A Series of Novel Dendritic Salicylaldimine Iron Catalysts: Synthesis, Characterization, and Application in Ethylene Oligomerization

Article abstract of DOI:10.1134/S0036024418130162

Abstract: A series of novel dendritic salicylaldimine ligands and their corresponding iron complexes have been synthesized using 1.0 generation (1.0 G) dendritic macromolecules, salicylaldehyde and FeCl₂ · 4H₂O as raw materials. The structures of the ligands and iron complexes were characterized by FT-IR, ¹H NMR, UV, ESI-MS, and elemental analysis. The dendritic salicylaldimine iron complexes presented moderate catalytic activity in ethylene oligomerization and selectivity for higher carbon number olefins products (≥C₈) when they were activated with methylaluminoxane (MAO). The catalytic activity and product selectivity were related to the co-catalyst, solvent, bridged group length and reaction parameters such as reaction temperature, Al/Fe molar ratio and ethylene pressure. The complex C3 showed the highest catalytic activity (up to 13.75 × 10⁴ g (mol Fe h)^–1) and the selectivity of higher carbon number olefins (35.31%) at the optimized conditions. Moreover, the catalytic properties of dendritic salicylaldimine iron complex C3 was better than non-dendritic iron complex.

Full text of DOI:10.1134/S0036024418130162

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2018, Vol. 92, No. 13, pp. 2618–2627. © Pleiades Publishing, Ltd., 2018.
CHEMICAL KINETICS
AND CATALYSIS
A Series of Novel Dendritic Salicylaldimine Iron Catalysts:
Synthesis, Characterization, and Application
in Ethylene Oligomerization1
Jun Wanga, Yiteng Shanga, Na Zhanga, Cuiqin Lia, and Weiguang Shia,*
aProvincial Key Laboratory of Oil and Gas Chemical Technology, College of Chemistry and Chemical Engineering,
Northeast Petroleum University, Daqing, Heilongjiang Province, 163318 China
*e-mail: sswwgg2003@126.com
Received October 5, 2017
Abstract—A series of novel dendritic salicylaldimine ligands and their corresponding iron complexes have
been synthesized using 1.0 generation (1.0 G) dendritic macromolecules, salicylaldehyde and FeCl2 · 4H2O
as raw materials. The structures of the ligands and iron complexes were characterized by FT-IR, 1H NMR,
UV, ESI-MS, and elemental analysis. The dendritic salicylaldimine iron complexes presented moderate cat-
alytic activity in ethylene oligomerization and selectivity for higher carbon number olefins products (≥C8)
when they were activated with methylaluminoxane (MAO). The catalytic activity and product selectivity were
related to the co-catalyst, solvent, bridged group length and reaction parameters such as reaction tempera-
ture, Al/Fe molar ratio and ethylene pressure. The complex C3 showed the highest catalytic activity (up to
13.75 × 104 g (mol Fe h)–1) and the selectivity of higher carbon number olefins (35.31%) at the optimized
conditions. Moreover, the catalytic properties of dendritic salicylaldimine iron complex C3 was better than
non-dendritic iron complex.
Keywords: dendritic macromolecule, iron complex, ethylene oligomerization, catalytic activity, selectivity
DOI: 10.1134/S0036024418130162
INTRODUCTION
complexes could be as catalytic active site for ethylene
oligomerization with high catalytic activity and selec-
tivity for linear α-olefins [22–25].
Ethylene oligomerization is currently one of the
major industrial processes for producing linear α-ole-
fins, which are main industrial substance widely used
for preparation of detergents, lubricants field chemi-
cals, and as co-monomers for copolymerization [1–
3]. Since Ziegler’s original work on AlR3 catalysis of
ethylene oligomerization [4], developing new oligom-
erization catalysts with various transition metals such
as nickel, chromium, titanium, zirconium, and others
have been considerable sustained interest in both aca-
demic and industrial fields [5–11]. A great number of
published works concerning the oligomerization reac-
tion were devoted to nickel complexes [12–18]. How-
ever, in recent years, the iron and cobalt complexes
based on different structures are known as highly
active catalysts for the oligomerization of ethylene,
and such systems are currently the focus of much
research [19–21]. Compared with other transition
metals, iron is the most abundant transition metal on
the earth with the content of 5.1% after oxygen, silicon
and aluminium, and is one of the most inexpensive
and environmentally friendly metals. Moreover, iron
Schiff base ligands have been synthesized and used
in number of catalytic reactions [26]. In particular the
salicylaldimine ligands can coordinate with metals
through hard nitrogen and oxygen donor atoms, which
lead to better stabilization of metal complexes against
reduction and confers unusual thermal stability [27].
