Reaction of hexane, cyclohexane, and methylcyclopentane over gallium-, indium-, and thallium-promoted sulfated zirconia catalysts

Article abstract of DOI:10.1006/jcat.1998.2256

Sulfate-zirconia catalysts were prepared via the colloidal-sol-gel method using ZrOCl₂ as precursor. After precipitation and washing, the solids were peptized with a 1:1 mixture of CH₃COOH and H₂SO₄. The promoters were introduced as gallium, indium, or thallium nitrates into the solution containing the zirconium sols. After a gel formation period, the samples were dried and calcined. The catalysts were characterized by chemical analysis, nitrogen sorption isotherms, FTIR analysis of adsorbed pyridine and deuterated acetonitrile, XRD, laser Raman and FTIR spectroscopy and XPS. The catalytic tests were performed in a fixed-bed quartz microreactor using as reactant molecules hexane, cyclohexane, and methylcyclopentane. Characterization of the samples by the different techniques allowed us to propose that the main factors controlling the observed properties are the high sulfur content and the promoter effect. Low surface areas of the catalysts were related to the high sulfur contents. Sulfate species in different coordinations were indentified: isolated sulfate groups, polynuclear sulfate species, and sulfuric acid. Lewis acid sites as well as the polynuclear sulfate species determined the acidity of these solids. The presence of promoters contributed not only to the increase of the redox behavior of the catalysts, but also to a decrease of the population of polynuclear sulfates, in the order Tl > In > Ga. In the reaction conditions adopted, on unpromoted catalysts and (especially) Gapromoted catalysts, dehydroizomerization and ring isomerization were the principal reaction pathways for hexane and methylcyclopentane, and for cyclohexane, respectively. Tl was found to promote ring cleavage. These data are consistent with previous studies pointing to the dehydrogenation activity of Ga embedded in zeolite matrices.

Full text of DOI:10.1006/jcat.1998.2256

JOURNAL OF CATALYSIS 180, 66–84 (1998)
ARTICLE NO. CA982256
Reaction of Hexane, Cyclohexane, and Methylcyclopentane
over Gallium-, Indium-, and Thallium-Promoted
Sulfated Zirconia Catalysts
,1
ꢀ
V. Paˆ rvulescu, S. Coman,† V. I. Paˆ rvulescu,† P. Grange,‡ and G. Poncelet‡
ꢀ
Institute of Physical Chemistry, Spl. Independentei 202, Bucharest 77208, Romania; †Department of Chemical Technology and Catalysis,
Faculty of Chemistry, University of Bucharest, B-dul Republicii 13, Bucharest 70346, Romania; and ‡Unite´ de Catalyse et Chimie des
Mate´riaux Divise´s, Universite´ Catholique de Louvain, Place Croix du Sud 2/17, 1348 Louvain-la Neuve, Belgium
Received May 15, 1998; revised August 3, 1998; accepted August 4, 1998
lower alkanes. Recently, other solid catalysts such as sul-
fated alumina (6) and sulfated ceria (7) have been reported.
Many studies have focused on the preparation and charac-
terization of sulfated zirconia because these catalysts were
claimed to exhibit superacidic properties. However, this
point of view has recently been questioned (8, 9) and now
it is generally accepted that these solids are merely strong
acid catalysts.
Sulfate–zirconia catalysts were prepared via the colloidal-sol–gel
method using ZrOCl2 as precursor. After precipitation and wash-
ing, the solids were peptized with a 1 : 1 mixture of CH3COOH and
H2SO4. The promoters were introduced as gallium, indium, or thal-
lium nitrates into the solution containing the zirconium sols. After
a gel formation period, the samples were dried and calcined. The
catalysts were characterized by chemical analysis, nitrogen sorp-
tion isotherms, FTIR analysis of adsorbed pyridine and deuter-
ated acetonitrile, XRD, laser Raman and FTIR spectroscopy, and
XPS. The catalytic tests were performed in a fixed-bed quartz mi-
croreactor using as reactant molecules hexane, cyclohexane, and
methylcyclopentane. Characterization of the samples by the differ-
ent techniques allowed us to propose that the main factors con-
trolling the observed properties are the high sulfur content and the
promoter effect. Low surface areas of the catalysts were related
to the high sulfur contents. Sulfate species in different coordina-
tions were indentified: isolated sulfate groups, polynuclear sulfate
species, and sulfuric acid. Lewis acid sites as well as the polynuclear
sulfate species determined the acidity of these solids. The presence
of promoters contributed not only to the increase of the redox be-
havior of the catalysts, but also to a decrease of the population
of polynuclear sulfates, in the order Tl > In > Ga. In the reaction
conditions adopted, on unpromoted catalysts and (especially) Ga-
promoted catalysts, dehydroizomerization and ring isomerization
were the principal reaction pathways for hexane and methylcy-
clopentane, and for cyclohexane, respectively. Tl was found to pro-
mote ring cleavage. These data are consistent with previous studies
pointing to the dehydrogenation activity of Ga embedded in zeolite
Early in the nineties, a new class of sulfated metal ox-
ides was described by Hsu and co-workers (10–13) who
showed that promotion of sulfated zirconia with Fe and
Mn increased activity by nearly three orders of magnitude
and that these new catalysts were able to isomerize bu-
tane at temperatures as low as near room temperature. In
subsequent studies on the reaction of butane and propane,
Cheung et al. (14, 15) concluded that promoted sulfated zir-
conia is a stronger acid than sulfated zirconia. Davis and
co-workers (16, 17) and Srinivasan et al. (8) proposed that
the particular activity of these catalysts was not due to the
increased acidity, but instead to a modification of the elec-
tron acceptor ability of sulfated zirconia with, as a result,
the enhancement of the redox properties. The first step of
the reaction mechanism of butane consists in the dehydro-
genation of the alkane which, in the second step, leads to the
iso-alkane via a bimolecular mechanism. Lately, Ghenciu
and Farcasiu (18) provided evidence for the existence of
an additional radical pathway. The modification of the
ZrO2 surface by iron was further confirmed by Sikabwe
and White (19). Promotion of sulfated zirconia with Ni
was found by Alvarez et al. (20) to be less effective than
with Fe and Mn. Vera et al. (21) compared the activity
of several promoted sulfated zirconias in the isomeriza-
tion of butane and showed that promotion by Fe, Co, Cr,
W, Ni, or Pt gave more active systems than sulfated zirco-
nias, whereas Cu-, Zn-, or Cd- promoted catalysts were less
active.
matrices.
ꢁc 1998 Academic Press
INTRODUCTION
Some 20 years ago, sulfated oxides such as ZrO2 ꢂ SO42ꢃ
,
TiO2 ꢂ SO24ꢃ, SnO2 ꢂ SO24ꢃ, Fe2O3 ꢂ SO24ꢃ, and HfO2 ꢂ SO42ꢃ
(1–5) initiated a hot topic in catalysis, namely the syn-
thesis of new solid acid catalysts for the isomerization of
Another attempt to improve the activity of sulfated zir-
conia catalysts was made by Lonyi et al. (22), who sulfated
1 To whom correspondence should be addressed.
66
0021-9517/98 $25.00
c
Copyright ꢁ 1998 by Academic Press
All rights of reproduction in any form reserved.

