Highly selective chromium-based ethylene trimerisation catalysts with bulky diphosphinoamine ligands

Article abstract of DOI:10.1039/b412431f

In situ prepared chromium catalysts containing bulky diphosphinoamine (PNP) ligands, upon activation with MAO, are extremely efficient catalysts for the trimerisation of ethylene to 1-hexene. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b412431f

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COMMUNICATION
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Highly selective chromium-based ethylene trimerisation catalysts with
bulky diphosphinoamine ligands{
Kevin Blann,* Annette Bollmann, John T. Dixon, Fiona M. Hess, Esna Killian, Hulisani Maumela,
David H. Morgan, Arno Neveling, Stephanus Otto and Matthew J. Overett
Received (in Cambridge, UK) 12th August 2004, Accepted 25th October 2004
First published as an Advance Article on the web 10th December 2004
DOI: 10.1039/b412431f
attention towards the sterically encumbered ortho-alkyl substituted
derivatives 1–9 as ligands for chromium-based oligomerisation
catalysts. To our surprise, a number of these catalysts proved to be
highly active and selective towards ethylene trimerisation, despite
the above report to the contrary.10
In situ prepared chromium catalysts containing bulky dipho-
sphinoamine (PNP) ligands, upon activation with MAO, are
extremely efficient catalysts for the trimerisation of ethylene to
1-hexene.
While conventional ethylene oligomerisation reactions generally
produce a mathematical distribution of LAO’s (linear alpha
olefins),1 there has been considerable interest in the selective
oligomerisation of ethylene to specific carbon chain length olefins.
In particular, the selective trimerisation of ethylene to 1-hexene has
received a great deal of attention from the academic and industrial
community alike.2 The major use for 1-hexene is as a comonomer in
the production of linear low density polyethylene (LLDPE). The
majority of the known trimerisation catalysts are chromium-based,
including the Phillips pyrrolide system3 and the Sasol mixed
heteroatomic systems,4,5 although catalysts based on Ta6 and Ti7
have also been described. We have recently reported the unprece-
dented tetramerisation of ethylene to 1-octene catalysed by an
aluminoxane activated chromium–diphosphine catalyst system.8
We report herein a similar catalyst system with ligands of the type
Ar2PN(R)PAr2 (Fig. 1) containing alkyl substituents in the ortho
position. Upon activation with an aluminoxane, these catalysts are
both highly active and selective for the trimerisation of ethylene.9
The first account of the use of diphosphinoamine ligands (with
ortho-methoxy substituents, Fig. 1, R1–R4 5 OMe and R5 5 Me)
and Cr as catalysts for trimerisation was published by Carter
et al.10 It was proposed that the ortho-methoxy groups act as
pendant donors to the chromium metal center and are a
prerequisite for catalytic activity. This was postulated by the
authors after ligand 2 proved to be completely inactive towards
catalysis under the conditions employed.
The diphosphinoamine ligands 1–3 were synthesised using
literature procedures11 while the novel ligands 4–9 were prepared
via a similar two step route. The catalysts were prepared by stirring
the chromium precursor and ligand briefly in a solvent before
adding the mixture to a 300 ml Parr reactor containing solvent and
the aluminoxane. The oligomerisation reactions were subsequently
conducted isothermally with ethylene fed on demand. Ligand 1
was evaluated under two sets of conditions, namely 30 bar, 65 uC
and 45 bar, 45 uC. Our initial results with ligand 1 were
immediately promising (Table 1, entry 1) with a selectivity towards
the C6 fraction of 90% of which 99.5% was 1-hexene. An
encouraging feature was the distinctly lower C10 fraction (0.5%) as
compared with that reported by BP with the ortho-methoxy
equivalent (7–29%). This implies a reduction in the amount of
secondary trimerisation (ethylene 1-hexene co-trimerisation) and is
of importance for the process economics. Increasing the ethylene
pressure to 45 bar and reducing the temperature (entry 2) gave a
marked improvement in the activity of the catalyst (298800 g/g
Cr.h), but also an increase in the 1-octene selectivity (10.5%) at the
expense of 1-hexene (86%). The higher catalyst productivity under
these conditions corresponds with results obtained for the related
diphosphinoamine-based tetramerisation catalysts,8 and conse-
quently all subsequent catalytic runs were conducted under these
conditions.
Since the unsubstituted diphosphinoamine derived catalysts
yielded predominantly 1-octene,8 it was postulated that steric bulk
around the metal centre caused by the ortho-methyl substituents
was responsible for the increased selectivity towards 1-hexene. In
line with this reasoning, the bulkier ligands 2 and 3 with ortho-
ethyl and ortho-isopropyl groups respectively were also evaluated.
This premise proved correct, with the catalyst containing ligand 2
producing 93% 1-hexene (entry 4), while there was no further
improvement in moving to the even bulkier ligand 3 (entry 9). It is
also interesting to note that the choice of chromium precursor
[Cr(acac)3 vs. CrCl3(THF)3 vs. Cr(III) 2-ethylhexanoate – (entries
3–5)] did not make a substantial difference in the overall
selectivities. Similarly, this reaction can be conducted in both