Therefore the metal complexes with the Schiff base
ligands have been used extensively in ethylene oligom-
erization. We have reported the preparation of the
dendritic nickel and cobalt complexes containing
Schiff base ligands, and the research results showed
that the dendritic nickel and cobalt complexes exhib-
ited excellent catalytic properties for ethylene oligom-
erization [28, 29]. In this paper, we synthesized a series
of dendritic salicylaldimine iron complexes with 1.0 G
dendritic macromolecules, salicylaldehyde and fer-
rous chloride tetrahydrate as raw materials. We looked
into the possibility of these complexes as catalysts
being able to produce higher carbon number olefins
and also investigated the effects of solvent, co-catalyst,
and reaction conditions on ethylene oligomerization,
1
The article is published in the original.
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A SERIES OF NOVEL DENDRITIC SALICYLALDIMINE IRON CATALYSTS
2619
(57%); IR (KBr, cm–1): 3436 (s), 2946 (w), 1644 (s),
along with the effect of the structure of catalysts on
ethylene oligomerization.
1
1459 (m), 752 (m); H NMR (400 MHz, CDCl3,
ppm): δ: 8.354 (N=CH), 8.273 (nh), 7.285 (CH, ben-
zene), 7.233 (CH, benzene), 6.946 (CH, benzene),
6.853 (CH, benzene), 5.299 (OH), 3.932
(CH2N=CH), 3.685 (CH2, CH2NH), 3.521 (CH2N),
3.089 (CH2, bridged group), 2.296 (CH2CO); λmax
(nm): 214, 254, 323.
Dendritic salicylaldimine ligands L2 and L3 were
synthesized through the reaction between 1.0 G den-
dritic macromolecules (S2 and S3) and salicylalde-
hyde according to the method described in the litera-
tures [28, 29].
EXPERIMENTAL
Materials and General Considerations
All experimental works of air and moisture sensi-
tive compounds were performed under a nitrogen
atmosphere using standard Schlenk techniques. All
the solvents were analytical grade and dried by reflux-
ing over sodium/benzophenone and distilled under
nitrogen prior to use. MAO (10 wt % in toluene) and
ethylaluminum sesquichloride (EASC, 25 wt % in
cyclohexane) were offered by Sigma-Aldrich (China).
Toluene and methanol were purchased from Tianjin
Kermel Chemical Reagent Co., Ltd. (China). n-Hex-
ane was obtained from Tianjing Damao Chemical
reagent factory. Salicylaldehyde was provided by Tian-
jin Guangfu Fine Chemical Research Institute
(China). 1.0 G dendritic macromolecules with 1,2-
ethylenediamine, 1,4-butanediamine, and 1,6-hex-
anediamine as cores were synthesized according to
[30]. FeCl2 · 4H2O were purchased from Tianjin
Damao Chemical Reagent Factory (China).
Synthesis of Dendritic Salicylaldimine Iron Complexes
A methanol solution (15 mL) of dendritic salicylal-
dimine ligand L1 (4.65 g, 0.005 mol) was added to a
methanol solution (5 mL) of FeCl2 · 4H2O (3.98 g,
0.02 mol). The reaction mixture turned into dark red
immediatedly and was allowed to stir at 25°C for 24 h.
