Preparation of a novel catalyst UDCaT-5: Enhancement in activity of acid-treated zirconia - Effect of treatment with chlorosulfonic acid vis-a-vis sulfuric acid

Article abstract of DOI:10.1016/j.jcat.2004.02.021

UDCaT-5, a zirconia-based catalyst, with high sulfur content (9% w/w) but preservation of the tetragonal phase of zirconia was synthesized for the first time, by using chlorosulfonic acid as a new source for sulfate ions. UDCaT-5 was characterized by using elemental analysis, FTIR, ammonia-TPD, XRD, and BET surface area. Its catalytic activity and stability were evaluated and compared with S-ZrO₂ in three different reactions. The characterization and reaction studies show that UDCaT-5 exhibits more superacidity vis-a-vis conventional sulfated zirconia prepared by using sulfuric acid.

Full text of DOI:10.1016/j.jcat.2004.02.021

Journal of Catalysis 224 (2004) 218–223
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Research Note
Preparation of a novel catalyst UDCaT-5: enhancement in activity of
acid-treated zirconia—effect of treatment with chlorosulfonic acid
vis-à-vis sulfuric acid
Ganapati D. Yadav ∗ and Ambareesh D. Murkute
Department of Chemical Engineering, University Institute of Chemical Technology (UICT), University of Mumbai, Matunga, Mumbai 400 019, India
Received 15 November 2003; revised 12 February 2004; accepted 19 February 2004
Abstract
UDCaT-5, a zirconia-based catalyst, with high sulfur content (9% w/w) but preservation of the tetragonal phase of zirconia was synthesized
for the first time, by using chlorosulfonic acid as a new source for sulfate ions. UDCaT-5 was characterized by using elemental analysis, FTIR,
ammonia-TPD, XRD, and BET surface area. Its catalytic activity and stability were evaluated and compared with S–ZrO in three different
2
reactions. The characterization and reaction studies show that UDCaT-5 exhibits more superacidity vis-à-vis conventional sulfated zirconia
prepared by using sulfuric acid.
2004 Elsevier Inc. All rights reserved.
Keywords: Sulfated zirconia; Superacidity; UDCaT-5; Chlorosulfonic acid; Sulfuric acid
1. Introduction
can be achieved by changing the calcination temperature but
another property, noticeably crystallinity of zirconia, also
is affected [7,8]. Wender [8] reported that sulfated zirco-
nia, derived from impregnation with 0.25 M sulfuric acid
and calcined at 600 ◦C for 3 h, possessed 2.5% w/w sulfur
content (in the form of sulfate ions). It is also reported that
“percolation” with sulfuric acid of concentrations varying
from 0.05 to 2 M led to about the same amount of sulfur
in sulfated zirconia after calcination [9]. Morterra et al. [10]
have studied the effect of sulfate concentration on the Brøn-
sted acidity of the catalyst. Their results suggest an increase
in the Brønsted acidity with an increase in sulfate concen-
tration up to a certain maximum, after which the amount of
Brønsted acidity remains constant. This trend has also been
reported by Nascimento et al. [11]. Thus, sulfur in the form
of sulfate present above 4% w/w is lost during the thermal
activation and represents a thermally more labile fraction.
Fârcas¸iu and co-workers [12] reported that the sulfur con-
tent could be controlled and zirconia with high sulfur loading
can be achieved. Their report suggests that zirconia exhibits
a pure tetragonal phase at low values of sulfur content but at
a higher value (S > 4%), its crystallinity is strongly affected
and a monoclinic phase of zirconia is formed in addition to
the tetragonal phase. In another study [13], sulfated zirconia
was prepared by different procedures using a colloidal sol–
Zirconia, when modified with sulfate ions, forms a highly
acidic or superacidic material that exhibits superior catalytic
activity than 100% sulfuric acid in many reactions [1–4].
