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 hydr ogenation of xylose to xylitol. Kinetic measurements carried out in a labor atory 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, furfur al 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 throug h collapse of the pore structure and leaching of the promoter metal, Mo and alumina. Also, accumulation of organic species in the pores may slightly c ontribute to the deactivation process. Catalyst deactivation was more rapid in aqueous milieu than in water-ethanol solutions. The deactivation rate w as 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 fo r the generation of the main and by-products. The rate equations were based on a semi-competitive adsorption model for hydrogen and organic species. T he catalyst deactivation kinetics was described with a reversible semi-empi rical model, which lumped the physical and chemical reasons for deactivatio n to a simple two-parameter system. The deactivation model was combined wit h the rate equations and the model of the slurry reactor. The kinetic and d eactivation parameters were determined with a sequential technique, by usin g non-linear regression analysis. The model was able to reproduce the hydro genation behaviour of Raney nickel very well: it predicted the product dist ribution and the catalyst deactivation within a wide range of process param eters. (C) 2000 Elsevier Science B.V. All rights reserved.