Enhancement of catalytic activity for selective oligomerization of ethylene by microwave treatment

Article abstract of DOI:10.1134/S0965544112040123

It has been found that the activity of a catalyst for selective oligomerization (trimerization) of eth-ylene based on a chromium salt and organoaluminum compounds (OACs) can be substantially enhanced by the microwave irradiation of organoaluminum compounds directly prior to mixing them with the chromium salt and a pyrrole ligand. The effects of irradiation conditions and the nature of the reagents used have been studied. The catalyst obtained according to an optimal procedure outperforms known analogs in activity.

Full text of DOI:10.1134/S0965544112040123

ISSN 0965ꢀ5441, Petroleum Chemistry, 2012, Vol. 52, No. 4, pp. 253–260. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © T.M. Zilbershtein, A.A. Nosikov, A.I. Kochnev, M.V. Lipskikh, A.K. Golovko, 2012, published in Neftekhimiya, 2012, Vol. 52, No. 4, pp. 284–291.
Enhancement of Catalytic Activity for Selective Oligomerization
of Ethylene by Microwave Treatment
T. M. Zilbershteina, A. A. Nosikova, A. I. Kochneva, M. V. Lipskikha, and A. K. Golovkob
a OOO NIOST, Kuzovlevskii trakt 2, Tomsk, 634067 Russia
eꢀmail: ztm@niost.ru
b Institute of Petroleum Chemistry, Siberian Branch, Russian Academy of Sciences,
Akademicheskii pr. 3, Tomsk, 634055, Russia
Received January 12, 2012
Abstract—It has been found that the activity of a catalyst for selective oligomerization (trimerization) of ethꢀ
ylene based on a chromium salt and organoaluminum compounds (OACs) can be substantially enhanced by
the microwave irradiation of organoaluminum compounds directly prior to mixing them with the chromium
salt and a pyrrole ligand. The effects of irradiation conditions and the nature of the reagents used have been
studied. The catalyst obtained according to an optimal procedure outperforms known analogs in activity.
DOI: 10.1134/S0965544112040123
The production of ethylene copolymers subishi Chemical can be distinguished [4–5]. It was
that involve 1–2 to 8–10% light
α
ꢀolefins, mainly, found that the addition of Lewis acids that are not prone
hexeneꢀ1 and octeneꢀ1 in their manufacture is curꢀ to coordination, such as B(C6F5)3, makes it possible to
rently increasing. Most traditional ethylene oligomerꢀ increase the activity of the catalyst system. The Phillips
ization processes used for the manufacture of these and Mitsubishi catalytic systems are activated by readily
olefins results in a wide range of products with a total available alkylaluminum compounds, whereas other
yield of hexene and octene no more than 50%. In this designers use expensive methylaluminoxane (MAO) [6–
connection, active development of processes that will 8]. Of these, the ethylene trimerization system based on
yield hexeneꢀ1 from ethylene with a selectivity higher diphosphazane
than 90% or octeneꢀ1 with a selectivity of more than nyl)2PN(Me)P(
ligands
(e.g.,
(oꢀmethoxypheꢀ
o
ꢀmethoxyphenyl)2), chromium salts,
70% is under way. In the latter case, hexeneꢀ1 prevails and MAO is the most active among the currently known
in the byproducts.
relevant systems. Its activity reaches 1033 kg/(g Cr h) at
2 MPa of ethylene and a Cr : Al ratio of 1 : 300 [7]. Given
the difference in pressure used, this system is 30–40 times
more active than the Phillips catalyst per gram of chroꢀ
mium.
The first of these processes—selective oligomerizaꢀ
tion (trimerization) of ethylene to form hexeneꢀ1—was
implemented by Chevron Phillips in 2004 in Qatar.
