Preparation of chromium catalysts bearing bispyridylamine and its performance in ethylene oligomerization

Article abstract of DOI:10.1007/s11243-019-00333-3

A series of bispyridylamine ligands and chromium complexes designed for ethylene oligomerization have been synthesized, which was made up of both neutral organic ligand and inorganic anion. The compositions of these complexes have been fully characterized by spectroscopic and analytical methods. Catalysts (Cr1–Cr4) were evaluated in detail with methylaluminoxane (MAO) as an activator in the ethylene oligomerization. As a result, Cr1 achieved a higher catalytic activity of 2.93 × 10⁵g/(mol(Cr) h) and a higher selectivity of 55.34% toward the valuable C₈ compound using toluene as the solvent. The total selectivity to LAOs was 71.2%.

Full text of DOI:10.1007/s11243-019-00333-3

Transition Metal Chemistry
https://doi.org/10.1007/s11243-019-00333-3
Preparation of chromium catalysts bearing bispyridylamine and its
performance in ethylene oligomerization
Jun Wang1 · Jinyi Liu1 · Liduo Chen1 · Tianyu Lan1 · Libo Wang2
Received: 15 March 2019 / Accepted: 6 May 2019
© Springer Nature Switzerland AG 2019
Abstract
A series of bispyridylamine ligands and chromium complexes designed for ethylene oligomerization have been synthesized,
which was made up of both neutral organic ligand and inorganic anion. The compositions of these complexes have been
fully characterized by spectroscopic and analytical methods. Catalysts (Cr1–Cr4) were evaluated in detail with methyla-
luminoxane (MAO) as an activator in the ethylene oligomerization. As a result, Cr1 achieved a higher catalytic activity of
2.93×105g/(mol(Cr) h) and a higher selectivity of 55.34% toward the valuable C8 compound using toluene as the solvent.
The total selectivity to LAOs was 71.2%.
Introduction
olefns to carbonyl compounds and carboxylic acid deriva-
tives [6–11]. Chromium complexes are also potent catalysts
to the shorter chain oligomerization reactions [12–14].
Indeed, chromium as a metal center has been dominated for
ages in ethylene oligomerization [15]. Phillips catalyst is a
typical example, producing 1-hexene and 1-octene [16, 17].
In addition, diferent ligand structures play a diferent
role in controlling the catalytic performance and the prod-
uct selectivity in ethylene oligomerization. Ligands such as
N^N^N, N^N^O and C^N^C [18–20] determine the cata-
lytic behavior via stabilizing a particular oxidation state. In
order to improve the catalytic performances, N^N^N-cobalt
catalysts have emerged, bearing cycloalkyl-fused pyridines
[21]. Furthermore, systems based on bis(arylimino)pyridine
pincer ligands were capable of generating either oligomers
(mainly α-olefns) or highly linear polyethylene, depending
on the steric properties of the N-aryl groups [12–14, 22, 23].
Herein, the synthesis of a new family of N^N^N-
chromium catalysts bearing bispyridylamine ligands and
their catalytic performance in ethylene oligomerization is
reported. On activation with methylaluminoxane (MAO),
good activities were observed for all the chromium com-
plexes studied highly afording LAOs. Moreover, the efects
of substituent pair ligands, Al/Cr molar ratio, reaction tem-
perature, reaction pressure and the solvent types on their
catalytic activities and selectivities have been investigated
in detail.
Linear alpha olefns (LAOs) are of great importance, serv-
ing as intermediates and building blocks for the synthesis of
various chemical products. According to the carbon chain
length, olefns in the range C4–C20 are used to produce
polyethylene, synthetic lubricants, plasticizer, detergents
and some co-polymers. Continually increasing demand for
LAOs has led to a research interest in ethylene oligomeri-
zation technologies [1–3]. Complexes with late transition
metals were widely applied to ethylene oligomerization as
homogeneous catalysts, in which the most famous model
initially developed by Brookhart et al. [4, 5] was based on
2, 6-bis(imino)pyridines. Among the transition metal com-
plexes, chromium complexes supported by pincer ligands
have always attracted more attention as the catalyst due to a
wide range of applications spanning from hydrogenation of
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-019-00333-3) contains
supplementary material, which is available to authorized users.
