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Biphasic oligomerization of ethylene with nickel complexes immobilized in organochloroaluminate ionic liquids.

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Full Paper
Received: 11 May 2009
Revised: 18 July 2009
Accepted: 22 July 2009
Published online in Wiley Interscience 16 September 2009
(www.interscience.com) DOI 10.1002/aoc.1542
Biphasic oligomerization of ethylene
with nickel complexes immobilized
in organochloroaluminate ionic liquids
Lixia Peia,b , Xianmei Liua , Haiyang Gaoa∗ and Qing Wua
Ethylene was selectively oligomerized by nickel complexes such as (PPh3 )2 NiBr2 and (PPh3 )2 NiCl2 immobilized in
chloroaluminate ionic liquid in biphasic catalytic reactions. The influence of reaction parameters such as reaction media,
reaction temperature and Et2 AlCl : Ni molar ratio was also evaluated. Turnover frequency up to 24000 mol C2 H4 /(mol Ni h) was
achieved under mild reaction conditions (0.5 atm and 40 ◦ C). GC-MS analyses showed that the obtained oligomers completely
consist of C4 and C6. The olefinic products can be easily separated from the catalytic ionic liquid phase by simple decantation,
and the nickel catalyst can be reused without a significant decrease in turnover frequency and change of the distribution of the
c 2009 John Wiley & Sons, Ltd.
olefinic products. Copyright Keywords: ethylene oligomerization; biphasic catalysis; nickel catalyst; ionic liquid
Introduction
Appl. Organometal. Chem. 2009 , 23, 455–459
Experimental
General Remarks
All manipulations were performed by standard Schlenk techniques
under nitrogen atmosphere or in a glovebox. All solvents were
purified and dried by standard procedures and distilled under
nitrogen. 1-Methylimidazole (99%) and anhydrous AlCl3 were
purchased from Aldrich Chemical Co. NiBr2 and NiCl2 were
purchased from Acros Chemical Co. Diethylaluminum chloride
(Et2 AlCl) was obtained from the Institute of Chemical Engineering
(Shanghai), and diluted to 350 g/l solutions in n-heptane prior to
use. 1-Butyl-3-methylimidazolium chloride (BMIC) was purchased
from Chemer Chemical Co. (Hangzhou). Other commercially
available reagents were purchased and used as received.
∗
Correspondence to: Haiyang Gao, Institute of Polymer Science, School of
Chemistry and Chemical Engineering, No. 135 Xingangxi Road, Sun Yat-Sen
(Zhongshan) University, Guangzhou, Guangdong 510275, China.
E-mail: gaohy@mail.sysu.edu.cn
a DSAPM Laboratory, Institute of Polymer Science, School of Chemistry and
Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou
510275, China
b School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China
c 2009 John Wiley & Sons, Ltd.
Copyright 455
Ethylene oligomerization is one of the major industrial processes
for the production of higher olefins. The obtained oligomers can be
used as comonomers in polymerization, plasticizers and synthetic
lubricants. [P, O] Neutral catalysts currently used in industry for
the Shell Higher Olefin Process (SHOP)[1,2] were reported to be
effective ethylene oligomerization catalysts. SHOP is a biphasic
process, the catalyst being dissolved in butane-1,4-diol, and the
products form a second layer easily removed by phase separation.
In addition, ethylene oligomerizations catalyzed by soluble
transition metal complexes in homogeneous system have also
been studied[3 – 6] and good results have been obtained in terms
of activity and selectivity of higher olefins. However, in this
homogeneous system, the separation of the catalyst from the
reaction products has always been a problem. One of the most
attractive proposed solutions for overcoming this drawback is
to perform the oligomerization reaction in a biphasic solvent
system.[7 – 10] A considerable body of literature describing the
advantages of ionic liquids (ILs) as media for the catalytic
oligomerization of light olefins has appeared over the last
10 years.[11 – 18] The completely ionic nature of ionic liquids makes
them useful as solvents for highly charged complexes, and the
low vapor pressure of ionic liquids makes it possible to use them
in vacuum and as ‘green’ solvents in industrial processes.[19] 1,3Dialkylimidazolium organochloroaluminate-based systems have
become prominent ILs because of their favorable physical and
chemical properties, and their ease of synthesis. The good solubility
of nickel complexes and poor solubility of olefin products in polar
and non-coordinating ionic liquids enable the separation of the
reaction products by simple decantation, allowing easy recycling
of catalyst in the IL solution. Oligomerizations of ethylene,[20 – 24]
propylene[16,18,25,26] and 1-butene,[27 – 32] with nickel catalysts in
ionic liquids have been reported by several research groups over
recent years.