The precipitate was obtained after diethyl ether being
added to the mixture. The resultant precipitate was
then isolated by filtration, and washed with diethyl
ether to afford complex C1 as a red solid. Yield: 4.06 g
(78%) (Scheme 1); IR (KBr, cm–1): 3398 (s), 2928
(w), 1611 (s), 1546 (m), 764 (m); anal. calcd. for
C50H60N10O8Fe2: Fe, 10.76; found: Fe, 9.97; λmax
IR spectra were recorded in a KBr disc matrix
using a Bruker Vector 22 IR spectrophotometer over
the range of 4000–450 cm−1. 1H-NMR spectrum was
recorded with Bruker-400 MHz NMR at 400 MHz in
CDCl3 with tetramethylsilane (TMS) as an internal
reference. The UV–Vis spectra were carried out on a 929 [M-2Fe]+
(nm): 233, 256, 323; ESI-MS: m/z: 1042 [M+H]+,
.
UV-1700 ultraviolet-visible spectrophotometer with
methanol as solvent. Elemental analyses were carried
on the Optima 5300DV analyzer (America). Electro-
spray ionization mass spectrometry (ESI-MS) data
were collected on an micrOTOF-Q II mass spectrom-
eter. GC-MS analyses were performed on Agilent
equipped with a flame ionization detector (FID) and
a 30 m (0.25 mm i.d., 0.25 μm film thickness) DB-1
column. Gas chromatography (GC) analyses of oligo-
mers were performed on a Fuli GC9720 equipped with
a flame ionization detector (FID) and a 50 m (0.2 mm
i.d., 0.5 μm film thickness) HP-PONA column.
Complex C2 was prepared according to the proce-
dure used for the synthesis of complex C1 using ligand
L2 (4.79 g, 0.005 mol) and FeCl2 · 4H2O (3.98 g,
0.02 mol). Yield: 4.28 g (80%) (Scheme 1); IR (KBr,
cm‒1): 3398 (s), 2929 (w), 1607 (s), 1535 (m), 753 (m);
anal. calcd. for C52H64N10O8Fe2: Fe, 10.49; found: Fe,
9.85; λmax (nm): 234, 261 321; ESI-MS: m/z: 1069
[M+H]+, 957 [M-2Fe]+.
Complex C3 was prepared according to the proce-
dure used for the synthesis of complex C1 using ligand
L3 (4.92 g, 0.005 mol) and FeCl2 · 4H2O (3.98 g,
0.02 mol). Yield: 4.16 g (76%) (Scheme 1); IR (KBr,
cm‒1): 3397 (s), 2953 (w), 1611 (s), 1541 (m), 764 (m);
anal. calcd. for C54H68N10O8Fe2: Fe, 10.22; found: Fe,
9.64; λmax (nm): 236, 260, 323; ESI-MS: m/z: 1096
Synthesis of Dendritic Salicylaldimine Ligands
Methanol (30 mL) was added to the mixture of
1.0 G dendritic macromolecule S1 (1.55 g, 3 mmol)
and NaSO4 (3 g) under the nitrogen atmosphere, and
the solution of the mixture was stirred for 10 min at
25°C. Then salicylaldehyde (2.60 mL, 24 mmol) was
added dropwise after the solution being heated to 65°C
and was refluxed for 12 h. The dendritic salicylaldi-
mine ligand solution was obtained after removing
NaSO4 by filtration. The ligand solution crystallized
for 48 h at –30°C. A yellow precipitate was formed and 3 h prior to each run, and then cooled to the required
was isolated by filtration, washed three times with temperature. After evacuation and flushing with eth-
ether and dried in vacuum, affording dendritic salicyl- ylene three times, a typical reaction was performed by
aldimine ligand L1 as a yellow solid. Yield: 1.60 g introducing solvent (40 mL), the proper amount of
[M+H]+, 984 [M-2Fe]+.
General Process for Ethylene Oligomerization
All ethylene oligomerization tests were performed
in a 250 mL stainless steel reactor with magnetic stir-
ring, temperature controller and an internal cooling
system. The reactor was dried in an oven at 160°C for
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JUN WANG et al.
