# Combination of Chemo- and Biocatalysis: Conversion of Biomethane to Methanol and Formic Acid
## 3 Results and Discussion  
### Chemocatalytic Methane Activation--Formaldehyde Production  
Methane from synthetic biogas was transformed in the presence of equimolar amounts of carbon dioxide to formaldehyde. As catalyst VO\({}_{\mathrm{x}}\) species supported on SBA-15 (V/SBA-15) were prepared by a grafting procedure since this method provides highly dispersed vanadia species [48] which are selective for formaldehyde. The investigated catalyst possesses a vanadia content of 2.2 wt% as determined by ICP-OES. In the X-ray diffraction pattern (Figures S1 and S2) reflections for the (100), (110) and (200) planes were observed confirming the hexagonal structure of the pure support and the derived catalyst. Nitrogen sorption experiments (Figures S3 and S4) revealed a specific surface area of 581 m\({}^{2}\cdot\)g\({}^{-1}\) and a mean pore diameter of 5.1 nm of the fresh catalyst. Spent V/SBA-15 slightly loses surface area (of 507 m\({}^{2}\cdot\)g\({}^{-1}\)) whereas pore diameter is nearly not affected. Raman spectroscopic investigations on the V/SBA-15 catalyst as well as the pure support (Figure S5) revealed that in V/SBA-15 a band at 1035 cm\({}^{-1}\) appears, which can be assigned to the V=O stretching mode, while the signals from the support at 970 cm\({}^{-1}\) (v\({}_{\mathrm{s}}\)(Si-OH)) and 800 cm\({}^{-1}\) (v\({}_{\mathrm{s}}\)(Si-O-Si)) decreased, respectively [31; 49; 50; 51; 52; 53; 54], indicating the molecular attachment of VO\({}_{\mathrm{x}}\) species on the silica surface. Furthermore, the infrared spectrum of the catalyst (Figure S6) shows, beside the typical v\({}_{\mathrm{s}}\)(SiO-H) band, a small signal at 3660 cm\({}^{-1}\) which has been assigned to the v\({}_{\mathrm{s}}\)(VO-H) mode [55]. According to literature the latter species were responsible for a high selectivity towards formaldehyde [50]. The UV-vis DRS spectra (Figure S7) of fresh and spent V/SBA-15 indicate a high amount of monomeric and low oligomeric VO\({}_{4}\) species which are stable during reaction. Both the samples possess absorption maxima at 250 nm and an UV-vis edge energy (E\({}_{\mathrm{g}}\)) of 3.5 eV. The transparency above 400 nm is a measure for the absence of polymeric or particulate VO\({}_{\mathrm{x}}\) species [51; 52; 56]. Before integrating into the cascade, the performance of the chemocatalytic first stage was tested under different conditions to evaluate the optimal set of reaction parameters. For this purpose, the reaction temperature and the continuous total flow were varied from 575-650 \({}^{\circ}\)C and from 100-300 mL\(\cdot\)min\({}^{-1}\), respectively. The latter correspond to gas hourly space velocities (GHSVs) from 114,000-342,000 L\({}_{\mathrm{CH_{4}}}\cdot\)kg\({}_{\mathrm{cat}}\)\({}^{-1}\cdot\)h\({}^{-1}\). The resulting selectivity vs. conversion dependency is illustrated in Figure 1a and shows a typical behavior for this selective oxidation of methane which was normally carried out in the absence of carbon dioxide [6]. The observed methane conversion (\(\mathrm{X_{CH_{4}}}\)) ranges from 1.0 to 6.8% corresponding to a formaldehyde selectivity (\(\mathrm{S_{CH_{2}O}}\)) between 50.3 and 12.5%, respectively. Side products were mainly CO and 5-10% CO\({}_{2}\). These values are comparable to those reported earlier by our group [31; 33] for SBA-15 supported VO\({}_{\mathrm{x}}\) catalysts obtained from carbon dioxide free reactant gases. Figure 1b depicts the formaldehyde space-time yields (STY) obtained at different conditions. In general, the formaldehyde space-time yield grows continuously with an increasing gas flow. The behavior was observed by increasing the reaction temperature except at very low flow rates caused by overoxidation of produced formaldehyde. The resulting CH\({}_{2}\)O space-time yield ranges from 1.0 to 2.7 \(\mathrm{kg_{CH_{2}O}\cdot kg_{cat}}^{-1}\cdot\mathrm{h}^{-1}\). These values are lower compared to previous studies [32] because of the lower methane concentration in biogas instead of using pure methane/oxygen mixtures.