Understanding template removal from MFI zeolite membranes by low-temperature ozone calcination
MFI zeolite membranes are typically synthesized with the aid of an organic template, i.e., tetrapropylammonium hydroxide (TPAOH). Consequently, it is crucial to remove the template from the membrane and simultaneously minimize non-selective defects. Ozonation at low temperature has been known as a mild detemplation to produce high-selective zeolite membranes reproducibly. However, the microstructural evolution of zeolite membrane especially template molecules during ozonation remains unclear. In this study, the microstructural evolution of the TPAOH molecule in MFI zeolite during low-temperature ozonation was investigated. The thermogravimetric analysis (TGA) and N 2 adsorption results indicate the gradual reduction in the amount of template with increasing ozonation time. The Fourier-transform infrared (FT-IR) spectroscopy, in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and 13 C nuclear magnetic resonance (NMR) results suggest the formation of new carbon species in the final zeolite. After ozonation for 8 h, ca. 87 % in mass of the template was removed from the MFI zeolite membrane. In comparison with the MFI zeolite membranes treated with air calcination, the MFI zeolite membranes detemplated in O 3 for 8 h (MFI-O 3 _8 h) exhibited a superior separation factor of 801 in para-xylene/ortho-xylene (PX/OX) separation at 150 °C, indicating fewer defects existed in the membrane. The as-synthesized membrane exhibited good separation stability during the PX/OX separation. In addition, MFI zeolite membranes were synthesized on hollow fiber supports with a length of 40 cm. These membranes showed excellent performance for xylene isomer separation (PX/OX separation factor >90).
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