Effect of the doping agent nature on the characteristic and catalytic properties of aerogel zirconia catalysts doped with sulfate groups or heteropolytungstic acid

Article abstract of DOI:10.1016/j.materresbull.2015.07.012

Zirconia catalysts doped with sulfates or heteropolytungstic acid (HPW) prepared by sol-gel method and dried in supercritical conditions of solvent have been evaluated in n-hexane isomerization in the temperature range 150-220°C. Using HPW as dopant, the obtained catalyst is active at 220°C and selective towards isomers having higher Individual Octane Number, i.e., 2,2-DMB and 2,3-DMB compared with using sulfates as dopant. Based on N2-physisorption, XRD, SEM and TEM analyses, low specific area, particular morphology of grains and the lack of tetragonal ZrO₂ particles reduce the adsorption of n-hexane reactant and led to inefficient catalyst throughout the temperature range 150-200°C. The catalyst doped with sulfur deactivates at T > 200°C since agglomerates of zirconia particles (TEM and SEM results) favor the coke formation. Using HPW, agglomerates of W oxide as well as phosphate groups (revealed by TGA, FTIR, EDX and UV/visible DRS) favor the formation of more valuable isomers at 220°C.

Full text of DOI:10.1016/j.materresbull.2015.07.012

Accepted Manuscript
Title: Effect of the doping agent nature on the characteristic
and catalytic properties of aerogel zirconia catalysts doped
with sulfate groups or heteropolytungstic acid
Author: Samir Chakhari Mohamed Kadri Younes Alain Rives
Abdelhamid Ghorbel
PII:
S0025-5408(15)30033-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2015.07.012
MRB 8320
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Received date:
Revised date:
Accepted date:
7-4-2015
26-5-2015
13-7-2015
Please cite this article as: Samir Chakhari, Mohamed Kadri Younes, Alain
Rives, Abdelhamid Ghorbel, Effect of the doping agent nature on the
characteristic and catalytic properties of aerogel zirconia catalysts doped
with sulfate groups or heteropolytungstic acid, Materials Research Bulletin
http://dx.doi.org/10.1016/j.materresbull.2015.07.012
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1
Effect of the doping agent nature on the characteristic and catalytic
properties of aerogel zirconia catalysts doped with sulfate groups or
heteropolytungstic acid
a,*
a
b
a
Samir Chakhari , Mohamed Kadri Younes , Alain Rives , Abdelhamid Ghorbel
aUniversity of Tunis El Manar, Faculty of Sciences of Tunis, Chemistry Department, LR01ES08, Laboratory of
Materials Chemistry and Catalysis, 2092, Tunis, Tunisia.
b
University of Science and Technology of Lille, UMR CNRS 818, Unit of Catalysis and Solid State Chemistry,
Building C3, 59655 Villeneuve d'Ascq, France.
*
Corresponding author: E–mail address: samir.chakhari@gmail.com
Phone: +216 22 547 796
Fax: +216 71 537 688
Graphical abstract
Highlights
►
►
►
►
We focused on the selectivity and the octane number of each isomers
Aerogel zirconia solids issued from sulfurs or HPW differ noticeably in properties
Sulfates as dopant do not allow the control of selectivity towards valuable isomers
HPW is promising to obtain more valuable isomers at 220°C
Abstract
Zirconia catalysts doped with sulfates or heteropolytungstic acid (HPW) prepared by
sol–gel method and dried in supercritical conditions of solvent have been evaluated in n–
hexane isomerization in the temperature range 150–220°C. Using HPW as dopant, the
obtained catalyst is active at 220°C and selective towards isomers having higher Individual
Octane Number, i.e., 2,2–DMB and 2,3–DMB compared with using sulfates as dopant . Based
on N2–physisorption, XRD, SEM and TEM analyses, low specific area, particular
morphology of grains and the lake of tetragonal ZrO particles reduce the adsorption of n–
2

2
hexane reactant and led to inefficient catalyst throughout the temperature range 150–200°C.
The catalyst doped with sulfur deactivates at T>200°C since agglomerates of zirconia
particles (TEM and SEM results) favor the coke formation. Using HPW, agglomerates of W
oxide as well as phosphate groups (revealed by TGA, FTIR, EDX and UV/visible DRS) favor
the formation of more valuable isomers at 220°C.
Keywords: Aerogel zirconia; Heteropolytungstic acid; Research Octane Number (RON); n–
hexane isomerization.
1
. Introduction
There is increasing interest in the environmental protection, which encourage the
automobile and petrol industries to develop new catalysts and efficient processes for clean
fuel production. Isomerization of linear hydrocarbons to branched ones is a good alternative
to improve the quality of fuels [1] by increasing their Run Octane Number (RON) [2, 3]. The
n–hexane isomerization into different branched isomers, e.g., 2,2–dimethylbutane (2,2–
DMB), 2,3–dimethylbutane (2,3–DMB) and 2–methylpentane (2–MP) has been extensively
investigated in the Laboratory of Materials Chemistry and Catalysis (Laboratoire de Chimie
des Matériaux et Catalyse (LCMC)). Catalysts involved are acidic ones, e.g., zirconia doped
with sulfate groups and transition metals such as nickel, cerium and chromium [4–9]. Such
catalysts have been prepared by sol–gel method and dried either in supercritical condition of
solvents or in ordinary condition [4–9] since sol–gel route offers a better control of the
homogeneity of the prepared solids as regards to other methods [10-12]. For zirconia based
catalysts, the sol–gel method offered the opportunity to introduce in one step zirconium
alkoxide, metal precursor and sulfate groups as dopant in order to assure the homogeneity and
to control morphological and textural properties of the final materials [13, 14].The obtained
results showed that aerogels manifest developed textural properties as well as higher acidity.

