GB2155490A - Hydrogenation of polyunsaturated fatty acid residue-containing compositions using naphthalene chromium tricarbonyl catalyst - Google Patents

Abstract

Hydrogenation of polyunsaturated fatty acid residues of compositions such as polyunsaturated vegetable oils containing linoleic acid residues, linolenic acid residues and mixtures thereof and those containing methyl linoleate is carried out in a reactor having an inert inner surface by contacting with hydrogen at a temperature of from 80 DEG to 160 DEG C in the presence of a naphthalene chromium tricarbonyl catalyst of formula CrA(CO)3 wherein A is <IMAGE> and R<1> to R<8> are H or C1-4 alkyl or alkoxy.m

Description

SPECIFICATION Hydrogenation of polyunsaturated fatty acid residue-containing compositions using naphthalene chromium tricarbonyl catalyst TECHNICAL FIELD The present application relates to the hydrogenation of polyunsaturated fatty acid residuecontaining compositions, such as polyunsaturated vegetable oils and those containing methyl linoleate, using a naphthalene chromium tricarbonyl catalyst.

Polyunsaturated fatty acid containing compounds are frequently hydrogenated to their more oxidatively stable monounsaturated products. Particularly important examples are the glyceride and non-glyceride ester of linoleic acid (cis,cis-9, 1 2-octadecadienoic acid) and to a lesser extent linolenic acid (cis,cis,cis9, 1 2,1 5-octadecatrienoic acid). The hydrogenation of linoleic and linolenic acid-containing glycerides preverns oxidative flavor deterioration of a variety of polyunsaturated vegetable oils such as soybean oil, safflower oil and sunflower seel oil. Also, the hydrogenation of methyl linoleate (present in the mixed methyl esters of tall oil fatty acids) to methyl isooleates can be used to produce isooleic acid mixtures similar in properties to oleic acid.

In the case of polyunsaturated vegetable oils, nickel catalysts are typically used for partial hydrogenation. See Bailey's Industrial Oil and Fat Products, (3rd edition 1964), pp. 793 et seq.

However, these nickel catalysts do not selectively hydrogenate the polyunsaturated fatty acid residues to the desired cis-monounsaturated fatty acid residues. Instead, a significant amount of higher melting transunsaturated glycerides are formed due to isomerization of the polyunsaturated fatty acid residues. Because these partially hydrogenated vegetable oils are often used in cooking and salad oil preparation, a separate winterization step can be required to remove these higher melting transunsaturated glycerides.

Instead of nickel catalysts, benzene and methyl benzoate chromium tricarbonyl complexes have been used as catalysts to more selectively hydrogenate the polyunsaturated fatty acid residues of vegetable oils to provide less of the undesirable transunsaturated glycerides. See U.S. Patent 3,542,821 to Frankel, issued November 24, 1970. These catalysts have also been used for selective hydrogenation of methyl linoleate to the methyl isooleates. See Frankel et al., "Homogeneous Hydrogenation of Diolefins Catalyzed by Tricarbonyl Chromium Complexes: Selective 1,4 Addition of Hydrogenation," J. Org. Chem., Vol. 34 (1969), pp. 3930-36.

Even using these more selective benzene and benzoate chromium tricarbonyl catalysts, a significant amount of transunsaturated fatty acid residues are formed, typically from 39% for the partially hydrogenated vegetable oils (see U.S. Patent 3,542,821, supra at column 3, lines 3-5) and from 6-1 2% for the hydrogenation of methyl linoleate (See Frankel et al., supra at p. 3933). It is believed that the temperatures (typically 1 60'-1 75'C) employed for hydrogenation with these catalysts promote the isomerization of the polyunsaturated fatty acid residues to the respective transunsaturates. At lower temperatures such as 1 20'-1 40'C, isomerization of the polyunsaturated fatty acid residues is less favored.However, these benzene and benzoate chromium tricarbonyl complexes are less effective hydrogenation catalysts at these lower temperatures. It would therefore be desirable to find a catalyst which provides more effective hydrogenation of these polyunsaturated fatty acid residues at lower temperatures which do not favor isomerization.

BACKGROUND ART A. Hydrogenation of Dienes Using Naphthalene Chromium Tricarbonyl as the Catalyst.

