Chromium catalysts for ethylene trimerization/tetramerization functionalized with ortho-fluorinated arylphosphine ligand

Article abstract of DOI:10.1016/j.catcom.2018.12.010

A series of novel asymmetric bidentate phosphines with ortho?fluorine substituents was developed and characterized. In combination with chromium, the compound gave high activity to 1-hexene and 1-octene (approaching 3200 kg/g Cr/h) and high combined selec

Full text of DOI:10.1016/j.catcom.2018.12.010

CatalysisCommunications121(2019)15–18
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Chromium catalysts for ethylene trimerization/tetramerization
functionalized with ortho-fluorinated arylphosphine ligand
⁎
Hoseong Leea,b, , Yongnam Joeb, Hyoseung Parkb
a Department of Chemistry, College of Natural Sciences, Seoul National University, Gwanak-gu, Seoul 08826, Republic of Korea
b Department of Chemical R&D Center, SK Innovation, Yuseong-gu, Daejeon 34124, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of novel asymmetric bidentate phosphines with ortho‑fluorine substituents was developed and char-
acterized. In combination with chromium, the compound gave high activity to 1-hexene and 1-octene (ap-
proaching 3200 kg/g Cr/h) and high combined selectivity (C6-C8 > 95% in selected examples). With fluorine
atoms at the ortho-position, the compound could be operable at a higher temperature without activity loss.
Ethylene trimerization
Ethylene tetramerization
Ethylene oligomerization
Chromium
Homogeneous catalysis
1. Introduction
through isomerization or degradation pathway [23,24]. Recently, some
of ortho-functionalized aryl phosphine ligands were introduced to en-
Linear alpha-olefins (LAOs) are demanded as versatile raw materials
for copolymers, plasticizers, detergents, and synthetic lubricants [1–6].
LAOs are generally produced with a broad range of olefins from C4 to
hance catalytic performances on the oligomerization reaction. Sasol
patented withdrawing groups such as -F, -OCF3, -SCF3 on the ortho-aryl
phosphine of the PNP ligand systems (e.g., (Pho-F)(Ph)PN(iPr)PPh2 with
a productivity of 1900 kg/g Cr/h and a selectivity of towards 56% of 1-
hexene and 30% of 1-octene at 45 bar of ethylene, 100 °C in 27 mins of
retention time) [25,26]. In our recent work, it was reported that the
ortho-fluorinated aryl phosphine substituent increases catalytic activity
especially at the high temperature using a POSS (polyhedral oligomeric
silsesquioxane) introduced PNP ligand system (e.g., (Pho-F)(Ph)PN
(propylPOSS)PPh2 with an activity of 1619 kg/g Cr/h and a selectivity
of towards 37% of 1-hexene and 43% of 1-octene at 30 bar of ethylene,
100 °C in 15 mins of retention time) [22]. However, the reaction time
was not enough to properly evaluate the activity of the catalyst, and
also the role of fluorine atom could not be accurately investigated.
Besides, it stimulates one's curiosity what would happen if the concept
of ortho-fluorinated aryl phosphine ligand applied to PCCP ligand
structures which are known for the more active structure than the PNP
ligand system [20,21]. Consequently, further investigation was carried
out on the PCCP ligand system manipulating a bulkiness of ligand
backbone and controlling the number of the ortho-fluorinated aryl
moiety.
C
20 featured by Schulz-Flory distributions manufactured by Shell, Ineos,
SABIC, and Chevron-Phillips [7–9]. Among the LAOs, however, 1-
hexene and 1-octene are the most significant alpha-olefins for the ap-
plications as the comonomers of the linear low-density polyethylene
(LLDPE) which the demand is steadily increased. For this reason, pro-
ducing 1-hexene and 1-octene selectively by trimerization and tetra-
merization of ethylene, plenty of researches have been carried out
among the academic and industrial circles. Most widely known cata-
lytic systems are the combination of bidentate phosphine ligands with
chromium such as PNP [10–18], PN(C)nNP [19], and PCCP [20,21]. BP,
Sasol, and SKInnovation have developed independent technologies for
the ethylene trimerization and tetramerization as the representatives in
this industry. Although this selective oligomerization technology is
currently considered as reaching the threshold of commercialization,
some of the critical problems awaiting solution have remained. One of
the process issues is that an inevitably generated high molecular
polyethylene causes a severe line plugging into the process stream [22].