Ga-, In-, AND Tl-PROMOTED SULFATED ZIRCONIA
67
TABLE 1
Chemical Composition and Textural Characteristics of the Investigated Samples
Activated in Ar at 550ꢄC
Activated in air at 550ꢄC
Promoter
content
wt%
Sulfate
content
wt%
Average
pore
Sulfate
content
wt%
Average
pore
SO24ꢃ
Surface area
m2 ꢂ gꢃ1
SO24ꢃ
groups ꢂ nmꢃ2
Suface area
m2 ꢂ gꢃ1
groups ꢂ nmꢃ2
diameter, A
diameter, A
˚
˚
Sample
metal oxide
ZrO2 ꢂ SO24ꢃ
—
19.3
18.2
18.4
18.1
16.3
18.1
26.1
2.71
84
78
75
48
48
49
45
62
18.3
14.2
11.4
5.8
17.4
14.6
12.4
2.5
76
71
68
45
42
40
40
60
ZrO2 ꢂ Ga2O3 ꢂ SO24ꢃ
ZrO2 ꢂ In2O3 ꢂ SO24ꢃ
ZrO2 ꢂ Tl2O ꢂ SO24ꢃ
15.0
15.1
15.0
a mixture of titania–zirconia oxides, but in this case the pro- by inductively coupled plasma atomic emission spectrome-
moting effect was unclear.
try (ICP-AES) and the results are given in Table 1.
All these studies came to the conclusion that the reac-
tion of hydrocarbons with less than five carbon atoms oc-
curs via a bimolecular mechanism (23), while for those with
more than five carbon atoms, a monomolecular mechanism
would be involved (24, 25). In both cases, however, the de-
hydrogenating function of the catalysts appears to be very
important.
These results were the starting point of this study. In the
aromaticisation oflower alkanes, it hasbeen proved that the
insertion of Ga in the ZSM-5 structure had a promoting ef-
fect on the dehydrogenation activity (26–31). Attempts at
using In or Tl were also reported (32) but the results were
not as convincing. With this background, our study aimed
to investigate the effect of promoting ZrO2 ꢂ SO24ꢃ with Ga,
In, or Tl in the reaction of C6 hydrocarbons, namely hex-
ane, cyclohexane, and methylcyclopentane. The reaction of
these hydrocarbons over sulfated zirconia catalysts was pre-
viously reported in the literature (22, 33–36). The charac-
terisation of the structures resulting from the promotion
with Ga, In, or Tl was another target of the present inves-
tigation.
2.2. Catalysts Characterization
Samples were characterized by N2 sorption at 77 K, FTIR
analysis of adsorbed pyridine and deuterated acetonitrile,
XRD, laser Raman and laser FTIR spectroscopy, and XPS.
The nitrogen sorption isotherms at 77 K were obtained in
an ASAP 2010 sorptometer from Micromeritics, after out-
gassing the samples at 200ꢄC for 12 h.
The surface acidity was characterized by IR spectroscopy
of adsorbed pyridine (Py) and deuterated acetonitrile (DA)
(both with Aldrich purity). The spectra were recorded with
a Nicolet 60SX spectrometer, with a resolution of 2 cmꢃ1
.
Prior to adsorption of the bases, the powder samples were
calcined at 550ꢄC for 2 h under an air flow of 30 ml minꢃ1
.
Self-supporting wafers obtained by compression (about
12 mg cmꢃ2) were outgassed in the IR cell at 400ꢄC at a
residualpressure of0.1mPa. After adsorption ofthe probes,
the samples were purged for 2 h with He at 150ꢄC to remove
the weakly sorbed species.
XRD analysis of the samples was made with a SIEMENS
D-5000 diffractometer powered at 40 kV–50 mA, and
equipped with a variable-slit diffracted-beam monochro-
mator and scintillation counter. The diffractograms were
recorded in the range 0–80ꢄ 2ꢀ at a scanning speed of 0.5ꢄ
˚
METHODS
2.1. Catalysts Preparation
The catalysts were prepared via the colloidal-sol–gel 2ꢀ minꢃ1 using CuKꢁ radiation (ꢂ = 1.5418 A).
method. Briefly, 90 mmol ZrOCl2 ꢂ 6H2O were dissolved
The Raman spectra were recorded on a Dilor–Jobin
in water and precipitated with NH3 solution (18 wt% ) at Yvon-SPEX spectrometer equipped with an optical mul-
pH = 9.0. The precipitate was washed free of Clꢃ with dis- tichannel analyzer. The Raman spectra were excited with
tilled water and then peptized with a 1 M solution of a 1 : 1 the 488-nm line of an Ar+-ion laser. Self-supporting wafers
mixture of CH3COOH and H2SO4. Peptization was com- were prepared from powder samples preactivated for 6 h
pleted after 6 h. To the solution containing zirconium sols, at 550ꢄC in Ar or in air.
0.12M solution ofeither gallium, indium, or thallium nitrate
The XPS spectra were recorded using a SSI X probe
was added. Gelification was carried out at 60ꢄC for 3 weeks. FISONS spectrometer (SSX-100/206) with monochro-
The samples were dried overnight under vacuum and then mated AlKꢁ radiation. The spectrometer energy scale
calcined at 550ꢄC for 6 h in argon flow for the one part,ꢃo1r was calibrated using the Au 4f7/2 peak (binding energy
in air flow for the other part, using a ramp of 0.14ꢄC min
.
84.0 eV). For the calculation of the binding energies, the
The chemical composition of the samples was determined C 1s peak of the C–(C,H) component at 284.8 was used as an

ˆ
PARVULESCU ET AL.
68
internal standard. The composite peaks were decomposed
by a fitting routine included in the ESCA 8,3 D software.
2.3. Catalytic Tests
Catalytictestswere performed in vapour phase, in a fixed-
bed quartz microreactor operated at atmospheric pressure
using 0.4 g catalyst (calcined in situ in the conditions men-
tioned earlier), and as reactants hexane, cyclohexane, and
methylcyclopentane (allfrom Merck). The hydrocarbon/Ar
(1 : 1 ratio) gas mixture was preheated at 120ꢄC and then
passed over the catalyst bed at space velocity of 5.4 hꢃ1
.
The experiments were carried out in the temperature range
120–240ꢄC. One-line analysis of the gaseous effluent was
made in a gas Pye-Unicam 104 chromatograph. The reac-
tion rates were determined by extrapolating the declining
conversions to zero time on stream. The apparent activation
energies were obtained from the Arrhenius plots. TOF was
calculated as mmol hydrocarbon reacted/mmol sulfate/s.
RESULTS
3.1. Textural Characteristics
Figures 1 and 2 show the N2 adsorption–desorption
isotherms of the samples activated in argon and air, re-
spectively. The hystereses were of type II for the samples
activated in Ar, except for the Tl-containing sample, which
was of type III. According to Sing and Rouquerol (37), such
a hysteresis indicates a porous structure in which the adsor-
bent does not possess a well-defined mesopore structure.
The pore size distribution calculated from the desorption
branch (BJH method) was quite narrow for ZrO2 ꢂ SO42ꢃ
as well as for the samples containing Ga or In, whereas for
the Tl-promoted sample, a second maximum corresponding
FIG. 1. Nitrogen adsorption–desorption isotherms of samples acti-
vated in argon flow.
˚
to pores with 65-A diameter was observed. The values are
The spectra show the typical Py-IR absorption bands of
such materials (38–41) assigned to the 8a and 19b vibration
modes of adsorbed-Py forming Lewis-type adducts (Py-L
with bands at 1610 and 1448 cmꢃ1), and to the 8a and 19b
modes of Py in interaction with Brønsted acid sites (Py-B
with bands at 1640 and 1542 cmꢃ1). The intensity of these
bands changes from one catalyst to the next one, according
to the chemical composition and atmosphere in which these
samples were activated. The two other bands at 1577 and
1490 cmꢃ1 are assigned to Py adsorbed on either Lewis or
Brønsted acid sites.
given in the Table 1 together with the BET specific surface
areas. It is noticeable that all the BET surface areas were
almost identical to those calculated by the t-plot method,
confirming the absence of a microporous texture. The Tl-
containing sample exhibited lower BET surface area and
greater pores than the other samples.
Air activation gave solids with lower BET surface areas
than those activated in Ar. Near type III hystereses were
obtained for all the samples (Fig. 2). The textural charac-
teristics of the samples activated in air are given in Table 1.
A closer examination of these spectra shows several
features. For the samples activated in Ar (solid line) at
550ꢄC:
3.2. Measurements of Acidity by FT-IR Spectroscopy
of Adsorbed Probe Molecules
3.2.1. FT-IR Spectra of Adsorbed Pyridine
(i) The intensity of the band at around 1610 cmꢃ1 in-
Figure 3 shows the FT-IR spectra recorded at room tem- creases going from the bottom spectrum (sample contain-
perature in the region 1400–1700 cmꢃ1, after adsorption of ing Tl) to the top spectrum (sample without promoter). It
Py on the different sulfated-zirconia samples activated in seems that the replacement of Tl by Ga or In, and partic-
Ar (solid line) and in air (dotted line), respectively.
ularly the absence of promoter, leads to an enhancement

