Solvothermal assembly of copper­(II) cations and 5-(pyridine-3-yl)­isophthalate linkers bearing different position-substituted methyl groups afforded four ligand-induced metal–organic framework (MOF) isomers as a platform for investigating diverse selective gas adsorption properties and understanding the positional effect of methyl functionality. Single-crystal X-ray diffraction (SCXRD) analyses showed that, when the methyl substituent is at the para position with respect to the pyridinic N atom, the resultant framework compound ZJNU-27 features an eea -type topology, while the other three solids possess an isoreticular structure with an rtl -type topology when the methyl group is situated at the other positions. As revealed by N2 physi-adsorption measurements at 77 K, they exhibit moderate specific surface areas ranging from 584 to 1182 m2 g–1 and distinct degrees of framework flexibility, which are heavily dependent on the methyl position. Comprehensive gas adsorption studies show that they are capable of effectively separating three pairs of binary gas mixtures including C2H2–CH4, CO2–CH4, and CO2–N2 couples. Moreover, their uptake capacities and adsorption selectivities can be tailored by altering the methyl position. In addition, their framework hydro-stability is also influenced by the methyl position. Compared to ZJNU-27 and ZJNU-28, ZJNU-26 and ZJNU-29 exhibit poorer stability against H2O, although the methyl group is more close to inorganic secondary building units (SBUs), which are believed to originate from the steric effect of the methyl group. Overall, the four MOFs display the methyl position-dependent network architectures, framework flexibilities, and selective gas adsorption properties as well as hydrostabilities. The findings observed in this work not only demonstrate the importance of the positional effect of the functional group but also highlight that engineering the substituent position is a potential strategy for achieving the modulation of MOF structures and properties.

Structural and functional expansion of metal–organic frameworks (MOFs) is fundamentally important because it not only enriches the structural chemistry of MOFs but also facilitates the full exploration of their application potentials. In this work, by employing a dual-site functionalization strategy to lock the ligand conformation, we designed and synthesized a pair of biphenyl tricarboxylate ligands bearing dimethyl and dimethoxy groups and fabricated their corresponding framework compounds through coordination with copper(II) ions. Compared to the monofunctionalized version, introduction of two side groups can significantly fix the ligand conformation, and as a result, the dual-methoxy compound exhibited a different network structure from the mono-methoxy counterpart. Although only one almost orthogonal conformation was observed for the two ligands, their coordination framework compounds displayed distinct topological structures probably due to different solvothermal conditions. Significantly, with a hierarchical cage-type structure and good hydrostability, the dimethyl compound exhibited promising practical application value for industrially important C2H2 separation and purification, which was comprehensively demonstrated by equilibrium/dynamic adsorption measurements and the corresponding Clausius–Clapeyron/IAST/DFT theoretical analyses.

A great demand for high-purity C2 hydrocarbons calls for the development of chemically stable porous materials for the effective isolation of C2 hydrocarbons from CH4 and CO2. However, such separations are challenged by their similar physiochemical parameters and have not been systematically studied to date. In this work, we reported a cadmium-based rod-packing coordination framework compound ZJNU-140 of a new 5,6,7-c topology built up from a custom-designed tricarboxylate ligand. The metal–organic framework (MOF) features an aromatic-abundant pore surface, uncoordinated amine functionality, and self-partitioned pore space of suitable size. These structural characteristics act synergistically to provide the MOF with both selective recognition ability and the confinement effect toward C2 hydrocarbons. As a result, the MOF displays promising potential for adsorptive separation of C2–CH4 and C2–CO2 mixtures. The IAST-predicted C2/CH4 and C2/CO2 adsorption selectivities, respectively, fall in the ranges of 7.3–10.2 and 2.1–2.9 at 298 K and 109 kPa. The real separation performance was also confirmed by dynamic breakthrough experiments. In addition, the MOF can maintain skeleton intactness in aqueous solutions with a wide pH range of 3–11, as confirmed by powder X-ray diffraction (PXRD) and isotherm measurements, showing no loss of framework integrity and porosity. The excellent hydrostability, considerable uptake capacity, impressive adsorption selectivity, and mild regeneration make ZJNU-140 a promising adsorbent material applied for the separation and purification of C2 hydrocarbons.

