Keggin heteropolyacids supported on TiO2 used in gas-solid (photo)catalytic propene hydration and in liquid-solid photocatalytic glycerol dehydration

Article abstract of DOI:10.1016/j.cattod.2016.04.037

(Photo)catalytic propene hydration to 2-propanol and glycerol dehydration to acrolein were carried out by using Keggin heteropolyacids (HPAs) supported on TiO₂. Binary materials have been prepared by impregnation of H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀ and H₄SiW₁₂O₄₀, on TiO₂ Evonik P25. Moreover, a binary material consisting of H₄SiW₁₂O₄₀ and TiO₂ was prepared via a hydrothermal treatment and tested for the same reactions. All the materials were characterized by X-ray diffraction (XRD), scanning electron microscopy observations (SEM) coupled with energy dispersive X-ray (EDX) measurements, diffuse reflectance spectroscopy (DRS), Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The supported Keggin HPA species played a key role both for the catalytic and for the photo-assisted catalytic reactions due to their strong acidity and ability to form strong oxidant species under UV irradiation.

Full text of DOI:10.1016/j.cattod.2016.04.037

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Keggin heteropolyacids supported on TiO2 used in gas-solid
(photo)catalytic propene hydration and in liquid-solid photocatalytic
glycerol dehydration
Giuseppe Marcìa,∗, Elisa García-Lópeza, Vincenzo Vaianob, Giuseppe Sarnob,
Diana Sanninob, Leonardo Palmisanoa
a “Schiavello-Grillone” Photocatalysis Group, Dipartimento di Energia, Ingegneria dell’informazione e modelli Matematici (DEIM), Università di Palermo,
Viale delle Scienze, 90128 Palermo, Italy
b Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 84084 Fisciano, Salerno, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
(Photo)catalytic propene hydration to 2-propanol and glycerol dehydration to acrolein were carried out
by using Keggin heteropolyacids (HPAs) supported on TiO2. Binary materials have been prepared by
impregnation of H3PW12O40, H3PMo12O40 and H4SiW12O40, on TiO2 Evonik P25. Moreover, a binary mate-
rial consisting of H4SiW12O40 and TiO2 was prepared via a hydrothermal treatment and tested for the same
reactions. All the materials were characterized by X-ray diffraction (XRD), scanning electron microscopy
observations (SEM) coupled with energy dispersive X-ray (EDX) measurements, diffuse reflectance spec-
troscopy (DRS), Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The supported
Keggin HPA species played a key role both for the catalytic and for the photo-assisted catalytic reactions
due to their strong acidity and ability to form strong oxidant species under UV irradiation.
© 2016 Elsevier B.V. All rights reserved.
Received 9 February 2016
Received in revised form 10 April 2016
Accepted 30 April 2016
Available online xxx
Keywords:
Heteropolyacid
Propene
Glycerol
Photocatalysis
Polyoxometalate
1. Introduction
because their ground electronic state (the solubilized HPA) absorbs
light producing a charge transfer-excited state HPA*. HPA materials,
Polyoxometalates (POMs) are a wide class of discrete nano-
sized transition metal-oxygen clusters that can be divided in three
classes: heteropolyanions (HPA), isopolyanions (IPA) and Mo-blue
and Mo-brown reduced HPA centers [1]. The most explored POM
materials are the HPAs and it is convenient to classify them starting
from the symmetrical ‘parent’ polyanion, for instance the Keggin
when excited by light to HPA*, act as better oxidant and reduc-
tant species with respect to the corresponding ground states [4].
Indeed, HPA* can easily become HPA−, a “heteropolyblue” specie
by means of one (or more) electron transfer from other species [5].
Heteropolyblue species HPA− are relatively stable and are readily
re-oxidized. Enhanced degradation of organic compounds in the
UV/TiO2 process has been reported in the presence of supported
Keggin-type HPAs [6]. In a previous paper the catalytic and catalytic
photo-assisted activity of H3PW12O40 supported on TiO2, ZrO2 or
WO3 was studied and a beneficial role of the photo-catalytically
active support on the reaction rate was reported [7]. The signifi-
cant increase of reactivity was justified by considering the ability
of the semiconductor to transfer electrons from the conduction
band to the activated HPA* species. TiO2, in fact, directly transfers
photo-generated electrons from its conduction band to the interfa-
cial HPA with empty d orbitals and consequently the electron-hole
charge-pairs recombination is delayed [8]. Moreover, the disper-
sion of HPAs on solid supports with high surface area, is generally
useful for improving the accessibility to their acid sites and the
(photo)catalytic activity increases.
or Wells-Dawson structures among others [2]. The Keggin anion
{XM12O40
}
y− contains a heteroatom X in the XO4y− centre as PO4
3−
3−
or SiO4 and the so-called addenda atoms, commonly W or Mo.
3−
For instance, the structure of the PW12O40
anion consists of
a PO4 tetrahedron surrounded by four W3O9 groups formed by
edge sharing octhaedra. This cluster has a diameter of ca. 1.2 nm
[3]. The heteropolyacids with the Keggin structure are strongly
acidic and are remarkably stable particularly when deposited onto
an oxide surface. They are widely used in catalysis and homoge-
neous photocatalysis because are very soluble in polar solvents.
Heteropolyacids have been used as homogeneous photocatalysts
∗
Corresponding author.
E-mail address: giuseppe.marci@unipa.it (G. Marcì).
http://dx.doi.org/10.1016/j.cattod.2016.04.037
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: G. Marcì, et al., Keggin heteropolyacids supported on TiO2 used in gas-solid (photo)catalytic propene
hydration and in liquid-solid photocatalytic glycerol dehydration, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.037

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In this paper catalytic photo-assisted activities of H3PW12O40
,
Tsukuda et al. investigated the production of acrolein from glyc-
erol over silica-supported heteropoly acids [17]. In that study,
silicotungstic acid supported on silica with mesopores of 10 nm
showed the highest catalytic activity with the acrolein selectivity
<85 mol% at 275 ◦C and at ambient pressure.
Shen et al. studied the catalytic activities of silicotungstic,
phosphotungstic, and phosphomolybdic acids in the liquid phase
dehydration of glycerol to acrolein in a semi-batch reactor [18].
Silicotungstic acid exhibited high catalytic activity and the maxi-
mum yield of 78.6% was achieved when glycerol was completely
converted at the reaction temperature of 300 ◦C with the mole
ratio of silicotungstic acid to glycerol of 1·10−4:1. The order
of catalytic activities toward the formation of acrolein was sil-
icotungstic acid > phosphotungstic acid > phosphomolybdic acid,
revealing that the reaction was affected by the acidity and the sta-
bility of the heteropolyacids. Hydroxyacetone and acetic acid were
also detected with yields of less than 10%, respectively.
