Catalytic properties of ZrO2-SiO2: Effects of sulfation in the cyclohexene isomerization reaction

Article abstract of DOI:10.1006/jcat.1996.0223

Catalytic properties of ZrO₂-SiO₂ (14% molar) for cyclohexene conversion (skeletal isomerization and hydrogen transfer) have been studied. Textural and morphological properties were correlated to explain catalytic behavior. XRD, DTA-TGA, N₂ adsorption, DRIFT, and acidity measurements by pyridine and dimethylpyridine chemisorption were used to characterize samples. The effect of the sulfation process upon surface structure as well on catalytic activity/selectivity has also been studied. It has been shown that catalytic activities are increased on sulfated samples, but at the same time products of the hydrogen transfer reaction seem to be increased too, lowering the selectivity of the reaction; isomerization always predominates. Sulfation transforms this compound to a relatively selective isomerization catalyst exhibiting greater catalytic activity.

Full text of DOI:10.1006/jcat.1996.0223

JOURNAL OF CATALYSIS 161, 605–613 (1996)
ARTICLE NO. 0223
Catalytic Properties of ZrO2–SiO2: Effects of Sulfation in the
Cyclohexene Isomerization Reaction
1
ꢀ
ꢀ
ꢀ
J. A. Navı´o, G. Colo´n, M. Macı´as, J. M. Campelo, A. A. Romero, and J. M. Marinas
Instituto de Ciencias de Materiales, Centro Mixto Universidad de Sevilla–CSIC, and Departamento de Qu´ımica Inorga´nica, Facultad de Qu´ımica,
ꢀ
Universidad de Sevilla, E-41012 Sevilla, Spain; and Departamento de Qu´ımica Orga´nica, Facultad de Ciencias, Universidad de Co´rdoba,
Avda. San Alberto Magno, s/n, E-14004 Co´rdoba, Spain
Received July 20, 1995; revised October 16, 1995; accepted March 25, 1996
stability of this dispersion is very important if the catalyst
is to be regenerated by air burnoff (to remove nonvolatile
organic deposits).
Catalytic properties of ZrO2–SiO2 (14% molar) for cyclohex-
ene conversion (skeletal isomerization and hydrogen transfer)
have been studied. Textural and morphological properties were
correlated to explain catalytic behavior. XRD, DTA–TGA, N2
adsorption, DRIFT, and acidity measurements by pyridine and
dimethylpyridine chemisorption were used to characterize samples.
The effect of the sulfation process upon surface structure as well on
catalytic activity/selectivity has also been studied. It has been shown
that catalytic activities are increased on sulfated samples, but at the
same time products of the hydrogen transfer reaction seem to be in-
creased too, lowering the selectivity of the reaction; isomerization
always predominates. Sulfation transforms this compound to a rel-
atively selective isomerization catalyst exhibiting greater catalytic
ZrO2, in its tetragonal form, is stable above 1100–1200ꢁC
but not at room temperature (even when quenched). When
˚
ZrO2 is dispersed on silica, this results in 50–150 A crystal-
lites. These have the tetragonal structure (8) and appear to
be stable to 500ꢁC. Another author (9) discussed the nature
of “hydrous ZrO2” (10) prepared by various precipitation
techniques. This material is not monoclinic but cubic and
stable to about 650ꢁC.
We have shown (11, 12) that the physical and chemical
properties of dispersed ZrO2 on a SiO2 matrix are different
from those of bulk zirconia. Ishida et al. have recently re-
ported (13) that superacid properties, which were obtained
by well-dispersed ZrO2 on a SiO2 matrix, can be correlated
with the extent of ZrO2 crystallization.
c
activity. ꢀ 1996 Academic Press, Inc.
INTRODUCTION
Processed composites of x ZrO2–(100–x) SiO2 where the
value of x ranges between 3 and 100% have been prepared.
The textural properties of such samples have been widely
studied by us (11). The surface behavior is noteworthy—
the plot of specific surface area versus SiO2 composition
presents a “volcano” shape, with a maximum for x = 14%
molar. This value rose in a later crystallization kinetic study
of these samples (14). There is a limit in the crystallization
model at 11.5% molar. This was shown again by XPS (15),
where samples with a low content of ZrO2 (<14% molar)
showed no correspondence between EDAX analysis and
XPS analysis of the surface composition.
