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Indenyl-silica xerogels: new materials for supporting metallocene catalysts
Applied Catalysis A: General 220 (2001) 287–302
Indenyl-silica xerogels: new materials for supporting metallocene catalysts João H.Z. dos Santos a,∗ , Hoang T. Ban b , Toshiharu Teranishi b , Toshiya Uozumi b,1 , Tsuneji Sano b , Kazuo Soga b,2 b
a Instituto de Qu´ımica, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91509-900 Porto Alegre, RS, Brazil School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan
Received 12 January 2001; received in revised form 2 July 2001; accepted 6 July 2001
Abstract Inorganic–organic hybrid xerogels bearing indenyl groups on their surfaces have been synthesised by the sol–gel method. The hybrid xerogels were obtained by hydrolysis and polycondensation of bisindenyldiethoxysilane (Ind2 Si(OEt)2 ) and tetraethoxysilane, TEOS (Si(OEt)4 ) under two different conditions. Chemical structure was investigated by ultraviolet spectroscopy (UV–VIS), transmittance (FT-IR) and diffuse-reflectance (DRIFTS) infrared spectroscopy, magic angle spin nuclear magnetic resonance (MAS-NMR) and X-ray photoelectron spectroscopy (XPS). Xerogel texture and structure were analysed by nitrogen sorption based on the Brunauer–Emmett–Teller (BET) method, X-ray diffraction spectroscopy (XRD), and natural scanning electron microscopy (N-SEM). Under our experimental conditions, in alkaline milieu and 1:3 indenyl/TEOS ratio, a xerogel with higher indenyl content, but nonporous and showing agglomerate patterns was produced (xerogel I). Spherical particles of diameter about 6–8 m where obtained in the absence of catalyst, using 1:5 indenyl/TEOS ratio, higher temperature and shorter reaction time (xerogel II). Xerogel II was used as support for heterogeneous metallocene catalyst synthesis. 3 −1 Zr h−1 ) The resulting catalyst presented high Zr content (0.68 mmol Zr g−1 cat ) and high catalyst activity (40 × 10 kg PE mol in ethylene polymerisation. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Supported metallocene; Sol–gel; Polymerisation; Silica
1. Introduction The preparation, characterisation and application of organic–inorganic hybrid materials have become a quickly expanding field of research in material sci∗ Corresponding author. Tel.: +55-51-319-1499; fax: +55-51-319-1499. E-mail address: [email protected] (J.H.Z. dos Santos). 1 Present address: Research Initiative for Green Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1, Higashi 1, Tsukuba, Ibaraki-ken 305-8565, Japan. 2 Deceased.
ence. Controlled chemical synthesis and design of structure, porosity, and shape of porous inorganic materials is not enough to fulfil several of their potential applications, as in sensors, chromatography, or catalysis. A controlled processing of inorganic precursors in the presence of functionalising agents is necessary to achieve both tailored microstructure and deliberate position of reactive sites [1–3]. An approach to achieve this goal lies in the use of the sol–gel process either from sols of oxides or by hydrolysis and polymerisation of alkoxides [4,5]. The latter provides a convenient method for producing chemically homogeneous and uniform
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 7 3 0 - X
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multicomponent gels. The most effective way to prepare such hybrid gels consists in covalently binding organic groups to an inorganic matrix. Organosilicon precursors can be used in which the functional organic group is bonded through a stable Si–C link to the network-forming inorganic part of the molecule. The most common precursor for silica xerogels is tetraethoxysilane (TEOS), which is readily purified by standard techniques and presents a relatively slow rate of reaction. A functional organic group R may be introduced with organotrialkoxysilane, R Si(OR)3 , where R is commonly ethyl. Organotrialkoxysilanes are usually processed in combination with TEOS to minimise the effects associated with low cross-linking. Many examples in the literature describe the use of organotrialkoxysilanes whose organic part merely aims at controlling pore size and shape in resulting meso- and microporous silica. These are removed from the target material after preparation [6–8]. In many other examples, organic units are preserved, showing their characteristic chemical and physical properties at the surface of the materials prepared, such as chelating ability [2], optical behaviour [9–11], and catalytic properties [12], just to mention a few. In parallel with the development of advanced sol–gel methods for the preparation of novel materials, olefin polymerisation has undergone an extraordinary transformation in the last 15 years. Homogeneous metallocene catalyst systems have been shown to combine high activity with excellent stereoregularity in the polymerisation of olefins, leading to enormous investment in research and development by industrial and academic investigators [13–15]. Nevertheless, heterogeneous catalyst system are required for industrial gas-phase and slurry polymerisation processes. In current technology, many heterogeneous systems are achieved by catalyst immobilisation on a suitable carrier. Besides playing a role in process design, the catalyst support commonly acts as a template for polymer particle growth, controlling size and morphology. Immobilised metallocene catalysts should also be suitable for use in current heterogeneous processes (drop-in technology). Partial success has been achieved by using inorganic carriers, in particular SiO2 [16–18], as support for metallocene polymerisation catalysts. The main disadvantages of such systems comprise leaching of catalyst active sites (specially by methylaluminoxane (MAO)
during the polymerisation process), incomplete fragmentation of the carrier matrix (complete fragmentation with exposure of new active sites is necessary to improve productivity and polymer morphology), and a drastic loss in catalyst activity as compared to corresponding homogeneous systems (in fact, about only ca. 1% of the supported species are believed to be active). Several processes for metallocene immobilisation have been reviewed in the literature [19–21], and a complete description is beyond the scope of this paper. Most of the reported immobilisation routes are based on reactions between silanol groups (either right at the surface of a support or spaced through organosilicon fragments) or ligands from a cocatalyst (Me from alkylaluminum or MAO) and ligands from a metallocene (usually chloro). Immobilised metal loading is partially limited by the number of silanol groups available to reagent molecules [22] and by steric hindrance from ligands in the metallocene [23,24]. Besides, as a variety of binding modes are generally possible at the surface of a support, formation of inactive species is likely to occur. These should at least in part explain catalyst activity drop due to immobilisation. In a previous paper, we reported the characterisation of same supported metallocene prepared by the sol–gel method [25]. The presence of indenyl groups on the surface of the xerogels allowed metallation in a further step, generating in situ metallocene species. Such approach aims at increasing the metal content on the support, which in turn might lead to higher polymer productivity. In the present study, we focus on the synthesis of two hybrid silica xerogels containing indenyl (Ind) ligands by the reaction between TEOS and Ind2 Si(OEt)2 under two deliberative conditions. The effect of preparative conditions on the resulting structural, textural and morphological properties is discussed, based on their analyses by thermal (TGA), spectroscopic (FT-IR, DRIFTS, MAS-NMR, XPS, UV–VIS, XRD), microscopic (N-SEM) and volumetric (BET) complementary techniques. 2. Experimental 2.1. Materials Indene (+85%, Kanto Chemical Co., Japan) and tetraethoxysilane (extra pure reagent, Nacalai Tesque
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Inc., Japan) were purified by vacuum distillation. Toluene (extra pure grade, from Nacalai Tesque Inc., Japan), tetrahydrofuran (THF, first grade from Kanto Chemical Co., Japan), diethylether (first grade, from Wako Pure Chemical Industries Ltd., Japan), heptane (extra pure grade, from Nacalai Tesque Inc., Japan), 1-hexene (first grade, from Wako Pure Chemical Industries Ltd., Japan) were purified according to the usual procedures. Zirconium tetrachloride (ZrCl4 , +98%, Merck-Schuchardt, Germany), diethoxydichlorosilane (ShinEtsu, Japan) and butyllithium (n-BuLi, reagent grade in hexane solution, from Kanto Chemical Co., Inc., Japan) were used without further purification. Nujol (Wako Pure Chemical Industries) was purged by nitrogen bubbling for 6 h. Ethylene (polymerisation grade, Takachiho Trading Co., Ltd., Japan) was purified through columns of NaOH (extra pure grade, from Nacalai Tesque, Inc., Japan) and P2 O5 (+98.9%, from Wako Pure Chemical Industries Ltd., Japan). Methylaluminoxane (MAO, in toluene solution), trimethylaluminum (TMA) and triethylaluminum (TEA) were donated by Tosoh Akzo Co., Japan and were used without further purification. 2.2. Synthesis of bisindenyldiethoxysilane Into a 500 cm3 two-neck flask equipped with a dropping funnel and a magnetic stirring bar were introduced 132 mmol (15.3 g) of indene and 200 cm3 of heptane. Then, 85 cm3 of a 1.57 mol/l n-BuLi solution in hexane were added dropwise at 0◦ C. After total addition, the reaction mixture was stirred at 0◦ C for 1 h, and then at room temperature for 16 h. After solvent removal, the resulting solid was washed with three aliquots of 200 cm3 of heptane, and dried under vacuum. To the solid, 150 cm3 of diethylether were added, followed by dropwise addition of 66 mmol of (EtO)2 SiCl2 in 50 cm3 of diethylether at 0◦ C. The reaction mixture was then stirred at 0◦ C for 1 h, and at room temperature for 16 h. After removal, the resulting red solid was washed with three aliquots of 200 cm3 of heptane. Heptane was removed from the extracted orange solution and the resulting oil was distilled at 40◦ C and 0.3 mmHg. 1 H-NMR (CDCl3 , 300 MHz, 25◦ C): δ = 7.79–7.76 (d, 2H, H-4), 7.57–7.55 (d, 2H, H-7), 7.54–7.36 (t, 2H, H-5), 7.34–7.27 (t, 2H, H-6), 7.11–7.09 (dd, 2H, H-3), 6.76–6.74 (dd, 2H, H-2), 4.09–4.06 (m, 4H, O–CH2 –), 3.57 (s, 2H, H-1),
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Scheme 1.