Liu et al. studied Al2O3 supported silicotungstic acid
(H4SiW12O40/Al2O3) samples prepared by impregnation and
calcined at 350, 450, 550, 650 ◦C to study the structural evolution
of the H4SiW12O40 heteropolyacid and its effect on the catalytic
performance during glycerol conversion to acrolein in gas-solid
regime at 300 ◦C [19]. The glycerol conversion increased with acid
center concentration under the specified reaction condition. As
well, selectivity to acrolein increased with Brønsted/Lewis acid
ratio, suggesting the crucial role of Brønsted acid sites.
In this paper some bulk and surface properties of three het-
eropolyacid clusters supported on TiO2 materials have been studied
along with their use as heterogeneous photocatalysts in two reac-
tions, i.e. the propene hydration and the glycerol dehydration. The
investigation focuses how the (photo)reactivity can depend on the
type of HPA but also how the support-heteropolyacid interaction
can be important. As far as the glycerol dehydration is concerned,
it is worth nothing that, contrarily to what reported in studies on
thermal catalysis, in our UV irradiated system the reaction pro-
ceeded at very low temperature (35 ◦C).
H3PMo12O40 and H4SiW12O40 supported on TiO2 have been inves-
tigated for two kinds of reactions: i.e. the propene hydration
and the glycerol dehydration in gas-solid and in liquid solid reg-
imens, respectively. These reactions are of great interest from
the industrial point of view, particularly when carried out under
experimental mild conditions. The industrial propene hydra-
tion to 2-propanol is carried out at moderate temperatures (ca.
150–200 ◦C) and under pressure (2 MPa) in the presence of phos-
phoric acid supported on silica, strong acid resins or zeolites but
also heteropolyacids are industrially used for this reaction [9]. Con-
versely, the dehydration of 2-propanol to form propene is accepted
to be a good reaction to probe the acid character of a catalyst [10]. In
a gas-solid regime propene and/or propanone are formed at ambi-
ent pressure and moderate temperature (in the range 140–325 ◦C).
The selectivity to propene or propanone strongly depends on the
acidity-basicity of the solid catalyst. The propene hydration is not
an easy reaction to be carried out, because thermodynamically lim-
ited by the reverse reaction at high temperature. Intensive research
efforts have been also devoted to find new outlets for the high
amount of glycerol produced from the biodiesel process [11]. The
finding of novel conversion processes to transform glycerol into
other chemicals includes scientific efforts in the glycerol dehydra-
tion to form acrolein. 3-hydroxypropionaldehyde is obtained after
the direct glycerol loss of one water molecule. A further loss of
a water molecule gives rise to acrolein. Acrolein is an important
chemical used as a feedstock to produce acrylic acid, pharmaceu-
ticals or to treat fibers, and it is also an herbicide which controls
the growth of aquatic plants. It is currently produced by oxidation
of propene by heterogeneous catalysis. A maximum yield of 80% at
high propene conversion (90–95%) is reached by using commercial
catalysts based on Bismuth molybdates [12]. Synthesis of acrolein
from glycerol has been widely investigated since the 1960’s but
it has been mainly reported in patent literature. A strong catalyst
acidity seems to be necessary to perform the dehydration of glyc-
erol to acrolein in gas phase in the presence of zeolite [13] or in
supercritical water [14] by using various solid acid catalysts includ-
ing sulphates. Chai et al. reported the gas-phase dehydration of
glycerol to produce acrolein at 315 ◦C by using Nb2O5 [15,16]. Glyc-
erol conversion and acrolein selectivity of the Nb2O5 catalysts were
dependent on the fraction of strong acid sites (−8.2 ≤ H0 ≤ −3.0),
where H0 corresponds to the Hammet factor. The amorphous
catalyst prepared by calcination at 400 ◦C, having the highest frac-
tion of acid sites at −8.2 ≤ H0 ≤ −3.0, showed the highest mass
specific activity and acrolein selectivity (51 mol%). The other sam-
ples, having a higher fraction of either stronger (H0 ≤ −8.2) or
weaker acid sites (−3.0 ≤ H0 ≤ 6.8), were less effective for glycerol
dehydration and formation of the desired acrolein. The highest
selectivity was ca. 70 mol%. The catalysts having further stronger
acid sites (H0 ≤ −8.2) produce a lower selectivity (40–50 mol%) due
to a more severe catalyst coking. Authors evidenced that Brön-
sted acid sites presented better performance than Lewis acid sites.
Chai et al. proposed a mechanism where the reaction was initi-
ated by the dehydration involving either the central or a terminal
OH of glycerol which result in the parallel formation of two enol
intermediates. The enols would undergo rapid rearrangement to 3-
hydroxypropionaldehyde or 1-hydroxyacetone, respectively. The
3-hydroxypropionaldehyde would be very unstable at the reaction
temperature (315 ◦C) and it can easily give rise to a further dehydra-
tion to acrolein. A secondary hydrogenation reaction of the acrolein
product will lead to the formation of allyl alcohol. The unstable
intermediate 3-hydroxypropionaldehyde would also decompose
to acetaldehyde and formaldehyde, according to a reversed aldol
condensation; a follow up hydrogenation or decomposition of
formaldehyde would result in the formation of methanol or CO and
H2.
2. Experimental
2.1. Preparation of the binary HPA/TiO2 samples
The binary materials HPA/TiO2 were prepared by impregnat-
ing the support (commercial TiO2 Evonik P25) with an aqueous
solution containing the desired amount of the commercial HPA,
i.e. H3PW12O40, H3PMo12O40 or H4SiW12O40 (Aldrich) denoted
hereafter as PW12, PMo12 and SiW12, respectively. During the
preparation, each suspension was stirred at 50 ◦C for 1 h, and then
water was evaporated until dryness. The binary materials prepared
as above described are denoted hereafter as PW12/TiO2, PMo12/TiO2
and SiW12/TiO2. The HPA amount deposited was 50% in weight with
respect to the support corresponding to a molar percentage of 14%
of W and 86% of Ti (with respect to the sum W + Ti) in PW12/TiO2
and SiW12/TiO2 samples and 20% of Mo and 80% of Ti (with respect
to the sum Mo + Ti) in PMo12/TiO2 sample.
The theoretical coverage of the support has been calculated by
taking into account that the diameter of each anionic HPA cluster
is equal to ca. 10 Å and, consequently, by considering a roundish
shape of the heteropolyanion, its surface area results ca. 78.5 Å2
[3].