Ga-, In-, AND Tl-PROMOTED SULFATED ZIRCONIA
69
Activation of the same samples in air at 550ꢄC has the
following effects:
(i) In the case of the Lewis acid sites, air activation
leads to a very small shift (3–4 cmꢃ1) to higher wavenum-
bers of both bands assigned to adsorbed Py species, irre-
spective of the nature of the promotor. Also, a reduction
of the band intensities is noticed, which may suggest that
air activation facilitates the interaction of sulfate and of
polysulfate groups with the ZrO2 carrier with, as a result,
a decrease of the amount of the Brønsted sites and a small
increase of the strength of the Lewis sites.
One should note that for the air-activated samples,
the band assigned to Py-L is located at slightly higher
FIG. 2. Nitrogen adsorption–desorption isotherms of samples acti-
vated in air flow.
of the acidity of the surface Lewis acid sites. Such a be-
havior, according to Morterra et al. (42), could also be as-
sociated with the formation of polynuclear sulfates which
is precluded by the presence of Tl. The second band, due
to Py adsorbed on Lewis acid sites (1448 cmꢃ1), follows
similar behavior with the modification of the chemical
composition.
(ii) The intensity of the band at 1542 cmꢃ1 is also
sensitive to the chemical composition. The population of
Brønsted acidic sites is higher in the nonpromoted sample
than in the promoted ones. A similar observation was pre-
viously reported by Nascimento et al. (43) for increasing
sulfur contents. Indeed, these authors also observed a shift
in the band position with the modification of the sulfur con-
tent. This could be another indication that the presence of
promoters, and mainly Tl, inhibits the formation of some
complex sulfates which exhibit Brønsted acidity.
(iii) Generally, the band located around 1490 cmꢃ1 is
not discussed in the literature because it characterizes Py in-
teracting with both Lewis and Brønsted acid sites. However,
the variation of this band shows that promotion produces
an increase of the sum of both kinds of acid sites.
FIG. 3. FT-IR spectra of Py adsorbed on the different sulfated-
zirconia samples activated in Ar (solid line) and in air (dotted line).

ˆ
PARVULESCU ET AL.
70
wavenumbers compared with the spectra recorded for sam-
(i) The intensity of the band at around 2305 cmꢃ1 de-
ples activated in argon. The presence of promoters pro- creases in the order ZrO2 ꢂ SO24ꢃ > ZrO2 ꢂ Ga2O3 ꢂ SO24ꢃ
>
duces a shift to lower wavenumbers.
ZrO2 ꢂ In2O3 ꢂ SO42ꢃ > ZrO2 ꢂ Tl2O ꢂ SO24ꢃ. This decrease, to-
(ii) For the promoted samples, a relatively more pro- gether with a simultaneous shift of the band by about
nounced decrease of the band intensity is observed for the 10 cmꢃ1 to lower wavenumbers, suggests that promotion
bands assigned to Py adsorbed on Brønsted acid sites. But results in a lowering of the total amount of the acid sites
in this case, the shift occurs in the opposite direction, i.e., (Brønsted and Lewis) and of the average acid strength.
to lower wavenumbers. This behavior could indicate that
(ii) The interaction of the basic CN with acidic pro-
there is, indeed, a direct relation between the strength of tons present in sulfated oxides shifts the OH band to lower
Brønsted acid sites and the promoting species.
wavenumbers and causes band broadening. The area of this
broad band is also related to the promoter element.
(iii) Activation in air produces a small decrease in the
intensityofthe broad band between 3650and 2800cmꢃ1 and
of the one near 2305 cmꢃ1. However, no clear shift of these
bands was noticed compared with the spectra recorded for
samples activated in Ar, perhaps indicating some compen-
sation effect between the two types of acid sites (Brønsted
and Lewis).
3.2.2. FT-IR Spectra of Deuterated Acetonitrile
Figure 4 shows room temperature FT-IR spectra in the
region 2200–3800 cmꢃ1 of CD3CN adsorbed on the differ-
ent sulfated samples activated in Ar (solid line) and in air
(dotted line). The spectra show broad absorptions between
3650 and 2800 cmꢃ1 indicating an interaction of CD3CN
with acidic protons present at the surface of the samples.
The other band at 2305 cmꢃ1, according to van Santen and
co-workers (44, 45), corresponds to the C–N stretching fre-
quency of CN bonded either to Brønsted or to Lewis acid
sites. The other two bands at 2250 and 2112 cmꢃ1 are due
to the C–D stretching modes.
3.3. Structural Characterization
3.3.1. Raman Spectra
Raman spectra recorded for samples activated in Ar and
air are presented in Figs. 5 and 6, respectively. The spectra
show some clear differences and, therefore, they will be
discussed separately. There is already a good agreement in
the literature on the band assignment (46–48).
The examination of the spectra of Fig. 4 provides infor-
mation complementing those obtained from FT-IR of ad-
sorbed Py:
3.3.1.1. Samples activated in Ar. Samples activated in
==
Ar exhibit bands which are attributed to S–O and S O
stretching modes of the sulfate group, at 1017 and
1040 cmꢃ1, and at 1390 cmꢃ1, respectively. According to
Babou et al. (40, 49), the band at 1017 cmꢃ1 corresponds
to S–O stretching modes in isolated SO24ꢃ species, and the
one at 1040 cmꢃ1, to adsorbed (H2SO4) species. The band
located between 1117 and 1127 cmꢃ1 is assigned to S–O
stretching modes in bidentate sulfate species (40, 49). The
bands in the low frequency region (<700 cmꢃ1 ) are mainly
due to zirconia. Recordings of pure tetragonal and mono-
clinic phases allowed us to assign the bands in this region to
these two phases ( Fig. 5). No band that could be assigned
to Ga, In, or Tl oxide was found.
Summarizing, the samples activated in Ar contain both
tetragonal and monoclinic phases, and the sulfate groups
in these catalysts coexist both as isolated and bidentate
species, as well as deprotonated SO24ꢃ groups and H2SO4
adsorbed species. The differences in the relative intensity of
the bands are in direct dependence on the type of promoter.
3.3.1.2. Samples activated in air. Samples activated in
==
air exhibit bands in the region of the S–O and S O stretch-
ing modes, except for a new one, located at 1060 cmꢃ1
(Fig. 6). Accordingto Babou etal. (40, 49), thisband can also
be attributed to S–O stretching modes but in polynucleated
sulfate groups. Its presence indicates that calcination in air
FIG. 4. FT-IR spectra of adsorbed CD3CN on the different sulfated
zirconia samples activated in Ar (solid line) and in air (dotted line).

