Activation of methane

The activation of methane (and of saturated hydrocarbons) is a difficult task to achieve by coordination catalysis under homogeneous conditions, in fact, only 

Webster [38] pointed out that this type of activation is severely limited to D-H exchange and has suggested that there should be, some particular unknown mechanistic feature which prevents the extension of the reaction toward reagents other than H and D. 

The reaction probably proceeds througli a carbenoid mechanism. In fact, both heterogeneous and homogeneous catalysts are known to promote an H-D exchange between methane (or other alkanes) and DjO in the DjO-HOAc system. In the homogeneous phase, catalysis by (PtClj)-, DCIO4 and an arc-inatic additive (for example, pyrene) are used to stabilize the active species. 

One characteristic of the above alkane activation process is the occurrence of a muUiple exchange parameter M greater than unity (M 1.3 - 2) (Note that M = the initial entry rate of D into the alkane divided by the initial rate of H hydrocarbon disappearance). Therefore, each time an alkane is bound to a platinum atom, it suffers several exchanges before being released into the solution. The following scheme has been proposed in the literature to explain the results [31. 

It is generally accepted that Pt(II) is the active species for an H D exchange trecause Pi(IV) is axndinaiively saturated (18e shell), whereas Pi(0) is too weak an acceptor. The Hrst step of the C-H bond activation is a nucleophilic attack of the H bond on the inetal followed by an oxidative addition step. In fact soft ligands (which form more stable complexes with soft Pt(II) ions than with hard ones) inhibit the exchange reaction and the reactivity order paiallels the HSAB scale (Pearson). 

---

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

Rh(TMP)- under these conditions, and in fact the selective activation of methane in benzene solution is a distinctive and unusual feature of this system, given that aryl C—H activation ought to be thermodynamically favored over alkyl C—H activation. The proposed linear transition state proposed in Fig. 8 is the key to this different reactivity. The corresponding trimolecular transition state for an arene would be expected to be bent, and this would be precluded by the bulky TMP... 

Sen A (1999) Catalytic Activation of Methane and Ethane by Metal Compounds. 3 81-95 Sheldon RA, see Arends IWCE (2004) 11 277-320... 

The trend is illustrated for ammonia activation in Figure 1.17 [19]. In this figure, the activation energies of ammonia activation are compared for stepped and nonstepped surfaces of Pt. Similarly as also found for H2O activation [20], the dissociation barrier is found to be invariant to surface structural changes. This is very different compared to the earlier discussed activation of methane that shows a very strong structural dependence. 

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

The activation of methane by microwaves has long been a goal of scientists in attempts to convert this natural gas component into higher hydrocarbons valuable in petrochemistry and the chemical industry. Two pathways are being extensively investigated by research groups all over the world ... 

Methane has also been used as the reducing agent in the catalytic conversion of NO to N2 over Co-ZSM-5 zeolites [75] in the presence of oxygen. The high NO conversions (>70%) were achieved by microwave irradiation at 250-400 °C, whereas under similar conditions thermal runs failed to convert either NO or methane in significant amounts. The high activity and selectivity of the reduction of NO by methane achieved with microwave irradiation was probably because of the activation of methane to form methyl radicals at relatively low reaction temperatures. 

Acetic acid can be synthesized from methane using an aqueous-phase homogeneous system comprising RhCI as catalyst, CO and 02.17 Side-products included methanol and formic acid, although yields of acetic acid increased upon addition of either Pd/C or iodide ions. The active species is thought to be a CH3-Rh(l) derivative, formed from the C-H activation of methane. The activation of ethane was also achieved, although selectivities were lower, with products including acetic and propionic acids and ethanol (Equation (9)). 

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

Phosphine complexes are generally regarded more electron-rich than the corresponding ammine complexes, and which pathway is preferred under these electronic conditions has also been investigated. For trans-PtCl2(PH3)2, oxidative addition has been calculated to be much more favorable than the electrophilic pathway for the activation of methane... 

For each case we will also present catalytic analogues, namely (1) the activation of methane to form methanol with platinum, the reaction of certain aromatics with palladium to give alkene-substituted aromatics, and (2) the alkylation of aromatics with ruthenium catalysts, and the borylation of alkanes and arenes with a variety of metal complexes. 

To summarize, the activation of methane is slow and incomplete on the monohydride la, whereas it is fast and quantitative on the bishydride lb, suggesting... 

Another related problem is associated with over-oxidation of the substrate, in the extreme case resulting in complete combustion. In the case of methane oxidation by FeO", for example, the activation of methane occurs with about 10% of the gas-kinetic collision rate, whereas those of the putative oxygenation products CH3OH, CH2O, and HCOOH occur on every collision [60]. With regard to applied catalysis this would imply that the oxidation products are oxidized faster by about one order of magnitude compared to methane as the initial substrate. In the particular context of heterogeneous oxidation... 

We reported the use of M-heterocyclic carbene complexes (NHC) for the catalytic activation of methane [55,56]. We found that solutions of N-heterocyclic carbene complexes of palladium(II) in carboxylic acids catalyze the conversion of methane to the corresponding methylesters. The high thermal stability of palladium(II) carbene complexes could be shown for complex 18 (Scheme 22), which we also structurally characterized [120]. An extraordinary feature is the unprecedented resistance of the palladium-NHC-complexes 18-22 under the acidic oxidizing conditions which are necessary for the CH-activation and functionalization. 

The activation of methane in solution by an organometallic complex presents some experimental difficulties because any solvent that is likely to be chosen will be more reactive than methane. In addition, insolubility of the complex in liquid methane may preclude reaction with the pure hydrocarbon. These problems were overcome in the case of the reaction of CH4 with the iridium complex of Eq. 15.106 by taking advantage of the fact that the desired hydndo methyl complex Is thermodynamically more stable than other hydrido alkyl complexes. The methyl complex was produced by first creating a hydrido cyclohexyl complex and then allowing it to react with methane, m... 

Generalization of the experimental data lead to the conclusion that gas-phase oxidative activation of methane with hydrogen peroxide is highly effective in the presence of biomimic PPFe3+OH/AlSiMg in a limited time interval (high selectivity up to 97%, conversion exceeding 60 wt.%, relatively low temperature -180°C, atmospheric pressure) [89],... 

The activation of methane [1] is also included as one of the most desired yet not technically viable reactions. Abundant amounts of methane occur with crude oil and as gas in remote locations it is also produced in large quantities during hydrocarbon processing. A large fraction of this methane is flared, because economical use or transportation is not possible. This gas and the abundant resources of methane gas hydrates would make a very suitable feedstock for higher hydrocarbons, if its activation to produce molecules other than synthesis gas were feasible. Despite enormous fundamental and practical efforts [1-5], no applicable method has yet been found for creation of ethylene, methanol, or formaldehyde from methane. 

Oxidation reactions have been the focus of several researchers in recent years. Several pathways are available for the activation of methane with two predominant ones being partial oxidation via oxidative coupling and the other being formation of oxygenates. Sojka, Herman and Klier have recently reported that formaldehyde selectivity can be enhanced by using a doubly promoted (Cu-Fe) doped ZnO with yields of 76 g HCHO (kg cat.) 1h . 31 These reactions were carried out at 750°C at 2.5% conversion. Singly doped Cu-ZnO catalysts yielded CO2 and H2O via deep oxidation whereas singly doped Fe-ZnO primarily yielded HCHO. Doubly doped Cu-Fe-ZnO minimized the formation of C2 products and therefore, the Cu-Fe-ZnO catalyst decreased C2 products and enhanced HCHO formation. 

---