Preparation of TiO2-ZrO2 mixed oxides with controlled acid-basic properties

Article abstract of DOI:10.1016/j.molcata.2004.06.003

TiO₂-ZrO₂ mixed oxides were synthesized by the sol-gel method and evaluated in the isopropanol decomposition. Acidity determined by NH₃-TPD and FTIR-pyridine adsorption showed that in mixed oxides, the number of acid sites was increased, pyridine adsorption identified Lewis acid sites. Basic sites were identified by FTIR-CO₂ adsorption, suggesting the formation of mixed oxides with acid-basic properties. XRD spectra identified anatase in the TiO₂ rich region, amorphous material in the mixed oxide 50-50 TiO₂-ZrO₂ and tetragonal and monoclinic crystalline phases in the ZrO₂ rich region. Activity in the isopropanol decomposition showed a good correlation between the acid-based properties and the selectivity to propylene, acetone, and isopropyl ether. The latter was a product, which mainly depended on the acid sites density.

Full text of DOI:10.1016/j.molcata.2004.06.003

Journal of Molecular Catalysis A: Chemical 220 (2004) 229–237
Preparation of TiO –ZrO mixed oxides with
2
2
controlled acid–basic properties
a
a
a,b,∗
, J. Navarreteb
M.E. Manr ´ı quez , T. López , R. Gómez
a
Depto. Qu ´ı mica, Universidad Autónoma Metropolitana-Izt., A.P. 55-534, México D.F. 09340, Mexico
b
Instituto Mexicano del Petróleo, Lázaro Cárdenas 152, 07730 México D.F., Mexico
Received 14 May 2004; received in revised form 3 June 2004; accepted 3 June 2004
Available online 17 July 2004
Abstract
Titania–zirconia mixed oxides with various ZrO
2
content in TiO
surface areas (77–244 m /g) were obtained. Acidity determined by NH
the number of acid sites is dramatically increased; it varies from 173 ␮mol NH
FTIR-pyridine adsorption showed the presence of Lewis sites in the catalysts. Basic sites were identified by FTIR-CO
the formation of mixed oxides with acid–basic properties. XRD spectra identified anatase in the TiO rich region, amorphous material in
the mixed oxide 50–50 TiO –ZrO and tetragonal and monoclinic crystalline phases in the ZrO rich region. Activity in the isopropanol
2
(10, 50 and 90 wt.%) were prepared by the sol–gel method. High specific
-TPD and FTIR-pyridine adsorption showed that in mixed oxides
/g for TiO to 1226–1456 ␮mol NH /g for the mixed oxides.
adsorption, suggesting
2
3
3
2
3
2
2
2
2
2
decomposition showed a good correlation between the acid–basic properties and the selectivity to propene, acetone and isopropyl ether. The
latter was found as a product which mainly depends of the acid sites density.
©
2004 Elsevier B.V. All rights reserved.
Keywords: Titania–zirconia sol–gel; XRD-TiO2–ZrO2; Isopropanol dehydration; Titania–zirconia acid–base properties; Titania–zirconia catalysts;
Titania–zirconia FTIR; Titania–zirconia ammonia TPD
1
. Introduction
occurs [9]. The analysis of the products distribution,
propene, acetone and isopropylether, give helpful informa-
tion to understand the role of the acidity strength, acid sites
density and acid–basic properties of the catalysts [10–12].
Alcohols decomposition has been carried out in a large
number of catalysts showing strong or medium acidity, like
silicoaluminates or aluminas [13,14] in which the isopropy-
lether selectivity in minor to 10%, in these catalysts propene
is the major product. In catalysts with low acidity as ZrO2
[15] or TiO2 [16], and also in the so-called basic catalysts
as MgO [17,18], the main products are propene or acetone
and practically the formation of the ether is not observed.
Since selectivity in alcohols decomposition depends of
the strength and distribution of the acid sites, efforts have
recently made in order to design catalysts with controlled
acidity, and mixed oxides were prepared with these purposes.
However, by example in SiO2–MgO [19] or ZrO2–MgO [11]
showing basic properties the ether formation is not observed
in the isopropanol decomposition. On the other hand for the
Al2O3–MgO acid–basic mixed oxides isopropyl ether is not
obtained and the propene/acetone ratio strongly depends of
the preparation conditions [19,20].
