Catalytic properties of the thermal decomposition products of Zr(SO4)2 · 4H2o

Article abstract of DOI:10.1023/A:1015337031791

The catalytic activity of the thermal decomposition products of Zr(SO₄)₂ · 4H₂O in the reactions of 1-butene isomerization to 2-butenes, isobutanol dehydration, and n-butane skeletal isomerization was studied. Their behavi

Full text of DOI:10.1023/A:1015337031791

Kinetics and Catalysis, Vol. 43, No. 2, 2002, pp. 280–283. Translated from Kinetika i Kataliz, Vol. 43, No. 2, 2002, pp. 305–309.
Original Russian Text Copyright © 2002 by Kotsarenko, Shmachkova.
Catalytic Properties of the Thermal Decomposition
Products of Zr(SO ) · 4H O
4
2
2
N. S. Kotsarenko and V. P. Shmachkova
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received April 12, 2000
Abstract—The catalytic activity of the thermal decomposition products of Zr(SO ) · 4H O in the reactions of
4
2
2
1
-butene isomerization to 2-butenes, isobutanol dehydration, and n-butane skeletal isomerization was studied.
Their behaviors in typical acid reactions and in skeletal isomerization were found to be considerably different.
In the first two reactions, which occur with the participation of proton sites, the activity of zirconium sulfates
was an extremal function of hydrate calcination temperature. Zirconium sulfate calcined at 400–550°ë was the
most active catalyst. The reasons for such behavior are discussed. In the skeletal isomerization of n-butane,
crystalline zirconium sulfate was practically inactive, and it became active only after degradation. The results
suggest that the activation of n-butane molecules did not occur at proton sites.
INTRODUCTION
[6]. The samples were calcined in a drying oven at tem-
peratures lower than 200°ë for 10 h or in a muffle fur-
nace at higher temperatures for 4 h.
The samples were studied by thermal and X-ray dif-
fraction (XRD) analysis. The specific surface area and
catalytic activity of the samples were also measured.
Sulfated zirconia is classified with superacid solids
because of its ability to catalyze the skeletal isomeriza-
tion of alkanes at low temperatures (20–100°C) [1–3].
Despite the great number of relevant publications, the
mechanism of action of this class of catalysts is not
clearly understood. However, there is no doubt that the
The XRD analysis was performed on a DRON
interaction between catalyst components with the for- instrument using monochromatic CuK radiation. Dif-
α
mation of surface sulfates is responsible for the appear- fraction patterns were recorded on a recorder at a goni-
ance of high activity. In this context, it is of interest to ometer rate of 1 deg/min. Phases were identified using
study the activity of zirconium sulfate (a compound the ASTM file.
with known chemical composition and structure) and
its thermal decomposition products in the skeletal
isomerization of alkanes. The assumed superacidity
raises the question of the relationship between the cat-
alytic properties of sulfated zirconia and the properties
of well-studied catalysts in heterolytic reactions whose
rates depend on the strength of acid sites.
The specific surface areas of samples were deter-
mined by the low-temperature adsorption of nitrogen.
Catalytic properties in the reactions of 1-butene
isomerization to 2-butenes, isobutanol dehydration,
and n-butane skeletal isomerization were measured in a
gradientless flow reactor under standard conditions.
The isomerization of 1-butene was performed at 220°C
and an initial 1-butene concentration of 7 vol % in a
mixture with nitrogen. The activity was characterized
as the rate of reaction at 20% olefin conversion [4]. The
reaction rate of alcohol dehydration was measured at
In this work, we studied the catalytic properties of
the thermal decomposition products of Zr(SO ) · 4H O
4
2
2
in the reactions of 1-butene isomerization to 2-butenes,
isobutanol dehydration, and n-butane skeletal isomer-
ization. As found previously [4, 5], the first reaction pri-
marily occurs at proton sites, and the second occurs at
both proton sites and paired (aprotic–base) centers. The
rates of these reactions increase with the strength of
active centers. Changes in the activity of the test cata-
lysts in these reactions and in n-butane skeletal isomer-
ization can give us an insight into the mechanism of the
skeletal isomerization reaction.
