Hydrogenation linoleic esters

The hydrogen atoms of the -CH2- group located between the two double bonds of linoleic ester (Lin-H) are especially susceptible to abstraction by radicals. 

Hydrogen abstraction from another molecular of the linoleic ester... 

Figure 10.7 Autoxidation of a linoleic acid ester. In step 1 the reaction is initiated by the attack of a radical on one of the hydrogen atoms of the -CH2-group between the two double bonds this hydrogen abstraction produces a radical that is a resonance hybrid. In step 2 this radical reacts with oxygen in the first of two chain-propagating steps to produce an oxygen-containing radical, which in step 3 can abstract a hydrogen from another molecule of the linoleic ester (Lin-H). The result of this second chain-propagating step is the formation of a hydroperoxide and a radical (Lin ) that can bring about a repetition of step 2.

CATALYTIC HYDROGENATION OF LINOLEIC ESTERS REACTION COURSES ACCORDING TO THE HYPERBOLIC FORMULA... 

Fig. 92. Hydrogenation of linoleic esters at a relatively high temperature (i8o°C). 2% Ni-guhr, i8o°C, 1 atm.

The catalytic hydrogenation of linoleic esters has also been treated kinetically by Boelhouwer et al,88. If the velocity constants of the consecutive reactions are represented by kx and k2 and first order kinetics are assumed, the following equations hold ... 

We found previously (10) that in a natural mixture of mono-, di-, and triunsaturated fatty esters the hydrogenation of monoenes with Fe(CO)s was minor. Therefore, competitive hydrogenation studies were carried out with an equal mixture of methyl oleate and linoleate. Diene hydrogenation in such a mixture was indeed dominant (Figure 2). At O.IM initial concentration of Fe(CO)s the formation of stearate was a minor reaction the diene-Fe(CO)3 complex reached a maximum of 4% and remained constant. On the other hand, at 0.5M Fe(CO)5, stearate formation became a more important reaction diene-Fe(CO)3 reached a maximum of 7% and decreased during the course of hydrogenation. Free conjugated dienes were minor products. 

Hydrogenation of linoleic esters (Cig) a shell catalyst with Pd on activated carbon is recommended... 

The hydrogenation process has, therefore, become popularly known as fat hardening. It converts oils to solids, with convenient softening points, that resist oxidation and contain polyunsaturated linoleic esters that are felt to be nutritionally useful. Most fats can be synthesized in the body, except for those containing linoleic and linolenic acids, so these are the essential fatty acids that must be provided with food. 

A mixture of palladium chloride and triphenylphosphine effectively catalyzes carboxylation of linoleic and linolenic acids and their methyl esters with water at 110°-140°C and carbon monoxide at 4000 psig. The main products are 1,3-and 1,4-dicarboxy acids from dienes and tricarboxy acids from trienes. Other products include unsaturated monocar-boxy and dicarboxy acids, carbomethoxy esters, and substituted a,J3-unsaturated cyclic ketones. The mechanism postulated for dicarboxylation involves cyclic unsaturated acylr-PdCl-PhsP complexes. These intermediates control double bond isomerization and the position of the second carboxyl group. This mechanism is consistent with our finding of double bond isomerization in polyenes and not in monoenes. A 1,3-hydrogen shift process for double bond isomerization in polyenes is also consistent with the data. 

A significant portion of the neutral ethyl ether extract from the salts of carboxylated methyl linoleate consists of ,/3-unsaturated cyclic ketones. This material is produced in small but significant amounts (4-10%) (Table I) from the carboxylation of polyunsaturates but not from the corresponding monounsaturated fatty acids and esters (19). These ,/ -unsaturated cyclic ketones were identified spectroscopically (IR, UV, and NMR) as 4. This structural assignment was firmly established by mass spectral analyses before and after hydrogenation of the carbon-carbon double bond. 

The methyl ester of carboxyoctadecenoate 1 was also identified (about 10% by GLC) in the neutral ether extract from the salts of car-boxylated linoleate. Apparently some methanol is formed from H20 and CO under the conditions of hydrocarboxylation, and esterification of the monocarboxy acids occurs to a small extent. Double bond hydrogenation is another minor side reaction observed. Small amounts of carboxyocta-decanoate la detected in final hydrocarboxylation mixtures would arise from H2 produced by the water-gas reaction (CO + H20 = C02 + H2). 

An accurate investigation of these problems was, however, only possible after the development of reliable methods of analysis. Particularly Bertram s method55 for the determination of saturated fatty acids should be mentioned. Van Vlodrop70 proved its usefulness for the study of the hydrogenation of several fatty acid esters (oleic, linoleic, elaeostearic). 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

The catalyst system [Rh(acac)(CO)2]/biphephos shows high activity for isomerization with yields of 60% of branched isomers at 20 bar CO/H2 pressure and 115°C [10]. With this catalyst system, a 26% selectivity of linear aldehyde from ethyl oleate was observed. The selectivity for the n-aldehvde was higher at 34% for linoleic acid. A hydrogenation side product was observed in the reaction due to the isomerization of the double bond toward the ester group, where hydrogenation is favored. 

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