Alcohols dehydration is an important catalytic test to
identify acid and basic sites in heterogeneous catalysts
1,2]. To established strength, distribution and density
[
of acid sites, alcohols decomposition has been carefully
studied. Between them isopropanol [3,4], 1-butanol [5,6],
and 4-methyl-2-pentanol dehydration [7,8] are the reac-
tions most frequently reported. The main products for the
1-butanol dehydration are 1-butene and the corresponding
isomerized olefins, the formation of dibutylether was also
observed. For 4-methyl-2-pentanol dehydration, hexenes
and skeletal rearrangements were reported as products; in
this reaction the formation of ethers is not observed. On the
other hand, in spite of the difficult to correlate the catalysts
acidity with activity and selectivity, isopropanol decompo-
sition is a recommendable reaction, since dehydration, de-
hydrogenation and condensation reactions simultaneously
∗
Corresponding author. Tel.: +55 58044668; fax: +55 58044666.
E-mail address: gomr@xanum.uam.mx (R. G o´ mez).
1
381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.06.003

2
30
M.E. Manr ´ı quez et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 229–237
Table 1
2000 automated apparatus. The specific surface areas were
calculated from the isotherms using the BET method.
Experimental relationships used in the synthesis of the TiO2–ZrO2 mixed
oxides
TiO2–ZrO2 weight
ratio (wt.%)
Titanium butoxide
(mol)
Zirconium
butoxide (mol)
3.2. X-ray diffraction
1
5
9
0–90
0–50
0–10
0.05
0.25
0.45
0.29
0.17
0.03
Specimens were prepared packing the powder samples
in a glass samples holder. The X-ray diffraction spectra
were obtained with a Siemens D-5000 diffractometer, us-
ing Cu K␣ radiation at 40 kV and 30 mA, and a secondary
graphite monochromator. The measurements were recorded
In the present work TiO2–ZrO2 mixed oxides were syn-
thesized by the sol–gel method and evaluated in the iso-
propanol decomposition. The purpose is to obtain catalysts
showing acid and basic properties with high selectivity to
the formation of ethers, additives in use to increase gasoline
octane number. Nitrogen isotherms and X-ray diffraction
◦
in steps of 0.02 with a count time of 3 s in the 2θ range of
–90 .
◦
5
3
.3. TPD-NH3
(
XRD) were used to characterize textural and structural
◦
Samples were heated at 400 C under N2 flow during
h, then cooled to room temperature and saturated with
properties of the solids. FTIR-pyridine adsorption and
ammonia thermodesorption (TPD-NH3) were used to eval-
uate acidity; the basicity was characterized by means of
FTIR-CO2 spectroscopy.
1
NH3 (30 cc/min). The excess of ammonia was flushed out
with N2 flow during 30 min. Ammonia desorption was
accomplished by increasing the temperature at 10 C/min
◦
◦
until 500 C. An Altamira AMI-3 automated instrument
operating in the programmed thermodesorption mode was
used.
2
. Experimental
The precursors employed in the preparation of the sol–gel
catalysts were titanium n-butoxide (98%, Aldrich) and zir-
conium n-butoxide (99%, Aldrich) in n-butanol (Baker,
3
.4. FTIR-pyridine adsorption
Transparent pellets of the oxides were put in a stainless
9
9%). Reference titania was prepared as follows: 3.0 mol
steel sample holder with CaF2 windows, which allows vac-
uum in situ and thermal treatments. The samples was des-
orbed at 400 C during 2 h and then cooled to room tem-
perature. After, pyridine was introduced in to the cell and it
was maintained in contact with the sample for 30 min. The
excess of pyridine was eliminated in vacuum and pyridine
desorption was accomplished by heating from room temper-
of n-butanol was put in a glass system at low temperature
◦
(
0 C) with constant stirring. After, 0.5 mol of titanium
◦
n-butoxide were drop-wise added to the solution and then
the pH was adjusted to 3.0 with HNO3 (Baker 56 vol.%).
◦
The flask was put under reflux at 70 C and stirred for
5
h. Finally 3.3 mol of H2O were added to the reaction
system and the resulting suspension was maintained under
reflux and constant stirring until gelling was achieved. The
ZrO2 reference was synthesized in the same using way but
zirconium n-butoxide as zirconia precursor.
