Water Coordination, Proton Mobility, and Lewis Acidity in HY Nanozeolites: A High-Temperature H-1 and Al-27 NMR Study

摘要

A nanosized HY zeolite was synthesized and studied by means of H-1 and Al-27 NMR during thermal dehydration in the temperature range 20-600 degrees C. The nanozeolite is comprised of a mixture of well-crystallized ultrathin platelets and octahedral nanocrystals, dressed with pentacoordinated extraframework Al-(V). H-1 NMR spinlattice (T-1) and spinspin (T-2) relaxation measurements in combination with Al-27 3Q-MAS NMR reveal two different interaction paths between water molecules and the nanozeolite solid matrix: (i) water molecules strongly interacting with Al-(V) cations, indicated by the high T-1/T2 ratio, and (ii) water molecules with amply smaller T-1/T-2 ratio, interacting moderately with Al-(IV) and Al-(VI) cations. Relevant measurements on bulk HY rich in extraframework Al-(VI) show the presence of the second relaxation channel only, indicating that the enhanced water adsorption observed for the nanozeolite originates partly from its extended surface and partly from the Al-(V) decoration. Al-(IV) sites in the nanozeolite appear to be highly resilient during heating, even while the framework starts to collapse and Al-(VI) transforms to Al-(V). Finally, H-1 NMR shows that water protons interact particularly strongly with the Al sites in the nanozeolite at temperatures as high as 500 degrees C, unveiling the important role of the Al-(V) decoration on this nanocatalyst.