Urea Complexes for the Separation of Straight Chain Solvents

In a recent blog pertaining to solvent replacement, ”Solvent Replacement: the need to change solvent either from a reaction solvent to a crystallizing solvent or during reaction telescoping in a process” April 9^th 2007, KiloMentor suggested the possibility of using a high boiling n-paraffin, or dibutyl ether, or a polyethyleneglycol as a chaser and then removing that solvent as a urea inclusion complex.

I proposed this, not as an established or even exemplified procedure, but only as something that might be expected to work.  A paper has appeared, commenting again on the need and the difficulty of removing high boiling dipolar aprotic solvent residuals when isolating pure reaction products [Removal of Reaction Solvent by Extractive Work-up: Survey of Water and Solvent Co-extraction in Various Systems, Laurent Delhaye, Attilo Ceccato, Pierre Jacobs, Cindy Kottgen and Alain Merschaert. Organic Process Research & Development, 2007, 11, 160-164.] This article was published on the web

 http://pubs.acs.org/cgi-bin/abstract.cgi/oprdfk/2007/11/i01/abs/op060154k.html

 Perhaps one solution will be found using dipolar aprotic solvents that are effectively linear and longer than eight atoms because it is these molecules which can be cleaned out of the final product using urea complexes.  I would like to offer some further literature support for this idea now.

Urea complexes of polyethylene glycol, dibutyl ether, octadecane and diethylene glycol are known in the literature are made in the established way. Also the literature already provides experimental details for making urea complexes of the n-paraffins from light gas-oil and heavy gas-oil petroleum fractions. [Ind. Eng. Chem. Res. 1997, 36, 3110-3115. Separation and Characterization of Paraffins and Naphthalenes from FCC Feedstocks,  A.A. Lappas, D. Patiaka, D. Ikonomou and I.A. Vasalos]. The paper teaches the separation of the n-paraffin fraction from fluid catalytic cracking using urea. This teaching encourages one to suspect that n-paraffins, even when present as a substantial portion of a mixture as it would be if it were the residual solvent after a concentration, can separated from other mixture constituents.  Sufficient urea would be added along with a polar compound (activator ) such as water, aliphatic alcohol, or ketone which expedites the completion of formation.  Methanol is usually used.

The procedure provided in the paper is quoted:

“Separation of n-Paraffins by Urea Adduct Formation.

 The entire separation procedure for the non-aromatic fraction is described in Figure 1.  The typical removal procedure of the straight chain hydrocarbons (n-paraffins) from heavy or light gas-oil is

 (i) 15 g of urea and 5 g of HGO (or (LGO) aliphatic hydrocarbons (isolated by elution chromatography-ASTM D-2549) are placed in a 250 ml flask and stirred for 0.5 h at 55-60 C by adding 25 ml of methanol and

 (ii) the mixture is stirred for 1.5 h in room temperature and for 0.5 h at 10 C. The solid adduct is washed with hexane (60 ml) and filtered off.”

The commentary on this procedure in the paper was:

“….The key factor which affects the entire procedure is the effective contact between urea (or thiourea) and the paraffinic substances.  This contact is influenced by the amount of excess urea and methanol. The following excesses are necessary for satisfactory separation/;25 ml of methanol and 15 g of urea for paraffin separation …….The stirring of mixtures at some very specific temperatures is also very important.  The initial heating must be at 55 C for a period of 30 minutes.  This serves to increase the rate of adduction of the heavier n-paraffins through increased solubility and diffusion in the methanol-urea phase.  By decreasing the final adduction temperature to 10 C, the recovery of compounds such as C₁₃ and above is improved…..”

It would seem that this advice can be useful devising conditions to remove uniform molecular weight, high boiling, straight chain solvents.  In fact this should be a simpler case.  A single optimal temperature for adduct formation tailored to the particular solvent and another temperature to maximize yield for filtration could be expected to work well.  What only experimentation can discover is to what extent solutes, from any particular reaction mixture being isolated from the high boiling solvent, are selectively excluded from the urea complexes.

Besides the straight chain high boiling solvents already mentioned we can imagine diglyme, triglyme and tetraglyme behaving effectively the same way.

The following articles show examples of these molecules forming complexes.

Redlich, O.; Gable, C.M.; Dunlop, A.K.  and Miller, R.W..  Addition Compounds of urea and organic substances. J. Am. Chem. Soc. (1950), 72, 4`153-60

Topchiev, A.V.; Roozenberg, L.M.; Nechitailo, N.A.; and Terent’eva, E.M., Khurnal Neorganischeskoi Khimii (1956), 1, 1185-93. (Russian)

C.A. 49, 11559b.

Geiseler, Gerhard; Richter, Peter. Urea-adduct formation of position-isomeric n-alkane derivatives. Chem. Ber., (1960), 93, 2511-21.

Hild, Gerard. Macromolecular addition compounds. I. Research on urea (or thiourea) addition compounds with poly (oxyethylenes). Bulletin de la Societe Chimique de France (1969), (8), 2840-54.