 2p1/2, respectively [64].
3.2.5 SEM Images
The
SEM
images
of
the
nanocomposite
of
[Mn4(AsW9O34)2]10−/Tb/TiO2 with 10% loading level of
POMs (Fig. 5) reveal that nanocomposite remain relatively uni-
form regular spherical with particles diameter less than 20 nm.
3.2.6 EDX Spectra
In order to achieve information about the elemental com-
position and distribution on the surface of the sample of
[Mn4(PW9O34)2]10−/Tb/TiO2 with 10% loading level was
characterized by using energy dispersive X-ray analysis
(EDX). EDX analysis detects the presence of Mn, O, As,
Tb, W elements (Fig. 6). Based on the above results, it is
demonstrated that of [Mn4(AsW9O34)2]10− was immobi-
lized onto the surface of the Tb/TiO2 nanoparticles.
Fig. 2 The ATR FT-IR spectra of the prepared nanocopmpos-
ites a Tb/TiO2, b [Mn4(PW9O34)2]10−/Tb/TiO2 with 10% loading of
[Mn4(PW9O34)2]10− and c [Mn4(PW9O34)2]10−/Tb/TiO2 with 20%
loading of [Mn4(PW9O34)2]10−
loading levels of POMs are shown in Fig. 4. As expected,
Ti 2p, O 1s, C 1s, W 4f and Mn 2p peaks were seen in the
XPS survey spectra. From the XPS spectrum of Ti, peaks
corresponding to Ti-O are observed at 466.8 and 472 eV
can be attributed to the 2p3/2 and 2p1/2 respectively. The
peaks observed at 538 eV can be assigned to the bind-
ing energy of O 1s. The XPS peak for C1s (284.8 eV) is
due to the adventitious hydrocarbon from the XPS instru-
ment itself. The W 4f spectrum can be deconvoluted into
doublets. This doublet consists of W 4f7/2 line at 35.3 eV
and W 4f5/2 line at 37.1 eV, which are assigned to the W
in the W–O bond coniguration and typically observed
for the W6+ [63]. Due to very low Mn and Tb content
in [Mn4(AsW9O34)2]10−/Tb/TiO2 in comparison to Ti
4 Catalytic Application of POMs/Ln/TiO2
4.1 Catalytic Application of POMs/Ln/TiO2
in the Oxidation of Sulides to Sulfoxides
and Sulfones
The catalytic application of the sandwich-type poly-
oxometalates [M4(XW9O34)2]n− (M=Mn2+, Cu2+, Zn2+;
X=Ge4+, P5+, As5+) on Ln/doped TiO2 (Ln=Nd3+, Sm3+,
Tb3+, Dy3+) nanoparticles in the oxidation of sulide
using H2O2 as oxidant was investigated. Initially, in order
to investigation of catalyst efect, the oxidation of methyl
Fig. 3 XRD patterns of
Tb/TiO2 with 20% loading of [Mn4(PW9O34)2]10− and
[Mn4(AsPW9O34)2]10−/Tb/TiO2
with 20% loading of
[Mn4(AsPW9O34)2]10−
a
Tb/TiO2,
b
[Mn4(PW9O34)2]10−
/
c
Fig. 4 XPS survey spectra of [Mn4(AsW9O34)2]10−/Tb/TiO2 with
20% loading of [Mn4(AsW9O34)2]10−
1 3

S. Gholamyan et al.
Fig. 5 SEM images of [Mn4(AsW9O34)2]10−/Tb/TiO2 with 10% loading of [Mn4(AsW9O34)2]10−
Fig. 6 The EDX spectra of
[Mn4(AsW9O34)2]10−/Tb/
TiO2 with 10% loading of
[Mn4(AsW9O34)2]10−
phenyl sulide (as model reaction) was performed using
hydrogen peroxide in the presence of various catalysts in
CH3CN at room temperature (Table 1). The reaction was
incomplete in the absence of any catalyst and in the pres-
ence of TiO2, Tb/TiO2 and [M4(XW9O34)2]10− (X=P5+,
As5+; M=Cu2+, Zn2+) (Table 1, entries 1–4). No product
was obtained by using of homogeneous sandwich-type
polyoxometalate [Mn4(GeW9O34)2]12− (Table 1, entry
5). Using [Mn4(AsW9O34)2]10−/Tb/TiO2 as nanocatalyst
gave the complete conversion of methyl phenyl sulide
to methyl phenyl sulfoxide by surprisingly high chemo
selectivity in very short reaction time in CH3CN at room
temperature (Table 1, entry 6). Under the same reaction
conditions, only sulfone was obtained in the presence of
[Mn4(PW9O34)2]10−/Tb/TiO2 as catalyst (Table 1, entry 7).
