Carbamate Stabilities of Sterically Hindered Amines from Quantum Chemical Methods: Relevance for CO2 Capture

The influence of electronic and steric effects on the stabilities of carbamates formed from the reaction of CO₂ with a wide range of alkanolamines was investigated by quantum chemical methods. For the calculations, B3LYP, M11‐L, MP2, and spin‐component‐scaled MP2 (SCS‐MP2) methods were used, coupled with SMD and SM8 solvation models. A reduction in carbamate stability leads to an increased CO₂ absorption capacity of the amine and a reduction of the energy required for solvent regeneration. Important factors for the reduction of the carbamate stability were an increase in steric hindrance around the nitrogen atom, charge on the N atom and intramolecular hydrogen bond strength. The present study indicates that secondary ethanolamines with sterically hindering groups near the N atom show significant potential as candidates for industrial CO₂‐capture solvents.     

CO2 Capture with Silylated Ethanolamines and Piperazines

Amine treatment is commonly used to capture CO₂ from exhaust gases and from ambient air. The Si−N bond in aminosilanes is capable of reacting with CO₂ more readily than amines. In the current study we have synthesized trimethylsilylated ethanolamines, diethanolamines and piperazines and investigated their reaction toward CO₂. All products were characterized by ¹H, ¹³C, and ²⁹Si NMR, RAMAN spectroscopy as well as mass spectrometry. The product of a twofold CO₂-insertion into bis-trimethylsilylated piperazine was analysed by single-crystal X-ray diffraction. Furthermore, quantum chemical calculations (DFT) were used to supplement the experimental results. Geometry optimizations and NBO calculations for each starting material were carried out at the B3LYP level with different basis sets. DFT calculations at the B3LYP, WB97XD and M062x level were conducted for geometry optimization and frequency calculations to examine the thermochemical data. The calculations were carried out both for the gas phase and in solvent environment. The calculated reaction enthalpies varied between −37 and −107 kJ mol⁻¹, while experimental values around −100 kJ mol⁻¹ were determined.     

醇胺分子结构及其CO2捕集能力

研究了一乙醇胺及其衍生物的非环状醇胺的分子结构与其吸收捕集CO2性能之间的关系．通过实验测定了在313．15K和393．15K两个温度下,CO2分压从1kPa到700kPa条件下,CO2在不同醇胺水溶液中的溶解度,分析研究了羟基与烷基间的链长、取代烷基位置、氨基上烷基链数目及烷基羟基链数目、羟基位置和两个羟基的影响．结果表明,不同分子结构的醇胺水溶液对CO2的吸收能力有明显差别．根据CO2的捕集分离要求,选择合适的溶剂,可大幅度提高溶剂的CO2捕集能力．     

Gas‐Phase Infrared Spectroscopy of Substituted Cyanobutadiynes: Roles of the Bromine Atom and Methyl Group as Substituents

The IR spectra of 5‐bromo‐2,4‐pentadiynenitrile (Br?C≡C?C≡C?CN) and 2,4‐hexadiynenitrile (CH₃?C≡C?C≡C?CN), a compound of interstellar interest, have been recorded within the 4000–500 cm^?1 spectral region and calculated by means of high‐level ab initio and density functional calculations. Although the calculated structures of both compounds are rather similar, there are very subtle differences, mainly in the strength of the C≡C bond not directly bound to the substituent. These subtle bonding differences are reflected in small, but not negligible, differences in the electron density at the corresponding bond critical points, and, more importantly, are reflected in the IR spectra. Indeed, the IR spectrum for the bromine derivative presents two well‐differentiated strong bands around 2250 cm^?1, whereas for the methyl derivative both absorptions coalesce in a single band. These bands correspond in both cases to the coupling between C≡C and C≡N stretching displacements. A third, very weak, band also associated with C≡C and C≡N coupled stretches is observed for the bromine derivative, but not for the methyl one, owing to its extremely low intensity.     