Previously, (PPh3 )2 NiBr2 was reported to be activated by MAO
or MeAlCl2 to form a trimerization catalyst, and this gave a liquid
product containing 95% trimer with an very high activity at 30 bar
ethylene pressure.[33,34] In the present article, we report biphasic
oligomerization of ethylene with nickel complexes (Ph3 P)2 NiBr2
and (Ph3 P)2 NiCl2 immobilized in organochloroaluminate ionic
liquids. The influences of reaction parameters such as reaction
media, reaction temperature and Et2 AlCl : Ni molar ratio were
also studied in detail. The influence of these parameters on
isomerization and oligomerization selectivity has been evaluated.
L. Pei et al.
Synthesis of Ionic Liquid
Results and Discussion
1-Butyl-3-methylimidazolium organochloroaluminate was prepared by mixing 1.0 mol BMIC and 1.2 mol AlCl3 in an ice bath and
stirring for 12 h at room temperature.
The nickel catalyst precursors (PPh3 )2 NiBr2 (1) and (PPh3 )2 NiCl2
(2) chosen for this study were used in the oligomerization of
ethylene in a 1-butyl-3-methyl imidazolium-based ionic liquid and
organic solvents biphasic system. Influences of reaction media,
polymerization temperature, Al : Ni ratio and recycling batch on
ethylene oligomerization were investigated.
Preparation of Precursor
(PPh3 )2 NiBr2 (1) was prepared according to our previously reported
method[35] by refluxing NiBr2 (0.1 mol) and PPh3 (0.22 mol) in
anhydrous ethanol, then the dark green solution was hot filtered,
condensed and cooled to crystallization. The NiBr2 (PPh3 )2 was
collected by filtration and dried in vacuum, obtained as bright
green crystal in 91% yield. Anal. calcd for C32 H30 Br2 NiP2 : C 58.19,
H 4.07. Found: C 58.37, H 4.13.
The synthesis of (PPh3 )2 NiCl2 (2) was preformed as in the case
of (PPh3 )2 NiBr2 ; (PPh3 )2 NiCl2 was obtained as bright blue crystals
in 92% yield. Anal. calcd for C32 H30 Cl2 NiP2 : C 65.10, H 4.62. Found:
C 65.33, H 4.56.
Characterization
Elemental analyses were performed on a Vario EL microanalyzer.
ICP-AES measurements for determining the leaching of nickel
metal from ionic liquid phase were performed on an IRIS
Advantage (HR) spectrometer. Analyses of ethylene oligomers
were performed by GC-MS on a Finnigan Voyager GC-8000 TOP
gas chromatograph–mass spectrometer with DB-5MS GC column.
The following temperature program of the oven was adopted:
40 ◦ C for 2 min, then the temperature was increased by 5 ◦ C/min
heating to 110 ◦ C, increased by 15 ◦ C/min to 250 ◦ C, and then kept
for another 2 min.
Catalytic Runs
456
Ethylene oligomerization reactions were performed in a 50 ml
flask equipped with magnetic stirrer and a thermocouple, with
continuous feed of ethylene at 0.5 atm m pressure. Except for
0 ◦ C, which was maintained with an ice–water bath, reaction
temperatures were controlled with a thermostatic circulation bath.
