Sustainable hydrogenation of xylose to xylitol using nickel-alumina catalysts supported on hexagonal mesoporous silica in water vis-à-vis aqueous isopropanol as solvent

Hydrogenation of xylose to xylitol is commercially attractive. The use of water as well as aqueous isopropanol as solvent using a novel robust catalyst was targeted in this work. This work showed that a nickel-alumina catalyst supported on hexagonal mesoporous silica (HMS) can effectively transform lignocellulose-derived xylose fraction to the value-added product, xylitol. HMS support was modified by simultaneous impregnating nickel (6 wt%) and alumina (4 wt%) which was subsequently oxidized. The catalyst was evaluated for its hydrogenation ability of xylose to xylitol. Identical reactions were carried out with HMS, alumina, Al/HMS and Ni/HMS to ascertain the activity of each moiety in the reaction. Before the reaction, the catalyst was reduced in a tubular reactor in the presence of hydrogen. Reaction parameters such as speed of agitation, type of precursor, metal loading, catalyst loading, solvent, concentration and temperature were studied and optimized. At 130℃ and 20 bar H₂, initial xylose concentration of 0.2 mmol/mL with water: isopropanol (1:1 v/v) mixture as solvent, conversion of xylose was found to be 98 % with a selectivity of 97 % towards xylitol. Comparison was also made with water alone as a solvent. A kinetic study of the reaction was performed. The transformation of xylose to xylitol using this novel catalyst is a green process since it entails substantially lower metal loading when compared to the conventional Raney nickel catalysts, it does not involve the use of noble metals such as Pt, Pd, Rh or Ru, and also does not suffer from the problem of metal leaching, making it superior to those catalysts reported yet.