Superacidity and catalytic activity of sulfated zirconia

Article abstract of DOI:10.1021/jp002212o

The acidic property of sulfated zirconia, a so-called solid superacid catalyst, was precisely determined by ammonia temperature-programmed desorption with water vapor treatment and theoretical analysis. The desorption peak from the zirconia support was removed by water vapor treatment, and the generation of two types of acid sites was clarified. One kind of acid site is a Lewis type generated on a submonolayer species of the sulfate covering the surface; the surface concentration of the acid site was 0.5 atoms nm^-2, and the adsorption heat of ammonia was ca. 200 kJ mol^-1. This corresponds to -19 of the H₀ function, demonstrating superacidity. The other type of acid sites generated by loading excess sulfate possessed a high concentration (maximum, 2 nm^-2), an adsorption heat of ca. 160 kJ mol^-1, an H₀ of -12, and Bronsted nature. The former was active for the Friedel-Crafts-type alkylation of benzene with benzyl chloride in the liquid phase, and the latter was active for the skeletal isomerization of butane in gas phase.

Full text of DOI:10.1021/jp002212o

J. Phys. Chem. B 2000, 104, 10321-10328
10321
Superacidity and Catalytic Activity of Sulfated Zirconia
Naonobu Katada,* Jun-ichi Endo, Kei-ichi Notsu, Naoko Yasunobu, Norihiro Naito, and
Miki Niwa
Department of Materials Science, Faculty of Engineering, Tottori UniVersity,
4-101 Koyama-cho Minami, Tottori 680-8552, Japan
ReceiVed: June 20, 2000; In Final Form: August 9, 2000
The acidic property of sulfated zirconia, a so-called solid superacid catalyst, was precisely determined by
ammonia temperature-programmed desorption with water vapor treatment and theoretical analysis. The
desorption peak from the zirconia support was removed by water vapor treatment, and the generation of two
types of acid sites was clarified. One kind of acid site is a Lewis type generated on a submonolayer species
of the sulfate covering the surface; the surface concentration of the acid site was 0.5 atoms nm-2, and the
adsorption heat of ammonia was ca. 200 kJ mol-1. This corresponds to -19 of the H0 function, demonstrating
superacidity. The other type of acid sites generated by loading excess sulfate possessed a high concentration
(maximum, 2 nm-2), an adsorption heat of ca. 160 kJ mol-1, an H0 of -12, and Brønsted nature. The former
was active for the Friedel-Crafts-type alkylation of benzene with benzyl chloride in the liquid phase, and the
latter was active for the skeletal isomerization of butane in gas phase.
Introduction
those of alumina34 and alkaline zeolites.35 Even when the
indicator method is used, the results suggest that the acid
Sulfated zirconia1,2 shows remarkable catalytic activity for
reactions which are not catalyzed by conventional solid catalysts,
e.g., the skeletal isomerization of alkane,2 the alkylation of
alkane by alkene,3 the activation of ethane4 and methane,5 the
low-temperature cracking of polyolefin,6 and the Friedel-Crafts-
type alkylation7 and acylation8 of hydrocarbon with an inert
molecule such as carbon monoxide.9 Moreover, catalytic func-
tions have been improved by loading additional components
such as platinum,10 tungsten,11 iron, and manganese.12 Loading
sulfate and other multivalent anions, e.g., tungstate13 and
molybdate,14 on zirconia and basic metal oxides such as iron,15
titanium,16 and tin17,18 oxides also generates solid acid cata-
lysts.19
A superacid is defined to be an acid stronger than 100%
perchloric or sulfuric acid,20 and therefore, H0 < -11.9.21
Superacidity on sulfated zirconia has been claimed on the basis
of its changing the color of a Hammet indicator at pKa < -11.9.2
The high catalytic activity has been attributed to the supera-
cidity,22,23 and evidence for the catalysis of the superacid site
for the skeletal isomerization of butane based on activation
energy investigation has been shown.24 Moreover, it has been
reported on the basis of deactivation experiments that a relatively
strong acid site is responsible for the isomerization.25 Also, for
the synthesis of fine chemicals using sulfated zirconia as a
Friedel-Crafts-type catalyst, superacidity is considered to be
important.26 Some NMR studies indicate strong acidity.27,28
Electron spin resonance (ESR) spectroscopy supports the fairly
strong acidity also.29
strength of sulfated zirconia is similar to that of 100% sulfuric
acid.36 Quantum chemical calculation demonstrated that the acid
strength of sulfated zirconia was similar to that of sulfuric acid37
and weaker than that of zeolite.38 A combination of NMR and
IR techniques elucidated the complex property of the multiple
types of acid sites.39
Thus there is serious controversy over the interpretation of
the acidic property of sulfated zirconia, and this is further
complicated by the nature of sulfated zirconia being variable
according to the preparation method and conditions40 (especially
the quality and concentration of surface hydroxyl groups40,43),
as shown by the reports of various preparation techniques.41,42
Even a recent review could not determine whether sulfated
zirconia is a superacid.44
Researchers point out the shortages in the analytical methods
of acidity. Only a method using typical basesse.g., aminess
as probes can determine the absolute acid strength, which is
defined as the equilibrium constant of the reaction between acid
and base, for example, the H0 function.21 Therefore, spectro-
scopic methods cannot directly determine the strength of acidity.
