# Combination of Chemo- and Biocatalysis: Conversion of Biomethane to Methanol and Formic Acid
## 3 Results and Discussion
### Product Separation
#### 3.4.2 Ion Exchange and Extraction of Formic Acid  
The separation of formic acid is even more crucial since a decrease of the pH affects FDM activity (Figure S8). The high boiling point of formic acid and the fact that it is dissociated at the present low concentrations make distillation nearly impractical. Hence, we investigated possibilities of ion exchange (as used in industrial formic acid production) and extraction of formic acid.  
When ion exchange is used a buffer solution can no longer be utilized. To maintain a neutral pH value under these conditions, formate (and carbonate) species have to be directly separated. The formate adsorption was investigated using an ion exchanger (Purolite A111, Figure S9) and an 80 mM solution of formic acid. The loading over time and formic acid concentration are illustrated in Figure 6a.  
After 8 min half of the maximum loading (0.31 mmol\(\cdot\)g\({}^{-1}\) as given by the manufacturer) was reached followed by a decrease in the further loading rate. For regeneration of the ion exchanger a  
Figure 5: (**a**) Selectivity of different polydimethylsiloxane (PDMS) membranes at different concentrations of methanol. The pressure in the membrane module used in the mass spectrometer was 0.16 mbar. (**b**) Depletion of methanol concentration in the reentate using the pervaporation membrane M12. Operating pressure was 20 mbar at the permeate side, 25 \({}^{\circ}\)C in solution and a membrane area of 0.005 m\({}^{2}\).  
Figure 6: (**a**) Decrease of formic acid concentration in solution and loading of ion exchanger Purolite A111 over time. In the experiment, Purrolite A111 (51.1 g) was dispersed in 200 mL of 80 mM formic acid solution. (**b**) Partition coefficient for formic acid in different mixtures of diethylcarbonate (organic phase) and extracting agents tributulphosphate (TBP) and trioctylamine (TOA). Then, 15 mL of 0.45 g\(\cdot\)L\({}^{-1}\) formic acid solution was extracted with 15 mL of the respective mixture.  
basic treatment is required. However, FDM is not stable under basic conditions. In order to overcome this problem commercially available mixed bed filters can be used which allow a separation of different support materials.  
Besides ion exchange liquid/liquid extraction represents another option for the separation of formic acid which is also used in industry. For this purpose, different mixtures of tributylphosphate (TBP) or trioctylamine (TOA) in diethylcarbonate were investigated. The partition coefficients K\({}_{\rm D}\) were determined by extraction of 10 mM formic acid solutions. Prior to these experiments a possible inhibition of FDM by these extraction agents was tested revealing no effect on enzymatic activity. The K\({}_{\rm D}\) values for the different extraction mixtures are shown in Figure 6b. The K\({}_{\rm D}\) values increase with increasing concentration TBP from 0.23 (0% TBP) to 1.75 (100% TBP) whereas TOA mixtures perform best at low concentration (K\({}_{\rm D}\) 1.24 at 5%). Higher concentrations of TOA decrease the extraction efficiency up to 0.13 for 100% TOA.