Improved (photo)catalytic propene hydration in a gas/solid system by using heteropolyacid/oxide composites: Electron paramagnetic resonance, acidity, and role of water

Article abstract of DOI:10.1002/ejic.201601396

Binary materials composed of the oxides SiO₂, TiO₂ and N-doped TiO₂ and the Keggin heteropolyacid (PW₁₂) were prepared and physicochemically characterized. They were used as catalysts and photocatalysts for the hydration of propene to 2-propanol. The characterization of the samples, particularly the electron paramagnetic resonance (EPR) spectroscopy results and the acidity properties, were useful to explain the key role played by the PW₁₂ in the composite materials in the thermal and photoassisted catalytic processes. The simultaneous pres-ence of heat and UV light improved the activity of PW₁₂ in the thermal process, and the binary materials showed better (photo)catalytic activities than that of the bare PW₁₂ in almost all cases. For the first time, this work evidenced through EPR spectroscopy that the increase of reactivity under irradiation could be attributed to the ability of photoexcited PW₁₂ to trap electrons, particularly if the PW₁₂ is supported. Moreover, the effect of water on the reactivity was also studied.

Full text of DOI:10.1002/ejic.201601396

A Journal of
Accepted Article
Title: Improved (photo)catalytic propene hydration in gas-solid system
by using heteropolyacid/oxide composites: EPR, acidity and role
of water
Authors: Giuseppe Marci, Elisa Isabel Garcia-Lopez, Francesca Rita
Pomilla, Leonarda Liotta, Maria Cristina Paganini, Chiara
Gionco, Bartolomeo Megna, Elio Giamello, and Leonardo
Palmisano
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601396
Link to VoR: http://dx.doi.org/10.1002/ejic.201601396

10.1002/ejic.201601396
European Journal of Inorganic Chemistry
Improved (photo)catalytic propene hydration in gas-solid system
by using heteropolyacid/oxide composites: EPR, acidity and role
of water
Elisa I. García-López, [a] Giuseppe Marcì, *[a] Francesca R. Pomilla, [a] Leonarda F. Liotta, [b]
Bartolomeo Megna, [c] Maria C. Paganini, [d] Chiara Gionco, [d] Elio Giamello, [d] Leonardo
Palmisano. [a]
catalysts [3]. These compounds have been also used as
photocatalysts [2], as their ground electronic state can absorb
Abstract: Binary materials composed by oxides as SiO2, TiO2 and
N-doped TiO2 and the Keggin heteropolyacid (PW12) have been
prepared and physico-chemically characterized. They have been
used both as catalysts and photocatalysts for the hydration of
propene to 2-propanol. The characterization of the samples,
particularly the EPR results and the acidity properties, were useful to
explain the key role played by PW12 in the composite materials, both
in thermal and in photo-assisted catalytic processes.
light, producing
a charge transfer-excited state HPA*; for
instance tungstophosphoric acid, H3PW12O40 (PW12), absorbs at
λ=260 nm (4.8 eV). The excited species can be easily reduced
to form HPA-, the so-called “heteropolyblue” species, by means
of electron(s) transfer from another species [4]. By summarizing,
HPAs have been used not only as acid catalysts for reactions
such as esterification, transesterification, hydrolysis, Friedel-
Crafts alkylation and acylation, Beckmann rearrangement, but
also as oxidation catalysts for alkanes, aromatics, olefins,
alcohols [5]. Moreover, the acidic properties of HPAs have been
exploited to catalyse hydration and dehydration reactions [6].
For instance, propene hydration to obtain 2-propanol is a
reaction carried out industrially at moderate temperatures (ca.
150-200°C) and pressure (2 MPa) in the presence of an acid
catalyst and it has been the object of several patents [3]. The
achievement of this reaction under mild conditions is of great
interest and the use of HPAs could help to reach this scope. It is
reported that HPA based materials showed a significant catalytic
activity from ca. 100 °C, reaching a maximum effect at ca. 130°C.
Indeed, further increase of temperature induced a decrease of 2-
propanol formation rate due to the significant occurrence of the
reverse 2-propanol dehydration reaction [7]. Consequently,
propene hydration is not an easy reaction to be carried out,
because thermodynamically limited by the mentioned reverse
reaction that, anyway, can occur also at temperatures lower than
130 °C.
2-Propanol dehydration reaction is frequently used to investigate
on the acidic character of catalysts, as for instance the
supported heteropolyacids [8]. In a previous work some of us
reported a study on the role of the support of HPA in 2-propanol
(photo)catalytic dehydration to propene [9], concluding that the
type of support (semiconductor or insulator) differently affected
the reactivity of the composite material. Also for propene
hydration carried out both in catalytic and photocatalytic systems,
various insulator and semiconductor oxides have been used as
supports of HPA, playing the semiconductor a role in the
reaction mechanism when irradiated by light with appropriate
wavelengths, but experimental evidence by EPR of the electron
transfer to the HPA species had not been presented [10,11].
