Modeling adsorption properties on the basis of microscopic, molecular, and structural descriptors for nonpolar adsorbents.

We propose a method for analytically predicting single-component adsorption isotherms from molecular, microscopic and structural descriptors of the adsorbate-adsorbent system and concepts of statistical thermodynamics. Expressions for Henry's constant and the heat of adsorption at zero coverage are derived. These functions depend on the pore size, pore shape, chemical composition, and density of the adsorbent material. They quantify the strength of the solid-fluid interaction, which governs the low-pressure part of the adsorption isotherm. For intermediate and high pressures, the fluid-fluid interactions must also be taken into account. Both solid-fluid and fluid-fluid interactions are combined within the framework of the Ruthven statistical model (RSM). The RSM thus constructs theoretical adsorption isotherms that are entirely based on microscopic molecular and structural descriptors. The theoretical results that we obtained are compared with experimental data for the adsorption of pure CO2 and CH4 on all-silica zeolites. The developed methodology allows for the estimation of the optimum properties of a nonpolar adsorbent for the adsorption of CO2 in cyclic adsorption processes.

Quantification of the confinement effect in microporous materials.

The confinement effect plays a key role in physisorption in microporous materials and many other systems. Confinement is related to the relationship between the pore geometry (pore size and topology) and the geometry of the adsorbed molecule. Geometric properties of the porous solid can be described using the concepts of Gaussian and mean curvatures. In this work we show that the Gaussian and mean curvatures are suited descriptors for mathematically quantifying the confinement of small molecules in porous solids. A method to determine these geometric parameters on microporous materials is presented. The new methodology is based on the reconstruction of the solid's accessible surface. Then, a numerical calculation of the Gaussian and mean curvatures is carried out over the reconstructed mesh. On the one hand, we show that the local curvature can be used to identify the most favourable adsorption sites. On the other hand, the global mean curvature of the solid is correlated to the heat of adsorption of CO2 and CH4 on several zeolites and MOFs. A theoretical justification for this empirical correlation is provided. In conclusion, our methodology allows for a semi-quantitative estimation of confinement, applicable to any pore geometry, independent of the chemical composition, and without the need for applying a force field.

New insight into the ZnO sulfidation reaction: mechanism and kinetics modeling of the ZnS outward growth.

Zinc oxide based materials are commonly used for the final desulfurization of synthesis gas in Fischer-Tropsch based XTL processes. Although the ZnO sulfidation reaction has been widely studied, little is known about the transformation at the crystal scale, its detailed mechanism and kinetics. A model ZnO material with well-determined characteristics (particle size and shape) has been synthesized to perform this study. Characterizations of sulfided samples (using XRD, TEM and electron diffraction) have shown the formation of oriented polycrystalline ZnS nanoparticles with a predominant hexagonal form (wurtzite phase). TEM observations also have evidenced an outward development of the ZnS phase, showing zinc and oxygen diffusion from the ZnO-ZnS internal interface to the surface of the ZnS particle. The kinetics of ZnO sulfidation by H(2)S has been investigated using isothermal and isobaric thermogravimetry. Kinetic tests have been performed that show that nucleation of ZnS is instantaneous compared to the growth process. A reaction mechanism composed of eight elementary steps has been proposed to account for these results, and various possible rate laws have been determined upon approximation of the rate-determining step. Thermogravimetry experiments performed in a wide range of H(2)S and H(2)O partial pressures have shown that the ZnO sulfidation reaction rate has a nonlinear variation with H(2)S partial pressure at the same time no significant influence of water vapor on reaction kinetics has been observed. From these observations, a mixed kinetics of external interface reaction with water desorption and oxygen diffusion has been determined to control the reaction kinetics and the proposed mechanism has been validated. However, the formation of voids at the ZnO-ZnS internal interface, characterized by TEM and electron tomography, strongly slows down the reaction rate. Therefore, the impact of the decreasing ZnO-ZnS internal interface on reaction kinetics has been taken into account in the reaction rate expression. In this way the void formation at the interface has been modeled considering a random nucleation followed by an isotropic growth of cavities. Very good agreement has been observed between both experimental and calculated rates after taking into account the decrease in the ZnO-ZnS internal interface.

