Oligomerization of ethylene catalyzed by iron and cobalt in organoaluminate dialkylimidazolium ionic liquids

Article abstract of DOI:10.1007/s10562-010-0371-7

Cationic iron and cobalt acetonitrile cationic complexes were prepared and used as catalyst precursors in catalytic ethylene oligomerization. The effects of temperature and the nature and effects of the co-catalysts were evaluated. The iron and cobalt catalysts are less active, giving turnover frequencies of 20,000 to 30,000 h^-1 while the corresponding nickel analogs exhibit turnover frequencies of 215,000 h^-1. The iron and cobalt catalysts were more selective for the synthesis of 1-butene, especially at lower temperatures, compared to the nickel catalyst.

Full text of DOI:10.1007/s10562-010-0371-7

Catal Lett (2010) 138:50–55
DOI 10.1007/s10562-010-0371-7
Oligomerization of Ethylene Catalyzed by Iron and Cobalt
in Organoaluminate Dialkylimidazolium Ionic Liquids
•
Daniel Thiele Roberto Fernando de Souza
Received: 10 February 2010 / Accepted: 9 May 2010 / Published online: 28 May 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Cationic iron and cobalt acetonitrile cationic
complexes were prepared and used as catalyst precursors in
catalytic ethylene oligomerization. The effects of temper-
ature and the nature and effects of the co-catalysts were
evaluated. The iron and cobalt catalysts are less active,
giving turnover frequencies of 20,000 to 30,000 h-1 while
the corresponding nickel analogs exhibit turnover fre-
quencies of 215,000 h-1. The iron and cobalt catalysts
were more selective for the synthesis of 1-butene, espe-
cially at lower temperatures, compared to the nickel
catalyst.
particularly those in the range of C6–C12, still present a great
challenge. These systems pose several problems, including
that of selectivity. The major difficulty is catalyst death due to
deactivation during the separation of products from the cat-
alyst. Some approaches have been developed to overcome
this drawback. The most successful have been the chemical or
physical immobilization of coordination compounds, allow-
ing access to the advantages of heterogeneous catalysis, and
the use of liquid–liquid biphasic catalysts, using ionic liquids
as the immobilization phase for the transition metal catalysts.
Oligomerization reactions in which ionic liquids act as
the immobilization phase have been studied extensively for
homogeneous nickel catalysts. Aspects such as the influ-
ence of ionic liquid acidity, due to the variety of compo-
sitions that can be obtained in chloroaluminate ionic liquids
[7–9], additives [10, 11], variations in ionic liquids [12, 13]
and reactor design [14, 15] have been considered. Several
oligomerization catalysts were evaluated in ionic liquids,
with different organic moieties [8, 12, 13, 16–22] and
different counter-anions in cationic coordination com-
pounds [23].
Keywords Ethylene Á Oligomerization Á Iron Á
Cobalt Á Organoaluminate Á Ionic liquid
1 Introduction
Recent developments in the use of coordination com-
pounds as catalysts for the synthesis of linear a-olefins
were made possible by the suitable development of ligands
and the use of different metals. The advances achieved in
this field allow access to catalysts that permit selective
dimerization [1–3], trimerization [4], tetramerization [5],
and even specific oligomerization [6] of ethylene.
Many experiments have been performed to find appro-
priate conditions for the selective synthesis of linear a-
olefins. Nickel coordination compounds are still the most
successfully used complexes for oligomerization reactions
in ionic liquids [12], but their impact has been limited by
the isomerization of the produced olefins [24]. We thus
extended our studies of oligomerization reactions in ionic
liquids to iron and cobalt catalysts for which several
attractive descriptions have become available in recent
years. To our knowledge, no studies have been published
demonstrating the use of cobalt and iron catalysts for
oligomerization in ionic liquids. With this goal, two cat-
ionic coordination compounds with structures very similar
to known nickel analogs were prepared, and their catalytic
Although many types of oligomerization catalysts are
known, the selective production of linear a-olefins,
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-010-0371-7) contains supplementary
material, which is available to authorized users.
