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Exclusive capture of carbon dioxide (CO 2 ) from hydrocarbons via adsorptive separation is an important technology in the petrochemical industry, especially for acetylene (C 2 H 2 ) production. However, the physicochemical similarities between CO 2 and C 2 H 2 hamper the development of CO 2 -preferential sorbents, and CO 2 is mainly discerned via C recognition with low efficiency. Here, we report that the ultramicroporous material Al(HCOO) 3 , ALF, can exclusively capture CO 2 from hydrocarbon mixtures, including those containing C 2 H 2 and CH 4 . ALF shows a remarkable CO 2 capacity of 86.2 cm 3 g -1 and record-high CO 2 /C 2 H 2 and CO 2 /CH 4 uptake ratios. The inverse CO 2 /C 2 H 2 separation and exclusive CO 2 capture performance from hydrocarbons are validated via adsorption isotherms and dynamic breakthrough experiments. Notably, the hydrogen-confined pore cavities with appropriate dimensional size provide an ideal pore chemistry to specifically match CO 2 via a hydrogen bonding mechanism, with all hydrocarbons rejected. This molecular recognition mechanism is unveiled by in situ Fourier-transform infrared spectroscopy, X-ray diffraction studies, and molecular simulations.

Separating oxygen from air to create oxygen-enriched gas streams is a process that is significant in both industrial and medical fields. However, the prominent technologies for creating oxygen-enriched gas streams are both energy and infrastructure intensive as they use cryogenic temperatures or materials that adsorb N 2 from air. The latter method is less efficient than the methods that adsorb O 2 directly. Herein, we show, via a combination of gas adsorption isotherms, gas breakthrough experiments, neutron and synchrotron X-ray powder diffraction, Raman spectroscopy, and computational studies, that the metal-organic framework, Al(HCOO) 3 (ALF), which is easily prepared at low cost from commodity chemicals, exhibits substantial O 2 adsorption and excellent time-dependent O 2 /N 2 selectivity in a range of 50-125 near dry ice/solvent (≈190 K) temperatures. The effective O 2 adsorption with ALF at ≈190 K and ≈0.21 bar (the partial pressure of O 2 in air) is ≈1.7 mmol/g, and at ice/salt temperatures (≈250 K), it is ≈0.3 mmol/g. Though the kinetics for full adsorption of O 2 near 190 K are slower than at temperatures nearer 250 K, the kinetics for initial O 2 adsorption are fast, suggesting that O 2 separation using ALF with rapid temperature swings at ambient pressures is a potentially viable choice for low-cost air separation applications. We also present synthetic strategies for improving the kinetics of this family of compounds, namely, via Al/Fe solid solutions. To the best of our knowledge, ALF has the highest O 2 /N 2 sorption selectivity among MOF adsorbents without open metal sites as verified by co-adsorption experiments..

A variety of amine-impregnated porous solid sorbents for direct air capture (DAC) of CO 2 have been developed, yet the effect of amine-solid support interactions on the CO 2 adsorption behavior is still poorly understood. When tetraethylenepentamine (TEPA) is impregnated on two different supports, commercial γ-Al 2 O 3 and MIL-101(Cr), they show different trends in CO 2 sorption when the temperature (-20 to 25 °C) and humidity (0-70% RH) of the simulated air stream are varied. In situ IR spectroscopy is used to probe the mechanism of CO 2 sorption on the two supported amine materials, with weak chemisorption (formation of carbamic acid) being the dominant pathway over MIL-101(Cr)-supported TEPA and strong chemisorption (formation of carbamate) occurring over γ-Al 2 O 3 -supported TEPA. Formation of both carbamic acid and carbamate species is enhanced over the supported TEPA materials under humid conditions, with the most significant enhancement observed at -20 °C. However, while equilibrium H 2 O sorption is high at cold temperatures (e.g., -20 °C), the effect of humidity on a practical cyclic DAC process is expected to be minimal due to slow H 2 O uptake kinetics. This work suggests that the CO 2 capture mechanisms of impregnated amines can be controlled by adjusting the degree of amine-solid support interaction and that H 2 O adsorption behavior is strongly affected by the properties of the support materials. Thus, proper selection of solid support materials for amine impregnation will be important for achieving optimized DAC performance under varied deployment conditions, such as cold (e.g., -20 °C) or ambient temperature (e.g., 25 °C) operations.

Two-dimensional covalent organic frameworks (2D COFs) have been widely viewed as rigid porous materials with smooth and reversible gas sorption isotherms. In the present study, we report an unusual hysteresis step in the CO 2 adsorption isotherm of a 2D COF, TAPB-OMeTA. In situ powder X-ray diffraction (PXRD) measurements, computational modeling, and Pawley refinement indicate that TAPB-OMeTA experiences slight interlayer shifting during the CO 2 adsorption process, resulting in a new structure that is similar but not identical to the AA stacking structure, namely, a quasi-AA stacking structure. This interlayer shifting is responsible for the step in its CO 2 adsorption isotherm. We attribute the interlayer shifting to the interactions between COF and CO 2 , which weaken the attraction strength between adjacent COF layers. Notably, the repulsion force between the methoxy groups on the backbone of TAPB-OMeTA is essential in facilitating the interlayer shifting process. After further increasing the size of side groups by grafting poly(N-isopropylacrylamide) oligomers to the TAPB-OMeTA backbone via surface-initiated atom transfer radical polymerization (SI-ATRP), we observed a second interlayer shifting and two adsorption steps in the CO 2 adsorption isotherm, suggesting tunability of the interlayer shifting process. Density functional theory (DFT) calculations confirm that the quasi-AA stacking structure is energetically preferred over AA stacking under a CO 2 atmosphere. These findings demonstrate that 2D COFs can be "soft" porous materials when interacting with gases, providing new opportunities for 2D COFs in gas storage and separation.