Used in this process were the Phillips chromiumꢀ
based homogeneous catalysts, which are described
The role of the halogenꢀcontaining component in
mainly in patent literature [1–3]. The catalytic system the catalytic trimerization system was discussed in a
activated by alkylaluminum compounds is obtained by number of publications [9, 10]. In particular, it has
mixing, for example, chromium(III) ethylhexanoate been assumed that an increase in the activity of the
(
Cr(EH)3), a pyrrole ligand (e.g., 2,5ꢀdimethylpyrꢀ catalyst system when using a halide coꢀcatalyst can be
role, DMP), an organoaluminum activator (preferaꢀ due to a change in the structure (form) in which the
bly triethylaluminum, TEA) and a halogenꢀcontainꢀ activator occurs. It is known [11] that triethylalumiꢀ
ing cocatalyst (e.g,, diethylaluminum chloride num (TEA) in the free state and solutions, except for
(DEAC) or hexachloroethane). The typical ratio of very dilute solutions, occurs under normal conditions
components in the Phillips catalyst is Cr(EH)3
:
predominantly (>99%) as a dimer Al2Et6 with two
DMP : TEA : DEAC = 1 : 3 : 11 : 8. The activity of the bridging ethyl groups.
catalyst, according to patent data, is up to 157 kg of
Kinetic studies show that the active species in the
olefins/(gCr h) at 10 MPa of ethylene. At lower presꢀ
sures, a secondꢀorder dependence of the catalyst
activity on the pressure of ethylene is observed.
Thus, at 4.6 MPa, the activity is about 68.6 kg of oleꢀ
fins/(gCr h) [1].
reactions of TEA is usually the monomeric form [12].
According to Yang et al. [9], the formation of an unstaꢀ
ble complex of monomeric TEA with a halide can
increase its reactivity. In the catalyst system, TEA
plays the role of a deprotonating agent for the pyrrole
There is a large number of modifications to these catꢀ compound to give the pyrrolide anion, destroys the
alytic systems, among which the developments of Mitꢀ organic acid radical in the chromium salt, and reduces
253

254
ZILBERSHTEIN et al.
chromium(III). An enhancement in the reactivity of lyst activity and selectivity for ethylene trimerization
TEA can lead to an increase in the yield of species that by microwave irradiation, sets of experiments 1–3, 4–
are active in the reaction of selective oligomerization. 11, 12–14, 15–18, and 19–20 were carried out. The
In particular, the increased activity of TEA as a reducꢀ catalysts preparation and testing conditions were
ing agent can facilitate an increase in the concentraꢀ maintained to be identical within a set, except for the
tion of chromium(I) compounds, which appear to be conditions of microwave treatment of OAC and/or the
responsible for the activity of the catalyst in the trimꢀ catalyst system as a whole.
erization of ethylene [13].
As one of the ways to increase the activity of a catꢀ
alyst, microwave irradiation can be used. Thus, microꢀ
Catalyst Preparation Procedure
wave irradiation was used during a catalyst activation
reaction to increase the activity of a silicaꢀsupported
catalyst based on the cyclopentadienyl zirconium
complex and MAO [14]. However, we have not been
aware of any example of use of microwave treatment
for the preparation of olefin oligomerization catalysts.
The purpose of this work was to study the feasibility
of increasing the activity of an ethylene trimerization
catalyst system by microwave irradiation during prepꢀ
aration of the catalyst. The basis for these studies was
the assumption that microwave treatment can faciliꢀ
tate the monomerization of TEA, which would make
it possible to increase the activity of the resulting cataꢀ
lyst and to reduce the required pressure of the reactant
ethylene.
A weighed sample of chromium(III) ethylhexꢀ
anoate containing a required amount of chromium,
dimethylpyrrole (3 mol with respect to chromium),
and 5 mL of toluene were placed in a flask in which a
nitrogen atmosphere was created then. A solution of
TEA in heptane (154 mg/mL) with a required amount
relative to chromium was mixed with a calculated
amount of a cocatalyst. To this mixture, another 1 mL
of toluene was added in runs 1–3, 0.5 mL of toluene
was added in runs 4–7 and 9–11, and no toluene was
used in runs 8 and 12–20. The resulting OAC solution
was subjected to microwave irradiation within a speciꢀ
fied time (Table 1), and then added to a mixture of
Cr(EH)3, DMP, and heptane. In control runs (1, 4, 12,
16), the microwave treatment of the OAC was not
used, as well as in run 11.
After mixing all components, the catalyst in runs 3
and 11 was subjected to microwave irradiation for a
given time. Fifteen minutes after mixing the reagents,
the solvents were evaporated under vacuum at room
temperature (residual vapor pressure of 600–800 Pa).
