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

Abstract

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 °C. Both catalysts exhibit approximately linear relationships between rate and [H₂] at substoichiometric [H₂] with rate law orders of 0.66 ± 0.09 and 0.48 ± 0.07 for Hf and Zr, respectively; at higher [H₂], the rates approach zero-order in [H₂]. 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 [H₂] is pivotal in one of two plausible and competing rate-determining transition states-bimolecular metal-alkyl bond hydrogenolysis vs. unimolecular β-alkyl elimination. The Zr and Hf catalyst activation parameters, ΔH^‡ = 16.8 ± 0.2 kcal mol^-1 and 18.2 ± 0.6 kcal mol^-1, respectively, track the relative turnover frequencies, while ΔS^‡ = -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 °C, while time-on-stream data indicate multiday activities with an extrapolated turnover number ~92,000 at 150 °C for the Zr catalyst. This methodology is attractive for depolymerization catalyst discovery and process optimization.