On the other hand, the chemical nature of a solid surface may
change with its environment, and so, the indicator method in
solution may detect the property far from the working state of
the catalyst. In addition, it is generally difficult to obtain accurate
and quantitative results from the color of the indicator.
From these points of view, a method using a gaseous base
probe such as ammonia is promising. Temperature-programmed
desorption (TPD) of ammonia45 is one promising method and
has been applied to the sulfated zirconia catalyst.46-48 Corma
et al. reported that the sulfated zirconia showed a desorption
peak at quite a high temperature, 815 K,46 but it was soon
pointed out that because their measurements were carried out
using a thermal conductivity detector (TCD), the desorption of
the sulfur compound could possibly have disturbed the spec-
trum.49 Through careful measurements, the desorption peak at
However, there are papers which claim that the sulfated
zirconia has no superacidity30,31 or that catalytic activity is not
related to superacidity.32 The infrared (IR) and nuclear magnetic
resonance (NMR) spectra of carbon monoxide and hydrocarbons
adsorbed on the sulfated zirconia33 showed acidities weaker than
* Corresponding author. Phone/fax: +81-857-31-5684. E-mail:
katada@chem.tottori-u.ac.jp.
10.1021/jp002212o CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/13/2000

10322 J. Phys. Chem. B, Vol. 104, No. 44, 2000
Katada et al.
a lower temperature49 and, in addition, the desorption peak on
the zirconia support, which was usually large were observed.50
There is a hypothesis that the small difference in the peak
temperature between zirconia and sulfated zirconia shows strong
acidity.50 However, these small difference cannot explain the
drastic change of catalytic activity due to the loading of sulfate.
Moreover, the desorption temperature of the TPD spectrum does
not show the acid strength directly.51 Because of these shortages,
the ammonia TPD on these catalysts is not accepted widely. In
addition, some authors reported a completely different TPD
spectrum; Essayem et al. reported that no desorption of ammonia
from their sulfated zirconia was observed up to 900 K.52 The
TPD of pyridine has been applied to the sulfated zirconia,53,54
but it has the same problems.
and a flame ionization detector (FID), and the injections were
repeated several times until the adsorption was saturated. Then
gaseous ammonia (10 cm3, 0.409 mmol) was injected from the
septum to convert the adsorbed species into benzonitrile.
Benzaldehyde forms a stable benzoate anion on a basic metal
oxide such as zirconia in high surface concentrations but is not
adsorbed on acidic material. Because the benzoate anion is
almost completely converted into benzonitrile, the amount of
yielded benzonitrile must reflect the amount of adsorbed
benzoate species, and therefore, the coverage of the zirconia
surface by acidic material is calculated as follows:
exposure ) nitrile yield per BET surface area (BN) on
the catalyst/BN on zirconia (2.2 molecules nm-2) (1)
We have developed experimental and analytical methods to
determine the quantity and strength of acid precisely from the
TPD spectrum, mainly on zeolites. Water vapor treatment
removed the unnecessary peak from weakly held ammonia to
show the actual property of the acid sites.55,56 Theoretical
analysis based on derived equations helped to determine the
adsorption heat of ammonia from the TPD spectrum.57,58 These
methods clarified the acidic properties of various zeolites.45 Our
theory was confirmed to be applicable to a tungsta-zirconia
catalyst, and in addition, water vapor treatment selectively
removed the ammonia species on zirconia exposed surfaces but
did not affect the species adsorbed on the tungsta. Thus, the
acid quantity and strength of the tungsta-zirconia were first
determined; on the monolayer of tungsta, one acid site with the
ammonia adsorption heat of ca. 130 kJ mol-1 was generated
with four to five atoms of tungsten.59
In the present study, we applied these methods to the sulfated
zirconia catalysts to determine the acidic property on the basis
of preliminary experiments.60 The benzaldehyde-ammonia
titration (BAT) method,61,62 which measured the spreading of
acidic components on basic metal oxide, was also applied to
clarify the surface structure of the sulfate species. Moreover,
the determined properties were related with catalytic activities
for the skeletal isomerization of butane and Friedel-Crafts
alkylation of benzene with benzyl chloride.
coverage ) 1 - exposure
(2)
Temperature-Programmed Desorption. The TPD experi-
ments were carried out according to the method in our paper45
after the evacuation of the catalyst at 873 K in a quartz cell
designed by Amenomiya et al.,63 followed by the adsorption of
ammonia and water vapor treatment at 373 K. The temperature
was increased at the heating rate of 10 K min-1, and the
desorbed materials were analyzed with a mass spectrometer
(ULVAC UPM-ST-200P). The amount of ammonia was deter-
mined on the basis of the intensity of the fragment with m/e )
16 because the parent peak with m/e ) 17 was influenced by
the desorbed water. In the standard experiments, the catalyst
amount was 0.1 g, the flow rate of the carrier helium was 1
cm3 STP s-1 (ca. 41 µmol s-1), and the pressure in the catalyst
bed was 13.3 kPa, but these were varied in some experiments
to obtain the relationship between the peak maximum temper-
ature and experimental conditions; this relationship is necessary
to calculate the adsorption heat of ammonia.
