Liquid clathrates separations

Liquid clathrates offer a great advantage over solid-state separations (e.g. by formation of Hoffman-type inclusion compounds, Section 9.4) because of the extremely fast mixing kinetics, the avoidance of the need to wait for crystallisation to occur and the easy separation of the two liquid phases. It should also prove possible to run liquid clathrate separations in a continuous extraction manner. The avalues of a number of liquid clathrate-based separations have been reported and are summarised in Table 13.1. 

The strong affinity of ionic liquids for aromatics has been attributed to the formation of liquid clathrates (90-92). Liquid clathrates (93) are semi-ordered liquids containing complex salt hosts. They are formed by associative interactions between aromatic molecules and salt ions, which separate cation-anion-packing interactions to a sufficient degree that localized cage structures are formed. Although the aromatic compounds are highly soluble in the ionic liquid phase, the... 

Liquid clathrates are useful in a number of selective organic separations. 

The majority of the interest in liquid clathrates has centred around their potential applications in separation science, particularly in the separation of closely related species such as benzene and toluene, or xylene isomers (cf. zeolite-based methods, Section 9.2). Separation occurs as a result of the greater ability of one organic solvent over another to stabilise the liquid clathrate phase. Hence, in a... 

Clearly significant separations can be effected, especially between aromatics and aliphatics. The size basis for the effect is clear in the comparison of hexane and hexene results the presence of the 1-hexene double bond does not seem to enhance its inclusion into the liquid clathrate phase greatly. Also clear is the strong dependence on the dimensions of the anion, with the smaller, more compact materials giving better separations. 

Table 13.1 Liquid clathrate-based separations of 1 1 feed mixtures.

Formation of small, closed units of association e.g. dimers, trimers etc.) is not permitted. Liquid clathrates are useful in a number of selective organic separations. 

It should be noted that the formation of the liquid clathrate is a visually dramatic event. As the reaction proceeds, a separation of two liquid layers (liquid clathrate and excess aromatic) becomes obvious upon shaking, the layers resemble oil and water. 

The first examples of materials that supported liquid clathrate phase formation, in contact with aromatic hydrocarbons, were highly reactive air-sensitive salts containing alkylaluminum anions, first described by At-wood. " Their applications for separations of aromatics from hydrocarbons and in coal liquefaction were extensively explored. However, a drawback of the initial systems studied was their air-sensitivity and reactivity. 

A second method for the production of liquid clathrates involves the addition of 0.005 mol KN3 and 0.010 mol A1(CH3)3 to 0.10 mol toluene in an N2 atmosphere dry box. This procedure yields a liquid clathrate identical in composition to the one prepared in the previous method. Note that liquid clathrate formation is a visually dramatic event. As the reaction proceeds, separation of two distinct liquid layers becomes obvious. 

Since the initial discovery of liquid clathrate systems, hundreds of compounds having the structure M[Al2R6X] were found to exhibit liquid clathrate behavior.While these highly reactive air-sensitive salts are useful in such applications ranging from separations of aromatics from hydrocarbons to coal liquefaction, their air-sensitivity and reactivity posed drawbacks to further study. 

The potential application of liquid clathrates to separation science is of continuing interest, in particular, the separation of closely related aromatic hydrocarbons such as benzene and toluene, or o. m. and p-xylene Organic nonquaternary clathrate salts were successfully applied as useful materials for separating hydrocarbon feed streams into aromatics-rich fractions and aromatics-lean fractions... 

Liquid clathrates represent an interesting area of supra-molecular chemistry. Over the years, the move from air-sensitive to air-stable liquid clathrate systems greatly simplified their handling procedures. Also, with the emergence of air-stable liquid clathrate systems, their possible applications broadened beyond early useful applications of coal liquefaction and hydrocarbon separations to include aqueous separations and catalytic systems. 

A model for liquid clathrate behavior is presented in Fig. 15. It is believed that the ions interact in a cooperative manner. One cation may be associated with two or more anions and vice versa. The cation-anion interaction must be strong, or the ions will separate and a normal solution will result. The aromatic molecules are necessary constituents of the layerlike structure. They are guests, but... 

Of the several interesting applications of liquid clathrates is in the area of separations. Since the behavior is found for aromatic molecules but not for aliphatic ones, a separation is possible. It is possible to envision even difficult problems such as the separation of the xylene isomers being attacked by liquid clathrates. Liquid clathrates have also been reported to be useful as solvents for the liquefaction of coal. 

Atwood, J.L. and Atwood, J.D., Non-stoichiometric liquid enclosure compounds ( liquid clathrates ), in Inorganic Compounds with Unusual Properties, bl. 150, ed. R.B. King, American Chemical Society, Washington, DC (1976), pp. 112-127 (c)Atwood, J.L., Liquid clathrates, in Recent Developments in Separation Science, bl. 3, ed. N.N. Li, CRC Press, Cleveland (1977), pp. 195-209. 

Different types of molecules acting as mass-separating agents may be added to a liquid mixture in gaseous, liquid or solid form to form preferentially a solid phase of the added external agent molecules and the solvent/one of the solute species in the liquid mixture when cooled. The solid phase of the solvent/solute and the external added agent is generally called an inclusion compound (Atwood et ai, 1984). We now consider a variety of inclusion compounds adducts clathrates crown ethers cyclodextrins liquid clathrates. 

Analytically, the inclusion phenomenon has been used in chromatography both for the separation of ions and molecules, in liquid and gas phase (1,79,170,171). Peralkylated cyclodextrins enjoy high popularity as the active component of hplc and gc stationary phases efficient in the optical separation of chiral compounds (57,172). Chromatographic isotope separations have also been shown to occur with the help of Werner clathrates and crown complexes (79,173). 

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