**2.2 A low temperature catalytic route involving homogeneous and heterogeneous catalysis**

At moderate conditions, catalysts play an important role in the partial oxidation conversion of methane to methanol in terms of controlling the selectivity of the desired yield. Several catalysts have been investigated at 1 atm and mild temperatures.

In homogeneous systems, in the early 1970s, it was shown that methane could be converted to methanol by Pt(II) and Pt(IV) complexes because these complexes do not oxidise CH3OH to COx [20]. Since that breakthrough, several oxidation catalysts based on Pt(II), Pd(II) and Hg(II) salts have been proven to functionalize C-H bonds [21, 22], leading to good yields of partially oxidised products (Eq. (1)). For example, [(2,2′-bipyrimidine)PtCl2] catalyses the selective oxidation of CH4 in fuming sulphuric acid to give methyl bisulphate in a 72% one-pass yield at 81% selectivity based on methane. Methyl bisulphate is then hydrolysed to methanol (Eq. (2)).

#### **Figure 1.**

*Homogeneous gas-phase partial oxidation of methane from several studies: (1) Lott and Sliepcevich; (3) Tripathy; (4) Brockhaus; (6) Hunter; (8) Rytz and Baiker [18].*

#### **Figure 2.**

*Schematic diagram of the methane conversion via homogeneous gas phase reaction.*

$$\text{CH}\_4 + 2\text{H}\_2\text{SO}\_4 \rightarrow \text{CH}\_3\text{OSO}\_3 + \text{SO}\_2 + 2\text{H}\_2\text{O} \tag{1}$$

$$\text{CH}\_3\text{OSO}\_3 \star \text{H}\_2\text{O} \rightarrow \text{CH}\_3\text{OH} \star \text{H}\_2\text{SO}\_4\tag{2}$$

The major drawbacks of the liquid phase include not only the difficulty of separating the methanol product from the solvent but also solvents such as sulphuric acid need expensive corrosion-resistant materials and periodic regeneration of the consumed H2SO4. A new class of solid catalyst based on immobilised complexes has recently been reported for the direct low-temperature oxidation of methane to methanol [23]. This solid catalyst has a covalent triazine-based framework (CTF) with numerous accessible bipyridyl structure units, which should allow the coordination of platinum (**Figure 3a** and **b**) [23]. The performance of these new catalysts showed that the activity is maintained throughout several cycles, and selectivity for methanol formation above 75% could be reached.

In nature, methane monooxygenase enzymes (MMO) transform CH4 to CH3OH in water under ambient conditions [11]. A number of metal complexes have been proposed to mimic the chemistry of these enzymes [11, 24, 25], but the systems which generate active oxygen species capable of converting CH4 to CH3OH are yet to be created. In contrast to organometallic CH4 activation, MMO proceeds via a different mechanism by creating a very strong oxidising di-iron species able to attack a C-H bond in CH4. An essential feature of MMO is an active site containing two iron centres [11]. Metallophthalocyanines (MPc), and more specifically iron phthalocyanines (FePc), are good catalysts for clean oxidation processes. More specifically, FePc supported in μ-oxo dimeric form (Fe-O-Fe fragment) has better catalytic properties in CH4 conversion in the presence of hydrogen peroxide as an oxidant than its monomer counterpart (FePc). The heterolytic cleavage of the O-O bond in the FeIVNFeIIIOOH complex and the formation of very strong oxidising FeIVNFeV = O species are favoured in the presence of acid by the protonation of peroxide oxygen [11, 24, 25]. A new oxidation mechanism based on the use of metal clusters to harness the 'singlet oxene', the most reactive form of the oxygen atom, has recently been proposed [11, 26]. In this proposal, the key to oxygen insertion is a complex containing three copper atoms, in which the atomic charges vary. By synthesising a series of ligands to complex three copper atoms, mimicking the likely structure of the active site in pMMO, facile O-atom insertion into C-C and C-H bonds has been demonstrated in a number of simple organic substrates under ambient conditions of temperature and pressure. The ligands were designed to form the proper spatial and electronic geometry to harness a 'singlet oxene' [11, 25, 26]. It has been shown that the activity for methanol synthesis is 5 mol (CH3OH) kg (catalyst)<sup>−</sup><sup>1</sup> h<sup>−</sup><sup>1</sup> for sMMO as a complete enzyme with NADH present and this result represents the bench-market by which MMO catalysts should be judged. However, when NADH cofactor is removed, H2O2 can be used as the terminal oxidant with the enzyme but the catalytic activity decreases to 0.076 mol (CH3OH) h<sup>−</sup><sup>1</sup> kg (MMOH)<sup>−</sup><sup>1</sup> [27].

