Synthesis and characterization of ZrO2 as acid-basic catalysts: Reactivity of 2-methyl-3-butyn-2-ol

Article abstract of DOI:10.1006/jcat.1999.2418

Two ZrO₂-based catalysts have been synthesized by thermal treatment of a gel prepared from ZrOCl₂ · 8H₂O. The effect of different preparation parameters on the final properties of the solids was studied. Textural characterization was carried out from nitrogen adsorption/desorption isotherms. XRD and FT-Raman spectroscopy have been used to elucidate the physical state of the catalysts. Their acid-base properties were studied following temperature programmed desorption-mass spectrometry and diffuse reflectance infrared Fourier transform techniques, by using pyridine, 2,6-dimethylpyridine and carbon dioxide as probe molecules. Both solids were tested for the 2-methyl-3-butyn-2-ol reaction by using two different procedures: (a) in a microcatalytic pulse reactor and (b) through isotherm and temperature programmed surface reactions in a flow reactor. In addition, temperature programmed oxidation and temperature programmed desorption experiments revealed that catalyst deactivation was caused by carbon deposition on the catalytic surface. Amphoteric selectivity for both solids, leading to 3-hydroxy-3-methyl-2-butanone and 3-methyl-3-buten-2-one, depends on the reaction temperature; thus, it is 92% at 225°C for a catalyst calcined at 400°C (ZrO₂-400VAC).

Full text of DOI:10.1006/jcat.1999.2418

Journal of Catalysis 183, 240–250 (1999)
Article ID jcat.1999.2418, available online at http://www.idealibrary.com on
Synthesis and Characterization of ZrO2 as Acid–Basic Catalysts:
Reactivity of 2-Methyl-3-butyn-2-ol
M. Angeles Aramend´ıa, Victoriano Bora´u, Ce´sar Jime´nez, Jose´ M. Marinas, Alberto Marinas,
Andre´s Porras, and Francisco J. Urbano
Department of Organic Chemistry, Faculty of Sciences, University of Cordoba, Avda San Alberto Magno s/n, E-14004 Cordoba, Spain
Received July 22, 1998; revised January 5, 1999; accepted January 8, 1999
Zirconia is known to exist in monoclinic, tetragonal, and
cubic phases and as an amorphous solid.The crystal forms of
ZrO2 depend on how the hydroxide is prepared and treated.
Steaming of the hydroxide results in the formation of the
monoclinic form, while a vacuum treatment leads to the
tetragonal form (9). The tetragonal form presents better
textural and acid–base properties than the monoclinic and,
therefore, is the one most used in catalysis (10).
The studies on catalysis by solid acids are extremely nu-
merous (11, 12). Several reviews have recently been pub-
lished dealing with the use of solid acids as heterogeneous
catalysts for the preparations of fine chemicals (13–16). An
important characteristic of solid acids compared to liquid
acids is that solid acids encompass different populations of
sites differing in their nature and strength (17–21). Excel-
lent reviews on catalysis by solid bases have been published
(18–20). Finally, the cooperative behavior of surface sites
with acid–base properties is an essential requirement for a
wide range of catalytic reactions (21).
Two ZrO2-based catalysts have been synthesized by thermal
q
treatment of a gel prepared from ZrOCl2 8H2O. The effect of dif-
ferent preparation parameters on the final properties of the solids
was studied. Textural characterization was carried out from nitro-
gen adsorption/desorption isotherms. XRD and FT-Raman spec-
troscopy have been used to elucidate the physical state of the
catalysts. Their acid–base properties were studied following tem-
perature programmed desorption–mass spectrometry and diffuse
reflectance infrared Fourier transform techniques, by using pyri-
dine, 2,6-dimethylpyridine and carbon dioxide as probe molecules.
Both solids were tested for the 2-methyl-3-butyn-2-ol reaction by
using two different procedures: (a) in a microcatalytic pulse reac-
tor and (b) through isotherm and temperature programmed surface
reactions in a flow reactor. In addition, temperature programmed
oxidation and temperature programmed desorption experiments
revealed that catalyst deactivation was caused by carbon deposi-
tion on the catalytic surface. Amphoteric selectivity for both solids,
leading to 3-hydroxy-3-methyl-2-butanone and 3-methyl-3-buten-
2-one, depends on the reaction temperature; thus, it is 92% at 225 C
c
for a catalyst calcined at 400 C (ZrO2–400VAC).
