Metal-Organic Frameworks with Internal Urea-Functionalized Dicarboxylate Linkers for SO2 and NH3 Adsorption

Architectural and catalytic aspects of designer materials built using metalloligands of pyridine-2,6-dicarboxamide based ligands

This perspective presents the design, synthesis and crystal structures of a large number of architectures constructed using assorted metalloligands of pyridine-2,6-dicarboxamide based ligands. The metalloligands offered various appended functional groups, whereas design strategies included varying both their position and number. A combination of these parameters resulted in the development of assorted architectures including discrete trinuclear and tetranuclear complexes as well as 1D/2D/3D coordination polymers. The metalloligand strategy not only assisted in the construction of ordered crystalline materials with varied dimensionalities but also judiciously allowed the incorporation of Lewis acidic and redox-active secondary metals in the resultant architectures. As a result, such designer architectures illustrated their noteworthy role both as homogenous and heterogeneous catalysts in different organic transformation reactions.

Versatile Platform of Ion Conducting 2D Anionic Germanate Covalent Organic Frameworks with Potential for Capturing Toxic Acidic Gases

Anionic covalent organic framework is an emerging class of functional materials in which included ionic species of the opposite charges play an important role in the ion conduction and selective gas adsorption. Herein, we reported a facile method to construct a series of germanate-based anionic COFs (Ge-COFs) by reticulating dianionic hexa-coordinated GeO₆ nodes with anthracene building blocks adopting a hcb topology in an extended 2D framework. A systematic change of pore properties in Ge-COFs was observed through the incorporation of three different alkali metal cations: Li⁺, Na⁺, and K⁺. The intrinsically negatively charged backbone provides a host matrix with a homogeneous distribution of counter cations and poses variable and exciting features for gas adsorption and ionic conduction. Among the series, K⁺-based Ge-COF-K with a surface area of 1252 m²/g and pore volume of 0.84 cm³/g shows a maximum CO₂ uptake of 126 cm³/g (247.4 mg/g) at 273 K and 1 bar, an IAST selectivity of 140 over N₂. Ge-COF-K also exhibits a high SO₂ kinetic breakthrough capacity of 154 mg/g at low ppm of SO₂ concentration under ambient conditions among recently reported porous materials. Moreover, reasonably high lithium, sodium, and potassium ionic conductivities were observed with the values of 1.2 × 10^-4, 3.4 × 10^-5, and 2.2 × 10^-5 S/cm for propylene carbonate infiltrated Ge-COF-Li, Ge-COF-Na, and Ge-COF-K at 100 °C, respectively.

Sulfur dioxide removal: An overview of regenerative flue gas desulfurization and factors affecting desulfurization capacity and sorbent regeneration

Numerous mitigation techniques have been incorporated to capture or remove SO₂ with flue gas desulfurization (FGD) being the most common method. Regenerative FGD method is advantageous over other methods due to high desulfurization efficiency, sorbent regenerability, and reduction in waste handling. The capital costs of regenerative methods are higher than those of commonly used once-through methods simply due to the inclusion of sorbent regeneration while operational and management costs depend on the operating hours and fuel composition. Regenerable sorbents like ionic liquids, deep eutectic solvents, ammonium halide solutions, alkyl-aniline solutions, amino acid solutions, activated carbons, mesoporous silica, zeolite, and metal-organic frameworks have been reported to successfully achieve high SO₂ removal. The presence of other gases in flue gas, e.g., O₂, CO₂, NOx, and water vapor, and the reaction temperature critically affect the sorption capacity and sorbent regenerability. To obtain optimal SO₂ removal performance, other parameters such as pH, inlet SO₂ concentration, and additives need to be adequately governed. Due to its high removal capacity, easy preparation, non-toxicity, and low regeneration temperature, the use of deep eutectic solvents is highly feasible for upscale utilization. Metal-organic frameworks demonstrated highest reported SO₂ removal capacity; however, it is not yet applicable at industrial level due to its high price, weak stability, and robust formulation.

