Impact of temperature ramping rate during calcination on characteristics of nano-ZrO2 and its catalytic activity for isosynthesis

Article abstract of DOI:10.1016/j.molcata.2007.10.020

This paper studied the effect of temperature ramping rate during calcination on characteristics of nanoscale zirconia and its catalytic performance for isosynthesis. The physical properties, i.e. BET surface area, cumulative pore volume, cumulative pore diameter and the phase composition in zirconia, acid-base properties and surface properties such as Zr³⁺ quantity, were characterized. Increase in the temperature ramping rate of calcination resulted in a higher composition of the tetragonal phase, but it showed insignificant influence on the other physical properties. Considering the catalytic activity, the acid sites did not affect the activity, but the basic sites depended on the fraction of the tetragonal phase in zirconia which was related to the selectivity to isobutene. The intensity of Zr³⁺ on the surface varied with the change in the heating rate of calcination. Both the tetragonal phase composition in zirconia and the quantity of Zr³⁺ were the key factors affecting the selectivity to isobutene in hydrocarbons. Moreover, the maximum value of the product selectivity to isobutene on the ZrO₂ (5.0) catalyst was attained at the highest concentration of Zr³⁺.

Full text of DOI:10.1016/j.molcata.2007.10.020

Available online at www.sciencedirect.com
Journal of Molecular Catalysis A: Chemical 280 (2008) 35–42
Impact of temperature ramping rate during calcination
on characteristics of nano-ZrO and its catalytic
2
activity for isosynthesis
a
a
a
Watcharapong Khaodee , Bunjerd Jongsomjit , Piyasan Praserthdam ,
b
a,∗
Shigeo Goto , Suttichai Assabumrungrat
Center of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering,
Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
a
b
Department of Chemical Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan
Received 1 February 2007; received in revised form 16 October 2007; accepted 17 October 2007
Available online 23 October 2007
Abstract
This paper studied the effect of temperature ramping rate during calcination on characteristics of nanoscale zirconia and its catalytic performance
for isosynthesis. The physical properties, i.e. BET surface area, cumulative pore volume, cumulative pore diameter and the phase composition in
3
+
zirconia, acid-base properties and surface properties such as Zr quantity, were characterized. Increase in the temperature ramping rate of calcination
resulted in a higher composition of the tetragonal phase, but it showed insignificant influence on the other physical properties. Considering the
catalytic activity, the acid sites did not affect the activity, but the basic sites depended on the fraction of the tetragonal phase in zirconia which
3
+
was related to the selectivity to isobutene. The intensity of Zr on the surface varied with the change in the heating rate of calcination. Both
3
+
the tetragonal phase composition in zirconia and the quantity of Zr were the key factors affecting the selectivity to isobutene in hydrocarbons.
3+
Moreover, the maximum value of the product selectivity to isobutene on the ZrO
2007 Elsevier B.V. All rights reserved.
2
(5.0) catalyst was attained at the highest concentration of Zr .
©
Keywords: Isosynthesis; Tetragonal phase; ZrO2; Zr3+; Isobutene
1
. Introduction
(i) the chosen resource of isobutene production is renewable,
then being more green than the conventional petroleum sources,
which are about to face shortage in the near future, (ii) carbon
dioxide, a by-product of fermentation process, is substantially
consumed to produce syngas, thus reducing the CO2 emission to
the atmosphere and (iii) the ratio of carbon monoxide to hydro-
genof1:1forthesyngasfromfermentationofbiomassissuitable
for the reaction of isobutene synthesis.
As is well known, isosynthesis is the catalytic reaction that
converts syngas to branched chain hydrocarbons, especially
isobutane and isobutene. The early work [1] showed that suitable
catalysts forthe isosynthesis reactionare hardly reducible oxides
such as zirconia rather than other reduced transition metals. It
has been reported that zirconia is the most selective catalyst
for isosynthesis [2–6]. The effect of characteristics of cata-
lysts on their catalytic performance was investigated by some
researchers. For instance, the characteristic and catalytic perfor-
mance of the nanoscale zirconias prepared by various methods
At present, the octane enhancer demand is continuously
increasing with the increased fuel consumption. Isobutene is an
important raw material for the production of important octane
enhancers such as methyl tert-butyl ether (MTBE) and ethyl tert-
butyl ether (ETBE). Typically, isobutene is extracted from the
C4 stream in petroleum refining process; however, the supply of
isobutene from the petroleum products is possibly inadequate
in the near future. It is expected that an alternative source for
the production of isobutene needs to be explored. It is evident
that one of the promising sources for isobutene synthesis can
be provided by syngas derived from a renewable resource such
as biomass. This route is attractive due to the following reasons
∗
Corresponding author. Tel.: +662 218 6868; fax: +662 218 6877.
