the role of the support in the performance of grafted metallocene catalysts
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Full Paper
The Role of the Support in the Performance ofGrafted Metallocene Catalysts
Fernando Silveira, Maria do Carmo Martins Alves, Fernanda C. Stedile,Sibele B. C. Pergher, Joao Henrique Zimnoch dos Santos*
A series of supports – differing in their textural properties, the nature of their surface sites, andin their crystallinities – were investigated in the sequential grafting reaction of Cp2ZrCl2 and(nBuCp)2ZrCl2 mixtures. The catalyst systems were analyzed by Rutherford backscatteringspectrometry, AFM, EXAFS, and nitrogen adsorp-tion. All systems were shown to be active inethylene polymerization reactions when methyl-aluminoxane was used as the cocatalyst. Experi-mental results are discussed in terms of therelationships between the grafted metal content,the catalyst activity, the surface roughness andpolarity, the interatomic Zr–C and Zr–O distancesof the grafted species and the molecular weightsof the resulting polyethylenes.
Introduction
Plastics obtained by olefin polymerization represent the
largest volume of polymer produced and commercialized
by the world’s petrochemical industry. Among the
available catalyst systems, metallocenes have generated
increasing interest in both industrial and academic
research due to the new properties that such catalyst
systems can engender in polymers and, especially,
F. SilveiraBraskem S.A., III Polo Petroquımico, Via Oeste, Lote 05, Triunfo,95853-000, BrazilM. C. M. Alves, F. C. Stedile, J. H. Z. dos SantosInstituto de Quımica, UFRGS, Av. Bento Goncalves 9500, PortoAlegre 91509-900, BrazilFax: þ55 51 3308 7304; E-mail: [email protected]. B. C. PergherDepartamento de Quımica, Universidade Regional Integrada doAlto Uruguai e das Missoes (URI) – Campus Erechim, CP 743,Erechim, 99700-000, Brazil
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copolymers. It has been estimated that the polyolefin
market will grow by 64% over the next decade, that is, 3–
5% per year. It is believed that the amount of polyethylene
produced by metallocene catalysts will increase by 470%.
At present, the market for polyethylenes produced by
metallocenes represents ca. 5–6% of the polyolefin market,
which corresponds to 3–4 million tons of polymer per
year.[1]
Supported metallocenes have been widely investigated
in both the open and the patent literature.[2] Silica has been
themost widely studied support. Nevertheless, other types
of support have also been evaluated for use in the
immobilization of metallocenes with the aim of increasing
catalyst activity or generating different polymer proper-
ties. Some examples of the different supports are alumino-
silicates,[3] organotin-modified silica,[4] organosilicon-
modified magnesium-chloride-based supports, especially
in combination with Ziegler-Natta catalysts,[5] and, more
recently, nucleation agents, such as sorbitol[6] and carbon
nanotubes.[7] Among the organic supports, polystyrene
has been the most widely employed support. Examples of
DOI: 10.1002/mren.200900004 139
F. Silveira, M. C. M. Alves, F. C. Stedile, S. B. C. Pergher, J. H. Z. dos Santos
140
non-interacting polystyrene,[8] or spacer-modified[9] or
poly(ethylene oxide) monomethyl ether functionalized
polystyrene[10] can be found in the literature. The use of
nano-sized latex has also been reported as an alternative
organic support.[11]
As shown in Scheme 1, the supports investigated differ
in terms of their morphology, texture and surface
chemistry. In terms of morphology, such materials can
adopt granular, spherical or lamellar forms, which, in turn,
may affect the polymer morphology by means of
replication phenomena or extrusion polymerization.
Texture may influence the nature and distribution of
potential catalyst sites, as well as the accessibility of the
cocatalyst and monomer to the growing polymer chain.
Finally, supports differ with respect to the presence of
Lewis or Bronsted acidic and basic sites, which may affect
the amount of grafted material, the metallocene distribu-
tion and the reactivity of such surface species, as well the
characteristics of the resulting polymer in terms of
molecular weight, crystallinity and polydispersity.
