On the effect of the strength of acid sites in heterogeneous catalysts on the activity in the skeletal isomerization of n-butane

Article abstract of DOI:10.1023/B:KICA.0000038084.43894.cc

The catalytic activity of three groups of acid catalysts different in the nature and strength of acid sites in the skeletal isomerization of n-butane was studied. It was found that the strength of the sites did not correlate with the rate of the reaction.

Full text of DOI:10.1023/B:KICA.0000038084.43894.cc

Kinetics and Catalysis, Vol. 45, No. 4, 2004, pp. 554–557. Translated from Kinetika i Kataliz, Vol. 45, No. 4, 2004, pp. 587–591.
Original Russian Text Copyright © 2004 by Shmachkova, Kotsarenko, Paukshtis.
On the Effect of the Strength of Acid Sites
in Heterogeneous Catalysts on the Activity
in the Skeletal Isomerization of n-Butane
V. P. Shmachkova, N. S. Kotsarenko, and E. A. Paukshtis
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received March 28, 2003
Abstract—The catalytic activity of three groups of acid catalysts different in the nature and strength of acid
sites in the skeletal isomerization of n-butane was studied. It was found that the strength of the sites did not
correlate with the rate of the reaction.
INTRODUCTION
EXPERIMENTAL
The HZSM-5 zeolite (an SiO2/Al2O3 ratio of 50) was
prepared by hydrothermal synthesis and calcined at
500°C [10].
The H3PW12O40 heteropoly acid/Sibunit (henceforth
referred to as HPA), which was prepared and character-
ized by Paukshtis et al. [9], was used as a catalyst with
strong proton sites.
Crystalline aluminum sulfate and zirconium sulfate
were prepared by the calcinaton of the crystal hydrates
Al2(SO4)3 · 18H2O and Zr(SO4)2 · 4H2O, respectively,
which were preliminarily dried at 110°C, at 500–600°C
[13, 14].
Sulfated alumina (SO3/Al2O3) was prepared by the
incipient wetness impregnation of γ-Al2O3 with a sulfu-
ric acid solution [15].
Zirconium hydroxide and zirconium oxide were
used as parent compounds for the preparation of sul-
fated zirconia (SO3/ZrO2). Zirconium hydroxide was
obtained by precipitation from an aqueous solution of
zirconium oxychloride with an ammonia solution at
65–70°C and a constant pH 8. To prepare crystalline
zirconium oxide, the precipitate (hydroxide), which
was washed and dried at 110°C, was calcined at 500°C.
The hydroxide and the oxide were sulfated by incip-
ient wetness impregnation with a solution containing a
specified amount of sulfuric acid.
Fluorinated alumina and zirconia (F/Al2O3 and
F/ZrO2) were obtained by the incipient wetness impreg-
nation of corresponding oxides with an aqueous solu-
tion of ammonium fluoride [12].
Presently, the fact that the formation of carbenium
ions is a key step in the skeletal isomerization reactions
of alkanes is beyond question. These ions can be
formed via various reaction paths: the decomposition of
carbonium ions formed by the protonation of paraffins,
the elimination of a hydride ion from a paraffin mole-
cule at Lewis sites, and the formation of radical cations
followed by the elimination of a hydrogen atom. The
formation of carbenium ions with the participation of
proton and aprotic surface sites has been discussed
most intensively in the literature. Both types of these
sites occur on the surface of sulfated zirconium oxides,
which are the most active catalysts for low-temperature
skeletal isomerization [1–3]. However, their role in the
reaction is unknown. The results of studies of various
classes of catalysts for the low-temperature skeletal
isomerization of lower alkanes were presented in a
great number of publications. However, based on these
data, it is difficult to determine the role of different sites
because the strength and concentration of sites and the
activity in reactions are absent from publications [4–8].
The aim of this work is to compare the activity of a
wide range of acid catalysts, characterized in terms of
the strength and activity of sites, in the isomerization of
n-butane to isobutane. For solving this problem, we
used a uniform methodology for the characterization of
acid properties and measured the rates of reactions in
the same flow-circulation setup. The following three
groups of test catalysts with different acid properties
were chosen: (I) with only strong surface proton sites
(H3PW12O40 heteropoly acid/Sibunit [9]), (II) with
simultaneously occurring strong proton and Lewis sites
(ZSM-5 zeolite [10], crystalline aluminum sulfate [11],
crystalline zirconium sulfate; sulfated alumina [11],
sulfated zirconia [3], and fluorinated alumina [12]), and
(III) with only Lewis sites of different strength (fluori-
All of the samples after treatment with solutions
were dried in air and then at 110°C; thereafter, they
were calcined at different temperatures (from 400 to
600°C) in a flow of dry air.
The concentrations of sulfate sulfur in the sulfated
nated zirconia and zirconium oxides with tetragonal zirconia and alumina were determined. For this pur-
and monoclinic structures).
pose, the catalysts were dissolved in hydrofluoric acid
0023-1584/04/4504-0554 © 2004 MAIK “Nauka /Interperiodica”

