Zeolite cluster

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. 

The separation of a reactant system (solute) from its environment with the consequent concept of solvent or surrounding medium effect on the electronic properties of a given subsystem of interest as general as the quantum separability theorem can be. With its intrinsic limitations, the approach applies to the description of specific reacting subsystems in their particular active sites as they can be found in condensed phase and in media including the rather specific environments provided by enzymes, catalytic antibodies, zeolites, clusters or the less structured ones found in non-aqueous and mixed solvents [1,3,6,8,11,12,14-30],... 

Quantum Chemical Calculations on the Electronic Structure of Zeolite Clusters... 

Not all quantum chemical calculations on zeolite clusters involve necessarily millions of integrations, and in the case of iso-electronic chemical systems fulfilling certain geometrical criteria, almost trivial back-of-an-envelope type calculations can yield rigorous upper and lower energy bounds. Fortunately, some zeolite structural units fulfill these geometric criteria. 

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. 

Fig. 12. Minima and transition states on the reaction path of hydrogen exchange for methanol on a zeolite cluster. The upper and lower diagrams shown the equivalent neutral complexes, and the middle figure illustrates the transition state. Reprinted with permission from Ref. 221. Copyright 1995 American Chemical Society.

Fig. 16. Reaction coordinate for hydrogen exchange between methane and an acidic zeolite cluster. Arrows represent displacement vectors along the reaction coordinate. Reprinted with permission from Ref. 248. Copyright 1994 American Chemical Society.

Dehydrogenation of methane on a zeolite cluster has also been proposed to proceed via interaction of a CH fragment with the deprotonated zeolite lattice. DFT calculations performed with a 3T-atom cluster (248) and HF calculations with a lT-atom cluster (254) gave very similar results. The calculated transition state determined from the DFT calculations (248) that leads to dehydrogenation is shown in Fig. 17. 

The use of zeolite clusters in quantum chemical calculations has now progressed to quite a sophisticated level. Elementary steps of reaction mechanisms can now be characterized and the results used to distinguish which steps are the most plausible. Computational power is such that clusters and methods can avoid obvious pitfalls (too small a cluster, basis set, etc.). Several key concepts that have arisen from theoretical studies are illustrated in the preceding discussion. These include the following carbo-cations exist as parts of transition state structures, rather than as stable intermediates, and their stabilization is controlled by the zeolite lattice. The transition states are very different from the ground states to either side of them, and each different reaction has been shown to proceed via a different transition state. 

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 5.8. Calculated transition state geometry for H—D exchange between a zeolite cluster and methane.

Pople s CNDO/2 method and a hexagonal cluster model used in calculations to simulate charge densities Wiberg bond orders of Al - O and Si - O in the six - ring sites and the total energies of the zeolite clusters. 

The importance of using a specific force field to represent the zeolite cluster can be inferred from the comparison of the adsorption energies obtained at the MM and D-MM levels. The D-MM values are 2-3 kcal/mol higher than the ones obtained when the van Santen et al. [38,39] force field is used to describe the zeolite. 

Figure 5 Zeolite cluster with proton located on the Si-O-Al bridge.

Figure 29 Hydrogen bonded (a) and ion-pair (b) complexes of methanol molecule and zeolite cluster.

Hardness and Mulliken charges for the possible alkylating agents on a zeolite cluster with different trivalent cations. 

The DFT energies of the reaction on a 4T zeolite cluster are compared with the periodical electronic structure results for chabazite. 

Bare cluster or molecule Complex between zeolite cluster or molecule ... 

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