Heteropolytungstic acids incorporated in an ordered mesoporous zirconia framework as efficient oxidation catalysts

Article abstract of DOI:10.1039/c3ra46421k

Ordered mesoporous composite catalysts consisting of nanocrystalline tetragonal ZrO₂ and heteropolytungstic clusters, i.e. 12-phosphotungstic (PTA) and 12-silicotungstic (STA) acids, were prepared via a surfactant-assisted co-polymerization route. According to the X-ray diffraction, transmission electron microscopy and N₂ physisorption measurements, the resultant materials possess a well-defined mesoscopic order ranging from wormhole to hexagonal pore structure and exhibit large internal surface area (126-229 m² g^-1) and quite narrow pore size distribution (ca. 2.2-2.6 nm in diameter). Energy dispersive X-ray microanalysis and infrared spectroscopy confirms that the heteropoly clusters are well dispersed within the zirconia matrix, while preserving intact their Keggin structure. The inclusion of PTA and STA clusters in the mesoporous framework has a beneficial effect on the catalytic activity of these materials. Although zirconium oxide and heteropoly acids alone show little catalytic activity, the ZrO ₂-PTA and ZrO₂-STA heterostructures exhibit surprisingly high activity in hydrogen peroxide mediated oxidation of 1,1-diphenyl-2- methylpropene under mild conditions. Indeed, the mesoporous ZrO₂-STA composite sample loaded with 5 wt% STA shows a conversion rate that is 17 times higher than the mesoporous ZrO₂. The catalytic activity of these materials is related to the spatial distribution of heteropoly acids in zirconia matrix and possible synergistic interactions between the incorporated Keggin units and Zr(iv) oxohydroxide species.

Full text of DOI:10.1039/c3ra46421k

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Heteropolytungstic acids incorporated in an
ordered mesoporous zirconia framework as
Cite this: RSC Adv., 2014, 4, 8402
efficient oxidation catalysts†
a
b
a
*
Euaggelia Skliri, Ioannis N. Lykakis and Gerasimos S. Armatas
Ordered mesoporous composite catalysts consisting of nanocrystalline tetragonal ZrO2 and
heteropolytungstic clusters, i.e. 12-phosphotungstic (PTA) and 12-silicotungstic (STA) acids, were
prepared via
a surfactant-assisted co-polymerization route. According to the X-ray diffraction,
transmission electron microscopy and N2 physisorption measurements, the resultant materials possess a
well-defined mesoscopic order ranging from wormhole to hexagonal pore structure and exhibit large
internal surface area (126–229 m2
g
ꢀ1) and quite narrow pore size distribution (ca. 2.2–2.6 nm in
diameter). Energy dispersive X-ray microanalysis and infrared spectroscopy confirms that the heteropoly
clusters are well dispersed within the zirconia matrix, while preserving intact their Keggin structure. The
inclusion of PTA and STA clusters in the mesoporous framework has a beneficial effect on the catalytic
activity of these materials. Although zirconium oxide and heteropoly acids alone show little catalytic
activity, the ZrO2–PTA and ZrO2–STA heterostructures exhibit surprisingly high activity in hydrogen
peroxide mediated oxidation of 1,1-diphenyl-2-methylpropene under mild conditions. Indeed, the
mesoporous ZrO2–STA composite sample loaded with 5 wt% STA shows a conversion rate that is
17 times higher than the mesoporous ZrO2. The catalytic activity of these materials is related to the
spatial distribution of heteropoly acids in zirconia matrix and possible synergistic interactions between
the incorporated Keggin units and Zr(IV) oxohydroxide species.
Received 5th November 2013
Accepted 14th January 2014
DOI: 10.1039/c3ra46421k
www.rsc.org/advances
as an efficient strategy for achieving high catalytic activity and
reusability.5 Compared to bulk heteropoly acids, supported
Introduction
POM clusters have various advantages, which are their large
surface area, good accessibility to the reactants, and easier
separation from the reaction mixture.6 In addition, the metal
oxide surfaces could have a benecial effect in catalytic
processes through complementary or synergistic interactions
with the POM compounds. For example, recently we indicated
that incorporation of [PW12O40]3ꢀ Keggin clusters into the
cobalt oxide matrix can dramatically improve the catalytic
activity of mesoporous Co3O4 towards N2O decomposition.7
Therefore, the synthesis of high-surface-area mesoporous
materials that combine the catalytic activity of POMs with the
redox activity and durability of metal oxides is a major direction
in catalysis research.
Impregnation is one of the most attractive synthetic routes
towards mesoporous POM-based materials. This approach
involves inltration of soluble POM compounds within the pore
channels of a previously synthesized host material, such as
silica,8 alumina,9 titania and zirconia.10 Although this is a
simple yet efficient method, the impregnation process is oen
plagued by poor dispersion of POM units and pore blocking
effects of the inorganic support. In addition, the impregnated
POM clusters show moderately low chemical stability when
anchored on surfaces with strong basic character (e.g. Al2O3,
Polyoxometalates (POMs) are attractive metal-oxygen
compounds of the early transition metals (V, Nb, Mo, W) with
diverse molecular structures and adjustable functionality.
