Highly stable Fe/CeO2 catalyst for the reverse water gas shift reaction in the presence of H2S

This study focused on evaluating the catalytic properties for the reverse water gas shift reaction (RWGS: CO₂ + H₂ → CO + H₂O ΔH ⁰ = 42.1 kJ mol^-1) in the presence of hydrogen sulfide (H₂S) over a Fe/CeO₂ catalyst, commercial Cu-Zn catalyst for the WGS reaction (MDC-7), and Co-Mo catalyst for hydrocarbon desulfurization. The Fe/CeO₂ catalyst exhibited a relatively high catalytic activity to RWGS, compared to the commercial MDC-7 and Co-Mo catalysts. In addition, the Fe/CeO₂ catalyst showed stable performance in the RWGS environment that contained high concentrations of H₂S. The role of co-feeding H₂S was investigated over the Fe/CeO₂ catalyst by the temperature programmed reaction (TPR) of CO₂ and H₂ in the presence of H₂S. The result of TPR indicated that the co-feeding H₂S might enhance RWGS performance due to H₂S acting as the hydrogen source to reduce CO₂.

Hydrophobic Deep Eutectic Solvents as Greener Substitutes for Conventional Extraction Media: Examples and Techniques

Deep eutectic solvents (DESs) are multicomponent designer solvents that exist as stable liquids over a wide range of temperatures. Over the last two decades, research has been dedicated to developing noncytotoxic, biodegradable, and biocompatible DESs to replace commercially available toxic organic solvents. However, most of the DESs formulated until now are hydrophilic and disintegrate via dissolution on coming in contact with the aqueous phase. To expand the repertoire of DESs as green solvents, hydrophobic DESs (HDESs) were prepared as an alternative. The hydrophobicity is a consequence of the constituents and can be modified according to the nature of the application. Due to their immiscibility, HDESs induce phase segregation in an aqueous solution and thus can be utilized as an extracting medium for a multitude of compounds. Here, we review literature reporting the usage of HDESs for the extraction of various organic compounds and metal ions from aqueous solutions and absorption of gases like CO₂. We also discuss the techniques currently employed in the extraction processes. We have delineated the limitations that might reduce the applicability of these solvents and also discussed examples of how DESs behave as reaction media. Our review presents the possibility of HDESs being used as substitutes for conventional organic solvents.

A Prediction for the Conversion Performance of H2S to Elemental Sulfur in an Ionic-Liquid-Incorporated Transition Metal Using COSMO-RS

In the present study, the conversion performance of hydrogen sulfide (H2S) to elemental sulfur in ionic-liquid-incorporated transition metals (ILTMs) is predicted using a conductor-like screening model for realistic solvents (COSMO-RS). The predictions were made via the establishment of a correlation between the conversion performance and solubility of H2S in ionic liquids (ILs). All molecules involved were optimized at the DFT/TZVP/M06 computational level and imported on the COSMOtherm program at equimolar conditions. For validation purposes, the solubility of ILs was predicted at 1 bar pressure. Simple regression analysis was used to establish a relationship between the solubility and conversion performance of H2S. The results indicate that the solubility prediction of ILs is accurate (R2 = 93.40%) with a p-value of 0.0000000777. Additionally, the conversion performance is generally found to be dependent on the solubility value. Furthermore, 1-butyl-3-methylimidazolium chloride [bmim][Cl] was chosen as the base IL for incorporating the transition metal, owing to its solubility and selectivity to H2S. The solubility trend of ILTMs is found to follow the following order: [bmim][NiCl3] > [bmim][FeCl4] > [bmim][CoCl3] > [bmim][CuCl3]. According to the viscosity measurements of ILTMs, [bmim][NiCl3] and [bmim][FeCl4] exhibited the highest and lowest viscosity values, respectively. Therefore, [bmim][FeCl4] is a promising ILTM owing to its higher solubility and low viscosity for the application studied.

