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 C-13 scrambling reactions of labeled propane over Al2O3-promoted sulfated zirconia (SZA) catalyst under mild conditions (30-102degreesC). 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-d(6), monitored by in situ H-1 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 H-2 MAS NMR; (3) a intramolecular C-13 scrambling, by skeletal rearrangement process, favored at higher temperatures, monitored by in situ C-13 MAS NMR. The activation energy of C-13 scrambling was estimated to be very close to that of H-2 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.