Computer simulations zeolite development

The five years since last considering specifically recent developments in X-ray and neutron diffraction methods for zeolites [1] have witnessed substantial progress. Some techniques, such as high resolution powder X-ray diffraction using synchrotron X-rays, have blossomed from earliest demonstrations of feasibility to widespread and productive application. Others, such as neutron powder diffraction, have shown steady progress. For still others, notably microcrystal diffraction, a variety of circumstances have contributed to extended gestation periods. Additionally, opportunities scarcely considered earlier (such as single crystal Laue diffraction, and certain developments in computer simulations that complement diffraction work) now command broad attention and warrant the commitment of substantial further investment. 

CONTENTS Introduction, Thom H. Dunning, Jr. Electronic Structure Theory and Atomistic Computer Simulations of Materials, Richard P. Messmer, General Electric Corporate Research and Development and the University of Pennsylvania. Calculation of the Electronic Structure of Transition Metals in Ionic Crystals, Nicholas W. Winter, Livermore National Laboratory, David K. Temple, University of California, Victor Luana, Universidad de Oviedo and Russell M. Pitzer, The Ohio State University. Ab Initio Studies of Molecular Models of Zeolitic Catalysts, Joachim Sauer, Central Institute of Physical Chemistry, Germany. Ab Inito Methods in Geochemistry and Mineralogy, Anthony C. Hess, Battelle, Pacific Northwest Laboratories and Paul F. McMillan, Arizona State University. 

Many new adsorbents have been developed over the past 20 years including carbon molecular sieves, new zeolites and aluminophosphates, pillared clays and model mesoporous solids. In addition, various spectroscopic, microscopic and scattering techniques can now be employed for studying the state of the adsorbate and microstructure of the adsorbent. Major advances have been made in the experimental measurement of isotherms and heats of adsorption and in the computer simulation of physisorption. 

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. 

In this chapter we present the results of computer simulations of lineeir cind branched alkcines in the zeolite Silicalite. We focus on the development of the model and a detailed comparison with experimental data for the linear and brcinched alkcines. In addition we demonstrate that these isotherms can be described quantitatively with a dual-site Langmuir isotherm. 

Despite the enormous success of the zeolite membrane, practical application in a larger scale is still limited. Several factors such as cost of membrane development, reproducibility, long-term stability, and the method for the preparation of the defect-free membrane restrict its implementation in the industry. Molecular simulations become a powerful tool to predict the catalytic behavior of zeolite [14]. Compared to the experimental process, it is rapid and convenient, is cost effective, can handle more complex systems within a reasonable period of time, and results to better understanding of the system. Many computer simulation methodologies have been employed to understand the physicochemical properties of zeolite such as adsorption characteristics [15], diffusion and permeation [16], catalytic reaction [17], and also the nature of the acidic site [18,19]. The main concern of this work is to design a membrane using computer simulation methodology. 

There has been a phenomenal growth of interest in theoretical simulations over the past decade. The concomitant advances made in computing power and software development have changed the way that computational chemistry research is undertaken. No longer is it the exclusive realm of specialized theoreticians and supercomputers rather, computational chemistry is now accessible via user-friendly programs on moderately priced workstations. State-of-the-art calculations on the fastest, massively parallel machines are continually enlarging the scope of what is possible with these methods. These reasons, coupled with the continuing importance of solid acid catalysis within the world s petrochemical and petroleum industries, make it timely to review recent work on the theoretical study of zeolite catalysis. 

The zeolite framework was described by a specific force field developed by van Santen et al. [11] while the hydrocarbon molecules and their interaction among themselves and with the zeolite lattice were described by the generic force field Drdding n [12]. All the internal coordinates of the alkane molecules were allowed to fully relax. The nonbonded interactions (electrostatic and van der Waals) were computed for aU atoms within a cutoff-radius of 12A. Periodic boundary conditions were imposed along the three axes of the zeolite model to simulate an infinite crystal. 

Computer modeling techniques are a substantial aid in zeolite structure solutions or refinements, and a means of extracting structural insight from difiraction or other anal ftical experiments. Sorption results, particle shapes in some cases, diffraction or scattering data, as well as optical, NMB and EXAFS spectra can all be simulated based on an atomic structure and, conversely, analytical data of these various types can be used to guide the development or detailing of an appropriate structural model. 

Many of the earlier computations of thermodynamic parameters associated with hydrocarbon adsorption into zeolites entailed development of interatomic potentials so as to fit reasonably with one particular set of experimental data. As a result, although the correspondence between simulation and experiment was often reasonable [102], the transferability of the potential set from one zeolite composition to ano er or from one type of simulation to another was poor. In principle, if the parameterization truly describes the fundamental physics in an approximate way, it should be viable to develop a more generally applicable set of potentials. 

This conversational and somewhat subjective overview of computational approaches in zeolite chemistry has illustrated that the field is very diverse, and expanding rapidly. Modeling and simulation at the atomistic or electronic structural level clearly contribute at various levels to practical zeolite research and development programs. Characterization and zeolite physical and chemical property prediction are the most prominent application domains at present. 

In 2003, the Jilin group developed a computational methodology for the design of open frameworks with predefined pore geometry.[37] The concept of forbidden zone is introduced. The forbidden zone corresponds to a porous pattern inside which no T-atom can be placed. Compared with previous simulation methods, this method is much more straightforward and efficient, especially for the design of zeolite frameworks with desired pore geometry. 

Whereas Eq.(5.58) serves for the determination of local interactions between cluster models of a zeolite and interacting molecules, analytical expressions are needed for the interaction potential if one wishes to compute vibrational frequencies for purpose of comparison with experiment or if the potentials are to be used in Monte Carlo or molecular-dynamics simulation calculations. Sauer and co-workers developed such analytical potentials for the water-silica interaction system. The method makes use of the molecular electrostatic potential (MEP) maps and the functional form of EPEN/2 (Empirical Potential based on interactions of Electrons and Nuclei). EPEN/2 potential functions consist of a point-charge interaction term and... 

In summary, the direct quantum mechanical simulation of zeolite vibrational spectra is evidently a formidable task and is often severely hampered by limited computational resources. Pure ab initio methods are well-suited if local effects or groups with characteristic vibrational frequencies like Bronsted acidic OH groups are under study. In theoretical studies of vibrational spectra of zeolite frameworks and cations on extra-framework sites, QM calculations are of crucial importance in developing force field parameters which can be used in a subsequent step in MM, MD or NMA calculations. Due to the lack of sufficient exper-... 

The above example, the adsorption of chain molecules in the pores of a zeolite, is used to illustrate the problems that may occur if one uses conventional simulation techniques for more complex systems. Similar problems may occur in the simulation of phase equilibria of chain molecules, simulations of polymers, or liquid crystals. For many of these systems it is relatively straightforward to implement the force fields to simulate these systems however, the simulation times required to determine reliable equilibrium properties may be prohibitively long. These simulation times may even be so extreme that it cannot be expected that increasing computer power will be of any help. To be able to perform simulations on complex systems it is therefore important to develop novel algorithms that are orders of magnitude more efficient than the conventional algorithms. In this article such algorithms are discussed. 

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