The Effect of Sulphate Doping on Nanosized TiO2 and MoOx/TiO2 Catalysts in Cyclohexane Photooxidative Dehydrogenation

Abstract

The effect of sulphate doping of titania in promoting activity and selectivity of MoOx/TiO2 catalysts for the cyclohexane photooxidative dehydrogenation has been investigated in a gas-solid fluidized bed reactor. Sulphate and/or molybdate-modified titania catalysts were prepared by incipient wet impregnation of nanosized (5–10 nm crystallite size) samples. At 60% of titania surface coverage by MoOx, sulphate surface density was obtained up to 19 μmol/m2 without formation of MoO3. The catalysts were characterized by N2 adsorption-desorption at −196∘C, micro-Raman and UV-visible reflectance spectroscopy, thermogravimetric analysis coupled with mass spectroscopy (TG-MS), and mass titration. Unsulphated and sulphated titania are both active in cyclohexane total oxidation, but sulphate doping of titania has a detrimental effect on the reaction rate. On Mo-based catalysts, polymolybdate species enabled sulphated titania to convert cyclohexane to benzene (99% selectivity) and cyclohexene, reducing at zero the formation of CO2. Cyclohexane conversion to benzene is almost linearly dependent on sulphate surface density, resulting in enhanced yield to benzene. The enhanced photooxidative dehydrogenation activity and benzene yield by sulphate doping could be attributed to the increase of surface acidity and, as a consequence, of cyclohexane adsorption.

Full text

Page 1

Hindawi Publishing Corporation
International Journal of Photoenergy
Volume 2008, Article ID 258631, 8 pages
doi:10.1155/2008/258631
ResearchArticle
The Effect of Sulphate Doping on Nanosized TiO2and
MoOx/TiO2Catalysts in Cyclohexane Photooxidative
Dehydrogenation
P. Ciambelli, D. Sannino, V. Palma, and V. Vaiano
Dipartimento di Ingegneria Chimica e Alimentare, Universit` a di Salerno, 84084 Fisciano (SA), Italy
Correspondence should be addressed to P. Ciambelli, pciambelli@unisa.it
Received 30 August 2007; Accepted 29 December 2007
Recommended by Leonardo Palmisano
The effect of sulphate doping of titania in promoting activity and selectivity of MoOx/TiO2catalysts for the cyclohexane photoox-
idative dehydrogenation has been investigated in a gas-solid fluidized bed reactor. Sulphate and/or molybdate-modified titania
catalysts were prepared by incipient wet impregnation of nanosized (5–10 nm crystallite size) samples. At 60% of titania sur-
face coverage by MoOx, sulphate surface density was obtained up to 19 μmol/m2without formation of MoO3. The catalysts were
characterized by N2adsorption-desorption at −196◦C, micro-Raman and UV-visible reflectance spectroscopy, thermogravimet-
ric analysis coupled with mass spectroscopy (TG-MS), and mass titration. Unsulphated and sulphated titania are both active in
cyclohexane total oxidation, but sulphate doping of titania has a detrimental effect on the reaction rate. On Mo-based catalysts,
polymolybdate species enabled sulphated titania to convert cyclohexane to benzene (99% selectivity) and cyclohexene, reducing at
zero the formation of CO2. Cyclohexane conversion to benzene is almost linearly dependent on sulphate surface density, resulting
in enhanced yield to benzene. The enhanced photooxidative dehydrogenation activity and benzene yield by sulphate doping could
be attributed to the increase of surface acidity and, as a consequence, of cyclohexane adsorption.
Copyright © 2008 P. Ciambelli et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1. INTRODUCTION
Recently, growing interest has been addressed to novel pro-
cesses for the synthesis of chemicals by reversing the final
goal of the photocatalytic oxidation process—enhancing the
selectivity to partially oxidized products so much that the
process could be used for their production. Therefore, since
one of the main goals of the 21st century chemistry is to re-
place environmentally hazardous processes, selective photo-
catalytic partial oxidation can play a key role in this evolu-
tion, offering an alternative green route for the production
of organics.
Titanium dioxide is the semiconductor most used in
photocatalysis because of its high photocatalytic activity al-
lowing to realize the abatement of pollutants both in liq-
uid and gas phases [1–4]. It has been reported that the in-
troduction of foreign species into TiO2leads to a dramatic
change in activity of the photocatalytic process. In particular,
SO42−/TiO2shows higher photocatalytic activity than unsul-
phated titania for a large variety of organic compounds [5].
Fu et al. [6] showed that the improved photoactivity of sul-
phated titanium dioxide was due to a greater surface area as
well as a larger fraction of anatase phase which is more active
than rutile for the photocatalytic application. In the presence
of metal oxides such as WO3, the surface acidity of TiO2was
increased, resulting in enhanced photocatalytic activity [7].
The formation of acid sites increases the adsorption strength
of different organics, that it is believed to contribute to en-
hance the photocatalytic activity.
The most of studies on photocatalytic reactions for
organic synthesis deals with slurry systems. In contrast,
selective oxidation in gas phase with molecular oxygen is still
arealchallengetocurrentresearch.Fewexamplesofselective
photocatalytic oxidative conversion of organic substrates
in gas phase on modified titania-based catalysts have been
reported such as alkane and cycloalkane oxidation [8–10],
and olephin epoxidation [11, 12]. However, a significant
enhancement of catalytic activity and energy efficiency is
necessary for such application of photocatalysis. Recently, we
have shown that benzene is obtained with high selectivity by

