Adsorption and catalytic properties of sulfated aluminum oxide modified with cobalt ions

Article abstract of DOI:10.1134/S0036024417010150

The adsorption properties of sulfated aluminum oxide (9% SO₄ ^2-/γ-Al₂O₃) and a cobalt-containing composite (0.5%Сo/SO₄ ^2-/γ-Al₂O₃) based on it are studied via dynamic

Full text of DOI:10.1134/S0036024417010150

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2017, Vol. 91, No. 1, pp. 36–43. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © S.N. Lanin, A.A. Bannykh, E.V. Vlasenko, I.N. Krotova, O.N. Obrezkov, M.I. Shilina, 2017, published in Zhurnal Fizicheskoi Khimii, 2017, Vol. 91,
No. 1, pp. 40–48.
CHEMICAL KINETICS
AND CATALYSIS
Adsorption and Catalytic Properties of Sulfated Aluminum Oxide
Modified with Cobalt Ions
S. N. Lanin*, A. A. Bannykh, E. V. Vlasenko, I. N. Krotova, O. N. Obrezkov, and M. I. Shilina
Department of Chemistry, Moscow State University, Moscow, 119991 Russia
*
e-mail: SNLanin@phys.chem.msu.ru
Received December 28, 2015
2
−
Abstract—The adsorption properties of sulfated aluminum oxide (9% SO /γ-Al O ) and a cobalt-containing
4
2
3
2
−
composite (0.5%Сo/SO /γ-Al O ) based on it are studied via dynamic sorption. The adsorption isotherms
4
2
3
of such test adsorbates as n-hydrocarbons (C –C ), benzene, ethylbenzene, chloroform, and diethyl ether are
6
8
measured, and their isosteric heats of adsorption are calculated. It is shown that the surface sulfation of alu-
minum oxide substantially improves its electron-accepting properties, and so the catalytic activity of
2
4
−
SO /γ-Al O in the liquid-phase alkylation of benzene with octene-1 at temperatures of 25–120°C is one
2
3
order of magnitude higher than for the initial aluminum oxide. It is established that additional modification
of sulfated aluminum oxide with cobalt ions increases the activity of this catalyst by 2–4 times. It is shown
that adsorption sites capable of strong specific adsorption with both donating (aromatics, diethyl ether che-
mosorption) and accepting molecules (chloroform) form on the surface of sulfated γ-Al O promoted by
2
3
cobalt salt.
Keywords: sulfation, cobalt-containing composites, adsorption, catalysis
DOI: 10.1134/S0036024417010150
INTRODUCTION
rarely used in studies of catalytic reactions, but its use
in studying surface intermolecular interactions seems
to be very promising [7–12].
It is known that solid sulfated Periodic Table
Group IV metal oxides have a catalytic effect on many
electrophilic-type reactions and have in recent years
The aims of this work were to synthesize sulfated
2
4
−
been widely used as catalysts in different processes, aluminum oxide (9%SO /γ-Al O ) and a cobalt-
2
3
including alkylation reactions [1]. Specific features of
the formation of such catalysts during thermal treat-
ment and their textural characteristics and composi-
tion are considered in the literature. Ways of con-
trolling their acidic properties and catalytic activity,
and the mechanisms of Brønsted and Lewis acidity
upon the sulfation of different metal oxides, were stud-
ied in [2, 3]. Systems based on sulfated aluminum
oxides would seem to be promising. They are stable
under reaction conditions and can exhibit superacidic
2−
4
containing composite (0.5%Сo/SO /γ-Al O ) based
2
3
on it, and to study their adsorption properties and cat-
alytic activity in the liquid-phase alkylation of benzene
with octene-1.
EXPERIMENTAL
Synthesis of Samples
To synthesize sulfated samples, a γ-Al O support
2
3
properties and activity in acid catalysis reactions [4]. It (AOK-63-11, grade B; specific surface area (S ),
sp
is known that the efficiency of sulfated oxides grows
appreciably upon the introduction of transition metals
2
3
1
80 m /g; average pore volume, 0.29 cm /g; pore
diameter, 8.9 Å) was calcined at 600°C for 8 h, cooled,
(
Pt, Fe, Ni, Со, Mn) [5, 6]. However, the role of a
impregnated with an aqueous sulfuric acid solution
promoting transition metal additive in the catalytic
effect of systems based on anion-modified metal
oxides remains poorly understood.
