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.

The structure of random ethylene/propylene (EP) copolymers has been modeled using step polymerization chemistry. Six ethylene/propylene model copolymers have been prepared via acyclic diene metathesis (ADMET) polymerization and characterized for primary and higher level structure using in-depth NMR, IR, DSC, WAXD, and GPC analysis. These copolymers possess 1.5, 7.1, 13.6, 25.0, 43.3, and 55.6 methyl branches per 1000 carbons. Examination of these macromolecules by IR and WAXD analysis has demonstrated the first hexagonal phase in EP copolymers containing high ethylene content (90%) without the influence of sample manipulation (temperature, pressure, or radiation). Thermal behavior studies have shown that the melting point and heat of fusion decrease as the branch content increases. Further, comparisons have been made between these random ADMET EP copolymers, random EP copolymers made by typical chain addition techniques, and precisely branched ADMET EP copolymers.

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.

Layer-stacking structures are very common in two-dimensional covalent organic frameworks (2D COFs). While their structures are normally determined under solvent-free conditions, the structures of solvated 2D COFs are largely unexplored. We report herein the in situ determination of solvated 2D COF structures, which exhibit an obvious difference as compared to that of the same COF under dried state. Powder X-ray diffraction (PXRD) data analyses, computational modeling, and Pawley refinement indicate that the solvated 2D COFs experience considerable interlayer shifting, resulting in new structures similar to the staggered AB stacking, namely, quasi-AB-stacking structures, instead of the AA-stacking structures that are usually observed in the dried COFs. We attribute this interlayer shifting to the interactions between COFs and solvent molecules, which may weaken the attraction strength between adjacent COF layers. Density functional theory (DFT) calculations confirm that the quasi-AB stacking is energetically preferred over the AA stacking in solvated COFs. All four highly crystalline 2D COFs examined in the present study exhibit considerable interlayer shifting upon solvation, implying the universality of the solvent-induced interlayer stacking rearrangement in 2D COFs. These findings prompt re-examination of the 2D COF structures in solvated state and suggest new opportunities for the applications of COF materials under wet conditions.