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

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-K alpha) of the synthesized niobium oxide showed characteristic peaks at 2 theta = 22.7 degrees and 46.2 degrees 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 N-2 adsorption at low pressure (1.0 x 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 degrees C, and Bronsted 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.