# Combination of Chemo- and Biocatalysis: Conversion of Biomethane to Methanol and Formic Acid
## 3 Results and Discussion
### Biocatalytic Disproportionation of Formaldehyde  
The produced formaldehyde can be transformed into methanol and formic acid by FDM. The immobilization of FDM on specific carriers offers long term and continuous processing modes as well as simple product recovery. Hence, we first tested several carriers with different functional groups. The surface specific activity allows a comparison of the different carriers. Table 1 shows the calculated enzyme activity related to the surfaces of the carrier particles. As can be seen, the carriers with a quaternary ammonium compound as functional group showed the highest area-specific activity. With regards to long-term stability, the best results were achieved with the columns IB-150A and IB-C435. After 440 days and 29 measurements the residual activity of the enzyme on a polyacrylic carrier IB-150A with an epoxy group was still 40%. The carrier IB-C435 with carboxyl groups showed a half-life of 329 days after 348 days and 16 measurements. The carrier IB-150A had a lower half-life of 291 days compared to the carrier IB-C435 but a higher activity. We have chosen carrier IB-150A for further experiments since it offers sufficient initial activity at high long-term stability. Figure 1: (**a**) Formaldehyde selectivity (\(\mathrm{S_{CH_{2}O}}\)) vs. methane conversion (\(\mathrm{X_{CH_{4}}}\)) at total flow rates of 100–300 mL\(\cdot\)min\({}^{-1}\) (\(\Delta\)F = 50 mL\(\cdot\)min\({}^{-1}\)). The dotted lines represent different possible formaldehyde yields (Y). (**b**) Space-time yield (STY) of formaldehyde at different flow rates and temperatures. The activity of the immobilized FDM was tested in batch and compared to that obtained over free FDM. The reaction catalyzed by the free FDM was completed seven- to eightfold faster compared to the immobilized FDM what might be caused by mass transport limitations and damages or hindrance of the enzymes caused by the immobilization process. The maximal reaction rate \(\mathrm{v_{max}}\) of the free FDM reached a value of 44.7 mmol-min\({}^{-1}\) which was much higher than the \(\mathrm{v_{max}}\) of the immobilized FDM with 1.6 mmol-min\({}^{-1}\). The corresponding Michaelis constants \(\mathrm{K_{m}}\), the substrate concentration with half maximum reaction rate, were determined to a value of 50.9 mM and 41.2 mM, respectively. However, increasing concentrations of methanol and formic acid rapidly inhibit product formation (see K\({}_{\mathrm{i}}\) in Figure 2). In order to illustrate this serious problem, activity tests with the free enzyme at different starting concentrations of either methanol or formic acid, respectively, are shown in Figure 3. In the experiments with formic acid, a buffer was added to exclude inhibition by a lower pH value. The temporal decrease of the formaldehyde concentration with ongoing reaction time at different product concentrations is obvious. For both products, an inhibition effect was observed. While methanol completely inhibits the reaction at a concentration of 1.5 M an addition of formic acid already stops the reaction at a concentration of 0.7 M.