Effect of chemical modification on oligomerization capacity of alumina support for Pd-Al2O3 catalyst

Article abstract of DOI:10.1134/S1070427211080155

Effect of modification with an alkali metal on the acid properties of alumina supports and on the oligomerization capacity and working stability of Pd-Al₂O₃ catalysts was studied. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S1070427211080155

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 8, pp. 1385–1393. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A.A. Lamberov, I.F. Khalilov, I.R. IL’yasov, A.Sh. Bikmurzin, V.M. Shatilov, I.F. Nazmieva, 2011, published in Zhurnal Prikladnoi
Khimii, 2011, Vol. 84, No. 8, pp. 1320–1328.
ORGANIC SYNTHESIS AND INDUSTRIAL
ORGANIC CHEMISTRY
Effect of Chemical Modification on Oligomerization Capacity
of Alumina Support for Pd–Al O Catalyst
2
3
a
b
a
A. A. Lamberov , I. F. Khalilov , I. R. IL’yasov ,
b
b
b
A. Sh. Bikmurzin , V. M. Shatilov , and I. F. Nazmieva
a
Kazan (Volga Region) Federal University, Kazan, Tatarstan, Russia
b
Nizhnekamskneftekhim Open Joint-Stock Company, Nizhnekamsk, Tatarstan, Russia
Received August 28, 2010
Abstract—Effect of modification with an alkali metal on the acid properties of alumina supports and on the
oligomerization capacity and working stability of Pd–Al
2
O
3
catalysts was studied.
DOI: 10.1134/S1070427211080155
The industrial process for production of benzene
from the pyrocondensate at Nizhnekamskneftekhim
Open Joint-Stock Company (Etilen plant) is performed
by two-stage hydrogenation of the benzene–toluene–
xylene (BTX) fraction, followed by thermal hydro-
dealkylation and additional fine adsorptive purification
of the benzene fraction to remove microscopic
impurities of unsaturated and sulfur-containing com-
pounds on natural clays. The process of exhaustive
hydrogenation of diene and vinyl aromatic hydro-
carbons from the BTX fraction is performed in the first
stage with a palladium-on-alumina catalyst of LD-265
brand, manufactured by Axens company (France). The
catalyst has the form of a spherical catalytic system
with a grain diameter of 2–4 mm, based on palladium
modification, with a basic additive introduced into a
catalyst [1]. In the present study, we used as a support
δ-Al O modified with sodium hydroxide to make
2
3
lower the acidity. As shown previously [2], δ-Al O
2
3
has a low concentration of acid centers, which
provides formation of a lesser amount of oligomeric
deposits on its surface.
The goal of our study was to examine the effect of
modification with sodium of a Pd–Al O hydrogena-
2
3
tion catalyst on its acid–base properties, oligomeriza-
tion capacity, and operation stability.
EXPERIMENTAL
We determined the specific surface area and
porometric volume with a Micromeritics ASAP 2400
versatile analyzer. The specific surface area Ssp was
determined by the method of thermal desorption of
nitrogen with an accuracy of ±3%. Adsorption
isotherms were obtained at 77 K on degassing the
(
Pd 0.3 wt %) and supported by γ-Al O . This catalyst
2 3
enables hydrogenation of the least stable unsaturated
hydrocarbons: acyclic dienes (isoprene, trans-piperi-
lene), cyclodiolefins (cyclopentadiene, methylcyclo-
pentadiene, vinylcyclohexene, dicyclopentadiene),
alkenyl aromatics (styrene, methylstyrene), and a small
part of mono-olefins. However, the high concentration
of diene (5–6 wt %) and vinyl aromatic (6–8 wt %)
hydrocarbons in the BTX fraction leads to fast
deactivation of the first-stage hydrogenation catalyst
because of the occurrence of side catalytic reactions of
oligomerization of unsaturated compounds on acid–
base centers of aluminum oxide. As a result, the inter-
regeneration life of the catalyst becomes shorter.
–4
catalyst at 773 K to a residual pressure of 10 mm Hg.