These nanomaterials exhibit interesting physical properties
such as strong Brønsted acidity, fast and reversible multi-stage
redox activity and high thermal and hydrolytic stability, making
them suitable for catalytic applications.1 As a result, POMs have
been extensively investigated as catalysts in a variety of chemical
transformations and redox reactions. Examples include oxida-
tive hydration and esterication of olens, oxidation of alco-
hols2 and alkenes,3 and alkylation and acylation of
hydrocarbons.4
Over the past few years, extensive research efforts have been
made to immobilize POMs on a high-surface-area solid support
aDepartment of Materials Science and Technology, University of Crete, Voutes, 71003,
Heraklion, Greece. E-mail: garmatas@materials.uoc.gr
bDepartment of Chemistry, Aristotle University of Thessaloniki, University Campus,
54124, Thessaloniki, Greece
† Electronic supplementary information (ESI) available: N2 physisorption data for
meso-ZrO2, catalytic data for ZrSTA(5) in different media, time evolution proles of
1 oxidation over meso-ZrO2, ZrPTA(w) and ZrSTA(w) materials and PTA and STA
compounds, and catalytic data, N2 adsorption–desorption isotherms and EDS
and IR spectra of reused ZrSTA(5) catalyst. See DOI: 10.1039/c3ra46421k
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MgO),11 while they are prone to leaching in polar solvents when ZrO2, while 12-silicotungstic acid along shows negligible activity
anchored on weak acid or neutral supports (e.g. SiO2).12 Never- under similar conditions.
theless, both the unprecedented molecular transport through
nanopores and high hydrolytic stability of mesophase are
valuable prerequisites for heterogeneous catalysis. A promising
synthetic strategy to this end is the sol–gel inclusion of POMs in
Experimental
Materials
the pore walls of a mesoporous metal oxide material. This
approach entails cross-linking polymerization of metal oxide
and POM compounds in the presence of surfactants and allows
controlled synthesis of mesoporous heterostructures with
modulated POM/metal-oxide composition. Unlike to the
impregnated analogues, the POM compounds in these mate-
rials constitute an integral part of the inorganic framework,
forming rather a solid-solution structure.13 Because of the
excellent catalytic properties of POM clusters and the large pore
surface of the host structure, mesoporous solids of this type are
anticipated to have interesting perspectives in the applications
as catalysts, exhibiting high activity and reusability.14 While
silica-based POM composites are easy to realize,15 the synthesis
of mesoporous heterostructures from transition-metal oxide
and POM compounds still remains a challenge. The assembly of
such complex materials requires an adjustment of the interac-
tion between heteropoly acids and organic templates. Otherwise
disordered mesophases of organic–inorganic aggregates due to
the strong binding of heteropoly anions to the surfactant
micelles might be formed.16 Therefore, specic reaction condi-
tions according to the choice of metal sources, template mole-
cules, and POM anions are essential in order to achieve ordered
POM-based mesoporous frameworks.
Zirconia (ZrO2) is an important n-type semiconductor
material with great potential applications in redox and acid
catalysis.17 Recently, we demonstrated that well-ordered meso-
porous frameworks consisting of nanocrystalline ZrO2 and
Keggin-type H3PMo12O40 acids can be prepared via a so-tem-
plating chemistry.18 This work used non-ionic surfactant Plur-
onic P123 to direct the formation of hexagonal mesoporous
ZrO2-based structures. These materials showed remarkable
catalytic activity in H2O2-mediated oxidation of selected
alkenes, which is comparable to that of homogeneous
12-phosphomolybdic acids. Herein, we report on the synthesis
of a new type of mesoporous composite materials consisting of
zirconium oxide and 12-phosphotungstic (H3PW12O40, PTA) or
12-Phosphotungstic acid (PTA) and 12-silicotungstic acid (STA)
were purchased from Alfa Aesar. The solids were thermal treated
at 100 ꢁC for 2 h in air to eliminate excess of uncoordinated water
molecules. The weight proportion of water on dried samples was
determined by TGA analysis (Perkin Elmer Diamond), obtaining a
molecular formula of H3PW12O40$16H2O and H4SiW12O40$15H2O,
respectively. Block copolymer surfactant (HO(CH2CH2O)20C16H33,
Mn ꢂ 1124) designated as Brij-58, zirconium oxide dichloride
(ZrOCl2$8H2O), absolute ethanol (99.8%) were purchased from
Sigma-Aldrich and used with no further purication. The alkene
used as substrate was synthesized based on literature procedure.19
Synthesis of mesoporous ZrPTA and ZrSTA composites
Mesoporous zirconia and ZrO2–PTA and ZrO2–STA composite
materials were prepared as following: In a typical preparation, 2
g of Brij-58 was dissolved in 20 mL of ethanol, forming a clear
solution. Then, 3.2 g (10 mmol) of ZrOCl2$8H2O was added to
the surfactant solution and the mixture was stirred vigorously
for ꢂ3 h at room temperature. Aer this, an appropriate
amount of PTA or STA acids dissolved in 2 mL of ethanol was
added slowly to the surfactant solution and the mixture was
keep stirring for another 15 min. This solution was then poured
into a Petri dish and le under static conditions at 40 ꢁC for 3–4
days. The resulting solid product was subsequently dried at
100 ꢁC under vacuum for 12 h. Template was removed by
ꢀ1
ꢁ
calcination in air at 265 C with a heating rate of 0.5 ꢁC min
and held for 4 h and, then, at 350 ꢁC with a heating rate of 0.5 ꢁC
minꢀ1 and held for 6 h. A series of mesoporous ZrO2–PTA
(denoted as ZrPTA(w)) and ZrO2–STA (denoted as ZrSTA(w))
composites with polyoxometalate loadings (w) of about 2, 5, 10
and 20 wt% were prepared by using appropriate amount of
12-phosphotungstic and 12-silicotungstic acid, respectively.