1999 Academic
Due to the great number of reactions catalyzed by acid,
basic, or acid–basic sites, the determination of number,
strength, Brønsted or Lewis nature of the different acid–
basic sites of solid catalysts, and softness and hardness of
Lewis sites in these solid catalysts is very important. In a
Press
INTRODUCTION
The acid–base property is one of the most important sur- previous work, the acid characteristics of several catalysts
face chemical properties of metal oxide catalysts (1). In this were determined using a spectrophotometric procedure
sense, ZrO2 is a promising material as catalyst, due to its (22) with n-buthylamine in cyclohexane as probe molecule.
amphoteric character as well as its thermal stability (2). At Besides, temperature programmed desorption–mass spec-
present, it has been applied as a catalyst to many organic trometry (TPD–MS) and TPD–diffuse reflectance infrared
reactions such as esterifications and olefin hydrogenations Fourier transform (DRIFT) techniques were designed to
(3).
carry out the characterization of acid–basic sites. Test reac-
Textural and acid–base properties of ZrO2 depend tions are used as well to characterize acid–basic sites (23–
greatly on both the synthetic procedure and the calcina- 25). Thus, Lauron-Pernot et al. (26–28) and other authors
tion temperature (4). Modification of these properties by (29, 30) suggested the 2-methyl-3-butyn-2-ol (MBOH) re-
addition of alkaline metals (5) or sulfate ions (6, 7) as well as action in order to characterize acid–base properties of
by mixing with other metal oxides (8) has also been studied solids.
extensively.
In this work the textural and acid–base properties of
The normal procedure used to prepare ZrO2 consists of two different zirconia catalysts are studied. The effect of
calcination ofitshydroxide (hydroxylated gel), which ispre- the calcination temperature is discussed. Acid–base prop-
pared by hydrolysis of zirconium salts.
erties were studied by TPD–MS of preadsorbed probe
240
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c
Copyright
1999 by Academic Press
All rights of reproduction in any form reserved.

REACTIVITY OF 2-METHYL-3-BUTYN-2-OL
241
molecules such as pyridine (total acidity), 2,6-dimethyl- DRIFT Experiments of Preadsorbed Probe Molecules
pyridine (Brønsted acidity) and carbon dioxide (basicity).
Diffuse reflectance infrared (DRIFT) experiments were
conducted on a Bomen MB-100 instrument with an “envi-
ronmental chamber” (Spectra-Tech). It was operated at a
resolution of 8 cm 1 over the range 4000–400 cm 1 to gather
256 scans. In those experiments with preadsorbed mole-
cules, each catalyst was cleaned by passing an Ar stream of
50 mL/min at 100 C for 30 min. The solids were saturated
with the probe molecule and subsequently flushed with a
stream of pure Ar (50 mL/min) at the saturation tempera-
ture for 2 h in order to avoid physisorption. The solid was
then transferred to the DRIFT environmental chamber and
spectra were obtained at three or four different tempera-
tures (50, 100, 200, and 300 C). A temperature leveling time
of 20 min was used.
The reactivity of MBOH was used as a test reaction to com-
pare the activity of the catalysts by two different proce-
dures: (a) in a microcatalytic pulse reactor and (b) tem-
perature programmed surface reactions (TPSR) in a flow
reactor. In addition, isotherm reactions were carried out
in order to obtain information about the solids. Finally,
the catalyst deactivation was studied by techniques such
as temperature programmed oxidation (TPO) and temper-
ature programmed desorption (TPD) of strongly adsorbed
species.
EXPERIMENTAL
Catalyst Synthesis
Temperature-Programmed Desorption-Mass
Spectrometry Experiments
Zirconium hydroxide was prepared from 30 g of
ZrOCl2 8H2O (Merck Art. 8917) and 1.5 L of distilled wa-
q
ter. Next, NaOH 1 M was added until pH 6.8 was achieved.
The gel was aged for 72 h and then filtered and air-dried for
24 h and placed in an oven at 110 C overnight.
ZrO2 was obtained by calcination of the dried gel by heat-
ing in vacuo from room temperature to the final tempera-
ture (400 or 600 C) at a rate of 2 C/min. Then the final
temperature was kept for 5 h and finally the solids were
allowed to cool back to ambient temperature in a vacuum.
The solids were named ZrO2–400VAC and ZrO2–600VAC,
indicating their respective calcination temperatures.
TPD-MS experiments were carried out in an on-line de-
vice (see Fig. 1) with a VG Sensorlab quadrupole mass spec-
trometer. The optimum TPD conditions were as follows:
heating rate, 10 C/min; Ar flow rate, 50 mL/min. The mass
spectrometer, which was operated in the multiple ion mon-
itoring (MIM) mode, was programmed to perform six scans
per minute.