The residue in the flask was diluted with nꢀheptane to
4 mL. The prepared catalyst solution was then tested
in the ethylene trimerization reaction.
EXPERIMENTAL
All the solvents used in the study were dried by
refluxing over metallic sodium followed by distillation
from sodium hydride. A TEA solution in heptane was
prepared from a 25% TEA solution in toluene by
removing the solvent under vacuum (500–700 Pa),
and diluting the residue with heptane to a required
concentration. To prepare DEAC solutions, a
1.0 M DEAC solution in hexane was used. The
reagent 2,5ꢀdimethylpyrrole of 98% purity was used
without further purification. Anhydrous chroꢀ
mium(III) ethylhexanoate was prepared according to
a known procedure [15] with additional drying in a
Catalyst Testing Procedure
The reactor was charged with 200 mL of a solvent
(nꢀheptane in runs 1–18, cyclohexane in runs 19–20).
In runs 1–11, TEA was also added to mixture in the
reactor (Table 1). In runs 19–20, the solvent was satuꢀ
rated with hydrogen under a pressure of 100 kPa prior
to the addition of the catalyst. Through a flow meter,
the reactor was filled with a specified amount of ethylꢀ
vacuum of 200 Pa at 200 С.
°
The microwave irradiation of OAC solutions of was
conducted in a MARSꢀ5 microwave oven, manufacꢀ
tured by CEM, in the continuous mode at a rated
power of 400 W.
ene, and the temperature was raised to 80 С. Then,
°
All catalyst samples prepared using microwave the catalyst was introduced into the reactor at an
treatment were tested in the ethylene trimerization excess pressure of argon (runs 1–18) or ethylene (runs
reaction run in a special setup designed for this purꢀ 19–20) and the reaction was conducted; in the latter
pose. The setup included a highꢀpressure reactor of case, the pressure of 0.8 MPa during the course of the
0.5 L capacity equipped with pressure and temperaꢀ reaction was maintained by pumpingꢀin ethylene as
ture sensors, an oilꢀfilled cooling jacket connected to far as it was consumed. After a specified reaction time,
a thermostat, a 6ꢀmL chamber for catalyst loading 1 mL of isopropanol was added. The reactor was
under pressure, and an ethylene feed line. The setup cooled to room temperature, and the pressure was
was equipped with a computerꢀrun process control released. The resulting polymer was separated and
system, which made it possible to record sensor readꢀ dried for 8 h at 100 С. The reaction mixture was anaꢀ
°
ings and to control the reactor temperature, the ethylꢀ lyzed by gas chromatography on an Agilent 7890A
ene feed line, etc. The conditions of catalyst preparaꢀ chromatograph with a FID. A sample (size, 1 mm3)
tion and testing in different experiments are shown in was injected with a model 7683B (G2913A) autosamꢀ
Table 1. To study the possibility of increasing the cataꢀ pler. The evaporator for use with capillary columns
PETROLEUM CHEMISTRY Vol. 52
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ENHANCEMENT OF CATALYTIC ACTIVITY FOR SELECTIVE OLIGOMERIZATION
255
Table 1. Conditions of the preparation and testing of ethylene trimerization catalysts
Catalyst preparation conditions
Catalyst testing conditions
MW irradiation
delay after
MW irradiaꢀ
tion of OAC, mol/mol Cr mol/mol Cr mol/mol Cr C H , g
Run
time, min
cocatalyst,
TEA,
TEA,
charge
Cr, mg
time, min
2
4
min
OAC catalyst
1
2
6
0
3
3
0
1
6
6
6
6
6
0
0
0
0
10
0
0
0
0
0
0
0
6
–
<0.5
<0.5
–
–
12.8
12.8
12.8
15.5
15.5
15.5
15.5
15.5
15.5
15.5
15.5
50
4.2
4.2
4.2
8.5
8.5
8.5
8.5
8.5
8.5
8.5
8.5
0
32
30
15
30
16
16
16
16
16
16
16
16
16
16
16
12
15
12
30
30
30
6
–
32
32
37
37
37
37
37
37
37
37
32
37
37
37
37
37
37
0
3
6
–
4
4
DEAC (2.3)
DEAC (2.3)
DEAC (2.3)
DEAC (2.3)
DEAC (2.3)
DEAC (2.3)
DEAC (2.3)
DEAC (2.3)
5
4
<0.5
<0.5
<0.5
3
6
4
7
4
8
4
9
4
3
10
11
4
9
4
–
[a]
12
13
14
3
0
0
0
0
0
0
0
0
0
–
CHCl (2)
3
[a]
[a]
[b]
3
6
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
–
CHCl (2)
50
0
3
3
6
3
6
6
3
6
0
CHCl (2)
50
0
3
15
16
17
18
4
DEAC (3)
16
8
4
DEAC (5.3)
DEAC (12)
DEAC (12)
DEAC (14)
DEAC (14)
21
0
1
38
0
4
16
0
[c]
19
20
0.8
0.8
36
0
[c]
36
0
0
[a], heptane is used as a solvent in the catalyst preparation;
[b], irradiation of a TEA solution without CHCl as a cocatalyst;
3
[c], 5 equiv. of DMP relative to Cr are used; a constant ethylene pressure of 0.8 MPa.
