Highly Efficient Separation of C8 Aromatic Isomers by Rationally Designed Nonaromatic Metal–Organic Frameworks

Abstract

The separation of C8 aromatics (xylenes and ethylbenzene) remains one of the most challenging industrial separations due to their similar structures and properties. Suitable adsorbents that can distinguish the small differences among isomers are urgently demanded. Herein, we demonstrate a strategy to realize the precise discrimination of C8 aromatics by constructing a nonaromatic confined pore environment with mixed polycycloalkane-type ligands. The nonaromatic low-polar pore environment avoids strong convergent interactions between the framework and the common phenyl rings while creating possibilities to amplify the difference between host–guest/guest–guest interactions regarding the different methyl (ethyl) group positions of isomers. The resultant metal–organic framework, ZUL-C3, with either tetragonal or monoclinic lattice, exhibits outstanding separation performance for C8 aromatics, not only realizing the simultaneous separation of four isomers from each other but also setting a benchmark for the dynamic separation performance of OX/PX and OX/MX.

Integrating Self-Partitioned Pore Space and Amine Functionality into an Aromatic-Rich Coordination Framework with Ph Stability for Effective Purification of C2 Hydrocarbons

“…In light of these findings, the MOF chemistry of tricarboxylate ligands is worthy of further study. On the other hand, although the aliphatic ligands endow the resulting frameworks with some unique properties, as shown in recent research work, , the aromatic-rich ones are still the most widely used, presumably because they are easier to synthesize or commercially available. Integrating more conjugated aromatic rings into organic linkers can not only significantly strengthen the structural rigidity and stability of the resulting materials but also furnish the affinity toward guest molecules based on specific π···π and C–H···π interactions .…”

Combination of Low-Polar and Polar Binding Sites in Aliphatic MOFs for the Efficient C2H6/C2H4 Separation

“…Structural Characterization. ZUL-C3 was successfully synthesized according to our previous report, 12 and ZUL-C4 was synthesized by heating the mixture of DMF/ethanol solution of Ni(NO 3 ) 2 •6H 2 O, H 2 bpdc, and ted at 413 K for 24 h for the first time. The experimental PXRD data revealed that ZUL-C4 was isostructural with ZUL-C3.…”

“…Adsorption separation offers less energy-extensive technical way by circumventing phase changes during the separation process. , Tremendous efforts have been devoted to the development of ultramicroporous adsorbents that can realize the efficient separation of C 2 H 4 /C 2 H 6 , and more than 600 adsorbents have been reported. Most of them showed an adsorption preference of C 2 H 4 from the C 2 H 4 /C 2 H 6 mixture due to the larger quadrupole moment (1.50 × 10 –26 esu cm 2 for C 2 H 4 ; 0.65 × 10 –26 esu cm 2 for C 2 H 6 ) and more π electrons of C 2 H 4 that enabled the stronger interactions between the C 2 H 4 molecule and the open metal sites or highly polar groups on the adsorbent pore surface. − However, their similar molecular size tends to result in co-adsorption phenomenon, seriously reducing the product purity of C 2 H 4 .…”

Pore Environmental Modification by Alkoxy Groups in Pore-Space-Partitioned Metal–Organic Frameworks to Achieve Gas Uptake-Selectivity Balance

“…Based on the gas separation mechanism, the effectiveness of ligand modification for optimizing performance has been well demonstrated. − Among various functional groups, alkoxy groups exhibit outstanding availability. − Compared to Lewis basic groups, alkoxy groups have the following advantages: (1) good hydrophobicity; (2) less impact on the structure because of weak coordination with metal ions and solvent molecules; and (3) chemical diversity in space resistance, flexibility, ability to provide electrons, and so on . What is more, alkoxy groups have a multi-directional impact on material performance, especially in improving separation selectivity: (1) splitting pores or channels, creating a more suitable pore size for the entry of guest molecules with a proper size; (2) as electron-donating groups, realizing the regulation of the pore polarity and influencing the host–guest interaction between the framework and gas molecules with high polarizabilities and quadrupole moment, thereby improving the gas selectivity; − and (3) based on the flexibility, endowing MOFs with a distinctive “breathing” behavior or a “gate-opening” process, showing selectivity for specific gas molecules. − Definitively, introducing alkoxy groups is helpful for improving gas selectivity.…”