Prospects of Transition Interface Sampling simulations for the theoretical study of zeolite synthesis

The transition interface sampling (TIS) technique allows to overcome large free energy barriers within reasonable simulation time, which is impossible for straightforward molecular dynamics. Still, the method does not impose an artificial driving force, but it surmounts the timescale problem by an importance sampling of true dynamical pathways. Recently, it was shown that the efficiency of TIS to calculate reaction rates is less sensitive to the choice of reaction coordinate than those of the standard free energy based techniques. This could be an important advantage in complex systems for which a good reaction coordinate is usually very difficult to find. We explain the principles of this method and discuss some of the promising applications related to zeolite formation.Comment: 9 pages, accepted for publication in Phys. Chem. Chem. Phys. for the special issue of the CECAM workshop: Computational aspects of building blocks, nucleation, and synthesis of porous materials Aug. 29 2006 to Aug. 31 200

Determination of the nature of the Cu coordination complexes formed in the presence of NO and NH3 within SSZ-13

Ammonia-selective catalytic reduction (NH3-SCR) using Cu zeolites is a well-established strategy for the abatement of NOx gases. Recent studies have demonstrated that Cu is particularly active when exchanged into the SSZ-13 zeolite, and its location in either the 6r or 8r renders it an excellent model system for fundamental studies. In this work, we examine the interaction of NH3-SCR relevant gases (NO and NH3) with the Cu2+ centers within the SSZ-13 structure, coupling powder diffraction (PD), X-ray absorption spectroscopy (XAFS), and density functional theory (DFT). This combined approach revealed that, upon calcination, cooling and gas exposure Cu ions tend to locate in the 8r window. After NO introduction, Cu-ions are seen to coordinate to two framework oxygens and one NO molecule, resulting in a bent Cu-nitrosyl complex with a Cu-N-O bond angle of similar to 150 degrees. Whilst Cu seems to be partially reduced/changed in coordination state, NO is partially oxidized. On exposure to NH3 while the PD data suggest the Cu2+ ion occupies a similar position, simulation and XAFS pointed toward the formation of a Jahn-Teller distorted hexaamine complex [Cu(NH3)(6)](2+) in the center of the cha cage. These results have important implications in terms of uptake and storage of these reactive gases and potentially for the mechanisms involved in the NH3-SCR process

Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions

Zeolites are the workhorses of today's chemical industry. For decades they have been successfully applied, however many features of zeolite catalysis are only superficially understood and in particular the kinetics and mechanism of individual reaction steps at operating conditions. Herein we use state-of-the-art advanced ab initio molecular dynamics techniques to study the influence of catalyst topology and acidity, reaction temperature and the presence of additional guest molecules on elementary reactions. Such advanced modeling techniques provide complementary insight to experimental knowledge as the impact of individual factors on the reaction mechanism and kinetics of zeolite-catalyzed reactions may be unraveled. We study key reaction steps in the conversion of methanol to hydrocarbons, namely benzene and propene methylation. These reactions may occur either in a concerted or stepwise fashion, i.e. methanol directly transfers its methyl group to a hydrocarbon or the reaction goes through a framework-bound methoxide intermediate. The DFT-based dynamical approach enables mimicking reaction conditions as close as possible and studying the competition between two methylation mechanisms in an integrated fashion. The reactions are studied in the unidirectional AFI-structured H-SSZ-24, H-SAPO-5 and TON-structured H-ZSM-22 materials. We show that varying the temperature, topology, acidity and number of protic molecules surrounding the active site may tune the reaction mechanism at the molecular level. Obtaining molecular control is crucial in optimizing current zeolite processes and designing emerging new technologies bearing alternative feedstocks

Sorption and activation of hydrocarbons by molecular sieves

Substantial progress has been made recently in the understanding of sorption and activation of alkanes. This progress reflects the emergence of new theoretical and experimental results, leading to a more quantitative picture of the elementary steps involved in the ordering of alkanes in molecular sieves and their chemical interaction with the acid site. Conversion of n-alkanes over various zeolites is now well understood to depend mainly upon the concentration of reactants sorbed. The sorption enthalpy and entropy of these molecules are linearly related and this relationship is characteristic of a particular molecular sieve (compensation effect). The interfacial chemistry that alkanes and alkenes undergo involves ionic species only in their transition state, whereas the stable intermediates are covalently bound. This leads to a description of the chemical transformations that resemble nucleophilic and electrophilic substitutions

Conducting polymer wires in mesopore hosts

Nanometer-size conducting structures are of great interest in view of fundamental issues and potential applications. We explore the inclusion chemistry of conjugated polymers and graphite-like materials as a means to create such structures. Novel mesoporous materials with pore diameters in the 3 nm range (MCM-41) are used as hosts. Monomer molecules are introduced via vapor or solution transfer and polymerized either by included or external reagents. The properties of the conjugated systems are studied while encapsulated or after dissolution of the host. In the case of polyaniline formed on oxidation of aniline with persulfate, microwave absorption shows the presence of conducting filaments in the host channels. The above systems are compared with graphite-type material encapsulated in MCM-41 by first forming a precursor polymer such as polyacrylonitrile that is pyrolyzed at 500-800°C. These polymer chains are the first nanometer-size conducting filaments stabilized in a well-defined channel host

Sodium Diffusion through Aluminum-Doped Zeolite BEA System: Effect of Water Solvation

To investigate the effect of hydration on the diffusion of sodium ions through the aluminum-doped zeolite BEA system (Si/Al = 30), we used the grand canonical Monte Carlo (GCMC) method to predict the water absorption into aluminosilicate zeolite structure under various conditions of vapor pressure and temperature, followed by molecular dynamics (MD) simulations to investigate how the sodium diffusion depends on the concentration of water molecules. The predicted absorption isotherm shows first-order-like transition, which is commonly observed in hydrophobic porous systems. The MD trajectories indicate that the sodium ions diffuse through zeolite porous structures via hopping mechanism, as previously discussed for similar solid electrolyte systems. These results show that above 15 wt % hydration (good solvation regime) the formation of the solvation cage dramatically increases sodium diffusion by reducing the hopping energy barrier by 25% from the value of 3.8 kcal/mol observed in the poor solvation regime

Nitrido Zeolites. A Novel and Promising Class of Compounds

The synthetic approach to nitrido zeolites has been opened up by the synthesis and characterization of the P-N-sodalite class of compounds. A variety of compounds with a general formula M7_xH2 x[P1 2N24]X2-y with Μ = Zn2 + , Co2 + , Ni2 + etc. and X = F", CI", Br", Γ has been obtained. Striking features of these compounds are high chemical and thermal stability as well as an intense colour in some cases