Influence of the preparation and of the activation treatments on the catalytic activity of mechanical mixtures of sulfated zirconia and Pt/Al2O3

Article abstract of DOI:10.1006/jcat.1996.0301

Zirconium hydroxides with specific surface areas in the range of 60 to 300 m²/g were obtained by varying between 6 and 12 the final pH of precipitation of the gels and the nature of the precursor salt. These compounds were sulfated by impregnation with 15 and 35 ml of aqueous 0.5 MH₂SO₄. Their sulfur content was 3 to 5 wt%. Sulfation increases the thermal stabilities of the zirconias by nearly 200 K. The solids remain amorphous up to at least 673 K and then crystallize in the tetragonal and monoclinic phases. The properties of mechanical mixtures of the sulfated zirconias and a Pt/alumina catalyst were evaluated for the isomerization of n-hexane. The study of the conditions of activation show that the highest performances of these catalysts result from a pretreatment under hydrogen at 573 and 623 K when the amounts of H₂SO₄ added are, respectively, of 35 and 15 ml/g of zirconium hydroxide, whatever their specific surface areas. A close correlation is found between the activities of the catalysts and their specific surface areas. This suggests that the active sites are located at the low coordination sites of zirconia and that their efficiency increases with their dispersion. The acidity determined by IR spectroscopy of NH₃ adsorption is essentially of Lewis type after evacuation at 573 K. However, Bronsted acidity results from a reversible redox equilibrium between hydrogen and zirconia. The isomerization of n-hexane is considered to follow a classical bifunctional mechanism.

Full text of DOI:10.1006/jcat.1996.0301

JOURNAL OF CATALYSIS 163, 18–27 (1996)
ARTICLE NO. 0301
Influence of the Preparation and of the Activation Treatments
on the Catalytic Activity of Mechanical Mixtures
of Sulfated Zirconia and Pt/Al2O3
D. Tichit, D. El Alami, and F. Figueras1
Laboratoire des Mate´riaux Catalytiques et Catalyse en Chimie Organique, URA 418 CNRS, Ecole Nationale Supe´rieure
de Chimie, 8 Rue Ecole Normale, 34075 Montpellier Cedex, France
Received March 9, 1994; revised February 29, 1996; accepted May 31, 1996
On very acidic sulfated oxides a strong band near
1400 cmꢁ1 is observed by IR spectroscopy which is related
to the high catalytic activity. The similarity of this band with
those found in the covalent organic sulfates has led some
authors to propose a bidentate sulfate structure with two
Zirconium hydroxides with specific surface areas in the range
of 60 to 300 m2/g were obtained by varying between 6 and 12 the
final pH of precipitation of the gels and the nature of the precursor
salt. These compounds were sulfated by impregnation with 15 and
35 ml of aqueous 0.5 M H2SO4. Their sulfur content was 3 to 5 wt%.
==
S O bonds where the metal cation of the oxide is a Lewis
Sulfation increases the thermal stabilities of the zirconias by nearly
200 K. The solids remain amorphous up to at least 673 K and then
crystallize in the tetragonal and monoclinic phases. The properties
of mechanical mixtures of the sulfated zirconias and a Pt/alumina
catalyst were evaluated for the isomerization of n-hexane. The study
of the conditions of activation show that the highest performances
of these catalysts result from a pretreatment under hydrogen at 573
and 623 K when the amounts of H2SO4 added are, respectively,
of 35 and 15 ml/g of zirconium hydroxide, whatever their specific
surface areas. A close correlation is found between the activities
of the catalysts and their specific surface areas. This suggests that
the active sites are located at the low coordination sites of zirconia
and that their efficiency increases with their dispersion. The acidity
determined by IR spectroscopy of NH3 adsorption is essentially of
Lewis type after evacuation at 573 K. However, Brønsted acidity
results from a reversible redox equilibrium between hydrogen and
zirconia. The isomerization of n-hexane is considered to follow a
c
acid site (4–6). 18O exchange studies led Lavalley et al. (7, 8)
to propose the existence of a tricoordinated sulfate species
==
with a single S O bond which is transformed to a bidentate
species upon water addition by the breaking of a M–O–S
bond. This accounts for the high Brønsted acidity of these
compounds due to the formation of SOH groups. Quan-
tum chemical calculations show that the stronger acidity
should be of the Brønsted type (9), and protons with an acid
strength comparable to that of high-silica zeolites have in-
deed been observed on sulfated zirconia at 873 K and kept
in air (10).
During catalytic reactions a rapid deactivation occurs
which can be reduced by loading a metal, generally Pt, and
using H2 atmosphere (11–13). However, Garin et al. (14)
stabilized the activity of the ZrO2–SO24ꢁ compounds with-
out a metallic function, by merely adjusting the hydrogen
pressure.
classical bifunctional mechanism.