NH2
H2N
O
O
O
HN
HN
NH
HO
CHO
N
N
n
+
NH
O
H2N
NH2
O
O
HO
N
HN
HN
NH
N
N
OH
OH
N
N
n
HO
N
NH
O
O
O
O
O
HO
HO
N
N
HN
NH
N
OH
+ 2FeCl2 ⋅ 4H2O
N
N
n
HN
NH
O
N
OH
O
O
O
O
N
N
HN
HN
NH
NH
N
N
O
O
Fe
Fe
n
N
N
O
O
L1: n = 0; C1: n = 0
L2: n = 1; C2: n = 1
L3: n = 2; C3: n = 2
Scheme 1. Synthetic routes of dendritic salicylaldimine iron complexes.
co-catalyst (MAO or EASC) and the catalyst solution the desired time, the reaction was quenched by cool-
(10 mL, [Fe] = 5 μmol) were injected into the reactor ing the system to 0°C, depressurizing and adding
under a stream of ethylene and then the reactor was HCl/ethanol (10 wt %). The catalytic activity was cal-
immediately pressurized. Ethylene was continuously culated based on the increase of product weight. The
fed in order to maintain the ethylene pressure. After mixture was analyzed by GC or GC-MS to gain the
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A SERIES OF NOVEL DENDRITIC SALICYLALDIMINE IRON CATALYSTS
2621
C3
L3
C2
C1
L2
L1
4000
3000
2000
1000
4000
3000
2000
1000
ꢀ, cmꢁ1
ꢀ, cmꢁ1
Fig. 1. FT-IR spectra of the dendritic salicylaldimine ligands and iron complexes.
8
6
4
2
ppm
1
Fig. 2. H NMR spectrum of the dendritic salicylaldimine ligand L1.
ber at around 1611 cm‒1, which indicated that the
metal ions had coordinated with the imino nitrogen
atoms and the oxygen atoms of the ligands.
selectivity of catalysts by comparison to the standard
authentic samples.
1H NMR Spectroscopy. 1H NMR spectrum of the
dendritic salicylaldimine ligand L1 is shown in Fig. 2.
The signal for the protons in the methylene groups
which belonged to the bridged group could be found at
around 3.089 ppm. The signal for the protons in
α-methylene connected to tertiary amine group was
evidenced at around 3.521 ppm. The signal for the
protons in methylene connected to carbonyl group was
exhibited at around 2.296 ppm. The signal for the pro-
tons in methylene group connected to secondary
amine group was evidenced by the peak at around
3.658 ppm. The signal for the protons in the secondary
amine group was indicated at around 8.273 ppm. The
signal for the protons in the methylene connected to
imine and benzene occurred at around 8.354 ppm.
RESULTS AND DISCUSSION
Characterization of Dendritic Salicylaldimine Ligands
and Iron Complexes
FT-IR Spectroscopy. The FT-IR spectra of the
dendritic salicylaldimine ligands L1-L3 and the iron
complexes C1-C3 are shown in Fig. 1. As can be seen
from the spectra of the dendritic salicylaldimine
ligands, the characteristic peaks at around 2946 cm‒1
were designated to the stretching vibration of ‒CH2‒
groups. The peaks at approximately 1459 cm‒1
belonged to the stretching vibration of benzene ring.
The absorption peaks at around 3436 cm‒1 were due to
the overlap of stretching vibration of ‒NH‒ and ‒OH
groups. The peaks at around 1644 cm‒1 belonged to The signal for the protons in aryl rings occurred at
the stretching vibration of the C=N groups, which around 6.853–7.285 ppm. In addition, the signal for
indicated that the Schiff base reactions were proceed the protons in hydroxyl connected to benzene
between 1.0 G dendritic macromolecules and salicyla- occurred at around 5.299 ppm. Above signal for the
dehyde. Compared with the spectra of the dendritic protons further indicated that the Schiff base reaction
salicylaldimine ligands, the absorption peaks of C=N occurred between 1.0G dendritic macromolecule S1
stretching vibration were shifted to a lower wave num- and salicyladehyde.
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JUN WANG et al.
C3
L3
C2
C1
L2
L1
300
400
500
600
ꢂ, nm
200
300
400
500
600
ꢂ, nm
Fig. 3. UV spectra of the dendritic salicylaldimine ligands and iron complexes.
