# Combination of Chemo- and Biocatalysis: Conversion of Biomethane to Methanol and Formic Acid
## 2 Materials and Methods  
### Chemocatalysis  
Plain SBA-15 was synthesized as described by [PERSON] et al. [47]. In brief, 8 g Pluronic P123 (Aldrich, St. Louis, MO, USA, 1.4 mmol) was diluted in 240 g of 2 M HCl (Fisher Scientific, Pittsburgh, PA, USA) at 40 \({}^{\circ}\)C. Afterwards 17.35 g tetraethyl orthosilicate (Aldrich, 98%, 83.3 mmol) was added dropwise to the solution, which was further stirred for 24 h at 40 \({}^{\circ}\)C. Subsequently the resulting suspension was treated hydrothermally in an autoclave with a PTFE inlet at 100 \({}^{\circ}\)C. After 24 h the white solid was isolated and washed with deionized water until the filtrate was free of Cl\({}^{-}\). The solid residue was dried overnight at 70 \({}^{\circ}\)C and calcined for 16 h at 625 \({}^{\circ}\)C (heating rate 1 K\(\cdot\)min\({}^{-1}\)) in air.  
The functionalization of SBA-15 with VO\({}_{x}\) species was prepared as described by [PERSON] et al. [48]. 2.7 g VO(acac)\({}_{2}\) (Strem Chemicals, 98%, Newburyport, MA, USA, 10.2 mmol) was dissolved in 450 mL of dry toluene in argon atmosphere. To this solution 4.5 g SBA-15 which was dried overnight at 120 \({}^{\circ}\)C was added and stirred for 5 h at room temperature. Afterwards, the solid was isolated and washed four times with toluene, followed by drying at 70 \({}^{\circ}\)C overnight and calcination at 625 \({}^{\circ}\)C for 16 h in air (heating rate 1 K-min-1) yielding 4.4 g of a bright yellow solid. The resulting catalyst is denoted as V/SBA-15.  
Catalytic oxidation of methane was carried out in a horizontal fixed bed plug flow reactor consisting of a quartz tube with an inner diameter of 8 mm. Experiments were performed with 25 mg of the catalyst placed in the center of the quartz tube, fixed by quartz wool. Behind the catalyst bed the inner diameter of the reactor decreases to 4 mm. The oven temperature was controlled by a thermocouple in the middle of the catalyst bed. The catalytic performance was measured in a temperature range of 575-650 \({}^{\circ}\)C at atmospheric pressure for 30 min at each testing point after reaching steady state. The premixed reactant gas contained CH4, CO2 and O2 in a ratio of 9:9:1 (Air Liquide, N45, Dusseldorf, Germany) and the gas flow passing the reactor was controlled by a mass flow controller (MKS-Instruments, Andover, MA, US). The on-line gas analysis was carried out with a FT-IR spectrometer (Bruker Matrix-MG01, Ettlingen, Germany) and the Bruker software _Opus GA_ which uses reference spectra of the single compounds and a nonlinear fitting algorithm.  
Methane conversion (XCH4), formaldehyde yield (YCH2O), formaldehyde selectivity (SCH2O) and space-time yield (STYCH2O) were calculated according to Equations (5) to (8):  
\[X_{CH4}=\frac{c_{CH4}^{f}-c_{CH4}^{p}}{c_{CH4}^{f}}.100\% \tag{5}\]  
\[Y_{CH2O}=\frac{c_{CH2O}^{p}-c_{CH2O}^{f}}{c_{CH4}^{f}}.100\% \tag{6}\]  
\[S_{CH2O}=\frac{Y_{CH2O}}{X_{CH4}}.100\% \tag{7}\]  
\[STY_{CH2O}=\frac{\dot{m}_{CH2O}}{m_{cat}} \tag{8}\]  
where \(c_{i}^{f}\) and \(c_{i}^{p}\) denote the concentration of component \(i\) in the feed gas \(f\) and product gas \(p\), \(\dot{m}_{CH2O}\) the mass flow of formaldehyde and \(m_{cat}\) the mass of the catalyst.