Infrared Spectroscopy. The infrared (IR) spectrum was
collected on a sample disk (10 mg, 1 cm in diameter) set in an
in-situ cell. The sample disk was evacuated at 873 K for 1 h,
followed by the adsorption of pyridine vapor (ca. 400 Pa) at
373 K for 30 min and further evacuation at 573 K for 1 h. Then
the spectrum was collected with a JASCO FT/IR-5300 spec-
trometer.
Experimental Section
Catalytic Reaction. The skeletal isomerization of n-butane
into isobutane (2-methylpropane) was carried out at 423 K
through, the two experimental methods. In the pulse experi-
ments, a pulse of n-butane (53 µmol) was injected onto the
catalyst (0.25 g) pretreated at 873 K in 20 cm3 min-1 of flowing
helium, and the products were analyzed with an FID-GC
instrument with a column of VZ-7 (2 mm i.d., 4 m in length)
operated at room temperature. The activity is shown through
the yield of isobutane from the first pulse of n-butane. In the
closed circular experiments, a mixture of n-butane (0.264 mmol)
and helium (0.881 mmol) was introduced into the reactor (167
cm3) in which the catalyst (0.1 g) was set. The reaction rate
was determined from the observed linear relationship between
the isobutane concentration and reaction time at 0.5-3 h. All
the experiments were carried out under differential conditions
(conversion, <5%).
The alkylation of benzene with benzyl chloride into diphe-
nylmethane was carried out in liquid phase. A catalyst (0.1 g)
pretreated at 873 K in flowing nitrogen, benzene (45 mmol),
and benzyl chloride (5 mmol) was put into a flask in a drybox.
The mixture was stirred at 313 K for 2 h, and the products were
analyzed with an FID-GC with a column of OV-1 (2 mm i.d.,
2 m length) operated at 373-523 K. The activity is shown
through the amount of diphenylmethane formed.
Preparation of Zirconia and Sulfated Zirconia. Zirconium
oxynitrate [ZrO(NO3)2] was solved into a nitric acid solution,
and aqueous ammonia was slowly added under stirring to
precipitate zirconium hydroxide until the pH reached ca. 11.
The obtained solid was washed with water and calcined to
prepare zirconia at 573 K for 4 h under atmospheric conditions.
Sulfate was loaded by an impregnation method; 5 g of zirconia
was suspended in aqueous solutions containing various amounts
of sulfuric acid [shown by the weight ratio of SO42-/ZrO2 (wt
%) in the following descriptions] with a constant volume of
100 cm3, followed by drying on a hotplate. After drying for ca.
2 h, the obtained solid was calcined at 923 K for 4 h under
atmospheric conditions. The elemental analysis after calcination
was carried out with a Horiba EMIA-610 FA analyzer. BET
surface area was determined from the amount of nitrogen
adsorbed at 77 K under 30.39 kPa of nitrogen (P/P0 ) 0.3).
Benzaldehyde-Ammonia Titration. The coverage of the
surface by sulfate species was measured by the BAT method.62
After the sample was pretreated at 673 K in flowing oxygen, 1
mm3 (ca. 9.9 µmol) of benzaldehyde was injected from the
septum installed before the reactor at 523 K in helium flow.
The eluted aldehyde was monitored by a gas chromatograph
(GC) with a column of Silicone Grease 20%/Chlomosorb WAW

Sulfated Zirconia
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10323
Figure 3. Ammonia TPD spectra measured before (broken lines) and
after (solid lines) water vapor treatment over sulfated zirconia
Figure 1. BET surface area of sulfated zirconia after calcination at
923 K against sulfuric acid content in impregnated solution, shown by
the weight ratio (wt %) of SO42-/ZrO2.
impregnated from solutions with 0 (a), 1.9 (b), 3.7 (c), 10.7 (d), 15.4
2-
(e), 18.0 (f), 21.0 (g), and 24.0 wt % (h) of SO4
.
nm-2sof sulfur was loaded by the impregnation of diluted
sulfuric acid solution, whereas the loading increased by increas-
ing the sulfuric acid concentration to greater than the threshold
value.65
The surface structure was analyzed by the BAT method.62
The coverage by sulfate on zirconia was determined on the basis
of the selective adsorption of benzaldehyde on a basic metal
oxide such as zirconia, as shown in Figure 2. The coverage
reached about 90% at 3 wt % of the sulfur content in the
solution, where ca. 2.5 atoms nm-2 of the surface concentration
of sulfate species was observed.