Ga-, In-, AND Tl-PROMOTED SULFATED ZIRCONIA
71
FIG. 5. Raman spectra recorded for samples activated in Ar at 550ꢄC.
facilitates the condensation of sulfate species to polynu- the Raman bands attributed to the monoclinic phase are
cleated ones. It is interesting to note that the bands near observed.
1117 cmꢃ1 are better defined in the Ga-promoted and for
It appears that activation in air leads to a more hetero-
the unpromoted samples. In the low-frequency region, only geneous surface in which the population of the nucleated
FIG. 6. Raman spectra recorded for samples activated in air at 550ꢄC.

ˆ
PARVULESCU ET AL.
72
sulfate species is increased as in the case of the samples ac- both in Ar and in air, bands due to the tetragonal phase
tivated in Ar. The differences among the investigated sam- show up (bands at 588, 514, and 217 cmꢃ1 ).
ples appear in the relative intensity of the bands which are
directly related to the sulfur content.
The organic contaminants give rise to different absorp-
tions. A band attributed to polyenic cations is observed at
429–433 cmꢃ1. Also, an intense band at 1500–1517 cmꢃ1
==
is assigned to C C stretching vibrations. This band was
3.3.1.2. Samples exposed to hydrocarbon vapour.
Wafers of unpromoted and promoted catalysts (previously
calcined in air and in Ar in the conditions indicated ear-
lier) were exposed for 1 h at 140ꢄC to hexane, methylcyclo-
pentane, and cyclohexane, and the Raman spectra were
recorded after the cells were outgassed. Only the results
obtained for the Ga-promoted systems will be discussed.
The spectra of the samples activated in air and in Ar at-
mosphere and then exposed to the different hydrocarbons
are shown in Figs. 7 and 8, respectively. The spectra of the
samples treated with hexane and cyclohexane exhibit simi-
lar features for both air- and Ar-activated samples. The S–O
stretching modes assigned to isolated deprotonated sulfate
relatively more intense for the sample treated with cyclo-
hexane than with hexane, again independent of the activa-
tion atmosphere. According to Spielbauer et al. (47), the
frequency of this band indicates a more or less pronounced
aromatic character. It was also found that the intensity of
this band increased in the sequence ZrO2 ꢂ Ga2O3 ꢂ SO42ꢃ
>
ZrO2 ꢂ SO24ꢃ > ZrO2 ꢂ In2O3 ꢂ SO42ꢃ > ZrO2 ꢂ Tl2O ꢂ SO42ꢃ
.
The spectra recorded on samples exposed to methylcy-
clopentane differ from those recorded for either hexane or
cyclohexane. The band at 1517 cmꢃ1 is completely absent.
Clear differences are noted in the region of the sulfate
bands as a function of the calcination atmosphere and of
the nature of the organic molecule the samples were ex-
posed to. The spectra recorded for the sample activated in
Ar still show the band at 1019 cmꢃ1 due to nonprotonated
isolated SO24ꢃ groups. For the samples activated in air, this
band disappears but three components are clearly defined
in the region 1385–1430 cmꢃ1. In agreement with several
authors (7, 41, 50, 51), the band located at 1423 cmꢃ1 could
groups (at 1017 cmꢃ1) are absent, while well-defined S O
–
==
and S O stretching bands of adsorbed H2SO4 at 1040 and
1391 cmꢃ1 are observed, the latter probably overlapping
with other species. It is also worth noting that no bidentate
(or polysulfate) species are present (band at 1117 cmꢃ1).
Concomitant with the modifications occurring in the sul-
fate coordination, the spectra show that the structure of
zirconium oxide is also modified. Indeed, after activation
==
be assigned to S O stretching modes in disulfate species.
FIG. 7. Raman spectra of Ga-promoted sulfated zirconia activated in Ar, after exposure to hydrocarbons.

Ga-, In-, AND Tl-PROMOTED SULFATED ZIRCONIA
73
FIG. 8. Raman spectra of Ga-promoted sulfated zirconia activated in air, after exposure to hydrocarbons.
According also with the literature data (5, 7), the bands at phases corresponding to two different types of zirconium
1404, 1385, and 1131 cmꢃ1 are assigned to surface polyden- sulfate (ASTM 24-1492 and ASTM 24-1498) and to tetrago-
tate sulfate species whereas that at 1131 cmꢃ1 to bulk-like nal ZrO2 (ASTM 34-1484). In addition, peaks correspond-
species. It isworth mentioningthat the decomposition ofthe ing to ZrOS (ASTM 4-0897) and to an unknown phase
band centered at 1391–1394 cmꢃ1 in the spectra recorded (indicated by ? in the figures) were also found. No line cor-
after exposure to hexane and cyclohexane shows that this responding to Ga, In, or Tl compounds could be evidenced.
band is also present. But, according to the model proposed
Activation in air leads to more crystalline structures.
by Bensitel et al. (50), which is now generally accepted, it The same crystallographic phases were identified, except
is difficult to explain why the stretching S–O modes disap- for ZrOS, but the number of diffracting planes was much
==
peared while the stretching S O modes became more evi- increased. In addition, instead of tetragonal ZrO2, only the
dent. Another assignement for this band could be the one lines corresponding to monoclinic ZrO2 (ASTM 34-1484)
proposed by Spielbauer et al. (47), who attributed a band were observed. The complexity of the patterns of the pro-
at 1450 cmꢃ1 to increased aromatic character of the organic moted samples and the apparent peak splittings require a
contaminants. But in our case, the shape of the band as well more detailed crytallographic analysis which will be add-
as the missing band at 1517 cmꢃ1, which should also show ressed in further work. Perhaps the phases in presence
up for such species, does not support this assignment. In ad- could be mixtures of zirconium sulfate and promoter-
dition, Ga was found to have no influence on this species. containing zirconium sulfate.
Therefore, we suppose that, taking into account also the
In conclusion, activation in air produces a far more com-
high amount of sulfate in the investigated samples, differ- plex mixture of crystalline sulfate phases compared to acti-
ent monosulfate species co-exist and at least one of those vation in argon.
could correspond to protonated species.
3.3.3. XPS Spectra
3.3.2. XRD Analysis
The X-ray patterns of the sulfated oxide samples acti-
The XPS data are given in the Table 2, and typical XPS
vated in Ar and in air are shown in Figs. 9 and 10, re- spectra in the region of S2p are illustrated in Fig. 11. The
spectively. All the Ar-activated samples contain crystalline data of Table 2 allow us to stress several points.

