Reactions under Supercritical Conditions

One problem which had previously been ignored in the dispute on the stability of biomolecules under extreme conditions was the influence of the properties of supercritical water. Water becomes supercritical at temperatures above the critical 

One example will show the manifold types of reactions studied by Mok et al. (1989). Lactic acid decomposes in supercritical water to give acetaldehyde, which then reacts further it can also undergo dehydration to give acrylic acid, which is either hydrogenated to give propionic acid or decarboxylated to give ethene (Fig. 7.4). 

Broil et al. (1999) have provided detailed surveys of the variety of reaction mechanisms which can occur in supercritical water. It is possible that supercritical conditions were present in the vicinity of hydrothermal systems, where as yet unknown 

The question of the elimination of water in polycondensation reactions still provides an unsolved problem. Solutions are being searched for in many laboratories, for example in Italy Paly6 and Zucchi from the University of Modena consider it possible that limited regions where liquid or supercritical CO2 phases were present could have existed on the young Earth. Such regions, with non-aqueous media, could have been particularly favourable for some prebiotic reactions, such as those involving the elimination of water. Experiments to study this hypothesis are planned (Paly6 and Zucchi, 2002 Holm and Andersson, 1998). 

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Another approach is to omit the solvent andrun the reaction under supercritical conditions where potassium fluoride dissolves in the superheated reactant This approach is illustrated by the conversion of 2-chloromethoxy-l,l,l,3,3,3-hexa tluoropropane to 2-fluoromethoxy-l,l,l,3,3,3 hexafluoropropane with either potassium fluoride or sodium fluoride as the fluorine source (equation 31)... 

In this case water is effectively acting as a catalyst for the reaction by lowering the energy of activation. These catalytic water molecules are more likely to participate in the reaction under supercritical conditions because their high compressibility promotes the formation of solute-solvent clusters. 

The subject of chemical reactions under supercritical conditions is well outside the scope of matters of major concern to combustion related considerations. However, a trend to increase the compression ratio of some turbojet engines has raised concerns that the fuel injection line to the combustion chamber could place the fuel in a supercritical state that is the pyrolysis of the fuel in the line could increase the possibility of carbon formations such as soot. The... 

The Oppenauer Oxidation. When a ketone in the presence of an aluminum alkoxide is used as the oxidizing agent (it is reduced to a secondary alcohol), the reaction is known as the Oppenauer oxidation. This is the reverse of the Meerwein-Ponndorf-Verley reaction (19-36) and the mechanism is also the reverse. The ketones most commonly used are acetone, butanone, and cyclohexanone. The most common base is aluminum ferf-butoxide. The chief advantage of the method is its high selectivity. Although the method is most often used for the preparation of ketones, it has also been used for aldehydes. An iridium catalyst has been developed for the Oppenauer oxidation, and also a water-soluble iridium catalyst An uncatalyzed reaction under supercritical conditions was reported. 

Recently, the phase behaviors of some reaction mixtures have been studied. To explore the advantages of the reactions under supercritical condition or in the critical region, the critical parameters and phase behavior of the reaction mixtures should be considered, and reaction properties and the phase behavior of the reaction systems should be combined in tlie study.In this section, we discuss some work about how pressure and composition of a complex reaction system affect the chemical equilibrium, conversion and selectivity and reaction rate in different phase regions. 

To complete this chapter, we would like to mention that recent monographs have reviewed the use of in-situ spectroscopies for monitoring heterogeneously catalysed reaction under supercritical conditions, although very few studies in this field has been devoted to the study of the fluid-solid interface.182 The use of a multi-technique approach in order to maximise information under real, in-situ conditions has also been reviewed recently.183 The combined use of powerful spectroscopies with simultaneous on-line analysis of the catalytic activity of the sample will become more widespread in application allowing an interpretation of catalytic behaviour in terms of the physico-chemical properties of the solid. The next frontier in spectroscopic characterisation of metal catalysts will consist of time-dependent analysis of the gas/liquid-solid interface, particularly with a view to analyse short-lived intermediates during catalysed reactions and with the aim to determine the response of the catalyst surface and relate these responses to the physico-chemical properties of the solid. 