ꢀ 1996 Academic Press, Inc.
Spectroscopic studies of the acidity of platinum sulfated
zirconia catalysts have shown that Brønsted acid sites are
generated when heating under H2 flow with a concomi-
tant decrease of Lewis acidity. The explanation involves
the dissociation of molecular hydrogen on platinum and
spillover of the hydrogen atoms on to the ZrO2–SO24ꢁ sur-
face (11). Due to this effect, the activity was proposed to be
restricted to the interface between Pt and sulfated zirconia
(12). A bifunctional mechanism was also claimed in other
reports (15).
In addition to these determinations concerning the na-
ture of the active sites, several studies have revealed the in-
fluences of the atmosphere and temperature of treatment,
type of the oxide support, and amount of SO24ꢁ added, but
the role of several other parameters is still not well under-
stood. Moreover, the acid strength and stability of these
INTRODUCTION
High catalytic activities for the isomerization reactions of
alkanes are found with catalysts of sulfate-promoted metal
oxides such as Fe2O3, TiO2, and ZrO2, due to the enhance-
ment of their acidic properties. Superacid sites were even
proposed (1–4). These originate from the introduction of
sulfate ions exchanged on the support followed by a cal-
cination at a judiciously chosen temperature (5, 6). This
induces the transformation of the isolated sulfate ion to
catalytically active sites whose structure has been exten-
sively studied since the initial work of Hino et al. (2).
1
Present address: Institut de Recherches sur la Catalyse, CNRS, 2
Avenue Albert Einstein, F-69626 Villeurbanne Ce´dex, France.
18
0021-9517/96 $18.00
c
Copyright ꢀ 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

ACTIVATION OF SULFATED ZIRCONIA
19
catalysts are very dependent on their mode of preparation of Pt/Al2O3 catalyst (0.37% in weight Pt, H/Pt = 0.9).
and on activation conditions. Usual activation includes a Hydrogen used as carrier gas was saturated with n-hexane
treatment in oxidizing atmosphere, but the influence of a whose partial pressure was 41 Torr and passed over the cata-
pretreatment in a reducing atmosphere has been much less lyst with a weight hourly space velocity (WHSV) of 0.8 hꢁ1
.
investigated.
The analysis of the products was made on line with a Carlo
This paper is devoted to study of the activation conditions Erba chromatograph equipped with an OV1 capillary col-
of sulfated zirconia giving reproducible high catalytic activi- umn (50 m, 0.2 mm).
ties. The influence of the dispersion of the sulfate entities on
Characterization
the catalytic activity and thermal stability was investigated.
The enhancement in activity obtained by an increase of the
dispersion of the acid sites is well known in solid acids such
as zeolites and pillared clays (16, 17).
The samples were characterized by X-ray powder diffrac-
tion (XRD), thermogravimetric analysis (TGA), nitrogen
physisorption, and infrared spectroscopy.
The sulfation of zirconia or titania is generally performed
by adding aqueous sulfuric acid or ammonium sulfate. In
the present work we chose to add different quantities of
H2SO4 to zirconia supports of different surface areas. The
catalytic test used the skeletal isomerization of hexane.
Elemental analysis of S was performed on the solids by
the Service Central d’Analyse CNRS (Solaize, France), by
decomposition of the sample above 1273 K and potentio-
metric titration of SO2 trapped in a solution.
X-raydiffraction patternswere recorded on a CGR Theta
60 instrument using CuKꢀ1 radiation. The crystal phases
were identified using the ASTM 13-307 and 17-923 cards.
The crystallite size (D111) of the monoclinic and metastable
tetragonal phases was determined from the (111) reflection
by using the Scherrer relationship,
EXPERIMENTAL
Sample Preparation
Zr(OH)4 compounds were obtained in aqueous me-
dia by hydrolysis of ZrOCl2, 8H2O or ZrONO3, xH2O
salt solutions (0.4 M) with adjusted quantities of NH4OH
(6 M)(Table 1, in which Z-60, for instance, represents a solid
with a surface area of 60 m2/g). The precipitates were stirred
for 12 h at ambient temperature, filtered, and washed with
hot deionized water (340–360 K) to remove excess salts
(AgNO3 or (NH4)SO4 tests). The products were dried at
383 K for 24 h.
The sulfated compounds were obtained by impregnation
of the dried supports prepared as described above with
0.5M H2SO4 solution (15 or 35 ml/g support) and further
dried at 383 K for 24 h.
D111 = 0.91/(L2 ꢁ d2)1/2 cos ꢁ,
where L is the width of the (111) peak at half maximum and
d the width due to the broadening of the peak (reference
d = 0.133 for mica).
