Selectivity

The term selectivity, as commonly used in the edible oils and fats industry, relates to equation 2 (2).

Ki K2 K3

Linolenic linoleic oleic ->• stearic (2)

With some simplification, linolenic selectivity is defined as the ratio (Ln SR) of the rate of reaction of K1 to K2. With the same simplification, linoleic selectivity is defined as the ratio (Lo SR) of the rate of reaction of K2 to K:i. Because of its much greater utility and its ability to be manipulated, when the term selectivity is used in the edible oil industry, the speaker or writer is referring to linoleic selectivity unless explicitly indicated otherwise. Another way of defining linoleic selectivity is as the degree of preferential conversion of dienes to monoenes compared with the conversion of monoenes to saturates.

Although few trans isomers occur in nature, during partial hydrogenation of oils the double bond may be either saturated or isomerized while it is being adsorbed on the catalyst surface. Some positional and many geometric isomers are formed, basically by way of the Horiute-Polanyi mechanism. Elaidic, the trans form of octadecanoic (oleic) acid, is the most common trans isomer. Its historic utility in margarine formulation is based on its having a significantly higher melting point (43.7°C) than the cis form (16.3°C), while still being considerably lower melting than the completely saturated stearic (69.6°C). Since the solids content of a fat at any given temperature depends on the distribution of all the esters of a triglyceride, the ratio of trans to cis is the principal determinant of the slope of the melting curve in the important refrigerator to body temperature range.

The preferential selectivity of nickel catalysts for the hydrogenation of a linolenic ester in a triglyceride compared with a linoleic (Ln SR) has been shown to be about 1.8 to 2.3. Since changes in process conditions do not significantly change this ratio, evidently the diene and the triene are hydrogenated by the same mechanism. Although it would seem the presence of the third double bond should double the chance of hydrogenation of one of the double bonds in the triene compared with the diene, this has not been shown to be the case. However, when copper catalysts are used to hydrogenate soybean oil, as will be discussed later, linolenic selectivity ratios of 8 to 12 are found. Copper catalyst obviously operates somewhat differently from nickel. It is surmised that copper catalyst causes conjugation of the linolenate triene on the catalyst surface. Since conjugated trienes have been shown to react about 200 times faster than nonconjugated ones, they do not accumulate in the product—evidently hydrogenating to a conjugated diene before desorbing from the catalyst surface. The conjugated dienes are then further reduced to monoenes. The monoenes are not reduced to saturates by the catalyst since hydrogenation with copper catalyst must involve two or more double bonds.

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