An alternative binary material has been prepared starting from
a selected HPA (SiW12) and a TiO2 precursor, by using the fol-
lowing methodology. A solution was prepared by adding 7.5 ml of
titanium isopropoxide (TTIP) to 50 ml of 2-propanol, hence 6 ml
of HNO3 0.05 M and the appropriate amount of SiW12 (0.5440 g)
were added under vigorous stirring to the Ti containing solu-
Please cite this article in press as: G. Marcì, et al., Keggin heteropolyacids supported on TiO2 used in gas-solid (photo)catalytic propene
hydration and in liquid-solid photocatalytic glycerol dehydration, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.037

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3
tion. A gel was formed and it was successively introduced in an
autoclave internally covered by Teflon. The tightly closed system
was heated for 48 h at 200 ◦C, achieving a final pressure of ca.
10 bar. The solid collected from the autoclave was washed four
times with hot water, dried and annealed at 110 ◦C for 12 h. The
molar ratios of the reagents TTIP:2-propanol:HNO3:H2O:HPA were
155:4000:18:2000:1, corresponding to a molar percentage of 8%
of W and 92% of Ti (with respect to the sum W + Ti). This mate-
rial is denoted as SiW12/TiO2-H where H means hydrothermal.
The reason to prepare a selected HPA supported material under
hydrothermal conditions was justified because leaching of HPA
(from sample obtained by simple impregnation) has been often
observed, due to the high solubility of HPA in polar reaction media.
A markedly increased stability of the hydrothermally prepared
binary materials HPA-TiO2, instead, has been reported by Guo and
Hu [20], Ma et al. [21] and Sivakumar et al. [22]. For the sake of
comparison a TiO2 bare sample (named TiO2-H) was also prepared
by means of the above hydrothermal method.
photoreactor (the fixed bed height was ca. 0.3 mm). The gas feeding
the photoreactor consisted of propene and water with molar con-
centrations of ca. 40 mM and ca. 2 mM, respectively. A mass flow
controller allowed to feed gaseous propene, whereas water was
mixed with the propene stream by means of a home made infusion
pump. A porous glass septum allowed to distribute homogeneously
the inlet gaseous mixture. The flow rate of the gaseous stream for
the catalytic and catalytic photo-assisted runs was 20 cm3 min−1
.
All of the runs were carried out at atmospheric pressure. The reactor
and the pipes of the set-up to and from the reactor were heated by
an electric resistance and K-type thermocouples allowed to mon-
itor the temperature in the whole system. The temperature inside
the (photo)-reactor was maintained constant at 85 ◦C for both
catalytic and photocatalytic experiments. For the catalytic photo-
assisted runs the reactor was illuminated from the top with an UV
LED IRIS 40 with an irradiation peak centered at 365 nm. The irradi-
ance reaching the photoreactor, measured in the range 300–400 nm
with a UVX Digital radiometer, was equal to 45 mW cm−2. The runs
lasted ca. 5 h and samples of the reacting fluid were analysed by the
same gas chromatograph above described.
2.2. Characterization of the photocatalysts
Determination of BET specific surface area (SSA) was performed
by using a Flowsorb 2300 (Micromeritics). The crystalline phase of
the prepared materials was determined at room temperature by
powder X-ray diffraction analysis (PXRD) carried out by using a
Panalytical Empyrean, equipped with CuKa radiation and PixCel1D
(tm) detector. Scanning electron microscopy observations (SEM)
were carried out by using a FEI Quanta 200 F ESEM microscope,
operating at 30 kV on specimens coated with a layer of gold. More-
over, an electron microprobe used in an energy dispersive mode
(EDX) was employed to obtain information on the atomic content
of W or Mo and Ti on the samples. Raman spectra were recorded
on pure powdered samples packed into sample cups. Spectra were
recorded by a Reinshaw in-via Raman spectrometer equipped with
an integrated microscope and with a charged-coupled device (CCD)
camera. A He/Ne laser operating at 632.8 nm was used as the excit-
ing source. Diffuse reflectance spectra (DRS) were recorded in the
range 250–600 nm by using a Shimadzu UV-2401 PC instrument
with BaSO4 as the reference sample. Infrared spectra of the sam-
ples in KBr (Aldrich) pellets were obtained by using a FTIR-8400
Shimadzu spectrometer with 4 cm−1 resolution and 256 scans.
The acidity of the samples were studied by the dehydration
reaction of 2-propanol to propene. To this scope it was used a cylin-
drical Pyrex batch reactor (V = 130 ml, external diameter = 93 mm,
external height = 22 mm) provided with a silicon/teflon septum.
The photocatalyst (0.1 g) was placed on the bottom of the pho-
toreactor and N2 was fluxed for ca. 0.5 h to remove the oxygen.
Subsequently, 10 ␮l of liquid 2-propanol was injected and vapor-
ized into the reactor (2-propanol nominal initial concentration
equal to 1.0 × 10−3 M). After that, the reactor was heated at 80 ◦C
in an oven and the reacting fluid was analysed by withdrawing
gas samples from the photoreactor by means of a gas-tight syringe
(the runs lasted ca. 4 h). Substrate and propene concentrations
were measured by using a GC-17A Shimadzu gas chromatograph
equipped with a methyl siloxane (30 m × 320 ␮m × 0.25 ␮m) HP-1
column kept at 40 ◦C and a FID. The acidity of the samples was
correlated with the amount of propene formed (higher amount
indicating higher acidity).
2.3.2. Photocatalytic glycerol dehydration in liquid-solid regime
The photocatalytic dehydration of glycerol to acrolein was car-
ried out in liquid phase. The experiments were performed by using
a Pyrex cylindrical photoreactor (ID = 2.5 cm) equipped with a He
distributor device (QHe = 200 cm3 min−1, STP) that provided a con-
tinuous flow inside the reactor throughout the runs. A magnetic
stirrer maintained 0.1 g of photocatalyst suspended in 100 ml aque-
ous solution containing 0.4 mol of glycerol (Sigma-Aldrich). The
dosage equal to 1 g L−1 ensured that the overall photocatalysts
particles were effectively irradiated [23]. The suspension was left
under dark for 1 h to reach the adsorption-desorption equilibrium
of glycerol on the photocatalyst surface. Then the photoreactor was
externally irradiated for 65 min by four Philips Black Light UV tubes
(8 W each; emission spectrum centered at 365 nm) and the flux
impinging the external surface of the photoreactor was equal to ca.
51 mW cm−2. The temperature inside the photo-reactor reached
the value of 35 ◦C. The volatile reaction species formed during the
photoreaction in the liquid-solid system were stripped by a He
stream bubbled in the suspension. The composition of the reaction
products in the liquid phase was analysed by gas chromatography
(GC) with a Thermo TR-WaxMS column coupled to a quadrupole
mass detector (Trace MS; ThermoFinnigan). Moreover, the gaseous
products (CO2 and CO) in the He stream from the photoreactor
were also analysed. The temperature ramp for the GC–MS anal-
yses was 50–250 ◦C (10 ◦C min−1) followed by 1 min at isothermal
conditions.
