Synthesis and characterization of oriented MFI membranes prepared by secondary growth

MFI membranes were prepared as free-standing and supported films on porous alumina disks and nonporous substrates. They were synthesized using secondary growth of precursor layers. For self-supported films the first step was to prepare an alumina-silicalite composite film, while for supported films the substrate was first coated with layers of the nanocrystalline silicalite particles. During the following hydrothermal treatment, silicalite particles acting as seed crystals form a dense film, which consists of 0.5-100-μm-thick columnar, intergrown, preferentially oriented grains. The crystal orientation of the MFI films was examined using X-ray diffraction pole-figure texture analysis. The crystals were preferentially oriented with both sinusoidal and straight channel networks along directions nearly parallel to the membrane surface. The degree of orientation increased with increasing membrane thickness. Single gas permeances through thin, oriented membranes were measured. Apparent activation energies for permeation were 16, 24, 30, 22 and 26 kJ/mol for H₂, N₂, O₂, CH₄, and CO₂, respectively. Ideal selectivities for H₂/N₂, CO₂/CH₄, and O₂/N₂ were as high as 30, 10, and 3.5, respectively. Binary permeation measurements for the gas pairs CO₂/CH₄, O₂/N₂, and CO₂/N₂ revealed trends similar to those of single gas permeation and the properties are attributed to the membrane microstructure.