Mesoporous zirconia (meso-ZrO2) was also prepared following a
procedure similar to that described above, without addition of
heteropoly acids.
12-silicotungstic (H4SiW12O40
produce these ZrO2–POM composites with different loading of
, STA) acid components. To
Characterization
POM clusters (i.e. ranging from 2 to 20 wt%), we utilized evap- The X-ray diffraction (XRD) patterns were recorded on a PAN-
oration-induced cooperative self-assembly of Zr(IV) oxide analytical X'Pert Pro MPD X-ray diffractometer (45 kV and 40
˚
dichloride and heteropolytungstic acid compounds in the mA) using Cu Ka radiation (l ¼ 1.5418 A) in Bragg–Brentano
presence of the non-ionic surfactant polyoxoethylene cetyl ether geometry. Nitrogen adsorption and desorption isotherms were
(Brij-58) as structure-directing agent. The products possess a measured at 77 K using a NOVA 3200e volumetric analyzer
ZrO2–PTA and ZrO2–STA solid-solution structure with large and (Quantachrome, USA). Before analysis,ꢀ5the samples were
accessible surface area and narrow pore size distribution. More degassed at 150 ꢁC under vacuum (<10 Torr) for 12 h to
important, we also show that, although mesoporous zirconia remove moisture. The specic surface areas were calculated
and POM compounds alone have little catalytic activity, the using the Brunauer–Emmett–Teller (BET) method20 on the
ZrO2–POM composite frameworks exhibit surprisingly high adsorption data in the 0.05–0.25 relative pressure (p/po) range.
activity in oxidation of 1,1-diphenyl-2-methylpropene under The total pore volumes were derived from the adsorbed volume
mild conditions. Indeed, the ZrO2–STA sample loaded with 5 wt at p/po ¼ 0.95. The pore size distributions were determined by
% STA is about 17 times more active than pure mesoporous the NLDFT method21 based on the adsorption data, using a data
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kernel for silica matrix with cylindrical pores. Micropore
volume was estimated by the Dubinin–Radushkevich (DR)
method in the low-pressure range p/po ¼ 0.0001–0.15. Trans-
mission electron microscopy (TEM) was performed using a
JEOL model JEM-2100 electron microscope equipped with an
energy-dispersive X-ray spectroscopy (EDS) detector (Oxford
Instruments) and operated at an accelerating voltage of 200 kV.
Samples for TEM were prepared by suspending nely ground
powder in ethanol using sonication, followed by depositing a
drop of solution onto a Cu grid covered with carbon lm.
Infrared spectra were recorded on a Perkin Elmer Model Fron-
tier FT-IR spectrometer. Spectra were obtained on averaging 100
interferograms with resolution of 4 cmꢀ1. Samples were
prepared as KBr pellets.
Fig.
1
Low-angle X-ray diffraction patterns of mesoporous (a)
ZrPTA(w) and (b) ZrSTA(w) composite materials. The corresponding
XRD pattern for mesoporous zirconia (meso-ZrO2) is given in inset of
panel (a).