Nanocomposites of [M4(PW9O34)2]n−/Tb/TiO2 (M=Cu2+,
Zn2+) and [M4(AsW9O34)2]10−/Tb/TiO2 (M=Cu2+, Zn2+)
give relatively low yields of sulfones and sulfoxides,
respectively (Table 1, entries 8–11). Other nanocompos-
ites such as [Mn4(GeW9O34)2]12−/Tb/TiO2 is not efective
1 3

Immobilized Sandwich-Type Polyoxometalates [Mn4(XW9O34)2]n− on Tb-Doped TiO2…
Table 1 Investigate efect of
catalysts on the oxidation of
methyl phenyl sulide to methyl
Entry Catalyst (50 mg)
Solvent Time (min) Sulfoxide (%)a,b Sulfone (%)a,b
1
2
3
4
Catalyst-free
TiO2
CH3CN 2880
CH3CN 2880
CH3CN 2880
50
25
25
25
–
phenyl sulfoxide or to methyl
phenyl sulfone with H2O2 as
oxidant
25
25
–
Tb/TiO2
[M4(XW9O34)2]10−
M=Mn,Cu, Zn X=P, As
CH3CN
900
5
[M4(XW9O34)2]12−
M=Mn, Cu, Zn X=Ge
CH3CN 2880
–
–
6
[Mn4(AsW9O34)2]10−/Tb/TiO2 (10%) CH3CN
2
2
100
–
–
7
[Mn4(PW9O34)2]10−/Tb/TiO2 (10%)
[Cu4(AsW9O34)2]10−/Tb/TiO2 (10%)
[Zn4(AsW9O34)2]10−/Tb/TiO2 (10%)
[Cu4(PW9O34)2]10−/Tb/TiO2 (10%)
[Zn4(PW9O34)2]10−/Tb/TiO2 (10%)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
100
–
8
120
120
120
120
600
15
70
70
–
9
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
70
70
Trace
50
–
–
[Mn4(GeW9O34)2]10−/Tb/TiO2 (10%) CH3CN
[Mn4(PW9O34)2]10−/TiO2 (10%)
CH3CN
[Mn4(AsW9O34)2]10−/Nd/TiO2 (10%) CH3CN
[Mn4(AsW9O34)2]10−/Sm/TiO2 (10%) CH3CN
20
50
100
100
–
15
15
–
[Mn4(PW9O34)2]10−/Nd/TiO2(10%)
[Mn4(PW9O34)2]10−/Sm/TiO2(10%)
CH3CN
15
100
100
25
25
–
CH3CN
15
–
[Mn4(AsW9O34)2]10−/Dy/TiO2 (10%) CH3CN
120
120
12
25
25
100
–
[Mn4(PW9O34)2]10−/Dy/TiO2 (10%)
[Mn4(AsW9O34)2]10−/Tb/TiO2 (5%)
[Mn4(PW9O34)2]10−/Tb/TiO2 (5%)
CH3CN
CH3CN
CH3CN
12
100
–
[Mn4(AsW9O34)2]10−/Tb/TiO2 (20%) CH3CN
[Mn4(PW9O34)2]10−/Tb/TiO2 (20%)
CH3CN
2
100
–
2
100
aConversions determined by GC
bReaction conditions for the selective oxidation of sulides to sulfoxides/sulfones: methyl phenyl sulide
(1 mmol), 30% H2O2 aqueous solution (6 mmol), acetonitrile (3 mL) at room temperature
Table 2 Selective oxidation
of sulides to sulfones or
sulfoxides with hydrogen
peroxide catalyzed by
[Mn4(PW9O34)2]10−/Tb/TiO2
and [Mn4(AsW9O34)2]10−/Tb/
TiO2 respectively
Entry
Substrate
Sulfoxidea, b
Time (min)
Sulfonea, b
Yield (%)
80
Time (min)
Yield (%)
90
S
1
2
3
50
55
S
2
100
90
2
100
90
S
2
2
S
S
4
5
10
15
80
90
15
25
80
90
OH
OMe
O
6
15
90
25
90
Me
S
O
1
aAll the products are known and were characterized by IR and HNMR and by melting point comparison
with those of authentic samples [65, 66]
bReaction conditions for the selective oxidation of sulides to sulfoxides/sulfones: sulides (1 mmol), 30%
H2O2 aqueous solution (6 mmol), acetonitrile (3 mL) at room temperature
1 3

