Adsorption of benzene on acidified alumina

Abstract

U.v.–visible reflectance spectroscopy of adsorbed benzene has been used to study the surface modification of alumina containing platinum and chloride. On unchlorinated alumina there are a few sites strongly adsorbing benzene. After chlorination by CCl₄, the Lewis acidity of alumina becomes so strong that some adsorbed benzene molecules are ionized and cracked: further condensation occurs leading to coke precursors. Chlorination of platinumcontaining alumina leads to a support whose ionizing power depends upon the metal loading: a platinum–aluminum–chloride complex is formed by CCl₄ interaction with support and metal. This complex is adsorbed on the strongest Lewis acidic centre, preventing benzene ionization. Addition of HCl to a chlorided platinum alumina leads to a superacid solid-state catalyst. This support reacts immediately with adsorbed C₆H₆, giving the benzenium C₆H⁺₇ cation that slowly transforms into the bulky arenium cation.