Supported molybdenum oxide catalysts in ethylene polymerisation

Abstract

Molybdenum oxide supported on γ-alumina can be used as a catalyst for the polymerisation of ethylene. Calcined catalysts are activated by reduction and chloriding with certain compounds such as anhydrous HCl before use in a polymerisation. The activity is related to physical properties such as surface area, pore volume, and pore diameter. We have determined that the average oxidation state of molybdenum on calcined catalysts decreases with decreasing molybdenum content. We have confirmed that activity is related to the Mo 5+ concentration and shown that each Mo 5+ is involved in an active site. Chloriding does not affect the level of Mo 5+ but only enhances the activities of existing active species. The amount of Mo 5+ is dependent on the amount of MoO 3 originally on the catalyst and is only about 0.1% of the total molybdenum atoms. Use of oxalyl chloride during the chloriding step allows much of the original molybdenum to be removed without significantly lowering activity or Mo 5+. This result further shows that most of the molybdenum is involved in stabilising Mo 5+ during the reduction but is not involved in the active site. In addition to the chain addition reaction which predominates, side reactions occur at the catalyst surface which lead to a small amount of branching in the polymer and isomerisation of the terminal double bond formed in the chain transfer step.