Ga₂O₂²⁺ Stabilized by Paired Framework Al Atoms in MFI: A Highly Reactive Site in Nonoxidative Propane Dehydrogenation

Determination of the structure of Ga-containing catalytic sites in the propane dehydrogenation (PDH) reaction on Ga/H-ZSM-5 has been a long-standing challenge for understanding Ga-based catalysts but has been hampered by the complexity of the system. We employed quantitative pulse titration reactions to differentiate two types of Ga species based on their distinct redox properties: divalent Ga species balancing paired framework Al atoms and isolated Ga⁺ sites. While isolated Ga⁺ is redox-active, the divalent Ga species cannot be reduced or oxidized at reaction temperature (550 °C) by H₂ or O₂, respectively. Together with in situ infrared spectroscopic evidence, the highly active divalent Ga species is identified to be Ga₂O₂²⁺. The PDH activity of Ga₂O₂²⁺ is shown to be at least a factor of 18 higher than that of isolated Ga⁺, suggesting that increasing the density of the former could be an effective strategy in enhancing the catalytic performance. Interestingly, only 30% of Al pair sites observed through the Co²⁺ exchange protocol are able to stabilize Ga₂O₂²⁺, that is, the latter has a more stringent requirement for the proximity between the two adjacent framework Al atoms, thus providing an additional method to probe the Al distribution in zeolites.