Liquids, supercritical

Natural Products. Various methods have been and continue to be employed to obtain useful materials from various parts of plants. Essences from plants are obtained by distillation (often with steam), direct expression (pressing), collection of exudates, enfleurage (extraction with fats or oils), and solvent extraction. Solvents used include typical chemical solvents such as alcohols and hydrocarbons. Liquid (supercritical) carbon dioxide has come into commercial use in the 1990s as an extractant to produce perfume materials. The principal forms of natural perfume ingredients are defined as follows the methods used to prepare them are described in somewhat general terms because they vary for each product and suppHer. This is a part of the industry that is governed as much by art as by science. 

Opt for solventless reactions, recycle solvents, or use benign solvents (e.g., water, ionic liquids, supercritical media). 

A summary of the research activities of the last four years reveals three different important trends (a) The design of new ionic ligands for excellent catalyst immobilisation in ionic liquids and high regioselectivity (b) the successful application of cheap, halogen-free ionic liquids in the biphasic Rh-catalysed hydroformylation (c) the successful development of unusual multiphasic reaction concepts for Rh-catalysed hydroformylation, namely catalysis in ionic liquid/supercritical C02 and SILP-catalysts. 

Table 1. Typical liquid, supercritical fluid (SCF), and gas properties.

Solvent a liquid in which certain kinds of molecules dissolve. Although they typically are liquids with low boiling points, they may include high-boihng liquids, supercritical fluids, or gases. 

The presence of small amounts of water was found to be essential even for hydrophobic ionic liquids (284). When a-chymotrypsin (in the form of salt-free lyophilized powder) was applied for the transesterification of Ai-acetyl-L-phenyl-alanine ethyl ester with 1-propanol in the dry ionic liquids [BMIM]PFg and [OMIMJPFg, little enzymatic activity was observed. The maximum activity was observed when 0.5 vol% water was added to the ionic liquids. Supercritical CO2 was also sufficient to activate the enzyme in dry ionic liquids. The addition of water to the supercritical C02-ionic liquid system further enhanced the enzymatic activity. 

The possible alternatives to ozone-depleting or otherwise environmentally challenging organic solvents for chemical processes are abundant. We can begin a review by looking at five broad categories aqueous, ionic liquids, supercritical fluids, fluori-nated solvents, or solventless processes (llhnan, 1993). 

J. A. Laszlo, D. L. Compton, Chymotrypsin-catalyzed transestrification in ionic liquids and ionic liquid/ supercritical carbon dioxide. In Ionic Liquids, R. D. Rogers, K. R. Seddon, Eds., ACS Symposium Series Vol. 

Koppenhoefer, B., Nothdurft, A., Pierrot-Sanders, J., Piras, P.,Popescu, P, Roussel, C., Stiebler, M., and Trettin, U. 1993. CHIRBASE A graphical molecular database on the separation of enantiomers by liquid, supercritical fluid, and gas chromatography. Chirality 5 213-219. 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

To understand any extraction technique it is first necessary to discuss some underlying principles that govern all extraction procedures. The chemical properties of the analyte are important to an extraction, as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical fluid, or solid extractant used to effect a separation. Of all the relevant solute properties, five chemical properties are fundamental to understanding extraction theory vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water-soil environmental compartments, and the transport between immiscible phases during analytical extraction. 

Many of the same properties that make supercritical fluids advantageous in chromatography also enhance their ability to extract compounds from within a sample matrix.(4) Also, since the solubility of most compounds is dependent on the density of the supercritical fluid, selective extraction is possible. (J5) These properties are well known, and have been exploited in some cases where the extraction was formerly done with a liquid. In many cases, the quality of extract is higher, and extractions are of higher efficiency than with liquids. Supercritical fluids, especially CO, are also often less expensive, less toxic and less flammable than their organic liquid phase counterparts. 

Reetz, M.T., W. Wiesenhofer, G. Francio and W. Leitner, Continuous Flow Enzymatic Kinetic Resolution and Enantiomer Separation Using Ionic Liquid/Supercritical Carbon Dioxide Media, Advanced Synthesis Catalysis, 345, 1221-1228 (2003). 

In biphasic reactors or two-phase partitioning bioreactors (TPPB), the substrate is located mostly in the immiscible phase and diffuses to the aqueous phase. The enzyme catalyzes conversion of the substrate at the interface and/or in the aqueous phase. The product/s of the reaction then may partition to the organic phase. The system is self-regulated, as the substrate delivery to the aqueous phase is only directed by the partitioning ratio between the two phases and the enzymatic reaction rate [53]. The use of ionic liquid/supercritical carbon dioxide for enzyme-catalyzed transformation is gaining attention [69]. 

Although C02 is the most common solvent for supercritical extraction processes because of it s abundance, non-toxicity and non-flammability, other compounds may prove to be better solvents in certain instances. In choosing a solvent, a balance between solubility and selectivity has to be struck. In the case of solutes with a melting point well below the decomposition temperature, it is usually desirable to perform a liquid-supercritical fluid extraction to circumvent the problems associated with handling solids at high pressures. In... 

The safety system design is often poor as very few experimental validation with liquefied gases/supercritical fluids have been published until now. We did operate some measurements of flash discharge of liquid/supercritical CO2 from pressure vessels and proposed a simple model for mass flux evaluation [1] these results can be used for safety systems design. 

Table 1 groups together differing orders of magnitude of physical parameters for the three states of the same fluid. It should be noted that in spite of high densities (similar to liquids), supercritical fluids are only slightly viscous and, from this point of view, have similar properties to gases. 

In addition to fluorous solvents and ionie liquids, supercritical fluids sc-fluids, scf s), sueh as supercritical carbon dioxide (se-C02), constitute a third class of neoteric solvents that can be used as reaction media. Although sc-fluids have been known for a long time and have been advantageously used as eluants in extraction and chromatography processes (see Sections A.6 and A.7 in the Appendix), their application as reaction media for chemical processes has become more popular only during the last decade. Some of their physical properties and the supercritical conditions necessary for their existence have already been described in Section 3.2 (see Figure 3-2 and Table 3-4) see also references [209, 211-220, 224-230] to Chapter 3 for reviews on sc-fluids and their applications (particularly for SC-CO2 and SC-H2O). 

Motyl, K. M., Cleaning Studies of Metal Substrates Using Liquid/ Supercritical Fluid Carbon Dioxide, Rockwell International, Rocky Flats Plant, Golden, Colorado (1988)... 

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