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

摘要

The strength and distribution of Bronsted acidic protons in anhydrous phosphomolybdic acid (H3PMo_(12)O_(40), 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. Bronsted 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 Bronsted 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 (O_b) > edge-sharing (O_c) terminal (O_d) sites. Moreover, a tendency of hybridization among Bronsted protons residing at O_b and O_c sites of HPMo was inferred by experimental as well as theoretical ~(31)P chemical shifts of the adsorbed TMPO.