n-Butane conversion on sulfated zirconia: in situ 13C MAS NMR monitoring of the kinetics of the 13C-label scrambling and isomerization

Abstract

The kinetics of the conversion of 13 C-labeled n-butane adsorbed on sulfated zirconia (SZ) were monitored by in situ 13 C MAS NMR spectroscopy. Rate constants of n-to isobutane isomerization and of the 13 C-isotope scrambling from the primary to the secondary carbon atoms in n-butane were determined. The monomolecular scrambling of the 13 C-label in adsorbed n-butane has an activation energy of 17 ± 3 kcal mol )1 and occurs faster than the bimolecular process of n-butane isomerization which has an activation energy of 15.1 ± 0.2 kcal mol )1 . The transfer of the selective 13 C-label from the primary to the secondary carbon atom in the adsorbed n-butane seems to consist of two reaction steps: (i) a hydride abstraction by SZ leading to the formation of sec-butyl cations and (ii) a label scrambling in the sec-butyl cations. This two-step process with the formation of sec-butyl cations as intermediate increases the apparent activation energy for the 13 C-label scrambling, which is almost twice as large compared with the activation energy for carbon scrambling of sec-butyl cations in a superacidic solution.

Mechanism Studies of the Conversion of 13C‐Labeled n‐Butane on Zeolite H‐ZSM‐5 by Using 13C Magic Angle Spinning NMR Spectroscopy and GC–MS Analysis

“…The obtained kinetic data indicate that the 13 Clabel scrambling in n-butane is performed with activation energy of approximately 75 kJ mol À1 (E 1 , Figure 5), which is in good agreement with the value of E a = 71 kJ mol À1 determined for the monomolecular 13 C-label scrambling in nbutane adsorbed onto sulfated zirconia. [44] The activation energy value of approximately 71 kJ mol À1 (E 2 , Figure 5) obtained for the isobutane formation is rather close to the activation energy value of 58-63 kJ mol À1 reported earlier for the isobutane formation by a bimolecular reaction mechanism on solid acid catalysts. [20,44,45] The apparent activation energy of approximately 71 kJ mol À1 (E 4 , Figure 5) observed for the propane formation is much lower in comparison with the value determined for the propane formation from nbutane on zeolite H-ZSM-5 (E a = 133 kJ mol…”

“…[44] The activation energy value of approximately 71 kJ mol À1 (E 2 , Figure 5) obtained for the isobutane formation is rather close to the activation energy value of 58-63 kJ mol À1 reported earlier for the isobutane formation by a bimolecular reaction mechanism on solid acid catalysts. [20,44,45] The apparent activation energy of approximately 71 kJ mol À1 (E 4 , Figure 5) observed for the propane formation is much lower in comparison with the value determined for the propane formation from nbutane on zeolite H-ZSM-5 (E a = 133 kJ mol…”

Isotopic Labeling and Kinetic Isotope Effects

“…A number of organic substances labeled with 2 H, 3 H, 13 C, 14 C, 15 N, or 18 O isotopes are commercially available, but the rational application of tracer techniques often requires the synthesis of a labeled compound. Recent developments in NMR and mass spectrometry have proliferated the application of stable isotopes, which are much easier to handle than radioactive isotopes.…”

“…• the C 18 O formed from the carbon present in the MS chamber and on the filament, which represents 3.2% of 18 For mechanistic studies, both the determination of the labeled species (among all other reactants) and the labeling position within the molecule are important. By using the chemical shifts of NMRactive nuclei as fingerprints of the positional isotope distribution, NMR spectroscopy can be used to identify the chemical structure of intermediates and products of catalytic transformations.…”

“…Thus, in situ 13 C MAS NMR has not only the advantage of tracing the 13 C labels both in gaseous and adsorbed states during reaction, but also the ability to monitor the kinetics [13][14][15][16][17][18]. Thus, in situ 13 C MAS NMR has not only the advantage of tracing the 13 C labels both in gaseous and adsorbed states during reaction, but also the ability to monitor the kinetics [13][14][15][16][17][18].…”