It has been widely used to catalyze hydrocarbon isomer-
ization, methanol conversion to hydrocarbons, alkylation,
acylation, esterification, etherification, condensation, nitra-
tion, cyclization, etc. [5,6]. A variety of methods have been
used to synthesize sulfated zirconia, which differ mainly in
the type of precursors, precipitating agents, sulfation pro-
cedure, and calcination temperature. The catalytic activity
of sulfated zirconia is mainly dependent on the precursor,
type of sulfating agent, and calcination temperature. High
sulfur content in the form of sulfate ions is introduced with
H2SO4 as a sulfating agent in comparison with (NH4)2SO4,
(NH4)2S2O3, (NH4)2S, SO3, SO2, H2S, etc. [6]. Different
methodologies are used to increase the sulfur content in zir-
conia with crystalline stability toward the tetragonal phase of
zirconia but only a maximum of 4% w/w sulfur content was
achieved (Table 1) [7–14]. Some control of sulfate loading
*
Corresponding author.
E-mail addresses: gdyadav@yahoo.com, gdyadav@udct.org
(G.D. Yadav).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.02.021
2004 Elsevier Inc. All rights reserved.

G.D. Yadav, A.D. Murkute / Journal of Catalysis 224 (2004) 218–223
219
Table 1
Preparation procedure of S–ZrO
2
a
Sulfating agent
Preparation method
Impregnation method
Sulfur content
(% w/w)
Calcination temperature
Nature of the phases
T
Ref.
◦
( C)
0.25 M H SO
2.5
600
[8]
2
4
0.05 to 2 M H SO
Percolation
Percolation
Percolation
Percolation
1.48
0.81
0.45
0.16
620
620
620
620
T
T
T
T
[9]
[9]
[9]
[9]
2
4
(NH ) SO
4 2
4
(NH ) S O
4 2
2 3
(NH ) S
4 2
0.5 M H SO
Controlled impregnation
Controlled impregnation
Controlled impregnation
1.43
3.42
5.64
580
580
580
T
T
[12]
[12]
[12]
2
4
4
4
0.5 M H SO
2
0.5 M H SO
2
T + M
0.5 M H SO
2
Colloidal-sol–gel (using CH COOH
3
peptizing agent + 3 weeks aging)
4.6
650
T+M + Zr(SO )
[13]
4
4
4
4 2
0.5 M H SO
2
Colloidal-sol–gel (Zr(OH) reflux
2.2–3.7
650
T + M + Zr(SO )
[13]
4
4 2
in H SO )
2
4
0.5 M H SO
Immersing Zr(OH) in 0.5 M H SO
4
4%
9%
650
650
T
T
[14]
2
4
2
0.5 M solution of
Immersing Zr(OH) in 0.5 M
4
This work
chlorosulfonic acid and
ethylene dichloride
solution of chlorosulfonic acid
and ethylene dichloride
a
T, tetragonal phase, M, monoclinic phase.
gel technique and impregnation. The colloidal sol–gel tech-
nique leads to the formation of sulfated zirconia with a high
amount of sulfur content along with mono and polynucle-
ate sulfate species as well as supported sulfuric acid. All
previous literature suggests that S–ZrO2 has been prepared
so far with a maximum 4% w/w of sulfur with preservation
of the tetragonal phase of zirconia and above this value the
tetragonal phase is strongly affected. Thus, it will be most
advantageous to synthesize sulfated zirconia with a high
sulfur content particularly above 4% with a pure tetrago-
nal phase to exhibit high superacidity. We report, for the first
time, a preparation of sulfated zirconia with a high sulfur
content (9%), possessing better activity including preserva-
tion of its tetragonal phase by using chlorosulfonic acid as
a new source for sulfate ions. This new catalyst is desig-
nated as UDCaT-5 and the conventional sulfuric acid-treated
catalyst as S–ZrO2 in this investigation.
ous ammonia at room temperature at a pH of 9–10. The
precipitated zirconium hydroxide was washed with deion-
ized water until a neutral filtrate; the absence of chlorine ion
was detected by phenolphthalein and AgNO3 tests. A mater-
ial balance on Cl before and after precipitation and washing
shows no retention on the solid. Zirconium hydroxide was
dried in an oven for 24 h at 100 ◦C and was crushed to
100 mesh size.
