1H–31P C P/MAS NMR studies of mesostructured aluminophosphates

Abstract

Mesostructured AlPO's with different structures synthesized ia the cationic templating pathway have been characterized using ¹H–³¹P cross-polarization/magic angle spinning (CP/MAS) NMR. Hexagonal (Hex) materials have much faster CP kinetics than lamellar (L) materials, which is reflected in shorter cross-polarization times (T_PH) and proton relaxation times in the rotating frame (T_1ρH). T_PH increases with the increasing degree of condensation of the inorganic component, which is consistent with the increased ¹H–³¹P distance. The CP dynamics of P sites connected to the surfactant head groups are affected by the fast motions of the protons in the –N(CH₃)₃ group. Measurements of T_1ρH indicate the microdomain structure of Hex-1, Hex-2 and L3, and two components with different proton relaxation rates have been identified. The fast relaxing component is attributed to protons within the inorganic network (mainly water molecules and OH groups coordinated to octahedral Al sites), while the component with the slower relaxation rate represents ¹H atoms at the inorganic/organic interface and from the organic component. The slower CP kinetics in mesolamellar AlPO's are caused by the particular structure of the inorganic network, consisting of tetrahedral Al with only a few protons located mainly at the inorganic/organic interface, that participates in the polarization transfer.