Surface vs Homogeneous Organo-Hafnium Catalyst Ion-Pairing and Ligand Effects on Ethylene Homo- And Copolymerizations

Jialong Zhang, Alexander H. Mason, Alessandro Motta, Laryssa G. Cesar, Yosi Kratish, Tracy L. Lohr, Jeffrey T. Miller, Yanshan Gao, Tobin J. Marks

Research output: Contribution to journalArticlepeer-review

Abstract

Heterogeneous catalysts have long dominated polyethylene and polypropylene production, but understanding their catalysis is challenged by uncertainties in active site structures and percentages. Surface-bound organometallic catalysts are an emerging strategy to combine successful homogeneous catalysts having well-understood structures, relatively high percentages of active sites, and exceptional control of selectivity, with the attractions of surface catalysts, to transition promising homogeneous systems to large-scale heterogeneous ones. Nevertheless, surface-bound olefin polymerization catalysts typically produce ultrahigh Mw's but with low activity and comonomer selectivity. Here, we report the systematic synthesis and characterization of a series of pyridylamido-Hf complexes and their corresponding surface catalysts chemisorbed on sulfated alumina (AlS) and zirconia (ZrS). Comparative ethylene homo- and 1-octene copolymerizations reveal similar activity and 1-octene selectivity trends in the homogeneous and heterogeneous systems. For the surface pyridylamido-Hf catalyst series, large variations in activity (up to 10×) and 1-octene incorporation (up to 28×) are achieved by ligand and support manipulation. Interestingly, while the homogeneous catalysts exhibit positive comonomer effects in ethylene/1-octene copolymerization, the surface catalysts behave oppositely. Extended X-ray absorption fine structure (EXAFS) reveals significantly elongated Hf···O bond distances vs typical Hf-O covalent bonds (2.06 vs 1.97 Å). Density functional theory (DFT) analysis of the heterolytic ion pair separation enthalpies, olefin insertion energetics, and NBO/Bader charges also suggest electrostatic Hf cation-anionic support binding and catalytic patterns, which are modulated by the ion-pairing energetics and ligand architecture.

Original languageEnglish
Pages (from-to)3239-3250
Number of pages12
JournalACS Catalysis
DOIs
Publication statusAccepted/In press - 2021

Keywords

  • comonomer effects
  • hafnium
  • ion pairing
  • olefin polymerization
  • supported organometallics

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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