Intrinsic Kinetics of 9-Monoenic Fatty Acid Methyl Ester Hydrogenation over Nickel-Based Catalysts

The monoenic fatty acid methyl ester hydrogenation and isomerization over a supported nickel catalyst was studied at 0.02 ≤ P H 2 ≤ 0.50 MPa and 333 ≤ T ≤ 443 K. Batch hydrogenations at constant and variable hydrogen pressure were carried out to investigate the rate-determining steps. On the basis of the Horiuti−Polanyi mechanism, involving a half-hydrogenated surface intermediate, kinetic rate equations were systematically derived following the Langmuir−Hinshelwood−Hougen−Watson approach. Each set of rate equations was tested by means of a χ2 method on its ability to describe the experimental curves simultaneously. The χ2 was minimized by a stepwise optimization of model parameters. Furthermore, we used Bartlett's test to reduce the set of most-likely rate expressions. The statistically most significant model assumes the formation of the half-hydrogenated surface intermediate as the rate-determining step and an equilibrium associative hydrogen adsorption. The rate equations are R O = m c{[−( + K iso k̄ iso)C O + k̄ iso C E]/[C O + C E + (K S/K M)C S]}K H P H [mol/(m3 s)] and R E = m c{[−( + k̄ iso)C E + k̄ iso K iso C O]/[C O + C E + (K S/K M)C S]}K H P H [mol/(m3 s)] with , , and k̄ iso (units: [kgr/(kgNi s MPa)]) the formation rate constants of the half-hydrogenated intermediate in the hydrogenation of cis-monoene and trans-monoene and in the isomerization path, respectively. K iso is the equilibrium constant for the isomerization, K S /K M is the ratio of adsorption constants of saturated and monoene components, m c is the catalyst load [kgNi/mr 3], C O, C E, and C S are the concentrations of methyl oleate (cis), methyl elaidate (trans), and methyl stearate, respectively [mol/m3], K H is the adsorption constant of hydrogen [MPa-1], and P H 2 [MPa] is the hydrogen pressure. The activation energies of K H, K H, and k̄ iso K H are 32.2, 28.1, and 47.2 kJ/mol, respectively.