Single-site heterogeneous Cr-based catalyst for the selective trimerisation of ethylene

Article abstract of DOI:10.1039/b417938m

TAC-Cr³⁺/SiO₂ complexes are highly active and selective ethylene trimerisation catalysts and possess single-site catalytic behaviour, an unusual property for heterogeneous catalysts. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417938m

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COMMUNICATION
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Single-site heterogeneous Cr-based catalyst for the selective
trimerisation of ethylene
Cristina N. Nenu and Bert M. Weckhuysen*
Received (in Cambridge, UK) 26th November 2004, Accepted 26th January 2005
First published as an Advance Article on the web 8th February 2005
DOI: 10.1039/b417938m
3
+
TAC–Cr /SiO
2
complexes are highly active and selective
ethylene trimerisation catalysts and possess single-site catalytic
behaviour, an unusual property for heterogeneous catalysts.
a-Olefins, especially 1-hexene, have a high economic value since
they are used for, e.g., the production of linear low density
polyethylene. Many ethylene trimerisation catalysts are based on
homogeneous Cr complexes in which the Cr center is surrounded
1
2
3
4
5
by either nitrogen, phosphorus, oxygen, sulfur or carbon
donor atoms. Although notable progress has been made in
homogeneous catalysis, no efficient heterogeneous catalysts have
been reported to date. So far, neither trimerisation nor
polymerisation single catalysts have been straightforwardly able
to selectively produce and consecutively incorporate the trimers
into the polymeric structure. As a result, two different catalysts are
required as well as the co-feeding with a-olefin–ethylene streams
into the industrial reactors. In short, these drawbacks lead to a
lower economical efficiency and higher cost prices of polymers. In
our opinion, the simultaneous presence of a trimerisation site
adjacent to a polymerisation site on a single carrier oxide surface
can successfully overcome these problems. Consequently, a
heterogeneous one-pot catalytic system for the production of
linear low density polyethylenes makes redundant the use of
expensive feedstock of 1-hexene as copolymerization agent.
Accordingly, the trimer is in-situ produced by the trimerisation
site, while the polymerisation site sequentially builds it into the
polymeric chain. Trimerisation sites can be created by attaching an
electron donating ligand onto the regular reduced chromium
polymerisation sites.
Scheme 1 Molecular structure of a homogeneous TAC–CrCl
with R- different alkyl or aryl substituents, together with the proposed
molecular structure for the heterogeneous anchored complex 2.
3
complex
1
highly unsaturated Cr species of the Phillips catalyst. At a Cr :
TAC ratio of 1, the catalyst presents predominantly trimerisation
centres and has been chosen as a basis to gain insight into the local
structure. Special attention has been paid to safe handling the
moisture- and air-sensitive catalysts.
For reference purposes, we also made complex 1. Fig. 1
compares the electronic spectra of complex 1 and of the
heterogeneous system 2. The molecular structure of 1 has been
7
described in literature and consists of a regular octahedron
III
CrN Cl containing Cr , which gives rise to two well-defined d–d
3
3
2
1
transitions at 13 900 and 19 700 cm . The third allowed d–d
transition is overshadowed by an intense charge transfer transition
2
1
located at 38 000 cm . Species 2 has d–d absorption bands at
2
1
14 250 and 20 750 cm , indicating a strong structural resemblance
with 1. The small band shifts can be explained by the expected
replacement of the chloride ligands with silica oxygens. A shoulder
Here, we report on the first successful heterogeneous Cr-based
catalyst for the selective production of 1-hexene. The catalyst
consists of a Cr center attached via two silanol groups to an
amorphous silica carrier and surrounded by three nitrogen atoms
of a 1,3,5-tribenzylhexahydro 1,3,5-triazine (TAC) ligand. The
donating TAC ligand can form active trimerisation complexes with
6
different metals. The heterogeneous system 2 is more active than
7
its homogeneous counterpart 1 (Scheme 1) and shows a surprising
single-site catalytic behaviour for a heterogeneous system. This
single-site behaviour also offers a unique possibility to unravel the
molecular structure of the active site in great detail; a situation very
8
uncommon in heterogeneous catalysis.
