Process for the Reduction of Unsaturated Carboxylic Acids

It is known that unsaturated carboxylic acids may be reduced to an ester or to the corresponding alcohol. However, the reducing catalysts which have heretofore been employed are not selective in the hydrogenation process, and thus the reductive process usually results in eliminating the retention of the unsaturation in the carbon chain. The compound which is obtained is therefore a saturated ester or alcohol. This is true when utilizing catalysts such as a mixture of copper and chromium oxide or rhenium catalysts which may be used in either a supported or unsupported state or which may also contain a noble metal of Group VIII of the Periodic Table, such as platinum, palladium or ruthenium.

In many instances, it is desirable to retain the unsaturation of the carbon chain when obtaining either alcohols or esters of the starting unsaturated carboxylic acid. As will hereinafter be shown in greater detail, it has now been discovered that a process for effecting the reduction of unsaturated carboxylic acids may be effected by utilizing certain catalytic compositions of matter to obtain esters or alcohols of unsaturated carboxylic acids in which the double bonds present in the original acid are retained in the reaction product to a greater extent than without the catalyst modifications.

This invention relates to a process for the reduction of unsaturated carboxylic acids. More specifically, the invention is concerned with a process for treating unsaturated carboxylic acids of the type hereinafter set forth in greater detail to effect a reduction of the carboxyl moieties of the acids to an unsaturated ester and the corresponding alcohol, while inhibiting the reduction of the double bonds which are present in the carbon atom chain of the starting material.

Unsaturated acid esters, or alcohols, especially those which possess a relatively long carbon atom chain, will find a wide variety of uses in the chemical field. The unsaturated acid esters, as exemplified by oleyl oleate, may be used as a substitute for sperm whale oil which is becoming increasingly difficult to obtain. Sperm whale oil is used as a high grade lubricating oil for light machinery such as watches, clocks and scientific instruments as well as in heat-treating and rustproofing. In addition to use as a lubricant, the esters which are obtained according to the process of this invention may also be used in cosmetics such as perfumes, colognes, bath oils, soaps, powders, etc. This is especially true in the case of relatively long chain unsaturated esters.

It is therefore an object of this invention to provide a process for the reduction of unsaturated carboxylic acids.

A further object of this invention is to provide a process for the reduction of the carboxyl moiety of an unsaturated carboxylic acid with a concurrent inhibition of the reduction of the double bond of the starting material. In one aspect, an embodiment of this invention resides in a process for the reduction of an unsaturated carboxylic acid which comprises treating said acid in a reaction system in the presence of hydrogen and a reducing catalyst system comprising rhenium composited on a solid support and a phosphorous-containing modifier at treatment conditions, and recovering the resultant unsaturated product.

A specific embodiment of this invention is found in a process for the reduction of an unsaturated carboxylic acid which comprises treating oleic acid in a reaction system in the presence of hydrogen and a reducing catalyst system comprising rhenium composited on gamma-alumina, and hypophosphorous acid at a temperature in the range of from about 100.degree. to about 500.degree. C. and a pressure in the range of from about 100 to about 5000 pounds per square inch (psi) and recovering the resultant oleyl oleate, oleyl alcohol and geometric and positional isomers thereof.

Other objects and embodiments can be found in the following further detailed description of the invention.

As hereinbefore set forth, the present invention is concerned with a process for the reduction of an unsaturated carboxylic acid in which said acid is treated with hydrogen in the presence of a reduction catalyst system of the type hereinafter set forth in greater detail. By employing this catalyst system, and also by employing certain reaction conditions it is possible to obtain the resulting ester and/or alcohol in which the unsaturation in the carbon atom chain which is present in the starting material will be retained to a greater degree than is possible when utilizing other catalysts.