Cais et al., "The Catalystic Activity of Tricarbonyl Chromium Complexes of Phenanthrene, Naphthalene and Anthracene in the Hydrogenation of Dienes," Coord. Chem. Rev., Vol. 16, (1975), pages 27-34, discloses kinetic studies on the hydrogenation of dienes using arenechromium tricarbonyls as catalysts, including naphthalene chromium tricarbonyl. The hydrogenation of methyl sorbate, a conjugated diene, was carried out at temperatures ranging from 27 to 120"C in solvents such as decalin, tetrahydrofuran (THF) and acetone. In addition, naphthalene chromium tricarbonyl was used to catalyze the hydrogenation of 1,4-cyclohexadiene in acetone at a temperature of 40.5"C; the reaction rate and induction time data suggest that this reaction was extremely sluggish.See also Yagupsky et al., "Solvent-Assisted Regioselective and Stereospecific Hydrogenation of Dienes, at Ambient Temperatures and Pressures Catalyzed by (Naphthalene) Cr(CO)3; Nature of the Active Catalytic Species," Inorg. Chim. Acta, Vol. 12, (1975), pages L27-L28, which also discloses the hydrogenation of methyl sorbate at 30"C and one atmosphere total pressure using naphthalene chromium tricarbonyl as the catalyst.

Eden et al., "Stereospecific and Regioselective Hydrogenation of Bicyclo[2.2.1]Hepta-2,5 Diene and Related Systems Catalyzed by Tricarbonyl Chromium Complexes of Phenanthrene and Naphthalene," Israel J. Chem., Vol. 25, (1977), pp. 223-29, discloses the hydrogenation of norbornadiene and related bicyclic compounds using naphthalene chromium tricarbonyl as the catalyst. Nortricylene was the major product formed (82%) with the minor product being norbornene (18%). This reaction was carried out at a temperature of 30.2"C in THF.

B. Hydrogenation of Methyl Linoleate Using Benzene and Methyl Benzoate Chromium Tricarbonyl Complexes as Catalysts.

Frankel at al., "Homogeneous Hydrogenation of Diolefins Catalyzed by Tricarbonyl Chromium Complexes: Stereoselective 1,4 Addition of Hydrogenation," J. Org. Chem., Vol. 34, (1969), pp. 3930-36, discloses the hydrogenation of various dienes using benzene and methyl benzoate chromium tricarbonyl complexes as the catalysts. Methyl linoleate was hydrogenated to methyl cis-isooleates using both of these complexes under the following conditions: Temp. Time Isooleates Trans Complex ( C) (hr) % Unsaturates (%) methyl benzoate 175 3 94.8 12.4 benzene 165 8 79.0 6.1 See also Frankel et al., "Selective Homogeneous Hydrogenation of Triunsaturated Fats Catalyzed by Tricarbonyl Chromium Complexes," J. Am. Oil Chem. Soc., Vol. 49, (1972), pp.

70-74 (hydrogenation of methyl linolenate at temperatures of from 1 65'C-1 75'C in cyclohexane using methyl benzoate chromium tricarbonyl as the catalyst; transunsaturate content ranged from 1 7.8-39.5%); Frankel et al., "Homogeneous Catalytic Hydrogenation of Unsaturated Fats: Group VIB Metal Carbonyl Complexes." J. Am. Oil Chem. Soc., Vol. 46, (1969), pp. 256-61 (hydrogenation of methyl esters of soybean oil at temperatures of from 165 -175 C in hexane using benzene and methyl benzoate chromium tricarbonyi complexes as catalysts: transunsaturate content ranged from 3.5-1 5.4%); Frankel "Homogeneous Catalytic Conjugation of Polyunsaturated Fats by Chromium Carbonyls," J. Am.Oil Chem. Soc., Vol. 47, (1970), pp. 33-36 (conjugation of methyl linoleate in hexane at temperatures of from 165 -185 C using benzene and methyl benzoate chromium tricarbonyl complexes as catalysts); Frankel et al., "Homogeneous Hydrogenation of Diolefins Catalyzed by Tricarbonyl Chromium Complexes: Deuteration," J. Org. Chem., (1969), pp. 3936 42 (deuteration of methyl linoleate at temperatures of from 1 65'-1 75"C in cyclohexane using benzene and methyl benzoate chromium tricarbonyl complexes as catalysts).

