C−H Bond Activation by Sulfated Zirconium Oxide is Mediated by a Sulfur-Centered Lewis Superacid

Abstract

Sulfated zirconium oxide (SZO) catalyzes the hydrogenolysis of isotactic polypropylene (iPP, M_w=13.3 kDa, Đ=2.4, <mmmm>=94 %) or high-density polyethylene (HDPE, M_n=2.5 kDa, Đ=3.6) to branched alkane products. We propose that this reactivity is driven by the pyrosulfate sites SZO, which open under mild conditions to transiently form adsorbed SO₃ and sulfate groups. This adsorbed SO₃ is a very strong Lewis acid that binds ¹⁵N-pyridine or triethylphosphineoxide (TEPO) (ΔE_ads>−39 kcal mol⁻¹), reacts with Ph₃CH to form Ph₃C⁺, and mediates H/D exchange in dihydroanthracene-d₄. DFT studies show that pyrosulfate sites open with a modest 26.1 kcal mol⁻¹ barrier to form the adsorbed SO₃ and sulfate in the presence of a tetramer of propylene. Hydride abstraction from the tertiary C−H in this model is exothermic and subsequent β-scission forms cleaved products. Analysis of the energetics provided here brackets the hydride ion affinity (HIA) of the adsorbed SO₃ between 226.2 to 237.9 kcal mol⁻¹, among largest values reported for a formally neutral Lewis acid. This study explains how SZO, a classic heterogeneous catalyst, can form carbocations by a redox neutral hydride abstraction reaction by very strong Lewis sites.