Silica for the Gas-Phase Manufacture of Polyethylene

In 1988 Howard W. Turner reported the preparation of a solid catalyst component based on the reaction of (BuCpl ZrCl and methylalumoxane at room temperature. The solid was isolated by cooling the reaction vessel to -30°C for one hour and then decanting the liquid layer and washing the solid material with pentane to yield a glassy solid. The solid with an Al/ Zr ratio of 20 was examined as a homogeneous ethylene polymerization catalyst using toluene as a solvent. Catalyst activity was 164 g PE/g catalyst at 80°C, 35 psi ethylene in 10 minutes [45]. 

Lo and coworkers reported [46] a silica-supported catalyst that was utilized in a gas-phase fluidized-bed reactor process for the preparation of HOPE and LLDPE using (BuCp)2ZrCl2/MAO and a Davison Grade 955 silica (pore volume of 1.5 cc/g) that was previously calcined at 600 C for 12 hours. A typical catalyst formulation is shown in Table 4.5. 

The key feature of this catalyst formulation is the low Al/Zr molar ratio of 200 that was required to produce a high-activity, silica-supported catalyst which is readily adaptable to the gas-phase polymerization process. In addition, this silica-supported, single-site catalyst also provided polyethylene with acceptable molecular weight for commercial applications. The amoimt of catalyst that can be impregnated into the silica is limited by the pore volume of the silica, so that the relatively low Al/Zr ratio in this catalyst system could be completely impregnated into the silca pore structme. 

The data in Table 4.6 show that catalyst activity increases with polymerization temperature from 1,150 g PE/g catalyst at 65 C to 5,100 g PE/g catalyst at 77.5°C. However, very importantly, the polyethylene molecular weight may be controlled over the range necessary for the manufactme of commercial grades of polyethylene for industrial appHcations, as shown by Melt Index (MI values from 4.1 to 0.6. Isopentane or isopentane containing 0.3 ppm oxygen was needed to produce the relatively higher molecular weight products. 

Temp (°C) Isopentane psi (hexene/ethylene) gas phase molar ratio ethylene psi Zr ppm 2.16 kg Density g/cc Activity g PE/g 

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