Acidic Property. Figure 3 (broken line) shows the ammonia
TPD spectrum measured by the conventional procedures without
water vapor treatment. Both zirconia and the sulfur-loaded
samples showed the large desorption peak at ca. 500 K. On the
samples with the sulfur concentration of 2.2-2.6 atoms nm-2
(prepared from 2-15 wt % of the impregnation solution), an
additional peak appeared at quite a high temperature, ca. 750
K. The water vapor treatment drastically changed the TPD
spectra (Figure 3, solid lines) as follows:
Figure 2. Surface concentration of sulfate species determined by
elemental analysis (b) and coverage-measured by the BAT method
(9).
Results
(1) on zirconia, the peak was almost diminished, and
Structure. The original surface area of zirconia support,
which had calcined at 523 K, was ca. 150 m2 g-1. As shown in
Figure 1, calcination at 923 K significantly decreased it to ca.
20 m2 g-1, whereas relatively high surface area, 50-100 m2
g-1, was kept by loading sulfuric acid. The highest surface area
was obtained with 2.5-15 wt % of the impregnated sulfate.
The elemental analysis of the zirconia treated with sulfuric
acid followed by calcination at 923 K showed that a fraction of
sulfur was eliminated, probably during the high-temperature
calcination. Figure 2 shows the relationship between the surface
concentration of the remaining sulfur (amount of sulfur/BET
surface area) and the content of sulfate in the impregnated
solution. From 2 to 15 wt % of the used solution content, a
constant sulfur concentration of ca. 2.5 atoms nm-2 was yielded.
The surface concentration of loaded sulfur again increased when
the sulfuric acid content of the solution was increased to >15
wt %. This relationship between the sulfur loading and the
sulfuric acid content (concentration) in the impregnated solution
agrees with the previous reports; Nascimento et al. reported that
(2) on the samples with the sulfur concentration of 2.2-2.6
atoms nm-2 (prepared from 2-15 wt % of the impregnation
solution), the peak at 750 K was not affected, while the peak at
the low temperature was almost diminished.
(3) On the sample with an excess amount of sulfate (>7 atoms
nm-2), a peak was still observed at ca. 600 K.
Thus, the water vapor treatment indicates the generation of
two types of acid sites by loading sulfate. One is the very strong
site showing a peak at 750 K, which appeared with 2-15 wt %
of the impregnation solution, and the other is the weak acid
site showing a peak at 600 K, which appeared with >15 wt %.
As shown in Figure 4, the surface concentration of the stronger
acid site is determined from the peak area to be about 0.5 nm-2
.
The weak acid showed a maximum concentration of ca. 2 nm-2
with 20 wt % SO42-/ZrO2 of the impregnated solution, and the
higher loading decreased the acidity.
To analyze the desorption peak according to our theory, we
measured the TPD spectra with varying the W (catalyst amount,
almost constant sulfur content at ca. 1-1.5 × 10-4
g
kg)/F (flow rate of carrier gas, m3 s-1) from 1 to 50 kg s m-3 60
.
m-2snamely, 2-3 atoms nm-2swas obtained from sulfuric
acid solutions with various concentrations,64 and Faˇrcas¸iu et al.
found that approximately 4-5 µmol m-2snamely, 2.5-3 atoms
The peak temperature increased with increasing W/F ratio, and
an almost linear relationship was observed between the terms
ln Tm - ln(A0W/F) and 1/Tm, where Tm is the peak temperature

10324 J. Phys. Chem. B, Vol. 104, No. 44, 2000
Katada et al.
Figure 4. Acid concentration (b) and strength (2, adsorption heat of
ammonia) of sulfated zirconia.
Figure 6. Relative intensity of pyridine species adsorbed on Brønsted
acid sites in IR spectrum of pyridine adsorbed at 373 K, followed by
evacuation at 573 K.
Figure 5. Plots of ln Tm - ln(A0W/F) against 1/Tm on sulfated zirconia
2-
impregnated from a solution with 10 wt % of SO4
.
and A0 is the amount of desorbed ammonia, as shown in Figure
5. The meaning of this linear relationship will be discussed
below.
The amounts of Brønsted and Lewis acid sites were estimated
from the IR spectrum of adsorbed pyridine. The relative intensity
of pyridine species adsorbed on Brønsted acid site was shown
by the equation
Figure 7. Catalytic activity for the liquid-phase alkylation of benzene
with benzyl chloride into diphenylmethane.
showing the formation of a small amount of polymerized
species. The activity for the alkylation of benzene showed the
2-
maximum at 5-15 wt % of the SO4 loading, as shown in
Figure 7. Excess sulfate loading suppressed the activity.
