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

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• 作者：Murielle F. DelleyMarius-C. Silaghi ; Francisco Nuñez-Zarur ; Kirill V. KovtunovOleg G. Salnikov ; Deven P. Estes ; Igor V. Koptyug ; Aleix Comas-Vives ; Christophe Copéret
• 刊名：Organometallics
• 出版年：2017
• 出版时间：January 9, 2017
• 年：2017
• 卷：36
• 期：1
• 页码：234-244
• 全文大小：537K
• ISSN：1520-6041

文摘

We synthesized isolated Cr(III) sites on SiO₂−Al₂O₃ and Al₂O₃ by grafting and subsequent controlled thermal treatment of Cr(OSi(O^tBu)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.