Acetic acid is an organic compound with the chemical formula CH3COOH, an organic monobasic acid. Acetic acid is one of the most important organic acids. Acetic acid can be used as acidity regulator, pickling agent, flavor enhancer, acidulant, spice, etc. It is also a very good antimicrobial agent, mainly because it lowers the pH below that required for optimal growth of microorganisms. Acetic acid can also generate various derivatives, such as methyl acetate, ethyl acetate, propyl acetate, butyl acetate, etc., which can be used as an excellent solvent in the paint and paint industry. Cellulose acetate produced by the action of acetic anhydride and cellulose can be used to make films, spray paints, etc. It is also an indispensable raw material for industries such as dyes, spices, and pharmaceuticals, and is widely used as a solvent. It can also be used as a solvent and a raw material for the preparation of acetate and acetate.
In recent years, how to efficiently convert methane and other low-carbon hydrocarbons has gradually become a widely discussed topic in the chemical industry. However, due to the extremely high carbon-hydrogen bond energy in methane, activating methane and converting it into high value-added chemical products such as acetic acid has always been a difficult problem in industry and academia. Activation of methane usually requires high temperature and high pressure reaction conditions, which greatly reduces the economics of industrial production of acetic acid. At the same time, long-term uninterrupted high-temperature and high-pressure reactions are not conducive to the safe operation of chemical plants. In the academic field, researchers often use single-atom catalysts to activate methane. However, single-atom catalysts with low loadings have always had defects such as low overall conversion and easy sintering in the reaction. Therefore, the development of efficient and controllable methane conversion catalysts has always been a research hotspot in natural gas chemical industry.
The researchers report a method for the synthesis of highly loaded Rh-based single-atom catalysts for acetic acid synthesis. This approach utilizes porphyrin functional groups in metal-organic framework (MOF) materials to stabilize Rh single atoms. The high active center loading enabled the catalyst activity to reach 23.62 mol·gcat-1·h-1, setting a milestone in single-atom catalytic activity.
In addition, this study shows that the Rh single-atom catalyst synthesized by this method is extremely sensitive to light. Under the condition of light, the selectivity of catalyzed reaction to acetic acid reaches 66.4%. In contrast, the dark field reaction conditions were more favorable for the production of methanol (65% selectivity). In order to further verify the effect of light conditions, the researchers conducted periodic light on and off switching experiments, and found that the phenomenon of light to produce acetic acid and dark field to produce methanol is reversible and repeatable. At the same time, the researchers changed the light intensity and compared it with the production of methanol and acetic acid, and found that the production rate of acetic acid increased with the increase of light intensity, while the production rate of methanol decreased.
In order to further explore the influence of light on the nature of the catalyst, the researchers used density functional calculations to explore the structure-activity relationship of the catalyst from the atomic level. It is found that under dark field conditions, a structure in which Rh is embedded in the porphyrin plane will be formed, which is conducive to the production of methanol due to its lower methanol formation barrier. However, the light conditions will produce the Rh out-of-plane structure, which is conducive to the generation of acetic acid due to the lower carbonylmethyl coupling barrier.
This work sets a milestone in the activity of highly loaded Rh single-atom catalysts for the selective oxidation of methane. This method utilizes porphyrin groups in the MOF structure to stabilize single-atom Rh. At the level of reaction mechanism, this paper found that light can change the reaction selectivity. The absence of light favors the production of methanol, while the light condition favors the production of ethanol. This work plays an important role in promoting the research on single-atom catalysts to promote the efficient conversion of methane to acetic acid.
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Reference
- Selective Formation of Acetic Acid and Methanol by Direct Methane Oxidation Using Rhodium Single-Atom Catalysts
J. Am. Chem. Soc., 2023, 145, 11415–11419