ON THE EFFECT OF THE STRENGTH OF ACID SITES IN HETEROGENEOUS CATALYSTS
555
followed by the removal of the acid and the analysis of
the resulting solution by inductively coupled plasma
atomic emission spectrometry.
RESULTS AND DISCUSSION
The table summarizes the acid properties of the test
catalysts. It can be seen that both proton and Lewis sites
occurred on the surface of the catalysts; the strength
and concentration of these sites varied over a wide
range mainly depending on the chemical composition
of the catalysts.
In the series of the test catalysts, HPA is the stron-
gest Brönsted acid; the strength of its proton sites
(PA = 1120 kJ/mol) is noticeably higher than the
strength of sites in the other catalysts, including
HZSM-5. It is well known that the acidity of the latter
is close to the acidity of concentrated sulfuric acid.
Lewis sites are absent from the surface of HPA. Crys-
talline aluminum sulfate is the strongest Lewis acid.
X-ray diffraction analysis was performed on a
DRON instrument using monochromatic CuKα radia-
tion. Diffraction patterns were recorded with a chart
recorder at a goniometer rate of 1 deg/min. Phase iden-
tification was performed using the ASTM data file.
The specific surface areas of samples were deter-
mined from the low-temperature adsorption of nitro-
gen.
The acid properties were characterized with the use
of the IR spectroscopy of adsorbed CO and pyridine
[16, 17]. Before adsorption, the samples were evacu- Sulfated zirconium oxides are weaker Brønsted and
Lewis acids. Zirconium oxides are the weakest acids.
ated at specified temperatures in the range 400–600°C.
Pyridine was adsorbed at 150°C for 15 min at a vapor
pressure that corresponded to the saturated vapor pres-
sure at room temperature. Before measuring the spec-
Note that the given values of the strength of proton
sites in sulfated zirconium oxide and the zeolite are
similar. This result is different from published data [3]
tra, an excess of pyridine was removed by evacuation at because of the use of different probe molecules (CO
and Py) for testing the strength. The strength of the pro-
ton sites of zirconium sulfates determined with the use
of pyridine is much higher than that determined with
CO. The reason for this discrepancy is related to the
interaction of the interaction of Py and CO molecules
with surface hydroxyl groups to form a system of
hydrogen bonds; this conclusion was discussed by
Paukshtis [18].
150°C for 15 min. CO was adsorbed in small portions
at liquid nitrogen temperature to a total pressure of
10 Torr. The IR spectra of adsorbed pyridine and CO
were measured on an IFS-113v spectrometer (resolu-
tion, 4 cm–1; number of scans, 64).
The concentration of Lewis sites (NCO, µmol/m2)
was calculated from the intensity of the absorption
band of adsorbed CO, and the strength of these sites
was characterized by the position of the absorption
band maximum (ν(CO)) (the band shifted to the short-
wave region as the strength of the sites increased) [16].
The concentration of Brönsted sites (NH, µmol/m2) was
Based on the results obtained, we can arrange the
catalysts in accordance with the strength of sites. The
strength of Brönsted sites changes in the order HPA >
HZSM-5 = Zr(SO4)2 = SO3/ZrO2 = Al2(SO4)3 =
SO3/Al2O3 ≥ F/Al2O3, whereas the strength of Lewis
determined from the absorption band intensity of the pyri- sites changes in the order Al2(SO4)3 > HZSM-5 >
SO3/Al2O3 ≈ Zr(SO4)2 ≈ F/Al2O3 > SO3/ZrO2
≈
dinium ion. The strength of Brönsted sites (PA, kJ/mol)
was determined from the shift of the absorption band
and characterized by the value of proton affinity (PA) in
accordance with a published procedure [16, 17].
F/ZrO2 > ZrO2.
The study of the catalytic activity allowed us to con-
clude that isobutane is the main reaction product (the
selectivity is ~99%); propane and pentane (the ratio
between them is close to 1, and the total selectivity is no
higher than 0.5%) are formed in an insignificant
amount in the presence of active catalysts. The deacti-
vation of catalysts was observed in the course of the
reaction. The table summarizes the maximum rates of
reaction obtained with the test catalysts.
The activity of the catalysts in the skeletal isomer-
ization of n-butane was studied by a flow-circulation
method at 150°C and an initial n-butane concentration
of 24 vol % in a mixture with nitrogen. Before the reac-
tion, the samples were calcined in a catalytic reactor in
a flow of dry air at a specified temperature; next, the
temperature was decreased to 150°C in a flow of dry
and deoxygenated nitrogen. Then the reaction mixture
was supplied to the catalyst. The space velocity of n-
butane feeding was 2000 h–1. The reaction products
passed through the loop of a sampling valve of a chro-
matograph, and they were analyzed at regular intervals.
The degree of conversion and the rate of reaction were
determined based on the analysis of the reaction prod-
ucts. The activity in the skeletal isomerization was
characterized by the maximum rate of the isomeriza-
tion reaction reached under the specified test condi-
tions.
As can be seen in the table, the specific activity
changes in the order SO3/ZrO2-t-600 > SO3/ZrO2-m-
600 ≈ SO3/ZrO2-m-400 > F/ZrO2 ≈ Zr(SO4)2
>
Al2(SO4)2 ≈ SO3/Al2O3 > SO3/ZrO2-xra. HZSM-5,
F/Al2O3, HPA, and zirconium oxides were found inac-
tive in the reaction.
A comparison between the strength of proton sites
and the activity of catalysts indicates that there is no
correlation between these values. Moreover, the stron-
gest proton acid, HPA, is inactive in this reaction,
whereas fluorinated zirconia, which has no proton sites,
exhibits a noticeable activity. This means that the pro-
KINETICS AND CATALYSIS Vol. 45 No. 4 2004

