Fabrizio Mani

CO2 absorption by aqueous NH3 solutions : speciation of ammonium carbamate, bicarbonate and carbonate by a 13C NMR study

The absorption of CO2 in aqueous NH3 solutions occurs with high efficiency and loading capacity at room temperature and atmospheric pressure producing the ammonium salts of bicarbonate (HCO3−), carbonate (CO32−), and carbamate (NH2CO2−) anions. 13C NMR spectroscopy at room temperature has been proven to be a simple and reliable method to investigate the speciation in solution of these three ionic species. Fast equilibration of HCO3−/CO32− anions results in a single NMR peak whose chemical shift depends on the relative concentration of the two species. A method has been developed to correlate the chemical shift of this carbon resonance to the ratio of the two anionic species. Integration of the carbamate carbon peak provided the relative amount of this species with respect to HCO3−/CO32− pair. No other species was detected in solution by 13C NMR, and no solid compounds separated out under our experimental conditions. Finally, the relative amount of HCO3−, CO32−, and NH2CO2− in solution have been correlated to the molar ratio between free ammonia in solution and absorbed CO2.

A 13C NMR study of the carbon dioxide absorption and desorption equilibria by aqueous 2-aminoethanol and N-methyl-substituted 2-aminoethanol

The 13C NMR experimental study presented investigates the absorption of CO2 by a series of primary, secondary and tertiary alkanolamines in aqueous solution. The absorption experiments were made at room temperature with four different amine concentrations in the range 0.167–0.667 M (1.01–5.88 wt%). As inferred by 13C NMR spectral analysis, the formation of carbamate increases with increasing amine concentration following the order secondary amine < primary amine. Moreover, it has been shown that carbamate reduces the CO2 absorption efficiency. A considerable physical absorption (10–20%) contributes to the loading capacity of the amines and partially compensates for the yield of chemical capture, which turned out to be poorer than was expected theoretically. Quite unexpectedly, carbamate was also produced by an endothermic reaction during the thermal CO2 desorption process which regenerated the amines (primary and secondary amines). In the case of the secondary amine 2-(methylamino)ethanol (MMEA), the amount of carbamate at the end of the desorption process is greater than the amount found at the end of the absorption step, thus reducing the desorption efficiency of the secondary amine in comparison to both primary and tertiary amines. Five cycles of absorption–desorption tests were carried out to verify the feasibility of regenerated amines for reuse. Our results indicate that absorption efficiency and loading capacity of the regenerated amine solutions remain essentially constant during the second to the fifth absorption–desorption experiments, but they both decrease slightly when compared to the initial amine.

Continuous cycles of CO2 absorption and amine regeneration with aqueous alkanolamines: a comparison of the efficiency between pure and blended DEA, MDEA and AMP solutions by 13C NMR spectroscopy

This experimental study describes the performances of CO2 capture by aqueous solutions of pure alkanolamines (0.667, 1.33 and 2.00 M) 2,2′-iminodiethanol (DEA), N-methyl-2,2′-iminodiethanol (MDEA) and 2-amino-2-methy-1-propanol (AMP). The behaviour of some alkanolamine blends (2.00 M) has been also considered. In these experiments the CO2-loaded and the regenerated amine solutions were continuously circulated in a closed system between the absorber (set at 293 K) and the desorber (set at 363, 373 and 363–388 K). The absorption efficiency of the single amines at equilibrium is between 69 and 81% according to the desorber temperature and to the amine concentration. The CO2-amine reaction equilibria have been investigated by 13C NMR spectroscopy, which established the regeneration efficiency and the loading capacity for each single amine experiment. AMP displays the greatest absorption efficiency and MDEA the greatest regeneration efficiency at any amine concentration and desorber temperature. Blended AMP-MDEA and AMP-DEA systems (1/2 and 2/1 molar ratios) significantly enhance the absorption efficiency (in the range 7–14%) with respect to single amines under identical operating conditions. AMP-MDEA blends display better performances than AMP-DEA due to the lower efficiency of DEA carbamate in both CO2 absorption and amine regeneration. Owing to a higher thermal stability, AMP and MDEA solutions surpass DEA, as no degradation product has been detected by 13C NMR analysis after heating AMP and MDEA solutions at 403 K up to fourteen days.

