Direct Capture of CO2 from Ambient Air.

The increase in the global atmospheric CO2 concentration resulting from over a century of combustion of fossil fuels has been associated with significant global climate change. With the global population increase driving continued increases in fossil fuel use, humanity's primary reliance on fossil energy for the next several decades is assured. Traditional modes of carbon capture such as precombustion and postcombustion CO2 capture from large point sources can help slow the rate of increase of the atmospheric CO2 concentration, but only the direct removal of CO2 from the air, or "direct air capture" (DAC), can actually reduce the global atmospheric CO2 concentration. The past decade has seen a steep rise in the use of chemical sorbents that are cycled through sorption and desorption cycles for CO2 removal from ultradilute gases such as air. This Review provides a historical overview of the field of DAC, along with an exhaustive description of the use of chemical sorbents targeted at this application. Solvents and solid sorbents that interact strongly with CO2 are described, including basic solvents, supported amine and ammonium materials, and metal-organic frameworks (MOFs), as the primary classes of chemical sorbents. Hypothetical processes for the deployment of such sorbents are discussed, as well as the limited array of technoeconomic analyses published on DAC. Overall, it is concluded that there are many new materials that could play a role in emerging DAC technologies. However, these materials need to be further investigated and developed with a practical sorbent-air contacting process in mind if society is to make rapid progress in deploying DAC as a means of mitigating climate change.

Boron Trifluoride Gas Adsorption in Metal-Organic Frameworks.

Coordinatively unsaturated metal-organic frameworks (MOFs) were studied for boron trifluoride (BF3) sorption. MOF-74-Mg, MOF-74-Mn, and MOF-74-Co show high initial uptake (below 6.7 × 10-3 bar) with negligible deliverable capacity. The BF3 isotherm of MOF-74-Cu exhibits gradual uptake up to 0.9 bar and has a deliverable gravimetric capacity that is more than 100% higher than activated carbon. Two other Cu2+ MOFs, MOF-505 and HKUST-1, have slightly lower deliverable capacities compared to MOF-74-Cu.

Isostructural synthesis of porous metal-organic nanotubes.

Employment of semirigid double-hinged di-1,2,4-triazoles has led to the synthesis of an isostructural series of metal-organic nanotubes (MONTs). The ditriazole ligands adopt a syn conformation between rigid metal chains while an appropriate anion choice provides a "capping" of the metal ions, leading to MONT formation. This approach of utilizing a variety of both semirigid ligands and metals is the first general methodology to prepare this class of 1D nanomaterial. The local geometry at the metal center depends on the metal ion employed, with Cu(I) centers adopting a tetrahedral geometry, Ag(I) centers adopting a seesaw geometry, and Cu(II) centers adopting a square-pyramidal geometry upon MONT synthesis. The pore size of the MONTs is adjusted by changing the central portion of the double-hinged ligand, allowing for a predictable method to control the pore width of the MONT. The adsorption properties of MONTs as a function of pore size revealed selective uptake of CO2 and CH4, with copper MONTs exhibiting the highest uptake. In the case of the silver MONTs, an increase in pore width improves both gas uptake and selectivity.

Rotating phenyl rings as a guest-dependent switch in two-dimensional metal-organic frameworks.

A semirigid bis(1,2,4-triazole) ligand binds in a syn conformation between copper(I) chains to form a series of two-dimensional metal-organic frameworks that display a topology of fused one-dimensional metal-organic nanotubes. These anisotropic frameworks undergo two different transformations in the solid state as a function of solvation. The 2D sheet layers can expand or contract, or, more remarkably, the phenyl rings can rotate between two distinct positions. Rotation of the phenyl rings allows for the adjustment of the tube size, depending on the guest molecules present. This "gate" effect along the 1D tubes has been characterized through single-crystal X-ray diffraction. The transformations can also be followed by powder X-ray diffraction (PXRD) and solid-state (13)C cross-polarization magic-angle-spinning (CP-MAS) NMR. Whereas PXRD cannot differentiate between transformations, solid-state (13)C CP-MAS NMR can be employed to directly monitor phenyl rotation as a function of solvation, suggesting that this spectroscopic method is a powerful approach for monitoring breathing in this novel class of frameworks. Finally, simulations show that rotation of the phenyl ring from a parallel orientation to a perpendicular orientation occurs at the cost of framework-framework energy and that this energetic cost is offset by stronger framework-solvent interactions.

