Modeling, analysis, and characterization of zeolite MFI crystal and membrane growth
Zeolites are crystalline microporous aluminosilicates with periodic arrangements of cages and channels of nanometer dimensions. Their structure, stability, and activity have led to a broad variety of applications in industry. Recently, there has been a renewed interest in the development of zeolite membrane based separation processes. However, their applicability is limited by the inability to systematically and reproducibly control the membrane microstructure. In this dissertation, invaluable new insights have been developed regarding the growth of silicalite-1 crystals and membranes. Based on these insights, systematic new strategies are developed to alter aspects of the growth mechanisms at the molecular level to obtain the desired crystal morphology and membrane microstructure.
A novel non-destructive technique, fluorescence confocal optical microscopy (FCOM), is used to visualize the grain boundary network in MFI membranes. Using this technique, it is conclusively demonstrated that membranes with different crystallographic orientations, but similar intrazeolitic pathways, show dramatically different separation characteristics due to their grain boundary structures. Hence, the orientation of the crystals in the membrane influences the grain boundary structure and thereby impacts the molecular sieving capability and flux through the membrane. The growth of polycrystalline, faceted MFI films, starting from a randomly oriented seed layer, is modeled and simulated. In lieu of in-situ experimental techniques, which are not available, these simulations provide invaluable insights into the mechanisms that give rise to the membrane preferred orientation. In addition, this analysis also allows us to understand the interrelationship between individual crystal morphology and membrane microstructure.
A systematic study on the influence of synthesis conditions and different organic templates on the silicalite-1 crystal morphology conclusively demonstrates that it is possible to change the fundamental morphology of silicalite-1 crystals by changing the structure and properties of the structure directing agent. For the first time, a number of new silicalite-1 crystal morphologies are reported.
The modeling and simulation studies coupled with the ability to modify the crystal morphology are the basis for a new rational design methodology for obtaining membranes of a desired orientation and microstructure by tailoring the individual crystal morphology. This methodology is used to synthesize the first functional b-oriented MFI membranes.