2024 AIChE Annual Meeting
(380v) Low-Temperature Ozone Calcination for Improving P-/O-Xylene Separation in MFI Zeolite Membranes: Suppression of Defect Formation and Dominance of Wider Cracks
Undesirable mechanical stress on the zeolite membrane can harm its polycrystalline structure, leading to cracks. During calcination, necessary for removing organic structure directing agents (OSDAs), defects form due to thermal expansion differences between the membrane and support.
For convenience, membranes prepared conventionally, via RTP treatment, and ozone treatment are denoted M_CC, M_RTP, and M_O3, respectively.
Distinct p-xylene perm-selectivities were observed among M_CC, M_RTP, and M_O3. The disparity, particularly pronounced in M_CC, can be attributed to local distortions induced by p-xylene adsorption, facilitating non-zeolitic permeation, indicating detrimental effects of defects on permeation.
Using fluorescence confocal optical microscopy (FCOM), three defect types were identified: cracks, lateral discoidal defects, and grain boundaries.
Analyzing these defects revealed smaller cracks in M_RTP compared to M_CC. Lateral discoidal defects occurred only in the middle of M_RTP, and grain boundary defects were present in all membranes. Cracks seemed crucial for p-xylene selectivity, as M_CC and M_RTP, both containing cracks, exhibited lower selectivities than crack-free M_O3.
Notably, grain boundary defects were widespread in M_CC and M_O3, indicating dense distribution, whereas cracks were a minor fraction in M_CC compared to other defects.
Despite similar porosities, the contributions to final molar fluxes differed significantly. In M_CC, cracks contributed significantly more than grain boundaries to p- and o-xylene molar fluxes, emphasizing cracks' dominant role in determining permeation properties.
In summary, crack size predominantly influences permeation properties of MFI membranes.
For convenience, membranes prepared conventionally, via RTP treatment, and ozone treatment are denoted M_CC, M_RTP, and M_O3, respectively.
Distinct p-xylene perm-selectivities were observed among M_CC, M_RTP, and M_O3. The disparity, particularly pronounced in M_CC, can be attributed to local distortions induced by p-xylene adsorption, facilitating non-zeolitic permeation, indicating detrimental effects of defects on permeation.
Using fluorescence confocal optical microscopy (FCOM), three defect types were identified: cracks, lateral discoidal defects, and grain boundaries.
Analyzing these defects revealed smaller cracks in M_RTP compared to M_CC. Lateral discoidal defects occurred only in the middle of M_RTP, and grain boundary defects were present in all membranes. Cracks seemed crucial for p-xylene selectivity, as M_CC and M_RTP, both containing cracks, exhibited lower selectivities than crack-free M_O3.
Notably, grain boundary defects were widespread in M_CC and M_O3, indicating dense distribution, whereas cracks were a minor fraction in M_CC compared to other defects.
Despite similar porosities, the contributions to final molar fluxes differed significantly. In M_CC, cracks contributed significantly more than grain boundaries to p- and o-xylene molar fluxes, emphasizing cracks' dominant role in determining permeation properties.
In summary, crack size predominantly influences permeation properties of MFI membranes.