Insight into the Formation Mechanism of an Industrially Relevant Ziegler-Natta Catalyst

Abstract for AIChE 2017                                                                                                             Antoine Klaue  

Insight into the formation mechanism of an industrially relevant Ziegler-Natta catalyst

Antoine Klaue¹, Hua Wu¹, Massimo Morbidelli¹, Matthias Kruck², Nic Friederichs², Francesco Bertola²

¹Institute for Chemical and Bioengineering, department of Chemistry and Applied Biosciences, ETH Zürich, 8093 Zürich, Switzerland

²SABIC, Technology & Innovation, P.O. Box 319, 6160 AH Geleen, The Netherlands

Ziegler-Natta (ZN) type polymerization processes have become very mature in terms of engineering and optimization [1]. However, few studies in the open literature exist dealing with the detailed chemistry during the formation of the ZN catalyst particles [2], of which knowledge is important in controlling the size and morphology of the ZN catalyst particles, thus those of the final polymer beads. In this work, we take a typical industrial ZN recipe to monitor the formation mechanism of the ZN catalyst particle. The recipe is a simple one-pot process [3], where an excess amount of aluminum alkyl dichloride is added into a precursor composed of magnesium and titanium alkoxides dissolved in hexane under agitation, leading to precipitation of the ZN catalyst particles. Through a combined technique based on ICP-OES, we have monitored the partition of the key elements in the liquid and solid phases along the catalyst formation. It is found that the first main reaction is the rapid precipitation of alkoxymagnesium chloride (MgCl_x(OR)_2-x):   Mg(OR)₂ + AlRCl₂ à MgCl_x(OR)_2-x+AlRCl_2-x(OR)_x   Although the precipitation of alkoxytitanium chloride species (TiCl_y(OR)_4-y) also starts at the very beginning,   Ti(IV)(OR)₄ + AlRCl₂ à Ti(IV)Cl_y(OR)_4-y+AlRCl_2-y(OR)_y,   it proceeds at a much slower pace, such that the precipitation of MgCl_x(OR)_2-x completes much earlier than that of TiCl_y(OR)_4-y. In fact, according to SEM-EDX measurements, TiCl_y(OR)_4-y precipitates mainly on the surface of MgCl_x(OR)_2-x. We have also observed that at the later stage, further addition of aluminum alkyl dichloride leads to the formation of soluble bimetallic complexes of titanium and aluminum, leading to partial dissolution of titanium back to liquid phase, as shown in Figure 1. However, both the dissolved and precipitated complexes, once heated, lead to Ti(III)-type precipitation in the form of Ti(OR)_2-xCl_x+1.

Figure 1 Composition measurement of the liquid phase during the particle formation process by ICP-OES. The dashed line symbolizes the feed time.

[1] Böhm, L. L., Angew. Chem. Int. Ed. 2003, 42, 5010-2030

[2] Chien, J. C. W., J. of Polym. Sci., 1982, 20, 2019-2032

[3] Berger E., Derroitte, J.-L. (Solvay Cie, Brussels, Belgium). Polymerization of olefins,  U.S. Patent, August 26, 1975.