Frameworks cluster model

In this review, we focus on cluster models of Br0nsted acid sites, bridging hydroxyl groups that result from the incorporation of trivalent aluminum atoms into the siliceous framework during synthesis. These sites are by no means the only active sites within zeolites, but they are among the best characterized. 

These problems are considered within the framework of the cluster model. During simulation the impurity is placed in the substituting anion site. 

For Si(l 11)-(7 x 7), various possible physisorption and chemisorption geometries have been discussed in the framework of a Si25H24PH3 cluster model.54... 

Silvestrelli5 reached a similar conclusion for Si(001) -(2x1) interaction with ethanol in the framework of a periodic slab approach, where the Car - Parrinello method was used, instead of a cluster model. Quantitatively, substantial differences... 

Mikheikin ID, Abronin IA, Zhidomirov GM, Kazansky VB (1977) Calculations of chemadsorption and elementary events of catalytic reactions within the framework of a cluster model. II. Properties of surface hydroxyl groups of oxides, Kinetics and Catalysis 18 1580—1583... 

Beran et al. (Ill) have also considered a Si5Fe06(0H)12Na cluster modeling a six-membered ring with Fe atoms included in a zeolite framework. It was concluded that Fe and Al affect the properties of a zeolite in a qualitative similar manner, although the destabilizing effects of Al are stronger. 

The initial step of this investigation is to analyze the reaction pathways using the low computational cost cluster approach. The small cluster model aims to model a zeolitic Br0nsted acidic site, and has been demonstrated to fill successfully this task. On the other hand, a small cluster cannot describe the zeolite framework. By comparison of reaction pathways taking and not taking into account the zeolite framework, we will be able to evaluate this effect on reactivity. 

The overwhelming majority of the theoretical studies were performed on cluster models of the catalytic site, hi spite of the fact that the role of space confinement and the secondary interactions with the framework atoms is well-known, there are only a few electronic structure calculations on lattice models involving hydrocarbons, using either periodic DFT calculations, or embedding methods. In this brief account of the subject we attempt to overview some of the recent computational results of the literature and present some new data obtained from ab initio DFT pseudopotential plane wave calculations on Cl - C4 alkanes in the chabazite framework. 

The different contributions to the HF, BLYP and B3LYP interaction energy of CO on the Pt 3(7,3,3) cluster model representation of the Pt(lll) surface as obtained at the various steps of the CSOV method are reported on Figure 2. The most important result of this comparison is that the qualitative picture of the chemisorption bond arising from ab initio HF and DFT quantum chemical approaches is essentially the same the relative importance of the different mechanisms remaining unchanged. This is an important conclusion because it validates many previous analysis of the chemisorption bond carried out in the framework of Hartree-Fock cluster model wave functions. " ... 

The calculated shifts agree very well with experimental data (see Table 7), which justifies further analysis of these results. For instance, switching off selected components of the KSCED embedding potential and comparing the results obtained using cluster models of the zeolite framework of different sizes, made it possible to... 

VPI-5 is an aluminophosphate framework with very large one-dimensional pores defined by 18-member ring[22]. The crystal structure report[7j of synthesized VPI-5 revealed the possible role of water molecules as templates. The use of VPI-5 as a molecular sieve and as a catalyst primarily depend on the removal of the occluded water molecules without the destruction of the framework structure. Prasad et al.[23] reported from their TGA experiment that the seven distinct types of water molecules could be desorbed from VPI-5 in a step-wise fashion, in the temperature range of 35 to 60°C. The cluster model calculations[24] have revealed the actual electronic interaction of water molecules with VPI-5 framework. CG technique also indicated that the void volume in VPI-5 could be controlled by the partial removal of water molecules. 

On the contrary, the constrained cluster model does not allow two framework AIO4 tetrahedra to approach (Fig. 4b) and, therefore, the Cu2+ ion cannot be coordinated to four... 

Shubin A A, Zhidomirov G M, Kazansky V B and Van Santen R A (2003), DPT cluster modeling of molecular and dissociative hydrogen adsorption on Zn ions with distant placing of aluminium in the framework of high-silica zeolites , Catal Lett, 90, 137. 

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