In principle, these facts could lead us to assume that ZrO2
coats SiO2, forming a monolayer at an estimated value of
11.5% molar, approximately, with 14% being a compos-
ite with highest surface area and close to the monolayer
behavior. However, previous results reported by us (11,
15) showed that the ZrO2–SiO2 composite with 14% molar
of ZrO2, really are constituted by SiO2 particles partially
coated of circonia and a small amount of individual SiO2
and ZrO2 particles.
Zirconia, ZrO2, is an important material that finds ap-
plication in several fields. It has been reported that ZrO2
shows specific catalytic activities for the cleavage of C–H
bonds (1), the hydrogenation of 1,3-butadiene by molec-
ular hydrogen and hydrogen donor molecules such as cy-
clohexadiene (2, 4), and high selectivities for the formation
of 1-olefins from secondary alcohols (5) and isobutane in
the reaction CO + H2 (6). All this characteristic behavior of
ZrO2 is considered to be due to the acid–base bifunctional
catalysis.
Other authors (7) have recently reported in detail on the
properties of ZrO2 supported silica using a variety of ana-
lytical procedures (XPS, SIMS, IR, TPD). They found that
ZrO2 on silica, when prepared form Zr(OEt)4–acetic acid–
methanol solution, have a far better ZrO2 dispersion than
similarly prepared derived from aqueos solutions. Also, the
metal oxide dispersion of the ethoxide derived ZrO2/SiO2
was thermally (600–750ꢁC) far more stable than the nitrate
derived binary system. Maximizing the ZrO2 dispersion is
important since this probably enhances the catalyst’s activi-
ty and selecivity (e.g., rate and efficiency), while the thermal
In the present work we study the catalytic properties of
ZrO2–SiO2 (14% molar) in the cyclohexene isomerization
1 To whom correspondence should be addressed. Fax: 34-95-455 71 34.
605
0021-9517/96 $18.00
c
Copyright ꢂ 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

´
NAVIO ET AL.
606
reaction. We study the effects of calcination temperature sequentially cooled to 473 and 373 K, the catalyst remain-
andsulfategroupsontheacidsitesdistributionandcatalytic ing 30 min at this temperature. PY and DMPY were then
yield for that reaction.
introduced by bubbling a stream of dehydrated and de-
oxygenated N2 (20 ml minꢄ1) through the liquid (298 K)
and into the sample chamber containing the neat catalyst
sample at 373 K for 1 h. After 1 h of adsorption the sys-
tem was flushed with N2 to desorb the loosely bound PY,
and DMPY from the surface of catalyst and the respective
DRIFT were recorded. The samples were then heated in
a N2 stream to 473 and 573 K and the respective DRIFT
spectra were obtained. Samples were equilibrated for at
least 1 h at each temperature or reactant condition prior to
spectra measurement.
Surface acidity (Lewis and Brønsted sites) was measured
in a dynamic mode by means of the gas-phase (473–673 K)
adsorption of PY and DMPY using a pulse chromato-
graphic technique according to a method described else-
where (13).
EXPERIMENTAL
Sample Preparation
Amorphous powders ZrO2–SiO2 (14% molar) were pro-
cessed by a sol-gel chemical route. A solution of ammonia
in deionized water was added dropwise with continuous
stirring to a solution of ZrOCl2 ꢃ 8H2O (Fluka Chemika)
containing silica gel (Aldrich, grade 634) as a core matrix
in suspension. Details for the preparation can be found else-
where (11). As previously mentioned, this method does not
lead to a homogeneous system in wich ZrO2 coats SiO2 par-
ticles, in spite of that, the molar percent is close to the es-
timated value for the monolayer situation. These samples
were then calcined at various temperatures (773, 873, and
1073 K) for 2 h. We called these samples ZS773, ZS873,
and ZS1073, respectively. They were then submitted to a
sulfation process. A certain amount of calcined sample was
suspended in H2SO4 (1 N) solution at a rate of 3 ml per gram
of catalyst and left for 1 h with continuous stirring to reach
equilibrium. Sulfated samples were denominated ZS773S,
ZS873S, and ZS1073S. (For textural studies sulfated sam-
ples were calcined at 773 K for 2 h to get exact information
of surface at reaction conditions.)
