Study of n-hexane isomerization on Pt/SO4/ZrO 2/Al2O3 catalysts: Effect of the state of platinum on catalytic and adsorption properties

Article abstract of DOI:10.1134/S0023158410040191

The state of surface Pt atoms in the Pt/SO₄/ZrO ₂/Al₂O₃ catalyst and the effect of the state of platinum on its adsorption and catalytic properties in the reaction of n-hexane isomerization were studied. The Pt-X/Al₂O₃ alumina-platinum catalysts modified with various halogens (X = Br, Cl, and F) and their mechanical mixtures with the SO₄/ZrO₂/Al ₂O₃ superacid catalyst were used in this study. With the use of IR spectroscopy (CO_ads), oxygen chemisorption, and oxygen-hydrogen titration, it was found that ionic platinum species were present on the reduced form of the catalysts. These species can adsorb to three hydrogen atoms per each surface platinum atom. The specific properties of ionic platinum manifested themselves in the formation of a hydride form of adsorbed hydrogen. It is believed that the catalytic activity and operational stability of the superacid system based on sulfated zirconium dioxide were due to the participation of ionic and metallic platinum in the activation of hydrogen for the reaction of n-hexane isomerization.

Full text of DOI:10.1134/S0023158410040191

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 4, pp. 584–594. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © M.D. Smolikov, K.V. Kazantsev, E.V. Zatolokina, D.I. Kir’yanov, E.A. Paukshtis, A.S. Belyi, 2010, published in Kinetika i Kataliz, 2010, Vol. 51, No. 4,
pp. 608–618.
Study of
n
ꢀHexane Isomerization on Pt/SO4/ZrO2/Al2O3 Catalysts:
Effect of the State of Platinum
on Catalytic and Adsorption Properties
M. D. Smolikova, K. V. Kazantseva, E. V. Zatolokinaa,
D. I. Kir’yanova, E. A. Paukshtisb, and A. S. Belyia
a Institute of Hydrocarbon Processing, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia
b Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
eꢀmail: smolikov@ihcp.oscsbras.ru
Received February 19, 2009
Abstract—The state of surface Pt atoms in the Pt/SO4/ZrO2/Al2O3 catalyst and the effect of the state of platꢀ
inum on its adsorption and catalytic properties in the reaction of nꢀhexane isomerization were studied. The
Pt–X/Al2O3 alumina–platinum catalysts modified with various halogens (X = Br, Cl, and F) and their
mechanical mixtures with the SO4/ZrO2/Al2O3 superacid catalyst were used in this study. With the use of IR
spectroscopy (COads), oxygen chemisorption, and oxygen–hydrogen titration, it was found that ionic platiꢀ
num species were present on the reduced form of the catalysts. These species can adsorb to three hydrogen
atoms per each surface platinum atom. The specific properties of ionic platinum manifested themselves in the
formation of a hydride form of adsorbed hydrogen. It is believed that the catalytic activity and operational staꢀ
bility of the superacid system based on sulfated zirconium dioxide were due to the participation of ionic and
metallic platinum in the activation of hydrogen for the reaction of nꢀhexane isomerization.
DOI: 10.1134/S0023158410040191
INTRODUCTION
tures were used to explain the results of the extremely
low chemisorption activity of Pt in the Pt/SO4/ZrO2
sulfated system [5, 6]. The IRꢀspectroscopic studies of
the state of Pt with the use of CO adsorption also demꢀ
onstrated that platinum did not entirely occur in a
metal state and charged platinum particles were
formed in the course of reduction of the Pt/SO4/ZrO2
system [7, 8].
The isomerization reactions of С4–С6 light alkanes
are of considerable current interest. Catalysts that
have been commonly used previously, such as alumiꢀ
num chloride and liquid acids, do no meet environꢀ
mental, corrosion, and other requirements. Thereꢀ
fore, they should be replaced by solid acids. Zeolites
2−
and metal oxides modified with
ions (primarily,
SO4
There is no general agreement among authors as to
the role of various states of platinum in sulfate–zircoꢀ
nia catalysts in isomerization [5–13]. The high stabilꢀ
ity of platinumꢀcontaining catalysts was related to the
activation of hydrogen on platinum metal and the
hydrogenation of coke precursors [5]. The modificaꢀ
tion effect was explained by the formation of new proꢀ
ton sites [4, 13] as a result of the interaction of hydroꢀ
SO4/ZrO2) belong to solid acids; they exhibit superꢀ
acid properties and actively isomerize alkanes at a low
temperature [1, 2]. However, although the selectivity
of the process on SO4/ZrO2 is high, this system is less
active than Pt–Cl/Al2O3. Moreover, this catalyst is
rapidly deactivated with an irreversible loss of activity.
The addition of platinum increases the activity of the
SO4/ZrO2 catalyst and considerably enhances its staꢀ
bility in the reaction of С4–С6 paraffin isomerization
[3], which is usually performed in an atmosphere of
hydrogen at 150–250°С. It is evident that, under these
conditions, platinum can undergo reduction up to the
metal. The XPS data indicate that only 15% Pt
occurred in a metal state, and the major portion
remained oxidized even upon reduction with hydroꢀ
gen at 350°С [4]. According to other data, which were
also obtained by XPS, the main state of Pt was zeroꢀ
valent particles covered with a PtS layer after the
gen with platinum. A reaction scheme of nꢀhexane
isomerization was proposed [9–11, 13], in which platꢀ
inum was considered as a supplier of hydride ions to a
carbocation isomerized at an acid site. However, no
experimental evidence for the formation of hydrides
on platinum was presented in the cited publications.
