Methacrylic acid by carboxylation of propene with CO2 over POM catalysts — Reality or wishful thinking?

Abstract

Inspired by promising literature data on the production of methacrylic acid (MAA) via heterogeneous selective gas-phase oxidation of propene by CO2, we performed a comprehensive study including in situ FTIR spectroscopy, catalytic tests as well as thermodynamic calculations using the same type of catalysts to reproduce these very surprising data and to further improve this process. Exemplarily, pure and supported Keggin type polyoxometalates (POM) Ni3[PW12O40]2 and Ni3[PMo12O40]2 were treated in different feeds containing CO2, propene/He, and propene/CO2/He. It was found that CO2 is only chemisorbed as surface carbonates but does not act as oxidizing agent. Instead, this role is played by lattice oxygen from the POM catalysts which, however, does not lead to the desired MAA target product but only to various unselective oxygenates including COx. Thermodynamic calculations clearly showed that reaction of CO2 with propene to MAA is not possible. Thus, this study clearly rebuts previous literature data.