Methane oxidation on the positive side – a selective industrial route to methanesulfonic acid

Methane is a major component of natural gas and one of the most difficult molecules to activate because most of the product is based on carbon dioxide. The industrial conversion of methane into alcohol derivatives is usually based on a backdoor route beginning with carbon monoxide overoxidation. Although more direct approaches have been shown to be promising in highly acidic, small-scale environments, they are not quite cost-effective. In a recent study now published in Science, Christian Díaz-Urrutia and Timo Ott, R & D Department Grillo-Werke AG, describe a reaction at a pilot plant directly combining methane (CH₄) and sulfur trioxide (SO₃) in sulfuric acid (H₂SO₄) to form methanesulfonic (CH₄O₃S) acid without by-products. The reaction appeared to proceed via a cationic chain mechanism initiated by the addition of a low concentration of sulfonyl peroxide, propagated by methenium (CH).₃⁺) ^molecules.

Direct functionalization of methane to form value-added products is a challenge because of the potential overoxidation in many reaction environments and sulfonation is an attractive approach to achieving selectivity of interest. In practice, Díaz-Urrutia and Ott produced methanesulfonic acid (MSA) using only two main reagents; methane and sulfur trioxide. They achieved a selectivity and a 99% efficiency of MSA in the job. Scientists based the electrophilic initiator on a sulfonyl peroxide derivative, which they protonated under superacid conditions to produce a highly electrophilic oxygen atom capable of activating a CH bond of methane. They proposed mechanistic studies to support the formation of a methenium cation (CH₃⁺)^{as key intermediate during the reaction. The proposed method is scalable with series connected reactors to prospectively produce up to 20 metric tons of MSA per year.}

While large-scale fracturing techniques and biogas production have allowed access to large quantities of inactive methane, the most significant chemical transformation of methane remains confined to energy-demanding Fischer-Tropsch processes. At present, methane is processed industrially into Syngas, a mixture of carbon monoxide and hydrogen, to form useful products, including methanol and Fischer-Tropsch hydrocarbons, which are synthesized during from later stages. The production of syngas is, however, extremely expensive; The factories "MegaMethanol" or the Fischer-Tropsch pearl complex in Qatar exceed 10 million tons of the total annual production of hydrocarbons. Consequently, the direct conversion of methane into valuable products according to an economically viable technique is of extreme interest.

In this context, the potential of methane sulphonate (CH₄) to methanesulfonic acid (CH)₄O₃S, MSA) realized substantial attention because of the abundance of raw materials and the ability of its rapid integration into existing industrial chemical processes. MSA is biodegradable and non-oxidizing with potential applications in metal recycling, energy storage and biodiesel production. Earlier work on methanesulfonation has suffered from low yields and conversions due to radical recombination, resulting in undesirable by-products such as ethane, rendering the methods unsuitable for large-scale production. Technically, the balance between reactivity and selectivity required by an industrial process can be achieved by superacid chemistry. Díaz-Urrutia and Ott reported treatment of oleum (20 to 60% sulfur trioxide) with CH₄ at around 50⁰Using less than 1 mol% of the electrophilic initiator to form MSA with 99% yield and 99% selectivity.

CH_{4 (g)} + SO_{3 (l)} → CH₃SO₃H_(L)

Scientists first studied the reaction in a batch system in order to optimize the experimental conditions and better understand the reaction mechanism. For the electrophilic initiator, they used sulfuric polymethylsulfonylperoxide (MMSP) to improve the technical feasibility. For increased productivity, they used a four-liter reactor instead of a 400-ml reactor, because of the greater amount of CH₄ forming in the free space of the larger reactor. Scientists were able to maintain consistent amounts of methane throughout the reaction to obtain higher yields of MSA. They used an optimal temperature of 50⁰C to achieve more than 99% selectivity to MSA, whereas the previous radical pathways had similar results at higher temperatures (85⁰C) due to thermal decomposition of the electrophilic initiator to sulfonyl peroxide. Low temperature experiments could also offer high selectivity for conversion and MSA, but required longer reaction times. Díaz-Urrutia and Ott provided information to support a non-radical mechanism in the present work.

When the scientists examined the reaction profile of the experiment, they observed an induction period immediately after the addition of the electrophilic reactor, where the amount of MSA (product) was proportional to the amount initial MMSP (initiator). In the second step of the reaction profile, they observed the solubility of CH₄ decrease with increasing pressure in the reactor. The activation energy of the process was determined to be 111 ± 1 kJ / mol, which is similar to those reported previously. The cationic pathway described is produced under very specific conditions. The researchers achieved high selectivity through electronic modifications of electrophilic substitutions, as opposed to previously reported free radical-based atom abstraction reactions.

The first results being very promising, the scientists built a pilot plant and tested the economic and technical feasibility of producing MSA on an industrial scale. Díaz-Urrutia and Ott built the plant with a projected capacity of 20 metric tons / year of MSA, based on their laboratory batch reactions, and took into account the solubility and recycling of methane, as well as as the concentration of sulfur trioxide and methane. . This configuration allowed the scientists to constantly increase the concentration of MSA as the reaction mixture passed through the reactors. When they used gas chromatography with flame ionization detection (GC-FID) to control the samples, they did not detect the presence of higher alkanes in the recycled methane stream or any other radical product, which allowed its direct use the cascade reaction.

To obtain pure MSA, Díaz-Urrutia and Ott completed the process with a final distillation step. They then recycled the remaining mixture of sulfuric acid and MSA into the first reactor for the continuous regeneration of sulfur trioxide and sulfuric acid (SO₃ and H₂SO₄). Using the four reaction chambers in the configuration, the scientists were able to produce 200 kg of pure MSA per week, which is two to three metric tons in 80 days. In this way, the demonstrated combination of high selectivity, conversion and the economy of the atom has made the process ideal for large scale upgrading of readily available methane and sulfur trioxide reagents. .

If this new process of methanesulfonic acid becomes effective on the market, less expensive reagents can replace the currently used mineral acids. However, even if the production of MSA were to increase dramatically, the amount of methane consumed during the process would remain lower than that of the quantities burned. Nevertheless, the works of Díaz-Urrutia and Ott predict a new synthetic chemical process to synthesize an interesting chemical, allowing scientists to consider a range of value-added products derived from methane or higher alkanes using this route. the chemistry of superacids. future.

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