Catalytic reactions
The catalytic oxidation of 1,1-diphenyl-2-methylpropene was
carried out as follows. A mixture of substrate (0.1 mmol) and
catalyst (0.5 mmol) was dissolved in 3 mL of ethanol into a
5 mL glass reactor with an open Teon cap. The mixture was
heated at 50 ꢁC and the reaction was initiated by slowly adding
(within a period of 3 min) the oxidant H2O2 (3 equiv., 30% in
H2O) into the ethanol solution. The assignment of products
Table 1 Textural properties of mesoporous zirconia (meso-ZrO2) and
ZrPTA(w) and ZrSTA(w) composite materials
BET Surface Pore volumea Pore size Unit cell WTb
Sample
(m2 gꢀ1
)
(cm3 gꢀ1
)
(nm)
(nm)
(nm)
meso-ZrO2 118
0.06 (0.04)
0.07 (0.05)
0.07 (0.04)
0.09 (0.05)
0.09 (0.05)
0.08 (0.05)
0.08 (0.05)
0.11 (0.06)
0.12 (0.07)
2.5
2.4
2.3
2.3
2.3
2.5
2.6
2.6
2.6
4.3
4.5
4.6
5.1
5.6
4.7
5.1
5.8
6.2
1.8
2.1
2.3
2.8
3.3
2.2
2.5
3.2
3.6
was realized by a combination of gas chromatography – mass ZrPTA(2)
126
160
spectrometry (Shimadzu GCMS-QP2010 Ultra) and 1H and 13
C
ZrPTA(5)
ZrPTA(10) 207
ZrPTA(20) 210
NMR spectroscopy (Bruker AMX 300 MHz) by withdrawing
small aliquots from the reaction mixture. To establish the
identity of the products unequivocally, the retention times
and spectral data were compared to those of commercially ZrSTA(10) 229
available compounds. In all oxidations, the amount of POM in
ZrSTA(2)
ZrSTA(5)
140
155
ZrSTA(20) 218
a
composite materials remains similar (about 2 mol%). As
Cumulative pore volume at relative pressure p/p0
¼
0.95. In
b
parenthesis: the micropore volume obtained by the DR plots. The
framework wall thickness given by WT ¼ a0 ꢀ D, where D is the pores
size.
shown below, the sites located at or near the interface
between the POM cluster and zirconia surface are the active
species of the reaction and, therefore, mesoporous ZrO2–POM
composites act as catalysts and not as stoichiometric
reagents.
The wide-angle XRD patterns of the mesoporous ZrPTA(w)
and ZrSTA(w) suggested the presence of nanocrystalline ZrO2, as
indicated from the broad reections in the 2q ¼ 30–60ꢁ range
(Fig. 2). The XRD peaks can be assigned to the (011), (110), (012),
(112) and (121) Bragg reections of the tetragonal phase of ZrO2
(JCPDS card no. 50-1089). As for the domain size of ZrO2 crys-
tallites, it is difficult to estimate from the peak-width analysis of
X-ray diffractions because of the low intensity of the peaks.
However, their broadness obviously indicates crystallite size in
the nanoscale range; typically below 30 nm.22 We did not
observed any peak in the XRD patterns that could be assigned to
the crystalline phase of H3PW12O40 and H4SiW12O40
compounds. This observation supports the notion that both
PTA and STA heteropoly acids are homogeneously dispersed in
mesoporous structures. However, given that the lower limit of
detection of powder XRD is approximately 3–5 wt%, the pres-
ence of tiny amounts of nanometer-sized POM aggregates
within the samples cannot be preclude.
Results and discussion
Structural characterization
X-ray diffraction (XRD) and transmission electron microscopy
(TEM) experiments were performed to investigate the long-
range order of the pore structure of ZrPTA(w) and ZrSTA(w)
materials. As revealed by low-angle XRD (Fig. 1), the meso-
porous zirconia (meso-ZrO2) and composite ZrPTA(w) and
ZrSTA(w) samples show a relatively broad but well-dened
diffraction peak at low 2q scattering angles of 1–3ꢁ, which
according to transmission electron microscopy could be
assigned to the (100) Bragg reection of hexagonal p6mm
structure. The low intensity of this diffraction peak for the
composite samples with low loading amount of POMs (2 and
5 wt%) although suggests mesoscopic order, but with a small
coherent domain size. On the basis of the pore symmetry and
XRD data, it is possible to calculate the lattice constant (a0) of
hexagonal motif from the d-spacing (a0¼(2/O3)d100) of the (100)
diffraction. The results are given in Table 1.
Direct examination of mesoporous ZrSTA(5) and ZrSTA(10)
samples with TEM reveals extended mesoscopic pore ordering
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Fig. 3a and c). According to the wide-angle XRD data, the elec-
tron diffraction ring could be tentatively assigned to the over-
lapped pair of (101) and (002) reections of ZrO2 with tetragonal
structure. Energy dispersive X-ray spectroscopy (EDS) acquired
on TEM conrms the chemical composition of the as-prepared
materials. As shown in Fig 3b and d (insets), the EDS spectra
taken across a thin area of mesoporous structures indicate an
average atomic ratio of Zr : W close to 97.3 : 2.7 and 94.8 : 5.2
for ZrSTA(5) and ZrSTA(10), respectively. In excellent agreement
with the stoichiometry of reactions, these atomic ratios corre-
spond to a weight percentage of STA content at ꢂ5.3 wt% for
ZrSTA(5) and ꢂ10.1 wt% for ZrSTA(10). These results give
evidence of the molecular dispersion and complete incorpora-
tion of heteropoly clusters into the mesoporous ZrO2 structure.
