CHEMISTRY OF ORGANOCHROMIUM COMPLEXES ON INORGANIC OXIDE SUPPORTS .4. STUDY OF ETHYLENE POLYMERIZATION OVER CHROMOCENE ON SILICA CATALYSTS
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The activity of chromocene on silica catalysts in low pressure (<1 atm) polymerization of ethylene was studied under static conditions. The catalytic activity was dependent upon the ethylene pressure, the dehydration temperature of the silica, and the amount of the deposited Cr. The activity increased with ethylene pressure; however, it reached a maximum with respect to the latter two parameters. The most active catalyst was formed by depositing chromocene on silica, dehydrated at 450C, until the Cr loading reached 4 wt%. Carbon monoxide, which was found to be a poison for the catalyst, inhibited the initial step of the reaction by adsorbing on the active sites. The maximum amount of active Cr responsible for the polymerization, estimated by a titrative CO poisoning technique, was ca. 0.32% of the total Cr. As a result, a high turnover frequency of 79 ethylene molecules per second was observed at 40C and 100 Torr C2H4. The low-pressure, low-temperature (30C) polymerization yielded a linear polyethylene with a high degree of crystallinity and negligible amounts of internal saturation or methyl branching. Addition of hydrogen resulted in decreased activity; however, no apparent changes were found for the polymer or the catalyst. 1991, American Chemical Society. All rights reserved.
