PARTICLE-SIZE EFFECT ON THE KINETICS OF P-CHLORONITROBENZENE HYDROGENATION OVER PLATINUM ALUMINA CATALYSTS

Hydrogenation of p-chloronitrobenzene (CNB) has been studied, in methanol suspension at 303 K and atmospheric pressure, over a series of Pt/Al2O3 catalysts of varying dispersion (0.07 < H/Pt < 1.26). The kinetics obeys a modified Langmuir-Hinshelwood model with competitive adsorption between CNB and hydrogen. The adsorption competition no longer holds at high CNB concentration: on the Pt surface saturated by CNB, there are still Pt sites available for the chemisorption of the smaller hydrogen molecule. A rate law is then proposed which allows determination of both absorption and rate constants. The specific rate constant (expressed on a per surface Pt atom basis as a turnover frequency) increases by one order of magnitude when the Pt dispersion decreases. This behaviour is interpreted in terms of a transition state bearing a weak negative charge more stabilized on the large Pt particles. This hypothesis agrees with the effect of substituents on the aromatic ring. The conversion of CNB leads to p-chloroaniline (CAN) as main product, p-chloronitrosobenzene and p-chlorophenylhydroxylamine as intermediates, with nitrobenzene, aniline and chlorobenzene as byproducts. The product selectivity depends on the Pt particle size. At high conversion, the highest selectivity in CAN (> 98%) is reached on large Pt particles. This can be interpreted by a lower relative adsorption strength between CAN and CNB on poorly dispersed catalysts; CAN is then easily removed from the surface by CNB, which prevents hydrodechlorination.