SYNTHESIS OF LARGE OPTICALLY CLEAR SILICOALUMINOPHOSPHATE CRYSTALS WITH AFI STRUCTURE

The optimization of the hydrothermal SAPO-5 synthesis using the system Al2O3:P2O5:SiO2:TEA (triethylamine): H2O resulted in the formation of large crystals (average size of up to 220 x 50 mu m) of hexagonal rod -like morphology and high optical quality in high yields only, if an a luminum oxide hydrate sol or a pseudohydrargillite was used as aluminu m source. It was possible to control the morphology and the crystal si ze of SAPO-5 by the silica source used. Further, it was found that the silica in the synthesis gel drastically influences the rate of crysta l growth in the crystallographic c-axis. The amount of pyrogenic silic a acid used controls the morphology of the crystals, while the reactiv ity of the silica determines the crystal size obtained. With powder X- ray diffraction (XRD) and thermogravimetric analysis-differential ther mal analysis (TGA-DTA) it was demonstrated that silicon predominantly substitutes for phosphorus sites if a silicon molar fraction (Si/Al+PSi) of <0.06 is used in the synthesis gel.