The crystallization and the crystal phase transformation
were studied by calcination of the samples in situ under air
flow in an X-ray diffraction furnace CGR (system` e Barret-
Gerard). The patterns were recorded every 50 K. The heat-
ing rate was 5 K/min.
TGA was carried out on a Setaram TG 85 1000ꢂC mi-
crobalance operating in a flow of dry nitrogen (110 ml/min)
at a heating rate of 2 K/min.
Specific surface areas and micropore volumes were ob-
tained from the adsorption isotherms determined for N2 at
liquid nitrogen temperature using the BET and Dubinin
equations.
Catalytic Measurements
Catalytic properties were studied for n-hexane isomeri-
zation at atmospheric pressure in an all-glass microflow
reactor with a fixed bed, using a mechanical mixture pre-
pared by grinding 80 mg of sulfated zirconia and 160 mg
IR spectra were measured at 2 cmꢁ1 resolution on a
Nicolet 320 FT-IR spectrometer. Self-supported wafers of
about 20 mg/cm2 were used. Ammonia was chosen as probe
molecule to characterize the acidity. Infrared spectra were
recorded at room temperature in all cases.
TABLE 1
Synthesis Conditions and Crystallographic State
of the Zirconia Samples
RESULTS
Control of the
Characteristics of the Supports
Final pH of
precipitation precipitation
pH during Crystallographic
Support
Z-60
Z-207 ZrOCl2, 8H2O
Z-236 ZrONO3, xH2O
Z-300 ZrOCl2, 8H2O
Precursor
state
All samples were in the amorphous state before calcina-
tion as shown, for example, in the X-ray diffractogram of
Z-60 (Fig. 1). Their specific surface areas depend on the
starting salt (chloride or nitrate) and whether the pH
was thoroughly controlled during the NH4OH addition
ZrOCl2, 8H2O
6
12
10
10
No
No
Yes
Yes
Amorphous
Amorphous
Amorphous
Amorphous

20
TICHIT, EL ALAMI, AND FIGUERAS
FIG. 1. XRD profiles of Z-60 calcined in the X-ray diffraction furnace at increasing temperatures. T, tetragonal phase; M, monoclinic phase.
(Table 1). They increase from 60 to 300 m2 gꢁ1 with a related The tetragonal phase results from the topotactic crystal-
enhancement of the proportion of micropores (Table 2), lization of the amorphous gel. This metastable phase is
representing 60% of the pore volume for the Z-300 sup- indeed due to the lower surface energy of the related par-
port. Thermogravimetric analysis revealed a total weight ticles. When the temperature increase further, the tetrago-
loss in the range of 14 to 17% achieved at 773 K due to the nal crystallites grow and reach the critical size, generally
˚
dehydration and the dehydroxylation ofthe samples. An av- around 100 A, where they transform to the more ther-
erage composition of these precipitated gels between ZrO2, modynamically stable monoclinic phase (19, 20), there-
1.2H2O for Z-207 and ZrO2, 1.4H2O for Z-60 was deduced, fore always of greater size than the former. For example,
which is well under the value of n = 2 for Zr(OH)4.
in the case of Z-236 calcined at 823 K the average sizes
Upon thermal treatment crystallization occurs and the of the tetragonal and monoclinic crystals are respectively
˚
lines of the tetragonal and monoclinic phases successively 100 and 140 A. These phenomena induce a regular de-
appear in the X-ray diffractogram. A typical evolution of crease of the specific surface areas from 236 to 21 m2/g at
these patterns up to 950 K is reported in Fig. 1 for the Z-60 1023 K.
sample. The tetragonal phase is observed after calcination
at 723 K and both the tetragonal and the monoclinic phases
at 773 K in all samples. The relative amounts of these two
Characteristics of the Sulfated Samples
phases at a given temperature also depends, as with the spe-
The sulfated samples prepared by the impregnation of
cific surface area, on the preparation conditions of the gels. Z-60, Z-236, and Z-300 with 35 ml of H2SO4 (0.5 M) and of
These phenomena are in accordance with the changes Z-207 with 15 ml of H2SO4 (0.5 M) are hereafter denoted as
on thermal treatment described in the literature (18). SZ-60-35, SZ-236-35, SZ-300-35, and SZ-207-15 (Table 2).

ACTIVATION OF SULFATED ZIRCONIA
21
TABLE 2
Preparation Conditions and SO24ꢁ Content of the Sulfated Zirconia
H2SO4 retained
on the solid
Calcination
temp.
