Zeolites simulations

The configuration-bias Monte Carlo (CB-MC) technique (112) has also been extensively applied to characterize the sorption of alkanes, principally in silicalite (111, 156, 168-171) but also in other zeolites (172-174). Smit and Siepmann (111, 168) presented a thorough study of the energetics, location, and conformations of alkanes from n-butane to n-dodecane in silicalite at room temperature. A loading of infinite dilution was simulated, based on a united-atom model of the alkanes and a zeolite simulation box of 16 unit cells. Potential parameters were very similar to those used in the MD study of June et al. (85). As expected, the static properties (heat of adsorption, Henry s law coefficient) determined from the CB-MC simulations are therefore in close agreement with the values of June et al. The... 

Figure 6.5 a Product distribution for hydrocracking catalyzed by an ERI-type zeolite, showing the window effect observed by Chen et ai. b simulation of n-C- in an ERI cage (window size = 0.36 nm xO.51 nm). Thanks to Dr. David Dubbeldam for the zeolite simulation snapshot. 

The sodium-23 MASNMR spectra of the Y zeolite samples with these particular levels of cation exchange are shown in Figure 2, along with the spectrum of the Na-Y zeolite. Simulations of these spectra indicate that about 25, 10, and 20% of the integrated NMR intensity of the respective exchanged zeolites arises from residual Na cations in the supercages. The different Na NMR lines are best resolved in the case of the (NH, Na)-Y zeolites. 

The location of extra framework cations is a major problem in characterising zeolites. Simulation is becoming an increasingly powerful tool for the exploration and rationalisation of cation positions, since it not only allows atomic level models to be compared to bulk experimental behaviour, but can also make predictions about the behaviour of systems not readily accessible to experimental probing. In the first part of this work we use the Mott-Littleton method in conjunction with empirical potential energy functions to predict and explore the locations of calcium cations in chabazite. Subsequently, we have used periodic non-local density functional calculations to validate these results for some cases. 

Ab-initio density-functional molecular dynamics is used to characterize dynamical processes in zeolites. Simulations performed on the structure of gmelinite show that the proton transfer between the 0-sites is a spontaneous process enabled in both Na-free and Na-zeolite by just one water molecule adsorbed to the acid site. A proton attack of the acid zeolite at the hydrocarbon molecule is investigated at increased temperature of 700 K. In the protonated molecule a series of proton jumps are observed indicating their high mobility. 

Commercial SCR V205,W03/Ti02 Cu-ZSM zeolites Simulated flue gas N0,02,H20,NH3/N2 NO/Ar Deactivation of active V sites by covering with polyphosphoric acids Activity of Cu + sites depending on their location within the pore structure 42 43... 

The simplest extra-framework cation to employ in zeolite simulations is, of course, H+, and the placement of the ion can be problematic. In this work, we computed the electrostatic potential due to the zeolite framework and placed the H+ at the location of the lowest (i.e., most negative) value. Subsequently, the position of the proton was optimized while all other atoms were kept fixed at the experimental coordinates. The resulting structures were used to evaluate the FFs. 

The first analytical tool to assess tire quality of a zeolite is powder x-ray diffraction. A collection of simulated powder XRD patterns of zeolites and some disordered intergrowths togetlier witli crystallographic data is available from tlie IZA [4o]. Phase purity and x-ray crystallinity, which is arbitrarily defined as tlie ratio of tlie intensity of... 

Treacy M M J, Fliggins J B and von Ballmoos R 1996 Collection of Simulated XRD Powder Patterns for Zeolites 3rd revised edn (London Elsevier)... 

Fig. 8.22 Schetnatic structure of the zeolite silicalite showing the straight and zig-zag chaimels. (Figure adapted fron Smit B and JI Siepmann 2994. Simulating the Adsorption of Alkanes in Zeolites. Science 264 1118-1120.)...

Smit B and J I Siepmaim 1994. Simulating the Adsorption of Alkanes in Zeolites. Science 264 111 ... 

