Alkane C-H Bond Activation in Zeolites: Evidence for Direct Protium Exchange

摘要

The mechanism of alkane C-H bond activation in heterogeneous acid catalysis is unknown.~1H solid-state NMR techniques have been used to simultaneously detect the reactivity of both catalyst and alkane reactant protons in a true in-situ experimental design.Specifically,the activation of isobutane C-H bonds by the solid acid zeolite HZSM-5 is directly observed,and the rate of proton transfer between the solid catalyst surface and gaseous isobutane is quantitatively measured using isotopic ~1H/~2H exchange methods.An observable adsorption complex forms between the isobutane and the primary Bronsted acid site of ZSM-5,which leads to proton exchange between the zeolite surface and the isobutane methyl groups at temperatures (273 K) much lower than previously reported.The secondary acid site in ZSM-5 is less accessible to or less reactive with the isobutane molecule.Simultaneous detection of protium loss from the Bronsted acid site and protium gain by perdeuterated isobutane reveals a common rate constant equal to 4.1-4.6 x 10~(-4) s~(-1) at 298 K,but at lower temperatures,the transition between this and a much slower rate process is resolved.The measured activation energy for isobutane H/D exchange is 57 kJ/mol.In all experiments,the isobutane reagent was purified to eliminate any unsaturated impurities that might serve as initiators for carbenium-ion mechanisms,and the active catalyst was free of any organic contaminants that might serve as a source of unsaturated initiators.In total,our results are consistent with direct proton exchange between the zeolite surface and the methyl groups of isobutane.