Only isobutane was detected in the products of the reaction
of n-butane at 423 K, while the color of the catalyst became
black. As shown in Figure 8, the samples prepared from 0-15
wt % of the impregnated solution displayed low activity for
the skeletal isomerization in both the pulse and closed circular
experiments. The high activity was observed on the samples
prepared with a large amount of sulfuric acid, >20 wt %.
IB
(3)
IB + IL
where IB is the intensity of peak at 1545 cm-1, attributed to the
pyridinium cation adsorbed on the Brønsted acid site and IL is
the intensity at 1455 cm-1 of the pyridine molecule coordinated
to the Lewis acid site. As shown in Figure 6, with 2-15 wt %
of the sulfuric acid content of the impregnation solution, almost
no Brønsted acidity was observed, while 10-40% of Brønsted
acidity was observed with >20 wt %. In other words, Lewis
acidity was predominant at a surface sulfur concentration of
2.5 atoms nm-2. This is in agreement with the findings of
Nascimento et al. because they reported that the Lewis acid
was predominant at sulfur contents of <1-2 × 10-4 g S m-2
(1.8-3.6 atoms nm-2) while the higher content generated
Brønsted acidity.64
Discussion
Structure. The elemental analysis showed a constant sulfur
concentration of ca. 2.5 atoms nm-2, with 2-15 wt % of the
sulfur content in the impregnated solution. This points out that
a fraction of sulfate, whose concentration was 2.5 S nm-2, was
tightly anchored the zirconia surface, while excess sulfate was
eliminated probably during the high-temperature calcination. It
is estimated from the covalent and van der Waals radii of the
atoms that one SO4 tetrahedra occupies ca. 0.25 nm2 (Scheme
1). The concentration of sulfur on the surface saturated with
this species is 1/0.25 ) 4 nm-2. On the other hand, the kinetic
Catalytic Activity. The reaction of benzene with benzyl
chloride yielded diphenylmethane, but almost no other byprod-
ucts were detected. The material balance was >90%, whereas
the color of the catalyst became light-brown after the reaction,

Sulfated Zirconia
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10325
submonolayer species, as shown in Figure 1. In summary, the
submonolayer species of sulfate roughly covers the surface of
the samples prepared from 2-15 wt % of the solution.
On the other hand, the surface concentration of loaded sulfur
again increased when the sulfuric acid content of the solution
was increased to >15 wt %. From 15 to 20 wt %, a
monolayerlike structure is possible because the surface con-
centration of sulfur is close to 10 atoms nm-2; it is possible to
form a monolayer consisting of S-O-S bonds with such a
concentration. However, with excess sulfuric acid, the formation
of some zirconium sulfate salt is suggested from a concentration
of sulfur as high as 40 atoms nm-2. Indeed, it was observed
that a fraction of the zirconia solid was dissolved into the
solution and recrystallized by removing water via heating when
the sulfuric acid content in the impregnation solution was >20
wt %. It is suggested that some complicated reactions proceed
by impregnating sulfuric acid at such a high concentration.
TPD Spectrum. Diminishing the desorption peak on the
zirconia support by water vapor treatment has been discussed
on the bases of TPD and IR experiments.59 The Lewis acid site
generated on the basic metal oxide by the dehydration of surface
hydroxyl groups was probably inactivated by the rehydration
of the surface through water vapor treatment. On the contrary,
the amount and nature of the ammonia species (both coordinated
molecule and ammonium cation) adsorbed on WO3/ZrO2 were
unchanged by water vapor treatment. It is supposed that the
adsorbed species due to the zirconia surface was selectively
removed by water vapor treatment on the sulfated zirconia also.
In other words, the acid site, which had been generated by
loading sulfate and was responsible for the catalytic activities,
was selectively detected. Hereafter, the acidic property of the
sulfated zirconia will be analyzed on the basis of the water vapor
treatment method.
Figure 8. Catalytic activity for the skeletal isomerization of n-butane
into isobutane (2-methylpropane), as measured by closed circular (0,
solid line) and pulse (4, broken line) experiments.
SCHEME 1: Estimation of Surface Area Occupied by
2-
an SO4 Aniona
a The S-O covalent bond length is assumed to be 0.143 nm, the
O-S-O bond angle 109°, and the van der Waals radius of the oxygen
atom 0.140 nm. The shaded area is ca. 0.25 nm2
Acidic Property. On the catalysts covered by the submono-
layer of the sulfate species (2-15 wt % SO42-/ZrO2), a fairly
strong acid site with ca. 0.5 nm-2 of the surface concentration
was observed, while excess sulfate diminished the strong acid
site and generated a relatively weak but highly concentrated
(maximum, 2 nm-2) acid site. Thus, the acidic property seems
dependent on the structure suggested above.
