CHAPTER NINE
HYDROGENATION
OF FATS
H. J. DUTTON Northern Regional Research Laboratory, U.S. Department of Agriculture, Peoria, Illinois 61604
CONTENTS Page 351
I. PROLOG II. III.
351
INTRODUCTION COMPOSITION
OF COMMERCIAL
SHORTENINGS
351
IV. GEOMETRIC
ISOMERIZATION
353
POSITIONAL
ISOMERIZATION
357
V. VI.
362
KINETICS
VII.
MECHANISM
OF HYDROGENATION
367
VIII.
SIMULATION
BY ANALOG
370
IX. SOME PRACTICAL
COMPUTER
CONSIDERATIONS
372
X. EPILOG XI.
371
373
REFERENCES
349
HYDROGENATION
OF FATS
H. J. DUTTON NorthernRegionalResearchLaboratory,U.S. Departmentof Agriculture, Peoria,Illinois61604 I. PROLOG ADVANCDIN knowledge frequently follow developments in methodology. In hydrogenation research, technique and result are so inextricably intertwined that it is impossible to discuss what is new without discussing how it came about. In few fields of research can this casual relationship between methodology and new information be more clearly documented. This thesis inevitably becomes a recurring theme for any survey of the hydrogenation process. II. INTRODUCTION Since Sabatier’s discovery of the process for catalytic hydrogenation, some six decades have passed. In 1965, about 3 billion pounds of vegetable oils were hydrogenated in the United States and some 20 pounds consumed per capita. Contrasting with this wide, and apparently unquestioned, acceptance of hardened oils is the facetious observation that if the hydrogenation process were discovered today, it probably could not be adopted by the oil industry. Of course the basis for such a comment lies in the recent awareness of our prior ignorance concerning the complexity of isomers formed during hydrogenation and of their metabolic and physiological fate. Our knowledge of hydrogenation up to 1950 needs only to be covered briefly because it has been so completely reviewed and interpreted by A. E. Bailey. 6 Instead emphasis will be given the newer knowledge developed since that time, particularly geometric and positional isomerization, kinetics, mechanism, and simulation. III. COMPOSITION OF COMMERCIAL SHORTENINGS As background for a technical discussion of the hydrogenation process, one example43 illustrates the complexity in composition of the hydrogenated fats which we consume. Given in Table 1 are analyses on a sample of hydrogenated soybean oil shortening and on a hydrogenated-winterized vegetable oil which probably represent typical commercial products. The analyses in Table 1are those that may be directly determined on unfractionated oils. In schematic form (Fig. 1) is given a flow diagram for a fractionation-analysis scheme. Figure 2 gives detailed analytical information on the double bond distributions within the monoene-cis and tram fractions. From such data it is now possible to calculate a complete analysis of monoenoic fat acids in terms of 351
352
PROGRESS IN THE CHEMISTRY OF FATS AND OTHER LIP1DS
the geometric configuration and position of double bonds. For simplicity and because of biological significance, Table 2 contains only the proportions of cis-9-monoenoate (oleate), trans-9-monoenoate (elaidate), cis-9,cis-dienoate (linoleate), and cis, trans (dienoate). In the lower half of Table 2 is given the unsaturation at all other carbon atoms for corresponding cis and trans configurations of bonds. It would appear that the hydrogenated-winterized vegetable oil Table 1. Composition of Commercial Fat Products by Direct Analysis Hydrogenated winterized vegetable oil
Hydrogenated soybean oil
10.0 3.0 45.6 39.4 2.0 108.5 0.6 34.5 34.3 15.1 71.7
9.7 4.5 63.0 22.8 0.0 87.8 0.4 13.9 12.6 36.8 45.3
Gas-liquid chromatography analysis, % Palmitate Stearate Monoenoate Dienoate Trienoate Iodine value Conjugated diene, % Alkali conjugabl¢ diene, % Lipoxidase conjugable, % Isolated trans, % Azelaic acid on cleavage, mole %
had a maximum of 25.4 % true oleic acid and 30.0 % true linoleic acid compared to 45.6% monoenoate and 39.4% dienoate directly determined (Table 1). The hydrogenated shortening had the maximum of 19.6 % oleic acid and of 12.8 % linoleate contrasted with 63.0% monoenoate and 22.8 % dienoate. Geometric
r Original Methyl EstersI I CCDAcetoeltrile
Argnntatlol
_FIG. I.
Scheme for separation of esters by countcreurrent distribution (CCD). 63
and position isomers of the two naturally occurring oleic and linoleic acids account for differences in corresponding figures of composition. Somewhat comparable oleie and linoleie acid contents were obtained by chromatographic separations of mercuric adducts 4s into monoene-saturates and diene-triene fraction for similar products.
HYDP, OGENATION
353
OF F A T S
With this brief glimpse of what hydrogenation does to structure of the fats consumed in our diets, we now turn to examine the hydrogenation reaction to explain, in terms o f kinetics and mechanisms, the complexities of the geometric and positional isomerization observed. 80-i' [
NWVO cis
NSBO
I I
trans
cis
trons
; I fron s
O]
'°-1
I
n106
8 10 12 14 6
$ 10 12 14 6
8 10 12 14 6 Dlka¢
$ 10 12 14 6
8 10 12 14
F]o. 2. Monoenes derived from a hydrogenated soybean oil (HSBO) and a hydro8enated winterized vegetable oil (HWVO)."s Table 2.
Composition o f Fat Products by Fractionation Before Analysis
Umaturation at carbon 9 cb-Monoenoate tran~Monoenoate c/s,c/s-Dienoate Mono-trana-dienoate Unsaturation of all other carbons c/s-Monoenoate trans-Monoenoate cis,cis.Dienoate Mono-trans-dienoate
IV.
Hydrogenated winterized vegetable oil
Hydrogenated soybean oil
25.4 1.7 30.0
19.6 5.4 12.8 3.1
9.1 9.5 5.0
13.3 24.7 3.9 3.0
GEOMetRIC ISOMERIZATXO~q
Transformation of the naturally occurring cis configuration of bonds to trans in olive oil to yield a solid fat was described as early as 1819 by the French pharmacist Poutet. 56 The nature of the geometric isomerization was not understood until some 55 years later. Today, analogously, margarine and shortening manufacturers use those catalysts and reaction conditions that impart desired plasticity characteristics to their products guided largely by empirical experience. A modern analytical method for assaying, and thus more precisely controlling the trans content of hydrogenated fats, was described by Swern et al. ~ I in 1950.
