Quantum Chemical Study of Low-pressure Adsorption in Zeolitic Materials (original) (raw)
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Mixed Gas Equilibrium Adsorption on Zeolites and Energetic Heterogeneity of Adsorption Volume
Langmuir
A model for binary mixture adsorption accounting for energetic heterogeneity and intermolecular interactions is proposed in this paper. The model is based on statistical thermodynamics, and it is able to describe molecular rearrangement of a mixture in a nonuniform adsorption field inside a cavity. The Helmholtz free energy obtained in the framework of this approach has upper and lower limits, which define a permissible range in which all possible solutions will be found. One limit corresponds to a completely chaotic distribution of molecules within a cavity, while the other corresponds to a maximum ordered molecular structure. Comparison of the nearly ideal O2-N2-zeolite NaX system at ambient temperature with the system of O2-N2-zeolite CaX at 144 K has shown that a decrease of temperature leads to a molecular rearrangement in the cavity volume, which results from the difference in the fluid-solid interactions. The model is able to describe this behavior and therefore allows predicting mixture adsorption more accurately compared to those assuming energetic uniformity of the adsorption volume. Another feature of the model is its ability to correctly describe the negative deviations from Raoult's law exhibited by the O2-N2-CaX system at 144 K. Analysis of the highly nonideal CO2-C2H6-zeolite NaX system has shown that the spatial molecular rearrangement in separate cavities is induced by not only the ionquadrupole interaction of the CO2 molecule but also the significant difference in molecular size and the difference between the intermolecular interactions of molecules of the same species and those of molecules of different species. This leads to the highly ordered structure of this system.
An ab initio study of adsorption related properties of diatomic molecules in zeolites
A perturbational approach (M-method) for the calculation of the interaction energies of N 2 , O 2 , Ar and CO in zeolite cavities is presented. The calculation method is compared with the 'molecule in point charge environment' calculation and a full ab initio calculation tested along an axis connecting the center of the cavity with a site II cation. The Henry constants, heats of adsorption and separation constants for N 2 , O 2 and Ar in a NaY zeolite are obtained using the 'molecule in point charge environment' method and the present approximation. Different grid sizes were considered depending on the distance between adsorbing system and cation. A promising result, in particular for the separation constants, was found using the Van der Waals and ionic radii for the grid delimitation and the M-method calculated on the B3LYP/6-31G * level. CO was added as a final test system with lower symmetry.
Zeolites, 1983
Equations describing the adsorption of simple gases in zeolites are developed, on the base of the model of localized adsorption with interactions between nearest neighbours of admolecules. It is assumed, that the distribution of adsorption energy inside the zeolites can be approximated by a skew-Gaussian distribution, with a certain minimum and maximum value of the adsorption energy to be taken into account. The new theoretical approach used by us leads to an isotherm equation, which describes very well the experimental isotherms of NH3 on Na-X and Nail-Y, in a very large interval of temperatures. Our numerical analysis of these experimental isotherms suggests that the effects of the adsorption energy value on the vibrational partition function of NH3 admolecules are negligible. Also negligible are the effects of the interactions between N H 3 admolecules in these adsorption systems.
Modeling of Adsorption Properties of Zeolites: Correlation with the Structure
The Journal of Physical Chemistry B, 1997
The adsorption of N 2 and CO in Na X-zeolites has been studied for different framework structures and extraframework cation distributions. To this aim, the cation-molecule system modeling one site has been embedded in a set of external point charges which simulate the zeolite environment of the site and has been treated quantum chemically, using a method based on density functional theory. This procedure has been applied to the 64 cationic sites accessible for adsorption in a crystal unit cell of an ideal X-zeolite with a Si/Al ratio equal to 1. These calculations have shown that only a few cations are favorable for initial adsorption and that those cations are always of type III(III′). Their efficiency depends both on the framework geometry and on their location in the supercages. The analysis of the quantum chemical results in terms of a classical description involving electrostatic and induction interaction energies with the framework has led to the conclusion that the direction of the electric field vector created by the zeolite in the supercages is an important factor determining the zeolite adsorption properties.
Thermodynamic Study of Light Organic Molecules Adsorption onto ZK-4 Zeolite
ABSTRACT In this work, we were interested to the interactions of various light gases (critical temperature not exceeding 31 °C) with the zeolite ZK-4 and its varieties exchanged at temperatures close to ambient. The interest first practice since these gases are frequently encountered in industry, and most separations, that concern them by pressure swing adsorption (PSA) are carried at room temperature. By working in these conditions, we will be in the scope of application of Henry's law, which will allow us to better characterize the adsorption of the first molecules (gas-solid interaction). The thermodynamic study should enable us to identify the type of adsorption (localized, non-localized) by applying a theoretical model explaining the experimental results. The choice of the adsorbate was guided by the behavior of the introduced molecules toward the compensating cations present in the zeolite framework. These probe molecules having a specific interaction with zeolitic cations...
