Synthesis of [Al]-SSZ-31 Molecular Sieves Using [Al]-Beta Zeolite ([Al]-BEA) as Precursors

Abstract

Abstract The aluminosilicate version of SSZ-31 ([Al]-SSZ-31) zeolite was synthesized by a new route including phase-transformation, wherein an aluminosilicate analog of beta zeolite ([Al]-BEA) was used as a precursor and 1,1,1,8,8,8-hexaethyl-1,8-diazoniaoctane dihydroxide was used as a structure-directing agent (SDA). [Al]-BEA precursors synthesized by two routes, viz. the hydrothermal (HTS) and dry-gel conversion (DGC) method, were employed for the synthesis. [Al]-SSZ-31 with a low output SiO2/Al2O3 ratio crystallized only when the precursor (SiO2/Al2O3 = 30), prepared by DGC method, was employed. Precursors synthesized by the HTS method produced highly crystalline samples, however, only in the high silica region. In general, [Al]-SSZ-31 samples over a wide range of SiO2/Al2O3 ratios could be synthesized using a precursor route. During crystallization, a phase transformation from [Al]-BEA to [Al]-SSZ-31 was observed, indicating dissolution of the beta phase, followed by its redirection towards the SSZ-31 phase in the presence of SDA. An increase in gel water helped to produce pure [Al]-SSZ-31. [Al]-SSZ-31 was found to crystallize in NaOH/SiO2 window of 0.040–0.056. Scanning electron microscopy (SEM) showed plate-like crystals of about 5 μm in size. 13C CP MAS NMR indicated that the SDA was intact inside the pores during crystallization. Furthermore, the samples were also characterized by 29Si and 27Al MAS NMR in addition to X-ray diffraction (XRD), temperature-programmed desorption of ammonia (NH3-TPD), SEM, etc.