The support plays an important role in the development
of a supported metallocene catalyst. It may affect catalyst
activity by generating bidentate inactive grafted species or
by hindering a cocatalyst’s access to the potential catalyst
species, thereby hindering the catalyst’s activation.
Furthermore, the support can promote the generation of
Scheme 1. The potential effects of the support on supported metallo
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active sites with lower propagation rates resulting from
interactions with the support’s surface, or restrict mono-
mer access to the active sites, thereby hindering chain
propagation.[12] Generally, polymers produced with sup-
ported metallocenes have higher average molecular
weights, due to a reduction in the rates of the termination
reactions.[13] Clearly, the nature of the support plays a very
relevant role in the performance of the supported
metallocene.
We have been studying the heterogenization of
metallocenes for ca. fifteen years in terms of the influence
of experimental parameters (grafting time and tempera-
ture, contact time, solvent, etc) on the grafting reac-
tion;[14,15] the effect of alternative immobilization techni-
ques, such as the synthesis of supported metallocenes on
hybrid silicas,[16] in situ immobilization[17] or metallocene
encapsulation;[18] the nature of the surface species;[19,20]
and the effect of surface modification with compounds
such as organosilanes[21] or tin chloride.[22] More recently,
we have investigated the effect of the textural properties
of silicas produced by different techniques (aerogel,
xerogel, precipitated silicas, etc),[23,24] as well as the
effect of the support surface’s chemical composition
(alumino-silicates, silica-zirconia, magnesium silica sup-
ports) and its crystallinity (amorphous, lamellar and
crystalline materials)[25] on the catalysts’ performance.
cene catalysts.
DOI: 10.1002/mren.200900004
The Role of the Support in the Performance of Grafted Metallocene Catalysts
An investigation of the leachability in those supported
systems showed it to be negligible.[23,24]
In this paper, we systematize our previously reported
results with the aim of extracting some trends related to
the support’s texture and polarity, and to its effect on the
grafted metal content, the nature of the surface metallo-
cene species, the catalyst’s activity and the morphologies
and characteristics of the resulting polymers.
Experimental Part
Materials
All chemicals were manipulated under an inert atmosphere using
Schlenk techniques. Commercial silicas (Grace 560, Grace 550 and
Grace 480) were provided by Grace Chemical. Chrysotile was
kindly provided by SAMA (Goias, Brazil). SBA-15 was prepared at
the Instituto de Tecnologıa Quımica de Valencia (ITQ-UPV, Spain).
Methylaluminoxane (MAO)-modified silica (SMAO) with 23 wt.-%
Al/SiO2 (Witco) and alumina (INLAB, Brazil) were used without
further purification. MCM-41, MCM-22 and ITQ-2 were prepared
according to literature procedures.[26–28] Silica xerogel and silica-
zirconia were prepared, respectively, by hydrolytic and non-
hydrolytic sol-gel processes.[23] Silica aerogel was synthesized at
CENERG (France). Leached chrysotile was obtained according to a
procedure in the literature.[29] Furthermore, the catalysts
(nBuCp)2ZrCl2 (Aldrich) and Cp2ZrCl2 (Aldrich) and the cocatalyst
MAO (Witco; 10.0 wt.-% toluene solution) were used as received.
Toluene was deoxygenated and dried by standard techniques
before use. Ethylene and argon (White Martins) were passed
through molecular sieves (13 A) prior to use. Toluene was purified
by refluxing over sodium and distilled under nitrogen just before
use.
Synthesis of the Supported Hybrid Catalysts
All of the supports were activated under vacuum (P<10�5 bar) for
16 h at 450 8C. In a typical experiment, a toluene solution
of Cp2ZrCl2, corresponding to 0.25 wt.-% Zr/SiO2, was added to ca.