1.41–1.35 (m, 6H, O–CH2 –CH3 ). For the sake of clarity Scheme 1 presents the numbering for the proton of indenyl. The assignments were made based on the literature [26]. 2.3. Synthesis of indenyl-silicas In a three-neck flask equipped with a reflux condenser, a dropping funnel, and a magnetic stirring bar, Ind2 Si(OEt)2 and TEOS (1:3 ratio) were cohydrolyzed (H2 O/OEt = 1:1) in ethanol (30 ml). The hydrolysis water was added in alkaline milieu (0.01N NH4 OH). The reaction was carried on for 60 h at 30◦ C. In another route, the components ratio Ind2 Si(OEt)2 :TEOS was 1:5, with water corresponding to the half of the number of ethoxy groups. The reaction was carried out in toluene at 80◦ C for 6 h. After 1 h, the ethanol produced by the hydrolysis reaction was purged under nitrogen flow and trapped in an ice/water bath. Solid obtained by the first procedure will be referred as xerogel I, while in the other case by xerogel II. In both procedures, solvents were removed by vacuum distillation. The precipitates were allowed to dry from the sol and collected as powders. 2.4. Metallocene catalyst synthesis Xerogel II (0.46 g) was reacted with n-BuLi solution in excess (6.0 cm3 ) at room temperature. The reaction mixture was then kept under reflux for 8 h and then washed with three aliquots of 30 cm3 of hexane. To the resulting solid, 1.9 g ZrCl4 ·2THF in THF were added at room temperature. The resulting brown solid was washed with 6 aliquots of 30 cm3 of THF and dried under vacuum.
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2.5. Polymerisation procedure Into a 100 cm3 stainless steel autoclave equipped with a magnetic stirrer were introduced 20 cm3 of toluene, 10 cm3 of MAO or TMA solution (1.00 mmol cm−3 ) and 1 cm3 of catalyst suspension (0.01 mmol Zr cm−3 ). After stirring at room temperature for 30 min, the system was degassed at liquid nitrogen temperature. After introducing 0.29 mol of ethylene monomer (7 dm3 at STP), polymerisation temperature (70◦ C for 1 h). The polymerisation was terminated by adding acidified methanol. The precipitated polymers were washed with methanol and dried vacuum at 60◦ C for 6 h. 2.6. Xerogel characterisation Elemental analyses were carried out on a Yamaco, CHN Corder, model MT-5. The Zr loading on the catalyst system was determined by inductively-coupled plasma optical emission spectroscopy (ICP-OES, Seiko, SPS 7700). UV–VIS analysis was performed in a DW-2000 spectrometer (Sim-Aminco, USA) equipped with a beam scrambler, which diffuses the entering light to form a uniform field of illumination, eliminating any spatial differences between beams. In order to increase the sample transparency and the viscosity, the solids were mixed with Nujol to form a slurry. All the samples were prepared in a glove box in quartz cells (1.0 cm path length). The absorption spectra were recorded under dry N2 between 250 and 550 nm, having Nujol as reference. FT-IR absorption spectra in the range of 4000– 550 cm−1 were recorded with a Jasco FT-IR spectrometer (model Valor-III) with a resolution of ±4.0 cm−1 (16 scans). Samples were analysed as pellets in KBr. A commercial diffuse-reflectance accessory with thermal control was employed for DRIFTS measurements in a JIR-7000 Jeol Spectrometer. The samples were transferred under N2 atmosphere and evacuated before, during, and between analyses. Spectra were recorded at 40◦ C, coadding 500 scans at a resolution of ±4.0 cm−1 . These spectra were collected in reflectance units and submitted to Kubelka–Munk transform. MAS-NMR spectra were recorded with a Varian 400 MHz spectrometer operating at 100.7 MHz for
13 C
and 79.6 MHz for 29 Si. Magic-angle spinning was performed at 10–12 kHz (13 C) and 3.3 kHz (29 Si) at room temperature. Peak assignment was relative to external hexamethylbenzene (13 C) and tetramethylsilane (29 Si). The number of scans varied from 1000 to 3000. Prior to spectra recording, samples were transferred in a glovebox and packed into 7.0 mm diameter zirconia rotors. NMR parameters for 29 Si were 3–5 ms (contact time), with recycle delay of 100 s. For the CP MAS 13 C NMR a contact time of 2 ms and recycle delay of 6 s were chosen. Thermogravimetric analysis was performed on a Seiko TG/DTA 320. Samples weighing 10–12 mg were heated at 10◦ C min−1 from 25 to 500◦ C under nitrogen flow. Specific surface area was determined by the BET method from N2 adsorption data at −196◦ C using a Belsorp 28AS (Bell). The samples were outgassed at 110 or 400◦ C for 20 h before measuring the nitrogen adsorption. X-ray powder diffraction patterns of the samples were obtained for angles 1◦ < 2θ < 50◦ at a scan rate of 1◦ min−1 (2θ ) with a RINT-2100 (Rigaku) automated X-ray diffractometer using Cu K␣ (P = 40 kV, I = 30 mA). Scanning electron microscopy of the xerogels was carried out in a Hitachi S-4500 digital field emission natural scanning electron microscope (Natural SEM S-3500 N). A small amount of powder was placed on a clean glass slide. Double-sided carbon tape was placed on the surface of a SEM stub. The scanning was carried out at 50 eV. The X-ray photoelectron spectra (XPS) were obtained on a PHI 5600 Esca System ( Physical Eletronics), using monochromated Al K␣ radiation (1486.6 eV). Acquisition was carried out at room temperature in high-resolution mode (23.5 eV pass energy). For the C 1s, Si 2p, and O 1s regions. The samples were mounted on an adhesive copper tape as thin films. Samples were prepared in a glove box, introduced into a transfer chamber and then evacuated at 10−6 Torr in 90 min using a turbomolecular pump. During data collection, an ion-getter pump kept the pressure in the analysis chamber under 10−9 Torr. Analysis area was 800 m in diameter. Each sample was analysed at a 75◦ angle relative to the electron detector. For each of the XPS spectra reported, an attempt has made to deconvolute the experimental curve in a series of peaks representing photoelectron emission from atoms in different chemical environments. These peaks are described as a mixture of
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Gaussian (80%) and Lorentzian (20%) contributions to take instrumental error into account together with the characteristic shape of photoemission peaks. 2.7. Polymer characterisation Polymer melting points (Tm ) were measured by differential scanning calorimetry (DSC), in a Seiko DSC 220C. The analyses were performed with a heating rate of 10◦ C min−1 in the temperature range from 25 to 200◦ C. The heating cycle was performed twice, but only the results of the second scan were recorded. Molecular weight and molecular weight distribution of the polymers were measured at 145◦ C by gel-permeation chromatography (GPC, Senshu Scientific, SSC7100) using o-dichlorobenzene as solvent. Polymer morphology was analysed by N-SEM as previously described.
3. Results and discussion 3.1. Preparation and characterisation of the indenyl-silicas Preparative protocols for silica based on the sol– gel technique are well-studied and documented in the literature [3–5]. The sol–gel process proceeds by hydrolysis and condensation reactions, which are strongly influenced by precursor concentration,
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relative component concentration in the precursor mixture, solvent nature, H2 O to alkoxy-group ratio, temperature, and pH. The solvent not only homogenises the precursor mixture, but also affects the particleand network-forming reaction, due to its polarity and viscosity [3]. Moreover, the volume of the reaction solution as well as the precursor concentration in the starting solution might influence the density of the resulting material. As silica, water, and alkoxysilanes are unmiscible, a mutual solvent such as ethyl alcohol is usually employed as homogenising agent. Furthermore, in order to achieve rapid and complete hydrolysis, acid or base catalysts may be generally employed. In order to prepare the indenyl-containing silica, we choose some arbitrary reaction conditions, namely: 3:1 TEOS/Ind2 Si(OEt)2 ratio and water amount corresponding to total hydrolysis of ethoxy groups (1:1, H2 O/OEt ratio). In this case, in which there is the addition of base catalyst (0.01 mol l−1 NH4 OH solution), alkoxysilane hydrolysis and condensation reaction is accelerated relative to hydrolysis, thus leading to generate highly branched networks [4,5]. Elemental analysis of the resulting solid (xerogel I) indicated 28.13% C and 3.96% H. Besides elemental analysis, a set of complementary technique was used to prompt the chemical composition of the xerogel produced, namely ultraviolet– visible (UV–VIS), infrared (FT-IR), photoelectron (XPS), and magnetic resonance (13 C and 29 Si MASNMR) spectroscopies. Fig. 1 shows the UV–VIS
Fig. 1. UV–VIS spectrum of xerogel I in Nujol.
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spectrum of the hybrid xerogel I measured as a slurry in Nujol, taken from the 200 to 550 nm wavelength interval. A band centred at 287.1 nm is observed. For comparative reasons, the UV–VIS spectrum of Indene was taken in toluene solution (1.4 × 10−3 mol l−1 ). It shows an intense band at 285.0 nm, in accordance with data reported in the literature to structures alike [27]. Total carbon in the xerogels cannot be thoroughly attributed to indenyl groups. Silica gels from alkoxides contain considerable amounts of organic by-products from the precursors, products of the hydrolysis and polycondensation reactions [3]. Additionally, there are ethoxy groups on the surface of the gel. For instance, aerogels made from Si(OMe)4 under basic conditions and supercritically dried with methanol present 30% of their silicon atoms still carrying a methoxy substituent [28]. The presence of such groups can be easily detected by vibrational spectroscopy. Fig. 2 shows the transmission IR spectrum of the hybrid xerogel. The main feature of silica can be observed, namely a broad absorption band at 1100–1000 cm−1 wavenumber attributed to the asymmetrical longitudinal silica lattice vibration (ν Si–O–Si ). Broad band placed between 2000 and 1850 cm−1 corresponds to skeleton (overtone) vibrations. The band at 1632 cm−1 is probably attributed to adsorbed water (bending OH) [29].
