CHLORINATION OF ALUMINA - AN IR SPECTROSCOPIC STUDY

Chlorination of gamma-alumina with various reagents (tetrachloromethan e: hexachloroethane, hexachloropropanone, thionyl chloride and hydroge n chloride) was followed by IR analysis of the gaseous products. CCl4 and SOCl2 directly react with the surface at 550 K. Chlorination with C2Cl6 occurs at 650 K, together with decomposition into Cl-2 and C2Cl4 . Reaction with C3Cl6O first involves a decomposition yielding COCl2 a nd C2Cl6, which then react with alumina. In any case, chlorination con verts the surface hydroxyl groups as well as some oxide ions The surfa ce acidity of the various Al2O3/Cl samples was then studied by infrare d spectroscopy of adsorbed basic molecules. Whatever the chlorination procedure, pyridine detected Lewis acid centers of comparable strength and density. In addition, pyridinium ions appeared on alumina samples chlorinated with HCl, but the corresponding Bronsted acidity could no longer be evidenced when the solids were outgassed above 370 K prior to pyridine adsorption. Similarly, lutidine detected Lewis centers and the various samples could be ranked in order of their acidic strength . Moreover, lutidinium ions were found on most samples, and vanished a t 420 K. Thus, the occurrence of Bronsted sites is ascribed to some we akly bound HCl. The surface acidity is presumably very strong, as the solids catalyze the alkylation of isobutane with ethylene. Spectroscop ic and catalytic data were compared and the selectivity for monoalkyla tion (versus, for example, multiple alkylation) appears to correlate w ith the number of Bronsted sites.