A typical biphasic reaction was performed by introducing 10 µmol
of the nickel complex 1 or 2 in 3 ml 1-n-butyl-3-methylimidazolium
organochloroaluminate ionic liquid. The system was saturated
with ethylene, and then 27 ml organic solution (n-heptane or
toluene) of Et2 AlCl was added for the biphasic runs. After 0.5 h,
the stirring was stopped and the phases were allowed to separate.
Turnover frequency values were determined by the ethylene gas
consumption. The upper organic layer was removed by means of a
cannula, and then quenched with acidic ethanol. The sample of the
organic layer was filtered through a layer of Celite after drying over
anhydrous Na2 SO4 , and analyzed by GC-MS. When the preceding
oligomerization reaction was terminated by the closure of the
ethylene feeding, the n-heptane phase was decanted while the
ionic liquid phase containing the catalyst remained in the reactor.
The next recycling of the catalysis experiment was performed by
adding another 27 ml n-heptane containing quantified Et2 AlCl to
the reactor and feeding ethylene; the reaction was carried out
again as mentioned above. The same procedure was repeated in
the follow-up recycling.
www.interscience.wiley.com/journal/aoc
Ethylene Oligomerization in Various Media
In order to investigate the influence of reaction media on
ethylene oligomerization, the reactions were carried out in
n-heptane solvent, toluene solvent, n-heptane–ionic liquid and
toluene–ionic liquid systems.
Table 1 shows the results of the ethylene oligomerizations with
1 and 2 in different media in the presence of Et2 AlCl. Using
n-heptane or toluene as reaction medium, both complexes 1
and 2 exhibited low turnover frequencies (TOF). However, both
complexes were found to be highly active in the organic solvent–IL
biphasic systems. Note that ethylene oligomerization activities
in homogeneous media with nickel catalyst precursor 1 or 2
were significantly lower than activities achieved with these nickel
precursors immobilized in the ionic liquid. This result can be
ascribed to an increase of the electrophilic nature of the nickel
metal center by the weak coordinating ability of chloroaluminate
anions.[22] Moreover, the introduction of ionic liquid also affected
the distribution of olefinic products. It should be noted that
higher amount of trimers (C6) but lower amounts of α-olefin were
produced in the biphasic systems than those produced in organic
solvents. On the other hand, (PPh3 )2 NiBr2 (1) showed the higher
TOF than NiCl2 (PPh3 )2 (2), which may result from easy removal of
bromine atom from the nickel metal center.
In addition, when toluene instead of n-heptane was used as
co-solvent in the biphasic system, a slight increase in catalytic
activities was observed, which can be ascribed to an increase in
the concentration of ethylene monomer in solvent. In addition,
the contents of C6 olefins (49%) in the products obtained from nheptane–IL system are obviously higher than those (39%) from the
toluene–IL system, indicating that the chain transfer reaction more
easily happens in toluene solvent. This may be attributed to the
coordination of the AlCl3 to the aromatic ring. A similar effect has
already been observed for ethylene oligomerizations catalyzed
by other nickel complexes immobilized in ionic liquid.[16,22]
Considering the high content of C6, n-heptane was selected as a
practical co-solvent to further probe the ethylene oligomerization
behaviors.
Influence of Oligomerization Temperature
Temperature has an important influence on ethylene oligomerization; thus a series of experiments were carried out at different
temperatures (Table 2). Ethylene oligomerization reactions are
extremely exothermic, thus the internal temperature probably
shows a profile with a more or less important peak. Our experiments showed the error between the inner temperature of the
reactor and bath temperature was within 2 ◦ C. Therefore, it is
reasonable and convenient to use external temperature instead
of internal. With an increase in reaction temperatures from 0 to
60 ◦ C, an optimum TOF was observed at the temperature of 40 ◦ C
for a biphasic catalytic system. As shown in Table 2, reaction temperature has certain effect on oligomer distribution. At 0 ◦ C, the
c 2009 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2009, 23, 455–459
Biphasic oligomerization of ethylene with nickel complexes
Table 1. Oligomerization of ethylene catalyzed by 1,2–AlEt2 Cl in different media
Oligomer distributionb (mol%)
Run
1
2
3
4
5
6
7
8
Precursor
Media
TOFa
C4
C6
α-olefin
1
1
1
1
2
2
2
2
Toluene
Heptane
Toluene + IL
Heptane + IL
Toluene
Heptane
Toluene + IL
Heptane + IL
13 000
9 800
26 000
19 800
10 800
8 700
23 000
18 200
75
71
61
51
77
68
59
48
25
29
39
49
23
32
41
52
25
35
8
13
27
36
6
13
General conditions: 10 µmol precursor; reaction temperature, 20 ◦ C; reaction time, 0.5 h; Et2 AlCl : Ni = 400; ethylene meter pressure, 0.5 atm; media,
30 ml organic solvent or 27ml organic solvent + 3 ml IL.