The presence of indenyl and ethoxy groups can be verified by analysis of the 3100–2800 m−1 (ν C–H ) and 1500–1300 cm−1 (δ C–H ) regions. Bands at 3069 cm−1 can be attributed to indenyl group (aromatic ν C–H ), while bands at 2981, 2930 and 2902 cm−1 to asymmetric and symmetric (ν C–H ) from the aliphatic groups. In the 1500–1350 cm−1 region are found the CH2 scissors deformation and CH3 asymmetrical deformation, which are near the same position when the substituents are the same. These deformations can be the attribution to the band present at 1460 cm−1 . Moreover, such band might include the possibility of C–H bending from the aromatic ring. Bands at 1395 and 1370 cm−1 can be attributed to the symmetric CH3 umbrella deformation. A broad band centred at 3437 cm−1 (ν O–H ) indicates the presence of C–OH and/or Si–OH groups [30]. The Si–C (from indenyl group) might give rise to a band at 1125–1100 cm−1 which is due to a planar ring vibration having some Si–C stretching character [31]. This stretching vibration occurs at the same place as the silica Si–O–Si stretching. In order to better characterise the xerogel surfaces, vibrational analysis was also performed by DRIFTS. The information carried by the signal is related uniquely to surface groups (depth ≤ x nm for incidence at x0 ) [32], while in the case of conventional transmission infrared spectroscopy, the signal
Fig. 2. FT-IR spectrum for xerogel I in the region 4000–550 cm−1 .
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contains information from both the surface and the bulk of the sample. Consistently with restriction to the samples surface, the main features of silica are attenuated. In the aromatic ν C–H region, a second band can be seen at the lower wavenumber side of the band at 3064 cm−1 , probably indicating the presence of different indenyl species on the surface. The chemical composition of the hybrid xerogel was further investigated by MAS-NMR. The 13 C CP MAS-NMR spectrum (Fig. 3a) confirms the presence of ethoxy moieties: 60.7 ppm (O–CH2 –) and 19.1 (O–CH2 –CH3 ). The presence of a large band in the low field regions (signals centred at 142.1, 133.1, 110.6 and 77.2 ppm) indicates indenyl groups. The ethoxy-to-indenyl ratio roughly estimated from peak areas as 40/60.
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The 29 Si MAS-NMR spectrum gives information on the coordination of silicon atoms at the xerogel surfaces and in the bulk silica gel. Spectrum (b) of Fig. 3 indicates three different species, which appear at −48.6, −70.0 and −101.8 ppm. The first signal corresponds to D1 species (see Scheme 2). The signal at 101.8 ppm is due to silanol (Q3 ) and the shoulder at −109.0 ppm to siloxane (Q4 ) sites [33], confirming the presence of OH groups at the xerogel surface and in the Si–O–Si network, as also seen by IR analysis. The broadness of the signal which extends from −65 to −80 ppm might suggest the presence of T species, resulting from the hydrolysis of some Si–C bonds under basic conditions. Other species proposed in Scheme 2 are expected around −50 ppm (D2 ) and −95 ppm (Q2 ) [34]. The
Fig. 3. MAS-NMR spectra of xerogel I. (a)
13 C;
(b)
29 Si.
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Fig. 3. (Continued).
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Scheme 2.
broadness of the signal at −48.6 ppm could result from the presence of both D1 and/or D2 . The likely absence of bidentate species and large excess of monodentate species in comparison to tridentate ones suggests that the indenyl ligands play an important steric effect during condensation reactions, preventing the generation of polydentate species. Silica gel surface coverage can be determined according to the uniform globular model of Amati and Sz Kovats [35], in which total carbon elemental analysis data is correlated to the specific surface area of the support, normally determined by BET nitrogen adsorption. Prior to BET measurements, thermogravimetric (TG) analysis under N2 atmosphere was performed in order to determine the thermal stability of the xerogels. A small weight loss was observed between 30 and 150◦ C, probably attributed to liberation of water and ethanol, which were physically adsorbed on the xerogel surface. A second step in mass loss was observed for temperatures above 150◦ C and could be attributed to organic moiety decomposition, including alkoxy and indenyl groups. Taking into account TGA results, BET analysis was performed by submitting the sample to thermal pre-treatment at 110◦ C
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for 20 h. The sample was shown to be nonporous, exhibiting surface area under 10 cm2 g−1 . Loy et al. [36] showed that aerogels produced with precursors such as (RO)3 Si(CH2 )n –Si(OR)3 with n = 2, 4, 6, 8, 10, and 14 exhibited a progressive reduction of specific surface area with increasing spacer length. Moreover, in the case of tetradecylmethylene, the resulting aerogel was nonporous. Similar results have been reported for hybrid xerogels from dendrimers and arborols [37]. Thermal treatment at 450◦ C for 20 h was then performed in order to remove at least organic surface groups, but no increase in surface area was observed. Powder X-ray diffraction patterns showed no clear diffraction peaks in the range 2θ = 1–50◦ , indicating that the produced xerogel is amorphous, in accordance with data reported for silica gel [38]. The analyses showed a broad band corresponding to a d-spacing of about 3.8 Å, slightly longer than that generally observed for xerogels made from TEOS (3.6 Å). The morphology of xerogel I was analysed by SEM. As we can see in picture (a) of Fig. 4, the xerogel was shown to be constituted of conglomerates with estimated size of 60 ± 10 m. Base-catalysed condensation (as well as hydrolysis) should be directed toward the middles rather than the ends of chains, leading to more compact, highly branched species [4,5]. The dependence of microstructure on xerogel preparative conditions has already been reported in the literature [39]. Final gel microstructure has been related to water content in precursor sols. Gels made from water-richer sols (as in the present case) show coarser surface microstructure as compared to those produced from sols with lower water content. It is worth mentioning that the formation of spherical particles is extremely important from the polymerisation catalyst point of view, since a better morphology control of polymer particles results. The employed reaction conditions so far showed that by the sol–gel method, the Si–C bond can be preserved, generating a silica containing indenyl groups on the surface. We performed another attempt of indenyl-containing silica synthesis, using different reaction conditions (xerogel II). We performed the reaction in absence of any catalyst using 5:1 TEOS/Ind2 Si(OEt)2 ratio and using water amount corresponding to partial hydrolysis of ethoxy groups (3:6 H2 O/OEt). No ethanol was added to the alkoxide–water mixture, since the alcohol generated
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as a by-product of the hydrolysis reaction was sufficient to homogenise the initially two-phase system. In order to avoid the reaction equilibrium towards alkoxy species regeneration, produced ethanol was removed by distillation in a second step, as proposed in the literature [40]. Elemental analysis indicated 20.66% C and 3.46% H, i.e. ca. 25% less C than in the previous case. This
could be in part due to the comparatively lower amount of the C-rich precursor Ind2 Si(OEt)2 employed. In effect, the longer reaction time employed previously (60 h versus 6 h) aimed at guaranteeing the incorporation of the bulky Ind moieties. Schwertfeger et al. [28], under alkaline conditions, demonstrated that in the case of a mixture containing TEOS and R Si(OR)3 , where R = alkyl, aryl, the rate of hydrolysis and
Fig. 4. SEM micrographs: (a) xerogel I; (b) xerogel II; (c) polyethylene produced with xerogel II.
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Fig. 4. (Continued).
condensation of R Si(OR)3 is slower that that observed for the corresponding tetraalkoxysilane Si(OR)4 . The R -substituted silane units condense to the existing network much later, after hours or even days [41]. The same analytical techniques were employed for the characterisation of this xerogel. UV–VIS and FT-IR analysis showed similar results as in the proceeding case. 13 C CP-MAS-NMR roughly indicated a higher content of ethoxy groups (60%). According to 29 Si MAS-NMR, the signal content related to Q4 is much lower, suggesting a less branched structure. Under the same experimental conditions, BET measurement showed that this xerogel was seen to exhibit also an extremely low surface area (<5 m2 g−1 ) with an extremely high C constant value (71), which in part can be attributed to the surface polarity exerted by organic groups. Contrarily to the previous case, after thermal treatment at 450◦ C for 20 h, the xerogel was shown to have 226 m2 g−1 as surface area (Vp = 41.7 mm3 g−1 ; Rp = 0.95 nm), indicating the presence of nanopores. It is worth mentioning that heating at 450◦ C is not high enough to cause rearrangement of the silica network (tetrahedral rearrangement takes place at temperatures higher than 550◦ C) [42]. Therefore, the increase in surface area can be attributed to decomposition of steric demanding surface
groups (indenyl), liberating blocked pores, besides the formation of new pores by decomposition of ethoxy groups. Similar results have been reported in the literature for temperatures treatments between 300 and 500◦ C. The increase of specific surface area with temperature was attributed to the formation of new pores due to evaporation of physisorbed water and oxidation of residual organic groups in gels [39]. Since xerogel drying was made under vacuum, shrinkage and mostly destruction and collapse of an initially highly porous network may have occurred. The gelation rate also influences pore structure. Fast gelation (as expected for the present case) yields an open structure, because the particles are quickly connected and cannot undergo further rearrangments [3]. SEM imaging was also performed. In this case, the resulting xerogel (picture (b) of Fig. 5) appears as distributed spherical particles of size about 6–8 m. As already mentioned, from the polymerisation point of view, such support is much more suitable due to its roughly spherical shape. Nevertheless, the particle diameter is extremely small, if compared to xerogel. It is worth noting that from the technological point of view, bigger catalyst particles might be less prone to processing problems than very fine ones. It seems that structural variations (porosity, inner surface area) in functionalised xerogels are not
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Fig. 5. XPS spectra curve fitting. The small inserted peaks are the curve fit components, which are summed to obtain the smooth drawn line. The line with visible noise component is the experimental raw data. (a) Si 2p; (b) O 1s; (c) C 1s.
