1 Downloaded via 148.251.232.83 on July 10, 2018 at 19:07:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Present State and Main Trends of Research on Liquid-Phase Oxidation of Organic Compounds N. M. EMANUEL Institut of Chemical Physics, Academy of Sciences, Vorobyevskoye Chaussee 2-b, Moscow, U.S.S.R. Oxidative chain reactions of organic compounds are current targets of theoretical and experimental study. The kinetic theory of collisions has influenced research on liquid-phase oxidation. This has led to determining rate constants for chain initiation, branching, extension, and rupture and to establishing the influence of solvent, vessel wall, and other factors in the mechanism of individual reactions. Research on liquid-phase oxidation has led to studies on free radical mechanisms and the role of peroxides in their formation. >""phis symposium has been a very good, necessary, and important experiment, organized as a Faraday-type discussion. At the Faraday Discussion on oxidation in 1946, Eric Rideal said that in the field of kinetics we float over seas that are not to be found in maps. Things have changed drastically during these 20 years. Previously unknown reactions of chain generation, propagation, and termination were discovered during this period, and the over-all scheme of oxidation now contains many new elementary steps. Various efficient physical methods are now available. The ESR technique makes it possible to "see" the free radicals involved in gas- and liquid-phase oxidations. Great possibilities are opened up by the chemiluminescence technique, which, however, is not yet widely used. It also appears possible to understand the reasons for the strong effect of the dielectric constant on reaction rates. Moreover, this effect is found to obey laws that were previously considered valid only for ionic reactions i.e., the Kirkwood equation. The chemistry of initial and later stages of oxidation can be distinguished. In the later stages, the reaction proceeds, in fact, in a dif- 1
2 OXIDATION OF ORGANIC COMPOUNDS 1 ferent chemical system that has undergone great changes with time. Thus, at higher conversions, reaction control should be quite different from that for initial stages. This simple conclusion seems promising in that it should permit control of oxidation by changing the medium, introducing inhibitors, catalysts and other admixtures, as well as by changing temperature, pressure, and other similar factors. At present the slower oxidative chain reactions of organic compounds, primarily.hydrocarbons, are perhaps the most widespread targets of theoretical and experimental investigations of chain reactions. The significance of these investigations is enhanced by the fact that direct oxidation of hydrocarbons and other organic compounds possesses more than theoretical importance; it underlies many technological processes for producing valuable chemical products. Production of phenol and acetone is based on liquid-phase oxidation of isopropylbenzene. Synthetic fatty acids and fatty alcohols for producing surfactants, terephthalic, adipic, and acetic acids used in producing synthetic and artificial fibers, a variety of solvents for the petroleum and coatings industries these and other important products are obtained by liquid-phase oxidation of organic compounds. Oxidation processes comprise many parallel and sequential macroscopic and unit (or very simple) stages. The active centers in oxidative chain reactions are various free radicals, differing in structure and in reactivity, so that the "nomenclature" of these labile particles is constantly changing as oxidation processes are clarified by the appearance in the reaction zone of products which are also involved in the complex mechanism of these chemical conversions. The kinetic theory of collisions, which has been so effective in developing the kinetics of vapor-phase reactions, has substantially influenced research on the processes of liquid-phase oxidation and in describing these processes. It has been thought that the lack of laws on which to base liquid-state theory (in contrast to the well-developed kinetic theory of gases) would in principle severely limit the development of a quantitative theory of liquid-phase reactions. At present the characteristics of the liquid state are carefully considered in discussing the mechanism of intermolecular reactions, influence of the medium on reactivity of compounds, etc. This opened the possibility of ascertaining quantitative characteristics in the numerous individual reactions comprising the complex mechanism of oxidative chain reactions. Thus, rate constants have been determined for reactions with respect to their initiation and the branching, extension, and chain rupture, establishing specific details concerning the influence of solvents, reaction vessel surfaces, and other factors on the mechanism of individual reactions. Results of these investigations have
1. EMANUEL Liquid-Phase Oxidation of Organic Compounds 3 made it possible to lay out effective plans for novel industrial oxidation processes in modern petroleum chemistry. In developing oxidation processes a major source of free radical formation was found to be degenerate chain branching. Among the products derived from the branching were intermediate peroxides ROOH. Formation of radicals from the hydroperoxides proceeded not only by monomolecular breakdown of hydroperoxides: k ' ROOH RO* + OH, but also by interaction of two saturated molecules: ks ROOH + RH RO' + R' + H 20. In some cases the main portion of the final products is formed by decomposition of peroxides. This is the current aspect of the classic Bach- Engler peroxide theory with respect to the chain theory of oxidation processes. The mechanism of chain branching in the later stages of oxidation is more complex. For example, it is being demonstrated that intermolecular hydrogen bonds substantially influence the process by forming hydroperoxides and oxidation products among the molecules and by accumulating these in the reaction mixture. In studying the reaction mechanism for chain extension it is assumed, in line with the fundamental reaction of chain extension, k 2 R0 2' + RH ROOH + R* where k 2 is the rate constant for the chain extension reaction, that the peroxide radical R0 2' can often isomerize, accompanied by rupture of the C-C bond and by formation of carbonyl compounds and alcohol radicals. Analogous reactions involving radical decomposition were observed previously in vapor-phase oxidation studies. As oxidation processes were clarified, it was observed in other chain extension reactions that R0 2 radicals reacted with oxidation products: hydroperoxides, alcohols, and ketones. The high reactivity of hydroperoxides and alcohols strongly influences the mechanism of oxidation processes. Chain rupture results from recombination of R0 2 radicals. Research on liquid-phase oxidation processes has opened up a tremendous area for research on free radical mechanisms. Such research will be concerned, not with hypothetical particles like those studied a few years ago, but with thoroughly material active centers, amenable to investigation by experimental techniques currently in use. Problems of foremost importance are involved in correlating data on unit processes