As an implementable solution, it is considered operating at a high
temperature to dissolve generated polyethylene such as over 100 °C. To
apply this skill, it must be secured to be superb catalytic stability at the
high temperature; however, most of the conventional chromium based
oligomerization catalysts could not afford to maintain their catalytic
stabilities at higher reaction temperature because they often went
⁎ Corresponding author at: Department of Chemistry, College of Natural Sciences, Seoul National University, Gwanak-gu, Seoul 08826, Republic of Korea.
E-mail address: lhs0505@snu.ac.kr (H. Lee).
https://doi.org/10.1016/j.catcom.2018.12.010
Received 6 October 2018; Received in revised form 14 December 2018; Accepted 15 December 2018
Availableonline17December2018
1566-7367/©2018ElsevierB.V.Allrightsreserved.

H. Lee et al.
CatalysisCommunications121(2019)15–18
2. Experimental section
After the reaction, volatile was removed by vacuum drying, and the
mixture was cooled to room temperature and diluted to n-hexane: ethyl
acetate (9:1 v/v) solution followed by silica column chromatography. A
pure oil product Ar2PC(R) = CPAr2 could be obtained and analyzed by
NMR and HR-MS. Yields and characterization data are summarized in
Supplementary Information.
2.1. General conditions
All reactions were performed under an inert atmosphere using
standard Schlenk techniques. All solvents and gases were dried and
degassed using standard procedures. Chemicals were purchased from
Sigma Aldrich or Alfa Aesar and used without further purification un-
less otherwise stated. mMAO-3A was obtained from Akzo Nobel
Corporation as a 7% w/w solution in heptane. 1H, 13C, 19F, and 31P
nuclear magnetic resonance (NMR) spectra were recorded on a Bruker
AVANCE III HD 500 MHz spectrometer in CDCl3. Chemical shifts are
reported in ppm with the internal tetramethylsilane signal at 0.0 ppm
and the internal chloroform signal at 77.0 ppm as a standard. Elemental
analyses were performed with a CE Instruments/ThermoQuest Italia
Flash EA 1112 Series. Bruker Daltonics (Billerica, MA, USA) APPI 7 T
FT-ICR MS was used for (+) mode atmospheric pressure photoioniza-
tion analysis. Quantitative chromatographic analysis of the oligomer-
ization products was performed using an Agilent 7890A GC-FID with an
HP-PONA column (50 m × 0.20 mm). The reaction solvent, methylcy-
clohexane, was used as an internal standard. GPC analyses were per-
formed on a 1260 Infinity II GPC/SEC system equipped with a dual flow
refractive index (DRI) detector and a UV detector. The samples were
analyzed in 1,2,4-trichlorobenzene at 145 °C using a flow rate of
1.0 mL/min. All polymers were injected at a concentration of 1 mg/mL
in 1,2,4-trichlorobenzene, after filtration through a 2.5 μm syringe
filter. The average molar masses (number average molar mass Mn and
weight average molar mass Mw) and the molecular weight distribution
(Mw/Mn) were derived from the RI signal by a calibration curve based
on polystyrene standards. DSC was performed using a TA DSC model
Q200. The samples were analyzed under a nitrogen atmosphere ac-
cording to the following cycles: in the first cycle, the sample was heated
from 30 to 200 °C, at a heating rate of 10 °C/min, leaving the material at
200 °C for 1 min; the second cycle was done using a cooling rate of
10 °C/min, until 20 °C; in the third cycle, the sample was heated from
30 to 200 °C, at a heating rate of 10 °C/min.