* Jun Wang
wangjun1965@yeah.net
1
Provincial Key Laboratory of Oil & Gas Chemical
Technology, College of Chemistry & Chemical Engineering,
Northeast Petroleum University, Daqing 163318,
Heilongjiang Province, China
2
Daqing Chemical Research Center of Petrochemical
Research Institute, Daqing 163714, Heilongjiang Province,
China
Vol.:(0123456789)
1 3

Transition Metal Chemistry
then heating at 5 °C min−1 until 240 °C and leaving for
5 min, and 240 °C for sample injector.
Experimental
Material and instrumentation
Synthesis of ligands (L1–L4)
All manipulations of air- and moisture-sensitive compounds
were carried out under nitrogen atmosphere using standard
Schlenk techniques. Toluene and tetrahydrofuran (THF)
were refuxed over sodium benzophenone and distilled under
nitrogen prior to use. Methylaluminoxane (MAO, 10 wt% in
toluene) was bought from Sigma-Aldrich. 2-Phenoxyethyl-
amine, 2-(2-methoxyphenoxy) ethylamine, 2-chloropyridine,
2-chloro-4-methylpyridine and CrCl3(THF)3 were purchased
from Aladdin and used as received. Polymerization-grade
ethylene was obtained from Daqing Summit Specially Gases
(China). All other reagents were purchased from Tianjin
Damao Chemical Reagent.
Proligands L1–L4 were prepared as shown in Scheme 1.
Synthesis of N‑(2‑phenoxyethyl)‑N‑(pyridin‑2‑yl)
pyridin‑2‑amine (L1)
A solution of 2-phenoxyethylamine (1.31 mL, 10 mmol) in
methanol (10 mL) was added to a solution of triethylamine
(Et3N) (2.76 mL, 20 mmol) in methanol (10 mL) at 0 °C. A
solution of 2-chloropyridine (3.80 mL, 40 mmol) in metha-
nol (10 mL) was then added at 0 °C under N2. After being
stirred for 30 min, the reaction mixture was heated to 25 °C
and stirred for 3 days. Then, the solvent was evaporated
under reduced pressure to aford a yellow solid L1 washed
with methanol and dried under vacuum. Yield: 2.31 g (79%).
FTIR data (KBr, pellet, cm−1): υ 3335, 3057, 2922, 1573,
FTIR spectra were recorded on a Bruker Vector 22 FTIR
spectrophotometer from 4000 to 450 cm−1 using KBr pel-
lets. MS data were collected from a micrOTOF-Q II mass
spectrometer using electrospray ionization (ESI) as the ion
source. 1H NMR and 13C NMR spectra were obtained using
a Bruker DMX 400 MHz and 100 MHz instrument corre-
spondingly at ambient temperature with CDCl3 as the sol-
vent and tetramethylsilane (TMS) as an internal standard.
Elemental analysis was performed with a Foss-Heraeus 240
CHNO-Rapid Analyzer (Daqing Petrochemical Research
Center). Gas chromatography (GC) analysis of oligomers
was conducted on a Fuli GC 9720 using n-heptane as an
internal standard, equipped with a fame ionization detector
(FID) and a 50-m (0.2 mm id, 0.5 μm flm thickness) HP-
PONA column, operating at 50 °C for 5 min followed by
heating at 10 °C min−1 until 140 °C and leaving for 0 min,
1
1436, 1243. H NMR (400 MHz, CDCl3, ppm): δ 3.94 (t,
2H, CH2, J 47.8 Hz), 4.59 (t, 2H, CH2), 6.85 (t, 2H, Py–H),
6.87 (d, 2H, Py–H, J 7.1 Hz), 7.15 (d, 2H, Ph–H), 7.26 (t,
1H, Ph–H), 7.54 (t, 2H, Ph–H), 7.57 (t, 2H, Py–H), 8.33 (d,
2H, Py–H). 13C NMR (100 MHz, CDCl3, ppm): δ 42.83,
67.63, 115.72, 116.79, 117.17, 121.13, 130.02, 137.32,
147.14, 157.30, 159.39. ESI–MS (m/z): 291.16 [L1+H]+.
Anal. Calcd for C18H17N3O (Found) %: C, 74.23 (74.01); H,
5.84 (5.71); N, 14.43 (14.13).
