Changes in Surface Property and Catalysis of Mesoporous Nb₂O₅ from Amorphous to Crystalline Pore Walls

• Published in: Catalysis letters 2011-02, Vol.141 (2), p.283-292
• Main Authors: Tanaka, Manabu, Shima, Hisashi, Yokoi, Toshiyuki, Tatsumi, Takashi, Kondo, Junko N
• Format: Article
• Language: English
• Subjects: Acetonitrile; acidity; Adsorbed water; Aqueous solutions; Catalysis; Chemistry; Chemistry and Materials Science; Colloidal state and disperse state; Conversion; Crystal structure; Crystallinity; Crystallization; Cyclohexene; Epoxidation; Exact sciences and technology; General and physical chemistry; Hydrogen peroxide; Hydrophilicity; Industrial Chemistry/Chemical Engineering; Infrared spectroscopy; Mesoporous materials; Niobium oxides; Organometallic Chemistry; Oxidation; Physical Chemistry; Porous materials; Selectivity; Solvents; Surface physical chemistry; Surface properties; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 1011-372X; 1572-879X
• DOI: 10.1007/s10562-010-0458-1

The amorphous inorganic phase of an ordered amorphous mesoporous Nb₂O₅ with two dimensional hexagonal (2D-hex) structure was crystallized with maintaining the original well arranged porous structure. The difference in surface property between amorphous and crystalline Nb₂O₅ with similar ordered mesoporous structure was compared. It was found from water adsorption-desorption isotherms and observation by infrared (IR) spectroscopy that the amorphous sample was hydrophilic and that the surface OH groups were acidic. On the other hand, the OH groups on crystalline mesoporous Nb₂O₅ were non-acidic and inside the pores was less hydrophilic. The surface property was also compared by a catalytic reaction, oxidation of cyclohexe by an aqueous solution of H₂O₂. The high (95%) selectivity for 1,2-epoxycyclohexane was obtained at 40 °C for 2 h in methanol solvent over crystalline mesoporous Nb₂O₅ at 12% conversion, while amorphous mesoporous Nb₂O₅ showed high (68%) selectivity for 1,2-cyclohexanediol in acetonitrile solvent at 60 °C for 2 h at 22% conversion. The differences in selectivity and the optimal solvent between amorphous and crystalline samples were interpreted in terms of the acidic feature of surface OH groups and hydrophilicity. While similar selectivity was observed over non-porous crystalline Nb₂O₅, much higher conversion over crystalline mesoporous Nb₂O₅ was attained at the same surface area. Thus, an advantage of mesoporous structure is attributed to the higher contact time of molecules inside the pores to the catalyst surface than those outside the particles. Graphical Abstract The selective epoxidation of cyclohexene using H₂O₂ has been recognized as an ideal eco-friendly process. The present catalytic performance of crystalline mesoporous Nb₂O₅ is regarded as one of the best examples to demonstrate the advantage of mesoporous marerials. [graphic removed]