A quartz reactor (10 mm i.d., 200 mm long) was located
in the middle of a furnace. The temperature of the furnace
was controlled within an error of 1 C. The output of the
reactor was fitted on line to a VG Sensorlab mass spec-
trometer (Fisons Instrument, plc/VG quadrupoles) work-
ing in MIM mode. Monitored peaks and relative abun-
dance were selected from Eight Peak Index of Mass Spectra
(33).
Chemicals
2-Methyl-3-butyn-2-ol, pyridine and 2,6-dimethylpyri-
dine were obtained from Aldrich.
The amines used as probe molecules to determine the
acid properties of the solids were pyridine (pK = 5.25)
Physical Characterization
The textural properties of solids (specific surface area, and 2,6-dimethylpyridine (pK = 7.3). It is known that 2,6-
pore volume, and mean pore radius) were determined from dimethylpyridine is selectively adsorbed on Brønsted acid
nitrogen adsorption–desorption isotherms at liquid nitro- sites, but not on Lewis ones, because of the steric hindrance
gen temperature by using a Micromeritics ASAP-2000 in- of two methyl groups, whereas sterically nonhindered pyri-
strument. Surface areaswere calculated bythe BET method dine is adsorbed on both Brønsted and Lewis (34, 35). The
(31), while pore distributions were determined by the BJH peaks used to quantify pyridine were the base peak (m/z =
method (32). Prior to measurements, all samples were de- 79) and the secondary peak at m/z = 52 (80% relative
gassed at 350 C to 0.1 Pa.
abundance). The MSpeakschosen for 2,6-dimethylpyridine
X-ray investigation of the solids was carried out using were the base peak (m/z = 107) and the secondary peak at
a Siemens D-500 diffractometer provided with an auto- m/z = 66 (60% abundance).
matic control and data acquisition system (DACO-MP).
Carbon dioxide was the probe molecule used to deter-
The patterns were run with nickel-filtered copper radiation mine the basic properties of the catalyst. The gases used for
˚
( = 1.5405 A) at 35 kV and 20 mA; the diffraction angle the CO2 TPD–MS experiments were CO2 and 5% CO2 in
2 was scanned at a rate of 2 C/min.
argon, both of which were supplied by Sociedad Espan˜ola
FT-Raman spectra were obtained on a Perkin-Elmer de Ox´ıgeno S.A. (>99.999% ). Carbon dioxide was quanti-
2000 NIR FT-Raman system with a diode pumped NdYAG fied by the base (m/z = 44) and secondary peaks (m/z = 12,
laser (9394.69 cm 1). It was operated at a resolution of 10% abundance). Calibration was performed by injecting
4 cm 1 over the range 3600–200 cm 1 to gather 64 scans.
pulses of pure CO2 or 5% CO2 in argon.

´
ARAMENDIA ET AL.
242
Prior to the adsorption of any probe molecule, each cata-
TPSR were carried out in two temperature stages: an
lyst was cleaned by passing an Ar stream at 110 C at 50 mL/ isotherm period (200 C for 30 min) and a second stage in
min for 30 min. The solids were then saturated by pass- which the temperature was raised from 200 to 600 C at a
ing an amine/N2 or CO2/Ar stream (50 mL/min) at 25 and constant rate (10 C/min). The feed consisted of a nitrogen
50 C, respectively. Subsequently, a pure N2 or Ar stream flow (50 mL/min) passed through a saturator with MBOH
(50 mL/min) was passed at the saturation temperature for at 20 C. The whole process was monitored through the mass
2 h in order to remove any physisorbed molecules. Once a spectrometer.
stable baseline was obtained, chemisorbed amine or CO2
For the TPD experiments, as soon as the TPSR reaction
was desorbed by heating from saturation temperature to finished, the nitrogen flow was set to 50 mL/min N2 and
600 C in a programmed fashion. The selected mass peaks the catalyst was immediately cooled down to room tem-
were monitored throughout the process. Each experiment perature. Then the catalyst was kept at room temperature
used ca. 100 mg of fresh catalyst. Full details of the TPD– and nitrogen flow for at least 4 h to clean the surface from
MS method and equipment have been given elsewhere (36, physisorbed species. Finally, the temperature was raised at
37).
a constant rate (10 C/min) from room temperature up to
600 C. The whole process was monitored through the mass
spectrometer.
NMR Spectroscopy
TPO were carried out in a way similar to that of TPD
experiments, but using a 2% O2/Ar flow (50 mL/min) during
the temperature ramp.