ˆ
74
PARVULESCU ET AL.
FIG. 9. X-ray diffraction patterns of sulfated zirconia samples activated in Ar.
FIG. 10. X-ray diffraction patterns of sulfated zirconia samples activated in air.

Ga-, In-, AND Tl-PROMOTED SULFATED ZIRCONIA
75
TABLE 2
XPS Parameters of the Investigated Catalysts
ZrO2 ꢂ SO24ꢃ
ZrO2 ꢂ Ga2O3 ꢂ SO24ꢃ
ZrO2 ꢂ In2O3 ꢂ SO24ꢃ
ZrO2 ꢂ Tl2O ꢂ SO24ꢃ
Treated in Ar Treated in Air Treated in Ar Treated in Air Treated in Ar Treated in Air Treated in Ar Treated in Air
Species
S 2p
550ꢄC
550ꢄC
550ꢄC
550ꢄC
550ꢄC
550ꢄC
550ꢄC
550ꢄC
168.9
170.2
184.2
186.6
2.31
—
1.26
—
—
—
—
—
—
169.1
170.4
184.2
186.6
1.92
—
1.69
—
—
—
—
—
—
169.1
170.4
183.9
186.3
2.32
1.97
1.65
162.1
21.4
—
169.4
170.7
183.8
186.2
1.94
1.64
1.89
162.2
21.5
—
168.7
170.0
184.1
186.5
2.60
1.90
1.25
—
169.1
170.4
184.0
186.4
2.15
1.57
1.38
—
168.5
169.8
184.6
187.0
2.66
1.88
1.17
—
—
—
—
120.7
125.2
169.1
170.4
184.5
186.9
2.18
1.54
1.25
—
—
—
—
120.9
125.4
S 2p
Zr 3d5
Zr 3d3
S/Zr
S/(Zr + M)
S169/S170
Ga 3s
Ga 3d
In 3d5
In 3d3
Tl 4f 7
Tl 4f5
—
—
445.8
453.4
—
446.0
453.6
—
—
—
—
—
—
—
—
—
(i) State of zirconium. The values determined for zir- to shift the binding energy to higher values, which could
conium are in a rather large range of binding energies and be caused by the same interaction of sulfate groups with
correspond to Zr in the oxidation state (IV) (52). Promotion the support. At the same time, air activation provokes an
with Ga seems to generate more “acidic” zirconium species increase of the proportion of sulfur atoms belonging to the
compared with nonpromoted sulfated zirconia. Promotion
S2p component with binding energy of 169, which may also
with Tl has an opposite effect, whereas In has almost no reflect an increase ofthe population ofdeprotonated sulfate
effect. The same shift was observed both for the Zr 3d5 and species.
Zr 3d3 binding energies. Air activation seems to have no in-
fluence on the oxidation state of Zr compared with samples the XPS beam concerns a thickness that does not exce-
activated in Ar. ed 2.5 nm. The surface S/Zr atomic ratio was almost the
(iii) Sulfur-to-metal atomic ratio. The penetration of
(ii) State of sulfur. The binding energies of sulfur were same in the nonpromoted and the Ga-promoted samples.
recorded in the region corresponding to the 2p energies. For the samples containing the other promoters, this ra-
As shown in Fig. 11, the peak corresponding to these spe- tio was higher than those determined for the nonpromoted
cies exhibits a pronounced asymmetry. Decomposition of
this peak using the machine routine leads to two compo-
nents, the one with BE at around 169 eV and the other one
at around 170 eV. The correlation of the change of the ratio
of these species with those determined from FT-IR mea-
surements (Py) and Raman allows one to propose that the
first component (at around 169 eV) corresponds to depro-
tonated sulfate species, and the one near 170 eV, to proto-
nated species. The high ratio for the sample promoted with
Ga compared with the non-promoted sample indicates the
predominance of deprotonated sulfated species. Promotion
with In seems to have no effect on the sulfur state, while for
Tl, there is a tendency to a slight increase of the population
of protonated species.
The presence of promoters also results in a tendency to
shift the binding energies from lower values (Tl) to higher
values (Ga), which could indicate a more advanced inter-
action of the sulfate groups with the zirconium oxide sup-
port. The atmosphere in which the samples were activated
also brought about some modifications in the position and
FIG. 11. Typical XPS spectra in the region of S 2p.
shape of the S 2p peak. Indeed, calcination in air also tends

ˆ
PARVULESCU ET AL.
76
clopentane on the catalysts activated in Ar. Promotion with
Ga and In enhanced the selectivity to methylcyclopentane
compared with nonpromoted sulfated zirconia. The dif-
ference (100 ꢃ Sel.)% on Ga- and In-promoted catalysts
mainly represents the cracked products (% ). On nonpro-
moted and Tl-promoted sulfated zirconias, high yields of
branched C6 hydrocarbons were formed.
Figure 13 shows the variation of the dibranched-to-
monobranched hydrocarbon ratio (a) and of the isomerized
C6 hydrocarbons-to-cyclic hydrocarbons ratio (b), respec-
tively, as a function of conversion for both the catalysts ac-
tivated in air and Ar. Air activation favours isomerization
to dibranched hydrocarbons and depresses the dehydrocy-
clization activity.
It is worth noting that in the case of Ga, the reaction
rate was about five times higher than for the nonpromoted
catalyst. An increase of the reaction rate by about 2 was also
determined for the In-promoted catalyst. Introduction of Tl
had an opposite effect, namely a decrease of the reaction
rate relative to sulfated zirconia (Table 3).
FIG. 12. Variation of the selectivity to methylcyclopentane (᭿) and
of the TOF (᭜) for the catalysts activated in Ar (T, 140ꢄC; space velocity,
5.4 hꢃ1).
samples. However, the sulfur-to-total metal content ratio
was lower than the sulfur-to-zirconium ratio in non-pro-
moted samples, and this ratio decreased from the sam-
ple containing Ga to the one containing Tl. One should
note that the sulfur-to-zirconium atomic ratios differ from
those determined from the chemical analyses. Irrespec-
tive of the chemical composition, the value obtained from
XPS measurements exceeded those established by chemi-
cal analysis. From these results, it could be concluded that,
regardless of the chemical composition, sulfate groups are
majoritary on the external surface.
The activation energies given in Table 3 have small val-
ues indicating that adsorption is a determining step in the
reaction. Experiments carried out on catalysts with dif-
ferent particle sizes showed no modification of the reac-
tion rate, suggesting that the mass transfer processes are
not responsible for this behavior. In addition, the values
given in Table 3 were obtained at space velocities as high
as 5.4 hꢃ1
.
Figure 14 shows the variation of the selectivity and of the
TOF when methylcyclopentane was used as reactant. The
main reaction product was cyclohexane. The presence of In
and Tl brought about a decrease of the isomerisation activ-
ity to cyclohexane and an enhancement of the ring cleavage
activity, branched C6 molecules being the dominant prod-
ucts. The cracking products were formed in lesser amounts
3.4. Catalytic Tests
Dehydrocyclization of hexane was the main reaction oc-
curring on the investigated catalysts. Figure 12 shows the
variation of the TOF and of the selectivity to methylcy-
TABLE 3
Conversion of n-Hexane over Sulfated-Zirconium Catalysts
Activated in Ar, 823 K
Activated in air, 823 K
Selectivity, mol%
E
E
Selectivity, mol%
T
TOFa
ꢅ103
(kcal/
mol)
TOFa
ꢅ103
(kcal/
mol)
Sample
(K)
CH
MCP
Cb
(d /m)c
B
CH
MCP
Cb
(d /m)c
B
ZrO2 ꢂ SO24ꢃ
393
513
0.17
1.30
21.4
19.6
39.4
38.4
7.3
8.8
0.29
0.31
0.2
0.3
6.8
7.1
6.8
8.9
0.41
3.30
18.8
12.5
43.3
41.8
7.4
9.0
0.31
0.34
2.3
2.9
7.0
7.0
6.9
9.4
ZrO2 ꢂ Ga2O3 ꢂ SO24ꢃ
ZrO2 ꢂ In2O3 ꢂ SO24ꢃ
ZrO2 ꢂ Tl2O ꢂ SO24ꢃ
393
513
0.85
7.17
0.2
0.1
92.1
89.7
5.4
7.3
0.30
0.31
2.2
2.8
1.02
8.24
24.8
22.5
37.3
35.8
7.4
9.0
0.32
0.34
0.4
0.6
393
513
0.51
3.93
1.0
0.9
89.2
87.6
8.0
9.1
0.32
0.34
0.9
1.4
0.38
1.40
25.9
23.2
40.3
38.9
8.3
9.4
0.33
0.35
0.6
0.9
393
513
0.07
1.00
14.1
12.5
38.3
37.0
7.6
8.3
0.29
0.24
—
—
0.31
5.20
16.80
15.20
39.9
38.8
5.0
7.3
0.23
0.25
—
—
Note. Data recorded afꢃte1r 5-min reaction. CH, cyclohexane; MCP, metylcyclopentane; B, benzene.
a mol ꢂ (mol SO24ꢃ
)
ꢂ s
.
ꢃ1
b Cracked products, i.e., hydrocarbons containing <number of carbon atoms lower than 6.
c The molar ratio of di- and monobranched hexane isomers.

