Efficient conversion of CO2 to formic acid by formate dehydrogenase immobilized in a novel alginate–silica hybrid gel

The efficient utilization of CO2 will not only help to alleviate greenhouse effect, but also to obtain useful chemicals as well. A novel bio-pathway has been explored to convert CO2 into formic acid by the aid of formate dehydrogenase (FateDH) as the biocatalyst and reduced nicotinamide adenine dinuncleotide (NADH) as the terminal electron donor. In order to simplify subsequent separation of the enzyme and improve its catalytic stability, the enzyme FateDH was encapsulated in a novel alginate–silica (ALG–SiO2) hybrid gel. This hybrid gel was prepared by in situ hydrolysis and polycondensation of tetramethoxysilane (TMOS) in alginate solution followed by gelation of alginate with Ca2+. The leakage of the enzyme was significantly reduced by hybridization compared to pure alginate. The optimum reaction condition was found to be at pH 7.0 and 37 °C. Under these conditions, the highest yield of formic acid catalyzed by the immobilized FateDH was up to 95.6%, only a little lower than that of the free form enzymatic reaction (98.8%). The relative activity of immobilized FateDH after 10 cycles could be maintained as high as 69%. Storage stability test showed that the relative activity of FateDH in hybrid gel was about fourfold higher than that in pure alginate gel after being kept at 4 °C for 1 month.