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

Abstract

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.

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 non-aqueous solution, since neither carbonic acid nor free H 2 O is present to facilitate (bi)carbonate-forming reaction pathways, only carbamate and carbamic acid products are formed. 1,25 Aromatic carbamic acids of pyrroles and indoles are known, [40][41][42] and a handful of spectral studies on aliphatic and other aromatic carbamic acids formed from free amines and CO 2 under nonaqueous conditions have been reported. [43][44][45][46][47][48] This study is the first extensive structure-property NMR investigation of these species and the relevance of their equilibria with other reaction products for CO 2 capture processes.…”

“…consequence of these different reaction pathways, and the motivation for the amine work described in this and related publications, 1,49,50 is that theoretical CO 2 sorption capacity for an amine system is a function of the products that are formed. When ammonium carbamates are the preferred reaction product under non-aqueous conditions, the theoretical CO 2 sorption capacity is only 50 mol% per amine (2:1 amine:CO 2 ratio).…”

“…After normalizing for the number of carbons represented in the amine aliphatic region, the carbonyl integral was divided into the amine aliphatic integral to obtain the mole per cent CO 2 present per molecule and CO 2 /amine group ratio (correcting when necessary for 2 amines per molecule). 1 H NMR spectra were used for monitoring amine concentration in the solvent (amine/DMSO ratio). Both amine and solvent can gradually evaporate in the described flow-through system, especially during desorption at elevated temperatures.…”

“…The quantification of carbamate and carbamic acid species in solution using 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 9 of the free amine -NH 2 or -NRH before CO 2 addition is initially detected at low ppm shift (1.0 -3.0 ppm), but significantly shifts and splits (for primary amine -NH 2 ) into two peaks as CO 2 is added. These peaks typically appear above 8 ppm and between 6-7 ppm at equilibrium.…”

“…5-15.5), including commercially relevant alkanolamines, with carbon dioxide in water 1 . The existing literature on carbon capture with amines in non-aqueous solvents is sparse and fragmentary, and in almost all cases focuses on kinetics as opposed to the elucidation of mechanistic pathways.…”