E-mail address: Suttichai.A@chula.ac.th (S. Assabumrungrat).
1
381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.10.020

3
6
W. Khaodee et al. / Journal of Molecular Catalysis A: Chemical 280 (2008) 35–42
◦
◦
such as the precipitation method, the supercritical fluid dry-
ing method and the freeze-drying method for isosynthesis were
studied by Su et al. [4]. They found that the different prepara-
tion methods affected catalytic performance, and better product
selectivity of isobutene resulted from higher ratios of base to acid
sites on the catalyst surface. As reported by Maruya et al. [6], the
crystal phases such as monoclinic phase in zirconia influenced
catalytic performance. Moreover, the acidity and basicity could
play an important role on the catalytic performance because of
bifuntionality of zirconia [4,7–9]. In addition, effects of these
factors such as crystallite size, the phase composition and acid-
base sites of zirconia on catalytic performance were also studied
in our previous paper [10]. However, the studies of surface prop-
erties of catalysts such as acid-base sites related with the crystal
filter over the 2θ ranging from 20 to 80 . The crystal sizes of
the prepared catalysts were obtained by XRD line broadening
using Scherrer’s equation. The characteristic peaks at 2θ = 28.2
◦
◦
and 31.5 for (−1 1 1) and (1 1 1) reflexes, respectively, were
assigned to the monoclinic phase in ZrO2. The characteristic
◦
peak at 2θ = 30.2 for the (1 1 1) reflex in the XRD patterns
represented the tetragonal phase in ZrO2.
The percents of tetragonal and monoclinic phases in ZrO2
were calculated by a comparison of the areas for the character-
istic peaks of the monoclinic phase and the tetragonal phase. The
percent of each phase was determined by means of the Gaussian
areas h × w, where h and w are the height and half-height width
of the corresponding XRD characteristic peak as follows [4]:
3+
% monoclinic phase
phase of zirconia and the quantity of Zr on the surface were
ꢀ
3+
not investigated. From the study of Zr [11], it was found that
(h × w) monoclinic phase
3
+
=
ꢀ
Zr ion was related to the selectivity to isobutene in the reaction
on zirconia.
(
h × w) monoclinic and tetragonal phase
In this work, the effect of temperature ramping rate dur-
ing calcination on characteristics and catalytic performance for
isosynthesis of zirconia catalysts was investigated. The syn-
thesized nanoscale zirconia catalysts were prepared using the
precipitation method. Various physical characteristics of zirco-
nia catalysts such as the phase composition in zirconia, acid-base
% tetragonal phase
ꢀ
(
h × w) tetragonal phase
=
ꢀ
(
h × w) monoclinic and tetragonal phase
properties and surface properties including quantity of Zr3 on
the catalyst surface were determined. The obtained information
was useful for describing the change in catalytic performance
of the synthesized catalysts.
+
2.2.3. Transmission electron microscopy (TEM)
Catalyst crystallite size and the diffraction pattern were
obtained using a JEOL JEM-2010 transmission electron micro-
scope operated at 200 kV with an optical point to point resolution
of 0.23 nm at National Metal and Materials Technology Center
(MTEC). The sample was dispersed in ethanol prior to the TEM
measurement.
2
. Experimental
2
.1. Catalyst preparation
2
.2.4. Temperature-programmed desorption (TPD)
Temperature-programmed desorption of ammonia and car-
The nanoscale zirconia (ZrO2) was prepared by the precipi-
tation method. It was carried out by slowly adding a solution of
zirconium salt precursors such as zirconyl nitrate [ZrO(NO3)2]
bon dioxide (NH3- and CO2-TPD) was used to determine the
acid-base properties of catalysts. TPD experiments were car-
ried out using a flow apparatus. The catalyst sample (0.1 g) was
treated at its calcination temperature (450 C) in a helium flow
for 1 h and then saturated with a 15% NH3/He mixture or a pure
CO2 flow after cooling to 100 C. After purging with the helium
at 100 C for 1 h to remove weakly physisorbed NH3 or CO2,
the sample was heated to 450 C at a rate of 20 C/min in a
helium flow (50 cm /min). The amount of acid-base sites on the
catalyst surface was calculated from the desorption amount of
NH3 and CO2, respectively. It was determined by measuring the
areas of the desorption profiles obtained from a Micromeritics
ChemiSorb 2750 pulse chemisorption system analyzer. For the
broad desorption peak, it was separated into many sub-peaks by
using the Fityk program for peak fitting. All areas of sub-peaks
were summed to calculate the total amount of acid and base sites.
