MAS NMR study of the mechanism of H/D exchange for deuterated propane adsorbed on H-ZSM-5

The kinetics of hydrogen exchange between the molecules of propane-d8, propane-1,1,1,3,3,3-d6, propane-2,2-d2, and Brönsted acid sites of the zeolite H-ZSM-5 have been monitored in situ by 1H MAS NMR spectroscopy in the temperature range of 230–280 ◦C. The intramolecular hydrogen transfer was estimated by in situ 13C MAS NMR spectroscopy of the kinetics of 13C-label scrambling in adsorbed propane-2-13C. The hydrogen exchange was found to occur directly between methyl or methylene groups of the alkane and the zeolite acid sites via a pentacoordinated carbonium ion. The exchange with the methyl groups is faster than that with the methylene group. This accounts for the earlier observed regioselectivity of the hydrogen exchange for propane on acidic zeolites (J. Am. Chem. Soc. 117 (1995) 1135). The intramolecular hydrogen transfer between methyl and methylene groups is one order of magnitude slower than the hydrogen exchange of the both groups with the zeolite acid sites.  2005 Elsevier Inc. All rights reserved.