Enhanced CO2 solubility in hybrid MCM-41: molecular simulations and experiments.

Grand canonical Monte Carlo simulations are performed in a hybrid adsorbent model in order to interpret the CO(2) solubility behavior. The hybrid adsorbent is prepared by confining a physical solvent (OMCTS) into the pores of a mimetic MCM-41 solid support. As a result, simulated adsorption isotherms of CO(2) nicely match the experimental data for three distinctive systems: bulk solvent, raw MCM-41, and hybrid MCM-41. The microscopic mechanisms underlying the apparition of enhanced solubility are then clearly identified. In fact, the presence of solvent molecules favors the layering of CO(2) molecules within the pores; therefore, the CO(2) solubility in the hybrid adsorbent markedly increases in comparison to that found in the raw adsorbent as well as in the bulk solvent. In addition, a good understanding of confined solvents' properties and solid surface structures is essential to fully evaluate the efficiency of hybrid adsorbents in capturing CO(2). The sorbent-solid interactions along with the solvent molecular size's impact on CO(2) solubility are therefore investigated in this study. We found that an ideal hybrid system should possess a weak solvent-solid interaction but a strong solvent-CO(2) interaction. Besides, an optimal solvent size is obtained for the enhanced CO(2) solubility in the hybrid system. According to the simulation results, the solvent layer builds pseudomicropores inside the mesoporous MCM-41, enabling more CO(2) molecules to be absorbed under the greater influence of spatial confinement and surface interaction. In addition, the molecular sieving effect is clearly observed in the case of larger solvent molecular sizes.

High throughput screening of CO2 solubility in aqueous monoamine solutions.

Post-combustion Carbon Capture and Storage technology (CCS) is viewed as an efficient solution to reduce CO(2) emissions of coal-fired power stations. In CCS, an aqueous amine solution is commonly used as a solvent to selectively capture CO(2) from the flue gas. However, this process generates additional costs, mostly from the reboiler heat duty required to release the carbon dioxide from the loaded solvent solution. In this work, we present thermodynamic results of CO(2) solubility in aqueous amine solutions from a 6-reactor High Throughput Screening (HTS) experimental device. This device is fully automated and designed to perform sequential injections of CO(2) within stirred-cell reactors containing the solvent solutions. The gas pressure within each reactor is monitored as a function of time, and the resulting transient pressure curves are transformed into CO(2) absorption isotherms. Solubility measurements are first performed on monoethanolamine, diethanolamine, and methyldiethanolamine aqueous solutions at T = 313.15 K. Experimental results are compared with existing data in the literature to validate the HTS device. In addition, a comprehensive thermodynamic model is used to represent CO(2) solubility variations in different classes of amine structures upon a wide range of thermodynamic conditions. This model is used to fit the experimental data and to calculate the cyclic capacity, which is a key parameter for CO(2) process design. Solubility measurements are then performed on a set of 50 monoamines and cyclic capacities are extracted using the thermodynamic model, to asses the potential of these molecules for CO(2) capture.

Experimental and molecular simulation investigation of enhanced CO2 solubility in hybrid adsorbents.

Hybrid adsorbents are prepared by confining physical solvents (propylene carbonate, N-methyl-2-pyrrolidone) within the porosity of a solid support (alumina) using both wet and dry impregnation methods. The resulting hybrid solids are analyzed using characterization methods (N(2) adsorption isotherm, TGA) to ensure that a proper confinement of the solvent has been achieved. The hybrid adsorbents are then subsequently assessed for CO(2) capture by performing solubility measurements. An enhanced CO(2) solubility is observed with regard to the ones in the bulk solvent and in the raw solid. In a next step, grand canonical Monte Carlo simulations have been performed on a slit pore model to understand the microscopic mechanisms yielding the apparition of enhanced solubility. The presence of solvent molecules favors the layering of CO(2) within the pore, and the resulting local density profile is then markedly increased compared to one found in the raw adsorbent as more carbon dioxide molecules can be accommodated into the pore volume.

Exploration of molecular dynamics during transient sorption of fluids in mesoporous materials.

