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

Abstract

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)2L] (L = surface oxygen) sites as the major surface organometallic entities. Conversely, grafting on silica affords dinuclear species [(≡SiO)2Ga2(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 benchmar...