Scheme 1 Synthetic routes of
bispyridylamine ligands and
chromium complexes
1 3

Transition Metal Chemistry
Synthesis of N‑(2‑(2‑methoxyphenoxy)
ethyl)‑N‑(pyridin‑2‑yl)pyridin‑2‑amine (L2)
Py–H), 7.26 (d, 2H, Py–H), 7.32 (d, 2H, Ph–H, J 7.9 Hz),
7.66 (t, 2H, Ph–H), 7.68 (t, 2H, Ph–H), 7.70 (d, 2H, Ph–H),
8.40 (d, 2H, Py–H, J 19.6 Hz). 13C NMR (100 MHz, CDCl3,
ppm): δ 21.23, 42.83, 56.83, 67.57, 114.48, 115.20, 116.12,
116.65, 121.38, 122.22, 146.00, 149.34, 151.71, 152.19,
158.12. ESI–MS (m/z): 350.35 [L4+H]+. Anal. Calcd for
C21H23N3O2 (Found) %: C, 72.21 (71.98); H, 6.59 (6.26);
N, 12.03 (11.87).
This product was prepared by following a procedure similar
to what was described above for L1, starting from 2-(2-meth-
oxyphenoxy) ethylamine (1.50 mL, 10 mmol), 2-chloro-
pyridine (3.80 mL, 40 mmol) and triethylamine (2.76 mL,
20 mmol). L2 was obtained as a yellow oil. Yield: 2.43 g
(76%). FTIR data (KBr, pellet, cm−1): υ 3374, 3052, 2942,
1573, 1456, 1250. 1H NMR (400 MHz, CDCl3, ppm): δ 3.14
(t, 2H, CH2), 3.59 (s, 3H, Ph-OCH3), 3.75 (t, 2H, CH2, J
5.2 Hz), 6.90 (t, 2H, Ph–H), 7.07 (d, 2H, Py–H), 7.25 (d, 2H,
Ph–H), 7.26 (t, 1H, Ph–H), 7.32 (t, 2H, Ph–H), 7.34 (d, 2H,
Ph–H), 7.37 (t, 2H, Py–H), 8.09 (d, 2H, Py–H, J 4.5 Hz). 13C
NMR (100 MHz, CDCl3, ppm): δ 42.83, 56.83, 67.57, 85.28,
116.12, 116.65, 116.79, 117.17, 121.38, 122.22, 137.12,
147.14, 149.34, 151.71, 157.30. ESI–MS (m/z): 322.23
[L2+H]+. Elemental Anal. Calcd for C19H19N3O2 (Found)
%: C, 71.03 (70.89); H, 5.92 (5.68); N, 13.08 (12.86).
Synthesis of chromium complexes (Cr1–Cr4)
The chromium complexes Cr1–Cr4 were prepared as dis-
played in Scheme 1.
Synthesis of N‑(2‑phenoxyethyl)‑N‑(pyridin‑2‑yl)
pyridin‑2‑amino chromium chloride (Cr1)
A solution of L1 (0.32 g, 1 mmol) in THF (10 mL) was
added to a solution of CrCl3(THF)3 (0.37 g, 1 mmol) in THF
(10 mL). The mixture was stirred for 24 h at 40 °C under N2,
and then excess diethyl ether was poured into the mixture
to precipitate the complex. The precipitate was collected by
fltration, washed with diethyl ether and dried under vacuum
to give the product Cr1 as a green powder. Yield: 0.37 g
(81%). FTIR data (KBr, pellet, cm−1): υ 3308, 3082, 2979,
1641, 1436, 1147. ESI–MS (m/z): 450.96 [M+H]+. Anal.
Calcd for C18H17N3OCrCl3 (Found) %: C, 48.05 (47.86); H,
3.78 (3.46); N, 9.34 (9.02).
Synthesis
of 4‑methyl‑N‑(4‑methylpyridin‑2‑yl)‑N‑(2‑phenoxyethyl)
pyridin‑2‑amine (L3)
This product was prepared by following a procedure similar
to what was described above for L1, starting from 2-phe-
noxyethylamine (1.31 mL, 10 mmol), 2-chloro-4-methyl-
pyridine (3.50 mL, 40 mmol) and triethylamine (2.76 mL,
20 mmol). L3 was obtained as a yellow solid. Yield: 2.58 g
(81%). FTIR data (KBr, pellet, cm−1): υ 3284, 3052, 2948,
1592, 1417, 1276. 1H NMR (400 MHz, CDCl3, ppm): δ 2.33
(s, 6H, Py–CH3), 3.66 (t, 2H, CH2), 3.96 (t, 2H, CH2), 6.86
(d, 2H, Py–H), 6.91 (d, 2H, Py–H), 6.98 (d, 2H, Ph–H, J
11.1 Hz), 7.07 (t, 1H, Ph–H), 7.24 (t, 2H, Ph–H), 8.18 (d,
2H, Py–H, J 5.0 Hz). 13C NMR (100 MHz, CDCl3, ppm): δ
21.23, 42.83, 67.62, 114.48, 115.20, 115.72, 121.13, 130.02,
146.01, 152.19, 158.12, 159.39. ESI–MS (m/z): 321.16
[L3+H]+. Anal. Calcd for C20H21N3O (Found) %: C, 75.24
(74.96); H, 6.58 (6.25); N, 13.17 (12.94).