354
PROGRESS IN THE CHEMISTRY OF FATS AND OTHER LIPIDS
This rapid infrared spectrophotometric method, which measures absorbance at 10.3 microns, is now widely applied and is the basis of an official method of the American Oil Chemists' Society (Cd 14-61). Previously, attempts at the isolation of trans acids were made by fractional crystallization or Twitchell lead salt precipitation. By 1955, Allen and Kiess* were able to study cis and trans Table 3.
Isomerization Characteristics o f Hydrogenation Catalysts lsomerizatlon Description
Platinum oxide 5 % Platinum on carbon 5 % Platinum on alumina 5 % Palladium on carbon powder 47 % Nickel on carbon 23 ~0 Nickel in cottonseed ttakcs 5 % Palladium on alumina 59 % Nickel on kieselguhr 25 % Nickel in hardened oil 65 % Nickel on carbon 25 % Nickel (electrolytic) in cottonseed flakes 40% Nickel on carbon 30% Nickel in hardened oil
trans
Mean trans
%
%
6.6 6.3 7.5 8.2 8.4 8.3 18.2 17.4 18.0 18.1 18.2 18.4 20.5 16.7 19.1 18.7 20.5 19.5 20.0 21.1 21.7 19.7 22.1 19.7 22.8 20.9
6.45
Statistical significance*
7.85 8.35 17.80 18.05
18.30 18.60 18.90 20.00 20.55 20.70 20.90 21.85
* No significant difference exists between the mean values bracketed by lines. isomerization by infrared analysis, as well as to study the migration of bonds in oleic acid. They concluded that the isomers formed a 1:2 equilibrium mixture of cis and trans. Feuge and Cousins 29 comparing a variety of catalysts on methyl oleate concluded that the amount of trans isomers formed was not proportional either to the degree of hydrogenation or the amount of double bond migration. Methyl linoleate with its two cis bonds presents a more compficated isomerization problem. Allen and Kiess* found that nonselective hydrogenation results in low amounts of trans isomers, whereas selective hydrogenation produces
355
H Y D R O G E N A T I O N OF FATS
large amounts. Consideration of these geometric isomerizations for both linoleate and oleate led them to subscribe to the half hydrogenation-dehydrogenation reaction mechanism discussed later. Cousins, Guice and Feuge 21 studied transformations during hydrogenation of methyl linoleate, varying conditions of temperature, rate of hydrogen dispersion, type of catalyst, and catalyst concentration. They concluded that platinum and palladium produced much higher proportions of trans isomers than did nickel. A coordinated picture of bond isomerization was obtained in the course of surveying commercially available catalysts for linolenate selectivity. Platinum, 411
-
40.0--
30.0 --
>-- '!7
i
20.0 --
I0.0
Iselltod I~ ~'+
%>(+'® ooj,x~
h .:+
> ~ d ao '0
FIG. 3.
~s t.
Plot of isolated t r a n s (vertical axis) as temperature of catalytic hydrogenation and log of nickel concentration (horizontal axis) change. 4°
palladium and nickel catalysts were compared by varying parameters of temperature and catalyst concentration and by using an equal mixture of linoleate and linolenate. 4° Although the platinum catalyst described in Table 3 showed the lowest tendency to trans isomerization, followed by palladium, the nickel catalyst showed wide variation in tendencies to forming trans bonds depending on the form and support. The effect of parameters of concentration and temperature upon transformation with this same linoleate-linolenate equimixture system has been studied with an electrolytic nickel catalyst at atmospheric pressure conditions. The three-dimensional plot, shown in Fig. 3, agrees with observations in the literature that trans content increases with increasing temperature and is, to a lesser extent, affected by the catalyst concentration. The points for which data are given represent experimentally feasible conditions, i.e. those which result in reaction times between 5 min and 1 hr. Whereas experimental techniques to separate mixtures of isologous and homologous fatty acids have been well worked out, the same was not true for cis and trans isomers until recently. For this analysis countercurrent distribution, liquid-liquid extraction, and gas chromatography have all been employed with success. Although fractional crystallization is surprisingly effective in separating
356
P R O G R E S S IN T H E C H E M I S T R Y OF F A T S A N D O T H E R L I P I D S
pure elaidic acid from oleic acid, attempts to separate cis and tram isomers in the presence of position isomers give indeterminate results. Thus, compounds containing either cis bonds near the terminal methyl group or t r a m bonds in any position generally have high melting points and crystallize out together. Usually, fractional crystallization merely gives t r a m enrichments. ~9 With diene isomers, co-crystaUizations impair resolutions. On an analytical scale, 200-ft capillary gas chromatographic columns have been required to separate cis and t r a m isomers of oleate, linoleate, and linolenate. 1 8 . s 1, 6 6 . 6 Argentation systems in countercurrent distribution supplied the first, generally successful, separation procedure for cis and trans monoenes and for cis,cis,
J is I0
Cndlon Atoms In Dlbosk Add
FIo. 4. Dibasic acids from cis monoenes of hydrogenated methyl linolenate. Triplicate valuesfor each acid are shown.65 cis, trans, trans,cis, and trans,trans dienes. 2~ The applicability of this argentation
system and countercurrent distribution to the analysis of commercial fats has already been cited. .3 This system has the peculiarity of separating monoenes and dienes on the basis of both configuration of double bonds and position of double bonds in the chain. Thus, 9,15-cis, cis-octadecadienoic acid because of the increased argentation of widely separated bonds has nearly the same partition coefficient as does linolenate. 66 These argentation systems operate by forming pi complexes; the silver ions have a greater affinity for the cis than for the t r a m configuration of ethylenic bonds. The use of silver nitrate has been described for adsorption chromatography by De Vries, 77 later by Barrett, Dallas and Padley 8 for thin-layer chromatography, and more recently for an ionic exchange resin by Emken et aL 2a Of these chromatographic methods, Emken's system has the advantages of re-use, large capacity, and being amenable to refractometric monitoring, t 7 By means of argentation procedures, separation of isomers and the fractionation of hydrogenation products are carried one more step beyond that possible
H Y D R O G E N A T I O N OF FATS
357
with reversed phase columns alone. These chromatographic methods permit separation of the homologous series by length of carbon chain, and of the isologous series by number of double bonds, but do not separate the isomers within the isologous series differing in position and geometric configuration of bonds. An example of the application of the analytical procedure (Fig. 1) exploiting argentation to hydrogenated methyl linolenate and to the determination of bond isomerization and bond migrations is shown in Figs. 4 and 5. 6s It may be seen that the double bonds in the cisfraction tend to remain in their natural 9-, 12-, and 15-positions, whereas in the trans fraction the bonds are scattered widely up and down the carbon chain. This same conclusion has recently been reached for methyl oleate by Subbaram and Youngs ~° using silver nitrate in thin-layer 25
ii Col*boil Atoms Io Olbilslc Acids
FIO. 5.
