Using a Sulfur Tag for Separations both in the Lab and At Scale

In 1964, G.M.Badger, N. Kowanko and W.H. F. Sasse submitted a short communication to J. Chromatog. 13, (1964) 234 titled, Chromatography on a column of Raney cobalt. The small experimental section read as follows:

“The freshly prepared Raney cobalt (ca 7.5 g) was mixed with clean sand and packed into a chromatographic column (1.2 cm X 10 cm.). A mixture of isoeugenol (0.5 g) and 2,5-dimethylthiophene (0.5 g) was applied to the column and eluted with methanol ( a 3-ft head of liquid was required). Evaporation of the first fraction 930 ml) gave sulfur-free isoeugenol (0.477 g). Subsequent fractions contained only trace amounts of isoeugenol and were also sulfur-free. The dimethylthiophene was subsequently recovered by Soxhlet extraction of the cobalt-containing solid with methanol.” (my italics).

The discussion pointed out that active cobalt metal binds sulfur-containing compounds by chemisorption; however, unlike Raney nickel, cobalt has a much reduced tendency to desulfurize. Nevertheless, this binding is powerful, much stronger than simple adsorption, as the rigorous conditions described for removing the dimethylthiophene from the solid support attested.

What this suggested to me was that the method would not need to be conducted as a column chromatography. It would probably work simply by stirring the solid with a solution containing the sulfurous material, passing through filter aid, and washing. Thus, the method could separate sulfur-containing from sulfur-free materials by filtration as easily as an insoluble polymer is separated from a solution.

That  desulfurization under the conditions of separation is unlikely is further suggested by another paper [1960] by the same authors which contains the sentence “Desulphurisation with Raney cobalt was similar to that with W7-J Raney nickel in that, although little reaction occurred in boiling methanol, it was complete in diethyl phthalate at 220.”

It would seem that, besides obviously being able to separate the sulfur-containing from sulfur-free compounds, the technology should be adaptable to separate compounds that have been derivatized with a sulfur-containing reagent from compounds without such an appendage.

It might be that the method of recovery of the chemisorbed compound could be improved. Eluting with a solvent containing carbon disulfide or COS might speed the recovery without irreversibly contaminating the eluting solvent.

Also, a chemisorbant simpler to prepare than Raney cobalt might be available by reducing a cobalt salt with sodium borohydride to give a Cobalt boride analogous to the Nickel boride catalysts called P-1 and P-2 developed by H. C.Brown et al.