(11a) Insight into the Formation Mechanism of an Industrially Relevant Ziegler-Natta Catalyst

Authors: 
Klaue, A., ETH Zürich
Wu, H., ETH Zürich
Morbidelli, M., ETH Zürich
Kruck, M., SABIC
Bertola, F., SABIC

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 (MgClx(OR)2-x):

 

Mg(OR)2
+ AlRCl2 à MgClx(OR)2-x+AlRCl2-x(OR)x
 

Although
the precipitation of alkoxytitanium chloride species (TiCly(OR)4-y)
also starts at the very beginning,

 

Ti(IV)(OR)4
+ AlRCl2 à Ti(IV)Cly(OR)4-y+AlRCl2-y(OR)y,

 

it
proceeds at a much slower pace, such that the precipitation of MgClx(OR)2-x
completes much earlier than that of TiCly(OR)4-y. In
fact, according to SEM-EDX measurements, TiCly(OR)4-y precipitates
mainly on the surface of MgClx(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-xClx+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.