UV spectroscopy. The UV spectra of the dendritic the effect of the co-catalysts on catalytic activity and
salicylaldimine ligands and iron complexes are given oligomer distribution at the same ethylene oligomeri-
in Fig. 3. The UV spectra of the dendritic salicylaldi-
mine ligands in the methanol solution showed three
absorption bands at around 214, 254, and 323 nm. The
bands at around 214 nm should be ascribed to the R
band of n → π* transition for C=O, the bands at
around 254 nm were assigned to the B band of the
benzene ring, and the bands at around 323 nm
belonged to the R band of n → π* transition for C=N.
The UV spectra of iron complexes showed that the R
band of the C=O and the B band of the benzene ring
occurred slightly shift to around 234 and 260 nm,
respectively. Meanwhile, the visibly weak peaks of
n → π* transition for C=N at around 323 nm were
observed, indicating that the iron atom was involved in
coordination to the dendritic salicylaldimine ligands.
MS spectroscopy. The MS spectra of the dendritic
salicylaldimine iron complexes are given in Fig. 4. As
shown in Fig. 4, the molecular ion peaks of the den-
dritic salicylaldimine iron complexes C1, C2, and C3
appeared at m/z: 1042, 1069, and 1096, respectively.
The loss of 2Fe resulted in the formation of the base
peaks at m/z: 929, 957, and 984, corresponding to the
dendritic salicylaldimine ligands. Above data indi-
cated that the actual values were accordant with the
theoretical values.
zation reaction conditions and the research results are
listed in Table 1. As showed in Table 1, the co-catalyst
had a crucial effect on catalytic activity. Upon activa-
tion with MAO, the iron complexes C1, C2, and C3
showed lower catalytic activities (activity varying from
5.59 × 104 to 6.79 × 104 g(mol Fe h)–1). However, acti-
vation the iron complexes C1, C2 and C3 with co-cat-
alyst EASC instead of co-catalyst MAO, the catalytic
activities were higher than the catalytic system with
MAO as co-catalyst (activity varying from 24.35 × 104
to 39.37 × 104 g (mol Fe h)–1). As for higher activity
with co-catalyst EASC that possible reason due to the
electron shielding effect can be decreased and the
strong Lewis acid can increase the metal active sites to
generate the high catalytic activity. In addition, the co-
catalyst also had a significant effect on oligomer distri-
bution. When activated the iron complexes C1, C2,
and C3 with MAO, the oligomerized ethylene to
afford C4 as the major product and C6 and higher car-
bon number olefins. The observed selectivity with
MAO as co-catalyst would be consistent with a faster
C1
525
481
815
737
613
569
408
929
851
1042
Ethylene oligomerization. Dendritic salicylaldi-
mine iron complexes C1–C3 were evaluated as cata-
lysts for oligomerization of ethylene, using two co-cat-
alysts (MAO and EASC) and three solvents (toluene,
chlorobenzene and n-hexane) in attempt to generate
active catalysts. The research results showed that all
the complexes produced active catalysts for the oligo-
merization of ethylene and the reaction conditions,
solvent, co-catalyst, bridged group length as well as
catalyst structure had a significant effect on catalytic
activity and oligomer distribution.
400 500 600 700 800 900 1000 1100
C2
639
894
957
429
790
720
983
1035
510
832
555
939
1069
400 500 600 700 800 900 1000 1100
532
C3
731
746
419
895
449
654
844
984
579
914
1096
400 500 600 700 800 900 1000 1100 1200
Effect of co-catalyst on the catalytic activity and
oligomer distribution. The dendritic salicylaldimine
iron complexes C1, C2, and C3 were activated with
MAO and EASC in toluene solvent so as to research
Fig. 4. MS spectra of the dendritic salicylaldimine iron
complexes.
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A SERIES OF NOVEL DENDRITIC SALICYLALDIMINE IRON CATALYSTS
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Table 1. Effect of co-catalyst on the catalytic activity and oligomer distributiona
Oligomer distribution, %
Activity
104 g (mol Fe h)–1
Entry
Complexes Co-catalytic
d
e
C4
C6
C8
≤C6
C8–C18
1b
2c
3b
4c
5b
6c
C3
C3
C2
C2
C1
C1
MAO
EASC
MAO
EASC
MAO
EASC
6.79
39.37
6.12
33.19
5.59
52.7
12.8
60.8
8.6
78.4
11.7
9.9
19.8
21.5
14.7
12.6
13.6
62.6
32.6
82.3
23.3
91.0
25.3
18.3
5.3
8.6
7.1
6.3
5.9
37.4
67.4
17.7
76.7
9.0
24.35
74.7
a
b
c
d
e
Reaction condition: [Fe] = 5 μmol, [toluene] = 50 mL, time = 30 min, temperature = 15°C, Al/Fe = 300, pressure = 0.5 MPa.
