Zeolite-molecule interactions

A list of interatomic potential parameters commonly used for zeolite lattice simulation is given in Table 2. In subsequent sections, reference will be made to this table when presenting data or results in addition to parameters needed for the zeolite/molecule interactions. 

Atom-atom pair potentials for zeolite-molecule interactions 20 Monte Carlo and molecular dynamics simulation of adsorption and diffusion of hydrocarbons and rare gases in zeolites 34-37... 

The main calorimetric studies on adsorption of water and ammonia on TS-1 and silicalite-1 have been reported by Bobs et al. [64,83,84,86], while other contributions came from the Auroux group [92] and Janchen et al. [93]. Cor-ma s group has investigated the interaction of water on zeolite [39]. The most important conclusion from the available literature is that calorimetric data require a very careful analysis, as probe molecules interact both with the silanols of the internal hydroxyl nests (see Sect. 3.8) and with Ti(lV) species. 

Interaction of the CO molecule with CuX-FER zeolites (X is an alkali-metal or proton as a co-cation) was investigated by IR spectroscopy and DFT calculations. An absorption band at 2138 cm 1 observed in IR spectra of CO on CuK- and CuCs-FER zeolites was assigned to a new type of CO adsorption complex on heterogeneous dual cation sites. CO molecule interacts simultaneously with Cu+ and alkali metal cations (via C- and O-end, respectively) in this type of complex. Interaction of CO with the secondary (alkali metal) cation led to a slight destabilization of the carbonyl complex. 

Theoretical calculations and simulations using ah initio and density function theory (DFT) methodologies are also seeing increasing use. Combining these theoretical calculations with spectroscopic data can assist in the interpretation of the observed spectral features and an improved understanding of how a probe molecule interacts with the various types of sites in zeolitic systems. 

In diffusion-controlled permeation, the components in a mixture differ largely in size but none is excluded from the pore network [8]. Separation is determined by the relative diffusivity (mobility) of a component within the zeolite. Expression of this mechanism requires low to medium loadings of the pore network where molecule-molecule interactions do not hinder diffusion. At high loadings, the mobility of aU molecules is reduced and they become unable to pass one another in the pore network [8]. 

For multi-component systems it seems intuitive that single-component diffusion and adsorption data would enable one to predict which component would be selectively passed through a membrane. This is only the case where molecular sieving is observed for all other separations where the molecules interact with one another and with the zeolite framework their behavior is determined by these interactions. Differences in membrane properties such as quahty, microstructure, composition and modification can also play a large role in the observed separation characteristics. In many cases, these properties can be manipulated in order to tailor a membrane for a specific apphcation or separation. 

As the aforementioned example demonstrates, for strong-strong separations the selectivity can be dependent on the loading of the membrane. When the size of the adsorbed molecule is similar to the zeolite pore, at loadings near saturation the zeolite framework atoms will adjust to allow for entropically favorable packing. Under these conditions, constituents must compete with one another for adsorption sites and molecule-molecule interactions play a dominant role [33]. 

The number of strong sites can be estimated directly from the nnmber of sorbed basic molecules defined by the peak maximum, provided that one basic molecule interacts with one acidic site. Auroux and colleagues [162] observed that ammonia adsorption shifts from strong chemisorption for H-ZSM5 to a process controlled by physisorption (shorter x) for boron-modified zeolites. 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

MD simulations, complete with ghost particle insertions (160, 161), may be used to obtain static and dynamic information. (These particle insertions were performed after the MD runs and do not affect the calculations they merely probe the insertion of particles into the system.) The MD simulations performed by Snurr et al. (155) were slightly more expensive than the GC-MC calculations, but they produced similar isotherms and also yielded important information about the structure of the adsorbed fluid. The methane molecules appeared to behave like an ordered fluid at all concentrations, although the structure does change. This change reflects the changing importance of sorbate-sorbate and zeolite-sorbate interactions as a function of loading. 

The nature of the methanol-zeolite interaction has been shown to be sensitive to a number of parameters and as such has proved to be a good benchmark for judging the reliability of quantum chemical methods. Not only are there a number of possible modes whereby one and two molecules interact with an acidic site (245), the barrier to proton transfer is small and sensitive to calculation details. Recent first-principles simulations (236-238) suggest that the nature of adsorbed methanol may be sensitive to the topology of the zeolite pore. The activation and reaction of methane, ethane, and isobutane have been characterized by using reliable methods and models, and realistic activation energies for catalytic reactions have been obtained. 

In order to describe the adsorption and diffusion in the zeolites in the framework of a modified lattice-gas, which takes into account the crystalline structure of the zeolite, the interaction among adsorbed molecules and the possibility of a transition of adsorbed molecules among different adsorption sites in the same unit cell and different unit cells that follows the model description of molecular diffusion in zeolites were previously proposed [88,104],... 

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),... 

The results are interpreted in terms of the size of the diffusing molecule and the effect of the cation upon the pore size of the zeolite. Counter diffusion of the molecules studied occurs readily in the various forms of type Y zeolite, but molecule-molecule interactions between the counterdiffus-ing molecules have a pronounced effect upon the diffusion rate. 

Guest molecules may interact favorably with each other upon absorption within a zeolite. Whatever interactions a pure substance may have amongst its atoms or molecules, may be carried into the void space of a zeolite. Polar molecules may continue to orient favorably, hydrogen-bonding molecules may continue that interaction, and formal chemical bonds can reform within the zeolite. Van der Waals liquids may continue to be that upon sorption by a zeolite. 

We shall concentrate on computational studies of the interaction between the methanol molecule and the acidic proton of the bridging (A1 0-H Si) hydroxyl group in zeolites to exemplify the contribution of simulation techniques in understanding chemical reactivity of zeolites. This interaction is the initial step of the industrially important conversion of methanol to gasoline. Therefore, understanding this primary step at the microscopic level has a direct impact on our understanding (and possibly rationalization) of the process. Before considering the results of calculations, let us outline the experimental information available for these systems. 

F. Haase and J. Sauer, /. Phys. Chem., 98, 3083 (1994). H NMR Chemical Shifts of Ammonia, Methanol, and Water Molecules Interacting with Bronsted Acid Sites of Zeolite Catalysts Ab-Initio Calculations. 

A particular method for studying complexes of iron is Mossbauer spectroscopy [7]. For instance, a- and p modifications of Fe(Pc) were studied by this method [8], and the dioxygen derivatives, as well. It was found that the oxygen molecule interacts with the complex, and (p-Oxo)-bis-phthalocyaninato-iron(lll) is formed [9]. The stability of the complex depends strongly on the solvents, e.g. in presence of strong N-bases the Fe ll) form is restored at room temperature, even in presence of oxygen [10]. Further, the Y-zeolite encaged Fe(Pc) was also studied, namely stabilization of the pyridine complex, Fe(Pc)(Py)2, as well as the effects of preparation conditions and presence of various counterions in the framework (Na, K, Rb) on the formation of Fe(Pc)(Py)2 are reported [11-13]. 

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