Growth patterns and shape development of zeolite nanocrystals in confined syntheses

The effects of confinement on the morphological development of the zeolite silicalite-1 were studied during hydrothermal synthesis in three-dimensionally ordered macroporous (3DOM) carbon monoliths. By scheduling multiple infiltration/hydrothermal reaction (IHT) steps using precursor solutions with high (H) or low nutrient content (L) in specific sequences, it was possible to obtain various zeolite morphologies of interest for technological applications. The special morphologies are also functions of shaping and templating effects by the 3DOM carbon reactor and functions of limited mass transport in the confined reaction environment. IHT steps employing high nutrient concentrations favor nucleation, whereas those using low nutrient concentrations provide growth-dominant conditions. Observed product morphologies include polycrystalline sphere arrays for the sequence HHH..., single crystal domains spanning dozens of macropores for the sequence LLL..., and faceted silicalite-1 crystallites with dimensions less than 100 nm with the sequence HLLL.... Most of these crystallites have dimensions less than 100 nm and would be suitable building blocks for seeded zeolite membrane growth. Finally, the sequence LLL...H introduces a secondary population of particles with smaller size, so that the size distribution of zeolite crystallites in the combined population may be tuned, for example, to optimize packing of particles. Hence, by choosing the appropriate infiltration program, it is possible to control grain sizes in polycrystalline particles (spheres and opaline arrays of spheres), which alters the concentration of grain boundaries in the particles and is expected to influence transport properties through the zeolite.