(
с = 0.5–1.0 mol/L), dried for 16 h at room tempera-
ture and then for 8 h at 140°C, and calcined in a dry air
flow for 3 h at 450°C.
Among the physicochemical ways of studying the
nature of active sites of such catalysts, the most widely
used techniques are spectral, magnetic resonance and
electron microscopy. Inverse gas chromatography is SO
2−
4
Сo/SO /γ-Аl O samples were prepared via
incipient wetness impregnation of the sulfated
2
3
2
4
−
/γ-Аl O support with an aqueous cobalt nitrate
2 3
36

ADSORPTION AND CATALYTIC PROPERTIES OF SULFATED ALUMINUM OXIDE
37
Table 1. Characteristics of test adsorbates (where M is of adsorption (Q ) were determined at different sur-
st
molecular mass, μ is the dipole moment, α is the total
face coverages, and the contributions from dispersion
polarizability of a molecule, and AN and DN are the elec-
(
Q
) and specific (Q ) interactions to Q were also
disp spec st
tron-accepting and electron-donating energetic character-
istics of molecules) [8, 9]
estimated.
DN,
kJ/mol
α, Å3
Adsorbate
M
μ, D
AN
Estimating Catalytic Activity
Prior to each catalytic experiment, catalyst samples
were activated in an air flow for 0.5 h at 450°C. The
liquid-phase alkylation of benzene with octene-1 was
performed in ampoules in the temperature range of
n-C H
86.2
100.2
114.2
78.1
0
11.9
13.7
15.6
10.4
14.1
8.2
0
0
0
0
6
14
16
18
n-C H
0
7
n-С Н
0
0
0
8
25–120°C with constant stirring. A mixture of ben-
C H
0
0.4
–
8.2
–
6
6
zene, octene-1, and n-nonane at a volume ratio of
20 : 5 : 1 was used. n-Nonane was used as an internal
standard. Control experiments demonstrated its inert-
ness under reaction conditions. Octene-1 was sub-
jected to partial isomerization under reaction condi-
tions with a change in the double bond position. In
quantitative analysis, the formed isomers of octene-1
C H C H
5
106.2
119.4
79.1
0.59
1.15
1.15
6
5
2
CHCl3
C H ) O
0
23.0
3.9
(
9.0
80.3
2
5 2
solution, followed by dying for 16 h at room tempera- were considered as initial octene.
ture and for 8 h at 140°C. It was then calcinated in a
dry air flow for 3 h at 450°C.
The composition of alkylation reaction products
was determined via gas-liquid chromatography on a
The sulfur content in our samples was determined Crystal 5000 chromatograph equipped with a flame
via elemental analysis and the back titration of sulfate
ionization detector and a BP capillary column 25 m
groups. In the latter case, weighed samples of the sul- long (temperature programming regime, t = 85–
col
fated catalyst were allowed to stand in a 0.05 N sodium
hydroxide solution for 4 h, and then the NaOH solu-
tion’s concentration was determined via titration with
a sulfuric acid solution. According to the results from
the two independent methods, the content of sulfate
200°C).
In addition to gas-liquid chromatography, gas
chromatography–mass spectrometry was used in ana-
lyzing reaction solutions. Prior to analysis, the reac-
tion solution was separated from the catalyst. Spectra
were recorded on a Varian MAT 112S spectrometer. A
DB-1 capillary column 60 m long was used in the
mode of linear column temperature programming
from 40 to 250°C at a rate of 8 K/min. The energy of
the ionizing electrons was ∼70 eV. The spectra were
2
4
−
groups in the synthesized SO /γ-Аl O samples was
2
3
9.0 ± 0.5 wt %.
Adsorption Studies
The test adsorbates used to study the surface tex- processed using the MMS Labcom 2 data processing
ture and chemistry of the initial γ-Al O and the sul- system. The components in the reaction mixture were
2
3
identified using the NIST/EPA database.
fated samples synthesized on its basis were n-hexane,
n-heptane, n-octane, benzene, ethylbenzene, chloro-
form, and diethyl ether, the properties of which are
given in Table 1 [8, 9].
