Hydrolysis in supercritical water

Meyer JC, Marrone PA, Tester JW. Acetic acid oxidation and hydrolysis in supercritical water. AIChE J 1995 41(suppl 9) 2108-2121. 

Tucker, S. C. Gibbons, E. M. (1994) Theoretical Models of Anisole Hydrolysis in Supercritical Water. In Structure and Reactivity in Aqueous Solution D. G. Truhlar and C. J. Cramer, Ed. American Chemical Society Washington, Vol. 568 pp 196-211. 

Adschiri, T. Shibata, R. Arai, K. Phenol recovey by Bisphenol-A (BPA) tar hydrolysis in supercritical water. Sekiyu Gakkaishi. 1997, 40.291-297. 

The increased dissociation of water in conjunction with the increased association of the electrolyte in the supercritical region has a fundamental influence on chemical reactions. Some reactions such as hydrolysis become faster in supercritical water. For example, there are at least eight species (KC1, KOH, HC1, HOH, K+, Cl , H+, and OH ) for potassium chloride in supercritical water. 

Conversion of polymers and biomass to chemical intermediates and monomers by using subcritical and supercritical water as the reaction solvent is probable. Reactions of cellulose in supercritical water are rapid (< 50 ms) and proceed to 100% conversion with no char formation. This shows a remarkable increase in hydrolysis products and lower pyrolysis products when compared with reactions in subcritical water. There is a jump in the reaction rate of cellulose at the critical temperature of water. If the methods used for cellulose are applied to synthetic polymers, such as PET, nylon or others, high liquid yields can be achieved although the reactions require about 10 min for complete conversion. The reason is the heterogeneous nature of the reaction system (Arai, 1998). 

Superheated and supercritical water are used in several applications. Supercritical water is most often used in the destruction of organic wastes, including some chemical warfare agents, as an alternative to incineration (Katritzky et al., 1996 Sherman et al., 1998). Recent reports describe the use of both forms as a solvent and as a reactant in synthetic chemistry (Katritzky et al., 1996 An et al., 1997). Some of the reactions investigated include metal-mediated alkyne cyclizations, Pd-catalyzed al-kene arylations, aldol reactions, the Fischer indole synthesis, and hydrolysis reactions. Waterborne coatings and the destruction of wastes in supercritical water are fully... 

Non-catalytic reaction pathways and rates of reaction of diethyl ether in supercritical water have been determined in a quartz capillary by observing the liquid- and gas-phase XH and 13C NMR spectra.37 At 400 °C, diethyl ether undergoes, competitively, proton-transferred fragmentation and hydrolysis as primary steps. The former path generates acetaldehyde and ethane and is dominant over the wide water density range up to... 

This paper deals with the degradation of substances like PVC, Tetrabromobisphenol A, y-HCH and HCB in supercritical water. This process is called "Supercritical Water Oxidation", a process which gained a lot of interest in the past. The difference between subcritical and supercritical processes is easy to recognize in the phase diagram of water. The vapor pressure curve of water terminating at the critical point, i.e. at 374 °C and 221 bar. The relevant critical density is 0.32 g/cm3. This corresponds to approx. 1/3 of the density of normal liquid water. Above the critical point, a compression of water without condensation, i.e. without phase transition is possible. It is within this range that supercritical hydrolysis and oxidation are carried out. The vapor pressure curve is of special importance in subcritical hydrolysis as well as in wet oxidation. 

The use of supercritical fluids, including SCW and NCW, in inorganic materials synthesis and the preparation of nanoparticles was recently reviewed. The hydrolysis and dehydration of metal nitrates and metal organic precursors in supercritical water is also known as hydrothermal synthesis (Figure 4.15). 

