Covalent Immobilization of Polyoxotungstate on Alumina and Its Catalytic Generation of Sulfoxides

Article abstract of DOI:10.1002/chem.201601864

The structural and chemical stabilities of immobilized polyoxometalate (POM)-containing catalysts are crucial factors for their industrial application. An alumina supported POM catalyst is prepared by using a facile condensation reaction between the trilacunary POM Na₁₂[α-P₂W₁₅O₅₆]?24 H₂O (P₂W₁₅) and the hydroxy groups on the surface of γ-Al₂O₃spheres under acidic conditions. The heterogeneous catalyst P₂W₁₅?Al₂O₃is characterized by a wide variety of techniques and shows excellent stability and highly efficient reactivity and selectivity for the oxygenation of thioethers to sulfoxides, which are a very useful intermediate in organic synthesis and the industrial preparation of drugs. Furthermore, P₂W₁₅?Al₂O₃can be recycled and reused at least ten times without any observable loss of its catalytic efficiency, mainly due to the covalent immobilization and high dispersion of P₂W₁₅on the γ-Al₂O₃surface.

Full text of DOI:10.1002/chem.201601864

DOI: 10.1002/chem.201601864
Full Paper
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Polyoxometalates |Hot Paper|
Covalent Immobilization of Polyoxotungstate on Alumina and Its
Catalytic Generation of Sulfoxides
Lanlan Hong,[a] Pyaesone Win,[a] Xuan Zhang,[a] Wei Chen,*[a] Haralampos N. Miras,[b] and Yu-
Fei Song*[a]
Abstract: The structural and chemical stabilities of immobi-
lized polyoxometalate (POM)-containing catalysts are crucial
factors for their industrial application. An alumina supported
POM catalyst is prepared by using a facile condensation re-
action between the trilacunary POM Na12[a-P2W15O56]·24H2O
(P2W15) and the hydroxy groups on the surface of g-Al2O3
spheres under acidic conditions. The heterogeneous catalyst
P2W15ÀAl2O3 is characterized by a wide variety of techniques
and shows excellent stability and highly efficient reactivity
and selectivity for the oxygenation of thioethers to sulfox-
ides, which are a very useful intermediate in organic synthe-
sis and the industrial preparation of drugs. Furthermore,
P2W15ÀAl2O3 can be recycled and reused at least ten times
without any observable loss of its catalytic efficiency, mainly
due to the covalent immobilization and high dispersion of
P2W15 on the g-Al2O3 surface.
zeolites,[8] and activated carbons,[9] the produced composite
materials have tended to suffer from various issues that need
to be taken into consideration. For example, heterogeneous
catalysts based on the electrostatic interactions developed be-
tween the POM species and the supports suffer from stability
issues owing to POM leaching,[10–12] whereas the production of
heterogeneous catalysts under high temperature regimes (e.g.,
calcination) usually leads to structural rearrangements and
compromises the POMs’ structural integrity, which greatly
limits opportunities to exploit the wide range of functionalities
directly correlated to the POMs’ structural features.[6,13,14] There-
fore, utilization of robust support surfaces that preserve the
POM’s structural integrity for the development of heterogene-
ous catalysts for a wide range of processes is highly desirable.
The spherical g-Al2O3 exhibits remarkable physical properties
such as mechanical stability, well-defined pore structure and
high specific surface areas. For these reasons, it is widely em-
ployed in the chemical and petrochemical industries.[15–22]
Herein we report the preparation of the a composite material
by covalent immobilization of Na12[a-P2W15O56]·24H2O (P2W15)
POM clusters onto spherical g-Al2O3 particles (Scheme 1) and
we discuss its catalytic efficacy for the selective oxygenation of
thioethers.
