The origin of increased chemoselectivity of platinum supported on magnesium fluoride in the hydrogenation of chloronitrobenzene

A highly active and selective catalyst for the hydrogenation of ortho-chloronitrobenzene (o-CNB) to ortho-chloroaniline (o-CAN) was obtained by supporting platinum on magnesium fluoride. In the above reaction, carried out in a liquid phase at 343 K under 3 MPa H2, the Pt/MgF2 catalyst was fivefold more active (apparent rate = 9.26 mol CNB/mol Ptsurf s) than the analogous system with Al2O3 as a support (apparent rate = 1.83 mol CNB/mol Ptsurf s). At the same time its selectivity to o-CAN (96.6%) was higher than that of Pt/Al2O3 (79.8%). The FTIR studies of adsorbed nitrobenzene (NB) enabled to determine the adsorption mode via oxygen atoms of nitric group. A model of o-CNB adsorption on the platinum/support interface has been proposed, according to which o-CNB adsorption proceeds on Lewis acid centres located in the vicinity of platinum crystallites which are the centres for hydrogen adsorption and dissociation. The presence of interfacial active sites was additionally proved by FTIR studies of adsorbed carbon monoxide and TPR-H2 measurements.