Specific
Contribution of
micropores
(% )
H2SO4 added
surface area
Support
(K)
(m2 gꢁ1
)
ml
ꢂmol/m2
Catalyst
ꢂmol/m2
wt%
Z-60
NC
NC
NC
NC
823
60
207
236
300
171
40
25
45
58
60
35
15
35
35
35
35
292
36
74
58
102
SZ-60-35
SZ-207-15
SZ-236-35
SZ-300-35
SZ-171-35
SZ-40-35
16.2
4.4
7.5
5
5.5
43.7
9.6
8.9
17.3
14.7
9.2
Z-207
Z-236
Z-300
Z-171
Z-40
Unknown
17.1
Note, NC, noncalcined.
The quantities of SO24ꢁ ions retained, as determined by reported. The sulfated sample (SZ-207-15) remains in the
elemental analysis of S on the solids, are in the range from amorphous state while the parent zirconia (Z-207) support
8.8 to 17% in weight corresponding to surface densities in crystallizes in the tetragonal phase. An increase of nearly
the range of 5–43 ꢂmol/m2 (Table 2). Therefore the density 200 K in the thermal stability has thus been clearly evi-
of SO24ꢁ species decreases regularly from SZ-60 to SZ-236 denced in all samples.
and SZ-300 supports impregnated with the same amount of
The TGA study of SZ-207-15 and SZ-236-35 (Fig. 3)
H2SO4 (35 ml). Its influence on the catalytic activity is, as shows that, in addition to the weight losses in two steps
previously reported, a main objective of the present study. at low temperature attributed to the dehydration of the
The stabilizing effect of the SO24ꢁ ions on the zirconia sup-
port is illustrated in Fig. 2 where the X-ray diffraction pat-
terns of Z-207 and SZ-207-15 after calcination at 620 K are
FIG. 2. XRD profiles of (a) the support Z-207 and (b) the sulfated
SZ-207-15 sample, both calcined at 620 K.
FIG. 3. DTA–TGA curves of (a) SZ-207-15 and (b) SZ-236-35.

22
TICHIT, EL ALAMI, AND FIGUERAS
TABLE 3
support, a third peak is detected at 890 and 923 K. The cor-
responding weight losses of 5.85 and 11% for SZ-207-15
and SZ-236-35 could be related to the decomposition with
emission of SO2 of the sulfate species deposited on these
supports (respectively 8.9 and 17.3 wt% ).
Chen et al. (21) provided evidence for the formation of
Zr3+ species located at the surface of the crystallites after
desorption at high temperature and described the decom-
position of sulfates by the reaction
Variations of the Activities of Pt/SZ-207-15 vs Temperature
in the Reaction of Hexane after A and B Treatments
Activities
(10ꢁ7 mol gꢁ1 sꢁ1
Reaction
temperature
)
(K)
Treatment A
Treatment B
433
443
463
473
493
—
—
0.52
0.60
1.09
0.52
0.55
—
1.51
2.38
2Zr4+ + SO42ꢁ → 2Zr3+ + SO2 + O2.
The amount of Zr3+ detected by ESR represents about
10ꢁ9 mol/g Zr4+, which is to be compared with the present
loss of 10ꢁ4 mol/g of SO2. The formation of Zr3+ only cannot
then account for the weight loss, and it can be proposed that
part of the SO2 is formed by the reaction
0.5 ꢃ 10ꢁ7 mol gꢁ1
s
ꢁ1) of the same catalyst. The catalysts
were consequently always heated under a hydrogen stream
in the following experiments.
The influence of the activation temperature was in turn
investigated using two samples impregnated with different
amountsofSO24ꢁ, Pt/SZ-207-15and Pt/SZ-236-35, activated
between 473 and 773 K under H2 (Fig. 4). Their activities in
the conversion of hexane at 463 K, both in the range of 0.5
to 1.7 ꢃ 10ꢁ7 mol gꢁ1 sꢁ1, vary similarly with a net defined
maximum after activation at 573 and 623 K for Pt/SZ-236-35
and Pt/SZ-207-15. Moreover, the same maximum activity
of 1.7 ꢃ 10ꢁ7 mol gꢁ1 sꢁ1 is found in the two cases.
ZrSO4H → ZrOH + SO2 + 1/2 O2,
which could explain desulfation without reduction.
Influence of the Pretreatment Conditions
As mentioned above, the properties of sulfated zirconia
were evaluated in the metal–acid bifunctional isomeriza-
tion of hexane using a mechanical mixture of the samples
and Pt/Al2O3 as catalysts. In a previous study (22) the con-
version of alkanes was investigated on mechanical mixtures
of Pt/Al2O3 and dealuminated mazzite zeolites. The activity
appeared to be controlled by the acid isomerization above
1 m2 Pt/g of zeolite, for catalysts showing activities as high as
3.16 ꢃ 10ꢁ6 mol secꢁ1 gꢁ1. The present catalysts are slightly
less active, so all the samples were tested as mechanical mix-
These studies on the activation conditions provided evi-
dence that higher activities resulted from treatment under
hydrogen. The catalysts have a similar behavior in the tem-
perature range from 473 to 673 K. However, the maximum
activity is observed at a different temperature when the
content of SO24ꢁ is varied. The shift is 50 K toward lower
temperatures when the amount of H2SO4 used for impreg-
nation increases from 15 to 35 ml/g of support.
turesconstituted by160mgPt/Al2O3 and 80mgZrO2/SO42ꢁ
corresponding to 1 m2 Pt/g zirconia.