Introduction
Polyoxometalates (POMs) are a class of discrete anionic metal
oxides, often incorporating V, Mo, W, or Nb, among other
metals,[1] in their highest oxidation states. They exhibit unique
structural archetypes, as well as a wide range of sizes, unique
physical properties,[2] and applications[3] in areas as diverse as
catalysis, medicine, electrochemistry, photochromism, and
magnetism.[4] Additionally, their solution processability, due to
their high solubility,[1] low lattice energies,[2] and generally
weak interactions between the POM clusters and their associat-
ed counterions (e.g., NH4+, K+, Na+), render them ideal candi-
dates and useful components for the design of functional ma-
terials such as catalysts. More specifically, studies on POM-
based catalysis are mainly focused on the design of simplified
routes for the preparation of heterogeneous systems due to
catalyst recoverability, reusability and excellent chemical, struc-
tural and thermal stability.[3]
Immobilization and solidification of POM species on various
substrates are two widely applied approaches for the genera-
tion of multifunctional POM-based heterogeneous catalysts.[3,4]
Although great efforts have been made for the efficient immo-
bilization of POMs on supports such as SiO2,[5] Al2O3,[6] ZrO2,[7]
[a] L. Hong, P. Win, X. Zhang, Dr. W. Chen, Prof. Y.-F. Song
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology, Beijing 100029 (P. R. China)
E-mail: chenw@mail.buct.edu.cn
Results and Discussion
The P2W15ÀAl2O3 composite material was synthesized by immo-
bilization of Na12[a-P2W15O56]·24H2O clusters onto the surface
of the spherical g-Al2O3 particles under acidic conditions.[23,24]
In an effort to optimize the immobilization conditions, we im-
mobilized the POM clusters under conditions ranging from
pH 6 to pH 1 (Figure 1). The resultant solids were separated
from the reaction mixture by filtration and washed with 0.3m
songyufei@hotmail.com
songyf@mail.buct.edu.cn
[b] Dr. H. N. Miras
WestCHEM, School of Chemistry
University of Glasgow, Glasgow, G12 8QQ (UK)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201601864.
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a condensation reaction between the {P2W15} anions and the g-
Al2O3 surface according to the Equations (1) and (2):
þ
12À
12À
ð1Þ
ð2Þ
AlÀOH2 þ P2W15O56
Ð AlÀOH2þP2W15O56
AlÀOH2þP2W15O56
Ð AlÀOÀP2W15O56
þ H2O
12À
11À
The W=O and WÀO stretching and bending vibrations of the
P2W15 generally occur below 1000 cmÀ1, so it is difficult to
detect them by FT-IR spectroscopy, owing to the strong IR ab-
sorption of the g-Al2O3 supports in this region (see the Sup-
porting Information, Figure S2).[13,27] The XRD pattern of the
P2W15ÀAl2O3 composite is similar to that of Al2O3, indicating no
diffraction of the Na12[a-P2W15O56]·24H2O due to poor crystal-
linity and coexistence of other crystalline phases (see the Sup-
porting Information, Figure S3). From the Raman spectra of
P2W15ÀAl2O3 (Figure 2a), it can be seen that the g-Al2O3 sup-
Scheme 1. Synthesis of P2W15ÀAl2O3.
Figure 1. Solid-state 31P NMR spectra: a) Na12[a-P2W15O56]·24H2O; b–f) P2W15À
Al2O3 composite material prepared under different pH environments.
LiCl solution to remove the physically adsorbed {P2W15} clus-
ters.[25]
Figure 1a shows part of the solid-state 31P NMR spectrum of
Na12[a-P2W15O56]·24H2O, where the signals at d=À0.79 and
À15.77 ppm can be assigned to the nonequivalent phospho-
rus atoms P1 and P2, respectively.[26] Upon decrease of the pH
value from 6 to 1, the left shoulder peak centered at
À0.79 ppm decreases and splits into several peaks in the range
of À0.79 to À8.41 ppm, which is a potential indication of par-
tial decomposition at specific pH values of the cluster prior to
immobilization on the alumina surface. The signal at d=
À15.77 ppm, shifts gradually to À14.58 ppm, as a result of the
reaction between the {P2W15} anions and the g-Al2O3 surface.
