MCM-41 immobilised borate co-catalyst for metallocene catalyzed propene oligomerisation

Article abstract of DOI:10.1039/b302813e

Reaction of tris(pentafluorophenyl)boron with the silanol groups of MCM-41 resulted in the heterogeneous tris(pentafluorophenyl)borate anion. This immobilised weakly-coordinating anion retains metallocenes, thus yielding a heterogeneous propene oligomerisation catalyst. Activity comparable to the corresponding homogeneous catalyst can be obtained. The products typically consist of over 90% 1-alkenes with Flory-Schulz carbon number distribution.

Full text of DOI:10.1039/b302813e

MCM-41 immobilised borate co-catalyst for metallocene catalyzed
propene oligomerisation
Marc Kwanten, Brenda A. M. Carrière, Piet J. Grobet and Pierre A. Jacobs*
Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium.
E-mail: pierre.jacobs@agr.kuleuven.ac.be
Received (in Cambridge, UK) 12th March 2003, Accepted 29th April 2003
First published as an Advance Article on the web 21st May 2003
Reaction of tris(pentafluorophenyl)boron with the silanol
groups of MCM-41 resulted in the heterogeneous tris(penta-
fluorophenyl)borate anion. This immobilised weakly-coor-
dinating anion retains metallocenes, thus yielding a hetero-
geneous propene oligomerisation catalyst. Activity
comparable to the corresponding homogeneous catalyst can
be obtained. The products typically consist of over 90%
1-alkenes with Flory–Schulz carbon number distribution.
manipulations were carried out in an inert nitrogen atmosphere.
Prior to use, (Al–)MCM-41 with worm-like morphology (8 3
0.75 µm) was dehydroxylated at 540 °C under a helium flow for
a period of 8 h to remove the majority of the hydroxyl groups.
The IR spectrum shows only isolated silanol groups at 3745
cm21. Typically, 3 g of pretreated MCM-41 was then
suspended in 30 ml of dry toluene. To this suspension a pink 0.2
M solution containing an equimolar mixture of N,N-dimethyla-
niline and tris(pentafluorophenyl)boron in 10 ml toluene was
added dropwise under stirring. Addition was continued until
fast decoloration of the solution ceased, corresponding to
consumption of the surface hydroxyl groups. Filtering yielded a
white solid and a pink filtrate. The supported co-catalyst was
washed 3 times with 50 ml toluene and dried overnight in vacuo.
The amount of the covalently linked borane co-catalyst, by
using thermoanalysis, was calculated to be 0.18 mmol per g for
MCM-41 support. Typical IR stretches pointing to the chemical
link of the borane co-catalyst to the MCM-41 support appeared.
Bands at 1644, 1518, 1468 cm21correspond to the nCNC in
perfluorobenzene, while a peak at 976 cm21 can be attributed to
Since their discovery, metallocenes are known as very active
polymerisation catalysts producing polymers with a narrow
molecular weight distribution, also allowing controlled co-
polymerisation.1–3 For the synthesis of oligomers, their use
remains rather limited, although a large number of potential
products are possible such as long chain 1-alkenes for
surfactants, diesel alkanes, octane boosters and new mono-
mers.1 Most of the research has been performed to obtain a
better insight into the reaction mechanism for the polymer-
isation reactions.4 Metallocene catalysts have to be activated
with a large excess of expensive methylalumoxane (MAO).
Reduction of the amount of co-catalyst or its replacement by
another weakly coordinating anion still remains a challenge.
Boranes and borates, like tris(pentafluorophenyl)boron
(B(C6F5)3) and tetra(pentafluorophenyl)borate ((C6F5)4B2),
respectively, can be used as efficient co-catalysts for metal-
locenes.5 This communication reports on the design of a
heterogeneous oligomerisation metallocene catalyst produced
by linking the borane co-catalyst chemically to a MCM-41-type
support. Its reactivity/selectivity in the oligomerisation of
propene is illustrated and compared to the behaviour of the
corresponding homogeneous catalyst.
nB–O.
11B MAS NMR on the heterogeneous catalyst showed a
small resonance shift at 26.5 ppm compared to Et2O·BF3 as
reference. This resonance line is indicative of the presence of
B(IV).10 Sorption measurements on the support and co-catalyst
on the support showed a reduction of the pore size from 3.0 to
2.4 nm, respectively, pointing to intrapore fixation of the borate
compound.
Homogeneous as well as heterogeneous catalytic oligomer-
isation reactions were performed in a fed-batch stainless steel
reactor.†
Initially, the reaction conditions were optimised for homoge-
neous oligomerisation reactions with N,N-dimethylanilinium
tetra(pentafluorophenyl)borate as a co-catalyst (entry 4, Table
1). Because of the high reactivity of propylene as monomer,
reactions have to be performed at elevated temperatures (470
°C) to prevent polymerisation. At these temperatures, supple-
mentary addition of hydrogen as a chain terminating agent is no
longer required. As a consequence the amount of saturated
product oligomers can be kept very low. Addition of small
The high surface area and hexagonally ordered tubular pores
make MCM-41 a suitable host for anchoring the borane
complex. The borane co-catalyst attached to the pore walls,
remains accessible for the metallocene catalyst and retains it via
a weakly coordinative bond (Scheme 1).
MCM-41 or its Al-substituted homolog (Al–MCM-41)6,7 as
well as the heterogeneous metallocene catalyst8,9 were syn-
thesised according to modified literature procedures. All
Scheme 1 Heterogenisation of the borane co-catalyst on the MCM-41 support.
CHEM. COMMUN., 2003, 1508–1509 This journal is © The Royal Society of Chemistry 2003
1508

