Toward Single-Site, Immobilized Molecular Catalysts: Site-Isolated Ti Ethylene Polymerization Catalysts Supported on Porous Silica

Article abstract of DOI:10.1021/ja031725g

A new, general patterning methodology that may allow for the preparation of site-isolated organometallic catalysts on a silica surface is reported. The technique is demonstrated with Group 4 polymerization catalysts. The catalysts synthesized via the patterning method have up to a 10-fold increase in activity as compared to materials prepared by traditional techniques. In addition to supporting Group 4 polymerization catalysts, the patterned aminosilica is a possible support for other metal complexes, allowing for the synthesis of a wide array of immobilized single-site organometallic catalysts. Copyright

Full text of DOI:10.1021/ja031725g

Published on Web 02/21/2004
Toward Single-Site, Immobilized Molecular Catalysts: Site-Isolated Ti
Ethylene Polymerization Catalysts Supported on Porous Silica
Michael W. McKittrick and Christopher W. Jones*
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology,
311 Ferst DriVe, Atlanta, Georgia 30332
Received December 15, 2003; E-mail: cjones@chbe.gatech.edu
Scheme 1
Soluble transition metal complexes are nearly ideal catalysts
because of the versatile chemical properties that can be incorporated
into them - the symmetry, sterics, and electronics of the active
center can all be controlled using molecular design principles.
However, industrial implementation of catalytic technologies often
requires a solid catalyst. Thus, transition metal complex catalysts
have been immobilized on solid supports for decades.1 Usually,
the performance of the supported catalysts is significantly inferior
as compared to the homogeneous analogues. This is due to many
factors, with a primary cause being the formation of multiple types
of ill-defined metal species on the solid support. For example,
single-site transition metal complexes such as Group 4 metallocenes
and related complexes have been developed as revolutionary new
olefin polymerization catalysts.2 However, the preparation of well-
defined (single-site) solid-supported polymerization catalysts has
not progressed as rapidly.3 This is because synthetic protocols that
mediately adjacent to each other,8 as shown in Scheme 1. After
lead to the creation of uniform, isolated organometallic catalysts
on surfaces are not well-developed.4 Hence, a general methodology
for immobilizing metal complex catalysts while maintaining single-
site behavior would be a major advance. A potential technique is
demonstrated here with Group 4 olefin polymerization catalysts.
To illustrate the pitfalls in catalyst design, consider literature
reports of Ti and Zr constrained geometry-inspired catalysts
(CGCs)5 covalently supported on oxides. Two different covalent
immobilization strategies have been reported. In the first approach,
a derivatized organometallic precatalyst is contacted with a support.6
This method routinely leads to multiple types of surface species
due to solid-metal atom interactions. In the alternate approach,
the complex is assembled stepwise on the solid.7 This also can result
in multiple types of sites due to steric crowding on the support and
the heterogeneous nature of the support surface. From these reports,
it is clear that a method is needed to prepare catalysts with (i)
isolated sites, (ii) uniform sites, and (iii) structures that are amenable
to detailed characterization. To this end, here we report a new,
general methodology that can be utilized to create site-isolated
organometallic catalysts on a silica surface. A model silica support
is functionalized using a molecular patterning technique to incor-
porate isolated aminopropyl groups on the surface.8 Primary amine
groups can serve as versatile ligands or scaffolds for the assembly
of well-defined, isolated complexes on the surface. This methodol-
ogy is demonstrated here through the assembly of Ti CGCs5 on a
porous silica surface. These new catalysts are up to 10 times more
active than multisited catalysts that are prepared using traditional
techniques, perhaps due to the isolated nature of the sites.