Catalytic reaction. Catalytic activity and selectivity for
these samples in cyclohexene isomerization reaction was
determined in a pulse microreactor (4 mm i.d.), with pulses
of 1 ꢁl of pure cyclohexene (supplied by Merck), connected
to a gas chromatograph (HP 5890 II) with an FID detector
using N2 as carrier gas. A detailed description of the mi-
croreactor and the experimental procedure for measuring
the catalytic activity have been given elsewhere (18). Reac-
tion products were characterized by gas chromatography–
mass spectrometry.
Experimental Technique
Textural properties. Specific surface area and pore size
were obtained from N2 adsorption-desorption isotherms at
77 K, using a Micromeritics ASAP2000 analyzer. Pore size
distribution was calculated using the Barrett, Joyner, and
Halenda method, assuming cylindrical pore model (16).
DTA–TGA (differential thermal and thermogravimet-
ric analysis) diagrams were obtained simultaneously in
static air with a Setaram high-temperature thermal ana-
lyzer, Model 92-16, 18, at a heating rate of 8 K minꢄ1. Cal-
cined alumina was used as reference material. A furnace of
graphite heating element with alumina protection tube was
used.
RESULTS AND DISCUSSIONS
Textural Properties
The DTA–TGA profile of ZrO2–SiO2 presents a sin-
gle exothermic peak due to ZrO2 crystallization, centered
around 1173 K in this present sample (Fig. 1). So all the sam-
ples studied in this work might be amorphous. However,
X-ray diffraction diagrams were made for all samples to
ensure that the sulfation process and second heating treat-
ment do not affect the ZrO2 structure in any way, such as
other authors have reported (19, 20).
X-ray diagrams for ZS1073, ZS1073 (recalcined at 773 K,
2 h), ZS1073S, and ZS1073S (recalcined at 773 K, 2 h) show
amorphous patterns (Fig. 1) though with a certain short-
range order, probably due to their calcination temperatures
X-ray powder diffraction diagrams were obtained in a
Siemens D-501 diffractometer, using Ni-filtered Cu Kꢀ
radiation.
DRIFT and surface acidity. Spectra were recorded on (1073 K) being close to the temperature of the crystalliza-
an FTIR instrument (Bonem MB-100) equipped with dif- tion peak. Thus, we conclude that all the samples, ZS773,
fuse reflectance (Spectra Tech, Collector). A resolution of ZS873, and ZS1073 and the corresponding sulfated sam-
8 cmꢄ1 with 256 scans was averaged to obtain a spectrum ples, present no large-range rearrangement. Only these lat-
from 4000 to 400 cmꢄ1. KBr or nonsulfated samples were ter samples, shown in Fig. 1, present a certain short-range
used as reference spectra.
order but are still amorphous.
Surface acidity was determined using pyridine (PY) and
Specific surface areas were calculated by means of the
dimethylpyridine (DMPY) adsorption. Samples were pre- n-method (Sn) and BET method (SBET). As Table 1 shows,
viously heated at 573 K under dried N2 for 1 h and then sol-gel prepared samples present high surface areas (in the

CYCLOHEXENE ISOMERIZATION
607
FIG. 2. Pore volume distribution for ZrO2–SiO2 (14% molar) samples.
FIG. 3. DRIFT spectra of nonsulfated samples.
FIG. 1. DTA–TGA curve of ZrO2–SiO2 (14% molar) composite gel
powders. XRD diagrams (Cu Kꢀ radiation) for: (a) ZS1073; (b) ZS1073
calcined 773 K, 2 h; (c) ZS1073S; (d) ZS1073S calcined 773 K, 2 h.
range 400–333 m2/g). A slight decrease is observed in SBET
as the calcination temperature increases, although high sur-
face areas remain.
Forsulfatedsamplessurfaceareas follow almost the same
sequence, being in this case lower than for nonsulfated sam-
ples (possibly small pores collapsed after sulfate impregna-
tion). There is no appreciable change in the pore volume
(Fig. 2).
DRIFT Spectra
In Fig. 3 the spectra in the 4000–2500 cmꢄ1 region are
shown for the nonsulfated samples calcined at the indicated
temperatures. As can be see, the IR spectrum of the ZS500
shows a sharp peak at 3740 cmꢄ1 and more than one (broad)
band at 3580 and 3350 cmꢄ1
.