In recent years, considerable progress has been made in the development of novel porous materials with controlled architectures and pore sizes in the mesoporous range. An important feature of these materials is the phenomenon of adsorption hysteresis: for certain ranges of applied pressure, the amount of a molecular species adsorbed by the mesoporous host is higher on desorption than on adsorption, indicating a failure of the system to equilibrate. Although this phenomenon has been known for over a century, the underlying internal dynamics responsible for the hysteresis remain poorly understood. Here we present a combined experimental and theoretical study in which microscopic and macroscopic aspects of the relaxation dynamics associated with hysteresis are quantified by direct measurement and computer simulations of molecular models. Using nuclear magnetic resonance techniques and Vycor porous glass as a model mesoporous system, we have explored the relationship between molecular self-diffusion and global uptake dynamics. For states outside the hysteresis region, the relaxation process is found to be essentially diffusive in character; within the hysteresis region, the dynamics slow down dramatically and, at long times, are dominated by activated rearrangement of the adsorbate density within the host material.

Wetting of rings on a nanopatterned surface: a lattice model study.

We perform mean-field density functional theory calculations on a lattice model to study the wetting of a solid substrate decorated with a ring pattern of nanoscale dimensions. We have found three different liquid morphologies on the substrate: a ring morphology where the liquid covers the pattern, a bulge morphology where a droplet is forming on one side of the ring, and a morphology where the liquid forms a cap spanning the nonwetting disk inside the pattern. We investigate the relative stability of these morphologies as a function of the ring size, wall-fluid interaction, and temperature. The results found are in very good agreement with experiments and calculations performed on similar systems at a micrometer length scale. The bulge morphology has also been observed in Monte Carlo simulations of the lattice model. Our results show that (i) morphologies of wetting patterns previously observed on a much larger (microm) scale can also form on a nm length scale, (ii) whether or not this happens depends crucially on the size of the wettable pattern, and (iii) the wettable ring may only be partially wet by the bulge morphology of the fluid. This morphology is a result of a spontaneously broken symmetry in the system.

Collective dynamics near fluid phase transitions.

By means of molecular dynamics simulations, we calculate the intermediate scattering function F(k(axially),t) where k(||) is the wave number and t is the time. We focus on thermodynamic states in the vicinity of a fluid phase transition in bulk and confined systems which we locate in parallel Monte Carlo simulations in the grand canonical ensemble. As one approaches the limit of stability of the fluid (i.e., its spinodal) from either low- or high-density branches of a subcritical isotherm, F(k(axially),t) becomes increasingly long-range. The apparent lack of decorrelation in the metastable regime can be understood within the framework of a simple mean-field theory that links the long-range nature of F(k(axially),t) to a divergence of the ratio of isostress and isochoric heat capacities gamma. Our results suggest that as one approaches the spinodal the dynamic structure factor S(k(axially),omega) (omega frequency), which is related to F(k(axially),t) through a Laplace transformation, should undergo a qualitative change from the usual triplet of Brillouin and Rayleigh lines to a singlet (delta-function-like peak) centered at omega=0 for states directly at the spinodal. This qualitative change in S(k(axially),omega) should be measurable in scattering experiments thereby promoting more detailed insight into the phase behavior and thermodynamic stability of confined and bulk fluids.

Propagating hydrodynamic modes in confined fluids.

In molecular dynamics simulations in the microcanonical ensemble (MEMD) we calculate the intermediate scattering function F(k(||),t) for a "simple" fluid confined to nanoscopic slit pores with chemically homogeneous, planar substrate surfaces. Since system properties are translationally invariant in the x-y plane, we focus on the propagation of density modes parallel with the confining substrates by choosing a two-dimensional wave vector |k(||)|=k(||)=(k(x),k(y)) for our analysis. Within the framework of classical hydrodynamics, we develop conservation laws for z-averaged fluxes of heat and momentum. Using in-plane versions of the macroscopic stress tensor and internal-energy current as constitutive equations we derive an expression for F(k(||),t) in the hydrodynamic limit depending on the thermal diffusivity D(T), the sound attenuation coefficient Gamma, the in-plane adiabatic velocity of sound v(||), and the ratio of heat capacities at constant transverse stress and volume gamma. Through a fit of F(k(||),t) in the hydrodynamic limit and its associated memory function M(k(||),t) to MEMD data, reliable values for the set [D(T),Gamma,v(||),gamma] of material coefficients can be obtained. Variations in [D(T),Gamma,v(||),gamma] with s(z) may be correlated with variations in the solvation pressure -tau(zz)-P(b) with s(z) (tau(zz) is the stress exerted by the fluid along the surface normal and P(b) is the bulk pressure) and therefore linked to stratification of the confined fluid.