Published data on the state (oxidation number or
charge) of surface platinum atoms are considerably
different [4–8, 10]. There are almost no published
data concerning the adsorption of hydrogen, which is
reductive treatment of the catalyst. These special feaꢀ an important participant of isomerization reaction in
584

STUDY OF
n
ꢀHEXANE ISOMERIZATION ON Pt/SO4/ZrO2/Al2O3 CATALYSTS
585
Table 1. Characteristics of the SO4/ZrO2/Al2O3 acid catalyst
Chemical composition, wt % Texture characteristics
Xꢀray diffraction characteristics
**
Phase composition of ZrO2
, wt %
Crystallite size
Vpore
,
SBET,
m2/g
*
Reff
,
nm
SO4
6
ZrO2
70
Al2O3
24
cm3/g
**
of tꢀZrO2
,
nm
**
tꢀZrO2
mꢀZrO2
5
0.34
110
5
95
6
* R is the effective pore radius.
eff
** tꢀZrO and mꢀZrO are the tetragonal and monoclinic modifications, respectively.
2
2
the absence of which the activity of Pt/SO4/ZrO2 catꢀ
EXPERIMENTAL
alysts is unstable, on platinum.
Preparation of Catalysts
Theoretical works devoted to the quantumꢀchemiꢀ
cal studies of the state of a metal, in particular, Pt, in
heterogeneous catalysts have been published recently
Acid
catalysts
without
platinum
(the
SO4/ZrO2/Al2O3 system) and with supported platiꢀ
num (Pt/SO4/ZrO2/Al2O3), Pt–Х/Al2O3 alumina–
[14–18]. These publications considered the nature of platinum catalysts with various halogens (Br, Cl, and
F), and mechanical mixtures of the SO4/ZrO2/Al2O3
and Pt–Х/Al2O3 catalysts in a weight ratio of 1 : 1 were
used in this study.
bifunctional catalysts in a new way. An alternative
mechanism of alkane conversion was proposed withꢀ
out the participation of acid sites. The reactivity of Pt
in zeolites was calculated using the density functional
method [14, 15], and the possibility of converting
alkanes adsorbed immediately on platinum particles
was demonstrated. A special feature of these particles
is the ability to adsorb hydrogen in ratios of H/Pt > 1
[17–19].
The acid catalyst was prepared from an aqueous
solution of ZrO(NO3)2 with a concentration of 90 g/l
in terms of ZrO2. The precipitation of zirconium dioxꢀ
ide hydrate was performed at 60–65°С on the addition
of an ammonia solution to a zirconium salt solution
until pH 10–11. The resulting precipitate was washed,
dried at 120°С, and then treated with a 12% solution
of sulfuric acid. Aluminum hydroxide as a binding
agent was added to the predried material for molding
Ambiguous data on the state of Pt in sulfate–zircoꢀ
nia systems and its role in catalysis stimulated us to
continue a study on the effect of the electronic state of and strengthening the support granules. The washed
and dried (at 120°С) material was calcined at 650°С in
a flow of dry air to obtain crystalline zirconium oxide.
Zirconium dioxide in the acid catalyst after sulfation
and calcination contained 95% of the tꢀZrO2 tetragoꢀ
nal phase (Table 1).
To prepare a sample of Pt/SO4/ZrO2/Al2O3, the
acid catalyst was impregnated with chloroplatinic acid
to obtain a 0.4 wt % platinum content of the prepared
catalyst. Then, the sample was dried at 120°С and calꢀ
cined at 450°С in a flow of dried air.
surface Pt atoms on adsorption and catalytic properꢀ
ties in the reaction of ꢀhexane isomerization in an
atmosphere of hydrogen.
n
In preliminary experiments with model catalysts as
mechanical mixtures of an acid component (the
SO4/ZrO2/Al2O3 catalyst) and a metal component (Pt
on Al2O3 and SiO2 supports), Smolikov et al. [20]
found that catalysts with the oxidized platinum Pt2+
(systems based on Pt–Cl/Al2O3) exhibited high cataꢀ
lytic activity in nꢀhexane isomerization. The activity
The alumina–platinum catalysts modified with Br,
Cl, and F halogens (Pt–Х/Al2O3) were prepared in
level of catalysts with only Pt0 atoms (systems based on
Pt/SiO2) was incommensurably lower. A special feaꢀ
ture of Pt–Сl/Al2O3 catalysts is the stabilization of
Pt2+ ions as a [PtOxCly] surface complex [21].
accordance with the following procedure: ꢀAl2O3
γ
(total pore volume, 0.65 cm3/g; BET specific surface
area, 230 m2/g; and effective pore radius, 7.0 nm) was
used. The support was precalcined at 580°С and evacꢀ
uated to remove air from pores in order to prevent the
formation of cracks and a loss in the pore volume.