2.3. Preparation of chromium complexes
In a glovebox, each ligand (0.2 mmol) and CrCl3(THF)3 (0.2 mmol)
are charged in a flask followed by 2.0 mL of dichloromethane, and then
the solution was stirred for 1 h. The color of the reaction mixture
changed from purple to blue. After this, the solvent was removed under
reduced pressure, and the resultant solid was washed twice with 5 mL
of n-hexane. The product was dried under vacuum to yield chromium
complex. As followed in the previous literature, ligated chromium
complexes tend to form bimetallic structures maintaining the oxidation
state +3 [20]. The chromium complexes were characterized and
summarized in Supplementary Information.
2.4. Ethylene oligomerization
All runs for ethylene oligomerization were carried out in a 3 L of
Büchi stainless steel autoclave charged with methylcyclohexane (1 L)
and mMAO-3A (500 equivalent of Al/Cr molar ratio), then heated to
just below the desired reaction temperature. A synthesized chromium
complex (suspension of solution in 5 mL of methylcyclohexane) was
added to the autoclave, which was then pressurized with ethylene and
stirred at 600 rpm. Ethylene was fed on demand to keep the reactor
pressure constant, and the uptake was monitored using a mass flow
meter (MFC). After an hour, the autoclave was cooled to 0 °C and de-
pressurized slowly to atmospheric pressure. The product was quenched
by adding 2-ethyl hexanol (2.0 mL). The crude products were filtered
and analyzed using GC-FID (Typical GC-FID data in Fig. S1). The
polymeric products were recovered by filtration and dried overnight in
an oven at 100 °C.
2.2. General procedure for the synthesis of ligands
3. Results and discussion
A series of novel ligands were obtained by suitable modification of
literature methods (Scheme 1) [21,22]. n-Butyl lithium solution
(26.6 mL, 42.5 mmol, 1.6 M in hexane) was slowly added to a solution
of alkyne (50.0 mmol) in diethyl ether (42 mL) stirred at −78 °C under
nitrogen condition for 1 h. Chlorodiarylphosphine (38.3 mmol) was
slowly added and then stirred further 2 h. The mixture was naturally
raised to the room temperature and filtered through a silica pad and
dried in vacuo. Further purification was carried out by distillation or
column chromatography. A portion of synthesized alkynyl diaryl
phosphine compound A (0.5 mmol), copper(I) iodide (5 mol%) and
cesium carbonate (10 mol%) were dissolved in N,N-dimethylformamide
(1 mL) followed by dropwise of diarylphosphine (0.6 mmol, 1.2 eq),
then the mixture was stirred for 3 h after raised temperature to 90 °C.
3.1. Synthesis of catalysts and their catalytic activity screening result
In our previous work, it was found that catalytic activity was
maintained when the fluorine atom was introduced at the ortho-position
on aryl phosphine ligand even at the high temperature over 140 °C
without any thermally degradable signals such as excess polymer for-
mation or deactivation [22]. From the result, some changes of the de-
sign were carried out on the ligand structure manipulating a bulkiness
on the ligand backbone and introducing ortho‑fluorine atoms according
to a number or a position. Total 14 ligand structures and precatalysts
derived from CrCl3(THF)3 were synthesized. After formation of the
metal complex, ortho‑fluorine atoms would be expected to be able to
directly or indirectly affect the metal center during oligomerization
Scheme 1. Synthesis of asymmetric Ar2PC(R) = CPAr2 ligands.