300°ë and an initial alcohol concentration of 10–50 vol %
and characterized by the rate constant of reaction [5].
The reaction of n-butane skeletal isomerization was
studied at 150°ë and an initial n-butane concentration
of 24 vol % in a mixture with nitrogen. The activity in
the skeletal isomerization was characterized by a max-
imum reaction rate attained in the transformation of
n-butane under the test conditions.
Before testing, the catalysts were calcined in the
reactor in a flow of air at a given temperature and then
EXPERIMENTAL
–2
evacuated at a pressure of ~10 torr. Thereafter, the
The starting compound used for preparing anhy- setup was filled with dry nitrogen, and the temperature
drous crystalline zirconium sulfate was Zr(SO ) · was decreased to a reaction temperature in a nitrogen
4
2
4
H O synthesized according to the published procedure flow.
2
0
023-1584/02/4302-0280$27.00 © 2002 MAIK “Nauka /Interperiodica”

CATALYTIC PROPERTIES OF THE THERMAL DECOMPOSITION PRODUCTS
281
Effect of the calcination temperature of zirconium sulfate hydrate on the specific surface area (S) and the reaction rates (w)
of 1-butene isomerization, isobutanol dehydration, and n-butane isomerization
w, mol g–1 h–1
2
Calcination temperature, °C
S, m /g
1
-butene
isobutanol
n-butane
270
300
350
400
450
500
550
600
700
4.8
7.3
7.0
8.3
5.0
11
0.40
0.73
0.75
0.83
0.73
2.50
4.20
7.00
2.60
–
–
–
–
0.3
1.7
–
–
1.1 × 10–6
–
2.4
–
1.2 × 10–6
7.0 × 10–6
8.2 × 10–6
1.5 × 10–3
12
48
6.5
3.9
104
RESULTS AND DISCUSSION
In this respect, the behavior of zirconium sulfate is sim-
ilar to that of aluminum sulfate [7].
Owing to the different orders of reaction, the activi-
ties of catalysts were compared using the reaction rate
Endothermic effects at 185, 220, and 290°ë due to
water removal and an endothermic effect at 775°ë due
to sulfate decomposition were observed on heating
Zr(SO ) · 4H O. The decomposition began at 660°ë
(
w) measured under standard conditions at a 20% con-
4
2
2
version of 1-butene (see Experimental). The table sum-
marizes the results.
and ended at 820°ë. The weight losses up to 325°ë cor-
responded to the removal of 3.94 mol of water;
The reaction products are cis-2-butene and trans-2-
butene. The ratio between them depends only on the
conversion of 1-butene, and it is independent of both
0
.06 mol was removed at higher temperatures without
a pronounced effect.
According to XRD data, the crystal hydrate calcined the calcination temperature of the initial hydrate and
at 300°ë was anhydrous crystalline zirconium sulfate, the degree of catalyst deactivation. As the conversion of
whereas the calcination at 600°ë afforded a mixture of 1-butene increased, the concentration ratio between cis-
the crystalline sulfate and tetragonal zirconia. A sample 2-butene and trans-2-butene proportionally decreased
calcined at 700°ë was a mixture of tetragonal and mon- to an equilibrium value. The initial ratio of the above
oclinic zirconia with 4.3 wt % SO3.
linear function was 0.90 ± 0.05, as determined by
extrapolation to zero conversion. Thus, a mixture of
The table summarizes the specific surface areas of
the crystal hydrate calcined at different temperatures. It
can be seen that the surface area of zirconium sulfate
2
-butenes, which contained both of the isomers in
almost equal amounts, was the primary product of
1-butene isomerization on crystalline zirconium sulfate.