According to literature data (21, 22) the characteristic ab-
sorption band of the so-called isolated silanol groups (OH
stretch mode of free single and geminal hydroxyls) is found
TABLE 1
Textural Properties of ZrO2–SiO2 (14% Molar) Catalysts
Catalyst
SBET/m2 gꢄ1
Sn/m2 gꢄ1
Vp/ml gꢄ1
rp/nm
ZS773
ZS873
ZS1073
ZS773S
ZS873S
ZS1073S
400
351
333
321
315
318
381
336
316
305
301
309
0.64
0.65
0.56
0.53
0.63
0.57
3.2
3.7
3.4
3.3
4.0
3.6

´
NAVIO ET AL.
608
an appreciable decrease is observed at the lower wavenum-
ber bands of this region. This results clearly indicate that
calcination does not affect to the silanol groups.
Figure 4 shows the sulfated sample spectrum with non-
sulfated subtracted, in the hydroxyl band frequency region.
It shows the changes that appear after sulfation and recal-
cination at 773 K. Therefore, the decrease in this band may
be due to a combined effect of sulfate attachment and later
heating process at 773 K. The temperature effect could be
understood as a sintering process for which hydroxyl groups
can be removed, liberating water molecules. At the same
time, the effect of sulfate produces a displacement of these
hydroxyl groups, what combined with the thermal remov-
ing of the hydroxyl groups could lead to the collapse of the
surface, as it is indicated in Scheme 1. The differential band
centered around 3730 cmꢄ1 can be assigned to terminal Si–
OH removal and that at 3300–3380 cmꢄ1 can be assigned to
other different OH removal from the surface, mentioned
before. As can be observed, the intensities of this second
broad band is significantly lower than the 3730 cmꢄ1 band.
Due to the fact that the calcination does not affect the band
at 3740 cmꢄ1 as shown in Fig. 3, consequently the decrease
in the intensity at the differential spectra (Fig. 4) can be
associated to the effect of sulfate attachment instead of the
recalcination process.
FIG. 4. DRIFT spectra of sulfated samples ratioed automatically
against nonsulfated series. Negative peaks due to OH removal on ZrO2–
SiO2 samples: (a) ZS773S; (b) ZS873S; (c) ZS1073S.
=
The differential spectra of the SO4 region can be di-
vided into two parts (Fig. 5). The first part, located around
1365 cmꢄ1, is due to ꢂS (19, 20), and the second part,
==
O
in silica and silica mixed oxides centered at 3740 cmꢄ1. Thus
the peak at 3740 cmꢄ1, observed in the spectra of Fig. 3, must
be assigned to OH stretch of isolated Si–OH.
Ontheotherhand, thebroadbandatlowerwavenumbers
couldbeassignedtomorethanonetypeofOHgroupsinthe
mixed oxide, from different neighbor cations, depending on
the Zr/Si ratio at the heterogeneous surface. It should be
noticed that an increase in the calcination temperature re-
mains practically unmodified the peak at 3470 cmꢄ1, while
more complicated, corresponds to ꢂS–O. The band cen-
tered at 1360 cmꢄ1 may be assigned to isolated sulfates lo-
calized in the crystallographically defective configurations
(ꢂS O < 1375 cmꢄ1 (20)).
==
Surface Acidity Measurements
The adsorption of probe molecules is widely used as
direct measurement of surface acidity (24–29). PY and
SCHEME 1

CYCLOHEXENE ISOMERIZATION
609
STRUCTURE 1
From the DMPY adsorption results, it can be observed
that the number of Brønsted acid sites is significantly
greater for sulfated samples. Calcination temperature af-
fects Brønsted acid sites population (Scheme 1). Thus for
sulfated samples Brønsted sites decrease gradually, being
always greater than for nonsulfated samples. Sulfate groups
are linked to Brønsted acid sites which become fewer af-
ter calcination. However, this weakens the O–H bond and
therefore strengthens the acidity of remaining hydroxy
groups (Structure 1).
Pyridinium is adsorbed on Brønsted sites, forming pyri-
dinium ion (1535–1550 and 1640 cmꢄ1, approx.). PY is ad-
sorbed on Lewis sites, by donation of the lone electron pair
of nitrogen to the acid site (ꢃ donation to a coordinatively
unsaturated cation, 1447–1464 cmꢄ1) (24–26). The predom-
inant band that characterizes Lewis acid sites is located at
1451 cmꢄ1, while 1544 cmꢄ1 is the characteristic band for
Brønsted acid sites. On the other hand, Uytterhoeven et al.
have reported (27) that the 1493 cmꢄ1 band corresponds
to the Brønsted and Lewis chemisorption. So we must ex-
amine those bands to correlate the acid center behavior.