C. Hydrogenation of Polyunsaturated Oils Using Benzene and Methyl Benzoate Chromium Tricarbonyl Complexes as Catalysts U.S. Patent 3,542,821 to Frankel, issued November 24, 1970, discloses the partial hydrogenation of polyunsaturated vegetable oils such as soybean oil and safflower oil, and mixed methyl esters of such oils, using benzene and methyl benzoate chromium tricarbonyl complexes as catalysts. The hydrogenations were carried out at temperatures of from 1 45'-1 75'C for 2-6 hours (typically 4-6 hours at the lower temperatures).The trans unsaturate content of the partially hydrogenated oils ranged from 29.5% and more typically from 39%. See also Frankel, "Conversion of Polyunsaturates and Vegetable Oils to cis Monounsaturates by Homogeneous Hydrogenation Catalyzed with Chromium Carbonyls," J.

Am. Oil Chem. Soc., Vol. 47, (1970), pp. 11-14 (similar disclosure); Frankel et al., "Stereoselective Hydrogenation of Model Compounds and Preparation of Taylor-Made Glycerides with Chromium Tricarbonyl Complexes," J. Am. Oil Chem. Soc., Vol. 47, (1970), pp.

497-500 (hydrogenation of vegetable oils using benzene and methyl benzoate chromium tricarbonyl complexes as catalysts to make simulated olive oil, peanut oil and safflower oil).

DISCLOSURE OF THE INVENTION The present invention relates to a method for hydrogenating the polyunsaturated fatty acid residues of a polyunsaturated fatty acid residue-containing composition, in particular polyunsaturated vegetable oils and those containing methyl linoleate. Compositions suitable in this method comprise glycerides, non-glyceride esters, or mixtures thereof having polyunsaturated fatty acid residues. These polyunsaturated residues are selected from linoleic acid residues, linolenic acid residues and mixtures thereof. After being placed in a reactor having an inert inner surface, this polyunsaturated fatty acid residue-containing composition is contacted with hydrogen at a temperature of from about 80 to about 160"C in the presence of a catalytic amount of a naphthalene chromium tricarbonyl complex, [Cr(A)(CO)3, A being a naphthalene of formula:

wherein R1, R2, R3, R4, R5, R6, R7 and R8 are H, C14 alkyl, or C14 alkoxy. As a result, the polyunsaturated fatty acid residues of the composition are hydrogenated to the cismonounsatu- rates.

The method of the present invention has a number of advantages, especially with regard to prior art catalyzed hydrogenations of polyunsaturated vegetable oils and methyl linoleate.

Compared to prior art benzene and benzoate chromium tricarbonyl catalysts, the naphthalene chromium tricarbonyl catalysts used in the method of the present invention provide much faster rates of hydrogenation and function effectively at lower temperatures, in particular at temperatures of from about 120 to about 140"C. As a result, there is no measurable isomerization of the polyunsaturated fatty acid residues to the respective transunsaturates, i.e. less than about 1% transunsaturates are formed in the hydrogenation method of the present invention.

A. Polyunsaturated Fatty Acid Residue-Containing Compositions The compositions suitable for hydrogenation according to the method of the present invention comprise glycerides, non-glyceride esters, or mixtures thereof which have polyunsaturated fatty acid residues. As used herein, the term "glycerides" refers to monoglycerides, diglycerides, and especially triglycerides, or mixtures thereof. As used herein, the term "non-glyceride esters" refers to fatty acid esters which are not monoglycerides, diglycerides or triglycerides.As used herein, the term "polyunsaturated fatty acid residue" refers to that portion of the glyceride or non-glyceride ester which is a linoleic acid residue having the formula:

or a linolenic acid residue having the formula:

Compositions suitable in the method the present invention usually consist essentially of glycerides, non-glyceride esters or mixtures thereof which have at least about 20% polyunsaturated fatty acid residues. As used herein, the term "at least about 20% polyunsaturated fatty acid residues" refers to a composition wherein at least about 20% by weight of the fatty acid residues of the glycerides or non-glyceride esters are linoleic acid residues, linolenic acid residues, or mixtures thereof.Glycerides and non-glyceride esters having other fatty acid residues (e.g. oleic, palmitic, stearic) can be included in such compositions, so long as at least about 20% of the total fatty acid residues are polyunsaturated fatty acid residues.