Decomposition of methoxynaphthalene In supercritical water at 390 C occurs by proton-catalyzed hydrolysis and results In 2-naphthol and methanol as main reaction products. The rate of hydrolysis Is enhanced by dissolved NaCl. The dielectric constant and the Ionic strength of supercritical water was found to affect the hydrolysis rate constant according to the "secondary salt effect rate law, which commonly describes Ionic reactions In liquid solvents. In subcrltlcal water vapor the decomposition of the ether results In a mixture of cracking products and polycondensates, which Is characteristic for a radical type thermolysis. 

Carbon dioxide is not the only nonflammable, biocompatible, and widely available solvent. Let us not forget about water. In particular, supercritical water oxidation and related reactive processes have shown a tremendous capacity for reducing toxic chemicals to innocuous constituents. Akiya and Savage have authored a recent review. Of particular interest is the section on hydrolysis in SCF water. Numerous references are tabulated according to chemical family. With regard to the more toxic compounds, Klein et al. have been especially active for many years and Tester et al. have focused on alkyl halides. ... 

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). 

J. Schanzenbacher, J. D. Taylor and J. W. Tester, Ethanol Oxidation and Hydrolysis Rates in Supercritical Water, J. Supercrit. Fluids, 22, 139-147, (2002). 

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. 

Like the oxides, several metal sulfides have also drawn the attention of crystal growth experts. Thus important compounds like ZnS can be grown in electronic-grade quality in supercritical water [101,102]. Hydrolysis is occasionally observed under certain conditons, but is not generally a problem. A variety of other related compounds, such as 61283, Ag3AsSe3, and CdTe, have been prepared hydrothermally, but their growth chemistry has not been studied in great detail [103]. 

Another important area of polymer modification with subcritical and supercritical water is the hydrolysis of polycondensation polymers such as polyethylene terephthalate (PET), polyurethanes, and nylons for conversion to their monomers [ 37]. Specifically, in supercritical water, 91 % monomer recovery (terephthalic acid) is achieved at 400 °C and 400 bar in less than 15min reaction times [38]. Studies of these reactions using a hydrothermal diamond anvil cell to follow the phase changes during the reaction of PET... 

Another example of polymer modification reaction is the hydrolysis of cellulose in subcritical and supercritical water [40]. Cellulose is shown to hydrolyze rapidly (<1 s) in supercritical water in the absence of any catalysts to glucose, fructose, and oligomers (cellobiose, cellotriose, etc) with a hydrolysis product yield of about 75 % at 400 °C and... 

Hydrolysis in Supercritical and Near Critical Water Reactivity and Availability, Journal of Supercritical Fluids 5, 163-168... 

Luo, H. Tucker, S. C. (1996) A Continuum Solvation Model Including Electrostriction Application to the Anisole Hydrolysis Reaction in Supercritical Water, Journal of Physical Chemistry 100, 11165-11174... 

Figure 8. Free energy of activation curves AGa rc) for the anisole hydrolysis reaction in supercritical water at Tr = 1.01 and pr = 0.8. Computed without compression (white circles) and with compression (solid circles). Reprinted from Ret [30]. Copyright 1996 American Chemical Society.

Klein, M. T., Y. G. Mentha, and L. A. Toiry 1992, Decoupling substituent and solvent effects during hydrolysis of substituted anisoles in supercritical water . Ind. Eng. Chem. Res. 31, 182. 

Luo, H. and S. C. Tucker 1996, A continuum solvation model including elec-trostriction Application to the anisole hydrolysis reaction in supercritical water . /. Phys. Chem. 100, 11165. 

Holgate, H. R., Meyer, J.C. and Tester, J.W. (1995) Glucose hydrolysis and oxidation in supercritical water, AIChE Jourrud 41,637-648... 

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... 

Nagai, Y, Matubayasi, N. and Nakahara, M. (2005). Mechanisms and kinetics of noncatalytic ether reaction in supercritical water. 1. Proton-transferred fragmentation of diethyl ether to acetaldehyde in competition with hydrolysis, J. Phys. Chem. A, 109, pp. 3550-3557. 

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