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

Sulfated zirconium oxide (SZO) catalyzes the hydrogenolysis of isotactic polypropylene (iPP, M=13.3 kDa, Đ=2.4, =94 %) or high-density polyethylene (HDPE, M=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 SO and sulfate groups. This adsorbed SO is a very strong Lewis acid that binds N-pyridine or triethylphosphineoxide (TEPO) (ΔE>-39 kcal mol), reacts with PhCH to form PhC, and mediates H/D exchange in dihydroanthracene-d. DFT studies show that pyrosulfate sites open with a modest 26.1 kcal mol barrier to form the adsorbed SO 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 SO between 226.2 to 237.9 kcal mol, 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.