Role of Spatial Constraints of Brønsted Acid Sites for Adsorption and Surface Reactions of Linear Pentenes
| Autor: | Johannes A. Lercher, Stefan Schallmoser, Maricruz Sanchez-Sanchez, Gary L. Haller |
|---|---|
| Rok vydání: | 2017 |
| Předmět: |
chemistry.chemical_classification
Double bond 010405 organic chemistry Chemistry Hydrogen bond Enthalpy General Chemistry 010402 general chemistry Photochemistry 01 natural sciences Biochemistry Catalysis 0104 chemical sciences chemistry.chemical_compound Colloid and Surface Chemistry Adsorption Ferrierite Pentene Alkoxy group Brønsted–Lowry acid–base theory |
| Zdroj: | Journal of the American Chemical Society. 139(25) |
| ISSN: | 1520-5126 |
| Popis: | The Brønsted acid sites of H-ZSM-5 and ferrierite reversibly adsborb linear pentenes via hydrogen bonding, rapidly isomerizing the double bond. On H-ZSM-5, dimerization of adsorbed pentenes occurs at a slower rate and leads to pentyl ester covalently bound to the surface. Pentene adsorbed on zeolites with narrower pores, such as ferrierite, remained stable in a hydrogen-bonded state even up to 423 K. Comparing the differential heat of adsorption of 2-pentene on silicalite and ferrierite allowed for the determination of the enthalpy difference between physically adsorbed pentene in ZSM-5 and the localized hydrogen-bonded π-complex at Brønsted acid sites, -36 kJ/mol. The activation energy (35 kJ/mol) for dimerization is almost identical to this enthalpy difference, suggesting that the rate-determining step is associated either with the mobilization of π-bonded 2-pentene or with the equally large activation barrier to form an alkoxy group via a carbenium-ion transition state. In a closed system, the dimerization rate is first order in the concentration of the π-complex that is both in equilibrium with the mobile pentene phase and in production of the carbenium ion that reacts with the mobile pentene. Overall, the alkoxy group is -41 ± 7 kJ/mol more stable than physisorbed pentene, establishing a series of energetically well-separated groups of reactive surface species. |
| Databáze: | OpenAIRE |
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