◦
ature to 400 C. The spectra were obtained with a 710SX
Nicolet FTIR spectrometer.
3.5. FTIR-CO2 adsorption
TiO2–ZrO2 mixed oxides were synthesized following the
above described procedure. Appropriate amounts of the ti-
tanium and zirconium alkoxides were simultaneously added
◦
After desorption at 400 C, the sample was cooled to
◦
◦
−
100 C using liquid nitrogen. Then CO2 (30 Torr) was ad-
drop by drop to the n-butanol solution at 0 C and the pH
mitted into the cell. After 15 min in contact with the sam-
ple, the excess of CO2 was evacuated and the spectra were
obtained with a 710SX Nicolet FTIR spectrometer.
was adjusted at 3.0 with nitric acid. Then the mixture was
◦
put in reflux at 70 C, and the water for the hydrolysis was
added. The amounts for the two alkoxides used for the dif-
ferent synthesis are reported in Table 1. After gelling the
◦
samples were dried under vacuum for 24 h at 70 C. Finally
3.6. Catalytic activity
◦
the gels were annealed at 400 C during 4 h in air.
The decomposition of isopropanol (Baker, 99%) was de-
termined in a continuous flow reactor (5 ml) at low con-
version. The reaction conditions were: catalyst mass 50 mg,
isopropanol partial pressure 100 Torr, reaction temperature
3
. Characterization
◦
3
.1. Specific surface area
150 C, flow carrier nitrogen (1.8 l/h). The reaction products
propene, acetone and isopropylether were analyzed with a
VARIAN Star 3600CX gas chromatograph apparatus cou-
pled to the reactant system.
Textural properties were determined from the nitrogen
adsorption isotherms obtained with a Micromeritics ASAP

M.E. Manr ´ı quez et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 229–237
231
Table 2
4.2. XRD diffraction
Specific surface area and mean pore size diameter of sol–gel TiO2, ZrO2,
and TiO2–ZrO2 mixed oxides annealed at 400 C
◦
◦
XRD pattern for annealed samples at 400 C are shown
in Fig. 1. In the TiO2 spectrum the peak at θ = 25.5 corre-
sponds to the anatase crystalline phase with a crystallite size
2
Catalysts
BET area (m /g)
Pore size diameter (Å)
TiO2 (100)
110
153
244
128
157
71
52
23
44
60
TiO2–ZrO2 (90–10)
TiO2–ZrO2 (50–50)
TiO2–ZrO2 (10–90)
ZrO2 (100)
of 13 nm. In mixed oxides, in the TiO rich region anatase
phase is observed with a crystallite size of 13 nm, while
in TiO2–ZrO2 (50–50) the sample is amorphous. Finally
in the ZrO2 rich region and pure ZrO2, tetragonal (t) and
monoclinic (m) crystalline phases were observed [21–25].
2
The crystallite size for ZrO2 was 9 and 11 nm for (t) and
4
. Results and discussion
.1. Specific surface areas
The specific surface areas of the isolated oxides and mixed
◦
(m) phases, respectively. In samples annealed at 400 C, for
TiO2 rich region crystalline zirconia phases are not observed,
while for the ZrO2 rich region the inverse phenomenon oc-
curs i.e., crystalline titania phases are not observed. In both
4
4
+
cases an inhibiting crystallite size growth effect by Zr or
oxides are reported in Table 2. For TiO2 and ZrO2 the cor-
responding specific surface areas were 110 and 157 m /g,
respectively. In mixed oxides the BET specific surface ar-
eas varies in depending of the ZrO2 content, they were 153
4
+
Ti in TiO2 and ZrO2, respectively, may occur [26]. The
oxide at low content will be found as a large dispersed ox-
ide in the ZrO2 or TiO2 structure, respectively. We can ex-
pect that in such cases the crystallite size of the doping ox-
ide will be lower than 5 nm and not observable by XRD. In
the TiO2–ZrO2 (50–50) sample amorphous material is ob-
tained. According to Tanabe hypothesis [27] models I and II
could be the structures of TiO2–ZrO2 mixed oxides, where
TiO2 or ZrO2 are the major component, while the model
III will correspond to the 1:1 mixed oxide composition.