The percentage of glycerol conversion (X) and of acrolein yield
(Y) were used as the parameters to evaluate the photocatalysts
performance. They were calculated according to the following
equations:
nGL,in − nGL,un
X =
Y =
× 100
(1)
(2)
nGL,in
nAC
× 100
nGL,in
The percentage of selectivity (S) to acrolein was also calculated
as follows:
2.3. Reactivity experiments
nAC
S =
× 100
)
(3)
2.3.1. Photocatalytic propene hydration in gas-solid regime
(nGL,in − nGL,un
A cylindrical continuous Pyrex photoreactor horizontally posi-
tioned (diameter: 10 mm, length: 100 mm) was used and it
operated in gas-solid regime. The reactivity runs were carried out
with 0.5 g of powder by simply dispersing it as a thin layer inside the
where nGL,in is the glycerol initial moles, nGL,un the moles of glycerol
unreacted and still present in the reaction medium after the run and
nAC the moles of produced acrolein.
Please cite this article in press as: G. Marcì, et al., Keggin heteropolyacids supported on TiO2 used in gas-solid (photo)catalytic propene
hydration and in liquid-solid photocatalytic glycerol dehydration, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.037

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Table 1
tion peaks attributable to TiO2 and to the heteropolyacid. Fig. 1(B)
shows the diffractograms of the hydrothermically home prepared
bare TiO2 (TiO2-H) and of the composite SiW12/TiO2-H. In both the
diffractograms the presence of anatase phase can be noticed with-
out significant differences and any other peaks ascribable to the
heteropolyacid. This finding, confirmed by SEM study, as reported
in the following, can be explained considering a good dispersion
of the heteropolyacid in the material prepared hydrothermically.
However, the possibility that a fraction of HPA was included in the
core of the TiO2 material during the solvothermal preparation of
the composite, cannot be excluded.
SEM microphotographs of some selected HPA/TiO2 materials are
reported in Fig. 2(a–d). The morphology of the HPA/TiO2 samples
obtained by HPA impregnation is very similar to that of the bare
TiO2 P25 used as support (SEM picture of bare TiO2 P25 has been
already reported in a previous paper [24]), since the agglomerates
of these particles present the same shape and consist of nanopar-
ticles with similar sizes (ca. 50 nm).
BET specific surface areas (SSA) and band gap energies (Egap) of the photocatalysts.
Sample
B.E.T. SSA [m2 g−1
]
Egap [eV]
TiO2-Evonik P25
PW12/TiO2
PMo12/TiO2
SiW12/TiO2
TiO2-H
50
48
46
47
180
177
3.1 and 3.25
3.0
2.5 and 2.9
3.1
3.25
3.2
SiW12/TiO2-H
(A)
10.2
14.5
17.8
34.6
46.1
23.1
20.6
59.9
PW12/TiO2
A
R
TiO2 P25
A
Fig. 2(a) and (b) report pictures of PMo12/TiO2, selected as
representative of the impregnated samples, with two different
magnifications. As far as the SiW12/TiO2-H sample is concerned,
SEM analysis evidenced the nanostructured nature of the material;
indeed it consisted of big agglomerates of particles the dimension of
which ranges between 20 and 60 nm. Moreover, interstitial spaces
(6–10 nm) can be noticed between the particles.
A
A A
R
A
R
R
A, R
PW12
10
20
30
40
50
60
70
80
Table 2 reports the nominal and the average EDX values of
atomic percentage of W or Mo from HPA and the atomic percentage
of Ti from the support, calculated by taking into account the pres-
ence of thirteen water moles per mole of HPA [25]. The amounts
of tungsten or molybdenum from HPA measured by EDX analyses
resulted close to the nominal ones. Moreover a quite homogeneous
distribution of HPA on the TiO2 was evidenced.
2Θ [Degrees]
(B)
SiW12/TiO2-H
A
A
A
A
A
A
A
A
Fig. 3 shows the Raman spectra of bare HPA and supported
samples. The former shows the characteristic bands correspond-
ing to anatase and rutile crystalline phases of the TiO2. Raman
bands at 395, 516 and 637 cm−1 are clearly present in all the sup-
ported materials and assigned to modes of TiO2 anatase [26]. The
rutile phase shows peaks at 444 cm−1 and at 609 cm−1; however
in Fig. 3, the first one is present as a shoulder and the second is
masked by the anatase peak at 637 cm−1. Intense peaks at 142
and 144 cm−1, assigned to rutile and anatase, respectively, are
out of the range of the spectra reported in Fig. 3. For the three
HPA supported samples the characteristic bands attributable to the
vibrational modes of the heteropolyacid cluster are also present.
The bare PW12 Raman spectrum is reported in Fig. 3(A). It exhibits
a strong W O symmetric and asymmetric stretching modes at
1012 and 990 cm−1, respectively. The bands centered at 937 and
TiO2-H
10
20
30
40
50
60
70
80
2Θ [Degrees]
Fig. 1. XRD patterns of the photocatalysts: PW12, TiO2 P25, and PW12/TiO2 (A); TiO2-
H and SiW12/TiO2-H (B).
3. Results and discussion
890 cm−1 are attributed to W
O W asymmetric stretching. The
3.1. Physico-chemical characterization of the materials
weaker Raman bands centered at 530 cm−1 are characteristic of
asymmetric stretching vibration modes of bridging W
O W. The
Table 1 shows the BET specific surface area (SSA) of all the binary
materials along with the commercial support (SSA of all of the
HPAs used was ca. 10 m2 g−1). The binary materials prepared under
hydrothermal conditions showed higher SSA’s. As a general trend,
the surface areas of all of the materials are smaller than those of
the support.
In Fig. 1(A) the XRD diffractograms of PW12, TiO2 P25 and
PW12/TiO2 samples are reported. The heteropolyacid cluster
presents a crystalline structure characterized by several diffraction
peaks. In the diffractogramm of the commercial TiO2 P25, anatase
(A) and rutile (R) polymorphs can be identified. In the compos-
ite material diffraction peaks attributable to TiO2 are present along
with some others corresponding to the heteropolyacid cluster (ver-
tical lines).
group centered at 325 cm−1 are attributed to W O bending [27].