MOF-Based Adsorbents for Atmospheric Emission Control: A Review

This review focuses on the use of metal–organic frameworks (MOFs) for adsorbing gas species that are known to weaken the thermal self-regulation capacities of Earth’s atmosphere. A large section is dedicated to the adsorption of carbon dioxide, while another section is dedicated to the adsorption of other different gas typologies, whose emissions, for various reasons, represent a “wound” for Earth’s atmosphere. High emphasis is given to MOFs that have moved enough ahead in their development process to be currently considered as potentially usable in “real-world” (i.e., out-of-lab) adsorption processes. As a result, there is strong evidence of a wide gap between laboratory results and the industrial implementation of MOF-based adsorbents. Indeed, when a MOF that performs well in a specific process is commercially available in large quantities, economic observations still make designers tend toward more traditional adsorbents. Moreover, there are cases in which a specific MOF remarkably outperforms the currently employed adsorbents, but it is not industrially produced, thus strongly limiting its possibilities in large-scale use. To overcome such limitations, it is hoped that the chemical industry will be able to provide more and more mass-produced MOFs at increasingly competitive costs in the future.

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

CAU-10 is an efficient system for SO₂ adsorption, and its great recyclability is given by van der Waals interactions present within its pore.

Two urea-functionalized metal–organic frameworks based on a pillared-layer strategy for gas adsorption and separation

Two pcu type Cu-MOFs based on urea-functionalized ligands have been synthesized by a pillared-layer strategy. Compound 1 shows good adsorption and separation behaviors of CO₂, C₂H₆, and C₃H₈ over CH₄, compound 2 exhibits a gate-opening behavior for N₂ adsorption.

Synthesis of Dibenzo[d,f][1,3]Diazepines via Elemental Sulfur-Mediated Cyclocondensation of 2,2'-Biphenyldiamines with 2-Chloroacetic Acid Derivatives

The three-component reaction of 2,2'-biphenyldiamines with 2-chloroacetic acid derivatives and elemental sulfur was developed for the practical synthesis of unknown 2-carboxamide-substituted dibenzo[d,f][1,3]diazepines. This protocol is distinguished by efficiency in water and good tolerance to functional groups and can be adapted to a large-scale synthesis. The chemoselective preparation of a variety of 2-S,N,O-substituted dibenzo[d,f][1,3]diazepines was accomplished using the developed method.

Reversible coordinative binding and separation of sulfur dioxide in a robust metal-organic framework with open copper sites

Emissions of SO₂ from flue gas and marine transport have detrimental impacts on the environment and human health, but SO₂ is also an important industrial feedstock if it can be recovered, stored and transported efficiently. Here we report the exceptional adsorption and separation of SO₂ in a porous material, [Cu₂(L)] (H₄L = 4',4‴-(pyridine-3,5-diyl)bis([1,1'-biphenyl]-3,5-dicarboxylic acid)), MFM-170. MFM-170 exhibits fully reversible SO₂ uptake of 17.5 mmol g^-1 at 298 K and 1.0 bar, and the SO₂ binding domains for trapped molecules within MFM-170 have been determined. We report the reversible coordination of SO₂ to open Cu(II) sites, which contributes to excellent adsorption thermodynamics and selectivities for SO₂ binding and facile regeneration of MFM-170 after desorption. MFM-170 is stable to water, acid and base and shows great promise for the dynamic separation of SO₂ from simulated flue gas mixtures, as confirmed by breakthrough experiments.

Recent advances on porous organic frameworks for the adsorptive removal of hazardous materials

Environmental pollution is one of the most serious problems facing mankind today, and has attracted widespread attention worldwide. The burgeoning class of crystalline porous organic framework materials, metal-organic frameworks and covalent organic frameworks present promising application potential in areas related to pollution control due to their interesting surface properties. In this review, the literature of the past five years on the adsorptive removal of various hazardous materials, mainly including heavy metal ions, harmful gases, organic dyes, pharmaceutical and personal care products, and radionuclides from the environment by using COFs and MOFs, is summarized. The adsorption mechanisms are also discussed to help understand their adsorption performance and selectivity. Additionally, some insightful suggestions are given to enhance the performance of MOFs and COFs in the adsorptive removal of various hazardous materials.