Generic postfunctionalization route from amino-derived metal-organic frameworks.

This study deals with the development of a soft, generic, one-pot postfunctionalization method for metal-organic frameworks (MOFs) starting from compounds with an amino group on the linker. The first step consists of transforming the amino group into azide (N(3)) by an unconventional route using tBuONO and TMSN(3). In the same vessel, the desired functionalized MOF then is obtained by the Huisgen 1,3-dipolar cycloaddition of azides to alkynes, otherwise known as the "click" reaction. The method was applied to DMOF-NH(2) and MIL-68(In)-NH(2), which represent two distinct and important classes of MOF. For both, the functionalization was complete (>90% grafting) and the crystallinity was maintained. Thanks to the large diversity and availability of cyano- and acetylene-based chemicals, this method opens the door to tailor-made functionalized MOFs.

Adsorption of CO(2), CH(4), and N(2) on zeolitic imidazolate frameworks: experiments and simulations.

Experimental measurements and molecular simulations were conducted for two zeolitic imidazolate frameworks, ZIF-8 and ZIF-76. The transferability of the force field was tested by comparing molecular simulation results of gas adsorption with experimental data available in the literature for other ZIF materials (ZIF-69). Owing to the good agreement observed between simulation and experimental data, the simulation results can be used to identify preferential adsorption sites, which are located close to the organic linkers. Topological mapping of the potential-energy surfaces makes it possible to relate the preferential adsorption sites, Henry constant, and isosteric heats of adsorption at zero coverage to the nature of the host-guest interactions and the chemical nature of the organic linker. The role played by the topology of the solid and the organic linkers, instead of the metal sites, upon gas adsorption on zeolite-like metal-organic frameworks is discussed.

Anisotropic united-atoms (AUA) potential for alcohols.

An anisotropic united-atom (AUA4) intermolecular potential has been derived for the family of alkanols by first optimizing a set of charges to reproduce the electrostatic potential of the isolated molecules of methanol and ethanol and then by adjusting the parameters of the OH group to fit selected equilibrium properties. In particular, the proposed potential includes additional extra-atomic charges in order to improve the matching to the electrostatic field. Gibbs ensemble Monte Carlo simulations were performed to determine the phase equilibria, while the critical region was explored by means of grand canonical Monte Carlo simulations combined with histogram reweighting techniques. In order to increase the transferability of the model, only the parameters of the Lennard-Jones OH group have been fitted, the parameters of the other AUA groups are taken from previous works. Nevertheless, a good level of agreement was obtained for all compounds considered in this work. In particular, excellent results were obtained for the Henry constants calculation of different gases in alkanols.

An anisotropic united atoms (AUA) potential for thiophenes.

The optimization of the parameters of the Lennard-Jones (LJ) 12-6 potential of the sulfur atom in thiophene has allowed the AUA 4 potential to be successfully extended to alkyl and polythiophenes. Monte Carlo Gibbs ensemble and grand canonical simulations combined with histogram reweighting techniques have been performed to investigate the resulting phase equilibrium and the critical region of different molecules of this family in order to test the proposed potential. Excellent agreement with experimental densities, enthalpies of vaporization, and saturation pressures has been obtained in most of the cases. In particular, the critical point of our model for thiophene has been located with a statistical precision of less than 0.1% and is within 1% of the experimental value. The calculation of the critical points has been made through a recently implemented methodology based on the calculation of a fourth order cumulant (Binder parameter) combined with the use of finite size scaling methods, allowing the critical points to be located in a straightforward and accurate way.

Optimized intermolecular potential for aromatic hydrocarbons based on anisotropic united atoms. III. Polyaromatic and naphthenoaromatic hydrocarbons.