After impregnation on the TiO2, no significant shift was observed
in the PW12/TiO2 sample for the sharper and more intense bands at
1012 and 990 cm−1, whereas the Raman peaks corresponding to the
W
O W asymmetric and symmetric stretching vibration modes
were broadened and became relatively weaker with respect to the
PW12 spectrum. Consequently the resolution between these peaks
decreases. These findings can be attributed to H-bonding interac-
tion between the oxygen atom of the Keggin anion and the hydroxyl
groups on the TiO2 surface, as observed before [28]. The Raman
spectrum of bare PMo12, reported in Fig. 3(B), showed bands at
995 and 983 cm−1 that are assigned to symmetric and asymmetric
stretching of terminal oxygen Mo O. The weak band at 882 cm−1
can be assigned to Mo
band at 603 cm−1 to a combined stretching and bending of the
Mo Mo bonds [29,30]. In Fig. 3(B) the comparison between the
O Mo asymmetric stretching, whereas the
The XRD patterns of the other impregnated samples, not
reported for the sake of brevity, showed also the presence of diffrac-
O
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Fig. 2. SEM pictures of two selected samples, each with two different magnifications: PMo12/TiO2 (a) and (b) and SiW12/TiO2-H (c) and (d).
Table 2
Atomic percentage of Ti and W or Mo (with respect to the sum Ti + W or Ti + Mo), both nominal and measured by EDX analyses and theoretical HPA coverage on the support.
Sample
Ti atomic percentage
nominal
W or Mo atomic percentage
nominal
Theoretical coverage
HPA layers
EDAX
EDAX
PW12/TiO2
86
80
86
92
83
83
83
92
14
20
14
8
17
17
17
8
2.8
4.5
2.8
n.c.a
PMo12/TiO2
SiW12/TiO2
SiW12/TiO2-H
a
Not calculated because the preparation was carried out by using a different method with respect to the other samples reported in Table 2 (see Section 2).
Raman spectrum of the bare PMo12 heteropolyacid cluster with the
PMo12 impregnated on TiO2 evidences, analogously to what occurs
in the case of the PW12, that in the supported material both the
bands of TiO2 and those related to the heteropolyacid are present
without significant shifts with respect to the bare PMo12. Only
an enlargement of the bands due to an interaction between the
cluster and the support is evident. Fig. 3(C) illustrates the differ-
ences between the bare SiW12 cluster and the materials containing
TiO2. The Raman vibrational modes typically assigned to the Keg-
gin anion are those located at 1019, 981, 927, 881 cm−1, attributed
ration of the HPA/TiO2 composites. The arrangement structure of
the PW12 consists of a PO4 tetrahedron surrounded by four W3O9
groups formed by edge sharing octhaedra [33]. This arrangement
gives rise to four stretching bands between 1100 and 700 cm−1, the
fingerprint region for this kind of compounds. The presence of the
TiO2 cut-off at wavenumbers lower than 900 cm−1 does not allow to
clearly notice part of these range. The characteristic bands of Keggin
ions have been assigned according to Rocchiccioli-Deltcheff et al.
[34].
Fig. 4(A) shows the vibration modes of the bare PW12 cluster
to symmetrical and asymmetrical stretching W O and W
O
W
where the P-O stretching mode is observed at 1080 cm−1, W
O
modes [31]. Significant differences are evidenced in Fig. 3(C) among
the three samples. In fact, for the SiW12/TiO2 and SiW12/TiO2-H
materials a clear enlargement of the bands attributable to the vibra-
tional modes of SiW12 can be observed along with a shift, which
increases in the case of SiW12/TiO2-H. This indicates a different
interaction between the support and SiW12 in the TiO2 containing
materials [28,32]. The samples were also investigated by FTIR to
check the structural integrity of the Keggin unit after the prepa-
asymmetric stretching at 990 and 982 cm−1 and a peak at ca.
890 cm−1 along with a wide band centred at ca. 800 cm−1 which
can be attributed to two types of asymmetric stretching of W
units.
O W
The presence of two bands or a wide one can be explained with
the existence of two kinds of tungsten chains, i.e. W Oc W and
W
Ob W where the Oc oxygen atom is common for two [WO6]
octahedra in [W3O10] subunits, joined by Ob atoms. These main
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1012
990
(A)
637 (A)
637 (A)
937
890
395 (A)
516 (A)
516 (A)
(C)
444
(R)
PW12/TiO2
PW12
395 (A)
SiW12/TiO2 -H
961
250 350 450 550 650 750 850 950 1050
Raman shift [cm-1]
637 (A)
1000
(B)
444
(R)
SiW12/TiO2
395 (A)
516 (A)
1019
800
995
800
981
927
881
SiW12
250 350 450 550 650 750 850 950 1050
Raman shift [cm-1]
PMo12/TiO2
983
603
882
PMo12
250 350 450 550 650 750 850 950 1050
Raman shift [cm-1]
Fig. 3. Raman spectra of: PW12 and PW12/TiO2 (A); PMo12 and PMo12/TiO2 (B); SiW12, SiW12/TiO2 and SiW12/TiO2-H (C).
1067
870
963 975
(A)
(B)
SiW12/TiO2 -H
PMo12
SiW12/TiO2
PMo12/TiO2
930 980
890
780
990
982
SiW12
890
1080
PW12
PW12/TiO2
1020
700
800
900
1000
1100
700
800
900
1000
1100
Wavenumber [cm-1]
Wavenumber [cm-1]
Fig. 4. FTIR spectra of PW12, PW12/TiO2, PMo12 and PMo12/TiO2 (A); SiW12, SiW12/TiO2 and SiW12/TiO2-H (B).
and located at wavenumbers lower than 900 cm−1, does not allow
to clearly notice the last wide band attributable to the asymmetric
stretching vibrations of the skeletal bonds in PW12 are located at
the same wavenumbers when the cluster is loaded on the surface
of TiO2, indicating that the Keggin geometry of PW12 is preserved
in the binary material although the presence of the TiO2 cut-off
stretching of W
O W units. As far as the bare PMo12 cluster is
concerned, also in Fig. 4(A) a band attributed to the P–O stretching
(1067 cm−1) is displayed, along with the Mo O stretching (975 and
intense and broad vibration band, originated from Ti
O Ti bonds
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(a)
(B)
(A)
(a)
12
80
60
40
20
0
(b)
(c)
(b)
(d)
(e)
8
4
0
(c)
(d)
250
350
450
550
650
750
300
350
400
450
500
550
Wavelength [nm]
Wavelength [nm]
(C)
7
6
5
4
3
2
1
0
7
8
7
6
5
4
3
2
1
0
TiO2-H
6
5
4
3
2
1
0
SiW12/TiO2-H
PW12/TiO2
2
2.5
3
3.5
4
4.5
5
2
2.5
3
3.5
4
4.5
5
2
2.5
3
3.5
4
4.5
5
7
6
5
4
3
2
1
0
7
7
SiW12/TiO2
PMo12/TiO2
TiO2 P25
6
5
4
3
2
1
0
6
5
4
3
2
1
0
5
2
2.5
3
3.5
hν [eV]
4
4.5
2
2.5
3
3.5
hν [eV]
4
4.5
5
2
2.5
3
3.5
4
4.5
5
hν [eV]
Fig. 5. (A) DRS spectra of TiO2 P25 (a); PW12/TiO2 (b); SiW12/TiO2 (c); SiW12/TiO2-H (d) and PMo12/TiO2 (e); (B) absorbance spectra of PW12/TiO2 (a); SiW12/TiO2 (b);
SiW12/TiO2-H (c); PMo12/TiO2 (d); (C) Tauc plots for all of the photocatalysts used.
