Ethylene tetramerization: A new route to produce 1-octene in exceptionally high selectivities

Article abstract of DOI:10.1021/ja045602n

Linear α-olefins, such as 1-hexene and 1-octene, are important comonomers in the production of linear low-density polyethylene (LLDPE). The conventional method of producing 1-hexene and 1-octene is by oligomerization of ethylene, which yields a wide spectrum of linear α-olefins (LAOs). While there exists several processes for producing 1-hexene via ethylene trimerization, a similar route for the selective production of 1-octene has so far been elusive. We now, for the first time, report an unprecedented ethylene tetramerization reaction that produces 1-octene in selectivities exceeding 70%, using an aluminoxane-activated chromium/((R²)₂P)₂NR¹ catalyst system. Copyright

Full text of DOI:10.1021/ja045602n

Published on Web 10/20/2004
Ethylene Tetramerization: A New Route to Produce 1-Octene in Exceptionally
High Selectivities
Annette Bollmann,*,† Kevin Blann,† John T. Dixon,† Fiona M. Hess,† Esna Killian,†
Hulisani Maumela,† David S. McGuinness,‡ David H. Morgan,† Arno Neveling,† Stefanus Otto,†
Matthew Overett,† Alexandra M. Z. Slawin,§ Peter Wasserscheid,# and Sven Kuhlmann#
R & D DiVision, Sasol Technology (Pty) Ltd., 1 Klasie HaVenga Road, Sasolburg 1947, South Africa, Sasol
Technology U.K. Ltd., Purdie Building, North Haugh, St. Andrews, KY16 9ST, U.K., School of Chemistry,
UniVersity of St. Andrews, Purdie Building, St. Andrews, Fife, KY16 9ST, U.K., and Lehrstuhl fuer Chemische
Reaktionstechnik der UniVersitaet Erlangen-Nuernberg, Egerlandstrasse 3, D-91058 Erlangen, Germany
Received July 22, 2004; E-mail: Annette.bollmann@sasol.com
Scheme 1. Proposed Catalytic Cycle for Ethylene Trimerization
Linear R-olefins, such as 1-hexene and 1-octene, are used, among
other applications, as comonomers in the production of linear low-
density polyethylene (LLDPE). The conventional method of
producing 1-hexene and 1-octene is by oligomerization of ethylene,
which yields a wide spectrum of linear R-olefins (LAOs).1 A more-
selective route to 1-hexene is via trimerization of ethylene. In this
regard, a number of trimerization catalyst systems have been
developed,2 most of them based on chromium catalysts, including
the Phillips pyrrolide system,3 the Sasol mixed heteroatomic
systems,4,5 and the BP diphosphine system.6 However, from a
market perspective, an analogous ethylene tetramerization route to
1-octene is highly desirable but has been unknown to date.
Scheme 2. Ligands for Ethylene Tetramerization
The common belief is that ethylene tetramerization is improbable.
This is because if ethylene tetramerization were to proceed via a
similar mechanism to that proposed for ethylene trimerization
(Scheme 1),7 it would imply expansion of a seven-membered to a
nine-membered metallacycle intermediate. It has been argued that
this is unlikely to occur8,9 since the nine-membered ring is the least-
favored medium-sized ring and should thus be disfavored relative
to the seven-membered ring.9
We now, for the first time, report such an ethylene tetramerization
reaction that produces 1-octene in good selectivity. The catalyst
system for this transformation is an aluminoxane-activated chromium/
((R2)2P)2NR1 system, yielding 1-octene in selectivities up to 70%.
A number of the diphoshinoamine (PNP) ligands with various
substituents on both the N and P atoms were evaluated at two
different sets of reaction conditions. All of these gave active
catalysts for the production of 1-octene in good selectivity, while
concurrently producing varying quantities of 1-hexene, methylcyclo-
pentane, and methylene cyclopentane as the major side products.
Selected examples are provided in Table 1.
While the substituent on the N atom had little effect on the
1-octene selectivity, it profoundly affected both catalyst productivity
and 1-hexene selectivity (Table 1, entries 1-5). The highest
R-selectivity for C6 was obtained with isopropyl and cyclohexyl
substituents, indicating that R-branching appears to be crucial.
Variations of the substituents on the P atoms also yielded active
catalysts for the production of 1-octene in good selectivity (Table
1, entries 6, 7, 9, and 10). Ligands with bulkier aromatic
substituents, such as biphenyl and naphthyl groups, gave between
54 and 56% C8 products at 65 °C and 30 bar. Surprisingly, ligands
containing heteroaromatic substituents, such as thiophenyl groups,
yielded in excess of 60% C8 products at 45 °C and 45 bar. Even
Table 1. Ethylene Tetramerization with Ligands 1-11
entry
P
T
productivity
(g/g Cr/h)
PE
C6
1-C6
(%)
C8
1-C8
(%)
(ligand)
(bar)
(
°C)
(%)
(%)
(%)
1 (1)
2 (2)
3 (3)
4 (4)
5 (5)
6 (6)
7 (7)
8 (4)
30
30
30
30
30
30
30
45
45
45
45
45
45
65
65
65
65
65
65
65
45
45
45