The starting point of our study was a regularly reduced Phillips
2
21
3
21
catalyst (SiO
purity . 99.6%, 1 wt% Cr/SiO
assemble a trimerisation site by anchoring a TAC ligand onto
2
: 500 m g , 1.5 cm g , iso-electric point y3.2,
9
2
). It was modified afterwards to
Fig. 1 Electronic spectra of (a) complex 1 and (b) species 2 measured at
*b.m.weckhuysen@chem.uu.nl
room temperature.
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2
1
at around 28 000 cm can also be noted. It was also found that
species 2 can be transformed into complex 1 by treating the solid
with a saturated hydrochloric acid gas stream. Indeed, the
electronic maxima of an HCl-treated species 2 fairly correspond
with those of complex 1.
these results are presented. However, the noteworthy difference of
the two solid products consists in their nature. It is known that 1
14
forms high-density polyethylene, while species 2 generates only a
solid oligomer with an average molecular weight (M ) of
w
324 atomic units. DSC measurements showed no well-defined
EXAFS analysis indicated that 2 is linked to two oxygen atoms
˚
of silica at a distance of 1.91 A. Three nitrogen atoms of the TAC
˚
ligand were observed at a distance of 2.09 A, while one chlorine
melting temperature for the solid oligomer, but a continuously
melting transition from solid to liquid phase in complete agreement
13
1
with XRD that also shows a total lack of crystallinity. C and H
NMR studies confirmed these results. Both NMR spectra present
no relevant resemblance with the NMR spectra of polyethylenes
such as LLDPE or HDPE. We consider that 1-hexene and other
oligomers, in-situ formed, were partially sequentially incorporated
into the solid product. In this regard, the trimerisation activity of 2
can be even higher.
atom originating from a chlorinated solvent completes the
˚
coordination sphere at 2.28 A. Accordingly, the proposed structure
of 2 consists of a distorted octahedron of type CrN (Osilica) Cl
Scheme 1). The chloride ligand can easily be displaced during
3
2
10
(
reaction, most probably by the co-catalyst developing the
structural configuration required for the chromacycloheptane ring
formation, which is assumed to be an intermediate mechanistic
Very unexpected is the single-site behaviour of complex 2
indicated by the very low value of the polydispersity index
extracted from gel permeation chromatography measurements
1
1
step in the formation of 1-hexene.
Near edge X-ray absorption spectroscopy at Cr K-edge and Cr
II
2,3-edge proved that the starting chromium site, respectively Cr ,
L
shown in Fig. 2. Given a M
of 173 atomic units and a M
polydispersity index defined as M
n
(the average molecular number) value
of 324 atomic units, the
/M is 1.87. This remarkable
III
was converted to Cr upon heterogenisation of the TAC
w
10,12
ligand.
w
n
In order to evaluate the catalytic properties of 1 and 2, the
materials were activated with N,N-dimethylanilinium tetra-
feature was primarily unforeseen, especially because of the well-
known polydisperse nature of our starting material, the reduced
Phillips catalyst. Apparently, the coordination of the TAC ligand
has drastically narrowed the diversity of the Cr active sites
previously existing on the silica surface. Undeniably, it makes 2
noteworthy in heterogeneous catalysis so far as similar catalytic
(pentafluorophenyl) borate and triisobutyl-aluminium. Table 1
compares the catalytic activities of both materials under different
13
reaction conditions.
Accordingly, system 2 is highly active towards ethylene
trimerisation and produces significant amounts of 1-hexene at 25
and 90 uC. Comparison of the trimerisation turnover numbers
8
systems have been seldom reported.
One could argue that the catalytic activity of 2 is due to
homogeneous Cr species, eventually leached in the liquid solution.
Therefore, the heterogeneous nature of species 2 has been
(TONs), defined as mole of 1-hexene formed per mole chromium,
indicates that species 2 is three times more active than complex 1
under the same reaction conditions. Besides 1-hexene, both
systems produce a solid product and traces of heavier oligomers,
such as 1-octene, 1-decene, 1-tetradecene and 1-octadecene, but no
internal olefins. Therefore, the selectivity in 1-hexene has been
estimated towards the free 1-hexene present in the liquid phase.
