Search

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]