NMR studies of mixed amines

Ballard, Mat; Bown, Mark; James, Susan; Yang, Qi

doi:10.1016/j.egypro.2011.01.054

Cited by 27 publications

(35 citation statements)

References 17 publications

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“…CO 2 loadings above 1.0 per amine were also observed with EEA in EMIM-OAc at low amine concentrations. 28 However, at higher EEA concentrations in EMIM-OAc, total CO 2 /amine loading decreases, with the carbamic acid decomposing to carbamate species.…”

Section: Scheme 15 Intramolecular Stabilization Of a Carbamic Acid Bmentioning

confidence: 99%

“…Existing spectral studies of carbamic acids referenced earlier place the carbamic acid C=O resonance at 157-160 ppm. 24,[26][27][28] Thus, based on the 13 C NMR resonance of the equilibrium peak at 158.6 ppm, the DMAOP/CO 2 reaction product at the given conditions can be interpreted as a carbamic acid species in fast exchange (through proton transfer) with the carbamate (expected at 165-164 ppm) present in the solution at lower concentration. Integration of the C=O peak versus the DMAOP backbone carbons (Fig.…”

Section: Co 2 Absorptionmentioning

confidence: 99%

“…Recently developed NMR techniques have provided new fundamental insights into CO 2 -removal reaction mechanisms [27][28][29][30][31][32][33][34] . The use of titration 35,32 and FTIR for mechanistic studies has been reported 29,36-38 but quantification and real time / flow unit identification of unexpected species is cumbersome.…”

Section: Introductionmentioning

confidence: 99%

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In Situ 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

et al. 2015

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In a previous study, we reported the use of in situ 1 H-and 13 C-NMR to elucidate mechanistic pathways for the reaction of carbon dioxide with a broad range of amines (pKa ~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 CO 2 :amine stoichiometry), into the alkyl ammonium bicarbonate with a more beneficial 1:1 CO 2 :amine stoichiometry. This study has been extended herein to amines, amidines and guanidines dissolved in non-aqueous solvent systems such as dimethylsulfoxide, 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 CO 2 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 CO 2 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 CO 2 is the same as in aqueous solution -nucleophilic attack by the amine nitrogen on CO 2 . 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 pKa). In contrast, carbamic acid/zwitterion formation is favored by lower amine concentrations, higher CO 2 partial pressures, lower amine pKa, and selection of more polar organic solvents that promote hydrogen bonding. The new amine-CO 2 reaction pathways enabled here by the use of non-aqueous solvents introduce stabilizing interactions between the non-aqueous solvent and the amine-CO 2 reaction products, facilitating higher capacity and selectivity for carbon capture than in water solutions. The effects of temperature, amine basicity, solvent electronic structures, and concentration on amine-CO 2 reaction products (carbamic acid/zwitterion/carbamate and equilibria between neutral and ion-paired forms) are discussed in detail herein.

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Section: Scheme 15 Intramolecular Stabilization Of a Carbamic Acid Bmentioning

confidence: 99%

Section: Co 2 Absorptionmentioning

confidence: 99%

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In Situ 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

et al. 2015

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“…Alkanolamines and etheramines are preferred over the use of amines lacking polar functional groups, in order to facilitate high water solubility of the amine and its reaction products with CO 2 . Lower-pKa aromatic and nitrile-amines (7)(8)(9) and higher-pKa cyclic amines and guanidines (10)(11)(12) were also studied to probe larger effects of basicity.…”

Section: Experimental Designmentioning

confidence: 99%

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

et al. 2015

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A series of closely related primary, secondary and tertiary alkanolamine model compounds were monitored in real time in aqueous solution via in-situ nuclear magnetic resonance (NMR) spectroscopy while purging CO2-rich gas through the solution over a range of temperatures. The real-time in-situ 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 (pKa ~4.5-11) and guanidines (pKa ~ 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 on 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) due to 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 are their lower thermal stability (greater ease of desorption). Unexpectedly guanidines do not form bicarbonates directly; 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.

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“…Unlike the spectra of MOF NH2 before adsorption and after regeneration, that of MOF NH2 adsorbing CO 2 molecules show the distinct peak at 160.5 ppm. Because a carbamic acid C = O resonance is usually observed at 157–160 ppm and carbamate at 164–165 ppm 28 29 , the obseved peak might be the result of the equilibrium between carbamate and carbamic acid groups through proton transfer in the solution state. The formed carbamate groups can block the narrow pore to prevent the diffusion of the next CO 2 molecules.…”

Section: Resultsmentioning

confidence: 99%

Crystal-Size Effects on Carbon Dioxide Capture of a Covalently Alkylamine-Tethered Metal-Organic Framework Constructed by a One-Step Self-Assembly

Kim

Hyun

Lee

et al. 2016

Sci Rep

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To enhance the carbon dioxide (CO2) uptake of metal-organic frameworks (MOFs), amine functionalization of their pore surfaces has been studied extensively. In general, amine-functionalized MOFs have been synthesized via post-synthetic modifications. Herein, we introduce a one-step construction of a MOF ([(NiLethylamine)(BPDC)] = MOFNH2; [NiLethylamine]2+ = [Ni(C12H32N8)]2+; BPDC2− = 4,4‘-biphenyldicarboxylate) possessing covalently tethered alkylamine groups without post-synthetic modification. Two-amine groups per metal centre were introduced by this method. MOFNH2 showed enhanced CO2 uptake at elevated temperatures, attributed to active chemical interactions between the amine groups and the CO2 molecules. Due to the narrow channels of MOFNH2, the accessibility to the channel of CO2 is the limiting factor in its sorption behaviour. In this context, only crystal size reduction of MOFNH2 led to much faster and greater CO2 uptake at low pressures.

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NMR studies of mixed amines

Cited by 27 publications

References 17 publications

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

Crystal-Size Effects on Carbon Dioxide Capture of a Covalently Alkylamine-Tethered Metal-Organic Framework Constructed by a One-Step Self-Assembly

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