20
16
12
8
4
3
2
1
0
4
0
PMo12/TiO2
PMo12/TiO2
SiW12/TiO2
PW12/TiO2
PW12/TiO2
SiW12/TiO2-H
SiW12/TiO2
SiW12/TiO2-H
Fig. 6. 2-Propanol and isopropylether (DIPE) formation rates by using the various HPA supported materials for the catalytic (grey bars) and catalytic photo assisted (black
bars) propene hydration reaction.
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60
50
40
30
20
10
100
80
60
40
20
0
0
10
20
30
40
50
60
70
Irradiation time [min]
0
20
30
40
50
60
70
10
0
Fig. 7. Glycerol conversion (X%) versus irradiation time for runs carried out in the
presence of: PMo12/TiO2 (ꢀ); SiW12/TiO2 (ꢁ); PW12/TiO2 (᭿); TiO2 P25 (ᮀ); TiO2-H
(ꢀ) and in the absence of photocatalyst (᭹).
Irradiation time [min]
Fig. 8. Acrolein yield (Y%) versus irradiation time for runs carried out in the absence
of photocatalyst and in the presence of HPA/TiO2 or bare TiO2. Symbols as in Fig. 7.
963 cm−1). The peaks at ca. 870 and the wide band centred at ca.
3.2. Photocatalytic reactivity
790 cm−1 [34] attributable to two types of Mo
O Mo units are also
present. Also in this case the main vibration modes of the PMo12
cluster are not shifted when the heteropolyanion is supported on
TiO2.
3.2.1. Photocatalytic propene hydration in gas-solid regime
Fig. 6 shows 2-propanol and di-isopropylether formation rate
for catalytic and catalytic photo-assisted hydration of propene
and it can be noticed that the presence of light is beneficial for
the occurrence of the reaction. Indeed, in any case the catalytic
photo-assisted reaction proceeded at higher rate with respect to
the catalytic one. Moreover, the SiW12/TiO2-H sample prepared by
the hydrothermal method did not show appreciable activity both in
catalytic and catalytic photo-assisted reaction. As far as the forma-
tion of 2-propanol is concerned, the catalyst with the best catalytic
activity was the SiW12/TiO2 sample, whilst the sample with the
best catalytic photo-assisted activity was PMo12/TiO2. The forma-
tion rate of di-isopropylether was, in any case, much lower than
the formation of 2-propanol (ca. one order of magnitude) and the
catalyst showing the best activity both in the presence and in the
absence of light was the PMo12/TiO2 sample.
Preliminary tests carried out under the same experimental con-
ditions, but by using bare TiO2 as the catalyst, instead of the binary
materials, evidenced that TiO2 was inactive for both the catalytic
and catalytic photo-assisted propene hydration reaction. This find-
ing evidenced that the active species for the hydration reaction
are the heteropolyacid clusters. The catalytic and catalytic photo-
assisted activity of an HPA (PW12) supported on TiO2, ZrO2 or WO3
has been previously studied and a beneficial role of the light on
the reaction rate was observed [6–8]. In this paper it is reported
that also other heteropolyacids (SiW12 and PMo12) supported on
TiO2, show a significant increase of the (photo)-catalytic activity
with respect to the catalytic one in the hydration reaction. Also in
this case the acidity of the HPA is the most important factor for the
occurrence of the reaction. Moreover, the increase of the reactiv-
ity in the presence of UV light can be explained by considering, as
already evidenced, the ability of the semiconductor to transfer elec-
trons from the conduction band to the activated HPA* species [7,38].
Indeed, TiO2 directly transfers photo-generated electrons from its
conduction band to the interfacial HPA with empty d orbitals to
form HPA− species, and consequently the electron-hole recombi-
nation is delayed (successively HPA species can be regenerated by
following the reaction pathway reported in literature) [8]. Con-
sequently, the different behaviour of the various catalysts (in the
presence and in the absence of light) can be justified with the dif-
The FTIR spectra of bare SiW12, SiW12/TiO2 and SiW12/TiO2-
H are shown in Fig. 4(B). The bare SiW12 spectrum presents five
characteristic vibration modes located at 780, 890, 930, 980, and
1020 cm−1. The bands at 780 and 890 cm−1 correspond to stretch-
ing of W
O O
W chains, those at 930 cm−1 and 980 cm−1 to Si
and W O stretchings, respectively [34,35]. The 1020 cm−1 band
has been previously observed, however its assignement is unknown
[36] and it could be attributed to an impurity adsorbed on the acidic
sites of the HPA. In the spectrum of SiW12/TiO2 sample only the
band corresponding to the Si O stretching presents a shift with
respect to the bare SiW12. On the contrary, the location of the bands
in the SiW12/TiO2-H accounts for some distortion or modification
of the cluster in the sample in agreement with the Raman results.
Fig. 5(A) and (B) report the diffuse reflectance UV–vis spec-
tra (DRS) and the absorption (reported as F(Rꢁ )), respectively,
∞
of the bare TiO2 and HPA/TiO2 supported samples. The samples
are characterized by a charge transfer process from O 2p to Ti 3d
corresponding to the TiO2 band gap values (ca. 3.2 eV for anatase
and 3.0 eV for rutile). The band gap energies of the samples have
been estimated (see Table 1) from the tangent lines in the plots
of the modified Kubelka–Munk function, [F(Rꢁ )h␯]1/2, versus the
∞
energy of exciting light reported in Fig. 5(C) [36]. The presence of
the HPA gave rise to a slight decrease of the band gap energy only
for PMo12/TiO2 sample that presented a second band bap due to the
presence of the HPA. By taking into account all the above physico-
chemical characterization results, it can be concluded that the
primary Keggin structure of the HPA remained virtually unchanged
after the deposition of the cluster on the oxide surface except for the
hydrothermally prepared material, but at the same time, based on
the spectroscopic results, an interaction between the Keggin unit
and the TiO2 surface can be hypothesized.
As far as the tests devoted to understand the acidity of the sam-
ples are concerned, they indicate that the acidity decreased with the
∼
sequence: PW12/TiO2 SiW12/TiO2 > PMo12/TiO2 > SiW12/TiO2-H,
=
which corresponds both to the sequence of the catalytic activity
for propene hydration and, at least for the impregnated samples, to
that reported in the literature for the bare HPA species [37].