Table 1 indicates that 2 is very selective towards 1-hexene as
selectivity values around 91% were achieved. The enhanced
trimerisation activity of 2 at 25 uC compared to those of 1 could
be due to the distortion of the Cr coordination induced by the
support oxide. Only at 90 uC, the trimerisation selectivity of 2
sustains an appropriate comparison with the trimerisation
selectivity of 1.
Table 2 Polymerisation and oligomerisation activity of the homo-
geneous complex 1 and heterogeneous system 2 at 1 bar
Nature of
the solid
a
Solid
PA/OA
g mmol Cr
21 21
h
Temperature/uC product/g product
1
2
2
a
25
25
90
0.94
1.10
1.29
HDPE
Oligomer
Oligomer
1.74
4.40
4.64
PA-polymerisation activity of the homogeneous complex 1; OA-
oligomerisation activity of the heterogeneous system 2.
Striking differences between 1 and 2 have also been observed
with respect to the solid products. The heterogeneous system 2
forms up to three times more solid product compared to 1,
confirming once more its enhanced catalytic activity. In Table 2,
Table 1 Trimerisation activity of the homogeneous complex 1 and of
the heterogeneous system 2 at 1 bar
TON
Light
6
(mole 1-C /
a
a
b
1C6
Temperature/uC 1-Hexene /g oligomers /g mole Cr)
S
1
2
2
a
25
25
90
0.0216
0.0158
0.0012
0.0163
0.0015
0.0011
2.9
9.6
7.9
57.0
91.3
52.2
The amount of 1-hexene and of light oligomers was estimated in
liquid phase from GC measurements using cyclo-octane as internal
standard. Selectivity in 1-hexene was estimated in liquid phase as
b
the percentage amount of 1-hexene reported at the total amount of
the obtained products.
Fig. 2 Gel permeation chromatographic plot of the solid oligomer
obtained by 2 at 1 bar, 25 uC and a Cr:TAC ratio of 1.
1
866 | Chem. Commun., 2005, 1865–1867
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significant amounts; (3) the solid is three times more active than
its homogeneous counterpart; and (4) the heterogeneous system 2
is more active towards trimerisation of ethylene at low pressures
and temperatures. Besides the outstanding low dispersity of 2, the
well-defined unique structure of the active site makes the catalyst
an excellent system for further structural and in-situ mechanistic
investigations on ethylene trimerisation reactions. Furthermore, by
changing the Cr : TAC ratio we expect to be able to tune the
properties of the polymers made.
This work is supported by grants from ATOFINA Research
and NWO CW-VICI. The authors thank Ph. Bodart from
ATOFINA Research for his valuable comments and suggestions
and Prof. D. C. Koningsberger and F.M.F. de Groot for the
EXAFS analysis and soft X-ray analysis, respectively.
Fig. 3 Trimerisation and polymerization activity of the heterogeneous
system 2 as a function of the reaction pressure.
Cristina N. Nenu and Bert M. Weckhuysen*
Utrecht University, Debye Institute, Department of Inorganic Chemistry
and Catalysis, Sorbonnelaan 16, 3584 CA, Utrecht, The Netherlands.
E-mail: b.m.weckhuysen@chem.uu.nl; Fax: 31 30251 1027;
Tel: 31 30 253 6760
investigated in more detail. For this purpose, we have carried out a
catalytic reaction for 0.5 h with species 2, followed by the transfer
of the liquid to a second reactor under inert atmosphere. The reac-
tion was continued for another 0.5 h. The trimerisation activities in
both reactors were compared. The second reactor showed no
increased trimerisation activity, implying that the solution con-
tained no Cr–TAC complex leached from the first reactor.
Chemical analysis with atomic absorption spectroscopy confirmed
Notes and references
1
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N. Braussaud and K. L. Cavell, Organometallics, 2001, 20, 1247–1250.
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J. A. Labinger and J. E. Bercaw, J. Am. Chem. Soc., 2004, 126,
2
15
that no Cr was present into the liquid phase in both reactors.