2
was low (5–11 m /g). It noticeably increased after cal-
In the tested range of calcination temperatures, the
cination at temperatures higher than 550°ë because of
sulfate decomposition and zirconia formation. The
sample calcined at 700°ë, which had the lowest con-
centration of sulfate sulfur, exhibited the highest sur-
face area. Thus, the specific surface area can serve as a
criterion for the phase purity of zirconium sulfate.
activity referenced to 1 g of catalyst changed by a factor
of almost 20 and reached a maximum value in a sample
calcined at 600°ë (see table). The figure demonstrates
the specific activity as a function of the calcination tem-
perature of the initial hydrate. As can be seen, in this
case, the activity is an extremal function of hydrate cal-
The thermal decomposition products of zirconium cination temperature. However, in the range of exist-
sulfate hydrate exhibited a catalytic activity in the reac- ence of the crystalline sulfate (300–550°ë), this change
tion of 1-butene isomerization. In the course of reac- was not so high (by a factor of ~3.5). It follows that the
tion, an insignificant decrease in the activity (by no calcination temperature not only affected the catalytic
more than 5% in 1 h), which is typical of acid catalysts, properties by changing the specific surface area, but it
was observed. The hydrate calcination temperature also had a certain effect on the state of the surface and
affected the activity and the dependence of the reaction its active centers. It was found previously that double
rate on the steady-state concentration of 1-butene. The bond isomerization primarily occurs at proton sites and
order of reaction was equal to 0 on samples calcined at the reaction rate depends on their strength and concen-
2
70 and 300°ë, whereas the order of reaction was equal tration. This is a test reaction for surface proton sites
to 1 on samples calcined above 450°ë. Within the inter- [4]. Based on these facts, it is believed that residual
mediate range of sample calcination temperatures, the water has an inhibiting effect in samples calcined at
order of reaction on these samples varied from 0 to 1. lower temperatures, as was observed in aluminum sul-
KINETICS AND CATALYSIS Vol. 43 No. 2 2002

2
82
KOTSARENKO, SHMACHKOVA
w, mol m–2 h–1
decomposition products of zirconium sulfate to be sig-
nificantly different.
0
0
0
0
.4
.3
.2
.1
1
2
A possible reason for the lower activity of high-tem-
perature samples is butene oligomerization, which also
occurs at proton sites and depends on their strength.
This assumption is inconsistent with the behavior of a
wide variety of acid catalysts with different strengths of
acid sites in the above reaction. As catalysts do not
undergo deactivation instantaneously, their activity in
double-bond isomerization depends on the reaction rate
extrapolated to the onset of reaction. If it is believed
that the strongest sites are deactivated instantaneously,
it is unlikely that a relationship between the strength of
sites and the activity of a single site similar to that
described previously [13] can be obtained. This rela-
tionship also describes data obtained more recently
with sulfated zirconia and very strong solid acids, such
as heteropoly acids, the strength of proton sites in
which is 1120 kJ/mol on a PA scale.
0
2
00 300
400
500 600 700
T, °C
Effect of the temperature of zirconium sulfate calcination
on the specific activity in the reactions of (1) 1-butene
isomerization and (2) isobutanol dehydration.
Zirconium sulfate also exhibited high activity in the
reaction of isobutanol dehydration. Its behavior in this
reaction (the stability of its activity and the composition
of reaction products) is analogous to that of aluminum
sulfate [7]. The calcination temperature of the initial
crystal hydrate and the catalyst deactivation in the
course of reaction had no effect on the composition of
fate [7]. Water adsorbed on a proton site either fully
blocks it or dramatically decreases its strength. Both of
these processes can decrease the catalytic activity and
affect the kinetic characteristics of the reaction. It is
impossible to distinguish them based on the results of
this study.
A decrease in the specific activity of samples after high- the initially formed products. The reaction products are
temperature calcination is associated with sulfate degrada- all possible butenes; the fraction of normal olefins is
tion. The activity of a sample containing 4.3 wt % SO pre- 25–26%, and it is independent of both the temperature
3
–
2
–1
pared by sulfate calcination at 700°ë (0.025 mol m h ) of sample calcination and the conversion of the alcohol.