1H MAS NMR spectra were obtained at 400.13 MHz on
a Brucker ACP-400 spectrometer under an external mag-
netic field for 9.4 T. All measurements were made at room
temperature although the samples had different thermal
pretreatments which mentioned heating in a furnace for
24 h at 120 C and evacuation of the sample at 0.4 Pa at
temperatures between 300 and 600 C. The samples con-
tained in zirconia rotors were spun at the magic angle (54
RESULTS AND DISCUSSION
Physical Characterization of the Catalysts
The thermal analysis of the Zr(OH)2 gel showed a con-
440 relative to the external magnetic field) at 5 kHz. 1H spec- tinuous loss of water from 100 C. Between 300 and 450 C,
tra were recorded after applying an excitation pulse of 5/2 a second step takes place. This second event has been at-
(5 s) with a 2-s interval between successive accumulations tributed to the conversion of an amorphous material to a
to avoid saturation effects (38). Tetramethylsilane (TMS) crystalline phase (9). The overall mass loss was about 15%
was used as external standard for the proton. The number of the initial gel.
of accumulations for the proton were 200.
Figure 2 shows the X-ray diffraction patterns of the dried
gel (trace “a”) as well as those of both ZrO2 samples studied
in thiswork (traces“b” and “c” for the solidsZrO2–400VAC
and ZrO2–600VAC, respectively). Diffractogram “a” is typ-
ical of an amorphous solid, showing broad bands. On the
contrary, pattern “c” presented numerous sharp bands in-
dicating that ZrO2–600VAC is a crystalline solid. This pat-
tern corresponding to ZrO2–600VAC was found to be com-
posed of bands corresponding to two different crystalline
phases: a minor tetragonal phase and a main monoclinic
phase (baddeleyite). The tetragonalphase ismetastable and
reverts to the monoclinic, which is, therefore, the dominant
phase. Catalyst ZrO2–400VAC is not very crystalline, show-
ing weak bands corresponding to the tetragonal phase.
FT-Raman spectra confirm the results obtained from
XRD. Raman spectroscopic information is particulary valu-
able for distinguishing the different phases of zirconia. In
fact, tetragonal and cubic phase unambiguous assignment
is not possible from XRD alone (39, 40). Kilo et al. (39)
reported the Raman frequencies observed for the modifi-
cations of zirconia. The most striking difference between
Reactivity Tests
a. Reactions in microcatalytic pulse reactor. The cata-
lytic processes were carried out in a microcatalytic pulse
reactor (0.4 cm i.d. quartz tubular reactor) placed in the
injection port of a gas chromatograph (Hewlett Packard
Model 5890) equipped with a Supelcowax capillary column
(30 m length and 0.20 mm i.d.) and with a flame ionization
detector (FID). The catalyst was placed between two layers
of glass wool. Previous to each run, it was pretreated in situ
in the reactor at 200 C for 2 h under nitrogen (75 mL/min).
The reactor temperature was then decreased to the reac-
tion temperature. Data were collected over a wide range of
experimental conditions: MBOH pulses, 0.5–2 L; temper-
ature, 160–350 C; N2 flow, 80–110 mL/min. This procedure
was repeated several times in order to follow the deactiva-
tion occurring during the first pulses.
In separate experiments, the reactions were carried out
by using a GC-MS (HP5970 quadrupole mass spectrome-
ter) in order to characterize the reaction products.
the Raman spectra of stable monoclinic and metastable
1
b. Temperature-programmed
reaction
experiments. tetragonal zirconia is the presence of a band at ca. 380 cm
TPSR, TPD, and TPO were performed in the device shown in the spectrum of the former modification and a charac-
in Fig. 1.
teristic band at 255–265 cm 1 in the spectrum of the latter

REACTIVITY OF 2-METHYL-3-BUTYN-2-OL
243
FIG. 1. Experimental device.
modification. Figure 3 presents the Raman spectra corre- (41, 42). However, the shape of the hysteresis loops in the
sponding to the dried gel (trace “a”), as well as the catalysts isotherms of Figs. 4A and 4B changes from intermediate
ZrO2–400VAC and ZrO2–600VAC (traces “b” and “c,” re- between A and E to A type according to the classifica-
spectively). According to the XRD results, the Raman spec- tion of De Boer (43) (H1 and H2 following the IUPAC
trum of the dried gel correspond to an amorphous solid. On nomenclature). In a previous work (44), it was found that a
the other hand, the spectrum of the ZrO2–600VAC shows zirconium oxide calcined at 300 C (called ZrO2–300VAC)
characteristic peaks of the monoclinic phase, while those showed E type hysteresis, which is often associated with
1
typical of the tetragonal phase (e.g., that at 255–265 cm
)
“ink-bottle” pores, spheroidal cavities, or voids among
appear only as small shoulders. The ZrO2–400VAC shows close-packed spherical particles. On the other hand, A-type
poor crystallinity, which can be assigned to the tetragonal hysteresis (ZrO2–600VAC) is characteristic of cylindrical
phase.
