Three acetate-functional polyimides based on 2,2-bis(3-amino-4-hydroxy-phenyl)hexafluoropropane (APAF) and dianhydrides with different linking groups (-O-, -CO-, and -C(CF3)2-) were prepared, from which films were prepared and thermally rearranged (TR) to form poly(benzoxazole) dense membranes. The polymers were characterized by 1H nuclear magnetic resonance, Fourier transform infrared, differential scanning calrimetry, thermogravimetric analysis-mass spectrometry, and wide-angle X-ray diffraction. Transport properties of the TR membranes were investigated, and effects of the linking group and TR conversion rate on the membrane performance were discussed. Among the TR polymers studied, those bearing larger linking groups have higher d-spacing, fractional free volume, and gas permeability. The membrane permeability to gases increases with TR conversion rate and follows the order H2 > CO2 > O2 > N2 > CH4, except that CO2 surpasses H2 in APAF-6FDA-TR450. The CO2/CH4 separation performance of all of the TR polymers studied surpasses Robeson’s 1991 upper bound (J. Membr. Sci. 1991, 62 (2), 165–185), and APAF-6FDA-TR450 is most favorable, the performance of which is closest to the 2008 upper bound (J. Membr. Sci. 2008, 320 (1–2), 390–400).

Alcohol dehydrogenase (ADH) catalyzes the final step in the biosynthesis of methanol from CO2. Here, we report the steady-state kinetics for ADH, using a homogeneous enzyme preparation with formaldehyde as the substrate and nicotinamide adenine dinucleotide (NADH) as the cofactor. When changing NADH concentrations with the fixed concentrations of HCHO (more or less than NADH), kinetic studies revealed a particular zigzag phenomenon for the first time. Increasing formaldehyde concentration can weaken substrate inhibition and improve catalytic efficiency. The kinetic mechanism of ADH was analyzed using the secondary fitting method. The double reciprocal plots (1/v∼1/[HCHO] and 1/[NADH]) strongly demonstrated that the substrate inhibition by NADH was uncompetitive versus formaldehyde and partial. In the direction of formaldehyde reduction, ADH has an ordered kinetic mechanism with formaldehyde adding to enzyme first and product methanol released last. The second reactant NADH can combine with the enzyme–methanol complex and then methanol dissociates from it at a slower rate than from enzyme–methanol. The reaction velocity depends on the relative rates of the alternative pathways. The addition of NADH also accelerates the releasing of methanol. As a result, substrate inhibition and activation occurred intermittently, and the zigzag double reciprocal plot (1/v∼1/[NADH]) was obtained.

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 pH 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 pH is restrictive to the sequential reaction due to distinct divergence in appropriate pH range between its single reactions.●We study serial enzymatic reduction of CO2 and its single reactions.●Reverse reaction is severe in the conversion of CO2 to HCHO.●Increasing CO2 pressure and the mass ratio of FDH to FADH can promote the reduction.●The optimum pH varies for the reactions and is restrictive to sequential reduction.Download full-size image

•CA facilitated the enzymatic conversion of CO2 into formic acid, the reaction rate was increased by a factor of 4.2-fold.•The reaction conditions were optimized.•A higher concentration of CA caused the decrease in the overall reaction rate, increasing FDH/CA was favorable to the reaction.•PB was better than the other several systems, and pH value and the concentration needed be controlled strictly.•The maximum rate was achieved at 37 °C in 0.05 M PB with pH value of 7.0.Enzymatic reduction of CO2 to formic acid promises the production of high-value chemicals from greenhouse gases in the way of high selectivity and low energy consumption. However, slow hydration process of CO2 in reaction solution leads to low production rate of formic acid. Carbonic anhydrase (CA), a vigorous biocatalyst for CO2 hydration, has attracted much attention for its potential applications in CO2 capture and sequestration in recent years, while its use in biosynthesis of chemicals and fuels based on CO2 has hardly been reported. Herein we report the feasibility of CA in facilitating biosynthesis of formic acid from CO2 with the catalysis of formate dehydrogenase (FDH), and various reaction conditions were optimized for the first time. With the addition of CA, the substrate of FDH might transform from CO2 to more soluble HCO3−, together with the increase in the hydration rate of CO2, resulting in that the production rate of formic acid was increased by a factor of 4.2-fold. Due to the decrease in pH value induced by CA with the reaction proceeded, the ratio of two enzymes was important to coordinate the reaction rate and avoid the accumulation of hydrogen ions, at the same time, buffer system was critical to ensure the synergize effect of CA. It appeared to us that the introduction of CA represents a new bioprocessing strategy for efficient biotansformation of CO2 to formic acid and its further products.Download full-size image