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Chemistry Letters Vol.33, No.11 (2004)
A New Solid Superacid Catalyst Prepared by Doping ZrO2 with Ce and Modifying
with Sulfate Simultaneously
Jong Rack Sohn,ꢀ Jun Seob Lim, and Si Hoon Leey
Department of Applied Chemistry, Engineering College, Kyungpook National University, Taegu 702-701, Korea
yEnvironment Research Team, Research Institute of Industrial Science and Technology, Pohang 790-330, Korea
(Received June 30, 2004; CL-040764)
A new solid superacid catalyst, Ce–ZrO2/SO42ꢁ, having
high surface area and thermal stability even after calcination at
temperatures of 650–700 ꢂC, is prepared simply by doping ZrO2
with Ce and modifying with sulfate simultaneously. The role of
Ce is to form a thermally stable solid solution with zirconia and
consequently to give high surface area of the sample.
band at 1390 cmꢁ1 accompanied by broad and intense bands
below 1200 cmꢁ1 due to the overlapping of the ZrO2 skeletal vi-
bration, thereby, indicating the presence of differently adsorbed
species depending on the treatment conditions of the sulfated
sample.7,8
The strong intense band at 1390 cmꢁ1 after evacuation at
500 ꢂC is related to the superacidic property,7,8 which is attribut-
able to the double-bond nature of the S=O in the complex
formed by the interaction between Ce–ZrO2 and sulfate. The
Many kinds of solid acids have been found; their acidic
properties, their catalysis, and the structure of acid site have been
elucidated and those results have been reviewed by several
workers.1–3 The preparation and use of solid superacid catalysts
are active areas of research for isomerization, cracking, hydro-
cracking, dehydration, acylation, and a process from methanol
to gasoline, etc.2 Zirconium oxide, ZrO2 is a very interesting ma-
terial because of its thermal stability, its mechanical properties,
and its basic, acidic, reducing, and oxidizing properties.4 The po-
tential for a heterogeneous catalyst has yielded many papers on
the catalytic activity of sulfated zirconia materials.1–3,5 Sulfated
zirconia deped with Fe and Mn has been shown to be highly ac-
tive for butane isomerization, catalyzing the reaction even at
room temperature.6 However, it is known that for zirconia-sup-
ported catalyst its surface area and catalytic activity are decreas-
ing under the severe reaction condition such as high temperature
above 600 ꢂC. In this paper we report a new solid superacid cat-
alyst prepared by doping ZrO2 with Ce and modifying with sul-
fate simultaneously to improve catalytic activity and thermal sta-
bility. For the acid catalysis, the cumene dealkylation was used
as a test reaction.
2ꢁ
acid strength of Ce–ZrO2/SO4 samples after evacuation at
500 ꢂC for 1 h was also examined by color change method, using
Hammett indicator in sulfuryl chloride.9 The samples were esti-
mated to have Ho ꢃ ꢁ14:5, indicating the formation of super-
acidic sites. The infrared spectra of ammonia adsorbed on
5Ce–ZrO2/SO4 sample evacuated at 500 ꢂC for 1 h showed
2ꢁ
two characteristic bands (1444 and 1620 cmꢁ1) of ammonia
adsorbed on both Bronsted and Lewis acid sites, indicating the
¨
presence of both Bronsted and Lewis acid sites. So, we suggest
¨
2ꢁ
a model structure for the acid sites on Ce–ZrO2/SO4 as fol-
lowing scheme.
2ꢁ
Figure 1 shows the BET surface areas of 5Ce-ZrO2/SO4
and 10Ce–ZrO2/SO42ꢁ as a function of calcination temperature.
It was found surprisingly that the surface areas of catalysts cal-
The precipitate of Zr(OH)4 was obtained by adding aqueous
ammonia slowly into an aqueous solution of zirconium oxy-
chloride (Junsei Chemical Co.) at room temperature with stirring
until the pH of mother liquor reached about 8. The preparation of
catalyst doped with Ce and modified with sulfate simultaneously
was carried out by adding an acidic aqueous solution of cerium
.
sulfate [Ce(SO4)2 4H2O] to the Zr(OH)4 powder followed by
drying and calcining at high temperatures for 2 h in air. 5Ce–
2ꢁ
ZrO2/SO4 indicates the catalyst containing 5 wt % Ce(SO4)2.
Chemisorption of ammonia was also employed as a measure of
the acidity of catalysts.7 Cumene dealkylation was carried out at
400 ꢂC in a continuous flow, fused quartz reactor having an inter-
nal diameter of 11 mm. Catalytic activity for cumene dealkyla-
tion was represented as mole of benzene converted from cumene
per gram of catalyst.
In general, metal oxides modified with sulfate ion,7,8 fol-
lowed by evacuating above 400 ꢂC, exhibit a strong band as-
signed to an S=O stretching frequency at 1390–1370 cmꢁ1
The infrared spectrum of self-supported 5Ce–ZrO2/SO4 after
evacuation at 500 ꢂC for 1 h was examined. There was an intense
.
Figure 1. Variations of surface area of 5Ce–ZrO2/SO42ꢁ(a)
and 10Ce–ZrO2/SO42ꢁ(b) as a function of calcination tempera-
ture.
2ꢁ
Copyright ꢀ 2004 The Chemical Society of Japan