Investigation of slow molecular motions and chemical exchange in the high loaded form of p-xylene in ZSM-5 by two-dimensional 13C solid-state NMR exchange experiments

The slow molecular motions and exchange processes in the high loaded form of p-xylene in zeolite ZSM-5 have been investigated by a variety of solid-state NMR techniques. The C-13 CP MAS NMR spectra of [13C, CH3] p-xylene show four resonances in the slow exchange limit, reflecting the four inequivalent methyl positions in the two organic molecules in the asymmetric unit. From the known assignment of the silicon resonances in the MAS NMR spectrum, the four carbon signals were assigned from the cross-correlations in a H-1/C-13/ Si-29 2D CP TEDOR experiment. Using this information, the chemical exchange processes between the methyls was invesitgated by a series of 2D CP NOESY experiments with different mixing times at different temperatures. Two types of processes were identified: intramolecular exchanges with activation energies of 65 +/- 2 kJ/m for the molecule in the zigzag channel and 56 +/-13 kJ/m for the molecule in the straight channel (the substantial error in the latter value reflecting the very limited temperature range over which measurements can be made). There is also intermolecular exchange between the two molecules with a much higher energy barrier. Possible mechanisms for the exchanges and the implications of slow motional processes for the use of solid-state NMR for the determination of sorbate/frame work structures are discussed.