Document Type : Original research
Authors
Graduate School of Science & Engineering, Saitama University 255, Shimo-Okubo, Sakura-ku, Saitama-shi, Saitama, 338-8570 Japan
Abstract
MAO-free supported catalysts for ethylene polymerization were prepared by sequentially treating montmorillonites (MMTs) with R₃Al and then with zirconocene. MMTs possessing different physicochemical properties—such as surface area, acidity, and crystallinity—were employed for catalyst preparation. Three zirconocenes, Cp₂ZrCl₂, (1,3-Me₂Cp)₂ZrCl₂, and Me₂Si(Cp)₂ZrCl₂, were used. Acid treatment of raw MMT (Na⁺-montmorillonite) increased the surface area while decreasing both the Al content and crystallinity. The maximum surface area (316 m² g⁻¹) was obtained when the residual Al content was 18%. The effects of triethylaluminum (TEA) and triisobutylaluminum (TIBA) on catalyst performance were investigated. Although treatment of surface OH groups was necessary to obtain the zirconocene-supported catalysts with high activity, no significant difference in catalytic activity was observed between TEA- and TIBA-treated MMTs. In contrast, the type of R₃Al used as a scavenger during polymerization strongly influenced catalytic activity: catalysts employing TEA exhibited much lower activity than those using TIBA. Since TEA is a stronger Lewis acid than TIBA, it likely coordinates more strongly to the active sites, thereby acting as an inhibitor. Therefore, TIBA was employed in all subsequent experiments. The catalytic activity, based on catalyst weight, increased with both surface area and the amount of supported zirconocene, reaching a maximum when the residual Al content was 54%. A linear correlation between the activity (per catalyst weight) and the amount of supported zirconocene was observed for two zirconocenes, Cp₂ZrCl₂ and (1,3-Me₂Cp)₂ZrCl₂, indicating that the supported zirconocenes functioned uniformly as active species for ethylene polymerization. However, 5–7 μmol g⁻¹-cat of zirconocene formed inactive species in both catalysts. Unusual behavior was observed for the supported Me₂Si(Cp)₂ZrCl₂ catalysts: at higher residual Al content, the correlation was similar to that of the other two catalysts, whereas at lower residual Al content, most of the supported zirconocene formed inactive species, and only a small fraction acted as active sites. We speculate that Me₂Si(Cp)₂ZrCl₂ is difficult to activate, and that only the species formed by reaction of strong acid sites with R₃Al can specifically activate this complex.
Graphical Abstract
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Keywords
- metallocene catalyst
- ammonia adsorption
- layered aluminosilicate
- methylalumoxane free
- olefin polymerization
Main Subjects
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