According to the solid state 31P NMR spectra, the most effec-
tive immobilization of the lacunary Dawson anions onto the
alumina support takes place at pH 1. This is in agreement with
the Mulcahy’s report,[23] where the positive net surface charge
acquired at pH values below the isoelectric point (IEP) of the
support, attracts and stabilizes the anionic species from the
bulk solution. However, a simple electrostatic model cannot ac-
count for the adsorption of theses anions. It is suggested that
this is a two-step process whereby the electrostatic ion pairing
occurs first as soon as the pH value is below 8.6, the IEP of g-
Al2O3 (see the Supporting Information, Figure S1), followed by
Figure 2. a) Raman spectra of Al2O3, P2W15ÀAl2O3 and P2W15; b) W4f X-ray
photoelectron spectrum of P2W15ÀAl2O3.
port is not Raman active, allowing detection of all WÀO vibra-
tions.[28] The characteristic vibrations of bulk {P2W15} are located
at 955, 871, and 827 cmÀ1, whereas P2W15ÀAl2O3 gives rise to
Raman bands at 980, 909 and 839 cmÀ1, which can be assigned
to the W=O stretch bands and WÀOÀW stretching modes of
the immobilized cluster.[29] The Raman shifts are due to the
strong interactions between the trilacunary POM cluster and
the g-Al2O3 support, leading to a decrease in the cluster’s and/
or the alumina’s local symmetry.[30]
Additionally, X-ray photoelectron spectroscopy (XPS) was
used to determine the oxidation states of metal centers. More
specifically, the XPS spectrum for W4f (Figure 2b) revealed
two peaks at 34.8 and 36.2 eV,[31,32] in the energy region of
W4f7/2, and two peaks at 37.5, 39.5 eV,[33,34] in the energy
region of W4f5/2, which are consistent with a WVI oxidation
state. Thermogravimetric analysis of the P2W15ÀAl2O3 compo-
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site (see the Supporting Information, Figure S4) under nitrogen
in the temperature range of 25 to 8008C revealed two main
weight losses. The first loss of 1.9% in a temperature range of
25–2078C can be attributed to the loss of the absorbed water
and crystallization water molecules of the sample. The second
weight loss of 1.4%, between 207 and 5238C, can be assigned
to the loss of the constitutional water from P2W15ÀAl2O3.[35]
SEM images of the P2W15ÀAl2O3 composite material show
sheet-like morphologies that are similar to that of the bare
Al2O3 support (see the Supporting Information, Figures S5 and
S6). The HR-TEM images of the P2W15ÀAl2O3 composite material
revealed unambiguously the presence of {P2W15} clusters on
the g-Al2O3 support, in the form of dark dots 1–2 nm in size
(Figure 3). To further investigate the nature of the linkage be-
Figure 4. The N2 adsorption–desorption isotherm and pore size distributions
of Al2O3 (a) and P2W15ÀAl2O3 (b).
Figure 3. a–e) HR-TEM images of Al2O3 (a–c) and P2W15ÀAl2O3 (d,e); f) energy
dispersive spectrum of P2W15ÀAl2O3.
The pore size distribution curves of g-Al2O3 and P2W15ÀAl2O3
showed mean pore sizes of 42.74 and 30.93 nm, respectively,
indicating that the regular mesoporous channels of the g-Al2O3
support are retained in the corresponding P2W15ÀAl2O3 cata-
lysts. The pore sizes of P2W15ÀAl2O3 decreased with increased
P2W15 loading, which can be explained by the fact that the
pores are partially blocked by {P2W15} clusters.
tween the {P2W15} clusters and the g-Al2O3 support, we studied
the stability of the P2W15ÀAl2O3 composite in LiCl solution, ac-
cording to a previously reported procedure.[13] The P2W15ÀAl2O3
composite materials were dispersed in 0.3m LiCl solution
under vigorous stirring for 12 h, removed by filtration, and
dried under vacuum. The procedure was repeated three times;
close examination of the HR-TEM images obtained from the
collected and dried material (see the Supporting Information,
Figure S7) showed that the POM clusters remained immobi-
lized on the surface of g-Al2O3 and inductively coupled plasma
(ICP) analysis of the collected filtrates showed that no elemen-
tal W was present. The experimental data strongly suggest
that the {P2W15} clusters are not just physically adsorbed on
the g-Al2O3, since the presence of Li+ counterions would have
removed the clusters from the surface and extracted them into
the solution. Examination of the elemental composition of the
material by energy-dispersive spectroscopy (EDS) showed that
P and W atoms are present on the P2W15ÀAl2O3 composite ma-
terial (Figure 3 f) whereas X-ray fluorescence spectroscopy re-
vealed that the loading of {P2W15} on the g-Al2O3 support was
approximately 9.98%.