In this third article of the series, a new anisotropic united atoms (AUA) intermolecular potential parameter set has been proposed for the carbon force centers connecting the aromatic rings of polyaromatic hydrocarbons to predict thermodynamic properties using both the Gibbs ensemble and NPT Monte Carlo simulations. The model uses the same parameters as previous AUA models used for the aromatic CH force centers. The optimization procedure is based on the minimization of a dimensionless error criterion incorporating various thermodynamic data of naphthalene at 400 and 550 K. The new model has been evaluated on a series of polyaromatic and naphthenoaromatic hydrocarbons over a wide range of temperatures up to near-critical conditions. Vaporization enthalpy, liquid density, and normal boiling temperature are reproduced with good accuracy. The new potential parameters have also been tested successfully on toluene, 1,3,5-trimethylbenzene, styrene, m-xylene, n-hexylbenzene, and n-dodecylbenzene to demonstrate their transferability to alkylbenzenes.

A transferable force field to predict phase equilibria and surface tension of ethers and glycol ethers.

We propose a new transferable force field to simulate phase equilibrium and interfacial properties of systems involving ethers and glycol ethers. On the basis of the anisotropic united-atom force field, only one new group is introduced: the ether oxygen atom. The optimized Lennard-Jones (LJ) parameters of this atom are identical whatever the molecule simulated (linear ether, branched ether, cyclic ether, aromatic ether, diether, or glycol ether). Accurate predictions are achieved for pure compound saturated properties, critical properties, and surface tensions of the liquid-vapor interface, as well as for pressure-composition binary mixture diagrams. Multifunctional molecules (1,2-dimethoxyethane, 2-methoxyethanol, diethylene glycol) have also been studied using a recently proposed methodology for the calculation of the intramolecular electrostatic energy avoiding the use of additional empirical parameters. This new force field appears transferable for a wide variety of molecules and properties. It is furthermore worth noticing that binary mixtures have been simulated without introducing empirical binary parameters, highlighting also the transferability to mixtures. Hence, this new force field gives future opportunities to simulate complex systems of industrial interest involving molecules with ether functions.

Thermodynamic and transport properties of carbon dioxide from molecular simulation.

Monte Carlo and molecular dynamics simulations have been used in order to test the ability of a three center intermolecular potential for carbon dioxide to reproduce literature experimental thermophysical values. In particular, both the shear viscosity under supercritical conditions and along the phase coexistence line, as well as the thermal conductivity under supercritical conditions, have been calculated. Together with the already reported excellent agreement for the phase coexistence densities, the authors find that the agreement with experimental values is, in general, good, except for the thermal conductivity at low density. Although extended versions of the model were employed, which include an explicit account of bending and vibrational degrees of freedom, a significant difference was still found with respect to the reported experimental value.

Critical point estimation of the Lennard-Jones pure fluid and binary mixtures.

The apparent critical point of the pure fluid and binary mixtures interacting with the Lennard-Jones potential has been calculated using Monte Carlo histogram reweighting techniques combined with either a fourth order cumulant calculation (Binder parameter) or a mixed-field study. By extrapolating these finite system size results through a finite size scaling analysis we estimate the infinite system size critical point. Excellent agreement is found between all methodologies as well as previous works, both for the pure fluid and the binary mixture studied. The combination of the proposed cumulant method with the use of finite size scaling is found to present advantages with respect to the mixed-field analysis since no matching to the Ising universal distribution is required while maintaining the same statistical efficiency. In addition, the accurate estimation of the finite critical point becomes straightforward while the scaling of density and composition is also possible and allows for the estimation of the line of critical points for a Lennard-Jones mixture.

Histogram reweighting method for dynamic properties.

The histogram reweighting technique, widely used to analyze Monte Carlo data, is shown to be applicable to dynamic properties obtained from molecular dynamics simulations. The theory presented here is based on the fact that the correlation functions in systems in thermodynamic equilibrium are averages over initial conditions of trajectory functions, the latter depending on the volume of the system, the total number of particles, and the classical Hamiltonian. Thus, the well-known histogram reweighting method can be almost straightforwardly applied to reconstruct the probability distribution of initial states at different thermodynamic conditions, without extra computational effort. Correlation functions and transport coefficients are obtained with this method from few simulation data sets.