Combined Solid-State NMR and Theoretical Calculation Studies of Brønsted Acid Properties in Anhydrous 12-Molybdophosphoric Acid

Abstract

The strength and distribution of Brønsted acidic protons in anhydrous phosphomolybdic acid (H3PMo12O40, HPMo) have been studied by solid-state magic-angle-spinning (MAS) NMR, using trimethylphosphine oxide (TMPO) as the probe molecule in conjunction with density functional theory (DFT) calculations. Brønsted acid sties with strengths exceeding the threshold of superacidity (Zheng, A. et al. J. Phys. Chem. B 2008, 112, 4496) were observed for HPMo. In addition, the locations and adsorption structures of Brønsted protons on various oxygen sites in HPMo were also identified. The preferred location of the acidic proton was found to follow the trend: corner-sharing (Ob) > edge-sharing (Oc) ≫ terminal (Od) sites. Moreover, a tendency of hybridization among Brønsted protons residing at Ob and Oc sites of HPMo was inferred by experimental as well as theoretical 31P chemical shifts of the adsorbed TMPO.