,
Some knowledge on these systems could also be deduced
from the structural changes observed during the activa-
tion and the catalytic reactions. The SO24ꢁ content of the
used catalystsinitiallyreduced at the temperaturesreported
above are reported in Fig. 4. Fifty to 60 wt% of the SO42ꢁ
species are decomposed at 523 K, and then the amount de-
creases only slightly up to 673 K. Therefore, for a given
sample, the changes in activity observed between 520 and
673 K are not related to those of the S content but probably
to the nature of the sulfur species.
A small deactivation was noticed with time on stream. All
conversions were measured at the steady state achieved,
under our experimental conditions, after 30–40 min. Iso-
merization selectivity is higher than 90% depending on
the conversion level. The main products are isohexanes
(2- and 3-methylpentane) with small amounts of 2,3-
dimethylbutane and 2,2-dimethylbutane, not separated
from 3-methylpentane.
Before testing, the catalysts were activated under a gas
flow at a given temperature. The influence of a reducing
or oxidizing atmosphere was particularly investigated. Two
procedures were tested with Pt/SZ-207-15 catalyst:
SO24ꢁ species decomposed in great amount above 473 K
during the catalytic reaction are therefore considerably less
stable than previously reported in the TGA study where
the corresponding peaks were found at 923 and 890 K for
SZ-236-35 and SZ-207-15. This is due to the respective pro-
cedures involving a treatment under air during TGA and
under hydrogen during catalytic reaction. In the former
Treatment A: a calcination under dry air at 573 K for 4 h
followed by a treatment for 2 h at 673 K under H2 stream
(1 atm);
Treatment B: 2 h at 673 K under a hydrogen stream only.
As reported in Table 3, following the B treatment the the SO24ꢁ species are decomposed with emission of SO2.
activity is higher. A calcination step under dry air increases In the latter a metal-catalyzed reduction of SO2 can dis-
by nearly 30 K the threshold activity (arbitrarily fixed at place the equilibrium to lower temperatures. Whatever the

ACTIVATION OF SULFATED ZIRCONIA
23
FIG. 4. Variations of the activity and of the SO24ꢁ content of (᭹)(ꢀ)Pt/SZ-207-15 and ( )( )Pt/SZ-236-35 vs activation treatment under hydrogen,
in the reaction of hexane at 463 K.
procedure involved in the decomposition of SO24ꢁ, enough crystallizes above this temperature, as previously observed
sulfur remainson the zirconia to induce a dopingeffect. This after calcination of the sulfated zirconia in air (Fig. 2).
is illustrated in Fig. 5, giving the X-ray diffraction patterns
Moreover, from the catalysis experiments in the conver-
after catalytic reaction of Pt/SZ-207-15 activated at 673 and sion of hexane at 463 K reported in Fig. 4, it is also notice-
773 K. Sulfated zirconia is still amorphous at 673 K and able that Pt/SZ-236-35 and Pt/SZ-207-15 samples treated at
773 K are inactive. This could be attributed to the crystal-
lization of the support discussed above. Arata (5) reported
that superacid siteswere not created byimpregnation ofsul-
fate ionson the crystallized oxides. Thisidea isreinforced by
the comparison of the three catalysts Pt/SZ-300-35, Pt/SZ-
171-35, and Pt/SZ-40-35.
SZ-171 is obtained using a tetragonal support prepared
as described above by precipitation of ZrONO3, xH2O at
pH 12, followed by a calcination at 823 K. SZ-40 is obtained
using a monoclinic zirconia supplied by Degussa, the spe-
cific surface area of which is 40 m2/g (Table 2). The activities
of these catalysts for the isomerization reaction of hexane
at 463 K are reported in Table 4.
The activities of Pt/SZ-300-35 and Pt/SZ-171-35 show
that the support in the amorphous state gives the more
active catalyst. The monoclinic support leads to a lower
activity in spite of a high sulfur content.
