Selective oxidation of uronic acids into aldaric acids over gold catalyst

Herein, uronic acids available from hemicelluloses and pectin were used as raw material for the synthesis of aldaric acids. Au/Al₂O₃ catalyst oxidized glucuronic and galacturonic acids quantitatively to the corresponding glucaric and galactaric acids at pH 8–10 and 40–60 °C with oxygen as oxidant. The pH has a significant effect on the initial reaction rate as well as desorption of acid from the catalyst surface. At pH 10, a TOF value close to 8000 h⁻¹ was measured for glucuronic acid oxidation. The apparent activation energy E_a for glucuronic acid oxidation is dependent on the pH which can be attributed to the higher energy barrier for desorption of acids at lower pH.