Catalytic Synthesis of Tartaric Acid from Glucose and Gluconic Acid over AuPt/TiO2 Catalysts: Studies on Catalyst Structure–Performance Dependency

Abstract

Replacing fossil-derived tartaric acid (TA) with sugar-based TA provides a greener route for synthesizing bioplasticizers. However, the detailed mechanism involving C–H and C–C activation of sugar substrates in this area is still under debate. In this work, the plausible reaction mechanism for activation of −CHO (oxidation) and −COOH (decarboxylation) groups in glucose and gluconic acid molecules in the presence of bimetallic AuPt/TiO2 catalysts has been studied and revealed using the UV–vis technique. It is found that Pt and Au phases in bimetallic catalysts selectively promote C–H and C–C cleavage reactions through σ extraction (n−π*) and a π–π (π–π*) coordination mechanism, respectively. The Au2Pt2/TiO2 (Au/Pt: 1/1 atomic ratio) catalyst displays remarkable activity for primary conversion of glucose to gluconic acid (TOF ∼ 20,260 h–1 at 110 °C). Meanwhile, consecutive conversion of gluconic acid to TA can be selectively accelerated by the Au1Pt2/TiO2 (TOF: 3,159 h–1) catalyst. Thus, a record high TA production rate of 12.3 mol/h/molmetal was achieved in this work. Process design and the purification of product mixtures to obtain high quality TA were also proposed and validated at the laboratory scale. The outcome in this work will provide insights for aqueous phase oxidation for synthesis of various valuable sugar-derived carboxylic acids.