US2954403A - Purification of thiophenols by treatment with aluminum thiophenoxides - Google Patents

Description

5 Claims. 01. 260-609) This invention relates to the art' of separating thionited States PatcntfO Ice phenols from tar acids. More'particul'arly it mama;

purifying thiophenols contaminated with tar acids by selective reaction. of the tar acids with aluminum thiophenoxides to form aluminum phenoxides.

By tar acids I refer to those constituents present in lcoal-tar distillates, certain cracked petroleum distillates .and the like, often referred to collectively phenols, which are soluble in dilute caustic soda, giving sodium cresylate. I particularly refer to a mixture of the simpler monohydric phenols boiling below about 230 C.

and consisting almost entirely of a mixture of phenol, 7

.methylphenols (cresols) and. dimethylphenols (xylenols),

with lesser amounts occasionally present of ethylphenols land trimethylphenols.

At present, thiophenols are principally obtained from ftwo sources: as a by-product from the caustic extraction of petroleum distillates and by synthetic methods starting with benzene. In one widely used direct synthesis tech- ;nique, the benzene is converted to benzene sulfonyl chloride by treatment with a molar excess of chlorosulfonic acid. The product is then converted to thiophenol by reduction in the presence of a metal-acid system. While :the resulting product is of high purity, and particularly useful for paint, dyestuffs, and pharmaceutical applicaztions, it is relatively expensive. This prevents its wide- :spread use for many applications.

In obtaining thiophenols from petroleum distillates rersulting from oil-cracking processes, the tar acids and .thiophenols are recovered by extracting the petroleum distillate with aqueous caustic solution to produce Waterrsoluble tar acid salts. In this process of extraction, thiophenols present are also removed by the aqueous caustic :solution inasmuch as the thiophenols are even stronger :acids than the phenols or tar acids themselves, The quantity of thiophenols in the original source material varies widely, being sometimes as little as one percent by: weight of the phenols and ranging as high as 25 percent and above. The thiophenols consist principally of thiophenol itself and mixed thiocresols and thioxylenols.

In US. Patent 2,767,220, a process is set forth for purifying thiophenol-contaminated tar acids that are substantially free of'neutral hydrocarbon oils. In this process, the feedstock, consisting principally of tar acids and thiophenols, is contacted with aqueous methanol and with a low boiling parafiinic naphtha fraction in a continuous countercurrent extraction zone. The aqueous methanol; solution dissolves substantially all the tar acids, and the naphtha fraction dissolves the thiophenols. The naphtha fraction is distilled off to yield the thiophenols' as still bottoms. Inasmuch as the purification'of the tar acids is the desideratum of this process, the thiophenols recoveredfrom the para'fiinic naphtha fraction invariably contain from 2 to 20 percent tar acids by weight. Complete removal of the tar acids fro'mthe thiophenols does not take place even when the thiophenols are fractionally-distilled in a highly eflicient column, such as a 50-plate-packed; tower. Thiophenol from such a distillation contains aphours.

2,954,403 Patented Sept. 27, 1960 proximately 1.5 percent phenol. Efficient fractional distill'ation is likewise unsuccessful in eifecting a removal of the tar acids from the mixed thiocresols and thioxylenols. These thiophenols contain even higher amounts of closeboilin'gj tar acids. 2

The process of the present invention provides a.- thin-' phenol of improved purity with respect to tar acid con;

tent. In accordance with this invention, a mixture of 'thiophenols and tar acids is treated with a preformed aluminum salt of a thiophenol. A rapid and irreversible interchange of the aluminum thiophenoxide with the tar acids then occurs whereby the tar acids selectively form aluminum phenoxides. The thiophenols are then separated from the relatively. nonvolatile aluminum phenoxides, preferably 'by distillation. A preferred aluminum thiophenoxide reactant for use in the present invention is the aluminum salt of the thiophenol that is present. Thereby the only thiophenolic by-product of the interchange is the same as the thiophenol being purified.

It has been found that this method is suitable for rendering thiophenols containing tar acids in amounts upto 50' percent by weight of the mixture substantially free from tar acid contamination. The method is particularly preferred where the thiophenol has a tar acid content of up to20 percent by weight of the mixture. In general, where thiophenol itself is to be purified, the contaminant present is usually phenol. It is believed that an azeotrope is formed thereby preventing the separation of the thiophenol and phenol by conventional distillation techniques. Mixed thiocresols are usually contaminated with o-cresol. The thioxylenols generally contain a contaminating amount of close-boiling cresols and xylenols. This process is particularly useful for puritying tar acid-contaminated thiophenol, thiocresols, and thioxylenols.