Examples of unsaturated carboxylic acids which may be employed as starting materials to form the desired unsaturated esters will include those acids containing from 3 to about 22 carbon atoms, some specific examples of these acids being acrylic acids; the isomeric butenic acids such as crotonic acid, isocrotonic acid, vinyl acetic acid, methylacrylic acid; the isomeric pentenic acids such as tiglic acid, angelic acid, senecioic acid; the isomeric hexenoic acids; heptenoic acids; octenoic acids; nonenoic acids; decenoic acids; undecenoic acids; dodecenoic acids; tridecenoic acids; tetradecenoic acids; pentadecenoic acids; hexadecenoic acids such as hypogeic acid; heptadecenoic acids; octadecenoic acids such as oleic acid, elaidic acid; nonadecenoic acids; eicosenoic acids; erucic acid; brassidic acid, etc. It is to be understood that the aforementioned unsaturated carboxylic acids are only representative of the type of compounds which may be employed to form the desired esters, and that the present invention is not necessarily limited thereto.

The catalyst system which is employed to effect the reduction of the aforesaid acids to esters and/or alcohols while maintaining the unsaturation of the carbon chain to a greater degree than was previously possible will be a reducing catalyst composite comprising a rhenium compound composited on a solid support and a phosphorous-containing compound which acts as a modifier. The rhenium will be present on the solid support in a low valence oxidation state, usually in the form of rhenium oxide or metallic rhenium in an amount in the range of from about 0.1 to about 2% by weight of the composite. Examples of rhenium compounds which may be employed to form the desired catalyst will include rhenium trichloride, ammonium perrhenate, rhenium oxide, perrhenic acid, etc. The aforementioned rhenium compounds will be composited on a solid support which, in the preferred embodiment of the invention, comprises a relatively high surface area inorganic oxide. Examples of these inorganic oxides will include aluminas such as gamma-alumina, eta-alumina, theta-alumina, silica or mixtures of inorganic oxides such as alumina-silica, silica-zirconia, silicamagnesia, alumina-silica-zirconia, etc.

Examples of phosphorous-containing compounds which act as a modifier in the catalyst system and which will inhibit the double bond reduction during the reduction reaction, and which are present in the catalyst system preferably in an amount in the range of from about 0.1% to about 5% by weight of the catalyst, will include phosphorous acids such as orthophosphorous acid, pyrophosphorous acid, metaphosphorous acid, hypophosphorous acid, orthophosphoric acid, pyrophosphoric acid, metaphosphoric acid, hypophosphoric acid, etc; phosphorous salts such as phosphorus dichloride, phosphorus trichloride, etc.

The reducing catalyst system which is used in the process of the present invention may be prepared in any suitable manner. For example, one type of preparation which may be used comprises impregnating a solid support, such as gamma-alumina, with an aqueous solution of a rhenium compound such as perrhenic acid for a period of time which is sufficient to allow the deposition of the desired amount of rhenium on the solid support, that is, an amount sufficient so that the finished catalyst composite will contain from about 0.1 to about 2% rhenium. Following this, the phosphorous-containing compound may then be utilized to impregnate the catalyst composite under similar conditions so that the finished catalyst system will contain the desired amount of phosphorus, that is, from about 0.1 to about 5% phosphorus. Alternatively, a coimpregnation may be effected in which the solid support is coimpregnated with a rhenium salt and the phosphorous-containing compound for a period of time sufficient to deposit the desired amount of both rhenium and phosphorous compounds on the base. After recovery of the impregnated solid support, the composite is then calcined at a temperature which may range from about 250.degree. to about 750.degree. C. in a nitrogen atmosphere for a period of time which may range from about 0.5 hour up to about 4 hours in duration. Following this, if so desired, the calcined composite may then be subjected to a reducing treatment by adding the composite at a temperature within the range hereinbefore set forth, that is, from about 250.degree. to about 750.degree. C. in a hydrogen atmosphere for a period of time sufficient to reduce the rhenium to a low valence oxidation state.

