Physicochemical properties and isomerization activity of chlorinated Pt/Al2O3 catalysts

Abstract

Alumina-supported platinum catalysts with various metal contents have been prepared and chlorinated by CCl₄ at 473 and 573 K. Their acidity has been measured by pyridine adsorption and catalytic activity in n-butane isomerization. The state of platinum has been followed by infrared spectroscopy, whereas the modifications of the dispersion were measured by electron microscopy and by H₂–O₂ titrations.

The presence of metal in chlorinated alumina produces only small changes in acidity but a great enhancement in isomerization activity for samples chlorinated at 573 K. After chlorination, platinum is oxidized to platinum chloride, which reacts with AlCl₃ to produce the PtCl₂·2AlCl₃ complex. This complex is mobile on the support and agglomerates in large particles. During the isomerization of n-butane, platinum is reduced in the metallic state leading to very low metal dispersion. Very small platinum clusters are also present, they are able to perform the hydrogenation of butenes and to decrease the coke formation. Thus ageing is limited in comparison with pure alumina. The possible role of HCl leading to superacid species is also discussed.