Search

Carbon capture is one of the essential low-carbon technologies required to achieve societal climate goals at the lowest cost. Covalent organic frameworks (COFs) are promising adsorbents for CO2 capture because of their well-defined porosity, large surface area, and high stability. Current COF-based CO2 capture is mainly based on a physisorption mechanism, exhibiting smooth and reversible sorption isotherms. In the present study, we report unusual CO2 sorption isotherms featuring one or more tunable hysteresis steps with metal ion (Fe3+, Cr3+, or In3+)-doped Schiff-base two-dimensional (2D) COFs (Py-1P, Py-TT, and Py-Py) as adsorbents. Synchrotron X-ray diffraction, spectroscopic and computational studies indicate that the sharp adsorption steps in the isotherm originate from the insertion of CO2 between the metal ion and the N atom of the imine bond on the inner pore surface of the COFs as the CO2 pressure reaches threshold values. As a result, the CO2 adsorption capacity of the ion-doped Py-1P COF is increased by 89.5% compared with that of the undoped Py-1P COF. This CO2 sorption mechanism provides an efficient and straightforward approach to enhancing the CO2 capture capacity of COF– based adsorbents, yielding insights into developing chemistry for CO2 capture and conversion. [ABSTRACT FROM AUTHOR]

While nitrogen- (N-) rich porous carbons hold promise to enable efficient carbon capture from post-combustion flue gas via temperature swing adsorption, a widespread tradeoff of surface chemistry with textural properties has resulted in a lack of detailed understanding in the relationships between a carbon adsorbent's material properties and its propensity for selective CO2 adsorption. This study details a comprehensive screening of the CO2 and N2 adsorption performance for a set of turbostratic porous carbons with a wide range of microporosity (0–0.7 cm3 g−1) and N-content (0–22 at%). By employing a temperature-dependent dual site Langmuir isotherm model, the energetics and capacities of CO2 adsorption were related to the carbons' textural and surface chemical properties. The isosteric heats of adsorption were dependent on the N content, with the most N-rich carbons exhibiting zero-loading isosteric heats upwards of 45 kJ mol−1 compared to around only 20 kJ mol−1 for unfunctionalized carbons. On the other hand, the N2 adsorption was a strong function primarily of the surface area and microporosity. As a result, the selectivity and predicted carbon capture performance for a simplified temperature swing adsorption process model were shown to be highly tunable by enhancing the N content (and limiting the N2-accessible porosity) of the carbon adsorbents. This work provides useful structure–function relationships and helps to answer the longstanding question of the validity of N-incorporation for tuning the CO2 adsorption performance of N-rich porous carbon adsorbents. [ABSTRACT FROM AUTHOR]

Background information, materials/procedures, and results are provided for an undergraduate laboratory experiment on a photochemical ligand substitution reaction using sunlight. The experiment illustrates demonstration of photochemistry and basic mechanisms of transition metal reactions using inexpensive materials and is easily followed by infrared spectroscopy. (Author/JN)

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.

Potassium poly(heptazine imide) (KPHI) has recently garnered attention as a crystalline carbon nitride framework with considerable photoelectrochemical activity. Here, we report a Ca 2+ -complexed analogue of PHI: calcium poly(heptazine imide) (CaPHI). Despite similar polymer backbone, CaPHI and KPHI exhibit markedly different crystal structures. Spectroscopic, crystallographic, and physisorptive characterization reveal that Ca 2+ acts as a structure-directing agent to transform melon-based carbon nitride to crystalline CaPHI with ordered pore channels, extended visible light absorption, and altered band structure as compared to KPHI. Upon acid washing, protons replace Ca 2+ atoms in CaPHI to yield H + /CaPHI and enhance porosity without disrupting crystal structure. Further, these proton-exchanged PHI frameworks exhibit large adsorption affinity for CO 2 and exceptional performance for selective carbon capture from dilute streams. Compared to a state-of-the-art metal organic framework, UTSA-16, H + /CaPHI exhibits more than twice the selectivity (∼300 vs ∼120) and working capacity (∼1.2 mmol g -1 vs ∼0.5 mmol g -1 ) for a feed of 4% CO 2 (1 bar, 30 °C).