Synthesis of N‑(2‑(2‑methoxyphenoxy)
ethyl)‑N‑(pyridin‑2‑yl)pyridin‑2‑amino chromium chloride
(Cr2)
In a manner similar to that described for Cr1, Cr2 was
isolated as a green powder. Yield: 0.46 g (95%). FTIR
data (KBr, pellet, cm−1): υ 3284, 3045, 2962, 1649, 1423,
1134. ESI–MS (m/z): 481.23 [M + H]+. Anal. Calcd for
C19H19N3O2CrCl3 (Found) %: C, 47.55 (47.13); H, 3.96
(3.68); N, 8.76 (8.48).
Synthesis of N‑(2‑(2‑methoxyphenoxy)
ethyl)‑4‑methyl‑N‑(4‑methylpyridin‑2‑yl)pyridine‑2‑amine
(L4)
Synthesis of 4‑methyl‑N‑(4‑methylpyridin‑2‑yl)‑N‑
This product was prepared by following a procedure simi-
lar to what was described above for L1, starting from
2-(2-methoxyphenoxy) ethylamine (1.50 mL, 10 mmol),
2-chloro-4-methylpyridine (3.50 mL, 40 mmol) and trieth-
ylamine (2.76 mL, 20 mmol). L4 was obtained as a yellow
oil. Yield: 2.18 g (62%). FTIR data (KBr, pellet, cm−1): υ
3302, 3058, 2955, 1649, 1431, 1241. 1H NMR (400 MHz,
CDCl3, ppm): δ 2.40 (s, 6H, Py–CH3), 2.75 (t, 2H, CH2),
3.38 (s, 3H, Ph–OCH3), 3.68 (t, 2H, CH2), 7.23 (s, 2H,
(2‑phenoxyethyl)pyridin‑2‑amine chromium chloride (Cr3)
In a manner similar to that described for Cr1, Cr3 was
isolated as a green powder. Yield: 0.31 g (65%). FTIR
data (KBr, pellet, cm−1): υ 3308, 3076, 2966, 1566, 1436,
1140. ESI–MS (m/z): 479.34 [M + H]+. Anal. Calcd for
C20H21N3OCrCl3 (Found) %: C, 50.26 (50.01); H, 4.40
(4.18); N, 8.80 (8.68).
1 3

Transition Metal Chemistry
Synthesis of N‑(2‑(2‑methoxyphenoxy)
ethyl)‑4‑methyl‑N‑(4‑methylpyridin‑2‑yl)pyridine‑2‑amine
(Cr4)
(L3) and 2.75–3.68 ppm (L4). In addition, the proton signals
in the range of 7.15–7.68 ppm were ascribed to the benzene
ring. After substitution reaction with 2-chloro-4-methylpyri-
dine, the signals of protons at 2.33–2.40 ppm belonged to the
methyl of pyridine (L3 and L4), respectively. Furthermore,
the 13C NMR data (L1–L4) are shown in Supplementary
Figure S3.
In a manner similar to that described for Cr1, Cr4 was
isolated as a green powder. Yield: 0.33 g (65%). FTIR
data (KBr, pellet, cm−1): υ 3291, 3052, 2929, 1566, 1417,
1114. ESI–MS (m/z): 509.35 [M + H]+. Anal. Calcd for
C21H23N3O2CrCl3 (Found) %: C, 49.66 (49.35); H, 4.53
(4.25); N, 8.28 (7.99).
