Methane to acetic acid

Acetic acid is an important petrochemical industrially produced from methane in three steps via the Monsanto process. In this process, methanol and carbon monoxide yield acetic acid with a selectivity of 99% at a pressure of 30-60 bar in the temperature range between 150 and 200 °C. Since these both reactants are 

In their pioneering work, Periana et al. [88] reported that Pd(II)/H2S04 catalytic system catalyzed direct conversion of methane to methanol and acetic acid with a combined selectivity of 90% at 455 K in liquid sulfuric acid. It was concluded that carbon atoms in acetic acid originate from methane and methanol, with the latter being primarily formed from methane. The reaction is initiated by the electrophilic CH activation with Pd(II) to yield Pd-CHs species. The activation is accelerated by sulfuric acid. The primarily formed Pd-CHs species is further transformed to methanol with simultaneous reduction of Pd(II) to Pd(0). The reduced Pd species are also formed upon methanol oxidation to some CO species. The latter participate in the carbonylation of the Pd-CHs species. To close the catalytic cycle Pd(0) is oxidized to Pd(II) by sulfuric acid. Since gas-phase O2 did not influence the reaction rate and the selectivity, free radicals were excluded as reaction intermediates. 

in agreement with homogeneous direct methane oxidation to acetic acid, methane is initially oxidized to methanol, which can be oxidized to carbon monoxide. The formation of acetic acid takes place through carbonylation of methanol. 

In contrast to methane conversion to C2 hydrocarbons (Section 23.2) and methanol (Section 23.3), methane conversion to acetic acid is still in its infancy. This approach is by no means competitive to the present Monsanto technology. Successful methane conversion will require novel significantiy more active catalysts. 

Direct oxidative functionalization of methane to C2 hydrocarbons and oxygenates has some inherent obstacles, which are mainly due to the low reactivity of methane molecule compared to the desired products. Thus, reaction products are further oxidized to carbon oxides more easily than methane. This problem may be overcome when using soft oxidants like N2O, NO, or H2O2. However, their price in comparison to the price of the desired products should be taken into account. The application of solar energy for methane activation was not often and thoroughly tested to draw final conclusions and, therefore, deserves further investigating. 

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Considerable challenges still remain in the development of new carbonylation processes for acetic acid manufacture. For example, all of the current processes use iodide compounds, leading to corrosive HI and the need for expensive materials for plant construction. An iodide-free system could potentially impart considerable benefit. Other long term goals include the selective direct conversion of syn-gas or oxidative carbonylation of methane to acetic acid. Organometallic chemists are certain to play a crucial role if these targets are to be achieved. 

Oxidative carbonylation of methane to acetic acid is one of the pursued ways to solve the fundamental problem of direct methane utilization. Partly aqueous systems with RhCU-HCl-KI catalyst mixture were applied with some success for this purpose. However, the reaction proceeds faster in acetic acid as solvent, containing only a small percentage of water [34]. 

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). 

This direct, oxidative condensation of methane to acetic acid in one-pot could be competitive with the current three-step, capital intensive process for the production of acetic acid based on methane reforming to CO, methanol synthesis from CO, and generation of acetic acid by carbonylation of methanol. Key improvements required with the PdS04/H2S04 system, however, will be to develop more stable, faster, and more selective catalysts. Although it is possible sulfuric acid could be utilized industrially as a solvent and oxidant for this reaction, it would be desirable to replace sulfuric acid with a less corrosive material. This chemistry has recently been revisited, verified, and extended by Bell et al., who used Cu(II)/02 as the oxidizing system [22],... 

Pd (II) Catalyzed Oxidative Condensation of Methane to Acetic acid ... 

In addition to Sen s work on the rhodium-catalyzed oxidative carbonylation of methane, Grigoryan has also reported a similar reaction in acetic acid [45]. Predictably, the reaction rate is in-between that observed in pure water and in the perfluorocarboxylic acid-water mixture. Finally, Otsuka has reported the oxidative carbonylation of methane to acetic acid by rhodium-doped iron phosphate [46]. 

Progress has also been achieved toward the direct oxidation of methane to acetic acid, for example, by Periana and coworkers, who used a Pd(II) catalyst in concentrated sulfuric acid at 180 °C [191-193]. The results of isotopic labeling experiments were consistent with a mechanism in which oxidation of methanol (from methane) generates a "CO" species that carbonylates a Pd-methyl species formed by C-H activation. 

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. 

Although the hydrocarboxylation of methane to acetic acid has so far been unsuccessful [18, 19], other quite inert gaseous C2-C4 alkanes can be transformed into the corresponding Q+i carboxylic acids, when using the compounds 5, 6, 7, and 11-13 as catalysts or catalyst precursors. Owing to the presence of only primary carbon atoms, C2H6 is the least reactive alkane, the selective transformation of which to propanoic acid occurs with reasonable efficiency (up to 29% yield based on substrate) in the presence of 5 [13] (Scheme 3.6). 

An even cheaper feedstock than ethane is methane. A direct reaction from methane to acetic acid requires first methane oxidation to methanol followed by a carbonylation step. Catalyst systems based on Pd/Cu or RhCls have been reported to show good acetic acid jdelds in academic work, but for a commerdal process the reaction rates are still too slow. Nevertheless, due to the very attractive feedstock basis this route may become a threat to methanol carbonylation in the future if this problem can be convincingly overcome by further process and catalyst development. 

Lin, M. and Sen, A. (1994) Direct catalytic conversion of methane to acetic acid in an aqueous medium. Nature, 368, 613-615. 

Zerella, M. and Bell, A.T. (2006) Pt-catalyzed oxidative carbonylation of methane to acetic acid in sulfuric acid. 

A.T. (2004) Direct oxidation of methane to acetic acid catalyzed by Pd and Cu in the presence of molecular oxygen. 

Homogeneous, catalytic, oxidative coupling of methane to acetic acid in one step. Top. CataL, 32,169 174. 

Knobler, C.B., and Yaghi, OJVI. (2011) Metal-organic frameworks of vanadium as catalysts for conversion of methane to acetic acid. Lnorg. Chem.,... 

FIGURE 11.21 CANMET Integrated process for converting methane to acetic acid [282,283]. 

Fig. 7.39 C NMR of the crude reaction mixture from the oxidation coupiing of methane to acetic acid-cataiyzed reaction...

Fig. 7.40 Proposed tandem catalysis mechanism for the oxidative condensation of methane to acetic acid with Pd(ll)/H2S04.

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