The Initial Stages of Solid Acid-Catalyzed Reactions of Adsorbed Propane. A Mechanistic Study by in Situ MAS NMR

In situ solid-state NMR spectroscopy was employed to study the kinetics of hydrogen/deuterium exchange and scrambling as well as 13C scrambling reactions of labeled propane over Al2O3-promoted sulfated zirconia (SZA) catalyst under mild conditions (30−102 °C). Three competitive pathways of isotope redistribution were observed during the course of the reaction: (1) a regioselective H/D exchange between acidic protons of the solid surface and the deuterons of the methyl group of propane-1,1,1,3,3,3-d6, monitored by in situ 1H MAS NMR; (2) an intramolecular H/D scrambling between methyl deuterons and protons of the methylene group, without exchange with the catalyst surface, monitored by in situ 2H MAS NMR; (3) a intramolecular 13C scrambling, by skeletal rearrangement process, favored at higher temperatures, monitored by in situ 13C MAS NMR. The activation energy of 13C scrambling was estimated to be very close to that of 2H scrambling, suggesting that these two processes imply a common transition state, responsible for both vicinal hydride migration and protonated cyclopropane formation. All pathways are consistent with a classical carbenium ion-type mechanism.