Page 2

2International Journal of Photoenergy
Table 1: List of catalysts and their characteristics.
Catalyst
PC500
PC500S10
PC500S18
24MoPC500
17MoPC500S10
12MoPC500S18
∗Evaluated by TG-MS analysis.
MoO3wt%
—
—
—
23.8
16.6
11.7
SO∗
3wt%
0.55
11
19
0.3
10
18
Surface area m2/g
345
241
169
202
144
99
MoO3density μmol/m2
—
—
—
8.1
8.0
8.1
SO4density μmol/m2
0.17
4.8
11.7
0.15
7.3
18.9
photooxidative
MoOx/TiO2 catalysts in a gas-solid fixed [10] or, better,
fluidized bed reactor [13, 14]. The active phases are oc-
tahedral polymolybdate species on titania surface [10].
Moreover, we found that the presence of sulphate species
on the surface of different titanias (2.4 wt% maximum SO4
content) enhances the benzene yield as more as higher is the
sulphate content evidencing a synergic effect of sulphate and
molybdate species [15]. It was also shown that the presence
of sulphate increases surface acidity as the sulphate surface
densityincreases.TheadditionofsulphatetoMoOx/γ-Al2O3
catalysts was found to promote the selective mono-oxidative
dehydrogenation of cyclohexane to cyclohexene, demon-
strating the possibility of finely tuning the process selectivity
[16]. Maximum cyclohexane conversion and cyclohexene
yield of 11% were obtained for SO4 content of 2.4 wt%
at 120◦C. Physicochemical characterization of catalysts
indicated the presence of both octahedral polymolybdate
and sulphate species on alumina surface as previously found
for titania. Increasing sulphate load, thermogravimetry
evidenced the presence of up to three sulphate species of
different thermal stability. We concluded that the lower
activity observed at high sulphate content is likely due to
polymolybdate decoration by sulphates [16].
In this paper, we focus the attention on the effect of sul-
phate doping of nanosized TiO2 on the photocatalytic ox-
idative dehydrogenation of cyclohexane to cyclohexene and
benzene on MoOx/TiO2in a gas-solid fluidized bed reactor.
dehydrogenationof cyclohexane on
2.EXPERIMENTAL
Anatase titania with crystallites size ranging from 5 to
10nm (PC 500 provided by Millenium Inorganic Chemi-
cals, Stallingborough, United Kingdom) was used as cata-
lyst support. The preparation procedure for catalyst samples
(MoPCs) containing various amounts of molybdenum ox-
ide and sulphate consisted of two steps. The first step was
the impregnation of the support with an aqueous solution of
ammonium sulphate. The suspension was dried under stir-
ring at80◦C tocompleteremovalof excesswater.Thesample
turned into sulphate doped titania by calcination at 300◦C
for 3 hours. The second step was the impregnation of the
sulphated sample with an aqueous solution of ammonium
heptamolybdate (NH4)6Mo7O24·4H2O, drying at 120◦C for
12 hours and calcination in air at 400◦C for 3 hours.
Photocatalysts characterization was performed by dif-
ferent techniques. Thermogravimetric analysis (TG-MS) of
powder samples was carried out in air flow with a thermo-
analyzer (Q600, TA) in the range 20–1100◦C at 10◦C/min
heating rate. Specific surface area was evaluated by N2
adsorption-desorption isotherms at −196◦C with a Costech
Sorptometer 1040. Powder samples were treated at 180◦C
for 2 hours in He flow (99.9990%) before testing. Laser Ra-
man spectra of powder samples were obtained with a disper-
sive micro-Raman (Invia, Renishaw), equipped with 785nm
diode laser, in the range 100–2500cm−1Raman shift. UV-
Vis reflectance spectra were obtained with a Perkin Elmer
Spectrometer Lambda 35 using an RSA-PE-20 reflectance
spectroscopy accessory (Labsphere Inc., North Sutton, NH,
USA). All spectra were performed using an 8◦sample posi-
tioning holder, giving total reflectance relative to a calibrated
standard SRS-010-99 (Labsphere Inc.). The reflectance data
were reported as F(R∞) values from Kubelka-Munk theory
versus the wavelength [17]. Equivalent band gap determina-
tionsweremadebyplotting[F(R∞)∗hν]2versushν(eV)and
calculating the x intercept of a line passing through 0.5 <
F(R∞) < 0.8. The zero point charge (ZPC) of supports and
catalysts was determined by mass titration according to Noh
and Schwarz [18].
Photocatalytic tests were carried out feeding a fluidized
bed photoreactor [19] with 830 (stp) cm3/min N2 stream
containing 1000ppm cyclohexane, 1500ppm oxygen, and
1600ppm water. The composition of the inlet and outlet
gases was measured by an online quadrupole mass detec-
tor (TraceMS, ThermoFinnigan) and a continuous CO-CO2
NDIR analyser (Uras 10, Hartmann & Braun). The reactor
was illuminated by four UV light sources (eye mercury lamp,
125W) in a dark box. The temperature was set at 120◦C after
a screening in the range 40–160◦C [19]. In order to achieve
fluidization conditions, 14g of catalyst mixed with 63g of α-
Al2O3(Aldrich, Milano, Italy 10m2/g, 50μm Sauter diam-
eter) were loaded to the reactor. UV sources were activated
after the complete adsorption of cyclohexane on catalyst sur-
face in dark conditions.
3. RESULTS AND DISCUSSION
3.1.Catalystcharacterization
The nominal MoO3and sulphate content together with the
specificsurfacearea(S.S.A)ofthephotocatalystsarereported
in Table 1.