H2S-CO2 gas separation with ionic liquids on low ratio of H2S/CO2

Acid gas removal, especially H₂S and CO₂, is an essential process in natural gas processing. In this research, 1-butyl-3-methylimidazolium bromide [bmim][Br] ionic liquid was analyzed as a hydrophobic solvent with has high selectivity to H₂S as an environmentally friendly solvent to absorb acid gas from natural gas with low H₂S/CO₂ concentration in ambient temperature and pressure. The absorption performance of pure [bmim][Br] ionic liquid was compared with various amine solutions, such as monoethanolamine (MEA), triethanolamine (TEA), and methyldiethanolamine (MDEA), as well as the mixture of [bmim][Br]-MDEA with various concentration. As a result, pure [bmim][Br] ionic liquid had high selectivity to H₂S compared with conventional amine solutions. In addition, the mixture of [bmim][Br]-MDEA with the mass ratio of 1:3 provided the highest H₂S/CO₂ selectivity of 6.2 in certain absorption conditions due to free tertiary amine attached in the cations of ionic liquids that can attract more H₂S to its functional site.

Investigation on Protic Ionic Liquids as Physical Solvents for Absorption of NO at Low Pressures

Nitric oxide (NO) absorption in ionic liquids (ILs) is an interesting issue, but little attention has been focused on the removal of NO at low partial pressures. Herein, a series of protic ionic liquids (PILs) based on polyamines as the cation and hydroxybenzenes as the anion were prepared for capturing low-concentration NO (0-0.6 bar). Triethylenetetramine phenolate ([TETAH][PhO]) showed an excellent absorption performance, with low viscosity, fast absorption rate, and high absorption capacity. The experimental solubility data were fitted by the Krichevsky-Kasarnovsky (K-K) equation, and the absorption enthalpy (ΔH) of NO in [TETAH][PhO] was thus calculated to be -43.60 kJ/mol. Density functional theory calculations were further performed to better understand the interaction of [TETAH][PhO] with NO on the molecular level, and the results suggest that the weak interaction of NO with the PIL was induced by the presence of H protons. It is believed that this work may provide a new method for the efficient and reversible absorption of low-concentration NO.

CO2 separation by supported liquid membranes synthesized with natural deep eutectic solvents

Betaine-based natural deep eutectic solvents (NADESs), a new class of green solvents, were immobilized into a porous polyvinylidene fluoride (PVDF) support and evaluated for the separation of CO₂ from CO₂/N₂ and CO₂/CH₄ mixtures. Two types of NADESs were synthesized by mixing betaine (hydrogen bond acceptor-HBA) with malic acid and tartaric acid (hydrogen bond donors-HBD) respectively. FTIR and Raman spectroscopy were studied to confirm the synthesis and purity of the NADESs. The thermal strength of the NADESs was investigated using thermogravimetric analysis. The gas permeation results of the fabricated NADES-based-supported liquid membranes (NADES-SLMs) showed that the permeability of CO₂ increased from 25.55 to 29.33 Barrer on substitution of hydrogen bond donor from tartaric acid to malic acid. Similarly, the ideal CO₂/CH₄ selectivity varied from 51.1 to 56.4 as tartaric acid was replaced by malic acid as the HBD. The performance of NADES-SLMs was compared with the competing imidazolium-based-supported ionic liquid membranes, and proved NADES-SLMs as a promising alternative considering their green potential and comparable gas separation performance. The current effort for the exploitation of NADESs into PVDF membranes in this study is expected to open new routes for the efficient separation of CO₂ from the industrial gas mixture.

Study on the Desulfurization and Regeneration Performance of Functional Deep Eutectic Solvents

Four deep eutectic solvents (DESs) were synthesized, and 5-30% polyethylenimine (PEI) was added to make functional DESs (FDESs) for dynamic absorption experiments of hydrogen sulfide. The synthesized FDESs were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and nuclear magnetic resonance. The results demonstrated the successful synthesis of FDESs. The interaction between H2S and the FDESs was discussed at a molecular level via the quantum chemical calculations. It was noticed that FDESs prefer chemisorption on H2S. In this work, the 25% PEI/FDES@EG showed the highest desulfurization performance. The effects of H2S concentration and temperature on the desulfurization performance were investigated. It was found that a relatively low temperature (30 °C) was favorable for the absorption of H2S. The 25% PEI/FDES@EG could remove H2S efficiently over a low H2S concentration. Moisture played an important role in the FDES desulfurization system. The absorption/desorption cycle experiment indicated that the FDESs retain their good regeneration performance for at least five times.