D. Thiele Á R. F. de Souza (&)
Institute of Chemistry, UFRGS, Av. Bento Gonc¸alves, 9500,
P.O. Box 15003, Porto Alegre CEP 91501-970, Brazil
e-mail: rfds@iq.ufrgs.br
123

Oligomerization of Ethylene Catalyzed by Iron and Cobalt
51
behavior while changing the co-catalyst and reaction
temperature was evaluated. Their results were compared
with their nickel analogs, which have been well described
in the literature [17].
Chromatographic analyses were performed on a Varian
3400 CX, equipped with a Petrocol DH capillary column:
methyl silicone, 100 m long, i.d. 0.25 mm, and film
thickness 0.5 lm. Analysis conditions: 36 °C for 15 min
and heating rate of 5 °C/min until 250 °C. The productivity
is expressed as turnover frequencies, TOF, defined as mol
converted ethylene per mol pre-catalyst and by reaction
time (in h).
2 Experimental
All manipulations were performed under argon using
standard Schlenk tube techniques. All solvents were puri-
fied and dried by standard procedures and distilled under
argon.
3 Results and Discussion
The majority of oligomerization catalysts described in the
current literature concerns nickel complexes. Their studies
in ionic liquids are mostly concentrated in organoaluminate
dialkylimidazolium ionic liquids, resulting in very high
activity but also in high isomerization rates, giving mostly
internally branched olefins. To understand this behavior,
many aspects must be taken in account.
2.1 Pre-Catalyst Synthesis
The pre-catalysts [Fe(MeCN)6][BF4]2 1, [Co(MeCN)6]
[BF4]2 2 and [Ni(MeCN)6][BF4]2 3 were prepared as pre-
viously described [25]. A typical procedure for their
preparation consists of mixing 12 mmol metal (purchased
from Riedel) with 20 mmol NOBF4 (purchased from
Acros) in acetonitrile. The solution was stirred for 24 h.
The unreacted metal was magnetically removed, and the
acetonitrile was removed under reduced pressure. The
complex was recrystallized from acetonitrile/ethyl acetate.
After decantation, excess solvent was removed and the
complex was dried under reduced pressure.
This behavior has been attributed to the fact that the
active species in oligomerization can also catalyze side
reactions, such as isomerization and co-oligomerization.
Scheme 1 shows a general mechanism for oligomerization
and side reactions. In this scheme, oxidation states and the
different oligomers produced are not shown for the purpose
of clarity.
The desired reaction is shown in path 1, which is
responsible for the oligomerization of the primary prod-
ucts. The active species is regenerated after coordination of
the olefin, chain growth, and termination by b-elimination,
producing the primary oligomerization product. At this
point, the active species could be used again in an oligo-
merization catalytic cycle, or it could isomerize the olig-
omerization product via path 2 and/or co-oligomerize these
products through path 3.
2.2 Ionic Liquid Synthesis
For the synthesis of the ionic liquid employed in this study,
1-butyl-3-methylimidazolium chloride (BMIC) was slowly
added over AlCl3, which had been sublimed once prior to
use. The temperature was controlled in order to do not to
exceed 30 °C. To complete the ionic liquid synthesis, the
desired amount of co-catalyst, AlEtCl2, AlEt2Cl, AlEt3 or
MAO, was added, and the mixture was stirred overnight.
In the case of nickel-catalyzed oligomerization using
dialkylimidazolium organoaluminate ionic liquids as the
immobilization phase, the isomerization and co-oligomer-
ization cycles becomes of considerable importance com-
pared to the oligomerization cycle.
2.3 Catalytic Runs
The catalytic runs were performed using a Fisher-porter
200 mL, semi-continuous glass reactor equipped with a
magnetic stirrer, a thermocouple, a continuous feed of
ethylene at 10 bar, and 20 mL of 2,2,4-trimethylpentane.
The pre catalyst was transferred to the reactor, followed by
the addition of 20 mL of 2,2,4-trimethylpentane. After the
system was purged with ethylene, 3 mL of the ionic liquid
were added, starting the oligomerization reaction. The
reaction was stirred for 0.5 h. The temperature was con-
trolled by a thermostatic circulation bath. After the end of
the reaction, the reactor was cooled to -60 °C and the
organic phase was collected to immediate quantification of
the products, without neutralizing the catalyst.
