Zeolite adsorption, simulations method

Cation distribution in zeolitic structures is one of the key aspects to the understanding of the adsorption mechanisms and selectivities. Many experimental and simulation methods have been used to try to localise the cations. The present work confronts the different analytical methods and gives general distribution trends in accordance with results from the literature. 

The complexity of xylene adsorption over zeolites is too high to predict the selectivity from the chemical properties of the zeolite only (electronegativity of the cations, charge of the framework oxygens). The interactions between xylenes and the zeolite must necessarily be considered, which explains the important development of molecular simulation methods. This is supported by the work of V. Lachet et al. (18) who succeeded in reproducing the inversion of selectivity between KY and NaY with Grand Canonical Monte Carlo Simulations. 

Molecular simulation has now become powerfrd means for the study of adsorbed molecules in high silica zeolites, and GCMC method is especially useful for predicting adsorption equilibria. However, information on forcefield parameters and charges are often inadequate, even in systems where the structure is well known. 

The experimental methods of dilfraetion and spectroscopy are uniquely applicable to the study of crystalhne microporous solids and their chemistry. Nevertheless, there are important aspects of zeolite science that are not readily accessible to these techniques the species involved in nucleation and crystal growth, the structure of sites (often present at low concentration) that are active for adsorption and catalysis or the reaction intermediates present in catalysis. In these cases computational atomistic simulation offers great possibilities for improved understanding. Furthermore, many experimental measurements, such as calorimetric studies of heats of adsorption, and NMR or neutron scattering studies of dynamics, may be very expensive and time-consuming. Computer simulation methods, which promise to predict the performance of materials as adsorbents and catalysts rapidly and at reasonable expense, are therefore highly attractive. Excellent recent texts and useful reviews are available that deal with the simulation of microporous materials. Here I summarise the most widely used methods and the information they give. 

Other hand, have modeled adsorption in MCM-41-type zeolite with NLDFT methods. Their conclusion is that the adsorption branch corresponds to the spinodal condensation, i.e. metastable situation, and the desorption branch corresponds to the equilibrium capillary condensation/evaporation situation. Kowalczyk et al. [8], have calculated the hysteresis using a lattice density functional theory. The basis of their work stems from similar simulations by... 

The adsorption and diffusion properties of benzene are of immense interest in zeolite research aromatics play important roles in a number of zeolite-catalyzed processes. Theoretical simulations of benzene diffusion first began to be published in the late 1980s. The first studies evaluated and minimized the potential energy of a molecule such as benzene within the channels, a method less computationally demanding than the MD simulations that followed. Most recent studies have used the TST formalism. 

Yashonath etal. (46) used a Metropolis Monte Carlo method to simulate the infinite-dilution adsorption of methane in NaY zeolite. The lattice had a Si/Al ratio of 3.0 and was treated as rigid, whereas methane was modeled... 

Snurr et al. (192) used biased GC-MC simulations to predict isotherms, isosteric heats of adsorption, and locations of benzene and p-xylene at various concentrations. The suitability of a bias method is clear, at low coverages to prevent trial insertions overlapping with the zeolite walls and at high coverages to prevent overlap with other sorbate molecules. (Slightly different bias schemes were used for the two extremes of concentrations.) Interactions between sorbates and zeolites—both of which were considered to be rigid—were modeled with parameters from the literature (79, 87). Electrostatic interactions were included to account for the quadrupole moment of the sorbates. Sorbate-sorbate interaction parameters were taken from Shi and Bartell (194) for benzene and from Jorgensen et al. (195) for p-xylene. 

For the molecules investigated, the MD and MC methods furnish similar adsorption energies although the MD results are slightly better when compared to the experiments. Since the MC and MD simulations have been performed in different ensembles but with the same force field, the difference in the results of the two simulations may be partly due to finite size effects. However, temperature fluctuations during the MD simulations in the NVE ensemble may also contribute to this difference. For the linear alkanes the MM results are qualitatively correct but only when a specific force field is used to describe the zeolite. However, for the branched alkane the MM results are comparable to the MD and MC ones even when a generic force field, such as Dreiding n, is used to represent the zeolite. 

Recoveiy of solvent vapor (dicfaloromethane, CH2CI2) fiom air by pressure swing adsorption (PSA) was studied, using two columns packed with high silica zeolite and resin as adsorbent Gravimetric measurements wete made for dichloromethane on the adsorbents. Computer calculations were carried out to simulate the experimental results using the Stop-Go method to show the calculated results coincide well with experimental results. The method is useful to predict the performance of a solvent recovery system operated by PSA... 

In this study, equilibria and isosteric heat of adsorption for the system of chlorinated hydrocarbons and Y-type zeolite were obtained with gravimetric method and chromatographic method. By comparing an experiment result with a molecular simulation result, the validity of forcefield parameters and zeohte model was exartuned... 

In the grand-canonical Monte Carlo method, the system volume, temperature, and chemical potential are kept fixed, while the number of particles is allowed to fluctuate.There exist three types of trial move (1) displacement of a particle, (2) insertion of a particle, and (3) removal of a particle. These trial moves are generated at random with equal probability. The acceptance probability of the Metropolis method can be used for the trial moves of type (1). For the two other types, the acceptance probabilities are different. Regarding zeolites, an adsorption isotherm can be calculated with the grand-canonical Monte Carlo method by running a series of simulations at varying chemical potentials. 

Grand canonical ensemble Monte Carlo simulations of the adsorption properties of several model faujasite zeolites were performed using the statistical bias method. The results enable a better understanding of the effect of cation exchange in the selective adsorption of binary mixtures of para and meta xylene isomers. We predict that adding a small amount of water molecules could enhance the adsorption selectivity in favour ofp-xylene. 

In this paper, we present an exact calculation of the statistical mechanics of a lattice model of hydrocarbon adsorption in the quasi one-dimensional pores of zeolites, based on a matrix method that utilises the Constant Pressure partition. The model is tested on benzene adsorption, where it reproduces experimentally observed steps in isotherms. The model has been extended also to linear alkanes where it reproduces very accurately experimental adsorption isotherms as well as Monte-Carlo simulation results of ethane. 

Such methods of analysing and describing adsorption data have considerable merit in describing microporosity in porous carbons, which are not crystalline, or for microporous solids of unknown structure, but for zeolites of known structure they add little to our understanding. In such cases, the form of the adsorption isotherms can be modelled by computer simulation using Grand Canonical Monte Carlo methods. In this approach all the parameters are known or can be measured or calculated (see Section 4.5.1) so that the adsorption isotherm can be simulated using a physically well-characterised model. 

The methods of computer simulation of adsorption (and diffusion) in micro-porous solids were described in Chapter 4 a summary is given in Table 4.1. These techniques are now sufficiently well developed for physisorption that thermodynamic properties can be predicted routinely for relatively simple adsorbates, once the structural details of the host are known. Molecular mechanics using standard forcelields are very successful for zeolitic systems, which take into account dispersive interactions satisfactorily, but it is also possible to use higher level calculations. 

Diffraction studies have been used in a small number of cases to determine the minimum energy positions of adsorbed moleeules within cationic zeolites. These difficult and relatively expensive experiments have been used to establish computational simulation as a reliable method to predict such interactions without recourse to experimentation. Crystallographic X-ray studies of the adsorption of small moleeules sueh as CO and NO on cationic forms of zeolite are of particular relevance to study of eation-molecule interactions, as are neutron powder diffraction studies of the location of pyridine, coordinatively bound to potassium ions in zeolite K-L, and of benzene, bound to sodium ions in zeolite Na-X. ... 

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