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 H2O2 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]3PW12O40 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]3PW12O40 has been successfully used to catalyse the selective oxidation of primary alcohols to the corresponding carboxylic acids with H2O2 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.