556
SHMACHKOVA et al.
Acid properties of catalysts and their activity in the skeletal isomerization of n-butane
Tcalc
°C
,
NCO
,
PA,
NH,
w × 106,
No.
I
Catalyst
SBET, m2/g ν(CO), cm–1
µmol/m2
kJ/mol
µmol/m2
mol m–2 h–1
H3PW12O40/Sibunit
170
500
20
–
–
1120
1170
2.6
0.005
0
II HZSM-5
450
2195
2228
0.04–0.09
0.13–0.19
0.36
5.0%F/Al2O3
500
500
550
200
40
2196
2211
2223
0.43
0.32
0.18
1180
1170
1170
0.17
0.42
0.15
0.01
0.11
0.10
Al2(SO4)3
2217
2225
2232
1.75
1.50
0.50
13%SO3/Al2O3
240
2196
2209
2225
0.08
0.38
0.21
Zr(SO4)2
500
400
11
33
2198
2224
1.82
0.91
1170
1170
3.0
0.74
6.7
7.7%SO3/ZrO2-m
2181
2199
2205
2209
3.73
2.42
0.76
0.30
0.82
5.7%SO3/ZrO2-m
600
68
2196
2200
2205
2209
2.78
2.13
0.74
0.41
1170
0.34
7.5
8.0%SO3/ZrO2-xra
7.0%SO3/ZrO2-t
400
600
272
149
2199
2208
0.51
0.74
1170
1170
0.10
0.19
0.05
85
2190
2200
2210
1.21
2.50
0.41
III 2.0%F/ZrO2
500
430
900
44
150
50
2187
2195
2207
7.05
2.02
0.03
–
–
–
–
–
–
0.82
0
ZrO2-t
2180
2201
2203
0.77
0.55
0.32
ZrO2-m
2170
2183
2192
2.48
1.28
0.84
0
Note: m and t refer to zirconium oxide with monoclinic and tetragonal structures, respectively; xra is X-ray amorphous.
KINETICS AND CATALYSIS Vol. 45 No. 4 2004

