A Strong Support Effect in Selective Propane Dehydrogenation Catalyzed by Ga(i-Bu)3 Grafted onto γ-Alumina and Silica

The reactions of Ga(i-Bu)(3) (i-Bu = CH2CH-(CH3)(2)) with the dehydrated and partially dehydroxylated surfaces of alumina (Al2O3-500) and silica (SiO2-700) were studied by IR, high field solid-state NMR and EXAFS spectroscopies, as well as elemental analysis. Grafting onto Al2O3-500 occurs selectively by protonolysis at individual surface hydroxyl groups, resulting in the formation of mononuclear [( AlO)Ga(i-Bu)(2)L] (L = surface oxygen) sites as the major surface organometallic entities. Conversely, grafting on silica affords dinuclear species [( SiO)(2)Ga-2(i-Bu)(3)] by a combination of protonolysis and isobutyl group transfer to Si. Further evidence for the difference in nuclearity was obtained by analysis of the WT-EXAFS. The mononuclear alumina-supported Ga sites show much higher activity in propane dehydrogenation than their dinuclear silica-supported counterparts. The propane dehydrogenation reaction may require the presence of Al-O-Ga bonds to promote heterolytic C-H bond activation. Comparisons with benchmark catalysts and related systems show that the effect of the catalyst diluent is significant under the reaction conditions and must be carefully assessed in order to attribute reactivity correctly.