The porometric volume and the pore size distribution
were calculated by the standard Barrett–Joyner–
Halenda procedure [3]. The measurement accuracy
was ±13%.
We synthesized Pd/Al O catalysts on supports in
2
3
the form of δ-Al O grains produced by calcination in
2
3
air at 900°C (5 h) of two samples of industrial
pseudoboehmite extrudates having a monomodal pore
volume distribution over pore diameter, with a
Most frequently, the concentration and strength of
Brønsted and Lewis centers are lowered by chemical
1
385

1
386
LAMBEROV et al.
Table 1. Composition and physicochemical properties of the
BTX fraction
The components of the starting BTX fraction and
hydrogenizates were identified with a Perkin–Elmer
Clarus 500 chromato-mass spectrometer. The hydro-
carbon composition of the samples being analyzed was
determined with a Khromatek Kristall 5000.1 gas
chromatograph with a flame-ionization detector and a
nonpolar capillary column with a length of 100 m,
inner diameter of 0.25 mm, and 0.5-μm-thick fixed
phase SE-30.
Component
Content, wt %
Aromatic compounds, including:
benzene
76–83
51–56
toluene
20–25
xylene
5–6
The catalytic activity and the stability of the
catalysts were monitored by the diene and bromine
numbers in the catalyzate.
Styrene
6–8
Dienes, including:
cyclopentadiene
isoprene
5–6
IR spectra were recorded with a Shimadzu 8300
1.43–1.79
0.01–0.03
0.07–0.14
0.60–0.75
2.59–2.78
10.1–18.2
25.5–36.0
0.856–0.867
–1
Fourier spectrometer with a resolution of 4 cm and
50 accumulations of spectra. To record the spectra,
catalyst samples were preliminarily compacted into
trans-piperidine
hexadiene
–
3
binder-free pellets with a density of 7–17 mg cm ,
preliminarily reduced in a flow of hydrogen at 160°C
(
0.5 h) and calcined for 1 h in an IR cuvette at 500°C
methylcyclopentadiene
–4
–5
in a vacuum to a residual pressure of (10 –10 ) × 133 Pa
and cooled to –190°C with liquid nitrogen. The
temperature at which the IR spectra were measured
Diene number, g I
2
/100 g
/100 g
Bromine number, g Br
2
(
–190°C) is determined by the need to diminish to the
–
3
Density at 20°C, g cm , in conformity
with GOST (State Standard) 3900–85
maximum possible extent the noise associated with
thermal oscillations of atoms in the crystal lattice of
Al O and, simultaneously, to preclude condensation
2
3
2
–1
of CO probing molecules having a boiling point of
–191.5°C in pores of the alumina support, which
makes impossible identification of the characteristic
bands of acid centers.
maximum at 8–16 nm: H (S = 124 m g , V
=
por
3 –1
1
sp
3
–1
2
–1
0
.53 cm g ) and H (S = 188 m g , V 0.71 cm g ).
2 sp por
Prior to deposition of a palladium salt, aluminum
oxides were impregnated with alkali (NaOH) solutions
of various concentrations. The catalysts contained the
following amounts of sodium (wt %): 0.25 (sample
nos. 1 and 3) and 0.5 (sample nos. 2 and 4). On drying
the samples at 110°C, we deposited palladium
Brønsted (protic) acid centers were found from the
intensities of the absorption bands of hydroxy
(
bridging and terminal) groups, which appear in the
–1
range 3550–3796 cm .
acetylacetonate [Pd(AcAc) ] from benzene. The
2
content of palladium in the samples under study was
The characteristics and number of Brønsted and
Lewis acid centers were determined by using carbon
monoxide as a probing molecule [1] whose adsorption
was performed at pressures in the range (0.1–
0
.5 wt %. The catalysts were reduced in a reactor in
the atmosphere of hydrogen at 200°C for 4 h, with
subsequent drying at 300°C.
1
0) × 1233 Pa. The procedure used to determine the
The reaction of hydrogenation of the BTX fraction
concentration and strength of Lewis centers was
described in [2].
