Effect of anion species from Ca-sources on framework type of Na–Ca zeolites prepared at 80°C

Na–Ca zeolites were synthesized at 80°C from the mixtures of sodium orthosilicate as a Si-source, Al(OH)3 powder as a Al-source, and Ca-sources. A variety of calcium salts highly soluble in␣water were chosen in this work: CaCl2, CaI2, Ca(HCOO)2, Ca(CH3COO)2, and Ca(EtCOO)2 to investigate the effect of anion species from the Ca-sources on the framework type and crystallinity of Na–Ca zeolites. The crystal phase diagrams as functions of the molar ratio SiO2/Al2O3 and reaction period were determined for different Ca-source systems on the basis of the XRD patterns and SEM observations. The use of CaCl2 or CaI2 as a Ca-source led to faujasite, hydroxysodalite, and gismondine, whereas hydroxycancrinite was preferentially produced from the reactants including Ca(NO3)2 independent of SiO2/Al2O3 (=4–20) and reaction period (168–840 h). It was found that the presence of iodine anion strongly hinders the crystallization of Na–Ca zeolites, in particular, in a high SiO2/Al2O3 range. On the other hand, the systems containing organic carboxylates gave more complex results where faujasite, hauyne, gismondine, and hydroxycancrinite were formed. All the reaction systems containing these anion species, as well as NO3-, also underwent the dissolution–nucleation–growth processes after faujasite was yielded at the early stage, as a result, the zeolite framework type varied from faujasite to hydroxysodalite, hauyne, and finally gismondine or hydroxycancrinite.