Liquid-phase hydrogenation of citral over Pt/SiO2 catalysts 1. Temperatureeffects on activity and selectivity

Liquid-phase hydrogenation of citral (3,7-dimethyl-2,6-octadienal) over Pt/ SiO2 catalysts was studied in the temperature and pressure ranges 298-423 K and 7-21 atm, respectively. The reaction kinetics were shown to be free of artifacts arising from transport limitations and poisoning effects. The re action rate in hexane as the solvent exhibited an activity minimum at 373 K . The initial turnover frequency for citral disappearance over 1.44% Pt/SiO 2 catalyst at 20 atm H-2 pressure decreased from 0.19 s(-2) at 298 IL to 0. 02 s(-1) at 373 K, but exhibited normal Arrhenius behavior between 373 and 423 K with an activation energy of 7 kcal/mol. Reaction at 298 Ii produced substantial deactivation, with the rate decreasing by more than an order of magnitude during the first 4 h of reaction; however, reaction at temperatu res greater than 373 K exhibited negligible deactivation and a constant rat e up to citral conversions greater than 70%. These unusual temperature effe cts were modeled using Langmuir-Hinshelwood kinetics invoking dissociative adsorption of hydrogen, competitive adsorption between hydro gen and the or ganic compounds, and addition of the second hydrogen atom to each reactant as the rate-determining step. Decomposition of the unsaturated alcohol (eit her geraniol or nerol) was proposed to occur concurrently with the hydrogen ation steps to yield adsorbed CO and carbonaceous species which cause the d eactivation, bd at higher temperatures these species could be removed from the Pt surface by desorption or rapid hydrogenation, respectively. The acti vity minimum observed in the present study is attributed to the relative ra tes of the alcohol decomposition reaction and CO desorption, with the decom position reaction having an activation barrier lower than that for CO desor ption. (C) 2000 Academic Press.