Supercritical Water in Chemical Synthesis

In many organic reactions such as hydrolysis or certain rearrangements, water is the solvent and catalyst via self-dissociation, and sometimes also a reactant [11, 12]. The advantage of the use of water is that the addition of acids and bases may be avoided. This means that cleaning the effluent is easier and less expensive. The ionic product of water increases with pressure (under supercritical conditions) therefore reaction rates e.g. of acid- or base-catalyzed reactions also increase. On the other hand, the reaction of free radicals, which are undesirable during pyrolysis, decreases with pressure (see Introduction), thus high selectivities can be achieved. 

In organometallic reactions, water is a thermally very stable solvent, but may also be a reactant. In this case water is a non-polar solvent from a macroscopic point of view, and a polar molecule from a microscopic point of view. This opens new opportunities for unusual reactions. A particular advantage of carrying out those reactions which are usually performed in organic solvents in supercritical water, is that solubility only exists at supercritical conditions. Following the reaction and cooling to ambient conditions, water and organic compounds separate. No distillation or other expensive separation techniques are necessary. 

Near the critical point, water is a very variable solvent with respect to the solubility of salts. Inorganic compounds are completely soluble below the critical point and precipitate at slighdy higher temperatures. The variability of solvent properties opens the opportunity to generate crystals of defined size and morphology. 

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. 

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Supercritical water (SCW) presents a unique combination of aqueous and non-aqueous character, thus being able to replace an organic solvent in certain kinds of chemical synthesis. In order to allow for a better understanding of the particular properties of SCW and of its influence on the rate of chemical reactions, molecular dynamics computer simulations were used to determine the free energy of the SN2 substitution reaction of Cl- and CH3C1 in SCW as a function of the reaction coordinate [216]. The free energy surface of this reaction was compared with that for the gas-phase and ambient water (AW) [248], In the gas phase, an ion-dipole complex and a symmetric transition... 

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... 

More polymerization reactions carried out at supercritical conditions, select biomass conversion supercritical fluid technologies for hydrogen production, wider use of supercritical water oxidation processes, portfolio of self-assembly applications, a spate of opportunities in process intensification, many supercritical fluid aided materials synthesis applications, and numerous reactions for synthesis of specialty chemicals are expected for years to come. 

A variety of chemical and biological reactions involving supercritical fluid technology are being explored and developed. They include polymerization reactions, biomass conversion, hydrogen production, applications of supercritical water oxidation, self-assembly applications, synthesis of specialty chemicals, manufacture of materials with tailored properties, and much more. These developments and new ones are expected to mature and be commercially deployed in years to come. 

Supercritical or.near-critical water has found technical applications for hydrothermal syntheses, as discussed in detail in Chapter 4.1. Another more recent industrid application of chemical reactions in SCFs is the oxidative destruction of chemical wastes in SCH2O (SCWO, supercritical water oxidation). A detailed coverage of the large and prolific field of SCWO is outside the scope of this book on chemical synthesis. The extensive pilot plant activity, primarily by MODAR (now General Atomics) and Eco Waste Technologies, has recently been sununarized by Schmieder [171]. The first commercial plant was opened by Huntsman Chemical in collaboration with Eco Waste. 

The use of supercritical fluids as alternatives to organic solvents is revolutionising a huge number of important science areas (24). Scientific applications vary from established processes, such as the decaffeination of coffee and the extraction and synthesis of active compounds, to the destruction of toxic waste in supercritical water, the production of nanoparticles and new materials, to novel emerging clean technologies for chemical reactions and extraction. 

Methanation. The formation of methane and CO2 from biomass is a classical example of chemical disproportionation of the zero-valence carbon in biomass. The reaction is mildly exothermic and is the natural decomposition reaction of wet biomass in the absence of oxygen (anaerobic digestion of biomass). The reaction also proceeds at elevated temperatures (up to 400 °C) in supercritical water as a reaction medium [29]. Alternatively the reaction can be carried out in a two-stage process ofgasification ofbiomass to synthesis gas, followed by catalytic methanation at T < 400°C (BioUaz). 

The choice of reaction solvent is a critical factor for development of sustainable synthetic pathways to any chemical product in general, and for polymers in particular. To this end, it is important to try to avoid volatile organic solvents, chlorinated solvents, and solvents that can damage the environment (e.g., fluorinated hydrocarbons). The most widely used green solvents for polymer synthesis are water, ionic liquids, and supercritical CO2. In addition, polymerizations can often be performed solvent free. 

The concern over enviromnental pollution by volatile organic compounds (VOCs), which are widely used in chemical transformation in research laboratories and industries, has led to the realization of the importance of green alternatives such as solvent-free synthesis [1,2], use of water as solvent [3], supercritical carbon dioxide [4-6], and room-temperatme ionic liquids (RTILs) as reaction media [7-10]. Even though extensive investigations have shown that it may not be possible to replace the VOCs with the ionic liquids (ILs), the studies have revealed the importance and some interesting and imusual properties of these substances generating a great deal of attention worldwide [11-15]. 

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