Indeed, charge carriers formation, stabilization and reactivity at
the surface of semiconductor metal oxides strongly influence the
heterogeneous photo-assisted reactions. EPR is an useful
technique to monitor the charge separation processes; Grätzel
and Howe [12,13] and Micic et al. [14] monitored for the first time
both the excited electrons and holes upon irradiation of a
colloidal suspension of TiO2 in water. The same experiment can
The contemporary presence of heat and UV light improved the PW12
activity with respect to the thermal process, and the binary materials
showed a better (photo)catalytic activity than the bare PW12 in
almost all cases. This work, for the first time, evidenced by the EPR
study that the increase of reactivity under irradiation could be
attributed to the ability of photoexcited PW12 to trap electrons,
particularly when it is supported. Moreover, the role of water on the
reactivity was also studied.
Introduction
Polyoxometalates are molecular metal oxides with potential
application in various fields. Heteropolyacids (HPAs) are a class
of polyoxometalates with a heteroatom in the cluster structure
which forms the oxo-metallic cluster with the addenda metal
atoms [1]. Due to the tunable acid-base and redox properties,
HPAs have been used as catalysts both for homogeneous and
heterogeneous reactions. Many efforts have been devoted to the
heterogeneization of HPAs [2]. In liquid-solid regime their
separation and recovery are relevant problems for practical
application because they are soluble in polar solvents. Moreover,
due to the low specific surface areas (1÷10 m2 g-1) their
dispersion on supports with high surface area can increase the
accessibility of the reacting species to their acidic sites, and
consequently their catalytic activity as Brönsted acid and oxidant
[a]
E. García-López, G. Marcì, F.R. Pomilla, L. Palmisano
“Schiavello-Grillone” Photocatalysis Group. Dipartimento di
Energia, Ingegneria dell’informazione e modelli Matematici (DEIM),
Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy
giuseppe.marci@unipa.it
[b]
[c]
[d]
L.F. Liotta
Istituto per Lo Studio dei Materiali Nanostrutturati (ISMN)-CNR, via
Ugo La Malfa, 153, 90146 Palermo, Italy
B. Megna
Dipartimento Ingegneria Civile, Ambientale, Aerospaziale, dei
Materiali, Viale delle Scienze, 90128 Palermo, Italy
M.C. Paganini, [d], C. Gionco, [d] E. Giamello,
Dipartimento di Chimica, Università di Torino, via Giuria 7, 10125
Torino, Italy
Supporting information for this article is given via a link at the end
of the document.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201601396
European Journal of Inorganic Chemistry
be performed under vacuum, though less efficiently, in the case
of TiO2 based photoactive solids [15,16]. The direct charge
separation can be obtained by irradiating the solid in situ at the
temperature of liquid nitrogen to avoid the rapid recombination of
the carriers.
and only a slight decrease was observed. On the contrary, it is
remarkable that the SSA of the materials prepared by mixing the
Si or Ti alkoxydes with PW12, i.e. the PW12/SiO2 ex A and
PW12/TiO2 ex A samples, increased significantly compared with
the correspondent hydrothermally prepared bare oxide,
suggesting that the heteropolyacid was incorporated into the
porous structure of the binary system without reducing the
porosity of the support. Notably, the SSA of the PW12/SiO2 ex A
sample was much higher than that of SiO2, probably because
during the hydrolysis of TEOS (SiO2 precursor), the ethanol
formed was dehydrated due to the presence of the PW12, giving
rise to the evolution of ethene which in turn favoured the
formation of smaller mesopores on the sample which were
responsible for a higher total volume (see Table 1).
The evolution of propene gas produced by dehydration of 2-
propanol (deriving from hydrolysis of TTIP) in the presence of
PW12 during the preparation of the PW12/TiO2 ex A sample, was
also accountable for its higher SSA with respect to the bare
TiO2, and the obtained solid contained mainly small mesopores
with size close to micropores.
In this work some home-prepared TiO2, SiO2 and N-doped TiO2
samples were coupled with the Keggin heteropolyacid (HPA)
H3PW12O40 (PW12), forming mixed compounds. Two strategies
have been used for the heterogenization of PW12, i.e. the
deposition by impregnation of the oxide surface with an aqueous
PW12 solution and the formation of the PW12-oxide composite
under hydrothermal conditions. The research aimed to elucidate
the role of PW12 as catalyst and photocatalyst along with that
played by the type of support (insulator or semiconductor). The
(photo)-catalysts have been characterized by various physico-
chemical methods and the results used to explain the catalytic
and photocatalytic activities. In particular, EPR measurements
have been carried out in dark and under UV irradiation to
demonstrate the ability of the supported PW12 to be (photo)-
reduced. A correlation between activity and Brönsted acidity was
found and the influence of the amount of water vapour on the
catalytic activity was investigated.
All of the TiO2 based samples were nanostructured (see Table
1) and the size of the crystallites was in the same magnitude
order of the sample’s pores.
The SEM microphotographs (Figure S2) indicate that the
presence of the heteropolyacid did not affect the morphology of
the support for the samples prepared by impregnation, however
the hydrothermal preparation of the composites gave rise to
different morphologies with respect to those of the bare support.