The Fourier transform-infrared (FT-IR) spectra of meso-
porous ZrPTA(w) and ZrSTA(w) materials show a series of
absorption peaks in the 700–1100 cmꢀ1 region that are similar
to the spectra of pure heteropoly acids, i.e. H3PW12O40 and
H4SiW12O40. The IR spectra of high PTA-loaded ZrPTA(w) (w ¼ 5,
10 and 20 wt%) samples show the characteristic W]Od and W–
Ob–W (i.e., between the corner-sharing WO6 octahedral)
absorption bands of 12-phosphotungstic clusters at ꢂ949–952
and ꢂ865–872 cmꢀ1, respectively, consistent with the presence
of these clusters in composite materials (Fig. 4a).23 These
vibration bands are rather obscure on mesoporous ZrPTA(2)
due to the low content (2 wt%) of PTA clusters. In comparison
with the bulk PTA, the absorption peaks of W]Od and W–Ob–W
vibrations are remarkable red-shied (by over 30 cmꢀ1), prob-
ably due to the strong conjugated interactions between the
terminal (W]Od) and bridged (W–Ob–W) oxygen atoms of
(PW12O40)3ꢀ anions and zirconia matrix. The absorptions at
ꢂ1083 and ꢂ1026 cmꢀ1 can be assigned as P–Oa stretching
vibrational bands of the distorted PO4 central units in
[PW12O40]3ꢀ and lacunary [PW11O39]8ꢀ Keggin clusters, respec-
tively.24 Given the relative strong basic character of zirconia
surface, the formation of lacunary Keggin structures in these
samples is a distinct possibility. Correspondently, the IR
Fig. 2 Wide-angle XRD patterns of mesoporous (a) ZrPTA(w) and (b)
ZrSTA(w) composite materials. The XRD pattern of mesoporous
zirconia (meso-ZrO2) is also given in panel (a).
Fig. 3 Representative TEM images and the corresponding SAED
patterns (panel a and c, inset) taken along a thin area of the meso-
porous (a and b) ZrSTA(5) and (c and d) ZrSTA(10) samples. The insets
of panel (b) and (d) show the corresponding EDS spectra obtained on
TEM. The copper peaks result from the TEM grid.
with uniform pores. As seen in Fig. 3, the ZrSTA(5) shows a
poorly ordered, wormhole-like, pore structure with regular
pores, while the ZrSTA(10) shows discernible domains of
hexagonally arranged cylindrical pore channels, suggesting
p6mm pore structure; although small deformations of the
hexagonal honeycomb structure can be observed. Detailed
analysis of TEM images indicated that both samples have an
average pore-to-pore distance of ꢂ5 nm and pore diameter
of ꢂ2 nm. This gives estimation for the framework wall thick-
ness of about 3 nm, which is in good agreement with the results
of the XRD and N2 porosimetry (see below). Furthermore, apart
from XRD, the structure of crystalline phase was investigated by
selected-area electron diffraction (SAED). SAED characterization
suggests the poorly crystalline nature of zirconia, showing a
Fig. 4 (a) Infrared spectra of mesoporous zirconia (meso-ZrO2) and
ZrPTA(w) composite materials and 12-phosphotungstic acid (PTA)
compound. (b) Infrared spectra of mesoporous ZrSTA(w) composites
broad and diffusive Debye–Scherrer diffraction ring (inset of and 12-silicotungstic acid (STA) compound.
