Revealing the Bronsted\nAcidic Nature of Penta-Coordinated\nAluminum Species in Dealuminated Zeolite Y with Solid-State NMR Spectroscopy

The inevitable dealumination process of zeolite Y is\nclosely related\nto ultrastabilization, enhanced Bro̷nsted acidity, and deactivation\nthroughout its life cycle, producing complex aluminum and acidic hydroxyl\nspecies. Most investigations on dehydrated zeolites have focused on\nthe Bro̷nsted acidity of tetra-coordinated Al (Al(IV)) and Lewis\nacidity associated with tricoordinated Al (Al(III)) sites, which has\nleft the penta-coordinated Al (Al(V)) in dealuminated zeolites scarcely\ndiscussed. This is largely due to the oversimplified view of detectable\nAl(V) as an exclusively extra-framework species with Lewis acidity.\nHere we report the formation of Bro̷nsted acidic penta-coordinated\nAl species (Al(V)-BAS) in the dealumination process. Two-dimensional\n(2D) through-bond and multiquantum <sup>1</sup>H-{<sup>27</sup>Al}\ncorrelation solid-state NMR experiments demonstrate the presence of\na bridging Si–OH–Al(V) structure in dealuminated Y zeolites.\nDifferent from the conventional belief that water attack leads to\nthe breaking of zeolite framework Al–O bonds in the initial\nstage of zeolite dealumination, the observed Al(V) as a dealumination\nintermediate is directly correlated with a BAS pair because of the\ndirect dissociation of water on the framework tetrahedral aluminum\n(Al(IV)), thus bypassing the cleavage of Al–O bonds. <sup>1</sup>H double-quantum solid-state NMR experiments and theoretical calculations\nprovide further evidence for this mechanism, whereas pyridine adsorption\nexperiments confirm stronger acidity of Al(V)-BASs than the traditional\nbridging hydroxyl groups associated with Al(IV). We were also able\nto detect the Al(V)-BAS site from dealuminated SSZ-13 zeolite with\nCHA topology, suggesting that its creation is not specific to the\nframework structure of zeolites.