Table
Cp2ZrCl2
1
Homogeneous propylene oligomerisation reactions with
Activity/105 g
Table 3 Distribution of the propylene oligomers and dimer selectivity
a
Composition
(wt%)
entry 3a
entry 5a
entry 2b
entry 4b
Entry
Co-catalyst
Al : Zr
(mol Zr)21h21
a
Dimer
2M-1-C5N
2M-C5
2,3diM-1-C4 N
2M-2-C5 N
nC6
2-C6 N
4M-1-C5 N
Trimer
Tetramer
Pentamer
Hexamer
10.1
59.91
29.76
3.10
5.94
0.92
0
0.38
13.5
13.7
13.1
12.8
56.9
88.81
6.07
3.42
0.82
0.23
0.46
0.19
23.4
10.0
4.9
53.2
86.39
6.93
4.90
0.85
0.23
0.45
0.25
24.2
10.9
5.6
62.5
85.21
6.85
6.12
0.79
0.31
0.40
0.32
22.8
8.4
1
2
3
4b
5
TMA
MAO
B(C6F5)4
B(C6F5)4
B(C6F5)3
100
100
100
100
100
0.85
2.90
3.62
2.43
2.08
—
0.65
0.77
0.70
0.39
2
2
a Reaction conditions: t = 75 min; T = 90 °C; C3H6: 1400 ml min21; H2:
50 ml min21; 90 µmol Cp2ZrCl2, b 0 ml min21 H2.
3.6
1.4
amounts of hydrogen however, enhances the activity sig-
nificantly (entry 3, Table 1) probably due to reactivation of
dormant sites. Of the formed products 70% are alkene isomers
(Table 3). By altering reaction temperature, catalyst concentra-
tion, monomer feed and hydrogen supply, the oligomer
distribution can be varied within a whole range of a values,
varying from 0.8 for 11-mers to approximately 0.3 for
pentamers.
In all reactions, trimethylaluminum (TMA) is added to the
catalytic mixture to alkylate the metallocene catalyst and to
scavenge impurities present in solvent and gases using a Al : Zr
ratio of 100 (entry 1, Table 1). With the borate catalytic system
under current oligomerisation conditions, higher activities are
obtained than with MAO as a weakly coordinating anion
(compare entries 2 and 3, Table 1). Homogeneous reactions
with B(C6F5)3 as co-catalyst lead to a drop in activity and a-
factor (entry 5, Table 1), due to stronger coordination with the
metallocene.
2.5
3.1
a Table 1. b Table 2.
immobilized tris(pentafluorophenyl)borate. After reaction, the
filtrate of the anchored weakly coordinating anion did not reveal
any activity higher than that of the blank test (entry 1, Table 1)
upon metallocene addition, indicating the true heterogeneous
nature of the co-catalyst. Hot filtration (at a reaction tem-
perature of 90 °C) of the heterogeneous catalyst shows a degree
of Zr leaching of 25%; filtration at room temperature shows a
clear solution devoid of Zr.
An IAP-PAI grant on Supramolecular Catalysis as well as a
FWO grant (G.0279.00) is acknowledged. M.K. and P.J.G.
acknowledge a fellowship from IWT and a position as research
director from FWO, respectively.
The activities obtained with the heterogeneous catalyst are
lower than those of the homogeneous catalysts under the same
reaction conditions (compare entry 3, Table 1 with entry 1,
Table 2). Whereas in the homogeneous system, each B atom is
surrounded by 4 pentafluorinated phenyls, in the heterogeneous
system each has 3 such substituents. Furthermore, its linkage
via the surface oxygen influences the electron structure of the
co-catalyst and decreases its weakly coordinating properties
compared to the homogeneous case. The different chemical
environment is also reflected in the difference in a-factor
between the homogeneous and heterogeneous system (0.77
(Table 1, entry 3) against 0.41 (Table 2, entry 1)). Also in Table