Then, the support was impregnated with aqueous
solutions of HBr, HCl, and HF acids to obtain uniꢀ
form impregnation and homogeneous Pt distribution
over the volume of support grains. Platinum was supꢀ
ported from an aqueous solution of H2PtCl6 and
H2PtBr6 acids taken in amounts corresponding to
platinum concentrations in the prepared catalyst at a
level of 0.4–0.5 wt %. The support was impregnated
with a platinumꢀcontaining acid for 1 h with an expoꢀ
In this work, we studied the effect of the electronic
state of platinum on the adsorption of hydrogen and
the activity of the Pt/SO4/ZrO2/Al2O3 superacid cataꢀ
lyst in the reaction of nꢀhexane isomerization in an
atmosphere of hydrogen. To determine the effect of
the state of Pt, we used model mixtures of the
SO4/ZrO2/Al2O3 superacid catalyst with no platinum
with metal components—the Pt–Х/Al2O3 platinum
catalysts (where X = Cl, Br, and F), from which platiꢀ
num metal atoms were absent and the ionic state of
platinum was characterized as Pt2+
.
KINETICS AND CATALYSIS Vol. 51
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2010

586
SMOLIKOV et al.
Table 2. Characteristics of the Pt–X/Al2O3 (X = Br, Cl, and F) and Pt/SO4/ZrO2/Al2O3 catalysts
Sample
PtꢀBr/Al2O3
Pt content, wt %
Training conditions
Dispersity of Pt, %
0.44
0.47
0.42
0.40
О2 500°С, Н2 500°С
О2 500°С, Н2 500°С
О2 500°С, Н2 500°С
О2 450°С, Н2 270°С
91
100
100
16
PtꢀСl/Al2O3
PtꢀF/Al2O3
Pt/SO4/ZrO2/Al2O3
sure at 70–75°С. The impregnated support was sity of the spectra was expressed in Kubelka–Munk
washed with distilled water to remove unsorbed platiꢀ units. Before spectroscopic studies, the samples
num solution, dried at 120°С in air, and calcined at placed in a movable cell, activated in a vacuum, and
500°С in a flow of dried air. Before use, the prepared reduced in a special setup. The cell for spectroscopic
samples were reduced with hydrogen at 500°С. Table 2 measurements was made of quartz and equipped with
summarizes the characteristics of Pt–Х/Al2O3 and a window of CaF2. To study the state of platinum, CO
Pt/SO4/ZrO2/Al2O3 samples.
(30 Torr) was added to the pretreated and evacuated
samples (fraction of 0.25–0.75 mm) at a temperature
of 25°С. After measuring the spectrum, the sample
was evacuated at room temperature, and the spectrum
was measured once again.
Experimental Procedures
The catalytic studies of the reaction of
nꢀhexane
isomerization were performed in a flow system with an
isothermal catalytic fixedꢀbed tube reactor and a therꢀ
mocouple well arranged along the reactor axis. The
catalyst loading in the reactor was 2 cm3; the particle
size was 0.25–0.75 mm.
Before loading into the reactor, the Pt–Х/Al2O3
alumina–platinum catalysts were preactivated in an
individual setup in a flow of purified hydrogen with a
stepwise temperature increase to 500°С and kept for
1 h at this temperature. The SO4/ZrO2/Al2O3 acid catꢀ
alyst was calcined at 650°С in a flow of dry air before
preparing a mechanical mixture for catalytic tests. The
further activation of catalysts (and mixtures) was perꢀ
formed immediately in the catalytic setup in a flow of
The H/D isotope exchange was studied on a Shiꢀ
madzu 8300 spectrometer in the transmission mode
with the use of a mixture containing 97 vol % isotope.
The studies were performed in situ with the use of a
movable cell. For this purpose, a freshly prepared and
dried (at 120°С) sample was pressed without a binder
to form a pellet with a density of 20 g/cm3 and treated
in oxygen and then in a vacuum at 450°С and the iniꢀ
tial spectrum was measured. Thereafter, hydrogen was
added to the cell at a pressure of 100 Torr and the specꢀ
tra were measured while heating the sample in hydroꢀ
gen from room temperature to 250°С in steps of 50 K.
The isotope exchange was performed after cooling the
samples with adsorbed hydrogen by adding a mixture
with deuterium at room temperature followed by heatꢀ
ing to 250°С in steps of 50 K with measurement of IR
spectra.
purified hydrogen for 2 h at a temperature of 270°С
.
The catalytic reaction was performed at 140–400°С, a
1
pressure of 1.5 MPa, an LHSV of 2 h⎯ , and an Н2
/nꢀС6
molar ratio of 3 : 1.
An oxygen–hydrogen (О2–Н2) titration and oxyꢀ
gen adsorption procedure [22, 23] was used for
adsorption studies in order to evaluate the dispersion,
the amounts of adsorbed oxygen and hydrogen, and
the state of surface platinum atoms.
The phase composition of the catalysts was anaꢀ
lyzed on a DRONꢀ3 diffractometer using monochroꢀ
The raw material, chemically pure nꢀhexane dried
with molecular sieves NaX, was supplied with a disꢀ
pensing pump to a Tꢀshaped pipe for mixing with
hydrogen and then to the reactor. The reaction prodꢀ
ucts were analyzed on line using a Tsvetꢀ800 chroꢀ
matograph with a Petrocol DH 50.2 capillary column.
