Simulations and experiments on the growth and microstructure of zeolite MFI films and membranes made by secondary growth

The seeded growth of polycrystalline, faceted MFI films was simulated in two spatial dimensions using a front tracking technique. Our model simulates the growth of a zeolite membrane, starting from seeds with initial random orientation and position, by tracking the motion of all intersections where faces or vertices of a crystal grain meet different grains. The temporal evolution of film growth shows that grain boundaries are straight, but undergo abrupt changes in direction when two grains of different crystals meet. The computer simulations also clearly show that with increasing film thickness, a preferential orientation of the crystal grains develops, and the average grain size increases. The crystallographic orientation distribution (texture) analysis of the simulation results shows two peaks that sharpen during film growth. The first peak position is common to all idiomorphs and may be derived directly from the orientation of the largest radial vector (c-axis) perpendicular to the substrate. The second peak position is dependent on a growth rate parameter and is derived from the orientation of the second largest radial vector in two dimensions. While this second peak had been experimentally observed, a possible explanation for its origin was not reported until now. These simulations are in qualitative agreement with X-ray diffraction pole figure analysis performed on MFI membranes grown by secondary growth under hydrothermal synthesis. The same membranes were also analyzed using scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). The simulations are in good agreement with the experimental results obtained using SEM and LSCM.