SCHEME 2: Proposed Superacidic Species
To determine the acid strength from the TPD spectrum, we
will show what controls the TPD spectrum. The TPD experi-
ments are classified as63
(1) being controlled by kinetics, i.e., the activation energy is
high,
(2) being controlled by equilibrium, i.e., re-adsorption of
desorbed material freely occurs
(3) or being controlled by diffusion.
diameter of molecule is widely estimated to be 21/6 times larger
than the static diameter.66 The occupation area based on the
kinetic diameter should be 0.25 × 21/3 ) 0.31 nm2, and hence,
the concentration must be 3.2 nm-2. Because the observed
concentration of 2.5 nm-2 was smaller but not so far from the
estimated values of 3.2-4 nm-2, the surface is considered to
be roughly covered by isolated species of sulfate. The BAT
analysis evidenced that the surface was covered by the loaded
material with 2.5 S nm-2. From IR spectroscopy, the species
in Scheme 2 was proposed to reside on the sulfated metal
oxide.22,67 This isolated structure is consistent with the present
results, and therefore, we propose that the layer of this or similar
species covers the zirconia surface. Hereafter, it is termed the
“submonolayer” of the sulfate species. Probably because of the
strong interaction between the acid (sulfuric acid) and the base
(zirconia surface), the isolated submonolayer species is spon-
taneously formed. Such a spontaneous coverage68 has been
found on V2O5,69 MoO3,70,71 WO3,59,72 SiO2,73,74 and GeO2,75
loaded on Al2O3, SnO2, TiO2, and ZrO2.
The theoretical equation of the second case is
A0W
F
â(1 - θm)(∆H - RTm)
∆H
RTm
ln Tm - ln
)
+ ln
(4)
∆S
P0exp
(
)
R
where Tm is the peak maximum temperature (K), A0 is the
amount of adsorption (acid) site (mol kg-1), W is the amount
of sample used (kg), F is the flow rate of carrier gas (m3 s-1),
∆H is the enthalpy change (adsorption heat, J mol-1), R is the
gas constant (8.314 J K-1 mol-1), â is the heating rate (K s-1),
θm is the coverage of adsorption site at the peak maximum, P0
is the pressure of the standard conditions (1.013 × 105 Pa),
and ∆S is the entropy change (J K-1 mol-1).45 Here, the second
term of the right-hand side is almost unchanged by varying the
parameters within a practical range,57 and therefore, the term
In such a case, it is usually observed that covering the surface
with these modifiers prevents thermal sintering of the support.
We have already shown that the loaded material protected the
particle like an egg shell.76 This is consistent with the high-
BET surface area of the samples covered by the sulfate

10326 J. Phys. Chem. B, Vol. 104, No. 44, 2000
Katada et al.
ln Tm - ln(A0W/F) must show the linear relationship against
1/Tm if the second case is available. The measurements of TPD
spectra were performed by varying the W/F ratio, and the liner
relationship was obtained as shown in Figure 5. This demon-
strates that the present experiments are classified into the second
case, and the desorption process is controlled by the equilibrium
between gaseous and adsorbed ammonia
Because the neutralization of the solid acid by ammonia in an
aqueous medium (eq 8) is -(eq 5) - (eq 7), its Gibbs energy
change ∆G8° can be expressed by eq 9 if it is assumed that the
solid acid can react with the same strength as an acid in an
aqueous medium
A + NH3 (aq) f NH3-A
(8)
(9)
NH3-A a NH3 (g) + A
(5)
∆G08 ) -(∆H50 - T0∆S05) - ∆G70
The thermodynamic equilibrium constant K8 of reaction 8 at
temperature T0 can be expressed as follows:
where A is the solid acid. This phenomenon is commonly
observed on various zeolites45,57 and tungsta-zirconia cata-
lysts.59
a
NH3-A
0
0
0
0
0
0
Subsequently, the whole entropy change was calculated from
8
5
5
7
K8 )
) e-∆G /RT ) e(∆H -T ∆S +∆G )/RT
(10)
the slope and intercept of Figure 5 to be 190 J K-1 mol-1
.
aAa
NH3(aq)
Because the entropy change due to gas mixing was calculated
to be 85-105 J K-1 mol-1 from the pressures of ammonia and
the carrier gas,58 the entropy change with respect to the elemental
step of desorption, which must be the difference between the
where ai shows the thermodynamic activity of component i. The
activities of NH3-A and A are shown by θ (the coverage of
acid site by ammonia) and 1 - θ, respectively. The activity of
the aqueous ammonia can be shown by the concentration of
ammonia [NH3] in units of mol dm-3. Hence, the eqs 11 and
12 are derived as
whole and mixing entropies, should be 85-105 J K-1 mol-1
.