16

H. Lee et al.
CatalysisCommunications121(2019)15–18
Table 1
Ethylene oligomerization result.a
Ligand
Productivity (kg/g Cr/h)
C6 (%)
1-C6/C6 (%)
C8 (%)
1-C8/C8 (%)
C10-14 (%)
Polymer
(%)
1-C8/1-C6
L1
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
100 °C
60 °C
693
523
26.9
54.9
40.2
70.2
41.0
71.5
44.7
55.8
33.4
57.7
45.0
75.4
45.3
74.4
52.1
84.1
32.6
57.0
45.2
75.9
42.8
72.2
47.3
82.2
40.8
68.6
49.8
80.3
36.1
69.7
22.1
48.9
65.2
91.1
84.4
96.8
83.0
95.6
84.3
98.4
82.8
88.8
85.8
97.6
84.3
95.8
88.9
98.6
75.8
86.0
86.9
96.7
84.3
94.9
88.0
98.3
87.9
94.4
86.8
98.0
89.1
97.9
82.0
92.2
65.5
37.1
55.6
24.9
52.7
21.9
43.0
8.9
96.3
95.1
97.4
83.7
98.8
97.2
97.5
94.5
97.8
96.7
98.9
97.3
96.2
94.5
99.7
98.0
97.5
96.8
96.8
95.5
95.1
93.6
95.9
94.5
98.5
97.0
95.6
93.2
98.6
95.9
97.5
95.6
4.2
4.4
3.7
4.0
4.3
4.5
3.1
3.3
3.6
3.9
4.5
4.7
3.3
3.5
3.2
3.5
2.2
2.4
3.8
4.1
3.6
3.9
3.8
4.1
3.8
4.3
3.5
3.7
3.4
3.6
2.3
2.4
3.4
3.7
0.5
0.9
2.0
2.1
9.2
32.0
0.5
0.5
0.4
0.5
0.3
0.4
0.5
0.6
0.7
0.9
0.5
0.4
0.5
0.7
1.5
2.2
0.1
0.3
0.1
0.1
0.2
0.4
1.0
1.2
3.6
0.7
1.6
0.3
1.5
0.3
1.1
0.2
2.2
0.7
1.3
0.3
1.3
0.3
1.0
0.1
2.5
0.8
1.2
0.3
1.4
0.3
1.1
0.1
1.5
0.4
1.0
0.2
1.8
0.4
4.0
1.0
L2
2204
2140
1172
1142
1256
1241
1643
1156
3328
3246
1293
1278
2921
2914
511
L3
L4
L5
62.5
37.9
50.1
19.4
51.1
21.7
44.2
11.8
64.5
39.6
50.5
19.6
53.1
23.3
47.4
11.4
55.3
26.8
46.6
15.9
60.3
26.3
74.6
47.6
L6
L7
L8
L9
386
L10
2136
2068
1064
1008
1899
1875
1818
1387
1803
1663
1805
1353
354
L11
L12
L13
L14
Chiraphos
nBuPNP(o-F)
100 °C
335
a
Standard conditions: 3 L autoclave, 1 L of MCH, 2 μmol of precatalyst, 500 equiv. of mMAO-3A, reaction time 1 h, 30 bar of ethylene pressure.