Results and Discussion
Characterization of the (photo)catalysts
A physico-chemical characterization of the composites, reported
in detail in the supporting information, gives information on some
features of the (photo)catalysts. XRD measurements confirmed
the crystalline structure of the home prepared TiO2 and N-TiO2
supports, the amorphous structure of SiO2 and the good
dispersion of the HPA in all of the composite materials (see
Figure S1). Table 1 reports the size of the anatase crystallites
estimated by applying the Scherrer equation to the XRD
diffractograms along with the BET specific surface areas (SSA)
of all of the binary materials, the bare supports and PW12. As far
as the molar ratio PW12/oxide reported in Table 1 the choice
1/150 [mol/mol] was due to the fact that in this way, at least one
supported catalyst (PW12/N-TiO2 in this case) resulted covered
by a theoretical monolayer (see experimental section of the
manuscript). Consequently, in order to compare the reactivity of
the various samples we maintained the same PW12/oxide molar
ratio. We know that by changing the molar ratio PW12/oxide the
reactivity changes but the aim of this work was not to find the
best catalyst but to better understand, by EPR study, how the
presence of light could increase the reactivity of both bare and
supported PW12. SSA’s of the PW12 impregnated samples
decreased with respect to the bare commercial supports. This
finding can be attributed to the fact that some pores of the
supports were blocked, due to the presence of the PW12
clusters; indeed, the volume of the pores of the supported
samples was always smaller with respect to that of the bare
support. This phenomenon was evident in the case of PW12/SiO2
for which the SSA decreased from 121 to 73 m2·g-1, but also for
PW12/TiO2 with a decrease from 171 to 134 m2·g-1. The SSA of
N-TiO2 did not significantly change after the deposition of PW12,
TABLE 1. Specific surface area (SSA), average pore size, anatase crystalline
size, PW12/oxide ratio and band gap value of the samples
SSA
Average
pore size*
[nm]
Anatase
crystallite
size [nm]
PW12/oxide
[mol/mol]
Sample
E gap
[eV]
[m2·g-1]
PW12
SiO2
15
-
-
-
-
-
3.1
-
121
25.0
(0.64)
TiO2
171
87
7.5 (0.31)
7.5
9.9
-
-
3.2
N-TiO2
23.4
(0.61)
3.2
and
2.4
PW12/SiO2
73
26.3
-
1/150
3.3
(0.47)
PW12/TiO2
134
80
6.2 (0.24)
7.1
1/150
1/150
3.2
PW12/N-TiO2
18.4
(0.36)
11.0
3.2
and
2.4
PW12/SiO2 ex A
PW12/TiO2 ex A
338
199
8.9 (0.75)
3.6 (0.18)
-
1/150
1/150
3.2
5.6
3.2
* Pore’s volume [cm3 g-1] in brackets
Raman spectroscopy (Figure S3) evidenced that the structure of
the PW12 Keggin cluster remained almost unchanged after the
preparation of the impregnated composites: PW12/SiO2,
PW12/TiO2, PW12/N-TiO2 and PW12/SiO2 ex A materials. The
widening of some vibration modes can be attributed to a H-
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10.1002/ejic.201601396
European Journal of Inorganic Chemistry
bonding interaction between the oxygen atom of the Keggin
anion and the hydroxyl groups on the oxide surface. On the
contrary, a strong modification of the Keggin structure was
evidenced in the PW12/TiO2 ex A composite.
PW12/N-TiO2 catalyst, for which such peak slightly shifted at
lower temperature (590 °C). By summarizing, the only sample
that did not show the presence of medium or strong strength
Brönsted acid sites was PW12/TiO2 ex A which was the only
inactive sample for the hydration of propene, as reported in the
“catalytic and photocatalytic activity” section. For such sample
some interaction between the forming TiO2 and the PW12 during
the hydrothermal preparation, can be hypothesized in order to
explain the catalytic inactivity. Moreover, it cannot be excluded
that during the formation of the PW12-oxide composite under
hydrothermal conditions, the active PW12 is somehow buried
into the TiO2 matrix.
For the bare SiO2 support the main desorption occurred with a
broad and intense peak centred at 600 °C but a less
pronounced peak can be also observed at ca. 270 °C along
with a pronounced shoulder at around 385 °C. NH3-TPD
profile recorded for PW12/SiO2 sample seems an overlap of
the profiles of the two components, i.e. PW12 and SiO2,
indicating that the acidic sites characteristic of the two solids
were maintained in the supported sample. On the contrary, for
the PW12/SiO2 ex A only the peak at 600 °C, due essentially to
PW12, was observed more or less unchanged. Indeed, two
intense peaks appeared at 255 and 430 °C where the first one
could correspond to that at 270 °C and the other one to the
shoulder at ca. 385 °C observed for the bare SiO2. However,
these two peaks are very intense and this finding strongly
suggests that some interaction occurred between the forming
SiO2 and the PW12 during the hydrothermal preparation of
PW12/SiO2 ex A. This hypothesis is supported by the Raman
study.
