Niobic Acid Nanosheets Synthesized by a Simple Hydrothermal Method as Efficient Brønsted Acid Catalysts

This paper reports a novel bottom-up hydrothermal route for the synthesis of niobic acid nanosheets. This route is simpler and greener than the conventional top-down and multistep route for the synthesis of hydrated metal oxide nanosheets via exfoliation of layered compounds, which typically requires the use of bulky organic cations. We have clarified that the pH of the suspension for hydrothermal treatment, the hydrothermal temperature and time, and the presence of NH 4 + play roles in determining the morphology of the product. We propose that the nanosheet is formed from amorphous niobic acid nanoparticles through a dissolution–crystallization mechanism. The obtained niobic acid nanosheets are uniform with a thickness of ∼2 nm and uniquely possess mainly Brønsted acid sites. As compared to the conventional amorphous niobic acid and several other typical solid acid catalysts, the niobic acid nanosheet synthesized by our bottom-up method exhibits significantly higher activity and selectivity for the Friedel–Crafts alkylation of anisole with benzyl alcohol. We have further demonstrated that our niobic acid nanosheet is a water-tolerant and efficient catalyst for the hydrolysis of inulin, a polysaccharide-based biomass, into fructose.