The hydroxylated form is expected to show an amorphous
structure [28], as it can be observed in the XRD pattern of
Fig. 1.
2
2
(90:10), 244 (50:50) and 128 m /g (10:90). The largest area
was obtained in the mixed oxide showing an amorphous
form, as observed by XRD, Fig. 1. In depending of the
hydrolysis-condensation rate of the titanium or zirconium
alkoxides the particle agglomeration behavior could be af-
fected and the different textural properties of the mixed ox-
ides are a result of such effects. In example, the mean pore
size diameter calculated with the BJH method corresponds
in all the solids to mesoporous materials (23–71). As an ef-
fect of the competitive hydrolysis reactions the small pore
diameter was obtained in the mixed oxides.
4.3. Ammonia thermodesorption
TPD-ammonia thermograms shown a flat and broad NH3
desorption curve for pure TiO2 and ZrO2 (Fig. 2), indi-
cating that such solids have a broad acid sites distribution.
The maximum desorption peak for ZrO2 is observed around
◦ ◦
82 C, whereas for TiO2 the maximum is around 310 C.
1
On the other hand, the desorption curves corresponding to
the TiO2–ZrO2 mixed oxides are characteristic of solids hav-
ing sharp acid sites distribution. For the TiO2–ZrO2 (50–50
and 90–10) samples the maximum temperature desorption
◦
peak are around 176 C, while for the TiO2–ZrO2 (10–90)
◦
the peak is around 204 C. The number of acid sites was
calculated from the ammonia thermodesorption curves and
the values are reported in Table 3. A notable increase in
the number of acid sites can be observed in the mixed ox-
ides if they are compared with titania or zirconia. Values
of 1326, 1456 and 1226 ␮mol NH3/g corresponding, respec-
tively, to the 90–10, 50–50 and 10–90 TiO2–ZrO2 mixed
oxides are reported. For titania and zirconia the number of
acid sites were 173 and 138 ␮mol NH3/g respectively. Such
results clearly indicate that in the mixed oxides a big number
of acid sites were developed, suggesting the formation of a
great number of acids sites probably due to the substitution
of some titanium by zirconium atoms, or by the effect of
large dispersion of TiO or ZrO on the surface of the mixed
Fig. 1. XRD pattern of TiO2, ZrO2 and TiO2–ZrO2 mixed oxides annealed
at 400 C.
◦
2
2

2
32
M.E. Manr ´ı quez et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 229–237
Fig. 2. Ammonia TPD profiles for TiO2, ZrO2 and TiO2–ZrO2 mixed oxides.
oxide. Concerning the strength of the acid sites, usually
is accepted that desorption temperature can be related to
the strength of the sites. In the mixed oxides the maximum
desorption temperature for the TiO2–ZrO2 are found be-
50–50 and 10–90 wt.% are shown. The adsorption spectra
shown that pyridine is not totally desorbed at temperatures
of 400 C. The results also show that acid sites strength in
such samples is comparable between them. The number of
acid sites determined from pyridine adsorption is reported
in Fig. 4. It can be seen that the samples showing the higher
number of acid sites correspond to the mixed oxides. Such
results confirm that obtained from ammonia thermodesorp-
tion, where the isolated TiO2 and ZrO2 oxides have the
lowest number of acid sites.
◦
◦
tween 176 and 204 C, while for zirconia and titania they
◦
are around 182 and 310 C, respectively. With exception of
TiO2 we can say that the strength of the acid sites is of the
same order. The number of them is which notably differs
in the mixed oxides. Moreover form these results, we can
speculate about the absence of Brönsted sites, since it has
been reported that to Brönsted sites correspond desorption
◦
temperatures higher than 400 C, which is not the case in
4.5. FTIR-CO2 adsorption
our catalysts [29,30].
Conventionally to identify basic sites in solids CO2 ad-
4
.4. FTIR-pyridine adsorption
sorption is a recommendable technique. In depending of
the basic site strength adsorbed CO2 was identified as
Pyridine adsorption is a valuable method to determine the
−
1
1
monodentate (1578 and 1359 cm ) or bidentate carbon-
nature of acid sites. Absorption bands around 1605, 1575,
1
can be observed for TiO2, ZrO2 and TiO2–ZrO2 mixed
oxides. Brönsted acidity (1540 cm band) is not observed
in any of the samples, isolated or mixed oxides. In Fig. 3.
