Journal of Physical Chemistry, Vol.100, No.23, 9870-9880, 1996
Reverse Micelle Based Growth of Zincophosphate Sodalite - Examination of Crystal-Growth
Formation of zincophosphates from zinc and phosphate containing reverse micelles (water droplets in hexane) has been examined. The frameworks formed resemble those made by conventional hydrothermal synthesis. Dynamics of crystal growth are however quite different and form the main focus of this study. In particular, the formation of zincophosphate with the sodalite framework was examined in detail. The intramicellar pH was found to have a strong influence on crystal growth. Crystals with a cubic morphology were formed directly from the micelles, without an apparent intermediate amorphous phase over a period of 4 days by a layer-by-layer growth at the intramicellar pH of 7.6. At a pH of 6.8, an amorphous precipitate rapidly sediments in hours. Sodalite was eventually formed from this settled phase via surface diffusion and reconstruction within 4 days. With a rotating cell, it was possible to minimize sedimentation and crystals were found to grow epitaxially from the spherical, amorphous particles. Intermediate pH’s of 7.2 led to formation of aggregated sodalite crystals prior to settling, again without any indication of an intermediate amorphous phase. These diverse pathways were possible due to changes in intramicellar supersaturation conditions by minor changes in pH. In contrast, conventional syntheses in this pH range all proceeded by similar crystallization pathways through an amorphous gel. This study establishes that synthesis of microporous frameworks is not only possible in reverse micellar systems, but they also allow examination of possible crystallization pathways.
Keywords:IN-OIL MICROEMULSIONS;TEMPERATURE SYNTHESIS;SOLUBILIZED WATER;W/O MICROEMULSION;MICRO-EMULSIONS;ZEOLITE;PARTICLES;EXCHANGE;PH;SULFOSUCCINATE