Hydrothermal synthesis of octahedra-based layered niobium oxide and its catalytic activity as a solid acid

Abstract

Layered-structure-type niobium oxides were synthesized by the hydrothermal method by using ammonium niobium oxalate as a precursor. The X-ray diffraction pattern (Cu-Kα) of the synthesized niobium oxide showed characteristic peaks at 2θ = 22.7° and 46.2°, indicating the linear corner-sharing of NbO₆ octahedra in the c-direction. From Raman measurements, the layered-structure-type niobium oxide was composed of NbO₆ octahedra and {Nb₆O₂₁} pentagonal units. The presence of micropores was confirmed by N₂ adsorption at low pressure (1.0 × 10⁻⁶), indicating that the arrangement of the a–b plane was an interconnection of the crystal structure motifs of {Nb₆O₂₁} units and micropore channels but without long-range order (deformed orthorhombic). Ammonium cations and water were desorbed from the deformed orthorhombic niobium oxide by calcination at 400 °C, and Brønsted acid sites were formed. The deformed orthorhombic niobium oxide showed high catalytic activity as a solid acid compared to the catalytic activities of other crystalline niobium oxides. The order of catalytic activity for the alkylation of benzyl alcohol and anisole was deformed orthorhombic Nb₂O₅ > TT-Nb₂O₅, amorphous Nb₂O₅ ≫ T-Nb₂O₅, pyrochlore Nb₂O₅.