Page 3

P. Ciambelli et al.3
1020770 520
T (◦C)
27020
60
65
70
75
80
85
90
95
100
TG (%)
PC500
PC500S10
PC500S18
(a)
1020770520
T (◦C)
270 20
DTG (%/min, a.u.)
PC500S18
PC500S10
PC500
0.5%/min
(b)
Figure 1: TG (a) and DTG (b) curves of PC500, PC500S10, and PC500S18 catalysts.
Both SO4 and MoO3 surface densities were evaluated
with respect to the surface area values of the calcined cata-
lysts. All MoPCs were designed to reach about 60% mono-
layer coverage of titania surface (about 8μmol/m2).
Thermogravimetric curves of PC500, PC500S10, and
PC500S18 are reported in Figure 1.
Three main steps of weight loss are recognized: (i) the
first one associated with hydration water desorption, (ii) the
second one related to the removal of OH−surface groups
of titania, (iii) the last one attributed to the decomposi-
tion of sulphate species giving rise to gaseous SO3as iden-
tified from its characteristic fragments m/z = 48 and 64
(not reported). The latter step ranges from 690◦C to 840◦C
for PC500, while downshifts to 420–855◦C and 375–850◦C
for PC500S10 and PC500S18, respectively. By analyzing the
first derivative curves in the region of sulphate loss, only
one peak centered at 741◦C is observed on PC500, while
PC500S10 and PC500S18 show three peaks at 552, 599,
717◦C and at 477, 555, 730◦C, respectively. It has been re-
ported [20] that sulphated TiO2shows different regions of
sulphate loss, the lowest temperature peak being attributed
to a more weakly bonded sulphate species, the highest tem-
perature peak to a more strongly bonded sulphate. The
structure of metal oxide bonded sulphate is still subject
of investigation in the literature. Jin et al. [20] proposed
that sulphate would be coordinated to the titania surface
as bidentate anion, Bensitel et al. [21] and Waqif et al. [22]
described a structure in which sulphate, at lower content,
would be bonded to the metal through three oxygen atoms.
Moreover, the sulphate coordination to the surface depends
on the dehydration state of the surface and then on the
temperature. Increasing temperature and sulphate content,
polynuclear sulphates (polysulphates, S2O7−, etc.) can be
formed.
TG and DTG curves of 24MoPC500, 17MoPC500S10,
and 12MoPC500S18 are displayed in Figure 2. All the sam-
ples show a decomposition step starting from about 700◦C;
the onset of weight loss shifts slightly at higher tempera-
ture increasing the sulphate load. The estimated amount of
weight losses of the three samples well agrees to the nominal
MoO3content reported in Table 1, allowing the attribution
of this step to Mo oxide sublimation, reported to occur in
the range 800–1000◦C on MoOx/Al2O3catalysts [23].
The DTG curve of 24MoPC500 shows a single high tem-
perature peak, centered at 624◦C due to sulphate decompo-
sition. We attribute the decreasing of decomposition tem-
perature with respect to PC500 to the presence of MoOx
species, similarly as we previously found for WOx/TiO2cata-
lysts [24]. For 17MoPC500S10, in addition to a DTG peak at
about 584◦C two overlapping peaks appear, with maximum
at 466◦C and 525◦C, likely due to different surface sulphates.
SimilarDTGpeaks,centeredat455◦C,573◦C,and630◦C,are
shown by 12MoPC500S18.
UV-Vis DRS spectra of all the photocatalysts are re-
portedinFigure 3.Whensupportedonsulphatedtitania,Mo
