Mn-CUK-1: A Flexible MOF for SO2, H2O, and H2S Capture.

The environmentally benign metal-organic framework (MOF) CUK-1 based on 2,4-pyridine dicarboxylate has been prepared for the first time using Mn(II) as the inorganic node and water as the only solvent. Mn-CUK-1 shows reversible and efficient capture of H2O, SO2, and H2S. Compared to previously studied Co(II) and Mg(II) versions of the same MOF, Mn-CUK-1 also exhibited unique temperature-induced structural flexibility due to organic linker torsion, as detailed by variable-temperature single-crystal X-ray diffraction studies. Owing to this inherent solid-state flexibility, Mn-CUK-1 showed stepwise adsorption for polar gases, which induce structural deformations upon adsorption, while the nonpolar guest adsorbates were reversibly sorbed in a more classical manner. Notably, Mn-CUK-1 demonstrates the highest reported H2S capacity-to-surface area ratio among MOFs that are chemically stable toward this reactive acidic molecule. Moreover, Mn-CUK-1 displays exceptional structural stability in the presence of high relative humidity and corrosive gases and shows soft crystalline behavior triggered by changes in both the adsorption temperature and guest molecule identity.

SO2 Capture Using Porous Organic Cages.

We report the first experimental investigation of porous organic cages (POCs) for the demanding challenge of SO2 capture. Three structurally related N-containing cage molecular materials were studied. An imine-functionalized POC (CC3) showed modest and reversible SO2 capture, while a secondary-amine POC (RCC3) exhibited high but irreversible SO2 capture. A tertiary amine POC (6FT-RCC3) demonstrated very high SO2 capture (13.78 mmol g-1 ; 16.4 SO2 molecules per cage) combined with excellent reversibility for at least 50 adsorption-desorption cycles. The adsorption behavior was investigated by FTIR spectroscopy, 13 C CP-MAS NMR experiments, and computational calculations.

A Robust and Biocompatible Bismuth Ellagate MOF Synthesized Under Green Ambient Conditions.

The first bioinspired microporous metal-organic framework (MOF) synthesized using ellagic acid, a common natural antioxidant and polyphenol building unit, is presented. Bi2O(H2O)2(C14H2O8)·nH2O (SU-101) was inspired by bismuth phenolate metallodrugs, and could be synthesized entirely from nonhazardous or edible reagents under ambient aqueous conditions, enabling simple scale-up. Reagent-grade and affordable dietary supplement-grade ellagic acid was sourced from tree bark and pomegranate hulls, respectively. Biocompatibility and colloidal stability were confirmed by in vitro assays. The material exhibits remarkable chemical stability for a bioinspired MOF (pH = 2-14, hydrothermal conditions, heated organic solvents, biological media, SO2 and H2S), attributed to the strongly chelating phenolates. A total H2S uptake of 15.95 mmol g-1 was recorded, representing one of the highest H2S capacities for a MOF, where polysulfides are formed inside the pores of the material. Phenolic phytochemicals remain largely unexplored as linkers for MOF synthesis, opening new avenues to design stable, eco-friendly, scalable, and low-cost MOFs for diverse applications, including drug delivery.

Modulated self-assembly of three flexible Cr(III) PCPs for SO2 adsorption and detection.

Modulated self-assembly protocols are used to develop facile, HF-free syntheses of the archetypal flexible PCP, MIL-53(Cr), and novel isoreticular analogues MIL-53(Cr)-Br and MIL-53(Cr)-NO2. All three PCPs show good SO2 uptake (298 K, 1 bar) and high chemical stabilities against dry and wet SO2. Solid-state photoluminescence spectroscopy indicates all three PCPs exhibit turn-off sensing of SO2, in particular MIL-53(Cr)-Br, which shows a 2.7-fold decrease in emission on exposure to SO2 at room temperature, indicating potential sensing applications.

Detection of SO2 using a chemically stable Ni(II)-MOF.

The MOF-type Ni2(dobpdc) shows a high chemical stability towards SO2, high capacity for SO2 capture at low pressure (4.3 mmol g-1 at 298 K and up to 0.05 bar), and exceptional cycling performance. Fluorescence experiments demonstrated the SO2 detection properties of Ni2(dobpdc) with a remarkable SO2 detection selectivity. Finally, time-resolved photoluminescence experiments provided a plausible mechanism of SO2 detection by this Ni(II)-based MOF material.

Sulfur Dioxide Capture in Metal-Organic Frameworks, Metal-Organic Cages, and Porous Organic Cages.

