Sharing of little knowledge in the chemical industry – Chemical simulation of biological nitrogen fixation

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Nitrogen is one of the elements essential for the growth and development of all living organisms; it is an important component of nitrogen-containing biomolecules such as amino acids, proteins, and nucleic acids. Although nitrogen is abundant in the air, only certain microorganisms in nature are capable of efficiently converting nitrogen molecules in the air into ammonia under normal temperature and pressure, with an annual production volume reaching 200 million tons. In the chemical industry, the synthesis of ammonia from hydrogen and nitrogen using an iron catalyst takes place under high temperature and pressure. This not only requires a large amount of high-quality steel, but the annual production volume is only about 50 million tons, which is insufficient to meet the growing needs in various areas. Chemical simulation of biological nitrogen fixation, also known as biological nitrogen fixation, refers to the process of mimicking the action of nitrogenase, an enzyme with special catalytic capabilities found in microorganisms, which converts free nitrogen in the air into ammonia. It involves using chemical methods to fix nitrogen from the air under normal temperature and pressure or under milder conditions, thereby converting it into ammonia or nitrides. Research on biological nitrogen fixation is of great significance not only theoretically but also in practical production. Although this area of research has a history of over a hundred years, it remains one of the major fundamental research topics in chemistry that receives widespread attention internationally. A great deal of work has been done, and significant progress has been made. In nature, biological nitrogen fixation is essentially achieved by activating nitrogen molecules through coordination. In recent years, chemical simulations of nitrogen fixation have mainly focused on three steps: using transition metal complexes to coordinate with nitrogen molecules and form molecular nitrogen complexes, thereby weakening the chemical bonds in those molecules; using reducing agents to supply electrons to the coordinated nitrogen, thus breaking its chemical bonds; and allowing H+ to combine with the negatively charged nitrogen to form ammonia. The chemically simulated biological nitrogen fixation systems that have been studied can be roughly divided into the following 3 categories: nitrogen fixation systems in aqueous solutions. Chemical simulated nitrogen fixation systems in aqueous solutions have been extensively studied. However, these systems are far from the catalytic activity of nitrogenase and have no industrial practical value. Among them, the chemical nitrogen fixation simulation system proposed by Schrauzer (G. N. Schrauzer) and others in 1970 is relatively close to the structure of the nitrogenase active center, providing important information for elucidating the structure and function of this active center. Nitrogen fixation systems in organic solvents. In 1970, E. E. Van Tamelen and others studied the reaction between molecular nitrogen and alkyl oxides of Ti(II); the resulting molecular nitrogen complexes could be reduced to release hydrazine or ammonia. In 1975, J. Chatt and others in the UK synthesized cis-Molecular nitrogen complexes (M=Mo or W, R being alkyl or aryl). Such compounds can be reduced to ammonia under anaerobic conditions at room temperature. They also discovered molecular nitrogen organophosphine complexes of molybdenum and tungsten, which can be reduced to ammonia in acidic media. All of these can be carried out at room temperature and pressure, with a Grignard reagent as the reducing agent. Electron donor-acceptor complex systems. Electron donor-acceptor complexes are a class of complexes composed of molecules that readily donate electrons and molecules that readily accept electrons. In 1969, Hita Honda and others synthesized such complexes. They evaporated the transition metal phthalocyanine complex MPc (M being a transition metal, Pc being phthalocyanine) onto a film of metallic sodium to obtain Na+(MPc)-. It is a class of electron donor-acceptor complexes. Under normal pressure and at 250°C, passing nitrogen and hydrogen over the Na+(MPc)-solid surface can yield trace amounts of ammonia. In recent years, significant progress has been made in the study of electron donor-acceptor complexes. Tanmaru and others replaced phthalocyanines with compounds possessing large π-electron conjugation systems, allowing them to interact with transition metal chlorides and alkali metals (sodium or potassium), thereby obtaining electron donor-acceptor complexes that exhibit certain catalytic activity in the synthesis of ammonia. As discovered by Ozaki and others, the use of the activated carbon-RuCl3-K system results in a significantly higher efficiency in ammonia synthesis under normal pressure and at 290°C compared to conventional industrial catalysts. Since 1972, our country has also carried out a lot of work in this area. Such as the studies on the activated carbon-Fe-K system and the molten iron-K system. Their catalytic activity for the nitrogen fixation reaction is higher than that of conventional industrial catalyst systems and electron donor-acceptor complex systems. Overall, chemical simulation of biological nitrogen fixation has seen significant progress in current research, but it has not yet met the requirements for industrial application, and further study is still needed.