(
0.15 M) into a well-stirred precipitating solution of ammonium
hydroxide (NH4OH) (2.5 wt%) at room temperature. The pH of
the solution was carefully controlled at 10. The resulting pre-
cipitate was removed, and then washed with deionized water.
◦
◦
◦
The obtained sample was then dried overnight at 110 C and
◦
◦
calcined at 450 C for 3 h at various temperature ramping rates
such as 1.0, 2.5, 5.0, 7.5 and 10.0 C/min. Zirconia catalysts pre-
◦
◦
◦
3
pared by using these temperature ramping rates were denoted as
ZrO2 (1.0), ZrO2 (2.5), ZrO2 (5.0), ZrO2 (7.5) and ZrO2 (10.0),
respectively.
2
2
.2. Catalyst characterization
.2.1. N2 physisorption
Measurements of BET surface area, cumulative pore volume
and average pore diameter were performed by the N2 physisorp-
tion using a Micromeritics ASAP 2020 surface area and porosity
analyzer.
2
.2.5. Electron spin resonance spectroscopy (ESR)
Electron spin configuration was detected by using electron
spin resonance spectroscopy (ESR) (JEOL model JES-RE2X)
at the Scientific and Technological Research Equipment Center
(STREC), Chulalongkorn University. The sample was degassed
before measurement at room temperature.
2
.2.2. X-ray diffraction (XRD)
The XRD spectra of catalysts were measured by a SIEMENS
D5000 X-ray diffractometer using Cu K␣ radiation with a nickel

W. Khaodee et al. / Journal of Molecular Catalysis A: Chemical 280 (2008) 35–42
37
2
.2.6. X-ray photoelectron spectroscopy (XPS)
The XPS spectra and the binding energy of ZrO2 catalysts
were determined using a Kratos Amicus X-ray photoelectron
spectrometer. The analyses were carried out with Mg K␣ X-ray
−
6
source under a working pressure of 1 × 10 Pa at current of
2
7
2
0 mA and 12 kV, resolution of 0.1 eV/step, and pass energy at
5 eV. The binding energy was calibrated using the C 1s peak at
85.0 eV as reference.
2
.3. Reaction study
◦
Isosynthesis was carried out at 400 C and atmospheric
pressure in a fixed-bed quartz reactor fed with a mixture of
3
CO/H2/N2 = 10/10/5 cm /min. The catalyst sample of 2 g was
used in each run. The reactor effluent samples were taken at
intervals of 1.5 h and analyzed using the gas chromatography
technique. Thermal conductivity detectors (TCD) with molecu-
lar sieve 5A and Porapak-Q columns was used to detect the CO
and CO2, respectively. A flame ionization detector (FID) with a
VZ-10 column was used to detect the light hydrocarbons such
as C1–C4 hydrocarbons. The steady-state rate for all catalysts
was obtained after 20 h.
Fig. 1. XRD patterns of ZrO2 catalysts calcined at various temperature ramping
rates.
