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 tem peratures in both dry and humid atmospheres. To investigate the struct ural basis for this unusual stability, a structure refinement of calci ned SAPO-40 (Pccn, a=22.2638(5), b=13.7788(2), c=14.1137(2)Angstrom) u sing 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 calci nation indicates that the unit cell expansion (1.5% along x and 0.6% a long y) is due to small shifts in the positions of all atoms in the fr amework rather than by any local distortion involving only a few atoms . Diffuse electron density was found in the pores although several dif ferent calcination procedures were tried. This may indicate that the c alcination was incomplete, but is more likely to be caused by the pres ence 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 st ructural model and refining their population parameters. The scatterin g power corresponds to about 90 electrons per unit cell. (C) 1997 Else vier Science B.V.