(
Table 1) was performed in a flow-through reactor
3
with a volume of 50 cm at a pressure of 4 MPa,
–
1
volumetric flow rate of the raw material of 2.5 h , and
For qualitative determination of deposits on the
surface of H and H supports and catalysts, the
–
1
hydrogen flow rate of 500 ml min . The supports H
1
1
2
and H were tested at 50°C (22 h), and the catalytic
samples were preliminarily extracted with chloroform.
The extract obtained was evaporated in a flow of
nitrogen and the dry residues were analyzed by IR
spectroscopy on a Perkin–Elmer Spectrum 100 Fourier
spectrometer.
2
system nos. 1–4, at 30 to 50°C (60 h). Prior to catalytic
tests, the samples were preliminarily reduced in a flow
of hydrogen for 5 h at 160°C, 3.0 MPa, and volumetric
–
1
flow rate of hydrogen of 300 ml min .
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 8 2011

EFFECT OF CHEMICAL MODIFICATION
1387
a
Table 2. Calculated values of the Thiele modulus ψ
–3
–5
3
–1
2
–1
φ
fr
,
D
eff × 10 ,
m h
s
u ×10 ,
Sample
V
p
, cm g
S
sp, m g
d
av, Å
3
–3
2
–1
–2 –1
ψ
cm cm
mol m h
Н
1
(δ-Al
no. 1 [0.5% Pd/Н
no. 2 [0.5% Pd/Н
(δ-Al
2
O
3
)
2.56
1.04
0.75
0.53
0.60
0.41
0.26
1
(0.25% Na)]
(0.5% Na)]
1.84
1.30
2.37
1.61
1.04
0
0
.53
.71
124
188
164.5
216.4
0.3498
0.4801
1
8
.98
Н
2
2 3
O )
no. 3 [0.5% Pd/Н
no. 4 [0.5% Pd/Н
2
(0.25% Na)]
(0.5% Na)]
2
a
V
p
, total porometric volume; φfr, porosity: fraction of the volume of a porous body, occupied by pores (relative free volume); Deff
,
effective diffusion coefficient; and u , specific catalytic activity.
s
Table 3. Effect of the composition of the alumina support and Pd/Al
2
O
3
catalyst on the oligomerization capacity of the
samples under study
–1
Formation rate of oligomers, μg gca t– 1
h
Sample
–
2
–1
desorbed into the liquid phase (%)
0.1 (0.1)
adsorbed on the surface (%)
75.0 (99.9)
total (μg m h )
Н
1
75.1 (0.19)
no. 1
no. 2
20.0 (37.0)
30.2 (79.3)
0.4 (0.6)
34.0 (63.0)
7.9 (20.7)
69.2 (99.4)
15.1 (32.0)
3.6 (11.8)
54.0 (0.44)
38.1 (0.31)
69.6 (0.14)
47.2 (0.25)
30.4 (0.16)
Н
2
no. 3
no. 4
32.1 (68.0)
26.8 (88.2)
It is known that the course of side reactions in
which unsaturated hydrocarbons are oligomerized is
affected by the texture-related and acid-base properties
of supports. The main contribution to processes in
which densification products of hydrogenation
catalysts are formed is made by the concentration and
strength of surface Brønsted and Lewis acid centers of
aluminum oxides, accessible to reacting molecules.
catalytic tests under laboratory conditions, we
calculated the Thiele modulus whose values (0.26–
1.04) indicate that the oligomerization reaction of
unsaturated hydrocarbons occurs on the samples under
kinetic control in the absence of a diffusion hindrance
from the porous structure of the alumina supports
(Table 2). This enables correlation of data on the
formation rate of oligomeric deposits with the
concentration of Brønsted and Lewis acid centers.
Based on the results of analyses of the texture
characteristics (specific surface area S , average pore
According to the results of laboratory catalytic tests
(Table 3), the total formation rates of oligomers on the
sp
diameter d ) of the supports and data obtained in
av
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 8 2011

1
388
LAMBEROV et al.
A
no. 1
Aromatic ring
no. 2
Oligomers
of styrene
Oligomers
of diCPD
–1
ν, cm
Fig. 1. IR spectra of dry residues of chloroform extracts from the surface of the H
and (ν) wave number; the same for Fig. 2.