This value is almost same as those observed for various
zeolites45,57 and tungsta-loaded zirconia59 (95-100 J K-1 mol-1
)
and, moreover, for the entropy change of the vaporization of
ammonia (97 J K-1 mol-1) and various liquids (80-110 J K-1
mol-1).77 These agreements suggest a common rule that controls
the TPD process on solids; the entropy change of desorption
and vaporization is mainly determined by the free-volume
change, in agreement with Trouton’s rule.45 From these
observations, we can hereafter determine the adsorption heat
of ammonia from the TPD spectrum by applying the same
method58 to zeolites and tungsta-zirconia, which assumes
equilibrium control and the constant entropy change of 95 J
0
0
0
0
0
θ
) e(∆H -T ∆S +∆G )/RT
(11)
(12)
5
5
7
(1 - θ)[NH3]
0
0
0
0
0
θ
) (1 - θ)e(∆H -T ∆S +∆G )/RT
5
5
7
[NH3]
When an equilibrium (eq 13) between an acid AH and a small
amount of base indicator B exists, the H0 function is defined
as21
K-1 mol-1
.
AH + B a A- + BH+
[BH+]
H0 ) pKa - log
[B]
(13)
(14)
On the basis of this theory, the acid strength of sulfated
zirconia catalyst was calculated as shown in Figure 4. In the
region of the 2-15 wt % of the sulfate content in the
impregnated solution where the submonolayer formed, a high
adsorption heat of ammonia (ca. 200 kJ mol-1) was obtained.
The adsorption heat of the weaker acid site generated with >15
wt % of the impregnated solution was ca. 160 kJ mol-1. These
values can be compared with those on other solid acids. Even
on the weaker acid site, the adsorption heat is higher than those
on any aluminosilicate zeolites (ca. 145, 130, 120, and 110 kJ
mol-1 on mordenite, ZSM-5,58 â-zeolites,78 and Y-zeolites,56,79
respectively), which are known as some of the strongest solid
where Ka is the acid dissociation constant of the conjugated
acid (BH+) of the base B, and pKa is -log Ka. Therefore, we
can express the H0 of the solid acid as the eq 15 on the basis of
the reaction between solid acid and a small amount of ammonia
in an aqueous solution (eq 8)82
θ
H0 ) pKNH + - log
(15)
4
[NH3]
acids; the strength of the exceptionally strong (ca. 170 kJ mol-1
)
acid site ascribed to an extraframework cation55,80 is comparable
to the strength of the weaker acid site on the sulfated zirconia.
Next, we will derive the H0 scale from the adsorption heat
of ammonia. On the assumption that changes in the standard
enthalpy and entropy of the adsorption of ammonia (5) are
∆H05 and ∆S05, respectively, and ignoring their temperature
dependence, we can express the standard Gibbs energy change
of reaction (5), ∆G05, at T0 ) 298 K as
+
where KNH is the acid dissociation constant of ammonium ion
4
in water. Substituting eq 12 into eq 15 derives
∆H05 - T0∆S05 + ∆G07
H0 ) pKNH + - log(1 - θ) -
(16)
RT0 ln 10
4
Because the amount of ammonia was assumed to be small, the
term 1 - θ is close to 1. Substituting pKNH ) 9.25,83 T0 )
+
4
298 K, ∆S5° ) 95 J K-1 mol-1, ∆G7° ) -9.85 kJ mol-1, and
∆G05 ) ∆H05 - T0∆S05
(6)
R ) 8.314 J K-1 mol-1 into the eq 16 derives the relationship
between the H0 function and the adsorption heat of ammonia
On the other hand, the standard Gibbs energy changes for the
formation of gaseous and aqueous ammonia are -16.65 and
-26.50 kJ mol-1, respectively, and therefore, the Gibbs energy
change of reaction (7), ∆G07, is 16.65 - 26.50 ) -9.85 kJ
(∆H5°, J mol-1
)
H0 ) -1.75 × 10-4∆H5° + 15.9
(18)
mol-1 81
.
Equation 17 is the relationship between the adsorption heat
of ammonia and the H0 function on the solid acid where the
entropy change of ammonia desorption is 95 J K-1 mol-1
.
NH3 (g) + aq f NH3 (aq)
(7)

Sulfated Zirconia
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10327
This theory suggests that the stronger acid site with 200 kJ
mol-1 of the adsorption heat of ammonia corresponds to H0 )
-19.1. According to Conant and Hall,20 an acid stronger than
100% perchloric acid or sulfuric acid (H0 ) -11.9) is a
superacid. Therefore, this type of acid site on the submonolayer
species of sulfate loaded on zirconia is a superacid. This H0
value agrees well with the method of indication, which showed
H0 < -16.04 on the sulfated zirconia calcined at 923 K.22 It is
noteworthy that the number of this type of acid site (see Figure
4, ca. 0.5 nm-2) was much smaller than the number of the sulfate
species (2.5 nm-2). Only a fraction of the submonolayer species
seems to act as an acid site. As shown in Figure 6, the nature
of this superacid site is mainly Lewis type. It is suggested that
the isolated species in Scheme 1 is the Lewis-type superacid
site, as previously proposed.19
surface concentration of acid sites such as >1.5 nm-2 is
considered to allow access via the multiple acid sites. We have
also observed that a Brønsted acid site with relatively high
strength (130 kJ mol-1 of the adsorption heat of ammonia) on
the tungsta monolayer was active for this reaction (the reaction
temperature was 623 K) and that the surface concentration of
acid site was high, ca. 1.5 nm-2 59
Because of the difficulty
.
taking accurate measurements of surface acidity, no researcher
has noticed the high density of the acid site on the sulfated
zirconia, but this is possibly the reason of the high activity for
the skeletal isomerization.
