J. Phys. Chem. B, ASAP Article 10.1021/jp0614087 S1520-6106(06)01408-8
Acidity of Mesoporous MoOx/ZrO2 and WOx/ZrO2 Materials: A Combined Solid-State NMR and Theoretical Calculation Study
Jun Xu, Anmin Zheng, Jun Yang, Yongchao Su, Jiqing Wang, Danlin Zeng, Mingjin Zhang,* Chaohui Ye, and Feng Deng*
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Insitute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, P. R. China, and Department of Chemistry, Wuhan University of Science and Technology, Wuhan 430081, P. R. China
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 1H MAS NMR experiments clearly revealed the presence of two different types of strong Brnsted acid sites on both MoOx/ZrO2 and WOx/ZrO2 mesoporous materials, which were able to prontonate adsorbed pyrine-d5 (resulting in 1H NMR signals at chemical shifts in the range 16-19 ppm) as well as adsorbed trimethylphosphine (giving rise to 31P NMR signal at ca. 0 ppm). The 13C NMR of adsorbed 2-13C-acetone indicated that the average Brnsted 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 Brnsted 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.