Hydrothermal synthesis of W–Ta–O complex metal oxides by assembling MO6 (M = W or Ta) octahedra and creation of solid acid

• Layered-structure-type tungsten and tantalum oxides (W–Ta–O) were synthesized.
• The synthesized W–Ta–O samples were the same layered-type materials.
• Strong acid sites were generated by ion exchange from Na+ to NH4+.
• We revealed the relationship between the strong acid sites and catalytic activity.
• The crystalline motif of W–Ta–O was an interconnection of MO6 and pentagonal units.

Layered-type tungsten and tantalum oxides (W–Ta–O) were synthesized by the hydrothermal method. The synthesized W–Ta–O showed characteristic peaks at 2θ = 22.7° and 46.2° in an X-ray diffraction pattern (Cu Kα), indicating linear corner sharing of MO6 (M = W, Ta) octahedra in the c-direction. The same layered-type materials were obtained with a wide range of W and Ta composition ratios using soluble Lindqvist-type tantalum polyoxometalate (Na8(Ta6O19)·24.5H2O). Na+ cations of as-synthesized W–Ta–O were replaced with NH4+ and then calcined at 500 °C to form Brønsted acid sites. The catalytic activity of W–Ta–O increased with increasing W ratio, suggesting that strong acid sites were generated. From Raman and adsorption measurements of W–Ta–O with various crystalline structures, it was revealed that the crystalline motif of W–Ta–O in the a–b plane was an interconnection of MO6 (M = W, Ta) octahedra and {M6O21} pentagonal units and micropore channels, but without long-range order.

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