Dibasic acids from t r a n s - m o n o e n e s of hydrogenated methyl linolenate. Value for each acid from runs 1, 2, and 3 are shown from left to right. 6s
chromatography and studying the hydrogenation of methyl petroselenate, methyl oleate, and methyl erucate. Their results also concur with those by argentation countercurrent distribution and show the ratios of trans to cis were greater than the two-to-one ratio found previously. It appears then, that the reactions of geometric isomerization are inherently related to those of positional isomerization. V.
POSITIONALISOMER/ZATION
In contrast to the virtual absence of techniques for separating and analytically determining geometric isomers before 1950, methodology was early available, though inadequate, for determining positional isomerization. Oxidative cleavage procedures were used historically to determine structure and the double bond locations in oleic and other fatty acids. The problem of modern research lay then, not so much in the approach of oxidative cleavage, but in the quantitative separation of those multiple fragments generated from catalytically hydrogenated fats whose double bonds have been distributed up and down the carbon chain. The story of advances in the field of positional isomerization, then, is a B
358
PROGRESS IN THE CHEMISTRY OF FATS AND OTHER LIPIDS
story of improvement in the quantitation of procedures for isolating and separating cleavage products. In 1913, Lewkowitch suggested that isoleic acid, formed during catalytic reduction, was a positional isomer of oleic acid. Formation of isomers with unsaturation at the 8,9- and 10,11-positions was indicated by Moore in 1914, by Hilditch and Vidyarthi in 1929, and by Bauer and Kallis in 1934, but their results must necessarily be considered as preliminary, lacking the certainty of correct qualitative identification and requiring greater quantitative resolution of cleavage fragments. At the time of Bailey's review in 1950,~ it was very apparent to workers in the field that considerable migration of the bonds was occurring during the hydrogenation of fats, and this knowledge was indicated by laborious isolations with inadequate fractional crystallization procedures. It was not until liquid-liquid partition chromatographic procedures developed by Gordon, Martin, and Synge a3 were used for separating the oxidatively cleaved dibasic acid fragments 9. 35 that the true extent of bond migration became known. A series of papers then flowed into scientific journals. Knegtel et al., . 5 Boelhouwer et al., t 2 Allen and Kiess a and Feuge and Cousins 39 described bond migration in methyl oleate. The shifting was observed to be equaUy toward and away from the carboxyl end of the molecule at the early stages of hydrogenation. It results in a "normal" distribution, which is broadened with increasing temperature (Fig. 6) or with increasing degree of hydrogenation. These observations of symmetry of bond distribution, however, are currently disputed. Subbaram and Youngs 7° find the distribution for methyl petroselenate, oleate, and erucate is symmetrical, but Allen 2 finds that bonds tend to accumulate away from the carboxyl group, as shown in Fig. 6 for the highest temperatures. He proposes that the greater reactivity of bonds in the 2-position from the carboxyl groups results in their selective reduction and, therefore, an apparent accumulation of double bonds toward the alkyl end of the fatty acid. The positional isomerization during hydrogenation of methyl linoleate has been studied by Allen and Kiess, 4 Takemura and Goldblatt 73 and Cousins. 22 Similar migrations of bonds and geometric isomerizations were observed for methyl linoleate but are necessarily more complex than those in methyl oleate. The higher the temperature and the more selective the conditions of hydrogenation, the greater the migration. The bimodal distribution for the naturally occurring linoleate bonds broadens as the hydrogenation proceeds finally to form a single peak distribution. 22 For the hydrogenation of methyl linolenate and linseed oil, considerable attention has been given isolinoleic acid. This acid, experimentally defined as one whose double bonds are nonconjugable by alkali isomerization under the spectrophotometric method of Mitchell, Kraybill, and Zscheile, 53 was believed to be 9,15-cis,cis-linoleic acid. As late as 1951, Rebello and Daubert 57, 5s studying the structure of isolinoleic acids from methyl linolenate had to rely upon fractional crystallization and the Twitchell lead-salt-alcohol method
359
H Y D R O G E N A T I O N OF FATS
which they found unsuitable due to the presence of "solid isolinoleic" acid in the "solid" acids separated by this procedure. They did, however, find evidence for three isomers, 8,14-, 9,14-, and 10,14-isolinoleic acid, of the 15 possible positionally isomeric acids and thus elucidated the composition of Lemon's earlier isolinoleic acid, s° which he presumed to be the 9,15 isomer. By contrast to these uncertainties of identification, 9,15-1inoleic acid has now been prepared in high purity by argentation countercurrent distribution from the diene fraction of hydrazine-reduced linolenic acid. On selenium isomerization those bonds of cis,eis configuration can be separated from the trans,trans and from the cis-trans and trans,cis. ~ s 8O 7O
1
SO SO
•,c 30 2O 10
o[
7
I 9 10 II 12 Positlu of Dtukle Buds
13
14
1 ~ . 6. Distribution of double bonds at different hydrogenation temperatures for methyl oleate: Run 1, 90°C; run 2, II0°C; run 3, 170°C; and run 4, 200°C. 29
Determining double bond positions in monoenoic fatty acids is a problem quite simple compared to that of determining bonds in polyunsaturated fatty acids. While the bond closest to the carbonyl group is definitely located on the basis ofanalysis of dibasic acids produced by oxidative cleavage, the location of a second bond demands that in addition the monobasic acids originating with the alkyl end of the molecule be recovered quantitatively. In the older structure determination for fatty acids, little attention was given to the percentage of theoretical recovery for a particular split product. Mere isolation and identification of products sufficed to establish structure. However, when unsaturation is distributed in all possible positions up and down on the chain in varying degrees, yields cannot be ignored, and quantification of cleavage and recovery is mandatory. The structure of monoenoic acids may now be determined by a variety of methods involving periodate permanganate 61 or ozone 63 and the split products may be recovered and assayed as acids, aldehydes, or alcohols. The analysis of the cleavage products may be by liquid-liquid or gas-liquid phase chromatography.
