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Zeolite Structures (zeolite + structure)
Selected AbstractsHypothetical Uninodal Zeolite Structures: Comparison of AlPO4 and SiO2 Compositions Using Computer Simulation.CHEMINFORM, Issue 13 2004Alexandra Simperler No abstract is available for this article. [source] Rational design of shape selective separations and catalysis: Lattice relaxation and effective aperture sizeAICHE JOURNAL, Issue 3 2010Chrysanthos E. Gounaris Abstract Gounaris et al. presented a computational method that can be used for the quick screening of zeolite structures and provide predictions regarding which of them have the potential to exhibit high selectivity among a set of molecules of interest. This article builds upon this earlier work and furthers our understanding of diffusion processes in zeolites and other microporous metal oxides. We first present an augmented formulation to account for the flexibility of the zeolitic portal and conduct an analysis to assess the effect of varying the parameters of the associated quadratic potential. We then introduce a methodology to map the energetic landscape, identify all locally optimal conformations, and probabilistically account for the multiplicity of conformers. Finally, we conduct sensitivity analysis on the effective size of the aperture, and show how the methodology can be fine,tuned through experimental observations. A comprehensive database of 290 molecules of industrial interest and a total of 123 different zeolite structures were used in this study. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Solving the crystal structures of zeolites using electron diffraction data.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2008The maximum-entropy and likelihood method for solving zeolite crystal structures from electron diffraction data is modified to use potential-map-density histograms as an additional figure of merit. The experimental histogram is compared to an idealized one (based on known zeolite structures) using Pearson and Spearman correlation coefficients. These supplement the use of log-likelihood estimates as figures of merit to select the optimal solution from a collection of phase sets. The method has been applied with success to seven zeolite and one inorganic crystal structures that have varying associated data quality. The technique works easily even with two-dimensional data sets of less than 50 unique diffraction data and a resolution of less than 2,Å. The method is very fast, and the computer time needed on a modest PC was never more than a few minutes. [source] Loading Dependence of Self-Diffusivities of Gases in ZeolitesCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 9 2007R. Krishna Abstract Experimental data on the self-diffusivities, Di,self, of a variety of gases (CH4, N2, Kr, C2H6, and C3H8) in three different zeolites, LTA, FAU, and MFI, show different dependences on the molar loading, qi. In LTA, Di,self appears to increase with qi for all molecules except N2. In FAU and in MFI the Di,self shows a sharp decrease with increasing qi. In order to gain insights into the causes behind the loading dependences, molecular dynamics (MD) simulations were carried out to determine the self-diffusivities of seven gases (CH4, N2, Kr, C2H6, C3H8, Ar, and Ne) in six different all-silica zeolite structures (MFI, AFI, FAU, CHA, DDR, and LTA). The simulation results show that the variation of Di,self with qi is determined by a variety of factors that include molecular size and shape, and degree of confinement within the zeolite. For one-dimensional channels (AFI) and intersecting channel structures (MFI), the Di,self invariably decreases with increasing qi. For zeolite structures that consist of cages separated by windows (FAU, CHA, DDR, LTA), the size of the windows is an important determinant. When the windows are wide (FAU), the Di,self decreases with qi for all molecules. If the windows are narrow (CHA, DDR and LTA), the Di,self often exhibits a sharp increase with qi, reaches a maximum and reduces to near-zero values at saturation. The sharpness with which Di,self increases with qi, is dictated by the degree of confinement at the window. Weakly confined molecules, such as Ne, do not exhibit an increase of Di,self with qi. [source] | ||||||||||