Catalytic oxidation of organic compounds under mild condi-
tions are of both considerable intellectual interest and poten-
tial utility.[36,37] Catalytic oxidation of organic sulfides remains
a topical interest due to the versatile utility of both sulfoxides
and sulfones in organic synthesis.[38–40] Although a multitude of
oxygen donors are available, utilization of H2O2 as a “green”
oxygen donor has become increasingly prominent.[41–44] In this
work, catalytic oxidation of thioanisole in the presence of
P2W15ÀAl2O3 as catalyst and H2O2 as oxidant was carried out
(Scheme 2). The selectivity of sulfoxide products and the con-
version of the initial sulfides were quantified by using gas
chromatography (GC; see the Supporting Information, Figur-
1
es S8 and S9) and H NMR spectroscopy of the reactant (thioa-
nisole) and the product upon completion of the reaction
(Figure 5). Reactivity data for the oxidation of thioanisole in
CH3OH at 258C in the presence of the {P2W15} and P2W15ÀAl2O3
and in the absence of the P2W15ÀAl2O3 are summarized in
N2 adsorption–desorption isotherms of g-Al2O3 and P2W15À Table S2 (see the Supporting Information). In the presence of
Al2O3 (Figure 4 and Table S1 in the Supporting Information) are
classified as type IV with a clear H1-type hysteresis loop ac-
cording to the IUPAC classification, indicating mesoporosity.
the P2W15ÀAl2O3 catalyst, conversion and selectivity were im-
proved considerably. This indicates that the P2W15ÀAl2O3 is an
effective catalyst for the oxygenation of thioanisole by H2O2. To
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thioanisole concentration at time t, respectively. By using Equa-
tions (3) and (4), the rate constant k of the oxidation reaction
was determined to be À0.082 minÀ1.The oxidation of thioani-
sole and subsequent conversion to the product (sulfoxide) can
be completed in about 35 min.
ÀdCt
ð3Þ
ð4Þ
¼ kCt
dt
Scheme 2. Selective oxidation of sulfides to sulfoxides by using the P2W15À
C0
ln ¼ kt
Ct
Al2O3 composite as catalyst.
As such, the catalyst shows high catalytic efficiency for the
oxidation of sulfides to sulfoxides, and the catalytic reaction
exhibits pseudo-first-order kinetics for the oxidation of thioani-
sole with 99% selectivity for sulfoxides (R2 =0.99). In an effort
to confirm that the catalyst is truly heterogeneous, we re-
moved the catalyst from the reaction mixture when the con-
version reached a value of 60% after 15 min. Real time moni-
toring of the reaction mixture by GC showed that the sub-
strate catalytic conversion stopped immediately. The addition
of the catalyst back to the reaction mixture restarted the reac-
tion (see the Supporting Information, Figure S13).
After confirming the stability and efficiency of the composite
material, we investigated the general applicability and selectiv-
ity of the as-prepared catalyst of P2W15ÀAl2O3. We utilized
a series of thioether-based compounds as substrates in catalyt-
ic oxygenation reactions under the same conditions (Table 1).
Alkyl aryl thioethers and 4-pentamethylene sulfides could be
selectively fully converted into the desired product. However,
2-bromothioanisole underwent relatively low conversion,
which might be due to increased steric hindrance and/or elec-
trophilic effects (Table 1, entry 4). Interestingly, the phenyl allyl
Figure 5. 1H NMR (CDCl3) spectra of the reactant (thioanisole) and the prod-
uct (sulfoxide). Experimental conditions: Thioanisole (1 mmol), H2O2
(1 mmol), catalyst (2.5 mmol), CH3OH (200 mL), 258C, 35 min.
optimize the reaction conditions, various solvents, reaction
times, and amounts of H2O2 and catalyst were investigated
(see the Supporting Information, Figures S10–S12 and
Table S3). The determined optimal conditions were found to
be the following: thioanisole (1 mmol), H2O2 (82 mL), CH3OH
(200 mL). Experiments to determine the kinetic parameters for
the catalytic oxidation of thioanisole were carried out under
these conditions.
