Cluster models zeolites

Fig. 20. Optimized BLYP/DNP transition state for benzene H/D exchange on a zeolite cluster model. (Reprinted with permission from Beck et al (81). Copyright 1996 American Chemical Society.)...

Figure 2.7 (A) Optimized equilibrium configurations of free TMPO, TEPO, TBPO, and TOPO probe molecules. Selected interatomic distances (in A) are indicated (B) correlations of calculated chemical shift of adsorbed RsPOH" complexes and proton affinity (PA) predicted based on the 8T zeolite cluster models [25]. Reproduced with permission from ACS.

NBO. The zeolite model used in the calculation was the cluster model shown in Figure 7.10. The results are shown in Table 7.2. In the Ag-zeolite cluster model, H is used to terminate the structure and does not have a long-range influence on the bonding of Ag. 

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

Zygmunt, S. A., Mueller, R. M., Curtiss, L. A., Iton, L. E., 1998, An Assessment of Density Functional Methods for Studying Molecular Adsorption in Cluster Models of Zeolites , J. Mol. Struct. (Theochem), 430, 9. 

Table 2. Geometric parameters of binuclear iron core cluster models. Oz corresponds to the oxygen of the zeolite. Within the brackets are indicated the considered configurations (a, b, c or d) of Figures. 2i and 2ii...

Fig. 4 Proposed defect cluster model in as-made zeolites with quaternary ammonium cations as structure directing agents (SDAs) hydrogen bond distances of 1.68 A are determined experimentally from the H NMR chemical shift of 10.2 ppm X and Y are atoms not further specified in the SDA the interaction between the SDA and the SiO- group is assumed based on bond valence arguments (see text)...

Mo(CO)6 and Co(CO)3NO NaY zeolite Adsorption from vapor phase and H2S treatment Intrazeolite Co2Mo2S i clusters, model hydrodesulfuration catalyst [25]... 

J. T. Fermann and S. Auerbach, Modeling proton mobility in acidic zeolite clusters II. Room temperature tunneling effects from semiclassical theory, J. Chem. Phys. 112 (2000), 6787. 

The previous two sections of this review deal with classical simulation methods. A description of the activation of adsorbates by acidic sites, together with any bond breaking or bond formation that may take place, is the realm of quantum mechanical (QM) simulations. These types of calculations are particularly well-suited to zeolite-adsorbate systems when the cluster approximation is used. The active acidic site in the zeolite is modeled by a molecular cluster, formed by cutting out a small portion of... 

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. 

Of course, certain features of overall kinetics are inaccessible via a cluster model method, such as the influence of pore structure on reactivity. The cluster model method cannot integrate reaction rates with concepts such as shape selectivity, and an alternative method of probing overall kinetics is needed. This has recently been illustrated by a study of the kinetics of the hydroisomerization of hexane catalyzed by Pt-loaded acidic mordenite and ZSM-5 (211). The intrinsic acidities of the two catalysts were the same, and differences in catalyst performance were shown to be completely understood on the basis of differences in the heat of adsorption of hexene, an intermediate in the isomerization reaction. Heats of adsorption are strongly dependent on the zeolite pore diameter, as shown earlier in this review (Fig. 11). 

Figure 7 shows DFT calculations (BLYP/DNP) of the geometries of acetone and mesityl oxide on a cluster model of the Brpnsted site in zeolite... 

DNP level found no evidence for a stable benzenium cation in contact with a cluster modeling the zeolite conjugate base site. We were able to locate a transition state for benzene H/D exchange as shown in Fig. 20, which is similar to the transition state for methane H/D exchange on zeolites (121). These transition states clearly show that hydrogen exchange is a concerted process. 

Fig. 1. Model of rhodium dicarbonyl complex on dealuminated Y zeolite, as determined by IR and EXAFS spectroscopies and density functional theory. The Rh atom, near the upper center of the figure, has two CO ligands bonded to it, pointing upward, and two oxygen atoms of the zeolite lattice below. An Al atom is located between these two oxygen atoms. The dangling atoms of the cluster model of the zeolite are capped by hydrogen atoms for the calculation (Goellner, Gates, etal., 2000).

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