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, Mn = 13.3 kDa, Đ = 2.4, <mmmm> = 94 %) or high-density polyethylene (HDPE, Mn = 2.5 kDa, Đ = 3.6) to branched alkanes. We propose that this reactivity is driven by the pyrosulfate sites SZO, which open under mild reaction conditions to transiently form adsorbed SO3 and sulfate. This adsorbed SO3 is a very strong Lewis acid that binds 15N-pyridine or triethylphosphineoxide (TEPO) (DEads > -39 kcal mol-1), reacts with Ph3CH to form Ph3C+, and mediates H/D exchange in dihydroanthracene-d4. DFT studies show that pyrosulfate sites have a modest 26.1 kcal mol-1 barrier to form the adsorbed SO3 and sulfate in the presence of a propylene tetramer. Hydride abstraction from the tertiary C-H in this model is exothermic and subsequent b-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 neutral Lewis acid. This study explains how SZO, a classic heterogeneous catalyst whose activity is often attributed to Brønsted acidity, can form carbocations by a redox neutral hydride abstraction reaction by very strong Lewis sites.