FTIR-pyridine adsorption spectra for TiO2–ZrO2 90–10,
−
ates (1672, 1243 and 1053 cm ) [32,33]. Additionally,
absorption bands have been reported at 1630, 1430, 1408
−
1
490 and 1444 cm characteristic of Lewis acid sites [31]
−
1
and 1221 cm
assigned to bicarbonates formed by the
−
1
interaction between OH groups and CO2. In particular for
−
1
TiO2 a band at 2338 cm is observed and assigned to CO2
linearly adsorbed on the Ti4 ions [34].
+
In Fig. 5a, for TiO2 it can be observed the absorption
−
1
bands at 1670 and 1600 cm corresponding to CO2 ad-
sorbed on basic sites. On the other hand, in TiO2–ZrO2
Table 3
Ammonia adsorption on TiO2, ZrO2, and TiO2–ZrO2 mixed oxides
(
1
1
90–10) and 10–90 mixed oxides additional bands at 1539,
441 and 1250 cm for the former and at 1612, 1565, 1363,
357 and 1259 cm for the latter can be observed (Fig. 5b
and c), respectively. These bands suggest the creation of
new basic sites by the incorporation of ZrO2 into the TiO2.
However, in the 50–50 mixed oxide the spectra (not shown)
Catalysts
␮mol NH3/g
␮mol/m2
−1
−
1
TiO2 (100)
173
1.57
8.67
5.97
9.58
0.88
TiO2–ZrO2 (90–10)
TiO2–ZrO2 (50–50)
TiO2–ZrO2 (10–90)
ZrO2
1326
1456
1226
138
−1
only the absorption bands at 1284 and 1268 cm assigned

M.E. Manr ´ı quez et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 229–237
233
Fig. 4. Pyridine (␮mol/g) adsorption on TiO2, ZrO2 and TiO2–ZrO2
◦
sol–gel samples annealed at 400 C.
Ti4 ions. The band appears after annealing the sample at
+
◦
4
00 C, and resists vacuum desorption, i.e. the samples ad-
sorb CO2 from the ambient and its intensity does not evolve
after CO2 exposure.
It has been reported that in ZrO2 strong CO2 adsorption
occurs. In monoclinic zirconia the CO2 adsorption corre-
−1
sponds to carbonates (1600 and 1430 cm ). On the other
hand in tetragonal ZrO2 the CO2 adsorption can be as
−
1
bidentate form (1672, 1243 and 1053 cm ) [35]. The CO2
absorption spectrum on ZrO2 is shown in Fig. 6. Absorp-
tion bands assigned to monoclinic zirconia crystalline phase
are observed. Note that the CO2 absorption assigned to
tetragonal phase are not present. X-ray diffraction, however,
identify the corresponding peaks to both monoclinic and
tetragonal zirconia crystalline phase. The absence of CO2
absorption bands identifying basic sites in tetragonal phase
suggested that in mixture of (t) and (m) phases, the CO2 ab-
sorption is selective to basic sites on (m) phases. The inten-
sity of these bands confirms the basic proprieties of ZrO2.
In Fig. 7 [36], the CO2 coordination is illustrated. When
CO2 is unidentate coordinated this type of sites correspond
to strong basic sites (I). In bidentate coordination sites cor-
respond to medium basic strength (II). Finally if CO2 is co-
ordinated like carbonate this adsorption is assigned to weak
basic sites (III). From Fig. 6, we can observe that the mixed
oxides have in large proportion medium basic sites.
Fig. 3. FTIR-pyridine adsorption on: (a) TiO2–ZrO2 (10–90), (b) TiO2–
ZrO2 (50–50) and (c) TiO2–ZrO2 (90–10) thermally annealed at 400 C.
◦
to the interaction between CO2 and basic OH groups of the
solid were observed.
