Synthesis of large optically clear silicoaluminophosphate crystals with AFI structure

Abstract

The optimization of the hydrothermal SAPO-5 synthesis using the system Al 2O 3:P 2O 5:SiO 2:TEA (triethylamine): H 2O resulted in the formation of large crystals (average size of up to 220 × 50 μm) of hexagonal rod-like morphology and high optical quality in high yields only, if an aluminum oxide hydrate sol or a pseudohydrargillite was used as aluminum source. It was possible to control the morphology and the crystal size of SAPO-5 by the silica source used. Further, it was found that the silica in the synthesis gel drastically influences the rate of crystal growth in the crystallographic c-axis. The amount of pyrogenic silica acid used controls the morphology of the crystals, while the reactivity of the silica determines the crystal size obtained. With powder X-ray diffraction (XRD) and thermogravimetric analysis-differential thermal analysis (TGA-DTA) it was demonstrated that silicon predominantly substitutes for phosphorus sites if a silicon molar fraction ( Si Al + P + Si ) of <0.06 is used in the synthesis gel.