Oxazolidinones are useful heterocyclic compounds in organic synthesis. They have a wide range of applications in asymmetric syntheses as chiral reagents and, since they have good antibacterial properties, in medicinal chemistry [53]. Oxazolidinones can be synthesized in traditional solvents such as acetonitrile [54] or DMF [5 5], but it is more environmentally friendly to use scC02 [56]. In the reaction an internal propargyl alcohol, carbon dioxide, and a primary amine participate in a cycloaddition reaction under supercritical conditions to give 4-alkylene-l,3-oxazoli-din-2-ones (Equation 4.30). 

Tahara reported [67] the design of a flow system for obtaining ultrafast measurements, including picosecond-TR, in SCFs. Such flow systems should allow wider use of TR to monitor chemical reactions under supercritical conditions. 

Enzymes are not catalytically active if water is completely absent. The often cited explanation is that at least a monolayer of water per enzyme molecule is necessary to keep the enzyme active [40]. Apparently, the essential noncova-lently bound water maintains the enzyme s native protein structure. In an enzymatic reaction under supercritical conditions, the water partitions between the enzyme, the enzyme support and the reaction mixture. In an essentially non-aqueous system, the existing water partitions preferrably to the solvent with increasing hydrophilicity. If there is little water in the system and if the solvent is relatively hydrophilic, the solvent may strip the essential water from the enzyme, making it inactive. When Zaks and Klibanov first noted that enzymes were more active in hydrophobic solvents than in hydrophilic organic solvents. 

Reactions under supercritical conditions have been used for large scale industrial production for most of the 20th century, but the application of supercritical fluids (SCFs) in the chemical synthesis of complex organic molecules or specialized materials is only just emerging. Research in this field has been particularly active in the last decade of this century. This book is intended to introduce the reader to the wide range of opportunities provided by the various synthetic methodologies developed so far. 

The utilization of biocatalysts other than hydrolytic enzymes has also been investigated. Alcohol dehydrogenase is used in the asymmetric reduction of ketones to yield optically active secondary alcohols (Scheme 51). " Although the productivity (substrate concentration) is low, high yields and excellent enantiomeric excess are obtained. On the other hand, carboxylation of pyrrole is efficient in SCCO2 (Scheme 52). The reaction under supercritical conditions is 12 times faster than that... 

Recently, reactions under supercritical conditions have been investigated. For example, styrene oxide is converted to styrene carbonate 32 at 150°C in the presence of dimethyl formamide without a catalyst (Scheme 56) ", but substrates are rather limited and propylene oxide does not react under similar conditions. On the other hand, solid catalysts on a fixed-bed flow reactor are being invesfigafed as a replacement for batch processes. " The reactivity of solid catalysts is usually lower than... 

Carbon dioxide is the supercritical solvent that is most commonly used in homogeneous catalytic reactions. In addition to being environmentally acceptable (nontoxic, nonflammable), inexpensive, and available in large quantities, carbon dioxide does not participate in most reactions. It also has an ambient critical temperature. Although, supercritical carbon dioxide is more effective in dissolution of non-polar, nonionic and low molecular mass compounds, addition of co-solvents enhances the solubility of many otherwise insoluble compounds in supercritical carbon dioxide. A recent review by Noyori et al. discusses homogeneous catalytic reactions under supercritical conditions. 

E. Salvador, M. J. Sanchez-Montero, and C. Izquierdo, C/H2O reaction under supercritical conditions and their repercussions in the preparation of activated carbon. J. Phys. Chem. C, 111, 14011—14020, 2007. 

Opportunities for Oxidation Reactions under Supercritical Conditions... 

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