Synthesis of formic acid from CO ₂ catalyzed by formate dehydrogenase immobilized on hollow fiber membrane

Formate dehydrogenase (FDH) was covalently attached to surface-modified polyethylene (PE) hollow fiber membrane. The effects of inlet way of CO ₂, pH value, the type of buffer solution, and the concentration of the reduced coenzyme nicotinamide adenine dinucleotide (NADH) on the synthesis of formic acid from CO ₂ were investigated. The catalytic performance of PE-supported FDH and its reusability were studied. The results showed that the CO ₂ bubbling method was superior to pressing method. Phosphate buffer was more favorable to the formation of formic acid than Tris-HCl and triethanolamine-HCl buffer. Immobilized FDH was less sensitive to pH than free FDH, and the optimum pH value was 6.0 for both systems. With the increase of NADH concentration, the initial reaction rate ascended while the yield tended to decrease. Enzyme activity reached 0.246 and 0.138 mmol/(L·h), respectively, for free and immobilized FDH when using 100 mmol/L of NADH. The stability was greatly improved after immobilization. The activity of PE-attached FDH only decreased by 4% while that of free enzyme dropped to 50% of initial activity after stored in phosphate buffer at 4°C for two weeks. Furthermore, immobilized FDH exhibited outstanding reusability, which almost kept original activity after undergoing 10 cycles.