(756a) Kinetic Role of Water on Evolution of Extra Framework Al Sites and Their Influence on Alkane Cracking

The presence of moisture is known to promote the hydrolysis of the framework Al atoms to create extra framework Al (EFAl). These species in isolation generally are not believed to participate in cracking reactions, but their presence in close proximity to framework Bronsted acid sites (BAS) may manipulate the adsorbed and transition states of molecules bound to them. Modifications to the confining environment near a framework site due to either Al or other species have often been suggested as responsible for increased reaction rates. The role that water plays on the generation and activity of these sites is quite complex.

At high temperatures or in the presence of a condensed liquid phase, water may hydrolyze framework sites to create EFAl, which may become mobile under severe conditions to generate Al₂O₃ clusters. In this contribution, we use a unique pulse reactor to expose a MFI catalyst with specific concentrations of framework density and extra framework species to pulses of gas phase water. Sequential micropulses of steam do not significantly alter the framework BAS density, but the conversion clearly is enhanced due to the migration of EFAL. We carry these experiments out at varying temperatures and decouple the activation energy barriers on isolated BAS sites and on the highly active sites, and use these intrinsic reaction rates to estimate the kinetic barriers associated with formation of these sites due to EFAl mobility. Initially after water vapor treatment, the activity for hexane cracking is diminished, but sequential drying to remove bound water significantly enhances cracking rates. These results help explain some of the discrepancies regarding the role of water on cracking activity, as although it can accelerate the formation of more active reaction environments, it may also strongly bind to active sites and inhibit rates if the water is present under reaction conditions.