1
support and catalyst nos. 1 and 2. (A) Absorption
surface of unmodified supports H and H are 75.1 and
contribution of oligomers desorbed from the catalyst
surface increases to 37.0–79.3 and 68.0–88.2%, respec-
tively. An increase in the content of sodium in the
catalysts to 0.25 and 0.5 wt % is accompanied by a
decrease in the amount of oligomers adsorbed on the
surface by factors of 2.2–4.6 and 9.5–19.2, respectively.
1
2
–
1
6
9.6 μg gcat h , respectively. The fraction of hydro-
carbons desorbed into the liquid hydrocarbon phase is
–1
at a minimum for both the supports (0.1–0.4 μg g h ).
cat
In oligomeric compounds desorbed from the surface of
the supports into the liquid phase of the hydrogenizate,
we identified mostly derivatives of cyclopentadiene
An IR spectroscopic analysis of dry residues of
chloroform extracts (Figs. 1 and 2) demonstrated that
the extracts are composed of oligomers of styrene (λ =
(
CPD) and methylcyclopentadiene and copolymers of
styrene with CPD and isoprene.
–
1
Calculations of the oligomerization capacity of the
699 cm ), dicyclopentadiene (λ 1339, 1253, 899,
–
1
Pd/Al O3 catalyst samples under study (Table 3)
721 cm ), and aromatic hydrocarbons (λ 3056, 1602,
2
–
1
demonstrated that the total formation rate of
oligomeric compounds decreases, compared with that
of the initial alumina supports, from 75.1 to 38.1 and
1493 cm ) on the surface of worked-out supports, and
are represented by compounds of oligomers of styrene
–
1
–1
(λ 756, 699 cm ) and alkylbenzene (λ (810, 766 cm )
–
1
from 69.6 to 30.4 μg gcat h . In this case, the
on the surface of the catalysts. The largest amount of
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EFFECT OF CHEMICAL MODIFICATION
1389
A
Aromatic ring
no. 3
no. 4
Oligomers
of diCPD
–1
ν, cm
2
Fig. 2. IR spectra of dry residues of chloroform extracts from the surface of the H support and catalyst nos. 3 and 4.
deposits on the samples under study corresponds to
oligomers of styrene. The intensity of the absorption
bands in the IR spectra characterizing the amount of
deposits on the catalyst surface is the highest for
sample nos. 1 and 3, compared with catalyst nos. 2 and 4.
oxidation state (absorption bands at ν 2091 and
CO
–
1
1950 cm ), and, therefore, the attachment firmness of
oligomers will be determined by the acid-base
characteristics of aluminum oxide.
The acid-base properties of Pd/Al O catalysts were
2
3
It is known that the attachment firmness of
oligomers on the surface of a catalyst is determined by
its acid-base characteristics and by the charge of
palladium. For example, surface atoms of palladium in
an electron-deficient state can firmly adsorb
unsaturated hydrocarbons, thereby accelerating the
deactivation of the catalyst [4]. However, according to
IR spectroscopic data for adsorbed CO, the catalyst
samples synthesized on H and H supports have the
studied by IR spectroscopy. It is known that surface
hydroxy groups, whose proton affinity (PA) is 1380–
1440 kJ mol , can initiate growth of an oligomer chain
[1, 2, 5–10]. An estimation of the total Brønsted
acidity by the method of H-bonds, with the absorption
–
1
band intensities of the complexes AlOH...CO at ν
=
CO
–
1
2152–2162 cm used, demonstrated (Table 4) that,
among the samples obtained, the highest concentration
of acid centers is observed for catalyst nos. 1
1
2
–
1
–1
form of palladium particles with diameters of 5.9 (nos.
and 2) and 7.7–8.0 nm (nos. 3 and 4) in the zeroth
(39.1 μmol g ) and 3 (34.3 μmol g ). Sample nos. 2
1
and 4 have the minimum concentration of these centers
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390
LAMBEROV et al.
