Zeolites quantum mechanical studies

The main result of extensive simulations of A1 placement in the FAU-framework topology is that random insertion of A1 into the structure, subject to Loewenstein s rule and to a weaker second neighbor Al-Al repulsion term, does not reproduce the measured Si-nAl distribution patterns [4]. The details of the aluminum distributions are therefore determined by additional or different factors. This is consistent with Melchior s model of FAU-framework construction from pre-formed 6-iing units [47,48], The simulation results also highlight the likely limitations of quantum mechanical studies of aluminum T-site preferences. If the factors controlling the aluminum distributions in zeolites X and Y are also at work in other systems, purely energetic arguments will likely have limited direct relevance for application to real materials. 

R. Millini, G. Perego, and K. Seiti, Stud. Surf. Sci. Catal., 84,2123 (1994). Ti Substitution in MFI Type Zeolites A Quantum Mechanical Study. 

Torres F J, Vitillo J G, Civalleri B, Ricchiardi G and Zecchina A (2007a), Interaction of H2 with alkali-metal-exchanged zeolites a quantum mechanical study , J Phys Chem C, 111, 2505. 

Sauer J (1992) Quantum mechanical studies of zeolites. In Catlow CRA (ed) Modelling of structure and reactivity in zeolites. Academic Press Inc., San Diego, p 183... 

From quantum mechanical calculations. By solving the Schrodinger equation using various approximations, we can obtain forces between different atoms and molecules. These forces can be fitted into a force field. This usually works very well for intra-molecular bonded interactions like bond-stretching, bond-bending, and torsion interactions, but less well for van der Waals interactions. Note that hydrocarbon-zeolite interactions are dominated by van der Waals interactions (see, for example, ref. [24] and chapter 4). Recently, there have been several quantum-mechanical studies of water and methanol in Sodalite [25,26] using the Car-Parrinello technique [27],... 

The currently available quantum chemical computational methods and computer programs have not been utilized to their potential in elucidating the electronic origin of zeolite properties. As more and more physico-chemical methods are used successfully for the description and characterization of zeolites, (e.g. (42-45)), more questions will also arise where computational quantum chemistry may have a useful contribution towards the answer, e.g. in connection with combined approaches where zeolites and metal-metal bonded systems (e.g. (46,47)) are used in combination. The spectacular recent and projected future improvements in computer technology are bound to enlarge the scope of quantum chemical studies on zeolites. Detailed studies on optimum intercavity locations for a variety of molecules, and calculations on conformation analysis and reaction mechanism in zeolite cavities are among the promises what an extrapolation of current developments in computational quantum chemistry and computer technology holds out for zeolite chemistry. 

Two methods for including explicit electrostatic interactions are proposed. In the first, and more difficult approach, one would need to conduct extensive quantum mechanical calculations of the potential energy variation between a model surface and one adjacent water molecule using thousands of different geometrical orientations. This approach has been used in a limited fashion to study the interaction potential between water and surface Si-OH groups on aluminosilicates, silicates and zeolites (37-39). 

Abstract The chemical activation of light alkanes by acidic zeolites was studied by a combined Classical Mechanics/Quantum Mechanics approach. The diffusion and adsorption steps were investigated by Molecular Mechanics, Molecular Dynamics and Monte Carlo simulations. The chemical reactions step was studied at the DPT (B3LYP) level with 6-31IG basis sets and 3T and 5T clusters to represent the acid site ofthe zeolite. 

In principle, the diffusion steps (a) and (e) could be studied through molecular dynamics simulations as long as rehable forces fields are available to describe the zeolite structure and its interaction with the substrates. Also, if the adsorption takes place without charge transfer between the reagents/products and the zeolite, steps (b) and (d) could also be investigated either by molecular dynamics or Monte Carlo simulations. Step (c) however can only be followed by quantum mechanical techniques because the available force fields cannot yet describe the breaking and formation of chemical bonds. 

Among the chemical reactions of interest catalyzed by zeolites, those involving alkanes are specially important from the technological point of view. Thus, some alkane molecules were selected and a systematic study was conducted, on the various steps of the process (diffusion, adsorption and chemical reaction), in order to develop adequate methodologies to investigate such catalytic reactions. Linear alkanes, from methane to n-butane, as well as isobutane and neopentane, chosen as prototypes for branched alkanes, were considered in the diffusion and adsorption studies. Since the chemical step requires the use of the more time demanding quantum-mechanical techniques, only methane, ethane, propane and isobutane were considered. 

Contrary to the previous steps of the catalytic process, we cannot use force-field-based techniques because the available force fields are unable to describe the breaking and formation of chemical bonds. Thus, the chemical reaction step must be investigated by quantum mechanical techniques. Right away this imposes some limitations on the size of the cluster to be used in the calculations. In principle, since the catalytic sites are well localized within the zeolite framework, one should expect the chemical reactions to occur at very locahzed points of the zeoUtic structure. Thus, one could think of representing the acid sites by much smaller clusters than the ones used in the diffusion and adsorption studies. 

The diffusion, adsorption and chemical steps for the dehydrogenation and cracking reactions of light alkanes catalyzed by zeolites were studied using a combined classical mechanics (MM, MD and MC) / quantum mechanics approach. 