show that the fraction of the tetragonal phase increases with the
increased temperature ramping rate during calcination. Accord-
ing to monoclinic-tetragonal phase transformation of zirconia,
◦
the tetragonal phase should be formed above 1170 C [12], but
the zirconia prepared by precipitation from aqueous salt solu-
tion can be formed as a metastable tetragonal phase at low
temperature as seen in this work. Moreover, the transforma-
tion of the metastable tetragonal form into the monoclinic form
was probably due to the lower surface energy of the tetrago-
nal phase compared to monoclinic phase [13,14]. In fact, phase
transformation of catalyst can take place by varying not only
the calcination temperature but also the heating rate of cal-
cination. In this case, lower temperature ramping rates would
result in better heat distribution over the surface and longer con-
tact times. Thus, this may contribute to higher stabilized crystal
phase of ZrO2, leading to more monoclinic phase present. Fur-
thermore, the tetragonal phase that appeared was attributed to
the size effect. Garvie [15] reported that the tetragonal phase
appeared below a critical crystallite size about 30 nm and above
that point the tetragonal could not be stabilized at room temper-
ature. The average crystallite sizes of each phase being present
3
. Results and discussion
3
.1. Physical properties
By using different temperature ramping rates during calcina-
tion, both crystal phase and crystallite size of the synthesized
nanoscale ZrO2 catalysts were changed. The XRD spectra of all
ZrO2 catalysts are illustrated in Fig. 1. It shows that ZrO2 cat-
alysts with various heating rates of calcination exhibit similar
XRD peaks at 2θ = 28.2 and 31.5 assigned to the monoclinic
phase and at 2θ = 30.2 assigned to the tetragonal phase. The
◦
◦
◦
characteristic peak areas of monoclinic and tetragonal phases
(Fig. 1) indicated that the latter is more dominant than the
former upon increasing temperature ramping rate during cal-
cination. In addition, the phase composition of each ZrO2
catalyst can be calculated as shown in Table 1. The results
Table 1
Characteristics of ZrO2 catalysts calcined at various temperature ramping rates
d
Acid sitese
(␮mol/g)
Base sitesf
(␮mol/g)
Catalysts
BET S.A.a
Cumulative pore
volume (cm /g)
Average pore
diameter (nm)
Crystal size (nm)
Crystal phase
2
b
3
c
(
m /g)
Mg
Th
ZrO2 (1.0)
ZrO2 (2.5)
ZrO2 (5.0)
ZrO2 (7.5)
ZrO2 (10.0)
92
0.169
0.172
0.199
0.191
0.139
4.9
4.7
5.4
5.0
3.7
9.8
8.3
71% M + 29% T
54% M + 46% T
57% M + 43% T
45% M + 55% T
15% M + 85% T
389
403
413
387
428
188
277
278
224
379
100
103
106
100
11.4
8.7
7.5
6.5
11.3
8.6
7.5
9.0
a
Error of measurement ± 5%.
b
c
d
e
f
BJH desorption cumulative volume of pores between 1.7 and 300 nm diameter.
BJH desorption average pore diameter.
Determined by XRD line broadening using Scherrer’s equation [24].
Measured by NH3-TPD.
Measured by CO2-TPD.
Monoclinic phase in ZrO2.
Tetragonal phase in ZrO2.
g
h

3
8
W. Khaodee et al. / Journal of Molecular Catalysis A: Chemical 280 (2008) 35–42
Fig. 2. TEM micrograph of ZrO2 (1.0) catalyst and its TEM micrograph with electron diffraction mode.
in all catalysts were calculated using the XRD line broadening
regarding to its characteristic peaks and also listed in Table 1,
which indicates the values ranging between 7 and 12 nm. There-
fore, all ZrO2 catalysts had mixed phases of monoclinic and
tetragonal phases because their crystallite sizes were less than
ture ramping rates during calcination are shown in Figs. 3 and 4,
respectively. The amounts of acid and base sites summarized in
Table 1 were calculated from the area below the curve of TPD
profile. The characteristic peaks of these profiles are assigned to
their desorption temperatures indicating the strength of Lewis
surface sites. From NH3-TPD results Ma et al. [16] have reported
3
0 nm. Moreover, it was revealed that ZrO2 catalysts in this
◦
work were in the nanoscale. However, for better understand-
ing on the crystallite size, it should be determined by means of
the TEM technique. Apparently, TEM can provide the image
of particle characteristics in terms of the crystallite size or the
particle size and geometry as well. The TEM micrographs of
all catalysts are similar and the typical micrograph of the ZrO2
that NH3 desorption peaks located at ca. 200 and 300 C for
ZrO2 catalysts are corresponding to weak acid sites and moder-
ate acid sites, respectively. Moreover, both peaks of monoclinic
ZrO2 exhibit slightly higher amount of acid sites compared
to the tetragonal ZrO2. In this work, all NH3-TPD profiles in
Fig. 3 exhibit similar desorption profiles consisting mainly of
weak acid sites. It can be observed that the amount of acid
sites is in the range of ca. 387–428 ␮mol/g as listed in Table 1
indicates insignificant difference. The acidity of these catalysts
slightly changes when the fraction of the tetragonal phase in
ZrO2 increases as illustrated in Fig. 5.
(
1.0) catalyst is illustrated in Fig. 2. The TEM images clearly
indicated that all ZrO2 catalysts were in nano-size. In addition,
TEM with the electron diffraction mode can determine the crys-
tallographic structure of catalyst. From the electron diffraction
results (Fig. 2), it is revealed that the nanoscale ZrO2 catalyst is
polycrystallite.
