Synergistic Effect of Electron-Deficient Ru⁰ and Lewis Acid Sites on Ru/TiO₂ for Efficient Reductive Amination of Furfural via Promoting Schiff Base Ammonolysis

Ru-based catalysts are widely used for the reductive amination of carbonyl compounds. However, challenges remain, particularly in achieving high catalytic activity under mild conditions. In this study, we report a strategy to simultaneously modulate the electron density of Ru0 and the RuOx/Ru0 ratio by systematically adjusting the calcination temperature. The catalyst calcined at 200 °C demonstrated remarkable performance, achieving a 97.8% yield of furfurylamine at 30 °C, significantly outperforming most previously reported catalysts. Furthermore, it showed compatibility with a range of carbonyl compounds. At 200 °C calcination, the reduced electron density of Ru0 weakened its electron-donating ability, thereby facilitating H2 activation. Meanwhile, this treatment yielded the highest RuOx content and thereby generated the greatest Lewis acid site density, enhancing the adsorption and activation of NH3, furfural, and the Schiff base. Mechanistic studies reveal that the synergistic effect between electron-deficient Ru0 and Lewis acid sites significantly reduced the apparent activation energy for the ammonolysis of Schiff base intermediates, the rate-determining step in reductive amination. This work provides an effective strategy for tuning both the electronic structure of the metal and the acid properties of the catalyst, highlighting the crucial role of metal-acid synergy in promoting Schiff base ammonolysis during reductive amination.