Synthesis and characterization of DNL-6, a new silicoaluminophosphate molecular sieve with the RHO framework

A new silicoaluminophosphate molecular sieve with the RHO framework (designated as DNL-6, Dalian National Laboratory Number 6) has been hydrothermally synthesized for the first time with diethylamine (DEA) template in the presence of cetyltrimethylammonium bromide (CTAB). Influence factors, such as CTAB amount and silica content in the starting gel, crystallization time and temperature, were investigated. It was observed that DEA together with CTAB were necessary for the formation of pure RHO phase, which appeared in a narrow range of the starting gel compositions and synthesis conditions. Only DEA and water molecules were found in the as-synthesized DNL-6 without any existence of CTAB. All Si atoms incorporated into the framework with Si(4Al) environment at a high amount close to the theoretical maximum value. DNL-6 molecular sieve possessed high micropore surface area (777 m2/g) and large micropore volume (0.36 cm3/g) with good thermal and hydrothermal stability. Crystallization course was studied by monitoring solid products at different crystallization times with XRD, SEM, XRF, NMR and CHN elemental analysis. It is proposed that DNL-6 crystals were more likely formed directly from the crystallization of liquid phase rather than from the transformation of amorphous materials, and Si atoms directly incorporated into the DNL-6 framework from the very beginning of the crystallization via a SM2 mechanism.

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► DNL-6, a novel SAPO molecular sieve with RHO topology has been synthesized.
► High thermal/hydrothermal stability, large micropore surface area and pore volume.
► Solution-mediated transport mechanism.
► Si content in crystals increased gradually with the crystallization time.