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Fig. 5. (Continued).
induced as much by the properties of the organic ligand as by the effect of the presence of an organosilane on the sol–gel process itself. It is believed that the alkoxyorganosilane basically acts as a co-solvent in the first reaction step, therefore diluting the tetraalkoxysilane and leaving a larger amount of water and catalyst available for reaction, resulting in larger particles [3]. On the other hand, morphology (pore size, surface) is mostly dependent on the nature of precursors than on reaction conditions. Some studies involving the synthesis of aerogels showed that the length of alkyl ligands in compounds such as (RO)3 SiR decisively is accelerated relative to hydrolysis. The rate of condensation enhances with increasing number of siloxane linkages. Thus, highly branched networks with ring structures are formed, generating therefore large, bulkier and ramified polymers. Both hybrid xerogels were further analysed by XPS. Fig. 5 presents the XPS spectra of the different components of xerogel I. From the Si 2p spectrum (Fig. 5a), Si atoms in the sample bulk (103.3 eV) can be distinguished from those on the surface, which are chemically bound to the organic ligands (101.1 eV). The species at high and low binding energies correspond, respectively, to 79 and 21% (see Table 1) of the Si atoms on the probed region of both xerogels. For xerogel II, the low binding energy component of the Si
Table 1 XPS data of the xerogel components Photoelectron Xerogel I Si 2p O 1s C 1s Xerogel II Si 2p O 1s C 1s a b
BEa (eV)
Relative elemental abundance (%)
FWHMb (eV)
103.3 101.1 533.6 531.8 285.0 287.0
79.0 21.0 88.0 12.0 95.9 4.1
2.3 2.3 2.1 2.2 2.1 1.3
103.3 101.8 533.6 531.6 285.0 287.3
79.0 21.0 77.0 23.0 95.7 4.3
2.7 3.7 2.4 3.1 2.7 1.4
Reference: Si 2p from SiO2 (103.3 eV). Full width at half maximum intensity.
2p spectrum is centred at 101.8 eV. Such behaviour might be probably due to a lower electron density on Si atom from the O groups (ethoxy groups) which are in a higher amount in xerogel II. According to the literature [43,44] species such as O2 SiC2 on xerogel surfaces can be distinguished by XPS analysis. In order to verify the possibility of
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distinguishing in XPS spectra silicon atoms bonded to oxygen in the network (as in Si–O–Si) from those bonded to surface ethoxy groups (Si–O–C) or Ind (Si–C), a silica containing exclusively ethoxy groups on its surface was prepared by Si(OEt)4 grafting and analysed according to the procedure above. Analyses showed that the best fitting corresponded to the deconvolution of two signals: 103.3 and 101.7 eV, corresponding roughly to 90 and 9% of total area, being the lowest BE signal attributed to surface (Si–O–C) ethoxy groups. Thus, under our analytical experimental conditions, we could not distinguish Si atoms bonded to organic or to alkoxy moieties. Part (b) of Fig. 5 shows the O 1s XPS spectrum of xerogel I. The signal centred a 533.6 eV binding energy corresponds to silica bulk oxygen, while oxygen from ethoxy groups appears at 531.8 eV. The low BE component is shifted to 531.6 eV in the case of xerogel II. Moreover, the low BE peaks for xerogels I and II show, respectively, full width at half maximum intensity (FWHM) of 2.2 and 3.1 eV, suggesting a larger heterogeneity of ethoxy species on xerogel II. The low BE signal corresponds to 12% of the O 1s spectrum for xerogel I and to only 23% for xerogel II, in accordance to the increased amount of ethoxy groups as previously pointed out. The high resolution of XPS analysis of the C 1s peak from xerogel I (Fig. 5c) indicates that 95.9% of this peak is contributed by C–C bonds of hydrocarbons (285.0 eV BE). The remaining is due to atoms presenting a C–O bond (287.0 eV BE). 3.2. Catalyst preparation and olefin polymerisation According to these results, under the two different experimental conditions employed in the present work, xerogel containing indenyl groups can be obtained. Since the xerogels presented water, alcohol, and ethoxide groups on its surface, xerogel II was initially treated with n-BuLi in excess to consume them. In spite of the lower area, we could not perform a thermal treatment to increase it due to stability of the indenyl groups bond to silica surface. Metallation with ZrCl4 ·2THF generated zirconocene supported systems containing 0.68 mmol Zr g−1 cat . Such metal content is about seven times higher than those usually
obtained by metallocene grafting on silica [45,46]. This catalyst amount is about 15% higher than that obtained in the case of in situ metallocene supported synthesis [47]. The approach of metallation of a support already containing the metallocene ligands seems to provide a route to increase the catalyst metal content, overcoming the limitation of number and accessibility of silanol groups. The characterisation of this catalysts by further complementary spectroscopic techniques, namely UV–VIS, XPS, SEM, inductively-coupled plasma-atomic emission spectroscopy (ICP-AES), electron probe microanalysis (EPMA), matrix-assisted laser desorption/ionisation time-of-flight mass spectroscopy (MALDI-TOF-MS) is reported elsewhere [25]. Ethylene homopolymerisation tests were performed using Al/Zr = 1000 (Table 2). The catalyst system showed much higher activity — 40 × 103 kg PE mol−1 Zr h−1 — than ordinary supported systems under similar polymerisation conditions [48,49]. Moreover, such supported systems seems to be thermally stable, since no significant loss in activity could be observed for polymerisation performed at 120◦ C. The generation of supported surface species might avoid eventual bimolecular deactivation reaction which are more prone to take place in the case of homogeneous systems. Low activity was observed in the case of polymerisation with TMA. It is worth mentioning that metallocene catalysts are not usually active in the presence of common alkylcatalysts. On the other hand, as in the present case, some supported metallocene catalysts have shown to be active in the presence of cocatalysts such as TMA, TEA or TIBA [50]. Copolymerisation tests were performed with 1-hexene. The higher activity is typical of comonomer effect, in which the higher solubility of the copolymer in the reaction milieu allows higher diffusion rates of the monomers onto the active sites, therefore, enhancing the copolymerisation activity. The high molecular weight observed in the case of copolymer might suggest low comonomer incorporation. The resulting (co)polymers showed high melting temperatures (close to 143.5◦ C). Molecular weight are very high, typically observed in the case of supported metallocene. This fact has been attributed to the blocking of one of the sides of the active site by the support, hindering the deactivation step. In other
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Table 2 Catalyst activity and resulting polymer properties of supported metallocene prepared with xerogel IIa Cocatalyst
Activity (103 kg PE mol−1 Zr h−1 )
Mw (10−6 )
Mw /Mn
Tm (◦ C)
MAO MAOb TMA MAOd MAOe
40 35 5 105 55
1.0 1.0 nsc 0.2 0.2
3.8 3.5 ns 3.4 3.3
141.8 140.5 141.6 141.8 140.3
Polymerizations were performed in toluene at 70◦ C, with [Zr] = 0.01 mmol ml−1 [Al/Zr] = 1000. 120◦ C. c Not soluble. d Copolymerisation with 0.26 mol cm−3 of 1-hexene. e Copolymerisation with 1.30 mol cm−3 of 1-hexene. a
b
words, the -elimination transfer between two metallocene centres is hindered, resulting in a larger growth of the polymer chain, and so in a higher molecular weight [51]. Metallocene catalysts usually produce polymer with polydispersion index around 2.0 The relative broad molecular weight distribution suggest a heterogeneity in the nature of the active sites, which was confirmed by XPS measurements [52]. Catalyst replication was evaluated by performing the ethylene homopolymerisation reaction at atmospheric pressure by bubbling ethylene at 30◦ C and low Al/Zr ratio (500), for 10 min. Under these polymerisation conditions (Fig. 4c), we could observe that the roughly spherical morphology of the support is partially replicate during the polymerisation.
4. Conclusion Hybrid xerogels containing indenyl groups on their surface can act as support for the in situ synthesis of supported metallocene catalysts. Experimental parameters influence in the nature of the surface species, as well as in the texture and morphology of the support. However, a systematic study is necessary to better evaluate, understand and control the influence of the several experimental parameters, in order to increase the particle diameter and indenyl content, improving the spherical morphology. Further experiment are in progress to study the effect of experimental parameters on xerogel synthesis, as well as on final zirconocene catalyst preparation.
Acknowledgements Financial support for Prof. J.H.Z. dos Santos has been provided by the Japan Society for Promotion of Science (JSPS). References [1] R. Corriu, C. Chuit, G. Dubois, C. Reye, R. Guilard, S. Brandes, in: Proceedings of the International Workshop on Si Chemistry Polymers, Ishikawa, Japan, 29–31 May 1999, p. 97. [2] U. Schubert, N. Hüsing, in: Proceedings of the International Workshop on Si Chemistry Polymers, Ishikawa, Japan, 29–31 May 1999, p. 91. [3] N. Hüsing, U. Schubert, Angew. Chem. Int. Ed. Engl. 37 (1998) 22. [4] C.J. Brinker, G.W. Scherer, Sol–Gel Science, The Physics and Chemistry of Sol–gel Processing, Academic Press, London, 1990, Chapter 3, p. 97. [5] E.F. Vansant, P. Van der Voort, K.C. Vrancken, Stud. Surf. Sci. Catal. 93 (1995) 15. [6] B. Boury, P. Chevalier, R.J.P. Corriu, P. Debid, J.J.E. Moreau, M.W. Chi Man, Chem. Mater. 11 (1999) 281. [7] T. Sugizaki, O. Moriya, Y. Nakamura, T. Endo, M. Oikawa, T. Kageyama, Polym. J. 31 (1999) 625. [8] J. Wen, B. Dhandapani, S.T. Oyama, G.L. Wilkes, Chem. Mater. 9 (1999) 1968. [9] H.K. Kim, S.-J. Kaug, S.-K. Choi, Y.-H. Min, C.-S. Yoon, Chem. Mater. 11 (1999) 779. [10] G.-H. Hsiue, R.-H. Lee, R.-J. Jeng, Polymer 40 (1999) 6417. [11] C. Sanchez, F. Ribot, B. Lebeau, J. Mater. Chem. 9 (1999) 35. [12] B. Cetinkaya, T. Seckin, L. Özdemir, B. Alici, J. Mater. Chem. 8 (1998) 1835. [13] J. Scheirs, W. Kaminsky (Eds.), Metallocene-based Polyolefins, Vol. 1, West Sussex, 2000.
