In this work, ZIF-300 and ZIF-301 derivatives with tunable ellipsoidal cage sizes were synthesized by adjusting the ratio between the small-sized mIm and large-sized bbIm/cbIm ligands. During ZIFs synthesis, when the amount of the fixed ligand is equimolar to Zn²⁺, adjusting the content of the other ligand does not yield a continuously tunable structure. In contrast, maintaining an excess of one ligand relative to Zn²⁺ and varying the content of the other ligand enables the formation of ZIF derivatives with continuously tunable chemical compositions and ellipsoidal cage sizes. This tunability is likely due to the increased concentration of the fixed ligand, which enhances its competitive coordination ability within the ZIF framework. Consequently, ZIF-300 and ZIF-301 derivatives with adjustable long-axis cage sizes ranging from 6.5 Å to 9.6 Å were successfully prepared. Notably, the small cage-sized ZIF-300-S1 (6.5 × 6.1 Å2) and ZIF-301-S1 (6.5 × 6.6 Å2) exhibited exceptional molecular length separation of acetone (6.4 × 5.8 Å2) and butanol (8.4 × 5.5 Å2). Liquid-phase adsorption and breakthrough experiments confirmed their separation performance, and GCMC simulations provided further insights into the separation mechanism.
Zhenggang Wang,Jianwei Yuan,Renhao Li,Haipeng Zhu,Jingui Duan,Yanan Guo,Gongping Liu,Wanqin Jin
Compared with traditional separation methods, membrane-based technology shows advantages in energy efficiency for natural gas purification. To overcome the performance trade-off of conventional polymeric membranes, in this study, ZIF-301 MOF was employed as novel filler to fabricate 6FDA-DAM polyimide mixed-matrix membranes (MMMs) for CO 2 /CH 4 separation. A solvent exchange approach was proposed to remove the high-boiling-point solvent used for fabricating the MMMs. The microstructures and physico-chemical properties of the ZIF-301 fillers and ZIF-301/6FDA-DAM MMMs were characterized by SEM, EDX, XRD, IR, TGA, DSC and high-pressure sorption isotherms. The CO 2 /CH 4 transport properties through the MMMs were evaluated by permeation measurement at various feed pressures and analyzed by solution-diffusion model. The results showed that the incorporation of ZIF-301 into 6FDA-DAM enhanced both the CO 2 permeability and CO 2 /CH 4 selectivity, attributing to the preferential CO 2 sorption and aperture molecular sieving effect of the ZIF-301 filler. The MMM with optimal ZIF-301 loading of 20 wt% exhibited CO 2 permeability of 891 barrer and CO 2 /CH 4 of 29.3 that surpassed the 2008 Robeson upper-bound, suggesting that ZIF-301 based membrane could be a potential candidate for natural gas purification.
Few studies have reported on the continuous evolution of dual-linker zeolitic imidazolate frameworks’ (ZIFs) structure and morphology during the crystal growth process. Herein, we report the synthesis of a novel ZIF material with CHA topology (ZIF-301-eIm) via the combination of a small-sized 2-ethylimidazole (eIm) with the large-sized 5-chlorobenzimidazole ligand. A series of derivative materials with distinct structures and morphologies were obtained via two pathways: (1) insufficient amount of eIm with prolonged crystallization time (pathway A) and (2) sufficient amount of eIm with prolonged crystallization time (pathway B). Various characterization techniques revealed the continuous evolution of structure and morphology during the crystal growth process. Insufficient amount of eIm and crystallization time (crystallization pathway A) led to ZIF-301-eIm derivatives with defective and open structures alongside an aggregated morphology of nanoparticles. Prolonging the crystallization time allowed small-sized eIm ligands to gradually fill into the framework, resulting in the formation of ZIF-301-eIm-A5 characterized by complete but dense structures with a perfect polyhedral morphology. Remarkably, a sufficient amount of eIm during synthesis (crystallization pathway B) formed ZIF-301-eIm-B1 with a similar structure and morphology to ZIF-301-eIm-A5 in just 1 day. ZIF-301-eIm-B3, with intact, dense structures, exhibits superior acetone/butanol separation performance compared to ZIF-301-eIm-A3 due to small pore windows and large cages facilitating selective adsorption of acetone through exclusion separation.
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