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 = CH 2 CH-(CH 3 ) 2 ) with the dehydrated and partially dehydroxylated surfaces of alumina (Al 2 O 3−500 ) and silica (SiO 2−700 ) were studied by IR, high field solid-state NMR and EXAFS spectroscopies, as well as elemental analysis. Grafting onto Al 2 O 3−500 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.

The Comparison between Single Atom Catalysis and Surface Organometallic Catalysis

“…It was found that grafting on alumina occurs mostly as in mononuclear gallium sites while grafting on silica results in dinuclear species. 113 SOMC and EXAFS spectroscopy also shine when crystalline supports with well-defined surface chemistry are used to isolate single atoms. Gates et al have notably investigated two types of supports: nonporous nanoparticles with preferentially exposed facets (e.g., MgO, TiO 2 ) and crystalline microporous solids (zeolite and metal organic framework).…”

Heterogeneous alkane dehydrogenation catalysts investigated via a surface organometallic chemistry approach

“…parallels what is found for Cr (vide supra). 58,80 Note however that Al 2 O 3 also displays a considerable PDH activity and selectivity (ca. 4% conversion, E40% total activity after 20 hours onstream).…”

Facilitating the reduction of V–O bonds on VOx/ZrO2 catalysts for non-oxidative propane dehydrogenation

“…This phenomenon is similar to that of the GaO x catalytic system, where the activity is not a consequence of Ga nuclearities, but of the Ga-O-support interaction. 34 To further evaluate the intrinsic activity in C-H activation, propane temperature-programmed surface reaction (C 3 H 8 -TPSR) experiments were carried out over 1VZr, ZrO 2 and 1VAl. The temperature where the C 3 H 8 signal begins to drop is determined as the C-H activation temperature.…”