Zeolites geometry-optimized cluster model

Fig. 5. (a) Site II cation on a six-membered oxygen ring as the basic unit on types A and X zeolites. T denotes Si or Al. (b) Geometry-optimized cluster model to represent the chemistry of Ag/zeolite. 

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. 

Cluster model scheme and periodical method were used in the molecular model calculations of active sites of zeolite catalysts results of both approaches are presented and discussed in this review. In cluster models of zeolite structures hydrogen boundary atoms (H ) were used to saturate dangling bonds of the Si and Al atoms. Definite restrictions were imposed on the optimization of positions of these boundary H atoms. In the optimization, the geometry of an appropriate fragment of zeolite lattice was taken from the experimental X-ray diffraction data [7]. Only Si-H and Al-H bond distances were optimized, while the positions of other atoms (except M), as well as directions of 0-H bonds, were kept frozen. The M ion was allowed to move freely in the structure. 

It is also worth to mention that exothermic heats of formation for similar O3 and O4 type moieties were observed for the reactions of the MeOxMe (X = 1—2) species with molecular oxygen (Table 11.9) [58, 59]. Close similarities are revealed between the O4 geometries obtained in our slab and the ones noted in the cluster models and periodic models of zeolites despite of more variable geometries in the last case. The closeness between bond lengths ( 0.02 A) with the similar O4 structures optimized at the PW91/PAW level for the Ca204 species in CaMOR zeolite is even surprising. 

The geometries of C2H4, AgX (where X = halide), and Ag-zeolite are optimized first, using STO-3G and then at the 3-21G levels. The cluster model (Kassab et al., 1993 Hill and Sauer, 1995) is used to represent the chemistry of zeolite, shown in Figure 8.3. The optimized zeolite cluster shows a tilt of Ag toward the alumina tetrahedral. The adsorbate and adsorbent are then combined into a single molecule, thereby optimizing its geometry. 

A small fragment representing only three tetrahedral centers (i.e.. Si or Al) had been chosen to represent the zeolite in the model calculations. Since the geomeby reorganizes significantly (Scheme 2) around the adsorption/reaction site, full geometry optimization of the cluster is necessary. 

The question of methanol protonation was revisited by Shah et al. (237, 238), who used first-principles calculations to study the adsorption of methanol in chabazite and sodalite. The computational demands of this technique are such that only the most symmetrical zeolite lattices are accessible at present, but this limitation is sure to change in the future. Pseudopotentials were used to model the core electrons, verified by reproduction of the lattice parameter of a-quartz and the gas-phase geometry of methanol. In chabazite, methanol was found to be adsorbed in the 8-ring channel of the structure. The optimized structure corresponds to the ion-paired complex, previously designated as a saddle point on the basis of cluster calculations. No stable minimum was found corresponding to the neutral complex. Shah et al. (237) concluded that any barrier to protonation is more than compensated for by the electrostatic potential within the 8-ring. 

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