Based on CO2 desorption peaks, the weak base sites, mod-
erate base sites and strong base sites can be identified [16]. All
The other physical properties of ZrO2 catalysts characterized
by means of N2 physisorption such as BET surface area, cumula-
tive pore volume and average pore diameter are also summarized
in Table 1. These nanoscale ZrO2 catalysts had specific surface
2
areas ranging between ca. 92 and 106 m /g. For cumulative pore
volume and average pore diameter, no significant difference was
observed in all ZrO2 catalysts, except for ZrO2 (10.0) where the
decrease in both properties was evident. This was probably due
to sintering of catalysts when a high temperature ramping rate
during calcination was applied. Therefore, it can be concluded
that the temperature ramping rate during calcination does not
significantly affect these physical properties corresponding to
their crystallite sizes.
3
.2. Acid-base properties
The acid-base properties of the catalysts were measured by
using NH3- and CO2-TPD techniques, respectively. The NH3-
and CO2-TPD profiles of ZrO2 catalysts with various tempera-
Fig. 3. NH3-TPD profiles of catalysts.

W. Khaodee et al. / Journal of Molecular Catalysis A: Chemical 280 (2008) 35–42
39
3
.3. The existence of Zr3+ on the surface of ZrO2
XPS measurement is performed to identify valence electron
or oxidation state of ion on the surface of catalysts. In this
work, zirconium ion on the surface of ZrO2 was investigated.
A survey scan is performed before the elemental scan which
was carried out for C 1s, O 1s, and Zr 3d. The binding energy
of each element was referenced by the C 1s peak position at
2
85.0 eV. A high-resolution scan of Zr of ZrO2 (1.0) sample
showed the double peaks of Zr 3d consisting of Zr 3d5/2 and Zr
d3/2 peaks (Fig. 6). The other ZrO2 samples (not shown) exhib-
3
ited similar results as seen in Fig. 6. It was found that there were
four sub-peaks for the Zr 3d peak after deconvolution by fitting
with Gaussian–Lorentzian shapes using a VISION 2 software
equipped with XPS. The two major peaks at position of 182.3
and 184.8 eV were assigned to Zr 3d and Zr 3d3/2 peaks of
5/2
Fig. 4. CO2-TPD profiles of catalysts.
4+
Zr , respectively, as reported early [17–19]. The binding ener-
gies for two small (minor) peaks shifted towards lower binding
energies compared with the major peaks. The peak position of
kinds of base sites were present in the tetragonal ZrO2 whereas
only weak and moderate base sites were observed on the mono-
clinic ZrO2. For CO2-TPD profiles (Fig. 4), the CO2 desorption
peaks at low temperature appear in all profiles, suggesting that
all ZrO2 catalysts have weak base sites. However, the increase
in temperature ramping rate during calcination results in higher
desorption temperature and areas under its curve as well. The
basicity of these catalysts increases with increase in the frac-
tion of the tetragonal phase in ZrO2 as shown in Fig. 5. As a
result of previous work [10], the presence of more tetragonal in
ZrO2 is attributed to higher basicity of catalysts, especially for
moderate and strong base sites. Therefore, the highest basic-
ity of ZrO2 (10.0) is due to the highest tetragonal phase in
ZrO2.
0
Zr [19] is at 178.5 eV. Therefore, those small peaks can be
attributed to peaks of zirconium ion between 0 and 4 because
the positions of those peaks are in the range of peak position
0
4+
of Zr and Zr . However, those small peaks were shifted by
.2–0.5 eV, which was only a little decrease in binding energies
0
that might cause a little change in valence electron of zirconium.
Therefore, those small peaks can be presumably assigned to the
3
+
4+
Zr peak. In fact, Zr is zirconium-based ion on the surface of
3+
synthesized ZrO2. However, Zr should have more chances to
2
+
+
be detected among other zirconium ions, such as Zr , Zr and
0
Zr .
Therefore, XPS measurements of ZrO2 samples showed the
existence of Zr3 on their surface. However, the ESR technique
is a promising method for quantifying the relative concentra-
tion of Zr3 on the ZrO2 surface, as reported in several papers
+
In this work, all ZrO2 catalysts have similar physical prop-
erties such as BET surface area. Thus, acid-base properties of
these catalysts are independent of the physical properties, but it
can be varied with the composition of phases in ZrO2 resulting
from various heating rates during calcination.
+
[
20–22] because it can certainly identify the peak position of
3
+
only unpaired valence electron of ZrO2 especially that of Zr .
For paired electron of ZrO2 such as Zr , it cannot be detected by
means of ESR. Therefore, the ESR technique was used to com-
4+
Fig. 5. Relationship between amount of acid sites and base sites and percent of
tetragonal phase in ZrO2.