1.0 g of the pre-activated support and stirred for 30 min at room
temperature. The solvent was then removed under vacuum
through a fritted disk. A toluene solution of (nBuCp)2ZrCl2,
corresponding to 0.75 wt.-% Zr/SiO2, was added, and the resulting
slurry was stirred for at least 30 min at room temperature, then
filtered through a fritted disk, and the solids were washed with
toluene (10�2.0 cm3) and dried under vacuum for 4 h.
The same procedure was carried out on all of the activated
supports, such as MCM-22, ITQ-2, SBA-15, silica-MAO, alumina,
silica-zirconia produced by the non-hydrolytic sol-gel route, as
well as on natural chrysotile. The resulting supported catalyst
systems were named M22, IT2, S15, SMAO, ALU, NHI and nCR,
respectively, after catalyst grafting. For the silicas, the label
corresponds to the commercial source: G56 for silica Grace 560,
SMAO for MAO-modified silica; or to the synthetic route: AER for
aerogel, HID for xerogel produced by the hydrolytic sol-gel route.
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Characterization of Supported Catalysts
Rutherford Backscattering Spectrometry (RBS)
Zirconium loadings in the catalysts were determined by RBS
using Heþ beams with 2.0 MeV of energy incident on
homogeneous compressed (12 MPa) tablets of the catalyst
material. The method is based on the determination of the
number and energy of the detected particles that are elastically
scattered by the Coulombic field of the atomic nuclei in the target.
In this study, the Zr/Si or Zr/Al atomic ratios were determined by
the heights of the signals corresponding to each of the elements in
the spectra and converted to a wt.-% of Zr/SiO2 or Al2O3. For an
introduction to thismethod and applications of this technique, the
reader is referred elsewhere.[30]
Nitrogen Adsorption/Desorption Isotherms
Samples were degassed (10�2 mbar) at 120 8C (silica) or 85 8C(supported catalysts) for 8 h. Adsorption/desorption nitrogen
isotherms were measured at �196 8C in a Gemini 2375 (Micro-
meritics) instrument. Specific surface areas (SBET) were determined
by the Brunauer-Emmett-Teller equation (P/P0¼0.05–0.35). The
mesopore size and distribution were calculated by the Barrett-
Joyner-Halenda (BJH) method using the Halsey standards.
Desorption branches were used.
Atomic Force Microscopy (AFM)
Images of the supported catalyst surfaces were obtained using a
Nanoscope IIIa1 atomic force microscope, manufactured by Digital
Instruments Co., using the contact mode technique with silicon
nitride probes. The WS M 4.0 software from Nanotec Electronic
S.L.[31]wasused to process the images and to calculate the roughness
of the surface (RMS). The samples were compressed in the form of
tablets and fragments of roughly 16mm2were used for the analysis.
Extended X-Ray Absorption Fine Structure (EXAFS)
The EXAFS measurements were performed around the Zr K edge
(E¼17 998 eV) using the Si(220) channel-cut monochromator at
the XAFS 1 beamline (LNLS, Campinas, Brazil). The spectra were
collected in fluorescencemode using one ionization chamber filled
with argon and a Si(Li) detector. In order to perform the EXAFS
experiments, the supported metallocene powder was compacted
into a pellet and covered with Kapton1 tape. All of the
manipulations were performed in a dry box to avoid any
oxidation reactions. The EXAFS spectra were acquired from
17900 to 18900 eV in 3 eV intervals. Several scans were averaged
in order to improve the signal-to-noise ratio.
The IFEFFIT analysis package[32] and the Winxas program[33]
were used to analyze of the EXAFS data. The EXAFS signals
between 1.0 and 10.0 A�1 were Fourier transformed with a k1
weighting and a Bessel window. Structural parameters were
obtained from least-squares fitting in k and R space using
theoretical phase shift and amplitude functions deduced from the
FEFF7 code.[29] The input for the FEFF7 code was provided by
the ATOMS program.[34] In the fitting procedure, the amplitude
reduction factor (S20) was close to 1.0 for all samples, and the
threshold energies (E0) for the Zr–C and Zr–O pairs were �7.5 and
�3.5 eV, respectively.
