ON THE ENHANCED DEHYDROGENATION VERSUS CRACKING ABILITY OF AN ACIDIC GALLIUM MFI - A TENTATIVE ACID-BASE INTERACTION-MODEL

The hydrogenation and cracking properties of an authentic H(Ga)-MFI are investigated in a true kinetic regime. The cracking of propane over the H(Ga)-MFI decreased dramatically in comparison to results obtained over a comparable H(Al)-MFI. Conversely, the dehydrogenation of propane was very much increased. The Bronsted acid sites associated with the framework gallium seem to operate in a similar manner to those associated with framework aluminium in H(Al)-MFI. However, since their strength is attenuated, their greater dehydrogenation activity is ascribed to their preferred reaction with a C-H sigma-bond of propane to form the carbonium ion with respect to a C-C sigma-bond, while the reverse situation prevails with the stronger acid sites of H(Al)-MFI. A kind of soft-soft and hard-hard acid-base interaction is thought to govern the formation of carbonium ions having distinct structures which lead, respectively, to cracking or dehydrogenation.