Acidity of mesoporous MoOx/ZrO2 and WOx/ZrO2 materials:: A combined solid-state NMR and theoretical calculation study

The acidity of mesoporous MoOx/ZrO2 and WOx/ZrO2 materials was studied in detail by multinuclear solid-state NMR techniques as well as DFT quantum chemical calculations. The H-1 MAS NMR experiments clearly revealed the presence of two different types of strong Brlnsted acid sites on both MoOx/ZrO2 and WOx/ZrO2 mesoporous materials, which were able to prontonate adsorbed pyrine-d(5) (resulting in H-1 NMR signals at chemical shifts in the range 16-19 ppm) as well as adsorbed trimethylphosphine ( giving rise to P-31 NMR signal at ca. 0 ppm). The C-13 NMR of adsorbed 2-C-13-acetone indicated that the average Brlnsted acid strength of the two mesoporous materials was stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which was in good agreement with theoretical predictions. The quantum chemical calculations revealed the detailed structures of the two distinct types of Brlnsted acid sites formed on the mesoporous MoOx/ZrO2 and WOx/ZrO2. The existence of both monomer and oligomer Mo ( or W) species containing a Mo-OH-Zr ( or W-OH-Zr) bridging OH group was confirmed with the former having an acid strength close to zeolite HZSM-5, with the latter having an acid strength similar to sulfated zirconia. On the basis of our NMR experimental and theoretical calculation results, a possible mechanism was proposed for the formation of acid sites on these mesoporous materials.