45
45
45
26500
43600
8050
11700
8570
52600
30800
272400
17200
4400
26200
24800
8800
1.4 24.8 39.4 59.0 94.1
0.3 24.9 54.7 58.1 96.8
0.5 32.1 86.1 59.4 99.3
0.3 32.7 86.5 60.6 99.2
0.4 27.3 67.8 61.6 97.8
1.3 26.0 38.7 54.2 93.4
1.8 22.9 38.6 56.1 95.3
1.1 16.9 70.3 68.3 98.8
2.4 16.6 42.5 60.3 96.6
13.6 16.8 64.6 45.2 97.4
8.0 25.2 69.6 58.8 98.4
35.1 19.7 38.2 39.2 96.6
9 (8)
10 (9)
11 (10)
12 (11)
13 (12)b
6.7 18
46.0 61.6 96.5
a All reactions were conducted in a 300 mL Parr reactor; % ) weight
%. Standard reaction conditions: 0.033 mmol Cr(THF)3Cl3 (entries 7-11;
Cr(acac)3), 2 equiv of ligand, 300 equiv of MAO, 100 mL of toluene, 30
min. b The preformed complex 12 containing ligand 5 was used.
the use of ligands with alkyl substituents, such as ethyl groups,
furnished more than 45% C8 products at 45 °C and 45 bar.
Finally, the scope of the ligands was extended to systems outside
the PNP configuration. It was found that a PNNP ligand (10) and a
† Sasol Technology.
‡ Sasol Technology, U.K.
§ University of St. Andrews.
# Universitaet Erlangen-Nuernberg.
9
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J. AM. CHEM. SOC. 2004, 126, 14712-14713
10.1021/ja045602n CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Ethylene Tetramerization with Ligand 4a
[Cr]
productivity
(g/g Cr/h)
PE
C6
1-C6
(%)
C8
1-C8
(%)
entry
(mmol)
activator
equiv
solvent
(%)
(%)
(%)
1
0.033
0.033
0.033
0.033
0.02
MAO
MAO
EAO/TMA
MAO/SiO2/TMA
MMAO-3A
MAO
300
300
1000/250
340/100
300
toluene
toluene
toluene
toluene
toluene
cyclohexane
272400
285200
27400
102000
285100
591000
1.05
0.07
0.10
1.20
0.1
16.9
16.6
17.8
19.5
16.0
14.1
70.3
65.6
68.3
69.8
66.4
63.0
68.3
68.9
71.6
68.6
70.7
67.4
98.8
98.5
98.5
98.7
97.9
98.7
2b
3
4
5
6
0.01
200
1.0
a Standard reaction conditions: Cr(acac)3 as Cr precursor, 1.35 equiv of ligand ((Ph2P)2NiPr), 45 °C, 45 bar, 30 min. b When 2.5 bar of H2 was added.
complex display a distorted octahedral geometry. The Cr-Cr bond
distance of 3.611(2) Å is indicative of little or no metal-metal
bonding. Compound 12, when activated with MAO in toluene, gave
good C8 selectivities at a reasonable catalyst productivity (Table
1, entry 13).
In conclusion, we have now reported an unprecedented ethylene
tetramerization reaction that produces 1-octene in selectivities
exceeding 70%. A variety of PNP and related diphosphine ligands,
in combination with Cr(III) compounds activated by aluminoxanes,
were found to be very active and efficient catalysts for this purpose.
Productivities in excess of 500 000 g/g Cr/h could be achieved.
Further research on kinetic and fundamental aspects of this reaction
Figure 1. Molecular structure of [Cr(PNP)Cl2(µ-Cl)]2. Selected bond
is ongoing.
distances and angles: Cr(1)-P(1) 2.578(2) Å; Cr(1)-P(2) 2.419(2) Å;
Acknowledgment. The authors thank Sasol Technology (Pty)
P(1)-Cr(1)-P(2) 66.62(7)°; P(1)-N(1)-P(2) 106.5(3)°.
Ltd. for permission to publish this work.
simple carbon-bridged diphosphine system, such as diphenylphos-
phinoethane (dppe) (11), were also active for tetramerization.
It is also noteworthy that the use of the chromium/(Ph2P)2NiPr
system and methylaluminoxane (MAO) as an activator led to
substantially higher productivities at 45 bar and 45 °C (Table 1,
entries 4 and 8). This may be attributed to greater catalyst stability
and higher ethylene solubility under these conditions. This system
was thus chosen for further investigations at these reaction
conditions (Table 2). The addition of 2.5 bar of hydrogen to the
system resulted in a remarkable decrease in solids formation, with
little effect on selectivities and productivity (Table 2, entries 1 and
2). Other activators, such as modified methylaluminoxane (MMAO-
3A), ethylaluminoxane (EAO), and silica-supported MAO in
combination with TMA, also proved to be suitable co-catalysts for
this system.Further extensive optimization showed that catalyst
productivities in excess of 500 000 g/g Cr/h could be achieved in
paraffinic solvents, such as cyclohexane.
While catalyst preparations were typically conducted in situ (i.e.,
the ligand, Cr(III) precursor, and activator were added separately
to the reactor), it was also possible to conduct catalytic reactions
using preformed PNP-Cr(III) complexes. For example, a (Ph2P)2N-
(Ph)-Cr(III) chloride complex was synthesized. An X-ray structure
determination10 of this complex revealed the chloride-bridged dimer
[Cr(PNP)Cl2(µ-Cl)]2 (12). The chromium centers of the binuclear
Supporting Information Available: Preparation of 1-9 and 12.
Experimental details for tetramerization experiments. Full product
distribution for entry 1, Table 1. Crystallographic data for 12. This
material is available free of charge via the Internet at http://pubs.acs.org.
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