Metal-Organic Gels Based on a Bisamide Tetracarboxyl Ligand for Carbon Dioxide, Sulfur Dioxide, and Selective Dye Uptake

A metal-organic gel (metallogel) based on the new tetracarboxyl ligand N¹, N⁴-(diterephthalic acid)terephthalamide in combination with chromium(III) has been converted into its xero- and aerogel and demonstrated to have excellent specific sorption properties for dyes in its metallogel state, where fuchsine is adsorbed faster than the two other dyes, calcein and disulfine blue, and for water, sulfur dioxide and carbon dioxide in its xero- and aerogel state. The metallogel showed very good shape retention and could be extruded from molds in designed shapes. In a rheology experiment, the storage modulus was determined to be 1440 Pa, and the metallogel is elastic up to 3 Hz, breaking at strains higher than 0.3%. Additional metallogels utilizing the same ligand with a wide range of metal ions (Al(III), Fe(III), Co(III), In(III), and Hg(II)) have also been synthesized, and the aluminum and mixed aluminum-chromium derivative were also converted into its aerogel. The highly porous Cr, Al, and AlCr metal-organic aerogels proved stable against water vapor in a physisorption experiment and were used to model breakthrough curves for SO₂/CO₂ gas mixtures with the idealized adsorbed solution theory from their physisorption isotherms. The breakthrough simulation utilized SO₂/CO₂ equivalencies from a real world application and showed effective retention of SO₂ from the gas mixture. Furthermore, the materials in this work exhibit the highest SO₂ uptake values for metal-organic aerogels so far (up to 116.8 cm³ g^-1, or 23.4 wt %).

Metal-Organic Frameworks with Potential Application for SO2 Separation and Flue Gas Desulfurization

Sulfur dioxide (SO₂) is an acidic and toxic gas and its emission from utilizing energy from fossil fuels or in industrial processes harms human health and environment. Therefore, it is of great interest to find new materials for SO₂ sorption to improve classic flue gas desulfurization. In this work, we present SO₂ sorption studies for the three different metal-organic frameworks MOF-177, NH₂-MIL-125(Ti), and MIL-160. MOF-177 revealed a new record high SO₂ uptake (25.7 mmol·g^-1 at 293 K and 1 bar). Both NH₂-MIL-125(Ti) and MIL-160 show particular high SO₂ uptakes at low pressures ( p < 0.01 bar) and thus are interesting candidates for the removal of remaining SO₂ traces below 500 ppm from flue gas mixtures. The aluminum furandicarboxylate MOF MIL-160 is the most promising material, especially under application-orientated conditions, and features excellent ideal adsorbed solution theory selectivities (124-128 at 293 K, 1 bar; 79-95 at 353 K, 1 bar) and breakthrough performance with high onset time, combined with high stability under both humid and dry SO₂ exposure. The outstanding sorption capability of MIL-160 could be explained by DFT simulation calculations and matching heat of adsorption for the binding sites O_furan···S_SO2 and OH_Al-chain···O_SO2 (both ∼40 kJ·mol^-1) and O_{furan/carboxylate}···S_SO2 (∼55-60 kJ·mol^-1).

Fluorinated MOF platform for selective removal and sensing of SO2 from flue gas and air

Conventional SO₂ scrubbing agents, namely calcium oxide and zeolites, are often used to remove SO₂ using a strong or irreversible adsorption-based process. However, adsorbents capable of sensing and selectively capturing this toxic molecule in a reversible manner, with in-depth understanding of structure–property relationships, have been rarely explored. Here we report the selective removal and sensing of SO₂ using recently unveiled fluorinated metal–organic frameworks (MOFs). Mixed gas adsorption experiments were performed at low concentrations ranging from 250 p.p.m. to 7% of SO₂. Direct mixed gas column breakthrough and/or column desorption experiments revealed an unprecedented SO₂ affinity for KAUST-7 (NbOFFIVE-1-Ni) and KAUST-8 (AlFFIVE-1-Ni) MOFs. Furthermore, MOF-coated quartz crystal microbalance transducers were used to develop sensors with the ability to detect SO₂ at low concentrations ranging from 25 to 500 p.p.m.