pores. It seems, therefore, that the zirconia samples pre-
Nitrogen adsorption–desorption isotherms obtained for pared are essentially mesoporous and contain spherically
both zirconium oxides (ZrO2–400VAC and ZrO2–600VAC) shaped pores which are gradually smoothed into cylindri-
are shown in Fig. 4. Both catalysts present isotherms with cally shaped pores on calcination (4).
closed and well defined hysteresis loops. The isotherms are
Surface area, pore volume and mean pore radius are
Type IV in the Brunauer, Deming, Deming, and Teller compiled in Table 1. The effect of the calcination tem-
(BDDT) classification associated with mesoporous solids perature on surface area is dramatic as reported in the

´
ARAMENDIA ET AL.
244
FIG. 3. FT-Raman spectra corresponding to zirconium dried gel
(trace a), as well as the catalysts ZrO2–400VAC and ZrO2–600VAC (traces
b and c, respectively).
Acid–Basic Properties
FIG. 2. X-ray diffraction patterns corresponding to ZrOH gel (a) and
Pyridine (PY), 2,6-dimethylpyridine (DMPY), and car-
bon dioxide were used as probe molecules to determine
overall acidity, Brønsted acidity, and overall basicity of the
catalysts from TPD–MS of preadsorbed probe molecules.
Table 2 presents the results obtained for the solids used in
this study. The solid calcined at 400 C presents higher acid-
ity and basicity than that calcined at 600 C. However, cata-
lyst ZrO2–600VAC exhibits a smaller but stronger basic-site
population than ZrO2–400VAC. In conclusion, it is clear
that calcination temperature is a crucial factor in obtaining
ZrO2-based solids with good acid–base properties.
the catalysts ZrO2–400VAC (b) and ZrO2–600VAC (c).
bibliography (2). The BET surface area decreases from 110
to 28 m2/g when the calcination temperature goes from 400
to 600 C. Mercera et al. (4) identified two possible processes
as being responsible for the changes occurring in the pore
structure and surface area on increasing calcination tem-
perature: crystallite growth and the accompanying phase
transformation on the one hand and sintering between the
crystallites (neck formation and growth) on the other.
The temperature causes severe loss of surface area, as
seen in Table 1, as well as transformation in the structure of
the solid, as proved by XRD and FT-Raman experiments.
The catalyst ZrO2–400VAC is an almost amorphous, high
surface area solid, whereas ZrO2–600VAC exhibits a low
surface area and a monoclinic structure.
This TPD–MS method allows us to determine, in addition
to the number of basic and acid sites, the relative strength
of those sites. Figures 5, 6, and 7 show the TPD–MS pro-
files obtained for pyridine, 2,6-dimethylpyridine, and CO2
TABLE 2
Acid–Basic Properties Determined by Temperature-Program-
med Desorption-Mass Spectrometry (TPD–MS) for the Solids Used
in This Work
TABLE 1
Textural Properties of the Catalysts
1
SBET
(m2 g
Vp
(mL g
rp
Acidity ( mol g
)
1
1
˚
Catalyst
Precursor
)
)
(A)
Basicity
mol g 1),
CO2
Brønsted
DMPY
Lewis
PY–DMPY
Total
PY
(
q
ZrO2–400VAC
ZrO2–600VAC
ZrOCl2 8H2O
110
28
0.15
0.11
19
155
Catalyst
q
ZrOCl2 8H2O
ZrO2–400VAC
ZrO2–600VAC
43
25
16
5
59
30
104
22
Note. Specific surface area (SBET), pore volume (Vp), and pore mean
radius (rp) obtained for the solids used in this work.