In my copending application Serial No. 747,589, filed July 10, 1958, are described various techniques for preparing aluminum thiophenoxide salts. In general, a preferred method involves preparing an aluminum amalgam, preformed or formed in situ. In a preferred method for preforming an active aluminum amalgam,

granular aluminum (20-30 mesh) is cont-acted'with a saturated ethereal solution of mercuric chloride for about 0.5 minute at the boiling point of the solution. The

' grams of an aluminum amalgam prepared as above were rapidly transferred to a 5-liter flask fitted with a stirrer, a thermowell, and an efficient reflux condenser. Thiophenol (98 percent assay), 2960 grams, was added, and the apparatus was purged with nitrogen. Reaction commenced at 100 C. and became extremely vigorous at 121 C. The temperature rapidly rose to C. even though theheating mantle was removed. Gas evolution ceased after 20 minutes.

A total of 2131 grams of thiophenol was then recovered by distillation on aste'am bath, with stirring, at 15 mm. The residual was cooled and treated with 500 ml. of hexane. Stirring was continued for one hour, and the slurry was rapidly filtered. The cake Was washed with a small amount of hexane and then was transferred to a vacuum desiccator for dryingat 5 mm. for several Undue contact with air was avoided'throughout the process. The dried cake, a grayish-white powder,

aluminum phenoxide.

such as mercuric chloride or cyanide, together with the thiophenol whose aluminum salt is to be prepared. A temperature between 30 and 175 C. is preferred for the reaction betweenthe aluminum and the thiophenol. A reaction product of aluminum and a thiophenol may also be prepared by reacting aluminum directly with a thiophenol in the absence of a mercury compound. However, to accomplish this, reaction at superatmospheric pressure at a temperature between 200 and 400 C. is required.

Upon addition of the aluminum thiophenoxide salt to the mixture of thiophenol and tar acid, preferably in an amount sufficient to provide at least /3 gram atom of aluminum per gram mole of phenol present in the thiophenol, an almost immediate exothermic reaction occurs. If moisture is present, additional amounts of aluminum thiophenoxide are required. However, an excess of aluminum thiophenoxide above the stoichiometric amount is ordinarily not required for substantially complete removal of the tar acids if moisture is not present.

If addition of aluminum thiophenoxide per se, for

' example, is made at room temperature to a mixture of thiophenol and phenol, the solid aluminum thiophenoxide almost immediately begins to dissolve in the mixture of thiophenol and phenol. Although aluminum thiophenoxide is not very soluble in cold thiophenol, a suflicient amount dissolves to react with phenols present to form Since this reaction is exothermic, the evolution of heat further promotes solution of the thiophenoxide. This reaction is therefore seen as capable of occurring at a temperature range of from to 200 C., particularly because of its exothermic nature. The

' greater the proportion of phenols present, the more rapid is the reaction. Also, the reaction is initiated at a lower temperature and proceeds more vigorously than when a lower concentration of phenol is present. The reaction is ordinarily complete within several seconds to several minutes, depending upon the initial temperature employed. In the course of the reaction, no evolution of hydrogen occurs, but an interchange of aluminum occurs so that upon use of a stoichiometric amount of aluminum thiophenoxide, all the tar acids present are combined with the aluminum.

This procedure offers the advantage that temperatures 0 above those used for vacuum distillation are not required. Further, no hydrogen evolution occurs in situ during formation of the aluminum phenoxide. Also the only by-product of the interchange is a thiophenol. By

a thiophenol in the presence of an inorganic mercury salt.

Where thesubsequent presence of the mercury may be undesirable, the aluminum thiophenoxide may still be prepared in the presence of the mercury salt, and most of the formed mercury may then be readily removed by simple mechanical separation. traces of mercury, the thiophenoxide may be dissolved in thiophenol, and the mercury separated therefrom by filtration or high-speed centrifugation. The thiophenol is then removed by distillation. Or the aluminum thiophenoxide containing traces of. mercury may be used as such, the mercury being subsequently removed from distillate fractions of the purified thiophenol.

The following examples illustrate this invention but are not intended as limitations thereof:

Example 1 Reaction of aluminum thiocresoxide with thiocresolphenol mixture.-An amount of 465 grams of mixed thiocresols free from tar acids and 1.80 grams of mercuric chloride were heated to 165 C. Fifteen grams of granular aluminum and an additional 0.40 gram of mercuric chloride were added in portions to maintain a vigorous 89 percent.

the excess thiocresols. removed by distillation under vacuum. To a mixture reaction. The reaction was complete after one hour. Approximately 208 grams of the thiocresols originally present combined with the added aluminum to form aluminum thiocresoxide.

The reaction mixture was then cooled to 105 C., and 126 grams of phenol was added. The temperature rose sharply to 129 C. because of the exothermic nature of thereaction that occurred. The resulting dark, clear solution was maintained at 100 C. for one hour; the pressure was then gradually reduced until 405 grams of distillate was collected. The distillate consisted of 100 percent thiocresol as determined by vapor-phase chromatography. No phenol was detectable. The latter, however, was readily recovered from the residual aluminum salts by hydrolysis.

