Nickel complexes incorporating pyrazole-based ligands for ethylene dimerization to 1-butylene

Article abstract of DOI:10.1016/j.jorganchem.2015.04.041

In this study we synthesized and characterized bis(3,5-dimethylpyrazolyl)dimethylsilane (L1) and bis(3,5-dimethylpyrazolyl)phenylphosphine (L2) and then prepared bidentate (NN) nickel complexes by reacting L1 or L2 with NiBr₂(DME). In combination with methylaluminoxane, diethylchloroaluminum or ethyldichloroaluminum (EADC) as co-catalyst, L1/Ni and L2/Ni complexes exhibited moderate catalytic activities (240-1310 kg/mol_Ni h) and high selectivities (up to 93.2%) for ethylene dimerization toward 1-butylene. The catalytic performance was substantially affected by the co-catalyst type and ligand environment, especially the bridge atom.

Full text of DOI:10.1016/j.jorganchem.2015.04.041

Accepted Manuscript
Nickel Complexes Incorporating Pyrazole-Based Ligands for Ethylene Dimerization to
1-Butylene
Tao Wang, Bo Dong, Yan-Hui Chen, Guo-Liang Mao, Tao Jiang
PII:
S0022-328X(15)00253-3
10.1016/j.jorganchem.2015.04.041
JOM 19027
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 30 December 2014
Revised Date: 25 February 2015
Accepted Date: 6 April 2015
Please cite this article as: T. Wang, B. Dong, Y.-H. Chen, G.-L. Mao, T. Jiang Nickel Complexes
Incorporating Pyrazole-Based Ligands for Ethylene Dimerization to 1-Butylene, Journal of
Organometallic Chemistry (2015), doi: 10.1016/j.jorganchem.2015.04.041.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Nickel Complexes Incorporating Pyrazole-Based Ligands for Ethylene
Dimerization to 1-Butylene
Tao Wang a, Bo Dong a, Yan-Hui Chen a, Guo-Liang Mao b, Tao Jiang a, b,
The nickel complexes with MAO, EADC or DEAC showed moderate activity
for ethylene dimerization toward 1-C4. The catalysts showed higher catalytic activity
with DEAC than other cocatalysts. The catalysts give the highest selectivity when
MAO use as cocatlyst.

ACCEPTED MANUSCRIPT
Graphical Abstract
Nickel Complexes Incorporating Pyrazole-Based Ligands for Ethylene
Dimerization to 1-Butylene
Tao Wang a, Bo Dong a, Yan-Hui Chen a, Guo-Liang Mao b, Tao Jiang a, b,
Ni complexes incorporating pyrazole-based ligands with different co-catalyst for ethylene dimerization.

ACCEPTED MANUSCRIPT
Nickel Complexes Incorporating Pyrazole-Based Ligands for
Ethylene Dimerization to 1-Butylene
Tao Wang a, Bo Dong a, Yan-Hui Chen a, Guo-Liang Mao b, Tao Jiang a, b,
1
a College of Material Science and Chemical Engineering, Tianjin University of Science & Technology, Tianjin
300457, China
b College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing 163318, China
Abstract
In this study we synthesized and characterized bis(3,5-dimethylpyrazolyl)dimethylsilane (L1) and
bis(3,5-dimethylpyrazolyl)phenylphosphine (L2) and then prepared bidentate (N^N) nickel complexes
by reacting L1 or L2 with NiBr2(DME). In combination with methylaluminoxane,
diethylchloroaluminum or ethyldichloroaluminum (EADC) as co-catalyst, L1/Ni and L2/Ni complexes
exhibited moderate catalytic activities (240-1310 kg/molNiꢀh) and high selectivities (up to 93.2%) for
ethylene dimerization toward 1-butylene. The catalytic performance was substantially affected by the
co-catalyst type and ligand environment, especially the bridge atom.
Keywords: Ethylene oligomerization; 1-Butylene; Pyrazole; Nickel complexes
Corresponding author. E-mail address: jiangtao@tust.edu.cn.
Fax: +86-22-60602936; Tel: +86-22-60601230.
1