2 a clear effect of the kind of support is seen on a (compare
entries 1, 3 and 4).
During MCM-41 synthesis, an Al source was added to the
synthetic mixture, yielding a support with (Lewis) acid
properties (after calcination at 540 °C). Enhanced catalytic
activity comparable to the homogeneous reaction with B(C6F5)3
as co-catalyst was obtained, without influencing the oligomer
distribution (entry 2, Table 2). Thus it seems possible to create
the heterogeneous borane co-catalyst without losing significant
activity, by using an appropriately modified support. It is clear
that a support with Lewis acidic properties such as dehydroxy-
lated Al-MCM-41, enhances the non-coordinating properties of
Notes and references
†
The homogenous or heterogenous borate co-catalyst (0.09 mmol) was
dissolved/suspended in 20 ml toluene and transferred under an inert
atmosphere to a 600 ml stainless steel Parr reactor filled with 260 ml of dry
toluene. Next, the metallocene Cp2ZrCl2 (0.09 mmol), alkylated with TMA
(Al : Zr = 100), was dissolved in 20 ml toluene and added to the reaction
mixture.
Prior to use, all solvents were carefully dried over 3 Å molecular sieves.
All gases were first deoxygenated over an oxy-trap and dried over molecular
sieves. The catalyst and solvent were added batchwise, while the gases were
continuously flowing through the reactor, entering at the bottom: propylene
(1400 ml min21), methane (491 ml min21) as internal standard, hydrogen
(50 ml min21) as chain transfer and nitrogen (40 ml min21). Reactions were
performed at an overall pressure of 0.7 MPa. The unreacted gases were
analysed on-line allowing the calculation of conversion and activity in time.
After the reaction, residual TMA is decomposed with a 1 M HCl acid
solution prior to analysis of the reaction mixture. Reaction selectivity of the
propene oligomerisation is determined by the growth factor a according to
Flory–Schulz kinetics:
log(Wm/m) = (m 2 1)loga + 2log(1 2 a)
in which Wm is the oligomer weight fraction with oligomerization degree
m.
1 W. Kaminsky, Macromol. Chem. Phys., 1996, 197, 3907.
2 M. R. Ribeiro, A. Deffieux and M. F. Portela, Ind. Eng. Chem. Res.,
1997, 36, 1224.
Table
Cp2ZrCl2
2
Heterogeneous propylene oligomerisation reactions with
3 C. Jenny and P. Maddox, Curr. Opin. Solid State Mater. Sci., 1998, 3,
94.
a
4 L. Resconi, I. Camurati and O. Sudmeijer, Top. Catal., 1999, 7, 145.
5 M. O. Kristen, Top. Catal., 1999, 7, 89.
6 M. Grün, K. K. Unger, A. Matsumoto and K. Tsutsumi, Microporous
Mesoporous Mater., 1999, 27, 207.
Activity/105 g
(mol Zr)21h21
Entry
Support
a
1
2
3
4
MCM-41b
Al-MCM-41b,c
MCM-41d
Silicab
1.45
2.06
1.77
1.47
0.41
0.41
0.52
0.32
7 M. T. Janicke, C. C. Landry, S. C. Christiansen, D. K. Kumar, G. D.
Stucky and B. F. Chmelka, J. Am. Chem. Soc., 1998, 120, 6940.
8 J. F. Walzer Jr. US Patent
1999:686672).
5 972 823/1999 (CAPLUS AN
9 M. Bochmann, G. J. Pindado and S. J. Lancaster, J. Mol. Cat. A: Chem,
1999, 146, 179.
10 K. Musikabhumma, T. P. Spaniol and J. Okuda, Macromol. Chem.
Phys., 2002, 203, 115.
a Reaction conditions: t = 75 min; T = 90 °C; C3H6: 1400 ml min21; H2:
50 ml min21; 90 µmol Cp2ZrCl2. b Dried at 540 °C. c Prepared in the same
conditions as MCM-41 with a Si : Al ratio of 8. d Dried at 170 °C.
CHEM. COMMUN., 2003, 1508–1509
1509