Fig. 6. XPS spectrum of Zr 3d of ZrO2 (1.0).

4
0
W. Khaodee et al. / Journal of Molecular Catalysis A: Chemical 280 (2008) 35–42
rapidly decreases beyond that value. Early researches [22,23]
suggested that the Zr3 center to ESR can be described as the
oxygen coordinatively unsaturated zirconium sites on the ZrO2
surface. Inaddition, theyproposedthattheremovalofthesurface
+
3
+
hydroxyl accounted for the formation of the new Zr sites. It
is possibly due to the presence of hydroxyl groups combined in
a position of coordinatively unsaturated sites resulting in less
3+
3+
Zr intensity. Therefore, the changes of Zr intensity in this
case may be attributed to loss of the surface O atoms, especially
hydroxyl groups, on the ZrO2 surface. Low heating rates during
calcination can remove the hydroxyl group more than the high
heating rate because the former had long times for releasing
hydroxyl group compared to the latter. As seen at calcination
◦
temperature ramping rates of 7.5 and 10 C/min, the relative
intensity of Zr3 obviously decreased. This is possibly due to
the effect of fast increase in temperature influencing more than
the effect of longer contact times. More hydroxyl groups were
removed from the surface of ZrO2 catalyst. Moreover, Fig. 9
shows the relationship between the tetragonal phase in ZrO2
+
Fig. 7. ESR spectrum of ZrO2 (1.0) catalyst.
pare the quantity of Zr3+ in ZrO2 as described in the following
section.
and Zr3 intensity. The intensity of Zr has the highest value at
ca. 43% of the tetragonal phase in ZrO2, then decreases almost
linearly upon increasing the tetragonal phase in ZrO2.
+
3+
3
.4. Intensity of Zr3+ in ZrO2
A spin of unpaired electron was detected by means of ESR
to identify defect center of zirconia, assuming the existence of
3
.5. Catalytic performance
3
+
3+
Zr sites. The Zr signals present at g⊥∼1.975 and g||∼1.957
The ZrO2 catalysts with various temperature ramping rates
3+
were very close to the positions of Zr on the ZrO2 surface
observed by many researchers [20–22]. Only g⊥ was considered
in this work due to the apparent signal. The ESR spectra for
all ZrO2 catalysts are similar and the typical ESR spectrum of
the ZrO2 (1.0) catalyst is shown in Fig. 7, but the intensity of
during calcination were tested for their isosynthesis activity and
selectivity at 400 C, atmospheric pressure and CO/H2 of 1.
◦
The steady-state rate was reached after 20 h. The results of the
catalytic activities calculated from the product formations are
summarized in Table 2. The amount of Zr3 is a major fac-
tor affecting the product selectivity to isobutene, as shown in
Fig. 10. It is obvious that the ZrO2 (5.0) exhibited the highest
concentration of Zr , corresponding to the highest selectivity
to isobutene in hydrocarbons. Considering the results obtained
with temperature ramping rates during calcination of 1.0, 2.5
+
3
+
Zr for all of them is significantly different. The peak height
3+
of the Zr signal (g⊥∼1.975) is attributed to the quantity of
3
+
3+
Zr . For comparison of the Zr intensity, the weight of each
ZrO2 catalyst tested with ESR must be equal. The relative ESR
intensity for various ZrO2 catalysts is shown in Fig. 8. It is found
that quantity of Zr3 is different for the catalysts with various
temperatures ramping rate during calcination. The result shows
3+
+
◦
and 5.0 C/min, it can be concluded that the relative intensity of
Zr is related to the heating rate of calcination. Although ZrO2
3+
3
+
that Zr gradually increases with the increase in the heating rate
of calcination up to the highest intensity at ca. 5 C/min, and then
◦
Fig. 9. Relationship between quantity of Zr3+ and percent of tetragonal phase
in ZrO2.
Fig. 8. Relative ESR intensity of various ZrO2 catalysts.