The optical absorption properties of the binary materials, related
to their electronic structure, were investigated by UV-Vis diffuse
reflectance spectra (DRS) (Figure S4). The obtained Eg values
in the Tauc plots, reported in Figure S4 are reported in Table 1.
The acidic properties of the samples were evaluated by NH3-
TPD experiments carried out from room temperature up to
600 °C. Such high temperature was chosen in order to detect
also the contribution of strong acid sites that are known to
desorb NH3 above 550 °C. According to the literature it seems
that Keggin acid clusters still kept their heteropolyoxoanion
structure at least up to 550 °C [17,18].
In order to known the fate of the PW12 structure of our samples
treated at 600 °C, we recorded the Raman spectra after the
TPD measurements. These results indicated that the
heteropolyoxoanion structure was maintained only in the case
of samples in which PW12 was supported on TiO2.
Nevertheless, the PW12 structure instability at temperatures
higher than 550 °C did not negatively affect the TPD results
obtained for both bare and SiO2 supported PW12 samples.
Indeed, for these samples, the loss of the heteropolyoxoanion
structure occurred between 550 °C and 600 °C. Consequently,
the ammonia evolution observed at temperatures higher than
550 °C indicates, anyway, the presence of strong acid sites
even if the heteropolyanion structure is damaged.
The NH3-TPD experiments provided information on the total
acidity of the catalysts without distinguishing between Brönsted
acid sites (typical of bare PW12 and SiO2) and Lewis acid sites,
characteristic of defective TiO2 support. The amount of
EPR spectra of PW12, the bare oxides and the mixed
compounds based on TiO2 are reported in Figure 2 A, while
PW12, the bare oxides and the mixed compounds based on SiO2
are reported in Figure 2 B. In both figures, the first spectra are
related to PW12 and the pure oxides. In particular, in Figures 2 A
and 2 B spectrum (a) is related to the PW12 sample and it does
desorbed NH3 (ppm/g PW12) allowed for
a quantitative
evaluation of the number of active sites, while the temperature
of desorption is an indication of the strength of the acid sites.
Figure 1 reports the NH3-TPD profiles obtained for TiO2 and
SiO2 supported PW12 samples (A and B, respectively), and for
TiO2 and SiO2 bare supports (C and D, respectively). For the
sake of comparison, NH3-TPD profile of bare PW12 is also
reported in Figure 1 (A and B). No NH3 desorption occurred at
temperature below 200 °C, suggesting the absence of weak
acid sites [19,20]. According to our previous results, NH3
desorption in the range 200-550 °C was attributed to medium
strength acid sites, whilst desorption peaks observed at
temperatures higher than 550 °C were assigned to strong sites.
As far as the NH3-TPD curves of the various TiO2 supported
PW12 samples are concerned, it can be observed that the
ammonia desorption depended on the type of TiO2 and on the
preparation method of the catalyst. In particular, a broad peak
centred between 350-410 °C was observed for the PW12/TiO2
ex A, PW12/TiO2 and PW12/N-TiO2 samples. In addition to such
features, attributed mainly to the Lewis acid sites typical of TiO2
(see Figure 1 (C)) [21], a shoulder probably due to the medium
strength Brönsted acid sites of supported PW12 was detected at
around 490 °C and 470 °C, for the PW12/TiO2 and PW12/N-TiO2
samples, respectively. Notably the peak at 600 °C
corresponding to the strong Brönsted acid sites typical of bare
PW12 was not observed for the previous samples, except for the
not show any paramagnetic species.
255 °C
410 °C
35000
28000
21000
14000
7000
0
600
500
400
300
200
100
0
600
430 °C
600 °C
(B)
(A)
10000
7500
5000
2500
0
(d)
500
400
300
200
100
0
(c)
470 °C
490 °C
(c)
325 °C
(b)
590 °C
600 °C
(b)
280 °C
280 °C
(a)
(a)
0
20
40
60
80
0
20
40
60
80
Time [min]
Time [min]
380 °C
600
500
400
300
200
100
0
18000
15000
12000
9000
6000
3000
0
600
600 °C (D)
600
500
400
300
200
100
(C)
500
400
300
200
100
0
270 °C
385 °C
(a)
(a)
80
0
0
20
40
60
80
0
20
40
60
Time [min]
Time [min]
Figure 1. Temperature programmed desorption curves of ammonia (NH3-TPD)
for the as prepared samples: (A) PW12 (a); PW12/N-TiO2 (b); PW12/TiO2 (c);
PW12/TiO2 ex A (d); (B) PW12 (a); PW12/SiO2 (b); PW12/SiO2 ex A (c); (C) TiO2
(a) and (D) SiO2 (a).
Spectra (b) and (c) in Figure 2 A are referred to N-TiO2 and TiO2
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10.1002/ejic.201601396
European Journal of Inorganic Chemistry
oxides, and in both cases paramagnetic species are present. In
the case of N-TiO2 the spectrum has been deeply described
elsewhere [22] and the signal can be attributed to the presence
of NO• trapped species adsorbed on the surface of the catalyst,
due to some residual nitrogen compounds formed during the
synthesis process. In the case of TiO2 sample, the EPR signal is
due to the presence of Ti3+ ions formed during the synthesis.
