Fine-Tuning Catalysts

ELECTRON-DONOR components of commercial Ziegler-Natta polymerization catalysts dictate the structure of the catalyst's active site, according to a new study (J. Catal. 2008, 257, 81). The study provides new avenues for probing details of catalytic reaction mechanisms and may lead to more active and chemically selective catalysts for tailoring molecular properties of polymers.

Ziegler-Natta catalysts drive commercial polyolefin production. They consist of titanium tetrachloride supported on magnesium dichloride crystallites and include aluminum alkyl cocatalysts and organic electron-donor compounds. Although the well-studied catalytic materials mediate polyethylene and polypropylene production on an industrial scale, some aspects of the catalytic system remain poorly understood.

For example, researchers have speculated that the electron-donor compounds may play a role in enhancing polymerization by controlling the geometry of MgCl₂ crystals and the orientations of their external surfaces. That proposal is based on the fact that although all of a crystal's facets are similar in chemical composition, subtle differences in their structures often lead to vast differences in their catalytic properties. In the case of Ziegler-Natta catalysts, little evidence has been available until now of the cause-and-effect relationship between electron donors, crystal geometry, and polymerization.

By using microscopy methods to monitor catalyst preparation and olefin polymerization, Hans J. W. Niemantsverdriet, Adelaida Andoni, and Peter C. Thüne of Eindhoven University of Technology, in the Netherlands, and coworkers obtained images that establish that relationship.

Specifically, the team showed that polymerization occurs strictly along the perimeter of the crystals (like the narrow edge of a coin) and that the nature of the electron donor dictates the structure of these catalytic hot spots. For example, diethers lead exclusively to hexagonal crystals with 120º edge angles and so-called (110) edge facets, which are active and stereoselective catalytic surfaces. In contrast, monoesters and diesters lead to square and hexagonal crystals (90º and 120º edge angles) and a combination of (110) and (104) edge facets, reducing catalytic activity and broadening the molecular weight distribution of the polymer.