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F. Silveira, M. C. M. Alves, F. C. Stedile, S. B. C. Pergher, J. H. Z. dos Santos
142
Polymerization Reactions
Polymerizations were performed in toluene (0.15 L) in a 0.30 L
Pyrex glass reactor connected to a constant-temperature circulator
equipped with mechanical stirring and inlet ports for argon and
monomers. For each experiment, the catalyst system (a mass
corresponding to 10�5 mol � L�1 of Zr) was suspended in toluene
(0.01 L) and transferred into the reactor under argon. The
polymerizations were performed under an atmospheric pressure
of ethylene at 60 8C for 30 min at an Al/Zr ratio of 1 000:1, using
MAO as the cocatalyst. Acidified ethanol (HCl) was used to quench
the reactions. The reaction products were separated by filtration,
washed with distilled water and finally dried under reduced
pressure at 60 8C.
Polyethylene Characterization
The molar masses and molar mass distributions were determined
with a Waters CV plus 150C high-temperature gel permeation
chromatography (GPC) instrument, equipped with a viscosimetric
detector, and three Styragel HT-type columns (HT3, HT4 and HT6)
with an exclusion limit of 1�107 for polystyrene. 1,2,4-
trichlorobenzene was used as the solvent with a flow rate of
1 cm3 �min�1, and the analyses were performed at 140 8C. Thecolumns were calibrated with polystyrenes that had standard
narrow molar mass distributions and with linear low-density
polyethylenes and poly(propylene)s. Scanning electron micro-
scopy (SEM) experimentswere carried out on a JEOL JSM/6060. The
powders were initially fixed on a carbon tape and then coated
with gold by conventional sputtering techniques.
Table 1. Grafted metal content, textural characteristics and catalytcharacteristics for several supported metallocenes.[22,24]
Supported catalyst Grafted metal content Surface area
wt.-%a) m2 � g�1
G56 0.51 200� 3
G55 0.33 244� 2
G48 0.35 249� 3
HYD 0.50 341� 0
AER 0.15 462� 2
M41 0.84 768� 1
lCR 0.20 226� 0
NHI 0.41 18.0� 0
SMAO 1.00 99� 0
ALU 0.90 103� 1
S15 0.42 463� 0
IT2 0.61 478� 0
M22 0.51 313� 0
nCR 0.21 14� 0
a)Expressed relative to Zr/SiO2 or Al2O3.
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Results and Discussion
Table 1 shows some data on the supported metallocenes,
such as their grafted metal content, their textural
characteristics, their catalytic activity in ethylene poly-
merization and the resulting polymer characteristics. More
information can be found elsewhere.[23,25] According to
Table 1, the grafted metal content varied from 0.15 to
1.00 wt.-% Zr/SiO2 or Zr/Al2O3, depending on the support,
while their textural characteristics were very distinct. As
shown in Figure 1, grafted metal content was expressed as
a Zr density by taking into account the immobilized metal
content (determined by RBS) and the specific surface area
(determined by the BET method, after nitrogen adsorp-
tion).
Figure 1 clearly shows that three catalyst categories can
be identified: i) catalysts NHI (which possesses 12.9 wt.-%
Zr/SiO2 incorporated into the silica backbone) and nCR
(which possesses 10.4 wt.-% Mg); ii) catalyst ALU, which
uses alumina as a support; and, iii) catalysts containing
silica or alumino-silicates as their support. The latter
group, independent of the specific support, has Zr densities
near 0.2 nm�2. It is likely that the steric repulsion caused
by the metallocene itself may restrict the number of metal
centers grafted to the OH-group-bearing surface. As
already shown by IR spectroscopy, the surface reaction
between metallocene centers and the silanol groups on a
silica surface is affected by steric repulsion from the
ic activity in ethylene polymerization and the resulting polymer’s
Pore diameter Catalyst activity Mw Mw=Mn
A kgPE �mol�1Zr �h�1 kg �mol�1
145 4 280 423 2.0
128 2 220 338 2.1
118 5 310 258 1.9
39 1 680 356 3.0
58 3 537 343 2.0
24 880 484 1.9
39 1 927 378 2.1
36 720 306 2.4
84 6 560 388 2.2
42 853 351 2.2
50 3 200 273 2.5
14 1 260 477 2.0
19 1 467 275 2.1
39 400 254 2.0
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The Role of the Support in the Performance of Grafted Metallocene Catalysts
Figure 1. Zr density as function of surface area for several sup-ported metallocenes.