To push forward the structural development and fully explore the potential utility, it is highly desired but challenging to regulate in a controllable manner the structures and properties of MOFs. In this work, we reported the structural and functional modulation of Cu(II)-tricarboxylate frameworks by employing a strategy of engineering the functionalities and their positions. Two pairs of unsymmetrical biaryl tricarboxylate ligands modified with a methyl group and a pyridinic-N atom at distinct positions were logically designed and synthesized, and their corresponding Cu(II)-based MOFs were solvothermally constructed. Diffraction analyses revealed that the variation of functionalities and their positions furnished three different types of topological structures, which we ascribed to the steric effect exerted by the methyl group and the chelating effect involving the pyridinic-N atom. Furthermore, gas adsorption studies showed that three of them are potential candidates as solid separation media for acetylene (C 2 H 2 ) purification, with the separation potential tailorable by altering functionalities and their locations. At 106.7 kPa and 298 K, the C 2 H 2 uptake capacity varies from 64.1 to 132.4 cm 3 (STP) g −1 , while the adsorption selectivities of C 2 H 2 over its coexisting components of CO 2 and CH 4 fall in the ranges of 3.28−4.60 and 14.1−21.9, respectively.

The exploration of reticular chemistry pertaining to rod-packing metal–organic frameworks and the development of solid adsorbents for multitask hydrocarbon separations are two active topics of current research. In this study, an In-organic coordination framework compound denoted ZJNU-121 was assembled from a custom-designed bithiophene-functionalized tetracarboxylate ligand. The title compound not only exhibited a rare two-way rod-packing pattern but also displayed the impressive capability of capturing C2 hydrocarbons from CH4 and CO2 to achieve C2/C1 and C2/CO2 separations. Under ambient conditions, the ideal adsorbed solution theory-predicted adsorption selectivities for equimolar mixtures fall in the range of 8.4–13.8 (C2H n /CH4) and 2.3–3.4 (C2H n /CO2). Such separation potentials were also ascertained by the column breakthrough experiments. Furthermore, the in-depth theoretical insight unveiled the crucial role of active sites such as thiophene sulfur, carboxylate oxygen, and the bridging μ2-OH group in capturing C2 hydrocarbons in preference to CO2 and CH4. Additionally, the structural integrity of ZJNU-121 can be retained in aqueous solutions with pH values varying from 3 to 11 for 24 h at ambient temperature, as verified by the preservation of PXRD patterns, textural characteristics, and static/dynamic adsorption behaviors. This research not only enriched the architectural diversity of rod MOFs but also reported a robust adsorbent with good application prospects for diversified hydrocarbon separations.

Three isostructural metal-organic frameworks (MOFs) based on dicopper paddlewheels and dimethyl-functionalized linear diisophthalate ligands were solvothermally constructed, with the aim to investigate their performance regarding acetylene (C 2 H 2 ) storage as well as separation and purification and to further understand the positional effect of the weakly polarized methyl functionality. Isotherm measurements and IAST selectivity analyses revealed that the three compounds exhibited the promising potential for storing C 2 H 2 and recovering C 2 H 2 from carbon dioxide (CO 2 ) and methane (CH 4 ) at ambient conditions. Furthermore, their C 2 H 2 adsorption properties can be tailored and optimized by altering the relative position of the methyl substituents immobilized in the organic linkers. At 298 K and 1 atm, the C 2 H 2 uptake capacities vary from 173.0 to 183.6 cm 3 (STP) g À 1 , while the C 2 H 2 /CH 4 and C 2 H 2 /CO 2 adsorption selectivities are in the range of 24.5-26.6 and 4.34-4.70 for the equimolar gas mixtures. The ortho-dimethyl-modified MOF outperformed better than the other two MOF isomers. The results indicate that rational arrangement of functional groups can to some extent improve gas adsorption properties. Although the property discrepancy caused by the less-polar methyl group is not as large as that by Lewis-basic nitrogen atoms, this is the first experimental demonstration of the positional effect of methyl group on C 2 H 2 adsorption.