Interfacial nanoarchitectonics for ZIF-8 membranes with enhanced gas separation

Abstract

Metal-organic framework (MOF) membranes are potentially useful in gas separation applications. Conventional methods of MOF membrane preparation require multiple steps and high-pressure conditions. In this study, a reliable one-step interfacial synthesis method under atmospheric pressure has been developed to prepare zeolitic imidazolate framework-8 (ZIF-8) membranes supported on porous α-Al₂O₃ disks. To obtain optimal ZIF-8 membranes, three reaction parameters were investigated, namely, reaction temperature, reaction time, and concentration of the organic linker (i.e., 2-methylimidazole). The growth of ZIF-8 membranes under various parameters was evaluated by field-emission scanning electron microscopy, and the optimal synthesis conditions were determined (i.e., 80 °C for 12 h in 50 mM of 2-methylimidazole). The as-synthesized ZIF-8 membranes were then applied to CO₂/N₂ gas separation, which exhibited a maximum separation factor of 5.49 and CO₂ gas permeance of 0.47 × 10^-7 mol·m^-2·s^-1·Pa^-1.

Keywords: ZIF membranes; defect-free; gas separation; interfacial synthesis; metal-organic frameworks.

Figure 1

(a) XRD patterns, (b) FE-SEM image, (c) side view, and (d) top view of the free-standing ZIF-8 thin films.

Figure 2

XRD patterns of simulated ZIF-8 crystal, porous α-Al₂O₃ disk, and ZIF-8 membranes fabricated at 40, 80, and 120 °C.

Figure 3

Top-view SEM images of ZIF-8 membranes fabricated at (a) 40 °C, (b) 80 °C, and (c) 120 °C, after 12 h. Cross-sectional EDS mappings of ZIF-8 membranes fabricated at (d) 40 °C, (e) 80 °C, and (f) 120 °C, after 12 h (red represents Zn).

Figure 4

Top-view SEM images of ZIF-8 membranes obtained after (a) 3 h, (b) 12 h, and (c) 15 h at 80 °C. Cross-sectional EDS mappings of ZIF-8 membranes obtained after (d) 3 h, (e) 12 h, and (f) 15 h at 80 °C.

Figure 5

Top-view SEM images of ZIF-8 membranes fabricated with 2-methylimidazole/1-octanol solution concentration of (a) 25 mM, (b) 50 mM, and (c) 400 mM after 12 h at 80 °C. Cross-sectional EDS mappings of ZIF-8 membranes fabricated with 2-methylimidazole/1-octanol solution concentration of (a) 25 mM, (b) 50 mM, and (c) 400 mM after 12 h at 80 °C.

Figure 6

(a) Cross-sectional view FE-SEM image of the synthesized ZIF-8 membrane. (b) Cross-sectional view EDS mapping of the synthesized ZIF-8 membrane (red represents Zn; blue represents Al).

Figure 7

(a) Top-view and (b) cross-sectional SEM images of ZIF-8 membranes fabricated via a counter-diffusion synthesis method (50 mM of 2-methylimidazole at 80 °C for 12 h).

Figure 8

Gas separation performance (CO₂/N₂) of the ZIF-8 membranes on α-Al₂O₃ disks (red stars, this work) compared with (a) ZIF-8 membranes on α-Al₂O₃ disks (blue squares) [–48] and ZIF-8 membranes on other substrates (green squares) [–59], and (b) pure ZIF-8 on substrates (turquois squares) [–59], MOF membranes supported on substrates (blue squares) [,–84], and MOF mixed matrix membranes (brown squares) [–97].