was a G3440A 113 injection port: temperature, 240
°
С
;
Measurement of the OAC Solution Temperature
after Microwave Irradiation
pressure, 111 kPa; septum airflow rate, 0.05 mL/s;
split ratio, 200 : 1. The mixture of components was
separated on an HPꢀ5 capillary column coated with a
nonpolar stationary phase (5% of diphenylpolysiloxꢀ
ane and 95% of dimethylpolysiloxane) of a 30 m
length, a 0.32 mm inner diameter, and a film thickness
A 3 : 1 TEA–DEAC mixture as 4 mL of solution in
a 2 : 1 toluene–hexane blend with an aluminum conꢀ
centration of 1.68 mol/L was placed in a plastic
syringe with a waxꢀplugged hole in the wall for insertꢀ
ing an electronic thermometer. The mixture of the
OACs was irradiated for 6 min. Then the temperature
in the syringe was measured with an ETSꢀD5 therꢀ
of 0.25
µ
m. Analysis conditions: helium the carrier
gas, column temperature programming from 50 to
120°
280
С
at a heating rate of
at a rate of 30°C/min.
7 C/min and from 120 to
°
mometer (accuracy: 0.1
°
С) by forcing the plug into
°
С
the syringe. The temperature of the solution 40 s after
PETROLEUM CHEMISTRY Vol. 52
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2012

256
ZILBERSHTEIN et al.
Table 2. Results of testing for catalysts 1–11
Amount in product, wt %
C without
Irradiation Cr/DMP/ Activity[a],
Run
Polymer,
wt %
time, min TEA/DEAC kg/(g Cr h)
6
1ꢀC 1ꢀC /C
C
C
C
C
12+
6
6
6
4
8
10
1ꢀC
6
1
2
3
4
5
6
0
1 : 3 : 17 : 0
1 : 3 : 17 : 0
3.6
7.8
66.3
85.0
86.7
88.0
96.5
93.6
94.4
94.4
93.6
93.0
94.9
92.0
1.2
1.9
0.9
0.3
7.3
1.9
1.3
1.8
2.7
0.4
5.5
11.7
9.8
7.7
3.1
4.8
4.4
4.5
5.2
5.6
4.4
6.1
2.5
2.3
2.3
4.6
5.3
3.2
2.1
2.7
3.1
3.8
5.8
13.6
14.8
18.2
5.5
4.6
8.0
14.4
0.8
3.6
8.0
6.5
4.9
4.5
2.5
1.7
0.55
0.20
0.21
0.31
0.16
0.09
0.04
0.21
0.10
0.67
0.10
2.5
63.2
56.5
85.7
70.6
73.6
76.7
75.6
73.8
81.6
70.0
[b]
2.5(+10) 1 : 3 : 17 : 0
6.5
0
1 : 3 : 24 : 2.3
1 : 3 : 24 : 2.3
1 : 3 : 24 : 2.3
1 : 3 : 24 : 2.3
1 : 3 : 24 : 2.3
1 : 3 : 24 : 2.3
1 : 3 : 24 : 2.3
1 : 3 : 24 : 2.3
4.5
1
9.3
8.3
6
18.7
21.6
11.2
13.8
5.1
8.9
[c]
7
6
8.9
[d]
[d]
[e]
8
9
6
6
9.8
10.3
7.3
10
6
[b]
11
0(+6)
9.0
10.9
[a], taking into account only liquid reaction products;
[b], microwave irradiation time for all components of the catalyst after mixing is given in the brackets;
[c], without toluene during irradiation of TEA/DEAC;
[d], delay 3 min between irradiation of TEA/DEAC and their mixing with Cr(EH) /DMP;
3
[e], delay 9 min between irradiation of TEA/DEAC and their mixing with Cr(EH) /DMP.
