报告主题：Protonation of C3-C5 Alkenes in H-FER Zeolites – Ab Initio Studies
报告人：任清华副教授 （上海大学）
报告时间：2019年10月12日（周六）9:30
报告地点：校本部G507
主办部门：理学院化学系
报告摘要：
Proton-exchanged zeolites are successfully used as catalysts for hydrocarbon transformation reactions such as skeletal isomerization of linear alkenes. The activity of these catalysts is based on the acidity of their Brønsted sites and their ability to donate a proton to an alkene, which is transformed into a positively charged carbenium ion. These carbenium ions are supposed to play a key role in the isomerization of alkenes. However, there is experimental [1] and computational [2] evidence that small protonated olefins do not exist as free carbenium ions, but rather as alkoxides bound to the aluminosilicate framework. Whether protonated olefin intermediate exist as alkoxides or as carbenium ion species inside zeolites as intermediates has been extensively debated in the literature. Ab initio quantum chemistry can answer the aforementioned question, but is useful only when the relative energies can be predicted with chemical accuracy (4 kJ/mol). [3] We use a hybrid MP2: (PBE+D2) + ΔCCSD(T) method to determine relative stabilities of possible intermediates for alkene protonation in zeolite H-FER, that is, π – adsorption complexes of different isomers of propene, butene and pentene, all kinds of alkoxides formed upon chemisorption of these alkenes, and the corresponding carbenium ions. Enthalpies and Gibbs free energies are calculated for 323K and 623K from harmonic wavenumbers. The primary and secondary alkoxide species are the most energetically stable chemisorbed structures. The corresponding π - complexes have very similar energies. The primary carbenium ions are not stationary points on the potential energy surface, whereas the secondary and tertiary butyl and pentyl cations are meta-stable species, but their energies are much higher than that of the corresponding alkoxides and π – complexes. The calculated adsorption enthalpy of 2-pentene over H-FER is in good agreement with the experimental value. At 623K, the entropic contributions included in the Gibbs free energies strongly destabilizes the tightly bound C3-C5 alkoxide species, whereas the mobile π – complexes are thermodynamically more stable than the alkoxide species and the carbenium ions.
References: [1] J. F. Haw; J.B. Nicholas; T. Xu; L.W. Beck; D.B. Ferguson;Acc. Chem. Res. 29(1996) 259 [2] V. Nieminen; M. Sierka; D. Y. Murzin; J. Sauer;J. Catal.231 (2005) 393 [3] M. Rybicki and J. Sauer; J. Am. Chem. Soc. 140(2018) 18151
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