Computational Support for Phillips Catalyst Initiation via Cr-C Bond Homolysis in a Chromacyclopentane Site

Anthony Fong, Craig Vandervelden, Susannah L. Scott, Baron Peters

Research output: Contribution to journalArticlepeer-review

Abstract

Using density functional theory, we examine a possible homolysis initiation mechanism for the Phillips catalyst, starting from CrII sites exposed to ethylene. Spin-crossing in an abundant quintet bis(ethylene) CrII site leads to cycloaddition to form a chromacyclopentane site. One Cr-C bond then homolyzes to generate a tethered n-butyl radical: [Cr(CH2)3CH2]. If the radical attaches to a nearby inorganic Cr site, it yields two alkylCrIII sites capable of Cossee-Arlman polymerization. The overall computed barrier for this initiation process is 132 kJ/mol, which is comparable to the 120 kJ/mol value that we estimated from reported initiation times in industrial reactors. Poisson statistics suggest that this mechanism could activate ∼35% of Cr sites on a commercial catalyst with a loading of 0.4 Cr/nm2. Pairwise Cr grafting, amplification by complementary initiation reactions, or the creation of dangling bonds that form as the silica support fractures, might explain the apparent increase in per-site activity at lower Cr loadings.

Original languageEnglish (US)
Pages (from-to)1728-1733
Number of pages6
JournalACS Catalysis
Volume8
Issue number3
DOIs
StatePublished - Mar 2 2018
Externally publishedYes

Keywords

  • Phillips catalyst
  • density functional theory
  • homolysis
  • initiation
  • polymerization

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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