US2954404A

US2954404A - Purification of thiophenols by treatment with aluminum and magnesium alkoxides

Info

Publication number: US2954404A
Application number: US807281A
Authority: US
Inventors: Robert J Laufer
Original assignee: Consolidation Coal Co
Current assignee: Consolidation Coal Co
Priority date: 1959-04-20
Filing date: 1959-04-20
Publication date: 1960-09-27
Anticipated expiration: 1977-09-27

Description

United States i latent PURIFICATION OF THIOPHENOLS BY TREAT- MENT WITH ALUMINUM AND MAGNESIUM ALKOXIDES Robert J. Laufer, Pittsburgh, Pa., assignor to Consolidation Coal Company, Pittsburgh, Pa., a corporation of Pennsylvania This invention relates to the art of separating thiophenols from tar acids. More particularly it relates to purifying thiophenols containing tar acids by selective reaction of the tar acids with alkoxides'of aluminum and magnesium.

By tar acids I refer to those constituents present in coal-tar distillates, certain cracked petroleum distillates. and the like, often referred to collectively as 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, methylphenols (cresols) and dimethylphenols '(xylenols), with lesser amounts occasionally present of ethylphenols and trimethylphenols.

At present, thiophenols are principally obtained from two 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 technique, 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 usefulfor paint, dyestuffs and pharmaceutical applications, it is relatively expensive. This prevents its wide-spread use for many applications.

In obtaining thiophenols from petroleum distillates resulting from oil-cracking processes, the tar acids and thiophenols are recovered by extracting the petroleum distillate with aqueous caustic solution to produce watersoluble 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 paraflinic 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 recovered from the parafiinic naphtha fraction invariably contain from 2 to 20 percent tar acids by weight. Complete removal of the tar acids from the thiophenols does not take place even when the thiophenols are fractionally distilled in a highly eflicient column, such as a SO-plate packed tower. Thiophenol from such a distillation contains approximately 1.5 percent phenol. .Eflicient fractional distillation is likewise unsuccessful in effecting removal of the tar acids from the mixed thiocresols and thioxylenols. These thiophenols contain even higher amounts of close=boiling tar acids.

Accordingly, it is an object of the present invention'to provide a thiophenol of improved purity with respect to tar acid content. It is a further object to provide a method for obtaining thiophenols free from tar acids that is readily adaptable to existing techniques for extracting tar acids from petroleum distillate fractions.

In accordance with this invention, a mixture of thin phenols and tar acids is treated with an alkoxide selected from the group consisting of aluminum alkoxides and magnesium alkoxides so that tar acids present selectively react with the alkoxide to form the'corresponding phenoxide. The thiophenols are then separated from the relatively nonvolatile phenoxides, preferably by distillation. Preferred alkoxides, or alcoholates, are'those formed by reacting aluminum or magnesium metalwith a lower monohydroxy paraflinic alcohol having from 1 to 4 carbon atoms in its chain. Preferred alcohols include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, secbutyl, and tert-butyl alcohols. Magnesium methoxide a particularly preferred reactant.

It has been found that this method is suitable for rendering thiophenols containing t-ar acids in amounts up to 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 to 20 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 a separation of the thiophenol and phenol by conventional distillation techniques. Mixed thiocresols are usually contaminated with o-cresol. The thioxylenols generally contain contaminating amounts of close-boiling cresols and xylenols. This process is particularly useful for purifying tar acidcontaminated thiophenol, thiocre'sols, and thioxylenols.

The alkoxides of aluminum and magnesium are readily prepared by reacting the metal, preferably in granular form and moisture-free and oxide-free, with a stoichiometric excess of the alcohol, preferably under reflux con ditions. Several methods are known for preparing magnesium and aluminum alkoxides. The primary alcohols are the more reactive, there generally being a marked decrease in the reactivity of the hydroxyl hydrogen of the alcohol in proceeding from a primary to secondary to tertiary alcohol. Thus with the tertiary alcohols the temperature must be increased and the reaction time lengthened. While in some instances it may be desirable to catalyze the reaction by amalgamating the magnesium or aluminum, this may introduce subsequent problems of removal of the mercury. Other catalytic techniques are'therefore preferred. Aluminum isopropoxide, which is commercially available, may be readily prepared by treating aluminum wire or'foil with anhydrous isopropyl alcohol, using carbon tetrachloride as catalyst. The contents are protected from moisture by a mercury trap or a calcium chloride drying tube. The mixture is then refluxed until all the aluminum has dissolved. The resulting solution is then distilled under reduced pressure.