DMPY is specifically adsorbed on Brønsted sites due to a
steric effect, and due to its higher basic strength it will de-
tect the weakest acid centers (28) with bands located at 1628
and 1648 cmꢄ1 for Brønsted sites and 1465 and 1580 cmꢄ1
for Lewis sites (29).
After PY adsorption in nonsulfated samples bands ap-
peared at 1647 cmꢄ1 (B), 1601 cmꢄ1 (L), 1.574 cmꢄ1 and
1547 cmꢄ1 (B), and 1489 cmꢄ1 (B + L), and 1447 cmꢄ1 (L)
(Fig. 6A); these latter ones are very intense. Increasing tem-
perature and times of adsorption most affected bands lo-
cated at 1447 and 1489 cmꢄ1 (Fig. 6A)—in other words,
Lewis acid sites. That is, Brønsted sites are stronger than
Lewis sites. Figure 7A shows spectra for sulfated samples.
In this case the bands assigned to Brønsted sites appear with
higher intensities. The presence of bands at 573 K/60 min
suggest that these Brønsted sites also appear with higher
acid strength.
FIG. 5. DRIFT spectra of sulfated samples [(a) ZS773S; (b) ZS873S;
(c) ZS1073S] showing ꢂS O and ꢂS O regions.
=
–
DMPY have been used in this study. They specifically ad-
sorb on acid sites (Lewis and Brønsted).
Table 2 shows quantitative results of the adsorption of
PY and DMPY. From PY adsorption, it is worth noting the
sulfate effects. After sulfation treatment, the acid sites un-
dergo a small decrease. Moreover, for sulfated samples the
number of stronger acid sites is higher than for nonsulfated
sites (56% for ZS773S vs 36% for ZS773). Therefore, sul-
fate groups not only modify the number of acid sites but also
increase the strongest acid sites population by inductive ef-
fect (Structure 1). It can be observed that with calcination
temperature, for all samples, acid sites are lost, remaining
the strongest.
TABLE 2
Surface Acidity of ZrO2–SiO2 (14% Molar) Catalysts
PY/ꢁmol gꢄ1
DMPY/ꢁmol gꢄ1
Catalyst
573 K
673 K
673 K
ZS773
ZS873
ZS1073
ZS773S
ZS873S
ZS1073S
87
76
52
62
59
51
32 (36%)a
24 (31%)
26 (50%)
35 (56%)
53 (89%)
39 (76%)
6
13
3
43
38
20
With respect to DMPY adsorption (Figs. 6B and 7B),
bands are observed at 1640 cmꢄ1 (B), 1601 cmꢄ1 (L),
1582 cmꢄ1 (L), and 1462 cmꢄ1 (L). Bands centered at
1601 and 1582 cmꢄ1 disappear, giving a broad band of low
a Percentage of acid site present after evacuation at 673 K respect to
initial population at 573 K.

´
NAVIO ET AL.
610
FIG. 6. DRIFT spectra of (A) PY and (B) DMPY on ZS773 sample. The sample was dried at 573 K under N2, cooled to 373 K, and then loaded
with PY or DMPY and flushed with N2 for 30 min (a). The temperature was then increased to 473 K and the catalyst remained, in N2 stream, at 473 K
for 30 min (b); 1 h (c); 2 h (d); and 3 h (e).
intensity located around 1560 cmꢄ1 . At the same time the test for these catalysts, due to its capability in character-
1640 cmꢄ1 band decreases to lower wavenumbers.
izing the acid strength of protonic sites (30). The reaction
Sulfated samples (Fig. 7B) present DMPY adsorption path is shown in Scheme 2.
bands centered at 1636 cmꢄ1, 1582 cmꢄ1 (with a shoulder
The catalytic runs have been carried out in the absence of
at 1600 cmꢄ1), and 1462 cmꢄ1. Only the 1636 cmꢄ1 band diffusional influences (boundary layer, internal, and exter-
remains on increasing temperature and times of adsorption, nal mass transfer process) through the choice of the suitable
located finally at lower wavenumbers (Fig. 7B) and with operating variables, specifically space velocity (changing
higher intensity than in nonsulfated samples.