It is also contemplated that the catalyst which is used in the process of the present invention may be prepared in a continuous manner of operation. When such a type of operation is employed, the solid support material comprising an inorganic oxide of the type hereinbefore set forth in greater detail which may be of any desired shape such as pellets, spheres, globules, rods, etc. is continuously passed through an aqueous solution of a rhenium-containing compound at a predetermined rate of speed in order that the predetermined amount of rhenium may be impregnated on the support. The support, after passage through the solution, is continuously withdrawn and passed to a calcination zone wherein it is treated at an elevated temperature, in the presence of air, within the range hereinbefore set forth. After completion of the calcination period, the rhenium-impregnated material is then passed through a second impregnation bath comprising a solution of a phosphorous-containing compound. Alternatively, it is also contemplated that the rhenium and the phosphorous-containing compound may be coimpregnated on the solid support in a single impregnation zone following which the impregnated solid support is calcined and thereafter, if so desired, subjected to a reducing step in which the impregnated support is continuously passed through a reducing zone at an elevated temperature while being subjected to a hydrogen flow. Following the reduction, the composite is continuously withdrawn and recovered. The reduction process of the present invention which results in the obtention of esters and alcohols which still possess the unsaturation of the starting materials and which are recovered in an amount greater than that which was hereinbefore obtained may be effected in either a batch or continuous type operation. When utilizing a batch type operation, a quantity of the unsaturated carboxylic acid, which is used to undergo esterification or to obtain an alcohol, is placed in an appropriate apparatus which is pressure-resistant in nature, such as an autoclave of the rotating, mixing or stirring type. In addition, the particular catalyst hereinbefore described is also added to the apparatus in an amount in the range of from about 25:1 to about 5:1 grams of acid per gram of catalyst. After pressuring the apparatus to an initial operating pressure, the apparatus is then heated to the desired operating temperature and maintained thereat for a predetermined period of time. The operating conditions which are employed to effect the desired reduction process will include a temperature in the range of from about 100.degree. to about 500.degree. C. and superatmospheric pressures ranging from about 100 to about 5000 psi for a period of time which may range from about 0.5 up to about 10 hours or more in duration, the reaction time being determined by the particular unsaturated carboxylic acid undergoing reduction as well as the reaction temperature and amount of pressure which is employed during the reaction. The superatmospheric pressures which are employed may be afforded by hydrogen alone or, if so desired, the amount of hydrogen present may afford only a partial pressure, the remainder of the desired operating pressure being afforded by the presence of an inert gas such as nitrogen, helium, argon, etc. in the reaction apparatus. Upon completion of the desired reaction period, the hydrogen charge is discontinued as is the heat treatment, and after the reaction vessel or apparatus has returned to room temperature, the excess pressure is discharged, the apparatus is opened, and the reaction mixture is recovered therefrom. The thus recovered mixture may then be filtered to separate the catalyst system from the reaction product, the latter then being subjected to conventional means of separation to recover the desired ester and/or alcohol.

It is also contemplated within the scope of this invention that the reduction process may be effected in a continuous manner of operation. When such a type of operation is employed, a reaction vessel containing the reduction catalyst system is maintained at the proper operating conditions of temperature and pressure, the unsaturated carboxylic acid which is to undergo reduction is continuously charged to the reaction vessel where it is contacted with the catalyst system in the presence of hydrogen which is also continuously charged to the reactor. After passage through the reaction vessel for a predetermined period of time, the reactor effluent is continuously withdrawn from the reaction vessel and subjected to conventional means of separation whereby the desired ester or alcohol of the unsaturated carboxylic acid which still possesses the unsaturation of the starting material, is separated and recovered, while any unreacted starting materials, both gaseous and liquid in nature, after being dried to remove the water formed during the reaction, are recycled to the reaction vessel to form a portion of the feedstock.

It is contemplated that the continuous method of operation may be effected in various ways. For example, the reduction catalyst may be positioned in the reaction vessel as a fixed bed, and the unsaturated carboxylic acid undergoing reduction is passed over the bed in either an upward or downward flow. Another method of effecting the reaction is to employ the catalyst system as a moving bed in the reaction vessel and having the unsaturated carboxylic acid and the catalyst system pass through the reaction vessel either concurrently or countercurrently to each other. Likewise, if so desired, a slurry-type of operation may be employed in which the reduction catalyst system is carried into the reaction vessel as a slurry in the unsaturated carboxylic acid.

The following examples are given for purposes of illustrating the process of the present invention utilizing the particular reducing catalyst system. However, it is to be understood that these examples are given merely for purposes of illustration and that the present invention is not necessarily limited thereto.

