In Situ ¹H and ¹³C MAS NMR Kinetic Study of the Mechanism of H/D Exchange for Propane on Zeolite H−ZSM-5

The kinetics of hydrogen (H/D) exchange between Brønsted acid sites of zeolite H−ZSM-5 and variously deuterated propanes (propane-d8, propane-1,1,1,3,3,3-d6, propane-2,2-d2) have been monitored in situ by 1H MAS NMR spectroscopy within the temperature range of 503−556 K. The contribution of intramolecular hydrogen transfer to the H/D exchange in the adsorbed propane was estimated by monitoring the kinetics of 13C-labeled carbon scrambling in propane-2-13C in situ with 13C MAS NMR at 543−573 K. Possible mechanisms of the exchange have been verified on the basis of the analysis of the variation of protium concentration in both the methyl and the methylene groups of propane in dependence of the reaction time. The main route of the exchange consists of a direct exchange of the acidic OH groups of the zeolite with either the methyl groups or the methylene group presumably with a pentacoordinated carbonium ion intermediate. The assumption that the intramolecular H scrambling between the methyl groups and the methylene group of propane via carbenium-ion-type intermediates is the fastest process among the other possible routes (J. Catal. 2003, 215, 122) does not account for the experimental kinetics of H/D exchange for propanes with different initial contents and locations of deuterium in a propane molecule. The rate constant (k3) for intramolecular H/D exchange between the methyl and the methylene groups is 4−5 times lower compared to those of the direct exchange of both the methyl (k1) and the methylene (k2) groups with Brønsted acid sites of the zeolite, the k1 being ca. 1.5 times higher than k2. At lower temperature (473 K), the exchange is slower, and the expected difference between k1 and k2 is more essential, k1 = 3k2. This accounts for earlier observed regioselectivity of the exchange for propane on H−ZSM-5 at 473 K (J. Am. Chem. Soc. 1995, 117, 1135). Faster direct exchange with the methyl groups compared to that with the methylene groups was attributed to a possible, more spatial accessibility of the methyl groups for the exchange. Similar activation energies for H and C scramblings with a 2 times more rapid rate of H scrambling was rationalization by the proceeding of these two processes through an isopropyl cation intermediate, as in classical carbenium ion chemistry.