The mechanical mixtures were prepared in an agate
mortar by mixing and grinding a metal component
(Pt–Х/Al2O3 catalysts) and an acid component (the
SO4/ZrO2/Al2O3 catalyst) in a weight ratio of 1 : 1.
Then, the resulting powder was pressed at 9 MPa and
ground in a mortar, and a fraction of 0.25–0.75 mm
was taken for catalytic tests.
mated Сu
К
radiation and a ꢀfilter.
β
α
The pore structure parameters of the samples were
measured on a Sorptomaticꢀ1900 instrument using the
isotherms of N2 adsorption at 77 K.
RESULTS AND DISCUSSION
nꢀHexane Isomerization
The IR spectra were measured in the region of
4000–1000 cm–1 on a Shimadzu 8300 spectrophoꢀ
tometer equipped with a DRSꢀ800 diffuse reflectance
Table 3 and Figs. 1 and 2 summarize the results of
attachment. A resolution of 4 cm–1 was used with the the catalytic tests of superacid catalysts
averaging of 50–100 accumulated spectra. The intenꢀ
(SO4/ZrO2/Al2O3 and Pt/SO4/ZrO2/Al2O3), mixed
KINETICS AND CATALYSIS Vol. 51
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STUDY OF
n
ꢀHEXANE ISOMERIZATION ON Pt/SO4/ZrO2/Al2O3 CATALYSTS
587
Table 3. Isomerization of nꢀhexane on SO4/ZrO2/Al2O3 and mixtures with Pt–X/Al2O3 (X = Br, Cl, and F)
Yield of hydrocarbons, wt %
Converꢀ
Selectivity
for isoꢀC6, wt %
Catalyst
T, °C
sion, %
C1–C4
isoꢀC5
n
ꢀ
C5
Σ
DMB**
Σ
MP**
140*
140
160
180
200
220
89.5
4.3
5.3
0
1.3
0
0.3
0
40.2
0.6
0.8
1.9
1.7
2.0
42.5
3.3
92
90
95
95
94
90
5.9
0
0
0
4.8
SO4/ZrO2/Al2O3
12.9
11.6
15.1
0.2
0.4
0.9
0
0.1
0.2
0
10.3
9.3
0
0.4
11.6
140
160
180
200
220
87.3
87.0
87.1
86.9
87.9
3.2
2.0
2.9
0.5
0.6
1.4
5.0
1.0
0.1
0.3
0.4
1.6
34.4
29.9
29.5
26.2
22.0
45.7
53.6
54.1
52.9
38.3
92
96
96
91
69
Pt/SO4/ZrO2/Al2O3
2.5
5.4
21.1
180
220
260
22.7
53.8
74.7
0.2
2.8
0.1
1.0
4.0
0
3.3
8.2
18.7
41.4
43.8
97
92
73
SO4/ZrO2/Al2O3 +
PtꢀBr/Al2O3
0.1
1.0
14.8
10.8
180
220
260
30.7
66.0
83.5
0.3
3.6
0
0.1
0.2
1.4
4.9
11.9
13.4
24.9
49.1
40.5
97
92
65
SO4/ZrO2/Al2O3 +
PtꢀCl/Al2O3
1.0
4.7
23.3
180
220
260
34.2
68.0
87.0
0.3
3.6
0
0.1
0.2
1.5
5.0
12.1
13.7
25.4
50.1
41.4
89
91
63
SO4/ZrO2/Al2O3 +
PtꢀF/Al2O3
1.0
4.8
23.8
* Characteristics measured in a 10ꢀmin experiment; in the other experiments, the characteristics were measured after 30 min.
** DMB and MP refer to dimethylbutanes and methylpentanes, respectively.
systems (SO4/ZrO2/Al2O3 + Pt–Х/Al2O3), and Pt– isomerization temperature to 200°С affected only
Х/Al2O3 catalysts.
The SO4/ZrO2/Al2О3 system initially exhibited
high catalytic activity in the reaction of nꢀhexane
slightly process parameters: the yield of hexane isoꢀ
mers was retained at a level of 79 at 91% selectivity.
The same dependence was also observed in cracking
products: the yields of С1–С4 and С5 products
increased only slightly to 200°С. An increase in the
temperature above 200°С considerably decreased the
yield of isomers to 60.3%, and selectivity dramatically
decreased to 69% because of the development of
cracking reactions. Thus, the addition of platinum to
the catalyst considerably increased conversion and the
yield of isomers.
isomerization; the yield of hexane isomers after a
10ꢀmin experiment was 82.7% (Table 3). As the experꢀ
iment time was increased to 30 min, activity characꢀ
teristics considerably decreased. The yield of isomerꢀ
ization products at 140–160°С was about 4–5% at
90–95% selectivity (Table 3). An increase in the temꢀ
perature to 220°С increased the yield of isomerization
products to 12–13%; however, selectivity for isohexꢀ
anes decreased because of an increase in the yield of
cracking products.