Removal of SO₂ from flue gas is of prime importance to avoid its poisoning of CO₂-seperating agents. Here, the authors demonstrate that two fluorinated metal–organic frameworks selectively remove SO₂ from synthetic flue gas and can sense SO₂ with p.p.m.-level detection using quartz crystal microbalance transducers.

Efficient Conversion of CO2 via Grafting Urea Group into a [Cu2(COO)4]-Based Metal-Organic Framework with Hierarchical Porosity

The assembly of mixed [1,1';3',1'']terphenyl-4,5',4''-tricarboxylic acid (H₃TPTC) and [1,1'-biphenyl]-4,4'-dicarboxylic acid (H₂BPDC), 2,2'-diamino-[1,1'-biphenyl]-4,4'-dicarboxylic acid (H₂BPDC-NH₂), or 6-oxo-6,7-dihydro-5H-dibenzo[ d, f][1,3]diazepine-3,9-dicarboxylic acid (H₂BPDC-Urea) with Cu²⁺ ion generated the corresponding copper-paddlewheel-based metal-organic framework (MOF) [Cu₅(TPTC)₃(BPDC)_0.5(H₂O)₅] (1), [Cu₅(TPTC)₃(BPDC-NH₂)_0.5(H₂O)₅] (1-NH₂), or [Cu₅(TPTC)₃(BPDC-Urea)_0.5(H₂O)₅] (1-Urea). They are isostructural with hierarchical porosity, consisting of zero-dimensional cage (19.2 Å × 18.9 Å) and one-dimensional pillar channel (29.7 Å × 15.1 Å) in a manner of face sharing. Platon analyses revealed the porous volume ratios are 80.2%, 80.0%, and 77.8% for 1, 1-NH₂, and for 1-Urea, respectively. Thermogravimetric measurements suggested 53, 51, and 48 wt % guest molecules in 1, 1-NH₂, and 1-Urea, respectively. 1-NH₂ and 1-Urea were precisely functionalized via the introduction of amino and urea functional groups into the pillar channels. The constructed MOF 1-Urea, incorporating both exposed copper active sites and accessible urea functional groups to substrates, presents high efficiency on catalytic CO₂ cycloaddition with propene oxide to produce cyclic carbonate in the yield of 98% with a TOF value of 136 h^-1 at 1 atm and room temperature. This synergic effect provides a new strategy for designing high-efficient catalysts for CO₂ chemical conversion under ambient conditions.

Exploration of tetra-branched multiple-site SO2 capture materials

Efficient exploration of the configuration space of the reaction complexes consisting of multi-branched structures and SO₂ molecules.

Nitrogen-Doped Microporous Carbons Derived from Pyridine Ligand-Based Metal-Organic Complexes as High-Performance SO2 Adsorption Sorbents

Heteroatom-doped porous carbons are emerging as platforms for gas adsorption. Herein, N-doped microporous carbon (NPC) materials have been synthesized by carbonization of two pyridine ligand-based metal-organic complexes (MOCs) at high temperatures (800, 900, 1000, and 1100 °C). For NPCs (termed NPC-1- T and NPC-2- T, where T represents the carbonization temperature), the micropore is dominant, pyridinic-N and other N atoms of MOC precursors are mostly retained, and the N content reaches as high as 16.61%. They all show high Brunauer-Emmett-Teller surface area and pore volume, in particular, NPC-1-900 exhibits the highest surface areas and pore volumes, up to 1656.2 m² g^-1 and 1.29 cm³ g^-1, respectively, a high content of pyridinic-N (7.3%), and a considerable amount of SO₂ capture (118.1 mg g^-1). Theoretical calculation (int = ultrafine m062x) indicates that pyridinic-N acts as the leading active sites contributing to high SO₂ adsorption and that the higher content of pyridinic-N doping into the graphite carbon layer structure could change the electrostatic surface potential, as well as the local electronic density, which enhanced SO₂ absorption on carbon edge positions. The results show great potential for the preparation of microporous carbon materials from pyridine ligand-based MOCs for effective SO₂ adsorption.