a
Table 4. Positions of absorption band peaks in IR spectra of OH groups and their concentration in Pd/Al
2
O
3
catalysts
–1
N, μmol g (%) for indicated catalyst
–
1
–1
Type of center
ν
OH, cm
PA, kJ mol
no. 1
no. 2
no. 3
14 (8.6)
no. 4
9 (8.7)
B
B
B
B
B
B
1
2
3
4
5
6
[Al
[Al
[Al
[Al
[Al
[Al
5
6
6
6
6
4
(–OH–)Al
(–OH–)Al
(–OH–)Al
(–OH–)Al
OH]
4
]
4
]
5
]
6
]
3663
3688
3710
3736
3759
3787
1380–1440
1380
24 (10.4)
45 (19.6)
23 (10.0)
78 (33.9)
37 (16.1)
23 (10.0)
230 (1.85)
14 (10.2)
29 (21.2)
19 (13.9)
40 (29.2)
18 (13.1)
17 (12.4)
137 (1.10)
36.5 (22.5)
13 (8.0)
25 (24.3)
8 (7.8)
1410
1440
64 (39.4)
23 (14.2)
12 (7.4)
38 (36.9)
14 (13.6)
9 (8.7)
1550–1590
1490
OH]
–
1
–2
ΣN, μmol g (μmol m )
162.5 (0.86)
103 (0.55)
–1
Concentration of Brønsted centers as indicated by IR absorption spectra of CO (νCO = 2157–2159 cm )
–
1
–2
ΣN, μmol g (μmol m )
39.1 (0.32)
24.1 (0.19)
34.3 (0.18)
19.7 (0.10)
a
6 5 4
PA, enthalpy of the reaction of proton detachment from the acid residue; Al , Al , Al , octa-, penta-, and tetrahedrally coordinated
aluminum atoms, respectively; concentration of Brønsted acid centers at a frequency of stretching vibrations of the complexes Al–
–
1
OH···CO, νCO = 2157–2159 cm .
2 3
Table 5. Lewis acid centers of Pd/Al O catalysts
–1
N
CO, μmol g (%), for indicated catalyst
–
1
Q
kJ mol
СО,–1
Type of center
νСО, cm
no. 1
no. 2
no. 3
no. 4
L
1
weak
2182–2183
2201–2206
27–30
35–38
550 (95.2)
500 (98.3)
460 (96.0)
416 (99.1)
L
2
of medium
27.6 (4.8)
8.9 (1.7)
19.4 (4.0)
3.9 (0.9)
strength
–
1
–2
ΣNСО[L
1
+ L
2
], μmol g (μmol m )
577.6 (4.66)
508.9 (4.10)
479.4 (2.55)
419.9 (2.23)
Concentration of medium-strength Lewis centers
–
1
–2
NСО[L
2
], μmol g (μmol m )
27.6 (0.22)
8.9 (0.07)
19.4 (0.10)
3.9 (0.02)
–
1
(
24.1 and 19.7 μmol g , respectively). According to
frequencies νCO revealed two types of aprotic centers
IR spectroscopic data for adsorbed carbon monoxide,
the amount of proton-donor groups is correlated with
L1–2 of different strengths, with heats of carbon mon-
oxide adsorption, QCO = 27–38 kJ mol (Table 5).
–
1
–1
the concentration of hydroxy groups at ν = 3688 cm
OH
Upon an increase in the sodium concentration from
–
1
(
PA = 1380 kJ mol ) (calculated from direct-
0
.25 to 0.5 wt %, the content of Lewis centers with
observation IR spectra). In all cases, the relative
decrease in the concentration of Brønsted centers upon
an increase in the content of sodium from 0.25 to
–1
QCO ≥ 35.0 kJ mol , which can activate oligo-
merization of unsaturated hydrocarbons [9–12],
–1
decreases by 68% (from 27.6 to 8.9 μmol g ) in
0
.5 wt % is about 40%.
sample nos. 1 and 2, and by 80% (from 19.4 to
–
1
An IR spectroscopic study of the Lewis acidity of
3.9 μmol g ) in sample nos. 3 and 4, prepared on the
the catalysts by analysis of the stretching vibration
H support (Table 5).
