Crystal growth of faujasitic microporous zincophosphate crystals using reverse micelles as reactants

The present study deals with the crystallization of zincophosphate-X (ZnPO- X) from zinc- and phosphate-containing reverse micelles. ZnPO-X has the sam e topology as faujasite (zeolites X,Y). Reverse micelles are microdroplets of water stabilized in a hydrocarbon medium with the help of surfactants. S uccessful synthesis of ZnPO-X was possible using reverse micelles made with cationic surfactant dimethyldioctylammonium chloride (DODMAC), l-decanol a s cosurfactant and isooctane as solvent, The composition of the reactants i n the water pools of the reverse micelles was about four times more dilute than that in a conventional hydrothermal synthesis. The dilution is necessa ry since a significant fraction of the water in the reverse micelles is use d up for hydrating the headgroups of the surfactant. The crystals appear wi thout the formation of an intermediate amorphous phase. Upon mixing the zin c and phosphate reverse micelles, collisions between them result in exchang e of reactants. We propose that only a small fraction of these reverse mice lles have the proper supersaturation conditions for nucleation of ZnPO-X. T hese nuclei then grow into crystals by incorporating nutrients from non-nuc leated reverse micelles. In a typical synthesis, only about 15-20% of the p ossible yield of ZnPO-X was obtained and the rest of the material remained suspended in the organic phase. This clear suspension was an excellent seed solution for ZnPO-X, even from reverse micelle compositions that would not produce ZnPO-X. Using this seed solution and reaction columns of different lengths, crystals of different sizes could be obtained. Reverse micelles a s reactants appear to provide a novel way to control supersaturation and th ereby influence crystal growth of microporous structures in ways that canno t be readily done with conventional procedures of synthesis of these import ant materials.