The lattice energy of zeolites

In order to quantify this relation one needs information on the lattice energies of zeolites as a function of lattice topology and composition. 

In the next sections calculations of the lattice energies of zeolites via several different computational approaches will be reported. Each approach is approximate each has its own advantage(s). 

A number of techniques have been employed to model the framework structure of silica and zeolites (Catlow Cormack, 1987). Early attempts at calculating the lattice energy of a silicate assumed only electrostatic interactions. These calculations were of limited use since the short-range interactions had been ignored. The short-range terms are generally modelled in terms of the Buckingham potential,... 

To begin with, we shall discuss computations of the lattice energies of aluminum-free zeolites. We shall then go on to discuss the lattice energies of aluminum-containing zeolites. We shall conclude this paper with a discussion of the relevance of the results obtained to zeolite synthesis. 

The interatomic potentials define the force field parameters that contribute to the lattice energy of a relaxed or energy minimized structure. The fundamental question is how reliable is a force field The force field used in evaluating a potential function must be consistent and widely applicable to all similar systems. It must be able to predict the crystal properties as measured experimentally. Two main approaches, namely empirical and semi-empirical, are usually employed in the derivation of potential parameters. Empirical derivations involve a least square fitting routine where parameters are chosen such that the results achieve the best correlation with the observed properties. The semi-empirical approach uses an approximate formulation of the quantum mechanical calculations. An example of such an approximation is the electron gas method [57] which treats the electron density at any point as a uniform electron gas. The following is the analytical description of the potential energy function and interatomic potentials we recommend for use in simulation of zeolites and related system. 

Physicochemical properties of L zeolites and of clinoptilolite were studied by adsorption, chromatographic, spectral, and ther-mogravimetric methods. The sodium form of L zeolite is characterized by better adsorption with respect to water and benzene vapor and by higher retention volumes of C C hydrocarbons and CO than potassium and cesium forms. The activation energy of dehydration determined by the thermogravimetric method decreases on going from the sodium to cesium form of L zeolite. When calcium is replaced by potassium ions in clinoptilolite, the latter shows a decreased adsorption with respect to water vapor. The infrared spectra of the L zeolite at different levels of hydration show the existence of several types of water with different bond characters and arrangements in the lattice. 

It follows from the calculations on the lattice energy as a function of Al/Si ratio that synthesis of new wide-pore zeolites would require a different strategy. We have found that increasing... 

Here, is the potential energy of ith molecule located at u inside the zeolite cavity interacting with the zeolite lattice — r l) is the potential energy of interaction of the ith and /th molecules located, respectively, at the points and In the calculations of Vi) = 1 (jr — r j),... 

Another family of potential models takes into account the covalent character of chemical bonds in zeolite lattices more explicitly and uses for the PES representation a set of potential functions like those employed in molecular mechanics (MM) force fields for organic molecules. Such a force field describes the potential energy of the system in two terms ... 

Figure 2 The initial (I) and final (F) positions of the diffusion path as well as the minimum energy location for 1,2-DCB in zeolite-L. The active site of zeolite-L lattice which surrounds 1,2-DCB is marked by a circle.

The geometry of the molecules were obtained by force field energy minimisation. The total energy of these molecules are also given in Table 1. The adsorption energy values calculated for CI2 and monochloroacetic acid are reasonable chemical values, whereas the adsorption energy of 1,2-DCB is an overemphasised value, which may be due to the inadequate representation of VDW forces between phenyl ring and the zeolite lattice. 

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