Detitanation of MgCl₂‐supported Ziegler‐Natta catalysts for the study of active sites organization

Abstract

Thermal treatments under vacuum of conventional supported Ziegler-Natta precatalysts (MgCl 2 /TiCl 4 /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 8C). 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.

Strong activation of MgCl2‐supported Ziegler‐Natta catalysts by treatments with BCl3: Evidence and application of the “cluster” model of active sites

“…Considering active sites at the precatalyst surface according to the “cluster” model,10 we aimed to modify their organization to change their properties (activity, selectivity). From this idea, we envisaged to replace some titanium atoms of clusters by another element.…”

“…We have recently demonstrated that there exists no simple ways to separate the different families of active sites at the catalyst surface by various chemical or thermal modifications and thus to approach a “single‐site” behavior from a “multisite” catalyst 9, 10. From this observed collective behavior, we assumed a new model of active sites based on a “cluster” organization of titanium atoms at the catalyst surface 10…”

DFT Study of Lewis Base Interactions with the MgCl2 Surface in the Ziegler−Natta Catalytic System: Expanding the Role of the Donors

“…The commonly accepted roles [29][30][31][32] of the internal and external donors during the polymerization appear to be to bind to the (110) lateral cuts of MgCl 2 and thereby provide (i) stability to the more acidic, four-coordinate (110) MgCl 2 surfaces and (ii) stereoregulating control to active titanium catalysts adjacently bound to the same (110) MgCl 2 surface. Therefore, the manner in which the donor can bind to the MgCl 2 surface is a question of significance.…”

Influence of Internal Donors on the Performance and Structure of MgCl2 Supported Titanium Catalysts for Propylene Polymerization

“…Spitz et al recently reported that internal donors might not have any direct role in the active site formation. [13] Thus, the role of internal donors for stabilizing MgCl 2 crystallites and blocking nonstereospecific/low-productivity sites became very critical.…”