3
On the other hand, the reactivity of sol–gel derived zirconia depended on several parameters
such as the preparation method, the alcogel drying mode and the doping agents (sulfates,
tungstates,..) which generates an acidity required in n–hexane isomerization [15–18, 9].
Though zirconia aerogels doped with sulfate groups are known as suitable for n–hexane
isomerization, and as such are not definitely commercialized, it would be interesting, at least
from the scientific viewpoint, to investigate other dopant agents, e.g., heteropolytungstic acid
(
HPW) in order to enrich reaction products with isomers having higher Individual Octane
Number. Table 1 represents the Research Octane Number (RON) of n–hexane as well as each
produced isomers [19–21].
Table 1: Research Octane Number (RON) of reactant and different reaction products (of Ref. [19–21]).
In the Table 1, it is seen that 2,2–DMB and 2,3–DMB are desired products while 2–MP,
having the lower ION, should be avoided.
HPW are reported as efficient dopant in many catalyst formulations and several investigations
focused in the isomerization of n–pentane and n–hexane [22–28]. In this study, we choose to
investigate n–hexane isomerization in the presence of aerogel zirconia modified either with
HPW or sulfates, and follow the contribution of each dopant in the improvement of catalytic
properties in term of the selectivity toward 2,2–DMB and 2,3–DMB. Different
characterization techniques have been employed in order to study the physicochemical
properties of the prepared solids. Textural study has been performed by N
2
physisorption at
–
196°C, while the stability of materials has been studied by thermal analyses (TGA/TDA).
Structural properties have been studied using X-ray diffraction (XRD) and Fourier Transform
Infrared Spectroscopy (FTIR), while the morphology has been evaluated by Transmission
Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). UV/visible Diffuse
Reflectance Spectroscopy (UV/visible DRS) has been employed to study the electronic

4
transitions of metals throughout the different catalysts, while the acidity has been evaluated
using catalytic isopropanol dehydration.
2
. Experimental
2
.1. Catalysts preparation
The aerogel sulfated zirconia catalyst was prepared as follows: zirconium (IV)
propoxide precursor (Aldrich, 70% in propanol) was dissolved in 1–propanol (Aldrich 99.7%)
4
+
to order to obtain [Zr ] = 1 M. The solution was maintained under stirring for 1 hour at
ambient temperature before adding concentrated sulfuric acid (molar ratio S/Zr = 0.5). To
form the alcogel, the hydrolysis has been performed by adding (dropwise) distilled water
(
molar ratio H O/Zr = 3). Once the gel is formed, it is dried in an autoclave under
2
supercritical conditions of 1–propanol (T = 263.6 °C, P = 51 bar). The obtained aerogel is
stored as AZrS, where A stands to Aerogel, Zr is Zirconia and S is the Sulfate dopant. The
same way was used to prepare zirconia doped heteropolytungstic acid (HPW) with the same
content of dopant and hydrolysis ratio. The obtained solid was denoted as AZrH, where H
stands to HPW. The obtained solid was denoted as AZrH, where H stands to HPW.
AZrS500 and AZrH500 catalysts were respectively obtained after calcinations of AZrS and
3
AZrA solids under oxygen stream (30 cm /min, 1°C/min) for 3h at 500°C.
2
.2. Catalysts characterization
adsorption–desorption at –196°C were performed with a Micromeritics ASAP 2020
N
2
apparatus after outgassing the solids at 200°C for 3 hours. Specific surface area was
determined by BET method, microporous volume by t-plot method and porous volume is the
volume adsorbed at P/P°=0.98. XRD measurements were performed on an X’Pert Pro X-ray
diffractometer from PANalytical with CuKα radiation (λ=1.54060 Å), generator setting of

5
4
0kV and 40mA, a scanning speed of 0.05°/min, and a scanning region of 2‒70°. The
diffractometer was operated at 1.0° diverging and 0.1° receiving slits and a continuous
intensity trace was recorded as function of 2θ. Structural data identification was performed
using EVA software. TGA and TDA analyses have been performed using a Labsys Evo
Thermogravimetric analyzer from Setaram. The thermal treatment was performed under
3
constant air flow (30 cm /min) from ambient temperature for up to 1000°C (10°C/min). FTIR
–
1
spectroscopy analysis was performed in the spectral range 400–4000 cm using a Perkin
‒
1
Elmer XT spectrophotometer, a spectral resolution of 4 cm and accumulating 200 scans. The
catalyst was mixed with KBr and pressed to obtain a pellet. SEM images were obtained with a
Cambridge Instruments Stereoscan 120 at an accelerating voltage of 10kV. UV/Visible DRS
spectra were recorded at room temperature in the range 900–200 nm on a Perkin Elmer
Lambda 45 spectrophotometer equipped with a diffuse reflectance attachment and BaSO as
4
reference material. The DR spectra were converted to the Kubelka–Munk function in order to
convert reflectance data into pseudo absorbance. TEM measurements were performed in a
JEOL JEM-2000 FX Electron Microscope (200kV) with an Energy Dispersive X-ray (EDX)
Spectroscopy. The catalyst samples were suspended and dispersed by ultrasonic treatment in
acetone. A drop of the fine suspension was placed on a copper TEM grid, which was then
loaded into the microscope.
The relative acidity of the catalysts was evaluated using the 2–propanol dehydration reaction
between 150°C and 250°C. Mixture of nitrogen and isopropanol (P = 20 Torr) was conducted
in a continuous down–flow fixed–bed reactor over 100 mg of catalyst. The inlet rate into the
3
reactor was 25 cm /min and the gas products were supplied automatically to Alpha M.OOS
PR2100 gas chromatography equipped with a HP–innowax column and analyzed by a Flame
Ionization Detector (FID). n–hexane isomerization was carried out over mechanical mixtures
obtained by grinding together during 10 min, an equal weight (100 mg) of the synthesized

