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Abstract The defect structure in a c -out-of-plane oriented MFI membrane was quantitatively analyzed by processing images obtained by fluorescence confocal optical microscopy (FCOM). The MFI membranes were placed in contact with a dye solution at a fixed concentration (1 mM) for 2, 4, and 8 d and at different concentrations (0.01, 0.1, and 1 mM) for 8 d. This approach led to the identification and understanding of two types of defects (cracks and grain boundary defects). The representative quantitative properties (porosity and tortuosity) relevant to the defects were obtained via the image processing. Furthermore, the estimation of the defect sizes was complemented by the use of a one-dimensional permeation model for the molar flux of the p -/ o -xylene components across the c -oriented MFI membrane. Using this combination, we found that although the amount of defects in the whole zeolite membrane was close to ~ 1%, they provided non-selective, facile pathways that accounted for ~ 58% of the total molar flux of faster permeating p -xylene. Surprisingly, despite the lower density (pixel-based area fraction) of cracks, wider cracks (~ 7.8–8.2 nm) accounted for the much higher molar flux of permeation components compared to major, but narrower grain boundary defects (~ 1–2 nm). Indeed, the cracks mainly deteriorated a p -/ o -xylene separation performance of the c -oriented MFI membrane. Finally, we found that the crack size increased in the presence of p -xylene so that the molar flux of o -xylene in the binary mixture was significantly increased, thus reducing the p -/ o -xylene separation performance. This behavior was ascribed to the flexible MFI zeolite structure after the adsorption of p -xylene. Graphical abstract fx1 Highlights • Defects in a MFI zeolite membrane were quantitatively analyzed by a FCOM technique. • Two types of defects (cracks and grain boundary defects) were investigated. • The defects with coverage of less than 1% significantly reduced membrane performance. • Fewer but wider cracks had a stronger influence on the final membrane performance. • Structural transformation of the MFI zeolite due to p -xylene opened defect spaces. [ABSTRACT FROM AUTHOR]

Microporous chabazite (CHA) zeolite is very promising for CO 2 capture because of its appropriate pores with molecular dimensions for the preferential adsorption of CO 2 molecules. Herein, CHA type zeolite particles and membranes were prepared by using a seeded growth method in the absence of an organic structure directing agent (OSDA) or template. After substantial effort to find appropriate and reliable conditions for obtaining continuous CHA type zeolite membranes, it was recognized that the formation of these membranes is a highly sensitive function of the Si/Al ratio in the synthetic precursor. Using the appropriate Si/Al ratio of ~50, OSDA-free CHA type zeolite membranes were manufactured with high reproducibility. The resulting OSDA-free CHA type zeolite membranes showed maximum CO 2 /N 2 and CO 2 /CH 4 separation factors of ~12.5 ± 3.8 and ~28.8 ± 6.9, respectively, with a moderate CO 2 permeance of ~1 × 10 −7 mol m −2 s −1 Pa −1 . Notably, under more realistic wet conditions (i.e., in the presence of H 2 O vapor), the separation performance at temperatures above 75 °C was comparable to that obtained under dry conditions, although permeation was hindered below 50 °C, apparently due to the strong adsorption of H 2 O vapor. [ABSTRACT FROM AUTHOR]

We report that low-temperature ozone calcination allowed MFI type polycrystalline zeolite membranes to maximize their p -/ o -xylene separation factor (as high as ca. 2000) by suppressing defect formation. Conventional high-temperature calcination and rapid thermal processing, which provide poor and marked p -/ o -xylene separation abilities, respectively, were used for comparison. The corresponding defect structures were quantitatively analyzed by image processing of fluorescence confocal optical microscopy images combined with membrane permeation modeling, revealing the main defects (grain boundary defects and cracks) and their tortuosity, porosity, and size. To the best of our knowledge, we, for the first time, demonstrated that in contrast to common belief, the minor portion of wider cracks instead of the major portion of narrower grain boundary defects determined the final permeation rates. Specifically, the MFI membrane prepared by high-temperature calcination contained many grain boundary defects (narrow; ca. 1 nm) and few cracks (wide; ca. 20 nm) that accounted for ca. 0.1% and 99.8%, respectively, of the slowly permeating o -xylene molar flux. In contrast, the ozone-treated MFI membranes, which only possessed grain boundary defects, achieved the high p -/ o -xylene separation performance, underlining the need for the selective reduction in the number of wider cracks rather than ubiquitous, narrow grain boundary defects. [Display omitted] • MFI films were activated by slow calcination, rapid thermal processing, and ozone. • Low-temperature ozone treatment eliminated cracks (not grain boundary defects). • Ozone-treated MFI membranes showed extremely high p -xylene perm-selectivities. • Defects were quantitatively analyzed, yielding their tortuosity, porosity, and size. • Small number of wide cracks (≥10 nm) negated the intrinsic molecular sieving. [ABSTRACT FROM AUTHOR]

A SSZ-13 zeolite (CHA type zeolite having a pore size of 0.37 × 0.42 nm2) can separate CO 2 (0.33 nm) from larger molecules (N 2 (0.364 nm) or CH 4 (0.38 nm)) because of the molecular size differences. However, methods to control the non-zeolitic defects of SSZ-13 membranes are lacking. Here, we demonstrate that rapid thermal processing (RTP) of as-synthesized SSZ-13 films tunes their microstructural defect properties and increases their CO 2 separation ability. The maximum CO 2 /N 2 separation factor (SF) at 30 °C increased from ~2.9 to ~4.8 due to RTP because the defective region was reduced. Furthermore, the addition of water vapor (the third main component of coal-fired power plant flue gas) to the feed markedly improved the CO 2 /N 2 SF of the RTP-treated SSZ-13 membrane; from ~4.3 at 50 °C (a representative flue gas stream temperature) under dry conditions to ~10.1 under wet conditions (vs. ~1.6 across the conventionally calcined counterpart). Furthermore, the less-defective RTP-treated SSZ-13 membranes achieved a CO 2 /CH 4 SF as high as ~43.7 under wet conditions at 50 °C. Fluorescence confocal optical microscopy analyses complemented with the permeation modeling revealed that the reduced defect size after RTP (~2.2 nm against ~3.9 nm for the conventionally calcined counterpart) improved the CO 2 permselectivity, even though the defect porosities were low (~0.1%). Image 1 • Rapid thermal processing (RTP) of SSZ-13 films improved the CO 2 permselectivities. • The improved separation performance was attributed to the reduced defect size. • Despite the comparable degree of defects, their size determined the performance. • Such improvement was pronounced with respect to water vapor-containing feeds. • Robust CO 2 permselectivities were achieved using the RTP-treated SSZ-13 membranes. [ABSTRACT FROM AUTHOR]