Three catalysts, UDCaT-5a, UDCaT-5, and UDCaT-5b,
were synthesized: (i) UDCaT-5a, Zr(OH)4 immersed in
6 cm3/g of 0.5 M solution of chlorosulfonic acid and eth-
ylene dichloride solution; (ii) UDCaT-5, Zr(OH)4 immersed
in 15 cm3/g of 0.5 M solution of chlorosulfonic acid and
ethylene dichloride; (iii) UDCaT-5b, Zr(OH)4 immersed in
50 cm3/g of 0.5 M solution of chlorosulfonic acid and eth-
ylene dichloride. All materials were immersed for 5 min in
the solution and then without allowing moisture absorption
were kept in an oven and the heating was started to evaporate
the solvent and then to a temperature of 120 ◦C after about
30 min. These materials were kept in oven at 120 ◦C for 24 h
and calcined at 650 ◦C for 3 h to get the active catalysts.
Sulfated zirconia (S–ZrO2) was prepared by immersing
Zr(OH) in 15 cm3/g of 0.5 M of sulfuric acid followed
by drying at 120 ◦C for 24 h and calcination at 650 ◦C for
3 h [14].
2. Experimental
2.1. Chemicals
Zirconium oxychloride, aqueous ammonia solution, sul-
furic acid, toluene, and benzyl chloride were procured
from M/s. s. d. Fine Chemicals (India). p-tert-Butylcyclo-
hexanol (PTBCH) and chlorosulfonic acid were purchased
from Lancaster and Spectrochem (India), respectively.
n-Hexane was procured from Merck (Germany).
The molarity of solution of chlorosulfonic acid in ethyl-
ene dichloride used to get UDCaT-5 was the same as that of
aqueous solution of sulfuric acid used to get S–ZrO2.
2.3. Reaction procedure and analysis
2.2. Catalyst preparation
Zirconium hydroxide was prepared by hydrolysis of
aqueous zirconium oxychloride (ZrOCl2·8H2O) with aque-
Vapor-phase isomerization of n-hexane was conducted in
a downflow fixed-bed reactor at WHSV of 3 h−1 at 250 ◦C

220
G.D. Yadav, A.D. Murkute / Journal of Catalysis 224 (2004) 218–223
using H2 as carrier gas and the effluent stream was ana-
lyzed online by GC (GC1000 Chemito) equipped with a
capillary column and FID detector. Liquid-phase reactions
were conducted in a glass reactor of 5 i.d. and 10 cm height
with four glass baffles and a four-bladed disk turbine im-
peller located at a height of 0.5 cm from the bottom of the
vessel and mechanically agitated with an electric motor. In
a typical alkylation reaction, 0.5 mol toluene was reacted
with 0.05 mol benzyl chloride with a catalyst loading of
0.018 g/cm3, 1000 rpm, and 90 ◦C. Esterification reactions
were conducted with 0.02 mol PTBCH and 0.60 mol acetic
acid with a catalyst loading of 0.025 g/cm3 at a speed of
1000 rpm at 90 ◦C and analyzed by a Chemito gas chro-
matograph equipped with a stainless-steel column (diameter
1/8 inches and length 4 m) packed with liquid stationary
phase of 10% OV-17 on Chromosorb WHP, and a FID de-
tector.
other compounds may evolve from the surface than just the
original probe molecule. In some cases probe or the sulfate
decomposes when acidity of metal ion promoted S–ZrO2
is determined by ammonia-TPD [16,17]. However, decom-
position of sulfate is observed above 600 ◦C during TPD
analysis for unpromoted sulfated zirconia by using NH3 as
a probe [18,19]. We used ammonia-TPD (Autochem 2920,
Micromeritics) to determine the acid strength of the conven-
tional sulfuric acid-treated zirconia (S–ZrO2) and chlorosul-
fonic acid-treated zirconia materials, UDCaT-5a, UDCaT-5,
and UDCaT-5b. Fig. 1A depicts that S–ZrO2 exhibits two
peaks, one at 180 ◦C and another at 450 ◦C corresponding to
intermediate and strong acid strengths. Similar types of in-
termediate and strong acid strengths are reported for S–ZrO2
by others [18,19]. Corma et al. [20] have reported genera-
tion of a superacidic peak at 550 ◦C in ammonia-TPD for
S–ZrO2 calcined at 550 ◦C for 3 h. It is well known that
the calcination temperature is a very important parameter
and S–ZrO2 calcined at 550 ◦C has a different acid strength
distribution than that calcined at 650 ◦C. Thus S–ZrO2 syn-
thesized and calcined at 550 ◦C by Corma et al. exhibits
superacidic centers at 550 ◦C in ammonia-TPD. The total
number of acid sites of S–ZrO2 (0.433 mmolg−1) prepared
by our procedure was almost the same as that of S–ZrO2
(0.420 mmolg−1) prepared by Corma et al. [20]. This con-
firms that the acid strength of S–ZrO2 prepared by us also
possesses the same type of acidity reported in the literature.
