Influence of Catalyst Porosity on Ethylene Polymerization

The structure and porosity of Cr/silica catalysts has a strong influence on its activity in ethylene polymerization and on the character of the polymer it produces. In this study, silicas of widely varying physical structure were chosen so that the influence of surface area, pore volume, pore diameter, and coalescence could be independently investigated by monitoring the surface activity, the polymer molecular weight (MW), MW distribution, melt flow, and the amount of long-chain branching (LCB). The results are discussed with respect to (1) fragmentation of the silica during polymerization, and (2) egress of polymer from pores inside the resulting fragments. Pores of narrow diameter were found to inhibit polymer egress, resulting in lower surface participation, which in turn raised the molecular weight. Pores of wide diameter were found to produce relatively constant surface participation and polymer molecular weight, but increased the amount of LCB in the polymer. Variations in MW are seen as a function of the amount of "crowding" within the pore, whereas variations in LCB are seen as a function of the number of active sites within the pore cavity. The physical principles observed from Cr/silica catalysts were found to be independent and additive to other (chemical) influences from Cr/silica catalysts. Moreover, these physical influences apply to other supports and to metallocene catalysts, as well.