S. Gholamyan et al.
(Table 1, entry 12). Nanocomposite of [Mn4(PW9O34)2]10−
/
and 6). Whereas employing [Zn4(PW9O34)2]10−/Tb/TiO2
as catalyst the desired product was obtained in less than
70% yield (Table 3, entry 7). Other sandwich-type poly-
oxometalates such as [Mn4(GeW9O34)2]12−/Tb/TiO2 are not
efective and do not show any catalytic activity (Table 3,
entry 8). These results indicate that manganese ion in
[Mn4(XW9O34)2]10− (X=P5+, As5+) with loading on Tb/
TiO2 plays signiicant catalytic role in oxidation of various
substrates with hydrogen peroxide.
TiO2 (without Tb metal) is not able to produce selec-
tive product and give the mixture sulfoxide and sulfone
(Table 1, entry 13). When [Mn4(PW9O34)2]10−/TiO2 was
doped with lanthanide metals like samarium and neo-
dymium, the reactions were performed in the longer times
(Table 1, entries 14–17) and changing the metal to Dyspro-
sium was not efective for the reaction (Table 1, entries 18
and 19).
The efect of diferent loadings of [Mn4(XW9O34)2]n−
(X=P5+, As5+) in oxidation of methyl phenyl sulide
under the same reaction conditions was also investigated.
A 10% loading of [Mn4(XW9O34)2]n− (X=P5+, As5+) need
for the complete conversion of methyl phenyl sulide to
methyl phenyl sulfoxide or sulfone. With 5% loading of
[Mn4(XW9O34)2]n− (X=P5+, As5+), reactions were per-
formed in the longer times (Table 1, entries 20 and 21) and
when 20% loading of [Mn4(XW9O34)2]n− (X=P5+, As5+)
was used no signiicant changes in the catalytic properties
of nanocomposite was observed (Table 1, entries 22 and
23). The results indicate that the best loading level is 10%,
which is also the saturation value of loading level. At load-
ing levels 20% and higher, the excess POMs is easy to drop
from the composite during the reaction [58].
Finally, in order to extend the scope, oxidation of sev-
eral alcohols with hydrogen peroxide were investgated in
the presence of [Mn4(PW9O34)2]10−/Tb/TiO2 as catalyst
in water at 80 °C. The expected products were obtained
with high yields in short times (Table 4). As shown in
Table 4, various types of primary benzylic alcohols,
including those with both electron-withdrawing and
electron-donating groups, were selectively converted
to the corresponding carbonyl compounds in excellent
yields (Table 4, entries 1–3). The primary benzylic alco-
hols with electron-with drawing groups show less reac-
tivity and the oxidation of them require longer reaction
times (Table 4, entries 1–3). Benzhydrol and 1-pheny-
lethanol as model for secondary benzylic alcohols were
converted to the corresponding carbonyl compounds as
well (Table 4, entries 4 and 5). Comparison between our
obtained results with the best of the well-known cata-
lytic systems on the oxidation of methyl phenyl sulphide
and benzyl alcohol with H2O2 as oxidant have been
given in Tables 5 and 6 (details given in the Supporting
Information).
In order to generalize the scope of the reaction, vari-
ous aromatic and aliphatic sulides were subjected to oxi-
dation using hydrogen peroxide in the presence of the
catalytic amount of [Mn4(PW9O34)2]10−/Tb/TiO2 and
[Mn4(AsW9O34)2]10−/Tb/TiO2 in CH3CN at room tem-
perature. We have found that the expected products were
obtained with high yields in short reaction times (Table 2).
