570
INDUSTRIAL A N D ENGINEERING CHEMISTRY
black and lampblack almost completely inhibit the oxidation of the oil during the first few hours of exposure. This effect is due to the fact that the pigments adsorb not only a portion of the lead drier but also the small amount
Vol. 18, No. 6
of autocatalytic oxidation product that is formed a t the beginning of the oxidation. +Carbon pigments have little effect upon the rate of oxidation of linseed oil containing manganese drier.
Heterogeneous Catalysis' 111-Hydrogenation of Cottonseed Oil with Platinum By A. S. Richardson and A. 0. Snoddy THE PROCTER & GAMBLECo.,
TVORYDALE,
OHIO
H E glycerides of cottonseed oil are derived from linoleic The mere fact that the average cottonseed oil is about 75 acid, oleic acid, and solid saturated acids, chiefly per cent derived from liquid fatty acids has never been a palmitic. Hydrogenation of such an oil may con- serious handicap to its use in edible fat. The average edible ceivably involve any combination of the following three fat should and does contain a substantial excess of liquid changes: (1) linoleic to oleic acid; ( 2 ) oleic to stearic acid; constituents. However, the fact that about 65 per cent of (3) linoleic to stearic acid. The problem of the distribution this liquid fatty acid consists of linoleic acid renders cottonof the total hydrogenation of cottonseed oil among these seed oil extremely susceptible to oxidation and resulting ranthree changes belongs to a general field of investigation which cidity. Hence the possibility of converting linoleic to the has appropriately been described by various investigators as relatively stable oleic acid by selective hydrogenation was the problem of "selective hydrogenation." even more attractive to the far-sighted manufacturer than the The previous literature on the selective hydrogenation of formation of solid saturated from unsaturated acid. fatty oils has been reviewed in the earlier papers of this series.2 Burchenal's discovery was followed, in turn, by industrial T h e present paper deals with the selective action of platinum development and by a series of investigations which show that black in the hydrogenation hydiogenation i n h e r e n t l y of cottonseed oil, but betends to be selective, but fore proceeding to a discusnot completely so. Many The predominating importance of selective hydrosion of the e x p e r i m e n t a l conditions in practice may genation in industry is pointed out. The hydrogenadata involved it will be in interfere with or defeat the tion of cottonseed oil with platinum catalyst has been o r d e r t o correct rather purpose of selective hydroinvestigated at temperatures varying from 40' to 240" C. widespread misunderstandIf this were not genation. The preferential conversion of linoleic acid to the oleic ing of the fundamental purso, it is aImost inconceivabIe acid stage and also the formation of solid unsaturated pose of hydrogenation. that selective features of acid is favored by increasing temperature over practihydrogenation should have cally the whole of the range investigated. HydrogenaPractical Importance of so long been unnoticed. Selective Hydrogenation tion of cottonseed oil with platinum catalyst does not The most important facappear to be so selective as with nickel catalyst. tor to be controlled is the Contrary to the impresend point of hydrogenation. sion which the casual reader For instance, at a more or may gain from a survey of the- published papers on selective hydrogenation of oils, the less well-defined critical point hydrogenation of vegetable oil subject is not primarily a theoretical one. It is a subject in containing linolein ceases to involve primarily the conversion which practical development on a large scale preceded any of linolein to olein and results in substantial increase of stearin content. Carried beyond that point, hydrogenation becomes scientific literature. The pioneers3 in the field of hydrogenation of oils regarded effectively less selective. Operating conditions may also render their process merely as a means of converting liquid un- the process of hydrogenation effectively nonselective. For insaturated to solid saturated fatty acids-for example, oleic stance, unhydrogenated oil may be swept through the system or linoleic to stearic acid. As long as that idea prevailed in into the finished product. This is particularly likely to occur industry, hydrogenation was making important but com- in a continuous process, but may also occur in a batch process paratively slow progress. The chief impetus to industrial in which pockets of unhydrogenated oil persist throughout development was the discovery by Burchena14 that hydro- the reaction period as a result of incomplete agitation. Temperature, perhaps, is the most important factor which genation could be controlled so as to produce a product especially suited for edible purposes, in which the principal has been shown to affect the inherent selective tendency chemical change is the selective conversion of the highly of hydrogenation of oils in the presence of nickel catalyst. unsaturated linoleic acid to the less unsaturated oleic acid, Moore, Richter, and Van Arsdelb found that the selective conversion of linolein to olein was favored by increasing temwith the formation of relatively little stearic acid. perature in the hydrogenation of cottonseed oil. Richardson, 1 Presented as a part of the Symposium on Cotton and Its Products Knuth, and Milligan2 have made the same observations on and Vegetable Oils before the joint session of the Divisions of Agricultural several oils in the temperature range 150" to 200" C., but have and Food Chemistry, Biological, Cellulose, and Industrial and Engineering Chemistry at the 71st Meeting of the American Chemical Society, Tulsa, obtained conflicting results above 200" C.
