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 SO 3 and sulfate groups. This adsorbed SO 3 is a very strong Lewis acid that binds 15 N‐pyridine or triethylphosphineoxide (TEPO) (Δ E ads >−39 kcal mol −1 ), reacts with Ph 3 CH to form Ph 3 C + , and mediates H/D exchange in dihydroanthracene‐ d 4 . DFT studies show that pyrosulfate sites open with a modest 26.1 kcal mol −1 barrier to form the adsorbed SO 3 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 3 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.