the catalyst weight and the carrier gas flow) and particle
size (below 0.7 mm); furthermore, the amount of catalyst
corresponded to similar area loaded in the reactor. The
catalytic runs have also been carried out at different weight
ratios of catalyst from the cyclohexene introduced, showing
Catalytic Study
The cyclohexene (CHE) isomerization reaction (cyclo-
hexene skeletal isomerization, CSI) was used as catalytic
FIG. 7. DRIFT spectra of (A) PY and (B) DMPY on ZS773S sample. The sample was dried at 573 K under N2, cooled to 373 K, and then loaded
with PY or DMPY and flushed with N2 for 30 min (a). The temperature was then increased to 473 K and the catalyst remained, in N2 stream, at 473 K
for 30 min (b); 1 h (c); 2 h (d); and 3 h (e); and finally at 573 K for 1 h (f).

CYCLOHEXENE ISOMERIZATION
611
SCHEME 2
that the fractional conversion of reactant to products was Student’s t test of significance showed that those are signi-
independent of the pressure, which determined the first- ficant levels over 1%. At least three measurements where
order reaction process. This behavior also ensured linear used to calculate each value of Ka. All values are repro-
chromatography in the pulse mode, i.e., ensuring equilib- ducible to within about 7%.
rium chromatography.
For nonsulfated samples it is worth noting that Ka for CSI
In the absence of mass transfer process, the conversion of decreases drastically with calcination temperature, having
cyclohexene (which was maintained below 20% level) fol- its lowest value at 873 K. In contrast, sulfated samples ex-
lowed the requirement of Bassett–Habgood kinetic treat- periment a great increase, without too much difference be-
ment (31) for the first-order kinetic process in which the tween samples.
rate determining step is the surface reaction:
There is a great controversy about the nature of the
promoter for the acid–base catalytic activity (23, 32). We
observe that Ka depends on the presence of S O groups
==
ln[1/( 1 ꢄ X ) ] = RTKa(W/F),
[1]
rather than on the calcination process. As previously men-
tioned, the incorporation of sulfate ions on these sam-
ples enhances the acidic strength of the remaining hydroxy
groups and consequently this effect may lead to an increase
in the catalytic activity for CSI as is readily observed.
For the HT reaction, lower Ka values are observed
for both nonsulfated and sulfated samples, with the same
tendency—a gradual but considerable decrease with tem-
perature.
where X is the conversion, Ka is the aparent rate constant
of the surface process, W is the catalyst weight, and F is the
flow rate of carrier gas.
The ln[1/(1ꢄX)] vs Fꢄ1 plots according to Eq. [1], for the
conversion of cyclohexene at various flow rates of the ni-
trogen carrier gas (20–40 ml ꢃ minꢄ1) and different reaction
temperatures are linear and also pass through the origin,
indicating a good fit of the data to Eq. [1] and, therefore,
the first-order process in the cyclohexene conversion.
As reaction products for cyclohexene isomerization, 1-
and 3-methylcyclopentene (1-MCPE and 3-MCPE respec-
tively) can be obtained, 1-MCPE being the main product.
The hydrogen transfer reaction (HT) also appears as a com-
petitive reaction, giving as secondary products cyclohexane
(CHA) and methylcyclopentane (MCPA) (see Scheme 2).
Table 3 shows activation and kinetic data for the isomer-
ization reaction of cyclohexene (CSI). The apparent rate
constant Ka at 523 K gives direct measurement of the de-
gree of reaction. Activation parameters (Ea and ln A) have
been obtained by the Arrhenius equation; experimental
selectivities (SCSI) are also included in Table 3. Table 4 con-
tains the same variables for HT.
OPE (optimum performance envelope) curves (33, 34)
that show the product distribution in the reaction for each
TABLE 3
Aparent Rate Constant (Ka), Activation Parameters (Ea, ln A),
and Experimental Selectivities (SCSI) for CSI on ZrO2–SiO2 (14%
Molar) Catalysts
Ka/106 a
Catalyst
(mol atmꢄ1 gꢄ1 sꢄ1
)
Ea/kJ molꢄ1
ln A
SCSI
ZS773
ZS873
ZS1073
ZS773S
ZS873S
ZS1073S
42.9
3.1
6.6
518.1
465.1
411.4
52.3
80.7
52.5
32.6
42.9
37.9
2.0
5.9
1.1
85.9
86.5
64.1
66.4
84.3
85.5
ꢄ5.7
2.2
Position of the 98% confidence limit lines and the value of
the coefficient of determination (always over 0.99) for the
regressions are used to check the adequacy of the data. A
1.0
a At reaction temperature of 523 K.

