Catalytic synthesis of lactic acid from cellulose over easily-prepared niobium-doped titania by solution combustion synthesis

Catalytic activity of doped (5, 10, 15, 20, 25% Nb) and undoped TiO₂, in the conversion of cellulose into lactic acid, was assessed in autoclave conditions. The catalysts were synthesized by solution combustion synthesis and characterized by several techniques such as: XPS, SEM-EDX, Py-FTIR, NH₃-TPD, Raman, N₂ adsorption/desorption isotherms and XRD. The insertion of niobium into titania structure did not lead to any phase segregation of the two oxides (TiO₂ and Nb₂O₅) and didn't even generate Brønsted acid sites on the surface of titania, not even for niobium loading of 25 wt%, leading instead to an increase in Lewis acidity highlighted by an increase in lactic acid yield from 6 to 14 % (over 25%Nb-TiO₂) for a small catalyst-to-cellulose mass ratio of 1:10. On the other hand, for high catalyst-to-cellulose mass ratio up to 2:1, the lactic acid yield was 29.8%. Moreover, due to high water tolerance, the 25%Nb-TiO₂ catalyst could be recycled three times to convert cellulose without significantly losing its catalytic activity.