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Indenyl-silica xerogels: new materials for supporting metallocene catalysts João H.Z. dos Santos a,∗ , Hoang T. Ban b , Toshiharu Teranishi b , Toshiya Uozumi b,1 , Tsuneji Sano b , Kazuo Soga b,2 b
a Instituto de Qu´ımica, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91509-900 Porto Alegre, RS, Brazil School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan
Received 12 January 2001; received in revised form 2 July 2001; accepted 6 July 2001
Abstract Inorganic–organic hybrid xerogels bearing indenyl groups on their surfaces have been synthesised by the sol–gel method. The hybrid xerogels were obtained by hydrolysis and polycondensation of bisindenyldiethoxysilane (Ind2 Si(OEt)2 ) and tetraethoxysilane, TEOS (Si(OEt)4 ) under two different conditions. Chemical structure was investigated by ultraviolet spectroscopy (UV–VIS), transmittance (FT-IR) and diffuse-reflectance (DRIFTS) infrared spectroscopy, magic angle spin nuclear magnetic resonance (MAS-NMR) and X-ray photoelectron spectroscopy (XPS). Xerogel texture and structure were analysed by nitrogen sorption based on the Brunauer–Emmett–Teller (BET) method, X-ray diffraction spectroscopy (XRD), and natural scanning electron microscopy (N-SEM). Under our experimental conditions, in alkaline milieu and 1:3 indenyl/TEOS ratio, a xerogel with higher indenyl content, but nonporous and showing agglomerate patterns was produced (xerogel I). Spherical particles of diameter about 6–8 m where obtained in the absence of catalyst, using 1:5 indenyl/TEOS ratio, higher temperature and shorter reaction time (xerogel II). Xerogel II was used as support for heterogeneous metallocene catalyst synthesis. 3 −1 Zr h−1 ) The resulting catalyst presented high Zr content (0.68 mmol Zr g−1 cat ) and high catalyst activity (40 × 10 kg PE mol in ethylene polymerisation. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Supported metallocene; Sol–gel; Polymerisation; Silica
1. Introduction The preparation, characterisation and application of organic–inorganic hybrid materials have become a quickly expanding field of research in material sci∗ Corresponding author. Tel.: +55-51-319-1499; fax: +55-51-319-1499. E-mail address: [email protected] (J.H.Z. dos Santos). 1 Present address: Research Initiative for Green Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1, Higashi 1, Tsukuba, Ibaraki-ken 305-8565, Japan. 2 Deceased.
ence. Controlled chemical synthesis and design of structure, porosity, and shape of porous inorganic materials is not enough to fulfil several of their potential applications, as in sensors, chromatography, or catalysis. A controlled processing of inorganic precursors in the presence of functionalising agents is necessary to achieve both tailored microstructure and deliberate position of reactive sites [1–3]. An approach to achieve this goal lies in the use of the sol–gel process either from sols of oxides or by hydrolysis and polymerisation of alkoxides [4,5]. The latter provides a convenient method for producing chemically homogeneous and uniform
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 7 3 0 - X
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multicomponent gels. The most effective way to prepare such hybrid gels consists in covalently binding organic groups to an inorganic matrix. Organosilicon precursors can be used in which the functional organic group is bonded through a stable Si–C link to the network-forming inorganic part of the molecule. The most common precursor for silica xerogels is tetraethoxysilane (TEOS), which is readily purified by standard techniques and presents a relatively slow rate of reaction. A functional organic group R may be introduced with organotrialkoxysilane, R Si(OR)3 , where R is commonly ethyl. Organotrialkoxysilanes are usually processed in combination with TEOS to minimise the effects associated with low cross-linking. Many examples in the literature describe the use of organotrialkoxysilanes whose organic part merely aims at controlling pore size and shape in resulting meso- and microporous silica. These are removed from the target material after preparation [6–8]. In many other examples, organic units are preserved, showing their characteristic chemical and physical properties at the surface of the materials prepared, such as chelating ability [2], optical behaviour [9–11], and catalytic properties [12], just to mention a few. In parallel with the development of advanced sol–gel methods for the preparation of novel materials, olefin polymerisation has undergone an extraordinary transformation in the last 15 years. Homogeneous metallocene catalyst systems have been shown to combine high activity with excellent stereoregularity in the polymerisation of olefins, leading to enormous investment in research and development by industrial and academic investigators [13–15]. Nevertheless, heterogeneous catalyst system are required for industrial gas-phase and slurry polymerisation processes. In current technology, many heterogeneous systems are achieved by catalyst immobilisation on a suitable carrier. Besides playing a role in process design, the catalyst support commonly acts as a template for polymer particle growth, controlling size and morphology. Immobilised metallocene catalysts should also be suitable for use in current heterogeneous processes (drop-in technology). Partial success has been achieved by using inorganic carriers, in particular SiO2 [16–18], as support for metallocene polymerisation catalysts. The main disadvantages of such systems comprise leaching of catalyst active sites (specially by methylaluminoxane (MAO)
during the polymerisation process), incomplete fragmentation of the carrier matrix (complete fragmentation with exposure of new active sites is necessary to improve productivity and polymer morphology), and a drastic loss in catalyst activity as compared to corresponding homogeneous systems (in fact, about only ca. 1% of the supported species are believed to be active). Several processes for metallocene immobilisation have been reviewed in the literature [19–21], and a complete description is beyond the scope of this paper. Most of the reported immobilisation routes are based on reactions between silanol groups (either right at the surface of a support or spaced through organosilicon fragments) or ligands from a cocatalyst (Me from alkylaluminum or MAO) and ligands from a metallocene (usually chloro). Immobilised metal loading is partially limited by the number of silanol groups available to reagent molecules [22] and by steric hindrance from ligands in the metallocene [23,24]. Besides, as a variety of binding modes are generally possible at the surface of a support, formation of inactive species is likely to occur. These should at least in part explain catalyst activity drop due to immobilisation. In a previous paper, we reported the characterisation of same supported metallocene prepared by the sol–gel method [25]. The presence of indenyl groups on the surface of the xerogels allowed metallation in a further step, generating in situ metallocene species. Such approach aims at increasing the metal content on the support, which in turn might lead to higher polymer productivity. In the present study, we focus on the synthesis of two hybrid silica xerogels containing indenyl (Ind) ligands by the reaction between TEOS and Ind2 Si(OEt)2 under two deliberative conditions. The effect of preparative conditions on the resulting structural, textural and morphological properties is discussed, based on their analyses by thermal (TGA), spectroscopic (FT-IR, DRIFTS, MAS-NMR, XPS, UV–VIS, XRD), microscopic (N-SEM) and volumetric (BET) complementary techniques. 2. Experimental 2.1. Materials Indene (+85%, Kanto Chemical Co., Japan) and tetraethoxysilane (extra pure reagent, Nacalai Tesque
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Inc., Japan) were purified by vacuum distillation. Toluene (extra pure grade, from Nacalai Tesque Inc., Japan), tetrahydrofuran (THF, first grade from Kanto Chemical Co., Japan), diethylether (first grade, from Wako Pure Chemical Industries Ltd., Japan), heptane (extra pure grade, from Nacalai Tesque Inc., Japan), 1-hexene (first grade, from Wako Pure Chemical Industries Ltd., Japan) were purified according to the usual procedures. Zirconium tetrachloride (ZrCl4 , +98%, Merck-Schuchardt, Germany), diethoxydichlorosilane (ShinEtsu, Japan) and butyllithium (n-BuLi, reagent grade in hexane solution, from Kanto Chemical Co., Inc., Japan) were used without further purification. Nujol (Wako Pure Chemical Industries) was purged by nitrogen bubbling for 6 h. Ethylene (polymerisation grade, Takachiho Trading Co., Ltd., Japan) was purified through columns of NaOH (extra pure grade, from Nacalai Tesque, Inc., Japan) and P2 O5 (+98.9%, from Wako Pure Chemical Industries Ltd., Japan). Methylaluminoxane (MAO, in toluene solution), trimethylaluminum (TMA) and triethylaluminum (TEA) were donated by Tosoh Akzo Co., Japan and were used without further purification. 2.2. Synthesis of bisindenyldiethoxysilane Into a 500 cm3 two-neck flask equipped with a dropping funnel and a magnetic stirring bar were introduced 132 mmol (15.3 g) of indene and 200 cm3 of heptane. Then, 85 cm3 of a 1.57 mol/l n-BuLi solution in hexane were added dropwise at 0◦ C. After total addition, the reaction mixture was stirred at 0◦ C for 1 h, and then at room temperature for 16 h. After solvent removal, the resulting solid was washed with three aliquots of 200 cm3 of heptane, and dried under vacuum. To the solid, 150 cm3 of diethylether were added, followed by dropwise addition of 66 mmol of (EtO)2 SiCl2 in 50 cm3 of diethylether at 0◦ C. The reaction mixture was then stirred at 0◦ C for 1 h, and at room temperature for 16 h. After removal, the resulting red solid was washed with three aliquots of 200 cm3 of heptane. Heptane was removed from the extracted orange solution and the resulting oil was distilled at 40◦ C and 0.3 mmHg. 1 H-NMR (CDCl3 , 300 MHz, 25◦ C): δ = 7.79–7.76 (d, 2H, H-4), 7.57–7.55 (d, 2H, H-7), 7.54–7.36 (t, 2H, H-5), 7.34–7.27 (t, 2H, H-6), 7.11–7.09 (dd, 2H, H-3), 6.76–6.74 (dd, 2H, H-2), 4.09–4.06 (m, 4H, O–CH2 –), 3.57 (s, 2H, H-1),
289
Scheme 1.
