Catalysis of Organic..

192Modeling Mass Transfer HydrogenationThe hydrogenation steps can be preceded or succeeded by isomerization steps,which typically require the presence of hydrogen in the liquid phase, becausehydrogen plays a role for the reaction mechanism, even though it is not consumed inthe isomerisation itself. Lack of hydrogen on the catalyst surface typically leads tosevere deactivation.The model systems considered in the present work are hydrogenation of citraland xylose on nickel catalysts. The complete reaction schemes are displayed in Fig.1. In an abbreviated form, the schemes can be presented as follows:OHD-XyluloseOOHCH HO2OHOHIsomerization=OOHHOOHhydrogenationhydrogenationhydrogenationCH 2OHHOOHOHisomerizationCH 2OHD-arabinitolCH 2OHOHHOOHCH 2OHIsomerizationOHOOHOHOHOOHOOHOHOHLactoseHOOOHOHHOHOOHOOOHOHLactuloseH 2H 2H 2H 2H 2OHOHOHOHO O OHHOOHOHLactulitolOHOHOHOHOHO OHOOHOHOHLactitolOHOHOHOHHOOHSorbitolOHOHD -Xylose(aldehyde form)XylitolOHHOOHOHOHGalactitolFigure 1. Reaction scheme for hydrogenation of citral, xylose and lactose.Kinetic experiments were carried out isothermally in autoclave reactors of sizes500 ml and 600 ml. The stirring rate was typically 1500 rpm. In most cases, thereactors operated as slurry reactors with small catalyst particles (45-90 micrometer),but comparative experiments were carried out with a static basket using large trilobiccatalyst pellets (citral hydrogenation). Samples were withdrawn for analysis (GC forcitral hydrogenation and HPLC for lactose hydrogenation). The experimental detailsas well as qualitative kinetics are reported in previous papers of our group Kuusistoet al. (17), Aumo et al. (5).Modelling resultsBased on experiments carried out with small catalyst particles under vigorousstirring, experimental data representing intrinsic kinetics were obtained. Rateexpressions based on the principle of an ideal surface, rapid adsorption anddesorption, but rate-limiting hydrogenation steps were derived. The competiveness