T
Okla., April 5 to 9, 1926. Richardson, Knuth, and Milligan, THISJOURNAL, 16, 519 (1924); 17, 80 (1925). a Leprince and Sieveke, German Patent 141,029 (August 14, 1902); Normann, British Patent 1515 (January 21, 1903). 4 U. S. Patent 1,135,351 (application filed November 10, 1910, patent granted April 13, 1915).
Scope of Present Work
The present work was undertaken a t the suggestion of H. J. Morrison for the immediate purpose of extending the &THISJOURNAL, 9, 451 (1917).
June, 1926
I.VDUSTRIAL A,VD ENGINEERING CHEMISTRY Temp.
c.
a
40 70 100 120 140 160 180 200 220 240 Original oil.
Oil 104.6O 75.8 74.9 74.2
--
(3.J
73.3 73.0 74.9 79.4 76.8 77.4
Table I-Hydrogenation of Cottonseed Oil with P l a t i n u m -1, DDINE VALUE-CO31POSITIO.N Mixed Solid Unsaturated Solid acids Saturated acids acids acids Per cent acids 109.4 2.7 148.4 27.1 26.3 79.3 5.4 125.1 38.9 36.6 (8.3 5.8 124.9 39.9 37.3 77.6 13.4 118.7 40.7 34.6 79.0 12.2 119.7 39.3 34.0 76.8 14.4 116.4 40.6 34.1 76.4 15.7 114.9 40.6 33.5 78.3 18.6 116.5 41.4 32.8 83.1 18.6 122.2 40.3 32.0 80.3 20.6 117.1 40.6 31.4 81.0 23.7 119.6 43.8 32.3
knowledge of the effect of temperature on selective hydrogenation to cover the case of cottonseed oil hydrogenated with platinum catalyst. The use of this catalyst readily allows a range of hydrogenation temperature considerably greater than is practical with the use of nickel. The results have a greater interest than their bearing on the effect of temperature on selective hydrogenation, as will be pointed out.
Discussion of Results
If hydrogenation were completely selective in the sense of preferential conversion of linoleic to oleic acid, none of the hydrogenated products within the range of iodine value .given in Table I would show any appreciable change in solid saturated fatty acids and the net result of hydrogenation would be a decrease in linoleic content with a corresponding increase in oleic acids (no distinction being made between solid and liquid oleic, since either might be produced by selective hydrogenation of linoleic acid). As a matter of fact, this condition does not even approximately hold for any of the products obtained. If all products shown had been hydrogenated to the same iodine value, the degree to which hydrogenation was selective in each case could be determined by comparing directly the increase in solid saturated acid, this increase being greatest for the least selective conditions of hydrogenation. Since iodine values of the finished products are not quite uniform, a better basis of comparison is given in Table 11,which shows the net change of linoleic, oleic, and stearic acid content and J. A m . Chem. Soc., 44, 1397 (1922); 46, 1071, 2171 (1923). THISJOURNAL, IS, 806 (1921). 8 I b i d . , 16, 520 (1924). 6
7
Black O F -MIXED
Oleic 25.6 36.6 36.0 38.7 39.2 40.3 41.2 39.0 35.6 39.0 34.2
ACIDS,P E R CENT-
“Iso-oleic” 0.8 2.3 2.6 6.1 5.3 6.5 7.1 8.6
8.3 9.2
11.5
Linoleic 47.3 24.5 24.1 20.6 21.5 19.1 18.2 19.6 24.1 20.4 22.0
shows also the ratio of increase in oleic to increase in stearic. As hydrogenation becomes increasingly selective this ratio becomes larger. It is assumed, of course, that all increase in saturated acids is due to stearic. * Table 11-Distribution a
Experimental