1.41–1.35 (m, 6H, O–CH2 –CH3 ). For the sake of clarity Scheme 1 presents the numbering for the proton of indenyl. The assignments were made based on the literature [26]. 2.3. Synthesis of indenyl-silicas In a three-neck flask equipped with a reflux condenser, a dropping funnel, and a magnetic stirring bar, Ind2 Si(OEt)2 and TEOS (1:3 ratio) were cohydrolyzed (H2 O/OEt = 1:1) in ethanol (30 ml). The hydrolysis water was added in alkaline milieu (0.01N NH4 OH). The reaction was carried on for 60 h at 30◦ C. In another route, the components ratio Ind2 Si(OEt)2 :TEOS was 1:5, with water corresponding to the half of the number of ethoxy groups. The reaction was carried out in toluene at 80◦ C for 6 h. After 1 h, the ethanol produced by the hydrolysis reaction was purged under nitrogen flow and trapped in an ice/water bath. Solid obtained by the first procedure will be referred as xerogel I, while in the other case by xerogel II. In both procedures, solvents were removed by vacuum distillation. The precipitates were allowed to dry from the sol and collected as powders. 2.4. Metallocene catalyst synthesis Xerogel II (0.46 g) was reacted with n-BuLi solution in excess (6.0 cm3 ) at room temperature. The reaction mixture was then kept under reflux for 8 h and then washed with three aliquots of 30 cm3 of hexane. To the resulting solid, 1.9 g ZrCl4 ·2THF in THF were added at room temperature. The resulting brown solid was washed with 6 aliquots of 30 cm3 of THF and dried under vacuum.
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2.5. Polymerisation procedure Into a 100 cm3 stainless steel autoclave equipped with a magnetic stirrer were introduced 20 cm3 of toluene, 10 cm3 of MAO or TMA solution (1.00 mmol cm−3 ) and 1 cm3 of catalyst suspension (0.01 mmol Zr cm−3 ). After stirring at room temperature for 30 min, the system was degassed at liquid nitrogen temperature. After introducing 0.29 mol of ethylene monomer (7 dm3 at STP), polymerisation temperature (70◦ C for 1 h). The polymerisation was terminated by adding acidified methanol. The precipitated polymers were washed with methanol and dried vacuum at 60◦ C for 6 h. 2.6. Xerogel characterisation Elemental analyses were carried out on a Yamaco, CHN Corder, model MT-5. The Zr loading on the catalyst system was determined by inductively-coupled plasma optical emission spectroscopy (ICP-OES, Seiko, SPS 7700). UV–VIS analysis was performed in a DW-2000 spectrometer (Sim-Aminco, USA) equipped with a beam scrambler, which diffuses the entering light to form a uniform field of illumination, eliminating any spatial differences between beams. In order to increase the sample transparency and the viscosity, the solids were mixed with Nujol to form a slurry. All the samples were prepared in a glove box in quartz cells (1.0 cm path length). The absorption spectra were recorded under dry N2 between 250 and 550 nm, having Nujol as reference. FT-IR absorption spectra in the range of 4000– 550 cm−1 were recorded with a Jasco FT-IR spectrometer (model Valor-III) with a resolution of ±4.0 cm−1 (16 scans). Samples were analysed as pellets in KBr. A commercial diffuse-reflectance accessory with thermal control was employed for DRIFTS measurements in a JIR-7000 Jeol Spectrometer. The samples were transferred under N2 atmosphere and evacuated before, during, and between analyses. Spectra were recorded at 40◦ C, coadding 500 scans at a resolution of ±4.0 cm−1 . These spectra were collected in reflectance units and submitted to Kubelka–Munk transform. MAS-NMR spectra were recorded with a Varian 400 MHz spectrometer operating at 100.7 MHz for
13 C
and 79.6 MHz for 29 Si. Magic-angle spinning was performed at 10–12 kHz (13 C) and 3.3 kHz (29 Si) at room temperature. Peak assignment was relative to external hexamethylbenzene (13 C) and tetramethylsilane (29 Si). The number of scans varied from 1000 to 3000. Prior to spectra recording, samples were transferred in a glovebox and packed into 7.0 mm diameter zirconia rotors. NMR parameters for 29 Si were 3–5 ms (contact time), with recycle delay of 100 s. For the CP MAS 13 C NMR a contact time of 2 ms and recycle delay of 6 s were chosen. Thermogravimetric analysis was performed on a Seiko TG/DTA 320. Samples weighing 10–12 mg were heated at 10◦ C min−1 from 25 to 500◦ C under nitrogen flow. Specific surface area was determined by the BET method from N2 adsorption data at −196◦ C using a Belsorp 28AS (Bell). The samples were outgassed at 110 or 400◦ C for 20 h before measuring the nitrogen adsorption. X-ray powder diffraction patterns of the samples were obtained for angles 1◦ < 2θ < 50◦ at a scan rate of 1◦ min−1 (2θ ) with a RINT-2100 (Rigaku) automated X-ray diffractometer using Cu K␣ (P = 40 kV, I = 30 mA). Scanning electron microscopy of the xerogels was carried out in a Hitachi S-4500 digital field emission natural scanning electron microscope (Natural SEM S-3500 N). A small amount of powder was placed on a clean glass slide. Double-sided carbon tape was placed on the surface of a SEM stub. The scanning was carried out at 50 eV. The X-ray photoelectron spectra (XPS) were obtained on a PHI 5600 Esca System ( Physical Eletronics), using monochromated Al K␣ radiation (1486.6 eV). Acquisition was carried out at room temperature in high-resolution mode (23.5 eV pass energy). For the C 1s, Si 2p, and O 1s regions. The samples were mounted on an adhesive copper tape as thin films. Samples were prepared in a glove box, introduced into a transfer chamber and then evacuated at 10−6 Torr in 90 min using a turbomolecular pump. During data collection, an ion-getter pump kept the pressure in the analysis chamber under 10−9 Torr. Analysis area was 800 m in diameter. Each sample was analysed at a 75◦ angle relative to the electron detector. For each of the XPS spectra reported, an attempt has made to deconvolute the experimental curve in a series of peaks representing photoelectron emission from atoms in different chemical environments. These peaks are described as a mixture of
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Gaussian (80%) and Lorentzian (20%) contributions to take instrumental error into account together with the characteristic shape of photoemission peaks. 2.7. Polymer characterisation Polymer melting points (Tm ) were measured by differential scanning calorimetry (DSC), in a Seiko DSC 220C. The analyses were performed with a heating rate of 10◦ C min−1 in the temperature range from 25 to 200◦ C. The heating cycle was performed twice, but only the results of the second scan were recorded. Molecular weight and molecular weight distribution of the polymers were measured at 145◦ C by gel-permeation chromatography (GPC, Senshu Scientific, SSC7100) using o-dichlorobenzene as solvent. Polymer morphology was analysed by N-SEM as previously described.
3. Results and discussion 3.1. Preparation and characterisation of the indenyl-silicas Preparative protocols for silica based on the sol– gel technique are well-studied and documented in the literature [3–5]. The sol–gel process proceeds by hydrolysis and condensation reactions, which are strongly influenced by precursor concentration,
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relative component concentration in the precursor mixture, solvent nature, H2 O to alkoxy-group ratio, temperature, and pH. The solvent not only homogenises the precursor mixture, but also affects the particleand network-forming reaction, due to its polarity and viscosity [3]. Moreover, the volume of the reaction solution as well as the precursor concentration in the starting solution might influence the density of the resulting material. As silica, water, and alkoxysilanes are unmiscible, a mutual solvent such as ethyl alcohol is usually employed as homogenising agent. Furthermore, in order to achieve rapid and complete hydrolysis, acid or base catalysts may be generally employed. In order to prepare the indenyl-containing silica, we choose some arbitrary reaction conditions, namely: 3:1 TEOS/Ind2 Si(OEt)2 ratio and water amount corresponding to total hydrolysis of ethoxy groups (1:1, H2 O/OEt ratio). In this case, in which there is the addition of base catalyst (0.01 mol l−1 NH4 OH solution), alkoxysilane hydrolysis and condensation reaction is accelerated relative to hydrolysis, thus leading to generate highly branched networks [4,5]. Elemental analysis of the resulting solid (xerogel I) indicated 28.13% C and 3.96% H. Besides elemental analysis, a set of complementary technique was used to prompt the chemical composition of the xerogel produced, namely ultraviolet– visible (UV–VIS), infrared (FT-IR), photoelectron (XPS), and magnetic resonance (13 C and 29 Si MASNMR) spectroscopies. Fig. 1 shows the UV–VIS
Fig. 1. UV–VIS spectrum of xerogel I in Nujol.
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spectrum of the hybrid xerogel I measured as a slurry in Nujol, taken from the 200 to 550 nm wavelength interval. A band centred at 287.1 nm is observed. For comparative reasons, the UV–VIS spectrum of Indene was taken in toluene solution (1.4 × 10−3 mol l−1 ). It shows an intense band at 285.0 nm, in accordance with data reported in the literature to structures alike [27]. Total carbon in the xerogels cannot be thoroughly attributed to indenyl groups. Silica gels from alkoxides contain considerable amounts of organic by-products from the precursors, products of the hydrolysis and polycondensation reactions [3]. Additionally, there are ethoxy groups on the surface of the gel. For instance, aerogels made from Si(OMe)4 under basic conditions and supercritically dried with methanol present 30% of their silicon atoms still carrying a methoxy substituent [28]. The presence of such groups can be easily detected by vibrational spectroscopy. Fig. 2 shows the transmission IR spectrum of the hybrid xerogel. The main feature of silica can be observed, namely a broad absorption band at 1100–1000 cm−1 wavenumber attributed to the asymmetrical longitudinal silica lattice vibration (ν Si–O–Si ). Broad band placed between 2000 and 1850 cm−1 corresponds to skeleton (overtone) vibrations. The band at 1632 cm−1 is probably attributed to adsorbed water (bending OH) [29].