a
In units of mol C2 H4 /(mol Ni h).
b Determined by GC-MS.
Table 2. Ethylene oligomerization with 1–Et2 AlCl in biphasic system
Table 3. Results of ethylene recycling oligomerization with 1 precursor in biphasic system
Oligomer distributionb (%)
◦
Run
T ( C)
Al : Ni
TOFa
C4
C6
α-olefin
9
4
10
11
12
13
14
15c
0
20
40
60
40
40
40
40
400
400
400
400
800
1200
1600
800
11 200
19 800
24 000
16 200
21 000
14 000
10 200
13 000
60
51
33
48
31
58
73
25
40
49
67
52
69
42
27
75
18
13
6
3
8
12
14
23
Run
General conditions: 10 µmol precursor; reaction time, 0.5 h; ethylene
meter pressure, 0.5 atm; media, 27 ml organic solvent + 3 ml IL;
cocatalyst: AlEt2 Cl.
a In units of mol C H /(mol Ni h).
2 4
b
Determined by GC-MS.
c MAO as cocatalyst.
obtained oligomerization products contain the highest C4 fraction
and α-olefin. However, oligomers obtained at 40 ◦ C contain the
lowest C4 fraction and the highest C6 fraction (67%), indicating
that the activation energy for the chain transfer reaction is not
much higher than that for the propagation for the catalyst systems
of the present study.[36]
Influence of Et2 AlCl : Ni Ratio
Appl. Organometal. Chem. 2009, 23, 455–459
TOFa
1st
2nd
3rd
19 800
16 700
15 600
99.6
98.7
98.2
Oligomer distributionb (%)
C4
C6
α-olefin
51
53
54
49
47
46
13
13
14
General conditions: 10 µmol complex; reaction temperature, 20 ◦ C;
reaction time, 0.5 h; Et2 AlCl : Ni = 400; ethylene meter pressure, 0.5
atm; media, 27 ml n-heptane + 3 ml ionic liquid.
a In units of mol ethylene/(mol Ni h).
b Determined by ICP-AES.
c
Determined by GC-MS.
higher contents of C6 and α-olefin were produced. This might
result in the strong Lewis acid of Et2 AlCl and coaction between
ionic liquid and cocatalyst. De Souza has also reported the role of
aluminum species for catalytic activity and selectivity of ethylene
oligomerization in acidic ionic liquids.[30] A similar effect has
already been observed for ethylene oligomerizations catalyzed
by nickel complexes or ethylene polymerization[24] catalyzed by
Cp2 TiCl2 immobilized in ionic liquid.[37]
Ethylene Recycling Oligomerization
The high activity and stability of the biphasic catalytic system
allows us to study its recycling ability. The catalytic system could
be reused in successive ethylene oligomerization cycles upon
supply of Et2 AlCl. At the end of the first batch, stirring was stopped
to allow the separation of catalyst and oligomers solution. The
second batch was performed by adding small amount of Et2 AlCl
and another fresh 27 ml of n-heptane. The same procedure was
repeated in the third batch. The results of the recycling ethylene
oligomerization are shown in Table 3.
In this manner, ionic liquid phase containing 1 could be reused
without a significant decrease in TOF after recycling three times.