Excess isopropyl alcohol distills over first, after which the condenser is removed, since air cooling is suflicien't for the condensation of aluminum isopropoxide. Other lower alcohol alkoxides may be similarly prepared charged to a batch still, and at least one third of a gram atom, based on aluminum or magnesium content, is added as an aluminum alkoxide or magnesium alkoxide per gram mole of tar acid present in the charge. An excess of available aluminum or magnesium above the stoichiometric amount is ordinarily not required for substantially complete removal of the tar acids if moisture is not present. The presence of excess aluminum alkoxide beyond that necessary for reaction with the tar acids and moisture is preferably avoided as the aluminum alkoxides are less reactive than the magnesium alkoxides with respect to thiophenols, and may undergo a slow interchange with the thiophenols thereby continuing to generate the related alcohol even while the thiophenol is being distilled. This may contaminate the thiophenol distillate and require removal of the alcohol by further fractionation of the tar acid-free thiophenol. The charge is heated at atmospheric pressure, preferably to a temperature between 100 and 220 C., depending on the specific thiophenolic and phenolic contaminants that are present. The lower molecular weight phenols and thiophenols readily react at the lower end of the temperature range.

- A preferred reaction temperature range for a thiophenol-phenol mixture in which vigorous reaction occurs is between 135 and 165 C. For mixed thiocresols, a range'between 140 and 185 C. is preferred. The mixed thioxylenols have a preferred reaction range between 160 and 200 C. Where substantial amounts of tar acids are present, in excess of 20 percent by weight, vigorous reaction occurs at the lower end of the range and even atlower temperatures. Because of the reaction between the magnesium and aluminum alkoxides with the tar acid is an exothermic one, these alkoxides may be added to a phenol-thiophenol mixture at room temperature, or lower, the heat of reaction gradually raising the temperature of the system. Upon addition to the mixture, the solid alkoxide almost immediately begins to dissolve therein, the evolution of heat promoting its solution. This re action may therefore occur over a temperature range from to 200 C. because of its exothermic nature. The reaction is ordinarily completed within several seconds to several minutes, depending upon the initial temperature employed and the relative amount of tar acid present. In' the course of the reaction there is no evolution of hydrogen. Rather an interchange of aluminum or magnesium occurs, so that upon use of an effective, i.e., moisture-free, stoichiometric amount of aluminum alkoxide or magnesium alkoxide, all the tar acids present are combined with the aluminum or magnesium. Higher reaction temperatures above room temperature are ordinarily employed only because of the subsequent temperature requirements for distillation of the particular thiophenol being purified. Distillation is preferably carried out under vacuum at pressures of 50-100 mm. Hg.

7 This process is particularly applicable to purifying thiophenols containing tar acids as recovered from the paraflinic naphtha fraction obtained from the doublesolvent extraction method of treating caustic-extracted petroleum distillates, as set forth in US. Patent 2,767,- 220. However, the process is equally applicable to refining thiophenols contaminated with other substances in addition to tar acids in that these other contaminants are removed by fractionation either before or after treatment of the thiophenol with the alkoxide.

The process is also considered applicable to the removal of moisture, particularly in trace amounts, as well as phenols, from thiophenols. It has been found that, in general, aluminum and magnesium alkoxides become selectively less reactive in going from water to phenols to thiophenols. Inasmuch as an alcohol is formed when moisture and tar acids are removed by reaction with the alkoxide, the latter should be selected so that a desiredalcohol is formed. Either the quantity and particular alcohol, formed should be noncontaminating, or

"ide is formed. This may then be partially separated from the thiophenols by filtration or centrifugation. However, it is ordinarily preferred to separate the thiophenols from the realtively nonvolatile aluminum phenoxides by distilling oflf the thiophenols from the mixture without any prior cooling of the mixture.

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

EXAMPLE 1 Aluminum isopropoxide reaction A synthetic mixture of 9.8 grams (0.104 mole) of phenol and 88.8 grams of thiophenol was treated with 15.2 grams (0.074 mole) of aluminum isopropoxide. The aluminum isopropoxide used as catalyst was a commercially obtained sample. The temperature was raised to C. over a period of 0.5 hour. The pressure was then reduced to 50 mm. Hg, and a 94 percent recovery of phenol-free thiophenol was achieved. The initial dis- 'tillate consisted of a mixture of thiophenol and all of the isopropyl alcohol formed.

Excess amounts of the aluminum isopropoxide beyondthat necessary for reaction with the tar acids and moisture undergo a slight interchange with the thiophenols thereby regenerating isopropyl alcohol. This alcohol may distill over with the thiophenols in detectable though very small amounts. Use of a smaller excess of reagent where the'. feed composition is accurately known would minimize this. Excess alcohol present in the initial distillate may be removed by refractionating the tar acid-free thiophenols.

EXAMPLE 2 Magnesium methoxide reaction An amount of 15.5 grams (0.63 gram atom) of 70-80 mesh granular magnesium and a stoichiometric quantity of dry methanol were refluxed for 4.5 hours. A vigorous reaction occurred at that time to yield a gray, freeflowing powder consisted of magnesium methoxide and unreacted magnesium. Hydrogen gas was evolved in the process. The initiation period of the reaction is considerably diminished if a small amount of the preformed alkoxide product is added to the reaction mixture. Although magnesium metal per se has not been found to react with a mixture of thiophenols and tar acids, the unreacted magnesium metal mixed with its methoxide salt will be activated and react.