´
NAVIO ET AL.
612
but with low selectivity for CSI, the products of HT appear-
TABLE 4
ing in even greater amount than 3-MCPE for ZS773S. In
this sense, ZS1073S presents the most activity and selectiv-
ity for cyclohexene isomerization to 1-MCPE.
Aparent Rate Constant (Ka), Activation Parameters (Ea, ln A),
and Experimental Selectivities (SHT) for HT on ZrO2–SiO2 (14%
Molar) Catalysts
Ka/106 a
CONCLUSIONS
Catalyst
(mol atmꢄ1 gꢄ1 sꢄ1
)
Ea/kJ molꢄ1
ln A
SHT
ZS773
ZS873
ZS1073
ZS773S
ZS873S
ZS1073S
6.9
0.5
3.7
228.1
77.0
48.7
36.8
59.3
33.3
26.7
19.0
23.5
ꢄ3.4
ꢄ0.9
ꢄ10.2
ꢄ2.2
ꢄ5.1
ꢄ4.2
14.1
13.4
35.9
33.5
15.7
14.5
ZrO2–SiO2 (14% molar) has been used as isomerization
catalyst. The effect of calcination temperature and the pre-
sence of sulfate groups on the catalyst surface have been
studied.
The presence of sulfate groups enhances the surface acid-
ity, especially Brønsted acid sites possibly responsible for
catalytic activity. Though there is great controversy on the
origin of the catalytic activity, we have shown that for
ZrO2–SiO2 samples studied here, the sites responsible for
the increasing activity in the isomerization reaction are
Brønsted acid sites, as can be inferred from PY and DMPY
chemisorption studies and relative catalytic tests.
It has been shown that the sulfation process on dif-
ferent calcined samples has no marked effect on textural
properties (SBET, pore distribution) but has so on acidic
properties—as much in the type of acid sites as in the
strength of these sites. Surface acidity is a direct cause of
catalytic activity. For cyclohexene isomerization, the calci-
nation temperature has no great effect on catalytic activity.
Thus conversions and selectivities vary over a short range,
always being a minimum for ZS873.
For sulfated samples, conversions and selectivity have
considerable variations. Conversions are in all cases greater
than for nonsulfated samples and the selectivity decreases
due to the hydrogenation reaction that takes place with iso-
merization products (more important in ZS773S). In par-
ticular, sample ZS1073S seems to be the most active and
selective for CSI to 1-MCPE.
a At reaction temperature of 523 K.
total conversion (XT molar%) have been plotted. From
these plots the following results may be pointed out. Con-
versions for the first series of samples are relatively low
(from 14 to 9%, lowest value for ZS873, 4%), the main
product being 1-MCPE (75% with respect to 3-MCPE).
HT is more outstanding in CHE than in isomerization prod-
ucts since MCPA is produced in a very low percentage. For
ZS873 there is a deactivation at pulses beyond 523 K.
In the sulfated samples, conversions are much higher
(about 40% for ZS773S and with a minimum using ZS873S,
24%). In this case HT for ZS773S is considerably higher, in
the sense that CSI takes place to 66%; a great amount of
MCPA is observed (Fig. 8). In other words, hydrogenation
is important for the products of the isomerization reaction
(1- and 3-MCPE), but not for the reagent (CHE).
For ZS873S and ZS1073S the CSI reaction appears with
83 and 85%, respectively, versus HT, but again MCPA has
molarpercentageshigherthanfornonsulfatedsamples. The
deactivation process is important for these samples, taking
place in pulses at above 323 K. It is worth noting that sul-
fated samples are more reactive than nonsulfated samples
ACKNOWLEDGMENTS
The authors acknowledge financial support by a NATO grant (HTECH.
CRG 931258, 1993) and financial support from the “Direccio´n General de
Investigacio´n Cient´ıfica y Te´cnica” (DGICYT), Projects PB92-0816 and
PB93-0917.
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