grafted metallocene, which hinders further reaction
between the metallocenes in solution and the remaining
silanol groups on the surface.[20]
A higher concentration of Zr per square nanometer
could produce catalysts having higher activity due to
either the greater quantity of potential catalyst (metal)
centers or the higher number of catalyst sites, which make
it more robust with respect to catalyst poisoning. Figure 2
shows catalyst activities expressed as function of Zr
density.
It indicates that the supported catalysts’ activities vary
significantly, although the majority of the supports have
similar Zr densities. Catalysts NHI, nCR and ALU – despite
having a high Zr density on the support – showed the
lowest catalytic activities. It is worth noting that a higher
Zr density may result in catalytic centers that are closer
Figure 2. Catalytic activity of the supported metallocenes as afunction of Zr density.
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together; this proximity may promote the formation of
bimolecular metallocene species, which are known to be
inactive in olefin polymerization reactions.[35]
A large number of ions associated with bimolecular
species were detected by matrix-assisted laser desorption/
ionization time-of-flight mass spectrometry (MALDI-TOF-
MS) analysis of catalyst supports having pore diameters
smaller than 100 A.[24] In such pores, the proximity of
intraglobular OH groups makes it easier to generate
bimolecular species on the surface, as shown in Scheme 2.
Catalyst activity can also be affected by the surface’s
roughness. As previously proposed, small diameter pores
may act as obstacles on the surface, imposing a diffusion
constraint on both the monomer and the cocatalyst.[36,27]
The higher the proportion of small diameter pores on the
surface, the greater the number of obstacles and, conse-
quently, the lower the catalyst activity. Obstacles on the
surface of the support afford it roughness. AFM allows us
to estimate this, expressed as the root-mean-square (RMS)
roughness, for the support surface on the nanometer scale.
Figure 3 demonstrates the correlation between catalyst
activity in ethylene polymerization and the surface
roughness of the different supported catalyst systems
investigated.
As shown, greater surface roughness correlates with
lower catalyst activity. It is worth noting that SMAO
exhibited the smoothest surface and afforded the highest
catalyst activity. This type of support is known to produce
very active catalysts. Its high activity has been attributed
to the nature of the surface species generated. It is believed
that the MAO creates a liquid layer that allows the
metallocene species to disperse over the surface and
renders the distribution of the surface species much more
Scheme 2. Proposed nature of grafted species in small diameterpores.
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F. Silveira, M. C. M. Alves, F. C. Stedile, S. B. C. Pergher, J. H. Z. dos Santos
Figure 3. Surface roughness (RMS) of the supported metallocenecatalysts and its effect on catalyst activity in ethylene polymeri-zation.
144
homogeneous.[38] This work contributes to the field by
highlighting another reason why metallocenes supported
on MAO-modified silicas always exhibit a high activity. It
seems that coating a silica surface, for example with MAO,
promotes the generation of a smoother support surface,
which, in turn, aids the diffusion of both themonomer and
the cocatalyst.
