Superheated water compound

Superheated water at 100°-240 °C, with its obvious benefits of low cost and low toxicity, was proposed as a solvent for reversed-phase chromatography.59 Hydrophobic compounds such as parabens, sulfonamides, and barbiturates were separated rapidly on poly(styrene-divinyl benzene) and graphitic phases. Elution of simple aromatic compounds with acetonitrile-water heated at 30°-130 °C was studied on coupled colums of zirconia coated with polybutadiene and carbon.60 The retention order on the polybutadiene phase is essentially uncorrelated to that on the carbon phase, so adjusting the temperature of one of the columns allows the resolution of critical pairs of... 

Most recently, a further study has been performed using superheated-water HPLC with NMR and MS to analyse a mixture of sulphonamides [68]. The chromatography was performed as before with D20-phosphate buffer (pD 3.0) as eluent. A temperature gradient from 160 to 200 °C at 2°C min-1 was employed. A mixture of four sulphonamides, i.e. sulacetamide, sulphadiazine, sulfamerazine and sulfamethazine, was separated in this system with UV, NMR and MS detection. It rapidly became clear from a study of the spectroscopic data that while sulfacetamide and sulfadiazine gave the expected NMR and mass spectra, those for sulfamerazine and sulfamethazine did not. These compounds gave spectra that were 3 and 6 mass units higher than expected,... 

These investigations of the use of superheated water with NMR spectroscopy are still at an early stage and have not yet been applied to real problems. Many questions remain to be answered concerning the suitability of this chromatography for thermally labile compounds, and with the current stationary phases available the technique is probably limited to moderately polar compounds. However, the technique is readily implemented and may in time prove to be a useful addition to the armoury of HPLC-NMR methods. 

Siskin, M. and A.R. Katritzky, Reactivity of Organic Compounds in Superheated Water General Background, Chemical Reviews, 101, 825-836 (2001). 

The principal advantages in the use of superheated water are that it is relatively easy to attain and the back-pressures required on the column are small. Thus even a modest length of narrow bore tubing can be employed to provide sufficient resistance to prevent boiling in the column and at these pressures many conventional spectroscopic flow cells can be used. Because of the high temperatures, there have been concerns about the thermal stability of the analytes, but of the numerous examples, there have been few reports of instability or a tendency for accelerated hydrolysis or oxidation, of the reported examples, only aspirin has hydrolyzed. Compounds which might be expected to be labile to oxidation or hydrolysis, such as the paraben antioxidants, have chromatographed without problems even up to 200°C [59]. 

TABLE 18-2. Pharmaceutical Compounds Separated Using Hot and Superheated Water... 

Reversed-phase separations using superheated water have not found any major applications so far, but a wide range of low molecular mass polar compounds have been separated at modest temperatures with relatively simple and rugged instrumentation. It has potential for wider use in laboratories that wish to reduce consumption of organic solvents. In addition, superheated water should be compatible with on-column preconcentration techniques for trace analysis... 

Most of the work described below is on a laboratory scale and some of it is directed towards chemical analysis. However, some larger-scale processes are under consideration, mainly for environmental reasons. Chemical reactions, which bring about modification of the extracted compounds, can occur during extraction. In some cases this is an advantage, e.g. when pollutants are destroyed or flavors produced. The work related to these is described in so far as it is publicly available. Some important features of superheated water are first briefly discussed. 

Partly because of its fall in polarity with increased temperature, superheated water can dissolve organic compounds to some extent, especially if they are shghtly polar or polarizable, as are aromatic compounds. The solubihty of an organic compound in superheated water is often many orders of magnitude higher than its solubility in water at ambient temperature for two reasons, one being the polarity change. 

The extraction of polyaromatic hydrocarbons from soil and urban particulates by superheated water was reported in 1994 [17]. Extraction of compounds up to ben-zo[a]pyrene was virtually complete in 15 min at 250°C, with a flow rate of 1 ml mim and a sample of 0.5 g. Good but less complete results were obtained when extracting urban air particulates. The pressure did not influence the extraction behavior, provided it was sufficient to maintain water as a Hquid. The extraction of polychlorinated biphenyls from soil and a river sediment was also found to be complete in 15 min at 250°C [18]. Work with a wider range of compounds showed that extraction was class selective [6, 19], with phenols and Hghter aromatics being extracted at 50 to 150°C, polyaromatic hydrocarbons and lighter ahphatics at 250 to 300°C, but the heavier ahphatics only removed by steam at 250 to 300°C. This selectivity has been compared to other extraction methods [20]. The extraction of agrochemicals from soil has also been studied [6]. 

