Competition of FAU and LTA in the synthesis system (TMA, Na)2O–Al2O3–SiO2–H2O

Eight initially clear solutions (with a general formula based on molar ratios of the components, xNaOH:(5.86 − x)TMAOH:1.0Al2O3:3.40SiO2:370H2O:19.6EtOH:6.0iso-PrOH, x = 0.06–0.86) with various sodium concentrations were prepared for synthesis of FAU/LTA zeolites at 98.0 °C. The synthesis process was studied using in situ calorimetry and the solid products were characterized using XRD, SEM and TG–DSC. At low sodium concentration (x = 0.06), the product is nearly pure FAU while with increasing x from 0.06 to 0.86, LTA gradually becomes dominant. In addition, the sodium concentration has a strong influence on crystal size but limited influence on the chemical composition of the resulting zeolite(s). At x = 0.86, the conversion yields of both silica and alumina are about 50%. When x decreases from 0.86 to 0.06, both conversion yields rapidly decrease to only about 4%. In contrast, the conversion yield of sodium remains at a much higher level (75–100%). There appears to be a maximum crystallization rate at x = 0.16–0.26. Crystallization of both FAU and LTA zeolites is exothermic and the crystallization enthalpy for FAU is higher in magnitude than for LTA.