Typical Catalysts
The following is a typical recipe for the preparation of a supported Ziegler-Natta catalyst, with the aim of highlighting the steps and the changes that occur in the compounds used as the various steps are carried out.
All operations are performed under a controlled atmosphere (free of H2O and O2) in a glove box and Schlenk flasks.
1) Dehydration:
1.a) Start with 75 grams of pure Cl2Mg;
1.b) It is treated for 30 minutes with a stream of pure ClH (g) at 380 C; at the end of this process, approximately 8 ml of H2O is recovered.
1.c) The temperature is raised to 430 C in an argon atmosphere, and the assembly is held at that temperature for 1 hour, followed by 5 hours under vacuum.
The anhydrous Cl₂Mg obtained in this way is called Support I
2) Addition of ethyl benzoate (EB) and grinding:
2.a) To achieve a partial modification of the catalytic substrate (support in the broad sense, but with an impact on the stereospecificity of the resulting catalyst), EB is added at a ratio of 0.155 moles of EB per mole of Cl₂Mg.
2.b) It is ground in a “ball” mill loaded with cylindrical porcelain balls.
2.c) The ground product is washed five times with n-heptane and thoroughly dried.
The partially modified Cl2Mg obtained in this way is called Support II.
3) Addition of p-cresol:
3.a) To complete the modification of the catalytic substrate in order to improve the stereospecificity of the resulting catalyst, p-cresol (PC) is added at a ratio of 0.5 moles of PC per mole of Cl₂Mg. The addition is carried out by adding the modifier drop by drop to a suspension of Support II in triple-distilled anhydrous n-heptane over the course of one hour under continuous stirring.
3.b) The solid product is washed with fresh n-heptane and dried. The Cl2Mg modified in this way is referred to as Support III.
4) Addition of Triethylaluminum (TEA):
4.a) Partial activation of the catalytic substrate, to which the transition metal will later be added, is achieved by adding TEA at a ratio of 2 moles of TEA per mole of PC. Once the activator has been added, the mixture is stirred for one hour. During this operation, one mole of C2H6 is released for every mole of PC (this is verified by collecting and subsequently analyzing the gases released).
4.b) The solid product is washed, recovered by filtration, and dried.
The modified and partially activated Cl2Mg obtained in this way is called Support IV.
5) Addition of Titanium Chloride:
5.a) Finally, the transition metal is added by reacting, at 100 C, 10 ml of Cl4Ti per 1.55 grams of Support IV suspended in 5 ml of n-heptane.
5.b) The solid product is washed, recovered by filtration, and dried.
The resulting solid is the catalyst. Final activation takes place in the polymerization reactor through the addition of the corresponding cocatalyst.