Molybdenum oxide with a varied valency ratio to enable selective d-galactose epimerization to d-talose†

Abstract

D-Talose, classified as a rare and expensive sugar molecule, is gaining attraction due to its antimicrobial and anti-inflammatory properties. Its production is widely investigated by adopting biological enzymes, which is costly. However, alternative chemical methodologies have reported its formation as a side product and in minor amounts. We report for the first time its significant synthesis using D-galactose (which comprises whey and hemicellulose) by employing a finely tuned molybdenum oxide (MoO₃) solid acid catalyst. The nitric acid treatment of MoO₃ modulated the valency ratio in Mo species (Mo^5+/6+), resulting in an improved Lewis acidity with up to 199 μmol g⁻¹ acidic sites and porosity of up to 48% relative to the pristine MoO₃, attributed to the generated oxygen vacancies. Combined together these have assisted in an augmented D-talose synthesis with as high as 25% yield, 70% selectivity and 98% carbon balance in a water medium under modest reaction conditions (120 °C and 30 min). As proposed, Mo's interaction with D-galactose to form a Mo–sugar complex has influenced the C1–C2 carbon shift to yield D-talose. Furthermore, the typical isotopic labelling NMR characterization has confirmed the Bílik mechanism of C2-galactose epimerization. Overall, the heterogeneous catalytic setup represents a sustainable and feasible method for producing rare sugar for food additive and pharma applications.