US2785199A - Purification of toluic acids - Google Patents

Description

United States Patent PURIFICATION 9F TOLUIC ACIDS Chester M. Himel, Menlo Park, Calii, assignor to Richfield Oil Corporation, Los Angeles, Cali'r., a corporation of Delaware No Drawing. Application July 1, 1952, Serial No. 296,716

3 Claims. (Cl. 260-525) My invention relates to the production of toluic acids in a purified form. More specifically, my invention is concerned with the separation of close-boiling neutral materials from toluic acids produced by the air oxidation of xylenes.

Toluic acids can be prepared by the oxidation of xylenes with air or other oxygen-rich gas. One commercial source of xylenes is to be found in cracked or reformed petroleum oils. Thus, petroleum naphtha which is rich in dimethyl cyclohexanes is dehydrogenated in the presence of hydrogen in accordance with the hydroforming or platforming processes to yield xylenes and other compounds, depending upon the purity of the naphtha fraction processed with respect to content of dimethyl cyclohexanes. The toluic acids produced are associated with neutral impurities which have boiling points close to those of toluic acids and which are therefore difficult to separate from the toluic acids.

My application Serial No. 126,240, now abandoned, and that of Lawrence M. Richards, Serial No. 126,237, now abandoned, both filed November 8, 1949, relate to the production of phthalic acids by the mild oxidation of xylenes to produce toluic acids and a further, more vigorous oxidation of toluic acids to produce phthalic acids, utilizing petroleum xylene fractions as starting materials. In these applications, a petroleum xylene fraction is first subjected to direct, liquid phase oxidation with air in the presence of a catalyst which is soluble in the reaction mixture, such as a cobalt, manganese or cerium soap, particularly the toluate soap. Reaction temperatures of from about 130 C. to about 190 C. are useful. The air oxidation is carried to partial completion, that is, from about 5 percent to about 30 percent by weight of oxygen is absorbed, based on the aromatic content of the petroleum xylene fraction. Such mild partial oxidation, while yielding a lower conversion of the petroleum xylene fraction, favors the production of toluic acids from the xylenes and acetophenone from any ethyl benzene present.

Acetophenone and partial oxidation products such as tolualdehydes are conveniently separated by fractional distillation. When such a distillation is made, there is produced a small fraction of neutral oxidation products which distills between the distillation range of tolualdehydes and the distillation range of the toluic acids. The fraction containing the toluic acids is the last fraction taken 0E overhead. This fraction, however, also contains varying amounts of neutral materials, depending upon the reaction conditions used in the air oxidation and the handling conditions during the preliminary distillation stages. These neutral materials or neutral oils present in the toluic acid fraction are possibly condensation products of acetophenone and tolualdehydes as well as other oxidation by-products and, as has been stated, have a boiling range close to, or within, the boiling range of the toluic acid isomers. The toluic acid fraction produced from the distillation has a light yellowgreen color and a characteristic odor. Analyses indicate ice that it contains from about to about 98 percent of toluic acids and traces of benzoic acid, with the remainder being the neutral oils mentioned above.

in accordance with my invention, 1 have found that the materials which contaminate the distilled toluic acids are also responsible for the discoloration and the odor of the acids. Moreover, I have found that such contaminants are quite soluble in saturated hydrocarbons having from 3 to 8 carbon atoms per molecule, and that toluic acids are soluble in such hydrocarbons to only a limited extent. I therefore utilize such hydrocarbons to purify the toluic acids produced by the air oxidation of the petroleum xylene fractions by simply washing the crude distillate composed of the crystalline toluic acids and associated materials with one or more hydrocarbons falling within the class stated above.

This can be done in various ways. For example, the neutral impurities can be solvent extracted by intimately contacting the crude, crystalline toluic acid fraction with propane, butane, pentane, isopentane, hexanes, heptanes, cyclohexane, methyl cyclopentane, dimethyl cyclopentanes, methyl cyclohexane, dimethyl cyclohexanes, or a mixture of these compounds. On the other hand, a process of combined solvent extraction and crystallization can be carried out by charging the molten toluic acid fraction directly from the distillation apparatus or from a toluic acids melt tank into a well-agitated excess of one or more of the solvents just mentioned. After filtration of the crystalline, purified toluic acids, the solvent containing dissolved impurities can be recovered for further use by distillation, in accordance with well known procedures, leaving the impurities as a kettle residue in the solvent recovery still. Preferably I use hydrocarbons having from 4 to 6 carbon atoms because of the better recoveries of toluic acids and the ease of recovery of the solvent. The hydrocarbons which I use are preferably essentially saturated, but they may be in admixture with small amounts of monoolefins of the same range of carbon content.

The following examples illustrated in detail the practice of my invention, and are to be considered not limitative thereof.

Example I A commercial xylene fraction having the following analysis:

was used (1500 grams) to charge a stirred autoclave, along with 45 grams of cobaltous toluate catalyst. The reactor was stirred and pressured to 500 p. s. i. g. with air and heated to 150 C. When reaction temperature was reached, 40 cu. ft. of air (measured at room temperature and pressure) was passed into the reactor over a 39-minute period. Efiluent gases were released through a back pressure regulator which maintained the pressure in the autoclave at 500 p. s. i. g. The efliuent, which weighed 1700 grams, was processed to remove the catalyst and water.

