Unravelling the Reactivity of Framework Lewis Acid Sites towards Methanol Activation on H-ZSM-5 Zeolite with Solid-State NMR Spectroscopy.

Understanding the acid sites on zeolites is vital for the establishment of catalyst structure-activity relationship and exploration of their potential applications in heterogeneous catalysis. Here, we report the identification of active framework Lewis acid sites on ZSM-5 zeolites. The structures of framework-associated tri-coordinated Al that is bonding with hydroxyl groups are determined by using one- dimensional (1D) P and two-dimensional (2D) P-{ Al} NMR spectroscopy of trimethylphosphine oxide probe molecule. 2D C-{ Al} NMR correlation experiments allow the observation of favorable formation of methoxy species on the framework-associated Al Lewis acid sites in methanol reaction at low temperature, which is corroborated by density functional theory calculations. These methoxy species contribute to the further conversion of methanol to hydrocarbons as active C1 species. The results provide new insights into the Lewis acidity of zeolites.