Sulfated and phosphated H-type niobate nanotubes as solid acid catalysts

Highly pure sodium niobate nanotubes (NaNbNTs) were prepared by treating nonporous Nb₂O₅ powder with 1.0 M NaOH solution at 423 K. SEM revealed that these NaNbNTs formed nanotube-bundles with diameters of 50-250 nm and lengths of several microns. A TEM image of nanotubes indicated that they had outer diameters of 15-20 nm and inner pore diameters of 3-4 nm. BET surface area and pore volume of NaNbNTs were 65 m² g^-1 and 0.17 mL g^-1, respectively. Treatment with diluted phosphoric or sulfuric solutions transformed NaNbNTs into phosphate- or sulfate-promoted protonated niobate nanotubes (PO₄^-3/HNbNTs and SO₄^-2/HNbNTs), which had a surface area of ∼80 m² g^-1, a pore volume ∼ 0.26 mL g^-1 and a surface that was densely covered with acid sites (1.0-1.5 mmol g^-1). Owing to the high density of its acid sites, SO₄^-2/HNbNTs outperformed a superacidic sulfated metal oxide like SO₄^-2/HfO₂ in catalyzing the formation of cyclic acetals from carbonyl compounds and ethylene glycol. TPD/NH₃ investigations indicated that the acid site density in PO₄^-3/HNbNTs was even higher than in SO₄^-2/HNbNTs, but its activity in catalyzing the formation of cyclic acetals was lower. These results reveal that acid sites in PO₄^-3/HNbNTs had a severe steric effect owing to the thick surface phosphate layer, permitting only the adsorption of small molecules. Large molecules, like heptanal, may not be easily adsorbed on a surface that is densely populated with PO₄^-3 groups, causing PO₄^-3/HNbNTs to have a lower activity than SO₄^-2/HNbNTs, which had a lower surface population of equally bulky SO₄^-2 groups.