Study on the energy distribution of active sites in heterogeneous Ziegler-Natta catalysts (in English)
Polimery 1998, No 4, 239
A method was developed to study the nonuniformity of active centers in heterogeneous Ziegler-Natta type catalysts. The method is to analyze by mass spectrometry the products of thermal destruction (resulting on thermo-programmed desorption) of the catalyst at the stage of olefin polymerization (when the polymer chain connected with the active site is still relatively short). The products to be examined were desorbed from the surface of the SiO2/TiCl4/Et2AlCl catalyst heated from room temperature to 500°C, before and after the gas-phase polymerization of ethylene that was carried out under the conditions guaranteeing short macromolecular chains to be obtained on the catalyst surface. Two distinct maxima were found on the TPD curve over 180—210°C and 280—320°C with the catalyst submitted to a short-term polymerization; they were ascribed to thermal destruction of the active Ti—C bond, accompanied by evolution of hydrocarbon molecules (PE) of varying chain lengths (up to 14 mers in the macromolecule). This fact allows to infer that the catalyst examined contained at least two types of active centers differing in the activation energy of thermal destruction of the Ti—C bond. Full analysis of the two maxima in the TPD curve allowed to obtain the distribution pattern of the energy of activation of thermal destruction, which shows the active centers in the catalyst investigated to be heterogeneous.
Keywords: ethylene polymerization catalyst, active center types, thermal destruction products, distribution of thermal destruction activation energy