The presence of indenyl and ethoxy groups can be verified by analysis of the 3100–2800 m−1 (ν C–H ) and 1500–1300 cm−1 (δ C–H ) regions. Bands at 3069 cm−1 can be attributed to indenyl group (aromatic ν C–H ), while bands at 2981, 2930 and 2902 cm−1 to asymmetric and symmetric (ν C–H ) from the aliphatic groups. In the 1500–1350 cm−1 region are found the CH2 scissors deformation and CH3 asymmetrical deformation, which are near the same position when the substituents are the same. These deformations can be the attribution to the band present at 1460 cm−1 . Moreover, such band might include the possibility of C–H bending from the aromatic ring. Bands at 1395 and 1370 cm−1 can be attributed to the symmetric CH3 umbrella deformation. A broad band centred at 3437 cm−1 (ν O–H ) indicates the presence of C–OH and/or Si–OH groups [30]. The Si–C (from indenyl group) might give rise to a band at 1125–1100 cm−1 which is due to a planar ring vibration having some Si–C stretching character [31]. This stretching vibration occurs at the same place as the silica Si–O–Si stretching. In order to better characterise the xerogel surfaces, vibrational analysis was also performed by DRIFTS. The information carried by the signal is related uniquely to surface groups (depth ≤ x nm for incidence at x0 ) [32], while in the case of conventional transmission infrared spectroscopy, the signal
Fig. 2. FT-IR spectrum for xerogel I in the region 4000–550 cm−1 .
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contains information from both the surface and the bulk of the sample. Consistently with restriction to the samples surface, the main features of silica are attenuated. In the aromatic ν C–H region, a second band can be seen at the lower wavenumber side of the band at 3064 cm−1 , probably indicating the presence of different indenyl species on the surface. The chemical composition of the hybrid xerogel was further investigated by MAS-NMR. The 13 C CP MAS-NMR spectrum (Fig. 3a) confirms the presence of ethoxy moieties: 60.7 ppm (O–CH2 –) and 19.1 (O–CH2 –CH3 ). The presence of a large band in the low field regions (signals centred at 142.1, 133.1, 110.6 and 77.2 ppm) indicates indenyl groups. The ethoxy-to-indenyl ratio roughly estimated from peak areas as 40/60.
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The 29 Si MAS-NMR spectrum gives information on the coordination of silicon atoms at the xerogel surfaces and in the bulk silica gel. Spectrum (b) of Fig. 3 indicates three different species, which appear at −48.6, −70.0 and −101.8 ppm. The first signal corresponds to D1 species (see Scheme 2). The signal at 101.8 ppm is due to silanol (Q3 ) and the shoulder at −109.0 ppm to siloxane (Q4 ) sites [33], confirming the presence of OH groups at the xerogel surface and in the Si–O–Si network, as also seen by IR analysis. The broadness of the signal which extends from −65 to −80 ppm might suggest the presence of T species, resulting from the hydrolysis of some Si–C bonds under basic conditions. Other species proposed in Scheme 2 are expected around −50 ppm (D2 ) and −95 ppm (Q2 ) [34]. The
Fig. 3. MAS-NMR spectra of xerogel I. (a)
13 C;
(b)
29 Si.
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Fig. 3. (Continued).
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Scheme 2.
broadness of the signal at −48.6 ppm could result from the presence of both D1 and/or D2 . The likely absence of bidentate species and large excess of monodentate species in comparison to tridentate ones suggests that the indenyl ligands play an important steric effect during condensation reactions, preventing the generation of polydentate species. Silica gel surface coverage can be determined according to the uniform globular model of Amati and Sz Kovats [35], in which total carbon elemental analysis data is correlated to the specific surface area of the support, normally determined by BET nitrogen adsorption. Prior to BET measurements, thermogravimetric (TG) analysis under N2 atmosphere was performed in order to determine the thermal stability of the xerogels. A small weight loss was observed between 30 and 150◦ C, probably attributed to liberation of water and ethanol, which were physically adsorbed on the xerogel surface. A second step in mass loss was observed for temperatures above 150◦ C and could be attributed to organic moiety decomposition, including alkoxy and indenyl groups. Taking into account TGA results, BET analysis was performed by submitting the sample to thermal pre-treatment at 110◦ C
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for 20 h. The sample was shown to be nonporous, exhibiting surface area under 10 cm2 g−1 . Loy et al. [36] showed that aerogels produced with precursors such as (RO)3 Si(CH2 )n –Si(OR)3 with n = 2, 4, 6, 8, 10, and 14 exhibited a progressive reduction of specific surface area with increasing spacer length. Moreover, in the case of tetradecylmethylene, the resulting aerogel was nonporous. Similar results have been reported for hybrid xerogels from dendrimers and arborols [37]. Thermal treatment at 450◦ C for 20 h was then performed in order to remove at least organic surface groups, but no increase in surface area was observed. Powder X-ray diffraction patterns showed no clear diffraction peaks in the range 2θ = 1–50◦ , indicating that the produced xerogel is amorphous, in accordance with data reported for silica gel [38]. The analyses showed a broad band corresponding to a d-spacing of about 3.8 Å, slightly longer than that generally observed for xerogels made from TEOS (3.6 Å). The morphology of xerogel I was analysed by SEM. As we can see in picture (a) of Fig. 4, the xerogel was shown to be constituted of conglomerates with estimated size of 60 ± 10 m. Base-catalysed condensation (as well as hydrolysis) should be directed toward the middles rather than the ends of chains, leading to more compact, highly branched species [4,5]. The dependence of microstructure on xerogel preparative conditions has already been reported in the literature [39]. Final gel microstructure has been related to water content in precursor sols. Gels made from water-richer sols (as in the present case) show coarser surface microstructure as compared to those produced from sols with lower water content. It is worth mentioning that the formation of spherical particles is extremely important from the polymerisation catalyst point of view, since a better morphology control of polymer particles results. The employed reaction conditions so far showed that by the sol–gel method, the Si–C bond can be preserved, generating a silica containing indenyl groups on the surface. We performed another attempt of indenyl-containing silica synthesis, using different reaction conditions (xerogel II). We performed the reaction in absence of any catalyst using 5:1 TEOS/Ind2 Si(OEt)2 ratio and using water amount corresponding to partial hydrolysis of ethoxy groups (3:6 H2 O/OEt). No ethanol was added to the alkoxide–water mixture, since the alcohol generated
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as a by-product of the hydrolysis reaction was sufficient to homogenise the initially two-phase system. In order to avoid the reaction equilibrium towards alkoxy species regeneration, produced ethanol was removed by distillation in a second step, as proposed in the literature [40]. Elemental analysis indicated 20.66% C and 3.46% H, i.e. ca. 25% less C than in the previous case. This
could be in part due to the comparatively lower amount of the C-rich precursor Ind2 Si(OEt)2 employed. In effect, the longer reaction time employed previously (60 h versus 6 h) aimed at guaranteeing the incorporation of the bulky Ind moieties. Schwertfeger et al. [28], under alkaline conditions, demonstrated that in the case of a mixture containing TEOS and R Si(OR)3 , where R = alkyl, aryl, the rate of hydrolysis and
Fig. 4. SEM micrographs: (a) xerogel I; (b) xerogel II; (c) polyethylene produced with xerogel II.
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Fig. 4. (Continued).
condensation of R Si(OR)3 is slower that that observed for the corresponding tetraalkoxysilane Si(OR)4 . The R -substituted silane units condense to the existing network much later, after hours or even days [41]. The same analytical techniques were employed for the characterisation of this xerogel. UV–VIS and FT-IR analysis showed similar results as in the proceeding case. 13 C CP-MAS-NMR roughly indicated a higher content of ethoxy groups (60%). According to 29 Si MAS-NMR, the signal content related to Q4 is much lower, suggesting a less branched structure. Under the same experimental conditions, BET measurement showed that this xerogel was seen to exhibit also an extremely low surface area (<5 m2 g−1 ) with an extremely high C constant value (71), which in part can be attributed to the surface polarity exerted by organic groups. Contrarily to the previous case, after thermal treatment at 450◦ C for 20 h, the xerogel was shown to have 226 m2 g−1 as surface area (Vp = 41.7 mm3 g−1 ; Rp = 0.95 nm), indicating the presence of nanopores. It is worth mentioning that heating at 450◦ C is not high enough to cause rearrangement of the silica network (tetrahedral rearrangement takes place at temperatures higher than 550◦ C) [42]. Therefore, the increase in surface area can be attributed to decomposition of steric demanding surface
groups (indenyl), liberating blocked pores, besides the formation of new pores by decomposition of ethoxy groups. Similar results have been reported in the literature for temperatures treatments between 300 and 500◦ C. The increase of specific surface area with temperature was attributed to the formation of new pores due to evaporation of physisorbed water and oxidation of residual organic groups in gels [39]. Since xerogel drying was made under vacuum, shrinkage and mostly destruction and collapse of an initially highly porous network may have occurred. The gelation rate also influences pore structure. Fast gelation (as expected for the present case) yields an open structure, because the particles are quickly connected and cannot undergo further rearrangments [3]. SEM imaging was also performed. In this case, the resulting xerogel (picture (b) of Fig. 5) appears as distributed spherical particles of size about 6–8 m. As already mentioned, from the polymerisation point of view, such support is much more suitable due to its roughly spherical shape. Nevertheless, the particle diameter is extremely small, if compared to xerogel. It is worth noting that from the technological point of view, bigger catalyst particles might be less prone to processing problems than very fine ones. It seems that structural variations (porosity, inner surface area) in functionalised xerogels are not
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Fig. 5. XPS spectra curve fitting. The small inserted peaks are the curve fit components, which are summed to obtain the smooth drawn line. The line with visible noise component is the experimental raw data. (a) Si 2p; (b) O 1s; (c) C 1s.