6
samples and Pt/Al
2
O
3
(0.35 wt.% Pt). Promotion with platinum [29] or other transition metals
[
30] increases the stability of the catalyst towards deactivation and improves the selectivity
towards di-branched isomers according to a metal–acid bifunctional mechanism. Prior the use,
Pt/Al was reduced at 400°C for 5 h under H . The mixture (Pt/Al + a synthesized
2
O
3
2
2
O
3
sample) was pretreated under He flow at 150°C for 30 minutes then exposed to n–hexane
3
stream (20 Torr) diluted with hydrogen (total flow rate: 30 cm /min). The hydrogen saturated
with n–hexane at 0°C was passed over the catalyst and the results were collected after 5
minutes. Reaction products were analyzed by online gas chromatography using FID and a 4-
m 1/8″ column packed with 10 % squalane in Spherosil. The temperature of column was fixed
at 70 °C and those of injector and detector are both fixed at 110 °C.
Catalytic results were expressed as follows:
n y
i i
Selectivity towards the product (i): S (%) 
100
i
n y

i i
i
Where y
i
is the mole fraction of product, n is the number of carbon atoms in each molecule of
i
reaction product.
3
. Results and discussions
3
.1. Textural properties
N
2
adsorption–desorption isotherms of AZrS and AZrH solids are presented in Fig.1
(
A (a) and (b)).
Fig.1: N adsorption–desorption isotherms and Pore size distribution of (a) AZrS and (b) AZrH solids
2
AZrS solid shows a combination of type II and type IV isotherms, characteristic of a
mesoporous material (Figure 1 A (b)), with a H3 hysteresis loop which corresponds to
particles with sheet structure [31]. However, the catalyst doped with heteropolytungstic acid
manifests the superposition of type I and type II isotherms, according to Brunauer et al. as

7
well as IUPAC classification [32, 33], and a small hysteresis (Figure 1 A (b)). Textural
properties of prepared solids are shown in Table 1. The pore size distribution curves obtained
by BJH method from adsorption branch are presented in Fig. 1 (B (a) and (b)).
Table 2: Textural properties
According to Table 2, the catalyst containing sulfate groups exhibited low microporous
volume when compared to AZrH. This result indicate that the micropores of AZrS solid were
evacuated from micropores during the drying procedure at 260°C, while heteropolytungstic
acid exhibits a better affinity towards the alkoxide, mainly during the condensation step, and
diffuse into zirconia matrix and fill micropores inside the autoclave. Average pores diameters
are 126 and 31 Å, respectively for AZrS and AZrH catalysts. These results agreed the BJH
porous distribution illustrated in Fig. 1 (B (a) and (b)). In fact, the pore size distribution is
monomodal, centered at around 135 Å, for the catalyst issued from sulfate groups, while
heteropolytungstic acid led to heterogeneous distribution with pores centered at 31 and 450 Å.
3
.2. XRD and thermal analysis results
XRD patterns of uncalcined AZrS and AZrH catalysts are given in Figure 2a and 2b,
respectively. Aerogel AZrS solid exhibits a ZrO
clear) diffraction peaks at 2θ = 30°, 50° and 60° even if no calcinations has been preformed. It
was well established that the tetragonal ZrO phase is developed in doped zirconia catalysts
by the introduction of the sulfate groups [35–39, 7, 9]. However, in the Figure 2b it is seen
less intense diffraction peaks of tetragonal ZrO phase. XRD patterns of calcined AZrS and
AZrH samples, i.e., AZrS500 and AZrH500 are presented in Figure 3a and 3b. In this plots, it
is seen intense diffraction peaks of tetragonal ZrO phase for the catalyst issued from sulfate
groups, while weaker peaks were detected for AZrH500 catalyst. In addition, we note
diffraction peaks of monoclinic ZrO phase in the diffractogram of AZrH500. These results
2
tetragonal phase characterized by broad (and
2
2
2
2

8
indicate HPW groups are decomposed or change in structure [34] during drying procedure
and/or during calcination, which inhibit the development of tetragonal ZrO
2
phase. The
correlation of these results with those of the textural study allows deducing that the
development of monoclinic ZrO
2
phase over AZrH induces a decrease in specific surface
area.
Fig.2 : XRD patterns of (a) AZrS and (b) AZrH
Fig. 3 : XRD patterns of (a) AZrS500 and (b) AZrH500
It seems that the thermal stability of the dopant is a crucial parameter which affects the
crystallographic structure of zirconia. To confirm this idea, we present in Figure 4 (A and B)
the thermal analysis results of AZrS and AZrH solids, respectively. In the case of AZrS solid,
a weight loss of 4% was observed between 60°C and 270°C (endothermic peak at 150°C),
which results from desorption of weakly adsorbed water as well as from dehydroxylation of
zirconium hydroxide into zirconium oxide. From 250°C to 600°C, a slight weight loss of
~
10% would correspond to ZrO transition phase [40, 41]. However, a third region in the
2
curve began at 650 °C and 760 °C with a weigh loss of 13%, evidencing the decomposition of
sulfate species [42, 43]. TGA/TDA curves of AZrH solid exhibits three stage weight loss: the
first one observed between 60 and 270°C (10%) corresponds to the loss of physisorbed water
on the structure (endothermic process at 130° C). The second stage weight loss of 11% in the
temperature range 270–610°C is attributed to volatilization of residual zirconium propoxide
groups [44–47]. The final weight loss stage (14%) between 650 and 800°C is due to the
breakdown of tungsten and phosphorus oxides in catalyst [48]. These two stage weight losses
are accompanied by two exothermic and endothermic peaks (at 340°C, 720°C, respectively).
Thermal analysis results confirm that HPW groups are thermally stable up to 250°C while
sulfate groups are stable for up to 650°C. The departure of HPW groups overlaps with ZrO
2
phase transition but during calcination, zirconia matrix retains an insignificant part of these