Finally, we will discuss on the disagreement among the
papers. The presence of the two or more kinds of acid sites on
the sulfated zirconia in the present study is in agreement with
the previous papers,30,39,86 and this must disturb the quantitative
characterization of the acidic property. Moreover, the presence
of the superacid site was indicated in relatively early works;
for example, Hino and Arata et al., reported the generation of
superacidity by pouring sulfuric acid solution into zirconia
placed on a filter paper.2 On the basis of the present study, we
conclude that only the submonolayer species of sulfate is
supposed to be bonded on the zirconia surface through such a
procedure. Recently, there has been a trend to optimize the
preparation method of the catalyst to obtain an active catalyst
for the skeletal isomerization.40-43 It is noteworthy that the two
important papers which claim the weak acidity of sulfated
zirconia stand on the commercially utilized sample.30,31 The
optimized preparation method possibly results in the generation
of a weaker Brønsted-type acid site but no superacidity, which
was observed with excess sulfate loading (>15 wt %) in the
present study.
On the other hand, the strength of the weaker acid site
generated by excess sulfate corresponds to H0 ) -12, and
hence, it is not a superacid but similar in strength to the 100%
sulfuric acid. There are papers which claim that the acid strength
of sulfated zirconia is similar to that of 100% sulfuric acid on
the bases of the indicator method36 and the quantum chemical
calculation.37 The surface concentration of this type of acid site
was quite high, ca. 2 nm-2 (Figure 4), and its maximum was
observed at 20 wt % of the sulfate content in the impregnated
solution, i.e., with a sulfur surface concentration of ∼10 atoms
nm-2. The high concentration of the acid site is similar to those
observed on the multivalent anion-loaded oxide catalysts; ca.
1.5-2 nm-2 of Brønsted acid site was generated by loading
the monolayers of tungsta on zirconia59 and molybdena on tin
oxide84 with ca. 5 nm-2 of the loaded metal atom. Similar to
these cases, it is suggested that the monolayerlike species of
sulfate consisting of S, O, Zr, and H atoms generates Brønsted
acidity.
Conclusions
(1) A submonolayer of sulfate, e.g., the isolated species in
Scheme 1, is spontaneously formed by the impregnation of
sulfuric acid on zirconia.
(2) The water vapor treatment of ammonia TPD changed the
spectrum, and this clarified the generation of acid sites by
loading sulfate.
Catalytic Activity. As shown in Figure 7, the activity for
the alkylation of benzene with benzyl chloride was generated
by the impregnation of 5-15 wt % of sulfate. From these results,
it is proposed that this Friedel-Crafts-type reaction is catalyzed
by the Lewis-type superacid site on the submonolayer species
of sulfate.
(3) On the submonolayer species of sulfate, ca. 0.5 nm-2 of
a Lewis-type superacid site with an ammonia adsorption heat
of ca. 200 kJ mol-1 of and a H0 function of -19, was generated.
This type of acid site catalyzes Friedel-Crafts type alkylation
using benzyl chloride.
(4) On the samples with excess sulfate loading, highly
concentrated (maximum 2 nm-2) Brønsted acid sites with ca.
160 kJ mol-1 of the adsorption heat and -12 of the H0 function
were observed. This type of acid site was active for the skeletal
isomerization of butane at 423 K.
In contrast to this simple conclusion, the explanation for the
skeletal isomerization of butane is complex. In both of the pulse
and closed circular experiments, which must show the initial
and static activities, respectively, the activity was observed at
>15 wt % of the sulfate content in the impregnated solution.
Therefore, the superacidity may not be necessary for this
reaction, at least at 423 K. With a high amount of sulfate, >15
wt %, Brønsted acidity was observed, and this type of acid site
is probably one of the strongest Brønsted acids among solid
acid catalysts. It is possible that (strong) Brønsted acidity is
important for this reaction.25 However, this explanation is not
enough because a strong Brønsted acids such as mordenite or
ZSM-5 zeolite never shows high activity for the skeletal
isomerization; more precisely, these zeolites are known as
cracking catalysts, but the selectivity for the isomerization is
low. Therefore, another explanation is needed for the extremely
high activity on the sulfated zirconia. One possible solution is
that the copresence of Brønsted and Lewis acid sites, as
observed, enhances the activity, and this is in agreement with
the postulation that a pair of Brønsted and Lewis acid sites
concertedly works as the active site.85 Another explanation is
that the activity is much enhanced by the high surface
concentration of the acid site at a maximum of ca. 2 nm-2. If
the concerned mechanism is important, as above, multiple acid
sites must access an intermediate molecule at once. A high
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