Platinum dioxide ( Pt02.H20) was prepared according to the method of Adams.6 A mixture of 200 gram3 of refined cottonseed oil and 0.1 gram of platinum dioxide was heated to reaction temperature in a Pyrex flask through which a slow stream of electrolytic hydrogen was being passed. As soon as reaction temperature was reached the mixture was mechanically agitated. Hydrogenation was effected a t temperatures varying from 40’ to 240” C., the iodine value of the finished products varying from 73 to 79.4. Temperature was held to within =t2’ C. of the reaction temperature recorded. All hydrogenated products were examined by the Twitchell’ method for the separation of solid from liquid fatty acids. From (1) the weight of the solid fatty acid fraction, (2) the iodine value of the solid fraction, and (3) the iodine value of the mixed fatty acids, the composition of the mixed fatty acids of each hydrogenated product was Calculated according to the method previously outlined by Richardson, Milligan, and Knuth.8 The results are shown in Table I. It should be understood that the term “iso-oleic” acid refers to the unsaturated fatty acids present in the solid fraction and is not meant to convey any impression whatsoever regarding the number of isomeric forms which may be present in this material.
571
a
Temp. Decrease in O C. % linoleic 22.8 40 23.2 70 100 26.7 25.8 120 140 28.2 29.1 160 180 27.7 23.2 200 26.9 220 25.3 240 Including solid isomers.
of Change after Hydrogenation b C Increase in Increase in Ratio % oleica % stearic b:c 12.5 10.3 1.2 12.2 11.0 1.1 18.4 18.1
20.4
21.9 21.2
17.5 21.8 19.3
8.3
7.7 7.8 7.2 6.5 5.7 5.1 6.0
a.2 2.4 2.6 3.0 3.3
3.1
4.3 3.4
The most striking conclusion from the results obtained is that hydrogenation with platinum is by no means so selective as with nickel catalyst. I n fact, the highest ratio of increase in oleic to increase in stearic found in Table I1 is less than the lowest ratio calculated for any sample of cottonseed oil hydrogenated to within the same range of iodine values with nickel catalyst by Richardson, Milligan, and Knuth.2 At first glance, one might question the appropriateness of speaking a t all of the selective action of platinum catalyst a t the lower temperatures studied, but careful inspection will show that in all cases the amount of stearic formed is substantially less than would be expected if hydrogenation of linoleic to oleic and of oleic to stearic proceeded continuously a t rates proportional to the concentration of the reacting acids. Hence even hydrogenation with platinum black is selective to a considerable degree. The results obtained indicate that increasing temperature favors selective hydrogenation. Such irregularities as will be noted are probably matters of experimental error and certainly of minor importance as compared with the total drift from the lowest to the highest temperatures studied. While hydrogenation with nickel has been preferred in industry to hydrogenation with platinum on the basis of cost, it is highly probable that the base metal catalyst would in the long run be preferred on the basis of quality of hydrogenated oil obtained, even if the two catalysts were available a t the same cost. Partial hydrogenation with platinum to a given melting point is calculated to produce a product more susceptible to oxidation than hydrogenation with nickel. It is important to note that this difference would be particularly great if the comparison were between a product hydrogenated with platinum a t low temperature and one hydrogenated with nickel at high temperature, whereas the chief supposed advantage of platinum catalyst is the possibility of operating efficiently a t very low temperatures. The effect of temperature on the formation of solid unsaturated acids is even more striking than the effect of temperature on the degree to which hydrogenation is selective. The amount of “iso-oleic” acid obtained by hydrogenation a t 40’ C. was barely beyond expeiimental error, but the amount steadily increased, with increasing temperature of hydrogenation, to 11.5 per cent a t 240’ C.
