On the nature of extra-framework aluminum species and improved catalytic properties in steamed zeolites

Steamed zeolites have improved catalytic properties for hydrocarbon activation (alkane cracking reaction as well as alkane dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was speculated to be related to increased Bronsted acidity during dealumination. We now prove that during steaming aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano)facets of hydroxylated transition-alumina surfaces. Bronsted acidity of zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O5Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al1O5 sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying reason behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on penta Al(V)1O5 sites of aluminum oxide clusters confined in zeolitic pores. Furthermore, slightly decreased number of adjacent Al framework sites (due to Al dislodgement from the framework) decreases the coking activity, prolonging catalyst lifetime. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites. Furthermore, our findings suggest that similar La2O3 clusters exist for La-containing zeolites and the origin of their cracking activity promotion should be similar.