1
304–1305.
P. A. White, J. Calabrese and H. Theopold, Organometallics, 1996, 15,
473–5475.
As can be seen in Table 1, trimerisation activity of species 2 is
affected by the higher reaction temperatures leading to a marked
decrease of the selectivity in 1-hexene. Therefore, to preserve the
high selectivity of 2 in 1-hexene, it is recommended to work at low
temperatures. On the other hand, working at higher pressures in
an autoclave reactor is not beneficial for the trimerisation system 2
either. Large amounts of solid products are formed and the free
3
4
5
D. S. McGuinness, P. Wasserscheid, W. Keim, D. Morgan, J. T. Dixon,
A. Bollmann, H. Maumel, F. Hess and U. Englert, J. Am. Chem. Soc,
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J. T. Dixon, V. Gokul, L. Griesel, C. Grove, F. Hess, H. Maumela and
L. Pepler, Appl. Catal., A, 2003, 255, 355–359.
2
5
6
1-hexene which can be retrieved in the liquid phase becomes
M. Haufe, R. D. Kohn, G. Kociok-Kohn and A. C. Filippou, Inorg.
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Kohn and M. F. Mahon, J. Chem. Soc., Dalton Trans., 2002,
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E. Groppo, C. Lamberti, S. Bordiga, G. Spoto and A. Zechina, Chem.
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practically a by-product (Fig. 3). Under these new reaction
conditions, the solid product definitely presents a polymeric nature
and it presents different characteristics than the heavy oligomer
obtained at low pressure. In other words, at high pressures, the
trimerisation system 2 is gradually transformed into a polymerisa-
tion system which can still produce 1-hexene, although not in high
amounts. A similar dual catalytic behaviour was observed also for
complex 1 which, under certain reaction conditions, could produce
large amounts of polyethylene and can mimic the catalytic
7
8
9
14
behaviour of the Phillips catalyst in the homogeneous phase.
10 Cr K-edge XANES/EXAFS measurements in fluorescence mode were
carried out at beamline E4, HASYLAB, Hamburg (Germany). EXAFS
data analysis was done using XDAPW2 programme. Theoretical
references generated by FEFF8 programme as well as experimental
references were used to achieve the fit of the experimental data.
11 J. R. Briggs, J. Chem. Soc., Chem. Commun., 1989, 11, 674–675;
R. Emrich, O. Heinemann, P. W. Jolly, C. Kruger and G. P.
J. Verhovnik, Organometallics, 1997, 16, 1511–1513.
2 Cr L2,3-edge XANES measurements were carried out at BESSY, Berlin,
Germany. The theoretical spectrum of Cr L2,3-edge XANES estimated
with CowanMCD programme was compared with the experimental one.
3 Catalytic tests were carried out in toluene, in a glass reactor, under a
continuous flow of ethylene. The reaction time was 1 h and 20 eq.
Al(iBu)3 were added for all catalytic tests as well as 0.04 mmol of
N,N-dimethylanilinium tetra(pentafluoro-phenyl) borate.
No experimental evidence was found that the TAC ligand is
leaching at high pressures reconverting species 2 in a regular
Phillips polymerisation site. In this view, the heterogeneous system
2
can be considered an active and highly selective trimerisation
catalyst only at low pressure. Furthermore, the reaction conditions
can be used to turn the catalytic behaviour of 2 towards the desired
direction. Finally, we observed that the trimerisation activity of
system 2 could be maintained for more than 3.5 h. Futher studies
will elaborate on the long term activity and stability of this new
catalytic system.
1
1
It can be concluded that (1) an ethylene polymerisation catalyst
was successfully converted into a truly heterogeneous single-site
ethylene trimerisation catalyst upon heterogenisation of a TAC
ligand; (2) this catalyst can selectively produce 1-hexene in
1
4 R. D. Kohn, M. Haufe, S. Mihan and D. Lilge, Chem. Commun., 2000,
927–1928.
5 The 357 nm absorption band of chromium was used for quantification
with a detection limit of 5 ppb Cr.
1
1
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Chem. Commun., 2005, 1865–1867 | 1867