3
the end of irradiation was 32.4
°
С
at an ambient temꢀ mation of the catalyst. In this case, considerable
warming of the catalytic system was noted. No appreꢀ
perature of 26.7
°
С.
ciable warming was observed during the microwave
irradiation of a TEA solution: the temperature rise did
RESULTS AND DISCUSSION
not exceed
6°С.
Experiments without Cocatalyst
The testing results given in Table 2 for the obtained
catalysts show that in runs 2 and 3, the activity of the
catalysts prepared with the use of microwave irradiaꢀ
tion is almost two times that in run 1. In this case, the
reaction selectivity for hexeneꢀ1 remained at low levꢀ
els. In run 3, the selectivity even decreased owing to
the formation of a greater amount of heavier byprodꢀ
ucts. It can be concluded that microwave irradiation of
the TEA solution promotes the activity, rather than
selectivity, of the catalyst whereas additional microꢀ
wave irradiation of the entire catalyst system slightly
reduces both the activity and selectivity of the catalyst.
Assuming that the catalyst activity and selectivity
can be increased by microwave irradiation of TEA
without the use of halogenꢀcontaining components,
we carried out runs 1–3 without a cocatalyst. The
component ratio of the Cr(EH)3 : DMP : TEA catalyst
system was 1 : 3 : 12.8. To eliminate the influence of
possible impurities on the catalyst, additional 4.2 mol
TEA/mol Cr were added into the reactor prior to the
introduction of the catalyst. The catalyst was prepared
according to the general procedure, but TEA in runs 2
and 3 was subjected to microwave irradiation for
2.5 min and then was immediately mixed with
Cr(EH)3 and DMP in toluene. In run 3, in addition,
the resulting catalyst system was irradiated for 10 min
immediately after mixing the reagents in order to
Effect of Microwave Irradiation Time
In the subsequent experiments, a cocatalyst was
assess the effect of microwave irradiation on the forꢀ used to increase the selectivity of the reaction for hexꢀ
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ENHANCEMENT OF CATALYTIC ACTIVITY FOR SELECTIVE OLIGOMERIZATION
257
Table 3. Results of testing of catalysts containing CHCl3
Amount in product, wt %
MW irradiꢀ
Run ation time,
Cr/DMP/ Activity[a],
Polymer,
wt %
C withꢀ
TEA/CHCl kg/(g Cr h)
6
3
1ꢀC 1ꢀC /C
C
C
C
C
12+
min
6
6
6
4
8
10
out 1ꢀC
6
12
13
14
0
1 : 3 : 50 : 2
1 : 3 : 50 : 2
1 : 3 : 50 : 2
5.5
12.4
24.7
81.6
96.0
96.1
90.0
2.1
1.7
0.6
3.4
3.5
8.6
4.8
2.3
2.1
5.2
4.5
5.3
2.9
1.1
5.8
0.55
0.20
0.21
[b]
6
[c]
86.9
77.6
6
[a], taking into account liquid products alone;
[b], microwave irradiation of TEA in hexane, then mixing it with CHCl and addition to Cr(EH) /DMP;
3
3
[c], microwave irradiation of TEA and CHCl in hexane, then addition to Cr(EH) /DMP.