TABLE 4
Activities of Pt/SZ-300-35, Pt/SZ-171-35, and Pt/SZ-40-35
in the Reaction of Hexane at 463 K
SO24ꢁ before
Crystallographic
phase
activation
(ꢂmol/m2)
Activity
(10ꢁ7 mol gꢁ1 sꢁ1
)
Samples
Pt/SZ-300-35
Pt/SZ-171-35
Pt/SZ-40-35
Amorphous
Tetragonal
Monoclinic
5
2.75
1.58
1
FIG. 5. XRD profiles of Pt/SZ-207-15 activated under hydrogen at
(a) 773 K and (b) 673 K after catalytic reaction of hexane. (᭡) Pt/Al2O3
catalyst.
5.52
43.75

24
TICHIT, EL ALAMI, AND FIGUERAS
FIG. 6. Variations of activities vs temperature of pretreatment for (᭹) Pt/SZ-300-35, ( ) Pt/SZ-236-35, and (᭡) Pt/SZ-60-35 in the reaction of
hexane at 463 K.
Influence of the Surface Area of the Zirconia
The threshold activity is 30 K higher for Pt/SZ-300-35 than
for Pt/SZ-60-35, and the activity of the former is then one
order of magnitude higher at 473 K. The selectivities of
the products obtained with Pt/SZ-300-35 and Pt/SZ-236-
35 show that these catalysts have essentially an isomeriza-
tion activity. Cracked productsare obtained onlyat reaction
temperatures above 463 K.
The activities of Pt/SZ-300-35, Pt/SZ-236-35, and Pt/SZ-
60-35 catalysts with decreasing densities of SO24ꢁ species
were compared in the conversion of hexane at 463 K.
As previously shown, a weak maximum appears after
treatment at the same temperature of 573 K with these
supports impregnated with the same amount of 35 ml of
H2SO4 (0.5 M) (Fig. 6). This result obtained with zirconia of
different specific areas confirms that the optimal activation
temperature is mainly related the quantity of H2SO4 added.
Infrared Study
An infrared study was performed on Pt/SZ-236-35 sam-
The respective activities of the three samples pretreated ple to characterize the sulfur species and define the nature
at 573 K were determined at different temperatures (Fig. 7). of the acid sites using ammonia as probe molecule.
FIG. 7. Variations of activities vs reaction temperature in the reaction of hexane for (᭹) Pt/SZ-300-35, ( ) Pt/SZ-236-35, and (᭡) Pt/SZ-60-35
pretreated at 573 K.

ACTIVATION OF SULFATED ZIRCONIA
25
The wafer was pretreated in situ at 573 K for 2 h in a
hydrogen flow in the IR cell, as in the optimum activation
treatment of the catalyst. The spectrum in the range from
1200 to 1800 cmꢁ1 is reported in Fig. 8a. A band is observed
at 1364 cmꢁ1 ascribed to the fundamental frequency of the
asymmetric stretching vibration of SO24ꢁ anions. It is gen-
erally found between 1375 and 1390 cmꢁ1 on several oxide
supports (4). The low frequency shift of this band leads us
to ascribe it to isolated surface SO24ꢁ localized in a crystal-
lographically defective configuration (23). These sites are
FIG. 9. IR spectra of Pt/SZ-236-35 outgassed at (a) 373 K, (b) 423 K,
(c) 473 K, and (d) 573 K.
present in great amount in the samples with high specific
surface areas.
==
Upon NH3 adsorption the S O peak of Pt/SZ-236-35
moves from 1364 to 1300 cmꢁ1, as observed by Lee and Park
(24) for SO24ꢁ–Fe2O3 catalysts. At the same time a broad
band appears at 1618 cmꢁ1 due to ammonia coordinated to
Lewis acid sites (Fig. 8b). Therefore the mechanical mixture
possesses only Lewis acid sites after activation in hydrogen.
33 Torr of hydrogen was then introduced into the cell at
ambient temperature (Fig. 8c). The intensity of the Lewis
band increases and a new band related to ammonium
ions chemisorbed on Brønsted acid sites appears at 1466
cmꢁ1. By heating at 463 K under hydrogen (Fig. 8d) the
intensity of the Brønsted acid band increases greatly. At
==
the same time the S O stretching frequency is shifted to
1257 cmꢁ1
.
An evacuation was performed on this sample up to 573 K
(Fig. 9). Above 373 K, a great decrease of the Brønsted
acidity and a shift of the S–O peak from 1257 to 1273 cmꢁ1
is observed. NH3 adsorbed on Lewis sites disappears after
==
evacuation at 573 K where the S O band returns to its
FIG. 8. IR spectra of Pt/SZ-236-35 (a) treated 2 h under hydrogen
at 573 K, (b) after NH3 adsorption, (c) after introduction of 33 Torr of
hydrogen, and (d) after heating at 463 K.
original position of 1364 cmꢁ1. The peak at 1466 cmꢁ1 char-
acteristic of Brønsted acidity disappears after evacuation

26
TICHIT, EL ALAMI, AND FIGUERAS
above 473 K. The surface sulfur species responsible for the ences the structure of zirconia and increases its thermal sta-
initial Lewis acidity of these sulfated zirconia undergoes bility by at least 200 K. This suggests that the SO24ꢁ species
then reversible changes upon hydrogen adsorption.
may diffuse into the structure. Crystallization into tetrago-
nal and monoclinic phases occurs after activation at 773 K.
