Water coordination, proton mobility, and Lewis acidity in HY nanozeolites: A high-temperature ¹H and ²⁷Al NMR study

A nanosized HY zeolite was synthesized and studied by means of ¹H and ²⁷Al NMR during thermal dehydration in the temperature range 20-600 °C. The nanozeolite is comprised of a mixture of well-crystallized ultrathin platelets and octahedral nanocrystals, dressed with pentacoordinated extraframework Al^(V). ¹H NMR spin-lattice (T₁) and spin-spin (T₂) relaxation measurements in combination with ²⁷Al 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 T₁/T₂ ratio, and (ii) water molecules with amply smaller T₁/T₂ 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, ¹H NMR shows that water protons interact particularly strongly with the Al sites in the nanozeolite at temperatures as high as 500 °C, unveiling the important role of the Al^(V) decoration on this nanocatalyst.