2
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EFFECT OF CHEMICAL MODIFICATION
3
Table 6. Catalytic characteristics and oligomerization activity of samples of a Pd/Al O catalyst
1391
2
Diene number, g
/100 g, in the
–
1
–1
2
N[B ]
N[L
2
]
ΣN[B
2
+ L
2
]
Formation rate, μg gcat h , of oligomers
I
2
operation period
Catalyst
no.
–
1
–2
desorbed into the adsorbed on the
liquid phase (%) catalyst surface (%)
Final
initial (4 h)
μmol g (μmol m )
total
(60 h)
1
2
3
4
39.1 (0.32) 27.6 (0.22) 66.7 (0.54)
24.1 (0.19) 8.9 (0.07) 33.0 (0.26)
34.3 (0.18) 19.4 (0.10) 53.7 (0.28)
19.7 (0.10) 3.9 (0.02) 23.5 (0.12)
20.0 (37.0)
30.2 (79.3)
32.1 (68.0)
26.8 (88.2)
34.0 (63.0)
7.9 (20.7)
15.1 (32.0)
3.6 (11.8)
54.0
38.1
47.2
30.4
0.80
0.63
0.72
0.68
3.70
0.90
1.98
0.73
–
1
An estimation of the total content of Brønsted
oligomers (from 7.9–34.0 to 3.6–7.9 μg gcat h ).
–
1
–1
(
PA = 1380 kJ mol ) and Lewis (Q > 35 kJ mol )
However, for sample nos. 1 and 3 (sodium content
CO
acid centers capable of initiating formation carbocation
structures demonstrated that the acidity of the catalysts
decreases on passing from sample no. 1 to catalyst nos.
0
.25 wt %), the 29.7% decrease in the concentration of
–
1
Lewis centers (from 27.6 to 19.4 μmol g ) does not
lead to a proportional lowering of the amount of
adsorbed oligomers, which falls by 55.6% (from 34.0
to 15.1 μg gcat h ). In this case, the fraction of
oligomers desorbed into the liquid phase is approxi-
–
1
3
, 2, and 4 from 66.7 to 53.7, 33.0, and 23.5 μmol g ,
respectively (Table 6). This behavior is correlated with
the total formation rate of the oligomers (Fig. 3).
–1
–
1
The formation rate of oligomeric compounds
adsorbed on the support surface (Table 6, Fig. 4),
which provide stable operation of the catalysts, is
largely determined by the concentration of aprotic L2
centers (compared with the total content of Brønsted
and Lewis centers). For example, on passing from
sample no. 1 to sample nos. 2–4, the 1.2-fold decrease
mately 1.6 times larger for sample no. 3 (32.1 μg g h ),
cat
–1
compared with catalyst no. 1 (20.0 μg gcat h ).
Probably, the amount of oligomers formed on the
surface decreases out-of-proportion on passing from
catalyst no. 1 to catalyst no. 3 because catalyst no. 1
has a higher surface concentration of aprotic L centers
(0.22 μmol m ), in contrast to the catalytic system no.
3 (0.10 μmol m ), which lead to a higher adsorption
probability of unsaturated compounds, with their
subsequent oligomerization on the support surface.
2
–
2
–
2
in the concentration of L Lewis centers (from 8.9–
2
–
1
2
1
7.6 to 3.9–19.4 μmol g ) is accompanied by a
.2 times decrease in the formation rate of surface
–1
–1
2 2
ΣN[B + L ], μmol g
2
ΣN[L ], μmol g
Fig. 3. Effect of the total content of Brønsted and Lewis
Fig. 4. Effect of the concentration of aprotic L centers, ΣN
2
acid centers, ΣN[B
oligomers, ΣW.
2
+ L
2
], on the total formation rate of
[L ], on the formation rate of oligomers, Wads.surf, adsorbed
2
on the catalyst surface.
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LAMBEROV et al.
