Pathways of the Chemical Reaction of Carbon Dioxide with Ionic Liquids and Amines in Ionic Liquid Solution

This paper focuses specifically on certain ionic liquids that are capable of acting as chemisorbents for CO₂ at ambient pressure and temperature. This low-pressure approach based on chemical reactivity is more effective than traditional physical absorption/solubility approaches for CO₂ capture in ionic liquids for higher pressure carbon capture. We describe a class of imidazolium ionic liquids bearing a relatively acidic hydrogen atom at C-2, which upon initial abstraction develops a nucleophilic carbon atom that is carboxylated by CO₂. Basicity of the anion plays a role in the ability to remove the acidic hydrogen to generate the nucleophilic carbon. The yield of carboxylated ionic liquid is not affected by non-aqueous co-solvents but changes as a function of the CO₂ partial pressure, solution temperature, and presence of H₂O in solution. CO₂ chemisorption by ionic liquids is particularly efficient in the presence of a non-nucleophilic nitrogenous base that serves to promote ionic liquid carboxylation and stabilize the carboxylic acid product as a salt. Selected ionic liquids are able to stabilize the formation of amine carbamic acids in the ionic liquid solution. In this case, each amine captures up to 1 CO₂ molecule, which is beneficial for the overall CO₂ capacity in the solution. Carboxylation of the ionic liquids themselves is lower because the basic anion of the ionic liquid also stabilizes N-carboxylated products. <i>In situ</i> ¹³C and ¹H nuclear magnetic resonance (NMR) spectroscopy using a built-in micro reactor was used to provide real-time insights on CO₂–ionic liquid and CO₂–amine reaction pathways and product speciation under various conditions

<i>In Situ</i> Nuclear Magnetic Resonance Mechanistic Studies of Carbon Dioxide Reactions with Liquid Amines in Aqueous Systems: New Insights on Carbon Capture Reaction Pathways

A series of closely related primary, secondary, and tertiary alkanolamine model compounds were monitored in real time in aqueous solution via <i>in situ</i> nuclear magnetic resonance (NMR) spectroscopy while purging CO₂-rich gas through the solution over a range of temperatures. The real-time <i>in situ</i> spectroscopic monitoring of this reaction chemistry provides new insight about reaction pathways through identification of primary products and their transformations into secondary products. New mechanistic pathways were observed and elucidated. The effects of CO₂ loadings, relative absorption and desorption kinetics, pH, temperature, and other critical features of the amine/CO₂ reaction system are discussed in detail. The effect of amine basicity and structure on these parameters was further elucidated by studying complementary electron-rich and -poor amines (p<i>K</i>_a ∼ 4.5–11) and guanidines (p<i>K</i>_a ∼ 14–15). While tertiary amines act only as simple proton acceptors, primary and secondary amines function as both bases and nucleophiles to form carbamates and (bi)­carbonates, whose product ratio is a function of both reaction conditions and amine steric and electronic properties. Water is also acting as a Lewis base by hydrolysis of carbamate species into bicarbonate, which results in a more beneficial 1:1 CO₂/amine ratio. Primary and secondary amines tend to react with CO₂ similarly at different CO₂ partial pressures, showing weak pressure dependence upon CO₂ loading; in contrast, reaction efficiencies of tertiary amines, which generally form less stable carbonate and bicarbonate products, are a strong function of CO₂ pressure. Primary and secondary amines capture significantly less CO₂ per mole of amine than tertiary amines (lower CO₂ loading capacities) because of the formation of carbamate species. Their faster reaction rates with CO₂ and high capture efficiencies at low CO₂ partial pressures are advantageous. In contrast, tertiary amines more effectively react with CO₂ at lower temperatures, capturing up to 1 CO₂ per amine; initially and unexpectedly, carbonate and bicarbonate species are initially formed simultaneously. Even at high pH, carbonates evolve into a final bicarbonate product. The secondary benefit of forming bicarbonates is their lower thermal stability (greater ease of desorption). Unexpectedly, guanidines do not form bicarbonates directly; the reaction proceeds via exclusive initial formation of the guanidinium carbonate. In summary, varying amine basicity leads to significant changes in the carbamate/(bi)­carbonate equilibrium and stability of reaction products

<i>In Situ</i> Nuclear Magnetic Resonance Mechanistic Studies of Carbon Dioxide Reactions with Liquid Amines in Non-aqueous Systems: Evidence for the Formation of Carbamic Acids and Zwitterionic Species

