Metal-Catalyzed C鈥揅 Bond Cleavage in Alkanes: Effects of Methyl Substitution on Transition-State Structures and Stability

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• 作者：David W. Flaherty ; David D. Hibbitts ; Enrique Iglesia
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：July 9, 2014
• 年：2014
• 卷：136
• 期：27
• 页码：9664-9676
• 全文大小：708K
• ISSN：1520-5126

文摘

Methyl substituents at C鈥揅 bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C₂鈥揅₈ alkanes on Ir, Rh, Ru, and Pt catalysts. C鈥揅 cleavage transition states form via equilibrated dehydrogenation steps that replace several C鈥揌 bonds with C-metal bonds, desorb H atoms (H*) from saturated surfaces, and form 位 H₂(g) molecules. Activation enthalpies (螖H) and entropies (螖S) and 位 values for ³C鈥?sup>xC cleavage are larger than for ²C鈥?sup>2C or ²C鈥?sup>1C bonds, irrespective of the composition of metal clusters or the cyclic/acyclic structure of the reactants. ³C鈥?sup>xC bonds cleave through 伪,尾,纬- or 伪,尾,纬,未-bound transition states, as indicated by the agreement between measured activation entropies and those estimated for such structures using statistical mechanics. In contrast, less substituted C鈥揅 bonds involve 伪,尾-bound species with each C atom bound to several surface atoms. These 伪,尾 configurations weaken C鈥揅 bonds through back-donation to antibonding orbitals, but such configurations cannot form with ³C atoms, which have one C鈥揌 bond and thus can form only one C鈥揗 bond. ³C鈥?sup>xC cleavage involves attachment of other C atoms, which requires endothermic C鈥揌 activation and H* desorption steps that lead to larger 螖H values but also larger 螖S values (by forming more H₂(g)) than for ²C鈥?sup>2C and ²C鈥?sup>1C bonds, irrespective of alkane size (C₂鈥揅₈) or cyclic/acyclic structure. These data and their mechanistic interpretation indicate that low temperatures and high H₂ pressures favor cleavage of less substituted C鈥揅 bonds and form more highly branched products from cyclic and acyclic alkanes. Such interpretations and catalytic consequences of substitution seem also relevant to C鈥揦 cleavage (X = S, N, O) in desulfurization, denitrogenation, and deoxygenation reactions.