REACTIVITY OF 2-METHYL-3-BUTYN-2-OL
245
FIG. 4. Nitrogen adsorption–desorption isotherms obtained at liquid nitrogen temperature for the catalysts ZrO2–400VAC (A) and ZrO2–600VAC
(B) (᭹, adsorption band; ᭺, desorption band).
preadsorbed on both ZrO2 based catalysts. As far as overall is higher than that of ZrO2–600VAC, being Brønsted sites
acidity (Fig. 5) is concerned, catalyst ZrO2–400VAC shows higher than Lewis ones for both solids. The range of acid
a single peak of desorption of pyridine at 135 C, whereas strength is broader for ZrO2–400VAC than for ZrO2–
ZrO2–600VAC exhibits not only this peak but also another 600VAC. The change on calcining in the number of acid
one at 161 C. The peak at 135 C coincides with the peak sites but not in their types can be mainly assigned to the
shown in the DMPY TPD–MS profile (Fig. 6) and there- dramatic reduction in surface area rather than to morpho-
fore is assigned to Brønsted sites. The peak at 161 C does logical changes in the structure.
not appear in the profile corresponding to DMPY TPD–MS
The effect of calcination on the basicity is much more in-
(Fig. 6); thus, it is assigned to Lewis acid sites. From Figs. 5 tense than on surface acidity. CO2 TPD–MS profiles (Fig. 7)
and 6 we can conclude that the acidity of ZrO2–400VAC indicate that the solid calcined at 400 C (ZrO2–400VAC)
FIG. 5. Temperature programmed desorption–mass spectrometry
profiles of preadsorbed pyridine over the catalysts ZrO2–600VAC (a) and
ZrO2–400VAC (b).
FIG. 6. Temperature programmed desorption–mass spectrometry
profiles of preadsorbed 2,6-dimethylpyridine over the catalysts ZrO2–
600VAC (a) and ZrO2–400VAC (b).

´
ARAMENDIA ET AL.
246
On the other hand, the DRIFT spectra of preadsorbed
CO2 allow research on the different types of surface species
formed upon CO2 adsorption on ZrO2-based catalysts (45–
47). DRIFT spectra corresponding to the solid ZrO2–
600VAC are of poor quality due to the small number of
basic sites on this catalyst. The main species observed on
the surface are unidentate carbonate (bands at 1304 and
1404 cm 1) and bidentate carbonate (at 1632 cm 1), the last
being the most abundant surface species. A detailed study
of surface species after carbon dioxide adsorption on dif-
ferent oxide type catalysts has been published in a previous
work (37).
1H MASNMR
1
Figure 8 shows the H MASNMR spectra for catalysts
ZrO2–400VAC and ZrO2–600VAC after treatment for 24 h
in a furnace at 120 C (a and b) and at room temperature
(c and d). It was found that ZrO2–400VAC is the catalyst
that shows the greater number of OH groups. However, a
wider range of distribution is observed in ZrO2–600VAC.
The small number of sites at = 0.85 and = 1.71 displayed
by ZrO2–600VAC could correspond to the few but strong
basic sites detected for this catalyst by CO2–TPD–MS.
FIG. 7. Temperature programmed desorption–mass spectrometry
profiles of preadsorbed CO2 over the catalysts ZrO2–600VAC (a) and
ZrO2–400VAC (b).
has not only a greater number of basic sites but also a differ-
ent distribution of strength as compared to ZrO2–600VAC.
In this sense, ZrO2–400VAC presents basic sites of low
strength (desorbing CO2 in the range 100–200 C). How-
ever, the solid calcined at 600 C (ZrO2–600VAC) has lost
those weak basic sites, showing only a slight increase of the
baseline in the range 350–650 C related to a broad popu-
lation of basic sites of great strength. The reason for this
change in the acid–base properties—more marked on basic
sites than on acid sites—may be related to a morphological
change in the catalyst crystal phase (from amorphous and
tetragonal to monoclinic) rather than to the mere reduction
in surface area. The nature of the acid sites was the same in
both solids; only their numbers differed. On the other hand,
basic sites on the two solids differed not only in number and
strength, but also in nature.
The study of surface acid and basic properties in both
ZrO2-based catalysts is completed with the DRIFT spectra
of pyridine and carbon dioxide adsorbed on the catalysts at
room temperature.
Marinas et al. (34) found that pyridine is adsorbed on
Brønsted and Lewis acid sites showing bands at around
1450, 1490, and 1590 cm 1. The first and third ones corre-
spond to the adsorption on Brønsted sites while the second
is assigned to adsorption on Lewis acid sites. Both solids
exhibit all three bands in agreement with data from PY and
DMPY TPD–MS experiments that indicate that catalysts
present both Brønsted and Lewis acid sites. Spectra ob-
tained at temperature over ambient also confirm that the
acid strengths of the two catalysts are similar since infrared
bands disappear at about the same temperature for both.
FIG. 8. 1H NMRMAS spectra of zirconia based catalysts: (a) ZrO2–
400VAC heated at 120 C in a furnace. (b) ZrO2–600VAC heated at 120 C
in a furnace. (c) ZrO2–400VAC at room temperature. (d) ZrO2–600VAC
at room temperature.