Table 1.1 Configurationally biased Monte Carlo simulations of the adsorption enthalpies of hydrocarbons for two zeolites.

This is the reason that for complex cracking reactions in zeohtes the product pattern can be predicted from a simulation of the free energies of the corresponding intermediate molecules in the zeolite [11]. 

ABSTRACT Zeolite Y modified with chiral sulfoxides has been foimd catal rtically to dehydrate racemic butan-2-ol enantioselectively depending on the chiral modifier used. Zeolite Y modified with R-l,3-dithiane-1-oxide shows a higher selectivity towards conversion of S-butan-2-ol and the zeolite modified with S-2-phenyl-3-dithiane-1-oxide reacts preferentially with R-butan-2-ol. Zeolite Y modified with dithiane oxide demonstrates a significantly higher catalsdic activity when compared to the unmodified zeolite. Computational simulations are described and a model for the catalytic site is discussed. 

Szanyi et al., using again in situ IR-TPD coupled skills, studied the effect of acid sites on the catalytic activities of a series of H+-modified Na-Y zeolites in the non-thermal plasma assisted NO reduction reaction using a simulated diesel engine exhaust gas mixture. The acid sites were formed by NH ion exchange and subsequent heat treatment of a NaY zeolite. The catalytic activities of these H+- modified NaY zeolites... 

Figure 5. Top Adsorption isotherms of C02 for 1-en at the indicated temperatures. Bottom Adsorption-desorption cycling of C02 for 1-en showing reversible uptake from (a) simulated air (0.39 mbar C02 and 21% 02 balanced with N2) and from (b) simulated flue gas (0.15 bar C02 balanced with N2). (c) time-dependent C02 adsorption for porous materials (A = 1-en, B = mmen-Mg2(dobpdc), C = 1, D = Mg-MOF-74, E = Zeolite 13X, F = MOF-5). (d) C02 adsorption ratio of 1-en in flue gas (after 6 min exposure to 100% RH at 21 °C) to 1-en in flue gas (Adapted from [192]).

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. 

Many experimental and, more recently, simulation methods have been put to use to try to localise the cations in faujasite (figure 1) in different situations hydrated or dehydrated zeolites, zeolites saturated with organic molecules, e.g. benzene, toluene, xylene. The four techniques that are described below have been used in more than 90% of all published works to detect and localise extraframework cations in faujasite type zeolites. 

Monte Carlo simulations and energy minimization procedures of the non-bonding interactions between rigid molecules and fixed zeolite framework provide a reasonable structural picture of DPP occluded in acidic ZSM-5. Molecular simulations carried out for DPB provide evidence of DPB sorption into the void space of zeolites and the preferred locations lay in straight channels in the vicinity of the intersection with the zigzag channel in interaction with H+ cation (figure 1). 

Fig. 2 Experimental analysis of the chemical shift anisotropy of high-silica ZSM-5 zeolite, (a) 29Si MAS NMR and (b) extracted CSA lineshapes from a two-dimensional CSA recoupling sequence dashed lines are simulated lineshapes. Adapted with permission from [79]. Copyright 2008 American Chemical Society...

Fig. 5 (a) Structure directing agent (SDA) in the synthesis of zeolite ITQ-3 N quaternary nitrogen A-I carbon atoms of SDA, l,3,3,6,6-pentamethyl-6-azonium-bicyclo[3.2.1]octane some 13C line assignments were not unequivocally possible, so these are left open the orientation of the dipole moment was calculated with a semiempirical AM-1 simulation with the origin at the center of gravity of the molecular cation (b) 13C H 27A1 REAPDOR experiment on synthetic zeolite ITQ-3. Adapted from [204]... 

Later in the simulations, the zeolite analcime (NaAlSi206 H20) begins to form, largely at the expense of micas, which serve as proxies for clay minerals. In the NaOH flood, the overall reaction (expressed per formula unit of analcime) is,... 

In part because of the open crystal structure and resulting low density characteristic of zeolite minerals, analcime is the most voluminous reaction product in the simulations. 

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