Monomer transport influences in the nascent polymerization of ethylene by silica-supported chromium oxide catalyst

Transport influences during nascent ethylene polymerization over Phillips chromium oxide--silica catalyst were evaluated by using pulse solid--gas chromatography. Measures of effective intraparticle diffusivity and tracer-accessible intraparticle void space were made using helium as a tracer. Measures of surface reaction rate and equilibrium adsorption were made using ethylene monomer as the tracer. Quantitative site poisoning using oxygen was performed. The Kubin--Kucera model was used for the transport measurements; the Suzuki--Smith model was used for the kinetic analysis. The Thiele modulus for ethylene polymerization at 50 deg C, 1 atm of monomer pressure, and yields less than 0.1 g of polymer/g of catalyst is estimated to be 1.5. This value increases as polymer is formed inside the catalyst pores. Significant intraparticle concentration gradients and transport control may exist even at low yields. Without particle fracture, the rate of polymerization, which would occur mainly at the particle surface, would be inhibited. The rate of adsorption of monomer over active chromium oxide is very fast relative to the surface reaction. The apparent first-order polymerization rate constant at these conditions is roughly 0.06 s exp --1 . The estimated activation energy for the surface insertion step is 14 kcal/mol. The equilibrium adsorption constant is six; the free energy for reversible adsorption is --3 kcal/mol. The effective intraparticle monomer diffusivity prior to particle fragmentation and accumulation of polymer is 2.5 x 10 exp --4 cm exp 2 /s, a very low value when compared to that estimated by using effective medium transport theory. Graphs. 52 ref.--AA