To determine the effect of the state of platinum on
activity, we performed experiments on
nꢀhexane
The
introduction
of
platinum
into isomerization on catalyst mixtures: a Pt–X/Al2O3 aluꢀ
SO4/ZrO2/Al2О3 resulted in a considerable increase in mina–platinum catalyst was added to a lowꢀactivity
the operational stability of the Pt/SO4/ZrO2/Al2О3 SO4/ZrO2/Al2О3 acid component in a weight ratio of
catalytic system. The yield of isomerization products 1 : 1 (Fig. 1). Catalysts with various halogens (X = Br,
was as high as 83.5–83.6% at 96% selectivity for isoꢀ Cl, and F) were specially chosen to change the state of
hexanes. The reaction products contained 2ꢀmethylꢀ a platinum site according to published data [21].
pentane, 3ꢀmethylpentane, deep isomerization prodꢀ Before considering the experiments with mixtures, it is
ucts (2,2ꢀ and 2,3ꢀdimethylbutanes), and С1–С5 necessary to explain the choice of components for
cracking products (Table 3). An increase in the these mixtures. The Pt–X/Al2O3 (where X = Br, Cl,
KINETICS AND CATALYSIS Vol. 51
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2010

588
Yield of isoꢀC6, wt %
SMOLIKOV et al.
Yield of isoꢀC6, wt %
100
80
60
40
20
0
70
60
50
40
30
20
10
0
3
2
5
2
4
6
3
1
1
140 180 220 260 300 340 380 420
, °C
T
350
400
450
500
, °C
T
Fig. 1. The temperature dependence of the yield of hexane
isomers for SO /ZrO /Al O and
Pt/SO /ZrO /Al O catalysts and Pt–X/Al O
(
1
3
)
(2)
+
4
2
2 3
4
2
2
2 3
SO /ZrO /Al O mixtures (X = (
3
) Br, (
4
) Cl, and (
5
) F).
Fig. 2. The temperature dependence of the yield of hexane
isomers for Pt–X/Al O catalysts: X = ( ) Br, ( ) Cl, and
(3) F.
4
2
2 3
(6
) Data for a mixture of Pt/SiO + SO /ZrO /Al O were
1
2
2
4
2
2
3
2 3
taken from [20].
and F) alumina–platinum systems themselves are
active isomerization catalysts. Figure 2 shows the temꢀ
perature dependence of the yields of hexane isomers
obtained in this work on the above catalysts. For
isomerization on alumina–platinum systems, optiꢀ
mum temperatures are higher than 400°С, which is
characteristic of highꢀtemperature isomerization in
accordance with a bifunctional mechanism [24]. In a
classical form, the bifunctional mechanism implies
the independent actions of metal and acid sites. Previꢀ
ously [21, 22, 25, 26], it was found that, in the Pt–
Х/Al2O3 catalysts modified with chloride and bromide
ions, Pt atoms were stabilized by chemical interaction
with the surface sites of alumina—surface Al3+ cations
(L refers to Lewis sites)—and halide ions to form the
Let us return to the results shown in Fig. 1 for
mixed catalysts. In a preliminary study, Smolikov et al.
[20] found that the mixtures containing an
SO4/ZrO2/Al2O3 acid component and a Pt/SiO2 cataꢀ
lyst, in which platinum occurred only in a metal state
(
Pt0), exhibited activity over the temperature range of
300–400°С, that is, in the region close to that of highꢀ
temperature isomerization [24]. In the mixed catalysts
based on Pt/Al2O3, which are active in a lowꢀtemperꢀ
ature isomerization region [20], ionic platinum speꢀ
cies, which are stabilized as the [PtOxCly] complex in
Pt–Cl/Al2O3 catalysts, should be present in addition
to Pt0 atoms.
As can be seen in Table 3, the mixed catalysts with
ionic platinum occupy an intermediate position
between the individual SO4/ZrO2/Al2О3 and
Pt/SO4/ZrO2/Al2О3 superacid catalysts in terms of
conversion and the yield of isomers. The addition of an
acid component to the alumina–platinum catalyst
shifts the reaction to the range of temperatures much
lower than 380–420°С, which is characteristic of a
classical bifunctional mechanism of isomerization on
Pt–Х/Al2O3 (cf. Fig. 2). The temperature range of a
maximum yield of hexane isomers on the mixed cataꢀ
lysts containing [PtOxХy] complexes is 200–240°С
(Fig. 1). The experimental results indicate that the
ionic state of Pt plays an important role in the catalysis
of hexane isomerization both in the case of alumina–
platinum catalysts (bifunctional catalysis) and in the
presence of the SO4/ZrO2/Al2О3 superacid system.