was close to the activity of sulfated zirconia containing The reaction rate per 1 g of catalyst is an extremal func-
the same amount of sulfate sulfur calcined at 500°ë tion of hydrate calcination temperature. It reached a
–
2
–1
(
0.03 mol m h ) [8]. A decrease in the activity in the maximum in a sample calcined at 600°ë. The maxi-
reaction that occurs at proton sites after the high-tem- mum difference in the activity within the tested range
perature treatment of sulfates is due to sulfate degrada- of calcination temperatures was by a factor of ~20
tion (after thermal treatment at 600°ë, a low-tempera- (see table).
ture tetragonal modification of zirconia was present in
The figure demonstrates the specific activity as a
the sample along with the crystalline sulfate, whereas
function of hydrate calcination temperature. It can be
the residual concentration of sulfate sulfur after treat-
seen that the specific activity of calcination products
ment at 700°ë was 4.3 wt %), which decreased the con-
remained constant over a wide range of hydrate calci-
centration of proton sites with no change in their qual-
nation temperatures (400–550°ë). The activity dramat-
ity. This assumption is based on the following facts:
ically decreased at both higher and lower temperatures
(1) The activity (and, consequently, strength) of pro-
of calcination. A decrease in the activity after high-tem-
perature treatment was due to sulfate degradation and
the formation of a zirconium oxide, which is inactive in
ton sites in anhydrous crystallineAl, Ga, and Zr sulfates
depends only slightly on the nature of the cation [9].
(
2) In anhydrous crystalline Al (SO ) and sulfated this reaction. It is likely that a crystal hydrate also
2 4 3
aluminum oxides containing different amounts of sul- occurred at a low temperature. In the case of this reac-
fate sulfur calcined at 400–650°ë, the activity of proton tion, the specific activities of the sulfate calcined at
sites has a constant value regardless of composition and 700°ë and sulfated zirconia [8] are similar.
calcination temperature. On a proton affinity (PA) scale,
Thus, anhydrous crystalline zirconium sulfate is a
the strength of these proton sites is 1170 kJ/mol [10].
highly active catalyst in the reactions of 1-butene
(
3) It was found by the IR spectroscopy of
isomerization and isobutanol dehydration. A common
feature in the behavior of the above catalyst in these
reactions is that the specific activity is an extremal
function of sulfate calcination temperature (figure).
The most probable reason for the inconstancy of the
adsorbed CO and pyridine that the strength of proton
and aprotic sites in sulfated zirconium oxides pre-
pared by different procedures is close to that in crys-
talline zirconium sulfate [11, 12].
Considering these facts, one would hardly expect specific activity in the region of the crystalline sulfate
the strength of proton sites in the high-temperature is the presence of a small amount of firmly bound water
KINETICS AND CATALYSIS Vol. 43 No. 2 2002

CATALYTIC PROPERTIES OF THE THERMAL DECOMPOSITION PRODUCTS
283
on the surface. Water, which is a stronger base than reaction rate should be low because of a low basicity of
1
-butene, forms hydrogen bonds with S–OH groups, the reactant. However, this assumption is inconsistent
with a dramatic increase in the activity of sulfate ther-
mal decomposition products, which have a much lower
concentration of proton sites, as evidenced by a lower
activity in the reactions of 1-butene isomerization and
isobutanol dehydration. Judging by the invariability of
the composition of alcohol dehydration products, apro-
tic sites did not qualitatively change upon the thermal
decomposition of crystalline zirconium sulfate. The
concentration of strong acid sites can only decrease
with decreasing sulfate sulfur content of the sample.