To illustrate the advantage of utilizing a phosphorous-containing compound as a modifier for a catalyst system, a catalyst was prepared by impregnating 100 grams of alumina with 200 cc of an aqueous ammonium perrhenate solution to afford a 1% rhenium-to-base ratio. The impregnation was allowed to proceed for a period of 4 hours following which the impregnated alumina was recovered, calcined at a temperature of 500.degree. C. in an air atmosphere for a period of 1 hour and thereafter reduced in a hydrogen atmosphere at a temperature of 500.degree. C. for an additional period of 1 hour.

A feedstock comprising 200 grams of oleic acid and 10 grams of the catalyst prepared according to the above paragraph was charged to a 1 liter stirred autoclave which was sealed and flushed with hydrogen. The autoclave was pressured to about 100 psig with hydrogen and heated to a temperature of 310.degree. C. Upon reaching the desired operating temperature, the autoclave was further pressured to 1000 psig with hydrogen and the reaction was allowed to proceed for a period of 4 hours while maintaining the temperature at 310.degree. C., the pressure at 1000 psig, and stirring the autoclave at a rate of 1100 rpm. At the end of the 4 hour period, heating was discontinued and, after the autoclave had returned to room temperature, the excess pressure was discharged and the autoclave was opened. The reaction mixture was recovered and separated from the catalyst by filtration. Analysis of the product by means of GC and Iodine Value disclosed that there had been a 97% reduction of the carboxyl moiety with a corresponding 90% saturation of the double bond of the carbon chain.

This example illustrates the ability of the catalyst system of the present invention to inhibit the reduction of the double bond while allowing the carboxyl moiety to be reduced. Seventy-five cc of the catalyst prepared in Example I above was further impregnated with 75 cc of an aqueous hypophosphorous acid solution to afford a 0.9% phosphorus-to-base ratio. The resulting catalyst system was recovered and reduced under a hydrogen atmosphere at a temperature of 525.degree. C. for a period of 2 hours.

To a stirred autoclave was added 200 grams of oleic acid and 10 grams of the catalyst system prepared according to the above paragraph. The oleic acid was treated in a manner similar to that set forth in Example I above, that is, by utilizing a reaction temperature of 310.degree. C., a pressure of 1000 psig of hydrogen while stirring the autoclave at 1100 rpm for a period of 4 hours. At the end of the 4 hour period, heating was discontinued and after the autoclave had returned to room temperature, the excess pressure was vented. The autoclave was then opened and after the reaction product had been separated from the catalyst by filtration, the product was analyzed. Analysis showed that there had been a 96% reduction of the carboxyl moiety with only a 70% saturation of the double bond of the carbon chain, thus permitting the recovery of oleyl oleate, oleyl alcohol as well as the geometric and positional isomers thereof.

In this example, a catalyst system similar to that utilized in Example II above was prepared, the difference being that a sufficient amount of aqueous hypophosphorus solution was utilized so that the finished catalyst system contained a 1.8% phosphorus-to-base ratio. Again, oleic acid was treated with this catalyst system utilizing similar reaction conditions including a reaction temperature of 310.degree. C., a pressure of 1000 psig of hydrogen and a reaction period of 4 hours. Analysis of the reaction product which was recovered disclosed that there had been an 87% reduction of the carboxyl moiety with a 62% saturation of the double bond of the molecule. The use of a catalyst system comprising rhenium impregnated on a base such as alumina with a phosphorous-containing compound as a modifier thereof illustrates the fact that this catalyst system will inhibit the saturation of the double bond of the unsaturated acid when compared to a catalyst system such as rhenium composited on alumina without the presence of a phosphorous-containing compound.

In a manner similar to that hereinbefore set forth, the treatment of other unsaturated acids such as hypogeic acid, erucic acid, crotonic acid, hexenoic acid, etc. utilizing a catalyst system comprising rhenium and a phosphorous-containing compound such as orthophosphorous acid or phosphorus trichloride and utilizing reaction conditions similar to those hereinbefore set forth of temperature, hydrogen pressure and time may result in the production of unsaturated esters and alcohols such as hypogeyl hypogeate, hypogeyl alcohol, erucyl erucate, erucyl alcohol, crotonyl crotonate, crotonyl alcohol, hexenyl hexenate, hexenyl alcohol, as well as geometric and positional isomers thereof.