9
instable groups. Low amounts of HPW retained in zirconia matrix undergo a developed
microporosity, while sulfate groups stabilized inside the support reduce the microporosity.
Fig. 4: TGA and DTA curves of freshly prepared (A) AZrS and (B) AZrH solids.
3
.3. FTIR spectroscopy
FTIR spectra of the uncalcined and calcined AZrS solids are given in Figure 5a and
b, respectively. Spectrum of AZrS solid presents bands at 3429 cm–1 attributed to OH
5
–
1
vibration of adsorbed water [49, 50] and a band located at 1631 cm , characteristic of non
–
1
–1
undissociated water molecules (HOH) [50]. The bands situated at 610 cm and 468 cm
would correspond to crystalline zirconia [50].
–
1
The bands characteristic of sulfate groups are situated at 1086, 1124 and 1402 cm . The two
first ones are assigned to asymmetric and symmetric stretching frequencies of S=O bonds
[
51–53], while the later corresponds the asymmetric stretching mode of O=S=O [54]. The
calcination of AZrS solid manifests on the FTIR spectrum of AZrS500 a decrease in the
–
1
intensities of 1086, 1124 and 1402 cm bands. In agreement with XRD and thermal analysis
results, the persistence of sulfate groups would inhibit the tetragonal–monoclinic phase
transition.
Fig.5: IR spectra of (a) AZrS and (b) AZrS500 solids.
FTIR spectra of uncalcined and calcined AZrH solids are given in Figure 6a and 6b
–
1
respectively. The spectrum of fresh AZrH solid exhibits bands at 1125 and 1131 cm ,
attributable to the vibrations of P=O and W=O, respectively [55]. Upon calcination, the band
–
1
–1
situated at 1131 cm decreases in intensity while the one situated at 1125 cm disappeared.
This different results confirm the fact that HPW species are less-stable in oxidative

10
atmosphere but tungsten species would persist either at higher temperatures (650°C, in Fig.
b) or at exposure to oxygen at 500°C for couples of hours (Fig. 6b).
5
Fig.6: IR spectra of (a) AZrH and (b) AZrH500 solids.
3
.4. Scanning electronic microscopy
SEM micrographs are shown in Figure 7a and 7b respectively for AZrS and AZrH
catalysts. SEM image of AZrS clearly show agglomerates of spherical elementary particles, in
which, many interstices are created. Nevertheless, AZrH contains narrow slits and an almost
smooth surface. According to thermal analysis results (Fig. 4B), the departure of HPW
molecules and the hydroxyl-to-oxide phase transition occurred together in the temperature
range 200–500°C at a relatively pronounced rate. At the contrary, the low rate of hydroxyl-to-
oxide phase transition observed in Fig. 4A over AZrS solid between 200 and 650°C
evidencing a slow departure of water molecule. Thus, less residual water is expected in the
case of AZrH solid (see weight losses between 200 and 500°C), while AZrS retain much
water molecules between 200 and 650°C. In studies which report on crystal growth, for
example Cr
2
3
O [56], it has been reported that hydrate water provides surface hydroxyls that
act as binders making aggregates.
Fig.7. SEM images of (a) AZrS and (b) AZrH.
3
.5. Transmission electronic microscopy and EDX
TEM results of AZrS and AZrH solids are presented in Figure 8a and 8b, respectively. The
TEM image of AZrS catalyst reveals “apparent spongy” zirconia particles. Nevertheless,
stacked oxide particles were observed at the surface of AZrH catalyst (Fig. 8b). The EDX
analysis of the catalysts AZrS (Figure 9 a) of randomly selected area detects O, Zr, and S

11
elements, indicating a low dispersion of the dopant elements. For AZrH displays the existence
of O, Zr, P and W elements, it can be seen that the W compounds were present in various
forms resulting in different locations upon the energy levels (Figure 9 b), which confirms
HPW is well dispersed and suggesting probably the presence of tungsten oxides. FTIR
–
1
spectrum of AZrH500 confirm the persistence of W=O band at 1131 cm , while DTA curve
(
Fig. 4B) revealed an endothermic phenomenon at 650°C which probably corresponds to the
crystallization of W oxide.
Fig.8: TEM images of (a) AZrS and (b) AZrH.
Fig.9: EDX spectra of (a) AZrS and (b) AZrH
3
.6. UV/Visible Diffuse Reflectance Spectroscopy
UV/Visible DRS plots are compiled in Figure 10a and 10b, respectively for AZrS and
AZrH solids.
Fig. 10: UV/Visible DR spectra of (a) AZrS and (b) AZrH solids
UV/Visible spectrum of AZrS solid (Fig. 10a) exhibited bands at 210, 230 and 280
2–
4+
nm, which correspond to O → Zr charge transfer transitions in quadratic ZrO phase,
2
corresponding to the excitation of electrons from the valence band (having O 2p character) to
the conduction band (having Zr 4d character). The coordination of zirconium in oxides varies
generally from six-fold to eight-fold [57]. On the other hand, the spectrum of AZrH solid
2
−
(
Fig. 10a) shows bands at 200 and 250 nm which correspond to the charge-transfer of O →
6
+
W . In the Keggin unit, 200 and 250 nm bands belongs to electronic transfer throughout
W=O and W–O–W bonds, respectively [58, 59]. The higher energy absorption band can be
attributed to the p