PW12/TiO2 ex A (line d), PW12/N-TiO2 (line e) and PW12/TiO2 (line
f) samples, show a signal in the region at g > 2.0 typical of holes
stabilized on oxygen ions (i.e. O− ions) in the solid [12,13]. In the
case of line (e), the signal due to the hole is covered by the
more intense signal due to the NO• species already described.
No signals are present in Figure 2 B for the PW12, SiO2,
PW12/SiO2 and PW12/SiO2 ex A samples.
materials based on SiO2.
No effects upon irradiation was observed for the bare PW12
sample. In the case of N-TiO2 (Figure 3 A, line b) the irradiation
simply produced a lessening of the intensity of the original
spectrum, indicating the desorption of the NO• species from the
catalyst’s surface. In the case of TiO2 (Figure 3 A, line c) the
irradiation did not bring to a deep change in the EPR spectra
already present in the as prepared sample, but the Ti3+ signal
increased in intensity, indicating the photo-formation of stabilized
electrons.
For all of the other samples the effect of UV irradiation
presented a common feature: the signal typical of holes (O−
ions) grew a little [23]. The formation of holes in all systems
caused by irradiation with UV light indicated that a charge
separation was actually induced in the solids by photons. In the
case of PW12/N-TiO2, the presence of NO• species hindered the
detectability of the O- ions signal. Photo-promoted electrons,
however, were produced in all cases but they were not
observable in terms of reduction of Ti(IV) present in the solid.
Anyhow, for the PW12/N-TiO2 and PW12/TiO2 (Figure 3 A, lines e
and f) and PW12/SiO2 ex A (Figure 3 B, line d) samples, a huge
signal at high field and g1=1.83/g2=1.78, g1=1.83/g2=1.79 and
g1=1.87/g2=1.80 can be respectively observed. This signal that
was observed also for PW12/SiO2 (in that case it was less
intense) can be correlated to the change of colour (turning from
white/yellow to blue) of the samples which occurred under
irradiation and it is strictly connected to the presence of PW12 in
the compounds. A possible interpretation of this behaviour is
that the photo-excited PW12 (that is a very oxidant species)
The spectra shown in Figure 3, obtained in the same conditions
as those mentioned above, but after UV irradiation under
vacuum, are more informative.
Indeed, in the case of a generic semiconductor oxide, the
reaction occurring after irradiation is:
MeOX + hν → (MeOx)* + e−(CB) + h+
(1)
(VB)
If the two photo-promoted charge carriers are stabilized by the
solid at different sites of the oxide, usually a cation for the
electron and an oxide anion for the hole, the formation of a
reduced form of the cation and the formation of the
paramagnetic and EPR active O− species (O2−+ h+→ O−) can be
observed.
trapped electrons from the oxide anion of the solid reducing to
-
form the heteropolyblue PW12 species (O2− + PW12* → O−
+
(A)
/10
(B)
PW12-). The photo-excited PW12* species, that in the reaction
environment could also trap electrons from propene [19], was
probably responsible for the photocatalytic activity as the
samples showing this signal were the most active ones in the
photocatalytic propene hydration experiments (vide infra).
(a)
(b)
(a)
(b)
(c)
NO
3+
Ti
(c)
(d)
(A)
/5
(B)
(a)
(b)
NO
NO
(a)
(b)
(e)
(f)
/10
(d)
3+
-
Ti
O
-
O
/10
(c)
(d)
310 320 330 340 350 360 370
B [mT]
310 320 330 340 350 360 370
B [mT]
(c)
(d)
g=1.87
NO
Figure 2. EPR spectra of the “as prepared” samples recorded at 77K: (A)
PW12 (a); N-TiO2 (b); TiO2 (c); PW12/TiO2 ex A (d); PW12/N-TiO2 (e); PW12/TiO2
(f) and (B) PW12 (a); SiO2 (b); PW12/SiO2 (c); PW12/SiO2 ex A (d).
g=1.83
(e)
(f)
-
O
g=1.80
g=1.78
Under irradiation it can be considered that also PW12 could be
excited by forming the PW12* that is a very strong oxidant
species which can easily trap electrons from other species
present in the reacting medium [3].
We followed (Figures 3 A and B) the charge separation for all of
the samples described before by irradiating with the full lamp
spectrum (including UV frequencies) the solids kept under
vacuum. Again, Figure 3 A is related to the samples PW12, and
the materials based on TiO2, while Figure 3 B is related to
310 320 330 340 350 360 370 380 390
B [mT]
310 320 330 340 350 360 370 380 390
B [mT]
Figure 3. EPR spectra of samples after irradiation with UV light under vacuum,
recorded at 77K: (A) PW12 (a); N-TiO2 (b); TiO2 (c); PW12/TiO2 ex A (d);
PW12/N-TiO2 (e); PW12/TiO2 (f) and (B) PW12 (a); SiO2 (b); PW12/SiO2 (c);
PW12/SiO2 ex A (d).
