INFLUENCE OF THE CHOICE OF THE TEMPLATE ON THE SHORT-TERM AND LONG-TERM STABILITY OF SAPO-34 ZEOLITE

SAPO-34 (chabasite structure) is studied with regard to decomposition of the template [tetraethylammonium hydroxide (TEAOH) or morpholine] and subsequent hydration-dehydration cycles over a period of few hours up to 2 years. XRD, IR, Al-27 and Si-29 MAS NMR, and N-2 adsorption an used to characterize the related structural changes. The template decomposition results into shifts in Al-27 and Si-29 MAS NMR peaks reflecting its interaction with the framework. Hydration of the template-free materials leads at room temperature to a breaking of Si-OH-Al bonds, giving a partial loss of crystallinity and the formation of Q(1), Q(2), and Q(3) Si species (Si(OT)(n)(OH)(4-n)). Dehydration of the materials after about 1 or 2 day hydration gives reversibly the. initial crystallinity, the Si environment, and the porosity for the template-free materials. Long-term hydration, up to 2 years, shows that zeolites synthesized with morpholine as a template do not recover their full crystallinity and eventually none of their porosity. By contrast, TEAOH materials-are more stable, keeping up to 80% of the starting crystallinity and 70% of their porosity. The influence of the template is correlated to the formation, during synthesis of Si islands with TEAOH, while morpholine gives a larger part of isolated Si. This restricts in the TEAOH sample the number of Si-OH-Al bonds, i.e., of points where water attacks the framework.