Ga-, In-, AND Tl-PROMOTED SULFATED ZIRCONIA
77
FIG. 13. Variation of the dibranched-to-monobranched hydrocarbon ratio (a) and of the isomerized C6 hydrocarbons-to-cyclic hydrocarbon ratio
(b) as a function of conversion for the catalysts activated both in Ar and air (T, 160ꢄC; ♦, ᭺, ᮀ, 4, catalysts activated in Ar; ᭜, ᭹, ᭿, ᭡, catalysts
activated in air).
than in the reaction of hexane. At the same time, the re-
action rate was one order of magnitude lower than those
found for hexane. Again, the Ga-containing catalyst exhib-
ited a high reaction rate while the Tl-promoted sample had
the lowest value. The activity of the In-promoted catalyst
was inferior to that of the pure sulfated zirconia. The activa-
tion energies were in the same range of values, also pointing
to adsorption as the rate determining step (Table 4).
Figure 15 shows the variation of the dibranched-to-
monobranched hydrocarbon ratio (a) and of the isomerized
C6 hydrocarbons-to-cyclohexane ratio (b), respectively, as
a function of conversion for the catalysts activated both
in air and Ar. As for hexane, air activation of the cata-
lystsfavoursthe isomerization to dibranched hydrocarbons.
The increase of the conversion corresponds to a slight in-
crease of ring opening. This is more evident for In- and
Tl-promoted catalysts.
FIG. 14. Variation of the selectivity to cyclohexane (᭿) and branched
C6 hydrocarbons (ᮀ), and of the TOF (᭜) for the catalysts activated in
In the reaction of cyclohexane, cyclic products, namely
methylcyclopentane, also predominated. A high selectivity Ar (T, 140ꢄC; space velocity, 5.4 hꢃ1).

ˆ
78
PARVULESCU ET AL.
TABLE 4
Conversion of Metylcyclopentane over Sulfated-Zirconium Catalysts
Activated in Ar, 823 K
Activated in air, 823 K
E
E
Selectivity, mol%
Selectivity, mol%
T
TOFa
ꢅ104
(kcal/
mol)
TOFa
ꢅ104
(kcal/
mol)
Sample
(K)
CH
n-C6
Cb
(d /m)c
CH
n-C6
Cb
(d /m)c
ZrO2 ꢂ SO24ꢃ
393
513
0.50
3.32
85.2
84.8
1.2
1.7
1.7
4.0
0.28
0.30
6.3
6.5
6.8
7.1
0.61
4.30
87.4
86.8
1.9
1.8
2.2
4.3
0.31
0.32
6.5
6.7
7.2
7.4
ZrO2 ꢂ Ga2O3 ꢂ SO24ꢃ
ZrO2 ꢂ In2O3 ꢂ SO24ꢃ
ZrO2 ꢂ Tl2O ꢂ SO24ꢃ
393
513
1.08
7.61
88.4
86.0
2.6
2.8
0.7
3.4
0.20
0.24
1.48
11.08
89.9
87.6
4.2
4.4
1.1
3.9
0.21
0.24
393
513
0.37
2.85
10.2
9.1
4.3
4.8
1.6
3.1
0.26
0.28
0.51
4.44
11.8
10.6
5.1
5.1
1.9
3.5
0.28
0.29
393
513
0.26
0.78
8.1
6.4
3.4
3.7
1.9
4.3
0.31
0.32
0.41
3.79
9.1
7.9
3.9
3.7
2.6
4.7
0.32
0.34
Note. Data recorded afꢃte1r 5-min reaction. CH, cyclohexane; n-C6, the sum of n-hexane and n-hexene.
a mol ꢂ (mol SO24ꢃ
)
ꢂ s
.
ꢃ1
b Cracked products, i.e., hydrocarbons containing a number of carbon atoms lower than 6.
c The molar ratio of di- and monobranched hexane isomers.
to methylcyclopentane was again achieved on the Ga- the observed properties are the high sulfur content and the
promoted catalyst (Fig. 16). However, even with In and promoter effect. It has been shown in the literature that
Tl, these compounds were predominant. The dependence the development of the acid sites in these catalysts requires
of the reaction rate on the catalyst nature showed the an amorphous structure of zirconia (53–56). Such materials
same features as those observed for methylcyclopentane, are easily synthesised by the sol–gel method and, in partic-
but the values were slightly smaller (Table 5). A similar ular, by the colloidal sol–gel approach, which is a variant
behavior is also noticed from the dependence of the of the sol–gel route. In addition, this method allows us to
selectivity on the conversion (Fig. 17). Again the increase achieve a homogeneous distribution of the components, in
of the conversion is accompanied by a slight increase of the occurrence of the promoters.
the noncyclic hydrocarbons.
Generally, the surface area of sulfated zirconia ob-
tained via the one-step sol–gel polymeric method exceeds
120 m2 gꢃ1. In the case of the colloidal sol–gel route, lower
surface areas are consistent with sulfur content as high as
14 wt% SO24ꢃ. But this behavior also needs to be related
to the preparation procedure. Tichit et al. (57) obtained
DISCUSSION
The characterization of the samples with the different
techniques indicates that the main factors which control
TABLE 5
Conversion of Cyclohexane over Sulfated-Zirconia Catalysts
Activated in Ar, 823 K
Activated in air, 823 K
Selectivity, mol%
E
E
Selectivity, mol%
T
TOFa
ꢅ104
(kcal/
mol)
TOFa
ꢅ104
(kcal/
mol)
Sample
(K)
MCP
n-C6
Cb
(d /m)c
MCP
n-C6
Cb
(d /m)c
ZrO2 ꢂ SO24ꢃ
393
513
0.13
1.20
50.9
48.1
6.0
6.4
4.9
6.8
0.29
0.30
7.4
7.3
7.4
8.5
0.19
2.10
58.8
54.1
6.0
6.3
5.0
8.4
0.30
0.32
8.1
7.8
8.4
8.7
ZrO2 ꢂ Ga2O3 ꢂ SO24ꢃ
ZrO2 ꢂ In2O3 ꢂ SO24ꢃ
ZrO2 ꢂ Tl2O ꢂ SO24ꢃ
393
513
0.36
3.23
70.9
68.4
2.2
2.4
3.8
6.6
0.26
0.27
0.40
4.18
74.4
70.9
1.9
2.2
4.4
7.3
0.27
0.29
393
513
0.09
1.06
51.2
47.7
6.0
6.4
6.6
8.8
0.29
0.30
0.14
1.75
54.0
49.2
5.8
7.1
5.4
9.6
0.31
0.34
393
513
0.07
0.90
38.3
36.6
3.2
6.6
3.6
6.6
0.20
0.23
0.19
0.95
40.1
39.0
5.2
5.5
3.7
7.1
0.19
0.22
Note. Data recorded afꢃte1r 5-min reaction. MCP, methylcyclopentane; n-C6, the sum of n-hexane and n-hexene.
a mol ꢂ (mol SO24ꢃ
)
ꢂ s
.
ꢃ1
b Cracked products, i.e., hydrocarbons containing a number of carbon atoms lower than 6.
c The molar ratio of di- to monobranched hexane isomers.