The reaction of CrCl3(THF)3 with ligands (L1–L4) in
THF at 40 °C aforded the corresponding chromium com-
plexes (Cr1–Cr4) isolated as green solids in moderate to
good yields. The synthesized complexes were character-
ized by a combination of FTIR spectra and ESI–MS. From
FTIR spectra of complexes (Supplementary Figure S1),
we could know that the absorption peaks became broad
and distinct. The band (C=N) is red-shifted from 1573 to
1553 cm−1 because the coordination between chromium ion
and nitrogen atom saps the strength of double bond C=N and
enhances the C–H bond bending vibration of carbon atom.
In addition, the ESI–MS of Cr1–Cr4 showed a molecu-
lar ion peak [M + 1]+ at m/z 450.96, 481.23, 479.34 and
509.35 (Supplementary Figure S4). Both 5- and 6-coordi-
nated Cr(III) complexes are known [24–26], and our results
of characterization consist of the 5-coordinated structure of
Cr1–Cr4 proposed in Scheme 1.
General procedure for ethylene oligomerization
A 250-mL stainless steel autoclave equipped with mechani-
cal stirring, internal temperature control and continuous feed
of ethylene was employed for the reaction. The autoclave
was placed under vacuum and replaced by ethylene for three
times. When the desired reaction temperature was reached,
a typical reaction was performed by introducing solvent, the
proper amount of co-catalyst and the catalyst solution (the
total volume was 50 mL). Then, the autoclave was immedi-
ately pressurized to the desired level, maintained at this level
with constant feeding of ethylene. After the reaction was
carried out for the required period, the reactor was cooled
with water bath and the reaction solution was quenched with
10 wt% HCl in ethanol. The distribution of the oligomers
was analyzed by GC when compared with the standard
authentic samples.
Evaluation of the chromium complexes as catalysts
for ethylene oligomerization reactions
After activation with MAO, the detailed selective ethylene
oligomerization reaction of the complexes Cr1–Cr4 was
evaluated. Good catalytic activity and selectivity to 1-octene
were shown. Various LAOs can be produced by inserting
ethylene into the cationic chromium species, depending on
the rates of chain propagation and chain transfer. Oxidative
coupling of two ethylene molecules with the active catalytic
metal gives a metallacycle intermediate. According to the
currently accepted metallacycles mechanism, the postulated
mechanism is shown in Scheme 2 [27–30].
Results and discussion
Characterization and synthesis of ligands
and chromium complexes
Ligands L1–L4 were readily synthesized by the substitu-
tion reaction of the corresponding primary amines and the
corresponding pyridine compounds in refuxing methanol
(Scheme 1). These ligands were characterized by FTIR,
1H NMR spectroscopy, 13C NMR spectroscopy and ele-
mental analysis. In the FTIR spectra of the synthesized
ligands (Supplementary Figure S1), the bands around
3302–3305 cm−1 were assigned to the –OH stretch due
to the contaminate water in the KBr. The bands around
3052 cm−1 were due to the C-H of pyridine ring stretch
vibration. The bands (C=N and C–N) of pyridine ring were
observed at 1566–1649 cm−1 and 1243–1276 cm−1, respec-
tively. Bands at 1417–1456 cm−1 were assigned to pyridine
rings. At room temperature, the 1H NMR spectra of L1–L4
in CDCl3 exhibited resonances in the region where the pro-
tons on pyridine are attributed to δ 6.87–8.33 ppm (Supple-
mentary Figure S2). The signals of methylene protons were
3.94–4.59 ppm (L1), 2.97–3.75 ppm (L2), 3.66–3.96 ppm
Efect of N‑substituent on catalytic activity and product
selectivity
To explore the infuence of the N-substituent on the catalytic
activity and product selectivity, four catalysts were synthe-
sized and the investigated results are listed in Table 1. As can
be seen in Table 1, their catalytic activities showed the order
Cr1>Cr2>Cr4>Cr3. Complexes Cr1 and Cr2 showed
better activities than the corresponding analogous complexes
Cr3 and Cr4, which can be ascribed to detrimental efect
due to the presence of an additional electron donating pyr-
idine-based methyl group [31]. In addition, the chromium
catalysts with more bulky steric hindrance showed higher
selectivity to high carbon number olefns (≥C8), illustrating
1 3

Transition Metal Chemistry
Scheme 2 Postulated mechanism of ethylene oligomerization
Table 1 Efects of solvent and
