Morphological modifications of Cr/SBA-15 and Cr/Al-SBA-15 ethylene polymerization catalysts: Influence on catalytic behaviour and polymer properties

Abstract

Mesostructured Phillips catalysts (Cr/SBA-15 and Cr/Al-SBA-15) with different morphologies were synthesized and tested in ethylene polymerization. Morphological properties of mesoporous supports were modified by changing some synthesis variables like stirring rate, concentration of silica source, mixture acidity and the type of cosolvent molecule. Obtained solids were calcined and characterized by N 2 adsorption–desorption, XRD, 27Al NMR and SEM. By decreasing the stirring rate during the synthesis, the size and shape of SBA-15 particles changes from small fibre-like to larger irregular agglomerates. As well, a higher amount of sphere-like particles were obtained by increasing TEOS/P123M ratio. On the contrary, for Al-SBA-15 material, agglomerated particles were obtained even at high stirring rate, but no sphere-like particles were observed by increasing the TEOS/P123M ratio. The increase of HCl/P123M ratio clearly modifies the SBA-15 particle morphology but the structure becomes disordered. Cosolvent incorporation in the SBA-15 synthesis produces a swelling effect in the order trimethylbencene > n-decane > toluene. Only the addition of toluene affects SBA-15 morphology from sphere-like to fibre-like particles. Chromium catalysts prepared by impregnation of SBA-15 supports synthesized at lower stirring rates (50 rpm) present higher activity in ethylene polymerization and produce polymers with larger particle size. On the other hand, Cr/SBA-15 catalysts lead to polyethylenes with slightly lower molecular weight and melting point than traditional Phillips catalyst like Cr/SiO 2.