71Ga NMR Signatures of Lewis and Brønsted Acid Sites in Gallium Silicates Evidenced and Deciphered upon Interaction with Probe Molecules

Determining the structure of Ga surface sites in Ga-based materials used in catalysis is critical for understanding their reactivity. However, obtaining molecular-level insights into their structures remains challenging. Here, using solid-state nuclear magnetic resonance (NMR) spectroscopy (71Ga, 31P, and 15N), two different families of acid sites are identified in a gallosilicate material (Ga@SiO2), prepared via surface organometallic chemistry and used as an efficient heterogeneous propane dehydrogenation catalyst or catalyst support. Through probe molecule studies using pyridine (py) and triphenylphosphine oxide (TPPO), coupled with the synthesis of well-defined molecular analogs of surface sites and computational modeling on related cluster models using density functional theory, the 71Ga NMR signatures and the molecular structures of two types of sites, namely, Brønsted acid sites and Lewis acid sites, are identified. These tetracoordinate sites can be described as Ga sites bound to three anionic siloxide surface ligands along with a silanol or a siloxane bridge, each associated with specific NMR parameters, which change markedly upon interaction with py or TPPO, thereby enabling their structural characterization.