4 OXIDATION OF ORGANIC COMPOUNDS I and on free radical mechanisms with identification of free radicals and their characteristic properties. Widely varied types and classes of organic compounds come under one or another of the prospective problems in constructing a general kinetic theory of unit mechanisms in oxidative chain reactions among these compounds, elucidating specific characteristics, correlating their properties with various functional groups, etc. There seems to be special promise in oxidizing liquefied hydrocarbon gases at temperatures and pressures approximating critical levels. That such reactions are highly effective is attested, for example, by the liquidphase oxidation of butane, one of the simplest and most efficient methods of producing acetic acid and methyl ethyl ketone. Isomerization and decomposition of peroxide radicals reactions which obviously do occur and which exert significant influence depend on the nature of the reaction vessels surface. For example, when butane is oxidized in a glass reactor, products formed by breakdown of R0 2 radicals are completely absent, whereas in a steel reactor they correspond to 20 mole % of the reacted butane. The quantity of these products rises to 35 mole % when the reactor is filled with metal packing shapes. Formation of products containing less than four carbon atoms is not related to the catalytic activity of the metal on the decomposition of hydroperoxides. Hence, the liquid-phase oxidation of hydrocarbons involves heterogeneous catalytic reactions of isomerization and decomposition of peroxide radicals, proceeding on the reactor surface. Results of these investigations demonstrate that changes of the reactor surface can be an effective method for directing chemical reactions. Thus, developing a theory of how heterogeneous factors influence liquidphase chain reactions is one of the important lines of advancement in this area. Only a few years ago it was thought, almost a priori, that there are practically no heterogeneous factors in liquid-phase oxidation and that liquid-phase processes differ from vapor-phase processes in this respect. Greater possibilities for discovering new kinetic phenomena and designing novel technological processes are opened up by combinations of chemical reactions depending on the utilization of free radicals and intermediate compounds formed in one of the reactions occurring in a system in order to obtain products of their interaction as active components with other components of the reaction mixture. On the basis of this principle, a process involving oxidation of unsaturated hydrocarbons and other organic compounds, more readily oxidized than alkenes, contribute substantially to solving problems in direct single-stage production of propylene and higher alkylene oxides. Upon oxidizing ethene, propene, or isobutene together with aldehydes, alkylated aromatic hydrocarbons, methyl ethyl ketone or other
1. EMANUEL Liquid-Phase Oxidation of Organic Compounds 5 compounds, the products include, along with hydroperoxide conversion products (acids, ketones, alcohols) some alkylene oxides. Continued investigation revealed that the principal epoxidizing agents for combined oxidation of unsaturated compounds and aldehydes are not the corresponding peracids, but the radicals of acyl peroxides. Including mixed systems among the research on the mechanism of liquid-phase oxidation reactions aids subsequent development of the chain theory and undoubtedly contributes to practical chemistry. An alluring field of research is the mechanism of action of oxidation inhibitors. This research will undoubtedly yield in the near future a theory for inhibition of undesirable oxidation processes. The relatively stable free radicals observed on such inhibition display extremely interesting properties. Of great interest are the effects of synergism, of inhibitor mixtures, and of mixtures of inhibitors with catalysts. A strictly quantitative and elegant description of all these phenomena may be made within the scope of the chain theory for slow oxidation. It has always been considered that the condition of the reactor wall is less important for liquid-phase processes than for gas-phase reactions. Now there are numerous examples of marked wall effects which induce essentially new chemical results in liquid-phase oxidations. Hence, the parts played by reactor walls, by solid surfaces, and by other solid catalysts in liquid-phase oxidations should be considered as one of the most important remaining problems. We are on the right path in attempting to establish the kinetics of oxidation reactions in flow systems. This is the scientific basis of continuous processes in chemical industry and an invaluable source of additional information on reaction mechanisms. Research on oxidation problems is in progress in nearly every country. Many projects for directed research on oxidation processes have been objects of sustained, organized international cooperation. The quantity of information on theoretical and applied aspects in this area grows ever larger. This symposium shows that research on oxidation processes constitutes a fertile field, tremendously rich in possibilities. This is particularly true of complex multicomponent chemical systems, where particularly great progress is expected. At present science is equipped with adequate experimental techniques for solving the problems. This symposium constitutes an important step in advancing chemical kinetics and will undoubtedly exert significant influence on future research on oxidation reactions and their practical applications. Finally, I wish to express my sincerest thanks to Dr. Mayo and the Program Chairman, Dr. Mill for the great amount of work they did in connection with the symposium as well as for their kind hospitality.