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100
90
80
70
60
50
40
30
20
10
0
ferent ability to transfer electrons, mainly depending on the nature
of the supported HPA. The most efficient catalyst (PMo12/TiO2)
reached a quantum efficiency of ca. 2% under the experimental
conditions used. The quantum efficiency was determined by con-
sidering only the increase of the reacted molecules in the presence
of light with respect to the corresponding catalytic run. It is impor-
tant to underline that the increase of the reactivity obtained by
irradiation, in some cases, was very high and in order to obtain
the same increase of reactivity by thermal catalysis it would be
necessary to raise the reactor temperature of ca. 15–20 ◦C.
X (Conversion)
S (Selectivity)
3.2.2. Photocatalytic glycerol dehydration in liquid-solid regime
Preliminary runs carried out by using HPA/TiO2 samples in the
absence of light indicated that the catalysts were completely inac-
tive. Fig. 7 shows glycerol conversions versus irradiation time for
runs carried out both in the absence and in the presence of photo-
catalyst. The photocatalytic conversion of glycerol increased always
by increasing the irradiation time.
SiW12/TiO2 PMo12/TiO2 PW12/TiO2
TiO2 P25
TiO2-H
no catalyst
Fig. 9. Glycerol conversion (X%) and acrolein selectivity (S%) after 65 min of irra-
diation in the presence of HPA/TiO2 photocatalysts, bare TiO2 samples and in the
absence of photocatalyst.
The control test carried out in the absence of catalyst but in
the presence of UV light evidenced that the photolysis process was
slightly effective for glycerol conversion, reaching a value of about
20% after 65 min of irradiation. A quite similar photo-reactivity was
also observed in the presence of the two bare TiO2 samples although
the conversion of glycerol reached about 35% when TiO2 P25 was
used as the photocatalyst. On the contrary, all of the HPA/TiO2 pho-
tocatalysts exhibited higher photocatalytic activity. In particular,
it was found that the photocatalytic activity of PMo12/TiO2 was
higher than that of SiW12/TiO2 and PW12/TiO2. In summary, the
photocatalytic activities of glycerol dehydration in the liquid phase
were in the order PMo12/TiO2 > SiW12/TiO2 > PW12/TiO2 > TiO2
P25 > TiO2-H. Notably, no tests were carried out in the presence
of bare HPA because all of the bare HPA samples used are soluble
in water and this work aims to study the photoreactivity only in
heterogeneous systems.
Formation of acrolein (its yield is reported in Fig. 8) along
with glyceraldehyde and dihydroxyacetone was observed during
the photo-conversion of glycerol. For all of the tested catalysts,
the acrolein production increased with the irradiation time and it
showed to be higher than the value obtained in the experiments
carried out in the absence of catalyst or in the presence of bare
TiO2 samples. HPA/TiO2 photocatalysts were able to promote the
dehydration of glycerol and the higher acrolein yield was achieved
with SiW12/TiO2 and PMo12/TiO2 samples.
100
90
SiW12/TiO2
80
70
60
50
40
SiW12/TiO2-H
30
20
10
0
0
10
20
30
40
50
60
70
Irradiation time [min]
Fig. 10. Glycerol conversion (X%) as a function of irradiation time for runs carried
out in the presence of SiW12/TiO2 or SiW12/TiO2-H photocatalysts.
100
By using PMo12/TiO2, the acrolein yield was higher than that
observed in the presence of SiW12/TiO2, for an irradiation time
lower than 35 min. On the contrary, the reactivity order changed for
irradiation times higher than 35 min. This result could be explained
by considering that the glycerol conversion on PMo12/TiO2 is
almost complete already after 35 min of irradiation and therefore
the reaction of acrolein formation slows down.
90
80
70
60
50
40
30
20
10
0
SiW12/TiO2-H
SiW12/TiO2
Fig.
9 shows the glycerol conversion and selectivity to
acrolein, respectively, obtained after 65 min of irradiation by using
HPA/TiO2, TiO2 samples and in the absence of photocatalyst. As
it can be noticed the selectivity to acrolein was quite different
depending on the type of supported HPA, increasing from about
20% for PW12/TiO2 to about 72% for SiW12/TiO2. On the contrary
the selectivity to acrolein obtained with the bare TiO2 samples and
in homogeneous conditions showed to be similar (ca. 6%) and very
low. Therefore, the photocatalyst able to produce acrolein with the
highest selectivity was SiW12/TiO2. Consequently, it was chosen to
investigate the influence of the preparation methods on the photo-
activity for glycerol dehydration. To this aim a comparison was
done with SiW12/TiO2-H. The results in terms of glycerol conversion
and acrolein selectivity versus irradiation time for SiW12/TiO2 and
SiW12/TiO2-H samples are reported in Figs. 10 and 11, respectively.
0
10
20
30
40
50
60
70
Irradiation time [min]
Fig. 11. Acrolein selectivity (S%) as a function of irradiation time for runs carried
out in the presence of SiW12/TiO2 or SiW12/TiO2-H photocatalysts.
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For both photocatalysts, glycerol conversion and acrolein yield
increased with irradiation time but the photocatalytic performance
in terms of glycerol conversion was higher for the SiW12/TiO2
sample. Despite the glycerol conversion in the presence of
SiW12/TiO2-H photocatalyst was lower than that obtained with
SiW12/TiO2 (Fig. 10), the acrolein selectivity (Fig. 11) increased from
about 70% (SiW12/TiO2) to 96% (SiW12/TiO2-H), underlining the
importance of the preparation method. The different selectivity was
probably due to a lower oxidizing ability of the sample obtained via
hydrothermal route (see the photocatalytic results on TiO2 P25 and
TiO2-H, Fig. 7). In particular, glycerol conversion in the presence of
TiO2 P25 was higher than that obtained with TiO2-H, whereas the
acrolein yield was higher by using the latter photocatalyst (Fig. 8).
As a consequence the oxidation products (dihydroxyacetone and
glyceraldehyde) were obtained in larger amount when TiO2 P25
was used.
On the contrary, in the presence of light, this photo-catalyst
turns the most active, probably due to its lower band-gap with
respect to the other samples, and this circumstance allows to
exploit more efficiently the light emitted by the lamp. Regarding the
SiW12/TiO2-H sample, it resulted almost inactive for the propene
hydration reaction and this finding could be related to the distortion
of the SiW12 cluster in the binary material, evidenced by RAMAN
and FT-IR investigations, that inhibits the HPA acid sites responsible
for the reaction.
In the case of the reaction system working in liquid-solid phase,
the conversion of glycerol, differently from the previous described
system, gives rise to the formation of one dehydration compound
(acrolein) and also to the formation of two oxidation products
(glyceraldehyde and dihydroxyacetone) and it should be consid-
ered to better understand the global reactivity of the samples.