W. Khaodee et al. / Journal of Molecular Catalysis A: Chemical 280 (2008) 35–42
41
Table 2
The catalytic performance of ZrO2 catalysts calcined at various temperature ramping rates during calcination in the isosynthesis
a
−1 −1
b
Catalysts
CO conversion (%) Reaction rate (␮mol kg cat
s
)
Product selectivity in hydrocarbons (mol%)
Product selectivity (mol%)
C1
C2
C3
i-C4H8
HCc
CO2
ZrO2 (1.0)
ZrO2 (2.5)
ZrO2 (5.0)
ZrO2 (7.5)
2.90
3.97
1.91
1.82
97.3
133.0
63.9
61.0
105.5
6.0
7.6
3.8
3.8
7.7
5.6 (60.4)
9.0 (62.8)
3.8 (65.1)
3.7 (66.9)
7.3 (60.2)
11.1 (87.9)
14.2 (85.5)
9.5 (93.5)
9.5 (94.0)
11.5 (86.1)
77.3
69.2
82.9
82.9
73.4
53.6
47.4
67.6
68.4
52.6
46.4
52.6
32.4
31.6
47.4
ZrO2 (10.0) 3.15
◦
Reaction conditions were at 400 C, 1 atm and CO/H2 = 1.
a
Steady-state of reaction was reached at 20 h.
Parentheses are percent of olefins present in products.
b
c
Total hydrocarbons.
Zr3+ ion and the selectivity to isobutene for the reaction over
ZrO2. Those results showed that Zr ion might be involved in
CO hydrogenation, possibly suggesting the mechanism of the
catalytic reaction via a surface species Zr(III)(CO)2.
(
5.0) and ZrO2 (7.5) resulted in the highest product selectivity
3+
3+
to isobutene, the two catalysts have different Zr intensities.
ZrO2 (7.5) have lower quantity of Zr3 than ZrO2 (5.0). Thus,
both catalysts would have another factor affecting the selectiv-
ity to isobutene in hydrocarbons, which may be attributed to
the presence of the tetragonal phase in ZrO2. Increase in the
tetragonal phase in ZrO2 probably results in higher selectiv-
ity to isobutene in hydrocarbons. Therefore, the increase in the
tetragonal phase in ZrO2 (7.5) leads to the high selectivity to
isobutene in hydrocarbons even at a lower content of Zr . No
direct relationship between the quantity of Zr3 or the fraction of
thetetragonalphase in ZrO2 and isobutene selectivitywas found.
It can be mentioned that the effects of amounts of Zr3 and the
tetragonal phase present in ZrO2 can be superimposed on each
other. Thus, the observed selectivities to isobutene were roughly
equal. By comparison of the ZrO2 (7.5) and ZrO2 (10.0) perfor-
mance, it can be proposed that the effect of Zr3 intensity is more
pronounced than that of the tetragonal phase. As a result, the lat-
ter exhibits lower selectivity to isobutene in hydrocarbons. The
presence of the tetragonal phase in ZrO2 is apparently related to
the amount of acid and base sites as reported by Khaodee et al.
+
It can be concluded that selectivity to isobutene in hydrocar-
bons is dependent on the intensity of Zr3 upon the temperature
+
◦
ramping rate during calcination less than 5.0 C/min. For heating
◦
rates of calcination more than 5.0 C/min, the product selectivity
to isobutene depends on not only the quantity of Zr but also
on the tetragonal phase in ZrO2. The direct relationship between
3
+
3+
+
3+
Zr intensity or fraction of the tetragonal phase in ZrO2 and the
selectivity to isobutene in hydrocarbons might not be observed
for these catalysts. Therefore, the key factor affecting the prod-
uct selectivity consists of intensity of Zr3 and the tetragonal
phase composition in ZrO2. In addition, the tetragonal phase
in ZrO2 was dependent on the basicity of catalysts. Thus, base
properties results in the product selectivity to isobutene as seen
in the previous works [4,7–9].
+
+
+
In addition, the selectivity to total hydrocarbons as reported
in Table 2 show similar trend to that for the selectivity to
isobutene in hydrocarbons. The amount of acid sites is not
directly related to the activity. The tendency of the activity of
ZrO2 catalysts is opposite to that of the product selectivity to
isobutene; for instance, activity decreases with increasing selec-
tivity to isobutene in hydrocarbons. Table 2 shows that ZrO2
[
10]. Therefore, there were many factors affecting the product
3+
selectivity to isobutene. However, the intensity of Zr and the
tetragonal phase in ZrO2 were dominant factors. Based on the
results of Li et al. [11], there was a close linear relation between
(2.5) exhibits the highest activity among all ZrO2 catalysts.