Of the polyunsaturated fatty acid residue-containing compositions consisting essentially of glycerides, the polyunsaturated vegetable oils are most preferred. Representative examples of polyunsaturated oils for which the method of the present invention is suitable include corn oil, cottonseed oil, linseed oil, peanut oil, safflower oil, sesame seed oil, sorghum oil, soybean oil, and sunflower seed oil. The method of the present invention is especially suitable for those polyunsaturated vegetable oils having at least about 50% polyunsaturated fatty acid residues, in particular, corn oil, soybean oil, safflower oil and sunflower seed oil.

Of the compositions consisting essentially of non-glyceride esters, the preferred ones are those containing linoleate and linolenate esters having the following formulas:

wherein R is a hydrocarbyl group which can be easily removed by hydrolysis. Suitable hydrocarbyl groups include C,C4 alkyl groups and phenyl groups. A particularly preferred non-glyceride ester is methyl linoleate (R is methyl). A suitable source of methyl linoleate is the mixture of methyl non-glyceride esters obtained after esterifying the fatty acids present in tall oil with methanol, using an acid catalyst such as sulfuric acid. Such a mixture usually contains at least about 30% methyl linoleate. Typically, methyl linoleate (99% purity) can be hydrogenated according to the method of the present invention to at least about 95% methyl isooleates.As used herein, the term "isooleates" refers to a mixture of 9-octadecenoate, 10-octadecenoate, ll-octadecenoate and 12-octadecenoate, typically in a 1:1:1:1 (equimolar) ratio.

B. Naphthalene Chromium Tricarbonyl Catalyst In the method of the present invention, a naphthalene chromium tricarbonyl catalyst is used.

As used herein, the term "naphthalene chromium tricarbonyl catalyst" refers to chromium tricarbonyl complexes of formula (Cr(A)(CO), A being a naphthalene of formula:

wherein R1, R2, R3, R4, R5, R6, R7 and R8 are hydrogen (H), C14 alkyl, or C1-C4 alkoxy.

Preferred naphthalenes are those wherein R', R2, R3, R4, R5, Rub, R7 and R8 are H, methyl, or methoxy. Naphthalene (R', R2, R3, R4, R5, R6, R7, and R8, are each H), 2-methoxynaphthalene, and 2,3-dimethylnaphthalene are especially preferred for catalysts used in the method of the present invention.

A catalytic amount of the naphthalene chromium tricarbonyl catalyst is used in the method of the present invention. What is "a catalytic amount" can vary depending upon the type of naphthalene chromium tricarbonyl catalyst used, the polyunsaturated fatty acid residue-containing composition being hydrogenated, the particular reaction conditions during hydrogenation (e.g., temperature), the degree of hydrogenation desired, and like factors. An amount of from about 0.5 to about 20 mole percent is usually suitable for hydrogenation. However, an amount of from about 5 to about 1 5 mole percent typically provides optimum hydrogenation of the polyunsaturated fatty acid residues.As used herein, mole percentages given for the naphthalene chromium tricarbonyl catalyst are based on the amount of the polyunsaturated fatty acid residuecontaining composition being hydrogenated.

The naphthalene chromium tricarbonyl catalysts used in the method of the present invention can be prepared by refluxing the particular naphthalene with chromium hexacarbonyl in a high boiling solvent such as heptane or di-n-butyl ether. See Cais et al., "The Catalytic Activity of Tricarbonyl Chromium Complexes of Phenanthrene, Naphthalene and Anthracene in the Hydrogenation of Dienes," Coord. Chem. Rev., Vol. 16, (1975), p. 29. See also Mahaffey et al., "(n6- Arene)tricarbonylchromium Complexes," Inorganic Synthesis, Vol. 19, (1979), pp. 154--58, which discloses a synthesis procedure for simple arene (benzene) chromium tricarbonyls which can be modified to prepare naphthalene chromium tricarbonyls.A general synthesis procedure for these naphthalene chromium tricarbonyl catalysts is as follows: In a 500 ml round bottom flask, 10 g of chromium hexacarbonyl and 1.1 equivalents of the particular naphthalene are suspended in 300 ml of di-n-butyl ether and 30 ml of tetrahydrofuran (THF) as the solvent. The flask is equipped with an air condenser and a water-cooled Friedricks condenser. This system is vented through an oil bubbler and purged with dry, purified nitrogen gas. The contents of the flask are heated to reflux for 1 2 to 24 hours and become deep orange to deep red as the reaction proceeds. At the end of the reaction, the solvent, unreacted naphthalene, and unreacted chromium hexacarbonyl are removed in vacuuo. The naphthalene chromium tricarbonyl formed is then purified, if necessary, by recrystallization from hexane.

