Interaction pathway of chloride ions with γ-Al2O3: surface acidity and thermal stability of the Cl/γ-Al2O3 system

Abstract

The promoting effect of adsorbed chlorine ions (Cl^–) on the surface acidity of γ-Al₂O₃ has been monitored by zero point charge (z.p.c) and catalytic activity measurements in cyclohexene isomerization. A progressive decrease of the z.p.c. values (7.8–4.8) with increasing extent of chloride adsorption (0–1.77 wt. %) has been found. A linear relationship between catalytic activity and z.p.c. is discussed. The thermal stability (300 ⩽T/°C ⩽ 500) of the Cl/γ-Al₂O₃ system has been investigated by dechlorination tests and surface area measurements. The fundamental role of the surface dehydroxylation in the mechanism of chloride loss and BET surface area decay is pointed out. The kinetics of the chloride loss are well described by a first-order reaction rate (d[Cl]_surf/dt=–k_T[Cl]_surf). Steam markedly enhances the dechlorination rate without altering the activation energy of the process (E_a= 6.2 kcal mol^–1).