Halo-, hydrido- and dinitrogen-complexes of iron(II) with tritertiary phosphines

Abstract From the five-coordinate iron(II) complexes [FeXL] BPh 4 , (L = tris(2-diphenylphosphinoethyl)amine, np 3 , and tris(2-diphenylphosphinoethyl)phosphine, pp 3 ) hydrido and hydrido dinitrogen complexes have been obtained; these have formula [FeH(pp 3 )]BPh 4 and [FeHN 2 L]BPh 4 (L = np 3 , PP 3 . In the latter complexes the dinitrogen molecule may be replaced by other neutral ligands such as CO, CH 3 CN, C 6 H 5 CN.

From greenhouse gas to feedstock: formation of ammonium carbamate from CO2 and NH3 in organic solvents and its catalytic conversion into urea under mild conditions

The capture of carbon dioxide by ammonia in both aqueous and non-aqueous solutions was investigated at atmospheric pressure and 273 K under different operating conditions. The CO2 capture is fast and efficient ranging between 78 and 99%, depending on both the NH3 concentration and the solvent nature. The precipitation of solid mixtures of ammonium bicarbonate, ammonium carbonate and ammonium carbamate occurred in ethanol–water solution. Selective precipitation of ammonium carbamate was achieved by reacting gaseous CO2 and NH3 in anhydrous ethanol, 1-propanol or N,N-dimethylformamide (DMF) in a flow reactor that operates in continuous. In the second step of the process, the pure ammonium carbamate is used to produce urea with good yield (up to 54% on carbamate basis) at 393–413 K in the presence of inexpensive Cu(II) and Zn(II) catalysts. The yield of urea depends on several factors including the catalyst, the reaction temperature and the reaction time. Identification and quantification of urea in the reaction mixtures was obtained by analysis of its 13C NMR spectrum. A preliminary mechanistic interpretation of the catalytic reaction is also briefly presented and commented.

The Role of Zinc(II) in the Absorption–Desorption of CO2 by Aqueous NH3, a Potentially Cost-Effective Method for CO2 Capture and Recycling

The absorption of CO2 by aqueous NH3 solutions has been investigated at atmospheric pressure and 0 degrees C. The CO2 absorption is fast and occurs with high efficiency (88-99%). The maximum CO2-removal efficiency increases slightly with the NH3 concentration. Addition of zinc(II) salts (as chloride, nitrate or sulfate) to the NH3 absorbent solution increases the overall CO2-absorption capacity without appreciably affecting the removal efficiency. Stripping of pure CO2 from HCO3(-) solutions is achieved by adding the calculated amount of ZnII salts, which under ambient conditions lead to rapid release of about 30-35% of the initially captured CO2. At the same time, about 65-70% of the captured CO2 is transformed into solid basic zinc carbonates. The recovery of these valuable solid products and the release of only 1/3 of free CO2 at room temperature and pressure reduces the cost of the overall process of CO2 capture, making it a potentially attractive method for CO2 capture on a larger scale.

Improved Solvent Formulations for Efficient CO2 Absorption and Low‐Temperature Desorption

This experimental study describes efficient CO₂ capture by 2-amino-2-methyl-1-propanol (AMP)/piperazine (PZ) in ethylene glycol monoethyl ether (EGMEE, 2-ethoxyethanol) containing approximately 15 wt % of water. In these experiments, the solvent is continuously circulated between the absorber (packed-bed reactor at 30, 40, or 45 °C) and the desorber (at 80, 85, or 90 °C). The CO₂ -solvent reaction equilibria have been investigated by using ¹³C NMR spectroscopy, which provides confirmatory evidence that the formation of mono- and biscarbamate derivatives of PZ accounts for most of the CO₂ absorbed by the AMP/PZ/EGMEE/H₂O blend. The solid-state structures of AMP carbamate and of the carbonate salt of protonated AMP have been determined by using XRD. Both AMPCO₂(-) and CO(3)(2-) species completely convert to the monoalkyl carbonates on dissolving the respective salts in methanol, ethanol, or ethylene glycol.