It is worth noting that many of these reactions are lim-
ited by mass transport [26]. This probably contributes to
the low selectivity for a-olefins for such oligomerization
reactions. The amount of the active species that cannot
follow path 1, due to the low availability of ethylene, fol-
lows path 2 or 3, consuming path 1 products. This mass
control does not exclude the contribution of the nature of
the catalysts themselves to the amount of substrate fol-
lowing the parallel path 2 and path 3 mechanisms.
Bis(imino)piridyl iron or cobalt has been successfully
used as oligomerization catalysts [6], attracting great
attention to iron and cobalt complexes for this reaction.
The outstanding performance of such complexes makes
123

52
D. Thiele, R. F. de Souza
Scheme 1 Mechanism for oligomerization of ethylene and possible side reactions
The iron/AlEt2Cl system (entries 4 to 6) and the iron/
MAO system (entries 10 to 12) in chloroaluminate ionic
liquid demonstrate an exponential dependency of activity
with temperature. For these co-catalysts, no induction time
was observed for the oligomerization, so the observed
dependence of oligomerization activity with the tempera-
ture is due to the enhancement of the reaction rate with
temperature. The calculation of apparent Arrhenius acti-
vation energies, plotting the logarithm of the turnover
frequency with the reciprocal of the absolute temperature,
gives values of 16 kcal/mol for the system iron/AlEt2Cl
and 19 kcal/mol for the system iron/MAO. In the case of
the nickel system, it was not possible to determine the
activation energy due to the strong variation in the internal
temperature in the reactor. The values observed for the iron
and cobalt complexes can be compared with those avail-
able in the literature concerning nickel oligomerization or
polymerization complexes, such as the values of 16.8 to
17.5 kcal mol-1 reported by Ziegler and co-workers [28],
or that of 16.3 kcal mol-1 reported by Zhang and Lana
[29].
Scheme 2 Structure of the pre-catalysts studied in this work [25]
them attractive subjects to be tested in ionic liquid biphasic
catalysis conditions. For comparative studies, the cationic
transition metal coordination compounds shown in
Scheme 2 were chosen.
These coordination compounds were selected because
the analogous nickel coordination compound is well known
in the literature [17], and the analog was extensively
described as being favorably used for comparison to the
target cobalt and iron catalysts. These complexes have
been evaluated in ethylene oligomerization, varying the
reaction temperature and the nature of the associated co-
catalyst, as shown in Table 1.
3.1 Ethylene Oligomerization with Iron
The oligomerization reactions performed using AlEtCl2
and AlEt3 as co-catalysts show a different pattern for the
dependence of activity with temperature. For the iron
systems, the activity grows with an increase in the tem-
perature from 10 °C to 30 °C (entries 1 and 2 for AlEtCl2
as co-catalyst, entries 7 and 8 for AlEt3), but decreases for
further increases of the temperature to 50 °C (entries 2 to 3
for AlEtCl2 and 8 to 9 for AlEt3).
The iron compound, [Fe(MeCN)6][BF4]2, 1, is active under
all conditions of oligomerization tested in this work. As
shown in Table 1, entries 1–12, the oligomerization
activity depends strongly on the nature of the co-catalyst as
well as on the oligomerization temperature. Compared to
the nickel analog, [Ni(MeCN)6][BF4], 3, the iron catalyst
shows oligomerization activities ca. one order of magni-
tude lower than the nickel catalyst, as seen comparing
entries 1–3 for iron with entry 24 for nickel with AlEtCl2,
or entries 4–6 for iron with entry 25 for nickel with AlEt2Cl
as co-catalyst. As previously observed for nickel catalyst
[27], more acidic co-catalysts, such as AlEtCl2 and AlEt2Cl
(entries 1 to 6), show higher activity than less acidic co-
catalysts (entries 7 to 12), like AlEt3 and MAO.