ACCEPTED MANUSCRIPT
1. Introduction
Linear α-olefins are normally used for the preparation of branched polyethylene and other
chemicals, such as detergents, synthetic lubricants and plasticizer alcohols. Oligomerization of ethylene
to linear α-olefin have played an important role in the industrial process. Ethylene dimerization and
oligomerization reactions were typically carried out using homogeneous late-transition-metal catalysts,
particularly nickel-based catalysts [1-6]. A number of selective ethylene dimerization catalysts based on
nickel complexes have been extensively studied and developed [7-9]. Nickel complexes that based on
various chelating-ligand topologies were studied most frequently [10-14]. Besides the choice of the
metal center, the desired catalytic selectivity can be tuned by designing ancillary ligands with the
desired structural and electronic properties to assist the generation and stabilization of active species
for selective ethylene oligomerization [15-18]. Usually, pyrazole-based ligands are used as either
terminal or bridging ligands with bidentate chelation at each metal center. Currently, pyrazole ligands
research mainly focus on bridging pyrazole atoms (such as carbon, silicon and boron on bridging the
bridge bidentate or tridentate ligands) or changing the substituents of the ligands. It will change the
space structure of ligand, coordination ability and influence the metal central atoms, changing the
catalytic performance [19-21]. Oliveira and his coworkers have reported that the tridentate catalyst
which was formed with pyrazol ligand (NNN or NSN) and Ni gave high activity and selectivity for
ethylene dimerization to 1-butene. They also found that changing the substituent of pyrazole would
change the catalyst performance effectively, increasing the electronic properties of substituents could
effectively increase the activity of the catalyst [22]. Ainooson and his coworkers reported that catalysts
that consisted of 3,5-dimethyl pyrazole bidentate ligands with iron, cobalt, nickel or palladium in the
ethylene oligomerization showed different catalytic activities. The nickel complexes with
ethyldichloroaluminum (EADC) possessed the highest catalytic activity and selectivity for the ethylene
oligomerization to butenes and hexenes [23]. We are interested in the design of new pyrazole ligands
used in Ni-based catalysts for ethylene dimerization to 1-butylene with high catalytic activity,
selectivity and long lifetime. Like S, C and B atom, phosphine or silicon were often used in the design
of novel ligands as connecting heteroatoms[24]. However, corresponding bidentate pyrazole ligands
featuring Si and P bridging atoms have never been explored previously in conjunction with transition
metals for preparing catalysts used for ethylene oligomerization.
Herein, we report the synthesis and characterization of nickel complexes with bidentate
bis(3,5-dimethylpyrazolyl)dimethylsilane (L1) or bis(3,5-dimethylpyrazolyl)phenylphosphine (L2)
ligands, as well as their catalytic behavior for ethylene dimerization toward 1-butene. We also discuss
the effects of the reaction temperature, the Al-to-Ni molar ratio and the co-catalysts types on the
catalytic activity and selectivity of these complexes.
2. Experimental
2.1 Materials
3,5-Dimethylpyrazole, dichlorodimethylsilane, dichlorophenylphosphine, n-butyllithium (2.4
mol/L in hexanes), and NiBr2(DME) were purchased from Aldrich and used as received.
Polymerization-grade ethylene was obtained from Tianjin Summit Specialty Gases (China).
Methylaluminoxane (MAO, 1.4 mol/L in toluene), diethylchloroaluminum (DEAC, 1.4 mol/L in
toluene), EADC (1.4 mol/L in toluene), triethylaluminum (TEAL, 1.4 mol/L in toluene),
trimethylaluminium (TMAL, 1.4 mol/L in toluene), triisobutylaluminium (TIBA, 1.4 mol/L in toluene)
were purchased from Albemarle (USA). Toluene, tetrahydrofuran (THF), dichloromethane and
2