3
3
eneꢀ1. In runs 4–11, the catalyst system consisted of the formation of liquid products, since the liquid :
Cr(EH)3, DMP, TEA, and DEAC in the 1 : 3 : 15.5 : polymer product ratio increases in direct proportion to
2.3 ratio. In addition, 8.5 equivalents of TEA was the total activity of the catalyst.
introduced into the reactor prior to charging with the
catalyst. The catalyst preparation procedure did not
Chloroform as a Cocatalyst
involve microwave irradiation in run 4, the mixture of
TEA and DEAC was subjected to microwave irradiaꢀ
tion for 1 min before mixing with Cr(EH)3 and DMP
in run 5, and the irradiation time was increased to
6 min in run 6. Run 7 was similar to run 6, but the OAC
solutions to be irradiated did not contain toluene.
To examine the effect of microwave irradiation
when halogenꢀcontaining organic cocatalysts are
involved, runs 12–14 have been carried out in which
chloroform was used. The component ratio Cr(EH)3
:
DMP : TEA : CHCl3 was 1 : 3 : 50 : 2, In run 12,
microwave irradiation was not used in the catalyst
preparation, a mixture of a TEA solution and chloroꢀ
form was added to Cr(EH)3 and DMP in toluene. In
run 13, a solution of TEA in heptane was subjected to
microwave irradiation for 6 min before mixing with
chloroform, and the resulting mixture was immediꢀ
ately mixed with the chromium salt and DMP. In run
14, a TEA solution and chloroform in heptane was
subjected to microwave irradiation for 6 min before
mixing with Cr(EH)3 and DMP in toluene. The testꢀ
ing of catalysts 12–14 showed that microwave radiaꢀ
tion in this case also has a positive effect (Table 3).
However, there is a significant difference in product
composition between runs 13 and 14. In the former
case, the TEA alone was irradiated and an approxiꢀ
mately twofold increase in catalytic activity compared
to run 12 and a slight increase in the hexeneꢀ1 selectivꢀ
ity was observed. In run 14, in which the solution of
TEA together with CHCl3 was subjected to microwave
irradiation, the activity increased by a factor of 4
(compared with run 12), but the reaction selectivity
simultaneously decreased. It is also noteworthy that
the formation of butenes in run 14 substantially
decreased as compared with runs 12 and 13.
A comparison of runs 4–6 shows that microwave
irradiation increases the activity of the catalyst. A
longer microwave irradiation time in run 6 (6 min)
compared with run 5 (1 min) provides a more active
catalyst. The presence or absence of toluene in the
OAC solution has no significant influence on the
observed effect.
Relaxation Time of the Microwave Irradiation Effect
To elucidate the issue of relaxation time for the
effect of alteration in the OAC properties by microꢀ
wave irradiation, we performed. runs 8–10. For evaluꢀ
ating the change with time in the effect observed after
microwave irradiation of OAC, a delay between the
end of 6ꢀminute microwave irradiation of a solution of
TEA/DEAC and mixing it with Cr(EH)3 and DMP
was made during the preparation of the catalyst sysꢀ
tem. For runs 2, 3, and 5–7 the interval between the
end of irradiation and mixing did not exceed 30 s. The
delay was 3 min in runs 8 and 9, and it was increased to
9 min in run 10. The results of testing the catalysts in
the trimerization reaction showed that the catalytic
activity decreased with the increasing delay time. The
conclusion is that the observed effect of microwave
irradiation on the OAC is temporary and, as the time
of delay between the end of irradiation and mixing
with the chromium salt increases, it decreases until
complete disappearance.
Microwave Irradiation after Mixing All Catalyst
Components
In run 11, the microwave treatment of the
Note that in the case of the catalysts prepared with TEA/DEAC mixture was not practiced. before mixing
the use of microwave treatment, the proportion of the with the chromium salt and DMP Instead, the final
byproduct polymer decreases. Apparently, this is mixture was subjected to microwave irradiation for
mainly due to an increased activity of the catalyst in 6 min immediately after mixing the components of the
PETROLEUM CHEMISTRY Vol. 52
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258
ZILBERSHTEIN et al.
The selectivity in runs 4–14 with the use of DEAC
%
and CHCl3 as cocatalysts is significantly higher than in
runs 1–3 in the absence of a coꢀcatalyst, in agreement
with published data. However, there is no unambiguꢀ
ous correlation between the microwave treatment and
selectivity in these experiments. That is, it can be conꢀ
cluded that the TEA : cocatalyst ratio should be
reduced for improving the selectivity of the reaction.