RESULTS AND DISCUSSION
Adsorption properties were studied via dynamic
sorption on a Crystallux-4000M chromatograph
equipped with a catharometer as a detector. Physico-
chemical measurements were made using glass col-
umns 20 cm long with inner diameters of 2 mm.
Adsorption Properties
n-Hexane, benzene, ethylbenzene, and chloro-
form adsorption isotherms, measured on the initial
and sulfated γ-Al O and the γ-Al O based cobalt-
2
3
2
2
3
−
Helium was used as the carrier gas at a flow rate of containing composite (0.5%Сo/SO /γ-Al O ) before
4
2
3
3
0 mL/min. Prior to measurements, the adsorbent
and after catalytic experiments, are shown in Figs. 1–4.
The initial regions of the isotherms are convex with
respect to the adsorption axis and belong to type II of
the classification in [11]. The isotherms confirm the
existence of strong adsorption sites and (probably)
was conditioned in the chromatograph’s column in a
carrier gas flow at 200°C for 3 h. The volumes of the
introduced adsorbate samples were varied from 0.5 to
10 μL.
Adsorption isotherms were measured at tempera- residual surface microporosity. The adsorption of the
tures of 100, 110, and 120°C for all of the above adsor- selected test adsorbates on the initial γ-Al samples
bates (except ethylbenzene), and at 150, 160, and and all the modified γ-Al O based composites falls as
O
3
2
2
3
1
70°C for С Н С Н . Adsorption isotherms were cal- the temperature rises at all equilibrium pressures, tes-
6
5
2
5
culated according to Glukauf [10]. The isosteric heats tifying to the physical character of adsorption.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 1 2017

3
8
LANIN et al.
а, μmol/m2
а, μmol/m2
2
2
3
.0
.5
.0
.5
0
1
2
3
1
4
2
.0
.0
0
1
1
0
4
1
0
40
80
120
160
0
50
100
150
200
р, mmHg
р, mmHg
Fig. 2. С Н adsorption isotherms at 110°C on composites
6
6
Fig. 1. n-С Н adsorption isotherms at 100°C on com-
6
14
2
−
2−
posites (1) 0.5%Co/SO2 /γ-Al O , (2) 0.5%Co/SO /γ-
−
2−
(1) 0.5%Co/SO4 /γ-Al2O3, (2) 0.5%Co/SO4 /γ-Al2O3
4
2
3
4
2
−
2
−
after catalysis, (3) SO4 /γ-Al2O3, and (4) γ-Al2O3.
Al O after catalysis, (3) SO /γ-Al O , and (4) γ-Al O .
2
3
4
2
3
2 3
а, μmol/m2
а, μmol/m2
2
1
1
3
2
2
.0
.0
0
2.0
3
4
1
1.0
0
0
50
100
150
200
р, mmHg
0
10
20
30
40
50
60
р, mmHg
Fig. 4. С Н С Н adsorption isotherms at 140°C on com-
6
5
2
5
Fig. 3. СНСl adsorption isotherms at 100°C on the com-
3
posites (1) 0.5%Co/SO2 /γ-Al O , (2) 0.5%Co/SO /γ-
−
2−
2
−
2−
4
2 3
4
posites (1) 0.5%Co/SO /γ-Al O , (2) SO /γ-Al O ,
4
2
3
4
2 3
2
−
and (3) γ-Al O .
Al O after catalysis, (3) SO /γ-Al O , and (4) γ-Al O .
2 3 2 3 2 3
4
2
3
2
In contrast to the initial support, diethyl ether was
The specific surface areas (S, m /g) and monolayer
capacity аm were calculated from the n-hexane
adsorption isotherms at 100°C by the BET equation,
and the parameters of the microporous structure of
samples were estimated from the benzene adsorption
isotherms using the Dubinin–Radushkevich equation
2
−
irreversibly chemosorbed both on SO /γ-Al O sulfated
4
2
3
2
4
−
aluminum oxide and on the 0.5%Co/SO /γ-Al O
3
2
cobalt-containing composite based on it. The adsorp-
tion of all the test adsorbates falls at all temperatures of
measurement and the same equilibrium pressures on
[
12]. The micropore radius was calculated from the
characteristic adsorption energy (E ) dependence
2
4
−
the composites in the order 0.5%Co/SO /γ-Al O >
0
2
2−
3
2
−
0
0
.5%Co/SO /γ-Al O (after catalysis) > SO /γ- according to the equation R = 12/Е [13]. Calculation
4
2
3
4
Al O > γ-Al O .
results are listed in Table 2. From Table 2, we can see
2
3
2
3
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ADSORPTION AND CATALYTIC PROPERTIES OF SULFATED ALUMINUM OXIDE
Table 2. Parameters of the microporous structure of the
39
ln a = ln k + nln p.