Capture and release of CO2 by polyamidine

By CO₂ bubbling into an aqueous dimethyl sulfoxide solution, the polyamidine ( Poly‐Amd ) prepared from hexamethylenediamine and triethyl orthoacetate was converted to the bicarbonate salt, Poly‐Amd H·HCO₃, which formed aggregates. Conversely, the aggregates disappeared upon Ar bubbling to release the captured CO₂, reverting to Poly‐Amd completely. Polyethyleneimine ( PEI ) also reversibly captured and released CO₂ molecules in solution by bubbling CO₂ and Ar, respectively, in an alternate manner. No appreciable difference was observed between Poly‐Amd and PEI in CO₂ capture in solution. The binary system consisting of Poly‐Amd and polyethylene glycol ( PEG ) captured CO₂ efficiently at ordinary pressure and reached a stationary state within 200 h, at which 66% of the amidine groups bound CO₂ molecules, which was released upon exposure to a N₂ flow. In contrast to the binary system with Poly‐Amd, the binary system of PEI with PEG did not capture CO₂ efficiently, and only 5.7% of the amino groups bound CO₂ molecules after 600 h. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3404–3411     

The Transition States for CO2 Capture by Substituted Ethanolamines

Quantum chemical studies are used to understand the electronic and steric effects on the mechanisms of the reaction of substituted ethanolamines with CO₂. SCS‐MP2/6‐311+G(2d,2p) calculations are used to obtain the activation energy barriers and reaction energies for both the carbamate and bicarbonate formation. Implicit solvent effects are included with the universal solvation model SMD. Carbamate formation is more favorable than bicarbonate formation for monoethanolamine (MEA) both kinetically and thermodynamically. Increase of the steric hindrance on the C atoms around the N atom in substituted ethanolamines favors bicarbonate formation over carbamate formation with lower activation barriers and thereby higher reaction rates. In contrast, substitution by an N‐methyl or N‐ethyl group on MEA leads to a lower activation barrier for both carbamate formation and bicarbonate formation. As a result, higher reaction rates are expected as compared to MEA, and therefore these compounds have significant potential as industrial CO₂ capturing solvents.     

Carbon Dioxide Capture and Use: Organic Synthesis Using Carbon Dioxide from Exhaust Gas

A carbon capture and use (CCU) strategy was applied to organic synthesis. Carbon dioxide (CO₂) captured directly from exhaust gas was used for organic transformations as efficiently as hyper‐pure CO₂ gas from a commercial source, even for highly air‐ and moisture‐sensitive reactions. The CO₂ capturing aqueous ethanolamine solution could be recycled continuously without any diminished reaction efficiency.     

CO2 in supramolecular chemistry: preparation of switchable supramolecular polymers

CO2 gas was used to construct novel types of supramolecular polymers. Self-assembling nanostructures 11 and 13 were prepared, which employ both hydrogen bonding and dynamic, thermally reversible carbamate bonds. As precursors, calixarene ureas 1 and 2 were synthesized, which strongly aggregate/dimerize (K(D)>/=10(6) M(-1) per capsule) in apolar solution with the formation of self-assembling capsules 7 and linear polymeric chains 8, respectively, and also possess "CO2-philic" primary amino groups on the periphery. CO2 effectively reacts with molecules 7 and 8 in apolar solvents and cross-links them with the formation of multiple carbamate salt bridges. Oligomeric aggregate 11 and three-dimensional polymeric network 13 were prepared and characterized by 1H and 13C NMR spectroscopy. The morphology of supramolecular gel 13 was studied by scanning electron microscopy. Addition of a competitive solvent destroyed the hydrogen bonding in assembling structures 11 and 13, but did not influence the carbamate linkers; carbamate salts 12 and 14, respectively, were obtained. On the other hand, thermal release of CO2 from 11 and 13 was easily accomplished (1 h, 100 degrees C) while retaining the hydrogen-bonding capsules. Thus, three-dimensional polymeric network 13 was transformed back to linear polymeric chain 8 without breaking up. Encapsulation and storage of solvent molecules by 11 and 13 was demonstrated. This opens the way for switchable materials, which reversibly trap, store, and then release guest molecules. A two-parameter switch and control over hydrogen bonding and CO2-amine adducts was established.     

