Revealing the contributions of homogeneous and heterogeneous catalysis to isomerization of d-glucose into d-fructose in the presence of basic salts with low solubility

Abstract

Isomerization of D-glucose (Glc) into D-fructose (Fru) presents an important step in the catalytic valorization of cellulosic biomass. However, a rational catalyst design for isomerization poses a challenge. In this work, we studied the catalytic activity of basic salts with low solubility Li₂CO₃, MgCO₃, Li₃PO₄, SrCO₃, CaCO₃, BaCO₃, and Mg₃(PO₄)₂ for Glc isomerization into Fru. In bulk water, these materials generate OH⁻via partial dissolution and protonation of the anions. The catalysts were tested for isomerization using 10 wt% aqueous Glc solution at 60 and 80 °C. The initial rate of Fru formation r_0,Fru shows an excellent correlation with the initial pH values of the reaction mixtures, indicating in situ generated OH⁻ anions as catalytically active species. Filtration and contact tests were performed and their limited applicability for catalysis by bases with low solubility was shown. Li₂CO₃ showed the highest catalytic activity for the isomerization, resulting in 25% Fru yield in 10 minutes at 80 °C. The selectivity of the isomerization depends on the catalyst nature. The highest selectivity for Fru formation was observed in the presence of MgCO₃, giving rise to 27% Fru yield at 80 °C. MgCO₃ and MgO could be recycled without loss of activity.