Aluminosilicates surface clusters

A simple cluster model of a bridged hydroxyl group in a zeolite is cluster 3. Such a cluster with A = H was used by Chuvylkin et al. (70) as early as 1975 to discuss the properties of possible intermediate structures in the catalytic isomerization of butenes on aluminosilicate surfaces in terms of CNDO/2 approximation. Mikheikin et al. (34) have used a similar cluster with terminal pseudo-atoms A to study the Bronsted acidity of zeolites and its dependence on the Si/AI ratio. 

Amorphous aluminosilicates represent a wide variety of systems with surfaces whose states are strongly dependent on the biography of a system. The very specificity of the amorphous structures and the more limited possibility of application of physical methods to their study results in much poorer knowledge about the structures of the amorphous aluminosilicate surfaces than in the case of crystalline systems. This makes quantum-chemical treatment considerably more qualitative in this case. Cluster models of active sites appear here mainly a priori and experimentally independent and provide, to some extent, an additional way of studying such systems. 

One more type of cluster can be obtained by intracluster saturation of the broken bonds (41). This model was used in the comparative description of the surface centers of aluminophosphates and aluminosilicates (see Section V). 

There are three traditional structures usually adopted as probable BASs in amorphous aluminosilicates a water molecule coordinated by an electron-acceptor center (I), a bridged OH group (II), and a surface H30 + ion (III) (125,126). The catalytic activity of these sites is obviously determined by their properties and surface concentrations. Pelmenshchikov et al. (127) have attempted to compare these characteristics for the above types of BAS in aluminosilicates in terms of the cluster approach. For this purpose they considered a sequence of states of the model fragment of a dehydroxylated surface plus two water molecules (Fig. 15). State S0 corresponds to a dehydroxylated surface, states S, Sn, and Sm represent the sites of the I, II, and III types and states Sla and SIla correspond to centers I and II at a higher coverage. The relative energies of these structures obtained using the CTP scheme and the CNDO/BW technique are presented in Fig. 15. The relative surface density of the sites, og(nJnf), was estimated as the relative probability of their occurrence ... 

Kramer and co-workers used ab initio calculations of H4TO4 (T = Si, Al, P) clusters to derive parameters for the rigid ion potential model. The potential energy surface of the clusters was scanned along two modes of distortion, and the resulting potential curves were fitted using Eq. [15]. The set of parameters was refined by the use of experimental data on a-quartz. This procedure resulted in a parameterization that well reproduced both structure and elastic moduli of silicates, aluminosilicates, and aluminophosphates. Subsequently, this approach was extended to protonated forms of zeolites. ... 

Si 3H CP/MAS-NMR was used to probe interactions of transition metal carbonyl clusters (Ru3H(CO)M, Os2FI(CO)ii, Co(CO)4 ) deposited in the mesoporous aluminosilicate material MCM-41.637 A 29Si MAS-NMR study has been made of rhodium-amine complexes on Si02 surfaces.638 31P CP/MAS-NMR spectroscopy was able to characterise Cu6(TePh)6(PPh2Et)5 clusters in the pores of MCM-41.639... 

---