ACCEPTED MANUSCRIPT
n-hexane were dried and degassed prior to use.
2.2 Synthesis of ligands
All air- and/or water-sensitive reactions were conducted under a dry N2 atmosphere in oven-dried
flasks using standard Schlenk techniques or in a glove box.
2.2.1 Bis(3,5-dimethylpyrazolyl)dimethylsilane (L1)
2.4 M n-BuLi solution in hexanes (0.44 mL, 1.05 mmol) was added dropwise to a solution of
3,5-dimethylpyrazole (96.1 mg, 1.0 mmol) in n-hexane (50 mL) at –78 °C. The mixture was stirred for
4 h, resulting in a light-yellow cloudy solution. Dichlorodimethylsilane (60.28 ꢁL, 0.5 mmol) was
added to the above solution, and then the mixture was stirred for 24 h. The precipitated LiCl salt was
filtered off and the filtrate was concentrated to give a white residue. Recrystallization
(benzene/n-hexane) provided L1 in 90% yield. 1H NMR (δ, ppm, CDCl3, TMS): 0.939 (s, 6H, CH3Si),
1.890 (s, 6H, (CH3)2pz), 2.250 (s, 6H, (CH3)2pz), 5.869 (s, 2H, H-4 in (CH3)2pz). 13C NMR (δ, ppm,
CDCl3): 1.65, 16.11, 17.88, 27.52, 28.55, 108.58, 109.88, 136.62, 137.49, 145.81, 147.07.
2.2.2 Bis(3,5-dimethylpyrazolyl)phenylphosphine (L2)
Triethylamine (3.05 mL, 22 mmol) was added to a solution of 3,5-dimethylpyrazole (1.9226 g, 20
mmol) in THF (100 mL). Then dichlorophenylphosphine (1.4 mL, 10 mmol) was added dropwise to
the solution at 0 °C. The mixture was stirred for 12 h, resulting in a white cloudy solution, and then the
mixture was stirred for 12 h at 50 °C. The precipitate was filtered off and the filtrate was concentrated
to give white oil. The resulting oil was redissolved in petroleum ether and filtered. The solvent was
1
removed in vacuo giving the resulting product as an oil which provided L2 in 94% yield. H NMR (δ,
ppm, CDCl3, TMS): 2.216 (s, 6H, CH3), 2.387 (s, 6H, CH3), 5.898 (s, 2H, CH), 7.208–7.355 (m, 5H,
Ph). 13C NMR (δ, ppm, CDCl3): 12.99, 13.98, 108.75, 128.44, 128.49, 129.53, 129.54, 130.69, 130.89,
135.38, 148.60, 148.76, 153.45, 153.52. 31P NMR (δ, ppm, CDCl3, H3PO4): 59.74.
2.3 Synthesis of metal complexes
Nickel complexes were prepared by mixing equimolar amounts of the metal halide and the ligands
in THF; they were isolated as diamagnetic solids in yields of 76.9% and 72.0%. These complexes are
insoluble in nonpolar solvents (e.g., n-hexane, cyclohexane), but can be dissolved in polar solvents
(e.g., toluene, THF).
2.3.1 L1/NiBr2 complex
A freshly prepared solution of L1 (273.2 mg, 1.1 mmol) in toluene was added to a solution of
NiBr2(DME) (308.6 mg, 1.0 mmol) in THF (10 mL). After the resulting dark blue solution had been
stirred for 24 h, the solvent was evaporated using a cold trap and vacuum pump and then the solid
residue was washed three times with n-hexane. Drying in vacuo provided a dark blue powder. Yield:
428.8 mg, 0.77 mmol, 76.9%. EI-MS: m/z 465 [M+]. C12H20Br2N4NiSi: Calcd. C 30.87, H 4.32, N
12.00; Found: C 30.59, H 4.44, N 12.25.
2.3.2 L2/ NiBr2 complex
A freshly prepared solution of L2 (328.2 mg, 1.1 mmol) in THF was added to a solution of
NiBr2(DME) (308.6 mg, 1.0 mmol) in THF (10 mL). After the resulting purple solution had been
stirred for 24 h, the solvent was evaporated using a cold trap and vacuum pump and then the solid
residue was washed three times with n-hexane. Drying in vacuo provided a dark blue powder. Yield:
437.0 mg, 0.72 mmol, 72.0%. EI-MS: m/z 515[M+]. C16H19Br2N4PNi: Calcd. C 37.18, H 3.71, N 10.84;
Found: C 37.35, H 3.66, N 10.69.
3