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299
Fig. 5. (Continued).
induced as much by the properties of the organic ligand as by the effect of the presence of an organosilane on the sol–gel process itself. It is believed that the alkoxyorganosilane basically acts as a co-solvent in the first reaction step, therefore diluting the tetraalkoxysilane and leaving a larger amount of water and catalyst available for reaction, resulting in larger particles [3]. On the other hand, morphology (pore size, surface) is mostly dependent on the nature of precursors than on reaction conditions. Some studies involving the synthesis of aerogels showed that the length of alkyl ligands in compounds such as (RO)3 SiR decisively is accelerated relative to hydrolysis. The rate of condensation enhances with increasing number of siloxane linkages. Thus, highly branched networks with ring structures are formed, generating therefore large, bulkier and ramified polymers. Both hybrid xerogels were further analysed by XPS. Fig. 5 presents the XPS spectra of the different components of xerogel I. From the Si 2p spectrum (Fig. 5a), Si atoms in the sample bulk (103.3 eV) can be distinguished from those on the surface, which are chemically bound to the organic ligands (101.1 eV). The species at high and low binding energies correspond, respectively, to 79 and 21% (see Table 1) of the Si atoms on the probed region of both xerogels. For xerogel II, the low binding energy component of the Si
Table 1 XPS data of the xerogel components Photoelectron Xerogel I Si 2p O 1s C 1s Xerogel II Si 2p O 1s C 1s a b
BEa (eV)
Relative elemental abundance (%)
FWHMb (eV)
103.3 101.1 533.6 531.8 285.0 287.0
79.0 21.0 88.0 12.0 95.9 4.1
2.3 2.3 2.1 2.2 2.1 1.3
103.3 101.8 533.6 531.6 285.0 287.3
79.0 21.0 77.0 23.0 95.7 4.3
2.7 3.7 2.4 3.1 2.7 1.4
Reference: Si 2p from SiO2 (103.3 eV). Full width at half maximum intensity.
2p spectrum is centred at 101.8 eV. Such behaviour might be probably due to a lower electron density on Si atom from the O groups (ethoxy groups) which are in a higher amount in xerogel II. According to the literature [43,44] species such as O2 SiC2 on xerogel surfaces can be distinguished by XPS analysis. In order to verify the possibility of
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distinguishing in XPS spectra silicon atoms bonded to oxygen in the network (as in Si–O–Si) from those bonded to surface ethoxy groups (Si–O–C) or Ind (Si–C), a silica containing exclusively ethoxy groups on its surface was prepared by Si(OEt)4 grafting and analysed according to the procedure above. Analyses showed that the best fitting corresponded to the deconvolution of two signals: 103.3 and 101.7 eV, corresponding roughly to 90 and 9% of total area, being the lowest BE signal attributed to surface (Si–O–C) ethoxy groups. Thus, under our analytical experimental conditions, we could not distinguish Si atoms bonded to organic or to alkoxy moieties. Part (b) of Fig. 5 shows the O 1s XPS spectrum of xerogel I. The signal centred a 533.6 eV binding energy corresponds to silica bulk oxygen, while oxygen from ethoxy groups appears at 531.8 eV. The low BE component is shifted to 531.6 eV in the case of xerogel II. Moreover, the low BE peaks for xerogels I and II show, respectively, full width at half maximum intensity (FWHM) of 2.2 and 3.1 eV, suggesting a larger heterogeneity of ethoxy species on xerogel II. The low BE signal corresponds to 12% of the O 1s spectrum for xerogel I and to only 23% for xerogel II, in accordance to the increased amount of ethoxy groups as previously pointed out. The high resolution of XPS analysis of the C 1s peak from xerogel I (Fig. 5c) indicates that 95.9% of this peak is contributed by C–C bonds of hydrocarbons (285.0 eV BE). The remaining is due to atoms presenting a C–O bond (287.0 eV BE). 3.2. Catalyst preparation and olefin polymerisation According to these results, under the two different experimental conditions employed in the present work, xerogel containing indenyl groups can be obtained. Since the xerogels presented water, alcohol, and ethoxide groups on its surface, xerogel II was initially treated with n-BuLi in excess to consume them. In spite of the lower area, we could not perform a thermal treatment to increase it due to stability of the indenyl groups bond to silica surface. Metallation with ZrCl4 ·2THF generated zirconocene supported systems containing 0.68 mmol Zr g−1 cat . Such metal content is about seven times higher than those usually
obtained by metallocene grafting on silica [45,46]. This catalyst amount is about 15% higher than that obtained in the case of in situ metallocene supported synthesis [47]. The approach of metallation of a support already containing the metallocene ligands seems to provide a route to increase the catalyst metal content, overcoming the limitation of number and accessibility of silanol groups. The characterisation of this catalysts by further complementary spectroscopic techniques, namely UV–VIS, XPS, SEM, inductively-coupled plasma-atomic emission spectroscopy (ICP-AES), electron probe microanalysis (EPMA), matrix-assisted laser desorption/ionisation time-of-flight mass spectroscopy (MALDI-TOF-MS) is reported elsewhere [25]. Ethylene homopolymerisation tests were performed using Al/Zr = 1000 (Table 2). The catalyst system showed much higher activity — 40 × 103 kg PE mol−1 Zr h−1 — than ordinary supported systems under similar polymerisation conditions [48,49]. Moreover, such supported systems seems to be thermally stable, since no significant loss in activity could be observed for polymerisation performed at 120◦ C. The generation of supported surface species might avoid eventual bimolecular deactivation reaction which are more prone to take place in the case of homogeneous systems. Low activity was observed in the case of polymerisation with TMA. It is worth mentioning that metallocene catalysts are not usually active in the presence of common alkylcatalysts. On the other hand, as in the present case, some supported metallocene catalysts have shown to be active in the presence of cocatalysts such as TMA, TEA or TIBA [50]. Copolymerisation tests were performed with 1-hexene. The higher activity is typical of comonomer effect, in which the higher solubility of the copolymer in the reaction milieu allows higher diffusion rates of the monomers onto the active sites, therefore, enhancing the copolymerisation activity. The high molecular weight observed in the case of copolymer might suggest low comonomer incorporation. The resulting (co)polymers showed high melting temperatures (close to 143.5◦ C). Molecular weight are very high, typically observed in the case of supported metallocene. This fact has been attributed to the blocking of one of the sides of the active site by the support, hindering the deactivation step. In other
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Table 2 Catalyst activity and resulting polymer properties of supported metallocene prepared with xerogel IIa Cocatalyst
Activity (103 kg PE mol−1 Zr h−1 )
Mw (10−6 )
Mw /Mn
Tm (◦ C)
MAO MAOb TMA MAOd MAOe
40 35 5 105 55
1.0 1.0 nsc 0.2 0.2
3.8 3.5 ns 3.4 3.3
141.8 140.5 141.6 141.8 140.3
Polymerizations were performed in toluene at 70◦ C, with [Zr] = 0.01 mmol ml−1 [Al/Zr] = 1000. 120◦ C. c Not soluble. d Copolymerisation with 0.26 mol cm−3 of 1-hexene. e Copolymerisation with 1.30 mol cm−3 of 1-hexene. a
b
words, the -elimination transfer between two metallocene centres is hindered, resulting in a larger growth of the polymer chain, and so in a higher molecular weight [51]. Metallocene catalysts usually produce polymer with polydispersion index around 2.0 The relative broad molecular weight distribution suggest a heterogeneity in the nature of the active sites, which was confirmed by XPS measurements [52]. Catalyst replication was evaluated by performing the ethylene homopolymerisation reaction at atmospheric pressure by bubbling ethylene at 30◦ C and low Al/Zr ratio (500), for 10 min. Under these polymerisation conditions (Fig. 4c), we could observe that the roughly spherical morphology of the support is partially replicate during the polymerisation.
4. Conclusion Hybrid xerogels containing indenyl groups on their surface can act as support for the in situ synthesis of supported metallocene catalysts. Experimental parameters influence in the nature of the surface species, as well as in the texture and morphology of the support. However, a systematic study is necessary to better evaluate, understand and control the influence of the several experimental parameters, in order to increase the particle diameter and indenyl content, improving the spherical morphology. Further experiment are in progress to study the effect of experimental parameters on xerogel synthesis, as well as on final zirconocene catalyst preparation.
Acknowledgements Financial support for Prof. J.H.Z. dos Santos has been provided by the Japan Society for Promotion of Science (JSPS). References [1] R. Corriu, C. Chuit, G. Dubois, C. Reye, R. Guilard, S. Brandes, in: Proceedings of the International Workshop on Si Chemistry Polymers, Ishikawa, Japan, 29–31 May 1999, p. 97. [2] U. Schubert, N. Hüsing, in: Proceedings of the International Workshop on Si Chemistry Polymers, Ishikawa, Japan, 29–31 May 1999, p. 91. [3] N. Hüsing, U. Schubert, Angew. Chem. Int. Ed. Engl. 37 (1998) 22. [4] C.J. Brinker, G.W. Scherer, Sol–Gel Science, The Physics and Chemistry of Sol–gel Processing, Academic Press, London, 1990, Chapter 3, p. 97. [5] E.F. Vansant, P. Van der Voort, K.C. Vrancken, Stud. Surf. Sci. Catal. 93 (1995) 15. [6] B. Boury, P. Chevalier, R.J.P. Corriu, P. Debid, J.J.E. Moreau, M.W. Chi Man, Chem. Mater. 11 (1999) 281. [7] T. Sugizaki, O. Moriya, Y. Nakamura, T. Endo, M. Oikawa, T. Kageyama, Polym. J. 31 (1999) 625. [8] J. Wen, B. Dhandapani, S.T. Oyama, G.L. Wilkes, Chem. Mater. 9 (1999) 1968. [9] H.K. Kim, S.-J. Kaug, S.-K. Choi, Y.-H. Min, C.-S. Yoon, Chem. Mater. 11 (1999) 779. [10] G.-H. Hsiue, R.-H. Lee, R.-J. Jeng, Polymer 40 (1999) 6417. [11] C. Sanchez, F. Ribot, B. Lebeau, J. Mater. Chem. 9 (1999) 35. [12] B. Cetinkaya, T. Seckin, L. Özdemir, B. Alici, J. Mater. Chem. 8 (1998) 1835. [13] J. Scheirs, W. Kaminsky (Eds.), Metallocene-based Polyolefins, Vol. 1, West Sussex, 2000.
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