Radiotracer Studies of the Chlorination of gamma-Alumina and the Catalytic Dehydrochlorination of 1,1,1-Trichloroethane

The chlorination of calcined gamma-alumina samples using anhydrous gaseous [36Cl]-chlorine labelled hydrogen chloride, anhydrous gaseous [36Cl]-chlorine labelled carbon tetrachloride, and anhydrous gaseous [14C]-carbon labelled carbon tetrachloride has been studied together with the dehydrochlorination of 1,1,1 trichloroethane over the chlorine promoted gamma-alumina surface at 293K. The uptake of chlorine by the calcined gamma-alumina materials in conjunction with the B. E. T areas of the calcined materials has shown that up to 50% more surface chlorine resulted from the carbon tetrachloride treatment of a given gamma-alumina sample calcined above 373 K compared with the corresponding hydrogen chloride treatment. It has been shown that treatment of the gamma-alumina material with anhydrous gaseous hydrogen chloride resulted in labile surface chlorine. This treated surface was shown to be catalytically inactive towards the dehydrochlorination of 1,1,1 trichloroethane at 293K. Treatment of calcined gamma-alumina with carbon tetrachloride at 500K resulted in two forms of surface chlorine being produced one of which was not labile and consisted of ~40% of the total chlorine present at the surface. Catalytic activity towards the dehydrochlorination of 1,1,1 trichloroethane at 293K was shown to occur at surface sites which possessed non-labile surface chlorine . The labile properties of the catalytically inactive surface chlorine allowed the selective labelling of the catalytically active site with [36Cl]-chlorine. This selective labelling technique has shown that the dehydrochlorination reaction of 1,1,1 trichloroethane at 293K over the chlorine promoted gamma-alumina surface was consistent with a concerted intramolecular dehydrochlorination mechanism. Ab-initio SCF-UHF MO calculations using SV-321 G basis sets to cluster models of the aluminium environments present at the surface of gamma-alumina suggest that the active site may consist of a co-ordinatively unsaturated dichlorinated aluminium environment. The SCF-UHF MO calculations showed that a stabilization of the LUMO orbital energy occurred by the addition of chlorine to the gamma-alumina cluster model together with a lowering of the Fermi level of the cluster. The ab-initio calculations also show that the Bronsted acid character of a protonated cluster was enhanced by the addition of chlorine, as measured by the energy required to deprotonate the cluster, and that a strong Bronsted acid site required a near neighbour Lewis acid aluminium environment.