ligand types on catalytic activity
and product selectivity
Entry
Solvent
Activity (105g/ Product selectivity (wt%)
(mol(Cr) h))
α-olefns (wt%)
C4
C6
8.14
C8
C10–C18
Cr1
Cr1
Cr1
Cr2
Cr2
Cr2
Cr3
Cr3
Cr3
Cr4
Cr4
Cr4
Toluene
2.93
2.52
1.08
2.61
2.15
1.02
2.09
1.16
0.42
1.53
1.45
0.55
27.29
41.27
75.35
21.72
29.88
80.53
25.60
32.27
36.74
9.24
55.34
6.94
3.62
52.37
8.74
2.89
38.08
14.80
2.67
15.59
11.48
3.74
9.23
2.95
13.41
6.69
4.35
8.92
21.10
23.76
35.17
27.08
10.68
25.62
71.20
46.02
79.39
60.51
88.78
85.49
51.98
50.52
47.93
60.73
55.42
43.99
Methylcyclohexane
Cyclohexane
Toluene
Methylcyclohexane
Cyclohexane
Toluene
Methylcyclohexane
Cyclohexane
Toluene
Methylcyclohexane
Cyclohexane
48.84
7.62
19.22
57.03
7.66
15.22
29.17
25.42
48.09
55.29
39.46
22.55
31.18
Reaction conditions: catalyst (5 µmol), n(Al)/n(Cr) (300), reaction temperature (50 °C), ethylene pressure
(1.0 MPa), solvent (total volume: 50 mL) and reaction time (30 min)
the protection aforded to the active site by the sterically
bulky substituent [32]. So, we chose Cr1 as the next research
catalyst at length [33, 34].
co-catalyst. The fnal results are displayed in Table 1. Under
the same reaction conditions, the highest catalytic activity
was observed in toluene compared with cyclohexane and
methylcyclohexane, which could be due to the maximum
polarity of toluene and the optimum solubility of chromium
catalyst in toluene. On the other hand, the selectivity to C8
was the highest with toluene as the solvent. The oligomers
were mainly C4 and C6 in other solvent. As a result, we chose
toluene as the solvent.
Efect of solvent on catalytic activity and product selectivity
Considering the efect of solvents on catalytic activity and
product selectivity, three solvents (toluene, cyclohexane,
and methylcyclohexane) were used to study with MAO as
1 3

Transition Metal Chemistry
Efect of reaction temperature on catalytic activity
and product selectivity
in ethylene pressure, reaching a maximum (2.93 × 105g/
(mol(Cr) h)) at 1.0 MPa. An elevated ethylene pressure led
to an increase in the ethylene concentration in the solvent,
leading to an increase in the chain propagation rate and thus
inducing increased catalytic activity. But higher ethylene
pressure made the active site distortion. The selectivity to
C4 and C6 increased at the cost of C8 with the increasing eth-
ylene pressure, which may be due to the faster rates of β-H
elimination than propagation in the oligomerization process.
The efect of reaction temperature on catalytic activity and
product selectivity is shown in Table 2. As we know from
Table 2, the catalytic activity and product selectivity were
strongly afected by reaction temperature. Elevating the reac-
tion temperature from 20 °C to 60 °C, the catalytic activity
increased initially with temperature and reached a maximum
(2.98×105g/(mol(Cr) h)) around 40 °C. However, the selec-
tivity to α-olefns and C8 was higher at 50 °C than at 40 °C.
As reaction temperature increased further from 40 °C to
60 °C, a sharp decrease in catalytic activity was observed,
which was attributed to both the instability of the active spe-
cies [35–37] and the lower solubility of ethylene in toluene
at higher temperatures [38]. Moreover, higher temperatures
resulted in higher rates of catalyst deactivation as well. The
selectivity to C8 showed the same tendency as the tempera-
ture-increasing activity at the expense of C4 and C6 due to a
faster chain transfer and termination at higher temperatures.
Efect of Al/Cr molar ratio on catalytic activity and product
selectivity
The efect of Al/Cr molar ratio on catalytic activity and prod-
uct selectivity was investigated thoroughly, and the results
are listed in Table 4. As shown in Table 4, typically the
runs were performed in toluene under 1.0 MPa of ethylene
pressure over 30 min with the molar ratio varied from 100
to 900. The optimum activity was observed as 2.93×105g/
(mol(Cr) h) with a molar ratio of 300. The selectivity to C8
was found to increase by more than half from 12.48 to 45.34
on changing the molar ratio from 100 to 300, which can be
ascribed to increased chain propagation and active sites. On
the contrary, the higher MAO could also interfere with the
formation of the over-reduced chromium species and make
the active species deactivated [39], indicating the decrease
of catalytic activity and the selectivity to C8.
