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

Kinetics of a double-bond-shift reaction, hydrogen exchange, and C-13-label scrambling were monitored in situ by H-2, H-1, and C-13 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 Bronsted 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 C-13-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 C-13-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. (C) 2004 Published by Elsevier Inc.