Influence of Bronsted Acid Site Proximity on Alkane Cracking in MFI Zeolites

Extra-framework aluminum (EFAL) species generated during the hydrolysis of framework Al are believed to play a crucial role in the activity of alkane cracking on zeolites. The presence of these EFAL species can enhance the reactivity of the adjacent Bri nsted acid sites (BAS), resulting in the formation of the synergistic BAS-EFAL sites. Recently, we have demonstrated that water treatment in a pulse reactor can facilitate the mobility of existing EFAL species in the absence of framework hydrolysis. Here, we investigate the role of framework Al location and proximity on the generation of synergistic sites in MFI zeolites. Correlations are developed between the cracking rates and the rate enhancement under pulsing water treatment with the concentration of proximate framework Al species. In addition, it is illustrated that sodium and calcium can titrate proximate framework Al, which we reveal to be important precursors to synergistic site formation. This allows quantification of the number of synergistic sites, as well as the conclusion that these highly active sites are likely located at pore intersections within the MFI framework. In addition, the analysis of experimental and theorical transition-state energies suggests that these EFAL species facilitate n-hexane cracking by lowering activation enthalpies.