Crystal phase-mediated oxidative dehydrogenation of lactic acid to pyruvic acid on MoO3

Abstract

Sustainable production of pyruvic acid was investigated in the presence of MoO3 catalyst through oxidative dehydrogenation of lactic acid with air as oxidizing agent. α-MoO3 can be easily transformed to h-MoO3 under the assistance of monovalent cation, in which Na+ offered the strongest ability. It is found that the reactivity of oxidative dehydrogenation of lactic acid depends on crystal phase of MoO3, and h-MoO3 displays far better activity than α-MoO3. Structure characterizations reveal that h-MoO3 mainly exposes (110) plane while α-MoO3 mainly exposes (010) plane. Adsorption energy of lactic acid molecule on (110) plane with -11.04 eV is far lower than (010) plane with +4.73 eV, suggesting that lactic acid molecule is easily adsorbed on (110) plane, rather than (010) plane. Kinetic study shows that the activation energy is 14.23 kJ/mol on h-MoO3, lower than that on α-MoO3 with 23.15 kJ/mol. XPS results reveal that the binding energy of Mo 3d in h-MoO3 is higher than α-MoO3, suggesting a stronger oxidative ability. Furthermore, h-MoO3 has a larger valence band value of 3.42 eV than α-MoO3 with 3.35 eV, also demonstrating a stronger oxidative ability. H2-TPR results show that the reduction peak at low temperature of around 500 °C is stronger over h-MoO3 than α-MoO3, suggesting that surface-crystal oxygen of h-MoO3 is easilier departed during the oxidative dehydrogenation of lactic acid. At 230 °C, h-MoO3 achieved 82.3% of pyruvic acid selectivity at LA conversion of 82.0%, far higher than α-MoO3 with 38.2% of pyruvic acid selectivity at LA conversion of 42.4%. This work provided a novel strategy for boosting the oxidative dehydrogenation of lactic acid to pyruvic acid by crystal phase-mediated reactivity, and achieving sustainable production of pyruvic acid.