The xylene solution was then divided into a 300-g-ram aliquot which was distilled to give 196 grams of xylene, 36 grams B. P. 70-130 C./5 mm. and 65 grams of toluic acid fraction B. P. 130-5 C./5 mm. The toluic acid fraction was a light yellow color and had a neutral equivalent of 146, indicating the presence of 7 percent neutral impurities. The yellow solid was ground and extracted once with cc. of petroleum ether (B. 35-60 C.) to give a pure white residue which had a neutral equivalent of 140. A second extraction gave 55 grams of white crystals having a neutral equivalent of 136, corresponding to pure toluic acids.

Example 11 4 The pentane treatment not only efiectively removedcontaminating materials from this product but also allowed separation into two fractions having different para-toluic acid content. This is of value in the preparation of a feed stock for the production of pure para-toluic acid.

While the foregoing is illustrative of the practice of this invention, the. actual washing. or extraction step is susceptible to obvious variations. Thus, the relative amounts of crude toluic acids: and solvent medium-used is not critical, but rather can be varied widely; In actualpractice, the relative amounts of crude ltoluic acids and solvent medium used will depend uponthe extent to which the toluic. acidsarecontaminated, upon the particular solvent used, and upon other factors. Fun-ther- Start 15!; Extraction 2nd Extraction Sample gms. N. E color gms. N. color gms; N. E. color 100 Mi. yellow s0. 137 white. 76. 136 white. 100 144' brown. 85 140' tan 75' 138 beige. 100 140 ll,.bl'.OW11' 83' 137' vvhite 74 136 white. 100 136.5 In. yellow as 7 136 white After a third extraction of sample B, there remained M! 2,5 more, although the extraction was carried out at room;

grams of acids which had a neutral equivalent of 136 andlwhichwere beige in'color. A150,, a- SSS-gram sample of A was then extracted with 358 ml.. of pentane to give 295 grams of white crystals with a neutral equivalent of 136.5. 7

Example 111 A toluic acid product from a low xylene content charge stock was subjected to a preliminary distillation and. gave 11 pounds of toluic acid fraction contaminated with aliphatic acids as well as the usual neutral oils. The product wasl melted and poured into 9 liters ofpentanewith stirring. The first crystalline fraction removed amounted to 3.5: pounds and had the following analysis:

Neutral equivalent 136.5 Benzoi'c' acid percent 2.0 OrtlIo-rto'luic acid do 5.0 Meta-tol'uic acid do 55-56 Para-toluic acid do- 37-38 Upon further cooling of the mixture, a second crystalline fraction was recovered and amounted to 6.0' pounds having the following analysis:

Neutral equivalent 136.5 Benzoic acid "percent" 3.0 Ortho-toluic acid r do 13.0 Meta-toluic acid do 59-60 Para-ttoluic acid do 24-25 temperature in the examples, it is not absolutely neces: sary that that bethe case, temperaturesbotheabove and below room temperature being operable. It ispreferred, however, that the extraction be carried. out. at artemperature not in excess. of about 50 C., in order that. excessive amounts of the acid will not be dissolved in the-solvent, and, for the same. reason and also in order to minimize solvent and solvent. handling costs, the amount of solvent used should: not. be too large. These considerations will be readily understood by those skilled in the 7 art.

I claim:

1. In a method for the purification of ttoluic, acids prepared by the liquid phase air oxidation of xylenes, the steps, comprising subjecting the reaction product to a distillation operation to remove xylenes and obtain toluic acids admixed with neutral materials having a boiling range close to that of the acids, crystallizing toluic acids, and washingithe crystallized acids with a saturated liquid hydrocarbon having from 3 to 8 carbon atoms per molecule at a temperature not in excess of about 50 C.

2. A method as in claim 1 in which the hydrocarbon is pentane.v

3. A method as in claim 1- in which the hydrocarbon is petroleum ether.

References Citedin the file of this patent.

UNITED STATES PATENTS" Great Britain May 24,

Claims (1)

1. IN A METHOD FOR THE PURIFICATION OF TOLUIC ACIDS PREPARED BY THE LIQUID PHASE AIR OXIDATION OF XYLENES, THE STEPS COMPRISING SUBJECTING THE REACTION PRODUCT TO A DISFILLATION OPERATION TO REMOVE XYLENES AND OBTAIN TOLUIC ACIDS ADMIXED WITH NEUTRAL MATERIALS HAVING A BOILING RANGE CLOSE TO THAT OF THE ACIDS, CRYSTALLIZING TOLUIC ACIDS, AND WASHING THE CRYSTALLIZING ACIDS WITH A SATURATED LIQUID HYDROCARBON HAVING FROM 3 TO 8 CABRON ATOMS PER MOLECULE AT A TEMPERATURE NOT IN EXCESS OF ABOUT 50*C.