Mechanistic routes of low temperature alkane activation over zeolites

Two reaction pathways for the hydrogen/deuterium exchange of iso- and n butane over zeolites MFI and BEA are proposed both having as key step the alkane dehydrogenation. The difference in the apparent activation energies over the two zeolites suggests marked differences in the nature of this step. For H-MFI the absolute rate of H/D exchange increased in parallel with the concentration of Brønsted acid sites. The H/D exchange is suggested to occur via the protonation of alkene intermediately formed by protolytic dehydrogenation. The Lewis acid sites accelerate by (a non-catalytic) hydride abstraction leading to sorbed hydrogen (atoms) and alkoxy species on the surface. Even traces of olefins accelerate the H/D exchange dramatically emphasizing their important role for the H/D exchange of alkanes.