Yields of the catalyst can range from 60 to 95% depending on the reaction time and the particular naphthalene.

C. Reaction Conditions During Hydrogenation Normally, the polyunsaturated fatty acid residue-containing composition is liquid at ambient temperatures (20"--25"C). For example, polyunsaturated vegetable oils such as soybean oil and methyl linoleate are all liquids. Accordingly, a reaction mixture suitable for hydrogenation can be formed by simply mixing together the liquid polyunsaturated fatty acid residue-containing composition and the naphthalene chromium tricarbonyl catalyst. If desired, nonpolar, nonreactive solvents can be used. Examples of suitable solvents for the method of the present invention include aliphatic hydrocarbon solvents such as hexane, heptane and the like.Examples of solvents which should not be used include arene (aromatic) solvents such as benzene, ketone solvents such as acetone, tetrahydrofuran, ethyl acetate and the like which can exchange with naphthalene and thus become bound to the chromium tricarbonyl catalyst. In the case of polyunsaturated vegetable oils, hydrogenation is typically carried out in the absence of solvent, i.e. as a neat reaction.

To effect hydrogenation of the polyunsaturated fatty acid residues, the reaction mixture is contacted with hydrogen. The hydrogen pressure is generally not critical to the method of the present invention as long as it is greater than the partial pressure of any solvent being used. A hydrogen pressure of at least about 50 psi is usually suitable in the method of the present invention. Typically, the hydrogen pressure is from about 100 to about 500 psi.

While being contacted with hydrogen, the reaction mixture is heated to a temperature of from about 80 to about 160"C. Temperatures much below about 80'C do not provide effective hydrogenation of the polyunsaturated fatty acid residues. Temperatures much above about 160"C can cause significant amounts of trans-unsaturates to be formed due to isomerization of the polyunsaturated fatty acid residues. Temperatures of from about 120 to about 140"C typically provide optimum hydrogenation of the polyunsaturated fatty acid residues without significant formation of fransunsaturates, i.e. typically less than about 1 % transunsaturates are formed.

Hydrogenation is carried out for a period of time sufficient to provide the desired degree of hydrogenation of the polyunsaturated fatty acid residues. The degree of hydrogenation desired often depends on the polyunsaturated fatty acid residue-containing composition being hydrogenated and the particular use of the hydrogenated composition. For compositions containing methyl linoleate, such as the mixed methyl esters of tall oil which are hydrolyzed to obtain isooleic acids, a higher degree of hydrogenation is desired. For example, methyl linoleate is typically hydrogenated for from about 1 to about 2 hours to provide at least about 95% methyl isooleates. For polyunsaturated vegetable oils such as soybean, safflower and sunflower seed oils which require oxidative flavor stability, partial hydrogenation is usually sufficient.For example, these oils, which typically contain at least about 50% polyunsaturated fatty acid residues, are usually partially hydrogenated for from about 1 to about 2 hours to provide from about 20 to about 30% polyunsaturated fatty acid residues.

