Mechanism of formation and structure of active centres formed during fixation of organometallic catalysts on support surface
Mechanism of formation and structure of active centres formed during fixation of organometallic catalysts
on support surface
Polimery 1989, No 6-7, 293
Model studies of the process of deposition from gas phase of the components of organometallic catalysts — TiCl4, AlEt3, and AlEt4Cl — on aerosil (supoprt) surface have been carried out with a precise adsorption method. Two variants of catalyst synthesis, differing in the sequence of deposition of the organoaluminium and titanium compound on the supoprt. The thus-prepared supported complexes have been used as catalysts of the gas-phase polymerization of ethylene and propylene. It has been found that in the case of SiO2/TiCl4-AlEt2Cl or AlEt3 system, titanium is present on the surface in the form of isolated complexes and no ТiСlз phase is formed. Such complexes are active in the polymerization of ethylene and have low activity in the polymerization of propylene, leading to a high-molecular-weight polypropylene containing to 30% atactic fraction. The olefin polymerization process in this case is a typical ionic-coordinate process. A change in the sequence of deposition of components of the catalytic system leads to the formation of different active centres. In the case of SiO2/Et3Al-TiCl4 system there is formed on the support surface the brown form of TiCl3-phase. This catalyst is several times more active both in the polymerization of ethylene and propylene. Its annealing at a temperature of 200°C gives the violet form of TiCl3, which is even more active in the polymerization of olefins. The substitution of Et2AlCl for AlEt3 in this catalyst results in the formation of cationic active centres. These centres are inactive in the polymerization, of ethylene and are highly active in the polymerization of propylene with the formation of low-molecular-mass irregular polypropylene. Thus, it has been found, that depending on the method of synthesis of a supported organometallic catalyst, three types of active centres can be obtained, differing in their character, activity, and stereospecifity.
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