US2872481A

US2872481A - Process for preparing low molecular carboxylic acid anhydrides

Info

Publication number: US2872481A
Application number: US545477A
Authority: US
Inventors: Vogt Wilhelm
Original assignee: Knapsack AG
Current assignee: Knapsack AG; Knapsack Griesheim AG
Priority date: 1954-11-13
Filing date: 1955-11-07
Publication date: 1959-02-03
Anticipated expiration: 1976-02-03
Also published as: FR1138448A; GB812963A

Description

Feb. 3, 1959 w. VOGT 2,872,481

PROCESS FOR PREPARING LOW-MOLECULAR CARBOXYLIC ACID ANHYDRIDES Filed Nov. 7, 1955 A 4 iner'f gas phase rich in wa ter mixture containing mhydp'de INVENTO R WILHELM I/OGT WATToRNEYs trite PROCESS FOR PREPARING LOW MOLECULAR 'CARBOXYLI-C ACll) ANHYDRIDES Wilhelm Vogt, Knapsack, near Koln, Germany, assignor to Knapsack-Griesheim Aktiengesellschaft, Knapsack, near Koln, Germany, a corporation of Germany Application November 7, 1955, Serial No. 545,477

Claims priority, application Germany November 13, 1954 6 Claims. (Cl. 260-546) Now I have found that monobasic saturated aliphatic carboxylic acids with 2 to 4 carbon atoms, can be dehydrated in a simple manner to yield the corresponding 7 anhydrides without intermediate formation of ketene, by heating the carboxylic acids in the liquid phase under the vapor pressure of the substances present in the reaction mixture, i. e. under a pressure of about 2 to about 50 atmospheres (gauge pressure), preferably about 10 to about 30 atmospheres (gauge pressure), and at temperatures ranging from about 180 C. to the critical temperature, suitably to about 270 C., preferably from about 200 C. to about 250 C. The upper limit of the superatmospheric pressure applied is determined by the vapor pressure of the corresponding carboxylic acids and of that of an entrainer, if used, which vapor pressures correspond to the critical temperature of these compounds or a temperature of about 270 C. The water which has been formed is suitably distilled off in admixture with carboxylic acid at the head of a distilling column, while the carboxylic acid anhydride, diluted with non-reacted carboxylic acid, remains in the distilling vessel.

To the carboxylic acid there may advantageously be added the known agents for entraining the water formed, especially aliphatic, aromatic or hydroaromatic hydrocarbons such as benzene, n-hexane or cyclohexane or, if desired, carboxylic acid esters which, under the conditions applied, are still sufiiciently stable, for example those obtained from acids and alcohols containing up to carbon atoms each, such as ethylacetate, propylacetate, butylacetate, propylpropionate, ethylpropionate, methylpropionate, methylbutyrate, ethylbutyrate or propylbutyrate. Such entraining agents should be used as form azeotropic mixtures with water, are immiscible with Water and suitably have a boiling point at least 20 C. below that of the acid to be obtained, so that they are present at the head of the column in a high concentration. Furthermore,,the condensate of the azeotropic mixture should possibly separate, at least in certain temperature ranges, into two phases of which only the one rich in water is drawn otf continuously, while the other one poor in water is partly or, advantageously, totally returned into the column.

In the preparation of acetic anhydride, for example,

theprocess of the invention is suitably carried out at a Patent 0 temperature within the range of C. and approximately the critical temperature of the acetic acid or the entraining agents, advantageously below about 270 C., the pressure being 5 to 50 atmospheres (gauge pressure) corresponding to the vapor pressures of the components. The process is accompanied by little decomposition only, particularly at low and medium temperatures--provided the walls of the apparatus are made of such a material as has no or only a small catalytic eifect on the decomposition of the organic materials, for example copper, enameled iron, silver, tantalum or, if desired CnNi-Mo steeland therefore leads to high yields of anhydride.

