Synthesis and characterization of zeolite films and membranes

Sammanfattning: In this work, a seeding technique was used to synthesize films and membranes of FAU, LTA and MFI type zeolites. In the first part, hydrothermal growth was performed without organic template molecules, which resulted in template-free zeolite films and membranes. The samples were characterized by Scanning Electron Microscopy, X-ray Powder Diffraction and permeation measurements with gaseous probe molecules. Thin films of FAU-type zeolite were prepared on polished single crystals. The thickness and morphology of the films could be controlled by varying the synthesis conditions. Preparation of LTA-type membranes was also attempted. However, the membranes cracked during drying at temperatures above room temperature. Template free MFI membranes with higher quality could be prepared. These membranes had a maximum separation factor alpha of 17.8 at 220°C for a n-butane/i-butane mixture. Cracks formed at temperatures higher than 250°C. Crack formation in zeolite membranes at high temperatures has also been reported byseveral other groups. Since no model for the crack formation process has been established in the literature, the second part of this work was devoted to study crack formation and to develop a model. Relatively thick (ca 1800 nm) alpha-alumina supported MFI films, prepared using organic template molecules (TPA+), were selected for the study since data on crack formation in the form of SEM images and permeation measurements for these membranes had been obtained in earlier work by the group. These membranes were further studied by in-situ High Temperature X-ray Powder Diffraction experiments in the present work. In addition, MFI powder and a blank alpha-alumina support were also investigated. Data were collected with the aid of a Synchrotron radiation facility as well as with a conventional laboratory instrument for the temperature cycle 25-500-25°C. The Rietveld method was used to determine the unit cell parameters of MFI and alpha-alumina as well as the TPA+ occupancy of MFI. The out-of- plane strain (i.e. strain in the direction perpendicular to the film surface) in the film and the support was calculated. In addition, the microstructure of the support was investigated by pattern decomposition and Williamson-Hall plots. In agreement with previous reports in the literature, it was found that the TPA-MFI structure contracts as a consequence of template removal and possibly also a structure intrinsic mechanism and the alpha-alumina support expands. Hence, a large thermal expansion mismatch occurs in the membranes during heating. An overall out- of-plane compressive strain was observed for the MFI film during heating, which indicates an in-plane tensile stress (i.e. in the direction parallel to the film surface) in the film. This result was explained by the larger expansion of the support, compared to the film. The alpha-alumina support was also found to be under an overall out-of-plane compressive strain at non-ambient temperatures, presumably due to zeolite in the pores of the support. The microstrain for the MFI coated alpha-alumina support increased during heating, and remained during cooling, which indicate the formation of structural defects in the support. Based on these results and results from earlier work, a model for crack formation was proposed: In the thick films (ca 1800 nm) studied in the present work, the crystals are well intergrown. During heating, the MFI crystals contracts and the alpha- alumina support expands. Consequently, a thermal stress develops in the composite which eventually leads to formation of cracks. In addition, part of the stress is also released via formation of structural defects in the alpha-alumina support. In thinner films (ca 500 nm), the crystals are less well intergrown and the thermal expansion mismatch between the crystals in the film and the support leads to opening of grain boundaries in the film rather than cracks.