Interestingly, oxidation of methyl 2-hydroxyethyl sulide
proceeded chemo selectively without oxidation of the
hydroxyl group (Table 2, entry 4).
4.3 Reactions Mechanism
In the studied the catalytic oxidation system with hydrogen
peroxide as oxidant usually two homolytic and heterolytic
4.2 Catalytic Application of POMs/Tb/TiO2
in Oxidation of Benzylic Alcohols
Table 3 Investigate efect of catalysts on the oxidation of benzyl
alcohol (1 mmol) to benzaldehyde using H2O2 (6 mmol) in H2O
(1 mL) at 80°C
In this work, the sandwich-type polyoxometalates
[M4(XW9O34)2]n− (M=Mn2+, Cu2+, Zn2+, X=Ge4+, P5+,
As5+) on Tb-doped TiO2 nanoparticles catalysts were
used for the oxidation of benzylic alcohols using hydro-
gen peroxide. The oxidation of benzyl alcohol was selected
as the model reaction and the efect of various catalysts
were investigated in water at 80°C (Table 3). The reaction
was failed in the absence of any catalyst and in the pres-
ence of TiO2, Tb/TiO2 and [M4(XW9O34)2]10− (M=Cu2+,
Zn2+, X=P5+, As5+) (Table 3, entries 1–3). No product
was obtained by using of homogeneous sandwich-type
polyoxometalate [Mn4(GeW9O34)2]12− (Table 1, entry 4).
When the same reaction was carried out in the presence
of [Mn4(XW9O34)2]10−/Tb/TiO2 (X=P5+, As5+), benzal-
dehyde was produced with 100% yield (Table 3, entries 5
Entry Catalyst 20% (40 mg)
Time (h) Benza-
ldeyde
(%)a
1
2
3
Catalyst-free
Tb/TiO2
[M4(XW9O34)2]10−
M=Mn, Cu, Zn; X=P, As
28
28
15
40
10
30
4
[M4(XW9O34)2]12−
M=Mn, Cu, Zn; X=Ge
15
–
5
6
7
8
[Mn4(PW9O34)2]10−/Tb/TiO2 (10%)
[Mn4(AsW9O34)2]10−/Tb/TiO2 (10%)
[Zn4(PW9O34)2]10−/Tb/TiO2 (10%)
5
5
5
100
100
70
[Mn4(GeW9O34)2]12−/Tb/TiO2 (10%) 10
20
aConversions determined by GC
1 3

Immobilized Sandwich-Type Polyoxometalates [Mn4(XW9O34)2]n− on Tb-Doped TiO2…
Table 4 Selective oxidation of alcohols using H2O2 catalyzed by
the actual role of [Mn4(AsW9O34)2]10−/Tb/TiO2 is not clear,
[Mn4(PW9O34)2]10−/Tb/TiO2 (10%) in H2O
however on the basis of previously reported mechanism, we
can suggest the reaction mechanism as: irstly, hydrogen
peroxide reacts with the immobilized polyoxometalate to
form the active POM form (peroxo species) which subse-
quently oxidizes the substrate molecule to yield the prod-
uct through the heterolytic cleavage of the O–O bond [49,
68–74]. According to the suggested mechanism, the role of
hetero atoms in POMs (P or As) can be explained. More
active peroxo species were formed with more electronega-
tive hetero atom which followed by complete oxidation of
substrate molecule.
Entry
1
Substrate
Time (h)
5
Yield (%)
90
OH
Cl
2
3
1
4
100
95
OH
MeO
O
OH
O
4.4 Catalyst Recovery and Reuse
4
5
4
5
90
90
OH
The catalyst recovery and reusability are the two most
important features for many catalytic processes. The reus-
ability of [Mn4(PW9O34)2]10−/Tb/TiO2, was investigated
for oxidation of benzyl alcohol and methyl phenyl sulide
as a model substrate using H2O2. In both cases, after the
completion of the reaction, the catalyst was easily separated
from the reaction mixture by centrifuge, washed with H2O
and dichloromethane to remove the residual product and
reused for subsequent experiments under similar reaction
conditions. As shown in Fig. 7 the catalyst was reused at
least ive times with little loss of activity.
