Nature and catalytic role of extra-framework aluminum and partially coordinated aluminum in MFI zeolite

Abstract

Zeolites are extensively employed in the chemical and petroleum industries as an acid catalyst for cracking, isomerization, and alkylation reactions. Recently, they have also gained attention as potential catalysts for biomass conversion. The activity of Brønsted acid sites (BAS) contained within the microporous zeolite channels is known to be affected by the location and local environment that stabilizes reaction intermediates and transition states. Additionally, it has been reported that presence of extra-framework Al (EFAL) and partially coordinated Al (PFAL) species can significantly alter reaction rates. However, the mechanism behind these enhancements and the catalytic roles of individual EFAL and PFAL species are yet to be fully understood. In this project, we utilized Density Functional Theory (DFT) to elucidate the locations and local environments of active sites in H-ZSM-5. We also propose the mechanisms behind the synergistic effects generated by the BAS and EFAL/PFAL in proximity. Moreover, the individual catalytic effects of different EFAL and PFAL on alkane cracking reactions were investigated. The theoretical calculations provide precise assignments of NMR spectra for acid sites and their surrounding in zeolites. The well-known signal 4.2 - 4.5 ppm is attributed from BAS at the intersection and main channels (straight and sinusoidal channels). The broad signal 5-9 ppm is assigned for framework BAS at 5-MR/6-MR or silanol groups in silanol nests for the first time. It is noteworthy that the broad signal 11-18 ppm rise from BAS forming hydrogen bonds with adjacent EFAL/PFAL species. Additionally, for different EFAL and PFAL species, the 27Al NMR chemical shift values are in the broad range of 55-85 ppm, which is rational to the expansion of aluminum peak up to 85 ppm for steamed samples. The synergistic sites are formed via the hydrolysis of Al framework atoms and migration of EFAL in the presence of water. During the initial stages of hydrolysis, incomplete hydrolysis of BAS site pairs creates BAS/PFAL synergistic sites. The formation of synergistic sites in pulsed water treatment is not related to dealumination but rather involves the migration of EFAL to BAS to generate synergistic effect. In both propane and hexane conversions, the cracking reactions are more influenced by EFAL and PFAL species than the dehydrogenation reaction. The charge distributions reveal that EFAL and PFAL interact with transition states, resulting in lower activation barriers. This work elucidated the underlying mechanism of EFAL and PFAL for enhancing alkane conversion and pave the way to investigate the function of broader extra-lattice species such as rare earth and phosphorous in zeolite.