Formation and characterization of gaseous adducts of carbon dioxide to magnesium, (CO2)MgX- (X=OH, Cl, Br)

A good fix: the structure and chemical reactivity of a reduced form of CO(2) bonded to magnesium, XMg(η(2)-O(2)C)(-), is reported. Upon reaction with water it loses CO, while it adds CH(3) upon reaction with alkyl halides, thereby signifying nucleophilicity of the carbon atom in XMg(η(2)-O(2)C)(-) in S(N)2 reactions.     

Computer‐Assisted Design of Imidazolate‐Based Ionic Liquids for Improving Sulfur Dioxide Capture,Carbon Dioxide Capture,and Sulfur Dioxide/Carbon Dioxide Selectivity

A new strategy involving the computer‐assisted design of substituted imidazolate‐based ionic liquids (ILs) through tuning the absorption enthalpy as well as the basicity of the ILs to improve SO₂ capture, CO₂ capture, and SO₂/CO₂ selectivity was explored. The best substituted imidazolate‐based ILs as absorbents for different applications were first predicted. During absorption, high SO₂ capacities up to ≈5.3 and 2.4 mol mol_IL⁻¹ could be achieved by ILs with the methylimidazolate anions under 1.0 and 0.1 bar (1 bar=0.1 MPa), respectively, through tuning multiple N ⋅⋅⋅ S interactions between SO₂ and the N atoms in the imidazolate anion with different substituents. In addition, CO₂ capture by the imidazolate‐based ILs could also be easily tuned through changing the substituents of the ILs, and 4‐bromoimidazolate IL showed a high CO₂ capacity but a low absorption enthalpy. Furthermore, a high selectivity for SO₂/CO₂ could be reached by IL with 4,5‐dicyanoimidazolate anion owing to its high SO₂ capacity but low CO₂ capacity. The results put forward in this work are in good agreement with the predictions. Quantum‐chemical calculations and FTIR and NMR spectroscopy analysis methods were used to discuss the SO₂ and CO₂ absorption mechanisms.     

Boron‐Functionalized Graphene Oxide‐Organic Frameworks for Highly Efficient CO2 Capture

The capture and storage of CO₂ have been suggested as an effective strategy to reduce the global emissions of greenhouse gases. Hence, in recent years, many studies have been carried out to develop highly efficient materials for capturing CO₂. Until today, different types of porous materials, such as zeolites, porous carbons, N/B‐doped porous carbons or metal‐organic frameworks (MOFs), have been studied for CO₂ capture. Herein, the CO₂ capture performance of new hybrid materials, graphene‐organic frameworks (GOFs) is described. The GOFs were synthesized under mild conditions through a solvothermal process using graphene oxide (GO) as a starting material and benzene 1,4‐diboronic acid as an organic linker. Interestingly, the obtained GOF shows a high surface area (506 m² g⁻¹) which is around 11 times higher than that of GO (46 m² g⁻¹), indicating that the organic modification on the GO surface is an effective way of preparing a porous structure using GO. Our synthetic approach is quite simple, facile, and fast, compared with many other approaches reported previously. The synthesized GOF exhibits a very large CO₂ capacity of 4.95 mmol g⁻¹ at 298 K (1 bar), which is higher those of other porous materials or carbon‐based materials, along with an excellent CO₂/N₂ selectivity of 48.8.     

Photocatalytic Conversion of CO(2) in Water over Layered Double Hydroxides

Have a bit of bubbly: Significant amounts of CO and O(2) gas are evolved in the photocatalytic conversion of CO(2) over layered double hydroxides (LDHs) in water. A simple mixture of the same metal hydroxides, which has the same constituent elements of the LDH, shows low activity for CO and O(2) evolution. Dissolved CO(2) gas was shown to be the source of carbon in the reaction over LDHs in water.     