(2)
2
−
initial γ-Al O , sulfated γ-Al O , and 0.5%Co/SO /γ-
2
3
2
3
4
The isosteric heats of adsorption (Q ) were calculated
st
Al O composite before and after the heterogeneous cata-
2
3
from the set of these dependences for three tempera-
2
lytic alkylation reaction*
tures (R = 0.999). The isosteric heats of adsorption at
2
surface coverage а = 0.35 μmol/m were used to deter-
2
0
S, m /(g am,
С Н ) μmol/g kJ/mol
E ,
Adsorbent
R, nm mine the contributions from dispersion (Q ) and
disp
6
6
specific (Q ) interactions. The calculation results
spec
are given in Table 3.
γ-Al O
147
73
276
249
227
218
10.3
13.3
14.4
12.1
1.16
2
3
2
−
The dependences of chloroform Q on the surface
st
9
0
0
%SO /γ-Al O
0.90
4
2
−
3
coverage in the initial monolayer area of
2
.5%Co/SO /γ-Al O
67
0.83
0.99
4
2
2
3
2−
2−
0
.5%Co/SO /γ-Al O and 9%SO /γ-Al O are
4
2 3 2 3
4
−
.5%Co/SO /γ-Al O
64
4
2
3
plotted in Fig. 5 as examples. The data shown in Fig. 5
display the dependence typical of all the studied sys-
tems when the highest heats of adsorption correspond
(
after catalysis)
0
*
S is the specific surface area, a is the monolayer capacity, E is
m
the characteristic energy of adsorption, and R is the pore radius to the most active surface sites, which are the first to be
calculated using the Dubinin–Radushkevich equation [12].
occupied by adsorbate molecules. As they are occu-
pied, adsorption occurs on less active sites, and the
heats of adsorption gradually fall.
that the specific surface area of sulfated γ-Al O
2
3
The contribution from the energy of specific inter-
shrinks after the introduction of 9 wt % of sulfate
groups, while additional modification with cobalt salts
has virtually no effect on its value. The specific surface
area of the sulfated samples is known to strongly
depend on the conditions of synthesis, among which
the thermal treatment of oxide systems before and
after the introduction of sulfate groups can be of cru-
cial importance. In some cases, the specific surface
action (Q ) was determined as the difference
spec
between isosteric heats of adsorption Q of a polar
st
adsorbate and a hypothetical n-alkane with the same
total polarizability α. From Table 3 containing
isosteric heats of adsorption Q and specific interac-
st
tion contributions Q
for the adsorption of test
spec
adsorbates in the region of low coverage at а =
2
area of sulfated aluminum oxide can grow slightly in 0.35 μmol/m , it can be seen that the sulfated alumi-
comparison to the initial support [4, 5, 14], but the num oxide and composite surfaces are characterized
surface modification of γ-Al O with sulfate anions by high adsorption heats and exhibit stronger specific-
2
3
ity in comparison with γ-Al O . The heats of adsorp-
usually reduces it [4, 15, 16].
2
3
tion of n-alkane characterizing nonspecific dispersion
interactions on sulfated aluminum oxide and the com-
posite grow slightly, compared to the initial γ-Al O .