this functionalization, any additional, unreacted silanols on the
surface are covered via treatment with hexamethyldisilazane
yielding material (3). The imine bond is then selectively hydrolyzed
to remove the trityl groups from the functionalized surface, leaving
aminopropyl species on material (4). A final capping step is used
to cover any additional silanol groups that might be produced in
the hydrolysis step, giving material (5). At every stage of the surface
functionalization, the materials have been characterized by a battery
of techniques including solid-state 29Si and 13C NMR, nitrogen
physisorption, and FT-Raman spectroscopy. Using these methods,
the proposed structure of the solid at each stage of the synthesis
has been verified to be as described in Scheme 1.8 Probe reaction
studies of the amine-functionalized silica materials indicate that
the amine groups on the surface of the patterned material behave
chemically as if they are isolated and fully accessible.8
This silica material with isolated amine sites can be easily utilized
as a scaffold for a variety of organometallic catalysts. For
preparation of a CGC,5 treatment of functionalized silica with
chlorodimethyl-(2,3,4,5-tetramethyl-2,4-cyclopentadien-1-yl) silane
yields a CGC precursor ligand on the surface, material (6).9 This
species is then treated with tetrakis(diethylamino)titanium in toluene
for 24 h under reflux to metalate the surface ligands. The resulting
immobilized titanium species [material (7)] is then treated with 5
equiv of trimethysilyl chloride to yield the more stable and easily
characterizable titanium chloride complex [material (8)]. This
synthetic protocol is shown in Scheme 2. Materials (6) and (8) were
characterized by 13C CPMAS NMR to verify the structure of the
surface species (Supporting Information). Elemental analyses
yielded metal loadings with a Ti-N ratio of nearly 1, with literature
methods resulting in substantially less Ti incorporation (∼0.5 Ti/
N) due to the presence of multiple types of surface ligands.7 This
stoichiometric metalation is consistent with the preparation of well-
defined surface species.
A patterning molecule has been designed and synthesized that
allows for the effective spacing of aminosilane groups on a silica
surface. The patterning molecule (1) is contacted with hexagonal
mesoporous silica materials such as SBA-15 with an average pore
diameter of ∼50 Å. The large trityl groups on the patterning agent
prevent incorporation of the silane on the surface at sites im-
9
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J. AM. CHEM. SOC. 2004, 126, 3052-3053
10.1021/ja031725g CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Catalytic Polymerization with Silica-Supported CGC/BARF Catalystsa
Ti loading
(mmol/g of cat)
temp
(°C)
polymerization
time (min)
alkyl
aluminum
activity
(kg of PE/mol of Ti‚h)
Tm
(°C)
M
w
entry
catalyst
Al:Ti
(polymer)
PDI
3.1
1.9
2.8
ND
2.5
broad
2
1
2
3
4
5
6
7
8 - patterned
8 - patterned
9 - control
10 - control
10 - control
11 - control
11 - control
0.38
0.38
1.6b
0.17
0.17
0.53
0.53
25
25
25
25
25
25
25
10
10
10
10
10
10
10
400
400
400
400
400
400
400
TMA
TIBA
TIBA
TMA
TIBA
TMA
TIBA
28.7
24.8
19.8
4.2
5.1
2.7
134.1
133.8
132.8
131.5
132.6
133.5
133.1
660 000
1 000 000
470 000
ND
620 000
500 000
1 000 000
1.5
a TMA ) trimethylaluminum; TIBA ) triisobutylaluminum. b Homogeneous catalyst.
Scheme 2
it may limit unwanted interactions of the complex with the oxide
surface. Addition of a preformed complex to silica (control 10)
likely results in the formation of multiple types of metal sites.6
The patterning protocol developed may prevent such surface-metal
interactions (silanols are capped), allowing for a more well-defined,
active material. Continuing work is focusing on a full molecular
level characterization of the new supported species to fully elucidate
the cause of the promising catalytic results.
In summary, a new, general patterning methodology that can be
utilized to create organometallic catalysts on a silica surface that
may be site-isolated is reported here. Ti-CGC-inspired sites are
prepared on this support and are demonstrated to be more active
than covalently tethered catalysts prepared via traditional techniques.
The isolated aminosilica scaffold is a versatile support for metal
complex immobilization and may allow for the preparation of a
wide array of single-site immobilized organometallic catalysts.