Table 1. Oxidation of various sulfides to the corresponding sulfoxides.[a]
Reaction kinetics for the oxidation of thioanisole are present-
ed in Figure 6, where the (Ct/C0) is plotted against the reaction
time. C0 and Ct are the initial thioanisole concentration and the
Entry
1
Substrate
Product
Conv. [%][b]
Sel. [%][c]
94
99
2
3
90
98
99
97
4
5
34
94
100
97
6
7
63
96
92
91
[a] Experimental conditions: Substrate (1 mmol), 30% H2O2 (1 mmol),
P2W15ÀAl2O3 (2.5 mmol), CH3OH (200 mL), 258C, 35 min; [b] conversion of
sulfides; [c] selectivity towards sulfoxides.
Figure 6. Catalytic conversion of thioanisole and ln(Ct/C0) as functions of re-
action time. Reaction conditions: Thioanisole (1 mmol), H2O2 (1 mmol),
CH3OH (200 mL), 258C, 35 min.
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thioether underwent 63% conversion into the desired sulfox-
ide alongside an unknown byproduct (Table 1, entry 6).
the catalytic system. Moreover, the P2W15 clusters are distribut-
ed uniformly onto the g-Al2O3 surface as evidenced by HR-TEM
studies. Highly efficient and selective oxygenation of thioethers
can be achieved by the P2W15ÀAl2O3 in the presence of H2O2
with 99% selectivity of sulfoxides and 94% conversion in
35 min. The P2W15ÀAl2O3 can be separated easily by filtration
and reused at least ten times without obvious decrease in cat-
alytic efficiency. The facile preparation, reusability, and efficacy
of the heterogeneous P2W15ÀAl2O3 catalyst provide great po-
tential for industrial applications. Finally, the straightforward
covalent attachment of the appropriate choice of the polyoxo-
metalate core (architecture and composition) opens the door
for further exploration and design of multifunctional catalytic
materials tailored for specific applications. Our current research
effort is focused on exploring the potential of the family of
POMÀAl2O3 catalysts for the development of chiral sulfoxides.
The main advantage of using a heterogeneous catalysts in
a liquid-phase reaction is the ease of separation and recycling
of the catalyst. To test whether any catalyst leaching occurred,
we filtered off the catalyst after each cycle, washed it with ace-
tone, dried and then added it into a fresh reaction mixture.
The solid-state 31P NMR (Figure 7a) and Raman spectra (Fig-
ure 7b) and the HR-TEM image (Figure 7c) of the recovered
Experimental Section
Chemicals and Materials
All chemicals were of analytical grade and were used as received
without any further purification. Thioether compounds were pur-
chased from Alfa Aesar. LiCl, HCl, KNO3, HNO3, and NaOH were pur-
chased from Beijing Chemical Reagent Company (Beijing, China).
Acetone, methanol, ethanol, acetonitrile, and ethyl acetate were
purchased from J&K Chemical Ltd. Spherical g-Al2O3 was purchased
from Shandong Benten Chemical Corporation. Deionized water
from a Millipore water purification system was used throughout
the experiments. Na12[a-P2W15O56]·24H2O (P2W15) was prepared ac-
cording to a reported procedure.[25]
Figure 7. a) Solid state 31P NMR spectra and b) Raman spectra of the fresh
and recycled P2W15ÀAl2O3 after 10 cycles; c) HR-TEM image of the recycled
catalyst after 10 cycles; d) recycling experiments for selective oxidation of
sulfides to sulfoxides catalyzed by P2W15ÀAl2O3. Experimental conditions: Thi-
oanisole (1 mmol), H2O2 (1 mmol), catalyst (2.5 mmol), CH3OH (200 mL), 258C,
35 min.
Instruments
Solid-state NMR experiments were carried out at 10 kHz for 31P on
a Bruker Avance 300MHz solid-state spectrometer equipped with
a commercial 5 mm MAS NMR probe. FT-IR spectra were collected
in transmission mode by using a JASCO FT-IR 410 spectrometer or
a JASCO FT-IR 4100 spectrometer (wavenumbers n are given in
catalyst after 10 cycles were almost the same as those of the
fresh one, indicating the stability of the catalyst. The aged
P2W15ÀAl2O3 catalytic system can be reused for the oxidation
of thioanisole, promoting the oxidation of sulfides to sulfox-
ides, at least ten times without significant decrease of its cata-
lytic efficiency (Figure 7d). The catalytic efficiencies of various
previously reported systems for the oxidation of sulfide sub-
strates are summarized in Table S4 (see the Supporting Infor-
mation).The high efficiency and selectivity of the P2W15ÀAl2O3
catalyst, along with its quick and easy preparation in a single
step and lower consumption of H2O2 during the catalytic con-
version of the substrates (Table S4, entry 14), render it an ideal
candidate for larger scale application.
