Supported metallocene on mesoporous materials
Graphical abstract
A series of hybrid supported catalysts was prepared by sequentially grafting Cp2ZrCl2 and (nBuCp)2ZrCl2 (1:3 ratio) onto alumino-silicates, alumina and chrysotile. The supported catalysts were characterized by Rutherford backscattering spectrometry, atomic force microscopy and nitrogen adsorption grafted metal content. Catalyst activity and polyethylene molecular weight were shown to be dependent on textural properties of the supports.
Introduction
The discovery of metallocene methylalumoxane catalyst systems has opened a frontier in the areas of organometallic chemistry, polymer synthesis, and processing. Metallocene-based polymers, ranging from crystalline to elastomeric materials, have been commercially available since about 1991. Most early applications have been in specialty markets in which value-added and higher priced polymers can compete. Impact strength and toughness, better melt characteristics, and improved clarity in films are some of the properties of the polymers produced by such catalyst systems. Nevertheless, homogeneous metallocene catalysts are in most cases unsuitable for the production of polyethylene or polypropylene on industrial scale. In order to use them in the existing technical processes, metallocenes have to be supported [1].
Silica has been the most studied support, both in open and patent literature. Other supports, among them mesoporous materials, such as MCM (mobil composition of matter) and SBA (Santa Barbara amorphous), have been less investigated. Examples of MCM-41 are relatively more numerous [1] and include the preparation of syndiotactic polypropylene [2] of nano-polyethylene fibers [3], [4] through ethylene extrusion polymerization using the mesoporous of the support, or as support for the immobilization of alkylaluminum [5]. A new bis(indenyl) zirconocene bearing a pendant Si–Cl group has been grafted onto MCM-41, SBA-15, MCM-48, which were shown to be active for the production of isotactic polypropylene [6]. Chromium alkyl complex Cp*Cr(pyridine)(methyl)2 was grafted on MCM-22 via methane elimination [7], which was active on ethylene polymerization, using MAO as the cocatalyst. SBA-15 was used as the support for zirconocene in situ synthesis [8] and for Cp2ZrCl2 grafting [9], [10]. Sulfonic acid functionalized SBA-15 silica was recently proposed as a methylaluminoxane (MAO)-free cocatalyst/support for ethylene polymerization [11]. The (nBuCp)2ZrCl2 [12] and Cp2ZrCl2 [13] were grafted on alumina for the production of polyethylene, exhibiting low activity.
In a previous work, we reported the effect of grafting Cp2ZrCl2 and (nBuCp)2ZrCl2 on the same support (silica) at different order and molecular ratios, on the catalyst activity and on the polymer properties. The best catalyst system was that obtained by grafting Cp2ZrCl2 followed by (nBuCp)2ZrCl2 in a 1:3 ratio [14], [15]. In a subsequent paper, both catalysts were grafted on silica supports produced by sol–gel (xerogel and aerogel) and precipitation methods [16].
In the present study, we comparatively investigated the effect of different supports, namely, MCM-41, MCM-22, ITQ-2, SBA-15 and chrysotile (native and leached) on the heterogeneization of a mixture of Cp2ZrCl2 and (nBuCp)2ZrCl2 grafted on the support in a 1:3 ratio in terms of catalyst activity and polymer properties. For comparative reasons, commercial silica and alumina were also investigated. ITQ-2 is a zeolitic material described as a delaminated zeolite [17]. Chrysotile is a kind of natural nano-fibriform mineral, containing silica and brucita. As far as we know, these two supports have not been investigated in the heterogeneization of metallocenes. Chrysotile has been used as support for metalloporphyrins catalysts which were active for oxidation of cyclohexane [18]. ITQ-2 has been used as support for Mn(III) salen in the production of chiral epoxides [19].
The supported catalysts were characterized by Rutherford backscattering spectrometry (RBS), atomic force microscopy (AFM) and nitrogen adsorption. The resulting hybrid supported catalysts were evaluated in ethylene polymerization, with MAO as the cocatalyst. The polymers were characterized by gel permeation chromatography (GPC) and differential scanning calorimetry (DSC).
Section snippets
Materials
All the chemicals were manipulated under inert atmosphere using the Schlenk technique. Chrysotile was gently provided by SAMA (Goiás, Brazil), SBA-15 was prepared at Instituto de Tecnología Química de Valencia-ITQ-UPV (Spain) and MCM-41 was prepared at Japan Advanced Institute of Science and Technology-JAIST (Japan). Silica (Grace 956), alumina (INLAB), (nBuCp)2ZrCl2 (Aldrich), Cp2ZrCl2 (Aldrich) and MAO (Witco, 10.0 wt.% toluene solution) were used without further purification. Pure grade
Results and discussion
Mesoporous silicates bear surface silanol (Si-OH) groups which can be present in high concentration, like in amorphous silica. Such groups can act as grafting sites for metallocene: silanol groups are capable of reacting with sequestering agents such as organometallic chlorides and alkoxides, with the elimination of one or more of the original ligands. In the present case, zirconocenes are grafted on the mesoporous supports by elimination of the chloride ligand with hydrogen atoms from the
Conclusions
Less common supports such as alumino-silicates or chrysotile were shown to afford active catalyst after metallocene grafting. In terms of catalyst activity, the resulting supported catalysts are less active than produced by grafting on silica surface. Nevertheless, particular textural characteristics afford peculiar properties to such systems. The possibility of grafting within or outside the pores seems to influence catalyst activity. For instance, S15 with the larger pore allows the
Acknowledgments
The authors thank Ipiranga Petroquímica for GPC analysis, and CNPq for the financement. Mr. William Bretas Linares from SAMA is specially thanked for providing chrysotile samples.
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