Hydrogenation according to the method of the present invention is carried out in a reactor which has an inert inner surface. As used herein, the term "reactor which has an inert inner surface" refers to a reactor which has an inner surface that does not interfere with or deactivate the naphthalene chromium tricarbonyls used as hydrogenation catalysts. Suitable reactors include those made totally out of an inert material such as glass reactors or those which have an inert inside lining, such as glass-lined or polytetrafluoroethylene (Teflon)-lined reactors. In addition to higher temperatures, it is also believed that prior art benzene and benzoate chromium tricarbonyl catalysts require unlined steel reactors in order to effectively catalyze the hydrogenation of polyunsaturated vegetable oils or methyl linoleate.For example, it has been found that there is no measurable yield of methyl isooleates after 2 hours when methyl linoleate is hydrogenated in a glass-lined reactor using either benzene or methyl benzoate chromium tricarbonyl as the catalyst. By contrast, the naphthalene chromium tricarbonyls used in the method of the present invention are effective hydrogenation catalysts in a glass or glass-lined reactor, i.e. when free of contact with iron. Indeed, it has been found that unlined, stainless steel reactors can cause deactivation of the naphthalene chromium tricarbonyls during the hydrogenation of methyl linoleate.

Conversion of Methyl Isooleates to Isooleic Acids In the case of polyunsaturated vegetable oils used in edible oil applications such as cooking oil and salad oil preparation, the glycerides are used as is after hydrogenation. However, in the case of hydrogenated compositions containing methyl isooleates such as the hydrogenated mixed methyl esters of tall oil (typically greater than 95% methyl isooleates), the non-glyceride esters are typically hydrolyzed to yield isooleic acids. Hydrolysis of the esters can be achieved using standard saponification reaction conditions. For example, the hydrogenated composition can be mixed with an ethanolic solution of base such as sodium or potassium hydroxide and water. This solution is then refluxed for a period of time sufficient to hydrolyze the esters to yield the respective fatty acids, in particular the isooleic acids.This hydrolyzed solution is neutralized with acid and then extracted with an organic solvent to partition the fatty acids, including the isooleic acids, into the organic layer. The organic solvent can be evaporated to yield a crude mixture containing a high percentage of isooleic acids. If desired, this crude mixture can be fractionally distilled to yield a purer mixture of isooleic acids. Commercial hydrolyzers can also be used to obtain isooleic acids from the hydrogenated composition.

Specific Illustrations of the Hydrogenation of Methyl Linoleate Using Naphthalene Chromium Tricarbonyl Catalysts The following examples of the hydrogenation of methyl linoleate (greater than 99% pure) to methyl isooleates using naphthalene chromium tricarbonyls as the catalyst are used to illustrate the method of the present invention: All hydrogenations were carried out in a reactor which was either: (1) a 50 ml Griffen-Worden (G-W) glass pressure vessel shaken in a thermostatically controlled oil bath; or (2) a 300 ml Autoclave Engineers (AE) rocking autoclave fitted with a glass liner which was electrically heated and controlled. A 0.5 9 (G-W) or 2 g (AE) portion of a reaction mixture containing 0.03 to 0.04 M methyl linoleate in hexane and a measured amount of catalyst was placed in the reactor.The reaction mixture was then placed under a hydrogen atmosphere at the appropriate hydrogen pressure. The reaction mixture was stirred under this hydrogen atmosphere at the desired temperature for 2 hours. At the end of this 2 hour period, a sample was taken from the reaction mixture for analysis by gas chromatography to determine the yield of methyl isooleates and by quantitative IR to determine the level of transunsaturates.

A. Catalyst The yield of methyl isooleates from the hydrogenation (200 psi hydrogen pressure, G-W reactor) of methyl linoleate at 135on for 2 hours using different naphthalene chromium tricarbonyl catalysts (10 mole percent) is shown in the following table: Catalyst Isooleates (%) Trans-unsaturates (%) Naphthalene 90 < 1 2,3-Dimethylnaphthalene 65 < 1 2-Methoxynaphthalene 70 < 1 1-Methoxynaphthalene 30 < 1 1,4-Dimethylnaphthalene 30 < 1 1-Methyinaphthalene 35 < 1 2-Methylnaphthalene 15 < 1 2,6-Dimethoxynaphthalene 8 < 1 B.Reaction Temperatures The yield of methyl isooleates from the hydrogenation (200 psi hydrogen pressure, G-W reactor) of methyl linoleate using naphthalene chromium tricarbonyl (10 mole percent) as the catalyst for 2 hours at different reaction temperatures is shown in the following table: Temperature ("C) Isooleates (%) Trans-unsaturates (%) 100 20 < 1 120 50 < 1 135 90 < 1 160 95 1-2 C.Hydrogen Pressure The yield of methyl isooleates from the hydrogentation of methyl linoleate using naphthalene chromium tricarbonyl (10 mole percent) as the catalyst at 135"C for 2 hours at different hydrogen pressures is shown in the following table: Hydrogen Pressure (psi) Isooleate (%) Trans-unsaturates (%) 50* 90 < 1 100** 90 < 1 200** 90 < 1 500** 90 < 1 2000"" 90 < 1 * G-W reactor ** AE reactor D.Catalyst Levels The yield of methyl isooleates from the hydrogenation (200 psi hydrogen pressure, G-W reactor) of methyl linoleate at 135"C for 2 hours using different levels of naphthalene chromium tricarbonyl as the catalyst is shown in the following table: Catalyst Level (mole%) Isooleates (%) Trans-unsaturates ( ,ó) 0.5 3.5 < 1 5.0 35 < 1 10.0 70 < 1 20.0 95 < 1 Comparision of Naphthalene Chromium Tricarbonyl to Prior Art Benzene and Methyl Benzoate Chromium Tricarbonyls as Hydrogenation Catalysts.

