Influence of metal-support interactions on the kinetics of liquid-phase citral hydrogenation

The kinetics of liquid-phase hydrogenation of citral (3,7-dimethyl-2,6-octadienal) on PuTiO2 catalysts were studied between 298-423 K and 7-21 arm H-2 and compared to those reported earlier for Pt/SiO2 catalysts. The kinetic data were shown to be free of transport limitations by application of the Madon-Boudart test and the Weisz-Prater criterion. Near zero- and first-order kinetics were observed for the initial rate of citral hydrogenation over the Pt/TiO2-LTR (T-red = 473 K) and Pt/SiO2 catalysts with respect to citral concentration and hydrogen pressure, respectively. In contrast, each dependency dropped by about one order with Pt/TiO2-HTR (T-red = 773 K) catalysts as they were negative first-order on citral concentration and near zero-order on hydrogen pressure. Furthermore, the initial rates over Pt/TiO2-LTR and Pt/SiO2 exhibited an activity minimum as temperature increased whereas conventional Arrhenius behavior was exhibited by Pt/TiO2-HTR with an activation energy of 18 kcal/mol. Pt/TiO2-LTR and HTR catalysts initially exhibited 90% selectivity to the unsaturated alcohol as compared to 40% for hydrogenation over Pt/SiO2. Metal-support interactions (MSI) resulted in a dramatic enhancement in specific activity at 373 K, 20 atm H-2 and 1M citral in hexane as Pt/TiO2-HTR exhibited a turnover frequency of 1.0 compared to 0.02 s(-1) for Pt/TiO2-LTR and 0.004 s(-1) fur Pt/SiO2. The reaction kinetics with Pt/TiO2-HTR in the differential conversion regime were described by a conventional Langmuir-Hinshelwood model assuming quasi-equilibrium for reactant adsorption, competitive adsorption between citral and hydrogen, and addition of the first H atom as the rate determining step. The reaction rate at higher conversions was modeled by invoking a decarbonylation reaction similar to that proposed earlier for this reaction over PVSiO2 catalysts to explain any observed deactivation. (C) 2000 Elsevier Science B.V. All rights reserved.