Superheated and supercritical water

With the emergence of SWE as an alternative to SFE opportunities exist for combining derivatisation reactions with aqueous extractions. Although extractions using superheated and supercritical water yield pleasing results, many instrumental problems will have to be overcome before this technique is ready to leave the (academic) research laboratories [77]. This approach might play a significant role in future analytical extractions. 

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... 

Water heated above 100 °C (superheated) and water heated above its critical point (supercritical) are often considered too acidic for synthesis. Superheated and supercritical water are well known for their ability... 

Smith, R. M., Extractions with superheated water, J. Chromatogr. A, 975, 31-46, 2002. Hawthorne, S. B., Yang, Y., and Miller, D. J., Extraction of organic pollutants from environmental solids with sub- and supercritical water, Anal. Chem., 66, 2912-2920, 1994. 

An area of study related to this topic is the use of subcritical, but superheated water as a mobile phase for chromatographic separations [78], These separations use water heated to 100-220°C and pressures up to 50 bar, avoiding problems due to hydrolysis and oxidation, which is common when supercritical water is used. Although this is a new area of investigation, several reports on the hyphenation of HPLC using... 

Hua et al. (1995) proposed a supercritical water region in addition to two reaction regions such as the gas phase in the center of a collapsing cavitation bubble and a thin shell of superheated liquid surrounding the vapor phase. Chemical transformations are initiated predominantly by pyrolysis at the bubble interface or in the gas phase and attack by hydroxyl radicals generated from the decomposition of water. Depending on its physical properties, a molecule can simultaneously or sequentially react in both the gas and interfacial liquid regions. 

Supercritical steam is actually supercritical water (SCW) because at supercritical pressures, the fluid is considered as a single-phase substance. However, this term is widely (and incorrectly) used in the hterature in relation to supercritical steam generators and turbines. Superheated vapor is a vapor at pressures below the critical pressure, but at temperatures... 

Carbon dioxide and water are the most commonly used SCFs because they are cheap, nontoxic, nonflammable and environmentally benign. Carbon dioxide has a more accessible critical point (Table 6.13) than water and therefore requires less complex technical apparatus. Water is also a suitable solvent at temperatures below its critical temperature (superheated water). Other fluids used frequently under supercritical conditions are propane, ethane and ethylene. 

Water reaches supercritical conditions at 373.9 °C (Table 6.13) but it becomes a suitable solvent at 200-350 °C and at pressures generated solely by the expansion of the liquid medium, about 20-100 bar (subcritical or superheated water). 

With an increased interest and awareness of the impact of society and industry on the environment, there has been a significant attempt in recent years to reduce or replace the usage of organic solvents. Much early work in this area concentrated on the application of supercritical and subcritical carbon dioxide, but in recent years superheated (or subcritical/pressurized hot) water (SHW) has become of interest for both chromatography and extraction [43,54], The earliest work was reported by GuUlemin et al. [55], who used the term thermal aqueous liquid chromatography. As well as using SHW for the separation of... 

Brogle H. CO2 as a solvent its properties and uses. Chem Ind 1982 19 385-390. Clifford AA, Basile A, Jiminez-Carmona MM, Al-Saidi SHR. Extraction of natural products with superheated water. Proceedings of the 6th Meeting on Supercritical Fluid Chemistry and Materials, 1999 485-490. 

Even superheated (to 260 °C) or supercritical (to 400 °C) water was employed in the Heck reaction with several catalyst precursors and aryl halides with styrene. However, all conversions show large amounts of side products and the yields were in the 5-30% range, indicating radical intermediates and by-products from decomposition of the arene starting material [15]. 

When water is used as the solvent, PLE is referred to as superheated water extraction, subcritical water extraction (SWE), or pressurized (hot) water extraction (PWE). Hot water is very effective as an extraction solvent for PAHs from soil and sediment. Superheated water is water above the boiling point but below the supercritical point, and under sufficient pressure to maintain... 

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