Ga-, In-, AND Tl-PROMOTED SULFATED ZIRCONIA
79
FIG. 15. Variation of the dibranched-to-monobranched hydrocarbon ratio (a) and of the isomerized C6 hydrocarbons-to-cyclohexane ratio (b) as
a function of conversion for the catalysts activated both in Ar and air (T, 160ꢄC; ♦, ᭺, ᮀ, 4, catalysts activated in Ar; ᭜, ᭹, ᭿, ᭡, catalysts activated
in air).
catalysts with similar sulfur content (about 16 wt% SO42ꢃ
)
also account for the quasi-absence of micropores. Summa-
by impregnation of high surface area zirconia with H2SO4 rizing, the solids prepared via the colloidal sol–gel method
in which zirconia keeps its texture. Low-surface-area cata- have a mesoporous texture in which part of the sulfate is
lysts were also obtained by Fa˚rcasiu and Li (33) using a as supported sulfuric acid which may possibly fill or block
percolation method. In our case, the rather low surface ar- the micropores. The shape of the adsorption–desorption
eas and the high sulfur-to-zirconium ratios obtained by XPS isotherms corresponds to type III solids, except for the
compared with the values established by chemical analysis Tl-containing sample activated in Ar, which exhibits type II
led us to suppose that sulfate species primarily form some hysteresis.
polymeric superficial sites. Moreover, if one compares the
Before calcination, the solids are X-ray amorphous, re-
density of the SO24ꢃ groups per nm2, values as high as 10 gardless of the sulfur content. Calcination at 550ꢄC induces
compared with 2–2.5 determined by Bensitel et al. (50) and the structuration of these materials and the X-ray patterns
Morterra et al. (42) appear to be unusual. Therefore, one show peaks corresponding to two different zirconium sul-
may assume that part of the sulfate in these catalysts exists fates as well as to zirconium oxides. This means that even in
as sulfuric acid supported on these materials, which could conditions of sol–gel preparation, a perfectly homogeneous

FIG. 16. Variation of the selectivity to methylcyclopentane (᭿) and branched C6 hydrocarbons (ᮀ), and of the TOF (᭜) for the catalysts activated
in Ar (T, 140ꢄC; space velocity, 5.4 hꢃ1).
FIG. 17. Variation of the dibranched-to-monobranched hydrocarbon ratio (a) and of the isomerized C6 hydrocarbons-to-methylcyclopentane ratio
(b) as a function of conversion for the catalysts activated both in Ar and air (T, 160ꢄC; ♦, ᭺, ᮀ, 4, catalysts activated in Ar; ᭜, ᭹, ᭿, ᭡, catalysts
activated in air).

Ga-, In-, AND Tl-PROMOTED SULFATED ZIRCONIA
81
structure is far from being achieved. No lines correspond- of the sulfate is eliminated. In all the cases, a temperature of
ing to Ga, In, or Tl species were noticed but, instead, reflec- 550ꢄC was found to determine the strong acidic properties.
tions due to an unidentified phase (? in Figs. 7 and 8) were
detected.
It emerges from the literature that the transformation
of linear (29, 57, 61) and cyclic (62, 63) C6 hydrocarbons
Raman spectroscopy provided additional information on requires strong acid sites. In this study, hexane, methylcy-
the nature of the sulfate species. Bands attributable to S-O clopentane, and cyclohexane were converted over the dif-
==
and S O stretching modes of sulfate groups in different ferent catalysts investigated. These results are in agreement
coordination (40, 46–48) were observed: isolated sulfate with numerous published works which underline the strong
groups, polynuclear sulfate species, and sulfuric acid (Figs. 5 acid character of sulfated zirconias, in spite of differences
and 6). The last two species were clearly identified. In ad- in the activities according to the hydrocarbon and the cata-
dition, the band attributed to polynuclear species contains lyst promoter. Also, the aromatization function of Ga in-
more than one component, which presumably indicates the troduced in ZSM-5 zeolites has been stressed in the litera-
existence of different polynuclear species with different ture. The catalytic functions responsible for the activity of
polymerization degrees.
(Ga, H)-ZSM-5 or (In, H)-ZSM-5 as dehydrocyclization
Acidity is the most important function of sulfated zirco- (aromatization) catalysts are acidity and redox properties
nia and it is evidently related to the presence of SO4 groups. of Ga or In. The oxidation state of these elements can cycle
Indeed, zirconium oxides exhibit only weak acidic proper- between +3 and +1 during reaction, whereas Tl+ cannot.
ties, irrespective of their synthesis method (58). Structural The results obtained will be discussed with respect to the
characterizations indicate a very heterogeneous surface: nature of the hydrocarbon.
isolated and polymeric sulfate species as well as zirconium
oxide being equally accessible. In addition, supported sul-
furic acid is also very probably present.
(a) Hexane
FTIR spectra of adsorbed bases showed the presence of
The analysis of the reaction products indicates that on the
both Brønsted and Lewis sites, whatever the catalyst com- non-promoted and the Tl-promoted catalysts, hexane was
position. The presence of promoters brought about a de- transformed according to three different reactions: chain
crease of the population of polynuclear sulfates, in the se- isomerization, cyclization, and cracking. On Ga- and In-
quence Tl > In > Ga. Also, the amount of Brønsted acid promoted sulfated zirconias, only cyclization and cracking
sites was lowered in the promoted samples.
reactions were evidenced. These reactions require the pres-
The nature of the Brønsted acidity in these catalysts is ence of acid sites but with different strengths. In addition,
still a debated question. Kno¨zinger and co-workers (46, 47) except for the Tl-promoted catalysts, small amounts of ben-
and Kustov et al. (59) attribute the Brønsted acidity to iso- zene were also detected.
lated monosulfate species, while Morterra et al. (42, 60) and
It is now accepted that the first step in the isomerization
Platero et al. (41) consider that these sites should be asso- reaction on these catalysts is the dehydrogenation of the hy-
ciated with disulfate species. These different authors, how- drocarbon. The hydroisomerization mechanism suggested
ever, agree on the role of residual water in determining the can be divided in (de)hydrogenation, protonation, and iso-
Brønsted acidity. In our catalysts, the Brønsted sites could merization steps(64). A bifunctionalityin the isomerization
be due to “hydrated” polynuclear species but also to sup- of pentane was also reported by Salmones et al. (65). In our
ported sulfuric acid. In addition, the water which generates case, the disappearence of the Raman bands correspond-
the Brønsted acidity could be held in the system by the same ing to deprotonated sulfate species and the increase of the
supported species. These results when related with those intensity of the bands corresponding to protonated species
obtained from Raman spectroscopy allow us to speculate also support this mechanism. The modification of the XPS-S
that, in these catalysts, Lewis acidity results, as generally 2p (169 eV) to the S 2p (170 eV) signal ratio is consistent
outlined in the literature, from the inductive effect exerted with the same process. The role of dehydrogenation has
by strongly bonded sulfate groups (mono- or polynuclear also been clearly proved by Ghenciu and Fa˚rcasiu (66) and
species). As a consequence, the model for the acid sites in Srinivasan et al. (8) using benzene as a probe molecule.
such a case is quite complicated.
They explained this process as a one-electron oxidation of
The atmosphere in which the catalysts were activated was the alkane, which implies the oxidation ability but does not
found to influence both the density and, to a lesser extent, require superacidity. The first step is a slow one and the
the strength of the acid sites. Activation in air provoked a initiation time, also observed by Lonyi et al. (22) and
slight decrease of the sulfur content. This decrease is con- Fa˚rcasiu et al. (34), could correspond to the dehydrogena-
comitant with a slight diminution of the surface area and tion step.
with an increase of the acid strength. Therefore, it may as-
The formation of polyenic cation species which may be
sumed that the presence of air favours the nucleation of the assumed from the Raman band located at 433 cmꢃ1 is
superficial sulfate groups. During this process, a small part another argument in favor of this process. Alkylsulfates