N-Aryl and N-Alkyl Carbamates from 1 Atmosphere of CO2

We have successfully isolated and characterized the zinc carbamate complex (phen)Zn(OAc)(OC(=O)NHPh) ( 1 ; phen=1,10-phenanthroline), formed as an intermediate during the Zn(OAc)₂/phen-catalyzed synthesis of organic carbamates from CO₂, amines, and the reusable reactant Si(OMe)₄. Density functional theory calculations revealed that the direct reaction of 1 with Si(OMe)₄ proceeds via a five-coordinate silicon intermediate, forming organic carbamates. Based on these results, the catalytic system was improved by using Si(OMe)₄ as the reaction solvent and additives like KOMe and KF, which promote the formation of the five-coordinated silicon species. This sustainable and effective method can be used to synthesize various N-aryl and N-alkyl carbamates, including industrially important polyurethane raw materials, starting from CO₂ under atmospheric pressure.     

Halogen‐Free Synthesis of Carbamates from CO2 and Amines Using Titanium Alkoxides

A direct synthesis of carbamates from amines and carbon dioxide in the presence of Ti(OR)₄ (R=n Bu ( 1 ), Me ( 2 ), Et ( 3 ), n Pr ( 4 )) was investigated. Aniline was reacted with titanium n ‐butoxide ( 1 ) in the presence of carbon dioxide (5 MPa) to give the corresponding n ‐butyl N ‐phenylcarbamate (BPC) in nearly quantitative yield (99 %) within 20 min. Furthermore, 1 could be regenerated upon reaction with n ‐butanol during water removal. The recovered 1 could then be reused in a subsequent reaction.     

Chiral Organocatalysts in Enantioselective CO2 Utilization Reactions

The development of efficient CO₂ utilization reactions has gained a significant amount of attention in recent years. Although transformations of CO₂ to produce basic chemicals have been extensively investigated, the development of catalytic enantioselective CO₂ utilization reactions for the preparation of fine chemicals remains limited at this stage. Several excellent methods for catalytic enantioselective CO₂ utilization using chiral metal complex catalysts have been reported. Many researchers have also focused on developing organocatalyzed approaches to enantioselective CO₂ utilization, and several excellent examples have appeared in recent years. Herein, recent advances in catalytic enantioselective CO₂ utilization reactions using chiral organocatalysts are reviewed to provide a forecast for organocatalyzed enantioselective CO₂ utilization research.     

New mechanistic insight into the coupling reactions of CO2 and epoxides in the presence of zinc complexes

Coupling reactions of CO(2) and epoxide to produce cyclic carbonates were performed in the presence of a catalyst [L(2)ZnX(2)] (L=pyridine or substituted pyridine; X=Cl, Br, I), and the effects of pyridine and halide ligands on the catalytic activity were investigated. The catalysts with electron-donating substituents on pyridine ligands exhibit higher activity than those with unsubstituted pyridine ligands. On the other hand, the catalysts with electron-withdrawing substituents at the 2-position of the pyridine ligands show no activity; this demonstrates the importance of the basicity of the pyridine ligands. The catalytic activity of [L(2)ZnX(2)] was found to decrease with increasing electronegativity of the halide ligands. A series of highly active zinc complexes bridged by pyridinium alkoxy ions of the general formula [((mu-OCHRCH(2)L)ZnBr(2))(n)] (n=2 for R=CH(3); n=3 for R=H; L=pyridine or substituted pyridine) were synthesized and characterized by X-ray crystallography. The dinuclear zinc complexes obtained from propylene oxide adopt a square-planar geometry for the Zn(2)O(2) core with two bridging pyridinium propoxy ion ligands. Trinuclear zinc complexes prepared from ethylene oxide adopt a boat geometry for the Zn(3)O(3) core, in which three zinc and three oxygen atoms are arranged in an alternate fashion. These zinc complexes bridged by pyridinium alkoxy ions were also isolated from the coupling reactions of CO(2) and epoxides performed in the presence of [L(2)ZnBr(2)]. Rapid CO(2) insertion into the zincbond;oxygen bond of the zinc complexes bridged by pyridinium alkoxy ions leads to the formation of zinc carbonate species; these which yield cyclic carbonates and zinc complexes bridged by pyridinium alkoxy ions upon interaction with epoxides. The mechanistic pathways for the formation of active species and cyclic carbonates are discussed on the basis of results from structural and spectroscopic analyses.