Thus, the activation of an n-butane molecule occurs via
a mechanism other than the protonation of reactant
which are active in the reaction, and 1-butene cannot
cleave these bonds. The removal of water by increasing
calcination temperature increased the activity. A maxi-
mum activity can also be attained by long-term calcina-
tion at a lower temperature, as was found with alumi-
num sulfate [7]. The basicity of the alcohol is higher
than the basicity of water. Therefore, the alcohol can
displace water from a hydrogen-bonded complex fol-
lowed by protonation, and the reaction of alcohol dehy-
dration can occur. However, this does not exclude the
possibility of crystal hydrate formation in the fine pores
of zirconium sulfate in the course of alcohol dehydration. molecules. Activation sites are almost absent from the
surface of crystalline zirconium sulfate. They are
Note that, in terms of specific activity, crystalline
formed on the surface of sulfated oxides prepared by
zirconium sulfate calcined at 400–550°ë is the most
the thermal decomposition of crystalline zirconium sul-
active catalyst among the catalysts tested previously in
these reactions [4, 5]. Crystalline aluminum sulfate is
closest in activity in the reaction of 1-butene isomeriza-
tion. The maximum activities of the above catalysts dif-
fered by a factor of 1.7, whereas the concentration of
proton sites on aluminum sulfate was higher by a factor
of 1.6, as measured using the IR spectra of protonated
pyridine [9, 10]. The activity of a proton site on crystal-
line zirconium sulfate was higher than the activity of a
site on aluminum sulfate by a factor of 2.7. It follows
that the strengths of proton sites on both of the crystal-
line sulfates differed only slightly, and zirconium sul-
fate has no strong (superacidic) sites, which are
assumed to occur on sulfated zirconia.
fate or by the sulfation of a hydroxide followed by high-
temperature calcination.
REFERENCES
1
2
. Arata, K., Adv. Catal., 1990, vol. 37, p. 165.
. Song, X. and Sayari, A., Catal. Rev.–Sci. Eng., 1996,
vol. 38, p. 329.
3. Corma, A., Chem. Rev., 1995, vol. 95, p. 597.
4
. Malysheva, L.V., Kotsarenko, N.S., and Paukshtis, E.A.,
Kinet. Katal., 1980, vol. 21, no. 2, p. 536.
5
. Kotsarenko, N.S. and Malysheva, L.V., Kinet. Katal.,
1
983, vol. 24, no. 4, p. 877.
The samples calcined at 400–700°ë were taken for
testing the activity in the reaction of n-butane skeletal
isomerization. The table summarizes the results. As can
be seen, the activity of samples calcined at 400–600°ë
was insignificant and much lower than the activity
of sulfated zirconium oxides, which is ~(6–15) ×
6. Handbuch der práparativen anorganischen Chemie,
Brauer, G., Ed., Stuttgart: Enke, 1978, vol. 4, p. 1470.
7. Kotsarenko, N.S., Shmachkova, V.P., and Mastikhin, V.M.,
Kinet. Katal., 1998, vol. 39, no. 4, p. 575.
8
9
. Shmachkova, V.P. and Kotsarenko, N.S., Kinet. Katal.
2002 (in press).
–
3
–1 –1
1
0 mol g h . The activity in this reaction appeared
. Shmachkova, V.P., Kotsarenko, N.S., Malysheva, L.V.,
and Paukshtis, E.A., Kinet. Katal., 1990, vol. 31, no. 4,
p. 1021.
only after calcination at 700°ë, when the sulfate under-
went degradation (the residual concentration of sulfate
sulfur was 4.3%). This activity was comparable to the
activity of sulfated zirconia in a tetragonal modifica-
tion.
1
1
0. Malysheva, L.V., Shmachkova, V.P., Kotsarenko, N.S.,
and Paukshtis, E.A., Kinet. Katal., 1991, vol. 32, no. 4,
p. 940.
1. Paukshtis, E.A., Shmachkova, V.P., and Kotsarenko, N.S.,
The behaviors of zirconium sulfate in typical acid
reactions (1-butene isomerization and isobutanol dehy-
React. Kinet. Catal. Lett., 2000, vol. 71, no. 2, p. 385.
dration) and in the skeletal isomerization of n-butane 12. Shmachkova, V.P., Paukshtis, E.A., Kotsarenko, N.S.,
et al., Catal. Lett., 2002 (in press).
were different, and they should be discussed in terms of
the possible mechanism of action. If the skeletal
isomerization of butane takes place at proton sites, the
1
3. Malysheva, L.V., Paukshtis, E.A., and Kotsarenko, N.S.,
Kinet. Katal., 1990, vol. 31, no. 1, p. 247.
KINETICS AND CATALYSIS Vol. 43 No. 2 2002