H-1 SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE MOBILITY OF THE TETRAPROPYLAMMONIUM TEMPLATE IN A PURELY SILICEOUS MFI-TYPE ZEOLITE

Dynamics of tetrapropylammonium (TPA) cation occluded during the synth esis in a siliceous MFI zeolite is investigated by H-1 broad-line nucl ear magnetic resonance (NMR) methods. Second moments M(2), spin-lattic e relaxation times T-1 and T-1 rho are measured in a large temperature domain. To allow comparison, similar measurements are also reported i n bulk tetrapropylammonium bromide (TPABr). Whereas methyl reorientati on at low temperature and tumbling of the cation in the plastic phase are observed in crystalline TPABr in accordance with published studies , a new slower motion which could not be identified is observed below the phase transition. Such a motion is much more clearly shown by the existence of a minimum of T-1 rho in a quenched sample. Our measuremen ts in the zeolite demonstrate that the TPA template exhibits a larger and more complex mobility below 378 K. Beyond the fast methyl reorient ation, the results disclose a motion which probably involves the entir e propyl arms inside the channels. So the zeolite framework seems to m ake such a kind of motion easier. On the contrary, even at 450 K, the highest temperature investigated, the tumbling which would necessitate exchange of the propyl arms between the channels is not observed. A s low motion, responsible for a decrease of T-1 rho above 350 K, could n ot be identified. While a simple correlation time is sufficient to des cribe the relaxation time dependences in TPABr, a distribution (such a s Williams-Watts) is required to account for those in the zeolite.