Detitanation of MgCl₂-supported Ziegler-Natta catalysts for the study of active sites organization
2008
Ribour, David | Monteil, Vincent | Spitz, Roger
Thermal treatments under vacuum of conventional supported Ziegler-Natta precatalysts (MgCl₂/TiCl₄/Dibutylphthalate) were conducted to gradually remove titanium to modify the active sites distribution. Only limited detitanations of precatalysts were achieved paying attention not to chemically alter the internal donor (T < 150 °C). Used in combination with the required cocatalyst and external donor in the propylene slurry polymerization, the modified precatalysts exhibited a drop of activity versus decreasing titanium content but the distributed polymer properties are almost not affected (a slight narrowing of molecular weight distribution was observed). After a titanium chloride secondary impregnation (possibly done in presence of an additional Lewis base), activity resumed but polymer properties are once again unchanged. These findings highlight the difficulty to separate the different families of active sites and lead us to propose a cluster organization of titanium active sites. Active sites are composed of titanium clusters having a size distribution at the precatalyst surface, possessing a critical operating size and operating collectively in polymerization.
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