5. Catalytic activity
Note that the band at 2338 cm 1 is observed in the
TiO2–ZrO2 mixed oxides (Fig. 5b and c). As established
before this band corresponds to CO2 linearly adsorbed on
−
The isopropanol decomposition has been extensively re-
ported as a valuable catalytic test to analyze the acid–basic
properties of heterogeneous catalysts [37,38]. Different

2
34
M.E. Manr ´ı quez et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 229–237
ads/CO2
blank
2
1
1
0
0
.0
.5
.0
.5
.0
ZrO2
1
602
1431
1
269
1
545
2
000
1800
1600
1400
1200
1000
-
1
Wavenumver cm
Fig. 6. FTIR-CO2 adsorption on ZrO2.
Fig. 7. CO2 coordination on basic sites: (I) monodentate, (II) bidentate
and (III) carbonate.
mechanisms have been reported to explain the role of acid
and basic sites in the isopropanol decomposition giving as
main products the olefin, acetone and isopropylether. In
general is conventionally accepted that acid sites are the
responsible for the dehydration activity giving propene.
A mechanism is suggested for the dehydration reaction in
Fig. 8b, where strong acid sites and weak basic sites are
involved. In similar way the formation of the dehydro-
genated product (acetone) acid and basic sites are required
(Fig. 8a). However, in this case acid sites with moderate
strength and strong basic sites are required. On the other
hand, for the formation of the ether only acid sites of mod-
erated strength are involved (Fig. 8c). The formation of
isopropylether depends then of the number of acid sites
preferentially than their strength [39]. In Table 4, the activity
Table 4
Activity and selectivity for the isopropanol decomposition on TiO2, ZrO2
and TiO2–ZrO2
Catalyst
Ra
×10 mol/s g)
Xa
Selectivity (%)
6
(
Propene Acetone Isopropylether
TiO2 (100) 3.01
4.67 53
3.35 51
26
19
22
31
TiO2–ZrO2
90–10)
TiO2–ZrO2
(50–50)
2.16
0.85
2.27
(
1.32 30
3.52 48
4.54 46
18
16
34
53
36
20
Fig. 5. FTIR-CO2 adsorption on: (a) TiO2–ZrO2 (10–90), (b) TiO2–ZrO2
◦
(
90–10), (c) TiO2, annealed at 400 C.
TiO2–ZrO2
(10–90)
ZrO2 (100) 2.9

M.E. Manr ´ı quez et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 229–237
235
Fig. 8. Mechanisms proposed for the 2-propanol decomposition.
1
2
3
)TiO2
8
6
4
2
0
)TiO -ZrO (90-10)
2
2
)TiO -ZrO (50-50)
2
2
4)TiO -ZrO (10-90)
2
2
5
)ZrO2
3
)
1
)
2
)
5
)
4
)
0
20
40
60
80
100
120
Time (min)
Fig. 9. Isopropanol decomposition in function of time for TiO2, ZrO2, and TiO2–ZrO2 mixed oxides.

2
36
M.E. Manr ´ı quez et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 229–237
and selectivity for isopropanol decomposition are reported.
It can be seen that the conversion of isopropanol is almost
of the same order for all the catalysts (<5%), then we have
differential reactor conditions. Only a moderated deactiva-
tion was observed in these conditions and is of the same
order in all catalysts (Fig. 9), then the effects in selectivity
will not be discussed in terms of deactivation side reactions.
In Table 4, we can see that the highest selectivity to
propene (53%) corresponds to the TiO2 catalyst, to which
also corresponds the ammonia desorption peak at highest
temperature. However, it cannot be possible to establish a
correlation between the number of acid sites or the ammonia
desorption temperature peaks for all of them are in agree-
ment with the results previously reported in this reaction
hand in the ZrO2 rich sample (10–90) and pure ZrO2 a mix-
ture of tetragonal and monoclinic crystalline phases were
observed. Activity in the isopropanol decomposition is of
the same order for al the catalysts. Nevertheless, selectivity
to propene, acetone and isopropylether varies in function of
the acid–basic properties. In mixed oxides is obtained the
great amount of isopropylether. The selectivity is found as
a function of the number of acid sites (mainly density than
strength of them). These results showed that by the sol–gel
method it is possible to prepare catalysts with controlled se-
lectivity to propene, acetone or isopropylether in depending
of the relative amounts of TiO2 and ZrO2 in the mixed ox-
ides.