Capture, storage and subsequent controlled release or transformation of sulfur dioxide (SO2 ) in mild conditions is still a challenge in the material science field. Recent advances in the use of porous materials have demonstrated good SO2 capture, particularly in metal-organic frameworks (MOFs), metal-organic cages (MOCs), and porous organic cages (POCs). The striking feature of these porous materials is the high SO2 uptake capacity in reversible settings. A partially fluorinated MIL-101(Cr) is stand-alone material with the highest SO2 uptake in chemically stable MOFs. Likewise, metal-free adsorbents like POCs exhibits a reversible SO2 uptake behavior. The SO2 adsorption characteristics of these three structurally and functionally unique adsorbent systems are highly dependent on the binding sites and mode of binding of SO2 molecules. This Review has highlighted the preferential binding sites in these materials to give a full perspective on the field. We anticipate that it will offer valuable information on the progress made towards improving SO2 capture by hybrid systems.

SO2 capture and detection using a Cu(II)-metal-organic polyhedron.

The SO2 adsorption-desorption capacity at room temperature and 1 bar of the metal-organic polyhedron MOP-CDC was investigated. In addition, the qualitative solid-state absorption-emission properties of this material (before and after SO2 exposure) were measured and tested, and it demonstrated remarkable capability for SO2 detection. Our results represent the first example of fluorimetric SO2 detection in a MOP.

SO2 Capture and Oxidation in a Pd6L8 Metal-Organic Cage.

The facile and green preparation of novel materials that capture sulfur dioxide (SO2) with significant uptake at room temperature remains challenging, but it is crucial for public health and the environment. Herein, we explored for the first time the SO2 adsorption within microporous metal-organic cages using the palladium(II)-based [Pd6L8](NO3)36 tetragonal prism 1, assembled in water under mild conditions. Notably and despite the low BET surface area of 1 (111 m2 g-1), sulfur dioxide was found to be irreversibly and strongly adsorbed within the activated cage at 298 K (up to 6.07 mmol g-1). The measured values for the molar enthalpy of adsorption (ΔHads) coupled to the FTIR analyses imply a chemisorption process that involves the direct interaction of SO2 with Pd(II) sites and the subsequent oxidation of this toxic chemical by the action of the nitrate anions in 1. To the best of our knowledge, this is the first reported metal-organic cage that proves useful for SO2 adsorption. Metallosupramolecular adsorbents such as 1 could enable new detection applications and suggest that the integration of soft metal ions and self-assembly of molecular cages are a potential means for the easy tuning of SO2 adsorption capabilities and behavior.

Capture of toxic gases in MOFs: SO2, H2S, NH3 and NO x.

MOFs are promising candidates for the capture of toxic gases since their adsorption properties can be tuned as a function of the topology and chemical composition of the pores. Although the main drawback of MOFs is their vulnerability to these highly corrosive gases which can compromise their chemical stability, remarkable examples have demonstrated high chemical stability to SO2, H2S, NH3 and NO x . Understanding the role of different chemical functionalities, within the pores of MOFs, is the key for accomplishing superior captures of these toxic gases. Thus, the interactions of such functional groups (coordinatively unsaturated metal sites, µ-OH groups, defective sites and halogen groups) with these toxic molecules, not only determines the capture properties of MOFs, but also can provide a guideline for the desigh of new multi-functionalised MOF materials. Thus, this perspective aims to provide valuable information on the significant progress on this environmental-remediation field, which could inspire more investigators to provide more and novel research on such challenging task.

SO2 Capture by Two Aluminum-Based MOFs: Rigid-like MIL-53(Al)-TDC versus Breathing MIL-53(Al)-BDC.

Metal-organic frameworks MIL-53(Al)-TDC and MIL-53(Al)-BDC were explored in the SO2 adsorption process. MIL-53(Al)-TDC was shown to behave as a rigid-like material upon SO2 adsorption. On the other hand, MIL-53(Al)-BDC exhibits guest-induced flexibility of the framework with the presence of multiple steps in the SO2 adsorption isotherm that was related through molecular simulations to the existence of three different pore opening phases narrow pore, intermediate pore, and large pore. Both materials proved to be exceptional candidates for SO2 capture, even under wet conditions, with excellent SO2 adsorption, good cycling, chemical stability, and easy regeneration. Further, we propose MIL-53(Al)-TDC and MIL-53(A)-BDC of potential interest for SO2 sensing and SO2 storage/transportation, respectively.

Reversible and efficient SO2 capture by a chemically stable MOF CAU-10: experiments and simulations.

The adsorption of sulphur dioxide (SO2) in CAU-10 is obtained with the use of advanced experimental and computational tools to gain insight into the molecular mechanisms responsible for the adsorption of SO2. It is shown that the adsorption by CAU-10 is highly energy efficient and that van der Waals interactions are the driving force that controls adsorption in this system.