Regulation on Topological Architectures and Gas Adsorption for Cadmium-Azolate-Carboxylate Frameworks by the Ligand Flexibility

Abstract

Metal-azolate-carboxylate frameworks have great potential for gas adsorption applications because azole ligands with a small size easily construct small pore MOFs with bare −N­(H) donor sites. However, most of the currently reported metal-azolate-carboxylate frameworks were constructed by individual triazole or tetrazole and carboxylic acid; there are still few MOFs based on polytetrazole ligands. Herein, two novel mixed-linker Cd-tetrazolate-carboxylate frameworks, [Cd4­(BDC)3­(HBTT)­(H2O)2] n (SNNU-17, BDC = 1,4-terephthalic acid, H3BTT = 1,3,5-tris­(2H-tetrazol-5-yl)­benzene) and {[Cd2­(HDC)­(HBTT)­(H2O)]} n (SNNU-18) (BDC = 1,4-terephthalic acid and HDC = trans-1,4-cyclohexanedicarboxylic acid), have been synthesized. Because BDC is a rigid ligand, whereas HDC is flexible, the resulting two MOFs have completely different structures, and their percentage of uncoordinated N-donor sites increased from 33% to 50% for SNNU-17 and SNNU-18. It is worth noting that, although SNNU-18 has almost no N2 adsorption at 77 K, the adsorption capacity of CO2 and C1/C2 hydrocarbons at 298 K is capable with SNNU-17 which illustrated the potential application of a flexible MOF in gas adsorption and separation. In addition, because the pore size for SNNU-17 is 6.0 × 6.0 Å, smaller than that for SNNU-18 (11.0 × 11.0 Å), the breakthrough curves indicated that SNNU-17 has good C2H2/CO2 and C2 hydrocarbon or CO2 over CH4 dynamic separation performance. The promising gas separation performance was further supported by the GCMC simulations.

Isoreticular Double Interpenetrating Copper–Pyrazolate–Carboxylate Frameworks for Efficient CO2 Capture

“…In the meantime, FJU-28a is capable of inducing an abrupt increase in C 2 H 2 , C 2 H 4 , and C 2 H 6 uptake at 273 K, which confirmed the existence of structural transformations (Figure S12). These results showed that the replacement of ligands can effectively regulate the structural robustness and microporous characteristics, which would synergistically improve the ability for CO 2 capture . Although the CO 2 uptake of FJU-27a was lower than those of some of the most famous MOFs, ,,, it was certainly comparable to those of some of the well-known MOFs reported earlier − (Table S5).…”

“…These results showed that the replacement of ligands can effectively regulate the structural robustness and microporous characteristics, which would synergistically improve the ability for CO 2 capture. 54 Although the CO 2 uptake of FJU-27a was lower than those of some of the most famous MOFs, 13,20,55,56 it was certainly comparable to those of some of the well-known MOFs reported earlier 57−60 (Table S5). In sharp contrast, the N 2 adsorption capacities of FJU-27a and FJU-28a at 296/273 K (1 bar) were 4.5/8.0 and 1.8/3.3 cm 3 g −1 , which gave the CO 2 /N 2 uptake ratio of 5.3 and 1.9, respectively.…”

Acetylene Separation by a Ca-MOF Containing Accessible Sites of Open Metal Centers and Organic Groups

“…The selectivity values for the varied ratios of C 2 H 2 –C 2 H 4 and C 2 H 2 –CO 2 mixtures were 2.8 and 1.7, respectively (Figure b). 1a exhibits high C 2 H n –CH 4 and CO 2 –CH 4 selectivities, and the values surpass some reported MOF values (Figure c), such as [Ni­(dpip)] (C 2 H 2 –CH 4 : 18.5, C 2 H 4 –CH 4 : 7.5, and CO 2 –CH 4 : 9.1), SNNU-17 (C 2 H 2 –CH 4 : 20.3, C 2 H 4 –CH 4 : 9.4, and CO 2 –CH 4 : 12), and Cu 0.5 (tatp) 0.5 (C 2 H 2 –CH 4 : 23, C 2 H 4 –CH 4 : 11.3, and CO 2 –CH 4 : 6.5) . Moreover, although the selectivities of 1a for separating mixtures of C 2 H 2 –C 2 H 4 and C 2 H 2 –CO 2 are lower than some advanced materials, its lower Q st for C 2 H 2 is also beneficial to the regeneration of adsorbents (Figure d).…”

A Multinary Metal–Organic Framework with Divided Linkers for C2H2/CO2 Separation

“…At 100 kPa and 298 K, the C 2 H 2 /CO 2 selectivity in the equimolar binary mixture is 3.3, implying the applicability of CuZn 3 (PDDA) 3 (OH) for C 2 H 2 purification (Figure c). Despite that the calculated selectivity is lower than several MOFs with high-density open metal sites (NKMOF-1-Ni, 30; UTSA-74, 9; and JNU-2, 3.5) and MOFs with fluorinated pillars (SIFSIX-Cu-TPA, 5.3; TiFSIX-2-Cu-i, 6.5), they are superior to many notable MOFs, such as FJU-36 (2.8), ST- sod -Co/Ti (1.7) and SNNU-17 (1.2) . This enhanced selectivity can be attributed to the uncoordinated pyrazyl N atoms in the MOF backbone that facilitate the strong interactions with C 2 H 2 .…”

“…Despite that the calculated selectivity is lower than several MOFs with high-density open metal sites (NKMOF-1-Ni, 30; 35 UTSA-74, 9; 36 and JNU-2, 3.5 37 ) and MOFs with fluorinated pillars (SIFSIX-Cu-TPA, 5.3; 38 TiFSIX-2-Cu-i, 6.5 39 ), they are superior to many notable MOFs, such as FJU-36 (2.8), 40 ST-sod-Co/Ti (1.7) 41 and SNNU-17 (1.2). 42 This enhanced selectivity can be attributed to the uncoordinated pyrazyl N atoms in the MOF backbone that facilitate the strong interactions with C 2 H 2 . Grand canonical Monte Carlo (GCMC) simulation was further applied to elucidate the interaction sites in the MOF.…”