The adsorption isotherms of the test adsorbates are
perfectly described by the empirical Freundlich equa-
tion [17] for a non-uniform surface:
2
3
The immobilization of both sulfate groups and metal
ions reduces the effective diameter of the support’s
pores, due to the blocking of pore inlet openings. The
adsorption potential then grows, due to the imposition
n
a = kp
or, in the linear form,
(1)
Table 3. Isosteric heats of adsorption (Q , kJ/mol) of test adsorbates and the contributions of dispersion (Qdisp, kJ/mol)
st
2
and specific (Q , kJ/mol) donor–acceptor interactions to them at а = 0.35 μmol/m on initial and sulfated γ-Al O and
spec
2
3
2
4
−
0
.5%Co/SO /γ-Al O , before and after the heterogeneous catalytic alkylation reaction
2
3
2
−
2
4
−
2−
0.5%Co/SO4 /γ-Al O
2 3
γ-Al O3
2
9%SO /γ-Al O
0.5%Co/SO /γ-Al O
2
3
4
2
3
(
after catalysis)
Adsorbate
Qst
39
43
47
50
57
78
60
Qdisp
Qspec
Qst
41
46
51
57
47
Qdisp
Qspec
Qst
40
45
50
57
79
Qdisp
Qspec
Qst
40
43
46
58
47
77
83
Qdisp
Qspec
n-С Н
6
14
16
18
n-С Н
7
n-С Н
8
С Н
36
31
34
44
14
26
44
16
37
31
20
16
36
30
21
49
38
34
36
44
20
13
41
39
6
6
CHCl3
C H ) O
(
Chemosorption
47
Chemosorption
46 41
2
5 2
С Н C H
5
67
20
87
6
5
2
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 1 2017

4
0
LANIN et al.
Q, kJ/mol
of the potential field of closely packed pore walls, thus
raising the heat of adsorption.
From Table 3, we can see that diethyl ether, the
molecules of which have predominantly electron-
donating properties, is irreversibly chemosorbed on
the surface of sulfated aluminum oxide. The adsorp-
1
00
tion of electron-accepting molecules (CHCl ) is char-
3
6
0
0
acterized by a drop in Q values, relative to the initial
st
γ-Al O . These results point to a considerable increase
2
3
in the number of electron-accepting (acidic) sites on a
surface of aluminum oxide after the introduction of
sulfate groups and subsequent calcination, which
agrees with the data in [18, 19] on the acidity of sul-
fated oxides.
1
2
2
0
50
100
150
200
а, μmol/g
However, the immobilization of even small
amounts of transition metal cations (less than
0.5 wt %) on a surface of sulfated aluminum oxide
Fig. 5. Isosteric CHCl adsorption heat vs. surface cover-
3
changes the adsorption properties of its surface con-
siderably. As can be seen from Table 3, the irreversible
chemosorption of diethyl ether on the cobalt-contain-
2
−
2−
age on (1) 0.5%Co/SO /γ-Al O and (2) SO /γ-Al O .
4
2
3
4
2 3
2
−
ing Co/SO /γ-Al O samples is similar to the adsorp-
4
2
2
3
interaction grows after the sulfation and subsequent
promotion of the γ-Al O surface with cobalt, from
1 kJ/mol on the initial sample to 17 kJ/mol on
−
tion on SO /γ-Al O , indicating that the acidic prop-
2
3
4
2
3
1
erties of surfaces of sulfated oxides not only remain the
same after modification with cobalt salts, but are likely
to increase. At the same time, the heat of adsorption of
2
−
Co/SO /γ-Al O . In addition, energy contribution
4
2
3
∆
Q
on the cobalt-modified samples grows appre-
spec
2
4
−
chloroform on the Co/SO /γ-Al O samples consid- ciably after introducing an alkyl substituent into the
2
3
benzene ring. ∆Q
has close values for benzene on
spec
erably exceeds Q on the sulfated and initial γ-Al O .
st
2
3
different oxide systems (∼11–17 kJ/mol), while con-
The data of Table 3 indicate that the heats of adsorp-
tion rise for СНСl due to an increase in the contribu- tribution ∆Q
= Q
– Q
for ethylben-
n-С8Н18
3
spec
С6Н5С2Н5
tion from the energy of specific interaction (Q ) to
2−
4
spec
zene on 0.5%Co/SO /γ-Al O (37 kJ/mol) greatly
exceeds the corresponding values obtained on γ-Al O
2
3
the total energy of adsorption, and are 16 and
2
3
49 kJ/mol for the sulfated and cobalt modified sam-
2
4
−
ples, respectively. The observed adsorption effects on and SO /γ-Al O (13 and 16 kJ/mol, respectively).
2
3
2
4
−
As can be seen from Tables 3 and 4, the heat of adsorp-
tion of aromatic compounds in this case grows, due
largely to an increase in the energy contribution from
specific interactions in which transition metal cations
also participate.
the Co/SO /γ-Al O samples are apparently due to
2
3
specific interactions between transition metal ions and
halogenated hydrocarbon molecules, for which the
formation of molecular complexes was determined
earlier via IR spectroscopy and confirmed theoreti-
cally [20].
