Methanol from methane and oxygen

Methane and molecular oxygen give methanol as the main carbon product at a once-through conversion high enough to matter, in a catalytic reactor held at steady state, and the methanol is not an artifact of an empty tube.

Methanol is made industrially by reforming methane to syngas and then hydrogenating carbon monoxide, at high temperature and in a plant that cannot sit on a remote gas well. A direct oxidation that kept most of the carbon in methanol at a few percent conversion per pass would be the number a process evaluation can compare with that plant. High selectivity at a conversion near zero, and high conversion at selectivity near a third, are both real results. Neither is that number.

Dummer, Willock, He, Howard, Lewis, Qi, Taylor, Xu, Bethell, Kiely, and Hutchings reviewed the target in Chemical Reviews (doi:10.1021/acs.chemrev.2c00439 (opens in a new tab)). Summarizing technical evaluations collected by Foulds and Gray, they report a consistent view that about 80 percent selectivity at 5 percent once-through conversion, or about 70 percent at 10 percent conversion, would be approximate parity with the syngas route. Their own reading is that a selectivity of about 75 percent at a once-through conversion of at least 5 percent is the minimum that could compete, and that this joint outcome surpassed what had been reliably shown with molecular oxygen when the review was written. The same article describes a reasonably reproducible gas-phase result of about 40 to 60 percent methanol selectivity at about 5 percent methane conversion, with a limiting once-through methanol yield of about 2.5 mol percent, in premixed systems. It also notes that if only one oxygen atom of O₂ is incorporated into methanol and no sacrificial reductant is added, the carbon-selectivity ceiling of that stoichiometry is 80 percent, from 5 CH₄ + 4 O₂ → 4 CH₃OH + CO₂ + 2 H₂O. A higher carbon selectivity requires a path that puts both oxygen atoms into methanol, or an accounting different from that stoichiometry.

Three later reports move one coordinate and not both.

Chauhan, Rana, Gupta, Kalita, Thadhani, and Manna oxidized methane with O₂ over an iron site in a metal–organic framework, in a batch reactor at 125 °C for 40 hours, and reported 94 percent selectivity in the liquid phase and about 90 percent overall selectivity to methanol (doi:10.1038/s41467-024-54101-8 (opens in a new tab)). The charge in that experiment is given as 34 mmol of methane, and the carbon balance accounts for 0.58 mmol of methane consumed (0.572 mmol of carbon-containing products, 98.6 percent balance). Dividing 0.58 by 34 gives 1.7 percent conversion. That division is done here from those two reported quantities; the paper does not print “1.7 percent” as the conversion. The same section describes 0.5 mmol of methanol with 5.8 μmol of iron, which is about 86 turnovers over the 40 hours, again calculated here from those two quantities. A conversion of about 13 percent in the same paper belongs to a two-temperature sequence aimed at acetic acid, not to the methanol condition. The reactor is a batch autoclave, not a steady continuous window.

Li, Shi, and co-workers ran ceria under continuous flow at atmospheric pressure (doi:10.1021/acscatal.5c05829 (opens in a new tab)). At 350 °C and the shorter contact time they examined, methane conversion was 3.8 × 10⁻⁴ percent. Methanol accounted for 80 percent of the carbon-containing products at 350 °C, and a stability test held about 85 percent selectivity and a methanol rate of about 3.3 μmol per gram of catalyst per hour over 55 hours. Lengthening the contact time raised conversion to 3.4 × 10⁻² percent and cut the methanol rate from 3.3 to 0.9 μmol g⁻¹ h⁻¹ by over-oxidation. Selectivity near 80 percent and a conversion of 5 percent are not the same experiment.

Nallapareddy and Underwood, using a non-thermal plasma and active product removal, report methanol selectivity above 30 percent at methane conversion above 50 percent, and a single-step methanol yield of 21.4 percent, near atmospheric pressure (doi:10.1021/acssuschemeng.4c01595 (opens in a new tab)). That is the high-conversion, moderate-selectivity corner. It is not 80 percent selectivity.

No combination opened for this problem is 80 percent carbon selectivity to methanol at 5 percent once-through conversion with O₂, in a steady window, above an empty reactor, at a turnover that rules out a stoichiometric oxidant.

The oxidant is O₂, diluted as the procedure specifies. No hydrogen peroxide, nitrous oxide, or other terminal oxidant is used, and no sacrificial reductant is added. The reactor is continuous. The reported point is a steady window of at least 2 hours, not the integral of a pulsed stoichiometric reaction and not a sealed batch held overnight.

Methane conversion is the moles of methane consumed divided by the moles of methane fed in that window. Carbon selectivity to methanol is the moles of carbon in methanol divided by the moles of carbon in all detected carbon-containing products, both measured by calibrated gas chromatography. Methanol dissolved in a liquid phase, or trapped by an adsorbent and then released, is quantified and included. An empty reactor of the same material, volume, temperature, pressure, and flow is run the same day.

The process is catalytic. If the catalyst contains a metal, the moles of methanol in the window are at least 100 times the moles of metal in the bed. If it contains no metal, the moles of methanol are at least 100 times the moles of catalyst, using the formula weight the procedure states for the molecule or the repeat unit. A bed that states no formula weight cannot use the metal-free rule, because the turnover would not be a number. A ¹³CH₄ experiment accounts for the carbon in the methanol.