Moreover, in the case of TiO2 based materials, the electrons
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10.1002/ejic.201601396
European Journal of Inorganic Chemistry
excited in the conduction band could be scavenged by PW12,
forming again the heteropolyblue PW12 species. Notably, each
dehydration gave rise to ethene and propene, respectively, and
their presence can explain the high pressure (ca. 10 and 25 bar
-
PW12* can trap more than one electron but this does not change
our interpretation of the new EPR signal reported in Figure 3 A
(lines e and f) and Figure 3 B (line d).
for the PW12/SiO2 ex A and PW12/TiO2 ex A samples,
respectively) reached in the autoclave during the preparation of
the samples.
This insight can explain why the photo-promoted electrons are
not available for reducing the metal oxide cations. Finally all of
the samples were irradiated with UV light in the presence of 20
mbar of molecular oxygen, and in all cases, except for the bare
oxides, the expected trace of the EPR signal of a superoxide
600
500
−
400
anion (O2 ) adsorbed on a surface metal cation and produced
292
−
according to the following reaction e−+ O2(gas)→ O2
was not
286
284
(ads),
300
observed [24,25]. The EPR spectra obtained after the irradiation
with oxygen (not shown for the sake of brevity) are very similar
to that reported in Figure 2 relative to the samples “as prepared”.
It can be hypothesized the occurrence of a reaction between
molecular oxygen and the reduced form of PW12 that gave rise
to the formation of a diamagnetic, EPR inactive, species.
208
220
192
206
180
200
100
0
70
62
PW12
PW12/SiO2ex A PW12/N-TiO2 PW12/SiO2 PW12/TiO2
Catalytic and photocatalytic activity
Figure 4. 2-Propanol formation rate per gram of PW12 present in the various
solids, in the catalytic (white) and photocatalytic (grey) propene hydration
reaction. Water concentration: 2 mM.
Neither catalytic nor photocatalytic activity were observed in the
presence of bare oxides, whilst PW12 was active both
catalytically and photocatalytically. The presence of PW12 was in
any case essential for the occurrence of propene hydration.
Indeed, no reaction occurred in the presence of bare oxide
samples, i.e. SiO2, TiO2 or N-TiO2.
2-Propanol formation rate by using the binary materials as
catalysts and photocatalysts, in the presence of 2 mM water
vapour concentration is reported in Figure 4. The reaction rate is
reported per gram of PW12, which is the active
catalyst/photocatalyst.
The presence of ethene and propene not only can account for
the high specific surface area and porosity of these two
composites, but also it produced effects on the nature of PW12.
The Keggin structure of PW12 was strongly compromised during
the PW12/TiO2 ex A sample preparation, as evidenced by FTIR
and Raman spectra, and the Brönsted acidity totally lost. On the
contrary, both Keggin structure and Brönsted acidity were
maintained in the case of PW12/SiO2 ex
A sample. By
As far as the thermal catalytic activity is concerned, the bare
PW12 material was the least active solid. When PW12 was
dispersed on the support surface its performance improved. The
PW12 impregnated on SiO2 resulted the most active catalyst,
followed by the PW12/SiO2 ex A sample which was prepared
under hydrothermal conditions. The PW12/TiO2 was also active
but, on the contrary, the PW12/TiO2 ex A sample which was
hydrothermally prepared resulted completely inactive. The
activity of PW12/TiO2-N was similar to that obtained by using
PW12/TiO2. The different Lewis or Brönsted acidity accounts for
the different catalytic activity of these materials (see NH3-TPD
results). The presence of the impregnated PW12 allowed a larger
surface of contact between the inorganic cluster and the
reagents which favoured the reactivity. The acidity is an
important feature as propene hydration occurred by means of an
acid-base mechanism involving dioxonium ions placed between
PW12 anions [26]. In all cases, the binary materials showed
higher catalytic and photocatalytic activities than the bare PW12,
with the exception of the PW12/TiO2 ex A. The hydrothermal
conditions used to obtain the PW12/oxide ex A composites
determined significant differences between PW12/SiO2 ex A and
PW12/TiO2 ex A. In fact, PW12/SiO2 ex A was one of the most
active material, whilst PW12/TiO2 ex A was inactive. This finding
can be justified by considering the strong Brönsted acidity of
PW12 which, under hydrothermal conditions, dehydrated ethanol
and 2-propanol deriving from hydrolysis of TEOS and TIIP
(precursors for SiO2 and TiO2, respectively). The alcohol
considering the experiments carried out under irradiation, the
activity slightly increased with respect to the catalytic runs with
the exception of the run carried out in the presence of PW12/SiO2
ex A sample, for which, instead, a significant increase of 2-
propanol formation rate was observed. Both catalytic and
photocatalytic reaction rates were higher than those observed in
a previous work [19].
