Isomerization during hydrogenation. III. Linoleic acid

Abstract

SummaryThe isomerization that takes place during the catalytic hydrogenation of linoleic acid and methyl linoleate producescis andtrans 9, 10, 11, and 12 monoenes. The double bond at the 12 position appears to hydrogenate slightly faster than that in the 9 position. More octadecenoic acids with double bonds at the 10 or 11 positions are produced during a selective (high temperature, low pressure) hydrogenation than during a non‐selective process.Although the degree of selectivity of the hydrogenation is determined by the amount of isomerization of the original pentadiene system to a conjugated diene, only part of the methylene‐interrupted diene goes through this type of isomerization even during a highly selective hydrogenation.The half hydrogenation‐dehydrogenation reaction mechanism is applied to explain the simultaneous positional and geometrical isomerizations.

Heterogeneous Catalysis in Oleochemistry

“…Carbon-carbon double bonds first undergo chemisorption at active surface metal sites by their π-system(s) with consequential rehybridization of the sp 2 carbon atom orbitals to sp 3 , resulting again in two σ -like Ni-C bonds [125]. Both mechanisms are in agreement with the different known adsorption states of hydrogen on metal surfaces [127]. The specific interaction between metal active sites and carbon-carbon double bonds can be best explained by electron transfer from carbon-carbon π-orbitals into unoccupied d-orbitals enhanced by back-bonding due to transfer of electrons from occupied metal d-orbitals into antibonding π-orbitals of the double bond.…”

“…In the oleochemical industry, the catalytic system for more then 10 decades has been powdered nickel, finely dispersed as a slurry, since the first patent on liquid-phase hydrogenation of oleic acid [157] was granted to Normann in 1903 [115,127,141,[158][159][160][161]. It provides activity and considerable selectivity S I at low cost.…”

“…The platinum group metals platinum, palladium [167], rhodium and ruthenium and combinations of these and other elements [168] are known to confer additional activity, selectivity (both S I and S i , also if other functional groups such as carboxy groups in fatty triglycerides or fatty acids are present), stability and lifetime in hydrogenation of oleochemical derivatives [115,127,143,144,[169][170][171][172][173]. Their better selectivity in slurry hydrogenation is mainly a result of their higher activity, which allows hydrogenation to be run at 50-120 • C compared with 130-230 • C in the case of nickel, thus favoring geometric isomerization (see Section 14.10.4.4.3).…”

Dehydration of Castor Oil

“…These volume differences are presumed because selectivity in hydrogenation is obtained at low pressure and maximum conjugation preceeds highest selectivity (Allen et al, 1956). These volume differences are presumed because selectivity in hydrogenation is obtained at low pressure and maximum conjugation preceeds highest selectivity (Allen et al, 1956).…”

Kinetic Model for Soybean Oil Hydrogenation Using Loop Reactor.

“…However, the authors did not discuss the formation of trans-acids. Allen & Kiess ( 1956), in a series of their research reports, discussed the isomerization of fatty acids during hydrogenation precisely and proposed the reaction sequences to explain the formation of the geometric and posltional isomers. Those models, the sequences, reaction rate, or rate equation in those studies, however, did not explain the hydrogenation totally but did focus on some aspects of the reaction.…”