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analysis of ZrSTA(w) showed absorption peaks at ꢂ1024, ꢂ952 [PW12O40]3ꢀ and [SiW12O40]4ꢀ anions are possibly engaged in
and ꢂ870 cmꢀ1, which are reasonably assigned to the Si–Oa, specic van der Waals and/or hydrogen bonding interactions with
W]Od and W–Ob–W stretching vibrations of the [SiW12O40]4ꢀ the surfactant micelles. Therefore,
a ZrO2–POM/surfactant
Keggin cluster, respectively (Fig. 4b).25 The remarkable red shi composite phase can be stabilized, leading in self-assembly of
of the W]Od and W–Ob–W absorption bands (ca. 15–30 cmꢀ1
these hybrid complexes into three-dimensional ZrO2–POM mes-
)
compared with those for pure STA, suggests that these clusters ostructures with long-range order. This assumption is also
may also engages in strong interactions with the Zr–OH species consistent with the structural deformation in low POM-loaded
through the W]Od terminal and W–Ob–W bridging bonds. The ZrPTA(w) and ZrPTA(w) (w ¼ 2 and 5 wt%) samples recommended
strong absorptions at ꢂ756 cmꢀ1 and ꢂ623 cmꢀ1 are attributed by XRD and TEM results. To acquire more information of the
to the stretching vibrations of Zr–O and Zr–O–Zr of tetragonal porous structure of composite materials, the Dubinin–Radush-
zirconia, respectively.26
kevich (DR) micropore analysis was applied in the low relative
The nanoporosity of the ZrPTA(w) and ZrSTA(w) hetero- pressure region (p/po < 0.15). By means of this method, the
structures was determined by nitrogen physisorption micropore volume is estimated to be in range of 0.05–0.07 cm3
measurements. As shown in Fig. 5, all samples display a gꢀ1 that is coincided with a structural microporosity of about 50
combination of type I and type IV adsorption isotherms asso- to 60%. These data, along with the type-I shape of N2 adsorptions,
ciated with a distinct condensation step at relative pressure readily conrm to the presence of interconnected pore network
(p/po) ꢂ0.15–0.2, characteristic of mesoporous solids with constructed from both mesopore and micropore channels. Table
uniform pores.27 Analysis of the adsorption data with the Bru- 1 summarizes all the textural parameters of mesoporous zirconia
nauer–Emmett–Teller (BET) method indicated large surface (meso-ZrO2) and ZrPTA(w) and ZrSTA(w) composite materials.
areas in the range of 126–229 m2 gꢀ1 and total pore volumes in
The pore size distribution calculated from the adsorption data
range of 0.07–0.12 cm3 gꢀ1. It can be observed that the surface based on the non-local density function theory (NLDFT) is quite
area of these materials is strongly related to the POM loading narrow, indicating a uniform pore structure with an average pore
and it increases as the POM fraction in the pore walls increases, diameter of ꢂ2.2–2.6 nm. Since the mesoporous materials with
see Table 1. This is quite peculiar considering the heavy different POM content show similar pore size, the continuous
elements of heteropoly acids (W) that composed the frame- increase in unit cell size (ao) of ZrPTA(w) and ZrSTA(w) samples
works. It is worth noting that mesoporous ZrO2 exhibits a implies an relative increase of framework wall thickness. From a
slightly lower BET surface area (118 m2 gꢀ1) and pore volume combination of NLDFT pore sizes and XRD lattice constants, we
(0.06 cm3 gꢀ1), see ESI, Fig. S1.† This signies the role of het- obtained a pore wall thickness from 2.1 to 3.3 nm for ZrPTA(w)
eropoly acids in self-assembly processes, in which the and from 2.2 to 3.6 nm for ZrSTA(w). Based on similar calcula-
tions, the framework wall thickness of mesoporous zirconia
(meso-ZrO2) was estimated to be ca. 1.8 nm. The increase of the
wall thickness with subsequent increase of the POM loading is
consistent with the integration of these heteropoly acids into the
framework of composite materials. This is in agreement with the
XRD and TEM-EDS data.
Catalytic oxidation
The catalytic activity of mesoporous ZrPTA(w) and ZrSTA(w)
composites was evaluated in oxidation of 1,1-diphenyl-2-
methypropene (1) as a test reaction,19 using hydrogen peroxide
as the oxidant (Scheme 1). The selective oxidation of alkene (1)
under mild conditions is a challenging issue in synthesis of
organic ne chemicals.28 All reactions were performed in
ethanol that gave the best results in terms of catalytic activity.
The optimized reaction conditions was obtained by using
ZrSTA(5) catalyst for the oxidation of 1 in different media. As
seen in supporting Fig. S2,† the conversion of 1 can be improved
when the reaction performed in ethanol (>99%), while oxidation
of 1 in other solvents such as methanol and acetonitrile
although proceeded fast, but in lower conversion yields (i.e.
93% and 87%, respectively). Notably, the reaction proceeds at a
negligible rate (<2% conversion) in toluene, probably due to the
low solubility of H2O2 and hydrophilic character of the catalyst.
Next, efforts were directed to investigate the catalytic
Fig. 5 N2 adsorption–desorption isotherms at 77 K for mesoporous
performance of composite catalysts featuring different POM
loading. The oxidations of 1 using hydrogen peroxide as oxidant
ZrPTA(w) and ZrSTA(w) materials. Insets: the corresponding NLDFT
pore size distributions.
