INFLUENCE OF SUPPORT AND METALLIC PRECURSOR ON THE HYDROGENATION OF P-CHLORONITROBENZENE OVER SUPPORTED PLATINUM CATALYSTS

Hydrogenation of p-chloronitrobenzene (CNB) has been studied, in methanol suspension, at 303 K and atmospheric pressure, over alumina, magnesia, titania and graphite supported platinum catalysts. The catalysts were prepared by using anionic, cationic or organometallic platinum precursors. The nature of the precursor does not modify markedly the catalytic properties of platinum for that reaction. However, the inhibiting effect of p-chloroaniline (CAN), the main product formed on CNB hydrogenation is the highest on Pt/Al2O3 prepared from the cationic precursor. For similar sizes of the Pt particles, the greatest influence on activity and selectivity is observed when titania is used as carrier. There is a tenfold increase of turnover frequency on Pt/TiO2 compared to Pt/Al2O3. At high CNB conversion (> 98%), the yield of CAN increases from 85.2% on Pt/Al2O3 to 99.3% on Pt/TiO2 reduced at high temperature (773 K). The improvement of CAN selectivity stems mainly from enhancement of the relative reactivity between hydrogenation of the nitro group and hydrogenolysis of the C-Cl bond of CAN. It is proposed that the migration of suboxide TiO(x) species (x < 2) on to the Pt particles, in a strong metal/support interaction state, is responsible for this behaviour. A schematic model of the reaction site is presented, in which the migrating TiO(x) adspecies on Pt activate the N=O bond which becomes highly susceptible to hydrogen attack.