Efficient CO2 capture by non-aqueous 2-amino-2-methyl-1-propanol (AMP) and low temperature solvent regeneration

An experimental study describes the reversible fixation of CO2 by non-aqueous solutions of 2-amino-2-methyl-1-propanol (AMP). The captured CO2 is stored in solution as AMP carbamate and alcohol carbonates in different relative amounts as a function of the CO2–AMP ratio. The identification and the quantification of the species in solution were obtained from 13C NMR spectroscopic analysis. The bench-scale experiments of CO2 fixation and solvent regeneration have been carried out in a continuous cycle where the CO2-loaded and regenerated solutions are continuously circulated between the desorber and absorber. The carbonated solutions are decomposed in the desorber at 80–90 °C to regenerate the free amine for its reuse, affording CO2 absorption efficiency over 90%. The replacement of water with an organic solvent and the relatively low temperature of the desorption-regeneration step, could have the potential of reducing some disadvantages of aqueous absorbents, namely the amine loss by degradation and evaporation, equipment corrosion and energy cost of the regeneration step, yet preserving the high efficiency of aqueous amines.

Copper(II) complexes with 2,2′-biimidazole. Preparation and crystal structure determination of a complex of stoichiometry Cu1.5Cl3(H2bim)2 (H2bim=2,2′-biimidazole)

Abstract By reacting 2,2′-biimidazole and copper(II) chloride in aqueous HCl we obtained the complex CuCl2(H2bim) as the main product and a compound with stoichiometry Cu1.5Cl3(H2bim)2 as a byproduct. The structure of the latter compound has been determined by X-ray analysis: monoclinic, a= 794.0(3), b=3146.8(6), c=722.9(4) pm, β= 114.2(1)°, space group P21/c. The compound actually contains two species, namely [Cu(H2bim)2]Cl2 and [CuCl2(H2bim)] in a 1:2 molar ratio.

Synthesis and coordinating properties of the functionalized macrocyclic ligand 1,4-bis(1-methylimidazol-2-ylmethyl)-7-carboxymethyl-1,4,7-triazacyclononane. Crystal structures of cobalt(III), nickel(II) and zinc(II) complexes

Abstract The synthesis of the new unsymmetrically derivatized macrocycle 1,4-bis(1-methylimidazol-2-ylmethyl)-7-carboxymethyl-1,4,7-triazacyclononane (LH) is reported. The potentially hexadentate ligand yields cobalt(III), nickel(II) and zinc(II) complexes of formulae COL (PF6)2 (1), NiLPF6 · CH3OH (2) and ZnLClO4 · H2O (3), which have been isolated in the solid state and characterized. The nickel complex is remarkably stable towards ligand dissociation in water solution even in the presence of a large excess of CN−. The structures of the complexes have been determined by single-crystal X-ray analyses. Compound 1 crystallizes in the triclinic space group P1, a = 7.397(2), b = 10.960(4), c = 17.420(5) A, α=101.51(3), β = 91.85(4), γ = 98.24(3)°, Z = 2. Compound 2 is monoclinic P21/a, a = 14.094(15), b = 11.045(4), c = 16.905(5) A, β = 105.54(7)°, Z = 4; 3 is triclinic P1, a = 7.903(4), b = 9.218(5), c = 16.996(10) A, α = 98.51(6), β = 96.76(8), γ = 101.98(7)°, Z = 2. In each complex cation the metal atom is six-coordinated, being surrounded by the three macrocycle nitrogens, the two imidazole nitrogens and one carboxylate oxygen atom, with a coordination geometry which is approximately trigonal antiprismatic in 1 and 2 and trigonal prismatic in 3.