Changes in Surface Property and Catalysis of Mesoporous Nb 2 O 5 from Amorphous to Crystalline Pore Walls
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Changes in Surface Property and Catalysis of Mesoporous Nb2O5 from Amorphous to Crystalline Pore Walls Manabu Tanaka • Hisashi Shima • Toshiyuki Yokoi Takashi Tatsumi • Junko N. Kondo
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Received: 30 May 2010 / Accepted: 27 September 2010 / Published online: 17 November 2010 Ó Springer Science+Business Media, LLC 2010
Abstract 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.
M. Tanaka H. Shima T. Yokoi T. Tatsumi J. N. Kondo (&) Chemical Resources Laboratory, Tokyo Institute of Technology, R1-10, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan e-mail: [email protected]
Keywords
Mesoporous materials Oxidation Acidity
1 Introduction The popularity of metal oxides in recent synthetic research on mesoporous materials seems to be increasing, due to the notable catalytic applications in comparison to previous synthetic research, and several successful results have been reported [1]. Crystallization of as-synthesized amorphous mesoporous metal oxides dramatically changes the electronic and surface structures in addition to stabilization of mesoporous structures. The stability of mesoporous silica samples against hydrothermal conditions was improved by inclusion of protozeolitic inorganic phase in frameworks [2–4] by mixing templates for zeolite and mesoporous structures. The presence of aluminum atoms in ptotozeolitic mesoporous silicas provides acid catalytic property. In the case of metal oxides, the specific magnetic [5], electronic [6], and semiconducting [7] properties of meta
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