Raman spectroscopy and ab initio investigations of transient complex formation in CO2-benzene mixtures

Fick Diffusion Coefficient in Binary Mixtures of [HMIM][NTf2] and Carbon Dioxide by Dynamic Light Scattering and Molecular Dynamics Simulations

Dynamic light scattering (DLS) experiments and equilibrium molecular dynamics (EMD) simulations were performed in the saturated liquid phase of the binary mixture of 1-hexyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide ([HMIM][NTf₂]) and carbon dioxide (CO₂) to access the Fick diffusion coefficient (D₁₁). The investigations were performed within or close to saturation conditions at temperatures between (298.15 and 348.15) K and CO₂ mole fractions (x_CO2) up to 0.81. The DLS experiments were combined with polarization-difference Raman spectroscopy (PDRS) to simultaneously access the composition of the liquid phase. For the first time in an electrolyte-based system, D₁₁ was directly calculated from EMD simulations by accessing the Maxwell-Stefan (MS) diffusion coefficient and the thermodynamic factor. Agreement within combined uncertainties was found between D₁₁ from DLS and EMD simulations for CO₂ mole fractions up to 0.5. In general, an increasing D₁₁ with increasing x_CO2 could be observed, with a local maximum present at a CO₂ mole fraction of about 0.75. The local maximum could be explained by an increasing MS diffusion coefficient with increasing x_CO2 over the entire studied composition range and a decreasing thermodynamic factor at x_CO2 above 0.7. Finally, PDRS and EMD simulations were combined to investigate the influence of the fluid structure on the diffusive process.

Mutual and Thermal Diffusivities as well as Fluid-Phase Equilibria of Mixtures of 1-Hexanol and Carbon Dioxide

This work contributes to an improved understanding of the fluid-phase behavior and diffusion processes in mixtures of 1-hexanol and carbon dioxide (CO₂) at temperatures around the upper critical end point (UCEP) of the system. Raman spectroscopy and dynamic light scattering were used to determine the composition at saturation conditions as well as Fick and thermal diffusivities. An acceleration of the Fick diffusive process up to CO₂ mole fractions of about 0.2 was found, followed by a strong slowing-down approaching vapor-liquid-liquid equilibrium or critical conditions. The acceleration of the Fick diffusive process vanished at temperatures much higher than the UCEP. Experimental Fick diffusivity data were compared with predictions from equilibrium molecular dynamics simulations and excess Gibbs energy calculations using interaction parameters from the literature. Both theoretical methods were not able to predict that the thermodynamic factor is equal to zero at the spinodal composition, stressing the need for new methodologies under such conditions. Thus, new sets of temperature-dependent interaction parameters were developed for the nonrandom two-liquid model, which improve the prediction of the Fick diffusion coefficient considerably. The link between the Fick diffusion coefficient and the nonrandomness of the liquid phases is also discussed.

Confined toluene within InOF-1: CO2 capture enhancement

The confinement of small amounts of toluene demonstrated an enhanced CO₂ capture for InOF-1 as a result of a bottleneck effect and synergistic interactions.

Enhancement of CO2 Uptake and Selectivity in a Metal-Organic Framework by the Incorporation of Thiophene Functionality

The complex [Zn₂(tdc)₂dabco] (H₂tdc = thiophene-2,5-dicarboxylic acid; dabco = 1,4-diazabicyclooctane) shows a remarkable increase in carbon dioxide (CO₂) uptake and CO₂/dinitrogen (N₂) selectivity compared to the nonthiophene analogue [Zn₂(bdc)₂dabco] (H₂bdc = benzene-1,4-dicarboxylic acid; terephthalic acid). CO₂ adsorption at 1 bar for [Zn₂(tdc)₂dabco] is 67.4 cm³·g^–1 (13.2 wt %) at 298 K and 153 cm³·g^–1 (30.0 wt %) at 273 K. For [Zn₂(bdc)₂dabco], the equivalent values are 46 cm³·g^–1 (9.0 wt %) and 122 cm³·g^–1 (23.9 wt %), respectively. The isosteric heat of adsorption for CO₂ in [Zn₂(tdc)₂dabco] at zero coverage is low (23.65 kJ·mol^–1), ensuring facile regeneration of the porous material. Enhancement by the thiophene group on the separation of CO₂/N₂ gas mixtures has been confirmed by both ideal adsorbate solution theory calculations and dynamic breakthrough experiments. The preferred binding sites of adsorbed CO₂ in [Zn₂(tdc)₂dabco] have been unambiguously determined by in situ single-crystal diffraction studies on CO₂-loaded [Zn₂(tdc)₂dabco], coupled with quantum-chemical calculations. These studies unveil the role of the thiophene moieties in the specific CO₂ binding via an induced dipole interaction between CO₂ and the sulfur center, confirming that an enhanced CO₂ capacity in [Zn₂(tdc)₂dabco] is achieved without the presence of open metal sites. The experimental data and theoretical insight suggest a viable strategy for improvement of the adsorption properties of already known materials through the incorporation of sulfur-based heterocycles within their porous structures.

