Nb-Doping-Induced Transformation of Lewis and Brønsted Acidic Sites on Sulfonated TiO₂ for Highly Efficient HMF Synthesis

The presence and optimal balance of Lewis and Brønsted acid sites in solid acid catalysts are crucial for their effectiveness in heterogeneous catalytic reactions. However, achieving this balance has proven to be a challenging task. In this study, the impact of niobium doping on sulfonated TiO₂ catalysts was systematically discovered, both through theoretical modeling and experimental analysis. The introduction of Nb caused a shift in the proportion of Lewis and Brønsted acid sites, resulting in a remarkable improvement in fructose conversion rates. Among the catalysts tested, the S-TiNb9 catalyst exhibited the highest reaction rate, achieving a conversion rate of 1.63 mM·min^-1, which is 9 times faster than that of TiO₂ catalysts (0.19 mM·min^-1). Detailed characterization techniques and density functional theory analysis revealed that the presence of Nb on the catalyst surface led to the formation of the Nb-SO₄ structure, enhancing the acidity of Nb atoms rather than Ti-HSO₄. This coexistence of Lewis and Brønsted acidic sites facilitated an alternative pathway for fructose dehydration. The findings of this study represent a significant advancement in the field of acid catalysis by providing a straightforward and practical approach for creating highly efficient catalysts and alternative reaction pathways.