Solid-state NMR studies of the organic template in mesostructured aluminophosphates

摘要

Solid-state NMR has been used to monitor the conformation and mobility of the organic surfactant occluded in aluminophosphate-based mesocomposites with hexagonal (Hex) and lamellar (L) structures. At ambient temperature hexagonal aluminophosphates (AlPO's) show predominant disordered gauche conformation of the aliphatic chains. On the other hand, in the mesolamellar L1 and L3 the surfactant is in the ordered all-trans conformation, while in L2 it is in the gauche conformation despite the well-defined mesolamellar structure. The temperature of the all-trans double left right arrow gauche transition increases in the series: Hex-1, Hex-2 < L2 < L3 < L1, which reflects the decrease of the bulk mobility of the surfactant species. Different segments of the surfactant have different mobilities depending on the type of mesoscopic organization and the conformation of the aliphatic chain. Thus, mobilities of N-methylene and the adjacent methylene groups as well as that of the N-methyl groups located at the inorganic/organic interface depend on the conformation. On the other hand, H-1-C-13 CP dynamics and C-13 T-1 rho relaxation of the inner-chain carbons are determined by the mesophase structure. 2D-WISE and H-1 MAS NMR confirm that the surfactant species are more rigid in the trans-dominant mesocomposites (L1 and L3). The much narrower wide-line H-1 spectra and the much longer T-2 relaxation times for L2 than Hex-1 and Hex-2 contradict the trends derived from the variable temperature studies and H-1-C-13 CP dynamics. This effect is explained in terms of the local and bulk mobility. While the former is reflected in the 2D-WISE spectra, the latter detemines the temperature of the all-trans double left right arrow gauche transition and T-1 rho relaxation of C-13. [References: 29]