Phase-Transformation Synthesis of SAPO-34 and a Novel SAPO Molecular Sieve with RHO Framework Type from a SAPO-5 Precursor

SAPO-34 and a novel SAPO molecular sieve with the RHO framework (designated as DNL-6) were synthesized using SAPO-5 (the denser phase) as the precursor and diethylamine (DEA) as the template. The entire transition process had been investigated by XRD, SEM, XRF, and NMR spectroscopy, which clearly revealed a solution-mediated transport mechanism attributed to the transformation from SAPO-5 precursor to SAPO-34 via oscillating phases [SAPO-5 (FD = 16.9 T/nm3) => SAPO-34 (15.1) + DNL-6 (14.5) → DNL-6 → SAPO-34]. The initially formed SAPO-34 was different from the final one in this transformation due to very different percentages of organic inclusions, indicating that the host−guest interactions played important roles for the phase selectivity during the synthesis. DNL-6, obtained as an intermediate of the transformation process, possessed good thermal stability, large microporous surface area (724 m2/g), and high micropore volume (0.36 cm3/g). Rietveld refinement showed that the framework of calcined DNL-6 had typical features of the RHO structure with a high symmetry [I23, a = 15.08429(9) Å], composed of a body-centered cubic arrangement of α cages linked via double 8-rings.