360
P R O G R E S S IN THE C H E M I S T R Y OF FATS AND OTHER L I P I D S
A quantitative procedure applicable to a mixture of dienoic acids has been developed at the Northern Laboratory and tested in many applications. It involves oxidative cleavage with periodate permanganate; performance of all solvent evaporation steps under alkaline conditions to retain the volatile shortchain acids as soaps; conversion of the salts to methyl, ethyl, or butyl esters as may be dictated by the volatility of the fragments; and quantitative determination of the monofunctional and difunctional cleavage products by temperatureprogrammed gas chromatography. 41 A microozonolysis procedure for the quantitative determination of unsaturation has been described that employs a stainless steel loop reactor held in and
Ft~. 7.
Microreactor for ozonization pyrolysis chromatography." 3
1. Stainless-steel needle, 20 gauge. 2. Copper-encased stainless-steel U-tube, ~ in. o.d. 3. Thermocouple attachment. 4. Swagelok tee. 5. Silicone septum fitting. 6. Side-arm connection to 6-way valve. 7. Soldering gun. 8. Manual on-off switch.
heated by a soldering gun 2a (Fig. 7). Samples of 0.5 to 5 ~1 in size may be successively ozonized, thermally cleaved, and injected into a gas chromatograph without sample transfer and attendant losses. Some of the newer physical methods of analysis promise rapid procedures for determining double bond location. Nuclear magneticresonance gives much information directly upon unfractionated molecules. Among the important analyses provided by the spectrum for methyl linolenate, illustrated in Fig. 8, are the total unsaturation, equivalent to iodine value from the measurement of the olefinic protons; a measure of the amount of pentadiene structure from dialpha-olefinic methylenes; i.e., equivalent to alkali-conjugation spectrophotometry; and from the total proton content, the average molecular weight either of glycerides or of the derived esters. 37 Although the determinations as listed may be made by other analytical procedures, the advantage of nuclear magnetic resonance resides perhaps, then, in its speed and ease of application. However, resonance measurements do provide one unique analysis not otherwise
HYDROGENATION
361
OF FATS
available or with difficulty; namely, a direct display of the amount of 15,16 double bond present. This method depends on the shift downfield of the absorption by terminal methyl protons which have beta-olefinic protons. 32 Fractionation methodology for geometric isomers is relatively adequate for the isolation and analysis of hydrogenated fats as explained above, but procedures for the separation of positional isomers in unsaturation are essentially ''Y
:
':
"
•
?--
'~
-"
:
7
-1
'~
~.,,, I , *~
t
V
--
/
[]
,
•
^/
/_,,,,~__.z" ......
.__M.. 0
+
I0
I
,,
~/ _ ." 8.0
7.0 IS
Symbol I
O
6.0
5.0 12
4.0
prlt 161 .. 9
Strmctwro ~ - Olofinlc motl0yl protons
PPN II
He.Protons
O.t7
3 6
O
a - Oloflnlc motbylono protons •
'/.25
A
DI .a. oloflmlc m0otkllomo protoos
3.80
4
]
01ofloic protons
ca 5.38
6
V
Metl0onty protons
3.67
3
Insolatod 0notl0yleoe protoos
1.33
10 32
• Also Ioclodos nnotlmylooo edle¢oot to torbooyl FIG. 8.
Nuclear ma~ctic resonance speetrurn of methyl linolenat¢ with key to protons and resonating groups. 3s
nonexistent. In those rare instances in which isomers have been isolated from mixtures, repeated recrystallizations are performed with or without complexing agents and are accompanied by inevitable cocrystallization, inclusions, and the resultant loss of quantitation. Even by chromatography on a silver-treated macroreticular resin the separation of 15-cis-octadecenoate from 9-octadecenoate requires repeated passes through a column. 64 The admitted lack of adequate methodology for separating positional isomers in unsaturation does not, however, preclude the possibility of their analytical determination. Illustrative of the applicability of recent developments in
362
PROGRESS IN THE CHEMISTRY OF FATS AND OTHER LIPIDS
methodology for double bond location and of the new information provided are the data for hydrogenated methyl linolenate given in Figs. 4 and 5. ~s From these data a complete analysis of individual monoenoic isomers may be calculated. Thus one may specify how much of each isomer at a given bond position and configuration is present in the mixture. Data in Figs. 4 and 5 were obtained from hexane-acetonitrilc and hexane-methanolic silver nitrate systems as successive immiscible solvent pairs in a countercurrent distribution operation. It is apparent for the three runs represented, which employed 0.5 ~o nickel-onKieselguhr catalyst, atmospheric hydrogen pressure, and 140°C, that in the cis-monoenes (47.4 ~o), bonds remain in their original naturally occurring 9,12and 15-positions whereas in the trans-monoenes (52.6 ~), bonds are widely distributed up and down the carbon chain. VI.
