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 same topology as faujasite (zeolites X,Y). Reverse micelles are microdroplets of water stabilized in a hydrocarbon medium with the help of surfactants. Successful synthesis of ZnPO-X was possible using reverse micelles made with cationic surfactant dimethyldioctylammonium chloride (DODMAC), l-decanol as cosurfactant and isooctane as solvent, The composition of the reactants in the water pools of the reverse micelles was about four times more dilute than that in a conventional hydrothermal synthesis. The dilution is necessary since a significant fraction of the water in the reverse micelles is used up for hydrating the headgroups of the surfactant. The crystals appear without the formation of an intermediate amorphous phase. Upon mixing the zinc and phosphate reverse micelles, collisions between them result in exchange of reactants. We propose that only a small fraction of these reverse micelles have the proper supersaturation conditions for nucleation of ZnPO-X. These nuclei then grow into crystals by incorporating nutrients from non-nucleated reverse micelles. In a typical synthesis, only about 15-20% of the possible 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 as reactants appear to provide a novel way to control supersaturation and thereby influence crystal growth of microporous structures in ways that cannot be readily done with conventional procedures of synthesis of these important materials.