Tailoring supported olefin polymerisation catalysts using non-equilibrium plasmas

Abstract

Supported olefin polymerisation catalysts are used to produce more than 20 million tonnes of polyethylene a year. In this thesis, the application of non- equilibrium plasmas to Phillips, Ziegler-Natta, and metallocene catalysts is described. A Cr(acetate)/silica Phillips catalyst precursor was activated using either thermal, plasma, or combined activations. Oxygen plasma activation was found to completely oxidise the acetate ligands, but left a low chromium dispersion and high hydroxyl population on the support. This large hydroxyl population caused the resulting catalyst to display a low activity. Plasma dehydroxylation of the silica support was then studied with the aim of increasing the activity of the plasma activated catalyst. Non-isothermal CF4 plasma treatment of mesoporous silica decreased the total hydroxyl population to a level comparable to a 773 K thermal treatment. These optimum conditions were then applied to the catalyst precursor, which in combination with oxygen plasma activation, produced an active polymerisation catalyst. Also, it has been found that combined thermal and plasma activations produce catalysts of lower activity than when solely calcined, but the resulting polymers have a narrower molecular weight distribution. Next, it was shown that Zeigler-Natta catalyst supports can be prepared by ccl(_4) plasma chlorination of a dibutylmagnesium/silica precursor. This approach offers the benefits of fast reaction times and less chemical waste compared to conventional solution phase chlorination. Finally, the replacement of conventional inorganic supports by polymer analogues has been investigated. It has been shown that plasma fluorination can be used to passivate the internal pores of high surface area polystyrene beads, thereby providing an ideal inert high surface area medium for high activity metallocene catalysts. Overall, this work has demonstrated how non-equilibrium plasmas can be highly effective at chemically modifying porous media.