However, the textural properties of the support do not
only affect catalyst activity by means of steric
effects. Cp2ZrCl2 and (nBuCp)2ZrCl2 possess Zr–C inter-
atomic distances of 2.34 and 2.35 A, respectively.[39] In the
corresponding supported systems, this distance contracts
to between 1.5 and 2.2 A, which suggests that the pore
diameter of the support may influence the structure of
Figure 4. Influence of pore diameter on the mean Zr–C inter-atomic distance in the grafted metallocene species.
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metallocene catalyst within the pores. Figure 4 shows the
correlation between the Zr–C interatomic distance and the
pore diameter.
Iler[40] proposed that, in silicas with pore diameter
smaller than 100 A, the negative curvature keeps silanol
groups closer, favoring the formation of hydrogen bonds
and, therefore, increasing the stability of silanol groups
against dehydroxylation. The presence of a larger number
of silanol groups can favor their interaction with the
grafted zirconocene species. IR bands assigned to the
interaction between anisole and surface silanol groups
have already been reported in the literature[41] and
attributed to the interaction between hydroxyl groups
and the p-electron system of the aromatic ring. The
interaction of intraglobular silanol groups within small
diameter pores could cause grafted species to becomemore
closely packed and, thus, increase the Zr–C distance of the
cyclopentadienyl rings in the supported metallocenes due
to the kind of interaction mentioned above. Such an
increase would influence the olefin coordination and chain
propagation steps, which in turn would affect the overall
catalyst activity. This proposition is depicted in Scheme 3.
Figure 5 shows the relationship between Zr–C intera-
tomic distance and catalyst activity; increases in the Zr–C
distance are correlated with a reduction in catalyst
activity.
The polarity of the support can also influence the
performance of the immobilizedmetallocenes. Polarity can
be estimated by nitrogen adsorption by determining the
constant C in the BET method.[42] Figure 6 shows the
relationship between the support’s polarity (determined
by BET) and the Zr–O distance (determined by EXAFS).
Two opposing trends are observed: for silica-based
catalysts, an increase in surface polarity engenders an
Scheme 3. Proposed interaction between grafted metallocenespecies within small diameter pores.
DOI: 10.1002/mren.200900004
The Role of the Support in the Performance of Grafted Metallocene Catalysts
Figure 6. Influence of surface polarity on the mean Zr–O inter-atomic distance in the grafted metallocene species.
Figure 5. Correlation between Zr–C interatomic distance andcatalyst activity.
increase in the Zr–O interatomic distance; however,
supports made from alumino-silicates, alumina, silica-
zirconia and natural chrysotile show the opposite trend –
with these support materials increases in surface polarity
result in decreased catalyst activity. It is noteworthy that
the second group of supports has significantly higher C
values (greater than 100), while the silica-based materials
have lower values (70–100). The increase in polarity in the
second group of supports is probably caused by the
presence of Lewis acid sites (e.g., Zrþ and Alþ), and may
affect the oxygen bonding in the surface–O–Zr (from the
zirconocene) triad. In the case of silica-based supports, the
Zr–O interatomic distances were between 1.98 and 2.10 A.
It is worth noting that the constant C in the BET equation is
related to the adsorption heat. A large value of C may be
observed when the pore size is small. Nevertheless, taking
into account only pore size, one cannot account for the
trends observed in Figure 6 (see Table 1). The textural
properties of the support could also cause the differences
observed in the mean Zr–O interatomic distances in the
surface-supported metallocene species.
The support may also affect the morphology of the
polymer produced. In the case of commercial supported
Figure 7. Micrographs of polymers produced by the M22 and G48 catalyst systems.
Ziegler-Natta catalysts, it is well known
that the spherical polymer formed is a
morphological result of the spherical
magnesium chloride adduct morphol-
ogy. The metallocenes grafted onto
alumino-silicates catalyze a process of
‘‘polymerization through extrusion,’’ in
which the polymer chain grows out of
the mesopores up to the particle’s sur-
face. This type of polymerization often
produces polymers in fiber form.[37,43,44]
In our studies, M22 catalysts did indeed
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produce nanofibers, as shown in Figure 7. Similar
morphologies were observed for NHI, M41 and lCR
catalysts. The width of the nanofibers varied between
12.5 and 39.9 nm.