In Table 4, contributions ∆Q
= Q (arom.) –
spec
st
Q (n-alkane) are compared to the data obtained for
st
An increase in the number of specific adsorption
sites after surface modification of sulfated aluminum
oxide with cobalt ions is also indicated by a rise in ben-
zene and ethylbenzene adsorption on them, relative to
the initial samples (see Table 3). The differences
the adsorption of test molecules on cobalt-containing
samples of nonsulfated γ-Al O , i.e., 5%Со/γ-Al O
2
3
2
3
and 5%СоО/γ-Al O , synthesized using the sol-gel
2
3
method [21]. According to the data in [21], the transi-
tion metal contained in such composites is present on
the surface of γ-Al O predominantly in the form of
between the isosteric heats of adsorption (∆Q ) of
st
2
3
aromatic hydrocarbons and n-alkanes on the surfaces
of different adsorbents (∆Q = Q (arom.) – Q (n-
metallic cobalt nanoparticles (in 5%Со/γ-Al O sam-
2
3
spec
st
st
ples) or cobalt oxide nanoparticles (in 5%СоО/γ-
alkane)) are given in Table 4. ∆Q
н-C6H14
benzene and n-hexane heats of adsorption and char-
acterizes the energy contribution from specific inter-
actions between the π-electron bonds of molecules
and active sites on the surfaces of sorbents to the total
= (QC6H6 –
) corresponds to the difference between the
spec
Al O composites). From Table 4, we can see that
2
3
Q
∆Q for the cobalt-containing composites prepared
spec
from nonsulfated γ-Al O are low for benzene adsorption
2
3
(
10 and 5 kJ/mol) and 20 and 11 kJ/mol for ethylbenzene
adsorption, much lower than on the cobalt-containing
2
4
−
energy of adsorption of benzene. From Table 4, we can sulfated aluminum oxide (0.5%Co/SO /γ-Al O ).
2 3
see that the contribution from the specific energy of A similar picture is observed for chloroform adsorp-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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ADSORPTION AND CATALYTIC PROPERTIES OF SULFATED ALUMINUM OXIDE
41
Table 4. The differences between the isosteric heats of adsorption (ΔQ ) of aromatic hydrocarbons, diethyl ether, and
st
2
n-alkanes on initial and modified γ-Al O at surface coverage a = 0.35 μmol/m
2
3
ΔQ = QC6H6
ΔQ = QC6H5C2H5 ΔQspec = QC6H5С2H5 ΔQ = Q(C2H5)2O
Adsorbent
Qspec(CHCl3)
–
Qn-C6H14
– Qn-C8H18
– QC6H6
– Qn-C5H12
γ-Al O3
11
16
17
18
5
13
16
37
37
11
20
2
10
20
19
14
18
43
26
16
49
13
11
17
2
2
−
9
0
0
5
%SO /γ-Al O
Chemosorption
4
2
3
2
−
.5%Co/SO /γ-Al O
Chemosorption
4
2
2
3
−
.5%Co/SO /γ-Al O*
40
28
27
4
2
3
%СоО/γ-Al O3
2
5%Со/γ-Al O3
10
2
*
After catalysis.
tion, where specific interaction contribution Q
is bilization of transition metal cations on a surface of
spec
only 17 and 11 kJ/mol for 5%Со/γ-Al O and sulfated oxide, can make a substantial contribution to
2
3
its catalytic activity. The data given below on the cata-
lytic activity of the studied systems confirm this
hypothesis.
5
%СоО/γ-Al O , respectively, against 49 kJ/mol for
2
−
3
2
0
.5%Co/SO /γ-Al O . With respect to diethyl ether,
4
2
3
the nonsulfated cobalt-containing samples behave like
specific adsorbents instead of chemisorbents
(
∆Q = 27–28 kJ/mol).
