Crystallographic site probing in the triclinic CHA-like AlPO4 by Al-27 2D quintuple-quantum NMR

The triclinic CHA-like precursor of AlPO4-34 was prepared in the presence of HF by using pyridine as a template. The three resonance lines corresponding to one 6-coordinated (Al(1)) and two 4-coordinated Al atoms (Al(2) and Al(3)) expected from a previous structure analysis are clearly visible on the well resolved Al-27 2D-quintuple-quantum NMR spectrum. The actual isotropic chemical shifts of the three lines were easily determined from the 2D spectrum; the corresponding values are -3.7 ppm (Al(1)), 46.7 ppm (Al(2)) and 52.5 ppm (Al(3)) with respect to Al(H2O)(6)(3+). On the contrary, the two tetrahedral sites could not be differentiated by the MAS technique; only two lines were observed, one in the octahedral region (Al(1)) and one in the tetrahedral region (Al(2))+Al(3)) with a 1:2 intensity ratio.