Indeed, most of the catalysts cited in the literature show
a maximum in activity after a thermal treatment in air at
about 823 K where they have been converted to the tetrago-
nal and monoclinic forms, and it is then difficult to appreci-
ate the importance of the original crystalline phase of zirco-
nia for acid catalysis. By contrast, the catalysts of this work
are active after reduction at lower temperatures and do not
crystallize below 673 K. The comparison (Table 4) of an
amorphous and a tetragonal ZrO2 containing similar densi-
ties of sulfate anions shows that the nature of the crystalline
phase has only a limited influence on catalytic activity.
For sulfated samples used as acid catalysts it seems now
established that activity passes through a maximum as
a function of the temperature of calcination, because an
optimum amount of sulfate is required at the surface. In an
oxidizing atmosphere the decomposition of sulfates occurs
at relatively high temperature, about 873 K, according to
several authors (5, 6). In H2, however, sulfur is partially lost
by reduction, and the optimum temperature of activation is
then shifted to much lower values. Catalytic activity is not
related only to the respective quantities of SO24ꢁ retained
by the support because catalysts with similar densities but
DISCUSSION
The purpose of this work is to investigate the influence of
preparation on the properties of sulfated zirconia, and some
information can be obtained on the mechanism of incorpo-
ration of sulfur. Only a small fraction of the sulfate ions of
the solution used for sulfation, about 5–10% , are trapped
by the solid. The comparison of two samples SZ-207-15 and
SZ-236-35, of comparable surface area and treated with dif-
ferent amounts of H2SO4, shows that the amount of sulfate
retained by the solid increases with the concentration of
acid. An equilibrium thus exists. Pure zirconia has a basic
character (25) and is therefore an anion exchanger. Sul-
fate anions can be introduced by one of the two possible
equilibria:
ZrOH + HSOꢁ4 → ZrSO4H + OHꢁ
or
Zr–O ꢀ O
2 ZrOH + SO24ꢁ
→
\Sꢀ + 2 OHꢁ.
Zr–Oꢀ \\O
This equilibrium is displaced in acid medium since OHꢁ prepared with 15 ml (Pt/SZ-207-15) and 35 ml of H2SO4
anions are then neutralized by protons, and this accounts (Pt/SZ-300-35) show different behaviors. This might be re-
for the influence of the precursor, ammonium sulfate or lated to the poisoning effect of SO2 on the metallic function,
sulfuric acid, reported by many authors (3, 5).
but since rather similar quantities of SO2 were released
It can be pointed out that for a constant acid solution, the at 573 and 623 K for different quantities of H2SO4 added
fraction of sulfate anions retained by noncalcined solids is (35 and 15 ml) this hypothesis then appears less probable.
higher for high surface areas and decreases when zirconia
For a given catalyst the variation of activity for treatment
is calcined at 820 K. The basic sites of oxides are usually temperatures between 473 and 673 K is accompanied by a
related to defects, edge or corner atoms, which are more smooth variation of the S content. These phenomena sug-
numerous on small particles and disappear upon thermal gest that the increase in activity is related to a change in the
treatment (26, 27). This proposal, established with MgO nature of the sulfur species. For pretreatments in oxidizing
and CaO, would account for the influence of surface area atmosphere, Chen et al. (21) also reported previously that
on the sulfation of zirconia.
the nature of the sulfur species is more important than the
The acid strength of sulfated zirconia is still a matter of S content for samples also prepared by impregnation with
debate. A super-acid character evidenced by some authors H2SO4 ofnoncalcined zirconia gels. Morterra etal. (23) have
(6, 11–13) was denied by other (28). The results reported shown that the polynuclear sulfates are decomposed and
in this paper show that the catalytic properties of sulfated yield some isolated sulfates when the calcination tempera-
zirconia are related to their preparation method and acti- ture of sulfated zirconia increases. This could account for
vating treatments. For zirconium oxide it was reported that the increase in activity observed in our case. On the other
acidic and basic properties change with the pretreatment hand, when the calcination temperature increases further,
temperature and that both the acid and basic sites of this the isolated sulfates such as the low coordination sites dis-
oxide act in many catalytic reactions (29). When sulfate appear and this could be easily related to the decrease in
anions are introduced in variable amount onto zirconium activity observed.
oxide the properties of the acid catalysts thus obtained are
related to the pretreatments.