[9]. Concerning the formation of acetone the catalyst show-
ing the highest selectivity was pure ZrO2 (34%). This result
confirms the basic character given to ZrO2. In TiO2–ZrO2
mixed oxides, the formation of acetone is of the same order
in all of them 18, 16, and 16% for the 90–10, 50–50 and
Acknowledgements
We acknowledge to the Conacyt by the support giving to
this study. M.E. Manriquez by the scholarship RE-111634.
1
0–90 TiO2–ZrO2 solids, respectively.
For isopropylether formation, in TiO2 and ZrO2 the se-
References
lectivity is of the same order in both catalysts 22 and 20%,
respectively. In Table 3, we can see that the number of acid
sites is the lowest in both oxides 173 and 138 ␮mol NH3/g,
respectively. The ether formation, however, is the higher in
the mixed oxides (31–53%), if compared with TiO2 or ZrO2.
In Tables 3 and 4, it can be seen that the ether formation
and the number of acid sites follow a similar sequence. For
[1] J.A. Linnekoski, A.O.I. Krause, A. Holmen, M. Kjetsa, K. Moljord,
Appl. Catal. A Gen. 174 (1998) 1.
[2] L. de Mourges, F. Peyron, Y. Trambouze, M. Prettre, J. Catal. 7
(1967) 117.
[3] A. Corma, V. Fornés, F. Rey, J. Catal. 148 (1994) 205.
[
4] A.A. Aramend ´ı a, V. Borau, C. Jiménez, J.M. Marinas, A. Porras,
F.J. Urbano, React. Kinet. Catal. Lett. 53 (1994) 397.
1
326, 1456, and 1226 ␮mol NH3/g acid sites concentration
the selectivity was 31, 53 and 36%, respectively. Mixed ox-
[
[
[
5] P. Berteau, B. Delmon, Appl. Catal. 70 (1991) 307.
6] P. Berteau, S. Ceckiewicz, B. Delmon, Appl. Catal. 31 (1987) 361.
7] M.G. Cutrufello, I. Ferino, V. Solinas, A. Primavera, A. Trovarelli,
A. Auroux, C. Picciau, J. Phys. Chem. 1 (1999) 3369.
ides also show the highest acid site density (8.6, 5.9 and
2
9
.5 ␮mol NH3/m ) in TiO2 and ZrO2 the density was 1.5
2
[8] A. Auroux, P. Artizzi, I. Ferino, R. Monaci, E. Rombi, V. Solinas,
Micropor. Mater. 11 (1997) 117.
and 0.88 ␮mol NH3/m . The agreement with the number but
not with the strength (NH3-TPD) of the acid sites and the
selectivity to isopropylether is a good support for the pro-
posed mechanism represented in Fig. 8. The big number of
acid sites obtained in the TiO2–ZrO2 mixed oxides prepared
by the co-gelling of the respective alkoxides, is a result con-
firming that in both oxides a large number of new acid cen-
ters were generated.
[
9] J.A. Wang, X. Bokhimi, O. Novaro, T. López, F. Tzompantzi, R.
Gómez, J. Navarrete, M.E. Llanos, E. López-Salinas, J. Mol. Catal.
A Chem. 137 (1999) 239.
[10] R. Valarivan, C.N. Pillai, C.S. Swamy, Reat. Kinet. Catal. Lett. 53
1994) 429.
(
[
11] M.A. Aramend ´ı a, V. Boáu, I.M. Garc ´ı a, C. Jiménez, A. Marinas,
J.M. Marinas, A. Porras, F.J. Urbano, Appl. Catal. A 184 (1999) 115.
12] K. Arata, H. Sawamura, Bull. Chem. Soc. Jpn. 48 (1975) 3377.
13] H. Knözinger, H. Bül, K. Kochloefl, J. Catal. 24 (1972) 57.
14] H. Közinger, A. Schengllia, J. Catal. 33 (1974) 142.
[
[
[
[
15] N. Satoh, J.I. Hayashi, H. Hattori, Appl. Catal. A Gen. 202 (2000)
6
. Conclusions
207.
[
16] G. Cai, Z. Lin, R. Shi, C. He, L. Yang, C. Sun, Y. Chang, Appl.
