Rietveld refinement of the calcined form of SAPO-40

In contrast to many aluminophosphate-based molecular sieves, calcined SAPO-40 ([Si₇Al₂₉P₂₈O₁₂₈]) appears to be remarkably stable at high temperatures in both dry and humid atmospheres. To investigate the structural basis for this unusual stability, a structure refinement of calcined SAPO-40 (Pccn, a = 22.2638(5), b = 13.7788(2), c = 14.1137(2)Å) using the Rietveld method was performed (R_wp = 0.080, R_exp = 0.043 and R_F = 0.043). Comparison of the framework structure before and after calcination indicates that the unit cell expansion (1.5% along x and 0.6% along y) is due to small shifts in the positions of all atoms in the framework rather than by any local distortion involving only a few atoms. Diffuse electron density was found in the pores although several different calcination procedures were tried. This may indicate that the calcination was incomplete, but is more likely to be caused by the presence of silica and/or alumina species resulting from a reorganization of the framework during the calcination. The extra-framework material was described by including three additional oxygen positions in the structural model and refining their population parameters. The scattering power corresponds to about 90 electrons per unit cell.