(153b) Alkane Cracking and Dehydrogenation In Zeolites

Acid base catalyzed alkane activation in zeolites is a key reaction to utilize light alkanes. Activation is preceded by the polarization of the C-H bond by bridging hydroxy groups or Lewis acid sites. Within zeolite pores, alkanes interact with well defined three dimensional sites allowing simultaneous interaction with the molecule. Such multi-point interactions can lead to very effective polarization of bonds even at low temperatures. The polarization by multivalent Lewis acid sites (such as La3+ or Al3+) is by far stronger than by bridging hydroxy groups leading to cleavage of hydrogen from alkanes already at mild reaction temperatures. Carbenium ions are the main reacting species at these mild conditions and chain reactions involving hydride transfer dominate after the initiation. The difference to the high temperature route of dehydrogenation via protolytic alkane activation is related to the different polarizations in the adsorbed state. The potential to the elementary steps in alkane conversion will be discussed.