REACTIVITY OF 2-METHYL-3-BUTYN-2-OL
247
(boundary layer, internal and external mass transfer pro-
cesses) through the choice of suitable operating variables,
especially space velocity (changing the catalyst weight and
the carrier gas) and particle size. Furthermore, the cata-
lytic runs have been carried out at different weight ratios of
catalyst to the substrate introduced (pulses in the range 0.2–
1.5 L), showing that conversion was almost unchanged,
which determines the first-order reaction process.
Hence, in absence of diffusional influences and in the
range 160–230 C, the requirements of Bassett–Habgood
kinetic treatment (48) for first-order kinetic processes are
satisfied. Thus, the rate determining step is the surface re-
action,
ln[1/1 Xp] = RTK(W/F),
where Xp is MBOH conversion, K the pseudokinetic con-
stant, W the catalyst weight, and F the flow-rate of the
carrier gas. On the contrary, as the temperature rises, very
high conversions and several secondary products, such as
trimethylbencenes or 3,5,5-trimethyl-2-cyclohexen-1-one,
are found and the Bassett and Habgood equation is not
fulfilled. In the range 160–200 C, in which the Bassett and
Habgood equation is followed, the change in amphoteric
selectivity (Samp = SHMB + SMIPK) and relative selectivity
(Srel = SMIPK/SHMB) with the temperature and the carrier
gas flow (from 80 to 120 ml/min) was studied. In all cases, Srel
increases drastically with the temperature, mainly due to
the increase in 3-methyl-3-buten-2-one (MIPK), as changes
in 3-hydroxy-3-methyl-2-butanone (HMB) are not signifi-
cant. At the same temperature, that selectivity increases as
the carrier gas flow decreases. It is important to point out
that the first pulse gives very high conversion with MBYNE
as the main product and a low ratio of MIPK. During the
other four pulses, MBYNE disappears and growing quan-
tities of HMB are formed as MIPK hardly changes.
Results found for a range of temperatures between 225
and 300 C and a total flow of 75 mL/min are shown in
Table 3. Total conversion and selectivity in MBYNE+
prenal (acid sites), acetone+acetylene (basic sites), as well
as MIPK and HMB (acid–base pair sites) as a function of
FIG. 9. 1H NMRMAS spectra of zirconia heated in vacuo for 5 h at
300, 400, 500, and 600 C. The scale is the same for all the spectra.
1H MASNMR spectra for zirconia heated in vacuo at tem-
peratures between 300 and 600 C are shown in Fig. 9. It
is important to emphasize that even though the number of
OH groups decreases as the temperature increases, these
groups are detected at temperatures as high as 600 C.
Reactivity of 2-Methyl-3-butyn-2-ol
The MBOH reaction has been proposed by several au-
thors as a test reaction in order to characterize a solid as
acid, basic, or amphoteric. When MBOH makes contact
with a solid, it can undergo three different reactions, lead-
ing to the main products shown in Scheme 1.
SCHEME 1. Overall reaction scheme (26). MBOH, 2-methyl-3-butyn-
2-ol; MBYNE, 3-methyl-3-buten-1-yne; PRENAL, 3-methyl-2-buten-1-al;
Reactions in microcatalytic pulse reactor. The catalytic
runs were carried out in absence of diffusional influences HMB, 3-hydroxy-3-methyl-2-butanone; MIPK, 3-methyl-3-buten-2-one.

´
248
ARAMENDIA ET AL.
TABLE 3
Comparison between Selectivity to Both ZrO2 Based Catalysts, at Three Different Temperatures
Temperature ( C)
225
SHMB
250
300
Acid sites/Basic
sites
Catalyst
SMIPK
Samp
Srel
SMIPK
SHMB
Samp
Srel
SMIPK
SHMB
Samp
Srel
ZrO2–400VAC
ZrO2–600VAC
0
0
91.75
87.91
91.75
87.91
0
0
23.62
17.21
50.29
60.45
73.91
77.66
31.96
22.16
26.01
28.93
12.35
33.33
38.36
62.26
67.81
46.47
0.50
1.36
Note. SMIPK, selectivity to MIPK; SHMB, selectivity to HMB; Samp, amphoteric selectivity (Samp = SMIPK + SHMB); Srel, relative selectivity (Srel
MIPK/Samp). Acid sites/Basic sites: ratio between both populations.
