Methyl Oleate Isomerization and Hydrogenation over Ni/α-Al2O3: A Kinetic Study Recognizing Differences in the Molecular Size of Hydrogen and Organic Species

Abstract

The kinetics of the hydrogenation and cis/trans isomerization of methyl oleate on a Ni/a-Al2O3 catalyst was studied in the absence of mass-transport limitation, at 398K=T=443Kand370kPa = PH2 = 650 kPa. On the basis of the Horiuti–Polanyi mechanism, involving a sigma half-hydrogenated surface intermediate, a kinetic model was derived in the framework provided by the Langmuir–Hinshelwood–Hougen–Watson formalism, using the advanced concept of semi-competitive adsorption. The classical LHHW rate equations for competitive and non-competitive adsorption between the hydrogen and large organic species were matched as asymptotic cases. Statistical results clearly demonstrated the inadequacy of the model approaching non-competitive adsorption to describe the experimental data, but the residual sum of squares between experimental data and model predictions was insufficient to discriminate between the kinetic models based on competitive and semi-competitive adsorption. However, the model considering semi-competitive adsorption gave additional indication that the adsorbed molecules of cis- and trans-methyl oleate could cover up to eleven surface sites, which is in excellent agreement with a rough estimate from primary molecular modeling. This feature seems to be the most fascinating result, since it is factual and unattainable from the classical LHHW approaches. Results and distinctive features characterizing this advanced approach are highlighted. Some insights to improve parameter estimation and adsorption model discrimination are also pointed out.