100
95
90
85
80
75
70
65
60
Optimization of Catalyst System Composition
10
TEA
8
6
4
2
0
/
DEAC
Runs 15–18 on the optimization of the composiꢀ
tion of the catalyst system to enhance its activity and
selectivity have shown that in the case of microwave
treatment of TEA and DEAC, the optimal TEA :
DEAC ratio is 3.2 (Fig. 1). This is significantly higher
than the typical ratio for the Phillips catalyst (1.3–1.4)
[1]. At larger values of the ratio, the activity and selecꢀ
tivity of the reaction decreases; at lower values, the
selectivity further increases, but the activity of the catꢀ
alyst substantially decreases. The general character of
the dependence of the activity and selectivity of the
system upon the TEA/chlorine compound ratio is
similar to published results for the system with 1,1,1ꢀ
trichloroethane as a coꢀcatalyst [9]. Table 4 shows the
results of testing of catalysts 19 and 20 prepared with
and without the use of microwave irradiation, respecꢀ
tively. In contrast to previous experiments, these tests
were performed at a constant pressure of 0.8 MPa of
ethylene maintained to compensate for its consumpꢀ
tion. The TEA/DEAC ratio was close to the optimal
value; in addition, to increase the activity of the cataꢀ
lyst, the solvent was saturated with hydrogen before
the reaction. The plot of ethylene consumption during
the reaction is shown in Fig. 2. The activity and selecꢀ
tivity in run 19 of the catalyst prepared using microꢀ
wave irradiation of the OACs, was more than 1.5 times
higher than the activity in run 20 for the catalyst preꢀ
pared without microwave treatment.
у = –2.4251
R
x
= 0.9501
+ 101.03
Selectivity for 1ꢀС
Purity, 1ꢀС in С
6
2
6
6
у = –0.6121x + 100.91
R
Linear
Selectivity for 1ꢀС )
2
= 0.9698
(
6
Linear
Purity,1ꢀС in С )
(
6
6
Fig. 1. Effect of the TEA/DEAC ratio on the activity and
selectivity of the catalyst.
catalyst. The testing of the catalyst obtained in this
manner also showed an approximately twofold
improvement in activity compared with the control
run 4. However, the activity of the catalyst in run 11 is
almost twice lower than in run 6, in which the
TEA/DEAC mixture only was subjected to microwave
irradiation for 6 min and is comparable with the result
of run 5 with a TEA/DEAC irradiation time of 1 min.
Note that the composition of the products formed in
runs 5 and 11 is similar.
It is known from the patent data that the formation
of the Phillips catalytic system takes place within 5–
15 min after mixing the components [3]. It can be
assumed that in run 11, as well as in the experiments
with the irradiation of OAC only, the impact of the
microwave radiation on the OAC that has not yet
reacted with the chromium salt plays the determining
role. In the case of run 5, complete conversion of OAC
to the activated form does not occur for 1 min of
microwave irradiation. Thus, it may be assumed that
Cr(EH)3 and DMP interact with partially activated
Nature of Microwave Irradiation Effect
To study the nature of the action of microwave
radiation on OACs, additional experiments were perꢀ
formed. At an ambient temperature of 26.7
temperature of the 3 : 1 mixture of TEA and DEAC
at 40 s after the end of microwave
°
С, the
OACs in these cases and, consequently, the properties solutions was 32.4
С
°
of the resulting catalytic systems turn out to be similar. treatment. Since the OAC solution was added in an
Table 4. Results of testing of the optimized catalysts
Amount in product, wt %
MW irradiaꢀ
Cr/DMP/ Activity[a],
TEA/DEAC kg/(g Cr h)
Polymer,
wt %
Run tion time,
min
C without
6
1ꢀC 1ꢀC /C
C
C
C
C
12+
6
6
6
4
8
10
1ꢀC
6
19
20
6
0
1 : 5 : 36 : 14
1 : 5 : 36 : 14
101.7
51.0
89.9
83.3
99.3
97.0
0.9
1.8
0.6
2.6
0.3
0.4
6.9
8.4
1.4
3.5
0.05
0.06
[a], taking into account only liquid reaction products.
