A procedure to find thermodynamic equilibrium constants for CO2 and CH4 adsorption on activated carbon

Adsorption Equilibrium and Diffusion of CH4, CO2, and N2 in Coal-Based Activated Carbon

Coal-based activated carbon is an ideal adsorbent for concentrating CH₄ from coalbed methane and recovering CO₂ from industrial waste gas. In order to upgrade the environmentally protective preparation technology of coal-based activated carbons and clarify the adsorption equilibrium and diffusion rules of CH₄, CO₂, and N₂ in these materials, we prepared granular activated carbon (GAC) via air oxidation, carbonization, and physical activation using anthracite as the raw material. Also, we measured the adsorption isotherms and adsorption kinetic data of GAC by the gravimetric method and characterized its surface chemical properties. According to the results, GAC had abundant micropore structures with a pore size mainly in the range of 5.0-10.0 Å, and its surface was covered with plentiful oxygen-containing functional groups. The specific pore structure and surface chemical properties could effectively improve the separation and purification effects of GAC on CH₄ and CO₂. In the temperature range of 278-318 K, the equilibrium separation of CH₄/N₂ by GAC with a coefficient between 3 and 4 could be achieved. Also, the CO₂/CH₄ separation coefficient decreased with the increase in temperature but remained around 3. The bivariate Langmuir equation could describe the adsorption behaviors of GAC on CH₄/N₂, CO₂/N₂, and CH₄/CO₂. With the increase in the concentrations of CH₄ and CO₂ in the gas phase, the difference between the adsorption capacity of CH₄ or CO₂ and that of N₂ became greater. The change of the gas ratio did not affect the characteristics of preferential adsorption of CH₄ and CO₂. At different temperatures (278, 298, and 318 K), the diffusion coefficients of CH₄, N₂, and CO₂ at various pressure points showed predominately a small variation without an obvious trend. These results demonstrated that the separation of CH₄/N₂, CO₂/N₂, and CH₄/CO₂ by the activated carbon could only rely on the equilibrium separation effect rather than the kinetic effect.

Solvation shell thermodynamics of extended hydrophobic solutes in mixed solvents

The ability of various cosolutes and cosolvents to enhance or quench solvent density fluctuations at solute–water interfaces has crucial implications on the conformational equilibrium of macromolecules such as polymers and proteins. Herein, we use an extended hydrophobic solute as a model system to study the effect of urea and methanol on the density fluctuations in the solute’s solvation shell and the resulting thermodynamics. On strengthening the solute–water/cosolute repulsive interaction, we observe distinct trends in the mutual affinities between various species in, and the thermodynamic properties of, the solvation shell. These trends strongly follow the respective trends in the preferential adsorption of urea and methanol: solute–water/cosolute repulsion strengthens, urea accumulation decreases, and methanol accumulation increases. Preferential accumulation of urea is found to quench the density fluctuations around the extended solute, leading to a decrease in the compressibility of the solvation shell. In contrast, methanol accumulation enhances the density fluctuations, leading to an increase in the compressibility. The mode of action of urea and methanol seems to be strongly coupled to their hydration behavior. The observations from this simple model is discussed in relation to urea driven swelling and methanol induced collapse of some well-known thermo-responsive polymers.

SJ, van der Vegt NFA. Characterizing Polymer Hydration Shell Compressibilities with the Small-System Method

The small-system method (SSM) exploits the unique feature of finite-sized open systems, whose thermodynamic quantities scale with the inverse system size. This scaling enables the calculation of properties in the thermodynamic limit of macroscopic systems based on computer simulations of finite-sized systems. We herein extend the SSM to characterize the hydration shell compressibility of a generic hydrophobic polymer in water. By systematically increasing the strength of polymer-water repulsion, we find that the excess inverse thermodynamic correction factor (Δ1/Γs∞) and compressibility (Δχs) of the first hydration shell change sign from negative to positive. This occurs with a concurrent decrease in water hydrogen bonding and local tetrahedral order of the hydration shell water. The crossover lengthscale corresponds to an effective polymer bead diameter of 0.7 nm and is consistent with previous works on hydration of small and large hydrophobic solutes. The crossover lengthscale in polymer hydration shell compressibility, herein identified with the SSM approach, relates to hydrophobic interactions and macromolecular conformational equilibria in aqueous solution. The SSM approach may further be applied to study thermodynamic properties of polymer solvation shells in mixed solvents.

CO2 separation using composites consisting of 1-butyl-3-methylimidazolium tetrafluoroborate/CdO/1-aminopyridinium iodide

1-Aminopyridinium iodide (iodine salt) was used in CO₂ separation composites consisting of CdO and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM⁺BF₄⁻). Using iodine salt, the separation performance was largely improved. The CO₂/N₂ selectivity was 64.6 and the permeance of CO₂ gas was 22.6 GPU, which was about twice that of BMIM⁺BF₄⁻/CdO composites without addition of iodine salt. These results were due to the both effect of iodine salt on the transport of the N₂ molecules by the cyclic ring compound and the promoting transport of CO₂ molecules by the amine groups. Moreover, the oxide layer on the surface of the CdO could enhance the CO₂ solubility, resulting in the enhancement of separation performance. The mechanical and chemical properties were measured using SEM, Raman, TGA and FT-IR. The cross-section of coated membranes was confirmed by SEM. The coordinative interactions of iodine salts with BMIM⁺BF₄⁻/CdO composite were observed by Raman.

Novel Porous Carbons Derived from Coal Tar Rejects: Assessment of the Role of Pore Texture in CO2 Capture under Realistic Postcombustion Operating Temperatures

Activated carbons (ACs) are among the most commonly used sorbents for CO₂ capture because of their high surface areas and micropore volumes, which depend on precursor and activation methods. In this study, we evaluated different ACs obtained from a low-value fraction of liquid-derived coal pyrolysis, namely phenolic oil, which was used as gel precursor before carbonization and KOH activation. CO₂ capture performances were determined at temperatures between 25 and 120 °C, with CO₂ concentrations ranging from 5 to 90 vol %. The most efficient sample captured 2.86 mmol of CO₂/g AC at 25 °C and 1 bar, which is a highly competitive capture capacity, comparable to previously reported values for ACs without any modification/functionalization. Finally, their thermal stability and cyclability (i.e., for a minimum of six adsorption-desorption cycles) were evaluated. CO₂ uptake was not affected by desorption temperature after six adsorption-desorption cycles. On the basis of the results obtained in this work, the role of the textural properties into the CO₂ capture at realistic postcombustion temperatures and partial pressures was elucidated. In particular, we concluded that CO₂ adsorption performance was more related to the volume of the narrowest pores and to the average pore size than to the surface area.