Since the reaction velocity is sufficiently high at tem-' peratures from about 180 C. to about 270 C., dehydration catalysts can be dispensed with; they may, however, be concomitantly used in some cases.

An apparatus suitable for use in carrying out the process of the invention is illustrated diagrammatically by way of example in the accompanying drawing:

The discontinuous distillation can be carried out in a small copper apparatus consisting of a still 1 and a column tube 2 filled with packing bodies and suitably well protected against loss of heat. From condenser 3, arranged above the tube, the condensate runs ofi and is returned into the column by pipe 4. In this pipe a separator 5 is installed from which in the case of disstillations carried out with entrainers the phase rich in water, 'for example the phase having a greater specific weight, can be drawn olf. The distilling apparatus is connected with a pressure vessel 6 which has about 10 times the volume of the apparatus and can be filled, before being operated, with a compressed inert gas, for example nitrogen, to about working pressure.

Depending on the working conditions used in the discontinuous distillation carried out with or without an entrainer, the formation of anhydride practically stops when a certain conversion has been attained. No anhydride is formed at all when already before the distillation such a quantity of anhydride is added to the distillation mixture as would be formed as a maximum after a prolonged time of distillation under the same conditions but without addition of an anhydride.

When the conversion of, for example, acetic acid into acetic anhydride is plotted against the time the first part of the curve rises in approximately a straight line. The slope of the curve gradually decreases. When using benzene as entrainer and proceeding at 265 C. under a pressure of 30 atmospheres (gauge), the extent of conversion amounts to about 5 percent after 3 hours of distillation and to about 8.4 percent after 10 hours of distillation under the same conditions. When operating at 210 C. under a pressure of 10 atmospheres (gauge-' pressure) the extent of conversion amounts to about 3.4

percent after 6 hours. With the use of n-heXane as entrainer, the conversion amounts to about 6 percent after heating at 260 C. for 8 hours under a pressure of 27 atmospheres (gauge pressure).

The above described gradually decreasing slope of the curve becomes understandable if the total dehydration process is assumed to be divided into .two stages. The process of the first stage of the chemical conversion of acetic acid into anhydride and water stops when in the second stage, i. e. during the physical accumulation of water in the distilling column, for example by separation of a phase rich in water in the condensate, the quantity of water removed from the column is not greater than thatreturned to the column by the phase poor in water. In a case of a distillation without an entrainer, the conversion is practically finished when the reaction water is present at the head of the column in such a concentration that it would be necessary to. carry along all the acid present in the apparatus in order to remove the water. As results from this, the conversion into anhydride depends on the efiicacy of the column so that columns of the highest possible efiiciency must be used in order to obtain higher yields. of' anhydride.

When. proceeding under such conditions that no condensate consisting of two. phases is formed, for example when operating without an entraining agent, the hydrous product distilling off. at the head of. the pressure column can be freed from water by' known: methods in a second distillation column under normal pressure. The re sultant anhydrous carboxylic acid can be returned into the pressure column by means of a pump. In'a continuous process the anhydride-containing product (I) runningv off. from the distillation: vesseliof the pressure column can. be separated into carboxylic acid and an: hydride under normal pressure in a third distillation column. The carboxylic acid. separated during; this pro= cedure can also be returned into the pressure column and, if desired, the anhydride can be distilled once more. The quantities of heat necessary'for heating: the second and third column may be provided for the'major part by the heat set free, at thehigh working temperature of the pressure column, in the condenser of the latter and from the heat contained in the product I.

The process described above is particularly suitable for producing acetic anhydride. It may, however, be used in the same manner for obtaining other anhydrides of fatty acidsof low molecular weight such aspropionic.

or butyric acid anhydride. To this end thereaction is carried out within. the range of temperature applied in the manufacture of acetic anhydride andgenerallytakes a more rapid course than the preparation of acetic anhydride. The working pressure is correspondingly lower. When starting from an acid. mixture, mixtures'of anhydride can also be obtained. Uniform anhydrides are; of course, more desired in industry.