This behavior is often observed in the case of nickel
complexes [30], and has generally been attributed to the
formation of species with lower inherent activity or, more
probably, to the partial decomposition or deactivation of
the catalyst. A decrease in the ethylene concentration in
solution at this oligomerization temperature, which would
induce indirect catalyst decay, cannot be ruled out.
123

Oligomerization of Ethylene Catalyzed by Iron and Cobalt
53
Table 1 Biphasic oligomerization with iron, cobalt, and nickel catalysts dissolved in butyl-methyl-imidazolium organoaluminate ionic liquids
Entry
Catalyst (lmol)
Temperature (°C)
Co-catalyst
[Al]/[M]d
TOF (h-1
)
C4 (wt%)
C4
C6 (wt%)
CC8 (wt%)
1-C4
1
1/21.5
1/18.5
1/20.4
1/19.9
1/22.3
1/20.2
1/18.3
1/16.0
1/15.1
1/16.6
1/13.8
1/18.1
2/16.5
2/12.7
2/12.3
2/14.3
2/18.2
2/16.9
2/16.3
2/14.0
2/18.4
2/15.0
2/13.8
3/21.5
3/16.3
1/18.3
2/12.7
4/18.1
5/13.0
10
30
50
10
30
50
10
30
50
10
30
50
10
30
50
10
30
50
10
30
50
10
50
10a
10b
30c
30c
30
30
AlEtCl2
AlEtCl2
AlEtCl2
AlEt2Cl
AlEt2Cl
AlEt2Cl
AlEt3
120
140
120
130
110
130
140
160
170
160
200
150
150
200
200
180
140
150
160
180
140
180
200
120
156
140
200
140
140
1010
16400
12300
560
98.2
86.0
81.1
98.7
95.8
79.0
99.1
89.3
92.8
100
86.7
42.1
40.1
91.2
78.4
27.5
91.9
47.1
57.7
100
1.8
13.2
17.4
1.3
–
0.8
1.5
–
2
3
4
5
2800
19500
980
4.2
–
6
19.0
0.9
2.0
–
7
8
AlEt3
14000
8100
70
10.7
6.9
–
9
AlEt3
0.3
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
MAO
–
MAO
415
97.7
91.2
97.9
87.4
66.7
98.9
95.4
81.1
97.3
92.5
91.1
97.2
98.0
42.6
68.1
88.3
76.5
73.3
76.1
95.8
66.7
96.8
48.8
21.0
94.8
77.7
37.6
98.7
95.3
95.8
98.9
96.0
14.5
18.4
63.3
87.0
27.9
29.3
2.3
–
MAO
4620
230
8.5
0.3
–
AlEtCl2
AlEtCl2
AlEtCl2
AlEt2Cl
AlEt2Cl
AlEt2Cl
AlEt3
2.1
14100
33200
960
12.5
30.0
1.1
0.1
3.4
–
4500
19500
140
4.6
–
16.9
2.7
2.0
–
AlEt3
400
7.5
–
AlEt3
350
8.9
–
MAO
130
2.8
–
MAO
668
2.0
–
AlEtCl2
AlEt2Cl
AlEtCl2
AlEtCl2
AlEtCl2
AlEtCl2
215000
159000
348
36.2
26.6
11.7
23.5
24.2
21.4
21.5
5.3
–
88
–
25900
21800
2.5
2.5
Ionic liquid BMIC:AlCl3:co-catalyst; molar ratio 1:1: 0.22; 30 min; 10 bar ethylene; 1 [Fe(MeCN)6][BF4]2; 2 [Co(MeCN)6][BF4]2; 3
[Ni(MeCN)6][BF4]2 4 FeCl2; 5 CoCl2
a
Internal temperature increased to 85 °C
Internal temperature increased to 65 °C
Without ionic liquid
b
c
d
Imprecision in [Al]/[M] ratio is 30
The effect of the ionic liquid in the oligomerization
reaction catalyzed by cationic iron complex can be seen
comparing entries 2 with 26. It is remarkable the increase
in activity of catalyst 1 upon addition of ionic liquid, from
348 h-1 to 16400 h-1. This enhancement shows the high
ability of the ionic liquid to solvate the iron compound. A
catalytic run performed with the organic phase obtained
from the reaction of entry 2, after the separation from the
ionic liquid, shows no further products formation, sug-
gesting that the catalyst is quantitatively retained in the
ionic liquid and the reaction occurs exclusively in the ionic
liquid. The comparison of the catalytic behavior in entries
2 and 28 demonstrates that pre catalyst 4 is more active
than 1, but the absence of ligands results in considerably
lower selectivity to a-olefins.