–d
charge-transfer transitions of the O
t
→ W bonds (O : terminal oxygens
t
linked to a lone tungsten atom in the oxygen sites in the Keggin anion [61,4]) , whereas the

12
lower energy absorption band is assigned to the p
W bonds (O : 12 O in W–O –W bridges, between two different W
sites in the Keggin anion [60, 61,4]).

–d

charge-transfer transitions of the O
b
→
b
b
b
3
O13groups in the oxygen
3
.7. Test of acidity: Isopropanol conversion
The acid properties of prepared catalysts were evaluated in isopropanol dehydration
reaction in the temperature range 150–250 °C. Propylene and acetone are the main reaction
products. It is believed that such products are formed through dehydration reaction over acid
sites for propylene, and dehydrogenation reaction over basic sites for acetone. Fig. 11
represents the isopropanol conversion and propylene selectivity as a function of reaction
temperature.
Fig.11: Isopropanol conversion and propylene selectivity as a function of reaction temperature over (a) AZrS
and (b) AZrH.
The selectivity towards propylene would reflect the acidic character of the
investigated catalysts. According to Fig 11, isopropanol is totally converted into propylene
over AZrS catalyst at 250°C. At this temperature, AZrH catalyst attains only 64% of
selectivity towards propylene with 80% of conversion. In summary, AZrS is a more acidic
catalyst, while AZrH would contain more basic sites required to produce acetone via
dehydrogenation.
3
.8 Catalytic test: n–hexane isomerization
Catalytic performances of prepared catalysts are complied in Table 3. According to
the illustrated data, AZrS solid is active and selective towards 2,3–DMB and 2–MP in the
temperature range 150–200°C. However, for 2,2–DMB, the selectivity is very low (5% at
1
2
50°C). On the other hand, the selectivity towards 2,3–DMB increases slowly from 17 to
4°C between 150 and 200°C and rises from 45 to 53% for 2–MP in the same temperature

13
range. AZrS solid seems to be more selective towards 2–MP than 2,3–DMB (at 150°C, 17%
vs. 45%, respectively for 2–MP and 2,3–DMB). AZrH solid shows a different behaviour
throughout the temperature range 150–200°C. In fact, this solid does not convert n–hexane
into the desired products, but 2,2–DMB and 2,3–DMB are the main isomers produced at
2
20°C.
Table 3: n–hexane isomerization: catalytic results
The obtained results have a marked impact in the catalytic conversion of n–hexane
into 2–MP, 2,2–DMB and 2,3–DMB. First, AZrS solid manifests a deactivation at
temperatures above 200°C due to a significant coking at the surface, which persists even at
temperatures required to burn the coke. Second, in the temperature range 150–200°C, this
catalyst led essentially to the isomer having the lower Individual Octane Number (ION of 2–
MP is 73, Table 1), while the formation of 2,2–DMB isomer, having higher ION (92) has
been inhibited in the whole investigated temperature range. Finally, Pt/Al
mixture, required for n–hexane deshydrogenation, is active only at temperatures higher than
20°C (precisely 240°C).
2
O
3
mechanical
2
These unwanted behaviors have been avoided with zirconia doped with heteropolytungstic
acid. In fact, no deactivation was observed at higher temperatures. On the other hand, mains
products obtained with AZrH are isomers with higher ION.
In the sol–gel preparation, HPW dopant, with Keggin-type structure, increases during
condensation step the space between alkoxide molecules. The available space contributes
after 1-propanol evacuation to a developed microporosity. However, sulfur groups evacuated
from zirconia alcogel (in the form of SO ) decreases the available space between alcoxide
2
chains and result in a mesoporous solid with higher specific surface area and lower
microporosity.

14
The uniform departure of sulfur groups during evacuation influences the shape and the size of
pores. Such a uniform pore size distribution (135 Å diameter) as well as spherical
agglomerates (with interstices as revealed by SEM) would influence the n–hexane adsorption
during catalytic test. Nevertheless, heating HPW with Keggin type structure inside the
autoclave followed by the solvent evacuation led essentially to particular texture,
heterogeneous pores (in size as well as in distribution) and particular morphology (narrower
2
slits with smooth surface). The texture (specific surface area = 235 m /g) and the morphology
of AZrH grain would reduce the adsorption of reactant during catalytic test.
The freshly prepared AZrH solid, issued directly from the autoclave, contains small fractions
of tetragonal ZrO
in air (thermal analysis, Fig. 4B) led to the departure of HPW groups at temperatures above
50°C. At 220°C, the temperature in which AZrH solid is active in n–hexane isomerization,
only physisorbed water was evacuated. In the case of AZrS solid, additional thermal
phenomenon was observed below 250°C which corresponds to ZrO phase transition. In fact,
residual zirconium hydroxides would rather transform into ZrO during test that in autoclave.
Prior the catalytic test, the treatment performed under hydrogen (from ambient for up to
50°C) could be described by our thermal analysis performed in air stream, namely if AZrH
2
as revealed in XRD diffractograms (Fig. 2b). However, heating AZrH solid
2
2
2
2
solid exhibited only dehydration thorough this temperature range.
The freshly prepared AZrS solid contain sulfate groups as revealed by FTIR (Fig. 5a), while
AZrH solid, directly issued from autoclave, contains phosphate groups (FTIR Fig. 6a) as well
as W species (FTIR (Fig. 6a), TEM (Fig. 8b), EDX (Fig. 9b) and UV/visible DRS (Fig. 10b).
Phosphate groups would be reduced under hydrogen stream during the test pretreatment.
However, the dehydration of AZrH solid prior the test (even under hydrogen stream) would
increase the agglomeration of W oxide since H
particles.
2
O molecules behave as bridges to oxide

