# Combination of Chemo- and Biocatalysis: Conversion of Biomethane to Methanol and Formic Acid
## 1 Introduction  
Nowadays, methane is mainly used as a source of energy [1] and its direct chemical utilization is restricted to the formation of (i) syngas [2, 3] or (ii) hydrogen cyanide via the Andrussov or Degussa process [4] as well as (iii) for the production of methyl halides or carbon disulfide [5]. Oxygenates from methane, like methanol, formaldehyde or formic acid, are conventionally being produced via the syngas route because the direct production routes are rather limited. Homogeneous, heterogeneous and enzymatic direct catalytic conversions of methane primarily focus on the target molecule methanol. However, all of these approaches have certain restrictions like batchwise production, low productivity or selectivity owing to the nature of the applied chemistry [6]. The direct chemocatalytic synthesis of methanol from methane can be divided into homogeneous and heterogeneous methods. Among homogenous catalytic approaches the Periana system, [(bypm)PtCl\({}_{2}\)] (bypm: bipyrimidine), converts methane in eleum directly to methyl bisulfate as a methanol derivative with a productivity of 0.115 kgcH\({}_{3 OH}\)-L\({}^{-1}\).h\({}^{-1}\)[7]. The major drawback of this reaction is the need for sulfur trioxide as transfer oxidant which makes it difficult to run the process in larger scale. [PERSON] and coworkers have found that K\({}_{2}\)[PtCl\({}_{4}\)] exhibits a higher productivity (0.48 kgcH\({}_{3 OH}\)-L\({}^{-1}\).h\({}^{-1}\)), but SO\({}_{3}\) concentration has to be even increased [8]. Milder oxidants, like oxygen or hydrogen peroxide have been investigated, too, but to the detriment of very low productivities [9, 10, 11]. In order to overcome limitations in productivity when using milder oxidants, the groups of [PERSON] andStrassner have recently shown that the concept of redox mediation could be a future way in activating alkanes [12; 13]. Heterogeneous catalytic approaches work with immobilized Pt(II) species on N-rich covalent triazine frameworks (CTF), but this may also include eleum as solvent/oxidant [14]. Alternatively, milder oxidants have been used by the group of [PERSON] which applied gold-palladium alloy nanoparticles for the oxidation with H\({}_{2}\)O\({}_{2}\)[15] or aqueous Au-Pd colloids for the oxidation with O\({}_{2}\) in the presence of H\({}_{2}\)O\({}_{2}\)[16]. Obviously, the oxidation of methane with oxygen is still challenging. Several efforts have been made utilizing Cu sites incorporated in zeolites to activate methane and form thermoxide species [17]. However, a subsequent hydrolyzing step with water is required in order to close the reaction cycle [18]. Recently and perhaps revolutionarily, [PERSON] et al. opened the possibility to transform methane to methanol by an anaerobic oxidation with water over copper-exchanged mordenite zeolite [19]. Bicostasis shows that the activation of gas phase oxygen and its further reaction with methane is principally feasible. Enzymes classified as methane monooxygenases (MMO) [20; 21; 22; 23], found in methanotrophic bacteria, do not only produce methanol selectively, but they also work under mild conditions of bacterial living. In soluble MMO (sMMO) di-iron clusters with bis-\(\upmu\)-oxo diamond core structure are discussed as active sites [21] whereas in particulate MMO (pMMO) a tri-copper center catalyzes the reaction [24]. Synthetic approaches mimicking the active site of the latter have already been successfully demonstrated [25; 26; 27]. Although the direct conversion of methane to methanol is possible using such natural or biomimetic structures, conversion rates and turn over numbers, respectively, are rather low and chemocatalytic activation of methane remains preferred. With regards to thermodynamic data [28] the fundamental problem of chemocatalytic approaches can be expressed as sensitivity of oxygenates against over-oxidation (see Equations (1)-(4)).