Mechanism and Kinetics of CO₂ Adsorption on Surface Bonded Amines

The impact of H₂O on the mechanism and kinetics of CO₂ adsorption by amine-impregnated SBA-15 has been investigated. The H₂O-free adsorption mechanism proceeds predominantly via formation of carbamates stabilized between two amine groups. Bicarbonates and surface stabilized carbamates were formed on hydrated sorbents. Film diffusion limitations were not observed during adsorption of CO₂ at high amine loadings. Gaseous water decreased the adsorption rate but increased the maximum equilibrium uptake as well as the uptake before breakthrough of the reactor bed. A water film is formed on the adsorbent particles in gas streams containing more than 5 vol %, gaseous H₂O limiting the interaction of CO₂ with the active amine sites and constraining the rates of adsorption. The higher uptake of CO₂ in the presence of H₂O vapor is a result of a change in the adsorption mechanism that increases the amine efficiency, thus leading to a higher adsorption capacity. The adsorption was fully reversible for all adsorbents at a maximum desorption temperature of 100 °C

Role of Amine Functionality for CO₂ Chemisorption on Silica

The mechanism of CO₂ adsorption on primary, secondary, and bibasic aminosilanes synthetically functionalized in porous SiO₂ was qualitatively and quantitatively investigated by a combination of IR spectroscopy, thermogravimetry, and quantum mechanical modeling. The mode of CO₂ adsorption depends particularly on the nature of the amine group and the spacing between the aminosilanes. Primary amines bonded CO₂ preferentially through the formation of intermolecular ammonium carbamates, whereas CO₂ was predominantly stabilized as carbamic acid, when interacting with secondary amines. Ammonium carbamate formation requires the transfer of the carbamic acid proton to a second primary amine group to form the ammonium ion and hence two (primary) amine groups are required to bind one CO₂ molecule. The higher base strength of secondary amines enables the stabilization of carbamic acid, which is thereby hindered to interact further with nearby amine functions, because their association with Si–OH groups (either protonation or hydrogen bonding) does not allow further stabilization of carbamic acid as carbamate. Steric hindrance of the formation of intermolecular ammonium carbamates leads to higher uptake capacities for secondary amines functionalized in porous SiO₂ at higher amine densities. In aminosilanes possessing a primary and a secondary amine group, the secondary amine group tends to be protonated by Si–OH groups and therefore does not substantially interact with CO₂

Tunable Water and CO₂ Sorption Properties in Isostructural Azine-Based Covalent Organic Frameworks through Polarity Engineering

The use of covalent organic frameworks (COFs) in environmental settings such as atmospheric water capture or CO₂ separation under realistic pre- and post-combustion conditions is largely unexplored to date. Herein, we present two isostructural azine-linked COFs based on 1,3,5-triformyl benzene (AB-COF) and 1,3,5-triformylphloroglucinol (ATFG-COF) and hydrazine building units, respectively, whose sorption characteristics are precisely tunable by the rational design of the chemical nature of the pore walls. This effect is particularly pronounced for atmospheric water harvesting, which is explored for the first time using COFs as adsorbents. We demonstrate that the less polar AB-COF acts as a reversible water capture and release reservoir, featuring among the highest water vapor uptake capacity at low pressures reported to date (28 wt % at <0.3 <i>p</i> <i>p</i>₀⁻¹). Furthermore, we show tailored CO₂ sorption characteristics of the COFs through polarity engineering, demonstrating high CO₂ uptake at low pressures ( <1 bar) under equilibrium (sorption isotherm) and kinetic conditions (flow TGA, breakthrough) for the more polar ATFG-COF, and very high CO₂ over N₂ (IAST: 88) selectivity for the apolar AB-COF. In addition, the pore walls of both COFs were modified by doping with metal salts (lithium and zinc acetate), revealing an extremely high CO₂ uptake of 4.68 mmol g⁻¹ at 273 K for the zinc-doped AB-COF

Lewis–Brønsted Acid Pairs in Ga/H-ZSM‑5 To Catalyze Dehydrogenation of Light Alkanes

The active sites for propane dehydrogenation in Ga/H-ZSM-5 with moderate concentrations of tetrahedral aluminum in the lattice were identified to be Lewis–Brønsted acid pairs. With increasing availability, Ga⁺ and Brønsted acid site concentrations changed inversely, as protons of Brønsted acid sites were exchanged with Ga⁺. At a Ga/Al ratio of 1/2, the rate of propane dehydrogenation was 2 orders of magnitude higher than with the parent H-ZSM-5, highlighting the extraordinary activity of the Lewis–Brønsted acid pairs. Density functional theory calculations relate the high activity to a bifunctional mechanism that proceeds via heterolytic activation of the propane C–H bond followed by a monomolecular elimination of H₂ and desorption of propene