KINEIaCS
Whereas the magnitude of absolute reaction rate constants tells whether a given reaction will be completed within practical time limits, it is the ratio of specific rate constants for the concurrent and consecutive reactions that determines the character of the product mixture. In a simplified example from fat hydrogenation, it is the ratio of the rate for saturation of the ethylenic double bonds to the rate at which the double bonds isomerize from cis to trans configuration that affects the plasticity of a shortening. As a second illustration, much attention in practical hydrogenation processes is given to the relative rate with which linoleate is hydrogenated to oleate compared with the rate by which the oleate goes to stearate. Frequently the operator searches for conditions which will give him maximum oleate concentration with minimum saturates present. Much confusion surrounds the term "selectivity". To the commercial manufacturer it connotes melting point plasticity characteristics which are indirectly related to relative rates of production of saturates and trans isomers and the rate of reduction of polyunsaturates. In this chapter the rate reduction of linoleate to oleate is referred to specifically as linoleate selectivity. There is another important aspect of selectivity stemming from the need to remove linolenic acid from soybean oil to improve its flavor stability. This kind of selectivity, the relative rate of linolenate hydrogenation compared to the rate of linoleate hydrogenation, is referred to here as linolenate selectivity. It becomes apparent, therefore, that a description of a hydrogenating system can be made in terms of specific reaction rate constants and that the specification of rate constant ratios is in essence a specification of the product. The kinetic description of the consecutive reactions of hydrogenation was the subject of the last of a series of papers written by A. E. Bailey. 7 He summarized the knowledge of the kinetics of hydrogenation well in a chapter on hydrogenation in 1950. 6 Disagreement as to the order of the rate of reaction existed before and at the time of his work and persists today. The observed zero-order rates are generally explained as being for those reactions that are limited by hydrogen solubility or rate of stirring. When such reactions occur, plots o f iodine value versus hydrogenation time, which are linear on semilogarithmic
H Y D R O G E N A T I O N OF FATS
363
paper, are thought to reflect the first-order rate of formation of the adsorption complex between catalyst, hydrogen, and fatty acid. The decreasing rates of hydrogenation with time, which are frequently observed, are considered to reflect the changing fatty acid composition of the hydrogenating mixture and some forms of catalyst deterioration or poisoning. Thus Swicklik et aL ~2 studying the hydrogenation of triolein, found it necessary to multiply the rate constant by a function of time (1 + wt) to describe progressive deterioration of the catalyst. By means of infrared analyses and hydrogen uptake they proposed to define a system in which oleate isomerizes reversibly to trans configuration and in which both cis and trans forms reduce to stearate. Similarly, Vandenbeuvel ~5' 76 studied the hydrogenation of methyl oleate and linoleate, but his interpretation attempted to make use of the knowledge that the reactions involved take place on the surface of the catalyst and as such are governed by the laws of adsorption. To avoid the considerable amount of analytical work involved in the time-course sampling procedure described and used so successfully by Bailey and his coworkers, Vandenheuvel attempted to interpret directly the kinetics of hydrogenation by observing only the rate of uptake of the hydrogen. Boelhouwer et aL ' 3 presented a mathematical treatment for the hydrogenation of linoleate to describe graphically linoleate selectively. Considering the reaction kz
Linoleate
k2
~-Oleate
~ Stearate
he developed the limits for (a) nonselective hydrogenation, i.e. k2 is Very large compared to kl ; for (b) selective hydrogenation, kl is very large compared to k2; and for (c) a random hydrogenation, where the ratio rates are proportional to the number of double bonds present, namely 2 to 1. A grid with these limits and with intermediate ratios of kl to k2 was presented. Plotted on this graph were the data for the hydrogenation of linoleic acid with various catalysts and concentrations of catalysts. This procedure gives a rapid and visual measure of linoleate selectivity of the catalyst. A simplified procedure of evaluating catalysts for linolenic selectivity has been devised, 2s based on kinetic theory. It requires the analysis for linolenate after reaction of 0.5 mole of hydrogen with an equal mixture of methyl linoleate and methyl linolenate. This degree of hydrogenation (0.5 mole) corresponds to the crossover point between oleate and linolenate curves, and in fact to minimize error, values for linolenate and oleate determined by gas-liquid chromatography are averaged. The ratio of specific reaction rates is then read directly from a mathematically derived curve. Based on this procedure, commercial catalysts composed of platinum, palladium, and nickel metals were evaluated for linolenate selectivity.39 These data are plotted in Fig. 9 against the percentage trans formed in the same samples and indicate that selectivity and isomerization characteristics of catalysts are not significantly correlated. The effect of the parameters of concentration of catalyst and of temperature was investigated with an electrolytic nickel catalyst chosen for its high selectivity
364
PROGRESS IN THE CHEMISTRY OF FATS AND OTHER LIPIDS
and relatively low yield of t r a n s products. Influence of the parameters on the t r a n s content, shown in Fig. 3, demonstrates the expected isomerizing tendency with temperature. As anticipated, rate of hydrogen adsorption increases with 24
® ® ®
20
®
®
®
r-
_o o []
8
® Nickel ~ Palladium [] Platinum
[] [] []
i
I 1.0
[
,
l
i
I 2.0
i
l
n
I
3.0
K = ktdkto
FIG. 9. Plot of iinolenate selectivity ratio K to the percentage isolated trans in
methyl esters for various hydrogenation catalysts. 39
~.0
1.1 0.$ 0.4
l
i
....
•
...-"
~
oo
oo~
os Fxo. 10. Plot of linolenate selectivity ratio K (vertical axis) vs. temperature and log of nickel concentration (horizontal axis) change. 4° both increasing catalyst concentration and increasing temperature. Unfortunately, the linolenate selectivity diagrammed in Fig. 10 appears to be little influenced by either temperature or the catalyst. "°
365
HYDROGENATION OF FATS
Bailey 7 presented kinetic evidence that the hydrogenation of linolenate proceeds by the following scheme: Linoleate Linolenate
> Oleate
~,Stearate
Isolinolate which implies that the reduction reaction is more complex, rather than being the simple kinetics of consecutive reactions: Linolenate -----> Linoleate - - ~ Oleate -----> Stearate The scheme also suggests that account must be taken of the pathway oflinolenate through isolinoleate to oleate, and of the direct reduction from linolenate to oleate. Confirmation of his theory is to be found in the data of Scholfield et al., 6s who used the equal mixture of linoleate and linolenate but added a radioactively C14-1abeled methyl linoleate. The resultant kinetic data of both radioactive and inactive molecules can be fitted only if one calculates that isolinoleate is formed and that the "oleate shunt" occurs (Fig. 11). It was calculated that 30~o of the linolenate goes through linoleate to oleate, that 41 ~o passed through to isolinoleate, and that 29 ~o went directly from linolenate to oleate. The "oleate shunt" results from linolenate, once adsorbed on the catalyst, being hydrogenated, desorbed, readsorbed, and reduced yet a second time. Alternatively, adsorbed linolenate might experience multiple reduction before desorption. By contrast, the hydrazine reduction of this mixture did not require the postulation of any linolenate to oleate shunt in order to fit data from both inactive and radioactive materials. The relative reactivities of double bond positions in the carbon chain have been the subject of considerable conjecture and conflicting experimental data. It is generally believed that the rate of hydrogenation increases as the double bond is moved farther away from the carboxyl group. Certain data of C0usins 21 on the relative rates of hydrogenation of 9 and 12 bonds in linoleate support this view. During the Wilzbach noncatalytic tritiation of methyl linoleate, the ratio of reaction rates of the 12,13 to the 9,10 double bond was 1.4 to 1.42 Most recently Allen 2 studied the rate of hydrogenation of a mixture containing methyl petroselenate, methyl oleate, and methyl cis-12,13-octadecenoate; however, he found no difference in the rate for two different hydrogenation catalysts. Kinetic problems involving reactivities of intermediates, isomers (positional and geometric), and isologs are particularly amenable to the radio-tracer approach. Obtaining these data is greatly facilitated by a technique of microvapor-phase hydrogenation.2~ This procedure provides a variety of kinetic information with great speed. The micro-hydrogenator comprises an accessory which is substituted for the nut and septum of the injection port of a gas chromatograph. Hydrogen used as the carrier gas passes first through the small reactor
366
PROGRESS
IN T H E C H E M I S T R Y
OF F A T S A N D
OTHER
LIPIDS
column packed with catalyst and then into the chromatographic column. Samples may be injected with a microsyringe at specified distances in the catalyst column thereby varying the column depth. Depth of column thus also has the dimension of reaction time. The partially hydrogenated sample issuing from the microvapor hydrogenator is immediately led into the chromatographic column where it is separated into its components that are analyzed by thermoconductivity and radioactivity detectors. I00
8O
70
i
;"
60-
"5/unL'"N'" •
d
I
I
i.i.
