**3.5 Challenges of unconventional ways to convert methane to methanol**

The unconventional technologies such as plasma, photocatalysts, supercritical and biological are long-term processes, and still away from practical. The methane oxidation under plasma conditions is considered as a clean method as there are no harmful emissions produced such as CO2 and CO [80]. The plasma reactor is simple, benign and cheap. However, the productivity of methanol is low due to the limitation of methane solubility in the reaction medium at ambient conditions.

Photocatalysis technology is an attractive way to convert methane to methanol, as the basic requirement for this method is the use of three abundant reactants of light, water and methane. Despite the two decades of work on photocatalysis, the selectivity of methanol is still low [68, 70].

Supercritical water oxidation is an efficient process to treat a variety of hazardous and non-hazardous wastes. However, there are some factors that limit the application of this technology in methane oxidation such as the complication of the reactor design, the high temperature used and the high corrosion rate when using halogens such as chlorine for some waste treatment [60, 81].

The use of membrane technology for methane conversion to methanol is feasible due to the advantage of the effective separation of methane and methanol. However, some challenges still exist for large scale application: first, the relatively high energy requirement for large scale plant, second, the low tolerance of polymerbased membrane to high temperatures and chemicals, third, the high conversion of methane will produce different organic compounds, and that might cause swelling or breakage of the membrane [60, 81].
