THERMAL AND HYDROTHERMAL STABILITY OF THE SILICOALUMINOPHOSPHATE SAPO-40

Thermal and hydrothermal stability of SAPO-40 were studied by X-ray po wder diffraction (XRD) and magic-angle spinning nuclear magnetic reson ance (MAS-NMR). For the X-ray study, the hydrothermal treatments were carried out, in situ, in a high-temperature chamber. The SAPO-40 struc ture shows a remarkable stability to high temperatures under both dry and humid air. At high temperatures a reorganisation of silicon occurs , but the long-range order is only lost above ca. 1000 degrees C. Chan ges in the XRD pattern induced by water at room temperature are comple tely reversible upon re-heating at 200 degrees C, as well as the chang es in the coordination and local environments of aluminium and phospho rus observed by NMR. For silicon however, some changes, that are obser ved upon hydration, remain after dehydration.