**1. Introduction**

The global consumption of energy and hydrocarbon-related commodities will continue to increase as the world population increases. The three sources of energy oil, gas, and coal are still dominating over 80% of the global energy matrix. However, natural gas is considered as the bridge fuel between the fossil fuel of today and the renewable fuel of tomorrow. It is cheap, more abundant than oil, and has lower CO2 emissions compared with oil and coal. These factors place natural gas, and by extension methane, as a principal candidate for replacing petroleum as a chemical feedstock and addressing various environmental issues. Natural gas is a flammable substance obtained from oil or gas fields and coal mines. At present, the confirmed natural gas reserves have a total volume of 187 trillion metric, of which 24.8% is found in the Middle East, 30.4% in Europe and Eurasia, 8.4% in the Asia Pacific region, 7.5% in Africa, 6.8% in North America and 4.1% in Middle and South America [1–5]. Natural gas is typically used as a fuel for power generation and for domestic heating. In 1971, global primary energy consumption was based on 48% oil, 29% coal and 18% natural gas. However, in 2015, the consumption of 13.1 billion tonnes (oil equivalent) of fuel was based on 33% oil, 30% coal and 24% natural gas [1], reflecting a shift from oil to natural gas. This transition from oil to natural gas consumption is expected to gradually increase until 2035 [1–3].

Natural gas resources are located in remote areas, and its utilisation is affected by high transportation costs. Therefore, conversion of natural gas to high value chemicals is the most promising solution. Methane and ethane are the main components of natural gas; they are stable and have no functional group, magnetic

moment or polar distribution to facilitate chemical attacks. The C-H bonds of these light hydrocarbons are strong and require high reaction conditions to be activated.

One of the most challenging processes in the chemical industry is the conversion of natural gas or methane to methanol, which is an important intermediate source of energy in our daily lives. Methanol can be used as a convenient energy storage material, a fuel, and a feedstock to synthesise hydrocarbons which mankind get from fossil fuel nowadays [2, 3] One of the importances of methanol comes from its direct use as a fuel or blending with gasoline to improve the octane number although it has half the volumetric energy density (15.6 MJ/L) relative to gasoline (34.2 MJ/L) and diesel (38.6 MJ/L) [4–6]. There had been 15,000 methanol-powered cars during the 1990s granted by the Environmental Protection Agency (EPA), but the use was discontinued due to an increased natural gas price [7]. Methanol is also a key feedstock for chemical manufacturing. The most major derivatives from methanol are formaldehyde, acetic acid, methyl tertiary butyl ether (MTBE) and dimethyl ether (DME). In recent years, methanol to hydrocarbons (MTH) research has been growing rapidly including methanol-to-gasoline (MTG) and methanol-to olefins (MTO) technology [8–10].

In industries, an indirect route for the conversion of natural gas to methanol is used. In this reaction, methane is first converted to synthesis gas by steam reforming, and the synthesis gas is then converted to methanol. However, the production of syngas is an energy-intensive process, which is operated between 800 and 1000°C, and more than 25% of the feed (natural gas) has to be burned to provide the heat of reaction. The direct conversion of methane to methanol in a single step without going through the reforming step is a desired alternative to the current technology [2, 4, 5]. In spite of the fact that there are no actual plants yet for the process of direct methane to methanol (DMTM), previous experimental and theoretical works have demonstrated the feasibility of this route [2, 4, 5]. Here, this chapter will mainly focus on the recent efforts on the direct conversion of methane to oxygenates.
