Olefin Polymerization from the Gas Phase

点击翻译按钮获取中文摘要

The majority of olefin polymerization processes are cataytic, most often involving a heterogeneous catalyst. The polymerization reaction occurs at some active site on the catalyst either on its surface or within its pores. This means that monomer has to contact the catalyst and that the polymer grows either on the surface or within the pores of the catalyst. The transport processes involved include monomer from the bulk phase diffusing through a polymer layer to an acitve site and the heat of reaction(the reactions are exothermic) diffusing out through this same polymer layer. The ractions are thus temperature related and transport limited. The properties of the product polymer are therefore controlled by the pore structure of the catalyst. As the reaction proceeds, polymer created in the internal pores of the catalyst increasingly blocks transport paths generating gradients in monomer concentration and temperature. Polymerization rate decreases and the molecular weight of product polymers vary greatly. The catalyst ultimately fragments, exposing more surface area as well as more accesss to previoulsy inaccessible pores. New sites are exposed, transport path length is reduced, and polymerization is then accelerated. The facture occurs due to confined growth and expansion of polymer chains in narrrow, friable channels of the catalysts and is an autocatalytic phenomena. This fracture rate depends on the catalyst pore structure which is thus of significant importance in the molecular weight distribution and yield of polyolefins. Gas phase polymerization has the advantage that: (1) low capital investment is required, (2) it is a solvent free process (i.e. distillation, drying and solvent rcovery are not necessary), and (3) therefore operating costs and energy consumption are low. The PIs plan to continue to experimentally study the effects of thethree-dimensional pore structure on the gas-phase polymerization of olefins. Pore structure will be characterized by physical adsorption, X-ray tomography and porosimetry. Extended N2 adsorption and 129 XeNMR will be employed to charactized the microporosity of the catalyst/polymer system and the rate by which the monomer diffuses to the active surface. X-ray imaging in two and three demensions will be developed to determine the spatial distribution of the catalyst within the growing polymer. The results should yield a better fundamental understanding of the interaction of kinetics and transport phenomena and the design of not only better catalysts, but whole reactors as well.