Local Deprotonation Enables Cation Exchange, Porosity Modulation, and Tunable Adsorption Selectivity in a Metal–Organic Framework

Abstract

This account demonstrates that under regulated synthetic conditions the protonated carboxyl sites in a neutral metal−organic framework (MOF), known as MOF-324 [formulated as Zn 3 OH(PzC) 2 (HPzC), H 2 PzC = 4-pyrazolecarboxylic acid], can undergo complete deprotonation, yielding NH 4 @ZnPzC [formulated as NH 4 •Zn 3 OH(PzC) 3 ]. This modified, anionic framework with a pcu-g net is thus capable of postsynthetic cation exchange, which is highly modular, encompassing organic ammonium (Me 3 NH + , Et 3 NH + ), main-group metal ions (Li + , Mg 2+ ), and even lanthanide ions (Eu 3+ , Tb 3+ ). The present approach is shown to be versatile and efficient in regulating porosity, fine-tuning gas adsorption properties, and endowing other functionalities such as liquid-phase adsorptive separation (benzene/cyclohexane). In particular, the selective adsorption behaviors of CO 2 over N 2 in this system have been studied in detail, targeting optimized CO 2 /N 2 selectivity, which are evaluated through uptake capacity, isosteric heat, and ideal adsorbed solution theory. Besides the competent CO 2 /N 2 selectivity (e.g., 50.8 for Li@ZnPzC at initial CO 2 /N 2 = 15:85), the current approach, when compared with the popular strategy of amine-grafting on exposed metal sites, shows the advantages of (i) modular porosity and adsorption property, (ii) facile exchange processes which are quantifiable and would not severely block the pore windows, and (iii) mild isosteric heat which is required for subsequent CO 2 release. It is worthy to note the regulation of local deprotonation can be regarded as a general approach for MOF chemistry and functionalization, given that there are plenty of reported MOFs bearing such "open protonated sites".

Tuning the Molecular and Cationic Affinity in a Series of Multifunctional Metal–Organic Frameworks Based on Dodecanuclear Zn(II) Carboxylate Wheels

“…The compound 1 with the largest aperture of channels and the minimum presence of the dangling hydrocarbon groups demonstrates the predominant benzene sorption. The benzene/cyclohexane selectivity in vapor phase was 20:1, which is quite promising, although, lower than the best benzene/cyclohexane selectivity values reported in the literature. , For the liquid-phase benzene/cyclohexane adsorption experiments the obtained selectivity value was as high as 92:1, clearly outperforming two known examples 32:1 and 14:1 …”

“…77,78 For the liquid-phase benzene/cyclohexane adsorption experiments the obtained selectivity value was as high as 92:1, clearly outperforming two known examples 32:1 79 and 14:1. 80 In order to unveil the nature of such a strong preference of 1 toward the benzene, we performed the single-crystal synchrotron X-ray diffraction analysis of crystals 1 immersed in pure benzene for several days. The diffraction data of 1• (benzene) 3 revealed two crystallographically independent positions of guest benzene molecules with optimized s.o.f.…”

Mixed-Linker Isoreticular Zn(II) Metal–Organic Frameworks as Brønsted Acid–Base Bifunctional Catalysts for Knoevenagel Condensation Reactions

“…For most of the documented MOFs with [Zn 3 (pyrazolate) 3 (μ 3 -OH)] SBUs, the cavities are usually <10 Å in size. − Interestingly, the combinations of the linear and cyclic trinuclear Zn­(II) SBUs connected by two linear ditopic ligands with different lengths give two types of cavities in JNU-401 , i.e., a large cavity with a window size of 15.0 Å (cavity A) and a small cavity with a window size of 7.0 Å (cavity B) (Figure c and Figure S7). For cavity A, two six-connected cyclic Zn 3 SBUs and three eight-connected linear Zn 3 SBUs are linked together by six L NA 2– /HL NA – linkers, forming the hexahedral cage.…”

Tuning Electrical‐ and Photo‐Conductivity by Cation Exchange within a Redox‐Active Tetrathiafulvalene‐Based Metal–Organic Framework

“…Recently, several applications of PSM (cation‐ or anion‐exchange) have been reported. For example, the reported cations involved in the exchange strategy are organic ammonium (Me 3 NH + , Et 3 NH + ), main‐group metals (Li + , Mg 2+ ), trivalent lanthanides (Eu 3+ , Tb 3+ ), and metal complex ([Cd(DMF) 6 ] 2+ ) . These reported works indicate that the pore size, catalytic activity, luminescent properties and gas and solvent absorption behaviors of the materials can be modulated .…”