Deactivation kinetics of Mo-supported Raney Ni catalyst in the hydrogenation of xylose to xylitol

The activity of a Mo-promoted Raney nickel catalyst was studied in the hydrogenation of xylose to xylitol. Kinetic measurements carried out in a laboratory scale pressurised slurry reactor (40-70 bar H-2 and 80-130 degrees C) with recycled catalysts revealed that the catalyst deactivates during the use, but an asymptotic activity level is finally attained. Water and water-ethanol mixtures were used as solvents. The formation kinetics of the main product, xylitol as well as the by-products, xylulose, D-arabinitol, furfural and xylonic acid were registered quantitatively in the experiments. Catalyst characterisation studies carried out with nitrogen adsorption, XRD, ESCA-XPS and gravimetric reduction with hydrogen suggested that the main reasons for the deactivation is the decay of accessible active sites through collapse of the pore structure and leaching of the promoter metal, Mo and alumina. Also, accumulation of organic species in the pores may slightly contribute to the deactivation process. Catalyst deactivation was more rapid in aqueous milieu than in water-ethanol solutions. The deactivation rate was retarded, if the catalyst was treated with ethanol at elevated hydrogen pressure and temperature between the hydrogenation experiments. A rate model based on plausible surface reaction mechanisms was proposed for the generation of the main and by-products. The rate equations were based on a semi-competitive adsorption model for hydrogen and organic species. The catalyst deactivation kinetics was described with a reversible semi-empirical model, which lumped the physical and chemical reasons for deactivation to a simple two-parameter system. The deactivation model was combined with the rate equations and the model of the slurry reactor. The kinetic and deactivation parameters were determined with a sequential technique, by using non-linear regression analysis. The model was able to reproduce the hydrogenation behaviour of Raney nickel very well: it predicted the product distribution and the catalyst deactivation within a wide range of process parameters. (C) 2000 Elsevier Science B.V. All rights reserved.