INFRARED-SPECTROSCOPY OF TRIMETHYLALUMINUM AND DIMETHYLALUMINUM CHLORIDE ADSORBED ON ALUMINA

The species formed by adsorption of trimethylaluminum and dimethylalum inum chloride on gamma-alumina that has been dehydroxylated at 400 K a re identified using infrared spectroscopy. Gas-phase trimethylaluminum dimer is proposed to adsorb via reaction of the bridging methyl group s forming adsorbed dimethylaluminum. A small amount of dimer is also f ound on the surface immediately following exposure to trimethylaluminu m, but this is removed by prolonged evacuation at room temperature. Ad sorbed mono- and dimethylaluminum species are distinguished by their c orresponding peak profiles in the methyl bending region where, in the latter case, a splitting is found due to coupling between adjacent met hyl species. Relative dimethyl- and monomethylaluminum coverages can b e monitored as a function of annealing temperature where it is found t hat dimethylaluminum converts to monomethylaluminum by reaction with s urface hydroxyl groups and evolving methane. The remaining methyl grou p in the monomethylaluminum surface species reacts in a similar manner with surface hydroxyl species to form methane but is found to be sign ificantly less labile than the dimethylaluminum species. A similar rea ction pathway is also found for dimethylaluminum chloride. The surface species formed from dimethylaluminum chloride, however, react more ra pidly than those formed by trimethylaluminum adsorption.