**3.3 Conversion of methane to methanol using supercritical water**

The supercritical water oxidation (SCWO) is a reaction that occurs in water at elevated temperatures and pressures above the thermodynamic critical point of the

### **Figure 5.**

*Graphical representation of reaction of conversion of methane to methanol via photocatalysts [73].*

mixture. Under the supercritical fluid conditions, the properties of water such as viscosity and dielectric constant can be adjusted between high gas-like diffusion rates and high liquid-like collision rates by varying pressure and temperature [60]. The catalytic oxidation of methane was examined over Cr2O3 under supercritical water conditions, and it was found that this catalytic system under supercritical conditions enhances the conversion rate of ethane and promotes the selectivity of methanol [74]. Another study investigated the isothermal conditions with a laminar reactor in SCWO for the direct partial oxidation of methane to methanol. They achieved a methanol selectivity of 35% at a conversion of 3% at temperatures of 400–410°C [75]. Savage PE et al., [76] have examined two types of reactors, glasslined reactors and stainless steel reactors. A parametric study has been conducted using both reactors, and the glass lined reactor showed higher conversion of methane to methanol.

### **3.4 Conversion of methane to methanol using membrane technology**

Membrane technology has been used for methane conversion to methanol using membrane reactor at moderate conditions. The advantage of using a membrane reactor is the fact that it can perform two functions at once, reaction and separation. The membrane can be classified based on the type of materials and porosity. The membrane can be made either by polymeric or organic materials with different porosity [60]. The organic membrane has advantages over the polymer in terms of the tolerance to chemical and temperature effects. Moreover, the organic membrane is mainly composed of metallic or ceramic materials and has greater physiochemical stability. Two research works studied the methane oxidation to produce methanol using Methylosinus trichosporium OB3b with a high concentration of Cu2+ and they found that the optimization of the conversion rate was positively affected by several parameters including the temperature, pH and concentrations of sodium formate, phosphate buffer and cyclopropanol [77, 78]. In another study, the methane oxidation was carried out using a membrane reactor where the methane and oxygen were introduced by two separate dense silicone tubes. A high methanol production of 1.12 g/L and 60% methane conversion were reported [79].
