kilomentor

Separating Sulphur-containing from Sulphur-free Compounds both in the Lab and At Scale

kilomentor | 06 November, 2012 14:18

Mahendar Velpuri recently asked  in the Custom Organic Synthesis and Process Chemistry Forum on LinkedIn
http://www.linkedin.com/groupItem?view=&gid=1061737&type=member&item=181862674&qid=5a7a2e3c-b873-4179-b40f-be79d4205731&trk=group_most_popular-0-b-ttl&goback=%2Egmp_1061737

  how to remove dimethylsulfide from an organic oily compound when he had already tried column purification and solvent stripping.

There were what I think simpler suggestions than Kilomentor’s but the question reminded me that a blog could be written about the method of separating sulphur containing from non-sulphur containing compounds that I was invoking, since it could be applied to a range of situations and it seems never to have resurfaced in the literature since that first reference in the 60s.
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 read as follows:

“The freshly prepared Raney cobalt (ca 7.5 g) was mixed with clean sand and packed into a chromatofraphic 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 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 material. Nevertheless, this binding is powerful,much stronger than simple adsorption, as the rigorous conditions described for removing the dimethylthiophene from the solid phase 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, filtering 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 a 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 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 ireversible 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.


 

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