Ethylene polymerization over Cr/MCM-41 and Cr/MCM-48 catalysts prepared by chemical vapor deposition

Cr-containing catalysts were prepared by chemical vapor deposition (CVD) of chromyl chloride (CrO2Cl2) vapor on two mesoporous molecular sieves (purely silica MCM-41 and MCM-48), and on amorphous silica. The catalysts were characterized with BET, inductively coupled plasma-atomic emission (ICP-AES), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), X-ray diffraction (XRD), FTIR and were evaluated for the polymerization of ethylene to produce high density polyethylene (HDPE). The supports had the following order of surface area: MCM-48 > MCM-41 ≫ SiO2, but Cr/MCM-41 and Cr/SiO2 showed significantly higher Cr concentration than Cr/MCM-48, which suggested that pore size/volume/structure had strong effect on the diffusion of CrO2Cl2 molecules inside the supports. HDPE yield per unit catalyst mass was found to decrease in the following order: Cr/MCM-41 > Cr/SiO2 ∼ Cr/MCM-48, which was ascribed to the higher Cr content of Cr/MCM-41. Polymers produced with the CVD prepared Cr/MCM-41 and Cr/SiO2 exhibited significantly different particle morphology, crystallinity, and crystal size. With the increase of support pretreatment temperature, Cr concentration and polymer yield decreased, probably due to the decrease of silanol concentration.