4
–1
10 g (mol Fe h)
.
Determined by GC.
Low carton number olefins.
Higher carton number olefins.
Table 2. Effect of solvent on the catalytic activity and oligomer distributiona
Oligomer distribution, %b
Activity
Entry Complexes
Solvent
Toluene
n-Hexane
Chlorobenzene
Toluene
n-Hexane
Chlorobenzene
Toluene
n-Hexane
Chlorobenzene
104 g (mol Fe h)–1
c
d
C4
C6
C8
≤C6
C8–C18
1
2
3
4
5
6
7
8
9
C3
C3
C3
C2
C2
C2
C1
C1
C1
6.79
1.29
20.57
6.12
52.7
66.8
83.7
60.8
69.4
85.8
78.4
74.2
87.6
9.9
23.0
7.2
21.5
20.6
9.5
12.6
18.9
10.2
62.6
89.8
90.9
82.3
90.0
95.3
91.0
93.1
97.8
18.3
9.8
3.5
8.6
6.7
4.0
6.3
5.2
2.1
37.4
10.2
9.1
17.7
10.0
4.7
1.02
13.59
5.59
1.01
9.0
6.9
10.67
2.2
a
Reaction condition: [Fe] = 5 μmol, [solvent] = 50 mL, time = 30 min, Al/Fe = 300, temperature = 15°C, ethylene pressure =
0.5 MPa.
b
Determined by GC.
Lower carton number olefins.
Higher carton number olefins.
c
d
rate of β-hydrogen elimination relative to that of chain vent in ethylene oligomerization reactions, complexes
growth. When activated the iron complexes C1, C2, C1, C2, and C3 were further investigated in n-hexane
and chlorobenzene, which were compared with the
previous results in toluene and the results are listed in
Table 2. The catalytic activities decreased under simi-
lar conditions in the order of n-hexane < toluene <
chlorobenzene. Because complexes C1, C2, and C3
have poor solubility in n-hexane the, so catalytic activ-
ities were the lowest than those in toluene and chloro-
benzene. This was because chlorobenzene and toluene
has bigger polarity than n-hexane, and they favour
having good solubility of the complexes in ethylene
oligomerization reaction. In addition, it is interesting
to note that the selectivity of higher carbon number
olefin was higher with toluene as solvent than n-hex-
and C3 with EASC in toluene solvent produced a
small amount of oligomers C4, C6, and C8, as well as
predominantly alkylated-toluenes of the pre-formed
oligomers (C4 and C6 as minor products). The above
experiment results showed that although the catalytic
activities were higher when EASC was used as co-cat-
alyst, the products were mainly alkylated–toluenes of
the pre-formed oligomers. Comprehensive consider-
ation, co-catalyst MAO was selected for all further
investigations.
Effect of solvent on the catalytic activity and oligo-
mer distribution. The solvent can influence the cata-
lytic activity and product distribution due to the solu- ane and chlorobenzene. Therefore, further catalytic
bility of the catalyst itself. Considering the kind of sol- studies were performed with toluene as solvent.
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JUN WANG et al.
