Selective capture of CO2, which is essential for natural gas purification and CO 2 sequestration, has been reported in zeolites, porous membranes, and amine solutions. However, all such systems require substantial energy input for release of captured CO2, leading to low energy efficiency and high cost. A new class of materials named metal-organic frameworks (MOFs) has also been demonstrated to take up voluminous amounts of CO 2. However, these studies have been largely limited to equilibrium uptake measurements, which are a poor predictor of separation ability, rather than the more industrially relevant kinetic (dynamic) capacity. Here carbon dioxide capture ͉ dynamic adsorption ͉ reticular chemistry S elective removal of CO 2 from gaseous mixtures is of paramount importance for the purification of fuel gases such as methane and acetylene and because of the imminent problem of anthropogenic CO 2 emissions. Effective systems for CO 2 removal must combine high selectivity and capacity with minimal energetic input to liberate the captured CO 2 . Materials presently used are amine solutions, zeolites, and porous membranes, but all fall short in one or more of these categories (1). To date, metal-organic frameworks (MOFs) have been shown to exhibit exceptional CO 2 storage capacity under equilibrium conditions where pure CO 2 is introduced into the pores (2-6). However, their capacities are dramatically reduced when exposed to mixtures of gases under dynamic conditions, as would be the case in power plant f lue gas and methane mining applications. A useful measure of dynamic separation capacity is obtained by exposing the material to mixed gas streams and detecting the appearance or ''breakthrough'' of CO 2 from the material. Here, we report that a MOF replete with open magnesium sites, Mg-MOF-74 [Mg 2 (DOT); DOT: 2,5-dioxidoterephthalate], has excellent selectivity, facile regeneration, and among the highest dynamic capacities reported for CO 2 in porous materials. Specifically, when Mg-MOF-74 is subjected to a gas stream containing 20% CO 2 in CH 4 , a percentage in the range relevant to industrial separations, it captures only CO 2 and not CH 4 . The pores retain 89 g of CO 2 per kilogram of material before breakthrough, a value higher than any other achieved in MOFs, and that rivals the highest capacities in zeolites. Remarkably, 87% of the captured CO 2 can be liberated at room temperature, and the remaining amount can be completely removed by mild heating (80°C). Based on this favorable performance, we assert that MOFs represent a competitive class of materials for efficient CO 2 capture, and that Mg-MOF-74 strikes the right balance between high capacity and heat of adsorption, notwithstanding the great opportunities available for functionalizing such MOFs for even further improved performance.In previous work, it has been shown that coordinatively unsaturated (open) metal sites in MOFs can be prepared by removal of coordinated solvent molecules (7,8). Whereas f lexible molecular or polymeric structures rearran...

Covalent organic frameworks (COFs) are porous crystalline materials composed of light elements linked by strong covalent bonds. A number of these materials contain a high density of Lewis acid boron sites that can strongly interact with Lewis basic guests, which makes them ideal for the storage of corrosive chemicals such as ammonia. We found that a member of the covalent organic framework family, COF-10, shows the highest uptake capacity (15 mol kg⁻¹, 298 K, 1 bar) of any porous material, including microporous 13X zeolite (9 mol kg⁻¹), Amberlyst 15 (11 mol kg⁻¹) and mesoporous silica, MCM-41 (7.9 mol kg⁻¹). Notably, ammonia can be removed from the pores of COF-10 by heating samples at 200°C under vacuum. In addition, repeated adsorption of ammonia into COF-10 causes a shift in the interlayer packing, which reduces its apparent surface area to nitrogen. However, owing to the strong Lewis acid-base interactions, the total uptake capacity of ammonia and the structural integrity of the COF are maintained after several cycles of adsorption/desorption.

Metal-organic frameworks (MOFs) with the Zr 6 O 4 (OH) 4 secondary building unit (SBU) have been of particular interest for potential commercial and industrial uses because they can be easily tailored and are reported to be chemically and thermally stable. However, we show that there are significant changes in chemical and thermal stability of Zr 6 O 4 (OH) 4 MOFs with the incorporation of different organic linkers. As the number of aromatic rings is increased from one to two in 1,4-benzene dicarboxylate (UiO-66, ZrMOF-BDC) and 4,4 0 -biphenyl dicarboxylate (UiO-67, ZrMOF-BPDC), the Zr 6 O 4 (OH) 4 SBU becomes more susceptible to chemical degradation by water and hydrochloric acid. Furthermore, as the linker is replaced with 2,2 0 -bipyridine-5,5 0 -dicarboxylate (ZrMOF-BIPY) the chemical stability decreases further as the MOF is susceptible to chemical breakdown by protic chemicals such as methanol and isopropanol. The results reported here bring into question the superior structural stability of the UiO-67 analogs as reported by others. Furthermore, the degradation mechanisms proposed here may be applied to other classes of MOFs containing aromatic dicarboxylate organic linkers, in order to predict their structural stability upon exposure to solvents.

Sorbent-assisted water harvesting from air represents an attractive way to address water scarcity in arid climates. Hitherto, sorbents developed for this technology have exclusively been designed to perform one water harvesting cycle (WHC) per day, but the productivities attained with this approach cannot reasonably meet the rising demand for drinking water. This work shows that a microporous aluminum-based metal-organic framework, MOF-303, can perform an adsorption–desorption cycle within minutes under a mild temperature swing, which opens the way for high-productivity water harvesting through rapid, continuous WHCs. Additionally, the favorable dynamic water sorption properties of MOF-303 allow it to outperform other commercial sorbents displaying excellent steady-state characteristics under similar experimental conditions. Finally, these findings are implemented in a new water harvester capable of generating 1.3 L kgMOF–1 day–1 in an indoor arid environment (32% relative humidity, 27 °C) and 0.7 L kgMOF–1 day–1 in the Mojave Desert (in conditions as extreme as 10% RH, 27 °C), representing an improvement by 1 order of magnitude over previously reported devices. This study demonstrates that creating sorbents capable of rapid water sorption dynamics, rather than merely focusing on high water capacities, is crucial to reach water production on a scale matching human consumption.