Catalytic Properties
spec
The catalytic properties of the synthesized sulfated
aluminum oxide and cobalt-containing composite
samples were estimated from their catalytic activity in
the model reaction of benzene alkylation with olefins
at temperatures of 25–120°C. The liquid-phase reac-
tion between benzene and octene-1 barely occurs on
the initial aluminum oxide in the studied range of tem-
peratures: the octene-1 conversion does not exceed
3% after 3 h at 120°C. Neither is a catalytic reaction
observed under these conditions on the 5%Сo/γ-Al O
The specific adsorption sites found on the studied
sulfated systems could play an important part when
exhibiting their catalytic activity in the alkylation reac-
tion. This is confirmed in particular by data obtained
for the samples after catalytic reactions in the adsorp-
tion of test adsorbate molecules. From Tables 3 and 4,
we can see that Q and contributions Q for the test
st
spec
2
−
adsorbates on the 0.5%Co/SO /γ-Al O sample fall
4
2
3
slightly after catalysis, compared to the initial
.5%Co/SO /γ-Al O sample, and contribution samples obtained via modification of the initial
2
3
2
−
0
∆
4
2
3
γ-Al O with cobalt salts. In contrast, the immobiliza-
2
3
Qspec (Q
–Q
) of 19 kJ/mol is also reduced.
C H C H
6
5
2
5
6
C H
6
tion of sulfate groups on the surface of γ-Al O greatly
2
3
In contrast to the chemosorption of diethyl ether on
the sulfated composites, the adsorption of diethyl
ether is observed both on the surface of the sample
increases the support’s activity: octene-1 conversion
2
4
−
on the 9%SO /γ-Al O samples reaches 30–35% in
2
3
after catalysis and on the initial γ-Al O (see Table 3),
2
3
1 h even at room temperature. The benzene alkylation
reaction in this case is selective and produces such lin-
ear monoalkyl benzenes (LABs) as 4-phenyl-, 3-phe-
nyl-, and 2-phenyloctane. Their major fraction (more
than 50%) is 2-phenyloctane, a target reaction product.
testifying to the participation of the most active elec-
tron-accepting surface sites in catalysis.
Considerable changes were also observed in chloro-
form adsorption. As can be seen from Table 3, a strong
reduction in the heat of adsorption is observed after the
The surface modification of sulfated aluminum
oxide with cobalt salts in amounts of 0.5 wt % consid-
erably enhances the activity of these catalysts.
2
−
catalytic reaction on the 0.5%Co/SO /γ-Al O sam-
4
2
3
ples, due primarily to a drop in the energy of specific
2
4
−
interactions between the chloroform and the adsorp- Octene-1 conversion on the 0.5%Co/SO
/γ-Al O
2 3
tion sites based on cobalt atoms.
sample was 90% even at room temperature. Our data
on process conversion and selectivity on the
A surface of sulfated aluminum oxide modified
2
−
with cobalt cations thus contains different sites capa- Co/SO /γ-Al O samples at different temperatures
2 3
4
ble of the specific adsorption of both donating mole- are given in Table 5. It can be seen that octene-1 con-
cules (adsorption of aromatics, chemosorption of version grew when the reaction temperature was raised
diethyl ether) and accepting molecules (chloroform). to 80°C, but the selectivity to LABs did not change
It is apparently these sites, which form after the immo- appreciably, and the yield of 2-phenyloctane, the most
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 1 2017

4
2
LANIN et al.
Table 5. Conversion (η) and selectivity to LABs (S ) and
At the same time, the character of the reaction on
1
2
-phenyloctane (S ) in the reaction of benzene alkylation
2−
2
the SO /γ-Al O samples without cobalt differs from
4
2
3
2
4
−
with octene-1 on the Co/SO /γ-Al O sample at different the one considered above. From Fig. 5, we can see that
2
3
temperatures
2−
alkene-1 conversion on SO /γ-Al O remained
4
2
3
Тreact, °C
η, %
S1, %
S2, %
nearly the same as the process time was increased
from 30 to 90 min) and grew slightly when the content
(
25
50
80
89.8
94.8
99.3
91
90
88
51.7
51.1
50.9
of catalyst in the reaction mixture was increased. Sim-
ilar patterns testify to the existence of a limited number
of sites that are active in the catalytic process at low
temperatures (295–313 K) on sulfated aluminum
oxide. In contrast, the nearly linear dependence
between the rate of the alkylation reaction and the
important product, remained at a level of more than mass content of the catalyst observed for the
5
0%. The content of subsequent products of alkyla-
2−
0
.5%Co/SO /γ-Al O samples demonstrates the key
4
2
3
tion, i.e., dioctylbenzenes, did not exceed 8–10%.
role of the transition metal, which forms catalytically
It should be noted that the surface modification of active sites on the sulfated support surface.