An additional set of propene hydration experiments were carried
out in the presence of higher amount of water vapour (10 mM
instead of 2 mM). The reaction rate generally decreased when
10 mM water vapour was used (Figure 5) instead of 2 mM
(Figure 4), both in the absence and in the presence of light. The
abatement of the catalytic 2-propanol formation rates can be
explained by taking into account the properties of the
heteropolyacid (PW12), used as bare or supported
(photo)catalyst. It is well known the ability of the heteropolyacid
species to absorb polar substrates giving rise to catalytic
reactions in a peculiar phase known as “pseudo-liquid” phase,
as extensively reported by Misono et al. [17,27]. Polar species,
as 2-propanol, can be solubilized in the water molecules present
in bulk PW12. The heteropolyacid nanometric clusters
(considered as the primary structures) can assemble to form a
secondary structure which incorporates water molecules. The
catalytic behaviour of PW12 strongly depends on the
modifications of the secondary structure, and in particular on its
Brönsted acidity which is influenced by water. The hydration
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10.1002/ejic.201601396
European Journal of Inorganic Chemistry
state of the secondary structure determined not only the
accessibility of the reacting molecules, but also the number and
the strength of the surface acid sites, i.e. the H5O2 species [9,
played by PW12 on SiO2, TiO2 and N-doped TiO2, both in the thermal
and photo-assisted catalytic processes. EPR spectroscopy indicated
the likely occurrence of electron-hole transfer between the
heteropolyacid and the oxide before and during the UV irradiation.
As far as the acidity measurements are concerned, it seems that the
+
18].
presence of strong Brönsted acidic sites are needed for the
600
occurrence of propene hydration and their amount is directly
proportional to the reactivity of the catalysts.
528
500
400
The contemporary presence of heat and UV light improved the PW12
activity with respect to the thermal process, and the binary materials
appeared in almost all cases to be more (photo)active than the bare
PW12. This work, for the first time, evidenced by the EPR study, that
the increase of reactivity under irradiation can be attributed to the
ability of photoexcited PW12 to trap electrons, particularly when it is
supported. Moreover, the role of water on the reactivity was studied
and it was observed that the rate of 2-propanol formation decreased
by increasing the water content in the catalytic experiments while it
increased under irradiation in the presence of PW12/TiO2 or
PW12/SiO2 ex A samples.
300
232
175
200
100
0
138
109
91
86
83
33
6
PW12
PW12/SiO2ex A PW12/N-TiO2 PW12/SiO2 PW12/TiO2
Figure 5. 2-Propanol formation rate per gram of PW12 present in the various
solids, in the catalytic (white) and photocatalytic (grey) propene hydration
reaction. Water concentration: 10 mM.
The presence of 2-propanol molecules in the pseudo liquid-
phase, favoured by the increased amount of water vapour in the
reacting stream (10 mM), appeared to be not beneficial because
2-propanol molecules could be subjected to dehydration on the
PW12 acidic sites to form again propene.
As far as the photocatalytic experiments are concerned, it can
be observed that the presence of light was beneficial, and the
activity always increased, particularly when PW12/TiO2 and
PW12/SiO2 ex A were used. Notably, the percentage of increase
of reactivity observed under irradiation was higher when the
concentration of water was 10 mM.
Probably the presence of light, in addition to the effect of exciting
the PW12 species (responsible for the increase of reactivity as
mentioned above) caused also a displacement of H2O from the
secondary Keggin structure to the support. In particular, this
water displacement could be more important in the case of the
samples with higher surface area, which can disperse water on
its surface more easily. It has been reported that the amount of
H2O in the secondary structure is one of the key factors
influencing the acidity, and hence the (photo)catalytic reactivity
of PW12 [9]. It is possible to envisage that in the case of
PW12/TiO2 and PW12/SiO2 ex A (samples possessing the highest
surface areas) the amount of water reached an optimal
Experimental Section
Preparation of the (photo)catalysts
SiO2 was obtained by using tetraethylortosilicate (TEOS) as the
precursor. An aqueous TEOS solution (0.101 mol of TEOS in 50 ml of
water) has been hydrothermally treated (24 h at 200 °C) and the obtained
solid filtered and dried overnight at 40 °C. TiO2 was obtained by using
titanium tetraisopropoxide (TTIP), that was hydrolysed in water (0.105
mol of TTIP in 50 ml of water). The resulting suspension was
hydrothermally treated for 24 h at 200 °C, filtered and dried overnight at
40 °C. A N-doped TiO2 (in the following N-TiO2) was prepared via a sol-
gel method, by mixing a solution of TTIP in isopropyl alcohol (7.5 ml + 7.5
ml) with an aqueous solution of 0.27 g of NH4Cl in 7.5 ml of distilled water.
The mixture was kept under stirring at room temperature until the
achievement of complete hydrolysis. The gel prepared in this way was
left aging for 15 h at room temperature, subsequently dried at 70 °C for 4
h and then calcined in air at 500 °C for 2 h.
