Jp. Mikkola et al., Deactivation kinetics of Mo-supported Raney Ni catalyst in the hydrogenation of xylose to xylitol, APP CATAL A, 196(1), 2000, pp. 143-155
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.