=
S
temperature for catalyst ZrO2–400VAC is represented in at the expense of MIPK, which only needs the addition of
Fig. 10. a water molecule to produce one of HMB, but instead at
From the study of Table 3, it can be concluded that within the expense of MBYNE which can lead to HMB by the
this range of temperatures, ZrO2–400VAC is the catalyst addition of two molecules of water (see Scheme 2).
with higher amphoteric selectivity at a lower temperature
Temperature-programmed experiments. To initially
check the activity of our catalysts for MBOH decomposi-
tion several experiments at programmed and isothermal
temperature (TPSR and ITSR) were carried out. The
results confirmed those obtained from the pulse reaction.
Moreover, TPD and TPO experiments performed after
each reaction gave some important information about the
causes of deactivation of the catalysts. MBOH conversion
at 250 C on catalyst ZrO2–600VAC is represented in Fig. 11.
Like in pulse reactions, there is a fast and strong initial
deactivation, after which the catalyst reaches stability.
TPD and TPO experiments carried out after isothermal
reaction at low temperature have proved that deactivation
is caused by strong retention of surface species (mainly
MIPK). As the reaction temperature increases, the amount
of MIPK retained by the catalyst decreases. At 600 C, no
retained species are detected by TPD–MS experiments.
and, at the same time, the one for which the selectivity de-
creases with the temperature at a faster rate. Srel and total
conversion increases with the temperature. On the other
hand, some experiments were carried out adding 2% water
to MBOH, finding that the relative selectivity in HMB dou-
bles the selectivity in MIPK. This increase is not produced
FIG. 10. Totalconversion and selectivityin MBYNE+prenal(᭹), ace-
tone+acetylene (᭿), MIPK (4), and HMB (ᮀ) as a function of tempera-
ture for catalyst ZrO2–400VAC.
SCHEME 2. Reaction scheme for the two main products (MIPK and
HMB) over acid–base pair sites.

REACTIVITY OF 2-METHYL-3-BUTYN-2-OL
249
Regarding these experiments, and since deactivation in-
creases with temperature, it seems that deactivation at high
temperatures (about 400 C) is due to carbon deposition
while at low temperatures (around 300 C), both carbon de-
position and fundamentally strongly adsorbed reactants are
what cause catalyst deactivation.
CONCLUSIONS
The results obtained in this work and the above interpre-
tation allow us to draw the following conclusions:
(a) The calcination temperature is crucial to determine
the structural, textural, and surface chemical properties of
the ZrO2 obtained. Calcination at 400 C led to an almost
amorphous solid, whereas calcination at 600 C gave a mix-
ture of tetragonal and monoclinic solid. A dramatic reduc-
tion in the surface area for the ZrO2–600VAC solid was
obtained. Calcination at 600 C led to a reduction in the
number of acid sites without any change in the strength dis-
tribution. However, calcination at 600 C induced changes
in the basic strength of the remaining sites: there were fewer
but stronger sites than in the solid calcined at 400 C. The
aciditylost could be due to surface area reduction bythe sin-
tering of the crystallites, while phase transformation (from
amorphous to crystalline) is responsible for the change in
the strength distribution of basic sites.
FIG. 11. Deactivation curve at 250 C on catalyst ZrO2–600VAC in an
IR-MS experiment.
TPO experiments carried out after temperature pro-
grammed reaction (final temperature 600 C) have proved
that deactivation is caused by carbon deposition in both
catalysts. The CO2 area as well as the regeneration temper-
ature were very similar in both catalysts (see Fig. 12).
(b) The effect oftemperature on selectivityfor both cata-
lysts must be taken into account in order to use the MBOH
reaction asa catalytictest. Thus, the range from 160to 210 C
was the most appropriate.
(c) Relative selectivity increases drastically with the
temperature, mainly due to the increase in MIPK since
changes in HMB are not significant. At the same temper-
ature, that selectivity increases as the carrier gas flow de-
creases.
(d) Isotherm reaction profiles showed an initial deacti-
vation period that depended on the reaction temperature.
TPD and TPO experiments performed after an isother-
mal run indicated that deactivation at high temperatures
(about 400 C) was related to carbon deposition, while at
low temperatures (around 200 C), both carbon deposition
and strongly adsorbed species caused catalyst deactivation.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the funding of this research by
the Junta de Andaluc´ıa and the DGESIC in the framework of Project
PB97-0446. The staff at the Mass Spectrometry and RMN Services of
the University of Co´rdoba are also acknowledged for their kind technical
assistance in the experiments.
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FIG. 12. Temperature programmed oxidation (TPO) profiles ob-
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formed after keeping the catalyst under TPSR of MBOH. CO2 (m/z = 44)
and O2 (m/z = 32) signals.
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