15
It seems that polymeric HPW molecules inhibit the transformation of Zr hydroxides into ZrO
2
inside the autoclave, while sulfur compounds are required for this crystallographic phase
transition. Prior the catalytic test, the hydrogen stream does not affect the catalytic behavior of
AZrS solid. Nevertheless, a developed specific area, as well as empty spaces inside spongy
zirconia encourage n–hexane adsorption, but behave as nests which favors the coke formation
namely above 200°C when residual Zr hydroxides start to transform into ZrO .
2
In the case of AZrH, phosphate and tungsten species persist after solvent evacuation. On the
other hand, dehydration of catalyst under hydrogen prior the catalytic test would encourage
agglomeration of W oxide and the reduction of phosphate groups. Tungsten oxide (and
probably phosphates) are required for n–hexane isomerization into 2,2–DMB and 2,3–DMB
at temperatures above 200°C (and lower than the temperature in which HPW decomposes),
while the formation of tetragonal ZrO
2
, inhibited in autoclave due to the size of HPW, is
required to transform n–hexane into more (and less) valuable isomers in the temperature range
1
50–200°C.
4
. Conclusion
Aerogel zirconia catalysts doped with sulfates or heteropolytungstic acid (HPW)
have been evaluated in n–hexane isomerization. For catalyst doped with heteroplytungstic
acid, HPW molecules with significant size inhibit the condensation of Zr alkoxide and
contributes after the evacuation of 1-propanol to a developed microporosity, lower
crystallinity, heterogeneous pores (in size as well as in distribution) and particular
morphology (narrower slits with smooth surface) as revealed by N
SEM and TEM analyses. Low specific area, particular morphology of grains and the lack of
tetragonal ZrO particles reduce the adsorption of n–hexane reactant and led to inefficient
catalyst throughout the temperature range 150–200°C. Nevertheless, at 220°C, agglomerates
2
–physisorption, XRD,
2

16
of W oxide as well as phosphate groups (TGA, FTIR, and UV/visible DRS) favor the
formation of more valuable isomers (2,2–DMB and 2,3–DMB). Using sulfates as dopant, the
higher crystalline solid, issued from autoclave, exhibited developed textural properties (XRD
and N
2
–physisorption results) which encourage n–hexane adsorption over strong acidic sites
(
isopropanol dehydration test) and led to (un)desired isomers between 150 and 200°C.
However, at temperatures above 200°C, the coking occurs due to the particular morphology
of sulfur doped solid (agglomerates of zirconia particles, as TEM and SEM revealed). In the
future, it would be interesting to investigate HPW xerogels (in order to encourage ZrO
2
formation) and the doping of sulfated aerogel zirconia with HPW by impregnation (or the
doping of HPW xerogels with sulfur).
Acknowledgments
The first author (Samir Chakhari) gratefully acknowledges the support of University
of Science and Technology of Lille (France) the Unit of Catalysis and Solid State Chemistry
of Lille (UCCS) as part of a research internship program, funded by the Laboratory of
Materials Chemistry and Catalysis (LCMC) and the University of Tunis El Manar, Tunisia (3
grants), of which he contributed to this work during his stay at UCCS.

17
References
[
[
[
[
1] D. Seddon, Catal. Today. 15 (1992) 1–21.
2] M. Busto, L.A. Dosso, C.R. Vera, J.M. Grau, Fuel Process. Technol. 104 (2012) 128–135.
3] F. G. Gault. Adv Catal. 30 (1981) 279–284
4] S. Chakhari, M. K. Younes, A.Rives, A. Ghorbel, J. Sol-Gel Sci. Technol. 69 (2014) 378–
3
85.
5] N. Kamoun, M.K. Younes, A. Ghorbel, A.S. Mamede, A. Rives, J. Porous Mat. 19 (2012)
75–381.
[
3
[
[
(
[
(
[
[
[
6] S. Raissi, M.K. Younes, A. Ghorbel, J. Porous Mat. 17 (2010) 275–281.
7] I Mejri, M.K. Younes, A. Ghorbel, P. Eloy, E.M. Gaigneaux, Stud. Surf. Sci. Catal.174
2008) 493–496.
8] Mejri I, Younes MK, Ghorbel A, Eloy P, Gaigneaux EM, Stud. Surf. Sci. Catal. 162
2006) 953–960.
9] R. Akkari, A. Ghorbel, N. Essayem, F. Figueras, Appl. Catal. A: Gen. 328 (2007) 43–51
10] F. Schmidt, Appl. Catal. A: Gen. 221 (2001) 15–21.
11] W. Huang, J. Yang, C. Wang, B. Zou, X. Meng, Y. Wang, X.Cao, Z.Wang, Mater. Res.
Bull. 47 (2012) 2349–2356.
[
[
[
12] D. Nedumaran, A. Pandurangan, Mater. Res. Bull. 61 (2015) 1–8.
13] O.V. Melezhyk, S.V. Prudius,V.V. Brei, Micro. Meso. Mat. 49 (2001) 39–44.
14] M. Signorettoa, E. Ghedini, F. Menegazzo, G. Cerrato, V.Crocella, C.L. Bianch, Micro.
Meso. Mat. 165 (2013) 134–14.