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

### **Figure 3.**

*(a) Bipyrimidyl Pt(II) complex used in the oxidation of methane to methyl bisulphate in concentrated sulphuric acid. (b) Covalent triazine-based framework (CTF) with numerous accessible bipyridyl structure units which are suitable to coordinate the Pt(II) complex.*

In heterogeneous catalytic systems, many attempts have been conducted for the oxidation of methane. In most cases, SiO2 was used as a support with different metals, and O2 as oxidant. It was claimed that [28, 29] with a similar condition, HCHO might be produced with one-pass yield from 0 to 4%. However, high one pass HCHO yields were reported in some publications, but the results were not confirmed by other groups. It is stated that the results by different researchers have always been quite different, and some of them were even contradictory to one other [25, 30]. For instance, in one published work, a high selectivity (90%) to oxygenates (CH3OH + HCHO) was obtained at CH4 conversion of 20–25% at 873 K in an excess amount of water vapour over MoO3/SiO2 catalysts prepared by a sol-gel method [31]. Another work conducted the experiments under similar reactions with MoO3/SiO2 catalyst prepared by a similar method, but the yield of oxygenates was not greater than 4% [25]. Another example of contradiction showed that by using N2O as an oxidant, CH3OH could be achieved with a noticeable selectivity in the presence of H2O over MoO3/SiO2 [32]. Results published by other groups used

similar catalytic systems, but the results showed no detectable formation of CH3OH [33, 34]. Metal-containing zeolites such as Co, Cu, and Fe have been studied at low temperature in batch mode [35, 36]. The direct conversion of methane to methanol over this metal-containing zeolite consists of three steps: (1) formation of active species by calcinations in air, (2) reaction of the active species with methane at low temperature and (3) extraction of methanol, using a polar protic solvent [37]. However, these catalytic systems are not yet a continuous process as an extraction procedure for methanol is required [36, 37]. A series of catalysts based on MoO3 and WO3 were studied, and the WO3-based catalysts were less effective for the production of methanol. The Ga2O3/MoO3 catalyst showed the maximum methanol yield [38].

In a series of seminal publications, Hutchings and co-workers demonstrated that Fe-ZSM-5- and Fe-Cu-ZSM-5-based zeolites and Au-Pd supported on titania could activate methane at temperatures under 100°C using aqueous hydrogen peroxide as the terminal oxidant [39, 40]. The initial product of reaction was shown to be methyl hydroperoxide which subsequently reacted to yield methanol and formic acid. **Figure 4** shows the time on line activity over Au-Pd/TiO2.

The turnover frequencies based on Fe were high with albeit very low conversions. More recently, Al-Shihri et al. [41] showed that the reaction pathway of oxidation in aqueous hydrogen peroxide over ZSM-5 catalysts followed the reaction sequence CH4 → CH3OOH → HCHO → formic acid. Although at the reaction conditions used the formaldehyde was oxidised rapidly to formic acid, it was also converted to low molecular weight polyoxomethylene polymer. Similar findings were achieved in preliminary results using Au-Pd catalysts. However, the use of aqueous hydrogen peroxide to oxidise methane is unlikely to prove economic unless its parallel catalysed decomposition into oxygen and H2O can be supressed. Thus, the development of a viable liquid phase process based on the use of aqueous hydrogen peroxide as the terminal oxidant would be challenging.