Mechanical mixtures with ionic platinum are active
over a much lower temperature range, as compared
[PtOxХy] surface complex. Thus, by introducing variꢀ
ous halogen atoms into the catalyst, we purposefully
changed the chemical composition of platinumꢀconꢀ
taining sites. Replacing chlorine by bromide or fluoꢀ
ride ions, we obtained considerable changes in the catꢀ
alytic activity of the alumina–platinum samples
(Fig. 2). The yield of hexane isomers reached maxiꢀ
mum values of 40–45, 50–55, and 60–65% for cataꢀ
lysts in the order Br
→
Cl
→
F, respectively. The temꢀ
perature range of maximum activity decreased in the
same order. Thus, the experimental results suggest
that, in terms of the bifunctional action mechanism,
the state of platinumꢀcontaining sites in the [PtOxХy
surface complexes exerted a considerable effect on the
activity of the catalyst in the reaction of ꢀhexane
isomerization: the activity of catalysts increased with
increasing halogen electronegativity in the order Br
Cl F. This is also evidenced by a decrease in the
]
n
with that for mixtures based on Pt/SiO2 (
Pt0 metal
atoms). However, a pronounced effect of the nature of
a halogen is not observed in the mixed catalysts. It is
likely that the mechanism of the reaction on mixed
→
→
temperature of maximum activity from 450–470 to
390–420°С in the above order of catalysts.
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589
F(R)
units
(a)
0.3
0.2
0.1
0
1800
1900
2000
2100
2200
2300
(
b)
0.4
0.3
0.2
0.1
0
1700
1800
1900
2000
2100
2200 2300
1
Wavenumber, cm−
Fig. 3. Diffuse reflectance IR spectra of CO adsorbed on (a) Pt–Cl/Al O and (b) Pt–F/Al O reduced with H at 500°C.
2
3
2
3
2
catalysts is different from the classical bifunctional form on oxidized platinum sites in the IR spectra. We
mechanism, which is characteristic of alumina–platiꢀ can conclude that platinum supported on aluminum
num systems [24]; correspondingly, platinum plays oxide was incompletely reduced in the alumina–platꢀ
another role in this case.
inum catalysts used in this study even after training in
Н2 at 500°С
.
After the adsorption of CO on the
Pt/SO4/ZrO2/Al2O3 catalyst reduced in Н2 at 270°С
State of Platinum in the Catalysts
Figure 3 shows the IR spectra of CO adsorbed on (Fig. 4), the spectrum exhibited absorption bands with
alumina–platinum catalysts. After the deconvolution maximums at 2099, 2116, 2151, and 2202 cm–1. The
of the spectra, individual components manifested lastꢀnamed band is characteristic of CO complexes
themselves as bands at 2094 and 2100 cm–1, which can with Lewis acid sites (LASs) on the surface of
be attributed to platinum metal bearing a small charge SO4/ZrO2/Al2O3. The band at 2151 cm–1 can be
δ
+. A distinctive feature of halogenꢀcontaining cataꢀ ascribed to linear CO complexes with Pt ions, which
lysts is the presence of absorption bands at 2123, 2128, can be the constituents of (SO3–O)–Pt surface comꢀ
2167, and 2172 cm–1 due to CO adsorbed in a linear plexes [7]. The frequencies of 2116 and 2099 cm–1 can
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adsorption properties of platinum in the
F(R) units
Pt/SO4/ZrO2/Al2O3 and Pt–X/Al2O3 catalysts with
the use of О2 chemisorption and (О2–Н2) titration.
0.10
0.05
0
Adsorption Properties of Platinum in Catalysts
Previously [21–23], we successfully applied
chemisorption and titration methods to study the
adsorption properties of platinum ions in [PtOxCly
]
and [PtOxBry] complexes as the constituents of aluꢀ
mina–platinum catalysts for catalytic reforming.
Table 4 summarizes the absorption of oxygen accordꢀ
ing to (О2–Н2) titration and О2 chemisorption data,
which were obtained for the catalysts in the determiꢀ
nation of the dispersity (the number of surface atoms)
of platinum in accordance with an adsorption proceꢀ
dure [22, 23]. Based on the adsorption data, we calcuꢀ
lated the amounts of adsorbed hydrogen per surface
platinum atom in each particular catalyst. A wellꢀ
known rule, according to which the amount of oxygen
consumed for titration is equal to the sum of the
amounts of oxygen chemisorbed on platinum and
consumed for the interaction with hydrogen adsorbed
on platinum with the formation of water, was used for
the calculations:
1800 1900 2000 2100 2200 2300 2400
1
Wavenumber, cm−
Fig. 4. Diffuse reflectance IR spectra of CO adsorbed on
Pt/SO /ZrO /Al O reduced at 270°C.
4
2
2 3
be attributed to the vibrations of CO adsorbed in a linꢀ
ear form on Pt + and Pt0 particles, respectively. A shift
δ
of the absorption band of adsorbed CO from 2070–
2080 cm–1, which are typical values of the metal, to
2099 cm–1 suggests platinum metal, which is strongly
affected by SO4/ZrO2 acid sites [7, 8].
OT = OC + 1/2HC,
where OT is the consumption of oxygen in titration,
(atom O)/(atom Ptt); OC is the consumption of oxyꢀ
gen in chemisorption on platinum, (atom O)/(atom
Ptt); HC is the amount of chemisorbed hydrogen on
platinum, (atom H)/(atom Ptt) (here, Ptt refers to the
total number of platinum atoms in the catalyst).