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Scheme 1. Structures of complex L1/Ni and L2/Ni
2.4 General oligomerization procedure
Ethylene oligomerization runs were performed in the Lab of Grest transparent glass reactor.
Reactor heated in an oil bath，which was equipped with a magnetic stirrer bar, a thermocouple, a
pressure meter, and needle valves for injections. The glass reaction vessel was dried in an oven at
105 °C for 2 h prior to each run, purged under vacuum to remove air, and then brought back to
atmospheric pressure using high-purity N2 and ethylene; this cycle was repeated three times. The
reactor was then charged with solvent and stirred magnetically under an atmosphere of ambient
ethylene. After the desired reaction temperature had been established, the co-catalyst and metal
complexes were injected into the reactor. With stirring, the reactor was brought to the working pressure
and fed continuously with ethylene until the end of each reaction was reached (generally after 30 min).
The reactor was cooled to 0 °C and the excess ethylene was bled off. The samples of the organic layer
were taken and analyzed through Agilent gas chromatography (GC) with flame-ionization detection
(FID). The polymer was isolated by filtration, washed with ethanol and then dried at 50°C in a vacuum
oven.
3. Results and discussion
3.1 Effect of reaction temperature on catalytic properties for the oligomerization of ethylene
From the Table 1 we can see that the catalytic activity of the both of the catalysts initially
increased upon increasing the reaction temperature, reached maximum near 30 °C and then decreased
significantly as the temperature increasing. It indicates that the reaction temperature has an obvious
influence on the catalytic activity for the ethylene oligomerization. Higher oligomerization temperature
will resulted in lower activities for both homogeneous and supported complexes due to decreasing
ethylene solubility in toluene and increasing deactivation rate of active sites at high temperature [25].
The selectivity of the complex in ethylene oligomerization is substantially affected by the ligand
environment. In most cases, the selectivity toward butene of the L2/NiBr2 complex was higher than
that of the L1/NiBr2 complex. The introduction of Phenyl substituent as compared to methyl groups on
the pyrazolyl groups led to noticeable decrease in catalytic activity but higher C4 selectivity. This
observation suggests that the chelating ring size of the bidentate bis(pyrazolyl) ligand determines the
formation of an active catalyst.
For the two nickel complexes activated with MAO, the selectivity to C4 is high, attaining
24.8-92.7% of the total amount of olefins formed in the oligomerization reactions. At the 30 °C, the
L2/NiBr2 complex gives the highest selectivity (92.7%) to C4 with no polymer was detected.
4