The immobilized Ti-CGC5 precatalyst derived from the patterned
Acknowledgment. C.W.J. thanks the NSF for support through
the CAREER program (CTS-0133209). M.W.M. thanks the Mo-
lecular Design Institute (N00014-95-1-1116, Office of Naval
Research) for partial support through a graduate fellowship. We
thank the Coughlin group at UMass for GPC analysis.
silica support (8) was evaluated in the catalytic polymerization of
ethylene. In addition, three control catalysts were prepared for
comparison. The first control was the homogeneous Ti-CGC
precatalyst (9).6 A second control was prepared via addition of a
preformed complex to silica as previously reported (10).6 Catalyst
(11) was assembled using the same protocol as was used for the
patterned catalyst (8), with the exception that the complexes were
assembled on a densely functionalized aminosilica surface. The
precatalysts were contacted with a toluene solution containing tris-
(pentafluorophenyl)borane and an alkylaluminum (trimethylalumi-
num or triisobutylaluminum) as activator and were subsequently
exposed to 60 psi of ethylene pressure at 25 °C. The produced
polymers were analyzed by GPC and TGA-DSC. The catalytic
results are described in Table 1.
Supporting Information Available: Synthetic protocols for Ti
catalysts, polymerization procedures, and 13C NMR spectra (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) Hartley, F. R. Supported Metal Complexes: A New Generation of Catalysts;
Dordrecht, Boston, 1995.
(2) Britovsek, G. J. P.; Gibson, V.; Wass, D. F. Angew. Chem., Int. Ed. 1999,
38, 428.
(3) Hlatkey, G. G. Chem. ReV. 2000, 100, 1347.
(4) Well-characterized, well-behaved oxide-supported transition metal complex
catalysts are rare, see, for example, early transition metals: (a) Nicholas,
C. P.; Ahn, H. S.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 4325. Late
transition metals: (b) Jones, M. D.; Raja, R.; Thomas, J. M.; Johnson, B.
F. G.; Lewis, D. W.; Rouzaud, J.; Harris, K. D. M. Angew. Chem., Int.
Ed. 2003, 42, 4326. Surface organometallic chemistry: (c) Coperet, C.;
Chabanas, M.; Saint-Arroman, R. P.; Basset, J. M. Angew. Chem., Int.
Ed. 2003, 42, 156.
(5) The catalysts are referred to here as “CGC-inspired” because there is no
direct evidence here or elsewhere6,7,9 that the immobilized catalysts have
the same structure as homogeneous CGC catalysts.
(6) Galan-Fereres, M.; Koch, T.; Hey-Hawkins, E.; Eisen, M. S. J. Organomet.
Chem. 1999, 580, 145.
(7) (a) Juvaste, H.; Iiskola, E. I.; Pakkanen, T. T. J. Organomet. Chem. 1999,
587, 38. (b) Juvaste, H.; Pakkanen, T. T.; Iiskola, E. I. Organometallics
2000, 19, 4834. (c) Juvaste, H.; Pakkanen, T. T.; Iiskola, E. I. Organo-
metallics 2000, 19, 1729. (d) Juvaste, H.; Iiskola, E. I.; Pakkanen, T. T.
J. Mol. Catal. A 1999, 150, 1.
(8) (a) McKittrick, M. W.; Jones, C. W. Chem. Mater. 2003, 15, 1132. (b)
The amine scaffolds behave chemically as if they are site-isolated.
However, it has not been proven that every amine site is isolated.
(9) Additional metalation strategies could be employed: Kasi, R. M.;
Coughlin, E. B. Organometallics 2003, 22, 1534.
The patterned precatalysts show significantly higher productivity
than the control materials. Furthermore, the patterned catalyst
exhibits increased activity even when compared to the homogeneous
analogue (catalyst 9). There are several possible explanations for
the observed improvement in performance as compared to solid
catalysts made via traditional techniques. It is clear that the
patterning process allows for more efficient assembly of surface
species based on the essentially quantitative yield of each step of
the synthesis, something that is unique as compared to previous
reports.7 Furthermore, if the sites are more uniform and significantly
more isolated than sites on densely functionalized materials (control
11), the patterned sites may be more accessible for activation by
the borane/alkylaluminum cocatalysts and also could more easily
incorporate monomer. In addition, the spectroscopic results and the
quantitative assembly of the supported complex imply that it is
less likely that there are non-CGC metal species on the patterned
catalyst. Another potential advantage of the patterning protocol is
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