PETROLEUM CHEMISTRY Vol. 52
No. 4
2012

ENHANCEMENT OF CATALYTIC ACTIVITY FOR SELECTIVE OLIGOMERIZATION
259
3
C H , cmм
2
4
40000
35000
30000
25000
20000
15000
10000
5000
Run 19
Run 20
0
00:00:00 0:05:00 0:10:00 0:15:00 0:20:00 0:25:00 0:30:00 0:35:00
Time
Fig. 2. Ethylene consumption during runs 19 and 20.
amount of 10–15% of the volume of the reaction mixꢀ mium salt. The observed effect is temporary in nature
ture the fact that during the catalyst preparation, we
may actually rule out that the observed effect is due to
dielectric heating of the reactants by the action of
microwave field.
and weakens with the increasing time between microꢀ
wave irradiation of the OACs and mixing them with the
chromium salt. Very low heating of the OAC solution
during the irradiation allows the thermal nature of the
effect of microwave treatment to be disregarded. The
cryoscopic data indicate a partial temporary monoꢀ
merization of TEA by microwave irradiation.
To verify the hypothesis of possible monomerizaꢀ
tion of TEA under the action of microwave irradiaꢀ
tion, the cryoscopic determination of the molar mass
(MM) of TEA was performed 6 min after microwave
irradiation of the TEA solution in toluene. The meaꢀ
sured MM value was 160 15 g/mol (temperature
It has been shown that an ethylene trimerization
catalyst with a higher activity can be obtained by
microwave irradiation of a TEA and cocatalyst soluꢀ
tion and immediate mixing of the irradiated solution
measurement 0.1
value at 3 min after exposure was 202
perature measurement with accuracy 0.01
°
С
) 1.5 min after irradiation. This
5
g/mol (temꢀ
°
С). The with a chromium salt and DMP. As prepared with the
determination of the molar mass of TEA 15 min after
irradiation for the same two solutions yielded results
use of microwave irradiation of the TEA and DEAC
solution, the Cr/DMP/TEA/DEAC catalyst with a
component ratio of 1 : 5 : 36 : 14 optimized in activity
and selectivity is characterized by an order of magniꢀ
tude higher activity than the similar Phillips catalyst.
This makes it possible to make the reaction conditions
substantially milder in terms of ethylene pressure,
namely, to lower it from 4.5–5.5 to 0.8–1.2 MPa. For
industrial implementation, lowering the pressure leads
to improvement in the economics of the process by
substantially reducing capital expenditures. In addiꢀ
tion, the use of the available alkylaluminum comꢀ
pounds instead of expensive MAO further improves
the efficiency of the catalyst prepared by the method
developed in this study.
240 20 and 224
5 g/mol, respectively. The latter
values correspond to the MM of the TEA dimer
(228.32 g/mol). The values of MM in the first minutes
after irradiation are intermediate between the MM of
the monomer (114.16 g/mol) and the dimer of TEA.
Thus, the cryoscopic data indicate a partial monomerꢀ
ization of TEA by microwave irradiation. The differꢀ
ence in MM value at 1.5 and 3 min after exposure, as
well as the return in 15 min to the value of the MM
characteristic of the dimer, shows the temporary
nature of the monomerization of TEA. This is consisꢀ
tent with the results of runs 8–10 demonstrating that
the effect of increasing catalyst activity using irradiꢀ
ated OACs is observed within a few minutes after irraꢀ
diation and rapidly decreases with the increasing time
between the end of microwave irradiation and mixing
OACs with the chromium salt.
ACKNOWLEDGMENTS
Thus, it has been found that the activity of the ethꢀ
ylene trimerization catalyst can be substantially
enhanced by microwave irradiation of organoalumiꢀ
num compounds as components of the catalytic sysꢀ
The authors thank OAO Sibur Holding for financꢀ
ing the works and permission to publish and
M.A. Lebedeva, a junior research fellow at OOO
tem, immediately before their mixing with the chroꢀ NIOST, for performing gas chromatographic analyses.
PETROLEUM CHEMISTRY Vol. 52
No. 4
2012

260
ZILBERSHTEIN et al.
8. T. Yoshida, T. Yamamoto, H. Okada, and H. Murakita,
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2012