lrt
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^ 2U ~
,• • s" •
.. ,!1" oo ql,. 'li • ¢ 2.5
,"
,,~e~ •
....... •
,
•
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'.
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,Xo.oo*."
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., .-°",,.
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000
IAo.oono
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•
• sS
/
era
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0/0
......... .
...... . •
I
"~ Dlomo"
] .............. Trlome
2.0
1.5
Number of Deiblo hmds
FIG. 1l. Catalytic hydrogenation of a linolenate-linoleate mixture with C t 4 laheled linoloatc calculated on the basis that isolinoleate is formed and that an oleate shunt OccurS. 42
The kinetic pattern for the liquid-phase hydrogenation of methyl linoleate and methyl linolenate shown in Fig. 11 required several weeks to record. By contrast, the data of Fig. 12 were collected in the course of an afternoon's work with the microvapor-phase hydrogenator. 54 Differences between kinetic patterns for the nickel catalyst of Fig. 12 and for the copper chromite catalyst of Fig. 13 are readily apparent. In the first place, copper chromite will simply not hydrogenate beyond the monoene level and the isolinoleic acid formed, i.e. 9,15-octadecadienoate with its isolated double bond, is as unreactive as is oleate. Secondly, the linolenate selectivity of the copper catalyst is approximately twice that for the nickel catalyst. This striking reactivity for linolenate over linoleate is that very selectivity characteristic being sought so diligently to solve the commercial problem of stabilizing soybean oil. Further, by
HYDROGENATION
367
OF F A T S o
100
/
/ woos
/
/
/
/
/
/
/
/
/
/
A/
/
t Saturate/
/f
/ Saturate,•
Q
/
I
js*
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2.5
,.
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s"
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®
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2.5
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Avoralo No. of Donble Bonds
Fzo. 12.
Ilass
100
0,,.\
/
/" /...--
Monocle/
/
ill
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2.5
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/ f 1.0
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2.0 Averap He. of Donliin Bonds
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FIn. 13. Kinetic curves simulated and drawn by an analog computer for data from microvapor-phase hydrogenation over copper chromite catalyst (left--unlabeled methyl linolenate, right--labeled methyl linoleate). 5" removing linolenic acid and yet not producing the saturated fats, a salad oil nonclouding at refrigerator temperature is produced with potentially greater oxidation and flavor stability.
VII. M E C H A N I S M OF H Y D R O G E N A T I O N Concepts of the hydrogenation mechanism have been refined since Bailey's review.6 His description of the mechanism involved the simultaneous adsorption of hydrogen and fattyacid double bond on an active sitcof thc catalyst.
368
P R O G R E S S IN THE C H E M I S T R Y OF FATS A N D O T H E R L I P I D S
In 1950, Blekkingh 11 suggested how by partial hydrogenation and partial dehydrogenation (steps shown below)a double bond can shift along the whole chain rather quickly.
-- CH~.CH--CH2--CH2
-
CH~--CH~CH--CH
2-
--CH2--CH2--CH
~CH--
He also proposed a concerted mechanism for the elaidinization of fatty acids along with bond migration. Allen 4 described the process as the half hydrogenation-dehydrogenation and presented data in its support. Since the halfhydrogenated form is believed to possess free rotation, it is apparent in reforming the bond by dehydrogenation that (a) the bond may be formed in a different position than originally found, and (b). the bond may be of either cis or trans configuration. This theory was also used to explain double bond migration and geometric isomerization in methyl linoleate. An e~lier mechanism of catalyst action proposed by Horiuti and Polanyi in 1934 a6 provides the concept of mono- and diadsorbed species such as the half hydrogenation-dehydrogenation theory would require. This mechanism enjoys popularity among catalyst chemists, particularly in the hydrocarbon field 14 and ha~ recently been invoked to explain tritium, hydrogen, and deuterium exchanges during catalytic reduction of methyl oleate. When 9-octadecenoic acid-Ha-9,10 is reduced with hydrogen, radioactivity appears in gas phase. ~° When a catalytic deuteration of methyl oleate is carried out, hydrogen is observed to be released into the deuterium gas phase by mass spectrometry. Within the deuterated stearates formed, individual species may have from 0 to 30 ator~s of deuterium exchanged for the hydrogen atoms attached to the carbon. 24 Even more surprising was the observation that the methyl octad~cenoate recovered after hydrogenation was only half completed and had deuterium contents ranging from 0 atoms up to 10 atoms per molecule. 6° Thus exchange but not hydrogenation was taking place during catalytic adsorption and desorption. An extension of the Horiuti-Polanyi mechanism as applied to catalytic deuteration is presented in Fig. 14. It serves to show (a) how deuterium may be adsorbed and exchanged without catalytic hydrogenation, (b) how the bond migrates up and down the chain, and (c) how cis and trans isomers containing a .distribution of deuterium may be desorbed from the catalyst. It also explains how the methyl stearate may have more than two deuterium atoms present and how even with 100~o deuterium in gas phase some stearate molecules may contain no deuterium. A concerted and systematic study of bond migration, geometric isomerization, and deuterium distribution during the catalytic deuteration of methyl oleate has been made. 62 Interpretation and digital computer simulation of the reactions are based upon a further extension of the model depicted in Fig. 14.
