MFI membranes of different Si/Al ratios for pervaporation and steam permeation

Abstract

For pervaporation and steam permeation, in one- and two-step crystallizations, respectively, two kinds of MFI membranes of different SiO 2/Al 2O 3 ratio on a mesoporous alumina ceramic support have been prepared. In the one-step crystallization, using TPA-OH as template, a supported silicalite membrane was obtained by in situ crystallization. In the two-step crystallization, by first attaching seed crystals to the ceramic surface with electrostatic forces and then crystallizing a continuous MFI layer in a template-free synthesis, a ZSM-5 membrane was prepared. The membranes have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) in combination with electron dispersive X-ray (EDX) line scan as well as by single gas and binary mixture permeation. In pervaporation and steam permeation measurements of water/iso-propanol and water/methanol mixtures, the silicalite membrane showed a hydrophobic and the ZSM-5 membrane a hydrophilic separation behavior. However, only the silicalite membrane obtained by one-step crystallization allows shape selective permeation such as the separation of n/iso-butane and methanol/MTBE mixtures. In the case of the polycrystalline ZSM-5 membrane with columnar microstructure, a superposition of mass transport through the crystal boundaries and the crystals takes place.