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Table 2 Selective oxidation of 1 with H2O2 catalyzed by mesoporous
zirconia (meso-ZrO2) and ZrPTA(w) and ZrSTA(w) composites as well
as homogeneous PTA and STA compoundsa
Conversionb
(%)
Relative yieldc
(1a : 1b : 1c : 1d)
Kinetic constant
Catalyst
(k) (hꢀ1
)
Scheme 1 Oxidation of 1 over mesoporous ZrPTA(w) and ZrSTA(w)
composite catalysts.
meso-ZrO2
ZrPTA(2)
ZrPTA(5)
ZrPTA(10)
ZrPTA(20)
PTA
ZrSTA(2)
ZrSTA(5)
ZrSTA(10)
ZrSTA(20)
STA
25
78
87
38
5
<1
65
>99
40
8
10 : 15 : 59 : 16
6 : 13 : 58 : 23
6 : 29 : 41 : 24
7 : 32 : 37 : 24
12 : 32 : 36 : 20
0.010
0.048
0.064
0.012
0.001
was monitored by CG-MS analysis and the results are shown in
Fig. 6 and S3 of the ESI.† It is evident that the catalytic activity
for both ZrPTA(w) and ZrSTA(w) composites is strongly related
to the amount of the incorporated POM clusters. Namely, the
2–5 wt% POM-loaded samples showed a moderate-to-high
conversion of 1, while composite materials containing large
amount of heteropoly acids (10–20 wt%) exhibited a dramatic
drop in conversion yield. It is noteworthy that employing pure
mesoporous zirconia and heteropoly acids as catalysts gave
poor activity under similar reaction conditions (ꢂ25% and less
than 2% in 30 min, respectively). Based on the 1 consumption,
the ZrSTA(5) catalyst achieves the highest activity, giving >99%
conversion of oxidative products in only 30 min. Meanwhile,
control experiment showed that in the absence of catalyst no
reaction takes places. The present data suggest that, apart to the
surface area, the elemental PTA/ZrO2 and STA/ZrO2 composi-
tion has a profound inuence on the catalytic oxidation
processes. It seems that ZrSTA(w) and ZrPTA(w) are really
synergistic catalysts, wherein Keggin anions and Zr(IV) oxo-
hydroxide (ZrOx(OH)y) species potentially act as cooperative
catalytic sites for the activation of C]C bonds of 1 substrate.
6 : 14 : 56 : 24
8 : 14 : 48 : 30
6 : 25 : 48 : 21
12 : 25 : 41 : 22
0.041
0.168
0.017
0.003
<2
a
Reaction conditions: 0.1 mmol of 1, 0.5 mmol of catalyst, 0.3 mmol of
b
H2O2 (30 wt% in water), 3 mL ethanol 50 ꢁC, 30 min. Based on the
c
consumption of alkene 1 measured by GC. Products relative yield
determined by GC-MS using nonane as internal standard.
This is a different from the case of pure ZrO2 and hetero-
polytungstic acids.
Table 2 summarizes the results of the catalytic oxidation of
1 on heterogeneous ZrPTA(w) and ZrSTA(w) as well as homoge-
neous PTA and STA catalysts. In all cases the corresponding
epoxide 1b and ketone 1c were formed as major oxidative prod-
ucts, accompanying with small amount of the ketone 1a and diol
1d. Benzophenone 1a is a cleavage product of the corresponding
1,2-dioxitane alkene, which is a non stable intermediate.19 While,
a notable amount of ketone 1c should be formed through 1,2-
phenyl shi29 of the corresponding diol 1d, as veried by GC-MS
and H1 NMR measurements,30 as well as by control catalytic
experiments.31 As seen in Table 2, all the ZrPTA(w) and ZrSTA(w)
catalysts promote the oxidation of 1 alkene with moderate-to-
good chemoselectivity (36–58%) for 1c.
The time evolution of 1 conversion can be reasonably
accounted by a pseudo-rst order reaction model. This is
reasonable if we account the excess of H2O2, so that its concen-
tration could be considered constant during the reaction. The
Fig. 7 shows the plots of ln(Ct/Co) (where, Co is the concentrations
of 1 at the initial state of reaction and Ct is the concentration at
the time t) as a function of time, by which the reaction rate
constants (k) as a slope of the linear ts were obtained. The
corresponding k values, shown in Table 2, indicate higher reac-
tion rate on ZrSTA(5) compared to the other catalysts, despite the
less ordered mesostructure and relative low surface area. Notably,
the catalytic oxidation rate over ZrSTA(5) is about 17 times greater
than mesoporous ZrO2 sample (0.168 hꢀ1 vs. 0.010 hꢀ1) under
similar conditions. The enhancement of catalytic activity over
ZrSTA(5) can be regarded as a result of synergistic interactions
between the [SiW12O40]4ꢀ anions and ZrO2 matrix, as previously
discussed. Nevertheless, the well dispersion of POM units and
the open-framework structure are important virtues, which may
also account for the high reactivity of these materials.
Fig. 6 Oxidation of 1 catalyzed by heterogeneous mesoporous ZrO2
(meso-ZrO2), ZrPTA(w) and ZrSTA(w) materials and homogeneous PTA
(H3PW12O40) and STA (H4SiW12O40) compounds. Experimental
The ZrPTA(5) catalyst is stable and it can be reused for the next
catalytic run without signicant loss in catalytic activity. Aer
conditions: 0.1 mmol 1, 0.5 mmol catalyst, 3 equiv. H2O2 (30% in
water), 3 mL ethanol, 50 ꢁC, 30 min.