OH
CH
3
6
5
100
H
O
The FT-IR spectra of [Mn4(PW9O34)2]10−/Tb/TiO2
nanocomposite with 10 and 20% loading of POM after ive
consecutive runs shows that the nanocomposite preserves
its fundamental structure. We can expect the catalyst with-
out signiicant structural changes show the activity in com-
parison to activity of catalyst in the irst run. The FT-IR
spectra of [Mn4(PW9O34)2]10−/Tb/TiO2 after ive consecu-
tive runs has been added to the supporting information
(Fig. S2).
EDX analysis of [Mn4(PW9O34)2]10−/Tb/TiO2 and
[Mn4(PW9O34)2]10−/Tb/TiO2 with 10% loading of POMs
on Tb/TiO2 after ive consecutive runs detect the pres-
ence of Mn, O, P, Tb, W and Mn, O, As, Tb, W elements
cleavage of the O–O bond have been reported [67]. In order
to study the reaction mechanism of oxidation of alcohols
and sulides catalyzed by [Mn4(PW9O34)2]10−/Tb/TiO2 and
[Mn4(AsW9O34)2]10−/Tb/TiO2 in the presence hydrogen
peroxide as oxidant, 2,2′-Azobis(isobutyronitrile) as radical
scavenger was added. The results conirmed that addition
of 2,2′-Azobis(isobutyronitrile) has no signiicant efect on
the yield of the product or reaction times. On the basis of
above result, we can conclude a heterolytic cleavage of the
O–O bond for hydrogen peroxide in this work. Although,
Table 5 Comparison of the activity of various catalyst with [Mn4(AsW9O34)2]10−/Tb/TiO2 and [Mn4(PW9O34)2]10−/Tb/TiO2 on the oxidation of
methyl phenyl sulide with H2O2 as oxidant
Entry
Catalyst
Condition
H2O2 (mmol)
Time (min)
Conversion (%)
sulfoxide/sulfone
References
a
1
2
3
4
5
6
7
[Mn4(AsW9O34)2]10−/Tb/TiO2(10%)
[Mn4(PW9O34)2]10−/Tb/TiO2(10%)
[γ-PW10O38V2(μ-OH)2]3−
(PyH)H3PMo11VO40
CH3CN. r.t
6
2
2
100/–
–/100
98/2
–
a
CH3CN. r.t
6
–
CH3CN/t-BuOH. 20°C
CH3CN. 40°C
CH3CN. 70°C
1,4-Dioxane. 60°C
CH3CN. 50°C
1
30
30
60
30
60
[36]
[16]
[37]
[39]
[60]
2–20
20
1
100/–
88/11
–/99
H3PMo12O40
[(C18H37)2(CH3)2N]7 PW11O39
TBA10[Fe4(PW9O34)2]
2.5
–/99
aReaction conditions as exempliied in the experimental procedure
1 3

S. Gholamyan et al.
Table 6 Comparison of the
activity of various catalyst with
[Mn4(PW9O34)2]10−/Tb/TiO2 on
the oxidation of benzyl alcohol
(1 mmol) to benzaldehyde using
H2O2 as oxidant
Entry
Catalyst
T(°C)/solvent
Time (h)
Benzal-
deyde (%)
References
a
1
2
3
4
5
6
[Mn4(PW9O34)2]10−/Tb/TiO2
H3PMo12O40
80/H2O
5
100
84
–
70/solvent free
100/H2O
5
[43]
[51]
[52]
[53]
[67]
H3PW12O40-NH2-IL-SBA-15
[PMo10V2O40]5−SBA-15
SiW11Zn
6
71
100/CH3CN
90/H20
12
9
98
100
100
1-POM(Zn)
Relux/CH3CN
4.5
aReaction conditions as exempliied in the experimental procedure
Fig. 7 The recycling experi-
/
ment of [Mn4(PW9O34)2]10−
Tb/TiO2 for a oxidation of
methyl phenyl sulide (1 mmol)
to methyl phenyl sulfone using
H2O2 in CH3CN at room
temperature and b oxidation of
benzyl alcohol (1 mmol) to ben-
zaldehyde using H2O2 (6 mmol)
in H2O at 80°C
respectively (Fig. S3). Analysis shows that unchanged
structure of synthesis nanocomposites after ive consecu-
tive runs.
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