C–H Activation on Co,O Sites: Isolated Surface Sites versus Molecular Analogs

Abstract

The activation and conversion of hydrocarbons is one of the most important challenges in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated on silica surfaces are known to catalyze such processes. The mechanisms of these processes are currently unknown but are thought to involve C-H activation as the rate-determining step. Here, we synthesize well-defined Co(II) ions on a silica surface using a metal siloxide precursor followed by thermal treatment under vacuum at 500 °C. We show that these isolated Co(II) sites are catalysts for a number of hydrocarbon conversion reactions, such as the dehydrogenation of propane, the hydrogenation of propene, and the trimerization of terminal alkynes. We then investigate the mechanisms of these processes using kinetics, kinetic isotope effects, isotopic labeling experiments, parahydrogen induced polarization (PHIP) NMR, and comparison with a molecular analog. The data are consistent with all of these reactions occurring by a common mechanism, involving heterolytic C-H or H-H activation via a 1,2 addition across a Co-O bond.

Cobalt‐Based Catalyst Supported on Different Morphologies of Alumina for Non‐oxidative Propane Dehydrogenation: Effect of Metal Support Interaction and Lewis Acidic Sites

“…[37] At 300°C, one weak peak at 1580 cm À 1 appeared which belongs to the carbonaceous products. [39] As reported previously by many studies in the presence of hydrocarbons, the peaks observed between 1585-1560 cm À 1 belongs to carbonaceous products [38] Furthermore, upon switching propane with He gas at 550°C (Figure 10(c)), the bands at 2967 and 1370 cm À 1 disappeared and a broad peak at 1585 cm À 1 still appeared.…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[89] Calculations performed on Cr III /SiO 2 also suggest that b-alkyl elimination from an M-propyl intermediate can lead to ethylene and methane-the cracking products. [89,91,93,94,96] Para-hydrogeninduced polarization (PHIP) experiments for Cr and Co have indicated that p-H 2 could be incorporated into propene in ap airwise fashion, consistent with the hydrogenation and dehydrogenation involving heterolytic H À Ha nd C À Hs plitting at an M À O site. [89,91,93,94,96] Para-hydrogeninduced polarization (PHIP) experiments for Cr and Co have indicated that p-H 2 could be incorporated into propene in ap airwise fashion, consistent with the hydrogenation and dehydrogenation involving heterolytic H À Ha nd C À Hs plitting at an M À O site.…”

“…[88,89] Achromium aluminate material free of surface Si sites, Cr-2/Al 2 O 3 ,has also been prepared by the grafting of Cr(k 2 -Al(OiPr) 4 ) 3 (Cr-2)o nto Al 2 O 3-700 followed by thermal treatment at 400 8 8C( Scheme 2d). [89] Similarly,monomeric Ga III [90] and Co II [91] sites supported on silica have also been prepared by using this approach (Scheme 2). Thep resence of monomeric sites was established through av ariety of spectroscopic techniques,i ncluding aW avelet transform analysis of the EXAFS data.…”

“…[88,89,91,93,96] Ther ate-limiting step involves heterolytic C À H bond activation across an MÀO bond, thereby resulting in the formation of an M-propyl species.S ubsequent b-hydride elimination results in the formation of propylene.A fter release of propylene,h ydrogen elimination from the resulting M(-H)···OH intermediate completes the catalytic cycle for PDH. [88,89,91,93,96] Ther ate-limiting step involves heterolytic C À H bond activation across an MÀO bond, thereby resulting in the formation of an M-propyl species.S ubsequent b-hydride elimination results in the formation of propylene.A fter release of propylene,h ydrogen elimination from the resulting M(-H)···OH intermediate completes the catalytic cycle for PDH.…”

“…[88] Thehydrogenation of propene,the microscopic reverse of dehydrogenation, has also been studied to gain mechanistic insight to the PDH reaction over isolated metal sites. [89,91] Thel ow yield of product incorporating p-H 2 (< 5%)h as been attributed to significant relaxation of the p-H 2 by the investigated paramagnetic metal centers. [89,91] Thel ow yield of product incorporating p-H 2 (< 5%)h as been attributed to significant relaxation of the p-H 2 by the investigated paramagnetic metal centers.…”