aromatic and paraffinic solvents (see entries 3 and 6) using
different aluminoxane-based activators (see entries 3, 7 and 8). The
best trimerisation result was obtained with ligand 2 at 45 bar and
45 uC (entry 4) with a selectivity towards 1-hexene of 93% and an
After the successful application of unsubstituted diphosphino-
amine ligands for the tetramerisation of ethylene,8 we turned our
Fig. 1
{ Electronic supplementary information (ESI) available: experimental
details of the new ligand syntheses and catalyst screening runs. See http://
www.rsc.org/suppdata/cc/b4/b412431f/
*blannk@sasol.com
620 | Chem. Commun., 2005, 620–621
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Table 1 Ethylene oligomerisation resultsa
Entry
(Ligand)
Pressure C2
(bar)
Temperature Time
(uC) (min) (g/g Cr h)
Productivityh
PE C6
(wt%) (wt%)
a-C6 selectivity C8
(%) (wt%)
a-C8 selectivity C10
(%)
(wt%)
1
2b
3b
4c
5d
6e
7f
(1)
30
45
45
45
45
45
45
45
45
45
45
45
45
45
45
65
45
45
45
45
45
45
45
45
45
45
45
45
45
45
30
13
10
15
10
10
30
30
20
30
18
30
30
18
14
53 730
298 800
324 110
161 660
305 720
131 250
197 020
35 170
100 840
96 940
37 470
26 460
52 360
2.5
3.3
3.4
0.6
1.0
8.5
5.7
2.2
2.4
12.0
8.3
9.0
3.9
2.8
0.7
90.4
86.0
90.7
93.0
92.0
85.9
87.0
89.7
92.9
41.5
17.1
29.8
38.3
59.1
27.1
99.5
99.1
99.7
99.8
99.8
99.7
99.6
99.6
99.3
81.9
55.3
26.0
39.1
94.1
71.9
5.8
10.5
4.2
3.6
3.9
5.3
5.5
5.4
2.7
41.9
66.0
47.6
49.1
34.1
63.4
.99
.99
.99
.99
.99
.99
.99
.99
0.5
0.3
1.5
1.1
3.0
1.0
1.0
1.1
1.4
1.2
1.4
2.8
2.0
1.7
1.5
(1)
(2)
(2)
(2)
(2)
(2)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
8g
9
93.8
10b
11
12
13
14
15
a
98.3
98.2
94.9
95.9
.99
98.0
110 010
159 300
Standard reaction conditions unless otherwise stated: 0.033 mmol [Cr(acac)3], 2 eq. ligand, 300 eq. MAO, 100 ml toluene as solvent.
0.02 mmol [Cr(acac)3]. 0.033 mmol CrCl3(THF)3. 0.033 mmol Cr(III) 2-ethylhexanoate. 100 ml cyclohexane as solvent. 0.01 mmol
b
c
d
e
f
g
h
[Cr(acac)3], 300 eq. MMAO-3A. 1000 eq. EAO and 250 eq. TMA. Calculated according to run times. Standard run time: 30 min or time
taken to fill the reactor (10–15 min).
Kevin Blann,* Annette Bollmann, John T. Dixon, Fiona M. Hess,
Esna Killian, Hulisani Maumela, David H. Morgan, Arno Neveling,
Stephanus Otto and Matthew J. Overett
Sasol Technology (Pty) Ltd, R&D Division, 1 Klasie Havenga Road,
activity approaching 162 000 g/g Cr h. The total quantity of useful
liquid products (i.e. 1-hexene + 1-octene) was 96.6%!
We were also interested in the effect of changing the number of
ortho-substituents on the aromatic rings: all indications were that
removing steric bulk would lead to a decrease in 1-hexene
selectivity and an increase in 1-octene. The partially ortho-
substituted ligands 4–9 were thus tested. Removal of only one
ortho-methyl group caused a dramatic shift in product selectivity
(ligand 4, entry 10), with the C8 selectivity increasing to 42% and
the C6 fraction decreasing to 42%. The selectivity to 1-hexene in
the C6 fraction was concommitantly reduced to 82%. The other
major C6 components were methylcyclopentane and methylene-
cyclopentane, consistent with our observations of the tetramerisa-
tion system.8 Ligands with only two ortho-methyl substituents, the
unsymmetrical ligand 5 and its symmetrical counterpart 6, both
afforded catalysts favouring the formation of 1-octene (entries 11
and 12). Ligand 7, the ortho-ethyl substituted analogue of 6,
showed a similar tendency (entry 13). However, on exchanging the
N-methyl group for an N-isopropyl moiety (ligand 8, entry 14) a
change in selectivity back towards 1-hexene was apparent. This can
be explained by a translated increase in the steric effect of the ethyl
substituents, caused by the greater bulk of the isopropyl group.
Interestingly, the 1-hexene selectivity in the C6 fraction was also
significantly improved (entry 13 vs. entry 14). An N-isopropyl
ligand with one ortho-ethyl group (ligand 9, entry 15) gave a
C8-selective catalyst, in line with its reduced steric demand.
In conclusion we have demonstrated that, under the appropriate
conditions, pendant coordination is not a prerequisite for selective
ethylene trimerisation with diphosphinoamine-based catalyst
systems. We have further shown that steric demand plays a
crucial role in determining the C6 and C8 selectivities. These new
catalyst systems are highly active and may be modified to give
various ratios of 1-hexene to 1-octene as required without
significant by-product formation.
Sasolburg, 1947, South Africa. E-mail: blannk@sasol.com;
Fax: 27 (0)11 522 3190; Tel: 27 (0)16 960 5408
Notes and references
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D. H. Morgan, J. Organomet. Chem., 2004, 689, 3641.
3 W. K. Reagan, T. M. Pettijohn and J. W. Freeman, US Pat., US 5 523
507, 1996 (to Phillips Petroleum Company).
4 D. S. McGuinness, P. Wasserscheid, W. Keim, J. T. Dixon,
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8 K. Blann, A. Bollmann, J. T. Dixon, A. Neveling, D. H. Morgan,
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and M. J. Overett, Pat. Appl., WO 2004/056479, 2002 (to Sasol
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and M. J. Overett, Pat. Appl., WO 2004/056477, 2002 (to Sasol
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The authors thank Sasol Technology (Pty) Ltd for permission to
publish this work.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 620–621 | 621