For a given quantity of H2SO4 impregnated onto zirco-
nia the activity is higher for the higher values of the surface
The thermal pretreatment changes the amount of sulfate area. This suggests that the active species are preferentially
remaining at the surface and induces a decrease of the sur- formed at the low coordination sites located at defects, cor-
face area and a crystallization of the oxide. Sulfation influ- ners, and edges of the crystals. These sites are isolated and

ACTIVATION OF SULFATED ZIRCONIA
27
in weak interaction, like acid sites in dealuminated zeolites,
which is a favorable situation for high acidity.
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127 (1992).
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150, 143 (1994).
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The catalysts resulting from the mixture of platinum on
alumina and sulfated zirconia are active for the isomer-
ization of hexane. This can be considered as experimental
evidence that the reaction obeys a metal–acid bifunctional
mechanism involving successive steps on metallic and acid
sites (30). The alkane dehydrogenates into olefin on Pt and
then diffuses to the acid sites where isomerization occurs.
The isomerized olefins are hydrogenated on the Pt sites and
the alkanes are desorbed. It is well established that activity
and selectivity in alkane hydroconversion on a bifunctional
catalyst depend on the balance between the hydrogenating
function and the acidic function (30). In the presence of
Pt in excess the limiting step is the acid-catalyzed isomeri-
zation of the intermediate olefin. This mechanism implies a
protonation of the olefin, and so should work better in the
presence ofprotons. The originalsulfated zirconia isa Lewis
acid, but the partial reduction of zirconia assisted by the
dissociation of hydrogen on Pt leads to its transformation
into a Brønsted acid as revealed by IR spectroscopy. The
following equilibrium is involved on the zirconia support:
Zr4+ + 1/2 H2 → Zr3+ + H+
[1]
This is supported by the observation that the Brønsted
acidity created by interaction with H2 increases with the
temperature of adsorption. The Brønsted sites are probably
the active sites of the metal–acid bifunctional mechanism
of the isomerization reaction.
21. Chen, F. R., Coudurier, G., Joly, J-F., and Ve´drine, J. C., J. Catal. 143,
616 (1993).
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69, 341 (1991).
However, in the absence of hydrogen, the number of acid
sites resulting from hydration decreases with the tempera-
ture of evacuation (11) or shows a high thermal stability
(31). Kustov et al. (10) reported that the intensities of the
related bands started to decrease only after heating above
820 K. Here, Brønsted acidity decreases after evacuation
above 473 K, which is far too low for dehydroxylation of
the surface. A second mechanism accounting for this phe-
nomenon is the reversal of equilibrium (1). In that case H+
ions are reduced to H2 and the surface is reoxidized. This
equilibrium can account for the positive order relative to H2
observed by Iglesia et al. (32), since in that case the activity
would be proportional to the concentration of protons and
thus to the pressure of hydrogen.
23. Morterra, C., Cerrato, G., and Bolis, V., Catal. Today 17, 505 (1993).
24. Lee, J. S., and Park, D. S., J. Catal. 120, 46 (1986).
25. Cotton, F., and Wilkinson, G., “Advanced Inorganic Chemistry,” 4th
ed. Wiley-Interscience, New York, 1980.
26. Garrone, E., and Stone, F. S., “Proceedings, 8th International
Congress on Catalysis, Berlin, 1984.” Dechema, Frankfurt-am-Main,
1984.
27. Tanabe, K., Misono, M., Ono, Y., and Hattori, H., in “New Solid Acids
and Bases, Their Catalytic Properties,” p. 38, Kodansha-Elsevier,
Tokyo, 1989.
28. Srinivasan, R., and Davis, B. H., in “Symposium on Alkylation, Arom-
atization, Oligomerization and Isomerization of Short Chain Hydro-
carbons over Heterogeneous Catalysts,” Vol. 36, p. 635. Preprints,
ACS Petroleum Division, New York City Meeting, August 25–30,
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29. Tanabe, K., Mater. Chem. Phys. 13, 347 (1985).
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33. Boulet, M., Bourgeat-Lami, E., Fajula, F., Des Courie`res, T., and
Garrone, E., “Proceedings, 9th International Zeolite Conference,
Montreal 1992” (R. von Ballmoos, J. B. Higgins, and M. M. J. Treacy
Eds.), Butterworth-Heinemann, Stoneham, 1993.
In conclusion, it can be remarked that these sulfated zir-
coniasexhibit catalyticactivitiesfor n-hexane isomerization
comparable to those of dealuminated mazzite (33) or BEA
(34). Thus they appear to be strong acids, but not super-
acids, at least in our operating conditions.
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