2
4
−
SO /γ-Al O with transition metal cations not only
The data obtained in this work on the adsorption of
2
3
2
4
−
appreciably increases the yield of alkylation products,
but also considerably changes the character of the cat-
alytic reaction. The data on olefin conversion in the
liquid-phase benzene alkylation reaction on modified
samples of different composition, depending on the
mass content of the catalyst in the reaction mixture,
test molecules also indicate that 0.5%Co/SO /γ-
Al O contains adsorption sites that greatly increase
2
3
contributions ΔQ to the heat of adsorption of aro-
spec
matic hydrocarbons and halide-containing molecules
(
particularly chloroform). The formation of specific
2
4
−
are plotted in Fig. 6. It can be seen that the rate of the adsorption sites on a Co/SO
/γ-Al O surface con-
2 3
reaction (alkene conversion over a certain period of taining sulfate anions is apparently due to the possibil-
2
4
−
ity of stabilizing the transition metal in the form of cat-
ions on this surface. Preliminary data from the IR
spectroscopy of adsorbed carbon monoxide have
shown that the electron state of the transition metal on
time) on the 0.5%Co/SO /γ-Al O cobalt-contain-
2
3
ing catalysts grows in an almost linear fashion as the
content of the catalyst in the reaction mixture rises.
Such dependences are observed for times close to the
onset of the reaction and its end (30 and 90 min, a Co/SO
2
4
−
/γ-Al O surface is close to that of ion-
2 3
2+
respectively).
exchange cations (Co ) on the surfaces of alumosili-
cate zeolites [22, 23] and differs substantially from the
state of cobalt in the oxides (СoО) formed on surfaces
of nonmodified γ-Al O . This agrees with the known
η, %
2
3
2
1
literature data for some other sulfated oxides modified
with transition metals [15, 24]. The state of a transition
metal on a surface of sulfated oxide and its role in
alkylation reaction catalysis will be considered in more
detail in a separate work.
90
70
50
30
CONCLUSIONS
Using the dynamic sorption of test molecules of
different classes of organic compounds, it was estab-
lished that the surface sulfation of aluminum oxide
leads to the growth of specific intermolecular sorbate–
sorbent interactions; greatly increases the electron-
accepting properties of its surface; and, upon the sur-
face modification of sulfated aluminum oxide with
cobalt(II) ions, improves both its electron-accepting
and electron-donating properties. The catalytic activ-
ity of sulfated aluminum oxide in the liquid-phase
alkylation of benzene with long-chain olefins is an
order of magnitude higher than for initial aluminum
oxide. Additional modification of sulfated aluminum
oxide with cobalt ions increases the catalyst’s activity
4
3
8
10
12
14
16
18
W
cat, wt %
Fig. 6. Octene-1 conversion in the liquid-phase benzene
alkylation reaction vs. catalyst content in the reaction mix-
2
−
ture at 295 K on samples (1, 2) Co/SO /γ-Al O3 and
4
2
−
(
3, 4) SO2 /γ-Al O for (1, 3) 30 and (2, 4) 90 min of the
4
2 3
reaction.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 91
No. 1
2017

ADSORPTION AND CATALYTIC PROPERTIES OF SULFATED ALUMINUM OXIDE
43
by 2–4 times, with the retention of high selectivity 11. S. Gregg and K. Sing, Adsorption, Surface Area and
Porosity (Academic, New York, 1982).
toward LABs and 2-phenyloctane.
1
2. M. M. Dubinin, Adsorption and Porosity (Khimiya,
Moscow, 1972) [in Russian].
ACKNOWLEDGMENTS
This work was supported by the RF Presidential
1
3. G. M. Plavnik and M. M. Dubinin, Russ. Chem. Bull.
15, 597 (1966).
Council for the State Support of Leading Scientific 14. D. Mao, W. Yang, J. Xia, B. Zhang, G. Lu, J. Mol.
Schools, grant no. NSh 8845.2016.3.
Catal. A: Chem. 250, 138 (2006).
1
5. J. C. Martın, S. B. Rasmussen, S. Suarez, et al., Appl.
Catal. B: Environ. 91, 423 (2009).
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Translated by E. Glushachenkova
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 1 2017