Table 3. Influence of reaction conditions on the catalytic activity and oligomer distributiona
Oligomer distribution, %b
Activity
104 g(mol Fe h)–1
Entry
Al/Fe
T, °C
P, MPa
c
d
C4
C6
C8
≤C6
C8–C18
1
2
3
4
5
6
7
8
9
300
300
300
300
300
500
700
1000
500
500
500
500
15
25
35
45
15
15
15
15
15
15
15
15
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.1
0.5
0.7
1.0
6.79
6.47
5.61
52.7
51.9
47.8
45.9
52.7
59.0
61.0
66.5
65.5
59.0
53.6
49.3
9.9
21.8
25.1
23.1
9.8
14.7
21.4
19.2
19.4
14.7
16.2
15.4
62.6
73.7
72.9
69.0
62.5
73.7
82.4
85.7
84.9
73.7
69.8
64.7
18.3
7.6
7.9
11.1
18.3
10.2
9.2
7.1
8.7
10.2
12.1
18.3
37.4
26.3
27.1
31.0
37.5
26.3
17.6
14.3
15.1
26.3
30.2
35.3
5.31
6.79
8.34
8.76
13.82
6.34
8.34
11.98
13.75
10
11
12
a
b
c
d
Reaction condition: [Fe] = 5 μmol, [toluene] = 50 mL, time = 30 min, MAO as co-catalyst.
Determined by GC.
Lower carton number olefins.
Higher carton number olefins.
The length of bridged group had a significant influ- sition of the active species and it will quicken deacti-
ence on the catalytic activities and oligomer distribu- vated ratio of catalyst [31, 32]. Meanwhile, ethylene
tion, which can be seen from Tables 1 and 2. It was solubility decreased at higher temperature. All the
apparent that the catalytic active increased with the results indicated that although enhanced reaction
increase of bridged group length. The complex C3 temperature were excepted to result in overall higher
with the longest bridged group length displayed the propagation and transfer rates, chain propagation rate
highest catalytic activity compared to the complexes and chain transfer rate did not present linear relation-
C1 and C2 (Tables 1 and 2). This could be explained ship, which could result in that the oligomer distribu-
that the solubility of the complex increased with the tion did not present obvious change rule with the
increase of the bridged group length, which led to the increase of reaction temperature.
increase of the concentration of the effective active
species in the catalyst system. Not only the catalytic
activity was sensitive to the length of bridged group,
but also the selectivity of higher carbon number olefins
was significantly affected by the length of bridged
group. The concentration of higher carbon number
olefins increased with the increase of the bridged
group length.
The effect of the Al/Fe molar ratio on the catalytic
activity and oligomer distribution were further investi-
gated with complex C3. The catalytic activities
increased with increasing of the Al/Fe molar ratio in
the range 300 to 1000 (Table 3, entries 5–8), and the
highest activity of 13.82 × 104 g/(mol Fe h) was
observed at an Al/Fe molar ratio of 1000 at 15°C.
Higher Al/Fe molar ratio resulted in increased cata-
lytic activity, which possibly due to that MAO scav-
enged adventitious water and impurities in the solvent
at low Al/Fe molar ratio and the iron complex
required more co-catalyst to be activated to produce
more active species, which make more ethylene
monomer react with active species. Generally higher
Al/Fe molar ratio decreases the formation of higher
carbon number olefins (Table 2, entries 5–8). This
trend would be attributed to increased chain transfer to
the co-catalyst or greater chain termination due to
increased catalytic activities.
Influence of reaction conditions on the catalytic
activity and oligomer distribution. It is well known that
the reaction conditions have significant effect on cat-
alytic activity and oligomer distribution. The complex
C3 was investigated under varying conditions, such as
reaction temperature, ethylene pressure and the
amount of MAO in order to probe the effects of reac-
tion parameters on the catalytic activity and oligomer
distribution. The oligomerization results are summa-
rized in Table 3. As revealed in Table 3, the reaction
temperature significantly affected the catalytic activity
and oligomer distribution. Increasing the reaction
The influence of ethylene pressure on the catalytic
temperature from 15 to 45°C led to decrease of the cat- activity and oligomer distribution was studied by vary-
alytic activity (Table 3, entries 1–4). With increased ing ethylene pressure from 0.1 to 1.0 MPa with com-
temperature, which might be attributable to decompo- plex C3 (Table 3, entries 9–12). From the results, an
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A SERIES OF NOVEL DENDRITIC SALICYLALDIMINE IRON CATALYSTS
2625
O
O
O
O
N
N
HN
HN
NH
NH
N
N
O
O
Ni
Ni
N
N
2
O
O
O
O
C4
O
O
N
N
HN
HN
NH
NH
N
N
O
O
Co
Co
N
N
2
O
O
C5
N
Fe
Ph
Ph
P
N
CH3
N
O
Cl
Cl
H3C
C6
Fig. 5. The structures of dendritic nickel, cobalt complexes and (pyrazolyl)-(phosphinoyl) pyridine iron complex.
