X-RAY-DIFFRACTION AND SOLID-STATE N.M.R. STUDIES OF AEL MOLECULAR-SIEVES - EFFECT OF HYDRATION

Interpretation of X-ray powder diffraction spectra by using profile refinement techniques allowed the determination of (i) the positions of P, Al and O atoms in AEL-type materials (AlPO4-11, SAPO-11) and (ii) the effect of adsorbed water molecules on the geometry of the previously evacuated AEL framework. The as-synthesized sample (hereafter labeled SYN form) and the 550°C calcined form (CAL form) crystallize in the acentric Ima2 space group with 3P + 3Al sites per asymmetric unit, whereas the rehydrated material (WAT form) adopts the acentric Pna21 space group with 5P + 5Al sites per asymmetric unit. In MAS-n.m.r. spectroscopy, a broad 31P peak observed for the SYN and the dehydrated CAL forms is centered near δ = -30.7 ppm and is interpreted as the evelope of three overlapping components with 2:1:2 intensity ratios (8P1:4P2:8P3). For the hydrated WAT form two peaks centered at δ = -23 and -29 ppm having an intensity ratio of 1:4 have been attributed to the 4P1 and the 4P2 + 4P3 + 4P4 + 4P5 sites, respectively. The first peak corresponds to the P1 species, which is attached to OH and/or water molecules, as shown by proton cross-polarization experiments. The second broad peak, which corresponds to the remaining P atoms, is interpreted as four overlapping peaks of equal intensity. Water molecules were shown by X-ray diffraction experiments to form dimeric clusters and to be located close to P1 in the elliptical channels. It is suggested that this atom could be substituted by Si atoms in the SAPO-11 samples, as supported by the observed 1:4 intensity ratio and by the fact that the intensity of the component corresponding to P1 decreases with increasing Si content. © 1990.