EFFECT OF ALKALI-METAL DOPING ON THE PERFORMANCE OF CR SILICA CATALYSTS IN ETHYLENE POLYMERIZATION/

Sodium contaminants are known to promote sintering of silicas during c alcination at high temperatures such as 900 degrees C. This study exam ines the effect of doping the Phillips Cr/silica polymerization cataly st with larger amounts of various alkali metals followed by calcinatio n 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 p rocess about 300-400 degrees C lower in temperature. Both Cr/silica an d Cr/silica-titania catalysts were similarly affected. The effect on t he physical properties of the catalyst and on catalyst performance dur ing the polymerization of ethylene have been investigated. Activity, m elt index potential, and polymer molecular weight distribution were al l influenced to varying degrees, depending on the amount of alkali met al added. These responses were then compared to that of sintering the undoped catalyst at higher temperatures. (C) 1998 Academic Press.