Flor R. Siperstein

Comparison of experimental techniques for measuring isosteric heat of adsorption

Experimental measurements of heats of adsorption published in the literature are often in disagreement; differences of 10–20% are common. The three most widely used experimental methods are: (1) differentiation of adsorption isotherms at constant loading; (2) measurement of adsorption isosteres; (3) calorimetry. Results from these methods were compared for the systems nitrogen on CaA, oxygen on CaA, and carbon dioxide on NaX. Although the same materials and similar degassing procedures were used for all experiments, calorimetric heats are about 2 kJ/mol higher than the heats from isoteric measurements. Additional experiments are needed to bring these methods into exact agreement.

Molecular modeling of freezing of simple fluids confined within carbon nanotubes

We report Monte Carlo simulation results for freezing of Lennard-Jones carbon tetrachloride confined within model multiwalled carbon nanotubes of different diameters. The structure and thermodynamic stability of the confined phases, as well as the transition temperatures, were determined from parallel tempering grand canonical Monte Carlo simulations and free-energy calculations. The simulations show that the adsorbate forms concentric molecular layers that solidify into defective quasi-two-dimensional hexagonal crystals. Freezing in such concentric layers occurs via intermediate phases that show remnants of hexatic behavior, similar to the freezing mechanism observed for slit pores in previous works. The adsorbate molecules in the inner regions of the pore also exhibit changes in their properties upon reduction of temperature. The structural changes in the different regions of adsorbate occur at temperatures above or below the bulk freezing point, depending on pore diameter and distance of the adsorbate molecules from the pore wall. The simulations show evidence of a rich phase behavior in confinement; a number of phases, some of them inhomogeneous, were observed for the pore sizes considered. The multiple transition temperatures obtained from the simulations were found to be in good agreement with recent dielectric relaxation spectroscopy experiments for CCl(4) confined within multiwalled carbon nanotubes.

Cosurfactant and cosolvent effects on surfactant self-assembly in supercritical carbon dioxide

The impact of alcohol additives on the self-assembly of surfactants in supercritical carbon dioxide is investigated using lattice Monte Carlo simulations. We observe that all studied (model) alcohols reduce the critical micelle concentration. The reduction is stronger the longer the hydrocarbon chain of the alcohol, and the higher the alcohol concentration. Short-chain alcohols are found to concentrate in the surfactant layer of the aggregates, replacing surfactant molecules and leading to a strong decrease of the aggregation number and a large increase of the number of aggregates. On the other hand, only a small number of alcohol molecules with longer chain length are found in the aggregates, leading to a slight increase in the aggregation number. However, structural properties such as size and density profiles of aggregates at the same aggregation number are not influenced markedly. Consequently, short-chain alcohols act as cosurfactants, directly influencing the properties of the aggregates, while alcohols with longer hydrocarbon chains work as cosolvents, altering the properties of the solvent. However, the transition between both extremes is gradual.

Thermal properties of supercritical carbon dioxide by Monte Carlo simulations

We present simulation results for the volume expansivity, isothermal compressibility, isobaric heat capacity, Joule-Thomson coefficient and speed of sound for carbon dioxide (CO 2 ) in the supercritical region, using the fluctuation method based on Monte Carlo simulations in the isothermal-isobaric ensemble. We model CO 2 as a quadrupolar two-center Lennard-Jones fluid with potential parameters reported in the literature, derived from vapor-liquid equilibria (VLE) of CO 2 . We compare simulation results with an equation of state (EOS) for the two-center Lennard-Jones plus point quadrupole (2CLJQ) fluid and with a multiparametric EOS adjusted to represent CO 2 experimental data. It is concluded that the VLE-based parameters used to model CO 2 as a quadrupolar two-center Lennard-Jones fluid (both simulations and EOS) can be used with confidence for the prediction of thermodynamic properties, including those of industrial interest such as the speed of sound or Joule-Thomson coefficient, for CO 2 in the supercritical region, except in the extended critical region.

Dual Control Cell Reaction Ensemble Molecular Dynamics:A Method for Simulations of Reactions and Adsorption in Porous Materials

We present a simulation tool to study fluid mixtures that are simultaneously chemically reacting and adsorbing in a porous material. The method is a combination of the reaction ensemble Monte Carlo method and the dual control volume grand canonical molecular dynamics technique. The method, termed the dual control cell reaction ensemble molecular dynamics method, allows for the calculation of both equilibrium and nonequilibrium transport properties in porous materials such as diffusion coefficients, permeability, and mass flux. Control cells, which are in direct physical contact with the porous solid, are used to maintain the desired reaction and flow conditions for the system. The simulation setup closely mimics an actual experimental system in which the thermodynamic and flow parameters are precisely controlled. We present an application of the method to the dry reforming of methane reaction within a nanoscale reactor model in the presence of a semipermeable membrane that was modeled as a porous material similar to silicalite. We studied the effects of the membrane structure and porosity on the reaction species permeability by considering three different membrane models. We also studied the effects of an imposed pressure gradient across the membrane on the mass flux of the reaction species. Conversion of syngas (H2/CO) increased significantly in all the nanoscale membrane reactor models considered. A brief discussion of further potential applications is also presented.