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Table 1 The Effect of Reaction Temperature on Catalytic Properties of Ni complexes
Selectivity (wt%)
Activity
Catalyst
T (°C
)
=
=
=
(kg/molNiꢀh)
C4
α-C4 cis-C4 trans-C4
C6
C8
PE
n.d.
n.d.
n.d.
n.d.
8.6
20
30
40
50
60
70
20
30
40
50
60
70
270
340
252
151
127
84
74.6
90.4
87.1
86.9
68.7
39.8
77.6
92.7
90.2
89.4
81.6
24.8
64.3
83.8
75.5
72.3
48.2
16.9
66.5
84.4
81.2
74.6
62.4
8.7
6.3
4.3
4.0
2.3
4.9
6.7
8.9
10.6
4.9
2.7
3.5
6.2
8.7
8.6
23.1
8.5
2.3
1.2
1.8
1.7
3.1
3.3
2.0
1.3
1.0
1.2
1.5
2.6
6.7
11.1
11.8
19.7
10.3
20.4
6.1
L1/Ni
8.0
11.6
12.3
6.2
46.7
n.d.
n.d.
n.d.
n.d.
8.5
203
240
185
125
116
70
5.6
5.5
8.7
L2/Ni
8.7
9.4
10.5
7.5
8.3
8.8
63.8
Reaction Conditions: c(Cat)=0.384 mmol/L, n(Al)/n(Ni)=800, P=0.8 MPa, t=30 min, solvent=20 mL (toluene); MAO used as
co-catalyst.
3.2 Effect of Al/Ni molar ratio on catalytic properties
Table 2 displays the influence of Al/Ni molar ratio on the catalytic activity and selectivity of the
catalysts. Upon increasing the Al/Ni molar ratio from 200 to 800, all of the catalytic activity and
selectivity toward butene increased significantly. This trend could be attributed to increased chain
transfer to the co-catalyst or greater chain termination due to increased catalytic activities [26]. When
the Al/Ni molar ratio increased from 800 to 1000, the activity and the selectivity to C4 decreased. The
decrease of activity and selectivity could be expected because the excessive MAO may have interfered
with the formation of active metal species and/or caused their over-reduction [27].
Table 2 The Effect of Al/Ni Molar Ratios on Catalytic Properties of Ni complexes
Selectivity (wt%)
Al/Ni
Activity
Catalyst
=
=
=
(mol/mol)
(kg/molNiꢀh)
C4
α-C4
58.3
63.5
69.3
83.8
73.5
72.4
75.1
79.5
84.4
71.4
cis-C4
trans-C4
7.3
C6
C8
PE
n.d.
n.d.
n.d.
n.d.
3.2
200
400
600
800
1000
200
400
600
800
1000
87
76.3
78.3
86.2
90.4
83.9
87.0
87.2
88.8
92.7
84.5
10.6
7.4
9.4
4.3
6.7
8.8
7.7
6.3
5.6
9.6
16.9
19.0
11.8
8.5
2.8
2.7
2.0
1.2
1.4
1.6
1.6
1.0
1.3
1.3
264
304
340
373
116
140
203
240
247
7.4
L1/Ni
7.6
2.3
3.7
11.5
11.4
9.2
5.8
n.d.
n.d.
n.d.
n.d.
5.3
4.4
L2/Ni
3.0
10.2
6.1
2.7
3.5
8.9
Conditions: c(Cat)=0.384 mmol/L, T=30 oC, P=0.8 MPa, t=30 min, solvent=20 mL (toluene); MAO used as co-catalyst.
3.3 Effect of the reaction time on catalytic properties
5