species absorption bands are difficult to distinguish, as pre-
viously reported [16]. However, the presence of MoO3crys-
tallites (main band at 360nm) on the catalysts was not de-
tected. Sulphate doping increases the overall UV absorption
of all the samples (Figure 3), correspondent to a slight in-
crease in equivalent band gap energies, reported for MoPCs
in Table 2. At similar sulphate level, the partial surface cov-
erage with MoOxleads to a small decrease in the equivalent
band gap energies with respect to the relevant support.

Page 4

4International Journal of Photoenergy
1020770 520
T (◦C)
270 20
TG (%, a.u.)
24MoPC500
17MoPC500S10
12MoPC500S18
10%
24.7%
16.8%
11.6%
(a)
1020 770520
T (◦C)
27020
DTG (%/min, a.u.)
12MoPC500S18
17MoPC500S10
24MoPC500
0.5%/min
(b)
Figure 2: TG (a) and DTG (b) curves of 24Mo PC500, 17MoPC500S10, and 12MoPC500S18 catalysts.
600 400 200
Wavelength (nm)
F(R) (a.u.)
12MoPC500S18
17MoPC500S10
24MoPC500
PC500S18
PC500S10
PC500
50
Figure 3: UV-Vis DRS spectra of PC500, PC500S10, PC500S18,
24MoPC500, 17MoPC500S10, and 12MoPC500S18 catalysts.
Increasing the sulphate content of titania, the values
of ZPC (Table 2) decrease from 5.8 for PC500 to 2.2 for
PC500S18. Also the presence of molybdenum confers, in all
cases,strongeracidity,indicatedbytheZPCvaluesofMoPCs
catalysts, as reported in [15].
The Raman spectra of PC and MoPC samples are re-
ported in Figures 4(a) and 4(b), respectively. PC500S10 and
PC500S18 exhibit a band around 986cm−1assigned to sym-
metric stretching mode ν1 of sulphate belonging to point
group Td [25]. The signal at about 1012cm−1for PC500S10
and at 1018cm−1for PC500S18 is due to the S=O bonds of
isolated sulphates [26].
Broad bands around 1050 and 1180cm−1are due to re-
moval of degeneration of ν3 mode, owing the symmetry
lowering with the coordination to the titania surface hy-
droxyl groups as bidentate anion. The upshift of S=O Ra-
man band could indicate the strengthening of double bond
due to the formation of polynuclear sulphates [25]. On
MoPCs, the absence of bulk molybdena bands at 819 and
995cm−1[27] for all Mo-based catalysts confirms the ab-
sence of MoO3crystallites. Moreover, a complex band with
several contributions in the range 940–1030cm−1is ob-
served. For 24MoPC500 the maximum is at 984cm−1, with
shoulders at 960 and 967cm−1. For 17MoPC500S10 the
complex band is sharpened, with an additional strong signal
at 992cm−1, overimposed to 970, 980, and 984cm−1shoul-
ders. For 12MoPC500S18, 978 and 982cm−1signals with a
maximum at 996cm−1are present, characteristic of poly-
molybdenyl species [28]. The Mo=O asymmetric stretching
modes of hydrated octahedrally polymeric species lay in the
range 940–988cm−1[29] with nuclearity ranging between
7 and 12. Wachs [28] reported that in hydrated conditions
Mo8O264−presents a Raman Mo=O band at 963cm−1on ti-
tania. The presence of polymeric octahedral species and sul-
phatecanbethusrecognised.984and996cm−1signalscould
be ascribed to the surface sulphates, considering the absence
of 820cm−1band of MoO3crystallites. Bands at 993, 1009,
and 1111cm−1have been assigned to the S–O stretching vi-
brations of SO42−species on CeO2[30].The structureof sul-
phate species responsible for the band at 993cm−1cannot
be definitely determined and thus requires further investiga-
tions.

