Reactions in supercritical water

This section is not intended to be a comprehensive review of reaction studies in supercritical water, but to illustrate the special properties of the medium described above and make some further general points by means of examples from the literature. The current position of research on supercritical water reactions and processes can be more comprehensively seen from a collection of abstracts from the 3rd International Symposium on Supercritical Fluids which took place in Strasbourg in October 1994. There were some 10 papers and posters on the subject presented at the Symposium, which are summarised in Tome 3 of the Proceedings, edited by Brunner and Perrut [13]. 

The hydrolysis of aniline to phenol in supercritical water by Patat [14] is accredited as the earliest reported study of a simple reaction in a supercritical medium. The reaction is catalysed by hydrogen ions formed from dissociation of phosphoric acid. Close to the critical point of water aniline has limited solubility, but as the conditions of the flow reactor were raised towards 450°C and 710 bar, the reaction rate was observed to increase by an order of magnitude. In his view, mechanism was unaffected by the phase change it was from the enhanced solubility of aniline and thus better contact between reactants and catalyst that the rate improvement was derived. 

Later studies showed that the mechanism of reactions, in particular ionic versus free-radical, could vary. Townsend [15] has studied the reaction of a series of coal model compounds (alkyl-aryl hydrocarbons and ethers) in supercritical water. For the hydrocarbons a free-radical pyrolysis route does not take advantage of the medium. However, for the ethers enhanced rates of reaction through a hydrolysis route occurs. As a result of different possible pathways, decomposition products of some organics in supercritical water have been shown by several workers to vary with solvent strength. In the absence of water, Pr(H20) = 0, pyrolysis is dominant and yields a variety of products including polycondensates. The main products of decomposition of neat methoxy 

Studies of salt effects on reactions in supercritical water are continuing, both for mechanistic reasons and because salts will be present in any process. The kinetics of salt deposition from reactions and the morphology of the salts produced are also a subject of study [19]. Reactions of inorganic compounds are also being studied in supercritical water using spectroscopic techniques [20]. 

Studies of the reaction mechanisms for a number of simple compounds in supercritical water have been carried out, but even these seem to be complex. For example the oxidation of carbon monoxide can involve 21 elementary reactions [23]. Computer modelling is required and programs designed for combustion processes are sometimes used. The rates of oxidation of carbon monoxide are substantially lower than those predicted by gas-phase models and the proportion of hydrogen produced by the concurrent water-gas shift reaction is unexpectedly high. The differences are explained in terms of lower diffusivities compared with the gas phase, as a result of solvent cages . Because of difficulties with fundamental studies, much work on reactions is empirical and directed towards particular processes. Some of this work relevant to toxics destruction is given in the next section. 

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Diels-Alder reactions in supercritical water have also been investigated.57 Kolis has shown that Diels-Alder reactions of dienes with various electron-poor dienophiles can be performed in supercritical water with high yields of the desired product without the addition of... 

Hu and Truhlar have recently reported a modeling transition state solvation at a single-water representation [295]. Recent experimental advances leading to the study of SN2 reactions of gas-phase microsolvated clusters which can advantageously been studied with ab initio electronic theory. These experiments and theoretical studies are quite relevant to chemical reactions in supercritical water. 

P. E. Savage, Organic Reactions in Supercritical Water , Chem Rev. 1999, 99, 603-621, and references cited therein. 

Savage, P.E., Organic Chemical Reactions in Supercritical Water, Chemical Reviews, 99,603-622 (1999). 

Savage, P.E. 1999. Organic chemical reactions in supercritical water. Chem. Rev. 99 603-621. 

Another class of extreme environments being used for synthetic applications are near critical and supercritical fluids. A number of reviews have looked at reactions in supercritical water.Brill and coworkers have developed experimental devices in which to perform infrared and Raman... 

In this study, thus, we selected sugi as softwood and buna as hardwood species. Just for comparison, cellulose and hemicellulose (glucomannan, xylan) and lignin (milled wood lignin) were also studied for their reaction in supercritical water. 

Homogeneous reactions carried out at supercritical conditions within water are organo-metallic reactions and Diels-Alder reactions. Reactions in supercritical water are well studied and will be described in the following section. ... 

Examples of the homogeneous reactions in supercritical water are included in Table 2. Use of acid or base catalysts enhances the rates of some of the... 

J. M. Ploeger, P. A. Bielenberg, J. L. DiNaro-Blanchard, R. P. Lachance, J. D. Taylor, W. H. Green and J. W. Tester, Modeling Oxidation and Hydrolysis Reactions in Supercritical Water—Free Radical Elementary Reaction Networks and Their Applications, Combust. Sci. and Tech., 178, 363-398 (2006). 

Scheme 7.2 Copper-free Sonogashira coupling reaction in supercritical water...

In a very early study Patat (1945) investigated the hydrolysis of aniline to phenol in a water-based acidic solution in near-critical and supercritical water (Tc = 374.2°C, Pc = 220.5 bar). Phosphoric acid and its salts are used as the catalyst for this reaction. The reaction proceeds extremely slowly under normal conditions and reaches equilibrium at low conversion levels. For these reasons, Patat chooses to study the reaction in supercritical water to temperatures of 450°C and to pressures of 700 bar in a flow reactor. He finds that the reaction follows known, regular kinetics in the entire temperature and pressure space studied and the activation energy of the hydrolysis (approximately 40 kcal/mol) is the same in the supercritical as well as in the subcritical water. He suggests that the reaction is catalyzed by hydrogen ions formed from dissolution of phosphoric acid in supercritical steam. Very small amounts of phosphoric acid and the salts of the phosphoric acid are dissolved in the supercritical steam and are split into ions. Patat lists several dissolution constants for primary ammonium phosphates in supercritical steam. In this instance, the reaction performance is improved when the reaction is operated homogeneously in the mixture critical region and, thus, in intimate contact between the reactants and the catalyst. 

A variety of reactions in aqueous media can be accelerated by the addition of acids or bases. Here examples of reactions are given, which proceed at very high reaction rates under conditions of high ionic product of water without addition of acids or bases. These reactions usually show the highest reaction rates in near-critical water, at the maximum of the ionic product. Undesirable side reaction such as C-C scissions occur at low densities to a certain extent. Decarboxylation is also reported as a side reaction for organic reactions in supercritical water. 

Nagai, Y., Morooka, S., Matubayasi, N., and Nakahara, M., Mechanisms and kinetics of acetaldehyde reaction in supercritical water Noncatalytic disproportionation, condensation,... 

Luo, H. and Tucker, S. C. (1997) A Compressible Continuum Model Study of the Chloride plus Methyl Chloride Reaction in Supercritical Water, J. Phys. Chem. B, 1063-71. 

Balbuena, P. B. Johnston, K. P. Rossky, P. J. (1994) Molecular Simulation of a Chemical Reaction in Supercritical Water, Journal of the American Chemical Society 116, 2689-2690... 

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