"Lo SR, linoleic selectivity ratio.
"Lo SR, linoleic selectivity ratio.
without affecting the energy factors of the reaction or being consumed in the reaction. Thus, the catalyst enters into the reaction over and over again, and a relatively small amount may be capable of transforming very large amounts of feedstock into reacted product. In the hydrogénation of vegetable oils, the concentration of nickel employed as a catalyst does not exceed a few hundredths of a percent. Furthermore, one catalyst may differ from another in its relative effect on alternate reaction rates. The hydrogénation of fats and oils furnishes examples of such specificity of catalyst action. Thus, the addition of 1 mol of hydrogen to a linoleic acid chain in a glyceride molecule may yield either a normal oleic ester or isomeric forms of it. Some nickel catalysts are more inclined than others to produce the isomeric forms.
Heterogeneous catalysts are the most important in industry generally. In edible oil hydrogénation they are used exclusively. In heterogeneous catalysis it is assumed the reaction proceeds through the formation of unstable intermediate compounds or adsorption complexes, in which the catalyst is temporarily combined with one or more of the reactants. If such compounds do exist, it is probable that in most cases they are not definite chemical combinations but consist merely of strongly bound molecules of the re-actant held to the catalyst surface by secondary valence forces, or by complexing. In any event, it is essential that they be unstable. That is, capable of being either decomposed or desorbed, to permit reaction to proceed according to the scheme shown in equation 3.
adsorption catalyst + reactants -* catalyst-reactant complex desorption
-»• reaction products + regenerated catalyst (3)
Although homogeneous catalysts for fats and oils hydrogénation have been studied, there is no current commercial interest in pursuing them. This is principally because of legal/environmental reasons. For homogeneous catalysts to be legally permitted in edible oil hydrogénation, they would need to achieve generally regarded as safe (GRAS) status. This would require either proof of their complete removal after use, or long-term human feeding tests demonstrating their harmlessness. In either case, the cost is too high and the potential liability too great to be considered seriously by either oil processors or catalyst manufacturers. Since enzyme catalysts might have the possibility of more easily being recognized as GRAS, they could be a future possibility.
Commercial hydrogénation catalysts are made by first combining nickel with other elements, such as in nickel oxide, nickel hydroxide, nickel carbonate, nickel formate, or nickel-aluminum alloy, and then reducing the compound to regain a portion of the nickel (now in a catalytic state) in metallic form. Considerable evidence indicates that the hydrogénation of an ethylenic compound must be preceded by two-point adsorption of the carbon atoms on either side of the double bond. This requirement imposes certain dimensional limitations on the space lattice of any catalyti-cally active metal. Actually, the metals that are at all effective in the hydrogénation of double bonds, such as
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