Ligand Conformation Fixation Strategy for Expanding the Structural Diversity of Copper-Tricarboxylate Frameworks and C₂H₂ Purification Performance Studies

Abstract

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.

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

“…In relation to the C 2 H 4 /CH 4 and CO 2 /CH 4 separation, the corresponding values are 0.40/0.79/1.13 mmol g –1 and 0.18/0.96/1.05 mmol g –1 . In terms of breakthrough interval time, the performance of SNNU-10-1 is like some MOFs, such as ZJNU-133 . It is worth noting that there are roll-ups on the breakthrough curve of CH 4 because of the competitive adsorption.…”

“…The framework formula of SNNU-10- n is [Mg 3 (μ 3 -OH) (DMOBDC) 3 (TPT)], where Mg 3 -based metal trimers are linked by framework-forming DMOBDC and pore-partitioning ligands TPT. All-Mg 2+ ions give an negative trimer [(M 2+ ) 3 (μ 3 -OH)­(COO) 6 ], and SNNU-10- n are determined as anionic frameworks which are charge balanced by dimethylammonium cations (Me 2 NH 2+ ) generated by in situ decomposition of DMA solvent molecules. − , …”

“…29−34 SNNU-10-1−4 all have the potential to realize CO 2 /CH 4 separation, and SNNU-10-3 has the highest CO 2 /CH 4 selectivity of 21.6 which can be compared with those of the reported MOFs, such as SNNU-13 (21.6), 25 ZJU-16a (11.0), 53 and ZJNU-133 (3.4, 298 K). 56 However, compared with C 2 H 2 /C 2 H 4 , the improvement of CO 2 /CH 4 brought from the introduction of alkoxy groups is not significant. In a word, the introduction of alkoxy groups has optimized the separation performance to a certain extent and is more effective for C 2 H 2 /CH 4 and C 2 H 4 /CH 4 selectivity improvement.…”

“…In terms of breakthrough interval time, the performance of SNNU-10-1 is like some MOFs, such as ZJNU-133. 56 It is worth noting that there are roll-ups on the breakthrough curve of CH 4 because of the competitive adsorption. C 2 H 2 , C 2 H 4 , and CO 2 molecules have a stronger affinity with MOFs, which will replace CH 4 molecules adsorbed in the frameworks and leads to a higher concentration of CH 4 molecules at the outlet than that in the feed gas.…”

“…Fourier transform infrared (FT-IR) spectra (KBr pellets) were recorded on a Nicolet Avatar 360 FT-IR spectrometer in the range of 4000−400 cm −1 . The powder X-ray diffraction patterns (PXRD) were measured by a Rigaku DMAX 2500 powder diffractometer at room temperature, using Cu Kα operating in [39][40][41]54 As shown in Figure S1a, each Mg center is of six-coordinated octahedral geometry with four oxygen atoms from four different 1,4-benzenedicarboxylate ligands, one μ 3 -OH, and one nitrogen atom from a TPT ligand. Three Mg atoms form [Mg 3 (OH)(COO) 6 ] trimers with six carboxylate groups of six DROBDC ligands and three pyridine N-donors of three TPT ligands.…”