3. Results and discussion
3.1. Characterization
3.1.1. Ammonia-TPD
Temperature desorption using ammonia as a probe has
been proposed as a measurement of the total acidity and acid
strength distribution in solids [15]. It has been reported that
Fig. 1. (A) Ammonia-TPD of S–ZrO and UDCaT-5. (B) FTIR of S–ZrO and UDCaT-5.
2
2

G.D. Yadav, A.D. Murkute / Journal of Catalysis 224 (2004) 218–223
221
In the case of UDCaT-5a well-defined peaks were ob-
tained at 210 and 450 ◦C which indicate that it possesses
strong and very strong acid strength, respectively. Fur-
ther, the total number of acid sites of UDCaT-5a was
0.403 mmolg−1 which was about 7% lower than that for
S–ZrO2 (0.433 mmolg−1). However, UDCaT-5a possesses
a higher acid strength than S–ZrO2 which is evident from the
higher NH3 desorption temperature for the former. It should
be noted that a lower volume (6 cm3/g) of 0.5 M solution
of chlorosulfonic acid and ethylene dichloride was used to
treat Zr(OH)4 for preparation of UDCaT-5a than the vol-
ume (15 cm3/g) of 0.5 M sulfuric acid use to treat Zr(OH)4
for preparation of sulfated zirconia (S–ZrO2). A completely
different type of spectra was obtained for UDCaT-5 and
UDCaT-5b vis-à-vis S–ZrO2. The most striking feature of
the spectra of the UDCaT-5 and UDCaT-5b is the generation
of an additional peak at 550 ◦C in both. Corma et al. [20]
assign this peak to superacidic centers of S–ZrO2 calcined
at 550 ◦C. It is thus inferred here that there is a gener-
ation of superacidic centers in UDCaT-5 and UDCaT-5b,
even though they were calcined at 650 ◦C for 3 h and the
same peak is not observed in S–ZrO2 calcined at 650 ◦C.
The total acid sites of each UDCaT-5 (0.584 mmol g−1) and
UDCaT-5b (0.623 mmolg−1) are much higher than that of
S–ZrO2 (0.433 mmol g−1) prepared by us and by Corma
et al. (0.420 mmol g−1). The acidity of both UDCaT-5 and
UDCaT-5b is dependent on the molar strength of chloro-
sulfonic acid and is more superacidic than S–ZrO2. Hence,
it was thought desirable to characterize UDCaT-5 through
FTIR, XRD, BET surface area, pore-size analysis, and ele-
mental analysis and its activity was compared with that of
S–ZrO2.
3400 cm−1 (not shown) corresponds to the stretching vibra-
tion νOH of the hydroxyl group. IR bands between 700 and
415 cm−1 are characteristics of crystalline zirconia. It may
thus be concluded that during the calcination at 650 ◦C, con-
densation of the hydroxyl group of Zr(OH)4 occurs leading
to a crystalline zirconia without any change of the nature of
sulfate species. The IR spectra of the S–ZrO2 show a similar
pattern as UDCaT-5, indicating the presence of the biden-
tate chelating sulfate group coordinated to zirconia [6,14].
IR spectra confirm that chlorosulfonic acid is decomposed
during calcination at 650 ◦C and sulfate ions are retained on
the surface of the UDCaT-5.
3.1.4. X-ray diffraction
Powder XRD (Phillips PW1729) was used to elucidate
the crystalline phase of sulfated zirconia and UDCaT-5
(Fig. 2A). It is generally observed that pure zirconia trans-
forms into a monoclinic phase from a tetragonal phase above
the calcination temperature of 600 ◦C [6]. This transfor-
mation is prevented in both S–ZrO2 and UDCaT-5 due to
doping of SO42−. Preservation of the tetragonal crystalline
phase, which is essential for catalytic behavior in sulfated
zirconia, has been well explained in earlier literature [6] but
this phenomenon was also evident in the case of UDCaT-5.
