X–H Bond Activation on Cr(III),O Sites (X = R, H): Key Steps in Dehydrogenation and Hydrogenation Processes

Abstract

We synthesized isolated Cr­(III) sites on SiO2−Al2O3 and Al2O3 by grafting and subsequent controlled thermal treatment of Cr­(OSi­(OtBu)3)3(THF)2 and Cr­(Al­(OiPr)4)3 on alumina. CrO x /Al2O3 was obtained from incipient wetness impregnation of Al2O3 with CrO3 in H2O followed by calcination, as carried out for the synthesis of industrial Cr-based dehydrogenation catalysts. These materials were characterized by IR, EPR, XAS, and by the adsorption of the probe molecules CO and pyridine, and the results were compared to previously reported isolated Cr­(III)/SiO2. All of these materials were active in propane dehydrogenation at 550 °C, where higher TOFs were obtained for Cr­(III)/SiO2-Al2O3 and Cr­(III)/Al2O3 than for CrO x /Al2O3 and Cr­(III)/SiO2. For mechanistic studies the reverse reaction, propene hydrogenation, was studied. Here, the order of reactivity was opposite that of dehydrogenation, with CrO x /Al2O3 and Cr­(III)/SiO2 being more active in hydrogenation than Cr­(III)/SiO2-Al2O3 and Cr­(III)/Al2O3. Kinetic analysis and labeling studies with D2 showed that the rate law is in all cases first order in H2 partial pressure but had different dependence on propene partial pressure from catalyst to catalyst. We found small normal kinetic isotope effects of 1 ≤ KIE ≤ 2, activation enthalpies up to 40 kJ mol–1, and large negative activation entropies between −100 and −194 J K–1 mol–1 for the different Cr catalysts. We also performed parahydrogen-induced polarization (PHIP) experiments, which showed that H2 addition to propene proceeds, at least in part, via a pairwise mechanism. The experimental data for propene hydrogenation suggests adsorption/desorption pre-equilibria of H2 (or D2) and propene followed by H2 activation and insertion of propene. DFT computations for propane dehydrogenation and propene hydrogenation on Cr­(III) on a periodic alumina model show that a mechanism involving X–H activation (X = R, H) is energetically feasible with activation enthalpies and entropies that are comparable to the experimentally determined values.

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.…”

“…[88,89,91,93,96] 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. [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] 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.…”

“…[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.…”