HYDROGENATION OF FATS
369
From consideration of the Horiuti-Polanyi mechanism it would appear that it is not obligatory for polyenoic acids to go through a conjugated intermediate as earlier proposed. 3° The small amount of conjugation detected is not necessarily an argument against its involvement in the hydrogenation. It may be that the conjugated isomer reacts and hydrogenates much more rapidly than the unconjugated form and thus its small composition is no indication of its importance as an intermediate. A definitive answer may be provided by labeling experiments. II
I0
9
"(i)
I
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,,,,
, Y
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, ,
"xM
I¢is )
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FIG. 14. Extension of Horiuti-Polanyi mechanism applied to catalytic deuteration of methyl oleate.6°
Conjugated dienes, however, are apparently not a major pathway for the homogeneous hydrogenation of linoleic acid with iron pentacarbonyl. 3x Instead, the conjugated diene complexed with iron carbonyl is not only catalytically active, but is itself reduced to yield a monoene. It is hoped that these homogeneous catalytic reducing systems will comprise model systems for heterogeneous catalysis in which intermediates may be added, isolated, and characterized. Halpern 34 has postulated the following reaction sequence shown in Fig. 15 for the reduction of fumaric acid with ruthenium chloride. In this mechanism the half-hydrogenated form is strongly reminiscent of the monoadsorbed species of the Horiuti-Polanyi mechanism. The homogeneous reduction of fatty acids with hydrazine differs in mechanism so greatly from metalcatalyzed homogeneous and heterogeneous catalysis 6s as to have little value as
370
PROGRESSIN THE CHEMISTRY OF FATS AND OTHER LIPIDS
a model; rather hydrazine reduction comprises a contrasting example of corn= plete nonselectivity of attack, i.e. the specific reaction rates are proportional to the number of double bonds present in the fatty acid and the attack at any specific bond of the three in methyl linolenate is relatively undirected or nonselective. In homogeneous hydrogenation with pentacyano cobaltate) apparently two conjugated bonds of t r a n s configuration conjugated with a carbonyl or a third double bond are required for reduction to take place, e.g. sorbic acid. ? s With sorbic acid, 95 % of the hydrogen attack is at the 4,5 rather than the 2,3 bond. s2 This reaction exemplifies the degree of selectivity required for the practical solution of the pressing flavor problem of soybean oil. , ~.e
TuB.,, )
T
'
)
',,
+li+
B\ ==c/A + A:
\~ ifast)
FIG. 15. Mechanism for homogeneous hydrogenation of fumaric acid with ruthenium catalyst, s"
VIII. SIMULATIONBY ANALOG C O M P U T E R From the review of current research, one is impressed with the complexity and multiplicity of lines of new evidence on mechanisms now emerging. Simplifying theories are needed to correlate the diverse information--a model such as was formulated by Bailey when he considered the complexity of the effects of hydrogenation parameters upon catalyst performance. Looked at, as he suggested, from the viewpoint of surface coverage on the catalyst particle, temperature, hydrogen pressure, influence of stirringrate, and catalyst concentration all fallinto unified perspective. N o w the Horiuti-Polanyi mechanism as extended collates observations of deuterium exchange and of geometric and positional isomerization. It may well be that future simplificationmay come from analog and digitalcomputer simulation of thismodel. Indeed, thisis one function of simulation, to extend processes of reasoning through and beyond human limitations for extrapolation. Simulation of reaction kinetics on an analog computer consists of first setting up a hypothetically chemical model of the mechanism which one believes to exist. If this model can be mathematically expressed, the second phase, constructing of the electricalanalog model, and the third phase, programming of the computer, are greatly facilitated.Fourth, one allows the computer to run, drawing the kinetic curves as specified by the rate constants that the operator
HYDROGENATION OF FATS
371
manually adjusts. These constants are altered until the curves drawn by the computer pass through and approximate the experimental data. If the data can be satisfactorily matched, one gains confidence that one's model is compatible with the chemical system. One then reads out the values of the specific reaction rate coefficients which gave the desired match. For nickel catalysts all pathways shown in the model given below were used to some degree; for the copper chromite catalyst the model scheme and the corresponding rate coefficients were as follows: s 4 ~, Linoleote Linolenate /
k~
k7
-x
~ Oleale k.k~.steorote
7 . k,, ke.k3.k,,~, k6.~,*8.*3 :0.0
ke =i.0
k~ =l.S6 k4 =).06
Simulation of the kinetics of these consecutive reactions of hydrogenation on an analog computer facilitates complicated calculations and determines specific rate constants as already demonstrated. 16' s4 Albrecht and Wisniak 1 programmed an analog computer for the hydrogenation of methyl oleate and arrived at a specification of selectivity in terms of specific rate constants for isomerization of trans to eis and cis to trans and for hydrogenation of trans to saturate and cis to saturate. With an analog computer the determination of specific rate constants and their ratios is readily performed in a matter of minutes, a calculation which would require days of work by arithmetic trial-anderror procedures. Further, proposed mechanisms of reaction schemes may be tested, retained, or discarded depending on their conformity to experimental data, or required modifications of the schemes may frequently be made and tested "on the spot" with only minor changes in the program. The experimental design for either liquid phase or the microvapor-phase systems, 68 which incorporates two individual fatty acid components, one radioactively labeled, permits comparing rates under identical reaction conditions. This design and the use of the analog computer goes far toward the previously stated objective of characterizing hydrogenation catalysts in terms of the specific rate constants for the reactions they promote. I'X.