In the preparation of TiO2, ZrO2 and TiO2–ZrO2 mixed
Catal. A Gen. 125 (1995) 29.
oxides by co-gelling the corresponding alkoxides, solids
with high specific surface were obtained. Acidity determined
by NH3-TPD and FTIR-pyridine adsorption showed that in
mixed oxides the number of acid sites is notably increased,
pyridine adsorption identify Lewis acid sites. By FTIR-CO2
the formation of monodentate and bidentate carbonates were
identified. Thus in this catalysts acid and basic sites co-exist.
[17] J.A. Wang, X. Bokhimi, O. Novaro, T. López, R. Gómez, J. Mol.
Catal. A Chem. 145 (1999) 291.
[18] T. López, R. Gómez, J. Navarrete, E. López-Salinas, J. Sol–Gel Sci.
Technol. 13 (1998) 1043.
[
19] T. López, M.E. Manr ´ı quez, R. Gómez, A. Campero, M.E. Llanos,
Mater. Lett. 46 (2000) 21.
[20] J.P. Dalmon, D. Bernard, J.M. Bonnier, Farad. Trans. I 70 (1974)
021.
[21] H. Matsuhaschi, M. Hinio, K. Arata, Appl. Catal. 59 (1990) 203.
2
◦
XRD spectra of samples treated at 400 C shows anatase as
[22] T. Lopez, M.E. Manriquez, R. Gomez, X. Bokhimi, J. Solid State
Chem. (in press).
the predominant crystalline phase in TiO2 and in the rich
TiO2 mixed oxide (90–10). In the mixed oxide at 50–50
composition the sample is almost amorphous. On the other
[23] D. Das, H.K. Mishra, A.K. Dalai, K.M. Parida, Appl. Catal. A Gen.
243 (2003) 271.

M.E. Manr ´ı quez et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 229–237
237
[
24] M.E. Manr ´ı quez, T. López, D.H. Aguilar, P. Quintana, in: T. Lopez,
D. Avnir, M. Aegerter (Eds.), Emerging Fields in Sol–Gel Sci-
ence and Technology Materials, Kluwer Academic Publishers, 2003,
p. 254.
[31] R. Barthos, F. Lónyi, J. Engelhardt, J. Valyon, Top. Catal. 10 (2000)
79.
[32] D.J. Suh, T.J. Park, Chem. Mater. 14 (2002) 1452.
[33] G. Martra, Appl. Catal. A Gen. 200 (2000) 275.
[34] M.A. Aramend ´ı a, V. Borau, C. Jiménez, J.M. Marinas, J.R. Ruiz,
F.J. Urbano, Appl. Catal. A Gen. 244 (2003) 207.
[35] K. PoKrovski, K.T. Jung, A.T. Bell, Langmuir 17 (2001)
4297.
[36] M.A. Aramend ´ı a, V. Borau, C. Jiménez, J.M. Marinas, J.R. Ruiz,
F.J. Urbano, Appl. Catal. A Gen. 244 (2003) 207–215.
[37] B. Shi, B.H. Davis, J. Catal. 157 (1995) 359.
[38] V.K. D ´ı ez, C.R. Apestegu ´ı a, J.I. Di Cosimo, Catal. Today 63 (2000)
53.
[
[
[
[
25] J.A. Wang, O. Novaro, X. Bokhimi, T. Lopez, R. Gómez, J. Phys.
Chem. B 105 (2001) 9692.
26] Y.M. Wang, S.W. Liu, M.K. Lu, S.F. Wang, F. Gu, X.Z. Gai, X.P.
Cui, J. Pan, J. Mol. Catal. A Chem. 215 (2004) 137.
27] K. Tanabe, T. Sumiyoshi, K. Shibata, Bull. Chem. Soc. Jpn. 47
(
1974) 1064.
28] I. Wang, W. Chang, R. Shiau, J. Wu, C. Chung, J. Catal. 83 (1983)
28.
4
[29] H. Zou, Y.S. Lin, Appl. Catal. A Gen. 265 (2004) 35.
[
30] D. Das, H.K. Mishra, A.K. Dalai, K.M. Parida, Appl. Catal. A Gen.
[39] P. Berteau, M. Ruwet, B. Delmon, Bull. Soc. Chim. Belg. 94 (1985)
11.
2
43 (2003) 271.