Treatment of polymers by superheated water at high temperatures can result in their decomposition/hydrolysis to monomers or lower oligomers [6]. At lower temperatures the polymer is stable and the small amounts of monomers, initiators, low oligomers and other small compounds can be extracted from polymers with superheated water [6]. For example, at 200°C styrene, alkylbenzene contaminants and styrene dimers were extracted from polystyrene without destroying the polymer. Although stable at 200°C, at 250°C polystyrene was decomposed into substituted benzenes [6]. 

Very often compounds being extracted by superheated water react in the medium by hydrolysis or otherwise. It is know from other studies involving pure contaminants that they will react, for example chlorinated hydrocarbons are often dechlo-rinated and converted into hydrocarbons. In other cases benign materials are obtained from pollutants. In the extraction of the explosives TNT, RDX and HMX from contaminated soil, decomposition occurs non-dramatically and completely to benign substances [48]. These compounds contain an oxidative reagent within the molecule. Soil obtained from a bomb disposal site contaminated with 120 000 ppm (12%) of TNT, after treatment in a static ceU at 275°C for 1 h, contained only 2 ppm and the water remaining 4 ppm. Dioxins in contaminated soil treated for 4 h at were found to be reduced by 99.4%, 94.5% and 60% at temperatures of 350°C, 300°C and 150°C, respectively [49]. 

Of course many other compounds are also extracted from plants by superheated water. This gives rise to problems in isolahng desired compounds from the extract for a parhcular process, as discussed in Section 3.8.3.3. It has been shown, for example, that anhoxidants are extracted from rosemary [59]. Experiments were carried out between 25 and 200°C and showed high selectivity of extrachon towards compounds exhibihng high anhoxidant achvity. 

It has been found that the total amount of fragrance compounds obtained by superheated water extrachon was greater than that that could be obtained by steam dishllahon. This could be because penetradon of the plant material is better with hot water under pressure, as proposed above. As an example of these comparisons, the work on wild marjoram Thymus mastichina) [60] will be briefly outlined. Steam dishllahon was carried out over 3 h, whereas superheated water extrachon at 150°C was completed in 15 min. The authors used about 10.5 hmes more water per gram of plant material in superheated water extrachon than in steam dishlla-... 

Analysis of plants normally involves a sample preparation stage such as extraction or distillation followed by analysis with gas chromatography or liquid chromatography. The common methods used currently for the isolation of essential oils from natural products are steam distillation and solvent extraction (Ozel Kaymaz, 2004). Losses of some volatile compounds, low extraction efficiency, degradation of xmsaturated compounds through thermal or hydrolytic effects, and toxic solvent residue in the extract may be encountered with these extraction methods. Recently, more efficient extraction methods, such as supercritical fluid extraction (SFE) (Simandi et al., 1998) and accelerated solvent extraction (ASE) (Schafer, 1998) have been used for the isolation of organic compounds from various plants. Subcritical or superheated water extraction (SWE) is non-toxic, readily available, cheap, safe, non-flammable and is a recyclable option. 

Compounds with Superheated Water, Accounts in Chemical Research 29, 399-406... 

Wet Oxidation Wet oxidation is a form of hydrothermal treatment. It is the oxidation of dissolved or suspended components in water using oxygen as the oxidiser. It is referred to as wet air oxidation (WAO) when air is used. Oxidation reactions occur in superheated water at a temperature above the normal boiling point of water (100°C), but below the critical point (374°C). The system must be maintained under pressure to avoid excessive evaporation of water. This is done to control energy consumption due to the latent heat of vaporisation. It is also done because liquid water is necessary for most of the oxidation reactions to occur. Compounds that would not oxidise under dry conditions at the same temperature and pressure oxidise under wet oxidation conditions. 

Katritzky, A.R. and Allin, S.M., Aquathennolysis reactions of organic compounds with superheated water, Acc. Chem. Res., 1996,29, 399 06. 

Siskin, M. and Katrizky, A.R., Reactivity of organic compounds in superheated water general background, Chem. Rev., 2001, 101(4), 825-835. 

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