The number of hydrogen atoms adsorbed by a surꢀ
face platinum atom can be obtained by relating the
values of H/Ptt to the dispersity of platinum (in terms
of a surface Pts atom). Data in Table 4 indicate that, in
the series of Pt–X/Al2O3 catalysts, the ability of surꢀ
face Pt atoms to adsorb hydrogen increased in the
The above spectroscopic characteristics clearly indiꢀ
cate that ionic platinum was present in the test catalytic
systems. In the alumina–platinum systems, ionic platiꢀ
num was identified as the [PtOxХy] complex bound to
LASs. In the superacid catalyst, the ionic form of Pt
hypothetically occurred in the neighborhood of Lewis
sites as structures like (SO3–O)–Pt [7]. According to
our data [21, 23], the [PtOxCly] surface complexes
exhibited specific properties; in particular, they
adsorbed hydrogen with the stoichiometry H/Pt = 2.
It is likely that the high adsorption capacity of ionic
platinum for hydrogen can explain the high catalytic
activity of ionic platinum in the isomerization reaction
in an atmosphere of hydrogen. If the reaction was perꢀ
formed in an inert gas (nitrogen or helium) atmoꢀ
sphere, rapid deactivation occurred in a few minutes
halogen order Br
acid properties of
→
Cl
→
F. It is well known that the
γ
ꢀAl2O3 were enhanced in the same
order upon the addition of halogens [24]. For the
Pt/Al2O3 systems modified with weakꢀ and mediumꢀ
analogously to the case that the acid component with acidity halogens (bromine and chlorine), specific
no platinum was used for isomerization. Therefore, to chemisorption values were similar: H/Pts = 1.89–2.00.
explain the role of various states of Pt, we studied the Upon modification with fluorine, which is a strongꢀ
Table 4. Adsorption characteristics of Pt in catalysts: dispersity, oxygen consumption upon chemisorption (OC) and titration
(OT), and the amount of adsorbed hydrogen
Oxygen absorption, (atom O)/(atom Ptt
)
Dispersity
Pts/Ptt, %*
Sample
H/Ptt
H/Pts
OT
OC
Pt–Br/Al2O3
91
100
100
16
1.37
1.50
1.79
0.24
0.51
0.50
0.60
0.02
1.72
2.0
1.89
2.0
Pt–Cl/Al2O3
Pt–F/Al2O3
2.38
0.44
2.38
2.75
Pt/SO4/ZrO2/Al2O3
* Pt and Pt are the amount of surface platinum atoms and the total amount of platinum atoms, respectively.
s
t
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591
F(R) units
0.025
0.020
0.015
0.010
0.005
0
3
2
1
−0.005
1600
1800
2000
2200
2400 2600
1
Wavenumber, cm−
Fig. 5. Diffuse reflectance IR spectra of Pt hydride on (
1) Pt–Cl/Al O , (2) a mixture (Pt–Cl/Al O + SO /ZrO /Al O ), and
2 3 2 3 4 2 2 3
(3
) Pt/SO /ZrO /Al O .
4
2
2 3
acidity halogen, the chemisorption capacity for an absorption band at 2089 cm–1 was recorded. After
hydrogen increased to H/Pts = 2.38. On going to the the addition of an H2–D2 isotope mixture (100 Torr)
Pt/SO4/ZrO2/Al2O3 superacid system, a further containing 97% D2 to the measurement cell and a
increase in the amount of adsorbed hydrogen to 30ꢀmin exposure at room temperature, the next specꢀ
H/Pts = 2.75 was observed.
trum was recorded, in which the absorption band at
2089 cm–1 retained its position and intensity. Noticeꢀ
able changes in this band were detected after the stepꢀ
wise heating of the sample to 200°С in steps of 50 K in
an atmosphere of the H2–D2 mixture. The H–D
The IRꢀspectroscopic study of hydrogen adsorpꢀ
tion on the surface of platinumꢀcontaining catalysts
with various states of platinum demonstrated the inhoꢀ
mogeneity of adsorbed hydrogen. Figure 5 shows difꢀ
fuse reflectance IR spectra in the region of 1700–
2600 cm–1. The spectra exhibited absorption bands at
2025, 2049, and 2091 cm–1, which can be attributed to
the hydride forms of adsorbed hydrogen. The spectra
were recorded after heating and keeping the catalysts
in hydrogen at elevated temperatures (300°С for the
Pt–Cl/Al2O3 catalyst and 250°С for the sulfate–zirꢀ
conia (including mixed) systems). The corresponding
absorption bands were not detected without heating in
an atmosphere of hydrogen. This fact suggests an actiꢀ
vated character of the formation of hydride hydrogen
species. In this case, the absorption band frequencies
were different to suggest another character of the
resulting hydride ions. At the same time, we can note
a clearly pronounced tendency: the positions of
absorption bands due to hydride ions shifted to a highꢀ
frequency region on going to the superacid system.
The formation of hydride ions on platinum upon
the adsorption of hydrogen was supported by the H/D
exchange. Figure 6 shows the IR spectra obtained
exchange occurred only at a temperature of 100°С
:
the absorption band at 2089 cm–1 shifted toward the
region of low frequencies to 1481 cm–1, which correꢀ
sponds to the Pt–D isotope substitution for Pt–H.
