Method for Purifying Pentachlorophenol

Pentachlorophenol is a useful antimicrobial agent which has been found to decompose readily in the presence of metals and metal chlorides and heat. In addition the by-products of the method of manufacture, as well as the metal and heat produced decomposition products are undesirable both from the aesthetic viewpoint and the environmental viewpoint. Pentachlorophenol without these by-products and/or decomposition products is substantially environmentally satisfactory.

Therefore, it would be advantageous to provide an inhibitor for crude pentachlorophenol which will permit the pentachlorophenol to be distilled from its by-products and simultaneously reduce, if not substantially eliminate, the decomposition during distillation of these by-products and/or the pentachlorophenol.

In accordance with the present invention crude pentachlorophenol, prepared for example in the presence of a Friedel Crafts catalyst such as aluminum chloride (AlCl.sub.3), can be effectively distilled in a metal still by adding to the crude pentachlorophenol a hydroxyl or polyhydroxyl containing hydrocarbon compound which may contain a plurality of ether linkages. Good results have been obtained when from about 0.1 to about 10 percent by weight and preferably from about 0.3 to 3 percent by weight hydroxyl compound, based upon the weight of the crude pentachlorophenol, is added to the crude pentachlorophenol during distillation. The hydroxyl containing compound may be anhydrous or may contain water, i.e. 0 to 90% aqueous solutions. In some instances for reasons yet unknown, better results are achieved when water is present.

The class of hydroxyl hydrocarbon compounds which are suitable for use in accordance with the present invention are:

A. the sugars

B. the polyhydric alcohols

C. polyglycols

D. ethers of polyglycols

E. mixtures of the above with water.

Exemplary of the compounds within the enumerated classes are sorbitol, ethylene glycol, propylene glycol, pentaerythritol, tetraethylene glycol, polyethylene glycols of molecular weights 200 to 400, polyglycerol, polyol 80 (80% glycerine ca 20% dimethylol dioxanes) and the like.

A 500 ml distillation flask was prepared containing 28.9 gms nickel having a surface area of 132 in.sup.2, 2.48 gms 316 stainless steel also having a surface area of 132 in.sup.2 and 0.45 gms iron having a surface area of 1.32 in.sup.2. To such prepared flask was added 500 gms of commercial pentachlorophenol prills and 2 gms polyethylene glycol having an average molecular weight of 200. The pentachlorophenol was melted at 200.degree. C. and distilled at 75 mm Hg through a 1" .times. 7" column packed with 0.16 inch 316 stainless steel protruded packing plus 0.7 gms (2.9 in.sup.2) iron protruded packing. The distillation took one hour. There was collected 439.5 gms (87.9% of the pentachlorophenol 500 gms feed. The decomposition of the pentachlorophenol was followed by scrubbing the exit gas from the pump with 1 N NaOH and back-titrating with HCl. The residue, 60.5 gms was heated to 270.degree. C. for 4 hours to simulate plant conditions in a reboiler under extreme temperature conditions. The decomposition of the residue was followed by scrubbing the exit gas with 1 N NaOH and back-titrating with HCl. The residue on heating for 4 hours at 270.degree. C. decomposed but much less rapidly than uninhibited pentachlorophenol.

In a similar manner other hydroxyl containing hydrocarbons were tested. The results are set forth below. The grade of pentachlorophenol employed is identified by the superscript-footnote notations.