December 5, 2011
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Now you can convert alkanes directly to carboxylic acids. The direct, selective conversion of lower alkanes to oxygenated derivatives is a long-standing goal of the petrochemical industry. Because of the relatively low reactivity of alkanes, however, most reactions have low conversions and lead to a mixture of products. To overcome this difficulty, producers steam-crack lower alkanes to ethylene and propylene at high cost. Once the olefins are in hand, many processes are available to convert them to the standard array of petrochemical monomers and intermediates.
A. Pombeiro and co-inventors disclose a method for carbonylating lower alkanes under mild conditions to carboxylic acids that contain an additional carbon atom. Their technique uses a simple solvent system (H2O–MeCN) and relatively low temperatures (50–60 °C), and it works with or without a metal catalyst.
In one example, 1.0–1.5 mmol propane is mixed with 1.0–2.0 mmol K2S2O8, 2.0–3.0 mL H2O, and 2.0–4.0 mL MeCN. The reactor is pressurized with 20–40 atm CO, and the mixture is magnetically stirred and heated for 2–6 h at 25–60 °C. The yield of C4 carboxylic acids is 34.4%: 29.8% isobutyric acid and 4.6% butyric acid.
The inventors also found that a copper catalyst can dramatically improve yields for certain hydrocarbon conversions. In the case of cyclohexane, for example, adding a tetracopper triethanolaminate complex improves the cyclohexanecarboxylic acid yield from 12.3% without the catalyst to 72.3% with it. For propane, however, the yield improves only marginally, from 34.4 to ≈38%, when the copper catalyst is used. (Instituto Superior Tecnico [Lisbon, Portugal]. US Patent 8,058,472, Nov. 15, 2011; Jeffrey S. Plotkin)