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Table 3 reveals that the reaction time has a slight effect on the activity of the catalysts. The
catalytic activity increased a little upon increasing the reaction time. The reaction time significantly
affected the selectivity to 1-butene. The selectivity to C4 olefins is high and stable (87.0-93.2%).
However, by varying the reaction time from 15 to 60 min, chain isomerization transfer happened which
was always relative to chain propagation. Based on these results, it was found that increasing the
reaction time resulted in higher quantity of trans-C4 and cis-C4 owing to the isomerization process
involving α-C4.
Table 3 The Effect of Reaction Time on Catalytic Properties of Ni complexes
Selectivity (wt%)
Activity
(kg/molNiꢀh)
281
Catalyst t(min)
15
=
=
=
=
C4
α-C4 cis-C4 trans-C4
C6
C8
C10
87.0
90.4
89.8
89.8
88.2
92.7
92.8
83.2
83.8
76.6
72.6
86.2
84.4
77.3
2.5
4.3
8.3
7.3
1.3
5.6
8.6
11.1
1.3
2.3
4.9
10.1
0.7
2.7
6.9
9.4
11.2
8.5
1.8
1.2
1.6
1.2
1.4
1.3
1.1
1.2
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
30
340
L1/Ni
45
363
13.3
13.0
10.4
6.1
60
15
387
217
30
240
L2/Ni
45
253
6.1
60
260
93.2 72.8
5.6
Reaction Conditions: c(Cat)=0.384 mmol/L, T=30 °C, n(Al)/n(M)=800, P=0.8 MPa, solvent=20 mL (toluene), MAO was used as
co-catalyst.
3.4 Effect of Co-catalysts on Catalytic Properties
Compared with the MAO system, DEAC system and EADC system performed a better activity.
The selectivity toward 1-butene with EADC as activator was larger than that of DEAC, but the
catalytic activity was opposite. The obvious reason for performance of two types of co-catalysts
probably lies in the composition of the chlorine in the co-catalysts [22]. For the TEAL, TMAL and
TIBA system, the activities was obviously different with the corresponding MAO system. It was
possibly because the difference of the alkyl-metal bond formed by alkylation might result in the
different dimerization activity of Ni(II) catalyst. On the other hand, the different three-dimensional
structure of co-catalysts might also be the reason of their unique performance, the influence of space
steric hindrance of aluminum was very obvious. The selectivities of 1-butene synthesized by ethylene
dimerization catalyzed by DEAC, EADC, TEAL, TMAL or TIBA systems were lower than that by
MAO
system.
The
catalytic
selectivity
to
1-butene
were
in
order
of
MAO>EADC>TEAL>TIBA>TMAL>DEAC. From the Table 4, it was found that 1-octene was
obtained in the MAO system, but not in the other co-catalyst systems. It indicated that the co-catalyst
has an important effect in catalytic systems. The relationship between activator structure and its
performance needs to be researched further.
6

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Table 4 The Effect of Co-catalysts on Catalytic Properties of Ni complexes
Selectivity (wt%)
Activity
Catalyst Co-catalyst
=
=
=
(kg/molNiꢀh)
C4
α-C4 cis-C4 trans-C4
C6
C8
MAO
340
961
575
29
90.4
61.4
88.4
82.7
76.4
77.8
92.7
60.5
83.4
80.1
76.3
78.3
83.8
53.6
82.7
76.3
63.6
55.5
84.4
54.4
77.9
75.2
65.5
61.5
4.3
5.1
3.1
4.6
7.5
11.7
5.6
3.7
3.4
3.0
5.9
9.7
2.3
2.6
2.5
1.7
5.3
8.6
2.7
2.3
2.2
2.0
4.9
7.1
8.5
1.2
n.d.
n.d.
n.d.
n.d.
n.d.
1.3
DEAC
38.6
11.6
17.3
23.6
22.2
6.1
EADC
L1/Ni
TEAl
TMAl
TIBA
MAO
DEAC
6
26
240
1310
554
57
39.5
16.6
19.9
23.7
21.7
n.d.
n.d.
n.d.
n.d.
n.d.
EADC
L2/Ni
TEAl
TMAl
TIBA
7
49
Reaction Conditions: c(Cat)=0.384 mmol/L, T=30 oC, n(Al)/n(Ni)=800, P=0.8 MPa, t=30 min, solvent=20 mL (toluene).
Oligomerization using EADC instead of MAO as co-catalyst was systematically studied. By
comparing the results with those using MAO as co-catalyst, it produced in general much more active
systems (164-605 Kg/molNiꢀh), along with lower selectivity for butene (74.2-89.0%) (seen in Table 5).
By comparing the catalytic performance of L1/NiBr2 complexes and L2/NiBr2 complexes, it can
be concluded that the structure of the ligand has important impact on the activity and selectivity of the
catalyst. Firstly, electron donating methyl substituents could be more expected to stabilize the active
cationic species or facilitate its formation than the phenyl substituents. It is possible to enlarge the
number of active sites or even to make the coordination of the counterion less tight, thus enhancing the
overall activity [28]. Secondly, the atomic radius of Si is longer than that of P atom, but the
electronegativity of Si is lower than that of P. These differences in the physical properties of the
bridging atoms resulted in different bite angles of the ligands. Bite angle effects are usually rationalized
as a combination of steric effects (related to ligand-ligand and ligand-substrate repulsion) and
electronic effects (related to the bite angle of the donor orbitals to the metal center); which affected the
strength of metal hybridization and, consequently, the metal orbital energies and catalytic reactivities.
7

