Transition-State Enthalpy and Entropy Effects on Reactivity and Selectivity in Hydrogenolysis of n-Alkanes
详细信息 查看全文
- 作者：David W. Flaherty ; Enrique Iglesia
- 刊名：Journal of the American Chemical Society
- 出版年：2013
- 出版时间：December 11, 2013
- 年：2013
- 卷：135
- 期：49
- 页码：18586-18599
- 全文大小：718K
- 年卷期：v.135,no.49(December 11, 2013)
- ISSN：1520-5126
文摘
Statistical mechanics and transition state (TS) theory describe rates and selectivities of C鈥揅 bond cleavage in C2鈥揅10 n-alkanes on metal catalysts and provide a general description for the hydrogenolysis of hydrocarbons. Mechanistic interpretation shows the dominant role of entropy, over enthalpy, in determining the location and rate of C鈥揅 bond cleavage. Ir, Rh, and Pt clusters cleave C鈥揅 bonds at rates proportional to coverages of intermediates derived by removing 3鈥? H-atoms from n-alkanes. Rate constants for C鈥揅 cleavage reflect large activation enthalpies (螖H, 217鈥?57 kJ mol鈥?) that are independent of chain length and C鈥揅 bond location in C4+ n-alkanes. C鈥揅 bonds cleave because of large, positive activation entropies (螖S, 164鈥?59 J mol鈥? K鈥?) provided by H2 that forms with TS. Kinetic and independent spectroscopic evidence for the composition and structure of these TS give accurate estimates of 螖S for cleavage at each C鈥揅 bond. Large differences between rate constants for ethane and n-decane (108) reflect an increase in the entropy of gaseous alkanes retained at the TS. The location of C鈥揅 bond cleavage depends solely on the rotational entropies of alkyl chains attached to the cleaved C鈥揅 bond, which depend on their chain length. Such entropy considerations account for the ubiquitous, but previously unexplained, preference for cleaving nonterminal C鈥揅 bonds in n-alkanes. This mechanistic analysis and thermodynamic treatment illustrates the continued utility of such approaches even for hydrogenolysis reactions, with complexity seemingly beyond the reach of classical treatments, and applies to catalytic clusters beyond those reported here (0.6鈥?.7 nm; Ir, Rh, Pt).