An attractive alternative approach is to couple in situ direct generation of hydrogen peroxide from hydrogen and oxygen with methane oxidation in a tandem process. Au-Pd catalysts have proved to be highly active for the direct hydrogen peroxide synthesis reaction and capable of enhancing the tandem catalytic oxidation of alcohols, especially using nanostructured oxide supports [42, 43]. However, while the production rate of hydrogen peroxide is high, the achievable

#### **Figure 4.**

*Time on line activity reaction temperature: 50°C, [H2O2]: 5000 μmol, solvent: H2O, 10 mL. Catalyst: 1.0 × 10<sup>−</sup>5 mol of metal, 28 mg 2.5 wt% Au-2.5 wt% Pd/TiO.*

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

concentration in the liquid phase remains low due to the catalysed decomposition of the formed hydrogen peroxide. This means that a tandem process in the liquid phase is more likely to find application in the selective oxidation of high value chemicals. For direct selective oxidation of light hydrocarbons to oxygenated compounds, a gas phase continuous process based on the use of heterogeneous catalysts would be more attractive. In a tandem oxidation process, the oxidant would be oxygen, air or N2O mixed with hydrogen to generate surface hydroperoxy in situ by the surface reaction of hydrogen-oxygen.

In a preliminary study, Al-Shihri et al. demonstrated that Au-Pd catalysts were able to catalyse the gas phase direct selective oxidation of methane at moderate conditions using the tandem synthesis of hydrogen peroxide from hydrogen-oxygen mixtures. The products of reaction were trapped and found to be methylhydroperoxide, polyoxomethylene, and a small amount of formic acid. Based on analogy with the liquid phase reaction sequence described above [41], the production of polyoxomethylene would be expected to involve initially the formation of formaldehyde as a reaction intermediate, although none was detected. This aspect is currently being investigated prior to publication of these exciting new results. The production of methyl hydroperoxide, formaldehyde and polyoxomethylene from methane is highly desirable. Polyoxomethylenes are valuable polymeric materials and also potential hydrogen storage materials; methyl hydroperoxide can be utilised to form methanol or react to form other compounds. In our preliminary study, the most promising Au-Pd catalysts were based on the use of nanostructured oxide supports. However, the catalysts were prepared by simple impregnation and were far from optimum in terms of metal particle dispersion and degree of Au-Pd alloy formation. These factors are important in the activity, selectivity, and maximising the selectivity in the use of the hydrogen, that is, avoiding direct combustion to water.

Shan et al. showed that mononuclear rhodium species supported by zeolite or titanium dioxide in aqueous solution can convert methane to methanol and acetic acid with high selectivity, using oxygen and carbon monoxide under mild conditions [44]. In a recent study, the direct conversion of methane to methanol was investigated using experimental and computational study. The results of this study showed that low Ni loadings on a CeO2(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts [45].

Gold-based catalysts have also shown interesting performance for the activation of C-H bond in alkane selective oxidation with dioxygen. A particular focus has been put on the synthesis of cyclohexanone and cyclohexanol. Zhao and co-workers [46] first applied gold catalysis in the activation of cyclohexane: Au/ZSM-5 and Au/MCM-41 favoured selectivity around 90% and conversions of 10–15% at 150°C, even though a loss in both activity and selectivity after their reuse is a drawback for industrial application. Two recent studies on the selective cyclohexane oxidation were performed by tailoring a supported gold on different materials, namely amorphous silica doped with titania and alumina prepared by a modified direct anionic exchange method [47].

In the direct gas phase oxidation of methane to methanol, no noble metal except Pd was investigated, and there was no promising results obtained when Pd was used and that might be due to the excessive interaction between Pd and the supports [25, 48]. Therefore, in order to overcome the extent of interaction between Pd and the support, and to increase selectivity toward methanol, bimetallic systems seem to be a more promising solution. Great success has been achieved in a variety of catalytic processes by combining two metallic elements in bimetallic catalysts, such as the platinum-iridium (Pt-Ir) system for petroleum reforming, platinum-tin (Pt-Sn) for alkane dehydrogenation, the nickel-gold (Ni-Au) system for steam reforming of alkanes, and the palladium-gold (Pd-Au) for selective oxidations [49].

### **2.3 Challenges in technologies for the conventional methods**

The industrial production of methanol is executed via indirect way, in which methane is first converted to synthesise gas in highly intensive energy step. The synthesis gas is then converted to methanol. The intensive energy synthesis gas step occurs in operational plant at pressure range between 200 and 600 psi and temperature range between 700 and 1000°C [50]. This step is responsible for 60% of the capital cost of the plant. Therefore, the direct conversion of methane to methanol is highly desirable. Several approaches have been investigated and reported; however, no breakthrough has been achieved yet.