The crystal structure of the sulfated zirconia is also affected
by the sulfur content and the low or medium sulfate content
zirconia crystallizes only in the tetragonal form, whereas for
sulfate content zirconia (S > 4%), it exists in a monoclinic
form, in addition to the major tetragonal form [12]. How-
ever, the crystal structure of the zirconia is not affected due
to the high sulfur content in UDCaT-5 and the quantity of
the tetragonal phase of zirconia in UDCaT-5 is the same as
that in S–ZrO2.
3.1.2. Elemental analysis
It was expected that during the thermal treatment of
UDCaT-5, chlorosulfonic acid would decompose to sulfuryl
chloride, pyrosulfuryl dichloride, sulfuric acid, sulfur diox-
ide, chlorine, and water [21]. The elemental analysis showed
a complete absence of Cl species and that 9% w/w S as sul-
fate was retained on the surface of UDCaT-5. This implied
that sulfuryl chloride and pyrosulfuryl dichloride were not
formed on the surface of the UDCaT-5. In contrast, only 4%
of S in the form of sulfate ion is retained on S–ZrO2 when
sulfation of zirconia is done through sulfuric acid.
3.1.5. BET surface area and pore-size analysis
The BET surface area (ASAP 2010, Micromeritics) of
UDCaT-5 is lower than that of S–ZrO2 (Table 2, S. No. 2).
Fârcas¸iu et al. [12] had observed that the surface area of
S–ZrO2 gradually increases at a low sulfate content up to
4% w/w (119 m2/g) but it decreases abruptly at the maxi-
mum sulfate content of 5.64% (71 m2/g) due to migration
of sulfate ions to the bulk phase of zirconia. A similar rea-
son could have been ascribed in our case. However, XRD
results indicate that the tetragonal phase of zirconia, which
is essential for exhibiting superacidity, is retained even af-
ter high sulfate ions were introduced on the surface of the
UDCaT-5. Furthermore, migration of the sulfate group into
the bulk phase of the zirconia would have resulted in ex-
tensive formation of a monoclinic phase of zirconia and
zirconium sulfate [12], but it was not observed in the XRD.
Therefore, the retention of high sulfate ions on the surface of
the UDCaT-5 without changing the crystalline phase of the
zirconia must follow a different mechanism. It is reported
that the precursor Zr(OH)4 should be made more reactive
to obtain a high sulfate ion content on zirconia and it is ac-
complished by prolonged heating of suspension containing
3.1.3. FTIR
The IR spectra (Shimadzu) of UDCaT-5 shows a pattern
2−
of four bands assigned to bidentate SO4 ions in C2ν sym-
metry with ν3 at 1218, 1152, and 1066 and ν1 at 997 cm−1
coordinated to zirconia (Fig. 1B). The absence of a band at
1400 cm−1 indicates that there is no formation of polynu-
2−
clear sulfates S2O7 on the surface of UDCaT-5. The band
at 1631–1642 cm−1 is attributed to δO–H-bending frequency
of water molecules associated with the sulfate group [22],
suggesting that chlorosulfonic acid is decomposed during
calcination of the UDCaT-5. An additional broad band at

222
G.D. Yadav, A.D. Murkute / Journal of Catalysis 224 (2004) 218–223
Fig. 2. XRD of S–ZrO and UDCaT-5. (B) Time stream of isomerization of n-hexane on S–ZrO and UDCaT-5.