Naphthalene chromium tricarbonyl was compared to prior art benzene and methyl benzoate chromium tricarbonyl as hydrogenation catalysts. The yield of methyl isooleates from the hydrogenation (200 psi hydrogen pressure, G-W reactor) of methyl linoleate using each catalyst (10 mole percent) at different reaction temperatures for 2 hours is shown in the following table: Reaction Catalyst Temp. ("C) Isooleates (%) Trans-unsaturates (%) benzene 120 0 135 0 - 160 0 - benzoate 1 20 0 135 0 - 160 0 naphthalene 1 20 50 < 1 135 90 < 1 160 95 1-2

Claims (11)

1. A method for hydrogenating the polyunsaturated fatty acid residues of a polyunsaturated fatty acid residue-containing composition, which comprises the steps of: (1) placing in a reactor which has an inert inner surface a polyunsaturated fatty acid residuecontaining composition comprising glycerides, non-glyceride esters, or mixtures thereof, having polyunsaturated fatty acid residues selected from linoleic acid residues. linolenic acid residues and mixtures thereof; and (2) contacting the polyunsaturated fatty acid residue-containing composition with hydrogen at a temperature of from 80 to 160"C in the presence of a catalytic amount of [Cr(A)(CO)3], A being an naphthalene of formula:

wherein R1, R2, R3, R4, R5, R6, R7 and R8 are H, C,C4 alkyl, or C14 alkoxy, to hydrogenate the polyunsaturated fatty acid residues.

2. A method according to claim 1 wherein the polyunsaturated fatty acid residue-containing composition contains at lest 20% by weight of polyunsaturated fatty acid residues.

3. A method according to either one of claims 1 and 2 wherein R1, R2, R3, R4, R5, R6, R7 and RB are H, methyl, or methoxy.

4. A method according to any one of claims 1-3 wherein A is selected from naphthalene, 2-methoxynaphthalene, and 2, 3-dimethylnaphthalene.

5. A method according to any one of claims 1 1 where the temperature during said contacting step is from 120 to 140"C.

6. A method according to any one of claims 1-5 wherein said contacting step is carried out at an hydrogen pressure of at least 50 psi.

7. A method according to claim 6 wherein said contacting step is carried out at an hydrogen pressure of from 100 to 500 psi.

8. A method according to any one of claims 1-7 wherein the amount of (Cr(A)(CO)3) is from 0.5 to 20 mole percent, based on the amount of polyunsaturated fatty acid residuecontaining composition.

9. A method according to Claim 8 wherein the amount of (Cr(A)(CO)3) is from 5 to 1 5 mole percent.

1 0. A method according to any one of claims 2-9 wherein the polyunsaturated fatty acid residue-containing composition has at least 30% methyl linoleate.

11. A method according to claim 4 wherein the polyunsaturated fatty acid residuecontaining composition is a polyunsaturated vegetable oil containing at least 50% polyunsaturated fatty acid residues.

1 2. A method according to claim 11 wherein the polyunsaturated vegetable oil is selected from corn oil, soybean oil, safflower oil and sunflower seed oil.

1 3. A method according to any one of claims 1-12 wherein the inert inner surface of the reactor is glass.