Carboxylation of methane

Table 3 Carboxylation of methane, ethane, and propane with COa...

Scheme 5. Carboxylation of methane by VO(acac)2, VO(acac)2 (0.05 mmol), CH (5atm), CO (20atm), K2S2Os (lOmmol),...

Acetic acid is an important commodity chemical, and the direct carboxylation of methane with C02 is of obvious industrial interest. This reaction has been observed with a [Pd(0Ac)2]/[Cu(0Ac)2]/02/CF3C00H system, albeit with low yields.83... 

As we have noted, most carboxylations and decarboxylations occur by way of anionic intermediates, and the stabilities of these anions are important in determining rates of carboxylations and decarboxylations. The simplest conceivable case is formation of acetic acid from methane and CO2. Methane has a pAa of approximately 48 14) consequently, removal of a proton from methane is precluded in nature and acetic acid is not formed in nature by the carboxylation of methane, nor is acetic acid decarboxylated. Instead, other substrates are used wherein the pA a is more favorable. 

These oxidative carbonylations also occur with alkanes, although with low conversions of alkane and relatively low turnover numbers. Fujiwara reported that cyclohexane reacts with CO and K S Oj in the presence of palladium acetate and copper acetate in trifluoroacetic acid to form cyclohexane carboxylic acid with about 20 turnovers and about 4% yield based on alkane (Equation 18.24). Fujiwara also reported the carboxylation of methane with KjSjOj as oxidant with V(0)(acac)j as catalyst (Equation 18.25). This reaction occurred in 93% yield based on methane and with 18 turnovers. Sen reported a palladium(II)-catalyzed oxidative carbonylation of methane with hydrogen peroxide as the oxidant. Subsequently, he showed that RhClj catalyzes the conversion of methane, CO, and oxygen to acetic acid at 100 °C in water (Equation 18.26). Periana has reported a somewhat related transformation of methane to acetic acid, although the reaction is conducted in the absence of CO, and both carbon atoms of the acetic acid arise from methane (Equation 18.27). In this case, the CO appears to arise from oxidation of the methaiie, as shown in Scheme 18.5. 

TABLE 2. Carboxylation of Methane, Ethane, and Propane by Various Catafyst Systems ... 

Scheme 22.4 Carboxylation of methane and ethane catalyzed by scorpionate V or Re complexes.

Nizova, G.V., Shul pin, G.B., Suss-Fink, G., and Stanislas, S. (1998) Carboxylation of methane with CO or CO2 in aqueous-solution catalyzed by vanadium complexes. Chem. Commun., 1885-1886. 

Among other remarkable developments, it should be noted that the carboxylation of methane can be coupled with oxidation by potassium peroxysulfate in trifluoro-acetic acid to yield acetic acid, the reaction being catalyzed by Cu(OAc)2 ... 

The direct carboxylation of methane to acetic add, a process of great industrial interest, has been achieved in low yield using a two step process with Rh and Pd catalysts [65]. 

Fig. 4. Proposed catalytic cycle for the hydroxylation of methane by MMO. The reductase and B components may interact with the hydroxylase in one or more steps of the cycle. Protons are shown in the step in which intermediate Q is generated, but other possibilities exist (see Fig. 3 and the text). The curved line represents a bridging glutamate carboxylate ligand.

Gaseous alkanes such as methane, ethane, and propane were also carboxylated to give acetic, propionic, and butyric acids, respectively, as shown in Table 3 102,103,103a Ethane and propane were best carboxylated by the mixed catalyst of Pd(OAc)2 and Cu(OAc)2, while methane was not effectively carboxylated by the same catalytic system. In the case of methane, Cu(OAc)2 gave the best result among the catalysts employed. However, the yield of acetic acid based on methane is low (Equation (78)). 

The best characterized of the BMMs are the sMMOs (Figure 13.24), which are the only members of the family capable of activating the inert C-H bond of methane, one of the most difficult reactions in nature to achieve. Like most members of the BMM superfamily, sMMO requires three protein components, the hydroxylase MMOH, which contains the carboxylate-bridged diiron centre, a regulatory protein MMOB and a [2Fe-2S]- and FAD-containing reductase (MMOR) which shuttles electrons from NADH to the diiron centre. 

The reactivity of the methyl complexes 14a and 15 with carboxylic acid was examined to provide carboxylato complexes as useful precursors for enzyme models. For example, a slight excess of acetic acid does cleave the Zn-CHa bond by elimination of methane and yields [Zn(bd bpza)02CCH3] (17) (Scheme 16) and [Zn(bpa ) O2CCH3] (18). 

Oxidation by flame treatment for polyolefins exposure to a flame of methane, propane or butane and oxygen in excess for a very short time (less than 0.2 seconds) to create oxidation and reactive sites such as hydroxyl, carbonyl, carboxyl... Particularly used for polyethylene and polypropylene. 

The simplest carboxylic acid is formic acid (more systematically methanoic acid) HCOOH. Formic acid contains a single carbon atom and can be thought of as a derivative of methane or methanol obtained by 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. 

A different approach, the direct conversion of methane to acetic acid, has been revealed.178,179 The process is an oxidative carboxylation in the presence of RhCl3 in an aqueous medium under mild conditions (100°C). 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

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