cmÀ1
; intensities are denoted as wk=weak, sh=sharp, m=
medium, br=broad, s=strong). XRD measurements were per-
formed on a JCN UItima III X-ray diffractometer. XPS data were ob-
tained from a Thermo-Fisher Scientific ESCALAB 250 X-ray photo-
electron spectrometer. TGA measurements were performed on
a DTG-60 A analyzer from the Shimadzu Corporation, under nitro-
gen atmosphere with a temperature increase of 108CminÀ1 be-
tween 25 and 8008C. Laser Raman spectra were obtained by using
a LabRAM ARAMIS Raman spectrometer. Scanning emission micro-
scope (SEM) images were collected on a Zeiss Supra 55 VP field-
emission scanning electron microscope. High-resolution transmis-
sion electron microscopy (HR-TEM) images were obtained with
a JEOL JEM-3010 microscope operated at 300 kV. BET data were
obtained by using a Bei Shi De 3H-2000PS2 specific surface area
and pore size analyzer. X-Ray fluorescence spectra were obtained
by using a XRF-1800 with a scanning rate of 208minÀ1. The content
of sulfides and sulfoxides were analyzed on an Agilent 7820 A GC
system using a 30 m 5% phenylmethyl silicone capillary column
with an ID of 0.32 mm and 0.25 mm coating (HP-5).
Conclusion
The immobilization of P2W15 clusters on spherical g-Al2O3 parti-
cles results in the formation of P2W15ÀAl2O3 as a new heteroge-
neous catalyst, in which strong covalent bonds are formed be-
tween the [a-P2W15O56]12À clusters and the hydroxyl groups of
the spherical g-Al2O3 support inducing exceptional stability to
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Spherical g-Al2O3 was activated at 9008C under atmospheric pres-
sure for 24 h, then cooled to room temperature in accordance with
the procedure reported by Cargnello et al.[45] In a reaction vessel,
Na12[a-P2W15O56]·24H2O (1.66”10À4 mol) was dissolved as a clear
solution in deionized water (50 mL) without CO2, and then g-Al2O3
spheres (1 g) were added to the solution. The pH of the solution
was adjusted to 1.00 carefully by dropwise addition of 4m aqueous
HCl. The reaction mixture was stirred at room temperature for 1 h
and then at 908C for 3 h.[46] The reaction mixture was cooled to
room temperature and the product was removed by filtration,
washed with 0.3m LiCl solution (3”100 mL), and dried under
vacuum at 608C for 24 h.
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The isoelectric point (IEP) of the g-Al2O3 support was determined
by a batch equilibration method, as described in the literature.[47]
The spherical g-Al2O3 support (25 mg) was dispersed in 0.1m aque-
ous KNO3 (25 mL) with vigorous stirring for 60 min. Six bottles of
the resultant solution were prepared and the pH values of the bot-
tles were adjusted to 3, 4, 5, 6, 7, 8, 9, 10, and 11 by addition of
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Catalytic test
The oxidation reaction was carried out in a 25 mL flask with mag-
netic stirring. A typical optimized procedure was as follows: Thioa-
nisole (1 mmol), H2O2 (1 mmol), CH3OH (200 mL), P2W15ÀAl2O3
(2.5 mmol; 9.98% loading of P2W15 on Al2O3) were added to the
flask. The reaction time was counted after addition of the catalyst
(T=258C). Before GC measurement, the reaction mixture was
quenched by adding the deionized H2O (1 mL) and diethyl ether
(6 mL). After stirring for 1 min, it was filtered through a 0.22 mm mi-
crofilter, and the filtrate was analyzed by GC. The yield was deter-
mined by using reference standards. No increase of conversion can
be found after 35 mins. After completion of the reaction, the cata-
lyst was recovered by filtration, washed with the acetone (20 mL),
and dried in the oven at 608C.
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This research was supported by the National Basic Research
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Science Foundation of China (U1407127, U1507102), and Fun-
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Keywords: alumina
·
immobilization
·
oxidation
·
polyoxometalates · supported catalysts
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