Effect of support structure on the activity of Cr/nanosilica catalysts and the morphology of prepared polyethylene

Phillips-type catalysts are responsible for the commercial production of more than one-third of all polyethylene sold worldwide. Many types of chromium-based catalysts are used in the Phillips polymerization process. Ordered mesoporous silica structures were synthesized using various surfactant species. Chromium nitrate nonahydrate (Cr(NO3)(3)center dot 9H(2)O) complex was grafted onto the surface of pure silica and was used for ethylene polymerization. The materials were characterized using X-ray diffraction, nitrogen adsorption-desorption, inductively coupled plasma optical emission spectroscopy, thermogravimetric analysis and Fourier transform infrared spectroscopy. In the as-synthesized materials, Cr3+ is present as a surface species in pseudo-octahedral coordination. After calcination, Cr3+ is almost completely oxidized to Cr6+, which is anchored onto the surface in various oxidative states. The catalyst polymerization activity is dependent on the chromium loading, the pre-calcination temperature and the support properties. In particular, the chromium catalyst prepared using spherical SBA-15 is more active than the other catalysts investigated. Porous and nano-fibrous polyethylene samples were prepared using various silica-supported chromium catalytic systems. Differential scanning calorimetry results show that the melting point of samples produced with the SBA-15-supported catalyst is higher than that of samples produced with Cr/SiO2 under the same conditions, which could be related to the existence of an extended-chain structure. (C) 2010 Society of Chemical Industry