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Table 5 The Effect of EADC on the Catalytic Properties of Ni complexes
Selectivity (wt%)
Al/Ni
(mol/mol)
200
T
(oC)
30
30
30
30
30
30
30
30
30
30
Activity
(kg/molNiꢀh)
164
Catalyst Co-catalyst
=
=
=
C4
α-C4 cis-C4 trans-C4
C6
C8
EADC
EADC
79.1
84.4
87.0
88.4
89.0
74.2
77.4
79.8
83.4
84.3
63.3
71.6
76.8
82.7
81.2
58.6
64.4
71.0
77.9
78.5
9.2
7.7
6.2
3.1
4.3
8.4
7.3
5.5
3.4
3.1
6.6
5.0
4.0
2.5
3.6
7.2
5.8
3.3
2.2
2.7
20.9
15.6
13.0
11.6
11.0
25.8
22.6
20.2
16.6
15.8
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
400
297
L1/Ni
L2/Ni
EADC
EADC
EADC
EADC
EADC
EADC
EADC
EADC
600
418
800
575
1000
200
605
201
400
298
600
415
800
554
1000
589
Reaction Conditions: c(Cat)=0.384 mmol/L, P=0.8 MPa, t=30 min, solvent=20 mL (toluene).
4. Conclusion
Two new nickel complexes based on bidentate ligands featuring Si and P bridging atoms (L1 and
L2, respectively) have been prepared and evaluated for ethylene oligomerization under MAO, DEAC,
EADC, TEAL, TMAL and TIBA activation. With the increase of temperature, catalyst activity and
selectivity decreased, becausing of the decreased ethylene solubility in toluene and increased
deactivation rate of active sites. The excessive co-catalysts may have interfered with the formation of
active metal species and/or caused their over-reduction, which will decrease the catalyst activity and
selectivity. Increasing the reaction time has a little influence on the activity of the catalysts. But the
isomerization process involving α-C4 made the higher quantity of trans-C4 and cis-C4, thus the
o
selectivity of the α-C4 decreased. From the research, we get the best reaction conditions (30 C,
n(Al)/n(Ni)=800, 30 min) for the catalysts. The DEAC-activating systems and EADC-activating
systems give higher activity for the ethylene oligomerization, but the selectivity of DEAC is the lowest.
The catalytic selectivity to 1-butene are in order of MAO>EADC>TEAL>TIBA>TMAL>DEAC.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (U1162114),
Program for New Century Excellent Talents in University (NCET-07-0142) and the Provincial Key
Laboratory of Oil & Gas Chemical Technology (HXHG2012-04).
Open Access
This article is distributed under the terms of the Creative Commons Attribution License which
permits any use, distribution, and reproduction in any medium, provided the original author(s) and the
source are credited.
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Highlights
ꢀ Bidentate nickel complexes were evaluated as ethylene dimerization catalysts.
ꢀ Pre-catalysts activated with MAO, Et2AlCl and EtAlCl2 co-catalysts.
ꢀ Activity and selectivity were temperature, Al/Ni ratio and co-catalysts dependent.
ꢀ Ligand substituents and bridging atoms had a great influence on the activity.