(608c) Role of Bronsted Acid Site Proximity in the Generation of Synergistic Sites for Alkane Cracking Conversion in MFI Zeolites
AIChE Annual Meeting
Thursday, November 11, 2021 - 1:06pm to 1:24pm
The presence of extra framework Aluminum (EFAL) species in the proximity of Bronsted acid sites (BAS) can modify the confining environment surrounding the active sites, leading to the rate enhancements during alkane cracking. It is well known that EFAl species can be generated during the hydrolysis of framework Al atoms under the steaming treatment at high temperatures. We have recently demonstrated that water treatment in a pulse reactor can be used to decouple the mobility of Al species from the framework Al hydrolysis. The role of water under these conditions is believed to facilitate the mobility of existing EFAL species leading to the formation of the highly active BAS-EFAL sites. Other factors such as the proximity and location of active sites can affect the ability of the zeolite structure to stabilize these EFAL species to form the synergistic sites. In this contribution, we investigate the role of proximate framework acid sites on the generation of synergistic sites in different MFI zeolite catalysts varying Si/Al ratios. We draw correlations between the rates of hexane cracking and the rate enhancement due to pulsing water treatment with the number of BAS in proximity. Moreover, we reveal how sodium can selectively exchange with precursors for these highly active sites, as reflected by significant shifts in cracking behavior observed after pulsed water treatments. This evidence suggests that sodium selectively titrates the environments required to generate these synergistic sites after Al migration. This could be explained by either inhibition of EFAL mobility or selective exchange of framework environments that serve as precursors to synergistic sites. The experiment results are in line with DFT calculations implying the preferential exchange of sodium at paired Al sites. These findings reveal the critical role of Al in pairs on the alkane cracking rate via the generation of highly active sites.