Influence of the Choice of the Template on the Short- and Long-Term Stability of SAPO-34 Zeolite

Abstract

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, 27Al and 29Si MAS NMR, and N2 adsorption are used to characterize the related structural changes. The template decomposition results into shifts in 27Al and 29Si MAS NhfR peaks reflecting its interaction with the framework. Hydration of the template-free materials leads at room temperature to a breaking of Si-OH-A1 bonds, giving a partial loss of crystallinity and the formation of Q', Q2, and Q3 Si species (Si(OT),(OH)+,). 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- A1 bonds, Le., of points where water attacks the framework.