2
2
Table 2
Acidity and activity of catalysts
S. No.
Parameters
Catalysts
S–ZrO
UDCAT-5a
UDCAT-5
UDCAT-5b
2
−1
1
2
3
Acidity by NH -TPD (mmol g
)
0.433
103
2
0.403
−
0.584
83
4
0.623
−
3
2
−1
BET surface area (m g
)
Isomerization of n-hexane,
conversion % (after 20 min)
−
−
4
5
6
Benzylation of toluene, initial activity
8
−
−
−
15
97
76
−
−
−
6
−1 −1
×10 (mol s
g )
Benzylation of toluene, conversion of
BzCl % after 1 h
79
53
Esterification of PTBCH, conversion of
PTBCH % after 4 h
Zr(OH)4 at reflux in H2SO4 [13]. Also a prolonged contact
of Zr(OH)4 with sulfuric acid and use of peptizing agent
such as acetic acid for 3 weeks are reported to result in
a high sulfate content [13]. We reasoned that to introduce
a higher sulfate content, chlorosulfonic acid could be used
as the sulfating agent since it would be more strongly ad-
sorbed on the Zr(OH)4 than sulfuric acid and it would be
possible to avoid the use of a peptizing agent with extensive
heating under reflux conditions. Further, chlorosulfonic acid
would be decomposed during calcination into sulfuryl chlo-
ride, pyrosulfuryl dichloride, sulfuric acid, and higher poly-
acids leaving more sulfur in the samples than that in sulfuric
acid-treated zirconia. In fact, preparation of UDCaT-5 using
chlorosulfonic acid showed that there was no formation of
the polynucleate sulfate ion and also there was no Cl residue
in the sulfate ions of UDCaT-5, which was confirmed by
the FTIR spectra and elemental analysis. Thus, we infer that
this decomposition process occurs with a corresponding de-
crease in surface area and not due to migration of sulfate
ions into the bulk phase of zirconia. Another argument in
favor of this supposition is that the XRD of both catalyst
S–ZrO2 and UDCaT-5 contain monoclinic and tetragonal
phases. The diffractograms show the same amount of tetrag-
onal phase of zirconia in both samples.

G.D. Yadav, A.D. Murkute / Journal of Catalysis 224 (2004) 218–223
223
3.2. Catalytic activity and stability of UDCaT-5
4. Conclusion
We report, for the first time, that chlorosulfonic acid treat-
ment of zirconia resulted into formation of a novel catalyst,
named UDCaT-5 which is superior to sulfuric acid-treated
S–ZrO2. Despite the fact that the same strength and volume
of chlorosulfonic acid was used to get UDCaT-5 as those
used for sulfuric acid to get S–ZrO2, UDCaT-5 contains
more sulfate ions, is more stable, and also is more active in
comparison with S–ZrO2. The characterization and reaction
tests support the above results.
3.2.1. Isomerization of n-hexane
The initial activity of UDCaT-5 was twice that of S–ZrO2
in n-hexane isomerization (Table 2, S. No. 3 and Fig. 2B).
Although UDCaT-5 deactivates completely after 1 h, its de-
activation is much slower than that of S–ZrO2 which de-
activates completely in 20 min. The high catalytic activ-
ity of UDCaT-5 in the isomerization of n-hexane is due
to the high acid strength of UDCaT-5. The selectivity of
2-methylpentane, 3-methylpentane, 2,2-dimethylbutane,and
2,3-dimethylbutane is together 77% for both catalysts. The
acidic catalysts require metal doping in isomerization reac-
tions to overcome deactivation and it is a matter of a separate
study.
Acknowledgments
G.D.Y. acknowledges funding from the Darbari Seth
Professorship Endowment and NMITLI programme of the
CSIR, New Delhi (2001-03).
3.2.2. Benzylation of toluene with benzyl chloride
It was observed that the initial rate of reaction with
UDCaT-5 was almost twice than that with S–ZrO2 (Table 2,
S. Nos. 4 and 5). The reusability of the catalyst was also
tested thrice in the case of the benzylation of toluene. It was
observed that UDCaT-5 shows the same activity after the
third use which confirms the stability of the UDCaT-5 even
in the presence of the corrosive HCl generated in situ.
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acid
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water, the pH of the suspension decreases quickly due to
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identification of sulfate ions in the aqueous phase [19]. The
same test was conducted for UDCaT-5 for fresh and reused
catalyst by pH measurement and it was found that there
was no leaching of sulfate ions from UDCaT-5. PTBCHA
is widely used in perfumery along with ionones, cedar-
wood products, and floral and nonfloral perfume chemicals
and is a very versatile perfume material for soap and fra-
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with acetic acid was chosen to study the efficacy and stability
of the UDCaT-5 in the presence of water generated in situ.
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to S–ZrO2 (Table 2, S. No. 6) for this reaction. In order to
study the stability of the UDCaT-5 the catalysts was reused
thrice and it was found that UDCaT-5 retained its activity
in all three experiments. Thus, there was no leaching of the
sulfate ion from UDCaT-5 and it is a stable catalyst in com-
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