The above experiments demonstrated that hydroꢀ
gen adsorbed on platinum was inhomogeneous in its
composition. The amount of adsorbed hydrogen for
the Pt–X/Al2O3 catalysts corresponded to an atomic
ratio of 2 : 1 between hydrogen and platinum, whereas
this ratio was about 3 : 1 for the Pt/SO4/ZrO2/Al2O3
superacid system. The found activated adsorption of
hydrogen was responsible for the appearance of
hydride ions on ionic platinum atoms. It is likely that
the proton formed upon the heterolytic dissociation of
hydrogen diffused to the support (hydrogen spillover).
Effect of the State of Platinum on Adsorption
and Catalytic Properties
The experimental results obtained in this work
upon the H2–D2 isotope exchange on the surface of concerning hydrogen adsorption (Table 4) suggest that
Pt/SO4/ZrO2/Al2O3. After a preliminary exposure of the high specific chemisorption of hydrogen in cataꢀ
the sample in hydrogen at 250°С followed by cooling lysts was related to the presence of ionic platinum. The
and evacuation at 20°С, the IR spectrum containing disperse platinum metal adsorbs hydrogen with the
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592
SMOLIKOV et al.
Absorbance
0.020
0.015
0.010
0.005
0
D2 100°C
D2 25°C
H2 25°C
−
0.005
1400
1600
1800
2000
2200 2400
1
Wavenumber, cm−
Fig. 6. IR spectra illustrating H/D exchange on Pt/SO /ZrO /Al O at 25 and 100°C.
4
2
2 3
stoichiometry H/Pt = 1, as substantiated in a number catalysts; it is responsible for the blocking of strong
of publications [22, 23], whereas the occurrence of acid sites by carbon deposits. Allyl and polyenyl catꢀ
ionic platinum in Pt/Al2O3 catalysts is currently ions and polycyclic aromatic compounds, which are
undoubted [21]. The experimental results suggest that formed from alkenes as byꢀproducts of the isomerizaꢀ
the Pt/SO4/ZrO2/Al2O3 superacid catalyst exhibited tion reaction, act as these compounds [2]. The reacꢀ
the highest capacity of platinum to accumulate hydroꢀ tion performed in an atmosphere of hydrogen comꢀ
gen (H/Pts = 2.75). Note that the quantitative data on pletely suppressed the formation of compounds of this
the adsorption of oxygen and hydrogen on this catalyst kind because platinum metal provided the adsorption
given in Table 4 can be considered as evidence for speꢀ and supply of atomic hydrogen to acid sites followed
cial properties of platinum in this system, which are by the hydrogenation of polycyclic aromatic comꢀ
different from the properties of Pt ions in alumina– pounds formed at the active sites.
platinum systems. First, a higher adsorption capacity
According to IRꢀspectroscopic data, the state of
for hydrogen was observed. Second, a low chemisorpꢀ
platinum in a superacid catalyst is characterized by the
tion capacity for oxygen (O/Ptt = 0.02) was found,
superposition of three characteristic frequencies of
which can be considered as a sign of a higher oxidation
2099, 2116, and 2151 cm–1, which belong to metal
state of platinum, as compared with alumina–platiꢀ
δ
+
atoms (Pt0), (Pt–H) structures, and Pt ions in strucꢀ
num catalysts.
tures like (SO3–O)–Pt, respectively. With the use of
The high chemisorption capacity for hydrogen H/D isotope exchange, we found that hydride ions
(
H/Pts = 2.75) formally allows for the adsorption of were formed upon the adsorption of hydrogen on platꢀ
hydrogen in both molecular and atomic forms. The inum with an absorption band at 2091 cm–1. The forꢀ
molecular adsorption of hydrogen on platinum metal mation of platinum hydrides exhibited an activated
can occur only at low temperatures of about –173°С character, and it was observed on catalysts containing
[27], whereas hydrogen is adsorbed on platinum metal ionic platinum (Fig. 4). Thus, it is reasonable to relate
at temperatures higher than 0°С in an atomic form. the appearance of hydride ions to the adsorption of
Therefore, in the isomerization reaction performed on hydrogen on platinum ions. This is also evidenced by
the Pt/SO4/ZrO2/Al2O3 superacid catalyst over the the dependence of the absorption frequency of Pt⎯
H–
temperature range of 120–220°С under standard conꢀ hydrides on the structure of surface complexes (like
ditions, the adsorption of hydrogen should be considꢀ (SO3–O)–Pt or [PtOxCly]) containing ionic Pt, which
ered to occur by a dissociative mechanism with the were identified in alumina–platinum and superacid
formation of atomic hydrogen on Pt0 as the constituꢀ catalysts. It is believed that the adsorption of hydrogen
ent of platinum metal particles with a characteristic on platinum ions at elevated temperatures occurred by
COads absorption frequency of 2099 cm–1. Platinum a heterolytic mechanism with the formation of Pt+–
metal plays an important role in the catalyst. Coke forꢀ H– and the proton O–H+ on the support, and the forꢀ
mation is a reason for the deactivation of superacid mation of new or the regeneration of spent Brønsted
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acid sites (BASs) can occur upon the dissociation of sulfated zirconium dioxide for lowꢀtemperature
hydrogen on surface platinum complexes. The interꢀ isomerization.
action of acid site protons with metal particles explains
the appearance of an absorption band at 2116 cm–1 in
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