No 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.

The dealumination of BEA zeolite by treatment with concentrated nitric acid is evidenced by X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. Two-dimensional 27Al 3Q and 5Q magic-angle-spinning (MAS) NMR allow the detection of two kinds of tetrahedral Al atoms whose relative amounts depend on the Si/Al ratio and which correspond to two specific T-sites. 29Si MAS NMR and 1H MAS NMR measurements confirm these results. 29Si MAS NMR spectra evidence two resonances at around −114 and −111 ppm ascribed to Si sites of the Si(OSi)4 environment of two different crystallographic sites. Moreover, the presence of Si atoms associated with hydroxyl groups is confirmed by a resonance at −102 ppm when 1H−29Si CP is applied. The Brønsted and Lewis acidic sites in dealuminated BEA zeolites are evidenced by FTIR spectra of adsorbed pyridine which show two kinds of bridging hydroxyl groups (Si−O(H)−Al) of different acid strength. 51V MAS NMR confirms the incorporation of vanadium atoms into vacant T-atom sites of a fully dealuminated SiBEA zeolite leading to two kinds of tetrahedral V(V) sites (δiso = – 708 and – 766 ppm), with a V=O double bond and linked by V−OSi bonds to the framework. The two types of tetrahedral V(V) sites are in line with the two kinds of tetrahedral Al sites initially present in the zeolite. Moreover, the two bands at 3620 and 3645 cm−1 suggest that VSiBEA also contains V(V) sites with V(V)O−H groups, which exhibit Brønsted acidic character as shown by FTIR of adsorbed pyridine. Possible ways for the formation of tetrahedral V(V) in the BEA structure are proposed.