Minghui He

Selective adsorption of C2H2 and CO2 from CH4 in an isoreticular series of MOFs constructed from unsymmetrical diisophthalate linkers and the effect of alkoxy group functionalization on gas adsorption

For acetylene production and natural gas purification, the development of porous materials exhibiting highly selective C2H2/CH4 and CO2/CH4 separations is very important but remains a major challenge. In this work, we employed three unsymmetrical diisophthalate ligands to construct an isoreticular series of copper-based MOFs exhibiting highly selective adsorption of C2H2 and CO2 from CH4 under ambient conditions. The gravimetric uptake capacities at 298 K and 1 atm vary from 171.7 to 200.4 cm3 (STP) per g for C2H2, and from 104.1 to 115.6 cm3 (STP) per g for CO2. The IAST adsorption selectivity is in the range of 27.6–34.5 for an equimolar C2H2/CH4 mixture, and 5.73–7.14 for an equimolar CO2/CH4 mixture at 298 K and 1 atm. These values are among the highest reported for MOFs constructed from bent diisophthalate ligands under the same conditions. The effect of alkoxy group functionalization on gas adsorption was also explored, revealing that compared to the parent compound, the alkoxy group functionalized MOFs exhibit a reduced uptake capacity but an improved adsorption selectivity. This work demonstrates that the three MOFs are promising materials for C2H2/CH4 and CO2/CH4 separations, and provides a fundamental understanding of alkoxy group functionalization on gas adsorption properties.

Three isoreticular MOFs derived from nitrogen-functionalized diisophthalate ligands: exploring the positional effect of nitrogen functional sites on the structural stabilities and selective C2H2/CH4 and CO2/CH4 adsorption properties

Understanding the structure–property relationship is conducive to the rational design and synthesis of porous MOFs with better performance. In this work, we constructed three isoreticular MOFs derived from quinoline-functionalized bent diisophthalate ligands as a platform to investigate the positional effect of nitrogen functional sites on the structural stabilities and gas adsorption properties of the resulting MOFs. N2 adsorption experiments performed at 77 K revealed that the three MOF compounds exhibited different stabilities against the framework desolvation, which we think is attributed to different accessibility degrees of nitrogen functional sites leading to different interactions between the frameworks and activated solvent molecules. In addition, as a consequence of the incorporation of nitrogen functional sites into the framework, the most stable MOF ZJNU-84 outperformed its parent compound ZJNU-71 in terms of C2H2 and CO2 uptake capacities and C2H2/CH4 and CO2/CH4 adsorption selectivities, indicating its promising potential for the selective separation of C2H2 and CO2 from CH4. However, the enhancement is not as remarkable as expected due to a low degree of accessibility of nitrogen functional sites in the framework.

Exploring the Effect of Ligand-Originated MOF Isomerism and Methoxy Group Functionalization on Selective Acetylene/Methane and Carbon Dioxide/Methane Adsorption Properties in Two NbO-Type MOFs

Investigation of the impact of ligand-originated MOF (metal-organic framework) isomerism and ligand functionalization on gas adsorption is of vital importance because a study in this aspect provides valuable guidance for future fabrication of new MOFs exhibiting better performance. For the abovementioned purpose, two NbO-type ligand-originated MOF isomers based on methoxy-functionalized diisophthalate ligands were solvothermally constructed in this work. Their gas adsorption properties toward acetylene, carbon dioxide, and methane were systematically investigated, revealing their promising potential for the adsorptive separation of both acetylene/methane and carbon dioxide/methane gas mixtures, which are involved in the industrial processes of acetylene production and natural gas sweetening. In particular, compared to its isomer ZJNU-58, ZJNU-59 displays larger acetylene and carbon dioxide uptake capacities as well as higher acetylene/methane and carbon dioxide/methane adsorption selectivities despite its lower pore volume and surface area, demonstrating a very crucial role that the effect of pore size plays in acetylene and carbon dioxide adsorption. In addition, the impact of ligand modification with a methoxy group on gas adsorption was also evaluated. ZJNU-58 exhibits slightly lower acetylene and carbon dioxide uptake capacities but higher acetylene/methane and carbon dioxide/methane adsorption selectivities as compared to its parent compound NOTT-103. By contrast, enhanced adsorption selectivities and uptake capacities were observed for ZJNU-59 as compared to its parent compound ZJNU-73. The results demonstrated that the impact of ligand functionalization with a methoxy group on gas adsorption might vary from MOF to MOF, depending on the chosen parent compound. The results might shed some light on understanding the impact of both ligand-originated MOF isomerism and methoxy group functionalization on gas adsorption.

A NbO-type MOF based on an aromatics-rich and N-functionalized diisophthalate ligand for high-performance acetylene storage and purification

A porous MOF ZJNU-93 based on a custom-designed diisophthalate ligand bearing an aromatic-rich backbone and N-functionalized sites was solvothermally synthesized and fully characterized. Single-crystal X-ray diffraction studies revealed that ZJNU-93 is a three-dimensional (4,4)-connected NbO-type network composed of dicopper paddlewheel units and organic linkers as 4-connected building blocks. Furthermore, its gas adsorption properties with respect to C2H2, CO2, and CH4 were systematically studied, revealing its promising utility as an adsorbent for the storage as well as separation and purification of C2H2. More specifically, the C2H2 uptake capacity reaches 164.0 cm3 (STP) g−1, while the estimated C2H2/CO2 and C2H2/CH4 adsorption selectivities are as high as 4.5 and 34.4 at 298 K and 1 atm. ZJNU-93 represents a rare MOF displaying high C2H2 storage and separation capacities at the same time.

Structural diversities and gas adsorption properties of a family of rod-packing lanthanide–organic frameworks based on cyclotriphosphazene-functionalized hexacarboxylate derivatives

Solvothermal reactions of cyclotriphosphazene-functionalized hexacarboxylate derivatives bearing different substituents (methoxy, chloro, bromo and methyl) with Ho(NO3)3·6H2O gave rise to four new three-dimensional lanthanide metal–organic frameworks (Ln-MOFs). Single-crystal X-ray diffraction analyses reveal that they all feature rod-shaped Ho-carboxylate chains as inorganic secondary building units but display substituent-driven structural diversities instead of the isoreticular structures. N2 adsorption and desorption studies show that among these Ln-MOFs, only the methoxy-modified Ln-MOF ZJNU-63 after activation exhibits permanent porosity. Furthermore, the gas adsorption properties of ZJNU-63 with respect to C2H2, C2H4, C2H6, CO2, and CH4 were systematically investigated, revealing its possible potential for natural gas purification. Although the separation performance is not very impressive compared to the other reported MOFs, ZJNU-63 presents a rare example of permanently porous Ln-MOFs constructed from flexible organic ligands displaying selective gas adsorption. Besides, ZJNU-63 can adsorb a large amount of C4 hydrocarbons with uptake capacities ranging from 58.8 to 75.7 cm3 (STP) g−1 at 298 K and 1 atm.