Regioselective H/D isotope exchange and skeletal rearrangement reactions of propane over strong solid acids

The regioselectivity of the hydrogen/deuterium exchange reaction of propane over D2O-exchanged ZSM-5 zeolite, unpromoted (SZ), and Al2O3-promoted sulfated zirconia (SZA) using a batch recirculation reactor was studied by means of 1H and 2H liquid state NMR spectroscopy. Low-temperature H/D exchange, below 100 °C, between acidic deuterium of the solid surface and propane was observed only on sulfated zirconia-based catalysts involving specifically the methyl protons. Detection of deuterium in the methylene position however occurred at higher temperatures as a consequence of skeletal rearrangement of the intermediate carbenium ion, evidenced by the 13C scrambling in propane 1-13C as well as by the 2H scrambling in 1,1,1,3,3,3 propane-d6 and 2,2-propane-d2. The regioselectivity of the initial H/D exchange and the label redistribution observed at higher temperatures support the assumption that the exchange between propane and catalyst acidic surfaces proceeds via carbenium-ion-type intermediates and exclude possible pentavalent carbonium ion intermediates under these conditions.