increase in ethylene pressure from 0.1 to 1.0 MPa led effect on the catalytic activity. The dendritic nickel
to increased catalytic activities from 6.34 × 104 to
13.75 × 104 g/(mol Fe h). The reason was that the eth-
ylene solubility dependence on pressure. In addition,
we also found that a change of ethylene pressure had
an effect on the product distribution. With increasing
of ethylene pressure, the formation of the higher car-
bon number olefins was easier. Higher composition of
higher carbon number olefins at higher pressures sug-
gested that increasing ethylene pressure had a heavier
impact on the chain propagation than on the β-H
elimination in the oligomerization process.
Influence of catalyst structure on the catalytic activity
and oligomer distribution. The influence of catalyst
structure on the catalytic activity and oligomer distribu-
tion was studied using the dendritic iron complex C3,
dendritic nickel complex C4 (Fig. 5), dendritic cobalt
complex C5 (Fig. 5), and (pyrazolyl)-(phosphinoyl)
pyridine iron complex C6 (Fig. 5) which was synthe-
sized by Nyamato et al. [33] and the results are sum-
complex C4 displayed higher catalytic activity com-
pared to dendritic cobalt complex C5 and dendritic
iron complex C3. One plausible reason for this trend
could be attributable to that the electron density of
nickel atom is larger than cobalt atom and iron atom,
and the nickel active center was easy to form and had
better stability than cobalt center and iron center
under the MAO activation. In addition, as can be seen
from table 4, the ligand type also had a considerable
impact on the catalytic activity. It was apparent that
the catalytic activity of dendritic iron complex C3 was
higher than (pyrazolyl)-(phosphinoyl) pyridine iron
complex C6. The finding was due to that the dendritic
iron complex C3 had double the amount of iron active
centers than the (pyrazolyl)-(phosphinoyl) pyridine
iron complex C6. There were much more ethylene
molecules reaction with iron active centers in unit
time when the dendritic iron complex C3 was used.
We also observed the dependence of oligomer dis-
marized in Table 4. The metal atom had a significant tribution on catalyst structure. The dendritic nickel
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2626
JUN WANG et al.
Table 4. Effect of catalyst structure on the catalytic activity and oligomer distributiona
Oligomer distribution, %b
Activity
105 g(mol Fe h)–1
Entry
Complexes
c
d
C4
C6
C8
≤C6
C8–C18
1
2
3
4
C3
C4
C5
C6
1.22
2.37
1.89
1.10
59.4
45.9
32.5
94.0
20.2
17.2
11.6
6.0
79.6
63.1
44.1
9.7
35.3
20.4
–
20.4
36.9
55.9
–
100.0
a
b
c
d
Reaction condition: [Fe] = 10 μmol, [toluene] = 80 mL, time = 30 min, temperature = 30°C, Al/M = 250, pressure = 1.0 MPa.
Determined by GC.
Lower carton number olefins.
Higher carton number olefins.
and cobalt complexes preferred to higher carbon num-
ber olefins compared with the dendritic iron complex.
For example, the dendritic iron complex gave the
selectivity of 20.47% for higher carbon number ole-
ACKNOWLEDGMENTS
We acknowledge the funding from the Natural Sci-
ence Foundation of China (no. 21576048), Petroleum
Innovation Foundation of China (no. 2014D-5006-
fins, and the dendritic nickel and cobalt complex gave 0503) and Youth Science Foundation of Northeast
Petroleum University (2013NQ130).
the selectivity of 36.95% and 55.83% for higher carbon
number olefins, respectively. However, the selectivity
of higher carbon number olefins of the dendritic iron
complex was higher than the (pyrazolyl)-(phosphi-
noyl) pyridine iron complex, which indicated that the
dendritic metal catalysts had good application pros-
pect in the field of ethylene oligomerization.
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