The iron catalyst shows high selectivity for ethylene
dimerization, in the range 79.0% to 100%, with low
amounts of trimers, C6 (0.9%, entry 7 to 19%, entry 6), and
tetramers, C8 (maximum of 2.0% in entry 6). The high
selectivity of the iron catalyst towards dimerization is
123

54
D. Thiele, R. F. de Souza
attributed to the higher chain end rate (b-elimination
reaction) as compared to the chain growth rate, an intrinsic
characteristic of these catalytic species.
varying from 66.7% to 97.9% for AlEtCl2 (entries 15 and
13), from 91.1% to 97.3% for AlEt3 (entries 21 and 19) or
even from 97.2% to 98.0% for MAO (entries 22, 23).
The selectivity for 1-butene with the cobalt catalyst is
similar to that of the iron catalyst with respect to temper-
ature. The tendency of higher isomerization with cobalt
catalysts as compared to iron catalysts was also described
in the homogeneous system [32].
Otherwise, the selectivity towards trimers depends
strongly on the temperature. For the system 1/AlEt2Cl, the
amount of C6 grows from 2.0% at 10 °C to 19.0% at
50 °C. The growth in the selectivity towards hexenes can
be attributed to the co-oligomerization of the primary
products, butenes, with ethylene. Increasing the tempera-
ture increases both the formation of C4 and the formation
of co-oligomerization products [31].
4 Conclusion
When AlEt3 or MAO is used as a co-catalyst, the effects
of the temperature are less pronounced than with the other
co-catalysts studied in this work. Despite the low activity
of the iron catalyst as compared to the nickel catalyst, the
former resulted in systems more selective towards 1-butene
than the latter.
Iron and cobalt catalysts have been shown to promote
ethylene oligomerization in chloro-aluminate ionic liquids.
The activity of these catalysts is lower than for analogous
nickel catalysts, but they exhibit higher selectivity towards
dimerization than the nickel analogs, enabling selective
access to 1-butene. The highest activity for the iron system
was 19,500 h-1, while the highest activity for the cobalt
system was 33,200 h-1. In conditions of low activity, both
systems showed high selectivity to 1-butene, higher than
86%. High activity generally corresponds to low selectiv-
ity, which shows the compromise between these concurrent
tendencies. This suggests that the reaction suffers from
mass transfer limitations. Olefins such hexenes or higher
alkenes are mainly internal and branched. These olefins are
mainly formed from co-oligomerization reactions, and their
amounts are dependent on temperature.
3.2 Ethylene Oligomerization with Cobalt
The complex [Co(MeCN)6][BF4]2, 2, combined with
AlEtCl2 or AlEt2Cl shows better performance than asso-
ciation with AlEt3 or MAO. The systems 2/AlEtCl2 or
AlEt2Cl give TOF of 33000 and 19500 h-1, entries 15 and
18 respectively, and TOF around 350 and 668 h-1, entries
21 and 23 respectively, for AlEt3 or MAO. Very similar
behavior has been previously described for nickel com-
plexes in homogeneous media [27].
It is worth noting the different effects of temperature on
the activity of the different co-catalysts. The activity with
the cobalt catalyst was very low at 10 °C for all co-cata-
lysts, showing a TOF of less than 1000 h-1. The increase
of oligomerization temperature for the systems with AlE-
tCl2 or AlEt2Cl as co-catalysts resulted in a substantial
increase in activity. For AlEt3 or MAO as co-catalysts, the
effects of temperature on the activity are less pronounced.
This clearly demonstrates the need for a stronger Lewis
acid as co-catalyst for the activation of compound 2 for the
oligomerization of ethylene.
Acknowledgment The authors thank to CNPq for financial support
of this work.
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