Effect of alkali metal doping on the performance of Cr/silica catalysts in ethylene polymerization

摘要

Sodium contaminants are known to promote sintering of silicas during calcination at high temperatures such as 900 degrees C. This study examines the effect of doping the Phillips Cr/silica polymerization catalyst with larger amounts of various alkali metals followed by calcination at milder temperatures, like 500-700 degrees C. The presence of 0.8 mmol of extra alkali metal per gram of catalyst shifts the sintering process about 300-400 degrees C lower in temperature. Both Cr/silica and Cr/silica-titania catalysts were similarly affected. The effect on the physical properties of the catalyst and on catalyst performance during the polymerization of ethylene have been investigated. Activity, melt index potential, and polymer molecular weight distribution were all influenced to varying degrees, depending on the amount of alkali metal added. These responses were then compared to that of sintering the undoped catalyst at higher temperatures. (C) 1998 Academic Press. [References: 16]