The following examples serve to illustrate the invention, they are, however, not intended to limit it thereto. They are chiefly concerned with the discontinuous mode of, executing the'process according to the invention. It should be mentioned that by returning the recovered carboxylic acids and entrain'ing agents according to-the continuous modeof executing the process of the invention described above substantially the total amount of carboxylic acid used can be converted'into the corresponding anhydride.

Example 1 In a distilling apparatus made of copper and resistant to compression and which consists of a still having a capacity of 3 litres and a packed column, 2' metres long and 25 millimetres in diameter, containing filling bodies, a mixture of 1000 grams of acetic acid and 800 grams of benzene is boiled under a pressure of nitrogen of atmospheres (gauge pressure), corresponding to a boiling temperature of 210 C., and the volatile constituents consisting of acetic acid, water and benzene are distilled off. By cooling to about 5 C., a condensate is formed which seperates into two phases, of which the one having the lower specific gravity consists chiefly of benzene, in addition to j percent of acetic acid, while the phase having the higher formed during the formation of anhydride. The lower phase which is rich in water is drawn off continuously. After distilling for 6 hours and returning, per

specific gravity contains in addition to 66 per ent of acetic acid, the'water' action water is nearly completed.

l- Example 2 Example 3 in: a manner analogous to that described in Example 1 a mixture of acetic acid and benzene of the same composition as in Example 1 is boiled at 265 C. under pressure of inert gas of 30 atmospheres (gauge pressure). After 10 hours 8.4- percent of the acetic acid used has been converted into anhydride. In an apparatus constructed for use on a half-technical scale, the acetic acid and the benzene-are returned in the original relative proportions into the pressure columns, after the acetic acid and the benzene have been separated from the water in known? manner inv additional distillation columns under ordinary pressure. In this manner substantially nearly all the acetic. acid used can be gradually converted into theanhydride by being repeatedly recycled and by further additions of acetic acid and benzene.

Example 4 in a manner analogous to that described in Example 1 a mixture of 1000 grams of acetic acid and 600 grams of n-hexane is boiled at 212 C. under pressure of nitrogen of'1'2 atmospheres (gauge pressure) the starting material being. returned to the still. After 12 hours of distillation the separation of the second phase of the condensate which phase. has the higher specific gravity and contains about 68. percent of acetic acid in addition to water, takes only a slow course. After 20 hours 5.2 percent of the acetic acid. used has been converted into acetic anhydride.

Example 5 The mixture of acetic acid and n-hexane described in Example 4 is boiled at 260 C. under a gas pressure of 27 atmospheres (gauge pressure) the starting material being returned to the still. After 20 hours, the extent of conversion into acetic anhydride amount to 7.8 percent. By-returning'the acetic acid and the n-hexane to the reaction process' in the: manner described at the end of Example 3, almost the whole amount of the acetic acid used can be converted into anhydride.

Example 6 In'the column described in Example 1 a mixture of 1000 grams of acetic acid and 600 grams of cyclohexane is distilled at 250 C. under a gas pressure of 17 atmospheres (gauge pressure); the speed of recycling being 3 litres per hour. The condensate remains one-phased and about percent of it is acetic acid. Within the course 0134 hours 1100 grams of distillate pass over during which procedure-3 percent of the acetic acid used is converted into acetic anhydride.

Example 7 In the column described in Example 1', 1900 grams of acetic acid are distilled at 230 C. under a pressure of inert gas of 10 atmospheres (gauge pressure) without addition of an entrainer. The speed of recycling amounts to 3 litres per hour. About 1200 grams of distillate containing; the reaction water are-obtained Within the course off; hours. 2.2 percent of the acetic acid use is converted into: acetic anhydride.

Example 8 As'described in Example 7 in the course of 4 hours a fraction of 1000 grams is distilled oti from 1900 grams of weakly hydrous acetic acid at a boiling temperature of 250 C. under a pressure of inert gas of 17 atmospheres (gauge pressure) without the addition of an entrainer,

the speed of recycling amounting to 3 litres per hour. In addition to the Water content of the starting material, the said fraction contains 17 grams of reaction water. 4 percent of the acetic acid used has been converted into acetic anhydride.

