Loading...
Thumbnail Image
Publication

ETHYLENE POLYMERIZATION USING THE PHILLIPS CATALYST

Abstract
The Phillips catalyst, chromate on silica support, is widely used to manufacture polyethylene. It is activated in an oxygen-rich environment before conducting polymerization. The polymerization involves an initial period of inactivity called the induction period, during which active site formation occurs, and ethylene acts as a reductant. To avoid this period, carbon monoxide is employed as reductant, but it increases the operational costs. Shortening this period has operational advantages, and understanding the redox chemistry might help in tailoring the catalyst. In this thesis, the effect of residual oxygen from the preceding catalyst activation step is investigated and the use of aldehydes as alternative reductants to ethylene or carbon monoxide is explored. Oxygen concentration lower than 100 ppm influenced the time to onset and the time to reach maximum conversion, while the impact on overall activity was not discernible. The ethylene exposure per chromium until maximum conversion increased with increase in residual oxygen concentration. The choice of aldehydes as reductants was motivated by literature associating formate with the active catalyst and by detection of formaldehyde. A series of aldehydes were used, including, formaldehyde, acetaldehyde, propionaldehyde, isobutyraldehyde, pivaldehyde, benzaldehyde and p-anisaldehyde. While carboxylate formation was consistent for all the aldehydes tested, the alkyl chain was oxidized whereas the aromatic ring stayed intact. Formaldehyde was least reactive of all the aliphatic aldehydes tested; however, formaldehyde-treated catalyst was active after a temperature-programmed desorption to 300 ℃. All the other aliphatic aldehydes required a temperature of 600 ℃. Induction period was absent for these aldehyde pre-treated catalysts, and a higher initial activity relative to pristine catalyst was observed. Thus, formaldehyde pre-treatment is one way of generating active sites at a temperature lower than typically employed for carbon monoxide. Polymerization was conducted in a plug flow reactor to gain spatial information about the reaction chemistry. For a downflow reactor, the reaction front moved from top to bottom for aldehydes whereas the polymerization proceeded from bottom to top. Model olefins tested to understand this polymer formation provided limited insights. Thus, this work provides fundamental insights into using aldehydes as a means for generating an active polymerization catalyst.
Type
campusone
article
dissertation
Date
2023-05-26
Publisher
Rights
License
Research Projects
Organizational Units
Journal Issue
Embargo
Publisher Version
Embedded videos
Collections