**2. Conventional catalytic approach to convert methane to oxygenates**

### **2.1 Gas phase reaction based on homogeneous radical mechanism**

This reaction is a free radical conducted under high temperature and pressure. The thermodynamic and kinetics studies identified the partial oxidation of methane as the rate limiting step due to the formation of methyl radical [7, 11]. Many studies with different oxidants have been conducted in this route. Babero et al. studied the partial oxidation of methane at 500 C temperature using nitrogen oxide as an oxidant [12]. Another study compared between oxygen and nitrogen oxide for the partial oxidation of methane in the gas phase [13]. The effect of adding small quantities of hydrocarbons such as ethane was investigated to promote the activation of methane and increase the selectivity of methanol [14]. Pressure is one of the most important factors which has a pronounced effect on the selectivity of methane oxidation. Dozens of studies have been performed in attempts to promote the selectivity toward oxygenates using high pressures and temperatures [8, 9, 15]. The results of these studies show that a conversion of 5–10% and a methanol yield of 30–40% can be achieved at a temperature of 723–773 K and pressures of 30–60 bar in the gas phase reaction. There are several works that investigated the reactor design and modifications. Zhang et al. designed a new tubular reactor based on quartz-line and stainless-steel line. The reaction was conducted at a temperature of 430–470°C and 5.0 Mpa pressure, and a high yield of methanol was obtained [9].

*Advances in Selective Oxidation of Methane DOI: http://dx.doi.org/10.5772/intechopen.86642*

The methane conversion to methanol was also conducted in the absence of catalysts at high reaction conditions. Methanol yields as high as 7–8% are obtained in the absence of catalysts operating at 350–500°C and 50 bar [10, 11, 16]. As reactor inertness is essential for obtaining good selectivity to methanol, the feed gas should be isolated from the metal wall by using quartz and Pyrex glass-lined reactors [17]. A typical experimental conversion-selectivity plot for the gas-phase partial oxidation of methane is shown in **Figure 1** [18]. This plot ably demonstrates that any improvement in the direct conversion of methane to methanol must come from the enhancement of selectivity without reducing the conversion per pass. The Huels process uses cold-flame burners operating at 60 bar, with a selectivity of 71% to methanol and 14% to formaldehyde, and a recycle ratio of 200 to 1 [8].

The suggested mechanism for the direct conversion of methane to methanol via homogeneous gas phase reactions is shown in **Figure 2** [19].
