Separation of organic compounds from mixtures by clathration
This invention relates to the separation of organic compounds from mixtures by clathration. 
     At present xylene isomers are separated one from another by clathration using as hosts for example, Werner complexes such as Ni(SCN) 2 .4(α-phenylethylamine) or phosphonitrile derivatives such as p,p-(o-phenylenedioxy)-phosphonitrile trimer. 
     Host compounds have now been found which by clathration can effect the separation of various organic compounds, including the separation of p-xylene from mixtures thereof with other isomers, the degree of separation being relatively increased. 
     Accordingly, the present invention comprises a method for the separation of organic compounds by clathration in which there is used as a host a compound of formula I: ##STR2## wherein: Ar 1  Ar 2  and Ar 3  Ar 4 , which may be identical or different, represent aromatic groups; X and Y, which may be identical or different, represent P, As or Sb or P, As or Sb to which O, S or Se are bound; 
     R represents a bond joining X and Y, a substituted or unsubstituted alkylene group, a substituted or unsubstituted arylene group, a substituted or unsubstituted alkenylene group or a substituted or unsubstituted alkynylene group. 
     Certain compounds of Formula I are described in &#34;Organic Phosphorus Compounds&#34; Editors: Kosolapoff and Maier; John Wylie and Sons, New York, 1972: Vol 1 pp 1-287 (tertiary phosphines), Vol 3 pp 341-500 (phosphine oxides) and Vol 4 pp 1-73 (phosphine sulphides, selenides and tellurides). The compound (I) in which Ar 1  to Ar 4  ═Ph, X and Y═P═Se and R═--(CH 2 ) 2  --, which is of particular interest, is disclosed by Philip Nicpon and Devon W. Meek in Inorganic Chemistry, 5 (7), 1297 (1966) and may be prepared by the method described therein. 
     The present invention comprises in a further aspect those compounds of Formula I which are novel. 
     In host compounds of particular interest, Ar 1  -Ar 4  represent substituted or unsubstituted aryl groups e.g. phenyl groups. 
     R moreover preferably represents an alkylene group, --(CH 2 ) n  -- wherein n is preferably 1-3 and X,Y which are preferably identical represent the group P═Se or P═S in preferred compounds. 
     The present invention finds particular application in the separation of hydrocarbons from one another and from other compounds. 
     Accordingly, the present invention further comprises a method for the separation by clathration of a hydrocarbon from a mixture comprising the hydrocarbon, in which the mixture is treated with a host compound of Formula I so that said hydrocarbon forms an adduct with the host compound. In general the hydrocarbon is subsequently recovered from the adduct although the process may, if desired, be applied to the removal of an unwanted component of a mixture therefrom. 
     The present process is applicable to the separation by clathration of aromatic hydrocarbons, notably mononuclear aromatic hydrocarbons and is of particular interest for the separation of isomers of the following compounds from one another and, if desired, from other compounds: xylenes, ethyltoluenes, cymenes, diethylbenzenes, chlorotoluenes, dichlorobenzenes, toluidines, nitrotoluenes, methylanisoles and cresols. 
     The process is particularly suitable for the separation of xylene isomers from one another by clathration of p-xylene and for the separation of xylene isomers and in particular p-xylene from ethylbenzene. 
     The present process of the present invention may be carried into effect by dissolving the compound of Formula I in the mixture, if necessary by either adding a solvent or raising the temperature of both. The mixture may then be cooled and, if desired, seeded to precipitate the adduct. 
     In order to simplify the separative procedure however it is generally preferable for the guest hydrocarbon to be absorbed by the host compound. 
     Accordingly the present invention comprises, in a further aspect, a method for the separation by clathration of a hydrocarbon from a mixture comprising the hydrocarbon, in which the mixture is treated with a host compound of formula I, said host compound and mixture being disposed in separate phases, so that said hydrocarbon forms an adduct with the host compound. 
     The host compound is generally present as a solid phase or a component thereof and the mixture preferably provides a liquid phase although if desired the mixture may be dissolved in a suitable solvent providing the liquid phase, the host compound being insoluble in said solvent. Absorption of the guest hydrocarbon from the vapour phase is however envisaged. 
     The host compound is preferably conditioned prior to use by formation of an adduct with the guest hydrocarbon which is to be separated from the mixture and subsequent removal of the guest hydrocarbon therefrom. In the conditioning step the adduct may be formed for example by precipitation from solution as hereinbefore described. 
     Recovery of the guest compound may be achieved, for example, by heating the adduct, if necessary at reduced pressure or, after separation of the adduct from the mixture, by dissolution thereof in a suitable solvent such as chloroform. 
     The present invention is illustrated by the following Examples: 
    
    
     EXAMPLE 1 
     (No co-solvent) 
     3 g of (I) [Ar 1  to Ar 4  ═Ph, X and Y represent P═Se, R═--(CH 2 ) 2  --]were dissolved in 25 ml. of an equivolume mixture of o-xylene, m-xylene, p-xylene and ethylbenzene at 383° K. The solution was allowed to cool at approximately 0.5°-1.0°/min. Seeding with crystal fragments which contained p-xylene initiated crystallisation when the temperature was 360° K. 2.9 g of adduct (1)-p-xylene was isolated. The adduct was purified by decantation then evacuation at room temperature. 
     The separated hydrocarbon was recovered from the adduct by heating the adduct at reduced pressure. 
     The isolated hydrocarbon was analysed by i.r. absorption methods and shown to consist of 95.5% p-xylene, 2.5% ethylbenzene, 1.2% m-xylene and 0.8% o-xylene (all values subject to an error ±0.1%). 
     EXAMPLE 2 
     (With CHCl 3  as co-solvent) 
     2g of (I) [Ar 1  to Ar 4  ═Ph, X and Y represent P═Se, R═--(CH 2 ) 2  --] were dissolved in 25 ml. of an equivolume mixture of o-xylene, m-xylene, p-xylene, ethylbenzene and chloroform at 363° K. This solution was allowed to cool at approximately 0.5°-1.0°/min. No seeding was employed. 1.9 g of adduct (I) p-xylene was isolated. 
     This was purified by decantation and evacuation at room temperature to remove surface liquid. 
     The separated hydrocarbon was recovered from the clathrate inclusion compound as in Example 1 (see above). Analysis by  1  H n.m.r. spectroscopy indicated that the separated hydrocarbon consisted of p-xylene (95% or better) with trace quantities only of ethylbenzene, o-xylene, or m-xylene. 
     EXAMPLE 3 
     (Absorption Method) 
     1.90 g of (I) [Ar&#39; - Ar&#34;═Ph, X═Y═(P═Se), R═(CH 2 ) 2  ] was stirred for 20 h. under a nitrogen atmosphere in 8 ml of an equivolume mixture of o-xylene, m-xylene, p-xylene and ethylbenzene at 295° K. 1.85 g of adduct (I)-p-xylene was isolated after filtration, n-pentane washing, and evacuation at room temperature. 
     The separated hydrocarbon was recovered from the adduct by heating the adduct at reduced pressure. 
     The isolated hydrocarbon was analysed by i.r. absorption methods and shown to consist of 96.8% p-xylene, 2.0% ethylbenzene, 0.7% m-xylene, and 0.5% o-xylene (all values subject to an error =/±0.1%).