Demystifying group-4 polyolefin hydrogenolysis catalysis: Gaseous propane hydrogenolysis mechanism over the same catalysts

• 成果类型: Article
• 署名作者: Mason, Alexander H.; Motta, Alessandro; Kratish, Yosi; Marks, Tobin J.
• 署名单位: Northwestern University; Northwestern University; Sapienza University Rome
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-11847
• DOI: 10.1073/pnas.2406133121
• 发表日期: 2024-07-23
• 关键词: plastic waste; polymerization; zirconium; complexes; chemistry; hydrides; pet

摘要

A kinetic/mechanistic investigation of gaseous propane hydrogenolysis over the single-site heterogeneous polyolefin depolymerization catalysts AlS/ZrNp2 and AlS/HfNp2 (AlS = sulfated alumina, Np = neopentyl), is use to probe intrinsic catalyst properties without the complexities introduced by time- and viscosity-dependent polymer medium effects. In a polymer-free automated plug-flow catalytic reactor, propane hydrogenolysis turnover frequencies approach 3,000 h(-1) at 150 degrees C. Both catalysts exhibit approximately linear relationships between rate and [H-2] at substoichiometric [H-2] with rate law orders of 0.66 +/- 0.09 and 0.48 +/- 0.07 for Hf and Zr, respectively; at higher [H-2], the rates approach zero-order in [H-2]. Reaction orders in [C3H8] and [catalyst] are essentially zero-order under all conditions, with the former implying rapid, irreversible alkane binding/activation. This rate law, activation parameter, and DFT energy span analysis support a scenario in which [H-2] is pivotal in one of two plausible and competing rate-determining transition states-bimolecular metal-alkyl bond hydrogenolysis vs. unimolecular beta-alkyl elimination. The Zr and Hf catalyst activation parameters, Delta H double dagger = 16.8 +/- 0.2 kcal mol(-1) and 18.2 +/- 0.6 kcal mol(-1), respectively, track the relative turnover frequencies, while Delta S double dagger = -19.1 +/- 0.8 and -16.7 +/- 1.4 cal mol(-1) K-1, respectively, imply highly organized transition states. These catalysts maintain activity up to 200 degrees C, while time-on-stream data indicate multiday activities with an extrapolated turnover number similar to 92,000 at 150 degrees C for the Zr catalyst. This methodology is attractive for depolymerization catalyst discovery and process optimization.