Efect of ethylene pressure on catalytic activity and product
selectivity
The efect of ethylene pressure on catalytic activity and
product selectivity is shown in Table 3. From Table 3, we
can know that the ethylene pressure played an important role
in the catalytic activity and product selectivity. The catalytic
activity frst increased and then decreased with the increase
Table 2 Efects of temperature
on catalytic activity and product
selectivity
Entry
Temperature Activity (105g/
Product selectivity (wt%)
α-olefns (wt%)
(°C)
(mol(Cr) h))
C4
C6
C8
C10–C18
1
2
3
4
5
20
30
40
50
60
1.67
2.36
2.98
2.93
2.14
32.06
29.04
24.78
27.29
45.79
12.01
9.74
17.16
8.14
52.30
58.04
51.60
55.34
33.01
3.63
3.18
6.46
9.23
6.97
70.08
72.06
53.86
71.20
59.87
14.23
Reaction conditions: catalyst (5 µmol), n(Al)/n(Cr) (300), ethylene pressure (1.0 MPa), solvent (toluene,
50 mL) and reaction time (30 min)
Table 3 Efects of ethylene
pressure on catalytic activity
and product selectivity
Entry
Pressure (MPa)
Activity (105g/ Product selectivity (wt%)
(mol(Cr) h))
α-olefns (wt%)
C4
C6
C8
C10–C18
1
2
3
4
5
0.5
1.0
1.5
2.0
2.5
1.13
2.93
1.89
1.77
1.32
40.79
27.29
42.22
24.39
61.78
15.31
8.14
15.29
27.99
13.99
19.11
55.34
32.09
25.85
15.58
24.79
9.23
10.40
21.77
8.65
63.53
71.20
67.13
41.94
76.17
Reaction conditions: catalyst (5 µmol), n(Al)/n(Cr) (300), reaction temperature (50 °C), solvent (toluene,
50 mL) and reaction time (30 min)
1 3

Transition Metal Chemistry
Table 4 Efects of Al/Cr molar
radio on catalytic activity and
product selectivity
Entry
N(Al)/n(Cr)
Activity (105g/
(mol(Cr) h))
Product selectivity (wt%)
α-olefns (wt%)
C4
C6
C8
C10–C18
1
2
3
4
5
100
300
500
700
900
1.52
2.93
2.12
1.56
1.51
61.17
27.29
63.55
68.99
66.76
14.37
8.14
9.96
15.01
12.34
12.48
55.34
19.99
13.39
11.73
11.98
9.23
6.50
2.61
9.17
73.67
71.20
80.95
80.24
77.39
Reaction conditions: catalyst (5 µmol), reaction temperature (50 °C), ethylene pressure (1.0 MPa), solvent
(toluene, 50 mL) and reaction time (30 min)
11. Hu P, Diskin-Posner Y, Ben-David Y, Milstein D (2014) ACS
Catal 4:2649–2652
Conclusion
12. Flisak Z, Sun WH (2015) ACS Catal 5:4713–4724
A series of short-chain ligands were synthesized and char-
acterized, to aford selective ethylene oligomerization chro-
mium catalysts, producing the selectivity of 55.34% to C8
and activity of 2.93×105g/(mol(Cr) h) with toluene as the
solvent at the temperature of 50 °C, the ethylene pressure
of 1.0 MPa and the molar ratio of 300. In this system, the
efects of reaction temperature, ethylene pressure, Al/Cr
molar ratio, solvents and the ligand types were investigated
fully. The oligomerization results showed that Cr1 had the
highest selectivity and catalytic activity than Cr2–Cr4 under
the same conditions in most cases. The selectivity to LAOs
could reach 71.2%.
13. Zhang W, Sun WH, Redshaw C (2013) Dalton Trans
42:8988–8997
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ysis by nitrogen-based metal complexes I, state-of-the-art and
perspectives. Springer, Berlin. https://doi.org/10.1007/97890
48138159
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Supporting information summary
The characterization data of short-chain ligands and chro-
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Acknowledgements The authors would like to thank the National Key
R & D Program of China (No. 2017YFB0306701).
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