It has been reported that enzymatic-catalyzed reduction of CO2 is feasible. Most of literature focuses on the conversion of CO2 to methanol. Herein we put emphasis on the sequential conversion of CO2 to formaldehyde and its single reactions. It appears that CO2 pressure plays a critical role and higher pressure is greatly helpful to form more HCOOH as well as HCHO. The reverse reaction became severe in the reduction of CO2 to formaldehyde after 10 h, decreasing HCHO production. Increasing the mass ratio of formate dehydrogenase to formaldehyde dehydrogenase could promote the sequential reaction. At concentrations of nicotinamide adenine dinucleotide lower than 100 mmol·L-1, the reduction of CO2 was accelerated by increasing cofactor concentration. The optimum p H value and concentration of phosphate buffer were determined as 6.0 and 0.05 mol·L-1, respectively,for the overall reaction. It seems that thermodynamic factor such as p H is restrictive to the sequential reaction due to distinct divergence in appropriate p H range between its single reactions.