The replication of a spherical morphology has been
reported previously.[16] In contrast, supports with unde-
fined morphology or porosity produce polymers with
irregular morphology, as shown in the case of the G48
catalyst.
The Zr–O interatomic distance of the grafted metallo-
cene seems to correlate with the properties of the polymer
it produces. Scheme 4 shows the correlation between
mean Zr–O interatomic distance and themolecular weight
(Mw) of the resulting polyethylene.
It has been suggested in the literature that blocking one
of the catalyst’s sides by grafting the metallocene to a
support disfavors the termination step of the polymeriza-
tion, which causes an increase in the final molecular
weight.[12] As a result, polymers produced by supported
metallocenes usually have a much higher molecular
weight than polymers produced by homogeneous analogs.
As shown in Scheme 4, an increase in the Zr–O interatomic
distance results in polyethylene with a lower molecular
weight. In other words, when themetallocene species feels
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F. Silveira, M. C. M. Alves, F. C. Stedile, S. B. C. Pergher, J. H. Z. dos Santos
Scheme 4. Correlation between the mean Zr–O interatomic distance in the grafted species and the molecular weight of the resultingpolyethylene.
146
less of the surface’s steric effect (i.e., increasing Zr–O
distance), the chain termination step is likely to be favored,
thereby causing a decrease in the resulting polymer’s
molecular weight. Clearly, when designing a supported
metallocene catalyst, the choice of support may tune the
molecular weight or morphology of the resulting poly-
ethylene.
Despite the potential for heterogeneity intrinsic to a
surface, which might contain several distinct species that,
in turn, might be capable of producing polymers of
different molecular weights, we observed relative uniform
values of polydispersity. As shown in Table 1, the
polydispersity of the polymers varied from 1.9 to 3.0
and no clear indications of bimodality were observed in
the chromatograms.
Conclusion
The internal environment of the support’s pores affects the
molecular structure of the metallocenes grafted into the
pores. This can be seen in the reduction of the interatomic
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distances between the coordination sphere and the Zr
center observed in supported catalysts, as compared to the
distances determined for the analogous homogeneous
catalysts. This change seems to be caused by the polarity of
the support’s surface. Intraglobular silanol groups occur-
ring at a relatively high density within small diameter
pores may interact with the grafted zirconocene centers,
increasing the distance between the carbon atoms of the
aromatic rings and the metal center. This increase in the
Zr–C interatomic distance is accompanied by a loss in
catalyst activity. In addition to facilitating the generation
of inactive bimolecular species and increasing the rough-
ness of the surface, which retards the diffusion of both
monomer and cocatalyst, small pores alter the nature of
the metallocene species grafted to the surface, further
reducing catalyst activity.
In summary, the textural properties of a support may
influence catalyst activity through the roughness exhib-
ited by the surface or by altering the structure of the
grafted metallocenes, which, in turn, may affect the
molecular weight of the resulting polyethylenes. Further-
more, the support can enforce a morphology on the
DOI: 10.1002/mren.200900004
The Role of the Support in the Performance of Grafted Metallocene Catalysts
polymer produced, as is the case with the nanofibers
produced by alumino-silicate crystalline supports. Ulti-
mately, the surface area of the support, which is an
integral textural property in the development of supported
catalysts, does not affect the amount of metallocene
grafted to the support, but alters the nature of the catalytic
species.
Acknowledgements: Thisworkwas partially funded by CNPq. Theauthors are thankful to LNLS for measurements performed in theEXAFS beamline (Project D04B XAFS1#5839). William BretasLinares from SAMA is especially thanked for providing thechrysotile samples.
Received: January 26, 2009; Revised: February 28, 2009; Accepted:March 3, 2009; DOI: 10.1002/mren.200900004
Keywords: AFM; EXAFS; mesoporous materials; silica; supportedmetallocene
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