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.
Bibliographical noteFunding Information:
This work was supported by the Mid-Career Researcher Program ( 2020R1A2C1101974 ) through the National Research Foundation of Korea , funded by the Korea government . In addition, this work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government ( MOTIE ) (no. 20202020800330 ). TEM characterizations were conducted at the KBSI.
Multiple types of defects can be formed in zeolite membranes. In particular, the narrow gaps between the intergrown grains constituting the zeolite membrane lead to the formation of grain boundary defects . In addition, undesirable mechanical stress on the zeolite membrane can damage the polycrystalline zeolitic structures, thus yielding cracks . In general, calcination, which is required to remove organic structure directing agents (OSDAs) in the zeolite framework, induces the formation of defects in the zeolite membrane owing to thermal contraction/expansion difference between the zeolite membrane and the support [ 22–25]. Interestingly, rapid thermal processing (RTP) treatment has been found to be effective for reducing the degree of non-zeolitic defect formation [19, 25–27]. In addition, low-temperature calcination (approximately, 200–250 °C) assisted by ozone (O3) has been demonstrated to minimize the defect formation for various types of zeolite membranes (e.g., MFI, CHA, and DDR) because of the reduced thermal contraction/expansion difference between the zeolite membrane and the support [11,18,28]. Despite such improvements, the corresponding defect structures and their effects on membrane performance are poorly understood.As-synthesized MFI zeolite membranes were fabricated using the seeded growth method according to a previous study . First, globular-shaped MFI seed particles having a size of 100 nm were prepared following the previously reported synthetic protocol . In addition, one side of an α-alumina disc support was polished and flattened using a polisher (GLP-AP105, GLP Korea). The polished α-alumina disc (thickness: 2 mm, diameter: 20 mm, porosity: 32%, and mean pore diameter: 0.15 μm) was sandwiched between two cover glasses (22 mm in diameter) and held in a custom-made Teflon holder. The disc-holding Teflon holder was transferred to a glass reactor containing anhydrous toluene (40 mL, 99.8%, Sigma Aldrich) and the MFI seed particles (0.05 g). Then, the glass reactor was sealed with Parafilm and placed in an ultrasonic bath (UC–10P, JEIO TECH, South Korea). The glass reactor was sonicated for 20 min to disperse the MFI seed particles and deposit them on the polished α-alumina disc support. Subsequently, the seed-deposited α-alumina disc was rinsed in ethanol quickly to remove any unwanted seed agglomerates weakly deposited on the disc support. Finally, the seed-deposited α-alumina disc was calcined at 450 °C for 4 h at a ramp rate of 1 °C·min−1 in air flow (100 mL min−1).The secondary growth of a seed layer comprising densely deposited MFI particles on the α-Al2O3 disc support (Fig. S2) led to a well-intergrown MFI zeolite membrane (Fig. 1). The membrane properties (e.g., morphology, thickness, and out-of-plane orientation) of M_CC and M_RTP were consistent with those reported in previous studies [17,22,26,36]. In this study, the three types of MFI membranes (i.e., M_CC, M_RTP, and M_O3) had comparable, continuous membrane surfaces at SEM resolution, regardless of the calcination method used (Fig. 1a1-c1). However, membrane continuity was not always homogeneous throughout the membrane thickness. For example, membrane continuity was preserved throughout the thickness of M_CC (Fig. 1a2) and M_O3 (Fig. 1c2), whereas lateral discoidal defects were observed in the middle of M_RTP (Fig. 1b2; indicated by green arrows), possibly as a result of local stress concentration during the rapid heating step of the RTP treatment . In addition, it was noted that the preferential c-out-of-plane orientations of the three types of MFI membranes was comparable and well preserved irrespective of the calcination method (Fig. 1a3-c3), indicating that their membrane structures were similar.The FCOM images were further processed to visualize the 3D defect structure of the three types of MFI membranes and obtain the corresponding quantitative structural properties. Furthermore, we estimated the contributions of the defects to the final p- and o-xylene molar fluxes across the MFI membranes effectively by combining the quantitative structural properties of the defects and the 1D permeation model to account for the p-/o-xylene molar fluxes. Notably, a large portion of the grain boundary defects contributed little or did not contribute to the total p-xylene molar fluxes. In contrast, a small portion of wide cracks (approximated to be 19–20 nm for M_CC in this study) contributed significantly to the permeation rate of the p-xylene molar flux as a result of their providing non-zeolitic, defect pathways. For the slowly permeating o-xylene that determined the final separation performance, the relative contributions of the grain boundary defects (assumed to be 1 nm) to the final o-xylene molar fluxes were 0.08% for M_CC, whereas those of cracks having estimated sizes of 20.2 nm were extremely high (99.82%). In contrast, the notably crack-free MFI membrane (i.e., M_O3) remarkably exhibited intrinsic p-xylene perm-selectivities (max p-/o-xylene SF of 2000) as a result of the minimized non-selective permeation through the relatively wide defects (i.e., cracks) in M_CC. This suggests that reducing the defect size, rather than the number or density (represented by the area fraction) of defects, is critical for preserving the intrinsic separation performance of the MFI membrane. Furthermore, wide cracks in M_CC could be visually demonstrated at SEM and TEM resolution. Because M_O3 showed the high p-/o-xylene SF, we are currently attempting to fabricate crack-free MFI membranes having high p-xylene permeance by reducing the membrane thickness and using asymmetric tube supports as a next step toward practical applications.This work was supported by the Mid-Career Researcher Program (2020R1A2C1101974) through the National Research Foundation of Korea, funded by the Korea government. In addition, this work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (no. 20202020800330). TEM characterizations were conducted at the KBSI.
© 2022 Elsevier B.V.
- Fluorescence confocal optical microscopy (FCOM)
- Grain boundary defect
- Xylene isomer separation
- Zeolite membranes
ASJC Scopus subject areas
- Materials Science(all)
- Physical and Theoretical Chemistry
- Filtration and Separation