Carbon selectivity to methanol is greater than 80 percent at a methane conversion of at least 5 percent, the methanol yield exceeds the empty-reactor yield measured side by side, the turnover rule is met, and the ¹³C experiment agrees.

Dummer and co-workers state that when only one oxygen atom of O₂ is incorporated and methane is the only reductant, the maximum theoretical methanol selectivity is 80 percent. Sitting the pass mark on that ceiling would grade the stoichiometric limit of the one-oxygen path. The pass mark is therefore selectivity greater than 80 percent, which is a pass mark defined here, together with the 5 percent once-through conversion those authors treat as the meaningful floor. The parity pair of about 80 percent at 5 percent remains the comparison in the sources. None of the 2024–2025 results cited above occupies both greater-than-80-percent selectivity and 5 percent conversion under the reactor rules in the specification. The turnover of 100 and the 2-hour window are pass marks defined here, so that a stoichiometric metal–oxo disguised as a catalyst, a metal-free bed with no stated formula weight, or a 40-hour batch at about 1.7 percent conversion, does not pass.

• The empty reactor is mandatory. Homogeneous gas-phase oxidation is real; Dummer and co-workers describe reproducible methanol selectivities around 40 to 60 percent at about 5 percent conversion in premixed systems, with once-through yields near 2.5 mol percent. A filled reactor that does not beat the empty tube fails.
• ¹³CH₄ labeling: the ¹³C fraction in the methanol matches the feed within the precision of the measurement.
• A methane-free oxygen run produces no methanol.
• Carbon balance over the window closes to within 10 percent relative. A missing 30 percent of the carbon fails the run even if the detected products look clean. The 10 percent closure is a pass mark for analytical honesty.

Heterogeneous or soluble catalysts, aqueous or gas-phase operation, non-thermal plasma, and any temperature or pressure the apparatus can hold, provided the oxidant is O₂, no sacrificial reductant is added, and the empty-reactor control is run at those same conditions. A methanol-adsorption strategy is allowed if the methanol is then recovered and quantified in the same window.

Hydrogen peroxide, periodate, nitrous oxide, or other stoichiometric oxidants. A reductant such as NADH, hydrogen, or carbon monoxide whose job is to spare one oxygen atom. A stoichiometric metal–oxo reagent, even if it is called a catalyst. Looping unreacted methane in the conversion arithmetic so that a low once-through conversion looks like a high one. Quoting a yield from a single liquid extraction with no gas-phase analysis. Reporting a batch integral in place of the 2-hour continuous window. Pairing a selectivity measured at one contact time with a conversion measured at another.

Selectivity to methanol is at least 80 percent at a methane conversion of at least 1 percent, in the same 2-hour continuous window, the empty-reactor and isotope controls pass, and the turnover rule is met. One percent conversion at that selectivity is a lesser pass mark. It is not a solution. The review treats conversion near 5 percent as the meaningful floor. A 40-hour batch near 1.7 percent conversion does not meet the window, so it is not partial success on these terms either.

Dummer, Willock, He, Howard, Lewis, Qi, Taylor, Xu, Bethell, Kiely, and Hutchings, Chemical Reviews, 2023, 123, 6359–6411, doi:10.1021/acs.chemrev.2c00439 (opens in a new tab). The parity pair of about 80 percent selectivity at 5 percent conversion, the competitive reading of about 75 percent at no less than 5 percent conversion, the gas-phase selectivities of about 40 to 60 percent at about 5 percent conversion, the once-through yield near 2.5 mol percent, and the 80 percent ceiling of the monooxygenase stoichiometry are taken from that review. The underlying 1995 Foulds and Gray paper was not re-read here; the parity pair is cited as Dummer and co-workers report it.

Chauhan, Rana, Gupta, Kalita, Thadhani, and Manna, Nature Communications, 2024, 15, doi:10.1038/s41467-024-54101-8 (opens in a new tab). About 90 percent overall methanol selectivity and 94 percent liquid-phase selectivity, 40-hour batch, 34 mmol of methane charged, 0.58 mmol consumed. The 1.7 percent conversion and the roughly 86 turnovers are calculated here from those reported quantities, as stated above.

Li, Shi, and co-workers, ACS Catalysis, 2025, 15, 20496–20511, doi:10.1021/acscatal.5c05829 (opens in a new tab). Continuous-flow ceria. The conversions 3.8 × 10⁻⁴ percent and 3.4 × 10⁻² percent, the 80 percent share of carbon-containing products, the roughly 85 percent selectivity, and the 3.3 and 0.9 μmol g⁻¹ h⁻¹ rates are taken from that paper.

Nallapareddy and Underwood, ACS Sustainable Chemistry & Engineering, 2024, 12, 9144–9155, doi:10.1021/acssuschemeng.4c01595 (opens in a new tab). Selectivity above 30 percent at conversion above 50 percent, and a methanol yield of 21.4 percent.

The turnover floor, the 2-hour window, the carbon-balance tolerance, and the partial-success conversion are pass marks defined here.