The selectivity of the hydrogenation of linseed oil
We have invest igated the se lec t iv i ty of the hydrogenat ion of l inseed oil, which is a mix tu re of t r i g l y c e r ides of fatty acids of different degrees of unsaturat ion linolenic, l inoleic, and ole ic and sa tura ted fatty acids . Depending on the conditions of hydrogenat ing oils and fats , the p roce s s may go according to the following scheme , given in the l i t e r a tu re [1]:
Some additional notes on the kinetics and theory of fatty oil hydrogenation
Summary1.Equations are given for estimating from the composition of oil samples the relative reaction rates of the different unsaturated fatty acids in an oil subjected to catalytic hydrogenation.2.Application of the equations to data from the hydrogenation of cottonseed oil reveals that the ratio of reaction rates, linoleic acid to oleic acid, varies from about 4 to 1 in very non-selective to about 50 to 1 in very selective hydrogenation. Re-examination of analytical data on two series of linseed oils hydrogenated selectively and non-selectively showed the following relative reaction rates for oleic, isolinoleic (9: 10, 15: 16 octadecadienoic), linoleic, and linolenic acids, respectively: non-selective, 1, 2.5, 7.5, 12.5; selective, 1, 3.85, 31, 77. In the non-selective hydrogenation of the oil, 24% of the linolenic acid reacting went to linoleic acid, 65% to isolinoleic acid, and 11% directly to oleic acid. In the selective reaction the corresponding figures were none to linoleic acid, 54% to isolinoleic acid, and 46% to oleic acid. The behavior of soybean oil hydrogenated selectively was quite similar to that of linseed oil.3.The results are discussed in relation to the theory of catalytic hydrogenation. They indicate that the solution of hydrogen in the oil and the adsorption of unsaturated oil on the catalyst are the two steps which are controlling with respect to the reaction rate. It is suggested that the hydrogen pressure, the degree of hydrogen dispersion through the oil, the catalyst concentration, and the temperature all affect the selectivity of the reaction through their influence on the concentration of hydrogen in the reaction zone, with selectivity being favored by a low concentration.
Solvent hydrogenation of cottonseed oil
Refined and bleached cottonseed oil was dissolved in a solvent (hexane, isopropyl alcohol, or di-isopropyl ether) and was then hydrogenated in a dead-end hydrogenator. Hydrogenation runs were conducted at temperatures from 115 to 145°C., at hydrogen partial pressures from 44 to 74 p.s.i.a., with catalyst concentrations varying from 0.05 to 0.40% nickel, and at high rates of agitation to climinate mass-transfer resistances. A series of hydrogenation runs was also made in which no solvent was used.The rates of hydrogenation for the various series of runs were in the same order of magnitude but decreased in the following order: nonsolvent, hexane, isopropyl alcohol, and di-isopropyl other runs. Selectivity and isomerization were low in all cases and essentially identical for solvent and nonsolvent runs.The rate of hydrogenation increased in all cases with higher catalyst concentrations. For the isopropanol runs, the reaction rate was maximum as a function of temperature at about 135°C. In the case of the other solvents, the rate of hydrogenation increased with increased temperature in the range from 115 to 145°C., but the rate increases of the solvent runs were less than those of the nonsolvent runs.