Supercritical fluids ammonia

Supercritical Fluid Chromatography. Supercritical fluid chromatography (sfc) combines the advantages of gc and hplc in that it allows the use of gc-type detectors when supercritical fluids are used instead of the solvents normally used in hplc. Carbon dioxide, -petane, and ammonia are common supercritical fluids (qv). For example, carbon dioxide (qv) employed at 7.38 MPa (72.9 atm) and 31.3°C has a density of 448 g/mL. 

Supercritical fluid solvents have been tested for reactive extractions of liquid and gaseous fuels from heavy oils, coal, oil shale, and biomass. In some cases the solvent participates in the reactions, as in the hydrolysis of coal and heavy oils with water. Related applications include conversion of cellulose to glucose in water, dehgnincation of wood with ammonia, and liquefaction of lignin in water. 

Most supercritical fluid chromatographs use carbon dioxide as the supercritical eluent, as it has a convenient critical point of 31.3°C and 72.5 atmospheres. Nitrous oxide, ammonia and n-pentane have also been used. This allows easy control of density between 0.2g ml-1 and 0.8g ml-1 and the utilization of almost any detector from liquid chromatography or gas chromatography. 

There is no doubt that these applications will grow in the future and that the range of supercritical fluids used (carbon dioxide and methanol modified carbon dioxide, nitrogen dioxide, ammonia, fluoro-hydrocarbons) will increase as will the combination of this technique with mass spectrometric identification of separated compounds. 

The first use of supercritical fluid extraction (SFE) as an extraction technique was reported by Zosel [379]. Since then there have been many reports on the use of SFE to extract PCBs, phenols, PAHs, and other organic compounds from particulate matter, soils and sediments [362, 363, 380-389]. The attraction of SFE as an extraction technique is directly related to the unique properties of the supercritical fluid [390]. Supercritical fluids, which have been used, have low viscosities, high diffusion coefficients, and low flammabilities, which are all clearly superior to the organic solvents normally used. Carbon dioxide (C02, [362,363]) is the most common supercritical fluid used for SFE, since it is inexpensive and has a low critical temperature (31.3 °C) and pressure (72.2 bar). Other less commonly used fluids include nitrous oxide (N20), ammonia, fluoro-form, methane, pentane, methanol, ethanol, sulfur hexafluoride (SF6), and dichlorofluoromethane [362, 363, 391]. Most of these fluids are clearly less attractive as solvents in terms of toxicity or as environmentally benign chemicals. Commercial SFE systems are available, but some workers have also made inexpensive modular systems [390]. 

Liquefied or Supercritical Cases as Solvents for Electrolytes For very special applications, where the increased efforts for low temperature and/or pressurized cells are acceptable, liquefied gases, for example, sulfur dioxide or ammonia, can be interesting solvents for electrolytes (see e.g. [3a]). Supercritical fluids show remarkable properties that are very different from other solvents. Detrimental to electrochemistry is that especially the dielectric constant in the supercritical state becomes low. For supercritical carbon dioxide, no supporting electrolyte with sufficient conductivity is known. 

The single largest use of ammonia is its direct apphcation as fertdizer, and in the manufacture of ammonium fertilizers that have increased world food production dramatically. Such ammonia-based fertilizers are now the primary source of nitrogen in farm soils. Ammonia also is used in the manufacture of nitric acid, synthetic fibers, plastics, explosives and miscellaneous ammonium salts. Liquid ammonia is used as a solvent for many inorganic reactions in non-aqueous phase. Other apphcations include synthesis of amines and imines as a fluid for supercritical fluid extraction and chromatography and as a reference standard in i N-NMR. 

Other low-temperature studies have been motivated by the desire to characterize and understand processes occurring in unusual media. For example, the use of liquid ammonia [8-10] and liquid sulfur dioxide [11-13] naturally requires reduced temperatures unless high pressures are used, as is done for electrochemistry in supercritical fluids [14]. Frozen media are interesting systems in terms of mass transport phenomena and microstructural effects. Examples include glasses of acetonitrile and acetone [15], frozen dimethyl sulfoxide solutions [16,17], and the solid electrolyte HC104 5.5 H20 [18-20]. 

To access the supercritical fluid state, we must have conditions in excess of the critical temperature and pressure. Given the rating of the autoclave, ammonia would not be suitable because one could not access the supercritical state as a result of the pressure limitation. Methylamine would not be suitable for a room temperature extraction because its Tc is too high. Either methane or tetrafluoromethane would be suitable for this application. 

In this study, C16-C32 wax distillate is separated into n-paraffin and denormal oil fractions by vapor phase contacting with type 5A molecular sieves. 2,2,4-Trimethylpentane (isooctane), 2,2-dimethylbutane, and 2-methylpentane are each used as the supercritical fluid. Recovery of n-paraffins from the molecular sieves is accomplished by contacting with nitrogen or ammonia gas. 

Since the early days of SFC, there always has been a desire to extend the useful range of the technique to more polar molecules. A similar type of desire exists in SFE. The hope for achieving efficient extractions of polar molecules from polar as well as non-polar substrates can only be realized with the use of more polar primary supercritical fluids or by the use of modifiers. Many of the more primary supercritical fluids that exists namely, ammonia or water, are not effectively usable in the analytical laboratory due to instrumental as well as safety restrictions, therefore, the need to do more research on the use of modifiers in SFE is greatly necessitated. Based upon the limited study that was done within the scope of this chapter, a few conclusions can be drawn. These conclusions are summarized in Figure 16. 

Absorption spectra of 2-nitroanisole in supercritical C02, N20, Freon-13, ammonia and C02-methanol mixtures were obtained on a Cary model 1605 spectrophotometer operated in the dual beam mode. The gases used as supercritical solvents were of the highest purity available from the supplier (Matheson) and were further filtered prior to use. The mixed solvent system of C02-methanol was obtained from Scott Speciality Gases (15.4 wt% methanol), and other mixtures were made in the laboratory. Spectra of 2-nitroanisole in n-pentane, methanol, tetrahydrofuran and acetonitrile (Burdick A Jackson) were obtained using quartz cells with a 1-cm light path and with a pure solvent blank in the reference beam. Vapor phase and supercritical fluid spectra were obtained using an air reference. 

Crooks RM, Bard AJ. Electrochemistry in near-critical and supercritical fluids, nitrogen heterocycles nitrobenzene, and solvated electrons in ammonia at temperatures to 150°C. J Phys Chem 1987 91(5) 1274. 

In this chapter, we have reviewed some of our own work on solvation properties in supercritical fluids using molecular dynamics computer simulations. We have presented the main aspects associated with the solvation structures of purine alkaloids in CO2 under different supercritical conditions and in the presence of ethanol as co-solvent, highlighting the phenomena of solvent density augmentation in the immediate neighborhood of the solute and the effects from the strong preferential solvation by the polar co-solvent. We have also presented a summary of our results for the structure and dynamics of supercritical water and ammonia, focusing on the dielectric behavior of supercritical water as functions of density and temperature and the behavior of excess solvated electrons in aqueous and non-aqueous associative environments. 

Other supercritical fluids can be used for chemical reactions, such as supercritical ammonia in the synthesis of labeled guanidines. ... 

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