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

• Hydrothermal treatment of Nb2O5 powder in 1.0 M NaOH at 423 K produces NaNbNTs.
• Acid treatment of NaNbNTs with H2SO4 and H3PO4 solutions produces PO4−3PO4−3/HNbNTs and SO4−2SO4−2/HNbNTs.
• Acid site densities in PO4−3PO4−3/HNbNTs and SO4−2SO4−2/HNbNTs are twice that in most solid acids.
• Due to a steric influence of the surface phosphate layer, PO4−3PO4−3/HNbNTs is much less active than SO4−2SO4−2/HNbNTs.
• SO4−2SO4−2/HNbNTs is more active than the superacidic SO4−2SO4−2/HfO2 in catalyzing the formation of cyclic acetal.

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 (PO4−3PO4−3/HNbNTs and SO4−2SO4−2/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, SO4−2SO4−2/HNbNTs outperformed a superacidic sulfated metal oxide like SO4−2SO4−2/HfO2 in catalyzing the formation of cyclic acetals from carbonyl compounds and ethylene glycol. TPD/NH3 investigations indicated that the acid site density in PO4−3PO4−3/HNbNTs was even higher than in SO4−2SO4−2/HNbNTs, but its activity in catalyzing the formation of cyclic acetals was lower. These results reveal that acid sites in PO4−3PO4−3/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 PO4−3PO4−3 groups, causing PO4−3PO4−3/HNbNTs to have a lower activity than SO4−2SO4−2/HNbNTs, which had a lower surface population of equally bulky SO4−2SO4−2 groups.

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