Entropic effects and isosteric heats of nitrogen and carbon dioxide adsorption on chabazite zeolites
Microporous and Mesoporous Materials, 2010
Equilibrium adsorption isotherms for CO 2 and N 2 were measured on a fully exchanged potassium chabazite (KCHA) zeolite, and partially exchanged sodium-chabazite (NaCHA) and lithium-chabazite (LiCHA) zeolites. This work presents integral thermodynamic functions and isosteric heats of N 2 and CO 2 adsorption equilibria on a fully exchanged potassium chabazite (KCHA) zeolite, and compares these to those functions measured on sodium exchanged chabazite (NaCHA) and lithium exchanged chabazite (LiCHA) zeolites. The thermodynamic quantities were calculated from equilibrium isotherms as function of loading. Adsorption of N 2 on KCHA was considered with distinct layers in inter-crystal spaces and mesopores volume, while CO 2 adsorption in KCHA cavities was more readily interpreted as fluid-like volume filling mechanism. The molecules comprising this fluid (CO 2) are in the dispersion energy field of the crystals and of each other, bounded also by considerable field-gradient quadrupole energy. In all cationic forms the adsorption equilibrium loading and differential enthalpies for CO 2 were considerably higher than those for N 2. The CO 2 integral entropy showed a rapid decreasing trend close to the saturation capacity of the cavities, demonstrating an appreciable loss of molecule degrees of freedom. The results of this work provide useful thermodynamic information that help in understanding the adsorption of N 2 and CO 2 on a fully exchanged KCHA zeolite.
Adsorption Equilibrium of Binary Mixtures in Zeolites and State of Adsorbed Phase
Adsorption, 1999
The criterion of ideal behavior of a mixture of a few molecules within a separate zeolite cavity is formulated on the basis of the statistical thermodynamics. The criterion determines the dependence of the Helmholtz free energy, internal energy, and entropy of a molecular aggregate on the ratio of the number of molecules of components 1 and 2. The similarity between this criterion and the criterion of ideal behavior for bulk solutions is shown. Expressions of excess thermodynamic functions of the molecular mixture in a cavity are obtained. The negative magnitude of these excess functions is proposed to be due to rearrangement of molecules under influence of energetic heterogeneity. The calculation procedure of the excess functions has been demonstrated for the system CO 2 -C 2 H 6 -zeolite NaX, the information of both isotherms and isosteric adsorption heats being used simultaneously. The approach offered allows the state of adsorbed mixture in a separate cavity to be analyzed from pure-component and multicomponent experimental data.
Applications of molecular simulations for separation and adsorption in zeolites
Microporous and Mesoporous Materials, 2017
Zeolites are fascinating and versatile materials which are vital for a wide range of industries, due to their unique structural and chemical properties, which are the basis of applications in gas separation, ion exchange, and catalysis. Given their economic impact, there is a powerful incentive for smart design of new materials with enhanced functionalities for maximizing their application performance. This review article intends to summarize the published reports on the applications of molecular simulation in adsorption, separation and diffusion. The theoretical aspects, adsorption thermodynamics, adsorption isotherm were comprehensively studied in relation to the adsorption applications and how the adsorbates' characteristics influence the adsorption. This review comprehensively discusses the theoretical and computational aspects of diffusion of pure components, long chain hydrocarbons or mixture diffusion, including the molecular dynamics simulations and kinetic Monte Carlo. Furthermore, the cation-zeoliteadsorbate interactions are thoroughly examined so as to elucidate the role of cations in zeolites applications and how the cation exchange influences structural dynamics and properties of zeolites. This study also focuses on the role of cations in gas/liquid adsorption and separations.
Isosteric Heats of Adsorption of N2O and NO on Natural Zeolites
Journal of the Mexican Chemical Society, 2019
We studied the capacities of three natural zeolites to adsorb N2O or NO using a glass high-vacuum volumetric system that permitted characterization of the energetics of the adsorption process. Adsorption equilibrium data were analyzed using the classical Freundlich equation and the Dual–Langmuir model. We employed the Clausius–Clapeyron relationship to calculate the isosteric heats of adsorption using the equilibrium data of the isotherms measured at 273.15 K and 293.15 K. The isosteric heats of reversible adsorption of both gases were smaller than the heats of total adsorption. The interaction energy of N2O with mordenite was larger than the interaction energies of N2O with either erionite or clinoptilolite. The interaction energy of NO was found to be largest with erionite.