Water-assisted catalytic transfer hydrogenation of guaiacol to cyclohexanol over Ru/NiAl2O4

Extensive use of grey hydrogen in traditional hydrodeoxygenation (HDO) severely reduces the sustainable development of lignin valorization processes. Conversely, the catalytic transfer hydrogenation (CTH) mediated by green hydrogen is an alternative strategy that promotes the sustainability of in-situ hydrogenation of lignin. In this work, a bifunctional catalyst (Ru/NiAl2O4) was developed to assist the proposed green hydrogen-driven cosolvent system composed of isopropanol and water for the CTH of guaiacol. Results indicate that 95% conversion of guaiacol and 80% cyclohexanol selectivity can be achieved at 200 ℃ within 3 h with a high water/isopropanol ratio (6/4), significantly increasing the greenness and sustainability. Furthermore, the recycling experiments verify the stability of the catalyst's active sites. Analysis of the mechanism suggests that the high efficiency of such system can be attributed to the activation of metal and acid dual functional sites of Ru/NiAl2O4 catalyst on aromatic rings and C-O bonds. The acceleration of the in-situ hydrogen supply is due to hydrogen exchange between water and isopropanol. The participation of water in the reaction is also demonstrated by the presence of deuterium in the product after the reaction in D2O. This work provided the insights into the development of a strategy for the production of cyclohexanol from lignin derivatives by an in-situ CTH system.