Example 2 Conversion of aluminum thiocresoxide to aluminum 0-cres0xide.--A mixture of 188 grams (0.474 mole) of aluminum thiocresoxide, prepared by the reaction of aluminum and mixed thiocresols, and 208 grams (1.92 moles) of o-cresol was distilled to a pot temperature of 200 C. at 5 mm. Hg. The overhead product contained 156 grams (1.26 moles) of thiocresols and 66 grams (0.612 mole) of o-cresol. The nonvolatile residue weighed 172 grams. Thus the conversion of aluminum thiocresoxide to an aluminum salt of o-cresol was at least Example 3 Exothermic and irreversible nature of aluminum thiophenoxide interchdnge.-Excess tar acid-free mixed thiocresols, 465 grams (3.74 moles), were reacted with 15.00 grams (0.556 gram-atom) of aluminum in the presence of mercuric chloride. On completion of the reaction, the temperature Was lowered to 105 C. at which point the aluminum thiocresoxide formed a light gray slurry in The unreacted thiocresols were consisting of 248 grams (2.00 moles) of dry mixed thiocresol isomers and 126 grams (1.34 moles) of dry phenol was then added the previously prepared aluminum thiocresoxide. A dark, clear solution formed instantly and the temperature rose sharply to 129 C., thus indicating a significantly exothermic reaction.

(In a separate experiment, a similar temperature rise occurred on the addition of o-cresol to aluminum thiophenoxide and thiophenol.) The reaction mixture was maintained at 100 C. for one hour. Finally, 409 grams (3.29 moles) of pure thiocresols Was recovered by a Vigreaux distillation of the reaction mixture to a pot-temperature of 105 C. at 10 mm. Hg. All of the phenol remained in the pot residue despite its lower boiling point (105 C./50 mm.-Hg) as compared to that of the thiocresols (108 C./50 mm. Hg). The result is interpreted to indicate that the interchange of phenol with aluminum thiocresoxide is complete, and, under the distillation conditions, essentially irreversible.

To remove remaining.

The mechanism that occurs in the reactions of this invention is considered a'highly complex one. While I do not desire to have the scope of this invention re- I stricted by any explanation profiered, it is considered num phenoxides. This interchange occurs somewhat surprisingly despite the fact that thiophenol is a considerably stronger acid than phenol, and hence thiophenol might ordinarily be expected to be more reactive than phenol. However, as shown herein, once this interto specific preferred embodiments, I do not desire to be limited by the illustrative examples given or the specula tive mechanisms postulated for this reaction. The scope of this invention accordingly should be determined in accordance with the principles and claims thereof.

I claim:

1. The process for purifying a tar acid-contaminated thiophenol which comprises reacting a mixture of a thiophenol and a tar acid with an aluminum thiophenoxide to react selectively with the tar acid to form an aluminum salt thereof and a thiophenol of said thiophenoxide, and separating the thiophenol from said salt.

2. The process for recovering a thiophenol in substantially pure form from a mixture containing a thiophenol and a tar acid which comprises adding to said mixture under reactant conditions an amount of an aluminum thiophenoxide suflicient to provide at least /3 gram atom of aluminum per mole of tar acid present to react selectively with the tar acid to form an aluminum salt thereof and a thiophenol of said thiophenoxide, and distilling off the thiophenol substantially free from tar acid.

3. The process according to claim 2 wherein said 6 aluminum thiophenoxide is the aluminum salt of a thiophenol present in said mixture.

4. The process for recovering a thiophenol in substantially pure form from a mixture containing a major portion of a thiophenol and a minor portion of a tar acid which comprises reacting an amount of an aluminum thiophenoxide sufiicient to provide at least /3 gram atom of aluminum per mole of tar acid present with said mixture at a temperature between 0 and 200 C. at which a reaction occurs and an aluminum phenoxide and a thiophenol are formed, and distilling off the thiophenol from the mixture in substantially pure form.

5. The process for recovering a thiocresol in substantially pure form from a mixture containing a major portion of a thiocresol and a minor portion of cresol which comprises adding to said mixture under reactant conditions an amount of aluminum thiocresoxide suflicient to provide at least A gram atom of aluminum per mole of cresol present to react selectively with the cresol to form aluminum cresoxide and thiocresol, and distilling off the thiocresol substantially free from cresol.

No references cited.

Claims (1)

1. THE PROCESS FOR PURIFYING A TAR ACID-CONTAMINATED THIOPHENOL WHICH COMPRISES REACTING A MIXTURE OF A THIOPHENOL AND A TAR ACID WITH AN ALUMINUM THIOPHENOXIDE TO REACT SELECTIVELY WITH THE TAR ACID TO FORM AN ALUMINUM SALT THEREOF AND A THIOPHENOL OF SAID THIOPHENOXIDE, AND SEPARATING THE THIOPHENOL FROM SAID SALT.