The roles for a Lewis base and MgCl2 in third-generation Ziegler-Natta catalysts

Abstract

Activity of a titanium catalyst supported on a bimetallic magnesium-aluminium system, involving a Lewis base [MgCl 2 (THF) 2 /Al(C 2 H 5 ) 2 Cl], was studied in ethylene polymerization, and the effect of the catalyst composition on the properties of the final polymer produced was investigated. Analysis and discussion of the findings covering also some part of the kinetic study, resulted in defining the roles for MgCl 2 and a Lewis base in the third-generation Ziegler-Natta catalysts. MgCl 2 forms a bimetallic complex with an organoaluminium compound, which involves also a Lewis base. Its reaction with TiCl 4 yields a very active catalyst wherein the Mg : Ti molar ratio is close to 1. This means that MgCl 2 is involved in creating catalytic active sites. The structure of these catalytic sites were suggested. Hence, the role for MgCl 2 can be twofold: it is a component of a catalytic active site, and it is a support if Mg : Ti ú 5. It was found in the study that the catalytic function of MgCl 2 can be maintained while its supporting function can be omitted. Thus, MgCl 2 can be substituted for Al 2 O 3 and the catalytic system obtained will have the same activity.

Olefin Polymerisation Catalysts

“…The effect of using AlEt 2 Cl to isolate the titanium compound from direct contact with the MgCl 2 was thus beneficial. This means that TiCl 4 which undergoes reduction is not incorporated in the crystallographic structure of the MgCl 2 , as was previously supposed (42,43) , but forms a titanium-aluminium active centre by reaction with the organoaluminium compound included in the structure of MgCl 2 (35,36) .…”

Titanium catalyst (TiCl4) supported on MgCl2(THF)(AlEt2Cl)0,34 for ethylene polymerization

“…The synthesis of magnesium-aluminium bimetallic complexes in the presence of a Lewis base was reported earlier [6,7]. Our previous investigations [8,9] of the role AlEt /TiCl mole ratio 3 4 Therefore, according to the model of the crystal structure of Ziegler-Natta catalysts proposed by Natta, Patat and Sinn [10,11], the studied MgCl 2 (THF)(AlEt 2 Cl) 0.34 complex support reduces immobilized TiCl 4 to TiCl 3 and forms bimetallic aluminium-titanium catalytic active sites [formula (II)]. Simultaneously, the binding of THF to the magnesium support weakens.…”

“…1). The important feature of the studied catalysts is the activator/catalyst molar ratio (AlEt 3 /TiCl 4 < 1000, Table 2), smaller than those previously applied during investigations of the similar magnesium-aluminium complex support at the laboratory scale made catalysts (Al/Ti ≈ 2000-10 000) [1,2,9]. Therefore, there is no need to apply a great excess of the activator to maintain the high activity of the catalyst, which is an important economical factor.…”

Investigation of Alkoxysilanes in the Presence of Hydrogen with Ziegler-Natta Catalysts in Ethylene Polymerization

“…Among various R1R2Si(OMe)2 types of external donor, cyclohexylmethyldimethoxysilane (CHMDMS) is more frequently used due to its effective performance to enhance the isotacticity of polypropylene without sacrificing the catalyst activity [2]. By the way, ethanol is one kind of Lewis base, which is usually used to improve MgCl2 crystal structure [16]. As mentioned previously, Lewis bases were studied in many aspects in propylene polymerization.…”