Synthesis and Characterization of Templated Mesoporous Materials Using Molecular Simulation

Abstract Lattice Monte Carlo simulations are used to calculate equilibrium properties of surfactant-solvent-silica liquid-crystal systems under no-polymerization conditions. The formation of a high-surfactant high-silica concentration phase in equilibrium with a dilute phase is observed when the surfactant-silica interactions are stronger than the surfactant-solvent interactions. Different silica structures that are similar to the M41 family are observed, depending on the overall concentration of the system. The formation of a hexagonal phase is favored at a surfactant/silica ratio of 0.2, whereas a lamellar phase is observed a surfactant/silica ratio of 1. Argon adsorption properties on a model porous structure of the MCM-41 type prepared using this mimetic simulation protocol are calculated using grand canonical Monte Carlo simulation. Heats of adsorption are calculated from fluctuations in the energy and number of molecules [1] following the work of Nicholson and Parsonage [Computer Simulation and the Statistical Mechanics of Adsorption (Academic Press, London), 1982, p 97 8 pp]. A decrease in the heats of adsorption for coverage less than one statistical monolayer is evidence of surface heterogeneity. The results are in qualitative agreement with experimental measurements for argon on MCM-41.

Long range corrections for computer simulations of adsorption

Long range corrections are routinely applied to simulations of bulk fluids by assuming that the radial distribution function is unity beyond a certain cutoff radius for pairwise interactions. Similar long range corrections for gas-solid interactions in adsorption frequently are ignored because of the anisotropic structure of the solid. However, the error associated with assuming isotropy beyond the cutoff radius is small compared with the magnitude of the long range correction. The long range correction to the Henry constant for a cutoff radius of 13 Å is 14% for CH4 and 70% for SF6 for adsorption in silicalite at 298 K. The large errors incurred by neglecting long range corrections can be concealed by increasing the well depth of the gas-solid interaction, but this approximation reduces the accuracy and portability of the potential parameters. Consistency in the cutoff radius is more important than the inclusion or neglect of long range corrections to the energy.

Characterization of adsorbents by energy profile of adsorbed molecules

Abstract The energy profile, which is the energy of the adsorbed phase as a function of loading, is essential for characterizing the equilibrium behavior of adsorbents. The energy of the adsorbed phase is measured directly by calorimetry or indirectly by differentiating adsorption isotherms at constant loading. The adsorption isotherm and energy profile are sufficient for the determination of thermodynamic variables such entropy and provide a basis for predicting the behavior of nonideal, multicomponent mixtures from single-gas isotherms.

A Monte Carlo study of capillary condensation of krypton within realistic models of templated mesoporous silica materials

We report molecular simulations of Kr adsorption at 87 and 100 K in three atomistic silica mesopores with an average pore diameter of 6.4 nm: (a) a pore that keeps the morphological features of a MCM-41 mesoscale model, generated from lattice Monte Carlo simulations mimicking its fabrication process; (b) a smooth, regular cylindrical pore; and (c) a cylindrical pore with constriction. Surface roughness and structural defects significantly affect Kr adsorption: marked differences were observed in the adsorption isotherms, isosteric heat curves and pore filling mechanisms for the three pore models. Our results suggest that the molecular-level surface disorder for the first pore model is too high, but its roughness at larger length scales (10-50 A), as determined from simulated SANS spectrum, is in agreement with experimental results. The dense phase of Kr inside the three pore models exhibits a liquid-like global structure, even though the temperatures considered are well below the bulk triple point.

Pore size distribution in microporous carbons obtained from molecular modeling and density functional theory

We present the pore size distribution (PSD) for activated carbons obtained from lignin pyrolysis and KOH activation. A comparison of the PSDs obtained from integral inversion of the adsorption isotherms generated using molecular simulation and density functional theory are compared in order to identify the differences observed when using different models to represent the adsorbate: single or two center Lennard-Jones models. Fair agreement is obtained when pores larger than 2 nm are present, despite the different models used. Only important differences are observed for porous carbons with pore sizes smaller than 1 nm.