The increase in the global atmospheric CO 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 CO capture from large point sources can help slow the rate of increase of the atmospheric CO concentration, but only the direct removal of CO from the air, or "direct air capture" (DAC), can actually reduce the global atmospheric CO concentration. The past decade has seen a steep rise in the use of chemical sorbents that are cycled through sorption and desorption cycles for CO 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 CO 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.

No abstract

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.

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.

The potential of using an amine-functionalized metal organic framework (MOF), mmen-M(dobpdc) (M = Mg and Mn), supported on a structured monolith contactor for CO 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-Mg(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 Mg(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 CO adsorption behavior of the monolith-supported mmen-Mg(dobpdc) material is assessed using 10% CO in helium and 100% CO, demonstrating a CO 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.

Macrocyclic tetraimidazolium diborate ligand precursors with two different ring sizes have been synthesized by ring-forming reactions between diimidazoles and haloboranes. Deprotonation of the macrocyclic tetraimidazoliums with n-butyllithium followed by the addition of divalent metal salts of palladium or nickel leads to macrocyclic tetracarbene complexes with an 18-atom macrocycle, but not the 16-atom variant. These neutral palladium and nickel complexes are the first examples of macrocyclic tetracarbene diborate complexes, and unlike their cationic counterparts, they are highly soluble in nonpolar solvents. All macrocyclic tetraimidazoliums and their corresponding metal complexes were characterized by single-crystal X-ray diffraction and spectroscopic techniques.

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.

The MIL-53 class of metal−organic frameworks (MOFs) has recently generated interest as potential adsorbents for xylene mixture separations. Cost-effective separation of xylene isomers is challenging owing to the similarity in their molecular structures, kinetic diameters, and boiling points. Here we report a systematic experimental and computational study of xylene isomer adsorption in MIL-53 adsorbents, focusing particularly on the effects of different metal centers, determination of separation properties with industrially relevant quaternary liquid-phase C 8 aromatic feeds, and a predictive molecular simulation methodology that accounts for all relevant modes of MIL-53 framework flexibility. Significant scale-up of MIL-53 synthesis was carried out to produce high-quality materials in sufficient quantities (300−500 g each) for detailed measurements. Single-component adsorption simulations incorporating the MIL-53 "breathing" and linker flexibility effects showed good agreement with experimental isotherms. Upon the basis of these results, three materialsMIL-53(Al), MIL-53(Cr), and MIL-53(Ga)were selected for detailed quaternary liquid breakthrough measurements. High o-xylene quaternary selectivity was obtained from all of the MIL-53 materials, with MIL-53(Al) being the most selective. Better packing efficiency of o-xylene and its preferred interactions with the MIL-53 framework are hypothesized to lead to high selectivity. Predictions from flexible-structure multicomponent adsorption simulations showed good overall agreement with experiment. This is, to the best of our knowledge, the first experimental report on the xylene adsorption characteristics of MIL-53 materials under industrially relevant operating conditions. In addition, it is also the first attempt to develop computational methods that account for various flexibility modes in MIL-53 materials for adsorption simulations. This has significant broader applications for the successful prediction of adsorption properties of larger molecules (such as C 8 aromatic isomers) in flexible MOFs.