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12-phosphotungstic (PTA) or 12-silicotungstic (STA) acid
compounds through a surfactant-templating approach. The as-
prepared materials possess ZrO2–PTA and ZrO2–STA framework
composition with different POM loadings, i.e. ranging from 2 to
20 wt%. Characterization by TEM and N2 physisorption showed
a mesoporous structure with large internal surface area (126–
229 m2 gꢀ1) and narrow-sized mesopores (ca. 2.2–2.6 nm).
Moreover, XRD, EDS and IR measurements indicated that the
Keggin structure of the incorporated clusters was preserved
intact into the mesoporous frameworks, forming rather a solid
solution with the ZrO2 species. Although ZrO2 and heteropoly
acids alone have little catalytic activity, ZrPTA(w) and ZrSTA(w)
composites exhibit excellent activity in 1,1-diphenyl-2-methy-
propene oxidation, using hydrogen peroxide as oxidant. Unlike
the specic surface area, the framework composition of these
materials has a dominant effect on the catalytic performance,
indicating higher catalytic activity for ZrSTA(w) composite
sample loaded with 5 wt% of STA clusters. Remarkably, the
ZrSTA(5) showed much higher activity than mesoporous
zirconia and 12-silicotungstic acids alone, which is reected in
the about 17-fold increase of the conversion rate compared to
mesoporous ZrO2; the homogeneous STA catalyst showed
negligible catalytic activity under similar conditions. Further-
more, the ZrSTA(5) composite demonstrated very good dura-
bility and recyclability without decomposition of STA clusters
and degradation of mesostructure. These results suggest that
these complex materials should have great application potential
in oxidation catalysis, including size-selective oxidation of
alkenes.
Fig. 7 Kinetic analysis of the 1,1-diphenyl-2-methylpropene oxidation
over heterogeneous mesoporous zirconia (meso-ZrO2), ZrPTA(w) and
ZrSTA(w) and homogeneous PTA (H3PW12O40) and STA (H4SiW12O40
compounds. The corresponding red lines are fit to the data.
)
catalysis, the solid ZrPTA(5) was separated from the reaction
mixture by ltration, washed with fresh ethanol several times and
dried at ambient conditions, and used for the next catalytic cycle.
The results in Fig. S4 of the ESI,† show that the consumption of 1
remains as high as ꢂ96% and ꢂ99% aer 30 min and 2 h
reaction time, respectively, i.e. very close to the conversion levels
achieved by the fresh catalyst. Nitrogen physisorption measure-
ments on reused sample gave no sings of decomposition of
mesostructure, indicating surface area of 149 m2 gꢀ1 and total
pore volume of 0.08 cm3 gꢀ1 (Fig. S5†). Moreover, elemental EDS
microanalysis and IR spectroscopy showed no change in the
chemical composition and Keggin structure of the incorporated
[SiW12O40]4ꢀ clusters of reused sample, thereby, conrming high
durability of recyclability. In particular, the EDS spectra showed
an average Zr/W atomic ratio of ꢂ97.3 : 2.7, which corresponds to
a STA content of ꢂ5.1 wt%, in accordance with the nominal
composition of the as-prepared catalyst, see ESI, Fig. S6.† The IR
spectrum displayed the characteristic Si–Oa, W]Od and W–Ob–
W vibration bands at ꢂ1024, ꢂ941 and ꢂ882 cmꢀ1 respectively,
reecting the Keggin structure of the 12-silicotungstic acids
(Fig. S7 of the ESI†). Nevertheless, minimal decomposition or
leaching of incorporated STA clusters (on the basis of EDS and IR
analysis limitations) by the excess of H2O2 cannot be excluded.32
Acknowledgements
Financial support by the Greek Ministry of Education under
Excellence grant (ARISTEIA-2691) and THALES project (MIS
377064) are kindly acknowledged.
Notes and references
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performed the catalytic reaction using 0.1 mmol of 1, 0.5
mmol of ZrSTA(5) catalyst (containing 2 mol% STA) and 3
equiv. H2O2 (30% in water) in 3 mL ethanol for 20 min.
Aer this, the reaction mixture was ltrated over a Celite-
silica pad to remove H2O2. The ltrate (containing the
oxidative products 1a, 1b, 1c and 1d), was then subject to
reaction at 50 ꢁC for another 2 h (with the addition of
0.5 mmol of catalyst) and the product ratios were analyzed
by GC-MS. No signicant changes were observed for the
amounts of 1a and 1b, while an increase of the 1c/1d
product yield ratio from 1.7 to 3.2 was obtained.
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