Page 5

P. Ciambelli et al.5
Table 2: Equivalent band gap energy, ZPC values, and amount of adsorbed cyclohexane in dark conditions for all catalysts.
Catalyst
PC500
PC500S10
PC500S18
24MoPC500
17MoPC500S10
12MoPC500S18
Equivalent band gap energy eV
3.3
3.3
3.4
3.0
3.2
3.3
ZPC pH unit
5.8
4.1
2.2
3.4
2.2
1.2
Adsorbed cyclohexane μmol/m2
0.048
0.034
0.026
0.09
0.14
0.22
120011001000
Raman shift (cm−1)
900
Counts (a.u.)
PC500S18
PC500S10
PC500
(a)
12001100 1000
Raman shift (cm−1)
900
Counts (a.u.)
12MoPC500S18
17MoPC500S10
24MoPC500
(b)
Figure 4: Raman spectra of PC500, PC500S10, PC500S18 (a) and of 24MoPC500, 17MoPC500S10, 12MoPC500S18 (b) catalysts.
The upshift of Mo=O band up to 980cm−1at increasing
sulphate surface density indicates a progressive polymeriza-
tion of MoOxspecies.
3.2.Photocatalytictests
Preliminary tests carried out in absence of UV light evi-
denced that no reaction occurred in dark conditions. Cyclo-
hexane conversion and CO2outlet concentration on PC500,
PC500S10, and PC500S18 as a function of irradiation time
are reported in Figure 5. CO2was the only product detected
in the gas phase (100% selectivity), reaching steady state val-
ues after about 30 minutes. Cyclohexane conversion to CO2
decreases with the sulphate content.
Foralltitania catalysts,thetotaloxidation ofcyclohexane
to CO2and H2O occurs without formation of by-products.
Moreover, from Figure 6 it is evident that both cyclohex-
ane consumption and CO2formation rates decreased with
the sulphate surface density as previously observed in a gas-
solid fixed bed photo reactor [15]. This behavior may be ex-
plained taking into account the reaction mechanism of gas-
solid photocatalytic oxidation of cyclohexane to CO2on ti-
tania catalyst [31]. In the photocatalytic process, in fact, the
surface hydroxyl groups trap the charge transfer to produce
very reactive surface hydroxyl radicals. However, the pres-
ence of sulphate, grafted to the surface by reaction with hy-
droxyl groups, decreases their surface concentration, result-
ing in reduced activity. In addition, it is worthwhile to note
that for total oxidation of hydrocarbons on titania the rehy-
droxylation of the surface is an essential step. Xie et al. [32],
studyingthegas-solidphotocatalyticoxidationofheptaneon
sulphated and unsulphated titania in the presence of water,
found that the presence of SO42−was detrimental to the re-
hydroxylability of the catalyst.
Cyclohexane conversion and benzene outlet concentra-
tion as a function of irradiation time on MoPCS catalysts
are reported in Figure 7. The analysis of outlet products dis-
closed the presence of benzene and cyclohexene, the absence
of carbon dioxide and the stability of catalyst activity during
the photocatalytic test. The trend of cyclohexene concentra-
tion(notreported)wassimilartothatofbenzenebutitscon-
centration was very low (<1ppm). It is evident that both cy-
clohexane totalconversionandpartial conversiontobenzene
are strongly dependent of sulphate and molybdate content.