Photocatalytic conversion of lactic acid to malic acid through pyruvic acid in the presence of malic enzyme and semiconductor photocatalysts

Abstract

The photocatalytic fixation of CO₂ in pyruvic acid to yield malic acid has been achieved in TiO₂ microcrystal and CdS particle suspensions using malic enzyme as the catalyst, methyl viologen as the electron mediator and either 2-mercaptoethanol or lactic acid as the hole scavenger. The evaluation of the rate-determining step in the photocatalytic fixation reaction was made at the TiO₂ microcrystal for the case of 2-mercaptoethanol as the hole scavenger. The use of lactic acid as the hole scavenger resulted in the selective photoproduction of pyruvic acid at the CdS photocatalyst, a fraction of which was then converted into malic acid by reductive CO₂ fixation at the malic enzyme. However, it has been found that lactic acid causes a decrease in the catalytic activities of the enzyme, the degree depending on the relative concentrations of malic enzyme and lactic acid.