18
[
[
15] Y. Y. Huang, B. Y. Zhao, Y. C. Xie, Appl. Catal. A: Gen. 172 (1998) 327–331.
16] A.V. Ivanov, T.V. Vasina, V.D. Nissenbaum, L.M. Kustov, M.N. Timofeeva, J.I.
Houzvicka, Appl. Catal. A: Gen. 259 (2004) 65–72.
17] A. Gherib, A. Aouissi, A.Rives, M. Fournier, R. Hubaut, Chin. J. Catal. 28 (2007) 1041–
046.
[
1
[
[
[
18] G. D. Yadav, J. J. Nair, Micro. Meso. Mat. 33 (1999) 1–48
19] Modern Petroleum Technology; 5th Eds. Part II; G.D Hobson, Wiley 1984, p. 786.
20] P. B. Weisz, in: D. D. Eley, P.W. Selwood, P.B. Weisz (Eds.), Advances in Catalysis and
Related Subjects, vol. 13, Academic Press, New York, 1963, p. 157.
[
21] N. Nikolaou, C.E. Papadopoulos, I.A. Gaglias, K.G. Pitarakis, Fuel 83 (2004) 517–523.
[
[
[
22] N. Viswanadham, L. Dixit, J.K. Gupta, M.O. J. Mol. Catal. A: Chem. 258 (2006) 15.
23] M.N. Yoshioka, T. Garetto, D. Cardoso, Catal. Today. 107–108 (2005) 693–698.
24] N. Vu, J. vanGestel, J.P. Gilson, C. Collet, J.P. Dath, J.C. Duchet, J.Catal. 231 (2005)
4
68–479.
[
[
[
[
[
25] M.G. Falco, S.A. Canavese, N.S. Fígoli, Catal. Commun. 2 (2001) 207–211.
26] M. Risch, E.E. Wolf, Catal. Today. 62 (2000) 255–268.
27] R.A. Asuquo, G. Edermirth, J.A. Lercher, J.Catal. 155 (1995) 376–382.
28] H. Liu, G.D. Lei, W.M.H. Sachtler, Appl. Catal. A: Gen 146 (1996) 165–180.
29] G. Resofski, M. Muhler, S. Sprenger, U. Wild, Z. Paal, Appl. Catal. A: Gen. 240 (2003)
7
1–81.
[
[
30] J.A. Moreno, G. Poncelet, Appl. Catal. A: Gen. 210 (2001) 151–164.
31] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T.
Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619.
[
[
32] S. Brunauer, L.S. Deming, W.S. Deming, E. Teller, J.A.C.S. 62 (1940) 1723–1732.
33] IUPAC recommendations, Pure Appl. Chem. 66 (1994) 1739–1758.

19
[
34] A. El-Aziz, A. Said, M.M.M. Abd El-Wahab, A.M. Alian, J. Chem. Technol. Biotechnol.
2 (2007) 513–523.
8
[
[
(
[
[
[
35] B.M. Reddy, P.M. Sreekanth, P. Lakshmanan, J. Mol. Catal. 237 (2005) 93–100.
36] M. Signoretto, F. Pinna, G. Strukul, G. Cerrato, C. Morterra, Catal. Lett. 36
1996)129–133.
37] B. H. Davids, Keogh RA, Srinivasan R, Catal. Today. 20 (1994) 219–256.
38] L. Ben Hamouda, A. Ghorbel, F. Figueras, Stud. Surf. Sci. Catal. 130 (2000) 971–976.
39] V.K. Ivanov, A.Y. Baranchikov, G.P. Kopitsa, S.A Lermontov, L.L. Yurkova, N.N
Gubanova, O.S. Ivanova, A.S. Lermontov, M.N. Rumyantsevac, L.P. Vasilyeva, M. Sharp,
P.K. Pranzas, Y. D. Tretyakov, J. Solid State Chem.198 (2013) 496–505.
[
40] J.R. Mawdsley, J.D. Carter, D.J. Myers, M.A. Lewis, T.R. Krause, Int. J. Hydrogen
Energy. 37 (2012) 11004–11011.
41] X. Bokhimi, A. Morales, O. Novaro, T. Lopez, R. Gomez, Polyhedron. 19 (2000) 2283–
287.
[
2
[
[
42] P. Bautista, M. Faraldos, M. Yates, A. Bahamonde, Appl. Catal., B 71 (2007) 254–261.
43] G.X. Yu, X.L. Zhou, F. Liu, C.L. Li, L.F. Chen, J.A. Wang, Catal. Today. 148 (2009)
7
0–74.
[
[
44] D .M. Avila, E.N.S. Muccillo, Thermochim. Acta 256 (1995) 391–398.
45] M Picquart, T Lopez, R Gomez, E Torres, A Moreno, J Garcia, J. Therm. Anal. Calorim.
7
6 (2004) 755–761.
46] J. Rivera De la Rosa, A. Hermandez, F. Rojas, J.J. Ledezma, Colloids and Surfaces A.
Physicochem. Eng. Aspects. 315 (2008) 147–155.
47] D. Wattanasiriwech, S. Wattanasiriwech, R. Stevens, Mater. Res. Bull. 41 (2006) 1437–
446.
48] T. Okuhara, N. Mizuno, M Misono, Adv. Catal. 41 (1996) 113–252
[
[
1
[