reaction. A screening test for the ethylene oligomerization was carried
out at the 60 °C and 100 °C respectively, and the screening result was
summarized in Table 1. A kinetic behavior was also measured by
ethylene uptake profile (Fig. S2). Most of the ortho-fluorinated ligands
showed similar tendency and no rapid decrease of ethylene uptake was
observed at high temperature. It was observed that the productivity was
increased when ortho‑fluorine atom introduced to the near side from the
bulkiness control substituent (L1 < L2, L5 < L6, L9 〈L10), whereas
fair-to-middling or slightly higher activity was shown to the far side
(L1 < L3, L5≒L7, L9 < L11, L13≒L14). In the case of the or-
tho‑fluorine atom was substituted to both sides, the activity was also
effectively increased compared to the non‑fluorine control group
(L5 < L8, L9 〈L12). In the case of L4, in which the smallest bulkiness
alkyl and both side fluorine substituted, a significant amount of
polymer was unexpectedly produced as a by-product. This phenomenon
became worse as the temperature increased (9.2% at 60 °C, 32.0% at
100 °C). It is presumably due to the combined effects which are a steric
effect of the substituted pendant fluorine atoms on both sides and an
excessive free volume given by poor bulkiness. Some of the produced
polymers were analyzed by GPC (Fig. S3) and DSC (Fig. S4). The mo-
lecular weight of L4, which had a higher polymer yield by side reaction,
was in the range about 250,000 to 640,000, and the molecular weight
distribution of the resulting polymer was considerably wide. Therefore
presumed to be the main reason for the possibility of melting polymer
by raised temperatures. The resulting polymer was also analyzed to
HDPE (high-density polyethylene), which was generally crystalline, and
this meant the product 1-hexene or 1-octene did not participate in the
reaction. As the main feature in this system, it was observed that the
activity was maintained with increasing reaction temperature in the
experimental group in which the ortho‑fluorine atom was substituted,
while the activity systematically decreased in the control group (See the
graphical data in Fig. S5 to comprehend the trend. Productivity drop on
L1 by 25%, L5 by 30%, L9 by 24%, L13 by 24%, and Chiraphos by
25%). As a result of the experiment, it was considered that the bulkiest
t-butyl substituent on the ethene backbone supposed to be the most
efficient ligand system with an ortho‑fluorine atom on the same side to
the t-butyl substituent. The activity of this possible structure might be
quite predictive; however, realistic evaluation of the target ligand could
not be achieved because it was not synthesized in spite of many trials,
probably due to the steric hindrance. Consequently, a trend of catalytic
activity by changing substituent on the backbone tended to be increased
by bulkiness (n-butyl≒phenyl < i-propyl < t-butyl). This was con-
sidered as a typical characteristic on the bidentate phosphine ligand
series [21,27,28]. In case of introducing a bulky alkyl substituent, the
movement and rotation of the neighboring aryl moiety would be lim-
ited, thus could affect on the reaction rate [28]. In the case of L6, which
has a reduced bulkiness and ortho-fluorinated substituent at the same
direction, it showed the best activity among the entries, and this ac-
tivity was considered as the best performance among the existent PCCP
ligand series. These results showed 2- to 7-fold greater activity based at
100 °C compared to other ligand systems (Chiraphos or ortho-fluori-
nated PNP). Therefore, it can be stated the newly developed catalyst
series has an advantage in the production of selective LAOs (especially
1-hexene and 1-octene) showing relatively high activities at the high
temperatures. It was noticeable that the product ratio for 1-octene
versus 1-hexene tends to be somewhat lower than the entry of ortho-
fluorinated PNP (4.0 for o-FPNP, 1.3 for L6 at 60 °C, and 1.0 for o-FPNP,
0.3 for L6 at 100 °C). It is corresponede that the small bite angle pro-
vides more tetramerization selectivity than trimerization selectivity. In
general, the P-Cr-P bite angle in the PNP system is about 67o, whereas
the bite angle in the PCCP system is about 80o [29]. Nevertheless, total
obtained 1-octene yield produced by L6 was comparatively high en-
ough to the conventional system.
17

H. Lee et al.
CatalysisCommunications121(2019)15–18
Fig. 1. 1) Weakly attractive and electrostatic interactions between the ortho-F and β-H in FI Catalyst, 2) in Chromium complex, 3) Alkyl‑aluminum promotes catalytic
activity with coordination of a fluoride atom.
3.2. Investigation into a role of the ortho-fluorine substituent
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Financial support from SKInnovation and unstinted advice from
professor Soon Hyeok Hong are kindly acknowledged.
Appendix A. Supplementary material
Crystal information files (cif), molecular structural data, and ex-
perimental details. See doi: https://doi.org/10.1016/j.catcom.2018.12.
010 contains the supplementary crystallographic data for compound 1.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_
request/cif. Supplementary data associated with this article can be
found in the online version, at https://doi.org/10.1016/j.catcom.2018.
12.010.
18