Thus, modification of a support with sodium in
oligomeric compounds and the catalyst is not
deactivated (Table 6).
amount of 0.25 to 0.5 wt % causes a decrease in the
surface concentration of Lewis centers and leads to a
low adsorption probability of unsaturated hydro-
carbons on aprotic centers, with their subsequent
oligomerization. In this case, the increase in the
amount of oligomeric compounds desorbed into the
liquid phase may be due to hydrogenation of the
oligomers, followed by their desorption into the
hydrocarbon flow, as indicated by the data in Table 3.
The fraction of oligomers desorbed from the catalyst
surface is approximately 1.6 times smaller for sample
Thus, modification of alumina supports with 0.5 wt %
sodium favors an increase in the stability of hydro-
genation of diene and vinyl aromatic hydrocarbons in
the BTX fraction.
CONCLUSIONS
(
1) To optimize the chemical composition and
synthesis technology of catalysts for hydrogenation of
diene and vinyl aromatic hydrocarbons in the benzene–
toluene–xylene fraction, the effect of the active
component (palladium) and acid characteristics of
alumina supports modified with alkaline additives on
the oligomerization activity and stability of alumina-
palladium catalysts was studied.
–
1
no. 3 (32.1 μg g h ), compared with catalyst no. 1
cat
–
1
(
20.0 μg gcat h ). For catalytic system nos. 2 and 4,
which contain 0.5 wt % sodium, the amounts of
desorbed oligomers are approximately the same: 30
–
1
and 27 μg g h , respectively.
cat
The change in the activity of the catalysts was
evaluated during 60 h by the decrease in the
concentration of diene hydrocarbons in hydrogenizates
and by the diene number (Table 6). In the initial period
of the tests (4 h), samples exhibited a high activity in
hydrogenation of diene hydrocarbons, with the diene
number being approximately the same for all the
(2) It was shown that deposition of palladium on
aluminum oxide leads to hydrogenation of
densification products on its surface, which leads to a
faster formation of oligomers desorbed into the liquid
phase, which are oligomers of styrene and dicyclo-
pentadiene.
(
3) It was found that the amount of oligomeric
samples (0.63–0.80 g I /100 g). This indicates that
2
compounds being formed is determined by the total
sodium additives have no effect on the adsorption
properties of palladium and, accordingly, on the
activity of the catalysts. However, catalyst nos. 1 and
–1
concentration of Lewis (Q
> 35 kJ mol ) and
CO
–1
Brønsted (PA = 1380 kJ mol ) centers. An increase in
the content of these centers on the catalyst surface
from 23.5 to 66.7 μmol g is accompanied by a rise by
2
, which are modified with 0.25 wt % sodium and have
–1
high concentrations of Lewis centers, are the least
stable in the reaction of hydrogenation of diene and
alkenyl aromatic hydrocarbons contained in the BTX
fraction. For these samples, the content of diene
hydrocarbons in the hydrogenation reaction products
increases after 60 h of operation (as evidenced by the
diene number) by factors of 4.6 and 2.75, respectively.
This indicates that the hydrogenation activity of these
samples decreases because of the formation of surface
oligomers, which shield active centers. The highest
stability and activity among the samples studied is
observed for catalysts containing 0.5 wt % sodium.
The concentration of diene hydrocarbons in hydro-
genation reaction products increases for the catalyst
–1
2
3.6 μg gcat
h
of the reaction rate of oligomer
formation.
(
4) It was shown that strong aprotic centers with
–
1
QCO > 35 kJ mol determine the amount of oligomeric
compounds adsorbed on the support surface and the
duration of the inter-regeneration life of the catalysts.
(
5) It was demonstrated that introduction of sodium
into a catalyst in an amount of up to 0.5 wt % makes
lower the concentration of aprotic L centers with Q
>
CO
2
–1
3
5 kJ mol , which, in turn, causes a decrease in the
formation rate of surface oligomers from 15.1–34.0 to
–1
3
.6–7.9 μg gcat h and leads to a high stable activity of
hydrogenation of diene and vinyl aromatic hydrocarbons.
prepared on an H support (0.5 wt %) during 60 h of its
1
operation by only a factor of 1.4 because of the
decrease in the activity of hydrogenation of un-
saturated hydrocarbons. For the sample synthesized
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