20
[
[
[
[
(
[
[
49] M. Signoretto, F. Pinna, G. Strukul, P. Chies, J.Catal. l167 (1997) 522–532.
50] F. Babou, G. Coudurier, J. Vedrine, J.Catal. 152 (1995) 341–349.
51] T. Yamaguchi, K. Tanabe, Y.C. Kung, Mater. Chem. Phys. 16 (1986) 67–77.
52] A. Corma, V. Fornes, M.I. Juan-Rajadell, J.M. Lopez Nieto, Appl. Catal. A: Gen. 116
1994) 151–163.
53] D. Xu, H. Ma, F. Cheng, Mater. Res. Bull. 53 (2014) 15–20.
54] M. Bensitel, O. Sauer, J.C. Lavelly, B.A. Morrow, Mater. Chem. Phys. 19 (1988) 147–
1
56.
55] A.V. Ivanov, T.V. Vasina, V.D. Nissenbaum, L.M. Kustov, M.N. Timofeeva, J.I.
Houzvicka, Appl. Catal. A: Gen. 259 (2004) 65–72.
[
[
[
56] L. Li, Z.F. Yan, G.Q. Lu, Z.H. Zhu, J. Phys. Chem. B 110 (2006) 178–183.
57] E. Rodriguez-Castellon, A. Jimenez-Lopez,P. Maireles-Torres, D.J. Jones, J. Roziere,M.
Trombetta,G. Busca, M. Lenarda, L. Storaro, J. Solid State Chem. 175 (2003) 159–169.
58] L.F. Chen, J.A. Wang, L.E. Noren, J. Aguilar, J. Navarrete, P. Salas, J.A. Montoya, P.
Del Angel, J. Solid State Chem. 180 (2007) 2958–2972.
[
[
[
59] L.R. Pizzio, C.V. Caceres, M.N. Blanco, Appl. Catal. A: Gen. 167 (1998) 283–294.
60] J.Y. Niu, K.H. Wang, H.N. Chen, J.W. Zhao, P.T. Ma, J.P. Wang, M.X. Li, Y. Bai, D.B.
Dang, Cryst. Growth Des. 9 (2009) 4362–4372.
[
(
61] N. Essayem, A. Holmqvist , P. Y. Gayraud, J. C. Vedrine, Y. Ben Taarit, J.Catal. 197
2001) 273–280.
Figure Captions
Fig.1: N adsorption–desorption isotherms and Pore size distribution of (a) AZrS and (b)
2
AZrH solids.
Fig.2: XRD patterns of (a) AZrS and (b) AZrH

21
Fig.3: XRD patterns of (a) AZrS500 and (b) AZrH500.
Fig. 4: TGA and DTA curves of freshly prepared (A) AZrS and (B) AZrH solids
Fig.5: IR spectra of (a) AZrS and (b) AZrS500 solids.
Fig.6: IR spectra of (a) AZrH and (b) AZrH500 solids.
Fig.7: SEM imagess of (a) AZrS and (b) AZrH.
Fig.8: TEM images of (a) AZrS and (b) AZrH.
Fig.9: EDX spectra of (a) AZrS and (b) AZrH
Fig. 10: UV/visible DR spectra of (a) AZrS and (b) AZrH solids.
Fig.11: Isopropanol conversion and propylene selectivity versus reaction temperature over (a)
AZrS and (b) AZrH catalysts.
Table. 1 Research Octane Number (RON) values of reactant and different reaction products (of Ref. [19–
2
1]).
Compound
n–hexane
RON
26
2
2
2
,2–DMB
,3–DMB
–MP
93
95
73
2
,2–DMB 2,2–dimethylbutane, 2,3–DMB 2,3–dimethylbutane 2–MP 2–methylpentane
Table. 2 Textural properties.
2
a
b
3
c
3
d
3
e
Catalyst
AZrS
S
BET (m /g)
D
p
(Å)
V
total (cm /g)
V
micro (cm /g)
Vmeso (cm /g)
323
235
126
31
0.35
0.20
0.08
0.12
0.27
0.07
AZrH
a
Calculated by the BET (Brunauer–Emmett–Teller) equation.
BJH (Barret–Joyner–Hallender) desorption average pore diameter.
Calculated at the relative pressure of 0.98.
b
c

22
d
e
Determined by t-plot model.
Calculated by the difference of Vtotal and Vmicro; Vmeso = (Vtotal-Vmicro).
Table. 3 n–hexane isomerization: catalytic results.
Catalysts
AZrS
AZrH
Selectivity (%)/T(°C)
150
5
175 200 220
150
0
175
200 220
2
2
2
,2–DMB
,3–DMB
–MP
0
0
0
0
0
0
0
0
0
0
0
24
32
0
17
45
19
50
24
53
0
0
2
,2–DMB 2,2–dimethylbutane, 2,3–DMB 2,3–dimethylbutane 2–MP 2–methylpentane
Fig. 1

23
Fig.1
Fig.2

24
Fig.3
Fig. 4

25
Fig.5
Fig.6

26
Fig.7
Fig.8

27
Fig.9
Fig. 10

28
Fig.11