Example 9 In the column described in Example 1, 1000 grams are distilled ofi from 1800 grams of propionic acid, without addition of an entrainer at 235 C., under a pressure of inert gas of 8 atmospheres within the course of 2 hours. The speed of recycling amounts to 3 litres per hour. 6.7 percent of the propionic acid used is converted into propionic acid anhydride which remains in the still mixed with the starting product. It can be easily obtained in a pure form from the content of the still by fractional distillation under reduced pressure. By returning the propionic acid to the reaction process in the manner described at the end of Example 3, more than 90 percent of the propionic acid used is converted into anhydride.

Example 10 In the column described in Example 1, 1800 grams of butyric acid are distilled at 230 C. under a pressure of nitrogen of 3.5 atmospheres (gauge pressure). 7 The speed of recycling amounts to 2 litres per hour. Within 1 hour, 3.4 percent of the butyric acid is converted into anhydride after 1400 grams of distillate have passed over. The resulting anhydride can be obtained easily in a pure form from the content of the still by distillation under reduced pressure. In this case, too, the major part of the butyric acid can be converted into the anhydride in the manner described at the end of Example 3.

I claim:

1. A process of preparing acid anhydrides which comprises heating monobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atoms under a pressure of about 2 to about 50 atmospheres and at a temperature of between about 180 C. and about 270 C. and distilling the water formed from the reaction mixture.

2. A process of preparing acid anhydrides which comprises heating monobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atoms under a pressure of about 2 to about 50 atmospheres and at a temperature of between about 180 C. and about 270 C. in the presence of a water entrainer for the water formed in the process and distilling the water and water entrainer mixture from the reaction mixture.

3. A process of preparing acid anhydrides which comprises heating monobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atoms under a pressure of about 2 to about atmospheres and at a temperature of between about C. and about 270 C. in the presence of a hydrocarbon water entrainer for entraining the water formed in the process, distilling the water and water entrainer mixture from the reaction mixture and reconducting Water entrainer and acid, distilled over with the water, into the reaction zone.

4. A process of preparing acid anhydrides which comprises heating monobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atoms at a pressure of about 2 to 50 atmospheres and at a temperature of between about 200 C. and about 250 C. in the presence of a hydrocarbon water entrainer, for entraining the water formed in the process, distilling the water and water-entrainer mixture from the reaction mixture and reconducting water entrainer and acid distilled over with the water, into the reaction zone, the pressure resulting from the vapour pressures of the components used in the process.

5. A process of preparing acid anhydrides which comprises heating monobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atoms under a pressure of about 2 to about 50 atmospheres and at a temperature of between about 180 C. and about 270 C., distiiling the water formed from the reaction mixture and reconducting acid, distilled over with the water, into the reaction zone.

6. A process of preparing acid anhydrides which comprises heating monobasic saturated aliphatic carboxylic acids of 2 to 4 carbon atoms at a pressure of about 2 to 50 atmospheres and at temperatures between about 200 C. and about 250 C., distilling the water formed from the reaction mixture and reconducting acid, distilled over with the water, into the reaction zone, the pressure resulting from the vapor pressures used in the components of the process.

Dreyfus Nov. 19, 1929 Schleichler et al July 12, 1931

Claims

1. A PROCEEE OF PREPARING ACID ANHYDRIDES WHICH COMPRISES HEATING MONOBASIC SATURATED ALIPHATIC CARBOXYLIC ACIDS OF 2 TO 4 CARBON ATOMS UNDER A PRESSURE OF ABOUT 2 TO ABOUT 50 ATMOSPHERES AND AT A TEMPERATURE OF BETWEEN ABOUT 180* C. AND ABOUT 270* C. AND LISTILLING THE WATER FORMED FROM THE REACTION MIXTURE.