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