The reagent as prepared above was added to a synthetic mixture of 31.0 grams of phenol (0.33 mole) and 301 grams of thiophenol. The pressure was reduced to 50 mm. and the distillation was carried out. Methanol was removed followed by 96 percent of phenol-free thiophenol.

It has been found that the magnesium alkoxides are much more reactive than the aluminum alkoxides with respect to the thiophenol present. Thus unlike aluminumisopropoxide, which reacts very rapidly with tar acids and much more slowly with thiophenols, magnesium methoxide interchanges rapidly and irreversibly with tar acids and almost as rapidly with thiophenols. no magnesium methoxide remains unreacted in the still pot even where a large excess is employed, sinceany excess is converted to the thiophenoxide salt.

formed is readily removed in the distillation fore-cut;'

Hence Where. an excess of magnesium methoxide is used, the methanol succeeding thiophenol fractions are therefore free of methanol. The thiophenol recovered is both tar acidfree and alcohol-free.

In general, While higher alcohol derivatives of magnesium and aluminum may be employed, it is preferred to employ the alkoxides of lower aliphatic alcohols, those having from 1 to 4 carbon atoms. Use of these lower alcohols offers the advantage that the boiling point of the alcohol is generally Well below that of the thiophenol in which it may be present. Thereby a separation of the alcohol in a fore-cut distillation is readily attained. The use of an alcohol having a higher boiling point than the thiophenol being purified would be undesirable. Furthermore, because the metal, namely the aluminum and magnesium, is essentially the elfective reactant, the use of lower-alcohol-derived alkoxides allows a greater proportion of metal present for a given molecular weight of alkoxide. For example, one pound of magnesium methoxide is equivalent to 3.15 pounds of aluminum isopropoxide because of the lower chemical equivalent weight of the magnesium methoxide. Thus aluminum isopropoxide priced at 30' cents per pound would be conomically equivalent to magnesium methoxide priced at one dollar per pound.

The interchange reaction that occurs between the magnesium and aluminum alkoxides with the thiophenol and phenol is considered a highly complex one. While I do not desire that the scope of this invention be restricted by any explanation profiered, it is considered apparent that the formation of the aluminum and magnesium phenoxides proceeds at a much more rapid rate than that of the corresponding aluminum and magnesium thiophenoxides. The magnesium thiophenoxides are, however, much more rapidly formed than aluminum thiophenoxides under corresponding conditions. Also, tar acids appear much more reactive than alcohols with respect to aluminum and magnesium. Thus aluminum and magnesium phenoxides are much more readily formed than the corresponding alkoxides, and hence displacement of the aluminum and magnesium from these alcoholates is essentially irreversible. Under the conditions used, the reaction is therefore highly selective to the removal of phenol from a phenol-thiophenol mixture. This selectivity phenomenon occurs despite the fact that thiophenol is a considerably stronger acid than phenol, and hence the thiophenol might ordinarily be expected to be more reactive than the phenol. However, as shown herein, not only is the reaction selective, but apparently the aluminum and magnesium will react with the phenol to almost the complete exclusion of the thiophenol as long as any phenol is present.

While this invention has been described with respect to specific preferred embodiments, I do not desire to be limited by the illustrative examples given or by the speculative mechanisms postulated for this reaction, but the scope of this invention should be determined in accordance with the objects 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 alkoxide selected from the group consisting of aluminum and magnesium alkoxides to react selectively with the tar acid to form a corresponding metal salt thereof and an alcohol of said alkoxide, and separating the thiophenol from said salt.

2. The process according to claim 1 wherein said metal alkoxide is a salt of a monohydroxy parafiinic alcohol having from 1 to 4 carbon atoms.

3. 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 an amount of a monohydroxy parafiinic alkoxide having from 1 to 4 carbon atoms and selected from the group consisting of aluminum and magnesium alkoxides sulficient to provide at least gram atom of metal per mole of tar acid present to react selectively with the tar acid to form a corresponding metal salt thereof and an alcohol of said alkoxide and recovering the thiophenol from the mixture in substantially pure form free from tar acid and alcohol.

4. The process according to claim 3 wherein said alkoxide is aluminum isopropoxide.

5. The process according to claim 3 wherein said alkoxide is magnesium methoxide.

6. The process for recovering thiophenol per se in substantially pure form from a mixture containing thiophenol and phenol which comprises adding under reactant conditions an amount of magnesium methoxide sufiicient to provide at least /3 gram atom of metal per mole of phenol present to react selectively with the phenol to form magnesium phenoxide and methyl alcohol, distilling said mixture to recover substantially all said methyl alcohol as a fore-cut, and continuing said distillation to recover thiophenol in substantially pure form free from phenol and methyl alcohol.

No references cited.