The development of catalyst systems for the direct low-temperature oxidation of methane to methanol has been one of the major challenges in catalysis over the last decades.

—Ferdi Schüth

The bonds in methane are very strong (binding energy of 435 kJ mol^-1) and difficult to break. In addition, under the reaction conditions required, methanol has the tendency to react further to form carbon dioxide. The process thus requires not only highly active but also highly selective catalysts.

Earlier catalysts targeted at this process, however, mostly suffered from irreversible reduction and bulk metal formation, together with consequently poor selectivity, the authors noted.

One breakthrough was the development of a platinum complex by a research group led by Roy Periana. This complex catalyzes the low-temperature oxidation of methane in concentrated sulfuric acid at temperatures around 200 °C to form methyl sulfate—which can be converted into methanol—in good yield and high selectivity.

Despite highly promising results, the complex separation and difficult recycling of this dissolved catalyst, among other things, hampered the commercial application of this process. Development proceeded to the pilot-plant stage before being abandoned.

A solid catalyst that can be easily separated could make such a process viable on a small scale, making possible the efficient, decentralized consumption of natural gas.

—Ferdi Schüth

The German researchers have now been able to develop such a solid catalyst, whose high reactivity and selectivity, and its stability through numerous recycling steps, have raised prospects for its industrial implementation.

The development is based on the recent discovery of a new class of high-performance polymer frameworks. Polymerization of a ring-shaped molecule, an aromatic nitrile, results in a network known as a “covalent triazine-based framework”, abbreviated as CTF. The materials are thermally stable up to 400 °C and resist strongly oxidizing conditions, which made them appear promising as a solid matrix for methane oxidation along the lines of Periana’s work, the authors wrote.

Loading this substance with platinum results in a highly active, easily separated, and recyclable catalyst.

The platinum-modified material was tested in the direct methane oxidation in concentrated sulfuric acid according to the conditions described by Periana et al. In principle, utilization of sulfuric acid and sulfur trioxide as oxidants, as schematically described in Equations (a)–(d), would allow design of a continuous process. All process steps, including methane oxidation to methyl bisulfate (a), hydrolysis to form free methanol (b), and reoxidation of SO₂ (c) could be integrated in such a system. A solid catalyst, with its advantages of easy separation and recyclability, would facilitate the implementation of such processes to allow efficient conversion of natural gas on-site.

—Palkovits et al.

CH₄ + H₂SO₄ + SO₃ → CH₃OSO₃H + H₂O + SO₂ (a)

CH₃OSO₃H + H₂O → CH₃OH + H₂SO₄ (b)

SO₂ + ½O₂ → SO_{3 (c)}

ΣCH₄ + ½O₂ → CH₃OH (d)

Resources

• Regina Palkovits, Markus Antonietti, Pierre Kuhn, Arne Thomas, Ferdi Schüth (2009) Solid Catalysts for the Selective Low-Temperature Oxidation of Methane to Methanol. Angewandte Chemie International Edition 48, No. 37, 6909-6912, doi: 10.1002/anie.200902009