ˆ
PARVULESCU ET AL.
82
have been proposed as intermediates by Kazansky (67) and accounted for from the Raman and XPS results. Indeed,
Cheng and Ponec (68). the Raman spectra recorded after exposure of the samples
The importance of calcination and the necessity for Lewis to methylcyclopentane still showed bands attributable to
acid sites in the reaction of aliphatic hydrocarbons on sul- unprotonated sulfate groups, and the intensity of the band
fated zirconias have been intensely discussed in the litera- attributed to polyenic species was much reduced. Besides,
==
ture. Most of the Lewis sites are generated during calcina- no band related to C C stretchingvibrationsofcompounds
tion, and their presence appears to be very important to the with an increased aromatic character (at about 1517 cmꢃ1
)
first dehydrogenation step.
Low activation energies were found to be the conse-
was observed.
The XPS ratio of the S 2p components at 169 and 170 eV
quence of the strong adsorption of the reactants and re- was closer to the values recorded on fresh catalysts than
action intermediates, which would be the determining step to those of samples exposed to hexane. The XPS sulfur
in this reaction. A similar conclusion has been previously content of the samples exposed to methylcyclopentane was
drawn by Adeeva et al. (45) who stressed that “the extraor- also higher than those determined in the case of hexane.
dinary activity of these catalysts is likely to be caused by a
good stabilization of the surface intermediates.”
The very low activation energies again point to the fact
that the determing step is not the reaction but the adsorp-
But the strongadsorption also causesa rapid deactivation tion. The activation energies were determined from the
of the catalysts. Raman spectra showed that after reaction, TOF dependence on temperature. The values reported by
and concomitant with the strong increase of the protonated Fa˚rcasiu et al. (34) are more rigorous because they distin-
sulfate species on the catalysts surface, bands due to organic guished isomerization and deactivation. However, at the
species appear. The mechanism through which this process temperatures at which the reactions were carried out, the
occurs is probably similar to the one proposed by Signoretto activation energies should be lower.
et al. (69). Bands assigned to polyenic and aromatic species
(c) Cyclohexane
were found in the Raman spectra of the samples exposed to
hexane vapour. Small amounts of benzene were also found
among the reaction products. Kno¨zinger and co-workers
(46, 47) proved that these speciescontribute to the catalyst’s
deactivation.
Another cause of deactivation could result from a par-
tial loss of sulfur. XPS data showed that after reaction, the
S/Zr atomic ratio was significantly lower in the case of the
catalysts with high sulfur content than before reaction. For
catalysts containing less sulfur, this decrease was not as im-
portant. Similar conclusions were drawn very recently by
Vaudagna et al. (36).
The reaction of cyclohexane was intermediate between
hexane and methylcyclopentane. Cyclohexane reacted fol-
lowing the similar routes as methylcyclopentane: isomeriza-
tion, ringopening, and cracking. The reactivitieswere found
to be slightly smaller than for methylcyclopentane. Methyl-
cyclopentane was the dominant reaction product, irrespec-
tive of the catalyst nature and reaction conditions, which
allows us to suppose that on these catalysts, these reactions
are reversible. The higher activation energies compared
with those determined in the case of hexane or methyl-
cyclopentane indicate that adsorption probably plays a less
important contribution.
The differences observed between the catalysts activated
in argon and in air could be accounted for by the fact that
for the latter ones, more deprotonated and dimeric species
exist.
CONCLUSION
Sulfatation of zirconium hydroxide requires an amor-
phous reactive precursor. Both sulfated zirconia and Ga-,
(b) Methylcyclopentane
Except for the Ga-promoted catalyst, methylcyclopen- In-, or Tl-promoted sulfated zirconias correspond to such
tane has also been found to react differently on the in- solids. A sulfate concentration as high as 50 wt% deter-
vestigated catalysts, namely, via three routes: isomerization mines a particular texture of these materials characterized
(first to cyclohexane), ring opening (possibly accomplished by a pore diameter in the mesoporous range. The sulfate
by a second isomerization process to mono- and dibranched groups in these catalysts are in different coordinations: iso-
molecules), and cracking. Again, on the Ga-promoted cata- lated species, polynucleated species, and supported acid.
lyst, only ring isomerization and cracking were evidenced. As a consequence, both Lewis and Brønsted acid sites with
On those catalysts and in our experimental conditions, ac- different strengths coexist. The amount of water necessary
tivation of methylcyclopentane is more difficult and this to promote Brønsted acidity could correspond to that re-
could explain the low conversions (compared with those tained by supported H2SO4. Activation in air was found to
obtained for hexane) and reaction rates, and the low crack- “catalyze” the formation of the polynucleate species and,
ing contents in the reaction products.
as a consequence, to increase the acid strength. Promotion
Dehydrogenation occurs to a small extent and only on resulted in an increase of the Lewis acid sites population in
nonpromoted and Ga-promoted catalysts, which may be the order Ga > In > Tl.

Ga-, In-, AND Tl-PROMOTED SULFATED ZIRCONIA
83
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