Functionalized Covalent Triazine Frameworks for Effective CO2 and SO2 Removal

Building novel frameworks as sorbents remains a highly significant target for key environmental issues such as CO₂ or SO₂ emissions from coal-fired power plants. Here, we report the construction and tunable pore structure as well as gas adsorption properties of hierarchically porous covalent triazine-based frameworks (CTF-CSUs) functionalized by appended carboxylic acid/sodium carboxylate groups. The densely integrated functionalities on the pore walls bestow strong affinity to the as-made networks toward guest acid gases, in spite of their moderate Brunauer-Emmett-Teller surface areas. With abundant microporosity and integrated carboxylic acid groups, our frameworks deliver strong affinity toward CO₂ with considerably high enthalpy (up to 44.6 kJ/mol) at low loadings. Moreover, the sodium carboxylate-anchored framework (termed as CTF-CSU41) shows an exceptionally high uptake of SO₂ up to 6.7 mmol g^-1 (42.9 wt %) even under a low SO₂ partial pressure of 0.15 bar (298 K), representing the highest value for a scrubbing material reported to date. Significantly, such pore engineering could pave the way to broad applications of porous organic polymers.

Role of Filler Porosity and Filler/Polymer Interface Volume in Metal-Organic Framework/Polymer Mixed-Matrix Membranes for Gas Separation

Metal-organic frameworks (MOFs) and inorganic fillers are frequently incorporated into mixed-matrix membranes (MMMs) to overcome the traditional trade-off in permeability ( P) and selectivity for pure organic polymer membranes. Therefore, it is of great interest to examine the influence of porous and nonporous fillers in MMMs with respect to the possible role of the polymer-filler interface, that is, the void volume. In this work, we compare the same MOF filler in a porous and nonporous state, so that artifacts from a different polymer-filler interface are excluded. MMMs with the porous MOF aluminum fumarate (Al-fum) and with a nonporous dimethyl sulfoxide solvent-filled aluminum fumarate (Al-fum(DMSO)), both with Matrimid as polymer, were prepared. Filler contents ranged from 4 to 24 wt %. Gas separation performances of both MMMs were studied by mixed gas measurements using a binary mixture of CO₂/CH₄ with gas permeation following the theoretical prediction by the Maxwell model for both porous and nonporous dispersed phase (filler). MMMs with the porous Al-fum filler showed increased CO₂ and CH₄ permeability with a moderate rise in selectivity upon increasing filler fraction. The MMMs with the nonporous Al-fum(DMSO) filler displayed a reduction in permeability while maintaining the selectivity of the neat polymer. A linear dependence of log P versus the reciprocal specific free fractional volume (sFFV) rules out a significant contribution from a void volume. The sFFV includes the free volume of the polymer and the MOF, but not the polymer-filler interface volume (so-called void volume). The sFFV for the MMM was calculated between 0.23 cm³/g for a 24 wt % Al-fum/Matrimid MMM and 0.12 cm³/g for a 24 wt % Al-fum(DMSO)/Matrimid MMM. The negligible effect of an interface volume is supported by a good matching of theoretical and experimental density of the Al-fum and Al-fum/(DMSO) MMMs which gave a specific void volume below 0.02 cm³/g, often even below 0.01 cm³/g.

Acid- and base-stable porous mechanically interlocked 2D metal–organic polyrotaxane forin situorganochlorine insecticide encapsulation, sensing and removal

The encapsulation and removal of extremely toxic dieldrin by compound1.

Designing tri-branched multiple-site SO2 capture materials

Tri-branched species with multiple isolated reactive sites are proposed for high and uniform SO₂ capture.