Moreover, the product distributions were not much affected by the
recycling of the catalytic system. GC-MS showed that the contents
of C6 and α-olefin were substantially similar within an experimental
error range. ICP-AES measurements for determining the leaching
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
457
Et2 AlCl can play the dual roles of metal activator (cocatalyst)
and formation of the alkylchloroaluminate anions. In general, a
threshold amount of Et2 AlCl was needed for activation of the
complexes. As shown in Table 2, with an increase in Et2 AlCl : Ni
ratio, the TOFs for ethylene oligomerization increased and then
decreased, and the highest TOF was observed when Et2 AlCl : Ni
ratio was 800. In addition, Et2 AlCl : Ni ratio also influenced the
distribution of the olefinic products. With an increase in Et2 AlCl : Ni
ratio, the contents of C6 increased and then decreased. The
highest content of C6 reached 69% when the Et2 AlCl : Ni ratio was
800, while the lowest content of α-olefin was determined. When
MAO instead of Et2 AlCl was used as cocatalyst in the biphasic
system, an obvious decrease in TOF was observed. However,
4
16
17
Batch
Catalyst
preserving in
ILc (%)
L. Pei et al.
Conclusions
2
In the presence of Et2 AlCl, nickel complexes (PPh3 )2 NiBr2 (1) and
(PPh3 )2 NiCl2 (2) show high TOF for ethylene oligomerization in
biphasic media consisting of ionic liquid and organic solvent. The
TOF depends on reaction parameters such as reaction media,
reaction temperature and Et2 AlCl : Ni molar ratio. High selectivity
towards C4–C6 olefins can be obtained. Contrary to organic
solvents, the ionic liquid is able to stabilize and immobilize the
nickel active species. The ionic liquid phase containing the catalyst
can be recycled and reused at least three times without much
change in the activity and the composition of the products. The
results clearly confirm that this biphasic catalytic process is a useful
technique to improve the catalytic performance for the ethylene
oligomerization.
3
12
9
1
4
1.4
1.6
56
7
8
1.8
2.0
2.2
2.4
Retention time (min)
10
11
2.6
2.8
Acknowledgments
Figure 1. Typical chromatogram of oligomers obtained by complex 1 in
heptane–IL biphasic catalytic system (Table 1, run 4). (Solvent peak was
omitted).
of nickel metal from ionic liquid phase were performed. In ICP-AES
analyses of the organic solvent phase separated from the three
cycle runs, only 1.8% of nickel metal was leached out to the organic
solvent after three cycles.
Composition of Oligomers
GC-MS analyses of the oligomerization products showed that the
oligomers produced by the nickel catalysts were composed of
olefins including linear α-olefins, branched olefins and internal
olefins. A representative GC-MS chromatogram of the oligomers
obtained by 1 was shown in Fig. 1 and the content of each fraction
was also listed in Table 4.
For C4 composition, the content of 1-butene could reach
7 mol%; the other was 2-butene (cis- and trans- structures) and
no 2-methylpropylene was detected. For C6 composition, the
olefins fraction contained 1-hexene, 2-hexene, 3-hexene (cis- and
trans-), 3-methylpentene-2 (cis- and trans-), 3-methylpentene1, 2-methylpentene-1 and 2-ethylbutene-1. Except for olefins,
no amount of alkane was detected. This result indicates that
the termination of the grown chain was β-H elimination. The
total content of α-olefin (1-hexene, 2-ethylbutene-1, 2-methylpentene-1 and 3-methylpentene-1) was 13 mol%, the content
of 1-hexene only was 2 mol% and branched olefins and internal
olefins were the main products. The composition analyses indicate
that the nickel active species catalyze olefin oligomerization and
isomerization simultaneously.[25,27] Similar oligomerization results
were observed with other nickel complexes.[20,24]
The financial support by NSFC (Projects 20 604 034, 20 674 097 and
20 734 004) and the Science Foundation of Guangdong Province
(Project 8 251 027 501 000 018 and 06 300 069) is gratefully acknowledged.
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