Ammonia Capture in Rhodium(II)-Based Metal–Organic Polyhedra via Synergistic Coordinative and H-Bonding Interactions

Abstract

Ammonia (NH 3 ) is among the world's most widely produced bulk chemicals, given its extensive use in diverse sectors such as agriculture; however, it poses environmental and health risks at low concentrations. Therefore, there is a need for developing new technologies and materials to capture and store ammonia safely. Herein, we report for the first time the use of metal−organic polyhedra (MOPs) as ammonia adsorbents. We evaluated three different rhodium-based MOPs: [Rh 2 (bdc) 2 ] 12 (where bdc is 1,3-benzene dicarboxylate); one functionalized with hydroxyl groups at its outer surface [Rh 2 (OH-bdc) 2 ] 12 (where OHbdc is 5-hydroxy-1,3-benzene dicarboxylate); and one decorated with aliphatic alkoxide chains at its outer surface [Rh 2 (C 12 O-bdc) 2 ] 12 (where C 12 O-bdc is 5dodecoxybenzene-1,3-benzene dicarboxylate). Ammonia-adsorption experiments revealed that all three Rh-MOPs strongly interact with ammonia, with uptake capacities exceeding 10 mmol/g MOP . Furthermore, computational and experimental data showed that the mechanism of the interaction between Rh-MOPs and ammonia proceeds through a first step of coordination of NH 3 to the axial site of the Rh(II) paddlewheel cluster, which triggers the adsorption of additional NH 3 molecules through H-bonding interaction. This unique mechanism creates H-bonded clusters of NH 3 on each Rh(II) axial site, which accounts for the high NH 3 uptake capacity of Rh-MOPs. Rh-MOPs can be regenerated through their immersion in acidic water, and upon activation, their ammonia uptake can be recovered for at least three cycles. Our findings demonstrate that MOPs can be used as porous hosts to capture corrosive molecules like ammonia, and that their surface functionalization can enhance the ammonia uptake performance.

Porous Metal‐Organic Cages Based on Rigid Bicyclo[2.2.2]oct‐7‐ene Type Ligands: Synthesis, Structure, and Gas Uptake Properties

“…Structured hysteresis loops of this sort have been observed in metal‐organic cages before, [3e,8b,9] where they may be associated with structural changes being induced by incorporation of the gas molecules. There are also examples of hysteresis due to enhanced interactions between cages and the sorbates [5b,19] . However, as in references 5b and 19, the exact structural origin of the step and hysteresis observed in 3 a is unclear.…”

Transformation of Type III to Type II Porous Liquids by Tuning Surface Rigidity of Rhodium(II)‐Based Metal‐Organic Polyhedra for CO2 Cycloaddition

“…Notably, such peak shifting (δ) was not observed from the mixture of neat RhMOP and IL‐Br (Supporting Information, Figure S8), which substantiates that tuning the surface rigidity of cages enhances solubility. In addition, solubility of functionalized cages in IL‐Br is illustrated in the magnified FTIR spectra of type II PILs when an absorption peak at 1718 cm −1 appeared from the stretching of C=O bond which belongs to the −COOH group of the bdc ligand (Supporting Information, Figure S27) [34] . The dissolution of diz ‐ and CF 3 ‐Py ‐RhMOP in IL‐Br also vary its fluidic nature as revealed by the viscosity measurements of the obtained type II PILs (Figure 3e).…”

“…In addition, solubility of functionalized cages in IL-Br is illustrated in the magnified FTIR spectra of type II PILs when an absorption peak at 1718 cm À 1 appeared from the stretching of C=O bond which belongs to the À COOH group of the bdc ligand (Supporting Information, Figure S27). [34] The dissolution of diz-and CF 3 -Py-RhMOP in IL-Br also vary its fluidic nature as revealed by the viscosity measurements of the obtained type II PILs Angewandte Chemie (Figure 3e). Similarly, the measured densities of PIL-dizand PIL-CF 3 -Py-RhMOP found to be slightly higher than neat IL, indicating the existence of permanent cavities of porous cages in IL-Br (Supporting Information, Table S1).…”

Transformation of Type III to Type II Porous Liquids by Tuning Surface Rigidity of Rhodium(II)‐Based Metal‐Organic Polyhedra for CO2 Cycloaddition

“…In addition, solubility of functionalized cages in IL-Br is illustrated in the magnified FTIR spectra of type II PILs when an absorption peak at 1718 cm À 1 appeared from the stretching of C=O bond which belongs to the À COOH group of the bdc ligand (Supporting Information, Figure S27). [34] The dissolution of diz-and CF 3 -Py-RhMOP in IL-Br also vary its fluidic nature as revealed by the viscosity measurements of the obtained type II PILs…”