As far as the preparation of the composite materials is concerned, two
sets of PW12/support samples were prepared. The first set of composites
were prepared by impregnating SiO2, TiO2 or N-TiO2 with an aqueous
solution containing H3PW12O40·26 H2O (Aldrich, reagent grade). The
obtained slurries were therefore dried overnight at 70 °C. The PW12:oxide
molar ratio was 1:150 and these binary materials, where the theoretical
PW12 coverage on the surface of the oxide was 0.4, 0.3 and 1.0, have
been labelled as PW12/SiO2, PW12/TiO2 and PW12/N-TiO2, respectively.
The theoretical coverage of the support was estimated by taking into
account the value of its specific surface area and the diameter of the
PW12 anionic cluster (ca. 10 Å) [27]. Consequently, the surface area of a
roundish heteropolyanion can be considered equal to ca. 78.5 Å2. The
second set of materials was prepared by a hydrothermal treatment (24 h
at 200 °C) of a solution obtained by dissolving the alkoxide precursor of
SiO2 (0.101 mol of TEOS) or TiO2 (0.105 mol of TTIP) in 50 ml of an
aqueous solution of PW12 (also in this case the molar ratio PW12:oxide
was 1:150). The solids obtained after the hydrothermal treatment of the
suspensions were filtered and dried and they were labelled as PW12/SiO2
ex A and PW12/TiO2 ex A, respectively.
concentration, causing
production.
a
strong increase of 2-propanol
Conclusions
Composite materials of SiO2-, TiO2- and N-doped TiO2 and the
Keggin heteropolyacid (HPA) H3PW12O40 (PW12) have been
prepared and physico-chemically characterized. They have been
used both as catalysts and photocatalysts for the hydration of
propene to 2-propanol in a gas-solid system at atmospheric pressure
and 85°C. The characterization of the samples, particularly the EPR
results and the acidity properties, were useful to explain the key role
Characterization of the binary HPA/oxide samples
Bulk and surface characterizations were carried out to study some
physicochemical properties of the composites. The crystalline structure
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10.1002/ejic.201601396
European Journal of Inorganic Chemistry
of the samples was determined at room temperature by powder X-ray
pre-treatment of the catalysts was carried out under dry N2 at 85°C for 1
h. The photo-assisted reactions were performed by illuminating the
reactor by two UV LED IRIS 40 with an irradiation peak centred at 365
nm. The irradiance reaching the photoreactor was 0.50 W. The
temperature of the photoreactor was held constant at 85°C also during
the irradiation. Samples of the reacting fluid were analyzed by a
Shimadzu 17A gas chromatograph equipped with a FID and an Alltech
AT-1 column (30 m, 0.53 mm, 2.65 µm) operating at 40 °C.
diffraction analysis (PXRD) carried out by using
a Panalytical
Empyrean apparatus, equipped with CuKɑ radiation and PixCel1D (tm)
detector. Scanning electron microscopy (SEM) was performed using a
FEI Quanta 200 ESEM microscope, operating at 20 kV on specimens
upon which a thin layer of gold had been evaporated. An electron
microprobe used in an energy dispersive mode (EDX) was employed
to obtain information on the actual PW12 content in the samples and to
evaluate its dispersion on the support. Specific surface area and
porosity were determined in accordance with the standard Brunauer-
Emmet-Teller (BET) method from the nitrogen adsorption-desorption
Keywords: EPR, heteropolyacids, photocatalysis, propene,
isotherm using
a Micromeritics ASAP 2020. Studies on the
hydration, 2-propanol
preservation of the structure of the PW12 cluster after the deposition
and the (photo)reactivity experiments were carried out by vibrational
spectroscopy. FTIR spectra of the samples in KBr (Aldrich) pellets
were obtained by using a FTIR-8400 Shimadzu spectrometer with 4
cm-1 resolution and 256 scans. Raman measurements were performed
on pure powdered samples. Spectra were recorded by a Reinshaw in-
via Raman 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 exciting source. In order to calculate the
band gap of the samples, 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.
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programmed desorption of ammonia experiments (NH3-TPD) by using
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conductivity detector (TCD), a quadrupole mass (QM) spectrometer
(Ther-mostar, Balzers) and an ultraviolet gas analyzer (ABB, Limas
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with propene by means of a home assembled infusion pump. The total
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10.1002/ejic.201601396
European Journal of Inorganic Chemistry
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10.1002/ejic.201601396
European Journal of Inorganic Chemistry
Entry for the Table of Contents
FULL PAPER
Heterogeneous photocatalysis by
(Photo)catalysis, hydration, heteropolyacids.
heteropolyacids
.
E. García-López, G. Marcì*, F.R. Pomilla, L.F.
Liotta, B. Megna, M. C. Paganini, C. Gionco, E.
Giamello, L. Palmisano
Page No. – Page No.
Improved (photo)catalytic propene hydration
in gas-solid system by using
heteropolyacid/oxide composites: EPR,
acidity and role of water
This work reports the preparation and
characterization of binary materials composed
by SiO2, TiO2 and N-doped TiO2 and the Keggin
heteropolyacid. The catalysts were used for the
hydration of propene to 2-propanol and for the
first time it was evidenced by an EPR study that
the increase of the reactivity under irradiation
can be attributed to the ability of photoexcited
PW12 to trap electrons.
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