In situ monitoring of n-butene conversion on H-ferrierite by ¹H, ²H, and ¹³C MAS NMR: kinetics of a double-bond-shift reaction, hydrogen exchange, and the ¹³C-label scrambling

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• 作者：Stepanov ; Alexander G. ; Arzumanov ; Sergei S. ; Luzgin ; Mikhail V. ; Ernst ; Horst ; Freude ; Dieter
• 关键词：n-Butene ; H-FER zeolite ; Kinetics ; H/D exchange ; Double-bond-shift reaction ; ¹³C-label scrambling ; Reaction mechanism ; ¹³C MAS NMR spectroscopy
• 刊名：Journal of Catalysis
• 出版年：2005
• 期刊代码：97_00219517
• 类别：ce
• 出版时间：January 1, 2005
• 卷：229
• 期：1
• 页码：243-251
• 文件大小：303 K

摘要

Kinetics of a double-bond-shift reaction, hydrogen exchange, and ¹³C-label scrambling were monitored in situ by ²H, ¹H, and ¹³C MAS NMR for n-but-1-ene adsorbed on the zeolite ferrierite under batch reactor conditions at 290–373 K. A double-bond-shift reaction, the fastest among the three reactions studied, can be monitored provided that 97%of Brønsted acid sites are substituted by Na cations. The activation energy for this reaction was found to be 9.8 kcal mol⁻¹. Hydrogen exchange with protons from the zeolite is observed for both methene and methyl groups of n-but-2-ene, formed from the initial n-but-1-ene. The terminal olefinic CH₂ group of n-but-1-ene is involved in the exchange, providing the pathway for the exchange into the methyl group of the n-but-2-ene, mainly observed in the spectrum in accordance with thermodynamic equilibrium between n-but-1-ene and n-but-2-ene. This offers similar apparent activation energies of about 7 kcal mol⁻¹ for the exchange into methene and methyl groups of n-but-2-ene. The ¹³C-label scrambling in n-but-2-ene is indicative of sec-butyl cation formation from the olefin in the zeolite framework, which can be formed as a small quantity of transient species not detectable by NMR but providing the label scrambling. The apparent activation energy for the ¹³C-label scrambling was found to be 21±2 kcal mol⁻¹, which is three times higher compared with the activation energy for the label scrambling in sec-butyl cation in a superacidic solution.