SOME PRACTICAL CONSIDERATIONS
The "hardening of fats" by hydrogenation remains a largely empirical process in which the manufacturer is primarily guided by the requirements of the product in terms of its inherent physical properties. To produce such diverse materials as margarine stock a vegetable shortening, a plastic shortening, or a liquid cooking oil, he must control a variety of parameters in the reaction. These may
372
P R O G R E S S I N T H E C H E M I S T R Y OF F A T S A N D O T H E R L I P I D S
include the particular properties of the hydrogenation catalyst itself, its metal composition, its isomerizing tendency, its support, and the method of preparation; time and rate of reaction; degree of hydrogen absorption; pressure; temperature; stirring rate; and catalyst concentration, to name a few. In Europe diverse animal, fish, and vegetable oils serve as starting materials and are blended with a variety of hydrogenated stocks to give the desired physical properties and economic advantages to the marketed product. In the United States the practice is to hydrogenate a single oil or limited mixture of oils to a hardened stock which unblended has the desired physical properties. The American practice stems necessarily from the fact that only soybean and cottonseed oils are produced in sufficient volume to supply domestic demand. Whatever specific advantages the minor oils----corn, safflower, sesame, or sunflower--may have to the consumer, production statistics stipulate that the nation shall be fed primarily on soybean oil and to a lesser extent with cottonseed oil. The current trend toward greater use of liquid shortenings, salad, and cooking oils has raised a major problem for catalyst research on soybean oil. Whereas in well-hardened products the 7 to 8 % linolenic acid of soybean oil is largely converted to stable monoenoic and dienoic isomers, in unhardened or mildly hydrogenated soybean oil sufficient linolenic acid may remain to pose a stability problem for higher temperature cooking oil applications. The solution to this problem is simple in theory but difficult in practice. The answer is to reduce the 15,16 double bond selectively thus converting linolenic acid to linoleic acid. In practice bonds other than at the 15,16 position are reduced also, and the residual bonds are isomerized both in position and geometric configuration. Furthermore, if the hydrogenation process is to be most effective, it not only must selectively attack linolenyl groups, but also must not produce saturated fat (high linoleate selectivity), which will cause clouding of an oil stored in household refrigerators or which will require the manufacturer to crystallize out these components before packaging. Much of the basic research already described was occasioned by the need for more fundamental information on the hydrogenation reaction in order to produce a stable liquid cooking oil from soybeans. Surveys of commercially available hydrogenation catalysts 39. s 9.79 indicate that none possess the required linolenate and linoleate selectivity. Simulation on an analog computer showed that to produce an oil by hydrogenation alone comparable in linolenic acid content (ca. 2~o)to commercial salad oil from hydrogenated winterized soybean oil, a linolenate selectivity of 4 is required. 1s The linolenate selectivity of commercial catalysts ranges in the area of 1.5 to 2.5. Concurrently in the United States, The Netherlands, and Japan, research is underway to investigate selectivity of copper-containing catalysts. Such catalysts were first described by Adkins and have already found commercial use in conversion of glycerides to fatty alcohols and in hydrogenation of acetylenes and diolefins in the presence of monoolefins, s Patents have been issued in Japan, in Britain, as well as in the United States, for reducing polyunsaturates in fish oils and in soybean oil. 19. 20.49.74 Work in The Netherlands 44 confirmed a high selectivity for copper-containing catalysts as also did work in the States. 46' 5s
HYDROGENATION OF FATS
373
Simultaneous studies in both the United States and Holland continue to support the efficacy of selective copper catalysts in improving the flavor stability of liquid soybean cooking oil. ss. 47 Linolenate selectivity ratios of 10-14 are observed and virtually no saturate is formed. If these laboratory operations are confirmed and established as practical on commercial scale, a selective hydrogenation catalyst has been found and stable liquid soybean oil not requiring winterization may become increasingly used in the world dietary patterns. X.
EPILOG
Along with the prolog, the homologs, the isologs, and the analogs, an appropriate epilog may conclude that: "Every advance in scientific knowledge is first an advance in technique". XI.
REFERENCES
1. ALBRIGHT,LYLE, F. and WIS~AK, JAIME, J. Am. Oil Chemists' Soc. 39, 14-19 (1962). 2. ALLEN,R. R., J. Am. Oil Chemists' Soc. 41, 521-523 (1964). 3. ALLm%ROBERTR. and K I ~ , ARTHURA., J. Am. Oil Chemists' Soc. 32, 400---405 (1955). 4. ALLEN,ROBERTR. and Kmss, ARTHURA., J. Am. Oil Chemists' Soc. 33, 355-359 (1956). 5. ARNOLD, MELVXN R., U.S. Patent No. 3,098,882 (July 23, 1963). Company, Chemetron Corp., Chicago. 6. BArnEY,A. E., Industrial Oil and Fat Products. Interscience Publishers, Inc., New York, 1951. 7. BAILEY,A. E., J. Am. Oil Chemists' Soc. 26, 644-648 (1949). 8. BARmrrr, C. B., DALLAS,M. S. M. and PADLEY,F. B., J. Am. Oil Chemists' Soc. 40, 580-584 (1963). 9. BEOEMANN,P. H., KEPPLY, J. G. and BOEKENOGEN,H. A., Rec. Tray. Chim. 69, 439-456 (1950). 10. BITr,ma, E. D., SELKE, E., ROHWEDDER, W. K. and DtrrroN, H. J., J. Am. Oil Chemists' Soc. 41, 1-3 (1964). 11. BLEKKrNGH,J. J. A., Bull. soc. chim. France, 278-282 (1950). 12. BOELHOUWER,C., GERCKENS,J., Lm, ONG TIAN and WATERMAN,H. I., J. Am. Oil Chemists' Soc. 30, 59-61 (1953). 13. BOELHOUWEa,C., SrCELDERWAARD,J. and WATERMAN,H. I., 3". Am. Oil Chemists" Soc. 33, 143-146 (1956). 14. BOND, G. C. and WELLS,P. B., Advan. Catalysis, 15, 92-221 (1964). 15. BtrrrEarELD, R. O., private communication. 16. BUTTERFIELD,R. O., BITNER,E. D., SCHOFIELD,C. R. and Du'I-I'ON, H. J., J. Am. Oil Chemists' Soc. 41, 29-32 (1964). 17. BtJw'~RFELD, R. O. and DUTTON, H. J., Anal Chem. 36, 903, 906 (1964). 18. Bu rr~RFIELD, R. O., SCHOLFmLD,C. R. and DtrrroN, H. J., J. Am. Oil Chemists" Soc. 41, 397-400 (1964). 19. CASTLES,S., British No. 973,957 (November 4, 1964). Nikki Chem. Co., Ltd. 20. COOK, J. S., British No. 932,991 (July 31, 1963). Company, Asani Electro. Chem., Ltd. 21. COUSINS, C. R., G~CE, W. A. and FEUOE, R. O., J. Am. Oil Chemists" Soc. 36, 24-28 (1959). c
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HYDROGENATION OF FATS
375
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