In a previous study, we reported the use of <i>in situ</i> ¹H and ¹³C nuclear magnetic resonance (NMR) to elucidate mechanistic pathways for the reaction of carbon dioxide with a broad range of amines (p<i>K</i>_a ∼ 4.5–15.5), including alkanolamines of commercial interest, in water. In the aqueous systems of that study, water most importantly functions as a Brønsted acid/Lewis base and, as the amine is consumed and pH decreases, hydrolyzes the initially formed carbamate species (1:2 CO₂/amine stoichiometry), into the alkyl ammonium bicarbonate with a more beneficial 1:1 CO₂/amine stoichiometry. This study has been extended herein to amines, amidines, and guanidines dissolved in non-aqueous solvent systems, such as dimethyl sulfoxide, sulfolane, toluene, 1-methyl-2-pyrrolidinone, and the ionic liquid 1-ethyl-3-methyl-imidazolium acetate. The use of non-aqueous organic solvents shuts off some CO₂ reaction pathways available in aqueous solution. However, more importantly, it opens up new possibilities and reaction pathways for amine-based carbon capture. Two important aqueous system pathways are eliminated: the direct hydration of CO₂ with tertiary amines or guanidines to form bicarbonates and the hydrolysis of carbamates at lower pH to form bicarbonates. In non-aqueous solution, the initial step for the reaction of primary and secondary amines with CO₂ is the same as in aqueous solution: nucleophilic attack by the amine nitrogen on CO₂. However, additional mechanistic pathways are enabled in non-aqueous solvents, particularly the stabilization of carbamic acid(s) (rather than carbamates) products in certain organic solvents. The formation of carbamates requires no water and is favored by higher amine concentrations and basicities (higher amine p<i>K</i>_a). In contrast, carbamic acid/zwitterion formation is favored by lower amine concentrations, higher CO₂ partial pressures, lower amine p<i>K</i>_a, and selection of more polar organic solvents that promote hydrogen bonding. The new amine–CO₂ reaction pathways enabled here by the use of non-aqueous solvents introduce stabilizing interactions between the non-aqueous solvent and the amine–CO₂ reaction products, facilitating higher capacity and selectivity for carbon capture than in water solutions. The effects of the temperature, amine basicity, solvent electronic structures, and concentration on amine–CO₂ reaction products (carbamic acid/zwitterion/carbamate and equilibria between neutral and ion-paired forms) are discussed in detail herein

<i>In Situ</i> Nuclear Magnetic Resonance Mechanistic Studies of Carbon Dioxide Reactions with Liquid Amines in Mixed Base Systems: The Interplay of Lewis and Brønsted Basicities

A new approach to non-aqueous CO₂–amine carbon capture has been elucidated on the basis of the utilization of a combination of a nucleophilic amine CO₂ sorbent (Lewis base) with a second, non-nucleophilic Brønsted base, a “mixed base” system. The nucleophilic amines, typically alkanolamines, e.g., ethanolamine, react directly with CO₂ in the gas stream, while the typically stronger nitrogenous Brønsted non-nucleophilic proton-acceptor base, e.g., a guanidine, then forms a more stabilized mixed carbamate reaction product. The proper choice of these bases allows for tailoring absorbent structure and properties and reaction conditions (<i>T</i> and <i>P</i>) to specific applications. Significant increases in absorption capacity are achieved because CO₂ capture ratios greater than 1:1 on a molar basis (CO₂ per amine group) can be obtained, resulting also in enhanced cyclic regeneration efficiency. In non-aqueous solutions, primary amines are carboxylated by reaction with one or two CO₂ molecules, forming either mono- or di-N-carboxylated products. These carbamic acids, unstable in aqueous media, are then stabilized as guanidinium carboxylates. A total of 2 mol of CO₂ per mol of a primary alkanolamine is thereby captured. In addition, under non-aqueous conditions, the hydroxyl group of alkanolamine reacts with CO₂ (O-carbonation) to form an alkylcarbonic acid that is subsequently stabilized by forming the corresponding alkylbicarbonate salt on reaction with a guanidinine. Each hydroxyl group thereby also absorbs up to 1 mol of CO₂. Thereby, enhanced capacity is achieved at both basic N and OH sites of monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), etc. These products may be decomposed by thermal treatment or CO₂ partial pressure decrease to liberate CO₂ and regenerate the liquid sorbent suitable for reuse in carbon capture operations

Tunable Interlayer Shifting in Two-Dimensional Covalent Organic Frameworks Triggered by CO2 Sorption

10.1021/jacs.2c08214Journal of the American Chemical Society1444420363–2037

Interlayer shifting in two-dimensional covalent organic frameworks

10.1021/jacs.0c03691Journal of the American Chemical Society1423012995-1300

Covalent organic framework atropisomers with multiple gas-triggered structural flexibilities

10.1038/s41563-023-01523-2Nature Materials22636–64

Linear Copolymers of Ethylene and Polar Vinyl Monomers via Olefin Metathesis−Hydrogenation: Synthesis, Characterization, and Comparison to Branched Analogues

Materials structurally equivalent to copolymers of ethylene and <10 mol % of vinyl alcohol, vinyl acetate, methyl acrylate, <i>tert</i>-butyl acrylate, and acrylic acid were synthesized via ruthenium-catalyzed olefin metathesis copolymerization followed by polymer hydrogenation. These polymers differ from previous metathesis-derived linear polyethylene analogues in that they do not have precise sequence distributions, and therefore, they serve as superior models for chain-addition functional polyethylenes. The ester group-substituted polymers display rapid decreases in melting temperature and heat of fusion with increasing comonomer contents irrespective of the identity of the functional group, consistent with these groups being equally excluded from the crystal lattice. The alcohol-substituted polymers, however, show higher melting temperatures and a weaker property dependence on comonomer content as compared to the other polymers. Thus, the behavior of alkyl branch-free functionalized polyethylenes is consistent with the large body of work involving the effects of functional substituents on polyethylenes containing alkyl branches