Effects of solvation on the framework of a breathing copper MOF employing a semirigid linker.

A semirigid di-1,2,4-triazole ligand leads to formation of the MOF [Cu(2)(L)(2)(SO(4))(Br)(2)]·xH(2)O (1). The framework structure of 1 flexes reversibly upon removal or addition of water to form semihydrated ([Cu(2)(L)(2)(SO(4))(Br)(2)]·4H(2)O) and dehydrated ([Cu(2)(L)(2)(SO(4))(Br)(2)]·0H(2)O) MOFs, 1' and 1″, respectively. Single-crystal X-ray analysis demonstrated that the 2-butene subunit of the ligand rotates between two positions for 1 and 1', causing a change in the solvent-accessible volume in the framework. This double hinge within the semirigid ligand is a built-in breathing mechanism and suggests a novel approach for general synthesis of breathing MOFs.

Monolith-Supported Amine-Functionalized Mg2(dobpdc) Adsorbents for CO2 Capture.

The potential of using an amine-functionalized metal organic framework (MOF), mmen-M2(dobpdc) (M = Mg and Mn), supported on a structured monolith contactor for CO2 capture from simulated flue gas is explored. The stability of the unsupported MOF powders under humid conditions is explored using nitrogen physisorption and X-ray diffraction analysis before and after exposure to humidity. Based on its superior stability to humidity, mmen-Mg2(dobpdc) is selected for further growth on a honeycomb cordierite monolith that is wash-coated with α-alumina. A simple approach for the synthesis of an Mg2(dobpdc) MOF film using MgO nanoparticles as the metal precursor is used. Rapid drying of MgO on the monolith surface followed by a hydrothermal treatment is demonstrated to allow for the synthesis of a MOF film with good crystallite density and favorable orientation of the MOF crystals. The CO2 adsorption behavior of the monolith-supported mmen-Mg2(dobpdc) material is assessed using 10% CO2 in helium and 100% CO2, demonstrating a CO2 uptake of 2.37 and 2.88 mmol/g, respectively. Excellent cyclic adsorption/desorption performance over multiple cycles is also observed. This is one of the first examples of the deployment of an advanced MOF adsorbent in a scalable, low-pressure drop gas-solid contactor. Such demonstrations are critical to the practical application of MOF materials in adsorptive gas separations, as structured contactors have many practical advantages over packed or fluidized beds.

Efficient CO2 Capture by a 3D Porous Polymer Derived from Tröger's Base.

A 3D Tröger's-base-derived microporous organic polymer with a high surface area and good thermal stability was facilely synthesized from a one-pot metal-free polymerization reaction between dimethoxymethane and triaminotriptycene. The obtained material displays excellent CO2 uptake abilities as well as good adsorption selectivity for CO2 over N2. The CO2 storage can reach up to 4.05 mmol g-1 (17.8 wt %) and 2.57 mmol g-1 (11.3 wt %) at 273 K and 298 K, respectively. Moreover, the high selectivity of the polymer toward CO2 over N2 (50.6, 298 K) makes it a promising material for potential application in CO2 separation from flue gas.

Utilizing a copper MOF as a reagent in a solvent mediated reaction to form a topologically distinct MOF.

Employing a semi-rigid di-1,2,4-triazole ligand leads to the formation of new MOFs [Cu(4)(L)(4)(SO(4))(4)]·4[Cu(H(2)O)(6)(SO(4))] (3) and [Cu(6)(L)(3)(SO(4))(5)(OH)(2)(H(2)O)(6)]·13H(2)O (4). The frameworks can be synthesized independently, but a reaction occurs in water wherein kinetic product 3 is used as a reagent to synthesize the topologically distinct thermodynamic product 4.