4. Conclusions
The effect of temperature ramping rates during calcination
on the characteristics and catalytic performance for isosynthe-
sis of nanoscale zirconia catalysts was investigated. The mixed
phase of tetragonal and monoclinic phases of zirconia catalysts
(
29–85% T) could be achieved when the heating rate of calcina-
◦
tion was varied in the range of 1.0–10.0 C/min. The tetragonal
phase composition increased with increasing temperature ramp-
ing rate during calcination. For the other physical properties such
as BET surface area, cumulative pore volume and pore diameter,
there was insignificant effect found based on the change in the
temperature ramping rate. For the surface properties of zirconia
catalysts, the intensity of Zr3 changed with the varying heating
+
Fig. 10. Relationship between intensity of Zr3+ along with selectivity of
isobutene in hydrocarbons and temperature ramping rates during calcination.
rate of calcination. The results showed that ZrO (5.0) had the
2

4
2
W. Khaodee et al. / Journal of Molecular Catalysis A: Chemical 280 (2008) 35–42
highest product selectivity to isobutene in hydrocarbons due to
[8] Y. Li, D. He, Y. Yuan, Z. Cheng, Q. Zhu, Fuel 81 (2002) 1611.
9] Y. Li, D. He, Q. Zhu, X. Zhang, B. Xu, J. Catal. 221 (2004) 584.
[10] W. Khaodee, B. Jongsomjit, S. Assabumrungrat, P. Praserthdam, S. Goto,
Catal. Commun. 8 (2007) 548.
the maximum quantity of Zr3 present in the sample. However,
+
[
both of the tetragonal phase in zirconia and intensity of Zr3+
influenced the selectivity to isobutene. ZrO2 (2.5) showed the
highest activity among all zirconia catalysts.
[
[
[
[
11] W. Li, Y. Yin, L. Feng, P. Zheng, Acta Physico—Chimica Sinica 12 (1996)
1074.
12] P.D.L. Mercera, J.G. van Ommen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H.
Ross, Appl. Catal. 71 (1991) 363.
13] E. Tani, M. Yoshimura, S. Somiya, J. Am. Ceram. Soc. 66 (1982)
Acknowledgements
1
1.
14] M.I. Osendi, J.S. Moya, C.J. Sena, J. Soria, J. Am. Ceram. Soc. 68 (1985)
35.
[15] R.C. Garvie, J. Phys. Chem. 82 (1978) 218.
The support from the National Research Council of Thailand
(
NRCT) is greatly appreciated. The authors would like to thank
1
Assistant Professor Joongjai Panpranot and Assistant Professor
Okorn Mekasuwandamrong for useful discussion.
[
[
[
[
[
16] Z.-Y. Ma, C. Yang, W. Wei, W.-H. Li, Y.-H. Sun, J. Mol. Catal. A: Chem.
27 (2005) 119.
17] S. Ardizzone, C.L. Bianchi, M. Signoretto, Appl. Surf. Sci. 136 (1998)
13.
18] S. Tsunekawa, K. Asami, S. Ito, M. Yashima, T. Sugimoto, Appl. Surf. Sci.
52 (2005) 1651.
2
References
2
[
[
[
[
1] I. Wender, Fuel Proc. Tech. 48 (1996) 189.
2
2] H. Pichler, K.H. Ziesecke, Brennst. Chem. 30 (1949) 13.
3] H. Pichler, K.H. Ziesecke, B. Traeger, Brennst. Chem. 30 (1949) 333.
4] C. Su, J. Li, D. He, Z. Cheng, Q. Zhu, Appl. Catal. A: General 202 (2000)
19] C.D. Wagner, W.M. Riggs, L.E. Davis, Handbook of X-ray Photoelectron
Spectroscopy, Eden Prairie, Minesoda, 1979, p. 55344.
20] C. Moterra, E. Giamello, L. Orio, M. Volante, J. Phys. Chem. 94 (1990)
8
1.
5] C. Su, D. He, J. Li, Z. Chen, Q. Zhu, J. Mol. Catal. A: Chem. 153 (2000)
39.
6] K. Maruya, T. Komiya, T. Hayakawa, L. Lu, M. Yashima, J. Mol. Catal. A:
Chem. 159 (2000) 97.
3111.
[
[
[
[
[
[
[
21] H. Liu, X. Zhang, Q. Xue, J. Phys. Chem. 99 (1995) 332.
22] Q. Zhao, X. Wang, T. Cai, Appl. Surf. Sci. 225 (2004) 7.
23] M. Anpo, T. Nomura, Res. Chem. Intermed. 13 (1990) 195.
24] H.P. Klug, L.E. Alexander, X-rayDiffractionProceduresforPolycrystalline
Amorphous Materials, Wiley, New York, 1974.
1
7] Y. Li, D. He, Z. Cheng, C. Su, J. Li, Q. Zhu, J. Mol. Catal. A: Chem. 175
(
2001) 267.