Simulation of adsorption of liquid mixtures of N2 and O2 in a model faujasite cavity at 77.5 K

Abstract

Grand canonical Monte Carlo simulations of adsorption of N 2 and 02 and their mixtures in a model zeolitic cavity 14 A in diameter were performed at 77.5 K for pressures ranging from zero up to saturation, where the adsorbed phase is in equilibrium with coexisting vapor and liquid phases. The same intermolecular potential functions were employed for gas-gas interactions in the vapor, liquid, and adsorbed phases. The gas-solid interaction potential includes dispersion-repulsion energy, induced electrostatic energy, and an ion-quadrupole term to model the interaction of the electric field in zeolites like NaX with polar molecules like N 2 . The simulation of the coexisting vapor and liquid phases reproduces the saturation properties of pure liquid oxygen and nitrogen at 77.5 K. Activity coefficients in the adsorbed phase derived from simulations as a function of cavity filling and composition show negative deviations from Raoult's law, even though the non-idealities in the bulk liquid phase have the opposite sign. The simulation of the surface excess isotherm for adsorption from liquid mixtures exhibits preferential adsorption of N 2 and has the commonly-observed quadratic shape skewed toward the more strongly adsorbed component. Micropore condensation is observed for oxygen but not for nitrogen. The condensation of oxygen is similar to a first order phase transition but because of the small number of molecules that can fit into a micropore, coexistence of the two phases is replaced by oscillations between gas-and liquid-like densities.

Adsorption of CH4−CF4 Mixtures in Silicalite: Simulation, Experiment, and Theory

“…This was explained with a partial exclusion of the larger molecules from portions of the micropore volume. A negative deviation from Raoult's law in the adsorbed mixture was also found in a simulation of adsorption of liquid N 2 and O 2 in a model NaX zeolite cavity at 77.5 K …”

“…This was explained with a partial exclusion of the larger molecules from portions of the micropore volume. A negative deviation from Raoult's law in the adsorbed mixture was also found in a simulation of adsorption of liquid N 2 and O 2 in a model NaX zeolite cavity at 77.5 K. 16 Adsorption of an equimolar gas mixture of nitrogen and methane in zeolite Y was studied by Maddox and Rowlinson, 17 who used a two-site LJ model for nitrogen and a five-site LJ model for methane. They found that methane is adsorbed preferentially and that selectivity was almost independent of the pressure.…”

Grand Canonical Monte Carlo Simulations of Nonrigid Molecules: Siting and Segregation in Silicalite Zeolite

“…Molecular simulation offers an attractive means to investigate siting and segregation effects in molecular sieves because a single simulation can yield both the macroscopic adsorption thermodynamics and the microscopic siting of the molecules. Adsorption isotherms and heats of adsorption have been calculated for a variety of systems using grand canonical Monte Carlo (GCMC) simulations. − Many of these studies have focused on silicalite, which has a set of straight channels intersected by a set of zigzag channels. Several groups have seen that small molecules, such as methane or xenon, prefer the channels over the intersections. , June et al found that at low loading linear alkanes adsorb in the channels but branched alkanes prefer the roomier channel intersections.…”

Critical Point Corrections for Two- and Three-Dimensional Systems

“…Modeling phase behavior of fluids at interfaces − is necessary to understand molecular mechanisms of surface tension in liquids and adsorption behavior at surfaces and interfaces. , Important features of interfaces include density gradients and oscillatory density profiles near the surfaces of liquids − and the interrelationship between two- and three-dimensional behaviors. − The role of dimensionality is not limited to geometry of the system but also includes correlation effects on heterogeneous surfaces , and in nanopores. − Dimensionality also affects critical phenomena including long-range critical fluctuations and universal scaling laws…”