Although there are many ways to describe a zeolite system, models are based either on classical mechanics, quantum mechanics, or a mixture of classical and quantum mechanics. Classical models employ parameterized interatomic potentials, so-called force fields, to describe the energies and forces acting in a system. Classical models have been shownto be able to describe accurately the structure and dynamics of zeolites, and they have also been employed to study aspects of adsorption in zeolites, including the interaction between adsorbates and the zeolite framework, adsorption sites, and diffusion of adsorbates. The forming and breaking of bonds, however, cannot be studied with classical models. In studies on zeolite-catalyzed chemical reactions, therefore, a quantum mechanical description is typically employed where the electronic structure of the atoms in the system is taken into account explicitly. 

Quantum mechanical approaches using a cluster approximation have been used extensively to determine proton affinities.The influence of the local composition on the structure and proton affinity of a zeolite was studied... 

Another important zeolite-catalyzed chemical reaction is the decomposition of NO. Cu-exchanged zeolites, especially Cu-ZSM-5, have been shown to catalyze the decomposition of NO in the presence of hydrocarbons and excess oxygen. The increasing awareness of the detrimental effects of automobile exhaust has prompted several theoretical studies on the active site and reaction mechanism. ° Cu-ZSM-5 was described using an empirical force field and energy minimization to locate the copper ions in ZSM-5. Isolated copper atoms and copper clusters were found in the micropores, mostly associated with framework aluminium species. A cluster of two copper ions bridged via an OH species not part of the zeolite framework ( extra-framework ) was proposed as the active site. Quantum mechanical cluster calculations were carried out to study the elementary steps in the NO decomposition. A single T-site model was used to represent the zeolite framework. 

The choice of a theoretical model usually depends on the goals of the study, but sometimes other considerations, such as computer resources available, can also play a role in this selection process. A study of zeolite framework acidity or catalytic activity requires an explicit consideration of the electronic structure of the system, and quantum mechanical (QM) models are best suited for such investigations. Since the high CPU demands greatly limit the size of the systems that can be simulated with quantum mechanical models, the zeolite framework is often represented in these simulations by a cluster that presumably resembles the active site. The cluster approximation has the obvious drawback that influences of the crystal lattice are neglected. 

The irradiation of acetanilide in the cavity of zeolites (X, Y and P) results in the formation of o-aminoacetophenone as the principal product. A laser flash study of the kinetics of rearrangement of the triazine derivative (212) into the isomerised product (213) has been reported. In particular the 1,3-hydrogen migration was studied to establish the degree of quantum mechanical tunnelling involved. The results obtained indicate that tunnelling at two vibrational levels is involved. 

We are concerned with the kinetics of zeolite-catalyzed reactions. Emphasis is put on the use of the results of simulation studies for the prediction of the overall kinetics of a heterogeneous catalytic reaction. As we will see later, whereas for an analysis of reactivity the results of mechanistic quantum-chemical studies are relevant, to study adsorption and diffusion, statistical mechanical techniques that are based on empirical potentials have to be used. 

While these computational studies [34] were awaiting publication, a neutron diffraction study on H-SAPO-34 provided evidence for a protonated water molecule [38]. Whereas comments in the more popular press [39] stressed the apparent disagreement with previous calculations ( much of the confusion about how zeolites work stems from quantum calculations ) and used the entertaining title Quantum mechanics proved wrong , a comment to the original paper in the same issue... 

The zeolite matrix allows the preparation of dispersions with distinctly different and narrow particle size distributions in a range where size dependent electronic properties can be studied (quantum size particles). Quantum-mechanical calculations suggest that the energy level of the first excited state of the exciton increases with decreasing particle size of the semiconductors in correspondence with the experimentally observed blue-shift of the optical absorption edge (refs. 8-10). 

The advantages of IR and Raman spectroscopy and INS lie in the fact that they provide information about microporous materials on a molecular level. However, the utilization of vibrational spectroscopic techniques necessitates the reliable assignment of vibrational transitions to particular forms of normal modes in relation to a given structure. Already in the case of medium-sized molecules studied purely on an empirical basis, this leads to unbridgeable difficulties. Force field and quantum mechanical methods can significantly contribute to obtain this information about the dynamic behavior and allow a more sophisticated interpretation of the experimental data. Thus, besides the development achieved over the last years in the field of experimental techniques, substantial progress in describing vibrational spectra of zeolites and adsorbate/zeolite systems on a theoretical basis has been made. 

In summary, a high diversity of potential models for molecular mechanics calculations of zeolites hitherto exists. From the theoretical point of view, an appropriate force field should be able to predict structures and vibrations with similar accuracy. On the other hand, the structure of a system under study is determined by the energy minimum, whereas normal modes are dependent on the curvature (second derivative) of the potential energy surface. Consequently, force fields obviously successful in predicting structural features might not automatically be appropriate for simulating vibrational spectra. The only way to overcome this difficulty is to include experimental spectroscopic data into the parametrization process [60]. Alternatively, besides structures and energies a matrix of force constants obtained in quantum mechanical calculations can be included into the quantum mechanical data base used to tune the parameters of the potential function [51]. 

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-... 

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