The homogeneous gas phase partial oxidation has the potential to replace the industrial method. In a technical evaluation study of this method, it was shown that a methanol selectivity of over 70% at 15% methane conversion can be achieved. However, the low conversion of methane per pass and relatively low methanol selectivity is still observed in most of the academic reports [51, 52]. The problem is due to kinetic and thermodynamic reasons [53]. This way requires a pressure of around 10 atm and temperature (1000°C) to activate methane and convert it to methanol with reasonable selectivity. The C-H bond in methane (440 kJ/mol) is stronger than the same bond in methanol (389 kJ/mol). That means at high temperatures, the methanol is more reactive than methane, which might lead to the decomposition of methanol to low grade product [52, 54–56]. In addition, the gas phase homogeneous is a free radical reaction, which means that it is difficult to control the process on the large scale [51, 54, 57].

The catalysts play an important role in activating methane at low reaction conditions and produce methanol with low byproducts [58, 59]. Two advantages of this method are the reduction of energy consumption used for methane conversion to methanol, and the low concentration of CO2 produced in this process [58, 59]. The important factor in this process is to find catalysts that could activate methane at moderate conditions and convert it selectively to methanol. Although intensive work has been reported, no catalytic system has achieved the target conversion and selectivity.

A low temperature homogeneous catalyst in solutions is another way to convert methane to methanol at low temperatures. However, two challenges of this method is first the introduction of the catalysts with reasonable reactivity and selectivity that also tolerates oxidising and protic conditions [11]. The second challenge is the use of acid as a solvent such as sulphuric acid, which is applied in many studies. The main disadvantage of using sulphuric acid as solvent is the difficulty in separating the methanol from the solvent. Moreover, the acid might corrode and poison the catalysts through the reaction [11].

In nature, methane monooxygenases (MMOs) demonstrate high activity for methanol synthesis with a production rate of 5 mol (CH3OH) kg (catalyst)-1 h<sup>−</sup><sup>1</sup> at ambient conditions. However, this method is still not practical yet due to the difficulty in purifying these proteins and the further oxidation reaction of methanol to formaldehyde.

## **3. Unconventional approach to convert methane to oxygenates**

### **3.1 Conversion of methane to methanol via plasma technologies**

Plasma can be used in many applications including oxidation of methane to methanol [60]. In plasma, the oxidation of methane to methanol can be conducted under atmospheric gas pressure. Plasma is often referred to as the fourth state of

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

matter, and it includes several components: positive ions, negative ions, electrons and neutral species. Plasma technology can be classified into thermal plasma and non-thermal plasma [61]. Thermal plasma can be described as a gas consisting of electrons, highly excited atoms, ions, radicals, photons and neutral particles, while electrons that have much higher energy than other surrounding particles populate non-thermal plasma. Okazaki et al. [62] reported that the conversion of methane to methanol was achieved using non-equilibrium plasma chemical reactions under atmospheric pressure by ultra-short pulsed barrier discharge in an extremely thin glass tube reactor. Various designs for plasma reactors for the oxidation of methane have been proposed to enhance the selectivity toward methanol. For example, in thermal plasma reactors, the dielectric barrier discharge (DBD) reactor was used for the synthesis of methanol from methane. This reactor was able to reduce the required temperature and pressure needed [63]. Another reactor is a new nonthermal discharge micro-reactor, which is used for a single-step, non-catalytic, direct and selective synthesis of methanol via methane partial oxidation at room temperature [64]. The non-thermal plasma can be developed by integrating the reactor with catalyst to improve the activity and selectivity of methane oxidation. In a recent study, the Cu-doped Ni was loaded on CeO2, which led to enhance the selectivity of methanol until 36% [65]. In another study, multicomponent catalysts were combined with plasma in two different approaches, in-plasma catalysis (IPC) and post-plasma catalysis (PPC), for achieving high levels in both methane conversion and aimed methanol selectivity through the synergetic effect of the Fe2O3 <sup>−</sup>CuO/γ-Al2O3 catalyst [66].