Hydrophobic Functional Deep Eutectic Solvents Used for Efficient and Reversible Capture of CO2

CO2 emission from flue gas is an important issue threatening human survival. Deep eutectic solvents (DESs), which have many unique properties, have been studied for CO2 capture. However, water can be absorbed by DESs during the absorption of CO2, which may increase the energy cost during the desorption of CO2. In this work, a new kind of hydrophobic functional DES formed by polyamine hydrochloride and thymol was synthesized and used for CO2 capture. It had been found that these DESs could efficiently capture CO2 even at low partial pressures. The CO2 capacity of [TEPA]Cl-thymol (n [TEPA]Cl/n thymol = 1:3) was high up to 1.355 mol CO2/mol DES at 40 °C and 101.3 kPa. Interestingly, these DESs were still hydrophobic after saturated with CO2. The CO2 absorption capacity increased with a decrease of temperature and an increase of CO2 partial pressure. Regeneration results showed that no obvious loss in the capacity could be found after five absorption/desorption cycles of these DESs. The Fourier transform infrared (FT-IR) spectra indicated that CO2 could interact with amino in the DESs by the formation of carboxylate. Moreover, the equilibrium constant and Henry's law constant in chemical absorption and physical absorption were studied.

Theoretical study on the absorption of carbon dioxide by DBU-based ionic liquids

In this article, 20 ns molecular dynamic (MD) simulations and density functional theory (DFT) were used to investigate the absorption of CO2 molecules by some functionalized 1,8-diazabicyclo[5,4,0]-udec-7-ene (DBU)-based ILs. According to the MD results, the highest coordination number for NC is observed in the case of [DBUH+][Im-], which indicates that the functionalization of the imidazole anion by different alkyl groups decreases the interaction ability of the anion with CO2 molecules. The addition of water molecules to the ILs decreases the ability of the anion to interact with CO2 because of the hydrogen bond formation between the imidazole anions and water. Two different paths were proposed for CO2 absorption by the ILs, and the effect of alkyl groups on the kinetics and thermodynamics of the reaction was analyzed by using the M06-2X functional at the 6-311++G(d,p) level of theory in the gas phase and water. On the basis of the results, CO2 absorption is more favorable in [DBUH+][Im-], thermodynamically. Kinetic parameters show that the alkylation of the imidazole anion by ethyl, propyl, iso-propyl, and phenyl groups decreases the rate of CO2 absorption, because of the steric and electron-withdrawing effect of different alkyl groups. In the presence of water molecules, the lowest activation Gibbs energy is related to [DBUH+][Im-], which confirms the greater ability of this IL in CO2 absorption.

Self-enhancement of CO reversible absorption accompanied by phase transition in protic chlorocuprate ionic liquids for effective CO separation from N2

An efficient strategy for the high-capacity capture of CO is reported, and a phase change in protic chlorocuprate ionic liquids (PCILs) from liquid to solid is found during CO absorption. The highest CO capacity is 0.96 molCO molIL-1, being at least 150 times higher than that in [BMIM][PF6]. Both absorption and membrane permeation reveal that the PCILs are potential for the selective separation of CO from N2.

Catalyst-free N-formylation of amines using BH3NH3 and CO2 under mild conditions

The catalyst-free N-formylation of amines using CO₂ as the C₁ source and BH₃NH₃ as the reductant has been developed for the first time. The corresponding formylated products of both primary and secondary amines are obtained in good to excellent yields (up to 96% of isolated yield) under mild conditions.

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 molSO2  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.