Some reflections on the current state of Cr-based polymerization catalysts
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oduction It has been 62 years since the discovery of chromium polymerization catalysts, known as Phillips catalysts, which were used commercially only five years later. Chromium catalysts now account for some 40% of the world’s high density polyethylene production. Phillips catalysts have evolved significantly since those early days to the point where those early catalysts are hardly recognizable today. Although chromium catalysis is often described as “mature” in the industry literature, research in the field continues in an effort to introduce new applications and solve new problems.1–11 In fact, many changes have occurred to Phillips catalysts in the last decade, and in this article, some of the progress that has been recently made is described from an industrial perspective, and a list is provided of some of the current challenges facing the continued development of chromium catalysts.
Polymerization mechanism Although hundreds of papers have been written about the mechanism of active site formation and polymerization on the Phillips catalyst, much of the process is still unclear. What makes the Phillips catalyst more difficult to understand than a Ziegler catalyst is the self-alkylation of the chromium. A Cr(VI) site is reduced by ethylene to an uncertain valence, becoming alkylated in the process, and leaving oxygenated ligands whose final disposition (perhaps desorption) is uncertain.1
Recent work12–19 has provided new information, some of which is possibly inconsistent20 with the long accepted view of Cr(VI) being reduced to Cr(II) followed by the reversible desorption of formaldehyde.1,21,22 The imagined process of adsorption and desorption of aldehydes on Cr(II) could not be duplicated. In fact, the possibility of reduction by alkanes was instead also raised in the industrial setting as another route to an active catalyst.20 These new findings merely emphasize how hazy our understanding is of the process. Actually this issue is not unique to chromium oxide catalysts. Other examples include chromocene catalysts, diarene chromium (0) on acidic supports, titanium (II) oxide, bis-cyclooctatrienyl titanium and bis-cyclooctatrienyl zirconium on supports, Cr(C7H7)(C7H10) on supports, nickel oxide, and others. Another new discovery—the addition of titania shutting down comonomer incorporation—is also seemingly mysterious (see the section on short-chain branching [SCB] later in the text), but with great commercial implications. Thus, the mechanism of polymerization will undoubtedly be an area of continued study.
Governmental regulations In the United States and Europe, hexavalent Cr has been considered as potentially carcinogenic, and regulations have recently become increasingly severe (especially for Cr(VI) but for Cr in general). This applies to transportation of Cr(VI), disposal
Max McDaniel, Chevron-Phillips Chemical Company, Phillips Research Center, Bartlesville, OK; [email protected] DOI: 10.1557/mrs.2013.48
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MRS BULLETIN • VOLUME 38 • MARCH 2013 • www.mrs.org/bulletin
© 2013 Materials Research Society
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