ON THE EFFECT OF THE STRENGTH OF ACID SITES IN HETEROGENEOUS CATALYSTS
557
tonation of butane with the formation of a carbonium the structure of this center. It is likely that this unique
situation occurs in sulfated zirconia with the tetragonal
structure. To understand this situation, the catalytic
behavior of systems with similar properties should be
studied. We intend to perform such a study.
ion and its conversion into a carbenium ion does not
occur at the surface of the test catalysts. At the same
time, the strict dependence of the rate of reaction on the
strength of sites was observed in the reactions that
occur with the participation of proton sites [19]. The
absence of a dependence of the rate of the low-temper-
ature skeletal isomerization reaction of alkanes on the
strength of proton sites suggests that proton sites do not
participate in a limiting step of the reaction.
REFERENCES
1. Morterra, C., Cerrato, G., Pinna, F., Signoretto, M., and
Strukul, G., J. Catal., 1994, vol. 149, p. 181.
2. Morterra, C., Cerrato, G., Pina, F., and Signoretto, M.,
The inactivity of catalysts simultaneously contain-
ing proton and Lewis sites, for example, zeolite and
crystalline aluminum and zirconium sulfates, contra-
dicts the published concept on the necessary participa-
tion of both types of sites in the activation of an alkane
molecule.
Spectrochim. Acta., 1999, vol. 55, p. 95.
3. Paukshtis, E.A., Shmachkova, V.P., and Kotsarenko, N.S.,
React. Kinet. Catal. Lett., 2000, vol. 71, p. 385.
4. Jin, T., Yamaguchi, T., and Tanabe, K., J. Phys. Chem.,
1986, vol. 90, p. 4797.
5. Arata, K., Adv. Catal., 1990, vol. 37, p. 165.
The abstraction of a hydride ion is a possible mech-
anism of the activation of an alkane molecule at a Lewis
site. However, the low activity of aluminum sulfate,
which has the strongest Lewis sites, does not support
this assumption. Modified zirconium oxides are most
active in this reaction. At close strengths and concentra-
tions of both Brønsted and Lewis sites, their specific
activity changed by more than three orders of magni-
tude. Note that this difference manifests itself in cata-
lysts with close chemical compositions. As can be seen
in the table, the nature of the modifying anion, the crys-
tal structure of zirconium oxide, and the temperature of
calcination are the most important factors affecting
activity. Moreover, the activity of sulfated zirconium
oxides in this reaction essentially depends on the sul-
fate content [20, 21]. However, it is not related to the
presence of a crystalline phase of zirconium sulfate.
The activity of zirconium sulfate is insignificant; how-
ever, upon the thermal decomposition of this com-
pound, it dramatically increased and approached the
activity of the sulfated oxide with the tetragonal struc-
ture [14]. Hence, it follows that either the strength of
Lewis sites is not a crucial factor in this process or the
Lewis sites with a very narrow strength distribution are
required for the reaction to occur. It is likely that the
additional participation of other sites is required for the
activation of an alkane molecule; that is, the active cen-
ter is a complex formation whose constituents can be
aprotic, proton, and basic sites. In these centers, geo-
metric and energetic factors can play a considerable
role because not only the geometric similarity of a com-
plex active center and the reactant molecule but also an
optimum energy of their interaction are required for the
reaction to occur. In this case, the rate of reaction would
be expected to be a sharp extremal function of the
strength of active centers because changes in the nature
of the anion and the cation, as well as insignificant
changes in the crystal lattice, could result in changes in
6. Chen, F.R., Goudurier, G., Joly, J.-F., and Vedrine, J.C.,
J. Catal., 1993, vol. 143, p. 616.
7. Corma, A., Chem. Rev., 1995, vol. 95, p. 597.
8. Song, X. and Sayari, A., Catal. Today., 1996, vol. 38,
no. 3, p. 329.
9. Paukshtis, E.A., Duplyakin, V.K., Finevich, V.P.,
Lavrenov, A.V., Kirillov, V.L., Kuznetsova, L.I.,
Likholobov, V.A., and Bal’zhinimaev, B.S., Stud. Surf.
Sci. Catal., 2000, vol. 130, p. 2543.
10. Makarova, M.A., Paukshtis, E.A., Thomas, J.M., Will-
iams, C., and Zamaraev, K.I., J. Catal., 1994, vol. 149,
p. 36.
11. Malysheva, L.V., Shmachkova, V.P., Paukshtis, E.A., and
Kotsarenko, N.S., Kinet. Katal., 1991, vol. 32, no. 4,
p. 940.
12. Kuklina, V.I., Levitskii, E.A., Plyasova, L.M., and
Zharkov, V.I., Kinet. Katal., 1972, vol. 13, no. 5, p. 1269.
13. Kotsarenko, N.S., Shmachkova, V.P., and Mastikhin, V.M.,
Kinet. Katal., 1998, vol. 39, no. 4, p. 575.
14. Kotsarenko, N.S. and Shmachkova, V.P., Kinet. Katal.,
2002, vol. 43, no. 2, p. 305.
15. Shmachkova, V.P., Kotsarenko, N.S., Moroz, E.M., and
Mastikhin, V.M., Kinet. Katal., 1991, vol. 32, no. 1,
p. 144.
16. Paukshtis, E.A. and Yurchenko, E.N., Usp. Khim., 1983,
vol. 52, no. 4, p. 426.
17. Paukshtis, E.A., Infrakrasnaya spektroskopiya v getero-
gennom kislotno-osnovnom katalize (Infrared Spectros-
copy in Heterogeneous Acid–Base Catalysis), Novosi-
birsk: Nauka, 1992.
18. Maksimov, G.M., Paukshtis, E.A., Budneva, A.A., Mak-
simovskaya, R.I., and Likholobov, V.A., Izv. Ross. Akad.
Nauk, Ser. Khim., 2001, no. 4, p. 563.
19. Malysheva, L.V., Paukshtis, E.A., and Kotsarenko, N.S.,
Kinet. Katal., 1990, vol. 31, p. 247.
20. Parera, J.M., Catal. Today, 1992, vol. 15, p. 481.
21. Shmachkova, V.P. and Kotsarenko, N.S., Kinet. Katal.,
2002, vol. 43, no. 4, p. 595.
KINETICS AND CATALYSIS Vol. 45 No. 4 2004