Consequently, the conversion of glycerol resulted the sum of two
different kinds of reactions (dehydration and oxidation) because in
the liquid-solid system the presence of water favoured also the oxi-
dation reaction of glycerol, although the reaction was carried out
in the absence of molecular oxygen.
It can be concluded that the presence of the heteropolyacid in
the binary materials gave rise under irradiation to the dehydra-
tion of glycerol while the presence of TiO2 to oxidation reactions
to glyceraldehyde and dihydroxyacetone. Only in the case of the
SiW12/TiO2-H sample the oxidant properties of TiO2 were almost
totally depressed probably because of the lower oxidizing ability
for the sample obtained via hydrothermal route.
Moreover, as in the case of propene hydration, the dehydra-
tion of glycerol that involves the acid sites of HPA was depressed
for the SiW12/TiO2-H sample, confirming the negative effect of the
strong interaction between SiW12 and TiO2. Anyway, it is interest-
ing to note that the selectivity to acrolein observed in the presence
of SiW12/TiO2-H sample was close to 100% because the oxidation
reactions were completely inhibited. By comparing the conversion
of glycerol and the selectivity to acrolein, it can be hypothesized
the following oxidant ability of the tested samples:
By considering these results the dehydration reaction of glycerol
to acrolein was strongly favoured for SiW12/TiO2-H with respect to
SiW12/TiO2.
As far as the temperature is concerned, the hydration reaction
of propene in gas-solid phase and the dehydration reaction of glyc-
erol in liquid-solid phase were differently influenced. In particular
in gas-solid phase the hydration of propene started, in appreciable
amounts, only at temperatures equal or higher than 85 ◦C. More-
over the presence of light increased the reaction rate but it is
important to underline that the only presence of light was not able
to sustain the reaction. On the contrary, the dehydration reaction
of glycerol in liquid-phase proceeded at low temperature (35 ◦C)
but only in the presence of light.
As far as the samples reactivity is concerned, the order of reac-
tivity for the propene hydration to 2-propanol in gas phase was:
SiW12/TiO2 > PW12/TiO2 > PMo12/TiO2 » SiW12/TiO2-H under dark
conditionsandPMo12/TiO2 > SiW12/TiO2 > PW12/TiO2 » SiW12/TiO2-
H in the presence of light. It was the same observed for the glycerol
conversion under irradiation in liquid phase, but in this case also
the SiW12/TiO2-H sample showed an appreciable reactivity. The
order of reactivity to produce acrolein, instead, was the follow-
ing: PMo12/TiO2 > SiW12/TiO2 > SiW12/TiO2-H > PW12/TiO2 after
35 min of irradiation and SiW12/TiO2 > PMo12/TiO2 > SiW12/TiO2-
H > PW12/TiO2 before 35 min of irradiation, although the first two
samples showed very similar yield (Fig. 8). Please note that acrolein
yield for the run carried out in the presence of SiW12/TiO2-H was
not reported in Fig. 8, but it can be easily calculated by multiply-
ing glycerol conversion by acrolein selectivity (data reported in
Figs. 10 and 11). Notably only the hydration reaction of propene to
2-propanol and small amounts of di-isopropylether (without for-
mation of oxidation products) were observed in gas-solid phase.
In liquid-solid phase also the formation of oxidation products
(glyceraldehyde and dihydroxyacetone), instead, was observed in
some cases in addition to the dehydration of glycerol to acrolein.
Moreover, not quantified, small amounts of CO2 and CO were
found as volatile species (see Section 2). The exclusive formation
of acrolein (with a selectivity close to 100%) was observed only
in the presence of SiW12/TiO2-H. The formation of small amonts
of oxidation species was observed also in the runs carried out in
homogeneous conditions.
TiO2P25 > TiO2-H > PW12/TiO2 > PMo12/TiO2 > SiW12
/TiO2 > SiW12/TiO2-H
Anyway, because the reaction was carried out in the absence
of O2, the oxidant properties of the powders were, in some cases,
depressed and consequently their acid-base characteristics were
privileged. The increase in acrolein selectivity is attributed to the
presence of HPA on TiO2 surface in accord with a previous work
about the liquid phase catalytic dehydration of glycerol to acrolein
[18]. It was suggested that the secondary hydroxyl group of glyc-
erol was firstly protonated, then dehydration occurred to form
3-hydroxypropanal that in turn dehydrated to acrolein [17,39,40].
However, 3-hydroxypropanal was not detected under UV irradia-
tion, indicating that it was rapidly dehydrated to acrolein [18].
4. Conclusions
(Photo)catalytic propene hydration to 2-propanol and pho-
tocatalytic glycerol dehydration to acrolein reactions have been
successfully carried out by using Keggin heteropolyacids (HPAs)
supported on TiO2. Binary materials have been prepared by impreg-
nation of H3PW12O40, H3PMo12O40 and H4SiW12O40, on TiO2
Evonik P25. Moreover, a binary material consisting of H4SiW12O40
and TiO2 was prepared via a hydrothermal treatment and tested
for the same reactions.
By considering the information deriving from bulk and textu-
ral characterization of the photocatalysts it is possible to explain
the different reactivity observed for the various samples in the two
kinds of systems. As far as the reaction of propene hydration is
concerned, the reactivity of the impregnated samples was quite
similar under dark condition, but the PMo12/TiO2 catalyst showed
the lowest reactivity probably because of the high theoretical cov-
erage onto the TiO2 surface that this sample presents with respect
to the others (4.5 vs. 2.8; see Table 2).
The hydration reaction of propene in gas-solid phase and the
dehydration reaction of glycerol in liquid-solid phase were differ-
ently influenced by the temperature. In particular in gas-solid phase
Please cite this article in press as: G. Marcì, et al., Keggin heteropolyacids supported on TiO2 used in gas-solid (photo)catalytic propene
hydration and in liquid-solid photocatalytic glycerol dehydration, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.037

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the hydration of propene started, in an appreciable amount, only at
temperatures equal or higher than 85 ◦C. Moreover the presence of
light increased the reaction rate but the only presence of light was
not able to sustain the reaction. On the contrary, the dehydration
reaction of glycerol in liquid-phase proceeded at low temperature
(35 ◦C) but only under irradiation. The Keggin HPA species played
a key role in (photo)catalytic reactions, due to their strong acidity
and their ability to form strong oxidant species gave rise to redox
reactions under UV irradiation.
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Acknowledgements
The authors wish to thank MIUR (Rome), Università degli Studi
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Please cite this article in press as: G. Marcì, et al., Keggin heteropolyacids supported on TiO2 used in gas-solid (photo)catalytic propene
hydration and in liquid-solid photocatalytic glycerol dehydration, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.037