Synthesis of formaldehyde from CO2 catalyzed by the coupled photo-enzyme system.

[Display omitted] • CO 2 was converted into formaldehyde by the coupled photo-enzyme catalysis. • The enzyme reduction of CO 2 is predominant compared to the photo-reduction. • EDTA is more efficient than H 2 O while the decomposition cannot be excluded. • For the sequence enzymatic reaction, the second step is rate-limiting. • The yield of formaldehyde can reach 3.83% with the addition of 1 mM NAD+. Using the reduced nicotinamide adenine dinucleotide (NADH) as coenzyme, formate dehydrogenase (FDH) and formaldehyde dehydrogenase (FADH) can catalyze the reduction of CO 2 to formaldehyde. In this study, a hollow fiber membrane module was used as the enzyme-bearing reactor and gas distributor, and then coupled with a UV/TiO 2 photocatalytic NADH regeneration system. First, photocatalytic CO 2 reduction with varied pH value and electron donor was studied as a control. Then, CO 2 reduction catalyzed by the photo-enzyme coupled system was investigated and the limiting step of the reactions was analyzed. At last, the ratio of FDH and FADH, and cofactor concentration were optimized, and the operation stability was examined as well. The results show that the optimal pH value for the production of formaldehyde by photocatalysis and photo-enzyme coupling catalysis is respectively 6.5 and 7.0 with H 2 O as electron donor. At pH 7.0, the initial reaction rate is 0.035 and 0.104 mmol/(L·h), and the formaldehyde production after 5 h is 0.117 and 0.393 mmol/L, proving the enzymatic reduction of CO 2 is predominant in the coupled system. With EDTA as electron donor, the optimal pH value is 5.5 for the coupled system, and the formaldehyde production is much higher than that with H 2 O at all pH values, however, part of them might come from EDTA. For the sequence reactions, the second step is a rate-limiting step, and cofactor regeneration is vital for enzyme catalysis. The optimal loading ratio of FDH and FADH is 1:0.3, and with the addition of 1 mmol/L NAD+ or NADH, the yield of formaldehyde can reach 3.83% and 6.47% after 4 h. The photo-enzyme coupled system shows a good manipulation stability during 48 h. [ABSTRACT FROM AUTHOR]