Carbon dioxide (CO₂) adsorption is enhanced via dipolar interactions of CO₂ with the sulfur center of thiophene. In situ X-ray diffraction experiments, theoretical studies, and breakthrough separation data support the superior CO₂ adsorption properties of [Zn₂(tdc)₂dabco] compared with the analogue [Zn₂(bdc)₂dabco] (H₂tdc = thiophene-2,5-dicarboxylic acid; H₂bdc = terephthalic acid; dabco = 1,4-diazabicyclooctane).

Understanding the mechanism of CO2 capture by 1,3 di-substituted imidazolium acetate based ionic liquids

Efficient CO₂ capture by ionic liquids needs a thorough understanding of underlying mechanisms of the CO₂ interaction with ionic liquids, especially when it involves chemisorption.

Assessing the non-ideality of the CO2-CS2 system at molecular level: a Raman scattering study

The dense phase of CO2-CS2 mixtures has been analysed by Raman spectroscopy as a function of the CO2 concentration (0.02-0.95 mole fractions) by varying the pressure (0.5 MPa up to 7.7 MPa) at constant temperature (313 K). The polarised and depolarised spectra of the induced (ν2, ν3) modes of CS2 and of the ν1-2ν2 Fermi resonance dyad of both CO2 and CS2 have been measured. Upon dilution with CO2, the evolution of the spectroscopic observables of all these modes displays a "plateau-like" region in the CO2 mole fraction 0.3-0.7 never previously observed in CO2-organic liquids mixtures. The bandshape and intensity of the induced modes of CS2 are similar to those of pure CS2 up to equimolar concentration, after which variations occur. The preservation of the local ordering from pure CS2 to equimolar concentration together with the non-linear evolution of the spectroscopic observables allows inferring that two solvation regimes exist with a transition occurring in the plateau domain. In the first regime, corresponding to CS2 concentrated mixtures, the liquid phase is segregated with dominant CS2 clusters, whereas, in the second one, CO2 monomers and dimers and CO2-CS2 hetero-dimers coexist dynamically on a picosecond time-scale. It is demonstrated that the subtle interplay between attractive and repulsive interactions which provides a molecular interpretation of the non-ideality of the CO2-CS2 mixture allows rationalizing the volume expansion and the existence of the plateau-like region observed in the pressure-composition diagram previously ascribed to the proximity of an upper critical solution temperature at lower temperatures.

Significant cation effects in carbon dioxide-ionic liquid systems

Carbon dioxide–ionic liquid systems are of great current interest, and significant efforts have been made lately to understand the intermolecular interactions in these systems. In general, all the experimental and theoretical studies have concluded so far that the main solute–solvent interaction takes effect through the anion, and the cation has no, or only a secondary role in solvation. In this theoretical approach it is shown that this view is unfounded, and evidence is provided that, similarly to the benzene–CO₂ system, dispersion interactions are present between the solute and the cation. Therefore, this defines a novel site for tailoring solvents to tune CO₂ solubility.

Phase behavior of liquid crystals with CO2

Liquid crystals are being considered as novel process solvents for CO(2) capture. The solubility of CO(2) is higher in the isotropic phase than in the structured (e.g., nematic) phase. CO(2) can be captured in the isotropic phase, and regeneration of the solvent is achieved by cooling down the mixture a few degrees until a phase transition to the structured phase occurs. This CO(2) capture process has the potential to consume less energy than the conventional amine-based processes. To address the potential of liquid crystals to efficiently capture CO(2), experimentally obtained P,T-phase diagrams of five liquid crystals with 5 mass % CO(2) are reported. The liquid crystals used in this study are 4'-(pentyloxy)-4-biphenylcarbonitrile, 4'-pentyl-4-biphenylcarbonitrile, 4-ethyl-4'-propyl-bicyclohexyl, 4-propyl-4'-butyl-bicyclohexyl, and 4'-(octyloxy)-4-biphenylcarbonitrile. It is found that a weakly polar liquid crystal had a higher CO(2) solubility than apolar and more polar liquid crystals.