High-Selective Oxidation of n-Propanol to Propionic Acid Catalysed by Amino-Acid-Based Polyoxometalates

Background: Propionic acid as a very valuable chemical is in high demand, and it is industrially produced via the oxo-synthesis of ethylene or ethyl alcohol and via the oxidation of propionaldehyde with oxygen. It is urgent to discover a new preparation method for propionic acid via a green route. Recyclable amino-acid-based organic-inorganic heteropolyoxometalates were first used to high-efficiently catalyse the selective oxidation of n -propanol to propionic acid with H 2 O 2 as an oxidant. Result: A series of amino-acid-based heteropoly catalysts using different types of amino acids and heteropoly acids were synthesized, and the experimental results showed proline-based heteropolyphosphatotungstate [ProH] 3 PW 12 O 40 exhibited excellent catalytic activity for the selective catalytic oxidation of n-propanol to propionic acid owing to its high capacity as an oxygen transfer agent and suitable acidity. Under optimized reaction conditions, the conversion of n -propanol and the selectivity of propionic acid reached 88% and 75%, respectively. Over four cycles, the conversion remained at ˃80%, and the selectivity was ˃60%. [ProH]3PW12O40 was also used to catalyse the oxidations of n -butanol, n -pentanol, n -hexanol, and benzyl alcohol. All the reactions had high conversions, with the corresponding acids being the primary oxidation product. Conclusions: Proline-based heteropolyoxometalate [ProH] 3 PW 12 O 40 has been successfully used to catalyse the selective oxidation of primary alcohols to the corresponding carboxylic acids with H 2 O 2 as the oxidant. The new developed catalytic oxidation system is mild, high-efficient, and reliable. This study provides a potential green route for the preparation propionic acid.