Journal
ACS CATALYSIS
Volume 8, Issue 3, Pages 1728-1733Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.7b03724
Keywords
Phillips catalyst; polymerization; initiation; density functional theory; homolysis
Categories
Funding
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under the Catalysis Science Initiative [DE-FG-02-03ER15467]
- National Science Foundation [CBET-1605867]
- Directorate For Engineering
- Div Of Chem, Bioeng, Env, & Transp Sys [1605867] Funding Source: National Science Foundation
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Using density functional theory, we examine a possible homolysis initiation mechanism for the Phillips catalyst, starting from Cr-II sites exposed to ethylene. Spin-crossing in an abundant quintet bis(ethylene) Cr-II site leads to cycloaddition to form a chromacyclopentane site. One Cr-C bond then homolyzes to generate a tethered n-butyl radical: [Cr(CH2)(3)CH2 center dot]. If the radical attaches to a nearby inorganic Cr site, it yields two alkylCr(III) 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 similar to 35% of Cr sites on a commercial catalyst with a loading of 0.4 Cr/nm(2). 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.
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