Ethylene polymerization over supported organochromium catalysts /
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| Other Authors: | , , |
| Format: | Thesis Book |
| Language: | English |
| Published: |
1990.
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| Subjects: | |
| Online Access: | ProQuest, Abstract Link to OAKTrust copy |
| Abstract: | Supported organochromium catalysts were characterized by FTIR and EPR. The results show that most of the surface chromium complexes on the chromocene-derived catalysts agglomerated to form polynuclear chromium clusters. These agglomerates, which adsorbed CO to form predominantly bridging carbonyl complexes, were inactive for ethylene polymerization. The active site is suggested to be the isolated CpCr complex which accounted for less than 0.32% of the total Cr. The maximum amount of chromium complexes which could be deposited on the support decreased with increasing dehydration temperature. The bis(benzene)chromium complex was oxidized to a (C6H6)2Cr+ ion upon deposition on oxide supports. The maximum amount of chromium complexes on this catalyst was dependent upon the acidity of the support, with acidic supports having a higher concentration. The kinetic profiles of supported organochromium catalysts varied considerably. The chromocene-derived catalysts had a very short induction period followed by a constant activity and then a slow decay in activity. By contrast, the supported bis(arene)chromium catalysts showed no induction period and a higher initial activity; however, the activity decreased very rapidly. The activity of the chromocene-derived catalysts was dependent upon the Cr loading and the dehydration temperature of silica. The most active catalyst was prepared by depositing chromocene onto silica dehydrated at 450°C until a 4 wt% Cr loading was achieved. The acidity of the support had negligible effects on the catalytic activity of the chromocene-derived catalysts. In contrast, the activity of the supported bis(arene)chromium catalysts was highly dependent upon the acidity of the support. Polymer properties were also varied by the type of supported organochromium catalysts. The chromocene-derived catalysts produced a linear polyethylene with a high degree of crystallinity and negligible branching or unsaturation, whereas the supported bis(benzene)chromium catalysts yielded a polymer with a significant amount of methyl branching and unsaturation. The different polymer characteristics suggest variations in polymerization mechanisms, which apparently correlated well with the different kinetic behaviors. Most of the chromium complexes on the chromocene-derived catalyst remained coordinatively unsaturated and adsorbed probe molecules to form various coordination complexes, which provided valuable information regarding the nature of the catalysts. |
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| Item Description: | Typescript (photocopy). Vita. "Major subject: Chemistry." |
| Physical Description: | xvi, 217 leaves : illustrations ; 29 cm |
| Bibliography: | Includes bibliographical references. |